Investigating active dynamics of contractile actomyosin gels with micro particle image velocimetry (micro-PIV) analysis.

Micro-particle image velocimetry (micro-PIV) is a powerful imaging method for resolving flow fields in microscopic systems, but its application to rapidly deforming gels requires key modifications. Here, we adapt micro-PIV to quantify contraction dynamics of active actomyosin gels without invasive tracer beads by tracking local myosin-generated actin inhomogeneities. We show that accurate displacement measurements depend critically on optimizing the time interval over which displacements are computed and the number of frames used in ensemble correlation, relative to the poroelastic timescale and stage of contraction. To assess reliability, we quantify four complementary metrics: displacement vector fields, displacement-magnitude maps, radial profiles of radial and tangential displacement components, and ensemble-averaged orientation with rotational measures. Incorporating the elastic response of the gel, we extract radial strain profiles under an axisymmetric approximation and demonstrate robustness for irregular geometries and off-center contractions. Across conditions, we observe common radial signatures consisting of inward radial contraction with peak displacement at intermediate radii and effective radial stretching near the boundary. These deformation fields provide a basis for inferring spatial and orientational distributions of motor-generated active stresses using appropriate constitutive models. Our approach advances quantitative analysis of active poroelastic materials and has broad applications in biomaterials design, cytoskeletal dynamics, and morphogenesis.