Powering Up Textiles
New technological developments are providing opportunities for on-body energy harvesting integration into textiles.
By Jesse S. Jur, Ph.D., and Ryan D. Hodges, Ph.D.
Imagine a future in which as soon as you get dressed in the morning, sensors inside your shirt power up and automatically begin streaming physiological and environmental data to your cell phone. Throughout the day, you can check how active you are or if you are dehydrated. The phone could automatically monitor personal exposure to particulates or harmful gases and identify hazardous working conditions or stream real-time electrocardiogram signals during your intramural soccer game. These types of systems could provide much more information than today’s wearables, and with no additional steps required beyond going about your normal routine. Advancements in low-power electronics and improved on-body energy harvesting are allowing researchers to develop these systems and tread new ground in the field of wearable electronics.
One example of a flexible, textile-based energy-harvesting solution is the wristband shown here. The infrared image, inset, shows the level of thermal energy in the hand.
The concept for self-powered wearable systems is the focus of a National Science Foundation Nanosystems Engineering Research Center on Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) at North Carolina State University in Raleigh, N.C.
Wearable Electronics And The Importance Of Power
As evidenced by the current number of fitness- and health-monitoring wearables on the market, there is an increasing interest in knowledge about a person’s wellness and environment. The majority of these devices are iterations of wristbands that rely on batteries that require periodic charging. In general, the battery-charging requirements of these devices influence the resulting form factor and affect user interaction with the device. Harvested power can be used to recharge batteries or supercapacitors to increase the lifetime of devices, eliminate external recharging, and increase the number of sensors one could integrate. This is important for increasing the ease of use of the device, an important human factor in device acceptance — think of not having to plug in your cell phone!
In applications for which the battery can be reduced in size or removed altogether, the physical form factor can be altered significantly, which can lead to innovative applications. In a similar way, reducing the size can alter prior assumptions about form factor, for example, placement on the body. Textile-based systems can be engineered to be flexible and comfortable and thus applied anywhere on the body, unlike some of the rigid form factors currently available. Moreover, advancements in low-power electronics and communication are providing newfound opportunities in the ability to use harvested power from the body to support low-battery or battery-free sensor systems. But, placement of the energy harvesting on the body is critical, and textile platforms may provide the answer.
The battery-powered FitBit Flex wireless activity and sleep wristband is one example of the many popular fitness-monitoring wristband platforms.
Harvesting Power From The Body
Energy from the body comes in many different forms, each of which can be harnessed with varying efficiencies (See Table 1). In general, on-body power can be harnessed in two types: thermal energy and mechanical energy. It is noted that power conversion losses and size limit the total useable power to microwatts or low milliwatts in most cases. Most commercial electrical systems available are unable to be supported by these low powers; thus, this development is linked closely to advancements in low-power electronics.
Thermal Energy: For localized transfer of thermal energy to useable power, researchers have been exploring devices called thermoelectric generators. These devices rely on the Seebeck effect, in which an applied temperature gradient across a material causes diffusion of charge carriers from the hot to the cold side. This creates a separation of charge within the material and, as a result, an electric voltage. When a thermoelectric material is placed between hot and cold surfaces, power is generated. Most thermoelectrics are designed for high temperature differences, but some useable power can be formed at the low temperature differences that exist between the body and the localized environment. For on-body applications, the placement and integration method of the thermoelectric is very important. The transfer of heat energy to electrical energy requires efficient heat collection from the body and efficient heat rejection to the outside environment, with thermal isolation between the two. Therefore, it is ideal to place the thermoelectric generator on the body’s pulse-points, where blood vessels are close to the skin surface. In addition, a location needs to have easy access to the ambient air for a proper heat sink.
A main limitation of current thermoelectric devices as they apply to on-body energy harvesting is that the devices are manufactured using rigid ceramic plates, which limit the ability of these devices to maintain good skin contact when applied to areas of the body that are not flat. Textile-based solutions are currently under development that allow for these devices to be flexible, permitting more versatile applications. Such solutions have been documented by S.J. Kim, J.H. Wei and B.J. Cho in the journal “Energy and Environmental Science” (2014); and by A. Yadav, K.P. Pipe and M. Shtein in the “Journal of Power Sources (2008). In addition, textiles can assist in the ability to control and regulate the thermal resistance between the hot and cold sides of the materials. Textiles engineered to have a high thermal conductivity, through fillers or metallization, could be used as a heat sink to efficiently reject the heat to the outside environment. In addition, the porous structure of textiles can reduce the thermal conductivity of the isolation layer and create a more consistent temperature differential across the thermoelectric. All of these advances will yield an increased ability to integrate thermoelectric devices into textile-based platforms and increase the amount of power that can be generated.
Mechanical Energy: Conversion of mechanical energy into electrical power can be realized a number of ways. The most common form is the standard electric generator, which uses a gas motor to turn a magnetic rotator and induce current in an output coil. Similarly, there are on-body systems that employ the movement of permanent magnets through a coil. However, these systems are often bulky and heavy and are not easily integrated into an unobtrusive wearable system. On-body mechanical energy harvesters are more commonly realized through the use of piezoelectric materials, which create an electric voltage in response to deformation. This deformation can occur in many ways depending on the physical implementation mechanism. For example, piezoelectric material placed in the shoe, which is compressed with each step, or piezoelectric films which bend with the joints. Athletic compressive fabrics are ideally suited for piezoelectric integration. Research into textile-based piezoelectric that include piezoelectric coated fibers or nanowires lead to the possibility of your entire shirt generating electric power every time you move.
Textiles offer key strategies to the advancement of wearable electronics. Engineering the textile’s construction and materials as well as utilizing advancements in surface modifications all have the opportunity to influence the power-harvesting efficiencies and many levels of human factors on the device implementation. It is important to reiterate the synergy required between advancement in wearable electronics textile integration and development of associated low-power electronics. While the power levels are very small, advancements in low-power electronics are making these energy-harvesting modes a reality. In return, these systems have the ability to transform preconceptions in how sensing systems can be applied into different form factors and materials, including textiles.
Editor’s note: Jesse S. Jur, Ph.D., is an assistant professor of Textile Engineering, Chemistry and Science; and Ryan D. Hodges is a postdoctoral research scholar in the Department of Textile Engineering, Chemistry and Science at North Carolina State University, Raleigh, N.C.