Research News

Macroporous Membranes with HighlyOrdered and Three-DimensionallyInterconnected Spherical Pores**

By Sang Hyun Park and Younan Xia*

We have demonstrated the fabrication of highly ordered three-dimensional macroporous membranes with crystallineassemblies of monodispersed mesoscale particles (~200 nm to ~10 mm in diameter) as templates. We filled the void spacesamong the particles with a UV-curable liquid prepolymer by capillary action. Subsequent solidification of the prepolymer anddissolution of the particles yielded a membrane film consisting of a highly organized and three-dimensionally interconnectedframework of spherical pores. The pores are uniform in size, with dimensions defined by the diameter of the particles. Thepores are completely exposed on both top and bottom surfaces of the membrane film. In addition to those applicationsassociated with conventional macroporous materials, the macroporous membranes fabricated here are potentially useful asphotonic bandgap structures or as supports for the fabrication of diffractive optical sensors due to their specified and highlyordered structures.

1. Introduction

Macroporous (pore sizes ³50 nm[1]) membranes withhighly ordered three-dimensional (3D) porous structuresare technologically important for a variety of applications.In addition to their conventional uses as supports incatalysis,[1,2] as filters in separation,[1,3] and as buildingblocks in tissue engineering,[4] these membrane films alsoexhibit interesting optical properties such as photonicbandgaps and optical stop-bands due to their spatiallyperiodic structures.[5] A number of methods have beendeveloped for producing such kinds of membranes, includ-ing those based on selective etching (electrochemicaletching of alumina or silicon,[6] chemical etching ofglasses,[7] and ion-track etching of polymers[8]), those basedon self-assembly of block copolymers,[9] and those based onreplica molding against various kinds of templates.[10]

Methods based on selective etching usually generatestraight, one-dimensional channel structures and have beenvery successful in the manufacture of commercial mem-brane films.[6±8] Methods based on self-assembly of blockcopolymers provide an elegant and efficient route to

macroporous films with a regular array of spherical pores.[9]

Although these pores are fully opened on the surfaces ofthe film, they are isolated from each other in the bulk.Methods based on replication take advantage of templatestructures that can be easily assembled from a variety ofspecies, such as aggregates of surfactant molecules, airbubbles, solid particles, and bacteria.[10] These methods aresimple and straightforward; they are capable of producingcomplex, 3D macroporous membranes with tightly con-trolled pore sizes and pore structures.

Recently, we demonstrated a simple and practicalprocedure for forming crystalline assemblies of mesoscalespherical particles over relatively large areas.[11] Theseassemblies could be subsequently used as sacrificialtemplates[10,12] to generate macroporous membranes oforganic polymers or inorganic ceramics containing spher-ical pores. The pores in each film are uniform in size, andtheir dimension can be precisely controlled in the rangefrom ~100 nm to ~10 mm. These pores are completelyexposed on both top and bottom surfaces of the membranefilm with a surface density of ³108 cm±2; they areinterconnected into a three-dimensional frameworkthrough circular ªwindowsº in the bulk.

2. Formation of Templates from MesoscaleParticles

Figure 1A shows the schematic procedure that we usedto produce cubic-close-packed (ccp) templates from aque-ous dispersions (~0.05 wt.-%) of monodispersed mesoscale

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[*] Prof. Y. Xia, Dr. S. H. ParkDepartment of Chemistry, University of WashingtonSeattle, WA 98195-1700 (USA)

[**] This work was supported in part by a New Faculty Award from theDreyfus Foundation, a subcontract from the AFOSR MURI Center(F49620-96-1-0035) at the University of Southern California, and start-up funds from the University of Washington. This project used theMicrofabrication Laboratory at the Washington Technology Center(WTC). YX thanks Prof. Larry Dalton for helpful discussions.

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(~0.2 to ~10 mm in diameter) spherical particles such aspolystyrene beads, polycarbonate beads, or silica col-loids.[11] When an aqueous dispersion of beads was injectedinto the cell, a positive pressure of nitrogen was appliedthrough the glass tube to force the solvent (water) to flowthrough the channels (with a cross-sectional height of h).Because the beads are larger than the cross-sectional heightof the channels, they were retained in the cell, and wereassembled into a ccp lattice under continuous sonication. Inthe ccp assembly, the (111) face is parallel to the surfaces ofthe glass substrates, and the beads are in physical contactrather than being separated by a certain distance.[11]

Using this procedure, crystalline assemblies of 0.48 mmpolystyrene beads can be produced in a 12 mm thick cellover an ~1 cm2 area within 24 h. The surface topology andthe number of layers of the crystalline assembly can also betightly controlled.[11] By changing the thickness (H) of thecell, or the diameter (D) of the particles, or both, we havebeen able to continuously change the number of layers ofthe crystalline assembly of polymer beads from one up to atleast fifty. The smallest diameter of particles that can bedirectly assembled in the cell is determined by the cross-

sectional height of the channels: the value of D must belarger than h to retain the beads in the cell. Although theminimum depth (hmin) of channels that can be routinelygenerated using conventional photolithography is approxi-mately 0.2 mm, we were also able to assemble polymerbeads as small as ~60 nm in diameter. In this case, weconsecutively injected dispersions of polymer beads withdecreasing sizes into the cell, and the ccp lattice of bigbeads provided a barrier to retain the small beads injectedafterwards.[11]

3. Fabrication of Highly Ordered 3DMacroporous Membranes

Figure 1B illustrates the procedure that we used togenerate membranes with highly ordered 3D networks ofspherical pores.[12] The void spaces among the beads of acrystalline assembly were filled (via capillary action) with aliquid precursor (such as UV-curable polyurethanes, UV-curable polyacrylates, thermally curable epoxies, or sol-gelmaterials).[13] The precursor liquid should be selected suchthat it does not swell or dissolve the beads. After solidifyingthe precursor material, the beads were dissolved byimmersing the sample in an appropriate etching solution.The resulting film consists of a 3D network of sphericalpores interconnected by circular ªwindowsº. The mem-brane fabricated using this procedure has a very preciselycontrolled pore size, which is defined by the diameter of thebeads. In general, the diameter of pores on the surface ofthe membrane is approximately half of the diameter of thebeads used to form the crystalline assembly (Fig. 1B).

Figure 2 shows the scanning electron microscopy (SEM)images of a polyurethane membrane film before thepolystyrene beads are dissolved. The crystalline assemblywas formed in a 12 mm thick cell by 0.48 mm polystyrenebeads. The bright dots correspond to bumps on the surfaceof the film formed by the underlying polystyrene beads.The ccp lattice of polystyrene beads is essentially main-tained in this process because the beads are confined by thewalls of the cell and the polyurethane liquid precursor usedhere does not swell or dissolve the polystyrene beads.

Figure 3 shows the SEM images of a membrane film thatwas fabricated from the composite film shown in Figure 2by dissolving the polystyrene beads in toluene. Thismacroporous membrane has pretty good mechanicalstability and free-standing films as large as 1 cm2 havebeen fabricated. Figures 3A and 3B are top-view images ofthe membrane film.[12] The pores are fully open on both topand bottom surfaces of the film with a diameter of~250 nm; they are packed into an hexagonal array with asurface density as high as ~4 ´ 108 cm±2. Figure 3Csimultaneously displays the top layer, the bottom layer,and the cross section of the membrane film. Figure 3Dshows the SEM image of the cross section at a highermagnification. These SEM images strongly support the

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Fig. 1. A) Schematic outline of the experimental procedure that was used toform crystalline assemblies from mesoscale spherical particles. B) Schematicillustration of the procedure that uses crystalline assemblies of polystyrenebeads as templates to fabricate highly ordered 3D membrane films (cross-sectional view).

illustration shown in Figure 1B. The membrane is a trulythree-dimensional one: the spherical pores in the bulk ofthe membrane are connected to each other through verysmall circular ªwindowsº (as indicated by an arrow inFig. 3D). Based on the ideal model of cubic close packing,each spherical pore should be connected to six other poresin the same plane and three other pores above and belowthe plane. The porosity the membrane should be greaterthan 74 % if we consider the ªwindowsº that connect thespherical pores into the three-dimensional structure.

Figure 4 shows the SEM images of another polyurethanemembrane that was fabricated from a crystalline assemblyof ~0.2 mm polystyrene beads. The surface density of poresis ~2 ´ 109 cm±2 and the diameter of each pore on thesurface is ~100 nm. By making templates from particlessmaller than 100 nm, we believe that this procedure can beextended to the fabrication of mesoporous membranes inwhich the pores are smaller than ~50 nm.

4. Future Work

We have demonstrated a convenient and versatile routeto macroporous membranes with precisely controlled poresizes. The pores are completely exposed on both top andbottom surfaces of films and are arranged into a highlyordered 3D structures. The procedure demonstrated herecan be applied to a variety of materials, including organicpolymers, inorganic ceramics (such as SiO2, TiO2, andZrO2),[10] and even metals (for example, in combinationwith electroless plating of silver[14]); the only requirementseems to be the availability of a liquid precursor that doesnot swell or dissolve the template. Although this proce-dure may lack the characteristics required for massproduction, it can efficiently produce membrane filmshaving highly ordered porous structures that are useful fora wide variety of applications. For example, thesemembrane films can be used as photonic bandgapmaterials that are extremely difficult to fabricate usingother techniques when the feature sizes approach themicrometer scale.[5,15] The photonic bandgap structure canbe controlled by changing the refractive index of theprecursor material. These three-dimensional open struc-tures can be used as substrates (with appropriate surfacemodifications[16±18]) to fabricate new types of opticalsensors or prototype sensors with enhanced sensitiv-ities.[18,19] These highly specified and organized membranescan also be used as templates to generate three-dimen-sional complex structures of various functional (for

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Fig. 2. SEM images of a composite film consisting of cured polyurethane and acrystalline assembly of 0.48 mm polystyrene beads. The photoresist films usedin the present study were~12 mm thick. The film was sputtered with a thin layerof Au before taking SEM images. Each bright spot corresponds to the bumpformed by an underlying polystyrene bead.

Fig. 3. A,B) Top-view SEM images of a polyurethane membrane that wasfabricated with a ccp lattice of 0.48 mm polystyrene beads as the template.C,D) Cross-sectional SEM images of the same membrane. In this 3Dmacroporous membrane, each spherical cage is connected to adjacent cagesthrough very small ªwindowsº (indicated by an arrow). The largest free-standing membrane that we have been able to fabricate using this procedure is~1 cm2 in area; this result clearly demonstrates the structural integrity of thepolymer membrane.

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example, optically or magnetically active) materials thateither cannot or cannot easily be fabricated usingconventional photolithographic techniques.[20]

±[1] R. R. Bhave, Inorganic Membranes: Synthesis, Characteristics and

Applications, Van Nostrand Reinhold, New York, 1991. D. W.Schaefer, MRS Bull. 1994, 19(4), 14.

[2] P. T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature 1994, 368, 321.[3] H. W. Ballew, Am. Biotechnol. Lab. 1997, May, 8.[4] J. A. Hubbell, R. Langer, Chem. Eng. News 1995, March 13, 42.[5] J. D. Joannopoulos, R. D. Meade, J. N. Winn, Photonic Crystals:

Molding the Flow of Light, Princeton University Press, Princeton, NJ1995.

[6] R. C. Furneaux, W. R. Rigby, A. P. Davidson, Nature 1989, 337, 147.[7] R. J. Tonucci, B. L. Justus, A. J. Campillo, C. E. Ford, Science 1992,

258, 783. D. H. Pearson, R. J. Tonucci, Adv. Mater. 1996, 8, 1031.[8] M. Yoshida, M. Asano, T. Suwa, N. Reber, R. Spohr, R. Katakai, Adv.

Mater. 1997, 9, 757.[9] G. Widawski, M. Rawiso, B. François, Nature 1994, 369, 387.

[10] D. J. LeMay, R. W. Hopper, L. W. Hrubesh, R. W. Pekara, MRS Bull.1990, 15(12), 19. W. R. Even, Jr., D. P. Gregory, MRS Bull. 1994, 19(4),29. D. Walsh, S. Mann, Nature 1995, 377, 320. A. Imhof, D. J. Pine,Nature 1997, 389, 948. S. A. Davis, S. L. Burkett, N. H. Mendelson, S.Mann, Nature 1997, 385, 420. G. A. Ozin, Acc. Chem. Res. 1997, 30, 17.O. D. Velev, T. A. Jede, R. F. Lobo, A. M. Lenhoff, Nature 1997, 389,447.

[11] S. H. Park, D. Qin, Y. Xia, Adv. Mater. 1998, 10, 1028. S. H. Park, Y.Xia, unpublished.

[12] S. H. Park, Y. Xia, Chem. Mater. 1998, in press.[13] Y. Xia, G. M. Whitesides, Chem. Mater. 1996, 8, 1558.[14] Y. Xia, N. Venkateswaran, D. Qin, J. Tien, G. M. Whitesides, Langmuir

1998, 14, 363.[15] E. Yablonovitch, J. Opt. Soc. Am. B 1993, 10(2), 283.[16] F. Svec, J. M. J. FrØchet, Adv. Mater. 1994, 6, 242.[17] C. H. Bamford, K. G. Al-Lamee, Adv. Mater. 1994, 6, 500.[18] V. S.-Y. Lin, K. Motesharei, K. P. S. Dancil, M. J. Sailor, M. R.

Ghadiri, Science 1997, 278, 840.[19] J. H. Holtz, S. A. Asher, Nature 1997, 389, 829. J. H. Holtz, J. S. W.

Holtz, C. H. Munro, S. A. Asher, Anal. Chem. 1998, 70, 780.[20] See, for example: C. R. Martin, Acc. Chem. Res. 1995, 28, 61.

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Fig. 4. SEM images (A, top view; B, cross-sectional view) of a polyurethanemembrane that was fabricated with a ccp lattice of 0.23 mm polystyrene beadsas the template. As indicated by an arrow, each spherical cage is also connectedto adjacent neighbors through circular ªwindowsº.

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