A cost-effective two-step method for enhancing the hydrophilicity of PDMS surfaces

Transcription

1 BioChip J. (2014) 8(1): DOI /s Original Article A cost-effective two-step method for enhancing the hydrophilicity of PDMS surfaces Gymama Slaughter 1 & Brian Stevens 1 Received: 28 December 2013 / Accepted: 9 February 2014 / Published online: 20 March 2014 The Korean BioChip Society and Springer 2014 Abstract Poly(dimethylsiloxane) (PDMS) is commonly used for fabricating micro- and nanofluidic devices due to its low cost and ease of fabrication. The major disadvantage to using PDMS for these applications is its hydrophobic properties. The current methods that enhance the hydrophilicity of elastomers used for micro- and nanofluidic applications are intricate and cost-inefficient. This contribution demonstrates how the hydrophobic PDMS surface can be chemically modified in 5 minutes using a two-step silanization method in an oxygen-saturated environment to provide a highly stable hydrophilic surface for irreversible PDMS to PDMS bonding and PDMS to borosilicate glass bonding. The surface wettabilities of the modified and pristine PDMS were characterized over a wide range of time using static contact angle measurements. The modified PDMS surfaces exhibit enhanced stable hydrophilicity with a contact angle of 87 for 5 days. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectra of the modified and pristine PDMS surfaces confirmed the presence of hydroxyl groups of water molecules in the modified PDMS at 3150 cm -1 in addition to the characteristic peaks of pristine PDMS. This reliable silane beaker chemistry-based method allows easy bonding of the PDMS surfaces for liquid containment and irreversible bonding to borosilicate glass. This approach can effectively be adapted for enhancing the hydrophilicity of PDMS, which ensure relevance for current and future microand nanofluidic applications. 1 Bioelectronics Laboratory - Department of Computer Science and Electrical Engineering, University of Maryland Baltimore County, Baltimore MD, 21250, USA Correspondence and requests for materials should be addressed to G. Slaughter ( gslaught@umbc.edu) Keywords: Irreversible bonding, PDMS, Hydrophilic stability, Silanization, Oxygen saturation Introduction Recently elastomers and polymers have become popular in micro- and nanofluidic research because of their relatively low cost and their ease of use for rapid prototyping of fluidic devices 1. Of all polymers being used in the field today, PDMS has received significant attention because of its optical transparency, elasticity, and biocompatible properties. Besides being cost-effective, PDMS is highly chemical inert, permeable to gases and can be molded to exhibit relatively small micron sized features 2,3. However, PDMS is naturally hydrophobic, which makes it difficult to permanently bond to multiple types of substrates (i.e., PDMS, glass, etc.) in a variety of fluidic applications without modifying the surface of PDMS. The current methods available to make PDMS surfaces more hydrophilic include oxygen plasma treatment 4-7, ultraviolet treatment 8, chemical vapor deposition 9, metal and metal oxide coatings 10-12, layer by layer deposition 13-18, partial curing methods 19,20, suspended gel methods 21, silanization 22,23, dynamic surface modification 24-28, protein absorption 29-31, polymer grafting 32 and combinations of these methods Many of these methods 40,41 are very time consuming and expensive due to the cost of materials and facility usages. Oxygen plasma treatment is the most common method used to enhance the hydrophilicity of PDMS, however, it is not cost effective and many research laboratories using PDMS for rapid prototyping do not have access to a plasma asher or plasma cleaner. In addition, oxygen plasma treated PDMS surfaces revert back to a hydrophobic state

2 BioChip J. (2014) 8(1): Step 1) Silanization solution heated at 110 C. Step 2) Insert PDMS substrates into solution. Step 3) Bubble O 2 into solution. Step 4) Place PDMS together. Step 5) Bond the PDMS substrates for 30 minutes at 110 C. Step 6) PDMS substrate is then flip over to heat for an additional 30 minutes. Figure 1. (Process A) An illustration of box fabrication by casting PDMS on SU-8 mold structures and (Process B) PDMS Casting on silicon wafer to create lids. after 12 hours of treatment 42. A more cost effective approach to modifying the surface of PDMS is silanization. Silanization introduces hydroxyl groups on the surface of PDMS, which renders the surface of the PDMS hydrophilic 22,23. However, silanized PDMS surfaces also exhibit hydrophilic instability similar to that of oxygen plasma treatment for bonding two substrates together 23. To address the current hydrophillic instability of silanized PDMS surfaces, the present work exploits the use of a two-step surface modification method using chloromethylsilane in an oxygensaturated environment after a short chloromethylsilane treatment to enhance the hydrophilicity of PDMS surface beyond the few hours reported by other research groups 41,42. Results and Discussion The two IR spectra for the modified PDMS and the pristine PDMS display very similar spectra values. Figure 2 shows the IR spectra for the modified and pristine PDMS surfaces. The characteristic peaks for PDMS 43 at 2962, 1409, 1258, 1073, 1018, 844, 796, and 691 cm -1 are present in both of the modified and pristine PDMS. As illustrated with the lines, there are two IR bands of interest that appear in regions for stretching vibrations of the hydroxyl groups (-OH) and water molecules at 3048 cm -1 and 3150 cm -1, respectively of the modified PDMS. This demonstrates the surface characteristics of PDMS after the novel silanization treatment in an oxygen-saturated environment.

3 30 BioChip J. (2014) 8(1): Pristine PDMS Modified PDMS 90 % Transmittance Wavenumbers (cm -1 ) Figure 2. ATR-FTIR spectra for pristine and modified PDMS surfaces. The distinct changes in the IR spectra (peak shifting) in the hydroxyl and water stretching vibrations indicate that the modified PDMS surface undergoes covalent bond formation with the hydroxyl bond of water molecules. The Si-CH 3 groups are converted to Si- OH silanol groups using the silanization treatment, thereby rendering the surface temporary hydrophilic. However, the expected silanol peak (-OH group) 44 at 3678 cm -1 was not observed in the spectra possibly due to the attenuation of its peak intensity. This means there are water molecules and hydroxyl groups on the surface of the modified PDMS sample, which then allows for PDMS to exhibit hydrophilic properties. Therefore, there is a clear a difference between the two spectra demonstrating that the silanization in an oxygen-saturated environment method introduces hydroxyl groups of water molecules onto the surface of the PDMS sample. The presence of hydroxyl groups and water molecules on the surface of a substrate plays an important role in the wetting behavior of the surfaces. In recent years, there has been an increasing interest in the surface modification of the hydrophobic PDMS surfaces, due to their potential applications in micro- and nanofluidics prototyping 45. The wetting of the modified PDMS and pristine PDMS surfaces were investigated. The primary data collected were contact angle measurements of pristine PDMS and each consecutively modified PDMS samples to determine the degree of wetting when the PDMS samples interact with DI water. Small contact angles ( 90 ) correspond to high wettability, while large contact angles ( 90 ) correspond to low wettability 46. The results of the interactions of modified PDMS and pristine PDMS with DI water are depicted in Figure 3. Modified PDMS samples were prepared over the course of 10 days. Day 0 represents the samples prepared on the testing day and day 9 represent samples prepared 10 days before the contact angle measurements were acquired. The pristine PDMS exhibited very poor wettability with a static contact angle of 105, which is in agreement with the contact angle of PDMS 41. After the modification of the PDMS with chloromethylsilane (0.1 vol%) in an oxygen-saturated environment, the static contact angle decreases with the change in surface modification treatment and time, therefore demonstrating the high wettability of the PDMS surface upon treatment with this novel process. Over the 10 day period, the contact angle increased to its original hydrophobic state of 105 on day 6. A slight decrease was observed in the measured contact angle on days 5 and 7. This can be attributed to the migration of different molecular weight species across the surface and bulk PDMS interface. The silanization method for PDMS developed in an oxygen-saturated environment represents a new class of silanized PDMS with enhanced hydrophilic stability of up to 5 days and has the potential for applications in micro- and nanofluidics. One sample was modified with this novel method and then fully cured for 1 hour at 110 C (Figure 3: fully-cured PDMS). This demonstrates that fully curing the modified PDMS sample causes the modified sample to revert back to

4 BioChip J. (2014) 8(1): Contact angle (Degrees) Contact angle of treated PDMS over nine days Postheated PDMS Pristine PDMS Piecewise Interpolating Oxidized PDMS Data Days Figure 3. Contact angle measurements: The effect of silanization on enhancing the hydrophilicity of PDMS overtime. Bond area (%) PDMS-PDMS PDMS-Borosilicate glass Figure 4. Evaluation of bond strength using salient peel test. the original hydrophobic state. For micro- and nanofluidic applications, it is imperative to create strong irreversible bonds between PDMS samples as well as PDMS to glass substrates. Two PDMS surfaces were brought into contact with each other immediately following surface modification with chloromethylsilane (0.1 vol%) in an oxygen-saturated environment on a similarly modified borosilicate glass and fully cured for 1 hour at 110 C. To assess the bond strength of the modified PDMS samples, the amount of adhesiveness was characterized by a manual peel of the modified PDMS from the underlying PDMS layer and glass through a mechanical shear test. The bonding strength is represented as the percent area intact after peeling the top PDMS layer from the bottom PDMS or glass substrate. The area of the PDMS substrate before peeling was 1 cm 2 and a thickness of 1 mm. Figure 4 illustrates the results from the peel test, which show excellent bond quality for PDMS bonding with modified PDMS and very good quality for borosilicate glass samples. The peel test was conducted in 2mm Figure 5. Image of irreversible bond form between PDMS- PDMS bonded to borosilicate glass substrate after being in DI water for two weeks. triplicate and consistent bond areas were observed for the samples analysed. The PDMS to PDMS bonded samples demonstrate the best bonding quality compared to PDMS to borosilicate glass samples. The instability in bonding may be attributed to the different surface chemistry and topology between the modified PDMS and borosilicate glass (i.e., difference in the surface roughness). In addition, a fully cured, bonded and modified PDMS to PDMS to borosilicate glass was immersed in DI water to monitor the delamination of the samples bonded together. Figure 5 shows that the irreversible bond created between the two modified PDMS

5 32 BioChip J. (2014) 8(1): irreversible bond formed between the modified PDMS samples and the modified PDMS samples to glass. The resulting chemical changes in the PDMS sample upon modification were determined using ATR-FTIR, which confirmed the presence of hydroxyl groups of water on the surface of the modified PDMS. This approach is enabled by using simple 5 minutes silane beaker chemistry to modify PDMS surfaces for rapid microand nanofluidic device prototyping. The success of this approach is due to activating the PDMS for bonding surfaces together and for maintaining very good wettability of the surfaces modified. Future work will involve the mechanical characterization of the bond/ adhesion strength between adhered PDMS samples. surfaces and the glass surface and shows no sign of delamination after more than 2 weeks in DI water. Another approach involved the filling of a reservoir created using two modified PDMS samples with red colored DI water via a needle syringe. The two open holes were created for solution delivery into the reservoir and for venting. The two holes were sealed using marine epoxy and cured for 15 minutes. A leak test was performed and Figure 6 shows the bond formed is water tight and suitable micro- and nanofluidic device fabrication and no detectable leak was observed when immersed in DI water for 2 weeks. The resulting novel surface modification technique is an easily adaptable and cost-effective method for enhancing the hydrophilicity of PDMS to rapid prototype micro- and nanofluidic devices. Conclusions 2mm Figure 6. Leak test of fabricated reservoir components (box and lid structures) irreversibly bonded together filled with red colored DI water for two weeks. The feasibility to enhance the hydrophilicity of PDMS by utilizing the silanization in an oxygen-saturated environment method has been demonstrated. This novel approach is adaptable to perform the cost-inefficient oxygen plasma treatment using a range of silane in an oxygen-saturated environment. The modified PDMS surfaces showed very good hydrophilic stability for up to 5 days (74 to 87 ) and upon fully curing modified PDMS, the hydrophobic characteristic of PDMS was recovered. The bonded PDMS sample to glass and the bonded reservoir demonstrated a strong Materials and Methods Materials PDMS (SYLGARD 184: 2-compartment base and curing agent) was purchased from Dow Corning. Ethanol and chloromethylsilane were purchased from Sigma- Aldrich. Oxygen gas was purchased from Roberts Oxygen. Nexus 670 FTIR spectrometer was used to evaluate the changes on the surface of the pristine and modified PDMS samples. Replicate contact angle measurements were acquired using the ramé-hart 190 contact angle goniometer. PDMS casting and surface modification In order to optimize the silanziation protocol of PDMS for enhanced hydrophillicity, a two-part compartment PDMS with base and curing agent (10 : 1 ratio) was thoroughly mixed and degassed in a vacuum desiccator to remove all the bubbles from the mixture prior to casting. PDMS is casted on SU-8 structures (1 cm 1cm 1mm(L W H) square molds) made using UV lithography on a three inch silicon wafer. The SU- 8 structures were treated with one to three drops of chloromethylsilane at 80 C under vacuum to allow for easily pealing of PDMS from the SU-8 structures 28. The SU-8 structure with the casted PDMS was partially cured at 80 C for 1 hr. The casted PDMS was then removed from the SU-8 mold upon cooling to room temperature and diced into sample pieces consisting of boxes and lids as illustrated in Figure 1. Our goal to enhance the hydrophilicity of the PDMS surface takes advantage of a novel two-step silanization method described below. The process starts with creating hydroxyl groups (-OH) on the PDMS surface by immersing the PDMS sample in a preheated receiving solution composed of chlorotrimethylsilane (0.1 vol%) in ethanol, followed by heating of the PDMS

6 BioChip J. (2014) 8(1): sample in the receiving solution at 110 C for 5 minutes while pure oxygen is pumped into the solution at a flow rate of 0.01 sccm to facilitate the creation of hydroxyl groups on the surface of PDMS. In the absence of the oxygen-saturated environment, the PDMS samples failed to bond to each other or to glass substrates. The modified samples created using the two-step silanization method were stored in a dried petri dish to contain the samples until testing. Altogether, 10 PDMS sample surfaces were treated over the course of 10 days. A set of four PDMS samples were fully cure at 110 C for 1 hour to evaluate the bonding of PDMS to PDMS and to borosilicate glass. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy was used to provide chemical evidence for the introduction of hydroxyl groups of water molecules on the modified PDMS surface. 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