WO2016115640A1

WO2016115640A1 - Methods and apparatus for creation of wrinkles in three-dimensional surfaces, and compositions of matter resulting from same

Info

Publication number: WO2016115640A1
Application number: PCT/CA2016/050056
Authority: WO
Inventors: Minggan LI; Dae Kun HWANG; Janusz A. Kozinski
Original assignee: Li Minggan; Hwang Dae Kun; Kozinski Janusz A
Priority date: 2015-01-23
Filing date: 2016-01-22
Publication date: 2016-07-28

Abstract

The embodiments relate to methods for producing textures in a surface, apparatuses for performing such methods, and compositions of matter and products resulting from same. The method involves: exposing a photo-curable material comprising an uncured material to radiation, to produce a layer of partially-cured material between a cured region of the photo- curable material and a remaining uncured region of the photo-curable material, the cured region resulting from exposure of the photo-curable material to the radiation; substantially removing the remaining uncured region to reveal the surface, the surface being located on the layer of partially-cured material; and exposing the surface to charged ions, so that textures are formed therein. In some embodiments, the methods can be used to create textured particles.

Description

METHODS AND APPARATUS FOR CREATION OF WRINKLES IN THREE- DIMENSIONAL SURFACES, AND COMPOSITIONS OF MATTER RESULTING FROM

SAME

FIELD OF THE INVENTION

[0001] The present invention relates to methods and apparatus for material fabrication, and in particular, methods and apparatus for creating wrinkles in surfaces.

BACKGROUND

[0002] Surface wrinkles are ubiquitous in nature and are used as a strategy tool for efficient energy intake or for enhanced surface functionality. These naturally wrinkled systems can be found in various sizes and forms, mostly on non-planar structures, such as certain types of animal tissue, sea corals and plant leaves.

[0003] Artificially formed surface wrinkles which mimic such natural wrinkled systems at a micro/nano scale, may exhibit biophysical, mechanical, morphological, photonic, or surface properties. For example, such wrinkles may offer surface platforms for cell migration regulation, flexible electronics, solar cell, smart adhesive, tunable devices, colloid and protein assembly, and/or metrology methods.

[0004] Spontaneous surface wrinkling can be formed by the mechanical instabilities of a compressed thin film attached to a soft foundation. One strategy to construct surface wrinkles may be to thermally or mechanically expand a foundation and then deposit a thin layer of stiff materials onto the prestrained foundation, and optionally, modify the top skin layer with external sources such as flame, reactive ion etching, plasma, or UV ozone to increase the modulus of the skin layer. A subsequent release of the prestrained foundation yields compressive strain on the skin layer, resulting in surface wrinkles. [0005] Another strategy for surface wrinkling is to modify the skin layer without stretching the foundation. In this route, the skin layer swells when subject to solvent, ion beam, plasma, or laser, thereby creating surface wrinkles.

[0006] While both strategies can produce controllable patterned wrinkles, they are mainly deployed in flat or planar surfaces and not for fabrication of wrinkles in surfaces with three-dimensional contours such as raised bumps. (Such surfaces with three dimensional contours or features will be referred to below as "3D surfaces".) Depending on the contours of the 3D surfaces, the first strategy may introduce undesired localized stress concentrations on the 3D surface as it is globally stretched, with such localized stress concentrations negatively impacting uniform global wrinkle formation. The second strategy does not provide any 3D spatial control.

[0007] There is thus a need for improved methods of creating wrinkles on 3D surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Non-limiting examples of various embodiments of the present invention will next be described in relation to the drawings, in which:

[0009] FIG. 1 is an illustration of a close-up illustrative cross-section of an apparatus for performing photolithography used for the creation of wrinkles in a three dimensional surface, in accordance with at least one embodiment of the present invention;

[0010] FIG. 2 are illustrative cross-sections of a prepolymer solution, first subject to the lithography process of FIG. 1, then exposed to plasma, in accordance with at least one embodiment of the present invention;

[0011] FIG. 3 is a diagram illustrating the variation of monomer concentration percentage with the duration of ultraviolet (UV) exposure time according to a simulation, in accordance with at least one embodiment of the present invention; [0012] FIG. 4A is a diagram illustrating the variation of monomer concentration percentage with the proportion of channel height for various UV exposure times, in accordance with at least one embodiment of the present invention;

[0013] FIG. 4B is a diagram illustrating the variation in wrinkle characteristics with monomer concentration percentage, in accordance with at least one embodiment of the present invention;

[0014] FIG. 5 is a scanning electron microscopy (SEM) image showing use of a dark-field photomask during photolithography for producing positive 3D surfaces on which wrinkles can be created, in accordance with at least one embodiment of the present invention;

[0015] FIGS. 6A - 6B are SEM images showing use of a bright-field photomask during photolithography for producing negative 3D surfaces on which wrinkles can be created, in accordance with at least one embodiment of the present invention;

[0016] FIGS. 7A - 7B are close-up illustrative cross-sections of an apparatus for performing photolithography in a multi-stage photolithography process for creating different types of wrinkle characteristics on a 3D surface, in accordance with at least one embodiment of the present invention;

[0017] FIGS. 8A - 8B are SEM images showing 3D surfaces with different types of wrinkles achieved using the multi-stage lithography process of FIGS. 7A - 7B, in accordance with various embodiments of the present invention;

[0018] FIGS. 9A - 9B are SEM images showing wrinkled 3D posts that may be created using the multi-stage lithography process of FIGS. 7 A -7B, in accordance with various embodiments of the present invention;

[0019] FIGS. 10A - 10B are cross-sectional illustrations of a lithography process that employs off- focus UV to create features in the 3D surface, on which wrinkles can be created, in accordance with at least one embodiment of the present invention;

[0020] FIGS. IOC - 10D are SEM images showing wrinkles on 3D surfaces that may be created using the off-focus photolithography process of FIGS. 10A - 10B, in accordance with at least one embodiment of the present invention; [0021] FIGS. 11A - 11C are SEM images showing various wrinkled 3D surfaces that may be created using the methods described herein, in accordance with various embodiments of the present invention;

[0022] FIG. 12 is an SEM image showing a 3D surface with a grid of wrinkled posts being positioned on a smooth planar surface, in accordance at least one embodiment of the present invention;

[0023] FIGS. 13A - 13C are SEM images showing cells being attracted to the wrinkled posts of FIG. 12, and how cells may attach to wrinkled posts and bridge across multiple wrinkled posts, in accordance with various embodiments of the present invention;

[0024] FIGS. 14A - 14B are SEM images that show cells spreading on the base and not on posts when posts of a 3D surface are not wrinkled, in accordance with various embodiments of the present invention;

[0025] FIG. 15 is an illustrative perspective view an apparatus for performing photolithography used for the creation of wrinkles in a three dimensional surface, in accordance with at least one embodiment of the present invention;

[0026] FIGS. 16A - 16B are illustrations of different wrinkle characteristics that may be achieved when creating wrinkles in a three dimensional surface, in accordance with at least one embodiment of the present invention;

[0027] FIG.17 is a schematic for a setup for performing stop flow lithography (SFL) as a part of a process for creating textured particles, in accordance with at least one embodiment of the present invention;

[0028] FIG. 18 is an illustration of a close-up illustrative cross-section of an apparatus for performing SFL used for the creation of textured particles, in accordance with at least one embodiment of the present invention;

[0029] FIG. 19 is an illustrative cross-section of a particle produced by the SFL process of FIG. 18 being exposed to plasma to create a textured particle, in accordance with at least one embodiment of the present invention; [0030] FIG. 20 are SEM images showing textured particles having various shapes that may be created using the methods described herein, in accordance with various embodiments of the present invention;

[0031] FIGS. 21A - 21C are SEM images showing particles having various texture characteristics resulting from variations in duration of radiation exposure, in accordance with at least one embodiment of the present invention;

[0032] FIG. 2 ID is a diagram illustrating the variation in texture characteristics with duration of radiation exposure, in accordance with at least one embodiment of the present invention;

[0033] FIGS. 22A - 22D are SEM images showing particles having variations in texture characteristics resulting from variations of the monomer concentration used in the rinsing agent, in accordance with at least one embodiment of the present invention; and

[0034] FIGS. 23A - 23F are SEM images showing cells attaching to textured particles, in accordance with at least one embodiment of the present invention.

DETAILED DESCRIPTION

[0035] In a broad aspect of the invention, there is provided a method for producing textures in a surface, the method including: exposing a photo-curable material including an uncured material to radiation, to produce a layer of partially-cured material between a cured region of the photo-curable material and a remaining uncured region of the photo-curable material, the cured region resulting from exposure of the photo-curable material to the radiation; substantially removing the remaining uncured region to reveal the surface, the surface being located on the layer of partially-cured material; and exposing the surface to charged ions, so that textures are formed therein.

[0036] In some embodiments, the uncured material includes uncured monomers, the cured region includes polymers, and the partially-cured material includes partially-polymerized material. In some embodiments, the textures comprise wrinkles. [0037] In some embodiments, when performing the exposing of the photo-curable material to the radiation, the method further includes controlling a duration of time the photo-curable material is exposed to the radiation, to manipulate a gradient of uncured monomer concentration produced in the layer of partially-cured material.

[0038] In some embodiments, the exposing is performed during a photolithography process, and a photomask is used in the photolithography process to selectively limit exposure of the photo-curable material to the radiation, to control characteristics of the cured region.

[0039] In some embodiments, after the exposing, the method further includes performing an additional exposure of the photo-curable material to the radiation, wherein an alternate photomask is used during such additional exposure to create variations in heights of the cured region.

[0040] In some embodiments, a source of the radiation is used during the exposing, and the method further includes altering a focal plane of the source of the radiation to create a non-cylindrical beam of the radiation for curing the uncured material during the exposing, so that the cured region produced from the exposing is non-cylindrical.

[0041] In some embodiments, an additional source of the radiation is positioned proximate to the source of the radiation during the exposing, and the radiation from the source of the radiation and the additional source of the radiation overlap to cure the uncured material during the exposing.

[0042] In some embodiments, the method further includes exposing the textured surface to cells so that the cells attach to the textured surface.

[0043] In some embodiments, during the exposing, a layer of curation-inhibiting material is positioned between a source of the radiation and the photo-curable material, such that the cured region produced from the exposing forms a particle enveloped within the layer of partially-cured material.

[0044] In another broad aspect of the invention, there is provided a composition of matter including textures on a surface, the composition of matter having been generated by: exposing a photo-curable material including an uncured material to radiation, to produce a layer of partially-cured material between a cured region of the photo-curable material and a remaining uncured region of the photo- curable material, the cured region resulting from exposure of the photo-curable material to the radiation; substantially removing the remaining uncured region to reveal the surface, the surface being located on the layer of partially-cured material; and exposing the surface to charged ions, so that textures are formed therein.

[0045] In some embodiments, the uncured material includes uncured monomers, the cured region includes polymers, and the partially-cured material includes partially-polymerized material. In some embodiments, the textures comprise wrinkles.

[0046] In some embodiments, when performing the exposing of the photo-curable material to the radiation, there is a controlling of a duration of time the photo-curable material is exposed to the radiation, to manipulate a gradient of uncured monomer concentration produced in the layer of partially-cured material.

[0047] In some embodiments, the exposing is performed during a photolithography process, and a photomask is used in the photolithography process to selectively limit exposure of the photo-curable material to the radiation, to control characteristics of the cured region.

[0048] In some embodiments, after the exposing, there is an additional exposure of the photo- curable material to the radiation, and wherein an alternate photomask is used during such additional exposure to create variations in heights of the cured region.

[0049] In some embodiments, a source of the radiation is used during the exposing, and a focal plane of the source of the radiation is altered to create a non-cylindrical beam of the radiation for curing the uncured material during the exposing, so that the cured region produced from the exposing is non-cylindrical.

[0050] In some embodiments, an additional source of the radiation is positioned proximate to the source of the radiation during the exposing, and the radiation from the source of the radiation and the additional source of the radiation overlap to cure the uncured material during the exposing.

[0051] In some embodiments, cells are attached to the textured surface as a result the textured surface having been exposed to the cells. [0052] In some embodiments, during the exposing, a layer of curation-inhibiting material is positioned between a source of the radiation and the photo-curable material, such that the cured region produced from the exposing forms a particle enveloped within the layer of partially-cured material.

[0053] In another broad aspect of the invention, there is provided a product when made by the methods described herein.

[0054] In another broad aspect of the invention, there is provided an apparatus for producing partially-cured material, the apparatus including: a source of radiation; and a substrate for holding a photo-curable material including uncured material, wherein at least a portion of the photo-curable material is exposed to the radiation; wherein, upon exposure of the photo-curable material to the radiation, a layer of partially-cured material is produced between a cured region of the photo-curable material and a remaining uncured region of the photo-curable material, the cured region resulting from exposure of the photo-curable material to the radiation.

[0055] In some embodiments, a photomask is positioned between the source of the radiation and the substrate, the photomask being used to selectively limit exposure of the photo-curable material to the radiation, to control characteristics of the cured region.

[0056] In some embodiments, when exposing the photo-curable material to the radiation, the apparatus provides a mechanism for controlling a duration of time the photo-curable material is exposed to the radiation, to manipulate a gradient of uncured monomer concentration produced in the layer of partially-cured material.

[0057] In some embodiments, a focal plane of the radiation source is alterable to create a non- cylindrical beam of the radiation that the uncured material is exposed to, so that the cured region produced from the exposing is non-cylindrical.

[0058] In some embodiments, the apparatus includes an additional source of the radiation positioned proximate to the source of the radiation, wherein upon exposure of the photo-curable material to the radiation, the radiation from the source of the radiation and the additional source of the radiation overlap to cure the uncured material. [0059] In some embodiments, the apparatus includes a layer of curation-inhibiting material positioned between a source of the radiation and the photo-curable material, such that the cured region produced from the exposing forms a particle enveloped within the layer of partially-cured material.

[0060] Aspects and advantages of the present invention will be apparent in view of the description which follows. It should be understood, however, that the detailed description, while indicating specific embodiments of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

[0061] In the following description, details are set forth to provide an understanding of illustrative embodiments of the present invention. In some instances, certain steps and techniques have not been described or shown in detail in order not to obscure the invention. Those of skill in the art will understand that the following description of illustrative embodiments of the present invention does not limit the implementation of embodiments of the present invention.

Creation of Wrinkles on 3D Surfaces

[0062] From a high-level perspective, the methods herein include the following main steps: (i) performing photolithography on a photo-curable solution in a controlled manner so as to produce a layer of partially-polymerized material in between the cured body resulting from the UV exposure and the uncured monomers, (ii) removing the uncured monomers to reveal the layer of partially- polymerized material; and (iii) exposing the partially-polymerized material to plasma to form wrinkles on the surface of the cured body created from the photolithography.

[0063] Referring to FIG. 1, there is shown an illustration of a close-up cross-section of an apparatus for performing the photolithography process of the present methods, in accordance with at least one embodiment of the present invention. The apparatus may be set up to include a prepolymer solution 104 containing monomers (e.g., poly(ethylene glycol) diacrylate (PEG-DA) or other suitable photo- curable material) being placed between a substrate 106 (typically made of a material that allows UV radiation to pass through, such as glass) and an inert material such as polydimethylsiloxane (PDMS), a silicon-based polymer. As will be understood, employing PDMS in this manner can be understood to be employing a method of soft lithography.

[0064] Referring simultaneously to FIG. 15, shown there is another cross-section of an apparatus for performing photolithography used for the creation of wrinkles in a three dimensional surface, in accordance with at least one embodiment of the present invention. FIG. 15 shows an example zoomed-out perspective of the photolithography apparatus, the illustration showing an example relative scale amongst a PDMS slab 180, the channel or groove 102 formed within the PDMS slab 180, and a channel height 130 of the channel 102. As discussed below, the channel height 130 can be configured so as to define the height(s) of the polymerized 3D micr/nano structures that are desired to be created during the photolithography process.

[0065] PDMS is 0₂ permeable. As discussed below, the permeability of PDMS to 0₂ allows for a partially-cured layer to be formed during polymerization due to oxygen inhibiting the polymerization process. As used herein, the term "PDMS channel" refers to the channel 102 that may be created in the PDMS material, and which may be filled with prepolymer solution 104 during photolithography. While PDMS is discussed as an example material that may be used, other 0₂ permeable materials may also be used in an analogous manner to achieve the outcomes described herein. An example process for creating the PDMS channel 102 is discussed below in the Example Laboratory Setup section.

[0066] Referring back to FIG. 1, as shown, the PDMS is formed into a slit PDMS channel 102 having a channel height 130 that can house the prepolymer solution 104 during the photolithography process.

[0067] A photomask 108 may be provided to control the exposure of the UV radiation to the prepolymer solution 104. By projecting UV light 110 through areas allowed by the photomask 108, the portion of the prepolymer solution 104 exposed to the UV light 110 will become polymerized, so as to create the cured body 112 of polymers.

[0068] As will be understood by persons skilled in the art, a photoinitiator (not shown) may be added to the prepolymer solution 104 to facilitate the polymerization reaction. Once the photoinitiator is added to the prepolymer solution, exposure of the prepolymer solution 104 to the UV light 1 10 will create free radicals that will react with oxygen present in the prepolymer solution 104. Upon depletion of the oxygen, polymerization can proceed as the generated free radicals will crosslink monomers instead of react with the oxygen. Examples of photoinitiators which can be used include: 1-Hydroxy-cyclohexyl-phenyl-ketone (also known as IRGACURE™ 184, available from BASF SE of Ludwigshafen, Germany, referenced as "BASF" below), 2-Hydroxy-2-methyl-l- phenyl -1 -propanone (also known as DAROCUR 1 173, available from BASF), 2-Hydroxy-l-[4-(2- hydroxyethoxy)phenyl]-2-methyl-l-propanone (also known as IRGACURE™ 2959, available from BASF), 2-Benzyl-2-(dimethylamino)-l-[4-(4-mo holinyl)phenyl]-l-butanone (also known as IRGACURE™ 369, available from BASF), and 2-Methyl-l-[4-(methylthio)phenyl]-2-(4- morpholinyl)- 1-propanone (also known as IRGACURE™ 907, available from BASF).

[0069] After the UV light 1 10 is activated, the polymerization of the prepolymer solution 104 starts from the substrate 106 side of the prepolymer solution 104, and expands with the duration of UV exposure. During the polymerization process, a partially-cured-polymer (PCP) layer 1 14 is created at the point where oxygen in the prepolymer solution 104 is being depleted by the free radicals produced by the photoinitiator in response to the UV radiation 1 10. When the UV light 1 10 is deactivated, the polymerization ceases and the partially-cured-polymer (PCP) layer 1 14 remains at the interface between the cured body 1 12 of polymers and the remaining unpolymerized monomers in the prepolymer solution 104. This PCP layer 104 may contain a semi-crosslinked poly(ethylene glycol) (PEG) polymer network, with uncured monomers trapped inside the network. As will be discussed in detail below, exposure of this PCP layer 104 to plasma results in the creation of a textured surface (e.g., a surface having wrinkles) on the cured body 1 12 of polymers.

[0070] As noted, the initial presence of oxygen in the prepolymer solution 104 inhibits polymerization until the oxygen is gradually depleted. This depletion of oxygen is counteracted by the continuous diffusion of oxygen 1 16 from the surrounding environment via the PDMS channel 102. This may cause portions of the prepolymer solution 104 near or adj acent the PDMS channel 102 to be less susceptible to polymerization or remain unpolymerized. It may also contribute to a gradient in the amount of polymerization within the PCP layer 1 14, with the amount of polymerization decreasing as the distance from the substrate 106 increases. As will be discussed below with respect to FIGS. 3 and 4 A, the thickness and gradient of polymerization of the PCP layer 1 14 may also be affected by the duration of UV exposure. [0071] Referring to FIG. 2, shown there are illustrative cross-sections of a prepolymer solution, first subject to the lithography process of FIG. 1, then exposed to plasma, in accordance with at least one embodiment. Shown on the left is another close-up illustrative cross-sectional view of the photolithographic setup described above with respect to FIG. 1. The prepolymer solution 104 is polymerized by UV radiation 110 permitted through photomask 108 to create a cured body 112 of polymers and a partially-cured layer 114.

[0072] After completing the photolithography step, the PDMS channel 102 can be removed and the sample can be rinsed using a rinsing agent to remove the uncured monomers 104. In one example embodiment of the present invention, ethanol may be used as the rinsing agent. The rinsing may be performed in a measured, progressive manner (e.g., in a drop by drop basis) so as to not flush away, but retain the monomers trapped in the PCP layer 114.

[0073] For example, the rinsing agent (e.g., ethanol) may be introduced to the sample via a pipette in a drop by drop manner where each drop is 3 μΐ.. The entire volume of rinsing agent used to complete the rinsing, as well as the duration of time necessary to complete the rinsing, would depend on the amount of uncured prepolymerized solution 104 that needs to be removed. This, in turn, would vary based, for example, on the height of the PDMS channel 102, the duration of UV exposure time, and the amount of monomer residuals that is desired (as that impacts the nature of the wrinkles that will be formed). As discussed below in the Example Laboratory Setup section, a total volume of 300 - 600 μΙ_, of ethanol may need to be used during rinsing to reveal the underlying PCP layer 114. For example, to form the wrinkes shown in FIGS. 5, 6A, 6B, 8A, 8B, 9A, 9B, IOC, 10D, and 11 A-13C, the volumetric ratio of the amount of the rinsing agent (ethanol) used versus the amount of the pre-lithography prepolymer solution is about 100: 1.

[0074] As the purpose of the rinsing is to keep certain uncured monomers in the PCP layer 114 after washing away the uncured monomers 104, it will be appreciated by the person skilled in the art that the rinsing method may be modified while still achieving this effect. For example, it may be possible to use a different rinsing agent instead of ethanol, such as water or some other solution (e.g., a PEG concentrate solution). As such different rinsing agents may have different chemical properties, the duration and volume of rinsing agent expended during the rinsing process may be adjusted accordingly in order to achieve the desired effect. [0075] For example, as discussed below in relation to the creation of textured particles, in some embodiments, a percentage concentration of PEG-DA solution may be used as the rinsing agent to have better control of the rinsing process. In some cases, 5%, 10%, 25% PEG-DA solution may be used to help ensure that a corresponding percentage of monomers were retained on the surface (e.g., if a 10%) PEG-DA solution is used as the rinsing agent, 10%> monomer concentration may be retained on the surface; likewise, if a 20% PEG-DA solution is used as the rinsing agent, then 20% monomer concentration may be retained on the surface)

[0076] Once the desirable amount of the PCP layer 114 is revealed, it can be treated with plasma 202 to turn the uncured monomers 114 into a thin crust (skin layer) bearing in-plane compressive strains, so as to produce wrinkles on the surface of the underlying cured body 112 of polymers. While plasma is being described as being used herein, other highly charged ions can also be used to induce the reaction achieved with plasma. The plasma reacts with the monomers to crosslink them to form a wrinkled polymer by way of a radical reaction. As this happens, a volumetric expansion occurs to create wrinkles. Notably, whereas polymerization during photolithography only crosslinks monomers, it does not cause swelling in the manner that polymerization due to plasma exposure does. Thus, the polymerization performed during exposure of the PCP layer 114 to plasma causes the creation of wrinkles, whereas photopolymerization does not. As discussed below in the Example Laboratory Setup section, a plasma cleaner (e.g., model # PDC-32G available from Harrick Plasma of Ithaca, New York, USA) may be used during the plasma exposure process.

[0077] In the photolithography process, the characteristics of the photomask 108 define the area of prepolymer solution 104 that is exposed to UV light 110, and as a result, which portions of the polymer solution 104 may be polymerized to form the 3D microstructures underlying the PCP layer 114. At the same time, the duration of UV exposure impacts the amount of polymerization that occurs in these areas, and therefore affects the height of the resultant 3D microstructures. The duration of UV exposure may further impact monomer conversion rates in the PCP layer 114 (as is discussed below in relation to FIGS. 3 - 4B).

[0078] Together, the configuration of the photomask 108 and duration of UV exposure govern: (i) the configuration of the microstructures in a given 3D surfaces (e.g., where the 3D microstructures are located and/or how high they may rise), and (ii) the characteristics of the wrinkles on the 3D surfaces themselves (e.g., the wavelength of a given wrinkle). Using these factors, the formation of wrinkles on 2D surface (e.g., planar) or 3D surface (e.g., planar with various raised microstructure features) can be spatially tunable and controllable. The configuration of these two factors will now be discussed.

Modifying UV Exposure Duration to Control Monomer Residuals in the PCP Layer and Wrinkle Characteristics

[0079] Referring to FIG. 3, shown there is a diagram illustrating the variation of monomer concentration percentage with the duration of ultraviolet (UV) exposure time, in accordance with at least one embodiment of the present invention. The diagrams in FIG. 3 are generated from simulations based on mathematical models discussed below. As used herein, the term "monomer concentration percentage" refers to the ratio of the uncured monomers present at a given point in the sample (e.g., a given height) after UV exposure versus the uncured monomers present at the same point in the sample before UV exposure. Put another way, the term "monomer concentration percentage" as used herein can be considered to be a percentage of the original monomers that remain after UV exposure, at a given point or height in the sample.

[0080] As noted in relation to FIG. 1, in a PDMS channel 102, as UV light 110 is applied, free radicals produced by the photoinitiator crosslink the monomers, while oxygen 116 diffusing from walls of the PDMS channel 102 inhibits polymerization of the prepolymer solution 104. The mass transport equations for oxygen and monomer were used to obtain the expressions for the specie variation in time and space and the simulations were performed by solving these equations in the Comsol 4.3b modelling software (available from Comsol, Inc. of Burlington, Massachusetts, USA, referred to below as "Comsol"). The dimensionless oxygen concentration (σ) is:

(1)

where,

¹ 2k_tD₀ ' fc¾ [0₂,_e?i)]^{2 L J}

H is the channel height, [0_{2 eqb}] is the equilibrium concentration of oxygen in the precursor solution, k_t is the termination reaction constant, φ is the quantum yield of formation of free-radicals, and [PI] is photoinitiator concentrations. I₀ is the intensity of incoming light, ε is the extinction coefficient of photoinitiator. [0₂] is the concentration of oxygen, D₀ the diffusivity of oxygen in the precursor solution, and k₀ the oxidation reaction constant.

The dimensionless monomer concentration percentage (ξ) is:

- f = D_a (-a + ² + a /^'exp(- ?0), (2) where k_p is the polymerization reaction constant and [ ] is the concentration of monomer in the precursor solution, ξ = -^- with [ ₀1 is the initial monomer concentration. Ζ , = ^k°^{kp H} ^°²'^e(t^b^

[M₀] ^{L 0 J fl}2 2k_tD₀

The detailed derivation of the equations and the solutions of these equations can be found in N. Hakimi, S.S. Tsai, C.H. Cheng and D.K. Hwang, Advance Materials, 2014, Vol. 26, Iss. 9, pp. 1393-1398 ("Hakimi"), the contents of which are incorporated by reference. Most of the values of the parameters can be found in Hakimi, except the channel height (30 μπι).

[0081] Referring still to FIG. 3, a scale for the monomer concentration percentage is provided on the right-hand side, wherein a gradient of monomer concentration percentage from 0.80 to 1 is illustrated via a spectrum of cross-hatchings that represent monomer concentrations. While the gradient is shown in discrete cross-hatchings in FIG. 3 for the purpose of illustration, it will be understood by persons skilled the art that the gradient may in actuality be a continuous gradient without the illustrated delimited zones of monomer concentrations. These cross-hatchings are reflected on the five constituent diagrams of FIG. 3, each showing the thickness of the PCP layer 114 for a given duration of UV exposure. For the 300ms UV exposure duration, the PCP layer 114a is the thickest, and has the largest gradient of monomer concentration percentage. As the UV exposure duration increases to 400ms, 500ms, and 700ms, it can be seen that the corresponding PCP layers 114b, 114c, and 114d respectively get progressively thinner, with the gradient span in the monomer concentration percentage decreasing. It can also be seen that the cured polymer structure grows. At the 2s UV exposure duration, it can be seen that the modeled PCP layer 114e is the thinnest, with the narrowest of monomer concentration gradient.

[0082] FIGS. 4 A - 4B illustrate information relating to the application of the above-described wrinkling-fabrication method to a planar film with no underlying 3D microstructures created using a photomask 108. By not using a photomask 108 to create 3D microstructures, an understanding of the correlation between the operational conditions of the lithography process and wrinkle formation can be obtained. The example experimental conditions for the information in FIGS. 4A - 4B involve a PDMS channel being filled with a PEG-DA photocurable solution, so that UV exposure time can regulate the gradient of PEG-DA polymerization which, in turn, determines the thickness of the cured foundation 112 and the monomer concentration percentage in the PCP layer 114.

[0083] Referring to FIG. 4A, shown there is a diagram 402 illustrating the variation of dimensionless monomer concentration percentage (ξ) with the proportion of channel height for various UV exposure times, in accordance with at least one embodiment of the present invention. Referring briefly back to FIG. 1, the channel height 130 can be considered the distance between the substrate 106 and the PDMS material at any given point along the substrate 106.

[0084] As illustrated, the diagram shows a normalized channel height along the horizontal axis and the monomer concentration percentage along the vertical axis. The relationship between monomer concentration percentage versus normalized channel height is then plotted for each of five representative UV exposure durations during the lithography process.

[0085] The chart shows that monomer concentration percentages in the lower portion of the channel height (e.g., from 0 to 0.5 normalized channel height) are lower for a longer UV exposure durations (e.g., 90% of the original monomers remain for a UV exposure duration of 400ms, but 70% of the original monomers remain for a UV exposure duration of 700ms). This is due to the longer UV exposure duration causing a larger amount of polymerization, so as to crosslink more monomers and reduce the monomer concentration percentage.

[0086] As illustrated, it can also be seen that for each of the shorter durations (300ms, 400ms, 500ms, and 700ms), the curves have a similar shape in which each has a relatively consistent level of monomer concentration percentage for the lower portion of the channel height and at some point in the channel height, these curves turn upward and the monomer concentration percentage increases towards 100% of all original monomers remaining. These curvatures reflect the gradient in the PCP layer 1 14 discussed above with respect to FIG. 3.

[0087] For the longest UV exposure duration of 2s, the lengthy UV exposure would have caused a significant amount of the original monomers to be polymerized so that the percentage of original monomers remaining is low for a significant portion of the channel height. However, as discussed above in relation to FIG. 1, near the top of the channel height, due to diffusion of oxygen 1 16 through the PDMS material inhibiting further polymerization, the monomer concentration percentage becomes high near the top of the normalized channel height.

[0088] Notably, in regions where at least 2% of the monomers are polymerized, such regions would solidify into a gel so as not to be easily rinsed away. Put another way, such gel-like regions can be considered to be regions where monomer concentration percentage is lower than 0.98 (e.g., regions where less than 98% of the original monomers are remaining), and where there is more than 2% monomer conversion (1 - ξ ). The region(s) with less than 2 % monomer conversion (e.g., regions where 98%- 100% of the original monomers remain present) are still in liquid form and can be rinsed away easily.

[0089] Referring to FIG. 4B, shown there is a diagram 450 illustrating how wrinkle characteristics may vary with monomer concentration percentage, in accordance with at least one embodiment of the present invention. As discussed above, a longer UV exposure depletes more oxygen and results in the crosslinking of more monomers, leaving less unconverted monomers in the PCP layer 1 14. In some experiments, increasing the UV exposure time from 300 ms to 700 ms decreases the unconverted monomer concentration percentage from 40% to 6% in the skin layer after rinsing, and further increasing the exposure time to 2 s leads to zero monomer residual as measured by Attenuated Total Reflectance - Fourier Transform Infrared Spectroscopy (ATR-FTIR). When exposed to plasma, these different concentrations of the unconverted monomer on the surface turn into wrinkles with different wavelengths. The wavelength of the wrinkles, Λ, can be approximated by λ « 2π/ι the thickness of the skin layer, and E_s and Ef are the Young's

modulus of the skin layer and the foundation, respectively. As noted above, UV exposure time determines the thickness of the skin layer and Young's modulus of both the skin layer and the foundation. Thus, the UV exposure time may have a direct impact in determining the wavelength of the wrinkles.

[0090] In a set of example results shown in FIG. 4B, the insets show the variation of wrinkle wavelength with respect to UV exposure time. As shown, the monomer concentration percentage may decrease as exposure time increases, resulting in wrinkle wavelengths which range from 2 μιη to zero. For example, the inset 452 shows that for a monomer concentration percentage of between 35-45%, the wrinkle wavelength may be around 2 μιη. As monomer concentration percentage decreases from 25-30%, to 12-20%, and to 2-10%, the wrinkle wavelength may also correspondingly decrease from 1.5 μιη (inset 454), 1 μιη (inset 456), and 0.6 μιη (inset 458) respectively, eventually reaching a smooth surface with no wrinkles when the monomer concentration percentage is 0% (inset 460). It can also be seen from insets 452, 454, 456, 458, 460 that the wrinkles may exhibit various morphologies - random, labyrinth, dimples, and smooth.

Modification of Photomask during Photolithography to Control Characteristics of 3D Surface

[0091] Different photomask configurations can be used to create different underlying 3D microstructures on which wrinkles can be created.

[0092] Referring to FIG. 5, shown there generally as 500 is a scanning electron microscope (SEM) image showing use of a dark-field photomask during photolithography for producing positive 3D surfaces (e.g., a surface with bumps or protrusions from the plane of the substrate 106) on which wrinkles can be created, in accordance with at least one embodiment of the present invention. As shown, the photomask 502 may contain a number of evenly-spaced circular openings or apertures. Referring back briefly to FIG. 1, these holes allow UV radiation 110 through during the photolithography process, so as to polymerize the prepolymer solution 104 and create a cured body 112 on which the PCP layer 114 is formed. Use of the photomask 502 would produce the 3D surface shown in the SEM image 500 with cone-shaped 3D microstructures 504. In this way, a wrinkled 3D surface that is similar to morphologies of naturally wrinkled surfaces can be created. For example, the wrinkled 3D surface shown in FIG. 5 has characteristics similar to the leaf scales of the yellow pitcher plant. In nature, these scales play a key role in trapping insects. [0093] FIG. 6A - 6B are SEM images showing use of a bright-field photomask during photolithography for producing negative 3D surfaces (e.g., a surface with holes or pockets) on which wrinkles can be created, in accordance with various embodiments of the present invention.

[0094] Referring to FIG. 6A, shown there generally as 600a is an SEM image of a 3D microstructure that is a microwell that can be generated using the bright field photomask 602a. Using the photomask 602a, the portions of the prepolymer solution 104 exposed to the UV radiation 110 would surround the four dark squares in the photomask 602a, causing such portions to rise and create cured body 112 (as shown in FIG. 1). In doing so, a 3D microstructure that is the square- shaped microwell 604a may be created. In one example, with a 400ms single UV exposure, a wrinkle wavelength of (λ = 1.5 μπι) can be formed on the surface of these microwells 604a.

[0095] The morphologies of the wrinkles on these 3D surfaces may depend on the foundation geometry of the 3D microstructures created by the cured bodies 112. For rectangular holes with edge size L = 20 μπι such as is shown in FIG. 6A, the wrinkles align in a direction which makes it appear as if they are stacked hoops or rings that circumscribe the interior of the rectangular hole. In this way, wrinkles on 3D surfaces can be created that have characteristics similar to wrinkled 3D surfaces in nature. For example, the aligned wrinkles in a tubular structure shown in FIG. 6A resemble many internal surface morphologies of a digestive tract of an animal, such as the endoderm layer of a small intestine (e.g., the jejunum), in which the wrinkles provide sufficient area for nutrient absorption.

[0096] Referring to FIG. 6B, shown there generally as 600b is an SEM image of a 3D microstructure generated using the photomask 602b. The bright field photomask 602b is similar to the photomask 602a of FIG. 6A, except the dark squares are smaller so as to reduce the hole size to holes with an edge of L = 10 μπι. Also, in the embodiment of the present invention shown in FIG. 6B, the UV exposure time is set at 400 ms to keep the wrinkle wavelength the same as was in the embodiment of the present invention shown in FIG. 6A.

[0097] Due to the smaller, more condensed size of the dark squares, there may be diffusion of free radicals between the openings in the photomask 602b, so as to alter the shape profile of the polymerized hole that is created. For example, polymerization may occur near the glass substrate 106 due to this diffusion, so as to "fill in" the bottom of the well and form pitted surfaces. The effect of forming a pitted surface can be further amplified if an off-focus UV source is used, as is described below with respect to FIGS. 10A - 10D.

[0098] Additionally or alternatively, upon plasma treatment, the skin PCP layer 114 on the top surface of the edges of the small microwell may fold up to extend the height of the edge of the microwell. This may extend the depth of the microwell up to 5 μπι as is shown in FIG. 6B. Referring to FIGS. 16A and 16B, shown there generally as 1600 and 1605 are illustrations of a wrinkle in FIG. 16 A, and how the wrinkle may be folded up in FIG. 16B to, for example, form a vertical extension of the underlying polymerized material.

Multi-stage Photolithography of Varying UV Exposure to Create a 3D Surface with Multiple Wrinkle Characteristics

[0099] The simple 3D microstructures discussed so far can be created in a single UV polymerization step. In alternative embodiments of the present invention, a two (or more) stage photolithography process may be used to create wrinkles with different morphologies and characteristics on the same 3D surface.

[00100] FIGS. 7A - 7B are illustrations of close-up cross-sections of an apparatus for performing photolithography when employing a multi-stage photolithography process. FIG. 7A shows generally as 700 a first stage of the multi-stage lithography process conducted using a long UV exposure. As will be appreciated, the cross section shown in FIG. 7A is similar to that which is shown in FIG. 1, with the prepolymer solution 104 resting on a substrate 106 being polymerized by UV light 110 shone through a first photomask 708a. In the configuration of FIG. 7A, the long UV exposure will result in a cured body 112 that comes close to filling the entirety of a first height Hi 710 of the PDMS channel 702a. As discussed above, the long UV exposure will polymerize most of the monomers in the prepolymer solution 104 and leave a small or negligible PCP layer 714a. After completion of the first-stage of the multi-stage lithography process shown in FIG. 7A, the first PDMS channel 702a with height Hi 710 may be removed.

[00101] Referring now to FIG. 7B, shown there generally as 750 is a second stage of the multi-stage lithography process that may be conducted using a second PDMS channel 702b with height H₂ 715. Once the PDMS channel 702b is reflooded with additional prepolymer solution 104, a different photomask 708b can be used to controllably cure the additional prepolymer solution 104 that fills the taller PDMS channel 702b having height H₂ 715. As illustrated, the photomask 708b in the second stage is narrower than the photomask 708a used in the first stage, such that a narrower portion of the additional prepolymer solution 104 can be cured on top of the existing cured body 112, and the step-wise cured body 112 shown in FIG. 7B can be created. As discussed above, the shorter UV exposure duration in the second-stage of the multi-stage photolithography process will result in a larger amount of uncured monomers in the PCP layer 714b resting above the portion of the cured body 112 created in the second stage of the photolithography process, as compared to the small or negligible amount of uncured monomers in the PCP layer 714a created after the first stage of the multi-stage photolithography process.

[00102] After completion of the multi-stage photolithography process, the cured body 112 will thus have different PCP layers 714a, 714b with different thickness and monomer concentrations, depending on the length of UV exposure that gave rise to the PCP layers 714a, 714b. When the excess uncured monomers 104 are washed off, and these different PCP layers 714a, 714b are subject to plasma exposure, different wrinkle characteristics will thus be produced.

[00103] In this way, the correlation between exposure time and wrinkle characteristics can be exploited to construct heterogeneous wrinkles on a 2D step-wise microstructure by using a two-step UV polymerization. UV exposure time can be controlled for each step to yield different monomer concentrations on the trenches and ridges of a given 3D microstructure, thereby producing heterogeneous wrinkles of varied wavelengths and morphologies on the patterned surface after plasma treatment. While the example discussed herein has the longer UV exposure photolithography step preceding the shorter UV exposure step, the inverse may also be possible. It may also be possible to have more than two exposure steps, with varying combination of exposure times to generate the desired combination of 3D microstructures and wrinkle characteristics.

[00104] Referring to FIG. 8A, shown there generally as 800 is a SEM image showing the multiple wrinkle types that can be created using the multi-stage photolithography process of FIGS. 7A-7B. As shown, the inset 802a shows a pictorial representation of the stepwise cured body 112 created using the multi-stage photolithography process. As shown, the concentration of monomers on the lower ledge is small or negligible, and the PCP layer 814a on the higher ledge is shown as having a thicker PCP layer. When such layers are exposed to plasma, this results in the lower ledge having little to no wrinkles (shown as the smooth surface 860) and the higher ledge having considerably more wrinkles (shown as the wrinkled surface 852).

[00105] The sharp delineation between the lower and higher ledge may impact the characteristics of the wrinkles that are formed thereon. For example, the raised nature of the upper ledge may release or reduce the impact of the compressive deformation forces that acts orthogonal to the length of the ledge. As a result, the PCP layer 814a on the upper ledge may be subject to compressive forces along the length of the ledge to a greater degree, so as to cause wrinkles that are well ordered (e.g., relatively straight, aligned, or substantially parallel).

[00106] Referring to FIG. 8B, shown there generally as 850 is another example SEM image showing multiple wrinkle types being created using the multi-stage photolithography process of FIGS. 7A-7B. As illustrated, the inset 802b shows another pictorial representation of a stepwise cured body 112 created using the multi-stage photolithography process. As shown, the concentration of monomers on the lower ledge 814b is greater than that for the lower ledge shown in FIG. 8A (e.g., as a result of a shorter UV exposure duration), and the PCP layer 814a on the higher ledge is shown as having a similar thick PCP layer as was shown in FIG. 8A. When such layers are exposed to plasma, this results in the lower ledge (shown as the surface 855 with wrinkles of short wavelengths) having more wrinkles than the corresponding smooth lower ledge (surface 860) in FIG. 8A. However, as the UV exposure time of the lower ledge in FIG. 8B was still longer than the UV exposure time of the upper ledge, these wrinkles are of a narrower wavelength than the wrinkles present on the higher ledge (surface 852 in FIG. 8B).

[00107] Referring to FIGS. 9A - 9B, shown there generally as 900a, 900b are SEM images with wrinkled 3D posts that may be created using the multi-stage polymerization process of FIGS. 7A-7B. For positive 3D microstructures such as cones, by using the multi-stage polymerization process, PCP layers can be selectively retained only on the positive features, allowing for the creation of wrinkled post arrays with a smooth base (called a "dummy layer" below). To do so, the base layer can be exposed to UV for a long duration such as 2 seconds, and the posts are "installed" by a short UV exposure of 400 ms. As discussed above, this two-step process selectively will effectively leave a PCP layer 114 on the post surfaces (the "upper ledges" described above) and no residual monomer on the dummy layer (the "lower ledges" described above). This results in wrinkled post arrays on a smooth base upon plasma treatment.

[00108] On larger posts (e.g., where the bottom diameter D = 60 μιη), with a wrinkle wavelength of 1.5 μιη (a feature ratio of the post size to the wrinkle wavelength: R = 40), the stress conditions in the pre-wrinkled skin layer approaches the conditions of a planar foundation. This results in a labyrinth wrinkle mode 904a on a planar surface, which is shown in FIG. 9A. The inset 902a highlights the wrinkle morphology shown.

[00109] Generally, the patterns of the wrinkle alignment rely on the competition of the in- plane compressive stresses, which can be predicted using numerical methods by taking into account the 3D curvature, the thickness of the wrinkling layer, and the elastic modulus of both the skin and substrate layers.

[00110] As the feature ratio R decreases to 20 (e.g., λ = 1.5 μπι, D = 30 μιη), the curvature takes into effect and longitudinal wrinkles 904b dominate the pattern. This is shown in FIG. 9B. The inset 902b of FIG. 9B highlight the wrinkle morphology shown. This curvature effect may be more prominent on a sharp cone, which can be generated by lifting the UV focal-plane (e.g., by -30 μιη). Modifications to the shape of the cured body 112 (as shown in FIG. 1) achieved by shifting the UV focal-plane are discussed below.

[00111] The shape and characteristics of wrinkles and how they are affected by various factors are more fully described in N. Bowden, S. Brittain, A. G. Evans, J. W. Hutchinson, G. M. Whitesides, "Spontaneous formation of ordered structures in thin films of metals supported on an elastomeric polymer" Nature, 1998, 393, page 164, the contents of which are incorporated herein by reference.

Employing Off-focus a UV Light Source to Create Features in the 3D Surface on which Wrinkles can be Created

[00112] FIGS. 10A - 10B are cross-sectional illustrations of a lithography process that employs off-focus UV to create features in the 3D surface on which wrinkles can be created, in accordance with at least one embodiment of the present invention. In the embodiments of the present invention discussed above, the plane of focus of the UV source 110 has generally been assumed to coincide with the prepolymer solution 104. In this traditional setup, it can be expected that the exposure of any given area of the prepolymer solution will be uniform throughout the height of the prepolymer solution 104 (e.g., the volume of prepolymer solution 104 exposed to the UV light is cylindrical). However, if the plane of focus of the UV source is modified (e.g., shifted up or down), the UV light travelling through the prepolymer solution may not be uniform along any given height of the prepolymer solution 104.

[00113] Referring to FIG. 10A, shown there generally as 1000 is a cross-sectional illustration of a lithography process where the focal plane of the UV light 110 has been raised above the prepolymer solution 104. As illustrated, it can be seen that the UV light 1 10 travelling through the prepolymer solution 104 may non-cylindrical and conical in nature (presuming the UV source is circular), with a greater area of the lower portion of the prepolymer solution 104 being exposed to the UV light 110, and a smaller area of the upper portion of the prepolymer solution 104 being exposed to the UV light 110. This may result in a conical polymer shape being created as a polymerized feature.

[00114] Referring to FIG. 10B, shown there generally as 1050 is a cross-sectional illustration of a lithography process where two proximate UV sources 110a, 110b both have their focal planes lifted. As illustrated, it can be seen that the overlapping of the two UV light sources as they travel through the prepolymer solution 104 may result in a merged polymerized feature instead of individual features from each of the UV sources. Notably, the cured body 112 having the polymerized feature does not form a ' V shape where the two light sources converge. Instead, due to diffusion of the free radicals created by the UV light, a rounded corner between the two ridges is formed.

[00115] Applying a modification of the focal plane of the UV source may impact the shape of 3D microstructures discussed above. For example, when creating microwells in the context of FIGS. 6A and 6B, raising the focal plane of the UV light would result in the creation of conical microwells. [00116] Referring to FIG. IOC, shown there generally as 1000c is a SEM image of a rectangular conical microwell created using the photomask 1002c. Such a conical microwell 1004c can be created by lifting the UV focal plane by about 30 μπι, and keeping the UV exposure time at 400 ms and the hole size at L = 20 μιη. Similar to the microwell shown in FIG. 6A, the wrinkles in the inner surface of this microwell 1004c continue to be aligned in a circular "stacked rings" hoop direction.

[00117] In the context of the wrinkled posts discussed with respect to FIGS. 9A - 9B, lifting the focal plane may cause a thin partially polymerized film being formed on the dummy layer. Shown generally as lOOOd in FIG. 10D, this may result in wrinkles 1004d forming that are aligned perfectly in the longitudinal direction and on the dummy layer itself. The inset 1002d is provided to highlight the wrinkle morphology.

[00118] As will be understood to persons skilled in the art, various combinations of the above factors may be employed to create a variety of 3D microstructures with wrinkles thereon. FIGS. 11 A - 11C show various example 3D microstructures with wrinkles.

[00119] For example, using a combination of off-focus polymerization and designed photomask patterns may allow for the wrinkled 3D microstructures of FIGS. 11A and 11B to be created. Referring to FIGS. 11A and 11B, shown there generally are SEM images showing the wrinkled 3D surfaces that result when the photomasks 1102a and 1102b are used respectively. When off-focus UV light 110 (as shown in FIG. 1) is also used in combination with such photomasks, merging of polymerized features may occur to created pitted surfaces 1120a, 1120b.

[00120] Referring to FIG. 11C, shown there generally as 1110c is an SEM image illustrating an even more complex wrinkled microstructure that may be created using the two-step polymerization discussed above. As illustrated, a smooth dummy layer is first created, and then a wrinkled second layer having the appearance of a maze is then installed using the photomask 1102c. Attraction of Cells to Wrinkled Posts of a 3D Surface

[00121] 3D wrinkled microstructures may find various applications. For example, 3D wrinkled microstructures may provide a physical cue for 3D cell control.

[00122] Referring to FIG. 12, shown there generally as 1200 is an SEM image with a 3D surface having a number of evenly-spaced (15 μπι apart) wrinkled posts 1202 installed on a smooth planar surface. The inset 1204 shows a wrinkled post, which may be similar in appearance to the ones discussed above with respect to FIGS. 9A or 9B. The scale for the bar shown on the bottom- left corner is 100 μπι. To illustrate how cells may interact with the wrinkled posts, bovine fibroblasts may be randomly seeded onto the surface.

[00123] Referring to FIG. 13 A, shown there generally as 1310 is an SEM image illustrating the cells being attracted to the 3D wrinkled post after three days of culture. As shown, the cells spread and climb onto these wrinkled cones, so as to conform onto individual posts.

[00124] Referring to FIG. 13B, shown there generally as 1320 is an SEM image in another configuration where the posts are positioned closer together. When the posts are close enough together (e.g., 80 μπι), cells climb onto two posts and bridge together to form an overhanging connection in between them. For this bridging, the two wrinkled posts may experience a bending deformation. The amount of bending may be correlated so as to provide information about the mechanical properties of the cellular network or individual cells bridging the two posts.

[00125] Referring to FIG. 13C, shown there generally as 1330 is an SEM image in a further configuration where the posts are arranged in a lattice or grid shape, similar to FIG. 12. As shown, the cells have created two bridges, one horizontally between the bottom two posts, and one vertically between the posts on the right side of the image. Interestingly, cells did not bridge diagonally arranged posts. This suggests that there may be a critical length for cells to attach and spread.

[00126] In a square lattice or array of the wrinkled posts, cells that bridge the posts may be considered as display a subgraph pattern, mimicking a cellular network. This is shown with the insets 1302a, 1302b, and 1302c in each of FIGS. 13A, 13B, and 13C respectively. [00127] The behaviour exhibited by the cells is in contrast to behaviour exhibited by the cells on smooth posts created without wrinkles.

[00128] Referring to FIGS. 14A and 14B, shown there generally as 1400 and 1450 are SEM images of, respectively, an individual smooth post and an array of smooth posts, installed on a smooth dummy layer. The same cells as that which was seeded onto the wrinkled surfaces of FIGS. 13A-C were seeded onto these surfaces, and it was noted that the cells rest on the base and keep away from the smooth posts. This cellular behavior confirms that cells may sense the 3D wrinkled microstructures and can be guided by them, suggesting that the 3D wrinkled surface may provide a tool to study cellular behavior on 3D curvatures. The finding may be useful in various biotechnologies such as biomechanics, mechanobiology, and tissue engineering, where 3D cell regulation is used.

Other Applications for Wrinkled 3D Surfaces

[00129] The application of the 3D wrinkled platform described herein is not limited to biotechnology. For example, in various embodiments of the present invention, the wrinkled 3D surfaces of the present invention may potentially be used in photovoltaics, where the wrinkled 3D microstructures can enhance antireflection characteristics due to its 3D morphology, and also improve light trapping and absorption capacity due to the wrinkles. The efficiency of solar cells may be increased as a result.

[00130] In various embodiments of the present invention, the 3D wrinkled surfaces may potentially be used in the field of flexible electronics. For example, the 3D wrinkled microstructures may better tolerate mechanical deformation from any angle. This may allow devices incorporating 3D wrinkled microstructures to provide for all directional flexibility, instead of the more limited uniaxial or biaxial stretch that may be possible with devices that employ wrinkles on planar 2D surfaces.

[00131] In various embodiments of the present invention, the 3D wrinkled surface fabrication described herein may provide a model for wrinkle formation that mimics wrinkled 3D surfaces of natural organisms. When constructed, such models may allow for the study of natural organisms without requiring actual tissue of the natural organism itself.

Example Laboratory Setup for Creating Wrinkled Surfaces

[00132] This section provides additional information regarding an example laboratory setup that may be used to perform one or more of the methods described herein.

[00133] The PDMS Channel 102 of FIG. 1 may be created in the following way. A

Polydimethyl-siloxane (PDMS, Sylgard^® 184, available from Dow Corning of Midland, Michigan, USA) precursor was created with a mixing ratio of 10: 1 of PDMS to a curing agent. This precursor is conformed to a master with the desired channel height, and then partially cured for 20 minutes at 65 °C on a SU-8 photoresist (available from Microchem of Newton, Massachusetts, USA) positive relief pattern. Thereafter, a piece of glass, slightly wider than the channel, was placed on the partially-cured PDMS surface. Then, additional PDMS precursor was poured onto the surface and heated at 65 °C for another 2 h to bond the additional PDMS with the partially-cured PDMS with the glass trapped therein.

[00134] Inclusion of the glass or other similar rigid material in the making of the PDMS channel is not required. However, making the slit PDMS channel with the glass support inside strengthens the mechanical properties of the PDMS channel and helps to prevent sagging of the channel. This is desirable because the channel has an aspect ratio that is skewed in one dimension (e.g., the channel width is typically much larger than the channel height).

[00135] Photolithography was can be used to create the polymeric 2D planar surfaces and 3D microstructures with the conformal partially-cured-polymer layer. An ultraviolet (UV) light (305- 390 nm) with intensity of 280 mw/cm (as measured using the ACCU-Cal™ 50 UV intensity meter, available from Dymax Corporation of Torrington, Connecticut, USA) was used as the light source. For the planar wrinkled surface in FIG. 4B and the wrinkled microwells in FIGS. 6A, 6B, and IOC, a one-step photopolymerization was performed. An amount of 3 μΐ. poly(ethylene glycol) (700) diacrylate (PEG-DA 700, available from Sigma-Aldrich Inc. of St. Louis, Missouri, USA) was dispensed on a cleaned glass slide and then a slit PDMS channel with 60 μηι height and 8 mm width was placed on the top of the droplet to form a uniform liquid film.

[00136] A metal arc lamp was used as a UV source (Lumen 200 Fluorescence Lumination System, provided by Prior Scientific) and a UV shutter (Lambda SC SmartShutter^® Controller, available from Sutter Instruments of Novato, California, USA) was installed in the UV light path to control the UV exposure time. The intensity of the UV light through a 10x microscope objective is 280 mw/cm . The UV shutter was controlled by a program created in Labview (available from National Instruments of Austin, Texas, USA) through a digital controller (NI 9472, available from National Instruments, referred to above) to precisely control the UV exposure time. An inverted microscope Axio Observer (available from Carl Zeiss of Jena, Germany) was used as the photopolymerization platform. A UV filter set (11000v3, available from Chroma Technology Corp. of Bellows Falls, Vermont, USA) was used to filter the UV light source to obtain desired UV excitation for polymerization (305-390 nm). The transparency photomasks were designed with AUTOCAD 2011 and printed at a resolution of 25,000 dpi.

[00137] The UV exposure of 300, 400, 500, 700, and 2000 ms through a blank photomask was applied for the wrinkle wavelength characterization in FIG. 4B. A UV exposure of 400 ms through the corresponding bright field photomasks was applied for the fabrication of wrinkled microwells in FIGS. 6A, 6B, and IOC. In particular, the conical microwell in FIG. IOC was made by exposing the prepolymer film to the UV light while its focus plane was lifted -30 μπι (depth of focus is 135 μπι for the 10x objective with numerical aperture NA = 0.3).

[00138] After the UV exposure, the PDMS channel was taken off and samples were rinsed with 300 μΐ, ethanol to remove the un cured monomers on the surface while retaining the monomers trapped inside the partially-cured polymer network. The rinsing was carried out by a pipette in a drop-by-drop fashion as discussed above in relation to FIG. 2.

[00139] For the heterogeneous wrinkle patterns and the 3D microstructured surfaces, a two- step photopolymerization was used. The first step was to make a planar base and the second step was to create the ridges in FIGS. 8 A, 8B or the various 3D features in FIGS. 9 A, 9B, 10D, and 1 IAMB. [00140] The procedure of the first step is the same as the fabrication of the 2D planar wrinkled surface described above in the context of FIG. 4B, except that the PDMS channel was changed to a 30 μπι channel height (corresponding to height Hi 710 in FIG. 7 A) and the UV exposure time was varied. The smooth base surfaces shown in FIGS. 8 A (surface 860), 9, and 11 A - 14B were created by the UV exposure time of 2 s and the wrinkled base surface 855 in FIG. 8B was UV cured for 500 ms.

[00141] In the second step, another 3 μΐ. PEG-DA was dispensed onto the sample and the PDMS channel was replaced by one with various heights to create the 3D features. For the step-wise wrinkled surface in FIG. 8B, a 60 μπι height channel was used (corresponding to height H₂ 715 in FIG. 7B) and a UV exposure time of 300 ms was applied to make the ridges (both in FIGS. 9 A and 9B) by using a line-patterned mask.

[00142] For the post arrays, different channel heights (again, corresponding to height H₂ 715 in FIG. 7B) were used in different contexts. For the embodiments of the present invention shown in FIGS. 9 A and 10D, a channel height of 100 μπι was used. For the embodiment of the present invention of FIG. 5, a channel height of 80 μπι was used. For the embodiments of the present invention shown in FIGS. 9B, 13A - 14B, a channel height of 60 μπι was used. The UV exposure time of 400 ms was applied to all samples (except for smooth post arrays in FIGS. 14A-B) through suitable photomasks. In particular, the conical posts in FIG. 5 were polymerized by lifting the UV focus by -30 μπι. For the smooth post arrays in FIGS. 14A-B, the UV exposure time was 2 seconds. After photopolymerization, the samples were rinsed with 600 μΐ. of ethanol using the same drop-by-drop rinsing process, as described above.

[00143] All the samples were plasma treated for 5 s (in a Plasma Cleaner model # PDC-32G available from Harrick Plasma of Ithaca, New York, USA) in order to cure the monomers on the surface, which simultaneously harden and swell the skin layer and produce wrinkles on the 2D and 3D surfaces.

[00144] To obtain SEM images, samples were mounted on aluminium stubs, sputtered with 5 nm platinum and observed by a scanning electron microscopy (the Quanta 3D FEG Scanning Electron Microscope (SEM), available from FEI Co. of Hillsboro, Oregon, USA). [00145] To obtain measurements using ATR-FTIR, monomer concentrations in the partially- cured layers on planar samples after rinsing were measured using a Spectrum one FTIR (available from PerkinElmer Life and Analytical Science Inc., Waltham, Massachusetts, USA) and a Gateway ATR accessory kit with zinc selenide (ZnSe) crystal (available from Specac Ltd. of 97 Cray Avenue, Orpington, United Kingdom). Data was collected and analyzed using software Spectrum 6. Spectra were collected at 4 cm^"1 resolution with 64 scans between 630 and 4000cm^"1. Flat films with 3 cm length and 5 mm width were made in a 60 μπι high PDMS channel using UV exposure time of 300, 400, 500, 700 and 2000 ms, respectively. Films were rinsed by 900 ethanol in a drop-by-drop fashion and were placed on the crystal with the surface facing the crystal. A pressure clamp was used to ensure the full contact of the film and the crystal surface. The monomer concentration was calculated by comparing the peak areas of C=C double bonds (1635 cm^"1) of a film with before and after polymerization versus an internal standard , C-0 (1730 cm^"1) peak.

[00146] For the culturing of cells on the patterns in the embodiments of the present invention shown in FIGS. 13 and 14, bovine ligament fibroblasts were randomly seeded onto the samples at a cell density of 10⁴ cells/cm³. After 72 hours of culture, samples were fixed and dried for SEM imaging.

[00147] To obtain isolated cells, bovine ligament fibroblasts were isolated from the central ligament of the metacarpal-carpal joint of 12-18 month old calves. Harvested ligaments were cleaned of extraneous fat, minced into < 1 mm pieces, and then digested in Dulbecco's Modified Eagle Medium (DMEM) culture media containing 0.25% by volume collagenase A (available from Roche Diagnostics Corporation of Indianapolis, Indiana, USA) for 36 hours at 37°C. Viable cells, determined by Trypan blue dye exclusion (an Invitrogen™ product available from Thermo Fisher Scientific of Waltham, Massachusetts, USA), were seeded in culture flasks and maintained in DMEM medium containing 5% Fetal Bovine Serum (FBS, available from Sigma- Aldrich Inc. noted above) and antibiotics (100 U/mL penicillin, 100 mg/mL streptomycin) (an Invitrogen™ product, available Thermo Fisher Scientific noted above). Cell cultures were grown in an incubator maintained at 37°C and 95% relative humidity supplemented with 5% C0₂ by volume. Cells were cultured up to passage 5 for the experiments with media changes every 2-3 days.

[00148] To obtain cell cultures, substrates were soaked in deionized water for 2 days to remove uncured monomers. The substrates were then immersed in 70% ethanol for 30 minutes for sterilization, followed by washing with IX Phosphate Buffered Saline (PBS) (pH 7.4). Substrates were then placed in 24 well plates and left with complete medium (DMEM containing 5% FBS and antibiotics) for 30 minutes to allow for protein adsorption on the surface. Cells were seeded onto the substrates at a density of 10⁴ cells/cm³ in a small volume of complete media and allowed to attach for 4 hours. Following cell attachment, a 1 mL of complete media was added to the cultures. After 72 hours of culture, samples were rinsed with PBS (pH 7.4), fixed with 4% paraformaldehyde (available from BioShop Canada Inc. of Burlington, Ontario, Canada) overnight at 4°C and then dehydrated with graded ethanol solutions (70, 90, 95, and 100%). Cultures were then chemically dried by grading to 100% hexamethyldisilazane (33, 67, and 100%) available from Sigma-Aldrich Inc. (referred to above) followed by air-drying in a chemical hood overnight.

Creation of Wrinkled Particles

[00149] Particles with wrinkled surfaces may be widely found in the nature. For example, such particles include plant pollens, plant seeds (e.g., peppercorns and walnuts) and/or microorganisms (e.g., neutrophils), and they form various wrinkle morphologies and present in different sizes and shapes. These wrinkles with their enlarged surface areas provide enhanced survival tools for these natural particles, such as modulating pollen adhesion and hydration, and regulating cell signalling. This has led to a desire to create synthetic particles with similar wrinkled surface properties.

[00150] In the majority of synthetic wrinkled particles created, the particles are commonly synthesized based on spray or emulsion methods followed by wrinkling post-processes. The particles made by these methods are limited to spherical or spheroidal shapes. In addition, due to the batch processes, these methods typically have difficulty achieving size uniformity of the particles, which may be relevant in applications such as controlled drug delivery and colloidal stability. Furthermore, because of lack of geometry constraints arising from the shape limit, guiding spatial patterns of wrinkling on these spherical particles is challenging.

[00151] Traditional wrinkling post-processes are relatively lengthy, in the range of 100 - 200 minutes. This is because the process typically requires a chemical reaction for coating an additional layer of stiff material on the surface of the resulting particles. In addition to requiring time for wrinkling post-processing, this uniform coating makes it difficult to generate complex wrinkle morphologies (e.g., hierarchical wrinkles) on the particles, which requires multiple layers of materials with gradient physical properties in the skin to form different wavelengths of wrinkles.

[00152] The embodiments described herein for creating textured particles may alleviate one or more these shortcomings. For example, as discussed below, the wrinkling post-process described herein may be accomplished in the range of a few seconds to generate wrinkled non-spherical particles. Additionally, the present embodiments may allow the wrinkled particles to be designed with a particular particle size and shape, and also with hierarchy and tuneable wrinkle morphology. Without additional chemical treatments, these artificially wrinkled particles appear to show unique surface functions and promote cell attachment to the particles.

[00153] Referring to FIG. 17, shown there generally as 1700 is a schematic for a setup for performing stop flow lithography (SFL) as a part of a process for creating textured particles, in accordance with at least one embodiment of the present invention.

[00154] Instead of using a static photolithography process discussed above, a stop flow lithography (SFL) process may be used in some embodiments. As discussed below, in combination with the plasma exposure steps discussed herein, textured particles may be created. As will be understood by persons skillled in the art, microfluidic flow lithography involves exposing a prepolymer solution to a radiation source as the solution flows through a channel. In SFL, instead of having the solution flow continuously, the solution is stopped during the lithography process when particles are created. Then, upon completion of the lithography process, the formed particles are flushed and the cycle may be repeated.

[00155] As shown, a flow 1720 of photo-curable monomer solution 104 is stopped in a microfluidic channel 102. Then, a radiation (e.g., ultraviolet) 110 beam shone through a photomask 108 is exposed to the solution 104 to polymerize the monomers contained therein into shapes defined by the photomask 108. As will be understood by persons skilled in the art, the ultraviolet radiation source 110 is focused through the microscope objective 1705 onto the resting photo- curable monomer solution 104. [00156] Immediately after the particle formation, the flow 1702 of monomer solution 104 is resumed and the synthesized particles 1750 are discharged from the channel 102 for washing. In various embodiments, this process may then be repeated so particles may be generated in a continuous and high throughput fashion.

[00157] Referring to FIG. 18, shown there generally as 1800 is an illustration of a close-up illustrative cross-section of an apparatus for performing SFL used for the creation of textured particles, in accordance with at least one embodiment of the present invention. During the radiation- induced polymerization, oxygen diffusion 116 into the channel 102 through the PDMS walls 102a, 102b plays a role in the particle discharge and wrinkle formation. As UV 110 is exposed to the channel, polymerization starts from the middle of the channel height 130, where oxygen is depleted first by the UV initiated free radicals. The polymerized particles increase in size upwards towards the upper wall 102a of the channel as the UV 110 exposure continues. The presence of the lower wall 102b allowing oxygen 116 to be diffused into the channel 102 inhibits polymerization of the monomer solution 104 that is immediately adjacent to the lower wall 102b. This creates an uncured monomer layer (e.g., lubrication layers) at the bottom channel wall 102b to ensure the smooth discharge of the formed particles. This results in particles with a fully cured body 112 and a partially-cured-polymer (PCP) outer layer 114 being created. The PCP layer will typically have a polymerization gradient that varies in accordance with the duration of UV 110 exposure, and will serve as the skin layer later in a wrinkling process.

[00158] The PCP outer layer 114 of a particle may include a loose PEG polymeric network and uncrosslinked monomers trapped inside the network. Since these trapped monomers are not chemically bonded to the PCP network, they can be completely removed by a thorough wash or can be partially retained by a controlled wash using a washing agent that contains a known concentration of monomers— higher concentrations will leave more monomers trapped and lower concentrations will leave less monomers in the PCP layer 114. Additional details of the effect of the monomer concentration on the resultant texture characteristics are discussed below.

[00159] It was noted above that the rising process to remove the uncured monomers when creating textured surfaces may need to be performed in a measured, progressive manner so as not to flush away the partially-cured layer. While the SFL process for generating wrinkled particles involves a process that flushes out particles, it is noted that the solution used to flush the particles in the SFL process is the original prepolymer solution that still has a substantial amount of uncured monomers. Similarly, during the washing, solutions with a certain concentration of monomer can be used. Because the wash solution contains the same monomer, there will be some concentration of the monomer remaining on the surface after rinsing (in some embodiments, even if the wash is conducted in a vigorous manner), and the washing of the particles will not completely remove the uncured layer.

[00160] Referring to FIG. 19, shown there generally as 1900 is an illustrative cross-section of a particle produced by the SFL process of FIG. 18, being exposed to plasma to create a textured particle, in accordance with at least one embodiment of the present invention.

[00161] After a controlled wash, particles are subjected to plasma treatment 202 for the wrinkling post-process. This process densifies the PCP skin layer 114 with the retained monomers into a crust and simultaneously expands the layer by providing crosslinking and annealing while has little effect on the particle core. This mismatched deformation between the core layer 112 and crust layer, induced by the plasma, buckles the layer 114, to form wrinkled particles 1905. In some embodiments, this wrinkling formation process may be completed in five (5) seconds in the example setup discussed below.

[00162] Referring to FIG. 20, shown there generally are SEM images of textured particles having various shapes that may be created using the methods described herein, in accordance with various embodiments of the present invention. The scale bars shown in the bottom right hand corners of the SEM images are 20 um.

[00163] Various elements of the setup of FIG. 17 can be configured to manipulate the shapes of the particles created. For a given height 130 of a PDMS channel 102, features of a photomask 108 can define the 2D extruded shapes of particles. At the same time, the mask feature-size and the magnification of microscope objective 1705 may determine particle sizes. Thus, by simply selecting photomask 108 patterns and objectives, it is possible to produce particles with various 2D extruded shapes, or even 3D shapes. As illustrated in FIG. 20, various wrinkled particles 2004, 2008, 2012 can be obtained by using photomasks 108 with corresponding shapes 2002, 2006, 2010. At a small size scale, the free radical diffusion during UV exposure may introduce undesired polymerization around a designed particle and thus affect the final particles size. In the experimental setup of the present embodiments, a minimum feature size of 5 um was obtained.

[00164] Referring to FIGS. 21A - 21C, shown there generally are SEM images of particles having various texture characteristics resulting from variations in duration of radiation exposure, in accordance with at least one embodiment of the present invention. The scale bars shown in the bottom right hand corners of the SEM images are 2 um.

[00165] As noted, the duration of radiation exposure of the monomer solution may affect wrinkle morphology. During UV-polymerization for particle formation, oxygen inhibition causes a nonlinear profile of polymer conversion rate from the center (e.g., the cured region 1 12) to the outer layer 1 14 of the resulting particles. Thus, by carefully tuning UV exposure time, it may be possible to control the PCP layer 1 14 thickness of particles and their polymer conversion rate, which in turn determines the wrinkle wavelength upon plasma treatment. l (E \ ^1/3

[00166] The wavelength of the wrinkles, Λ, can be approximated by λ « Inhi^)¹/² ; where h is the thickness of the crust layer after plasma treatment, and E_s and £ are the Young's moduli of the crust layer and the particle core, respectively. In particle formation process, UV exposure time not only determines the ratio of (E_s/Ef), but also dominates the thickness (h) of the PCP and the crust layer after plasma treatment. Consequently, the UV exposure time enables us to control the wavelength of the wrinkles.

[00167] The wrinkle morphologies on the particles shown in FIGS. 21A - 21C were generated by tuning UV exposure time with the same washing solutions (5% PEG-DA water solution) and plasma treatment time. Specifically, FIGS. 21 A - 21C show the morphology of wrinkled particles and their corresponding wrinkle wavelengths by using the UV exposure time of 300, 700 and 1000 ms respectively; after washing with a 5% PEG-DA solution and plasma treatment for 5 sec. Shown inset as 2102, 2106, 21 10 in the top right hand corner of FIGS. 21 A - 21C are zoomed-out perspective images of the particles, while the main images 2104, 2108, 21 12 show characteristics of the wrinkles. [00168] Referring to FIG. 21D, shown there generally as 2150 is a diagram illustrating the variation in texture characteristics with duration of radiation exposure, in accordance with at least one embodiment of the present invention. As shown, the wavelengths 2104', 2108', 2112' for exposure times of 300, 700, and 1000ms respectively are illustrated. Each data point represents 10 wavelength measurements. As can be seen, as the UV exposure time increases from 300 ms to 1000 ms, the wavelength decreases from a micron scale of 1.3 μιη to submicron of 0.4 μιη. Along with the decrease of the wavelength, the amplitude of the wrinkles also decreases. The morphology change can be also observed which varies from a continuous to discontinuous pattern as the UV exposure time increases from 300 ms to 1000 ms, as shown in FIGS. 21 A- 21 C.

[00169] Referring to FIGS. 22A - 22D, shown there generally are SEM images showing particles having variations in texture characteristics resulting from variations of the monomer concentration used in the rinsing agent, in accordance with at least one embodiment of the present invention. Shown inset as 2202, 2206, 2210, 2214 in the top right hand corner of FIGS. 22 A - 22D are zoomed-out perspective images of the particles, while the main images 2204, 2208, 2212, 2216 show characteristics of the wrinkles. The scale bars shown in the bottom right hand corners of the SEM images is 2 urn for FIGS. 22A - 22C and lOum for FIG. 22D.

[00170] In addition to the UV exposure time, it may also be possible to control the morphology of wrinkles on particles by tuning the known concentration of monomers used in the washing agent. By using various concentrations of solutions for wash, different amounts of monomers can be retained in the PCP layer 114 so as to cause different morphologies of wrinkles after plasma treatment.

[00171] Specifically, in an embodiment, the UV exposure time was fixed at 200 ms and the resultant particles were washed with 0%, 5%, 10% and 25% of PEG-DA solutions in different respective situations. As a result, these particles display different morphologies after plasma treatment. When washed by 0% solution, the monomers in the PCP layer are completely removed, thus no wrinkles were formed and smooth surface is observed (see FIG. 22A). As the monomer concentration in the washing solution increased, the wavelength of wrinkles correspondingly increased and various texture patterns are formed. For the 5% PEG-DA solutions, it can be seen that some level of monomers are retained, and formation of wrinkle morphology on the particles are present (see FIG. 22B). For the 10% PEG-DA solution, further monomers are retained on the surface of the particle and better-formed wrinkle morphologies can be observed (see FIG. 22C).

[00172] When washed with a high concentration monomer solution (25%), excessive monomers are retained in the PCP layer and a hierarchical (nested) wrinkle morphology is developed (see FIG. 22D). In this latter scenario, the plasma treatment initially generates primary wrinkles on the surface and its amplitude quickly saturates; this saturated surface experiences further compressive stresses in the primary wrinkled crust and generates secondary wrinkles with a larger wavelength, forming a hierarchical (nested) wrinkle patterns on the surface.

[00173] In some embodiments described above, it was noted that ethanol or pure water may be used as the washing/rinsing agent to remove the uncured monomers. Ethanol and pure water may likewise be used as the washing agent when washing the particles created from the SFL process. However, if such washing agent is to be used, as discussed above, care will have to be employed to performing the rinse in a measured, progressive manner so as not to flush away, but retain the monomers trapped in the PCP layer 114.

[00174] Referring to FIGS. 23A - 23F, shown there generally as 2302, 2304, 2306, 2308,

2310, and 2312 respectively are SEM images showing cells attaching to textured particles, in accordance with at least one embodiment of the present invention. The scale bars shown in the bottom right hand corners of the SEM images are: 300 urn for FIG. 23 A; 40 urn for FIGS. 23B, 23C, 23E; and 20um for FIGS. 23D and 23F.

[00175] As noted above, the wrinkled surfaces produced in the present disclosure may allow biological cells to attach to them more easily. This phenomenon is also applicable to textured particles. In particular, in some embodiments, a simple physical modification on the particle surface can enhance cell attachment, without any intensive chemical modification.

[00176] In one embodiment, smooth and wrinkled particles of 40 μπι in diameter were used to compare the amount of cell attachment between the two types of particles. Both sets of particles were created using a UV exposure time of 300 ms. However, two different concentrations of washing solutions (0% and 10% monomer solutions) were used to create smooth and wrinkled particles respectively. Bovine fibroblasts were then randomly seeded onto both the smooth and wrinkled particles. The samples were then examined after three days of cell culture. In the case of the smooth particles, cells had spread onto the substrate and the particles were ignored (see FIGS. 23 A and 23B). Although some cell attachment to the smooth particles was observed, these cells only appear to reach to the edge of the particles by a small partial anchorage (see FIG. 23C).

[00177] However, in case of the wrinkled particle samples (a representative surface morphology of the wrinkled particles before cell seeding is shown in FIG. 23D), cells attach to and cover most part of the wrinkled particles (see FIG. 23E). With samples of 100 wrinkled and smooth particles, it was observed that about 70% of the wrinkled particles have cells attached to them, whereas cells attached to only 8% of the smooth particles. In addition, when cells attach to wrinkled particles, they climb and conform onto the particles, forming a cell wrapping unit (see FIG. 23F), which is not observed on the smooth particles.

[00178] This suggest that the wrinkles on particles can largely improve cell attachment and help to form cell wrapping particles, which may usually be achieved by complex chemistry modifications. Thus, the embodiments described herein may provide a simple route to promote cell attachment to particles by its wrinkled surface textures without chemical processing. This may be useful in a number of biomedical applications where particles are used as cell microcarriers such as cell delivery in cellular therapy, tissue formation and regeneration in tissue engineering, and cellular biophysical studies.

[00179] In addition to cell attachment, these wrinkled particles may provide useful functions in other biomedical applications. For example, in drug delivery, they may increase drug carrying efficiency by their enhanced surface area and reduce inter-particulate adhesion by the roughness of wrinkles, thus improving drug delivery performance. Furthermore, the size- and shape-controllable ability of the described methods may allow the particles to be designed in a manner that achieves optimal drug delivery performance.

[00180] By taking advantage of partial polymerization in SFL in combination with plasma treatment, the present embodiments for creating textured particles may allow for a simple and relatively-quick route for wrinkled particle fabrication. In some embodiments, variations in UV exposure time and monomer concentration of washing agent may affect the wrinkle morphologies created on the particles. In this manner, not only can the present embodiments allow for fabrication of wrinkled particles with designed shape and size, but it may also be possible to create wrinkles on the particle surface with tuned morphology and wavelength. In some embodiments, the wrinkling post-process is completed in a few seconds by using plasma treatment, which simplifies the process of wrinkled particle formation. In some embodiments, the surface wrinkles of the particles improve cell attachment to the particles without any chemical modifications. This may be beneficial to many biomedical applications where cell attachment to a surface or particle is desired, such as cell micro- carriers, cell physiological study and tissue engineering.

Example Laboratory Setup for Creating Wrinkled Particles

[00181] This section provides additional information regarding an example laboratory setup that may be used to perform one or more of the methods described herein for creating textured particles. Where the materials used have been previously referenced in the present disclosure, only a reference to their industrial name may be listed below.

[00182] The materials used include: PEG-DA 700, available from Sigma-Aldrich; and Darocur 1173 (available from Sigma-Aldrich) initiator are used for polymeric particles synthesis. 5% Darocur 1173 in PEG-DA 700 were used as the prepolymer solutions for the particle synthesis.

[00183] For the microfluidic device fabrication, a mixture of PDMS, Sylgard 184 (available from Dow Corning) and curing agent at a ratio of 10: 1 was prepared to make the microfluidic channels. The elastomer mixture was poured onto a SU-8 patterned silicon wafer (SU-8 photoresist, available from Microchem) and baked in an oven at 65 °C for 1 hour in order to mold the PDMS channels. The channels were then placed onto PDMS-coated glass slides, where the PDMS layer is partially cured at 65°C for 20 minutes. The assembled channels were then baked for another 1 hour for full cure of the PDMS channels and the coated layers.

[00184] For the photopolymerization setup, microparticles were polymerized by using SFL and designed photomasks. A metal arc lamp (Lumen 200, available from Prior Scientific) was connected to the Axio Observer (available from Carl Zeiss) inverted microscope to provide the UV source, and a UV shutter (Lambda SC, available from Sutter Instruments) was used to control the UV exposure. The prepolymer solution was supplied through a pneumatic tubing system, which consisted of a pressure regulator (Type 100LR, available from Control Air, of Amherst, H, USA), serially connected to a three-way solenoid valve (Model 6014, Burkert, Germany) and the PDMS channel. The UV shutter and the solenoid valve were controlled by a program in Labview (available from National Instruments) through a digital controller (NI 9472, available from National Instruments) to control UV exposure time and prepolymer flow cycle. The microscope equipped with 5x/0.13, lOx/0.3, and 20*/0.4 objectives (N-Achroplan, Ec plan-Neofluar and korr LD Plan- Neofluar, available from Carl Zeiss, of Jena, Germany) was used as the synthesis platform. The desired UV excitation (350 nm) required by polymerization was attained by filtering the UV light source through a UV filter set (11000v3, available from Chroma Technology Corp). AUTOCAD 2011 was used to design the transparency photomasks. Photomasks were printed at a resolution of 25 000dpi (available from CAD/Art Services, of OR, USA).

[00185] For the polymeric particle synthesis, monomer solution consisting of 95% PEG-DA

700 and 5% Darocur 1173 was supplied to the channel. When flow was fully stopped, UV exposure at different times through the designed photomasks and a 10X objective lens were applied to polymerize desired non-spherical particles. Particles remain in the prepolymer solution and are collected at the outlet. The solution is diluted with deionized (DI) water at different ratios to partially or completely remove the uncrosslinked monomers. After the dilution process, samples were loaded on glass slides and dried for plasma treatment.

[00186] For the wrinkle formation, before plasma treatment, the dried particle samples were transferred onto aluminum foil to ensure the plasma contact of the bottom of the particles. All samples were exposed to plasma for 5 seconds in a plasma cleaner (model #PDC-32G, available from Harrick Plasma, of Ithaca, NY, USA).

[00187] For the cell culture experiments, bovine ligament fibroblasts were randomly seeded onto the wrinkled and smooth particle samples at a cell density of 3 >< 10⁴ cells/cm³. After 72 hours of culture, samples were fixed and dried for SEM imaging.

[00188] All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually incorporated by reference. [00189] While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be appreciated by one skilled in the art, from a reading of the disclosure that various changes in form and detail can be made without departing from the true scope of the invention.

Claims

CLAIMS What is claimed is:

1. A method for producing textures in a surface, the method comprising:

exposing a photo-curable material comprising an uncured material to radiation, to produce a layer of partially-cured material between a cured region of the photo-curable material and a remaining uncured region of the photo-curable material, the cured region resulting from exposure of the photo-curable material to the radiation;

substantially removing the remaining uncured region to reveal the surface, the surface being located on the layer of partially-cured material; and

exposing the surface to charged ions, so that textures are formed therein.

2. The method of claim 1, wherein the uncured material comprises uncured monomers, the cured region comprises polymers, and the partially-cured material comprises partially-polymerized material.

3. The method of claim 1 or claim 2, wherein the textures comprise wrinkles.

4. The method of claim 2 or claim 3, wherein when performing the exposing of the photo- curable material to the radiation, the method further comprises controlling a duration of time the photo-curable material is exposed to the radiation, to manipulate a gradient of uncured monomer concentration produced in the layer of partially-cured material.

5. The method of any one of claims 1 to 4, wherein the exposing is performed during a photolithography process, and a photomask is used in the photolithography process to selectively limit exposure of the photo-curable material to the radiation, to control characteristics of the cured region.

6. The method of claim 5, wherein after the exposing, the method further comprises performing an additional exposure of the photo-curable material to the radiation, wherein an alternate photomask is used during such additional exposure to create variations in heights of the cured region.

7. The method of any one of claims 1 to 6, wherein a source of the radiation is used during the exposing, and the method further comprises altering a focal plane of the source of the radiation to create a non-cylindrical beam of the radiation for curing the uncured material during the exposing, so that the cured region produced from the exposing is non-cylindrical.

8. The method of claim 7, wherein an additional source of the radiation is positioned proximate to the source of the radiation during the exposing, and the radiation from the source of the radiation and the additional source of the radiation overlap to cure the uncured material during the exposing.

9. The method of any one of claims 1 to 8, further comprising exposing the textured surface to cells so that the cells attach to the textured surface.

10. The method of any one of claims 1 to 9, wherein during the exposing, a layer of curati on- inhibiting material is positioned between a source of the radiation and the photo-curable material, such that the cured region produced from the exposing forms a particle enveloped within the layer of partially-cured material.

11. A composition of matter comprising textures on a surface, the composition of matter having been generated by:

exposing the surface to charged ions, so that textures are formed therein.

12. The composition of matter of claim 11, wherein the uncured material comprises uncured monomers, the cured region comprises polymers, and the partially-cured material comprises partially-polymerized material.

13. The composition of matter of claim 11 or claim 12, wherein the textures comprise wrinkles.

14. The composition of matter of claim 12 or claim 13, wherein when performing the exposing of the photo-curable material to the radiation, there is a controlling of a duration of time the photo- curable material is exposed to the radiation, to manipulate a gradient of uncured monomer concentration produced in the layer of partially-cured material.

15. The composition of matter of any one of claims 11 to 14, wherein the exposing is performed during a photolithography process, and a photomask is used in the photolithography process to selectively limit exposure of the photo-curable material to the radiation, to control characteristics of the cured region.

16. The composition of matter of claim 15, wherein after the exposing, there is an additional exposure of the photo-curable material to the radiation, and wherein an alternate photomask is used during such additional exposure to create variations in heights of the cured region.

17. The composition of matter of any one of claims 11 to 16, wherein a source of the radiation is used during the exposing, and a focal plane of the source of the radiation is altered to create a non- cylindrical beam of the radiation for curing the uncured material during the exposing, so that the cured region produced from the exposing is non-cylindrical.

18. The composition of matter of claim 17, wherein an additional source of the radiation is positioned proximate to the source of the radiation during the exposing, and the radiation from the source of the radiation and the additional source of the radiation overlap to cure the uncured material during the exposing.

19. The composition of matter of any one of claims 11 to 18, wherein cells are attached to the textured surface as a result the textured surface having been exposed to the cells.

20. The composition of matter of any one of claims 11 to 19, wherein during the exposing, a layer of curation-inhibiting material is positioned between a source of the radiation and the photo- curable material, such that the cured region produced from the exposing forms a particle enveloped within the layer of partially-cured material.

21. A product when made by the method of any one of claims 1 to 10.

22. An apparatus for producing partially-cured material, the apparatus comprising:

a source of radiation; and

a substrate for holding a photo-curable material comprising uncured material, wherein at least a portion of the photo-curable material is exposed to the radiation;

wherein, upon exposure of the photo-curable material to the radiation, a layer of partially- cured material is produced between a cured region of the photo-curable material and a remaining uncured region of the photo-curable material, the cured region resulting from exposure of the photo-curable material to the radiation.

23. The apparatus of claim 22, wherein a photomask is positioned between the source of the radiation and the substrate, the photomask being used to selectively limit exposure of the photo- curable material to the radiation, to control characteristics of the cured region.

24. The apparatus of claim 22 or claim 23, wherein when exposing the photo-curable material to the radiation, the apparatus provides a mechanism for controlling a duration of time the photo- curable material is exposed to the radiation, to manipulate a gradient of uncured monomer concentration produced in the layer of partially-cured material.

25. The apparatus of any one of claims 22 to 24, wherein a focal plane of the radiation source is alterable to create a non-cylindrical beam of the radiation that the uncured material is exposed to, so that the cured region produced from the exposing is non-cylindrical.

26. The apparatus of any one of claims 22 to 25, further comprising an additional source of the radiation positioned proximate to the source of the radiation, wherein upon exposure of the photo- curable material to the radiation, the radiation from the source of the radiation and the additional source of the radiation overlap to cure the uncured material.

27. The apparatus of any one of claims 22 to 26, a layer of curation-inhibiting material is positioned between a source of the radiation and the photo-curable material, such that the cured region produced from the exposing forms a particle enveloped within the layer of partially-cured material.