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- ItemMechanical Robustness of Graphene on Flexible Transparent Substrates(Washington, DC : Soc., 2016) Kang, Moon H.; Prieto López, Lizbeth O.; Chen, Bingan; Teo, Ken; Williams, John A.; Milne, William I.; Cole, Matthew T.This study reports on a facile and widely applicable method of transferring chemical vapor deposited (CVD) graphene uniformly onto optically transparent and mechanically flexible substrates using commercially available, low-cost ultraviolet adhesive (UVA) and hot-press lamination (HPL). We report on the adhesion potential between the graphene and the substrate, and we compare these findings with those of the more commonly used cast polymer handler transfer processes. Graphene transferred with the two proposed methods showed lower surface energy and displayed a higher degree of adhesion (UVA: 4.40 ± 1.09 N/m, HPL: 0.60 ± 0.26 N/m) compared to equivalent CVD-graphene transferred using conventional poly(methyl methacrylate) (PMMA: 0.44 ± 0.06 N/m). The mechanical robustness of the transferred graphene was investigated by measuring the differential resistance as a function of bend angle and repeated bend–relax cycles across a range of bend radii. At a bend angle of 100° and a 2.5 mm bend radius, for both transfer techniques, the normalized resistance of graphene transferred on polyethylene terephthalate (PET) was around 80 times less than that of indium–tin oxide on PET. After 104 bend cycles, the resistance of the transferred graphene on PET using UVA and HPL was found to be, on average, around 25.5 and 8.1% higher than that of PMMA-transferred graphene, indicating that UVA- and HPL-transferred graphene are more strongly adhered compared to PMMA-transferred graphene. The robustness, in terms of maintained electrical performance upon mechanical fatigue, of the transferred graphene was around 60 times improved over ITO/PET upon many thousands of repeated bending stress cycles. On the basis of present production methods, the development of the next-generation of highly conformal, diverse form factor electronics, exploiting the emerging family of two-dimensional materials, necessitates the development of simple, low-cost, and mechanically robust transfer processes; the developed UVA and HPL approaches show significant potential and allow for large-area-compatible, near-room temperature transfer of graphene onto a diverse range of polymeric supports.
- ItemCytocompatible, Injectable, and Electroconductive Soft Adhesives with Hybrid Covalent/Noncovalent Dynamic Network(Weinheim : Wiley-VCH, 2019) Xu, Yong; Patsis, Panagiotis A.; Hauser, Sandra; Voigt, Dagmar; Rothe, Rebecca; Günther, Markus; Cui, Meiying; Yang, Xuegeng; Wieduwild, Robert; Eckert, Kerstin; Neinhuis, Christoph; Akbar, Teuku Fawzul; Minev, Ivan R.; Pietzsch, Jens; Zhang, YixinSynthetic conductive biopolymers have gained increasing interest in tissue engineering, as they can provide a chemically defined electroconductive and biomimetic microenvironment for cells. In addition to low cytotoxicity and high biocompatibility, injectability and adhesiveness are important for many biomedical applications but have proven to be very challenging. Recent results show that fascinating material properties can be realized with a bioinspired hybrid network, especially through the synergy between irreversible covalent crosslinking and reversible noncovalent self-assembly. Herein, a polysaccharide-based conductive hydrogel crosslinked through noncovalent and reversible covalent reactions is reported. The hybrid material exhibits rheological properties associated with dynamic networks such as self-healing and stress relaxation. Moreover, through fine-tuning the network dynamics by varying covalent/noncovalent crosslinking content and incorporating electroconductive polymers, the resulting materials exhibit electroconductivity and reliable adhesive strength, at a similar range to that of clinically used fibrin glue. The conductive soft adhesives exhibit high cytocompatibility in 2D/3D cell cultures and can promote myogenic differentiation of myoblast cells. The heparin-containing electroconductive adhesive shows high biocompatibility in immunocompetent mice, both for topical application and as injectable materials. The materials could have utilities in many biomedical applications, especially in the area of cardiovascular diseases and wound dressing.
- ItemA contact problem for viscoelastic bodies with inertial effects and unilateral boundary constraints(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2015) Scala, Riccardo; Schimperna, GiulioWe consider a three-dimensional viscoelastic body subjected to external forces. Inertial effects are considered; hence the equation for the displacement field is of hyperbolic type. The equation is complemented with Dirichlet and Neuman conditions on a part the boundary, while on another part the body is in adhesive contact with a solid support. The boundary forces acting on the latter part due to the action of elastic stresses are responsible for delamination, i.e., progressive failure of adhesive bonds. This phenomenon is mathematically represented by a nonlinear ODE which describes the evolution of the delamination order parameter z. Following the lines of a new approach introduced by the authors in a preceding paper and based on duality methods in Sobolev-Bochner spaces, we define a suitable concept of weak solutions to the resulting PDE system. Correspondingly, we prove an existence result on finite time intervals of arbitrary length.
- ItemFunnel-shaped microstructures for strong reversible adhesion(Hoboken, NJ : Wiley, 2017) Fischer, Sarah C.L.; Groß, Katja; Abad, Oscar Torrents; Becker, MIchael M.; Park, Euiyoung; Hensel, René; Arzt, EduardThe potential of a new design of adhesive microstructures in the micrometer range for enhanced dry adhesion is investigated. Using a two-photon lithography system, complex 3D master structures of funnel-shaped microstructures are fabricated for replication into poly(ethylene glycol) dimethacrylate polymer. The diameter, the flap thickness, and the opening angle of the structures are varied systematically. The adhesion of single structures is characterized using a triboindenter system equipped with a flat diamond punch. The pull-off stresses obtained reaches values up to 5.6 MPa, which is higher than any values reported in literature for artificial dry adhesives. Experimental and numerical results suggest a characteristic attachment mechanism that leads to intimate contact formation from the edges toward the center of the structures. van der Waals interactions most likely dominate the adhesion, while contributions by suction or capillarity play only a minor role. Funnel-shaped microstructures are a promising concept for strong and reversible adhesives, applicable in novel pick and place handling systems or wall-walking robots.
- ItemEngineering Micropatterned Dry Adhesives: From Contact Theory to Handling Applications(Weinheim : Wiley-VCH, 2018) Hensel, René; Moh, Karsten; Arzt, EduardReversible 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.
- ItemMonitoring the contact stress distribution of gecko-inspired adhesives using mechano-sensitive surface coatings(Washington D.C. : American Chemical Society, 2018) Neubauer, Jens W.; Xue, Longjian; Erath, Johann; Drotlef, Dirk-Michael; del Campo, Aránzazu; Fery, AndreasThe contact geometry of microstructured adhesive surfaces is of high relevance for adhesion enhancement. Theoretical considerations indicate that the stress distribution in the contact zone is crucial for the detachment mechanism, but direct experimental evidence is missing so far. In this work, we propose a method that allows, for the first time, the detection of local stresses at the contact area of biomimetic adhesive microstructures during contact formation, compression and detachment. We use a mechano-sensitive polymeric layer, which turns mechanical stresses into changes of fluorescence intensity. The biomimetic surface is brought into contact with this layer in a well-defined fashion using a microcontact printer, while the contact area is monitored with fluorescence microscopy in situ. Thus, changes in stress distribution across the contact area during compression and pull-off can be visualized with a lateral resolution of 1 μm. We apply this method to study the enhanced adhesive performance of T-shaped micropillars, compared to flat punch microstructures. We find significant differences in the stress distribution of the both differing contact geometries during pull-off. In particular, we find direct evidence for the suppression of crack nucleation at the edge of T-shaped pillars, which confirms theoretical models for the superior adhesive properties of these structures.
- ItemA weak formulation for a rate-independent delamination evolution with inertial and viscosity effects subjected to unilateral constraint(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2015) Scala, RiccardoWe consider a system of two viscoelastic bodies attached on one edge by an adhesive where a delamination process occurs. We study the dynamic of the system subjected to external forces, suitable boundary conditions, and an unilateral constraint on the jump of the displacement at the interface between the bodies. The constraint arises in a graph inclusion, while the delamination coefficient evolves in a rate-independent way. We prove the existence of a weak solution to the corresponding system of PDEs.
- ItemA study of the adhesive foot of the gecko: Translation of a publication by Franz Weitlaner(Milton Park : Taylor & Francis, 2015) Kroner, Elmar; Davis, Chelsea S.In recent years, hundreds of scientific studies have been published regarding gecko-inspired adhesives. The primary reason for this increasing interest lies in the unique properties which are combined in the adhesive system of the gecko: this natural system can quickly and repeatedly adhere to different surface chemistry and roughness without the use of adhesion-mediating fluids. Although these properties seem to be inconspicuous at first, there is no man-made system currently available which successfully combines all of these properties and competes with the biological adhesive system. However, there are many applications which may benefit from an artificial adhesion system inspired by geckos, ranging from climbing robots and handling systems to biomedical patches and household objects.
- ItemNumerical simulation of the edge stress singularity and the adhesion strength for compliant mushroom fibrils adhered to rigid substrates(Amsterdam : Elsevier, 2016) Balijepalli, R.G.; Begley, M.R.; Fleck, N.A.; McMeeking, R.M.; Arzt, E.Bio-inspired adhesion of micropatterned surfaces due to intermolecular interactions has attracted much research interest over the last decade. Experiments show that the best adhesion is achieved with compliant “mushroom”-shaped fibrils. This paper analyses numerically the effects of different mushroom shapes on adhesion to a rigid substrate. When a remote stress is applied on the free end of a fibril perfectly bonded to a rigid substrate, the resultant stress distribution along the fibril is found to change dramatically between the straight punch and mushroom fibrils. A singular stress field is present at the edge of the fibril where it contacts the substrate and, in this work, the amplitude of the singularity is evaluated for fibrils perfectly bonded to a flat substrate so that sliding cannot occur there. This exercise is carried out for fibril geometries involving combinations of different diameters and thicknesses of the mushroom cap. By assuming a pre-existing detachment length at the corner where the stress singularity lies, we predict the adhesive strength for various mushroom cap shapes. Our study shows that a smaller stalk diameter and a thinner mushroom cap lead to higher adhesive strengths. A limited number of results are also given for other shapes, including those having a fillet radius connecting the stalk to the cap. The results support the rational optimization of synthetic micropatterned adhesives.
- ItemFibrillar elastomeric micropatterns create tunable adhesion even to rough surfaces(Hoboken, NJ : Wiley, 2016) Barreau, Viktoriia; Hensel, René; Guimard, Nathalie K.; Ghatak, Animangsu; McMeeking, Robert M.; Arzt, EduardBiologically inspired, fibrillar dry adhesives continue to attract much attention as they are instrumental for emerging applications and technologies. To date, the adhesion of micropatterned gecko-inspired surfaces has predominantly been tested on stiff, smooth substrates. However, all natural and almost all artificial surfaces have roughnesses on one or more different length scales. In the present approach, micropillar-patterned PDMS surfaces with superior adhesion to glass substrates with different roughnesses are designed and analyzed. The results reveal for the first time adhesive and nonadhesive states depending on the micropillar geometry relative to the surface roughness profile. The data obtained further demonstrate that, in the adhesive regime, fibrillar gecko-inspired adhesive structures can be used with advantage on rough surfaces; this finding may open up new applications in the fields of robotics, biomedicine, and space exploration.