Fabrication of paper microfluidic devices using a toner laser printer

Abstract

This paper describes a method to fabricate microfluidic paper-based analytical devices (μPADs) using a toner laser printer. Multiple methods have been reported for the fabrication of μPADs for point-of-care diagnostics and environmental monitoring. Despite successful demonstrations, however, existing fabrication methods depend on particular printers, in-house instruments, and synthetic materials. In particular, recent discontinuation of the solid wax printer has made it difficult to fabricate μPADs with readily available instruments. Herein we reported the fabrication of μPADs using the most widely available type of printer: a toner laser printer. Heating of printed toner at 200 °C allowed the printed toner to reflow, and the spreading of the hydrophobic polymer through the filter paper was characterized. Using the developed μPADs, we conducted model colorimetric assays for glucose and bovine serum albumin (BSA). We found that heating of filter paper at 200 °C for 60 min caused the pyrolysis of cellulose in the paper. The pyrolysis resulted in the formation of aldehydes that could interfere with molecular assays involving redox reactions. To overcome this problem, we confirmed that the removal of the aldehyde could be readily achieved by washing the μPADs with aqueous bleach. Overall, the developed fabrication method should be compatible with most toner laser printers and will make μPADs accessible in resource-limited circumstances.

Fig. 1. (A) Schematic illustration of the fabrication of the toner-based μPAD; the top view and the cross-sectional view of the filter paper during the fabrication are presented (Step 1) Toner was printed on a filter paper to create the patterns of the microchannels. (Step 2) The patterned filter paper was then heated on a hotplate at 200 °C for 60 min to create hydrophobic patterns. (B) A plot showing the width of the toner gap at the front side before the reflow of the toner (with respect to the design width) (n = 4). (C) A plot of the actual channel widths measured from the front and back of the paper after the reflow of the toner (with respect to the design width) (n = 4).

Fig. 2. Dimensions of the toner-based channels. A solution containing red dye was deposited at the center of the eight microchannels (described in Fig. 1). The fluid flow along the hydrophilic region of the microchannel due to capillary action was visualized in red. Optical micrographs were taken at the front, the back, and the cross-section of the microchannels. The designed widths of the channels were (A) 100 μm, (B) 200 μm, (C) 300 μm, (D) 400 μm, (E) 500 μm, (F) 600 μm, (G) 700 μm, (H) 800 μm. Scale bar = 500 μm.

Fig. 3. Examples of toner-based μPAD patterned on the filter paper. (A) A well with eight channels. (B) A Y-shaped channel. (C) A channel with multiple regions for the reagents. (D) A 16-well paper plate. Scale bars = 10 mm.

Fig. 4. Colorimetric assay using the 96-well paper plate. (A) Samples of glucose (1.0–80.0 mg mL⁻¹) showing the color change to orange with Benedict's solution on the 96-well paper plate. (B) A plot showing the blue intensity from the RGB image of the μPAD with respect to the concentration of glucose (n = 4). (C) Samples of BSA (100–500 μg mL⁻¹) showing the color change due to the formation of blue tetrabromophenol blue (TBPB) complex. (D) A plot showing the calculated intensity at 620 nm estimated from the RGB image of the μPAD with respect to the concentration of BSA (n = 4). Scale bar = 10 mm.

Fig. 5. (A) Optical images of the wells of cellulose papers added with 10 μL of BCA assay and treated with D.I water (Group 3) and bleach (Group 4). Four groups of the cellulose samples were studied. Group 1: unheated pristine filter paper, Group 2: heated on a hot plate at 200 °C for 60 min, Group 3: heated and washed in D.I. water, and Group 4: heated and treated with bleach. Both the front side and the backside were shown for each group. (B) A graph showing the blue intensity of the wells. The lower blue intensity indicated the darker color of the well, suggesting the presence of the aldehydes in the well that produced the BCA complex via reduction of Cu²⁺ (n = 4).

Fig. 6. Benedict's test in toner-based μPAD. (A) The images of the paper wells in three conditions of the cellulose wells: (1) unheated (negative control for aldehydes), (2) heated, and (3) heated and washed with the bleach. (B) Graph of the blue intensities in the wells with respect the concentration of glucose. The coloration in the group of heated samples suggested the presence of the aldehydes from pyrolysis, which was effectively removed by washing the μPAD with the aqueous solution of the bleach (n = 4).