Plastic Surgery Research Council
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PSRC 60th Annual Meeting

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Electrical Impedance Spectroscopy as a Tool for Analysis of In Vivo Encapsulation of Muscle Electrodes
Michelle K. Leach, PhD, Stephanie A. Goretski, Melanie G. Urbanchek, PhD, Paul S. Cederna, MD, Nicholas B. Langhals, PhD.
University of Michigan, Ann Arbor, MI, USA.

PURPOSE:Chronically implanted electrodes are prone to encapsulation by fibrous scar tissue which can severely decrease functionality. Electrical impedance spectroscopy (EIS) can be used to monitor the extent of this encapsulation in vivo. The majority of the literature on this subject is focused on electrodes implanted in brain. Here, we examine changes in impedance of an electrode implanted on muscle.
METHODS:Two stainless steel pad electrodes were sutured to the epimysium of the left extensor digitorum longus (EDL) muscle of the rat hind limb. The electrode wires were tunneled subcutaneously and secured to a head cap. Prior to implantation, the electrodes were characterized in a physiological saline bath in a 3-electrode configuration with a platinum counter electrode and an Ag/AgCl reference electrode using a Gamry Potentiostat. One hour following surgery, electrical impedance spectroscopy (EIS) was performed over a frequency range of 1 Hz to 100 kHz at an amplitude of 100 uV using a stainless steel needle reference electrode placed subcutaneously in the left hind limb and a counter electrode placed in the tail.
RESULTS:The shape of the Bode plot was similar at all implantation times (see inset in Figure 1A). Impedance decreased rapidly with increasing frequency in the low frequency range and then leveled off. However, as implantation time increased, the impedance curves shifted upwards in the high-frequency range, which is indicative of an increase in encapsulation (Figure 1A). Similarly, an arc appeared in the high-frequency portion of the Nyquist diagram (Figure 1B) as implantation time increased that can be attributed to the extracellular matrix and cellular membranes, which are well fit by a model consisting of a parallel combination of a resistance and a capacitance, respectively.
CONCLUSION:Similar changes in impedance were observed by EIS of an electrode chronically implanted in muscle as have been reported in the literature for electrodes in brain. Chronically implanted electrodes should exhibit long term stability. A thorough understanding of the foreign body response to the electrode is therefore crucial. These preliminary results indicate EIS is a sufficient tool to monitor and quantify muscle electrode encapsulation in vivo over time.
Acknowledgements:This work was sponsored by the Defense Advanced Research Projects Agency (DARPA) MTO under the auspices of Dr. Jack Judy through the Space and Naval Warfare Systems Center, Pacific Grant/Contract No. N66001-11-C-4190.


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