This article is based on a presentation given at a workshop on redefining the word biocompatibility. It suggests that it would be better to base the definition on the application of a device and not on the material.

Exploring the rationale for change I am writing this article on the way to a workshop in Seattle, Washington State, USA, organised by a biomaterials scientist, Buddy Ratner. The University of Washington in Seattle now hosts one of the world's largest biomaterials and tissue-engineering groups. The theme of the workshop is biocompatibility, with the subtext: the need to alter the concepts and definitions of biocompatibility in the changing medical device and tissue-engineering world. As the originator of the definition of biocompatibility that is now widely used, I was invited to address the meeting on whether or not I believe that the definition needs changing. There is a need to preserve the original basis of the definition, but to move to redefining biocompatibility in relation to devices rather than the material. This extract from my presentation¹ outlines the rationale for change and some proposed new definitions.

Current definition The definition of biocompatibility that is contained in my Dictionary of Biomaterials² is:

The ability of a material to perform with an appropriate response in a specific application.

This was based on the principles that a biomaterial has to perform and not simply exist, that it has to be associated with the most appropriate response to ensure satisfactory performance. The definition recognises the important fact that the response to a material will vary from one situation to another and that the appropriateness may vary. For example, those who design material surfaces to be attractive to cells to encourage their proliferation and expression of extracellular matrix proteins may find success under some circumstances, but failure under others.

This definition has found favour in many places, especially because it moved the focus of biocompatibility away from the concept that biocompatibility is equivalent to biological safety, which implies that the only issue is the quality "of not having toxic or injurious effects on biological systems," which is the classic definition in, for example Dorland's Medical Dictionary (W.B. Saunders Company, Philadelphia, Pennsylvania, USA). There have equally been some who have objected to the definition on the grounds that it is too vague and does not help anyone to fully understand the subject.

Moving forward I propose that the definition is still relevant today, but it needs amendment or supplementation. Most people discussing biocompatibility 15 years ago had long-term implantable devices in mind; a few were more concerned about extracorporeal devices and some with short-term invasive systems. No one was talking about tissue engineering and few were interested in invasive sensors, microelectromechanical systems and implantable drug delivery systems. The main difficulties we have today with the definition of biocompatibility is that the applications are so varied that there may be little commonality with the appropriateness of the responses. Thus, we need to differentiate the different types of application and modify our definitions accordingly.

Biocompatibility of long-term implantable devices In preparing for a paper titled, "The Inert-Bioactivity Conundrum," which I presented in 2002,³ I looked at two aspects with the benefit of hindsight: the nature of the materials now being used in long-term implantable devices on a commercial basis, and the experiences, good and bad, that had led to this evolution. The conclusions were clear. A small group of materials, most of which have been around for many years, although subject to some refinement, constitute the bulk of today's materials. This group includes titanium and cobalt-chromium alloys and platinum group metals, carbon, alumina, silicone elastomers, polytetrafluoroethylene, polyester textiles, polymethylmethacrylate and polyethylene. They are selected for their respective applications primarily on mechanical performance together with the fact that they are the most corrosion- or degradation-resistant materials in their class. There have been attempts over the years to introduce new biomaterials, for example, many composites have been examined. We wait to see how good fibre-reinforced polyetheretherketone (PEEK) is, and await the long-term performance of PEEK itself. However, most of these new biomaterials have not stood the test of time. In spite of their excellent all-round characteristics and versatility, we have yet to see long-term performance with flexible polyurethanes because of their ultimate degradability, although sufficiently biostable flexible polyurethane is still achievable.

My conclusion is quite stark. Choose biomaterials for long-term medical devices on the basis of optimal mechanical or physical functionality and optimal inertness and you have the best chance of success. Let me qualify this statement by the following caveats. First, by inertness I mean the greatest resistance to a corrosion, degradation or leaching process, coupled with maximal biological inertness, recognising of course that inertness is an imprecise term and that nothing is truly inert. Second, there have been many attempts to improve the biocompatibility of long-term implantable devices with surface coatings. In the context of bone biocompatibility, these attempts have involved the so-called bioactive coatings based on calcium phosphate ceramics, some of which may be deemed successful on the basis that bone adaptation takes place faster than it does on the underlying substrate such as titanium. There are also many ways in which the titanium itself can be surface treated to improve this speed of osseointegration. I acknowledge that this speed of attachment is an important issue in, for example, the clinical handling of patients receiving dental implants, but I do not believe that this is a major advance in the development of biocompatibility. I go one step further. I believe that the concept of surface treatments on materials for long-term implantable devices is largely based on false premises and rarely do they lead to better clinical performance. Surface coatings are susceptible to delamination or exfoliation, which decreases biocompatibility, and surface coatings have a tendency to become masked by adsorbed species, which limits their effectiveness.

So where does this leave us with the definition of biocompatibility with respect to long-term implantable medical devices? Let me suggest the following:

The biocompatibility of a long-term implantable medical device refers to the ability of the device to perform its intended function, with the desired degree of incorporation in the host, without eliciting any undesirable local or systemic effects in that host.