## 3 Lower Respiratory-Physiology-On-A-Chip
### 3D Lower Respiratory-Physiology-On-A-Chip
#### Microfluidic Chip-Based Air-Liquid Interface Respiratory Model  
Appropriate tissue and organ models are needed to accurately represent the structure and function of the human respiratory microenvironment, but most in vitro respiratory models do not properly reproduce important in vivo microenvironmental characteristics (such as biological, physical properties). Transwell membrane inserts are widely used to establish co-cultures of epithelial cells grown on membranes of the ALI and airway constituent cells such as SMCs or vascular endothelial cells. However, transwell membranes are made of synthetic materials such as polyester or polyethylenederephthalate, which cannot mimic the stiffness and properties of natural pulmonary ECM. Fluid flow simulations involving air and blood circulation in respiratory systems have limitations of being difficult to reproduce in conventional culture dishes or transwell culture methods. Recently, the focus has been on creating more complex in vitro respiratory models with greater physiological similarity to the actual in vivo environment. In addition, research on respiratory system is being conducted using microfluidic devices, which can utilize laminar flow to provide nutrients and various flow-induced physiological stimuli to cells. [PERSON] et al. [55] cultivated human alveolar basal epithelial cells in a transwell and microfluidic system. They compared the ability to maintain monolayer integrity of epithelial cells and to produce surfactants between ALI culture and immersion culture. In that study, epithelial cell junctions formed more tightly in ALI cultures than immersion cultures, and mechanical stimulation by fluid flow in the microfluidic system ALI cultures showed an improvement in epithelial monolayer formation and reduced surface tension because of increase surfactant production in epithelial cells compared to conventional transwell ALI cultures. In addition, [PERSON] et al. [35] developed microfluidic respiratory chips for more complex lung microenvironments, such as intercellular and cell-ECM interactions, through co-culture of airway epithelial cells (EC) and airway SMCs. They adjusted the mixing ratio of type 1 collagen and Matrigel to establish an optimal EC-SMC co-culture condition for long-term culture of cells more than 31 days in the microfluidic chips. Recently, these microfluidic chips have been used in various studies including efforts to mimic respiratory diseases, mechanistic studies, and drug response. [PERSON] et al. [56] reproduced the physiological and pathological liquid plug flow found in the human airway epithelium using a microfabricated airway system integrated with computerized air-liquid biphasic microfluidics. They demonstrated that the propagation and rupture of liquid plugs, which simulated the reopening of closed airways in the absence of pulmonary surfactant, cause mechanical stress on harmful fluids, resulting in serious damage to small airway epithelial cells; in addition, explosive pressure waves generated by plug rupture were shown to facilitate detection of mechanical cell damage using a stiff sound. In addition, [PERSON] et al. [57] proposed a biomimetic microsystem that reconstructs the lung microenvironment to monitor the role of eosinophil cationic protein (ECP) in lung inflammation. ECP induces airway epithelial cells to express CXCL12 and stimulates the migration of fibroblasts toward the epithelium. By reproducing the flows that mimic the native physiological action, they were able to demonstrate that CXCL12-CXCR4 stimulation mediates the extracellular outflow of ECP-induced fibroblasts in lung inflammation. [PERSON] et al. [34] developed a smoking airway-on-a-chip by connecting a device allowing the system to \"breathe\" cigarette smoke across a small airway-on-a-chip made from cultured living human bronchial epithelial cells from normal and COPD patients, and used it to study the association between smoking and COPD.