In this paper, we determine the smallest feature size that enables fluid flow in microfluidic paper-based analytical devices (µPADs) fabricated by laser cutting. The smallest feature sizes fabricated from five commercially available paper types: Whatman filter paper grade 50 (FP-50), Whatman 3MM Chr chromatography paper (3MM Chr), Whatman 1 Chr chromatography paper (1 Chr), Whatman regenerated cellulose membrane 55 (RC-55) and Amershan Protran 0.45 nitrocellulose membrane (NC), were 139 ± 8 µm, 130 ± 11 µm, 103 ± 12 µm, 45 ± 6 µm, and 24 ± 3 µm, respectively, as determined experimentally by successful fluid flow. We found that the fiber width of the paper correlates with the smallest feature size that has the capacity for fluid flow. We also investigated the flow speed of Allura red dye solution through small-scale channels fabricated from different paper types. We found that the flow speed is significantly slower through microscale features and confirmed the similar trends that were reported previously for millimeter-scale channels, namely that wider channels enable quicker flow speed.

Fabrication method for compact and microscale features in paper-based analytical devices using a CO ₂ laser and foil backed paper.

In this paper we experimentally examine evaporation flux distributions and modes of interfacial energy transport for continuously fed evaporating spherical sessile water droplets in a regime that is relevant for applications, particularly for evaporative cooling systems. The contribution of the thermal conduction through the vapor phase was found to be insignificant compared to the thermal conduction through the liquid phase for the conditions we investigated. The local evaporation flux distributions associated with thermal conduction were found to vary along the surface of the droplet. Thermal conduction provided a majority of the energy required for evaporation but did not account for all of the energy transport, contributing $64\pm 3\%, 77\pm 3\%$, and $77\pm 4\%$ of the energy required for the three cases we examined. Based on the temperature profiles measured along the interface we found that thermocapillary flow was predicted to occur in our experiments, and two convection cells were consistent with the temperature distributions for higher substrate temperatures while a single convection cell was consistent with the temperature distributions for a lower substrate temperature.

In this paper, we present a microfluidic paper-based analytical device (μPAD) with a gold nanosensor functionalized with α-lipoic acid and thioguanine (Au–TA–TG) to detect whether the arsenic level of groundwater from hand tubewells in Bangladesh is above or below the World Health Organization (WHO) guideline level of 10 μg/L. We analyzed the naturally occurring metals present in Bangladesh groundwater and assessed the interference with the gold nanosensor. A method was developed to prevent interference from alkaline metals found in Bangladesh groundwater (Ca, Mg, K and Na) by increasing the pH level on the μPADs to 12.1. Most of the heavy metals present in the groundwater (Ni, Mn, Cd, Pb, and Fe II) did not interfere with the μPAD arsenic tests; however, Fe III was found to interfere, which was also prevented by increasing the pH level on the μPADs to 12.1. The μPAD arsenic tests were tested with 24 groundwater samples collected from hand tubewells in three different districts in Bangladesh: Shirajganj, Manikganj, and Munshiganj, and the predictions for whether the arsenic levels were above or below the WHO guideline level agreed with the results obtained from laboratory testing. The μPAD arsenic test is the first paper-based test validated using Bangladesh groundwater samples and capable of detecting whether the arsenic level in groundwater is above or below the WHO guideline level of 10 μg/L, which is a step towards enabling the villagers who collect and consume the groundwater to test their own sources and make decisions about where to obtain the safest water.

In this paper, we report the development of semiquantitative counting-based lateral flow assay (LFA)-type microfluidic paper-based analytical devices ($\mu$ PADs) to analyze samples at submicroliter volumes. The ability to use submicroliter sample volumes is a significant advantage for $\mu$ PADs since it enables enhanced multiplexing, reduces cost, and increases user-friendliness since small sample volumes can be collected using methods that do not require trained personnel, such as finger pricking and microneedles. The challenge of accomplishing a semiquantitative test readout using submicroliter sample volumes was overcome with a counting-based approach. In order to use submicroliter sample volumes, we developed a flow strategy with a running liquid to facilitate flow through the assay. The efficacy of the devices was confirmed with glucose and total human immunoglobulin E (IgE) tests using 0.5 $\mu$ l and 1 $\mu$ l of sample solutions, respectively. Semiquantitative results were generated to predict glucose concentrations in the range of 0–12 mmol/l and IgE concentrations in the range of 0–400 ng/ml. The counting-based approach correlates the number of dots that exhibited a color change to the concentration of the analyte, which provides a more user-friendly method as compared with interpreting the intensity of a color change. The devices reported herein are the first counting-based LFA-type $\mu$ PADs capable of semiquantitative testing using submicroliter sample volumes.