# Selector+
## 1 Introduction
QC systems to report the health state of cells are time-consuming and dependent on the ability of operators. Differences in the culture medium, percentage of serum, growing cell density, type of plastic, training of operators, and much more can affect the quality and vitality of cells, in addition to the risk of microbial contamination. Proliferative capacity is one of the characteristics measured to assess the cells' quality and vitality. However, new quality systems need to be implemented to fulfil the lack of QC for these types of cells. Analytical methods applied for QC aspects, such as chromatography-like technology for small molecules, could be a powerful tool for QC in ATMPs. Microfluidic systems have been used lately for bio-analytes and single-cell analysis, and among them, field-flow fractionation (FFF) has proven its capacity to analyze, discriminate, and separate a wide size range of biological samples based on their native physical characteristics with high resolution and throughput, and to correlate physical properties to biological cell activity [8; 9; 10; 11; 12; 13; 14; 15; 16]. Among FFF techniques, the gravitational field-flow fractionation (GrFFF) variant is the most employed for whole-cell analysis with advantages related to the simple instrumental set-up of the separative device which allows its easy decontamination and guarantees sterile and clean conditions. In GrFFF, the separation occurs in a capillary channel due to the combined action of the flow of the mobile phase and Earth's gravity field [17; 18]. Despite the interesting potentialities of GrFFF, the technique shows some limitations, particularly related to the low throughput for adherent cells. We developed a novel method, non-equilibrium Earth-gravity-assisted dynamic fractionation (NEEGA-DF) to work with adherent cells to avoid their attachment to capillary walls [19]. Cells with different physical characteristics acquire different velocities inside the capillary channel and elute at different times. An automated instrument was developed to implement the NEEGA-DF method (Celector(r), Stem Sel srl, Bologna, Italy). We proved that different subpopulations could be defined in amniotic fluid stem cells, and we could select the most potent cells and identify the mesenchymal stem cell component in fresh bone marrow samples [20; 21]. In this work, we demonstrated for the first time the use of Celector(r) as a quality control system for cell culture procedure standardization in order to monitor changes in cell behavior during in vitro culturing. The instrument is equipped with a micro-camera for cell detection, and specifically designed software for image acquisition, post-processing, and data analysis was employed. The output of the instrument is a multiparametric fractogram representing number, size, and shape of the eluted cells as a function of fractionation time, and it is the fingerprint of the cell sample. This instrument possesses two functions, as a QC device and as a sorter to enrich the homogenous target cell population. We analyzed two types of cell lines, the promyeloblasts HL60 and the colorectal cancer cells SW620, as models of in vitro expansion. Both cell lines were monitored for their doubling time, and each passage was analyzed by the instrument to obtain the cell viability and dimensions, the presence of dead cells, and the evolution of possible heterogeneity of the cell population.