Obtaining the maximum contact surface area between two substrates is one of the best understood ways to enhance adhesion them. Often the design and nature of adhesives means that they will automatically, perhaps even unstoppably, flow towards this goal. Now US researchers have developed polymers where the contact area with their substrate can controlled through exposure to an external stimulus - more specifically water1. In fact, the contact surface area increases because the polymers surface becomes more wrinkled. Could this discovery deliver previously untold versatility to adhesives? Using surface patterns to control adhesion has become increasingly fashionably, largely due to the fascination with the spoon-shaped nanoscale hairs geckos use to stick to walls. But mimicking these patterns is challenging, and in 2008 a group of researchers in Massachusetts developed wrinkled polymers with the hope of providing easier fabrication1. Now, researchers have developed what they call a responsive surface-wrinkled adhesive (RSA) that wrinkles as it swells in water.2 They synthesised the RSA by photopolymerization from 74 wt % polyethylene glycol methyl ether acrylate, 24 wt % acrylic acid, 2 wt % polyethylene glycol dimethacrylate, and 1 wt % photoinitator. The scientists then irradiated the mixture with ultraviolet light, moulding the RSA into hemispheres mounted on glass substrates.
Exposing the resulting surfaces directly to water causes them to swell within five seconds, but because theyre secured to glass they wrinkle up. The wrinkling remains stable as long as the surface remains hydrated, but disappears when the surface is dried, and can cycle between the two states. However, the distance between peaks of the wrinkles - the wavelength, in effect - increases from 80 μm initially to 540 μm ultimately with prolonged exposure to water.
While the increase in surface area could clearly be useful if the polymer were adhesive, the wrinkling behaviour alone says little about the materials adhesion. So, the US researchers introduced their adhesive to a hydrogel made from gelatin. To do this they compressed their hemispheres into the gelatin surface and withdrew them to gauge the level of adhesion provided. allowing them to remain for a set period of time before doing so. As the RSA rests against the gelatin hydrogel, it absorbs water from it and consequently develops wrinkles like it did in isolation. By assessing tensile force, the researchers determined that as the wavelength of the wrinkles increases, the more firmly the RSA sticks to the hydrogel. In pressure-sensitive adhesives (PSAs) tack development enhances adhesion through viscous flow of mobile polymer chains, truly delivering interfacial contact with rough surfaces. The time-scale that tack normally enhances adhesion over is around 0.01 seconds for gelatin, and 0.1 seconds for PSAs. By contrast, wrinkling appeared to continue improving adhesive strength of the bond between gelatin and the RSA even after 250 seconds. This, the researchers claim, indicates that tack is not the primary adhesion mechanism in their system. Instead, adhesion grows with the length of the perimeter of the contact area between the RSA and the hydrogel. The force needed to separate these surfaces is directly proportional to the length of this perimeter. Consequently, when the perimeter lengthens through wrinkling so too does the force needed to separate the surfaces. In effect, as they are peeled apart, any time that the point of separation encounters a wrinkle it gets pinned there, as separation has to be freshly initiated in a different direction instead of proceeding continuously as it would on a smooth surface. There is a clear similarity between the gelatin hydrogel and biological systems like the human body. Consequently, the researchers suggest that this mechanism of bonding could inspire alternative approaches for developing adhesives for biomedical tapes that must interface with hydrated, compliant materials such as soft tissues. With medical adhesives being able to successfully command premium prices, RSAs could prove quite lucrative were it to fill an unmet need in this sector. Potentially, they could even be used to produce bandages that adhere ever tighter the more blood theyre exposed to and then become less adhesive as the bleeding wound heals. But could this adhesion mechanism really be successfully used in a practical setting? Please share your thoughts using the tools below. 本站所有信息与内容，版权归原作者所有。网站中部分新闻、文章来源于网络或会员供稿，如读者对作品版权有疑议，请及时与我们联系，电话：025-85303363 QQ：2402955403。文章仅代表作者本人的观点，与本网站立场无关。转载本站的内容，请务必注明"来源：林中祥胶粘剂技术信息网（www.adhesive-lin.com）".
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