Microscale creep and stress relaxation experiments with individual collagen fibrils

Nanoscale macromolecular biological structures exhibit time-dependent mechanical behavior, yet a quantitative understanding of their creep and stress relaxation behavior remains elusive, largely due to experimental challenges in attaining sufficient spatial and temporal resolution and control of stress or strain in conditions that guarantee their molecular integrity. To address this gap, an experimental methodology was developed to conduct creep and stress relaxation experiments with individual mammalian collagen fibrils. An image-based edge detection method, implemented with high magnification optical microscopy and combined with closed-loop proportional–integral–derivative (PID) control, was implemented and calibrated to apply constant force or stretch ratio to individual collagen fibrils via a Microelectromechanical Systems (MEMS) device. This experimental methodology allowed for real-time control of uniaxial tensile stress or strain with ~25 nm displacement accuracy. The overall experimental system was tuned to apply step inputs with rise times below 0.5 s, less than 5% overshoot, and steady-state error smaller than 1%. Three collagen fibrils with diameters in the range 101–121 nm were subjected to creep and stress relaxation tests in the range 4–20% engineering strain, under partially hydrated conditions. The collagen fibrils demonstrated non-linear viscoelastic behavior that was described well by the adaptive quasi-linear viscoelastic model. The results of this study demonstrated for the first time that mammalian collagen fibrils, the building blocks of connective tissues, exhibit nonlinear viscoelastic behavior in their partially hydrated state.