Switchable Adhesives for Multifunctional Interfaces

Abstract

The reliable bonding or attachment of materials is critical in many industries, is a common necessity of daily life, and is key to functionality in numerous biological settings. [1][2][3][4][5][6][7][8][9][10][11] Adhesives Joining two materials with an adhesive is an extremely common activity, but is most often irreversible. However, emerging and current applications ranging from consumer and medical products to soft robotics and wearable bio-monitoring devices demand strong adhesives which can be easily removed on-demand and subsequently reused. This requires new paradigms in adhesive science and engineering, resulting in the emergence of new classes of multifunctional switchable adhesives. Here summarized is the current state of the art in switchable adhesive systems, focusing on how devices fit into the basic science of adhesion while providing performance metrics for comparison across current approaches. Using fracture mechanics as a guide, systems are classified as functioning under one of three basic mechanisms: near-interface, contact area, or mechanical. These mechanisms must be initiated or "triggered" by a specific input, which can include mechanical, electromagnetic, fluidic, or thermal stimuli. Triggers and adhesion switching performance are compared through the use of a "switching ratio," the interfacial energy release rate, and an estimate of mechanism timing. Finally, it is discussed that how the fundamental mechanisms relate to challenges and opportunities for switchable interfaces in future applications and adhesive systems.

Switchable Adhesion of Micropillar Adhesive on Rough Surfaces

“…The complicated hierarchical setae on gecko's toe pads guarantee the proper contact on rough surfaces, generating van der Waals force large enough to support the body weight of geckos . Inspired by gecko setae, artificial micro‐ and nanopillar adhesives have been widely studied . The array of pillars has an effective elastic modulus much smaller than the inherent elastic modulus of the material, allowing the pillar array to form reliable contacts on counterpart surfaces.…”

“…Numerous studies on gecko‐inspired reversible adhesions on smooth surface have been reported . The reversibility of adhesion is usually achieved by regulating the contact area via deflection of pillars and deformation of adhesives that are triggered by external stimuli like magnetic field, heating, preload, and electricity .…”

“…Humidity and temperature are also used to regulate the storage modulus of pillar arrays ( E ), allowing the change of contact area and therefore the regulation of reversible adhesion. To evaluate the reversible adhesion, three parameters, including adhesion force capacity ( F C ), “switching ratio” between the on/off states of adhesion, and switching time, have been proposed . Since the strong adhesion and easy detachment are always on the opposite sides, it is difficult to realize high adhesion strength and large switching ratio at the same time .…”

“…To evaluate the reversible adhesion, three parameters, including adhesion force capacity ( F C ), “switching ratio” between the on/off states of adhesion, and switching time, have been proposed . Since the strong adhesion and easy detachment are always on the opposite sides, it is difficult to realize high adhesion strength and large switching ratio at the same time . For example, high adhesion much larger than gecko adhesion (such as, 140 kPa, 280 kPa) was realized in shape memory polymer while the switching ratio was only 7 and 5.62 .…”

Rapid and continuous regulating adhesion strength by mechanical micro-vibration

“…Besides, the surface microstructures are susceptible to environmental and intrinsic surface forces, which limits the durability. Considering that the apparent adhesion strength can be influenced by the real contact area, the contact region size 17 , and the adhesion work 20 , adhesion switching has been proposed by either controlling the load path and failure mode of the interface or constructing a "smart" interface such as a phase transition interface 6,20 . The adhesion switching triggers, such as mechanical 12,15,[21][22][23][24][25][26][27][28][29][30] , electro/magnetic 14,16,31 , light [32][33][34] , fluid [35][36][37][38] , and thermal [39][40][41][42][43][44] stimulations, have been suggested.…”