A Review on the Use of Hydroxyapatite-Carbonaceous Structure Composites in Bone Replacement Materials for Strengthening Purposes

Abstract

Biomedical materials constitute a vast scientific research field, which is devoted to producing medical devices which aid in enhancing human life. In this field, there is an enormous demand for long-lasting implants and bone substitutes that avoid rejection issues whilst providing favourable bioactivity, osteoconductivity and robust mechanical properties. Hydroxyapatite (HAp)-based biomaterials possess a close chemical resemblance to the mineral phase of bone, which give rise to their excellent biocompatibility, so allowing for them to serve the purpose of a bone-substituting and osteoconductive scaffold. The biodegradability of HAp is low (Ksp ≈ 6.62 × 10^-126) as compared to other calcium phosphates materials, however they are known for their ability to develop bone-like apatite coatings on their surface for enhanced bone bonding. Despite its favourable bone regeneration properties, restrictions on the use of pure HAp ceramics in high load-bearing applications exist due to its inherently low mechanical properties (including low strength and fracture toughness, and poor wear resistance). Recent innovations in the field of bio-composites and nanoscience have reignited the investigation of utilising different carbonaceous materials for enhancing the mechanical properties of composites, including HAp-based bio-composites. Researchers have preferred carbonaceous materials with hydroxyapatite due to their inherent biocompatibility and good structural properties. It has been demonstrated that different structures of carbonaceous material can be used to improve the fracture toughness of HAp, as they can easily serve the purpose of being a second phase reinforcement, with the resulting composite still being a biocompatible material. Nanostructured carbonaceous structures, especially those in the form of fibres and sheets, were found to be very effective in increasing the fracture toughness values of HAp. Minor addition of CNTs (3 wt.%) has resulted in a more than 200% increase in fracture toughness of hydroxyapatite-nanorods/CNTs made using spark plasma sintering. This paper presents a current review of the research field of using different carbonaceous materials composited with hydroxyapatite with the intent being to produce high performance biomedically targeted materials.

Keywords: carbon; crack bridging; fracture; fracture mechanics; graphene; hydroxyapatite; nanotechnology; strengthening; toughening.

Figure 1

A microstructural representation of bone with size scales.

Figure 2

Picture depicting fracture mechanics as an integrated blend of applied stress, flaw size analysis, and fracture toughness.

Figure 3

A typical representation of strengthening and toughening by fibres; (A,C) represent two views of crack in a body being perturbed by fibres, (B,D) represents the see-through image of the body presented in A and C, for a better understanding of the reinforcement effects.

Figure 4

A series of SEM images of surface morphology of high temperature sintered bovine rib bone; (A) a particle of bovine bone-derived (CNF)/hydroxyapatite (HAp), (B,C) presents one of the interconnected pores, (C–E) presents surface imaging of a HAp particle & (F) depicts surface sub-micron cracks on the surface of HAp. These materials were generated by one of the review’s authors (Siddiqui).

Figure 5

A broad classification of Carbon and its structures (0D, 1D and 2D means zero dimensional, one dimensional and two dimensional, respectively).

Figure 6

Some important carbonaceous structures, (a) Graphene, (b) Fullerene C₆₀, (c) Diamond, (d) NanoCone, (e) Graphite, (f) Single-wall Carbon Nanotube, and (g) Multiwall Carbon Nanotube.

Figure 7

Scheme for making graphene oxide and reduced graphene oxide from graphite.

Figure 8

A general classification of several factors needs to be considered when designing an implant material.