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Multi-step dielectrophoresis for separation of particles

Student: Bui Tuan Anh ( 裴俊英 )Professor: Yi – Chu HsuClass: Nano-MEMS

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CONCEPT1. Introduction2. Theory of method3. Experiments and calculations4. Results and dicussion5. Conclusion

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1. IntroductionSeparation of micro particles with different properties is an issue of great importance.

One of most common methods is flow cytometry

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This paper proposed a separation method based on principle of Dielectrophoresis (DEP).

-+-++++

+---- -

+---

+++

(+)(-)-

Positive DEP

--

++++

+---

-+

---

+++(+) (-)

-Negative DEP

Particles in micro-flow channel will be trapped and released in a number of steps. Each step will improve resolution.

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2. Theory of method2.1. DEP mobilityIf the medium is moving with velocity uf, then the total velocity utot of particles is:

uDEP is the velocity of a particle induced by DEP.

is the DEP mobility. is the gradient of the squared electrical field.

2DEP DEPu E

tot f DEPu u u

DEP2E

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A spherical particle with radius r, in the medium with viscosity , the DEP mobility is: is the real part of the complex effective polarisability of the particle.

is conductivity and is permittivity

2

( )18DEPrR

( )R

2 ( ) ( )3

2 ( 2 ) ( 2 )p m p m

mp m p m

f if i

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2.2. Multi-step DEP trappingThe trap-release step incluse 3 phase:

Particles X

Particles YFlow

AC

Particles X Particles Y

Flow

AC

a) b)

c)

L0

L0

Electrode array

Electrode array

Phase a: Particles are focused in the middle of channelPhase b: Trap Phase c: Release

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+ -

Electric fieldThe particles are trapped and released from electrode.

If the difference in mobility bteween the particles to small, using one trap and release step will not be enough to achieve compete separation. Besides that, it’s difficult to trap the particles with only single pair of electrodes.

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Electrode array (connected to AC)

L0

Particles X Particles Y

Flow

After several steps the separation is complete.

Therefore, an array of interdigitated electrodes will be utilized.

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To quantify the level of fractionation in this method, a DEP resolution factor RDEP between two particle populations A and B is define as:

3DEP

A B

dR

w w

d is the distance between the two centers of each particle populationsw is the distance between the particles most far apart within each population.(wide of population)

Two particle populations are completely separated if RDEP=1.5.

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The resolution will continue to increase as a direct function of the number of steps. Hence, the DEP resolution RDEP can be expressed as:

DEP R DEPR C N

Where the CR is a constant value reflecting the separation increase in each step. And NDEP is a number of steps.

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3. Experiments and calculationsExperimental conditions:Particles: Polystyrene micro beadsFlow channel: micro-flow channel

Number of eletrode: 15 electrodes /1 stepVelocity of flow is 50. Voltage phase angle between two adjacent electrodes is 180° ( ).Width of one electrode and distance between two adjacent electrode is 0.5 L0.

Width>>L0

Height=L0

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To icrease the trapping efficiency, and to extend the separation range, superpositioned AC field are utilized. This means that the arrays of interdigitated electrodes are used both at top and bottom of the channel.Then the velocity of the particles in the trapping phase is calculated:

where upDEP ,unDEP are the velocity induced by positive DEP and negative DEP, respectively.

tot f pDEP nDEPu u u u

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In the release phase, both arrays of electrodes push the particles to the middle of the channel, then velocity become:

tot f nDEPu u u

In this research, the radius of polystyrene beads is set to 0.05 L0. Then, the maximum DEP mobility of polystyrene particle is 1.2x10-18m4/V2s at low frequencies(<5kHz).

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Frequency (Hz)

1.5

0.5

-0.5

-1

0

-1.5

102 104 106 108 1010DEP

mob

ility

[m4 /V

2 s] x10-18

At low frequency (<5kHz), the positive DEP motion will be increased. And at high frequency it could be reduced. For negative DEP, a suitable frequency would be 5MHz.

2DEP DEPu E

tot f DEPu u u

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4. Results and discussionA difference in size will have largest influence on DEP mobility

00

1

0.5

1

2

1.5

2.5

2 3 4

5%

2%

1%

Res

olut

ion

Number of steps

2

( )18DEPrR

Separate particles with a 5% size difference, only 2 step are used to obtain a complete separation (RDEP>1.5). For 2% size difference, we need 4 step, while 8 would be required to separate particles with 1% difference in size.

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When size difference is 0.2%, number of steps we need is about 200 steps.

1800

1

0.5

2

1.5

2.5

200 220

0.2%

Res

olut

ion

Number of steps15

0

1

0.5

2

1.5

2.5

20 25

0.5%

Res

olut

ion

Number of steps

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Differences in Conductivity have a lower impact on DEP mobility than differences in size.

Res

olut

ion

Number of steps

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

00 1 2 3 4

42% 32%

19%

If differences in conductivity is reduced to 18%, no separation at all is achiveved in the first trapping phase, and then any separation would be impossible.

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Also with a differences in permittivity of 62%, it is a limitation of separation.

00

1

0.5

1

1.5

2 3 4

203% 83%

62%R

esol

utio

n

Number of steps

2

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5. Conclusions-The separation method based on repetitive DEP trapping and release in a flow system.-The resolution RDEP is a funtion of the number of steps.-Calculations for some model particles showed that it should be to obtain a complete separation for a 0.2% size difference after about 200 steps.-The limiting value of diffrences in conductivity is 18% that separations can not performed.

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The power of multi-step DEP could be of great interest, not only for fractionation of particles, but also for measuring changes in surface conductivity.

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Thanks for your attention.