Where the silicone meets the wrist: direct over molding of biometric sensors on a wearable band

The wearables market is highly saturated with activity and heart rate monitors that are prone to mimickry between various vendors. Advances such as cuffless blood pressure monitoring based on electrocardiography (ECG) and photoplethysmography (PPG) are gaining traction as novel wearable device offerings, albeit with concerns pertaining to robustness, reproducibility, motion compensation and affordability. The prohibitively higher cost of this newer device category is associated with the quantity and quality of on-board custom components along with their complex assembly and integration requirements. Here, such limitations are addressed by offering multiple sensing modalities including activity monitoring from a tri-axis accelerometer, optical PPG utilizing four light emitting diodes, two photodiodes, and single-lead ECG utilizing 10 μm-thick, screen printed Silver-Silver Chloride electrodes in a cost-effective, Bluetooth-enabled wearable wristband. Blood pressure is measured by simultaneous ECG and PPG measurements to derive the pulse transit time. The device is powered by a rechargeable 3.7V Lithium-Ion battery with 20mAh capacity. Data are transferred to either Android or iOS applications, which then based on application programming interfaces communicate with a cloud back-end for long-term data storage and retrieval of archival parameters. The added uniqueness of this wristband is that it incorporates direct over molding of the on-board electronics and assembled printed circuit boards (33 x 22.8 x 10.8 mm³) using novel low temperature (100°C) liquid silicone rubber with specialized high accuracy mixing equipment. Our approach sharply contrasts with conventional silicone which cures at temperatures reaching 150°C and higher, thereby endangering electronic components during the over molding processes.