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Polymer Microsieves Manufactured by Inkjet Technology Stephan F. Jahn,† Lutz Engisch,† Reinhard R. Baumann,*,† Susann Ebert,‡ and Werner A. Goedel*,‡ Institute for Print and Media Technology, Chemnitz UniVersity of Technology, Reichenhainer Strasse 70, 09126 Chemnitz, Germany, and Department for Physical Chemistry, Chemnitz UniVersity of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany ReceiVed September 3, 2008. ReVised Manuscript ReceiVed October 15, 2008 Liquid sessile drops can be used as sacrificial templates for the creation of pores in polymeric microsieves. Using inkjet printing, we deposit sessile drops of a water-based liquid onto a hydrophobic solid support and cover them with a thin liquid layer of a polymer solution in such a way that the sessile drops penetrate through the top interface of this layer. The liquid layer is solidified, and the sessile drops imprint their shape into it, acting as templates for the creation of pores. Finally, the polymer layer is separated from the substrate, and a freely suspended polymer microsieve is obtained.
Introduction Porous polymer membranes find a wide range of applications in technical, medical, and biological processes, including applications such as scaffolds for living cells1,2 and optoelectronic3 and microelectronic devices.4 One of the most technologically important applications is their use as filtration media for separation and purification5 (e.g., the recovery of solid products), gas and liquid purification, and sterile filtration. Preparation methods for porous membranes are, for example, phase inversion, track etching, and extrusion processes.6,7 To obtain high selectivity and low flow resistance of the membranes, one requires a uniform pore size distribution and a high pore density. Conventional filtration media usually possess a wide pore size distribution and a thickness that is considerably larger than the pore size and thus show considerable flow resistance and limited selectivity. On the other hand, one can prepare membranes of uniform pore size and a thickness comparable to the size of the pores s so-called microsieves s by photolithography.8,9 Using interference lithography or sacrificial layers, membranes with pore widths well below optical resolution can be made.10,11 In general, the * Corresponding authors. (R.R.B.) E-mail:
[email protected]. Phone: +49 371 531-35843. (W.A.G.) Phone: +49 371 53131713. † Institute for Print and Media Technology. ‡ Department for Physical Chemistry. (1) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S.-I.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng., C 1999, 10, 141–146. (2) Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2363–2375. (3) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802–804. (4) Hedrick, J. L.; Miller, R. D.; Hawker, C. J.; Carter, K. R.; Volksen, W.; Yoon, D. Y.; Trollsås, M. AdV. Mater. 1998, 10, 1049–1053. (5) Gates, B.; Yin, Y.; Xia, Y. Chem. Mater. 1999, 11, 2827–2836. (6) (a) Pusch, W.; Walch, A. Angew. Chem. 1982, 94, 670–695. (b) Pusch, W.; Walch, A. Angew. Chem., Int. Ed. Engl. 1982, 21, 660–685. (7) Paul, D. Chem. Unserer Zeit 1998, 32, 197–205. (8) (a) van Rijn, C. J. M.; Elwenspoek, M. C. In Proceedings of the Workshop of Micro Electro Mechanical Systems; IEEE: Amsterdam, 1995; Vol. 8, pp 3-87. (b) Kuiper, S.; van Rijn, C. J. M.; Nijdam, W.; Elwenspoek, M. C. J. Membr. Sci. 1998, 150, 1–8. (c) van Rijn, C. J. M.; Veldhuis, G. J.; Kuiper, S. Nanotechnology 1998, 9, 343–345. (d) van Rijn,C. J. M. Nano and Micro Engineered Membrane Technology; Membrane Science and Technology Series 10; Elsevier: Amsterdam, 2003. (9) Yang, X.; Yang, J. M.; Wang, X.-Q.; Meng, E.; Tai, J.-C.; Ho, C.-M. Proceedings of the IEEE 11th Annual International Workshop; MEMS: Heidelberg, Germany, 1998.. (10) Kuiper, S.; van Wolferen, H.; van Rijn, C.; Nijdam, W.; Krijnen, G.; Elwenspoek, M. J. Micromech. Microeng. 2001, 11, 33–37. (11) Desai, T. A.; Hansford, D.; Ferrari, M. J. Membr. Sci. 1999, 159, 221.
limited thickness of the microsieves renders them fragile. Thus, they need a supporting structure. The preparation of microsieves using a lithographic process offers an easy way to prepare hierarchically structured membranes that comprise microsieve parts as well as mechanically more sturdy supportive structures and allows each of the pores of the desired membrane to be given a defined position. Unfortunately, however, lithography is comparatively elaborate and expensive. Inkjet printing is a feasible tool for positioning tiny volumes of a liquid precisely and quickly onto a substrate and has become a common tool for many technical applications. If the printed liquids comprise nonvolatile solids or can be solidified, then inkjet printing can be used for manufacturing planar as well as 3D structures.12-19 One challenge in the preparation of 3D structures is the preparation of holes. Inkjet printing can be used to remove polymeric material to create isolated holes and arrays of dimples or grooves in a polymeric film.12-16 On the other hand, in microlithography one has successfully deposited sacrificial material and used it as template for voids within 3D structures.11 This intermediate deposition of sacrificial material might as well be utilized for the creation of pores and microsieves via inkjet technology. The aim of this article is to introduce into inkjet technology the use of sacrificial liquid sessile drops for the creation of pores and microsieves (Figure 1). Using inkjet printing, we deposit sessile drops of a water-based liquid onto a hydrophobic solid support and cover them with a thin liquid layer of a polymer solution. The liquid layer is solidified by the evaporation of the solvent, and the sessile drops imprint their shape into it, acting as templates for the creation of pores. Finally, the polymer (12) Kawase, T.; Sirringhaus, H.; Friend, R. H.; Shimoda, T. AdV. Mater. 2001, 13, 1601–1605. (13) de Gans, B.-J.; Hoeppener, S.; Schubert, U. S. AdV. Mater. 2006, 18, 910–914. (14) Bonaccurso, E.; Butt, H.-J.; Hankeln, B.; Niesenhaus, B.; Graf, K Appl. Phys. Lett. 2005, 86, 124101-1–124101-3. (15) Karabasheva, S.; Baluschev, S.; Graf, K Appl. Phys. Lett. 2006, 89, 0311101–031110-3. (16) Khan, F.; Zhang, R.; Unciti-Broceta, A.; Dı´az-Mocho´n, J. J.; Bradley, M. AdV. Mater,. 2007, 19, 3524–3528. (17) Gorand, Y.; Pauchard, L.; Calligari, G.; Hulin, J. P.; Allain, C. Langmuir 2004, 20, 5138–5140. (18) Lee, M.; Dunn, J. C. Y.; Wu, B. M. Biomaterials 2005, 26, 4281–4289. (19) Pfister, A.; Walz, U.; Laib, A.; Mu¨lhaupt, R. Macromol. Mater. Eng. 2005, 290, 99–113.
10.1021/la802860n CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
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Langmuir, Vol. 25, No. 1, 2009 607 Laborchemie Apolda GmbH, concentration 50 g L-1 to 100 g L-1), is dispensed onto these water drops with a pipet. The solution is allowed to distribute itself on the substrate, surrounding the droplets and solidifies within 3 to 10 min after application because the solvent evaporates (temperature between 22 °C and 25 °C, relative humidity values between 28% and 35%). The resulting polymer layer is either manually peeled off of the substrate or the substrate is removed by etching with hydrochloric acid (36.5%, Bayer Material Science). Surface tension was measured with the pendant drop method using an OCA30 optical contact angle measuring instrument from Dataphysics. Contact angles were measured by the sessile drop technique using the G2 contact angle measurement system (Kru¨ss). Viscosities were measured with a Hoeppler viscosimeter. Optical micrographs were obtained with an Axioskop 40 Pol microscope from Zeiss. Scanning electron microscopy was performed with a NanoNovaSEM from Philips. The height of a polymer membrane was determined with an 8 M DEKTAC profilometer from Veeco. Size exclusion chromatography was carried out using a PL-GPC 50 Plus (Polymer Laboratories, Varian, Inc.) with THF as the solvent and polystyrene standards utilizing universal calibration to obtain Mw and Mn for PMMA.
Results and Discussion Figure 1. Inkjet fabrication method for polymer microsieves. Table 1. Physical Properties of the Water/Ethylene Glycol Mixture (30/70 vol%) physical property
value
surface tension advancing contact angle on modified aluminum foil (air/aqueous phase/substrate)a receding contact angle on modified alumina foil (air/aqueous phase/substrate)a advancing contact angle on modified aluminum foil (chloroform/aqueous phase/substrate)b receding contact angle on modified aluminum foil (chloroform/aqueous phase/substrate)b viscosity (25 °C) density boiling point
46.1 × 10-3 N m-1 75° ( 1.6° 55° ( 2.5° 85° ( 1.7° 69° ( 6.8° 6 mPa s 1.1 g cm-3 120 °C-125 °C
a Angle between the interface (air/aqueous phase) and the interface (aqueous phase/alumina) at the position where both meet the three-phase contact line. b Angle between the chloroform /aqueous phase interface and the aqueous phase/alumina interface at the position where both meet the three-phase contact line. In both cases, the term ‘advancing’ is the equivalent of moving the three-phase contact line in such a way that the area of the interface (aqueous phase/alumina) is enlarged.
layer is separated from the substrate, and a freely suspended polymer microsieve is obtained.
Experimental Section As a solid substrate we used commercial aluminum foil (15 µm thickness, Alupro), surface-functionalized via reaction with a hydrophobic silane (exposure to the vapor of 1,1,1,3,3,3-hexamethyldisilazane (HMDS, Merck) at 23 °C for 240 min). Using the piezoelectric driven inkjet system Dimatix Materials Printer (DMP 2831, Fujifilm Dimatix, Inc.) with its 10 pL printheads (DMC11610), a multiplicity of droplets of mixtures of deionized water and ethylene glycol (technical grade, BASF) are applied onto the solid substrate in a predefined pattern and allowed to merge into arrays of individual sessile drops (customized waveform, 1 kHz). Table 1 shows the physical properties of the mixture. A solution of polymethyl methacrylate (PMMA molar mass Mn ) 64 000 g mol-1, Mw/Mn ) 1.85, 170 µL cm-2), dissolved in chloroform (technical grade,
In our experiments, we printed a multiplicity of primary droplets onto a hydrophobized aluminum foil and allowed them to merge into larger sessile drops. We then added a solution of polymethyl methacrylate (PMMA) dissolved in chloroform and allowed this solution to distribute itself on the substrate, surrounding the droplets and to solidify within 3 to 10 min as a result of the evaporation of the solvent. The resulting polymer layer is either manually peeled off of the substrate or the substrate is removed by etching with hydrochloric acid. For the printing of the sacrificial sessile drops, we initially used pure water. However, pure water was not suitable because of its high vapor pressure (2340 Pa at 20 °C) and consequently its fast evaporation. With respect to the printing of membranes, the printed droplets should not evaporate within 10 min. Pure ethylene glycol has a much lower vapor pressure than water (5.3 Pa at 20 °C) but is soluble in chloroform and for that reason cannot be used as an ink for printing the sessile drops. Thus, we finally settled on a mixture of water and ethylene glycol (70 vol% ethylene glycol) as ink. The mixture of water and ethylene glycol that was used shows the best compromise between volatility and solubility (Table 1), and the printed sessile drops were found to be stable against evaporation for at least 10 min and did not dissolve in chloroform. Finally, other physical properties such as viscosity show that the mixture is printable with an inkjet printer. The printer was set to deposit individual droplets of 10 pL volume each onto the substrate following a predefined pattern of pixels within a rectangular lattice. To vary the volume of the sacrificial sessile drops, individual droplets were printed into arrays of neighboring pixels at such a close distance that they finally merged. As an example, the pattern used for the deposition of sessile drops consisting of 24 individual droplets is shown in Figure 2a. In Figure 2b, the positions of individually deposited droplets are represented by circles of a hypothetical diameter that could be expected if no fusion occurred. As can be seen, these hypothetical footprints within each array overlap, thus the desired fusion of these 24 individual droplets into a single sacrificial template is highly probable. Patterns used for the generation of smaller sacrificial templates were composed of corresponding smaller arrays that were in general designed to resemble as close as possible a filled circle. Using this procedure, sacrificial templates were inkjet printed, and the above-described process was used to obtain membranes that initially adhered to the aluminum foil but were easily separated
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Figure 2. Microsieve manufactured by inkjet technology (240 pL per cast drop): (a) print pattern (bitmap file) where each black pixel indicates the position of an individual droplet to be deposited by the printhead, with each pixel having a width of 20 µm; (b) same pattern as shown in part a, where each circle indicates the position at which an individual droplet is placed and its expected footprint assuming no fusion of the droplets; these hypothetical footprints overlap, thus fusion into larger sessile drops is to be expected; (c) overview of polymer membrane finally obtained (light microscopy, top view, top illumination); and (d) detail showing two pores from the bottom side (SEM image).
from it. After separation from the substrate, the membranes were self-supporting, stayed flat if released, and were able to sustain gentle pulling and bending by hand. A close-up microscopy image of such a membrane is shown in Figure 2, parts c (an overview seen from the top) and d (an enlarged image seen from the bottom) and reveals that indeed pores were formed. A comparison between Figure 2, parts a and c shows that each of the arrays of individual droplets indeed merged into a single template and that the spacing between these arrays was large enough to prevent coalescence beyond the borders of each array. The center of each pore is identical to the center of each of the arrays of the predefined pattern. Thus, by printing arrays one can accurately position pores in the membranes. As required for a microsieve, the pores are uniform in shape and size, and the pore diameter is in the same size range as the membrane thickness. Deviating from the pixelated pattern used as the input file, we find that the pores have a smooth, round appearance. The enlarged SEM picture of single pores (Figure 2d) shows that the pore walls are smooth and concave. This shape is in accordance with the assumption that the individual printed droplets merged to form a single sessile drop with a circular footprint that served as templates for the pores. It is very likely that the centers of the holes of the membrane are at exactly the same position as the centers of the printed patterns. We cannot completely exclude, however, that either the merging of individual droplets or the application of the polymer solution caused a systematic shift of all positions. Given the fact that there usually is hysteresis between the advancing and receding contact angles, it is reasonable to expect that a process involving a lateral movement of the sessile drops finally would give rise to noncentrosymmetric pores, showing a receding contact angle on one side and an advancing contact angle on the other. Because we observe centrosymmetric pores (Figure 2d), we consider a lateral movement of the sessile drops during the production
process unlikely. Figure 2d reveals as well that the shape of the pore is more complicated than a simple spherical section. Whereas most of the pore resembles a section of a sphere, there is a thin polymeric lamella or “iris” at the top of the pore. If polymer solutions are concentrated to dryness, then one commonly observes the formation of a solid or highly viscous skin at the surface, well before the remainder of the solution is solidified. This skin formation often is responsible for structures formed by drying polymer solutions, which deviate significantly from the shape expected from the equilibrium capillary surfaces of liquids.17 When the polymer solution is applied to the substrate, the volume per area is high enough to cover the sessile drops completely. During evaporation of the chloroform, the layer thickness shrinks, and the final thickness calculated from the volume of polymer per area is smaller than the height of the sessile drops. Thus, at some point the sessile drops have to penetrate the upper interface. It is very likely that at this point a skin has been formed that is only partially displaced outward as a result of the penetration of the drop through the upper interface. Thus the “iris” that is visible in Figure 2d most probably is reflecting this polymeric skin. As can be seen from the overview pictures (Figures 2c and 3), inkjet printing enables us to position the templating drops as well in a predefined hierarchical pattern. Areas fully covered with dense arrays of porous and nonporous areas can be individually arranged into the membrane. In this way, it is possible to combine the advantageous properties of microsieve filtration and supportive structures that yield mechanical stability. The membranes shown in Figures 2 and 3 are uniform in thickness. It is worth pointing out that an additional level of hierarchy may easily be added in the same process by using the inkjet printer in a conventional way to print thick supportive structures on an even larger scale on top of the structures shown here.
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Figure 3. Microsieve manufactured by inkjet technology with porous and nonporous areas (light microscopy, top view, top illumination).
Figure 4. Model of a sessile drop used as a template for a pore in a polymer membrane.
Figure 5. Pore diameter vs drop volume.
To use these membranes as filters for liquids or gases, it is important to control the pore size. The printed sacrificial sessile drop consists of a multiplicity n of single inkjet-printed droplets, and thus it is comparatively easy to vary the volume of the sessile drop. To obtain a rough estimate of the expected pore diameter, we use the following model: we consider the liquid sessile drop geometrically to be a spherical cap with a static contact angle φ and a height H that is (partially) embedded in a liquid layer of thickness h (Figure 4 and Supporting Information). According to this simple model, the theoretical final pore diameter d can be deduced by simple geometric considerations, which are given in detail in Supporting Information, from its height H and the contact angle at the liquid/liquid/solid interface φ and is given by
d)2·
( 1 - Hcos φ ) - (1 - Hcos φ - H + h) 2
2
(1)
The height of the sessile drop is proportional to the cubic root of its volume V:
(
H) 3
3V 3 π -1 1 - cos φ
)
(2)
In doing so, we neglect the influence of gravity on the shape of the sessile drop, which can safely be done in the size range
in which we are interested.20 We neglect as well, however, the need to balance tensions at the liquid/liquid/gas three-phase contact line and, maybe even more important, the skin formation and resulting complex shape distortions that usually occur if polymer solutions are dried.17 The first of these two neglected effects will give rise to pores that are wider and the second one, to pores that are not as wide as predicted by our simple estimation. Figure 4 and eqs 1 and 2 suggest that the pore diameter is adjustable by changing the volume of the sessile drop or the height of the surrounding polymer film. The polymer height in principle is adjustable by a variation of the PMMA concentration in the chloroform. More straightforward is a variation of the volume of the sessile drop that can easily be done by applying various numbers of single droplets per sessile drop. Thus, we conducted a series of experiments by printing sessile drops that consisted of 1 to 24 single droplets (i.e., their theoretical volume ranged between 10 and 240 pL) while keeping the amount and concentration of the applied polymer solution and thus the membrane thickness constant. (The membrane thickness was 21 µm.) In Figure 5, the experimentally obtained pore diameters are plotted versus the calculated volume of the sessile drop and compared to the theoretical prediction.21 The pore size increases with increasing drop volume. The mean values for the pore size range between 19 µm (V ) 10 pL) and 86 µm (V ) 240 pL). With the exception of the first three data points, the measured pore sizes are astonishingly close to the theoretical curve calculated from the simple model using eqs 1 and 2. Deviations are probably based on the additional forces discussed above, but they might also be due to a volume reduction of the sessile drops arising from partial evaporation. It is conspicuous that the sessile drops in particular, which consist of few droplets, cause larger pores than calculated. The reason for this is likely the so-called first drop problem (i.e., the first drops of an inkjet firing sequence are inconsistent in their formation).22 Thus, the real volumes of the sessile drops corresponding to the first three data points in the diagram are probably significantly larger than calculated by simply summing the number of drops. With the current setup, it is possible to prepare microsieves with pore sizes down to 20 µm. The minimal pore size depends mainly on the droplet size of the inkjet printhead. Thus, the use of a printhead or a printing system that generates smaller droplets would lead to smaller pore sizes. For the size selectivity of the membrane, it is crucial to obtain a narrow distribution of pore sizes. Figure 6 displays the diameter distribution of 150 pores imprinted by sessile drops with a volume of 130 pL each. The pores were generated during one printing process. The mean value is 58.7 µm, and the standard deviation is 2.6 µm (4.4%). The fluctuations arise mainly from the inhomogeneity of the polymer surface and the evaporation of the cast drops. In our experiments, the polymer solution was applied (20) Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces,; Wiley-VCH : Weinheim, Germany, 2003; pp 10-12. (21) To calculate the theoretical prediction, a value for the contact angle φ is needed. We assume that the moment the drops are printed their shape is consistent with the advancing contact angle air/aqueous phase/substrate. When the drops are covered with the polymer solution, the contact angle might change to the receding contact angle polymer solution/aqueous phase/substrate if this is larger than the original one obtained during printing or to the advancing contact angle polymer solution/aqueous phase/substrate if this is smaller than the original one. To be precise, this contact angle needs to be estimated for exactly the composition at which the polymer solution vitrifies or forms a gel. This is not available. Thus, as an approximation we choose to use the contact angle of chloroform/aqueous phase/substrate. The advancing contact angle of air/aqueous phase/substrate is between the advancing and the receding contact angle of chloroform/aqueous phase/substrate. We thus assume that the contact line is pinned during application of the polymer solution and thus use the advancing contact angle of air/aqueous phase/substrate for our calculations. (22) Dong, H.; Carr, W. W. ReV. Sci. Instrum. 2006, 77, 085101.
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Figure 7. Pore diameter drift for pores made of 240 pL sessile drops. Figure 6. Pore size distribution of 150 pores made of 130 pL sessile drops.
manually: The PMMA solution starts flowing from one location on the substrate. The layer thickness and evenness depend only on its flowing characteristics. The resulting polymer height deviates by (4.5 µm from its mean value. From eqs 1 and 2, it follows that for cast drops with V ) 130 pL and a polymer layer with h ) 20 ( 4.5 µm the pore size varies between 61.8 and 73.1 µm (mean value 68.3 µm). The calculated pore size deviations due to the polymer height variations are comparable to the experimentally obtained variations for the 150 pores in Figure 6. This inhomogeneity in membrane thickness could be reduced by applying the polymer solution using a doctor blade or printing it with a second inkjet printhead. The printed sessile drops stay stable on the substrate for a few minutes before being embedded in the polymer solution. Because of the high content of ethylene glycol in the mixture, the evaporation of water is slow. However, it is not completely suppressed. During the time gap tgap between the printing of the sessile drops and the application of the polymer solution, evaporation of the sessile drops occurs. Figure 7 shows this effect for pores made of 240 pL sessile drops. There is a drift in pore diameter of about 1 µm/min. This problem could be reduced by a faster printing system, by the use of a second printhead that applies the polymer solution with a constant time gap following the printing of the sessile drops, or by an intelligent design of the print pattern that reduces the printing time, respectively. The intelligent design of print patterns means that the printed dots should be preferably arranged in the x direction (moving direction of the printhead) and not in the y direction (moving direction of the substrate). For a printout of a line of dots in the x direction and using one nozzle, the DMP printhead makes one movement in the x direction. However, the same pattern in the y direction leads to as many printhead
movements as dots that have to be printed and, consequently, to a much longer printing time. By printing the polymer solution with a secondary printhead synchronously with a constant time gap following the printing of the sessile drops, this evaporation effect could be avoided completely.
Conclusions We have developed a new technology for the preparation of structured polymer microsieves. Inkjet printing was used as a tool to deposit sacrificial sessile drops as templates for pores. The position of each pore is easily selectable through the print pattern. To counteract the fast evaporation of pure water, a mixture of water and ethylene glycol (30/70 vol%) was used as an ink to print the sessile drops. It was possible to vary the pore size through the variation of the polymer height and the drop volume. Pore sizes between 19 and 86 µm were achieved. Pore sizes below 10 µm seem to be possible, and a high uniformity of pore diameters is achievable. To increase the mechanical stability, a hierarchical structure could be implemented by arranging porous and nonporous areas. Acknowledgment. We thank M. Hietschold, G. Baumann, M. Knieriem, and S. Schulze, Department for Solid Surfaces Analysis, Chemnitz University of Technology, for support in scanning electron microscopy and S. Spange and F. Riedel, Department for Polymer Chemistry, Chemnitz University of Technology, for support in conducting gel permeation chromatography. Supporting Information Available: Sketch of the geometrical parameters of the embedded sacrificial sessile drop that are used in the calculations. This material is available free of charge via the Internet at http://pubs.acs.org. LA802860N
