Towards a standard method for mechanically characterizing soft organs
The month of October was focused on the mechanical characterization of soft organs. The characterization of healthy and diseased tissues is important in the field of tissue engineering and regenerative medicine to establish reference values of important mechanical properties (such as elastic/shear modulus, ultimate strength, ultimate strain) for the development of lab-grown human tissues or alternative biomaterials and therapies. Furthermore, the ex vivo mechanical characterization of treated tissues can give complementary information to standard imaging techniques in the course of in vivo studies of new drugs and technologies.
The mechanical characterization of soft tissues, however, is not straightforward. The measurement of their viscoelastic properties on standard mechanical testers, when possible, is often laborious and requires important customization steps (see our previous publication for specific examples). This leads to significant variation in the final method applied and in the properties reported in the literature. In order to illustrate the discrepancy one can find in the literature, the illustration below gathers measured elastic or shear modulus data of porcine lung tissues reported in published studies.
As shown in the image, lung tissues have been tested with ultrasound, micro-indentation, cavitation rheology, SAOS rheometry, uniaxial tension, magnetic resonance elastography, and viscoelastic testing of bilayered materials (VeTBiM). These many methods were found in just six studies so this list is still not complete. The range of values reported for the elastic and shear modulus of lung tissues can vary by a factor of 4. Furthermore, elastic modulus should be significantly higher than shear modulus (for example, the Young modulus equals three times the shear modulus) which is not really observed in these studies. It is possible to see that comparison among literature is challenging due to the inherent variations of the applied methods.
The lack of a standard method to measure the viscoelastic properties of soft organs impairs the development of the field. The quest for a robust method that can provide consistent data is therefore extremely valuable. Researchers need to know what they should target in terms of mechanical properties during the development of bioengineered tissues, novel biomaterials and therapies. For example, a heart valve implant produced with hard plastic will not perform as a soft and flexible natural valve. If this difference in viscoelastic properties is converted into reliable and standardized numbers, the arrival of these novel technologies into the market will definitely be facilitated.
 Cui, J., Lee, C. H., Delbos, A., McManus, J. J., & Crosby, A. J. (2011). Cavitation rheology of the eye lens. Soft Matter, 7(17), 7827-7831.
 Jansen, L. E., Birch, N. P., Schiffman, J. D., Crosby, A. J., & Peyton, S. R. (2015). Mechanics of intact bone marrow. Journal of the mechanical behavior of biomedical materials, 50, 299-307.
 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.
 Mariappan, Y. K., Kolipaka, A., Manduca, A., Hubmayr, R. D., Ehman, R. L., Araoz, P., & McGee, K. P. (2012). Magnetic resonance elastography of the lung parenchyma in an in situ porcine model with a noninvasive mechanical driver: Correlation of shear stiffness with trans‐respiratory system pressures. Magnetic resonance in medicine, 67(1), 210-217.
 Hutchens, S. B., & Crosby, A. J. (2014). Soft-solid deformation mechanics at the tip of an embedded needle. Soft Matter, 10(20), 3679-3684.
 Rheolution Inc. (2021). Viscoelastic properties of porcine and sheep soft organs.