Hemp-Based Microfluidics

Abstract

Hemp is a sustainable, recyclable, and high-yield annual crop that can be used to produce textiles, plastics, composites, concrete, fibers, biofuels, bionutrients, and paper. The integration of microfluidic paper-based analytical devices (µPADs) with hemp paper can improve the environmental friendliness and high-throughputness of µPADs. However, there is a lack of sufficient scientific studies exploring the functionality, pros, and cons of hemp as a substrate for µPADs. Herein, we used a desktop pen plotter and commercial markers to pattern hydrophobic barriers on hemp paper, in a single step, in order to characterize the ability of markers to form water-resistant patterns on hemp. In addition, since a higher resolution results in densely packed, cost-effective devices with a minimized need for costly reagents, we examined the smallest and thinnest water-resistant patterns plottable on hemp-based papers. Furthermore, the wicking speed and distance of fluids with different viscosities on Whatman No. 1 and hemp papers were compared. Additionally, the wettability of hemp and Whatman grade 1 paper was compared by measuring their contact angles. Besides, the effects of various channel sizes, as well as the number of branches, on the wicking distance of the channeled hemp paper was studied. The governing equations for the wicking distance on channels with laser-cut and hydrophobic side boundaries are presented and were evaluated with our experimental data, elucidating the applicability of the modified Washburn equation for modeling the wicking distance of fluids on hemp paper-based microfluidic devices. Finally, we validated hemp paper as a substrate for the detection and analysis of the potassium concentration in artificial urine.

Keywords: diagnostics; hemp; microfluidics; paper; urine diagnostics.

Figure 1

Hemp paper lifecycle. (a) The hemp plant can be cultivated on either side of the equator, preferably between 25th and 55th latitude parallels [31]. Seeds, leaves, and stalk are the three main parts of the hemp plant. (b) After cultivation, the hemp stalk should be split apart from roots and leaves. The stalk is comprised of two parts: woody core (hurd) and bast fiber. Bast fiber possesses 57–77% cellulose content and tougher fibers compared to hurd, with lower cellulose content (40–48%) [31,33]. (c) The retting process breaks the chemical bonds that keep hurd and bast fiber together [34,35]. The separated bast fiber, hurd, and green microfibers have different applications [15]. Bast fiber can be used to fabricate ropes, textiles, cloths [16], concrete, composites [18], and reinforced plastics [36,37,38,39]. Hurd has applications in the production of paper [18], fuel [20], hempcrete [40,41], fiberboard [42,43], insulations [17,18], pet litter [19], garden products [19], and food preservation. Moreover, bionutrients, beauty products, and pet wellness feedstock can be derived from hemp seeds and green microfiber [15]. (d) Besides cellulose, lignin is one of the most important constituents of hemp. Pulping is the process of degrading semicellulose and lignin to small, water-soluble molecules that can be washed out [44]. Although chemical pulping removes lignin and other impurities more efficiently, the use of chemicals raises concerns regarding environmental issues [31,44]. Mechanical pulping, on the other hand, decreases the use of chemicals, but the quality of the resultant pulp is not as good as with chemical pulping [45]. Biopulping is an emerging technique that can substantially address the drawbacks of both chemical and mechanical pulping [44]. Chemical pulping can be carried out by kraft, soda or soda anthraquinone, neutral sulfite, and acidic sulfite processes [31]. Stone groundwood, refiner mechanical (RMP), thermomechanical (TMP), and chemimechanical (CTMP) are the most common mechanical pulping methods [45]. (e) Even after the pulping process, the pulp contains remnants of lignin and diverse chromophoric compounds [31]. In order to produce high-quality, brighter, whiter, softer, and highly absorbent papers, a bleaching step is necessary to remove the remaining lignin [45]. This process can be performed using different chemicals, namely, chlorine, sodium hypochlorite, caustic soda, chlorine dioxide, oxygen, ozone, hydrogen peroxide, peracetic acid, and enzymes [31]. Unbleached pulps can be used in the production of linerboard, boxboard, and grocery bags. Bleaching of mechanical pulps results in lower-grade papers, turning to yellowish colors over time, suitable for newspapers and pocket fabrication. However, bleaching of chemical pulps does not induce a yellowish paper problem [45]. (f) The paper-making step includes screening, cleaning, and deforming the bleached pulp into paper rolls using pressing rollers. Screening and cleaning are performed to remove unwanted oversized particles (debris) from paper pulp. Debris refers to shives (small fiber bundles that have not been separated by mechanical or chemical pulping), chop (oversized particles), and knots (uncooked particles) [45]. (g) The finishing step is performed to prepare papers with the desired size and design. (h) Hemp paper can be recycled up to 8 times, compared to 3 times for wooden paper. Firstly, used papers are rewetted and returned to pulp form. Then, physical objects (e.g., staples) are separated by a mechanical process (e.g., magnets). The next step is deinking papers using chemicals to produce cleaner pulp. The deinking step can be omitted from the recycling process in the case that low-quality paper is needed (e.g., carton and corrugated board production). Finally, the repulped paper can be mixed with virgin pulp to be bleached to reenter the paper-making step [45].

Figure 2

The water-resistant capability of markers and characterization of plotting speed and pattern dimensions. (a) Images of 4 mm-in-diameter patterns with Comix and Deli brand markers. From top to bottom, images are arranged to show the front and the back of the hemp papers (only with inks). In order to achieve water-resistant patterns, ink should penetrate to the backside of the paper, preventing the diffusion of applied samples. The bottom rows show results from spotting red aqueous food dye. (b) Characterization of the influence of different plotting speeds (left to right), markers, and numbers of passes on the water-resistant capacity of hemp paper. A red aqueous solution was applied to test the plotted patterns. (c) Performance chart for different plotting speeds, markers, and numbers of passes; summary of part b. (d) Images of patterns plotted with Comix fine tip, with 2 passes at various plotting speeds, to find the thinnest water-resistant boundary thickness. The top image is plotted patterns, and the bottom image is the same patterns after spotting dye. (e) Testing multiple circle diameters with Comix fine tip at speed of 20% and 2 passes to find the smallest achievable pattern, without filling by ink. The top image is plotted patterns, and the bottom image is the same patterns after spotting dye. (f) Inset: representative image for measuring thickness, and the inner and outer radii of the plotted circles at eight different points (at angles of 0–315° with an interval of 45°) with MATLAB. The mean and standard deviation of inner and outer radii of the patterns plotted at different plotting speeds with 2 passes using the Comix fine tip (n = 3).

Figure 3

Physical characterization of hemp paper and Whatman grade 1 (WG1) paper. (a) SEM image with 300× magnification, showing the surface and cross-section of hemp paper. (b) The cross-section of hemp paper with 2250× magnification. SEM images of the surface of hemp paper with (c) 500× magnification and (d) 5000× magnification. (e) SEM image of surface and cross-section of WG1 paper with 250× magnification and (f) 750× magnification. (g) SEM image of the surface of WG1 paper with 250× magnification and (h) 750× magnification [88]. Images of a water droplet (i) on hemp paper, and (j) WG1 paper; substrates were coated by Comix ink. (k) Quantitative results for contact angles for hemp paper and WG1 paper. The contact angle of water on hemp paper is 4% less than that on WG1 paper, demonstrating comparable hydrophobic features of hemp paper and WG1 paper. Error bars represent standard deviation (n = 3).

Figure 4

Fluid dynamic characterization of hemp paper and Whatman grade 1 (WG1) paper at room temperature. (a) Wicking distances for hemp paper and WG1 paper for various channel widths from 1.0 to 5.0 mm (WG1′s result is from the article [91]). (b) Wicking distances in a channel with multiple numbers of branches, from 1 to 4, within 60 s for WG1 (in accordance with [91]) and 180 s for hemp paper. The width of the channel in each branch was 2 mm, and 20 μL sample volume was applied. (c) The effect of fluid viscosity on wicking distance in hemp and WG1 strips (2 mm in width). Fluid viscosities were arranged according to [89] by changing the amount of d-glucose in DI water. (d) Images of a four-channel, 2 mm-in-width hemp paper strip, where a red aqueous solution was used to show fluid diffusion as a function of time. Error bars represent standard deviation (n = 3).

Figure 5

A comparison of experimental results and mathematical model. (a) Representative images of hydrophobic boundary and cut boundary channels for both WG1 and hemp paper. The channel widths were 2 mm. (b) Comparison of experimental results for wicking length with the mathematical model using Equations (1) and (2). Markers show the experimental data over 5 min. The dotted and solid lines represent the best-fit curve for estimating the constants k and β, respectively, for both papers. A 2 mm paper channel width was used in all experiments (n = 3).

Figure 6

K+ cation measurement in artificial urine (pH 6) using PBFI potassium-sensitive dye as a fluorescent probe. Calibration curve for K+ ions on the hemp paper matrix at a constant probe concentration of 25 μM in DMSO (PBFI (λex/λem: 360/450 nm)). Error bars represent the standard error of the mean (n = 6).