Synthesis techniques, characterization and mechanical properties of natural derived hydroxyapatite scaffolds for bone implants: a review

Hydroxyapatite (HAp) with good mechanical properties is a promising material meant for a number of useful bids in dentistry and orthopedic for biomedical engineering applications for drug delivery, bone defect fillers, bone cements, etc. In this paper, a comprehensive review has been done, by reviewing different literatures related to synthesis techniques, mechanical properties and property testing, method of calcination and characterization of hydroxyapatite which are product of catfish and bovine bones. The discussion is in relations of the obligatory features vital to attain the best properties for the envisioned bid of bone graft. The process approaches that are capable of fabricating the essential microstructure and the ways to advance the mechanical properties of natural mined HAp are reviewed. The standard values for tensile strength were found to be within the range of 40–300 MPa, compressive strength was 400–900 MPa, while Elastic modulus was 80–120 GPa and fracture toughness was 0.6–1 MPa m1/2 (Ramesh et al. in Ceram Int 44(9):10525–10530, 2018; Landi et al. in J Eur Ceram Soc 20(14–15):2377–2387, 2000; Munar et al. in Dent Mater J 25(1):51–58, 2006). Also, the porosity range was 70–85% (Yang et al. in Am Ceram Soc Bull 89(2):24–32, 2010), density is 3.16 g/cm3 and relative density is 95–99.5% (Ramesh et al. 2018; Landi et al. 2000; Munar et al. 2006). The literature revealed that CaP ratio varies in relation to the source and sintering temperature. For example, for bovine bone, a CaP ratio of 1.7 (Mezahi et al. in J Therm Anal Calorim 95(1):21–29, 2009) and 1.65 (Barakat et al. in J Mater Process Technol 209(7):3408–3415, 2009) was obtained at 1100 °C and 750 °C respectively. Basic understanding on the effect of adding foreign material as a strengthening agent to the mechanical properties of HAp is ground factor for the development of new biomaterial (Natural hydroxyapatite, NHAp). Therefore, it is inferred that upon careful combination of main parameters such as compaction pressures, sintering temperatures, and sintering dwell times for production natural HAp (NHAp), mechanical properties can be enhanced.


Introduction
Concerning human body infirmity of restoring bone faults, the use of non-natural bone tissue has gotten much consideration in recent times. Novel group of bio-scaffolds are the suitable method for faulty bone tissue. Bone fracture at times emerge from accident, genetic or disease [1]. Numerous researches have been steered to produce porous biomaterials scaffold using different approaches for clinical use [2][3][4]. Biomaterials used in this category of application should be bioactive and made of synthetic or natural polymers, ceramics, and metals with sufficient porosity to ensure blood circulation inside human body using software and anatomical model [5][6][7][8][9][10]. Ceramic materials including hydroxyapatite (HAp) and others like biphasic calcium phosphate (CaP), bio-glasses (BG), and β tricalcium phosphate (βTCP) have been used in bone tissue engineering for implant [11][12][13][14]. Bioceramics are of great importance in tissue engineering. Bioceramics are ceramics materials used in tissue engineering for restoration, repair and replacement of bones [15]. Tissue engineering pushes for a material with elementary features of biocompatibility, biodegradability and ideal morphology that can serve for bone substitutes in the field of bone implants [16]. It have been proved in tissue engineering that hydroxyapatite (HAp) is the best sure system of calcium phosphate. This attest to its affinity for bone tissue and similarity to natural bone minerals, therefore, it is considered to be the most commonly used in bone restoration [17]. Compared with other materials like polymers and improve the new group of calcium silicate (CS) scaffold. Thus, the use of HAp in the repair and reconstruction of bones has been able to open up thrilling prospects for the orthopedic and dentistry over other methods. Therefore, the study on microstructural developments is imperative in developing materials with superior mechanical properties that is acceptable [43]. Gross et al. [44] applied Instrumented nanoindentation to determine the mechanical properties of plasma sprayed hydroxyapatite-coated implants from different commercial vendors. Abifarin et al. and Akpan et al. [21,45] in different studies agreed that HAp from Fish and Bovine bones have excellent characteristics of biocompatibility and biodegradability because of their physicochemical behaviours similar to that of normal human bone HAp.
Akpan et al. [46] optimized and evaluated HAp synthesised from from catfish bones to achieve optimum handling situations and workable instructive methods for the research of hydroxyapatite scaffolds for tissue engineering. Studies to improve the properties of HAp derived from natural sources have been widely researched in the past years by numerous researchers, essentially due to the fact that these materials are readily available, thereby reducing the cost of producing HAp which is very important in tissue engineering. Enough methods have been used to advance biomaterial mechanical properties such as compressive strength, fracture toughness, porosity and tensile strength [47][48][49][50][51][52].
For improved mechanical properties, from the open literature reviewed, no investigation has been made regarding the physical, mechanical and chemical properties of animal matrix of non-separated biowastes and catfish bones derived hydroxyapatite (% wt, as shown in the graphical abstract). The availability of these bones as the natural means of HAp makes the study interesting. Hence, this study aims to accomplish a review to improve the mechanical properties of HAp (made from Cow and fish bones, by mixing them in a proportion, see the graphical abstract)-processing, mechanical properties evaluation and characterization in order to produce a scaffold that will be use for the following biomedical applications, i.e. as bone defect fillers and tissue engineering scaffolds, implants coating, pins for anchoring tooth implant and drug delivery systems (Fig. 1).

Formation of hydroxyapatite
Hydroxyapatite (Fig. 2a) is a calcium phosphate mineral found in the human bone. The lattice-like ( Fig. 2b) organization of hydroxyapatite crystals accounts for the firmness of bones. Hydroxyapatite is commonly used in the fabrication of bone void plaster products that affords scaffolding for bone development to complement a parent or host bone in orthopedic techniques [54][55][56]. Hydroxyapatite which has chemical configuration of (Ca 10 (PO 4 ) 6 (OH) 2 ), is harmless, biocompatible, osteoconductive, non-immunogenic and gives undeviating ties with active materials. It also have Ca/P ratio of 1.66. Hydroxyapatite (HAp) can be produced from natural and synthetic sources [20,21,57,58]. Hence, both natural and synthesized types of HAp are largely used as bio-ceramic in form of particulates, coatings etc. for hard tissue repair. It has been extensively reported that HAp has like chemical constituents with the inorganic part in animal tissues and this has resulted in its preferred biocompatibility attributes such as harmlessness, no inflammatory effects etc. [19,20,59]. Sobczak et al. [60,61] reported that the procedure to produce HAp powders from wastes is interesting because of its economic value and environmental benefits and that the HAp is not easily overruled by human tissues due its natural likeness to human bone. Sahmani et al. [44], added titanium oxide (TiO2) nanoparticles to nanoclay (NC) composed with NaCl microparticles to build a porous scaffold using space holder procedure. Their result proposed that the scaffold ought to have an organized permeable structure besides its porosity size (> 80%) for improved cell seeding and the development of tissue.
Scaffold forms the base for bone growth and helps cell bond, development and variation in orthopedic operations. Producing scaffold with satisfactory porous geometry and right compressive strength is contentious amongst biomaterial engineers and medical orthopedic surgeons [63]. Aghdam [64]. Saber-Samandari & Gross investigated the fitness of nanoindentation to encompass the study of the crosssection to manifold points for a more comprehensive understanding into disparities through the thickness of the coating. Multiple indentations with a separation of 4 μm exhibited a transition within the thickness of the coating. Nanoindentation testing revealed a non-uniform stress state throughout the HAp coating. Micro-Raman Spectroscopy (MRS) (Fig. 3) revealed dehydroxylation of the powder during the traverse in the thermal zone. [65].
In another study by Saber-Samandari & Gross, nanoindentation was adopted to determine the hardness and elastic modulus on the surface of well spread solidified droplets at the hydroxyapatite coating surface. The Micro RMS (Fig. 4) and X-ray diffraction (Fig. 5) confirmed a larger degree of dehydroxylation for the smaller particles also showing a lower elastic modulus This shows the influence of particle size and possibly dehydroxylation of hydroxyapatite on the mechanical properties of the coating surface [66].

Preparation of hydroxyapatite from cow bone
The extraction of HAp from bovine bone have been stated by many researchers. The bone part shown in Fig. 6a is usually used because its likeness to human bone in HAp composition [45]. Ravaglioli and Krajewski reported that the extraction methods affects the Ca/P proportion, dimension, natures and crystalline segments of Ca-P of the extracted HAp [57,71]. Some researchers agreed that washing and removal of dirt, fats, protein, and additional constituents like marrows and spineless tissues should be done before extraction. The pretreatment involves boiling the bones in water for up to 8 h or more to eliminate organic constituents on the bone [21,22,72,73].
Other initial treatment method that are extensively adopted is cleaning the osteo with surfactant and alkali solutions to confiscate the soft tissues [74]. To have pure synthesized HAp, Absorption Spectrometry (AAS) is normally employed to investigate the concentration of heavy metals in the tap water specimen [21]. According to Abifarin et al. [21], the concentration in portions per million (ppm) of hefty metals like Copper (Cu), Iron (Fe) and Zinc (Zn) were in minor aggregates and is in agreement as claimed that concentrations of weighty metal ions trace in running water are commonly negligible [59,75].
Some methods like thermal decomposition, calcination or its combination with one or other methods have been employed to extract HAp from bovine bone [21,76]. In the calcination process, to remove some organic materials such as proteins, oils, fat, pathogen, etc., the bones are heated in the furnace at an elevated temperature of up to 1400 °C [76]. Abifarin et al. synthesized HAp from nonseparated animal bone. In his experiment, he cleaned the bones (Fig. 6a) with running water before boiling the samples for 3 h to deproteinize them. After boiling, there was 10CaO + K 2 HPO + 4H 2 O → Ca 10 PO 4 6 (OH) 2 + 12KOH another washing before drying at 150 °C for 8 h. Thereafter, it was made to powders and calcined underneath atmospheric state with an electric furnace at 900 °C at a ramp rate of 5 °C/min with 2 h of water-logged time and permitted to furnace cool. The samples at 900 °C presented high crystallinity of 86.15% deprived of impurities. The XRD (Fig. 7) reflections characteristics showed a hexagonal phase HAp. According to their result, at sintering temperatures range of 900-1100 °C for 2 h, the crystallinity of the HAp segments in the samples became extra apparent with growing shrill and thin reflections indicating the consequence of sintering proceeding the change of RB to pure HAp without the presence of decomposed products and the Ca/P proportion of 1.58 was obtained. The FT-IR information showed phosphate and hydroxyl peaks in the thermally treated samples and all the samples formed distinctive extending manners of O-H bands at about 3417 cm −1 which were noticed in all FT-IR spectra of HAp [21].
The process of combining alkaline heat treatment and calcination was employed towards mining HAp from pig bones. The pig bones were immersed in 4 M of sodium hydroxide (NaOH) at 100 °C for 24 h. The practice was  redefined again with a fresh NaOH solution and washed until the deposit attained a pH value of 7 and then dried. The samples were calcined at 800 °C and 1200 °C. But at 800 °C, a pure HAp phase was produced, while at 1200 °C, there was a mixture of HAp and CaO [77,78]. At calcination temperature of above 700 °C, there were traces of CaO [12]. The above method was used to extract 70-180 nm nanometer HAp from pig bones, but the shape of the particles was different (irregularly and plate-shaped HAp) [77,78]. The Ca/P ratios of both samples were 1.709 [78] and 1.72 [77], and the Ca/P ratio is above the stoichiometric ratio. Another method employed to extract HAp from bovine bones was combination of calcination and vibration grinding [72]. The calcination temperature was 800 °C for 3 h before the numerous grinding periods. Two grinding periods used were: ball milling (24 h) and vibration milling for altered intervals [72]. This combination of methods gave a needle-shaped HAp ( Fig. 8) with a diameter of less than 100 nm and a Ca/P ratio of 1.66 [72]. The calcination method can convert HAp into new calcium phosphate phases, such as β-TCP, at elevated temperatures. Though, alkaline heat treatment systems, ionic liquid pretreatment and enzymatic hydrolysis systems produce pure phase HAp with minor crystallinity likened to the calcination system. Generally, record of the ways used in this review study can yield nano-scale HAp with altered morphologies.

Preparation of hydroxyapatite from catfish bone
The increase in fish consumption all over the globe has caused major surge in fish remains creation in the nature of bones (Fig. 6b). The retrieval of fish scales and bones permits the production of HAp and decrease in fish remains in the fishery production [79]. The fish bone is filled with calcium, phosphate, and carbonate which makes it important for the mining of HAp [76]. Fish bone apatite is produced after processing the fish wastes through a series of steps (Table 2) [21,22,45]. The fish bones can be either mechanically-, enzymatically-, or thermally-treated before using them as an absorbent in the batch and column experiments [80]. With good technique, the mechanically and enzymatically treated fish bone apatite yielded greater dissolved organic carbon (DOC) and biochemical oxygen needs (BON) with concentrations than those produced by thermal treatments [80]. In the above study, mechanical treatment was done by eliminating the skin and organic measures of the fish by slicing, pressing, searing and hotair drying. Enzymatic treatment was done using an enzymatic digestion route that detached all bio organics [80]. Thermal treatment was done by assigning roughly 1 kg of untouched mechanically treated apatite in a muffle furnace for 24 h and heating it at a high temperature [80]. Just as in the preparation of HAp from bovine bone, pretreatment is also done before the production of HAp from fish bone. Soaking the bone in a small concentration of hydrochloric acid (0.1-1 M HCl) to deproteinize it and then wash in tap water and boiling has been used by different researchers to remove flesh and debris from the fish bone [34,79,[81][82][83][84]. Natural apatite from fish bone can be used as a supplementary for HAp for remediating aqueous heavy metals [85]. It was found that fish bone apatite has the capacity of removing various metals and radionuclides including Pb 2+ , Cu 2+ , Cd 2+ and Ni 2+ to below visible levels. Chromium removal by fish bone apatite from groundwater has been studied by Ozawa and Suzuki [62]. Lead, Pb removal by fish apatite has been investigated by Ozawa and Kanahara [86]. Wright et al. established that by using fish bone apatites, the remediation cost was $40 per 3,785,411.78 L of water per mg/L of metal, and for soils it was $20-$30/yd 3 . Thus, fish bone can serve as a costeffective substitute for HAP for use in PRBs employed for treating metal plumes [87].
The combined alkaline heat treatment and calcination used to obtain HAp showed that the HAp has pure crystallinity having 76.62 nm particle size and 1.62 Ca/P stoichiometric ratio. In this technique, fish scales were first of all soaked in 1 M of NaOH. Thereafter, calcium chloride dehydrate (CaCl 2 ·2H 2 O) was employed to treat it at 75 °C for the compensation of calcium absent in the fish scales. The treated fish scale is then heated at 800 °C under 1 h [79]. The calcination of HAp at 600-1000 °C produces pure HAp with high crystallinity. 1.63 Ca/P stoichiometric ratio was observed in the sample heated at 600 °C, and is a little bit below the HAp Ca/P stoichiometric ratio. Furthermore, from the elemental analysis, the sample heated at 600 °C showed that there are Magnesium, Mg ions in the sample, and these Mg ions act as micronutrients for tissue metabolism [81]. A thermally treated fish scales at 1000 °C, which was pulverized for 16 h with a pulverizer to obtain pure phase HAp with 1.71 Ca/P stoichiometric ratio. The morphology of the HAp structure shows an almost spherical shape having 76.62 nm particle size [88]. The crystals size of hydroxyapatite can be preserved when alkali heat treatment technique is employed, which tend to coalesce during heating. Hydroxyapatite with Ca/P proportion of 1.65 that is handy to the stoichiometric HAp has been produced by calcination, but a higher Ca/P stoichiometric ratio is observed for HAp derived through alkaline heat treatment [62]. The calcination and alkaline hydrolysis systems was used to mine HAp from fish bones at 900 °C for 5 h. Alkaline heat treatment technique is when the fish bones are soaked in 2 M NaOH and then heated around 250 °C for about 5 h. The precipitate is then cleaned and accustomed with water to a neutral pH, and dried afterwards. Nevertheless, the crystallinity of HAp produced by calcination is upper linked with the one produced when alkaline heat treatment is employed. When comparing alkaline heat treatment method, the HAp obtained by calcination has a larger particle size [89]. Also, HAp was extracted using alkaline heat treatment from fish scales, although in two stages. The samples were first treated using 5% of NaOH to soak them in the solution for 5 h. The treated samples were sieved and dried; after which the treated dehydrated samples were immersed again in 50% of NaOH solution for 1 h. The chemically treated residue was sieved and rinsed with deionized water to achieve neutral pH value, and was dried afterwards. This technique produced a highly pure nano HAp with a Ca/P stoichiometric ratio of 1.66. The crystallinity of HAp made by alkaline heat treatment ( Fig. 9) is smaller than that of HAp produced by calcination [84].
Ozawa and Suzuki [62] employed thermal decomposition system to formulate HAp from fish bone. First, the flesh on the fish was cleaned with the aid of a strong water jet after steaming in water for 1 h. The cleaned samples were later dehydrated and calcined at a temperature of 600-1300 °C in air. According to their result, at calcination temperature of 800-1200 °C, highly crystalline HAp was gotten and this HAp was seen cohabited with a tricalcium phosphate (TCP) gotten when the calcination temperature was 1300 °C.

Characterization of bovine and catfish bone derived hydroxyapatites
Hydroxyapatite characterization is necessary to investigate the material structure and properties. Characterization can be chemical or physical. The characterization of HAp is exceptionally proficient and appropriate to analyse HAp to meet the set standard for a broad range of application in biomedical. Akpan et al. [45] adopted the procedure used by Abifarin et al. [21] to clean the bovine and fish bones. The cleaned samples were flame cooked in an open air and subjected to sintering temperature of 900 °C for 2 h at a heating rate of 5 °C/min. Afterward, the produced HAp powders were pound in a steel mortar and sieved with a filter of 300 μm mesh. The SEM (Fig. 10) result for the hydroxyapatite derived from Catfish bone (CB) showed a typical flower-like microstructure which comprises flakes of petal-like alignment, while rod-shaped particles are manifest for the bovine bone (BB) hydroxyapatite. They reported that the images showed the presence of extra naked pores in the CB and the micro pore amounts decline with apparent density. The CB showed higher porosity (Fig. 11) than the BB. In this research, the FT-IR (Fig. 12) spectra attained from the CB and NB powders displayed a  Hosseinzadeh et al. [90] used the thermal decomposition method to extract HAp from bovine bone, in which the sample was heated at two varried temperatures (750 °C and 850 °C) for 6 h, and two unit dimensions (ie good (< 420 μm), HAp are rough particles (420-500 μm) at 750 °C and 850 °C respectively were obtained. From the XRD results, bone meal heated to 750 °C indicated unadulterated HAp, while at 850 °C, the binding of β-tricalcium phosphate (β-TCP) and HAp were detected. Due to the presence of β-TCP, the Ca/P ratio of the powder heated to 850 °C decreased to 1.5. Ayatollahi et al. [73] again used heat treatment to mined HAp from bovine bones and calcined it at 900 °C for 2 h. The calcined bones were ground by means of a high-energy planetary ball mill to shrink the size to 30 nm. The XRD outcomes gave unadulterated HAp phase at 900 °C.
Londo ~ no-Restrepo et al. [91] employed both calcination and hydrothermal methods to mine HAp from bovine bones. Hydrothermal technique is used to remove some protein and fat from the bones. Thereafter, the powder is calcined at a temperature range of 700-1100 °C. The morphology of HAp changes with the calcination temperature (for example, lopsided at 700 °C and hemispherical at 800 °C). Dehydroxylation of HAp was formed at temperatures above 800 °C. Sun et al. [74] also used calcination technique to produce HAp from cow bones through thermal treatment at 750 °C for 2 h. This process led to the realization of irregular shaped HAp with 20-100 μm size range and a Ca/P ratio > 1.67. Because there were some ions, namely; carbonate was exchanged within the HAp. The Ca/P ratio is usually above 1.67. The XRD pattern indicated that there was formation of HAp and β-TCP when thermally treated. Abifarin et al. [21] proposed a data set, which introduced the morphological characteristics, basic structure and functional groups of hydroxyapatites (HAp) synthesized from non-separated biological waste (animal bone) through an improved and convenient heat treatment method. The temperature reaches 1100 °C. The elemental configuration obtained by the weight and atomic analysis of all samples by EDX provided information  about the conversion of calcium to phosphate into apatite. The FTIR (Fig. 14) figures exhibited the phosphate and hydroxyl crests in the heat-treated samples, which produced a characteristic elongating mode of O-H band at almost 3417 cm −1 , which is discernable in all the FTIR spectra of HAp. Saber-Samandari et al. [92] used Micro-Raman Spectroscopy to show how the coating process affects the characteristics of hydroxyapatite. The HAp was sieved to a particle size of 30 ± 10 µm. Splats were heat-treated to crystallize any remaining amorphous phase. FTIR spectroscopy of the powder showed a clear hydroxyl vibration peak, at both 630 cm −1 and 3570 cm −1 (Fig. 15b). The second peak was located on a broad band between 2500 and 3600 cm −1 , showing asymmetrical and symmetrical extending vibrations of adsorbed water. The most intense absorption bands of hydroxyapatite were situated at 1029 cm −1 and 1092 cm −1 , matching to the asymmetric extending modes of PO4 3− , while the peaks at 961 and 471 cm − 1 derived from the symmetric stretching and bending mode of PO4 3 − , respectively, and this showed that Raman spectroscopy is more appropriate. The XRD of the powder showed a single-phase hydroxyapatite (Fig. 15a). The most concentrated hydroxyapatite (211) peak is located at 31.8 2θ.
Saber-Samandari et al. [93] studied, the morphology of solidified droplet "splats" using Scanning Electron Microscopy (SEM). The topography of splats sprayed onto substrates at room temperature (25 °C) and preheated to 100 °C and 300 °C was investigated (Fig. 16). The splat shape was found to be strongly dependent on substrate preheating temperature. A very early stage of re-crystallization was detected using Transmission Electron Microscopy (TEM) for splats deposited onto 300 °C preheated substrates. TEM in conjunction with Focused Ion Beam (FIB), revealed the splats' ultra-micro-structure. Splats deposited onto the substrate at 300 °C showed generally well-adherent inter faces. TEM dispersion (Fig. 17) in conjunction with Focused Ion Beam (FIB) (Fig. 18), revealed the splats' ultra-micro-structure.
Saber-Samandari & Gross [Q] characterized the deposits of calcium phosphate produced by thermal printing in terms of structure, topography and mechanical properties. Hydroxyapatite was molten and directed to (a) a titanium target in relative motion and (b) stationary titanium substrates preheated to 100 °C and 350 °C. Scanning electron microscopy showed round-like deposits, but high resolution profilometry measured the profile. Micro-Raman spectroscopy and X-ray diffraction characterized the surface for structure. The characterization and testing approach is useful for hemispherical deposits produced by printing, coatings (laser ablation, thermal spraying, simulated body fluid) and melt extrusion elements in scaffolds.

Mechanical properties of hydroxyapatite
The mechanical properties of the composite biomaterials, HAp is the most alike to those of the human bone tissue [94,95]. Therefore, their mechanical properties are of huge importance. Mechanical properties of some metals, ceramics and biomaterials classification are presented in Tables 3, 4 and 5 respectively. Also, some consideration in materials for biomedical application is shown in Table 6, while some organic and inorganic composite for biomedical application is given in Table 7. Among these biomaterials, hydroxyapatite is the most widely studied bioactive and biocompatible material. However, it has lower young's modulus and fracture toughness with brittle nature [21,45,96]. Hence, it is required to produce a biomaterial with good mechanical properties. As it relates to bio-ceramics, the mechanical properties that are of interest includes: Hardness testing, compressive strength evaluation, Fracture toughness testing, Young Modulus evaluation, Brittle index. Elastic modulus describes the stiffness of the bioceramic within the elastic range when tensile and compressive loads are applied [21,96,97]. The strength of bio-ceramics is an important mechanical property because they are brittle. Therefore, in this case, nano-indentation test is carried out to determine the elastic modulus of these nano materials. Hardness is quite important for comparing the properties of biomaterials. It is used for finding the suitability of the clinical importance and utilization of the biomaterials [96][97][98][99][100][101][102]. HAp in its bulk contains calcium and phosphate, having stoichiometric ratio 1.67 Ca/P. It has been determined that this ratio can effectively promotes bone regeneration [76]. Natural HAp in most cases does not give exact stoichiometric ratio due to the fact that the ratio is not chemically controlled. It also contains insufficient calcium or phosphorus [105]. Calcium position is the supreme vacancy in HAp, in which cations such as Na + , Mg 2 + and Al 3 + are replaced on calcium position, while carbonate ion replaces phosphate or hydroxide ion, and fluoride ion can replace hydroxide ion [105].
Some requirements are needed in the classification of any material for biomedical applications. First, the material should have good biocompatibility, which means that the material should not be alien to the host organism. Second, the material should have high biodegradability, meaning that the materials should be harmlessly degraded in the host organism to restore natural functions. Third, the material should have mechanical integrity so that the load-bearing structure can be replaced. Currently, many types of ceramics (zirconia, bioglass, alumina and hydroxyapatite), metals and metal alloys are adopted for load-bearing applications. Such as dental  replacements and replacements for bone connections or medical purposes [106]. Hence, biomaterials mechanical properties are essential most especially when applied in human load bearing area [107]. Among these biomaterials, hydroxyapatite (HAp) is the most widely studied biomedical material. An increasing research is geared towards its mechanical properties improvement because of it is brittle and low Young's modulus and fracture toughness [22,26]. To the researcher, the mechanical properties design data sets the standard for the material or product [17,71]. The heat treatment process and the working temperature can be used to alter the mechanical possessions. Some studies on the Ca/P ratio of HAp from different sources is shown Table 8 [108]. The present study intends to produce HAp from bovine and catfish bones before mixing them in different proportions (%wt) for further investigation to improve the mechanical properties of the produced HAp. Amirhossein et al. [108] produced natural nanohydroxyapatite through calcination of raw turkey femurbone waste powder at 850 °C after ball milling the powder. The compressive strength results (Fig. 19a) (Fig. 19b). The results revealed that the strength and hardness of the samples increased by increasing the sintering temperature up to 1150 °C.
Chen et al. [107] viewed the consequence of sintering temperature on the microstructure, densification, grain size/boundary, calcium/phosphorus (Ca/P) of biocompatible hydroxyapatite (HAp) ceramics prepared by cold isostatic pressing, i.e. the influence of ion ratio and mechanical properties. It was observed that as the sintering temperature rose from 900 to 1200 °C, the mechanical hardness increased from 166 kgf/mm 2 (about 1.63 GPa) to 497 kgf/mm 2 (about 4.87 GPa) and started to decline at 1300 °C.
Aarthy et al. [113] produced HAp from goat bone. The goat bone sample was subjected to thermal analysis (Fig. 20) in order to ascertain its thermal behaviour. They sintered the Vickers hardness (HV) values of the HAp samples at different temperatures which increases with increase in sintering temperature. The initial HV value was calculated to be 164.3 MPa at 1100 °C (HAp1), and increases to 169.5 MPa at 1200 °C (HAp2), 245.3 MPa at 1300 °C (HAp3) and lastly scopes the maximum of 301.3 MPa at 1400 °C (HAp4) (Fig. 21).
Foroughi et al. [114] produced HAp powder from bovine bones by heat treatment at 900 °C. Then he used the polyurethane squeegee duplication method to produce HAp scaffolds. In order to upturn the mechanical possessions of the stent, poly-3-hydroxybutyrate (P3HB) was coated on the stent for 30 s and 1 min. The compressive strength and moduli of the scaffold obtained with 50 wt% HAp are 1.46 and 21.27 MPa, respectively, which makes it suitable for biomedical engineering applications. Chang et al. [115] introduced nano-hydroxyapatite (nHAp) into a chitin and the result was dissolved in NaOH/urea aqueous solution at little temperature. He then prepared a new hybrid hydrogel, and considered its compressive strength. The compressive strength of the chitin/nHAP hybrid hydrogel (gel 2) was 274 kPa, which is higher than   [116] used a simple hydrothermal method to synthesize hydroxyapatite (HA)-graphene nanosheets (GNs) composite system in situ. It was observed that the reduced graphene oxide (rGO) enhanced hydroxyphosphorus Greystone nanotube (nHA) composite material. The amalgamation route was executed by hot isostatic persuasive (HIP) at 160 MPa. He studied the microhardness of 0 wt%, 0.5 wt%, 1 wt% and 1.5 wt% of the reduced graphene oxide (rGO) hydroxyapatite (HAp)-graphene nanosheets (GNs). He recorded the microhardness values (MPa) of (322 ± 8), (363 ± 5), (425 ± 4), (381 ± 7); and the fracture toughness (GPa) of (0.81 ± 0.05), (0.95 ± 0.03), (1.31 ± 0.07) and (1.51 ± 0.05), respectively. Kim et al. [117] shaped a chitosan composite scaffold with great strength and controllable pore structure through uniformly isolated nano-sized hydroxyapatite powder (HAp content increased to 70 wt%). The compressive strength was studied. The compressive strength Packaging, secondary processability  Obada et al. [120] executed a laboratory study on the mechanical properties of kaolin reinforced hydroxyapatite (K-HAp) made from animal osteo under varied sintering conditions. HAp particles were produced by a simple heat treatment way and reinforced with 15 wt% selected kaolin (HAp/15 wt% BK) using sol-gel means to mix them. The HAp and K-HAp-derived scaffolds are made by cold pressing at a compaction pressure of 500 Pa. In open air, the stent was sintered at 900 °C, 1000 °C and 1100 °C for a residence time of 2 h, and its mechanical properties was studied. At the supreme sintering temperature (1100 °C), the hardness of the bracket was 0.93 and 1.09 GPa. Under the compaction pressure of 500 Pa, the compressive strength of KHAp was 7.84 MPa, and the compressive strength of non-reinforced HAp matrix was 0.69 MPa. The research results proved that the mechanical properties of the synthetic kaolin-enhanced HAp are suitable for human trabecular bones compared to the scaffold produced at a low pressure of 500 Pa.
Saber-Samandari and Gross [121] studied the micromechanical properties of single crystal hydroxyapatite by nanoindentation (at the base and side) to investigate the hardness, elastic modulus and fracture toughness of the trivial samples. The hardness and elastic modulus were measured using the standard method projected by Oliver and Pharr [122]. The hardness and elastic modulus figures obtained revealed greater figures for the base (7.1  Saber-Samandari and Gross [123] in another study observed the crack resistance of Amorphous calcium phosphate (ACP) to assist know its activity in organic creatures and help in the proposal of CaP-based implants by satiating droplets to formulate a bulk sample and separate splats. The result proved that ACP has a lesser hardness and elastic modulus than HAp. Fracture toughness was established for fused silica (FS) to authenticate the method. In the experiment, a value of 0.6 ± 0.1 MPa m 1/2 was gotten for FS, whereas for the Sintered HAp (0.2 µm grain size) a fracture toughness of 0.9 ± 0.1 MPa m 1/2 was obtained. Likened to splats on the non-preheated substrate, a splat on the 100 °C preheated substrate has a lower toughness (0.31 ± 0.06 MPa m 1/2 ), and an even lower figure on the 350 °C substrate (0.17 ± 0.04 MPa m 1/2 ), Fig. 22.
Joneidi et al. [125] with space holder approaches conducted an experiment on bio-nanocomposite scaffold (BS). The response of the bio-nanocomposites buckle under load was examined with the aid of an Abaqus software. The results showed (Fig. 23) that the sample containing 15 wt% additives exhibited preferred mechanical property than the sample without additives. Similarly, the elastic modulus of the specimens rises twice with the introduction of additive (from 60 to 145 MPa). Likewise, the scaffold bio-nanocomposite did not come back to its unique state due to the elasticity of the ceramics and its deformation at the plastic region which was permanent and irreversible.
Obada et al. designed strontium (Sr) doped HAp through solution combustion. A low compaction pressure system was used for pelletizing the powders (Sr-HAp) to assess the physical and mechanical properties of the pellets. The hardness and fracture toughness were 0.38 GPa and 0.82 MPa m 1/2 and 0.20 GPa and 0.67 MPa m 1/2 for the doped and undoped Sr-HAp respectively. The SEM images suggested that strontium co-existed with the calcium ion in hydroxyapatite due to the presence of irregular bead like structures on a micro-scale. The Sr-HAp pellets were stable in Phosphate Buffer Saline solution [126].
Yang et al. [127] fabricated a novel co-dispersed Fe 3 O 4 -GO nanosystem through electrostatic self-assembly of positively charged Fe 3 O 4 (pFe 3 O 4 ) on negatively charged GO nanosheets. Afterward, the nanosystem was fused into poly L-lactic acid (PLLA) scaffolds constructed by means of selective laser sintering. Additionally, the nanosystem also revealed a harmonious improvement result on the mechanical properties of scaffolds, since the pFe 3 O 4 loaded on GO enhanced the effectiveness of stress transfer in matrix. The tensile stress and compressive strength of scaffolds were increased by 67.1% and 132%, respectively (Fig. 24a). The tensile stress and strain  of the 0NC scaffold was 8.2 MPa and 12.1%, respectively (Fig. 24a). Shuai et al. produced a mace-like nanosystem to advance their dispersion by in-situ development of Ag nanoparticles on cellulose nanocrystal (CNC@Ag-NS) and was later added into poly-L-lactide (PLLA) scaffolds. A homogeneously dispersed result was observed in the scaffold. Promisingly, the mechanical properties of the scaffolds were strongly higher than that of PLLA/CNC scaffolds, owing to the mace-like CNC@Ag nanosystem improved the load transfer efficiency in the scaffold [128].

Physical properties of hydroxyapatite
The integrity of HAp can be assessed based on its mechanical (as discussed above) and physical properties (density and porosity). The slope of mechanical characteristics for sintered HAp differs in density and strain [129][130][131]. Though mechanical reliability is vital, porosity of scaffolds is equally a serious influence. It is so because, porosity marks the transfer of nutrients, metabolic wastes, etc. in cell culture [132]. Basically, microstructures of HAp are shaped by a bunch of defectiveness which have different forms, dimensions and porosity [120]. Some of the physical properties of HAp are shown in Table 10 below, while  Table 11 displays the bio mechanical properties of bone. It has been formerly stated that porosities in the range of 40-90% supports osteointegration on the implant surface and boosted bond of implants with bone [133].
Joneidi et al. [125] produced a porous scaffold for bone substitute using space holder methods. It was observed that the specimen without additive brought down the porosity value because of rise it the mass percentage of the metallic stage. Correspondingly, the porosity of the specimens was in the range of 70-80%. Equally, Table 12 showed that as the MNPs was introduced to the based bioceramics the Poisson Ratio rises alike to the density value.
Obada et al. [120] produced HAp from Bovine bone. With Eq. 1 below, the the density of HAp determined, using 3.16 g/cm 3 as the theoretical density of HAp [20,135,136].
From the result, it was observed that the porosity (which varied from 46.9 ± 0.10 to 60.2 ± 0.76%) of sintered samples decreased with rising in sintering temperature. (2) Density (g∕cm3) = mass volume   Yang et al. [137] examined a novel nanophase ceramic for improved drug transporter. With space holder technique, the bulk of the study was the proposal of the scaffold to produce a plan with suitable porosity in the array of 70-85% alongside acceptable compressive strength, that was estimated by the scope of the cavity, bending properties, fusion of magnetite nanoparticles, porosity and rate of degradability.

Summary
A careful attention from different researches exposes the process of extracting HAp from bovine or catfish bone, standard methods of testing the mechanical properties and characterization techniques that guarantee acceptable HAp with improved mechanical and physical properties. This is shown in Fig. 4. Note that the cost of electricity tariff (62.33 Naira per kwh) in Nigeria today discourages the pretreatment process used by Abifarin et al. [21] and Obada et al. [22,120] as this will increase the cost of producing natural hydroxyapatite in large quantity. Therefore, burning in open with charcoal will bring down the cost of producing hydroxyapatite. It was observed that HAp can have a hexagonal shape. Also, HAp is brittle in nature, but its biodegradable characteristics makes it suitable for biomedical application [73].

Conclusion
Hydroxyapatite (HAp) with good mechanical properties are of great use and important applications in dentistry and orthopedic for biomedical engineering. This review paper chiefly ezzmphases on the use of bovine and catfish wastes to extract HAp. In this paper, the production steps of HAp from bovine and catfish wastes were classified as follows: soaking the bio-wastes in water, washing, boiling for hours, oven drying at certain temperature (example 150 °C), thermal stability analysis, calcination, crushing with mortar and pestle, and sieving with a sieve. The mechanical testing and characterization come and the above steps. Apatites have arrangements and properties comparable to human bones. In this review, there is a discussion on the treatment methods capable of producing hydroxyapatite with the desired mechanical and physical characteristics for the applications in tissue engineering scaffolds for bone implants. It was observed that the calcination method can convert HAp into other calcium phosphate phases, such as β-TCP, at high temperatures. However, it is concluded that the alkaline heat treatment methods, ionic liquid pretreatment and enzymatic hydrolysis methods produce pure phase HAp, but have lower crystallinity matched to the calcination routine. Record of the procedures used in this review study can produce nano-scale HAp with altered morphologies. The standard values for tensile strength were found to be within the range of 40-300 MPa, compressive strength was 400-900 MPa, while Elastic modulus was 80-120 GPa and fracture toughness was 0.6-1 MPa·m 1/2 [20,135,136]. Also, the porosity range was 70-85% [138], density is 3.16 g/cm 3 and relative density is 95-99.5% [20,71,95,104,136,[138][139][140][141][142][143][144][145]. The literature revealed that CaP ratio varies in relation to the source and sintering temperature. For example, for bovine bone, a CaP ratio of 1.7 [110] and 1.65 [111] was obtained at 1100 °C and 750 °C respectively. The inference is that if the main parameters of natural HAp (NHA) fabrication such as compaction pressure, sintering temperature and sintering residence time are carefully controlled, they may lead to mechanical enhancement of NHA-based bioceramics. It is desirable to make full use biogenic materials to derive HAp with optimal properties and proper synthesis methods in producing HAp for application tissue engineering to meet the increasing clinical requirements. With good technique, the mechanically and enzymatically treated fish bone apatite yielded higher dissolved organic carbon (DOC) and biochemical oxygen need (BON) concentrations than those produced by thermal treatments.
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