REPOSITORY > RESULTS

Abstract

Modified hydroxyapatite with antibacterial properties and high biocompatibility with mammalian
cells is very desirable in many biomedical applications. The increasing problem of
bacterial resistance against conventional antibiotics calls for investigation of alternative
antimicrobials, such as metal ions and nanomaterials. In this thesis, different modifications
of hydroxyapatite based on gallium and gold are explored to obtain new antibacterial
materials with low toxicity: incorporation of Ga3+ ions, attachment of Ga nanoparticles
to hydroxyapatite, attachment of functionalised gold nanoparticles on the Ga(III)-doped
hydroxyapatite and attachment of functionalised Ga/Au core/shell nanoparticles onto hydroxyapatite.
The physicochemical properties, ion-release characteristics in physiological
conditions, antibacterial performance against P. aeruginosa, E. coli and S. epidermidis,
and in-vitro cytotoxicity for mammalian cells (L929 mouse fibroblasts, IMR-90 human
lung fibroblasts and U2OS osteosarcoma cells) are presented. Incorporation of Ga(III)
into hydroxyapatite occurs in a thin amorphous surface layer of hydroxyapatite by substitution
of Ca2+. Different morphologies were obtained by adding Ga3+ ions at different
stages of the hydroxyapatite synthesis and different Ga(III) contents by varying the Ga3+
concentration. The release of Ga3+ ions from the Ga(III)-containing hydroxyapatite depended
on the overall amount of Ga in the material, while the biological response was
mediated by the composition of the amorphous surface layer. The mainly Ca-phosphate
amorphous surface layer resulted in a low antibacterial efficiency, while the mainly Gaphosphate
amorphous surface layer resulted in a high antibacterial efficiency, but also cytotoxicity.
The widest concentration range (from 0.3 g/L to at least 1 g/L) of antimicrobial
action against P. aeruginosa and low toxicity to mammalian cells was obtained at 4 wt%
of Ga(III) and by adding Ga3+ ions during transformation of the precursor octacalcium
phosphate/hydroxyapatite crystals to the final hydroxyapatite crystals. Nanocomposites
of Ga nanospheres on hydroxyapatite laths/plates with different Ga content (6 to 32 wt
%), different average Ga particle size (15 to 40 nm) and different width of Ga particle
size distribution were also prepared and two different synthesis methods were compared.
Nanocomposites with optimum properties were obtained by an ultrasonic emulsification
method without using any surfactants and the antibacterial action was found to be primarily
a result of the dissolution of the Ga nanoparticles and consequent local release of
Ga3+ ions. The nanocomposite containing 16 wt % of Ga NPs with diameters of 2211 nm
exhibited better antibacterial properties against P. aeruginosa and lower in-vitro cytotoxicity
for human lung fibroblasts IMR-90 and mouse fibroblasts L929 (efficient antibacterial
action and low toxicity from 0.1 to 1 g/L) than a comparable nanocomposite of hydroxyapatite
and silver nanoparticles (efficient antibacterial action and low toxicity from 0.2 to
0.25 g/L). Addition of a gallium component (incorporation of Ga(III) into the disordered
surface layer or attachment of Ga NPs) provided the hydroxyapatite with antibacterial
action against P. aeruginosa, but not against E. coli and S. epidermidis, whereas the
functionalised nanogold component provided the antibacterial properties against E. coli
and S. epidermidis but not against P. aeruginosa. Both antibacterial components were successfully combined in a nanocomposite composed of the optimum hydroxyapatite with
incorporated Ga3+ ions and Au nanoparticles functionalized with arginine attached to its
surface. This material is efficient against E. coli, S. epidermidis and P. aeruginosa and
has a wide concentration range of antibacterial efficiency with low cytotoxicity (from 0.2 to
0.75 g/L). Such nanocomposites with combined antimicrobial effect present an innovative
way for prevention and cure of bacterial infections and seem a promising human-friendly
alternative for antibiotics and Ag-based antibacterial materials in treatments of infections
by bacteria resistant to antibiotics.