Bioinspired Surface Design for Magnesium Alloys with Corrosion Resistance

Abstract

Magnesium alloys are regarded as potential candidates in industrial and biomedical applications because of their excellent mechanical properties and biodegradability. However, the excessive degradation rate of magnesium alloys can cause a premature disintegration of mechanical integrity, which is the main bottleneck that limits applications. Inspired by nature, various novel surface designs provide a clever strategy to regulate the corrosion behavior of magnesium alloys. This review extensively discusses bioinspired surface designs to reduce corrosion resistance and realize functionalization, so as to offer new ideas with great potential for biomedical applications. Future research on corrosion resistance is expected to benefit greatly from the bioinspired surface designs.

1. Introduction

Today, more than 200 years after the first isolation of elemental magnesium, biodegradable magnesium-based metals have attracted extensive attention [1,2,3,4]. The mechanical performances of magnesium alloys, such as plasticity, stiffness, and processability, are better than those of the bioresorbable polymers, such as polylactic acid employed in several clinical applications, due to the peculiarities of the metal properties [5,6]. Additionally, the density of magnesium alloys ranges from 1.73 to 1.85 g cm⁻³ and is similar to cortical bone (1.75 g cm⁻³) [7,8]. As a result, it is anticipated that magnesium alloys may replace other materials [9,10,11,12].

Since the most negative self-corrosion potential is so low (−2.37 V), magnesium alloys can naturally dissolve and adsorb after being implanted in the body [13,14]. After two weeks, bladder, liver, and renal tissues show no adverse effects from the generated Mg²⁺, which is mostly used by the organism [15,16,17]. However, the fundamental obstacle preventing more widespread use in industrial and medical settings is the rapid rate of deterioration of magnesium alloys. When magnesium alloys are exposed in a physiological environment, it is primarily degraded by electrochemical reactions, which produce magnesium hydroxide, hydrogen, and other compounds [18,19,20]. The corrosion of magnesium alloys also worsens due to the adsorption of biomolecules, including protein, small-molecule amino acids, and lipids [21,22]. Hydrogen gas produced by corrosion results in a somewhat alkaline atmosphere [23]. During the reaction process, a layer of magnesium hydroxide can also be produced to form the magnesium alloys’ surface coat, the structure of which is flimsy and porous [24,25]. The corrosion medium can be further etched via these openings to create a new magnesium matrix, creating a pit that is corrosive and releases a lot of hydrogen gas [26].

Nature has supplied us with a multitude of inspirations [27,28] to overcome the difficulties in the design of protective surfaces for magnesium alloys. The bioinspired surface design used in this review is primarily focused on enhancing the corrosion resistance of magnesium alloys (Figure 1). An introduction to the corrosion mechanism of magnesium alloys is presented in the first part. The methods for creating bioinspired solid surface designs and liquid surface designs are shown in the next section. The conclusions are then offered, along with a brief prognosis of the development’s course.

2. Corrosion Mechanism

According to thermodynamics, the system’s free energy occurs throughout the corrosion process [29,30]:

$$\Delta G=-nF{E}_0$$

(1)

where $\Delta G$ denotes the change in the interfacial Gibbs free energy, E₀ the cell’s standard potential, F the Faraday’s constant, and n the quantity of electrons transferred during the process. The system experiences spontaneous oxidation when the Gibbs free energy is negative. Magnesium alloys quickly oxidize when in contact with water and liberate hydrogen gas from the water because of their weak thermodynamic stability [31]. Following are expressions for the corrosion process of magnesium alloys [32,33,34]:

$$2Mg\to M{g}^{+}+2e^{-},$$

(2)

$$2M{g}^{+}+2H_{2}O\to 2M{g}^{2+}+2O{H}^{-}+H_2,$$

(3)

$$2H_{2}O+2e^{-}\to 2O{H}^{-}+H_2,$$

(4)

$$2M{g}^{2+}+4O{H}^{-}\to 2Mg{(OH)}_2,$$

(5)

$$2MgO+H_{2}O\to Mg{(OH)}_2,$$

(6)

Equations (2)–(6) show that preventing magnesium alloys from contacting water helps to inhibit corrosion. Therefore, building a physical barrier on the surface of magnesium alloys exhibits great corrosion protection potential for magnesium alloys [35,36]. In comparison, bare magnesium alloys without a physical barrier are completely exposed to the corrosive solution. It is generally believed that boosting the compactness, stability, and thickness of the physical barrier surface can be an efficient strategy to increase the magnesium alloys’ anticorrosion property from the standpoint of surface design.

Bioinspired surface design is a useful strategy for improving magnesium alloys’ anticorrosion property by forming a protective layer to keep magnesium alloys from coming into direct touch with external aqueous solution, preventing the excessive and rapid deterioration of magnesium alloys [37,38,39,40]. As shown in Figure 2, the number of scholarly works on magnesium alloy corrosion has increased dramatically during the previous two decades. However, the number of articles begins to decline in 2017, indicating that certain limitations in magnesium corrosion prevention may exist. In comparison, the number of publications on bioinspired surface design has increased significantly over the last 20 years, demonstrating that bioinspired surface design has garnered a lot of attention.

3. Bioinspired Solid Surface Design

Bioinspired solid-based surface treatments on magnesium alloys not only serve as anticorrosion design techniques, but also increase biocompatibility. For corrosion prevention, several solid surface-based protective barriers are used, including the bioinspired surface structure and bioinspired surface modification.

3.1. Bioinspired Surface Structure

Several research groups sought to build a biomimetic structure of a lotus leaf to generate a superhydrophobic surface on magnesium alloys, which is a viable approach for improving corrosion resistance [41,42,43,44,45] (Figure 3). The process is based on the perception that the surface air film can keep magnesium alloys from coming into direct contact with external aqueous solution [46,47]. Due to the tiny solid–liquid interfacial area on top of the protrusions, the liquid on top of this bioinspired surface is separated by trapped air. This can greatly limit the interaction between the corrosive species and the magnesium alloys and display the anticorrosion property. Several ways for endowing the bioinspired surface structure of magnesium alloys with corrosion resistance functions have been demonstrated [48,49,50,51,52].

The bioinspired surface structure can be obtained by eliminating layers from the surface of magnesium alloys using the etching process [50,54,55]. The air regions on the bioinspired surface cannot be replaced by corrosive species, and the “air” parts of the surface are deemed fully non-wetting, which increases the corrosion resistance of magnesium alloys. Liu et al. [56] created a bioinspired surface by assembling peony-like micro-nano-size hierarchical structures on magnesium alloy surfaces and altering them in an ethanol solution containing fluorosilane for 12 h at ambient temperature. Low sliding angles (<5°) made it easy for water droplets to readily slide off. After 180 days of air exposure, the resulting surface’s water contact angle changes very minimally, indicating a stable bioinspired surface structure and strong corrosion resistance. Liang et al. [57] created a bioinspired surface structure on AZ31 magnesium alloys with a contact angle (CA) of 154° and a sliding angle (SA) of 7.1° (Figure 4). When compared to the bare specimen, the bioinspired surface’s corrosion potential E_corr is moved positively from −1.59 to −1.16 V, boosting its corrosion resistance. After 12 h of immersion in 3.5% NaCl solution, the charge transfer of the bioinspired surface is larger than that of the untreated magnesium alloys, exhibiting the long-term durability in NaCl solution.

The electrodeposition method of synthesis enables the creation of bioinspired surface structures with clearly defined morphologies and variable sizes, merely by adjusting the experimental synthesis parameters without the use of a template [58,59]. It has also been applied to the creation of bioinspired surfaces on magnesium alloys. Liu et al. [60] used an electrodeposition approach coupled with immersion in stearic acid solution to create a stearic acid/CeO₂ bioinspired surface on AZ31B magnesium alloys. With a high water contact angle (more than 158°) and low sliding angles (less than 2°), the modified surface demonstrated good superhydrophobic and self-cleaning properties. The I_corr for the bioinspired surface is 1.14 × 10⁻⁶ A cm⁻² compared to 4.7 × 10⁻⁴ A cm⁻² for the untreated surface, suggesting that the bioinspired surface greatly enhanced the magnesium alloys’ anticorrosion property. The I_corr of the bioinspired surface increased to 1.56 × 10⁻⁴ A cm⁻² even after 100 h of immersion in corrosive solution (Figure 5a), which is still lower than the untreated surface and demonstrates anticorrosion performance. Liu et al. [61] used one-step electrodeposition to create a bioinspired surface structure on Mg-Mn-Ce magnesium alloys. Compared to the value for the substrate, the I_corr of the bioinspired surface is more than two orders of magnitude lower. The bioinspired surface considerably improved the magnesium alloys’ anticorrosion property in NaCl solution, as shown by the increase in E_corr and decrease in I_corr (Figure 5b). It was demonstrated that the CA of the bioinspired surface maintained above 150° after 400 mm abrasion by dragging the bioinspired surface on 1000# sandpaper in one direction at a pressure of 1.3 kPa, while CA of the bioinspired surface was lowered to 147.91° after abrasion for 500 mm, showing good mechanical durability to some extent.

The hydrothermal synthesis approach typically employs a sealed system to develop a bioinspired surface structure on the magnesium alloys’ surface by managing the controllable parameters. Feng et al. [62] utilized a one-step hydrothermal synthesis approach to create a bioinspired surface structure on AZ91 magnesium alloy (Figure 5c). After hydrothermal treatment, the I_corr drops from 1.48 × 10⁻⁴ A cm⁻² to 3.32 × 10⁻⁷ A cm⁻². The I_corr of the bioinspired surface drops by nearly three orders of magnitude when compared to the unmodified surface, demonstrating the bioinspired surface’s better anticorrosion property in NaCl solution. Only a minor quantity of corrosion products was discovered on the bioinspired surface of the magnesium alloy after soaking in NaCl solution for 20 h, showing that the bioinspired surface was mildly damaged and could significantly increase corrosion resistance.

3.2. Bioinspired Surface Modification

Compared with the bioinspired surface structure, bioinspired surface modification has been widely used as a protective surface for magnesium alloys owing to its affordability, versatility, and applications [63]. A quick and low-cost approach for creating the anticorrosion coating on magnesium alloys was proposed by Li et al. [64]. With a CA of 154° and a SA of 7.1°, this bioinspired surface displayed exceptional superhydrophobicity. When compared to the unmodified magnesium alloys, the I_corr of the bioinspired surface significantly drops from 1.65 × 10⁻⁴ A cm⁻² to 1.73 × 10⁻⁶ A cm⁻², or around two orders of magnitude (Figure 6). After being immersed in 3.5 weight percent NaCl for 336 h, the capacitive loop’s diameter is noticeably larger than that of the unmodified magnesium alloys, demonstrating that the bioinspired surface modification dramatically increases the magnesium alloys’ anticorrosion performance.

Although many bioinspired solid surfaces have been proposed for various biomedical uses, only a few have been used in clinical settings due to their poor biocompatibility. With inspiration from the natural ceramic structure, a high-quality ceramic surface is designed onto magnesium alloys, combining the mechanical strength of the metal with the superior biological properties of ceramics. This surface design can not only minimize the rate of corrosion of magnesium alloys in the biological environment, but it can also incorporate materials that can encourage bone formation to reduce cytotoxicity in the membrane layer and make it biologically active. This is a very promising bioinspired surface design for medical implantable metal materials.

The introduction of multi-functionalities through the use of bioactive coatings is another appealing technique to increase corrosion protection’s effectiveness [65,66,67,68]. Bioactive coatings, including polylactic acid (PLA), polycaprolactone (PCL), chitosan (CS), polydopamine (PDA), and other biopolymers, are widely used in biomedical applications [69]. It can provide an adsorption center through electronic atoms and deposit magnesium alloy surfaces to hinder surface corrosion [70,71,72,73,74]. Construction of bioactive coatings can be realized by molecular interaction, such as electrostatic interaction or intermolecular force between functional molecular groups and the surface of the magnesium alloy. Marine mussels can cling to “non-adherent” magnesium alloys for corrosion protection because their foot protein has a high concentration of the amino acids, lysine and 3, 4-dihydroxy-L-phenylalanine [75,76]. By using layer-by-layer assembly on AZ31 magnesium alloy, Wang et al. [77] created a nanocomposite bioinspired surface. The formation of an abnormally dense layer adjacent to the surface, with a thickness of around 100 nm, results in the material having a high degree of protection against external molecules such as water or corrosive ions (Figure 7a).

To further improve cell adhesion, proliferation, and migration, the bioinspired polydopamine surface can also operate as a flexible substrate for the following surface-mediated reactions [78,79,80,81]. For instance, Zhou et al. [65] created a bioinspired protective surface on AZ31 magnesium alloy based on the hydrothermal treatment of hydroxyapatite and polydopamine. Compared to bare AZ31 (I_corr = 3.81 × 10⁻⁴ A cm⁻²), the bioinspired surface showed the smallest I_corr (2.47 × 10⁻⁶ A cm⁻²) and a high E_corr (−1.50 V), indicating a lower corrosion rate. Due to the immobilization of hydroxyapatite, the bioinspired surface promoted the proliferation of osteoblasts and the cell survival rate reached 120%. Therefore, the bioinspired surface is more suitable for the growth and proliferation of osteoblasts (Figure 7b).

In order to better improve the preparation, design, and performance, the layer-by-layer preparation approach extends the bioinspired solid surface design from 2D to 3D [82,83]. Chu et al. [84] proposed a comparable structure on Mg-Zn(6 wt%)-Ca(0.5 wt%) magnesium alloy modified with decreased graphene oxide/bistriethoxysilylethane composites for corrosion protection, drawing inspiration from the “bricks and mortar” structure of nacre. Due to the improved barrier effect and less galvanic corrosion caused by reduced porosity and flaws, the bioinspired surface has a high continuity and integrity and corrodes at a pace that is in orders of magnitude lower than that of naked magnesium alloy (Figure 8). The wear rate of untreated magnesium alloy is high, which is 3.5 × 10⁻³ mm³ N⁻¹ m⁻¹. It remarkably decreases to 3.98 × 10⁻⁵ mm³ N⁻¹ m⁻¹ with the bioinspired surface, indicating the good anti-wear protection. Luo et al. [85] employed tannic acid containing Mg²⁺ ions to create a bioinspired surface on AZ31 magnesium alloy for better corrosion protection and high biocompatibility, which was inspired by the “tea stain” development process. Pan et al. [86] presented a chemically built bioinspired surface mixed with graphene oxide and heparin that worked as a compact barrier against corrosion spread. The multilayer coating can also greatly minimize hemolysis and platelet adhesion and activation, resulting in good blood compatibility.

Bioinspired solid surfaces provide effective corrosion protection for magnesium alloys because a barrier layer of air film can keep magnesium alloys from coming into direct touch with external aqueous solution. It also can greatly limit the adsorption of bovine serum albumin and fully inhibit platelet adhesion and activation by influencing protein adsorption. However, only a handful have been used in clinical settings, since the single function of bioinspired solid surfaces is incompatible with biomedical applications. For example, superhydrophobic surfaces, which are often utilized to increase magnesium alloys’ anticorrosion property, are not favorable to cell attachment or cell growth. The optimal biomedical magnesium alloys should be multifunctional, enhancing not just anticorrosion characteristics, but also cell adhesion and proliferation. In fact, integrating many bioinspired capabilities onto a solid surface is rather difficult. That is, bioinspired solid surfaces still have a long way to go before they can be considered anticorrosive with good biocompatibility.

4. Bioinspired Liquid Surface Design

The specific properties of liquid interfaces operate as a physical barrier to increase long-term corrosion resistance [87]. The bioinspired liquid surface design of magnesium alloys is inspired by the natural Nepenthes pitcher plant, which can successfully isolate substrate corrosion by forming a stable immobilized liquid overlayer. Due to the capillary force and the van der Waals force, liquids are mobile and have unpredictable shapes on a solid surface, and they can be filled in micro-/nanoscale structures [88,89]. Three conditions must be met when creating magnesium alloys with a liquid surface [90]:

• The functional liquid must be stabilized on the magnesium alloys’ surfaces;
• The functional liquid must preferentially wet the surfaces of the magnesium alloys over an aqueous solution;
• The functional liquid and aqueous solution must be immiscible.

4.1. Bioinspired Surface Structure

To satisfy the first principle, the creation of micro-/nanostructures on magnesium alloys is employed to build the solid substrate with a tiny rough texture, improve the specific surface area, and enhance the storage of functional liquid [91,92]. Pant et al. [93] investigated the influence of several etching degrees of surface roughness on the stability of bioinspired liquid surfaces and discovered that a nanoscale roughness of 24.5 nm considerably improves functional liquid stability. Since surfaces with modest nanoscale roughness have higher adherence to functional liquids. Smaller and bigger rough surfaces exhibit functional liquid loss. The cloak effect easily occurs when there is insufficient functional liquid on the surface. Nanoscale roughness may be more favorable to the formation of stable functional liquid on magnesium alloy surfaces without im-pairing droplet sliding. On the one hand, nanoscale voids offer capillary force to lock the functional liquid without interfering with droplet mobility on the surface owing to excessive roughness. When compared to micrometer-scale holes, nanoscale rough structures generate more dense synaptic sites that can withstand the influence of external media and improve stability.

4.2. Bioinspired Surface Modification

To satisfy the second principle, the magnesium alloys’ surface and functional liquid must match the physicochemical property to form a stable working system. Pretreatment with specific low surface energy groups promotes functional liquid distribution and storage in micro-/nano structures [94]. Common modification compounds, such as silane coupling agent, which connects magnesium alloys via silica-hydroxyl and organic matter, improve locking-liquid capability. Common low surface energy compounds, such as fluorosilane and fluorocarbon compounds (C₂F₈), can reduce the magnesium alloys’ surface energy. Meanwhile, fluorinated groups are compatible with fluorinated functional liquid, which can react with it to produce precursors or mutually soluble tiny molecules. The functional liquid can be held for a long time, protecting magnesium alloys against corrosion over the long term.

In order to demonstrate whether the aqueous solution does not replace the functional liquid, the theoretical model is introduced in terms of the minimization of a system’s free energy [90]. Specifically, configuration 1 refers to the state where the magnesium alloys’ surface is infused with functional liquid, and the aqueous solution is floating on top of it (E₁). Configurations 2 and 3 refer to the states where the magnesium alloys’ surface is completely wetted by functional liquid (E₂) and aqueous solution (E₃), respectively. To discover whether the surface of magnesium alloys has a greater affinity for the functional liquid than the aqueous solution, one should satisfy ΔE_A = E₃ − E₁ > 0 and ΔE_B = E₃ − E₂ > 0 (Figure 9). ΔE_A and ΔE_B can be further expressed as:

$$\Delta E_A=R({\gamma }_f\cos {\theta }_f-{\gamma }_{aq}\cos {\theta }_{aq})-{\gamma }_{faq},$$

(7)

$$\Delta E_B=R({\gamma }_f\cos {\theta }_f-{\gamma }_{aq}\cos {\theta }_{aq})+{\gamma }_f-{\gamma }_{aq},$$

(8)

where R is the magnesium alloys’ surface roughness factor, ${\gamma }_f$ the surface tension of the functional liquid, ${\gamma }_{aq}$ the surface tension of aqueous solution, ${\gamma }_{faq}$ the interfacial surface tension between the functional and aqueous solution, ${\theta }_f$ and ${\theta }_{aq}$ the contact angles of the functional liquid and aqueous solution on magnesium alloys’ surface, and the subscripts f and aq the functional liquid and aqueous solution, respectively.

Preston et al. [95] and Anand et al. [96] analyzed and summarized four cases of unsuccessful designs in the interaction between the functional liquid and aqueous solution:

• The surface tension of functional liquids is much lower than that of water, leading to water cloaking and the gradual loss of functional liquid by entraining as the droplet falls off the surface. The cloak effect is common when there is inadequate functional liquid on the surface. The spreading coefficient provides the cloaking criterion [95]:

  $$S_{faq}={\gamma }_{aq}-{\gamma }_{faq}-{\gamma }_f<0,$$

  (9)

  where ${\gamma }_{aq}$ denotes the surface tension of the aqueous solution; ${\gamma }_{faq}$ the interfacial surface tension between the functional liquid and aqueous solution; ${\gamma }_f$ the surface tension of the functional liquid, and the subscripts f and aq the functional liquid and aqueous solution, respectively.

2.

The surface energy of the functional liquid is higher, so the aqueous solution can spread out on the functional liquid and cover the magnesium alloys’ surface:

$$S_{aqf}={\gamma }_f-{\gamma }_{aqf}-{\gamma }_{aq}<0,$$

(10)

where ${\gamma }_{aqf}$ denotes the interfacial surface tension between aqueous solution and functional liquid.

3.

The functional liquid cannot completely infiltrate the magnesium alloys’ surface.

4.

The aqueous solution shows a higher affinity for the magnesium alloys’ surface and can replace the functional liquid. Similarly, the spreading coefficient of functional liquid on magnesium alloys when in contact with air or aqueous solution can be calculated:

$$S_{fm}={\gamma }_m-{\gamma }_{fm}-{\gamma }_f>-{\gamma }_{f}R,$$

(11)

$$S_{fm(aq)}={\gamma }_{aqm}-{\gamma }_{fm}-{\gamma }_{faq}>-{\gamma }_{mf}R,$$

(12)

where ${\gamma }_m$ denotes the surface energy of magnesium alloys, ${\gamma }_{fm}$ the interfacial surface tension between the functional liquid and magnesium alloys, ${\gamma }_{aqm}$ the interfacial surface tension between the aqueous solution and magnesium alloys, and the subscripts f, aq, and m the functional liquid, aqueous solution, and magnesium alloys, respectively. According to the three bioinspired liquid surface criteria, the magnesium alloys are closer to the functional liquid when ${\gamma }_{faq}(R-1)/(R-\phi )<S_{fm(aq)}<0$, where φ represents the fraction of the magnesium alloys’ surface area filled by the functional liquid.

If criterion (9) is not met, the water droplet is “cloaked,” or coated with a thin layer of functional liquid (Figure 10a), which can progressively deplete the functional liquid as water droplets depart. If condition (10) is not satisfied, the aqueous solution forms a film on top of the functional liquid (Figure 10b). To guarantee that the functional liquid remains infused in the magnesium alloys’ surface, criteria (11) and (12) must be satisfied. If $S_{fm}\ge 0$ or $S_{fm(aq)}\ge 0$, the functional liquid thoroughly wets the magnesium alloys. Otherwise, if criteria (11) or criteria (12) are still met, but $S_{fm}<0$ or $S_{fm(aq)}<0$, a portion of the magnesium alloys comes into touch with the water droplets (Figure 10c,d). Therefore, it can be seen that the design of bioinspired liquid surfaces has harsh conditions. The rough structure that does not comply with the specifications or an inappropriate functional liquid cannot meet the constraints of the above formula. As a result, the stability or protective effect of the bioinspired liquid is reduced, resulting in the loss of functional liquid or erosion of the magnesium alloys by external aqueous solution.

4.3. Functional Liquid

To satisfy the third principle, the functional liquid used must be chosen carefully to guarantee that the final infused surface has appropriate qualities. With these fundamental characteristics in mind, it is evident that there is no optimal functional liquid for all scenarios. Rather, the selection of functional liquid depends on the application of the surface. The functional liquid often has the following characteristics [97]:

• Low surface tension, allowing it to spread readily and penetrate roughness.

The low surface tension of the functional liquid is almost a necessary condition for bioinspired liquid surface design. The surface tension of the typical functional liquid is summarized in the literature [95,98]. Functional liquid with too low surface tension, on the other hand, is detrimental to interface stability. Sett et al. [98] observed that when perfluoropolyether oil and silicone oil come into contact with water droplets, a cloak occurs, resulting in functional liquid loss.

2.

Low vapor pressure (<1 Pa), so as not to evaporate fast.

Functional liquid with low vapor pressure (perfluoropolyether krytox is 10⁻⁸ Pa, mineral oil 10⁻² is Pa, silicone oil is 10⁻¹ Pa) can be stored for a long time. However, the majority of functional liquids are composed mostly of carbon compounds, which can lead to the breakdown of functional liquid in high-temperature or UV-radiation environments. Zhang et al. [99] detected that the functional liquid progressively evaporates, and the bioinspired liquid surface loses its function at 65–75 °C, causing the surface contact angle to rise continually. In contrast, the ionic liquid is more stable than the perfluoropolyether and retains steady liquid surface performance under harsh environments such as 250 °C, strong UV irradiation, and high vacuum conditions.

3.

Chemical inertness, in that it is not quickly destroyed when exposed to other chemicals.

Sett et al. [98] investigated the mutual solubility of flurane, silicone oil, mineral oil, and ionic liquid in different erosive liquids (such as water, ethanol, hexane, toluene, and ethylene glycol). According to studies, the chemical inertia of perfluoropolyether oil is the best, which does not react with external liquids, except for perfluorhexane with a comparable molecular structure. Ionic liquids, unlike non-polar fluorinated oil, silicone oil, and mineral oil, are not mutually soluble with water, but rather with alcohols, ethylene glycol, and toluene [100]. Except for water and perfluorohexane, silicone oil has the lowest chemical inertia and is nearly insoluble with other liquids. Even an inert functional liquid, however, can cause dissolution and failure due to molecular diffusion in the long-term immersion process of an ambient liquid [101]. Howell et al. [102] investigated and determined the solubility of krytox, a widely used perfluoropolyether functional liquid, to be 52 ± 46 ng cm⁻² after 16 h of static immersion. As a result, the slow diffusion cannot be ignored for the seeming stability of the functional liquid layer.

4.

Wide range of viscosity, with <100 cSt being the most typical; this value is neither too low to delay functional liquid depletion nor too high to hasten functional liquid infusion.

The viscosity of functional liquid influences the movement of the ambient medium on the functional liquid surface. When water droplets collide with the surface of the functional liquid, the interface shear force between the low viscosity functional liquid and the water droplets is negligible, allowing water droplet slide [103]. Too low viscosity, on the other hand, can compromise the stability of the functional liquid, leading to functional liquid loss.

To summarize, we believe that when selecting a functional liquid, elements such as solubility, surface energy, viscosity, and volatility should be considered extensively, and that a suitable functional liquid should be chosen in conjunction with the real service environment. Surface energy and viscosity that are too low do not promote the creation of a stable bioinspired liquid surface. In order to achieve a specified level of performance, a specific index of functional liquid must be used.

4.4. Application

The functional liquid on magnesium alloys possesses fluidity due to capillarity, which could fill in scratches and cure damage, as well as self-healing properties, of which the bioinspired liquid surface can keep the good super-slippery property under the condition of magnesium alloy damage, so as to avoid the loss of anti-corrosion and anti-biological adhesion [104,105,106]. Xiang et al. [107] prepared a superhydrophobic coating with a porous structure by means of electroplating and chemical replacement, and injected functional liquid on the surface to obtain a bioinspired liquid surface. It is found that the scratch narrowing of the bioinspired liquid surface damaged by the knife almost disappears after 2 h placement, which indicates that the functional liquid injected into the microporous surface can penetrate into the damaged cracks to plug the scratch, showing a good self-healing property. In addition, electrochemical testing and immersion in sodium chloride solution revealed that the bioinspired liquid surface of the scratch had self-healing properties.

Bioinspired liquid surface design also opens up new avenues for creating multifunctional surfaces that benefit from both long-term anticorrosion and biocompatibility. The essence of bioinspired liquid surface is that the air layer in micro/nano structures is replaced by functional liquid, which makes the water slide on the surface and cannot directly contact the magnesium alloys, giving the bioinspired liquid surface excellent corrosion resistance. Zhang et al. [44] created a two-layered surface for enhancing the anticorrosion performance of AZ31B magnesium alloy. The functional liquid was injected into the porous top layer, while the compact underlayer served as a corrosion barrier (Figure 11a). Magnesium alloys lower the I_corr of the useful liquid surface by six orders of magnitude. The solid superhydrophobic surface can operate for 3 days after soaking in sodium chloride solution, and the functional liquid surface can work for at least 15 days. The results reveal that the functional liquid surface outperforms the solid surface in terms of corrosion inhibition. Jiang et al. [108] created an anticorrosion system on AZ91D magnesium alloy that included a plasma electrolytic oxidation (PEO) film, hydroxide coating, and a functional liquid. Figure 11b depicts the anticorrosion system protection mechanism. The PEO coating formed in situ on the magnesium alloys’ surface can provide a modest corrosion barrier. The sandwiched LDH film performed ternary functions, such as loading inhibitor, functional liquid anchoring, and PEO defect sealing. The functional liquid surface was water-repellent and self-reparable in the event of surface damage. Chloride ions can cause molybdate to be released in the LDH membrane, preventing further corrosion. This system can be immersed in sodium chloride solution for 20 days.

In the process of employing magnesium or magnesium alloys, constructing a bioinspired liquid surface can effectively solve the problem of corrosion resistance. Due to its fluidity, the bioinspired liquid surface can reduce the formation of defects and repair possible damage when compared to a bioinspired solid surface. In addition, it endows them with unique abilities, including antibiotic adhesion and antibacterial properties, to meet market applications that are always improving. The potential for bioinspired liquid surfaces in anticorrosion applications is anticipated to be the future study area in this field.

5. Conclusions and Prospects

The use of bioinspired surface designs to increase magnesium alloys’ anticorrosion performance is a successful strategy. Bioinspired solid surface design, including bioinspired surface structure and bioinspired surface modification, often generates a physical barrier on the surface of magnesium alloys to improve its anticorrosion performance. The use of bioinspired liquid surface design to produce a resilient and self-reparable functional liquid barrier paired with active corrosion inhibition endows the magnesium alloys with excellent anticorrosion performance. To date, bioinspired surface design can now increase not just the magnesium alloys’ anticorrosion performance, but also their biocompatibility. However, it is very difficult to achieve a controllable corrosion rate of biodegradable, biomedical magnesium alloys as implant materials, since the protective qualities of various bioinspired surfaces on magnesium alloys are not well understood. As a result, systematic and long-term comparison and assessment are required in future study to explain the benefits and drawbacks of each bioinspired surface. Moreover, fabricating an ideal medical degradable magnesium alloy is still a challenge, in combination with corrosion resistance, self-degradation, biocompatibility, and drug release properties. Therefore, further study is needed to develop a novel bioactive surface for magnesium alloys. There is no doubt that bioinspired surface design advances both basic research and material therapy development.