CaPs can provide ions to reconstruct damaged enamel. Remineralization solutions containing calcium and phosphorus ions are usually used in remineralization experiments, which must be replaced or replenished on a regular basis. Some stable CaPs materials can provide ions required for an extended time. Amorphous calcium phosphate (ACP), tricalcium phosphate (TCP), and nano-hydroxyapatite (nHA) are common CaPs materials used for remineralization. The type and size of the CaPs crystals can influence the ion supply capacity and the depth of ion entry into the lesion. Therefore, the mineralization effects of these materials are differrent.

ACP, the precursor phase of biogenic HA of bone and tooth, is the basic mineralization unit in the biological mineralization process (Gelli et al., 2019). Aqueous ACP solutions contain abundant Ca²⁺ and PO₄ ³⁻ ions, which form highly hydrated clusters. The structure and composition of the crystalline phase change after further aggregation of clusters until the thermodynamically stable crystalline HA (alkaline conditions) or carbon brushes (acidic conditions) formed (He et al., 2020). Usually, such reaction time is fast in the absence of external interference. Only ACP solutions failed to restore enamel (Shao et al., 2019). Therefore, enamel remineralization requires ensuring the stability of ACP in solution and prolonging its phase transition time. Acidic groups, such as carboxyl and phosphoric groups, can bind calcium ions in solution, preventing Ca²⁺ and PO₄ ³⁻ from aggregating. Organic compounds with carboxyl or phosphate groups are the most common ACP stabilizers. It is a good method to use amino acids such as aspartate (Asp), glutamate (Glu), citrate (Delgado-López et al., 2014; Iafisco et al., 2015), and the phosphate stabilizer triethylamine (Shao et al., 2019) to maintain the size of ACP particles, ensuring ion supply in the subsequent mineralization process. In addition, the casein phosphopeptide (CPP) that containing four to seven phosphate groups can attach to ACP nanoclusters, forming CPP-ACP. CPP-ACP complexes have been used as common additives for caries prevention. Furthermore, CPP-ACP in combination with fluoride show advantages in remineralization of existing lesions (Bijle et al., 2018; Tao et al., 2018). However, CPP-containing products should be used with caution in individuals with lactose intolerance issues.

TCP can be divided into α-TCP and β-TCP according to the crystal form. β-TCP is often used in dental materials because of its great biodegradability and biocompatibility. When exposed to acid, β-TCP degrades to release ions for enamel restoration. After surface functionalization by carboxylic acid and surfactants, functionalized TCP (fTCP) can prevent fluoride from binding with calcium ions on the enamel surface prematurely to build a low-dose fluoride release system (Karlinsey and Pfarrer, 2012; Shen et al., 2018; Viana et al., 2020). After combining with fumaric acid, fTCP can show significantly higher calcium bioavailability than β-TCP and better remineralization of subsurface enamel damage (Karlinsey et al., 2010).

nHA is a bioactive and biocompatible material with a small particle size of 10–20 nm in diameter and 60–80 nm in length (Huang et al., 2011). The nanometer size enables nHA to penetrate deeper lesion layers through large lesion pores and repair enamel damage (Juntavee et al., 2018; Bossu et al., 2019; Memarpour et al., 2019). However, high-concentrating nHA tend to self-aggregate into large-sized nHA, which can affect the amount and depth of nHA entering the lesion (Huang et al., 2009). As a carrier, the gel effectively extends the contact time between the active ingredient and the enamel, allowing nHA to fill the small holes and depressions. Both silica-based glycerol hydrogel (Khonina et al., 2020) and carbomer-based gel (Sari et al., 2021) containing nHA can repair damaged enamel.

Fluorinated compounds have been commonly utilized since the previous century to reverse or prevent enamel defects from spreading. Consequently, the global incidence rate of dental caries has decreased dramatically (Jokstad, 2016; Clark et al., 2020). Fluoride reduces demineralization by altering enamel solubility (Lynch et al., 2004). Fluoride and calcium ions are more strongly bound than hydroxyl groups. Therefore fluoride can replace hydroxyl to form fluorapatite (FAP), which has high acid resistance and poor solubility (Clark et al., 2020). Fluorides, on the other hand, can promote remineralization by encouraging Ca²⁺ in saliva to attach to the tooth surface. In addition, fluoride can reduce the adhesion and growth of germs by blocking the activities of numerous enzymes.

Fluoride is primarily ingested through drinking water (75%). Fluoridation of home water is a typical measure to prevent dental caries in many countries, and it can successfully reduce the incidence of dental caries. Fluoride can also be found in a variety of oral care treatments and dental materials, including sodium fluoride (NaF), stannous fluoride, silver diamine fluoride, acidulated phosphate fluoride, ammonium fluoride, and others (Barrera-Ortega et al., 2020). Fluoride sustained-release ability could be altered by combining fluoride ions with different cations and complexing it with different organic molecules. In toothpaste and rinses, polyvalent fluorides with tin and titanium as cations exhibit excellent corrosion resistance (Zanatta et al., 2020). This is due to the fact that they can not only produce CaF₂ on the enamel surface, but can also generate metallic precipitates on the enamel surface, which contributes to the reduction of calcium ion loss when subjected to external erosion.

However, fluorides have caused certain issues when they are used. Fluoride tends to develop a disordered layered structure of remineralization layer, which is considerably different from natural enamel. The mechanical characteristics of the remineralized layer can be weakened by these disordered formations. Organic compounds like amelogenin can help minimize the occurrence of disordered structures in the reaction system (Yu et al., 2019) (Figure 2). Moreover, excessive fluoride use can result in hazardous effects like dental fluorosis and skeletal fluorosis (Philip, 2019). It also has the possibility to make cariogenic bacteria resistant, diminishing the effectiveness of follow-up prophylaxis (Liao et al., 2017). Fluorinated hydroxyapatite is rapidly formed in the superficial enamel layer of very concentrated F⁻ solutions, preventing Ca²⁺ and PO₄ ³⁻ from penetrating deeper into the lesion. As a result, subsurface enamel lesions can fail to mineralize adequately. Therefore, fluoride slow-release systems made of copolymer acrylic reservoirs and glass ionomer cement can be great alternatives for promoting enamel mineralization by extending the trailing effect (Chong et al., 2018).

Fluoride compounds, as traditional enamel remineralization materials, have a relatively well-studied mineralization mechanism, which facilitates the development of novel fluoride-mediated remineralization systems. However, the functions of fluoride compounds are still need to be improved, and thus, blends or composites of fluoride compounds, which can combine multifunction together to achieve satisfactory clinical results, are greatly needed in the future.

Magnesium presents in the hard tissues of the body. In enamel, the content of Mg²⁺ is ranging from 0.2 to 0.5 wt%. Mg²⁺ is present near the grain boundaries as an intergranular phase of Mg substituted amorphous calcium phosphate (Mg-ACP) (La Fontaine et al., 2016). Such amorphous phases have been proved to make a significant impact on the mechanical characteristics and wear resistance of enamel (Gordon et al., 2015). Mg²⁺ slows crystal growth by competing with calcium ions at the growth point during mineralization, affecting the production of apatite (Ren et al., 2010; Abdallah et al., 2016). As a result, Mg²⁺ can act as a competitive inhibitor to guide narrower crystal columns, which promotes a highly ordered arrangement and increases mineralized tissue hardness. As the concentration of Mg²⁺ on the enamel surface increases, the nano-hardness of the enamel rises dramatically (Kis et al., 2021). Layer by layer mineralization process is used to create multilayer arrays of enamel-like FAP/polymer nanocomposites controlled by Mg²⁺ (FPN-M) at room temperature (Li Y. et al., 2021) (Figure 3). In the presence of Mg²⁺, the single nanorods are refined in size and a highly compact array is formed, eventually, (FPN-M)_n exhibits excellent mechanical strength and transparency. The present researches have demonstrated Mg²⁺ have great importance during the process of enamel remineralization, therefore, more and more attention should be paid to Magnesium related materials. Besides, further understanding of the relationship between Mg²⁺ and biomineralization can help develop strategies to improve the mechanical properties of mineralized tissues and improve the functions of repaired tooth enamel.

Amino acid molecules contain different amounts of amino and carboxyl groups. Depending on the isoelectric point, amino acids can be classified as acidic, neutral, and basic amino acids. Among them, acidic amino acids are negatively charged in weakly acidic solutions, which can influence the nucleation, crystallization, growth, and crystal transformation of HA. Glu and Asp can operate as soft templates, connecting two calcium ions diagonally to generate ordered HA crystals that parallel to the enamel column while stabilizing calcium ions in the solution. The crystals on the enamel surface can grow more ordered with the amino acid concentration rises. Asp and Glu is used to deposit the CaCO₃ layer as a sacrificial template on the enamel’s surface (Wu et al., 2015). The acidic amino acids then absorbed phosphate and carbonate ions, depositing HA into the CaCO₃ layer to form the rod crystal. Glycine (Gly), a highly hydrophilic amino acid, can also be used as a biological additive to create enamel-like structures (Tao et al., 2007). According to the molecular dynamics experiment, Gly exhibits the same adsorption abilities and coverage in all directions of the crystal surface, which maintains HA’s c-axis propensity (Pan et al., 2007). In the carboxymethyl chitosan-stabilized ACP remineralization system, a rod-shaped crystal layer is successfully produced in artificial caries when Gly is introduced to the system, whereas the system without Gly fail (Wang et al., 2017). Arginine (Arg), a basic amino acid, positively affects pH homeostasis, bacterial ecology, and pathogenicity. Arg is metabolized in oral biofilms to produce ammonia mainly through the internal arginine deiminase system (ADS) of bacteria (Streptococcus sanguis and Streptococcus). Ammonia produced under this pathway has a significant pH-raising effect, while inhibiting tooth demineralization by neutralizing acids in the peripheral environment (Bijle et al., 2021b). It also facilitates the formation of arginine-friendly microorganisms while disrupting the internal homeostasis of caries-causing bacteria (Nascimento et al., 2019). The combination of Arg and fluoride can create a pH-responsive fluoride pool that inhibit acid production and has potential synergistic effects in maintaining a healthy oral microbial balance (Agnello et al., 2017; Bijle et al., 2021a). The pool can also significantly improve the fluoride uptake and surface hardness of damaged enamel compared to fluoride alone (Zheng et al., 2015; Bijle et al., 2020).

Enamel matrix proteins (EMPs) and proteases control the formation of enamel (Jia et al., 2020; Shin et al., 2020). EMPs govern the parallelism between the glazing columns and organize them in a dense and slender hexagonal prism structure at the micro-level by regulating the creation and structure of HA crystals (Bartlett, 2013; Uskokovic, 2015; Bai et al., 2020). These highly co-oriented glaze columns give enamel its remarkable shear strength and make it resistant to everyday abrasion (Yeom et al., 2017). Over 90% of the EMPs consists of amelogenin. Amelogenin can be enzymatically processed into different peptides. These peptides undergo a change in spatial conformation, manifested by α-helix unraveling and β-sheet and β-turns formation, at which point amyloid-like aggregation occur in the proteins (Carneiro et al., 2016; Bai et al., 2020). Then, they self-assemble into oligomers and nanospheres (Fang et al., 2011; Engelberth et al., 2018; Bai et al., 2020). These oligomeric nanospheres further form nanochains that concentrate Ca²⁺ and PO₄ ³⁻ in the peripheral matrix, generating mineralized precursors during enamel development, which then serve as templates to guide the crystal phase transition, eventually generating HA (Gil-Bona and Bidlack, 2020) (Figure 4). The enamel columns then elongate in one direction to form a hexagonal prismatic structure (Jokisaari et al., 2019).

Enzymes are critical requirements for enamel biomineralization. Enzymes activate the biological function of amelogenin and degrade organic matter in the matrix until a sufficiently hard tissue formed (Prajapati et al., 2016). Matrix metalloproteinase 20 (MMP-20) cleaves amelogenin, and the product peptide controls the lengthening and growth of crystal nucleus and induces HA mineralization (Fukae et al., 1998; Nagano et al., 2009; Gil-Bona and Bidlack, 2020). Addition of MMP-20 to full-length porcine amelogenin can promote neatly aligned bundles of enamel-like HA, whereas in the absence of MMP-20, only ACP particles seen (Kwak et al., 2016). Another important enzyme is Kallikrein-related peptidase 4 (KLK4). During enamel maturation, KLK4 degrades the organic matrix in the mineral (Smith et al., 2017; Sari et al., 2021). The width and thickness of the microcrystals can increase when proteins are removed from mature enamel. If the enzyme is deficient, the enamel will undergo hypoplasia (Simmer et al., 2009; Smith et al., 2017).

In-depth studies of the enzymatic cleavage products revealed three major functional domains of the amelogenin (Mukherjee et al., 2019; Dissanayake et al., 2020). N-terminal: a hydrophobic tyrosine-rich N-terminal region, known as tyrosine-rich amelogenin peptide (TRAP), is critical in the directed assembly of amelogenin (Buchko et al., 2018). The central region: the central hydrophobic proline-rich region is mainly composed of X-Y-proline (X and Y are usually glutamine) repeat motifs, which is rich in β-sheets and β-turns. The C-terminal: a highly hydrophilic domain contains a large number of acidic amino acid residues. These residues could combine with Ca²⁺ to provide nucleation sites and bind to the (100) face of octacalcium phosphate (OCP), the intermediate sub-stable phase in early enamel, thereby govern the direction in which the enamel column extends (Wu et al., 2017) (Figure 5). Leucine-rich amelogenin peptide (LRAP), which contains two self-assembled domains of full-length amelogenin, is the most common alternative splicing product of amelogenin (Xia et al., 2016; Green et al., 2019). It is discovered that depending on the phosphorylated version of the peptide on serine 16, LRAP can perform distinct activities. Phosphorylated LRAP (+P) inhibits calcium phosphate crystallization and stabilizes ACP, whereas LRAP (–P) directs the production of aligned enamel crystals (Yamazaki et al., 2017; Le Norcy et al., 2018). In vitro, LARP and PP_i are used to remineralize the eroded enamel, and acicular HA crystals are successfully regenerated on the surface (Kwak et al., 2017). Inspired by these structural domains, amelogenin analogs are designed and synthesized to induce in vitro bionic remineralization. After grafting different fragments, these synthetic functional peptides can be easier to obtain and show certain functional enhancements, such as adsorption ability.

Non-amelogenins work in early enamel formation, including enamelin and tuftelin. Enamelin acts as a transport and nucleation protein that affects amelogenin to regulate early enamel development (Bartlett et al., 2006; Lacruz et al., 2017; Yan et al., 2017). Tuftelin is an acidic protein produced by ameloblasts during the early stages of enamel formation. It is concentrated near the dentin-enamel intersection, in which enamel mineralization begins. The tuftelin-derived peptide (TDP) is created based on the structure of tuftelin. The group repaired by TDP demonstrate comparable enamel hardness and lesion depth healing results after pH cycling to NaF groups (Ding et al., 2020).

Functional peptides inspired by bioproteins can in some ways replicate the unique functions of these bioproteins, as well as easier access. Assembly of these peptides with different functions can produce multifunctional peptides, such as peptides with high enamel binding and remineralization capacity or peptides with antibacterial and remineralization activities (Table 1).

Alendronate (ALN), a powerful bone resorption inhibitor with a high affinity for HA, is used to treat and prevent osteoporosis (Chen S. et al., 2020). The phosphate of ALN exchanges with the phosphate of HA in enamel, forming coordination chains that tightly bind it to the enamel surface (Palazzo et al., 2007). Therefore, ALN can act as a “glue” in the mineralization system to increase the adsorption of materials. ALN modified poly (amino amine) dendrimers (Wu et al., 2013) and carboxymethyl chitosan (Wang et al., 2017) can significantly increase their absorption on the enamel surface. In addition, after forming a HA layer around the ALN-modified polyacrylic acid (PAA), the outer layer of HA is zinc-modified to synthesize ZHA@ALN (Xu et al., 2020). Once ZHA@ALN dissolved by acid, a substantial number of calcium, phosphorus, and zinc ions can be released for remineralization and sterilizing. The inner layer of ALN-PAA adheres quickly to the enamel surface due to the ALN. Then, PAA serves as an antifouling layer with 75.05% bacteriostatic efficiency.

Non-collagenous proteins (NCPs) stabilize crystal precursors during dentin and bone collagen mineralization. NCP analogs, such as polyaspartic acid (pAsp), polyglutamic acid (PGA), and biocompatible polymers polyacrylic acid (PAA), contain a large number of carboxyl groups. These polymers can be used to create induced liquid precursors by stabilizing calcium ions. PGA and pAsp can also bind to calcium ions on the enamel surface, strengthening adhesion and providing nucleation sites (Ustriyana et al., 2020a; Ustriyana et al., 2020b). It has been discovered that the α-helical of pAsp or PGA can promote HA crystal nucleation by templating Ca²⁺ distribution. The HPO and -COO- can work together to attract Ca²⁺ and form stable Ca²⁺ triangles, which can develop into the nucleation core of ACP (Zeng et al., 2021). PAA can also chelate with Ca²⁺ while maintaining liquid phase stability and transporting ions continuously for subsequent biomineralization (Chen R. et al., 2020; Xu et al., 2020; Li N. et al., 2021). Furthermore, PAA can direct the transformation of ACP to form acicular microcrystals (Wang et al., 2018).

Poly (amino amine) (PAMAM) is a kind of synthetic protein with a dendritic structure. PAMAM can self-assemble into nanospheres, nanochains, and nanoribbons (Yang et al., 2015). Grafting different groups such as -NH₂, -COOH and -OH onto PAMAM can improve its ability to bind Ca²⁺or promote its adsorption on the enamel surface. The mineralization effects decrease in the order of -NH₂ > -COOH > -OH (Fan et al., 2020). This is because positive charged PAMAM-NH₂ can be more adsorbed on the negatively charged enamel surface. In addition, the adherence of S. mutans is also evaluated. Both PAMAM-COOH and PAMAM-NH₂ are shown to be effective in forming a smooth remineralized layer and minimizing S. mutans adherence (Jia et al., 2020). Grafted with SN15, SN15-PAMAM can increase adsorption on the enamel surface and achieve 90% higher mineralization effect than the control group. (Gao et al., 2020).

Polydopamine (PDA), the polymer of dopamine that rich in amino and phenolic groups, shows great hydrophilicity. It has been used as a functional agent to increase the wettability and biocompatibility of substrate (Barclay et al., 2017; Ghorbani et al., 2019). After being immersed in PDA solution, a dense film can be formed on the surface of the material in a short time (Kaushik et al., 2020). The film contains a large number of charged groups, to which calcium and phosphorus ions will be attracted and form a stable bond (Ryu et al., 2010; Murari et al., 2020). It is also observed that the HA crystals on the PDA modified enamel surface accumulated more closely, suggesting that PDA might help in inducing uniform crystal nucleation (Zhou et al., 2012). This may be because PDA can increase surface hydrophilicity, decrease the interfacial energy, and accelerate crystallization speed of HA (Qu et al., 2020). Based on the super adhesion of PDA, HA layer can be synthesized on the subsurface of different materials after being modified (Chen et al., 2019; Wong et al., 2022).

Biopolymers, including proteins and polysaccharides, have been used for the bionic formation of HA. Most of these polymers are mostly used in mineralized systems in the form of gels. All of these biopolymers show excellent non-immune and biocompatible properties, meanwhile with the advantages of easy storage and clinic application.

Chitosan (CS), a cationic polysaccharide, can rarely produce allergic or inflammatory reactions in humans (Younes and Rinaudo, 2015). Therefore, CS has been used to construct organic templates and scaffolds, which can ensure the bioactivity of peptides for mineralization guidance (Ruan et al., 2016; Ren et al., 2018). Furthermore, CS is also able to prevent bacteria from adhering and reproducing. The adherence of S. mutans may be decreased by 94.91% by employing CS alone (Ren et al., 2019). This is because chitosan is positively charged. When CS comes into touch with the negatively charged bacterial wall, the structure of the bacterial wall would be disrupted. Simultaneously, positive charged CS can adhere to the negatively charged enamel surface, preventing acid erosion (He et al., 2019; Boda et al., 2020). In addition, the antibacterial function can be enhanced when CS paired with fluoride (Wassel and Khattab, 2017; Ren et al., 2019). Carboxymethyl chitosan (CMC), formed by CS carboxylation, also has excellent ACP stabilization and can promote the formation of enamel remineralization layers. (Chen et al., 2015; Xiao et al., 2017). The mineralization system using chitosan as a gel carrier is summarized in Table 3.

Agarose is a natural polysaccharide with -OH groups that can form a thermally reversible gel. Agarose aqueous gel is widely used in medical systems such as mineral regeneration and drug delivery (Zarrintaj et al., 2018). The abundant hydroxyl groups in agarose molecules have a strong mutual attraction with Ca²⁺, allowing agarose to control the formation of nano ACP precursors and act as an ion reservoir to transport mineral precursors to the enamel surface for mineral mesomorphic transformation. The average elastic modulus and nano hardness of enamel increased significantly to 89.46 ± 11.82 and 3.04 ± 0.75 GPA after 6 days of the interaction of agarose gel with 500 ppm F (Cao et al., 2014a). When chitosan is added to the agarose aqueous gel, the groups between the two gels are cross-linked with each other, forming a fiber structure together with calcium ions, which can further simulate the protein matrix for enamel repair. The regeneration layer is 7.5–8.5 μm thick and regained 77.4% of the natural enamel’s microhardness (Musat et al., 2021). Agarose can also be combined with amelogenin to form oriented hexagonal prism enamel columns on the enamel surface (Cao et al., 2014b).

Gelatin is a peptide molecular polymer. Gel peptides in gelatin can form salt bonds with phosphate groups on the surface of apatite, causing enamel-like minerals to regenerate. The limited directional diffusion of ions in the gel environment promotes heterogeneous nucleation, resulting in a crystal with a clear structure (Zhang et al., 2010). Using the bionic double-layer gel system assisted by anodic aluminum oxide, it is possible to successfully prepare HA crystals with good orientation (Chen et al., 2019).

Silk fibroin (SF) governs the synthesis of calcium carbonate in mollusks and the creation of animal shells. SF contains a large number of β-sheets, which are rich in acid aspartic acid and have a high affinity for calcium ions. In the rotary thermal evaporation approach, SF serves as a template to guide heterogeneous nucleation of HA and mineral layers with natural enamel-like shape, organization, and mechanical characteristics swiftly formed (Wang S. et al., 2020).

Abalone water-soluble matrix (AWSM) plays an important role in the formation abalone shells. The proportion of organic matter and inorganic minerals in abalone shells is very similar to enamel (95% calcium carbonate and less than 5% organic matter). Therefore, AWSM can promote the formation of crystals. High AWSM concentration can increase the content of calcium and phosphate in the mineralized layer and promote to form a parallel, dense and highly ordered structure (Wen et al., 2016).

Carboxy betaine (CB) polymers are amphoteric polymers with functional carboxyl and quaternary ammonium groups. The carboxyl groups can serve as Ca²⁺ and PO₄ ³⁻ deposition sites, whereas the positive quaternary amino group has bactericidal properties (Xu et al., 2018). Furthermore, CB can resist bacterial adhesion via electrostatically induced hydration. ACP can be stabilized by amphoteric ionic poly (carboxy betaine acrylamide) (PCBAA). PCBAA/ACP nanocomposites contribute to the growth of HAP in the damaged sublayer of enamel by blocking spontaneous ion conversion on the enamel surface while releasing sufficient ions (He et al., 2022). Thus, PCBAA/ACP nanocomposites performed admirably in both remineralization (10.08 μm thick remineralized layer in mice intraoral for 14 days) and antimicrobial experiments (almost no bacterial adherence). ACP and poly (vinylpyrrolidone) nanofibers are mixed, and making electrospun mats. The mats can be hydrated to form a gel in the savila containing fluoride. Then, calcium and phosphorus ions crystallize under the guidance of fluorine ions to form HA (Fletcher et al., 2011).