Bioinspired fibrillar adhesives with shape-controlled off-center caps for switchable and directional adhesion

Most natural fibrillar adhesive structures feature spatula-shaped caps as a common adhesive element. The spatula-shaped tips endow adhesive fiber arrays with on demand adhesion, allowing them to maximize adhesion in a given direction and facilitate effortless detachment. Replication of off-center spatula ends synthetically, a key contributor to directional adhesion, remains a fabrication challenge. In this work, a method to feature synthetic microfiber adhesives with off-center caps is proposed. This new method decouples the shape and placement dependency between the cap and the stem, providing endless possibilities in positioning of the tip with respect to the stem and the two-dimensional shape and the material of both the cap and the stem. Experiments of directional adhesion were carried out to demonstrate switchable adhesion relevant for pick & place type applications, and the ability to generate high friction and easy removal. Fibers with off-center caps were shown to promote easy detachment after strong adhesion in both straight and bent stalks. Straight fibers with off-center caps showed 3-to-5-fold reduction in adhesion when pulled diagonally instead of vertically from the adhering surface. Bent fibers on the other hand provided up to 3 times more maximum shear than adhesion, when fibers are dragged laterally against the cap overhang. These results show the versatility and the potential of the proposed fabrication method in designing application specific directional and switchable adhesives.