Long-term Cycling of Surfactant Films in Wilhelmy Balance

Surface properties of porcine surfactant were observed during long-term cycling in Wilhelmy bal- ance. Various amounts of surfactant were applied onto the surface as dry particles or droplets, or were sus- pended 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 sur- factant, but the "stable period" during which minimum surface tension of the compressed film remained below 5 mNim was significantly longer for the dry material. For surfactant suspensions the period of film "refine- ment", 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 sur- factant, used successfully in clinical trials for treatment of neonatal respiratory distress syndrome, were evalu- ated in the same system suspended in the hypophase at a concentration of 0.5 mgiml. 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, meco- nium, and bilirubin, but the specific inhibitory activity was significantly higher for bilirubin than for the other substances tested.