**3.3 Composites**

The composites are a blend of ceramics-polymers, polymer-polymer; and can incorporate biomolecules, carbon nanotubes and metals [1, 3].

Various ceramics, polymers as well as composites are subjected to functionalization of their surfaces to improve their compatibility with the biological microenvironment. In order to modify the surface hydrophobicity of polymers, to make them more hydrophilic and biocompatible, various surface modification techniques are employed [30, 48].

Graft polymerization technique: Polymer grafting and graft polymerization are achieved by chemical, photochemical, plasma induced and enzymatic grafting methods [30].

Nanoindentation method: This method is used to increase roughness by micropatterning to promote cell adhesion, but it is difficult to implement on large scale [30].

Surface modification by self-assembled monolayer formation: Metal surface modified by ligands through metal ligand bond formation [30].

Corona discharge: Electrically induced stream of ionized air is bombarded on the polymeric surface resulting in generation of oxygen containing functional groups. As this method does not operate in vacuum, it is prone to contamination by local moisture and humidity [30].

Flame treatment: Bombardment of polymer surface with ionized air resulting in surface functionalization of top several layers of polymer with hydroxyl, aldehyde and carboxylic functional groups. Although the method increases printability, wettability and adhesiveness of the polymer surface, it reduces the optical clarity [30].

UV irradiation: UV irradiation of polymer results in generation of reactive sites and can initiate graft polymerization of bioactive molecules such as N-vinyl pyrrolidinone [30].

Wet Chemical treatment: treatment of polyethylene and polypropylene surfaces with concentrated acid such as chromic acid in presence potassium permanganate and concentrated sulfuric acid results in development of reactive oxygen of functional group. This method generates hazardous chemical waste and surface etching and is therefore difficult to scale up [30].

Plasma treatment: Plasma is a high energy state of matter in which gas is partially ionized into charged particles, electrons and neutral molecule. Such ions when bombarded on a polymer surface results in functionalization of molecules in contact [30].

Graphene coating: Graphene is a 1-atom thick film with a honeycomb structure and is composed of carbon atoms created by sp2 hybridization [10]. The materials of graphene family have been widely applied in diverse medical applications owing to their nanoscale size photoluminescence properties, large specific surface area

and anti-bacterial activity [49]. High elasticity and flexibility of graphene and its derivatives (graphene oxide (GO) and reduced graphene oxide (rGO)) presents them as a promising mechanical filler for biomaterials [34]. It is widely used as surface modification coating or dopant in scaffolds, to enhance biocompatibility and promote osteogenic differentiation of stem cells [44].
