Switchable Adhesion via Subsurface Pressure Modulation

A Low-Voltage, High-Force Capacity Electroadhesive Clutch Based on Ionoelastomer Heterojunctions

Electroadhesive devices with dielectric films can electrically program changes in stiffness and adhesion, but require hundreds of volts and are subject to failure by dielectric breakdown. Recent work on ionoelastomer heterojunctions has enabled reversible electroadhesion with low voltages, but these materials exhibit limited force capacities and high detachment forces. It is a grand challenge to engineer electroadhesives with large force capacities and programmable detachment at low voltages (<10 V). In this work, tough ionoelastomer/metal mesh composites with low surface energies are synthesized and surface roughness is controlled to realize sub-ten-volt clutches that are small, strong, and easily detachable. Models based on fracture and contact mechanics explain how clutch compliance and surface texture affect force capacity and contact area, which is validated over different geometries and voltages. These ionoelastomer clutches outperform the best existing electroadhesive clutches by fivefold in force capacity per unit area (102 N cm-2 ), with a 40-fold reduction in operating voltage (± 7.5 V). Finally, the ability of the ionoelastomer clutches to resist bending moments in a finger wearable and as a reversible adhesive in an adjustable phone mount is demonstrated.

Smart Manipulation of Complex Optical Elements via Contact-adaptive Dry Adhesives

The implementation of complex, high-precision optical devices or systems, which have vital applications in the aerospace, medical, and military fields, requires the ability to reliably manipulate and assemble optical elements. However, this is a challenging task as these optical elements require contamination-free and damage-free manipulation and come in a variety of sizes and shapes. Here, a smart, contact-adaptive adhesive based on magnetic actuation is developed to address this challenge. Specifically, the surface bio-inspired adhesives made of fluororubber facilitate contamination-free and damage-free adhesion. The stiffness modulation of packaged magnetorheological grease based on the magnetorheological effect endows the smart adhesive with a high conformability to the optical elements in the soft state, a high grip force in the stiff state, and the ability to quickly release the optical elements in the recovered soft state. The smart adhesive provides a versatile solution for reliably and quickly manipulating and assembling multiscale optical elements with planar or complex 3D shapes without causing surface contamination or damage. These extraordinary capabilities are demonstrated by the manipulation and assembly of various optical elements, such as convex/concave/ball lenses and extremely complex-shaped light guide plates. The proposed smart adhesive is a promising candidate for conventional optical element manipulation technologies.

Bio-Inspired Soft-Rigid Hybrid Smart Artificial Muscle Based on Liquid Crystal Elastomer and Helical Metal Wire

Artificial muscles are of significant value in robotic applications. Rigid artificial muscles possess a strong load-bearing capacity, while their deformation is small; soft artificial muscles can be shifted to a large degree; however, their load-bearing capacity is weak. Furthermore, artificial muscles are generally controlled in an open loop due to a lack of deformation-related feedback. Human arms include muscles, bones, and nerves, which ingeniously coordinate the actuation, load-bearing, and sensory systems. Inspired by this, a soft-rigid hybrid smart artificial muscle (SRH-SAM) based on liquid crystal elastomer (LCE) and helical metal wire is proposed. The thermotropic responsiveness of the LCE is adopted for large reversible deformation, and the helical metal wire is used to fulfill high bearing capacity and electric heating function requirements. During actuation, the helical metal wire's resistance changes with the LCE's electrothermal deformation, thereby achieving deformation-sensing characteristics. Based on the proposed SRH-SAM, a reconfigurable blazed grating plane and the effective switch between attachment and detachment in bionic dry adhesion are accomplished. The SRH-SAM opens a new avenue for designing smart artificial muscles and can promote the development of artificial muscle-based devices.

Biomimicking interfacial fracture behavior of lizard tail autotomy with soft microinterlocking structures

Biological soft interfaces often exhibit complex microscale interlocking geometries to ensure sturdy and flexible connections. If needed, the interlocking can rapidly be released on demand leading to an abrupt decrease of interfacial adhesion. Here, inspired by lizard tail autotomy where such apparently tunable interfacial fracture behavior can be observed, we hypothesized an interlocking mechanism between the tail and body based on the muscle-actuated mushroom-shaped microinterlocks along the fracture planes. To mimic the fracture behavior of the lizard tail, we developed a soft bilayer patch that consisted of a dense array of soft hemispherical microstructures in the upper layer acting as mechanical interlocks with the counter body part. The bottom control layer contained a microchannel that allowed to deflect the upper layer when applying the negative pressure, thus mimicking muscle contraction. In the microinterlocked condition, the biomimetic tail demonstrated a 2.7-fold and a three-fold increase in adhesion strength and toughness, respectively, compared to the pneumatically released microinterlocks. Furthermore, as per the computational analysis, the subsurface microchannel in the control layer enabled augmented adhesion by rendering the interface more compliant as a dissipative matrix, decreasing contact opening and strain energy dissipation by 50%. The contrasting features between the microinterlocked and released cases demonstrated a highly tunable adhesion of our biomimetic soft patch. The potential applications of our study are expected in soft robotics and prosthetics.

Smart Adhesives via Magnetic Actuation

Smart adhesives possess a wide range of applications owing to their reversibly and repeatedly switchable adhesion in transfer technology. Despite recent advances, it still remains a technical and scientific challenge to achieve strategies for rapidly tunable adhesion in a noncontact manner. In this study, a smart adhesive to achieve dynamically tunable adhesion is developed. Specifically, a mushroom-shaped adhesive with a magnetized tip is actuated to reversibly and rapidly transform the morphology via magnetic actuation. The smart adhesive has two working modes, namely, selective pickup mode and pick-and-place mode. In the selective pickup mode, the external magnetic field is applied and the tip undergoes bending deformation. Changes in tip morphology allow for a reversible switch of the adhesion between "turn on" and "turn off." In the pick-and-place mode, the external magnetic field is applied when the target object needs to be released. Upward bending deformation of the micro-beam, a part of the tip, creates an initial crack at the edge of the adhesion interface. The propagation of the edge crack modulates the adhesion from strong to weak and the target object is instantly released. The proposed smart adhesive may be of interest for practical applications demanding highly precise and swiftly controlled movements.

Bioinspired Smart Materials With Externally-Stimulated Switchable Adhesion

Living organisms have evolved, over billions of years, to develop specialized biostructures with switchable adhesion for various purposes including climbing, perching, preying, sensing, and protecting. According to adhesion mechanisms, switchable adhesives can be divided into four categories: mechanically-based adhesion, liquid-mediated adhesion, physically-actuated adhesion and chemically-enhanced adhesion. Mimicking these biostructures could create smart materials with switchable adhesion, appealing for many engineering applications in robotics, sensors, advanced drug-delivery, protein separation, etc. Progress has been made in developing bioinspired materials with switchable adhesion modulated by external stimuli such as electrical signal, magnetic field, light, temperature, pH value, etc. This review will be focused on new advance in biomimetic design and synthesis of the materials and devices with switchable adhesion. The underlying mechanisms, design principles, and future directions are discussed for the development of high-performance smart surfaces with switchable adhesion.

Self-Cleaning Performance of the Micropillar-Arrayed Surface and Its Micro-Scale Mechanical Mechanism

The exceptional adhesive ability of geckos remains almost uninfluenced by contaminated surfaces, showing that the adhesion system of geckos has self-cleaning properties. Although there have been several studies on the self-cleaning performance of geckos and gecko-inspired synthetic adhesives, the microscale mechanical mechanism of self-cleaning is still unclear. In the present study, a micropillar-arrayed surface is fabricated using a template molding method to investigate its self-cleaning performance in a load-pull contact process. The effects of preload, microparticle size on self-cleaning properties are studied. The self-cleaning efficiency of the micropillar-arrayed surface is found to increase with an increase in the microparticle size. For large and small microparticles, self-cleaning efficiency increases with an increase in the preload. For medium microparticles, self-cleaning efficiency first increases and then decreases as the preload increases. The mechanical mechanism underlying such self-cleaning performance is further elucidated; it is mainly attributed to the competition among the elastic energy stored in the micropillars induced by the bending deformation, the interfacial adhesion energy between the microparticles and the deformed micropillars, and the interfacial adhesion energy between the microparticles and substrate, as well as the varying contact states between the microparticles and the deformed micropillars. The preload can not only change the contact states between the microparticles and the micropillar-arrayed surface but also influence the bending elastic energy stored in the micropillars. The results obtained in the present study can help deeply understand the self-cleaning mechanism of micropillar-arrayed surfaces as well as provide a guideline for designing functional surfaces with high self-cleaning efficiency.

Adhesion performance study of a novel microstructured stamp for micro-transfer printing

Micro-transfer printing is an effective method that enables the integration of micro-scale heterogeneous materials for flexible electronics. As the key component of micro-transfer printing equipment, the stamp is adopted to pick up and print microdevices due to its reversible and controllable adhesion. In this paper, we propose a novel microstructured stamp based on the bionic theory, which consists of a microchamber and four microchannels. A theoretical model about the pressure change of the gas in the microchamber is established and the effects of compression distance and pull-up velocity on the pull-off force of the stamp are investigated. The performance test results show that the pull-off force of the stamp can be controlled by both the compression distance and the pull-up velocity. Finally, micro-transfer printing operations of microdevices with different sizes, shapes and materials are realized based on the proposed microstructured stamp. The results show that the proposed microstructured stamp exhibits good performance in the transfer printing of microdevices, and provides a new way for the design of microstructured stamps for micro-transfer printing without an extra excitation system.

Adhesion of flat-ended pillars with non-circular contacts

Fibrillar adhesives composed of fibers with non-circular cross-sections and contacts, including squares and rectangles, offer advantages that include a larger real contact area when arranged in arrays and simplicity in fabrication. However, they typically have a lower adhesion strength compared to circular pillars due to a stress concentration at the corner of the non-circular contact. We investigate the adhesion of composite pillars with circular, square and rectangular cross-sections each consisting of a stiff pillar terminated by a thin compliant layer at the tip. Finite element mechanics modeling is used to assess differences in the stress distribution at the interface for the different geometries and the adhesion strength of different shape pillars is measured in experiments. The composite fibrillar structure results in a favorable stress distribution on the adhered interface that shifts the crack initiation site away from the edge for all of the cross-sectional contact shapes studied. The highest adhesion strength achieved among the square and rectangular composite pillars with various tip layer thicknesses is approximately 65 kPa. This is comparable to the highest strength measured for circular composite pillars and is about 6.5× higher than the adhesion strength of a homogenous square or rectangular pillar. The results suggest that a composite fibrillar adhesive structure with a local stress concentration at a corner can achieve comparable adhesion strength to a fibrillar structure without such local stress concentrations if the magnitude of the corner stress concentrations are sufficiently small such that failure does not initiate near the corners, and the magnitude of the peak interface stress away from the edge and the tip layer thickness are comparable.