Difference between revisions of "Biomimetic self-assembly of helical electrical circuits using orthogonal capillary interactions"
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Latest revision as of 03:06, 29 November 2011
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


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.