1. Introduction

Ages defined by their prevalent materials seem now to be passing with disconcerting speed. The Stone Age can be stretched, depending on one's terms of reference, over 20 millennia; the Bronze Age lasted for over 20 centuries. But one could argue that the Plastic Age began and ended during the twentieth century, and the Silicon Age could be over within the lifetime of many of those who saw its dawning. If the current time represents the Age of Materials, as some have suggested, that is surely because the appearance and demise of new materials is happening at a phenomenal rate. All of this leaves us decidedly myopic when trying to gaze into the crystal ball of the future.

Discussions of future technologies-indeed, of the future in general-must either be prosaic extrapolations of current capabilities, or science fiction. This article will err on the side of the former, while acknowledging that for each ensuing decade the probability that it will overlook something of great importance is at least doubled.

This can be illustrated with reference to the case of carbon nanotubes (described in more detail in Section 9). Before the 1990s, no one had even postulated that these super-strong filaments of graphite-like carbon exist. That in itself is worth remarking on, since it is commonplace now for theorists to speculate about materials that might exist, to calculate their putative properties in inordinate detail. But even though the soccer-ball-shaped carbon molecules called buckminsterfullerene (now one of a family of hollow "fullerenes") were predicted in the 1970s and discovered in 1985, no one extrapolated to tubular versions. Yet in the 8 years that have now passed since the discovery of carbon nanotubes, they have furnished an entirely new arena of research, have eclipsed the fullerenes as the most promising and interesting form of "nanostructured" carbon (that is, a form of pure carbon sculpted at the nm scale), and are now discussed in the context of molecular wires for microelectronics, single-molecule transistors, ultra-strong carbon fibers, probosci for the most powerful microscopes, high-capacity storage cylinders for hydrogen fuel, etc.) If written 10 years ago, (1990), this article would have missed them totally.

Specific materials systems aside, one can be little more confident in making some prognostication of the possible future trends in materials science and engineering. First-and this is not a semantic detail-the discipline will need a new name. Already there is concern that this stolid, post-Second World War label does scant justice to a field that embraces (amongst others) cell biology, computer science, geophysics, organic chemistry and mechanical engineering. It is truly now a discipline that surveys the behavior of matter in all its guises, short of the extreme energy scales that remain the domain of particle physics. Two of the prime reasons for this expansion of breadth are associated with scale and function.

The materials devised and created today have important structural features on all length scales between the atomic and the macroscopic (the "everyday" scale discernible to the human eye). Sometimes just a single scale tends to dominate: for the nm-scale crystals that act as "quantum dots" for optoelectronic information-processing devices (which employ both light and electricity as their input and output), it is the size of the crystals that sets the wavelength of light absorbed or emitted. In other cases, the material might acquire important properties from structures over a range of scales. This is typical, for example, of natural materials such as bone, wood and shell-all of them sophisticated composite materials whose superior properties make them attractive models for synthetic products.

One of the key concepts under the umbrella of scale is hierarchical structure, which implies that the structure is defined over a multiplicity of length scales. The architectural prototype is the Eiffel Tower, which gains a high strength-to-density ratio by a repeated application of the triangulated crossbeam principle (which an engineer knows as a Warren's truss) over four distinct length scales. (Here, as so often, nature points the way: Warren's truss is seen in the metacarpal bones of a vulture's wing.) But a hierarchical structure need not simply repeat the same motif at increasing magnification: in bone, for example, each level of structural organization bears no resemblance to the one before.

Traditional methods of materials synthesis will not generally allow for simultaneous control of structure over several length scales. At scales below ~1 mm, this sort of control requires various kinds of chemical expertise. Making nm-scale particles of some inorganic material might involve finesse in colloid science. Organic chemists might then be able to advise on how to attach molecular linkers to the surface of the particles so as to join them together. In any event, this sort of microscopic manipulation is a far cry from the hot-pressing techniques with which a ceramics technologist might be familiar.

The second consideration is function, and this forces us to re-evaluate the whole meaning of the term "material." Colloquially, it conjures up some substance that performs a structural role: a steel girder, a cement bridge, a cotton sheet. But increasingly, materials are acting like machines: they do things. Materials that emit light, that swell and contract when prompted, that stimulate bone growth or release drugs-all are active substances, qualitatively unlike the passive materials that have traditionally formed the core of the discipline. It then becomes a matter of opinion where "material" ends and "device" begins, or what distinguishes a materials researcher from a computer engineer or a biomedical scientist.

Many of the advanced materials in development today betray a desire to be invisible-not to muscle in on daily life like a garish plastic chair or a chrome-plated staircase, but to carry out their role as inobtrusively as possible. Vibration sensors and switches made from "smart" materials that respond to stimuli in their environment will make aircraft cabins quieter without anyone knowing they are there. Fractured bones will be held in place not by metal plates that will forever trigger airport alarms, but by sutures that slowly dissolve as the bone regrows. Cumbersome and eye-fatiguing televisual screens might give way to electronic ink: microstructured particles that redistribute themselves on a flat white field, looking almost indistinguishable from ink on paper. The world of advanced materials has its Herculean aspects, such as ceramics that withstand awesome temperatures; but it is more often a Lilliputian and modest neighborhood where the job gets done more cleanly and quietly.

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