D. Koh, A. Wang, K.W. Oh
University at Buffalo,
Keywords: PDMS, flexible electrode fabrication, bio-potential electrode
Summary:This paper reports an application of the Chemical-Free Metal Polydimethyloxane (PDMS) Thermal Bonding (CFMPTB) in the fabrication of a bio-potential electrode, a flexible Ag/AgCl electrode. Current bio-potential electrode is called wet electrode because it uses electrolyte gel as an interlayer between electrode and skin . However, the electrolyte gel can cause skin irritation and not suitable for wearable health monitoring devices. As a result, an alternative electrode, dry electrode, is widely studied to replace the wet electrode, and a key factor of dry electrode is the flexibility of the electrode . Ag/AgCl electrode is the most popular electrode used in bio-signal measurement however, the fabrication of a flexible Ag/AgCl electrode is challenging. This can be overcome by CFMPTB which enables the fabrication of a flexible Ag/AgCl by growing a AgCl layer on the transferred Ag layer. CFMPTB is a thermally induced process that enhances the adhesion between metal and PDMS in the PDMS curing process without any extra treatment . In CFMPTB, the PDMS curing conditions were modified to create a strong adhesion between metal and PDMS, and this can be used in the fabrication of a flexible electrode by transferring an electrode structure, such as a silver (Ag) layer, onto PDMS. Then, a layer of AgCl layer can be grown on the transferred Ag layer by a bleach bath. To fabricate a flexible Ag/AgCl electrode, a 1.5 µm-thick Ag layer (Figure 1) was transferred onto a 500 µm-thick PDMS substrate then it was merged into a bleach bath at 25⁰C for 30 minutes to grow a AgCl layer. Next, as a characterization of the electrode, the impedance of the electrode was measured at frequencies from 10 – 1000 Hz, which includes the frequency range of the bio-signal. The results show that the impedance is generally 90 Hz and some of them are ~20 kΩ at ~100 Hz (Figure 2). The impedance is high due to the cracks on the electrode and the crack is created in the fabrication process. The cracks can be minimized by the optimizing the thickness of PDMS and the transferred silver and the impedance can reach below 1 kΩ at the frequency range of the bio-signal. Once the process is optimized, the electrodes will be integrated into a VR headset for EOG monitoring. The form pad of a VR headset will be replaced by a pad with flexible Ag/AgCl electrodes integration as shown in Figure 3. It will provide faster and easier EOG monitoring without skin irritation.