Viscoelasticity: The Forgotten Property of Living Tissues
Team Dynamic Matrices | 6 January 2026
Press a finger into your forearm and hold it. The resistance you feel in the first instant is not the resistance you feel two seconds later; the tissue yields, fluid moves, fibres slide. Now press a block of cured silicone the same way. The resistance is constant. Almost every material used to study mechanobiology for thirty years behaved like the silicone. Every tissue those studies were meant to represent behaves like the arm. This post is about that gap: what viscoelasticity is, why it was ignored, and what the clinical evidence says about it.
Why it was forgotten
The reason is practical, not scientific. Polyacrylamide gels (Pelham and Wang, PNAS, 1997) and PDMS elastomers were easy to make at defined stiffness, easy to functionalise, and easy to measure. They are also nearly purely elastic. Engler et al. (Cell, 2006) used polyacrylamide to show stiffness directs MSC fate, and a generation of researchers followed with the same material. Convenience became an assumption: that a purely elastic substrate at the right stiffness is a fair model of tissue. It omits a property every tissue has.
What viscoelasticity is
A viscoelastic material combines an elastic response (energy stored, returned) with a viscous one (energy dissipated). Hold it at fixed deformation and the stress required to hold it decays over time; that is stress relaxation. Apply constant stress and it keeps deforming; that is creep. Both come from the same physics: physical crosslinks that unbind and rebind, fibres that slide, water that flows through the network.
For a cell, the consequence is that pulling on the matrix is a time-dependent measurement. The cell pulls, waits, and feels how quickly the resistance fades. A fast-relaxing matrix lets the cell cluster ligands and spread; a slow-relaxing one resists. Chaudhuri et al. (Nature, 2020) reviewed how this drives spreading, proliferation, differentiation and migration; our cell fate post walks through the experiments.
Representative values for native tissues
Values vary with method and species, but the pattern is consistent: soft tissues relax on timescales of seconds to minutes, and the relaxation timescale is independent of stiffness.
| Tissue | Approximate stiffness | Stress relaxation half-time | Source |
|---|---|---|---|
| Brain | 0.1 to 1 kPa | ~10 s to minutes | Budday et al., 2017; Chaudhuri 2020 |
| Liver | 1 to 3 kPa | tens of seconds | Chaudhuri 2020 |
| Adipose | 1 to 3 kPa | tens of seconds | Chaudhuri 2020 |
| Muscle | 10 to 15 kPa | seconds to a minute | Chaudhuri 2020 |
| Breast tumour vs normal | 4 to 12 kPa vs ~0.2 to 1 kPa | tumour relaxes slower | Levental 2009; Sinkus 2005 |
| Fibrotic lung vs normal | rises 2 to 5x | relaxation largely preserved | Booth et al., 2012 |
Compare to a covalent PEG gel: half-time effectively infinite.
Where the clinical evidence points
Cancer. Tumours are stiffer than surrounding tissue, which is the basis of palpation and elastography. They also relax more slowly. MR elastography studies of breast lesions (Sinkus et al., Magnetic Resonance Imaging, 2005) found that malignant tumours differ from benign ones in viscous as well as elastic parameters, and that the viscous component improved discrimination. In vitro, slow relaxation combined with stiffness promotes invasive phenotypes; plasticity enables protease-independent invasion (Wisdom et al., Nature Communications, 2018).
Fibrosis. Booth et al. (American Journal of Respiratory and Critical Care Medicine, 2012) measured decellularised fibrotic human lung and found stiffness rose several-fold while the tissue stayed viscoelastic. Stiffness and relaxation moved independently, which is a direct demonstration that they are separate variables with separate biology.
Development and ageing. Tissue viscoelasticity changes across morphogenesis (Elosegui-Artola et al., Nature Materials, 2023, showed the tissue-to-matrix relaxation ratio patterns branching) and with age, as crosslinking increases and relaxation slows. Cells in a developing embryo experience a matrix whose relaxation is a regulated variable, not a constant.
The research gap
Most 3D culture systems still cannot vary relaxation independently of stiffness. Natural gels couple everything to protein concentration; covalent synthetic gels are elastic; basement membrane extracts are whatever the lot is. So the clinical observations above, that relaxation changes in disease independently of stiffness, cannot be reproduced in the dish, and cannot be tested as causes rather than correlates.
That is the gap we designed DyNAtrix to close. Its DNA crosslinks set relaxation over five orders of magnitude while polymer content sets stiffness, so a fibrotic condition (stiff, viscoelastic), a tumour-like condition (stiff, slow-relaxing) and a healthy condition (soft, fast-relaxing) can be built from one material and compared (Peng et al., Nature Nanotechnology, 2023).
References
- Pelham RJ, Wang Y. Cell locomotion and focal adhesions are regulated by substrate flexibility. Proc Natl Acad Sci USA 94, 13661–13665 (1997).
- Engler AJ et al. Matrix elasticity directs stem cell lineage specification. Cell 126, 677–689 (2006).
- Chaudhuri O et al. Effects of extracellular matrix viscoelasticity on cellular behaviour. Nature 584, 535–546 (2020).
- Budday S et al. Mechanical characterization of human brain tissue. Acta Biomater 48, 319–340 (2017).
- Levental KR et al. Matrix crosslinking forces tumor progression by enhancing integrin signaling. Cell 139, 891–906 (2009).
- Sinkus R et al. Viscoelastic shear properties of in vivo breast lesions measured by MR elastography. Magn Reson Imaging 23, 159–165 (2005).
- Booth AJ et al. Acellular normal and fibrotic human lung matrices as a culture system for in vitro investigation. Am J Respir Crit Care Med 186, 866–876 (2012).
- Wisdom KM et al. Matrix mechanical plasticity regulates cancer cell migration through confining microenvironments. Nat Commun 9, 4144 (2018).
- Elosegui-Artola A et al. Matrix viscoelasticity controls spatiotemporal tissue organization. Nat Mater 22, 117–127 (2023).
- Peng Y-H et al. Dynamic matrices with DNA-encoded viscoelasticity support the development of organoids and stem cells. Nat Nanotechnol (2023).
FAQs
No. A material can be soft and elastic (a PEG gel) or stiff and viscoelastic (fibrotic lung). They are independent axes.
MR elastography and shear-wave ultrasound elastography both extract viscous and elastic parameters from wave propagation through tissue. The viscous component is increasingly used diagnostically for liver fibrosis and breast lesions.
It depends on the cell type and question, which is exactly why you need to be able to vary them separately. Screen both at a few levels rather than assuming.