Nicholas John Crowther

Poly(vinyl alcohol) Hydrogel as a Biocompatible Viscoelastic Mimetic for Articular Cartilage

The prevalence of suboptimal outcome for surgical interventions in the treatment of full‐thickness articular cartilage damage suggests that there is scope for a materials‐based strategy to deliver a more durable repair. Given that the superficial layer of articular cartilage creates and sustains the tribological function of synovial joints, it is logical that candidate materials should have surface viscoelastic properties that mimic native articular cartilage. The present paper describes force spectroscopy analysis by nano‐indentation to measure the elastic modulus of the surface of a novel poly(vinyl alcohol) hydrogel with therapeutic potential as a joint implant. More than 1 order of magnitude decrease in the elastic modulus was detected after adsorption of a hyaluronic acid layer onto the hydrogel, bringing it very close to previously reported values for articular cartilage. Covalent derivatization of the hydrogel surface with fibronectin facilitated the adhesion and growth of cultured rat tibial condyle chondrocytes as evidenced morphologically and by the observance of metachromatic staining with toluidine blue dye. The present results indicate that hydrogel materials with potential therapeutic benefit for injured and diseased joints can be engineered with surfaces with biomechanical properties similar to those of native tissue and are accepted as such by their constituent cell type.

Complexation between polyoxyethylene and polymethacrylic acid—the importance of the molar mass of polyoxyethylene

Abstract The effect of the molar mass of polyoxyethylene, POE, upon the mechanism of complexation with polymethacrylic acid, PMAA, has been examined, using pH, density, calorimetric and cloud point studies. Complexation has been found to occur, even for POE of molar mass as low as 200, further, the mechanism appears to be more complex than a simple 1:1 base mole pairing of oxyethylene units (OE) with methacrylic acid units (MAA) by a hydrogen bond mechanism. When the molar mass of POE is

INFLUENCE OF COMPOSITION AND SEGMENT DISTRIBUTION UPON LOWER CRITICAL DEMIXING OF AQUEOUS POLY(VINYL ALCOHOL-STAT-VINYL ACETATE) SOLUTIONS

Abstract Cloud point data are presented for a series of 2% w/v solutions of poly(vinyl alcohol- stat -vinyl acetate) copolymers in water and a series of 0.15 M electrolyte solutions; the copolymers were of various compositions and segment length distributions. A relationship has been found showing that the relative sizes of the hydroxyl and acetate sequences, rather than their relative amounts, are the dominant factors in the determination of the lower critical demixing behaviour of this important polymer.

Factors influencing the aqueous solution behaviour of vinyl alcohol with vinyl acetate copolymers

Abstract Variations, as a function of temperature, are reported in the magnitude of values for the partial molal volume, partial molal expansibility, flowing volume and surface pressure for dilute solutions of a series of poly(vinyl alcohol-co-vinyl acetate) copolymers of various vinyl acetate contents and acetate sequence length. The temperature range investigated, from 5° to 30°, showed that, for all the examples examined, a general behaviour pattern was observed, indicating the predominance of hydrophilic interactions at temperatures below approximately 10°, with hydrophobic interactions predominating between 10° and 20°. The variations can be related to the detailed microstructure of the polymer backbone.

The volumetric behaviour of poly(vinyl alcohol) in aqueous solution

High-precision density measurements of aqueous solutions of a narrow molecular mass fraction of poly(vinyl alcohol) prepared by 80% alcoholysis of poly(vinyl acetate), over an extended range of concentration and temperature, have revealed a complex pattern of solution behaviour. The concentration dependence of the partial molal volume and apparent molal expansibility indicate that two processes occur in solution; a conformational change at low concentrations (ca. 0.05% w/w) and an aggregation at higher concentrations (ca. 0.2% w/w). The concentration at which these processes occur varies with temperature showing a minimum in the region 14–18 °C. The volumetric data presented suggest that the temperature dependence of these two processes is a function of the solvent–solvent interactions and the minimum at 14–18 °C reflects a critical region for water over this temperature range.

Rheological and densitometric studies of poly(propylene oxide)–water mixtures over a range of temperatures

Rheological and densitometric data are reported for aqueous solutions of poly(propylene oxide), molar mass 400 (PPO 400), over the concentration range 0–1 weight fraction and a temperature range 4.8–55 °C. Negative excess partial specific volumes were found, which varied as a function of temperature and concentration of PPO and which were found to correlate with the viscoelastic behaviour of the solutions. The PPO molecule, in the pure phase, was found to undergo a change of conformation at 30 °C.

Microengineered Surface Topography Facilitates Cell Grafting from a Prototype Hydrogel Wound Dressing with Antibacterial Capability

Skin wounds derive therapeutic benefit from redeployment of dermal tissues, whether as split‐thickness allo‐ and autografts or as biological dressings comprising cultured cells. However, the clinical outcome is strongly influenced by the techniques used for cell/tissue grafting and also the microbiological status of the wound. Here we report that microtopography incorporated into the surface of a novel polymeric material, derivatized with fibronectin to promote attachment and encourage motility, improved the efficiency of cell transfer onto de‐epithelialized human skin ex vivo. The microtopography had two functions, first as a conduit for migrating cells to cross between the vehicle and recipient surface and second to shield adherent cells from destruction by mechanical shearing during handling and application. Quantitative analysis showed that topographic projections (columns) rather than recesses (pits) in the hydrogel surface achieved the highest efficiency of cell transfer. In order to address the crucial relevance of microbiological contamination to the success of wound grafting, the effect of iodine on several common bacterial pathogens was examined using an XTT+CQ10 kinetic cell viability assay. Increasing concentrations of iodine initially stressed and after 0.5% v/v were subsequently bacteriocidal for Gram‐negative Pseudomonas aeruginosa and Escherichia coli and Gram‐positive Bacillus subtillis and Staphylococcus aureus. Slightly higher doses of iodine (approx 1–1.5% v/v) were required to kill HaCaT cells outright, but for both pro‐ and eukaryotes the major determinant of cytotoxicity was absolute dose rather than duration of exposure. Iodine delivered by the hydrogel at low concentration was bacteriostatic but not apparently cytotoxic to epithelial cells as measured by MTT end‐point cell viability assay. Zone of inhibition studies confirmed that bacteriocidal quantities of neomycin, phenol red, and silver could also be delivered using the same hydrogel. This research suggests that grafting cell‐based biological dressings to wounds using a topographically modified hydrogel dressing capable of simultaneous reducing the microbiological threat to a successful outcome may be a realistic clinical proposition.

The temperature dependent contributions of component dyads to the partial molal volumes of a series of poly(vinyl alcohol-vinyl acetate) copolymers in aqueous solution

Values for the partial molal volumes (φv) of a series of poly(vinyl alcohol-co-vinyl acetate) copolymers, obtained from the same parent poly(vinyl acetate) but of differing degrees of hydrolysis and block size distribution, were obtained from high precision density measurements. An analysis of the data is presented in which the contributions of (vinyl acetate/vinyl acetate), (vinyl acetate/vinyl alcohol) and (vinyl alcohol/vinyl alcohol) dyads to the value of φv, are determined. The results appear to show that temperature dependent hydrophobic effects upon φv, are initiated by the (vinyl acetate/vinyl alcohol) dyads, not by the (vinyl acetate/vinyl acetate) dyads, as might be expected.

Hydrophobic interactions in dilute solutions of poly(vinyl alcohol)

High-precision density and light-scattering measurements in water and 10–3 mol dm–3 electrolyte solutions of samples of 80 and 90% poly(vinyl alcohol)(PvOH) over an extended range of concentration and temperature have revealed a complex pattern of solution behaviour; this report is restricted to the behaviour of solutions at concentrations below 0.15% w/v.Partial molal volume (V2°) and expansibility (Ē2°) studies at infinite dilutions in water show the existence of a very strong temperature dependence in these partial molal properties; V2° for 80% hydrolysed PvOH decreases by 40% between 10 and 15 °C, rising by almost the same amount between 15 and 18 °C, becoming virtually constant at higher temperatures. Ē2° studies suggest that increasing hydrophobic hydration occurs between 10 and 15 °C, which results in a rapid conformational change in the molecule between 15 and 18 °C, producing a structure which is predominantly hydrated in the hydrophilic sense. At temperatures above 18 °C gradual erosion of this hydration appears to take place.Corresponding studies on a sample of 90% hydrolysed PvOH show broadly similar behaviour, but modified by the presence of a smaller proportion of hydrophobic vinyl acetate groups; for example, the conformation change with temperature is not as abrupt as in the case of 80% hydrolysed PvOH.Similar studies in 10–3 mol dm–3 solutions of LiCl, Li2SO4, NaCl and KCl illustrate the large ionic effects upon the hydrophobically dictated conformational change; these effects compare remarkably well with the calculated thermodynamic property changes, associated with salt effects, in the folding process of the globular protein ribonuclease.Particle size and second virial coefficent values obtained from light-scattering studies confirm the behaviour pattern observed.

Light-scattering studies of aggregation in aqueous solutions of poly(vinyl alcohol co-vinyl acetate) copolymer

Light-scattering studies are presented for a series of aqueous solutions of a poly(vinyl alcohol co-vinyl acetate) copolymer, of varying concentration, over the temperature range 5–30 °C. The data indicate the presence of an aggregation process which occurs over a narrow concentration range; the data further indicate that the temperature-dependent conformational change in the macromolecule, which has been reported previously, is also reflected in the aggregation process.