Problems in the use of polymeric materials in medical and biomedical applications

Lamination for subdermal implant fixation

Thirty-six aluminum oxide laminated discs were implanted into 12 young rabbits (18 with a 0.5-mm porous layer and 18 with 1 mm) to determine whether implants that are porous only on one side could fixate to subcutaneous tissue. After 3 months, discs were encased within thin pouches (0.02-0.14 mm) of fibrous connective tissue, as would have been expected of a completely porous implant. Solid sides showed no, while the porous sides showed little, attachment to pouches. Forty-seven percentage (17) of the discs had moved 1.4 +/- 0.8 cm beyond the 4.7 + 1 cm they had moved due to normal skin growth, while two others had moved between 6.2 and 6.5 cm beyond this measure. The proportion of 1 mm porous layer discs migrating within subcutaneous tissue was no greater than the proportion of 0.5 mm layer discs migrating (p = 0.15). Porous layer height and disc migration did not affect the attachment strength of pouch to surrounding tissues (68 +/- 23 N, p = 0.34). Pouch thickness, which has been associated to the level of applied forces in other studies, increased with migration distance (p = 0.054). Results indicate that one-sided porous discs are likely easier to retrieve than completely porous ones, but cannot be prevented from migrating in loose tissue of young animals. Data is being used to design subdermal radio frequency devices for endangered marine animals.

An approach to the soft tissue/synthetic material interface

It is argued that chemical modification of soft tissue implants in the hope of obtaining an associated tissue response is unlikely to succeed as a method for studying the fundamentals of implant/tissue interactions. An alternative approach is proposed which places greater emphasis on the interfacial interactions (such as protein adsorption) which occur after implantation, in a manner paralleling current advances in knowledge of the blood/material interface. From simple arguments, it is proposed that the observed similarities in soft tissue response of hydrophobic materials may result from irreversible protein adsorption, and that if unusual tissue responses are possible they are likely to be found only with hydrophilic implants. The possiblity of a critical hydrophilic/hydrophobic character which an implant must possess for essentially irreversible protein adsorption is also discussessed.

Quantitative cell culture biocompatibility testing of medical devices and correlation to animal tests

The biocompatibility of a wide variety of biomaterials was quantitatively assessed, in a physiologically normal environment" as to cytotoxicity induced in WI-38 cells by cell culture medium extracts. Materials tested included PVC plastic, rubber, silicone rubber, polyethylene, polypropylene, acetal, polyurethane, Teflon, nylon, epoxy, and polystyrene. Cell culture test results were correlated to U.S.P. animal tests. Potential test artifacts, lead, barium, cadmium, and endotoxin were tested for cytotoxicity in WI-38 cells. Cell culture methods yielded more positive tests, particularly rubber, PVC plastic and silicone rubber compounds, than observed in U.S.P. animal tests. Positivity in animal tests did not correlate quantitatively to cytotoxic titers in cell culture. Discrepancies between cell cultures tests and animal tests, specifically rubber compounds, were attributable, in some instances, to differentials in elution efficiency between saline, cottonseed oil, and complete MEM cell culture medium. In other instances, particularly PVC plastics, differences between cell culture and animal test results were due to an inherent difference in the two indicator systems to respond to specific toxic moieties.

Pharmacological properties of the plasticizer, acetyl N-tributyl citrate, and its extraction from poly(vinyl chloride) tubing

Aqueous extracts of a PVC transfusion tubing contracted isolated guinea-pig ilea. A pharmacological and chemical study (infrared spectroscopy, gas-liquid chromatography) showed that this was due to extraction of the plasticizer, acetyl N-tributyl citrate (ATBC). The concentrations of ATBC to produce contractions of the isolated guinea-pig ileum (from 1 mug/ml) can be compared with those achieved after recirculating tubing under conditions simulating use (up to 6 mug/ml). In assessing the toxicity of plastic tubing, there is a need to determine extraction of consituents under conditions simulating use and the effects of such extracts on a variety of tissues.