Investigation on the formation mechanisms of hydrogels made by combination of γ‐ray irradiation and freeze‐thawing

Hydrogels based on poly(vinyl alcohol) (PVA)/water‐soluble chitosan (ws‐chitosan)/glycerol were prepared by γ‐ray irradiation, freeze‐thawing, and combination of γ‐ray irradiation and freeze‐thawing, respectively. The influence of freeze‐thawing cycles, the irradiation doses, and the sequence of freeze‐thawing and irradiation processes on the rheological, swelling, and thermal properties of these hydrogels was investigated to evaluate the formation mechanisms of hydrogels made by combination of irradiation and freeze‐thawing. For hydrogels made by freeze‐thawing followed by irradiation, the physical crosslinking is destroyed partially while chemical crosslinking is formed by irradiation. However, the chemical crosslinking density reduces with the increase of freeze‐thawing cycles. Hydrogels made by irradiation followed by freeze‐thawing bear less degree of physical crosslinking with the increase of irradiation dose for the increased chemical crosslinking density. It is found that these hydrogels own larger swelling capacity and better transparent appearance than those made by freeze‐thawing followed by irradiation. Moreover, the former hydrogels have larger mechanical strength than the latter at low freeze‐thawing cycles. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal and rheological properties of poly(vinyl alcohol) and water‐soluble chitosan hydrogels prepared by a combination of γ‐ray irradiation and freeze thawing

Poly(vinyl alcohol) (PVA)/water‐soluble chitosan (ws‐chitosan) hydrogels were prepared by a combination of γ‐irradiation and freeze thawing. The thermal and rheological properties of these hydrogels were compared with those of hydrogels prepared by pure irradiation and pure freeze thawing. Irradiation reduced the crystallinity of PVA, whereas freeze thawing increased it. Hydrogels made by freeze thawing followed by irradiation had higher degrees of crystallinity and higher melting temperatures than those made by irradiation followed by freeze thawing. ws‐Chitosan disrupted the ordered association of PVA molecules and decreased the thermal stability of both physical blends and hydrogels. All the hydrogels showed shear‐thinning behavior in the frequency range of 0.2–100 rad/s. Hydrogels made by freeze thawing dissolved into sol solutions at about 80°C, whereas those made by irradiation showed no temperature dependence up to 100°C. The chemical crosslinking density of the hydrogels made by irradiation followed by freeze thawing was much greater than that of hydrogels made by freeze thawing followed by irradiation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation and properties of PVA/PVP hydrogels containing chitosan by radiation

Radiation can induce chemical reactions to modify polymers even when they are in the solid state or at a low temperature. Radiation crosslinking can be easily adjusted by controlling the radiation dose and is reproducible. The finished product contains no residuals of substances required to initiate the chemical crosslinking, which can restrict its application possibilities. In these studies, hydrogels for wound dressing were made from a mixture of chitosan and polyvinyl alcohol (PVA)/poly‐N‐vinylpyrrolidone (PVP) by freezing and thawing, gamma‐ray irradiation, or combined freezing and thawing and gamma‐ray irradiation. The physical properties of the hydrogel, such as gelation, water absorptivity, and gel strength, were examined to evaluate the usefulness of the hydrogels for wound dressing. The PVA/PVP composition was 60:40, PVA/PVP–chitosan ratio was in the range 9:1–7:3, and the concentration of, PVA/PVP–chitosan as a solid was 15 wt %. A mixture of PVA/PVP–chitosan was exposed to gamma irradiation doses of 25, 35, 50, 60 and 70 kGy to evaluate the effect of irradiation dose on the physical properties of hydrogels. Water‐soluble chitosan was used in these experiment. The physical properties of the hydrogels, such as gelation and gel strength, were higher when the combination of freezing and thawing and irradiation were used rather than just freezing and thawing. The PVA/PVP–chitosan composition and irradiation dose had a greater influence on swelling than gel content. Swelling percent increased as the composition of chitosan in PVA/PVP–chitosan increased. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1787–1794, 2002     

Preparation and characterization by radiation of poly(vinyl alcohol) and poly(N‐vinylpyrrolidone) hydrogels containing aloe vera

In these studies, hydrogels for wound dressings were made from a mixture of aloe vera and poly(vinyl alcohol) (PVA)/poly(N‐vinylpyrrolidone) (PVP) by freezing and thawing, γ‐Ray irradiation, or a two‐step process of freezing and thawing and γ‐ray irradiation. We examined the physical properties, including gelation, water absorptivity, gel strength, and degree of water evaporation, to evaluate the applicability of these hydrogels for wound dressings. The PVA:PVP ratio was 6:4, the dry weight of aloe vera was in the range 0.4–1.2 wt %, and the solid concentration of the PVA/PVP/aloe vera solution was 15 wt %. We used γ radiation doses of 25, 35, and 50 kGy to expose mixtures of PVA/PVP/aloe vera to evaluate the effect of radiation dose on the physical properties of the hydrogels. Gel content and gel strength increased as the concentration of aloe vera in the PVA/PVP/aloe vera gels decreased and as radiation dose increased and the number of freeze–thaw cycles was increased. The swelling degree was inversely proportional to the gel content and gel strength. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1477–1485, 2003     

Controlled mechanical and swelling properties of poly(vinyl alcohol)/sodium alginate blend hydrogels prepared by freeze–thaw followed by Ca2+ crosslinking

Poly(vinyl alcohol) (PVA)/sodium alginate (SA) blend hydrogels have immense potential for use as functional biomaterials. Understanding of influences of processing parameters and compositions on mechanical and swelling properties of PVA/SA blend hydrogels is very important. In this work, PVA/SA blend hydrogels with different SA contents were prepared by applying freeze–thaw method first to induce physical crosslinking of PVA chains and then followed by Ca²⁺ crosslinking SA chains to form interpenetrating networks of PVA and SA. The effects of number of freeze–thaw cycles, SA content and Ca²⁺ concentration on mechanical properties, swelling kinetics, and pH‐sensitivity of the blend hydrogels were investigated. The results showed that the blend hydrogels have porous sponge structure. Gel fraction, which is related to crosslink density of the blend hydrogels, increased with the increase of freeze–thaw cycles and strongly depended on SA content. The SA content exerts a significant effect on mechanical properties, swelling kinetics, and pH‐sensitivity of the blend hydrogels. The number of freeze–thaw cycles has marked impact on mechanical properties, but no obvious effect on the pH‐sensitivity of the PVA/SA blend hydrogels. Concentration of CaCl₂ aqueous solution also influences mechanical properties and pH‐sensitivity of the blend hydrogel. By altering composition and processing parameters such as freeze–thaw cycles and concentration of CaCl₂ aqueous solution, the mechanical properties and pH‐sensitivity of PVA/SA blend hydrogels can be tightly controlled. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Medicated wound dressings based on poly(vinyl alcohol)/poly(N‐vinyl pyrrolidone)/chitosan hydrogels

Poly(vinyl alcohol)/poly(N‐vinyl pyrrolidone) (PVP)/chitosan hydrogels were prepared by a low‐temperature treatment and subsequent ⁶⁰Co γ‐ray irradiation and then were medicated with ciprofloxacin lactate (an antibiotic) and chitosan oligomer (molecular weight = 3000 g/mol). The gel content, swelling ratio, tensile strength, and crystallinity of the hydrogels were determined. The effects of the chitosan molecular weight, the low‐temperature treatment procedure, and the radiation dosage on the hydrogel properties were examined. The molecular weight of chitosan was lowered by the irradiation, but its basic polysaccharide structure was not destroyed. Repeating the low‐temperature treatment and γ‐ray irradiation caused effective physical crosslinking and chemical crosslinking, respectively, and contributed to the mechanical strength of the final hydrogels. The incorporation of PVP and chitosan resulted in a significant improvement in the equilibrium swelling ratio and elongation ratio of the prepared hydrogels. The ciprofloxacin lactate and chitosan oligomer were soaked into the hydrogels. Their in vitro release behaviors were examined, and they were found to follow diffusion‐controlled kinetics. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2453–2463, 2006     

Dual stimuli responsive glycidyl methacrylate chitosan‐quaternary ammonium hybrid hydrogel and its bovine serum albumin release

A new family of cationic hybrid hydrogels from two new positively charged aqueous soluble precursors, glycidyl methacrylate‐chitosan (GMA‐chitosan), and 2‐(acryloyloxy) ethyl trimethylammonium (AETA), was developed via a simple photocrosslinking fabrication method. These hybrid hydrogels have pendant quaternary ammonium functional groups on the AETA segments. The chemical composition of GMA‐chitosan/AETA hybrid hydrogels were characterized by Fourier transform infrared spectroscopy and their mechanical, swelling, and morphological properties were examined as a function of the composition of the hybrids as well as the effect of pH and ionic strength of the surrounding medium. GMA‐chitosan/AETA hybrid hydrogels show a porous network structure with average pore diameter 20–50 μm. The compression moduli of these hybrid hydrogels ranged from 27.24 to 28.94 kPa, which are significantly higher than a pure GMA‐chitosan (17.64 kPa). GMA‐chitosan/AETA hybrid hydrogel shows pH/ionic strength responsive swelling behavior because of the presence of the positive charge pendant groups. These hybrid hydrogels showed a sustained BSA protein release and a significantly lower initial burst release than a pure GMA‐chitosan hydrogel. The two aqueous soluble precursors and the cationic charge characteristics of the resulting GMA‐chitosan/AETA hybrid hydrogels may suggest that this new family of biomaterials may have promising applications as the pH responsive protein drug delivery vehicles. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3736–3745, 2013     

Characterization of hydrogels based on chitosan and copolymer of poly(dimethylsiloxane) and poly(vinyl alcohol)

Copolymers composed of poly(vinyl alcohol) (PVA) and poly(dimethylsiloxane) (PDMS) were crosslinked with chitosan to prepare semi‐interpenetrating polymer network (IPN) hydrogels by an ultraviolet (UV) irradiation method for application as potential biomedical materials. PVA/PDMS copolymer and chitosan was cast to prepare hydrogel films, followed by a subsequent crosslinking with 2,2‐dimethoxy‐2‐phenylacetophenone as a nontoxic photoinitiator by UV irradiation. Various semi‐interpenetrating polymer networks (semi‐IPNs) were prepared from different weight ratios of chitosan and the copolymer of PVA/PDMS. Photocrosslinked hydrogels exhibited an equilibrium water content (EWC) in the range of 65–95%. Swelling behaviors of these hydrogels were studied by immersion of the gels in various buffer solutions. Particularly, the PCN13 as the highest chitosan weight ratio in semi‐IPN hydrogels showed the highest EWC in time‐dependent and pH‐dependent swelling. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2591–2596, 2002     

A novel pH‐sensitive and freeze‐thawed carboxymethyl chitosan/poly(vinyl alcohol) blended hydrogel for protein delivery

BACKGROUND: Blended hydrogels are widely applied in medical fields. They can provide many advantages, such as biocompatibility and biodegradability. Many materials and methods are used to obtain blended hydrogels. In this work, carboxymethyl chitosan (CMCS) and poly(vinyl alcohol) (PVA) blended hydrogels were prepared using the freezing and thawing technique. The properties of the hydrogels prepared, i.e. gel fraction, swelling and pH‐responsive behaviors, were investigated. RESULTS: The gel fraction increased with increasing time of freezing and thawing as determined through gravimetric analysis. It was also found that the equilibrium degree of swelling improved obviously due to the addition of CMCS compared to pure PVA hydrogel. The blended hydrogel with composition CMCS/PVA 80/20 (by weight) possessed the highest swelling ratio. The results of the influence of pH values on the swelling behavior showed that minimum swelling ratios of the hydrogels occurred near the isoelectric point of CMCS. Protein release studies were performed under various pH conditions: the release was much slower under acid than under basic conditions. The release showed a burst in the first 15 h and then steadily increased. CONCLUSION: The addition of CMCS can improve the physical properties of pure PVA hydrogels and provide pH sensitivity. It is concluded that PVA hydrogels containing CMCS could be potentially applied as oral delivery systems for protein drugs. Copyright © 2009 Society of Chemical Industry     

Preparation and characterization by radiation of hydrogels of PVA and PVP containing Aloe vera

In these studies, hydrogels for wound dressing were made from a mixture of Aloe vera, poly(vinyl alcohol) (PVA) and poly(N‐vinylpyrrolidone) (PVP) by freeze‐thaw, gamma‐ray irradiation, or a two‐step process of freeze‐thaw and gamma‐ray irradiation. Physical properties, such as gelation, water absorptivity, gel strength and degree of water evaporation were examined to evaluate the applicability of these hydrogels to wound dressing. The PVA:PVP ratio was 6:4, and the dry weight of Aloe vera was in the range of 0.4‐1.2 wt %. The solid concentration of PVA/PVP/Aloe vera solution was 15 wt %. Mixtures of PVA/PVP/Aloe vera were exposed to gamma irradiation doses of 25, 35 and 50 kGy to evaluate the effect of irradiation dose on the physical properties of the hydrogels. Gel content and gel strength increased as the concentration of Aloe vera in PVA/PVP/Aloe vera decreased and as irradiation dose increased and freeze‐thaw was repeated. Swelling degree was inversely proportional to gel content and gel strength. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1612–1618, 2004     

Self-assembly, drug-delivery behavior, and cytotoxicity evaluation of amphiphilic chitosan-graft-poly(1,4-dioxan-2-one) copolymers

Allyl glycidyl ether (AGE)-functionalized chitosan (CS-AGE), a macromolecular crosslinker, was synthesized and then copolymerized with N-isopropylacrylamide (NIPAAm) monomer under UV irradiation to produce hydrogels. The allylated chitosan and the resulting hydrogels were characterized by ¹?H NMR and FT IR, respectively. The interior morphologies of the hydrogels were investigated by scanning electron microscopy (SEM) after freeze drying them in the equilibrium state in buffer solution at pH 2.0. Their swelling kinetics were found to be sensitive to both temperature and pH, so it was possible to modulate the swelling by adjusting the pH or the temperature of the medium containing the hydrogel and the proportion of the CS derivative with respect to the NIPAAm monomer. Rheological measurements were utilized to investigate the mechanical properties of the hydrogels. The in vitro release profiles of the model drugs methyl orange (MO) and bovine serum albumin (BSA) from the hydrogels were also examined. The results revealed that the drug release rate could be tuned by adjusting the pH of the medium and the hydrogel composition.     

PVA hydrogels loaded with a Brazilian propolis for burn wound healing applications

PVA hydrogels offer many suitable characteristics for burn wound dressings. However, unmodified PVA gels do not act against infection. Propolis is a natural antimicrobial agent suitable for incorporation into PVA gels. PVA–propolis gels were produced by freeze–thawing method, and their microstructure, mechanical, and swelling properties (in standard PBS and a PBS‐based solution with pH 4.0) were characterized. The propolis release profiles and the gel's antibacterial and cytotoxicity properties were also investigated. The presence of propolis in the gels interfered with the PVA crystallization profile and with the mechanical properties. All samples swelled at least 400% in both media. The propolis was mostly released to the media in the first day of immersion. PVA–propolis gels with concentrations of 15% propolis or more were active against the gram‐positive bacterium Staphylococcus aureus, which is associated with initial colonization of the wound. All PVA–propolis samples acted as barriers to microbial penetration. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42129.     

Novel preparative method for porous hydrogels using overrun process

Porous poly(vinyl alcohol) (PVA) hydrogels were prepared using the overrun process which is usually used in manufacturing ice cream. The pores in the hydrogel formed exhibit dual‐pore structure due to the injection of air bubbles and ice recrystallization. Morphological investigation revealed that overrun‐processed hydrogels had closed pore structures and that their pore size and size distribution had been influenced by the impeller rate and concentration of polymer solution. The closed‐pore structure was reformed into interconnected open‐pore structure at lower concentrations of the solution. The freeze–thawing process, which takes place in PVA cross‐linking, has no effect on the bubble structure, but removes the small pores formed during ice recrystallization. Besides the swelling ratio of overrun‐processed PVA hydrogels is increased tenfold in comparison with non‐porous hydrogels. Overrun‐processed hydrogels showed more rapid swelling kinetics than freeze‐dried and film‐like hydrogels due to their larger surface area. In the future, the overrun process can be applied to prepare porous scaffolds containing proteins, such as growth factors and other cytokines, without denaturation, because it operates at a low temperature. Copyright © 2004 Society of Chemical Industry     

Preparation and characterization of biocompatible spongy cryogels of poly(vinyl alcohol)–gelatin and study of water sorption behaviour

Porous biocompatible spongy hydrogels of poly(vinyl alcohol) (PVA)–gelatin were prepared by the freezing–thawing method and characterized by infrared and differential scanning calorimetry. The prepared so‐called ‘cryogels’ were evaluated for their water‐uptake potential and the influence of various factors, such as the chemical architecture of the spongy hydrogels, pH and the temperature of the swelling bath, on the degree of water sorption by the cryogels was investigated. It was found that the water sorption capacity constantly decreased with increasing concentration of PVA while initially an increase and thereafter a decrease in swelling was obtained with increasing amounts of gelatin in the cryogel. The water sorption capacity decreased with an increase in the number of freeze–thaw cycles. The hydrogels were also swollen in salt solutions and various simulated biological fluids and a fall in swelling ratio was noticed. The effect of the drying temperature of the cryogel on its water sorption capacity was also investigated, and a decrease in swelling was obtained with increasing temperature of drying. The biocompatibility of the prepared materials was assessed by in vitro methods of blood‐clot formation, platelet adhesion, and per cent haemolysis. It was noticed that with increasing concentration of PVA and gelatin the biocompatibility increased, while a reduced biocompatibility was noted with an increasing number of freeze–thaw cycles. Copyright © 2005 Society of Chemical Industry     

On physical and antibacterial properties and drug release behavior of poly(vinyl alcohol) hydrogels: effect of drug loaded chitosan nanoparticles

This paper deals with influence of chitosan nanoparticles (CNPs) loaded by tetracycline, as a drug, on the physico-mechanical and antibacterial properties as well as drug release behavior of poly(vinyl alcohol), PVA, hydrogels prepared by electron beam irradiation. The formation of spherical chitosan particles in nanoscale size prepared by an ionic gelation method was confirmed by FTIR and UV spectroscopy, and scanning electron microscopy analyses. The drug release kinetic studies from drug loaded chitosan nanoparticles (DLCNPs) at pH = 7.4 revealed a linear and steady release behavior over long period of time. The theoretical analysis of the swelling kinetic data, using Peppas’s model showed that the swelling kinetic is governed by Fickian diffusion for all the prepared hydrogels, however, the water diffusion coefficient, and therefore, the swelling content were lower for the hydrogels loaded with DLCNPs as compared to the ones with the neat drug. In agreement with these results, the hydrogels containing DLCNPs exhibited a more controlled drug release behavior with significantly stronger antibacterial activity. The tensile mechanical properties of the hydrogels not affected by the DLCNPs were found to be suitable for wound dressing applications.     

Cellular PVA hydrogels produced by freeze/thawing

Cellular poly(vinyl alcohol) (PVA) hydrogels that exhibit enhanced swelling kinetics from an initial dry state were prepared by freezing and thawing techniques in the presence of NaCl. Gels that were dried immediately after preparation demonstrated a dual‐sorption effect upon swelling. Gels that were exposed to a 10‐day washing period to remove NaCl and excess PVA chains before drying showed an increased initial rate of swelling and overall water content. Freeze/thawed, cellular PVA gels showed overall enhanced swelling with increased mechanical strength over traditional gels prepared by chemical or irradiative crosslinking techniques. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 2075–2079, 2000     

Preparation and antibacterial effects of PVA‐PVP hydrogels containing silver nanoparticles

The poly(vinyl alcohol)/poly(N‐vinyl pyrrolidone) (PVA–PVP) hydrogels containing silver nanoparticles were prepared by repeated freezing–thawing treatment. The silver content in the solid composition was in the range of 0.1–1.0 wt %, the silver particle size was from 20 to 100 nm, and the weight ratio of PVA to PVP was 70 : 30. The influence of silver nanoparticles on the properties of PVA–PVP matrix was investigated by differential scanning calorimeter, infrared spectroscopy and UV–vis spectroscopy, using PVA–PVP films containing silver particles as a model. The morphology of freeze‐dried PVA–PVP hydrogel matrix and dispersion of the silver nanoparticles in the matrix was examined by scanning electron microscopy. It was found that a three‐dimensional structure was formed during the process of freezing–thawing treatment and no serious aggregation of the silver nanoparticles occurred. Water absorption properties, release of silver ions from the hydrogels and the antibacterial effects of the hydrogels against Escherichia coli and Staphylococcus aureus were examined too. It was proved that the nanosilver‐containing hydrogels had an excellent antibacterial ability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 125–133, 2007     

聚乙烯醇/膨润土杂化水凝胶的力学性能和溶胀行为

利用冷冻-解冻法制备了聚乙烯醇/膨润土杂化水凝胶. X射线衍射结果表明,膨润土以剥离形式分布在水凝胶基体中. 研究结果表明,与纯PVA5水凝胶相比,经过5个冷冻-解冻循环制备的含2%(w)膨润土的杂化水凝胶的拉伸模量、拉伸强度和断裂伸长率分别增加了44.0%, 74.2%和25.2%,而溶胀行为与5个循环的纯水凝胶相近. 含0.5%(w)膨润土的杂化水凝胶的拉伸模量和拉伸强度高于基体水凝胶,其在溶胀400 min时的溶胀度高于所有的样品.     

Swelling and drug‐release behavior of the poly(AA‐co‐N‐vinyl pyrrolidone)/chitosan interpenetrating polymer network hydrogels

A series of novel hydrogels were prepared from acrylic acid (AA), N‐vinyl pyrrolidone (NVP), and chitosan by photopolymerization. The swelling behavior, gel strength, and drug release behavior of the poly(AA/NVP) copolymeric hydrogels and corresponding interpenetrating polymer network hydrogels were investigated. Results showed that the swelling ratios for the present hydrogels decreased with an increase of NVP content in the gel, but the gel strength increased with an increase of NVP content in the gel. Results also showed that the drug‐release behavior for the gels is related to the ionicity of drug and the swelling ratio of the gel. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2135–2142, 2004     

Porous boron nitride nanofibers/PVA hydrogels with improved mechanical property and thermal stability

Polyvinyl alcohol (PVA) hydrogel is a promising material possessing good chemical stability, high water absorption, excellent biocompatibility and biological aging resistant. However, the poor mechanical performance of PVA hydrogel limits its applications. Here we report the utilization of one-dimensional (1D) BN nanofibers (BNNFs) as nanofillers into PVA matrix to prepare a novel kind of BNNFs/PVA composite hydrogel via a cyclic freezing and thawing method. For comparison, the composite hydrogels using spherical BN nanoparticles i.e. BN nanospheres (BNNSs) as fillers were also prepared. The mechanical properties, thermal stabilities and swelling behaviors of the composite hydrogels were investigated in detail. Our study indicates that the mechanical properties of the hydrogels can be improved by adding of BNNFs. After loading of BNNFs into PVA with content of 0.5?wt%, the compressive strength of the composite hydrogel increases by 252% compared with that of pure PVA hydrogel. The tensile performance of BNNFs/PVA composite hydrogels has also been improved. Impressive 87.8% increases in tensile strengths can be obtained with 1?wt% BNNFs added. In addition, with the increase of BNNFs content, the thermal stability and the swelling ratio of hydrogels are increased gradually. The swelling ratio of hydrogel increases by 56.3% with only 1?wt% BNNFs added. In comparison, the improvement effects of the BNNS fillers on the mechanical strengths and swelling ratios are much weaker. The enhanced effects of BNNFs can be ascribed to the strong hydrogen bond interaction between BNNFs and PVA. The high aspect ratios of the nanofibers should also be took into account.