A compact micro-load compression testing device for stiffness characterization of soft biological tissues

Mechanical stiffness is a critical physical property of soft biological tissues that is closely associated with physiological and pathological states. However, the quantitative mechanical characterization of small and soft samples remains challenging due to the limited sensitivity and poor portability of conventional compression testing systems. Here, we present a compact, benchtop micro-load compression testing platform designed for the evaluation of small, soft biological tissues. The device enables reproducible measurements under millinewton-level loading and micrometer-scale displacement while maintaining a simple, portable configuration. Using collagen- and gelatin-based gel samples with spherical and cylindrical geometries as biomimetic models, we demonstrate the system's performance across different materials, temperatures, and sample geometries. To address the predominance of nonlinear deformation in soft materials, we introduce a model-free stiffness index, which is defined as the local slope of the semi-logarithmic nominal stress–strain relationship. This approach allows for a robust relative comparison of mechanical responses without assuming specific constitutive models or material homogeneity. The proposed platform combines practical operability with analytical robustness, providing a versatile tool for the rapid mechanical screening of soft tissues and biomimetic materials. Its compact design and compatibility with controlled environments make it ideal for applications that require the flexible, high-sensitivity mechanical assessment of limited, fragile biological samples.