AN INEXPENSIVE METHOD FOR PDMS-PDMS BONDING

IN FABRICATION OF MICROFLUIDIC DEVICE

Koh Kai Seng,The University of Notitngham Malaysia Campus

The Second Conference on Advances in Microfluidics and Nanofluidics and Asian-Pacific International Symposium on Lab on Chip5th-7th January 2011, Singapore

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TABLE OF CONTENT

Introduction Experiments Result and discussion Conclusion

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INTRODUCTION

Poly(dimethylsiloxane) (PDMS) has been widely used in microfluidic applications.

Sealing has been the key challenges in microfluidic fabrication. Reversible bonding, irreversible bonding.

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Reversible bonding Irreversible bonding

Reusable. Not reusable.

Able to sustain low pressure Able to sustain relatively high pressure (200-700kPa)

Natural bonding Surface property modification.

Conventional sealing techniques require Costly equipments.(Oxygen plasma) Time consuming. (Partial curing technique) Involve complicated steps. (Oxygen plasma) Reliability and repeatability are always at the

centre of argument.

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Piranha solution is used because More economical, Time saving, Involve less complicated steps. Relatively high reliability and repeatability

BONDING MECHANISM

First stage: Dehydration of hydrogen and oxygen into water

unit.

Second stage: H2SO4 + H2O2 → H3O

+ +H2O4- + O∙Reactive atomic

oxygen species

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+

Bonding mechanism:

Si O Si O Si

CH3

n

PDMS surface

O∙

O∙ → H2O → OH- + OH-

CH3O·

OH-

OH

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EXPERIMENT METHOD

Sealing procedure: PDMS samples with strings embedded in it were

solidified. The samples were cut into 2cm × 2cm square piece

each.

Piranha solution concentrated Sulfuric acid (96wt%) & Hydrogen peroxide(30wt%) at 3:1 weight ratio.

Interfaces of two PDMS are drenched into the solution for a define interval.

Next, both surfaces attached and bounded together followed by thermally curing of 70°C for 60 minutes.

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RESULT & DISCUSSION

2 methods are used to test bonding power:• Tensile strength test• Leakage test

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Drenching time (sec)

Tensile strength (kPa)

1 2 3 AveageStandard

Deviation (%)

0 20 13 24 19.00 29.30

15 69 59 72 66.67 10.21

30 102 94 100 98.67 4.22

45 189 200 217 202.00 6.98

60 125 132 124 127.00 3.43

75 32 50 54 45.33 25.85

Table 1: Tensile strength (kPa) against drenching time (sec) at 3:1 weight ratio of Piranha solution

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Table 2: Calibration of tensile strength of string (kPa)

MaterialTensile strength (kPa)

1 2 3 Average

String 500 473 486 486.33

PIRANHA SOLUTION USING HNO3

(65WT%) + H2O2 AT 3:1 WEIGHT RATIO

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Drenching time (sec)

Tensile strength (kPa)

1 2 3 Aveage

Standard Deviation

(%)

0 20 25 24 23.00 11.50

15 32 28 34 31.21 10.21

30 48 44 47 46.19 4.22

45 59 62 58 59.45 3.43

60 88 94 102 94.57 6.98

75 72 87 72 76.82 11.22

Table 3: Tensile strength (kPa) against drenching time (sec) at 3:1 weight ratio of Piranha solution

Figure 1: Graph average tensile strength (kPa) against drenching time (sec)

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Sustaining time of the PDMS-PDMS interfaces is within the range of 20-30 seconds.

0 10 20 30 40 50 60 70 800

50

100

150

200

250

Graph Tensile strength (kPa) against sealing time (sec)

Surfuric acid(96wt%)Nitric acid(65wt%)

Drenching time, (sec)

Tensile s

trength

,(k

Pa)

Optimum drenching time is 45 seconds. Reaction ends at around 65±5seconds.

Pure Sulfuric acid remains. (limiting reactant is Hydrogen peroxide). PDMS surface will be attacked.

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Leakage test Water is pumped into closed channels for 5

minutes at 5,20,50µl/min. No leakage detected!!

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CONCLUSION

An inexpensive polymer-based sealing technique, using piranha solution (3:1 weight ratio) is demonstrated and quantitative result is shown.

This irreversible bonding techniques is able to sustain a mechanical strength of (200±20kPa).

Water is injected into closed microchannels at different flowrates and no leakage occurred up to a flowrate 0.3ml/hr.

In future work, optimization of the bonding strength using the same method will be investigated.

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THANK YOU FOR YOUR ATTENTION.

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