Laminates and Templates

Biomineralization33 is precisely controlled by complex templating relationships and coordinated secretory processes that are ultimately encoded in genes. During formation of abalone shell, for example, secreted proteins self-assemble into two-dimensional polyanionic b-pleated sheets that serve as templates for the nucleation and epitaxial growth of calcium carbonate crystalline domains (Figure 10). A microlaminate composite is formed that has exceptional regularity, strength, and crystalline ordering. The organizing organic polymers typically contribute less than 1% of the composite material by weight, yet the material's strength and fracture resistance far exceed those of the crystals themselves. The unique properties of biosynthetic microlaminates are due to the organic matrix layers' capacity for flexible deformation and to the retardation of crack propagation at each mineral-organic interface.

The biological templating process that controls the structure of molluscan shell has been mimicked in the formation of sub-micron structures built up on tubules. The cylindrical structure of a tubule is intrinsically interesting as a template. It leads directly to a large degree of shape anisotropy, which can be tuned by adjusting the dimensions of the tubules either by manipulating the molecular structure of the protein or other molecule attached to the substrate or by changing the processing conditions under which the tubules form.

A beautiful example of the use of spontaneously self-assembled mesoscopic ordering to produce new materials is the use of the lyotropic hexagonal phase as a template to make mesoporous solids. These materials have periodic arrays of nanometer-sized pores of unprecedented regularity and density. Mesoporous metallic surface films have also been generated via molecular templating, by employing the self-assembled two-dimensional crystalline protein arrays of bacterial cell walls as patterning elements.

Biomotors

Much of the molecular transport in biological systems occurs not by diffusion but by active transport by biomotors.34 Such systems do not conform to our definition of self-assembly because the biomotors are driven by chemical energy input

Figure 10

Scanning electron micrograph of the growth front of abalone shell. The aragonite (CaCO3) crystals in the shell form very regular stacking and interdigitating plates. A combination of polyanionic proteins and matrix proteins produces this hierarchical structure, which gives the shell its unique optical properties and its increased strength. (Courtesy of A. Belcher, D. Morse, and G. Stucky, University of California, Santa Barbara.)

33  

See footnote 11.

34  

“Seventh Biophysical Discussions: Molecular Motors: Structure, Mechanics and Energy Transduction,” Biophysical Journal 68 (April 1995).



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