**1. Introduction**

Sustainable development of modern biotechnology in the field of regenerative and reconstructive bone surgery is governed by the quality of the available biomaterials, a specific class of systems with a unique set of physico-chemical and mechanical characteristics (chemical inertness, microstructural diversity, mechanical strength, fracture resistance, and durability), as well as biocompatibility (non-toxicity, bio-inducivity, bio-conductivity, and bio-resistivity) [1,2]. The combination of such properties in one product is a challenging scientific and technological problem in the search for affordable raw materials and simple processing technologies to fabricate the final products.

Calcium monosilicate, β-wollastonite (CaSiO3), is actively studied at the moment due to its applications in traumatology, orthopedics, dentistry, maxillofacial surgery, and other areas of medicine for the recovery, replacement, and reconstruction of the damaged tissue in a living organism [1,3–14]. Owing to β-wollastonite's ability to activate the growth of the apatite layer on its surface due to the

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pronounced osteoconduction and bio-resorption via exchange of Ca2+ and SiO3 2− with the bioorganic medium, the material is promising as an artificial bone substitute [15,16]. In terms of chemical bonding, wollastonite ceramic is close to the inorganic bone matrix and has no toxic e ffect on the body. In addition, it is corrosion-resistant, thermally stable, and chemically inert or bioactive under long-term exposure in bioorganic environments [17]. In order to achieve maximum similarity to the functional and structural parameters of the bone tissue, β-wollastonite can be modified with hydroxyapatite (HAp), which is a complete analogue of a living bone [14,18,19].

Chemical composition is an important, but not the only characteristic, of bioceramics implants. Their uniqueness to a large extent is determined by a combination of structural and mechanical characteristics. Porous and mechanically stable ceramics represents a model of cellular "spongiose" material that serve as a matrix for ingrowth (implantation) of the bone tissue during osteointegration, a process of recovering lost tissular structures in the living organism in the presence of the implant. Osteointegration intensity depends on pores presented in the implant, particularly, on their size, quantity, and interconnectivity. The bio-integration process, which is based on the reproduction of osteogenic cells, requires large macropores, sized 100–135 μm, as well as on the thin submicron and nanosized pores that are commensurate with blood plasma proteins for their e ffective adsorption. Stability of such a matrix has to be optimal for the uniform distribution of mechanical load between natural and artificial bone that avoids the possibility of excess decomposition of the bone tissue [20,21]. Thus, it is obvious that load-resistant ceramics with a hierarchical pore size distribution is necessary for practical medicine.

From an implant manufacturing standpoint, fabrication of the β-wollastonite powder and its composites with the HAp-required characteristics and properties is not di fficult. This can be done by sol-gel, hydrothermal, and precipitative synthesis technologies [6,22–30]. These approaches are easy to implement and allow to vary the size and shape of the crystallites and to tailor the surface morphology. On the contrary, to manufacture the bulk ceramics of the required geometry, characteristics, and properties is far more challenging due to the rigid thermal conditions of the powder treatment during consolidation into dense ceramics. During the fabrication of wollastonite ceramics, most of the conventional sintering methods do not provide the preservation of the porous structure of the wollastonite [31] due to the negative impact of the heat treatment on the composition and structure of the final product. The reason lies in the phase instability of the HAp above 1000 ◦C, as well as the destruction of the porous volume and the activation of grain growth, which leads to distortion or destruction of the porous frame in the solid body and negatively a ffects the final properties of the biocomposite [14,32,33].

The problems in the synthesis of porous ceramics described above are solved by using the technology of spark plasma sintering (SPS) [34–38]. The unique mechanism of powder consolidation in this technology is based on the spark plasma current flowing through the sample under pressure, which provides a number of advantages over traditional methods, because in this case ceramic wollastonite with a tailored microstructure and exceptional mechanical characteristics is rapidly formed [39–41]. The structural strength of the SPS ceramics is achieved without the need for additional reinforcement components that contaminate the final product. Our early studies have identified the above-described prospects of SPS application for the synthesis of nanostructured bioceramic wollastonite [42–45] with its bioactive properties being assessed "in vivo" [46]. Additionally, these studies showed several original methods for developing a porous structure of the ceramics that is similar to the texture of bone tissue by introducing various porous templates. However, the technology of spark plasma sintering–reactive synthesis (SPS-RS) should be considered even more promising for the production of innovative ceramics. SPS-RS is based on the chemical interaction between the starting components of the sintering mixture under the influence of spark plasma, resulting in a new type of the final product [47–49]. This approach allows one to directly obtain di fferent materials with unique properties based on multi-component ceramic systems. The literature substantiates the advantages of SPS-RS compared to conventional SPS and hot pressing [41]. SPS-RS e fficiency for bioceramic

synthesis is based on the local character of the chemical interaction between the components of the reaction mixture (resulting in biocomponents) under the spark plasma heating, taking place on the interparticle contacts.

Such local heating favors the formation of the fine-crystal phases of the biocomponent, which will be more biologically compatible or resorbed depending on its chemical composition. Additionally, microlocal heating allows the reactions to proceed at lower temperatures; thus, not disrupting the metastability of the substances and increasing the limit of their thermodestruction. We have recently explored the feasibility of this approach for fabrication of ZrO2 ceramics containing the bioactive phosphate compounds obtained "in situ" under SPS-RS conditions [50]. For the synthesis of ceramic HAp-containing wollastonite, similar studies have not been conducted before.

In this regard, the work intends to study the way to obtain a crystalline ceramic HAp–wollastonite composite via solid phase transformation of amorphous xonotlite and "in situ" interaction of the reaction mixture (CaO and CaHPO4) under SPS conditions. Additionally, the way to tailor the porous structure of the ceramics using pore-forming templates has been investigated. Microbial tests were conducted to assess the possible risks of an infectious process caused by bacterial contamination of the ceramics.

The proposed non-standard SPS-RS approach can pave the way to fabrication of biocompatible ceramics for bone tissue engineering; thus, contributing another flexible strategy for the synthesis of biomaterials.

#### **2. Materials and Methods**
