Collagen-immobilized poly(vinyl alcohol) as an artificial cornea scaffold that supports a stratified corneal epithelium

The cornea is a transparent tissue of the eye, which is responsible for the refraction of incoming light. Both biological corneal equivalents and synthetic keratoprostheses have been developed to replace donor tissue as a means to restore vision. However, both designs have drawbacks in terms of stability and biocompatibility. Clinically available synthetic devices do not support an intact epithelium, which poses a risk of microbial infection or protrusion of the prosthesis. In the present study, type I collagen was immobilized onto poly(vinyl alcohol) (PVA-COL) as a possible artificial cornea scaffold that can sustain a functional corneal epithelium. Human and rabbit corneal epithelial cells were air-lift cultured with 3T3 feeder fibroblasts to form a stratified epithelial layer on PVA-COL. The epithelial sheet expressed keratin 3/12 differentiation markers, the tight junction protein occludin, and had characteristic microvilli structures on transmission electron microscopy. Functionally, the stratified epithelium contained normal glycogen levels, and an apical tight-junction network was observed to exclude the diffusion of horseradish peroxidase. Furthermore, the epithelium-PVA-COL composite was suturable in the rabbit cornea, suggesting the possibility of using PVA-COL as a biocompatible material for keratoprosthesis.     

Porous nano-hydroxyapatite/poly(vinyl alcohol) composite hydrogel as artificial cornea fringe: characterization and evaluation in vitro

A nano-hydroxyapatite/poly(vinyl alcohol) (n-HA/PVA) composite hydrogel was employed as artificial cornea fringe to improve biocompatibility for the firm fixation between material and surrounding host tissues. The morphology and swelling behavior, as well as mechanical strength of the fringes were characterized. The results showed that the n-HA/PVA fringes had interconnective porous structure, high water content and good mechanical properties. With the aid of cell culture observed by inverted microscopy, scanning electron microscopy (SEM) and MTT test, it was concluded that PVA hydrogel modified with n-HA can improve biocompatibility and has no negative effects on the corneal fibroblasts in vitro. These findings indicate that the porous n-HA/PVA fringe can allow invasion and proliferation of cells, and can function as a fringe for artificial cornea.     

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.     

Oxidation of poly(vinyl alcohol) with an oxoammonium salt

Oxidation of poly(vinyl alcohol) (PVA) with 1-oxo-4-methoxy-2,2,6,6-tetramethylpiperidinium chloride (oxoammonium salt, 1 ) prepared by a one-electron oxidation of the corresponding nitroxyl radical ( 2 ) was carried out. PVA with a degree of polymerization (DP) of 300 and a degree of saponification (DS) of 88 mol-% was oxidized with 1 in the presence of magnesium perchlorate in N-methyl-2-pyrrolidone (NMP) to obtain a polymer containing 66 mol-% of ketone units. The oxidation was found to be dependent on solvent and additive and further, independent of DP but dependent on DS of PVA. In the case of PVA with extremely low (10 mol-%) or high (98,5 mol-%) DS, no or little oxidation took place. The highest ketone content was obtained in a polymer with a DS of 88 mol-%. Moreover, the ketone content of the oxidized PVA could be controlled by the amount of 1 .     

Interaction between poly(L-lysine) and sulfated poly(vinyl alcohol)

Ionic polymer-polymer interaction was studied in aqueous solution for poly(L -lysine) (PLL) and sulfated poly(vinyl alcohol) (PVS) as functions of pH, the degree of sulfation, the functional unit mole ratio of the two polymers and temperature by means of circular dichroism and viscosity measurements. In all the cases studied, strong inter-polymer complexes were formed at the functional unit mole ratio (VS)/(LL) higher than 1. Although PLL itself is well known to take the α-helical conformation at such a high pH as 11, the PLL conformation in the PLL/PVS complexes did not depend on pH but on the degree of sulfation: at room temperature, PLL took random coil conformation in PLL/PVS-25 (25: degree of sulfation in mole-%) and PLL/PVS-30, and the α-helical conformation (helicity of 70%) in PLL/PVS-46 and PLL/PVS-95. Models for the complex structures are postulated. Methanesulfonic acid did not influence the conformational transition of PLL, supporting that a polymer effect took place in the complex formation between PLL and PVS. Thermal effect on the PLL conformation in the complex is also discussed.     

Nanofibrous scaffold from electrospinning biodegradable waterborne polyurethane/poly(vinyl alcohol) for tissue engineering application

A series of nanofibrous scaffolds, free of organic solvents, are prepared by electrospinning biodegradable waterborne polyurethane (BWPU) emulsion blending with aqueous poly(vinyl alcohol)(PVA). Tuning the proration of BWPU to PVA, various nanofibers with diameter from 370 to 964 nm are obtained. Strong intermolecular interaction existing between them benefits to the electrospun of BWPU emulsion, which is demonstrated by dynamic thermomechanical analysis and Fourier transform infrared spectroscopy. The nontoxic nanofibrous scaffolds with porous structure, which is similar to the natural extracellular matrix, favor to the attachment and proliferation of the L929 fibroblasts. Thus, the scaffolds are promising to be used as biomaterials for many natural tissues repair.     

Synthesis of poly(vinyl alcohol) having a thiol group at one end and new block copolymers containing poly(vinyl alchohol) as one constitent

Poly(vinyl alcohol) (PVA) and poly(vinyl acetate) having a thiol group at one end were synthesized by free-radical polymerization of vinyl acetate in presence of thioacetic acid as a chain transfer agent followed by treating with sodium hydroxide/methanol and ammonia, respectively. It was found that block copolymers containing the PVA sequence as one constituent were obtained by the redox-initiated polymerization of some vinyl monomers, for example, acrylic acid (AA) and acrylamide (AAm), using PVA having a thiol end group as reductant and an oxidant such as potassium bromate and potassium persulfate.     

Mechanical properties and biocompatibility of soluble eggshell membrane protein/poly(vinyl alcohol) blend films

Poly(vinyl alcohol) (PVA) has been blended with soluble eggshell membrane protein (SEP) to improve the mechanical properties of SEP film that is brittle. Tensile strength and elongation-at-break increase with increasing amount of PVA. When the SEP/PVA proportion is 1:1, a strong and flexible film is obtained, having a tensile strength of 22.7 MPa and elongation-at-break of 106%. Although scanning electron microscopy observation of the freeze-fractured cross-section shows microphase separation, interaction between SEP and PVA exists, as revealed by FT-IR. NIH3T3 cell culture demonstrates that SEP/PVA blend films with up to 50% of PVA show biocompatibility comparable to pure SEP film.     

Mechanical properties and biocompatibility of soluble eggshell membrane protein/poly(vinyl alcohol) blend films

Poly(vinyl alcohol) (PVA) has been blended with soluble eggshell membrane protein (SEP) to improve the mechanical properties of SEP film that is brittle. Tensile strength and elongation-at-break increase with increasing amount of PVA. When the SEP/PVA proportion is 1:1, a strong and flexible film is obtained, having a tensile strength of 22.7 MPa and elongation-at-break of 106%. Although scanning electron microscopy observation of the freeze-fractured cross-section shows microphase separation, interaction between SEP and PVA exists, as revealed by FT-IR. NIH3T3 cell culture demonstrates that SEP/PVA blend films with up to 50% of PVA show biocompatibility comparable to pure SEP film.     

Nucleation of particles in seeded emulsion polymerization of vinyl acetate with poly(vinyl alcohol) as emulsifier

Seeded emulsion polymerizations of vinyl acetate with and without poly(vinyl alcohol) were carried out in a batch reactor. Two types of poly(vinyl alcohol) (different molecular weights but the same degree of hydrolysis) have been investigated. In the absence of poly(vinyl alcohol) no new nucleation was observed, while in its presence new particles were formed, giving a bimodal particle size distribution. The number of new particles was greater with the higher-molecular-weight poly(vinyl alcohol). Compared with emulsifier-free seeded emulsion polymerization, the reaction rate increased when the higher-molecular-weight poly(vinyl alcohol) was used. However, the polymerization rate decreased when the lower-molecular-weight poly(vinyl alcohol) was applied. It was also shown that for seeded emulsion polymerization of vinyl acetate in the absence of poly(vinyl alcohol), the kinetics follows Smith-Ewart case III theory. These observations are discussed and a theory is proposed to explain the effect of macromolecular emulsifier on the nucleation of particles in seeded emulsion polymerization. This theory involves nucleation of new particles via precipitation of “copolymer” poly(vinyl alcohol)-poly(vinyl acetate) radicals in the water phase.     

Chromophoric poly(vinyl alcohol derivative)s, 1. Synthesis and spectroscopical characterization of some poly(vinyl alcohol)s with alkoxyazobenzenecarbonyl substituents

The synthesis of new poly[(vinyl 4′-alkoxyazobenzene-4-carboxylate)-co-(vinyl alcohol)]s 4a–c by Einhorn esterification of poly(vinyl alcohol) with 4′-alkoxyazobenzene-4-carbonyl chlorides 3a–c prepared in two steps from 4′-hydroxyazobenzene-4-carboxylic acid ( 1 ) is described. The degree of esterification is found to be about 85 mol-%. Infrared, ¹H and ¹³C nuclear magnetic resonance and ultraviolet spectroscopical data of the obtained polymers are determined.     

Infrared analysis on blends of poly(3-hydroxybutyric acid) and stereoregular poly(vinyl alcohol): influence of tacticity of poly(vinyl alcohol) on crystallization of poly(3-hydroxybutyric acid)

The extens of crystallization of poly(3-hydroxybutyric acid) (P(3HB)) in blends of P(3HB) with stereoregular poly(vinyl alcohol) (PVA) have been studied by means of infared spectroscopy. The degree of crystallinity of P(3HB) was found to decrease by blending with atactic PVA when PVA was the predominant component of the blend. In P(3HB) blends with highly syndiotactic PVA, a significant suppression of P(3HB) crystallization was observed in a wider range of blend composition than that in P(3HB)/atactic PVA blends. These results are consistant with previous results obtained by means of solid-state ¹³C nuclear magnetic resonance and differential scanning calorimetry. In blends with highly isotactic PVA, P(3HB) was found to crystallize in the same way as in pure P(3HB).     

Islet-encapsulation in ultra-thin layer-by-layer membranes of poly(vinyl alcohol) anchored to poly(ethylene glycol)-lipids in the cell membrane

The microencapsulation of islets of Langerhans (islets) in a semipermeable membrane, i.e., the creation of a bioartificial pancreas, has been studied as a safe and simple technique for islet transplantation without the need for immunosuppressive therapy. The total volume of the implant tends to increase after enclosure of the islets in the semipermeable membrane, which limits transplantation sites. Thus, ultra-thin membranes are required for clinical applications. Here, we propose a novel method to encapsulate islets in an ultra-thin membrane of poly(vinyl alcohol) (PVA) anchored to a poly(ethylene glycol) (PEG)-phospholipid conjugate bearing a maleimide group (Mal-PEG-lipids, PEG Mw: 5000) in the cell membranes of islets. When Mal-PEG-lipids were added to an islet suspension, they spontaneously formed a thin layer on cells of the outer layer of islets. The PEG-lipid layer on the islets was covered by a PVA monolayer, and the PVA membrane was further reinforced by using the layer-by-layer method with thiol/disulfide exchange reactions. No practical volume increase in islets was observed after microencapsulation by this method. In addition, encapsulation of the islet surface in PVA membranes did not impair insulin release in response to glucose stimulation.     

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.     

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.     

Effect of poly(ethylene glycol) on the photochemical immobilization of an enzyme in photocrosslinkable poly(vinyl alcohol)

A novel method was applied to improve the activity of enzymes immobilized in photocrosslinked poly(vinyl alcohol). Whereas the addition of glycerol to photosensitive poly(vinyl alcohol), bearing a styrylpyridinium group, diminishes the rate of photocrosslinking as well as the mechanical strength of the insolubilized gel, no considerable decrease in the rate of photoreaction was observed when poly(vinyl alcohol) was mixed with poly(ethylene glycol). This difference is based on the phase separation of the two water-soluble polymers, resulting in the formation of a porous structure of the photocrosslinked gel. Immobilization of invertase in the photocrosslinked gel in the presence of poly(ethylene glycol) changes the activity, because of the case of diffusion of the substrate into the porous matrix.     

Polypeptide‐synthetic polymer hybrids, 2. Miscibility of poly(vinyl alcohol) with polysarcosine

Transparent blend films were prepared from poly(vinyl alcohol) (PVA) and polysarcosine (PSA), i. e. poly(N‐methylglycine). Miscibility between PVA and PSA was investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). It was revealed that PVA is miscible with PSA having a DP of 103 with contents of up to 40 wt.‐%. Molecular weight dependence of the miscibility was observed, since PVA shows miscibility with PSA having a DP of 309 with contents of up to 20 wt.‐%. Segmental interaction between PVA and PSA was investigated by Fourier transform infrared (FT‐IR) spectroscopy. While the carbonyl stretching band of PSA appears at 1 660 cm^–1, the peak maxima of the PVA/PSA blend samples were observed at 1 649–1 651 cm^–1. The shift of the bands to lower wavenumbers is explained by the formation of hydrogen bonds between hydroxyl groups of PVA and the carbonyl oxygen in PSA. Furthermore, thermogravimetric (TG) analysis of the blend film proved that the thermal stability of PVA was improved by blending with PSA. The shift of the degradation temperature to a higher temperature region is discussed on the basis of hydrogen bonding interaction between the two components in the miscible state.     

Investigation of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) blends, 3. Permeation properties of polymer blend membranes

Dense membranes made of poly(vinyl alcohol) (PVA) and poly(N-vinyl-2-pyrrolidone) (PVP) blends of different compositions were studied in gas-sweeping permeation of a water-ethanol mixture containing 10 wt.-% of water. When the PVP content increases, the steady-state permeation flux and the water content in the permeate show sudden changes at ca. 30–40 wt.-% of PVP in the membrane: the flux increases drastically, while the water selectivity of the membrane falls to a much lower level. The diffusion coefficients of both water and ethanol, as determined from the permeation flux changes in the transient regime, exhibit a large increase when the PVP content exceeds 30 wt.-%, and this accounts for the flux increase. The fall in the permeation selectivity is shown to be due to a strong decrease in the sorption selectivity. The threshold of the change corresponds approximately to the PVP content at which PVA crystallites no longer exist in the blend. A further study on the states of water in the blend membranes by differential scanning calorimetry made it possible to relate these changes with a change in the interactions of water with hydrophilic sites in the polymer blend when the PVP content changes.     

Preparation and solution properties of microhydrogels from poly(vinyl alcohol)

Microgels were synthesized by intramolecular crosslinking of poly(vinyl alcohol) single chains with glutaraldehyde in dilute aqueous solutions (concentration c = 0,1 g/100 mL). The degree of crosslinking was varied from X = 0 to X = 150 (X is the number of crosslinker molecules per chain). The reaction products were characterized by static and dynamic light scattering and viscosity measurements. A clear shrinking of molecular dimensions could be observed with increasing degree of crosslinking, since the radius of gyration R_g, and with that, the p-parameter (p = R_g/R_h) decrease with the number of intramolecular crosslinks. A similar behavior is seen from viscosity measurements. The intrinsic viscosity [η] decreases with increasing X-parameter and depends only very weakly on the temperature which is an indication of a loss in flexibility and therefore for a relatively fixed structure.