Structural design of wearable electronics suitable for highly-stretched joint areas

Developing new generation wearable electronics with skin-like stretchability and conformability has become the research hotspot in many areas, such as bio-sensoring, communication and robotics. In this study, a novel design and manufacturing method was proposed to develop wearable electronics, which could perform a similar deformation as the target skin surface during the rotation of joint. Considering the nonuniform auxetic deformation behaviour of the skin around joint areas, the designed electronics should also have the same Poisson's ratio (PR) distribution. Re-entrant honeycomb structure was chosen to obtain tuneable PR, by changing the interior angel of each unit cell. By mapping unit cells with different PR to the target surface, the obtained structure would have the same mechanical property and deformation behaviour as the covered skin. Consequently, there would be little sheer stress at the connection areas between wearable electronics and human skin, which would surely increase the attachment stability. Besides, the conformability of the device could also be generated. PloyJet process was introduced to fabricate the flexible substrate, and silver ink was printed directly on the substrate with direct-ink-writing (DIW) technology. Conductive wire with 'wavy' structure was manufactured by this PolyJet-DIW process, and it had shown a much better stretchability than the common straight-line structure. Furthermore, wearable electronics that suitable for a person-specific elbow was developed. Both the conformability and conductivity were tested, and all results had shown much better performance than traditional wearable electronics with uniform structures.