Sensors 2008. 8 2321 3. Fabrication In this effort, room-temperature-vulcanizing(RTv) liquid rubber of polyurethane was selected to form the elastomeric layer. This material offers mechanical robustness such as high tear and abrasive resistances, chemical resistance, and controllability of its softness over a wide range. It has been extensively used in medical implant applications [21] and was also used to fabricate micro/nanostructures for MEMs applications [22-24]. Other rubber materials such as polydimethylsiloxane(PDMS )that are formed with low-viscosity liquids are also potential candidates for the elastomer layer. Of course, mechanical properties such as plasticity limit, thermal expansion coefficient, would play a role in the final selection, as would considerations about manufacturing and Integration The fabrication process is illustrated in Figure 3. As mentioned earlier, two capacitive plates were patterned with HEDM using a Panasonic MG-ED72W system(step 1). The base and top plates were cut from type -304 stainless-steel sheets with thickness of 200 um and 50 um, respectively, using cylindrical electrodes with 190-um diameter(Figure 4a). The base plate was still connected to the original sheet through two tethers after the machining as shown in Figure 4a. a two-part polyurethane RTV liquid rubber(Poly 74-20, part A: polyurethane pre-polymer, part B: polyol, Polytel Development Co., PA, USA)with the softener(part C: plasticizer), which is vulcanized to very soft (20 Shore A)and robust rubber, was used to form the intermediate polymer layer. The softness of the rubber can be adjusted by changing the proportion of the softener to be mixed. This effort used a formulation of part A B: C=1: 1: 1. The mixed solution was applied to the upper surface of the base plate (step 2), and then the top plate was placed on it( step 3). In this step, the top plate is self-aligned to the base due to surface tension of the solution. After curing, the device was released as shown in Figure 4b by mechanically breaking the tethers(step 4). The measured thickness of the cured polyurethane layer was approximately 38 um. The thickness of the layer can be adjusted by controlling the amount of the solution to be applied. (In large scale production, many of the kinds of parameters that are used to control the thickness of photoresist in photolithography polymer viscosity, substrate spin speed, etc can be used in this context as well. )Finally, the device was coupled with an inductive coil: For the device in Figure 1b, the coil was formed by winding an enamel-coated copper wire(AWG 36, 40 turns)directly on the sensor and bonding the terminals on separate stainless-steel plates with conductive adhesive(step 5). The fabricated L-C tank shown in Figure 4c was measured to have nominal capacitance of 6.3 pF and inductance of 640 nH Measured resonant frequency and quality factor of the tank, which were probed via test leads shown in Figure 4c, were 106 MHz and 1 respectively. The measured resonant frequency of the tank is close to the theoretical frequency of about 80 MHz that is obtained from the measured capacitance and inductance of the tank he capacitive structure was also coupled with and centered in a larger circular coil(5-mm diameter, 5 turns) formed using AWG 40( 80 um) enamelled copper lead. This configuration was selected for preliminary wireless testing to enlarge the magnetic coupling coefficient [25] between the device and the external antenna/coil while reducing the negative impact of eddy current generated in the stainless-steel plates. The use of conducting adhesive between the stainless-steel plates(without surface preparation)and copper leads of the coil with a conductive adhesive provided high contact resistance between them. This caused the low quality factor mentioned earlier, which limits theSensors 2008, 8 2321 3. Fabrication In this effort, room-temperature-vulcanizing (RTV) liquid rubber of polyurethane was selected to form the elastomeric layer. This material offers mechanical robustness such as high tear and abrasive resistances, chemical resistance, and controllability of its softness over a wide range. It has been extensively used in medical implant applications [21] and was also used to fabricate micro/nanostructures for MEMS applications [22-24]. Other rubber materials such as polydimethylsiloxane (PDMS) that are formed with low-viscosity liquids are also potential candidates for the elastomer layer. Of course, mechanical properties such as plasticity limit, thermal expansion coefficient, would play a role in the final selection, as would considerations about manufacturing and integration. The fabrication process is illustrated in Figure 3. As mentioned earlier, two capacitive plates were patterned with μEDM using a PanasonicTM MG-ED72W system (step 1). The base and top plates were cut from type-304 stainless-steel sheets with thickness of 200 µm and 50 µm, respectively, using cylindrical electrodes with 190-µm diameter (Figure 4a). The base plate was still connected to the original sheet through two tethers after the machining as shown in Figure 4a. A two-part polyurethane RTV liquid rubber (Poly 74-20, part A: polyurethane pre-polymer, part B: polyol, Polytek Development Co., PA, USA) with the softener (part C: plasticizer), which is vulcanized to very soft (<20 Shore A) and robust rubber, was used to form the intermediate polymer layer. The softness of the rubber can be adjusted by changing the proportion of the softener to be mixed. This effort used a formulation of part A:B:C=1:1:1. The mixed solution was applied to the upper surface of the base plate (step 2), and then the top plate was placed on it (step 3). In this step, the top plate is self-aligned to the base due to surface tension of the solution. After curing, the device was released as shown in Figure 4b by mechanically breaking the tethers (step 4). The measured thickness of the cured polyurethane layer was approximately 38 μm. The thickness of the layer can be adjusted by controlling the amount of the solution to be applied. (In large scale production, many of the kinds of parameters that are used to control the thickness of photoresist in photolithography – polymer viscosity, substrate spin speed, etc. – can be used in this context as well.) Finally, the device was coupled with an inductive coil: For the device in Figure 1b, the coil was formed by winding an enamel-coated copper wire (AWG 36, 40 turns) directly on the sensor and bonding the terminals on separate stainless-steel plates with conductive adhesive (step 5). The fabricated L-C tank shown in Figure 4c was measured to have nominal capacitance of 6.3 pF and inductance of 640 nH. Measured resonant frequency and quality factor of the tank, which were probed via test leads shown in Figure 4c, were 106 MHz and 1.9 respectively. The measured resonant frequency of the tank is close to the theoretical frequency of about 80 MHz that is obtained from the measured capacitance and inductance of the tank. The capacitive structure was also coupled with and centered in a larger circular coil (5-mm diameter, 5 turns) formed using AWG 40 (φ 80 μm) enamelled copper lead. This configuration was selected for preliminary wireless testing to enlarge the magnetic coupling coefficient [25] between the device and the external antenna/coil while reducing the negative impact of eddy current generated in the stainless-steel plates. The use of conducting adhesive between the stainless-steel plates (without surface preparation) and copper leads of the coil with a conductive adhesive provided high contact resistance between them. This caused the low quality factor mentioned earlier, which limits the