Pulmonary surfactant is a complex lipoprotein mixture secreted into the alveolar lumen by type 2 pneumocytes, which is composed by tens of different lipids (approximately 90% of its entire mass) and surfactant proteins (approximately 10% of the mass). It is crucially involved in maintaining lung homeostasis by reducing the values of alveolar liquid surface tension close to zero at end-expiration, thereby avoiding the alveolar collapse, and assembling a chemical and physical barrier against inhaled pathogens. A deficient amount of surfactant or its functional inactivation is directly linked to a wide range of lung pathologies, including the neonatal respiratory distress syndrome. This paper reviews the main biophysical concepts of surfactant activity and its inactivation mechanisms, and describes the past, present and future roles of surfactant replacement therapy, focusing on the exogenous surfactant preparations marketed worldwide and new formulations under development. The closing section describes the pulmonary surfactant in the context of drug delivery. Thanks to its peculiar composition, biocompatibility, and alveolar spreading capability, the surfactant may work not only as a shuttle to the branched anatomy of the lung for other drugs but also as a modulator for their release, leading to innovative therapeutic avenues for the treatment of several respiratory diseases.

中文翻译：

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肺表面活性剂：具有救生治疗应用的独特生物材料。

肺表面活性剂是由2型肺细胞分泌到肺泡腔内的复杂脂蛋白混合物，由数十种不同的脂质（约占其总质量的90%）和表面活性蛋白（约占总质量的10%）组成。它通过在呼气末将肺泡液体表面张力的值降低到接近于零，从而避免肺泡塌陷，并为吸入的病原体建立化学和物理屏障，从而对维持肺稳态至关重要。表面活性剂含量不足或其功能失活与多种肺部疾病直接相关，包括新生儿呼吸窘迫综合征。本文回顾了表面活性剂活性及其失活机制的主要生物物理概念，并描述了过去，表面活性剂替代疗法的当前和未来作用，重点关注全球销售的外源性表面活性剂制剂和正在开发的新制剂。结尾部分描述了药物递送背景下的肺表面活性剂。由于其独特的成分、生物相容性和肺泡扩散能力，表面活性剂不仅可以作为其他药物进入肺部分支解剖结构的穿梭机，还可以作为药物释放的调节剂，从而为治疗几种呼吸道疾病。