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systems such as the biotin-avidin couple may be used as controllable strong adhesives. Colloidal dispersions have also been used as a solvent for self-assembling lamellar phases of surfactants. In particular, superparamagnetic grains have been used to allow orientation of the smectic liquid crystal phase in magnetic fields as low as a few gauss. The physics of the phase behavior of these amphicolloids is just now beginning to be studied. It is likely that there will be increasing activity in the study of colloid-surfactant interactions. Other studies, 43 mostly carried out in Europe, have investigated colloidal particles in polymer solutions. They have demonstrated that under appropriate conditions the particles may adsorb on the polymers, forming “pearl necklaces” that have the potential to combine the excellent mechanical properties of polymers with useful electronic or optical properties of the solids. In the future, we may expect biomolecular applications based on such “necklaces.”
Biosynthesis of Polymers
Although there has been impressive progress in the development of new polymeric materials by biosynthetic pathways, important challenges in synthesis remain. Artificial DNAs, especially those with highly repetitive sequences, can be so unstable in microbial hosts that they are rapidly deleted from the cellular population or are rearranged. Artificial messenger RNAs can be rapidly degraded or translated into protein with unacceptably low efficiency. Even after a protein product is formed, it may be toxic to the host cell or subject to rapid metabolic turnover. Bioprocess engineering for the separation and recovery of products from genetic engineering is, at present, a time-consuming and expensive step that is often rate-limiting. Further work on each of these problems will be required to make protein biosynthesis a tool of broad utility in materials science and technology.
Processing of Biosynthetic Polymers
As novel in vivo synthesized polymers become increasingly available, researchers need to address processing issues. The principles of self-assembly imply slow processing in order to achieve equilibrium. Practical situations may demand assisted assembly, however, and indeed the modification of self-assembly by external controls may in certain cases prove beneficial. Alternatively, the use of processing techniques that have worked well with synthetic polymers may prevent realization of the inherent properties present in precision-built biosynthetic materials. Processing of biosynthetic materials demands detailed investigation.
In order to realize the potential of self-assembled tubules for applications such as those mentioned in the “Status” section, the dimensions of the tubules need to be engineered for optimal performance for each application. Such an effort will require a better fundamental understanding of basic tubule phenomena. In addition, the cost of the tubule-based approach must be consistent with operational requirements and the competitive marketplace.
A high priority should be assigned to learning to control the processes by which multicomponent self-assembling systems evolve for homogeneous states.
Multicomponent self-assembly can yield structure at several length scales. The patterns that can be formed by a suitable quench from an initially homogeneous state can vary over several orders of magnitude in size. This variability arises from the many possible combinations of phase transition mechanisms (e.g., spinodal decomposition versus nucleation and growth), including phase transitions of one or more components. For example, the transition of one of the components from liquid to liquid
For example, B. Cabane and R. Duplessix, Colloids and Surfaces 13:19 (1985).