Formation and manipulation of ferrofluid droplets with magnetic fields in a microdevice: a numerical parametric study

Advanced strategies in high-throughput droplet screening for enzyme engineering

Enzymes, as biocatalysts, play a cumulatively important role in environmental purification and industrial production of chemicals and pharmaceuticals. However, natural enzymes are limited by their physiological properties in practice, which need to be modified driven by requirements. Screening and isolating certain enzyme variants or ideal industrial strains with high yielding of target product enzymes is one of the main directions of enzyme engineering research. Droplet-based high-throughput screening (DHTS) technology employs massive monodisperse emulsion droplets as microreactors to achieve single strain encapsulation, as well as continuous monitoring for the inside mutant library. It can effectively sort out strains or enzymes with desired characteristics, offering a throughput of 108 events per hour. Much of the early literature focused on screening various engineered strains or designing signalling sorting strategies based on DHTS technology. However, the field of enzyme engineering lacks a comprehensive overview of advanced methods for microfluidic droplets and their cutting-edge developments in generation and manipulation. This review emphasizes the advanced strategies and frontiers of microfluidic droplet generation and manipulation facilitating enzyme engineering development. We also introduce design for various screening signals that cooperate with DHTS and devote to enzyme engineering.

Droplet-Based Microfluidics: Applications in Pharmaceuticals

Droplet-based microfluidics offer great opportunities for applications in various fields, such as diagnostics, food sciences, and drug discovery. A droplet provides an isolated environment for performing a single reaction within a microscale-volume sample, allowing for a fast reaction with a high sensitivity, high throughput, and low risk of cross-contamination. Owing to several remarkable features, droplet-based microfluidic techniques have been intensively studied. In this review, we discuss the impact of droplet microfluidics, particularly focusing on drug screening and development. In addition, we surveyed various methods of device fabrication and droplet generation/manipulation. We further highlight some promising studies covering drug synthesis and delivery that were updated within the last 5 years. This review provides researchers with a quick guide that includes the most up-to-date and relevant information on the latest scientific findings on the development of droplet-based microfluidics in the pharmaceutical field.

Exploiting ferrofluidic wetting for miniature soft machines

Miniature magnetic soft machines could significantly impact minimally invasive robotics and biomedical applications. However, most soft machines are limited to solid magnetic materials, whereas further progress also relies on fluidic constructs obtained by reconfiguring liquid magnetic materials, such as ferrofluid. Here we show how harnessing the wettability of ferrofluids allows for controlled reconfigurability and the ability to create versatile soft machines. The ferrofluid droplet exhibits multimodal motions, and a single droplet can be controlled to split into multiple sub-droplets and then re-fuse back on demand. The soft droplet machine can negotiate changing terrains in unstructured environments. In addition, the ferrofluid droplets can be configured as a liquid capsule, enabling cargo delivery; a wireless omnidirectional liquid cilia matrix capable of pumping biofluids; and a wireless liquid skin, allowing multiple types of miniature soft machine construction. This work improves small magnetic soft machines' achievable complexity and boosts their future biomedical applications capabilities.

Self-Transportation of Superparamagnetic Droplets on a Magnetic Gradient Slippery Surface with On/Off Sliding Controllability

Recently, research about droplet self-transportation on slippery surfaces has become a hotspot. However, to achieve on/off sliding control during the self-transportation process is still difficult. Herein, we report a magnetic slippery surface, and demonstrate on/off sliding control during the self-transportation of superparamagnetic droplets. The surface is prepared through integrating a substrate that has a gradient magnetic region with a layer of paraffin infused hydrophobic SiO₂ nanoparticles. On the surface, a superparamagnetic droplet is pinned at room temperature (about 25 °C), while it can self-transport directionally as the temperature is increased to about 70 °C. When the temperature is cooled down again, the droplet would return to the pinned state, indicating that on/off sliding control during the self-transportation process can be achieved. Furthermore, based on the excellent controllability, controllable coalescence of two droplets from opposite direction is displayed, demonstrating its potential application in numerous areas.

Asymptotic shape of isolated magnetic domains

We investigate the energy of an isolated magnetized domain Ω ⊂ R n for n = 2 , 3 . In non-dimensionalized variables, the energy given by E ( Ω ) = ∫ R n | ∇ χ Ω |   d x + ∫ R n | ∇ h Ω | 2   d x penalizes the interfacial area of the domain as well as the energy of the corresponding magnetostatic field. Here, the magnetostatic potential h Ω is determined by Δ h Ω = ∂ 1 χ Ω , corresponding to uniform magnetization within the domain. We consider the macroscopic regime | Ω | → ∞ , in which we derive compactness and Γ -limit which is formulated in terms of the cross-sectional area of the anisotropically rescaled configuration. We then give the solutions for the limit problems.

On-demand ferrofluid droplet formation with non-linear magnetic permeability in the presence of high non-uniform magnetic fields

The magnetic actuation of ferrofluid droplets offers an inspiring tool in widespread engineering and biological applications. In this study, the dynamics of ferrofluid droplet generation with a Drop-on-Demand feature under a non-uniform magnetic field is investigated by multiscale numerical modeling. Langevin equation is assumed for ferrofluid magnetic susceptibility due to the strong applied magnetic field. Large and small computational domains are considered. In the larger domain, the magnetic field is obtained by solving Maxwell equations. In the smaller domain, a coupling of continuity, Navier Stokes, two-phase flow, and Maxwell equations are solved by utilizing the magnetic field achieved by the larger domain for the boundary condition. The Finite volume method and coupling of level-set and Volume of Fluid methods are used for solving equations. The droplet formation is simulated in a two-dimensional axisymmetric domain. The method of solving fluid and magnetic equations is validated using a benchmark. Then, ferrofluid droplet formation is investigated experimentally, and the numerical results showed good agreement with the experimental data. The effect of 12 dimensionless parameters, including the ratio of magnetic, gravitational, and surface tension forces, the ratio of the nozzle and magnetic coil dimensions, and ferrofluid to continuous-phase properties ratios are studied. The results showed that by increasing the magnetic Bond number, gravitational Bond number, Ohnesorge number, dimensionless saturation magnetization, initial magnetic susceptibility of ferrofluid, the generated droplet diameter reduces, whereas the formation frequency increases. The same results were observed when decreasing the ferrite core diameter to outer nozzle diameter, density, and viscosity ratios.

Novel Approaches Concerning the Numerical Modeling of Particle and Cell Separation in Microchannels: A Review

The demand for precise separation of particles, cells, and other biological matter has significantly increased in recent years, leading to heightened scientific interest in this topic. More recently, due to advances in computational techniques and hardware, numerical simulations have been used to guide the design of separation devices. In this article, we establish the theoretical basis governing fluid flow and particle separation and then summarize the computational work performed in the field of particle and cell separation in the last five years with an emphasis on magnetic, dielectric, and acoustic methods. Nearly 70 articles are being reviewed and categorized depending on the type of material separated, fluid medium, software used, and experimental validation, with a brief description of some of the most notable results. Finally, further conclusions, future guidelines, and suggestions for potential improvement are highlighted.

Continuous-Flow Magnetic Fractionation of Red Blood Cells Based on Hemoglobin Content and Oxygen Saturation—Clinical Blood Supply Implications and Sickle Cell Anemia Treatment

Approximately 36,000 units of red blood cells (RBCs) are used every day in the U.S. and there is a great challenge for hospitals to maintain a reliable supply, given the 42-day expiration period from the blood donation date. For many years, research has been conducted to develop ex vivo storage solutions that limit RBC lysis and maintain a high survival rate of the transfused cells. However, little attention is directed towards potential fractionation methods to remove unwanted cell debris or aged blood cells from stored RBC units prior to transfusion, which could not only expand the ex vivo shelf life of RBC units but also avoid adverse events in transfused patients. Such fractionation methods could also limit the number of transfusions required for treating certain pathologies, such as sickle cell disease (SCD). In this work, magnetic fractionation is studied as a potential technology to fractionate functional and healthy RBCs from aged or sickle cells. It has been reported that during ex vivo RBC storage, RBCs lose hemoglobin (Hb) and lipid content via formation of Hb-containing exosomes. Given the magnetic character of deoxygenated- or met-Hb, in this work, we propose the use of a quadrupole magnetic sorter (QMS) to fractionate RBCs based on their Hb content from both healthy stored blood and SCD blood. In our QMS, a cylindrical microchannel placed inside the center of the quadrupolar magnets is subjected to high magnetic fields and constant field gradients (286 T/m), which causes the deflection of the paramagnetic, Hb-enriched, and functional RBCs from their original path and their collection into a different outlet. Our results demonstrated that although we could obtain a significant difference in the magnetic mobility of the sorted fractions (corresponding to a difference in more than 1 pg of Hb per cell), there exists a tradeoff between throughput and purity. Therefore, this technology when optimized could be used to expand the ex vivo shelf life of RBC units and avoid adverse events in transfused individuals or SCD patients requiring blood exchange therapy.

Splitting dynamics of ferrofluid droplets inside a microfluidic T-junction using a pulse-width modulated magnetic field in micro-magnetofluidics

Micro-magnetofluidics offers a promising tool for better control over the ferrofluid droplet manipulation which has been vastly utilized in biomedical applications in recent years. In this study, the ferrofluid droplet splitting under an asymmetric Pulse-Width-Modulated (PWM) magnetic field in a T-junction is numerically investigated using a finite volume method and VOF two-phase model. By utilizing the PWM magnetic field, two novel regimes of ferrofluid droplet splitting named as Flowing through the Same Branch (FSB) and Double Splitting (DS) have been observed for the first time. In the FSB regime, the daughter droplets move out of the same microchannel outlet, and in the DS regime, the droplet splitting occurs two times which results in generating three daughter droplets. The main problem related to the asymmetric droplet splitting under a steady magnetic field is daughter droplet trapping. By using a PWM magnetic field, this issue is resolved and the trapped/escaped regions are obtained in terms of the duty cycle and dimensionless magnetic field frequency. The effects of six important dimensionless parameters on the splitting ratio, including magnetic Bond number, duty cycle, dimensionless magnetic field frequency, capillary number, dimensionless mother droplet length, and dimensionless dipole position are investigated. The results showed that the splitting ratio increases with increasing magnetic Bond number or duty cycle, or decreasing the dimensionless magnetic field frequency. Eventually, a correlation is offered for the splitting ratio based on the dimensionless variables with an average relative error of 2.67%.