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. Keywords—on-body sensing; inductive power; wireless power transfer; healthcare; safety compliance INTRODUCTION The popularity of on-body sensing devices is increasing rapidly. Within research there has been a corresponding increase in studies developing and using sensors in areas such as healthcare and activity monitoring, e.g. [1]-[5]. Like many others we believe that the potential of on-body sensing is significant. However, to fully exploit this potential there is a key challenge that must be addressed: that of power. On-body devices are typically powered from a battery that must be replaced or recharged on a regular basis. This basic requirement places significant operating constraints on devices. For example, human intervention is required to recharge or change the battery and this is a well-known cause of frustration. At the very least it leads to down-time between charges. When users place a high value on a device (e.g. a mobile phone) they will generally accept the effort involved in recharging. This is not always the case when devices are considered less important or have been deliberately designed not to play a prominent role in a person's day-today routine (e.g. passive health sensors). In many cases people simply forget to recharge devices for long periods or stop using them altogether. For designers of on-body sensors this leads to a difficult decision. A wide range of power management techniques can