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Mechanical properties are critical in regulating pathophysiological cell behavior via mechano-transduction. Although biological tissues are generally regarded as viscoelastic, mechanobiology studies mainly focused on characterizing tissue elasticity and investigating cell behavior as a function of substrate stiffness. Moreover, mechanical properties are often derived using bulk testing techniques, likely being poorly representative of the local biomechanical environment felt by cells. Aiming at characterizing micro-mechanical viscoelastic properties at typical cell length-scales in physiological-like conditions, we present here a new testing approach to perform dynamic nano-indentation measurements in liquid (e.g. PBS, culture medium) at controlled temperature (e.g. 37 °C).
Our method involves a customized version of Optics11’s PIUMA Nanoindenter, which is based on a unique ferrule-top opto-mechanical cantilever force transducer operated by a z-axis piezoelectric motor, and which has been modified to enable user-defined load- and displacement-controlled measurements (e.g. creep, stress-relaxation, DMA and constant strain rate tests). Moreover, a temperature (T) sensor has been integrated with that of the PIUMA sample stage to control the actual sample T via a new master-slave control loop. In this study, we characterized the viscoelastic properties of PDMS samples, gelatin hydrogels at different temperatures and concentrations, single smooth muscle cells, and healthy and infarcted myocardial tissue samples. Experimental data within the linear viscoelastic region were fitted to Generalised Maxwell models, deriving instantaneous and equilibrium elastic moduli, and characteristic relaxation times.
This method could be beneficial for better investigating soft tissues/(bio)materials mechanics and for designing new mechano-mimetic substrates for tissue engineering, disease modelling and cell mechanobiology studies.
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