The tissues of the body exist in an internal environment separated from the one outside. In higher animals, this separation is maintained by an outer surface of skin and in some internal organs like the lungs and gastro-intestinal tract by mucous membrane. Any alien material that breaches this barrier is handled by a family of immuno-protective cells and cell-products. Any implant placed in the body has to have the requisite material properties to remain compatible with these tissue responses. These biocompatible properties and the requisite mechanical and fatigue characteristics for each situation are common to all implants. Implanted stimulation electrodes also require to have certain surface and electrical properties to provide safety and longevity during use.
The tissue responses to electrodes and to electrical stimulation on the brain, nerves and muscles are presented in this section.