Stretchable devices such as soft robots and wearable healthcare devices are assembled with the help of various modules having different material characteristics like softness or rigidity. However, the commercial pastes presently being applied for connecting the modules are usually unable to transmit mechanical and electrical signals with reliability when they become deformed or break easily. For creating a reliably functioning device, module connectors (interfaces) have to be custom-built with sufficient strength to perform the tasks for which they are built. Easily assembling stretchable devices while retaining their strength and reliability under stress still remains a challenge that has limited their development.

Now, an international team led by researchers from Nanyang Technological University, Singapore (NTU Singapore, Nanyang Ave, Singapore) has developed a universal connector for assembling stretchable devices simply and quickly. Their BIND interface (biphasic, nano-dispersed interface) simplifies the assembly of stretchable devices and also offers an excellent mechanical and electrical performance. Similar to building structures using Lego blocks, it is possible to assemble high-performing stretchable devices by just pressing together any module bearing the BIND interface. This easy and simple method of connecting electronic modules could allow producers to assemble future stretchable devices by using ‘plug-and-play’ components based on their designs.

The researchers developed the BIND interface by thermally evaporating metal (gold or silver) nanoparticles to create a robust interpenetrating nanostructure inside a soft thermoplastic generally used in stretchable electronics (styrene-ethylene-butylene-styrene). This nanostructure offers continuous mechanical as well as electrical pathways, enabling modules with BIND connections to remain robust despite being deformed. The team conducted experiments in which the modules joined by the interface demonstrated an excellent performance. During stretching tests, the modules could be stretched up to seven times their original length before finally breaking. In addition, the electrical transmission of the modules remained robust up to 2.8 times the original length when stretched. The researchers also evaluated the interfacial toughness of the BIND interface by using a standard Peel Adhesion Test, in which the adhesive strength between two modules is put to test by pulling it apart at a constant speed at 180°. In the case of encapsulation modules, the researchers found the BIND interface to be 60 times tougher than conventional connectors.

Additionally, the researchers demonstrated the feasibility of its use in real-life applications by building stretchable devices using the BIND interface, and then testing them on rat models and human skin. The recordings from the stretchable monitoring device attached to rat models displayed reliable signal quality despite interferences with the wirings like touching or tugging. The device stuck on human skin managed to collect high-quality electromyography (EMG) signals which measure the electrical activity generated in muscles during contraction and relaxation, even underwater. The research team has filed an international patent and is now developing a more efficient printing technology to expand the material choice and final application of their innovation. This will accelerate its transition from the laboratory to the designing and manufacturing of products.

“Our breakthrough innovation makes it very 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,” said Chen Xiaodong, lead author of the study. “Moreover, we do away with the cumbersome process of building customized interfaces for specific systems, which we believe will help accelerate the development of stretchable devices.”

“These impressive results prove that our interface can be used to build highly functional and reliable wearable devices or soft robots,” added Dr. Jiang Ying, Research Fellow at the NTU School of Materials Science & Engineering. “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.” 

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