Electrical properties of implant encapsulation tissue

Area of Science:

• Biomedical Engineering
• Materials Science
• Neuroscience

Background:

• Chronic implantation of electrodes in biological tissue can lead to the formation of encapsulation tissue.
• The electrical properties of this surrounding tissue are critical for the performance of implanted electrodes.
• Understanding tissue resistivity is essential for predicting device function and potential alterations in electric fields.

Purpose of the Study:

• To determine the electrical properties, specifically resistivity, of encapsulation tissue formed around chronically implanted electrodes.
• To investigate the relationship between encapsulation tissue morphology and its electrical resistivity.
• To assess the impact of encapsulation tissue resistivity on the electric fields generated by implanted electrodes.

Main Methods:

• Implantation of two four-electrode arrays (epoxy or silicone rubber) in six adult cats for 82 to 156 days.
• In vivo measurement of tissue resistivity using the four-electrode technique.
• In vitro measurement of tissue impedance using a four-electrode cell to analyze frequency-dependent resistivity.

Main Results:

• Encapsulation tissue formation significantly increased tissue resistivity around the electrode arrays.
• Silicone rubber encapsulation tissue exhibited higher resistivity (627 ± 108 Ω·cm) independent of frequency.
• Epoxy encapsulation tissue showed frequency-dependent resistivity, decreasing from 454 ± 123 Ω·cm to 195 ± 88 Ω·cm between 10 Hz and 100 kHz.

Conclusions:

• The resistivity of encapsulation tissue is significantly influenced by its morphology and the implant material.
• The observed increase in tissue resistivity can substantially alter the electric field distribution around chronically implanted electrodes.
• These findings highlight the importance of considering encapsulation tissue electrical properties in the design and application of neural implants.