A PLASMA SEPARATION DEVICE BASED ON

CENTRIFUGAL EFFECT AND ZWEIFACH-FUNG EFFECT1

Zhaoxin Geng1, 2, 3, Lingqian Zhang1※, Yanrui Ju1, Wei Wang1 and Zhihong Li1＃ 1National Key Laboratory of Nano/Micro Fabrication Technology, Institute of Microelectronics,

Peking University, CHINA 2School of Information Engineering, Minzu University of China, CHINA

3Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors,

Chinese Academy of Sciences, CHINA

ABSTRACT

A microfluidic chip used in separation of plasma from whole blood, which combines, the Zweifach–Fung bifurcation law,

centrifugation and diffuser-nozzle effect, is presented. The characteristics of this device were investigated at different flow

rates. The efficiency of plasma separation in this divice reached more than 95 percent at a flow rate 0.0628μl/s. The simple

planar structure and high throughput offered by this passive microfluidic approach make it attractive for clinical application

and integration with microfluidic chip.

KEYWORDS: Microfluidic, Separation, Plasma

INTRODUCTION

Human blood plasma separation has direct, obvious applications in human health monitoring, disease diagnostic and

theranostics. Meanwhile, separation of plasma from whole blood is a strategic preliminary step in the preparation for biologi-

cal analysis on-chip because the blood biochemical tests are performed on cell-free serum or plasma. Therefore, by separating

blood into its basic components of plasma and cells, the reaction can be optimized for better results. Recently, some miniatur-

ized separation devices developed by using microfabrication and microfluidic technology were applied in plasma separation,

those device based on different theories such as micro-gap filter [1], capillary-driven [2], cofferdam [3], planar microfilters

[4],T-shaped channels [5], micropost arrays [6]. However, there are some drawbacks such as low purity of plasma, little sepa-

rated volume of plasma from whole blood, low throughput, which blocked the plasma separation chip to be integrated with

other microdevices on a chip and to be applied in the medical test.

With development of microfabrication technology, Microfluidic chips have proven as ideal tools to precisely handle small

volumes of samples, such as plasma. Additionally, microfluidic devices can form part of portable systems for point-of-care or

in-the-field detection. Therefore, a plasma separation chip is presented to increase the purity and throughput, which combines

with Zweifach–Fung bifurcation law, centrifugal effect and diffuser-nozzle effect.

APPROACH

Figure 1: Sketch of assembled separation

structures.

Figure 2: Microchannel of separation structures. (A)Top-view of the separa-

tion structure，1diffuser-nozzle, 2inlet, 3plasma outlet, 4 cells outlet, 5 inner

channel, 6 outer channel; (B)Illustration of Zweifach-Fung effect and centrif-

ugal effect.

Microfluidic devices, consisting of a network of channels with critical dimension comparable to human arterioles

(~90μm), are ideal platforms for the study of the plasma separation. To improve the purity of plasma and separation efficien-

cy and to solve the issues of clogging, bubbles and size in separation devices, a novel structure, which is applied to separate

plasma with simple process, high separation efficiency and high throughput, is shown in Fig. 1 and 2. This separation chip

consists of two Archimedean spiral microchannels with different length and width. The two Archimedean spiral microchan-

nels are connected with diffuser-nozzle channel. The angle ( ) between the diffuser-nozzle channel and microchannels is 45

※The author have a contribution equally to the first author. ＃Z. Li, Tel: +86-10-62758911-14, email: zhhli@pku.edu.cn.

978-0-9798064-4-5/µTAS 2011/$20©11CBMS-0001 224 15th International Conference onMiniaturized Systems for Chemistry and Life Sciences

October 2-6, 2011, Seattle, Washington, USA

degree and the angle ( ) of diffuser-nozzle is 30 degree. In this structure, the blood cells were first separated to the outside of

the main microchannel as a result of the centrifugal effect (shown in Fig. 2(B) and Fig. 3 (A)). Then the blood plasma flow to

the inner channel at branching points and the blood cells keep their direction as a result of the Zweifach–Fung effect. The

Zweifach-Fung bifurcation law is illustrated in the Fig. 3(B). At ranching points of microchannel bifurcation, micro-particles,

eg. erythrocytes, always enter the mcirochannel with the higher flow rate. This effect occurs mainly for the following reasons:

First, the side of particles that faces the branch with the higher flow rate is subject to lower pressure, therefore the particles

are drawn into that branch. Second, different flow rates subject the particles to different shear forces. Meanwhile, the diffus-

er-nozzle structure could improve the throughput of plasma from whole blood microchannel to plasma microchannel (shown

in Fig. 3(C)). These three kinds of effects ensure the stability and separation efficiency of this microfluidic device.

Figure 3: Theory of plasma separation. (A) Zweifach-Fung bifurcation law, centrifugal effect and diffuser-nozzle; (B)

Illustration of Zweifach-Fung effect and the distribution of the streamline and shear forces; (C) the distribution of pres-

sure and streamline in diffuser-nozzle structure. Neglecting the colliding of the cells, single-phase flow simulation about this structure is carried out to verify its feasibil-

ity. The streamline diagram shows that the fluid can be accumulated by these Y-shaped bifurcations, and most of the fluid

goes to the inner flow channel through diffuser-nozzle microchannel (Fig. 4). After measuring average velocity, the flow rate

of the two outlets reaches 1.157μL/s and 0.099μL/s when the inlet flow rate is 1.256μL/s. Therefore, the separation ratio

reaches 92.2%. The vector diagram shows that the fluid of the inner channel will not go to the outer channel for the centrifu-

gal effect (Fig. 5). As shown in Fig. 6, differential pressure distributions of the inner and outer channels at the connections

(diffuser-nozzle channel) are different. The pressure difference indicates that the first loop does not contribute to separate the

plasma from whole blood. Therefore, the first loop of the separation device only has one Archimedean microchannel and

there is no the diffuser-nozzle channel in the first loop. Although the loop number of Archimedean microchannel is less than

that of Y-shaped bifurcation, the pressure difference increases tremendously after Archimedean microchannel turning 500

degree, therefore, the separation ratio keeps a high value.

Figure 4: Streamline diagram of fluid in

the microchannel

Figure 5: Vector diagram of fluid in the

microchannel

Figure 6: Graph of differential

pressure

EXPERIMENT

The simple fabrication process of chip is described as follows. A silicon wafer was first spin-coated with positive photo-

resist (AZJ304, 3000 rpm), pre-baked at 100℃ for 15min, exposed for 30s, developed for 40s and hard baked at 120℃ for

30min, forming two Archimedean microchannels, diffuser-nozzle channels and inlet/outlet reservoirs. After photolithogra-

phy, the pattern was transferred onto the silicon substrate by deep reactive ion etching. All the channels were around 100μm

in depth after the etching process. PDMS materials (Sylgard 184) were prepared by mixing pre-polymer and curing agent

with weight ratio of 10:1. They were degassed prior to pouring an adequate amount onto a silicon substrate. The PDMS pre-

polymer was cured in an oven at 80℃ for 1h. After curing, the PDMS layer was peeled off and drilled to form one inlet port

and two outlet ports. Lastly, the silicon wafer with the microstructure and the PDMS layer with inlet/outlet were bonded to-

gether. The packaged device was shown in Fig. 7. The size of the fabricated chip is 1 cm × 1 cm.

To investigate the relationship between the separation effect and inlet flow rate, the experiments were carried out at dif-

ferent flow rates. The simulation results show that trend of flow field distribution are almost the same in different inlet ve-

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locity. The separation ratio keeps more than 90% and decreases with the increase of the inlet velocity (Table1). When the in-

let velocity is larger than 3.5μl/s, the centrifugal effect becomes significant and the fluid of inner channel flows to the outer

channel. Meanwhile, combining test results and simulation results, the cycles of the Archimedean microchannel were

changed to investigate the separation effect and the structure optimization. The results show that the separation ratio does not

change much when increasing or decreasing the cycle of the channel.

Figure 7: The pack-

aged separation de-

vice

Figure 8: A compar-

ison of fluid from the

two outlets

Figure 9: Microscope photos of the sample in the blood counting chamber

(A) The plasma outlet fluid with a diluting ratio of 1:20 (B) The blood cell

outlet fluid with a diluting ratio of 1:500 (C) Inlet fluid with a diluting ratio

of 1:500

RESULTS AND DISCUSSION

The blood, which was diluted with physiological saline with ra-

tio (volume) 1:20, was pumped in this separation device at different

flow rates. The separated results show in Fig. 8. After diluting, 1μL

samples was taken at each output before and after separation with

different ratios and spread on the blood counting chamber. To count

the amount of the cells, the sample in the blood cell outlet was di-

luted with ratio 1:500. The results detected under the microscope

were shown in Fig. 9. The calculation results show that the cell den-

sity of the inlet fluid, plasma outlet fluid and blood cell outlet fluid

are 2.075×108/μL, 8.960×107/μL and 1.041×109/μL. The number of

the cells in blood cell outlet is prominently larger than that in plas-

ma outlet. Meanwhile, no blood cell in plasma outlet was detected

with the commercial device; therefore, these results mean that the

purity of purity is very high.

CONCLUSION

A novel separation chip combining the Zweifach–Fung bifurca-

tion law, centrifugation and diffuser-nozzle effect was presented to

realize a continuous microfluidic centrifugation-Zweifach–Fung-in-

chip, together with the manufacturing methods and preliminary tests

of the devices. Multi-separation methods，which are applied simultaneously in one chip, could improve the purity of the

plasma and throughput and could reduce the bubbles in the microchannel. The developed design is planar and thus is simple

to fabricate and integrate with other LOC components with the help of different manufacturing methods. And mass-

manufacturability of the blood plasma separation system is studied.

ACKNOWLEDGEMENTS

This work was supported by National Science Foundation of China (No. 60976085), China Postdoctoral Science Founda-

tion funded project (No.20090460497) and Fundamental Research Funds for the Central Universities (No. 0910KYQN65).

REFERENCES

[1] X. Chen, D. Cui, J. Chen, Design, “Fabrication and characterization of nano-filters in silicon microfluidic channels

based on MEMS technology”, Electrophoresis, 30, 3168–3173, (2009).

[2] Yu Chang Kim, Seung-Hoon Kim, et al. “Plasma extraction in a capillary-driven microfluidic device using surfactant-

added poly(dimethylsiloxane)”, Sensors and Actuators B: Chemical, 145, 861–868, (2010).

[3] Z.Geng, Z. Xu, W. Wang, W. Su, Z. Li, “Separation of Blood on a Chip Utilizing Spiral Micorchannel with Fence and

Cofferdam as Filtration Structures”, ICSICT2010, shanghai, 1856-1858, (2010).

[4] T.A. Crowley, V. Pizziconi, “Isolation of plasma from whole blood using planar microfilters for lab-on-a-chip applica-

tions”, Lab Chip, 5922–929, (2005).

[5] S. Yang, A. Undar, J.D. Zahn, “A microfluidic device for continuous, real time blood plasma separation”, Lab Chip, 6

871–880, (2006).

[6] J.A. Davis, D.W. Inglis, et al., “Deterministic hydrodynamics: taking blood apart”, Proc. Natl. Acad. Sci. U.S.A. 103,

14779–14784, (2006).

Table 1: The relationship of inlet flow rate, plasma

outlet flow rate and separation ratio

Inlet Flow

Rate (μL/s)

Plasma outlet

Flow Rate (μL/s)

Separation ra-

tio

0.0628 0.0597 95.1%

0.314 0.295 94.0%

0.628 0.585 93.2%

0.942 0.872 92.6%

1.256 1.157 92.1%

1.570 1.441 91.8%

1.884 1.724 91.5%

2.198 2.007 91.3%

2.512 2.289 91.1%

2.826 2.571 91.0%

3.140 2.853 90.9%

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