OLEDs are considered to be particularly suitable for wearable devices or human touch devices. They can be processed on various flexible substrates at low temperature, such as plastics, which is very useful for realizing light sources as light as goose feather, flexible, stretchable and conformal objects. In fact, most of the display devices used by wearable devices such as smart watches use plastic active matrix OLED because of its lightweight and thin appearance. OLEDs applications in wearable devices are not limited to the display of smart watches. In fact, OLEDs can also play an important role in the field of mobile medicine, and its form of wearable devices is particularly popular. For example, health monitoring sensors measure important human signals, such as heartbeat and blood oxygen levels, as well as light patches for advanced wound care or skin care, as shown in Figure 1. Here we will review the highlights and key issues of these OLEDs in wearable healthcare applications.
Figure 1 potential application of OLEDs in wearable medicine
OLEDs for wearable pulse oximetry sensor
Cardiovascular circulation is the most important biological activity to maintain life. Therefore, it is very important to monitor it in an appropriate way. In many cases, the measurement of these activities is limited to care in hospitals, but it is rapidly expanding to the field of mobile or personal health care. In particular, the progress of wearable electronic devices makes it possible to measure daily activities from health tracking to biological signal monitoring of infants, patients with chronic diseases or patients discharged after surgery. In cardiovascular monitoring methods, light plethysmography (PPG) signal and blood oxygen saturation (SpO2) level are measured by noninvasive light using light-emitting devices and photodetectors. Since organic technology has good compatibility with wearable or human body attachable form factors, it is natural to think that organic light emitting diodes (OLEDs) and organic photodiodes (OPDS) can be used as light sources and detectors of wearable PPG and / or SpO2 sensors. At present, some research groups have proved that OLED technology is a feasible and promising wearable health monitoring technology.
PPG and SpO2 signal detection principle
The signal in the PPG sensor is photon absorption adjusted according to the change of blood vessel volume caused by cardiovascular circulation (Fig. 2a). Generally speaking, PPG sensors can be divided into transmission type (T type) and reflection type (R type) according to the configuration between light source and photodetector. In type T, the light source and photodetector are located on the object of interest (such as fingers, earlobes, etc.). On the other hand, in the R-type PPG sensor, the light source and photodetector are placed side by side. R-type PPG sensors are often used in smart watches or wristbands because most body parts (such as wrists) are too thick to use T-type PPG sensors (see Figure 2b).
SpO2 refers to the ratio of oxygenated hemoglobin (HBO2) to the total amount of non oxygenated hemoglobin (HB) in the blood. Through the calibration process, it can be estimated by comparing the absorption difference of HBO2 and Hb at two different wavelengths. Therefore, SpO2 sensor is composed of two light sources with different wavelengths and a shared photodetector, and the two light sources are turned on alternately. Because it is essentially equivalent to a PPG sensor, SpO2 sensor can also provide PPG signal in addition to two different light sources. Therefore, SpO2 sensor is also called “pulse oximeter” or “pulse oximeter sensor”.
Organic pulse oxygen sensor
Based on the inherent advantages of organic technology realized by flexible films necessary for wearable devices, some research groups are committed to developing organic PPG or pulsed blood oxygen measurement sensors to replace discrete components based on Si and III-V composite semiconductors. As shown in Fig. 3a, a PPG sensor based on conjugated polymer OLED and OPD integrated together. Further studies demonstrated that the combination of solution treated green and red organic light emitting diodes with polymers, OPDS, can measure SpO2 values and PPG signals (Fig. 3b). Unlike commercial pulse oximeters using red and near infrared (NIR) LEDs, green and red light sources are used in their research because NIR OLEDs are still in its infancy compared with light sources in the visible spectrum. After that, a system was proposed, including an organic pulse oximetry (OPO) sensor head, a custom designed CMOS IC chip and a button battery (Fig. 3C). The patch is based on 5.5cm × 2.5cm polyethylene terephthalate film. It has small molecular OLEDs with green and red phosphorescent emitters to operate effectively. Based on the small molecule blend volume heterojunction, the OPD is placed in the middle position adjacent to OLEDs for T-type operation. EQE up to 20% and response rate with Si photodiodes (PDS) and relatively low arterial oxygen saturation drive current data are measurable 240 µ a green OLED and 300 µ a red OLED. After that, the application of ultra-thin packaging technology further improved the potential of OPO sensor in wearable devices, resulting in a light weight, highly conformal and almost imperceptible pulse oximeter. The polyimide planarized p-xylene film was used as the substrate, and the multilayer barrier layer with alternating two positions of p-xylene and Zion was used for passivation (Fig. 3D). Total thickness of prepared OPO sensor 3 μ m。 In addition, the recent progress in the manufacturing of print based OPO sensors illustrates the potential of OPO for low-cost implementation of OPO technology. In this work, two different functional materials are printed simultaneously in one step. They used the hydrophobicity of self-assembled monolayers (SAMs) to distinguish the surface to be patterned from the surface not to be patterned. O2 plasma is used to spatially selectively define SAMs. As shown in Fig. 3E, through the proposed blade coating process, the electron barrier layer PEDOT: PSS and each emission layer are formed at the same time. Therefore, green and red polymer light emitting diodes (PLEDs) are realized at the same time. They placed a silicon-based PD beside the green and red folds and obtained an R-type pulse oximeter, which showed that PPG signal and SpO2 value could be extracted from the back of the hand (see Figure 3e).
The early research of OPO sensor mainly focused on the display of formal factors or process advantages of organic technology; However, these sensors are used for wearable devices, where the battery capacity is quite small. This may limit the actual deployment of OPO technology, because if continuous monitoring can be realized all day, the benefits of wearable health monitoring devices will be maximized. Therefore, we re examined the overall design of R-type pulse oximeter. Instead of their typical side-by-side layout, we make maximum use of photons from light sources. Considering absorption and scattering, the propagation of wavelength dependent light in human skin is optically analyzed (FIG. 4A). The degree of freedom formed in the organic mode is then used to determine the pulse oximetry sensor with optimized sensor layout, so that the number of wasted photons can be minimized while still meeting the size constraints and the requirements of signal-to-noise ratio (SNR). As shown in Figure 4b, the shape of OPD is like a digital character “8”, surrounded by red and green circular OLEDs, and the driving power of R-type OPO sensor is as low as tens of microwatts. This is an order of magnitude reduction compared with the R-type pulse oximeter on the market (Fig. 4C).
Another reason for the improved efficiency of this sensor is due to the refractive index matching between the substrate of the sensor device and human skin. Generally speaking, due to the action of TIR, only 30% to 35% of the total photons generated in the substrate are emitted at the bottom. When the index between substrate and skin matches, there is no air gap between them, and a large number of substrate limited photons are coupled with skin. This is not only conducive to low-power operation, but also to prevent light from being directly coupled from OLED to OPD without passing through human skin. By comparing the two structures, one is OLED wound on OPD, the other is OPD wound on OLED. Experiments show that the power consumption of the latter is 80 times lower than that of the former. This shows that the benefits of this technology for wearable OPOs may not be limited to shape factors. But it can also significantly reduce energy consumption, both of which are the key to the successful promotion of wearable technology.
Efforts are being made to diversify the functions of OPO sensors. For example, a group of R-type OPO sensors are realized through printing technology, in which each unit is composed of a red OLED, a NIR OLED and two OPDS. The sensor array fabricated on pen substrate can realize two-dimensional oxygenation mapping. Therefore, oxygen saturation mapping sensor can be used not only for real-time monitoring of chronic patients, but also for postoperative recovery management. Similarly, a patched OPO sensor may be used to monitor sleep apnea, which is the cause of about 38000 deaths in the United States each year. In order to reduce the possibility of false positives and improve sleep quality while continuously monitoring patient status, the patch can be configured to include two or more different types of sensors, such as respiratory detector. In addition, if the ECG sensor is combined to estimate blood pressure through the pulse conversion time difference between ECG and ECG, it may make telemedicine more accurate and PPG signal possible.
Wearable OLED patch for advanced wound care
Photobiological regulation (PBM) refers to the biological changes caused by the interaction between molecules in cells or tissues and light. The therapeutic effect of PBM has been found in ancient times, but recently it has become popular with the development of various light sources such as laser and led. For example, the photons absorbed by the chromophore called cytochrome c oxidase will lead to luminescent chemical reactions, promote the synthesis of adenosine triphosphate and provide energy for various processes of organisms. This process can help cells grow more effectively, so that the tissue damage of the wound can heal faster. So far, these phototherapies have installed light sources in hospitals, which often limits the frequency and duration of PBM treatment for each patient. In this sense, OLEDs is very useful in the form of wearable and patch, because it can be used for personal health care. Even in the hospital environment, patients with deep trauma do not need to move to the position where the PBM light source is located; Instead, the patient can apply a PBM light patch to the wound on his or her bed, just like an adhesive bandage. The characteristics of OLED as an area light source are considered to be very beneficial. It produces uniform light output without generating too much heat, which is very important to prevent heat damage to cells and tissues.
Recognizing this potential, people worked hard to develop OLED based phototherapy technology and proposed an OLED based PBM device (Fig. 5a-c). As a surface light source, OLED produces uniform light output at an irradiance of 10MW cm − 2. Effective wavelength tuning can be achieved through microcavity effect. Figure 5 B, C shows an obvious wound healing effect.
For the development of this patch OLED, forms such as fabrics will be very useful because they can be stretched in any direction, and because of the inherent free space in the fabric, they can easily conform to objects with various shapes. Based on this point of view, someone has proved an efficient fabric based pholeds (Fig. 6). Through finite element simulation, it is verified that the fabric substrate has good flexibility (Fig. 6b). Compared with plastic substrate, fabric has complex stress distribution due to its single fiber. Because the fabric is wound by independent fibers, the woven fabric contains some spaces, which constitute multiple neutral axes and a zero stress region. That is, the woven fabric substrate distributes mechanical stress according to the weaving pattern, which makes the fabric substrate more mechanically robust than the plastic film, as shown in Fig. 6B. The obtained OLED shows high performance in terms of maximum brightness and current efficiency, exceeding 35000 CD m − 2 and 70 CD a − 1.
In the process of developing practical wearable devices, one of the biggest challenges is to ensure a washable packaging barrier. It is generally believed that the degradation of Al2O3 during washing is the main reason for the degradation of typical multilayer barrier performance. In order to avoid this deterioration, it is proposed to use SiO2 – polymer composite as the cover layer of Al2O3 in the packaging layer. They confirmed that the proton deproton reaction between SiO2 polymer complexes can inhibit the degradation of packaging barrier in aqueous environment (Fig. 7a). With this method, the water vapor transmittance (WVTR) can be maintained for 7 days (Fig. 7b). Using the proposed packaging barrier, devices fabricated on real textile substrates can show stable operation even when exposed to water (Fig. 7C).
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