Simultaneous measurement of erythrocyte deformability and blood viscoelasticity using micropillars and co-flowing streams under pulsatile blood flows

Quantification of Blood Viscoelasticity under Microcapillary Blood Flow

Blood elasticity is quantified using a single compliance model by analyzing pulsatile blood flow. However, one compliance coefficient is influenced substantially by the microfluidic system (i.e., soft microfluidic channels and flexible tubing). The novelty of the present method comes from the assessment of two distinct compliance coefficients, one for the sample and one for the microfluidic system. With two compliance coefficients, the viscoelasticity measurement can be disentangled from the influence of the measurement device. In this study, a coflowing microfluidic channel was used to estimate blood viscoelasticity. Two compliance coefficients were suggested to denote the effects of the polydimethylsiloxane (PDMS) channel and flexible tubing (C₁), as well as those of the RBC (red blood cell) elasticity (C₂), in a microfluidic system. On the basis of the fluidic circuit modeling technique, a governing equation for the interface in the coflowing was derived, and its analytical solution was obtained by solving the second-order differential equation. Using the analytic solution, two compliance coefficients were obtained via a nonlinear curve fitting technique. According to the experimental results, C₂/C₁ is estimated to be approximately 10.9-20.4 with respect to channel depth (h = 4, 10, and 20 µm). The PDMS channel depth contributed simultaneously to the increase in the two compliance coefficients, whereas the outlet tubing caused a decrease in C₁. The two compliance coefficients and blood viscosity varied substantially with respect to homogeneous hardened RBCs or heterogeneous hardened RBCs. In conclusion, the proposed method can be used to effectively detect changes in blood or microfluidic systems. In future studies, the present method can contribute to the detection of subpopulations of RBCs in the patient's blood.

Biosensing of Haemorheological Properties Using Microblood Flow Manipulation and Quantification

The biomechanical properties of blood have been used to detect haematological diseases and disorders. The simultaneous measurement of multiple haemorheological properties has been considered an important aspect for separating the individual contributions of red blood cells (RBCs) and plasma. In this study, three haemorheological properties (viscosity, time constant, and RBC aggregation) were obtained by analysing blood flow, which was set to a square-wave profile (steady and transient flow). Based on a simplified differential equation derived using a discrete circuit model, the time constant for viscoelasticity was obtained by solving the governing equation rather than using the curve-fitting technique. The time constant (λ) varies linearly with respect to the interface in the coflowing channel (β). Two parameters (i.e., average value: <λ>, linear slope: dλdβ) were newly suggested to effectively represent linearly varying time constant. <λ> exhibited more consistent results than dλdβ. To detect variations in the haematocrit in blood, we observed that the blood viscosity (i.e., steady flow) is better than the time constant (i.e., transient flow). The blood viscosity and time constant exhibited significant differences for the hardened RBCs. The present method was then successfully employed to detect continuously varying haematocrit resulting from RBC sedimentation in a driving syringe. The present method can consistently detect variations in blood in terms of the three haemorheological properties.

Comparing of Frequency Shift and Impedance Analysis Method Based on QCM Sensor for Measuring the Blood Viscosity

Blood viscosity measurements are crucial for the diagnosis of cardiovascular and hematological diseases. Traditional blood viscosity measurements have obvious limitations because of their expensive equipment usage and large sample consumption. In this study, blood viscosity was measured by the oscillating circuit method and impedance analysis method based on single QCM. In addition, the effectiveness of two methods with high precision and less sample is proved by the experiments. Moreover, compared to the result from a standard rotational viscometer, the maximum relative errors of the proposed oscillating circuit method and impedance analysis method are ±5.2% and ±1.8%, respectively. A reliability test is performed by repeated measurement (N = 5), and the result shows that the standard deviation about 0.9% of impedance analysis is smaller than that of oscillating circuit method. Therefore, the impedance analysis method is superior. Further, the repeatability of impedance analysis method was evaluated by regression analysis method, and the correlation coefficient R2 > 0.965 demonstrated that it had excellent reproducibility.

Microfluidic Quantification of Blood Pressure and Compliance Properties Using Velocity Fields under Periodic On–Off Blood Flows

To monitor variations of blood samples effectively, it is required to quantify static and dynamic properties simultaneously. With previous approaches, the viscosity and elasticity of blood samples are obtained for static and transient flows with two syringe pumps. In this study, simultaneous measurement of pressure and equivalent compliance is suggested by analyzing the velocity fields of blood flows, where a blood sample is delivered in a periodic on-off fashion with a single syringe pump. The microfluidic device is composed of a main channel (mc) for quantifying the equivalent compliance and a pressure channel (pc) for measuring the blood pressure. Based on the mathematical relation, blood pressure at junction (Px) is expressed as Px = kβ. Here, β is calculated by integrating the averaged velocity in the pressure channel (<Upc>). The equivalent compliance (Ceq) is then quantified as Ceq = λoff · Q0/Px with a discrete fluidic model. The time constant (λoff ) is obtained from the transient behavior of the averaged blood velocity in the main channel (<Umc>). According to results, Px and Ceq varied considerably with respect to the hematocrit and flow rate. The present method (i.e., blood pressure, compliance) shows a strong correlation with the previous method (i.e., blood viscosity, elasticity). In conclusion, the present method can be considered as a potential tool for monitoring the mechanical properties of blood samples supplied periodically from a single syringe pump.

Experimental Investigation of Air Compliance Effect on Measurement of Mechanical Properties of Blood Sample Flowing in Microfluidic Channels

Air compliance has been used effectively to stabilize fluidic instability resulting from a syringe pump. It has also been employed to measure blood viscosity under constant shearing flows. However, due to a longer time delay, it is difficult to quantify the aggregation of red blood cells (RBCs) or blood viscoelasticity. To quantify the mechanical properties of blood samples (blood viscosity, RBC aggregation, and viscoelasticity) effectively, it is necessary to quantify contributions of air compliance to dynamic blood flows in microfluidic channels. In this study, the effect of air compliance on measurement of blood mechanical properties was experimentally quantified with respect to the air cavity in two driving syringes. Under periodic on–off blood flows, three mechanical properties of blood samples were sequentially obtained by quantifying microscopic image intensity (<I>) and interface (α) in a co-flowing channel. Based on a differential equation derived with a fluid circuit model, the time constant was obtained by analyzing the temporal variations of β = 1/(1–α). According to experimental results, the time constant significantly decreased by securing the air cavity in a reference fluid syringe (~0.1 mL). However, the time constant increased substantially by securing the air cavity in a blood sample syringe (~0.1 mL). Given that the air cavity in the blood sample syringe significantly contributed to delaying transient behaviors of blood flows, it hindered the quantification of RBC aggregation and blood viscoelasticity. In addition, it was impossible to obtain the viscosity and time constant when the blood flow rate was not available. Thus, to measure the three aforementioned mechanical properties of blood samples effectively, the air cavity in the blood sample syringe must be minimized (V_{air, R} = 0). Concerning the air cavity in the reference fluid syringe, it must be sufficiently secured about V_{air, R} = 0.1 mL for regulating fluidic instability because it does not affect dynamic blood flows.

Simultaneous measurement method of erythrocyte sedimentation rate and erythrocyte deformability in resource-limited settings

OBJECTIVE

The individual effects of plasma and red blood cells (RBCs) on the biophysical properties of blood can be monitored by measuring the erythrocyte sedimentation rate (ESR) and RBC deformability simultaneously. However, the previous methods require bulky and expensive facilities (i.e. microscope, high-speed camera, and syringe pump) to deliver blood or capture blood flows.

APPROACH

To resolve these issues, a simple method for sequential measurement of the ESR and RBC deformability is demonstrated by quantifying the cell-free volume (V _CF ), cell-rich volume (V _CR ), and blood volume (V _B ) inside an air-compressed syringe (ACS). A microfluidic device consists of multiple micropillar channels, an inlet, and outlet. After the ACS is filled with air (V _air   =  0.4 ml) and a blood sample (V _B   =  0.6 ml, hematocrit  =  30%) sequentially, the ACS is fitted into the inlet. The cavity inside the ACS is compressed to V _comp   =  0.4 ml after closing the outlet with a stopper. A smartphone camera is employed to capture variations in the V _CF , V _CR , and V _B inside the ACS. The ESR index suggested in this study (ESR _PM ) is obtained by dividing the V _CF (t  =  t ₁) with an elapse of t ₁. By removing the stopper, ΔV _B (ΔV _B   =  V _B [t  =  t ₁]  -  V _B ) is obtained and fitted as a two-term exponential model ([Formula: see text]. As a performance demonstration, the proposed method is employed to detect an ESR-enhanced blood sample, homogeneous hardened blood sample, and heterogeneous blood sample.

MAIN RESULTS

From the experimental results, it is found that the proposed method has the ability to detect various bloods by quantifying the ESR _PM and two coefficients (a, b) simultaneously.

SIGNIFICANCE

In conclusion, the present method can be effectively used to measure the ESR and RBC deformability in resource-limited settings.

Blood Viscoelasticity Measurement Using Interface Variations in Coflowing Streams under Pulsatile Blood Flows

Blood flows in microcirculation are determined by the mechanical properties of blood samples, which have been used to screen the status or progress of diseases. To achieve this, it is necessary to measure the viscoelasticity of blood samples under a pulsatile blood condition. In this study, viscoelasticity measurement is demonstrated by quantifying interface variations in coflowing streams. To demonstrate the present method, a T-shaped microfluidic device is designed to have two inlets (a, b), one outlet (a), two guiding channels (blood sample channel, reference fluid channel), and one coflowing channel. Two syringe pumps are employed to infuse a blood sample at a sinusoidal flow rate. The reference fluid is supplied at a constant flow rate. Using a discrete fluidic circuit model, a first-order linear differential equation for the interface is derived by including two approximate factors (F₁ = 1.094, F₂ = 1.1087). The viscosity and compliance are derived analytically as viscoelasticity. The experimental results showed that compliance is influenced substantially by the period. The hematocrit and diluent contributed to the varying viscosity and compliance. The viscoelasticity varied substantially for red blood cells fixed with higher concentrations of glutaraldehyde solution. The experimental results showed that the present method has the ability to monitor the viscoelasticity of blood samples under a sinusoidal flow-rate pattern.

Microfluidic-Based Biosensor for Sequential Measurement of Blood Pressure and RBC Aggregation Over Continuously Varying Blood Flows

Aggregation of red blood cells (RBCs) varies substantially depending on changes of several factors such as hematocrit, membrane deformability, and plasma proteins. Among these factors, hematocrit has a strong influence on the aggregation of RBCs. Thus, while measuring RBCs aggregation, it is necessary to monitor hematocrit or, additionally, the effect of hematocrit (i.e., blood viscosity or pressure). In this study, the sequential measurement method of pressure and RBC aggregation is proposed by quantifying blood flow (i.e., velocity and image intensity) through a microfluidic device, in which an air-compressed syringe (ACS) is used to control the sample injection. The microfluidic device used is composed of two channels (pressure channel (PC), and blood channel (BC)), an inlet, and an outlet. A single ACS (i.e., air suction = 0.4 mL, blood suction = 0.4 mL, and air compression = 0.3 mL) is employed to supply blood into the microfluidic channel. At an initial time (t < 10 s), the pressure index (PI) is evaluated by analyzing the intensity of microscopy images of blood samples collected inside PC. During blood delivery with ACS, shear rates of blood flows vary continuously over time. After a certain amount of time has elapsed (t > 30 s), two RBC aggregation indices (i.e., S_EAI: without information on shear rate, and erythrocyte aggregation index (EAI): with information on shear rate) are quantified by analyzing the image intensity and velocity field of blood flow in BC. According to experimental results, PI depends significantly on the characteristics of the blood samples (i.e., hematocrit or base solutions) and can be used effectively as an alternative to blood viscosity. In addition, S_EAI and EAI also depend significantly on the degree of RBC aggregation. In conclusion, on the basis of three indices (two RBC aggregation indices and pressure index), the proposed method is capable of measuring RBCs aggregation consistently using a microfluidic device.

In vitro and ex vivo measurement of the biophysical properties of blood using microfluidic platforms and animal models

Haemorheologically impaired microcirculation, such as blood clotting or abnormal blood flow, causes interrupted blood flows in vascular networks. The biophysical properties of blood, including blood viscosity, blood viscoelasticity, haematocrit, red blood bell (RBC) aggregation, erythrocyte sedimentation rate and RBC deformability, have been used to monitor haematological diseases. In this review, we summarise several techniques for measuring haemorheological properties, such as blood viscosity, RBC deformability and RBC aggregation, using in vitro microfluidic platforms. Several methodologies for the measurement of haemorheological properties with the assistance of an extracorporeal rat bypass loop are also presented. We briefly discuss several emerging technologies for continuous, long-term, multiple measurements of haemorheological properties under in vitro or ex vivo conditions.

Simultaneous measurement of blood pressure and RBC aggregation by monitoring on–off blood flows supplied from a disposable air-compressed pump

A simple method for simultaneously measuring RBC aggregation and blood pressure is demonstrated by analyzing blood flows supplied from a disposable air-compressed pump.

A Disposable Blood-on-a-Chip for Simultaneous Measurement of Multiple Biophysical Properties

Biophysical properties are widely used to detect pathophysiological processes of vascular diseases or clinical states. For early detection of cardiovascular diseases, it is necessary to simultaneously measure multiple biophysical properties in a microfluidic environment. However, a microfluidic-based technique for measuring multiple biophysical properties has not been demonstrated. In this study, a simple measurement method was suggested to quantify three biophysical properties of blood, including red blood cell (RBC) deformability, RBC aggregation, and hematocrit. To demonstrate the suggested method, a microfluidic device was constructed, being composed of a big-sized channel (BC), a parallel micropillar (MP), a main channel, a branch channel, inlet, and outlets. By operating a single syringe pump, blood was supplied into the inlet of the microfluidic device, at a periodic on-off profile (i.e., period = 240 s). The RBC deformability index (DI) was obtained by analyzing the averaged blood velocity in the branch channel. Additionally, the RBC aggregation index (AIN) and the hematocrit index (HiBC) were measured by analyzing the image intensity of blood flows in the MP and the BC, respectively. The corresponding contributions of three influencing factors, including the turn-on time (Ton), the amplitude of blood flow rate (Q₀), and the hematocrit (Hct) on the biophysical indices (DI, AIN, and HiBC) were evaluated quantitatively. As the three biophysical indices varied significantly with respect to the three factors, the following conditions (i.e., Ton = 210 s, Q₀ = 1 mL/h, and Hct = 50%) were maintained for consistent measurement of biophysical properties. The proposed method was employed to detect variations of biophysical properties depending on the concentrations of autologous plasma, homogeneous hardened RBCs, and heterogeneous hardened RBCs. Based on the observations, the proposed method exhibited significant differences in biophysical properties depending on base solutions, homogeneous hardened RBCs (i.e., all RBCs fixed with the same concentration of glutaraldehyde solution), and heterogeneous hardened RBCs (i.e., partially mixed with normal RBCs and homogeneous hardened RBCs). Additionally, the suggested indices (i.e., DI, AIN, and HiBC) were effectively employed to quantify three biophysical properties, including RBC deformability, RBC aggregation, and hematocrit.

Microfluidic-Based Measurement Method of Red Blood Cell Aggregation under Hematocrit Variations

Red blood cell (RBC) aggregation and erythrocyte sedimentation rate (ESR) are considered to be promising biomarkers for effectively monitoring blood rheology at extremely low shear rates. In this study, a microfluidic-based measurement technique is suggested to evaluate RBC aggregation under hematocrit variations due to the continuous ESR. After the pipette tip is tightly fitted into an inlet port, a disposable suction pump is connected to the outlet port through a polyethylene tube. After dropping blood (approximately 0.2 mL) into the pipette tip, the blood flow can be started and stopped by periodically operating a pinch valve. To evaluate variations in RBC aggregation due to the continuous ESR, an EAI (Erythrocyte-sedimentation-rate Aggregation Index) is newly suggested, which uses temporal variations of image intensity. To demonstrate the proposed method, the dynamic characterization of the disposable suction pump is first quantitatively measured by varying the hematocrit levels and cavity volume of the suction pump. Next, variations in RBC aggregation and ESR are quantified by varying the hematocrit levels. The conventional aggregation index (AI) is maintained constant, unrelated to the hematocrit values. However, the EAI significantly decreased with respect to the hematocrit values. Thus, the EAI is more effective than the AI for monitoring variations in RBC aggregation due to the ESR. Lastly, the proposed method is employed to detect aggregated blood and thermally-induced blood. The EAI gradually increased as the concentration of a dextran solution increased. In addition, the EAI significantly decreased for thermally-induced blood. From this experimental demonstration, the proposed method is able to effectively measure variations in RBC aggregation due to continuous hematocrit variations, especially by quantifying the EAI.