PSI - Issue 82

Tsanka Dikova et al. / Procedia Structural Integrity 82 (2026) 9–15 Dikova et al. / Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction The bioceramics are a special class of inorganic, non-metallic materials especially designed for biomedical applications. They can be bioinert, bioactive, or bioresorbable (Abd El-Hamid (2025), Tanvir et al. (2024). The bioinert ceramics, such as alumina and zirconia, do not interact with surrounding tissues and are mainly applied when stability and durability are required. Hydroxyapatite (HA) and bioactive glasses are bioactive ceramics that interact with tissues and promote bone regeneration and repair. The bioresorbable ceramics have the ability to degrade in the body and be replaced by the natural tissues. A typical representative of this group is β tricalcium phosphate (β-TCP). Bioceramics are widely used as scaffolds in various fields of general and dental medicine: orthopedics, implantology, regenerative therapy, controlled drug delivery, etc. (Abd El-Hamid (2025), Tanvir et al. (2024). The hydroxyapatite with chemical formula Ca 10 (PO 4 ) 6 (OH) 2 is a major natural inorganic component of the bone and is characterized by excellent bioactivity, biocompatibility and osteoconductivity (Chinnasami et al. (2023), Cinici et al. (2024). The listed properties and the stoichiometric similarity with the minerals naturally present in the hard tissues, define HA as one of the most important materials for production of porous scaffolds for bone tissue regeneration. Various techniques and processes are used to produce bone scaffolds of bioceramics: freeze-drying, solvent casting and particulate leaching, gas foaming, phase separation, electrospinning, 3D printing, etc. (Chinnasami et al. (2023) Cinici et al. (2024). Freeze-drying, also called lyophilization or ice-templating, provides a simple, inexpensive, and environmentally friendly way to fabricate porous 3D scaffolds with controllable porosity. The method has been used to fabricate porous structures from polymers and ceramics (Fereshteh (2018), Zerankeshi et al. (2022). The production of porous bioceramic scaffolds by the freeze-drying method consists of four main stages: 1) preparation of a colloidal suspension of ceramic particles and pouring the suspension into a mold; 2) freezing of the suspension at a controlled rate; 3) after freezing, the samples are removed from the mold and subjected to lyophilization. By sublimating the solvent under vacuum, the stresses and shrinkage that occur in normal drying and can lead to cracks and deformations are avoided; 4) in the final stage, the dried samples are sintered to obtain porous scaffolds with improved strength, hardness and desired porosity (Yin et al. (2021), Deville et al. (2006), Deville (2008). The microstructure and properties of the final scaffold are mainly determined by the suspension parameters (particle size and concentration and colloidal stability of the suspension), the freezing conditions (speed and direction) and the sintering conditions (temperature and time) (Diaz et al. (2023), Liu et al. (2015). Proper selection of process parameters allows for the production of bioceramic scaffolds with interconnected gradient channels that mimic the porous structure of natural bone (Bai et al. (2015), Landi et al. (2008). The interconnectivity of the pores and their sizes are of essential importance. For successful bone regeneration, scaffolds with interconnected pores with sizes from 20 to 400 μm are required (Mukasheva et al. (2024). The amount and size of the ceramic particles play a key role in the microstructure, overall porosity and mechanical properties of the freeze-dried scaffolds. Increasing the concentration of particles in the suspension leads to a decrease in the porosity of HA scaffolds. Fu et al. (2008-1) found that the porosity of scaffolds made with 5, 10 and 20 % HA content was 85, 70 and 55%, respectively. Farhangdoust et al. (2013) confirmed this relationship when studying HA scaffolds with powder content from 7 to 37.5 %. The results show a linear decrease in total porosity from 87 to 45%. The decrease in porosity leads to a sharp increase in compressive strength, from 0.4 MPa to ~60 MPa. Zamanian et al. (2014) have shown that the compressive strength of HA scaffolds with larger particle sizes is higher as a result of lower total porosity. Using suspensions of 15% HA powder with different particle sizes (3.9 and 1.69 µm), scaffolds with porosity from 57 to 83% and compressive strengths between 15 and 1.7 MPa were obtained. Fu et al. (2008-2) studied the influence of sintering temperature on the microstructure and mechanical properties of scaffolds made from 20% HA. They sintered samples at temperatures from 1250°C to 1375°C. With increasing temperature, the porosity slightly decreased, from 55% at 1250°C to 52% at 1350°C. The compressive strength increased with increasing sintering temperature and reached a maximum value (12 MPa) at 1350 °C. Zamanian et al. (2013) found that the compressive strength of HA scaffolds tripled (from 2 to 8 MPa) as the sintering temperature