Progress on Microfluidic Blood Cell Counting Techniques

The blood system serves important functions in the transportation of substances, maintenance of the internal environment homeostasis, and defensive protection. The blood primarily consists of plasma and blood cells. Numerous diseases that affect the human body can be reflected by changes of blood indicators. For instance, anemia is associated with an abnormally low number of red blood cells (RBCs), leukemia and liver cirrhosis are characterized by an abnormally high number of white blood cells (WBCs), and thrombosis is featured by an abnormally high number of platelets (PLTs) [-]. The blood routine is the most fundamental blood test item, determining the health of the blood by tracking changes in the quantity and distribution of blood cells. The RBC counts, WBC counts, WBC differential counts, hematocrit, and PLT are part of the indicators in the blood routine report, determined by a blood cell analyzer. Blood cell analyzer has replaced all other options as a particular solution for routine blood testing due to its high accuracy. However, blood routine tests can only be conducted in hospitals due to the high cost, large size, and difficulty in maintenance of blood cell analyzers, making it inconvenient to meet the needs of point-of-care testing and other scenarios. Therefore, recent development of blood analysis equipment has been focusing on miniaturization and portability [].

Microfluidics is an emerging technology that enables precise control of minute liquid volumes for biochemical analysis. In recent years, with the rise of microfluidic technology, microfluidic chips have become an important tool to support the development of miniaturization and portability of blood cell detection equipment due to their high bioaffinity, low sample consumption, quick reaction, and small size [-]. The principles of blood cell detection based on microfluidic chips include electrical impedance (Coulter’s principle) and light scattering (flow cytometry principle), and the combination of the two [,]. Additionally, differential counting of blood cells using image recognition has grown in popularity as a field of study with the rise of machine learning [].

This paper discusses the above-mentioned three methods for differential counting of whole blood cells and provides an overview of the latest research progress in whole blood cell analysis utilizing microfluidic chips.

During studying automatic blood detection, American scientists Wallace H. Coulter and Joseph R. Coulter Jr. came up with the electrical impedance method in 1956 [,]. Therefore, this approach is also named the Coulter principle.shows the schema of electrical impedance approach.

The diluted blood cell sample solution is centrifuged to ensure that the blood cells are equally dispersed before pouring it into a non-conductive container. The blood cell sample suspension should be placed in an insulated test tube with an aperture. Then, positive and negative electrodes are be placed on either side of the aperture, and power is provided by a continuous current source to create an electrically sensitive area. Due to the different conductivities of the blood cells and the solution, as the blood cells pass through the aperture under the influence of negative pressure, the voltage between the two electrodes will change, and a pulse signal will be generated [,,]. The size of the identified particles directly affects the amplitude of the pulse signal. When non-metallic particles pass through the aperture, the voltage change can be described using the Maxwell’s electromagnetic field theory as [,,]:

where ρ_e is the resistivity of the solution; I is the loop current; d is the diameter of the particle to be detected; D is the diameter of the aperture or the size of the electrically sensitive area. When measuring WBCs (diameter range 7-20 μm), the diameter of the aperture should be 70-100 μm, and when measuring RBCs (diameter range 6-9 μm) and PLTs (diameter range 1-8 μm), the diameter of the aperture should be about 50 μm.