A biomaterial is a synthetic material used to replace part of a living system or a material meant to be in contact with living tissue. In this sense, biomaterials can be categorized into polymers, liposomes, micelles, dendrimers, and calcium phosphate (CaP) nanoparticles, where each will show a different type of bioactivity. Hydroxyapatite (HAP) has been the gold standard in the biomedical field due to its composition and similarity to human bone. HAP nanoparticles have been used as vehicles for delivery due to their affinity to DNA, proteins, several drugs, and proper release activity.
1. Interactions in Drug Delivery with HAP Nanoparticles
HAP nanoparticles have been used as vehicles for delivery due to their affinity to DNA, proteins, several drugs, and proper release activity (
Table 2)
[1]. Although there are no conclusions on which HAP nanostructure is more suitable for which kind of molecule, the adsorbed amount is a function of the functional groups interactions, surface area, porosity, pH, and the surrounding medium
[2][3]. Regarding the porosity and pore structure effect on nanoparticles’ ability to adsorb and retain the cargo, it is known that mesoporous inorganic materials with high pore volume and adequate pore size are able to adsorb higher amounts of therapeutic molecules and ensure a sustained release
[2][4]. Although the synthesis process influences pore size and structure, factors such as the amount and size of the pores are primarily a function of the composition of the raw materials and the sintering conditions. A sintering process up to 1350 °C maintains the HAP phase while promoting pore formations
[3]. However, sintering procedures at higher temperatures makes other phases such as TCP appear due to HAP decomposition. Where the TCP band appears at a sintering temperature of 1400 °C in a study by Sofronia et al.
[5]. Regarding the effect of particle size, it is known that for solid drug delivery systems, it has a strong impact on its dissolution and drug absorption. Given the large surface area of nanoparticles, an increase in the bioavailability of poorly soluble drugs can be seen in several studies
[6][7][8]. A study to correlate the influence of particle size specifically with HAP was performed by Rouahi et al., where they found that HAP nanopowders containing 100 nm particles adsorbed more proteins than 1 µm
[9]. This was directly attributed to the difference in superficial surface area, where smaller nanoparticles had higher superficial surface areas than micro-scale HAP particles. The results of these studies suggest that a higher superficial surface area leads to higher protein adsorption, which is also reported in other studies
[10].
2. Proteins
As it is essential to tailor the characteristics of HAP nanoparticles to control the affinity of the cargo with the delivery system, several authors have studied the relationship between different physicochemical properties and protein absorption on the surface
[11][12][13][14]. CaPs are able to adsorb more protein than other materials, as calcium and phosphate ions are present as preferential binding sites for proteins. Several authors studied the protein adsorption potential of HAP powders treated with heat, where it was found that there are two main correlations between the superficial surface area and the protein adsorption capacity of HAP: the higher the superficial surface area, the higher the protein adsorption; however, when HAP is sintered, intergranular microporosity is formed and less proteins can be adsorbed
[9][15]. In a study by Rouahi et al., an HAP powder with agglomerated granules and a low value of surface area due to the partial fusion of the particles was synthesized. For the FTIR characterization, although the HAP composition was confirmed, the formation of TCP was not observed even though the sample was treated with high heat, which is contrary to other studies displayed in
Table 1, where Sofronia et al. obtained TCP after sintering at 1400 °C
[5]. However, the carbonate peak located around 1500 cm
−1 disappeared after the heat treatment. Nevertheless, the authors explained that the heat treatment did affect the protein adsorption potential and that the slight difference between samples was due to the surface area values, where the original samples that were not sintered had higher surface area values than those that underwent heat treatment. Finally, the ceramics prepared from the sintered samples had higher microporosity and intergranular microporosity, which explained the higher values of cells attaching on the surface. According to the previous study, increasing the surface area results in higher protein adsorption. Furthermore, as the microporosity decreases, the lower the protein adsorption and cell attachment rates become
[5].
Table 1. Vibration frequencies in FTIR spectrums from different HAP samples sintered and treated at different temperatures.
43]. This allows the use of HAP delivery systems for the attachment of regulatory sequences and movement across the cell membrane. One of the main disadvantages of using HAP as a delivery system is that the sintering process can cause the agglomeration of particles, which in the case of gene delivery decreases its transfection efficacy. In a study by Han et al., well-dispersed HAP nanoparticles were obtained by a simple ultrasound-assisted precipitation method with the assistance of glycosaminoglycans
[44]. The nanocrystalline nature of the particles was confirmed by the broadening and merge of the three major peaks (211), (112), and (300) at around 2θ = 30, which are characteristic of HAP. They were also able to confirm the presence of carbonate ions due to the peaks at 603 and 567 cm
−1, which are phosphate bands appearing in different sites
[44]. The size for the rod-like particles was about 20 × 50 nm and had a zeta potential of −60.9 mV, which improved stability, as mentioned in previous sections. The authors also mentioned that the acoustic cavitation caused by the ultrasound processing dispersed the HAP nanoparticles. The addition of glycosaminoglycans improved the electrostatic interaction between their negatively charged groups and the calcium ions of HAP, resulting in the overall negative charge of HAP nanoparticles. A novel strategy for gene therapy involves the use of biominerals through the nucleation of HAP on a DNA template. The rationale behind this stems from the relation between DNA and HAP in other biological systems and a strong interaction between both materials. As a specific binding activity of HAP exists for DNA, these kinds of complexes are less susceptible to degradation by serum and nucleases
[45]. In a study by Bertran et al., DNA was encapsulated into HAP nanoparticles through the fabrication of nanocapsules and crystalline nanorods with DNA inside. The experiments suggested that HAP grew around the DNA matrix
[45].
Table 24. Drug delivery applications for HAP and its respective cargos.
|
Cargo |
Heat Treatment |
Size |
Potential |
SSA |
Porosity |
Pore Volume |
Morphology |
Amount Adsorbed |
Application |
Reference |
|
|
(°C) |
(nm) |
(mV ) |
(m2g−1) |
(%) |
(cm3g−1) |
(-) |
(mg) |
|
|
|
Fibrinogen |
|
|
|
- |
2.53 |
2.39 |
- |
Spheres |
2.93 mg/m2 |
|
|
|
Insulin |
80 overnight |
60 |
|
|
|
|
|
2.24 mg/m2 |
Diabetes |
[4] |
|
Col-I |
|
|
|
|
|
|
|
|
1.12 mg/m2 |
|
|
|
BSA |
1250 |
4 h |
1000 |
−37 |
0.9 |
micropores |
- |
Granules |
65.7 µg/mL |
- |
[9] |
|
BSA |
1000 |
15 h |
100 |
0 |
25.4 |
micropores |
- |
Granules |
78.3 µg/mL |
|
|
BSA |
600 |
3 h |
39 |
−0.55 |
40 |
- |
- |
- |
4.0 mg/m2 |
|
|
Proteins |
MG |
600 |
3 h |
39 |
−0.55 |
40 |
- |
- |
- |
1.0 µg/m2 |
Blood compatibility |
[20] |
BSA |
700 |
3 h |
43 |
−0.9 |
20 |
- |
- |
- |
9.8 mg/m2 |
|
MG |
700 |
3 h |
43 |
−0.9 |
20 |
- |
- |
- |
1.5 µg/m2 |
|
|
|
BSA |
600 |
4 h |
32 |
- |
73 |
- |
- |
|
89 µg/mg |
Delivery |
[19] |
|
BSA |
700 |
4 h |
36 |
- |
66 |
- |
- |
|
85 µg/mg |
|
|
|
Cyt c |
60 |
3 h |
60 × 30 |
−24 |
96 |
|
0.79 |
Rod |
60 µg/mg |
|
|
|
MGB |
|
|
|
|
|
|
|
|
43 µg/mg |
Delivery |
[17] |
|
BSA |
|
|
|
|
|
|
|
|
78 µg/mg |
|
|
Peptides |
APWHLSSQYSRT |
1350 |
1 h |
- |
- |
0.05 |
- |
- |
Granules |
1 nmol |
|
|
|
STLPIPHEFSRE |
|
|
- |
- |
|
|
|
|
2.4 nmol |
Delivery |
[15] |
|
VTKHLNQISQSY |
|
|
- |
- |
|
|
|
|
2.5 nmol |
|
|
Drugs |
Doxorubicin |
100 |
24 h |
400 × 600 |
- |
163.2 |
mesopores |
0.53 |
Oval |
3 × 10 5 mol/g |
Breast cancer |
[36] |
|
Ibuprofen |
1000 |
2 h |
79 |
- |
- |
- |
- |
Plates |
- |
Arthritis |
[38] |
|
Cisplatin |
80 |
93 × 29 |
- |
96.8 |
- |
- |
Plates |
2.4 mg/g |
Cancer |
[41] |
|
Ampicilin |
1200 |
1 h |
8–9 ×103 |
- |
- |
Mesopores |
|
Spheres |
6.5 mg/g |
Bacterial infection |
[42] |
DNA |
Fish sperm DNA |
80 |
1.5 h |
20 |
- |
- |
- |
- |
Spheres |
11 µg/mg |
Gene therapy |
[45] |
|
EGFP-N1 pDNA |
170 |
2 h |
40–60 |
- |
- |
- |
- |
Rod |
0.02 µg/ug |
Gene therapy |
[46] |
|
CDglyTK |
35 |
72 h |
23–34 |
+16.8 |
- |
- |
- |
Feather |
- |
Antitumor |
[47] |
Sintering Temperature |
- |
- |
60 °C |
600 °C |
950 °C |
1250 °C |
1350 °C |
Sample |
Natural HAP (cm−1) |
Si-HAP (cm−1) |
HAP-p (cm−1) |
HAP (cm−1) |
HAP (cm−1) |
HAP (cm−1) |
HAP (cm−1) |
|
1540 |
- |
1540 |
1462.48 |
1456 |
- |
1540 |
C-O |
1548 |
- |
- |
- |
- |
- |
- |
|
1418 |
- |
- |
1418.2 |
876 |
- |
- |
C=O |
1653 |
- |
1650 |
1621.94 |
- |
- |
- |
C-N |
1560 |
- |
1560 |
- |
- |
- |
- |
N-H |
1560 |
- |
1560 |
- |
- |
- |
- |
Peptide |
- |
- |
1400 |
- |
- |
- |
- |
P-O |
1087 |
1089 |
1020 |
1100 |
1091 |
1100 |
1087 |
O-H |
3569 |
3570 |
- |
3571 |
3572 |
3575 |
- |
SiO44− |
- |
881 |
- |
- |
- |
- |
- |
- |
498 |
- |
- |
- |
- |
- |
Reference |
[5] |
[16] |
[17] |
[18] |
[5] |
[9] |
[15] |