Pulmonary emphysema, a form of Chronic Obstructive Pulmonary Disease (COPD), is characterized by irreversible enlargement of lung airspaces, accompanied by the destruction of alveolar walls, and loss of elasticity, all of which contribute to alterations in respiratory mechanics and a drastic impairment in pulmonary function [1], [2], [3]. Despite being recognized as a preventable and treatable condition by the Global Initiative for Chronic Obstructive Lung Disease [4], COPD remains a major public health problem worldwide, ranking as the third leading cause of death in 2019 with 3.23 million deaths and a prevalence of 10% [5]. One explanation for its high prevalence is the lack of diagnostic methods that enable early detection and treatment [6], [7], which motivates the study of the remodeling process and microstructural changes occurring in the lung tissue during pathogenesis [8].

Animal experimentation has been a critical resource in investigating pulmonary emphysema, as it offers a controllable setting for pathophysiological studies. Among current small-animal models available, elastase-induced emphysema is the most popular as it mimics the protease and anti-protease activity imbalance observed in emphysematous lungs [9], [10]. Further, by varying the dose and time after instillation, elastase-induced emphysema displays a marked dose-response relationship in terms of alveolar structure, extracellular matrix composition, and lung function [11], [12], [13]. This dose dependence offers to control the severity of emphysema in terms of alveolar airspace enlargement, potentially allowing the study of different stages of the disease. From a structural and functional perspective, elastase-induced emphysema in small animals has been crucial in evaluating alterations in alveolar morphology and respiratory mechanics [14], [15], [16]. In effect, histology studies that assess alveolar enlargement measured in terms of mean linear intercept (LM) have associated an increased respiratory-system compliance to increased LM values in the lungs of elastase-treated rats after three weeks of development time [12]. Despite these results, it is important to remark that pathological structural changes in alveolar tissue may not always alter lung mechanics, as respiratory system compliance in elastase-treated rats does not experience significant changes before three weeks of development when compared to normal lungs [17], [18].

Although extensive research has been conducted on the lung mechanics of emphysematous lungs, the investigation of tissue properties in lungs from emphysema animal models has received far less attention [8]. One indirect approach has been the mechanical characterization of normal lung tissue samples that are treated ex vivo with elastase baths [19], [20]. By comparing the mechanical behavior of lung strips under uniaxial tests, studies have shown significant differences between normal and elastase-treated tissues in terms of stress-strain curves. While these studies confirm a softer mechanical response in elastase-treated tissue samples, the ex vivo approach is limited in its ability to capture the actual tissue properties in emphysematous lungs, which are controlled by a progressive elastin degradation and structural remodeling with regional variations within the lungs [8], [21], [22].

In this work, our objective is to characterize the mechanical response and alveolar morphology of lung tissue in an elastase-induced multiple-dose emphysema animal model. We further analyze how tissue and whole-organ properties vary when using different elastase doses, and their possible associations. Our contribution aims to describe the relationship between alveolar microstructure and tissue properties for varying degrees of disease severity, contributing to the discovery of sensitive biomechanical markers that can be useful in the detection of disease onset and progression.