Poets and philosophers have, through the ages, viewed organisms as the embodiment of the mysterious “Vital Force,” a unique non-earthly element required for the functioning of life processes.

Biologists have seen, in living organisms, an adaptive, self-reproducing, evolving collection of molecules acting solely according to the laws of chemistry and physics.

Historians speak of the iron or bronze ages and, more recently of the plastics (polymers) and the silicon ages. Materials science departments speak of metals, alloys, ceramics, and perhaps polymers—but not of genes.

The “common man” has, it must be admitted, seen living organisms as a source of useful and important materials—wood for building; cotton, silk, and other fibers for textiles; horn, shell, and bone for tools and weapons; fats for lubricants; fur for clothing

But, in fact, few of us now think of materials when we think of living things. Neither do we think of DNA, protein, and carbohydrates when we think of materials.

No, biologists have not been blackballed by materials scientists, chemists, and physicists. Until recently, they neither understood the processes by which life produces its materials nor even conceived of manipulating those processes to tailor the properties of the materials to our needs. Only within the past few years has the “biological revolution” expanded our understanding of the molecular basis for biological phenomena and our ability to control them. It is only now, for the first time, that one can point to a legitimate field of science based on mimicking, adapting, and controlling biological systems with the goal of producing novel materials with important, unique, and useful properties.

Product Concept: Fluid packaging device.

Description: A package is required to contain approximately 50 ml (55–60 grams) of a viscous, 2-phase, thermally labile, aqueous medium containing proteins, carbohydrates, salts, nucleic acids and fats. A seamless ellipsoidal container is anticipated having a maximum radial dimension of 45 mm and a maximum axial dimension of 60 mm. The container wall must be permeable to respiratory gases and water vapor. External surface texture must be minimal. White coloration is acceptable, but the ability to incorporate solid or patterned surface coloration is desirable. The package wall must weigh approximately 5 g, the material must contain at least 0.3 g of bioavailable calcium, and it must be recyclable and biodegradable. The container must withstand 400-g drop test impacts.

Processing: All materials must be maintained and the container fabricated by aseptic processing in neutral-pH, aqueous medium at less than 40°C and with a unit production time of less than 24 hours.

Unit cost: Filled unit containers must be produced for approximately $4.00/100 units, with approximately $1.00/100 units allocated to container components and assembly.

Laboratory Solution: Not achievable in the laboratory.

Biological Solution: The eggshell.

In its composition, architecture, and function, the eggshell is an elegant solution to the design problem posed above—a multilayered bioceramic composite, consisting of a mineral phase (primarily calcite, CaCO3) deposited on and within a complex organic phase (the matrix). The eggshell provides a unique microenvironment for housing embryonic development. It protects the embryo, regulates respiratory gases, water and ions, and provides calcium for embryonic skeletal development.

The impetus for a biomimetic approach to mineralization stems from the need for increasingly sophisticated materials showing greater efficiency, specialization, and optimization—properties that ultimately depend on the control of molecular and supramolecular structure, and hence on methods of predictive chemical fabrication. Biomineralization is of central importance to the development of new approaches in materials science because, as discussed in the preceding article by Fink, the formation of bioinorganic materials, such as bones, shells, and teeth is highly regulated and responsive to the surrounding environment in a manner not achieved by conventional synthetic routes. Some possible areas of overlap are shown in Figure 1. As in the other areas of biomaterials discussed in this and next month's issue of the MRS Bulletin, there are two potential connections between the natural processes of biomineralization and the synthetic demands of materials science; first, the commercial exploitation of biologically derived, tailored materials, and second, the assimilation and adaptation of biological concepts and mechanisms into “artificial” materials design and synthesis. The former is an extension of biotechnology, by which microbial systems could be utilized to produce mineral powders. Some of the possible processes have been discussed elsewhere. In general, the use of biological sources is only applicable where the high production costs are offset by a marketable specialty product. While this is feasible for organic-based products such as polyhydroxybutyrate (see next month's MRS Bulletin) it imposes a severe limitation when we transfer the approach to biomineralization.

An obvious parallel of structure and function exists between a rhinoceros and a tank, and between a beetle shell and the skin of an aircraft. We can also draw comparisons at the microstructural level between these biological and synthetic materials. Significant differences also exist, however, and the rigid biological materials such as bone and shell have much to teach us. In particular, they are distinctly composite structures. Although they bear loads in much the same way as synthetic composites or ceramics, they have far more complex architectures.

The goal in considering the group of mineralized biological materials as described, for example, in the article by Fink et al. in this issue, and in devising modifications of them, which is the focus of this article and of Mann's, is to learn to devise arrangements of synthetic materials that work more efficiently than the homogeneous substances of simple composites that we use now. In addition to designing better microstructural arrangements we may also learn, again by analogy to the biological materials, how best to process these structures and how to recycle them after use.

Biological structural materials are optimized for their high strength- or stiffness-to-weight ratio. Achieving this in synthetic materials for nonbiological application, for example in cars and airplanes, would be of obvious value. Our own interest here has focused on cuticle and bone as models for our synthetic work. Another property of biomineralized materials, for example biological ceramics, is their increased toughness. In this case we will discuss tooth enamel mimicking.

Silks produced by silkworms (e.g., Bombyx mori) and orb-web weaving spiders (e.g., Nephila clavipes) are essentially pure protein, that is, complexes of amino acid polymers. They are the most common fibers spun by biological systems. There has been a long-standing interest in the use of these and similar fibers in textiles, cables, fiber reinforcement in composites, in addition, for example, to cross hairs in optical instruments, and fishing nets. Both nylon, a homo-polymer of the amino acid glycine, and Kevlar, a polymer of a nonnatural aromatic amino acid, can be considered modified, synthetic versions of silk and are used for some of the applications mentioned above. The potential for genetic manipulation, through recombinant DNA technology, of the natural biosynthetic process for these natural proteins (see the article by Cappello in this issue) has renewed interest in the production of new silklike proteins.

The natural silks are characterized by a β-sheet secondary structure which is stabilized by interchain hydrogen bonds and intersheet hydrophobic interactions (Figure 1). Silks can be considered block copolymers, with crystalline domains consisting of short side chain monomers (the amino acids glycine, alanine, and serine) interspersed in amorphous domains consisting of bulkier side chain amino acids.

This family of fibers is naturally tailored to perform functions such as catching prey (orb web) or serving as a barrier against environmental challenges (cocoon). The domestic silkworm (B. mori) produces only one type of silk, cocoon silk, at only one stage in its lifecyle, during the fifth larval instar just before molt to the pupa. The silk is produced in modified salivary glands and spun from the mouth.

A goal of polymer chemistry is to control the final properties of a processed material by controlling the chemistry and physics of the polymer chain. Since the final formulated materials' properties are a complex function of chemical and physical interactions at both the micro and macro scales, it would be desirable to influence those interactions at the level of the molecular chemistry of the chain. However, in order to affect those macromolecular associations that occur via chemical functionalities separated over long distances, a chemistry is needed that can specify not only variability in chemical composition along the chain but can also specify the positions at which this chemical variability must occur over the entire length of the chain—several hundred nanometers.

Protein-based materials in nature are formulated from protein chains which have evolved chemical compositions that through the specific sequence of their amino acid monomer constituents can result in diverse materials such as ultrahigh strength fibers, silks and collagens, and soft, ultradurable elastomers, elastin. As described in the article by Kaplan in this issue, the fundamental six amino acid repeat of the crystalline fraction of Bombyx mori silk fibroin is composed of essentially three amino acids, glycine (G), alanine (A), and serine (S), in a 3:2:1 ratio. The strength of the fiber is derived from these crystallized peptide blocks. They are positioned to a great extent as tandem repeats of the sequence GAGAGS.

The total world production of water-soluble polymers is estimated to be greater than five million tons per year. Water-soluble polymers are most conveniently described according to their origin in three classes (see Structures 1-6):

∎ Natural polymers, including starch (1) and cellulose (2);

∎ chemically modified natural polymers, including, for example, hydroxyethyl starch (3) and cellulose acetate (4); and

∎ synthetic polymers, the most important of which are polyacrylamide (5) and polyvinyl alcohol (6), (commonly composed of both alcohol and acetate groups as shown). The widespread use of these materials is due to both their availability and the range of useful physical properties found in the various natural and chemically modified natural polymers.

Of the commercial water-soluble polymers, approximately 50–80% are based on natural polysaccharide materials. One of the primary reasons that these materials find such widespread use is the dramatic response of their properties to changes in their functionality and stereochemistry: chemical modification or the combination of polysaccharides with other polymeric materials has yielded materials whose applications range from explosives to food additives. Although efforts directed at controlling the properties of polysaccharides has resulted in a wide variety of useful materials, we felt control of the composition of carbohydrate-based polymers at the molecular level would provide materials with properties superior to those derived from natural and chemically modified polysaccharide materials.

Our approach for the preparation of new carbohydrate-based materials is to use the carbohydrate as a template for the introduction of desired functionality with complete regiochemical and stereochemical control by both chemical and enzymatic methods (Scheme I).