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Review

Advances in Schiff Base and Its Coating on Metal Biomaterials—A Review

by
Zhiqiang Zhang
1,†,
Qingya Song
1,†,
Yubin **
1,
Yashan Feng
2,
**gan Li
1,* and
Kun Zhang
3,*
1
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
2
Zhengzhou Railway Vocational & Technical College, Zhengzhou 450000, China
3
School of Life Science, Zhengzhou University, Zhengzhou 450001, China
*
Authors to whom correspondence should be addressed.
†
Joint first authors.
Metals 2023, 13(2), 386; https://doi.org/10.3390/met13020386
Submission received: 22 December 2022 / Revised: 6 February 2023 / Accepted: 10 February 2023 / Published: 13 February 2023
(This article belongs to the Special Issue Advances in Stability of Metallic Implants)
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Abstract

:
In recent years, metal biomaterials have emerged one after another, and have many excellent properties, playing a great role in medicine. However, these coatings cannot meet the medical needs in every aspect. Schiff base is an important organic synthetic reagent and liquid crystal material in organic chemistry. It mainly refers to a class of organic compounds containing imine or azomethine characteristic groups (-RC=N-). It has important anti-tumor, anti-virus, antifungal and antibacterial activities. Based on the excellent properties of Schiff base, the coatings made of Schiff base can improve the bioactivity of materials, which have a good development prospect in medicine. In this paper, the preparation methods and properties of Schiff base and many advantages of Schiff base coatings are reviewed. The research on the modification of coatings or functional membranes by Schiff base and Schiff base reaction, as well as the extensive application of special Schiff base coatings in many fields such as anti-corrosion, antibacterial, flame retardant, etc., are carried out. Suggestions for further research on Schiff base coatings on metal biomaterials are put forward.

1. Introduction

Biomaterials are materials used to repair or replace diseased or damaged parts of the human body. The importance of biomaterials has increased significantly with population growth and aging, as it contributes to improving the quality of life and prolonging life spans. The surface structure of biomaterials is the key to control their interaction with the biological environment [1]. In recent years, metal biomaterials have been widely used in medical treatment, and various coatings have also brought great improvement to the functions of materials. So far, high requirements have been proposed for the preparation of uniform and corrosion resistant metallic material coatings [2].
Using the composite technology of different kinds of conventional materials to generate new practical functions is one of the strategies to accelerate the development of the next generation of biomaterials [3]. Schiff bases are a class of important bases in organic chemistry, with anti-tumor, anti-virus, antifungal and antibacterial activities [4]. Biomaterials can be ceramics, polymers or metals. Although ceramics and polymers are widely used in various medical applications, metal implants still occupy a dominant position because of their advantages, such as ease of manufacture [5]. In recent years, it has been applied in medicine and other fields, and has good prospects for development. This paper introduces Schiff base and its advantages as coatings.
In some environments, many biological macromolecules will be decomposed, but the synthetic biological composites are stable [6]. The biocompatibility of materials depends on their surface characteristics, such as chemical composition, charge density, wettability and topography, which directly affect tissue reactions, such as inflammatory reaction and tissue regeneration [7]. Among metal materials, 316L stainless steel is widely used as a temporary biological material due to its excellent mechanical properties, sufficient biocompatibility, high corrosion resistance, low inherent toxicity, availability and low economic cost. However, the interface between metal materials and electrolytes is prone to electrochemical corrosion [8], which has a great impact on the performance and service life of metals. In order to solve these problems, scientists have studied various types of coatings for metal surfaces. A modified coating composed of two Schiff base complexes (Cr3+and Ni3+) and SiO2 nanoparticles was constructed, which greatly enhanced the protective property and mechanical resistance of carbon steel on the surface of carbon steel [9].
The coatings on these metal surfaces not only beautify materials, but also reduce corrosion and wear of materials. Therefore, modern research mainly focuses on anti-corrosion coatings on metal surfaces [10]. However, corrosive substances cannot be completely blocked by the coating, and more or less corrosive substances will enter the metal/coating interface. Therefore, people try to mix and improve the coating in a variety of ways to provide better shielding against corrosive media [8]. In general, corrosion inhibition mainly depends on the physical and chemical properties of the inhibitor molecule, the π orbital characteristics of the electron donor and the electronic structure of the inhibitor. Schiff bases have been reported as useful corrosion inhibitors in metal and metal alloy corrosion media, which can be easily synthesized from relatively cheap materials. However, it is well known that the behavior of inhibitors depends on the interaction between functional groups in their molecules and metal surfaces. The inhibition efficiency of Schiff base is attributed to the existence of (-RC=N-) group and the electronegativity of any other heteroatoms (such as nitrogen, sulfur and oxygen) in the molecule. The high corrosion inhibition is caused by the lone pair electrons of N and the electronic distribution of the double bond of the imine unit [11]. Schiff base can stabilize the corrosion inhibition of steel through its good solubility and polar nitrogen atom, and with the increase of Schiff base ligand concentration, the protection ability of steel in corrosive media is improved [12]. The study of Schiff base coating is helpful to further explore the potential role and value of Schiff base. In this study, we will review the advances in Schiff base and its coating for the metal biomaterials application (Figure 1).

2. Structure, Properties and Classification of Schiff Bases

Schiff bases are called “privileged ligands”. They can bind with almost all metal ions and stabilize the oxidation state of metal ions under various useful conditions. This is why most metal ion Schiff base complexes have high activity. Schiff bases are also an important class of bases in organic chemistry. They have been widely studied because of their synthetic flexibility, selectivity, sensitivity to central metal atoms, structural similarity with natural biological compounds, and the presence of imino (-N-CH-) groups [4]. Schiff base with a methylene imino structure is widely used in many biological activities and industrial applications, such as antibacterial, anti-tumor, antioxidant, anti-inflammatory, antifungal, etc. Schiff base ligands are widely used in the development of inorganic chemistry and coordination chemistry because they can form complexes with metal ions. Moreover, Schiff bases will not change their original performance due to high temperature and pressure. Some Schiff bases show good catalytic performance at high temperature, which makes the application fields and researchability of Schiff bases more extensive.
Schiff base ligands, as bidentate compounds, coordinate with metal ions through methylene nitrogen and phenoxy centers in the form of single deprotonation [13]. Derivatives of Schiff bases and their complexes have also been proved to have many functions. Schiff base transition metal complexes are considered to be a kind of coordination compound with great potential. In recent years, they have been widely used in biochemistry, analysis, antibacterial agents, and other fields, especially in pharmaceutical chemistry, because of their advantages in antibacterial effect [14]. In addition, because the Schiff base of salicylaldehyde and its derivatives can coordinate with some metal ions, especially copper ions, salicylaldehyde assembled on the electrode surface by Schiff base reaction can be used as a copper ion sensor and its related applications, such as electrocatalysis of benzoquinone [15].
Schiff bases are easy-to-synthesize ligands formed by condensation of aldehydes and imines. They are often used as efficient catalysts for various reactions in homogeneous and heterogeneous catalytic reactions. For example, the Schiff-base-mediated gold nano catalyst directly catalyzes the hydrogenation of carbon dioxide to generate formate [16]. In recent years, the self-healing properties of Schiff bases have received extensive attention, but many self-healing reactions require external stimuli, such as light, heat, and pH. The Schiff base bond is one of the few covalent bonds which can repair itself without external stimulation. In addition, when exposed to a liquid environment, water can promote the kinetic properties of Schiff base reactions [17]. Schiff bases are a class of important bases in organic chemistry, with anti-tumor, anti-virus, antifungal, and antibacterial activities [4]. Schiff bases are a class of very famous organic molecules with a variety of structures and properties, as displayed in Table 1 [18].
Another key point to note is that Schiff bases containing oxygen, nitrogen and carbonyl groups have been used as drugs. It is reported that Schiff bases have biological activity against bacteria and fungi due to their biochemical, clinical and pharmacological characteristics. The bioactivity of Schiff base mainly depends on the imine group. Therefore, nitrogen atoms may participate in the process of forming hydrogen bonds with active centers of cell components and interfere with normal cell processes [19]. In addition, the unique molecular structure of Schiff base compounds can be used to modify coatings on metals to obtain better physical and chemical properties [20].

3. Synthesis of Schiff Base

The synthesis of Schiff bases is catalyzed by various types of chemical catalysts [21]. The imino group is considered as the basic characteristic of Schiff base, which has interesting biological significance and is found to be responsible for biological activities, such as bactericidal activities. Because the structural characteristics of Schiff bases provide target-specific recognition sites, the combination of Schiff bases and nanomaterials shows synergistic benefits, which has attracted extensive attention from researchers [18].
Gossypol is an effective enantiomer and has good therapeutic prospects for various types of cancer. Racemic gossypol can be separated from cottonseed, and its enantiomer can be purified by using the diastereomeric resolution technology of methyl tryptophan hydrochloride. This method is environmentally friendly and will not produce many pollutants. Schiff base can also be obtained by modifying high molecular weight and low molecular weight chitosan with phenol condensation reaction [22].
The synthesis of Schiff bases requires the presence of organic bases as catalysts. FeCu@N-doped carbon is an efficient catalyst, which can be used to convert amines and alcohols into Schiff bases. Bifunctional cobalt/zinc mesoporous silica nanoparticles can also be used to synthesize Schiff bases from aromatic amines and benzyl alcohols. The reaction temperature was 120 °C, and the reaction was carried out in the presence of air flow for 3 h. The mixture of P2O5 and Al2O3 was used to catalyze the synthesis of Schiff base from carbonyl compounds and primary amines under solvent-free conditions [4].
Many of the most abundant polysaccharides based on monosaccharides can be simply modified with a large number of aldehyde groups, such as dialdehyde cellulose (DAC). Due to the existence of a large number of active aldehyde groups on the skeleton, these modified polysaccharides can be used to prepare Schiff base complexes [23]. In addition, polyshev bases can be prepared by a solvent-free method. A mixture of bifurfural (0.526 mmol) and diamine (0.526 mmol) was stirred at 140 °C in a 25 mL double neck flask under nitrogen for 3 h. After the reaction mixture was heated under vacuum at 140 °C for 3 h, the solid obtained by washing with methanol was heated under vacuum at 80 °C to remove residual methanol. The resulting solids were collected as yellowish brown solids or reddish brown materials [24]. Common preparation method of Schiff base: in methanol medium, the mixed solution is stirred continuously for 2 h with a magnetic stirrer at 80 °C, cooled to separate yellow orange crystal products, washed with cold methanol and dried, and the purity is identified by thin layer analysis [25].
Toluidine (0.01 mol) was mixed with azo coupled 4—(3-formyl-4-hydroxy radical) diazoyl-n-5 (5-methylisoxazol-3-yl) benzenesulfonamide (0.01 mol) in equal molar ratio (1:1). The reaction mixture was mixed with 3~4 mL methanol and irradiated in a microwave oven. The reflection time was 4 min and the microwave power was 90 w. Orange products were recrystallized with hot ethanol, and finally dried with anhydrous CaCl2 in a dryer under reduced pressure (pollution-free, environmental protection, low cost, high yield, simple operation) [26]. In addition, in some experiments, the Schiff base was synthesized with 0.2 mol/L L-cysteine and 0.2 mol/L glucose [20].

4. Schiff Base Reaction

The Schiff base reaction is widely used in chemistry because of its mild reaction conditions and high reaction rate. In recent years, Schiff base complexes have attracted extensive attention in biochemistry and biomedicine due to their unique properties [15]. The Schiff base reaction is a feasible surface modification strategy, because it has many advantages: (1) mild reaction conditions; (2) simple process; (3) the by-products are harmless; (4) high yield; and (5) the product is easy to separate [18].
The Schiff base reaction was proposed by German chemist Hugo Schiff in 1864. It refers to the reaction between a class of compounds containing aldehydes (or ketones) and amino groups to produce imino groups (-C=N-), which enable them to simultaneously have fluorescence emission and bonding ability [27]. Schiff bases and their derivatives can be easily synthesized through simple chemical reactions, which have better inhibition on metals and alloys than other organic molecules in acidic media [28].
First of all, Schiff base bonding makes the films have high stability and able to withstand harsh conditions. Secondly, the formation of in situ Schiff base avoids the additional post-treatment process and improves the stability of the multilayer. Third, the Schiff base reaction can be carried out in both aqueous and organic solutions. This allows the materials to be incorporated, dissolved only or used only in nonaqueous solutions. Fourth, the Schiff base reaction leaves redundant active groups (aldehydes and amino groups) in the multilayer, which can further react with other substances to adjust the product characteristics to adapt to different applications [15].

5. The Application of Schiff Base on Biomaterials

5.1. Schiff Base and Nano Materials

In recent years, the cross application of Schiff base chemistry and nanomaterials has become increasingly widespread. Nanomaterials have unique physical and chemical properties, and can play a synergistic role with Schiff base to prepare modified electrodes. The reasonable design of Schiff base structure can improve the selectivity to specific ions, and can be used as an excellent ionic group to enhance the electrochemical response of the electrode. In addition, Schiff base nano sensors are widely used in biology, industry, environment, food and other fields [18]. Polymer Schiff base with adjustable redox voltage is considered as a promising negative electrode material for sodium ion batteries [29].
The Schiff base reaction is used to prepare large-area polymer nanofilm (PTF) with adjustable thickness. The PTF contains a polymer chain network of conjugated Schiff bases, provides micropores, and is decorated with a combination bag arranged in space, which may adsorb uranium in octahedral geometry [30].
Schiff bases can also stabilize the activity of several metals in various oxidation states, so as to control the activity change of metals in the catalytic conversion process. It is also widely used in industry, including as a dye and pigment [31]. The new modified Schiff base nanoparticles obtained by crosslinking chitosan with bis (4-formyl-2-methoxyphenyl carbonate) have free radical scavenging ability and anti-cancer properties. The carbonate skeleton and part of the nanoparticles are hydrolyzed under the targeted carcinogenic microenvironment, releasing vanillin and chitosan, and enhancing the anti-cancer activity [32]. Chemical modification of chitosan with aromatic aldehydes such as vanillin, salicylaldehyde, and provanillin can prepare stable Schiff base ligands [33].
Polylysine and catechol can also be modified by the Schiff base reaction in one step, improving the adhesion of polylysine to inorganic particles and surfaces [34]. Dopa catechol can be prepared by Schiff base reaction. Its binding with metal ions is site specific. This preparation method is inspired by the formation of the cuticle before and after the formation of mussels, and a hard but scalable metal surface cross-linked protein composite coating with layered structure is prepared [35].

5.2. Schiff Base and Chitosan

Chitosan is a common natural polysaccharide, which is widely distributed in the skeleton of insects and crustaceans in the form of acetyl (chitin). The hydroxyl and amino groups (nucleophilic groups) on the chitosan chain provide suitable positions for various attractive chemical transformations. Although chitosan exists widely and its cost is low, it has moderate thermodynamic properties in epoxy ammonia system. The preparation of biological aromatic amines by Schiff base reaction and structure is a good solution under the premise of simple synthesis and renewability. Among them, polyelectrolyte chitosan Schiff bases (PECSBs) are very effective as antifouling agents at high levels of eutrophication, and have strong inhibitory effects on the sedimentation of brown algae, green algae and red algae [36].
Chitosan has different functional groups, has a good structure for preparing new derivatives [37], and a variety of magnetic adsorbents [38]. Carboxymethylation of chitosan can not only improve the hydrophobicity of polymers, but also eliminate the problem of water solubility [39]. Glycans and their derivatives have some unique properties in the field of pharmacy and pharmaceutical chemistry [40]. Chitosan derivatives are widely used in key pharmaceutical compounds due to their non-toxic and biodegradable properties. Chitosan Schiff base and its metal complexes can improve the biological activity of chitosan [41]. Chitosan Schiff base containing aromatic aldehyde is of great importance to the biology of pathogenic bacteria, fungi and multi-drug-resistant bacteria. It is reported that the biological potential of chemically modified chitosan in antibacterial and antifungal effects has been improved. Chitosan Schiff base fused with medicinal plants has a good inhibitory effect on experimental pathogens and multi-drug-resistant bacteria [42]. In addition, graphene oxide can be linked to chitosan through ester bonds to further modify chitosan Schiff base. This modification increases the number of metal ion binding sites [43].
The accurate determination of chitosan content is of great significance to the quality control of chitosan, so people pay more and more attention to it. Two chitosan Schiff base derivatives, chitosan-Schiff base bearing benzaldehyde (BCSB) and chitosan-Schiff base derivatives bearing propionaldehyde (PCSB) were synthesized according to the average degree of deacetylation of chitosan with benzaldehyde and propionaldehyde. The total mass of glucosamine hydrochloride obtained by hydrolysis of all Schiff base products was calculated, and then the theoretical mass of chitosan was deduced through reverse calculation. Finally, the sample mass of the Schiff base reaction was combined with the theoretical mass of chitosan to obtain the content of chitosan. This method is accurate and simple, and it provides a good idea and method for the determination of chitosan content [44].
An environment-friendly Schiff base, salicylaldehyde chitosan Schiff base, was synthesized from chitosan and salicylaldehyde. It can be adsorbed on the steel surface in the form of protonation as an inhibitor, and it is a new corrosion inhibitor in the oil and gas industry [45]. As a new adsorbent, Schiff base grafted graphene oxide magnetic chitosan was used to extract and quantify lead ions in blood samples by dispersive magnetic solid phase extraction [46]. Chitosan-coated manganese Schiff base complex supported on magnetic iron oxide nanoparticles is a promising nano catalyst [47].
Vanillin and trans cinnamaldehyde combine with chitosan through the Schiff base reaction and reductive amination reaction. It was used as an active coating on the surface of fresh cut muskmelon. The modified polysaccharide produces a stable and good adhesive coating, which does not affect the appearance of the product, and can be used as an active edible coating and a transport carrier directly used in food [48].
Cellulose-based materials are widely used in the biomedical field. However, the lack of antibacterial activity of cellulose fibers will inevitably lead to the damage of journals in preventing bacterial infection. In this study, cellulose Schiff base ligands were prepared by selective oxidation of cellulose fibers, introduction of aldehyde groups, and the Schiff base reaction of aldehyde groups and amino groups on glycine. A new type of cellulose Schiff base copper complex was synthesized by complexing copper ions with Schiff base ligands. The prepared composite greatly enhances the antibacterial property of cellulose fiber [49].
According to the research, researchers have prepared a new injectable hydrogel with biocompatibility and biodegradability through the Schiff base reaction, which is used for soft tissue adhesion, hemostasis and bone repair materials [50]. This study provides a strategy for the design and preparation of rapid in situ forming hydrogels. Through the in situ formation reaction of Schiff base, hydrogels extracted from natural polysaccharides can be modulated and prepared for biomedical applications, such as soft tissue adhesives and hemostatic agents in the future [51].
Double Schiff base bonds can restrict the intramolecular movement of single cluster horizontal surface motifs. Double Schiff base linkage can induce high brightness luminescence of gold nanoclusters at single cluster level in aqueous solution. Dialdehydes are commonly used to construct different building units of nanoscale structures with high precision and good controllability. Dialdehydes react with amino groups to form imines, which induce the crosslinking of each component. In conclusion, dialdehyde mediated cross-linking strategy is particularly attractive for water-soluble gold nano carbon [52].
Schiff base compounds used in the field of flame retardancy have also received extensive attention because they can cross link and generate nitrogen containing hexatomic rings, which is conducive to the formation of a stable cross-linking network at high temperatures, thus producing high residual carbon and giving it inherent flame retardancy [53].
Monolithic porous phosphorus containing organic porous polymers (PPOPs) with imine chain can be synthesized in one step by using Schiff base polycondensation reaction. They have regular three-dimensional network structure and good thermal insulation and flame retardancy [54].

5.3. Schiff Base and Epoxy Resin

Epoxy resin is a kind of key polymer material, which is used in coatings because of its excellent chemical stability and good mechanical properties. Epoxy resins are usually used for protection in corrosive environments. Their resistance to different corrosives depends on the type of resin, the type and amount of hardener, and the curing temperature [55]. However, the epoxy resin coating has high flammability, which will cause huge economic losses and personnel losses during use [56]. According to research, curing epoxy resin based on a Schiff base polymer has excellent adhesion to steel. For example, using protocatechol to prepare high-performance Schiff base epoxy resin, while maintaining excellent chemical resistance and thermal stability, gives it strong biological activity, degradability, excellent fire safety and antibacterial ability [53] (Figure 2).

5.4. Schiff Base and Others

Structure determines performance. Based on its special structure, Schiff base has many unique properties. Schiff bases, as one of the most commonly used dynamic covalent bonds, have become an important research field due to their structural diversity and the simplicity of condensation of carbonyl compounds with primary amines. Moreover, Schiff bases can exchange with each other under external stimuli such as temperature and pH to trigger network topology rearrangement, thus endowing cross-linked polymers with many unique dynamic properties [57]. In addition, Schiff base metal complexes exhibit different biological and catalytic activities in different conversion processes [58], for example, a new Schiff base modified gold nano catalyst (its catalytic performance has a large size dependence) [16]. The Schiff base reaction is used to determine the binding sites of histones in nucleosomes and regulate RNA transcription [59]; It can also induce metal ions to achieve the detection purpose [60]. A new type of Schiff base ligand lanthanide complex was constructed by Schiff base reaction, which has low toxicity and high biological activity (anti-tumor, antibacterial, anti-HIV, anti-inflammatory and anti-cancer) [25].
Because most biomolecules contain amino groups in their structures, aldehyde-mediated Schiff base reactions have been widely used in the preparation of microarrays. The membrane that immobilizes the active enzyme structure on the solid surface through the interaction of Schiff bases can not only be used as a bioreactor, but is also often used as a biosensor for the determination of various biomolecules [15]. The transition of the unsaturated π bond in conjugation after Schiff base reaction leads to a certain self-fluorescence property of the product [61]. Schiff bases and their complexes are widely used as probes for ion detection in chemistry, environment, biology and other fields [18]. For example, the Schiff base exchange reaction is used to construct a fluorescent probe under the action of a catalyst. This probe is used for efficient and rapid detection of dichloromethane in aqueous solution [62]. At the same time, the donor receptor (D-A) and Schiff base coupling organic structure can also be used to reduce the infrared emissivity, thereby enhancing the infrared stealth performance of the polymer.

6. Schiff Base Coating

Based on the deep understanding of the molecular structure, preparation process and extensive application of Schiff base, Schiff base coatings have been widely used in the field of surface modification of biomedical materials. On the one hand, it can be used as a metal biomaterial coating to improve its molecular binding ability, and on the other hand, it can be used with other biomolecules to improve the biological function of the metal biomaterial surface.

6.1. Metal Coating

The Schiff base coating has excellent corrosion resistance [63]. Schiff base compounds are condensation products of amines and ketones/aldehydes. Recent publications show that these compounds are increasingly used as corrosion inhibitors for various metals (such as steel, aluminum and copper), especially in acidic environments [64]. Schiff base compounds are usually adsorbed on metal surfaces to block active corrosion sites to inhibit metal corrosion [65]. Schiff bases are introduced as a group of well-defined self-assembled monolayer-forming materials as corrosion inhibitors for steel, zinc and copper in different corrosion environments [66]. Moreover, some studies show that the inhibition efficiency of Schiff bases is much higher than that of corresponding aldehydes and amines [65]. A new type of Schiff base has been constructed to inhibit the corrosion of concrete reinforcement. The analysis of electrochemical and thermodynamic results shows that the protection process of this Schiff base inhibitor conforms to the Langmuir isotherm [67]. A “semi amphiphilic” copolymer/guanine conjugate was prepared by Schiff base reaction. Using the pH response characteristics of the dynamic ammonia bond, guanine, as a corrosion inhibitor, can be stably wrapped and continuously released when corrosion occurs [68]. A new type of Schiff base silicon oxide anti-corrosion coating was constructed, as an effective anti-corrosion barrier, which slowed down the corrosion and pitting potential of the alloy [69]. In addition, inspired by the natural modules of bacterial secretions, animal and plant extracts, Liu et al. synthesized Schiff compounds through tobramycin (TOB) of streptomyces and protocatechualdehyde (PR) of black fern. In addition, a dynamic self-renewing Schiff base metal composite coating (Fe/TOB-PR) n was prepared by a layer-by-layer self-assembly (LBL) method. It has been proved to be a universal coating that can be attached to different types of substrates, thus providing a research idea for the preparation of environment-friendly biological antifouling coatings [70].
Too fast degradation has always been the bottleneck of further application of magnesium (Mg) alloys in the biomedical field [71]. Although a large number of Mg alloy corrosion resistant coatings have been developed, there is not yet a coating that is completely suitable for Mg alloys (Figure 3) [72]. Recently, a Schiff base synthesized from amino acid and natural product paeonol was proved to have a good corrosion inhibition function for Mg alloy (Figure 4) [73], and was made into a coating for a Mg alloy substrate by electrostatic spraying (Figure 5) [74]. This Schiff base can chelate with the Mg ions degraded by Mg alloy and prevent the release of degradation products, thus the corrosion resistance of Mg alloy has been improved (Figure 6). This research also demonstrated that the coating made of the mixture of several single Schiff bases can better improve the corrosion resistance of Mg alloys, but its mechanism is still unclear.
At present, biodegradable metals have been widely studied in the field of biomedical devices and have shown great application potential, such as iron-based cardiovascular stents [75], zinc-based bone implants [76], etc. Wherein, Mg alloy is one of the biodegradable metals with the fastest degradation rate, which has seriously affected its further clinical application [77]. For inhibiting the degradation rate of Mg alloy, many kinds of coatings have been reported at present, including magnesium fluoride (MgF2) [78], magnesium hydroxide [79], etc. However, the preparation time of these coatings is too long. Mg alloy small devices, such as stents, can easily lose their precise structure or mechanical properties due to excessive corrosion during long-term acid or alkali passivation. In addition, it is reported that the Mg2+ produced by Mg alloy degradation can regulate the conversion of inflammatory M1 type macrophages to anti-inflammatory M2 type macrophages, and also promote the growth and function of vascular endothelial cells, but this function is limited by the concentration of Mg2+ released by Mg alloy [80]. This discovery proves that Mg alloy, as a vascular stent material, not only plays its mechanical properties and biodegradability, but the degradation products also can be used as an active factor in the treatment of diseases. Thus, the amino acid-paeonol Schiff base can be used as the exclusive coating of vascular stents due to its biological safety, sprayability and good corrosion resistance, so as to regulate the concentration of Mg2+ released by controlling degradation to meet the needs of various cells. Other advantages of Schiff base as the exclusive coating of Mg alloy for vascular stents are also summarized in Table 2.

6.2. Biological Coating

Metal coatings also affect the biocompatibility of materials, so Schiff base coatings are also commonly used in implant materials. Surface modification of immobilized biomolecules has been widely demonstrated to enhance the biocompatibility of cardiovascular implants. Anti-CD133 and fucoidan were chemically fixed on the polydopamine (PDA) membrane known for its stability and endothelial cell (EC) compatibility (Michael addition and Schiff base reaction) to make a coating. In vitro experiments on New Zealand white rabbits showed that the coating had good blood compatibility and was expected to be applied to the surface modification of cardiovascular biomaterials [81]. Through the Schiff base reaction and Michael addition reaction, a functional coating with polymethylmethacrylate as the substrate was constructed, which reduced the protein surface adsorption capacity of the substrate and greatly improved the antifouling capacity [82]. Similarly, this reaction is used to prepare poly(vinylidene fluoride) (PVDF) membrane with underwater super oil repellency effect, which is used for environmental protection one-step modification research of efficient emulsion separation [83]. Schiff base prepared from glucose and amino acid is used to modify Ca-P coating. The modified Ca-P coating has fine particles, low surface roughness and high adhesion, which greatly improves its degradation in vitro [20]. Schiff base compounds were synthesized with p-phenylenediamine as crosslinking agent and reacted with polylactic acid (PLA) to reduce the brittleness of PLA biological composite coatings and improve their antibacterial activity [84]. Composite Schiff base coated samples have high resistance and good corrosion resistance in electrochemical polarization reaction. Schiff base coating can also promote endothelial cell growth and inhibit platelet adhesion/activation [74].
Poly-l-lysine-3,4-dihydroxybenzylaldehyde (PLL-DHBA) films can be prepared by cathodic electrophoretic deposition (EPD) based on the Schiff base reaction of amino groups of PLL monomers and aldehyde groups of 3,4-dihydroxybenzaldehyde (DHBA) molecules. Nanocomposite coatings with dual micro nano morphology have been developed for orthopedic and dental coatings. This work paved the way for the development of the next generation of biomedical implant coatings [34]. A new chitosan Schiff base and its Fe2O3 nanocomposite, as a natural adsorbent with low cost, good biocompatibility and biodegradability, have good performance in removing methyl orange [85]. It has a good prospect to be used on the surface of metal biomaterials. The new modified Schiff base nanoparticles obtained by crosslinking chitosan with bis (4-formyl-2-methoxyphenyl carbonate) have free radical scavenging ability and anti-cancer properties. The carbonate skeleton and part of the nanoparticles are hydrolyzed under the targeted carcinogenic microenvironment, releasing vanillin and chitosan, and enhancing the anti-cancer activity [32]. Chemical modification of chitosan with aromatic aldehydes such as vanillin, salicylaldehyde and provanillin can prepare stable Schiff base ligands [33].
Natural hydrogels have been widely studied in the field of biomedicine because of their good biocompatibility and biodegradability due to their structural similarity to the extracellular matrix of primary tissue. However, they are often vulnerable to mechanical failures. A new hydrogel was constructed by Schiff base reaction. Due to the existence of dynamic transfer base bonds, it has pH responsive swelling behavior and good mechanical properties, and has excellent self-healing and thixotropy [86]. In addition, it was found to be pH responsive and injectable. This hyaluronic acid gel has broad application prospects in drug release, biological printing, intelligent robots and tissue regeneration [87]. A kind of adhesive smart hydrogel inspired by double cross-linked mussel has good mechanical properties, stronger antibacterial and angiogenesis properties on the basis of maintaining the above properties, and is used for the treatment of chronic infected diabetes wounds [88]. Through the in situ formation reaction of Schiff base, hydrogels extracted from natural polysaccharides can be modulated and prepared for biomedical applications such as soft tissue adhesives and hemostatic agents in the future [51]. Nano-emulsion-loaded hydrogel coating is used to inhibit bacterial toxicity and the formation of solid surface biofilm [89]. The chitosan bound hydrogel has potential research value in the treatment of hepatocellular carcinoma [90].
A complex of Schiff base and copper metal was synthesized, and the modified coating was prepared by adding it to polyurethane varnish. It has good flame retardancy and antibacterial properties, and does not affect the flexibility, hardness and adhesion of polyurethane varnish [26].

7. Conclusions

I. Schiff bases are widely studied due to their synthetic flexibility, selectivity, sensitivity to central metal atoms, structural similarity with natural biological compounds, and their (− N-CH -) groups.
II. Schiff bases can be applied for surface modification of biomedical metals as functional coatings to improve the biocompatibility, comprehensive mechanical properties, biological activity and corrosion resistance.
III. An amino acid—paeonol Schiff base can endow the surface of Mg alloy with self-healing function and promote the growth of endothelial cells. Spraying this Schiff base on the surface in a few minutes is expected to make a special coating of Mg alloy for vascular stents.
IV. Many Schiff bases and their compound coatings still have unclear application mechanisms, which need further exploration.

Author Contributions

Conceptualization, J.L. and K.Z.; methodology, J.L.; investigation, Z.Z., Q.S. and Y.F.; writing—original draft preparation, Z.Z., Q.S. and Y.J.; writing—review and editing, J.L. and K.Z.; supervision, J.L.; funding acquisition, J.L., Y.F. and K.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number U2004164; the Key Scientific and Technological Research Projects in Henan Province, grant number 222102310234 and 222102230025.

Data Availability Statement

Not applicable.

Acknowledgments

Table 1 reprinted (adapted) with permission from Ref. [18], Copyright {2022} American Chemical Society; Figure 2 reproduced from Ref. [53] with permission from Elsevier B.V. (License Number: 5452300869071).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Summary of Schiff bases in this review.
Figure 1. Summary of Schiff bases in this review.
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Figure 2. (a) A synthetic route of Schiff base compounds (PH-DDM, PDE and WPDE) and (b) schematic diagram [53].
Figure 2. (a) A synthetic route of Schiff base compounds (PH-DDM, PDE and WPDE) and (b) schematic diagram [53].
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Figure 3. Classification of surface coatings according to their compositions [72].
Figure 3. Classification of surface coatings according to their compositions [72].
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Figure 4. Molecular structures of three different kinds of Schiff base: PCLys, PCGly and PCMet [73].
Figure 4. Molecular structures of three different kinds of Schiff base: PCLys, PCGly and PCMet [73].
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Figure 5. Schematic diagram of electrostatic spraying [74].
Figure 5. Schematic diagram of electrostatic spraying [74].
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Figure 6. Schematic diagram of corrosion inhibition mechanism of Schiff base coating.
Figure 6. Schematic diagram of corrosion inhibition mechanism of Schiff base coating.
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Table
Table 1. Classification of Schiff base [18].
Table 1. Classification of Schiff base [18].
TypeExamples
BidentateMetals 13 00386 i001
TridentateMetals 13 00386 i002
TetradentateMetals 13 00386 i003
PolydentateMetals 13 00386 i004
Table
Table 2. Properties comparison of corrosion resistance coatings on magnesium alloy surface.
Table 2. Properties comparison of corrosion resistance coatings on magnesium alloy surface.
TypeSchiff Base CoatingMgF2Magnesium Hydroxide
Corrosion resistance+++
Shorter preparation time+−−
Better biocompatibility+−−
Surface self-healing+−−
“+” indicated possessing or better, “−” indicated not possessing or worse.
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Zhang, Z.; Song, Q.; **, Y.; Feng, Y.; Li, J.; Zhang, K. Advances in Schiff Base and Its Coating on Metal Biomaterials—A Review. Metals 2023, 13, 386. https://doi.org/10.3390/met13020386

AMA Style

Zhang Z, Song Q, ** Y, Feng Y, Li J, Zhang K. Advances in Schiff Base and Its Coating on Metal Biomaterials—A Review. Metals. 2023; 13(2):386. https://doi.org/10.3390/met13020386

Chicago/Turabian Style

Zhang, Zhiqiang, Qingya Song, Yubin **, Yashan Feng, **gan Li, and Kun Zhang. 2023. "Advances in Schiff Base and Its Coating on Metal Biomaterials—A Review" Metals 13, no. 2: 386. https://doi.org/10.3390/met13020386

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