Medical Applications of Implantable Polyurethanes: Current Issues

Abstract

For three decades, polyurethanes have been studied and used as components of implantable medical devices. The polyurethane family of synthetic polymers offers the most versatility in physical and chemical properties yet attained. The most valuable forms of implantable polyurethanes historically have been as elastomers and coatings. These uses exploit polyurethanes’ unique combination of toughness, biocompatibility, biostability and surface functionality.

Early investigators of polyurethanes for implantable devices recognised the superior physical properties and biocompatibility of polyurethanes. For bone fixation⁽¹⁾ and device coatings⁽²⁾, initial results were very promising. They were not aware, however, of the great diversity among polyurethanes in chemical properties, particularly hydrolytic stability. In fact, for implantable applications, certain polyester urethanes which can degrade in vivo with severe pathological consequences⁽³⁾ were chosen.

The failures of these devices in chronic implantation led to a widespread opinion, which persisted well into the 1970s, that polyurethanes had ‘doubtful qualities’ relative to the ‘current status and future development of implant surgery’, ‘although some recent success'⁽⁴⁾ had been found.

These successes were based on studies performed on polyether urethanes which displayed the requisite hydrolytic stability along with mechanical properties comparable to the polyester urethanes. This second wave of studies, begun in the mid- to late-1960s, certified the value of segmented polyether urethanes for long-term implantation. Two products of that era, Ethicon's Biomer⁽⁵⁾ (an analogue of DuPont's Lycra T-126 Spandex) and Avco Everett's Avcothane (currently Kontron's Cardiothane)⁽⁶⁾, provide most of the dynamic components of today's cardiac assist and total artificial heart devices.

This article gives details of the structures and properties, requirements and applications, biocompatibility and biostability of polyurethanes used currently in implantable medical devices.