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Cells sense and transduce mechanical forces to regulate diverse biological processes, yet the mechanical stimuli that initiate these processes remain poorly understood. In particular, how nuclear and cytoplasmic deformations respond to external forces is unclear. Here, we developed a microscopy-based technique to quantify the extensional uniaxial strains of the nucleus and cytoplasm during cell stretching, enabling direct measurement of their bulk mechanical responses. Using this approach, we identified a previously unrecognized inverse relationship between nuclear and cytoplasmic deformation in epithelial monolayers. We demonstrate that nucleo-cytoskeletal coupling, mediated by the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, regulates this anti-correlation (Pearson correlation coefficient approx. 0.3). Disrupting LINC abolished this relationship, revealing its fundamental role in intracellular deformation partitioning. Furthermore, we found that cytoplasmic deformation is directly correlated with stretch-induced nuclear shrinkage, suggesting a mechanotransduction pathway in which cytoplasmic mechanics influence nuclear responses. Lastly, multivariable analyses established that intracellular deformation can be inferred from cell morphology, providing a predictive framework for cellular mechanical behaviour. These findings refine our understanding of nucleo-cytoskeletal coupling in governing intracellular force transmission and mechanotransduction. 

Viscoelastic shells subjected to a pressure loading exhibit rich and complex time-dependent responses. Here we focus on the phenomenon of pseudo-bistability, i.e. a viscoelastic shell can stay inverted when pressure is removed, and snap to its natural shape after a delay time. We model and explain the mechanism of pseudo-bistability with a viscoelastic shell model. It combines the small strain, moderate rotation shell theory with the standard linear solid as the viscoelastic constitutive law, and is applicable to shells with arbitrary axisymmetric shapes. As a case study, we investigate the pseudo-bistable behaviour of viscoelastic ellipsoidal shells. Using the proposed model, we successfully predict buckling of a viscoelastic ellipsoidal shell into its inverted configuration when subjected to an instantaneous pressure, creeping when the pressure is held, staying inverted after the pressure is removed, and eventually snapping back after a delay time. The stability transition of the shell from a monostable, temporarily bistable and eventually back to the monostable state is captured by examining the evolution of the instantaneous pressure–volume change relation at different time of the holding and releasing process. A systematic parametric study is conducted to investigate the effect of geometry, viscoelastic properties and loading history on the pseudo-bistable behaviour. This article is part of the theme issue 'Probing and dynamics of shock sensitive shells'. 

Abstract In response to external stimuli, such as heat, light, or magnetic fields, stimuli-responsive soft materials can change their current configuration to a new equilibrium state through non-equilibrium kinetic processes, including reaction, diffusion, and viscoelastic relaxation, which generates novel spatiotemporal shape-morphing behavior. Using a photothermal shape memory polymer (SMP) cantilever beam as a model system, this work analytically, numerically, and experimentally studies its non-equilibrium kinetic processes and spatiotemporal bending under light illumination. We establish a thermomechanical model for SMPs capturing the concurrent non-equilibrium processes of heat transfer and viscoelastic relaxation, which induces inhomogeneous temperature and strain distributions through the thickness of the beam, resulting in its bending and unbending. By varying the key dimensionless parameters, we theoretically and experimentally observe different types of bending dynamics. Moreover, our theory takes into consideration changes in the angles of incidence caused by extensive beam bending, and demonstrates that this effect can dramatically delay the bending due to reduction of the effective light intensity, which is further validated experimentally. This work demonstrates programmable and predictable spatiotemporal morphing of SMPs, and provides design guidelines for SMP morphing structures and robots.