Table 3.

Studies on HAp synthesis from CB (HAp group) and its characterisation using various methods.

Study ID	Aim of the Study	The Methods			Finding/s	Limitation
		Preparation of CB	Experimental or Commercial Materials	Characterisation
[56]	To create a hydrogel composite with HAp-based gelatin and genipin as the crosslinking agent.	1. Researchers washed CB to remove odour and contaminants and then calcine. Phosphoric acid, sodium dodecyl sulphate (SDS), and ammonium hydroxide (AH) were added and dried. 2. Researchers created a gelatin hydrogel composite with HAp, crosslinked with genipin, and dried scaffolds (1 × 1 × 1 cm3).	Experimental materials	1. Swelling behaviour. 2. Degradation behaviour. 3. MTT cytotoxicity assay. 4. XRD, FTIR, SEM, TEM, and TGA.	1. The cylindrical and needle-shaped CB-HAp can be used as a filler when a hydrogel is formed using gelatin as the matrix. 2. Hydrogels with less than 1% HAp are biocompatible with low SDS and are ideal scaffolding materials.	Not mentioned.
[57]	To create HAp, nano-powders using CB, mussel shells (MSs), chicken eggshells (ESs), and synthetic bioinspired amorphous calcium carbonate ACC.	1. ESs, CBs, and MSs were washed, boiled, dried, ground, and sieved to remove particles larger than 300 µm 2. HAp of ESs, CBs, MSs, and ACC via wet mechano-synthesis with +(NH4)2HPO4 or H3PO4 was used to obtain CaP:1.67, then ball-milling and oven drying were performed for 24 h. 3. HAP uniaxial pressing and sintering produced ACC-800, ES-900, MS-1000, sHA-1100, CB-900, and CB-1100.	Commercial materials	1. XRD. 2. ICP/OES. 3. SEM. 4. Cytotoxicity test. 5. LDH cytotoxic assay. 6. Confocal analyses for cell adhesion.	1. Bioactive Ca/P nanomaterials can be produced by synthesising nanocrystalline HAp from CBs, ESs, MSs, and ACC and consolidating them between 800 and 1100 °C. 2. Mg2+ in ES-derived HA and Sr2+ in CB-derived HAp affect the crystalline phases in addition to Ca/P. 3. Materials produced with good cell adhesion qualities and no cytotoxic effect are appropriate for bone regeneration.	Not mentioned.
[58]	To prepare HAp nanocomposites (HAp-NC) using an oil bath-mediated synthesis method and study their mechanical and biological properties.	1. The CB lamellar part was cleaned with water, acetone, and ethanol to remove impurities and then dried in a hot air oven. The powder was milled with a high-energy ball mill and then 0.6 M of (NH4)2HPO4, pH 8 to 12, was added and mixed, washed, dried, and sintered. 2. Graphene oxide (GO), carbon nanotubes (CNTs), multi-walled carbon nanotubes (GONRs), and silver nanoparticle (Ag NP) nanocomposites were added with different concentrations of 1, 3, and 5 wt%.	Experimental material	1. XDR, FTIR, and SEM. 2. MTT assay. 3. Hemolysis. 4. Antimicrobial activity. 5. Vicker’s hardness test. 6. Bioactivity test. 7. Drug loading and release study.	1. Oil-path-synthesised HAp-NC with rod-like morphology, carbon, and Ag NPs increased crystallite and particle size but decreased hardness due to agglomeration above 5 wt% carbon. 2. Ag NP-containing nanocomposites have higher inhibition for E. coli and S. aureus. 3. The matrix’s sheet-like structure controls lidocaine release. 4. Biocompatibility may make this nanocomposite suitable for load-bearing biomedical applications.	Not mentioned.
[59]	To synthesise an injectable bone-active hydrogel containing hyaluronic acid (HA) and silk FIB to mimic the extracellular matrix (ECM).	1. Bombyx Mori cocoon dissolving yielded silk fibroin (FIB) solution. 2. CB-treated HT was heated to remove organic materials. Then, 0.6 M NH4H2PO4 was sealed in a Teflon lining, heated in a furnace, washed, and ground into a powder 3. CB HAp and hyaluronic solution were stirred, then sonicated, centrifuged, and oven-dried. 4. FIB-HA-HAp crosslinking was performed.	Experimental material	1. FTIR, XRD, TGA, and SEM. 2. DLS measured the size and Zeta-Sizer. 3. Viscosity analysed. 4. Biological and antibacterial tests.	1. Silk fibroin HA-HAp hydrogel is biocompatible and natural. 2. Porous, interconnected, and viscous HA-HAp-FIB cell communication promotes osteoblast attachment without cell toxicity. 3. HA-HA-FIB hydrogel outperforms other hydrogels in mechanical strength. 4. At 15 μg/mL concentration, S. aureus is inhibited more effectively than E. coli.	Not mentioned.
[60]	To investigate ball-milling biogenic CaCO3 sources such as mussel shells (MS), eggshells (ES), and CB in aqueous circumstances at low temperatures to produce HAp.	Researchers washed, boiled, dried, ground, and sifted the following ingredients using a centrifugal mill: (1) (NH4)2HPO4, with an initial pH of 8.5; (2) ammonium phosphate dibasic and hydroxide, with an initial pH of 13; and (3) H3PO4, with a starting pH of 3.2. After aqueous milling, the slurry was dried at different temperatures.	Experimental materials	1. XRD. 2. FTIR. 3. FE_SEM. 4. TGA. 5. ICP/OES.	1. Bone-like carbonate apatite synthesis was achieved. 2. CB bone aragonite rapidly converted to HAp in acidic aqueous environments. 2. The flake-like crystals were nanometric. 3. Powder may have the potential to be bone tissue engineering biomaterials.	Not mentioned.
[32]	To use Arrhenius kinetics to study how aragonite from CB is transformed into HAp based on temperature and reaction time.	1. To eliminate organic compounds from CB, heat at 350 °C for three hours and add 0.6 M NH4H2PO4 solution to achieve Ca/P = 1.67. 2. The teflon-lined stainless steel pressure vessel was heated from 140 to 220 °C for 20–48 h in an electric furnace, rinsed, and dried at 110 °C.	Experimental material	1. SEM. 2. XRD. 3. FTIR. 4. Crystallisation kinetics	1. Aragonite entirely became HAp over time. 2. The hydrothermal treatment temperature preserved aragonite’s interconnecting porous architecture, enhancing HA production. 3. One-dimensional development governed by diffusion is the best way to forecast HA crystallisation and the morphology gives a 3D structure for bone tissue creation.	Not mentioned.
[61]	To study the hydrothermal conversion of CB aragonite to HAp at 200 °C for 1–48 h.	1. CB pieces were heated to 350 °C for 3 h to eliminate organic matter, then 0.6 M of NH4H2PO4 solution was added to set Ca/P = 1.67. A 200 °C Teflon-lined stainless steel pressure vessel was employed. 2. Researchers washed and dried the HAP at 110 °C.	Experimental material	1. XDR, FTIR, SEM. 2. DSC-TGA. 3. Hg intrusion porosimeter to detect CB porosity.	1 Complete conversion of CB aragonite to HAp was achieved in 48 h with NH4H2PO4 at 200 °C. 2. Hydrothermal treatment converts aragonite to HAp while preserving interconnecting channels and maintaining a plate- and needle-like crystal morphology.	Not mentioned.

Abbreviations: ICP/OES, Inductively Coupled Plasma Optical Emission spectroscopy; TEM, transmission electron microscope; FESEM, field emission scanning electron microscopy; XRD, X-ray diffraction; TGA, thermal gravimetric analysis; SEM, scanning electron microscopy; FTIR, Fourier-transform infrared.