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Electrophoresis 2015, 36, 889–892

Lijun Sun1 Yong Luo1 Zhigang Gao1 Weijie Zhao1 Bingcheng Lin1,2 1 State

Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian, China 2 Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

Received July 25, 2014 Revised December 8, 2014 Accepted December 8, 2014

Short Communication

Easy-to-fabricate thin-film coating on PDMS substrate with super hydrophilicity and stability With the fast expansion of microfluidic applications, stable, and easy-to-fabricate PDMS surface coating with super hydrophilicity is highly desirable. In this study, we introduce a new kind of copolymer-based, single-layer thin-film coating for PDMS. The coating can exist in air at room temperature for at least 6 months without any noticeable deterioration in the super hydrophilicity (water contact angle 7°), resistance of protein adsorption, or inhibition of the EOF. In addition, this coating enables arbitrary patterning of cells on planar surfaces. Keywords: PDMS / Surface coating / Super hydrophilicity



DOI 10.1002/elps.201400366

Additional supporting information may be found in the online version of this article at the publisher’s web-site

Applications of microfluidics have exploded in recent years in many research fields ranging from biomedicine and chemistry to physics and informatics. [1] Researchers have been overcoming many challenges in order to step across the proof-of-concept threshold. One of the challenges is to find suitable material for fabricating applicable microfluidic devices [2]. PDMS [3] is among the current most popular choices due to its distinctive advantages, including elasticity, transparency, ease of fabrication, low cost, and biocompatibility; however, the strong hydrophobicity of PDMS surface results in severe difficulties in manipulating fluid in microchannels. Tremendous efforts have been taken to change PDMS surface from hydrophobic to hydrophilic. The approaches employed include physical adsorption, [4–7] sol-gel chemistries, [8, 9] ultraviolet (UV) grafting [10], layer-by-layer assembly [11–13], plasma polymerization [14], UV-mediated graft polymerization [15], electrostatic layerby-layer self-assembly [16], linking by platinum-catalyzed hydrozilization [17], tethering via a swelling-deswelling process [18], atom-transfer radical polymerization [19], and photo-induced radical polymerization [20], etc. Though versatile and workable, these approaches generally encounter

Correspondence: Professor Yong Luo, State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116012, China E-mail: [email protected] Fax: +86-411-84986360

Abbreviations: AHPCS, allylhydridopolycarbosilane; DMA, dimethylmethacrylate; GMA, glycidyl methacrylate; UV, ultra violet; WCA, water contact angle  C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a challenge that the hydrophilicized surface tends to recover its original hydrophobicity, caused by the migration of uncured PDMS oligomers from the bulk to the surface and the rearrangement of highly mobile polymer chains featuring Si–OH bonds toward the bulk at room temperature. Therefore, much endeavor has been taken to find ways to create hydrophilic coatings with long-term stability to meet the needs of real-world microfluidic applications. Polymer grafting is a promising way to create stable coatings by forming a thin film on the PDMS surface, which can hardly be affected by uncured PDMS oligomers and mobile PDMS polymer chains in the bulk. Lillehoj et al. attached a PEG layer on the PDMS surface, and the PEG-modified PDMS surface showed long-term stability maintaining a water contact angle (WCA) ⬍22° for 47 days at room temperature under atmospheric conditions [21]. Makamba et al. created hydrophilic PDMS surfaces by using electrostatic selfassembly of polyethyleneimine and poly(acrylic acid) on top of a hydrolyzed poly(styrene-alt-maleic anhydride) base layer adsorbed on the PDMS surface. This three-layer thin film can resist hydrophobicity recovery in air over five months of the study with a WCA of 20° [16]. Li et al. grafted allylhydridopolycarbosilane (AHPCS) onto a plasma pretreated PDMS surface, and the resulting AHPCS film was converted into a silica film via hydrolysis in NaOH solution. The WCA after hydrolysis decreased to 35°, and the EOF was maintained for 3 months of measurement [22]. These coatings work in a long period, but fall in short of moderate hydrophilicity and lengthy procedure to graft polymer films on PDMS.

Colour Online: See the article online to view Figs. 1 and 2 in colour.

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In this communication, we introduce a new kind of thin-film coating that demonstrates excellent performance of long-term hydrophilicity, resistance of protein adsorption, and inhibition of EOF, as well as ease of fabrication. The thin film is made from dimethylmethacrylate (DMA) copolymerized with poly(glycidyl methacrylate) (GMA). DMA renders the polymer film hydrophilicity, while the epoxy group of GMA reacts with the silanol and carboxyl groups on the PDMS surface to fasten the polymer film on the surface. The coating requires only a simple two-step procedure: (1) Fill an oxygen plasma-treated PDMS microchannel with the DMA-GMA copolymer solution for 15 min; (2) Draw the copolymer solution out of the microchannel and leave the microchannel in an oven at 75°C for 30 min. The DMA-GMA copolymer solution is prepared as the following: Mix DMA and GMA in water, and degas the solution thoroughly by argon bubbling for 10 min. Add 0.1% TEMED v/v and 0.1% K2S2O8 w/v to the solution to initiate the polymerization for 30 min. The copolymer solution is then extensively dialyzed against water by 12 000 molecular weight cut off dialysis membranes to remove unpolymerized monomers and small polymer molecules for 2 days with water changed every 12 h. The existence of the copoly(DMA-GMA) film on the PDMS surface was verified by FT-IR spectroscopy. As shown in Fig. 1A, the appearance of the peak at 1625 cm-1 (C=O stretching) serves as unambiguous evidence because there is no C=O in PDMS, while the peak at 3451 cm–1 (–OH stretching) suggests the polymer film is covalently bonded to the native PDMS surface because the hydroxyl groups can only be produced when the epoxy groups in the polymer film reacts with the silanol and carboxyl groups on the PDMS surface. The morphology of the copoly(DMA-GMA) film on the PDMS surface was characterized by tapping-mode atomic force microscopy. Figure 1B clearly shows the copoly(DMAGMA) film lays on the right part of PDMS surface, and the average thickness of the film was measured to be 5.2 nm. The WCA of the copoly(DMA-GMA) film was measured to be 7° (Fig. 1C), which was much lower than other types of polymer film reported previously [16, 21, 22]. This small WCA was maintained for 6 months during the study. The electrokinetic stability was tested over the same period of 6 months by measuring the EOF at four different pH values (3.0, 5.0, 7.0, and 9.0). Nearly constant EOF mobilities of (3.5 ± 0.4) × 10−5 cm2 v−1 s−1 were obtained in all of these measurements, which implies that the copoly(DMAGMA) film inhibits the EOF and is highly stable. The high stability of the copoly(DMA-GMA) film can be explained by the fact that GMA and DMA are cross-linked to form a thin film that is covalently bonded on the PDMS surface firmly. Figure 2A compares the nonspecific protein adsorption in native and copoly(DMA-GMA)-coated PDMS microchannels. The fluorescent intensity of native microchannel flushed by BSA-FITC is 20.1 ± 3.5; the fluorescent intensity of native microchannel flushed by LYZ-FITC is 20.1 ± 2.5. As compared, the fluorescent intensity of coated microchannel flushed by BSA-FITC is 2.1 ± 0.9; the fluorescent intensity of  C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electrophoresis 2015, 36, 889–892

Figure 1. (A) FT-IR spectra of native PDMS and copoly(DMAGMA)-coated PDMS, recorded by Continuum Infrared Microscope (Thermo Nicolet, MA, USA) coupled with a liquid nitrogen cooled MCT detector and a KBr crystal beamsplitter. The spectra were an average of 50 scans at a resolution of 0.4 cm−1 . (B) Atomic force microscopy image of a coated PDMS surface on which the right part was modified with the copoly(DMA-GMA) film. The equipment used was a Digital Instruments Multi-mode Scanning Probe Microscope (SPM) with a Nanoscope 3A controller (Santa Barbara, CA) operating in tapping mode at a scan rate of 2 Hz using an etched silicon probe. The coated PDMS surface exhibits distinctive hill-like features, representing copoly(DMAGMA) chains, whereas the native PDMS surface exhibits a smooth profile. (C) The WCA (␪) of the copoly(DMA-GMA) film was measured by an OCA 20 Optical Contact Angle Measuring Device (Dataphysics, Germany). A 2-␮L droplet of deionized water was used for each measurement, and three measurements were taken for each surface.

coated microchannel flushed by LYZ-FITC is 2.0 ± 0.4.These data clearly indicate that the copoly(DMA-GMA) film is able to suppress the adsorption of BSA and lysozyme (LYZ). Figure 2B demonstrates the copoly(DMA-GMA) coating’s application in microchip electrophoresis. Two model proteins, www.electrophoresis-journal.com

Electrophoresis 2015, 36, 889–892

Figure 2. (A) Fluorescence image of proteins adsorptions in native and copoly(DMA-GMA)-coated PDMS microchannels. Channel sides are marked with white dashed lines. BSA and LYZ are labeled in 100 mM sodium bicarbonate (pH 8.3) by FITC at a molar ratio of 1:1 with gentle shaking at 4°C overnight, and then purified by Sephadex G-25 spin column. PDMS microchips, both modified and unmodified, were filled with a 2 mg/mL protein solution in 100 mM phosphate buffer (pH 7.4), incubated for 20 min at 25°C, and then rinsed with 5 mL phosphate buffer. Fluorescence images were recorded with a fluorescence microscope (IX-71, Olympus, Japan). (B) Separation of fluorescently labeled proteins in a PDMS microchannel coated with a copoly(DMAGMA) film. The microchannel was 50 ␮m deep and 76 ␮m wide. A home-made confocal LIF system was used. The sample was a mixture of LYZ and ribonuclease A (RNa) (100 ␮g/mL each) in water. Injection conditions: float injection, 500 V/cm, 1-cm injection distance, 20-s injection time. Separation conditions: 400 V/cm, 2-cm separation distance; 30-mM phosphate sodium buffer, pH 9.0.

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Figure 3. Cell patterning on PDMS surface. Copolymer coatings were firstly patterned on the PDMS surface. Then HUVEC cells were applied on the surface, and they were selectively attached on the bare area of the surface.

This work was financially supported by the General Project of the Education Department of Liaoning Province, China (No. L2012013), National Key Scientific Instrument and Equipment Development Project, China (No. 2011YQ03012404) and State Project For Essential Drug Research and Development, China (No. 2013ZX09507005001). The authors have declared no conflict of interest.

References lysozyme (LYZ), and ribonuclease A (RNa), are separated with high efficiency in a copoly(DMA-GMA)-coated PDMS microchip. This separation is otherwise impossible with native PDMS microchips due to the irreversible adsorption of both proteins on the native PDMS surface. Another important application of the copoly(DMA-GMA) coating is cell patterning. The coating resists cell adhesion to enable cell patterning on planar surfaces. Figure 3 shows such an example, in which cells could only grow on the bare PDMS surface and the coated areas were left empty. Another observation in Fig. 3 is that the coating enables precise control of the distances between two cell populations, making this a potentially powerful tool for studies of cell–cell communications. In conclusion, we have developed a new kind of PDMS surface coating featuring the copoly(DMA-GMA) film. The copoly (DMA-GMA) film coating has four distinctive advantages: (1) ease of fabrication; (2) long-term super hydrophilicity; (3) high resistance of protein adsorption; (4) sufficient inhibition of EOF. It also enables arbitrary patterning of cells on planar surfaces with precisely controlled distances of cell populations. This technical innovation is straightforward to follow and has wide applications in the field of microfluidics.  C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Easy-to-fabricate thin-film coating on PDMS substrate with super hydrophilicity and stability.

With the fast expansion of microfluidic applications, stable, and easy-to-fabricate PDMS surface coating with super hydrophilicity is highly desirable...
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