Hydrogel biomineralisation
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.
Biomineralisation explained
Professor Kate McGrath, of the MacDiarmid Institute, explains the process by which organisms that have hard structures such as bone and shell deposit mineral-rich hard tissues. The process is called biomineralisation. In the case of shellfish like pāua, it involves the laying down of a combination of the inorganic mineral calcium carbonate with biological components such as proteins and carbohydrates.
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.
Soft contact lens
Soft contact lenses are hydrogels with high water content. The polymeric material or plastic part of the lens has an open structure capable of holding large amounts of water.
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.
Chitosan and chiton molecules
The hydrogel chitosan is a polymer similar to cellulose. It is made from chitin.
There are a number of steps involved in mimicking the calcium carbonate biomineralisation process. In one method chitosan hydrogel is a scaffold within which crystallisation of calcium carbonate is induced.
The artificial biomineralisation process
These are the key steps in promoting the precipitation of calcium carbonate within the chitosan framework.
Further research is being conducted to improve control of the crystallisation process as well as experimenting with calcium phosphate (bone mineral) precipitation.
Future applications
Long-term benefits of biomineralisation research
In this video clip, Professor Kate McGrath, a prior Director of the MacDiarmid Institute, outlines some of the long-term benefits of biomineralisation research. Implant technologies in the human health area and new materials with unique mechanical and/or electrical properties are possible targets for future research.
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.
