"Halloysite nanotubes/hydroxyapatite nanocomposites as hard tissue substitutes: effect on the morphology, thermomechanical behavior and biological development of aliphatic polyesters and polymethacrylates"

Abstract

Bone pathology entails an important average of physical disability, being bone tissue regeneration one of the most actively researched fields in Tissue Engineering. Accordingly, large efforts are focused on the research of novel bioabsorbable materials as a prosthesis with stiffness values similar to that of the host tissue capable of fulfilling the requirements for bone fracture remodelling. This work is focused on studying mechanical properties and cell interaction as a function of the chemical structure and hydrophobicity of different bioabsorbable polymers compared with non-absorbable polymers. Hydroxyapatite (HA) and Halloysite nanotube (HNTs) were used as fillers in order to grant better cell attachment, proliferation and differentiation along with hydrophobicity behaviour. For this end, it was aimed to firstly monitor the effects of bioactive fillers on the structural properties of bioabsorbable Polycaprolactone (PCL). Thus, mechanical and thermal properties of PCL were studied by modifying the additivation percentage of the bioactive fillers HA and HNTs. This preliminary study allowed to understand the synergic effect among the polymeric matrix and the functional groups present on the additives chemical structure by establishing the additivation threshold and optimizing the additivation rate. In general terms, a noticeable improvement of mechanical properties was achieved with the simultaneous addition of the two fillers. Additionally, taking advantages of the HNTs nano-tubular shape, those were studied as drug carrier structures and their loading and release ability with curcumin was monitored. Knowing in one side that HA promotes the formation of a layer of new bone composed of biological apatite and collagen; and in the other side, that HA and HNTs alter hydrophobicity behaviour; morphological properties supplied by both fillers were studied and compared among different pairs of polymers with similar chemical natures but different hydrophobicity. Accordingly, the hydrophobic polyester PCL was modified by its blending with Polylactic acid (PLA) and combined with HA nanoparticles and HNTs. On the other hand, the hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) was copolymerized with ethyl methacrylate (EMA) and also combined with HA and HNTs. Thus, the effect of both fillers was studied on Hydroxyapatite nucleation, distribution of the fillers into the polymeric matrices and PCL/PLA degradation rate. Therefore, it was observed that introduction of HA in polymers with moderately hydrophilic character induce a higher rate of hydroxyapatite nucleation and a faster degradation rate. However, HNTs tends to form big aggregates when the hydrophilic character increases, driving to crack initiation sites and failure of the material. The completion of this study was accomplished by means of monitoring cell viability, proliferation, and morphology on the two pairs of polymers varying the polymer's chemical surface by blending hydrophilic and hydrophobic polymers, copolymerizing monomers of opposite natures, and/or loading the polymer matrix with nanoparticles such as HA or HNTs. Polymer surface wettability is known to affect cell attachment and can be enhanced by modifying the polymer with HA and HNT. In this way, improvement in cell viability with the addition of HA and HNTs was observed due to the generation of new reactive sites with Ca2+ and PO4 3? groups present in HA, and silanol groups (Si-OH) located at the surfaces of HNTs. Thus, it was concluded that on hydrophobic materials, due to a faster arrival rate of proteins, those compete for surface absorption driving to low interaction sites between cells and polymer surface showing a round shape. However, on hydrophilic materials, the highly solvated surface at initial stage limits protein arrival and allowed protein rearrangement and spreading over the surface promoting cell adhesion and proliferation with better cytoskeleton spreading.