## 1 Introduction  
Titanium alloys are widely used in medical implants such as artificial implants, sensors, stents, etc. They offer outstanding biocompatibility which is a major factor in their successful usage [1]. However, owing to the complex environment in vivo, the surface properties of implants require further improvements. Surface modification can be used to polish the surface of the titanium alloy, without destroying its bulk characteristics [2, 3, 4]. Surface modification strategies vary based on the application, especially considering biofouling and tribology. Polymer-based modifications are usually performed on surfaces that are prone to biofouling [5]. The hydrophilic property of modified coatings is the main reason for the adsorption resistance of biomolecules [6], such as poly(ethylene glycol) [7, 8, 9], and poly(sodium styrene sulfonate) [10]. Operational wear on artificial joints inside the human body can lead to implant failure. Consequently, techniques to improve the tribological properties of titanium alloys have received significant attention, and surface modifications using inorganic, organic polymer, and composite coatings have been proposed. Inorganic coatings can improve the wear resistance owing to the increased hardness with the coatings. Surface modification techniques such as oxidation treatment [11, 12, 13, 14, 15] and spraying [16, 17] can provide hard inorganic coatings. Diamond-like carbon (DLC), TiN, oxide ceramic coatings, etc., have also been studied. A complex of two-dimensional carbon materials with hard ceramic coatings is the most conventionally used method to reduce friction [18]. However, the main disadvantage of this method is that the coating is eventually stripped away due to the stress difference between two materials. Consequently, oxide coatings have become increasingly attractive, as the formation process of oxide coatings increases the adhesion between the coating and the substrate [19]. To solve the coating stripping problem, several researchers have recommended organic polymer coatings as they are linked to the metal surface by covalent bonds and have excellent behavioral properties. Furthermore, polymer coatings offer better biocompatibility compared to inorganic coatings. Hydrophilic polymer poly (2-methoxylooxyethyl phosphorylcholine) (PMPC) brushes were grafted on to a porous metal surface and were found to significantly reduce the friction coefficient [20; 21]. [PERSON] used ultraviolet irradiation to graft PMPC on to the surface of a selective laser-melted titanium alloy. The modified alloy protected the implant from scratches at high loads [22]. In addition to its hydrophilic properties that prevent biofouling, poly(ethylene glycol) (PEG) has excellent tribological properties, and can be applied in water-based lubricants in the human body [23]. Poly(vinylphosphonic acid) (PVPA) coatings offer excellent friction reduction. The friction coefficient of PVPA coatings is less than 0.01, which indicates the superlubricity state [23; 24]. PVPA-modified titanium is velocity independent and can be used in different implants [25]. The layer-by-layer (LbL) assembly technique can be used to construct multilayer polymer films on titanium. Modified titanium with thicker polyelectrolyte multilayers has superior tribological properties [26]. Despite good anti-frictional properties, a critical disadvantage of soft polymer organic coatings is their poor bearing capacity. The synergy of inorganic and organic materials can help reduce their individual deficiencies. Biphasic structures consisting of both soft and hard substances can provide high load bearing capacity and low friction. However, for in vivo applications, several important factors must be investigated such as the influence of the inorganic coating morphology on polymer linkage, feature matching of the various materials, cooperation mechanism between materials, etc.