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

[[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


By Alex Epstein
----

===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==
[[Image:biomimetic1.png|thumb|left|300px|'''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.]]

[[Image:biomimetic2.png|thumb|right|300px|'''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.