The study of adhesion phenomena has established a potent and very broad technology nowadays, offering innovative solutions to a wide range of applications (such as, packaging and consumer goods; automotive, aircraft and metal processing; electronics; medicine and hygiene; building and furniture, etc), and providing craftsmen and private consumers with easy-to-use technically-advanced products. The diversity of adhesive formulations is enormous and can be classified in many different ways: bonding mechanism (physical hardening, chemical curing, pressure sensitive,..), chemistry employed (cyanoacrylates, polyurethane or epoxy resins,..), or adhesion force and sensibility (soft, structural, thermosensitive,..). For every application in which two elements need to be fastened together, a specific adhesive can in principle be formulated.
On the other hand, the development of materials with adaptive behaviour or reversible connectivity using the tools and concepts of supramolecular chemistry is receiving increased attention during the last years. In nature, we can find many examples including molecular recognition processes through non-covalent interactions, and, in some of them, adhesion is the resulting property. Noncovalent interactions represent versatile design elements that are central in biological adhesion processes, where adaptive and reversible connectivity is required. Examples include cell and bacterial adhesion to surfaces, and a broad range of adhesive animal secretions (mussels, tubeworms). Such phenomenon has inspired researchers to employ similar functionalities into synthetic materials. In this endeavour, non-covalent interactions, particularly hydrogen-bonding interactions, are receiving special attention due to their reversible, dynamic bonding vs debonding, surface selectivity, etc. Thus, the link between supramolecular chemistry and adhesion science is an excellent approach to overcome some drawbacks of synthetic adhesives and create, in a bioinspired fashion, new adhesive materials with unprecedented properties.
In the CompAd project, we have been working in the development of a novel adhesive technology that ultimately targets the commercialization of adhesive coatings where bonding: 1) occurs under a pressure-sensitive contact mechanism; 2) can be classified as weak; 3) is reversible; and 4) takes place between two complementary surfaces. The idea behind CompAd is based in chemically mimicking the operation of materials like magnets or Velcro® by using, in a bioinspired approach, complementary noncovalent interactions between molecules. In these currently existing materials, bonding and debonding processes can be repeated multiple times by applying weak mechanical forces. But most importantly, bonding only occurs when the two complementary surfaces are brought into contact, while none of them is self-adhesive. In this PoC Project we aimed to bond the scientific bases, technical know-how and expertise acquired during the last years in the ERC Starting Grant PROGRAM-NANO project, with a novel potential applied technology that aspires to provide industry and society with an unconventional chemical adhesive product. Such technology could potentially be used in any application that requires relatively weak, reversible and complementary adhesion, and several industries might show interest in it: 1) Packaging (e.g. sealable food bags and boxes), 2) Textiles (e.g. substitution of buttons in shirts, or Velcro® in pockets), 3) Global Hygiene (e.g. diapers and adult incontinence items), 4) Toys (e.g. in puzzles or construction kits), 5) Decoration (e.g. wall paper and sticky ornaments), 6) Health (e.g. medical tape applications).
Concretely, in CompAd we made use of hydrogen-bonding and ionic interactions between complementary chemical moieties (called stickers) attached to hydrophobic polymer surfaces. For the preparation of the required two complementary polymer films, the stickers and polymers were synthesized separately, and then coupled together through a simple grafting reaction. Preliminary adhesion tests confirm the superior bonding between complementarily functionalized polymer fims with respect to non-functionalized or the identically functionalized polymer films.
