Poly(vinyl alcohol) hydrogels as soft contact lens material

A new type of soft contact lens was developed from the poly(vinyl alchol) (PVA) hydrogel prepared by a low temperature crystallization technique using a water-dimethyl sulfoxide mixed solvent. The PVA contact lens materials had a water content of 78% and a tensile strength of 50 kg/cm², five times as strong as that of commercial poly(2-hydroxyethyl methacrylate) soft contact lens. The amount of proteins adsorbed to the PVA soft hydrogel material was half to one thirtieth of that of conventional soft contact lenses. Histological and scanning electron microscopic observation of rabbit eyes which had worn the PVA soft contact lens for 12 weeks showed no difference in corneal epithelium and cell arrangement in the corneal epithelium from the non-wearing eyes.     

Poly(vinyl alcohol)-acrylamide hydrogels as load-bearing cartilage substitute

Poly(vinyl alcohol) (PVA) has been advanced as a biomaterial for the fabrication of medical devices to be used as synthetic articular cartilage because of its viscoelastic nature, high water content, and biocompatibility. Key material requirements for such devices are high creep resistance to prevent mechanical instability in the joint and high water content to maintain a lubricious surface to minimize wear and damage of the cartilage counterface during articulation. The creep resistance of PVA hydrogels can be increased by high temperature annealing; however this process also collapses the pores, reducing the water content and consequently reducing the lubricity of the hydrogel surface [Bodugoz-Senturk H, Choi J, Oral E, Kung JH, Macias CE, Braithwaite G, et al. The effect of polyethylene glycol on the stability of pores in polyvinyl alcohol hydrogels during annealing. Biomaterials 2008;29(2):141-9.]. We hypothesized that polymerizing acrylamide (AAm) in the pores of the PVA hydrogel would minimize the loss of lubricity during annealing by preventing the collapse of the pores and loss of water content. Increasing AAm content increased porosity and equilibrium water content and decreased the coefficient of friction, tear strength, crystallinity, and creep resistance in annealed PVA hydrogels.     

Poly(vinyl alcohol) membranes for adhesion prevention

The abnormal joining of anatomic structures after abdominal and pelvic surgery can lead to such major complications as bowel obstruction or infertility. Poly(vinyl alcohol) (PVA) membranes and hydrogels were placed over the injured tissue to act as a physical barrier and prevent such adhesions from occurring in a rabbit sidewall model. The membranes were sutured into place to prevent their slipping or curling on the moist tissue. Various in vitro experiments (including testing for swelling and mechanical strength) were conducted in order to better understand the behavior of these membranes in the wound. The results showed that both the PVA membranes and PVA hydrogels significantly reduced the number and severity of adhesions in the rabbit sidewall model, and even indicated a distinct improvement over SEPRAFILM as antiadhesion barriers. Contact-angle measurements were taken in order to evaluate the surface properties of the membranes and hydrogels. Three approaches were taken to render the membranes more bioadhesive, and forego the need for future additional suturing: imprinting a texture onto the membrane, coating the membrane with carboxy methyl cellulose (CMC), and producing bi-layered, porous PVA membranes through a process of lyophilization. Though the surface of the PVA hydrogels is more hydrophilic than the surface of the PVA membranes, neither would adhere untreated to moist tissue. However, all three approaches aimed at improving their bioadhesion yielded excellent results and demonstrated that PVA could indeed be considered a viable method of adhesion prevention.     

Poly(vinyl alcohol) gels as photoacoustic breast phantoms revisited

A popular phantom in photoacoustic imaging is poly(vinyl alcohol) (PVA) hydrogel fabricated by freezing and thawing (F-T) aqueous solutions of PVA. The material possesses acoustic and optical properties similar to those of tissue. Earlier work characterized PVA gels in small test specimens where temperature distributions during F-T are relatively homogeneous. In this work, in breast-sized samples we observed substantial temperature differences between the shallow regions and the interior during the F-T procedure. We investigated whether spatial variations were also present in the acoustic and optical properties. The speed of sound, acoustic attenuation, and optical reduced scattering coefficients were measured on specimens sampled at various locations in a large phantom. In general, the properties matched values quoted for breast tissue. But while acoustic properties were relatively homogeneous, the reduced scattering was substantially different at the surface compared with the interior. We correlated these variations with gel microstructure inspected using scanning electron microscopy. Interestingly, the phantom's reduced scattering spatial distribution matches the optical properties of the standard two-layer breast model used in x ray dosimetry. We conclude that large PVA samples prepared using the standard recipe make excellent breast tissue phantoms.     

Cell-matrix adhesions on poly(vinyl alcohol) hydrogels

Cell-matrix adhesions regulate cell morphology, intracellular signaling, gene expression, and phenotype. Understanding how different methods of attaching matrix proteins to substrates affect the molecular arrangement of these adhesions offers the possibility of controlling cell function and architecture. The goal of this study was to visualize and quantify the cell-matrix adhesions formed by human fibroblasts on the matrix protein fibronectin covalently attached to poly(vinyl) alcohol (PVA) hydrogels. These adhesions were then compared with the cell adhesions formed in routine cell culture on fibronectin noncovalently coated onto glass coverslips or those formed on fibronectin covalently immobilized onto glass coverslips. Cell adhesions were characterized by immunofluorescence confocal microscopy utilizing paxillin as a marker for focal adhesions and alpha(5) integrin as a marker for fibrillar adhesions. As expected, distinct focal and fibrillar adhesions were observed in routine cell culture on coverslips coated noncovalently with fibronectin. Cells cultured on fibronectin covalently linked to PVA demonstrated diminished spatial separation of paxillin and alpha(5) integrin, accompanied by a reduction in fibrillar adhesions and fibronectin fibrillogenesis. Cells on fibronectin covalently immobilized on glass displayed the strongest marker colocalization and the most complete loss of fibrillar adhesions and lack of fibrillogenesis. These results indicate that fibronectin-conjugated PVA promotes the formation of cell adhesion structures intermediate in composition between those formed on noncovalently attached and covalently immobilized fibronectin. Furthermore, they imply that bioactive polymers can selectively induce specific cell-matrix adhesions, a characteristic that may have consequences in various tissue-engineering applications.     

Poly(vinyl alcohol) gels for use as tissue phantoms in photoacoustic mammography

Materials for solid photoacoustic breast phantoms, based on poly(vinyl alcohol) hydrogels, are presented. Phantoms intended for use in photoacoustics must possess both optical and acoustic properties of tissue. To realize the optical properties of tissue, one approach was to optimize the number of freezing and thawing cycles of aqueous poly(vinyl alcohol) solutions, a procedure which increases the turbidity of the gel while rigidifying it. The second approach concentrated on forming a clear matrix of the rigid poly(vinyl alcohol) gel without any scattering, so that appropriate amounts of optical scatterers could be added at the time of formation, to tune the optical properties as per requirement. The relevant optical and acoustic properties of such samples were measured to be close to the average properties of human breast tissue. Tumour simulating gel samples of suitable absorption coefficient were created by adding appropriate quantities of dye at the time of formation; the samples were then cut into spheres. A breast phantom embedded with such 'tumours' was developed for studying the applicability of photoacoustics in mammography.     

Poly(vinyl alcohol) microspheres with pH- and thermosensitive properties as temperature-controlled drug delivery

One of the most important inconveniences of the pH- and temperature-sensitive hydrogels is the loss of thermosensitivity when relatively large amounts of a pH-sensitive monomer are co-polymerized with N-isopropylacrylamide (NIPAAm). In order to overcome this drawback, we propose here a method to prepare thermosensitive poly(vinyl alcohol) (PVA) microspheres with a higher content of carboxylic groups that preserve thermosensitive properties. Moreover, PVA possesses excellent mechanical properties, biocompatibility and non-toxicity. PVA microspheres were obtained by suspension cross-linking of an acidified aqueous solution of the polymer with glutaraldehyde. Poly(N-isopropylacrylamide-co-N-hydroxymethyl acrylamide) (poly(NIPAAm-co-HMAAm)), designed to have a lower critical solution temperature (LCST) corresponding to that of the human body, was grafted onto PVA microspheres in order to confer them with thermosensitivity. Then, the pH-sensitive functional groups (COOH) were introduced by reaction between the un-grafted OH groups of PVA and succinic anhydride. The pH- and temperature-sensitive PVA microspheres display a sharp volume transition under physiological conditions around the LCST of the linear polymer. The microspheres possess good drug-loading capacity without losing their thermosensitive properties. Under simulated physiological conditions, the release of drugs is controlled by temperature.     

Synthesis and Characterization of High-Transparent Poly(vinyl alcohol)/Poly(vinyl pyrrolidone)(PVA/PVP) Hydrogels

In this paper, transparent poly(vinyl alcohol)/poly(vinyl pyrrolidone)(PVA/PVP) hydrogels were prepared by using the solvent of dimethyl sulfoxide(DMSO) aqueous solution and the method of freeze/thawing. The effect of PVP interaction, mechanical property and transparence of PVA/PVP hydrogel was investigated and evaluated by FT-IR analysis, mechanical test machine and UV spectrophotometer. The results showed that when the content of PVP was 8% in the DMSO aqueous solution, the highest transparence of PVA/PVP hydrogel was obtained, with excellent tensile strength values higher than 3.5 MPa. Therefore, the PVA/PVP hydrogel composite has a potential to be used as the transparent core of a novel artificial cornea.     

Structural and functional characterisation of poly(vinyl alcohol) and heparin hydrogels

Synthetic scaffolds show great promise for use in tissue engineering due to their ability to mimic some aspects of the extracellular matrix, however, their use has been hindered by the lack of inherent recognition sites that are required for protein and cell interactions. Heparan sulfate (HS), a glycosaminoglycan polysaccharide present in the basement membrane and on the cell surface, binds growth factors and cytokines and enhances the signalling of these ligands by forming complexes with their receptors. This study focuses on the formation of photopolymerised hydrogels derived from methacrylated macromers of poly(vinyl alcohol) (PVA) and heparin, with the aim of imparting the growth factor activation property of heparin to the synthetic scaffolds. It was shown that the methacrylate group attachment on heparin did not result in the fragmentation of heparin molecules, and that the biological activity of the methacrylated heparin was preserved as determined by tests on its anticoagulation properties and ability to signal fibroblast growth factor-2 (FGF-2). The addition of heparin into the PVA hydrogels resulted in an increase in mass swelling ratio from 5.8 for pure PVA to 6.5 and 6.6 for PVA/heparin co-hydrogels of 19/1 and 17.5/2.5 (w/w) compositions, respectively. It is believed that heparin molecules can be added into a synthetic PVA scaffold without adversely affecting the structural and mechanical stability of the PVA scaffold. The tensile moduli of the co-hydrogels remained close to that of PVA hydrogels (61 kPa), even up to 2.5% heparin composition (PVA/hep 17.5/2.5). Finally, the co-hydrogels were found to retain the growth factor signalling activity of heparin at equilibrium.     

Poly(vinyl alcohol) and poly(vinyl pyrrolidone) blended films for local nitric oxide release

The nitric oxide (NO) donor S-nitrosoglutathione (GSNO) was incorporated in solid polymeric films of poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP) and blended PVA/PVP. These matrices were found to provide a great stabilization effect on the thermal decomposition of GSNO, leading to 8-16-fold reduction in the first-order rate constants of NO release, compared to aqueous GSNO solutions. PVA/PVP-GSNO released 90% of the NO supply, over a time period of 24h at 37 degrees C. Differential scanning calorimetry has confirmed the miscibility between the two polymeric components. Stress-strain analysis has shown an improvement of the mechanical property of PVA films in the PVA/PVP blend, which leads to an increase of 25% in the stress at break. Scanning electron microscopy has shown that the PVA/PVP-GSNO blend leads to a smooth coating of metallic surfaces. These properties, allied to the already known good biocompatibility of PVA and PVP, makes GSNO-containing PVA and PVA/PVP blend films good candidates for the local and controlled release of NO in target areas.     

Microstructure of poly(vinyl alcohol) hydrogels investigated with differential scanning calorimetry

The physical state of water in poly(vinyl alcohol) (PVA) hydrogels was studied with differential scanning calorimetry to elucidate their microstructure. PVA hydrogels were prepared after three different methods, viz. chemical crosslinking with glutaraldehyde, annealing of dried film and crystallization at low temperature. The water in PVA hydrogels could be classified into three types: free water, intermediate water and bound water. The concentration of bound water is practically not dependent on the total water content of the hydrogels when prepared by low-temperature crystallization or annealing of the PVA film. The gel prepared by low-temperature crystallization contained less bound water than the gel obtained by annealing of a cast film. Furthermore, the concentration of bound water increases as the degree of polymerization (DP) of PVA increases, whereas the reverse tendency was observed for the intermediate water. From these results it was concluded that the low-temperature crystallization method results in a larger free space between the PVA microcrystallites and a larger size of microcrystallites than the annealing method. It is also likely that PVA of higher DP provides a larger amount of free PVA chains which are not involved in microcrystallites.     

Microwave‐Induced Poly(ionic liquid)/Poly(vinyl alcohol) Shape Memory Composites

Recently, different stimuli triggered shape memory polymer composites (SMPC) have received much research interest. However, most shape memory effects (SME) in polymer composites are mainly caused by introducing different functional inorganic fillers into polymer matrix. In the present work, the special structure of poly(ionic liquids), the heating effect of microwaves (MW), and the mechanism of SMPs are combined to design MW induced SMPC completely based on polymers, poly(ionic liquid)/poly(vinyl alcohol) (PIL/PVA) polymer networks. PILs contain anion cation pairs, which have strong MW absorption capability due to their dielectric properties. Polymer composites absorb MW energy under irradiation and generate MW heating, which could provide the required energy for shape recovery. The functional ionic liquid monomer (MIL) styrene imidazole ([VBBI][BF₄]) is synthesized and polymerized. Then, PIL is mixed into PVA solution, which is crosslinked with glutaraldehyde to form P[VBBI][BF₄]/PVA polymer networks. ¹H NMR is used to characterize the structure of the synthesized styrene imidazole ILM and PIL. A bending test is applied to quantitatively characterize SMEs, which are affected by the PIL content in the composites and the applied MW power output. The present work combines the advantages of microwaves with PILs, opening up an efficient and environmentally friendly field for SMPs.

     

Molecular design of biodegradable functional polymers, 4 Poly(vinyl alcohol) block as biodegradable segment

The minimum structure of a poly(vinyl alcohol) block as a biodegradable segment in the polymer chain was estimated with respect to block length and its stereochemical configuration. It was found that poly(vinyl alcohol) block with a chain length of more than 3 monomeric units and having an isotactic structure is quickly biodegraded, in contrast to an atactic block having the same chain length, by poly(vinyl alcohol) (PVA) assimilating microbes and PVA-dehydrogenase. It was also found that the substrate specificity of PVA-dehydrogenase obtained from both Alcaligenes faecalis KK314 and Pseudomonas sp. 113 P3 is quite similar.     

Preparation and characterization of alkylated poly(vinyl alcohol) hydrogels using alkyl halides

A poly(vinyl alcohol) hydrogel coated onto polyethylene was partially alkylated by reaction with an alkylhalide (C⁴, C⁸, or C¹⁸) in the presence of a deprotonating agent (sodium ethoxide or potassium tertbutoxide). Surface coverage determined by X-ray photoelectron spectroscopy (XPS) was respectively ~ 34, 25, and ~ 8% for the C₄, C₈, and C₁₈ modified surfaces. Statistically significant differences were observed in the fraction of C₈ and C₁₈ grafted alkyl groups as a function of depth (i.e. take-off angle) indicating the presence of a verticle composition gradient. All three surfaces showed maximal surface coverage of alkyl groups after 1 h reaction. At this reaction time, no further coverage was observed beyond a base/PVA ratio twenty times greater than the stoichiometric ratio. The advancing contact angle data exhibted an increase in hydrophobicity that correlated with the degree of coverage obtained by XPS: 90 ± 1, 83 ± 0.5, and 71 ± 1 deg for C₄, C₈ and C₁₈ alkylated PVA, and 55 ± 2 deg for PVA respectively. Large contact angle hysteresis was observed on all three surfaces consistent with surface heterogeneity.     

Friction and wear behavior of poly(vinyl alcohol)/poly(vinyl pyrrolidone) hydrogels for articular cartilage replacement

Many hydrogels have been proposed as articular cartilage replacements as an alternative to partial or total joint replacements. In the current study, poly(vinyl alcohol)/poly(vinyl pyrrolidone) (PVA/PVP) hydrogels were investigated as potential cartilage replacements by investigating their in vitro wear and friction characteristics in a pin-on-disk setup. A three-factor variable-level experiment was designed to study the wear and friction characteristics of PVA/PVP hydrogels. The three different factors studied were (a) polymer content of PVA/PVP hydrogels, (b) load, and (c) effect of lubricant. Twelve tests were conducted, with each lasting 100,000 cycles against Co-Cr pins. The average coefficient of friction for synovial fluid lubrication was a low 0.035 compared with 0.1 for bovine serum lubrication. Frictional behavior of PVA/PVP hydrogels did not follow Amonton's law of friction. Wear of the hydrogels was quantified by measuring their dry masses before and after the tests. Higher polymer content significantly reduced the wear of hydrogel samples with 15% PVA/PVP samples, showing an average dry polymer loss of 4.74% compared with 6.05% for 10% PVA/PVP samples. A trend change was observed in both the friction and wear characteristics of PVA/PVP hydrogels at 125 N load, suggesting a transition in the lubricating mechanism at the pin-hydrogel interface at the critical 125 N load.     

Silk fibroin/poly(vinyl alcohol) photocrosslinked hydrogels for delivery of macromolecular drugs

Hydrogels are three-dimensional polymer networks widely used in biomedical applications as drug delivery and tissue engineered scaffolds to effectively repair or replace damaged tissue. In this paper we demonstrate a newly synthesized cytocompatible and drug releasing photo-crosslinked hydrogel based on poly(vinyl alcohol) methacrylate and silk fibroin which possesses tailorable structural and biological properties. The initial silk fibroin content was 0%, 10%, 20%, 30%, 40% and 50% with respect to the weight of poly(vinyl alcohol) methacrylate. The prepared hydrogels were characterized with respect to morphology, crystallinity, stability, swelling, mass loss and cytotoxicity. FITC-dextrans of different molecular weights were chosen as model drugs molecules for release studies from the hydrogels. The hydrogels containing different silk fibroin percentages showed differences in pore size and distribution. X-ray diffraction analysis revealed that amorphous silk fibroin in poly(vinyl alcohol) methacrylate is crystallized to β-sheet secondary structure upon gelation. The sol fraction increased with increasing fibroin concentration in the co-polymer gel (from 18% to 45%), although the hydrogel extracts were non-cytotoxic. Similarly, the addition of silk fibroin increased water uptake by the gels (from 7% to 21%). FITC-dextran release from the hydrogels was dependent on the silk fibroin content and the molecular weight of encapsulated molecules. The study outlines a newer type of photo-crosslinked interpenetrating polymer network hydrogel that possess immense potential in drug delivery applications.     

Tissue reactions induced by modified poly(vinyl alcohol) hydrogels in rabbit cornea.

The purpose of this study was to compare the tissue reactions induced by modified poly(vinyl alcohol) (PVA) hydrogels with other materials implanted in the rabbit cornea. PVA hydrogels with and without covalently bonded collagen were compared to ethylene-vinyl alcohol copolymer, and polysulfone discs. These materials were implanted in the rabbit corneal stroma. The resultant tissue reactions were histologically and clinically studied. The PVA hydrogels had high flux rates of glucose and L-lactic acid in vitro. Covalent immobilization of collagen onto their surface was found to render them more biocompatible. However, excessive ulceration and vascularization of the cornea still occurred in vivo, possibly because of extremely little blinking of the rabbits, which might cause desiccation of the cornea.     

人工髓核材料-聚乙烯醇水凝胶的溶胀性能研究

利用冷冻-解冻法制得聚乙烯醇(PVA)水凝胶弹性体,研究了其用于人工髓核材料的溶胀特性,以及聚乙烯醇浓度、溶胀温度、溶胀体系的pH值对其溶胀性能的影响,采用扫描电镜对其微观形貌进行了观察,并对其溶胀动力学进行了探讨。结果表明,聚乙烯醇水凝胶是一种多孔网状结构,网络孔径大小与水凝胶中聚乙烯醇的含量有关;增加聚乙烯醇的浓度,提高溶胀温度以及溶胀体系的pH值,其平衡溶胀率减小;通过溶胀动力学方程对其溶胀过程进行了描述,水凝胶中聚乙烯醇的含量,试样尺寸以及溶胀体系的pH值,是溶胀速率快慢的重要影响因素。     

Poly(vinyl alcohol) cryogel phantoms for use in ultrasound and MR imaging

Poly(vinyl alcohol) cryogel, PVA-C, is presented as a tissue-mimicking material, suitable for application in magnetic resonance (MR) imaging and ultrasound imaging. A 10% by weight poly(vinyl alcohol) in water solution was used to form PVA-C, which is solidified through a freeze-thaw process. The number of freeze-thaw cycles affects the properties of the material. The ultrasound and MR imaging characteristics were investigated using cylindrical samples of PVA-C. The speed of sound was found to range from 1520 to 1540 m s(-1), and the attenuation coefficients were in the range of 0.075-0.28 dB (cm MHz)(-1). T1 and T2 relaxation values were found to be 718-1034 ms and 108-175 ms, respectively. We also present applications of this material in an anthropomorphic brain phantom, a multi-volume stenosed vessel phantom and breast biopsy phantoms. Some suggestions are made for how best to handle this material in the phantom design and development process.