Respiratory Physiology & Neurobiology 155 (2007) 255–267 Effect of humidity on lung surfactant films subjected to dynamic compression/expansion cycles Edgar J. Acosta a,∗ , Roya Gitiafroz a , Yi Y. Zuo b , Zdenka Policova b , Peter N. Cox c , Michael L. Hair b , A. Wilhelm Neumann b a Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ont., M5S 3E5 Canada b Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ont., M5S 3E5 Canada c Department of Critical Care Medicine, The Hospital for Sick Children, 555 University Avenue, Toronto, Ont., M5S 3E5 Canada Accepted 26 June 2006 Abstract The surface activity of bovine lipid extracted surfactant (BLES) preparations used in surfactant replacement therapy is studied in dynamic film compression/expansion cycles as a function of relative humidity, surfactant concentration, compression rate, and compression periodicity. BLES droplets were formed in a constrained sessile droplet configuration (CSD). Images obtained during cycling were analyzed using axisymmetric drop shape analysis (ADSA) to yield surface tension, surface area, and drop volume data. The experiments were conducted in a chamber that allowed both humid (100% RH), and “dry” air (i.e. less than 20% RH) environments. It was observed that in humid environments BLES films are not stable and tend to have poor surface activity compared to BLES films exposed to dry air. Further analysis of the data reveal that if BLES films are compressed fast enough (i.e. at physiological conditions) to avoid film hydration, lower minimum surface tensions are achieved. A film hydration–relaxation mechanism is proposed to explain these observations. © 2006 Elsevier B.V. All rights reserved. Keywords: Dynamic cycling; Lung surfactant; Hysteresis; Compressibility; Humidity; Hydration 1. Introduction 1.1. Lung surfactants and the respiratory distress syndrome (RDS) The concept of “lung surfactants” was introduced by Pattle, who recognized that these surfactants are a complex mixture of phospholipids and proteins (Pattle and Thomas, 1961). Accord- ing to Pattle and earlier studies by Von Neergaard (1929) the surface tension (γ ) of the alveoli should be close to zero to avoid alveolar collapse due to the difference in Laplace pressure (P L ) among alveoli of different size. In general, it is considered that surface tensions near 1 mJ/m 2 or less are reached at the end of the expiration in healthy individuals (Hall et al., 1993; Sch ¨ urch et al., 1989). Respiratory distress syndrome (RDS) is a clinical condition defined by the onset of poor blood oxygenation due to lung injury, and lack or malfunction of lung surfactant (Petty, ∗ Corresponding author at. Tel.: +1 416 946 0742; fax: +1 416 978 8605. E-mail address: acosta@chem-eng.utoronto.ca (E.J. Acosta). 2003). RDS is typically classified into neonatal RDS (nRDS) and acute RDS (ARDS) (Petty, 2003; Notter, 2000; Zuo and Neumann, 2005a). With the introduction of lung surfactant replacement therapy, the rate of mortality of nRDS patients decreased from 50% to approximately 20% (Notter, 2000). Unfortunately, lung surfac- tant therapy has not been effective in ARDS patients (∼40%), where surfactant inhibition is associated with the pathology (Petty, 2003; Notter, 2000; Taeusch, 2000; Zasadzinski et al., 2001). Thus, the importance of pursuing a deeper understanding of how the surface activity is affected by the composition of the lung surfactant preparation, the presence of various inhibitors, and the role of the gas-phase (i.e. air humidity, carbon dioxide in air, air pollutants, etc.). Recent studies from our group have shown that the rate of formation of BLES (a lung surfactant preparation) films and the stability of these films under compression are affected by humid air (Zuo et al., 2005b, 2006). Here the effect of humidity on BLES preparations is further explored in dynamic cycling exper- iments that mimic physiological respiration parameters such as surface area reduction and respiration periodicity. 1569-9048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.resp.2006.06.009