Introduction

While applications of organic light emitting diodes (OLEDs) in display and lighting technologies have reached the commercial market, a current and promising trend in application of OLEDs is in wearable technology1,2. This emerging trend of using OLEDs in wearable technology is due to the unique property of OLEDs in being highly flexible and stretchable3,4,5, and therefore, having the potential to conform to any shape and size of the human body. OLEDs are also very thin and light weight and could be fabricated with simple solution processing or ink-jet printing methods6,7,8. Further, OLED emission is easily tuneable and therefore, is particularly effective as a light source. Another major advantage of OLEDs is homogenous illumination over a large area9, making it a suitable light source for optical sensing with small implants which tend to move with time leading to misalignment of sensing elements. The printability of OLEDs also opens up possibilities to design integrated complex and compact devices.

Applications of OLEDs in wearable technology are mainly as a light source where emission from an OLED needs to penetrate past the skin barrier1,2. In particular, accurate measurements have been reported for cutaneous sensors with OLEDs, such as pulse oximetry. Such applications require OLED light to penetrate the skin, which is a highly inhomogeneous medium with a very complex multi-layered structure. Optical properties of human skin such as reflection, absorption and scattering have been reported earlier10,11,12,13. It is well known that, even within the visible light wavelengths, longer wavelengths penetrate deeper than shorter wavelengths. Additionally, highly coherent and collimated beams of light, such as light provided by a laser, have deeper penetration in tissues as compared to incoherent sources14. On the other hand, light from OLEDs are highly incoherent and non-monochromatic, and OLEDs have a wide electroluminescence (EL) spectrum. Recently, we reported dynamic colour tuning of OLED emission by using an anisotropic thin film filter15. However, to realise the full potential of OLEDs in biometric and therapeutic applications, understanding and quantification of transmission of OLED emission through skin is essential.

In this report, we describe the changes in intensity and EL spectrum of emission from yellow and red OLEDs when it penetrates human skin. We find that the intensity of emission reduces and the EL spectrum widens significantly, as it passes through skin. Reduction in intensity is obvious since skin absorbs, reflects and scatters light. However, the significant widening in EL spectrum is seen only for OLED emissions. We also compared our results with conventional LEDs. These results provide guidelines for the application of OLEDs in wearable technologies.

Results

Simple solution processed OLEDs with yellow and red emissions were used for this study. The structure of OLEDs along with materials used and current-voltage-luminance characteristics are provided in Fig. S1. The EL spectra of the OLEDs are shown in Fig. 1a. The EL of both yellow and red OLEDs has a full width half maximum (FWHM) of around 90 nm, which is typical of fluorescent OLEDs15,16. EL spectra of yellow and red commercial LEDs used for comparative study are shown in Fig. 1b. LEDs with two types of encasing, a rounded top and a flat top, were used (inset of Fig. 1b). A flat top LED will have an emission profile closer to that of an OLED, which has a Lambertian emission profile15. The ELs of yellow flat (YF) and yellow round (YR) LEDs are almost exactly the same, whereas EL of red round (RR) LED is red-shifted as compared to red flat (RF) LED. For both red and yellow LEDs, the FWHM is ~10–14 nm, which is considerably narrower in comparison to FWHM of OLEDs. To investigate changes in EL spectrum and intensity when OLED light passes through skin, the human skin sample is placed directly on top of the OLEDs, which has a flat surface (Fig. 1c). For LEDs, a holder was designed to ensure that only the top of the LEDs are in contact with the skin, while ensuring the skin stays flat (Fig. 1d). Details of the experimental set up can be found in Fig. S2.

Figure 1
figure 1

(a) Electroluminescence spectra of yellow and red OLEDs. (b) Electroluminescence spectra of yellow and red LEDs with rounded/dome and flat tops. Inset: Schematic of dome and flat tops of LED. (c) Schematic of an OLED pixel on substrate and OLED emission passing through skin. (d) Schematic of LEDs in contact with skin in the experimental set-up.

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Changes in intensity of OLED emission

Figure 2a,b show typical emission intensity and intensity of emission through skin of yellow and red OLEDs, respectively, as a function of input current of the OLED. The intensity of emission is presented as current of the photodiode that was used in the measurement. The corresponding luminance and voltage for the input currents used can be seen from the current-voltage-luminance characteristics shown in Fig. S1. As seen in Fig. 2a,b, the intensity of OLED emission increases almost linearly with input current, a typical characteristic of OLEDs17. The intensity of OLED emission through skin also varies linearly with applied current, indicating that the percentage of OLED light transmission through skin is independent of light intensity. As such, percentage of OLED light transmitted through skin is constant for all intensities of emission tested (Fig. 1a), which is consistent with an earlier report showing that light penetration depth is independent of light intensity18. Like OLEDs, similar characteristics of linear increase in intensity of emission and emission through skin, and constant percentage of transmitted light were seen for LEDs, as shown in Fig. 2c,d for YF LED and RF LED, respectively. The characteristics for YR and RR LEDs are included in Fig. S3. The intensities of light emission tested for LEDs are kept in the same range as for OLEDs (Fig. 2a–d, Fig. S3).

Figure 2
figure 2

Intensity of emission, emission through skin and percentage of transmission through skin as a function of applied current for (a) yellow OLED, (b) red OLED, (c) YF LED and (d), RF LED. (e) Percentage of light transmitted through skin for OLEDs and LEDs for skin samples from anterior wrist. (f) Ray traces of emission at the edges of OLED pixel with and without skin. (g) Skin on top of a masked OLED and ray traces of emission at the edges of a masked OLED pixel with skin. (h) Percentage of light transmitted through skin for masked OLEDs and LEDs for skin samples from inner elbow.

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Average percentage of light transmitted for skin samples taken from the anterior wrist are shown in Fig. 2e, with standard deviation as error. The average was calculated over various intensities of OLED emission and across all points tested on the skin samples. For yellow OLEDs, ~25% of emitted light is transmitted through skin while for red OLEDs ~46% of emission gets transmitted (exact values are provided in Table S1). The higher transmission of light for red OLED is expected since longer wavelengths are known to have better penetration through human tissues19. The same trend is also seen for transmission of emission from LEDs. However, the percentage of transmission for OLEDs is almost double compared to LEDs (Fig. 2e, Table S1) for both yellow and red, even though LEDs have a higher coherence length and are thus expected to have better penetration through skin15. Further development of spectral narrowing techniques of OLEDs will lead to an increase in the applicability of OLEDs in optoelectronic wearable devices and sensors.

In summary, we report our findings on transmission of OLED light through human skin for yellow and red OLEDs. The intensity attenuation of OLED light passing through skin is comparable with light from commercial LEDs passing through skin. If the OLEDs are not masked, edge emissions coupling to skin leads to higher intensity of OLED light going through skin. ELs of OLEDs widen significantly upon passing through skin, making considerable changes in chromaticity co-ordinates. Our findings are very topical and relevant with the current trend in OLED research for applications in wearable electronics as a light source.

Methods

OLED fabrication and testing

Bottom emitting yellow and red OLEDs were fabricated using standard processes reported earlier15,16. Current-voltage-brightness measurements were recorded using a Keysight B2901A sourcemeter and a photodiode calibrated with a Konica Minolta CS 200 luminance meter. EL spectra of the OLEDs were measured using an Ocean Optics spectrometer (USB4000).

Preparation of skin samples

Sections of skin from the anterior wrist, inner elbow and shoulder regions taken from cadaveric donors were resected at the hypodermal layer and preserved at ~−20 °C until experimentation. The data presented here is for skin samples taken from one donor body to avoid major variations in skin pigmentation of different individuals. During experimentation, skin was thawed under refrigeration at ~4 °C, cut into ~1.5 cm × 1.5 cm segments and phosphate-buffered saline (PBS pH 7.4) was applied to prevent drying of the tissue. All skin samples were prepared and stored in the same condition during experiments and storage to maintain the same condition for hydration of skin.

Cadaveric skin samples were resected from donors at the Medical Engineering and Research Facility (MERF, QUT). Harvesting and use of cadaveric skin samples was supported by University Human Research Ethics Committee at Queensland University of Technology (QUT) (ethics approval number: 1600000449). The tissues were obtained through QUT’s body bequest program from donors who provided consent for use of their body for the advancement of science and medicine. All experiments were conducted in accordance with the guidelines provided by University Human Research Ethics Committee at QUT.

Skin thickness and pigmentation measurement

The thickness of the skin samples were measured using thin slices of skin placed under a microscope (Nikon SMZ745T). Skin pigmentation were measured using a skin tone matching device, Pantone x-rite RM200, and matched with skin tones from Pantone Skintone library.

Light transmission through skin samples

The skin samples were placed directly on the OLEDs and LEDs as illustrated in Fig. 1. Experimental set-up is shown in Fig. S2. The OLEDs and LEDs were driven by current using a Keysight B2962A sourcemeter. The light output of LEDs and OLEDs, with and without skin on top were recorded using a calibrated photodiode. Since the photocurrent of the photodiode increases linearly with the incident light, it allows to measure OLED intensities indirectly. ELs were collected using an Ocean Optics spectrometer USB 4000. The photodiode and optical fiber were always maintained at a fixed distance of 3 mm. This ensures that the skin is not in direct contact with either the photodiode or the optical fiber. For experiments with masked OLEDs, either a black tape (thickness = 300 μm) or stainless steel mask (thickness = 150 μm) was used to mask the OLEDs.

All measurements, OLED characterization and skin related measurements were done at ambient environment.