Millimeter-Scale Contact Printing of Aqueous Solutions using a Stamp Made out of Paper and Tape
The Whitesides lab has devoted great effort to develop paper-based microfluidic technology. Previous work adapted a commercial printer to achieve micro-scale resolution and accurate reagent delivery, the present paper demonstrates printing with using a low cost, easy-to-assemble device. The technology is equipped to deliver aqueous reagents with millimeter scale resolution. Applications include low cost medical diagnostic devices.
Materials and Methods
Devices were assembled with layers of patterned paper attached by layers of double-sided tape. "Ink", aka, the reagents of interest, flows from the rechargeable reservoir in the top paper layer. The ink then wicks through the sequence of paper layers, finally assuming the desired pattern on the stamp. A plexiglass piece provides mechanical support for the stamp. Printing is achieved by pressing the stamp to a fresh substrate, delivering the desired reagents. Stamps produce as many as 40 prints before recharging the reservoir, and one prototype was found to print 500 times without noticeable degradation. Test inks and substrates were "aqueous solutions of small molecules, proteins, and nucleic acids in different shapes and patterns on a wide variety of substrates including paper, glass, polystyrene (PS), nitrocellulose (NC) membranes, cellulose acetate (CA) membranes, hydrophilic polyvinylidene fluoride (PVDF) membranes and thin-layer chromatography (TLC) plates made with silica gel (as shown in ESIf )." Figure 1 shows the layout of an example device.
To test the quality of the printed patterns, circles were printed on chromatography paper using low-molecular-weight aqueous dye solutions. Circular spots with nominal diameters of 1mm were printed in a 4 second process (2 seconds of contact with the paper). The resulting spots had a mean measured diameter of 1.1 ± 0.2 mm, with a coefficient of variation of 18%. This was reduced to 11% when "wax printing" was used to establish a hydrophilic spot within a hydrophobic zone. More detail is show in Figure 2 below.
Quality could be increased by such measures as controlling contact time, ink-substrate wicking, fluid delivery, or using pre-defined, patterned substrates for printing. Regardless of future optimization, the device described is a successful low-cost, low-resolution printer. It is also an apt demonstration of the power of capillary and wetting forces. By further manipulating these interactions between the ink, paper channels, and substrate the authors could increase the precision and robustness of the printing method via chemistry alone.