# Self-Mixing Interferometry-Based Micro Flow Cytometry System for Label-Free Cells Classification
## 1 Introduction  
With the development of microfluidic and optoelectronics techniques, flow cytometry (FC) has become a fundamental tool for the characterization and the analysis of cells or other micro-scale biological entities. This technique has been widely used in the life science, clinical diagnosis and biotechnology for more than 50 years [1; 2]. Traditional cytometers retrieve the forward/side scattering properties of particles and/or the fluorescence emission intensity, thus requiring various devices such as a photomultiplier (PMT), multiple-wavelength light sources and spectroscopy. As a consequence, these systems are often bulky and mechanically complicated. Furthermore, the conventional FC techniques need tricky fluorescence labeling pretreatment step with specific expensive dyes, and the valuable cell samples can be destroyed. Therefore, a label-free cell assay undertaking non-invasive and fast identification of cells is of major interest. So far, there are three well-established methods mainly used as label-free microfluidic FC system. The first technique is optical flow cytometry (OFC) which is the most common non-intrusive cell interrogation method. Multiple implementations have been proposed such as light-scatteringimaging [3], reflectance confocal microscopy [4], holographic microscopy [5,6] and 2D scattering pattern imaging [7], diffraction phase microscopy [8], common-path interferometry [9] and single-shot color quantitative phase imaging [10] that have been demonstrated to operate label-free single-cell characterization. The second technique is the impedance-based cytometry (IFC) technique. By measuring the impedance change between two microelectrodes embedded inside the channel when the cells pass through the measurement volume, the dielectric properties of the cells and the internal components can be retrieved. IFC devices have various advantages over others, for example, they are self-contained, highly integrated in the micro channel and easy to operate [11, 12, 13]. The third label-free cytometry technique is photoacoustic cytometry (PAFC) [14]. The principle of PAFC is based on photoacoustic effect when the tissue absorbs a short laser pulse sequence, the photon energy transforms it into heat and then the ultrasonic acoustic waves are generated due to thermo-elastic effect. The time-resolved acquisitions of the acoustic waves are performed through an ultrasound transducer, and the properties of the cells can be extracted from the ultrasound signal analysis. The most important advantage of PAFC is in vivo measurement capability such as melanoma detection in skin [15] or red blood cell aggregate in vessels and even cancer cells in the bone tissue [16]. In spite of the outstanding performance and vast application domains of these three label-free FC techniques, there are still some limitations in practice: (1) expensive devices are required, such as high-speed camera for instantaneous imaging, and ultrasound transducer for acoustic wave detection; (2) most of these systems are mechanically complex, and difficult to integrate in the microfluidic chip. To solve the shortcomings mentioned, a simple, commercial, accurate and reliable single-cell detection method is still strongly desired. Self-mixing interferometry (SMI) as a new laser interferometry technique has been widely used in various metrology applications [17, 18, 19]. Considering the intrinsic advantages of SMI, such as compacity, low-consumption, self-alignment, and the same half-wavelength resolution as compared with the traditional two-path interferometry, SMI is adaptable for operation as a fully integrated micro-nano particle-sensing technique in microfluidic systems. However, to our knowledge, most of the existing related works have described the collective properties of a population of samples.