Effects of cleaning protocols on surface roughness and optical properties of thermoplastic orthodontic retainers

Abstract

Objectives

To investigate the impact of three commonly used cleaning methods—mechanical cleaning, chemical cleaning, and ultrasonic cleaning—on the optical transmittance and surface roughness of thermoplastic orthodontic retainers following coffee-induced staining.

Materials and methods

Standardized thermoplastic specimens were assigned to five groups (n = 10): an unstained control group, a coffee-stained group without cleaning, and three post-staining cleaning groups treated with either mechanical cleaning, chemical cleaning using Polident tablets, or ultrasonic cleaning. Optical transmittance was measured using ultraviolet–visible (UV–Vis) spectrophotometry. Surface roughness parameters (Ra, Rz, RSm) were analyzed via surface profilometer, and surface morphology was examined using scanning electron microscopy (SEM).

Results

Coffee staining significantly reduced transmittance and increased surface roughness, indicating adverse effects on both optical transmittance and microstructural integrity. Ultrasonic cleaning effectively restored transmittance and surface integrity to near-control levels, with minimal surface damage. Mechanical cleaning successfully removed surface stains but introduced linear abrasions and led to a significant decrease in RSm values. Chemical cleaning achieved moderate stain removal but left visible surface residues.

Conclusions

Among the cleaning protocols evaluated, ultrasonic cleaning demonstrated superior performance in stain removal and preservation of surface integrity. Mechanical cleaning was effective but caused surface wear, while chemical cleaning offered limited cleaning efficacy and left undesirable deposits.

Clinical relevance

Cleaning strategies for orthodontic retainers should balance stain removal efficiency with material preservation. Ultrasonic cleaning provides a clinically favorable option, offering effective cleaning with minimal adverse effects on surface characteristics.

Introduction

Orthodontic retainers are essential in maintaining tooth alignment during the post-treatment retention phase, serving to prevent relapse and ensure the long-term stability of orthodontic outcomes [1, 2]. Among them, removable thermoplastic orthodontic retainers have gained popularity due to their esthetic appeal, ease of use, and reduced potential for microbial adhesion compared to fixed appliances, thereby promoting better oral hygiene [3, 4]. However, their prolonged wear time subjects them to continuous exposure to complex intraoral conditions, which may compromise the material’s optical transmittance, and surface integrity. Factors such as exposure to dietary chromogens [5], variations in salivary composition, microbial colonization [6], and inappropriate cleaning practices [5, 7], can result in discoloration, increased surface roughness, and microstructural deterioration. These issues not only affect esthetic quality but also reduce patient comfort and compliance. Hence, establishing a safe, efficient, and material-compatible cleaning strategy is crucial for prolonging appliance lifespan and ensuring sustained clinical outcomes.

The most widely used cleaning strategies for retainers include mechanical cleaning, chemical cleaning, and ultrasonic cleaning [8]. Mechanical cleaning is practical and cost-effective for daily home use but may induce surface abrasion and microdamage due to frictional forces, potentially leading to irreversible changes over time [9]. Chemical cleaning typically involves oxidizing agents, enzymes, or surfactants to remove organic residues, with the added benefit of antimicrobial action. Nonetheless, inconsistencies in cleanser composition raise concerns regarding chemical compatibility and possible adverse effects on material stability. Studies have shown that various chemical agents can influence the transmittance, surface roughness, and flexural properties of thermoplastic materials [10–12], and in some cases, may accelerate polymer degradation [11, 13]. Ultrasonic cleaning, by contrast, utilizes high-frequency cavitation in a liquid medium to effectively dislodge particles and disrupt contaminant films. This technique is noted for its minimal impact on surface morphology and superior cleaning efficiency [8, 14], although its implementation in home settings remains limited by equipment availability.

Several studies have evaluated the effects of cleaning methods on microbial reduction, color retention, and mechanical performance. Investigations have compared the decontamination and de-staining efficacy of commercial cleansers [15], and others have quantified alterations in mechanical attributes such as hardness and modulus after repeated cleaning cycles [10, 13, 16]. However, these works predominantly focus on bulk material behavior and often overlook surface-specific characteristics. Few have adopted a comprehensive approach to evaluate the combined effects of staining and various cleaning methods on surface morphology, roughness, and optical transmittance—especially under simulated conditions replicating long-term clinical use.

Therefore, the current study introduces a standardized in vitro coffee-staining model to replicate long-term retainer discoloration and employs a consistent cleaning protocol to evaluate three commonly used cleaning methods—mechanical cleaning, chemical cleaning, and ultrasonic cleaning—on thermoplastic clear retainer films. The study targets three critical performance indicators: surface roughness, optical transmittance, and microstructural morphology. Roughness serves as a quantitative descriptor of surface quality; transmittance reflects the visual esthetics and functional transparency; and microstructure assessment via scanning electron microscopy (SEM) helps identify any cleaning-induced damage. By integrating these multi-dimensional parameters, the present findings provide useful reference data that may support further optimization of retainer maintenance approaches in clinical orthodontic practice. An overview of the experimental workflow is provided in Fig. 1, illustrating the staining, cleaning, and evaluation procedures.

Fig. 1.

Schematic overview of the experimental workflow. After coffee staining, orthodontic retainer membrane samples were cleaned using mechanical, chemical, or ultrasonic methods. The treated specimens were then analyzed for transmittance, surface roughness, and surface morphology

Materials and methods

Materials

Thermoplastic orthodontic retainers (PET-G, 0.040” × 5”, FDA-approved; JBC and Company, USA) were used to fabricate standardized specimens measuring 10 × 3 × 1.02 mm³. A total of 50 specimens were prepared for comparative evaluation across different cleaning protocols. To replicate pigment exposure from dietary sources, standardized instant coffee powder (Nescafé Classic, Nestlé, Switzerland) was used as the staining agent.

Cleaning interventions included: Mechanical cleaning using a soft-bristled toothbrush (Oral-B Soft, Procter & Gamble, USA) and fluoride toothpaste (Crest Cavity Protection, Procter & Gamble, USA); chemical cleaning using Polident denture cleansing tablets (GlaxoSmithKline, UK); and ultrasonic cleaning using an ultrasonic cleaner (KQ3200E, Kunshan Ultrasonic Instruments Co., China). All materials and equipment were standardized throughout the experimental process to ensure consistency and repeatability.

Experimental grouping

Samples were randomly allocated into five experimental groups (n = 10 per group):

• Group A (control): No staining or cleaning treatment.

• Group B (coffee-stained): Immersed in coffee solution without subsequent cleaning.

• Group C (coffee-stained + mechanical cleaning): Samples were stained and then subjected to simulated brushing.

• Group D (coffee-stained + chemical cleaning): Samples were stained and cleaned via chemical cleaning.

• Group E (coffee-stained + ultrasonic cleaning): Samples were stained and cleaned using ultrasonic technology.

Sample treatment protocols

Coffee staining

To simulate long-term accumulation of chromogens, a standardized staining protocol was implemented: Instant coffee (Nescafé Classic, Nestlé, Switzerland; 3 g) was dissolved in 250 mL boiling distilled water, stirred for 10 min, filtered and cooled to room temperature. Prior to immersion, specimens were rinsed under tap water to remove debris. Each sample was fully submerged in the prepared solution, which was renewed every 12 h for a total staining period of 60 h.

Mechanical cleaning simulation

A custom mechanical device was designed to simulate routine home brushing, as shown in Fig. 2. The apparatus consisted of a motor-driven arm equipped with soft-bristled toothbrushes (Oral-B Soft, Procter & Gamble, USA) that generated linear reciprocating motion under uniform vertical contact conditions. To ensure consistent brushing pressure, the brush head height and specimen mounting position were fixed throughout the procedure, maintaining identical contact across all samples. The brushing frequency was set at approximately 70–80 cycles/min, consistent with in-vitro brushing simulation protocols. Fluoride toothpaste (Crest Cavity Protection, Procter & Gamble, USA) containing hydrated silica as the main abrasive component was used to replicate typical home-care cleaning. Each specimen underwent 730 brushing cycles, representing an estimated equivalent of two years of daily single-session cleaning (one cleaning event per day) during typical removable retainer wear, which corresponds to the average recommended duration of removable retainer use in clinical practice [17]. In this context, one cycle was defined as a complete forward-and-back reciprocating motion of the brush head. The total brushing time per specimen was approximately 10 min, conducted continuously without interruption, and specimens were air-dried at room temperature between procedures to maintain consistent surface conditions.

Fig. 2.

Schematic illustration of the mechanical cleaning device for retainer membranes. (A) Side view of the brushing apparatus showing the motor-driven reciprocating mechanism. (B) Top view of the device illustrating the structural alignment of the brushing unit and support track. (C) Cleaning demonstration setup: the retainer membrane is fixed on the platform, and the brush head moves back and forth to simulate mechanical cleaning

Chemical cleaning simulation

Specimens in Group D were immersed once daily for 5 min in freshly prepared Polident solution over a 30-day period. The 5-minute duration was selected in accordance with the product instructions, which recommend soaking removable appliances in warm water for 3–5 min before brushing. This approach reflects a typical home-care practice and ensures better alignment with real-world cleaning behavior.

Ultrasonic cleaning simulation

Specimens in Group E underwent ultrasonic cleaning at room temperature twice daily for 3 min per session using an ultrasonic cleaner (KQ3200E, Kunshan Ultrasonic Instruments Co., China) operated at 20–40 kHz over a continuous 30-day period. The 3-minute duration was selected to reflect typical home-use ultrasonic cleaner cycles (typically 3–6 min) and was chosen to balance effective stain removal with minimal material alteration in the context of removable retainer maintenance.

Evaluation of material properties

Optical transmittance analysis

A UV-Vis spectrophotometer (wavelength set at 500 nm) was used to measure absorbance before staining, after staining, and following cleaning. Transmittance values were calculated to evaluate changes in optical transmittance.

Surface roughness assessment

Surface roughness was assessed using a surface profilometer (Contour GT-K, Bruker, USA) by scanning three random surface locations per specimen and calculating the mean values of average roughness (Ra), maximum height (Rz), and mean spacing of profile irregularities (RSm). The selection of these parameters is grounded in established measurement practices described in the literature, where Ruzova and Haddadi provide a comprehensive overview of Ra and Rz measurement protocols [18], and Šimunović et al. demonstrate the application of Ra in orthodontic-retainer cleaning studies [8]. Together, these references reinforce the methodological validity of the present investigation.

Scanning electron microscopy

Microstructural surface features were visualized using SEM (Hitachi SU3500, Hitachi Ltd., Japan) at an accelerating voltage of 10.00 kV and a magnification of 1000×. Representative surface regions were selected for image acquisition to assess surface integrity and residual deposits.

Statistical analysis

Statistical analyses were performed using GraphPad Prism 10.0 (GraphPad Software, San Diego, CA, USA). Data normality and homogeneity of variance were verified before analysis. One-way ANOVA followed by Tukey’s multiple comparisons test was applied for parametric data, while non-parametric methods were used where appropriate. Exact p-values are reported when available, and extreme values are presented as P < 0.0001 or P > 0.9999 according to the statistical software output. A significance threshold of P < 0.05 was considered statistically significant. All results are expressed as mean ± standard deviation (SD).

Results

Optical transmittance analysis

Figure 3 illustrates the effects of coffee staining and subsequent cleaning methods on the optical transmittance of orthodontic retainer membranes. Coffee staining (Group B) caused a pronounced reduction in transmittance compared with the untreated control (Group A, P < 0.0001), indicating severe optical degradation. Following cleaning, all three treatments—mechanical cleaning (Group C), chemical cleaning (Group D), and ultrasonic cleaning (Group E)—significantly improved transmittance relative to the stained group (P < 0.0001, P = 0.0035, and P = 0.0038, respectively). Among these, ultrasonic cleaning achieved the greatest recovery, showing no significant difference from the control group (P > 0.9999). Mechanical and chemical cleaning produced partial improvements but remained significantly lower than the control (P < 0.05). Exact P-values from Tukey’s post hoc test for all pairwise comparisons were as follows: A–B < 0.0001; A–C = 0.0035; A–D < 0.0001; A–E > 0.9999; B–C < 0.0001; B–D < 0.0001; B–E < 0.0001; C–D < 0.0001; C–E = 0.0038; D–E < 0.0001. Collectively, these results confirm that ultrasonic cleaning most effectively restores the optical transmittance of discolored retainer membranes while preserving the material’s transparency.

Fig. 3.

Transmittance changes of orthodontic retainer membranes after different cleaning treatments. (A) Sample images from each group: A—untreated control, B—coffee-stained, C—mechanical cleaning, D—chemical cleaning, E—ultrasonic cleaning. (B) Transmittance comparison among groups. Coffee staining significantly reduced transmittance, while cleaning treatments partially restored it, with ultrasonic cleaning showing the best recovery. Significant differences are indicated in the figure (P < 0.05)

Surface roughness analysis

Figure 4; Tables 1, 2 and 3 summarize the surface roughness parameters, including Ra (average roughness), Rz (maximum height), and RSm (mean spacing of irregularities). Coffee staining (Group B) markedly increased surface roughness compared with the untreated control (Group A), with Ra and Rz values reaching 11.11 ± 7.36 μm and 85.06 ± 38.26 μm, respectively (P < 0.0001). Following cleaning, all treatments—mechanical cleaning (Group C), chemical cleaning (Group D), and ultrasonic cleaning (Group E)—significantly reduced Ra compared with the stained group (P < 0.0001, P = 0.0001, and P < 0.0001, respectively). Ultrasonic cleaning achieved the greatest recovery, showing no significant difference from the control (P > 0.9999).

Fig. 4.

Surface roughness parameters of orthodontic retainer membranes after different cleaning treatments. (A) Average roughness (Ra); (B) maximum height difference (Rz); (C) mean spacing of surface irregularities (RSm). Coffee staining (Group B) significantly increased both Ra and Rz relative to the control (P < 0.0001). All cleaning methods effectively reduced surface roughness, with ultrasonic cleaning (Group E) showing the closest recovery to the control (P > 0.9999). A significant difference in RSm was found only between Groups C and E (P = 0.0286). Significant differences are indicated in the figure (P < 0.05)

Table 1.

Surface roughness (Ra) values of retainer membranes in five groups (µm)

Ra	Minimum	Maximum	Mean	Standard Deviation		Lower 95% CI of mean	Upper 95% CI of mean
Group A	0.03	0.10	0.06		0.03	0.03	0.09
Group B	3.42	21.27	11.11		7.36	3.38	18.84
Group C	0.26	0.75	0.50		0.51	0.23	0.27
Group D	0.77	1.39	0.63		0.99	0.25	0.73
Group E	0.11	0.31	0.21		0.22	0.09	0.13

Table 2.

Surface roughness (Rz) values of retainer membranes in five groups (µm)

Rz		Minimum	Maximum	Mean	Standard Deviation	Lower 95% CI of mean	Upper 95% CI of mean
Group A	0.72		3.53	1.72	1.24	0.42	3.02
Group B	38.51		137.80	85.06	38.26	44.90	125.20
Group C	2.17		7.19	5.02	4.70	2.39	2.19
Group D	7.81		18.75	10.94	10.74	4.02	6.52
Group E	1.48		6.91	5.43	3.78	2.16	1.51

Table 3.

Surface roughness (RSm) values of retainer membranes in five groups (µm)

RSm	Minimum	Maximum	Mean	Standard Deviation	Lower 95% CI of mean	Upper 95% CI of mean
Group A	156.08	515.78	340.63	148.85	184.43	496.84
Group B	188.06	374.07	309.18	67.31	238.54	379.81
Group C	57.46	161.25	103.83	48.68	52.74	154.92
Group D	96.42	390.54	185.49	105.62	74.65	296.33
Group E	102.35	603.74	349.76	221.4	117.42	582.1

A similar pattern was observed for Rz: Groups C, D, and E exhibited substantial decreases relative to Group B (P < 0.0001), and none differed significantly from Group A (P > 0.05). These findings indicate that ultrasonic cleaning effectively restored the surface integrity of stained retainers.

For RSm, the values varied considerably among groups (ranging from 103.83 ± 48.68 μm in Group C to 349.76 ± 221.4 μm in Group E). A statistically significant difference was observed only between Groups C and E (P = 0.0286), likely reflecting micro-scratches introduced by mechanical cleaning. No other intergroup differences were detected (P > 0.05). Collectively, these results demonstrate that ultrasonic cleaning most effectively preserves surface integrity and minimizes microabrasion, while mechanical cleaning may locally roughen the surface.

3D surface topography analysis

As shown in Fig. 5, the 3D surface images include optical views, pseudo-color height maps, and 3D reconstructions. Group A displayed a flat, homogeneous surface. Group B showed extensive surface cracking and localized elevation differences, reflecting residual pigment accumulation. Group C produced distinct directional scratches and moderate surface undulation. Group D resulted in uneven, rough surfaces with visible particulate residues, indicating incomplete stain removal. In contrast, Group E resulted in the smoothest surface, with minimal height variations and morphology closely resembling Group A. These topographic findings align with Ra and Rz trends.

Fig. 5.

Surface morphology (A–E), false-color height maps (A1–E1), and 3D topography maps (A2–E2) of orthodontic retainer membranes after different cleaning treatments. A–E represent the control group, stained group, mechanical cleaning group, chemical cleaning group, and ultrasonic cleaning group, respectively. Group B exhibited pronounced surface cracking and irregular height distribution. Group C showed regular scratches. Group D displayed surface particle deposits and uneven roughness. Group E had a relatively smooth and uniform surface, closely resembling the untreated control (Group A). Scale bar: 100 μm

Scanning electron microscopy observations

Figure 6 presents representative SEM micrographs highlighting the surface morphological characteristics of each group. Group A exhibited a smooth and intact surface with uniform texture. In contrast, Group B revealed extensive surface cracking and patchy deposits (red arrows), indicating pigment and contaminant accumulation following coffee staining. Group C displayed distinct linear abrasions (white arrows), consistent with the mechanical wear produced by toothbrush bristles. Group D showed incomplete stain removal, with residual film-like deposits and localized rough regions (yellow arrows), suggesting limited cleaning efficacy of chemical cleaning. Group E presented a smooth and homogeneous surface closely resembling the untreated control, confirming that ultrasonic cleaning effectively removed surface contaminants without inducing structural damage. These microstructural findings align with the profilometric results, further supporting the superior surface preservation achieved by ultrasonic cleaning.

Fig. 6.

SEM images of orthodontic retainer membranes after different treatments (A–E). Groups A–E correspond to untreated control, coffee-stained, mechanical cleaning, chemical cleaning, and ultrasonic cleaning, respectively. Red arrows indicate patchy surface deposits remaining after coffee staining; white arrows indicate linear abrasions formed after mechanical cleaning; yellow arrows indicate residual film-like stains after chemical cleaning. Scale bar: 20 μm

Discussion

Orthodontic retainers serve as the primary method for maintaining post-treatment alignment and preventing relapse [1, 2, 19]. Their physical and optical stability is critical for long-term effectiveness. Orthodontic retainers are typically fabricated from high-transparency thermoplastic polymers [16], including polyurethane (PU), polypropylene/polyethylene copolymers, polyethylene terephthalate glycol (PETG), polycarbonate, and ethylene-vinyl acetate (EVA). Among these, PETG and PU are preferred due to their excellent biocompatibility, thermoformability, and mechanical durability. These retainers are produced by heating thermoplastic sheets and vacuum forming them over plaster or 3D-printed dental models, resulting in close-fitting and highly transparent appliances [16, 20, 21]. However, these polymers are susceptible to discoloration, surface roughening, and reduced mechanical and optical transmittance due to environmental factors such as pigment accumulation, thermal changes, and suboptimal cleaning practices [10–12, 22–25]. Notably, although PETG is widely used in retainer fabrication, its response to cleaning and aging may vary significantly among different brands. A recent comparative study demonstrated that thermocycling and brushing affected the surface roughness and mass stability of PETG retainers to varying extents depending on formulation, indicating that cleaning recommendations may need to be customized for specific material types [26].

The methodological design in this study aimed to replicate realistic clinical scenarios that affect orthodontic retainer materials. Coffee was selected as the staining agent for its frequent use and strong colorant properties, which are known to affect both color and surface quality of dental polymers [5, 7, 27]. Its polyphenolic and tannin content can adhere to thermoplastic surfaces and degrade aesthetics, potentially reducing patient compliance and satisfaction. To replicate clinical discoloration, a standardized coffee-staining model was employed. We then assessed the efficacy of three cleaning methods in restoring surface integrity and optical clarity. These characteristics included optical transmittance, roughness, and surface morphology.

Surface roughness plays a vital role in determining microbial adhesion and patient comfort [7, 28]. Higher roughness increases the likelihood of plaque accumulation and may lead to oral soft tissue irritation [29, 30]. Proper cleaning can help mitigate these issues and maintain the functionality of retainers. A surface profilometer can detect specific roughness parameters (Ra, Rz, and RSm), providing micrometer-level measurements of surface topography. These techniques have been consistently used in evaluating surface changes in dental materials under mechanical or chemical stress [8]. Ra represents average roughness, Rz indicates peak-to-valley height, and RSm refers to the mean spacing of surface irregularities. These parameters reflect surface smoothness, surface uniformity, and potential for microbial interaction. Meanwhile, SEM provided complementary morphological evidence at nanometer-level resolution, enabling visual identification of pigment accumulation, surface discontinuities, and topographical changes. The combination of these techniques offered both quantitative and qualitative insights into optical degradation and surface condition.

Optical transmittance, referring to the ability of light to pass through retainer materials, serves as a key indicator of aesthetic quality and overall performance. Pigments in coffee, especially polyphenols and tannins, readily adhere to polymer surfaces and, under acidic conditions, induce staining and material swelling. These interactions significantly reduce transmittance and affect clinical aesthetics [7, 31, 32]. In this study, Group B (stained) exhibited the lowest transmittance levels (Fig. 2), confirming coffee’s strong staining potential.

Surface morphology observations revealed that Group A exhibited smooth surfaces, Group B showed discoloration and microcracks, Group C displayed abrasion marks, Group D had residual particulates, and Group E closely resembled the untreated control. These patterns affirm the superior efficacy of ultrasonic cleaning in maintaining surface integrity.

All three cleaning methods offered varying levels of improvement in surface properties. Ultrasonic cleaning most effectively restored transmittance, Ra, and Rz, with RSm values nearly matching the control group. It removed surface deposits via cavitation, without visible abrasion, thus preserving surface integrity. Profilometric and SEM analyses corroborated these findings. Consistent with previous investigations, ultrasonic cleaning has been reported to efficiently remove stains and organic residues from polymeric dental materials while maintaining surface smoothness [33], and to significantly reduce microbial biofilm formation even on roughened surfaces [34]. While mechanical cleaning reduced discoloration, it also caused visible scratches, as evidenced by SEM images. These abrasions were further supported by decreased RSm values. These changes compromise both aesthetics and long-term structural stability. Literature suggests that roughened surfaces promote bacterial colonization and plaque accumulation, underscoring the need for caution with mechanical cleaning [35, 36]. Chemical cleaning, employing oxidative agents such as sodium percarbonate along with enzymes and surfactants, provided moderate effectiveness. While able to remove loosely bound stains, it showed limited performance against dense or granular residues [37]. Group D retained visible granular residues (Figs. 5D and 6D). Similarly, transmittance was lower than in Groups C and E (Fig. 3), and Ra and Rz values were higher (Fig. 4), reinforcing the observation. These findings indicate that chemical cleaning produced a moderate cleaning effect—less effective than ultrasonic treatment but with fewer abrasive defects than mechanical cleaning.

Despite the methodological strengths of this study, several limitations should be acknowledged. First, mechanical cleaning was performed manually. Although a standardized protocol was followed, variations in applied force and motion may have introduced operator-dependent variability. Second, only one commercial chemical cleanser and a single concentration were tested. This may not fully represent the diversity of products and usage patterns in real-life settings. Third, the study employed coffee as the sole staining agent. While it is a commonly used chromogen in simulation studies, it does not account for the combined effects of other dietary stains (e.g., tea, red wine) or microbial biofilms. Finally, this was an in vitro study, and the oral environment—featuring salivary proteins, microbial activity, pH fluctuations, and mechanical wear—was not simulated. These factors may influence the cleaning efficacy and material degradation in clinical use. Future research incorporating more complex, dynamic, and long-term in vivo models is recommended to validate and extend the present findings.

Conclusions

This study provides a systematic comparison of three cleaning methods using a standardized coffee staining protocol combined with profilometric and SEM analysis. Ultrasonic cleaning demonstrated the most effective restoration of optical transmittance and surface integrity without causing mechanical damage. Mechanical cleaning showed partial effectiveness but resulted in surface abrasion, while chemical cleaning was less effective in removing tightly adhered residues. These findings underscore the importance of selecting appropriate cleaning strategies based on clinical context, material preservation, and patient compliance. A combined approach may further enhance long-term retainer hygiene and performance.

Author contributions

Conceptualization, Xiaoxian Huang and Ao Jia; Methodology, Ao Jia and Mengxia Peng; Validation, Xiaoxian Huang; Data curation, Jian Liang and Ziyi Wei; Formal analysis, Ao Jia; Writing—original draft, Ao Jia; Writing—review and editing, Hongshan Ge and Zhihua Li; Funding acquisition, Hongshan Ge and Zhihua Li. All authors read and approved the final manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant numbers 81960205 and 82460197.

Data availability

All data generated or analyzed during this study are included in this published article and its supplementary information files.

Declarations

Ethical approval

Not applicable.

Competing interests

The authors declare no competing interests.