Microfluidic Systems for Blood and Blood Cell Characterization.

A laboratory blood test is vital for assessing a patient's health and disease status. Advances in microfluidic technology have opened the door for on-chip blood analysis. Currently, microfluidic devices can reproduce myriad routine laboratory blood tests. Considerable progress has been made in microfluidic cytometry, blood cell separation, and characterization. Along with the usual clinical parameters, microfluidics makes it possible to determine the physical properties of blood and blood cells. We review recent advances in microfluidic systems for measuring the physical properties and biophysical characteristics of blood and blood cells. Added emphasis is placed on multifunctional platforms that combine several microfluidic technologies for effective cell characterization. The combination of hydrodynamic, optical, electromagnetic, and/or acoustic methods in a microfluidic device facilitates the precise determination of various physical properties of blood and blood cells. We analyzed the physical quantities that are measured by microfluidic devices and the parameters that are determined through these measurements. We discuss unexplored problems and present our perspectives on the long-term challenges and trends associated with the application of microfluidics in clinical laboratories. We expect the characterization of the physical properties of blood and blood cells in a microfluidic environment to be considered a standard blood test in the future.

Temperature Correction to Enhance Blood Glucose Monitoring Accuracy Using Electrical Impedance Spectroscopy.

Electrical methods are among the primarily studied non-invasive glucose measurement techniques; however, various factors affect the accuracy of the sensors used. Of these, the temperature is a critical factor; hence, the effects of temperature on the electrical properties of blood components are investigated in this study. Furthermore, the changes in the electrical properties of blood according to the glucose level are corrected by considering the effects of temperature on the electrical properties. An impedance sensor is developed and used to measure whole blood impedance in 10 healthy participants at various temperatures and glucose levels. Subsequently, the conductivities of the plasma and cytoplasm were extracted. Changes in the electrical properties of the blood components are then analyzed using linear regression and repeated measures ANOVA. The electrical conductivities of plasma and cytoplasm increased with increasing temperatures (plasma: 0.0397 (slope), 0.7814 (R2), cytoplasm: 0.014 (slope), 0.694 (R2)). At three values of increasing glucose levels (85.4, 158.1, and 271.8 mg/dL), the electrical conductivities of the plasma and cytoplasm decreased. These tendencies are more significant upon temperature corrections (p-values; plasma: 0.001, 0.001, cytoplasm: 0.003, 0.002). The relationships between temperature and electrical conductivity changes can thus be used for temperature corrections in blood glucose measurement.

Electrochemical Impedance Characterization of Blood Cell Suspensions-Part 2: Three-Phase Systems With Single-Shelled Particles.

The analyses presented in Part 1 are expanded to three-phase composite materials. The theory developed in Part 1 is used for analytical and numerical calculations of dielectric spectra. In this study, three-phase systems with single-shelled particles were considered. The disordered particle distribution, aligned orientation of particles, and particles placed in different lattice structures are studied. It is shown that both two-phase and three-phase composites exhibit ß-dispersion, while three-phase composites additionally exhibit δ-dispersion. This is the fundamental difference in the spectra for two- and three-phase materials. In the case of aligned orientation, the effective permittivity in the direction perpendicular to the spheroid plane exhibits a maximum at a volume fraction of approximately 0.5. Both two- and three-phase materials display this behavior. This may be of interest for the development of new metamaterials. The dielectric properties of composite materials with random distribution and periodic arrangement of particles differ significantly. The dielectric spectrum of erythrocyte suspension in plasma was measured by means of electrochemical impedance spectroscopy. The measurement system consisted of a small chamber with two planar electrodes placed at the bottom and an impedance analyzer. The dielectric properties of erythrocyte cytoplasm and membrane were numerically determined based on experimental data. The measurement of the dielectric properties of whole blood and blood components is very promising for various medical applications.

Electrochemical Impedance Characterization of Blood Cell Suspensions. Part 1: Basic Theory and Application to Two-Phase Systems.

Electrochemical impedance spectra of composite materials contain information on the topological arrangement, volume fraction, and shape of particles, as well as the dielectric properties of the matrix and particles. The objective of this study is to investigate how these parameters affect the dielectric spectrum and what reliable information can be extracted from experimental data. The main attention was focused on systems with dielectric behavior similar to that of human blood. Mostly plasma and erythrocytes determine the dielectric properties of whole blood. Erythrocytes suspended in plasma can be considered as three-phase systems with single-shelled particles. A theoretical approach based on the effective medium theory is developed for calculating the effective permittivity and conductivity of three-phase composites at a wide frequency range (from 0 to 1 GHz). A finite-difference method is applied to model three-dimensional periodic structures. A special case of two-phase materials is used to demonstrate the influence of the shape and arrangement of particles on dielectric properties. Theoretical and numerical approaches are applied to two-phase composites with spherical, spheroidal and biconcave particles and are compared with each other and with published data. It is shown that two-phase composites exhibit only ß-dispersion. In contrast to the quasi-static limit, the wide-bandwidth impedance spectroscopy makes it possible to distinguish between disordered and regular arrangements of spheroidal and biconcave particles. The results can be used to analyze the dielectric properties of blood, which is very promising for various medical applications. This study of two-phase composites can be further extended to three-phase composites.

A physiometer for simultaneous measurement of whole blood viscosity and its determinants: hematocrit and red blood cell deformability.

In this study, a microfluidic-based physiometer capable of measuring whole blood viscosity, hematocrit, and red blood cell (RBC) deformability on a chip is introduced. The physiometer consists of two major parts: a hydrodynamic component for whole blood viscosity measurement and an electronic component for hematocrit and RBC deformability measurement. In the hydrodynamic component, the whole blood is infused with phosphate buffered saline as a reference fluid for estimation of the whole blood viscosity. At a given flow rate, ten sets of whole blood viscosity readings are successfully obtained over a wide range of shear rates; this is achieved via a series of geometrically optimized microchannel arrays. In the electronic component, analysis of the whole blood impedance spectrum under flowing conditions reveals the electrical characteristics of the blood: the cytoplasm resistance (Rcytoplsm), plasma resistance (Rplasma), and RBC membrane capacitance (constant phase element). The hematocrit is estimated from Rcytoplsm and Rplasma, while the RBC deformation index is determined from the membrane capacitance change of the RBC. Each unique function is experimentally demonstrated and compared to the corresponding gold standard method. The whole blood viscosity measured using the physiometer is 0.8 ± 1.4% in normalized difference compared to that using a rotational cone-and-plate viscometer. For the hematocrit measurement, the coefficient of variation for the physiometer ranges from 0.3 to 1.2% which is lower than the one obtained from centrifugation. In the deformability measurement, there is a strong linear correlation (R2 = 0.97) between the deformation index acquired by image processing and the change in the membrane capacitance acquired by using the physiometer. The effects of the hematocrit and RBC deformability on the whole blood viscosity are also demonstrated. For simultaneous and reliable measurement on a chip, a physiometer equipped with a temperature-control system is prepared. Lab-made software enables the measurement of the three target indices and the temperature control in an automated manner. By using this system, the temperature is controlled to 36.9 ± 0.2 °C which greatly matches with the target temperature (37.0 °C) and it is varied from 25 °C to 43 °C. The developed physiometer is potentially applicable for a comprehensive analysis of biophysical indices in whole blood.

Effects of Aggregation on Blood Sedimentation and Conductivity.

The erythrocyte sedimentation rate (ESR) test has been used for over a century. The Westergren method is routinely used in a variety of clinics. However, the mechanism of erythrocyte sedimentation remains unclear, and the 60 min required for the test seems excessive. We investigated the effects of cell aggregation during blood sedimentation and electrical conductivity at different hematocrits. A sample of blood was drop cast into a small chamber with two planar electrodes placed on the bottom. The measured blood conductivity increased slightly during the first minute and decreased thereafter. We explored various methods of enhancing or retarding the erythrocyte aggregation. Using experimental measurements and theoretical calculations, we show that the initial increase in blood conductivity was indeed caused by aggregation, while the subsequent decrease in conductivity resulted from the deposition of erythrocytes. We present a method for calculating blood conductivity based on effective medium theory. Erythrocytes are modeled as conducting spheroids surrounded by a thin insulating membrane. A digital camera was used to investigate the erythrocyte sedimentation behavior and the distribution of the cell volume fraction in a capillary tube. Experimental observations and theoretical estimations of the settling velocity are provided. We experimentally demonstrate that the disaggregated cells settle much slower than the aggregated cells. We show that our method of measuring the electrical conductivity credibly reflected the ESR. The method was very sensitive to the initial stage of aggregation and sedimentation, while the sedimentation curve for the Westergren ESR test has a very mild slope in the initial time. We tested our method for rapid estimation of the Westergren ESR. We show a correlation between our method of measuring changes in blood conductivity and standard Westergren ESR method. In the future, our method could be examined as a potential means of accelerating ESR tests in clinical practice.

Universal curves for the van der Waals interaction between single-walled carbon nanotubes.

We report very simple and accurate algebraic expressions for the van der Waals (VDW) potentials and the forces between two parallel and crossed carbon nanotubes. The Lennard-Jones potential for two carbon atoms and the method of the smeared-out approximation suggested by Girifalco were used. It is found that the interaction between parallel and crossed tubes is described by two universal curves for parallel and crossed configurations that do not depend on the van der Waals constants, the angle between tubes, and the surface density of atoms and their nature but only on the dimensionless distance. The explicit functions for equilibrium VDW distances, well depths, and maximal attractive forces have been given. These results may be used as a guide for the analysis of experimental data to investigate the interaction between nanotubes of various natures.

Van der Waals interaction between two crossed carbon nanotubes.

The analytical expressions for the van der Waals potential energy and force between two crossed carbon nanotubes are presented. The Lennard-Jones potential between pairs of carbon atoms and the smeared-out approximation suggested by L. A. Girifalco (J. Phys. Chem. 1992, 96, 858) were used. The exact formula is expressed in terms of rational and elliptical functions. The potential and force for carbon nanotubes were calculated. The uniform potential curves for single- and multiwall nanotubes were plotted. The equilibrium distance, maximal attractive force, and potential energy have been evaluated.

Field enhancement factor and field emission from a hemi-ellipsoidal metallic needle.

We present an exact solution for the electrostatic field between a metallic hemi-ellipsoidal needle on a plate (as a cathode) and a flat anode. The basic idea is to replace the cathode by a linearly charged thread in a uniform electric field and to use a set of "image" charges to reproduce the anode. We calculate the field enhancement factor on the needle surface and ponderomotive force acting on the needle. Using the Fowler-Nordheim theory we obtain an exact analytical formula for the total current.