Radial Wicking of Biological Fluids in Paper

Electrokinetics of Erosive Seepage through Deformable Porous Media

Channelization and branching patterns frequently appear in porous structures as a result of fluid-flow-mediated erosion, which causes spatiotemporal changes in the medium. However, most studies on electrokinetic effects in porous media focus on the overall impact of the electric field on electrical double-layer formation in micropores and its influence on ionic transport, without addressing the spatiotemporal erosive characteristics and resulting porosity distribution. In this study, we explore the interplay between flow-induced shear stress and an external electric field on the dynamic evolution of porosity in deformable porous media using semi-analytical modeling. Our numerical simulations accurately predict the differences in porosity and erosive development when the electric field aligns with or opposes the flow, highlighting the importance of the direction of the external stimulus and not just its magnitude. Our findings establish a foundation for electric-field-mediated control of porous media properties and explain electrokinetic transport by considering dynamic porosity variations as a result of erosive deformation, an aspect previously unaddressed.

Low-Gradient Magnetophoresis of Nanospheres and Nanorods through a Single Layer of Paper

The possible magnetophoretic migration of iron oxide nanoparticles through the cellulosic matrix within a single layer of paper is challenging with its underlying mechanism remained unclear. Even with the recent advancements of theoretical understanding on magnetophoresis, mainly driven by cooperative and hydrodynamics phenomena, the contributions of these two mechanisms on possible penetration of magnetic nanoparticles through cellulosic matrix of paper have yet been proven. Here, by using iron oxide nanoparticles (IONPs), both nanospheres and nanorods, we have investigated the migration kinetics of these nanoparticles through grade 4 Whatman filter paper with a particle retention of 20-25 μm. By performing droplet tracking experiments, the real-time stained area growth of the particle droplet on the filter paper, under the influences of a grade N40 NdFeB magnet, were recorded. Our results show that the spatial and temporal expansion of the IONP stain is biased toward the magnet and such an effect is dependent on (i) particle concentration and (ii) particle shape. The kinetics data were first analyzed by treating it as a radial wicking fluid, and later the IONP distribution within the cellulosic matrix was investigated by optical microscopy. The macroscopic flow front velocities of the stained area ranged from 259 μm/s to 16 040 μm/s. Moreover, the microscopic magnetophoretic velocity of nanorod cluster was also successfully measured as ∼214 μm/s. Findings in this work have indirectly revealed the strong influence of cooperative magnetophoresis and the engineering feasibility of paper-based magnetophoretic technology by taking advantage of magnetoshape anisotropy effect of the particles.

Cellular aggregation dictates universal spreading behaviour of a whole-blood drop on a paper strip

HYPOTHESIS

The complex spreading dynamics of blood on paper matrix is likely to be quantitatively altered with variations in the fractional occupancy of red blood cells in the whole blood (haematocrit). Here, we presented an apparently surprising observation that a finite volume blood drop undergoes a universal time-dependent spreading on a filter paper strip that is virtually invariant with its hematocrit level within physiologically healthy regime, though distinctively distinguishable from the spreading laws of blood plasma and water.

EXPERIMENTS

Our hypothesis was ascertained by performing controlled wicking experiments on filter papers of different grades. Spreading of human blood samples of different haematocrit levels ranging between 15% and 51% and the plasma separated from therein were traced by combined high-speed imaging and microscopy. These experiments were complemented with a semi-analytical theory to decipher the key physics of interest.

RESULTS

Our results unveiled the exclusive influence of the obstructing cellular aggregates in the randomly distributed hierarchically structured porous pathways and deciphered the role of the networked structures of the various plasma proteins that induced hindered diffusion. The resulting universal signatures of spontaneous dynamic spreading, delving centrally on the fractional reduction in the interlaced porous passages, provide novel design basis for paper-microfluidic kits in medical diagnostics and beyond.

Spontaneous Imbibition in Paper-Based Microfluidic Devices: Experiments and Numerical Simulations

Microfluidic paper-based analytical devices (μPADs) have quickly been an excellent choice for point-of-care diagnostic platforms ever since they appeared. Because capillary force is the main driving force for the transport of analytes in μPADs, low spontaneous imbibition rates may limit the detection sensitivity. Therefore, quantitative understanding of internal spontaneous capillary flow progress is requisite for designing sensitive and accurate μPADs. In this work, experimental and numerical studies have been performed to investigate the capillary flow in a typical filter paper. We use light-transmitting imaging technology to study wetting saturation changes in the paper. Our experimental results show an obvious transition of a saturated wetting front into an unsaturated wetting front as the imbibition proceeds. We find that the single-phase Darcy model considerably overestimates the temporal wetting penetration depths. Alternatively, we use the Richards equation together with the two-phase flow material properties that are obtained from the image-based pore-network modeling of the filter paper. Moreover, we have considered a dynamic term in the capillary pressure due to strong wetting dynamics in spontaneous imbibition. As a result, the numerical predictions of spontaneous imbibition in the paper are significantly improved. Our studies provide insights into the development of a quantitative spontaneous imbibition model for μPADs applications.

Wash-free paper diagnostics for the rapid detection of blood type antibodies

Identification of specific antibodies in patient plasma is an essential part of many diagnostic procedures and is critical for safe blood transfusion. Current techniques require laboratory infrastructure and long turnaround times which limits access to those nearby tertiary healthcare providers. Addressing this challenge, a novel and rapid paper-based antibody test is reported. We validate antibody detection with reverse blood typing using IgM antibodies and then generalise the validity by adapting to detect SARS CoV-2 (COVID-19) antibodies in patient serum samples. Reagent red blood cells (RBC) are first combined with the patient plasma containing the screened antibody and a droplet of the mixture is then deposited onto paper. The light intensity profile is analyzed to identify test results, which can be detected by eye and/or with image processing to allow full automation. The efficacy of this test to perform reverse blood typing is demonstrated and the performance and sensitivity of this test using different paper types and RBC reagents was investigated using clinical samples. As an example of the flexibility of this approach, we labeled the RBC reagent with an antibody-peptide conjugate to detect SARS CoV-2 (COVID-19) antibodies in patient serum samples. This concept could be generalized to any agglutination-based antibody diagnostics with blood plasma.

Pattern formation in drying blood drops

Patterns in dried droplets are commonly observed as rings left after spills of dirty water or coffee have evaporated. Patterns are also seen in dried blood droplets and the patterns have been shown to differ from patients afflicted with different medical conditions. This has been proposed as the basis for a new generation of low-cost blood diagnostics. Before these diagnostics can be widely used, the underlying mechanisms leading to pattern formation in these systems must be understood. We analyse the height profile and appearance of dispersions prepared with red blood cells (RBCs) from healthy donors. The red cell concentrations and diluent were varied and compared with simple polystyrene particle systems to identify the dominant mechanistic variables. Typically, a high concentration of non-volatile components suppresses ring formation. However, RBC suspensions display a greater volume of edge deposition when the red cell concentration is higher. This discrepancy is caused by the consolidation front halting during drying for most blood suspensions. This prevents the standard horizontal drying mechanism and leads to two clearly defined regions in final crack patterns and height profile. This article is part of a discussion meeting issue 'A cracking approach to inventing new tough materials: fracture stranger than friction'.

Porosity Centrifuge: Determination of Pore Sizes of Swellable Porous Materials under Hypergravity

Porous materials are ubiquitous and essential for many processes in nature as well as in industry, and the need to produce them from renewable materials will definitely increase. A prominent example for such a fully recyclable and biogenic porous material is paper, a material that contains macropores formed in between the fibers as well as a large distribution of much finer pores on and within the fiber walls. While the determination of pore sizes is of central importance for the characterization of such materials, their determination is usually only possible with complex methodologies. The determination of pore sizes in the context of water has remained largely unsolved to date, in particular, if water-swellable materials are considered. Here, we introduce a completely new way of determining pore sizes of materials even under swelling conditions. Using a centrifugal device and studying the imbibition of water into paper at various centrifugal forces that oppose the capillary forces, we can access the mean pore size of different paper materials in an experimentally simple fashion. In addition, we can show that the pore size values obtained with our "centrifugal porosimetry" are consistent with the values obtained using other methods, usually much more involved methods. For this purpose, we measure well-characterized translucent macroporous materials using water, ranging from simple glass capillaries to standard filters and nitrocellulose membranes.

A rapid paper-based blood typing method from droplet wicking

Paper-based diagnostics are leading the field of low-cost, point of care analytical techniques.