Determining the physiological optimal surface viscosity

本研究的主要目标是首次使用我们实验室独有的活性微流变学技术确定肺表面活性剂的生理最佳表面粘度。我们假设在一种有效的肺表面活性剂中存在一个最佳的表面粘度，它既能快速吸附到空气-水界面，又能提供超低的表面张力。我们的目标是确定如何通过控制合成替代肺表面活性剂的胆固醇含量来达到最佳效果。与宏观流变仪相比，我们的微流变学技术的灵敏度提高了三个数量级，可以精确监测胆固醇存在时肺表面活性剂膜分子组织的变化，从而精确测量表面活性剂膜的表面粘度。最终，确定最佳胆固醇浓度将有助于更好地设计合成表面活性剂来治疗新生儿呼吸窘迫综合征（NRDS），并可能深入了解成人呼吸窘迫综合征（ARDS）中表面活性剂失活的原因。

The primary goal of this research is to determine, for the first time, the physiologically optimal surface viscosity of the lung surfactant using an active microrheology technique unique to our lab. We hypothesize that there exists an optimal surface viscosity in an effective lung surfactant that provides both rapid adsorption to the air-water interface and ultra-low surface tensions. Our goal is to determine how best to achieve this optimum by controlling the cholesterol fraction of a synthetic replacement lung surfactant. Three orders of magnitude increased sensitivity of our microrheology technique as compared to macroscopic rheometers allows precise monitoring of changes in the molecular organization of the lung surfactant film in the presence of cholesterol, enabling accurate measurements of surface viscosity of surfactant films. Ultimately, determining the optimal cholesterol concentration will enable a better design of synthetic surfactants to treat Neonatal Respiratory Distress Syndrome (NRDS) and may give insights into the causes of surfactant inactivation in Adult Respiratory Distress Syndrome (ARDS). We hypothesize that small fractions (1-5 wt. %) of cholesterol reduce the crystalline ordering of saturated lipids in lung surfactant monolayers, leading to a reduction in the shear viscosity, which enhances the surfactant's ability to flow and cover the alveolar interface. We also hypothesize that excess cholesterol ( >10 wt %) decreases the effectiveness of lung surfactants in ARDS by increasing the minimum surface tension of the interfacial film. This inability to reach ultra-low surface tensions is hypothesized to be a consequence of significantly reduced interfacial energy of the film (line tension). Low interfacial film energy can influence the mechanical cohesion in the surfactant film and lead to the failure of the film on compression, which ultimately causes the film to become unstable at lower surface tensions. Furthermore, lipid(cholesterol)- protein interactions can also alter these mechanical and structural properties by changing their molecular organization at the interface. By determining the mechanical properties of both model and clinically relevant surfactant film in the presence of physiological and elevated amounts of cholesterol, we can understand how increased cholesterol might lead to surfactant inactivation in ARDS and determine better replacement surfactants for treatment. The mechanical properties thus determined by the active microrheology technique will be correlated with isotherms, fluorescence microscopy, and grazing incidence synchrotron X-ray diffraction to determine how cholesterol alters the molecular packing of lung surfactant lipids, which determines the mechanical properties of monolayers.