Enhancement of Stability and Superhydrophilicity of Plasma-Modified Microfluidic Materials

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 (T_g
=130¬°C) [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 10û 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 µm. 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 62û), 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 C_1s and Si_2p 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 C_1s and Si_2p high resolution
spectra of the surface of COC coated with sputtered silica-like layer.

 

References

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Relevant Publications

 

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

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