BIOMATERIALS AND ARTIFICIAL ORGANS

Royal College of Surgeons, London, 7 March 1980

This meeting was organised by the Biomaterials Group of the Biological Engineering Society as one of their series of one day local meetings. The.first speaker, Dr. K. Gilding, Liverpool, presented a paper entitled Biocompatibility of Implant Materials. Reviewing the nature of the interactions taking place in viuo around an implant he pointed out that the essential nature of tissue materials was their hydrophilic nature. In order to promote hea- ling around an implant, it should be possible to introduce an active interaction between the implant material and the tissue. To achieve this, the surface characteristics of polymeric materials could be modified, the key property being identified as the hydrophobic/hydrophilic balance. In the joint ICI/ Liverpool University group it had been decided to concentrate on this aspect of polymeric implant materials. Copolymers of methyl methacrylate with hydroxy ethyl methacrylate (HEMA) to pro- duce a neutral system, dimethylamino ethyl metha- crylate (DMAEMA) to provide a positively charged system and acrylic acid (AA) to provide a negatively charged system were investigated. The mechanical mismatch between tissue and hard polymers was overcome by using a mat of the materials. Capsule thickness, collagen content and cell content were determined as an index of biocompatibility. Results were then presented for a series of polymers, having a range of charges and solubilities. Tracer techniques were also used to study protein binding to the poly- mer surface. Using fibrinogen, y-globulin and albu- min the competitive adsorption of these proteins was investigated. One result of this work has been the production of a synthetic collagen-type material in which the various charged units are represented by the monomers referred to above and this has been shown to be perfectly accepted by the tissues with no capsule formation being reported in 80% of the results obtained.

Dr. R. Van Noort of Sheffield speaking on Bio- logical Tissues for Heart Valve Replacement com- pared non-tissue and tissue valves. Typical examples of both types of valves were then described and their characteristics discussed. Although clinical work has been carried out using tissue materials, there remains a lack of information about their properties. Results on the variability of the strength of dura with age were presented. Glycerol treatment app- eared to remove the memory of previous stress- strain history from the material and hence provided a method of processing the material in order to evaluate different treatments given to it. The final

paper of the morning session was given by Professor J. Courtney and was entitled Materials for Artificial Kidneys. As well as various forms of cellulose of which the cuprammonium form is the most widely used, various synthetic materials have also been employed, including acrylonitrile, polycarbonate, polyamide, polysulphone and polyvinylalcohol. Evaluation of these materials is related to their permeability, mechanical properties and blood compatibility. He described methods for studying permeability of membranes that have been used at Strathclyde. A rocking test cell has been used for blood compatibility studies. The difficulty is always to separate evaluation of the membrane from evalua. tion of the device in which it will be used. In the second part of his paper, the author discussed the technique of haemoperfusion by which blood can be purified, for example, in cases of acute poison- ing. The method is not so acceptable for the use in renal failure because urea is not removed. The forms of charcoal used and the various polymer coatings that have been applied were then described. Haemo- perfusion resins have been used and some of the work that has been performed on these was then discussed.

The morning session thus provided a very useful introduction to the concepts involved in biocompa- tibility of polymeric materials and the materials developed for two specific fields of application.

In the afternoon session dealing with valves and dialysis systems the first paper by Mr. G. Harston from Sheffield was entitled Development of Por- table Dialysis Systems. One of the problems associated with the use of dialysis is the loss of mobility for the patient. There has thus been a considerable incentive to develop portable units. Since conventional units employ approximately 180 litres of dialysing fluid over a period of 6 hours, there must be a change in concept if porta- bility is to be the aim. He described the develop- ment of a system which uses only 17 litres, replaced twice during the period of dialysis, then gave results of a limited series of clinical trials involving dialysis extending over a two week period.

The next speaker, Mr. T. Feest, Exeter spoke from a clinician’s point of view and entitled his paper Dialysis as a Failure. He saw dialysis as a stage on the road to successful treatment with successful transplantation being the only really effective form of treatment of renal disease. He said that dialysis provides reasonable control over fluid electrolyte

J. Biomed. Engng. 1980, Vol 2, July 229

acid base balance, but is unselective with regard to the excretory function of the kidney and has no contribution to make to regulatory function and endocrine/anabolic function. He then discussed the problems that remain after dialysis and dealt with various types of dialysis disease. This included the role of aluminium present in the water supply in the incidence of encephalopathy. He discussed the work going on in peritoneal dialysis which is very encouraging, but still of limited scope. The use of chronic ambulatory peritoneal dialysis (CAPD) has changed the practice in the USA and is doing so increasingly in the UK. New membranes have not yet had much of an impact on clinical practice, although some new materials do show promise. He felt that the move was away from haemodialysis to peritoneal methods, particularly CAPD which becomes increasingly important as renal transplan- tation is more possible. It is, however, limited to one year duration. Increasing success of transplan- tation (80% success for a one year graft) is empha- sising the need for a system which will enable the patient to be established rapidly on dialysis and which has a low capital outlay involved. The use of reverse osmosis for water treatment has had the added advantage of improving the bacteriological situation.

The final speaker was Dr. M. Ionescu (Leeds), who spoke on the subject of Biophrosthetic Cardiac Valves and talked in particular of the work that he has done on the IS pericardial xenograft valve. This uses bovine pericardium frame mounted, which is

formaldehyde sterilized and gluteraldehyde stabi- lized. A total of 530 valves have been implanted with an overall mortality rate of 7% early and 9% late. (For class III and IV patients). The follow up was for 66 months in some of the cases. In all cases the patients were restored to class I or II post opera- tively. More than 90% of patients were free from subsequent valve dysfunction. In their practice no late anti-coagulants have been given, although in some cases they were given in the early post opera- tive stages. In 506 cases only 8 early emboli and one late embolus have been observed with no sequelae. He discussed the differences between this valve and other tissue valves in terms of the flow pattern through the leaflets. Haemodynamic studies have been carried out and the peak systolic gradient for this and other valves has been determined. The IS pericardial valve has a gradient 3 to 4 times lower than that shown with the others. This is because of the good functional orifice area compared to anulus area. Post operative haemodynamics have been measured for up to 68 months and no significant changes in the various parameters had been observed. The valve, he claimed,had proven durability for up to 9 years, representing 16 788 patient months. No significant histological changes had been observed.

This session also concentrated the attention of bio- materials scientists upon the clinical consequences of their work and highlighted the outstanding prob- lems to which future attention must be given.

G.W. Hastings

230 J. Biomed. Engng. 1980, Vol 2, July