(C-93) Analyzing the Effects of Helical Flow in Blood Vessels using Acoustofluidic-based Helical Flow Generator

Blood flow conditions are critical for blood vessels because atheroprone flows, such as low shear and disturbed flow, could be initiation factor of inflammation and atherosclerosis. On the other hand, helical flow is well-known as an atheroprotective flow because it prevents the accumulation of inflammatory factors such as enzymatic low-density lipoprotein (eLDL) and maintains the healthy condition of blood vessels due to high shear stress and efficient mass transportation. Although many efforts have been made to replicate this flow in vitro to prove its effects on blood vessels, progress has been hindered because of difficulties in generating and controlling microscale helical flow. In this research, we investigate the effects of helical flow in blood vessel using a dynamic flow generator based on surface acoustic wave (SAW) in a microfluidic device. Acoustic streaming is generated by SAW using an interdigital transducer (IDT). It rapidly develops within the fluid domain without directly contacting the medium, resulting in a steady and constant helical flow in the microscale channel. The unique window design allows for the generation of distinct unidirectional vortices, which can be controlled by adjusting the amplitude of the SAW.

The microfluidic chip was made with polydimethylsiloxane (PDMS) and SAW was applied on piezoelectric substrate (LiNbO₃). Human umbilical vein endothelial cells (HUVECs) were seeded inside the channel and diverse conditions of helical flow were applied. Biomarkers such as calcium influx, F-actin, VE-Cadherin and PECAM-1 were stained and analyzed to investigate the effects of helical flow. Acoustic streaming velocity field was shown using simulation with MATLAB and COMSOL Multiphysics and flow visualization with fluorescence particles.

Unidirectional vortex was successfully generated within the microfluidic channel. The unique window design made it possible to create clear vortex and avoid strong acoustic radiation force. This could be observed using fluorescence particles and computational simulations (Figure 1A). The vortex could be rapidly developed in the microscale channel, and a constant helical flow could be generated. To apply this system in culturing endothelial cells, a duty cycle was applied. The connection between cells differed with respect to the duty cycles, and ideal settings for this system were obtained (Figure 1B). F-actin and VE-Cadherin were well connected under specific duty cycles, and the direction of aligned F-actin could be successfully controlled by utilizing the amplitude of the surface acoustic wave (SAW). Helical flow was sensed by mechanosensitive ion channels such as Piezo1 (Figure 1C). Calcium ion flux differed under various flow conditions, and helical flow showed a similar intensity pattern to that of high shear flow. This phenomenon is due to the combined effect of shear stress and pressure, which are atheroprotective aspects of helical flow. Helical flow also decreases the adhesion of monocytes. There are several reasons for this, including vortex mixing and biochemical effects. The drag force due to the vortex is much larger than the force of gravity, resulting in effective decrease in adhesion and accumulation of monocytes. Helical flow also decreases the overexpression of PECAM-1, which is related to the adhesion site of platelets and monocytes. Overall, we successfully generated and controlled helical flow within the microscale channel and applied this system in dynamic cell culture. The atheroprotective aspects of helical flow was observed, and the results provide evidence to fabricate spiral-shape vascular grafts or stents that could create helical flow in blood vessels.

This work was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Korea government (MSIT) (no. 20008546), and the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2020R1A2C1100471 and 2020R1A5A8018367).