High capacity, easy release adhesives from renewable materials

Isosorbide-Based Acrylic Pressure-Sensitive Adhesives Through UV-Cured Crosslinking with a Balance Between Adhesion and Cohesion

The development of sustainable pressure-sensitive adhesives (PSAs) from natural biomass resources has attracted increasing attention owing to their non-toxic, biocompatible, and biodegradable features. In this study, a bio-based acrylic PSA with tunable adhesion and cohesion was synthesized by a selective chemical modification of isosorbide-5-acrylate (IA) and its copolymerization with butyl acrylate and acrylic acid through UV-curing crosslinking. During the UV-curing process, the synthesized isosorbide diacrylate ester (IDAE) served as the crosslinker, effectively improving the crosslinking degree of PSA. The impact of IA and IDAE on the mechanical properties of PSA was studied. Moreover, to achieve a balance between adhesion and cohesion, the optimal composition was identified. The addition of IA significantly enhances the stiffness of PSA. Furthermore, the combined effect of IA and IDAE improves the overall adhesion properties of the PSA. The optimal bio-based PSA demonstrates a peel force of 13.9 N/25 mm and a persistent time of 6820 min, promising to replace traditional petroleum-based PSAs.

E-Tattoos: Toward Functional but Imperceptible Interfacing with Human Skin

The human body continuously emits physiological and psychological information from head to toe. Wearable electronics capable of noninvasively and accurately digitizing this information without compromising user comfort or mobility have the potential to revolutionize telemedicine, mobile health, and both human-machine or human-metaverse interactions. However, state-of-the-art wearable electronics face limitations regarding wearability and functionality due to the mechanical incompatibility between conventional rigid, planar electronics and soft, curvy human skin surfaces. E-Tattoos, a unique type of wearable electronics, are defined by their ultrathin and skin-soft characteristics, which enable noninvasive and comfortable lamination on human skin surfaces without causing obstruction or even mechanical perception. This review article offers an exhaustive exploration of e-tattoos, accounting for their materials, structures, manufacturing processes, properties, functionalities, applications, and remaining challenges. We begin by summarizing the properties of human skin and their effects on signal transmission across the e-tattoo-skin interface. Following this is a discussion of the materials, structural designs, manufacturing, and skin attachment processes of e-tattoos. We classify e-tattoo functionalities into electrical, mechanical, optical, thermal, and chemical sensing, as well as wound healing and other treatments. After discussing energy harvesting and storage capabilities, we outline strategies for the system integration of wireless e-tattoos. In the end, we offer personal perspectives on the remaining challenges and future opportunities in the field.

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.

Recent advances in adhesive materials used in the biomedical field: adhesive properties, mechanism, and applications

Adhesive materials are natural or synthetic polymers with the ability to adhere to the surface of luminal mucus or epithelial cells. They are widely used in the biomedical field due to their unique adhesion, biocompatibility, and excellent surface properties. When used in the human body, they can adhere to an accessible target and remain at the focal site for a longer period, improving the therapeutic effect on local disease. An adhesive material with bacteriostatic properties can play an antibacterial role at the focal site and the adhesive properties of the material can prevent the focal site from being infected by bacteria for a period. In addition, some adhesive materials can promote cell growth and tissue repair. In this review, the properties and mechanism of natural adhesive materials, organic adhesive materials, composite adhesive materials, and underwater adhesive materials have been introduced systematically. The applications of these adhesive materials in drug delivery, antibacterials, tissue repair, and other applications are described in detail. Finally, we have discussed the prospects and challenges of using adhesive materials in the field of biomedicine.

Stiff skin, soft core: soft backings enhance the conformability and friction of fibre-reinforced adhesives

Biomimetic adhesives with a stiff fibre-reinforced base layer generate strong attachment, even without bioinspired micropatterning of the contact surface. However, current fibre-reinforced adhesive designs are still less versatile with respect to substrate variability than their biological counterparts. In this study, we enhance the comformability of a fibre-reinforced adhesive on curved substrates by adding bioinspired soft backings. We designed and fabricated soft backing variations (polyurethane foams and silicone hydroskeletons) with varying compressive stiffnesses that mimic the soft viscoelastic structures in the adhesive appendages of tree frogs, geckos and other animals. The backings were mounted on a smooth silicone layer enforced with a polyester mesh, and we experimentally investigated the contact area and friction performance of these adhesives on a curved substrate. The results show that the contact area and friction created by a fibre-reinforced adhesive with a soft backing in contact with a non-flat substrate scale inversely with backing stiffness. The integration of stiff fibre-reinforcement with a compressible backing represents an important step in bringing bioinspired adhesives out of the laboratory and into the real world, for example in soft robotic grippers. Moreover, our findings stimulate further research into the role of soft tissues in biological adhesive systems.

A mechanics-based approach to realize high–force capacity electroadhesives for robots

Materials with electroprogrammable stiffness and adhesion can enhance the performance of robotic systems, but achieving large changes in stiffness and adhesive forces in real time is an ongoing challenge. Electroadhesive clutches can rapidly adhere high stiffness elements, although their low force capacities and high activation voltages have limited their applications. A major challenge in realizing stronger electroadhesive clutches is that current parallel plate models poorly predict clutch force capacity and cannot be used to design better devices. Here, we use a fracture mechanics framework to understand the relationship between clutch design and force capacity. We demonstrate and verify a mechanics-based model that predicts clutch performance across multiple geometries and applied voltages. On the basis of this approach, we build a clutch with 63 times the force capacity per unit electrostatic force of state-of-the-art electroadhesive clutches. Last, we demonstrate the ability of our electroadhesives to increase the load capacity of a soft, pneumatic finger by a factor of 27 times compared with a finger without an electroadhesive.

A Bio-Based Supramolecular Adhesive: Ultra-High Adhesion Strengths at both Ambient and Cryogenic Temperatures and Excellent Multi-Reusability

Developing high-performance and reusable adhesives from renewable feedstocks is of significance to sustainable development, yet it still remains a formidable task. Herein, castor oil, melevodopa, and iron ions are used as building blocks to construct a novel bio-based supramolecular adhesive (BSA) with outstanding adhesion performances. It is prepared through partial coordination between melevodopa functionalized castor oil and Fe3+ ions. Noncovalent interactions between adherends and the catechol unit from melevodopa contribute to reinforcing adhesion, and the metal-ligand coordination between catechol and Fe3+ ions is utilized to strengthen cohesion. By combining strong adhesion and tough cohesion, the prepared BSA achieves an adhesion strength of 14.6 MPa at ambient temperature, a record-high value among reported bio-based adhesives as well as supramolecular adhesives to the best of knowledge. It also outperforms those adhesives at cryogenic temperature, realizing another record-high adhesion strength of 9.5 MPa at -196 °C. In addition, the BSA displays excellent multi-reusability with more than 87% of the original adhesion strength remaining even after reuse for ten times. It is highly anticipated that this line of research will provide a new insight into designing bio-based adhesives with outstanding adhesion performances and excellent multi-reusability.

Photo-Detachable Self-Cleaning Surfaces Inspired by Gecko Toepads

Strong, reversible, and self-cleaning adhesion in the toe pads of geckos allow the lizards to climb on a variety of vertical and inverted surfaces, regardless of the surface conditions, whether hydrophobic or hydrophilic, smooth or tough, wet or dry, clean or dirty. Development of synthetic gecko-inspired surfaces has drawn a great attention over the past two decades. Despite many external-stimuli responsive mechanisms (i.e., thermal, electrical, magnetic) have been successfully demonstrated, smart adhesives controlled by light signals still substantially lag behind. Here, in this report, we integrate tetramethylpiperidinyloxyl (TEMPO)-doped polydopamine (PDA), namely, TDPDA, with PDMS micropillars using a template-assisted casting method, to achieve both improved adhesion and self-cleaning performances. To the best of our knowledge, this is the first report on PDA being used as a doping nanoparticle in bioinspired adhesive surfaces to achieve highly efficient self-cleaning controllable by light signals. Notably, the adhesion of the 5% TDPDA-PDMS sample is ∼688.75% higher than that of the pure PDMS at the individual pillar level, which helps to explain the highly efficient self-cleaning mechanism. The sample surfaces (named TDPDA-PDMS) can efficiently absorb 808 nm wavelength of light and heat up from 25 °C to 80.9 °C in 3 min with NIR irradiation. The temperature rise causes significant reduction of adhesion, which results in outstanding self-cleaning rate of up to 55.8% within five steps. The exploration of the photoenabled switching mechanism with outstanding sensitivity may bring the biomimetic smart surfaces into a new dimension, rendering varied applications, e.g., in miniaturized climbing robot, artificial intelligence programmable manipulation/assembly/filtration, active self-cleaning solar panels, including high output sensors and devices in many engineering and biomedical frontiers.

The relationship between viscoelasticity and elasticity

Soft materials that are subjected to large deformations exhibit an extremely rich phenomenology, with properties lying in between those of simple fluids and those of elastic solids. In the continuum description of these systems, one typically follows either the route of solid mechanics (Lagrangian description) or the route of fluid mechanics (Eulerian description). The purpose of this review is to highlight the relationship between the theories of viscoelasticity and of elasticity, and to leverage this connection in contemporary soft matter problems. We review the principles governing models for viscoelastic liquids, for example solutions of flexible polymers. Such materials are characterized by a relaxation time λ, over which stresses relax. We recall the kinematics and elastic response of large deformations, and show which polymer models do (and which do not) correspond to a nonlinear elastic solid in the limit λ → ∞. With this insight, we split the work done by elastic stresses into reversible and dissipative parts, and establish the general form of the conservation law for the total energy. The elastic correspondence can offer an insightful tool for a broad class of problems; as an illustration, we show how the presence or absence of an elastic limit determines the fate of an elastic thread during capillary instability.

Substrate-Independent, Reversible, and Easy-Release Ionogel Adhesives with High Bonding Strength

It is highly desirable to develop reversible and easy-release adhesives with high bonding strength for a broad range of substrates, while the adhesion of low-surface-energy materials (e.g., polytetrafluoretyhylene, PTFE) is challenging. Herein, a substrate-independent ionogel adhesive is developed by blending an ionic liquid with the copolymer bearing charged segments. By regulating the viscoelastic properties of the ionogel, the adhesive and cohesive strength of the ionogel can be well balanced to maximize the bonding strength for different substrates. The as-developed ionogel exhibits high bonding strength (>0.3 MPa) for PTFE, plastics, metal, wood, and glass, because the variety of functional groups in the ionogel can form various supramolecular interactions with different substrates. The ionogel also exhibits reversible, easy-release, and reusable properties for multiple times of bonding and on-demand debonding without leaving obvious residues on the substrates. The ionogel has high potential for practical applications as temporal adhesives with high bonding strength.

Intrinsically reversible superglues via shape adaptation inspired by snail epiphragm

Adhesives are ubiquitous in daily life and industrial applications. They usually fall into one of two classes: strong but irreversible (e.g., superglues) or reversible/reusable but weak (e.g., pressure-sensitive adhesives and biological and biomimetic surfaces). Achieving both superstrong adhesion and reversibility has been challenging. This task is particularly difficult for hydrogels that, because their major constituent is liquid water, typically do not adhere strongly to any material. Here, we report a snail epiphragm-inspired adhesion mechanism where a polymer gel system demonstrates superglue-like adhesion strength (up to 892 N⋅cm^-2) that is also reversible. It is applicable to both flat and rough target surfaces. In its hydrated state, the softened gel conformally adapts to the target surface by low-energy deformation, which is locked upon drying as the elastic modulus is raised from hundreds of kilopascals to ∼2.3 GPa, analogous to the action of the epiphragm of snails. We show that in this system adhesion strength is based on the material's intrinsic, especially near-surface, properties and not on any near-surface structure, providing reversibility and ease of scaling up for practical applications.

Effects of Digit Orientation on Gecko Adhesive Force Capacity: Synthetic and Behavioral Studies

In this study we developed an analytical relationship between adhesive digit orientation and adhesive force capacity to describe the tendencies of climbing organisms that use adhesion for climbing to align their toes in the direction of loading, maximizing adhesive force capacity. We fabricated a multi-component adhesive device with multiple contact surfaces, or digits, to act as a model system mimicking the angular motion of a foot and found the synthetic experiments agree with the developed analytical relationship. In turn, we find that observations of gekkonid lizards climbing on vertical substrates correlate well with our analytical relationship; a reduction in toe spacing is seen on the forelimbs when the animals are facing up. Interestingly, the toes on the hindlimbs tend to have an increase in spacing, possibly a mechanism for stabilization rather than load-bearing.

Strong, Rebondable, Dynamic Cross-Linked Cellulose Nanocrystal Polymer Nanocomposite Adhesives

A series of strong, rebondable polydisulfide nanocomposite adhesive films have been prepared via the oxidation of a thiol-endcapped semicrystalline oligomer with varying amounts of thiol-functionalized cellulose nanocrystals (CNC-SH). The nanocomposites are designed to have two temperature-sensitive components: (1) the melting of the semicrystalline phase at ca. 70 °C and (2) the inherent dynamic behavior of the disulfide bonds at ca. 150 °C. The utility of these adhesives was demonstrated on different bonding substrates (hydrophilic glass slides and metal), and their bonding at both 80 and 150 °C was examined. In all cases, stronger bonding was achieved at temperatures where the disulfide bonds are dynamic. For high surface energy substrates, such as hydrophilic glass or metal, the adhesive shear strength increases with CNC-SH content, with the 30 wt % CNC-SH composites exhibiting adhesive shear strengths of 50 and 23 MPa for hydrophilic glass and metal, respectively. The effects of contact pressure and time of bonding were also investigated. It was found that ca. 20-30 min bonding time was required to reach maximum adhesion, with adhesives containing higher wt % CNCs requiring longer bonding times. Furthermore, it was found that, in general, an increase in contact pressure results in an increase in the shear strength of the adhesive. The rebonding of the adhesives was demonstrated with little-to-no loss in adhesive shear strength. In addition, the 30 wt % nanocomposite adhesive was compared to some common commercially available adhesives and showed significantly stronger shear strengths when bonded to metal.

Tree frog attachment: mechanisms, challenges, and perspectives

Tree frogs have the remarkable ability to attach to smooth, rough, dry, and wet surfaces using their versatile toe pads. Tree frog attachment involves the secretion of mucus into the pad-substrate gap, requiring adaptations towards mucus drainage and pad lubrication. Here, we present an overview of tree frog attachment, with focus on (i) the morphology and material of the toe pad; (ii) the functional demands on the toe pad arising from ecology, lifestyle, and phylogenetics; (iii) experimental data of attachment performance such as adhesion and friction forces; and (iv) potential perspectives on future developments in the field. By revisiting reported data and observations, we discuss the involved mechanisms of attachment and propose new hypotheses for further research. Among others, we address the following questions: Do capillary and hydrodynamic forces explain the strong friction of the toe pads directly, or indirectly by promoting dry attachment mechanisms? If friction primarily relies on van der Waals (vdW) forces instead, how much do these forces contribute to adhesion in the wet environment tree frogs live in and what role does the mucus play? We show that both pad morphology and measured attachment performance suggest the coaction of several attachment mechanisms (e.g. capillary and hydrodynamic adhesion, mechanical interlocking, and vdW forces) with situation-dependent relative importance. Current analytical models of capillary and hydrodynamic adhesion, caused by the secreted mucus and by environmental liquids, do not capture the contributions of these mechanisms in a comprehensive and accurate way. We argue that the soft pad material and a hierarchical surface pattern on the ventral pad surface enhance the effective contact area and facilitate gap-closure by macro- to nanoscopic drainage of interstitial liquids, which may give rise to a significant contribution of vdW interactions to tree frog attachment. Increasing the comprehension of the complex mechanism of tree frog attachment contributes to a better understanding of other biological attachment systems (e.g. in geckos and insects) and is expected to stimulate the development of a wide array of bioinspired adhesive applications.

Competing failure modes in finite adhesive pads

Thin adhesive pads used to attach objects to each other often fail catastrophically. Here we consider the nature of failure of such a pad under loading parallel to the adhesive substrate. To determine the modes of failure of the pad and to understand what limits its load bearing capacity, we conduct experiments with finite pads composed of a soft adhesive layer with a stiff backing and load them parallel to the surface of adhesion. We find that two different peeling mechanisms emerge as a function of the slenderness of the adhesive pad: an interfacial peeling mechanism that starts close to the pulling end for very long pads, and an unstable curling mechanism that starts at the opposite end for relatively short pads. A minimal theoretical framework allows us to explain our observations and reveals the adhesive bond stiffness as a dominant parameter in defining the peeling mode. A phase diagram that delineates the different regimes of peeling modes brings our experiments and theory together. Our results suggest that unstable peeling by curling may be more common than previously thought, and could perhaps occur naturally in such examples as the gecko foot.

Slanted Functional Gradient Micropillars for Optimal Bioinspired Dry Adhesion

For biologically inspired dry adhesives, the fibrillar structure of the surface requires sufficient flexibility to form contacts and meanwhile high rigidity to maintain stability. This fundamental conflict has greatly hindered the advance of synthetic adhesives toward mass-scale and practical applications, where adhesion is desired to be simultaneously strong, durable, directional, and roughness-adaptive. In this work, we overcome such a long-term challenge by developing fibrillar structures that combine both slanted geometry and gradient material of micropillars. The termed slanted functional gradient pillars (s-FGPs), fabricated by a magnetically assisted mold replication technique, exhibit flexible tips for contacts, gradually stiffened stalks for reinforcement, slanted structure to give rise to anisotropy, and high aspect ratio (AR) to facilitate surface adaptation. We demonstrate that the material and structure of the s-FGPs complement each other, synergetic effects of which result in a multifunctional combination of adhesion properties including high strength (∼9 N/cm² in shear), ultradurability (over 200 cycles of attachment/detachment without adhesion degradation), super anisotropy (anisotropic ratio of ∼7), and good adaptability to rough surfaces. The s-FGPs not only step forward the bioinspired adhesion toward optimized designs and performances for practical applications but may also open up other concepts for various high-AR and structurally stable fibrillar surfaces with emerging functionalities and applications in the fields of self-cleaning, superhydrophobicity, biosensors, energy harvesting, etc.

Revisiting the generalized scaling law for adhesion: role of compliance and extension to progressive failure

A generalized scaling law, based on the classical fracture mechanics approach, is developed to predict the bond strength of adhesive systems. The proposed scaling relationship depends on the rate of change of debond area with compliance, rather than the ratio of area to compliance. This distinction can have a profound impact on the expected bond strength of systems, particularly when the failure mechanism changes or the compliance of the load train increases. Based on the classical fracture mechanics approach for rate-independent materials, the load train compliance should not affect the force capacity of the adhesive system, whereas when the area to compliance ratio is used as the scaling parameter, it directly influences the bond strength, making it necessary to distinguish compliance contributions. To verify the scaling relationship, single lap shear tests were performed for a given pressure sensitive adhesive (PSA) tape specimens with different bond areas, number of backing layers, and load train compliance. The shear lag model was used to derive closed-form relationships for the system compliance and its derivative with respect to the debond area. Digital image correlation (DIC) is implemented to verify the non-uniform shear stress distribution obtained from the shear lag model in a lap shear geometry. The results obtained from this approach could lead to a better understanding of the relationship between bond strength and the geometry and mechanical properties of adhesive systems.

Draw-Spinning of Kilometer-Long and Highly Stretchable Polymer Submicrometer Fibers

A new method is developed to directly spinning perfectly uniaxial fibers in an ultrafast manner. Besides, this method can tune the fibers' diameter through adjusting processing parameters such as the feeding rate of precursors. Uniaxial nylon 66 fibers prepared via this method show superior mechanical properties due to the alignment in each level of the structure.

Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces

Geckos have developed foot pads that allow them to maintain their unique climbing ability despite vast differences of surfaces and environments, from dry desert to humid rainforest. Likewise, successful gecko-inspired mimics should exhibit adhesive and frictional performance across a similarly diverse range of climates. In this work, we focus on the effect of relative humidity (RH) on the "frictional-adhesion" behavior of gecko-inspired adhesive pads. A surface forces apparatus was used to quantitatively measure adhesion and friction forces of a microfibrillar cross-linked polydimethylsiloxane surface against a smooth hemispherical glass disk at varying relative humidity, from 0 to 100% (including fully submerged under water). Geometrically anisotropic tilted half-cylinder microfibers yield a "grip state" (high adhesion and friction forces after shearing along the tilt of the fibers, F_ad⁺ and F_∥⁺) and a "release state" (low adhesion and friction after shearing against the tilt of the fibers, F_ad^- and F_∥^-). By appropriate control of the loading path, this allows for transition between strong attachment and easy detachment. Changing the preload and shear direction gives rise to differences in the effective contact area at each fiber and the microscale and nanoscale structure of the contact while changing the relative humidity results in differences in the relative contributions of van der Waals and capillary forces. In combination, both effects lead to interesting trends in the adhesion and friction forces. At up to 75% RH, the grip state adhesion force remains constant and the ratio of grip to release adhesion force does not drop below 4.0. In addition, the friction forces F_∥⁺ and F_∥^- and the release state adhesion force F_ad^- exhibit a maximum at intermediate relative humidity between 40% and 75%.

A Review of the State of Dry Adhesives: Biomimetic Structures and the Alternative Designs They Inspire

Robust and inexpensive dry adhesives would have a multitude of potential applications, but replicating the impressive adhesive organs of many small animals has proved challenging. A substantial body of work has been produced in recent years which has illuminated the many mechanical processes influencing a dry adhesive interface. The especially potent footpads of the tokay gecko have inspired researchers to develop and examine an impressive and diverse collection of artificial fibrillar dry adhesives, though study of tree frogs and insects demonstrate that successful adhesive designs come in many forms. This review discusses the current theoretical understanding of dry adhesive mechanics, including the observations from biological systems and the lessons learned by recent attempts to mimic them. Attention is drawn in particular to the growing contingent of work exploring ideas which are complimentary to or an alternative for fibrillar designs. The fundamentals of compliance control form a basis for dry adhesives made of composite and “smart,” stimuli-responsive materials including shape memory polymers. An overview of fabrication and test techniques, with a sampling of performance results, is provided.

Optimizing Adhesive Design by Understanding Compliance

Adhesives have long been designed around a trade-off between adhesive strength and releasability. Geckos are of interest because they are the largest organisms which are able to climb utilizing adhesive toepads, yet can controllably release from surfaces and perform this action over and over again. Attempting to replicate the hierarchical, nanoscopic features which cover their toepads has been the primary focus of the adhesives field until recently. A new approach based on a scaling relation which states that reversible adhesive force capacity scales with (A/C)(1/2), where A is the area of contact and C is the compliance of the adhesive, has enabled the creation of high strength, reversible adhesives without requiring high aspect ratio, fibrillar features. Here we introduce an equation to calculate the compliance of adhesives, and utilize this equation to predict the shear adhesive force capacity of the adhesive based on the material components and geometric properties. Using this equation, we have investigated important geometric parameters which control force capacity and have shown that by controlling adhesive shape, adhesive force capacity can be increased by over 50% without varying pad size. Furthermore, we have demonstrated that compliance of the adhesive far from the interface still influences shear adhesive force capacity. Utilizing this equation will allow for the production of adhesives which are optimized for specific applications in commercial and industrial settings.