Liquid-metal-based stretchable bioelectronics can conform to the dynamic movements of tissues and enable human-interactive biosensors to monitor various physiologic parameters. However, the fluidic nature, surface oxidation, and low biostability of the liquid metals have limited the long-term use of bioelectronics. Here we have developed a rationally designed material engineering approach to overcome these challenges in liquid metal bioelectronics. To our knowledge, this is the first demonstration of stretchable, leak-free, and highly conductive gallium-based bioelectronic devices with exceptional biostability and electrochemical properties. We first utilized unique gallium oxide properties to create 3D microscale wrinkled structures on the gallium surface. Then, gold nanoparticles and biostable poly(3,4-ethylenedioxythiophene) were successively deposited on the wrinkled liquid metal surface. We demonstrated this multilayer encapsulation material could conform to the stretching deformation and showed excellent environmental stabilities while maintaining high electrical properties. Electromyographic measurements were used to evaluate the bioelectrical performance of the stretchable electronics, and the results demonstrated the encapsulated liquid metal device could outperform bare liquid metal devices. Finally, a sensory feedback study demonstrated our liquid metal bioelectronic device could record precise physiologic signals to control robots for mimicking dexterous hand gestures. This study opens the possibility of chronic liquid-metal-based stretchable bioelectronics.

Most neural stimulators do not have a high enough compliance voltage to pass current through the skin. The few stimulators that meet the high compliance voltage necessary for transcutaneous stimulation are typically large benchtop units that are not portable, and the stimulation waveforms cannot be readily customized. To address this, we present the design and validation of a portable, programmable, multichannel, noninvasive neural stimulator that can generate three custom bipolar waveforms at ± 150 V with microsecond temporal resolution. The design is low-cost, open-source, and validated on the benchtop and with a healthy population to demonstrate its functionality for sensory and motor stimulation. Sensory stimulation included electrocutaneous stimulation targeting cutaneous mechanoreceptors at the surface of the skin and transcutaneous nerve stimulation targeting the median nerve at the wrist. Both electrocutaneous stimulation on the hand and transcutaneous stimulation at the wrist can elicit isolated tactile percepts on the hand but changes in pulse frequency are more discriminable for electrocutaneous stimulation. Also, neuromuscular electrical stimulation of the flexor digiti profundus is evoked by applying electrical stimulation directly above the muscle in the forearm and to the median and ulnar nerves in the upper arm. Muscle and nerve stimulation evoked similar grip forces and force rise times, but nerve stimulation had a significantly slower fatigue rate. The development and validation of this noninvasive stimulator and direct comparison of common sensory and motor stimulation targets in a human population constitute an important step towards more widespread use and accessibility of neural stimulation for education and research.

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