Design of porous structure for bone tissue engineering

Tissue engineering approaches have gained increasing importance in recent years due to the significant drawbacks of conventional regenerative surgery. In this approach, cells cultured in a laboratory environment are seeded onto a biodegradable scaffold and implanted into the patient after tissue formation. However, the growth behavior of bone tissue on scaffolds has not been fully modeled. Bone tissue regeneration is a mechanobiological process. Nevertheless, most studies in this field focus primarily on biological processes, considering mechanical loading merely as an experimentally acknowledged factor without investigating its mechanisms of action. Tissue scaffolds are expected to provide adequate mechanical strength while allowing sufficient porosity for nutrient and waste diffusion, as well as fulfilling essential properties such as biocompatibility and biodegradability. Evaluating these features simultaneously during the design process significantly complicates the development of ideal tissue scaffolds. In this thesis study, the effects of different pore geometries and porosity ratios on the biodegradation behavior and bone regeneration of tissue scaffolds were examined under mechanical loading. Three different pore geometries were designed: 1.Basic Cube (BC): Comprising only horizontal and vertical struts, 2.Body-Centered Structure (BCS): Comprising only diagonal struts, 3.Body-Centered Cubic (BCC): Comprising horizontal, vertical, and diagonal struts. Additionally, scaffolds were designed with porosity ratios of 50%, 62.5%, and 75% by altering the cross-sectional dimensions of the struts. As a result, scaffolds with varying stiffness, specific surface area, and specific energy absorption values, which are directly influential in bone regeneration, were obtained. Experimental and finite element analyses revealed that BC scaffolds exhibited the highest stiffness values across all porosity ratios. The highest specific surface area was observed in BCC scaffolds with a porosity of 75%, while the highest specific energy absorption was obtained in BCS scaffolds with the same porosity ratio. To characterize the biodegradation behavior, two different numerical models were utilized, and biodegradation experiments were conducted under 4 N mechanical loading for 15, 30, 60, 90, and 120 days. The first-order kinetic model represented the second stage of biodegradation, while the diffusion-based model corresponded to the first stage, aligning with experimental results. The experiments demonstrated that mechanical loading had no significant effect on biodegradation, and the mechanical properties and weight of the scaffolds remained relatively unchanged. The highest water absorption was observed on day 60 in BCC scaffolds, indicating an advantage for cell seeding and adhesion. In vitro studies involved cell viability analysis and ALP enzyme activity measurements under 2 N mechanical loading for 50% and 75% porosity scaffolds. For 50% porosity, mechanical loading increased cell viability by 18 times in BCC scaffolds, whereas it led to 6 times increase in BCS scaffolds with 75% porosity. Mechanical loading had no significant effect on cell viability in BC scaffolds. Furthermore, ALP enzyme activity was unaffected by mechanical loading. Mineralization and micro-CT analyses were conducted under 1 N and 2 N mechanical loading for BCS and BCC scaffolds with 50% and 75% porosity. The highest mineralization value was observed in BCC scaffolds with 75% porosity under 2 N loading, with the highest specific surface area. Similarly, micro-CT scans indicated that the highest mineral content was achieved in BCS scaffolds with 75% porosity under the same loading condition, with the highest specific energy absorption. In vitro analyses confirmed the significant impact of mechanical loading, specific surface area, and specific energy absorption on bone regeneration. To numerically evaluate the effects of different design parameters, an in silico model was developed based on the finite element method. This model utilized strain energy density in elements representing bone tissue for bone remodeling. Coefficients used in the bone remodeling model were determined from normalized results obtained from mineralization and micro-CT analyses, ensuring consistency between in silico and in vitro findings. Thus, the development of bone tissue on scaffolds is modeled by using an in silico approach. In conclusion, tissue scaffolds with different design parameters were assessed through mechanical testing, biodegradation experiments, and in vitro and in silico analyses for bone tissue engineering applications. The obtained results identified that specific surface area and specific energy absorption are critical parameters for the success of tissue scaffold applications.