Computer Optics and Nanophotonics
Information Technology and Nanotechnology (ITNT-2016) 159
FABRICATION AND CHARACTERIZATION OF
PASSIVE MICROPUMP FOR MICROFLUIDICS
BASED DEVICES
Prerna Balyan
1,2
, Deepika Saini
1
, Supriyo Das
1
, Payal Verma
3
, Ajay Agarwal
1,2
1
CSIR – Central Electronics Engineering Research Institute, Pilani, India
2
Academy of Scientific & Innovative Research (AcSIR), New Delhi, India
3
Samara National Research University, Samara, Russia
Abstract. Point-of-care and low cost microfluidics platforms has found an ac-
celerated research focus. One of the essential elements of microfluidics is driv-
ing liquid flow in microchannels. Device discussed gives a way to pump liquid
passively through a microchannel. The present work represents fiber and paper
based passive micro-pumping of liquids through microfluidic devices. The po-
rous structure and network of capillaries inside the paper and fiber materials
support spontaneous liquid movement. Agarose gel coating is used with paper
in order to achieve variations flow rates. The effect of gel concentration on liq-
uid flow is studied. The concept can be used ubiquitously for microfluidics de-
vice application for its low-cost and is feasible to integrate with devices for low
resource settings.
Keywords: Microchannel, flow rate wicking.
Citation: Prerna Balyan, Deepika Saini, Supriyo Das, Payal Verma, Ajay
Agarwal. Fabrication and characterization of passive micropump for microflu-
idics based devices. CEUR Workshop Proceedings, 2016; 1638: 159-165. DOI:
10.18287/1613-0073-2016-1638-159-165
Introduction
The research in the field of MEMS and Microfluidics is being explored extensively. With
the successful development of devices such as microsensors [1-3], micro actuators, lab-on-
a-chip platforms, a transformation can be seen in the present technologies. Recently there
has been extensive research on paper and fiber based microfluidics [4, 5]. Paper and fiber
material has been reported as a convenient substrate for plethora of microfluidics applica-
tions [6, 7]. One of the essential elements of microfluidics is driving liquid flow in
microchannels. Due to scaling effects a large pressure force is required to move fluids
and/or suspended particles in microchannels. To move a fluid through microchannels
Computer Optics and Nanophotonics Prerna Balyan, Deepika Saini et al…
Information Technology and Nanotechnology (ITNT-2016) 160
various pumping mechanisms have been proposed in literature. A general classification
divides the micro-pumping based on active and passive methods. Active methods such as
electro-kinetic, electroosmotic, syringe pump, thermo-pneumatic etc. require external
energy/power source. On the other hand, passive methods of micro-pumping of liquids
utilize the surface properties and geometric effects at micro-scales such as capillary in-
duced pumping or manual hand-held-syringe.
While inherent capillary action inside micro-channels makes one-time flow through the
channel, or if the surface is wetting the capillary action will wet the channel spontaneously
once but to make the liquid flow continuously some external force is required. Xiaoze et
al. (2011) have proposed an evaporation-based Micropump and they have achieved con-
stant and continuous flow rates of ∼100 nl s−1. Researchers have used paper as the device
substrate hence liquid flows automatically, however for other microfluidics platforms
external or inbuilt micro pumping is required. Since materials with liquid absorption and
wicking capabilities have shown huge potential to be used in microfluidics area, various
modes to control and vary flow rates have been reported [8-11].
In present work the fluid flow is achieved via internal and external control. The internal
control consists of coating of hydrophilic material layer onto the microchannel, whereas
external control includes use of gel coated paper fixed in outlet to conduct a flow. The
coated paper and fiber materials are used that caused a negative pressure generation to
attain liquid flow in any microfluidic platform. Advantages offered by proposed method
is its simple fabrication and low cost, wide availability of pumping material, uubiqui-
tous use with broad range of flow rates achievable. Passive mode of operation elimi-
nates the external power requirements as well as aapplicability to both open and
closed channels.
1 Material preparation and fabrication
Desired flow rates are achieved by internal control and external control. In internal
control, channel surface modification causes a self-driven flow of liquid. External
control is achieved via connecting wicking material externally to the outlet of the
microfluidic channel. Work presented here uses liquid absorbing materials like cotton
fiber and cellulose paper to pump liquid through a polydimethylsiloxane (PDMS)
elastomer based microchannel. The microchannel is fabricated in PDMS substrate via
standard soft-lithography process, followed by plasma bonding of channel to glass
substrate. For liquid pumping 1 mm x 20 mm wide Whatman filter paper strips were
used as wicking substrate. The wicking strips were spin coated with Agarose gel.
The apparatus and method take advantage of the fact that liquids get absorbed/ wicked
spontaneously in some special type of materials. The properties and makeup of mate-
rial can be modified to obtain wide range of flow rates. A straight circular channel of
170 µm width and 2 cm length made up of PDMS was used for the proof of concept.
Other materials glass, Si, Polymers, metals can also be used to fabricate the device.
The outlet of the flowmeter was attached to the absorbing material and inlet was in-
jected with the test solution. As the device does not require a fixture and tubes for
flow injection, high magnification objectives could be used for the flow study. The
Computer Optics and Nanophotonics Prerna Balyan, Deepika Saini et al…
Information Technology and Nanotechnology (ITNT-2016) 161
liquid could flow continuously from inlet to outlet until the inlet was completely ex-
hausted. Introduction of more flow controlling channels upstream and/or could help
varying flow rates inside the main channel. The outlet wick creates a negative pres-
sure differential causing a continuous liquid flow. It is necessary to prefill the chan-
nel. If channel is hydrophilic (glass or silicon dioxide channels) the micro-channel get
wet spontaneously due to capillary action however a hydrophobic channel (PDMS)
needs to be filled. A hand held syringe may be used for channel priming with liquid.
Once the connection is established the sample analyte is introduced in the inlet reser-
voir. To measure flow rate either the liquid meniscus can be tracked or polymer beads
may be inserted and tracked. A video is recorded and frames can be generated using
any video analysis software. In the frames the beads/ meniscus crossing a graduation
mark is noted down and liquid velocity and flow rate is measured. Various absorbing
materials can be characterized for their liquid wicking ability by measuring the flow
rate of the meniscus/beads.
The flowrate variations were obtained by varying shape, size and concentration of gel
coating on the wicking strips. Different flow rates have been obtained at different
concentrations (in wt. %) of the agarose gel. The prepared liquid pumping material is
inserted inside the output reservoir directly (Fig. 1). Small particles (micro-beads or
blood cells) were added in very low concentrations and tracked for flow characteriza-
tion. The flow was analyzed under optical microscope; the video was captured with a
camera and was analyzed using ImageJ software. The method is very simple, cost
effective for microfluidics platforms in point of care diagnostics for low resource
settings applications.
2 Results and discussion
Paper and fiber based micropump carried out a continuous liquid flow through micro-
channels. Figure 1 depicts the overall scheme of liquid flow through a microchannel.
The flow pumping material was inserted in the outlet section of the microchannel.
The flow rate can be adjusted from very low to high value by changing the absorbing
material type, material makeup/constituents, material geometry, material physical
properties, material dimensions, and material assembly as well by devising new com-
binations of different materials.
Fig. 1. Flow channel scheme showing Inlet and Outlet reservoir connected through micro-
channel and a wicking material placed in the outlet reservoir
In this work following methods are used to obtain flow rate variations:
Width of the wicking strips: By increasing the size/diameter of fiber significant flow
rate variation was seen. Two sizes 1 mm x 20 cm and 5 mm x 20 cm wide wicking
strips were used for conducting liquid flow. The velocity of particles was found to be
Computer Optics and Nanophotonics Prerna Balyan, Deepika Saini et al…
Information Technology and Nanotechnology (ITNT-2016) 162
greater in case of wider wicking strip, Fig. 2. The reason may be availability of higher
surface area for wicking of liquid.
Fig. 2. Effect of width of wicking strip on solution flow. The flow rate is higher in wider sized
strips than the narrow strips
Fig. 3. Flow of polystyrene bead solution through the microchannel at different flow rates set
up by coating the pumping material with different agarose gel concentrations. Particle trajecto-
ry is visualized via subtracting image background using ImageJ software, (a) Uncoated Wick-
ing substrate, (b)
Wicking substrate + 1%
gel, (c) Wicking substrate + 5% gel and (d) Wicking
substrate + 10%. The flow rate decreases with the increase in gel concentration
(a)
(b)
(c)
(d)
Computer Optics and Nanophotonics Prerna Balyan, Deepika Saini et al…
Information Technology and Nanotechnology (ITNT-2016) 163
Agarose gel concentration: Agarose gel is a polymer with its constituent molecules
connected with abundance of hydrogen bonds. The concentration of gel amounts to
the pore size of the gel matrix which plays a crucial role in controlling the liquid flow.
Very high flow rates were observed with untreated wicking material. Uniformly sus-
pended polystyrene beads travelling at high speeds, appeared as long streaks during
the liquid flow (Fig. 3(a)). With increasing the gel concentrations for the coating the
beads travelled at slower rate (Fig. 3 (b)-(d)). This conforms to the potential of using
Agarose gel to vary the liquid flow rate through microchannels. Figure 4 represents
the average velocity of the tracked polystyrene beads through the microchannel.
Fig. 4. Effect of gel concentration on solution flow
Fig. 5. Measured contact angle for (a) uncoated PDMS surface (Contact angle 106.4°) and (b)
agarose coated PDMS surface (Contact angle < 40 ° (solution spreads immediately)
Internal Control: PDMS is widely being used for microfluidics applications; howev-
er its hydrophobicity limits the spontaneous flow of liquid. Coating of agarose gel
favors automatic flow of liquid through microchannel. The channel surface was spin
coated with agarose gel (Fig. 5(b)). The non-coated surface had a contact angle
around 106.5° (Fig 5(a)), while the coated channel showed an immediate spread of
liquid onto the surface (Fig. 5(b)). This coated layer of hydrogel causes immediate
liquid wicking through the walls of the channel, and since channel acts as a fine capil-
lary the wicking force generated at surface causes a bulk liquid flow.
In order to assess the conduction of flow. 1 µl of colored solution was introduced into
the inlets of coated and non-coated channels. A spontaneous liquid flow was observed
through the coated channel (Fig. 6(b)).