2, 3D Systems Inc., Rock Hill, South Carolina, United States
Flexible electronics are devices that can be bent, folded, stretched, or conformed regardless of their material composition without losing the electronic functionality. These electronics are employed in healthcare – designing stretchable electronic skins or lightweight smart sensors conformal to human body for biomonitoring and energy harvesting applications. Despite their increasing demands, only a handful of these devices have been commercialized due to the lack of novel functional materials available along with the complex fabrication mechanisms needed to process them. Unlike conventional silicon-based microelectronics manufacturing that is limited to rigid wafers, flexible electronics need to be incorporated onto plastics, paper, fibers and even biological tissues – necessitating low temperature processing. In addition, these devices need to be inexpensive and customizable according to an individual’s body needs with short manufacturing lead times. Polymers are the most common materials to be 3D printed today due to the simplicity of extruding them in molten form that quickly cools and hence solidifies. However, there is a great demand for developing methods to easily print metals. Current methods for additive manufacturing of metals tend to be prohibitively expensive, and use energy-intensive lasers at high sintering temperatures in excess of 800°C. Secondly, they need vacuum-like pressures to avoid oxidation while handling metal nanoparticles, leading to porosity in finished parts, low resolution and poor electrical conductivity, apart from having slow printing speeds. Finally, the operating procedures are impossible to integrate with various polymeric, organic, soft and biological materials. Here, we present a simple approach that utilizes low melting point gallium-based alloys that offer the electrical and thermal benefits of various metals like gallium and indium, combined with the ease of printing due to its low viscosity. Despite having high surface tension, these metals build mechanically stable structures due to the formation of a thin surface oxide. The oxide skin forms spontaneously in presence of air allowing us to direct-write planar as well as free-standing, out-of-plane conductive microstructures at room temperature, down to a resolution of ~10 microns, on-demand, using a pneumatic dispensing robot at relatively low pressures. We have demonstrated rapid prototyping of functional electronics such as flexible and stretchable antennas for defense communications, consumer-friendly electronic devices like laser pointers and inductive power coils for wireless charging of smartphones, and wearable thermoelectric generators for energy-harvesting applications. We have also exhibited the patterning of 3D multilayered microchannels with vasculature using liquid metals as a sacrificial template at room temperature that can be embedded in lab-on-a-chip devices to enable inexpensive fabrication of personalized healthcare sensors.