Exploring Spiral Microfluidic Channels: Secondary Dean Flow Induction and Micro-Particle Separation via a Progressively Narrowing Upper Channel

Abstract

Separating micro-particles or biological cells within a fluid has wide-ranging applications in environmental and medical fields. The concentration and separation of micro-plastics or micro-algae for water purification and the separation of tumor cells, blood, and plasma for cell analysis in the clinic are crucial for diagnosis and treatment. The microfluidic chip allows the handling of fluids on a small scale, enabling the miniaturization of devices. Additionally, microfluidic technology has been applied in various fields based on microfluidic dynamics to economically and precisely control micro-particles in fluid with low material consumption. However, existing technologies have limitations, such as a significant decrease in separation efficiency for samples with low throughput. This poses challenges in applying these methods to research involving large sample volumes or high-particle-density blood samples. This study proposes a microfluidic chip that utilizes the inertial effect to separate micro-particles in the fluid at a high throughput. The proposed chip aims to achieve separation across a range of particle sizes and is specifically designed for the rapid isolation of causative bacteria in blood, such as in the diagnostic process of sepsis. Sepsis is a severe condition where bacteria invade the body, triggering an excessive inflammatory response that can affect multiple organs. This leads to a significant decrease in the hourly survival rate by 9%. Therefore, rapid diagnosis and administering appropriate antibiotics for the causative bacteria are crucial. In clinical practice, blood culture tests are conducted to diagnose sepsis, which takes 2 to 5 days. Due to this extended timeframe, clinicians often prioritize prescribing broad-spectrum antibiotics effective against various bacteria without specific identification of the causative bacteria, potentially contributing to antibiotic resistance. To address this issue, there is a need for a technology capable of replacing the blood culture method for isolating bacteria in the bloodstream. However, blood contains approximately 5 billion red blood cells per milliliter, while bacteria are present in very low concentrations, ranging from 100 to 1000 CFU/mL, making their separation challenging due to their small size (1μm). Therefore, we aimed to use the proposed chip to concentrate and remove red blood cells from the blood, recovering the recovery of bacteria. The proposed chip enables particle separation using the secondary flow formed by the centrifugal acceleration towards the outer radius in a microscale curved channel. This is based on the mass conservation principle. A more robust secondary flow is induced by integrating a gradually narrowing upper channel with a rectangular cross-section lower channel, as used in previous studies. This enhancement results in improved particle separation efficiency under high flow conditions. The separation efficiency of particles with sizes of 1μm, 4.1μm, 6.3μm, and 15.0μm was evaluated. At the optimized flow rate of 25 mL/h, the 4.1μm particles showed a separation efficiency of 96.2%, indicating an 89.3% increase compared to the curved channel with a basic rectangular cross-section. For the particles with sizes of 6.3μm and 15.0μm, there was an increase in separation efficiency by 13.4% and 80.5%, respectively. This resulted in separation efficiencies of 93.3% and 90.1%, respectively. As part of a sepsis case study, the chip was applied to separate blood cells and recovered bacteria from the blood. Red blood cells (2~8μm) and white blood cells (14~20μm) move toward the inner wall of the channel, getting concentrated and removed at Outlet A. In contrast, bacteria (1μm), due to their tiny size, uniformly distribute within the channel with minimal inertial effects and are collected at both Outlets A and B. Therefore, only bacteria are recovered at Outlet B. Using a blood sample with a hematocrit of 2%, Escherichia coli (E. coli), was spiked at a concentration of 100-500CFU/mL. Subsequently, the performance of the decrescendo spiral channel, which combines a lower channel with a rectangular cross-section and an upper channel gradually narrowing, was evaluated based on the channel’s rotation in the optimized flow rate of 35 mL/h. As a result, 98.6% of red and white blood cells were effectively removed at Outlet A after four rotations in the Decrescendo curved channel. The bacterial recovery rate at Outlet B was 80.2%. The Decrescendo structure was employed to induce a more robust secondary flow, allowing for the rapid and efficient separation of particles of various sizes. Furthermore, the processing of 10 mL of whole blood, typically used for sepsis diagnosis in the clinic, was achieved within 7 hours, resulting in the recovery of over 80% of bacteria. These outcomes demonstrate the ability to separate particles of diverse sizes in various microenvironments and suggest potential benefits for expediting the sepsis diagnosis and treatment by reducing the time required for prescribing appropriate antibiotics.