Basic concepts of biological microparticles isolation by inertia spiral microchannels in simple terms: a review

The microfluidics separation has absorbed wide-ranging attention in recent years due to its outstanding advantages in biological, medical, clinical, and diagnostical cell studies. While conventional separation methods failed to render the acceptable performance, microfluidics sorting methods offer many privileges such as high-throughput, user-friendliness, minimizing sample volumes, cost-efficiency, non-invasive procedures, high precision, improved portability, quick processing, etc. Among the inertial microfluidics approaches such as the straight and curved microchannels, although the spiral microchannels, which are the sorts of passive separations, are complicated in concepts and geometries, they have demonstrated auspicious benefits for this purpose. Thus, numerous studies have strived to explain the principle of particle migrating and forces in these complex microchannels. However, a comprehensive understanding is still necessary. On the other side, it is manifest that the diagnosis and separation of circulating tumor cells from the blood are significant for targeted treatments of this detrimental disease. Therefore, this study aims to review the previous investigations and developments for understanding the circulating tumor cell separation using the spiral microchannels straightforwardly and profoundly. After elucidating the inertial microfluidics and their governing physics in simple terms, we provide insights about spiral microchannels' mechanism and concepts, the secondary flow, the cross-section effects on the separation processes, and finally, the future applications and challenges of this kind of inertial microfluidics. The investigations reveal that new approaches should be conducted to use the spiral microchannels with combined cross-sections. These kinds of microchannels with optimum size and shape of cross-sections can improve the performance efficiently.