English

• Orthodontic appliances can effectively correct dental and maxillofacial deformities, improve aesthetics [1,2]. However, appliances such as brackets inadvertently increase the risk of dental caries and gingivitis [3]. Their complex design makes cleaning difficult and triggers crystal biofilm formation due to bacterial aggregation and calculus deposition [4,5]. The crystal biofilm around brackets may lead to an increased risk of two kinds of diseases during orthodontic treatment: the activity of acid-producing bacteria (e.g., Streptococcus mutans, S. mutans) in the biofilm can lead to enamel demineralization and even dental caries; and the stimulus of bacteria and dental calculus to the gingival can lead to gingivitis [6,7]. To solve these urgent and critical problems, the advanced way is to inhibit crystal biofilm formation by modifying the appliance surfaces [8,9]. In recent years, most of the developed strategies mainly focus on inhibiting biofilm formation [10]. Bactericide strategies such as metals and their oxides [11–13], antimicrobial peptides [14] and quaternary ammonium salt [15] have been developed, but persistent bactericidal activity may induce adverse effects [16,17], and their biocompatibility requires further evaluation. Anti-adhesion coatings were developed such as superhydrophobic coatings [18–20] and hydrogel coatings [21–23]. Zwitterionic hydrogel (ZH) is an excellent anti-adhesion coating due to the hydration layer [24], but anti-adhesion alone is difficult to achieve long-term effectiveness [25,26]. Thus, combining the advantages of anti-adhesion and bactericidal activity may be a new strategy to effectively inhibit crystal biofilm formation. However, most studies ignored crystal deposition effect on bacterial aggregation. Inhibiting bacteria adhesion and crystal deposition in the early stage to reduce the formation of crystal biofilms are crucial for the prevention of demineralization, dental caries and gingivitis [27–29]. It is necessary to inhibit both bacterial deposition and crystal deposition to effectively inhibit the formation of crystal biofilms.

  Oral cavity is a special environment which is capable of generating various environmental stimuli such as pH value [30], enzyme [31] and temperature [32]. Among these stimuli, bacterial activity in dental plaque leads to a local decrease of pH [33]. The local pH under dense dental plaque can drop to 4.5–5.5 within hours after sugar intake [34]. Around orthodontic brackets, plaque accumulation can creates localized acidic niches [35]. Moreover, enamel demineralization and dental caries are closely related to the decrease of pH [36]. Therefore, pH is a special stimulus in the oral environment, enabling adjustment of the coating surfaces. Microcapsules are an unique class of materials which can respond to multiple stimuli such as pH [37]. They can encapsulate functional materials such as bactericides to enable on-demand control of their environmental concentration [38]. Therefore, integrating an anti-adhesion coating ZH with pH-responsive microcapsules (PRMs) encapsulating bactericide can combine the advantages of anti-adhesion and on-demand bacterial killing to inhibit crystal biofilm formation. This may be an effective way to solve the problems of dental caries and gingivitis during orthodontic treatment while maintaining good biosafety.

  Herein, we prepared a pH-responsive ZH coating containing PRMs on stainless steel (SS), displaying an effective inhibition of crystal biofilm formation by synergistic suppression of bacteria and stone (Fig. 1). ZH provides anti-adhesion properties via electrostatically induced hydration layers. The PRMs composed of chitosan (CS) and polylactic acid (PLA) encapsulating the bactericide (tea tree oil, TTO), can kill bacteria through on-demand release under acidic conditions induced by bacterial activity. The combination of anti-adhesion and controlled bactericidal properties effectively inhibited the formation of crystal biofilm. In an in vitro model, our coating extended the crystal biofilm inhibition effect from one day to five days. Therefore, this work provides a promising strategy to reduce the risk of dental caries and gingivitis during orthodontics treatment.

  Figure 1

  

  Figure 1.  Schematic of the zwitterionic hydrogel (ZH) coating containing pH-responsive microcapsules (PRMs), showing dual functions of anti-fouling and bacteria killing for orthodontics appliances.

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  As shown in Fig. S1 (Supporting information), we prepared the PRMs via the combination of oil-in-water emulsion method and solvent evaporation method [39]. Firstly, the water phase of CS and deionized water was mixed with the oil phase of PLA, TTO and dichloromethane (DCM) and then ultra sounded. Then, 100 mL of 1% polyvinyl alcohol (PVA) was add as a protective colloid to gain higher encapsulation efficiency [40]. The PRMs were gained after evaporating DCM from the emulsion, centrifugation, and freeze-drying sequentially. Next, the ZH+PRMs coating was prepared by incorporating PRMs with ZH precursor solution, and then applied onto the SS surface. Fig. 2a shows that the PRMs exhibit spherical shape and smooth surface. The TEM image (Fig. 2b) clearly displays a contrast difference between the shell and core regions, confirming the core-shell structure of the PRMs. PRMs exhibited a relatively uniform diameter distribution of an average diameter of 5.6 ± 2.1 µm (Fig. 2c). EDX analysis shows that PRMs surface is abundant of C and O elements, and the appearance of P element from TTO confirms the successful loading of bactericide TTO (Fig. 2d and Fig. S2 in Supporting information). The FT-IR spectrums were assessed to explain the composition of PRMs. As shown in Fig. 2e, 2957 and 2864 cm^-1 were the -CH₃ symmetric and asymmetric stretching vibration peaks of TTO [39], 1750 cm^-1 was the C= O stretching vibration peak of PLA [41], 1654 cm^-1 was the C=O stretching vibration peak in amide group of CS [42]. FT-IR results demonstrate the successful TTO encapsulation into PRMs. Therefore, these results provide conclusive evidence for the successful fabrication of PRMs and the encapsulation of TTO. Moreover, the TTO mass and drug loading efficiency were determined by dissolving PRMs in ethanol, followed by centrifugal cracking and UV spectrophotometry at 272.5 nm (characteristic absorption peak of TTO) [43]. The TTO loading efficiency was determined to be 52.1% ± 1.2%.

  Figure 2

  

  Figure 2.  Characterization of PRMs and their pH-responsive properties. (a) SEM characterization of PRMs. (b) TEM image confirmed the core-shell structure of the PRMs. (c) Particle size distribution of PRMs (D refers to diameter, N = 158). (d) EDX data of P element verifies successful bactericide loading. (e) FT-IR for PRMs, TTO, PLA, and CS confirms the composition of PRMs. (f) Quantitative release profile of PRMs. (g) Schematic, fluorescence images of PRMs pH responsive behavior. The insert shows the SEM images of PRMs after pH stimuli. Under pH 5, red color appeared around the PRMs compared to the original state. The results showed that DiI pre-stained TTO release rapidly via PRMs shell broken. In contrast, rare red color was obtained around the PRMs implies that PRMs kept its structural integrity under pH 7. The pH stimuli time is 10 min.

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  To assess the pH-responsive behavior of PRMs, we employed fluorescence and quantitative release test for release effect and SEM for morphological change under pH stimuli. Detailed experimental procedures are provided in Supporting information. For fluorescence test, 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI) was used to pre-stain the bactericide TTO [44,45]. As shown in Fig. 2g, the PRMs exhibited excellent pH-responsive performance. Under pH 5, a rapid increase of red color was observed around PRMs. SEM image displayed the shell broken of PRMs after stimuli under pH 5. In contrast, under pH 7 rare red color was obtained around the PRMs during the same observation period. SEM image after stimuli under pH 7 exhibits that PRMs kept its structural integrity. The quantitative release profile (Fig. 2f) demonstrates that rapid bactericide release in a short time can be observed under pH 5, with saturation reached within 1.5 h. The steady release of the bactericide was subsequently observed. Under pH 7, the bactericide concentration cannot reach saturation. The solubility of bactericide TTO in water is very low, suggesting there is limited release of surface-adsorbed TTO under pH 7 rather than bulk release of core-encapsulated TTO. Moreover, PRMs kept its pH-responsive properties after incorporating with ZH. As illustrated in Fig. S3a (Supporting information), ZH+PRMs exhibited faster TTO release for 3 cycles within 72 h under pH 5, while there is only a small amount of TTO under pH 7. Fig. S3b (Supporting information) shown the cumulated release within 3 cycles. Under pH 5, a considerable amount of TTO release can be observed each time after changing the solution, while under pH 7 minimal amounts of TTO release were observed after 40 h. The pH-responsive behavior of PRMs can be attributed to the structural damage of PRMs shell and the release of encapsulated TTO, enabling to control the concentration of bactericide. The shell breakage is due to pH-triggered PLA degradation, and CS dissolution [46].

  The effect of PRMs content on the structure and surface wettability was investigated by adjusting the concentration of PRMs. As illustrated in Fig. 3a, the cross-sectional morphology of ZH+PRMs was observed to confirm the successfully incorporation of PRMs and ZH. ZH has a typical porous network structure of hydrogel, and the PRMs are dispersed and preserved within the ZH network pores. To analyze the effect of PRMs concentration on the surface wettability of the ZH+PRMs coating, water contact angles (WCAs) was obtained (Fig. 3b). With the PRMs concentration increase from 0 to 15 wt%, the WCAs exhibited significant increase from 66.0° ± 3.1° to 84.2° ± 2.7°, especially after incorporating 15 wt% of PRMs. The surface roughness of ZH+PRMs coatings was quantitatively analyzed by confocal laser scanning microscopy (CLSM). The results demonstrated a concentration-dependent increase in surface roughness (Ra values), particularly at 15 wt% PRMs (Fig. S4 in Supporting information). The change in wettability may be attributed to the assembly of PRMs, which could lead to surface roughness and disrupt the intrinsic structure and hydrophilicity of the hydrogel networks [47]. To maintain the surface hydrophilicity of ZH while maximizing PRMs content, we mainly use 10 wt% of PRMs concentration for following tests.

  Figure 3

  

  Figure 3.  Characterizations and antibacterial property of ZH+PRMs. (a) Cross-sectional SEM images of ZH+PRMs demonstrates the successful incorporation of ZH and PRMs. The arrows indicate PRMs. (b) WCAs of ZH+PRMs demonstrate that surface hydrophobicity rose as PRMs concentration increased. (c) SEM images of adhesion behavior of S. mutans and E. coli on different substrate surfaces after 24 h incubation. (d) Quantitative analysis of S. mutans density. The significance analysis was compared with SS. (e) The antibacterial efficiency of ZH+10%PRMs coating against S. mutans and E. coli. The ZH+10%PRMs coating showed the best antibacterial effect. (f) Average ZOI of ZH+PRMs. The incorporation of PRMs significantly improved bactericidal property of the coatings. P < 0.05, **P < 0.01, ***P < 0.001.

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  In general, the insufficient interfacial binding strength between the hydrogel coating and solid substrate surface limits their applications [48]. To solve the problem, the ZH+PRMs coatings was grafted by the superwetting-assisted interfacial polymerization strategy, which can strongly enhance the interfacial adhesion strength and stability of hydrogels [49]. To test the interfacial adhesion strength between ZH+PRMs coating and SS, the shear force test was employed to measure the fracture strength of ZH+PRMs coating (Fig. S5a in Supporting information). The adhesion strength of the ZH+PRMs is of 204.2 ± 39.3 kPa with a bulk failure, indicating the interfacial adhesion strength between ZH+PRMs and SS is stronger than the bulk strength. The sand falling test was used to assess the impact-resistance of the ZH+PRMs coating (Fig. S5b in Supporting information). After 9 sand falling cycles (10 g sand each time, 0.1–0.2 mm in diameter, and 40 cm in height) [19], the contact angle remains 70.5° ± 6.8°. The wear resistance of ZH+PRMs coating was evaluated by simulated toothbrush test (Fig. S5c in Supporting information) [50]. The hydrophilicity of ZH+PRMs exhibited a slight decrease with the increase of toothbrushing cycles. Even after 500 brushing cycles, the WCAs still remains 72.0° ± 4.3°, indicating that the influence of toothbrushing is minimal. Thus, the ZH+PRMs coating showed apparent stability.

  To evaluate the antibacterial properties of ZH+PRMs coatings, we explored the effect of PRMs concentration from antibacterial adhesion test and bactericidal test. Two representative bacteria were selected for the following tests: Gram-positive S. mutans (the largest proportion and the most critical cariogenic bacteria in oral environment) and Gram-negative Escherichia coli (E. coli) to verify the universal antibacterial activity [51]. For antibacterial adhesion test, ZH incorporating with 0, 5, 10 and 15 wt% PRMs were detected to assess the effect of PRMs concentrations on antibacterial efficiency. SEM images were used to estimate the antibacterial efficiency of ZH+PRMs (Fig. 3c and Fig. S6 in Supporting information). Compared with SS, the bacteria density significantly decreased on the ZH and ZH+PRMs surfaces. Quantitative analyze shows that (Figs. 3d and e) as the PRMs content increased within a range (from 0 to 10 wt%), after 24 h incubation, the antibacterial efficiency significantly increased from 85.7% ± 3.5% to 91.3% ± 1.4% for S. mutans and 49.8% ± 7.3% to 95.2% ± 1.1% for E. coli. However, excessive PRMs (15 wt%) significantly reducing its anti-adhesive properties (only 52.3% ± 5.4% for S. mutans). More hydrophobic surface is disadvantageous to the formation of a hydrated layer, leading to increased bacterial adhesion [24]. For bactericidal test, the anti-bacterial circle test was employed. The bactericidal ability was evaluated based on the size of transparent zone of inhibition (ZOI). The ZH+10%PRMs performed best in antibacterial adhesion tests was selected to evaluate its bactericidal property. Negligible ZOI was observed around ZH, while the ZOI width expanded to 1.3 ± 0.3 mm for S. mutans and 2.3 ± 0.2 mm for E. coli after PRMs incorporation (Fig. 3f and Fig. S7 in Supporting information). The incorporation of PRMs introduces bactericidal properties to ZH. Results proved that ZH+PRMs coating combines the anti-adhesion properties from ZH and bactericidal effect from PRMs, effectively inhibits bacterial deposition.

  In order to further apply the anti-stone property of the ZH+PRMs coating, scaling deposition tests were carried out in a supersaturated mineral solution. The two main mineral components of dental calculus (i.e., calcium carbonate and calcium phosphate) were selected. Fig. 4a shows that few stone depositions was observed on both the ZH+PRMs and ZH surfaces. The XRD pattern of the deposited stone (Fig. S8 in Supporting information) exhibits disordered wide peaks, indicating the deposited stone is amorphous calcium phosphate (ACP) [52]. Furthermore, the stone mass gain (SMG) of the ZH+PRMs coatings was merely 5.3 ± 2.0 µg/cm² for CaCO₃, and 2.2 ± 1.5 µg/cm² for ACP, respectively (Fig. 4c). In contrast, lots of stone was deposited on the SS surface and the SMG values increased significantly to 177.4 ± 27.7 µg/cm² for CaCO₃, and 17.6 ± 5.1 µg/cm² for ACP, respectively. Compared with SS surface, the anti-stone efficiency of ZH+PRMs could attain ca. 97.0% for CaCO₃ and ca. 87.3% for ACP, greatly acquiring the stone accumulation problem. There is no significant difference between the anti-stone efficiency of ZH+PRMs and ZH, indicating that the incorporation of PRMs had no adverse effect for stone resistance of the coating. The ZH+PRMs coating can effectively inhibit stone deposition through the hydration layer introduced by zwitterionic hydrogel [53].

  Figure 4

  

  Figure 4.  Anti-calculus property and anti-biofouling property in an in vitro oral model of ZH-SS+PRMs. (a) Representative SEM and Ca EDX images of the ZH-SS+PRMs after 24 h scaling. (b) The optical images of flow cells containing ZH+PRMs coated SS after incubating in flowing artificial saliva (with S. mutans and sucrose). (c) SMG value of ZH+PRMs from two main minerals (i.e., CaCO₃ and ACP). (d) Light transmittance at 600 nm of flow cells containing ZH+PRMs and SS after incubating in flowing artificial saliva. The light transmittance of ZH+PRMs on day 5 was similar to that of SS on day 1, demonstrating that ZH+PRMs extends the crystal biofilm inhibition effect from day 1 to day 5. (e) The optical images of ZH+PRMs coated and SS brackets and corresponding fluorescence images after incubating with a saturated mineral solution containing bacteria. The deposited Ca²⁺ was stained by Calcein. The green color arrow headed in fluorescence images indicates deposited stone. (f) Biocompatibility of ZH+PRMs. n.s.: P > 0.1; **P < 0.01; ***P < 0.001.

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  Considering the complexity of the oral environment, an in vitro model is employed for anti-crystal biofilm test. To simulate the oral environment, artificial saliva added by S. mutans and sucrose was pumped into the flow cells with samples (i.e., SS and ZH+PRMs) at a rate of 0.5 mL/min [54]. As shown in Fig. 4b, the ZH+PRMs flow cell retained transparency with minimal cloudiness through day 5, evidencing the remarkable anti-crystal biofilm ability of the ZH+PRMs. Conversely, cloudiness was observed in the SS flow cell even after day 1 of incubation, indicating rapid crystal biofilm formation, and the cloudiness continuously intensified over time. To quantify the anti-crystal biofilm ability of ZH+PRMs, UV spectrum was employed to observe the light transmittance of solution in flow cells at the wavelength of 600 nm (Fig. 4d) [55]. The light transmittance of ZH+PRMs flow cell on day 5 was similar to which of SS flow cell on day 1, indicating that compare with SS, the ZH+PRMs coating can extend the anti-crystal biofilm effect from one day to five days. After 5-day incubation, the ZH+PRMs demonstrated an excellent anti-crystal biofilm efficiency of ca. 99.2% compared with the SS control. Moreover, the ZH+PRMs coating was prepared on commercial SS bracket surface and incubated with a mineralized solution containing bacteria. The deposited stone was stained by Calcein. As shown in Fig. 4e, few green colors were observed on ZH+PRMs surface, and lots of green colors were observed on SS surface. Compared with SS, the ZH+PRMs coating demonstrated excellent resistance to crystal biofilm formation. Therefore, the ZH+PRMs coating can efficiently inhibit the formation of crystal biofilm.

  Moreover, to evaluate the biocompatibility of the ZH+PRMs coating, cytotoxicity was performed using the CCK-8 test [56]. As shown in Fig. 4f, the viability of L929 fibroblasts for SS, ZH and ZH+PRMs remained 91.0% ± 12.3%, 87.5% ± 11.8% and 91.7% ± 14.2%, respectively. No significant difference in cytotoxicity was observed among SS, ZH, and ZH+PRMs, indicating that both the introduced ZH and PRMs exhibit excellent biocompatibility, comparable to that of SS, which has already been clinically applied.

  In summary, we have developed the ZH+PRMs coating which displays an effective inhibition of crystal biofilm formation by synergistic suppression of bacteria and stone. There are two main strategies for the coating: ZH provides anti-adhesion properties by forming a hydrated layer, while PRMs provide bactericidal properties by pH-triggered release of bactericide. The coating demonstrates significant anti-bacterial and anti-calculus capabilities with excellent biocompatibility, indicating its potential for clinical use. Therefore, this study provides a new idea for designing antifouling coatings of orthodontic appliances, which may help reduce the risks of demineralization, dental caries and gingivitis during orthodontic treatment.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Sirui Chen: Writing – original draft, Investigation. Ran Zhao: Writing – review & editing. Li Dong: Writing – review & editing. Qilin Liu: Writing – review & editing. Wei Chen: Writing – review & editing. Danna Liu: Writing – review & editing. Maosheng Ye: Writing – review & editing. Yingbo Li: Writing – review & editing. Qiong Nie: Writing – review & editing, Supervision, Funding acquisition, Conceptualization. Jingxin Meng: Writing – review & editing, Supervision, Funding acquisition, Conceptualization. Shutao Wang: Writing – review & editing.

  Acknowledgments

  This work is supported by the National Natural Science Foundation of China (Nos. 51973003 and 22275203) and the Beijing Natural Science Foundation (No. JQ23008).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111647.

  
  
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• 

  Figure 1  Schematic of the zwitterionic hydrogel (ZH) coating containing pH-responsive microcapsules (PRMs), showing dual functions of anti-fouling and bacteria killing for orthodontics appliances.

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  Figure 2  Characterization of PRMs and their pH-responsive properties. (a) SEM characterization of PRMs. (b) TEM image confirmed the core-shell structure of the PRMs. (c) Particle size distribution of PRMs (D refers to diameter, N = 158). (d) EDX data of P element verifies successful bactericide loading. (e) FT-IR for PRMs, TTO, PLA, and CS confirms the composition of PRMs. (f) Quantitative release profile of PRMs. (g) Schematic, fluorescence images of PRMs pH responsive behavior. The insert shows the SEM images of PRMs after pH stimuli. Under pH 5, red color appeared around the PRMs compared to the original state. The results showed that DiI pre-stained TTO release rapidly via PRMs shell broken. In contrast, rare red color was obtained around the PRMs implies that PRMs kept its structural integrity under pH 7. The pH stimuli time is 10 min.

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  Figure 3  Characterizations and antibacterial property of ZH+PRMs. (a) Cross-sectional SEM images of ZH+PRMs demonstrates the successful incorporation of ZH and PRMs. The arrows indicate PRMs. (b) WCAs of ZH+PRMs demonstrate that surface hydrophobicity rose as PRMs concentration increased. (c) SEM images of adhesion behavior of S. mutans and E. coli on different substrate surfaces after 24 h incubation. (d) Quantitative analysis of S. mutans density. The significance analysis was compared with SS. (e) The antibacterial efficiency of ZH+10%PRMs coating against S. mutans and E. coli. The ZH+10%PRMs coating showed the best antibacterial effect. (f) Average ZOI of ZH+PRMs. The incorporation of PRMs significantly improved bactericidal property of the coatings. P < 0.05, **P < 0.01, ***P < 0.001.

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  Figure 4  Anti-calculus property and anti-biofouling property in an in vitro oral model of ZH-SS+PRMs. (a) Representative SEM and Ca EDX images of the ZH-SS+PRMs after 24 h scaling. (b) The optical images of flow cells containing ZH+PRMs coated SS after incubating in flowing artificial saliva (with S. mutans and sucrose). (c) SMG value of ZH+PRMs from two main minerals (i.e., CaCO₃ and ACP). (d) Light transmittance at 600 nm of flow cells containing ZH+PRMs and SS after incubating in flowing artificial saliva. The light transmittance of ZH+PRMs on day 5 was similar to that of SS on day 1, demonstrating that ZH+PRMs extends the crystal biofilm inhibition effect from day 1 to day 5. (e) The optical images of ZH+PRMs coated and SS brackets and corresponding fluorescence images after incubating with a saturated mineral solution containing bacteria. The deposited Ca²⁺ was stained by Calcein. The green color arrow headed in fluorescence images indicates deposited stone. (f) Biocompatibility of ZH+PRMs. n.s.: P > 0.1; **P < 0.01; ***P < 0.001.

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