Measuring the viscoelastic properties of lungs through complementary techniques
In the development of new concrete materials that are more durable and environmentally friendly, civil engineers and scientists must determine its strength. One common technique that they perform is known as compressive testing. A cylindrical specimen made of fresh concrete is compressed at various ages between two plates. The strength of the concrete is the major factor that determines its quality, it is the basis of acceptance or rejection in construction. Insufficient strength can lead to costly, dangerous, and challenging repairs, or even failure of the construction. As concrete blocks and bricks are the structural elements of houses and buildings, tissues and organs are the structural and functional units of the human body. Therefore, determining their strength is also important to the development of biomaterials that are investigated to repair or replace them. To develop a better or a new medical implant, researchers need to know what they should target in terms of mechanical behavior.
There is not a standard mechanical testing method for soft tissues and organs. Most researchers customize the available mechanical testers and this is a major source of the wide results variation present in literature. This variation makes the understanding of the mechanobiological relationships challenging and impairs the development of biomaterials and bioengineered tissues. To assess how the Young modulus (common reported mechanical property) can vary among different techniques, a group of researchers from University of Massachusetts led by professor Shelly Peyton tested lung tissue using multiple characterization techniques such as micro-indentation, small amplitude oscillatory shear (SAOS), uniaxial tension, and cavitation rheology. In this study entitled “Cross-platform mechanical characterization of lung tissue”, the authors reported the average Young modulus obtained from various areas of the lung with each technique and their particularities .
Lung tissue has a delicate balance between strength and compliance which is essential for its repeated and massive expansion over the respiratory cycles. With the aim to detect and reduce the variations reported in the literature for this tissue, the first point evaluated by the authors was whether sample freezing (common preservation method) prior to the test can affect the final mechanical properties of the tissue. Fresh samples (not frozen) had a small but detectable higher value of Young’s modulus compared to samples frozen by different methods (liquid nitrogen, – 80 °C, and optimal cutting temperature medium). On the other hand, the effect of temperature (from 25 °C to 37 °C) had no significant effect on the samples’ moduli.
The authors then reported the average Young modulus of fresh lung specimens (4-20 specimens per lung in a total of 12 lungs) obtained using a novel technique (cavitation rheology) and three common methods found in literature (micro-indentation, SAOS and uniaxial tension). Briefly, cavitation rheology was performed using a custom-built instrument composed of a syringe pump, pressure sensors and a syringe needle responsible for injecting air inside the lung until a drop in pressure was attained (cavitation pressure). SAOS was performed using a standard rheometer with a flat plate geometry. Micro-indentation was done using a custom indenter with a flat cylindrical probe made from tool hardened steel and a deformable cantilever. Finally, uniaxial testing was carried out with a standard tensile tester (Fig. 1).
The authors observed that the modulus did vary depending on the testing method (even though they are within the same order of magnitude) and this is due to their inherent constraints (such as differences in sample preparation, sample geometry and region of the tissue that it had been taken, strain rate, temperature, and data processing). Cavitation rheology reported the stiffest moduli (6.1 ± 1.6 kPa), while SAOS led to the most compliant moduli (1.4 ± 0.4 kPa). The higher modulus obtained with the cavitation method was suggested to be the result of the fiber orientation during extension of the organ.
The group highlighted that micro-indentation, SAOS, and uniaxial generate a modulus for a small portion of the organ (small specimen excised from the organ) while in the cavitation method, almost the whole organ was used to obtain the modulus. Therefore, the authors believe that the last method generates a modulus that is more representative of the lung microstructure. Indeed, the heterogeneity of the lung tissue was seen with the higher range of measurements on individual samples using the cavitation rheology.
Overall, this study demonstrated the importance of recognizing that the little mechanical properties of soft organs reported in literature have inherent variations according to the testing method and customized protocol applied. A common method of characterization for soft organs is desirable to effectively facilitate the understanding of soft tissue mechanics and the development of biomaterials designed to repair or replace them.
 Polio, S. R., Kundu, A. N., Dougan, C. E., Birch, N. P., Aurian-Blajeni, D. E., Schiffman, J. D., … & Peyton, S. R. (2018). Cross-platform mechanical characterization of lung tissue. PloS one, 13(10), e0204765.