Home Electrospun thermoplastic polyurethane/nano-Ag-coated clear aligners for the inhibition of Streptococcus mutans and oral biofilm
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Electrospun thermoplastic polyurethane/nano-Ag-coated clear aligners for the inhibition of Streptococcus mutans and oral biofilm

  • Yun Zhang , Jiarong Yan , Lichao Yu , Yange Wu , Jiajia Shen , Jiayong Zhong EMAIL logo , Junlu Sheng EMAIL logo and Xuepeng Chen EMAIL logo
Published/Copyright: May 22, 2025
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Nanotechnology Reviews
From the journal Nanotechnology Reviews Volume 14 Issue 1

Abstract

The aim of this study is to develop a novel clear aligner coated with electrospun thermoplastic polyurethane (TPU) and a nano-silver-based antibacterial agent to inhibit Streptococcus mutans (S. mutans) growth and biofilm formation, addressing the risk of dental caries and enamel demineralization associated with bacterial accumulation on clear aligners. TPU/Ag-coated aligners were fabricated via electrospinning, incorporating silver nanoparticles (AgNPs) at concentrations of 0, 1, 2, and 5 wt%. The TPU/Ag-coated aligners were characterized for morphology, mechanical properties, adhesion stability, wettability, silver ion release kinetics, antibacterial efficacy, and cytotoxicity using human gingival fibroblasts. The TPU/Ag coatings exhibited uniform fiber morphology with AgNPs dispersed homogeneously. Mechanical tests revealed no significant differences in the modulus of elasticity between coated and uncoated aligners, although the breaking strength and elongation at break decreased slightly at higher AgNP concentrations. The TPU/Ag coating demonstrated good adhesion stability in both dry and wet conditions. Antibacterial assays showed a 5–6 log reduction in free S. mutans and a 44% reduction in biofilm metabolic activity for the 2 wt% group, with acceptable changes in mechanical properties and sustained silver ion release. Cytotoxicity assays confirmed biocompatibility, with cell viability >90% across all groups. The electrospun TPU/Ag-coated aligners with 2 wt% AgNPs effectively inhibit bacterial growth and biofilm metabolic activity without compromising mechanical integrity or biocompatibility. This approach offers a durable, clinically viable solution to mitigate caries risks during clear aligner treatment. Further in vivo studies are warranted to validate long-term safety and efficacy.

Graphical abstract

Keywords: clear aligner; electrospinning; thermoplastic polyurethane; S. mutans ; dental caries; nano-silver-based antibacterial agent

1 Introduction

Orthodontic treatment is gaining popularity as societal aesthetics and comfort expectations evolve, with clear aligners becoming a favored option for their unobtrusive and user-friendly design [1,2]. Clear aligners offer a discreet and comfortable alternative to traditional braces, potentially enhancing patient compliance due to their transparency and comfort [3,4]. They also require fewer clinical visits and reduced chair time, which highly appeal to busy workers and students [5,6,7]. However, the hygienic challenges, such as cariogenic bacteria accumulation and biofilm formation, have raised the risk of dental caries and tooth enamel demineralization [8,9,10,11].

In response to these concerns, the orthodontic community has been actively exploring suitable methods to integrate antimicrobial properties into clear aligners. Including nanoparticles within the aligner material is one approach that has demonstrated the potential to inhibit microbial growth [12]. To preserve the inherent physical and chemical attributes of clear aligners, considerable research has been dedicated to surface modifications that can confer antibacterial characteristics without compromising the aligners’ essential properties [13,14,15,16]. However, current techniques applied for nanoparticle integration in these researches, such as layer-by-layer deposition and chemical interaction, can be costly, time-consuming, and may not be universally applicable due to substrate or morphology specificity [17,18,19]. Interest in alternative fabrication techniques that are more efficient and versatile has been spurred.

Applying nanoparticles to the surface of clear aligners to create coatings that can prevent bacterial accumulation and biofilm development is still a significant challenge, while electrospinning stands out as an innovative technique. Recognized for its efficiency in preparing micro- or nano-sized fibers for various applications, electrospinning is used across different fields, including energy, electronics, environment, and biomaterials [20,21]. The unique features of electrospun nanofibers include high porosity, large surface area, and a straightforward fabrication process. Additionally, nanofibers have a high capacity for incorporating various biological materials or active substances, such as drugs, natural remedies, or metal nanoparticles [22,23]. It offers a simple and cost-effective method to produce nanofibers containing antibacterial agents, significantly enhancing the material’s antimicrobial capabilities without additional processing of the clear aligner diaphragm.

Sliver nanoparticles (AgNPs) are recognized for their broad-spectrum antimicrobial effects, which stem from their ability to disrupt bacterial cell walls and membranes, leading to cell death [24]. Their high surface area-to-volume ratio enhances cell membrane permeability and induces the production of reactive oxygen species, which interfere with bacterial DNA replication [25]. The application of AgNPs in dental care is well documented for its efficacy against a range of microorganisms [24,26].

This study used electrospinning to develop a durable antibacterial coating for clear aligners. Focusing on the coating stability and material performance, a nano-silver-based inorganic agent (NAFUR® RHA-T2) was used and the electrospinning process with thermoplastic polyurethane (TPU) was optimized. The silver-based agent was chosen as an antibacterial agent for its potent antimicrobial properties. TPU was applied as the base material of electrospinning, consistent with clear aligners’ raw material, which made the coating and the aligner diaphragm tightly bonded without additional treatment on the surface of the diaphragm and reduced the complexity of the diaphragm coating process. A TPU/Ag-coated clear aligner plastic (CAP) was prepared, and comprehensive evaluations of its properties were made in this study. The flowchart of this research is shown in Figure 1. Our study presents a novel approach to enhancing the functionality of clear aligners, aiming to improve patient outcomes by reducing the risk of dental caries and tooth enamel demineralization associated with aligner use.

Figure 1 Fabrication flowchart of TPU/Ag-coated CAP.
Figure 1

Fabrication flowchart of TPU/Ag-coated CAP.

2 Materials and methods

2.1 Materials and reagents

As the antibacterial agent, AgNPs (NAFUR® RHA-T2) were obtained from Shanghai Runhe Nano Material Sci. & Tech. Co., Ltd, China. TPU and CAP without any modification were purchased from Shanghai Smartee Denti-Technology Co., Ltd, China. The materials and reagents were used as received. N,N-Dimethylacetamide (DMAc) was purchased from Shanghai Lindi Chemical Reagent Co., Ltd., China.

2.2 Fabrication and characterization of TPU/DMAc solution and fabrication of TPU/Ag nanofibrous membrane-coated CAP

First, TPU elastomers were dissolved in DMAc and then stirred for 10 h to achieve uniform and transparent solutions. Subsequently, with the TPU polymer concentration fixed at 14 wt%, TPU/Ag electrospun solutions containing different AgNP contents (0, 1, 2, and 5 wt%) were prepared by adding AgNPs directly into the TPU/DMAc solutions. The colloidal mixtures were homogenized for 12 h of magnetic stirring to prepare the electrospinning solutions. Subsequently, the solutions were allowed to stand undisturbed until no visible bubbles remained prior to electrospinning processing. The electrical conductivity and viscosity of the spinning solutions were measured using a conductivity meter (FE38, Mettler-Toledo Group, Switzerland) and digital viscometer (NDJ-5S, Shanghai Changji Geological Instruments Co., Ltd., China). Measurements were taken with five replicates.

In the electrospinning process, the prepared solutions were delivered through a capillary tube fitted with a 0.5 mm diameter metal syringe needle using a DXES-3 electrospinning apparatus (Shanghai Oriental Flying Nano-technology, China). The flow rate of the solution was 3 mL h−1, and the applied voltage was 25 kV. The distance was 21 cm between the injection syringe and the collector covered with CAP. During electrospinning, the temperature and relative humidity were 23 ± 2°C and 45 ± 2%, respectively. The electrospinning time was about 30 min. The prepared nanofibrous membrane without AgNPs was defined as a TPU group, and the nanofibrous membranes with AgNPs were defined as TPU/Ag-x, where x was the various mass percents of AgNPs.

2.3 Characterization of CAP, TPU, and TPU/Ag-x diaphragms

2.3.1 Scanning electron microscopy (SEM) observation and surface element analysis

The surface morphologic structure of the nanofibrous membranes was studied using a SEM, (Apreo 2, Thermo Fisher Scientific, USA). The SEM analysis was conducted at an acceleration voltage of 20 kV with a working distance of 10.1 mm. The surface element analysis of the nanofibrous membranes was characterized through energy-dispersive spectroscopy (EDS) analysis (Super-X, Thermo Fisher Scientific, USA). For EDS measurements, the samples were mounted on conductive carbon tape and analyzed until achieving a characteristic X-ray count rate exceeding 200,000 counts per second (cps), maintaining an energy resolution of 125 eV at Mn Kα.

2.3.2 Tensile testing

Tensile tests were conducted according to the standard GB/T 1040.3-2006 for plastics. CAP and electrospun diaphragms were prepared first. The diaphragms were cut into dumbbell-shaped specimens using a pre-sized cutting machine. The tensile tests were performed using a Shimadzu AG-X Plus 50 kN universal testing machine at a speed of 2 mm/min and a gauge length of 25 mm. The modulus of elasticity, breaking strength, and elongation at break of the tested samples were recorded. These parameters of all electrospinning groups were compared with those of the CAP group to assess whether electrospinning would affect the tensile properties of the diaphragm. Experiments were conducted with three replicates.

2.3.3 Resistance to separation of the coating

A modified cross-cut test (referring to ISO-2409:2020, Paints and varnishes – Cross-cut test) was performed to evaluate the resistance of the TPU and TPU/Ag nanofibrous coating to separation from the clear aligner diaphragm in both dry and wet states. Referring to a previous study, a 6 × 6 mm square grid was cut into the TPU and TPU/Ag nanofibrous coatings using a multi-blade cutting tool with 1 mm blade spacing, deviating from the standard 5 × 5 mm grid. In the dry groups, the samples were kept for reserve use, while in the wet groups, the samples were soaked in 37°C double-distilled water for 30 min and then taken out to simulate the oral wet environment. An adhesive tape (#600, 3 M, USA) was then applied fully over the 6 × 6 square grid cut in a direction parallel to the cuts. The tape was pressed down and rubbed with a fingernail to ensure good adhesion to the coating. After 5 min, the tape was pulled off steadily in 1 s at an angle of 60° to the diaphragm, using a protractor to ensure precision. The cutting area was then observed under a stereomicroscope to evaluate the separation result of the coatings. Specifically, a detached cutting area was scored 0 point, irrespective of the size of the detached area, whereas a cutting area with fully retained coating was scored 1 point. The sum of these scores for each coating was recorded as the coating’s resistance ability to separation. Experiments were conducted with five replicates for both dry and wet groups.

2.3.4 Water contact angle (WCA) test

WCA was measured to characterize the surface wettability using a dynamic contact angle testing instrument (DSA-30, Beijing Liangshan Xincheng Technology, China). The analyzer was equipped with a microsyringe that could dispense 3 microliter volumes of water droplets. The WCAs were measured under normal atmospheric conditions at 23 ± 2°C. For each sample, measurements were taken at five distinct locations to ensure spatial representation, with subsequent data processing involving the calculation of both mean values and standard deviation.

2.3.5 Evaluation of silver ion release

Silver ion release from the TPU and TPU/Ag-x clear aligner diaphragms was measured as described in the following. The TPU, TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 diaphragms were cut into 2.0 cm × 2.0 cm squares and placed in a 6-well plate with the coated side facing up. About 10 mL of double-distilled water was added as the release medium in every well of the 6-well plates. The 6-well plates were then incubated at room temperature, and the leaching solution samples were collected at 24, 48, 96, 144, 192, and 240 h, after which 10 mL of double-distilled water was added each time. Silver ion concentration in the leaching solution samples were measured using inductively coupled plasma mass spectrometry (ICP-MS, iCAP RQ, Thermo, Germany) to track silver ion release from the diaphragm sample over the experimental period. A release curve was subsequently plotted based on the collected data. Experiments were conducted with three replicates for each group.

2.4 Evaluation of antibacterial properties

2.4.1 Colony‑forming unit assay

Streptococcus mutans (S. mutans, UA159) was obtained from the Key Laboratory of Oral Biomedical Research of Zhejiang Province, School of Stomatology, Zhejiang University School of Medicine, and cultured at 37°C micro-aerobically for 18 h in brain heart infusion (BHI, Sigma, USA) medium and adjusted to 1 × 106 CFU/mL with BHI medium for further usage. The CAP, TPU, and TPU/Ag-x diaphragms were cut into squares of 2.0 cm × 2.0 cm and put in a sterile 6-well plate with the coated surface facing up. The 6-well plate was placed on the counter of the biosafety cabinet (Thermo Scientific 1300, Thermo Fisher Scientific, USA) under the ultraviolet (UV) lamp, and the cover of the 6-well plate was opened to expose the coated surface of the diaphragm sample, and UV sterilization was performed for 30 min. After the UV sterilization of the diaphragm sample, 100 μL of 1 × 106 CFU/mL S. mutans solution was dripped on the surface of the sample coating, and a polypropylene (PP) plastic film of 1.8 cm × 1.8 cm was covered onto the droplet. The air between the surface and the film was gently squeezed out, and the S. mutans solution was spread to the edges of the PP plastic. The 6-well plates were incubated at 37°C micro-aerobically for 24 h. After incubation, 3 mL of PBS was added to the wells of the 6-well plates to flush off the bacteria on the samples and films. A 10-fold serial dilution of the eluted bacteria was prepared with PBS. About 100 μL diluted S. mutans solution was applied evenly on a BHI solid agar medium (BHI with 1.5 wt% agar) and incubated at 37°C micro-aerobically for 48 h to count the S. mutans colonies. Experiments were conducted with three replicates for each group.

2.4.2 Methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay

An MTT assay was performed to further evaluate the anti-biofilm ability of the TPU/Ag-coated clear aligners. S. mutans was cultured at 37°C micro-aerobically for 18 h in BHI medium and then adjusted to 1 × 106 CFU/mL with a sugar-containing BHI (BHI with 1 wt% sucrose, BHIS) medium for further use. The sterilized CAP and TPU/Ag-x were cut into squares of 1.1 cm × 1.1 cm and put into wells of a 24-well plate with the coated surface facing up, and 400 μL of the S. mutans solution mentioned earlier was dripped to immerse the coated surface of each sample. The 24-well plate was incubated at 37°C micro-aerobically for 24 h. After incubation, the samples were put into a new 24-well plate, and 1 mL of 0.5 mg/mL MTT solution (Beyotime, China) was dripped into each well of the 24-well plate to incubate with the biofilm at 37°C micro-aerobically for 4 h. After incubation, the MTT solution was replaced by 1 mL of dimethyl sulfoxide (DMSO, Sigma, USA). After shaking the 24-well plate on a table concentrator for 30 min, 200 μL of the DMSO solution from each well was piped into a well of a 96-well plate, and the absorbance of each well at 570 nm was measured using a spectrophotometer (Synergy H1, Bio-Tek Instruments, USA). The absorbance of each sample was the average of three repetitions. Experiments were conducted with five replicates for each group.

2.4.3 pH value evaluation

Given that cariogenic bacteria within biofilms ferment carbohydrates to produce organic acids, causing significant decreases in local pH levels, a pH evaluation around the biofilm was conducted to further assess the metabolic activity of the biofilms. The sample preparation followed the same procedure as in the MTT assay, with an additional group containing pure fresh BHIS medium as a natural control. Samples and BHIS medium were co-incubated in a 24-well plate at 37°C for 24 h microaerobic conditions. Then, the diaphragm samples were removed, and the pH value of the solution in each well was measured three times using a pH meter (FE28–FiveEasy Plus™, METTLER TOLEDO, USA) to calculate the average. Experiments were conducted with five replicates for each group.

2.5 Evaluation of cytotoxicity

2.5.1 Cell counting Kit‑8 (CCK-8) assay

The in vitro biocompatibility of the coatings was evaluated using the CCK-8 method. Primary human gingival fibroblasts (HGFs) were obtained from the gingival tissues of adolescents after alveolar surgery at Stomatology Hospital, School of Stomatology, Zhejiang University School of Medicine. The protocol was approved by the Ethics Committee of Hospital of Stomatology, Zhejiang University School of Medicine (No. 2023Y061). The following assay was performed using HGFs of the fourth to sixth generations: the CAP and TPU/Ag-x were cut into rectangular samples of 2.0 cm × 2.0 cm. Then, every sample was cut into small pieces and co-cultured with 2 mL alpha minimum essential medium (aMEM, Cytiva, USA) containing 10% fetal bovine serum (FBS, Gibco, USA) in a 5 mL centrifuge tube at 37°C with 5% CO2 atmosphere for 3 days to obtain sample extracts. HGFs were seeded in a 96-well plate for 1 × 104 cells per well and cultured by αMEM with 10% FBS at 37°C and 5% CO2 for further use. When the cell density reached about 70%, the culture medium was replaced by the sample extracts (100 μL per well, three wells for each extract), and cells with pure culture medium were served as the natural culture group (NC group). After incubating the 96-well plate for 24 h, 10 μL CCK-8 (Beyotime, China) solution was dripped into each well and incubated for another 2 h. The absorbance of each well at 450 nm was measured using a spectrophotometer (Synergy H1, Bio-Tek Instrument, USA).

The cell viability (%) was determined using the following equation:

Cell viability ( % ) = [ A ( test ) − A ( medium ) ] / [ A ( NC ) − A ( medium ) ] × 100 %

where A(test) represents the absorbance of all CAP and NRCAP groups. A(NC) is that of cells of the NC group. A(medium) stands for the absorbance of pure culture media without cells. For each group, the absorbance at 450 nm was the average of three repetitions. Experiments were conducted with five replicates for each group.

2.5.2 Live/dead cell staining analysis

Calcein AM/PI Cell Vitality/Cytotoxicity Assay Kit (Beyotime, China) was used for live/dead cell staining analysis to observe the cell morphology and the distribution of living and dead cells. The Calcein AM and PI provided in this kit were 1,000x solutions. The cell culture process was the same as the CCK‑8 assay. After co-incubating HGFs with the sample extracts in the 96-well plate for 24 h, the cells were stained by Calcein AM/PI staining solution for another 30 min. Fluorescence images were taken by an inverted fluorescence microscope (EVOS M5000, Invitrogen, USA). Three fields of view were randomly selected for each well at 100 times magnification to take images. Experiments were conducted with three replicates for each group.

2.6 Statistical analysis

The results were presented as the mean ± standard deviation. SPSS statistical software (IBM SPSS 22.0, Armonk, USA) was used for statistical analysis. All data were analyzed using one-way analysis of variance followed by Bonferroni’s multiple comparison tests. The significance level for all tests was set at α = 0.05.

3 Results

3.1 Characterization of CAP, TPU, and TPU/Ag-x

3.1.1 Morphological observation

The morphological structure and diameter distribution were characterized by SEM, as shown in Figures 2 and S1. As for the CAP, there were no nanofibers. After electrospraying TPU fibrous membranes on the CAP, there were obvious fibers. The diameters of TPU fibers were non-uniform with a wide diameter distribution ranging from 1,025 to 6,000 nm, and the average fiber diameter was approximately 2,500 nm (Figure 2f). Adding AgNPs with the concentration of 1 wt%, the fiber diameter decreased to 327 nm significantly, and the distribution of the fiber diameter changed to narrow down, which was due to the increased conductivity and viscosity with little change of the spinning solution (Table S1). Upon further increasing the AgNP content from 1 to 5 wt%, the average fiber diameter exhibited increasing tendency. The average diameters of the TPU/Ag-2 and TPU/Ag-5 fibrous membranes were approximately 494 nm and 701 nm, respectively. Besides, the agglomeration of AgNPs increased gradually. When the concentration of AgNPs was 5 wt%, the AgNPs appeared obvious agglomeration (Figure 2e).

Figure 2 SEM images of CAPs with different Ag concentrations: (a) CAP, (b) TPU, (c) TPU/Ag-1, (d) TPU/Ag-2, (e) TPU/Ag-5, and (f) average fiber diameter.
Figure 2

SEM images of CAPs with different Ag concentrations: (a) CAP, (b) TPU, (c) TPU/Ag-1, (d) TPU/Ag-2, (e) TPU/Ag-5, and (f) average fiber diameter.

3.1.2 Elemental analysis

The results of elemental mapping and energy-dispersive X-ray spectrum (EDX)-mapping measurements are shown in Figure 3. In these atomic mapping images, AgNPs were represented by yellow-green dots. As shown in Figure 3e, the homogeneous distribution of dots confirmed that the AgNPs were uniformly distributed on the nanofibers such that the particles were well dispersed uniformly on the nanofibrous membranes.

Figure 3 Elemental mapping of TPU/Ag-2-coated CAP surface: (a) C, (b) N, (c) O, (d) Si, and (e) Ag.
Figure 3

Elemental mapping of TPU/Ag-2-coated CAP surface: (a) C, (b) N, (c) O, (d) Si, and (e) Ag.

3.1.3 Tensile test

The tensile test results are presented in Figure 4, where the dimensions of the tensile specimen are illustrated in Figure 4a. From the experimental results, no significant differences in the modulus of elasticity were observed among the CAP, TPU, TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 groups (898.14 ± 21.55 N/mm2 for CAP, 895.50 ± 23.79 N/mm2 for the TPU group, 897.30 ± 21.11 N/mm2 for TPU/Ag-1, 896.51 ± 34.46 N/mm2 for TPU/Ag-2, and 874.52 ± 15.01 N/mm2 for TPU/Ag-5, p > 0.05) (Figure 4a). Similarly, no significant differences in breaking strength were found between the CAP group and the TPU, TPU/Ag-1, and TPU/Ag-2 groups (44.89 ± 1.76 N/mm2 for CAP, 44.16 ± 1.46 N/mm2 for the TPU group, 43.02 ± 0.94 N/mm2 for TPU/Ag-1, and 41.74 ± 0.66 N/mm2 for TPU/Ag-2, p > 0.05), while the TPU/Ag-5 group exhibited a slightly lower breaking strength compared to the CAP group (41.26 ± 1.01 N/mm2 for TPU/Ag-5, p < 0.05) (Figure 4b). However, there were no significant differences in breaking strength between the TPU, TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 groups (p > 0.05). In terms of elongation at break, no significant difference was found between the CAP and TPU groups (8.04 ± 0.19% for CAP and 7.98 ± 0.21% for TPU, p > 0.05), but the TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 groups showed a slightly lower elongation at break compared to the CAP and TPU groups (8.04 ± 0.19% for CAP, 7.22 ± 0.29% for TPU/Ag-1, 7.18 ± 0.20% for TPU/Ag-2, and 7.22 ± 0.29% for TPU/Ag-5, p < 0.05) (Figure 4c). Notably, there were no significant differences in elongation at break between the TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 groups (p > 0.05).

Figure 4 Tensile test of the CAP, TPU-coated clear aligner diaphragm, and TPU/Ag-x-coated clear aligner diaphragm: (a) specimen size, (b) modulus of elasticity, (c) breaking strength, and (d) elongation at break.
Figure 4

Tensile test of the CAP, TPU-coated clear aligner diaphragm, and TPU/Ag-x-coated clear aligner diaphragm: (a) specimen size, (b) modulus of elasticity, (c) breaking strength, and (d) elongation at break.

3.1.4 Resistance to separation of the coating

Figure 5a illustrates the schematic of the resistance to separation assessment. In the dry state, the results of resistance to separation of the TPU and TPU/Ag-x coatings from the clear aligner diaphragm are shown in Figure 5b. Although the TPU group had a relatively low score (26.00 ± 4.64), the other TPU/Ag-x groups achieved higher scores nearing the maximum (33.40 ± 2.61 for TPU/Ag-1, 35.00 ± 1.00 for TPU/Ag-2, and 35.80 ± 0.45 for TPU/Ag-5, p < 0.05), with no significant differences among the TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 groups (p > 0.05). Similarly, in the wet state (Figure 5c), despite the TPU group’s higher score (31.40 ± 2.51, p < 0.05) compared to the dry state, the TPU/Ag-x coatings still outperformed the TPU group (34.40 ± 1.14 for TPU/Ag-1, 35.00 ± 0.71 for TPU/Ag-2, and 35.60 ± 0.55 for TPU/Ag-5, p < 0.05), with no significant differences among the TPU/Ag-x groups.

Figure 5 Evaluation of the resistance to separation of the TPU and TPU/Ag-x coatings from the clear aligner diaphragm: (a) schematic diagram of the resistance to separation assessment, (b) score of resistance to separation of the coatings in dry state, and (c) score of resistance to separation of the coatings in wet state.
Figure 5

Evaluation of the resistance to separation of the TPU and TPU/Ag-x coatings from the clear aligner diaphragm: (a) schematic diagram of the resistance to separation assessment, (b) score of resistance to separation of the coatings in dry state, and (c) score of resistance to separation of the coatings in wet state.

3.1.5 WCA test

The WCA of the membranes is shown in Figure 6. The WCA of the CAP was 90.9°, exhibiting hydrophobicity. As for the TPU membranes, the WCA was 86.1o. After introducing AgNPs, the average WCA of TPU/Ag-1 fibrous membranes was 91o. With increasing the concentration of AgNPs, the contact angle of the membranes increased to 93o. There was no significant difference between the CAP group and TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 groups (90.9 ± 0.8 for CAP, 91.0 ± 1.5 for TPU/Ag-1, 92.3 ± 1.4 for TPU/Ag-2, and 93.0 ± 1.0 for TPU/Ag-5, p > 0.05).

Figure 6 WCA of the CAP and TPU/Ag fibrous membranes.
Figure 6

WCA of the CAP and TPU/Ag fibrous membranes.

3.1.6 Silver ion release

Silver ion release from the TPU, TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 clear aligners is shown in Figure 7. There were near no silver ions detected in the leaching solution samples collected from the TPU group. In the first 2 days, the release was rapid. In the following 8 days, ion release was kept in a slower rate as time goes. The silver ion release amount was higher in the groups of higher silver concentration groups. After 10 days, there was still silver ion release existing, and the concentration was 0.30 ppb, 2.16 ppb, and 28.25 ppb in TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5 groups, respectively.

Figure 7 Silver ion release curves for TPU and TPU/Ag-x clear aligner diaphragms.
Figure 7

Silver ion release curves for TPU and TPU/Ag-x clear aligner diaphragms.

3.2 Antibacterial evaluation

3.2.1 Antibacterial efficacy against S. mutans

Figure 8a presents the schematic of the antibacterial evaluation against S. mutans. As depicted in Figure 8b and c, the TPU group demonstrated a similar magnitude of S. mutans concentration to the CAP group (9.32 ± 0.21 log(CFU/mL) for CAP and 9.35 ± 0.16 log(CFU/mL) for TPU, p > 0.05). In contrast, all TPU/Ag-x groups exhibited significantly lower magnitude of S. mutans concentrations than both the CAP and TPU groups (4.00 ± 0.18 log(CFU/mL) for TPU/Ag-1, 3.69 ± 0.04 log(CFU/mL) for TPU/Ag-2, and 3.15 ± 0.34 log(CFU/mL) for TPU/Ag-5, p < 0.05). Notably, the TPU/Ag-5 group demonstrated the lowest S. mutans concentration among all groups (p < 0.05), while no significant difference was found between the TPU/Ag-1 and TPU/Ag-2 groups (p > 0.05).

Figure 8 Antibacterial efficacy against S. mutans: (a) schematic diagram of the CFU assay, (b) order of magnitude of CFU count of different groups, (c) 105 dilution of S. mutans colonies flushed off the CAP, (d) 105 dilution of S. mutans colonies flushed off the TPU group, (e) no dilution of S. mutans colonies flushed off the TPU/Ag-1, (f) no dilution of S. mutans colonies flushed off the TPU/Ag-2, and (g) no dilution of S. mutans colonies flushed off the TPU/Ag-5.
Figure 8

Antibacterial efficacy against S. mutans: (a) schematic diagram of the CFU assay, (b) order of magnitude of CFU count of different groups, (c) 105 dilution of S. mutans colonies flushed off the CAP, (d) 105 dilution of S. mutans colonies flushed off the TPU group, (e) no dilution of S. mutans colonies flushed off the TPU/Ag-1, (f) no dilution of S. mutans colonies flushed off the TPU/Ag-2, and (g) no dilution of S. mutans colonies flushed off the TPU/Ag-5.

3.2.2 Antibacterial efficacy against S. mutans biofilm

Figure 9a shows the schematic of the antibacterial evaluation against S. mutans biofilm. Figure 9b shows that, compared to the CAP group, the TPU group had a higher absorbance value at 570 nm, although not statistically significant (0.609 ± 0.054 for CAP, 0.735 ± 0.058 for the TPU group, p > 0.05). The absorbance value of the TPU/Ag-1 group at 570 nm (0.535 ± 0.067) was similar to that of the CAP group (p > 0.05) but lower than that of the TPU group (p < 0.05). The TPU/Ag-2 and TPU/Ag-5 groups had comparable absorbance values at 570 nm (0.358 ± 0.051 for TPU/Ag-2 and 0.357 ± 0.084 for TPU/Ag-5, p > 0.05), both significantly lower than that of the CAP, TPU, and TPU/Ag-1 groups (p < 0.05).

Figure 9 Antibacterial efficacy against S. mutans biofilm and pH value assay: (a) schematic diagram of the S. mutans biofilm culture, (b) MTT assay showed that the TPU/Ag-2 and TPU/Ag-5 groups had a significantly lower absorbance value than CAP, and (c) pH value assay showed that the TPU/Ag-2 and TPU/Ag-5 groups had a statistically higher pH value than CAP.
Figure 9

Antibacterial efficacy against S. mutans biofilm and pH value assay: (a) schematic diagram of the S. mutans biofilm culture, (b) MTT assay showed that the TPU/Ag-2 and TPU/Ag-5 groups had a significantly lower absorbance value than CAP, and (c) pH value assay showed that the TPU/Ag-2 and TPU/Ag-5 groups had a statistically higher pH value than CAP.

Figure 9c shows that all tested groups (CAP, TPU, TPU/Ag-1, TPU/Ag-2, and TPU/Ag-5) groups had a significant pH decrease compared to the BHIS group (p < 0.05). The CAP, TPU, and TPU/Ag-1 groups exhibited comparable pH levels (3.91 ± 0.02 for CAP, 3.89 ± 0.01 for TPU, and 3.91 ± 0.02 for TPU/Ag-1, p > 0.05). In contrast, the TPU/Ag-2 and TPU/Ag-5 groups demonstrated significantly higher pH values than the other groups (4.02 ± 0.02 for TPU/Ag-2 and 4.20 ± 0.11 for TPU/Ag-5, p < 0.05), with the TPU/Ag-5 group showing the highest pH value among all groups (p < 0.05).

3.3 Cytotoxicity evaluation

CCK-8 assay (Figure 10a) indicated that the cell viability of the NC group had no significant difference from all the CAP, TPU, and TPU/Ag-x groups (100% ± 9.32% for NC group, 97.51% ± 10.78% for CAP, 95.30% ± 10.31% for TPU, 90.49% ± 11.05% for TPU/Ag-1, 96.54% ± 4.74% for TPU/Ag-2, and 92.79% ± 13.97% for TPU/Ag-5, p > 0.05). Live/dead cell staining results (Figure 10b) show that almost no dead HGFs were visible in all groups.

Figure 10 Cytotoxicity evaluation: (a) CCK-8 assay revealed that the cell viability values of CAP and all TPU/Ag groups were similar to those of the NC group and (b) fluorescence microscopic observation of live/dead cell staining showed a large amount of live (green) cells and almost no dead (red) cells in all groups.
Figure 10

Cytotoxicity evaluation: (a) CCK-8 assay revealed that the cell viability values of CAP and all TPU/Ag groups were similar to those of the NC group and (b) fluorescence microscopic observation of live/dead cell staining showed a large amount of live (green) cells and almost no dead (red) cells in all groups.

4 Discussion

The clear aligner system has become increasingly popular due to its cosmetic properties and patient-friendly design [4]. However, concerns have been raised regarding the risk of tooth enamel demineralization and dental caries due to microbial accumulation on aligners [27,28]. Researchers are currently exploring two primary strategies for antibacterial dental appliances: enhancing hydrophilicity to deter bacterial adhesion and utilizing antibacterial agents for broader oral bacteria management [29]. Surface modifications, which add functionality without altering the core properties of biomedical devices, are a vital area of focus [30]. Antibacterial coatings on device surfaces have been identified as an effective method to combat bacterial adhesion and biofilm development [31,32]. To achieve antibacterial coating, various antimicrobial agents and fabrication strategies have emerged [33]. Nanomaterials, including silver, gold, zinc, and metal oxide nanoparticles, have been reported to show antimicrobial ability [34,35]. Currently, limited research focuses on antibacterial nanoparticle agents for antibacterial coating preparation on clear aligner surfaces for the inhibition of microbes.

This study introduces an electrospun antibacterial coating for clear aligners, leveraging the potential of a nano-silver-based inorganic agent to counteract bacterial colonization on aligner surfaces. Since the electrospinning technology uses the same material as the clear aligner as the electrospinning fluid, the coating and the aligner diaphragm can be tightly bonded without additional treatment on the surface of the diaphragm, which reduces the complexity of the diaphragm coating process. SEM observation confirmed the successful integration of AgNPs within the TPU nanofibers on clear aligners. However, increased AgNP concentrations led to particle agglomeration. To determine the distribution of AgNPs in the TPU/Ag composite fibers and ensure that the AgNPs were adhered to the surface of the TPU/Ag nanofibers, elemental mapping and EDX-mapping measurements were performed. As observed, AgNPs were uniformly distributed on the nanofibers, ensuring a consistent surface modification effect on the entire clear aligner surface.

As the AgNPs content increased, the conductivity and viscosity of the spinning solution rose significantly, affecting both the macroscopic morphology and fiber diameter. The TPU fibers exhibited a non-uniform diameter distribution, ranging from 1,025 to 6,000 nm, with an average of approximately 2,500 nm (Figure 2f). When 1 wt% AgNPs was added, the fiber diameter decreased significantly to 327 nm, and the distribution of the fiber diameter changed to narrow down, which was due to the increased conductivity and viscosity with little change of the spinning solution (Table S1). When the concentration of the AgNPs increased from 1 to 5 wt%, the corresponding average fiber diameters increased from 327 to 701 nm (Figure 2f).

Mechanical properties are key factors in the realization of orthodontic treatment effects through clear aligner treatment [12], and adhesion performance is directly related to the stability and durability of the coating on the clear aligner diaphragm. This study evaluated the mechanical properties and adhesion performance of TPU and TPU/Ag-x-coated clear aligner diaphragms in both dry and wet states. Results showed no significant difference in the modulus of elasticity among the CAP, TPU, and TPU/Ag-x-coated diaphragms. As the concentration of AgNPs increased, there was a minimal impact (below 10%) on breaking strength and elongation at break in some groups, which was within an acceptable range. Since the clear aligner mainly relies on its elasticity to apply the orthodontic force in practical applications, the nearly constant modulus of elasticity and slightly decreased breaking strength and elongation at break of the clear aligner diaphragm in this study indicate that electrospinning technology could add functionality without compromising the basic physical properties of the clear aligner diaphragms. Moreover, the adhesion performance test results were promising. Using a modified cross-cut test, the separation resistance of the TPU and TPU/Ag nanofibrous coatings from the clear aligner diaphragm in both dry and wet states was assessed. The TPU group had relatively low adhesion in the dry state but a significantly higher score in the wet state, possibly due to reduced tape adhesion in wet conditions. All TPU/Ag-x groups scored near the maximum in both the dry and wet states, indicating that the addition of AgNPs significantly enhanced the adhesion of the coating to the clear aligner diaphragm. This result is crucial for ensuring the long-term stability of the coating in the oral environment, as it reduces the risk of coating wear or detachment due to food intake and daily cleaning.

Oral biofilm, also called dental plaque, cannot be washed or rinsed off by water due to its soft, unmineralized nature and protective matrix [36]. Its formation on the clear aligners can compromise oral health [37]. Previous studies have linked the hydrophobicity of clear aligner surfaces to their cleanability and resistance to fouling, and the hydrophilic properties of the membrane surface greatly affect the adhesion of bacteria [16,38,39]. Therefore, the WCA of the membranes was characterized in this study. In the results, no significant change in surface wettability with the introduction of the antibacterial coating was found, suggesting that the electrospinning coating did not change the wettability of the pristine CAP.

Dental caries is closely associated with cariogenic bacteria [40]. Therefore, effectively controlling the proliferation of such bacteria is pivotal in preventing and managing dental caries [13]. S. mutans, a key player in caries development, is known for its acidogenic properties that can erode tooth enamel [41,42]. The results indicated that the TPU/Ag-coated clear aligners significantly reduced the number of free S. mutans by 5–6 orders of magnitude compared to the CAP and TPU groups, highlighting their potential in dental caries prevention.

The ability of cariogenic bacteria to form resistant biofilms compounds the challenge of managing dental caries because biofilms are complex bacterial communities less susceptible to antimicrobial treatments than free bacteria [43]. To evaluate the anti-biofilm property of the TPU/Ag-coated clear aligner diaphragms, an MTT assay for anti-biofilm assessment was conducted. The results revealed that, compared to the CAP group, the TPU group showed a similar metabolic activity of biofilms. The TPU/Ag-1 group, despite showing a strong antibacterial effect on free S. mutans, had a relatively poor inhibitory effect on biofilm. However, when the AgNP concentration reached 2 and 5 wt%, the TPU/Ag-coated clear aligners significantly reduced the metabolic activity of biofilms by 44%. This inhibition of both free S. mutans and biofilms indicated that the coating provided a protective barrier against bacterial growth and biofilm metabolic activity, which is consistent with the broad-spectrum antimicrobial properties of AgNPs.

Poor oral hygiene practices can lead to the fermentation of carbohydrates by cariogenic bacteria, resulting in the production of organic acids that cause demineralization of tooth enamel. Typically, the local pH level drops significantly in the presence of cariogenic bacteria, potentially plummeting to a pH of around 4 near the biofilms [29,44], as evidenced by the pH decrease in all tested groups compared to fresh BHIS in this study. Our findings also indicated a slight but statistically significant increase in pH with TPU/Ag-coated clear aligner diaphragms, particularly at higher AgNP concentrations, further confirming their antibacterial effect and potential to counteract the acidic conditions that promote caries formation. However, the minimal pH increase also suggested that a substantial bacterial population remained active. Based on the results of the anti-free S. mutans and anti-biofilm tests, it was inferred that the primary mode of antibacterial action of the coating was through direct contact, leading to bacterial inactivation. Despite SEM observations indicating AgNP agglomeration at higher concentrations, which might lower antibacterial effectiveness, the electrospun coating’s actual antibacterial efficacy rose with the AgNP concentration.

The release rate and the concentration of Ag ions were determined by the AgNP concentration and the nanofibers’ surface area [45,46]. As shown in Figure 7, Ag ion release decreased sharply after the first 48 h because AgNPs did not adhere completely to the nanofiber, but a part was only dispersed in the nanofiber membrane. Once the unstable AgNPs were depleted in 48 h, Ag ions were gradually released from the residual and deep-seated AgNPs. Accordingly, in the following 8 days, ion release was kept at a slower rate over time. The silver ion release amount was higher in the higher silver concentration groups. The continuous release of silver ions from the TPU/Ag-coated clear aligner was critical, ensuring a sustained antibacterial effect over time. This release profile was essential for maintaining the coating’s effectiveness against microbial challenges without the need for frequent recharge or additional treatments. The sustained release also implied a potential long-term benefit in reducing the risk of caries during the wearing duration of the clear aligner in orthodontic treatment, making the TPU/Ag-coated clear aligners good candidates as antibacterial aligners.

For biosafety evaluation, the extracts of CAP and TPU/Ag-x were obtained to assess their in vitro cytotoxicity. Since the clear aligners are most likely to contact the gingiva during wearing, HGFs were chosen as the experimental subject of the cytotoxicity evaluation. Figure 10 shows the viability of HGF cells in different groups. After co-incubation with the sample extracts, normal morphologies of HGF cells in all groups were maintained, and there were almost no dead cells in all groups. The in vitro cytotoxicity assessment revealed that the TPU/Ag coating was biocompatible with HGFs, indicating its safety in oral applications. This was a significant consideration, given the known potential toxicity of AgNPs even at low concentrations [46,47].

Compared to other antibacterial solutions such as mouthwash, tooth brushing, and fluoride use, TPU/Ag-coated clear aligners offer significant real-world orthodontic advantages. TPU/Ag coatings can attach directly to the clear aligner surface, providing a sustained antibacterial effect without the need for frequent reapplication or treatment. This reduces treatment inconvenience and complexity, improving patient acceptance and compliance. In addition, TPU/Ag-coated clear aligners can be used in combination with other antibacterial solutions to enhance the antibacterial and caries-preventive effect during the clear aligner treatment.

While the in vitro findings are promising, further in vivo studies are essential to evaluate the TPU/Ag-coated clear aligner’s performance under the dynamic conditions of the oral environment. In real-world orthodontic applications, clear aligners face challenges such as saliva interactions, dynamic oral forces, and prolonged bacterial exposure. Enzymes, proteins, and other components in saliva may interact with the coating on the surface of the clear aligner, affecting its stability and antibacterial effect. Therefore, the resistance of the coating to saliva is an important consideration. Dynamic oral forces, such as those generated by chewing and speaking, also have an impact on the coating. The coating needs to maintain its shape and position under the action of these forces to ensure the antibacterial effect. Although the TPU/Ag coating performed good mechanical properties and adhesion in this study, it still needs to withstand these dynamic forces and maintain stability and function in the mouth. In addition, prolonged bacterial exposure is another important challenge. The oral cavity has a complex microbial environment. In the long-term use of the TPU/Ag-coated clear aligner, it is necessary to continuously and effectively inhibit the growth of bacteria and the formation of biofilm. The coating’s silver ion release can maintain antibacterial effects over time and reduce bacterial accumulation risk, but this needs real-oral-environment testing. What is more, long-term use of TPU/Ag-coated aligners may raise some safety concerns. Despite showing good biocompatibility in this study, the effects of silver accumulation on oral tissue and systemic health during long-term use require further research. Additionally, the oral flora is a complex ecosystem maintaining oral health. Long-term silver ion release may alter the oral flora’s composition and balance, affecting oral health. Therefore, future studies should deeply evaluate the long-term safety of TPU/Ag-coated appliances, including their impact on oral flora and the potential risk of silver accumulation.

5 Conclusion

The integration of a silver-based antibacterial agent into clear aligners through electrospinning presents a novel strategy to combat dental caries. The same material as the clear aligners’ raw material was applied as the base material of electrospinning, making the coating and the aligner diaphragm tightly bonded without additional treatment on the surface of the diaphragm and reducing the complexity of the coating process. The findings indicate that the TPU/Ag-coated clear aligners effectively inhibit bacterial growth and biofilm metabolic activity, offering a durable and biocompatible solution without significantly affecting the mechanical properties of the clear aligner diaphragm. Among the various concentrations of AgNPs tested, the 2 wt% TPU/Ag-coated clear aligners emerged as the optimal formulation, exhibiting the most effective balance of properties. This optimal concentration provided a significant reduction in bacterial growth and biofilm formation, a sustained release of silver ions over time, and a minimal impact on the oral environment. For example, the 2 wt% TPU/Ag-coated clear aligners demonstrated a 5–6 orders of magnitude reduction in free S. mutans compared to the CAP and TPU groups, a 44% reduction in biofilm metabolic activity, and a slight increase in pH levels, indicating their potential to counteract the acidic conditions that promote caries formation. However, while the in vitro results are promising, future in vivo studies are essential to validate these benefits in practical orthodontic applications and to assess the long-term safety of TPU/Ag-coated appliances, including their impact on oral flora and the potential risk of silver accumulation.

# These authors contributed equally to this work and should be considered first co-authors.

Acknowledgments

The authors are grateful for the reviewer’s valuable comments that improved the manuscript.

  1. Funding information: This work was supported by the Key Program of Zhejiang Provincial Natural Science Foundation (LZ25H140001); the Fundamental Research Funds for the Central Universities (2023QZJH60); R&D Program of the Stomatology Hospital of Zhejiang University School of Medicine (RD2022DLYB03); the National Natural Science Foundation of China (No. 51803075); Zhejiang Provincial Natural Science Foundation of China (No. LY20E030010); Postdoctoral Science Preferential Funding of Zhejiang Province (No. ZJ2023134); Jiaxing Youth Science and Technology Talent Project (No. 2023AY40012); and Jiaxing Science and Technology Plan Project (No. 2023AD31058). Xuepeng Chen is sponsored by the Zhejiang Provincial Program for the Cultivation of High-level Innovative Health Talents.

  2. Author contributions: Yun Zhang is responsible for experimental data processing and article writing. Junlu Sheng is responsible for the preparation and physical characterization of electrospun TPU/nano-Ag-coated clear aligners. Yun Zhang and Junlu Sheng contributed equally. Jiarong Yan is responsible for directing the bacterial experiment. Lichao Yu is responsible for assisting with material preparation. Yange Wu is responsible for reviewing and editing. Jiajia Shen is responsible for experimental design. Jiayong Zhong is responsible for experimental design. Xuepeng Chen is responsible for experimental design and management and correspondence. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

  4. Ethical approval: The protocol was approved by the Ethics Committee of Hospital of Stomatology, Zhejiang University School of Medicine (No. 2023Y061).

  5. Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2025-01-17
Revised: 2025-03-27
Accepted: 2025-04-21
Published Online: 2025-05-22

© 2025 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  23. Thermal and stability analysis of silver–water nanofluid flow over unsteady stretching sheet under the influence of heat generation/absorption at the boundary
  24. Cobalt iron oxide-infused silicone nanocomposites: Magnetoactive materials for remote actuation and sensing
  25. Magnesium-reinforced PMMA composite scaffolds: Synthesis, characterization, and 3D printing via stereolithography
  26. Bayesian inference-based physics-informed neural network for performance study of hybrid nanofluids
  27. Numerical simulation of non-Newtonian hybrid nanofluid flow subject to a heterogeneous/homogeneous chemical reaction over a Riga surface
  28. Enhancing the superhydrophobicity, UV-resistance, and antifungal properties of natural wood surfaces via in situ formation of ZnO, TiO2, and SiO2 particles
  29. Synthesis and electrochemical characterization of iron oxide/poly(2-methylaniline) nanohybrids for supercapacitor application
  30. Impacts of double stratification on thermally radiative third-grade nanofluid flow on elongating cylinder with homogeneous/heterogeneous reactions by implementing machine learning approach
  31. Synthesis of Cu4O3 nanoparticles using pumpkin seed extract: Optimization, antimicrobial, and cytotoxicity studies
  32. Cationic charge influence on the magnetic response of the Fe3O4–[Me2+ 1−y Me3+ y (OH2)] y+(Co3 2−) y/2·mH2O hydrotalcite system
  33. Pressure sensing intelligent martial arts short soldier combat protection system based on conjugated polymer nanocomposite materials
  34. Magnetohydrodynamics heat transfer rate under inclined buoyancy force for nano and dusty fluids: Response surface optimization for the thermal transport
  35. Review Articles
  36. A comprehensive review on hybrid plasmonic waveguides: Structures, applications, challenges, and future perspectives
  37. Nanoparticles in low-temperature preservation of biological systems of animal origin
  38. Fluorescent sulfur quantum dots for environmental monitoring
  39. Nanoscience systematic review methodology standardization
  40. Nanotechnology revolutionizing osteosarcoma treatment: Advances in targeted kinase inhibitors
  41. AFM: An important enabling technology for 2D materials and devices
  42. Carbon and 2D nanomaterial smart hydrogels for therapeutic applications
  43. Principles, applications and future prospects in photodegradation systems
  44. Do gold nanoparticles consistently benefit crop plants under both non-stressed and abiotic stress conditions?
  45. An updated overview of nanoparticle-induced cardiovascular toxicity
  46. Arginine as a promising amino acid for functionalized nanosystems: Innovations, challenges, and future directions
  47. Advancements in the use of cancer nanovaccines: Comprehensive insights with focus on lung and colon cancer
  48. Membrane-based biomimetic delivery systems for glioblastoma multiforme therapy
  49. The drug delivery systems based on nanoparticles for spinal cord injury repair
  50. Green synthesis, biomedical effects, and future trends of Ag/ZnO bimetallic nanoparticles: An update
  51. Application of magnesium and its compounds in biomaterials for nerve injury repair
  52. Micro/nanomotors in biomedicine: Construction and applications
  53. Hydrothermal synthesis of biomass-derived CQDs: Advances and applications
  54. Research progress in 3D bioprinting of skin: Challenges and opportunities
  55. Review on bio-selenium nanoparticles: Synthesis, protocols, and applications in biomedical processes
  56. Gold nanocrystals and nanorods functionalized with protein and polymeric ligands for environmental, energy storage, and diagnostic applications: A review
  57. An in-depth analysis of rotational and non-rotational piezoelectric energy harvesting beams: A comprehensive review
  58. Advancements in perovskite/CIGS tandem solar cells: Material synergies, device configurations, and economic viability for sustainable energy
  59. Deep learning in-depth analysis of crystal graph convolutional neural networks: A new era in materials discovery and its applications
  60. Review of recent nano TiO2 film coating methods, assessment techniques, and key problems for scaleup
  61. Special Issue on Advanced Nanomaterials for Carbon Capture, Environment and Utilization for Energy Sustainability - Part III
  62. Efficiency optimization of quantum dot photovoltaic cell by solar thermophotovoltaic system
  63. Exploring the diverse nanomaterials employed in dental prosthesis and implant techniques: An overview
  64. Electrochemical investigation of bismuth-doped anode materials for low‑temperature solid oxide fuel cells with boosted voltage using a DC-DC voltage converter
  65. Synthesis of HfSe2 and CuHfSe2 crystalline materials using the chemical vapor transport method and their applications in supercapacitor energy storage devices
  66. Special Issue on Green Nanotechnology and Nano-materials for Environment Sustainability
  67. Influence of nano-silica and nano-ferrite particles on mechanical and durability of sustainable concrete: A review
  68. Surfaces and interfaces analysis on different carboxymethylation reaction time of anionic cellulose nanoparticles derived from oil palm biomass
  69. Processing and effective utilization of lignocellulosic biomass: Nanocellulose, nanolignin, and nanoxylan for wastewater treatment
  70. Retraction
  71. Retraction of “Aging assessment of silicone rubber materials under corona discharge accompanied by humidity and UV radiation”
https://doi.org/10.1515/ntrev-2025-0173
Keywords for this article
clear aligner; electrospinning; thermoplastic polyurethane; S. mutans; dental caries; nano-silver-based antibacterial agent
Creative Commons
BY 4.0

Articles in the same Issue

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  13. Green synthesis of silver nanoparticles using Ginkgo biloba seed extract: Evaluation of antioxidant, anticancer, antifungal, and antibacterial activities
  14. A numerical analysis of heat and mass transfer in water-based hybrid nanofluid flow containing copper and alumina nanoparticles over an extending sheet
  15. Investigating the behaviour of electro-magneto-hydrodynamic Carreau nanofluid flow with slip effects over a stretching cylinder
  16. Electrospun thermoplastic polyurethane/nano-Ag-coated clear aligners for the inhibition of Streptococcus mutans and oral biofilm
  17. Investigation of the optoelectronic properties of a novel polypyrrole-multi-well carbon nanotubes/titanium oxide/aluminum oxide/p-silicon heterojunction
  18. Novel photothermal magnetic Janus membranes suitable for solar water desalination
  19. Green synthesis of silver nanoparticles using Ageratum conyzoides for activated carbon compositing to prepare antimicrobial cotton fabric
  20. Activation energy and Coriolis force impact on three-dimensional dusty nanofluid flow containing gyrotactic microorganisms: Machine learning and numerical approach
  21. Machine learning analysis of thermo-bioconvection in a micropolar hybrid nanofluid-filled square cavity with oxytactic microorganisms
  22. Research and improvement of mechanical properties of cement nanocomposites for well cementing
  23. Thermal and stability analysis of silver–water nanofluid flow over unsteady stretching sheet under the influence of heat generation/absorption at the boundary
  24. Cobalt iron oxide-infused silicone nanocomposites: Magnetoactive materials for remote actuation and sensing
  25. Magnesium-reinforced PMMA composite scaffolds: Synthesis, characterization, and 3D printing via stereolithography
  26. Bayesian inference-based physics-informed neural network for performance study of hybrid nanofluids
  27. Numerical simulation of non-Newtonian hybrid nanofluid flow subject to a heterogeneous/homogeneous chemical reaction over a Riga surface
  28. Enhancing the superhydrophobicity, UV-resistance, and antifungal properties of natural wood surfaces via in situ formation of ZnO, TiO2, and SiO2 particles
  29. Synthesis and electrochemical characterization of iron oxide/poly(2-methylaniline) nanohybrids for supercapacitor application
  30. Impacts of double stratification on thermally radiative third-grade nanofluid flow on elongating cylinder with homogeneous/heterogeneous reactions by implementing machine learning approach
  31. Synthesis of Cu4O3 nanoparticles using pumpkin seed extract: Optimization, antimicrobial, and cytotoxicity studies
  32. Cationic charge influence on the magnetic response of the Fe3O4–[Me2+ 1−y Me3+ y (OH2)] y+(Co3 2−) y/2·mH2O hydrotalcite system
  33. Pressure sensing intelligent martial arts short soldier combat protection system based on conjugated polymer nanocomposite materials
  34. Magnetohydrodynamics heat transfer rate under inclined buoyancy force for nano and dusty fluids: Response surface optimization for the thermal transport
  35. Review Articles
  36. A comprehensive review on hybrid plasmonic waveguides: Structures, applications, challenges, and future perspectives
  37. Nanoparticles in low-temperature preservation of biological systems of animal origin
  38. Fluorescent sulfur quantum dots for environmental monitoring
  39. Nanoscience systematic review methodology standardization
  40. Nanotechnology revolutionizing osteosarcoma treatment: Advances in targeted kinase inhibitors
  41. AFM: An important enabling technology for 2D materials and devices
  42. Carbon and 2D nanomaterial smart hydrogels for therapeutic applications
  43. Principles, applications and future prospects in photodegradation systems
  44. Do gold nanoparticles consistently benefit crop plants under both non-stressed and abiotic stress conditions?
  45. An updated overview of nanoparticle-induced cardiovascular toxicity
  46. Arginine as a promising amino acid for functionalized nanosystems: Innovations, challenges, and future directions
  47. Advancements in the use of cancer nanovaccines: Comprehensive insights with focus on lung and colon cancer
  48. Membrane-based biomimetic delivery systems for glioblastoma multiforme therapy
  49. The drug delivery systems based on nanoparticles for spinal cord injury repair
  50. Green synthesis, biomedical effects, and future trends of Ag/ZnO bimetallic nanoparticles: An update
  51. Application of magnesium and its compounds in biomaterials for nerve injury repair
  52. Micro/nanomotors in biomedicine: Construction and applications
  53. Hydrothermal synthesis of biomass-derived CQDs: Advances and applications
  54. Research progress in 3D bioprinting of skin: Challenges and opportunities
  55. Review on bio-selenium nanoparticles: Synthesis, protocols, and applications in biomedical processes
  56. Gold nanocrystals and nanorods functionalized with protein and polymeric ligands for environmental, energy storage, and diagnostic applications: A review
  57. An in-depth analysis of rotational and non-rotational piezoelectric energy harvesting beams: A comprehensive review
  58. Advancements in perovskite/CIGS tandem solar cells: Material synergies, device configurations, and economic viability for sustainable energy
  59. Deep learning in-depth analysis of crystal graph convolutional neural networks: A new era in materials discovery and its applications
  60. Review of recent nano TiO2 film coating methods, assessment techniques, and key problems for scaleup
  61. Special Issue on Advanced Nanomaterials for Carbon Capture, Environment and Utilization for Energy Sustainability - Part III
  62. Efficiency optimization of quantum dot photovoltaic cell by solar thermophotovoltaic system
  63. Exploring the diverse nanomaterials employed in dental prosthesis and implant techniques: An overview
  64. Electrochemical investigation of bismuth-doped anode materials for low‑temperature solid oxide fuel cells with boosted voltage using a DC-DC voltage converter
  65. Synthesis of HfSe2 and CuHfSe2 crystalline materials using the chemical vapor transport method and their applications in supercapacitor energy storage devices
  66. Special Issue on Green Nanotechnology and Nano-materials for Environment Sustainability
  67. Influence of nano-silica and nano-ferrite particles on mechanical and durability of sustainable concrete: A review
  68. Surfaces and interfaces analysis on different carboxymethylation reaction time of anionic cellulose nanoparticles derived from oil palm biomass
  69. Processing and effective utilization of lignocellulosic biomass: Nanocellulose, nanolignin, and nanoxylan for wastewater treatment
  70. Retraction
  71. Retraction of “Aging assessment of silicone rubber materials under corona discharge accompanied by humidity and UV radiation”
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