Date Published: October 17, 2018
Publisher: Public Library of Science
Author(s): Samuel R. Polio, Aritra Nath Kundu, Carey E. Dougan, Nathan P. Birch, D. Ezra Aurian-Blajeni, Jessica D. Schiffman, Alfred J. Crosby, Shelly R. Peyton, Etienne Dague.
Published data on the mechanical strength and elasticity of lung tissue is widely variable, primarily due to differences in how testing was conducted across individual studies. This makes it extremely difficult to find a benchmark modulus of lung tissue when designing synthetic extracellular matrices (ECMs). To address this issue, we tested tissues from various areas of the lung using multiple characterization techniques, including micro-indentation, small amplitude oscillatory shear (SAOS), uniaxial tension, and cavitation rheology. We report the sample preparation required and data obtainable across these unique but complimentary methods to quantify the modulus of lung tissue. We highlight cavitation rheology as a new method, which can measure the modulus of intact tissue with precise spatial control, and reports a modulus on the length scale of typical tissue heterogeneities. Shear rheology, uniaxial, and indentation testing require heavy sample manipulation and destruction; however, cavitation rheology can be performed in situ across nearly all areas of the lung with minimal preparation. The Young’s modulus of bulk lung tissue using micro-indentation (1.4±0.4 kPa), SAOS (3.3±0.5 kPa), uniaxial testing (3.4±0.4 kPa), and cavitation rheology (6.1±1.6 kPa) were within the same order of magnitude, with higher values consistently reported from cavitation, likely due to our ability to keep the tissue intact. Although cavitation rheology does not capture the non-linear strains revealed by uniaxial testing and SAOS, it provides an opportunity to measure mechanical characteristics of lung tissue on a microscale level on intact tissues. Overall, our study demonstrates that each technique has independent benefits, and each technique revealed unique mechanical features of lung tissue that can contribute to a deeper understanding of lung tissue mechanics.
Lung tissue is highly elastic and mechanically robust over hundreds of millions of respiratory cycles. In order to properly ventilate the alveoli to facilitate gas exchange, it must maintain a delicate balance between strength and compliance to allow for these repeated, massive expansions. Lung parenchyma is the area of the lung that is involved with gas exchange, including the alveoli and smaller bronchioles, but excludes the large, cartilaginous bronchi. Lung parenchyma derives its mechanical integrity from the ECM, which is primarily composed of elastin, laminin, and collagen [1–4]. Others have shown that these structural proteins contribute to the mechanical properties of tissues , .
The mechanical properties of the lungs, such as compliance, are integral to this organ’s biological functions. However, the lung is a structurally and mechanically heterogeneous tissue, with regions of high and low stiffness and porosity, which are difficult to comprehensively capture. Significant research has been performed to understand lung mechanics to fully grasp this complexity and in an attempt to correlate lung structure and compliance with organ-, tissue-, and cell-scale behaviors. One challenge in achieving this goal is that studies have used a variety of different techniques quantified the compliance of lung tissue: from indentation on small, excised samples to elastography on of whole lungs in situ. Therefore, one of our goals was to take the first step in unifying a variety of different methods, we directly compared various testing modalities to determine how the reported modulus of lung tissue varies across them given the inherent constraints for each method: differences in sample preparation, temperature, and region of tissue tested. This effort will enable the field to compare measured and reported values, using different methods to quantify a tissue modulus, and will help to create a more accurate hydrogel model for simulating in vivo-like compliance of the lung. In doing this comparative study, our most significant findings are that the modulus of lung tissue does vary depending on the testing modality used (though each reported value is within the same order of magnitude), the order of testing does not significantly affect the reported modulus, freezing tissue samples has a modest, though detectable effect on the reported modulus, and cavitation rheology is the only method capable of reporting a localized in situ modulus.