Time dependent stress relaxation and recovery in mechanically strained 3D microtissues.

Matthew Walker,Andrew E Pelling,Michel Godin,James L Harden

doi:10.1063/5.0002898

Matthew Walker, Andrew E Pelling + Show 2 more

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https://doi.org/10.1063/5.0002898

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Abstract

Characterizing the time-dependent mechanical properties of cells is not only necessary to determine how they deform but also to understand how external forces trigger biochemical-signaling cascades to govern their behavior. At present, mechanical properties are largely assessed by applying local shear or compressive forces on single cells grown in isolation on non-physiological 2D surfaces. In comparison, we developed the microfabricated vacuum actuated stretcher to measure tensile loading of 3D multicellular “microtissue” cultures. Using this approach, we here assessed the time-dependent stress relaxation and recovery responses of microtissues and quantified the spatial viscoelastic deformation following step length changes. Unlike previous results, stress relaxation and recovery in microtissues measured over a range of step amplitudes and pharmacological treatments followed an augmented stretched exponential behavior describing a broad distribution of inter-related timescales. Furthermore, despite the variety of experimental conditions, all responses led to a single linear relationship between the residual elastic stress and the degree of stress relaxation, suggesting that these mechanical properties are coupled through interactions between structural elements and the association of cells with their matrix. Finally, although stress relaxation could be quantitatively and spatially linked to recovery, they differed greatly in their dynamics; while stress recovery acted as a linear process, relaxation time constants changed with an inverse power law with the step size. This assessment of microtissues offers insights into how the collective behavior of cells in a 3D collagen matrix generates the dynamic mechanical properties of tissues, which is necessary to understand how cells deform and sense mechanical forces in vivo.

Highlights

The requirements of crawling, dividing, and contracting require the cell’s cytoskeletal network of structural and motor proteins to be tremendously dynamic
After three days of static culture in the Microtissue Vacuum Actuated Stretcher (MVAS) device, fibroblasts cells had compacted the collagen matrix to form dense 3D microtissues suspended between the cantilevers (Fig. 1)
The measured stress relaxation and recovery of microtissues followed generalized stretch exponential behavior, differing from the power law rheology seen in isolated cells

Summary

Introduction

The requirements of crawling, dividing, and contracting require the cell’s cytoskeletal network of structural and motor proteins to be tremendously dynamic. This behavior is unique for other soft materials and gives cells and tissues their distinct elastic and dissipative properties. Defining these properties is necessary for understanding how cells deform and how cells sense and transduce external mechanical forces into biochemical signals that direct their behavior in vivo. In that regard, when cells are stretched or come into contact with a stiffer matrix, there are time scale-dependent conformational changes to adhesion and cytoskeletal protein networks, which, in turn, alter ligand-receptor binding affinities to trigger biochemical signaling cascades.[1,2] Through this regulation of biochemical signaling, mechanical forces have been linked to normal development and function, as well as disease progression, including bone, muscle, heart, and lung disorders and cancer.[3,4]

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