Fig. 1.

Comparison of typical compression isotherms of various synthetic, modified-natural, and natural surfactant films at room temperature. The two protein-free synthetic systems are pure DPPC and DPPC:POPG (7:3). The four clinical modified-natural surfactants are Survanta, Curosurf, Infasurf, and BLES. The native natural surfactant for comparison is bovine natural surfactant (BNS). All pulmonary surfactants were spread as monolayers to an initial surface pressure (π) of 1–3 mN/m prior to compression. Surfactant films were compressed at an identical rate of 20 cm²/min until film collapse. Four pressure-dependent regions are detected for the compression isotherms of protein-containing modified and natural surfactants. These are: Region I. Monolayer region at π≤40 mN/m; Region II. Monolayer-to-multilayer transition region at 40<π<50 mN/m; Region III. Multilayer region at π≥50 mN/m; and Region IV. Collapse region at 72 mN/m for all films but Survanta, which collapses at 62 mN/m. The structural nature of these four regions is revealed in Fig. 2 by AFM.