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

Abstract

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 (Vair, R = 0). Concerning the air cavity in the reference fluid syringe, it must be sufficiently secured about Vair, R = 0.1 mL for regulating fluidic instability because it does not affect dynamic blood flows.

Biosensing of Haemorheological Properties Using Microblood Flow Manipulation and Quantification

“…According to previous studies [ 23 , 30 , 38 , 39 ], based on the assumption that the time constant remained unchanged over the interface, the time constant was obtained by analysing the interface or fluid velocity and conducting a curve-fitting procedure. Previous studies reported that the flow conditions of the test fluid influence the time constant.…”

“…However, the compliance effect contributes significantly to the delay in the dynamic response. The air cavity inside the driving syringe caused an increase in the time constant [ 30 ]. Thus, the higher value of the time constant hindered the measurement of RBC aggregation under specific off-period flow conditions.…”

“…Based on Equation (9), λ t was expressed as a function of the interface ( β ). Unlike in previous studies [ 5 , 30 , 38 , 39 ], the present method does not assume that the time constant remains unchanged with respect to the interface. Thus, based on Equation (9), variations in λ t were obtained with respect to the interface under transient blood flow conditions (i.e., λ t = λ t [ β ]).…”

“…However, as the time constant contributes to gradually decreasing transient blood flow, the blood flow does not stop immediately. This makes it difficult to induce RBC aggregation [ 30 ]. Therefore, the AI is significantly influenced by the time constant.…”

Editorial for the Special Issue on Micro/Nano Devices for Blood Analysis, Volume II

“…Related to his other published works, Kang [9] also studied, under periodic on-off blood flows, the contributions of air compliance to the dynamic blood flows in microfluidic channels for the successful evaluation of the mechanical properties of blood samples. The author analyzed the image intensity and interface in a co-flowing channel and concluded that air cavities in the blood sample syringe contributed to delays in the transient behaviors of blood flows, hindering the quantification of RBCs' aggregation and blood viscoelasticity [9].…”