Dynamically Tunable Dry Adhesion via Subsurface Stiffness Modulation

Abstract

Tunable dry adhesion has different applications, including microtransfer printing, climbing robots, pick and place robots and gripping in manufacturing processes. This study investigates a new type of tunable adhesion mechanism in which the adhesion strength can be dynamically and reversibly tuned via subsurface stiffness modulation. In the current study, a novel soft gripper is introduced and demonstrated such that the adhesion strength can be switched between weak and strong states depending on the subsurface stiffness. The low and high stiffness correspond to weak and strong adhesion strengths, respectively. The proposed soft device is constructed out of a soft elastomer, polydimethylsiloxane (PDMS), which is embedded with a tunble stiffness core, Conductive Propylene Based Elastomer (CPBE). Stiffness of the core can be tuned via application of electrical voltage. Activation of the core with electrical voltage reduces the core stiffness and subsequently stiffness of the whole composite post. This reduction in the composite post stiffness results in a change in the stress distribution and required force for delamination at the adhered interface and finally a drop in the effective adhesion strength is achieved. ANSYS software is employed to conduct Finite Element (FE) simulations and derive the pattern of stress distribution at the interface. Pattern of crack initiation and propagation has also been experimentally video recorded for both activated and non-activated composite posts. Both experimental and simulation results show that when the post is not electrically activated (stiff) crack starts from the center and high adhesion strength is achieved. But when the post gets activated (compliant state) the interfacial crack starts from the edge and as a result adhesion strength is decreased. The adhesion of the composite posts with a range of dimensions and activation voltages has been characterized and it is shown that the adhesion can be reduced by as much as a factor of 6. As a demonstration for the applications of the proposed soft gripper, a variety of objects with different weights has been manipulated in the supplemental video. The proposed novel tunable dry adhesion mechanism uses subsurface stiffness modulation in which the adhesion strength is completely reversible and does not need a complicated activation mechanism.