Since the development of ultra-small electrical circuitry, molecular electronics and artificial neural networks, the search to find ways of providing electrical connections between such nanosize three-dimensional structures has become a priority. The use of conventional mechanical or photolithographic fabrication techniques is inadequate for direct linking of such structures. Connecting these “molecular wires”, by using various chemical techniques to direct the linkage such as conducting organic polymer strands, has been the most successful so far, but a general technique for connecting large numbers of “molecular wires”, independently of each other, has not yet been found.

Now, however, researchers at the University of California, San Diego, have demonstrated the ability to interconnect many individual platinum nodes in three dimensions by an electrochemical polymerisation technique (C. L. Curtis, J. E. Ritchie and M. J. Sailor, “Fabrication of Conducting Polymer Interconnects”, Science, 1993, 262, 2014–2016).

Their technique is based upon the ability of conductive polymers, such as poly(3-methylthiophene), to be electrochemically switched between electronically conducting and non-conducting states. Pairs of platinum wires in an arrav. immersed in a solvent of monomer/electrolyte, were independendy electrically linked by polymer dendrites on passing an alternating current between them. When an actively polymerising strand electrically contacts a non-conductive strand, the non-conductive strand switches into its conductive state in the region close to the connection. Further polymerisation then occurs in the contacted region to reinforce the connection. The process requires no external mechanical manipulation or lithographic patterning. By using a separate insulating step, sets of electrically independent nodes can be prepared. The insulation of the electrically active links was performed by subsequent electropolymerisation of 4-vinylpyridine or 2-methylthiophene or, more consistently, by dip-coating the connections into a tetrahydro-furan solution of polystyrene.

The polymer connections display several properties relevant to neural networks, fuzzy logic or other nanofabricated model systems. In theory, a large number of nodes could be connected by this method, with the strength of each connection being determined by the conductivity between the node and the network. Therefore, it is expected that with further research, this non-mechanical technique could lead to the construction of complex three-dimensional nanosize interconnected arrays.