Biomimetic self-assembly of helical electrical circuits using orthogonal capillary interactions

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Another review is: Biomimetic Self-Assembly of Helical Electrical Circuits Using Orthogonal Capillary Interactions

Original entry: Alexander Epstein, APPHY 226, Spring 2009

Biomimetic self-assembly of helical electrical circuits using orthogonal capillary interactions

Authors: David H. Gracias, Mila Boncheva, Osahon Omoregie, and George M. Whitesides

App. Phys. Lett, Vol. 80, no. 15, 2802-2804

Soft matter keywords

Self-assembly, biomimetics, capillary forces, polyurethane, multifunctional assemblies

Abstract from the original paper

This letter describes the biomimetic self-assembly of mm-sized polyhedra into helical aggregates. The system used two orthogonal, capillary interactions that acted in parallel. The design of the self-assembly process, and of the resulting structures, was modeled on the formation and structure of tobacco mosaic virus. The self-assembled, helical aggregates carried one, two, or four isolated, electrical circuits.

Soft matters

Fig. 1 Design of the basic unit of the helical assemblies. (a) Schematic drawing of a wedge-shaped polyhedron. (b) A drawing of an aggregate containing five wedges. The coplanar angles a and b have values of 66° and 30°, respectively. The arrows indicate the two possible binding sites for an incoming wedge. (c) A drawing of an aggregate containing six wedges. Helical assembly carrying one electrical circuit: (d) The pattern of copper used single-wire pattern. (e) The patterned wedge. (f) A single wedge after solder deposition, prior to assembly. (g) A photograph of a helix formed from 48 pieces. (h) Schematic diagram of the helical electrical circuit.
Fig. 2 Helical assemblies carrying several electrical circuits. (a) The pattern of copper used dual-wire pattern. (b) The patterned wedge. (c) A single wedge after solder deposition, prior to self-assembly. (d) A photograph of a helix formed from 20 pieces, carrying one pair of electrically isolated wires. (e) A photograph of two interdigitated helices, one formed from seven pieces, the other one from ten pieces. Each helix carries a pair of electrically isolated wires. (f), (g) Schematic diagram of the electrical circuits formed in (d) and (e), respectively.

The authors design and test a millimeter-scale system of 3-D self-assembling units that serve as analogues of the protein molecules in tobacco mosaic virus (TMV). The basic units of the self-assembling system they describe are wedge-shaped polyhedra made of polyurethane. They replicate the geometry of the units via PDMS mold from a machined metal master.

The orthogonal faces of the polyhedra carry patterns of solder and hydrophobic lubricant (perfluorodecalin, PFD); these patterns provide the information necessary for the self-assembly of the system. The geometry of the wedge (Fig. 1) was chosen to assemble into helices with defined handedness. After the component wedges are patterned, they are dipped in molten solder and dipped in PFD. The solder covers the copper pattern selectively, while PFD covers the hydrophobic faces of the wedges. Dual electrically isolated wires are also attached to each wedge and terminate at contact pads on each end.

Now the wedges are placed in an isodense solution and tumbled to let the capillary forces go to work. Pieces collide with each other and either drops of solder, or drops of PFD, coalesce. The two types of capillary interactions seem not to interfere.

Upon self-assembly, the polyhedra organize into a helix via two capillary forces: a strong capillary interaction between drops of molten solder--the free energy of the water–solder interface is 400 ergs/cm^2--and a relatively weak capillary interaction between drops of PFD--the free energy of the PFD–water interface is 50 ergs/cm^2. The system is designed in such a way that these two forces are orthogonal to each other, like the forces that generate the TMV helix. The resulting helices also link the electrical wires on the wedges via the contact pads, and the authors demonstrate the continuous electrical circuit (Fig. 2) formed by connecting the helix to an LED and battery.

Systems of orthogonal interactions are especially relevant for fabricating 3D, self-assembled aggregates that can act as components in functional devices. In order to perform a specific function, such assemblies must comprise complex, often asymmetrical, networks of connections providing structural connectivity between the elements, together with connections carrying the functional, e.g., electrical or optical, signals. The use of orthogonal forces in 3D makes possible the fabrication of multifunctional structures of higher complexity.