3.5.3. Nanosheets

Recently, regenerative medicine focused on the nanosheets applications owing to their excellent biocompatibility and unique mechanical and physicochemical properties. Two-dimensional (2D) structures of nanosheets (e.g., 1–100 nm thickness) are characterized by a large surface-to-volume ratio, ultrathin structure, and enhanced mechanical strength, which can be substituted with a large number of functional biomolecules [259–261]. They express a greater ability to interact with polymers through hydrophobic interaction, Van der Waals force, physical adsorption, and electrostatic attraction. The mechanical strength and biocompatibility of scaffolds can be improved by combinations of nanosheets with ceramics polymers [262].

Nanosheets are categorized into monolayered hydroxide nanosheets (MLDHs), polymeric nanosheets, metallic nanosheets, and nonmetallic nanosheets. Metallic and nonmetallic nanosheets are used for tissue engineering. They have desirable features for tissue engineering, such as biocompatibility, mechanical strength, and photothermal and colloidal stability [78,145,263,264]. Molybdenum disulfide (MoS2), manganese dioxide (MnO2), and magnesium phosphate (MgPO4) are frequently used metallic compounds, while the commonly used non-metallic components include graphene (GN), graphene oxide (GO), and black phosphorus (BP) [262,265].

Graphene oxide nanosheets (GOns) have improved mechanical ability, a large surfaceto-volume ratio, protein adsorption, and biocompatibility, all of which are important properties required in tissue engineering [266]. Surface roughness, protein absorption, hydrophilicity, and cell adhesion can be improved by adding extracellular matrix (ECM) components such as Col and HAp to the above nanostructured composites [267].

Molybdenum disulfide (MoS2) nanosheets exhibit excellent mechanical properties, including 300 GPa Young's modulus, a tensile strength of over 23 GPa, and excellent elasticity [266,268]. Graphene nanosheets are mostly used to strengthen HAp scaffolds. After nanosheets were incorporated into the scaffolds, the elastic modulus of the composite was increased by 40% to 141 ± 8.50 GPa and the fracture toughness was increased by 80% to 1.06 ± 0.03 MPa [269]. Plasma spray was used to create a graphene nanosheet (GNS) reinforced HA on Ti6Al4V substrate. The resulting GNS/HA composite coating has increased strength and toughness, with ~32.3% and ~54.7%, increases in fracture toughness and indentation yield strength, respectively [68]. The composite coating's improved strength and toughness were attributed to synergetic toughening and strengthening mechanisms such as load transfer, graphene nanosheet (GNS) pull-out, GNS inter-layer sliding, crack branching, and GNS bridging. Moreover, the frequent crack deflection when a crack comes into contact with GNS could tailor the trade-off between strength and toughness through crack branching and GNS bridging [68].

#### 3.5.4. Fibers

Fibers in ceramic matrix composites (CMC) help increase the fracture toughness, due to their excellent mechanical properties [270]. Different types of fibers on the basis of length, i.e., particulates and fiber network, continuous fibers, and short fibers, can be used for the processing of ceramic matrix composites. For bridging by brittle short fibers, an increase in interfacial shear forces is observed until it either causes the particle to break or debond from the matrix. This interfacial debonding, when followed by the subsequent frictional pulling out process, has a great impact on the toughness of the material. Hull and Clyne (1996) expressed the fracture energy related to fibers pull-out with the following formula:

$$
\Delta G\_{PillL-OUT} = \int\_0^l \frac{N\pi r x^2 \tau\_i}{l} dx = \frac{V\_f l^2 \tau\_i}{3r} \tag{1}
$$

where *G* represents the interfacial shear strength, *r* is fiber radius, *l* is pull-out length, and *N* is the number of fibers per unit area.

In bioceramics, the mechanism for fiber reinforcement involves fiber bridging the crack after its appearance due to stress, impeding its further propagation. Furthermore, the frictional sliding of fibers against the matrix during pullout further consumes the applied force that results in increased fracture toughness. The addition of different types of fibers (e.g., carbon, e-glass, aramid, and polyglactin) increased the strength of bioceramics and resulted in an increase of approximately two orders of magnitude in the fracture work [271].

Carbon fibers are the preferred choice among researchers compared to all other types of fibers due to their high strength-to-weight ratio, thermophysical properties, sorption, and high elastic modulus [272]. Carbon fibers are crystalline filaments of carbon that have a regular hexagonal pattern of carbon sheets. Moreover, due to their inherent biocompatibility (in vivo and in vitro), they are extensively used in the production of artificial heart valves, purulent wounds, in the treatment of bone fractures, and for making bio composites. Carbon fibers are produced by high temperature conversion during the pyrolysis of carbon-rich precursors.

The fracture toughness of bioceramic composites can be increased by adding carbon fibers. A 300% increase in fracture toughness of alumina-single-walled carbon nanotubes (SWCNTs) composites was reported [221]. In another report, a 69% improvement in fracture toughness for silica-CNT composites by loading only 0.05 wt % CNTs was obtained [273]. A significant increase in the fracture toughness of BaTiO3-CNTs composites was described when loading 0.5, 1, and 3 wt %, respectively [274]. Wang also reported a moderate improvement of 15% for ZrB2-SiC-multi-walled carbon nanotube (MWCNT) nanocomposite (2 wt %) [275]. He successfully manufactured composites comprising of micrometer-sized carbon fibers (CFs) and also made biocompatible nanocrystalline calcium hydroxyapatite that contained carbon fibers by 1.0, 2.0, and 5.0 wt %. Moreover, he reported the manufacturing of a HAp-carbon fiber composite via hot pressing by using high

temperature, pressure, and argon atmosphere. The resulting bioceramic composite had improved fracture toughness and strength [276]. In another instance, the microabrasion resistance of carbon fiber based reinforced and non-reinforced hydroxyapatite was worked on. Commercial grade Hap and carbon fibers were used by hot pressing. The researchers used a temperature of 1000–1150 ◦C and 25 MPa pressure with 15 min pressing time in an argon atmosphere. Most researchers have used the microhardness indentation method to the measure fracture toughness (*K*Ic) of carbon-based bioceramic composites due to the small sample sizes [277].

A chemical treatment performed to activate the fiber surface to improve the adhesion adhesion with surrounding matrix concerned the conditioning of the fibers surface, using molecules such as carboxylic acid, sulfuric acid, nitric acid, alkali, formaldehyde, and isocyanate [276,278–281].

The main drawbacks associated with the use of fiber-reinforced bioceramics include the tendency of fibers to agglomerate due to their high Van-der-Walls forces of interaction among carbon particles and light weight, and the low interfacial adhesion between the fibers and the matrix. This tendency to agglomerate has obstructed their application in various fields. In this context, surface functionalization/modification processes that can reduce this agglomeration tendency and increase the fiber–bioceramics interfacial adhesion through covalent or ionic bonding were proposed [276]. Several functionalization strategies were reported for fibers, including wet oxidation (oxidation using potassium permanganate, hydrogen peroxide, sulfuric acid, nitric acid, etc.), dry oxidation (oxidation by using plasma, air, ozone, etc.), surface adsorption, and encapsulation [282].

The oxidation of carbon fibers can be carried out in both wet and dry conditions [282]. Strong acids, such as H2SO4, HNO3 or a mixture of the two with a strong oxidant, i.e., KMnO4, is used for the wet oxidation of CFs and ozone or reactive plasma is used for the oxidation of CFs in dry conditions. Wet oxidation is the most cost-effective process for the surface modification of CFs. A few studies have indicated that the addition/activation of some functional groups on CF surface favors the bonding between bioceramics and carbon fibers; in particular, defects caused by oxidants on the surface of carbon fibers are stabilized by bonding with hydroxyl (-OH) or carboxylic acid (COOH) [282,283].

### **4. Processing Approaches towards Ceramic Toughening**

Several approaches have been developed to improve the mechanical properties of bioceramics [26]. The accurate processing of toughened bioceramic composites involves many steps, from raw materials to the semi-finished processing, including the synthesis of powders, controlled drying, calcination, the debonding of organic components, the addition of second phases, and thermal sintering [284]. The intrinsic features of the ceramic powders significantly influence each physical (e.g., density, porosity), microstructural (e.g., shape of grains, grain size, grain boundaries), mechanical (e.g., strength, hardness, toughness, resistance to fatigue failure,), and chemical (e.g., dissolution, hydrolysis) property of the final bioceramic composite scaffold.

Essential criteria for the effective preparation and reinforcement of bioceramic composites are the homogeneous mixing of the matrix and reinforcement phase and a controlled particle size distribution to optimize the packing density of particles while avoiding agglomeration.

The preparation of powders involves several approaches, classified into dry and wet chemical methods. The formulation technique has a significant impact on surface characteristics, powder morphology, stoichiometry, and crystallinity. Dry methods involve three main types of chemical reactions: thermal decomposition, oxidation/reduction, and solid-state reactions. In contrast, various methods can be used for the liquid or wet reaction of bioceramic powders such as hydrothermal synthesis, precipitation, liquid drying, and sol–gel synthesis [24,285]. The preparation of bioceramic powders, in particular hydroxyapatite, mainly involves wet chemical methods, especially hydrothermal synthesis and solid-state reactions [286].

A promising approach for the toughening of bioceramics is the addition of carbon fibers into the matrix. The manipulation of carbon fibers can be performed by the solution powder mixing technique to prepare polymer–carbon composite materials [287] or biomimetic mineralization to improve the biocompatibility and bone inductivity [83,288].

The consolidation of bioceramic scaffolds is modulated by thermal treatments capable of improving the interactions among the particles. Sintering is a high-temperature treatment that can compact the ceramic particles of a pre-shaped *green body* or powder to consolidate a solid structure [289]. The major goal of sintering is the densification; fine and uniform microstructure and bioceramics are typically sintered at temperatures ranging from 500 to 1200 ◦C. The high temperature of sintering provides adequate energy to force material transport processes such as the migration of grain boundaries via the diffusion of atoms or evaporation–condensation phenomena, with the aim of reducing the superficial energy of ceramic particles and eliminating the pores [290]. The sintering can be performed in different atmospheres, including inert gas or air [291].

Semi-finished processing techniques for bioceramic composites involve a myriad of techniques, including hand layup, spray up, injection molding, resign transfer molding, compression molding, filament winding, and pultrusion, according to the type of filler (particles, whiskers, and fibers) [175,176].

A recently reported approach also explored the possibility to increase the toughness of bioceramics by introducing a large and controlled density of dislocations, thus leading to local plasticity [292]. It was observed that conventional sintering, the standard densification method for ceramics, actually yields ceramics virtually free of dislocations and dislocation sources. In other words, the brittleness of ceramics appears as merely a consequence of the established conventional production method.

#### **5. Conclusions**

The limited toughness of bioceramics highlights a relevant clinical need, especially when the regeneration of load-bearing bone portions is required. Despite the multitude of approaches that have been explored in the past decades, further research is still needed to improve the performance of sintered bioceramics for clinical use. In particular, fiber reinforcement is a promising approach, even though some critical issues still remain, mainly related to the achievement of a strong interface between fibers and the surrounding matrix and to the thermal fiber decomposition. In this respect, processes based on the activation of the fibers' surface or dislocation-toughening have been proposed and are promising for improving the reinforcement–matrix interface. Relevant research targets for material scientists in the future will be to focus on new forming processes that can generate reinforced ceramics with tailored porous architecture, thus enabling advanced applications in bone surgery.

**Author Contributions:** Conceptualization, M.D., S.S. and A.T.; writing—review and editing, Z.A., M.D.; supervision, S.S. and M.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

