Thanks to the rapid development of wearable activity trackers, now we can track how many steps we walk every day. In fact, many of us are setting goal of daily step-count for a healthier life. The question being asked in the energy harvesting community is: ‘can we harvest enough energy from footsteps to power wearable devices?’
Most of the people in the energy harvesting community know that piezoelectric effect can directly convert kinetic energy into electrical energy. Many flexible polymer-based piezoelectric materials have the advantages of being lightweight, compliant, non-toxic, washable, and can be attached to textiles. These features are particularly useful for wearable applications where user-comfort and design are important. Since walking and running generate an abundant amount of mechanical energy, it is not surprised that footwear attracts most research interest for developing into an energy harvester. In the 1990s, prototypes of energy harvesting footwear have been developed, most commonly using piezoelectric materials such as PZT ceramic or PVDF polymer. At that time, the wearable devices were still not truly ‘wearable’ - they were large and power-hungry. The power generated from an EH footwear could hardly meet the power demand of the wearables, so their applications were quite limited. However, in recent years, the development in ultra-low-power electronic components, such as MEMs sensors, ZigBee and Bluetooth Low Energy (BLE) communications, have enabled the possibility of powering wearable devices using piezoelectric energy harvesters.
Researchers at the University of Southampton, UK, have designed the removable insole to collect the kinetic energy generated by the pressure of footsteps. This kinetic energy is then transformed into electrical energy that can be used to power small wearable devices. Unlike previous devices which mostly used PZT ceramic or less-efficient PVDF polymer, Southampton’s insole uses a ferroelectret material. This material is a flexible porous polymer that can store positive and negative charges in its internal voids after charging. Compressing or expanding a ferroelectret in its thickness direction will result in a change of its internal dipole moments in magnitude, thus the compensation charges in its surface electrodes change and will generate electric pulses. Its d33 after charging is comparable to PZT, and significantly higher than other piezoelectric polymers.
Dr Jerry Luo presented this EH footwear research at the Wearable Technology Show in London this March. Jerry demonstrated the ferroelectret material being used as a battery-free solution that can power wireless transmission of limited amounts of sensor data up to ranges of around 10 meters. This attracted interest from a number of wearable technology developers and also, footwear manufacturers who are looking to expand their business into the wearable market. The group is currently exploring this technology in the applications of identification, indoor tracking and activity measurement. One example is their current partnership with a local hospital to replace balance and foot pressure tests conducted on recovering patients in time-consuming clinics with continuous self-powered measurement. This research was recently reported by CNET news http://www.cnet.com/uk/news/battery-free-wearables-of-the-future-could-be-powered-by-your-footsteps/.
If you want to learn more about this and other recent energy harvesting advances and applications from industry and academia then you are invited to attend Energy Harvesting 2016. This is the annual UK Energy Harvesting Network event being held at Ambassadors Bloomsbury Hotel, London on May 11th. Please see the Network website for full details and agenda (http://eh-network.org/events/eh2016.php).