Mechanobiophysics of the alveolus – Reconstruction of strain and forces at the gas-blood barrier

Introduction: In the last years, the importance of mechano-chemical cellular responses for biology and pathobiology has been recognized. As cells sense their mechanical environment, they respond to extracellular forces. This is particularly important for lung epithelial and endothelial cells forming the air-blood barrier where they constantly get stretched during breathing. Mechanical stretch in the alveolus plays a key role in triggering the secretion of pulmonary surfactant which reduces surface tension at the alveolar air-liquid interface. However, an integrated research of the alveolar environment has been precluded due to the complex biophysical environment of the alveoli that has to be reconstructed. This includes a multiphase system with an air-liquid-interface, a co-culture model combining cell types with different mechanical properties, as well as the occurrence of cyclic mechanical stretch.

Methods: Biomimetic microfluidic devices have appeared to be a promising solution in this matter. These devices are mostly made of a transparent biocompatible polymer known as Polydimethylsiloxane (PDMS), and are fabricated using soft lithography. Cell growth and differentiation of primary lung epithelial cells on PDMS is currently under testing.

Results: At the moment, we are engineering a lung-on-a-chip prototype based on the publication by Huh (Huh, D. et al., 2010). The chip contains a porous PDMS membrane with a thickness of 10 µm, which separates the two central channels. On the apical side of the membrane, primary epithelial cells obtained from rat lung are cultured at the air-liquid-interface, while on the bottom side endothelial cells are perfused with media. The smaller side channels of the chip can be air-evacuated to stretch the membrane, and thereby approximately 10% cyclic strain can be applied to cells attached on the membrane.

Discussion: This lab-on-a-chip device will enable to explore distinct biomechanical response of different lung cells, under cyclic strain conditions combined with pathophysiological conditions. For that purpose, we will incorporate fluorescent microbeads into the porous membrane as subcellular force/strain sensor, which allows the evaluation of cell stiffness and viscosity by tracking and analyzing bead displacements (Das, T. et al., 2008). Alongside the sensory application, the lab-on-a-chip device can also be used for live cell imaging of surfactant release and adsorption at the air-liquid interface and it has the potential for drug delivery studies.