Composition and properties of nacre

Nacre is an primeval material with an iridescent shimmer, which forms the inner coating of certain mollusc shells (oysters, abalone, etc.). It dates back some 500 million years, and its complex composition presents surprising similarities and great complementarity with living tissue, particularly mammals.

Location of nacre

Pinctada Maxima

The shell (exoskeleton) of the bivalve mollusc comprises three layers:

  • An external prismatic called the periostracum
  • An intermediate Layer called prismatic calcitic
  • An internal layer: aragonitic flat “tablets” running parallel to the shell, bonded to the organic matter

This internal layer is known as nacre. Close examination reveals that it has a complex organo-mineral structure at a micrometric and nanometric level, and is composed of aragonite nanocrystals and an organic matrix.

Composition of nacre

Microstructure of nacre observed via electron microscopy

The composition of nacre is 95-97% mineral and 3-5% organic.

The close bonding of the mineral and organic structures of nacre give rise to an ultrastructure”, whereby the aragonite crystals are the bricks and the organic fraction is the mortar.

95-97% mineral

(Calcium carbonate = CaCO3)

The mineral components of nacre are needed for cell and tissue metabolism in mammals.
Almost all of these mineral components are found in mammals.

3-5% organic :

1. Proteins and free amino acids:

Proteins are either essential amino acids (which are not made by the human body, and must be included in the diet) or non-essential amino acids (which are made by the human body). The main amino acids present in the organic fraction of nacre play a key role in cell and tissue metabolism, in the formation, growth and repair of the body and play a part in cell ageing.

Proteins can combine with other molecules to form the following:

  • glycoproteins to “growth factor-like”
    In particular, the following glycoproteins are among those contained in nacre:

    • Proteoglycans: regulate an array of cellular activities such as cell proliferation, differentiation, adherence and migration.
  • Glycosaminoglycans (or mucopolysaccharides): complexes found in the composition of the cartilage and the hyaluronic acid in conjunctive tissue, among others
    Role: a lubricant that’s a component of the hyaluronic acid present in practically all of the tissues, owing to their high-water retention capacity
  • Metalloproteins: play a role in immune responses
    Role: prompting / accelerating biological reactions
  • Aminoglycosides with “antibiomimetic-like”properties
    Role: antimicrobial

2. Pigments, including:

  • Carotenoids

Role: natural yellow-red colouring pigments, which are responsible for protection against UV rays and the coloration of teguments (skin, nails, hair, claws, fur) + antioxidant.

  • Melanins

Role: dark-coloured biological pigments which are responsible for protection against UV rays and the coloration of teguments (skin, nails, hair, claws, fur) + antioxidant.

3. Lipids and fatty acids (mono- and polyunsaturated)

Role: lubricating, hydrating molecules found in the composition of cells and in the production of certain hormones. They help to retain the cell membrane’s structural and functional integrity

The organic fraction of nacre contains substances that stimulate osteogenesis during the formation process (Lopez et al., 1994):

  • Molecules that stimulate the activity of bone tissue, affecting the differentiation of bone-forming cells and mineralisation
  • “Signal” molecules which are active on osteoclastsand can activate osteogenesis of the bone marrow cells (Lamghari et al., 1999)

The organic and inorganic components of nacre affect epidermal regeneration and hydration of the skin:

  • Calcium enabling stimulation of the fibroblastsin the dermis (Morvan, 2009)
  • Components involved in cell adhesion and communication
  • Proteins involved in the synthesis of collagen types I and III (Lopez et al., 2000)
  • Lipids to strengthen the epidermal barrier (Rousseau et al., 2006)

Properties of nacre

Nacre presents multiple properties:

The biological, mechanical and physical properties of nacre

The biological properties of nacre have particular appeal when it comes to the repair and regeneration.

  • Biocompatibility: refers to a material’s ability to be accepted by the biological environment into which it is implanted, and to not cause damage to it. The by-products of the breakdown of nacre (water, CO2, Ca2+ are not toxic to the human body
  • Angiogenesis: the physiological process of forming and growing new blood vessels governed by growth factors

The mechanical properties of nacre have particular appeal when it comes to the repair and regeneration of hard tissue (bone):

  • Mechanical strength: outperforming bone, for example, hence the benefit of working on an alternative, with dental implants made of nacre
  • Elasticity: nacre outperforms human bone in terms of elasticity

The physical properties of nacre, too, are of particular appeal when it comes to the repair and regeneration of hard tissue (bone):

  • Osteoconduction: the physiological process enabling circulation of the cell elements needed to repair a loss of bone substance. nacre in powdered form, with set granulometric parameters, promotes the porosity needed for the circulation and proliferation of new vessels and of the cell elements needed to heal damaged tissu
  • Entering systemic local regulation mode

Nacre’s property enables the tissues in contact with it to get all the properties and functions that they need: no risk of overdose (the tissues only use what they need or excessive proliferation.