Guoqing Hu

Field-Free Isolation of Exosomes from Extracellular Vesicles by Microfluidic Viscoelastic Flows

Exosomes, molecular cargos secreted by almost all mammalian cells, are considered as promising biomarkers to identify many diseases including cancers. However, the small size of exosomes (30-200 nm) poses serious challenges in their isolation from complex media containing a variety of extracellular vesicles (EVs) of different sizes, especially in small sample volumes. Here we present a viscoelasticity-based microfluidic system to directly separate exosomes from cell culture media or serum in a continuous, size-dependent, and label-free manner. Using a small amount of biocompatible polymer as the additive in the media to control the viscoelastic forces exerted on EVs, we are able to achieve a high separation purity (>90%) and recovery (>80%) of exosomes. The proposed technique may serve as a versatile platform to facilitate exosome analyses in diverse biochemical applications.

Microfluidic based high throughput synthesis of lipid-polymer hybrid nanoparticles with tunable diameters

Core-shell hybrid nanoparticles (NPs) for drug delivery have attracted numerous attentions due to their enhanced therapeutic efficacy and good biocompatibility. In this work, we fabricate a two-stage microfluidic chip to implement a high-throughput, one-step, and size-tunable synthesis of mono-disperse lipid-poly (lactic-co-glycolic acid) NPs. The size of hybrid NPs is tunable by varying the flow rates inside the two-stage microfluidic chip. To elucidate the mechanism of size-controllable generation of hybrid NPs, we observe the flow field in the microchannel with confocal microscope and perform the simulation by a numerical model. Both the experimental and numerical results indicate an enhanced mixing effect at high flow rate, thus resulting in the assembly of small and mono-disperse hybrid NPs. In vitro experiments show that the large hybrid NPs are more likely to be aggregated in serum and exhibit a lower cellular uptake efficacy than the small ones. This microfluidic chip shows great promise as a robust platform for optimization of nano drug delivery system.

Numerical modeling of Joule heating effects in insulator-based dielectrophoresis microdevices

Insulator‐based DEP (iDEP) has been established as a powerful tool for manipulating particles in microfluidic devices. However, Joule heating may become an issue in iDEP microdevices due to the local amplification of electric field around the insulators. This results in an electrothermal force that can manifest itself in the flow field in the form of circulations, thus affecting the particle motion. We develop herein a transient, 3D, full‐scale numerical model to study Joule heating and its effects on the coupled transport of charge, heat, and fluid in an iDEP device with a rectangular constriction microchannel. This model is validated by comparing the simulation results with the experimentally obtained fluid flow patterns and particle images that were reported in our recent works. It identifies a significant difference in the time scales of the electric, temperature, and flow fields in iDEP microdevices. It also predicts the locations of electrothermal flow circulations in different halves of the channel at the upstream and downstream of the constriction.

Particle manipulations in non-Newtonian microfluidics: A review

Microfluidic devices have been widely used since 1990s for diverse manipulations of particles (a general term of beads, cells, vesicles, drops, etc.) in a variety of applications. Compared to the active manipulation via an externally imposed force field, the passive manipulation of particles exploits the flow-induced intrinsic lift and/or drag to control particle motion with several advantages. Along this direction, inertial microfluidics has received tremendous interest in the past decade due to its capability to handle a large volume of samples at a high throughput. This inertial lift-based approach in Newtonian fluids, however, becomes ineffective and even fails for small particles and/or at low flow rates. Recent studies have demonstrated the potential of elastic lift in non-Newtonian fluids for manipulating particles with a much smaller size and over a much wider range of flow rates. The aim of this article is to provide an overview of the various passive manipulations, including focusing, separation, washing and stretching, of particles that have thus far been demonstrated in non-Newtonian microfluidics.

Unveiling the Molecular Structure of Pulmonary Surfactant Corona on Nanoparticles

The growing risk of human exposure to airborne nanoparticles (NPs) causes a general concern on the biosafety of nanotechnology. Inhaled NPs can deposit in the deep lung at which they interact with the pulmonary surfactant (PS). Despite the increasing study of nano-bio interactions, detailed molecular mechanisms by which inhaled NPs interact with the natural PS system remain unclear. Using coarse-grained molecular dynamics simulation, we studied the interaction between NPs and the PS system in the alveolar fluid. It was found that regardless of different physicochemical properties, upon contacting the PS, both silver and polystyrene NPs are immediately coated with a biomolecular corona that consists of both lipids and proteins. Structure and molecular conformation of the PS corona depend on the hydrophobicity of the pristine NPs. Quantitative analysis revealed that lipid composition of the corona formed on different NPs is relatively conserved and is similar to that of the bulk phase PS. However, relative abundance of the surfactant-associated proteins, SP-A, SP-B, and SP-C, is notably affected by the hydrophobicity of the NP. The PS corona provides the NPs with a physicochemical barrier against the environment, equalizes the hydrophobicity of the pristine NPs, and may enhance biorecognition of the NPs. These modifications in physicochemical properties may play a crucial role in affecting the biological identity of the NPs and hence alter their subsequent interactions with cells and other biological entities. Our results suggest that all studies of inhalation nanotoxicology or NP-based pulmonary drug delivery should consider the influence of the PS corona.

Induced charge effects on electrokinetic entry flow

Electrokinetic flow, due to a nearly plug-like velocity profile, is the preferred mode for transport of fluids (by electroosmosis) and species (by electrophoresis if charged) in microfluidic devices. Thus far there have been numerous studies on electrokinetic flow within a variety of microchannel structures. However, the fluid and species behaviors at the interface of the inlet reservoir (i.e., the well that supplies the fluid and species) and microchannel are still largely unexplored. This work presents a fundamental investigation of the induced charge effects on electrokinetic entry flow due to the polarization of dielectric corners at the inlet reservoir-microchannel junction. We use small tracing particles suspended in a low ionic concentration fluid to visualize the electrokinetic flow pattern in the absence of Joule heating effects. Particles are found to get trapped and concentrated inside a pair of counter-rotating fluid circulations near the corners of the channel entrance. We also develop a dept...

Sheathless Focusing and Separation of Diverse Nanoparticles in Viscoelastic Solutions with Minimized Shear Thinning

Viscoelastic microfluidics becomes an efficient and label-free hydrodynamic technology to enrich and separate micrometer-scale particles, including blood cells, circulating tumor cells, and bacteria. However, the manipulation of nanoscale particles by viscoelastic microfluidics remains a major challenge, because the viscoelastic force acting on the smaller particle decreases dramatically. In contrast to the commonly used polymer solutions of high molecular weight, herein we utilize the aqueous solutions of poly(ethylene oxide) (PEO) of low molecular weight with minimized shear thinning but sufficient elastic force for high-quality focusing and separation of various nanoparticles. The focusing efficiencies of 100 nm polystyrene (PS) nanoparticles and λ-DNA molecules are 84% and 85%, respectively, in a double spiral microchannel, without the aid of sheath flows. Furthermore, we demonstrate the size-based viscoelastic separation of two sets of binary mixtures-100/2000 nm PS particles and λ-DNA molecules/blood platelets-all achieving separation efficiencies of >95% in the same device. Our proposal technique would be a promising approach for enrichment/separation of the nanoparticles encountered in applications of analytical chemistry and nanotechnology.

High-Throughput Particle Manipulation Based on Hydrodynamic Effects in Microchannels

Microfluidic techniques are effective tools for precise manipulation of particles and cells, whose enrichment and separation is crucial for a wide range of applications in biology, medicine, and chemistry. Recently, lateral particle migration induced by the intrinsic hydrodynamic effects in microchannels, such as inertia and elasticity, has shown its promise for high-throughput and label-free particle manipulation. The particle migration can be engineered to realize the controllable focusing and separation of particles based on a difference in size. The widespread use of inertial and viscoelastic microfluidics depends on the understanding of hydrodynamic effects on particle motion. This review will summarize the progress in the fundamental mechanisms and key applications of inertial and viscoelastic particle manipulation.

Joule heating effects on electroosmotic entry flow

Electroosmotic flow is the transport method of choice in microfluidic devices over traditional pressure‐driven flow. To date, however, studies on electroosmotic flow have been almost entirely limited to inside microchannels. This work presents the first experimental study of Joule heating effects on electroosmotic fluid entry from the inlet reservoir (i.e., the well that supplies fluids and samples) to the microchannel in a polymer‐based microfluidic chip. Electrothermal fluid circulations are observed at the reservoir‐microchannel junction, which grow in size and strength with the increasing alternating current to direct current voltage ratio. Moreover, a 2D depth‐averaged numerical model is developed to understand the effects of Joule heating on fluid temperature and flow fields in electrokinetic microfluidic chips. This model overcomes the problems encountered in previous unrealistic 2D and costly 3D models, and is able to predict the observed electroosmotic entry flow patterns with a good agreement.

Two mechanisms underlying the induction of long-term potentiation in motor cortex of adult cat in vitro

Long-term potentiation of synaptic transmission (LTP), as documented by the enhancement of evoked field potentials in layer III following stimulation of the underlying white matter, has been studied in slices of motor cortex from adult cats. With a 1 M NaCl-filled recording electrode, LTP was induced only in one out of eight slices. When the recording electrode in addition contained 5 mM bicuculline metiodide, LTP was obtained with a much higher rate of success (15/19), suggesting that reduction of GABAA receptor-mediated inhibition facilitated the induction of LTP in the motor cortex. Bath application of dl-2-amino-5-phosphonovaleric acid (APV, 100 μM) or Ni2+ (100 μM) significantly reduced the success rate for LTP occurrence (6/16 and 5/16, respectively); but when LTP was induced, it did not show significant change in magnitude and time course. In slices perfused with APV (100 μM) plus Ni2+ (100 μM), LTP induction was completely blocked (0/12). These results suggest that two different mechanisms may subserve LTP induction in the cat motor cortex: one is mediated by N-methyl-d-aspartate receptors and can be blocked by APV; the other may be mediated by low-threshold calcium channels and can be blocked by Ni2+.