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Self Assembly
In any living cell, nanoscale cellular machines spontaneously assemble themselves and drive the processes of life. Improvements in fabrication techniques are pushing the dimensions of electronic components to regimes beyond the reach of direct manipulation by human or machine. The functional self-assembly sub-group at the Whitesides Lab seeks to design self-assembling systems at a variety of scales, and to use these systems to form working devices that would be difficult (or practically impossible) to build with any other technique.
Self-assembly involves spontaneous organization of interacting components into an ordered aggregate or aggregates without direct human or mechanical interference. In the natural world, self-assembly occurs over a wide range of size-scales to create structures that display new properties not present in the original components. Self-assembly occurs both in systems at equilibrium - such as the crystallization of proteins or colloids - and in systems far from equilibrium - such as cellular replication of DNA. This work seeks to exploit the power of self-assembly to order small components into functional, three-dimensional structures in a parallel process.
Driving Forces
We have demonstrated the self-assembly of functional electronic devices with components as small as 100 microns on a side. Figure 1 describes a self-assembled GaAs display. Figure 2 describes 1560 silicon blocks self-assembled onto a flexible substrtate. To provide the interactions between components, our past work relied on the capillary interaction between menisci, drops of hydrophobic liquid, or pads of molten metal; more recent work has used electrostatic or magnetic interactions.
Perfecting Self-Assembly
Two main problems complicate the design of self-assembling systems: maximization of yield and fabrication of components. To improve the yield of self-assembly, we have demonstrated templating of the self-assembly process. Templating can include constraining the aggregation in a container of a particular shape, and tethering the components together on a flexible ribbon or polymer sheet. The strategy of confining components to a flat sheet is particularly interesting to us, because we can use photolithography and other established methods to fabrication components in two-dimensions, and then allow the sheet to fold spontaneously by self-assembly into a functional three-dimensional shape. Once we have techniques to pattern components for function and self-assembly in parallel, it will be easy to decrease the size of the components even further. To date, most functional assemblies have been composed of either very simple components (e.g., silicon blocks) or relatively large ones (mm-scale). A "synthetic" approach to the fabrication of individual components will lead to greater understanding of the self-assembly process, and to smaller, better devices.
Projects
Self-healing materials: Wires and bonds made from low-melting conductive alloys can spontaneously heal upon heating. It is possible to fix devices based on self-healing materials from the outside, with no disassembly required. Figure 3 describes a self-healing "spine"; Figure 4 describes a composite of a flexible polymer and molten metal, tat spontaneously folds into a helix. We have also used these techniques to describe a functional 3D sphere folded from a sheet by magnetic forces (Figure 5).
Folding tapes and sheets: Capillary forces between patterns of molten metal (or other liquid with high surface free energy) lead to ordered folded structures. As with proteins, the primary structure of the precursor - that is, the sequence and spacing of "monomers" with various sizes, hydrophobicity, or other forms of patterning - determines the structure of the final product. Unlike the synthesis of proteins, we are free to begin with either linear chains or flat sheets of unfolded components.
Passive electronic components: Self-assembly offers a potential method for reducing the footprint of passive components (capacitors, inductors, and resistors) on microchips. In experiments we have shown that the same components can assemble into different devices when these components are placed in different containers.
Plasticity and redundancy: In these experiments, we show that designing components that self-assemble to different products under different macroscopic conditions, or components with redundant elements, will lead to reconfigurable devices. Specifically, Shape-complementarity can improve the yield of self-assembly (Figure 6)
Three-dimensional recognition: Most photolithographic methods are optimized for the fabrication of two-dimensional patterns. Biological recognition depends on both chemical interactions and shape recognition. This work seeks to improve yield of self-assembly through improved design of high surface energy recognition patterns, and improved fabrication of three-dimensional components for shape-complementarity (Figure 7, Figure 8).
Select Publications
Boncheva, M. et al. "Magnetic Self-Assembly of Three-Dimensional Surfaces from Planar Sheets." PNAS 102, 3924-3929 (2005).
Boncheva, M., Bruzewicz, D. A. & Whitesides, G. M. "Millimeter-Scale Self-Assembly and Its Applications." Pure Appl. Chem. 75, 621-630 (2003).
Boncheva, M., Bruzewicz, D. A. & Whitesides, G. M. "Formation of Chiral, Three-Dimensional Aggregates by Self-Assembly of Helical Components." Langmuir 19, 6066-6071 (2003).
Boncheva, M. et al. "Plasticity in Self-Assembly: Templating Generates Functionally Different Circuits from a Single Precursor." Angew. Chem. Int. Ed. 42, 2644-2647 (2003).
Boncheva, M. & Whitesides, G. M. "Self-healing systems having a design stimulated by the vertebrate spine." Angew. Chem. Int. Ed. 42, 2644-2751 (2003).
Gracias, D. H. et al. "Forming Electrical Networks in Three Dimensions by Self-Assembly." Science 289, 1170-1172 (2000).
Jacobs, H. O. et al. "Fabrication of a Functional Cylindrical Display using Solder-Based Self-Assembly." Science 296, 323-325 (2002).
Wolfe, D. B. et al. "Mesoscale Self-Assembly: Capillary Interactions When Positive and Negative Menisci Have Similar Amplitudes." Langmuir 19, 2206-2214 (2003).
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