Piezoresistive polymer composites for cantilever readout

Microcantilever based biochemical sensors can be used for detection of surface stress changes, due to the adsorption of specic molecules on one side of the cantilever. The method is fast and label-free and due to the small dimensions, it opens the possibility of fabricating point-of-care measurement devices. Surface stress changes of a cantilever sensor can be detected by an integrated piezoresistive readout. The goal of this PhD thesis is to increase the sensitivity of polymer based cantilever sensors, by investigating new strain sensitive (piezoresistive) polymer materials, that can improve the piezoresistive readout. A two- and four-probe electrode chip, for measuring the strain sensitivity of the materials, have been designed and fabricated with standard cleanroom technology. A thin lm layer of polymer material is structured on the chips and by insertion in a four-point bending xture, the deposited thin lm can be strained, while measuring how the resistance changes. This allows the determination of the strain sensitivity of the materials. Three qualitatively dierent material types have been investigated: conductive polymer composites, an intrinsically conductive polymer and thin gold lms. Conducting polymer composites consisting of SU-8 (an epoxy based photoresist) and dierent concentrations of carbon- and silver nanoparticles have been investigated. For the carbon nanoparticle doped SU-8 composites, a positive piezoresistive eect was measured, with the largest eect towards the lower concentrations. No signicant piezoresistive eect was observed for the silver nanoparticle doped composites. Thin lm structures of the intrinsically conductive polymer, polyaniline, have been fabricated and a negative piezoresistive eect was observed. Thin gold lms were investigated, with the aim of measuring the piezoresistive eect in discontinuous gold lms. Various thin lm thicknesses were investigated and the piezoresistive eect was found to be close to the value of bulk gold.