# Pneumatically Actuated Microfluidic Platform for Reconstituting 3D Vascular Tissue Compression
## 1 Introduction  
In vivo, many cells in the human body continuously experience various mechanical stimuli. For example, endothelial cells (ECs) in blood vessels are exposed to varying mechanical stimuli, internally and externally. Mechanical stress causes EC phenotype alternation, vascular cytoskeletal remodeling, and disease progression [1]. Solid stress has a directional component as tissues have different structural characteristics depending on the location and orientation. The interactions between blood vessel networks and external solid stress remains comparatively under-investigated despite their importance to the fundamental understanding of many pathological conditions.  
Recent advancements in microfluidic microfabrication technologies have demonstrated the feasibility of generating in vitro vascular network models within microfluidic stromal and endothelialcell co-culture chips [2; 3; 4]. These microfluidic platforms successfully generated perfusable vascular lumina with in-vivo-like tissue features, junction protein expression, and barrier functions.  
While pre-existing vascular tissue culture platforms have achieved reliable vasculature generation, to date, there has been minimal research using a biomechanical approach based on a microfluidic device that is used for inducing biomechanical stresses on a vascular network within a controlled microfluidic setting.  
Quake valves are pneumatic microvalves made with polydimethylsiloxane (PDMS). These monolithic and elastomeric valves are easy to fabricate and can be utilized in flow control and microfluidic metering [5]. Several studies have incorporated Quake valves or pneumatically actuated valves to facilitate cell co-culture in both isolated and interacting culture environments by regulating the opening and closing of channels [6; 7; 8; 9]. [PERSON] et al. adopted Quake valves as a means of applying mechanical stress to mildly stretch the axons cultured in a microfluidic device [10]. The results showed similar responses to in vivo models, approving Quake valves as a reliable source of biomechanical stress.  
In this study, we proposed a novel multilayer microfluidic platform that incorporates a 3D perfusable in vitro blood vessel to mimic biomechanical stress in the range of 0.5 psi to 2.5 psi by utilizing Quake valves. Live/dead assays, reactive oxygen species (ROS) levels, and vascular permeability assays characterized the vascular tissue states within the compression platform. Experimental observations showed stress-dependent histological reactions of cell death, ROS level increases, and increased vascular leakiness in gradients that corroborated with the finite element analysis (FEA) models of stress distribution, indicating that the experimental and theoretical models of stress propagation within the in vitro vasculature was sound. These results emphasize the importance of solid-stress-induced blood vessel compression on vascular physiological and pathological conditions and suggest that controlling biomechanical stress can be an important factor in controlling vascular abnormalities.