## 1 Introduction  
Cells in vivo are exposed to various mechanical forces [1; 2; 3; 4; 5]. For example, endothelial cells, which line the interior surface of blood vessels and lymphatic vessels, are mainly exposed to three types of mechanical forces: (1) shear stress caused by blood shear flow; (2) cyclic stretch results from vasodilation; and (3) hydrostatic pressure from blood pressure [6; 7]. In vivo cells sense mechanical forces and have appropriate morphological responses, which plays an important role in health maintenance. For example, it has been reported that atherosclerosis is always located at the bifurcation of the artery, where the mechanical forces and cell morphology are different from those in uniform linear blood vessels [8; 9]. Over the past 30 years, many researchers have attempted to study the mechanisms by which cells perceive and respond to mechanical forces, which may provide new types of drug targets that are different from those discovered from chemical methods [10; 11; 12]. Cyclic stretch is an in vitro method to investigate the influence of mechanical forces on a population of cells [13; 14]. Figure 1a shows a diagrammatic sketch of a typical cell cyclic stretch system. Dimethylpolysiloxane (PDMS) is usually used as the material for the membrane in the commercially available stretch chamber, because it is deformable, biocompatible, and optically transparent [4; 15; 16]. At the beginning of the experiments, cells are cultured on an elastic membrane for 24 hours or longer before exposure to cyclic stretch, a process that enables the cells to adhere firmly onto the membrane [14; 17; 18; 19]. After that, the stretch chamber filled with 1\(\sim\)2-mL cell culture medium was set to one or two motorized stages, which cyclically stretched the chamber according to the experimental design of researchers [20]. The role of cyclic stretching in cell behavior was investigated through real-time visualization of the morphology of living cells and the internal structures during the process of cyclic stretching, because the cellular response to mechanical forces is a process ranging from less than 1 second to several days [21; 22; 23; 24; 25; 26; 27]. It has been reported that the size of focal adhesions in vascular smooth muscle cells increased significantly after exposure to cyclic stretch for 2 min [28]. [PERSON] et al. recorded cell behaviors every 5 min during an experimental process of 115 min and found that, when they measured the activity of RhoA (a small guanosine triphosphatase (GTPase) protein), it increased significantly after exposure to cyclic stretch for 10 min compared to that in the initial state [29]. [PERSON] et al. pointed out that the kinase activity of c-src increased and peaked at 15 min after an application of uni-axial cyclic stretch in human endothelial cells [30]. On the other hand, it has been reported that cells had an immediate viscoelastic response after exposure to mechanical forces, and had adaptive behavior within 15 s, or a shorter time (less than 1 s) [31; 32; 33]. It is not possible to obtain the rapid responses of cells to mechanical cyclic stretching by using phenomena observed at discrete time points. Further, instead of gradual cell changes, quick structural disruption, such as the sudden disassociation of a actin-myosin apparatus, may occur at any time point during the periods of experiments [35], the imaging timing cannot be designed prior to the experiments. Therefore, it is useful to improve the time resolution to figure out the continuous or sudden cell responses under cyclic stretch. In addition, a cyclic stretch system for real-time imaging without stopping the system is useful in studying the mechanical dynamics of some cellular components that are not suitable for immobilization, such as Ca\({}^{2+}\) and phospholipids [36; 37; 38]. Even today, the details of cell rapid responses induced by cyclic stretch have not been fully understood.