The Power of Touch

Fueled by innovations in a wide range of electronic components and computing units, today’s wearable devices are smaller and more powerful than ever before. As such, these wearables are not only capable of performing a number of useful tasks — like monitoring our health, improving our productivity, and enhancing the immersion of our entertainment experiences — but they are also able to transparently integrate into our daily lives without being cumbersome. Well … almost, anyway.

To varying extents, all of these otherwise fantastic wearable devices have an Achilles’ heel — power delivery. The batteries that they generally use are rigid and bulky, adding weight and making the devices uncomfortable to wear. Furthermore, the need for frequent recharges gets tiresome, and often causes users to toss their shiny new device in a drawer and forget about it. Worse yet, wearables that perform a continuous monitoring function — as is the case with a health tracker — must be removed to charge. This causes critical data collection functions to cease for hours at a time.

Researchers have attempted to solve this problem in a variety of ways. Energy harvesting is one popular approach. These techniques allow body motions to be converted into electrical energy to power the devices. However, the hardware tends to be bulky, and energy production is sporadic. Harvesting energy from solar cells or more exotic sources like RF energy have also been tried, but tend to produce vanishingly small amounts of power.

A team at the University of California, Los Angeles has come up with a very clever approach that leverages the power of mechanical energy harvesters to produce substantial amounts of energy, but moves them off-body for comfort. They built these energy harvesters into a variety of everyday objects, like wheels, doors, and cranks. Energy is transferred through the human body from these harvesters to the wearable device via capacitive coupling to enable charging without removing the device or taking any other intentional action.

The researchers call their power generation system Interaction-Power Stations. By normally interacting with everyday objects — perhaps opening a door — electrical energy is produced. That energy is then converted into high-frequency signals that can propagate through the human body on contact. A transmitter electrode on the harvester contacts a receiver electrode on the body to initiate a charge. The energy can then flow through the tissues of the body, as they are naturally somewhat electrically conductive.

The astute reader will immediately recognize that there must be a return path to complete the electrical circuit, and since the body cannot be partitioned into multiple signal lines, the Interaction-Power Stations approach appears to have a fatal flaw. To deal with this situation, the researchers introduced transmit and receive ground electrodes that are positioned just above the body and the harvesting device. This creates a separate return path, via capacitive coupling, to complete the circuit and allow electricity to flow to the wearable for charging purposes.

To validate their system, the team built three prototype devices. After equipping a bicycle, hand crank, and door with energy harvesters, they charged a capacitor on a wrist-worn device. These systems produced between 0.08 and 0.87 microjoules of power. This is far short of the 1.21 gigawatts one would need for time travel in a DeLorean, and well, frankly it is not a lot for most wearable devices either. But, it is a very interesting concept and a good starting point — few technologies arrive fully-formed. With a bit of work, Interaction-Power Stations might one day charge entire networks of on-body electronics.