Surface tension of pulmonary surfactant systems.

Description

Pulmonary surfactant facilitates respiration by reducing the surface tension in the alveoli. To study the surface activity of pulmonary surfactant films under physiologically relevant conditions, several experimental methods were developed to measure surfactant film interfacial tension with Axisymmetric Drop Shape Analysis (ADSA).Using a pendant drop method, the film formation of a therapeutic pulmonary surfactant, Bovine Lipid Extract Surfactant (BLES), was studied as a function of surfactant bulk concentration. It was found that the rate of surfactant film formation is strongly dependent on its bulk concentration. The addition of dextran, poly (ethylene glycol) (PEG) and poly (vinyl pyrrolidone) (PVP) in low concentration BLES significantly increases its rate of film formation. The effect of these polymeric additives is dependent on both polymer concentration and molecular weight, and can be explained by the depletion-attraction mechanism. For each polymer of a given molecular weight, there is an optimum concentration at which the effect of the polymer is the most significant.Inhibition of BLES by bovine serum albumin was investigated. In the presence of albumin, the surface activity of BLES is significantly hindered. Albumin competes with BLES during film formation, causing high film surface tension and reduced film stability. By promoting surfactant adsorption, the polymeric additives prevent surfactant inhibition by albumin.Using a captive bubble method, a surfactant film stability study was performed as functions of compression ratio, compression rate and surfactant bulk concentration. When the compression ratio is relatively low (i.e. <20%), the effect of compression rate and bulk concentration is insignificant. In particular, the surfactant film produces a reversible surface tension response during film compression and expansion. Film failure and collapse occur when the compression ratio becomes higher, resulting in surface tension hysteresis. In this case, a high compression rate and/or low surfactant concentration further reduces surfactant film stability. By enhancing surfactant readsorption, the polymeric additives studied in the film formation experiments also improve surfactant film stability.The findings on the effect of polymeric additives may pave the way for cheaper and more efficient pulmonary surfactant preparations used in surfactant replacement therapy.

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