## 1 Introduction  
Microfluidics is the science and technology of manipulating and processing colloidal systems including fluids and particles at the micro- or nano-scale [1]. With the rapid development of microfabrication technology during the past two decades, microfluidics technology has been integrated in small chips for realization of the lab-on-a-chip (LOC) system, where all laboratory tasks for material treatment and analysis can be performed in an automated manner. The integrative system has numerous advantages originating from scale-down compared to conventional macro-sized devices. Specifically, they offer low cost, short reaction time, small reagent and power consumption, less waste output, and high integration efficiency of microfluidic components for multi-parallel processing [1; 2; 3]. Those advantages have triggered wide applications in various research and industry fields, including life science [4; 5; 6], inkjet printing [7; 8; 9; 10], micro-air vehicles [11; 12; 13], automobiles [14; 15], robotics [16; 17], and microelectronics [18; 19; 20]. Among these fields, the largest beneficiary of microfluidics is biotechnology, which often requires precise manipulation and fast pretreatment of cell suspensions and accurate detection of target cells for biological assays. The ability to move motile or non-motile cells in a controlled fashion improved the efficiency and reliability of biological/clinical analysis and further enabled the development of high-performance miniaturized total analysis systems (\(\upmu\)TAS) [3; 5; 21]. Particularly, the trapping, isolation, and analysis of a single cell allowed resolution of many unsolved questions in biology by providing information about the physical and biological behaviors of individual cells in a cell population [2]. This led to the creation of new research fields such as single-cell genomics, transcriptomics, proteomics, and metabolomics, improving many existing clinical and biomedical applications [22; 23]. A main focus is on how precisely and fast cell manipulation can be performed in a non-invasive manner [3; 24; 25; 26]. To date, various microfluidic manipulation techniques to meet the requirements have been developed and integrated into a chip for high-resolution, high-throughput, and non-contaminating biological assays. The primary techniques are dielectrophoresis and electrophoresis utilizing interactions of polarity or gradients of applied electric fields and net charges of manipulated particles [27; 28; 29; 30], optical tweezers using the scattering force and the gradient force delivered from a focused laser beam [31; 32; 33; 34], magnetophoresis driven by application of magnetic field gradients to particles with magnetic susceptibility [35; 36; 37; 38], and acoustophoresis wherein particle motion is controlled using acoustic waves with varying pressure profiles [39; 40; 41]. Additionally, there also have been other microfluidic approaches using inertial focusing and hydrodynamic stretching [42], optical stretching [43], and pressure gradients [44; 45]. These techniques have been consistently applied for concentration, separation, sorting, isolation, and transport of cells [46; 47; 48; 49]. However, applications also revealed the drawbacks of each manipulation technique. For example, dielectrophoresis requires a complex 3D electrode structure for precise cell manipulation [5]. In addition, it is difficult to control nano-sized cells because the dielectrophoretic force is proportional to the volume of a manipulated object [50; 51]. Electrophoresis, another electric field-based technique, is applicable only for charged particles [28]. Accordingly, its application range for cell manipulation is not very wide [52; 53]. In addition, the electrophoretic migration speed of cells is low [24]. The use of optical tweezers is typically limited to manipulation of a single or a few cells because of the narrow beam waist of a focused laser [6; 27; 33; 54]. The high heat in the laser beam may cause cell damage and lysis [34].