Spin coating of hydrophilic polymeric films for enhanced centrifugal flow control by serial siphoning
In this paper, we implement rotational flow control on a polymeric microfluidic “lab-on-a-disc” platform by combining serial siphoning and capillary valving for sequential release of a set of on-board stored liquid reagents into a common (assay) channel. The functionality of this integrated, multi-step, multi-reagent centrifugal assay platform critically depends on the capability to establish very reproducible, capillary-driven priming of the innately only weakly hydrophilic siphon microchannels made from common poly(methyl methacrylate) (PMMA) substrates. Due to the relatively high contact angle of the native PMMA substrate, it was practically impossible to ensure sequential release of on-board stored reagents using the capillary-driven serial siphon valves. In this work, we demonstrate that spin-coated hydrophilic films of poly(vinyl alcohol) (PVA) and (hydroxypropyl)methyl cellulose (HPMC) provide stable contact angles on PMMA substrates for more than 60 days. The deposited films were characterized using contact angle measurements, surface energy calculations and X-ray photoelectron spectroscopy spectra. The PVA and HPMC films reduced the water contact angle of the PMMA substrate from 68° to 22° and 27° while increasing their surface energies from 47 to 62 and 57 mN m−1, respectively. On the centrifugal microfluidic platform, the films were validated to enable the effective and reproducible priming of the serial siphon microchannels at low rotational frequencies while ensuring that the in-line capillary valves are not opened until their respective burst frequencies are passed. Furthermore, the biocompatibility of the proposed surface modification method was examined, and the platform was used to run a sandwich immunoassay for the detection of human immunoglobulin G, and its performance was proven to be comparable to dynamic coating using surfactants.
KeywordsHydrophilic polymers Spin coating Centrifugal microfluidic platform Lab-on-a-disc Serial siphoning Capillary valving
This work has been supported partly by the FP-7 ENIAC programme CAJAL4EU, ERDF, Enterprise Ireland (Grant No. IR/2010/0002) and the Science Foundation of Ireland (Grant No. 10/CE/B1821). Authors would like to thank R. Monaghan for the PECVD APTES depositions.
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