As a representative of the third generation of medical metals, Magnesium (Mg) alloy is considered as the most capable metal in the field of orthopedics with its exceptional mechanical properties, degradability and biocompatibility [1], [2], [3]. Mg is an important inorganic constituent of the human body, usually stored in bone as Mg3(PO4)2 and MgCO3, in addition to being present in the cellular and humoral environment in ionic and other forms [4]. The normal daily intake of Mg in average adult is about 300–400 mg, and excess Mg can be excreted through the body's metabolic pathways [5]. As an orthopedic implant, the Mg ions released during the Mg alloy degradation process could effectively promote new bone and blood vessel regeneration [6], [7]. The degradation also avoids the trauma of secondary removal of traditional metals.

Nevertheless, rapid degradation of normal Mg alloys has led to many disadvantages, such as: the destabilization of mechanical properties, the rapid increase of pH value, and the generation of H2 capsules [8], [9]. Currently, there are two main approaches to improve Mg alloys’ stability, namely, elemental mix and surface coating [2], [3], [10]. Binary or ternary Mg alloys are formed by the addition of conventional metal elements, rare earth elements, or inorganic non-metallic elements, which modifies the Mg alloy lattice size or creates a second phase of alloys. Surface coating technology mainly adopts some chemical-physical deposition, polymer gel coating and gluing techniques to delay the degradation and enhance the biocompatibility and tissue interface bonding mechanics [11], [12], [13]. In addition to passive barrier coatings, active anti-corrosion coating strategies, such as intelligent self-healing coatings, were also important in Mg alloy coating design [14], [15], [16], [17]. Such designs were capable of repairing corrosive coatings to extend the service time of Mg alloys.

Calcium (Ca) and Zinc (Zn) are essential metallic elements for the human body and play an important role in human physical activity[18], [19]. Numerous studies have confirmed that the addition of < 3% Zn and the addition of < 1% Ca were capable to delay degradation and enhance biological activity [20], [21]. However, Mg-Zn-Ca alloys still suffer from excessive degradation rates. In our preliminary work, we designed four Mg-2Zn-Ca alloys with different Ca contents (wt%: 0%, 0.2%, 0.5% and 1%) and investigated their degradation in three different degradation solutions (Hanks’ solution, phosphate buffer solution and saline solution) [22]. The results confirmed that the degradation rate of Mg-Zn-Ca alloys was slowest in Hanks’ solution and fastest in saline solution. A small amount of Ca didn’t accelerate the degradation rate and enhanced the mechanical properties of Mg-Zn-Ca alloys. In addition, physiological mineralization occurred along with the degradation of Mg-Zn-Ca alloys. A comprehensive analysis of Ca content on the degradation and mineralization behavior of the alloy is of great interest to study. Hydroxyapatite (HA) serves as the main inorganic component in bone [23]. Given the structural instability of HA, the human body seldom has pure HA, which is usually replaced by elements such as F-, Sr2+, and CO32-. Compared with HA, FHA has superior mechanical strength, denser structure, better thermal stability, as well as good bioactivity and biocompatibility [24].

In this study, it was proposed to deposit FHA onto Mg-Zn-Ca alloys with different Ca contents by electrodeposition, in order to analyze the properties of FHA on Mg-Zn-Ca surfaces, mainly in microstructure, deposition composition, degradation behavior, bond strength, corrosion behavior and related cellular behavior for evaluation.