Long-term cycling of surfactant films in Wilhelmy balance

Surface properties of porcine surfactant were observed during long-term cycling in Wilhelmy balance. Various amounts of surfactant were applied onto the surface as dry particles or droplets, or were suspended in the hypophase, and the films generated by this material were subjected to 50% cyclic compression at a rate of 1 cycle per min. Film spreading was faster from a droplet than from a particle of lyophilized surfactant, but the "stable period" during which minimum surface tension of the compressed film remained below 5 mN/m was significantly longer for the dry material. For surfactant suspensions the period of film "refinement", defined as the number of cycles required to reduce minimum surface tension to a level below 5 mN/m, was inversely correlated with the concentration of surfactant in the hypophase. Thirteen batches of porcine surfactant, used successfully in clinical trials for treatment of neonatal respiratory distress syndrome, were evaluated in the same system suspended in the hypophase at a concentration of 0.5 mg/ml. Films adsorbed from these batches had a median refinement period of 4 cycles, and a median stable period of 2160 cycles (36 h). In the same assay system, surfactant activity was inhibited in a dose-dependent manner by serum, fibrinogen, meconium, and bilirubin, but the specific inhibitory activity was significantly higher for bilirubin than for the other substances tested.