Alveolar Microfluidic Systems for Study of Barrier Function, Cell Damage, and Migration at the Air-Blood Barrier.

Abstract

The exchange of oxygen and carbon dioxide occurs across the air-blood barrier (or alveolar-capillary barrier). This barrier must be sufficiently thin to allow the passive diffusion, yet sufficiently strong to maintain a dry alveolar environment. When solid and fluid mechanical stresses damage the air-blood barrier’s integrity, edema fills this normally air-filled alveolar environment and pathology results. The specific mechanisms by which these stresses impact the cells of the air-blood barrier remain poorly understood. The role of solid mechanical stress (cyclic stretch) has been explored through traditional, culture techniques, but only recently have microfluidic systems allowed systematic exploration on combined solid and fluid stresses. Although such systems can be tailored to the biological phenomena being studied, key design parameters include: (i) two-layered channel design (to mimic “alveolar” and “endothelial” compartments), (ii) ability to convey combined solid and fluid stresses, (iii) co-culture, and (iv) the integration of biological sensors to detect real-time changes.

A microfluidic “Alveoli-on-a-Chip” system was designed and fabricated. By varying the degree of fluid-filling within the “alveolar” channel, differential strain conditions were applied to alveolar epithelial cells. Experiments using this system, demonstrated significant increases in cell death and detachment in alveolar cell populations exposed to fluid and solid mechanical stresses compared to populations exposed solely to solid mechanical stresses.

Because nearly all pathological processes of alveoli alter barrier permeability, detection of changes to the integrity of this barrier is an essential feature in alveolar models. A technique for embedding Ag/AgCl recording electrodes within a two-layered PDMS microsystem, allowing impedance to be measured across a porous cell culture membrane was also developed. This fabrication technique eliminated the need for direct deposition of recording electrodes onto the elastomer, avoiding the frequent and deep cracking pattern resulting from the modulus mismatch between conductive metals and PDMS polymer.

The impact of mechanical stresses on the alveolar immune response was also studied by patterning alveolar macrophages onto confluent monolayers of alveolar epithelial cells using aqueous two-phase (ATPS) printing. Using this technique, increased migration rates in co-cultures experiencing physiologic stretch levels were demonstrated compared to migration in static cultures.