Bandages that do not stay stuck or that are painful to remove blatantly demonstrate where the adhesives that they use need to improve. While the pain caused by many conventional pressure-sensitive adhesives (PSAs) is obvious, the most important shortcoming is their inability to adhere in moist conditions. Thats according to French and Algerian researchers, who have produced nanoparticle-filled hydrogel PSAs that they claim may be the solution to this problem.
Hydrogels contain a large quantity of water, which allows them to tolerate the additional liquid from sweat, blood, mucous or other bodily fluids. While that makes them strong candidates for medical applications, their mechanical properties have typically been poor. However, in recent years four different stronger, more elastic, hydrogel structures have been developed. These include highly-water absorbing topological gels that have figure-of-eight cross-linkers that can slide along hydrogel polymer chains. In nanocomposite gels, which have already been demonstrated in medical PSAs, the cross-linking is typically achieved by clay nanoparticles. Interpenetrating polymer networks interweave highly cross-linked rigid and loosely cross-linked flexible polymers, while snake-cage gels trap linear polymer "snakes" with good mechanical properties in cross-linked hydrophilic polymer "cages". Now, the French and Algerian team have exploited a snake-cage nanocomposite structure, using an acrylamide (AM) and hydroxyethyl methacrylate (HEMA) copolymer as the cage. The copolymers were produced using classical radical polymerisation techniques, with N,N-methylene-bisacrylamide as the crosslinker. However, instead of using a linear polymer as the snake, they exploit monodisperse 500 nm diameter polystyrene nanoparticles, These are formed as a latex, stirred into the copolymer in a proportion of 26 per cent by volume to create the nanocomposite. The final PSA contains more than 60 per cent water. One key factor influencing this adhesives mechanical properties is whether the copolymer has a homogeneous or heterogeneous structure. HEMA reacts almost 20 times faster than AM, which can result in non-ideal copolymerization with large blocks of HEMA arising from extensive homopropagation. As well as other properties like its elastic modulus and maximum stress withstood, the researchers assessed the adhesives tack through the total work of adhesion (Wadh) needed to detach the adhesive sample from the surface. If HEMA was allowed to homopropagate, the adhesives elasticity, tack and firmness decreased, with the most drastic reduction caused when equal amounts of AM and HEMA are present. In this case the copolymer is highly heterogeneous, with large segments made up predominantly of either AM or HEMA, and adding polystyrene nanoparticles has little effect on homogeneity. Consequently, the researchers focussed on adhesives based on copolymers containing a maximum of 20 per cent HEMA.
Nevertheless, when tested on "human skin substitute" substrates, the nanoparticle-filled hydrogels always debonded without leaving residues behind, a property that is important in skin-contact applications. This remained true even when samples were kept in wet conditions for one month. Meanwhile,the Wadh needed to remove the nanocomposite adhesives from skin was the same magnitude as seen in existing commercially available products. In addition, the ability to control tack through the composition of the polymer network could also provide a way to remove the adhesives without damaging skin. Finally, the stress-strain curves in debonding were very different when comparing adhesion to skin or metal substrates. While for metal the bonds steadily reached a peak stress, after which stress fell similarly steadily, for skin they peaked and the fell to a plateau just below the peak. From there, the stress profile decreases rapidly to zero. The scientists believe the plateau-forming could be due to the differences in surface properties like roughness between the two different substrate types.
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