Dual-porosity micromodels for studying multiphase fluid flow in carbonate rocks.

Carbonate rocks usually present a wide variation in pore size within a sample and may contain macroscopic pores ranging from a few millimeters to microscopic pores smaller than one micrometer. Therefore, studying the fluid flow inside carbonates presents a challenging problem. This study proposes a methodology to create dual-porosity micromodels for studying single and two-phase fluid flow in multiscale, carbonate-like, rocks. For this purpose, a design technique for Rock-on-a-Chip (ROC) devices based on the Voronoi tessellation was extended to take into account bimodal pore size distributions, allowing the creation of a macroporous system made up of larger channels and vugs that can be filled with distinct microporosity types. The porous media thus generated were then employed to fabricate polymer micromodels by applying the soft lithography technique. Experimental and numerical results show that the microporosity can increase or reduce the permeability, depending on whether it is added to the grains and/or to the large channels. Even when the microporous matrix completely filled the large channels, the addition of vugs did not increase the permeability. Regarding two-phase fluid flow, the location of the steady-state fluids after drainage clearly depends on the proportion and spatial distribution of microporosity, as well as its type. For the micromodel with microporous grains, no significant amount of wetting fluid was displaced from the micropores. In contrast, when microporosities fill the large channels, the injected fluid forces the displacement of the wetting liquid from the micropores, although far from effectively. The novel approach presented in this work represents a step forward in the artificial generation of more representative micromodels for studying fluid flow at the pore scale.

Advances in Concentration Gradient Generation Approaches in a Microfluidic Device for Toxicity Analysis.

This systematic review aimed to analyze the development and functionality of microfluidic concentration gradient generators (CGGs) for toxicological evaluation of different biological organisms. We searched articles using the keywords: concentration gradient generator, toxicity, and microfluidic device. Only 33 of the 352 articles found were included and examined regarding the fabrication of the microdevices, the characteristics of the CGG, the biological model, and the desired results. The main fabrication method was soft lithography, using polydimethylsiloxane (PDMS) material (91%) and SU-8 as the mold (58.3%). New technologies were applied to minimize shear and bubble problems, reduce costs, and accelerate prototyping. The Christmas tree CGG design and its variations were the most reported in the studies, as well as the convective method of generation (61%). Biological models included bacteria and nematodes for antibiotic screening, microalgae for pollutant toxicity, tumor and normal cells for, primarily, chemotherapy screening, and Zebrafish embryos for drug and metal developmental toxicity. The toxic effects of each concentration generated were evaluated mostly with imaging and microscopy techniques. This study showed an advantage of CGGs over other techniques and their applicability for several biological models. Even with soft lithography, PDMS, and Christmas tree being more popular in their respective categories, current studies aim to apply new technologies and intricate architectures to improve testing effectiveness and reduce common microfluidics problems, allowing for high applicability of toxicity tests in different medical and environmental models.

Ultrasensitive microfluidic electrochemical immunosensor based on electrodeposited nanoporous gold for SOX-2 determination.

An ultrasensitive and portable microfluidic electrochemical immunosensor for SOX-2 cancer biomarker determination was developed. The selectivity and sensitivity of the sensor were improved by modifying the microfluidic channel. This was accomplished through a physical-chemical treatment to produce a hydrophilic surface, with an increased surface to volume/ratio, where the anti-SOX-2 antibodies can be covalently immobilized. A sputtered gold electrode was used as detector and its surface was activated by using a dynamic hydrogen bubble template method. As a result, a gold nanoporous structure (NPAu) with outstanding properties, like high specific surface area, large pore volume, uniform nanostructure, good conductivity, and excellent electrochemical activity was obtained. SOX-2 present in the sample was bound to the anti-SOX-2 immobilized in the microfluidic channel, and then was labeled with a second antibody marked with horseradish peroxidase (HRP-anti-SOX-2) like a sandwich immunoassay. Finally, an H2O2 + catechol solution was added, and the enzymatic product (quinone) was reduced on the NPAu electrode at +0.1 V (vs. Ag). The current obtained was directly proportional to the SOX-2 concentration in the sample. The detection limit achieved was 30 pg mL-1, and the coefficient of variation was less than 4.75%. Therefore, the microfluidic electrochemical immunosensor is a suitable clinical device for in situ SOX-2 determination in real samples.