Freestanding microscale 3D polymeric structures with biologically-derived shapes and nanoscale features
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Microscale polymeric structures with intricate three-dimensional (3D) shapes and nanoscale features were synthesized by using silica-based microshells of diatoms (unicellular algae) as transient scaffolds. Diatom microshells were immersed in dilute solutions of polymer precursors in volatile solvents. After extraction and solvent evaporation, the resulting thin films on the microshells were cross-linked to form rigid polymer coatings. Selective silica dissolution then yielded freestanding polymeric structures that retained the microshell shapes and fine features. By utilizing bioscaffolds capable of genetically precise and massively parallel replication, enormous numbers of polymeric micro/nanostructures with identical 3D shapes may be generated for various applications.
Significant global effort is underway to develop shapetailored polymeric microstructures with fine (down to nanoscale) features for medical, cosmetic, aerospace, environmental, agricultural, optical, fluidic, and microelectromechanical applications.1–10 Microfabrication routes based on layer-by-layer two-dimensional (2D) techniques, such as lithographic micromachining methods developed for the silicon microelectronics industry, are not particularly well suited for fabricating threedimensional (3D) polymeric structures with complex shapes (e.g., curved 3D surfaces) and with very fine (down to nanoscale) features.11,12 However, alternative approaches capable of direct 3D micro/nanofabrication must also be scalable to allow for high-throughput production.12 The often-conflicting requirements of precise 3D fabrication on a fine scale and continuous production on a large scale may be achieved through self-assembly processes. Nature is particularly adept at massively parallel nanoscale self-assembly. Numerous examples exist of natural microorganisms that direct the assembly of mineralized (bioclastic) nanoparticle-based structures with complex, but reproducible 3D shapes.13–17 Among the best-known and most striking examples are diatoms. Diatoms are single-celled algae that populate virtually every body of water on earth.16,17 Diatoms possess rigid cell walls
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0342 J. Mater. Res., Vol. 19, No. 9, Sep 2004
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(frustules) composed of amorphous silica nanoparticles.18 Each diatom species assembles a frustule with a unique 3D shape and with well-controlled patterns of fine features (pores, channels, and protuberances).16–18 Such morphological specificity is a strong indication that the frustule assembly process is under genetic control.19 The maximum dimensions of such frustules can range from 102 microns, whereas the fine, regular features distributed on the frustule wall possess characteristic dimensions of 101–102 nanometers.16,17 A wide diversity in frustule shapes and features can be found among the tens of thousands of diatom species that are known to exist, as is illustrated in the secondary ele
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