Gallium-Based Thin Films for Wearable Sensors and Electronic Skins

Integrated wearable electronics capable of providing biophysical information on complex and dynamic systems have attracted high interest in diverse fields such as healthcare, gaming and robotics. Wearable sensors can create artificial, electronic skins with human-like sensory capabilities, but also control next-generation robots based on flexible and compliant materials. One of the key challenges in wearable technology is the ability to combine the desired device functionality with a satisfactory degree of integration with the host. Gallium and gallium-based liquid metals have recently gathered attention for their excellent combination of stretchability and electrical conductivity. However, liquid metals feature complex physical and chemical properties that pose challenges to the integration of useful soft and stretchable electronics. In this thesis, different strategies were explored to pattern gallium-based thin films for wearable sensors and electronic skins. Microfabrication techniques were combined to surface engineering to form soft and stretchable liquid metal electronic conductors embedded in silicone substrates. In the first chapter I present intrinsically stretchable, biphasic solid-liquid thin film electronic conductors obtained by thermal evaporation of gallium on flat, gold pre-coated PDMS substrates. Thin (< 20 Όm), highly conductive (Rs = 0.5 Ω / sq.), stretchable (400 % strain) and durable (1 million cycles) electrical conductors were manufactured and implemented into wearable strain sensors for accurate monitoring of the finger movements of a human hand. Despite their excellent stretchability and high electrical conductivity, the thin films displayed a short functional lifetime, poorly reproducible and unstable electromechanical properties over time. Structure-property investigations attributed these limitations to the heterogeneous morphology of the thin films. In the second chapter I describe my approach to tackle the aforementioned challenges by introducing smooth and homogeneous, gallium-based thin films electronic conductors obtained by thermal evaporation of gallium on textured, gold pre-coated PDMS substrates. The surface coating and the topography of silicone substrates were engineered to form homogeneous thin films (1.5 Όm) on large-areas (> 10 cm2) with highly reliable, reproducible and stable electromechanical properties. Demonstrations included stick-on motion sensors that could reliably monitor the movements of a human hand and replicate it with a virtual model and the first integration of a sensory skin made of liquid metals with vacuum-powered soft pneumatic actuators. In the third chapter I introduce a new manufacturing process combining soft lithography, self-assembly and physical vapor deposition techniques to pattern gallium features with micron-scale lateral resolution, high surface density over large areas. The process enables a range of channel designs and geometries that can be used to form miniaturized conductors with unprecedented performance, such as micrometer wide stretchable liquid metal conductors with large form factor (= 10000), highly stretchable (100 %) and transparent (T > 89 %) electronic conductors, and miniaturized arrays with high density of capacitance for proximity sensing. In the future, the developments reported in this thesis may enable liquid-metal based devices that open up new avenues in diverse fields such as human-machine interfaces, soft robotics and healthcare.