Hydrocolloid‐Based Multiwavelength Stretchable QD‐OLED Patch for Simultaneous Neonatal Jaundice and Dermatitis Treatment with Biosignal Monitoring

Abstract

Conventional light‐emitting‐diode‐based light sources suffer from rigidity, localized heating, and poor adaptability to skin deformation, limiting their use in skin‐attached medical devices. To address these challenges, a quantum dot organic light‐emitting diode (QD‐OLED) architecture utilizing a blue OLED with QD for wavelength conversion is developed, achieving dual‐wavelength emission (470 and 630 nm) in a simplified structure. The use of hydrocolloid as a substrate ensures skin adhesion, moisture retention, and attachability. The device exhibits high mechanical resilience with 190% stretchability and stable optical performance under deformation for over 80 h at low temperatures (<39 °C). By combining photomedicine functionality with flexibility, the platform demonstrates 26% bilirubin degradation under blue light and 46% human skin keratinocyte (HaCaT) cell proliferation under red light. Furthermore, the integration of organic photodiodes enables real‐time biosignal detection, such as photoplethysmography, expanding the platform's potential in personalized healthcare systems. This study highlights the convergence of emerging electronic materials, QD‐OLED, stretchable substrates, and organic sensors as a viable route for next‐generation medical devices that integrate treatment, sensing, and skin‐conformal mechanics in a single patch‐type system.

Comparison of therapeutic and diagnostic applications of a surface‐emitting light source with multiwavelength emission characteristics.

1. Introduction

The rapid advancement of medical technology is blurring the boundaries between diagnosis and treatment, accelerating the integration of innovative healthcare platforms into everyday life. In particular, wearable optoelectronic skin‐attachable devices are emerging as next‐generation technologies that enable both precise diagnosis and real‐time therapy simultaneously^{[ 1} , ² , ³ , ^{4 ]} and the development of practical medical platforms integrating light‐emitting devices and electronic systems is gaining momentum.^{[ 5} , ⁶ , ⁷ , ^{8 ]}

However, current inorganic‐based light sources such as LASER,^{[ 9} , ^{10 ]} light emitting diode (LED),^{[ 11} , ^{12 ]} micro LED, and mini LED^{[ 13} , ^{14 ]} suffer from limited flexibility and point light source characteristics, making it difficult for them to conform to the curved surfaces of the human body.^{[ 11} , ¹² , ^{15 ]} In addition to their restricted emission areas, they generate significant localized heat, presenting structural limitations for use in skin‐attachable medical devices.^{[ 16} , ^{17 ]} These issues strongly highlight the need for new types of light sources to realize next‐generation smart medical systems.

To overcome these limitations, organic light‐emitting diodes (OLEDs) and quantum dot light‐emitting diodes (QLEDs) have been proposed as promising alternatives.^{[ 18} , ^{19 ]} These technologies offer structural flexibility and planar light‐emission characteristics, enabling uniform light distribution and low‐temperature operation, which are advantageous for creating stable therapeutic environments.^{[ 20} , ^{21 ]} Furthermore, their compatibility with various substrates such as paper, fabric, and polyethylene terephthalate (PET) allows for high scalability and wearability.^{[ 22} , ²³ , ^{24 ]} Owing to these advantages, OLEDs and QLEDs are rapidly gaining traction as biolight sources that go beyond traditional displays to integrate both therapeutic and sensing functions, positioning themselves as core technologies for future personalized healthcare systems.^{[ 25} , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ^{30 ]}

In addition, there is research on using electrolytes as a method to extend the lifespan of quantum dots in QLED,^{[ 31 ]} and research on developing high‐performance QLED with wavelength controllability through exciton control through quantum wells.^{[ 32 ]} Also, research on high‐performance phototransistors capable of optical detection using quantum dots has been reported.^{[ 33 ]}

Nevertheless, most OLED/QLED‐based research to date has focused on single‐wavelength emission or flexible display development. Efforts to develop platforms that simultaneously achieve multiwavelength emission and mechanical stretchability essential for diverse biomedical applications remain limited. Technological approaches for realizing multiwavelength emission have included stacked structures,^{[ 34} , ³⁵ , ^{36 ]} microcavity effects,^{[ 37 ]} and AC/DC voltage modulation.^{[ 38} , ³⁹ , ^{40 ]} Among these, quantum dots (QDs) have garnered particular attention due to their excellent wavelength tunability, narrow emission bandwidth, and capability to emit in the near‐infrared region.^{[ 41} , ^{42 ]} However, most of these technologies have inherent structural limitations in that their emission wavelengths are fixed after fabrication, posing challenges for adaptability and applicability in real clinical settings.

As a core technology for multiwavelength emission, quantum dot organic light‐emitting diode (QD‐OLED) structures combining OLED with wavelength‐converting QD have attracted increasing interest.^{[ 43} , ⁴⁴ , ^{45 ]} QD‐OLED utilizes a blue OLED light source, and the QD layer converts it into desired wavelengths (e.g., red or near‐infrared), thereby simplifying the fabrication process while enabling broadband emission.^{[ 46} , ⁴⁷ , ^{48 ]} This approach not only overcomes the narrow‐spectrum limitations of conventional QLED but also provides a foundational technology for developing multifunctional platforms capable of wavelength selection tailored to therapeutic purposes.^{[ 49 ]}

Meanwhile, to reliably accommodate the dynamic motions of the body and the irregular deformation of skin surfaces, not just flexibility but high stretchability is essential. Accordingly, various methods have been employed to impart stretchability to such devices. Approaches to impart stretchability to rigid devices such as displays or electronic components can be broadly categorized into material‐engineering and structural‐engineering methods.^{[ 50} , ^{51 ]} The material engineering method is a method of securing unique elasticity by making the parts that make up the device, such as light‐emitting layers or electrodes, with flexible materials. To achieve this flexibility and adherence to the skin surface, this study used a PET substrate grafted onto a hydrocolloid. This improves the adhesion between the OLED and the skin surface through the hydrocolloid and provides protection against wet‐dressing and bacteria. Furthermore, by grafting OLED onto a PET substrate, which has already been verified in various studies, phototherapy can be provided simultaneously.

The structural‐engineering approach, on the other hand, involves implementing specific geometric patterns or structural designs to introduce mechanical stretchability. This method allows stretchability to be imparted to substrates, electrodes, and devices that are otherwise flexible or possess low elastic moduli.^{[ 50} , ⁵² , ^{53 ]} Representative structural patterns include serpentine,^{[ 54} , ^{55 ]} island,^{[ 56 ]} and kirigami.^{[ 57} , ^{58 ]} The serpentine pattern enables free‐form geometrical deformation through its wavy structure; however, its low resilience makes it unsuitable for applications involving repeated stretching.^{[ 59 ]} The island pattern involves placing functional devices on rigid island structures, which are then connected by polymers with low elastic moduli. This design effectively disperses mechanical stress from external forces. Depending on the materials used, this structure can achieve high stretchability; nevertheless, further studies are required to ensure interfacial stability between the island and the surrounding polymer matrix.^{[ 60 ]}

By contrast, kirigami pattern is an origami‐inspired art technique where linear cuts allow 2D paper to stretch into 3D forms. This approach enables the transformation of a flat surface into complex 3D structures or imparts stretchability to otherwise rigid materials.^{[ 61} , ^{62 ]} Kirigami patterns have garnered attention for their ability to maintain optoelectronic and mechanical stability under repeated strain conditions.^{[ 63} , ^{64 ]} Recently, these patterns have been increasingly applied in the field of stretchable optics due to their high visibility and excellent stretchability characteristics.^{[ 58} , ⁶⁵ , ⁶⁶ , ^{67 ]}

Beyond mechanical stretchability and optical functionality, the development of skin‐attachable medical devices also requires functional adhesive structures that can remain stably attached to the skin over prolonged periods and promote physiological skin recovery. Hydrocolloids have emerged as promising materials that simultaneously satisfy these criteria for attachability and therapeutic support. Widely used in wound dressings, hydrocolloids offer multifunctional benefits such as infection prevention, exudate absorption, necrotic tissue removal, and protection against ultraviolet light and external irritation.^{[ 68} , ⁶⁹ , ⁷⁰ , ⁷¹ , ^{72 ]}

In this study, we propose a hydrocolloid‐based stretchable QD‐OLED light therapy platform in patch form, presenting a new direction for existing wearable optical device technologies. The implemented platform demonstrated a maximum stretchability of 190%, along with dual‐wavelength emission at 470 and 630 nm, effectively validating both blue‐light‐induced bilirubin degradation and red‐light‐induced cell proliferation. This system confirmed the feasibility of phototherapy for conditions such as neonatal jaundice and atopic dermatitis, with biological experiments revealing a 26% reduction in bilirubin and a 46% increase in human skin keratinocyte (HaCaT) cell proliferation. Furthermore, through the integrated organic photodiode (OPD), successful measurement of photoplethysmography (PPG) signal was achieved, marking a step beyond therapy toward real‐time biosignal monitoring and the collection of personalized medical data, thereby advancing the platform into a smart medical system. By combining QD‐OLED with hydrocolloids, the proposed platform enables a multifunctional therapeutic environment that supports both optical treatment and physical wound healing.

2. Results

2.1. Design of Multiwavelength Hydrocolloid‐Based QD‐OLED

As the possibility of combining commonly used light sources in daily life with wearable devices for biosignal monitoring or treatment of skin diseases has been reported, studies have been conducted to develop optic platforms capable of effective treatment and diagnosis.^{[ 6} , ²⁵ , ²⁶ , ^{73 ]} Most research has focused on identifying the diagnostic and therapeutic effects depending on the wavelength range and output intensity of the light source or maximizing the flexibility of the platform.^{[ 11} , ¹² , ²⁷ , ⁶⁷ , ⁶⁸ , ^{74 ]} However, previous research methods have shown a tendency to study wavelength control and flexibility separately, and studies on flexible platforms that simultaneously enable diagnosis and treatment using multiwavelengths emitted from a single light source are very limited.

There are various cases of using light sources in addition to dressings for wounds on the skin, etc. However, most studies including dressings do not use flexible, surface‐based, and wearable OLED, PLED, or QLED, but rather combine treatment with lasers.^{[ 75} , ⁷⁶ , ⁷⁷ , ⁷⁸ , ⁷⁹ , ^{80 ]} These studies provide wet dressing for various wounds, absorb wound exudates, block bacterial invasion, and provide phototherapy. However, these studies are unsuitable as wearable platforms due to the size of the equipment using lasers. Furthermore, studies using polyurethane foam dressings can provide wet dressing, bacterial blocking, and exudate absorption, but cannot simultaneously provide phototherapy due to the material's inherent permeability.^{[ 81 ]} A study applying phototherapy using polyurethane foam and wearable LED satisfies all of these conditions, but cannot absorb wound exudates.^{[ 82 ]} Furthermore, this study utilized a LED and a light guide plate to build the platform, which increases the size of the equipment, potentially causing inconvenience. Furthermore, the point‐light source nature of the device makes it difficult to provide uniform treatment. This is detailed in Table 1 .

Table 1.

Light‐based biomedical applications with wet dressing substrate. ◎ = very good, ° = good, △ = not bad, × = poor.

Dressing	Light source	Exudate absorption capacity	Life‐compatibility with light source	Light application	Reference no.
Hydrocolloid	QD‐OLED	°	Wearable, attachable	Diagnosis, phototherapy	This work
Hydrocolloid	LASER	°	–	–	[77]
Hydrogel	LASER	△	–	Phototherapy	[78, 79, 80]
Spongin‐like collagen	LASER	◎	–	Phototherapy	[81]
Polyurethane foam	Unavailable (transmittance)	◎	–	–	[82]
Polyurethane film	LASER LED	×	– Wearable (LED)	Phototherapy	[83, 84]

In this study, we propose a hydrocolloid‐based multiwavelength stretchable QD‐OLED by combining a lightweight and stretchable light source with multiwavelength characteristics and skin‐attachable medical material. This approach presents a new paradigm to existing research methods, and the device was developed in the form of a theragnosis patch‐type platform capable of both diagnosis and treatment in daily environments while addressing the chronic issues of phototherapy. To realize this, a lightweight and flexible blue OLED was combined with a QD film to develop a QD‐OLED that emits both red and blue wavelengths simultaneously, and by applying a kirigami structure and combining it with hydrocolloid, a medical patch material, multiwavelength characteristics for various skin disease treatments and stretchable as well as skin‐adhesive functions adaptable to dynamic skin were imparted. The hydrocolloid‐based multiwavelength stretchable QD‐OLED patch can be deformed while attached to the skin, and it is designed such that the adhesive side of the hydrocolloid and the emission direction of the OLED are aligned, allowing for simultaneous chemical and optical effects at the attachment site. In addition, among the components of the multiwavelength light emitted from the platform, the blue light converts bilirubin to its photoisomer lumirubin, contributing to jaundice treatment, while the red light promotes proliferation of HaCaT cells, confirming its potential for wound healing, psoriasis, and atopic dermatitis treatment (Figure 1 ). Studies using other OLEDs have generally shown poor skin adhesion. To verify this, we prepared Table S1 (Supporting Information) and performed a simple comparison.

Figure 1.

Concept and functional illustration of a hydrocolloid‐based stretchable QD‐OLED patch.

In this study, to achieve combined phototherapy at the attachment site, a light source capable of simultaneously emitting stable blue and red light was developed and integrated with medical materials. To secure blue and red wavelength bands, OLED, QD, and hydrocolloid were sequentially stacked (Figure 2a). The blue light emitted from the bottom‐emission‐structured blue OLED passes through the bottom QD film, and part of the blue light is converted into red light and emitted. In other words, red light converted through the QD film and unconverted blue light are emitted simultaneously, giving the device a multiwavelength characteristic from a single light source. A QD film was fabricated to exhibit a photoluminescence peak at 630 nm in the red spectrum upon excitation with blue light. The QD film with a peak wavelength of 630 nm was formed by encapsulating indium‐phosphide (InP)‐based QD in a resin, and the matrix layer, where color conversion takes place, has a thickness of ≈130 µm. To prevent oxidation due to oxygen and moisture penetration from the external environment, the quantum dots constituting the matrix layer are protected by a barrier structure composed of 200 nm thick Al₂O₃ and SiO _x deposited on a 12 µm thick PET substrate. Additionally, to ensure the uniformity of the red light emitted from the QD film, a SiO₂ diffusion coating was finally applied on the barrier layer to provide a uniform light scattering effect throughout the film. QD‐OLED with multiwavelength emission characteristics was designed to be attached to the skin by combining with hydrocolloid. To fabricate the designed patch, based on previous studies, the fluorescence emitter 1‐4‐di‐[4‐(N,N‐diphenyl)amino]styryl‐benzene (DSA‐Ph), which has been proven for emission and operation stability, was selected as a dopant for the high‐efficiency blue OLED.^{[ 83 ]} DSA‐Ph is doped at 3 wt% into 2‐methyl‐9,10‐di(2‐naphthyl) anthracene (MADN) to form the emissive layer of the OLED and exhibits dual peak wavelengths at 470 and 500 nm. However, the commonly used electrode material, indium tin oxide (ITO), in previous studies requires additional tuning due to its brittleness and broad wavelength characteristics. Therefore, in this study, electrode materials used in previous research were replaced, and simulations were conducted to secure a flexible light source and implement a deep blue OLED.

Figure 2.

Design and manufacture of a QD‐OLED for the treatment of jaundice and atopic dermatitis. a) Schematic diagram of the structure of the hydrocolloid‐based QD‐OLED platform (bottom emission). b) Energy level diagram according to the material and structure of the blue OLED. c) Emission image of the Ag‐based blue OLED and its emission under stretchable conditions, along with the emission image of the hydrocolloid‐based QD‐OLED and its emission under stretchable conditions. d) PL analysis results to examine the excitation–emission characteristics of the QD film. e) Comparison of light conversion ratios depending on the presence of the QD film and hydrocolloid. f) Spectral comparison between ITO‐based OLED, Ag‐based QD‐OLED, and hydrocolloid‐integrated QD‐OLED. g) Radiant emittance graph according to OLED driving voltage. h) Quantitative comparison of blue and red spectral output. i) Spectral analysis comparing peak wavelengths and FWHM measured in each device structure.

In previous studies, the transparent ITO was generally used as the anode electrode of blue OLED.^{[ 84} , ⁸⁵ , ^{86 ]} ITO is a 9:1 mixture of indium oxide (In₂O₃) and tin oxide (SnO₂), possessing high conductivity (10³–10⁴ S cm⁻¹) and high transmittance (80–90%) in the visible light region, and is manufactured as a transparent, colorless thin film. It is widely used as an electrode material due to its durability and compatibility with large‐area processes. However, due to its ceramic nature, it is not suitable for flexible electronic devices because of brittleness, high elastic modulus, and low elongation. Although we selected a blue fluorescent organic light‐emitting diode, which has been proven to be stable and reliable through prior research. While blue phosphorescent OLEDs are known to be more efficient than fluorescent materials, their instability limits their practical applications, with an LT₇₀ lifetime typically less than 1113 h at an initial luminance of 1000 cd m⁻². Conversely, blue fluorescent OLEDs, while lower in efficiency than phosphorescent materials, boast relatively excellent stability and material safety, making them suitable for this study.^{[ 87} , ^{88 ]}

To develop a flexible blue OLED, we selected Ag as an alternative electrode material to ITO. Ag has high electrical conductivity and can be used as a flexible electrode. Through optical simulation using MATLAB, it was confirmed that Ag‐based blue OLEDs have a peak wavelength at 470 nm and can induce resonance through interference phenomena (two‐beam and Fabry–Pérot interference) near this wavelength (Figure S1, Supporting Information). This confirmed that a strong microcavity effect can be formed between the transparent Ag electrode (25 nm) and a highly reflective aluminum (Al) electrode (Figures S2 and S3, Supporting Information). A microcavity refers to a structure where light is repeatedly reflected inside, leading to resonance at a specific wavelength that amplifies emission at that wavelength and suppresses others. The resonant wavelength is determined by the physical properties (e.g., refractive index, reflectance) and thickness of the organic layers and electrodes. To maximize the microcavity effect, the emission position of the internal light source in the device and Fabry–Pérot and two‐beam interfaces were adjusted through simulation. The structure was designed so that electrons and holes could move smoothly by considering the work functions and energy levels of each layer. Electrons and holes injected through the anode and cathode recombine in the emission layer (EML) to emit photons. These photons emit blue light due to the influence of the MADN:DSA‐Ph material (Figure 2b).

To achieve multiwavelength emission from a single light source, a precisely designed blue OLED was integrated with quantum dots to form a QD‐OLED structure. QDs are semiconductor nanocrystals that exhibit photoluminescence, converting absorbed high‐energy photons into longer‐wavelength photons with a narrow emission bandwidth. The peak emission wavelength is precisely controlled by the QD's size.^{[ 89} , ^{90 ]} The QD used in this study is an 8 nm InP quantum dot encapsulated in resin to form a film. The QD film was placed on the cathode side of the blue OLED so that the emitted blue light can excite the QD. Through this design, it was possible to realize both blue and red emissions in a single OLED device simply by applying the QD film to the blue OLED. The PLQY of commercial InP QD was measured to be 82.24% at an absorbance wavelength of 450 nm. To encapsulate these InP‐based QDs, we encapsulated them at a ratio of QD:encapsulating material = 1:5.5. The encapsulated QDs were then isolated in resin. At this point, the QDs were distributed in the resin at a concentration of 22.5%. The completed QD film is shown in Figure S4 (Supporting Information), and when the surface is magnified under a microscope, it can be confirmed that QD powder is included. After the QD‐OLED was fabricated through the above process, a kirigami structure was applied and transferred to hydrocolloid to produce a patch with flexibility, stretchability, and skin adhesion. The electrodes of the light sources in the fabricated patch are interconnected, enabling simultaneous operation without the need for individually connected power sources, and multiwavelength characteristics can be secured by simply applying the QD film (Figure 2c).

To analyze the excitation–emission characteristics of the QD and determine the wavelength and intensity of emitted light in response to specific incident wavelengths, photoluminescence (PL) analysis was conducted (Figure 2d). In this analysis, visible light in the 400–600 nm range was incident on the QD film in 20 nm intervals, and the emitted wavelength range was measured from 598 to 700 nm. When 440 nm blue light was incident, it was converted into light with the highest intensity, and the emitted red light had a peak at 628 nm. Additionally, incident light from 400 to 600 nm was partially converted to 624–630 nm.

To clearly analyze the actual conversion rate of blue light emitted from the Ag‐∖ electrode‐based blue OLED into red light as it passes through the QD film, the conversion ratio, defined as the proportion of blue light converted into red light via QD and hydrocolloid, was derived (Figure 2e). Final conversion ratios considering light attenuation through the hydrocolloid were calculated. The QD‐OLED and hydrocolloid QD‐OLED exhibited light conversion from 3.5 to 6.5 V in the range of 20.76–23.51%. Light conversion occurs exclusively in the QD film, and no conversion occurs in the hydrocolloid; thus, the conversion ratios of the QD‐OLED and hydrocolloid QD‐OLED are very similar (≈99.6%). However, the intensity of the converted light is slightly reduced by the hydrocolloid, and when accounting for light attenuation, the conversion ratio is 14.75% at 3.5 V and 18.81% at 6.5 V, showing a decrease of ≈4–6%. The degree of light attenuation by the hydrocolloid tends to decrease exponentially as the input voltage increases.

Through simulations, it was confirmed that the hydrocolloid‐based stretchable QD‐OLED patch platform with precisely designed optical functionality has two peak wavelengths in the blue and red regions. The spectrum of the ITO‐based blue OLED emitting a single blue band was compared, and unlike the broad spectrum of ITO, the developed device showed a distinct peak in the blue region and simultaneous acquisition of the red region converted through the QD film (Figure 2f).

The OLED‐based patch we developed begins to emit from 3 V in accordance with the diode characteristics and operates at voltages below 6.5 V. The optical properties of the Ag‐based blue OLED, QD‐OLED, and final platform of the hydrocolloid‐stretchable QD‐OLED combined patch were analyzed. The radiant emittance of the Ag‐based blue OLED in the blue region reached a maximum of 44.17 mW cm⁻² at 6.5 V. The blue OLED device exhibited an external quantum efficiency of ≈6.22% at a current density of 263 mA cm⁻² (Figure S5, Supporting Information). As such, sufficient blue OLED performance to be applied to various photomedicine has been secured. Based on this, QD‐OLED also obtained multiwavelength light suitable for application to photobiomodulation. On the other hand, the QD‐OLED and hydrocolloid‐based QD‐OLED exhibited multiwavelength emission of 10.38 and 8.31 mW cm⁻² at 6.5 V, respectively, securing suitable output intensities for medical applications via photobiomodulation (Figure 2g; Figure S6, Supporting Information).

To accurately confirm the individual emission characteristics of blue and red light within the multiwavelength emissions of QD‐OLED and hydrocolloid‐based QD‐OLED, radiant emittance was analyzed by dividing the wavelength bands. The blue region was defined as 380–580 nm and the red region as 580–780 nm. At 6.5 V, QD‐OLED exhibited a total light output of 10.38 mW cm⁻², comprising 6.94 mW cm⁻² red and 3.44 mW cm⁻² blue light. The hydrocolloid‐based QD‐OLED exhibited a total light output of 8.31 mW cm⁻², comprising 5.21 mW cm⁻² red and 3.10 mW cm⁻² blue light (Figure 2h).

The spectra of the blue region of the Ag‐based blue OLED and the blue and red regions of the QD‐OLED and hydrocolloid‐based QD‐OLED were visualized. The blue OLED exhibited a peak wavelength at 463 nm with a full width at half maximum (FWHM) of 22.26 nm. When QD and hydrocolloid were applied, two peak wavelengths at 464 and 636 nm were observed, with FWHM of 43.91 and 41.87 nm in the red region, respectively. This confirms that the emission spectrum does not change after transferring the QD‐OLED to the hydrocolloid (Figure 2i).

2.2. Fabrication of Hydrocolloid‐Based Stretchable QD‐OLED

The fabrication process of hydrocolloid‐based stretchable QD‐OLED and OPD optimized for comprehensive treatment is shown in Figure 3 . First, PET, a transferable ultrathin substrate, was patterned into a kirigami structure using a carbon dioxide (CO₂) laser cutter (Speedy 360, Trotec Laser Inc.). At this time, in the laser cutting process, the main process variables that can be controlled such as power, speed, and ppi were set to 11, 2.5, and 1000, respectively, to optimize the cutting quality. The substrate was prepared with a flexible base that can be stretched through a laser cutting process. The transferable PET substrate consists of a PET layer, an acrylic adhesive layer, and a release liner layer with a total thickness of 20 µm. It is lightweight (less than 0.017 g cm⁻²) and can be transferred even after OLED deposition. To fit the radiation area of the flexible structure, the QD film was also patterned through the laser cutting process (Figure 3a).

Figure 3.

Manufacturing process of the hydrocolloid‐based QD‐OLED platform. a) Laser‐patterned process. b) Planarization process. c) OLED/OPD evaporation process. d) Transfer process.

A planarization process was performed to uniformly deposit OLED and OPD on the prepared substrate. The planarization step is an important process to widen the range of substrates applicable to OLED deposited on nanometer‐thick films, and PET planarization technology has been widely studied and standardized. In this process, a laser‐patterned PET film was first attached to a guide glass. Subsequently, highly planarized substrates were fabricated by applying an SU‐8 solution, followed by spin coating and curing (Figure 3b). The SU‐8 photolithography process, involving UV‐induced acid generation, postexposure thermal activation, and subsequent cross‐linking, ensures structural stability and high‐resolution patterning, thereby enabling the stable and precise fabrication of QD‐OLED.^{[ 91 ]}

The curing process included soft baking for 1 min and 30 s at 95 °C, UV hard baking for 40 min, and a second soft baking for 1 min and 30 s at 95 °C. The process allowed the substrate to be flattened by achieving nanometer‐scale surface roughness while maintaining its elasticity and transferability.

Thereafter, OLED and OPD were fabricated by thermal evaporation of metal and organic materials under a 10⁻⁷ Torr vacuum. OLED and OPD were individually deposited on the prepared planarized stretchable PET substrate. Each organic material in the OLED deposited on the stretchable substrate formed an emission layer to form a blue OLED with a peak spectrum at 470 nm. To induce the cavity effect, Ag (25 nm) was used as a cathode and Al (100 nm) was used as an anode. The manufactured OPD was doped with organic materials to form a PN junction, and magnesium–silver alloy (Mg:Ag) and Al were used as electrodes (Figure 3c).

Blue OLEDs deposited on stretchable PET substrates were transferred to hydrocolloids to provide skin adhesion and create a humid environment on the attachment area. Our study utilized a kirigami structure to fabricate an island‐type electrode with elasticity (Figure S7, Supporting Information). Furthermore, QD films were strategically positioned between the nonadhesive regions of hydrocolloids and the light‐emitting surfaces of the OLED to facilitate multiwavelength emission. QD is a particle that converts an incident high energy wavelength into a low energy wavelength, and the wavelength emitted varies depending on the particle size. QD used in the platform is a film‐shaped particle with a diameter of 8 nm, which converts an incident spectrum with a peak wavelength of 470 nm to a peak wavelength of 630 nm. At this time, the produced OPD was inserted together, so that the multiwavelength light emitted from the QD‐OLED could be reflected by hemoglobin and then received by OPD (Figure 3d).

2.3. Mechanical Properties of Hydrocolloid‐Based Stretchable QD‐OLED Patch Platform

The structure of the hydrocolloid‐based stretchable QD‐OLED patch proposed in this study is schematically illustrated in Figure 4a. After OLED deposition, the OLED was transferred to a hydrocolloid substrate by utilizing the adhesive properties of the PET with transferability. Additionally, a QD film was inserted between the OLED and the hydrocolloid to further secure a peak wavelength at 630 nm. In this process, a PET‐based encapsulation film with an additional barrier layer is attached to the anode side of the OLED, placing the PET barrier structure on the outermost top surface of the patch platform, with the hydrocolloid and QD film positioned below. This creates a hydrocolloid–QD film–PET–OLED–PET encapsulation layer structure. This configuration prevents performance degradation due to moisture and oxygen penetration from both directions, thereby ensuring the long‐term stability of the device.

Figure 4.

Evaluation of stretchability of hydrocolloid‐based stretchable QD‐OLED patch. a) Structural diagram of a hydrocolloid‐based stretchable QD‐OLED. b) Hydrocolloid‐based stretchable QD‐OLED circuit diagram. c) Stretched along the x‐axis image of stretchable blue OLED and hydrocolloid‐based stretchable QD‐OLED. d) Stretchable blue OLED performance by degree of stretch (current density and luminance for voltage). e) Stretchable blue OLED performance with number of stretch and release (luminance and radiant emission for current density). f) Hydrocolloid‐based stretchable QD‐OLED (current density and luminance for voltage) according to the degree of stretch. g) Hydrocolloid‐based stretchable QD‐OLED performance with number of stretch and release (luminance and radiant efficiency for current density). h) Stretch degree light‐emitting image. i) Luminous image with the number of extensions. j) Various physical modifications of hydrocolloid‐based stretchable QD‐OLED.

In the platform designed in this study, a unique circuit structure was implemented in which all four OLEDs are driven by a common power supply based on diode characteristics, as shown in Figure 4b. The interconnections between each OLED were designed to have stretchable properties by undergoing laser cutting processes. Each OLED operates based on anode and cathode electrodes and receives a stable voltage from the common power source. Considering the forward voltage characteristics of the OLED, protective components such as resistors were arranged in parallel or series to prevent excessive current flow and to ensure each device operates under optimal conditions. Particularly, the stretchable interconnects in this circuit provide high mechanical stretchability, enabling the entire circuit to withstand bending, stretching, and twisting. This contributes to maintaining stable electrical performance of the platform, making it highly advantageous for applications in dynamic environments such as flexible displays and wearable electronic devices, and enhancing the overall system reliability and durability.

Figure 4c visually demonstrates the ability of the hydrocolloid‐based QD‐OLED patch to flexibly adapt to mechanical deformation. The stretchable QD‐OLED fabricated in this study was able to stretch up to 190%, while the four light‐emitting devices connected through stretchable electrodes maintained stable emission simultaneously. In this study, the strain (ε _s) of the flexible electrode was defined using the following equation (Equation (1))

$${\epsilon }_s=\frac{\mathrm{\Delta }l}{\begin{array}{c}{l}_0\end{array}}$$ (1)

where Δl represents the change in length, and l ₀ indicates the pitch of the stretchable QD‐OLED.

Generally, mechanical stretching can lead to performance degradation in electronic devices. However, the OLED patch developed in this study maintained stable emission characteristics even during deformation, thanks to the uniform stress distribution structure and high mechanical stretchability design. This result is a significant factor in expanding the potential applications in wearable displays and flexible optobiological fields.

For wearable devices and stretchable electronic devices, maintaining consistent luminance under tensile deformation is crucial. In this experiment, the luminance change of the stretchable QD‐OLED device was analyzed by gradually increasing the strain. Measurements were taken initially in the undeformed state, and subsequently at 25% increments, up to a maximum strain of 150%. The results showed that the luminance variation remained within ≈5% of the initial state to 150% strain (Figure 4d). Figure 4e,g shows the evaluation of luminance durability through repeated deformation. Although mechanical fatigue is typically a major factor in the performance degradation of devices, the developed device in this study maintained its initial luminance level even after 10 000 cycles of repeated deformation. This result proves that the stretchable device can be used over an extended period and retains high reliability even under repetitive stretching environments like human skin. The electroluminescence images according to stretch percentage and images before and after 10 000 stretching cycles are shown in Figure 4h,i, respectively, demonstrating the stability of electrical and optical performance under stress. These results particularly suggest the feasibility of long‐term OLED performance maintenance in stretchable environments such as wearable devices.

Figure 4j presents the evaluation of the OLED device's ability to continuously emit light under various physical deformation conditions. The durability of the device was tested by applying different types of physical deformation, including bending, twisting, and compression. The results showed that the OLED devices continued to emit light normally under each deformation environment. Notably, the device maintained structural stability not only under simple tensile deformation but also under complex deformation conditions, which is a crucial feature for application on dynamic skin surfaces. This characteristic of maintaining emission performance under diverse physical deformations further strengthens the potential of this technology as a next‐generation wearable and flexible electronic device.

2.4. Flexibility and Reliability of Hydrocolloid‐Based QD‐OLED

To verify the mechanical flexibility of the patch platform developed in this study, two tests were conducted. The first was the cantilever test. The mechanical flexibility of the platform was evaluated using the cantilever test (ASTM D:1388) (Figure 5a).^{[ 38 ]} The cantilever measurement method can easily assess the bending stiffness of thin, flexible devices. It is a simple and robust method that allows for the preservation of specimens after measurement, making it a nondestructive measurement method. It can effectively measure the inherent flexibility of materials with high accuracy^{[ 92 ]} and is actively being used in wearable research.^{[ 38} , ⁹³ , ^{94 ]} The sample slides back and forth along an inclined plane of the test apparatus set at an angle of 41.5°. As the sample moves, one side of the specimen bends downward due to gravity, and the length at which it touches the inclined plane, along with the width, thickness, and weight of the sample, are measured to determine the cantilever length. The cantilever length is defined as the distance from the edge of the sample to the point where it contacts the inclined plane. A shorter cantilever length indicates that less force is required to bend the sample to a specific radius, implying greater flexibility.

Figure 5.

Flexibility and reliability of hydrocolloid‐based stretchable QD‐OLED. a) Photograph and measurement method of cantilever length measurement. b) Measured cantilever length. c) Measured Young's modulus (cantilever tester). d) Measured Young's modulus (UTM) and measured picture. e) Lifetime of hydrocolloid‐based stretchable QD‐OLED by conditions. f) Stretchable blue OLED and hydrocolloid‐based stretchable QD‐OLED in water. g) Temperature measured by condition. h) Temperature measurement infrared image.

However, since the cantilever length does not consider the influence of the sample's weight or area, it has limitations in accurately determining flexibility. When samples with different weights are measured under the same conditions, variations in cantilever displacement may be attributed to mass differences rather than actual flexibility, affecting measurement reliability. Therefore, to enable a more quantitative, material‐property‐based evaluation, the Young's modulus of each platform was calculated using a mathematical approach. Young's modulus is a mechanical property that represents the ratio of stress to strain during deformation by external force, indicating the elasticity or resistance to deformation. This allows a quantitative comparison and analysis of the mechanical flexibility of each component. Calculations were performed using the following formula (Equation (2))^{[ 93 ]}

$$Youn{g}^{\prime }sModulus=\frac{12w{\left(l·f\left(\theta \right)\right)}^3\left(1-v^2\right)}{\begin{array}{c}{t}^3\end{array}}\left[pa\right]$$ (2)

$$\begin{array}{ccc}& & \left(w:\mathrm{weight},I:\mathrm{cantilever}\mathrm{length},f\left(\theta \right):1/2,t:\mathrm{thickness},v:\right.\hfill \\ & & \left.{\mathrm{Poisson}}^{\prime }\mathrm{s}\mathrm{ratio},v^2\to 0\right)\hfill \end{array}$$ (3)

For comparison with the hydrocolloid‐based stretchable QD‐OLED patch platform developed in this study, four additional samples were evaluated: A4 paper, PET film with guide PET, PET film with kirigami‐structured guide PET, and hydrocolloid without guide PET. The area and weight information of each sample are listed in Table S2 (Supporting Information). A4 paper is divided into two directions depending on the direction of pulp flow during the manufacturing process (Figure S8, Supporting Information). The direction of pulp flow is the machine direction (the long side of the paper), and the direction perpendicular to the machine direction is the cross‐machine direction (the short side of the paper). Due to this manufacturing method, paper is anisotropic, and the machine direction generally has lower flexibility than the cross‐machine direction. In this study, A4 paper cut in the cross‐machine direction was used for the cantilever test.

The cantilever length results obtained from the cantilever test are shown in Figure 5b. Bare PET exhibited low flexibility similar to paper, while PET with kirigami structure applied through laser cutting and hydrocolloid exhibited significantly enhanced flexibility. Figure 5c presents the results of the modulus of elasticity as a function of sample thickness, enabling a more quantitative evaluation of the platform's mechanical properties. The Young's modulus of bare hydrocolloid and hydrocolloid‐based QD‐OLED was 0.81 and 5.58 MPa, respectively, which is substantially more flexible compared to A4 paper (2.36 GPa) and bare PET (1.10 GPa). This indicates that the platform combining hydrocolloid and QD‐OLED retains high flexibility with minimal reduction due to integration, minimizing mechanical stress during bending and enabling adaptability to various physical deformations without restricting movement.

In addition to flexibility, the hydrocolloid‐based QD‐OLED patch platform also possesses stretchability. To evaluate this mechanical property, a second test tensile strength test using a Universal Testing Machine was conducted. In this test, strain, elastic modulus, and ultimate tensile strength (UTS) were quantitatively analyzed. The tensile test was conducted by securing the specimen between two clamps and applying a pulling force by moving the upper clamp at a constant rate. Stress–strain curves were obtained in real time by measuring the strain and corresponding stress. These curves are divided into elastic and plastic regions. In the elastic region, the material returns to its original state upon removal of the stress. The slope of the linear section between stress and strain is defined as the elastic modulus, which quantifies mechanical stiffness. Beyond the yield point, permanent plastic deformation occurs, and the material eventually fractures at the point of maximum stress (UTS). The mechanical stability and structural durability of the component are evaluated based on maximum tensile strength tests. The linearly increasing region of the strain/stress curve obtained through tensile strength tests represents the elastic regime. Using the slope of this region, we can calculate Young's modulus. Patterned PET with kirigami structure showed a maximum elongation of 379% and an elastic modulus of 0.54 MPa. Patterned PET stretched up to 369% with an elastic modulus of 1.24 MPa in that range. Bare hydrocolloid showed a maximum elongation of 3927% and an elastic modulus of 1.54 MPa at maximum stress, with an elastic modulus of 0.34 MPa at 369% strain. The hybrid structure combining both materials (this work) showed an elastic modulus of 0.60 MPa at 369% strain and 2.59 MPa at a maximum elongation of 3961%. This indicates that the hybrid structure exhibits mechanical behavior intermediate to patterned PET and hydrocolloid before PET fracture and becomes similar to hydrocolloid after PET failure. In addition, the Young's modulus calculated from the cantilever test was 5.58 MPa, consistent with the modulus measured in the PET region (5–6 MPa), suggesting that the patch platform maintains sufficient flexibility and durability under mechanical strain and provides structural stability in defining light emission positions.

Additionally, the elastic region, yield region, and plastic region were analyzed. In Figure S9a (Supporting Information), for the patterned PET, the kirigami pattern of the element is elongated in the initial 10% elongation region, and this is a section where no special tensile force is applied to the element. This section is an elastic region, which returns to its original state when the stress is released. In addition, a plateau region was observed in the 12–211% section where the stress remains constant up to a certain strain. After that, the element is elongated while maintaining elasticity up to the 340% section, and from the yield point, the pattern is gradually destroyed and enters the plastic region (Figure S9b, Supporting Information). After entering the plastic region, permanent deformation is maintained even after the stress is released, and it is gradually destroyed and does not return to its original state or separate.

Furthermore, in Figure S9c (Supporting Information), both the bare hydrocolloid and our final structure maintain the elastic region up to ≈20% of the area. Following this, the materials enter the plastic deformation region beyond their yield point. As depicted in Figure S9d (Supporting Information), the bare hydrocolloid fractured at 3927% elongation, whereas the hydrocolloid‐based stretchable QD‐OLED fractured at 3961% elongation.

To evaluate the long‐term usability of the OLED patch, the device lifetime was evaluated under various output conditions. Operation time was measured at output levels of 3, 4.5, and 6 mW cm⁻², and the lifetime was defined as LT₅₀ (time until luminance drops to 50% of its initial value). As a result, the OLED patch took 88, 82, and 46 h, respectively, to reach LT₅₀. It operated reliably for at least 40 h under all conditions, demonstrating its potential for prolonged light therapy in real medical environments (Figure 5e).

As the hydrocolloid and QD‐OLED used in the patch exhibit high waterproof performance, it is crucial to ensure the water resistance and long‐term reliability of the OLED. Therefore, encapsulation using PET was applied to reinforce durability against environmental exposure, as shown in Figure 4a. The platform with encapsulation was tested in deionized water to evaluate operational stability in water. The results confirmed that OLED operated continuously and stably in underwater conditions (Figure 5f), suggesting reliable long‐term use of the hydrocolloid‐based patch. Additionally, the functionality of stretchable electrodes without encapsulation was tested and shown in Figure S10 (Supporting Information), confirming mechanical stability in water even without a protective structure.

The hydrocolloid‐based stretchable QD‐OLED platform developed in this study was designed to provide simultaneous optical treatment and physical healing effects from the hydrocolloid while being attached to the skin. As wearable patches are intended for prolonged skin contact, thermal safety assessment is essential. Therefore, thermal behavior under OLED operation was analyzed. Heat generation was evaluated at maximum output (8 mW cm⁻²) and low output (2.5 mW cm⁻²), with surface temperature measured in real time using an infrared thermal camera. Initial temperature was around 21–22 °C, with a mean increase of ≈2.2 °C min⁻¹ during operation. At 2.5 mW cm⁻², thermal saturation was reached in about 2 min, with final saturation temperature at 26.8 °C (±0.1 °C). At 8 mW cm⁻², saturation occurred after about 9 min, with a saturation temperature of 38.4 °C (±0.1 °C). Notably, even under high output, the temperature remained within a safe range comparable to human body temperature (≈37 °C), and no excessive heat accumulation was observed. These results indicate that the platform operates within safe limits, below the threshold for low‐temperature burns (e.g., sustained exposure above 44 °C for over 6 h).^{[ 95 ]} This confirms the thermal stability of the hydrocolloid‐based stretchable QD‐OLED platform and its suitability for prolonged use as a wearable light therapy patch (Figure 5g). The combination of flexibility, water resistance, and thermal safety strongly suggests that this platform can be utilized as a practical wearable healthcare device.

2.5. Photodiagnosis and Photobiomodulation of Hydrocolloid‐Based Stretchable QD‐OLED Patch Platform with OPD

Through the preceding validations, it has been confirmed that the hydrocolloid‐based multiwavelength stretchable QD‐OLED platform developed in this study possesses electro‐optical properties and reliability. In order to effectively treat skin diseases such as atopic dermatitis using OLED, it is important to induce a photobiomodulation effect by irradiating light with low‐intensity energy to the skin. In this study, to demonstrate that a platform with dual‐wavelength emission characteristics from a single light source can be used for photomedical applications, in vitro experiments were conducted to verify the reduction of bilirubin concentration and the promotion of cell regeneration. Furthermore, for real‐time continuous biosignal monitoring, the platform was integrated with a PPG sensing module fabricated using organic materials.

The purpose of this study is to monitor the newborn's heart rate in real time and detect early signs of the transition of jaundice to nuclear jaundice, a severe neurological complication, by integrating PPG sensors into OLED‐based wearable patches. Approximately 30% of newborns with a bilirubin level of 25–30 mg dL⁻¹ may progress to kernicterus, a condition that causes severe neurological damage.^{[ 96 ]} This occurs when bilirubin crosses the blood–brain barrier and accumulates in the brain, leading to cardiovascular abnormalities such as bradycardia and blood pressure instability. Over 75% of infants with kernicterus die, and 80% of survivors suffer from sequelae such as intellectual disability, hearing loss, and limb paralysis, making early diagnosis critical.^{[ 97 ]} According to previous studies, it has been reported that nuclear jaundice causes damage to the central nervous system, including the autonomic nervous system, which can ultimately lead to cardiovascular dysfunction.^{[ 98} , ⁹⁹ , ^{100 ]} Based on this pathophysiological association, PPG‐signal‐based hemodynamic monitoring technology is applied to detect early signs of the autonomic nervous system and cardiovascular abnormalities that appear during the progression of jaundice, quantitatively assess the possibility of metastasis to nuclear jaundice, and is designed to enable rapid and preemptive clinical intervention. PPG is a noninvasive technique that detects changes in blood flow and enables real‐time monitoring of heart rate, vascular status, and blood circulation.^{[ 73 ]} However, conventional PPG sensors are made from inorganic materials and lack compatibility with the flexible OLED patch developed in this study. To address this, an OPD was fabricated using organic materials to ensure compatibility with the flexible and stretchable patch platform.

The light emitted from the hydrocolloid‐based QD‐OLED passes through the QD film and reaches the human body (wrist), where hemoglobin in the wrist blood vessels selectively absorbs and reflects the light. By irradiating light onto the skin through the QD‐OLED and analyzing changes in reflected light using the OPD sensor, the blood volume change can be measured to assess cardiovascular health (Figure 6a). The OPD, based on organic semiconductors, converts light into electrical signals and consists of an anode, hole transport layer, light‐absorbing layer, electron transport layer, and cathode. However, conventional OPD using electrodes such as ITO, IZO, and AZO have low inherent flexibility, making them physically incompatible with the OLED patch developed in this study. To overcome this, the electrode material was replaced with a Mg:Ag composed of 10% Mg and 90% Ag, used as the electrode material (Figure 6b). The detailed fabrication process and structure of the OPD are described in the Experimental Section. The thickness and energy levels of the light‐absorbing layer made from organic materials were adjusted to enable effective PPG sensing (Figure 6c). The fabricated OPD was integrated with the hydrocolloid OLED patch to continuously monitor biosignals when attached to the body (Figure 6d).

Figure 6.

Biosignal measurement using light and OPD of hydrocolloid‐based QD‐OLED. a) Schematic of the hydrocolloid‐based stretchable QD‐OLED with OPD. b) OPD structure. c) Energy level diagram according to the material and structure of the OPD. d) Actual image light emission (left) light emission (right). e) Absolute value characteristics of current density with voltage of OPD. f) Measured PPG signal (before extension). g) Measured PPG signal (after 100% extension).

As a result of measurements, under light irradiation from the QD‐OLED (light condition), a significant increase in current density with respect to voltage was observed. Especially in the reverse bias region, the increase in light‐induced current was prominent, indicating that the OPD effectively generated photocurrent (Figure 6e). As blood moves with the heartbeat, the amount of reflected light changes, and this change is used to measure the PPG signal. The reflected light is redirected to the OPD, which detects it and converts it into electrical signals. Notably, the light emitted from the QD‐OLED is internally reflected at the cathode surface of the OPD, minimizing light leakage and enabling more accurate PPG signal acquisition.

To verify this, a PPG signal measurement experiment was conducted, in which the QD‐OLED was driven at 100 Hz and 10 mA to obtain the PPG signal (Figure 6f). At this time, the hydrocolloid‐based QD‐OLED was set to an output of 3.1 mW cm⁻² and maintained stable emission characteristics. Additionally, the reflected light was measured without applying an additional bias to the OPD, using natural voltage variation. The experiment utilized blue (470 nm) and red (630 nm) light signals emitted from the QD‐OLED to acquire PPG signals exhibited periodic patterns consistent with heart rate variations (Figure 6e). During this process, it was confirmed that even with a 100% strain applied to the platform, the PPG sensing performance was maintained, indicating that the platform possesses high mechanical flexibility and enables stable PPG signal detection (Figure 6g). Additionally, Fast Fourier transform (FFT) analysis was performed on the acquired raw PPG signals to verify heart rate periodicity in the frequency domain (Figure S11, Supporting Information). In both the before stretch and after stretch FFT results, a distinct peak was observed around 1.199 Hz (corresponding to ≈71 BPM), indicating that the device reliably captures heart rate signals even under stretched conditions. These results demonstrate the ability of this platform to reflect physiological periodic changes. Moreover, signal quality measured from the OPD remained stable after repeated stretching and deformation, suggesting that the hydrocolloid‐based QD‐OLED is suitable for wearable devices capable of real‐time biosignal monitoring. Therefore, the OLED‐based wearable patch combined with an OPD‐based PPG sensor platform developed in this study offers a novel approach for real‐time heart rate monitoring and early diagnosis. By using OPD, the mechanical compatibility with the OLED patch was secured, overcoming the flexibility limitations of conventional inorganic PPG sensors and enabling stable PPG signal acquisition even in highly stretchable platforms.

As previously mentioned, this study designed a photobiomodulation platform for real‐time treatment of neonatal jaundice and atopic dermatitis. To verify its efficacy, experiments were conducted to confirm that the OLED patch developed in this study is effective in reducing bilirubin concentration for jaundice treatment and promoting skin cell proliferation. To demonstrate that the red light emitted from the wearable patch can be sufficiently applied for treating neonatal skin conditions such as atopic dermatitis and psoriasis, activation and cell proliferation experiments were conducted using keratinocyte HaCaT (American Type Culture Collection (ATCC) PCS‐200‐011) cells. The mechanism of red light therapy is illustrated in Figure 7b. When red light in the range of 600–700 nm is irradiated on the human body, it affects intracellular chromophores, which absorb the incident light and either store energy or induce chemical or physical changes. Chromophores are specific atoms or functional groups within molecules that absorb light and exhibit color, and the wavelength range absorbed by chromophores varies depending on the molecular chemical structure. In biological systems, chromophores absorb light and induce biochemical reactions. A representative chromophore, cytochrome c oxidase, absorbs red and near‐infrared light to activate mitochondria and facilitate the electron transport chain. In this process, the generation of metabolic substances such as reactive oxygen species (ROS, involved in intracellular signal transduction), adenosine triphosphate (ATP, an energy source required for biochemical reactions), and nitric oxide (NO, involved in vasodilation and increased blood flow) is enhanced, ultimately promoting tissue healing, pain relief, inflammation reduction, and nerve regeneration. Using this principle to proliferate keratinocyte cells enables the regeneration of the skin barrier, which restores defense mechanisms against bacterial substances from external environments and reduces the infiltration of external antigens such as allergens, thereby contributing to the improvement of skin diseases by preventing excessive immune activation.

Figure 7.

Biomedical applications for the treatment of neonatal jaundice and atopy dermatitis (the graphs are expressed as the mean ± standard deviation (SD) error (n = 4)). a) Conceptual schematic of blue light therapy and red light therapy by the hydrocolloid‐based QD‐OLED. b) Mechanism of red light treatment. c) Mechanism of blue light treatment. d) Photograph of the degree of cell division under a microscope (magnification: 10⁴). e) Degree of cell proliferation versus time (absorbance) (** ≤ 0.05 vs control). f) Degree of cell division versus time (number of cells). g) Degree of bilirubin level decrease versus time (absorbance) (*** ≤ 0.005 vs control).

The actual experiment was conducted in a 96‐well plate, and we applied a minimum controllable distance of 1 mm to simulate the skin adhesion environment. We also aimed to create an environment similar to real‐world application conditions while preventing contamination from direct contact with the hydrocolloid substrate. (This experimental method was also applied to the bilirubin reduction validation experiment.) In the experiment to confirm that the red light component of the multiwavelength light induces cell proliferation, the proliferation of light‐irradiated cells was compared with that of nonirradiated control cells. Data collected from four separate plates were averaged and analyzed for distribution. After HaCaT cells were cultured, the light of a single pixel emitted by the hydrocolloid‐based QD‐OLED was irradiated for 40 min at an intensity of 2.5 mW cm⁻². The energy capacity of the multiwavelength band irradiated toward the cells totaled 6 J cm⁻². At this time, the total energy of the red light contained in the multiwavelength corresponds to 3.77 J cm⁻² (red light 1.57 mW cm⁻² × 40 min = 3.77 J cm⁻²). To analyze the proliferation of HaCaT cells after light irradiation, measurements were taken every 24 h from 0 to 72 h, and cell proliferation was evaluated by measuring absorbance using cell counting kit‐8 (CCK‐8) solution.

Microscopic images (magnification: 10⁴) of cells with and without light exposure at each time point are presented in Figure 7d. Notably, under the condition of multiwavelength irradiation, clear differences in cell proliferation were observed, and it was visually evident that HaCaT cells exposed to light proliferated significantly more than the unexposed cells. This demonstrates that cells exposed to multiwavelength light for the same duration showed a higher proliferation rate compared to the control group without light exposure. To quantitatively analyze the degree of cell proliferation, changes in absorbance over time after light exposure were analyzed. The graph presented in Figure 7e compares the changes in average absorbance over time between the control group (nonirradiated cells) and the experimental group (cells exposed to QD‐OLED light). At the initial time point (0 h), both groups showed similar absorbance values (control: 0.3553, experimental: 0.3312), but a significant difference between the two groups appeared as time progressed. After 48 h, the absorbance increased to 0.6336 in the control group and 0.7728 in the experimental group, and after 72 h, it further increased to 0.7309 and 1.0128, respectively, confirming the metabolic activity‐enhancing effect of QD‐OLED light irradiation (Figure 7e). Additionally, to more precisely evaluate cell proliferation, the number of cells was directly counted at the same time points. According to the experimental results (Figure 7f), the initial average cell count was 2951 in the control group and 2650 in the experimental group, showing no significant difference; however, after 48 h, the control group had 6430 cells while the experimental group had 8169 cells, showing a considerable difference. Notably, after 72 h, the experimental group reached 11 170 cells, whereas the control group had only 7645 cells (p < 0.01). These results demonstrate that the multiwavelength light emitted from the QD‐OLED effectively promotes the survival and proliferation of HaCaT cells, supporting the photomedical bioactivation capability of the platform.

The mechanism of phototherapy by the composite wavelength light source emitted from the platform for the treatment of jaundice and atopic dermatitis is illustrated in a schematic diagram, with the mechanism of blue light therapy for jaundice shown in Figure 7c. Neonates possess erythrocytes with a notably shorter lifespan than those of adults. This physiological difference drives an increased rate of hemoglobin degradation, consequently resulting in elevated bilirubin production. Due to abnormalities in the metabolic process (production, metabolism, excretion), bilirubin can accumulate in the body. When bilirubin cannot be effectively excreted due to pathological causes, it accumulates in the skin and sclera, leading to jaundice.^{[ 101} , ^{102 ]} Due to the nature of bilirubin accumulated on the outer surface of the skin, treatment using blue light with low penetrability is possible. In this study, a blue light therapy method using an OLED patch was applied to treat jaundice, facilitating the conversion of bilirubin into photoisomers that can be excreted from the body.

This study evaluated the potential of the hydrocolloid‐based multiwavelength QD‐OLED phototherapy platform for jaundice treatment through in vitro experiments. Before conducting the experiment, to verify the stability of the fabricated device, the device was immersed in phosphate‐buffered saline (PBS), and the performance changes were measured (Figure S12, Supporting Information). The PBS immersion test results confirmed that the device performance remained nearly constant, within ≈10% of the initial value, after immersion in PBS. This suggests that dissolution by solvents such as PBS does not occur in actual use environments. In the experiment, a bilirubin solution at a concentration of 0.05 mg dL⁻¹ was prepared in a 96‐well plate, and multiwavelength light with a total light intensity of 2.5 mW cm⁻² emitted from the QD‐OLED was irradiated for 30 min at a time, for a total of 90 min. The multiwavelength light contained a blue light component with a peak wavelength of 470 nm, where a total amount of light energy delivered to the bilirubin was 5.02 J cm⁻² (0.93 mW cm⁻² × 90 min = 5.022 J cm⁻²). The experiment was conducted with the aim of verifying the effect of blue light on bilirubin degradation. To analyze the photodegradation effect of blue light, the absorbance of the bilirubin solution was measured at 30 min intervals, and the degree of bilirubin degradation was evaluated based on the concentration reduction rate over time. To simulate conditions close to actual skin attachment, the distance between the light source and the solution was set to 1 mm, reflecting the irradiation conditions in actual applications. To clearly compare the light irradiation effect of the hydrocolloid‐based multiwavelength QD‐OLED, both light‐exposed experimental groups and nonexposed control groups were established, and the experiments were conducted under identical conditions. All conditions were measured in four independent wells, and the collected data were averaged and statistically analyzed.

To highlight the excellence of this study, a brief comparison and analysis were conducted between single surface‐emitting devices, the results of which are presented in Table S3 (Supporting Information). This study stands out for its ability to achieve multiple applications while simultaneously ensuring high stretchability. As a result, it was confirmed that the blue light included in the multiwavelength light emitted from the QD‐OLED platform had a significant effect on reducing bilirubin concentration. After 90 min of light exposure, the experimental group showed an average reduction of ≈26% in bilirubin concentration compared to the control group (Figure 7g), demonstrating the therapeutic effect of the multiwavelength light emitted from the QD‐OLED light source, including blue light. Notably, despite the presence of multiple wavelengths in the hydrocolloid‐based QD‐OLED used in this experiment, the therapeutic effect of blue light was maintained, suggesting that the efficacy of specific wavelengths can be preserved even in a multiwavelength environment. These results indicate that the hydrocolloid‐based flexible and stretchable QD‐OLED light source platform has advantages in skin adhesion and treatment site selection compared to existing rigid light sources and has high potential as a next‐generation wearable phototherapy device for treating neonatal jaundice.

3. Conclusion

In summary, we confirmed the potential of using a hydrocolloid‐based stretchable QD‐OLED patch platform for wearable devices to treat neonatal jaundice and atopic dermatitis. To optimize the multiwavelength device for jaundice and atopic treatment, we fabricated a QD‐OLED with two peak wavelengths of 470 and 630 nm through a bottom‐emission structure, microcavity optical design by electrode replacement, and the application of QD films. Additionally, to enable real‐time attachment and use, we applied a kirigami structure, achieving a stretchability of 190%, and successfully developed a highly stretchable platform that integrates optical and chemical therapeutic effects through its combination with hydrocolloid, a widely used medical material. This platform demonstrated a maximum radiative output intensity of 8.31 mW cm⁻² and achieved a radiative intensity of 2.5 mW cm⁻² even at a low voltage of 5.7 V, proving its capability for focused treatment of jaundice and atopic dermatitis. The wearable OLED platform also exhibited thermal stability, ensuring that it does not cause low‐temperature burns even when operated at maximum radiative output voltage. Furthermore, under various radiative output condition of (3, 4.5, and 6 mW cm⁻²), the platform maintained stable operational reliability for more than 88 to 46 h under all conditions until reaching LT₅₀. Its low cantilever length and bending rigidity also demonstrated that it maintains high flexibility even when attached to the skin, allowing it to adapt to various physical deformations. Through multiple verifications, the platform's flexibility and reliability were confirmed, and the potential for phototherapy was also validated. Using the multiwavelength emissions of 470 and 630 nm from the platform, in vitro experiments demonstrated a 26% reduction in bilirubin levels and a 46% increase in HaCaT cell proliferation. These results confirm the feasibility of using this platform for the actual treatment of jaundice and atopic dermatitis.

Based on these findings, this study presents a new direction in the field of photomedical therapy for real‐time diagnosis and treatment of neonatal diseases, as well as in the integration of medical materials with optoelectronic devices. The proposed wearable healthcare platform is expected to be widely utilized in the medical industry.

4. Experimental Section

Information of Ultrathin PET

A 20 µm thick adhesive PET film (product name: Ultra‐thin Tape, JY‐Material Technology Co., Ltd., South Korea), a stretchable and adhesive substrate, was used. An adhesive PET film consisted of a PET layer, an acrylic adhesive layer, and a release liner layer.

Stretchable Substrate Fabrication

To fabricate a flexible and transferable OLED, a transferable ultrathin PET (20 µm) (JY‐Material Technology Co., Ltd., South Korea) substrate was fabricated to be stretchable by patterning the kirigami structure through a CO₂ laser (Trotec 360 Speedy, Trotec Co. LTD., Austria). The laser irradiation conditions were carried out at 11 power, 2.5 speed, and 1000 ppi. The ultrathin PET substrate was composed of three parts: release liner, acrylic adhesive, and PET, and the PET and acrylic adhesive parts were transferable parts, with a total thickness of 20 µm.

Stretchable OLED and OPD Fabrication

OLED was fabricated on stretchable adhesive PET substrates using Thermal Deposition Equipment (OLED Evaportator, SELCOS Co. LTD., South Korea) in a 3 × 10⁻⁷ Torr vacuum environment. Starting with Ag (25 nm) serving as a cathode, Cs₂CO₃ (1 nm) serving as an electron injection layer, bis(10‐hydroxybenzo[h]quinolinato) serving as an electron transport layer, bis(10‐hydroxybenzo[h]quinolinato)‐beryllium (10 nm) serving as an EML, MADN:p‐vis(p‐N,N‐di‐phenyl‐amino style) (25 nm, 3 wt%), N′‐naphthyl‐N,N′‐diphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine (40 nm), molybdenum trioxide (MoO₃, 5 nm) serving as a HIL, and finally aluminum (Anode) serving as an anode.

OPD was fabricated on adhesive PET substrates (20 µm) (JY‐Material Technology Co., Ltd., South Korea) using Thermal Deposition Equipment (OLED Evaportator, SELCOS Co. LTD., South Korea) in a 3 × 10⁻⁷ Torr vacuum environment. Starting with Mg:Ag (25 nm, 0.1 Å s⁻¹ (Mg), 0.9 Å s⁻¹ (Ag)) as the anode (Metal Chamber), MoO₃ (10 nm, 0.5 Å s⁻¹) as the hole transport layer (Organic Chamber), P‐type‐doped layer pentacene (75 nm, 0.5 Å s⁻¹) (Organic Chamber), and N‐type‐doped layer fullerene (C₆₀, 50 nm, 0.5 Å s⁻¹) (Organic Chamber) were sequentially deposited to form a depletion layer. Afterward, a hole blocking layer basocuproin (10 nm, 0.5 Å s⁻¹) (Organic Chamber) and Al (100 nm, 1.0 Å s⁻¹) as the cathode (Metal Chamber) were sequentially deposited in the OPD structure.

Device Characterization

The electrical characteristics of the OLED fabricated in this study were measured using a source meter (Keithley 2601, Keithley Inc., USA), while the optical characteristics were analyzed using a spectroradiometer (CS‐2000, Konica Minolta Inc., Japan). Also, QD films were measured with PL measurement equipment (HORIBA Jobin Yvon NanoLog, HORIBA Instruments Inc., Japan).

Cell Culture (In Vitro)

The HaCaT (ATCC PCS‐200‐011) used in this experiment were purchased from the ATCC (Manassas, VA, USA). The medium used for cell culture was Roswell Park Memorial Institute Medium (RPMI 1640; Gibco, Grand Island, NY, USA), supplemented with 10% fetal bovine serum (Gibco, USA) and 1% penicillin–streptomycin (Sigma Chemical Co., St. Louis, MO, USA). Cells were cryopreserved in liquid nitrogen until the experiment and were thawed and cultured one week before use. The cells were seeded in a 25 cm² flask with 4 mL of RPMI 1640 and incubated in a CO₂ cell culture chamber (MMM Group, Planegg, Germany) with 37 °C, 5% CO₂. When the cells occupied 80–90% of the bottom area of the 25 cm² flask, they were detached by adding 4 mL of trypsin–ethylenediaminetetraacetic acid (Gibco, USA) solution for 10 min. The detached cells were centrifuged at 1500 revolutions per minute (rpm) for 3 min, the supernatant was removed, and the cells were resuspended in 4 mL of Dulbecco's phosphate‐buffered saline (Gibco, USA), followed by another centrifugation at 1500 rpm for 3 min. After removing the supernatant, the cells were resuspended in RPMI 1640. The suspension was mixed with 10 µL in a 1:1 ratio with Trypan blue solution and the number of cells was measured using a microscope through a hemocytometer. Based on the measured number of cells, the number of cells was diluted in RPMI 1640 as many as required in the experiment and used.

CCK‐8 Cell Viability and Proliferation Assay

Cell proliferation was evaluated using the CCK‐8 assay kit (ab238538; Abcam Inc.). CCK‐8had low cytotoxicity and provided a convenient and sensitive colorimetric assay using [2‐(2‐methoxy‐4‐nitrophenyl)‐3‐(4‐nitrophenyl)‐5‐(2,4‐disulfophenyl)‐2H‐tetrazolium, monosodium salt]. For the cell proliferation experiment, HaCaT cells were seeded into a 96‐well plate (Daehan Sci. Co., South Korea) at 2 × 10³ cells per well and incubated in a CO₂ cell culture chamber for 24 h to allow stable attachment to the bottom of the 96‐well plate. Subsequently, 470 nm blue OLED, 630 nm red Minolta OLED (Konica Minolta Inc., Japan), and hydrocolloid‐based QD‐OLEDs were irradiated at 2.5 mW cm⁻² for 10 min to seeded cells and incubated in CO₂ cell culture chamber with 37 °C, 5% CO₂. After incubation, the media used were removed and replaced with RPMI 1640 100 µL. Then, 10 µL of CCK‐8 solution was added to each well and reacted in CO₂ cell culture chamber for 1 h with 37 °C, 5% CO₂. The number of cells was determined by measuring absorbance at 450 nm using a spectrophotometer (Multiskan FC; Thermo Fisher Scientific, Waltham, MA, USA) and substituting the measurements into HaCaT standard growth curves. In this way, the difference in cell proliferation according to the light source was confirmed by measuring at 24 h intervals from 0 to 72 h.

Cell Counting

The standard growth curve of HaCaT cells was used to calculate the number of cells. The absorbance values corresponding to 0, 2000, 4000, 6000, 8000, and 10 000 cells were measured in HaCaT samples without light irradiation (n = 4) to derive the absorbance expression for cell number. The standard growth curve of HaCaT was determined as y = 3 × 10⁻⁵ x − 0.0052. Using this equation, the number of cells at each time point was back‐calculated by substituting the measured absorbance values.

PPG Analysis

For PPG measurements, signals were collected from the in‐house fabricated QD‐OLED and OPD. These components, manufactured using Thermal Deposition Equipment (OLED Evaporator, SELCOS Co. LTD., South Korea), were connected to a custom‐designed PCB (Board Lab Co., South Korea) optimized for the OPD output. Subsequently, a low‐pass filter was applied using MATLAB (MathWorks, USA) to remove noise, and the final measured results were presented.

Temperature and Mechanical Analysis

To evaluate the temperature stability and flexibility of the fabricated devices (Hydrocolloid QD‐OLEDs), UTS measurement, cantilever test, and temperature measurement using an infrared camera were performed. For UTS measurement, substrates under various conditions were measured up to the yield point using UTStesters (34SC‐1, Instron Co., USA). When performing the cantilever test (Cantilever, Heungshin Tester Co., South Korea), each substrate was precisely cut into 1 cm × 4 cm, and Young's modulus was measured using the cantilever length. In addition, temperature measurements were performed for 15 min using a thermal imager (HT‐19, HTI Co., China).

Statistical Analysis

The in vitro experimental results conducted in this study were expressed as the mean ± standard deviation (SD). Statistical analysis was performed using four samples for each condition, and significance levels were evaluated using a two‐tailed t‐test. In all statistical analyses, a p‐value of less than 0.5 was denoted as *, less than 0.05 as **, and less than 0.005 as ***.

Encapsulation Process

An adhesive PET film was used to protect the OLED device from moisture. The transferable PET substrate consisted of a PET layer, an acrylic adhesive layer, and a release liner layer, with a total thickness of 20 µm (product name: Ultra‐thin Tape, JY‐Material Technology Co., Ltd., South Korea).

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

I.H.K. and Y.W.K. contributed equally to this work. I.H.K. performed conceptualization (Lead), formal analysis (Lead), investigation (Lead), visualization (Lead), writing – original draft (Lead). Y.W.K. performed data curation (Equal), formal analysis (Equal), investigation (Equal), methodology (Lead), writing – original draft (Equal). J.‐Y.J. performed formal analysis (Equal), methodology (Equal). Y.J.S. performed data curation (Supporting), methodology (Equal). C.L. performed methodology (Supporting). J.G.C. performed resources (Equal). O.K.K. performed resources (Equal), validation (Equal). K.H.K. performed resources (Equal). S.J.K. performed funding acquisition (Equal), resources (Equal), supervision (Supporting). E.‐S.C. performed funding acquisition (Lead), project administration (Equal), supervision (Equal), validation (Equal). Y.J. performed conceptualization (Lead), data curation (Lead), supervision (Lead), writing – original draft (Supporting), writing – review and editing (Lead).

Supporting information

Supporting Information

Data Availability Statement

The data shown in this article and/or the Supporting Information are available form all authors upon a request.