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Titanium is popular in dentistry as a substitute material for the noble and non-noble alloys because of its superior biocompatibility, good mechanical properties and high corrosion resistance (Francescantonio et al., 2012; Al-Helou, 2014; Ramesh et al., 2016). Prostheses with titanium have been indicated for a metallic framework in fixed partial denture, full crowns, prostheses with multiple units, adhesive prostheses, root posts and framework for dentures (Schneider et al., 2007).

Although titanium has many advantages, there are concerns regarding the weak bonding between luting materials and cast titanium when luting cements are indicated (Kılıçarslan et al., 2016). Many types of surface treatment have been proposed to improve the bond strength of titanium to cement. These treatments include sandblasting, silica coating, acid etching, and use of functional monomers (Veljee et al., 2015). However, the problems of achieving a durable and predictable bond between titanium and luting material still remain. Hence the development of surface modification is a real essential.

Recently, the adhesive strategies of the marine mussel adhesive proteins have attracted great attention owing to the amazing ability of mussels to adhere to various kinds of surfaces (Li et al., 2014). Natural adhesive proteins are found in the byssus threads of mussels, which they use to adhere to the surfaces of wet solids such as rocks to allow them to remain anchored despite strong waves. Mussel-inspired proteins have more adhesive potential than synthetic adhesives, as they have excellent tensile strength and flexibility. Mussel-inspired proteins can adhere to various surfaces such as glass, metal, plastics, and biological materials (Lee et al., 2014). Thus, this represents a very convenient and universal method for adding an organic layer to various materials including polymers, metals, and ceramics (Kang et al., 2013).

The current study was concerned with the bonding of resin cement to titanium surface and how we could improve the bond strength at the cement-titanium junction. Previous studies (Ho et al., 2015; Veljee et al., 2015) have used mechanical surface treatments to increase the bond strength at the cement‑titanium interface, but there are no studies, which have used of polydopamine coating treatments. Hence, the aim of this in vitro study was to use polydopamine coating treatment to increase the shear bond strength at the cement‑titanium interface reducing the chances of failure of the prosthesis.

Dopamine hydrochloride and Tris-HCl buffer were supplied by Sigma-Aldrich (USA). Ti6Al4V titanium alloy plates (diameter 10 mm × thickness 3 mm) were chosen as the substrates for modification.

The resin-based cements tested in this study are summarized in Table 1.

The strong bonding between titanium and resin plays an important role in the longevity of prosthesis. When titanium is used for a metallic substructure for restoration covered with resin with weak bond strength between layers, micro-leakage tends to appear on the metal-resin interface leading to failure of prosthodontic prostheses (Lee et al., 2009). To overcome these defects, sandblasting, electrolytic etching technique and chemical etching technique are used to obtain a micro-roughness on the metals. However, micro-leakage cannot be solved completely because all the techniques are based on mechanical bonding (Lee et al., 2009).

To solve the problem, pH-induced polymerization of dopamine have been introduced for surface modification (Lee et al., 2007).

Dopamine, which was inspired by the composition of adhesive proteins in mussels, was reported to form thin, surface-adherent polymer films onto virtually all material surfaces, in alkaline aqueous media. Only by a simple immersion of substrate in a dilute aqueous solution of dopamine, buffered to alkaline pH, could result in spontaneous deposition of a thin polydopamine film (Jiang et al., 2014). It has been widely used to prepare a variety of ad-layers, including self-assembled monolayers through deposition of long-chain molecular building blocks, multilayer films, surface modification of the inorganic materials, nano-capsules for drug delivery and bio-inert and bioactive surfaces via grafting of macromolecules (Wei et al., 2010).

Dopamine polymerization results in the formation of quinones by catechol oxidation or alkaline pH-induced oxidation. Quinones and their derivatives are highly chemically reactive; they are continuously oxidized, polymerized, or condensed via non-enzymatic processes, and are then processed by enzymes involved in melanin pigmentation (Jiang et al., 2014; Lee et al., 2014). The change in color of dopamine from light violet to dark green due to dopamine polymerization is shown in Fig. 1.

The formation of polydopamine coating on titanium has been confirmed by XPS analyses of the materials surfaces (Fig. 2). The absence of Ti2p peak from polydopamine-coated titanium surface indicates that the polydopamine coating completely has covered the titanium surface. More importantly, polydopamine coating exhibits strong reactivity towards various nucleophiles with amine and thiol groups due to the presence of quinone after polymerization. Thus, titanium surface containing large amounts of amine and thiol groups have been successfully conjugated through polydopamine coating for various applications, such as adhesion properties (Yu et al., 2013). In addition, when we analyzed the composition of the specimens using SEM, we observed irregularly-shaped polydopamine particles on the surfaces of the specimens coated with polydopamine.

When wettability was measured by adding water to the titanium specimens (Fig. 4), the contact angle of the polydopamine-coated specimen was 38.28°, lower than that of the non-coated titanium specimen. Lee et al., (2014) reported that increased wettability and decreased contact angle indicated a hydrophilic surface with a high surface energy, an excellent environment for adhesion properties (Yang et al., 2015). The surface contact angle has been recognized as one of the most governing and influencing parameters to control the bond characteristics, particularly the bond strengths, and the smaller the contact angle the higher the surface bond strength. Moreover, with a smaller contact angle, the bonding agent can penetrate easily into surface convex portions, resulting in a strengthening of the bond (Yoshida et al., 2005; Yang and Zhao, 2011). In this study, we observed that the specimens that underwent surface treatment had better wettability and stimulated greater shear bond strength than the non-modified specimens, which could result in increased adhesion properties of a material (Kang et al., 2013; Lee et al., 2015). Therefore, the approach described here may represent an enabling strategy to improve the performances of titanium by surface modification.

In summary, polydopamine-coated titanium was successfully prepared from a dopamine solution in 10 mM Tris-HCl buffer solution at pH 8.5. The formation of polydopamine coating on titanium has been confirmed by XPS analyses of the materials surfaces. The absence of Ti2p peak from polydopamine-coated titanium surface indicates that the polydopamine coating completely has covered the titanium surface. Using SEM, we determined that nanometer-sized bright granules were randomly distributed after coating. The static contact angles on non-coated titanium and polydopamine-coated titanium surfaces were 82.16° and 38.28°, respectively. Therefore, the findings from our research suggest that polydopamine coating results in greater hydrophilicity of titanium surface and enhancement of adhesion properties.