TECHNICAL NOTE
www.rsc.org/loc | Lab on a Chip
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
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{ 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.
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