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     

聚乙烯醇水凝胶自修复性能

高浓度聚乙烯醇（PVA）水凝胶具有一定的修复功能,但其自修复机理及制备工艺参数对其修复性能的影响缺乏研究。本文采用冷冻-解冻法制备了高浓度自修复PVA水凝胶,通过调整PVA水凝胶制备工艺参数（PVA分子量、PVA浓度、冷冻时间、解冻时间、冷冻-解冻次数、修复时间、冷冻温度等）得到了最佳工艺条件,分析了水凝胶自修复机理,并研究了PVA水凝胶的多次自修复性能。研究结果表明：相对分子质量大的PVA制备的水凝胶自修复性能好;其中冷冻时间为2h,解冻时间为1h,一次冷冻-解冻循环制备得到的水凝胶自修复性能最好,最佳修复时间为12h,能较好地进行反复自修复。指出水凝胶自修复性能主要是由其内部可逆氢键的相互作用形成的,其主要影响源于冷冻-解冻处理后水凝胶内部羟基含量及PVA分子的流动性。     

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 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     

Performance and characterization of irradiated poly(vinyl alcohol)/polyvinylpyrrolidone composite hydrogels used as cartilages replacement

In this article, hydrogels were prepared by compounding polyvinylpyrrolidone (PVP) with poly(vinyl alcohol) (PVA), which is used as artificial cartilages, by means of repeating freezing and thawing and irradiation, for improving their mechanical and surface lubricative properties. The structures and properties, including gel content, crystallized degree, elastic modulus, and frictional coefficients of the compound hydrogels with different PVP contents and irradiative conditions, were examined and compared. The existence of PVP macromolecules interfered with the crystallization of PVA hydrogels resulted in the decrease of gel contents and elastic modulus, as well as the unstable external frictional coefficient in water. After irradiation treatment, these performances increased with irradiation intensity in lower dose ranges. The solubility and exudation of PVP in water were prevented and reduced because of the chemical crosslink of PVA and PVP, and the lubricative properties of PVA/PVP hydrogels in water were improved. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

辐射交联PVA/PVP/胶原复合水凝胶的制备及性能研究

采用辐射交联与冻融循环相结合的方法,将胶原引入聚乙烯醇(PVA)/聚乙烯基吡咯烷酮(PVP)水凝胶体系,制备具有较高生物活性的PVA/PVP/胶原复合水凝胶。通过含水率、溶胀性能、力学性能及微观结构研究胶原对复合水凝胶结构与性能的影响,并优选最佳体系进行体外细胞毒性实验(MTT法)。研究结果表明,复合水凝胶具有均匀分布的三维多孔结构,胶原的添加增大水凝胶网络空间结构,其初始含水率达92%,并在10 h内达到溶胀平衡,但力学性能降低。辐射交联与冻融循环相结合的方法有利于提高胶原水凝胶制备效率,胶原结构不改变,其体外细胞存活率从PVA/PVP水凝胶的77.3%提高到93.8%,细胞相容性提高。     

Viscoelastic behaviour of self‐assembling polyurethane and poly(vinyl alcohol)

The rheological behaviour of polyurethane (PU) and poly(vinyl alcohol) (PVA) was investigated in aqueous solution and the hydrogel state. The dependence of viscosity on polymer concentration is discussed. The formation of supramolecular structures induced by temperature increase or shear conditions was evidenced. In PU solutions, as temperature increases, a self‐assembling process occurs due to hydrogen bonding and hydrophobic interactions determining a thermoreversible hydrogel formation. In creep and recovery tests, the weak PU network presents high elasticity only at low shear stress (below 10 Pa); it recovers only 15%–20% of strain above 40 Pa and the hydrogel structure fails at high shear stress (above 150 Pa). Also, PU hydrogel is not able to recover its structure after being submitted to successive low and high deformations. In PVA solutions, a shear induced aggregation was observed at 37 °C. PVA hydrogels obtained by the freezing–thawing method present high elasticity and stability due to the strong polymer–polymer interactions established between the polymer chains. Physical networks based on PU/PVA mixtures synergistically combine the characteristics of the two polymers, showing high elasticity when a shear stress up to 3000 Pa is applied during the creep test followed by a fast recovery of the hydrogel structure after exhibiting successive levels of deformation (self‐healing ability). Therefore, these hydrogels are suitable materials for tissue engineering applications. © 2019 Society of Chemical Industry     

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     

Photoactive antimicrobial PVA hydrogel prepared by freeze‐thawing process for wound dressing

A freeze‐thawing process was employed to produce both rose bengal (RB)/polyvinyl alcohol (PVA) and benzophenone (BP)/PVA hydrogels, respectively. Results indicated that only RB incorporated PVA (RB/PVA) could form hydrogel after undergoing three cycles of freeze‐thawing process; One of the cycles should be conducted by freezing at ?15°C ± 3°C for 18 h followed by thawing at 25°C for 6 h. The structural features and functional properties of the RB/PVA hydrogel were investigated by FTIR, XRD, SEM evaluations, and photo‐induced antimicrobial functions were examined as well. Release of RB from the RB/PVA hydrogel was examined by UV‐Vis spectroscopy. The freeze‐thawed RB/PVA hydrogel showed antimicrobial abilities against both E. coli and S. aureus under the exposure to fluorescence light as well as UVA (365 nm) light. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Development of poly(vinyl alcohol) hydrogel for waste water cleaning. I. Study of poly(vinyl alcohol) gel as a carrier for immobilizing microorganisms

The structures and properties of poly(viny1 alcohol) (PVA) hydrogel prepared by the freezing and thawing method and poly(ethy1ene glycol) and polyacrylamide hydrogels prepared by polymerization were compared in view of immobilizing microorganisms. Observation in a scanning electron microscope and measurement of the physicochemical properties revealed that the frozen PVA gel is superior in water content and oxygen permeability than the other gels. The PVA hydrogel has a high tensile and folding strength and good handleability and processability. The durability against abrasion and chemical resistance of PVA gel were also studied. Activated sludge was immobilized and fixed on PVA gel particles and incubated. This gel had a treating capacity for synthetic sewage of about two to three times that with the standard activated sludge process. © 1995 John Wiley & Sons, Inc.     

Preparation and characterization of dual responsive sodium alginate‐g‐poly(vinyl alcohol) hydrogel

Poly(vinyl alcohol) (PVA) was chosen as a controllable gelator to prepare sodium alginate (SA)‐based physically cross‐linked dual‐responsive hydrogel by three steps. First, polyvinyl acetate (PVAc) was grafted onto SA via radical copolymerization. Then, the copolymer was subsequently converted into SA‐g‐poly(vinyl alcohol) (SAPVA) by alcoholysis reaction. PVA content of SAPVA was tailored by controlling the graft percentage of PVAc, i.e. through varying the amount of vinyl acetate during copolymerization. Finally, SAPVA hydrogels were formed by freezing‐thawing cycles. The structure of the graft copolymers was verified with FTIR spectroscopy. X‐ray diffraction analysis results revealed that the crystallinity of SAPVA hydrogels depended on the PVA content of SAPVA. The swelling test showed that SAPVA hydrogels were pH‐responsive, and the swelling was reversible. SAPVA hydrogels also behaved electric‐responsive. In addition, the pH‐sensitivity of SAPVA hydrogels was able to be controlled with the composition of the hydrogels. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

Microstructural evaluation of poly(vinyl alcohol)‐based hydrogels obtained by freezing‐thawing technique: Thermal analysis and positron annihilation

In this work, semi‐interpenetrating polymer network (s‐IPN) hydrogels of poly(vinyl alcohol) (PVA) with different contents of water‐soluble sulfonated polyester (PES) were obtained by freezing and thawing cycles. The samples were characterized by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC). PALS was used to determine the average free volume radius through lifetime measures of the ortho‐positronium (o‐Ps). Degree of crystallinity of the PVA/PES hydrogels was evaluated using the melting enthalpy ratios between the samples and the 100% crystalline PVA. The results show that an increase on the PES content leads to a decrease on the degree of crystallinity of the samples, reflecting an increase on the lifetimes (τ₃). These structural changes could be interpreted as a result of different polymer‐polymer interactions between PVA and PES in the hydrogels. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Photosensitive antibacterial and cytotoxicity performances of a TiO2/carboxymethyl chitosan/poly(vinyl alcohol) nanocomposite hydrogel by in situ radiation construction

Nano‐TiO₂/carboxymethyl chitosan (CMCS)/poly(vinyl alcohol) (PVA) ternary nanocomposite hydrogels were prepared by freezing–thawing cycles and electron‐beam radiation with PVA, CMCS, and nano‐TiO₂ as raw materials. The presence of nano‐TiO₂ nanoparticles in the composite hydrogels was confirmed by thermogravimetry, Fourier transform infrared spectroscopy, and X‐ray powder diffraction. Field emission scanning electron microscopy images also illustrated that the TiO₂/CMCS/PVA hydrogel exhibited a porous and relatively regular three‐dimensional network structure; at the same time, there was the presence of embedded nano‐TiO₂ throughout the hydrogel matrix. In addition, the nano‐TiO₂/CMCS/PVA composite hydrogels displayed significant antibacterial activity with Escherichia coli and Staphylococcus aureus as bacterial models. The antibacterial activity was demonstrated by the antibacterial circle method, plate count method, and cell density method. Also, with the Alamar Blue assay, the cytotoxicity of the composite hydrogel materials to L929 cells was studied. The results suggest that these materials had no obvious cytotoxicity. Thus, we may have developed a novel, good biocompatibility hydrogel with inherent photosensitive antibacterial activity with great potential for applications in the fields of cosmetics, medical dressings, and environmental protection. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44150.     

Thermal transitions of poly(vinyl alcohol) hydrogel sensed by a fluorescent probe

Fluorescence from the fluorescein molecule has been used to study of phase transition processes in five poly(vinyl alcohol) (PVA) hydrogels in the temperature range 75–350 K. These hydrogels were prepared with PVA with molecular weight = 124,000–186,000, 99+% hydrolyzed, by repeated freezing–thawing cycles in five compositions: 9.9, 11.1, 13.3, 16.5, and 20.9 PVA w/w %. Plots of total fluorescence intensity versus temperature, normalized with the fluorescence signal at 75 K, and of the emission wavelength maximum versus temperature indicated that the dynamic of the guest deactivation was dependent on the hydrogel thermal transitions. These thermal transition processes were compared to three model systems: uranine (fluorescein di‐sodium salt) in water, fluorescein in water/ethanol (85%/15% v/v), and fluorescein/PVA homopolymer. Small spectral shifts were obtained for these model systems over the entire temperature range attributed to the hot band effect. Nevertheless, larger spectral shifts were obtained for hydrogels, indicating that shifts of the chemical dissociation equilibrium was occurring in this last case. Slope changes of both intensity and wavelength maximum plots versus temperature occurred at the same temperature values, and we attributed the onset of the hydrogel thermal transition as the sol–gel transition occurring at T ≅ 370 K; at 320 K we observed the glass transition temperature of PVA plasticized by water molecules; at T ≅ 240 ± 5 K we attributed the melting point of water molecules bonded to the PVA chains (freezable bond molecules); at T ≅ 270 ± 5 K we observed the melting point of free water molecules (bulk water); and, at 135–145 K the water glass transition. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 815–824, 2000     

Synthesis and swelling characteristics of pH and thermoresponsive interpenetrating polymer network hydrogel composed of poly(vinyl alcohol) and poly(acrylic acid)

The swelling behavior of novel pH- and temperature-sensitive interpenetrating polymer networks (IPNs) composed of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAAc) in water was investigated. The PVA/PAAc IPN hydrogels were synthesized by UV irradiation, followed by a repetitive freezing and thawing process by which PVA hydrogel networks were formed inside of cross-linked PAAc chains. The swelling behaviors of these IPNs were analyzed in buffer solution at various pH and temperature ranges. Swelling ratios of all IPNs were relatively high, and they showed reasonable sensitivity to both pH and temperature. Hydrogels showed both the positive and negative swelling behaviors depending on PAAc content. IPN46 showed the positive temperature-sensitive swelling behaviors and its stepwise changes in swelling ratio was about 1.8 and 2.0 obtained between 25 and 45°C at pH 7, and between pH 4 and 7 at 35°C, respectively. The positive temperature dependence is attributed to the formation and dissociation of hydrogen bonding complexes between PVA and PAAc. These IPNs are expected to show a pH- and temperature-sensitive drug release according to the stepwise behavior at this temperature region. © 1996 John Wiley & Sons, Inc.     

Silicon dioxide/poly(vinyl alcohol) composite hydrogels with high mechanical properties and low swellability

Poly(vinyl alcohol) (PVA) hydrogels with tissue-like viscoelasticity, excellent biocompatibility, and hydrophilicity have been considered as promising cartilage replacement materials. However, the low mechanical properties of pure PVA hydrogels limit their applications for bearing complicated loads. Herein, we report silicon dioxide (SiO₂)/PVA composite hydrogels fabricated by fabricated cyclically freezing/thawing the aqueous mixture of PVA and methyltrimethoxysilane (MTMS). MTMS hydrolyzes and forms SiO₂ particles in situ to reinforce PVA hydrogel. Meanwhile, silanol group condenses with hydroxyl groups of PVA and chemically bonds with PVA. The resulting SiO₂/PVA hydrogels exhibit much better mechanical properties than bare PVA hydrogel. In addition, the composite hydrogels keep very low swellable property. This prepared composite hydrogels are promising in a variety of biomedical applications such as artificial articular cartilage, drug delivery, and biosensors. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46895.     

Evaluation of two chemical crosslinking methods of poly(vinyl alcohol) hydrogels for injectable nucleus pulposus replacement

Low back pain caused by intervertebral disc degeneration is one of the most common spinal disorders among patients seeking medical treatment. The most common surgical treatments are spinal fusion and total disc arthroplasty, both of which are very invasive surgical procedures. Nucleus pulposus replacement is an earlier stage intervention for disc degeneration. One of the material classes being studied for this application is hydrogels: a three‐dimensional hydrated network of polymer(s), which mimics the mechanical and physiological properties of the nucleus. Poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), and poly(ethylene glycol) (PEG) hydrogels have previously been shown to be great candidate materials for injectable nucleus pulposus replacement, but have experienced issues with swelling and mass retention. The addition of chemical crosslinking to the PVA/PVP/PEG hydrogel system will allow tailoring of the swelling, mechanical, injectability, and mass loss properties of the hydrogel network. Two chemical crosslinking methods were evaluated for the PVA/PVP/PEG hydrogel system by characterizing the hydrogels with compression, swelling, and spectroscopy experiments. The results of these experiments led to the selection of the difunctional crosslinking strategy using PEG functionalized with terminal epoxide group (PEG diglycidyl ether) as the preferred crosslinking method. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40843.     

Properties of gels obtained by freezing/thawing of poly(vinyl alcohol)/water/dimethyl sulfoxide solutions

Melting points of poly(vinyl alcohol) (PVA) gels obtained by repeated freezing/thawing cycles of the solutions in mixed solvents of water and dimethyl sulfoxide (DMSO) were measured. PVA solutions in 1–30 and 90 vol % DMSO froze at ?40°C. The melting point of a gel increased with an increase in freezing/thawing cycles for all the solutions frozen and with an increase in freezing time in 10–30 vol % DMSO. The solutions in 1–5 vol % DMSO gelled even at very low polymer concentration of 1.0 g/dL by repeated freezing/thawing. The melting point of all the gels with a 5% polymer concentration reached ca. 73°C after 14 freezing/thawing cycles except for the gels in 90 vol % DMSO. However, the melting point of the gels with the same polymer concentration prepared at ?40°C using an unfrozen mixed solvent in 60 vol % DMSO was ca. 77°C and the largest. The maximum apparent enthalpy of fusion of the gels prepared by repeated freezing/thawing was ca. 40 kcal/mol. The gels in 90 vol % DMSO were transparent independent of cycles where as those in 1–80 vol % DMSO were opaque. The degree of opaqueness increased with decreasing DMSO content.     

Self‐healing hydrogel of poly(vinyl alcohol)/graphite oxide with pH‐sensitive and enhanced thermal properties

Supramolecular hydrogel is a fascinating polymeric material composed of three‐dimensional noncovalent networks with many outstanding properties, especially reversible relevant performances. A self‐healing supramolecular hydrogel of poly(vinyl alcohol)/graphite oxide, with reversible pH responsiveness and good thermal stability, was prepared. The morphology, functional group changes, swelling performance, thermal stability, rheological performance, and self‐healing property of the PVA/GO hydrogel were investigated. A probable mechanism between the components and potential applications were also examined in our study. The experimental results show that the PVA/GO hydrogel was not only self‐healable without external stimulus or addition of any healing agents, but also pH sensitive and with good thermal stability. Green ingredients (PVA and GO) and a simple synthesis method (a freezing/thawing treatment) may pose little threat to the environment and also promote the production of such hydrogels. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46143.