Epidermal electronic systems
Mike Fury, Techcet Group
Scientists have developed a new type of ultra-thin, self-adhesive electronics device that can effectively measure data about the human heart, brain waves and muscle activity - all without the use of bulky equipment, conductive fluids, or glues.
Presenting at the MRS Spring 2012 meeting in San Francisco, Nanshu Lu (now at UT Austin) of the John Rogers group at U Illinois Urbana discussed the groups' recent achievements in epidermal electronic systems. Micro-transfer printing is the method of choice for interconnecting small rigid silicon electronics elements with thin nanoribbons of silicon or metal. Depositing onto a pre-stretched elastomer substrate provides a resting state in which the interconnects are buckled or canted and can endure up to 100% elongation while imparting ???1% stress to the rigid circuit elements.
The trick of fabricating extremely thin silicon for flexibility applies to the PDMS polymer substrate as well when the objective is to apply the device to the skin and tolerate stretching and bending without adhesion loss. The thin polymer stability is maintained until it is applied using technology similar to that used in applying temporary tattoos. For some device types, the rigid silicon electronics can be eliminated by integrating the active device elements into the serpentine interconnects themselves. For development of integrated devices, functions that have been demonstrated include amplifiers, temperature sensors, strain gauges, solar power sources, induction couplers and wireless transmitters & receivers for device control. Current devices, however, use wires to connect to external control and power sources. The only three elements in contact with the skin are gold, silicon and polyimide, all of which are FDA approved.
Also presenting at MRS, Michael Melzer of IFW Dresden extended the family of stretchable electronics from silicon and optoelectronics to now include magneto electronics. Stretchable GMR multilayers are fabricated by depositing GMR thin films on a pre-strained PDMS substrate. Data indicates no loss of magnetic performance through this process to 2.5% strain even though resistance starts to rise above 1.6% strain. For greater detection sensitivity, stretchable spin valves were developed using the same process flow as for the GMR multilayers. After some refinement of the process, they were able to achieve 29% strain without losing functionality or sensitivity.
A Gaikwad of City College NY described a stretchable battery embedded in cloth with Zn and MnO2 as the active materials. Cracking and delamination due to flexing and stretching was addressed by embedding these materials in a non-conducting nylon mesh in an earlier version. In the new version, a silver coated nylon cloth is used as the substrate for the Zn electrode and separately for the MnO2 electrode. No delamination or electrical degradation was observed at 100% strain in either the x- or the y-direction. The capacity of 4 mAh/cm2 was maintained even with this stretching 100% level.
Solid State Technology, Volume 55, Issue 5, June 2012