A new load-controlled testing method for viscoelastic characterisation through stress-rate measurements

Viscoelastic or time-dependent behaviour is typical of several materials, including biological tissues. Thus, several testing methods have been developed to characterise material viscoelastic properties. In this study, we propose a new load-controlled testing approach, called the sigma-dot method (σ˙M), based on measurements at different constant stress-rates. The method can be implemented with either standard load-controlled testing devices or bioreactors with load-controlled mechano-actuation to perform in-situ viscoelastic characterisation during cell culture. Both polydimethylsiloxane and hydroxyapatite/gelatin composites were tested using the σ˙M, by performing measurements with the MechanoCulture TR- a load-controlled bioreactor - and a universal testing machine as control. The results show that the proposed method is effective for deriving sample viscoelastic parameters, independent of the testing device. The main advantage of the σ˙M with respect to classical step-response viscoelastic tests, such as creep and stress-relaxation, is its ramp stress input which is physically implementable and does not need any a priori determination of the sample linear viscoelastic region.