(K-413) Surface Characterization of Novel Polymers for use in Microfluidics

Point-of-care testing (POCT) aims to reduce time, cost, and resources needed to monitor and diagnose disease for a particular patient. We are developing integrated and mixed-scale fluidic systems that combine multiple task-specific modules to complete multi-step diagnostic assays that would otherwise require trained operators to run expensive and laborious laboratory procedures. The fluidic system can run autonomously by connecting the modules to a fluidic motherboard populated with valves, interconnects, pneumatic control channels, and a fluidic network (1). Fluidic motherboards typically consist of a thin elastomer membrane serving as a valve that is bonded to a rigid thermoplastic layer of fluidic channels. The most common material used for the elastomer membrane is polydimethylsiloxane (PDMS). However, PDMS has some limitations. Here, we present an alternative elastomer membrane, elastomeric cyclic olefin copolymer (eCOC, TOPAS E-140), and an alternative thermoplastic, polyethylene terephthalate glycol (PETG). eCOC is an elastomer blend of ethylene and norbornene that has an elongation at break of >500% and does not undergo hydrophobic recovery as seen with PDMS following UV-Ozone activation to reduce its water contact angle. PETG is shown to have stronger bonding with eCOC while not sacrificing rigidity or milling capability. Such properties make eCOC and PETG highly attractive. To quantify their material properties relevant for use in the microfluidics, surface and chemical properties were extensively characterized. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and water contact angle measurements were taken before and after surface activation, which involves subjecting the polymer to UV-Ozone radiation for activation.

Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) is a non-destructive method for identifying and distinguishing chemical species in a sample through their characteristic vibrational spectrum. Specifically, the ATR accessory probes the sample surface about 2 µm deep. Water Contact Angle (WCA) analysis is another method to characterize surfaces. WCA primarily provides a measure of wettability, hydrophobicity, and the strength of contact of a solid surface. In this work, these two methods were used to compare non-activated and activated samples of eCOC and PETG.  Surface activation of the samples was performed in a UV-Ozone (UV/O₃) cleaner from Jelight Co. Samples were exposed to 5-min, 10-min, or 15-min of UV/O₃ and compared to their pristine counterparts. For ATR-FTIR, three measurements were performed on one sample and a representative measurement was selected. For WCA, two samples were taken for each polymer and exposure time with three measurements on each sample totaling twelve measurements (two per water droplet) for each data point. An average and standard deviation was calculated – shown in figure below. All samples were cleaned and prepared in a procedure using water, Micro-90, Iso-propyl alcohol, and compressed air sprayer.

ATR-FTIR showed that UV/O₃ activation altered the surface chemistry of polymers, specifically it altered eCOC by creating carbonyl groups as seen by the formation of a peak at ~1700 cm^-1. UV/O₃ activation caused a roughly 5% difference in eCOC’s transmittance, much smaller in comparison to a ~30% drop in transmittance for the other materials tested (COC-8007, PDMS, COP, PET, and PC). This difference in transmittance, however, did not affect the change in WCA after UV-Ozone exposure. The oxidation from the UV-Ozone exposure resulted in a more energetic surface in all WCA tests, thereby increasing hydrophilicity and enhancing wettability of eCOC’s and PETG’s surface, which is attractive for microfluidic applications. Lastly, different eCOC manufacturing methods, suppliers, and thicknesses were also shown to have no differences in surface chemistry thus providing flexibility when choosing eCOC across various applications. Overall, all tests performed have confirmed both eCOC’s and PETG’s attractive properties, further showing their promise in microfluidics. 

National Institutes of Health (NIH) P41 EB020594) and NIH R01EB03157901A1

1.         Pahattuge, T. N.;  Freed, I. M.;  Hupert, M. L.;  Vaidyanathan, S.;  Childers, K.;  Witek, M. A.;  Weerakoon-Ratnayake, K.;  Park, D.;  Kasi, A.;  Al-Kasspooles, M. F.;  Murphy, M. C.; Soper, S. A., System Modularity Chip for Analysis of Rare Targets (SMART-Chip): Liquid Biopsy Samples. ACS Sensors 2021, 6 (5), 1831-1839.