With all of the ways that have been devised to harvest energy -- from the human heartbeat to the energy of a kicked soccer ball -- it seems that finding a way to use human movement to generate power would be the next logical step.
Now a sensor-enhanced fabric called "stretch sensors" designed by Danish company Danfoss PolyPower A/S can harvest the energy generated when someone engages in exercise or a sporting activity. The energy powers sensors in the fabric to provide people with information about their movement to a wireless device.
In this way, the fabric can be worn to help the person improve their performance in an activity, giving them information about their gait while running or helping them determine the correct angle of their elbow to improve their golf swing, as shown in this video:
The stretch sensors also can be used in other applications beyond human movement, such as to test the structural health of concrete beams or to test the strain on an underground storage facility for wind energy, the company said. Danfoss even claims this technology can one day be used to generate energy from ocean waves and is working on an installation of the material to do so.
There is a growing trend in the medical device industry in particular to use low-power sensors embedded in fabric to provide people with medical and health monitoring information either within the sensor or wirelessly to a device to promote wellness. Boston-based startup Rest Devices, for instance, has created a shirt with sensors to monitor people while they sleep to test for sleep apnea and other disturbances.
Because these sensors consume such little power, using an energy-harvesting design for medical sensors makes sense because it means they won't depend on a battery that might have to be replaced or recharged once it runs out. The idea, then, is that by using this type of design, the sensors can sustain their own power source indefinitely or as long as they are needed.
The material used by Danfoss in its stretch sensors works by using DEAP (Dielectric Electro Active Polymers), which can be used for actuation, sensing, and energy harvesting, according to the company. EAPs are polymers that change size or shape when stimulated by an electric field, and they come in two forms: dielectric or ionic.
Dialectric polymers can produce actuation by electrostatic forces caused by two electrodes squeezing the polymer. In this way, a DEAP becomes a capacitor that changes its capacitance when a voltage is applied by allowing the polymer to stretch, compressing in thickness and expanding in area due to the electric field, the company said.
Danfoss claims that because of this fabric design, the sensors can withstand a significant amount of strain, are resilient, and can sense the shape and structures over more than one dimension. However, a user may have to make some compensation for the effect of environmental wear and tear or moisture on the fabric, depending on how the stretch sensor is used.
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