2, 3D Systems Inc., Rock Hill, South Carolina, United States
In 2014, TIME Magazine heralded Additive Manufacturing / 3D printing as one of the 25 best inventions of all-time signifying that it can lead to a decentralized and highly customizable manufacturing technique in the future. 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. Currently available commercial methods for additive manufacturing of metals tend to be prohibitively expensive requiring upwards of $500,000 in capital investments, and use energy-intensive lasers with techniques that need high sintering temperatures in excess of 800°C. In addition, they need special environments including vacuum-like low pressures to avoid oxidation while handling metal nanoparticles, making it a very messy process leading to porosity in finished parts, low resolution and poor electrical conductivity due to presence of some non-sintered powder particles with organic viscous binders, apart from having slow printing speeds as compared to conventional subtractive manufacturing methodologies. Finally, the operating and processing procedures are almost impossible to be integrated with co-printing of various polymeric, organic, soft and biological materials on the same equipment. Here, we present an alternate but simple approach that utilizes low melting point gallium-based alloys as complements to the existing materials for 3D printing metals and co-printing them with polymers at room temperature. Gallium-based liquid metal alloys offer the electrical and thermal benefits of various metals like gallium and indium, combined with the ease of printing due to its low viscosity (~2x water). Despite having high surface tension (~10x water), these metals build mechanically stable structures due to the formation of a thin (~3 nm thick) surface oxide. The oxide skin is passivating, forms spontaneously in presence of air or dissolved oxygen on the surface of the metal and allows us to direct-write planar as well as free-standing, out-of-plane conductive microstructures down to a resolution of ~10 μm, on-demand, using a 4-axis pneumatic dispensing robot customized from a desktop CNC machine at relatively low pressures (~10s of kPa). We have demonstrated rapid prototyping of functional electronics such as flexible and stretchable antennas for radio-frequency defense communications, as well as consumer-based 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 these printed liquid metals as a sacrificial template at room-temperature that can be employed in numerous lab-on-a-chip devices to enable inexpensive fabrication of personalized healthcare sensors.