Passive Focusing Techniques of Particles in Microfluidic Device

Particle focusing in microfluidic device is a rapidly growing research field due to its wide applications in biology, chemistry, engineering, and medicine. Precise and high-throughput focusing can be a pivotal pretreatment step for counting, detection and separation application. Generally, focusing technologies can be divided into active, passive and sheath-assisted methods. The active and sheath-assisted methods rely on external energy field or sheath flow, which can accurately control the position of particles in microchannel, but require integrating other complex functional elements. Passive focusing uses fluid inertia, viscoelasticity, and other characteristics to control the equilibrium position of particles in the microchannel, and it has been proven to be a powerful tool due to the simplicity, label-free, biocompatible, contact-free, low-cost, and high throughout nature. A large amount of experimental and numerical work has been carried out on the passive focusing technology from the aspects of chip structure, microfluidic characteristics, and particle characteristics. In this review, the fundamental hydrodynamic forces and the basic principles related to the focusing mechanism were firstly briefly introduced and discussed. Then the state of the art of detailed passive focusing methods was presented. At last, the focusing methods were summarized and future development in this field was predicted.

Automated summary

Particle focusing in microfluidic devices has become a rapidly growing research field, with active, passive, and sheath-assisted methods being the main focusing technologies. Passive focusing, utilizing fluid inertia and other characteristics, has proven to be a powerful tool due to its simplicity, label-free nature, biocompatibility, contact-free operation, low cost, and high throughput.