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To provide a low-cost and durable platform to store the reaction mix for RNA-detection, glass fiber filter paper (glass microfiber paper 934-AH RTU, Whatman, GE healthcare, Germany) was chosen. To enable transport of the processed sample-containing fluid to the reaction surface, the glass filter paper was embedded in a filter paper sandwich. This sandwich is composed of layers as depicted in figure X (from left to right starting with top layer): One-sided adhesive tape (853 polyester tape, 3M), cellulose filter paper (Whatman® qualitative filter paper, Grade 1, Sigma-Aldrich), double-sided adhesive tape (467MP transfer-tape, 3M), filter paper containing holes for insertion of glass fiber paper and channels for fluid transport onto it, double-sided adhesive tape (3M), cellulose filter paper, one-sided adhesive tape.
The one-sided adhesive tapes serve to seal the paper sandwich, the top and bottom paper layers are implemented to avoid sticking of the tapes to the fragile glass fiber paper with loaded reaction mix (red). The middle cellulose filter paper is coated with hydrophobic wax (dark grey) applied with wax printer (Xerox ColorQube 8900) forming hydrophilic channels (light grey) enabling transport of fluid by capillary forces. For precise cutting of holes (white) into the layers of cellulose paper and adhesive tapes (blue), a laser cutter was used.
After assembly from bottom to top guided by orientation marks (6 small holes in every layer), sample can be applied via the inlets (holes on top and bottom in top 3 layers). To avoid interferences with the passing light through auto-fluorescence, layers were designed to have holes above and beneath the glass fiber paper. In a final product, the sealing top- and bottom-layer should be designed to have easily removable, pre-cut strip instead of the plain holes to avoid contamination even for a non-specialized end-user.
Composition of the Paper Sandwich.
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