INEB THESIS DEFENSE: Nanohydroxyapatite-based antibacterial surfaces to prevent biofilm associated biomaterials bone infection

INEB PhD student Liliana Grenho defended her thesis on May 6th, 2015, at FEUP, Faculty of Engineering of the University of Porto. The thesis ios untitled "Nanohydroxyapatite-based antibacterial surfaces to prevent biofilm associated biomaterials bone infection" and was a work in the area of Biomedical Engineering. Supervised by Maria Pia Ferraz from Universidade Fernando Pessoa, that is also an INEB collaborator and co-supervised by Fernando Jorge Monteiro, professor at FEUP and Group Leader of the Biocomposite Group at INEB.

In orthopaedics, as in many other medical fields, the use of implants has greatly improved the quality of life for an increasing number of patients. Nowadays, orthopaedic implants are used routinely worldwide for fixation of long bone fractures and non-union defects, for correction and stabilization of spinal fractures and deformities, for joint replacement, and for other orthopaedic and maxillofacial applications. Despite the rapid evolution of implant technologies and bone grafting techniques, there is still a great demand for novel and more sophisticated synthetic materials for bone applications. Among biomaterials used for bone-related applications, hydroxyapatite (HA) has received considerable attention due to its excellent bioactive and osteoconductive properties as it bonds to bone and enhances bone tissue formation. In particular, synthetic HA with crystals within the nanometer range (nanoHA) has superior functional properties due to its biomimetic chemistry and morphology when compared to the mineral phase of bone. However, adhesion of microorganisms on biomaterials with subsequent formation of antibiotic-resistant biofilms is a critical factor in implant-related infections and it is currently regarded as the most severe and devastating complication associated to the use of biomaterials. In this context, the main purpose of this work was the development of nanoHA based-anti-infective biomaterials to prevent or treat implant colonisation and, subsequent biofilm formation. To address this goal both inorganic and organic approaches were investigated. Initially a composite that combines the favourable biological characteristics of nanoHA and, simultaneously, possesses antimicrobial activity as expressed by ZnO was synthesized as dense discs and the primary objective was to determine whether the size of ZnO particles (from the micrometer scale down to the nanometer range), when incorporated into nanoHA, was playing an important role in inhibiting bacterial growth. The materials had robust in vitro antibacterial activity, with an inversely proportional relationship between particle size and antibacterial effect. Accordingly, the challenge of the following step was the continued research on nanoHA-ZnO composites but this time for the production of three-dimensional and interconnected porous granules. The experimental evidence supports the view that nanoHA-ZnO granules exhibited antibacterial activity not only in vitro but also in vivo, and the interconnected porous scaffolds support tissue recovery, with the presence of blood vessels and tissue regeneration in vivo. Regarding the organic approaches, two bee-derived natural extracts, namely green and red propolis, were adsorbed on nanoHA substrates and their antibacterial effectiveness was observed through the reduction of bacterial growth and biofilm formation while being non-cytotoxic to fibroblast cells. In a slightly different approach, the activity of three novel imidazole derivatives to prevent Candida spp biofilm formation as well as to eradicate pre-formed Candida spp biofilms on nanoHA substrate was studied. The antifungal agents displayed strong inhibitory effect on biofilm development as potent in vitro activity against sessile cells within biofilm. Overall, this work reached the aimed results and presents promising strategies that could improve the capacity to prevent or eradicate biofilms in medical devices. Nevertheless, comparing the results obtained with the various materials tested in this work, nanoHA-ZnO porous granules take the best position as candidates for translational research from benchside to bedside.