Biocompatibility

Biocompatibility is the ability of a material to perform with an appropriate host response in a specific application. (Williams, 1999) The critique against this definition usually boils down to the fact that it is not possible to make a single test that determines whether a material is biocompatible or not. Indeed, since the hemostasis of the immune response and repair functions in the body are so complicated it would seem odd that one can make one test to determine the biocompatibility of any given material. Sometimes one hears of biocompatibility testing that is a large battery of in vitro test that is used in accordance with ISO 10993 to determine if a certain material (or rather biomedical product) is biocompatible. These tests do not determine the biocompatibility of a material, but they constitute an important step towards the animal testing and finally clinical trials that will determine the biocompatibility of the material in a given application, and thus medical devices such as implants or drug delivery devices.

'''In short: there is no such thing as a universally biocompatible material but there are degrees of biocompatibility. '''

Three definitions of biocompatibility

 * 1. the ability of a material to perform with an appropriate host response in a specific application. – Williams' definition.
 * 2. the quality of not having toxic or injurious effects on biological systems. - Dorland's Medical Dictionary.
 * 3. comparison of the tissue response produced through the close association of the implanted candidate material to its implant site within the host animal to that tissue response recognised and established as suitable with control materials- ASTM

Comments on the above three definitions

 * 1.this is also referred to as the Williams' definition. It was defined in the European Society for Biomaterials Consensus Conference I
 * 2.the Dorland Medical definition is not recommended since it only defines biocompatibility as the absence of host response and does not include any desired or positive interactions between the host tissue and the biomaterials.
 * 3.the ASTM is not recommended since it only refers to local tissue responses, in animal models.

All these definitions deal with materials and not with devices. This is a drawback since many medical devices are made of more than one material. Much of the pre-clinical testing of the materials is not conducted on the devices but rather the material itself. But at some stage the testing will have to include the device since the shape, geometry and surface treatment etc of the device will also affect its biocompatibility.

Biocompatible
In the literature one quite often stumble upon the adjective form: biocompatible. But according to Williams dictionary this does make any sense because it is biocompatibility is contextual, i.e. much more than just the material itself will determine the clinical outcome of the medical device of which the biomaterial is a part. This also points to one of the weaknesses with the current definition because a medical device usually is comprise of more than one material.

Suggested sub-definitions
The scope of the first definition is so wide that D Williams tried to find suitable subgroups of applications in order to be able to make more narrow definitions. In the MDT article from 2003 the chosen supgroups and their definitions were:


 * Biocompatibility of long-term implanted devices
 * 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


 * Biocompatibility of short-term implantable devices
 * The biocompatibility of a medical device that is intentionally placed within the cardiovascular system for transient diagnostic or therapeutic purposes refers to the ability of the device to carry out its intended function within flowing blood, with minimal interaction between device and blood that adversely affects device performance, and without inducing uncontrolled activation of cellular or plasma protein cascades.


 * Biocompatibility of tissue-engineering products
 * The biocompatibility of a scaffold or matrix for a tissue-engineering products refers to the ability to perform as a substrate that will support the appropriate cellular activity, including the facilitation of molecular and mechanical signalling systems, in order to optimise tissue regeneration, without eliciting any undesirable effects in those cells, or inducing any undesirable local or systemic responses in the eventual host.

In these definitions the notion of biocompatibility is related to devices rather than to materials as compared to top three definitions.

There was a consensus conference on biomaterial definitions in Sorrento 2005 September 15-16, http://www.esb2005.it/satellite.html