Development of human respiratory airway models: A review

Pulmonary drug delivery has gained great interest as an important subject of research over the past decades given the lung diseases which are affecting millions of people suffer from these diseases. Drug delivery into the respiratory system is influenced by many anatomical and physiological factors such as lung morphometry, breathing patterns, fluid dynamics, particle properties, etc. The respiratory airway structure is one of these parameters which greatly influences the deposition pattern of inhaled drug particles. There have been a wide variety of major morphometric studies, conducted using cadavers to increase an understanding of the respiratory airway anatomy and provide important information for developing realistic airway models. Casting as one of the first methods, was utilized for morphometric studies providing a hollow model for in vitro investigations. The above-mentioned morphometric data were utilized to describe the first idealized airway model as a simple symmetric description of the branching airways, later followed by more realistic asymmetric models. However, even these asymmetric airway models were not good enough to reflect the anatomical complexities of the human respiratory airway and contained several major limitations which made them inefficient. Further attempts alongside with the progress of technology led to introduction of the stochastic and image-based models which provided more realistic and efficient tools for numerical and experimental investigations. The main objective of this study is to provide a comprehensive review about the development of different perspectives of the respiratory airway modeling over the past decades. The following sections will present useful information about anatomy of the human respiratory tract, and different viewpoints of the respiratory airway modeling, including their historical routes, strengths, and deficiencies.