Multilayer Films

Biomimetrics involves the study of synthetic structures that mimic or imitate structures found in biological systems [see Cooper (2000)]. It makes use of large-scale or supramolecular self-assembly to build up hierarchical structures similar to those found in nature. This approach has been applied to the development of techniques to construct films in a manner that imitates the way nature sequentially adsorbs materials to bring about the biomineralization of surfaces, as discussed below. The process of biomineralization involves the incorporation of inorganic compounds such as those containing calcium into soft living tissue to convert it to a hardened form. Bone contains, for example, many rod-shaped inorganic mineral crystals with typical 5nm diameters, and lengths ranging from 20 to 200 nm.

The kinetics for the self-assembly of many of these films involved in biomineralization can be approximately modeled as the initial joining together or dimerization of two monomers

with a low equilibrium constant K0 = CD/(CF)2, followed by the stepwise or sequential addition of more monomers

with a much larger equilibrium constant K — C„+1/CFC„. These two constants exercise control over the rate at which the reaction proceeds. For the case under consideration, KD < K, the concentration of free or unbound monomers CF always remains below a critical concentration C0= l/K, specifically, CF < C0. When the total concentration of free and clustered (i.e., bound) monomers CT satisfies die condition CT < C0, then the free monomer concentration CF increases with increases in CT. When a high enough concentration is provided so that CT becomes larger than the critical value (i.e., CT > C0), then the aggregate forms and grows for further increases in CT. In analogy with this model, self-assembly kinetics often involves a slow dimer formation step followed by faster propagation steps, with A"D < K.

There are many Cases of multilayer thin films in biology, such as structural colors in insects that change when the films are subjected to pressure, shrinking, or swelling. For example, scale cells from some butterflies can produce iridescent multicoloring affects due to optical interference of thin-film layers or lamellae formed from the secretion of networks of filaments that condense on cell boundaries.

The biomineralization of mollusc cells begins by laying down a sheet of organic material so that calcium carbonate can be deposited on its surface and in its pores, and C,aC03 layers can build up. Proteins from the mollusc shell containing high concentrations of particular amino acid residues control the form of the calcium carbonate layering, and these proteins can be altered to vary the layering morphology. Multiple layers either grow in sequences that are organic in nature or contain the ifaombohedral calcite form, or the orthorhombic aragonite variety of CaCC>3. It is possible to imitate some aspects of these natural biomineralization processes for the preparation of synthetic multilayer thin films, although the resulting films themselves do not closely resemble those in molluscs. For example, consider a positively charged substrate placed in a solution with a negative electrolyte, that is, a solution containing negative ions that can carry electric current. The positive substrate attracts die negative electrolyte, and the latter can adsorb on its surface, forming a structure called a pofyion sheet, as shown in Fig. 12.16. This sheet is rinsed and dried, and then placed into another electrolyte solution from which it adsorbs a second positive layer. The sequential adsorption process can be repeated, as indicated in Fig. 12.16, to form a multilayer of alternating positively and negatively

Figure 12.16. Sketch of the sequential adsorption process for the formation of a polyion film. The upper figure shows a positively charged substrate (left) that has adsorbed a negatively charged potyelectrolyte by being dipped into a negative electrolyte solution (center), and then adsorbed a positively charged layer from a positive electrolyte solution (right). The lower figure shows two additional steps in the sequential adsorption process. [From T. M. Cooper, in Nalwa (2000), Vol. 5, Chapter 13, p. 720.]

Figure 12.16. Sketch of the sequential adsorption process for the formation of a polyion film. The upper figure shows a positively charged substrate (left) that has adsorbed a negatively charged potyelectrolyte by being dipped into a negative electrolyte solution (center), and then adsorbed a positively charged layer from a positive electrolyte solution (right). The lower figure shows two additional steps in the sequential adsorption process. [From T. M. Cooper, in Nalwa (2000), Vol. 5, Chapter 13, p. 720.]

charged polyion sheets. The nature of the layering can be controlled by varying the types of electrolytes in the successive solutions.

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