Cantamessa, A. | Search bioRxiv

bioRxiv2024

The mineralization of osteonal cement line depends on where the osteon is formed

The cement line (CL) is a thin layer separating secondary osteons from interstitial bone and other osteons. It is assumed to play a significant role in bone fracture resistance, owing to its ability to deflect or arrest microcracks. Despite the possible role for bone quality, the CL is still one of the least understood microstructural features of bones, with unknowns on CL composition, mineralization, and mechanical properties. This study, focusing on CL mineralization, aims to elucidate the interplay between the mineral content of the CL and of adjacent bone tissue. Using quantitative backscattered electron imaging, osteons with different degrees of mineralization coming from human femoral samples were analyzed. We implemented a spatially resolved analysis of the mineral content in layers along the CL, considering both regions inside the osteon (i.e., formed soon after CL deposition) and outside (i.e., already present at the time of CL deposition). We found that the mineral content of the CL correlates strongly with the mineral content outside of the osteon, but not inside. Assuming the mineral content of the osteon as a proxy of its age, we demonstrate that not only the osteon, but also the CL increases its mineral content with time. However, the rate of increase is lower in the CL. Importantly, the specific value of the high initial mineral content of the CL depends on the mineral content of the local surrounding, in which the osteon was formed. Our findings highlight the central role of the local degree of mineralization of the bone around the osteon for building the CL.

bioengineering↗

bioRxiv2023

Rational positioning of 3D printed micro-bricks to realize high-fidelity, multi-functional soft-hard interfaces

AO_SCPLOWBSTRACTC_SCPLOWLiving organisms have developed design principles, such as functional gradients (FGs), to interface hard materials with soft ones (e.g., bone and tendon). Mimicking such design principles can address the challenges faced when developing engineered constructs with soft-hard interfaces. To date, implementing these FG design principles has been primarily performed by varying the ratio of the hard phase to that of the soft phase. Such design approaches, however, lead to inaccurate mechanical properties within the transition zone. That is due to the highly nonlinear relationship between the material distribution at the microscale and the macroscale mechanical properties. Here, we 3D print micro-bricks from either a soft or a hard phase and study the nonlinear relationship between their arrangements within the transition zone and the resulting macroscale properties. We carry out experiments at the micro- and macroscales as well as finite element simulations at both scales. Based on the obtained results, we develop a co-continuous power-law model relating the arrangement of the micro-bricks to the local mechanical properties of the micro-brick composites. We then use this model to rationally design FGs at the individual micro-brick level and create two types of biomimetic soft-hard constructs, including a specimen modeling bone-ligament junctions in the knee and another modeling the nucleus pulposus-annulus fibrosus interface in intervertebral discs. We show that the implemented FGs drastically enhance the stiffness, strength, and toughness of both types of specimens as compared to non-graded designs. Furthermore, we hypothesize that our soft-hard FGs regulate the behavior of murine preosteoblasts and primary human bone marrow-derived mesenchymal stromal cells (hBMSCc). We culture those cells to confirm the effects of soft-hard interfaces on cell morphology as well as on regulating the expression of focal adhesion kinase, subcellular localization, and YAP nuclear translocation of hBMSCs. Taken together, our results pave the way for the rational design of soft-hard interfaces at the micro-brick level and (biomedical) applications of such designs.

bioengineering↗