3.6. Other nanomachines
As we have said before, for reasons of space (and also for not going into too much detail) we must limit ourselves to describing a few examples of nanomachines, chosen for their innovative value or because they are particularly representative. We must also remember, however, that since the 1990s many molecular and supramolecular systems capable of behaving like molecular machines or motors have been designed, constructed and studied. Among them it is worth mentioning those that use biomolecules – in particular DNA and RNA – as molecular components.
The development of this category of artificial nanomachines is closely linked to a rapidly expanding research sector: that of DNA nanotechnology. In the early 1980s, scientists (notably Ned Seeman at the University of New York) realized that DNA molecules could be used to build nanostructures in a controlled manner. The chemical properties of DNA molecules can be accurately programmed by choosing the sequence of bases (adenine, guanine, cytosine and thymine). DNA can exist as single molecules, or pair off to form the famous double helix and other different structures. There is a large number of enzymes capable of cutting and sewing DNA molecules with great precision. These and other structural and functional features of DNA have allowed the creation of very complex nanostructures, such as the now famous origami.
Scientists interested in artificial DNA nanostructures soon realized that they could be modified in response to external stimuli, that is, they could behave as molecular machines. In most cases also the stimuli employed are represented by DNA molecules, although ions, small molecules, proteins (enzymes), light or electrical stimuli can be used. The operation of these nanomachines is too complex to be described quickly here. Suffice it to say that, with DNA, nanoscale switches, logic gates, memories, tweezers, linear and rotary motors, up to real programmable robots capable of moving and manipulating nanometric objects by executing instructions given from the outside, have been realized (Thubagere 2017).
Despite the scientific value of these results, the considerable complexity and very high cost of these systems has, for the time being, relegated them to laboratory curiosity. However, various experiments are underway for practical applications, mainly related to the development of systems for the targeted release of drugs.