Viscoelastic models and RHEOS
Power-law relaxation is common in soft and biological materials, but classical models capture it poorly. Fractional calculus describes a broad relaxation spectrum with few parameters. Our review and tutorial, rooted in work on model tissues, explains the approach (Bonfanti et al., Soft Matter 2020); it is now used from cell and tissue mechanics to geomechanics. We developed the framework with Alessandra Bonfanti and Louis Kaplan, and implemented it in the open-source Julia package RHEOS.jl (Kaplan, Bonfanti and Kabla, JOSS 2019). The package underpins our analyses of tissues (Bonfanti et al., Royal Society Open Science 2020; Duque et al., Nature Materials 2024; Tajvidi Safa et al., Communications Biology 2025), and later work extends the models to active materials (Tajvidi Safa et al., Royal Society Open Science 2024).