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May 2010

Biocompatibility is used extensively within biomaterial science, but there still exists a great deal of uncertainty about what it actually means and about the mechanisms that are subsumed within the phenomena that collectively constitute the biocompatibility. Biocompatibility has traditionally been concerned with aspects of biological, chemical and physical properties of the implant. This implies that the probes do not evoke a toxic or immunologic reaction, do not harm or destroy enzymes, cells or tissues, do not compress adjacent tissues inducing vascular problems and be able to remain for a long term within the organism without encapsulation or rejection.

To those who were developing and using the first generation of implantable devices (1940?1980) it was becoming increasingly obvious that the best performance was achieved with materials that were the least reactive chemically. However, while the surface composition of the implant is an important parameter, in some cases physical properties are the major determinants of biocompatibility.

Furthermore although it is widely accepted to define biocompatibility as “the ability of a material to perform with an appropriate host response in a specific application”, nowadays any neural probe is comprised of more than one material, therefore we have to move from a material base to an specific application base definition. Consequently a neural implant can be considered to be biocompatible if,

It performs its desired function without eliciting any undesirable local or systemic effect in the recipient of the implant or in the own implant materials.

It remains for a long term within the organism, entirely functional and with the desired degree of incorporation in the host.

This concept is not limited to minimize local lesions, but also encloses the whole behavior of the implant in its biological environment. Therefore three areas have to be considered, the “biosafety”, the “biofunctionality” and the “biostability”. Biosafety means that the implant does not harm its host in any way, biofunctionality is related with the ability of the device to perform its intended function, and biostability means that the implant must not be susceptible to attack of biological fluids, proteases, macrophages or any substances of the metabolism. In addition it should also be taken into account the “biotolerability” or the ability of the implant to reside in the body for long periods of time with only low degrees of inflammatory reaction. All these considerations imply extreme demands on stability and function of neural implants and place unique constraints on the architecture, materials, and surgical techniques used in the implementation of intracortical microelectrodes.

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