A highly soft, stretchable, and sticky polydimethylsiloxane-based elastomer is achieved by adding small fractions of an amine-based polymer. The modified elastomer is tuned with a simple mixing step and shows good processability for microstructure fabrication. The modified elastomer shows excellent compatibility with an epidermal strain sensor on human skin.

system. [11] Researchers have developed highly stretchable strain sensors made of compliant elastomers and various conductive materials, such as silver nanowire, [12] carbon nanotube (CNT), [13][14][15][16][17][18] carbon grease, [19] graphene, [20] graphite, [21] laser-carbonized polyimide, [22] conductive acrylic elastomer, [10] liquid metal, [23,24] ionic liquid, [25][26][27] and conductive fabric. [28] However, not all of these technologies can be manufactured in large scale at low cost.Here, we propose the use of carbon black (CB)-filled elastomer composites for highly stretchable strain sensors (up to 500%) that can be batch manufactured at low cost. CBs are a type of low-cost conductive nanoparticle, which, when used as a filler in an elastomeric matrix, enhances the mechanical strength, abrasion resistance, UV resistance, and light absorbency of the composite. [29][30][31] The CB-filled elastomer can be printed in large areas by means of a layer-by-layer process, [32] with good wettability and high adhesion to silicone surfaces. Mixing various types of CBs and elastomers [33] gives material designers flexibility to achieve high compliance and stretchability.Our layer-by-layer CB-filled elastomer fabrication process can be used to create resistive or capacitive sensors. [11] Resistive sensing relies on the piezoresistive effect and geometrical changes of electrodes, where mechanical strain causes a change in electrical resistivity. Capacitive sensing exploits changes of the capacitance between a pair of electrodes sandwiching a dielectric layer. Strain expands the area of the electrodes and reduces the thickness of the dielectric layer, leading to an increase of the capacitance. A recent review on strain sensors has pointed that resistive type strain sensors have high sensitivity but hysteresis and nonlinear response, while capacitive type strain sensors display excellent linearity and hysteresis performance but low sensitivity. [11] On the other hand, according to other literature, both resistive and capacitive type strain sensors show good linearity, low hysteresis, and repeatability. [10,13,15,28] Therefore, there is a lack of comprehensive knowledge of highly stretchable strain sensors that clarifies advantages and disadvantages of the two sensing methods. In addition, other characteristics, such as responses to different strain speed and temperature, have not yet been compared. This would result in difficulty when it is required to select an appropriate sensor The advent of soft robotics has led to the development of devices that harness the compliance and natural deformability of media with nonlinear elasticity. This has led to a need of batch-manufacturable soft sensors that can sustain large strains and maintain kinematic compatibility with the systems they track. In this article, an approach to address this challenge is presented with highly stretchable strain sensors that can operate at strains up to 500%. The sensors consist of a carbon black-filled elastomer composite that is batch manufacture...

Recently, microfluidic stretchable electronics has attracted great interest from academia since conductive liquids allow for larger cross-sections when stretched and hence low resistance at longer lengths. However, as a serial process it has suffered from low throughput, and a parallel processing technology is needed for more complex systems and production at low costs. In this work, we demonstrate such a technology to implement microfluidic electronics by stencil printing of a liquid alloy onto a semi-cured polydimethylsiloxane (PDMS) substrate, assembly of rigid active components, encapsulation by pouring uncured PDMS on-top and subsequent curing. The printing showed resolution of 200 μm and linear resistance increase of the liquid conductors when elongated up to 60%. No significant change of resistance was shown for a circuit with one LED after 1000 times of cycling between a 0% and an elongation of 60% every 2 s. A radio frequency identity (RFID) tag was demonstrated using the developed technology, showing that good performance could be maintained well into the radio frequency (RF) range.

Stretchable electronics offers unsurpassed mechanical compliance on complex or soft surfaces like the human skin and organs. To fully exploit this great advantage, an autonomous system with a self-powered energy source has been sought for. Here, we present a new technology to pattern liquid alloys on soft substrates, targeting at fabrication of a hybrid-integrated power source in microfluidic stretchable electronics. By atomized spraying of a liquid alloy onto a soft surface with a tape transferred adhesive mask, a universal fabrication process is provided for high quality patterns of liquid conductors in a meter scale. With the developed multilayer fabrication technique, a microfluidic stretchable wireless power transfer device with an integrated LED was demonstrated, which could survive cycling between 0% and 25% strain over 1,000 times.

Stretchable electronics and soft robotics have shown unsurpassed features, inheriting remarkable functions from stretchable and soft materials. Electrically conductive and mechanically stretchable materials based on composites have been widely studied for stretchable electronics as electrical conductors using various combinations of materials. However, thermally tunable and stretchable materials, which have high potential in soft and stretchable thermal devices as interface or packaging materials, have not been sufficiently studied. Here, a mechanically stretchable and electrically insulating thermal elastomer composite is demonstrated, which can be easily processed for device fabrication. A liquid alloy is embedded as liquid droplet fillers in an elastomer matrix to achieve softness and stretchability. This new elastomer composite is expected useful to enhance thermal response or efficiency of soft and stretchable thermal devices or systems. The thermal elastomer composites demonstrate advantages such as thermal interface and packaging layers with thermal shrink films in transient and steady-state cases and a stretchable temperature sensor.

Conventional thermoelectric generators (TEGs) are normally hard, rigid, and flat. However, most objects have curvy surfaces, which require soft and even stretchable TEGs for maximizing efficiency of thermal energy harvesting. Here, soft and stretchable TEGs using conventional rigid BiTe pellets metallized with a liquid alloy is reported. The fabrication is implemented by means of a tailored layer-by-layer fabrication process. The STEGs exhibit an output power density of 40.6 μW/cm at room temperature. The STEGs are operational after being mechanically stretched-and-released more than 1000 times, thanks to the compliant contact between the liquid alloy interconnects and the rigid pellets. The demonstrated interconnect scheme will provide a new route to the development of soft and stretchable energy-harvesting avenues for a variety of emerging electronic applications.

In order to make conductors with large cross sections for low impedance radio frequency (RF) electronics, while still retaining high stretchability, liquid-alloy-based microfluidic stretchable electronics offers stretchable electronic systems the unique opportunity to combine various sensors on our bodies or organs with high-quality wireless communication with the external world (devices/systems), without sacrificing enhanced user comfort. This microfluidic approach, based on printed circuit board technology, allows large area processing of large cross section conductors and robust contacts, which can handle a lot of stretching between the embedded rigid active components and the surrounding system. Although it provides such benefits, further development is needed to realize its potential as a high throughput, cost-effective process technology. In this paper, tape transfer printing is proposed to supply a rapid prototyping batch process at low cost, albeit at a low resolution of 150 μm. In particular, isolated patterns can be obtained in a simple one-step process. Finally, a stretchable radio frequency identification (RFID) tag is demonstrated. The measured results show the robustness of the hybrid integrated system when the tag is stretched at 50% for 3000 cycles.

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