Photolithographic structuring of soft, extremely foldable and autoclavable hydrophobic barriers in paper

Abstract

Microfluidic paper-based analytical devices (μPADs) offer the possibility to carry out laboratory test on a piece of paper. This enables on-site monitoring in regions with scarce laboratory infrastructure but also promises cost savings for health care systems in highly-developed regions. One key element of all μPADs are hydrophobic barriers which control the liquid flow during the analysis. There are different approaches to generating hydrophobic barriers such as, e.g., wax or polymer printing as well as lithographic techniques. However, all of these introduce stiff barriers into the otherwise soft and foldable paper which significantly limits its handling. In almost all cases, once the paper is folded strongly the barriers break and are no longer able to retain a liquid sample. In this paper, we present a method for structuring hydrophobic barriers by a light-based approach making use of a light-controlled locally confined silanization. This method combines the advantages of photolithography and 3D printing in terms of process speed and flexibility with a chemical modification technique which locally modifies the wetting behaviour of the paper instead of applying a physical bulk barrier. This allows generating hydrophobic barriers which retain the flexibility of the paper and can be freely folded without losing their liquid-retaining properties even after as many as 50 fold cycles. The structures produced in this way are highly chemically stable and can even be autoclaved. We demonstrate the suitability of this method in bioanalytics using an enzymatic assay demonstrating that the silanization chemistry does not impair the biocompatibility of the substrate.