General Computational Methodology for Modeling Electrohydrodynamic Flows: Prediction and Optimization Capability for the Generation of Bubbles and Fibers

Working Regime Criteria for Microscale Electrohydrodynamic Conduction Pumps

We investigated the microscale electrohydrodynamic (EHD) conduction pumps in a wide range of working regimes, from the saturation regime to the ohmic regime. We showed that the existing macro- and microscale theoretical models could not accurately predict the electric force of microscale EHD conduction pumps, especially for the cases of a strong diffusion effect. We clarified that the failure is caused by a rough estimate of the heterocharge layer thickness. We revised the expression of heterocharge layer thickness by considering the diffusion effect and developed a new theoretical model for the microscale EHD conduction pumps based on the revised expression of heterocharge layer thickness. The results showed that our model can accurately predict the dimensionless electric force of the microscale EHD conduction pumps even for the cases of a strong diffusion effect. Furthermore, we developed a working regime map of microscale EHD conduction pumps and found that the microscale EHD conduction pumps more easily fall into the saturation regime compared with the macroscale EHD conduction pumps due to the enhanced diffusion effect; in other words, the microscale EHD conduction pumps have a wider saturation regime. We showed that the conduction number C0 could not distinguish the working regime of the microscale EHD conduction pumps because it does not take the diffusion effect into account. By employing the revised expression of heterocharge layer thickness, we proposed a new dimensionless number, C0D to distinguish the working regimes of microscale EHD conduction pumps.

Lattice Boltzmann finite-difference-based model for fully nonlinear electrohydrodynamic deformation of a liquid droplet

Electric field-induced flows involving multiple fluid components with a range of different electrical properties are described by the coupled Taylor-Melcher leaky-dielectric model. We present a lattice Boltzmann (LB)-finite difference (FD) method-based hybrid framework to solve the complete Taylor-Melcher leaky-dielectric model considering the nonlinear surface charge convection effects. Unlike the existing LB-based models, we treat the interfacial discontinuities using direction-specific continuous gradients, which prevents the miscalculation arising due to volumetric gradients without directional derivatives, simultaneously maintaining the electroneutrality of the bulk. While fluid transport is recovered through the LB method using a multiple relaxation time (MRT) scheme, the FD method with a central difference scheme is applied to discretize the charge transport equation at the interface, in addition to the electric field governing equations in the bulk and at the interface. We apply the developed numerical model to study the different regimes of droplet deformation due to an external electric field. Similar to the existing analytical and other numerical models, excluding the surface charge convection (SCC) term from the charge transport equation, the present methodology has shown excellent agreement with the existing literature. In addition, the effect of SCC in each of the regimes is analyzed. With the present numerical model, we observe a strong presence of SCC in the oblate deformation regime, contrary to the weak effect on prolate deformations. We further discuss the reason behind such differences in the magnitude of nonlinearity induced by the SCC in all the regimes of deformation.

Combining Ultrasound and Capillary-Embedded T-Junction Microfluidic Devices to Scale Up the Production of Narrow-Sized Microbubbles through Acoustic Fragmentation

Microbubbles are tiny gas-filled bubbles that have a variety of applications in ultrasound imaging and therapeutic drug delivery. Microbubbles can be synthesized using a number of techniques including sonication, amalgamation, and saline shaking. These approaches can produce highly concentrated microbubble suspensions but offer minimal control over the size and polydispersity of the microbubbles. One of the simplest and effective methods for producing monodisperse microbubbles is capillary-embedded T-junction microfluidic devices, which offer great control over the microbubble size. However, lower production rates (∼200 bubbles/s) and large microbubble sizes (∼300 μm) limit the applicability of such devices for biomedical applications. To overcome the limitations of these technologies, we demonstrate in this work an alternative approach to combine a capillary-embedded T-junction device with ultrasound to enhance the generation of narrow-sized microbubbles in aqueous suspensions. Two T-junction microfluidic devices were connected in parallel and combined with an ultrasonic horn to produce lipid-coated SF₆ core microbubbles in the size range of 1-8 μm. The rate of microbubble production was found to increase from 180 microbubbles/s in the absence of ultrasound to (6.5 ± 1.2) × 10⁶ bubble/s in the presence of ultrasound (100% ultrasound amplitude). When stored in a closed environment, the microbubbles were observed to be stable for up to 30 days, with the concentration of the microbubble suspension decreasing from ∼2.81 × 10⁹/mL to ∼2.3 × 10⁶/mL and the size changing from 1.73 ± 0.2 to 1.45 ± 0.3 μm at the end of 30 days. The acoustic response of these microbubbles was examined using broadband attenuation spectroscopy, and flow phantom imaging was performed to determine the ability of these microbubble suspensions to enhance the contrast relative to the surrounding tissue. Overall, this approach of coupling ultrasound with microfluidic parallelization enabled the continuous production of stable microbubbles at high production rates and low polydispersity using simple T-junction devices.

Quercetin Covalently Linked Lipid Nanoparticles: Multifaceted Killing Effect on Tumor Cells

The encapsulation of hydrophobic drugs is a problem that many researchers are working on. The goal of this study is to achieve the delivery of hydrophobic drugs by means of prodrugs and nanoformulations for a stronger tumor cell-killing effect and explore related killing mechanisms. Lipophilic quercetin (Qu) was covalently linked to glyceryl caprylate-caprate (Gcc) via disulfide bonds-containing 3,3'-dithiodipropionic acid (DTPA) to synthesize novel lipid Qu-SS-Gcc. Qu-SS-Gcc lipid nanoparticles (Qu-SS-Gcc LNPs) were fabricated using the solvent diffusion technique. The intracellular release of Qu by cleavage of nanocarriers was determined by liquid chromatography and compared with the uptake of free Qu. Detection methods, such as fluorescent quantitation, flow cytometry, and western blot were applied to explore the action mechanism induced by Qu. It was revealed that Qu-SS-Gcc LNPs could be cleaved by the high concentrations of reduction molecules in MCF-7/ADR (human multidrug-resistant breast cancer) cells, followed by the release of Qu. The intracellular Qu content produced by dissociation of Qu-SS-Gcc LNPs was higher than that produced by internalization of free Qu. The resulting release of Qu exerted superior cell-killing effects on MCF-7/ADR cells, such as P-gp inhibition by binding to P-gp binding sites, blocking the cell cycle in the G2 phase, and causing cell apoptosis and autophagy. Moreover, it was revealed autophagy triggered by a low concentration of Qu-SS-Gcc LNPs was beneficial to cell survival, while at a higher concentration, it acted as a cell killer. Qu-SS-Gcc LNPs can realize massive accumulation of Qu in tumor cells and exert a multifaceted killing effect on tumor cells, which is a reference for the delivery of hydrophobic drugs.

Effectiveness of Oil-Layered Albumin Microbubbles Produced Using Microfluidic T-Junctions in Series for In Vitro Inhibition of Tumor Cells

This work focuses on the synthesis of oil-layered microbubbles using two microfluidic T-junctions in series and evaluation of the effectiveness of these microbubbles loaded with doxorubicin and curcumin for cell invasion arrest from 3D spheroid models of triple negative breast cancer (TNBC), MDA-MB-231 cell line. Albumin microbubbles coated in the drug-laden oil layer were synthesized using a new method of connecting two microfluidic T-mixers in series. Double-layered microbubbles thus produced consist of an innermost core of nitrogen gas encapsulated in an aqueous layer of bovine serum albumin (BSA) which in turn, is coated with an outer layer of silicone oil. In order to identify the process conditions leading to the formation of double-layered microbubbles, a regime map was constructed based on capillary numbers for aqueous and oil phases. The microbubble formation regime transitions from double-layered to single layer microbubbles and then to formation of single oil droplets upon gradual change in flow rates of aqueous and oil phases. In vitro dissolution studies of double-layered microbubbles in an air-saturated environment indicated that a complete dissolution of such bubbles produces an oil droplet devoid of a gas bubble. Incorporation of doxorubicin and curcumin was found to produce a synergistic effect, which resulted in higher cell deaths in 2D monolayers of TNBC cells and inhibition of cell proliferation from 3D spheroid models of TNBC cells compared to the control.

Kinetics of albumin microbubble dissolution in aqueous media

The effectiveness of microbubbles as ultrasound contrast agents and targeted drug delivery vehicles depends on their persistence in blood. It is therefore necessary to understand the dissolution behavior of microbubbles in an aqueous medium. While there are several reports available in the literature on the dissolution of lipid microbubbles, there are no reports available on the dissolution kinetics of protein microbubbles. Moreover, shell parameters such as interfacial tension, shell resistance and shell elasticity/stiffness which characterize microbubble shells, have been reported for lipid shells but no such data are available for protein shells. Accordingly, this work was focused on capturing the dissolution behavior of protein microbubbles and estimation of shell parameters such as surface tension, shell resistance and shell elasticity. Bovine serum albumin (BSA) was used as a model protein and microbubbles were synthesized using sonication. During dissolution, a large portion of the protein shell was found to disengage from the gas-liquid interface after a stagnant dissolution phase, leading to a sudden disappearance of the microbubbles due to complete dissolution. In order to estimate shell parameters, microbubble dissolution kinetic data (radius vs. time) was fit numerically to a mass transfer model describing a microbubble dissolution process. Analysis of the results shows that the interfacial tension increases drastically and the shell resistance reduces significantly, as protein molecules leave the gas-liquid interface. Furthermore, the effect of processing conditions such as preheating temperature, microbubble size, and core gas and shell composition on the protein shell parameters was also evaluated.