Tissue Response

The tissue capsule forming on the backside of the electrode, a silicone rubber surface facing away from the muscle was quite thin or not detectable. On the surface between the stimulating surface and the muscle the encapsulation layer had a mean thickness of 1.24 mm. This layer exhibited two sub regions, a collagen outer layer and a layer of granulation tissue adjacent to the electrode surface. The nature of the granulation tissue layer suggested the cause was a chronic mechanical irritation brought about by relative movement between the contracting (shortening) muscle tissue and the non-compressible silicone rubber of the electrode carrier.

Schmit, B. D., J. T. Mortimer. (1997) The tissue response to epimysial electrodes for diaphragm pacing in dogs. IEEE Trans Biomed Eng 44: 921-30

Tissue Reaction: Summary.
The mechanism of stimulation induced tissue damage is still unknown. The neural activity and the toxic products of electrochemical reactions are two strong candidates. Heating and poreation are generally not considered viable candidates for inducing tissue injury under conditions that most neural prostheses operate.
Stranded stainless steel intramuscular electrodes are the configuration tested most extensively.
For balanced charge biphasic stimuli, cathodic first, the charge density should be less than 40 µC/cm2 to avoid damage to the electrode (i.e. corrosion).
Monophasic cathodic stimulation can be tolerated if the average current density is less than 10 µA/mm2.
Imbalanced biphasic stimulation can be tolerated for net cathodic currents up to 35 µA /mm2, (125 µC/cm2 in the cathodic direction and 50 µC/cm2 in the anodic direction at 50 Hz.
When IM electrodes are withdrawn, the encapsulation tissue tears and no damage is done to the surrounding muscle tissue. Care must be exercised to avoid breaking the electrode during removal, i.e. pull very slowly and keep load on wires less than the yield strength.
Open helix leads are fully encapsulated with connective tissue, which mechanically locks them in the tissue. Clean leads develop a thin tight cell layer lining around the leads. The tight cell layer provides a barrier for bacterial infection. Care must be taken to avoid damaging the tight cell layer.