Energy harvesting is an important enabling technology necessary to unleash the next shift in mm-scale and μW power computing devices, especially for wireless sensor nodes. Energy harvesting could play an important role in biomedical devices where it extends the lifetime of the system. Furthermore, it eliminates the need for periodic maintenance such as exchanging or recharging the battery. This paper presents experimental results of thermal and vibration energy harvested from human body using the thermoelectric generator and the piezo electric harvester, respectively. Contemporary research revealed that most of the published data, including harvesters datasheets, are adjusted for industrial or laboratory-setting environment. This paper focuses on obtaining experimental data from the human body using off-the-shelf harvesters, and discrete electrical components. Our experimental results showed that for 9 cm ² area of thermoelectric generator, up to 20 μW of power can be generated at 22 ^° C room temperature. In addition, 0.5 cm ³ piezo electric harvester can generate up to 3.7 μW when running at 7 mi/h. These data correspond to a power density of 2.2 μW/cm ² and 7.4 μW/cm ³ for thermoelectric generator and piezo electric harvester, respectively. As such, the harvested energy from thermal and vibration of human body could potentially power autonomous wearable and implantable devices.

This paper introduces an efficient reconfigurable, multiple voltage gain switched-capacitor dc-dc buck converter as part of a power management unit for wearable electronics. The proposed switched-capacitor converter has an input voltage of 0.6 V to 1.2 V generated from an energy harvesting source. The switched-capacitor converter utilizes pulse frequency modulation to generate multiple regulated output voltage levels, namely 1 V, 0.8 V, and 0.6 V based on two reconfigurable bits over a wide range of load currents from 10 μA to 800 μA. The switched-capacitor converter is designed and fabricated in 65-nm low-power CMOS technology and occupies an area of 0.493 mm ² . The design utilizes a stack of MIM and MOS capacitances to optimize the circuit area and efficiency. The measured peak efficiency is 80% at a load current of 800 μA and regulated load voltage of 1 V.

This paper presents an area- and power-efficient dual-output switched capacitor (DOSC) dc-dc buck converter for wearable biomedical devices. The DOSC converter has an input voltage range between 1.05 and 1.4 V and generates two simultaneous regulated output voltages of 1 and 0.55 V. The DOSC consists of two main blocks: a switched capacitor regulator and an adaptive time multiplexing (ATM) controller. The switched capacitor regulator generates a single regulated voltage using pulse frequency modulation based on a predetermined reference voltage. In addition, the ATM controller generates two simultaneous output voltages and eliminates the reverse current using pulse width modulation during the switching between the output voltages. Addressing the reverse current problem is important to reduce the output voltage droop and improve the performance. The proposed converter supports load currents of 10-350 μA and 1-10 μA at load voltages of 1 and 0.55 V, respectively. The DOSC circuit is fabricated in 65-nm CMOS, and it occupies an active area of 0.27 mm ² . Measured results show that a peak efficiency of 78% is achieved at a load power of 300 μW.

This brief presents an improved zero current switching (ZCS) control for a high-gain inductor-based dc-dc converter targeting thermoelectric generator for wearable electronics. The proposed ZCS control is an all-digital circuit that utilizes a simple finite state machine and a 3-bit counter to locate the zero current point. In addition, an efficient push-pull circuit along with delay capacitance banks is used to tune the delay near the zero current point to reduce the estimated error. The proposed control circuit achieves 56 delay steps using 3 control bits while having a high resolution, which helps in maintaining the efficiency of the converter. The prototype chip is fabricated in 65-nm CMOS and occupies an area of less than 0.04 mm ² . The measured results of the converter confirm 81% peak efficiency at 55-μW output power and 50-mV input voltage.

A 0.5 V CMOS light-to-digital converter (LDC) based on a nonuniform quantizer and off-chip photodiode enables a photodiode bias current (Ibias) range spanning 4 nA to 3.5 μA while consuming less than 4 μW of power. Using an off-chip LED as a modulated light source, measurements with a photodiode current signal having modulation frequency of 1.2 Hz (72 beats per minute) and 0.5% peak-to-peak amplitude relative to I _bias performed at the low and high end of the Ibias range confirm over 30 dB of SNR for an integration bandwidth spanning 0.5 to 5 Hz. Using off-chip digital signal processing of the LDC output, instantaneous period jitter (a proxy for instantaneous heart rate) is measured to be less than 0.45% (rms) of the period, and the high sensitivity of the LDC allows detection of the heart-rate signal from a finger pressed against the off-chip photodiode using only ambient light. Key circuit components of the LDC include a wide range logarithmic digital-to-resistance converter (DRC) utilizing digital multibit ΔΣ modulation to achieve fine resolution and a nonuniform quantizer based on a laddered inverter quantizer (LIQAF) which also acts as a low-noise front-end amplifier and filter.

Emerging technology in semiconductor devices and circuits has enabled ultra-low power systems in medical, structural (bridges), and hard to reach places which fueled the energy scavenging research and its interface circuitry. The energy can be harvested from ambient sources such as vibration, wireless, thermal and solar. To harvest the energy, special interface circuit needs to be designed for suitable energy conversion such as DC-DC converters, power management unit, and storage elements. In this paper, we present a survey about the state-of-the-art thermal energy harvesting systems focusing on the interface circuitry, and explaining the tradeoffs in human body-based harvesting applications.