Microsolidics: Fabrication of Three-Dimensional Metallic Microstructures in Poly(dimethylsiloxane)

This Communication describes a method of fabricating complex metallic microstructures in 3D by injecting liquid solder into microfluidic channels, and allowing the solder to cool and solidify; after fabrication, the metallic structures can be flexed, bent, or twisted. This method of fabrication—which we call microsolidics—takes advantage of the techniques that were developed for fabricating microfluidic channels in poly(dimethylsiloxane) (PDMS) in 2D and 3D, uses surface chemistry to control the interfacial free energy of the metal– PDMS interface, and uses techniques based on microfluidics, but ultimately generates solid metal structures. This approach makes it possible to build flexible electronic circuits or connections between circuits, complex embedded or freestanding 3D metal microstructures, 3D electronic components, and hybrid electronic–microfluidic devices. There are several techniques for making metal microstructures in 3D. Electroplating and electroless deposition are routinely used to construct microstructures with metallic layers several nanometers to several microns thick in 2D or 3D. To generate solid replicas of 3D objects, several groups have developed a technique, referred to as “microcasting”, to form metals in order to fabricate microparts (e.g., posts and gears) with features as small as 10 lm and aspect ratios as high as 10 from steel, zirconia, and alumina. Techniques based on LIGA (Lithographie, Galvanoformung, und Abformung) produce even more complicated metallic objects by depositing a metal onto a molded polymer template that is subsequently removed to yield an open structure (such as a honeycomb arrangement of cells). In principle, these approaches can be used to pattern metals of any thickness to produce features with an aspect ratio that is larger than that produced using electroplating. Solder reflow is a standard technique in electronic packaging, and has recently been combined with micromolding in channels to form custom-made solder pieces for batches of chips. The technique has also been used to form 3D connections (e.g., bridging opposite sides of an electronic circuit board or substrate): Lauffer and co-workers and Ference and co-workers describe similar approaches to bridge electrical “islands” of metal by heating solid rods of solder “on chip”—the solder flows along trenches, holes, or metal strips (formed lithographically) and produces electrically conductive wires between the top and bottom surfaces of the substrate. A growing interest in flexible sensors and displays has fueled the development of polymer–metal composites. Research in this field includes composites of metal in PDMS with optical functions, conductive PDMS–carbon nanotube composites, substrates for surface-enhanced Raman spectroscopy, spherically curved metal oxide semiconductor field-effect transistors, and flexible gold–polymer nanocomposites as passive components. In addition to materials with electrical functionality on curved or bendable surfaces, some groups have demonstrated stretchable electronics by patterning stiff materials on compliant polymer surfaces. The stiff interconnects include silicon wires fabricated by Khang et al. and arrays of gold lines by Lacour et al. that can withstand up to 12 % strain. Although these methods for fabricating metal in 3D are useful in specific applications, many of them are limited or not applicable for general use. Fabrication of metal in 3D using electrodeposition and electroplating requires multiple lithography steps; this process is slow, costly, and the process is generally limited to patterning metals layer-by-layer on smooth, rigid substrates. LIGA and injection molding provide a useful, specialized approach to fabrication, but are limited by the cost of the equipment, high pressure (3–5 MPa for lowpressure powder-injection molding; higher for other techniques), shrinking of the molded metal upon cooling (typically 15–22 %), and the cost of materials for electroplating thick layers of highly conductive metals such as gold. Solder reflow is a useful method for forming electrical connections between metal features, but the technique requires patterning metal features prior to reflow, and cannot be easily applied to form intricate patterns in 3D. Polymer–metal composites show promise in the field of printable electronic devices. C O M M U N IC A IO N