398887 Enhancement of Stability and Superhydrophilicity of Plasma-Modified Microfluidic Materials

Monday, November 17, 2014
Galleria Exhibit Hall (Hilton Atlanta)
Lea Winter1, Bradley Da Silva1, Guillaume Schelcher1, Cedric Guyon1, Daniel Bonn2 and Michael Tatoulian1, (1)Institut de Recherche de Chimie Paris, Chimie ParisTech/CNRS, Paris, France, (2)Institute of Physics, University of Amsterdam, Amsterdam, Netherlands

Enhancement of Stability and Superhydrophilicity of Plasma-Modified Microfluidic Materials

Overview

Microfluidic devices represent integral means for research in diverse fields such as catalysis [1], biology, and physics, and they provide the medium for on-chip diagnostics. Microfluidics are most readily constructed from polymers, but most of the polymers used in this field are hydrophobic, and many microfluidics applications require hydrophilic or hydrophilic/hydrophobic-patterned [2] microchannel surfaces. The primary material used to manufacture microfluidic devices is polydimethylsiloxane (PDMS). Attempts to deposit a hydrophilic coating onto this hydrophobic polymer have proven unsuccessful, since the material exhibits hydrophobic recovery only in a day [3-7]. Three new polymers (COC, NOA and THV) were investigated as microfluidic materials, and all were shown to remain hydrophilic for at least two months following coating with a hydrophilic silica-like layer by plasma-enhanced chemical vapor deposition and sputtering.

Materials

Cyclic olefin copolymer (COC) is a thermoplastic copolymer composed of norbornene and ethylene groups. Due to its high glass transition temperature (Tg =130C) [4], COC is easily shaped and can withstand modifications such as photo-patterning, wet etching, and surface functionalization [5]. Norland Optical Adhesive (NOA81) is a thiolene-based photocurable resin that exhibits ideal properties for microfluidic applications [6]: it is optically transparent, dissolvable, safe, and biocompatible [7]. A new class of fluorinated polymer (THV) exhibits low surface energy and high chemical resistance, making it well-suited for droplet and organic solvent microfluidics [8]. Surface functionalization is difficult, though, because of the aliphatic carbon and fluorine atoms that contribute to the polymer backbone [10] (Fig. 1).

Methods & Results

The polymer surfaces were modified using two different plasma techniques: sputtering in a commercial reactor and plasma-enhanced chemical vapor deposition (PECVD) in a homemade reactor. Using a hexamethyldisiloxane precursor in the PECVD reactor, a silica-like coating was deposited on the surface of the polymer substrates. For the sputtering, a pure silica target was used. Wettability of the coated samples was measured using water contact angles (WCA), over an aging period of two months, in air. For all three samples coated using the PECVD method, the WCA remained under a value of 10 for the two-month period; in contrast, coated PDMS clearly recovered its original hydrophobicity for both plasma processes (Fig. 2).

Sputtered samples of COC and NOA81 also remained hydrophilic for the two-month period. Incorporation of silanol groups onto these polymer surfaces was further confirmed by FTIR and XPS analyses (Figs. 3, 4).

Ellipsometric measurements revealed that the sputtered silica layer was 30 nm in thickness, whereas the PECVD silica layer was 1 m. The sputtered layer on COC became slightly less hydrophilic over time, and THV was not successfully rendered hydrophilic by sputtering. The wettability of THV just after sputtering was significantly less than any other sample (WCA of 62), and the WCA value increased with time. It is hypothesized that the presence of fluorine atoms in the THV polymer backbone (as confirmed by XPS and FTIR analysis) prevent the sputtered silica layer from adhering, resulting in delamination of the hydrophilic coating. The significantly thicker PECVD layer may explain why coatings deposited by PECVD were more robust, combined with the effect of the low sticking coefficient for THV.

Conclusions

The successful coating of new polymeric materials with a durable hydrophilic coating provides an essential innovation for a range of studies involving microfluidics studies and medical applications.

Figure 1. XPS C1s and Si2p high-resolution spectra of the surface of sputtered silica-like coating on THV.

Figure 2. Wettability aging of plasma-deposited silica-like coating on polymers in air. PDMS by both deposition methods showed rapid hydrophobic recovery. Sputtering was ineffective at rendering THV hydrophilic. Enduring hydrophilic coatings were successfully deposited on COC and NOA81 by both methods, and PECVD was a successful method for depositing an enduring hydrophilic coating on THV.

Figure 3. FTIR-ATR spectra of the blank COC surface (continuous line) and the silica coated COC surface either functionalized using PECVD (red line) or sputtering (blue line).

Figure 4. XPS C1s and Si2p high resolution spectra of the surface of COC coated with sputtered silica-like layer.

References

1.     G. L. Chen, C. Guyon, Z. X. Zhang, B. Da Silva, P. Da Costa, S. Ognier, D. Bonn, M. Tatoulian, Microfluidics and Nanofluidics, 2014, 16, 141-148.

2.     E. Sollier, C. Murray, P. Maoddi, D. Di Carlo, Lab Chip, 2011, 11, 3752.

3.     J. Zhou, D. A. Khodakov, A. V. Ellis, N. H. Voelcker, Electrophoresis, 2012, 33, 89-104.

4.     D. Bodas, C. Khan-Malek, Sens. Act. B, 2007, 123, 368-373.

5.     J. L. Fritza, M. J. Owen, The Journal of Adhesion, 1995, 54, 33-4.

6.     V. Barbier, M. Tatoulian, H. Li, F. Arefi-Khonsari, A. Ajdari, P. Tabeling, Langmuir, 2006, 22, 5230-5232.

7.     S. Bhattacharya, A. Datta, J. M. Berg, S. Gangopadhyay, J. Microelectromechanical Sys., 2005, 14, 590-597.

8.     R.K. Jena, C.Y. Yue, Y.C. Lam, Microsystem Technologies, 2012, 18, 159.

9.     G. Schelcher, E. Martinez, S. Ognier, S. Cavadias, C. Guyon, L. Malaquin, P.Tabeling,, M. Tatoulian, Lab Chip, 2014, 14, 3037-3042.

10.  D. Bartolo, G. Degr, P. Nghe, V. Studer, Lab Chip, 2008, 8, 274–279.

11.  PH. Wgli, A. Homsy, N. de Rooij, Sens.and Act B, 2011, 156, 994-1001.

12.  S. Begolo, G. Colas, J.-L. Viovy, L. Malaquin, Lab Chip, 2011, 11, 508.

13.  Da Silva, B., Schelcher, G., Winter, L., Guyon, C., Bonn, D., Tatoulian, M. Submitted.

14.  S. Laib, B. D. MacCraith, Anal. Chem., 2007, 79, 6264-6270.

Relevant Publications

1.     Da Silva, B., Schelcher, G., Winter, L., Guyon, C., Tabeling, P., Bonn, D., Tatoulian, M. "Plasma surface modification of a new family of microfluidic chips for biological applications." European Cells and Materials (2013) 26 (6): 105.

2.     Da Silva, B., Schelcher, G., Winter, L., Guyon, C., Bonn, D., Tatoulian, M. "Enhancement of Stability and Superhydrophilicity of Plasma-Modified Microfluidic Materials." Submitted.


Extended Abstract: File Uploaded