Editorial for the Special Issue on Bioceramic Composites

This Special Issue on bioceramic composites and its published papers, addressing a number of current topics from industry and academia, are intended to be a reference for students and scholars in the field of biomaterials science, giving an insight into challenges and research topics in the field bioceramic composites. Moreover, its aim is to stimulate the interest of young and experienced researchers introducing novel research topics as inspiration for future work.

At present, bioceramic composites have two wide areas of application in the biomedical field. The first is in load-bearing devices, such as the joints of hip, knee, and shoulder joint replacements, as well as in dental implants. In this field, zirconia–alumina composites have attracted a great deal of attention because of their superior mechanical behavior, and new compositions are under development. The other field where bioceramic composites are intensively investigated is bone regeneration. Particular emphasis is given to calcium phosphates and silicates, as well as to doping with bioactive ions, aiming to enhance osteogenic ability and bioresorbability. On the other hand, hybrid biopolymer/ceramic materials mimicking the complex composition and multiscale structure of bone tissue are a new class of biomimetic materials that are very promising in regenerative medicine.

The paper by Piconi and Sprio provides the reader with an overview of the state of the art of bioceramic composites in orthopedics [1]. Their development is placed in a historical perspective, and the characteristics of the different materials now on the market are outlined. These themes are further developed by Burger and Kiefer [2], who offer an exhaustive presentation on the issues that led to the production of BIOLOXdelta^®, the ceramic composite that is today’s golden standard in the bearings for total hip replacements. Furthermore, they demonstrate improved mechanical properties using ceramic composites made of zirconia stabilized by an yttria and ceria matrix with dispersed alumina and hexagonal platelets, and highlight their potential for use in dental applications. Moreover, they underline the need for special raw materials to achieve the expected behavior, and the relevance of the raw material processing and of the feedstock for injection molding, today representing the standard for production of dental implants.

The relevance of the physical–chemical characteristics of the stabilizing and toughening additives to zirconia, as well as of their concentrations, was investigated by Magnani et al., with special attention to their influence on the mechanics of toughening and, hence, on the mechanical properties [3]. The study of the in situ toughening mechanism induced by the tetragonal–monoclinic (t–m) transformation of zirconia allowed modeling the opposite effect played by the grain size and the tetragonality of the zirconia lattice on the mechanical properties. In this way, the design of materials with customized properties such as fracture toughness and bending strength is feasible, opening new perspectives for the development of high-performance zirconia composites for orthopedic implants with high hydrothermal resistance.

The hydrothermal resistance of zirconia is one of the key issues for its use as a biomaterial. The spontaneous transition into the monoclinic toughening tetragonal phase, enhanced in a wet environment, can lead to the degradation of the mechanical properties of the ceramic. This is a relevant issue for load-bearing zirconia–alumina composites, where a higher monoclinic content than expected from in vitro simulations has been observed. Notwithstanding the number of explanations formulated thus far, none have been fully satisfactory. The preferred method to quantify the amount of transformed monoclinic zirconia is Raman spectroscopy. Nevertheless, many sources of errors may negatively affect Raman results, leading to errors in their interpretation. This issue is addressed by Porporati et al. [4], who put in evidence the critical aspects associated with the used equation for the calculation of the volume fraction of the monoclinic phase and with the definition of the related calibration coefficients. Raman spectroscopy is a delicate procedure that does not offer consistent and unique results for the analysis of surface degradation, and the need for a standard is clearly put in evidence by this paper. Solarino et al. [5] highlight the clinical relevance of the improvement in the outcomes of total hip replacements thanks to the use of bioceramic composite components. Implants with composite components are especially suited for high-demand patients, with no material-related side effects, no ceramic fracture, and no mechanical loosening of the implant components. Clinical cases that support this conclusion are described, as well as a comprehensive discussion of the literature.

Tavoni et al. provide a comprehensive overview of the state of the art of calcium phosphate (CaP) biomaterials, widely accepted today to promote the regeneration of bone tissue [6]. They discuss the reported strategies to develop and optimize bioceramics while also highlighting future perspectives in the development of bioactive ceramic composites for bone tissue regeneration. The co-existence of various factors such as the bioactive chemical composition, nanostructure, and bone-like mechanical performance is a major problem with ceramics due to the need for sintering and the difficulty of achieving complex bone-mimicking 3D structures. This paper puts in evidence how several technologies for the manufacturing of a highly porous bioceramic-based scaffold from traditional methods (partial sintering, replica method, sacrificial templates, and direct foaming, as well as various 3D printing technologies) do not result in the expected outcomes. The future of bioactive and effective bone scaffolds is strongly related to the development of new approaches that can generate bone scaffolds endowed with bone-mimicking features yielding an effective regenerative ability. Besides the biological aspects, a relevant issue of bone scaffolds is their intrinsic brittleness. This limits their applications, especially in the case of large bone defects in load-bearing sites. The work conducted in the past to develop processes enhancing both the strength and toughness of bioceramics is reviewed by Abbas et al. [7]. To this aim, fiber reinforcement is a promising approach, although further work is necessary to improve the fiber–matrix interface and control thermal fiber decomposition.

Ahlhelm et al. [8] report the manufacturing of load-bearing scaffolds with improved mechanical behavior. The process set up to obtain this result consists in a combination of additive manufacturing and freeze foaming. In this way, they obtained complex-shaped structural composites that not only unite the structural features of a real bone (dense and porous sections) but also reach similar and improved compressive strengths. This process has the potential for the production of bone replacements suitable for a number of bone defects, especially long-bone and load-bearing positions.

Hybrid nanostructured materials obtained by biomineralization are a special class of composites for bone regeneration, consisting in organic (e.g., polymer) and inorganic (e.g., hydroxyapatite) components. The process of biomineralization is described in detail in the paper by Campodoni et al. [9], who devoted special attention to calcium biomineralization, a process that can lead to highly biomimetic and biocompatible materials resembling natural hard tissues such as bones and teeth. Hints are offered on the numerous applications of biomimetic materials, whose behavior can be finely tuned by changing the environmental conditions (e.g., pH), doping ions, and organic network. The technologies to obtain hybrid scaffolds combining the tunable macro/microporosity and osteoinductive properties of ceramic materials with the chemical/physical properties of biodegradable polymers are the subject of the comprehensive review by Ozcan et al. [10]. The porosity of the scaffolds can be tuned for optimum results using conventional and additive manufacturing techniques and, more recently, 3D and 4D printing. The authors put in evidence that, facing the growing demand, only a limited number of biodegradable materials are currently available for the manufacture of materials and composites, particularly by 3D printing techniques. The conclusion highlights the need for research to develop new biomaterials for hybrid composites with adjustable properties that can restore functionality at the application site, providing optimum printability, mechanical stability, and better integration with the host.

The need for specific feedstocks for 3D printing of ceramics is also addressed by Magnani et al. [3]. The feedstock they prepared allowed the production of an alumina-toughened zirconia (ATZ) dental implant starting from a blend of selected ceramic precursors and a photopolymeric resin. This demonstrates the effectiveness of additive manufacturing in the small-batch production of medical devices with complex shapes using bioceramic composites.