Thiol–norbornene photoclick hydrogels for tissue engineering applications

Thiol–norbornene (thiol–ene) photoclick hydrogels have emerged as a diverse material system for tissue engineering applications. These hydrogels are crosslinked through light‐mediated orthogonal reactions between multifunctional norbornene‐modified macromers [e.g., poly(ethylene glycol) (PEG), hyaluronic acid, gelatin] and sulfhydryl‐containing linkers (e.g., dithiothreitol, PEG–dithiol, biscysteine peptides) with a low concentration of photoinitiator. The gelation of thiol–norbornene hydrogels can be initiated by long‐wave UV light or visible light without an additional coinitiator or comonomer. The crosslinking and degradation behaviors of thiol–norbornene hydrogels are controlled through material selections, whereas the biophysical and biochemical properties of the gels are easily and independently tuned because of the orthogonal reactivity between norbornene and the thiol moieties. Uniquely, the crosslinking of step‐growth thiol–norbornene hydrogels is not oxygen‐inhibited; therefore, gelation is much faster and highly cytocompatible compared with chain‐growth polymerized hydrogels with similar gelation conditions. These hydrogels have been prepared as tunable substrates for two‐dimensional cell cultures as microgels and bulk gels for affinity‐based or protease‐sensitive drug delivery, and as scaffolds for three‐dimensional cell encapsulation. Reports from different laboratories have demonstrated the broad utility of thiol–norbornene hydrogels in tissue engineering and regenerative medicine applications, including valvular and vascular tissue engineering, liver and pancreas‐related tissue engineering, neural regeneration, musculoskeletal (bone and cartilage) tissue regeneration, stem cell culture and differentiation, and cancer cell biology. This article provides an up‐to‐date overview on thiol–norbornene hydrogel crosslinking and degradation mechanisms, tunable material properties, and the use of thiol–norbornene hydrogels in drug‐delivery and tissue engineering applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41563.     

Dual‐Crosslinked Human Serum Albumin‐Polymer Hydrogels for Affinity‐Based Drug Delivery

A dual‐crosslinked in situ gelling drug delivery scaffold based on dextran (DEX), thiolated serum albumin, and poly(ethylene glycol) (PEG) is presented. Dextran–vinyl sulfone conjugates with varied molecular weight and degrees of substitution are synthesized by controlling the reaction time and temperature with divinyl sulfone. Dextran–human serum albumin (sHSA) hydrogels are prepared using a thiol‐vinyl sulfone Michael addition reaction with thiolated albumin as the crosslinker. Poly(ethylene glycol) dithiol is added as a third component to the crosslinked dextran–human serum albumin hydrogel to facilitate additional crosslinking, and reduce gelation time, while modulating the physicochemical properties of the Dex–sHSA–PEG network. The onset of gelation of the modular three‐component dual‐crosslinked hydrogel network ranges from 45 min to 1.5 h depending on gel constituent concentrations and the gelation temperature (25 or 37 °C). All gels remain stable for over a 25 d period under physiological conditions. In vitro drug release assays show that dual‐crosslinked Dex–sHSA–PEG hydrogels can deliver doxorubicin in a sustained manner over 7 d. Finally, a Tetrazolium‐based assay shows the biocompatible nature of the Dex–sHSA–PEG hydrogels and capacity to deliver doxorubicin successfully to MCF‐7 breast cancer cells.     

Poly(N‐isopropylacrylamide‐co‐poly(ethylene glycol))‐acrylate simultaneously physically and chemically gelling polymer systems

In an effort to create an in situ physically and chemically cross‐linked hydrogel for in vivo applications, N‐isopropylacrylamide (NIPAAm) was copolymerized with poly(ethylene glycol)‐monoacrylate (PEG‐monoacrylate) and then the hydroxyl terminus of the PEG was further modified with acryloyl chloride to form poly(NIPAAm‐co‐PEG) with acrylate terminated pendant groups. In addition to physically gelling with temperature changes, when mixed with a multi‐thiol compound such as pentaerythritol tetrakis 3‐mercaptopropionate (QT) in phosphate buffer saline solution of pH 7.4, this polymer formed a chemical gel via a Michael‐type addition reaction. The chemical gelation time of the polymer was affected by mixing time; swelling of the copolymer solutions was temperature dependant. Because of its unique gelation properties, this material may be better suited for long‐term functional replacement applications than other thermo‐sensitive physical gels. Also, the PEG content of this material may render it more biocompatible than similar HEMA‐based precursors in previous simultaneous chemically and physically gelling materials. With its improved mechanical strength and biocompatibility, this material could potentially be applied as a thermally gelling injectable biomaterial for aneurysm or arteriovenous malformation (AVM) occlusion. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Physically Linked PVA/P(AAm‐co‐SMA) Hydrogels: Tough,Fast Recovery,Antifatigue, and Self‐Healing Soft Materials

The application of traditional chemically crosslinked hydrogels is often limited by poor mechanical properties because of their own inhomogeneous network and irreversible crosslinking bonds. Herein, physical interactions are applied to crosslink the interpenetrating network hydrogel, i.e., hydrogen bonding and crystalline domain for polyvinyl alcohol network, and hydrophobic interaction inside micelle for poly (acrylamide‐co‐stearyl methyl acrylate) [P(AAm‐co‐SMA)] network. In this gel network system, reversible energy dissipation mechanism is realized by dissociation and reassociation of weak interactions including hydrogen bonding and hydrophobic interaction inside the micelle. Strong crystalline domains serve as permanent crosslinking interactions to maintain network integrity under large extension. As a result, the synergy of weak and strong interactions leads to tough, antifatigue, fast recovery, and self‐healing properties of the hydrogel. This proposed strategy of achieving versatile hydrogels can broaden the use of hydrogels into load‐bearing applications.     

Preparation and property of starch nanoparticles reinforced aldehyde–hydrazide covalently crosslinked PNIPAM hydrogels

Injectable, de‐crosslinkable, and thermosensitive hydrogels are obtained by hydrazide‐functionalized poly(N‐isopropylacrylamide) and aldehyde‐functionalized dextrin through in situ crosslinked method. Natural based and degradable starch nanoparticles (SNPs) are used as fillers in order to improve mechanical property of hydrogels. Internal morphology, dynamic modulus, thermosensitivity property, de‐crosslinking performance, drug release, and in vitro cytotoxicity of hydrogels are investigated. Results show that SNPs disperse well throughout hydrogel and have no significant influence on gelation time and de‐crosslinking performance. Elasticity property of composite hydrogel prepared from 9.0 wt % precursors with 1.5 wt % fillers is improved significantly by SNPs and maximum storage modulus reaches 399.2 kPa, but 89.6 kPa of unreinforced hydrogels. Hydrogels exhibit good thermosensitive performance at alternating cyclic temperature of 25 and 37 °C. Doxorubicin hydrochloride‐loaded hydrogels can release more than 25 days. No significant cytotoxicity to L929 fibroblast cells is observed through a CCK‐8 assay for hydrogels, precursors, and SNPs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45761.     

Studies on preparation and properties of porous biodegradable poly(NIPAAm) hydrogels

A series of biodegradable porous hydrogels, based on thermosensitive N‐isopropylacrylamide (NIPAAm) and biodegradable crosslinker‐polycaprolactone diacrylate (PCLdA) that was synthesized from polycaprolactone diol with acryloyl chloride were prepared by photopolymerization at low temperature. The effect of the crosslinker content and gelation method on the swelling behaviors and physical properties for the poly(NIPAAm) hydrogels was investigated. Results showed that the swelling ratio of the gel in deionized water decreased with an increase of the content of polycaprolactone (PCL) segment in the poly(NIPAAm) hydrogels. The properties of the gels crosslinked with PCLdA were compared with those crosslinked with N, N′‐methylenebisacrylamide (NMBA). The results showed that the critical gel transition temperatures (CGTT) of the gels crosslinked with PCLdA were lower than those of the gels crosslinked with NMBA due to the hydrophobicity of the PCL segment. The results also showed that the gels crosslinked with PCLdA had higher mechanical strength and crosslinking density than those gels crosslinked with NMBA. Comparing the porous gels with nonporous gels, the results showed that the swelling ratio and CGTT of the porous gels were higher than those of the nonporous gels, and the transition temperature curve was smoother for the porous gels. The porous gels also exhibited more rapid thermal response and faster degradation rates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Gelation and crosslinking characteristics of photopolymerized poly(ethylene glycol) hydrogels

The gelation and crosslinking features of poly(ethylene glycol) (PEG) hydrogels were scrutinized through the UV polymerization processes of poly(ethylene glycol) methacrylate (PEGMA) and poly(ethylene glycol) dimethacrylate (PEGDMA) mixtures. The real‐time evolutions of the elastic moduli of the prepolymerized mixtures with different crosslinking ratios of PEGMA and PEGDMA and the photoinitiator concentrations were measured during photopolymerization. The rheological properties were compared with other properties of the PEG hydrogels, including the relative changes in the C?C amounts in the mixtures before and after UV irradiation, water swelling ratio, gel fraction, mesh size, and mechanical hardness. As the portion of PEGDMA as a crosslinker increased, the final elastic modulus and gel fraction increased, whereas the swelling ratio and scratch penetration depth at the hydrogel film surface decreased because of the formation of compact networks inside the hydrogels. These results indicate that there was a good correlation between the rheological analysis for predicting the crosslinking transition during photopolymerization and the macroscopic properties of the crosslinked hydrogels. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41939.     

Effect of porosigen on the swelling behavior and drug release of porous N‐isopropylacrylamide/poly(ethylene glycol) monomethylether acrylate copolymeric hydrogels

A series of porous thermoreversible hydrogels were prepared from N‐isopropylacrylamide (90 mol %) and poly(ethylene glycol) methylether acrylate (10 mol %), which was derived from poly(ethylene glycol) monomethylether, N,N′‐methylenebisacrylamide, and porosigen, or poly (ethylene glycol) (PEG) with different molecular weights (MWs). The influence of pore volume in the gel on the physical properties, swelling kinetics, and solute permeation from these porous gels was investigated. The results show that the surface areas, pore volumes, and equilibrium swelling ratios for the porous gels increased with increasing MW of PEG, but the shear moduli and effective crosslinking densities decreased with increasing MW of PEG. The results from the dynamic swelling kinetics show that the transport mechanism was non‐Fickian. The diffusion coefficients of water penetrating into the gels increased with increasing pore volume of the gels. In addition, we also studied solute permeation through the porous gel controlled by temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5490–5499, 2006     

Preparation and properties of crosslinked chitosan thermosensitive hydrogel for injectable drug delivery systems

The aim of this study was to prepare and investigate the physical properties of a thermosensitive crosslinked chitosan pregel solution, and evaluate the in vitro release profiles of macromolecules from this sol–gel transition system. Chitosan and poly (vinyl alcohol) were used to form an interpenetrating polymeric network with glutaraldehyde as the crosslinker, and glycerophosphate (GP) was added to transform the pH‐dependent solutions into thermosensitive pH‐dependent solutions. Rheological study showed that the gelation was dependent on the crosslink degree and GP concentration of the solution. The crosslinked gel had excellent mechanic properties and no apparent “pores” and formed an integrated hydrogel texture according to scanning electronic micrograph. Gas chromatography test guaranteed the medication safety with no detection of glutaraldehyde remnants in the hydrogels. In vitro release study showed that the gelation does not significantly affect the macromolecules diffusion but the crosslinking degree does. These results indicated that the hydrogel formed an intensified three‐dimensional hybrid network with interpenetrating molecules, which effectively buffered or delayed the macromolecules diffusion. The hydrogels sustained the drug release over 30 days and could be potentially used as in situ gelling implants. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1892–1898, 2006     

Toughening Mechanism of Hydrophobic Association Hydrogels Reinforced by Latex Particles

In this investigation, the copolymer poly(n‐butyl acrylate)‐b‐PEG‐b‐poly(n‐butyl acrylate) hydrogel is synthesized by atom transform radical polymerization and chosen as a simplified hydrophobic association hydrogel model. After adding the latex particles, hydrophobic poly(n‐butyl acrylate) segments can self‐assemble in an aqueous solution, forming a hydrogel (LP‐Gel) model with network structure. Structures of this model system are investigated by small angle X‐ray scattering. Dynamics are investigated through a combination of linear viscoelastic and creep measurements. It is found that introduction of the latex particles increases the crosslinking density and the association energy compared to the particles‐free system. This simple LP‐Gel model reveals the toughening mechanism of latex particle‐modified hydrophobic association hydrogels.     

Effect of drug loading on the properties of temperature‐responsive polyester–poly(ethylene glycol)–polyester hydrogels

Hydrogels that can undergo gelation upon injection in vivo are promising systems for the site‐specific delivery of drugs. In particular, some thermo‐responsive gels require no chemical additives but simply gel in response to a change from a lower temperature to physiological temperature (37 °C). The gelation mechanism does not involve covalent bonds, and it is possible that incorporation of drugs into the hydrogel could disrupt gelation. We investigated the incorporation of drugs into thermo‐responsive hydrogels based on poly(?‐caprolactone‐co‐lactide)‐block‐poly(ethylene glycol)‐block‐poly(?‐caprolactone‐co‐lactide) (PCLA–PEG–PCLA). Significant differences in properties and in the response to incorporation of the anti‐inflammatory drug celecoxib (CXB) were observed as the PEG block length was varied from 1500 to 3000 g mol^?1. Linear viscoelastic moduli of a PCLA–PEG–PCLA hydrogel containing a 2000 g mol^?1 PEG block were least affected by the incorporation of CXB and this gel also exhibited the slowest release of CXB, so the incorporation of phenylbutazone, methotrexate, ibuprofen, diclofenac and etodolac was also investigated for this hydrogel. Different drugs resulted in varying degrees of syneresis of the hydrogels, suggesting that they interact with the polymer networks in different ways. In addition, the drugs had varying effects on the viscoelastic and compressive moduli of the gels. The results showed that the effects of drug loading on the properties of thermo‐responsive hydrogels can be substantial and depend on the drug. For applications such as intra‐articular drug delivery, in which the mechanical properties of the hydrogel are important, these effects should thus be studied on a case‐by‐case basis. © 2019 Society of Chemical Industry     

A comparative study of swelling properties of hydrogels based on poly(acrylamide‐co‐methyl methacrylate) containing physical and chemical crosslinks

Poly (acrylamide‐co‐methyl methacrylate) hydrogels of different ratios were prepared by using chemical and physical crosslinks to study the effect of nature of crosslinks on swelling behavior of hydrogels. The chemically crosslinked gels were prepared by using NN′‐methylene bis acrylamide, while physically crosslinked hydrogels were prepared by precipitation polymerization method, using dioxane as solvent. Detailed swelling kinetics such as swelling ratio, transport exponent n, diffusion coefficient D and the effect of pH on equilibrium swelling studies. The study revealed that the nature of crosslinks alter the swelling characteristics of the hydrogel. In chemically crosslinked hydrogels the water transport is Fickian in nature, while in the case of the physically crosslinked hydrogels the water transport mechanism is anomalous indicating major change in relaxation mechanism due to nature of crosslinks. The results also indicate that with increasing acrylamide content the swelling ratio of the hydrogels were also increased, but the transport exponent n remains nearly constant. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 779–786, 2003     

In situ crosslinked hydrogels formed using Cu(I)‐free Huisgen cycloaddition reaction

BACKGROUND: ‘Click’ chemistry, or the 1,3‐dipolar cycloaddition of organic azides with alkynes, has been evaluated for many biomedical purposes; however, its utility in crosslinking hydrogels in situ is limited by the toxicity of the requisite copper(I) catalyst. We report the first use of catalyst‐free Huisgen cycloaddition to generate crosslinked hydrogels under physiological conditions using multivalent azide‐functionalized polymers and an electron‐deficient dialkyne crosslinker. RESULTS: Water‐soluble azide‐functionalized polymers were crosslinked with an electron‐deficient dialkyne crosslinker to form hydrogels at physiological temperature without the addition of copper(I) catalyst. Crosslinking was confirmed using scanning electron microscopy, Fourier transform infrared and ¹H NMR analyses. Flow by vial inversion and dynamic rheological methodologies were implemented to evaluate gelation kinetics at 37 °C of variable polymer compositions, concentrations and stoichiometric ratios. Kinetic studies revealed gelation in as little as 12 h at 37 °C, although strong gels that withstand inversion were observed by 1–8 days. CONCLUSION: The ability to form hydrogel networks under mild conditions demonstrates the potential viability of the catalyst‐free ‘click’ crosslinking chemistry for in situ gelling and other biological applications. Further chemical modifications in the crosslinking moieties, as well as polymer and crosslinker conformations, are expected to enhance gelation kinetics to a more biomedically practical rate. Copyright © 2009 Society of Chemical Industry     

PH-induced structure change of poly(vinyl alcohol) hydrogel crosslinked with poly(acrylic acid)

The structure of the hydrogel of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) was investigated by small angle X-ray scattering (SAXS) of synchrotron radiation. A physically crosslinked blend gel, which was prepared by repetitive freezing and thawing of an aqueous solution of PVA and PAA, could be chemically crosslinked by esterfication of PVA with PAA even in the hydrogel state. The chemical crosslinking induced the destruction of physical crosslinks into a folded structure, indicating that the chemical crosslinking proceeds at the sites around the physical crosslinks that contain PVA and PAA in much higher concentration than other portion of the gel. The pH-induced structure changes of the PVA hydrogels, chemically crosslinked with poly(acrylic acid) (PAA) were investigated by SAXS on the samples of various chemical crosslinking time. The gels were shrunk at pH4, and swollen at pH8. The results of SAXS showed, that the Porod slope changed with chemical crosslinking time from -3.5 to ?2.9 at pH4, and from ?2.9 to ?2.4 at pH8. The results suggest that a folded structure as a structural domain, which is characterized by fractally rough interface, tends to change into the structure that corresponds to percolation cluster, particularly at pH8. The gels immersed in pH8 showed a remarkable structure change accompanying swelling. The results revealed that a conformational change of PAA chains, induced by the pH change, can be explained by the presence of a structural domain in the gel network, where both PVA chains and PAA chains get entangled and partially form a interpenetrating polymer network(IPN).     

Synthesis and swelling behavior of thermosensitive IPN hydrogels based on sodium acrylate and N‐isopropyl acrylamide by a two‐step method

A series of interpenetrating polymer network (IPN) hydrogels having higher swelling ratio (SR) and thermosensitivity were synthesized from sodium acrylate (SA) and N‐isopropyl acrylamide (NIPAAm) by a two‐step method. A series of the porous poly(sodium acrylate ‐co‐1‐vinyl–2‐pyrrolidone) [poly(SA‐co‐VP)], (SV), hydrogels were prepared from acrylic acid having 90% degree of neutralization and VP monomer in the first step. The second step is to immerse the SV dried gels into the NIPAAm solution containing initiator, accelerator, and crosslinker to absorb NIPAAm solution and then polymerized to form the poly(SA‐co‐VP)/poly(NIPAAm) IPN hydrogels (SVN). The effect of the different molar ratios of SA/VP and the content of NIPAAm on the swelling behavior and physical properties of the SVN hydrogels was investigated. Results showed that the SVN hydrogels displayed an obviously thermoreversible behavior when the temperature turns across the critical gel transition temperature (CGTT) of poly(NIPAAm) hydrogel. The pore diameter distributions inside the hydrogel also indicated that the pore sizes inside the SVN hydrogels were smaller than those inside the SV hydrogels. At the same time, the more proportion of SA was added into the hydrogel, the larger pore diameter of the SV hydrogel was formed. The results also showed that the SR decreased with an increase of the VP content in the SV hydrogel and more obviously decreased in the SVN hydrogels. The SVN networks also showed stronger shear moduli than SV hydrogels. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Hydrophobic association and temperature and pH sensitivity of hydrophobically modified poly(N‐isopropylacrylamide/acrylic acid) gels

Hydrophobically modified poly(acrylic acid/N‐isopropylacrylamide) gels were synthesized by the radical copolymerization of acrylic acid/N‐isopropylacrylamide with a small amount of the hydrophobic comonomer 2‐(N‐ethylperfluorooctanesulfoamido)ethyl acrylate, stearyl acrylate, or lauryl acrylate in tert‐butanol with ethylene glycol dimethacrylate as a crosslinker. Swelling kinetics and fluorescence measurements showed that the hydrophobic association ability of fluorocarbon groups was stronger than that of hydrocarbon analogues in modified hydrogels that contained both physical and chemical crosslinking networks. The effects of the fractions and the species of the hydrophobe on the gel swelling and pH and temperature sensitivity were studied. The results indicated that the swelling behavior and pH and temperature sensitivity of the gels were affected by the degree of hydrophobic modification. A hydrogel with a suitable 2‐(N‐ethylperfluorooctanesulfoamido)ethyl acrylate content (0.349 mol %) showed good pH and temperature sensitivity. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2406–2413, 2003     

Crosslinking structures of gelatin hydrogels crosslinked with genipin or a water‐soluble carbodiimide

It was suggested in our previous studies that carbodiimide‐ and genipin‐crosslinked gelatin hydrogels could be used as bioadhesives to overcome the cytotoxicity problem associated with formaldehyde‐crosslinked gelatin hydrogels. In this study, we investigated the crosslinking structures of carbodiimide‐ and genipin‐crosslinked gelatin hydrogels. We found that crosslinking gelatin hydrogels with carbodiimide or genipin could produce distinct crosslinking structures because of the differences in their crosslinking types. Carbodiimide could form intramolecular crosslinks within a gelatin molecule or short‐range intermolecular crosslinks between two adjacent gelatin molecules. On the basis of gel permeation chromatography, we found that the polymerization of genipin molecules could occur under the conditions used in crosslinking gelatin hydrogels via a possible aldol condensation. Therefore, besides intramolecular and short‐range intermolecular crosslinks, additional long‐range intermolecular crosslinks could be introduced into genipin‐crosslinked gelatin hydrogels. Crosslinking a gelatin hydrogel with carbodiimide was more rapid than crosslinking with genipin. Therefore, the gelation time for the carbodiimide‐crosslinked gelatin hydrogels was significantly shorter than that of the genipin‐crosslinked gelatin hydrogels. However, the cohesive (interconnected) structure of the carbodiimide‐crosslinked gelatin hydrogels was readily broken because, unlike the genipin‐crosslinked gelatin hydrogels, there were simply intramolecular and short‐range intermolecular crosslinks present in the carbodiimide‐crosslinked hydrogel. In the cytotoxicity study, the carbodiimide‐crosslinked gelatin hydrogels were dissolved into small fragments in the cultural medium within 10 min. In contrast, the genipin‐crosslinked gelatin hydrogels remained intact in the medium throughout the entire course of the study. Again, this may be attributed to the differences in their crosslinking structures. The genipin‐crosslinked gelatin hydrogels were less cytotoxic than the carbodiimide‐crosslinked gelatin hydrogels. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 4017–4026, 2004     

Preparation and characterization of PAM/SA tough hydrogels reinforced by IPN technique based on covalent/ionic crosslinking

In order to fabricate tough hydrogels with superior formability, polyacrylamide/sodium alginate (PAM/SA) interpenetrating polymer network (IPN) hydrogels were produced with ionically crosslinked SA interpenetrated in covalently crosslinked PAM. TGA results show that the heat resistance of PAM/SA IPN hydrogel is improved as compared to that of the individual component. Swelling studies indicate that increasing either chemical crosslinker content or ionic crosslinking via adding more N,N′‐methylenebisacrylamide (MBA) or SA results in lower ESR. It is concluded by tensile test that loosely crosslinked PAM coupled with tightly crosslinked SA improve mechanical strength for hydrogels based on covalent/ionic crosslinking. PAM/SA hydrogels via “one‐pot” method can form different complex shapes with mechanical properties comparable to conventional double network (DN) gels. The fracture strength of PAM_0.05/SA₂₀ reaches level of MPa, approaching 2.0 MPa. The work strives to provide method to tune mechanical and physical properties for hydrogels, which is hopefully to guide the design of hydrogel material with desirable properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41342.     

Effect of gelatin on the swelling behavior of organic hybrid gels based on N‐isopropylacrylamide and gelatin

Organic hybrid gels based on poly(N‐isopropylacrylamide) and a natural polymer, gelatin, were prepared through two‐step crosslinking with genipin or glutaraldehyde. The effects of the gelatin content on the swelling behaviors and physical properties of these hybrid gels were investigated. The results indicated that the swelling ratio decreased with an increase in the content of gelatin in these hybrid gels. The swelling ratio for the gel crosslinked by genipin was significantly smaller than that for the gel crosslinked by glutaraldehyde. The results also showed that the gel crosslinked with genipin had a higher crosslinking density and a higher gel strength. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1092–1099, 2005     

Hydrogels composite of poly(acrylamide‐co‐acrylate) and rice husk ash. I. Synthesis and characterization

Composite hydrogels of poly(acrylamide‐co‐acrylate) with rice husk ash (RHA) were synthesized and studies of the swelling variables were accomplished comparatively with commercial polyacrylamide gel and PAMACRYL, a poly(acrylamide‐co‐acrylate) hydrogel without RHA. FT‐IR and WAXS were the techniques employed for characterizing a series of hydrogel obtained by varying the percentage of RHA (1, 2, 5, 10, and 20 wt %) and the amount of crosslinking agent (0.05, 0.1, and 0.2 mol %) relative to sum of AAm and AAc. Superabsorbent hydrogel with W_eq > 800 g H₂O/g gel was obtained with percentage of 10 wt % of RHA and 0.1 of crosslinking agent mol %. The hydrogel showed to be sensitive to the pH variation and to the presence of salts. The hydrogels, even though submitted through cycles of drying and swelling, preserved their superabsorbent characteristics and demonstrated better water absorbance properties when compared with commercial polyacrylamide gel. The composite hydrogels of poly(acrylamide‐co‐acrylate) with RHA presented good characteristics to be applied as soil conditioner for using in agriculture. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011