Wet‐Responsive,Reconfigurable, and Biocompatible Hydrogel Adhesive Films for Transfer Printing of Nanomembranes

This study presents a wet‐responsive and biocompatible smart hydrogel adhesive that exhibits switchable and controllable adhesions on demand for the simple and efficient transfer printing of nanomembranes. The prepared hydrogel adhesives show adhesion strength as high as ≈191 kPa with the aid of nano‐ or microstructure arrays on the surface in the dry state. When in contact with water, the nano/microscopic and macroscopic shape reconfigurations of the hydrogel adhesive occur, which turns off the adhesion (≈0.30 kPa) with an extremely high adhesion switching ratio (>640). The superior adhesion behaviors of the hydrogels are maintained over repeating cycles of hydration and dehydration, indicating their ability to be used repeatedly. The adhesives are made of a biocompatible hydrogel and their adhesion on/off can be controlled with water, making the adhesives compatible with various materials and surfaces, including biological substrates. Based on these smart adhesion capabilities, diverse metallic and semiconducting nanomembranes can be transferred from donor substrates to either rigid or flexible surfaces including biological tissues in a reproducible and robust fashion. Transfer printing of a nanoscale crack sensor onto a bovine eye further demonstrates the potential of the reconfigurable hydrogel adhesive for use as a stimuli‐responsive, smart, and versatile functional adhesive for nanotransfer printing.     

A Highly-Adhesive and Self-Healing Elastomer for Bio-Interfacial Electrode

Stretchable electrodes are playing important roles in the measurement of bio-electrical signals especially in wearable electronic devices. These electrodes usually adopt commercial elastomers such as polydimethylsiloxane or polystyrene-ethylene-butylene-styrene as substrates, which result in poor stability and reliability due to weak interfacial adhesion between electrodes and human skin. Here, dopamine is introduced into the hydrogen bonding based elastomer as pendent groups. The elastomer shows both mechanical strength and adhesion strength at the same time. It exhibits high stress at break (1.9 MPa) and high fracture strain (5100%). Significantly, it exhibits a high adhesive strength (≈62 kPa) and underwater adhesive strength (≈16 kPa) with epithelial tissue. Thus, a stretchable bio-interfacial electrode is fabricated by spray-coating silver nanowires on the elastic substrate, which is stretchable, self-healable, and highly adhesive and suitable for electromyogram measurement.     

Bioinspired Microstructured Adhesives with Facile and Fast Switchability for Part Manipulation in Dry and Wet Conditions

The rapid growth in the miniaturized mechanical and electronic devices industry has created the need for temporary attachment systems that can carry out pick-and-place and transfer printing tasks for fragile and tiny parts. Current systems are limited by a fundamental trade-off between adhesive strength and state-changing trigger force, which causes the need for a rapidly switchable adhesive. In this study, an elastomeric microstructure is presented combining a trapezoidal-prism-shaped (TPS) and a mushroom-shaped microstructure, which overcomes the trade-off with the help of the TPS structure. The optimal design exhibits a strong adhesive strength of 87.8 kPa and a negligible detachment strength of <0.07 kPa with a low trigger shear stress of 10.7 kPa on smooth glass surfaces. The large tip-to-stem ratio (50 to 20 µm) enhances the suction effect, allowing the microstructure to maintain its adhesive performance even in wet conditions. Pick-and-place manipulation tasks of a single and an array of ultralight parts from micrometer to millimeter scales are performed to demonstrate the capability of handling fragile and tiny parts. Moreover, it demonstrates the ability to transfer parts across water and air interfaces. This proposed microstructure offers a facile solution for manipulating microscale fragile parts in dry and wet conditions.     

Barnacle Cement Proteins-Inspired Tough Hydrogels with Robust,Long-Lasting,and Repeatable Underwater Adhesion

The development of adhesives that can achieve robust and repeatable adhesion to various surfaces underwater is promising; however, this remains a major challenge primarily because the surface hydration layer weakens the interfacial molecular interactions. Herein, a strategy is proposed to develop tough hydrogels that are robust, reusable, and long-lasting for underwater adhesion. Hydrogels from cationic and aromatic monomers with an aromatic-rich composition inspired by the amino acid residuals in barnacle cement proteins are synthesized. The hydrogels are mechanically strong and tough (elastic modulus 0.35 MPa, fracture stress 1.0 MPa, and fracture strain 720%), owing to the interchain π–π and cation–π interactions. In water, the hydrogels firmly adhere to diverse surfaces through interfacial electrostatic and hydrophobic interactions (adhesion strength of 180 kPa), which allows for instant adhesion and reversibility (50 times). Moreover, the hydrogel shows long-lasting adhesion in water for months (100 days). Novel adhesive hydrogels may be useful in many applications, including underwater transfer, water-based devices, underwater repair, and underwater soft robots.     

Self‐Hydrophobization in a Dynamic Hydrogel for Creating Nonspecific Repeatable Underwater Adhesion

Adhesive hydrogels are widely applied for biological and medical purposes; however, they are generally unable to adhere to tissues under wet/underwater conditions. Herein, described is a class of novel dynamic hydrogels that shows repeatable and long‐term stable underwater adhesion to various substrates including wet biological tissues. The hydrogels have Fe³⁺‐induced hydrophobic surfaces, which are dynamic and can undergo a self‐hydrophobization process to achieve strong underwater adhesion to a diverse range of dried/wet substrates without the need for additional processes or reagents. It is also demonstrated that the hydrogels can directly adhere to biological tissues in the presence of under sweat, blood, or body fluid exposure, and that the adhesion is compatible with in vivo dynamic movements. This study provides a novel strategy for fabricating underwater adhesive hydrogels for many applications, such as soft robots, wearable devices, tissue adhesives, and wound dressings.     

Controllable Anisotropic Dry Adhesion in Vacuum: Gecko Inspired Wedged Surface Fabricated with Ultraprecision Diamond Cutting

Gecko adhesion has inspired the fabrication of various dry adhesive surfaces, most of which are developed to be used under atmospheric conditions. However, applications of gecko‐inspired surfaces can be expanded to vacuum and even space environment due to the characteristics of van der Waals interactions, which are always present between materials regardless of the surrounding environment. In this paper, a controllable, anisotropic dry adhesion in vacuum is demonstrated with gecko‐inspired wedged dry adhesive surfaces fabricated using an ultraprecision diamond cutting mold. The adhesion and friction properties of the wedge‐structured surfaces are systematically characterized in loading–pulling mode and loading–dragging–pulling mode. The surfaces show significant anisotropic adhesion (P_ad ≈ 10.5 kPa vs P_ad ≈ 0.7 kPa) and friction (P_f ≈ 50 kPa vs P_f ≈ 30 kPa) when actuated in gripping and releasing direction, respectively. The wedge‐structured surfaces in vacuum show comparable properties as exposed in atmosphere. A three‐legged gripper is designed to pick up, hold, and release a patterned silicon wafer in vacuum. The study demonstrates a green, high‐yield, and low‐cost method to fabricate a reliable and durable mold for gecko inspired anisotropic dry adhesive surfaces and the potential application of dry adhesive surface in vacuum.     

Elastic Energy Storage Enabled Magnetically Actuated,Octopus-Inspired Smart Adhesive

Octopus suckers offer remarkable adhesion performance against nonporous surfaces and have inspired extensive research to develop artificial adhesives. However, most of existing octopus-inspired adhesives are either passive without an actuation strategy or active but not energy efficient. Here, a novel design of a magnetically actuated, energy-efficient smart adhesive with rapidly tunable, great switchable, and highly reversible adhesion strength inspired by the elastic energy storage mechanism in octopus suckers is reported. The smart adhesive features two cavities separated by an elastic membrane with the upper cavity filled with magnetic particles while the lower one empty. The deformation of the elastic membrane can be actively controlled by an external magnetic field to change the cavity volume, thus generating a cavity-pressure-induced adhesion. Systematically experimental and theoretical studies reveal the fundamental aspects of design and operation of the smart adhesive and give insights into the underlying adhesion mechanisms. Demonstrations of this smart adhesive in transfer printing and manipulation of various surfaces in both dry and wet environments illustrate the potential for deterministic assembly and industrial or robotic manipulation.     

Reversible Elastomer–Fluid Transitions for Metamorphosic Robots

Endowing robots with reversible phase transition ability, especially between elastomer and fluid states, can significantly broaden their functionality and applicability. Limited attempts have been made to realize the reversible elastomer–fluid transition. Existing phase transition materials in robotics have over-hard (≈4 GPa) or over-soft (≈4 kPa) stiffness in the solid states, which should be further investigated to perform more compliant motions. To address these challenges, a reversible elastomer–fluid transition mechanism  enabled by magnetically induced hot melt materials (MIMMs) is presented. The transition principle is explained and material characterizations are conducted. MIMMs-based metamorphosic robots endow self-metamorphosing abilities, such as self-healing, spatial reshaping, self-division/assembly, and additive manufacturability. When interacting with external environments, MIMMs-based robots can perform further multifunctional abilities, such as collaborations for structure repairs, swimming by symbiosis with external objects, flowing through a narrow terrain by transiting to fluid, and working with elastomeric structures for stiffness-variable fluid soft actuators. The proposed elastomer–fluid transitions open a new path for robots to generate more flexible and metamorphosic motions, thereby addressing the cross-phase transformation challenges that soft robots face.     

Phototunable Underwater Oil Adhesion of Micro/Nanoscale Hierarchical‐Structured ZnO Mesh Films with Switchable Contact Mode

Controllable surface adhesion of solid substrates has aroused great interest both in air and underwater in solving many challenging interfacial science problems such as robust antifouling, oil‐repellent, and highly efficient oil/water separation materials. Recently, responsive surface adhesion, especially switchable adhesion, under external stimulus in air has been paid more and more attention in fundamental research and industrial applications. However, phototunable underwater oil adhesion is still a challenge. Here, an approach to realize phototunable underwater oil adhesion on aligned ZnO nanorod array‐coated films is reported, via a special switchable contact mode between an unstable liquid/gas/solid tri‐phase contact mode and stable liquid/liquid/solid tri‐phase contact mode. The photo‐induced wettability transition to water and air exists (or does not) in the micro/nanoscale hierarchical structure of the mesh films, playing important role in controlling the underwater oil adhesion behavior. This work is promising in the design of novel interfacial materials and functional devices for practical applications such as photo‐induced underwater oil manipulation and release, with loss‐free oil droplet transportation.     

Thermal Controlled Tunable Adhesive for Deterministic Assembly by Transfer Printing

Transfer printing techniques based on tunable adhesives that enable heterogeneous integration of materials in desired layouts are essential for developing existing and envisioned systems such as flexible electronics and micro-LED (µ-LED) displays. Here, a novel thermal controlled tunable adhesive, which not only has the ability of eliminating the interfacial adhesion for printing but also provides a new strategy for enhancing the interfacial adhesion for pick-up is reported. The tunable adhesive features cavities filled with air on the surface. This simple construct offers thermal controlled suction and thrust with their amplitudes on the order of a few tens of kPa within 100 °C temperature change, which enables a reliable damage-free transfer printing. Systematically theoretical and experimental studies reveal the underlying thermal induced pressure change mechanism and provide insights into the design and operation of the thermal controlled tunable adhesive. Demonstrations of this smart adhesive in manipulation of various surfaces and transfer printing of micro-scale Si inks and µ-LEDs illustrate its unusual capabilities for deterministic assembly by transfer printing.     

Self-Contained Underwater Adhesion and Informational Labeling Enabled by Arene-Functionalized Polymeric Ionogels

Adhesives and water exhibit a conflicting correlation as indicated by the failure of most synthetic adhesives in submerged and humid environments. Development of instant, strong, reversible, and long-lasting adhesives that can adhere to wet surfaces and function in underwater environments presents a formidable challenge, yet it is of paramount importance in biomedical and engineering applications. Herein, viscoelastic and moldable ionogels are developed based on synergistic engineering of aromatic substituents, fluorinated counterions, ionic building blocks, and 3D cross-linked networks. The molecular design and structural engineering result in a facile synthesis, two bonding methods (glue- and tape-type), and the combined mechanisms of enhanced adhesion and cohesion. The high underwater adhesion strength of over 8.9 MPa is among the best-performing tape-type underwater adhesives reported to date. A combination of excellent durability, reliability, deformation resistance, salt tolerance, water proof, antiswelling, and self-healing properties demonstrates the “self-contained” underwater adhesion. Furthermore, the extended π-conjugation of the aromatic pendant groups confers a new functionality to the ionogels – visible fluorescence, enabling intriguing applications such as underwater labeling, information encryption, and signal transmission. This study shines lights on the fabrication of ionogel-based adhesives and provides their future perspectives in underwater sealing, self-repair, crack diagnosis, and informational labeling.     

Gecko‐Inspired Controllable Adhesive Structures Applied to Micromanipulation

Gecko‐inspired angled elastomer micropillars with flat or round tip endings are presented as compliant pick‐and‐place micromanipulators. The pillars are 35 μm in diameter, 90 μm tall, and angled at an inclination of 20°. By gently pressing the tip of a pillar to a part, the pillar adheres to it through intermolecular forces. Next, by retracting quickly, the part is picked from a given donor substrate. During transferring, the adhesion between the pillar and the part is high enough to withstand disturbances due to external forces or the weight of the part. During release of the part onto a receiver substrate, the contact area of the pillar to the part is drastically reduced by controlled vertical or shear displacement, which results in reduced adhesive forces. The maximum repeatable ratio of pick‐to‐release adhesive forces is measured as 39 to 1. It is found that a flat tip shape and shear displacement control provide a higher pick‐to‐release adhesion ratio than a round tip and vertical displacement control, respectively. A model of forces to serve as a framework for the operation of this micromanipulator is presented. Finally, demonstrations of pick‐and‐place manipulation of micrometer‐scale silicon microplatelets and a centimeter‐scale glass cover slip serve as proofs of the concept. The compliant polymer micropillars are safe for use with fragile parts, and, due to exploiting intermolecular forces, could be effective on most materials and in air, vacuum, and liquid environments.     

Controllable Underwater Oil‐Adhesion‐Interface Films Assembled from Nonspherical Particles

A controllable underwater oil‐adhesion‐interface is presented based on colloidal crystals assembled from nonspherical latex particles. The underwater oil‐adhesive force of the as‐prepared film can be effectively controlled from high to low adhesion by varying the latex structures from spherical or cauliflower‐like to single cavity, which effectively adjusts the solid/liquid contact mode/wetting state of oil droplets on the films. This facile fabrication of functional films with special underwater oil‐adhesion properties based on a flexible design of a latex structure will offer significant insight for the design and creation of novel underwater antifouling materials.     

Asymmetric “Janus” Biogel for Human-Machine Interfaces

Human-machine interfaces (HMIs) are essential for effective communication between machines and tissues. However, mechanical and biological mismatches, along with weak adhesion between rigid electronic devices and soft tissue, often cause unreliable responses and affect the signal recording of HMIs. In this study, an asymmetrical “Janus” biogel patch with one side firmly adhering to tissues, and the other surface having little adhesion and minimal interactions with surrounding environments has been developed. A series of analytical, mechanical, and electrical tests are performed to investigate the “Janus” biogel patch as a functional and biocompatible HMI. It is found that the gallic acid-modified gelatin adhesive surface on one side exhibits body temperature-dependent tissue adhesion, enabling low modulus and seamless skin contact. The other side is a tough gelatin/glycerol gel layer, which is thermally welded into the adhesive layer and functions as an encapsulant to prevent external interference due to adhesion. The encapsulation layer also exhibits a low friction coefficient when wet and proves to be a reliable alternative barrier to conventional encapsulation materials. The scientific insights and engineering principles revealed in this type of “Janus” biogel will be applicable to a broad range of biomedical applications, such as epidermal adhesive electrodes or skin-adhesive wearable devices.     

Phase Change Hydrogels for Bio-Inspired Adhesion and Energy Exchange Applications

The adhesion strategies of the gecko's toe through surface adaptation of spatulas to increase contact area and the snail's epiphragm via dehydration-induced solidification to lock interfaces are combined to design a class of adhesion-switchable hydrogels. The hydrogels are made via incorporating CH₃COONa·3H₂O salt (SA) into polyacrylamide (PAM) aqueous networks to construct supersaturated and stimuli-responsive phase change materials (PAM-SA). The crystallization dramatically strengthens the mechanical properties, and tensile Young's moduli are 340.7 and 0.1 MPa for crystalline C-PAM-SA-120% and soft PAM hydrogel. As a result, PAM-SA-120% shows excellent adhesive performance (adhesion strength, 348 kPa) compared with PAM hydrogel adhesive (adhesion strength, 7 kPa). The stimuli-induced crystallization from H-PAM-SA-120% phase change hydrogels releases thermal controllably, which can be utilized for thermochromic materials and thermotherapy.     

Highly Permeable Skin Patch with Conductive Hierarchical Architectures Inspired by Amphibians and Octopi for Omnidirectionally Enhanced Wet Adhesion

Amphibian adhesion systems can enhance adhesion forces on wet or rough surfaces via hexagonal architectures, enabling omnidirectional peel resistance and drainage against wet and rough surfaces, often under flowing water. In addition, an octopus has versatile suction cups with convex cup structures located inside the suction chambers for strong adhesion in various dry and wet conditions. Highly air‐permeable, water‐drainable, and reusable skin patches with enhanced pulling adhesion and omnidirectional peel resistance, inspired by the microchannel network in the toe pads of tree frogs and convex cups in the suckers of octopi, are presented. By investigating various geometric parameters of microchannels on the adhesive surface, a simple model to maximize peeling strength via a time‐dependent zig‐zag profile and an arresting effect against crack propagation is first developed. Octopus‐like convex cups are employed on the top surfaces of the hexagonal structures to improve adhesion on skin in sweaty and even flowing water conditions. The amount of reduced graphene oxide nanoplatelets coated on the frog and octopus‐inspired hierarchical architectures is controlled to utilize the patches as flexible electrodes which can monitor electrocardiography signals without delamination from wet skin under motion.     

Water-Resistant Ionogel Electrode with Tailorable Mechanical Properties for Aquatic Ambulatory Physiological Signal Monitoring

Underwater electrocardiography (ECG) monitoring, which can monitor cardiac autonomic changes and arrhythmias during diving, is essential for sports management and healthcare. However, it is crucial yet rather challenging to achieve ECG monitoring in an aquatic environment because the interface electrodes may lose their functionality underwater. Here, an ionogel with tailorable mechanical properties is prepared by a facile one-step polymerization and used as water-resistant electrode. The Young's modulus and strain at break of the ionogel can be modulated in the range of 0.22–337 MPa and 349 to >10 000%, respectively. The hydrophobic polymer networks inside the ionogel endow this ionogel with excellent stability, adhesion, and self-healing ability underwater. The ionic conductivity imparted by the free ionic groups inside the ionogel allows the ionogel to detect and transmit physiological electrical signals. Compared with commercial gel electrodes, this ionogel electrode demonstrates better adhesion ability, conductivity, and stability underwater. The ionogel electrode can collect real-time ECG signals effectively both in the air and underwater, and the data can be used to warn users of the potential risk of a heart attack.     

A Conformable and Tough Janus Adhesive Patch with Limited 1D Swelling Behavior for Internal Bioadhesion

Advanced bioadhesion techniques have offered unprecedented opportunities for life-saving internal surgical procedures. However, most existing bioadhesives failed to rapidly establish long-lasting reliable biointerface and effectively reconstruct normal physiological function in the body fluid-rich and inevitable dynamic internal environment. Herein, a PEGylated poly(glycerol sebacate)-based Janus adhesive patch (PEGS-based JAP) is developed by integrating physically crosslinked guanidinylated PEGS (PEGSG) and acryloylated PEGS-chemically crosslinked poly(acrylic acid)-N-hydrosuccinimide ester (PEGSA-crosslinked PAAc-NHS) with a single-sided zwitterionic polymer-interpenetrated layer. The dry JAP can rapidly absorb the unpleasant interfacial water and effectively form strong physical interactions with wet tissues. Benefiting from the amphiphilic nature of PEGSG and a simple spatial-confined drying process, the JAP is allowed to resist excessive swelling and limitedly swell along one direction to avoid deterioration of the as-established conformable patch-tissue interface. Moreover, covalent interactions formed subsequently can further improve the adhesive strength and ensure a long-lasting reliable adhesion. Meanwhile, the notch-insensitive and puncture-resistant JAP can achieve tough adhesion to adapt to the dynamic conditions owing to the highly efficient energy-dissipation mechanism of the physically cross-linked network. Combining the above ideal features with the desirable postoperative anti-adhesion ability, the PEGS-based JAP is demonstrated to be a promising candidate for internal tissue adhesion and function reconstruction.     

Biomimetic Natural Biopolymer-Based Wet-Tissue Adhesive for Tough Adhesion,Seamless Sealed,Emergency/Nonpressing Hemostasis,and Promoted Wound Healing

Bioadhesives have been used in clinics among the most prospective alternatives to sutures and staples for wound sealing and repairing; however, they generally have inadequate adhesion to wet surfaces, improper mechanical strength, poor hemostasis, and cytotoxicity. To address these challenges, a robust wet tissue adhesive based on collagen and starch materials (CoSt) is designed in this study. CoSt hydrogels integrate the feature of drainage, molecular penetration and strengthen cross-linking similar to mussel, ivy, and oyster glues, which remove interfacial water quickly, reinforce tough dissipation and involve multiple reversible dynamic interactions. Therefore, they form strong adhesion and sutureless sealing of injured tissues, accompanying actuate robust biointerfaces in direct contact with tissue liquids or blood, resolving the crucial impediments with sutures and commercially accessible adhesives. The novel bioadhesive shows repeatable strong wet tissue adhesiveness (62 ± 4.8 KPa), high sealing performance (153.2 ± 35.1 mmHg), fast self-healing ability, excellent injectability, and shape adaptability. For different hemostatic needs in rat models of tail amputation, skin incision, severe liver, abdominal aorta, and transected nerve injuries, the CoSt hydrogel shows better hemostatic efficiency than fibrin glue because of the coordinate efficacy of tough wound sealing property, outstanding red blood cell arresting capability, and the activation of hemostatic barrier membrane. Moreover, in vivo investigation of the skin injury repair of the rat model validate that CoSt hydrogels accelerate wound healing and functional recovery via skin damage/defects. Tough wet adhesion, quick hemostasis, distinguished biocompatibility, suitability to match irregular-shaped target sites, and good wound healing promotion of the CoSt hydrogel makes it a prospective bioadhesive for various biomedical applications.     

In Situ Forming Dual-Conductive Hydrogels Enable Conformal,Self-Adhesive and Antibacterial Epidermal Electrodes

Conductive hydrogels (CHs) are regarded as one of the most promising materials for bioelectronic devices on human-machine interfaces (HMIs). However, conventional CHs cannot conform well with complex skin surfaces, such as hairy or wrinkled skin, due to pre-formation and insufficient adhesion; they also usually lack antibacterial abilities and require tissue-harm and time-consuming preparation (e.g., heating or ultraviolet irradiation), which limits their practical application on HMIs. Herein, an in situ forming CH is proposed by taking advantage of the PEDOT:PSS-promoted self-polymerization of zwitterionic [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) (SBMA). The hydrogel is formed spontaneously after injection of the precursor solution onto the desired location without any additional treatments. The as-prepared hydrogel possesses excellent elasticity (elastic recovery >96%), desirable adhesive strength (≈6.5 kPa), biocompatibility, and intrinsically antibacterial properties. Without apparent heat release (<5 °C) during gelation, the hydrogel can form in situ on skin. Additionally, the obtained hydrogel can establish tight contact with skin, forming highly conformal interfaces on hairy skin surfaces and irregular wounds. Finally, the in situ forming hydrogels are applied as conformal epidermal electrodes to record stable and reliable surface electromyogram signals from hairy skin (with high signal-to-noise ratio, SNR ≈ 32 dB) and accelerate diabetic wound healing under electrical stimulation.