Biodegradable and Bioactive Porous Polymer/Inorganic Nanocomposites Scaffolds for Biomedical Applications

With the aging of populations and prolonged life expectancy, there is an increasing demand for bone grafts or synthetic materials that can potentially replace, repair or regenerate lost, injured or diseased bone. Tissue engineering (TE) is one of the approaches being investigated to tackle this problem. In common TE strategies, a three-dimensional structure, termed “scaffold”, fabricated from a suitable artificial or natural material. In bone tissue engineering, a scaffolding material is used either to induce formation of bone from the surrounding tissue or to act as a carrier or template for implanted bone cells or other agents. To serve as a scaffold, the material must be biocompatible, osteoconductive, and osteointegrative, and have enough mechanical strength to provide structural support during the bone growth and remodeling. Several attempts have been successfully made to construct porous scaffolds with desired porosity and appropriate mechanical performance from inorganic materials such as bioactive ceramics and glasses, from biodegradable polymers and their composites. The focus of biomaterial design for tissue engineering applications has recently been directed towards bioactive components that facilitate biomaterial integration and native tissue regeneration at the implant site. During the last four decades, various materials known as ‘bioactive materials’ such as glasses, sintered hydroxyapatite, glass ceramics, composite materials, etc., have been synthesized and developed for medical applications. A significant characteristic of bioactive materials is their ability to bond with living bone through the formation of a hydroxyapatite (HA) interface layer. A recognized method to estimate the bone-bonding potential ability of material is simulated body fluid method (SBF), which involves immersing materials into SBF for bone-like apatite formation on its surface according to Kokubo et al. In other words, the behavior in vivo could be predicted by using SBF method in vitro. One remarkable success of bioactive ceramics as implant materials is the clinical use of sintered hydroxyapatite (HA) due to its bioactivity and osteoconductivity. However, the low fracture toughness of HA ceramic limits the scope of clinical applications. In recent years, more attentions have been focused on developing novel bioactive ceramics with improved properties. More recently, extensive interests have been shown in developing new bioactive inorganic materials containing CaO–SiO2 component for biomedical applications. Calcium silicate-based ceramics have received great attention as materials for bone tissue regeneration due to their excellent bioactivity. Compared to phosphate-based bioceramics, silicate bioceramics possess a wide range of chemical compositions and crystal structures, which contribute to their adjustable physicochemical properties, such as mechanical strength, bioactivity and degradation, providing them with suitable characteristics to be used as biomaterials. However, a major drawback of the CaSiO3 ceramics is their high dissolution rate, leading to a high pH value in the surrounding environment, which is detrimental to cells, which can be modified by incorporation of different elements such as Zn, Mg, Sr, Ti and Zr. In any case, the proposed approach can be extended to those more complex bioceramic compositions. In particular, due to the difficulties with sintering, silicate ceramics are generally obtained by complex techniques, such as the hydrothermal method, devitrification of glass, sol–gel processing, spark plasma-sintering, solution combustion processes etc. The sol–gel method is well suited for the preparation of complex ternary and quaternary silicate ceramics, as it allows for a precise control of the stoichiometry of the starting materials. However, it is of difficult industrialization, in the case of the fabrication of bulk components, because of the cost of the raw materials, the presence of large amounts of solvents and the associated drying problems. The current project is aiming at developing and fabricating of bioactive silicate-based ceramics from preceramic polymers (commercially available polymethylsiloxanes, silicones), and fillers (commercially available MgO, CaO, ZnO, TiO2, nano- and/or micro-particles), in the form of tablets, foams and 3D printed structures using additive manufacturing technology, to be used as bioactive scaffolds and biomaterials, thereby confirming that the proposed approach can be used to obtain components suitable for bone tissue regeneration. The incorporation of fillers, that generally can be passive or active, into the preceramic system is considered one of the most effective strategies to produce the silicate ceramics with different composition and structures as well as, to decrease the shrinkage and the formation of macro-defects in the produced ceramics. The approach of adding different oxide precursors (such as CaO and/or CaO, MgO and TiO2) as fillers enabled developing of different silicate bioactive ceramics (such as wollastonite (CaSiO3), hardystonite (Ca2ZnSi2O7), diopside (CaMgSi2O6) and sphene (CaTiSiO5)) as a result of the reactions between the preceramic polymers and these reactive fillers, occurring during the ceramization process and leading to the formation of specific crystalline phases with highly phase assemblage, that are known to be difficulty achievable by the conventional synthesis methods. A particular attention will be given to the production of open-celled porous components, to be employed as scaffolds for bone tissue engineering. These components will be prepared by various techniques, including unconventional direct foaming of silicone mixtures and additive manufacturing technology. Once the ceramic materials and scaffolds will be prepared, they will be fully characterized in terms of crystalline phase assemblage, physical and mechanical properties as well as microstructure analysis. The remarkable bioactivity of these scaffolds will be the main object of current investigations.