J.M. Courtney

Biomaterials for blood-contacting applications

Consideration of biomaterials for blood-contacting applications should take into account blood-biomaterial interactions, factors influencing the blood response and evaluation procedures. Examination of blood-biomaterial interactions indicates that relevant features are protein adsorption, platelet reactions, intrinsic coagulation, fibrinolytic activity, erythrocytes, leucocytes and complement activation. Factors influencing the blood response to a biomaterial in clinical application are the biomaterial structure, the presence of an antithrombotic agent, the patient status as determined by the disease and drug therapy, and the nature of the application. Evaluation options for biomaterials are clinical, in vivo, ex vivo and in vitro, with ex vivo and in vitro procedures relevant for biomaterial development.

Non-pressure treatment of hypertrophic scars☆

A silicone gel (Dow Corning X7-9119) has been successfully used in the management of hypertrophic scars. The gel softens and reduces scars in a shorter time period than pressure therapy. Relevant properties of the material and its mode of action have been investigated. The mode of action is unknown, but it is not due to pressure, temperature, oxygen tension or occlusion.

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.

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.

Hepatocyte culture between three dimensionally arranged biomatrix-coated independent artificial capillary systems and sinusoidal endothelial cell co-culture compartments.

Freshly isolated parenchymal liver cells, the hepatocytes, retain many of the metabolic activities of the tissue in vivo. As isolated cells, they are generally in a catabolic stage and therefore care is essential in fully maintaining their activity. Bioreactors are in development to address this problem. Since they enable an upscale of hepatocyte in vitro culture, they could be used as the main device in hybrid liver support systems. A basic problem in the development of special bioreactors for such systems is that with current techniques of hepatocyte culture, external hepatocyte function in vitro is limited to only a few weeks. In the majority of recently developed culture models for bioreactors, substrate exchange to the hepatocytes is performed by diffusion. This brings a remarkable change in the local situation for the cells in comparison to the in vivo situation. Even the gradients of oxygen and carbon dioxide, as well as the gradients of metabolites are not sufficiently considered in current culture models. An organisation of hepatocytes along such gradients is therefore ruled out, which impairs the typical metabolic function of hepatocytes dependent on their localisation in the sinusoid. A bioreactor concepts, which will allow the cells to operate under physiological flow conditions, may result in a macro environment more similar to the physiological situation. We have recently developed a culture model together with a newly developed bioreactor construction, which address this problem. This model has the following characteristics: perfusion of the cells between independent capillary membrane systems; identical units (Fig. 1) for few hepatocytes in parallel, analog to the liver lobuli; decentral metabolite inand outflow with low gradients; decentral oxygen supply and carbon dioxide removal with low gradients; cell adhesion on hepatocyte membranes coated with biomatrix and aggregation between the capillaries; co-culture compartment for sinusoidal endothelial cells; possibility of upscaling the construction to a large cell mass bioreactor for therapeutic liver support; possibility of further capillary functions, such as dialysis and heat-exchange for DMSO-freezing of the reactor with the cells. In several studies (1 ), this culture model has been investigated over a period of three weeks. With the present report, the initial results of a long-term study over seven weeks are described.

In vivo evaluation and comparison of collagen, acetylated collagen and collagen/glycosaminoglycan composite films and sponges as candidate biomaterials.

Native collagen, acetylated collagen, collagen/10% chondroitin sulphate, collagen/2.5% hyaluronic acid and collagen/20% hyaluronic acid were implanted both as film and as sponge into rat lumbar muscle for 7 and 14 d. After 7 d implantation, all materials elicited an acute inflammatory cell response characterized by numerous polymorphs and histocytes. The cell population after 14 d was principally mononuclear, i.e. leucocytes, neutrophils, macrophages, lymphocytes and fibroblasts. Both films and sponges followed a similar pattern. Native collagen elicited a subacute inflammatory response after 7 d. However, 14 d after implantation, a marked infiltration by neutrophils was apparent with subsequent degradation of existing collagen material. Acetylated collagen film evoked a much greater inflammatory cell response than native collagen. Both collagen/hyaluronic acid composites elicited a similar response. The collagen/10% chondroitin sulphate composite elicited the least inflammatory cell response at 7 d, whereas infiltration by host fibroblasts after 14 d implantation was clearly seen.

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.

The attachment and growth of an established cell line on collagen, chemically modified collagen, and collagen composite surfaces

The attachment and growth of an established cell line derived from mouse fibroblasts on collagen, chemically modified collagen, and collagen composite surfaces were compared. Tissue culture polystyrene dishes provided a suitable control. The substrates included native bovine dermal collagen, succinylated, acetylated and methylated collagen, and a series of composite materials formed from collagen and the glycosaminoglycans hyaluronic acid, chondroitin 4-sulphate and chondroitin 6-sulphate and the glycoprotein fibronectin. Attachment and growth of cells on each of these substrates were assessed by visual inspection under optical microscopy, by detachment of the cells using trypsinization and subsequent counting in a Coulter counter; and by 3H-thymidine incorporation studies. A very good correlation between the results was obtained by the three methods employed which showed that collagen, in comparison to polystyrene, is a relatively poor substrate for cellular attachment, growth and proliferation, but it may be improved by chemical modification and by incorporation of either fibronectin, chondroitin sulphate (5 and 10%), or low levels (less than 5%) of hyaluronic acid into the collagen matrix. Concentrations in excess of 5% hyaluronic acid into the collagen matrix, however, appeared to inhibit cellular attachment and growth and such materials provided a poorer substrate than native collagen.

Novel quantitative methods for the determination of biomaterial cytotoxicity.

Two novel methods for the determination of biomaterial cytotoxicity using cell culture are presented. The methods combine a standardized protocol for producing extracts from medical devices with either the established MTT assay or a new fluorimetric assay. The suitability of both methods for evaluating the toxicity of candidate materials was demonstrated by resolution of the differences in the toxic effects of serial dilutions of a PVC extract on BHK21 and HT1080 cells. The tests yield highly reproducible, quantitative results and can be applied to materials in the usual physical forms applicable to artificial organs.