Can intelligent components organize themselves into final products? asks S.Ananthanarayanan.

The ultimate examples of things that build themselves from basic components, of course, are living things. While living things grow from a single cell to their final form, doing something like this with materials in general has been the dream of material scientists, architects and engineers

Ran Niua, Chrisy Xiyu Du, Edward Esposito, Jakin Ng, Michael P. Brenner, Paul L. McEuen and Itai Cohen, from Cornell and Harvard Universities, report in the Proceedings of the National Academy of Sciences (PNAS), what looks like a positive step towards realizing this dream. The paper in the journal describes a set of centimeter-size discs, embedded with magnetic domains of just four kinds, which are able to fall into programmed patterns and shapes, just by being put together and shaken about.

The ability to follow rules in interactions with one another, so that they connect or repel in specific orientations, is a property of matter of very small dimensions. Atoms and molecules are a ready example. They form specific compounds simply by being in contact at the right temperature and pressure, and the chemical industry would not be possible if compounds had to be individually assembled. Forming specific links is also true of very small objects, which interact based on their shape alone or arrange themselves so that the arrangement is energetically the most efficient.

A much more powerful device of self-organization is what is followed by living things, which programme the interaction of their cells through the DNA. The DNA is a millions of units long chain molecule, whose segments contain the code for the production of specific proteins. This programming of cells guides different cells to interact and grow into the different organs and according to the shape and structure of the living thing.

The coding for the different proteins that cells produce comes about through a set of four kinds of ‘side chains’ that protrude, in succession, from the backbone of the DNA molecule. Triads, or sets of three side chains, are used to code for twenty components of proteins, called amino acids, and with different sequences of amino acids, different proteins can be constructed by the cell

DNA technology has been used in a ‘bottom-up fabrication approach’ in the laboratory, by exploiting the intrinsic properties of atoms and molecules and getting them to self-organise. Experiments using segments of DNA, or related molecules, have succeeded in creating some simple, self-constructing nano-structures, square, discs or stars, some 100 nanometers across. The field has grown to be known as ‘DNA Origami’ and complex shapes, like letters, a helicoid and a teddy bear, have been created. And larger and more complex, functional structures may soon be possible.

These successes, however, the PNAS paper says, are limited to very small sizes and involve complex and time-consuming processing. The structures themselves are not hardy, as it is with the help of thermal motion of atomic or molecular components that they are built, and are held together by weak, inter-molecular forces. The work the authors report is thus to explore the possibility of stronger and longer ranged boding interactions. One such, they say, is the force between components that consist of magnetic elements and they propose a method of self-assembly of a set of components that interact based on the magnetic information that is embedded within them.

The components are a set of discs, just 0.9 cm in diameter, which contain a pattern, an array, of a single, a pair or a triad of magnets, and capped on either side by spacers, as shown in the picture. The tendency of a panel to bind with another depends on the magnet pattern in the discs. Because the ratio of how strongly the discs bind depends only on the pattern, the behavior of such a set of discs remains the same even if their size is changed, all the way down to 10 nanometers (below that we cannot have the magnetic regions on the discs), the paper says.

The key behavior of the discs is that discs with similar patterns bind, or repel, most readily, and more readily when they are placed on a surface that is agitated, to help overcome friction with the surface. The attraction (or repulsion) is the strongest when the agitation just cancels gravity. And again, if the agitation is a lot stronger, it can cause bonded panels to separate. The agitation thus acts just like thermal agitation in a solution or a gas, the paper says, where gentle motion would promote reaction, but high temperature may result in dissociation.

The level and kind of interaction between panels depends both on the way the panel is oriented as well as what patterns of magnetism the panels have. This results in a scale of weaker to stronger interactions. The strongest boding is found between panels with the same pattern, except with the green panels, which bind more strongly with the black panels. The strength of binding varies according to the alignment of the patterns and again, the relative strengths changes when the degree of agitation is varied.

With the binding strength following a pattern, it becomes possible to build chains of panels, with successive panels in a given order. Variations were tried by creating panel pairs which looked different from different sides and again, at different levels of agitation, to produce differing patterns, including branching chains of panels.

The next thing tried was to linking panels by gluing them to an elastic backbone. Now, the order in which the panels were attached to the elastic specified the panels that would attach to the string, as in the case of the side chains along a single strand of DNA. Variations in the spacing of panels along the elastic could be made to promote bending of the chain and then the formation of shapes, like an S bend. And there could even be a spiral form, with control over the level of twist.

Yet another possibility was with elastic backbone in a two dimensional shape. Now, it became possible to vary the panels chosen, their orientation and spacing as well as the shape of the backbone. The second picture, which is taken from the paper, shows the shapes, a folded book, a tetrahedron, a cube and a bowl, generated by the panels that attached to the programmed backbone.

“These examples illustrate the potential for building hierarchically complex structures and the need for further development of systematic approaches for implementing these strategies, the paper says. Nanoscale magnetic elements and nanometer-thick elastic elements are now feasible and the platform demonstrated, at centimeter scale, could be repeated at the micro or nano scale. 3-D printing technology could be used to fabricate the magnetic elements and it is conceivable that magnetic element patterns could be written out by a computer. The technology would lead to micro-scale structures or even self-assembling of machines that can be controlled by external magnetic fields, the paper says.

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