Inertial microfluidic separation using curvilinear channels. (a) Separation of 10 μm, 15 μm, and 20 μm diameter particles in a spiral microchannel.63 Reproduced with permission from Kuntaegowdanahalli et al., Lab on a Chip 9(20), 2973–2980 (2009). Copyright 2009 Royal Society of Chemistry. (b) Fermat's spiral (double-spiral) channel used for tumor cell separation.101 Reproduced with permission from Sun et al., Lab on a Chip 12(20), 3952–3960 (2012). Copyright 2012 Royal Society of Chemistry. (c) Blood cell and particle separation in a spiral channel with a trapezoid cross section.102 Reproduced with permission from Wu et al., Anal. Chem. 84(21), 9324–9331 (2012). Copyright 2012 American Chemical Society. (d) A cascaded spiral microchannel99 and a spiral combining with an active lateral cavity acoustic transducer (LCAT) unit for blood cell separation.95 Reproduced with permission from Abdulla et al., Anal. Chem. 90(7), 4397–4405 (2018). Copyright 2018 American Chemical Society.99 Reproduced with permission from Nivedita et al., Analyst 142(14), 2558–2569 (2017). Copyright 2017 Royal Society of Chemistry.95 (e) A low-cost spiral channel by winding a square silica capillary into a helical form for particle focusing.87 Reproduced with permission from Wang et al., Biomicrofluidics 11(1), 014107 (2017). Copyright 2017 AIP Publishing. (f) Different curvilinear channels formed by winding soft Polydimethylsiloxane (PDMS) channels.107 Reproduced with permission from Xi et al., Proc. Natl. Acad. Sci. U. S. A. 114(40), 10590–10595 (2017). Copyright 2017 Authors licensed under a CC BY 4.0. Different forms of curved channels (g)–(i) other than spiral have also been developed for particle focusing.61,110,111 Reproduced with permission from Di Carlo et al., Proc. Natl. Acad. Sci. U. S. A. 104(48), 18892–18897 (2007). Copyright 2007 National Academy of Sciences. Reproduced with permission from Zhang et al., Sci. Rep. 4, 4527 (2014). Copyright 2014 Authors licensed under a CC BY 4.0. Reproduced with permission from Özbey et al., Sci. Rep. 6, 38809 (2016). Copyright 2016 Authors licensed under a CC BY 4.0.