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Thermally Controlled Fluidic Self-Assembly Ravi Sharma* Research and DeVelopment Laboratories, Eastman Kodak Company, Rochester, New York 14650 ReceiVed December 5, 2006. In Final Form: March 10, 2007 A novel approach for the fluidic self-assembly (FSA) of microparts in a multibatch process utilizing the thermal behavior of the carrier fluid as a means for selecting binding sites is presented. In the system studied, fluidic assembly takes place due to a capillary bridge between hexadecane deposited on a hydrophobic patch on a substrate and a hydrophobic surface on a micropart suspended in a carrier fluid. When desired, FSA of microparts is prevented by causing the surrounding carrier fluid to form a gel when heated, offering a method for directing self-assembly to sites that are not heated. It is shown that a suitable carrier fluid is 15 wt % Pluronic F127, which gels at about 40 °C when tested in the geometry used to demonstrate the concept. Experimental results demonstrating FSA and thermally controlled fluidic assembly (TCFSA) of plastic microparts dispersed in Pluronic F127 solution are presented. Potentially, TCFSA offers a general method for directed assembly as it relies on restricting the transport of microparts to a site rather than interfering with the fundamental attractive forces responsible for self-assembly.
Introduction In the past two decades, self-assembly has been used to describe the spontaneous attachment of molecules and millimeter- to micrometer-sized objects to a surface or to themselves.1-16 For example, self-assembly is used to describe events in layer-bylayer assembly,3,4 microcontact printing,5,6 and fluidic selfassembly (FSA) of microcomponents.12-38 Recently, there has been great interest in FSA because of its potential to reduce the * Corresponding author. E-mail:
[email protected]. (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (3) Decher, G. Science 1997, 277, 1232. (4) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150. (5) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (6) Xia, Y. N.; Whitesides, G. M. Agnew. Chem., Int. Ed. 1998, 37, 550. (7) Terfort, A.; Bowden, N. N.; Whitesides, G. M. Nature 1997, 386, 162. (8) Tien, J.; Breen, T. L.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 12670. (9) Breen, T. L.; Tien, J.; Oliver, S. R. J.; Hadzic, T.; Whitesides, G. M. Science 1999, 284, 948. (10) Gracias, D. H.; Tien, J.; Breen, T. L.; Hsu, C.; Whitesides, G. M. Science 2000, 289, 1170. (11) Syms, R. R. A.; Yeatman, E. M.; Bright, V. M.; Whitesides, G. M. J. Microelectromech. Syst. 2003, 12 (4), 387. (12) Whitesides, G. M.; Gryzbowski, B. Science 2002, 295, 2418. (13) Yeh, H.-J. J.; Smith, J. S. Sens. Mater. 1994, 6, 319. (14) Yeh, H.-J. J.; Smith, J. S. Fluidic self-assembly for the integration of GaAs light-emitting diodes on Si substrates. IEEE Photonics Technology Letters, June 1994; Vol. 6, pp 706-708. (15) Verma, A. K.; Hadley, M. A; Yeh, H.-J. J.; Smith, J. S. Fluidic selfassembly of silicon microstructures. Proceedings of the 45th Electronic Components and Technology Conference (Cat. No. 95CH3582-0), Las Vegas, NV, May 21-24, 1995; IEEE: New York, 1995; pp 1263-1268. (16) Yeh, H.-J. J.; Smith, J. S. IEEE Photonics Technol. Lett. 1994, 6, 705. (17) Srinivasan, U.; Howe, R. T.; Liepmann, D. J. Microelectromech. Syst. 2001, 10, 17. (18) Srinivasan, U.; Helmbrecht, M. A.; Rembe, C.; Muller, R. S.; Howe, R. T. Opt. Photonics News 2000, November, 21-24. (19) (a) Smith, J. S.; Yeh, H.-J. J. Method for fabricating self-assembling microstructures. U.S. Patent 5,545,291, August 13, 1996. (b) Gengel, G. W. Integrated circuit packages assembled utilizing fluidic self-assembly. U.S. Patent 6,417,025, July 9, 2002. (c) Smith, J. S.; Hadley, M. A.; Craig, G. S. W.; Nealy, P. F. Methods and apparatuses for improved flow in performing fluidic selfassembly. U.S. Patent 6,527,964, March 4, 2003. (d) www.alientechnology.com. (20) Srinivasan, U.; Helmbrecht, M. A.; Rembe, C.; Muller, R. S.; Howe, R. T. IEEE J. Sel. Top. Quantum Electron. 2002, 8, 4. (21) Esener, S. C.; Hartmann, D. SPIE Crit. ReV. Opt. Sci. Technol. 1998, 40, 13. (22) McNally, H. A.; Pingle, M.; Lee, S. W.; Guo, D. Bergstrom, D. E.; Bashir, R. Appl. Surf. Sci. 2003, 214, 109. (23) Bashir, R. Mater. Today 2001, November/December, 30-39.
Figure 1. Schematic of self-assembly facilitated by capillary forces in water. An attractive force (a) between hydrophobic coating (gray) on microparts and hydrophobic liquid (black) present on binding site facilitates binding (b) and alignment of micropart on binding site (c). (Adapted from refs 17 and 28) (Copyright 2001 IEEE).
cost of assembling components in fabricating, for example, electronic devices and sensors. One of the major technical barriers to low cost fabrication is the ability to cheaply and effectively integrate complex microsystems that may find use in telecommunications, display, chemical analysis, and biomedical instrumentation. The current trend toward flexible substrates adds (24) Murakami, Y.; Idegami, K.; Nagai, H; Kikuchi, T.; Yarnamura, A.; Yokohama, K.; Tamiya, E. Mater. Sci. Eng. C 2000, 12, 67. (25) Lee, S. W.; McNally, H. A.; Guo, D.; Pingle, M.; Bergstrom, D. E.; Bashir, R. Langmuir 2002, 18, 3383. (26) (a) Jackson, T. N.; Mayer, T. Electric field assisted assembly process. U.S. Patent 6,536,106, March 25, 2003. (b) Mayer, T.; Jackson, T. N.; Nordquist, C. D. Electro-fluidic assembly process for integration of electronic devices onto a substrate. U.S. Patent 6687987, Feb 10, 2004. (27) Sliwa, J. W., Jr. Method of coplanar integration of semiconductor IC devices. U.S. Patent 5,075,253, December 24, 1991. (28) Bo¨hringer, K. F.; Srinivasan, U.; Howe, R. T. Modelling of Fluidic Forces and Binding Sites for Fluidic Self-Assembly. Proceedings of the 14th International Conference on Micro Electromechanical Stems (MEMS), Interlaken, Switzerland, January 21-25, 2001; pp 369. (29) Xiong, X.; Hanein, Y.; Fang, J.; Wang, Y.; Wang, W.; Schwartz, D. T.; Bo¨hringer, K. F. J. Microelectromech. Syst. 2003, 12, 117. (30) Harsh, K. F.; Bright, V. M.; Lee, Y. C. Sens. Actuators, A 1999, 77, 237. (31) Yousaf, M. N.; Houseman, B. T.; Mrksich, M. PNAS 2001, 98, 52992. (32) Ozkan, M.; Esener, S.; Bhatia, S. Electric-field-assisted fluidic assembly of inorganic and organic materials, molecules and like small things including living cells. U.S. Patent 6,605,453, August 12, 2003. (33) Jacobs, H. O.; Tao, A. R.; Schwartz, A.; Gracias, D. H.; Whitesides, G. M. Science 2002, 296, 323. (34) Zheng, W.; Buhlman, P.; Jacobs, H. O. PNAS 2004, 101, 12814. (35) Zheng, W.; Jacobs, H. O. AdV. Mater. 2006, 18, 1387. (36) Zheng, W.; Chung, J.-H.; Jacobs, H. O. J. MEMS 2006, 15, 864-870. (37) Chung, J-H.; Zheng, W.; Jacobs, H. O. Programmable Reconfigurable Self-Assembly: Approaching The Parallel Heterogeneous Integration on Flexible Substrates; Proceedings of 18th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2005), Miami Beach, Florida, January 30 - February 3, 2005; pp 572-575.
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Figure 2. (a) Microparts-1 in carrier fluid flowing across substrate. Selected sites are heated, causing a protective gel to form. Assembly of microparts-1 only takes place on unprotected sites. (b) Microparts-2 in carrier fluid flowing over substrate after microparts-1 have assembled. Successive assembly of microparts-1 and -2 is complete.
further challenges to heterogeneous integration (the integration of microcomponents made by a variety of fabrication methods that are mutually incompatible) onto a single plastic sheet. Heterogeneous integration is a critical step in the fabrication of a variety of electronic devices including display devices. FSA continues to be investigated as a tool for assembling microcomponents into a functional microsystem as a means to overcome the high cost of “pick and place” assembly and the “sticking problem” (adhesion between micropart and manipulator) and other alternatives described by Cohn39 and Fearing.40 To accomplish efficient microassembly of a large number of components, parallel assembly methods have been investigated and implemented in manufacturing.19a-d One approach is to transfer microstructures between aligned wafers.41,42 Another approach is the FSA approach in which a large number of identical microparts is dispersed in a carrier fluid and flowed across specially designed binding sites. Several research groups have demonstrated FSA of microscale (submillimeter) and mesoscopic (∼1 µm) components.12-20 The self-assembly of micro- and mesoscopic components may be shape directed or directed by the spontaneous association of molecules (molecular selfassembly) present at the surface of the components. Shape controlled FSA has been developed into a manufacturing process utilized by the Alien Technology Corporation in the fabrication of low cost radio frequency identification (RFID) tags.19a-d In Alien Technology’s FSA process, specifically shaped semiconductor devices ranging from 10 µm to several hundred micrometers are suspended in a liquid (carrier fluid) and flowed over a surface, which has correspondingly shaped holes or receptors into which the devices settle. Certain limitations are (38) Barry, C. R., Hoon, C. J., Jacobs, H. O. Approaching Programmable Self-Assembly from Nanoparticle-Based Devices to Integrated Circuits. Proceedings of the Foundations of Nanoscience: Self-Assembled Architectures and DeVices (FNANO), Snowbird, UT, April 21-23, 2004. (39) Cohn, M. B. Assembly Techniques for Microelectromechanical Systems. Ph.D. Dissertation, University of California at Berkeley, Berkeley, CA, 1997. (40) Fearing, R. S. Survey of Sticking Effects for Micro-Parts; Proceedings of the IEEE International Conference on Robotics and Intelligent Systems (IROS), Pittsburgh, PA, August 7-9, 1995; pp 212-217. (41) Cohn, M. B.; Bohringer, K. F.; Noworolski, J. M.; Singh, A.; Keller, C. G.; Goldberg, K. Y.; Howe, R. T. Microassembly Technologies for MEMS. Proceedings of SPIE Micromachining and Microfabrication, Conference on Micromachining and Microfabrication Process Technology IV, Santa Clara, CA, Sept 21-22, 1998; pp 2-16. (42) Holmes, A. S.; Saidam, S. M. J. Microelectromech. Syst. 1998, 7, 416.
Figure 3. Site-selective assembly using TCSFA. A patterned heating block is used to provide heat to selected sites. In this case, assembly of microparts is caused by hydrophobic oil (e.g., hexadecane) deposited on a hydrophobic patch preferentially wetting and forming a capillary bridge with a hydrophobic surface on the micropart.
apparent: the chips have to be relatively large to settle, and if the assembly of more than one kind of chip is desired, siteselective assembly would require multiple unique sets of the device shape and holes. The assembly of components driven by molecular selfassembly, molecular attraction, or molecular recognition between two surfaces results in joining of the two complementary surfaces. For example, when components are in water, hydrophobic molecules present at a surface of one component will associate with hydrophobic molecules present at a surface of another component. Molecular recognition between complementary DNA strands and between avidin and biotin has also been used to selectively assemble components.21-23 In the work of Murakami et al.,24 magnetic forces cause microscopic metal disks to attach onto a substrate patterned with arrays of nickel dots. More recently, published studies describe electric field-mediated (electrophoretic) assembly of charged micromachined silicon islands.25,26 The assembly of large mesostructures is facilitated by the presence of liquid components that preferentially wet surfaces to be joined. Capillary attraction between the wetted surfaces allows for local positioning that minimizes interfacial free energy and results in relatively defect-free 2-D and 3-D structures.7-12 Figure 1 is a schematic depicting the assembly of a micropart on a binding site. Capillary attraction has enabled the integration of semiconductor IC devices,27 the assembly of micromirror arrays,17,18,20,28 and the assembly of light emitting diodes.29 Molten solder has also been used to assemble silicon structures.11,30,33-35 While a large number of driving forces has been utilized to drive self-assembly, the forces have only been applied to the
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Figure 4. (a) Micrograph showing iron particles being dragged above a cold spot heater. (b) Micrograph showing iron particles being forced to go around the hot spot heater indicating the presence of a gel in the heated area.
Figure 5. Selective coating of liquid adhesive (hexadecane) on FC722 patches when substrate is passed through hexadecane/F127 solution interface (adapted from ref 17) (Copyright 2001 IEEE).
case of parallel assembly for identical components in a single batch.29 However, in the fabrication of certain display devices and other functional microsystems, the integration of multiple microcomponent types may be desired. The patterning of different cell populations by electrochemical modulation,31 use of photosensitive electrodes to activate the assembly of charged microcomponents,32 and recent work published by Bo¨hringer et al.29 and Jacobs et al.36-38 are the only reports to date where an attempt was made to activate (or deactivate) binding sites for controlled multibatch self-assembly of microparts. In this paper, we describe a novel method for controlled multibatch selfassembly of microparts by capillary adhesion while utilizing the thermal behavior of the carrier fluid (as opposed to the thermal behavior of the binding site) to control microcomponent binding. TCFSAsThe Concept. A method for site-selective FSA is to prevent assembly on selected sites by blocking or masking the site. Conceptually, this may be achieved by the application of site-targeted heat that causes a carrier fluid to gel locally, creating a gel cap that prevents the binding site from receiving a device. When heat provided to a selected site is discontinued, a gel cap protecting a binding site reverts back to a carrier liquid. By successively applying heat to selected sites, different batches of microcomponents can be sequentially assembled in a massively parallel multibatch process onto a single substrate. Such carrier fluids have the property of a thermal gel (i.e., a liquid whose viscosity increases to the consistency of a gel upon heating). Binding sites may be selectively heated using a variety of methods such as resistive heaters, contact heating using patterned block heaters, and laser thermal scanning printheads. These methods for site-selective heating, to be discussed later, are compatible with current capabilities in web handling, patterned coatings (gravure printing and ink jet printing), dye chemistry, thermal management engineering, and laser thermal technology. A schematic describing thermally controlled FSA or TCFSA is depicted in Figure 2a,b. A pattern of binding site holes is prepared on a substrate, and a first suspension of microparts (denoted microparts-1) is flowed over the substrate. The holes are filled by the appropriately shaped microparts (reminiscent
of Alien Technology Corporation’s method for FSA19d) except for holes that are blocked by the presence of a gel cap that has been created by local heating. In Figure 2a, the heat source (not defined) is depicted as an arrow directing heat at the binding site. A gel cap created in the vicinity of the heated binding site prevents the heated site from receiving micropart-1. After the desired assembly of microparts--1 on the non-heated sites, the substrate is rinsed with clean carrier fluid to remove the unbound microparts-1 (heat is still on). A second type of microparts, microparts-2, may now be assembled by flowing a suspension of microparts-2 across the target substrate, as shown in Figure 2b. Microparts-2 will only fall into holes that are unoccupied or no longer heated. Clearly, if only two types of microparts are to be assembled, selective heating of binding sites is only required during assembly of first type of micropart. Furthermore, the TCFSA concept may be applied in the assembly of microparts utilizing any kind of attractive force (i.e., magnetic, molecular recognition, capillary force, etc). Capillary attraction between hydrophobic surfaces in FSA has been used to assemble microparts into mesostructures9 and substrates.17 The presence of a hydrophobic liquid serves to lubricate the local movement of microparts so that they orient and settle into a free energy minimum configuration. In contrast to shape recognition FSA, capillary attraction between the surfaces does not require (although it may be used in conjunction with) a hole or microcup. Patterned hydrophobic sites (potentially prepared by a variety of printing techniques such as gravure printing, microcontact printing,6 and ink jet printing) are coated with oil (an adhesive), and once binding takes place, the adhesive serves to stabilize the binding of the micropart.28 Site-selective assembly by capillary binding may also be achieved using TCFSA, as shown schematically in Figure 3. TCFSA requires a suitable thermally responsive fluid, and if it is to be used in conjunction with capillary adhesive, then the liquid adhesive has to be compatible with the carrier liquid. Thermally Responsive Carrier Fluid. Thermally reversible gel formation is a key parameter on which the TCFSA concept depends. The carrier fluid must gel upon application of local heat and return to a liquid upon removal of heat in a reasonable time (0.1-10 s). Gel transition temperatures most conveniently between 30 and 90 °C for aqueous based systems are desirable. Furthermore, for practical and economic reasons, the power requirements for gel formation should be minimized. The viscosity of the carrier fluid must be such that it does not severely impact settling times for the microparts into corresponding holes. Concentrated aqueous solutions of poly(ethylene oxide) and poly(propylene oxide) or PEO-PPO copolymer surfactants (poloxamers) are thermal gels with gel transition temperatures
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Figure 6. Schematic of apparatus used to demonstrate surface tension driven self-assembly of plastic microparts in F127 solution.
Figure 7. Schematic showing assembly of plastic microparts with hydrophobic ODT SAM coating.
Figure 8. Micrograph of self-assembled plastic microparts adhering to hexadecane (stained yellow with Orange OT). One of the microparts has been intentionally removed to reveal the hexadecane.
that can be tuned by concentration and type of poloxamer.43-46 Thermal gels of Pluronic F127 may be potentially useful as a carrier fluid for thermally controlled FSA or TCFSA. Recent work by Stoeber et al.46 confirmed that the F127 solution in microfluidic channels gels with a submillisecond response when heated with a resistive heater in the microfluidic channel. Stoeber et al. also provide the gel transition temperature as a function of F127 concentration. They found that a 15 wt % solution (43) Brown, W.; Schillen, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850. (44) Yu, G.-E.; Deng, Y.; Dalton, S.; Wang, Q.-G.; Attwood, D.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 2537. (45) Schmolka, I. R. J. Biomed. Mater. Res. 1972, 6, 571. (46) Stoeber, B.; Yang, Z.; Liepmann, D.; Muller, S. J. J. MEMS 2005, 14 (2), 207-213.
Figure 9. Schematic of apparatus for assembling plastic microparts.
had a gel transition temperature of approximately 25 °C. In our experiments, we found that 15 wt % of F127 was required to achieve rapid gel formation with an estimated gel transition temperature of 40 °C. The need for a higher F127 concentration may be due to excessive thermal loss in heating a small area in a large heat sink provided by the several milliliters of surrounding fluid (i.e., the apparent shift in gel transition temperature may be attributed to geometrical differences). Thermal gelation was demonstrated by studying the movement of magnetic iron filings when drawn over the heated site. In Figure 4a, an image of iron filings being drawn over a heater (heater is off) by a magnet is presented. In contrast, when the heater is energized, the iron filings are impeded from traveling across the heater (Figure 4b). These experiments provide evidence
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Figure 13. Top view image of a resistive heater glued to a PET sheet using silver-filled conductive epoxy (A, gray region). Copper wire leads (B) twist-tied to a constantin wire coil (not visible, drawn in as circle with broken line and copper wire loops extending to copper leads) can be seen on either side of the conductive epoxy.
Results and Discussion Figure 10. Substrate emerging from circulation apparatus described in Figure 9 with two blue microparts assembled. The center hexadecane binding site is not occupied because it was protected by an F127 gel when heated.
Figure 11. Close-up picture of self-assembled blue microparts (plastic microparts).
that gelation is achieved using a 15 wt % F127 solution when heated in the manner to be used in demonstrating TCFSA. Details on heater construction are presented in the Experimental Procedures. Practical questions dealing with the use of a concentrated surfactant solution as a carrier fluid in a self-assembly process utilizing hydrophobic attractions need to be answered. For example, while the F127 carrier fluid may be desirable for its thermal behavior, its potential to interfere with or smother hydrophobic attractions between adhesive oil on the binding site and hydrophobic surface on microparts was a concern. Thus, to demonstrate FSA using a concentrated F127 solution as a carrier liquid before attempting to demonstrate TCFSA using the F127 carrier fluid was first needed.
Demonstration of FSA of Microparts in Pluronic F127 Fluid using Hexadecane Binding Site. FSA of rectangular shaped hydrophobic poly(ethylene terepthalate) (PET) cut from a 100 µmthick sheet was attempted in a concentrated F127 solution. The preparation of the hydrophobic sites on PET as well as the deposition of hexadecane as an adhesive oil are presented in the Experimental Procedures (preparation of hexadecane binding site on PET). A pattern of six hydrophobic FC722 spots (each spot ∼0.5-1 mm diameter) was deposited on a piece of PET (approximately 1.5 cm × 1.5 cm) as described in the Experimental Procedures. The sample was then inserted into an alligator clip attached at the end of a stick that was glued to a bottle cap. The PET sample was then dipped into a F127 carrier liquid topped with hexadecane. A yellow dye, Orange OT, was added to the hexadecane to make it visible and able to be imaged. During the dipping process, the hydrophobic patterned sample passes through the hexadecane/ F127 interface, causing hexadecane to selectively coat the hydrophobic F722 spots (see schematic in Figure 5). The excess hexadecane floating on top of F127 was then carefully aspirated. ODT-coated plastic microparts were then introduced to the vial and capped with the cap and alligator clip assembly (Figure 6). The sample was then carefully shaken to suspend the plastic parts and allow them to collide and stick to the hydrophobic hexadecane binding sites. Hexadecane liquid selectively coating the hydrophobic binding sites (also called hexadecane binding sites) was used as the adhesive or capillary bridge to bind the microparts by preferentially wetting hydrophobic surfaces on them, as depicted in Figure 7. The result of the self-assembly process can be seen in Figure 8. In Figure 8, all six binding sites were occupied by plastic microparts, but for the purpose of visualizing the presence of hexadecane binding sites, one of the plastic microparts has been removed to reveal a yellow-stained hexadecane binding site on the top right section of the hexadecane binding site array.
Figure 12. Close-up picture of the completed self-assembly process. A red micropart occupies the center hexadecane binding site.
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Successful demonstration of FSA of microparts by hydrophobic attraction between hexadecane and hydrophobic surfaces in the F127 solution provided an important resultsif FSA could not be demonstrated using the F127 carrier fluid, there would be no point in continuing the work to demonstrate thermally controlled fluidic assembly using the same fluid system. Demonstration of Thermally Controlled Fluidic Assembly (TCFSA). To demonstrate the directed assembly of two types of microparts by TCFSA, two types of plastic microparts (blue and red microparts) were assembled sequentially on selected sites. A resistive heater was prepared on a sheet of PET, as described previously. A simple pattern of hexadecane binding sites with one binding site located on top of the heater was prepared on the PET by deposition of FC722 spots and by passing the sheet through a hexadecane/F127 interface as described previously. The sample was immersed in the F127 solution in a funnel as shown in Figure 9. The procedure used to prime and position the target substrate is described in the Experimental Procedures. After energizing the heater (3-4 V, ∼1.5 amps), the blue plastic microparts suspended in the F127 carrier fluid were pumped through the funnel. After a few minutes (typically 3-5 min), the pump was turned off, and the target substrate was inspected for self-assembly of blue microparts on the two outer binding spots as these were not heated. Figure 10 is an image of the substrate emerging out of the assembly funnel showing the blue microparts assembled on the two outer binding sites. A higher magnification image of the self-assembled blue chips is shown in Figure 11. Note the yellow-stained hexadecane that is visible through the blue micropart on the right (a fortuitous coincidence as the micropart should have been solid blue as is the one on the left). The blue microparts in the carrier fluid were replaced with red chips, and the process was repeated with heater off this time as we wanted a red micropart to bind to it. A high magnification picture (Figure 12) shows the successful self-assembly of a red micropart on the previously unoccupied center site. Figure 12 provides evidence for the site-directed sequential assembly of blue microparts followed by a red micropart by TCFSA.
Conclusion Several features distinguish this study from other studies in FSA. This work demonstrates, for the first time (1) random or stochastic fluidic assembly of microparts utilizing hydrophobic attractions in a concentrated non-ionic surfactant solution of F127. It is a necessary step in the demonstration of TCFSA using F127 carrier fluid. (2) Site-selective FSA of microcomponents suspended in a concentrated aqueous solution of F127 using TCFSA. TCFSA could potentially be used for high throughput web assembly of components. On the basis of preliminary calculations, the estimated hydrophobic binding energy is sufficiently robust to withstand dislodging forces caused by liquid drag on the components on a moving web. Estimated web speeds of between 1 and 3 ms-1 are tolerated depending on size of the micropart, interfacial tension of the capillary adhesive, viscosity of the liquid adhesive, and carrier fluid viscosity among other parameters. Furthermore, a thermal polymerizable glue in the adhesive liquid can be used to glue the assembled components if necessary.18,20 TCFSA may potentially be used to sequentially assemble arrays of, for example, red, green, and blue bichromic beads (Gyricon balls) or microcapsules47 in making a color reflective display. Other potential applications include the fabrication of integrated circuit devices requiring the assembly of different components (47) www.eink.com.
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and the assembly of substrates having different chemical receptors and biomarkers for detection of chemicals, proteins, DNA, and other biomolecules. Experimental Procedures Materials. PET (Kodak internal stock supply; ∼100 µm thick) with one surface treated with a submicrometer coating of gelatin was used as a substrate for the assembly of microparts. The gelatin coating renders the surface more hydrophilic than bare PET. A fluorinated polymer (FC722) solution obtained from 3M was used to coat plastic microparts and to make hydrophobic coatings on PET. Pluronic F127 obtained from BASF was dissolved in single distilled water (15% w/v) and used as the thermally responsive carrier fluid. Hexadecane (Aldrich) was used as the adhesive or glue to bind microparts by capillary attraction. Orange OT dye (Aldrich) was dissolved in the hexadecane to provide coloration and make it visible. Iron particles (electrolytically reduced, obtained from Matheson, Bell, and Coleman) were introduced to the F127 carrier fluid to probe and visualize the fluidity of the carrier fluid. 1-Octadecanethiol (ODT) obtained from Aldrich was used to create self-assembled monolayers (SAMs) on gold-coated PET. Ethanol and methanol obtained from Aldrich were used as solvents and for storing the microparts. A two part silver-filled conductive epoxy (Circuit Works Conductive Epoxy, part CW2400) was used to glue coil resistive heaters to PET. The resistive heaters were made using constantin wire. Preparation of Hydrophobic Gold-Coated PET Microparts. A 3 in. × 3 in. piece of wire was cleaned in an oxygen plasma48 and was coated with 250 Å of chromium by evaporation at 2 Å/s on the bare side (non-gel subbed side). The chromium was then over-coated with 500 Å of gold by evaporation at 2 Å/s. The chromium layer serves as an adhesion-promoting layer for the gold coating. Gold surfaces may be rendered hydrophobic by a SAM of 1-octadecanethiol.49,50 A solution of 1-octadecanethiol (ODT) was prepared by mixing 22 mg of ODT in 80 mL of absolute ethanol and stored in a tightly sealed bottle. Samples of plasma cleaned48 gold-coated PET were then introduced to the ODT solution between 1 and 24 h, after which they were removed and washed with methanol and dried in a stream of nitrogen before further use. The resulting ODT coating had a water contact angle of 106° and a hexadecane contact angle of 44°. ODT-coated microparts were prepared by simply cutting the hydrophobic gold coating into 1 mm sized pieces using scissors. The ODT on gold-coated PET microparts was used to demonstrate conventional FSA in F127 carrier fluid. Preparation of All-Plastic Microparts with FC722 Hydrophobic Coating. A second type of micropart was prepared using PET. A plasma cleaned48 PET sheet was coated with FC-722 on the hydrophilic side, making it hydrophobic. The other side (bare side) was then colored (blue and red) with a marker pen. Microparts were then cut out using a circle die cut hole punch (∼1.6 mm diameter) and stored in jar for later use in TCFSA experiments. Preparation of Resistive Heaters. Resistive heaters were prepared as described next. A length of constantin was wrapped around a sewing needle to form the coil, while copper wire leads were twisttied to opposite ends of the coil. The copper leads were used to minimize resistive heating by the leads. Finally, the coil heater was glued down to the bare side of the substrate using a two part conductive epoxy and cured in an oven at 105 °C for 5 min. As a result of gluing, the coil heater became a spot heater, as can be seen in Figure 13. The resistance of the spot heater varied between 2 and 5 Ω, most likely depending on the length of constantin wire and the volume of the conductive epoxy used. Visualization of Gelation of Pluronic F127. A resistive heater was built on a piece of PET (bare side). A small amount of F127 (48) Medium power setting, 30 s, 5 mTorr oxygen pressure, plasma cleaner (model PDC-23G made by Harrick). (49) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570. (50) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. AdV. Colloid Interface Sci. 1992, 39, 175.
Thermally Controlled Fluidic Self-Assembly was placed over the heater on the hydrophilic side of the PET, and a small amount of iron powder was introduced to the F127 solution. A magnet was glued onto a plastic strip and used to conveniently slide the magnet to and fro beneath the heater. We noted that iron particles followed the to and fro motion of the magnet dragging the surrounding liquid and could be used to probe the fluidity of the carrier fluid. The movement of the iron particles induced by the sliding magnet was recorded using a CCD camera equipped with a 5× magnification lens. Preparation of Capillary Adhesive Sites on PET. Hydrophobic sites on PET were prepared by depositing small amounts of FC722 using a micropipet dispensing tip. A small amount of FC722 was wicked into the pipet tip and then deposited at the desired location on the hydrophilic PET surface. Because of its wetting properties, FC722 wetted the gelatin-coated PET surface, forming a small spot of about 0.5 mm diameter when dried. The water contact angle on FC722 coating was 103°, and the hexadecane contact angle was 42°, which is a fairly comparable to those obtained on ODT SAM on gold-coated PET. The FC722 patch was used to prepare the hexadecane binding site as described next. Preparation of Hexadecane Binding Sites on PET. Srinivasan et al.17 have prepared hexadecane drops assembled on patterned hydrophobic sites. In this work, hexadecane was found to selectively wet the hydrophobic SAM coating, creating a pattern of hexadecane binding sites when a patterned substrate was passed through a hexadecane/water interface. We adapted this method for preparing hexadecane binding sites replacing water with F127 solution (Figure 5). Hydrophobic sites on PET were prepared by depositing small amounts of FC722 using a micropipet dispensing tip. A small amount of FC722 was wicked
Langmuir, Vol. 23, No. 12, 2007 6849 into the pipet tip and then deposited at the desired location on the hydrophilic PET surface. Because of its wetting properties, FC722 wets the gelatin-coated PET surface, forming a small spot of about 0.5 mm diameter when dried. The water contact angle on the FC722 coating was 103°, and the hexadecane contact angle was 42°, which is a fairly comparable to those obtained on ODT SAM on goldcoated PET. The FC722 patch was used to prepare the hexadecane binding site by preferential wetting.17 Apparatus for TCFSA. A schematic of the apparatus is shown in Figure 9. Prior to operation, about 150 mL of the F127 carrier fluid was poured into the lower funnel. The pump (ND 100 liquid diaphram pump, KNF Neuberger Inc.) was then primed with the carrier fluid. Colored plastic microparts were then added to the carrier liquid. The trajectory of the microparts emerging from the top funnel into the lower funnel was noted, and the pump was turned off. A pattern of three hexadecane binding sites prepared on PET support and with a resistive heater addressing the middle binding site was strategically placed in the lower funnel so that chips emerging from the top funnel would drift toward the binding sites once the pump was activated. The carrier fluid containing the microparts was pumped for approximately 5 min after which time the sample was inspected for the assembly of microparts.
Acknowledgment. I thank Frank Zendejas (Department of Electrical Engineering and Computer Science, University of California at Berkeley) for experimental work and Gil Hawkins (Printing Solutions Platform Center, Eastman Kodak Company, Rochester, NY) for his support. LA063516Q
