Reversibility adhesive

Human bodies are constantly exposed to a plethora of bacteria, viruses, and other inflammatory substances. To combat these infections and toxic agents, the body has developed a carefully regulated inflammatory response system. Part of that response is the orderly migration of leukocytes to sites of inflammation. Leukocytes literally roll along the vascular wall and into the tissue site of inflammation. This rolling movement is mediated by reversible adhesive interactions between the leukocytes and the vascular surface. 

Reversible and irreversible adhesion may be explained in terms of the primary minimum and F [Oliveira 1992]. Reversible adhesion will occur at F and irreversible adhesion at short distances and high negative values of F. The implications for the fouling process and deposit removal may be clearly identified. 

The pads on the feet and/or legs of insects or animals such as beetles, spiders and geckoes have nanoscale hairs that enable them to attach and detach from different kinds of surfaces, allowing them to walk on them. These provide natural examples of reversible adhesion or smart adhesion. It has been suggested that the adhesion arises from a combination of van der Waals attractive forces and capillary forces (depending on the humidity) [103]. Synthetic surfaces comprising arrays of 400 X 600 nm nanopillars have been produced that mimic the gecko-foot effect with reversible adhesion for at least a few contact cycles [103]. 

In vivo LTB4 causes a conspicuous and reversible adhesion of leukocytes to the endothelium in post-capillary venules [135], and the metabolites 2O-OH-LTB4 and 2O-COOH-LTB4 are approximately 50 times less potent than LTB4 itself [179]. LTC4 and LTD4 do not enhance leukocyte adherence in this way [135]. 

In a design approach unique among gecko-inspired adhesives, hiodegradable dextran-coated poly(glycerol-co-sebacate acrylate) has been used to create patterned arrays of microscale truncated cones, and used to improve attachment of adhesive tapes to tissue surfaces. While this biocompatible, elastomeric material is not intended for reversible adhesion and... 

It is not permissible to adopt data acquired from changes of the liquid resin s surface tension for solid state polymer. Zisman [72] does it supposing that the reversible adhesion work of the solid polymer must be close to that estimated for the liquid state. The conclusion follows from the assumption that the forces that act on the phase separation boundary spread out to a depth that does not exceed the size of some molecules. As a result, the interaction on the bound y cannot depend on the change of state of the substance. One must accept this because the determined v dues of the surface tension of solid polymers significantly exceed those for the liquid oligomers. If we deal with undercured products the v dues usually exceed those for the polymer surface tension acquired by wetting agent critic d surface tension methods. 

The conclusion was that adhesion of wet caldte was small and well controlled, such that reversible adhesion curves could be obtained with a sharp AFM probe tip, giving atomic resolution of the surface oxygen atoms. 

The first truly reversible adhesion cracking experiments were carried out by Obreimoff in 1930 on mica and by Johnson et al in 1971 using smooth elastic rubber spheres. The diameter of the black contact zone was measured in reflected light, and plotted against the applied force to compare with the thermodynamic cracking theory. The results were reasonably reversible and fitted the thermodynamic work of adhesion theory. These experiments are described more fully in Chapters 4 and 9. 

Red blood cells, erythrocytes, were used because of their low and reversible adhesion. Cells were prepared from three species, human blood from North Staffordshire Hospital, fresh horse blood in EDTA, and fresh rat blood from Central Animal Pathology Ltd. Each blood sample was washed six to seven times in phosphate buffered saline to remove the nonred-cell components, before suspending in physiological saline solution, then examined by both optical and Coulter tests. Each species of cell was treated in three ways to judge the effect of surface adhesion molecules by adding glutaraldehyde, fibronectin, and papain. 

Of course, the above experiments were devised to give weak and reversible adhesion in order to verify the theory. In engineering design, it is usual to go for more resistance to load, especially in the butt joint where the joint progressively requires more force as the adhesive thickness is reduced. For very thin adhesive layers, it was clear that the mechanism of failure was changing, and that cavitation could arise within the adhesive.Breakdown was observed within the adhesive layer, and ultimately this limited the load capability ofthejoint at a thickness near I pm. 

The zone of reversible adhesion (Zone II) will depend not only on the thickness of the layer of surfactant adsorbed on the surface, but also on the coverage of the surface with this layer, i.e., which is arbitrarily taken as unity when the area of the adsorbed surfactant layer is near saturation. 

Fig. VI.4. Adhesive interaction diagram for carbon black particles on steel surface (I) no adhesion (II) reversible adhesion, or unstable state (III) irreversible adhesion, or stable state Hq is the thickness of the adsorbed surfactant layer 5 is the degree of surface coverage A//tr=12.5.

Taken from Campo, A.D., Femandez-Blazquez, J.P., 2012. Bio-inspired reversible adhesives for dry and wet conditions. In Biomimetic Approaches for Biomaterials Development. Wiley-VCH Verlag GmbH Co. 

Alternatively, tree frogs could offer a valid solution for wet and reversible adhesion (Scholz et al., 2009). Their pads consist of regularly arrayed nanopillars of 300—400-pm diameters (Fig. 2.3(d)). Randomly distributed within the array are wide pores that secrete a mucus layer. To date, their properties and roles remain unclear. Wet adhesion is suggested to be attained by a combination of capillary, friction, and viscous forces. However, the entire mechanism by which the tree frog succeeds in adhesion under wet conditions remains uncertain. Therefore, reversible bioinspired wet adhesives are stiU much less advanced in their development than their dry adhesive counterpart. 

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