Dye deposition patterns obtained in line printing on macroporous membranes: Improvement of line sharpness by liquid redistribution

We report on dye deposition patterns obtained when printing dye-labeled protein solutions onto a macroporous membrane. A typical area of application would be the manufacturing of immunochromatographic tests, but the results bear significance for a wide range of printing processes. As the liquid is taken up by the membrane, the protein adsorbs non-specifically to the wetted membrane surface. The transport of protein to the pore surface involves spreading of a drop on top of the membrane, imbibition of liquid into the membrane, adsorption of protein at the pore surface, liquid redistribution from large to small pores, and liquid redistribution from the center of the wetted area to the rim. Two techniques were employed to monitor the transport of protein, which are, firstly, conventional optical imaging of the wet membrane, and, secondly, fluorescence microscopy on vertical cuts through dried samples. The latter technique visualizes protein deposition. It is conceptually related to the acquisition of breakthrough curves in chromatography. Comparison of the deposition patterns obtained with different amounts of proteins allows to reconstruct the flow pattern. We frequently find a brightening of the rim, which can be traced back to redistribution of liquid from the center to the edge during drying and to a preferential uptake of liquid across the rim of the drop. Small pores and slow drying are beneficial for the achievement of sharp printing patterns.