Dry Adhesives Research Articles

Bioinspired elastomer composites with programmed mechanical and electrical anisotropies

Concepts that draw inspiration from soft biological tissues have enabled significant advances in creating artificial materials for a range of applications, such as dry adhesives, tissue engineering, biointegrated electronics, artificial muscles, and soft robots. Many biological tissues, represented by muscles, exhibit directionally dependent mechanical and electrical properties. However, equipping synthetic materials with tissue-like mechanical and electrical anisotropies remains challenging. Here, we present the bioinspired concepts, design principles, numerical modeling, and experimental demonstrations of soft elastomer composites with programmed mechanical and electrical anisotropies, as well as their integrations with active functionalities. Mechanically assembled, 3D structures of polyimide serve as skeletons to offer anisotropic, nonlinear mechanical properties, and crumpled conductive surfaces provide anisotropic electrical properties, which can be used to construct bioelectronic devices. Finite element analyses quantitatively capture the key aspects that govern mechanical anisotropies of elastomer composites, providing a powerful design tool. Incorporation of 3D skeletons of thermally responsive polycaprolactone into elastomer composites allows development of an active artificial material that can mimic adaptive mechanical behaviors of skeleton muscles at relaxation and contraction states. Furthermore, the fabrication process of anisotropic elastomer composites is compatible with dielectric elastomer actuators, indicating potential applications in humanoid artificial muscles and soft robots.

Simply Formulated Dry Pressure-Sensitive Adhesives for Substrate-Independent Underwater Adhesion

Simply Formulated Dry Pressure-Sensitive Adhesives for Substrate-Independent Underwater Adhesion

Design of Microfabricated Mechanically Interlocking Metamaterials for Reworkable Heterogeneous Integration

Abstract Next-generation interconnects utilizing mechanically interlocking structures enable permanent and reworkable joints between micro-electronic devices. Mechanical metamaterials, specifically dry adhesives, are an active area of research which allows for the joining of objects without traditional fasteners or adhesives, and in the case of chip integration, without solder. This paper focuses on reworkable joints that enable chips to be removed from their substrates to support reusable device prototyping and packaging, creating the possibility for eventual pick-and-place mechanical bonding of chips with no additional bonding steps required. Analytical models are presented and are verified through finite element analysis (FEA) assuming pure elastic behavior. Sliding contact conditions in FEA simplify consideration of several design variations but contribute ∼10% uncertainty relative to experiment, analysis, and point-loaded FEA. Two designs are presented; arrays of flat cantilevers have a bond strength of 6.3 kPa, and nonflat cantilevers have a strength of 29 kPa. Interlocking designs present self-aligning in-plane forces that emerge from translational perturbation from perfect alignment. Stresses exceeding the material yield stress during adhesion operations present a greater concern for repeatable operation of compliant interlocking joints and will require further study quantifying and accommodating plastic deformation. Designs joining a rigid array with a complementary compliant cantilever array preserve the condition of reworkability for the surface presenting the rigid array. Eventual realization of interconnect technology based on this study will provide a great improvement of functionality and adaptability in heterogeneous integration and microdevice packaging.

A Tough Reversible Biomimetic Transparent Adhesive Tape with Pressure-Sensitive and Wet-Cleaning Properties.

Dry adhesives that combine strong adhesion, high transparency, and reusability are needed to support developments in emerging fields such as medical electrodes and the bonding of electronic optical devices. However, achieving all of these features in a single material remains challenging. Herein, we propose a pressure-responsive polyurethane (PU) adhesive inspired by the octopus sucker. This adhesive not only showcases reversible adhesion to both solid materials and biological tissues but also exhibits robust stability and high transparency (>90%). As the adhesive strength of the PU adhesive corresponds to the application force, adhesion could be adjusted by the preloading force and/or pressure. The adhesive exhibits high static adhesion (∼120 kPa) and 180° peeling force (∼500 N/m), which is far stronger than those of most existing artificial dry adhesives. Moreover, the adhesion strength is effectively maintained even after 100 bonding-peeling cycles. Because the adhesive tape relies on the combination of negative pressure and intermolecular forces, it overcomes the underlying problems caused by glue residue like that left by traditional glue tapes after removal. In addition, the PU adhesive also shows wet-cleaning performance; the contaminated tape can recover 90-95% of the lost adhesion strength after being cleaned with water. The results show that an adhesive with a microstructure designed to increase the contribution of negative pressure can combine high reversible adhesion and long fatigue life.

Theoretical optimization of functional graded micropillars for strong and durable bioinspired dry adhesion

Inspired by the prominent adhesion capability of gecko foot-hair, micro/nano-pillar array has been fabricated and widely explored as biomimetic dry adhesives. Pillars with graded material along their length have been proved to have great advantage in simultaneously raising adhesion and reducing clusterisation, yet the design of the graded micropillars lacks theoretical and quantitative guides. In this work, we first derived the theoretical expressions of critical aspect ratio and effective elastic modulus for graded micropillars composed of rigid bases and soft tips. Then, a Cohesive Zone Model (CZM) was constructed to simulate the lateral collapse of the two-step-graded micropillars with epoxy resin and polyurethane (typical materials adopted in the reported experiments) as the base and tip materials, respectively. Combining the numerical results and the theoretical ones, we found that when the length ratio of the rigid base to the micropillar increases, the ability of the pillars to resist structural collapse enhances (i.e., the critical aspect ratio raises), while the adhesion strength reduces (i.e., the effective elastic modulus increases). Considering the tradeoff between the structural stability and the adhesion strength, we found an optimum value of ∼0.65 for the length ratio of the rigid base to the whole micropillar. Our study establishes theoretical models for the critical aspect ratio and effective elastic modulus for the graded micropillars and provides quantitative guides to the design and optimization of bioinspired dry adhesives.

Addition and Correction to “Fluorocarbon Rubber-Based Inert Dry Adhesive for Applications under Harsh Conditions”

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionAddition and Correction to “Fluorocarbon Rubber-Based Inert Dry Adhesive for Applications under Harsh Conditions”Sung Ho LeeSung Ho LeeMore by Sung Ho Lee, Hyo Sung KimHyo Sung KimMore by Hyo Sung Kim, Hyun Woo SongHyun Woo SongMore by Hyun Woo Song, and Moon Kyu Kwak*Moon Kyu Kwak*[email protected]More by Moon Kyu Kwakhttps://orcid.org/0000-0002-8902-7685Cite this: ACS Appl. Polym. Mater. 2021, 3, 11, 6026Publication Date (Web):October 12, 2021Publication History Published online12 October 2021Published inissue 12 November 2021https://doi.org/10.1021/acsapm.1c01193Copyright © 2021 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views503Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (496 KB) Get e-Alerts Get e-Alerts

Load Sharing Design of a Multi-legged Adaptable Gripper With Gecko-Inspired Controllable Adhesion

Gecko-inspired controllable dry adhesion has promising applications in various fields such as transfer printing, advanced robotics, and space technology. In this study, we proposed a multi-legged self-adaptive gripper based on gecko-inspired controllable adhesion to manipulate both flat and curved objects. It consists of four symmetric adhesive units, each modulated by two adaptive-locking mechanisms for compression and rotation, respectively, and one peeling mechanism. The two adaptive-locking mechanisms adapt to surfaces with height and curvature differences to ensure intimate contact and then lock the adaption configuration to enable equal load sharing for strong attachment. The peeling mechanism rapidly peels the adhesive surfaces from the substrate for easy detachment. Therefore, this gripper can achieve controllable load sharing to reliably grasp and easily release smooth objects with various surface shapes. Furthermore, the effects of object posture and structural parameters on its grasping performance were analyzed, which can guide its optimal design by improving equal load sharing among adhesive units. This work provides an equal load-sharing strategy for multi-unit adhesive system design and demonstrates its potential for emerging industrial applications.

Mini Review: Comparison of Bio-Inspired Adhesive Feet of Climbing Robots on Smooth Vertical Surfaces.

Developing climbing robots for smooth vertical surfaces (e.g., glass) is one of the most challenging problems in robotics. Here, the adequate functioning of an adhesive foot is an essential factor for successful locomotion performance. Among the various technologies (such as dry adhesion, wet adhesion, magnetic adhesion, and pneumatic adhesion), bio-inspired dry adhesion has been actively studied and successfully applied to climbing robots. Thus, this review focuses on the characteristics of two different types of foot microstructures, namely spatula-shaped and mushroom-shaped, capable of generating such adhesion. These are the most used types of foot microstructures in climbing robots for smooth vertical surfaces. Moreover, this review shows that the spatula-shaped feet are particularly suitable for massive and one-directional climbing robots, whereas mushroom-shaped feet are primarily suitable for light and all-directional climbing robots. Consequently, this study can guide roboticists in selecting the right adhesive foot to achieve the best climbing ability for future robot developments.

Theoretical Limits in Detachment Strength for Axisymmetric Bi-Material Adhesives

AbstractReversible dry adhesives rely on short-ranged intermolecular bonds, hence requiring a low elastic modulus to conform to the surface roughness of the adhered material. Under external loads, however, soft adhesives accumulate strain energy, which release drives the propagation of interfacial flaws prompting detachment. The trade-off between the required compliance, for surface conformity, and the desire for a reduced energy release rate, for better strength, can be achieved with a bi-material adhesive having a soft tip and a rigid backing (RB). This design strategy is widely observed in nature across multiple species. However, the detachment mechanisms of these adhesives are not completely understood and quantitative analysis of their adhesive strength is still missing. Based on linear elastic fracture mechanics (LEFM), we analyze the strength of axisymmetric bi-material adhesives. We observed two main detachment mechanisms, namely (i) center crack propagation and (ii) edge crack propagation. If the soft tip is sufficiently thin, mechanism (i) dominates and provides stable crack propagation, thereby toughening the interface. We ultimately provide the maximum theoretical strength of these adhesives obtaining closed-form estimation for an incompressible tip. In some cases, the maximum adhesive strength is independent of the crack size, rendering the interface flaw tolerant. We finally compare our prediction with experiments in the literature and observe good agreement.

Machine Learning-Based and Experimentally Validated Optimal Adhesive Fibril Designs.

Setae, fibrils located on a gecko's feet, have been an inspiration of synthetic dry microfibrillar adhesives in the last two decades for a wide range of applications due to unique properties: residue-free, repeatable, tunable, controllable and silent adhesion; self-cleaning; and breathability. However, designing dry fibrillar adhesives is limited by a template-based-design-approach using a pre-determined bioinspired T- or wedge-shaped mushroom tip. Here, a machine learning-based computational approach to optimize designs of adhesive fibrils is shown, exploring a much broader design space. A combination of Bayesian optimization and finite element methods creates novel optimal designs of adhesive fibrils, which are fabricated by two-photon-polymerization-based 3D microprinting and double-molding-based replication out of polydimethylsiloxane. Such optimal elastomeric fibril designs outperform previously proposed designs by maximum 77% in the experiments of dry adhesion performance on smooth surfaces. Furthermore, finite-element-analyses reveal that the adhesion of the fibrils is sensitive to the 3D fibril stem shape, tensile deformation, and fibril microfabrication limits, which contrast with the previous assumptions that mostly neglect the deformation of the fibril tip and stem, and focus only on the fibril tip geometry. The proposed computational fibril design could help design future optimal fibrils with less help from human intuition.

Biopolymeric Anticorrosion Coatings from Cellulose Nanofibrils and Colloidal Lignin Particles.

This study presents a process for preparation of cellulose–lignin barrier coatings for hot-dip galvanized (HDG) steel by aqueous electrophoretic deposition. Initially, a solution of softwood kraft lignin and diethylene glycol monobutyl ether was used to prepare an aqueous dispersion of colloidal lignin particles (CLPs) via solvent exchange. Analysis of the dispersion showed that it comprised submicron particles (D = 146 nm) with spherical morphologies and colloidal stability (ζ-potential = −40 mV). Following successful formation, the CLP dispersion was mixed with a suspension of TEMPO-oxidized cellulose nanofibers (TOCN, 1 and 2 g·L–1) at a fixed volumetric ratio (1:1, TOCN–CLPs), and biopolymers were deposited onto HDG steel surfaces at different potentials (0.5 and 3 V). The effects of these variables on coating formation, dry adhesion, and electrochemical properties (3.5% NaCl) were investigated. The scanning electron microscopy results showed that coalescence of CLPs occurs during the drying of composite coatings, resulting in formation of a barrier layer on HDG steel. The scanning vibrating electrode technique results demonstrated that the TOCN–CLP layers reduced the penetration of the electrolyte (3.5% NaCl) to the metal–coating interface for at least 48 h of immersion, with a more prolonged barrier performance for 3 V-deposited coatings. Additional electrochemical impedance spectroscopy studies showed that all four coatings provided increased levels of charge transfer resistance (Rct)—compared to bare HDG steel—although coatings deposited at a higher potential (3 V) and a higher TOCN concentration provided the maximum charge transfer resistance after 15 days of immersion (13.7 cf. 0.2 kΩ·cm2 for HDG steel). Overall, these results highlight the potential of TOCN–CLP biopolymeric composites as a basis for sustainable corrosion protection coatings.

Achieving enhanced adhesion through optimal stress distributions

Dry adhesives that rely on surface force mediated adhesion, such as van der Waals forces, are important in applications ranging from robotics to manufacturing. The maximum theoretical adhesion strength of a contact is achieved when the stress is uniformly distributed over the entire contact area as the full potential of all the bonds at the interface is realized in this scenario. Most dry adhesive structures are composed of a tip layer that forms contact and a support structure that transfers load from the far field to this tip structure. Here, we determine the displacement distribution that must be applied on the tip layer to generate an optimum interfacial stress distribution. We realize this through a linear, closed-form optimization framework that uses data obtained from finite element analysis for a few basis cases. It was found that adhesion can be maximized by applying an optimum displacement on the tip layer that consists of uniform tension in the center, a peak tension between the center and the edge, and compression near the edge. The displacement applied on the tip layer of a mushroom-shaped, composite, and novel segmented composite structures are then analyzed and compared with the optimal case to guide the design of dry adhesives.

Self‐Adhesive Silicone Microstructures for the Treatment of Tympanic Membrane Perforations

Inspired by the gecko foot, polymeric microstructures have demonstrated reliable dry adhesion to both stiff objects and sensitive surfaces such as skin. Microstructured silicone patches are proposed, herein, for the treatment of tympanic membrane perforations with the aim of serving as an alternative for current surgical procedures that require anesthesia and ear canal packing. Sylgard 184 PDMS micropillars of 20 μm in diameter and 60 μm in length are topped by a Soft Skin Adhesive (SSA) MG7‐1010 terminal layer, of about 25 μm thickness. The adhesion is evaluated by specially designed tack tests against explanted murine eardrums and, for comparison, against a rigid substrate. Functional effects are evaluated using auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAE). The adhesion strength of the microstructure and unstructured controls to explanted murine tympanic membranes is comparable (typically 12 kPa), but the microstructured patches are easier to handle by the surgeon. For the first time, partial recovery of hearing performance is measured immediately after patch application. The novel patches adhere without the need for further fixation, removing the need for ear canal packing. The proposed material design holds great promise for improving clinical treatments of tympanic membrane perforations.

Making Contact: A Review of Robotic Attachment Mechanisms for Extraterrestrial Applications

The space environment offers unique challenges for robotic grippers and controllable attachment mechanisms. Applications include satellite and orbital debris capture, perching for free‐floating robots inside the International Space Station, climbing rocky surfaces in planetary exploration tasks, and grasping asteroids and other rough substrates for drilling and sample collection operations. Many traditional adhesive technologies, such as pressure‐sensitive adhesives, suction, and electromagnetism, are not tenable in the space environment due to the lack of an atmosphere (e.g., pressure‐sensitive adhesives outgas and suction requires a pressure differential) or suitable substrates (e.g., materials for space applications are rarely ferromagnetic). Instead, a number of other technologies have shown promise for space, including bio‐inspired fibrillar dry adhesives that offer controllable adhesion on both flat and curved smooth surfaces, electrostatic adhesives that work on smooth surfaces but also rougher surfaces and fabrics, and microspines for rocky and rough substrates. Herein, a review of these technologies, focusing on their operation and providing examples of the technologies applied to space and other extraterrestrial missions is presented.

Soft Gel-Filled Composite Adhesive for Dry and Wet Adhesion

Soft Gel-Filled Composite Adhesive for Dry and Wet Adhesion

Processing, Adhesion and Corrosion-inhibiting Properties of Poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylene vinylene], (MEH-PPV) on Aerospace Aluminum Alloys

Researchers at the Naval Air Warfare Center Weapons Division (NAWCWD) and Wright-Patterson Air Force Base (WPAFB) investigated poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylene vinylene], (MEH-PPV) for its potential corrosion-inhibition properties on aerospace aluminum alloy AA2024-T3. Solution processing of the polymer, as well as adhesion testing and accelerated weathering tests were performed on MEH-PPV full military aerospace coatings. Wet and dry tape adhesion testing, as well as pencil hardness, impact flexibility and pneumatic adhesion tensile test instrument (PATTI) testing were used to demonstrate the adhesion performance of MEH-PPV on aluminum substrates. The results showed that MEH-PPV had acceptable adhesion characteristics when compared to hexavalent chromium (Cr(VI)) based coatings in all of these tests. Accelerated weathering analysis was performed on MEH-PPV coatings to determine their corrosion protection and weathering resistance capabilities. These tests included neutral salt spray (NSS) exposure and xenon-arc lamp testing. The results showed that while MEH-PPV does not exhibit significant color change after 500 hours of xenon arc lamp exposure, the polymer has poor corrosion protection performance under aggressive salt environments.

Adhesive performance enhancement of the mushroom-shaped microstructured elastomer by atmospheric plasma treatment.

Bioinspired reversible adhesives that have been developed in the course of recent years have found several applications in robotics, transportation, and marine applications. One of their prominent features is strong reversible static adhesion. To fulfill the requirements of various applications, the static adhesive performance of these materials can be enhanced by modifying the material and surface properties. In this work, the mushroom-shaped adhesive microstructured surface was functionalized by atmospheric plasma treatment to enhance its adhesive performances. Through optimizing the duration of the treatment, the pull-off force increase of up to 60% can be reached after the treatment in comparison to the measurements performed on the same mushroom-shaped microstructured sample before the treatment. In comparison to the microstructured samples, the attachment of the unstructured sample made of the same silicone elastomer was enhanced by 16% after plasma treatment. The strong adhesion enhancement on the microstructured sample was attributed to the combination of the changed effective elastic modulus of the material and the specific detachment behavior of microstructures. These results are anticipated to contribute to the further development of bioinspired dry adhesives and may potentially widen their usage in various technological applications.

Fluorocarbon Rubber-Based Inert Dry Adhesive for Applications under Harsh Conditions

Artificial dry adhesives have been developed based on materials and production methods by many researchers. Nowadays, extensive research has attempted to apply dry adhesives to various industrial fields, such as glass transportation systems, with admirable performance. However, most artificial dry adhesives consist of polymers such as thermosetting polymers, ultraviolet curable polymers, and water-based polymers. These polymers exhibit low thermal resistance and outgassing under high-vacuum conditions. Hence, the usage of dry adhesives under high-temperature and vacuum conditions, such as semiconductor manufacturing processes, is critically limited. In this study, based on the contact mechanism, a dry adhesive for use under high-temperature and vacuum conditions is developed using fluorocarbon rubber with excellent thermal resistance and low outgassing properties. The fluorocarbon rubber dry adhesive (FDA) material shows a high pull-off strength of up to 24.1 N cm–2 and its performance is verified through various experiments. Glass transportation experiments using a robot arm and custom-built equipment are also conducted and the results confirm the feasibility of using FDA in industrial applications. The FDA is expected to be a useful alternative under high-temperature and vacuum conditions, such as those in the aerospace industry and display manufacturing processes.

Dual Adaptation of a Flexible Shape Memory Polymer Adhesive.

A shape memory polymer (SMP) adhesive forms a conformal and hermetic contact with a target surface at the soft, rubbery state and provides a high-strength dry adhesion at the rigid, glassy state. In particular, recent SMP adhesive studies show SMP's ability to adhere to various rough and even underwater yet rigid surfaces. However, achieving and retaining the strong adhesion on flexible target surfaces such as common fabrics has not been reported since a flexible target surface would easily be peeled off from an SMP adhesive, which is too rigid to accommodate the target surface flexing. Here, we introduce the dual adaptation of an SMP adhesive composed of a thin SMP layer and a backing fabric, which involves the shape adaptation to make a strong adhesive contact and the flexure adaptation to tolerate a target surface flexing. To discover the criteria for optimizing both shape and flexure adaptations, we present the theoretical rationale as well as the computational and experimental studies in this work. Based on the findings here, we design a thin SMP adhesive and demonstrate its dry and underwater adhesive performances on common clothes to highlight its potential applications.

Cutting to the Point: Directly Machined Metal Molds for Directional Gecko-Inspired Adhesives

Abstract Fabrication techniques for gecko-inspired adhesives generally target mold durability, adhesive performance, and process efficiency and simplicity. With these goals in mind, we present a micromachining process for creating reusable aluminum molds used to fabricate directional dry adhesives. The molds require deep, narrow and overhanging grooves to create sharp and angled adhesive features. This geometry precludes most traditional machining and lithographic material removal processes. The presented process is a hybrid of indenting and orthogonal machining, using a diamond-coated microtome blade as the tool. An finite element analysis reveals the local extent of work hardening as each groove is created, and helps to define a trajectory that reduces the effects of tool deflection and chip build up. The results of a series of experiments agree with predictions from the analysis and reveal a range of blade approach angles and a lower bound on groove spacing to achieve the desired geometry. This range is narrower than for molds machined from wax in previous work. Nonetheless, adhesive samples cast from the new metal molds achieve comparable performance to those previously cast from wax.