Available in press in the journal Materials Today: Proceedings, scientists have synthesized nanometric hydroxyapatite-silica composite using the sol-gel technique.
Study: Sol-gel synthesis of a nanometric hydroxyapatite-silica composite. Image Credit: AgriTech/Shutterstock.com
Hydroxyapatite in Biomedical Research
The strength and rigidity of human bones and teeth are primarily provided by inorganic components.
One of the major contributors is hydroxyapatite Ca10(PO4)6(OH)2, which is a form of calcium phosphate. This compound has been reported to be highly biocompatible, bioactive, and to exhibit significant osteoconductivity.
Because of these properties, scientists have synthesized synthetic hydroxyapatite (HA) for many biomedical applications, such as dental implants, coating on metal prostheses for orthopedics, filling material for bone injuries or defects, and drug delivery.
HA possesses low mechanical properties compared to natural bones; therefore, it cannot be utilized as load-bearing implants. Additionally, its low dissolution rate and reduced reactivity decelerate the rate of bone-bonding after implantation.
Silica in the Development and Repair of Bones
Previous studies have indicated the role of silica (Si) in the biomineralization of many organisms such as coral and diatoms. Human bones also comprise many minerals, including Si.
Research has indicated that aqueous Si (Si(OH)4) is associated with the mineralization and remodeling process of bones and, the presence of Si on a material’s surface may increase osteogenesis (development of bones).
Prior in vivo studies have also indicated that silica-based materials increase the rate of bone tissue repair.
These studies reported that pure hydrated silica gel could promote apatite formation on its surface in an activated body fluid and silanol groups (SiOH) stimulate apatite nucleation.
Development of HA-Si Composite Using the Sol-gel Technique
Scientists envisioned that the development of HA-Si composite could overcome the shortcomings of synthetic HA in terms of low solubility and bioactivity.
In the new study, researchers have used the sol-gel technique to prepare the HA-Si composite. They performed calcination of the as-prepared powders both in electric ovens and microwaves.
Importantly, researchers characterized the HA-Si composite material and evaluated its bioactivity and acid dissolution capacity.
Calcination of the raw powder in an electric oven at 600 °C for 4 hours, exhibited the presence of pure HA, which was confirmed using an X-Ray Diffraction (XRD) tool.
XRD data further confirmed that all samples calcined in microwaves and electric ovens are of the same nanometre size.
The main advantage of the nanometric size of the synthesized HA-Si is that it resembles the size of biological apatite.
A Fourier Transform Infrared Spectroscopy (FT-IR) analysis of HA and HA-Si composite was conducted.
In HA-Si composite analysis, researchers indicated that the stretching and flexing bands of the SiO4 groups overlap with the bands (e.g., Si-O-Si bonds) due to the PO4 group.
The elemental analysis of HA-Si composite, by Energy Dispersive X-ray Spectroscopy (EDS), revealed that it consists of calcium (Ca), phosphorus (P), oxygen (O), and Si.
Researchers also determined the porosity of the newly synthesized nanocomposite. They revealed that all samples had average porosity. However, the porosities of microwave heated samples were marginally lower than those calcined by the electric oven.
The XRD data of the HA-Si composite exhibited that Si acts as a protective layer of HA nanoparticles.
HA is amorphous, which implies the total dissolution of HA. Additionally, XRD patterns revealed a complete dissolution of HA nanoparticles.
Partial dissolution of HA was observed in microwave calcined samples and also silanol groups.
A previous study also reported the synthesis of nanocomposite using HA particles and SiO2 via the TEOS method. However, calcination of synthesized HA power was not conducted in this study.
The combination of the sol-gel technique and microwave heating method adopted in the new study resulted in an efficient coating of HA nanoparticles by SiO2.
These powders can form silanol groups, which are known to be suitable nucleation sites for biological HA.
SEM analysis of HA-Si revealed deposition of small spherical particles of HA on the surfaces of the samples.
In vitro bioactivity tests were conducted on microwave-heated samples, which revealed average bioactivity.
Additionally, a marginal increase in the pH values suggests a low ion exchange rate between the samples and the simulated body fluid (SBF) solution. This result indicates a good coating of HA particles by silica.
In this study, the authors developed a composite material using HA and SiO2 via the sol-gel method.
The as-prepared method was calcined in an electric oven for 4 hours and microwaves for 20 minutes.
HA decomposition was not observed at all studied temperatures. Importantly, the microwave calcined samples showed partial dissolution of HA and exhibited the formation of silanol groups, which is an important attribute for biomedical application.
Herradi, S. et al. (2022) Sol-gel synthesis of a nanometric hydroxyapatite-silica composite. Materials Today: Proceedings. Available at: https://www.sciencedirect.com/science/article/pii/S2214785322004680?via%3Dihub