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Volume 3
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Gels, Volume 3, Issue 1 (March 2017) – 11 articles

Cover Story (view full-size image): Physically crosslinked polymer-based responsive hydrogels with tunable properties can be formed by intermolecular polyionic complexation of oppositely charged repeating units (ion bonding). Two types of hydrogels are envisioned: (i) hydrogels formed by triblock copolymers bearing oppositely charged blocks (self-assembled network); and (ii) hydrogels formed by co-assembly of oppositely charged polyelectrolyte segments belonging to different macromolecules (co-assembled network). View Paper here
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1538 KiB  
Communication
Self-Assembly of Colloidal Nanocomposite Hydrogels Using 1D Cellulose Nanocrystals and 2D Exfoliated Organoclay Layers
by Takumi Okamoto, Avinash J. Patil, Tomi Nissinen and Stephen Mann
Gels 2017, 3(1), 11; https://doi.org/10.3390/gels3010011 - 17 Mar 2017
Cited by 7 | Viewed by 5603
Abstract
Stimuli-responsive colloidal nanocomposite hydrogels are prepared by exploiting non-covalent interactions between anionic cellulose nanocrystals and polycationic delaminated sheets of aminopropyl-functionalized magnesium phyllosilicate clays. Full article
(This article belongs to the Special Issue Stimuli-Responsive Gels)
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Graphical abstract
Full article ">Figure 1
<p>(<b>a</b>) Unstained TEM image of cellulose nanocrystal (CNC)-organoclay hydrogel; (<b>b</b>) photograph showing self-supported CNC:organoclay (1:0.13) colloidal nanocomposite hydrogel, inset showing schematic illustration of cross-linked network formed by non-covalent interactions between CNC (grey) and exfoliated organoclay sheets.</p> Full article ">Figure 2
<p>Rheometry studies showing (<b>a</b>) frequency sweep and (<b>b</b>) amplitude sweep curves for an as-prepared CNC–organoclay colloidal nanocomposite hydrogel (ratio 1:0.13); values for storage <span class="html-italic">G</span>′ (filled circles) and loss <span class="html-italic">G</span>″ moduli (open circles); (<b>c</b>) and (<b>d</b>) show frequency and amplitude sweep profiles, respectively, for a CNC–organoclay nanocomposite hydrogel (ratio 1:0.13) after exposure to gaseous ammonia (<span class="html-italic">G</span>′, filled triangles; <span class="html-italic">G</span>″, open triangles) and carbon dioxide (<span class="html-italic">G</span>′, filled squares; <span class="html-italic">G</span>″, open squares).</p> Full article ">Figure 3
<p>Cross-polarized microscopy images of (<b>a</b>) as-prepared (<b>b</b>) ammonia- and (<b>c</b>) carbon dioxide-treated CNC–organoclay colloidal nanocomposite hydrogels, scale bar = 100 μm.</p> Full article ">Figure 4
<p>(<b>a</b>) Ibuprofen release profiles from organoclay–ibuprofen nanocomposite pellet (triangles) and CNC–organoclay–ibuprofen colloidal nanocomposite hydrogels (circles); (<b>b</b>) powder X-Ray diffraction (PXRD) pattern of CNC–organoclay nanocomposite hydrogel (red), as-synthesized organoclay (black), organoclay–ibuprofen nanocomposite (blue), and CNC–organoclay–ibuprofen colloidal nanocomposite hydrogel (green).</p> Full article ">
1637 KiB  
Article
Mechanical, Swelling, and Structural Properties of Mechanically Tough Clay-Sodium Polyacrylate Blend Hydrogels
by Hiroyuki Takeno, Yuri Kimura and Wataru Nakamura
Gels 2017, 3(1), 10; https://doi.org/10.3390/gels3010010 - 25 Feb 2017
Cited by 37 | Viewed by 9644
Abstract
We investigated the mechanical, swelling, and structural properties of mechanically tough clay/sodium polyacrylate (PAAS) hydrogels prepared by simple mixing. The gels had large swelling ratios, reflecting the characteristics of the constituent polymer. The swelling ratios initially increased with the increase of the swelling [...] Read more.
We investigated the mechanical, swelling, and structural properties of mechanically tough clay/sodium polyacrylate (PAAS) hydrogels prepared by simple mixing. The gels had large swelling ratios, reflecting the characteristics of the constituent polymer. The swelling ratios initially increased with the increase of the swelling time, and then attained maximum values. Afterwards, they decreased with an increase of the swelling time and finally became constant. An increase in the clay concentration lead to a decrease in the swelling ratios, whereas an increase in the PAAS concentration lead to an increase in the swelling ratios. Tensile measurements indicated that the toughness for clay/PAAS (Mw = 3.50 × 106) gels was several hundred times larger than that of clay/PAAS (Mw = 5.07 × 105) gels, i.e., the use of ultra-high molecular weight PAAS is essential for fabricating mechanically tough clay/PAAS blend hydrogels. Synchrotron small-angle X-ray scattering (SAXS) results showed that the SAXS intensity measured at small scattering angles decreased with an increase in the clay concentration, indicating that the interparticle interactions were more repulsive at higher concentrations. The decrease of the scattering intensity at high clay concentrations was larger for the clay/PAAS (Mw = 5.07 × 105) gel system than for the clay/PAAS (Mw = 3.50 × 106) gel system. Full article
(This article belongs to the Special Issue Rheology of Gels)
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Figure 1
<p>A schematic representation for the preparation of clay/PAAS blend hydrogels.</p> Full article ">Figure 2
<p>Pictures of a 5 wt % clay/1 wt % PAAS3500K/0.5 wt % TSPP blend hydrogel. A pressed gel (<b>top</b>) and a bent gel (<b>bottom</b>).</p> Full article ">Figure 3
<p>Time course of the swelling ratios for clay/PAAS3500K hydrogels at various clay concentrations (<b>a</b>) and at various PAAS concentrations (<b>b</b>).</p> Full article ">Figure 4
<p>Storage modulus (<span class="html-italic">E</span>′) and loss modulus (<span class="html-italic">E</span>″) vs. frequency for a 12.5 wt % clay/PAAS3500k hydrogel.</p> Full article ">Figure 5
<p>Typical tensile stress-strain curves of clay/PAAS3500K and clay/PAAS507K gels at various clay concentrations.</p> Full article ">Figure 6
<p>SAXS curves for clay/PAAS3500K gels (<b>a</b>) and for clay/PAAS507K gels (<b>b</b>) at 5 wt % and 10 wt % clay concentrations.</p> Full article ">Figure 7
<p>The experimental structure factors of clay/PAAS3500K gels (<b>a</b>) and clay/PAAS507K gels (<b>b</b>) at 5 wt % and 10 wt % clay.</p> Full article ">
6229 KiB  
Article
Synthesis of Helical Phenolic Resin Bundles through a Sol-Gel Transcription Method
by Changzhen Shao, Jiangang Li, Hao Chen, Baozong Li, Yi Li and Yonggang Yang
Gels 2017, 3(1), 9; https://doi.org/10.3390/gels3010009 - 23 Feb 2017
Cited by 6 | Viewed by 5380
Abstract
Chiral and helical polymers possess special helical structures and optical property, and may find applications in chiral catalysis and optical devices. This work presents the preparation and formation process of helical phenolic resins through a sol-gel transcription method. A pair of bola-type chiral [...] Read more.
Chiral and helical polymers possess special helical structures and optical property, and may find applications in chiral catalysis and optical devices. This work presents the preparation and formation process of helical phenolic resins through a sol-gel transcription method. A pair of bola-type chiral low-molecular-weight gelators (LMWGs) derived from valine are used as templates, while 2,4-dihydroxybenzoic acid and formaldehyde are used as precursors. The electron microscopy images show that the phenolic resins are single-handed helical bundles comprised of helical ultrafine nanofibers. The diffused reflection circular dichroism spectra indicate that the helical phenolic resins exhibit optical activity. A possible formation mechanism is proposed, which shows the co-assembly of the LMWGs and the precursors. Full article
(This article belongs to the Special Issue Gels as Templates for Transcription)
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Full article ">Figure 1
<p>Molecular structures of gelators <span class="html-italic">LL</span>-<b>1</b> and <span class="html-italic">DD</span>-<b>1</b>.</p> Full article ">Figure 2
<p>FE-SEM (<b>a</b>,<b>b</b>); and TEM (<b>c</b>,<b>d</b>) images of left-handed (<b>a</b>,<b>c</b>); and right-handed (<b>b</b>,<b>d</b>) helical phenolic resin bundles.</p> Full article ">Figure 3
<p>DRUV-vis and DRCD spectra of left-handed and right-handed helical phenolic resin bundles.</p> Full article ">Figure 4
<p>FE-SEM images of the reaction mixture taken at different reaction times: (<b>a</b>) before the addition of formaldehyde; (<b>b</b>) 5 min after the addition of formaldehyde; (<b>c</b>) 10 min after the addition of formaldehyde; (<b>d</b>) 30 min after the addition of formaldehyde.</p> Full article ">Figure 5
<p>Schematic illustration of the formation of helical phenolic resin bundles.</p> Full article ">
6168 KiB  
Article
Carboxymethyl Cellulose-Grafted Mesoporous Silica Hybrid Nanogels for Enhanced Cellular Uptake and Release of Curcumin
by Neha Tiwari, Laxman Nawale, Dhiman Sarkar and Manohar V. Badiger
Gels 2017, 3(1), 8; https://doi.org/10.3390/gels3010008 - 22 Feb 2017
Cited by 24 | Viewed by 10763
Abstract
Mesoporous silica nanoparticles (MSNs) with ordered pore structure have been synthesized and used as carriers for the anticancer drug curcumin. MSNs were functionalized with amine groups and further attached with carboxymethyl cellulose (CMC) using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) coupling chemistry, which increased the hydrophilicity and [...] Read more.
Mesoporous silica nanoparticles (MSNs) with ordered pore structure have been synthesized and used as carriers for the anticancer drug curcumin. MSNs were functionalized with amine groups and further attached with carboxymethyl cellulose (CMC) using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) coupling chemistry, which increased the hydrophilicity and biocompatibility of MSNs. The functionalized MSNs (MSN-NH2 and MSN-CMC) were characterized using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS), N2 adsorption, X-Ray Diffraction (XRD), Thermo Gravimetric Analysis (TGA) and Fourier Transform Infrared Spectroscopy (FT-IR). The in vitro release of curcumin from the –NH2 and CMC functionalized MSNs (MSN-cur-NH2 and MSN-cur-CMC) was performed in 0.5% aqueous solution of sodium lauryl sulphate (SLS). The effect of CMC functionalization of MSNs towards cellular uptake was studied in the human breast cancer cell line MDA-MB-231 and was compared with that of MSN-NH2 and free curcumin (cur). Both MSN-NH2 and MSN-CMC showed good biocompatibility with the breast cancer cell line. The MTT assay study revealed that curcumin-loaded MSN-cur-CMC showed better uptake as compared to curcumin-loaded MSN-cur-NH2. Free curcumin was used as a control and was shown to have much less internalization as compared to the curcumin-loaded functionalized MSNs due to poor bioavailability. Fluorescence microscopy was used to localize the fluorescent drug curcumin inside the cells. The work demonstrates that CMC-functionalized MSNs can be used as potential carriers for loading and release of hydrophobic drugs that otherwise cannot be used effectively in their free form for cancer therapy. Full article
(This article belongs to the Special Issue Micro- and Nanogels)
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Graphical abstract
Full article ">Figure 1
<p>Transmission electron microscopy (TEM) images of (<b>a</b>) MSN; (<b>b</b>) MSN-NH<sub>2</sub> and (<b>c</b>) MSN-CMC.</p> Full article ">Figure 2
<p>(<b>a</b>) Particle size distribution of MSN and functionalized MSNs obtained by DLS experiments and (<b>b</b>) nitrogen adsorption-desorption isotherms of MSN, MSN-NH<sub>2</sub> and MSN-cur-CMC nanoparticles.</p> Full article ">Figure 3
<p>(<b>a</b>) X-Ray Diffraction (XRD) patterns of MSN and MSN-NH<sub>2</sub> and (<b>b</b>) Thermogravimetric Analysis (TGA) curves of MSN, MSN-NH<sub>2</sub> and MSN-CMC.</p> Full article ">Figure 4
<p>Fourirer Transform Infrared Spectroscopy (FT-IR) Spectra of MSN, MSN-NH<sub>2</sub> and MSN-CMC.</p> Full article ">Figure 5
<p>In vitro cumulative release (%) of curcumin from MSN-cur-NH<sub>2</sub> and MSN-cur-CMC in 0.5% sodium lauryl sulphate (SLS).</p> Full article ">Figure 6
<p>(<b>a</b>) % Cytotoxicity of MDAMB 231 cells incubated with MSN-NH<sub>2</sub> and MSN-CMC and (<b>b</b>) % Cytotoxicity of MDA-MB-231 cells incubated with free curcumin, MSN-cur-NH<sub>2</sub> and MSN-cur-CMC keeping the amount of curcumin same in all the samples (<span class="html-italic">x</span> axis represents concentration of free curcumin and curcumin incubated in MSN-NH<sub>2</sub> and MSN-CMC).</p> Full article ">Figure 7
<p>Intracellular uptake of –NH<sub>2</sub> and –CMC functionalized MSNs using fluorescence microscopy. Images of MDA-MB-231 incubated with 16 µg/mL of free curcumin, MSN-cur-NH<sub>2</sub> (GI<sub>50</sub> = 7 µg/mL) and MSN-cur-CMC (GI<sub>50</sub> = 1.5 µg/mL). Control refers to the non-treated MDA-MB-231 cells. Blue fluorescence is due to nucleus staining of cells with 4’,6-Diamidino-2-Phenylindole Dihydrochloride (DAPI) and green is due to fluorescence of curcumin release inside the cells effectively in MDA-MB-231 cancer cells, which is also in agreement with the MTT assay where the comparable % cytotoxicity in the cells is absent.</p> Full article ">Figure 8
<p>Apoptosis of MDA-MB-231 cells using fluorescence microscopy. Images of MDA-MB-231 incubated with 16 µg/mL of free curcumin, MSN-cur-NH2 (GI<sub>50</sub> = 7 µg/mL) and MSN-cur-CMC (GI<sub>50</sub> = 1.5 µg/mL). Control refers to the non-treated MDA-MB-231 cells. Blue fluorescence is due to nucleus staining of cells with DAPI and green fluorescence is due to staining of cells by annexin V-FITC.</p> Full article ">Figure 9
<p>Apoptotic ratios of free curcumin, MSN-cur-NH<sub>2</sub> and MSN-cur-CMC in 48 h.</p> Full article ">Scheme 1
<p>Synthesis of carboxymethyl cellulose (CMC)-grafted mesoporous silica nanoparticles (MSNs).</p> Full article ">Scheme 2
<p>Synthesis of curcumin-loaded carboxymethyl cellulose grafted MSN.</p> Full article ">
51502 KiB  
Article
Hydrogel Micro-/Nanosphere Coated by a Lipid Bilayer: Preparation and Microscopic Probing
by Sarah Rahni and Sergey Kazakov
Gels 2017, 3(1), 7; https://doi.org/10.3390/gels3010007 - 15 Feb 2017
Cited by 6 | Viewed by 6551
Abstract
The result of polymeric nanogels and lipid vesicles interaction—lipobeads—can be considered as multipurpose containers for future therapeutic applications, such as targeted anticancer chemotherapy with superior tumor response and minimum side effects. In this work, micrometer sized lipobeads were synthesized by two methods: (i) [...] Read more.
The result of polymeric nanogels and lipid vesicles interaction—lipobeads—can be considered as multipurpose containers for future therapeutic applications, such as targeted anticancer chemotherapy with superior tumor response and minimum side effects. In this work, micrometer sized lipobeads were synthesized by two methods: (i) mixing separately prepared microgels made of poly(N-isopropylacrylamide) (PNIPA) and phospholipid vesicles of micrometer or nanometer size and (ii) polymerization within the lipid vesicles. For the first time, a high vacuum scanning electron microscopy was shown to be suitable for a quick validation of the structural organization of wet lipobeads and their constituents without special sample preparation. In particular, the structural difference of microgels prepared by thermal and UV-polymerization in different solvents was revealed and three types of giant liposomes were recognized under high vacuum in conjunction with their size, composition, and method of preparation. Importantly, the substructure of the hydrogel core and multi- and unilamellar constructions of the peripheral lipid part were explicitly distinguished on the SEM images of lipobeads, justifying the spontaneous formation of a lipid bilayer on the surface of microgels and evidencing an energetically favorable structural organization of the hydrogel/lipid bilayer assembly. This key property can facilitate lipobeads’ preparation and decrease technological expenses on their scaled production. The comparison of the SEM imaging with the scanning confocal and atomic force microscopies data are also presented in the discussion. Full article
(This article belongs to the Special Issue Micro- and Nanogels)
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Graphical abstract
Full article ">Figure 1
<p>Schematic of the spherical lipid bilayer/hydrogel assemblies (lipobeads) with encapsulated drugs.</p> Full article ">Figure 2
<p>Comparison of the microstructures of bulk wet PNIPA-FA hydrogels prepared by thermal polymerization in water (<b>A</b>) or DMSO (<b>B</b>) and by UV polymerization in water (<b>C</b>) (scale bars = 5 µm).</p> Full article ">Figure 3
<p>Shrinking abilities <span class="html-italic">S<sub>V</sub></span> of the bulk PNIPA-FA hydrogels prepared by thermal and UV polymerization in different solvents.</p> Full article ">Figure 4
<p>The bright field (<b>A</b>,<b>B</b>) and fluorescence (<b>C</b>) images and size distribution (<b>D</b>) of PNIPA-FA hydrogel spheres after washing.</p> Full article ">Figure 5
<p>SEM micrographs of the PNIPA-FA hydrogel spheres filled with water (<b>A</b>,<b>B</b>) or ionic liquid (<b>C</b>) and deposited on the carbon (<b>A</b>) or aluminum (<b>B</b>,<b>C</b>) substrates of the SEM mount.</p> Full article ">Figure 6
<p>SEM micrographs of the wet lipobeads prepared by polymerization inside GMVs (<b>a</b>–<b>c</b>) or giant unilamellar vesicles (GUVs) (<b>d</b>–<b>f</b>) and deposited on the aluminum SEM mounting stub (scale bars = 10 µm).</p> Full article ">Figure 7
<p>The temporal changes in a lipobead morphology under the scanning electron beam (15 kV) in high vacuum (~50 Pa). The lipobead is prepared by polymerization inside a GUV and deposited on the carbon conductive tab preliminary adhered on the SEM mounting stub (scale bars = 20 µm).</p> Full article ">Figure 9
<p>Atomic force microscopy images (amplitude and height data) of a PNIPA nanolipobead resulted from polymerization within a large unilamellar vesicle (LUV) (scale bar = 100 nm, right insert is the height scale). The lipobead was deposited on mica as described in <a href="#sec4-gels-03-00007" class="html-sec">Section 4</a>.</p> Full article ">
1223 KiB  
Review
Hydrogels for Biomedical Applications: Their Characteristics and the Mechanisms behind Them
by Qinyuan Chai, Yang Jiao and Xinjun Yu
Gels 2017, 3(1), 6; https://doi.org/10.3390/gels3010006 - 24 Jan 2017
Cited by 762 | Viewed by 29835
Abstract
Hydrogels are hydrophilic, three-dimensional networks that are able to absorb large quantities of water or biological fluids, and thus have the potential to be used as prime candidates for biosensors, drug delivery vectors, and carriers or matrices for cells in tissue engineering. In [...] Read more.
Hydrogels are hydrophilic, three-dimensional networks that are able to absorb large quantities of water or biological fluids, and thus have the potential to be used as prime candidates for biosensors, drug delivery vectors, and carriers or matrices for cells in tissue engineering. In this critical review article, advantages of the hydrogels that overcome the limitations from other types of biomaterials will be discussed. Hydrogels, depending on their chemical composition, are responsive to various stimuli including heating, pH, light, and chemicals. Two swelling mechanisms will be discussed to give a detailed understanding of how the structure parameters affect swelling properties, followed by the gelation mechanism and mesh size calculation. Hydrogels prepared from natural materials such as polysaccharides and polypeptides, along with different types of synthetic hydrogels from the recent reported literature, will be discussed in detail. Finally, attention will be given to biomedical applications of different kinds of hydrogels including cell culture, self-healing, and drug delivery. Full article
(This article belongs to the Special Issue Hydrogels in Tissue Engineering)
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Graphical abstract
Full article ">Figure 1
<p>Supramolecular inclusion complex <b>1</b> formed from deoxycholate-β-CD derivative <b>2</b> and azobenzene-branched poly(acrylic acid) copolymer <b>3</b>. Reprinted from [<a href="#B34-gels-03-00006" class="html-bibr">34</a>] with permission from the American Chemical Society (2009).</p> Full article ">Figure 2
<p>The response of novel hybrid hydrogels containing ssDNA as a cross-linker to ssDNA. Reprinted from [<a href="#B36-gels-03-00006" class="html-bibr">36</a>] with permission from the American Chemical Society (2005).</p> Full article ">Figure 3
<p>Synthetically tractable click hydrogels for three-dimensional cell culture formed using tetrazine–norbornene chemistry. Reprinted from [<a href="#B71-gels-03-00006" class="html-bibr">71</a>] with permission from the American Chemical Society (2013).</p> Full article ">Figure 4
<p>Schematic representation of high concentration cationic gels adhered using 1.6 mg of Micromica supporting a tensile load of 10 kg. Reprinted from [<a href="#B81-gels-03-00006" class="html-bibr">81</a>] with permission from the American Chemical Society (2016).</p> Full article ">
151 KiB  
Editorial
Acknowledgement to Reviewers of Gels in 2016
by Gels Editorial Office
Gels 2017, 3(1), 5; https://doi.org/10.3390/gels3010005 - 11 Jan 2017
Viewed by 2980
Abstract
The editors of Gels would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2016.[...] Full article
3066 KiB  
Review
Thermoresponsive Gels
by M. Joan Taylor, Paul Tomlins and Tarsem S. Sahota
Gels 2017, 3(1), 4; https://doi.org/10.3390/gels3010004 - 10 Jan 2017
Cited by 139 | Viewed by 18287
Abstract
Thermoresponsive gelling materials constructed from natural and synthetic polymers can be used to provide triggered action and therefore customised products such as drug delivery and regenerative medicine types as well as for other industries. Some materials give Arrhenius-type viscosity changes based on coil [...] Read more.
Thermoresponsive gelling materials constructed from natural and synthetic polymers can be used to provide triggered action and therefore customised products such as drug delivery and regenerative medicine types as well as for other industries. Some materials give Arrhenius-type viscosity changes based on coil to globule transitions. Others produce more counterintuitive responses to temperature change because of agglomeration induced by enthalpic or entropic drivers. Extensive covalent crosslinking superimposes complexity of response and the upper and lower critical solution temperatures can translate to critical volume temperatures for these swellable but insoluble gels. Their structure and volume response confer advantages for actuation though they lack robustness. Dynamic covalent bonding has created an intermediate category where shape moulding and self-healing variants are useful for several platforms. Developing synthesis methodology—for example, Reversible Addition Fragmentation chain Transfer (RAFT) and Atomic Transfer Radical Polymerisation (ATRP)—provides an almost infinite range of materials that can be used for many of these gelling systems. For those that self-assemble into micelle systems that can gel, the upper and lower critical solution temperatures (UCST and LCST) are analogous to those for simpler dispersible polymers. However, the tuned hydrophobic-hydrophilic balance plus the introduction of additional pH-sensitivity and, for instance, thermochromic response, open the potential for coupled mechanisms to create complex drug targeting effects at the cellular level. Full article
(This article belongs to the Special Issue Stimuli-Responsive Gels)
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Full article ">Figure 1
<p>Gelling of injection below its upper critical solution temperature (UCST).</p> Full article ">Figure 2
<p>The UCST detail showing spinodal and binodal curves.</p> Full article ">Figure 3
<p>Relationship of the polymer form with temperature for a polymer showing UCST behaviour.</p> Full article ">Figure 4
<p>UCST, glass transition (Tg) and Berghmans point.</p> Full article ">Figure 5
<p>Gelatin coil and helix ‘crystalline’ regions.</p> Full article ">Figure 6
<p>Related EC-g-copolymer aqueous micelle systems that gel when aggregated beyond LCST or UCST respectively. Adapted from [<a href="#B124-gels-03-00004" class="html-bibr">124</a>].</p> Full article ">Figure 7
<p>Dually responsive (i.e., two sequential stages LCST) micelle formation from poly(mPEGV<sub>466</sub>)<sub>18</sub>-<span class="html-italic">b</span>-PNIPAm<sub>60</sub> in water. Adapted from [<a href="#B145-gels-03-00004" class="html-bibr">145</a>].</p> Full article ">Figure 8
<p>Unified LCST and USCT showing both theta points.</p> Full article ">Figure 9
<p>Hourglass pattern of some combined LCST and UCST behaviours.</p> Full article ">Figure 10
<p>Schizophrenic (reverse) PDEGEA-PMA micelles in ethanolic solution. Adapted from [<a href="#B152-gels-03-00004" class="html-bibr">152</a>].</p> Full article ">Figure 11
<p>Swelling characteristics of hydrogels.</p> Full article ">
5252 KiB  
Review
Responsive Hydrogels from Associative Block Copolymers: Physical Gelling through Polyion Complexation
by Christine M. Papadakis and Constantinos Tsitsilianis
Gels 2017, 3(1), 3; https://doi.org/10.3390/gels3010003 - 1 Jan 2017
Cited by 30 | Viewed by 8008
Abstract
The present review article highlights a specific class of responsive polymer-based hydrogels which are formed through association of oppositely charged polyion segments. The underpinning temporary three-dimensional network is constituted of hydrophilic chains (either ionic or neutral) physically crosslinked by ion pair formation arising [...] Read more.
The present review article highlights a specific class of responsive polymer-based hydrogels which are formed through association of oppositely charged polyion segments. The underpinning temporary three-dimensional network is constituted of hydrophilic chains (either ionic or neutral) physically crosslinked by ion pair formation arising from intermolecular polyionic complexation of oppositely charged repeating units (polyacid/polybase ionic interactions). Two types of hydrogels are presented: (i) hydrogels formed by triblock copolymers bearing oppositely charged blocks (block copolyampholytes), forming self-assembled networks; and (ii) hydrogels formed by co-assembly of oppositely charged polyelectrolyte segments belonging to different macromolecules (either block copolymers or homopolyelectrolytes). Due to the weak nature of the involved polyions, these hydrogels respond to pH and are sensitive to the presence of salts. Discussing and evaluating their solution, rheological and structural properties in dependence on pH and ionic strength, it comes out that the hydrogel properties are tunable towards potential applications. Full article
(This article belongs to the Special Issue Stimuli-Responsive Gels)
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Full article ">Figure 1
<p>pH dependence of: (<b>a</b>) the optical absorbance (black symbols), the zeta potential (blue symbols); and (<b>b</b>) of the reduced viscosity, η<sub>sp</sub>/<span class="html-italic">c</span>, for a APA<sub>1</sub> aqueous solution (0.2 wt %) at 25 °C. (<b>a</b>,<b>b</b>) Adapted with permission from [<a href="#B32-gels-03-00003" class="html-bibr">32</a>]. Copyright 2003 American Chemical Society. (<b>c</b>) Zero-shear viscosity as a function of polymer concentration; (<b>d</b>) Storage modulus <span class="html-italic">G′</span> (closed symbols) and loss modulus <span class="html-italic">G</span>″ (open symbols) as a function of frequency at different polymer concentrations: circles (3.5 wt %), triangles (4.5 wt %), squares (6.0 wt %). Adapted with permission from [<a href="#B33-gels-03-00003" class="html-bibr">33</a>]. Copyright 2004 American Chemical Society.</p> Full article ">Figure 2
<p>pH-dependent rheological properties of salt-free aqueous solutions of A(QP)A<sub>2</sub>. (<b>a</b>) <b>Left</b>: Apparent viscosity as a function of shear stress for A(QP)A<sub>2</sub> at <span class="html-italic">c</span> = 4 wt % and various pH conditions; <b>right</b>: photos showing free-standing gels at pH 3 and 4, whereas solution behavior is observed at other pH values; (<b>b</b>) pH dependence of zero-shear viscosity, η<sub>0</sub>, at <span class="html-italic">c</span> = 4 wt % and its precursor APA<sub>2</sub> at <span class="html-italic">c</span> = 1.2 wt % (<b>inset</b>); (<b>c</b>) Dynamic moduli, <span class="html-italic">G</span>′ (black symbols) and <span class="html-italic">G</span>″ (red symbols) versus frequency at different pH values of A(QP)A<sub>2</sub> at <span class="html-italic">c</span> = 4 wt % at the pH values given in the graphs. Adapted with permission from [<a href="#B34-gels-03-00003" class="html-bibr">34</a>]. Copyright 2014 American Chemical Society.</p> Full article ">Figure 3
<p>(<b>a</b>) Concentration dependence of the zero shear viscosity at the pH of maximum viscosity for A(QP)A<sub>2</sub> (closed symbols) and APA<sub>2</sub> (pH 2.9) (open symbols). Lines along the data guide the eyes and arrows indicate the gelation concentration; (<b>b</b>) Apparent viscosity versus shear stress of 1.6 wt % aqueous A(QP)A<sub>2</sub> solution at pH 3: increasing stress (black circles) and decreasing stress (red circles). Adapted with permission from [<a href="#B34-gels-03-00003" class="html-bibr">34</a>]. Copyright 2014 American Chemical Society.</p> Full article ">Figure 4
<p>SANS results from A(QP)A<sub>2</sub> and APA<sub>2</sub>. Data from [<a href="#B34-gels-03-00003" class="html-bibr">34</a>]. (<b>a</b>) Scattered intensity, <span class="html-italic">I</span>(<span class="html-italic">q</span>), as a function of the momentum transfer, <span class="html-italic">q</span>, of solutions of A(QP)A<sub>2</sub> at pD 7.0 (open blue circles), pD 5.0 (open green triangles) and pD 3.0 (open black squares), and APA<sub>2</sub> at pD 3.0 (closed black squares, shifted vertically for clarity). All scattering curves were measured at a concentration of 4 wt % and at 26 °C. The red lines are the model fits; (<b>b</b>) Corresponding nanostructures at the pH values given. Blue lines: QP, orange lines: A, yellow circles: counter ions. Average length scales are given. Adapted with permission from [<a href="#B34-gels-03-00003" class="html-bibr">34</a>]. Copyright 2014 American Chemical Society.</p> Full article ">Figure 5
<p>Effect of NaCl on the self-assembled hydrogels from A(QP)A<sub>3</sub> and APA<sub>3</sub>. (<b>a</b>) Zero shear viscosity, <span class="html-italic">η<sub>0</sub></span>, as a function of NaCl concentration of 3 wt % APA<sub>3</sub> aqueous solutions at pH 3.0; (<b>b,c</b>) SANS curves of solutions of APA<sub>3</sub> at pD 3.0 (<b>b</b>) and A(QP)A<sub>3</sub> at at pD 5.0 (<b>c</b>), both at 3 wt % in D<sub>2</sub>O and at 26 °C for different NaCl concentrations (symbols) together with the model curves (lines); and (<b>d</b>) sketch of the morphology at a NaCl concentration of 0.15 M. Adapted with permission from [<a href="#B35-gels-03-00003" class="html-bibr">35</a>]. Copyright 2015 American Chemical Society.</p> Full article ">Figure 6
<p>Snapshots from computer simulations of polyampholyte solutions PAA<sub>20</sub>-<span class="html-italic">b</span>-QP2VP<sub>172</sub>-<span class="html-italic">b</span>-PAA<sub>20</sub> (A(QP)A<sub>4</sub>) at a polymer concentration of 2.2 wt % in salt-free solution (<b>a</b>); and at salt concentrations of: 0.153 M (<b>b</b>); 0.305 M (<b>c</b>); and 0.61 M (<b>d</b>). A and QP monomers are shown in orange and blue color, respectively. Reprinted with permission from [<a href="#B35-gels-03-00003" class="html-bibr">35</a>]. Copyright 2015 American Chemical Society.</p> Full article ">Figure 7
<p>Co-assembled system formed by a triblock copolymer with negatively charged end blocks and: a neutral middle block (SES) (<b>a</b>); and AH homopolymer (<b>b</b>); (<b>c</b>) Small-angle X-ray scattering (SAXS) data for gels from SES and AH at 0.4 M KCl at 20 °C. The polymer concentrations are (from bottom to top) 1%, 4%, 6%, 12% and 20%. Lines are model fits. The curves are shifted vertically as indicated in the graph; (<b>d</b>) Schematic morphology diagram for gels from SES and AH in dependence on polymer concentration, <span class="html-italic">C</span>, and salt concentration [KCl]. Reproduced with permission from [<a href="#B38-gels-03-00003" class="html-bibr">38</a>]. Copyright 2011 Royal Society of Chemistry.</p> Full article ">Figure 8
<p>Schematics of the morphologies of solutions from SES and AH for excess negative charge, charge stoichiometry and excess positive charge for the concentrated regime. Adapted with permission from [<a href="#B39-gels-03-00003" class="html-bibr">39</a>]. Copyright 2012 Royal Society of Chemistry.</p> Full article ">Figure 9
<p>(<b>a</b>) Schematics of the polymers MEM and A; (<b>b</b>) storage modulus G′ and loss modulus G″ of polyion complexes flower micelles (55 mg/mL, <math display="inline"> <semantics> <mrow> <mi>r</mi> <mo>=</mo> <mn>1</mn> <mo>:</mo> <mn>1</mn> </mrow> </semantics> </math>, 150 mM NaCl, pH 6.2, 550 mM phosphate buffer) with increasing temperature and decreasing temperature, as indicated by the arrows; (<b>c</b>) ionic strength dependence (55 mg/mL, <math display="inline"> <semantics> <mrow> <mi>r</mi> <mo>=</mo> <mn>1</mn> <mo>:</mo> <mn>1</mn> </mrow> </semantics> </math> , pH 6.2): concentration of phosphate buffer 330 mM (closed squares) and 550 mM (closed circles); and (<b>d</b>) schematics of the solution of the polyion complexes micelles and the irreversible gel formation upon increasing temperature and ionic strength. Adapted with permission from [<a href="#B40-gels-03-00003" class="html-bibr">40</a>]. Copyright 2015 American Chemical Society.</p> Full article ">Figure 10
<p>(<b>a</b>) Co-assembled system formed by charged triblock copolymers, which were synthesized from the same precursor having a hydrophilic middle block and uncharged end blocks. Schematics of co-assembly of the triblock copolymers having oppositely charged end blocks; (<b>b</b>) chemical structures of the charged triblock copolymers having guadinium and sulfonate groups; and (<b>c</b>) phase diagram of mixtures of these triblock copolymers in dependence on polymer and NaCl concentration. BCC stands for body-centered cubic. Reprinted with permission from [<a href="#B42-gels-03-00003" class="html-bibr">42</a>]. Copyright 2013 American Chemical Society.</p> Full article ">Figure 11
<p>Phase diagrams of mixtures of triblock copolymers with a hydrophilic middle block and oppositely charged end blocks, calculated using the embedded fluctuation model. The morphologies are given in dependence on polymer number concentration and on the endblock fraction for different sets of parameters. <span class="html-italic">E</span> is the electrostatic strength parameter. DIS denotes a polymer rich homogeneous phase, DIL a nearly pure water phase, L lamellae, C hexagonally packed cylinders and S cubically packed spheres (body-centered or face-centered). Adapted with permission from [<a href="#B43-gels-03-00003" class="html-bibr">43</a>]. Copyright 2015 Royal Society of Chemistry.</p> Full article ">
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Review
Single-Handed Helical Polybissilsesquioxane Nanotubes and Mesoporous Nanofibers Prepared by an External Templating Approach Using Low-Molecular-Weight Gelators
by Jing Hu and Yonggang Yang
Gels 2017, 3(1), 2; https://doi.org/10.3390/gels3010002 - 1 Jan 2017
Cited by 11 | Viewed by 4780
Abstract
Chiral low-molecular-weight gelators (LMWGs) derived from amino acids can self-assemble into helical fibers and twisted/coiled nanoribbons by H-bonding and π–π interaction. Silica nanotubes with single-handed helices have been prepared using chiral LMWGs through sol–gel transcription. Molecular-scale chirality exists at the inner surfaces. Here, [...] Read more.
Chiral low-molecular-weight gelators (LMWGs) derived from amino acids can self-assemble into helical fibers and twisted/coiled nanoribbons by H-bonding and π–π interaction. Silica nanotubes with single-handed helices have been prepared using chiral LMWGs through sol–gel transcription. Molecular-scale chirality exists at the inner surfaces. Here, we discuss single-handed helical aromatic ring-bridged polybissilsesquioxane nanotubes and mesoporous nanofibers prepared using chiral LMWGs. This review aims at describing the formation mechanisms of the helical nanostructures, the origination of optical activity, and the applications for other helical nanomaterial preparation, mainly based on our group’s results. The morphology and handedness can be controlled by changing the chirality and kinds of LMWGs and tuning the reaction conditions. The aromatic rings arrange in a partially crystalline structure. The optical activity of the polybissilsesquioxane nanotubes and mesoporous nanofibers originates from chiral defects, including stacking and twisting of aromatic groups, on the inner surfaces. They can be used as the starting materials for preparation of silica, silicon, carbonaceous, silica/carbon, and silicon carbide nanotubes. Full article
(This article belongs to the Special Issue Gels as Templates for Transcription)
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Graphical abstract
Full article ">Figure 1
<p>Molecular structures of the low-molecular-weight gelators (LMWGs).</p> Full article ">Figure 2
<p>(<b>a</b>–<b>c</b>) Field-emission scanning electron microscopy (FESEM) and (<b>d</b>) transmission electron microscopy (TEM) images of left-handed multiple helical mesoporous 1,4-phenylene-silica nanofibers; (<b>e</b>) FESEM image of right-handed multiple helical mesoporous 1,4-phenylene-silica nanofibers. Reproduced with permission from [<a href="#B46-gels-03-00002" class="html-bibr">46</a>]. Copyright 2009 American Chemical Society.</p> Full article ">Figure 3
<p>(<b>a</b>) FESEM and (<b>b</b>) TEM images of helical 1,4-phenylene-bridged polybissilsesquioxane nanorods prepared using <b>L-4</b>; (<b>c</b>) FESEM and (<b>d</b>) TEM images of nanorods prepared using <b>D-4</b>. Reproduced with permission from [<a href="#B50-gels-03-00002" class="html-bibr">50</a>]. Copyright 2011 Royal Society of Chemistry.</p> Full article ">Figure 4
<p>FESEM images of (<b>a</b>,<b>b</b>) 4,4′-biphenylene bridged polybissilsesquioxane nanotubes; (<b>c</b>,<b>d</b>) carbon/silica nanotubes; and (<b>e</b>,<b>f</b>) carbonaceous nanotubes. The samples were prepared using (<b>a</b>,<b>c</b>,<b>e</b>) <b>LL-8</b> and (<b>b</b>,<b>d</b>,<b>f</b>) <b>DD-8</b>. Reproduced with permission from [<a href="#B47-gels-03-00002" class="html-bibr">47</a>]. Copyright 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA.</p> Full article ">Figure 5
<p>TEM images of the reaction mixture after (<b>a</b>) 0 s; (<b>b</b>) 90 s; (<b>c</b>) 3.0 min; and (<b>d</b>) 4.0 min. Reproduced with permission from [<a href="#B52-gels-03-00002" class="html-bibr">52</a>]. Copyright 2008 Royal Society of Chemistry.</p> Full article ">Figure 6
<p>Schematic illustration of formation of single-handed helical polybissilsesquioxane nanostructures. Formation of mesoporous nanofibers (Routes A–C) and that of nanotubes and double-twisted nanoribbons (Routes D–G).</p> Full article ">Figure 7
<p>Simulated circular dichroism (CD) spectrum of the 1,4-phenylene-bridged bis(silsesquioxane) dimer at the B3LYP/6-311++G** level with right-handed stacking of phenylene rings.</p> Full article ">Figure 8
<p>Simulated CD spectrum of the right-handed twisted and stacked biphenylene rings of the 4,4′-biphenylene-bridged bis(silsesquioxane) dimer.</p> Full article ">
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Review
Physicochemical Properties and the Gelation Process of Supramolecular Hydrogels: A Review
by Abdalla H. Karoyo and Lee D. Wilson
Gels 2017, 3(1), 1; https://doi.org/10.3390/gels3010001 - 1 Jan 2017
Cited by 55 | Viewed by 15458
Abstract
Supramolecular polysaccharide-based hydrogels have attracted considerable research interest recently due to their high structural functionality, low toxicity, and potential applications in foods, cosmetics, catalysis, drug delivery, tissue engineering and the environment. Modulation of the stability of hydrogels is of paramount importance, especially in [...] Read more.
Supramolecular polysaccharide-based hydrogels have attracted considerable research interest recently due to their high structural functionality, low toxicity, and potential applications in foods, cosmetics, catalysis, drug delivery, tissue engineering and the environment. Modulation of the stability of hydrogels is of paramount importance, especially in the case of stimuli-responsive systems. This review will update the recent progress related to the rational design of supramolecular hydrogels with the objective of understanding the gelation process and improving their physical gelation properties for tailored applications. Emphasis will be given to supramolecular host–guest systems with reference to conventional gels in describing general aspects of gel formation. A brief account of the structural characterization of various supramolecular hydrogels is also provided in order to gain a better understanding of the design of such materials relevant to the nature of the intermolecular interactions, thermodynamic properties of the gelation process, and the critical concentration values of the precursors and the solvent components. This mini-review contributes to greater knowledge of the rational design of supramolecular hydrogels with tailored applications in diverse fields ranging from the environment to biomedicine. Full article
(This article belongs to the Special Issue Colloid Chemistry)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Schematic representations of possible mechanisms of network formation in helical gels: gelation on: (<b>a</b>) the helical level; and (<b>b</b>) super-helical level. Reproduced from [<a href="#B68-gels-03-00001" class="html-bibr">68</a>] with permission. Copyright 1994 American Chemical Society.</p> Full article ">Figure 2
<p>Optical photos of the complexes of: (<b>A</b>) mPEG1.1K; and (<b>B</b>) mPEG2K with α-CD; and invertible supramolecular hydrogels formed by: (<b>C</b>) Ada-PEG1.1K; and (<b>D</b>) Ada-PEG2K and α-CD. Reproduced from [<a href="#B65-gels-03-00001" class="html-bibr">65</a>] with permission. Copyright 2008 American Chemical Society.</p> Full article ">Figure 3
<p>(<b>a</b>) Schematic representing dimer configurations of various amphiphilic systems (<b>1</b>–<b>6</b>); (<b>b</b>) Structures of the aromatic carbohydrate amphiphiles <b>1</b>–<b>6</b> containing different aromatic moieties (R<sub>1</sub> and R<sub>2</sub>) and either a galactosamine or glucosamine residues. R<sub>1</sub> and R<sub>2</sub> represent fluorene and naphthalene residues, respectively. Redrawn from [<a href="#B90-gels-03-00001" class="html-bibr">90</a>].</p> Full article ">Figure 4
<p>Chemical illustration of: (<b>a</b>) the host monomer (mono-Ac-βCD); (<b>b</b>) the guest polymer (AD<sub>x</sub>HA); and (<b>c</b>) the host–guest macromer (HGM). (<b>d</b>–<b>g</b>) Representation of various hydrogel/host–guest structures. Reproduced from [<a href="#B48-gels-03-00001" class="html-bibr">48</a>] with permission. Copyright 2016 American Chemical Society.</p> Full article ">Scheme 1
<p>Phase transition of a gel indicated as a reversible and discontinuous volume change in response to various stimuli. Redrawn from [<a href="#B67-gels-03-00001" class="html-bibr">67</a>].</p> Full article ">Scheme 2
<p>Born–Haber cycle showing separation of the actual (measured) enthalpy (ΔH<sub>actual</sub>) into intrinsic enthalpy (ΔH<sub>intinsic</sub>) and the enthalpy of solution values for the bound (ΔH<sub>s,b</sub>) and unbound species (ΔH<sub>s,u</sub>). The physical states of the reactants and products are not shown.</p> Full article ">Scheme 3
<p>Hypothetical structure of a polymer network showing a chitosan backbone (purple line) with hydrophilic PEG pendants (red lines) and CD pendants (toroid).</p> Full article ">
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