Hello Everyone,

I hope many of you have seen a mussel near salty water (sea/ocean) or near fresh water; some have tasted it too. For those who are unaware of it: “Mussel is a common name used for the bivalve molluscs with elongated and asymmetric shell covering the body.” Mussels have a unique property of adhering to the solid surface to be saved from the sea's next income waves. Besides its other benefits of being rich in nutrients and healthy snacks, mussels showed excellent potential for being used in several biomedical applications.

Mussels have the ability to withstand strong sea waves and keep themselves attached to the surface of their attachment site. This could only be made possible by their byssus (Mussel Holdfast), which functions as a bundle of threads adhered to the rock. These bundled threads generally have four parts: an attachment plaque, stem, thread, and root. The thread has two sections, distal and proximal portions: the distal thread end holds to the surface, while the proximal end is towards the stem, which is connected to the body part of the mussel. (ref) The retractor muscles within the mussel contain a byssal protein, which is generally called the “mussel protein”. The mussel foot comes out of the ventral shell gap and explores the surface within an average diameter of 8-10 centimeters. The choice of a smooth or rough surface is still a topic of investigation, like how it chooses where to attach and where not.

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Microstructure of Mussel Byssus

The mussel byssus plaques have a fine degree of hierarchical microstructure. Especially the distal core threads comprise a bundle of microfibers on the surface, which helps in the attachment. I will not go into more detail about the microstructures of byssus, rather will explain more about the protein involved in the adhesiveness and its applicability in biomedical research. The major protein associated with the byssal cuticles is mfp-1; the rest are calcium, iron, and fatty acid. The protein is of 108 kDa and has an overall negative charge (isoelectric point 10.5). The protein contains a tandemly repeated sequence of ten amino acids containing two unusual amino acids, hydroxyproline (Hyp) and 3,4-dihydroxyphenyl-L-alanine (Dopa). The presence of hydroxyproline in the sequence gives the liberty to form a left-handed helical structure, and further, the Dopa makes it prone to oxidation, especially at or above the physiological pH. To oxidize the protein, the pH must decrease beyond pH 5.5. The adhesion was triggered by the presence of a metal ion (Fe3+), which is mainly because of the Dopa-Fe complexation. Mussel byssus is known to adhere to wet surfaces, as they are designed with a hard but extensible coating, gradient porous core, fiber reinforcement, and abyssal thread collagen.

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Taking advantage of such an understanding of the mechanism of mussel byssus and their adherent property. Several research groups have proposed biomimetic constructs of mussel protein for the adherent property and their application in different biomedical applications. With the advent of mussel-inspired adherent protein, a new realm of adhesive materials with exceptional biocompatibility has been developed with the potential to transfigure research and the industrial future.

Here is an example of utilizing this mussel-inspired protein (DOPA) to be used as an adhesive to join the cracked bone. The Dopa derivates are combined with polyester [poly(β-amino ester)] to develop a tissue adhesive. The adhesive developed via this strategy showed the low cytotoxic nature of the material and a degradation rate of 58.5% of the total mass in a month. These materials also possessed a strong tensile strength of 2-3 megapascals, proving their applicability as bone glue. Besides biomedical applications, mussel proteins also showed their potential in underwater repair and electronic and automotive industries.