# Fabrication of a Promising Hierarchical Porous Surface on Titanium for Promoting biocompatibility
## 1 Introduction  
Metallic materials have been extensively applied in surgical implants [1; 2; 3; 4; 5]. Among them, titanium (Ti) and its alloys are widely adopted as metallic implants owing to their superior physical and chemical properties [6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. Ti and its alloys exhibit excellent corrosion (crevice corrosion and pitting) resistance. In addition, they are suitable for use in cooling water systems to clean polluted water. The excellent corrosion resistance of Ti and its alloys can be attributed to a dense oxide layer formation on their surface upon their exposure to atmospheric moisture [16; 17; 18]. This oxide film shows self-healing properties and is stable over a wide range of redox potentials. Moreover, the oxide layer is always present inoxidizing media such as human body fluids and can be reconstituted within milliseconds after being damaged [19]. Ti-based alloys are also resistant to chemical species associated with micro-organisms such as nitrites, ammonium, sulfides, and organo-sulfur compounds. They are also resistant to acidic conditions and aerobic activity of microbes. Hence, Ti and its alloys are promising materials for medical implants.  
The native oxide film also facilitates the adhesion and growth of micro and macro-foulants on the material surface. These microbes can form biofilms on the material surface and macro-organisms can improve the tissue healing efficiency of the implant material by reducing the healing time. However, amino acids and proteins in body fluids increase metal corrosion [20]. Ti-based alloys are not biotoxic. The release of metallic ions induces the osseeo/osteointegration and often results in the clinical failure of metallic implants. Although the release of metallic ions is harmful, metallic implants are suitable for clinical applications owing to their high strength. However, the anti-corrosion performance and biocompatibility properties of metallic implants should be promoted. The presence of a Ti oxide layer on the Ti-based implants is considered a prerequisite for their biocompatibility. This Ti oxide film induces the formation and nucleation of apatite. In addition, the formation of a porous Ti oxide layer on the Ti implant [10] caused bone ingrowth into the porous structure, inducing the implant to be morphologically fixed to bones as compared with other porous nanomaterials [21; 22].  
Recently, various techniques--including thermal oxidation, anodization, and sol-gel methods--have been extensively used to investigate for the formation of Ti oxide layers [23; 24]. Previous studies also reported that the formation of a nanoporous or hybrid porous (micro and nanoporous) Ti oxide layer on Ti surface not only enhanced cell adhesion and proliferation but also promoted the early stages of bone healing and osseointegration [10; 25]. Based on the improvement of Ti dental implant, herein, we developed a potential approach (hydrogen fluoride (HF) pretreatment and sodium hydroxide (NaOH) anodization) to form a hierarchical porous (micro and nanoporous) Ti oxide surface. The surface properties, microstructural characteristics, and cellular behaviors of the treated Ti specimens were investigated for dental and biomedical applications.