Editorial: Particle manipulation in microfluidic devices

Biological samples, and more in detail cell populations, are intrinsically heterogeneous, nevertheless standard approaches analyze the average properties of the entire cell populations, hindering single-cell specificity. Therefore, the development of alternative approaches single-cell investigation is a priority for human health, with several implications in diagnosis, screening as well as in patient monitoring and personalized drug optimization. This Research Topic composed of five contributions that identify, measure, and analyze sample heterogeneity to optimize system performances. An example is the work presented by Woo et al., who report the comparison between different protocols for tissue clearing, by Punching Assisted Clarity Analysis (PACA). Using this method, they have been able to compare the efficiency of more than 28 tissue clearing protocols in rodent brain samples. Given the sample heterogeneity, including differences in cell density and in neural and blood vessel networks, they have retrieved clear regional differences in tissue transparency that remained consistent across all tested protocols, irrespective of tissue thickness. Among different procedures to investigate sample heterogeneity, microfluidics is becoming a powerful instrumentation to target this goal (Yin and Marshall, 2012). Labon-a-chip technologies based on microfluidic networks are indeed major allies in singlecell analysis procedures (Haeberle and Roland, 2007). However, this requires the capability to assess particle manipulation, to sort, orient, align and stretch specimens in a controlled way. This comes together with the necessity of performing precise fluid control in terms of pressure, temperature, and fluidic resistance. The remaining four papers of this Research Topic cover these themes.