The advancement of three-dimensional (3D) printing techniques has introduced the concept innovation of orthopedic implants manufacturing. With the advent of 3D-printed Ti6Al4V implant (3DTi), several challenging clinical problems in the field of orthopedics are expected to be resolved, including the reconstruction of large segmental or complex-shaped bone defects [1], [2] and stress-shielding-induced adverse bone remodeling [3]. Meanwhile, the interconnected porous structures of 3DTi enable bone in-growth to enhance osseointegration [4]. In recent years, we have explored the clinical therapeutic effects of 3DTi. Results have proven the feasibility of maintaining the long-term stability of implant-bone complex [5], [6]. However, further work is required. Ideally, an orthopedic implant should possess excellent osteoconduction properties (supporting bone growth on the surface) as well as osteoinduction properties (stimulating osteoblast differentiation) [7]. However, the bioinert property of titanium significantly limits the osteoinduction. In addition, neovascularization is vital for osteogenesis and peri-implants osseointegration [8]. The porous architectures of 3DTi provide space for the in-growth of blood vessels. Meanwhile, the absence of stimulating factors usually leads to inadequate angiogenesis speed. Therefore, it is necessary to develop comprehensive strategies to further accelerate the osteogenesis and angiogenesis of 3DTi.

As previously described, the bone in-growth pattern of porous titanium alloy implants was mainly from the periphery towards central pores [9], [10]. The early improvement of osteoinduction of peripheral implants appears to be crucial for bone defect repair. In recent years, metallic ions (such as Cu2+, Mg2+ and Sr2+) have been widely accepted as solutions when regulating bone microenvironment [11], [12]. Among them, Cu2+ plays crucial roles in promoting angiogenesis and enhancing the osteogenic differentiation of bone marrow stromal cells (BMSCs) [13], [14]. Numerous Cu-doped implants were designed and the advantages of Cu in bone defect repair have been demonstrated [15], [16]. However, overabundance of Cu2+ involves in metabolic dysfunctions and the cytotoxicity of elevated concentrations of Cu2+ has caused much concern. Therefore, it is very important to have a tailored Cu-doping approach to realize sustained and safe ion release. Here, we proposed the Cu ion implantation onto the surface of 3DTi using the metal vapor vacuum arc (MEVVA) technique. The MEVVA technique was applied owing to its various advantages, including that it is a pure ion source that prevents the import of unexpected ingredients, controlled quantity and depth of implanted ions and the absence of interface and layer delamination [17]. To the best of our knowledge, we performed the first application of Cu ion implantation through MEVVA for the surface treatment of 3DTi. The influence of the quantities of implanted Cu ions on the cell viability, osteogenesis and angiogenesis was then evaluated.

A single strategy is usually inadequate to satisfy the need for functional improvement. The cell attachment and spreading on scaffolds are crucial for osteogenesis, which occurs much earlier than osteogenic differentiation. It is reported that treatment with Cu2+ is unhelpful for the proliferation of BMSCs even though it facilitates osteogenic differentiation [18]. Our recent experiment obtained similar results. Among the multiple factors influencing cell adhesion, the surface property plays an essential role except for material composition. Previously, we designed an omnidirectional ultraviolet radiator to achieve the ultraviolet (UV) photofunctionalization of 3DTi. The attachment and proliferation rate of BMSCs were significantly promoted after UV irradiation [19]. In fact, UV irradiation can reduce carbon contamination and enhance the surface hydrophilicity of 3DTi, which provides an appropriate microenvironment for cell attachment [20]. Nevertheless, it is necessary to determine whether a combination of Cu ion implantation and UV photofunctionalization can exert synergistic effects on the regulation of cell behavior.

In this study, we developed a functional 3DTi using Cu ion implantation combined with UV photofunctionalization. Subsequently, we confirmed its effects on improving cell attachment, osteogenesis and angiogenesis. In the long term, the novel functional 3DTi has good potential for clinical translation owing to its relative ease of fabrication and the need for fewer doping components which requires further clinical validation.