Recyclable Printed Liquid Metal Composite for Underwater Stretchable Electronics

Multifunctional stretchable conductors are crucial components in fully stretchable circuits for wearable bioelectronics. Conductive composites made from liquid metal (LM) fillers and polymer matrices have garnered significant interest due to their high electrical conductivity, adjustable mechanical properties, biocompatibility, and recyclability. Herein, a printable LM composite is developed using a custom-designed block copolymer to ensure electromechanical stability in both wet and dry conditions. The LM composite demonstrates high conductivity (around 10⁵ S m⁻¹), stretchability up to 500%, and maintains stable resistance with cyclic strain ranging from 0 to 50% for over 16 h, in both ambient and aqueous environments. Furthermore, bulk LM is successfully recovered from printed composites using green solvents, supporting the composite’s recyclability.

The synthesized DDMAT-terminated PAA and PASTA samples were examined in deuterated chloroform using H-NMR spectroscopy. Spectra were obtained using a Bruker AVANCE II 400 spectrometer operating at 400 MHz to confirm the chemical structures of DDMAT-terminated PAA and PASTA. For GPC sample preparation to determine polymer molecular weight distribution, PASTA was diluted to a concentration of 0.2 wt% in THF. The analysis was performed using a 1260 Infinity GPC system equipped with a WAT044228 Styragel HR 5 E column manufactured by Waters. THF was used as the eluent and was maintained at 30 °C at a flow rate of 1 mL min⁻¹. In this study, we achieved a polydispersity index (PDI) of 2 for free-radical polymerization. DSC analysis (Q2000, TA Instruments, USA) was conducted to determine the glass transition temperature (T_g) of the PASTA samples. ≈12 mg of the sample were loaded into a standard aluminum pan and subjected to a constant flow of nitrogen. To ensure the accurate representation of the thermal behavior without thermal history, the samples were initially cooled to −30 °C and then heated to 200 °C at a rate of 10 °C min⁻¹. Subsequently, a second heating cycle was performed from −30 to 200 °C at the same heating rate. The analysis was based on data collected during the second heating cycle. To prepare the samples for tensile tests, 50 wt% PASTA was dissolved in DMF. The solution was drop cast in a Teflon dog-bone mold and dried at 30 °C on a hotplate for 24 h. PASTA dog bone with ≈4.1 mm width, ≈10.3 mm length, and ≈1.2 mm thickness was obtained. The tensile tests were performed using a Mach-1 V500cst MA009 mechanical tester (Biomomentum Inc., Canada) at a stretching speed of 500 mm min⁻¹.