Abstract:
A wearable Flexible Hybrid Electronics (FHE) sensor for monitoring human heart rate, heart rate variability, and skin temperature is reported. The autonomous monitor includes: electrodes, thermistors, data acquisition, and processing and communication circuitry, in a wearable flexible patch with an operational time of over 40 hours. The copper device-interconnect circuits are first fabricated on both sides of a KaptonÒ substrate and interconnected by plated through hole vias using photolithographically patterned plated copper. The sensors’ pads and connecting circuitry are inkjet printed and sintered from precursor gold-nanoparticle ink. The ECG electrodes and the thermistor are on one surface (skin side) of the substrate and the electronics is attached to the opposite side. This FHE approach provides flexible wearability with silicon electronics processing power. Commercially available microprocessor and front end analog chips were down-selected for optimum ECG signal processing and minimum power consumption especially with respect to wireless communications. Front end filters detect contact errors and amplify the ECG signal, and the microprocessor digitizes and analyzes the ECG and thermistor signals. The microprocessor has built-in secure Bluetooth communications to a host computer. The ECG signals are recorded using inkjet printed Au gel based electrodes, and skin temperature is detected by semiconducting NiO nanoparticle screen printed thermistors. Detection and calculation of heart rate and heart rate variability have been demonstrated using certified archived human ECG signals, and the detected and archived values are in good agreement, demonstrating effective signal conditioning and ECG peak detection. High fidelity ECG signals have been recorded from human subjects that compare favorably with those recorded by rigid thicker belt worn commercial units. Custom software allows on-board ECG signal analysis eliminating the need to send raw data to the host. Thermistors have demonstrated high sensitivity and linear variations over the physiological range of human skin temperature, and are optimized to match the detection circuitry. An encapsulation layer protects them from downstream fabrication processing and from operational conditions. Power consumption is a challenge, necessitating the use of coin batteries, as is the robustness of copper traces and the copper/printed gold interfaces. Improvements in device robustness are being evaluated in collaboration with Lockheed Martin’s Advanced Technology Laboratories. Work is also ongoing to develop capacitive coupled ECG electrodes and their associated circuitry including flexible custom operational amplifiers. Human tests complied with a Binghamton University IRB protocol.
We acknowledge and thank the NanoBio Manufacturing Consortium (FA86501327311-7) and the US Air Force Research Laboratory for funding this work.