Design and self-assembly of open, regular, 3D mesostructures

Science

Volumn 284, Issue 5416, 1999, Pages 948-951

  Breen, Tricia L ^a   Tien, Joe ^a   Oliver, Scott R J ^a   Hadzic, Tanja ^a   Whitesides, George M ^a  

^a HARVARD UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALLOY;

ARTICLE; CHEMICAL STRUCTURE; CONFORMATION; CRYSTAL STRUCTURE; FILM; MOLECULAR DYNAMICS; PRIORITY JOURNAL; STRUCTURE ANALYSIS;

EID: 0033532287     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.284.5416.948     Document Type: Article

References (18)

1. N. Bowden, A. Terfort, J. Carbeck, C. M. Whitesides, Science 276, 233 (1997).
2. A. Terfort, N. Bowden, G. M. Whitesides, Nature 336, 162 (1997).
3. A. Terfort and G. M. Whitesides, Adv. Mater. 10, 470 (1998).
4. J. Tien, T. L. Breen, G. M. Whitesides, J. Am. Chem. Soc. 120, 12670 (1998).
5. A. W. Simpson and P. H. Hodkinson, Nature 237, 320 (1972).
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9. P. W. Green, R. R. A. Syms, E. M. Yeatman, J. Microelectromech. Syst. 4, 170 (1995).
10. We estimated the interfacial free energy of the alloy/ aqueous KBr interface using data from F. H. Howie and E. D. Hondros, J. Mater. Science 17, 1434 (1982); M. A. Carroll and M. E. Warwick, Mater. Sci. Technol. 3, 1040 (1987); and Handbook of Chemistry and Physics, D. R. Lide, Ed. (CRC Press, Boston, ed. 71, 1990), pp. 4-137 to 4-143. A liquid with a lower surface tension, such as the hydrophobic lubricant used in (2) and (4), was unable to generate capillary forces that were strong enough to hold objects together.
11. We estimated the interfacial free energy of the alloy/ aqueous KBr interface using data from F. H. Howie and E. D. Hondros, J. Mater. Science 17, 1434 (1982); M. A. Carroll and M. E. Warwick, Mater. Sci. Technol. 3, 1040 (1987); and Handbook of Chemistry and Physics, D. R. Lide, Ed. (CRC Press, Boston, ed. 71, 1990), pp. 4-137 to 4-143. A liquid with a lower surface tension, such as the hydrophobic lubricant used in (2) and (4), was unable to generate capillary forces that were strong enough to hold objects together.
12. We estimated the interfacial free energy of the alloy/ aqueous KBr interface using data from F. H. Howie and E. D. Hondros, J. Mater. Science 17, 1434 (1982); M. A. Carroll and M. E. Warwick, Mater. Sci. Technol. 3, 1040 (1987); and Handbook of Chemistry and Physics, D. R. Lide, Ed. (CRC Press, Boston, ed. 71, 1990), pp. 4-137 to 4-143. A liquid with a lower surface tension, such as the hydrophobic lubricant used in (2) and (4), was unable to generate capillary forces that were strong enough to hold objects together.
13. The designs selected for the components were used to fabricate machined aluminum masters; these masters were used to fabricate an elastomeric mold. We placed the masters in a petri dish, covered them with a liquid polydimethylsiloxane prepolymer (Sylgard 184, DuPont), and cured the prepolymer with heat (60°C, 30 min). Subsequent removal of the aluminum masters left an elastomeric mold consisting of wells in the shape of the aluminum masters. We filled the wells with a liquid PU prepolymer (NOA 73, Norland), cured the polymer by exposure to ultraviolet light (45 min), and removed the solid PU pieces. We patterned selected faces of these objects using adhesive copper foil (Scotch Brand Electrical Tape, 3M Corp., St. Paul, MN). This tape was cut to size and applied to the selected planes (or portions of these planes) of the polyhedra. This process was labor-intensive and limited the number of objects we were willing to examine in any experiment; it also limited these objects to sizes greater than 5 mm. The pattern of copper foil was coated with molten alloy (Bismuth Alloy 117, melting point: 47°C, Small Parts, Miami Lakes, FL) by dipping the pieces into a beaker containing molten alloy (under an aqueous acetic acid solution at pH 4 to dissolve oxidized alloy). Cooling left the faces of the copper foil coated with a film (-0.5 mm thick) of solid alloy. Self-assembly was allowed to take place in a 500-ml Morton flask filled with an aqueous solution of KBr. Acetic acid (3 ml) was added to dissolve oxide from the surface of the alloy. The flask and its contents were rotated horizontally with a rotary evaporator motor at 5 to 10 rpm and heated to 60°C in an oil bath. A typical assembly was complete after ∼1 hour of agitation. We reduced the intensity of agitation about halfway through the experiment to obtain complete assemblies of the component pieces by increasing the concentration and thus the density (ranging from 1.1 to 1.3 g/ml) of the aqueous KBr solution.
14. Fragility resulted from delamination of tape from polymer, not fracture of alloy connections.
15. We assembled each array twice (with the exception of the dodecahedra, which were assembled three times). With the exception of the dodecahedra, the arrays incorporated all the starting pieces; for the dodecahedra, an excess of pieces was required. If we broke apart an array during assembly with a brief increase in rotation speed, the resulting fragments reassembled into a crystalline, monolithic array in ∼1 hour.
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17. G. M. Whitesides, Sci. Am. 273, 146 (September 1995).
18. We thank S. Brittain for photography and K. Paul for discussions. Supported by the DARPA and the NSF. T.L.B. and S.R.J.O. thank the NSERC of Canada for postdoctoral fellowships; J.T. acknowledges the NSF for a predoctoral fellowship. T.H. was supported by the MRSEC program of the NSF under award number DMR-9400396.