Effects of extracellular matrix viscoelasticity on cellular behaviour

Abstract

Substantial research over the past two decades has established that extracellular matrix (ECM) elasticity, or stiffness, affects fundamental cellular processes, including spreading, growth, proliferation, migration, differentiation and organoid formation. Linearly elastic polyacrylamide hydrogels and polydimethylsiloxane (PDMS) elastomers coated with ECM proteins are widely used to assess the role of stiffness, and results from such experiments are often assumed to reproduce the effect of the mechanical environment experienced by cells in vivo. However, tissues and ECMs are not linearly elastic materials-they exhibit far more complex mechanical behaviours, including viscoelasticity (a time-dependent response to loading or deformation), as well as mechanical plasticity and nonlinear elasticity. Here we review the complex mechanical behaviours of tissues and ECMs, discuss the effect of ECM viscoelasticity on cells, and describe the potential use of viscoelastic biomaterials in regenerative medicine. Recent work has revealed that matrix viscoelasticity regulates these same fundamental cell processes, and can promote behaviours that are not observed with elastic hydrogels in both two- and three-dimensional culture microenvironments. These findings have provided insights into cell-matrix interactions and how these interactions differentially modulate mechano-sensitive molecular pathways in cells. Moreover, these results suggest design guidelines for the next generation of biomaterials, with the goal of matching tissue and ECM mechanics for in vitro tissue models and applications in regenerative medicine.

Figure 1|. Mechanical interactions between cells and extracellular matrices.

Cells interact with ECMs mechanically, including by pulling, often through actomyosin-based contractility coupled to the ECM through integrin-based adhesions, and pushing, often through actin polymerization and microtubules. The mechanical properties of ECMs mediate these interactions resulting in cell mechanotransduction and impacting cell behaviors.

Figure 2|. Biological tissues and extracellular matrices are viscoelastic and exhibit stress relaxation in response to a deformation.

a, Plot of loss modulus at ~1 Hz, a measure of viscosity (or dissipation), versus storage modulus at ~1 Hz, a measure of elasticity, for skeletal tissues, soft tissues, and reconstituted ECMs (rECMs). Grey dotted line indicates a loss modulus that is 10% of storage modulus. Data was taken from a set of randomly selected publications^,,,,–. Shear storage and loss moduli were converted to storage and loss moduli by assuming a Poisson ratio of 0.5, and thus multiplying by a factor of 3. b, Stress relaxation tests on the indicated tissues. Data from refs.^,,,,. Data for a and b result from various modalities of measurement (shear, compression, tension), various measurement tools (mechanical testers, nanoindentation, AFM, shear rheometry), and tissue of different animal origins (human, rat, mouse, bovine, sheep, porcine, canine).

Figure 3|. The molecular clutch model of mechanotransduction explains the impact of matrix viscoelasticity on cell spreading in 2D.

a, Schematic of molecular clutch model of mechanotransduction as applied to viscoelastic substrates. Adapted from Ref.. b, Molecular clutch model simulations predict optimal cell spreading when the timescale for stress relaxation is similar to the clutch binding timescale.

Figure 4|. Matrix viscoplasticity mediates mechanical confinement in 3D culture.

a, In confining 3D matrices, processes that involve volume change, morphological changes, or a combination of both are restricted. b, Confinement is governed by a combination of matrix pore size, matrix degradability, and matrix viscoplasticity. A sufficiently large value for any one of these properties releases confinement.

Figure 5|. Designing viscoelastic biomaterials for regenerative medicine.

a-b, Advanced imaging is utilized to detect the mechanical properties of the tissue, damaged and normal, in order to design materials with appropriate viscoelastic properties to guide the desired pattern of gene expression from interacting cells and morphogenesis. c-d, Introduction of the material, either alone or carrying various regeneration-promoting cargoes (e.g., cells) will then lead to (right panel) regeneration of the damaged tissue and reconstitution of function.

Box 2 Figure|

Strategies for forming hydrogels that are elastic, viscoelastic but not viscoplastic, or viscoelastic and viscoplastic.