Optical analysis for effective phototherapy in an LED oral care device

Abstract

This paper investigates the advanced capabilities of light-emitting diodes (LEDs) in oral care devices, emphasizing their versatility in wavelength control and ability to reach complex areas within the oral cavity. While LEDs enable precise dosage control and adjustable penetration depths, existing oral care devices are often limited to single-wavelength designs, primarily targeting anterior teeth whitening or lateral surfaces, thereby failing to provide comprehensive oral coverage. To address these limitations, this study introduces a novel LED-based oral care device integrating three distinct wavelengths: blue for antibacterial effects, green for anti-inflammatory effects, and red for preventive and therapeutic applications. Using computed tomography (CT) data, upper and lower dental arch trajectories were acquired to design a flexible printed circuit board (FPCB) that conforms to the natural curvature of the dental arch. Strategically placed LEDs on the FPCB ensure uniform light distribution and optimized irradiance across the entire oral cavity. This research systematically determines the optimal design parameters and operating conditions necessary for achieving appropriate irradiance density, including LED placement, operating time, and power control through driving current and duty cycles. The findings demonstrate a practical and effective approach to overcoming the current limitations of LED oral care devices, significantly enhancing their performance and applicability in dental phototherapy.

1. Introduction

Periodontal disease represents a significant challenge in dental health care, where outcomes substantially depend on patients’ ability to manage their oral hygiene effectively. This underscores the growing importance of reliable self-treatment methods. While most current self-treatment approaches in phototherapy are predominantly focused on low-power laser usage, the exploration of light emitting diode (LED) technology in this domain remains limited.

Although lasers have been widely used in biomedicine for years, their high energy and resultant heat generation pose risks of damaging deeper tissues, including periodontal ligaments and bone structures. In contrast, LEDs provide a safer alternative due to their minimal heat emission. Additionally, their long lifespan, compact size, and design versatility make LEDs particularly well-suited for addressing hard-to-reach areas in the oral cavity. This paper delves into the potential of LED technology as a viable alternative in periodontal self-care, offering safer, more adaptable, and effective solutions for managing dental health conditions [1,2].

Phototherapy, widely used in medical and dental fields, primarily employs the visible light spectrum, comprising three main wavelengths: blue (∼400 nm), green (∼500 nm), and red (∼600 nm). Each wavelength has distinct effects, with extensive research supporting their benefits.

Firstly, the blue wavelength region (∼400 nm) has shown significant potential in inhibiting bacterial growth. Bumah et al. [3] compared the antibacterial effects of hyperbaric oxygen and blue light at 470 nm in vitro. Their findings revealed that blue light at 470 nm suppressed the growth of Staphylococcus aureus by over 92%, outperforming hyperbaric oxygen. This substantial efficacy underscores the potential of blue light phototherapy in the medical field, especially in the fight against bacterial infections. Furthermore, Guffey [4] investigated the antibacterial effects of blue light at wavelengths of 405 nm and 470 nm. Their study, which varied irradiance densities from 1 to 15 J/cm², found that Staphylococcus aureus was inhibited at all levels of light exposure. In contrast, Pseudomonas aeruginosa required a minimum irradiance of 10 J/cm² to achieve over 90% inhibition. These results emphasize the importance of optimizing both wavelength and irradiance in phototherapy to develop effective antibacterial treatments. Additionally, it has been reported that blue light irradiation also has a germicidal effect on pathogenic oral bacteria. According to Soukos et al. [5], blue light irradiation with a fluence of 4.2 J/cm² killed cultures of Prevotella intermedia and P. nigrescens, while P. melaninogenica required 21 J/cm² for the same effect. Exposure to light with a fluence of 42 J/cm² resulted in a 99% reduction in P. gingivalis. This bactericidal effect of light irradiation is believed to be dependent on porphyrin, and porphyrin seems to be essential for the effective antimicrobial action of blue light. Utilizing this principle, the present study suggests that intraoral light exposure may be employed to control the growth of pathogenic bacteria in the oral cavity, potentially benefiting patients with periodontal disease.

Secondly, the use of the green wavelength in the 500 nm region has yielded the following results. As reported by Fushimi et al. [6], an in vitro study was conducted to investigate the anti-inflammatory effects of three wavelengths: red, blue, and green. The results indicated a significant reduction in the size of skin wounds after 7 days of exposure to the green wavelength, demonstrating an accelerated healing process when compared to the control group exposed to the blue wavelength. This implies that, in addition to red wavelengths, green wavelengths could also present a novel therapeutic approach for reducing inflammation. Similarly, a study conducted by Rohringer et al. [7] observed that red and green wavelengths notably increased the proliferation of human umbilical vein endothelial cells (HUVECs), while the blue wavelength had no such effect. Furthermore, Catao et al. reported that green LED light exhibits an anti-inflammatory effect on skin burns in rats. Notably, in certain variables, the green wavelength was found to be more effective than the red wavelength, which is well-known for its anti-inflammatory properties, in stimulating the proliferation and migration of HUVECs.

Thirdly, the outcomes obtained from the use of the red wavelength in the 600 nm region are as follows. Yamauchi et al. [8] conducted an analysis of the effects of the red wavelength on the inflammatory responses of human periodontal ligament stem cells. Their study revealed that irradiance at or above 6 J/cm² in the red wavelength led to an increase in adenosine triphosphate (ATP) levels and a decrease in the production of interleukin-6 (IL-6) and interleukin-8 (IL-8). These findings underscore the effectiveness of the red wavelength in the treatment of inflammation. In a similar line of research, Lim et al. [9] investigated the impact of the red wavelength on human gingival fibroblasts. Choi et al. [10] demonstrated that 635 nm LED irradiation significantly suppressed pro-inflammatory cytokines by regulating the MAPK signaling pathway, suggesting its potential clinical utility as an anti-inflammatory tool.

Based on the existing literature on the antibacterial, anti-inflammatory, and cell proliferation effects of the three aforementioned wavelengths of light, our research group conducted both in vitro and in vivo experiments to investigate the application of LED light with wavelengths of 470 nm, 525 nm, and 630 nm for oral health. In our previous studies, we demonstrated that the 470 nm wavelength exhibited significant antibacterial effects against various oral bacteria [11], while the 525 nm and 630 nm wavelengths were effective in reducing inflammation [12,13]. Furthermore, these effects were confirmed to be beneficial in a long-term periodontal disease model using a canine model [12,13].

Despite the growing interest in LED-based oral care devices, a significant research gap remains in this field. Existing studies, such as those by Chen et al. [2], have broadly categorized these devices into four types: flashlights, oral tips, fiber optics, and toothbrushes, focusing mainly on the advantages and disadvantages of each type. Further research, such as that by Guramet et al. [14] and Saritha et al. [15], has focused on the use of blue LEDs in dental resins, without exploring a wider range of applications. Additionally, Manphibool et al. have demonstrated the effectiveness of blue-light LED toothbrushes in reducing plaque and gingival inflammation in orthodontic patients with fixed appliances [16]. Moreover, Yoo et al. reported that LED irradiation at wavelengths of 632 nm and 870 nm promoted cell regeneration and facilitated scar treatment in animal studies. They further designed and proposed a U-shaped LED module tailored for dental applications to optimize light exposure [17]. Furthermore, commercially available LED-based oral care products, such as Lumoral (Koite Health, Finland) and DPL Oral Care (LED Technologies, Inc., USA) [18], have been introduced to the market and chare widely distributed for consumer use. However, research on devices that integrate multiple wavelengths for combined antibacterial and anti-inflammatory effects remains insufficient.

Addressing this gap, the primary objective of this study is to develop an LED-based oral care device specifically designed to accommodate the unique structure of the oral cavity. This development will facilitate further investigations into the biological effects of various LED wavelengths in human subjects. The device will incorporate advanced technical components, including the precise configuration of wavelength-specific light sources and optimized intraoral positioning. Ultimately, the LED oral care device developed through this research is expected to be applicable in future clinical settings, enabling the verification of its bio-efficacy and promoting its practical use in oral healthcare.

To develop an oral care device with specific wavelength ranges, we advanced the selection of LEDs and the configuration of LED arrays. The uniformity of light distribution and irradiance was evaluated as a function of the distance between the LEDs and the treatment area. Furthermore, a target irradiance density and the necessary total operation time—accounting for irradiance levels, duty cycle, single-use duration, and daily usage frequency—were established to ensure effective sterilization and antibacterial effects. Optical simulations were performed using Zemax OpticStudio, and data analysis was conducted with MathWorks’ MATLAB.

This study bridged the existing research gap by systematically exploring the integration of multi-wavelength LEDs in oral care devices. It provided a foundation for developing safer and more efficient self-treatment methods for periodontal care, with the potential to revolutionize daily dental hygiene management through advanced.

2. Method

2.1. Shape design of the LED oral care device

The LED oral care device is designed to illuminate all areas of the teeth within the oral cavity, ensuring comprehensive coverage of both the front and rear regions. To achieve this, the flexible printed circuit board (FPCB) is engineered to conform to the parabolic curvature of the dental arch, ensuring precise positioning and secure placement within the mouth. The trajectory of the dental arch was derived using CT data from 242 adult males aged 30–50 years (Fig. 1(A)).

Fig. 1.

Procedure for determining the dental arch trajectory. (A) Obtaining central tooth coordinates from CT-scanned adult dental arches. (B) Grouping and aligning coordinate data to estimate the curvature of the dental arch. (C) Obtained adult dental arch trajectory.

The central coordinates of each tooth were determined relative to the origin based on the CT data, creating individual data points along the dental arch. These points were divided into three groups—narrow, normal, and wide—and aligned to follow the curvature of the dental arch (Fig. 1(B)). A fourth-degree polynomial (ax⁴ + bx²) was then fitted to the data from each group using the least-squares method, as depicted in Fig. 1(B) [19]. The parabolic coefficients for the groups were: narrow group (a = 6.67 × 10⁻⁵, b = 0.0260), normal group (a = 4.86 × 10⁻⁵, b = 0.0185) and wide group (a = 2.99 × 10⁻⁵, b = 0.0175). Figure 1(B) presented the calculated dental arch trajectory for the three groups, with distributions of 29% for narrow (sky blue line), 47% for normal (purple line), and 24% for wide (green line).

The FPCB and mouthpiece were designed based on the normal group's dental arch trajectory to ensure broad usability (Fig. 1(C)). The FPCB was split into two sections to illuminate both the outer and inner surfaces of the teeth, as shown in Fig. 2. Since the upper and lower jaws were symmetrical, only one side was shown in the figure. ELVAX 210W (DOW Inc., USA) was selected as the molding material due to its successful completion of biological safety tests (ISO 10993-10:2021, ISO 10993-5:2009; certified by GLR Laboratories Pvt Ltd., India) and photobiological safety tests (IEC 62471; certified by SGS, USA). Additionally, this material offered excellent optical transparency, maintaining over 90% light transmittance in the visible spectrum at the mouthpiece molding thickness (93%@1.2 mm, 91%@1.5 mm), while providing the necessary flexibility for comfort and effective fitting. It was expected that more than 70% of adults would be able to comfortably use the device by utilizing the dental arch of the normal group and flexible materials. As the device was designed for general use, factors like malocclusion were not considered. To address these specific needs, personalized mouthpiece shapes would need to be developed.

Fig. 2.

Design of mouthpiece and FPCB shapes based on the adult dental arch trajectory.

2.2. Selection and modeling of LEDs

The device was designed to be positioned within the oral cavity, making it essential to select compact LEDs with high light output to minimize the sensation of a foreign object, enhance user comfort, and ensure effective adhesion to the teeth.

After exploring various LED chips available on the market, we selected the APHHS1005 product line from KINGBRIGHT. These LEDs were highly compact, with package dimensions of 1.0 × 0.5 mm and a thickness of less than 0.5 mm, making them particularly suitable for the development of wearable LED devices. From the APHHS1005 product line, we selected the APHHS1005 product line from KINGBRIGHT. These LEDs are highly compact, with package dimensions of 1.0 × 0.5 mm and a thickness of less than 0.5 mm, making them particularly suitable for the development of wearable LED devices. From the APHHS1005 product line, we chose specific models emitting blue (465 nm, spectral bandwidth Δλ = 25 nm), green (525 nm, Δλ = 20 nm), and red (630 nm, Δλ = 20 nm) wavelengths: APHHS1005LQBC/D-V, APHHS1005ZGCK, and APHHS1005SURCK, respectively. According to the datasheets, the blue and green LEDs have a beam viewing angle of 140°, while the red LED has a narrower viewing angle of 120°. Additionally, the red LED was expected to have a lower optical output compared to the other wavelengths due to its smaller emission area (LED chip), as illustrated in Fig. 3(B).

Fig. 3.

Spatial light distribution and dimensions of each LED. (A) Spatial light distribution for each LED. (B) Package dimensions of each LED.

Each LED was modeled in Zemax OpticStudio for non-sequential ray tracing, incorporating the spatial distribution and emission surfaces (LED chip) shown in Fig. 3(A). Since the simulation software could not process the spatial light distribution for all angles, data were entered at 10-degree intervals, and intermediate values were estimated through interpolation (Table 1). The resulting spatial distribution of the LED model in Fig. 3(A) was confirmed to closely match the spatial light distribution provided in the datasheet.

Table 1. Relative Spatial Light Distribution of Each LEDs.

	0°	10°	20°	30°	40°	50°	60°	70°	80°	90°
Blue (465 nm)	1	0.989	0.958	0.909	0.837	0.740	0.643	0.481	0.311	0.099
Green (525 nm)	1	0.989	0.958	0.909	0.837	0.740	0.643	0.481	0.311	0.099
Red (630 nm)	1	0.988	0.934	0.859	0.755	0.633	0.496	0.340	0.188	0.086

To evaluate the actual optical output power of each LED, we fabricated test boards, each equipped with a single LED for the corresponding wavelength (Fig. 4(A)). The test boards were constructed using metal PCBs to minimize heat damage caused by the relatively high currents applied during the testing process. We measured the optical power using an integrating sphere detector (819C/D, Newport) while varying the applied current from 10 mA to 70 mA with a constant current source (RD6006, RIDEN). Figure 4(B) showed the variations in optical output power at each wavelength as a function of the applied current.

Fig. 4.

Optical power measurement of each LED. (A) Optical power testing board. (B) Optical power measured as a function of current change.

The LED models were designed with initial optical power of 5.7 mW, 5.3 mW, and 2.7 mW for blue (465 nm), green (525 nm), and red (630 nm) wavelengths, respectively, and each model was set to trace 10⁶ rays per each LED. Each LED had a narrow bandwidth (Δλ = 20–25 nm), and since there are no optical elements such as lenses in front, wavelength-induced changes in the spatial light distribution within this bandwidth are negligible. Therefore, the LED models for the simulation were developed only for the central wavelength. The initial optical power used in the simulation was determined by measuring the output power of a single LED operating at 20 mA (Fig. 4(B)).

2.3. Establishment of design goals

Prior to performing the optical analysis for the placement of LED array, the following design targets were established for each wavelength: an irradiance density of 10 J/cm² and a uniformity greater than 50%. These targets were based on findings discussed in the introduction. Specifically, previous studies indicated that blue wavelengths with an irradiance above 10 J/cm² achieved over 90% antibacterial efficacy against Pseudomonas aeruginosa, and red wavelengths above 6 J/cm² were associated with increased adenosine triphosphate (ATP) levels [15].

Irradiance density was defined as the product of irradiance and operating time, making it crucial to account for the time factor. Although prolonged use of the oral care device could achieve the desired irradiance density, practical considerations such as the duration of a single use and the frequency of daily usage also needed to be carefully evaluated.

2.4. Configuration of the LED array

We assumed these LEDs were mounted on the FPCB and conducted optical ray tracing simulations to determine the optimal LED arrangement that met the irradiance and uniformity requirements of the LED oral care device.

The wavelength-specific arrangement of the FPCB was strategically designed to match the contact areas within the oral cavity. The LED models were positioned in different areas of the FPCB in a lattice pattern (Fig. 5), and their coordinates were input into the Lens Data Editor of Zemax OpticStudio. Specifically, blue (465 nm) LEDs, which are effective for antibacterial purposes, have been positioned to target the teeth (at the bottom of the FPCB), while green (525 nm) and red (630 nm) LEDs, known for their anti-inflammatory effects, have been placed to correspond to the gum areas (at the top of the FPCB). The device was also designed in two parts to illuminate the outside (lateralis group) and inside (medialis group) of the teeth separately, and the upper and lower jaws were assumed to be identical due to their symmetrical structure.

Fig. 5.

LED array configuration for the lateralis and medialis regions.

A total of 28 blue LEDs were installed, with 20 positioned on the outer side and 8 on the inner side at the bottom of the FPCB, ensuring optimal placement for targeting the teeth. Similarly, 28 green LEDs were evenly distributed, with 20 on the outer side and 8 on the inner side near the gums, to aid in the prevention of periodontitis. For red LEDs, which play a crucial role in addressing periodontal disease—a primary function of oral care devices—a total of 56 units were arranged in a grid pattern to effectively target both the teeth and gums. Specifically, 40 red LEDs were placed on the outer side and 16 on the inner side, resulting in twice the number of red LEDs compared to the other wavelengths. This increased quantity was implemented to compensate for the lower output power of red LEDs, ensuring adequate irradiance density (see Fig. 3 and 4(B)).

The distance between the LED and teeth in an LED oral care device can vary depending on factors such as age, gender, and the presence or thickness of molding material. To analyze the light irradiance distribution, a light detector sized 150 mm × 60 mm was placed in Zemax OpticStudio at distances ranging from 1 mm to 5 mm. Distances beyond this range were excluded from the analysis due to the practical gap between the LEDs and the teeth. The detector’s pixel resolution was set to 750 × 350 pixels, resulting in an area of 0.04 mm² per pixel.

The device utilized an optically transparent mouthpiece molding material (with light transmittance of 93%@1.2 mm, 91%@1.5 mm) to position the LED array along the arch trajectory, as shown in Fig. 2. This design ensured that the distance between the teeth and the LED remained above 1 mm, with the LED array maintained close contact with the teeth. The distance between the teeth and the LED array was generally observed to be around 2–3 mm, with this study offering data up to 5 mm to comprehensively evaluate light intensity and distribution across different oral cavity conditions.

Each LED array was arranged to operate either as a whole or in sections, allowing adaptability for various oral conditions depending on the type and location of the disease. For instance, wavelengths intended for inflammation relief were selectively applied to affected areas. The circled letters (ⓐ – ⓔ) in Fig. 5 showed the arrangement of LED sections that operated independently (Outer Left/Mid/Right, Inner Left/Right).

It is important to note that while the LED array was arranged in a 3D configuration along the dental arch, the simulation was simplified by using a 2D plane of the FPCB. A full 3D simulation would have required positioning the detector along the arch curve to maintain the correct distance between the LED and the teeth. As a result, the outcomes of both 3D and 2D analyses were anticipated to be similar. However, the use of the 2D optical detector enabled more efficient analysis and interpretation. This comparability is attributed to the fact that the LED and the detector consistently maintained a fixed distance, similar to the spacing between the LED light source and the target areas (teeth and gums). Additionally, the simulation excluded factors such as the light transmittance of the molding material and environmental conditions like moisture or saliva present in the oral cavity. These factors could increase optical scattering, potentially leading to reduced irradiance but improved uniformity.

3. Result

3.1. Irradiance analysis of the LED array

Irradiance was directly obtained from the detector data following the ray tracing simulation (Fig. 6). Although ray tracing was conducted at 1 mm intervals within the 1–5 mm range, only the results for distances of 1, 3, and 5 mm were presented.

Fig. 6.

Irradiance map at different LED-to-teeth distances for (A) blue, (B) green, and (C) red LED arrays.

The irradiance in Fig. 6 was calculated based on the pixel area. To determine the average irradiance in the illuminated region of the teeth, the target area first needed to be defined. This target area for irradiation was defined using the shape of the FPCB mounted with the LED array, as indicated by the red box in Fig. 6.

As explained in Section 2.4, the blue and green LEDs were strategically placed in different areas on the FPCB to separately illuminate the teeth and gums. However, in Fig. 6, the red box was positioned at the vertical center of each LED array, making the positions of the blue and green LEDs appear identical. This alignment occurred because the blue and green LED arrays were vertically realigned within the red box to ensure that the average irradiance was calculated over the same area. Otherwise, most of the rays would have fallen outside the red box for the off-centered blue and green LED arrays, leading to an unreasonably low average irradiance. It should be noted that the red box served solely as a mask for interpretation purposes; it was not a physical area on the FPCB and was not used as an input for the ray tracing simulation.

The irradiance values for each pixel within the red square (Fig. 6) were summed and then divided by the total number of pixels (35,663 pixels) to calculate the average irradiance for that area, as shown in Fig. 7(A). As the distance between the light source and the detector increased, more photons were recorded outside the defined area, resulting in a decrease in the average irradiance.

Fig. 7.

Average irradiance and illumination time needed to meet the target specifications based on the LED-teeth distance. (A) Averaged irradiance. (B) Illumination time required to achieve 10 J/cm² for each LED.

As described in Section 2.3, the irradiance density was obtained by multiplying the irradiance by the operating time. The required operating time to achieve the design target was calculated by dividing the target irradiance density of 10 J/cm² by the average irradiance (Fig. 7(A)). The corresponding operating times were illustrated in Fig. 7(B). The average irradiance (Fig. 7(A)) and operating time (Fig. 7(B)) exhibited similar trends across all three wavelengths. To compensate for the lower output power per LED in the red wavelength, twice as many LEDs were arranged in the red LED arrays compared to other wavelengths. This adjustment successfully achieved comparable irradiance levels across all wavelengths, even at varying depths, as evidenced by the data.

From Fig. 7(B), it was evident that approximately 16 minutes of operating time is required to achieve the target irradiance density at a 1 mm distance from the tooth. However, as the distance between the tooth and the LED increased, the required operating time also increased to maintain the same efficacy. Insufficient operating time at greater distances could have resulted in reduced sterilization and antibacterial effects. Given the design considerations of an oral care device, which needed to be held in the mouth during use, a session duration exceeding 16 minutes was deemed impractical. It is important to note that daily oral care devices were not designed to completely eradicate pathogenic oral bacteria or deliver anti-inflammatory effects in a single use. While increasing the light output power could have enhanced sterilization and anti-inflammatory effects within the same operating time, it also carried the risk of generating excessive heat, potentially compromising both device safety and user comfort.

A common solution to address heat generation was the used of pulsed driving for the LED arrays. The off-time between pulses allowed the LEDs to cool down, preventing excessive heat buildup and ensuring safer operation [20]. However, pulsed driving increased the operating time required to achieve the same irradiance density. Therefore, further research is needed to optimize this approach for various conditions, such as single-use duration and daily usage frequency. Addressing this challenge may require more extensive clinical studies, which beyond the scope of the optical analysis presented in this paper and are not explored further.

3.2. Uniformity analysis of the LED array

Light uniformity (U_o) was calculated as the percentage of the minimum irradiance (I_min) to the average irradiance (I_avg), as shown in Eq. (1).

$$U_o=\left(\frac{I_{min}}{I_{avg}}\right)\times 100$$ (1)

Line profiles, indicated by the white line in Fig. 8(A), were obtained to determine the minimum and average irradiance, which were then used to calculate the uniformity for each wavelength. The profiles were extracted separately for the outer and inner regions. Due to the red LED array being arranged in two layers, obtaining standard line profiles was challenging. To address this, profiles crossing both LED layers were extracted. Figure 8(B) and Fig. 9 displayed the extracted irradiance line profiles in 3D and 2D plots, respectively.

Fig. 8.

Configuration of the irradiance line profile for uniformity calculation. (A) The region for extracting the irradiance line profile from the irradiance map of each LED array is marked with a white line. (B) The extracted irradiance line profile is displayed in 3D, showing only the lateralis group.

Fig. 9.

Uniformity profiles shown on a 2D plane at 3 mm and 5 mm LED-tooth distances for each wavelength, for (A) the lateralis group and (B) the medialis group

Figure 10 presented the results of light uniformity calculations for (A) the lateralis LED group (outer part) and (B) the medialis LED group (inner part). The figure showed that a minimum distance of 2.5 mm was required in all cases to achieve a target uniformity of 50% or higher. In the lateralis group, uniformity exceeded 80% and stabilizes at 4 mm for all wavelengths. However, in the medialis group, the uniformity for the red wavelength exhibited a slight decrease as the distance increases beyond 4 mm. This decrease was attributed to the red LED array being arranged in two rows, which complicated obtaining a consistent uniformity profile along the LEDs and may not have accurately reflected an actual reduction in uniformity.

Fig. 10.

Uniformity with distance for different LED wavelengths in (A) the lateralis region and (B) the medialis group.

3.3. LED oral care device prototype

Figure 11 showed the LED oral care device developed based on the optical analysis presented above. An LED driver with an output current of 20.5 mA was designed, and the LED arrays were pulse-driven at a frequency of 1 kHz to reduce heat generation. We adjusted the duty cycle for each wavelength to ensure that the device operated at a temperature below 30°C at room temperature (25°C), protecting the teeth and gum tissues from heat damage when the mouthpiece was inserted into the oral cavity. This process was carried out by monitoring the temperature with a thermocouple tightly positioned in front of the light source and manually lowering the duty cycle of each LED wavelength. Through this process, the duty cycles for the blue, green, and red wavelengths were set at 75%, 75%, and 87%, respectively. We aimed to keep the irradiance density similar across all three wavelengths. The red wavelength required a higher duty cycle of 87% because its irradiance remained slightly lower than the other wavelengths, despite the number of red LEDs being doubled (see Fig. 9). The higher duty cycle was not considered due to safety concerns, as it would have caused the temperature of the red LEDs to exceed 30°C.

Fig. 11.

LED oral care device manufactured based on optical simulation results. (A) Mouthpiece unit including the LED driver. (B) Controller unit including the control board.

To assess heat generation, the LED array temperatures for each wavelength were measured using a thermocouple and data logger (Multi-Channel Recorder, Sefram). The thermocouple tip was placed directly on the LED emitting area after 5 minutes of aging to record the temperature under operating conditions. The stabilized temperatures were measured at 27.1°C, 26.1°C, and 29.2°C for the blue, green, and red wavelengths, respectively, with no further temperature changes observed. The detailed manufacturing methods for the LED driver and controller were not covered in this research, as the primary focus was on optical analysis.

4. Discussion

This study focused on the optical analysis of LED oral care devices designed for light-based phototherapy. The optical design and analysis were conducted to effectively arrange three types of LEDs with antibacterial and sterilizing effects, considering the distance between the teeth and the LED arrays, as well as light uniformity. It was observed that when the distance between the LEDs and all target areas exceeded 2.5 mm, the light uniformity reached at least 50%, and it stabilized at approximately 80% once the distance surpassed 4 mm. Therefore, maintaining a light irradiation distance between 2.5 mm and 4 mm was crucial to ensure a uniformity of 50% or higher.

Subsequently, irradiance maps were obtained for each wavelength, and the average irradiance within the area of interest was calculated. Achieving the required irradiance density of 10 J/mm², which depended on both irradiance and operating time, required at least 16 minutes of operation. It is noteworthy that pulsed driving of LEDs reduced heat generation but also lowered irradiation energy. Therefore, to maintain total irradiance, either the irradiation time per session or the number of daily uses needed to be increased. This oral care device is designed for daily use rather than achieving complete therapeutic effects from a single session. Thus, frequent use would result in better sterilization and anti-inflammatory effects. However, further research is necessary to establish specific guidelines for practical use, including single-use duration and daily usage frequency.

Based on the optical analysis results, we developed a prototype LED oral care device. The device operated with a 20.5 mA pulse current, with duty cycles of 75% for the blue and green LEDs and 87% for the red LEDs. Under these conditions, the operating temperatures at the LED contact surface stabilized below 30°C, ensuring a level suitable for safe human use.

The design parameters established in this study include the output power (current) of each LED, as well as their position and arrangement. While the spatial distribution characteristics served as input variables for the simulation, they were considered constant since they had been determined during the LED selection process. The final parameters were determined by considering the simulation results (irradiance, irradiance density, uniformity) based on LED output power and arrangement, along with measured heat generation and practical operating time. As this study did not employ computational optimization techniques, other design possibilities exist. The LED positions were manually arranged according to wavelength efficacy, and pulse width was adjusted manually through temperature monitoring. Therefore, the provided data can serve as a useful reference for developing various types of oral care devices. From an optical analysis perspective, both irradiance and irradiance density exhibited linear changes with variations in optical power, duty cycle, and operation time, while light uniformity remained unaffected by these parameters. Therefore, adjustments in these factors could be easily predicted based on the findings of this analysis.

Further research, including clinical studies, is needed to evaluate the device's temperature safety within the oral cavity and its antibacterial and anti-inflammatory effectiveness for daily use. However, these aspects fall outside the scope of the current optical analysis study and were not addressed.

Funding

National Research Foundation of Korea10.13039/501100003725 (2019R1A2C1084080, RS-2024-00336436); Korea Medical Device Development Fund10.13039/100019266 (RS-2022-00164620).

Disclosures

The authors declare no conflicts of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.