Professor Kate McGrath’s team of researchers at the MacDiarmid Institute are trying to replicate the way in which pāua Haliotis iris) lay down their shell material. Pāua shell is an arrangement of calcium carbonate crystal forms and organic macromolecules such as proteins, lipids and polysaccharides. It is the high strength, resistance to fracture and aesthetic value of these natural materials that has prompted scientists to try to replicate natural shell growth in the laboratory. The long-term goal of such research is to develop materials that might find application in such diverse areas as human health, electronics and non-hydrocarbon-based plastics.
Getting a material that is comparable to the natural biomineral does not necessarily involve attempting to produce it the same way that a biological organism does. It is thought that, with an actual biological organism, the proteins and carbohydrates are being synthesised simultaneously with the mineralisation process. To replicate that extremely complex system in the lab is not possible at this point in time. However, it is possible to utilise other techniques and other scientific knowledge and apply these. The outcome will not be identical but will capture the essential elements of the natural system.
Chitin to chitosan
In pāua shell nacre (also known as mother of pearl), the major organic material is a type of carbohydrate called chitin, and this is the starting point in the replication process. Chitin is not very soluble in water, and in order to improve this, it is converted to chitosan via a simple chemical reaction that simply modifies the chitin structure.
By carefully dissolving the chitosan in water, a jelly-like material forms, creating the framework that can support the mineralisation process. The structure of this jelly-like material, a type of hydrogel, can be likened to a household sponge with lots of pores and holes within a continuous solid system.
Soaking the hydrogel
By firstly soaking the hydrogel in an acidic solution of calcium ions and then in a solution of carbonate ions, precipitation of calcium carbonate can be initiated within the hydrogel matrix.
The addition of polyacrylic acid (PAA) to the system, in an attempt to replicate the presence of proteins that are in the native system, can greatly aid this precipitation process. Careful control of all of the soaking steps and the PAA addition allows manipulation of when and how much crystallisation occurs.
Structural analysis of the precipitate
In order to establish how this artificially formed material compares with the natural material, a series of physical analysis procedures need to be performed. First, the actual structure of the system is determined using a scanning electron microscope. This allows for analysis at the nanometer scale range and gives the researchers a good look at what has been precipitated.
Calcium carbonate has several crystalline forms, known as polymorphs, and two of them, calcite and aragonite, are found in pāua shell. Depending on the starting conditions, the calcium carbonate generally precipitates out as either calcite or aragonite. Calcite is the default form but aragonite is the one that, in nature, gives the nacre its strength and toughness. It is of interest, then, to establish which of the polymorphs is present, and this is achieved by using an analysis technique known as micro-Raman spectroscopy. This allows a particular crystal in the nanometre range to be probed and identified.
Finally, the sample is analysed using X-ray diffraction to confirm the presence of calcite and/or aragonite as well as their relative amounts.
Results to date
Results to date have been promising, and planning for the next phase is well advanced – this involves precipitating a form of calcium phosphate similar to that found in human bone and teeth. The end result of this type of research could well be the production of biocompatible hydrogels that can be mineralised when placed in the body to assist in the repair of bone and teeth.
Nature of science
Science is a blend of logic and imagination. These two principles are very much at play when it comes to developing systems and processes that attempt to mimic pāua shell biomineralisation