Mechanics and biomimicking of marine mussel plaques: the effects of rigidity and the surface texture pattern of the underlying substratum

How can a mussel survive the harsh marine environment including the dynamic forces exerted by the turbulent tidal currents and attacks from predators? If your answer is that the strong, rigid mussel shell may play an important protective role, then you would only be half right. The mussel also relies on hundreds of tiny collagen-rich threads on the surface of its shell, which are deposited radially around it. These threads adhere to various wet surfaces allowing the mussel to fix its position on objects such as rocks, piers and ships. The threads terminate in adhesive pads, the plaques, which operate as strong holdfasts to underlying surfaces: the detachment force of a plaque is approximately equivalent to the weight of an average mussel (~ 2 N). The unique adhesive structure of a mussel plaque consists of a dense outer protective cuticle layer and a low density, porous plaque core, which work with each other in a cooperative way. The plaque core also forms its internal microstructures in adaptive response to the surrounding physical environment. Hence, the conditions of the surfaces it adheres to, such as rigidity and surface texture pattern, can have significant reciprocal effects on the forming adhesive structure of a mussel plaque. Exactly how the tiny adhesive structure of a mussel plaque can lead to so strong an adhesion to various wet surfaces under the harsh marine environment remains a fascinating mystery. The interaction between a plaque and an underlying surface provides a good example of how distinct natural materials, in this case the cuticle layer, the plaque core and the substratum, can be joined to resist external loadings. Understanding of the physical mechanisms will bring new insights into the development of novel approaches to joining dissimilar materials, which has long been a challenge for man-made engineering systems. 

With advances in nanofabrication methods such as Electron Beam Lithography (EBL) as well as experimental approaches such as Nanofocus Computed Tomography (Nano CT) and Traction Force Microscopy (TFM), it has become possible to measure complex plaque-substratum interaction events through manipulating the rigidity and the surface texture pattern of the underlying substratum. In addition, using the latest 3D printing techniques, we can investigate the behaviours of the engineering systems that mimic the adhesive structure of a mussel plaque. This can then facilitate the further understanding of complex plaque-substratum interaction events. We believe that the principles learnt from plaque-substratum interactions can provide unique opportunities for further advancement of the well-established scientific concepts in man-made engineering systems.