**3. Bio-applications**

Mg alloys are considered as suitable candidate materials for biomedical applications due to their mechanical properties and their confirmed biocompatibility. In biological environments, magnesium alloys biodegrade with kinetics that depend on the surrounding tissue, eliminating secondary surgical procedure of implant removal [53]. The desirable Young's modulus of Mg alloys (41–45 GPa), which is close to the cortical bone (3–20 GPa), and the excellent ability of the Mg ions to promote bone regeneration make them attractive as orthopedic implants [54]. Currently, researchers are underway to improve bioresorbable cardiovascular stents based on Mg alloys, which are designed to provide short-term supporting structures and to combat coronary heart and peripheral artery diseases [55].

Soluble magnesium ions (Mg2+), hydroxide ions (OH− ), and hydrogen gas (H2) are well known for being the primary magnesium corrosion products. Many studies have confirmed that Mg2+ ions are essential for living cells and the excess can be excreted in the urine without causing damage to excretory organs such as the liver or the kidney. However, the rapid corrosion rate of Mg-based alloys in physiological conditions promotes an intense hydrogen evolution [56]. Hydrogen gas is nontoxic and is easily diffusible, but excessive corrosion leads to the formation of undesirable gas bubbles (emphysema) in surrounding soft tissue. The rapid evolution of H2 bubbles can get accumulated and form gas pockets, leading to intensifying necrosis and inflammation within the living tissues [57]. On the other hand, depending on the type of the implant, the excessive corrosion leads to secondary problems. In the case of orthopedic implants, an excessive corrosion can produce early losing mechanical strength properties avoiding the implant assist the fracture of the bone firmly at least in the early healing stages (typically 12 weeks) [58]. Moreover, the uncontrollable and uneven degradation behavior for a vascular implant will produce huge amounts of hydrogen within a short time disfavoring the healing of neovascularization tissues which easily result in restenosis. Studies have shown that the critical period of vascular healing normally ends 3 months after implantation [58].

Both orthopedic and vascular magnesium implants look promising, but these drawbacks limit their applications. Thus, the use of Mg alloys as biodegradable implants is still in its infancy due to its high susceptibility to corrosion.

## **3.1 The effect of a silane coatings to control Mg alloy degradation**

Since, silane coatings have demonstrated excellent biocompatibility, favorable cellular adhesion, and proper protein absorption, they have been employed as biofunctional coatings to control the high in-vivo degradation rate of Mg and its alloys. It has also been reported that organofunctional silane coatings do not cause adverse tissue reactions, and the degradation product (Si(OH)4) produced into the body can be easily eliminated through the renal system. For this reason, some researchers have developed different compositions of organo-inorganic silane coatings for this

application. For instance, Gaur et al. [59] studied the effect of a phosphonatosilane coating, trying to improve the corrosion resistance of Mg-6Zn-Ca magnesium alloy in a physiological environment. In this study, the authors used a phosphonate (silane diethylphosphatoethyltriethoxysilane (DEPETES)) and bis sulfur silane (bis-[3- (triethoxysilyl) propyl] tetrasulfide (BTESPT)) precursors to synthetize the silane coating**,** considering that both precursors were found to be nontoxic. The in-vitro investigation showed that the silane coating provided significant and durable corrosion resistance. Moreover, the presence of hydrated magnesium phosphate was also identified after 216 h of immersion test in m-SBF; component reported to support osteoblast formation and tissue healing. Two years later, the same authors [60] reported the preparation of other silane coating composition obtained by using GPTMS and MTEOS to improve the in-vitro corrosion resistance and biocompatibility of the Mg6ZnCa alloy. The results demonstrated that the deposition of a silica coating obtained by combining both precursors slow down the dissolution of a biodegradable magnesium alloy in the early stages (280 h), enhancing cells growth on the coated specimen. Furthermore, the formation of magnesium/calcium phosphate on the surface of the Mg alloys after immersion time showed good bioactivity and osteo conductivity of the coating. The results suggested that the sol-gel coating developed for the Mg6ZnCa alloy is a promising solution for biomedical application such as bio-absorbable surgical skin staples (needs to be removed after 10–12 days of postsurgery), micro-clips (needs to degrade within 2 weeks), and pins used in fingers dislocation or fracture that are predicted to heal quickly.

To enhance the corrosion resistance of magnesium alloys, a modified epoxysilane coating obtained by using GPTMS and diethylenetriamine (DETA) as organic cross-linker was also proposed by Zomorodian et al. [61]. Although hydrogen evolution, pH, and in-vitro cell culture tests were not carried out, the EIS data showed an improvement of the corrosion resistance properties in Hank's solution associated with a dense and homogenous coating deposited on the Mg alloy. On the other hand, Castro et al. [62] also investigated the corrosion degradation rate of Mg alloys (AZ31B and AZ91D) by the deposition of two different silica sols prepared with and without colloidal silica particles for biodegradable implant materials. The results showed that the corrosion resistance behavior of Mg alloys, characterized in SBF using three different in-vitro tests: hydrogen evolution, pH variation, and potentiodynamic curves, enhanced after the deposition of the silane coating that contains nanoparticles as cross-linked network reinforcement (**Figure 6**).

Recent studies consider the development of double nano-composite coatings [63], based on the first deposition of Mg(OH)2 or MgO enriched oxide layer and a subsequent deposition of a silane sol-gel coating, to achieve longer corrosion protection systems. Dou et al. [64] prepared double composite coatings using a conventional micro-arc oxidation process, and then the sol-gel technique. The in-vitro degradation performance of the composite coatings showed an improvement of the corrosion resistance properties by reducing the corrosion current density.

Different approaches have been considered to improve the biocorrosion resistance of the Mg alloys for cardiovascular stent application since it is a disease with high mortality and an increasing incidence [65]. For example, Liu et al. [66] reported the use of layer-by-layer self-assembly technique, based on the deposition of a first APTES-based silane coating followed by the deposition of a graphene oxide (GO) suspension. The results showed that the silane/GO composite coating improves the corrosion and wear resistance of Mg alloy, suggesting its use in biomedical fields as a vascular stent.

*The Role of Silane Sol-Gel Coatings on the Corrosion Protection of Magnesium Alloys DOI: http://dx.doi.org/10.5772/intechopen.102085*

### **Figure 6.**

*Variation of hydrogen evolution as a function of immersion time in SBF solution for coated and uncoated AZ31B and AZ91D substrates; MTL coating corresponds to the silane coating that contains nanoparticles and TG to the silane coating without nanoparticles. (reprinted with permission from Ref. [62], Springer Nature).*

## **3.2 Factors affecting the bio-functionality of silane coatings**

## *3.2.1 Cyto-compatibility*

The biocompatibility of Mg alloys is determined by the toxicity of the released corrosion products and the interaction effect between metal surface and living tissues. Not all the studies mentioned in the previous section included in-vitro cell viability tests as complementary information, necessary to determine the response of the silane coatings deposited on Mg alloys.

Although AZ91D showed a better corrosion resistance performance with respect to AZ31B Mg alloys, the AZ91D shows lower biocompatibility and bioactivity due to its higher Al content. To improve the cyto-compatibility of AZ91 Mg alloys, Witecka et al. [67] studied the effect of the deposition of different silane coatings on its surface; ethyltriethoxysilane (S1), 3-aminopropyltriethoxysilane (S2), 3-isocyanatopyltriethoxysilane (S3), phenyltriethoxysilane (S4), and octadecyltriethoxysilane (S5). S1 was used to introduce a simple polysiloxane precursor to the substrate; S2 and S3 were selected to introduce positive and negative charges on the alloy surface, and finally, S4 and S5 were chosen to examine the π electrons and the long alkyl chain effect on the surface. Cell culture experiments showed that the cyto-compatibility was not affected by the surface modification. However, Silane S1 was the only system able to improve cell growth during 7 days of incubation. Because cyto-compatibility is a basic and important parameter in the design of silane coating to bio application, other strategies have been considered to improve the bioactivity of Mg alloys, the deposition of sol-gel derived bioactive glasses coatings being one of them. These coatings based on pure silica, SiO2-CaO-P2O5 or SiO2-CaO, have been shown the largest level of bioactivity based on their reaction rate and bone binding ability. The bone-bonding ability occurs through the development of a biological apatite layer when the materials are exposed

### **Figure 7.**

*Schematic representation of the deposition sol-gel glass-like bioactive sol for enhancing the implant performance of AZ91D magnesium alloy (reprinted with permission from [70]; Elsevier).*

to body fluids or simulated body fluids. In-vivo studies have shown that biological species are incorporated into the silica-rich and apatite layers. Consequently, coatings react with the physiological fluid for obtaining an adequate interfacial bonding with bone by forming hydroxyapatite layer (HA). Their main applications are focused on bone repair and regeneration in the field of tissue engineering. Regarding the synthesis, some attempts have been made to obtain bio-glass silane coatings [68, 69]. Recently, Omar et al. [70] synthesized two compositions of bioactive silica-glasses, 58S and 68S, by using tetramethyl orthosilicate (TMOS), methyltriethoxysilane (MTES) and calcium l-lactate hydrate (**Figure 7**).

The lactate was used to avoid the use of calcium nitrate as a precursor due to the presence of nitrate residuals in the coatings is not beneficial to the body. The results showed that both coatings showed a quick apatite formation, good corrosion resistance properties, good cell adhesion, and proliferation, representing a promising coating system for degradable AZ91D implants.

Other strategies considered in the synthesis of silane-coatings to potentially improve the biocompatibility of Mg alloys consisted of the incorporation of hydroxyapatite nanoparticles. Nikbakht et al. [71] synthetized a modified silane coating with hydroxyapatite nanoparticles to promote biocompatibility and bone healing through producing calcium phosphorus-rich corrosion products. The results showed that a correct amount of hydroxyapatite nanoparticles not only helped to optimize the barrier properties of the silane coating, but also improved cell growth, especially the MG-63 osteoblastic.

## *3.2.2 Protein absorption-platelet adhesion*

The initial interaction of biomaterials with the biological environment is based on the absorption of protein on the surface and the interaction with ions and water molecules to form various reactive interfaces. Understanding protein adsorption mechanisms, kinetics, and thermodynamics are essential to improve the design of silane biocompatible coatings [72]. Appropriate protein adsorption on the modified Mg surface alloys is essential for application in bone tissue regeneration and the effective
