simultaneously continuous. The bilayer has been shown to be an infinite periodic minimal surface. It has cubic symmetry and long-range three-dimensional periodicity, even though the lipid molecules within the bilayer possess no positional order. Although the structures of biological cell membranes most often resemble those of the liquid-crystalline lamellar phase exhibited by phospholipids, there are several examples of organelles with structures very similar to bicontinuous cubic phases. Cell membranes resembling periodic minimal surfaces have been observed in cytoplasmic organelles such as mitochondria and chloroplasts. It has been suggested that in certain invertebrates the endoplasmic reticulum (a system of interconnecting membranes inside the cell) may exhibit gyroid (spiral) cubic structures.

Electron micrographs show that nature has elevated these structures to a high level of sophistication. The lamellar-like body of chloroplasts has revealed a structure in which the pore sizes of the two intertwining aqueous channels are different, suggesting a difference in osmotic pressure between them. It is believed that these two channels act as reaction chambers for the synthesis of chlorophyll. Even though the formation of these saddle surfaces in lipid-water systems is determined by the balance of forces between the polar head groups and the nonpolar tails, in biological membranes the periodic curvature may be due to the presence of some membrane-spanning proteins.

The bicontinuous cubic phases provide thermodynamically stable structures on the nanoscale whose characteristic size can be precisely controlled. They are not rigid, however. If these structures could be stabilized, they would provide continuous, triply-periodic pore space with very uniform nano-sized pores. Such structures could find many useful technological applications in such areas as controlled release and ultrafiltration. “Smart” release vehicles can be envisioned that would allow first-order drug release in response to stimuli. These phases can also be used as templates for synthesizing nanoporous materials and nanocomposites. Another important property of the cubic phase that can be harnessed is the large bilayer surface area that it provides (103 to 104 m2/g). Immobilization of proteins can be envisaged, either by covalent attachment to the head group or by simple incorporation into the bilayer, which could lead to the development of biosensors. It is believed that the functionality of the integral proteins will be optimal since the fluid bilayer provides an environment closest to the natural condition in vivo.

Fabrication of Devices by Self-Assembly37

The potential for fabrication of electronic devices by self-assembly has often been cited as a long-term goal of research in organic thin films. It is attractive to consider devices in which the components are individual molecules or molecular complexes self-assembled on substrates from solution or by deposition from interfaces. There are three steps that must be accomplished in order to achieve this goal. Functioning molecular units must be designed and synthesized, they must be organized on a surface into defect-free structures, and they must be interconnected to form functioning networks. While progress needs to be made in each of these areas, the last one is the most difficult and needs to be addressed in the long-term. It is clearly possible to make connections by photolithography, but this cannot be accomplished at the molecular scale. Methods based on scanning microscopy, especially with chemically active tips, may provide a solution, but it is difficult to envision how such a process can be carried out on a practical scale. A biomimicking process of self-assembly, in which connections are made by enzyme-like molecules that either form bonds or activate functional groups so that they can be photochemically linked, is more attractive.

37  

See footnote 7.



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