Sea creatures offer inspiration for novel means of adhesion

Nature has long proved a source of inspiration to scientists and engineers and this is as much the case in the arena of fastening and adhesives as anywhere. In fact, as scientific techniques advance, there increasingly exists the possibility of mimicking (or even directly appropriating) the mechanisms developed over millions of years of evolution, for engineering purposes. One such example can be found in the development of adhesives based on the humble Blue Mussel. The chemistry that lets mussels stick to underwater surfaces may also provide a highly adhesive wound closure and more effective healing from surgery. Mussels attach themselves to rocks with a fibrous appendage called the byssus. The individual byssal threads are stiff but stretchy, in order to dissipate the energy of crashing waves. They are produced by the mussel through a process not unlike injection moulding. Because they are constantly being blasted with water-borne debris, they have a protective outer cuticle. This cuticle is described as a biological polymer and while it exhibits epoxy-like hardness, it can also stretch up to 100% without cracking. Clearly, the potential uses of such adhesive capabilities underwater are many. Dr. Kaichang Li, for instance, was inspired by these proteins to create PureBond, a formaldehyde-free, soy-based adhesive used in wood glue that employs proteins similar to those found in mussels. Recent application However, a more recent application for these proteins has been found in the medical sector. In recent years, bio-adhesives, tissue sealants and haemostatic agents have become preferred products to control bleeding and promote tissue healing after surgery. The problem, however, is that many of them have side effects or other shortcomings, such as an inability to perform well on wet tissue. Clearly, the mussel has some value here. Jian Yang, associate professor of bioengineering at Penn State says: "There are sea creatures, like the mussel, that can stick on rocks and on ships in the ocean. They can hold on tightly without getting flushed away by the waves because the mussel can make a very powerful adhesive protein. We looked at the chemical structure of that kind of adhesive protein." Yang and his fellow researchers took the biological information and developed a wholly synthetic family of adhesives that incorporated the chemical structure from the mussel's adhesive protein into the design of an injectable synthetic polymer. The bio-adhesives, called iCMBAs, adhere well in wet environments, have controlled degradability, improved biocompatibility and lower manufacturing costs, putting them a step above current products such as fibrin glue and cyanoacrylate adhesives. Fibrin glues are fast acting and biodegradable but have relatively poor adhesion strength. They may also carry risk of blood-borne disease transmission and have the potential for allergic reactions due to animal-based ingredients. Cyanoacrylate adhesives or 'super glues' offer strong adhesion, rapid setting time and strong adhesion to tissue, but they degrade slowly and may cause toxicity, often limiting their use to external applications. Additionally, neither product is effective when used on wet tissue, a requirement of internal organ surgery, nor are there any current commercially available tissue adhesives or sealants appropriate for both external and internal use. The researchers tested the newly-developed iCMBAs on rats, using the adhesive and finger clamping to close three wounds for two minutes. Three other wounds were closed using sutures. The researchers reported their findings in a recent issue of Biomaterials. The iCMBAs provided 2.5 to eight times stronger adhesion in wet tissue conditions compared to fibrin glue. They also stopped bleeding instantly, facilitated wound healing, closed wounds without the use of sutures and offered controllable degradation. The iCMBAs are also non-toxic, and because they are fully synthetic, are unlikely to cause allergic reactions. Side effects were limited to mild inflammation. The iCMBAs could eventually be used in a wide range of surgical disciplines from suture and staple replacement to tissue grafts to treat hernias, ulcers and burns. Sticking with it Another sea creature to have inspired an adhesive application recently is the remora fish, which attaches itself to sharks for transportation, protection and food. It does this by using a suction disk located on the top of its head. However, the exact nature of this adhesive process has never been entirely clear. However, a study led by researchers at the Georgia Tech Research Institute (GTRI) provides details of the structure and tissue properties of the remora's unique adhesion system. The researchers plan to use this information to create an engineered reversible adhesive inspired by the remora that could be used to create pain- and residue-free bandages, attach sensors to objects in aquatic or military reconnaissance environments, replace surgical clamps and help robots to climb. "While other creatures with unique adhesive properties – such as geckos, tree frogs and insects – have been the inspiration for laboratory-fabricated adhesives, the remora has been overlooked until now," said GTRI senior research engineer Jason Nadler. "The remora's attachment mechanism is quite different from other suction cup-based systems, fasteners or adhesives that can only attach to smooth surfaces or cannot be detached without damaging the host." The remora's suction plate is a greatly evolved dorsal fin on top of the fish's body. The fin is flattened into a disk-like pad and surrounded by a thick, fleshy lip of connective tissue that creates the seal between the remora and its host. The lip encloses rows of plate-like structures called lamellae, from which perpendicular rows of tooth-like structures called spinules emerge. The intricate skeletal structure enables efficient attachment to surfaces including sharks, sea turtles, whales and even boats. To better understand how remoras attach to a host, Nadler and GTRI research scientist Allison Mercer teamed up with researchers from the Georgia Tech School of Biology and Woodruff School of Mechanical Engineering to investigate and quantitatively analyse the structure and form of the remora adhesion system, including its hierarchical nature. Results from the GTRI study suggest that remoras use a passive adhesion mechanism. This means that the fish do not have to exert additional energy to maintain their attachment. The researchers suspect that drag forces created as the host swims actually increase the strength of the adhesion. Dissection experiments showed that the remora's attachment or release from a host could be controlled by muscles that raise or lower the lamellae. New insights The researchers also developed a technique that allowed them to collect thousands of measurements from three remora specimens, which yielded new insight into the shape, arrangement and spacing of their features. First, they imaged the remoras in attached and detached states using microtomography, optical microscopy and scanning electron microscopy. From the images, the researchers digitally reconstructed each specimen, measured characteristic features, and quantified structural similarities among specimens with significant size differences. Detailed microtomography-based surface renderings of the lamellae showed a row of shorter, more regularly-spaced and more densely-packed spinules and another row of longer, less densely spaced spinules. A quantitative analysis uncovered similarities in suction disk structure with respect to the size and position of the lamellae and spinules despite significant specimen size differences. One of the fish's disks was more than twice as long as the others, but the researchers observed a length-to-width ratio of each specimen's adhesion disk that was within 16% of the average. The researchers are planning to conduct further tests to better understand the roles of the various suction disk structural elements and their interactions to create a successful attachment and detachment system in the laboratory.