This project focuses on two vibration non-idealities that affect the performance of vibration energy harvesters ^{[1] [2]} . Most vibration energy-harvesting research focuses on harvesting from a fixed-frequency single-harmonic vibration. Correspondingly, most harvesters exhibit a single mechanical resonance that couples to the vibration. Real vibration spectra, however, often exhibit multiple and/or non-stationary harmonics and random background vibrations. The inability to handle such non-ideal vibration sources has become a technological obstacle to the applicability of vibration energy harvesters. This work addresses the two major non-ideal vibration conditions illustrated in Figure 1: the non-stationary behavior of individual vibration harmonics, i.e., the slow drift of their frequencies over time; and the multi-harmonic character of the vibration spectra. It does so both theoretically and experimentally by modifying the behavior of a single-resonance vibration energy harvester through its electrical load ^[3] .

To support the experimental work, a vibration harvesting test bench is developed. The test bench is built around the V25W piezoelectric (PZT) energy harvester manufactured by Mide Technology. Figure 2 (a) shows the complete test bench; Figure 2 (b) shows the V25W PZT harvester mounted on a shaker table; and Figure 2 (c) shows the computer-based control and measurement system through which the test bed is excited and observed. It should be noted that the work pursued here uses a piezoelectric harvester out of convenience. The modifications studied here are applicable to vibration energy harvesters employing other energy conversion physics such as electric- and magnetic-based conversion.

1. S. Roundy, P. K. Wright, and J. Rabaey, “A study of low level vibrations as a power source for wireless sensor nodes,” Computer Communications, vol. 26, no. 11, pp. 1131-1144, 2003.
2. C. B. Williams and R. B. Yates, “Analysis of a micro-electric generator for microsystems,” Sens. Actuators A, Phys., vol. 52, no. 1-3, pp. 8-11, 1996.
3. V. R. Challa, M. G. Prasad, Y. Shi, and F. T. Fisher, “A vibration energy harvesting device with bidirectional resonance frequency tunability,” Smart Mater. Struct., vol. 17, no. 1, p. 015035, 2008.

Polymer materials doped with conductive particles exhibit piezoresistive properties. These materials are fabricated such that their conductivity changes with an applied compressive force. When compressed, the formation of percolation pathways allows increased electrical conduction through tunneling between the particles. This work explores and utilizes this property of composites to fabricate various devices with the ultimate goal of developing integrated flexible systems resembling sensory skins.

As a first generation of piezoresistive devices, a squeezable switch (squitch) is fabricated with a three-terminal configuration shown in Figure 1 ^[1] . In this study, the squitch is fabricated from a composite of polydimethylsiloxane doped with 60 wt% Ni microparticles that shows more than 5 orders of magnitude change in conductivity over a 20% strain (Figure 2). In the absence of an applied gate bias, the composite is a poor conductor. An applied gate voltage generates an electrostatic force between the source and the gate that compresses the composite, causing the squitch to conduct. To allow fabrication of reliable and reproducible devices, the composite needs to be engineered such that its mechanical properties are more stable. To achieve this goal, current research explores the effects of the type of polymer and conductive particles and the method of fabrication on the properties of the nanocomposite and performance of the squitch. The surfaces of the metal particles are chemically treated to allow better distribution in the polymer matrix while also chemically binding the particles to the polymer preventing particle migration over repeated use of the device. After the composite is optimized, future work will involve extending the squitch design to fabricate devices such as analog amplifiers, digital inverters, and various sensors and developing processes to allow large-area fabrication. The devices will then be integrated to develop artificial skins.

1. S. Paydavosi, F. Yaul, A. Wang, F. Niroui, V. Bulovic, J. H. Lang, “MEMS switches employing active metal-polymer nanocomposites,” in IEEE MEMS International Conference, 2012, pp. 180-183.

Nano-electromechanical systems (NEMS) are an emerging area of research with potential applications as low-power switches for electronic circuits. The proliferation of electronics in both stationary and portable applications demands the development of more energy-efficient devices than are currently available. While solid-state silicon MOS-based transistor circuits, the dominant technology in today’s electronics, have greatly reduced their power requirements by aggressive scaling, the concurrent increase in off-state leakage current limits their energy efficiency. In contrast, microelectromechanical relays have been demonstrated with zero off-state currents and abrupt switching characteristics ^{[1] [2]} . As these and other electromechanical devices are shrunk to the nanoscale, their actuation voltages, and hence power requirements, are expected to be reduced significantly.

Our group recently presented a three-terminal electromechanical switch based on a piezoresistive polymer nanocomposite as the active material ^[3] . The metal-polymer composite consisted of a polydimethylsiloxane polymer matrix doped with 60 wt% nickel particles. A schematic diagram of this squeezable switch, or “squitch,” is shown in Figure 1. In its initial state, the conductive metal particles are separated by the insulating polymer matrix. Thus, the active material is highly resistive, and little current flows through the device (in the “off” state). When compressed, the metal-metal distances decrease until the onset of tunneling allows current to flow from source to drain (“on” state). The first-generation squitch demonstrated transistor-like behavior with drain-source conduction modulation over 4 orders of magnitude when electromechanical force was applied. However, the large mechanical dimensions of this concept demonstration necessitated higher supply voltages than desired. Our current work focuses on incorporating the squitch concept into nanoscale devices by (a) developing improved device structures and fabrication methods and (b) exploring new materials such as ligand-coated nanoparticles and self-assembled monolayers as active materials.

Figure 1: Schematic diagram of squitch. Applying a voltage bias between gate and source generates an electrostatic force that compresses the active material and allows carriers to tunnel from source to drain. When the voltage bias is removed, the active material acts as a mechanical spring and recovers from its compressed state to cut off current flow through the device.

1. M. Spencer, F. Chen, C. C. Wang, R. Nathanael, H. Fariborzi, A. Gupta, K. Hei, V. Pott, J. Jaeseok, L. T.-J. King, D. Markovic, E. Alon, and V. Stojanovic, “Demonstration of integrated micro-electro-mechanical relay circuits for VLSI applications,” IEEE Journal of Solid-State Circuits, vol. 46, pp. 308-320, January 2011.
2. R. Parsa, M. Shavezipur, W. S. Lee, S. Chong, D. Lee, H. S. P. Wong, R. Maboudian, and R. T. Howe, “Nanoelectromechanical relays with decoupled electrode and suspension,” in IEEE 24th International Conference on Micro Electro Mechanical Systems, 2011, pp. 1361-1364.
3. S. Paydavosi, F. M. Yaul, A. I. Wang, F. Niroui, T. L. Andrew, V. Bulovic, and J. H. Lang, “MEMS switches employing active metal-polymer nanocomposites,” in IEEE 25th International Conference on Micro Electro Mechanical Systems, 2012, pp. 180-183.

A pressure sensor array is under development for unmanned undersea vehicles (UUV). This project is inspired by the lateral line sensory organ in fish, which enables some species to form three-dimensional maps of their surroundings ^{[1] [2]} . The canal subsystem of the organ can be described as an array of pressure-sensors ^[3] . The lateral line allows fish to perform a variety of actions, from tracking prey ^[4] to recognizing nearby objects ^{[2] [5]} .  Similarly, by measuring pressure variations on the surface of an UUV, an engineered pressure-sensor array supports the identification and location of obstacles for navigation.

To be compatible with the doubly-curved surface of a typical UUV hull, the pressure sensor array must be flexible.  Further, it is desirable that the array be amenable to wide-area fabrication. Correspondingly, the design pursued here is fabricated primarily from a PDMS polymer, some parts of which are doped with conducting nanoparticles so as to become piezoresistive.  As shown in Figure 1 below, a pressure sensor array consists of piezoresistive strain-gauges patterned onto PDMS membranes suspended over cavities formed in a PDMS substrate ^[6] .  The resistance of each strain gauge is measured using a four-point probe array with a common current source shared by all sensors. The strain-gauge resistance can be related to the deflection of its corresponding membrane, and hence the pressure difference across the membrane.  All cavities are connected together so that all pressure sensors have a common reference.

During the past year, flexible pressure sensor arrays were mounted on the side of a kayak for open-water tests, as shown in Figure 2a below. The pressure measurement from one sensor is shown in Figure 2b together with measurements from nearby commercial reference sensors.  The similarity of the measurements demonstrates the functionality of the PDMS pressure sensors in an uncontrolled environment.

1. J. C. Montgomery, S. Coombs, and C. F. Baker, “The mechanosensory lateral line system of the hypogean form of Astyanaxfasciatus,” Environmental Biology of Fishes, vol 62, pp 87-96, 2001.
2. C. von Campenhausen, I. Riess, and R. Weissert, “Detection of stationary objects by the blind cave fish Anoptichthys jordani (Characidae),” Journal of Computational Physiology A, vol 143, pp 369-374, 1981.
3. S. Coombs, “Smart skins: Information processing by lateral line flow sensors,” Autonomous Robots, vol 11, pp 255-261, 2001.
4. K. Pohlmann, J. Atema, and T. Breithaupt, “The importance of the lateral line in nocturnal predation of piscivorous catfish,” Journal of Experimental Biology, vol 207, pp 2971-2978, 2004.
5. T.J. Pitcher, B. L. Partridge, and C. S. Wardle, “A blind fish can school,” Science, vol 194, pp 963-965, 1976.
6. F. M. Yaul, V. Bulovic and J. H. Lang, “A flexible underwater pressure sensor array using a conductive elastomer strain gauge,” in Proc. IEEE MEMS Workshop, pp 500-503, Paris, France, January 29 – February 2, 2012.

Collaborators

• M.G. Allen, Georgia Inst. of Tech.
• J.G. Brisson, MIT
• V. Bulovic, MIT
• J. L. Kirtley, Jr, MIT
• D. J. Perreault, MIT
• A.H. Slocum, MIT
• C.R. Sullivan, Dartmouth
• M.S. Triantafyllou, MIT
• D.L. Trumper, MIT
• E.N. Wang, MIT

Graduate Students

• M. Araghchini, EECS
• K. Cao, EECS
• S. Chang, EECS
• M. E. D’Asaro, EECS
• N. Farve, EECS
• S. Hou, EECS
• F. Niroui, EECS
• M. Woo, MSE

Postdoctoral Associate

• A. Wang, MTL & RLE

Support Staff

• D. Bizi, Admin. Asst. I

Publications

A. Murarka, C. Packard, F. Yaul, J. H. Lang and V. Bulovic; Micro-contact printed MEMS; Proceedings: IEEE Workshop on Microelectromechanical Systems, 292-295, Cancun, Mexico, January 23-27, 2011.

M. Ryou, P. Cantillon-Murphy, D. Azagury, S. N. Shaikh, G. Ha, I. Greenwalt, M. B. Ryan, J. H. Lang and C. C. Thompson; Smart self-assembling magnets for endoscopy (SAMSEN) for transoral endoscopic creation of immediate gastrojejunostomy (with video); Gastro-intestinal Endoscopy, 73, 353-359, February 2011.

A. Sprunt, A. H. Slocum and J. H. Lang; Fracture fabrication of precision single-crystal silicon surfaces for MEMS devices; Precision Engineering, 35, 406-415, July 2011.

J. M. Allison, W. L. Staats, M. McCarthy,  D. Jenicek, A. K. Edoh, J. H. Lang, E. N. Wang and J. G. Brisson; Enhancement of convective heat transfer in an air-cooled heat exchanger using interdigitated impeller blades; International Journal of Heat and Mass Transfer, Online at doi: 10.1016/j.ij-heatmasstransfer.2011.06.023, July 2011.

M. D. Ilic and J. H. Lang; Complexity of voltage and reactive power dispatch in control centers: from analysis to on-line decision making; Proceedings: IEEE PES General Meeting, Paper 2011GM0998, Detroit, MI, July 24-29, 2011.

V. I. Fernandez, A. Maertens, F. M. Yaul, J. Dahl, J. H. Lang, and M. S. Triantafyllou; Lateral-line inspired sensor arrays for navigation and object identification; Marine Technology Society Journal, 45, 130-146, July/August 2011.

V. I. Fernandez, J. Dusek, J. Schulmeister, A. Maartens, S. Hou, K. Srivatsa, J. Dahl, J. H. Lang and M. S. Triantafyllou; Pressure sensor arraysto optimize high-speed performance of ocean vehicles; Proceedings: 11^th International Conference of Fast Sea Transportation (FAST 2011), Honolulu, HI, September 2011.

S. Sato, S. Jovanovic, J. H. Lang and Z. S. Spakovszky; Demonstration of a palm-sized 30-Watt air-to-power turbine generator; ASME Journal of Engineering for Gas Turbines and Power, 133, #102301/1-10, October, 2011.

M. Ilic, J. H. Lang, E. Litvinov, X. Luo, J. Tong, B. Fardanesh and G. Stefopoulos; Toward the Coordinated Voltage-Control-Enabled HV Smart Grids; Proceedings: IEEE PES Conference on Innovative Smart Grid Technologies (ISGT Europe), Manchester, UK, December 5-7, 2011.

S. Paydavosi, F. M. Yaul, A. I. Wang, F. Niroui, T. L. Andrew, V. Bulovic and J. H. Lang; MEMS switches employing active metal-polymer nanocomposites; Proceedings: IEEE Workshop on Micro-electromechanical Systems, 180-183, Paris, France, January 29 – February 2, 2012.

A. Murarka, S. Paydavosi, T. L. Andrew, A. I. Wang, J. H. Lang and V. Bulovic; Printed MEMS membranes on silicon; Proceedings: IEEE Workshop on Microelectromech-anical Systems, 309-312, Paris, France, January 29 – February 2, 2012.

F. M. Yaul, V. Bulovic and J. H. Lang; A flexible underwater pressure sensor array using a conductive elastomer strain gauge; Proceedings: IEEE Workshop on Micro-electromechanical Systems, 500-503, Paris, France, January 29 – February 2, 2012.

T. B. Peters, H. A. Kariya, D. F. Hanks, W. Staats, J. Allison, D. Jenicek, M. Cleary, M. McCarthy, J. H. Lang, J. G. Brisson and E. N. Wang; Fabrication and characterization of a high-performance air-cooled heat exchanger with an integrated multi-condenser heat pipe; Proceedings: 37th GOMACTech Conference, Las Vegas, NV, March 19-22, 2012.

F. M. Yaul, V. Bulovic and J. H. Lang; A flexible underwater pressure sensor array using a conductive elastomer strain gauge; IEEE/ASME Journal of Microelectromech-anical Systems. Accepted.