Bone is an important part of the human musculoskeletal system. However, when a non-union fracture occurs, the bone is unable to heal and needs clinical treatment from orthopaedic surgeon. This is a major problem which affects both the young and old population. The current “gold standard” treatment remains autograft where new bone is regenerated from the patients’ own tissue. However, limited source of autograft is available and the operation cost required for harvesting of autograft is high. In the past 25 years, bone tissue engineering has been proposed as an alternative for the current treatments. Bone tissue engineering aims to create a bone healing response in a precise anatomic area so that the tissue formed is integrated structurally with the surrounding skeleton and possesses the required biomechanical properties. Various biodegradable polyesters have been evaluated for tissue engineering applications including poly(3-hydroxybutyrate-co-hydroxyvalerate) (PHBV) and polycaprolactone (PCL). They are biocompatible but their surfaces are not biologically active. As a biomaterials surface influences biomolecule adsorption and cell adhesion, it is important to optimize the surface properties for any potential use in bone tissue engineering applications. Different surface modification methods have been used to improve the surface properties of polymers and they have demonstrated different degree of success.
In the current study, gamma irradiation induced graft copolymerization was performed on both PHBV and PCL. For PHBV, different grafting parameters including monomer concentration, irradiation dose and the solvent were studied on solvent cast films. It was found that the grafting of AeMA onto PHBV was successful as evident from XPS, FTIR and water contact angle measurement. The grafting was found to be predominantly in the amorphous region of PHBV as evident from DSC characterization. It was also observed that the monomer conversion was incomplete in methanol from 1H NMR.
The surface properties of 2D spun-coated and solvent cast PCL were evaluated in detail as both substrates were used for surface modification studies. It was found that solvent cast PCL displayed a higher crystallinity and larger spherulite size as compared to the spun-coated PCL. This is attributed to the longer time for evaporation of solvent to produce the solvent cast PCL. In addition, the surface of the solvent cast PCL was found to be more hydrophobic and rougher as compared to the spun-coated PCL. The cell viability was found to be similar on both substrates after 3 days. AAc grafting was conducted on the solvent cast and spun-coated films. Different grafting parameters including monomer concentration,
irradiation dose, use of homopolymerization inhibitor and solvent were investigated on the solvent cast films. The grafting extent could be manipulated by changing the AAc concentration and/or the solvent as evident from XPS and FTIR. It was shown from the data obtained using Raman spectroscopy that grafting was predominantly observed at the edge of the solvent cast films when water was used as the solvent. However, in the presence of methanol (10 or 100 %) PAA was observed across the thickness of the grafted films. This led to a significant decrease in the elastic modulus of the films as evident from nanoindentation using AFM. AAc grafting on spun-coated PCL films showed a decrease in the water contact angle but an increase in the surface roughness. AeMA grafting on spun-coated PCL was shown to introduce amine functionalities as evident from XPS analysis. A 2-step surface modification procedure was carried out in which AeMA was grafted in the initial step followed by AAc grafting. Both functionalities were introduced using this method.
Surface modification of 3D PCL scaffolds fabricated using the thermally induced phase separation (TIPS) method was conducted and it was shown that AAc grafting in a mixed solvent system enabled surface modification throughout the scaffold as evident from toluidine blue dye staining and XPS analysis. Similar degree of modification was observed from the edge to the centre of a cross section. The compressive modulus of the scaffolds was shown to decrease by 20 % after irradiation and AAc grafting but the crystallinity remained similar. XPS analysis of the PCL scaffolds after two grafting steps revealed that AAc grafting was evident at the exterior and the interior of the scaffolds with a similar level of AAc grafting obtained across the diameter of a cross section. AeMA was shown to be grafted exclusively at the exterior of the scaffolds using water as the solvent.
The current study has shown that the surface properties of both PHBV and PCL can be modified using gamma irradiation induced grafting. In addition, site specific surface modification as well as surface modification throughout a PCL scaffold can be achieved using appropriate grafting conditions. The surface modified substrates will be useful in potential bone engineering applications.