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Volume 3
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Gels, Volume 3, Issue 4 (December 2017) – 12 articles

Cover Story (view full-size image): Confocal laser scanning microscopy, as a non-destructive method, allows the study of macroporous cryogels or hydrogels in native strongly hydrated state (up to 99 % water) to analyse 3D morphology and textural characteristics in parallel to analyses with low-temperature 1H NMR spectroscopy, cryoporometry, relaxometry, etc. These give a deeper insight into interfacial phenomena, linked to the behaviour of bound water, that is very important in biomedical applications of the materials. View Paper here.
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7326 KiB  
Article
Improved Concrete Materials with Hydrogel-Based Internal Curing Agents
by Matthew J. Krafcik, Nicholas D. Macke and Kendra A. Erk
Gels 2017, 3(4), 46; https://doi.org/10.3390/gels3040046 - 25 Nov 2017
Cited by 51 | Viewed by 9404
Abstract
This research article will describe the design and use of polyelectrolyte hydrogel particles as internal curing agents in concrete and present new results on relevant hydrogel-ion interactions. When incorporated into concrete, hydrogel particles release their stored water to fuel the curing reaction, resulting [...] Read more.
This research article will describe the design and use of polyelectrolyte hydrogel particles as internal curing agents in concrete and present new results on relevant hydrogel-ion interactions. When incorporated into concrete, hydrogel particles release their stored water to fuel the curing reaction, resulting in reduced volumetric shrinkage and cracking and thus increasing concrete service life. The hydrogel’s swelling performance and mechanical properties are strongly sensitive to multivalent cations that are naturally present in concrete mixtures, including calcium and aluminum. Model poly(acrylic acid(AA)-acrylamide(AM))-based hydrogel particles with different chemical compositions (AA:AM monomer ratio) were synthesized and immersed in sodium, calcium, and aluminum salt solutions. The presence of multivalent cations resulted in decreased swelling capacity and altered swelling kinetics to the point where some hydrogel compositions displayed rapid deswelling behavior and the formation of a mechanically stiff shell. Interestingly, when incorporated into mortar, hydrogel particles reduced mixture shrinkage while encouraging the formation of specific inorganic phases (calcium hydroxide and calcium silicate hydrate) within the void space previously occupied by the swollen particle. Full article
(This article belongs to the Special Issue Polyelectrolyte Gels)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>A schematic of a covalently crosslinked polyelectrolyte hydrogel network in a dry state (<span class="html-italic">ξ</span><sub>1</sub>), swollen state (<span class="html-italic">ξ</span><sub>2</sub>) after adding water, and deswollen state (<span class="html-italic">ξ</span><sub>3</sub>) upon being exposed to a water and salt solution. The dry state is indicated with a lighter background colors. The approximate distance between crosslinks (mesh size) is indicated with <span class="html-italic">ξ</span>. The charged acrylic acid (AA) segments are indicated by is the dashed light red lines, while the uncharged acrylamide (AM) is indicated with dark blue lines. The covalent crosslinker (MBAM) is indicated with black squares, and counterions are indicated with red circles. The charges of each counterion are displayed within the circle. Monomer chemical structures (acrylic acid and acrylamide) are also provided. Not to scale.</p> Full article ">Figure 2
<p>Hydrogel swelling ratios as a function of time in cement pore solution are shown in (<b>a</b>). Strain over time (autogenous shrinkage) results for cement mortars with and without hydrogels indicated by (<b>b</b>). Water-to-cement (w/c) ratios are provided for each mortar specimen in addition to hydrogel composition (% AA). Data adapted from Krafcik and Erk, 2016 [<a href="#B32-gels-03-00046" class="html-bibr">32</a>].</p> Full article ">Figure 3
<p>Swelling ratios over time for 17% AA (<b>a</b>), 33% AA (<b>b</b>), 67% AA (<b>c</b>), and 83% AA (<b>d</b>) hydrogels immersed in solutions of sodium chloride, calcium nitrate, and aluminum sulfate.</p> Full article ">Figure 4
<p>Swelling ratios over time for all hydrogel compositions in two different aluminum sulfate solution concentrations, (<b>a</b>) 0.005 M and (<b>b</b>) 0.025 M.</p> Full article ">Figure 5
<p>Elastic modulus of large hydrogel pieces that were exposed to several concentrations of aluminum sulfate solutions for 96 h. Modulus was calculated from the small-strain linear regime of the stress-strain curve obtained from compression testing.</p> Full article ">Figure 6
<p>Elastic modulus as a function of time for all compositions of hydrogels immersed into 0.025 M aluminum sulfate solution. Higher uncertainties at later times are due to cracking, bulging, and other macroscopic deformations of the hydrogel specimen; modulus values after 24 h have been omitted from the graph.</p> Full article ">Figure 7
<p>Comparison of control paste with no hydrogels (<b>a</b>) and 3-day cured paste containing 17% AA hydrogels (<b>b</b>). Each paste had w/c = 0.35. Hydrogel voids are indicated with black circles, unhydrated cement grains are white, hydrated product is the gray matrix throughout the image.</p> Full article ">Figure 8
<p>3-day cured pastes containing 67% AA hydrogels (<b>a</b>). Hydrogel voids are circled in black. Some voids have hydrated product in them. A 17% AA hydrogel void with capillary porosity surrounding it (<b>b</b>). Hydrogel remains are traced in red and calcium hydroxide crystals are indicated in blue (<b>b</b> insert).</p> Full article ">Figure 9
<p>Percentage of total voids counted that contained either calcium hydroxide or some hydrated product (either calcium hydroxide and/or calcium-silicate-hydrate). As percent AA increases, the proportion of voids containing both calcium hydroxide and calcium-silicate-hydrate decreases dramatically.</p> Full article ">Figure 10
<p>Morphology map that relates structure and mechanical properties of hydrogels as a function of aluminum sulfate concentration and immersion time. Majority acrylamide hydrogels are summarized in (<b>a</b>) and majority acrylic acid hydrogels are summarized in (<b>b</b>).</p> Full article ">Figure 11
<p>A photograph of a majority acrylic acid hydrogel several centimeters in size swollen in 0.025 M aluminum solution. The specimen was sliced in half with a razor blade, revealing a hollow, water-filled interior. This water was not bound to any part of the hydrogel and could be poured out of the hollow shell onto the table.</p> Full article ">Figure 12
<p>Amount of dry hydrogel mass that was retained on each sieve for the hydrogels synthesized for this study. Size indicates the maximum opening of the wire sieve [<a href="#B32-gels-03-00046" class="html-bibr">32</a>].</p> Full article ">
3289 KiB  
Article
Effect of Shear History on Rheology of Time-Dependent Colloidal Silica Gels
by Paulo H. S. Santos, Marcelo A. Carignano and Osvaldo Campanella
Gels 2017, 3(4), 45; https://doi.org/10.3390/gels3040045 - 20 Nov 2017
Cited by 20 | Viewed by 7474
Abstract
This paper presents a rheological study describing the effects of shear on the flow curves of colloidal gels prepared with different concentrations of fumed silica (4%, 5%, 6%, and 7%) and a hydrophobic solvent (Hydrocarbon fuel, JP-8). Viscosity measurements as a function of [...] Read more.
This paper presents a rheological study describing the effects of shear on the flow curves of colloidal gels prepared with different concentrations of fumed silica (4%, 5%, 6%, and 7%) and a hydrophobic solvent (Hydrocarbon fuel, JP-8). Viscosity measurements as a function of time were carried out at different shear rates (10, 50, 100, 500, and 1000 s−1), and based on this data, a new structural kinetics model was used to describe the system. Previous work has based the analysis of time dependent fluids on the viscosity of the intact material, i.e., before it is sheared, which is a condition very difficult to achieve when weak gels are tested. The simple action of loading the gel in the rheometer affects its structure and rheology, and the reproducibility of the measurements is thus seriously compromised. Changes in viscosity and viscoelastic properties of the sheared material are indicative of microstructural changes in the gel that need to be accounted for. Therefore, a more realistic method is presented in this work. In addition, microscopical images (Cryo-SEM) were obtained to show how the structure of the gel is affected upon application of shear. Full article
(This article belongs to the Special Issue Rheology of Gels)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>(<b>a</b>) Typical thixotropic behavior of a rheological time dependent gel after the application of constant shear rates of 50 1/s and 100 1/s; (<b>b</b>) schematic of the approach used to determine the parameters defining the structural parameter <math display="inline"> <semantics> <mrow> <mi>λ</mi> <mrow> <mo>(</mo> <mrow> <mover accent="true"> <mi>γ</mi> <mo>˙</mo> </mover> <mo>,</mo> <mi>t</mi> </mrow> <mo>)</mo> </mrow> </mrow> </semantics> </math> from Equation (10). Data used are from a 5% silica gel.</p> Full article ">Figure 2
<p>Viscosity measurements of 6% gel. Three runs were carried out using the same sample: shear rate was increased from 1 to 1000 s<sup>−1</sup> (ramp up), varied from 1000 to 1 s<sup>−1</sup> (ramp down), and increased from 1 to 1000 s<sup>−1</sup> (ramp up 2).</p> Full article ">Figure 3
<p>Viscosity (mean) as a function of shear rate for 4%, 5%, 6%, and 7% gels. These curves describe the flow behavior of the sheared and time independent fluid. The Extended Herschel-Bulkley model was applied to describe the flow characteristics of the gels. Measurements were carried out in triplicate. Error bars shown in the plot were calculated using a 95% confidence interval determined with the software OriginPro 2016 (OriginLab Corporation, Northampton, MA, USA).</p> Full article ">Figure 4
<p>“Intact” and sheared gels: (<b>A</b>) shear stress versus shear rate of 6% gels. A typical curve of shear thinning fluid is only observed for the sheared gel (empty circle marks); (<b>B</b>) first normal stress versus shear rate of 6% gels. Filled squares show the decrease in normal stress of the “intact” gel at different shear rates. Empty squares show no variation of normal stress at different shear rates.</p> Full article ">Figure 5
<p>Cryo-SEM micrographs of 6% “intact” (<b>on the left</b>) and pre-sheared gels (<b>on the right</b>). Micrographs at different magnifications: 40 and 20 µm (from <b>top</b> to the <b>bottom</b>).</p> Full article ">Figure 6
<p>Viscosity of 4% “intact” gel as a function of time at different constant shear rates (50, 100, 500, and 1000 s<sup>−1</sup>).</p> Full article ">Figure 7
<p>(<b>A</b>) Experimental (symbols) and predicted viscosity (lines) data for 4% and 5% gels at 10 and 500 s<sup>−1</sup>. The 4% data is plotted in the left <span class="html-italic">Y</span>-axis and is represented by square symbols. The 5% data is plotted in the right Y-axis and is represented by diamond symbols; (<b>B</b>) calculated viscosity assuming <math display="inline"> <semantics> <mrow> <msub> <mi>λ</mi> <mi>e</mi> </msub> <mo>=</mo> <mn>1</mn> </mrow> </semantics> </math> for 4% gels at shear rates of 100 and 1000 s<sup>−1</sup>.</p> Full article ">
3768 KiB  
Article
Micromechanical Characterization of Hydrogels Undergoing Swelling and Dissolution at Alkaline pH
by Wei Hu, Francois Martin, Romain Jeantet, Xiao Dong Chen and Ruben Mercadé-Prieto
Gels 2017, 3(4), 44; https://doi.org/10.3390/gels3040044 - 18 Nov 2017
Cited by 5 | Viewed by 5025
Abstract
The swelling of polyelectrolyte hydrogels usually depends on the pH, and if the pH is high enough degradation can occur. A microindentation device was developed to dynamically test these processes in whey protein isolate hydrogels at alkaline pH 7–14. At low alkaline pH [...] Read more.
The swelling of polyelectrolyte hydrogels usually depends on the pH, and if the pH is high enough degradation can occur. A microindentation device was developed to dynamically test these processes in whey protein isolate hydrogels at alkaline pH 7–14. At low alkaline pH the shear modulus decreases during swelling, consistent with rubber elasticity theory, yet when chemical degradation occurs at pH ≥ 11.5 the modulus decreases quickly and extensively. The apparent modulus was constant with the indentation depth when swelling predominates, but gradients were observed when fast chemical degradation occurs at 0.05–0.1 M NaOH. In addition, these profiles were constant with time when dissolution rates are also constant, the first evidence that a swollen layer with steady state mechanical properties is achieved despite extensive dissolution. At >0.5 M NaOH, we provide mechanical evidence showing that most interactions inside the gels are destroyed, gels were very weak and hardly swell, yet they still dissolve very slowly. Microindentation can provide complementary valuable information to study the degradation of hydrogels. Full article
(This article belongs to the Special Issue Polyelectrolyte Gels)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Experimental setup for the dynamic mechanical characterization of hydrogels by indentation. (<b>a</b>) Right after the gel is immersed in water, and (<b>b</b>) after 180 min; (<b>c</b>) in pH 11.5 after 180 min and (<b>d</b>) in 0.1 M NaOH after 140 min. The weight of the gels, as well as the calculated overall volumetric swelling degree <span class="html-italic">Q</span>, is also shown. Note that there is extensive dissolution in (<b>d</b>).</p> Full article ">Figure 2
<p>An example of an indentation of a whey protein isolate (WPI) hydrogel swollen from 3 min in water. The contact point is estimated by robust regression at large force values (dashed line), e.g., 6–31 mN here. The apparent shear modulus is then calculated using Equation (1) at different indentation depths at 25 μm intervals, shown in the inset, only for the purpose of checking the mechanical homogeneity of the indented layer. Error bars show the 95% CI of the fitted G. The dashed line in the inset is the average <span class="html-italic">G</span> between 50–500 μm.</p> Full article ">Figure 3
<p>Height increase measured from the estimated contact point in swelling experiments at low alkaline pH or in water. Notice the significant variability of the dynamic swelling process between the several repeats shown, which masks the effect of the solution pH.</p> Full article ">Figure 4
<p>Calculated apparent shear modulus at different indentation depths in gels swollen at different conditions for different times. Dry gels refer to gel as prepared, i.e., before being submerged in solution. Points are the mean value of three indentations at different locations, error bars show the standard error. Note that as Equation (1) is only valid for homogeneous samples, <span class="html-italic">G</span> estimates with depth can only highlight qualitatively if substantial mechanical inhomogeneity occurs in the tested swollen layer.</p> Full article ">Figure 5
<p>Shear modulus during swelling experiments in different solutions normalized against the initial modulus of the dry unswollen gels (before being submerged in solution). Error bars show the standard deviation of the moduli calculated between 50–500 μm indentation depths for three replicates in different locations. Lines show several theoretical scaling laws for neutral polymer gels as a comparison.</p> Full article ">Figure 6
<p>Swelling at neutral pH and at 1 M NaCl. (<b>a</b>) Shear modulus with the indentation depth at different swelling times; (<b>b</b>) average modulus (&gt;50 μm indentation depth) and estimated gel height at different swelling times; (<b>c</b>) correlation between the shear modulus and the overall volumetric swelling ratio Q; the continuous line is the best power law fit. Error bars in (<b>a</b>) as in <a href="#gels-03-00044-f004" class="html-fig">Figure 4</a>, in (<b>b</b>,<b>c</b>) as in <a href="#gels-03-00044-f005" class="html-fig">Figure 5</a>.</p> Full article ">Figure 7
<p>Shear modulus and height of particulate gels swollen in water (filled points) and in 0.1 M NaCl (empty points). Particulate gels were made with 0.1 M NaCl, heated at 80 °C for 1 h. Note that both the modulus and the height change little with time. Error bars are as in <a href="#gels-03-00044-f005" class="html-fig">Figure 5</a>.</p> Full article ">Figure 8
<p>(<b>a</b>) Estimated height of the mechanically responsive gels (“hard” gels) at high (NaOH). Inset shows an example of the “weak” protein layer that forms during dissolution; (<b>b</b>) total height of gels from side microscopy measurements, including the “weak” layer shown in the inset. Empty points and dashed lines are used for repeated experiments.</p> Full article ">Figure 9
<p>Calculated shear modulus at different indentation depths in (NaOH) typical of dissolution experiments. Note that as Equation (1) is only valid for homogeneous samples, <span class="html-italic">G</span> estimates with depth can only highlight qualitatively if substantial mechanical inhomogeneity occurs in the tested swollen layer. Error bars are as in <a href="#gels-03-00044-f004" class="html-fig">Figure 4</a>.</p> Full article ">Figure 10
<p>Shear modulus in WPI gels undergoing dissolution at different (NaOH). Moduli were calculated as the average between 50–500 μm indentation depth. Empty points and dashed lines are used for repeated experiments. Error bars are as in <a href="#gels-03-00044-f005" class="html-fig">Figure 5</a>.</p> Full article ">Figure 11
<p>Typical loading-unloading indentations, continuous and dashed lines respectively. (<b>a</b>) Gels tested dry or swollen at low alkaline pH; (<b>b</b>) gels undergoing dissolution at high [NaOH]. For clarity, the force is normalized with the maximum force, shown at the top, and some data has been shifted horizontally to avoid overlapping.</p> Full article ">
1419 KiB  
Perspective
Why Hydrogels Don’t Dribble Water
by Gerald H. Pollack
Gels 2017, 3(4), 43; https://doi.org/10.3390/gels3040043 - 15 Nov 2017
Cited by 1 | Viewed by 4975
Abstract
Hydrogels contain ample amounts of water, with the water-to-solid ratio sometimes reaching tens of thousands of times. How can so much water remain securely lodged within the gel? New findings imply a simple mechanism. Next to hydrophilic surfaces, water transitions into an extensive [...] Read more.
Hydrogels contain ample amounts of water, with the water-to-solid ratio sometimes reaching tens of thousands of times. How can so much water remain securely lodged within the gel? New findings imply a simple mechanism. Next to hydrophilic surfaces, water transitions into an extensive gel-like phase in which molecules become ordered. This “fourth phase” of water sticks securely to the solid gel matrix, ensuring that the water does not leak out. Full article
(This article belongs to the Special Issue The Role of Water in the Properties of Hydrogels)
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Figure 1

Figure 1
<p>Polyacrylic acid gel immersed in microsphere suspension. Microspheres (right) excluded from a zone, labeled “exclusion zone” or “EZ” next to gel.</p> Full article ">Figure 2
<p>Diagrammatic representation of EZ water, negatively charged, and the positively charged bulk water beyond. Hydrophilic surface at left.</p> Full article ">Figure 3
<p>Buildup of honeycomb planes from bulk water (<b>top</b>, blue). Hydrophilic surface nucleates EZ growth, which progresses layer by layer.</p> Full article ">Figure 4
<p>The specter of the leaking gel—averted because EZ water layers stick to polymeric surfaces within the gel.</p> Full article ">Figure 5
<p>Flow in tunnel bored within polyacrylic-acid gel. EZ forms adjacent to gel material, while aqueous microsphere suspension resides in core. Microsphere suspension flows.</p> Full article ">
3423 KiB  
Article
The Influence of Differently Shaped Gold Nanoparticles Functionalized with NIPAM-Based Hydrogels on the Release of Cytochrome C
by Sulalit Bandyopadhyay, Anuvansh Sharma and Wilhelm Robert Glomm
Gels 2017, 3(4), 42; https://doi.org/10.3390/gels3040042 - 8 Nov 2017
Cited by 9 | Viewed by 5792
Abstract
Here, we report the synthesis and functionalization of five different shapes of Au nanoparticles (NPs), namely nanorods, tetrahexahedral, bipyramids, nanomakura, and spheres with PEG and poly (N-isopropylacrylamide)-acrylic acid (pNIPAm-AAc) hydrogels. The anisotropic NPs are synthesized using seed-mediated growth in the presence [...] Read more.
Here, we report the synthesis and functionalization of five different shapes of Au nanoparticles (NPs), namely nanorods, tetrahexahedral, bipyramids, nanomakura, and spheres with PEG and poly (N-isopropylacrylamide)-acrylic acid (pNIPAm-AAc) hydrogels. The anisotropic NPs are synthesized using seed-mediated growth in the presence of silver. The NPs have been characterized using Dynamic Light Scattering (DLS), zeta potential measurements, UV-Visible spectrophotometry (UV-Vis), and Scanning Transmission Electron Microscopy (S(T)EM). Cyt C was loaded into the PEG-hydrogel-coated AuNPs using a modified breathing-in method. Loading efficiencies (up to 80%), dependent on particle geometry, concentration, and hydrogel content, were obtained. Release experiments conducted at high temperature (40 °C) and acidic pH (3) showed higher release for larger sizes of PEG-hydrogel-coated AuNPs, with temporal transition from spherical to thin film release geometry. AuNP shape, size, number density, and hydrogel content are found to influence the loading as well as release kinetics of Cyt C from these systems. Full article
(This article belongs to the Special Issue Hydrogels for Drug Delivery)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>S(T)EM images of (<b>a</b>) AuNR; (<b>b</b>) PEG-hydrogel coated AuNR; (<b>c</b>) AuHex; (<b>d</b>) PEG-hydrogel coated AuHex; (<b>e</b>) AuBP; (<b>f</b>) PEG-hydrogel coated AuBP; (<b>g</b>) AuNM; (<b>h</b>) PEG-hydrogel coated AuNM; (<b>i</b>) AuNS; and (<b>j</b>) PEG-hydrogel coated AuNS.</p> Full article ">Figure 2
<p>(<b>a</b>) Hydrodynamic sizes and (<b>b</b>) zeta potentials of different shapes of AuNPs.</p> Full article ">Figure 3
<p>Variation of (<b>a</b>) hydrodynamic sizes and (<b>b</b>) zeta potentials of AuNPs with different concentrations of PEG. Variation of (<b>c</b>) hydrodynamic sizes and (<b>d</b>) zeta potentials of AuNPs with different concentrations of hydrogel.</p> Full article ">Figure 4
<p>(<b>a</b>) UV-Vis spectra for differently shaped AuNPs. Variation of UV-Vis spectra after coating with PEG followed by hydrogel coating for (<b>b</b>) AuNR; (<b>c</b>) AuHex; (<b>d</b>) AuBP; (<b>e</b>) AuNM; and (<b>f</b>) AuNS; Variation of (<b>g</b>) hydrodynamic sizes and (<b>h</b>) zeta potentials of differently shaped AuNPs measured at base and release conditions, respectively.</p> Full article ">Figure 4 Cont.
<p>(<b>a</b>) UV-Vis spectra for differently shaped AuNPs. Variation of UV-Vis spectra after coating with PEG followed by hydrogel coating for (<b>b</b>) AuNR; (<b>c</b>) AuHex; (<b>d</b>) AuBP; (<b>e</b>) AuNM; and (<b>f</b>) AuNS; Variation of (<b>g</b>) hydrodynamic sizes and (<b>h</b>) zeta potentials of differently shaped AuNPs measured at base and release conditions, respectively.</p> Full article ">Figure 5
<p>(<b>a</b>) Loading and (<b>b</b>) encapsulation efficiencies for differently shaped AuNPs for two different hydrogel concentrations. Release kinetics of Cyt <span class="html-italic">C</span> from differently shaped AuNPs for (<b>c</b>) 1.7 mg/mL and (<b>d</b>) 3.3 mg/mL hydrogel concentrations respectively. (<b>e</b>) Rate constants and (<b>f</b>) release exponents for Cyt <span class="html-italic">C</span> release from differently shaped AuNPs.</p> Full article ">
1562 KiB  
Review
Polyampholyte Hydrogels in Biomedical Applications
by Stephanie L. Haag and Matthew T. Bernards
Gels 2017, 3(4), 41; https://doi.org/10.3390/gels3040041 - 4 Nov 2017
Cited by 46 | Viewed by 7098
Abstract
Polyampholytes are a class of polymers made up of positively and negatively charged monomer subunits. Polyampholytes offer a unique tunable set of properties driven by the interactions between the charged monomer subunits. Some tunable properties of polyampholytes include mechanical properties, nonfouling characteristics, swelling [...] Read more.
Polyampholytes are a class of polymers made up of positively and negatively charged monomer subunits. Polyampholytes offer a unique tunable set of properties driven by the interactions between the charged monomer subunits. Some tunable properties of polyampholytes include mechanical properties, nonfouling characteristics, swelling due to changes in pH or salt concentration, and drug delivery capability. These characteristics lend themselves to multiple biomedical applications, and this review paper will summarize applications of polyampholyte polymers demonstrated over the last five years in tissue engineering, cryopreservation and drug delivery. Full article
(This article belongs to the Special Issue Hydrogels in Tissue Engineering)
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Figure 1

Figure 1
<p>Schematic representation of the impact that changes in pH and salt concentrations have on electrostatic interactions within a polyampholyte hydrogel. This figure is reprinted from Ref. [<a href="#B1-gels-03-00041" class="html-bibr">1</a>] with permission. Copyright 2013, Wiley Periodicals, Inc.</p> Full article ">Figure 2
<p>(<b>a</b>) General structure of a tough gel based on the sacrificial bond principle consisting of a highly stretchable matrix with a high density of brittle bonds; (<b>b</b>) Possible fracture processes of a single network gel; (<b>c</b>) Possible fracture processes of a sacrificial bond gel. The brittle bonds are widely ruptured prior to the macroscopic crack propagation around the crack tip (shadowed zone). This figure is reprinted from Ref. [<a href="#B27-gels-03-00041" class="html-bibr">27</a>] with permission. Copyright 2017, the Society of Polymer Science, Japan.</p> Full article ">Figure 3
<p>Average number of MC3T3-E1 cells (cells/mm<sup>2</sup>) that adhered to tissue culture polystyrene (TCPS) and TMA/CAA hydrogels with or without adsorbed or conjugated proteins. * Represents a statistically significant difference between the surfaces being compared (<span class="html-italic">p</span> &lt; 0.05). This figure is reprinted from Ref. [<a href="#B41-gels-03-00041" class="html-bibr">41</a>] with permission. Copyright 2013, American Chemical Society.</p> Full article ">Figure 4
<p>Quantitative viability results of mesenchymal stem cells (MSCs) after slow and fast vitrification with various VSs and different cooling speeds (<b>a</b>) immediately after warming and (<b>b</b>) after 1 day of culture. (<b>c</b>) Cell proliferation curves after slow vitrification at a cooling rate of 10.8 °C/min with various VSs (*** <span class="html-italic">p</span> &lt; 0.001). This figure reprinted from Ref. [<a href="#B43-gels-03-00041" class="html-bibr">43</a>] with permission. Copyright 2016, American Chemical Society.</p> Full article ">Figure 5
<p>Schematic depicting the release of caffeine, metanil yellow and methylene blue from TMA/CAA gels. This figure is reprinted from Ref. [<a href="#B52-gels-03-00041" class="html-bibr">52</a>] with permission. Copyright 2015, American Chemical Society.</p> Full article ">Figure 6
<p>Release profiles of coumarin 1 dye under different solvent conditions for PDDC capsules with (<b>a</b>) 3:1 triazine/DETA and (<b>b</b>) 1:1 triazine/DETA. (<b>c</b>) Control experiments: 1:1 triazine/DETA with TC. This figure is reprinted from Ref. [<a href="#B58-gels-03-00041" class="html-bibr">58</a>] with permission. Copyright 2017, American Chemical Society.</p> Full article ">
2076 KiB  
Article
Development of Novel N-isopropylacrylamide (NIPAAm) Based Hydrogels with Varying Content of Chrysin Multiacrylate
by Shuo Tang, Martha Floy, Rohit Bhandari, Thomas Dziubla and J. Zach Hilt
Gels 2017, 3(4), 40; https://doi.org/10.3390/gels3040040 - 22 Oct 2017
Cited by 14 | Viewed by 6729
Abstract
A series of novel temperature responsive hydrogels were synthesized by free radical polymerization with varying content of chrysin multiacrylate (ChryMA). The goal was to study the impact of this novel polyphenolic-based multiacrylate on the properties of N-isopropylacrylamide (NIPAAm) hydrogels. The temperature responsive [...] Read more.
A series of novel temperature responsive hydrogels were synthesized by free radical polymerization with varying content of chrysin multiacrylate (ChryMA). The goal was to study the impact of this novel polyphenolic-based multiacrylate on the properties of N-isopropylacrylamide (NIPAAm) hydrogels. The temperature responsive behavior of the copolymerized gels was characterized by swelling studies, and their lower critical solution temperature (LCST) was characterized through differential scanning calorimetry (DSC). It was shown that the incorporation of ChryMA decreased the swelling ratios of the hydrogels and shifted their LCSTs to a lower temperature. Gels with different ChryMA content showed different levels of response to temperature change. Higher content gels had a broader phase transition and smaller temperature response, which could be attributed to the increased hydrophobicity being introduced by the ChryMA. Full article
(This article belongs to the Special Issue Stimuli-Responsive Gels)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>HPLC chromatograms for Chrysin and chrysin multiacrylate (ChryMA).</p> Full article ">Figure 2
<p>Chemical structure of (<b>a</b>) Chrysin; (<b>b</b>) Chrysin-monoacrylate; (<b>c</b>) Chrysin-diacrylate.</p> Full article ">Figure 3
<p>Example polymerization scheme for synthesizing <span class="html-italic">N</span>-isopropylacrylamide (NIPAAm)-<span class="html-italic">co</span>-ChryMA gels.</p> Full article ">Figure 4
<p>Kinetic swelling behavior of NIPAAm-<span class="html-italic">co</span>-ChryMA gels.</p> Full article ">Figure 5
<p>Temperature dependent swelling profile of ChryMA hydrogels.</p> Full article ">Figure 6
<p>Reversible swelling behavior of NIPAAm-<span class="html-italic">co</span>-ChryMA hydrogels.</p> Full article ">Figure 7
<p>(<b>a</b>) Lower critical solution temperature (LCST) measurement of NIPAAm-<span class="html-italic">co</span>-ChryMA gels; (<b>b</b>) LCST as a function of ChryMA content.</p> Full article ">
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Review
Peptide-Based Physical Gels Endowed with Thixotropic Behaviour
by Nicola Zanna and Claudia Tomasini
Gels 2017, 3(4), 39; https://doi.org/10.3390/gels3040039 - 21 Oct 2017
Cited by 27 | Viewed by 7002
Abstract
Thixotropy is one of the oldest documented rheological phenomenon in colloid science and may be defined as an increase of viscosity in a state of rest and a decrease of viscosity when submitted to a constant shearing stress. This behavior has been exploited [...] Read more.
Thixotropy is one of the oldest documented rheological phenomenon in colloid science and may be defined as an increase of viscosity in a state of rest and a decrease of viscosity when submitted to a constant shearing stress. This behavior has been exploited in recent years to prepare injectable hydrogels for application in drug delivery systems. Thixotropic hydrogels may be profitably used in the field of regenerative medicine, which promotes tissue healing after injuries and diseases, as the molten hydrogel may be injected by syringe and then self-adapts in the space inside the injection site and recovers the solid form. We will focus our attention on the preparation, properties, and some applications of biocompatible thixotropic hydrogels. Full article
(This article belongs to the Special Issue Hydrogels Based on Dynamic Covalent Chemistry)
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<p>Gel–sol and sol–gel transitions of an organogel in 2-methoxyethanol. Image adapted with permission from reference [<a href="#B10-gels-03-00039" class="html-bibr">10</a>]. Copyright 2012 American Chemical Society.</p> Full article ">Figure 2
<p>Chemical structure of the gelator Fmoc-K(Fmoc). The highlighted regions indicate the corresponding possible interactions during self-assembly.</p> Full article ">Figure 3
<p>This cartoon diagram summarizes all the possible applications of the hydrogels prepared with the Fmoc-K(Fmoc) gelator. Image adapted with permission from reference [<a href="#B42-gels-03-00039" class="html-bibr">42</a>]. Copyright 2015 Royal Society of Chemistry.</p> Full article ">Figure 4
<p>Chemical structure of the amphiphilic gelator derived from lysine.</p> Full article ">Figure 5
<p>(<b>Left</b>) Time-dependent repetitive cycle of step-strain analysis of the hydrogel–AgNP soft nanocomposite with a 0.6 % <span class="html-italic">w/v</span> gelator concentration. (<b>Right</b>) Photographs of (<b>a</b>) AgNP-incorporated hydrogel in a syringe; (<b>b</b>) nanocomposite gel flowing through the needle of a syringe; (<b>c</b>) solution of AgNP-1 composite after syringe processing; and (<b>d</b>) AgNP-including hydrogel that is re-formed after shear thinning at room temperature for 2 min. Image adapted with permission from reference [<a href="#B45-gels-03-00039" class="html-bibr">45</a>]. Copyright 2014 John Wiley and Sons.</p> Full article ">Figure 6
<p>The simulative diagram of the self-assembly process of the hydrogelator based on (−)-menthol. Image adapted with permission from reference [<a href="#B47-gels-03-00039" class="html-bibr">47</a>]. Copyright 2014 Royal Society of Chemistry.</p> Full article ">Figure 7
<p>Chemical structure of FF-derived hydrogelators.</p> Full article ">Figure 8
<p>(<b>Left</b>) Chemical structures of peptides <b>P1</b>−<b>P5</b>. (<b>Right</b>) (<b>A</b>) Stimuli-responsive behavior of peptide <b>P3</b> hydrogels; (<b>B</b>) Injectable nature of peptide <b>P3</b> hydrogels; (<b>C</b>) Slow release of proflavine (yellow) and rhodamine (pink) from hydrogel matrix to the buffer layer after 16 h. Image adapted with permission from reference [<a href="#B51-gels-03-00039" class="html-bibr">51</a>]. Copyright 2017 American Chemical Society.</p> Full article ">Figure 9
<p>Time-dependence of the storage modulus. The sample was prepared from <span class="html-italic">cyclo</span>(<span class="html-small-caps">l</span>-<span class="html-italic">O</span>-hydroxyhexylaspartyl-<span class="html-small-caps">l</span>-phenylalanyl) in an ethanol/water mixture (20% ethanol/80% water) at 20 g·L<sup>−1</sup>. Image adapted with permission from reference [<a href="#B52-gels-03-00039" class="html-bibr">52</a>]. Copyright 2013 American Chemical Society.</p> Full article ">Figure 10
<p>Chemical structures of the peptides and an illustration of the injectable nature of the hydrogel. Image adapted with permission from reference [<a href="#B53-gels-03-00039" class="html-bibr">53</a>]. Copyright 2016 American Chemical Society.</p> Full article ">Figure 11
<p>(<b>Top</b>) The four images show the sequence followed to prepare the hydrogel bridge (length ≈ 4.0 cm). (<b>Bottom</b>) The four images demonstrate that the hydrogel has a self-healing property. The right-end image was taken after one week. Image adapted with permission from reference [<a href="#B57-gels-03-00039" class="html-bibr">57</a>]. Copyright 2016 Royal Society of Chemistry.</p> Full article ">Figure 12
<p>(<b>Left</b>) chemical structure of the gelators A–C, studied in this work. (<b>Right</b>) viability of embedded HGFs in hydrogels A, B, and C after seven days of culture. Data were expressed as relative percentage ± SD compared to control HGFs. Image adapted with permission from reference [<a href="#B59-gels-03-00039" class="html-bibr">59</a>]. Copyright 2016 American Chemical Society.</p> Full article ">Figure 13
<p>(<b>Left</b>) Chemical structures of bolaamphiphiles <b>1</b>, <b>2</b>, <b>3,</b> and <b>4</b>. (<b>Right</b>) TEM images of aqueous samples at 2% <span class="html-italic">w/v</span> obtained for bolaamphiphiles (<b>A</b>) <b>1</b> (scale bar: 100 nm), (<b>B</b>) <b>2</b> (scale bar: 100 nm), (<b>C</b>) <b>3</b> (scale bar: 100 nm)<b>,</b> and (<b>D</b>) <b>4</b> (scale bar: 200 nm). Image adapted with permission from reference [<a href="#B60-gels-03-00039" class="html-bibr">60</a>]. Copyright 2017 Elsevier.</p> Full article ">Figure 14
<p>Schematic representation of a nanocrystalline magnesium phosphate (NMP) hydrogel and its application for bone repair. Image adapted with permission from reference [<a href="#B76-gels-03-00039" class="html-bibr">76</a>]. Copyright 2016 American Chemical Society.</p> Full article ">Figure 15
<p>Schematics show how the VEGF mimic polypeptides self-assemble through hydrophobic packing and hydrogen bonding along the fiber axis, exposing the angiogenic domain. Image adapted with permission from reference [<a href="#B77-gels-03-00039" class="html-bibr">77</a>]. Copyright 2016 Elsevier.</p> Full article ">Figure 16
<p>Schematic representation of thixotropic nanocomposite formation from poly-<span class="html-small-caps">l</span>-lysine and nanosilicates. Image adapted with permission from reference [<a href="#B40-gels-03-00039" class="html-bibr">40</a>]. Copyright 2016 Elsevier.</p> Full article ">Figure 17
<p>Schematic of gel formation for a bolamphiphile peptide-based hydrogel with self-assembly process mediated by a lipase-catalyzed reaction. Image adapted with permission from reference [<a href="#B78-gels-03-00039" class="html-bibr">78</a>]. Copyright 2015 American Chemical Society.</p> Full article ">Figure 18
<p>2D cell culture using peptide-based hydrogels. (<b>A</b>) Phase contrast images of attached cells of SH-SY5Y cells and L929 cells grown for 24 h on glass substrates and P5 gel. Scale bar for all images are 100 mm. (<b>B</b>) SEM image depicting cell adhesion and spreading of SH-SY5Y on P5 hydrogel after 1 h (top) and 24 h (bottom) of incubation respectively. Scale bars are 1 mm (<b>top</b>) and 10 mm (<b>bottom</b>). Image adapted with permission from reference [<a href="#B79-gels-03-00039" class="html-bibr">79</a>]. Copyright 2015 Elsevier.</p> Full article ">Figure 19
<p>Fabrication of SF nanofibril-based hydrogels with an ECM-like structure. (<b>a</b>) Schematic representation of the procedure followed to prepare SF nanofibril-based hydrogels; (<b>b</b>) The resultant SF nanofibril solution with opalescence; (<b>c</b>,<b>d</b>) AFM and TEM images of SF nanofibrils; (<b>e</b>) SF nanofibril-based hydrogel; (<b>f</b>,<b>g</b>) SEM images of lyophilized SF nanofibril-based hydrogel with different magnifications, which show an ECM-like structure. Image adapted with permission from reference [<a href="#B80-gels-03-00039" class="html-bibr">80</a>]. Copyright 2016 Elsevier.</p> Full article ">Figure 20
<p>Kinetic of doxorubicin (DOX) release as a function of time. Image adapted with permission from reference [<a href="#B81-gels-03-00039" class="html-bibr">81</a>]. Copyright 2016 American Chemical Society.</p> Full article ">Figure 21
<p>Images of silk- hydroxyapatite hydrogel, its fiber-like structure (<b>left</b>) and its application in bone regeneration (<b>right</b>). Image adapted with permission from reference [<a href="#B82-gels-03-00039" class="html-bibr">82</a>]. Copyright 2017 American Chemical Society.</p> Full article ">Figure 22
<p>Schematic representation of the procedure followed to prepare silk fibroin nanofibril/nanoHAP films and the resultant photograph. Image adapted with permission from reference [<a href="#B83-gels-03-00039" class="html-bibr">83</a>]. Copyright 2016 Royal Society of Chemistry.</p> Full article ">
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Review
Properties of Water Bound in Hydrogels
by Vladimir M. Gun’ko, Irina N. Savina and Sergey V. Mikhalovsky
Gels 2017, 3(4), 37; https://doi.org/10.3390/gels3040037 - 19 Oct 2017
Cited by 186 | Viewed by 14248
Abstract
In this review, the importance of water in hydrogel (HG) properties and structure is analyzed. A variety of methods such as 1H NMR (nuclear magnetic resonance), DSC (differential scanning calorimetry), XRD (X-ray powder diffraction), dielectric relaxation spectroscopy, thermally stimulated depolarization current, quasi-elastic [...] Read more.
In this review, the importance of water in hydrogel (HG) properties and structure is analyzed. A variety of methods such as 1H NMR (nuclear magnetic resonance), DSC (differential scanning calorimetry), XRD (X-ray powder diffraction), dielectric relaxation spectroscopy, thermally stimulated depolarization current, quasi-elastic neutron scattering, rheometry, diffusion, adsorption, infrared spectroscopy are used to study water in HG. The state of HG water is rather non-uniform. According to thermodynamic features of water in HG, some of it is non-freezing and strongly bound, another fraction is freezing and weakly bound, and the third fraction is non-bound, free water freezing at 0 °C. According to structural features of water in HG, it can be divided into two fractions with strongly associated and weakly associated waters. The properties of the water in HG depend also on the amounts and types of solutes, pH, salinity, structural features of HG functionalities. Full article
(This article belongs to the Special Issue The Role of Water in the Properties of Hydrogels)
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<p>Confocal laser scanning microscopy (CLSM) images of HEMA-AGE hydrogel (sample A) in (<b>a</b>) hydrated and (<b>b</b>) dried states (scale bar 150 μm) with the pore (<b>c</b>) size and (<b>d</b>) wall thickness distributions (reproduced from Ref. [<a href="#B47-gels-03-00037" class="html-bibr">47</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 2
<p>Wall thickness distributions with (<b>a</b>) Fiji and (<b>b</b>) ImageJ, and (<b>c</b>) pore size distributions for HEMA-AGE HG A, B, C and D (<a href="#gels-03-00037-t001" class="html-table">Table 1</a>) (reproduced from Ref. [<a href="#B47-gels-03-00037" class="html-bibr">47</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 3
<p>PSDs calculated from the DSC data for HEMA-AGE (A, B, C, and D samples) and G gels (hydration <span class="html-italic">h</span> = <span class="html-italic">m</span><sub>w</sub>/<span class="html-italic">m</span><sub>d</sub> where <span class="html-italic">m</span><sub>w</sub> is the weight of water evaporated in DSC measurements up to 160 °C and <span class="html-italic">m</span><sub>d</sub> is the residual weight of heated sample) at (<b>a</b>) high and (<b>b</b>,<b>c</b>) low hydration (reproduced from Ref. [<a href="#B47-gels-03-00037" class="html-bibr">47</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 4
<p><sup>1</sup>H NMR spectra, of water adsorbed by gelatin gel recorded at different temperatures: (<b>a</b>) initial freeze-dried (0.3 wt % H<sub>2</sub>O) in CDCl<sub>3</sub> (solid lines) and in a mixture CDCCl<sub>3</sub>:CD<sub>3</sub>CN 3:1 at (<b>a</b>, dashed-dotted lines) 0.8 wt % and (<b>b</b>) 10 wt % of water. Signal at 0 ppm corresponds to tetramethylsilane added as a standard; signal at 7.2 ppm corresponds to residual CHCl<sub>3</sub> (reproduced from Ref. [<a href="#B47-gels-03-00037" class="html-bibr">47</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 5
<p><sup>1</sup>H NMR spectra, recorded at different temperatures, of water bound in gelatin gel at hydration <span class="html-italic">h</span> = 1 g per gram of dried gelatin in different media: (<b>a</b>) air (solid lines) and C<sub>6</sub>D<sub>6</sub>:CD<sub>3</sub>CN = 6:1 (dashed-dotted lines), (<b>b</b>) C<sub>6</sub>D<sub>6</sub> (solid lines) and CDCl<sub>3</sub>:CD<sub>3</sub>CN = 3:1 (dashed-dotted lines) (reproduced from Ref. [<a href="#B47-gels-03-00037" class="html-bibr">47</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 6
<p>(<b>a</b>) Amount of unfrozen water (C<sub>uw</sub>) as a function of temperature; (<b>b</b>) derivative dC<sub>uw</sub>/d (ΔG), and (<b>c</b>) pore size distribution (NMR cryoporometry) for G gel in different media (reproduced from Ref. [<a href="#B47-gels-03-00037" class="html-bibr">47</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 7
<p>Theoretical <sup>1</sup>H NMR spectra of water bound to models of partially hydrated gels with cross-linked HEMA-AGE (2373 atoms) with 1192 H<sub>2</sub>O, collagen (two triple coils (1639 atoms) and 1032 H<sub>2</sub>O) and fibronectin (8–9 Fn)—collagen (3200 atoms) with 827 H<sub>2</sub>O (geometry optimized with PM6 method).</p> Full article ">Figure 8
<p>Chemical shifts of water molecules in clusters: pure (curves 1, 2, and 4), with dissolved NaCl (curve 3) and bound to PVA fragments cross-linked by glutaraldehyde (curve 5). Computational models are based on calculations using PM6 and PM7 methods and correlation functions based on DFT and PM6 or PM7 calculations of the same water clusters (adapted from [<a href="#B13-gels-03-00037" class="html-bibr">13</a>]).</p> Full article ">Figure 9
<p>Amount of unfrozen water (<span class="html-italic">C<sub>uw</sub></span>) as a function of temperature; and changes in the Gibbs free energy of interfacial water versus <span class="html-italic">C<sub>uw</sub></span> at different concentrations of collagen in the hydrogel (adapted from [<a href="#B76-gels-03-00037" class="html-bibr">76</a>] with permission, Copyright 2006, Elsevier).</p> Full article ">Figure 10
<p>The free surface energy as a function of the collagen concentration in the CG hydrogel (adapted from [<a href="#B76-gels-03-00037" class="html-bibr">76</a>] with permission, Copyright 2006, Elsevier).</p> Full article ">Figure 11
<p>(<b>a</b>) Temperature dependence of the TSD current for the initial collagen hydrogel and “free” (bulk) water; (<b>b</b>) temperature dependences of the TSD current and the amounts of unfrozen water (<span class="html-italic">C</span><sub>uw</sub>) (NMR) for initial collagen HG (98.5 wt % of water) (<b>c</b>) distribution function of the activation energy of relaxation in these systems; and (<b>d</b>) incremental pore size distributions for the initial CG HG calculated on the basis of <sup>1</sup>H NMR and TSDC data (adapted from [<a href="#B76-gels-03-00037" class="html-bibr">76</a>] with permission, Copyright 2006, Elsevier).</p> Full article ">Figure 12
<p>Pore size distribution calculated using <sup>1</sup>H NMR cryoporometry and CLSM methods (reproduced from Ref. [<a href="#B44-gels-03-00037" class="html-bibr">44</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 13
<p>Diffusion kinetics through a collagen HG membrane (~1 mm in thickness) for (<b>a</b>) BPTI. Curve 1 is for an initial concentration of 1.23 mg/mL in the feeder cell (OD<sub>280</sub> = 0.09), curves 2–4 are for an initial concentration of 2.46 mg/mL (OD<sub>280</sub> = 0.18); curve 2—BPTI run after the first BPTI run; curve 3—BPTI run after Fg; curve 4—BPTI run after Fg and BSA; (<b>c</b>) BSA and BSA (with twice concentration) after the first BSA run, (<b>e</b>) Fg (initial concentration 1.7 mg/mL); curves (<b>b</b>,<b>d</b>,<b>f</b>) show the corresponding distribution functions of the diffusion coefficient <span class="html-italic">f</span>(<span class="html-italic">D</span>) for (<b>b</b>) BPTI, (<b>d</b>) BSA, and (<b>f</b>) Fg (reproduced from Ref. [<a href="#B44-gels-03-00037" class="html-bibr">44</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 14
<p>Changes in frequency (1) and auto-gain controller voltage (2) on two injections (A and B) of 0.1 mL aliquot of 3T3 fibroblast suspension (2.0 × 10<sup>6</sup> cell mL<sup>−1</sup>) upon an unsupported section of CG HG laid the surface of a 10 MHz gold coated crystal. Flow injection rate 0.01 mL min<sup>−1</sup>, 37 ± 0.1 °C, pH 7.2, PBS (reproduced from Ref. [<a href="#B44-gels-03-00037" class="html-bibr">44</a>] with permission from The Royal Society of Chemistry).</p> Full article ">Figure 15
<p><sup>1</sup>H NMR spectra of water bound to (<b>a</b>–<b>c</b>) CM1 and (<b>c</b>) CM2 (dot-dashed lines) at hydration <span class="html-italic">h</span> = 2.3 wt % in different media: (<b>a</b>) air (solid lines) CDCl<sub>3</sub> (dot-dashed lines), (<b>b</b>) CD<sub>3</sub>CN (solid lines) CD<sub>3</sub>CN:CDCl<sub>3</sub> = 1:2.6 (dot-dashed lines), (<b>c</b>) CD<sub>3</sub>CN:CDCl<sub>3</sub> = 1:5.</p> Full article ">Figure 16
<p>Temperature dependence of the amount of (<b>a</b>) SAW and (<b>b</b>) WAW and the relationships between changes in the Gibbs free energy and the amounts of SAW and WAW in HA/A-300 composites CM1 and CM2 (*).</p> Full article ">Figure 17
<p><sup>1</sup>H NMR spectra of CM1 with adsorbed aqueous solutions (150 mg/g) of (<b>a</b>) 18% HCl and (<b>b</b>) 16% H<sub>2</sub>O<sub>2</sub> in CDCl<sub>3</sub> medium.</p> Full article ">Figure 18
<p>(<b>a</b>,<b>c</b>,<b>e</b>) Relationships between the amounts of unfrozen water and changes in the Gibbs free energy and (<b>b</b>,<b>d</b>,<b>f</b>) the corresponding PSD (NMR cryoporometry with GT equation at <span class="html-italic">k</span><sub>GT</sub> = 67 K nm) for (<b>a</b>,<b>b</b>) MCC, (<b>c</b>,<b>d</b>) MCC/A-300 (5.6:1), and MCC/TiO<sub>2</sub> (3:1) (adapted from [<a href="#B13-gels-03-00037" class="html-bibr">13</a>]).</p> Full article ">Figure 19
<p>PSD (<span class="html-italic">k</span><sub>GT</sub> = 70 K nm) for A-300/PVP systems (adapted from [<a href="#B13-gels-03-00037" class="html-bibr">13</a>]).</p> Full article ">
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Article
Peptide Drug Release Behavior from Biodegradable Temperature-Responsive Injectable Hydrogels Exhibiting Irreversible Gelation
by Kazuyuki Takata, Hiroki Takai, Yuta Yoshizaki, Takuya Nagata, Keisuke Kawahara, Yasuyuki Yoshida, Akinori Kuzuya and Yuichi Ohya
Gels 2017, 3(4), 38; https://doi.org/10.3390/gels3040038 - 15 Oct 2017
Cited by 13 | Viewed by 8009
Abstract
We investigated the release behavior of glucagon-like peptide-1 (GLP-1) from a biodegradable injectable polymer (IP) hydrogel. This hydrogel shows temperature-responsive irreversible gelation due to the covalent bond formation through a thiol-ene reaction. In vitro sustained release of GLP-1 from an irreversible IP formulation [...] Read more.
We investigated the release behavior of glucagon-like peptide-1 (GLP-1) from a biodegradable injectable polymer (IP) hydrogel. This hydrogel shows temperature-responsive irreversible gelation due to the covalent bond formation through a thiol-ene reaction. In vitro sustained release of GLP-1 from an irreversible IP formulation (F(P1/D+PA40)) was observed compared with a reversible (physical gelation) IP formulation (F(P1)). Moreover, pharmaceutically active levels of GLP-1 were maintained in blood after subcutaneous injection of the irreversible IP formulation into rats. This system should be useful for the minimally invasive sustained drug release of peptide drugs and other water-soluble bioactive reagents. Full article
(This article belongs to the Special Issue Stimuli-Responsive Gels)
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<p>Structures of the polymers and polythiol used in this study.</p> Full article ">Figure 2
<p>Photographs of (<b>a</b>) <b>F(P1)</b> hydrogel containing GLP-1 and (<b>b</b>) <b>F(P1/D+PA<sub>40</sub>)</b> hydrogel containing GLP-1 after heating at 37 °C for 1 min and subsequent cooling at 4 °C for 1 min.</p> Full article ">Figure 3
<p>Cumulative release of GLP-1 (%) in vitro from <b>F(P1)</b> hydrogel containing GLP-1 (〇) and <b>F(P1/D+PA<sub>40</sub>)</b> hydrogel containing GLP-1 (<b>□</b>). The data are shown as the mean ± SD (<span class="html-italic">n</span> = 3).</p> Full article ">Figure 4
<p>Photographs of (<b>a</b>) <b>F(P1)</b> hydrogel containing GLP-1 and (<b>b</b>) <b>F(P1/D+PA<sub>40</sub>)</b> hydrogel containing GLP-1 during the in vitro release test.</p> Full article ">Figure 5
<p>Cell viability of L929 fibroblast cells incubated in the presence of <b>F(P1)</b> (<span style="color:red">●</span>), <b>F(P1/D)</b> (<span style="color:#04B45F">◆</span>), <b>F(PA)</b> (<span style="color:blue">▲</span>) and <b>F(P1/D+PA50)</b> (■) in E-MEM containing 10% Fetal calf serum (FCS) at 37 °C for 21 h.</p> Full article ">Figure 6
<p>(<b>a</b>) Active GLP-1 concentration in plasma after subcutaneous injection for 25 days. (<b>b</b>) Magnification of the area between Days 6 and 25. GLP-1 solution (×), <b>F(P1)</b> containing GLP-1 (●), <b>F(P1/D+PA40)</b> containing GLP-1 (■), and <b>F(P1/D+PA40)</b> without GLP-1 (◆). The data are shown as the mean ± SD (<span class="html-italic">n</span> = 3–6). * <span class="html-italic">p</span> &lt; 0.05 vs. <b>F(P1)</b> containing GLP-1.</p> Full article ">Figure 7
<p>Photographs of hydrogels (<b>a</b>) <b>F(P1)</b>, (<b>b</b>) <b>F(P1/D+PA<sub>40</sub>)</b> 25 days after subcutaneous injection in rats. The values indicate the number of rats harboring hydrogel/all rats.</p> Full article ">
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Article
Controlled Release of Vascular Endothelial Growth Factor from Heparin-Functionalized Gelatin Type A and Albumin Hydrogels
by Christiane Claaßen, Lisa Sewald, Günter E. M. Tovar and Kirsten Borchers
Gels 2017, 3(4), 35; https://doi.org/10.3390/gels3040035 - 9 Oct 2017
Cited by 32 | Viewed by 7407
Abstract
Bio-based release systems for pro-angiogenic growth factors are of interest, to overcome insufficient vascularization and bio-integration of implants. In this study, we investigated heparin-functionalized hydrogels based on gelatin type A or albumin as storage and release systems for vascular endothelial growth factor (VEGF). [...] Read more.
Bio-based release systems for pro-angiogenic growth factors are of interest, to overcome insufficient vascularization and bio-integration of implants. In this study, we investigated heparin-functionalized hydrogels based on gelatin type A or albumin as storage and release systems for vascular endothelial growth factor (VEGF). The hydrogels were crosslinked using carbodiimide chemistry in presence of heparin. Heparin-functionalization of the hydrogels was monitored by critical electrolyte concentration (CEC) staining. The hydrogels were characterized in terms of swelling in buffer solution and VEGF-containing solutions, and their loading with and release of VEGF was monitored. The equilibrium degree of swelling (EDS) was lower for albumin-based gels compared to gelatin-based gels. EDS was adjustable with the used carbodiimide concentration for both biopolymers. Furthermore, VEGF-loading and release were dependent on the carbodiimide concentration and loading conditions for both biopolymers. Loading of albumin-based gels was higher compared to gelatin-based gels, and its burst release was lower. Finally, elevated cumulative VEGF release after 21 days was determined for albumin-based hydrogels compared to gelatin A-based hydrogels. We consider the characteristic net charges of the proteins and degradation of albumin during release time as reasons for the observed effects. Both heparin-functionalized biomaterial systems, chemically crosslinked gelatin type A or albumin, had tunable physicochemical properties, and can be considered for controlled delivery of the pro-angiogenic growth factor VEGF. Full article
(This article belongs to the Special Issue Hydrogels for Drug Delivery)
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<p>Critical electrolyte concentration (CEC) staining of gelatin type A (10 wt %) and albumin (10 wt %) hydrogels without heparin or with heparin (1 wt %) that were crosslinked with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) (0.125 M). Hydrogels were washed for at least 5 h in PBS<sup>+</sup> before staining. Alcian blue solutions (0.05%, pH 5.8) with 0.06 to 0.9 M MgCl<sub>2</sub> were applied for staining. Hydrogels without heparin showed no dependence of staining on electrolyte concentration, indicating that the CEC for albumin and gelatin hydrogels was lower than 0.06 M MgCl<sub>2</sub>. For heparin-functionalized hydrogels, the CEC was 0.5 M MgCl<sub>2</sub>.</p> Full article ">Figure 2
<p>Results of the gravimetric determination of the gel yield of hydrogels. Hydrogels were prepared with gelatin type A or albumin (10 wt %), heparin (1 wt %), and different EDC concentrations. At 0.1 M EDC, no crosslinking was achieved for albumin-based gels. There was no significant influence of EDC concentration or biopolymer type on the gel yield (<span class="html-italic">p</span> &gt; 0.05; <span class="html-italic">n</span> = 3).</p> Full article ">Figure 3
<p>Swelling kinetics of gelatin (10 wt %) or albumin (10 wt %) hydrogels with heparin (1 wt %) crosslinked via EDC (0.125 M). The degree of swelling was determined at different time points and is denoted in % of the degree of swelling of the respective gels after 24 h. Both hydrogel types showed a fast water uptake. Gelatin-based gels passed through a hyper-swollen state after approximately 30 min, while albumin-based gels swelled continuously. Evaluation of the absolute numbers for the degrees of swelling for each gel revealed that after 60 min, no significant variation of degree of swelling (DS) was noticed. (<span class="html-italic">p</span> &gt; 0.05, <span class="html-italic">n</span> = 4).</p> Full article ">Figure 4
<p>Results of the gravimetric characterization of the hydrogels. Hydrogels were prepared with gelatin type A or albumin (10 wt %), heparin (1 wt %), and different EDC concentrations. At 0.1 M EDC, no crosslinking was achieved for albumin-based gels. The EDS decreased with increasing EDC concentration for both biopolymers (asterisks with solid lines: <span class="html-italic">p</span> &lt; 0.05; grey lines refer to gelatin, black line refers to albumin; <span class="html-italic">n</span> = 3), while the EDS of gelatin gels was significantly higher compared to the albumin gels in all cases (asterisk with dotted lines: <span class="html-italic">p</span> &lt; 0.01; <span class="html-italic">n</span> = 3).</p> Full article ">Figure 5
<p>Hydrolytic stability of hydrogels prepared with gelatin (10 wt %) or albumin (10 wt %) and heparin (1 wt %) with 0.125 M EDC. Hydrogels were incubated under the same conditions as for the release of growth factor. Gel yield (<b>A</b>) and DS (<b>B</b>) were determined at different time points and are denoted as percentage of the initial values in the figure. There was no significant decrease in gel yield for both biopolymers (<span class="html-italic">p</span> &gt; 0.05; <span class="html-italic">n</span> = 5); for the EDS a significant increase for albumin-based hydrogels was noticed (<span class="html-italic">p</span> &lt; 0.05; <span class="html-italic">n</span> = 5), while no significant effect for gelatin type A gels was found (<span class="html-italic">p</span> &gt; 0.3; <span class="html-italic">n</span> = 5).</p> Full article ">Figure 6
<p>Cumulative release profiles of hydrogels prepared with gelatin type A (10 wt %) or albumin (10 wt %) with heparin (1 wt %). Loading was achieved by swelling in vascular endothelial growth factor (VEGF)-solution (1 µg VEGF per mL; loading with 0.1 µg VEGF per mg hydrogel dry weight). VEGF concentrations were normalized to the total VEGF amount used for hydrogel loading. Crosslinking and loading conditions: (<b>A</b>) 0.125 M EDC/1 h; (<b>B</b>) 0.125 M EDC/3 h; (<b>C</b>) 0.15 M EDC/1 h; (<b>D</b>) 0.15 M EDC/3 h. Gelatin type A-based hydrogels showed a higher initial release but lower overall release compared to albumin-based gels. Gelatin gels showed a diffusion controlled release curve, while release curves of albumin seemed to be diffusion controlled the first days but were afterwards similar to a zero-order release (<span class="html-italic">n</span> = 3).</p> Full article ">Figure 7
<p>Results of key parameters for the release properties. Hydrogels were prepared with gelatin type A (10 wt %) or albumin (10 wt %) and heparin (1 wt %). For each composition, two EDC concentrations (0.125 M and 0.15 M) were used and hydrogels were loaded by swelling for 1 or 3 h in VEGF solution (1 µg VEGF per mL; loading with 0.1 µg VEGF per mg hydrogel dry weight). VEGF concentrations were normalized to the total VEGF amount used for hydrogel loading (<span class="html-italic">n</span> = 3). (<b>A</b>) Loading efficiency: albumin hydrogels had a higher loading efficiency compared to gelatin type A hydrogels (<span class="html-italic">p</span> &lt; 0.001). For both hydrogel types the loading efficiency showed a negative correlation with the loading time (<span class="html-italic">p</span> &lt; 0.01); (<b>B</b>) Burst release (release within 24 h): gelatin type A hydrogels had a higher burst release compared to albumin gels (<span class="html-italic">p</span> &lt; 0.001). For albumin hydrogels, crosslinker concentration showed a negative correlation with the burst release (<span class="html-italic">p</span> &lt; 0.01); (<b>C</b>) Cumulative overall release after 21 days: was higher for albumin gels compared to gelatin gels (<span class="html-italic">p</span> &lt; 0.001); (<b>D</b>) Average release rate between day 7 and day 21: release rates of all albumin gels were significantly higher compared to the corresponding gelatin type A-based gel (<span class="html-italic">p</span> &lt; 0.001).</p> Full article ">
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Review
Poly(N-isopropylacrylamide) and Copolymers: A Review on Recent Progresses in Biomedical Applications
by Sonia Lanzalaco and Elaine Armelin
Gels 2017, 3(4), 36; https://doi.org/10.3390/gels3040036 - 4 Oct 2017
Cited by 306 | Viewed by 20675
Abstract
The innate ability of poly(N-isopropylacrylamide) (PNIPAAm) thermo-responsive hydrogel to copolymerize and to graft synthetic polymers and biomolecules, in conjunction with the highly controlled methods of radical polymerization which are now available, have expedited the widespread number of papers published in the [...] Read more.
The innate ability of poly(N-isopropylacrylamide) (PNIPAAm) thermo-responsive hydrogel to copolymerize and to graft synthetic polymers and biomolecules, in conjunction with the highly controlled methods of radical polymerization which are now available, have expedited the widespread number of papers published in the last decade—especially in the biomedical field. Therefore, PNIPAAm-based hydrogels are extensively investigated for applications on the controlled delivery of active molecules, in self-healing materials, tissue engineering, regenerative medicine, or in the smart encapsulation of cells. The most promising polymers for biodegradability enhancement of PNIPAAm hydrogels are probably poly(ethylene glycol) (PEG) and/or poly(ε-caprolactone) (PCL), whereas the biocompatibility is mostly achieved with biopolymers. Ultimately, advances in three-dimensional bioprinting technology would contribute to the design of new devices and medical tools with thermal stimuli response needs, fabricated with PNIPAAm hydrogels. Full article
(This article belongs to the Special Issue Organogels for Biomedical Applications)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>(<b>A</b>) Evolution of published research articles about poly(<span class="html-italic">N</span>-isopropylacrylamide) (PNIPAAm) and its application in biomedical and other fields, per year; and (<b>B</b>) Percentage of PNIPAAm published research articles in biomedical field, classified by application. Database used for the bibliographic analysis: Scopus<sup>®</sup> (Elsevier, Amsterdam, The Netherlands).</p> Full article ">Figure 2
<p>(<b>A</b>) Chemical formula of PNIPAAm and (<b>B</b>) Representation of volume phase transition between coil (<b>left</b>) and globular (<b>right</b>) hydrogel conformations.</p> Full article ">Figure 3
<p>Representation of (<b>A</b>) the swollen PNIPAAm hydrosol in aqueous solution below <span class="html-italic">T</span><sub>c</sub> (32 °C) and (<b>B</b>) the shrunken dehydrated PNIPAAm hydrogel above <span class="html-italic">T</span><sub>c</sub> (32 °C). Adapted with permission from Reference [<a href="#B64-gels-03-00036" class="html-bibr">64</a>]. Copyright <sup>©</sup> 2015 Springer Science &amp; Business Media Singapore.</p> Full article ">Figure 4
<p>LCSTs of PNIPAAm (0.40 mg/mL) in water-<span class="html-italic">co</span>-nonsolvent mixtures. Adapted with permission from Reference [<a href="#B69-gels-03-00036" class="html-bibr">69</a>]. Copyright © 1991 American Chemical Society.</p> Full article ">Figure 5
<p>Model of competitive hydrogen-bond formation between polymer–water (<span class="html-italic">p</span>–<span class="html-italic">A</span>) and polymer–methanol (<span class="html-italic">p</span>–<span class="html-italic">B</span>) of PNIPAAm in both mixed solvents, proposed by Kojima and Tanaka. Reprinted with permission from Reference [<a href="#B67-gels-03-00036" class="html-bibr">67</a>]. Copyright <sup>©</sup> 2012 The Royal Society of Chemistry.</p> Full article ">Figure 6
<p>Sketch of subcutaneous injection of drug-delivery hydrogel containing bioactive molecules, with reversible sol–gel transition around the LCST point. <span class="html-italic">T</span><sub>c</sub> is the coil–globular critical temperature of a LCST hydrogel. Adapted with permission from reference [<a href="#B81-gels-03-00036" class="html-bibr">81</a>]. Copyright © 1997 Nature Macmillan Publishers Ltd. (Basingstoke, UK) 1997.</p> Full article ">Figure 7
<p>(<b>A</b>) Representation of the structure of poly(<span class="html-italic">N</span>-isopropylacrylamide)-grafted gelatin (PNIPAAm–gelatin) and (<b>B</b>) PNIPAAm–gelatin gel formation in rat subcutaneous tissue. Reprinted from reference [<a href="#B83-gels-03-00036" class="html-bibr">83</a>]. Copyright © 2004 The Japanese Society for Artificial Organs.</p> Full article ">Figure 8
<p>Light microscopy of the rabbit eye (<b>A</b>) retina and (<b>B</b>) anterior chamber after 6 months of intravitreal injection of PNIPAAm hydrogel. Ganglion cell layer and iris tissue are both facing upwards in (A) and (B). Reprinted with permission from reference [<a href="#B49-gels-03-00036" class="html-bibr">49</a>]. Copyright © 2016 Hindawi Publishing Corporation (Cairo, Egypt).</p> Full article ">Figure 9
<p>(<b>A</b>) Progressive mass loss of c-Dxt/PNIPAAm 8 xerogel films (■) in lysozyme/PBS and (●) in PBS only; (<b>B</b>) IOn vitro release of ornidazole from dextrin and c-Dxt/PNIPAAm 8; (<b>C</b>) In vitro release of ciprofloxacin from dextrin and c-Dxt/PNIPAAm 8. “Dxt” refers to dextrin biopolymer, whereas the letter “c” refers to covalently cross-linked hydrogel and the number 8 is related to the molar ratio described on <a href="#gels-03-00036-t001" class="html-table">Table 1</a> inside the article. Results represented are mean ± SD, <span class="html-italic">n</span> = 3. Reprinted with permission from reference [<a href="#B86-gels-03-00036" class="html-bibr">86</a>]. Copyright © 2015 American Chemical Society.</p> Full article ">Figure 10
<p>Scheme of P(NIPAAm-<span class="html-italic">co</span>-PAA) copolymer synthesis reported by Stayton and co-workers [<a href="#B98-gels-03-00036" class="html-bibr">98</a>].</p> Full article ">Figure 11
<p>Scheme of preparation of PNA-BAC/DOX nanogels for in vitro anticancer drug release, reported by Zhan et al. [<a href="#B102-gels-03-00036" class="html-bibr">102</a>]. Reprinted with permission from reference [<a href="#B102-gels-03-00036" class="html-bibr">102</a>]. Copyright © 2015 The Royal Society of Chemistry.</p> Full article ">Figure 12
<p>SEM micrographs of degraded (<b>A1</b>–<b>A3</b>) PNIPAAm-<span class="html-italic">co</span>-PCLDMA copolymer, (<b>B1</b>–<b>B3</b>) PNIPAAm-<span class="html-italic">co</span>-BACy, and (<b>A3</b>–<b>C3</b>) PNIPAAm-<span class="html-italic">co</span>-PCLDMA-<span class="html-italic">co</span>-BACy at (<b>A1</b>–<b>C1</b>) 0 days, (<b>A2</b>–<b>C2</b>) 10 days, and (<b>A3</b>–<b>C3</b>) 60 days after immersion on glutathione (GSH) at 37 °C. Reprinted with permission from Reference [<a href="#B77-gels-03-00036" class="html-bibr">77</a>]. Copyright © 2016 The Royal Society of Chemistry.</p> Full article ">Figure 13
<p>Chemical structures of P(NIPAAm-<span class="html-italic">co</span>-HEMA-<span class="html-italic">co</span>-DBA) triblock copolymer (<b>A</b>) before and (<b>B</b>) after hydrolysis of the hydrogel, as reported by Guan and co-workers [<a href="#B75-gels-03-00036" class="html-bibr">75</a>].</p> Full article ">Figure 14
<p>Scheme of the synthesis of hydrazide-functionalized precursor copolymers (PNIPAAm-<span class="html-italic">co</span>-ADH) (Route A) and aldehyde-functionalized precursor copolymers (PNIPAAm-<span class="html-italic">co</span>-oxoethyl methacrylate) (Route B). ADH is the acronym of adipic acid dihydrazide compound used as reversible and rapid functionalization of PNIPAAm oligomers. Adapted with permission from Reference [<a href="#B76-gels-03-00036" class="html-bibr">76</a>]. Copyright © 2012 American Chemical Society.</p> Full article ">Figure 15
<p>In vitro toxicity assays: (<b>A</b>) NIH 3T3 mouse fibroblasts and (<b>B</b>) RPE retinal pigment epithelial cells in the presence of (PNIPAAm-<span class="html-italic">co</span>-ADH) and (PNIPAAm-<span class="html-italic">co</span>-oxoethyl methacrylate). (<b>C</b>–<b>F</b>) In vivo toxicity assays of the hematoxylin-eosin stained sections of mouse subcutaneous tissue: (<b>C</b>) 6 wt % of PNIPAAm-<span class="html-italic">co</span>-ADH in PBS, after 48 h; (<b>D</b>) 6 wt % of PNIPAAm-<span class="html-italic">co</span>-oxoethyl methacrylate in PBS, after 48 h; (<b>E</b>) PNIPAAm in situ-formed hydrogel from 6 wt % of polymer precursor solutions in PBS, after 48 h; (<b>F</b>) PNIPAAm in situ-formed hydrogel from 6 wt % of polymer precursor solutions in PBS, after 5 months; Tissue labels on (F) are pertinent to all histological samples. Reprinted with permission from Reference [<a href="#B76-gels-03-00036" class="html-bibr">76</a>]. Copyright © 2012 American Chemical Society.</p> Full article ">Figure 16
<p>(<b>A</b>) Example of in vivo evolution of drug release from diblock copolymer PPS<sub>60</sub>-<span class="html-italic">b</span>-PDMA<sub>150</sub> and triblock copolymer of PPS<sub>60</sub>-<span class="html-italic">b</span>-PDMA<sub>150</sub>-<span class="html-italic">b</span>-PNIPAAm<sub>150</sub> injected onto BALB/c mice during 12 days; and (<b>B</b>) Quantification of drug release for both hydrogels over 14 days. Reprinted with permission from Reference [<a href="#B22-gels-03-00036" class="html-bibr">22</a>]. Copyright © 2014 American Chemical Society.</p> Full article ">Figure 17
<p>Graft copolymerization of NIPAAm and CNC via free radical polymerization employing ammonium persulfate (APS) as initiator and 1,2-di-(dimethylamino)ethane (TEMED) as accelerator (<b>up</b>) and schematic representation of metronidazole drug loading and release upon the action of the volume phase transition temperature (VPTT) at 37 °C (<b>down</b>). Adapted with permission from reference [<a href="#B52-gels-03-00036" class="html-bibr">52</a>]. Copyright © 2017 Polymers MDPI.</p> Full article ">Figure 18
<p>Scheme of the copolymerization reaction between ethylene glycol (EG) and ε-caprolactone (ε-CL) with glycidyl methacrylate (GMA) end-groups and 4-dimethylaminopyridine (DMAP) catalyst, followed by UV irradiation to obtain the interpenetrating polymer network (IPN)-hydrogel with grafted sodium alginate polysaccharide. Adapted with permission from Reference [<a href="#B109-gels-03-00036" class="html-bibr">109</a>]. Copyright © 2009 Elsevier Ltd. (Amsterdam, The Netherlands).</p> Full article ">Figure 19
<p>(<b>A</b>) Photographs of PNIPAAm-alginate IPN hydrogels; and (<b>B</b>,<b>C</b>) theophylline release profiles of distinct concentrations of sodium alginate on PNIPAAm hydrogel at pH 2.2 and pH 7.4, respectively. Reprinted with permission from Reference [<a href="#B110-gels-03-00036" class="html-bibr">110</a>] and Reference [<a href="#B74-gels-03-00036" class="html-bibr">74</a>], for (A) and (B,C), respectively. Copyrights © 2010 Society of Chemical Industry and © 2014 Wiley Periodicals Inc. (Seoul, Korea), respectively.</p> Full article ">Figure 20
<p>Representation of polymerization of NIPAAm by using β-barrel membrane protein (ferric hydroxamate uptake protein component A, FhuA) with ATRP initiating sites and “grafting from” strategy. Reprinted with permission from Reference [<a href="#B27-gels-03-00036" class="html-bibr">27</a>]. Copyright © 2016 Elsevier Ltd. (Amsterdam, The Netherlands).</p> Full article ">Figure 21
<p>Scheme of the synthesis of thiol-reactive PNIPAAm polymer (Polymer 2) and reversible addition fragmentation chain transfer (RAFT) polymerization with CP-MVP vault. DMF: <span class="html-italic">N,N</span>-dimethylformamide; AIBN: azobisisobutyronitrile; MES: 2-(<span class="html-italic">N</span>-morpholino) ethanesulfonic acid). Adapted with permission from Reference [<a href="#B118-gels-03-00036" class="html-bibr">118</a>]. Copyright © 2012 American Chemical Society.</p> Full article ">Figure 22
<p>SEM micrographs of PNIPAAm/SWCNT (on <b>left</b>) and PNIPAAm hydrogel (on <b>right</b>). Reprinted with permission from reference [<a href="#B43-gels-03-00036" class="html-bibr">43</a>]. Copyright from © 2014 Elsevier Ltd. (Amsterdam, The Netherlands).</p> Full article ">Figure 23
<p>(<b>A</b>–<b>C</b>) Macroscopic and microscopic images of PNIPAAm and PNIPAAm/SWCNTs hydrogels, (<b>D</b>) comparison of conductivities values of both materials, PNIPAAm and PNIPAAm/SWCNTs hydrogels, upon gelation. Reprinted with permission from Reference [<a href="#B43-gels-03-00036" class="html-bibr">43</a>]. Copyright from © 2014 Elsevier Ltd. (Amsterdam, The Netherlands).</p> Full article ">
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