Functional PDMS Composite Microbridges for Temperature Sensing Applications

Microstructures provide excellent sensing and actuating capabilities due to their high surface to volume ratio and have brought about important advancements in life sciences research [1-3]. The development of electrically-conductive polymer composites for such sensors and actuators as well as rapid fabrication techniques will result in microstructures with better transduction, low cost of production and flexibility [4, 5]. Conventional microfabrication techniques for such transducers are photolithography, reactive ion etching (RIE), laser ablation, and focussed ion beam etching. However these techniques are limited by the requirement of multiple processing steps (photolithography and RIE) or serial processing with specialized equipment (laser ablation and focussed ion-beam etching). In this work we report a low-cost, rapid and convenient technique to microfabricate electrically-conductive Iron-Polydimethylsiloxane (Fe-PDMS) microbridges using agar as the sacrificial material and further demonstrate the temperature sensing property of this polymer composite. Fabrication of the Fe-PDMS microbridges by the sacrificial agar technique is illustrated schematically in Fig. 1. A rectangular mold containing a 20mm×0.7mm×0.2mm channel and three pairs of sidewall through-holes (800µm in diameter) was manufactured via 3D printing (Fig.1a-i). Glass capillary guide rods with diameters of 65µm, 240µm and 350µm were inserted into the sidewall though-holes and passed through the channel (Fig.1a-ii). The guide rods functioned as master molds for the sacrificial agar, creating cavities for the Fe-PDMS composite to flow through. A 4% agar solution was prepared and poured into the rectangular mold (Fig.1a-iii). Following the room-temperature curing of the agar, the guide rods were removed horizontally and the agar replica was carefully de-molded, exposing the cylindrical cavities to be filled with Fe-PDMS composite (Fig.1a-iv). The agar replica was transferred into a petri dish (Fig.1a-v). The Fe-PDMS composite was prepared by mixing 80wt% iron particles (200 mesh size) with Sylgard 184 pre-polymer (10:1 elastomer-curing agent ratio). This composite was carefully casted into the aforementioned cylindrical cavities using assistive capillary flow. Undoped Sylgard 184 pre-polymer (10:1 elastomer-curing ratio) was then casted on the entire structure and cured at 37 o C for 24 hours (Fig. 1a-vi). The cured structure was then immersed in a 100˚C water bath (Fig. 1a-vii) to dissolve