Rapid Photoresponsive Switchable Pressure-Sensitive Adhesive Containing Azobenzene for the Mini-Light Emitting Diode Transfer Process

Adhesion Evolution: Designing Smart Polymeric Adhesive Systems with On-Demand Reversible Switchability

Smart polymeric switchable adhesives represent a rapidly emerging class of advanced materials, exhibiting the ability to undergo on-demand transitioning between "On" and "Off" adhesion states. By selectively tuning external stimuli triggers (including temperature, light, electricity, magnetism, and chemical agents), we can engineer these materials to undergo reversible changes in their bonding capabilities. The strategic design selection of stimuli is a pivotal factor in the design of switchable adhesive systems. This review outlines recent advancements in the field of smart switchable polymeric adhesives over the past decade with a focus on the selection of stimulus triggers. These systems are further categorized into one of four adhesion switching mechanisms upon initiation by a specific stimuli-trigger: (i) interfacial adhesion, (ii) stiffness, (iii) contact area, or (iv) suction-based switching. Evaluation of adhesion switching performance across systems is primarily made based on three key metrics: (i) maximum adhesion strength, (ii) switch ratio, and (iii) switch time. Different stimuli and mechanisms offer distinct advantages and limitations, influencing the performance characteristics and applicability of these materials across domains such as detachable biomedical devices, robotic grippers, and climbing robots. This review thus offers a perspective on the present advancements and challenges in this emerging field, along with insights into future directions.

Branched Oligomer-Based Reversible Adhesives Enabled by Controllable Self-Aggregation

Supramolecular adhesion material systems based on small molecules have shown great potential to unite the great contradiction between strong adhesion and reversibility. However, these material systems suffer from low adhesion strength/narrow adhesion span, limited designability, and single interaction due to fewer covalent bond content and action sites in small molecules. Herein, an ultrahigh-strength and large-span reversible adhesive enabled by a branched oligomer controllable self-aggregation strategy is developed. The dense covalent bonds present in the branched oligomers greatly enhance adhesion strength without compromising reversibility. The resulting adhesive exhibits a large-span reversible adhesion of ≈140 times, switching between ultra-strong and tough adhesion strength (5.58 MPa and 5093.92 N m-1) and ultralow adhesion (0.04 MPa and 87.656 N m-1) with alternating temperature. Moreover, reversible dynamic double cross-linking endows the adhesive with stable reversible adhesion transitions even after 100 cycles. This reversible adhesion property can also be remotely controlled via a voltage of 8 V, with a loading voltage duration of 45 s. This work paves the way for the design of reversible adhesives with long-span outstanding properties using covalent polymers and offers a pathway for the rational design of high-performance adhesives featuring both robust toughness and exceptional reversibility.

Thermoresponsive On-Demand Adhesion and Detachment of a Polyurethane-Urea Bioadhesive

The development of bioadhesives with strong adhesion and on-demand adhesion-detachment behavior is still critically important and challenging for facilitating painless and damage-free removal in clinical applications. In this work, for the first time, we report the easy fabrication of novel polyurethane-urea (PUU)-based bioadhesives with thermoresponsive on-demand adhesion and detachment behavior. The PUU copolymer was synthesized by a simple copolymerization of low-molecular-weight, hydrophilic, and biocompatible poly(ethylene glycol), glyceryl monolaurate (GML, a special chain extender with a long side hydrophobic alkyl group), and isophorone diisocyanate (IPDI). Here, GML was expected to not only adjust the temperature-dependent adhesion behavior but also act as an internal plasticizer. By simple adjustment of the water content, the adhesion strength of the 15 wt % water-containing PUU film toward porcine skin is as high as 55 kPa with an adhesion energy of 128 J/m2 at 37 °C. The adhesion strength dramatically decreases to only 3 kPa at 10 °C, exhibiting switching efficiency as high as 0.95. Furthermore, the present PUU-based adhesive also shows good on-demand underwater adhesion and detachment with a cell viability close to 100%. We propose that biomaterial research fields, especially novel PUU/polyurethane (PU)-based functional materials and bioadhesives, could benefit from such a novel thermoresponsive copolymer with outstanding mechanical and functional performances and an easy synthesis and scaled-up process as described in this article.

Plasmonic Hydrogel Actuators for Octopus-Inspired Photo/Thermoresponsive Smart Adhesive Patch

Octopuses are notable creatures that can dynamically adhere to a variety of substrates owing to the efficient pressure control within their suction cups. An octopus' suckers are sealed at the rim and function by reducing the pressure inside the cavity, thereby creating a pressure difference between the ambient environment and the inner cavity. Inspired by this mechanism, we developed a plasmonic smart adhesive patch (Plasmonic AdPatch) with switchable adhesion in response to both temperature changes and near-infrared (NIR) light. The AdPatch incorporates an elastic, nanohole-patterned elastomer that mimics the structure of octopus suckers. Additionally, a monolayer of gold nanostars (GNSs) is coated on the patch, facilitating a NIR light-responsive photothermal effect. A musclelike, thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel functions as a volumetric actuator to regulate cavity pressure. When exposed to heat or light, the PNIPAM hydrogel shrinks, enabling the AdPatch to achieve strong suction adhesion (134 kPa at 45 °C, 71 kPa at 85 mW cm-2). Owing to its capability to achieve light-triggered remote adhesion without the need for external pressure, the Plasmonic AdPatch can be employed to transfer ultrathin films and biosensors to fragile organs without causing damage.

Flexible Azo-Polyimide-Based Smart Surface with Photoregulatable Surface Micropatterns: Toward Rewritable Information Storage and Wrinkle-Free Device Fabrication

Stimulus-sensitive materials are of great fascination in surface and interface science owing to their dynamically tunable surface properties and/or morphologies. Herein, we have synthesized an azobenzene-containing polyimide (azo-PI) with enhanced chain flexibility for the fabrication of photosensitive surface patterns on a film/substrate wrinkle system or wrinkle-free devices. The phototriggered cis-trans isomerization kinetics of azobenzene groups in the novel azo-PI with various chain structures were systematically investigated. On the basis of the characteristics of stress relaxation that azobenzene reversible cis-trans isomerization induces in the wrinkled azo-PI film/substrate system, a variety of rewritable visual surface patterns with high resolution and a long legibility time (>30 days) could be easily constructed via visible-light irradiation, enabling the wrinkled azo-PI surfaces to be used as rewritable information storage media. Meanwhile, because of the visible-light irradiation strategy, these photoresponsive surfaces could find potential application in the fabrication of wrinkle-free flexible devices. This study not only sheds light on the influence of the azo-polymer chain structure on its photoresponsive behavior but also provides a versatile strategy for realizing tailor-made smart surface patterns on multilayer functional devices.

Realizing Vat-Photocycloaddition 3D Printing with Recyclable Synthetic Photorheological Silicone Fluids

Synthetic silicone rubbers are finding a broad spectrum of applications, yet there has been a demand for developing greener silicone rubbers with processability, recyclability, and reversible tunability in their mechanical properties. Here, a recyclable photorheological silicone fluid (RPSF) is developed, which realizes completely reversible wavelength-selective liquid-rubber conversion upon photoirradiation, relying on the reversible photocycloaddition of coumarin upon alternating irradiation of light with wavelengths of 365 nm (UV₃₆₅ ) and 254 nm (UV₂₅₄ ). Rheological studies demonstrate that the storage modulus of the developed RPSF increases by a factor of more than 100,000 upon UV₃₆₅ irradiation to reach 20-50 kPa, while it decreases to ca. 0.01 kPa upon UV₂₅₄ irradiation. The reversibility of the photocycloaddition of coumarin enables the application of RPSF as a photodismantlable adhesive. Furthermore, unprecedented vat-photocycloaddition 3D printing of silicone rubber is realized by taking advantage of the excellent photocurability, i.e., dramatic increase in viscoelasticity upon UV₃₆₅ irradiation. This article is protected by copyright. All rights reserved.

Thermoresponsive ionogels with switchable adhesion in air and aqueous environments induced by LCST phase behavior

The rapid development of wearable devices is in urgent demand for materials with switchable adhesion both in air and aqueous environments. Herein, we report a thermoresponsive ionogel with switchable adhesion against various substrates both in air and aqueous environments. The switchable adhesion of ionogels is realized by a phase separation induced collapse of the polymer network and the subsequent extrusion of ionic liquids (ILs) on ionogel surfaces. The hydrophobic poly(butyl acrylate) (PBA) network and ILs endow the ionogels with excellent water-resistance ability, which enables the application of ionogels in aqueous environments. As a result, the adhesion strength of ionogels against rubber can reach an on/off ratio of 75-fold (45 kPa versus 0.6 kPa) and 7.7-fold (21 kPa versus 2.7 kPa) in air and aqueous environments, respectively. By varying the ratio of two structurally similar ILs in their blends, the responsive temperature of ionogels can be tuned within a wide temperature range from 32 °C to 100 °C. Furthermore, we show a demonstration of an underwater on demand capture and release by taking advantage of the switchable adhesion of ionogels. These nonvolatile ionogels with tunable responsive temperatures and high on/off adhesion strength ratio both in air and aqueous environments show broad applications in the fields related to wearable devices, soft robots and submersible sensors.

Smart Responsive Azo-Copolymer with Photoliquefaction for Switchable Adhesive Application

The development and utilization of switchable adhesives are considered to be an essential target to solve the problems of their separation and recycling in some specific service environments, such as the preparation or repair process of electronic devices. Intelligent materials with controllable phase transition are utilized to fabricate switchable adhesives because of the significantly diverse adhesion strengths in different phase states. Photoresponsive azobenzene and its derivatives usually possess different melting temperatures (T_m) or/and glass transition temperatures (T_g) of the cis-trans isomers, which are beneficial to making the photoinduced solid-liquid phase transition for switchable adhesive application possible. Here, a novel three-component azo-copolymer (PNIM-Azo) with fast and reversible photoinduced solid-liquid phase transition has been designed and synthesized. PNIM-Azo possesses reversible bonding/debonding processes, resulting from the different adhesion strengths between trans-configuration PNIM-Azo in the solid state and cis-configuration in the liquid state. Moreover, by incorporating commercialized 2-methoxyethyl acrylate and N-isopropylacrylamide with O and N heteroatoms into the copolymer, the trans-configuration PNIM-Azo possesses the highest adhesion strength (∼11 MPa between two glass substrates) among all of the reported azobenzene-based switchable adhesives, which could be attributed to the increase in the entanglement effect because of the H-bond in the polymer chains formed by introducing heteroatoms. Our synthesized PNIM-Azo copolymer provides an alternative for designing and developing switchable adhesives with high adhesion strength for some electronic production processes.