Supplementary Figure 1: Mechanical terminology.

(a) The engineering strain ɛ is a measure of a material's deformation from a reference shape, as defined above for a material of length L₀ stretched to a length L by an external force. The engineering stress σ is a measure of the tension exerted within a material. It is a force per unit area, as defined above where W₀ is the cross-sectional area of the material at rest. (b) Elastic materials are characterized by a reversible relationship between stress and strain regardless of their deformation history. Materials are linear elastic when the stress varies linearly with the strain. In that case, the slope of the stress-strain relationship is a measure of a material's stiffness. (c) Many materials, including cells and tissues, exhibit time-dependent responses following application of deformation, something often referred to as a visco-elastic behavior. Gels, and to some extent living cells and tissues, can be described to the first order using standard viscoelastic solid models, though many more complex behaviours have been documented⁴⁶. One classic mechanical test is known as a stress relaxation test. In response to a step deformation, a viscoelastic material relaxes with a characteristic time τ above which the stress reaches an equilibrium value from which a stiffness can be defined. In contrast to viscoelastic materials, elastic materials subjected to a step deformation do not display relaxation. (d) Stress-strain relationship for a thin sheet of linear elastic material (PDMS) affixed to our force measurement device. (e) Stress-relaxation test for a thin sheet of PDMS affixed to our force measurement device. As PDMS is linear elastic, no relaxation can be detected following deformation.