Practical Considerations for Gallium-Based Liquid Metal for Stretchable Electronics: Metal Contacts and Strain Cycling

Gallium (Ga)-based liquid metals (LMs) are promising for stretchable electronics due to their metallic conductivity and liquid-like deformability. As a step toward utilizing Ga-based LM in commercial devices, we sought to address two practical considerations: (1) The stability and contact resistance of electrical contacts between solid metal and LM, and (2) the stability of LM in response to strain cycling. We studied the long-term (∼1000 h) interfacial stability between LM and conventional metals used in electronics, such as copper (Cu), tin (Sn), and silver (Ag), at elevated temperature (100 °C). The interface between Ga and Cu forms a thin layer of CuGa2 and Cu9Ga4 intermetallic that separates Ga from the Cu and thereby significantly slows down degradation and provides stable contact resistance relative to other common metals such as Sn or Ag, which dissolve rapidly into the Ga. Additionally, we demonstrated that LM wires maintain consistent resistance profiles during more than 15,000 strain cycles. The electromechanical response to strain cycling of the LM is much better than that of a commercial stretchable conductor (i.e., elastomeric composites containing Ag flakes). These results should lower the barrier to the practical implementation of LM in stretchable devices.