Wearable computing is an effort to make computers truly part of our everyday lives by embedding them into our clothing ( e.g. shoes) or by creating form factors that can be used like clothing ( e.g. sunglasses) [Starner et al, 1995]. This level of access to computation will revolutionize how computers are used. While the computational hardware has been reduced in size to accommodate this vision, power systems are still bulky and inconvenient. Even today's laptops and PDA's are often limited by battery capacity, output current, and the necessity of having an electrical outlet within easy access for recharging. However, if energy can be generated by the user's actions, these problems will be alleviated.
: Comparisons of common energy sources and computing power requirements.
At this point, a review of vocabulary and units is in order.
Energy is defined as the capacity to do work. For this paper, the
joule will be used as the standard unit of energy. A joule
(
) is the product of a force of one newton acting
through a distance of one meter. For reference, Table
1 compares some common sources of energy.
The calorie, which is 
joules, is also often used as a unit of
energy. However, in dietary circles, a Calorie refers to a
kilocalorie or 1,000 calories. Therefore, an average adult diet of
2,500 Calories translates to 
.
Power, often confused with energy,
is the time rate of doing work. Power can be measured in watts
(
), or joules per second. Table
1 also shows power requirements for common
computing devices. The reader should be aware that in some
literature, units of power are combined with units of time to indicate
energy. For example, watt seconds, watt hours, and kilowatt hours are
often used in favor of joule, kilojoule, and megajoule.
As shown by human powered flight efforts [Sullivan and Clancy, 1989], the human body is a tremendous storehouse of energy. For example, the energy obtained from a jelly doughnut is
This energy may be stored in fat at approximately
Thus, an average person of 68 kg (150 lbs) with 15% body fat stores energy approximately equivalent to
The body also consumes energy at a surprising rate, generally using between 70,000 and 1,400,000 calories per hour depending on the activity (see Table 2). In fact, trained athletes can expend close to 9.5 million calories per hour for short bursts [Morton, 1952]. On the other hand, the energy rate, or power, expended while sleeping is
Thus, the jelly doughnut introduced earlier would be ``slept off'' in 4.7 hours. If only a small fraction of such power could be harnessed conveniently and unobtrusively, batteries per se could be eliminated. However, difficulties arise from the acquisition, regulation, and distribution of the power.
Recent technology makes these tasks easier. Computers are now small enough to disappear into the user's clothing or body. With such small devices, the main power consumers, namely the CPU and storage, could be located near the implemented power source. Interface devices, such as keyboards, displays, and speakers, have limitations as to their placement on the body. However, these devices may communicate wirelessly via a ``body network'' as described by Zimmerman [Zimmerman, 1995]. They may generate their own power, share in a power distribution system with the main generator (wired or wireless), or use extremely long lasting batteries. Thus, depending on the user interface desired, wires may not be needed for power or data transfer amoung the components of a wearable computer.
In the following sections, power generation from breathing, body heat, blood transport, arm motion, typing, and walking are discussed. While some of these ideas are fanciful, each has its own peculiar benefits and may be applied to other domains such as medical systems, general consumer electronics, and user interface sensors. More attention is given to typing and walking since these processes seem more practical sources of power for general wearable computing.
Table: Human energy expenditures for selected activities. Derived
from [Morton, 1952].
