Modeling cortical bone adaptation using strain gradients

Multi-Gradient Bone-Like Nanocomposites Induced by Strain Distribution

The heterogeneity of bones is elegantly adapted to the local strain environment, which is critical for maintaining mechanical functions. Such an adaptation enables the strong correlation between strain distributions and multiple gradients, underlying a promising pathway for creating complex gradient structures. However, this potential remains largely unexplored for the synthesis of functional gradient materials. In this work, heterogeneous bone-like nanocomposites with complex structural and compositional gradients comparable to bones are synthesized by inducing strain distributions within the polymer matrix containing amorphous calcium phosphate (ACP). Uniaxial stretching of composite films exerts the highest strain in the center, which ceases gradually toward the sides, resulting in the gradual decrease of polymer alignment and crystallinity. Simultaneously, the center with high orientation traps most ACP during stretching due to the nanoconfinement effect, which in turn promotes the formation of aligned nanofibrous structures. The sides experiencing the least strain have the smallest amounts of ACP, characteristic of porous architectures. Further crystallization of ACP produces oriented apatite nanorods in the center with a larger crystalline/amorphous ratio than the sides because of template-induced crystallization. The combination of structural and compositional gradients leads to gradient mechanical properties, and the gradient span and magnitude correlate nicely with strain distributions. Accompanying bone-like mechanical gradients, the center is less adhesive and self-healable than the sides, which allows a better recovery after a complete cutting. Our work may represent a general strategy for the synthesis of biomimetic materials with complex gradients thanks to the ubiquitous presence of strain distributions in load-bearing structures.

Computation of physiological loading induced interstitial fluid motion in muscle standardized femur: Healthy vs. osteoporotic bone

BACKGROUND AND OBJECTIVES

Physiological loading-induced mechanical environments regulate bone modeling and remodeling. Thus, loading-induced normal strain is typically considered a stimulus to osteogenesis. However, several studies noticed new bone formation near the sites of minimal normal strain, e.g., the neutral axis of bending in long bones, which raises a question on how bone mass is maintained near these sites. Secondary mechanical components such as shear strain and interstitial fluid flow also stimulate bone cells and regulate bone mass. However, the osteogenic potential of these components is not well established. Accordingly, the present study estimates the distribution of physiological muscle loading-induced mechanical environments such as normal strain, shear strain, pore pressure, and interstitial fluid flow in long bones.

METHODS

A poroelastic finite element muscle standardized femur (MuscleSF) model is developed to compute the distribution of the mechanical environment as a function of bone porosities associated with osteoporotic and disuse bone loss.

RESULTS

The results indicate the presence of higher shear strain and interstitial fluid motion near the minimal strain sites, i.e., the neutral axis of bending of femoral cross-sections. This suggests that secondary stimuli may maintain the bone mass at these locations. Pore pressure and interstitial fluid motion reduce with the increased porosity associated with bone disorders, possibly resulting in diminished skeletal mechano-sensitivity to exogenous loading.

CONCLUSIONS

These outcomes present a better understanding of mechanical environment-mediated regulation of site-specific bone mass, which can be beneficial in developing prophylactic exercise to prevent bone loss in osteoporosis and muscle disuse.

BEM evaluation of surface octahedral strains and internal strain gradients in 3D-printed scaffolds used for bone tissue regeneration

Most of the mechnoregulatory computational models appearing so far in tissue engineering for bone healing predictions, utilize as regulators for cell differentiation mainly the octahedral volume strains and the interstitial fluid velocity calculated at any point of the fractured bone area and controlled by empirical constants concerning these two parameters. Other stimuli like the electrical and chemical signaling of bone constituents are covered by those two regulatory fields. It is apparent that the application of the same mechnoregulatory computational models for bone healing predictions in scaffold-aided regeneration is questionable since the material of a scaffold disturbs the signaling pathways developed in the environment of bone fracture. Thus, the goal of the present work is to evaluate numerically two fields developed in the body of two different compressed scaffolds, which seem to be proper for facilitating cell sensing and improving cell viability and cell seeding efficiency. These two fields concern the surface octahedral strains that the cells attached to the scaffold can experience and the internal strain gradients that create electrical pathways due to flexoelectric phenomenon. Both fields are evaluated with the aid of the Boundary Element Method (BEM), which is ideal for evaluating with high accuracy surface strains and stresses as well as strain gradients appearing throughout the analyzed elastic domain.