32.1 A Behind-The-Ear Patch-Type Mental Healthcare Integrated Interface with 275-Fold Input Impedance Boosting and Adaptive Multimodal Compensation Capabilities

Conventional brain activity monitoring devices including head bands and scalp caps are still not appropriate for daily-life monitoring applications because of their wearing discomfort and electrode-related artifacts. A proposed behind-the-ear (BTE) device, where motion artifacts are significantly reduced, would be a promising candidate for continuous mental healthcare platforms. This proposed BTE-based measurement allows multimodal bio-signals such as electroencephalogram (EEG), electrocardiogram (ECG), photoplethysmogram (PPG), and galvanic skin response (GSR), whereas most conventional brain devices only monitor EEG. However, these bio-signals become much weaker at the BTE location, and their signal quality is more important, being seriously degraded by artifact-related corruptions during daily activities. For this purpose, an offset-compensated auxiliary path (OCAP) and a dual-resolution external positive feedback loop (DR-EPFL) are proposed to boost the input impedance and to improve its AC characteristics further. For small-featured device sizes, the number of electrodes is reduced by utilizing a proposed GSR-embedded ECG readout structure where electrodes are shared and monitored together. For system-level feasibility, a direct-conversion PPG readout with a proposed parasitic capacitance compensation, a transcutaneous vagus-nerve stimulator $(\text{tVNS})$ with an electrode-monitored adaptive charge pump, and a high-resolution bio-impedance channel are integrated together. A BTE-based mental healthcare patch-type device prototype is also manufactured and its multimodal functionality is experimentally verified.