Capillary flow of blood in a microchannel with differential
wetting for blood plasma separation and on-chip glucose
detection
M. Sneha Maria,
1,2
P. E . Rakesh,
1
T. S . Chandra,
2
and A. K. Sen
1,a)
1
Department of Mechanical Engineering, Indian Institute of Technology Madras,
Chennai 600036, India
2
Department of Biotechnology, Indian Institute of Technology Madras, Chennai 600036,
India
(Received 9 June 2016; accepted 3 September 2016; published online 22 September 2016)
We report capillary flow of blood in a microchannel with differential wetting for
the separation of a plasma from sample blood and subsequent on-chip detection of
glucose present in a plasma. A rectangular polydimethylsiloxane microchan nel
with hydrophilic walls (on three sides) achieved by using oxygen plasma exposure
enables capillary flow of blood introduced at the device inlet through the micro-
channel. A hydrophobic region (on all four sides) in the microchannel impedes the
flow of sample blood, and the accumulated blood cells at the region form a filter to
facilitate the separation of a plasma. The modified wetting property of the walls
and hence the device performance could be retained fo r a few weeks by covering
the channels with deionised water. The effects of the channel cross-section, expo-
sure time, waiting time, and location and length of the hydrophobic region on the
volume of the collected plasma are studied. Using a channel cross-section of
1000 400 lm, an exposure time of 2 min, a waiting time of 10 min, and a hydro-
phobic region of width 1.0 cm located at 10 mm from the device inlet, 450 nl of
plasma was obtained within 15 min. The performance of the device was found to
be unaffected (provides 450 nl of plasma in 15 min) even after 15 days. The purifi-
cation efficiency and plasma recovery of the device were measured and found to be
comparable with that obtained using the conventional centrifugation process.
Detection of glucose at different concentrations in whole blood of normal and dia-
betic patients was performed (using 5 ll of samp le blood within 15 min) to demon-
strate the compatibility of the device with integrated detection modules. Published
by AIP Publishing. [
http://dx.doi.org/10.1063/1.4962874]
I. INTRODUCTION
Diagnostic tests provide critical information about the health status of an individual,
thereby helping health care providers and patients to make the right medical decisions. Such
tests often provide objective, quantitative measurements that inform every stage of care—pre-
vention, detection, diagnosis, treatment, and successful management of health conditions.
Diagnostic test results, including blood tests, inform approximately 70% of medical decisions.
1
Blood is truly a window for the health of the body. Though blood testing is not a substitute for
a professional medical diagnosis, it can be used to assess the general state of health, viz., levels
of hormones, glucose, minerals, and vitamin deficiencies, confirm the presence of a bacterial or
viral infection, inspect organ function, or screen for certain genetic conditions such as cystic
fibrosis or spinal muscular atrophy.
2
Blood, a complex non-Newtonian fluid, is comprised of
two components—cells (red blood cells (RBC), white blood cells (WBC), and platelets) sus-
pended in plasma. Cells comprise 45.7% of blood while plasma occupies 54.3% of blood.
a)
Author to whom correspondence should be addressed. Electronic mail: ashis@iitm.ac.in
1932-1058/2016/10(5)/054108/15/$30.00 Published by AIP Publishing.10, 054108-1
BIOMICROFLUIDICS 10, 054108 (2016)
Separation of plasma from blood cells (plasmapheresis) is the prior step for further biochemical
analysis
3
since most of the blood analyses are based on optical detection techniques and the
blood cells need to be removed to decrease the interference with the optical path, thereby
increasing assay sensitivity and reliability.
4
The conventional method of centrifuge based separation of plasma commonly needs millili-
ters of blood and a labor-intensive handling process in addition to the bulky apparatus. This
makes it difficult to conduct self-help and low-cost inspection of drug and other metabolites in
blood and hence unsuitable for regular tests on patients, and thus the sample pre-treatment step
becomes a bottleneck of the assay process.
5
Use of microfl uidics technology for the sample
preparation step would miniaturize and enable easier integration with detection modules. The
point-of-care (POC) diagnostic tests will then be made simpler and overcome the difficulties
with sample handling, transportation, and storage. Such devices will increase the quality, repro-
ducibility, and reliability of the assay results.
3
Extensive reviews are available which detail
about various particle separation methods.
6–14
Also, several works report plasma separation
from blood through a variety of techniques. Although active separation methods of plasma sep-
aration
15–19
are efficient in terms of plasma recovery, employment of external fields including
electric, magnetic, and acoustic energy limits their portability and cost. Such methods also
require longer residence time, which brings down the speed and throughput. Passive methods of
plasma separation by making use of the inherent properties of the fluid and the suspended par-
ticles are simpler and gentler to the cells. These methods also exhibit high throughput and are
cost effective in terms of fabrication, labor, and power.
20
The passive separation devices may
include filters,
21,22
microchannel bend structures,
20,23,24
or microposts
25
and require external
pressure sources (pumps) for driving the sample fluid, and thus may still be inherent in the
challenge of fabrication and operation.
26
Therefore, the use of such blood plasma separation
devices for point of care diagnostics applications is limited.
As an alternative, self-infusion of a blood sample into a microchannel device using capil-
lary force is more suitable for point-of-care (POC) applications. A summary of the various
works reported on a capillary-driven blood plasma separation device is presented in the
supple-
mentary materia l
, Table S1. Kim et al.
26
reported surfactant modified polydimethylsiloxane
(PDMS) and SU8 microstructures for driving capillary blood flow and a planar cross-flow filter
for plasma separation from whole blood but the plasma recovery was found to be very low
(20 nl plasma) due to the low velocity of the blood flow. Lee and Ahn
27
reported a modified
surface for achieving plasma separation. It involves successive treatment of Cyclic olefin co-
polymer (COC) with adhesion promoters and layer by layer coating of five patterned 12 nm sil-
ica nanobead structure as superhydrophilic surfaces for creating the asymmetric capillary effect
for the pumping and a patch of uncoated area for plasma separation. The plasma recovery of
the device is only 6.8%, and the use of silica nanoassembly makes the device fabrication very
complicated and expensive. Szydzik et al.
28
described dielectrophoresis enabled plasma separa-
tion from capillary-driven whole blood. Dielectrophoresis forces were used to unblock the filter
post structures to increase the extraction of plasma. This device gives better plasma recovery
compared with similar works, but the volume of plasma is still less (165 nl), and the micro-
electrodes and very shallow plasma extraction channels involve fabrication complexity. Dimov
et al.
4
used a filter trench to achieve sedimentation of RBC and a straight microchannel into
which the plasma flows from the trench. In addition, a region on the top glass slide overlapping
the trench was coated with a hydrophobic pen to improve separation. Son et al.
29
used a mem-
brane in addition to a vertical up-flow chamber to achieve plasma separation by filtration and
sedimentation. Similarly, Kuroda et al.
30
showed plasma separation with diluted blood
(Dilution—1:3) by ascension in a cylindrical channel. But, Dimov et al.,
4
Son et al.,
29
and
Kuroda et al.
30
used degasi ng of the channel using a vacuum desiccator to enable the flow of
the sample blood into the microchannel device. Although the plasma recovery was found to be
higher, the sample blood has to be introduced into the device immediately after the removal of
the device from the vacuum. The time gap between the time instants the device is taken out of
the vacuum and the sample blood is introduced into the device could affect the efficiency of
the device, which limits its use in POC applications.
054108-2 Maria et al. Biomicrofluidics 10, 054108 (2016)
Here, we present a simple capillary driven PDMS microchannel device with a hydrophobic
region for blood plasma separation and detection of glucose in the plasma of normal and dia-
betic patients. In contrast to other capillary driven blood plasma separation devices reported in
the literature, the proposed device is much simpler in terms of design and fabrication and offers
higher plasma separation and enhanced purity. The proposed device differs from the device
reported by Lee and Ahn,
27
in that the device is much simpler in terms of fabrication as
opposed to a very complicated procedure (layer by layer coating of a silica nanobead structure)
adopted by Lee and Ahn.
27
Also, unlike Lee and Ahn, various parameters have been investi-
gated systematically for improving plasma volume, i.e., channel width, length, hydrophilicity
levels, patch length, and position. The plasma separation efficiency (22.5%) is higher as com-
pared to that achieved by Lee and Ahn (6.8%). The hydrophilicity of channels was retained by
filling with deionised (DI) water, and the separation of plasma in such devices has been
reported after 5, 10, and 15 days. The performance of the device was found to be unaffected in
terms of plasma volume, separation time, and purification efficiency during this period. First, a
brief description of the device and operating principle is presented. Then, a brief theory of
capillary flow of blood in hydrophilic and hydrophobic regions is detailed. Further, device fab-
rication, materials and methods, and the experimental procedure are outlined. Finally, experi-
mental results for blood plasma separation and glucose detection are presented and discussed.
II. DEVICE DESCRIPTION AND PRINCIPLE
A schematic of the proposed capillary driven blood plasma separation device is shown in
Fig.
1. The device comprises a PDMS microchannel layer bonded with a PDMS coated glass
slide. The PDMS substrate, which forms the top and two side walls of the rectangular micro-
channel, is hydrophilic everywhere except the hydrophobic region. The PDMS coated glass sub-
strate which forms the bottom wall of the microchannel is hydrophobic throughout. Thus at the
hydrophobic region, all four walls of the microchannel are hydrophobic. When the sample
blood is introduced at the device inlet, due to the hydrophilic channel walls (on three sides)
and thus adequate Young-Laplace pressure, it flows into the microchannel due to capillary
action. At the hydrophobic region, due to large contact angles and small Young-Laplace pres-
sure, the motion of the capillary front is impeded. Similar to the phenomenon reported by Lee
and Ahn,
27
the hydrophobic region acts as a flow barrier, prevents the motion of the blood
cells, and leads to the accumulation of blood cells near the region (RBCs adhere to the PDMS
wall and accumulate due to the van der Waal interactions
31,32
). The aggregated blood cells near
the hydrophobic region act as a filter to facilitate the separation of plasma. Since the viscosities
of the blood cells and plasma are quite different,
33
the blood plasma moves at a higher velocity
as compared to the blood cells in the hydrophilic region. Thus while the blood cells stop at the
FIG. 1. Schematic of the capillary driven PDMS microchannel device with a hydrophobic region for the separation of
plasma from whole blood and subsequent detection of glucose.
054108-3 Maria et al. Biomicrofluidics 10, 054108 (2016)
hydrophobic region, due to higher velocity, the blood plasma flows past the hydrophobic region
and gets separated from the blood cells.
III. THEORY: CAPILLARY FLOW OF SAMPLE BLOOD
The sample blo od flowing in a rectangular microchannel at lower strain rates can be
assumed to be non-Newtonian and described using the Casson model
34
as follows:
ffiffiffi
s
p
¼
ffiffiffiffiffi
k
0
p
þ
ffiffiffiffiffiffiffi
k
1
_
c
p
; (1)
where s is the shear stress,
_
c is the shear rate, and k
0
and k
1
are constants. Yield stress of blood
k
0
¼0.004 Pa (Ref.
34) and k
1
¼ g
p
ð1 þ 0: 025H þ7:35 10
4
H
2
Þ, where hematocrit concentra-
tion H is taken as 43.8% and viscosity of plasma g
p
¼0.012 Pa s.
34
Since the aspect ratio
(width:height ratio) of the channel is very high, infinite parallel plate assumption is considered
to model the flow. The steady, fully developed Navier-Stokes equation is given as
@s
@y
¼
@P
@x
: (2)
In the capillary flow situation, the Young-Laplace pressure is the driving pressure, which can
be written as
P
0
¼ r
1
R
1
þ
1
R
2
; (3)
where r is the surface tension of blood (¼0.055 N/m),
35
and R
1
and R
2
are the radii of curva-
ture on the top and side walls of the channel. Due to the high aspect ratio of the channel, the
radius of curvature along the width is much higher as compared to that along the height. So
ð1=R
2
Þ is negligible. Therefore, P
0
can be written as
P
0
¼
2r cos h
h
; (4)
where h is the height of the channel. Since the channel outlet is open to ambient, the pressure
gradient along the channel
@P
@x
¼
P
0
x
¼ C: (5)
Next, we substitute the expression for pressure gradient, which is a constant C, from Eq.
(5) in
Eq.
(2) and express shear rate in terms of velocity gradient as
_
c ¼ @u=@y. Then, upon integra-
tion and by using the boundary conditions, u ¼ 0aty ¼ 0 and
du
dy
¼ 0,
du
dy
¼ 0aty ¼
h
2
, the
velocity profile u is obtained as
u ¼
1
k
1
C
y
2
2
þ 2k
0
C
h
2
y
4
3
ffiffiffiffiffi
k
0
p
C
Cy þ k
0
C
h
2
3=2
þ
4
3
ffiffiffiffiffi
k
0
p
C
k
0
C
h
2
3=2
!
:
Now, the average velocity u
avg
can be written as
u
avg
¼
1
h
ð
h
0
udy: (6)
If we neglect the insignificant terms (and retain only first two terms on the R.H.S.), we get
054108-4 Maria et al. Biomicrofluidics 10, 054108 (2016)
u
avg
¼
1
k
1
k
0
h þ
P
0
x
h
2
12
: (7)
The above equation gives the expression for the average velocity of the capillary meniscus
along the flow direction in the channel.
IV. EXPERIMENTS
A. Device fabrication
The device design was dr awn using AutoCAD LT 2008, which was printed on a flexi mask
at 40 000 dpi (Fineline Imaging, USA). A silicon wafer (Semiconductor Technology and
Application, Milpitas, USA) was cleaned using a hydrofluoric acid (HF) solution and deionised
(DI) water at 1:10 and kept in an oven at 120
C for 2 min. First, SU8 2075 (MicroChem Corp.,
Newton, USA), a negative photoresist, was spun coated onto the Si wafer with an acceleration
of 300 rpm/s. Then, the coated SU8 was soft baked at 65
C for 3–7 min and hard baked at
95
C for 6–30 min. Further, the baked SU8 was exposed to UV light (J500-IR/VISIBLE, OAI
Mask alligner, CA, USA) through the flexi mask. Next, post-exposure bake of the wafer was
performed at 65
C for 1–5 min and 95
C at 5–10 min. Then, the pattern was developed by
exposing it to SU8 developer and placing inside the oven at 120
C for 30 min. SU8 master pat-
terns of four different thicknesses, i.e., 50 lm, 100 lm, 200 lm, and 300 lm were fabricated
using the process parameters given in
supplementary material Table S2. For the fabrication of
the PDMS devices, PDMS monomer and the curing agent (Sylgard 184, Silicone Elastomer kit,
Dow Corning, USA) at a ratio of 10:1 (by weight) were mixed and degassed. The mixture was
poured onto the SU8 master and then placed in a vaccum oven at 65
C for 3 h for curing. The
baked PDMS was peeled off from the master mold and cut to size. Inlet and outlet holes of
around 3 mm diameter were punched using biopsy punches (Shoney Scientific, Pondicherry,
India). A small amount of the mixture was also poured onto a glass slide, spin coated
(spinNXG-P1, Apex Instruments, India) at 500 rpm for 30 s and baked. The PDMS channel
layer was exposed to the oxygen plasma (Harrick Plasma, Brindley St., USA) at power 11 W
for 0.5–2 min, while the PDMS coated glass slide was kept unexposed. Finally, the exposed
PDMS channel layer and the unexposed PDMS coated glass slide were bonded together by
applying a gentle pressure. In experiments employing the hydrophobic region, a thin strip of
tape (0.1 mm thick) of required length was placed on the PDMS channel layer across the chan-
nel at some distance away from the inlet and then the PDMS channel layer was exposed to
oxygen plasma. Thus, a small area of the channel is prevented from getting exposed to the oxy-
gen plasma, which forms the hydrophobic region. Finally, the thin tape was then removed from
the PDMS layer and bonded with the PDMS coated glass slide which was not exposed to oxy-
gen plasma. A photograph of the device and an optical image of the microchannel are depicted
in
supplementary material Figs. S1(a) and S1(b), respectively.
B. Materials and methods
1. Human blood sample
Samples of human blood from healthy donors were collected from a hospital (Institute
Hospital, IIT Madras) in vacutainers with 7.2 mg K2 Ethylene diamine tetraacetic acid (EDTA)
(BD, NJ, USA). The blood samples from different diabetic patients were also obtained from
our Institute Hospital after ethical clearance and used for the detection experiments.
2. Quantification of the plasma separation
a. Purification efficiency. The purity of the plasma from the reported device and centrifuged
blood was compared. For this, the difference in the gray scale intensity of the same section of
the channel with and without the plasma was measured in both the cases and compared. Since
054108-5 Maria et al. Biomicrofluidics 10, 054108 (2016)