Active microrheology is one of the main methods to determine the mechanical properties of cells and tissue, and the modelling of these viscoelastic properties is under heavy debate with many...

Tension and mechanical properties of muscle tissue are tightly related to proper skeletal muscle function, which makes experimental access to the biomechanics of muscle tissue formation a key requirement to advance our understanding of muscle function and development. Recently developed elastic in vitro culture chambers allow for raising 3D muscle tissue under controlled conditions and to measure global tissue force generation. However, these chambers are inherently incompatible with high resolution microscopy limiting their usability to global force measurements, and preventing the exploitation of modern fluorescence based investigation methods for live and dynamic measurements. Here we present a new chamber design pairing global force measurements, quantified from post deflection, with local tension measurements obtained from elastic hydrogel beads embedded in muscle tissue. High resolution 3D video microscopy of engineered muscle formation, enabled by the new chamber, shows an early mechanical tissue homeostasis that remains stable in spite of continued myotube maturation.

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration.

AbstractThe mechanical properties and tension of muscle tissue are tightly related to proper skeletal muscle function, which makes experimental access to the biomechanics of muscle tissue development a key requirement to advance our understanding of muscle function and development. Recently developed elastic in vitro culture chambers allow for raising 3D muscle tissue under controlled conditions and measurements of tissue force generation. However, these chambers are inherently incompatible with high resolution microscopy limiting their usability to global force measurements, and preventing the exploitation of modern fluorescence based investigation methods for live and dynamic measurements. Here we present a new chamber design pairing global force measurements, quantified from post deflection, with local tension measurements obtained from elastic hydrogel beads embedded in the muscle tissue. High resolution 3D video microscopy of engineered muscle development, enabled by the new chamber, shows an early mechanical tissue homeostasis that remains stable in spite of continued myotube maturation.

Understanding life is arguably among the most complex scientific problems faced in modern research. From a physics perspective, living systems are complex dynamic entities that operate far from thermo-dynamic equilibrium.1–3 This active, non-equilibrium behaviour, with its constant hunger for energy, allows life to overcome the ever dispersing forces of entropy, and hence drives cellular organisation and dynamics at the micrometer scale.4,5 Unfortunately, most analysis methods provided by the powerful toolbox of statistical mechanics cannot be used in such non-equilibrium situations, forcing researchers to use sophisticated and often invasive approaches to study the mechanistic processes inside living organisms. Here we introduce a new observable coined the mean back relaxation, that allows simple detection of broken detailed balance and full quantification of the active mechanics from passively observed particle trajectories. Based on three-point probabilities and exploiting Onsager’s regression hypothesis, the mean back relaxation extracts more information from passively measurements compared to classical observables such as the mean squared displacement. We show that it gives access to the non-equilibrium generating energy and the viscoelastic material properties of a well controlled artificial system, and, surprisingly, also of a variety of living systems. It thus acts as a new marker of non-equilibrium dynamics, a statement based on an astonishing relation between the mean back relaxation and the active mechanical energy. Combining, in a next step, passive fluctuations with the extracted active energy allows to overcome a fundamental barrier in the study of living systems; it gives access to the viscoelastic material properties from passive measurements.

Duchenne muscular dystrophy (DMD) represents the most common inherited muscular disease, where increasing muscle weakness leads to loss of ambulation and premature death. DMD is caused by mutations in the dystrophin gene, and is known to reduce the contractile capacity of muscle tissue both in vivo, and also in reconstituted systems in vitro. However, these observations result from mechanical studies that focused on stimulated contractions of skeletal muscle tissues. Seemingly paradoxical, upon evaluating bioengineered skeletal muscles produced from DMD patient derived myoblasts we observe an increase in unstimulated contractile capacity that strongly correlates with decreased stimulated tissue strength, suggesting the involvement of dystrophin in regulating the baseline homeostatic tension level of tissues. This was further confirmed by comparing a DMD patient iPSC line directly to the gene-corrected isogenic control cell line. From this we speculate that the protecting function of dystrophin also supports cellular fitness via active participation in the mechanosensation to achieve and sustain an ideal level of tissue tension. Hence, this study provides fundamental novel insights into skeletal muscle biomechanics and into a new key mechanical aspect of DMD pathogenesis and potential targets for DMD drug development: increased homeostatic tissue tension.

Living cells are complex entities that perform many different complex tasks with astonishing robustness. While the direct dependence of biological processes on controlled protein expression is well established, we only begin to understand how intracellular mechanical characteristics guide and support biological function. This is in stark contrast to the expected functional role that intracellular mechanical properties should have for many core cellular functions such as organization, homeostasis and transport. From a mechanical point of view, cells are complex viscoelastic materials that are continuously driven out of thermodynamic equilibrium, which makes both a physical measurement and mathematical modeling of its properties difficult. Here, we define a "mechanical fingerprint" that can not only characterize the intracellular mechanical state, but also carve out the mechanical differences between cell types with the potential to relate these to proper cell function. By analyzing the frequency-dependent viscoelastic properties and intracellular activity of cells using microrheology, we distilled the complex active mechanical state into just 6 parameters that comprise the mechanical fingerprint. The systematic investigation of the fingerprint illustrates a parameter tuning that can be explained by the functional cellular requirements. However, the full potential of the mechanical fingerprint is given by a statistical analysis of its parameters across all investigated cell types, which suggests that cells adjust mechanical parameters in a correlated way to position their intracellular mechanical properties within a well defined phase-space that is spanned between activity, mechanical resistance and fluidity. This paves the way for a systematic study of the interdependence of biological function and intracellular active mechanics.

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