J.D.S. Gaylor

The preclinical evaluation of the water vapour transmission rate through burn wound dressings

The control of evaporative water loss, following burn injury, is of major importance to the overall condition of the patient, whether this control is by natural eschar or by a dressing. It is therefore important to preclinically determine the water vapour transmission rate of these dressings, firstly to make comparisons between different materials and secondly to screen prototype materials, under controlled conditions. A preclinical (in vitro) technique is described and the results are given for several commercially available dressings which encompass foam, film and hydrogel material categories.

In vitro assessment of water vapour transmission of synthetic wound dressings

The water vapour transmission rate (WVTR) of 14 commercially available wound dressings (11 hydrocolloids, 2 hydrogels and 1 polyurethane film) was evaluated over 24 and 48 h periods using a modified ASTM standard method (ASTM E96-90). For the 48 h studies a novel microcomputer-controlled apparatus was employed. The dressings exhibited a wide range of WVTRs (76-9360 g m-2 d-1 at 24 h under forced air convection of 0.4 m s-1). The influence of air velocity of 0.4 m s-1 was not significant if the WVTR of the dressing was less than 880 g m-2 d-1 when measured under static air conditions. The influence of outer barrier layers and additional bandages on WVTR was also examined.

Burn wound dressings—a review

Man has dressed wounds since life began many millions of years ago. Since this time many materials have been devised for the intention of dressing wounds. This review indicates the vast range presently available, providing a starting point for those seeking information on this subject.

TECHNIQUES FOR MEASUREMENT OF OXYGEN CONSUMPTION RATES OF HEPATOCYTES DURING ATTACHMENT AND POST-ATTACHMENT

Three techniques for measuring oxygen consumption rate (OCR) of cultured cells relevant to the development of bioartificial liver devices are reported. In an oxystat apparatus, HepG2 cells immobilised on Cytodex 3 microcarriers at a concentration of 106 cells ml-1 had a mean OCR of 0.7 nmol s-1/106 cells. The OCR decreased with increasing cell density, a characteristic previously reported for other cell lines. Rat hepatocytes immobilised on single collagen layers in a flow cell and challenged with ammonia had a mean OCR of 0.59 nmol s-1/106 cells. A novel two-compartment oxystat system was used to determine the OCR of rat hepatocytes during the attachment phase. OCR declined from 1.0 nmol s-1/106 immediately after seeding to 0.7 nmol s-1/106 cells at nine hours. The low OCR for HepG2 reflects loss of certain oxygen dependent metabolic pathways. The OCR measured for rat hepatocytes during and post-attachment are significantly higher than those reported elsewhere and have major implications for the development of bioartificial liver devices.

Polymeric biomaterials: influence of phosphorylcholine polar groups on protein adsorption and complement activation.

The introduction to polymeric biomaterials of phosphorylcholine polar groups represents an approach towards the development of materials with improved blood compatibility. In this respect, two biomaterials, one a copolymer of butyl methacrylate and 2-methacryloyloxyethylphosphorylcholine (MPC), (poly(BMA-co-MPC) and the other, MPC-grafted Cuprophan, were examined with respect to their influence on protein adsorption and complement activation. Protein adsorption was studied by measurement of the adsorption of radiolabelled single proteins (albumin and fibrinogen), while complement activation was measured using radioimmunoassay for C3a des Arg. The investigation demonstrated that the polymers containing phosphorylcholine polar groups can achieve a marked reduction in protein adsorption and complement activation and supports the utilization of phosphorylcholine polar groups as a means of improving the compatibility of biomaterials for blood-contacting applications.

In vitro investigation of the blood response to medical grade PVC and the effect of heparin on the blood response.

This paper reports the results of an investigation into the blood response of polymers in vitro, using non-anticoagulated and heparinised blood and plasma. The materials studied were regenerated cellulose, (Cuprophan), an acrylonitrile-allyl sulphonate copolymer (AN69S), and medical grade polyvinyl chloride plasticised with di-2-ethyl-hexyl-phthalate (PVC/DEHP). Blood-material or plasma-material contact was achieved using a parallel plate flow cell, and C3a generation and FXII-like activity measured. The results of the study with non-anticoagulated human blood show that PVC/DEHP is a high complement activator. C3a concentration in the blood was higher after contact with PVC/DEHP than after contact with regenerated cellulose. The introduction of heparin in the blood induced complex alterations in the blood response. C3a generation could be elevated, decreased, or remain the same, depending on the material. The FXII-like activity on the surface of the PVC/DEHP after contact with plasma was also higher than the other two polymers. The introduction of heparin could increase or decrease FXII-like activity, depending on material. The patterns of response obtained with non-anticoagulated blood in vitro for AN69S and Cuprophan bore a strong resemblance with patterns of response obtained in the clinic, whereas those obtained with heparinised blood in vitro did not.

Water vapour transmission rates in burns and chronic leg ulcers: influence of wound dressings and comparison with in vitro evaluation

One of the main functions of wound dressings is to control water vapour transmission rate (WVTR) from wounded skin. In this paper, the influence of hydrocolloid, knitted viscose and gauze dressings was evaluated through in vivo measurement of WVTR in burns and chronic leg ulcers utilizing an evaporimeter. The results suggest that the evaporative water vapour loss from exposed skin wounds depends mainly on the wound depth, and that chronic leg ulcers have the same level of the WVTR as full thickness burns. Compared with the knitted viscose and gauze dressings, hydrocolloid dressing has a greater effect on reducing evaporative water loss, with WVTR being 20-30% of that of exposed wounds under the conditions used in this study. This result is in agreement with that obtained in an in vitro evaluation.

In vitro uptake and elimination of isoflurane by different membrane oxygenators

OBJECTIVE This study was designed to investigate the effect of membrane oxygenator design and composition on the uptake and elimination of isoflurane. DESIGN Prospective, in vitro laboratory study. SETTING Bioengineering laboratory. PARTICIPANTS Three types of membrane oxygenator were tested: the SM-35 (polydimethylsiloxane in sheet form), the CML (polypropylene in sheet form), and the SAFE II (polypropylene in hollow-fiber form). The oxygenators were incorporated into a standard cardiopulmonary bypass circuit. INTERVENTIONS Isoflurane was added to the oxygenator input gas and measured in exhaust gas and in (bovine) blood leaving the oxygenator at 1, 2, 3, 5, 7, 10, 15, and 20 minutes. The isoflurane vaporizer was then turned off, and samples were obtained at the same time intervals. The experiment was performed at 28 degrees C and 37 degrees C. MEASUREMENTS AND MAIN RESULTS Uptake and elimination of isoflurane were slower via the SM-35 compared with the CML and the SAFE II (p < 0.01). CONCLUSIONS If isoflurane is administered during cardiopulmonary bypass, knowledge of the influence of oxygenator membrane composition on its pharmacokinetics is essential if patient awareness and unexpected cardiovascular depression are to be avoided.

Biomaterials in cardiopulmonary bypass.

The improved utilization of biomaterials in cardiopulmonary bypass is dependent on polymer science and technology, procedures for blood compatibility assessment, optimization of biomaterial/antithrombotic agent combinations and the interpretation of clinical data.

An in vitro assessment of wound dressing conformability

An in vitro assessment technique has been developed to determine the conformability of wound dressings. The technique employed is based on an inflation technique which provides a measurement of the minimum radius of curvature which a specific dressing will adopt under pressure. A pressure of 40 mmHg was chosen as this had been shown to be the maximum tolerable pressure before the occurrence of tissue breakdown. This radius is then matched to the natural radii of the body surfaces and an assessment of conformability can be made. A series of commercially available dressings have been assessed with respect to their conformability, and to the enhancement of their conformability due to viscoelastic creep behaviour.