A Fully Flexible Temperature Sensor for Wearable Applications

This paper presents the design of a fully flexible temperature sensor for wearable applications. The sensor is implemented using the newly available flexible integrated electronic circuit (Flex-IC) technology from Pragmatic Semiconductor [1]. To address the design challenges due to the limited set of thin film components available in this non-CMOS technology, a counter-based time-to-digital conversion (TDC) technique has been developed, where the pulses provided by a temperature-dependent ring oscillator are gated and counted by a temperature-independent delay cell. The operation of the delay cell relies on the temperature coefficient cancellation of resistive components by a cross-coupled pair. A tunable calibration mechanism is also incorporated in the design to handle the process corners in a temperature range of 32 °C - 42 °C suitable for biological sensing. The presented sensor design exhibits a low time-jitter induced measurement error, with a 0.81 % probability of toggling the least significant bit (LSB) per sample, maintaining 0.1 °C precision and a zero-error range of 6.8 °C, while dissipating a power of 2.5 mW with an acquisition time of 3 ms. The chip layout measures 3.0 mm×3.0 mm with a die thickness of 30 μm. Compared to existing rigid and hybrid systems, the Flex-IC technology opens up opportunities for advancing conformal electronics, particularly for applications requiring continuous monitoring on dynamic body surfaces.