The role of smoking on the mechanical properties of the human lung

Abstract

The mechanical functions of the lungs are concerned with ventilating the alveoli and aiding venous return to the heart. The mechanical properties of the lung allow this to be fulfilled with a very small expenditure of energy. Smoking via chronic obstructive pulmonary disease (COPD) can affect this mechanical function through the alteration of the mechanical properties of the lung tissue. So far, no study has been conducted to experimentally compare the in vitro mechanical properties of the human lung tissue among the healthy nonsmokers and unhealthy smokers. Therefore, there is a paucity of knowledge on how the macro-mechanical properties of the lung tissue as a consequence of at least 7 years of smoking can alter. This study was, hence, aimed at performing a comparative study to compare the linear elastic and nonlinear hyperelastic mechanical properties of the healthy nonsmokers and unhealthy smokers’ lung using uniaxial tensile testing under two different loading directions, i.e., the axial and transversal. To do that, the COppm (Carbon Monoxide part per million) and %COHb (blood Carboxyhemoglobin) of 18 cadaveric individuals, including 9 nonsmokers and 9 smokers were measured. The COppm and %COHb were found to be 26 $\pm$ 1.58 (Mean $\pm$ SD) and 4 $\pm$ 1.16 for the smokers and 4.79 $\pm$ 0.25 and 1.27 $\pm$ 0.16 for the nonsmokers, respectively. The lung tissues were excised from the cadavers and mounted on the uniaxial tensile test machine under the loading rate of 5 mm/min. The stress-strain data of the tissues revealed the axial elastic modulus of 58 $\pm$ 4.21 and 142 $\pm$ 8.84 kPa for the nonsmokers and smokers’ lungs, respectively. Similarly, the transversal elastic modulus of 53 $\pm$ 7.16 and 127 $\pm$ 11.15 kPa were found for the nonsmokers and smokers’ lungs, respectively. The results revealed a significant difference between the axial and transversal mechanical properties of the nonsmokers and smokers’ lung tissues ( $p<$ 0.05, post hoc Scheffe method). The hyperelastic material coefficients of the lung tissues were also calculated and reported. These findings have implications not only for understanding the role of smoking on the mechanical properties of the lung tissue but also to give rise to novel therapeutic strategies for the management of the disease and prevention of smoking addiction.