A tuly batteryless wearable device is developed for the first time for continuous point-of-care testing of human vitals.

The device is enabled by a motion-adaptive heartbeat detection system-on-chip (SoC) completely powered by harvested energy from human body-heat. A unique low-power smart sensing architecture is presented to detect heartbeats from ECG signal in presence of motion-artifacts. The sensing electronics is powered by human body-heat using a portable thermoelectric generator (TEG) and energy-efficient power management circuits embedded in the SoC.

[published in ISSCC'20, JSSC'20]

Body-heat can be converted into electrical energy using thermoelectric generators (TEG) and can be used to power batteryless wearables. But tens of millivolts from a portable TEG is unsuitable to turn on electronics.

This work comprises multiple innovations in ultra-low voltage circuit architectures that assisted self-start of a power converter at the smallest supply voltage reported till date with fully on-chip circuits. A unique CMOS delay element is introduced to design an integrated ring oscillator, operational at 50mV supply. It is the smallest supply voltage for an on-chip ring oscillator reported till date and advances low-voltage operation of CMOS circuits closer to the fundamental limit. Also a novel switched-capacitor-based on-chip voltage multiplier is introduced that enablesvoltage boosting of the smallest input voltage.

[published in JSSC'19, CICC'18, ISCAS'18]

High conversion ratio DC-DC boost converters, required for energy harvesting from small voltage sources, suffers from low power efficiency resulting in a small usable power for the downstream electronics. 

In this work a single-inductor DC-DC boost converter architecture is proposed that is suitable for power extraction from low-voltage sources and is able to self-start at ultra-small supply voltage. It uses a dual-path voltage conversion technique with loss-optimized maximum power transfer regime to achieve the best efficiency at maximum power point. The converter achieved 2.5 times better power efficiency than prior art at low input voltages that makes it the smallest input voltage (3.5mV) operational DC-DC boost converter reported till date. The work addressed fundamental challenges of a TEG-based autonomous body-heat energy harvester. 

[published in JSSC'20, CICC'19]

Sensors in CMOS technology are immensely useful for portable applications not only because of the small feature size but also for easy integration with the sensing and processing circuits. 

In my doctoral research I have done extensive study on the noise-sensitivity trade-off of single photon avalanche diodes (SPADs) by fabricating different test stuctures in conventional CMOS process. I have also implemented on-chip power-management and sensing circuits to bias SPADs beyond the p-n junction breakdown voltage and sense the electrical signal changes on dectecting a photon to develop integrated high-dynamic range optical sensing system for biomedical applications. 

[published in IEEE Sensors J.'18, ISCAS'17, BioCAS'17, TCAS-I'19, TCAS-II'19]

Frequency/time-domain signal processing is a promising technique to reduce the power consumption of portable sensors and increase their lifetime.

In this area my research contributions led to the successful demonstrations of multiple low-power sensing applications. A frequency locked loop (FLL)-based circuit architecture is presented that utilizes duty-cycled resistance for low-power strain sensing. The sensor interface consumed 2500X lower power than a conventional current/voltage-based approach for sensing a 45X lower resistance. 

I have also contributed to develop a precise frequency synthesizer exploring the frequency dependent impedance mismatch between a capacitor and a resistor. 

Most recently, we demonstrated FLL-based time-domain signal processing architecture as an efficient circuit for low-power multi-modal (voltage/current/resistance) sensing.

[published in JSSC'22, BioCAS'22, TBioCAS'23, TBioCAS'23(x2)]