CASPER: Capacitive Serendipitous Power Transfer for Through-body Charging of Multiple Wearable Devices

Edward Wang, Manuja Sharma, Yiran Zhao, Shwetak Patel

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(Left) Prototype of a CASPER powered wound monitoring gauze pad worn on the user’s leg. The bed sheet acts as the body electrode, while the blanket acts as the floating electrode. (Right) The RX floating and body electrode feeds the 13.56MHz coupled signal into a multi-stage diode rectifier network for AC to DC conversion. A Zener diode prevents the power storage from over-voltage. Depending on the application, the power storage can be a simple capacitor or a solid state battery with a BQ25504 charge controller. A load can then be run from the stored DC power.

Abstract

We present CASPER, a charging solution to enable a future of wearable devices that are much more distributed on the body. Instead of having to charge every device we want to adorn our bodies with, may it be distributed health sensors or digital jewelry, we can instead augment everyday objects such as beds, seats, and frequently worn clothing to provide convenient charging base stations that will charge devices on our body serendipitously as we go about our day. Our system works by treating the human body as a conductor and capacitively charging devices worn on the body whenever a well coupled electrical path is created during natural use of everyday objects. In this paper, we performed an extensive parameter characterization for through-body power transfer and based on our empirical findings, we present a design trade-off visualization to aid designers looking to integrate our system. Furthermore, we demonstrate how we utilized this design process in the development of our own smart bandage device and a LED adorned temporary tattoo that charges at hundreds of micro-watts using our system.