TECHNICAL NOTE
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Capillary inserts in microcirculatory systems{ Javier Atencia and David J. Beebe* Received 4th October 2005, Accepted 6th January 2006 First published as an Advance Article on the web 20th January 2006 DOI: 10.1039/b514068d Microfluidic loops (i.e. closed fluid paths) pose specific practical challenges such as priming, introducing analytes or reagents in a controlled way and sampling products. In this technical note we address these three issues using a removable part of the microchannel that we call a ‘capillary insert’.
Introduction and theory Closed fluid paths are ubiquitous in nature where circulatory systems are essential to many organisms. Microfluidic loops are being employed for processes that need cycling such as PCR applications,1–3 enhanced sensing e.g. exposing a sample several times to a sensor,4 and for microfluidic controlled cell culture.5–7 Here we consider microfluidic loops with internal pumps, so that once filled they can function as self-contained systems. Microfluidic loops however have specific operational challenges due to the dominant phenomena at the microscale. A major challenge is priming or filling a microfluidic loop8 in part because of the difficulties in avoiding bubble formation. An additional challenge is sample handling. Microfluidic loops could enable processing of very small samples, but a convenient method of handling samples is required. There are several approaches to avoid bubble formation when filling a microfluidic loop. The first approach is shown in Fig. 1(a). The liquid is introduced through inlet A filling simultaneously both branches until it reaches the junction towards the outlet B. If the liquid reaches the outlet through one of the branches first, a bubble will form in the other branch clogging the flow. At the macroscale, tipping the channels to elevate the outlet might eliminate the bubble via gravity. However, at the microscale the force required to move the bubble is related to the capillary forces. The contact angle h is initially the same at both liquid gas interfaces with no difference in external pressure (Pa = Pb), see Fig. 2. However, if Pa ? Pb, the contact angle at both sides change, yielding a pressure that opposes movement. The maximum opposing pressure is given by the Laplace equation: P0 = cLG |cos ha 2 cos hr|/r
apply the critical pressure to the bubble in Fig. 1(a), that pressure must be applied between A and B. Thus, the same pressure would be applied to branches (X & Y) of the microchannel network. While the critical pressure in branch X
Fig. 1 (a) The fluid is introduced through the inlet of the microfluidic loop. If the liquid does not reach the outlet through branches X and Y simultaneously, one of them (Y in the figure) shortcuts the microfluidic circuit blocking the other branch (X) with an air bubble. (b) a removable capillary insert to transform a single channel into a microfluidic loop avoiding the previous filling problems.
(1)
where r is the radius of the capillary, cLG the surface tension at the liquid/gas interface, ha the advancing contact angle, and hr the receding contact angle, see Fig. 2. Table 1 presents values of the static contact angles in capillaries of different materials, and the critical pressure to start moving a bubble. In order to 2142 Engineering Centers Building 1550, Engineering Drive, Madison, WI 53706, USA. E-mail:
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[email protected]; Fax: +1 (608) 2659239; Tel: +1 (608) 262 2260 { Electronic supplementary information (ESI) available: Demonstration of a two step mixing operation using a capillary insert to close microfluidic loops and handle samples. See DOI: 10.1039/ b514068d
This journal is ß The Royal Society of Chemistry 2006
Fig. 2 If there is no difference of pressure Pa–Pb, both sides of the bubble are symmetrical with the same contact angle h. Otherwise the bubble loses its symmetry with two different contact angles ha and hr and yields to a threshold of pressure that must be applied in order to be able to start moving the bubble.
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Table 1 Calculations of the critical pressure of a static bubble in a cylindrical capillary P0/Pa ha/u
hr/u
W = 0.1 mm
W = 1 mm
77 ¡ 0.9a 746 74.6 PDMS 113 ¡ 1.6a 95 ¡ 17b 394 39.4 Teflon 118 ¡ 47b c c 0 6 0.6 Glass 25 ¡ 1 a cLG = 72.3 dynes cm21 for water at 25 uC. Static contact angles ha and hr, values from ref. 9. b values from ref. 10. c values from ref. 11. W capillary diameter; P0 (in Pa). Critical pressure before the bubble starts moving, calculated with eqn (1).
would slowly start moving the bubble, in the branch Y the same pressure would lead to large undesired flows. A second approach to overcoming bubbles involves prefilling the loop with CO2 and then introducing the liquid. Thus, any CO2 bubble in the microloop will dissolve into the liquid.12 However, the dissolved CO2 may change the pH of the liquid and could be an undesired side effect for some applications. Hansen et al. proposed a third solution for a similar problem, consisting in pressurizing both inlet and outlet simultaneously to evacuate the air bubble out to atmosphere through semipermeable walls of PDMS.8 In this case filling times can be as long as minutes and the approach is only valid for permeable materials. An alternative approach, also limited to devices made from permeable materials, consists in immersing the whole microfluidic device in buffer solution and exposing it to vacuum for several minutes.13 Still, with any of these methods: (1) applying pressure above the critical pressure, (2) prefilling with CO2, or (3) applying pressure at all ports above the atmospheric pressure or submerging the device in liquid and creating a vacuum, other problems arise such as loading the analytes in the microchannel in a controlled way, and sampling without contamination. In this technical note, we present an approach that simplifies these operations. We utilize a removable portion of the microchannel that we call a ‘capillary insert’, both for filling microfluidic loops without bubbles and for introducing and collecting samples from the loop.
Fig. 3 Capillary insert filled with dye, with small droplets at the tips.
composed of a single microchannel which is easily filled with liquid. The capillary insert (U-shaped capillary tubing shown in Fig. 3) is filled with water (or sample fluid) and then is used to connect both ends of the microchannel closing the loop to create a circulatory system. Fig. 4 shows a demonstration of a two step mixing operation (simulating a two step reaction) using a capillary insert to close microfluidic loops and to handle samples. The flow in the microfluidic loops is generated by the rotation of an impeller externally driven by a magnet. In Fig. 4(a) a capillary insert filled with blue dye is used to close microfluidic loop prefilled with yellow dye (details of the platform in ref. 14). We overfilled both the capillary insert and the microchannel in
Experimental The capillary inserts were fabricated using Ø 1000 mm Tubing (PTFE 20 TW Cole Parmer, Vernon Hills, IL), with internal diameter Ø 500 mm. Filling was manually performed using a syringe with a blunt needle. Diluted blue food color (McCormick&Co., Inc) was used to track the delivery of sample from the capillary inserts. The dimensions of the impellers—stainless steel stir bars—are 4000 mm 6 800 mm 6 120 mm with a 300 mm diameter hole drilled in the center. The details of the fabrication of the single loop platform are given elsewhere.14
Results Here we use removable tubing that will form part of the microfluidic loop, as shown in Fig. 1(b). The microdevice is 576 | Lab Chip, 2006, 6, 575–577
Fig. 4 (a) A microfluidic loop prefilled with dye is closed with a capillary insert filled with blue dye. (b) The impeller in the first loop extracts the contents of the capillary insert, mixing them with the contents in the microfluidic loop. (c) The capillary insert is removed from the first loop, sampling the results of the first reaction, and is inserted in the second microfluidic loop. (d) The impeller in the second loop is activated, extracting the contents of the capillary insert in order to be analyzed or to perform a second reaction.
This journal is ß The Royal Society of Chemistry 2006
order to have small droplets at all the ports (1, 2, 3 and 4 in Fig. 1(b)). When the tips of the capillary insert were pressed into the ports of the microchannel the droplets coalesced together avoiding bubble formation. The blue dye from the capillary insert was extracted and mixed with yellow dye from the microfluidic loop by the impeller (simulating a first reaction in a hypothetical two step reaction). After recirculating the mixture several times, the capillary insert was removed from the first loop and inserted in a second loop prefilled with water. Subsequently the sample carried in the capillary was extracted and delivered in the second microfluidic loop by a second impeller. The second loop could serve for analyzing the sample or for performing a second reaction (ESI, video 1{).
Discussion We made the following observations throughout the experiments: (1) The fluidic connection was established immediately in all the experiments, even when we pulled out the capillary insert and connected it back again. The key is the initial coalescence between the droplets of the capillary insert with those at the microchannel (1–3 and 2–4 in Fig. 1(b)). The droplets are subjected to large capillary forces in equilibrium with the antiadherent forces to the hydrophobic material (PDMS). The coalescence between droplets starts when they touch each other and a microfluidic bridge forms connecting them. However, the initial radius of curvature at the bridge is very small, yielding large capillary forces that tend to increase coalescence only opposed by the inertia and viscosity of the liquid. The transient dynamics and small size of the droplets result in a favourable and fast coalescence between droplets (e.g. the coalescence of two water droplets15 of 1 mm diameter takes 4 ms). (2) Two per cent of the total volume of dye in the capillary was lost when the capillary insert was pressed inside the PDMS holes. In a future design the amount of sample lost could be further minimized by creating a permanent microfluidic connection just with coalesced drops, without pressing the capillary insert inside holes (i.e. without direct capillary insert to channel connection, but only a fluid connection). (3) The dispersion forces between the drops at the tips of the capillary insert were higher than the weight of the drops. The maximum size for a hanging droplet without detaching from the capillary insert can be calculated16 from cLG sin h = Vrg + Fadh, where V is the volume of the droplet, r is the density of the liquid, g acceleration of gravity, and Fadh the adhesion force between solid and liquid. However, although it is possible to use large droplets at the tips of the capillary insert, small droplets are preferred because they hold more strongly to
This journal is ß The Royal Society of Chemistry 2006
the tips, and smaller droplets imply smaller volume loss during the microfluidic connection.
Summary In this manuscript we present a simple solution for working with microfluidic loops which we believe will open the door to many applications based on microcirculatory systems. The capillary insert solves three major problems when working with microfluidic loops: (1) filling a loop, (2) introducing a sample in a loop without sample loss or dilution in connectors and tubing, and (3) extracting a sample and handling it at the macro scale. We believe that the capillary inserts and derivatives will prove to be useful not only in self-contained microsystems, but also as efficient means to insert, extract and handle samples in other microfluidic applications—as an interface between the microscale and macroscale worlds.
References 1 J. F. Chen, M. Wabuyele, H. W. Chen, D. Patterson, M. Hupert, H. Shadpour, D. Nikitopoulos and S. A. Soper, Anal. Chem., 2005, 77, 658–666. 2 P. K. Yuen, G. S. Li, Y. J. Bao and U. R. Muller, Lab Chip, 2003, 3, 46–50. 3 Z. Y. Chen, S. Z. Qian, W. R. Abrams, D. Malamud and H. H. Bau, Anal. Chem., 2004, 76, 3707–3715. 4 R. G. H. Lammertink, S. Schlautmann, G. A. J. Besselink and R. B. M. Schasfoort, Anal. Chem., 2004, 76, 3018–3022. 5 A. Sin, C. F. Reardon and M. L. Shuler, Biotechnol. Bioeng., 2004, 85, 359–363. 6 W. Gu, X. Y. Zhu, N. Futai, B. S. Cho and S. Takayama, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 15861–15866. 7 F. K. Balagadde, L. C. You, C. L. Hansen, F. H. Arnold and S. R. Quake, Science, 2005, 309, 137–140. 8 C. L. Hansen, E. Skordalakes, J. M. Berger and S. R. Quake, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 16531–16536. 9 T. R. Kyriakides, K. J. Leach, A. S. Hoffman, B. D. Ratner and P. Bornstein, Proc. Natl. Acad. Sci. U. S. A., 1999, 96, 4449–4454. 10 A. M. Schwartz, C. A. Rader and E. Huey, in Contact angle, wettability, and adhesion, ed. R. F. Gould, American Chemical Society, Washington, DC, 1964. 11 T. Cubaud and C. M. Ho, Phys. Fluids, 2004, 16, 4575–4585. 12 R. Zengerle, M. Leitner, S. Kluge and A. Richter, in Proceedings IEEE Microelectromechanical Systems. 1995, 29 Jan.-2 Feb. 1995, IEEE, Allschwil, Switzerland, Elsevier, Amsterdam, Netherlands, 1995, pp. 340–343. 13 J. Monahan, A. A. Gewirth and R. G. Nuzzo, Anal. Chem., 2001, 73, 3193–3197. 14 J. Atencia and D. J. Beebe, Lab Chip, 2005, DOI: 10.1039/ b514070f. 15 M. M. Wu, T. Cubaud and C. M. Ho, Phys. Fluids, 2004, 16, L51–L54. 16 T. C. Ku, J. H. Ramsey and W. C. Clinton, IBM J. Res. Dev., 1968, 12, 441–447.
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