Overview of Biomineralization

Scientists and engineers have long been inspired by the beautiful structures and functional properties of the materials formed within living organisms. In particular, the hard tissues of organisms (e.g. bone, teeth, mollusk shells) are composed of minerals which are typically in close association with an organic polymeric phase, and thus are biocomposites. The mineral crystals that are formed by the organisms, called biominerals, frequently have shapes that are very different from the crystal habits produced inorganically. In fact, the biomineral crystals may not have a defined crystal habit at all; instead the crystals may be "molded" into elaborate structures which have non-faceted crystal surfaces, wherein the "mold" is a vesicular compartment within which the crystal is formed. Of course biominerals can not be melted at physiological temperature; thus this ability to "mold" (i.e. grow and stabilize) such energetically unfavorable crystalline structures within the physiological environment has intrigued the materials engineering community. In particular, there is a demand for low-temperature processing techniques with environmentally friendly materials, and particularly for biomaterials applications in which it is desirable to incorporate thermally sensitive organic and/or biological components into materials (e.g. bioreceptor proteins, enzymes for biocatalysis, live cells, etc.). The control of crystal shape is only one of the many puzzling features of biomineralization. Overall, it is seen that control over biomineral properties can be accomplished at a myriad of levels, including the regulation of particle size, shape, crystal orientation, polymorphic structure, defect texture, and particle assembly. In the latter case, cellular processes enable control in both the spatial and temporal domain in such a way that hierarchical composite structures can be built which increase the toughness and durability of the material, which is invaluable for load-bearing materials such as bones, teeth, mollusk shells, etc..

There are a variety of types of biominerals, as indicated in Table 1, and biominerals can be found in many phyla of plant and animal, thus biomineral systems offer ample opportunity for investigations directed at the development of biomimetic strategies for the synthesis and processing of particles, ceramic thin films, biomaterials, and hierarchically-structured composites.

Table 1. Variety of Biomineral Types and Biological Systems*

Biomimetic Engineering

Biomimetics is defined as microstructural processing techniques that either mimic or are inspired by biological processes. Seemingly, it would be difficult for the materials engineer to mimic complex cellular processes, however, the materials chemistry aspects of biomineralization can be studied by model systems, and utilized for biomimetic engineering. One particular aspect of interest to the materials chemist is the means by which these organisms use organic constituents to mediate the growth of the mineral phase. For example, macromolecular templates are used to direct the nucleation event, vesicular compartments to delineate particle size and shape, and solubilized proteins to regulate the kinetics of crystal nucleation and growth. In recent years, researchers have capitalized on some of these concepts to produce novel materials. For example, the self-organizing ability of amphiphilic molecules has been used to direct the nucleation and growth of inorganic materials precipitated in their presence, such as in the fabrication of mesoporous ceramic thin films, organized arrays of nanoparticles, and microlaminated structures, etc.. Biopolymers and their synthetic analogues are also being utilized in industrial processes requiring water treatment and particle manipulation. For example, polypeptides have been designed for use as biodegradable dispersants and flocculents, absorbents, anti-scalants, and crystal growth modifiers.

The manipulation of particles is of primary interest to the Engineering Research Center on Particle Science & Technology here at UF, and because biomineralizing organisms are masters at manipulating particles and interfaces, we are using a biomimetic approach to fabricate core-shell particles which encapsulate active agents (e.g. drugs, catalyst, pesticide) within a "soft" organic core. The goal is to provide controlled released of the active agent through the inorganic calcium carbonate shell, in which porosity of the shell is achieved by using self-assembling amphiphiles to template the deposition of the inorganic phase.

The low temperature aspects of biomimetic processing are particularly appealing for biomedical applications because thermally labile components can be incorporated to improve biocompatability, such as the addition of osteoinductive factors into calcium phosphate coatings and cements for bone/dental prosthetics. We are also examining the use of organic-inorganic interactions to generate iron oxide/polypeptide composite particles for applications in magnetically-guided drug delivery. Iron oxide nanoparticles, found in magnetotactic bacteria, are another example of biominerals that are well-regulated in biological systems. In a third project related to biomedicine, we are examining the role of macromolecular species in pathological bimineralization, such as occurs in the formation and/or aggregation of calcium oxalate crystals in kidney stones.

Polymer-Induced Liquid-Precursor (PILP) Processing

In a previous study by Gower , it was shown that acidic polypeptides can alter the crystallizing environment of calcium carbonates (in model systems) by interacting with the ionic species and inducing the phase-separation of a hydrated calcium carbonate/polypeptide liquid-precursor phase. The result of this interaction is to transform the solution crystallization process to a solidification process, in which the precursor can be deposited in or on substrates to produce mineral films, coatings, or spatially delineated structures (Figures). Current investigations include determining the mechanism responsible for this PILP process, as well as its generality with respect to other mineral systems. The evidence thus far seems to suggest that this liquid-precursor process could play a key role in the morphological control obtained in calcification processes . Therefore, it is our hopes that this work will contribute to unraveling mother nature's secrets for mediating particle properties, and that we can develop biomimetic processing strategies that enable the design of novel materials, such as engineered particulates, ceramic thin films, and hierarchically-structured composites.