Non-destructive assessment of multi-material micro-tissue mechanics reveals the critical role of rigidity gradients in tumour growth and pressure

Probing stiffness anisotropies in three-dimensional materials non-destructively is a major challenge in disciplines as diverse as aeronautics and medicine. While the former typically relies on various mechanical tests-such as tensile, compression, bending, and shear-performed on sample parts, the latter often employs acoustic techniques or wave propagation through matter. The choice of techniques depends on the size of the sample of interest and the desired resolution. In our case, to probe the mechanical properties of sub-millimetre micro-tissues, it is necessary to use methods with high resolution and as furtive as possible. We present a method based, 1/ on imaging the displacement of microbeads within a hydrogel resulting from the growth of a three-dimensional micro-tissue and, 2/ on finite element modelling of the deformations underlying bead displacements. This approach allows us to determine the elastic properties of the hydrogel and, in particular, to show that beyond a certain thickness, incomplete cross-linking of the hydrogel results in a stiffness gradient. We show that when the micro-tissue contacts with an immediately rigid alginate wall, the pressure exerted over time increases very rapidly, whereas when the micro-tissue encounters a substrate with a stiffness gradient, the pressure increase is more gradual. Uncovering this could provide a better understanding of the role of tumour microenvironment stiffness in metastatic escape processes. STATEMENT OF SIGNIFICANCE: Understanding factors modulating tumours growth is crucial for developing better cancer treatments. This study introduces a non-destructive method to assess the stiffness of subcomponents of a tissue avatar, a question unwieldly to tackle. The authors show that small changes in stiffness of the tumour-mimic surrounding tissue can strongly affects how the tumour cell aggregate grows. This is an outreach in cancer biology because it connects the mechanical environment of tissues to cancer behaviour in a original way. It provides a powerful tool for studying 3D biological systems and could help design more suited materials for biomedical research and therapy. This work is relevant for the fields of cancer biology, biomaterials, and tissue engineering.