Piezoresistive strain sensors, commonly known as resistance strain gauge, have many important applications. In this work, an alternative method to fabricate piezoresistive strain sensors directly on the structure of interest is demonstrated using a particle-free silver ink as the sensing material. The sensing material is first printed as a rectangular film on the structure of interest and a conductive serpentine pattern is generated by selective laser sintering. Only the material exposed to the focused laser is sintered and becomes conductive. The rest is washed-off by 1-dodecene solvent, leaving only the serpentine pattern, which serves as the piezoresistive strain sensor. This alternative method eliminates the need for a carrier or backing substrate and thus improves the mechanical coupling between the sensing material and the structure of interest. It also removes reinforcement effect due to the stiffness of the carrier substrate. Results from electrical characterization revealed that laser sintering power is a crucial parameter that influences fundamental properties of the sensing material such as electrical conductivity and work function. In addition, it was observed that there exists an optimum laser sintering power that results in a maximum gauge factor (GF). For strain sensors, the GF is the most important parameter because it is the measure of sensor sensitivity. When the particle-free silver ink was printed as a serpentine pattern followed by thermal sintering on a hot plate, a lower GF was measured. This shows that the alternative method to fabricate piezoresistive strain sensors is more attractive than printing the serpentine pattern then thermally sintering it.

In this work we build upon a previously published technique for printing dielectric ramps and printed RF interconnects across leveled surfaces to gain a better understanding of the effects that the dielectric material itself has on the conductivity of the printed conductive ink. The use of printed dielectric ramps, referred to as fillets, to assist in additively manufactured RF circuits and interconnects can be found throughout literature. One of the most widely used materials for these ramps, the UV-curable adhesive NEA-121, was found to exhibit physical changes when exposed to high curing temperatures and to have a significant effect on the conductivity of a wide variety of commercially available conductive ink materials; in some cases causing a 2x drop in conductivity compared with the expected conductivity reported by the manufacturer. We report on the conductivity effects from printing on the NEA-121 dielectric surface for three commercially available Ag inks for an RF circuit application and report the manufacturing techniques necessary to optimize both the dielectric ramp and the conductive ink performance.