What is a machine? And why do we need them? The scientific definition of a “machine” is any device that transmits or modifies energy. In common usage, the meaning is restricted to devices that have rigid moving parts that perform, or assist in performing some work (see Concise Oxford Dictionary), although animals, including humans, and plants can also be considered machines; even though they are a product of evolution rather than of design. Machines usually require some energy source (“input”), and always achieve some sort of work (“output”). A machine has a design and it is constructed following some processes. It also uses power and it operates according to information built into it when it is fabricated. Some machines are even used to construct or to replicate other machines.
There is no doubt that machines are an integral part of our daily lives. For centuries we have conquered the “human-scale” world by fabricating large machines (e.g., cars, computers, house appliances, telephones) that have facilitated global communication, transportation, and scientific advances, just to name a few. However, in the past decades we have paid considerable attention to the endless possibilities that the nano and molecular world have to offer. On December 29, 1959, physicist Richard Feynman in his famous lecture “There’s Plenty of Room at the Bottom” considered the possibility of direct manipulation of individual atoms as a more powerful form of synthetic chemistry than used at the time. Feynman suggested that it should be possible, in principle, to do chemical synthesis by mechanical manipulation, and he presented the possibility of building a tiny, swallowable surgical robot by developing a set of one-quarter-scale manipulator hands slaved to the operator’s hands to build one-quarter scale machine tools analogous to those found in any machine shop. This set of small tools would then be used by the small hands to build and operate ten sets of one-sixteenth-scale hands and tools, and so forth, culminating in perhaps a billion tiny factories to achieve massively parallel operations. As the sizes got smaller, they would have to redesign some tools because the relative strength of various forces would change. Gravity would become less important, while Brownian motion, surface tension, Van der Waals interactions, etc., would become more important. Feynman’s lecture at that moment in history was partly responsible for the beginning of a collective dialogue that explored the potential of manipulating the molecular world. Since then, scientists and engineers have sought nanotechnology as an alternative medium to solve many current problems in medicine (i.e., blood diseases, cancer, drug delivery), air and water pollution, sensors to detect molecules or particles, and to make devices smaller to conserve space, energy, materials, and money. But perhaps the most important challenge that nanotechnology faces is the creation of useful work by an object in a world dominated by randomness and uncertainty. Directed motion or propulsion is a difficult challenge that must be overcome in order to make many of these devices “come to life”.