Superior Low-Temperature Reversible Adhesion Based on Bio-Inspired Microfibrillar Adhesives Fabricated by Phenyl Containing Polydimethylsiloxane Elastomers

Gecko-inspired microfibrillar adhesives have achieved great progress in microstructure design and adhesion improvement over the past two decades. Space applications nowadays show great interest in this material for the characteristics of reversible adhesion and universal van der Waals interactions. However, the impact of harsh environment of space on the performance of microfibrillar adhesives, especially the extreme low temperature, is rarely addressed. Herein, microfibrillar adhesives fabricated by phenyl containing polydimethylsiloxane (p-PDMS) elastomers with superior low-temperature reversible adhesion is proposed. p-PDMS elastomers are synthesized through one-pot anionic ring-opening copolymerization, and the resulting elastomers become non-crystallizable with excellent low-temperature elasticity. Low-temperature adhesion tests demonstrate that the adhesion strength of microfibrillar adhesives fabricated by p-PDMS elastomers can be well maintained to as low as −120 °C. In contrast, the adhesion strength of pure PDMS microfibrillar adhesive reduces more than 50% below its crystallization temperature. The low-temperature cyclic adhesion tests further demonstrate that p-PDMS microfibrillar adhesives exhibit superior reversible adhesion compared to that of PDMS microfibrillar adhesives, owing to the sustainable conformal contact and even distribution of loads over repeated cycles. This study provides a new fabrication strategy for microfibrillar adhesives, and is beneficial for the practical application of microfibrillar adhesives.     

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.     

Ultra Rate-Dependent Pressure Sensitive Adhesives Enabled by Soft Elasticity of Liquid Crystal Elastomers

The fabrication of pressure sensitive adhesives (PSAs) using liquid crystal elastomers (LCEs), which are known for their excellent dissipation properties, is explored in this work. The adhesive properties of the PSAs are evaluated using the 180° peeling test at various conditions. The performance of the LCE adhesives is found to show significant rate and temperature dependence. When the adhesion energy is plotted against the rate, LCE shows an anomalously large power law exponent (n ≈ 1.17) compared to existing PSAs (n ≈ 0.1–0.6). The unusual rate sensitivity is hypothesized to originate from the synergy of soft elasticity and non-linear viscoelasticity. The adhesive properties at various rates and temperatures are correlated to the results from dynamic mechanical analysis. Moreover, the large strain stiffening behavior of LCE under uniaxial tension reveals the distinctive advantages offered by LCE as adhesives. Time-temperature superposition is used to obtain a master curve of adhesion energy that spans rates beyond typical experimental limits. The extreme rate dependence and the large strain stiffening of LCE yield a new category of adhesives that possess special properties, such as reversible adhesion and impact resistance, unlike traditional adhesives.     

Towards the Next Level of Bioinspired Dry Adhesives: New Designs and Applications

This Feature Article aims to highlight our recent efforts to develop more robust gecko‐inspired dry adhesives and their applications. Due to recent progress in micro‐ and nanofabrication techniques, it is possible to fabricate highly sophisticated multiscale, hierarchical structures using various polymer materials. In addition, the adhesion strength of synthetic dry adhesives has been shown to be similar to or exceed that of real gecko foot‐hair by several times. Therefore, it is timely and appropriate to drive the research of gecko‐inspired dry adhesives into a new epoch by investigating more robust dry adhesive structures, efficient detachment mechanisms, and new applications. In this Feature Article, we present a series of our recent achievements to overcome some of the limitations of gecko‐like hair structures such as rough surface adaptation, durability, and controlled geometry, with particular emphasis on materials issues and detachment mechanism. For potential applications, a clean transportation device and a biomedical skin patch are briefly described to expand the application realm from the well‐known wall climbing robot.     

Patterned Hydrogels for Controlled Platelet Adhesion from Whole Blood and Plasma

This work describes the preparation and properties of hydrogel surface chemistries enabling controlled and well‐defined cell adhesion. The hydrogels may be prepared directly on plastic substrates, such as polystyrene slides or dishes, using a quick and experimentally simple photopolymerization process, compatible with photolithographic and microfluidic patterning methods. The intended application for these materials is as substrates for diagnostic cell adhesion assays, particularly for the analysis of human platelet function. The non‐specific adsorption of fibrinogen, a platelet adhesion promoting protein, is shown to be completely inhibited by the hydrogel, provided that the film thickness is sufficient (>5 nm). This allows the hydrogel to be used as a matrix for presenting selected bioactive ligands without risking interference from non‐specifically adsorbed platelet adhesion factors, even in undiluted whole blood and blood plasma. This concept is demonstrated by preparing patterns of proteins on hydrogel surfaces, resulting in highly controlled platelet adhesion. Further insights into the protein immobilization and platelet adhesion processes are provided by studies using imaging surface plasmon resonance. The hydrogel surfaces used in this work appear to provide an ideal platform for cell adhesion studies of platelets, and potentially also for other cell types.     

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.     

Engineering Micropatterned Dry Adhesives: From Contact Theory to Handling Applications

Reversible adhesion is the key functionality to grip, place, and release objects nondestructively. Inspired by nature, micropatterned dry adhesives are promising candidates for this purpose and have attracted the attention of research groups worldwide. Their enhanced adhesion compared to nonpatterned surfaces is frequently demonstrated. An important conclusion is that the contact mechanics involved is at least as important as the surface energy and chemistry. In this paper, the roles of the contact geometry and mechanical properties are reviewed. With a focus on applications, the effects of substrate roughness and of temperature variations, and the long‐term performance of micropatterned adhesives are discussed. The paper provides a link between the current, detailed understanding of micropatterned adhesives and emerging applications.     

A Bioinspired Medical Adhesive Derived from Skin Secretion of Andrias davidianus for Wound Healing

Existing surgical tissue adhesives on the market cannot meet the desired demand for clinical operations due to their limited adhesivity or undesired cytotoxicity. A new bioadhesive is derived from the skin secretion of Andrias davidianus (SSAD). This bioinspired SSAD has significantly stronger tissue adhesion than the fibrin glue and improved elasticity and biocompatibility when compared to the cyanoacrylate glue both ex vivo and in vivo. Additionally, the SSAD‐based adhesive decreases skin wound healing time and promotes wound regeneration and angiogenesis. The SSAD‐based adhesive is completely degradable, strongly adhesive, and easily produced from a renewable source. Based on these favorable properties, the SSAD‐based bioadhesive demonstrates potential as a surgical bioadhesive for a broad range of medical applications.     

Tough Adhesion of Nucleobase‐Tackifed Gels in Diverse Solvents

Adhesive gels have attracted increasing attention in biological medicine and industrial fields. However, it remains a huge challenge to achieve robust adhesion in various nonpolar and polar solvents. Herein, a tough nucleobase‐tackified adhesive gel is successfully fabricated and exhibits strong adhesion to various materials in diverse solvents, including hexane, chloroform, dimethyl sulfoxide, ethanol, and water (seawater, high salt, acid, and alkali aqueous solutions). The adhesive gels possess high toughness and excellent resist fatigue as well as impressive nonswelling behavior in water or oil. This tough gel‐based adhesive holds great promise for various applications, such as battery adhesives, soft robots, wound dressing, wearable devices, and 3D printing in various environments. It is anticipated that this strategy will provide a novel route for fabricating the next generation of adhesive soft materials.     

Supramolecular β‐Sheet Suckerin–Based Underwater Adhesives

Nature has evolved several molecular strategies to ensure adhesion in aqueous environments, where artificial adhesives typically fail. One recently‐unveiled molecular design for wet‐resistant adhesion is the cohesive cross‐β structure characteristic of amyloids, complementing the well‐established surface‐binding strategy of mussel adhesive proteins based on 3,4‐l ‐dihydroxyphenylalanine (Dopa). Structural proteins that self‐assemble into cross β‐sheet networks are the suckerins discovered in the sucker ring teeth of squids. Here, light is shed on the wet adhesion of cross‐β motifs by producing recombinant suckerin‐12, naturally lacking Dopa, and investigating its wet adhesion properties. Surprisingly, the adhesion forces measured on mica reach 70 mN m^?1, exceeding those measured for all mussel adhesive proteins to date. The pressure‐sensitive adhesion of artificial suckerins is largely governed by their cross‐β motif, as evidenced using control experiments with disrupted cross‐β domains that result in complete loss of adhesion. Dopa is also incorporated in suckerin‐12 using a residue‐specific incorporation strategy that replaces tyrosine with Dopa during expression in Escherichia coli. Although the replacement does not increase the long‐term adhesion, it contributes to the initial rapid contact and enhances the adsorption onto model oxide substrates. The findings suggest that suckerins with supramolecular cross‐β motifs are promising biopolymers for wet‐resistant adhesion.     

Plant‐Inspired Pyrogallol‐Containing Functional Materials

Pyrogallol‐containing molecules are ubiquitous in the plant kingdom. The chemical synthesis of these molecules remains challenging. Thus, they are obtained via purification from heterogeneous mixtures of plant extracts. Previous studies have focused on their biological roles, such as antioxidants. Additionally, the molecules are used as ink colorants and in tanning processes for leather. Recently, many disciplines have paid attention to adhesiveness of pyrogallol‐containing molecules, including the control of interface properties in energy storage/generation and medical devices, as well as the changes in wettability related to membrane technologies. In particular, pyrogallol‐containing molecules act as “molecular glues,” binding to virtually all biomacromolecules, for example, DNA/RNA, soluble proteins, insoluble extracellular matrices, and peptides. Furthermore, the cohesion of pyrogallol by forming pyrogallol‐to‐pyrogallol covalent bonds is useful for the preparation of bulk hydrogels and thin films. The content of this review focuses on interactions with biomacromolecules used as molecular glues, used as modifiers in material‐independent surface chemistry, and applied as chemical moieties to form covalent linkages to fabricate hydrogels and related biomaterials. Future perspectives include the development of new pyrogallol‐containing materials, the understanding of chirality in adhesion, and the improvement of the mechanical stability for applications in various biomedical, energy, and industrial devices.     

Injectable Self-Healing Natural Biopolymer-Based Hydrogel Adhesive with Thermoresponsive Reversible Adhesion for Minimally Invasive Surgery

Biocompatible hydrogel adhesives with multifunctional properties, including injectability, fast self-healing, and suitable on-demand detachment, are highly desired for minimally invasive procedures, but such materials are still lacking. Herein, an injectable self-healing biocompatible hydrogel adhesive with thermoresponsive reversible adhesion based on two extracellular matrix-derived biopolymers, gelatin and chondroitin sulfate, is developed to be used as a surgical adhesive for sealing or reconnecting ruptured tissues. The resulting hydrogels present good self-healing and can be conveniently injected through needles. The strong tissue adhesion at physiological temperatures originates from the Schiff base and hydrogen bonding interactions between the hydrogel and tissue that can be weakened at low temperatures, thereby easily detaching the hydrogel from the tissue in the gelation state. In vivo and ex vivo rat model show that the adhesives can effectively seal bleeding wounds and fluid leakages in the absence of sutures or staples. Specifically, a proof of concept experiment in a damaged rat liver model demonstrates the ability of the adhesives to act as a suitable laparoscopic sealant for laparoscopic surgery. Overall, the adhesive has several advantages, including low cost and ease of production and application that make it an exceptional multifunctional tissue adhesive/sealant, effective in minimally invasive surgical applications.     

Metal‐Based Nanomaterials in Biomedical Applications: Antimicrobial Activity and Cytotoxicity Aspects

Microbial colonization on material surfaces is ubiquitous. Biofilms derived from surface‐colonized microbes pose serious problems to the society from both an economical perspective and a health concern. Incorporation of antimicrobial nanocompounds within or on the surface of materials, or by coatings, to prevent microbial adhesion or kill the microorganisms after their attachment to biofilms, represents an important strategy in an increasingly challenging field. Over the last decade, many studies have been devoted to preparing meta‐based nanomaterials that possess antibacterial, antiviral, and antifungal activities to combat pathogen‐related diseases. Herein, an overview on the state‐of‐the‐art antimicrobial nanosized metal‐based compounds is provided, including metal and metal oxide nanoparticles as well as transition metal nanosheets. The antimicrobial mechanism of these nanostructures and their biomedical applications such as catheters, implants, medical delivery systems, tissue engineering, and dentistry are discussed. Their properties as well as potential caveats such as cytotoxicity, diminishing efficacy, and induction of antimicrobial resistance of materials incorporating these nanostructures are reviewed to provide a backdrop for future research.     

Physical Double‐Network Hydrogel Adhesives with Rapid Shape Adaptability,Fast Self‐Healing,Antioxidant and NIR/pH Stimulus‐Responsiveness for Multidrug‐Resistant Bacterial Infection and Removable Wound Dressing

Developing physical double‐network (DN) removable hydrogel adhesives with both high healing efficiency and photothermal antibacterial activities to cope with multidrug‐resistant bacterial infection, wound closure, and wound healing remains an ongoing challenge. An injectable physical DN self‐healing hydrogel adhesive under physiological conditions is designed to treat multidrug‐resistant bacteria infection and full‐thickness skin incision/defect repair. The hydrogel adhesive consists of catechol–Fe³⁺ coordination cross‐linked poly(glycerol sebacate)‐co‐poly(ethylene glycol)‐g‐catechol and quadruple hydrogen bonding cross‐linked ureido‐pyrimidinone modified gelatin. It possesses excellent anti‐oxidation, NIR/pH responsiveness, and shape adaptation. Additionally, the hydrogel presents rapid self‐healing, good tissue adhesion, degradability, photothermal antibacterial activity, and NIR irradiation and/or acidic solution washing‐assisted removability. In vivo experiments prove that the hydrogels have good hemostasis of skin trauma and high killing ratio for methicillin‐resistant staphylococcus aureus (MRSA) and achieve better wound closure and healing of skin incision than medical glue and surgical suture. In particular, they can significantly promote full‐thickness skin defect wound healing by regulating inflammation, accelerating collagen deposition, promoting granulation tissue formation, and vascularization. These on‐demand dissolvable and antioxidant physical double‐network hydrogel adhesives are excellent multifunctional dressings for treating in vivo MRSA infection, wound closure, and wound healing.     

Deep Eutectic Solvents-Based Ionogels with Ultrafast Gelation and High Adhesion in Harsh Environments

Adhesive materials have recently drawn intensive attention due to their excellent sealing ability, stimulating advances in materials science and industrial usage. However, reported adhesives usually exhibit weak adhesion strength, require high pressure for strong bonding, and display severe adhesion deterioration in various harsh environments. In this study, instead of water or organic solvents, a deep eutectic solvent (DES) is used as the medium for the photopolymerization of zwitterionic and polarized monomers, thus generating a novel ionogel with tunable mechanical properties. Multiple hydrogen bonds and electrostatic interactions between DES and monomers facilitated ultrafast gelation and instant bonding without any external pressure, which has rarely been reported previously. Furthermore, high adhesion in different harsh environments (e.g., water, acidic and basic buffers, and saline solutions) and onto hydrophilic (e.g., glass and tissues) and hydrophobic (e.g., polymethyl methacrylate, polystyrene, and polypropylene) adherends is demonstrated. Also, the high stretchability of the ionogel at extreme temperatures (−80 and 80 °C) indicates its widespread applications. Furthermore, the biocompatible ionogel shows high burst pressure onto stomach and intestine tissues to prevent liquid leakage, highlighting its potential as an adhesive patch. This ionogel provides unprecedented opportunities in the packaging industry, marine engineering, medical adhesives, and electronic assembly.     

Cucurbit[n]uril Supramolecular Hydrogel Networks as Tough and Healable Adhesives

Supramolecular noncovalent interactions are widely found in natural adhesion phenomena to control macroscopic adhesion and accomplish a variety of complex functions. Such supramolecular adhesives could impart the interfaces with intriguing properties, e.g., energy dissipation and self‐healing, on account of their dynamic nature. Here, we demonstrate that cucurbit[8]uril (CB[8])‐based supramolecular hydrogel networks can function as dynamic adhesives for diverse nonporous (e.g., glass, stainless steel, aluminum, copper, and titanium) and porous substrates (wood and bone). Without any surface prefunctionalization or introduction of curing agents, these CB[8] hydrogel networks can be readily applied by curing around the softening temperature, forming a tough and healable adhesive interlayer. The ability to fabricate a robust sandwich model consisting of substrate–CB[8] hydrogel network–substrate enables a number of applications including stretchable and wearable electronics, hybrid systems for biomedical devices or tissue/bone regeneration.     

Highly Conductive,Flexible, Polyurethane‐Based Adhesives for Flexible and Printed Electronics

Flexible interconnects are one of the key elements in realizing next‐generation flexible electronics. While wire bonding interconnection materials are being deployed and discussed widely, adhesives to support flip‐chip and surface‐mount interconnections are less commonly used and reported. A polyurethane (PU)‐based electrically conductive adhesive (ECA) is developed to meet all the requirements of flexible interconnects, including an ultralow bulk resistivity of ≈1.0 × 10^?5 Ω cm that is maintained during bending, rolling, and compressing, good adhesion to various flexible substrates, and facile processing. The PU‐ECA enables various interconnection techniques in flexible and printed electronics: it can serve as a die‐attach material for flip‐chip, as vertical interconnect access (VIA)‐filling and polymer bump materials for 3D integration, and as a conductive paste for wearable radio‐frequency devices.     

Nanostructured Si/Organic Heterojunction Solar Cells with High Open‐Circuit Voltage via Improving Junction Quality

Nanostructured silicon (Si) can provide improved light harvest efficiencies in organic‐Si heterojunction solar cells due to its low light reflection ratio compared with planar one. However, the associated large surface/volume ratio of nanostructured Si suffers from serious surface recombination as well as poor adhesion with organics in organic‐Si heterojunction solar cells, which leads to an inferior open‐circuit voltage (V_oc). Here, we develop a simple and effective method to suppress charge recombination as well as enhancing adhesion force between nanostructured Si and organics by incorporating a silane chemical, namely 3‐glycidoxypropyltrimethoxydsilane (GOPS). GOPS can chemically graft onto nanostructured Si and improve the aqueous organic wetting properties, suppressing surface charge recombination velocity dramatically. In addition, this chemically grafted layer can enhance adhesion force between organics and Si. In such a way, a record V_oc of 640 mV associated with a power conversion efficiency of 14.1% is obtained for organic‐nanostructured Si heterojunction devices. These findings suggest a promising approach to low‐cost and simple fabrication for high‐performance organic‐Si solar cells.     

Controllable Particle Adhesion with a Magnetically Actuated Synthetic Gecko Adhesive

Particle capture and release using controllable adhesion is of growing interest in many fields including reusable adhesives, solar panel cleaning, micro‐manipulation and robot locomotion where current methods of particle management are not sufficient. Here controllable adhesion to glass spheres with a new magnetically actuated synthetic gecko adhesive made from a magnetoelastomer composite is demonstrated. Adhesion is controlled by changing the effective elastic modulus of the surface through actuation of micro surface features with an external magnetic field. A compliant mechanics and magnetic torque analysis explains this general principle and generalizes the results for various geometries. Results show sphere pull‐off forces can be increased 10‐fold by changing the ridge orientation via the external magnetic field, and that the effective elastic modulus can be changed from 65 kPa to 1.5 MPa. Particle transport and release of 500‐ and 1000‐micrometer glass spheres is also demonstrated.     

Bioinspired Adhesive Hydrogels Tackified by Nucleobases

Bioinspired strategies for designing hydrogels with excellent adhesive performance have drawn much attention in biomedical applications. Here, bioinspired adhesive hydrogels tackified by independent nucleobase (adenine, thymine, guanine, cytosine, and uracil) from DNA and RNA are successfully explored. The nucleobase‐tackified hydrogels exhibit an excellent adhesive behavior for not only various solid substrates (polytetrafluoroethylene, plastics, rubbers, glasses, metals, and woods) but also biological tissues consisting of heart, liver, spleen, lung, kidney, bone, and muscle. The maximum adhesion strength of A‐, T‐, G‐, C‐, and U‐tackified hydrogels on the aluminum alloy surface is 780, 166, 250, 227, and 433 N m^?1, respectively, superior to that of pure PAAm hydrogels (40 N m^?1) after adhesive time of 10 min. It is anticipated that bioinspired hydrogels will play a significant role in the applications of wound dressing, medical electrodes, tissue adhesives, and portable equipment. Moreover, the bioinspired nucleobase‐tackified strategy would open a novel avenue for designing the next generation of soft and adhesive materials.