Biomimetics: holy mother of pearl!

Marriyum Hasany introduces a new class of materials in mimicry of nature

Natural selection has allowed organisms to evolve into having various systems and attributes that have a great deal of efficiency and resistance towards harsh elements of the environment. Hence, scientists have been looking towards such systems to imitate their properties in synthesising new materials—a science referred to as biomimetics.

Examples of biomimetics are abundant, from the design of velcro being inspired by cockleburs attaching themselves to a dog to looking at the hydrophobic properties of lotus leaves to create a sealant with analogous properties. Now, research is being conducted to understand the structure of nacre, also known as Mother of Pearl, to eventually synthesise nacre-mimetic materials which would have the sought-after mechanical properties of their inspiration.

Found in the inner layer of mollusc shells, nacre has a rather aesthetically pleasing iridescent look that made its use historically popular in architecture, musical instruments and other decorative items. However, scientists around the globe find this material more fascinating for its strength and light weight rather than beauty.

Nacre is an organic-inorganic substance, made of polygonal aragonite (calcium carbonate) nanograins and ductile biopolymers, which respectively make up 95% and 5% of its volume. The aragonite nanograins are held together to form a single layer of nanoplates, and each plate is layered upon each other, with the adhesive biopolymers fastening the entire structure together. This forms a rather “brick and mortar” structure that gives nacre most of its strength, measured to be three times that of calcium carbonate.

Figure 1 Structure of Nacre

Research into nacre-mimetic synthesis is warranted due to its desirable properties including its high tensile strength, toughness, fracture resistance, lightweight and sustainability. In order to synthesise a nacre-like material, six main categories of techniques have been developed: conventional method for bulk ceramics, freeze casting, layer-by-layer deposition, electrophoretic deposition, mechanical assembly and chemical self-assembly —each with its own advantages and disadvantages.

Nacre’s properties allow it to have several applications, especially in biomedicine—it can be injected into bones to amend defects in bone substitution, and be used as a coating for metal implants. Additionally, when added to soft materials it increases their strength and elasticity without compromising on mass, and therefore can be used in construction and aerospace engineering.

Nacre-mimetic materials could be a valuable resource, so understandably many scientists are tirelessly researching techniques to improve and accelerate synthesis of the material. Who knows how long before this technology with origins in molluscs found by the sea, may be employed on a much larger scale.

From Issue 14