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Arrays of Ultracompliant Electrochemical Dry Gel Cells for Stretchable Electronics

Stretchable electronics is the building or embedding of electronic circuits and devices in compliant material. Substrate and interconnects should be made stretchable rather than flexible or rigid (as is the case in flexible electronics or printed circuit boards). Whereas application of flexible electronics is limited to flat substrates, stretchable electronics can cover moving parts, such as joints in robotic elements, and also curved substrates or unusual materials such as silk, paper, leather etc. Despite extensive efforts to advance stretchable electronics, including the integration of active components like diodes, transistors and integrated circuits, as well as sensors and actuators, surprisingly no solution for integrating a power supply into such electronic products has so far been found. Here we demonstrate dry gel cells that withstand stretch ratios up to 100% before failure; deliver open circuit voltages close to 1.5 V and short circuit currents up to 30mA, a lifetime of more than 1000 h and capacities of 3.5 mAh cm 2 active area. The fabrication process allows for mass production with roll-to-roll techniques based on printing and laminating. With compliant interconnects, the dry cells can be connected in series or in parallel to form arrays. Such arrays of gel cells allow the construction of self-powered stretchable electronic items. The future in electronics is flexible and stretchable, electronic items are thought to be used in settings were to date electronic functionalities are currently not available. In flexible electronics, an astonishing variety of devices have been demonstrated, including solar cells, active matrices of field-effect transistors and memories, integrated circuits, actuators, displays, and transponders. Flexible power supplies are also available, including printed batteries and supercapacitors. However, a similar maturity has not been achieved in stretchable electronics; there are no stand alone applications possible due to the lack of concepts for powering such items. Batteries are electrochemical cells that are used to convert stored chemical energy into electrical energy. Dry cells are common power sources in many household and industrial applications, being a multimillion dollar market. Zinc carbon, alkali manganese, or lithium ion cells are among the best known elements. Flexible batteries have been shown for a variety of basic cells; making these systems ultra-compliant is, however, a nontrivial task, since the batteries are not allowed to be internally short-circuited upon mechanical stretching. Therefore, designs currently available in printed batteries will not work for compliant ones. Supercapacitors may be an alternative to batteries in powering conformable electronics; first attempts have been reported with a maximum current below 1mA at a voltage of 1V, too low for practical purposes. Our concept for ultra-compliant and mechanically robust dry cells that can deliver power to stretchable electronics is based on the integration of highly elastic carbon black silicon oil paste electrodes into a stretchable acrylic elastomer (VHB 4910 from 3M, introduced for dielectric elastomer actuators). A plotted cell of zinc, carbon, and xanthan forms the cathode, whereas the printed anode consists of manganese dioxide, carbon, and electrolyte (NH4Cl, ZnCl2) pastes. The resulting gel cell is depicted in the scheme of Figure 1. To avoid intermixing of the chemicals upon stretching (which would cause internal short circuits and damage of the battery) it is crucial to laterally separate the two 1 cm Zn andMnO2 containing electrodes by a distance of 0.3 cm. Thus, high stretch ratios are achieved as documented below. A printed xanthan electrolyte gel is closing the power cell circuit. Finally, the stretchable power supply is sealed by lamination of another layer of the acrylic elastomer, resulting in a total thickness of 2mm for the whole element. Such batteries with a lateral separation of anode and cathode can easily be arranged in arrays; with elastic conductors they can be connected in parallel to enhance the output current or in series to enhance the voltage. Thereby, distributed power sources are easily generated to drive stretchable electronic devices. Different power levels are common in nowadays devices, flash memories for example require at least three power levels, one for reading and two for writing and erasing the memories. Charge pumps provide an interesting means for delivering different power levels from one supply, but they are currently unavailable in stretchable electronics. The concept of arrays of batteries in a single elastomer matrix is illustrated with the experiment depicted in Figure 2. Figure 2a shows a sketch of two gel cells connected in series to enhance the voltage. The two batteries power an SMD light emitting diode (green emitting SMD LED with an operating voltage of 2–2.6 V and a current consumption between 3 and 20mA), following the stiff island elastic interconnect approach. The whole circuit is subject to uniaxial stretching in Figures 2b–e and biaxial stretching in Figure 2f. Figure 2b shows a photo of the device before stretching, Figure 2c shows the dry gel cells stretched to 25% of their initial length, and Figure 2d proves the mechanical