Functional PDMS Composite Microbridges for Temperature Sensing Applications
Authors: Pallapa Manu , Leung Jacob C. K. , Rezai Pouya 
Published: February 2020

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 37oC for 24 hours (Fig. 1a-vi). The cured structure was then immersed in a 100˚C water bath (Fig. 1a-vii) to dissolve the sacrificial agar and dried to form the suspended microbridge structures (Fig. 1a-viii) before plasma bonding to another flat PDMS layer (Fig. 1a-ix). The scanning electron microscope images of the fabricated microbridges are shown in Fig. 2. The thickness of the fabricated suspended bridges were measured and compared against their respective guide rod thicknesses (Fig. 3). The average thicknesses of the 65µm and 240µm microbridges showed a high precision in fabrication with a standard deviation of ~12µm from the guide rods. The larger deviation of the 350µm microbridge may be attributed to the size range (1-75 µm) of the iron particles in the Fe-PDMS composite which is currently under investigation. Uniform particle size would ensure better consistency in microbridge thicknesses.
The temperature sensing ability of the Fe-PDMS composite was experimentally verified as well. A 24 AWG copper wire was used to provide electrical interconnection with the composite. The current-voltage (IV) characteristics of the Fe-PDMS composite in the input voltage range of 1-20V was measured by a Keithley 2410 source meter at four equilibrium temperatures of 45, 50, 60 and 70˚C applied externally via a hot plate. Each equilibrium temperature level produced a distinctive near-ohmic IV curve (Fig. 4a) with a positive correlation between the temperature and electrical conductivity. The obtained mean resistivities (Fig. 4b) imply a positive coefficient of resistance that is analogous to metals.
The sacrificial agar fabrication technique in conjunction with the properties of the developed Fe-PDMS polymer composite will be suitable for development of low-cost and flexible electrodes and microstructures in microfluidic devices for thermo-electric temperature sensing and actuating applications.

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Figure 1