## 4 Vascular Respiratory-Physiology-On-A-Chip
### Vascular Respiratory-Physiology-On-A-Chip  
#### 4.3.1 Lung-On-A-Chip  
Vascularized lung-on-a-chip research has been actively pursued for almost 10 years since the first microfluidic device was presented by [PERSON] and colleagues in 2010. The [PERSON] et al. [32] developed a microfluidic device consisting of two superimposed microfluidic channels separated by a microporous membrane. In this system, the upper chamber (ventilation) supports the growth of human lung alveolar epithelial cells, while the lower chamber (perfusion) cultivates pulmonary microvascular endothelial cells to mimic vasculature and provide a laminar flow. In addition, the membrane was periodically swelled and relaxed by hollow microchannels on both sides of the cell culture chamber to generate the mechanical strain by respiratory movements. Lung-on-a-chip-based research has demonstrated that periodic mechanical strain enhances the uptake of silica nanoparticles into epithelial and endothelial cells, which facilitates transport into the lower microvascular channels and increases lung toxicity and inflammatory responses. This model describes a biomimetic microsystem that reconstructs the alveolar-capillary interface of the human lung, providing particular utility by reproducing the integrated organ-level responses to inflammatory cytokines and bacteria infiltrated to the alveolar space. [PERSON] et al. [31] developed a lung-on-a-chip that mimics the microenvironment of the lung parenchyma by regenerating the alveolar barrier using a thin and flexible membrane co-cultured with epithelial and endothelial cells (Figure 1a). Primary human lung epithelial cells from patients and a bronchial epithelial cell line were used to demonstrate that mechanical stress significantly affects epithelial barrier permeability. In addition, when primary human lung alveolar epithelial cells were cultured in dynamic condition, the metabolic activity and expression of the inflammatory marker IL-8 were found to be significantly higher than cultured in static condition. This is the first case to investigate the effect of mechanical modification on primary cells from patients.  
The same group [72] successfully co-cultured patient-derived human primary alveolar type I and II-like epithelial cells and pulmonary endothelial cells on this respiratory lung-on-a-chip for over several days (Figure 1b). This has implications for the ability to mimic the in vivo environment using actual patient-derived lung epithelial cells. Recently, [PERSON] et al. [73] proposed an alveolar-on-a-chip that mimics pulmonary thrombosis (Figure 1c). This microfluidic alveolar-on-a-chip reproduces the role of platelets and endothelial cells in vivo, and it was shown that toxins do not act directly on the endothelium but indirectly stimulate vascular thrombosis by activating the alveolar epithelium. In addition, endothelial activation and inhibition of thrombosis by protease-activated receptor-1 (PAR-1) antagonists were analyzed to demonstrate the applicability of the system for new antithrombotic drug development studies. The vascularized lung-on-a-chip clearly mimics the pathological and biological characteristics of human lungs, and it is valuable as a tool for developing preclinical models to replace animal experiments as well as for screening new drugs related to lung disease (Table 3).