Biominerals are hybrid materials made from organic and inorganic components. For example, the nacre (mother of pearl) associated with pāua shell is formed from calcium carbonate, chitin and proteins.

By carefully studying the way in which calcium carbonate is precipitated during seashell formation (biomineralisation), materials scientists have devised ways of replicating this process artificially. With further development, some of these processes and resulting materials may find application in medical settings such as bone repair.

A group of scientists led by Professor Kathryn McGrath at the MacDiarmid Institute in Wellington have developed an artificial biomineralisation process. This process uses a chitin-derived hydrogel (chitosan) as the framework upon which calcium carbonate can precipitate out of solution in a controlled fashion.

Hydrogel basics

A gel is a jelly-like material that is made up of a liquid trapped in a solid framework. The gel is mostly liquid but behaves as a solid due to the cross-linked network present in the framework.

Hydrogels are gels where the liquid portion is water.

Hydrogels are commonly used in our modern society especially in medical applications. For example, hard to heal wounds are often dressed with a hydrogel dressing that can slowly release antibacterial chemicals as well as maintaining moisture levels. Soft contact lenses are made of hydrogel materials that can comfortably mould to the shape of the eye when worn, as well as correcting vision defects.

Depending on their structure and manufacture, hydrogels can be designed to absorb or release water. This makes them particularly useful in and around the household for such things as infant nappies, making jellies and jams, and slow-release garden fertiliser.

Chitosan and artificial biomineralisation

One type of hydrogel that has proved to be of use in artificial biomineralisation is chitosan. This hydrogel is made from chitin, which is a polymer similar to cellulose. Chitin is the main component of the exoskeletons of organisms such as crabs, crayfish and insects. Chitosan is biocompatible, non-toxic and biodegrades in the body to non-toxic components.

Structure of the chitosan molecule showing two of the de-acylated glucosamine units that repeat to form long chains in β-1,4 linkage

Structure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains in β-1,4 linkage

These are the key steps in promoting the precipitation of calcium carbonate within the chitosan framework:

Chitosan film is carefully laid down on to a glass slide.

The film is soaked in a calcium ion-rich solution and then in a carbonate ion-rich solution.

The slide is then placed in a solution known as Kitano solution, which is a special type of saturated calcium carbonate solution.

The addition of PAA (polyacrylic acid) to the solution was used to control not only the crystal growth but also the crystal structure .

Further research is being conducted to improve control of the crystallisation process as well as experimenting with calcium phosphate (bone mineral) precipitation.

Future applications

If artificial biomineralisation processes can be better controlled and understood, one of many future uses could be in human bone repair techniques. For example, a chitosan hydrogel pretreated with bone mineral ions such as phosphate and calcium could be injected into the damaged area. Precipitation of calcium phosphate could then be induced, and this would enhance the normal bone-healing processes. Quicker recovery, stronger repair and reduced pain may well be the outcomes of such a technique.

Nature of science

Sometimes we think that scientists follow a set scientific method with an aim, a prediction, an experimental method, data gathering and a conclusion. In artificial biomineralisation work, a great deal of careful and managed manipulation of the conditions, solution concentrations and additives is required to achieve deposition of crystals of calcium carbonate. There is no single scientific method.

Published 8 October 2012