Enhanced tissue integration of implantable electrodes for sensing, and stimulation, via radio frequency glow discharge
O'Connor, Laurie M.
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Biopotential electrodes are conductive materials that convert electronic currents to or from ionic currents for sensing, and stimulating specific tissue sites for medical applications. Implanted electrodes become “walled off” by the foreign body tissue reactions producing poorly attached scar capsules dominated by surrounding dense collagenous lamellae and source fibroblasts which are electrically resistive. The conductive interstitial fluid that is typical between an electrode and the resistive capsule allows spurious current paths. The insulating layer increases the distance between the electrode and the target sites and poor attachment often results in electrode migration within the host tissue. This investigation tested the hypothesis that surface-energy modulation of electrodes, via Radio Frequency Glow Discharge Treatment (RFGDT), can improve the performance of tissue-implantable electrodes by reducing the foreign body tissue reaction and enhancing interfacial bonding between the tissue and electrode material. Previously published findings were reproduced in a pilot study of explanted reference grade medical-grade methyl silicone (PDMS) and commercially pure titanium (cpTi) materials and their tissue capsules from 30-day subcutaneous exposures in Balb/C mice. The low-critical surface tension PDMS produced thick, dense, poorly attached scar capsules while the higher-surface-energy commercially pure titanium (cpTi) produced more cellular and strongly attached tissue layers difficult to delaminate from the biomaterial. For the main body of work, cpTi, capacitor-grade Tantalum (Ta), and synthetic heart valve-quality Pyrolytic Carbon (PyC) were evaluated, representative of potential high-surface-energy implant electrode materials. Their surface characteristics were determined as-manufactured and after Radio Frequency Glow Discharge Treatment (RFGDT) by Critical Surface Tension (CST) measurement, Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and Electron Spectroscopy for Chemical Analysis (ESCA). Representative tissue/electrode interfaces were created through subcutaneous implantation and harvest from Balb/C mice after 7, 28 and 56 days. The recovered, formalin preserved conductive implant/tissue specimens were examined by Electrical Impedance Spectroscopy (EIS) over the range from 0.1 Hz to 100,000 Hz, in Bode plot and Nyquist plot formats. After EIS the implants were extracted from the tissue for surface examination by SEM and the separated tissue capsules were evaluated by histological examination in hematoxylin and eosin (H&E)-stained light microscopic thin sections. The differential findings were that the RFGDT PyC and Ta implants had significantly increased interface resistance and capacitance over their sterilized-only controls, correlating with more flattened cellular layers retained on the RFGDT specimens. The cpTi specimens, earlier identified as promoting hard tissue-to-surface integration in dental implants, showed only marginal improvements in soft tissue attachment following RFGDT. These experimental findings were confirmed through equivalent circuit modeling by the conversion from a standard Randles model with constant phase elements, which described the pre-implant materials, to a modified Randles model with additional resistance and capacitance to describe the implants with well-integrated surface coatings. Future work with RFGDT-modified electrode materials must include actual signal acquisition/stimulation trials in implant host tissues, to assess possible improvements in electrical energy-transfer efficiency and battery lifetime extensions.