Fibroblast-driven compaction and tension generation in the collagen matrix leads to changes of the mechanical microenvironment that are important for the regulation of these cells’ differentiations and functions. Yet routine, region-specific measurements of elastic modulus of developing fibroblast-populated collagen matrices (FPCM) remain challenging. This work refined and validated an optical coherence tomography (OCT) based microindentation assay that uses a spherical glass bead as the indenter. The applied load is simply the bead’s weight minus buoyancy, so indentation force is known without complex calibration. OCT provides an accurate, detailed surface indentation profile of the developing FPCM in real time. Using the Hertz contact formula for the spherical-indentation model converts these OCT measurements into local elastic modulus of FPCM during cell-dependent compaction and tension generation. The method was verified through testing against reference silicone gels with known elastic moduli. This project also assessed practical operating ranges by varying bead weight to achieve relatively small indentation depths on soft matrices. Applied to FPCM, the assay revealed progressive stiffening (increased elastic modulus) together with thinning and volume loss while the attachment area remains largely constant, consistent with cell-driven compaction and remodeling. Aligning load and readout at the same plane enabled the capture of time-dependent deformation, probing of nonlinear elasticity at larger strains, and estimation of built-in in-plane tension. In summary, the OCT-based approach provides a minimally invasive, spatially resolved tool for quantifying elasticity of collagen matrices and related biomaterials, supporting studies of wound healing, fibrosis modeling, and tissue-engineering applications.
Pitzer et al. (Sun,) studied this question.