171 Current possibilities and future opportunities with three-dimensional lung ECM-derived hydrogels of porcine-sourced lung hydrogels were reported using parallel plate rheometry [50]. Other studies have utilized this method on alginate-porcine ECM [51], poly(ethylene glycol)(PEG)-murine ECM [52], and PEG-porcine ECM [53] hybrid hydrogels. LowLoad compression testing (LLCT) is another compression-based method [54] that has been used with lung ECM-derived hydrogels. Stiffness and viscoelastic stress relaxation capacity of human non-disease control, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) lung ECM-derived hydrogels have been reported; moreover, the mechanical properties of the hydrogels derived from these diseased lungs resembled such properties of the native tissues from which the ECMs were sourced [2]. Similarly, LLCT-measured stiffness and stress relaxation parameters of both native and chemically crosslinked porcine lung ECM-derived hydrogels were also reported [55, 56]. Lastly, atomic force microscopy (AFM), which is a more micro-level mechanical measurement based on indentation, was recently used to characterize Young’s modulus values of porcine lung ECM hydrogels [50]. Measuring the mechanical properties is not only useful for diseased environment characterization, but also for verification of the success of methodologies designed to alter such properties. While it is clear that the use of different concentrations of the starting ECM material (powder) [39] and adjusting the pepsin digestion duration (the essential step in generation of a pre-gel ECM-derived substrate) [57] influences the mechanical properties, one of the initial attempts to specifically modulate the mechanical properties of lung ECM-derived hydrogels was treating the porcine lung ECM with genipin to increase the stiffness [58]. This approach has been extended with thiol-functionalization [52, 53], alginate-reinforcing [51] or fibre crosslinking [56] to allow greater control over mechanical parameters in the lung ECM-derived hydrogels. The concepts of altering the mechanical properties, measuring and reporting these changes triggered in the lung ECM-derived hydrogels have been evolving as more novel tools are developed (Figure 1). However, mechanical characterization of lung ECM-derived hydrogels is far from completed. As of today, tensile testing or fatigue testing on such hydrogels have yet to be performed, although using polyacrylamide-ECM hybrid hydrogels these properties were characterized in an early study [59]. A thorough mechanical and cross-platform characterization of lung ECM-derived hydrogels has not been reported yet. Providing (the comparison of) such characterizations would help the field regarding the interpretation and comparison of different studies using different methods to measure similar parameters. As the field is new, establishing different methods and discussing their advantages and limitations will be important for being able to understand the 7
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