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Formation of more stable hydrophilic surfaces of PDMSby plasma and chemical treatments

Dhananjay Bodas, Chantal Khan-Malek *

FEMTO-ST, Dept LPMO CNRS UMR 6174, 32 Avenue de lObservatoire, Besancon Cedex 25044, France

Available online 23 February 2006

Abstract

In the present work hydrophilic stability of Sylgard 184 poly(dimethyl siloxane) (PDMS) was studied with the objective to create morestable hydrophilic surfaces. The surface modification of PDMS was carried out by conventional (oxygen plasma) and unconventionalplasma modification (2-step modification using oxygen and C2F6) processes and also by chemical grafting using oxygen plasma polymer-ization of 2-hydroxyethyl methacrylate (HEMA). The hydrophilic stability of the modified surfaces was monitored as a function of timeelapsed after treatment and quantified. The surfaces were characterized using static contact angle measurements and Attenuated TotalReflection Fourier Transform Infrared Spectroscopy (ATR-FTIR).2006 Elsevier B.V. All rights reserved.

Keywords: PDMS; Oxygen plasma treatment; 2-Step modification; HEMA grafting; Hydrophilic stability

1. Introduction

Poly(dimethyl siloxane) (PDMS) elastomer is the mostwidely and one of the most versatile material used in theconstruction of microfluidic devices, in particular for rapidprototyping. One of the reasons for the popularity of PDMSis the particularly straightforward manufacturing methods.PDMS has as essential characteristics that it is elastomeric,can be optically transparent, chemically inert, permeable togases and amenable to fabrication via rapid prototyping.However, in spite of the many advantages of PDMS, thesurface of PDMS is naturally hydrophobic and cannot beused as it is for a variety of applications. A number of efforts

have been made to modify the surface of PDMS micro chan-nels, in order to enhance hydrophilicity and EOF whichlimit its applicability for analytical devices[1].

It has long been recognized that in order to modify thesurface properties of PDMS, various techniques can be uti-lized that involve, physical or chemical treatments or a com-bination of both[24]. Particularly oxygen plasma has beenwidely used to modify the surfaces of PDMS (e.g., [5]).

However, the characteristics of oxygen-plasma-modified

PDMS surfaces gradually change during aging and the sur-face recovers its hydrophobicity after a short time as hasbeen observed by some groups[6]. The quick hydrophobicrecovery of PDMS surfaces associated with most of the cur-rently employed treatments is one of the current issues lim-iting the use of PDMS for microfluidic devices.

To overcome the problem of quick hydrophobic recov-ery, work on use of 2-step modification has been carriedout by Gilmor et al., [7] using various gases, e.g., SiCl4,CCl4, etc., after the oxygen plasma treatment. Also deposi-tion of 2-hydroxyethyl methacrylate (HEMA) film insteadof gases has been employed after the oxygen plasma treat-

ment to stabilize the hydrophilicity by Choi and Yang [8].In the present work, three methods were investigated tocarry out the surface modification of PDMS: (1) an oxygenplasma treatment; (2) a novel 2-step plasma modificationusing O2 and C2F6; and (3) surface grafting using HEMAand oxygen plasma[8].

2. Experimental section

A 2-component PDMS from Dow Corning (SYLGARD184) with a 10:1 base to curing agent mixing ratio was used.

0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.mee.2006.01.195

* Corresponding author. Tel.: +33 381 853 999; fax: +33 381 853 998.E-mail address:chantal.khan-malek@lpmo.edu(C. Khan-Malek).

www.elsevier.com/locate/mee

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The 2-components were thoroughly mixed and degassed invacuo to remove bubbles. The mixture was cured at a100 C temperature for 1 h. Three different processes wereexplored. All plasma treatments were conducted in a Plas-sys reactive ion etching (RIE) system using an RF powersupply of 13.56 MHz frequency for the plasma excitation.

For oxygen plasma modification, pressure of the systemand oxygen flow rate was kept constant at 100 lbar and20 sccm, respectively. RF plasma power was varied from50 to 150 W and also the time of plasma exposure (treat-ment time) was varied from 1 to 15 min.

The second process corresponds to a novel 2-step mod-ification carried out using a combination of O2 and C2F6.The first step consisted in the formation of functionalizedsilanol groups on the PDMS surface using pure oxygenplasma with a constant system pressure and oxygen flowrate of 100 lbar and 20 sccm, respectively. RF power of100 W was used for 30 s for the modification. In the nextstep, a 3:1 mixture of C2F6and oxygen was used at a con-

stant pressure of 60 lbar. RF power was varied from 100to150 W with a time variation of 515 min.

The third process involved a monomer HEMA, com-mercially available from Aldrich Chemicals. The modifica-tion was conducted in several steps. Firstly, oxygenmodification was carried out at a system pressure of100 lbar and flow rate 20 sccm, and an RF power of100 W was used for 30 s for the modification. HEMAwas then spin coated onto the oxygen modified PDMS sur-face at a spin speed of 1500 rpm for 15 s (thickness600 nm). The HEMA-coated PDMS was finally treatedwith oxygen plasma at a constant pressure of 100 lbar at

an oxygen flow rate of 20 sccm. RF power was varied from50 to 100 W with a time variation of 15 min.

ATR-FTIR spectra were recorded on a Nicolet 5-PCFTIR spectrometer to assess the changes on the surfaceafter different modifications. The FTIR spectra wererecorded within 5 min of surface modification. All the sam-ples were also characterized for hydrophilic stability usingcontact angle measurements. In our experiments, a manualcontact angle measurement setup was used having a errorof 2. Deionized water and diiodomethane (AldrichChemicals) droplets (2 lL) were delivered on the PDMSsurface by a calibrated syringe. The static contact angleswere measured before and after modification, as well as afunction of time in the time interval of 012 h, every5 min at the beginning and every hour thereafter.

3. Results and discussion

Fig. 1shows the ATR-FTIR spectra of pristine PDMSand PDMS modified with oxygen, O2+ C2F6and HEMA.The spectra for all treated samples were obtained within5 min of the modification process. Comparison of the pris-tine and modified PDMS samples with their spectral valuesand corresponding group assignments are shown in Table1. The ATR-FTIR data show characteristic peaks depend-

ing on the treatment used which confirms the surface mod-

ification in addition to the regular peaks of PDMS [710].The presence of a 3413 cm1 peak corresponding to thebroad absorption band of OH bond of water moleculesindicates changes in the surface characteristics of PDMSafter oxygen plasma treatment. These H2O molecules areadsorbed by the silanol interface of the surface of PDMSas a thin layer as a consequence of plasma treatment.The sharp peak corresponding to OH group of silanolmoeties expected at around 3750 cm1 [11] has not beenobserved in the present study because its intensity is orderof magnitudes lower. A peak at 1841 cm1 correspondingto CF is observed for the 2-step modified surface using

C2F6 indicative of modification. Similarly a peak at 3363,

4000 3500 3000 2500 2000 1500 1000 500

3413

(d)

(c)

(b)Transmission(%

)

Wave Number (cm-1

)

(a)

1404

1256

1015 790

6922

958

1841

1710

3363

1163

Fig. 1. ATR-FTIR spectra for: (a) pristine PDMS, (b) O2 plasmamodified, (c) 2-step O2+ C2F6plasma modified, and (d) HEMA modifiedPDMS.

Table 1Comparison of ATR-FTIR spectral values of PDMS with their groupassignments before and after modification

Assignments PristinePDMS

Oxygenmodification

2-Stepmodification

HEMAmodification

H2O 3413 OH 3363Methyl CH 2964 2959 2959 2959CF 1841 C@O 1710CH3 asymmetric

deform1408 1408 1408 1408

CH3 symmetricdeform

1259 1259 1259 1259

Primary alcohol 1163SiOSi

asymmetricdeform

1015 1015 1015 1015

SiC 851 851 851 851Si(CH3)2 790 785 790 790Surface 691 691 691 691

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1710, and 1163 cm1 observed in case of HEMA, corre-sponds to OH, C@O, and primary alcohol group respec-tively, which are characteristic peaks of HEMA.

The pristine PDMS shows its hydrophobic behavior andpoor wettability with a 100112water drop contact angle.After O2 modification, the contact angle decreases withplasma power and treatment time. Maximum hydrophilicstability without cracking the PDMS surface was achievedat 150 W RF power for 15 min. For the 100 W power con-tact angle increased rapidly within 60 min and recoveredwithin 2 days. Similar trend is observed for the

O2+ C2F6 modification. There is negligible decrease incontact angle with increase in power. At powers beyond150 W and longer time intervals the film showed changein color and decrease in transparency as a silica-like layerwas formed. Best results in terms of hydrophilic stabilitywere obtained for 150 W plasma power for 1 min. TheHEMA modified samples showed increase in hydrophilicstability with increase in RF power. Increasing powerbeyond 100 W had little effect on the hydrophilic stability.

Fig. 2shows the best results in terms hydrophilic stabil-ity for the three PDMS treatments. PDMS treated withoxygen plasma recovers from hydrophilicity to hydropho-

bicity faster compared to the O2+ C2F6and HEMA mod-ified PDMS. Recovery in case of oxygen plasma modifiedsample might be due to the migration of low molecularweight species from the surface to the bulk of PDMS.The PDMS modified with HEMA showed the lowest con-tact angle and kept the hydrophilicity over 10 days. Thewettability in case of HEMA may be attributed to carbonylgroups also seen in the ATR-FTIR spectrum. TheO2+ C2F6-modified PDMS did not show much lower con-tact angle but its hydrophilic stability, of over a week, wasmuch higher compared to the O2 plasma treated sample.This high stability can be attributed to stable hydrophilicCF groups on the surface.

4. Conclusion

The surface modification of Sylgard 184 PDMS wasstudied in terms of hydrophilic stability using three differ-ent modification processes. Oxygen modification showshydrophilic stability in the range of 2 days at the power

of 150 W and exposure time of 15 min. A novel 2-stepmodification using O2+ C2F6 shows a good hydrophilicstability in the range of a week (increase in contact anglefrom 64 to 85). The chemical modification using HEMAproved to be the most efficient in terms of hydrophilicstability with surfaces stable over 10 days (increase in con-tact angle from 7 to 44). The study shows in particularthe possibility of grafting long-term stable hydrophilicfunctional groups onto PDMS using oxygen plasma treat-ment of HEMA. These results may find use in the con-struction of microfluidic devices with PDMS as itincreases the time available for sealing, confers superiorwetting properties to the microchannels that facilitate

channel filling, and may prevent bio-fouling. The sealingof channels using HEMA/O2 plasma treated PDMS isunderway.

Acknowledgments

Dr. M. Fromm (LMN, Besancon) and Dr. J. Takadoum(ENSMM, Besancon) are respectively thanked for the helpwith recording FTIR spectra and for lending the contactangle measurement apparatus.The work was performedwithin the framework of the 4M Network of ExcellenceMulti Material Micro Manufacture: Technology and

Applications (4M) (EC funding FP6-500274-1;www.4m-net.org).

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0 1 2 3 4 5 6 7 8 9 10 11 12 13

0

10

20

30

40

50

60

70

80

90

O2

O2+C

2F

6

HEMA

WaterContact

angle()

Aging time (hour)

Fig. 2. Graph of water contact angle vs. aging time for PDMS modified

with oxygen plasma, O2+ C2F6 plasma and HEMA.

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