DOI: 10.1002/adfm.200500562
Smart Microfluidic Channels**
By Leonid Ionov,* Nikolay Houbenov, Alexander Sidorenko,Manfred Stamm, and Sergiy Minko*
1. Introduction
At a high surface-to-volume aspect ratio interfacial forces
may strongly contribute to mass transport and separation. This
effect is expressed in microfluidic devices.
[1–7]
Microscopic
chemical apparatuses (in other words, systems of microchan-
nels) that manipulate microquantities of liquid solutions allows
for substantial improvement of the efficiency of chemical
processes
[8–12]
for microanalytical applications,
[5,13,14]
high-
throughput devices,
[15,16]
and chemical processes on an indus-
trial scale.
[17]
Manipulation of liquids in small channels is very
important for the successful development of the rapidly grow-
ing field of microfludic sensors. One of the key problems in
these devices is the passage of liquids in a system of channels
that connect a number of microreactors. In these microreactors
reactant solutions are mixed, reacted, and analyzed. They
could also be separated into several flows (in different analyti-
cal schemes). A microfluidic laboratory uses a very similar
toolbox of chemical processes that is used for large-scale pro-
cesses: mixing, separation, dosing, extraction, filtration, heat-
ing, cooling, dialysis, and so forth. However, a substantial dif-
ference is that an appropriate modification of the channel
surface has a strong effect on chemical processes in microfluid-
ic devices.
The first analysis of the passage of liquids in narrow tubes
was published by Jamin.
[18,19]
A liquid drop in a capillary
(Fig. 1a) with a radius r forms meniscuses with angles h
1
and h
2
(h
1
can be equal to h
2
or they can be different, depending on
surface composition in the contact lines). The phases are desig-
nated as follows: 1 is the liquid, 2 is the fluid (liquid or gas),
and 3 is the solid; r
12
, r
23
, and r
13
are the interfacial tensions
between phases 1 and 2, 2 and 3, and 1 and 3, respectively. The
maximum pressure that can be withstood by a drop of liquid in
cylindrical capillaries is given by the expression:
P=2r
12
(cosh
R
– cosh
A
)/r, (1)
where P=P′ –P
0
,P′ is the external pressure, P
0
is the atmo-
spheric pressure, and h
R
and h
A
are the contact angles of the
receding and advancing ends of the drop, respectively, as
shown in Figure 1b. The external pressure P′ initially deforms
Adv. Funct. Mater. 2006, 16, 1153–1160 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1153
–
[*] Dr. L. Ionov, Dr. N. Houbenov, Prof. M. Stamm
Leibniz-Institut für Polymerforschung Dresden (IPF)
Hohe Strasse 6, 01069 Dresden (Germany)
E-mail: ionov@ipfdd.de
Prof. S. Minko
Department of Chemistry, Clarkson University
8 Clarkson Avenue, Potsdam, NY 13699-5810 (USA)
E-mail: sminko@clarkson.edu
Dr. A. Sidorenko
Bell Labs, Lucent Technologies
600 Mountain Avenue, Murray Hill, NJ 07974 (USA)
[**] The authors from IPF are grateful to DFG (SPP1164) and BMBF for fi-
nancial support. S. M. acknowledges support from the US ARO grant
W911NF-05-1-0339 and the NYSTAR Center for Advanced Materials
Processing at Clarkson University.
The walls of microfludic “smart channels” are coated by responsive mixed polymer brushes with a gradient of chemical compo-
sition. The concept of the smart channels is based on switching/adaptive behavior of the mixed polymer brushes where wetting
and nonwetting can be tuned upon interaction with liquids. This design of microfluidic channels brings new opportunities for
manipulating the passage of liquids in the channels. The developed approach allows us to fabricate three microfluidic elements
for separation, sensing, selection, and dosing microvolumes of liquids.
2
3
1
2
3
2
1
2
3
3
2
1
2
3
P P
P
PP
PP
σ
σ
σ σ
σ
σ
σ
σ
σσ
σσσσ
σσ
σσσσ
σ
σ
θ θ
θ θ
θ θ
θ θ
θ θ
θθ
r
σ
a)
b)
c)
Figure 1. Meniscuses formed by a liquid drop (1) in a cylindrical capillary
tube (3) in a fluidic medium (2): a) steady state, no flow, h
1
≈ h
2
; b) condi-
tions for moving the drop enforced by the external pressure P when
h
1
= h
R
, h
2
= h
A
, and h
R
< h
A
; c) conditions for moving the drop enforced by
the surface tension (r
13
and r
23
) gradient when h
R
> h
A
.
F
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L
L
P
A
P
E
R

the drop. Then the drop moves as soon as a critical value of
cosh
R
– cosh
A
is approached.
Various approaches were used to overcome the P′ value and
to guide liquid flow in microfluidic systems. For example,
Unger et al. developed an approach to fabricate a system of
on–off valves, switching valves, and pumps using soft lithogra-
phy. This method allows for the use of an external pressure to
control liquid flow in the microfluidic system.
[20]
A change in
the concentration gradient of species in the solution created by
electrochemical reactions, electroosmotic flow,
[21–24]
is widely
used to pump a liquid in narrow channels. A very interesting
example of the use of electrowetting to control liquid flow has
been demonstrated by Huh et al.
[25]
The authors used the
change of wetting properties of the substrate upon application
of an electric field to merge liquid streams on a hydrophobic–
hydrophilic patterned surface. Gallardo et al. used a gradient
of concentrations of surface-active species in the solution gen-
erated by electrochemical reaction to direct a liquid flow.
[26]
A
temperature gradient was used to direct a microscopic flow on
a selectively patterned surface.
[27]
Electric fields
[28]
and light
[29]-
can also be used to switch conformations of molecules on spe-
cially tailored surfaces and change the interfacial tension.
One of the many possible ways to manipulate drop passage
is the use of a gradient of wetting.
[30–38]
On a gradient surface
the surface tension ahead of the drop is different to that behind
the drop so that the advancing contact angle can be even small-
er than the receding contact angle (Fig. 1c). Thus the drop will
move in the direction of increasing wettability. A difference be-
tween h
R
and h
A
is typically around 10° or even bigger. Thus,
practically, the entire length of a capillary with a wetting gradi-
ent, which can force liquid to flow, is limited to several centi-
meters. This length may not be sufficient for many microfluidic
devices, however, the effect can be used for a local regulation
of flow or for the fabrication of valves or other smart de-
vices.
[34,35]
The smart effects are based on the local turning on
and turning off of the wetting gradient.
[39,40]
The interfacial ten-
sion can be changed precisely in front of or behind the liquid
drop by adsorption/desorption of surfactants.
[33]
Finally, the
chemical composition of the liquid drop can be used as an ex-
ternal signal to switch wetting.
[40,41]
Modification of the inner surface of walls of microchannels
with responsive thin films provides an alternative powerful
means to switch interfacial energy in response to external stim-
uli such as temperature
[42]
or pH.
[43–45]
For example, responsive
coatings grafted onto porous membranes were successfully
used to gate liquid flow.
[43,44,46,47]
In this paper we extend the
application of responsive coatings for the fabrication of smart
microfludic channels. We report here on the properties, fabri-
cation, and use of smart microfluidic channels based on respon-
sive gradient coatings constituted of mixed polymer brushes.
This approach offers new mechanisms for manipulating liquid
flow and separation, and for the self-adjustable smart dosing of
microvolumes of liquids.
Recently, thin polymer films constituted of two unlike poly-
mers tethered to the same substrate (mixed polymer brushes)
have been shown to be capable of switching wetting behavior
in response to external stimuli.
[41,48,49]
The switching mecha-
nism originates from the phase segregation of two polymers
when one of two polymers preferentially segregates to the top
of the brush.
[50]
The phase segregation can be tuned using ex-
ternal stimuli (solvent, pH, temperature). The mixed brush
prepared with a gradient of composition (ratio between two
unlike polymers) along the sample demonstrates a switching of
the wetting gradient.
[39,40]
In other words r
13
and r
23
vectors
(their magnitudes and directions) can be switched by a pH
change, for example. In this work we demonstrate several mi-
crofludic concepts that we have developed using the mixed
brushes with a composition gradient.
2. Results and Discussion
2.1. Microfluidic Separation
Separation of immiscible liquids in microquantities is an im-
portant step of the analysis of water-insoluble biomolecules.
Analysis of biological liquids explores an extraction of water-
insoluble substances using organic solvents. For example, the
analysis of holysterol can be performed using extraction with
organic solvents
[51]
(chloroform or toluene). In microfluidic de-
vices the extraction would be followed by a separation step in
which the organic solvent is separated from the water emulsion
and directed to the appropriate compartment for quantitative
measurements. We demonstrate here, how the separation of an
emulsion can be performed using the smart-channel concept.
An emulsion of two incompatible liquids enters a capillary with
gradually changing properties on the walls, for example, from hy-
drophobic on the left-hand side to hydrophilic on the right-hand
side. If liquid 1 and liquid 2 tend to interact preferentially with
hydrophobic and hydrophilic parts of the capillary, respectively,
the surface-tension gradient will force the separation of the liq-
uids. The liquids will flow in opposite directions as shown in Fig-
ure 2. Depending on the interaction of the liquids with the solid
substrate, wemay expect two situations. 1) The liquid–solid inter-
facial tension gradients for both liquids will be of opposite signs.
Each liquid will flow in the direction that corresponds to decreas-
ing liquid–solid surface tension. 2) The liquid–solid interfacial
tension gradient g will be formed for both the liquids in the same
direction. However, for one of two liquids the absolute value of
the gradient will be larger, for example g
1
>g
2
 Thus, if r
13
decreases in the x-axis direction, liquid 1 will flow in this direc-
tion filling up the channel and forcing liquid 2 to flow in the
opposite direction. Practically, for the separation of immiscible
water and an organic liquid, the water–solid interfacial tension
will contribute with a much larger fraction of the free-energy
change when compared to the organic–solid interaction term.
Thus, water will flow in the direction of the wetting gradient. The
organic liquid will be squeezed in the opposite direction.
The separation takes place if two liquids are in contact with
the gradient surface. The efficiency of the separation depends
on the ratio between the droplet size and the diameter of the
microfluidic channel. The larger the ratio is the better the sepa-
ration, because the droplets will be pinned by the surface
rather than trapped in the liquid flow.
1154 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 1153–1160
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L. Ionov et al./Smart Microfluidic Channels