Surface Modifications on Magnesium Alloys for Biomedical Applications: Comparison
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Magnesium (Mg) alloys have great potential in biomedical applications due to their incomparable properties regarding other metals, such as stainless steels, Co–Cr alloys, and titanium (Ti) alloys. However, when Mg engages with body fluids, its degradation rate increases, inhibiting the complete healing of bone tissue. For this reason, it has been necessary to implement protective coatings to control the rate of degradation. 

  • magnesium alloys
  • degradation
  • natural polymers
  • surface modification

1. Introduction

Implant devices are used to treat a fracture or osteoarthritis, and for other orthopedic fixations (spinal, knee, hip)[1]. In recent years, biodegradable alloy materials such as Fe, Zn, and Mg have received considerable attention due to their comparative advantages in the field of orthopedic implants concerning metals that are typically used (stainless steel, cobalt–chromium (Co–Cr) alloys, and Ti alloys)[2]. Biodegradable Mg-based alloys used as orthopedic implants stand out for their low density, their similar mechanical properties to native bone, and their excellent biocompatibility. Mg is present in several enzymatic reactions; it influences heart, neurological, and digestive functions, and it also promotes the growth of human bones. Under this perspective, Mg is considered an essential element for the biological functions of the human body and with great potential for its use in implants for osseointegration.
However, one of the major limitations of Mg alloys is their high corrosion rate compared to those reported for Zn and Fe[3]. Currently, most studies are focused on the control of its rapid degradation, where the use of ceramic coatings, polymeric coatings, or the combination of both (hybrid coatings) has been addressed. In the search for a coating not only as a layer to control implant degradation but also to contribute to the rapid generation of bone structures, the three most relevant concepts to consider are osteoinduction, osteoconduction, and osseointegration, according to its relationship with bone morphogenic proteins (BMP), bone growth factors, and direct bone anchorage, respectively. Conceptually, these three terms are defined as follows: osteoinduction refers to the process by which osteogenesis is induced, osteoconduction means that the bone grows on a surface, and osseointegration is defined as the contact interface between the living bone and implant, which is the direct anchorage of an implant by the formation of bony tissue around the implant[4].
It can be said that the success of a Mg-based implant is subject to the performance of the coating. In the case of Mg alloy implants for osseointegration, the coatings should be not only biocompatible but also biodegradable, osteoconductive, osteoinductive, and, if possible, with a porous structure similar to bones. Among the strategies to control its rapid degradation, most studies are focused on the use of ceramic coatings, polymeric coatings, or a combination of both (hybrid coatings). Generally, both inorganic coatings such as calcium phosphates (hydroxyapatite is the most common) and organic coatings (natural polymers) are biocompatible and biodegradable, have very good osseointegration, promote cell and bone growth on the surface of the implant, as well as repair and tissue growth. Ceramic coatings also offer chemical and mechanical properties close to the bone, with high solubility and bioresorbability, but, at the same time, they have poor mechanical properties (low tensile strength, low fracture toughness, and poor impact resistance) and brittle layers as their main drawbacks [5][6]. Brittle layers of ceramic could generate complications that lead to implant rejection due to poor osseointegration [7]. Deficiency at the bone–implant interface leads to aseptic loosening and can also generate implant debris, such as wear particles or coating detachments. In general, debris particles inhibit direct prosthesis–bone contact; cannot be easily phagocytosed, causing an inflammatory response; and are more susceptible to microbial colonization [8][9]. In this sense, an implanted device must retain its mechanical integrity until the bone tissue is recovered, and then gradually dissolve to be replaced by the osteogenic tissue [10].

2. Surface Modification

Mg implants must satisfy two fundamental characteristics to guarantee tissue healing: biocompatibility and resistance to degradation. Biocompatibility is a characteristic in which the material can be metabolized in the human body. An alternative to guarantee resistance to degradation is surface modification. It can be classified into three categories: chemical modification, physical modification, and a combination of these, which seeks the elimination of the shortcomings and supplementation of the advantages of the individual techniques; see Figure 21 [11].
Polymers 14 05297 g002 550
Figure 21. Methods of surface modification.

2.1. Chemical Modification

Conversion coatings are in situ chemical or electrochemical interactions of the metal scaffolds with the environment, which leads to the formation of a superficial layer of substrate metal oxides, chromates, phosphates, or other compounds that are chemically bonded to the surface [12][13]. These types of coatings are among the most cost-effective methods; therefore, they are widely used to provide a barrier between the metal and its environment [14].
Conversion coatings include chromates, phosphate/permanganate, and fluorides [13]. The content of chromates has been reported to cause environmental problems and human body toxicity [15]. Other techniques include alkaline treatment, acid etching, electron beam treatment, and micro-arc oxidation coating [11].

2.1.1. Fluoride Treatment

The fluoride chemical conversion process is considered a simple and low-cost technique to obtain protective coatings in Mg alloys [16]. Mg reacts with hydrofluoric acid (HF) to form MgF2 via a displacement reaction, forming a barrier coating on Mg with increased resistance to polarization [17]. Fluoride is an essential natural component in the human diet; it is required for normal dental and skeletal growth. Moreover, it is one of the few known agents that can stimulate osteoblast proliferation and increase new mineral deposition in cancellous bones  [16].
Several authors have studied biomedical coatings containing fluoride on Mg alloys. It has been observed that the coating process is usually performed by the immersion of Mg or Mg alloy substrates in 40% or 48% HF [18]. A chemical conversion treatment in HF was applied on the surface of an AZ31B Mg alloy to reduce its degradation rate. The biodegradation kinetics of the AZ31 alloy evaluated by EIS confirmed that the fluoride coating offered effective protection that delayed the degradation process [19][20]. Another important result was the corrosion products formed in the coated materials, which were compounds rich in calcium and phosphorus, necessary for bone health [19]. Furthermore, the bonding strength between the fluoride coating and AZ31B substrate was over 43.2 MPa, while the minimum bonding strength of 22 MPa is required for a coating on medical implants according to ASTM 1147-F [20].

2.1.2. Alkaline Treatment

Alkaline treatment consists of a passive layer coated on the Mg sample by using a NaOH solution [21]. It was reported that by immersing the implant surface in 1 M NaOH for 24 h, the reactivity of the surface was reduced due to the formation of a thin passive layer of Mg (OH)2 [22]. The research also investigated the corrosion behavior of heat- and alkali-treated magnesium samples in simulated body fluid (SBF). It showed that the mass of the samples remained almost constant during the 14-day tests, which implies that this treatment has good corrosion resistance. Moreover, the pH values increased slowly compared to those of the untreated sample [23].

2.2. Physical Modification

Physical modification techniques are less often used on metals such as Mg. They do not form chemical bonds between the substrate and the outer surface. The physical coatings are used to modify the structure by introducing physical processing techniques or new phases such as inorganic compounds and polymers. There are different techniques, such as laser modification, metal oxide implantation, organic coatings, and apatite coatings  [11].
One of the latest techniques used for surface modification is called directed plasma nano-synthesis (DPNS) and it seeks to provide a bioactive and bioresorbable interface for Mg foams. A study in which DPNS was used in open-porous Mg foams to modify the surface chemistries and topographies was published. It obtained a surface modification strategy that adjusts the interaction of the material and the environment without using a coating that affects the geometry and properties of the porous material [24].
A novel technique used before a natural polymer coating application is surface activation by ultraviolet ozone (VUV/O3). The fibroin-coated MgZnCa alloy prepared by O2 and VUV/O3 plasma activation showed improved storage properties and in vitro corrosion resistance compared to the bare MgZnCa alloy. Furthermore, it was confirmed that the structures prepared via the activation of VUV/O3 possessed enhanced biocompatibility, bioactivity, and biosafety in systematic cell adhesion and cytotoxicity experiments [25].

3. Natural Polymeric Coating

The use of coatings is effective in decelerating degradation and reducing hydrogen evolution [26]. Natural polymers are classified into proteins (Coll, GEL, albumin, and silk fibroin), polysaccharides (CS, alginate, and cellulose), and weak organic acids. These polymers have been studied as coatings on the surface of Mg and its alloys because they provide barriers to limit direct contact between the surface of Mg and the aqueous biological environment [27]. Natural polymers usually contain biofunctional molecules that guarantee bioactivity [28] and non-immunogenic properties [29]. Another advantage of natural coatings is the ability to add functional properties to the implant surface, adapt the composition and structure of the coating [27], serve as templates for cell attachment and growth, and foment cellular responses; they have been utilized in biomedical fields (e.g., soft tissue engineering) for a long period [10]. Modified polymeric coatings can act as host reservoirs for corrosion inhibitors and can provide more effective active corrosion protection with self-healing properties [30]. 

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