Christian M. Siket

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

Intrinsically stretchable and rechargeable batteries for self-powered stretchable electronics

Stretchable electronic circuits conform to irregular three dimensional surfaces. They are formed with soft materials and contain electronic circuits, sensors, and other components. We report on a soft matter based rechargeable electrochemical power storage element for such devices. The chemistry is based on a rechargeable alkaline manganese battery concept. The cells withstand more than 700 mechanical stretch relaxation cycles up to 25% strain, with an average cell capacity of 6.5 mA h. Combined with wireless power transmission or stretchable solar cells, the rechargeable battery can be used to store and supply energy in stretchable electronic devices.

Instant tough bonding of hydrogels for soft machines and electronics

A strategy for bonding water-rich hydrogels to diverse materials for electronic skins, energy storage, and soft optics is reported. Introducing methods for instant tough bonding between hydrogels and antagonistic materials—from soft to hard—allows us to demonstrate elastic yet tough biomimetic devices and machines with a high level of complexity. Tough hydrogels strongly attach, within seconds, to plastics, elastomers, leather, bone, and metals, reaching unprecedented interfacial toughness exceeding 2000 J/m2. Healing of severed ionic hydrogel conductors becomes feasible and restores function instantly. Soft, transparent multilayered hybrids of elastomers and ionic hydrogels endure biaxial strain with more than 2000% increase in area, facilitating soft transducers, generators, and adaptive lenses. We demonstrate soft electronic devices, from stretchable batteries, self-powered compliant circuits, and autonomous electronic skin for triggered drug delivery. Our approach is applicable in rapid prototyping and in delicate environments inaccessible for extended curing and cross-linking.

High‐Frequency, Conformable Organic Amplifiers

Large-bandwidth, low-operation-voltage, and uniform organic amplifiers are fabricated on ultrathin foils. By the integration of short-channel OTFTs and AlOx capacitors, organic amplifiers with a bandwidth of 25 kHz are realized, demonstrating the highest gain-bandwidth product (GBWP) reported to date. Owing to material and process advancements, closed-loop architectures operate at frequencies of several kilohertz with an area smaller than 30 mm(2) .

From Playroom to Lab: Tough Stretchable Electronics Analyzed with a Tabletop Tensile Tester Made from Toy-Bricks

Toy bricks are an ideal platform for the cost‐effective rapid prototyping of a tabletop tensile tester with measurement accuracy on par with expensive, commercially available laboratory equipment. Here, a tester is presented that is not only a versatile demonstration device in mechanics, electronics, and physics education and an eye‐catcher on exhibitions, but also a powerful tool for stretchable electronics research. Following the “open‐source movement” the build‐up of the tester is described and all the details for easy reproduction are disclosed. A a new design of highly conformable all‐elastomer based graded rigid island printed circuit boards is developed. Tough bonded to this elastomer substrate are imperceptible electronic foils bearing conductors and off‐the‐shelf microelectronics, paving the way for next generation smart electronic appliances.

Direct writing of anodic oxides for plastic electronics

Metal oxide thin films for soft and flexible electronics require low cost, room temperature fabrication, and structuring processes. We here introduce an anodic printing process to realize the essential building blocks of electronic circuitry, including resistors, capacitors, field-effect transistors, diodes, rectifiers, and memristors directly on imperceptible plastic substrates. Largely independent on surface properties, we achieve high-quality, few nanometer thin dielectric and semiconducting films even on rough substrates via localized anodisation of valve metals using a scanning droplet cell microscope. We demonstrate printing-like fabrication of 3D multilayer solid-state capacitors with a record-high areal capacity of 4 µF cm−2. Applicable to the whole class of valve metals and their alloys, our method provides a versatile fabrication technique for the circuits that empower the flexible and stretchable electronics of tomorrow.Flexible circuits: anodization makes them all!A simple concept of scanning head-guided anodization is shown to be highly expandable to fabricate various electronic components. A team led by Professor Siegfried Bauer from Johannes Kepler University Linz, Austria develops a universal and patternable printing protocol of anodic oxides for a full range of circuit components for flexible devices. The researchers employ a scanning droplet cell microscope to anodize the pre-deposited thin metal films to form dielectric layers with good control in both lateral dimension and vertical thickness. They demonstrate the versatility of the on-site anodization methods by fabricating oxides-based resistors, diodes, transistors and memristors, and multilayer capacitors with a record-high areal capacity of 4 µF cm−2. The approach is cheap, adaptable, and thus ideal for rapid-prototyping of metal oxides circuits for various applications.