Peter Huang

Direct measurement of slip velocities using three- dimensional total internal reflection velocimetry

The existence and magnitude of slip velocities between deionized water and a smooth glass surface is studied experimentally. Sub-micron fluorescent particles are suspended in water and imaged using total internal reflection velocimetry (TIRV). For water flowing over a hydrophilic surface, the measurements are in agreement with previous experiments and indicate that slip, if present, is minimal at low shear rates, but increases slightly as the shear rate increases. Surface hydrophobicity is observed to induce a small slip velocity, with the slip length reaching a maximum of 96 nm at a shear rate of 1800

Direct measurement of slip length in electrolyte solutions

\,{\rm s}^{-1}

The effects of hindered mobility and depletion of particles in near-wall shear flows and the implications for nanovelocimetry

. Issues associated with the experimental technique and the interpretation of results are also discussed.

Catch Strip Assay for the Relative Assessment of Two-Dimensional Protein Association Kinetics

Electrokinetic effects and electrostatic repulsion between tracer particles and glass surface have both been proposed as possible sources that would lead to false slip results obtained from velocimetry-based measurements. Using a three-dimensional total internal reflection velocimetry technique, we address such a concern by comparing the measured slip lengths between nonionic solutions and electrolyte solutions whose ionic concentrations have been predicted to reduce the electricity-induced slip effect to a submolecular level. It is observed that the presence of electrolytes has no effect on the measured slip lengths, suggesting that the observed slip velocities are most likely not due to electrostatic and electrokinetic effects, but are consequences of true boundary slip.

Confocal light scattering spectroscopic imaging system for in situ tissue characterization

The behaviour of spherical Brownian particles in a near-wall shear flow is explored using Langevin simulations and experimental measurements, focusing on the effects of anisotropic hindered particle mobility and the formation of a particle depletion layer due to repulsive forces. The results are discussed in the context of particle velocity distributions obtained by near-wall image-based velocimetry. It is observed that the shear force and dispersion dominate at high Peclet number ( Pe > 3), and the asymmetric shapes of particle velocity distributions are attributed to broken symmetry due to the presence of the wall. Furthermore, the excursions outside the observation depth between image acquisitions and the shear-induced slowdowns of tracer particles cause significant measurement bias for long and short inter-frame time intervals, respectively. Also impeding the measurement accuracy is the existence of a near-wall particle depletion layer that leads to an overestimation of the fluid velocity. An analytical protocol to infer the correct fluid velocity from biased measurements is presented.

Pulsations with reflected boundary waves: a hydrodynamic reverse transport mechanism for perivascular drainage in the brain.

Accurate interpretation of recruitment rate measurements of microscale particles, such as cells and microbeads, to biofunctional surfaces is difficult because factors such as uneven ligand distributions, particle collisions, variable particle fluxes, and molecular-scale surface separation distances obfuscate the ability to link the observed particle behavior with the governing nanoscale biophysics. We report the development of a hydrodynamically conditioned micropattern catch strip assay to measure microparticle recruitment kinetics. The assay exploited patterning within microfluidic channels and the mechanostability of selectin bonds to create reaction geometries that confined a microbead flux to within 200 nm of the surface under flow conditions. Systematic control of capillary action enabled the creation of homogeneous or gradient ligand distributions. The method enabled the measurement of particle recruitment rates (keff, s-1) that were primarily determined by the interaction of the biomolecular pair being investigated. The method is therefore well suited for relative measurements of delivery vehicle and cellular recruitment potential as governed by surface-bound molecules.

Confocal Backscattering-Based Detection of Leukemic Cells in Flowing Blood Samples

We report on the design and construction of a confocal light scattering spectroscopic imaging system aimed ultimately to conduct depth-resolved characterization of biological tissues. The confocal sectioning ability of the system is demonstrated using a two-layer sample consisting of a 200 microm thick cancer cell layer on top of a scattering layer doped with a green absorber. The measurement results demonstrate that distinct light scattering signals can be isolated from each layer with an axial and a lateral resolution of 30 and 27 microm, respectively. Such a system is expected to have significant applications in the areas of tissue engineering and disease diagnostics and monitoring.

Confocal backscattering spectroscopy for leukemic and normal blood cell discrimination.

Beta-amyloid accumulation within arterial walls in cerebral amyloid angiopathy is associated with the onset of Alzheimer’s disease. However, the mechanism of beta-amyloid clearance along peri-arterial pathways in the brain is not well understood. In this study, we investigate a transport mechanism in the arterial basement membrane consisting of forward-propagating waves and their reflections. The arterial basement membrane is modeled as a periodically deforming annulus filled with an incompressible single-phase Newtonian fluid. A reverse flow, which has been suggested in literature as a beta-amyloid clearance pathway, can be induced by the motion of reflected boundary waves along the annular walls. The wave amplitude and the volume of the annular region govern the flow magnitude and may have important implications for an aging brain. Magnitudes of transport obtained from control volume analysis and numerical solutions of the Navier–Stokes equations are presented.

The role of shear stress and altered tissue properties on endothelial to mesenchymal transformation and tumor-endothelial cell interaction

The prognostic value of assessing minimal residual disease (MRD) in leukemia has been established with advancements in flow cytometry and PCR. Nonetheless, these techniques are limited by high equipment costs, complex, and costly cell processing and the need for highly trained personnel. Here, we demonstrate the potential of exploiting differences in the relative intensities of backscattered light at three wavelengths to detect the presence of leukemic cells in samples containing varying mixtures of white blood cells (WBCs) and leukemic cells flowing through microfluidic channels. Using 405, 488, and 633 nm illumination, we identify distinct light scattering intensity distributions for Nalm‐6 leukemic cells, normal mononuclear (PBMC) and polymorphonuclear (PMN) white blood cells and red blood cells. We exploit these differences to develop cell classification algorithms, whose performance is evaluated based on simultaneous acquisition of light scattering and fluorescence flow cytometry data. When this algorithm is used prospectively for the analysis of samples consisting of mixtures of PBMCs and leukemic cells, we achieve an average specificity and sensitivity of leukemic cell detection of 99.6 and 45.2%, respectively. When we consider samples that include leukemic cells along with PMNs and PBMCs, which can be acquired using a simple red blood cell lysis step following venipuncture, the specificity and sensitivity of the approach decreases to 91.6 and 39.5%, respectively. On the basis of the performance of these algorithms, we estimate that 42 or 71 μL of blood would be adequate to confirm the presence of leukemia at an 80% power level in samples containing 0.01% leukemia to either PBMCs or PBMCs and PMNs, respectively. Therefore, light scattering‐based flow cytometry in a microfluidic platform could provide a low cost, highly portable, minimally invasive approach for detection and monitoring of leukemic patients. This could offer significant improvements especially for pediatric patients and for patients in developing countries.

Performance and scaling of an electro-osmotic mixer

Leukemia is the most common pediatric cancer and leading cause of cancer related deaths in children. Improvements in the assessment of leukemic cells have the potential to influence not only the diagnosis of leukemia, but also the risk assessment of patients during the course of the treatment, both of which are important for improving the cure rate for this disease. In this study, we report on the design and performance of a confocal laser based system built to collect backscattered light over a range of 26° at 405, 488, and 633 nm to discriminate leukemic cells from normal red blood cells (RBC) and white blood cells (WBC). The design of the system is based on the spectral differences observed from spectroscopy measurements with a similar system designed with a white light source. Significant differences are observed in the intensity and wavelength dependence of leukemic cells from normal RBC and WBC. Specifically, the distinct light scattering of RBC is due to hemoglobin absorption, allowing for its discrimination from leukemic cells, mononuclear, and polymorphonuclear WBC particularly at certain wavelengths. Meanwhile, the high scattering intensities of polymorphonuclear WBC reflect the intracellular complexity of these cells in comparison to the leukemic or normal lymphocytes. Additionally, the detected light scattering spectra for leukemic cells are consistently steeper in comparison to normal WBC, which we attributed to differences in the fractal organization of intracellular scatterers. Based on our findings, the system has potential applications in the detection and quantification of leukemic cells in blood either in vivo or in vitro, using microfluidic‐based systems, for disease monitoring.