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Biomed Microdevices (2015) 17: 4
DOI 10.1007/s10544-014-9913-x
Simple and low cost integration of highly conductive
three-dimensional electrodes in microfluidic devices
Srinivasu Valagerahally Puttaswamy & Peng Xue &
Yuejun Kang & Ye Ai
Published online: 28 January 2015
# Springer Science+Business Media New York 2015
Abstract This work presents a fast, simple, and cost-effective
technique for fabricating and integrating highly conductive
3D microelectrodes into microfluidic devices. The 3D electrodes are made of low cost, commercially available conductive adhesive and carbon powder. The device can be fabricated
by a single-step soft lithography and controllable injections of
a conductive composite into microchannels. Functioning of
the microfluidic device with 3D electrodes was demonstrated
through DEP particle switching as an example for a wide
range of microfluidic applications.
Keywords Microfluidics . Low-cost fabrication . 3D
Microelectrodes . Dielectrophoresis . Particle manipulation
1 Introduction
Three-dimensional (3D) microelectrodes offer numerous advantages in a wide range of microfluidic applications related
to electrically driven manipulation and detection of particles
and biological cells compared to traditional planar electrodes.
Notably, 3D electrodes are more advantageous than planar
electrodes for dielectrophoretic (DEP) manipulation of bioparticles. Electric fields generated by planar electrodes rapidly
Electronic supplementary material The online version of this article
(doi:10.1007/s10544-014-9913-x) contains supplementary material,
which is available to authorized users.
S. V. Puttaswamy : Y. Ai (*)
Pillar of Engineering Product Development, Singapore University of
Technology and Design, Singapore 138682, Singapore
e-mail: [email protected]
P. Xue : Y. Kang
School of Chemical and Biomedical Engineering, Nanyang
Technological University, Singapore 637459, Singapore
decay from the electrode surface, leading to inconsistent DEP
responses along the channel height direction (Xuan et al.
2010; Li et al. 2014). 3D electrodes can generate uniform
electric fields along the channel height and accordingly improve the efficacy of DEP manipulation technique. As a result,
3D electrodes have been productively employed in
microfluidic devices for various DEP applications, for example particle focusing (Choongho et al. 2005; Wang et al.
2007), particle trapping (Martinez-Duarte et al. 2010), particle
electro-rotation (Han et al. 2013; Benhal et al. 2014) and cell
separation (Park et al. 2005; Lewpiriyawong et al. 2011; Kang
et al. 2009; Li et al. 2013). Electrode design is also crucial for
cell lysis via electroporation. With the application of 3D electrodes, cells can experience a much more uniform electric field
during electroporation, which can substantially increase the
lysis efficiency compared to planar electrodes (Fox et al.
2006; Lu et al. 2006). Similarly, high aspect ratio 3D electrodes have been employed for electrofusion of liposomes
and cells with low power consumption (Tresset and Takeuchi
2004; Hu et al. 2011). 3D electrodes are also more preferred
than planar electrodes in microfluidic impedance cytometry
(Gawad et al. 2001). Impedance sensing by planar electrodes
highly depends on the cell position from the electrode surface,
which requires extra 3D focusing techniques to ensure a consistent cell translocation through planar electrodes (Watkins
et al. 2009). 3D electrodes can significantly reduce the positional dependence of cells in impedance cytometry, and thus
avoid complicated 3D focusing techniques (Ayliffe et al.
1999).
Different approaches for fabricating and integrating 3D
electrodes in microfluidic devices have been proposed.
Electroplating a thin metallic seed layer is a traditional technique to fabricate 3D electrodes integrated with
microchannels (Voldman et al. 2002; Wang et al. 2007). This
fabrication procedure involves photolithography, metal deposition and electroplating techniques, which require
4 Page 2 of 5
cumbersome processes, expensive material, expensive and
specialized equipments inaccessible to many research groups.
Doping of silicon (Hu et al. 2011) and pyrolysis of SU-8 photoresist (Martinez-Duarte et al. 2010; Jaramillo et al. 2010)
have also been used to fabricate 3D electrodes using special
recipes. Precise positioning of microfabricated conductive
strips (Kang et al. 2009) or conductive microspheres (Li et al.
2013) at the sidewall of microchannels has also been implemented to integrate 3D electrodes into microfluidic devices.
In recent years, there has been a growing interest in developing simple and accessible fabrication techniques for the
integration of 3D electrodes into microfluidics. Low melting
point metal alloy has been used to fabricate 3D electrodes in
microfluidic systems (So and Dickey 2011; Hallfors et al.
2013). However, it requires a precise control of temperature
during and after the fabrication process to maintain the quality
and functioning of electrodes. More recently, conductive polymer composites have been developed and employed as materials for 3D electrodes in microfluidic devices. In this technique, nanoparticles such as silver (Lewpiriyawong et al.
2010), carbon black (Deman et al. 2011; Blau et al. 2011;
Mustin and Stoeber 2012), carbon nanotubes (Pavesi et al.
2011), copper flake and nickel (Han-Sheng and Steven
2009; Li et al. 2010) are dispersed in polydimethylsiloxane
(PDMS), an insulating polymer material widely used in the
fabrication of microfluidic devices. Conductivity of these
polymer composite materials highly depends on the concentration of conductive nanoparticles dispersed in PDMS. In
order to achieve a high conductivity, the filler concentration
is maintained very high, resulting in a highly viscous composite. Therefore, fabrication of 3D electrodes requires sophisticated patterning of paste-like composites, as well as precise
alignment when integrated with microchannels. It is also
worth mentioning that uniform dispersion of nanoparticles in
PDMS is also crucial to synthesize highly conductive composites, which is typically implemented by multi-step, timeconsuming, chemical and physical dispersion techniques. In
particular, physical dispersion may involve some specialized
tools including high-speed shear mixer and ultra-fine ball
miller (Khosla and Gray 2012).
In this work, we present a fast, simple and low cost method
to reliably fabricate highly conductive 3D microelectrodes to
overcome current limitations. The conductive composite for
electrode fabrication is a mixture of silver conductive adhesive
and carbon nanopowder. Carbon nanopowder is mainly used
to increase the viscosity of the conductive composite to precisely control its flow in microfluidic channels. A channel
filled with conductive composite after complete curing serves
as a single 3D electrode. As the channel for composite injection is fabricated together with the fluidic channel, alignment
of 3D electrodes with respect to the fluidic channel is avoided.
Therefore, the device fabrication is a single-step soft lithography process with controllable injections. Compared to
Biomed Microdevices (2015) 17: 4
existing PDMS-based conductive 3D electrodes, the electrical
conductivity of our 3D electrodes is much higher as both the
two constituents are highly conductive. In addition, our method does not require complicated mixing procedure, which is
however critical for PDMS-based conductive composites. Finally, our conductive composite is injectable because of a
reasonable viscosity; while PDMS-based conductive composites are highly viscous and typically require sophisticated patterning for the fabrication of microscale 3D electrodes. A
microfluidic device with a 3D electrode array was fabricated
and demonstrated by negative DEP (nDEP) particle
switching. The fabricated microfluidic device can be further
applied for a wide variety of microfluidic applications involving particle manipulation and cell detection.
2 Device fabrication with integrated 3D electrodes
The microfabrication process of 3D electrodes integrated with
a microchannel is summarized and illustrated in Fig. 1. A
master mold made of a negative photoresist (SU-8 25,
MicroChem Corp., USA) with a height of 40 μm was fabricated on a silicon substrate using a standard photolithography
process (Fig. 1a). PDMS (Sylgard 184, Dow Corning Corp.,
USA) pre-polymer was mixed with its curing agent at a weight
ratio of 10:1, and then poured carefully onto the master mold
(Fig. 1b). After degassing in a vacuum chamber, PDMS mixture was cured by heating in an oven at 70 °C for 4 h. The
PDMS layer was peeled off from the master mold, and all the
inlet and outlet ports were punched with a 0.75 mm diameter
puncher (Fig. 1c). Subsequently, the cured PDMS layer was
permanently bonded onto a glass slide immediately after applying oxygen plasma treatment (Harrick Plasma Inc., USA)
for 60 s (Fig. 1d). This plasma treatment also makes the channel wall hydrophilic, which is favorable for introducing aqueous samples into the device. The complete device was connected to an external fluidic system via silicone tubes.
Two simple steps were followed to fabricate conductive 3D
electrodes. At first, silver conductive adhesive (Electrolube,
Singapore) and carbon nanopowder (<50 nm, Sigma-Aldrich
Pte. Ltd., Singapore) were mixed at a weight ratio of 10:1 and
stirred mechanically for 60 s. Secondly, the prepared mixture
was drawn into a plastic syringe and injected into the electrode
channels with a controlled flow rate (Fig. 1e). The addition of
carbon nanopowder maintains a reasonable viscosity of the
mixture so that the viscous effect can dominate over the inertial effect. Therefore, the injection can be stopped almost instantaneously, which enables a controllable stop of the mixture flow at the tip of the electrode channel. Therefore, the
electrode channel was entirely filled by the conductive mixture that was solidified with solvent evaporation immediately
after stopping injection to form desired 3D electrode (Fig. 1f).
The entire process takes less than 3 min to complete the
Biomed Microdevices (2015) 17: 4
Fig. 1 Schematic diagram of the
fabrication process. a SU-8
master mold fabricated on a
silicon substrate. b PDMS
polymer poured on the master
mold. c Cured PDMS with
microchannel patterns peeled
from the master mold. d
PDMS/glass bonding after
plasma treatment. e Injection of a
conductive composite into
electrode channels with a
controllable stop position. f
Fabricated 3D electrodes
integrated with a microchannel.
The PDMS layer is shown
transparent in step b and sketched
as open in subsequent steps to
clearly illustrate the fabrication
process
Page 3 of 5 4
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(b)
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fabrication of a single electrode. The solvent evaporation may
cause the 3D electrode to slightly shrink, which however will
neither compromise its conducting function nor cause any
liquid leakage problem. Figure 2a illustrates the fabricated
microfluidic device with eight 3D electrodes, and Fig. 2b
shows the microscopic image of a single 3D electrode in contact with the main channel.
Fig. 2 a Photograph of the
fabricated microfluidic device
with 3D electrodes. b
Microscopic image of a single 3D
electrode in the fabricated device.
c Schematic illustration of the
working principle of DEP
switching
3 Electrical characterization of 3D electrodes
A single straight microchannel was designed and fabricated to
characterize the electrical property of 3D electrodes. The conductive mixture was injected into the straight channel
(5000 μm long, 200 μm wide and 40 μm high) through the
inlet and it was allowed to fully solidify for a few minutes.
4 Page 4 of 5
Conductive probes were placed at the inlet and outlet served to
perform electrical measurement of the fabricated 3D electrode. The averaged value of electrical resistivity is 79.2±5×
10−6 Ω•m, which is as low as most metallic electrodes, because conductive silver and carbon nanoparticles are the major
constituents in the mixture. The electrodes are observed to be
intact under a wide range of flow rates and electrically stable
over different electric field conditions commonly used in
microfluidic applications. The electrical resistivity of the fabricated 3D electrodes was measured again after storing at
room temperature for 3 months to test its reliability. It was
found that the change in the electrical conductivity is less than
2 %. Highly conductive 3D electrodes are suitable for applications involving sensing or detection, which is a major drawback of some other reported 3D electrodes (Pavesi et al. 2011).
4 DEP particle switching
In order to validate the usability of the fabricated 3D electrodes, the device was applied for nDEP particle switching,
as depicted in Fig. 2c. A slower particle sample and a faster
sheath flow are introduced into the device through two different inlets. As a result, randomly distributed particles are focused to flow along the channel wall before entering the electrode region. When AC electric fields are generated across the
channel, particles exposed to the field experience an nDEP
force, which tends to push them away from the higher electric
field region near the electrode. Therefore, flow-through particles can be deflected to different outlets depending on the
magnitude of electric field applied.
Prior to the particle switching demonstration, the main
microfluidic channel was washed with 1 % surfactant of
Pluronic F-127 (Sigma-Aldrich Pte. Ltd., Singapore) for
30 min to prevent particle adhesion onto the channel wall.
Fluorescent polystyrene microbeads with a diameter of
10 μm (Polyscience Inc., USA) were uniformly suspended
in DI water by sonication. The particle sample and sheath flow
(DI water) were introduced into the channel by two separate
syringe pumps (New Era Pump Systems, USA) at a flow rate
of 0.2 μL/min and 0.6 μL/min, respectively. AC signals, generated by a function generator (Tektronix, USA) and amplified
by a power amplifier (OPHIR RF, USA), were applied on the
electrode array to generate electric fields across the main channel. Motion of microbeads was captured and recorded by a
CCD camera installed on a fluorescence microscope (Leica
Microsystems, Germany).
When the electric field was not activated, the randomly
distributed microbeads at the inlet were focused along the
top channel wall due to the faster sheath flow (Fig. 3a) and
continued to move towards outlet I as a result of the laminar
flow. When 40 Vpp AC signals at 5 MHz were applied on the
electrode array, the Clausius-Mossotti factor of 10 μm
Biomed Microdevices (2015) 17: 4
(a)
(b)
I
II
(c)
100 µm
III
Fig. 3 Superimposed trajectories of 10 μm fluorescent microbeads under
different electric fields. a No electric field. b The applied voltage is 40
Vpp at 5 MHz. c The applied voltage is 60 Vpp at 5 MHz. The particle
flow with a flow rate of 0.2 μL/min enters the electric field along the
sidewall of the main channel due to the hydrodynamic focusing by a
sheath flow with a flow rate of 0.6 μL/min
polystyrene microbeads suspended in DI water is −0.47, indicating an nDEP response. Accordingly, the nDEP force acting
on flow-through microbeads repelled them away from the
sidewall to move into outlet II (Fig. 3b). nDEP response was
used instead of positive DEP (pDEP) response in the demonstration, as nDEP force repels the particles away from electrodes. On the contrary, pDEP force attracts the particles towards their surface resulting in accumulation of particles
around electrodes even after turning off the electric field,
which should be avoided in particle switching. When the voltage amplitude was increased to 60 Vpp at the same frequency
(5 MHz), the nDEP force acting on microbeads became even
stronger, and further repelled the microbeads away from the
sidewall to move into outlet III (Fig. 3c). The nDEP particle
switching is shown in the supporting video.
5 Conclusions
We have developed and demonstrated a simple method to
fabricate and integrate highly conductive 3D electrodes into
microfluidic device, which does not require further
microfabrication steps apart from soft lithography technique
for the fabrication of microfluidic channels. The materials for
electrode fabrication are available at low cost and easy for
preparation. This method requires less than 3 min for each
electrode fabrication by a controllable injection of conductive
composite into a microchannel. The electrical conductivity of
the fabricated 3D electrode is comparable to conventional metallic electrodes, which may replace metallic planar electrodes
for rapid concept validation without accessing expensive deposition equipment. nDEP switching of microbeads using a
Biomed Microdevices (2015) 17: 4
3D electrode array has been demonstrated by the fabricated
microfluidic device. The device with 3D electrodes could be
further used in a wide variety of microfluidic applications, for
example, particle manipulation, cell lysis, cell fusion, and impedance cytometry for single cell analysis. Conclusively, this
technique does not need expensive materials, minimizes the
use of microfabrication facility, avoids tedious alignment at
microscale, and is thus suitable for low cost microfluidic applications using 3D electrodes.
Acknowledgment This work was supported by SUTD-MIT International Design Center (IDG11300101) and TL@SUTD Seed Grant (IGDS
S14 02011) awarded to Y.A.
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