Microfluidic paper based analytical devices (μPADs) are cost effective, simple and a flexible tools for diagnostic assays. One of the advantages that comes with using μPADs is that compared with conventional laboratory tests these assays require smaller sample volumes. Furthermore the assays can be carried out in every environment without the necessity to use high standard laboratories.
There are two key processes that are necessary for successful production of a μPAD. First, a hydrophobic boundary layer has to be implemented on the paper in order to control the fluid flow. Second, the biological reagents required have to be coupled to the paper for the specific assay.
In order to ensure proper function and controlled fluid flow on the μPAD it is necessary to create hydrophobic barriers. These hydrophobic barriers should stay intact even when the paper is bent because otherwise the sample will leak through the barrier causing the test to fail. However, commonly used wax barriers are not bendable. It is furthermore of great importance that the hydrophobic barrier is biocompatible, so that the barrier does not influence the biological detection reagent on the paper.
Our approach is based on a silanization method to implement a bendable hydrophobic barrier on papers, which are commonly used for the creation of μPADs. Furthermore the implemented barriers are biocompatible and do not influence the detection reagent.
Structuring hydrophobic layers on paper using lithography. In the first step the silane mixture (Dimethoxydimethylsilane, Triarylsoulfonium hexafluoroantimonate and bidestilled water) is poured over the paper. Then the silane is polymerized onto the paper using UV-light. After that the sample is washed with ethanol and dried at 60 °C
Bendable hydrophobic barriers: a) The silanized area is hydrophobic and slightly colored. As depicted dyed water cannot penetrate the paper and therefore forms droplets on the surface. On the untreated area (marked with black lines) the water soaked in. b)The structured paper is bendable and not stiff like the commonly used wax barrier. This allows the creation of foldable μPADs.
The second key process during the production of a μPAD is the immobilizing of proteins onto filter paper. For that we are using lithography and used for that a custom-built maskless projection lithography setup.To structure protein patterns lithographically the proteins have to be labeled with a fluorophore. We use a fluorescein/biotin conjugate for doing so. Upon irradiation the fluorophore will bleach and a short-lived photoradical will be created. This radical will then bind to the surface and thus immobilize the protein to the surface. However, in order to immobilize proteins onto a filter paper the latter has to be treated such that the surface serves as a radical acceptor.
Immobilization of proteins on pretreated filter paper using maskless photolithography. The filter paper is pretreated with a radical acceptor coating. Flourecein-5-Biotin (F5B) is introduced and immobilized using a maskless lithography setup. The pattern is visualized by staining it with Streptavidin-Cy3.
F5B patterned filter paper. The pattern was stained with Streptavidin-Cy3.The resulting images show the KIT logo and an emoticon.
T. M. Nargang, R. Dierkes, J. Bruchmann, N. Keller, K. Sachsenheimer, C. Lee-Thedieck, F. Kotz, D. Helmer and B. E. Rapp: "Photolithographic structuring of soft, extremely foldable and autoclavable hydrophobic barriers in paper", Analytical Methods, 2018 (in publication) | Link
T. M. Nargang, M. Runck, D. Helmer, B. E. Rapp: "Functionalization of paper using photobleaching: a fast and convenient method for creating paper-based assays with colorimetric and fluorescent readout", Engineering in Life Sciences, 2016 | Link
Books and Book chapters
T. Nargang, F. Kotz, B. E. Rapp: "Structuring unbreakable hydrophobic barriers in paper", talk, SPIE Photonics West, San Francisco, USA, 2018
T. M. Nargang, F. Kotz, N. Keller, D. Helmer, B. E. Rapp: “Rapid structuring of proteins on filter paper using lithography”, talk, SPIE Photonics West, San Francisco, USA, 2017
T. M. Nargang, E. Wilhelm, F. Kotz, D. Helmer, B. E. Rapp: “UV-light based structuring of unbreakable hydrophobic barriers on commonly used papers”, poster, 19th International Conference on Miniaturized Systems for Chemistry and Life Sciences (µTAS), Gyeongju, Korea, 2015
E. Wilhelm, T. M. Nargang, B. Waterkotte, B. E. Rapp: “Protein assay structured on paper by using lithography”, talk, SPIE Photonics West, San Francisco, USA, 2015
E. Wilhelm, C. Neumann, K. Sachsenheimer, K. Länge, B. E. Rapp: “UV-light structured silanization for selective bonding and fabrication of paper-based microfluidic channels”, poster, 18th International Conference on Miniaturized Systems for Chemistry and Life Sciences (µTAS), San Antonio, USA, 2014
T. M. Nargang, B. E. Rapp: "Chemical Modification of Paper for Analytical Devices", HEALTH Europa 5, Paneuropean Network, 2018. | Link