Assembling Lego-like stretchable electronic devices

An international team led by researchers from Nanyang Technological University, Singapore (NTU Singapore) has developed a universal connector to assemble stretchable devices simply and quickly, in a ‘Lego-like’ manner. Stretchable devices including soft robots and wearable healthcare devices are assembled using different modules with different material characteristics — some soft, some rigid and some encapsulated. However, the commercial pastes (glue) currently used to connect the modules often fail to transmit mechanical and electrical signals reliably when deformed, or break easily.

To create a reliably functioning device, module connectors (interfaces) must be custom-built with enough strength to perform their intended tasks. Making easily assembled stretchable devices without compromising strength and reliability under stress has been a challenge that limits their development. The NTU-led team reported in the journal Nature that their solution to this challenge, the BIND interface (biphasic, nano-dispersed interface), makes assembly of stretchable devices simple while offering enhanced mechanical and electrical performance.

Much like building structures with Lego blocks, high-performing stretchable devices can be assembled by pressing together any module bearing the BIND interface. This method of connecting electronic modules could form the basis of assembling future stretchable devices, in which producers ‘plug-and-play’ the components according to their designs.

The lead author of the study, Chen Xiaodong, President’s Chair Professor in Materials Science and Engineering at NTU Singapore, said the innovation makes it easy to form and use a stretchable device since it works like a ‘universal connector’. “Any electronic module bearing the BIND interface can be connected simply by pressing them together for less than 10 seconds. Moreover, we do away with the cumbersome process of building customised interfaces for specific systems, which we believe will help accelerate the development of stretchable devices,” Xiaodong said.

In experiments, modules joined by the interface showed excellent performance. When subjected to stretching tests, modules were able to withstand stretching of up to seven times their original length before breaking. The electrical transmission of modules also remained robust up to 2.8 times its original length when stretched. The BIND interface was also evaluated for its interfacial toughness using a standard Peel Adhesion Test, in which the adhesive strength between two modules is tested by pulling it apart at a constant speed at 180°. For encapsulation modules, researchers found the innovation to be 60 times tougher than conventional connectors.

Dr Jiang Ying, Research Fellow at the NTU School of Materials Science & Engineering, said these results prove that the interface can be used to build highly functional and reliable wearable devices or soft robots. “For example, it can be used in high-quality wearable fitness trackers where users can stretch, gesture and move in whichever way they are most comfortable with, without impacting the device’s ability to capture and monitor their physiological signals,” Ying said.

To demonstrate the feasibility of use in real-life applications, the researchers built stretchable devices using the BIND interface and tested them on rat models and human skin. When attached to rat models, recordings from the stretchable monitoring device showed reliable signal quality despite interferences on the wirings, such as touching and tugging. When stuck on the human skin, the device collected high-quality electromyography (EMG) signals that measure electrical activity generated in muscles during muscle contraction and relaxation, even underwater.

The BIND interface was developed by thermally evaporating metal (gold or silver) nanoparticles to form a robust interpenetrating nanostructure inside a soft thermoplastic commonly used in stretchable electronics (styrene-ethylene-butylene-styrene). The resulting nanostructure provides continuous mechanical and electrical pathways, allowing modules with BIND connections to remain robust even when deformed.

Professor Takao Someya from the University of Tokyo said this research represents an achievement in developing an electrical bonding technique with excellent elasticity, avoiding stress concentration at the interconnection between modules with different rigidity, thereby reducing noise generation. “Acceleration of industrial application of stretchable devices is anticipated with the establishment of an industrially scalable high-throughput manufacturing method,” Someya said.

According to Professor Shlomo Magdassi from The Hebrew University of Jerusalem, the new plug-and-play approach is significant because it will enable the rapid combining of different components by simple pressing to form devices with various functionalities and complexities, thereby accelerating development in the field of stretchable electronics.

An international patent has been filed for the innovation; the research team will now develop a more efficient printing technology to expand the material choice and final application of this innovation, to accelerate its translation from the lab to the design and manufacturing of products.

Top image caption: The BIND interface (biphasic, nano-dispersed interface) makes assembly of stretchable devices simple while offering excellent mechanical and electrical performance. Image credit: NTU Singapore.