Stretchable metallization technologies for skin-like transducers

The skin is not only the largest human organ, capable of accomplishing distributed and multimodal sensing functions. Replicating the versatility of skin artificially is a significant challenge, not only in terms of signal processing but also in mechanics. Stretchable electronics are an approach designed to cover human and artificial limbs and provide wearable sensing capabilities: motion sensors distributed on the hand of neurologically impaired patients could help therapists quantify their abilities; prostheses equipped with multiple tactile sensors could enable amputees to naturally adjust their grasp force. Skin-like electronic systems have specific requirements: they must mechanically adapt to the deformations imposed by the body they equip with minimal impediment to its natural movements, while also providing sufficient electrical performance for sensor transduction and passing electrical signals and power. A metallization ensuring stable conductivity under large strains is a prerequisite to designing and assembling wearable circuits that are integrated with several types of sensors. In this work, two innovative metallization processes have been developed to enable scalable integration of multiple sensing modalities in stretchable circuits. First, stretchable micro-cracked gold (Au) thin films were interfaced with gallium indium eutectic (EGaIn) liquid metal wires. The Au films, thermally evaporated on silicone elastomer substrates, combined high sheet resistance (9 to 30 Ohm/sq) and high sensitivity to strain up to 50%. The EGaIn wires drawn using a micro-plotting setup had a low gauge factor (2) and a low sheet resistance (5 mOhm/sq). Second, a novel physical vapor deposition method to deposit of thin gallium-based biphasic (solid-liquid) films over large areas was achieved. The obtained conductors combined a low sheet resistance (0.5 Ohm/sq), a low gauge factor (~1 up to 80% strain), and a failure strain of more than 400%. They could be patterned down to 10 µm critical dimensions. Skin-like sensors for the hand were assembled using the two processes and their capabilities were demonstrated. Thin (0.5 mm) silicone strips integrating EGaIN wires and micro-cracked Au strain gauges were mounted on gloves to encode the position of a biomimetic robotic finger and a human finger. In combination with soft pressure sensors, they enabled precise grasp analysis over a limited range of motion. Then, biphasic films were micro-patterned on silicone to assemble 50 µm thin epidermal strain gauges. The strain gauges were attached on a user's finger and accurately encoded fine grasping tasks covering most of the human hand range of motion. The biphasic films were also used to power wireless MEMS pressure sensors integrated in a rubber scaffold. The device was mounted on a prosthetic hand to encode normal forces in the 0 N to 20 N range with excellent linearity. The epidermal strain sensors are currently being used to quantify the tremors of patients with Parkinson's disease. In the future, the unique properties of the biphasic films could enable advanced artificial skins integrating a high density of soft transducers and traditional high-performance circuits.