TABLE 2.

MAO treatment of titanium and its alloys with different electrolyte compositions.

Different electrolyte systems	Electrolyte composition	Substrate	Surface morphology	Results of XRD	Outcome	References
Silicate electrolyte system	Na2 (EDTA), CaO and Ca(H2PO4)2, H2O	Pure titanium	Porous microstructure, the pore size is around 1–5 μm	Anatase and rutile	Grows fast and corrodes fast in SBF solution	Zhang et al. (2008)
	Na2SiO3·9H2O, (NaPO3)6, NaAlO2	Ti6Al4V discs	Nano-scale TiO2 grains, of different size, ranging from several nm to tens nm	Rutile and a small amount of anatase TiO2	The adhesion strength of coating interface is found to be about 70 MPa	Wang et al. (2006)
	Sodium silicate (Na2SiO3·9H2O) and calcium glycerol phosphate (C3H7CaO6P)	Ti6Al4V alloys	Calcium phosphate electrolyte produces a thicker, more compact MAO layer than silicate	The silicate electrolyte consists of TiO2,SiO2, Ti3(PO4)4, TiP2O7, and the calcium phosphate electrolyte comprisingTiO2, CaO, CaTiO3, Ti3(PO4)4, TiP2O7 and Ca2P2O7	The CaP apatites can integrate with human bone tissue and promote bone growth	Wang et al. (2020)
Phosphate electrolyte system	(NaPO3)6–NaF–NaAlO2	Ti6Al4V alloy	As treatment duration increases, coating development slows and roughens	Anatase, rutile and AlPO4 phases	The adhesion strength of substrate/coating interface is about 40 MPa	Wang et al. (2004)
	β-glycerophosphate disodium salt pentahydrate and calcium acetate monohydrate	Pure titanium plates	Macro-porous, Ca- and P-containing titania-based films were formed on the titanium substrates	Rutile and anatase	Ca- and P-containing, micro-arc oxidized titanium implants have the capability to induce bone-like apatite	Song et al. (2004)
	CaCl2, KH2PO4	Pure Ti	MAO micro-arcs decrease when CaCl2 concentration increases, while nanocrystals grow	XRD patterns didn’t show anatase or rutile titania (TiO2) production	First, a single MAO coating procedure was proposed to generate crystalline HAP coatings on Ti substrates	Kim et al. (2007)
	Citric acid, ethylene diamine, and ammonium phosphate	Ti6Al4V alloy	An HA crystalline peak could not be detected by XRD	Coated with TiO2 film and hydroxyapatite	Improved bioactivity, cell adhesion, and viability while retaining film-substrate bonding	Hong et al. (2011)
	H2SO4-H3PO4	Pure titanium and Ti6Al4V	Ti6Al4V has a cortical morphology with irregular worm-like slots, unlike MAO/Ti		MAO films were successfully produced on pure Ti and Ti6Al4V materials at 180 V. MAO substantially improved the corrosion resistance of untreated materials	Fazel et al. (2015)
	Na3PO4 and K3PO4	Pure titanium	K3PO4 electrolyte’s oxide layer was rougher than Na3PO4’s	Anatase and rutile crystalline phases	Attachment and multiplication of osteoblast cells to K3PO4’s oxide layer were better than in Na3PO4	Jung et al. (2014)
Aluminate electrolyte system	Aluminate solution	Ti6Al4V alloy	After MAO treatment, Ti6Al4V substrate microstructure is unaltered and no hardening zone is identified	TiO2 rutile and TiAl2O5 compounds	Nanohardness and elastic modulus rise from coating surface to inside	Wenbin et al. (2002)
	NaAlO2 electrolyte	Pure titanium	Increasing NaAlO2 lowers micropores, increases the quantity and size of sintered disks, and roughens the surface	Mainly composed of TiO2, rutile and anatase	The surface of the coating is rough, and the corrosion rate first decreases and then increases	Ping et al. (2016)
Sodium tetraborate electrolyte system	Na2B4O7·10H2O	Pure titanium slices	Cortex-like layers with pores and slots	Mostly rutile	Cortex-like coatings with interior pores and slots are more wettable than volcanic coatings	Liu et al. (2013)
	Li2B4O7,Na2B4O7 and K2B4O7	Pure titanium disks	Novel “cortex-like” micro/nano dual-scale structured TiO2 coating	Rutile with a little anatase	Promotes stem cell adhesion, spreading, and differentiation, and leads to excellent osseointegration	Li et al. (2018)
Phytic acid	Phytic acid, KOH, EDTA-Na2, Ca(CH3COO)2	Ti6Al4V plates	Typically porous structure	Anatase- TiO2,rutile-TiO2 and perovskite-CaTiO3 phases	Porous TiO2 ceramic layer containing calcium and phosphate was prepared by MAO on Ti6Al4V alloy	Qiao et al. (2016)
	EDTA-ZnNa2, KOH, and phytic acid	Ti6Al4V plates	Typical porous structure	Anatase and rutile	MAO coating combines Zn and P, and phytic acid concentration impacts Zn and P content, which is beneficial	Wang et al. (2018b)
	NaOH and Na12Phy	Ti6Al4V	Typical porous structure and the pore size is about 3 μm in diameter	Anatase TiO2	MTT tests showed good biocompatibility	Zhang et al. (2015)
	Phytic acid	Ti6Al4V alloys	Porous structure with tiny micropores and great hydrophilicity	Rutile, anatase, TiP2O7 as well as some OH- groups	MC3T3-E1 Pre-osteoblasts had excellent cytocompatibility in viability, adhesion, proliferation and differentiation	Wang et al. (2017)