Approaches for In Situ Monitoring of Matrix Development in Hydrogel-Based Engineered Cartilage

Near infrared (NIR) spectroscopy using a fiber optic probe shows great promise for the nondestructive in situ monitoring of tissue engineered construct development; however, the NIR evaluation of matrix components in samples with high water content is challenging, as water absorbances overwhelm the spectra. In this study, we established approaches by which NIR spectroscopy can be used to select optimal individual engineered hydrogel constructs based on matrix content and mechanical properties. NIR spectroscopy of dry standard compounds allowed identification of several absorbances related to collagen and/or proteoglycan (PG), of which only two could be identified in spectra obtained from hydrated constructs, at ∼5940 and 5800 cm^-1. In dry sample mixtures, the ratio of these peaks correlated positively to collagen and negatively to PG. In NIR spectra from engineered cartilage hydrogels, these peaks reflected higher collagen and PG content and dynamic modulus values, permitting the differentiation of constructs with poor and good matrix development. Similarly, the increasing baseline offset in raw NIR spectra also reflected matrix development in hydrated constructs. However, weekly monitoring of NIR spectra and the peaks at ∼5940 and 5800 cm^-1 was not adequate to differentiate individual constructs based on matrix composition. Interestingly, changes in the baseline offset of raw spectra could be used to evaluate the growth trajectory of individual constructs. These results demonstrate an optimal approach for the use of fiber optic NIR spectroscopy for in situ monitoring of the development of engineered cartilage, which will aid in identifying individual constructs for implantation. Impact statement A current demand in tissue engineering is the establishment of nondestructive approaches to evaluate construct development during growth in vitro. In this article, we demonstrate original nondestructive approaches by which fiber optic NIR spectroscopy can be used to assess matrix (PG and collagen) formation and mechanical properties in hydrogel-based constructs. Our data provide a cohesive molecular-based approach for in situ longitudinal evaluation of construct development during growth in vitro. The establishment of these approaches is a valuable step toward the real-time identification and selection of constructs with optimal properties, which may lead to successful tissue integration upon in vivo implantation.