General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Abstract-This paper presents a flexible 2.45-GHz wireless power harvesting wristband that generates a net dc output from a −24.3-dBm RF input. This is the lowest reported system sensitivity for systems comprising a rectenna and impedancematching power management. A complete system has been implemented comprising: a fabric antenna, a rectifier on rigid substrate, a contactless electrical connection between rigid and flexible subsystems, and power electronics impedance matching. Various fabric and flexible materials are electrically characterized at 2.45 GHz using the two-line and the T-resonator methods. Selected materials are used to design an all-textile antenna, which demonstrates a radiation efficiency above 62% on a phantom irrespective of location, and a stable radiation pattern. The rectifier, designed on a rigid substrate, shows a best-inclass efficiency of 33.6% at −20 dBm. A reliable, efficient, and wideband contactless connection between the fabric antenna and the rectifier is created using broadside-coupled microstrip lines, with an insertion loss below 1 dB from 1.8 to over 10 GHz. A self-powered boost converter with a quiescent current of 150 nA matches the rectenna output with a matching efficiency above 95%. The maximum end-to-end efficiency is 28.7% at −7 dBm. The wristband harvester demonstrates net positive energy harvesting from −24.3 dBm, a 7.3-dB improvement on the state of the art.

Cellular polypropylene (PP) ferroelectret is a thin and flexible cellular polymer foam that generates electrical power under mechanical force. This work investigates single and multilayer ferroelectret PP foams and their potential to supply energy for human-bodyworn sensors. Human foot-fall is emulated using an electrodynamic instrument, allowing applied compressive force and momentum to be correlated with energy output. Peak power, output pulse duration, and energy per strike is derived experimentally as a function of force and momentum, and shown to be a strong function of external load resistance, thus providing a clear maximum energy point. The possibility of increasing pulse time and reducing voltage to CMOS compatible levels at some expense of peak power is shown. To further increase the output power, multilayer ferroelectret is presented. The synchronized power generation of each layer is studied and illustrated using simulation, and results are supported by experiments. Finally, the energy output of single-layer and multi-layer ferroelectrets are compared by charging a capacitor via a rectifier. A ten-layer ferroelectret is shown to have charging ability 29.1 times better than that of the single-layer ferroelectret. It demonstrates energy output that is capable of powering the start-up and transmission of a typical low-power wireless sensor chipset.

This paper describes the design and operation of power conditioning system with maximum power transfer tracking (MPTT) for low-power electromagnetic energy harvesters. The system is fully autonomous, starts up from zero stored energy, and actively rectifies and boosts the harvester voltage. The power conditioning system is able to operate the harvester at the maximum power point against varying excitation and load conditions, resulting in significantly increased power generation when the load current waveform has a high peak-to-mean ratio. First, the paper sets out the argument for MPTT, alongside the discussion on the dynamic effects of varying electrical damping on the mechanical structure. With sources featuring stored energy, such as a resonant harvester, maximum power point control can become unstable in certain conditions, and thus, a method to determine the maximum rate of change of electrical damping is presented. The complete power conditioning circuit is tested with an electromagnetic energy harvester that generates 600 mV rm s ac output at 870 μW under optimum load conditions, at 3.75 m·s −2 excitation. The digital MPTT control circuit is shown to successfully track the optimum operating conditions, responding to changes in both excitation and the load conditions. At 2 V d c output, the total current consumption of the combined ancillary and control circuits is just 22 μA. The power conditioning system is capable of transferring up to 70% of the potentially extractable power to the energy storage.

Designers of on-body health sensing devices face a difficult choice. They must either minimise the power consumption of devices, which in reality means reducing the sensing capabilities, or build devices that require regular battery changes or recharging. Both options limit the effectiveness of devices. Here we investigate an alternative. This paper presents a method of designing safe, wireless, inductive power transfer into on-body sensor products. This approach can produce sensing devices that can be worn for longer durations without the need for human intervention, whilst also having greater sensing and data capture capabilities. The paper addresses significant challenges in achieving this aim, in particular: device safety, sufficient power transfer, and human factors regarding device geometry. We show how to develop a device that meets stringent international safety guidelines for electromagnetic energy on the body and describe a design space that allows designers to make trade-offs that balance power transfer with other constraints, e.g. size and bulk, that affect the wearability of devices. Finally we describe a rapid experimental method to investigate the optimal placement of on-body devices and the actual versus theoretical power transfer for on-body, inductively powered devices.

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A method for power sensing in low-power energy harvesting systems is presented. The technique utilises a small sense capacitor between the converter output and the main storage capacitor. The storage capacitor is disconnected from the converter for a short period of time and the change of sense capacitor voltage is measured. This change is proportional to the average output current of the converter for the period of the measurement, thus the relative output power can be determined using only voltage measurements. The method has been experimentally validated in a low-power energy harvesting system that employs a microcontroller-based control implementation.

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