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EETE APR 2014

WEARABLE electronics Body motion drives implantable energy harvester By Julien Happich Researchers from A*STAR’s Institute of Microelectronics (IME), Singapore, have demonstrated a new MEMS-based energy harvesting concept that could harness low frequency vibrations across a wide naturally occurring range of 0.1 to 100Hz to power miniature electronic devices. The new concept combines MEMS microfluidics and piezoelectric micro-belts that convert changes in pressure (from random real-world vibrational sources) into electricity. Under the alternating pressure waves from the harnessed vibrations, a pressurized fluid in micro-channels synchronizes the random input vibrations into pre-defined resonance frequencies that make the most of the piezoelectric elements for charge generation, despite the irregularity and randomness of the vibrations. MEMS energy harvesters are not new, but most concepts rely on vibrating piezoelectric cantilevers or micro-electrets, which only operate efficiently within very narrow frequency bands, if not only at one frequency. This limitation discards these concepts in most practical environments. Dr Alex Gu, Technical Director of IME’s Sensors and Actuators Microsystems Programme, found an interesting way to expand Fig. 1: (a) The flow-induced EH structure showing the fluid behaviour in the cavity, (b) without vortex shedding effect and (c) with bluff body and vortex shedding effects enhancing the Helmholtz resonance. the range of vibration frequencies within which the MEMS energy harvesters would be receptive. Relying on air for the working fluid, Dr Gu uses input and output cavities connected to micro-channels of specific geometries that leverage Helmholtz pressure resonance effects. The pressure oscillations are further enhanced by coupling the Helmholtz resonance with so-called vortex shedding – see figure 1, an oscillation of vortexes that occurs in a fluid when the flow has to pass by obstacles or a bluff body. The vortex shedding effect in the pressurized fluid is what synchronizes the pressure oscillations at pre-defined high frequencies to be harvested by a piezoelectric structure embedded into the microfluidic channels. “By transferring the low frequency input vibrational energy into a pressurised fluid, the fluid synchronizes the random input vibrations into pre-defined resonance frequencies, thus enabling the full utilization of vibrations from the complete low frequency spectrum”, explains Gu. In a paper published early February, the researchers Fig. 2: Fully integrated energy harvesting device with a polydimethylsiloxane (PDMS) cap. Fig. 3: Interior functional structures beneath the PDMS cap. highlight that because the operating frequency of the energy harvester is determined by the physical sizes of the cavity and orifice, it is independent of the input fluid flow rate. This greatly simplifies the ASIC design and simultaneously improves the energy storage efficiency. The paper reports a peak output power of circa 21μW. To demonstrate this new concept, the researchers have built an aluminium nitride (AlN) based energy harvester which delivered a record power density of 1.5mW/cm3 – see figures 2 and 3. As a comparison, the prototype would be capable of generating electricity equivalent to three commercial implantable lithium batteries over a 10-year period, claim the researchers (taking as a reference batteries with an energy density of 1.05W.h/cm3 occupying a volume of 2.34 cm3). Such a MEMS energy harvester could find applications in medical applications to power pace makers or implantable cardioverter defibrillators, removing the need for battery replacement altogether, but it could also find its way in automotive sensing, and wireless communication applications. Although this technology harvests sub-audio frequency vibration energy and uses microfluidic design principles with air as the working fluid, the lab’s current target applications do not include harvesting from noise environments. “In a practical implementation, as long as the pressure of the vibrational source is equivalent or higher than the driving pressure of the energy harvester, the vibrational source can be harnessed and converted into electricity by our device” told us Dr. Alex Gu. “For implantable applications, we could take advantage of the pressure differential between the inhale and exhale (~0.1-0.2 psi) pressure differential. This could take the form of a compressible bladder similar to a breast implant”, Gu detailed in an email. “During the inhale phase, the bladder is compressed, the self-contained air is pressurized to drive the energy harvester. With the help of a pair of check valves, the energy harvester could harvest twice during one breathing cycle, on both the inhale and exhale phases” he added. “For wearable applications, we can imagine harvesting heel-strike pressures or movement induced (0.1-2 g acceleration) pressure generation. Implementation of these ideas could take the form of smart-insole or shoes, like Nike Air, or button-size energy harvesters powering wearable body sensors network”, Gu concluded. The research institute is also simulating such MEMS with liquid as a working fluid. 40 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
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