Fabrication, Flow Control, and Applications of Microfluidic Paper-Based Analytical Devices.

Paper-based microfluidic devices have advanced significantly in recent years as they are affordable, automated with capillary action, portable, and biodegradable diagnostic platforms for a variety of health, environmental, and food quality applications. In terms of commercialization, however, paper-based microfluidics still have to overcome significant challenges to become an authentic point-of-care testing format with the advanced capabilities of analyte purification, multiplex analysis, quantification, and detection with high sensitivity and selectivity. Moreover, fluid flow manipulation for multistep integration, which involves valving and flow velocity control, is also a critical parameter to achieve high-performance devices. Considering these limitations, the aim of this review is to (i) comprehensively analyze the fabrication techniques of microfluidic paper-based analytical devices, (ii) provide a theoretical background and various methods for fluid flow manipulation, and iii) highlight the recent detection techniques developed for various applications, including their advantages and disadvantages.

Dynamics of a capillary invasion in a closed-end capillary.

The position of fluid invasion in an open capillary increases as the square root of time and ceases when the capillary and hydrostatic forces are balanced, when viscous and inertia terms are negligible. Although this fluid invasion into open-end capillaries has been well described, detailed studies of fluid invasion in closed-end capillaries have not been explored thoroughly. Thus, we demonstrated, both theoretically and experimentally, a fluid invasion in closed-end capillaries, where the movement of the meniscus and the invasion velocity are accompanied by adiabatic gas compression inside the capillary. Theoretically, we found the fluid oscillations during invasion at short time scales by solving the one-dimensional momentum balance. This oscillatory motion is evaluated to determine which physical forces dominate the different conditions, and is further described by a damped driven harmonic oscillator model. However, this oscillating motion is not observed in the experiments. This inconsistency is due to the following: first, a continuous decrease in the radius of the curvature caused by decreasing the invasion velocity and increasing pressure inside the closed-end capillary, and second, the shear stress increase in the short time scale by the plug like velocity profile within the entrance length. The viscous term of modified momentum equation can be written as K(8µl/rc(2))(dl/dt) by using the multiplying factor K, which represents the increase of shear stress. The K is 7.3, 5.1, and 4.8 while capillary aspect ratio χc is 740, 1008, and 1244, respectively.

Creating mini-pregnancy models in vitro with clinical perspectives.

During the last decade, organs-on-chips or organoids microphysiological analysis platforms (MAP) have garnered attention in the practical applications of disease models, drug discovery, and developmental biology. Research on pregnant women has firm limitations due to ethical issues; thus, remodelling human pregnancy in vitro is highly beneficial for treatment modality development via disease remodelling or drug monitoring. This review highlights current efforts in bioengineering devices to reproduce human pregnancy and emphasises the significant convergence of biology, engineering, and maternal-foetal medicine. First, we review recent achievements in culturing cells from tissues involved in pregnancy; specifically, trophoblasts from the placenta. Second, we highlight developments in the reconstitution of pregnancy-related female reproductive organs across several structural and functional interpretations. Last, we examine research on the fundamental comprehension of pregnancy-associated diseases to find bioengineering solutions. Recreating human pregnancy through an engineered model is naturally complex; nevertheless, challenges are inevitable to progress precision medicine.

Non-Hermitian chiral degeneracy of gated graphene metasurfaces.

Non-Hermitian degeneracies, also known as exceptional points (EPs), have been the focus of much attention due to their singular eigenvalue surface structure. Nevertheless, as pertaining to a non-Hermitian metasurface platform, the reduction of an eigenspace dimensionality at the EP has been investigated mostly in a passive repetitive manner. Here, we propose an electrical and spectral way of resolving chiral EPs and clarifying the consequences of chiral mode collapsing of a non-Hermitian gated graphene metasurface. More specifically, the measured non-Hermitian Jones matrix in parameter space enables the quantification of nonorthogonality of polarisation eigenstates and half-integer topological charges associated with a chiral EP. Interestingly, the output polarisation state can be made orthogonal to the coalesced polarisation eigenstate of the metasurface, revealing the missing dimension at the chiral EP. In addition, the maximal nonorthogonality at the chiral EP leads to a blocking of one of the cross-polarised transmission pathways and, consequently, the observation of enhanced asymmetric polarisation conversion. We anticipate that electrically controllable non-Hermitian metasurface platforms can serve as an interesting framework for the investigation of rich non-Hermitian polarisation dynamics around chiral EPs.

On-chip selection of adenosine aptamer using graphene oxide-coated magnetic nanoparticles.

Systematic evolution of ligands by exponential enrichment (SELEX) is a method that is generally used for developing aptamers, which have arisen the promising alternatives for antibodies. However, conventional SELEX methods have limitations, such as a limited selection of target molecules, time-consuming and complex fabrication processes, and labor-intensive processes, which result in low selection yields. Here, we used (i) graphene oxide (GO)-coated magnetic nanoparticles in the selection process for separation and label-free detection and (ii) a multilayered microfluidic device manufactured using a three-dimensionally printed mold that is equipped with automated control valves to achieve precise fluid flows. The developed on-chip aptamer selection device and GO-coated magnetic nanoparticles were used to screen aptamer candidates for adenosine in eight cycles of the selection process within approximately 2 h for each cycle. Based on results from isothermal titration calorimetry, an aptamer with a dissociation constant of 18.6 ± 1.5 µM was selected. Therefore, the on-chip platform based on GO-coated magnetic nanoparticles provides a novel label-free screening technology for biosensors and micro/nanobiotechnology for achieving high-quality aptamers.

Identifying Fibrillization State of Aß Protein via Near-Field THz Conductance Measurement.

Progressive Alzheimer's disease is correlated with the oligomerization and fibrillization of the amyloid beta (Aß) protein. We identify the fibrillization stage of the Aß protein through label-free near-field THz conductance measurements in a buffer solution. Frequency-dependent conductance was obtained by measuring the differential transmittance of the time-domain spectroscopy in the THz range with a molar concentration of monomer, oligomer, and fibrillar forms of the Aß protein. Conductance at the lower frequency limit was observed to be high in monomers, reduced in oligomers, and dropped to an insulating state in fibrils and increased proportionally with the Aß protein concentration. The monotonic decrease in the conductance at low frequency was dominated by a simple Drude component in the monomer with concentration and nonlinear conductance behaviors in the oligomer and fibril. By extracting the structural localization parameter, a dimensionless constant, with the modified Drude-Smith model, we defined a dementia quotient (DQ) value (0 < De < 1) as a discrete metric for a various Aß proteins at a low concentration of 0.1 µmol/L; DQ = 1.0 ± 0.002 (fibril by full localization, mainly by Smith component), DQ = 0.64 ± 0.013 (oligomer by intermixed localization), and DQ = 0.0 ± 0.000 (monomer by Drude component). DQ values were discretely preserved independent of the molar concentration or buffer variation. This provides plenty of room for the label-free diagnosis of Alzheimer's disease using the near-field THz conductance measurement.

Fabrication of Artificial Leaf to Develop Fluid Pump Driven by Surface Tension and Evaporation.

Plants transport water from roots to leaves via xylem through transpiration, which is an evaporation process that occurs at the leaves. During transpiration, suction pressure is generated by the porous structure of mesophyll cells in the leaves. Here, we fabricate artificial leaf consisting of micro and nano hierarchy structures similar to the mesophyll cells and veins of a leaf using cryo-gel method. We show that the microchannels in agarose gel greatly decrease the flow resistance in dye diffusion and permeability experiments. Capillary tube and silicone oil are used for measuring the suction pressure of the artificial leaf. We maintain low humidity (20%) condition for measuring suction pressure that is limited by Laplace pressure, which is smaller than the water potential of air followed by the Kelvin-Laplace relation. Suction pressure of the artificial leaf is maximized by changing physical conditions, e.g., pore size, wettability of the structure. We change the agarose gel's concentration to decrease the pore size down to 200 nm and add the titanium nano particles to increase the wettability by changing contact angle from 63.6° to 49.4°. As a result, the measured suction pressure of the artificial leaf can be as large as 7.9 kPa.

Continuous Purification of Colloidal Quantum Dots in Large-Scale Using Porous Electrodes in Flow Channel.

Colloidal quantum dots (QDs) afford huge potential in numerous applications owing to their excellent optical and electronic properties. After the synthesis of QDs, separating QDs from unreacted impurities in large scale is one of the biggest issues to achieve scalable and high performance optoelectronic applications. Thus far, however, continuous purification method, which is essential for mass production, has rarely been reported. In this study, we developed a new continuous purification process that is suitable to the mass production of high-quality QDs. As-synthesized QDs are driven by electrophoresis in a flow channel and captured by porous electrodes and finally separated from the unreacted impurities. Nuclear magnetic resonance and ultraviolet/visible/near-infrared absorption spectroscopic data clearly showed that the impurities were efficiently removed from QDs with the purification yield, defined as the ratio of the mass of purified QDs to that of QDs in the crude solution, up to 87%. Also, we could successfully predict the purification yield depending on purification conditions with a simple theoretical model. The proposed large-scale purification process could be an important cornerstone for the mass production and industrial use of high-quality QDs.

[Human multi-route exposure assessment of lead and cadmium for Korean volunteers].

OBJECTIVES: We wanted to evaluate the main route of exposure to lead and cadmium for the general population in Korea by using multi-route and multi-media exposure assessment METHODS: Samples of air at the homes, samples of the food and water and peripheral blood samples were collected from thirty volunteers living in Seoul (the metropolitan area), Yong-in (the suburban area) and Ansan (the industrial area) in 2001. Graphite furnace atomic absorption spectrometric methods were used for the determination of the Pb and Cd levels in the air, food, water and blood samples. RESULTS: The average intake of lead through the air, drinking water and food were 5.06 microg/day (26.3%), 0.002 microg /day (0.1%), and 16.4 microg/day (73.6%), respectively. The average intake of cadmium through the air, drinking water and food were 0.082 microg/day (0.9%), 0.001 microg/day (0.007%), and 12.61 microg/day (99.0%), respectively. The blood lead level was statistically higher in the male subjects than in the female subjects (3.39 and 2.22 microg/dl, respectively), and only gender was a significant variable on the multiple regression analysis for blood lead. CONCLUSIONS: This study showed that the intake of lead and cadmium through food was the major route of exposure. A policy to reduce the pollutants according to the exposure routes should be established. However, more studies will be needed to support these data for the general population.