Paper in its many forms is one of the most common materials of all cultures—and a hallmark of civilization when inks applied to it create religious texts, laws, legends, edicts and art. In modern times, paper-based technology is widely used in medical diagnoses, for hygiene, and as a means of testing water quality. Understanding liquid spread in the paradigm of diffusion, scientists can control it more precisely to expand applications of it in the refinement and creation of new products that entail liquid spreading through paper. For example, current markets have validated an important potential property of paper: acting as the essential building block of a rapid diagnostic kit in ultra-low-cost paradigm. Examples of this application include, pregnancy test strips; alkalinity or acidity tests of beauty and baby soaps using a paper strip; paper-strips for checking water quality; medical diagnosis aided by paper-strip tests of urine, saliva and blood. Another application of liquid spreading in paper can be in writing, drawing and painting. Paper is becoming popular among experts in miniaturization and micro fluidics because of its ability to transport fluids through capillary action. It is also biodegradable, biocompatible and low cost. Using paper, scientists are developing advanced analytical devices in association with fluorescence, colorimetric and electrochemical detection systems.
The theory of diffusion can be applied to liquid spreading through paper, too - a process at work in a range of everyday products, from ink pens to paper sampling patches for medical tests. We have produced a deeper conceptual grasp of how liquids spread through paper: liquid spreading in a paper is essentially random liquid motion through randomly distributed network of fibers. While diffusion is well known process, our elaboration of diffusion theory to paper-liquid interactions brings new theoretical detail. If random molecular motion gives rise to diffusion, can the liquid motion through the randomly distributed network of fibers constituting a paper be perceived as diffusive spreading? To find out, we mapped liquid spreading dynamics from a single fiber capillary to a network of the fibers, and then computed the resulting transport characteristics in a generalized unified perspective of diffusion. Our study reveals that, despite such diversified uses of paper interacting with liquids, there is a fundamental uniqueness of liquid spreading through paper leading toward a general and unified theory about it.
Its porosity makes paper easy to load a reagent needed for diagnostic tests. In fact, throughout the last decade, paper-based microfluidics has caught significant attention for a wide variety of applications ranging from disease diagnostics, water quality control, food quality monitoring to heavy metal ion detection.
Obviating the need of expensive fabrication techniques, we have developed a simple fabrication methodology for paper-based microfluidic devices by using an office printer and a hot-plate. The process relies on the melting of toner particles deposited over the paper surfaces. These particles eventually impregnate within the paper, and thus form the hydrophobic barrier. Accordingly, fluid flow is directed along a desired path, primarily by capillary action. To achieve better reproducibility and control over the fluid transportation through paper matrix, we have implemented electrically driven transport through paper channels, mediated by the use of graphite electrodes as deposited by pencil-sketches on paper. In this way, we have innovated a new branch of microfluidics, called as paper and pencil microfluidics.
We have further explored the paper-and-pencil device for clean energy generation by direct conversion of surface energy to electrical power by utilizing streaming potential effects. The primary advantages of such a platform are the self-propelling nature of the input flow through an exploitation of intrinsic capillary transport in paper pores (to this end, no syringe pump or equivalent actuation is necessary), and an explicit integrability with paper based diagnostic platforms for point of care applications. These features empower the device with a favourable functionality in extremely challenging and resource limited settings in an ultra-low cost paradigm.
The existence of electrokinetic phenomenon in paper substrates can be exploited for performing various processes, for e.g. manipulation/trapping of electrically responsive biomolecules, electrical separation/mixing of analytes, water quality monitoring, and microscale liquid handling. The paper-and-pencil based device can be integrated with a portable electronic platform which is capable of sensing and detection, integrated with a smart/low power sources and operational from environmentally challenged locations.
References