Dual-axis MEMS force sensors for gecko adhesion studies

Dual-axis piezoresistive microelectromechanical systems (MEMS) force sensors were used to investigate the effects of orientation angle on the adhesion of gecko hairs, called setae. These hairs are part of a fantastic, robust dry adhesive. Their adhesion is highly angle-dependent, with both the "pitch" and "roll" orientation angles playing a role. This anisotropy in adhesion properties is critical for locomotion, as it enables detachment of the gecko's foot with limited pull-off force. Many synthetic mimics of the gecko adhesive are isotropic. This work on the anisotropy of natural setae will inform future work on synthetic dry adhesives.;A dual-axis microscale force sensor was needed to study single seta adhesive forces, which are stronger parallel to a substrate than perpendicular. Piezoresistive silicon cantilevers that separately detect lateral and normal forces applied at the tip were used. The fabrication process and rigorous characterization of new devices are reported. A novel calibration method was developed that uses resonant frequency measurements in concert with finite element models to correct for the expected variability of critical dimensions. These corrected models were used to predict the stiffnesses of each cantilever, and thus improve the accuracy of force measurements made with these sensors. This calibration technique was also validated by direct measurement of the dual-axis cantilever stiffnesses using a reference cantilever.;The adhesion force of a single gecko seta is dramatically enhanced by proper orientation. The dual-axis cantilevers were used to measure two components of force between a substrate and a Gekko gecko seta. Lateral adhesion was highest with the stalk oriented parallel to the surface at 0° pitch. Adhesion decreased smoothly as the pitch angle of the stalk was increased, until detachment or no adhesion occurred at approximately 30°. To display enhanced adhesion, the splayed tuft at the end of the seta needed to be only grossly oriented towards the substrate. Moderate misalignment of roll angle (+/-40°) still produced high adhesion values.