Highly hydrophilic microfluidic device prototyping using a novel poly(dimethylsiloxane)-based polymeric mix

Abstract

We present a novel methodology to create in a simple, fast and cheap way an interpenetrating polymer network biomaterial, mixing 2-hydroxyethil methacrylate and poly(dimethylsiloxane), for long-lasting highly hydrophilic microfluidic device prototyping. The presented polymer could be potentially useful to develop point-of-care microfluidic diagnostic devices allowing blood displacement without exertion in microchannels while proving to have low biological analytes adhesion.

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It is often a goal in areas such as point-of-care (POC) instrumentation to move fluids inside a microfluidic device.^1,2 However, silicone elastomer polymers used in this kind of process suffer from an essential drawback: their surface, as well as their bulk, are hydrophobic and this hinders fluid motion and chip filling. The fabrication of hydrophilic polymers with similar properties to PDMS may considerably expand the scope of applications of microfluidics and facilitate the conception of devices having a biomedical scope.

Cross-linked poly(dimethylsiloxane) (PDMS) is a very versatile material that can be used in several biomedical applications due to its highly interesting biocompatible and mechanical properties.^3,4 Furthermore, its polymerizing conditions make it suitable for microfluidic devices prototyping.^5,6 In fact, it is the most actively developed, used and studied polymer for this kind of applications due to its advantages in the biomedical field.⁷

It is also a flexible material thanks to its Si–O bonds in the repeating unit of the molecule, so its mechanical properties are closely similar to a vein-like structure.⁸ Nevertheless, PDMS's surface energy is relatively low (around 20 mJ m⁻², much lower than other synthetic polymers).⁹ In order to use this material for biomedical and POC applications it is highly desirable to increase its wettability.

Although PDMS surface can be made hydrophilic through an oxygen plasma treatment, it will revert back to hydrophobic.^10,11 The relapse can be a major problem triggering unwanted phenomena such as undesired fluid motion or adsorption of proteins to the surface.¹² Furthermore, PDMS can absorb small hydrophobic molecules which can interfere in biosensors applications and may cause clotting when in contact with blood.¹³

So far, the toolbox of techniques used to make PDMS more hydrophilic (corona treatment, plasma treatment, laser treatment or surface grafting) fail to maintain a long-lasting condition and hydrophobic groups flourish back on its surface.^14–17 On the other hand, surface coatings or grafting with hydrophilic polymers may also increase manufacturing costs and require a more complicated procedure.¹⁸

Here we propose another method for modifying PDMS by creating an interpenetrating polymer network (IPN) using hidroxietilmetacrylate (HEMA) in a simple, fast and cheap way. Our work exploits the concept of polymer mixing to render this silicone elastomer hydrophilic.

HEMA is one of the most widely used hydrogels (therefore hydrophilic) in biomedical applications since its water content is similar to that of living tissues, it shows bio and blood-compatibility and it is resistant to degradation.^19–22

One of the advantages of using IPNs is the possibility to combine properties of different polymers, which usually are incompatible and can, in many cases, be an alternative to synthesizing a totally new polymer with customized properties. From the literature, there are some examples of studies of IPNs based on PDMS. Liu et al. described the preparation of poly(N-isopropyl acrylamide) with PDMS. The resulting IPN showed an increase in hydrophilicity in comparison to pure PDMS.²³ In another example, Turner et al. used another hydrophilic polymer, polymethacrylic acid and they formed an IPN by immersing the PDMS in the monomer followed by polymerization.²⁴ Contact angle measurement data in both cases showed a magnitude decrease down to around 60°. However, the synthesis in both cases was tedious and time-consuming.

Because PDMS has a low surface energy it must be combined with a hydrophilic monomer in order to turn it more hydrophilic. So, the present methodology shows that it is possible to prepare a hydrophilic HEMA/PDMS-IPN by using a simple two-step method that maintain, on one hand, the mechanical properties of PDMS while enhancing the wettability and the hydrophilicity of the polymer's surface.

The improvement in process comprises the idea of developing a polymer that could be used to prototype and manufacture microfluidic devices for biomedical applications plus having hydrophilic properties in order to facilitate the transport of fluids, like for example blood, therefore facilitating immunoassays or any other diagnostic tool in a simple way.

The methodology for preparing the mentioned polymer consists firstly in preparing a mixture of PDMS prepolymer (Dow Corning, Midland, USA) and HEMA (Sigma Aldrich) in a selected proportion of 5 : 1 w/w (different concentration were studied and results are shown in Fig. 1), that has to be stirred in air, temperature and atmospheric humidity conditions. Afterwards, PDMS curing agent is added and stirred with the HEMA/PDMS mixture in the same conditions, place the sample in a desiccator with vacuum applied to remove possible bubbles, and finally followed by thermal polymerization (70 °C, 2 hours). Morphological studies to characterize the mixing were done and homogeneity was confirmed (Fig. S1†). The chosen proportion fulfilled the criterion of being the one that presented a more evident decrease of hydrophobicity using the minor quantity of HEMA in order to maintain PDMS' properties.

Fig. 1 (A) Contact angle measurements (median) in different weeks and the (B) difference in terms of this measure between bare PDMS and PDMS/HEMA 5 : 1 (w/w).

Contact angles (θ_C) were measured in order to verify polymer's hydrophilicity. Sessile-drop method was used with an OCA contact angle system (Dataphysics, Filderstadt, Germany). At least five θ_C measurements were collected from the different samples.

θ_C measurements data of the surface of the HEMA/PDMS-IPN showed a decrease in its contact angle, from θ_C = 105° ± 2.8% of bare PDMS to an average value of θ_C = 60.7° ± 2.4% of the polymer described (two months tracing, Fig. 1A and difference between both scenarios in Fig. 1B and S2†).

The polymer presented is flexible (Fig. 2A), having close similar durable mechanical properties to PDMS, with a Young's modulus mean value of 3.9 MPa versus a mean value of 3.5 MPa of PDMS. Moreover, it is capable of creating stable bonds with glass by contact attachment (Fig. 2B), but not irreversible bonding through plasma-activated process. Besides being hydrophilic it is also able to reproduce small details being useful for lab-on-a-chip devices prototyping (Fig. 2C). However, a possible drawback is that is not as transparent as PDMS (HEMA/PDMS-IPN optical absorption coefficient at λ = 600 nm is 3.1 μm⁻¹, whereas PDMS value is 0.06 μm⁻¹), situation that could be a disadvantage in many situations, although its initial commitment is not to be used for standard microfluidic experimental but for POC. This optical property is aggravated with increased HEMA concentration.

Fig. 2 Polymer detail. (A) Polymer flexibility. (B) An example of a microfluidic device fabricated with the polymer attached to glass substrate. (C) Detail using an inverted optical microscope (Olympus IX71) of the polymer prototyping of the device.

In order to examine polymers applicability to diagnostic microfluidic devices, some trials where performed to evaluate the effectiveness of the HEMA/PDMS-IPN in moving human whole blood inside microchannels.

Human red blood cells were obtained from donors at the Blood and Tissue Bank (Barcelona), after written consent and in accordance with the ethics Committee protocols of the Blood and Tissue Bank. Upon receipt, cells were washed twice with incomplete RPMI and re-suspended at a 50% haematocrit in the same medium and used immediately for experimental assays.

For comparison purposes, eight microfluidic devices were fabricated, using either HEMA/PDMS-IPN (4) or bare PDMS (4) by casting the polymers in liquid form to cylindrical metal wires acting as a microchannel mold of 1 mm diameter. Once polymerized, they were punched (Harris Uni-Core 1 mm puncher) and contact-attached to a glass slide (Deltalab) for sealing purposes. A 1 ml syringe (BD Plastipak) was inserted into the access holes of the devices, which were slightly smaller than the outer diameter of the syringe to form a pressure seal, and 0.1 ml of blood sample was introduced to each device.

Blood motion behaviour inside the structures could be studied. As can be seen in Fig. 3A and ESI Movie S1,† blood was displaced from one end of the device fabricated with the HEMA/PDMS-IPN to the other without any effort or use of external pumping, just by slightly lifting one side of the device (something that does not occur with the PDMS one, as can be observed in Fig. 3B).

Fig. 3 (A) Blood sample flowing inside a chip fabricated with HEMA/PDMS-IPN without any exertion, something that does not happen in a (B) device fabricated with PDMS.

Prevention of cell and protein attachment in the polymers surface is critical for a good demeanour of the polymer's features in lab-on-a-chip apparatus useful for diagnostic purposes. To address this situation, NIH 3T3 fibroblast cultures were performed at 37 °C and 5% CO₂ in complete Dulbecco's modified Eagle's medium and afterwards cultured with the same medium in both HEMA/PDMS-IPN and bare PDMS (curing agent-to-PDMS ratio of 1 : 10) surfaces, as shown in Fig. 4A and B respectively.

Fig. 4 NIH 3T3 Fibroblast culture in the surface of (A) HEMA/PDMS-IPN and (B) bare PDMS. (C) HEMA/PDMS-IPN surface after rinsed process with media.

Results obtained after 24 hours of culture highlight the fact that cells are not capable of attaching and growing in the IPN surface (differences in morphology and proliferation can be observed in Fig. 4A and B when comparing both situations). After washing the surface with media, cells immediately detached from the HEMA/PDMS-IPN surface (Fig. 4C). Cell viability was confirmed using Trypan Blue exclusion assay.

Conclusions

In this work a new cheap methodology for improving the wettability of PDMS was thoroughly investigated.

To our knowledge, this is the first paper presenting a polymeric system including HEMA and PDMS as an IPN that can be prepared in a simple way, offering many potential future advantages in lab-on-a-chip application.

The experimental results presented show a decrease in PDMS's contact angle (from θ_C = 105° ± 2.8% of bare PDMS to θ_C = 60.7° ± 2.4%) and, therefore, in its capability of moving fluids within this polymer, capability that we consider to be of potential interest in the field of microfluidics.

Acknowledgements

This project was funded by the technology transfer program of the Fundación Botín. We thank David Izquierdo, Miriam Funes and IBEC Nanotechnology platform staff for technical help and Elena Martínez and Anna Lagunas for helpful discussions. We would like to specially thank David Caballero and Albert Garcia for their generous help. We thank Barcelona Centre for International Health Research (CRESIB) for providing the human blood samples.

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Footnote

† Electronic supplementary information (ESI) available: Supplementary Fig. S1, S2 and Movie S1. See DOI: 10.1039/c4ra14750b

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