Nonangled anisotropic elastomeric dry adhesives with tailorable normal adhesion strength and high directionality

We present how direction-sensitive biomimetic dry adhesives made from poly(dimethylsiloxane) may be tailored directly with standard lithographic techniques without requirements for angled fibers or caps. Specifically, it is possible to alter both 90° pull-off adhesion strength and directionality solely through the control over precise cap dimensions and offset, and the improved or reduced adhesion strength is determined primarily by the vulnerability of the fiber cap to peeling in different directions. Running experimental trials on two different variations of anisotropic adhesives and two isotropic adhesives reveal that in this method, directionality is due to reducing the adhesion strength in specific orientations rather than improving it in the strong direction, and this has implications for the maximum adhesion strengths that could be reached with direction-sensitive designs. Using a combination of microscale normal adhesion tests, load-drag-pull tests and macroscale peel strength tests, we reveal that peel strength is the primary factor that is necessary to characterize when manufacturing dry adhesives, as it will ultimately affect all other properties of interest. Our anisotropic adhesive variations have demonstrated peel strength ratios of approximately 15:1, making these versions some of the most directionally sensitive to date, while isotropic variations can demonstrate more than 600 kPa of adhesion strength in the normal direction.