Plastic substrate characteristics, good dimensional stability is the biggest challenge
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Most of the plastic substrate materials are organic polymers, which are in line with the concept of flexible displays. When selecting a plastic substrate material, its mechanical, optical or thermal properties must meet the requirements of the display. For example, to meet higher processing or operating temperatures, the coefficient of thermal expansion must be small; the light transmittance must be greater than 90%; good surface properties It is good for the formation of surface film and has certain resistance to some common organic solvents. In addition, if it is applied to a liquid crystal display, it must have a low birefringence. Table 1 is a plastic substrate which is currently used to make a flexible display. In the process of the display, since some steps need to be completed in a relatively high temperature environment, the material or dimensional stability of the plastic substrate at high temperatures becomes an extremely important reference condition.
The glass transition temperature (Tg) means that the substance is heated at a specific temperature, and the volume increases at a certain rate. When the temperature reaches the glass transition temperature, not only the molecular velocity increases but also the volume expansion curve is not continuous. For amorphous polymers, the behavior below the Tg is similar to glass, and when the temperature rises above Tg, it transforms into a soft rubber-like property. For plastic substrates of flexible displays, the glass transition temperature can be considered as the highest temperature that can be tolerated by dimensional stability in the manufacturing process. Even in Table 1, the glass transition temperature of some polymer substrates is greater than 200 ° C, but the thermal expansion coefficient is much larger (>50 ppm / ° C) than the glass substrate. In this case, dimensional stability becomes The biggest challenge in the flexible display process of plastic substrates. Excessive dimensional changes make the mask alignment extremely difficult, which also limits the size of the transistor design, while easily creating internal stresses between the organic and inorganic material layer interfaces, resulting in layer-to-layer peeling during bending.
At present, the solution to this problem is to heat-treat the plastic substrate. Before the process starts, the plastic substrate undergoes several thermal cycles to reduce the dimensional shrinkage after cooling to several ppm, and then carry out the subsequent process. In addition to dimensional stability, the permeability of water vapor to a typical plastic substrate at room temperature is about 0.1 to 1 g/m2/day, which is much larger than 10-5 g/m2/day of the glass substrate. The operating life of the organic light-emitting device is very sensitive to the presence of moisture and oxygen. Therefore, the barrier property of the plastic substrate can seriously affect the life of the organic light-emitting device. It is currently the main practice to plate the barrier layer on the plastic substrate to achieve the function of blocking water and oxygen. US Vitex Systems has developed a multi-layer structure consisting of a combination of inorganic and organic layers, called Barix Coating, which achieves moisture and oxygen permeability of less than 10-5g/m2/day and 10-3 cc/m2/ Day atm, which is currently the best process for blocking moisture and oxygen permeability in the literature.
Thin film transistor process on plastic substrate
According to the driving method, the display can be divided into passive matrix type and active matrix type. With the development of the display to the large size and high resolution, the active drive display has become the mainstream trend of the flat display. The active flexible display can be classified into thin film transistors such as amorphous silicon (A-Si), polycrystalline silicon (Poly-Si), and organic (Organic) in terms of materials. In terms of process methods, the current technology can be divided into two types, one is Direct Technology, which is to directly fabricate thin film transistors on plastic substrates, and the other is Transfer Technology.
Direct technology needs to be carried out in a low temperature process
The direct technology is limited by the heat resistance of the plastic substrate, and the entire process must be carried out at a low temperature so as not to damage the substrate. Table 2 compares the process temperatures of different thin film transistors. At present, FlexICs in the United States have successfully fabricated low-temperature polysilicon thin film transistor arrays on plastic substrates with a process temperature of less than 115 °C. Samaung Electrics manufactures a-Si TFTs on PES substrates with component mobility (Mobility) of 0.4 cm2/V-sec and process temperatures below 150 °C.
In addition, the selection of organic semiconductor materials for the fabrication of organic thin film transistors has also attracted many research institutions to invest in related research and development. In terms of molecular structure, organic semiconductor materials can be classified into small molecules and polymers. Pentacene is the most commonly used small molecule organic semiconductor material. It can be directly evaporated on plastic substrates at 80-100 ° C. Components with a mobility of 0.3-2.2 cm 2 /V-sec have been successfully fabricated, but the process is successful. The need to use expensive vacuum equipment, and the size of the transistor array can not be made is a problem that needs to be overcome.
Unlike small molecule organic semiconductor materials, high molecular organic semiconductor materials are soluble in some organic solvents, so they can be processed in liquid form. At present, the main polymer organic semiconductor materials include Dihexyl-hexithiophene (DH6T), Dihexylanthra-dithiophene (DHADT), Poly(3-hexythiophene) (P3HT), and Poly-9 (9dioctylfluorene-co-bithiophene) (F8T2). Among them, P3HT has attracted more attention due to its stability in the atmospheric environment and high mobility. The solution process is relatively simple and cost-effective, and is more in line with the process concept of flexible displays.
At present, the direct development of organic thin film transistors on plastic substrates by Inkjet Printing is the main development direction. Figure 1 is a schematic diagram of the inkjet process, and companies such as Lucent and DuPont have related research. Xerox also published in April 2004. A 208 × 208 OTFT array was fabricated on a PES substrate using its self-developed organic semiconductor (Inorganic Semiconductor Ink) with an inkjet method. Figure 2 is the molecular structure of Organic Semiconductor. The breakthrough of this research is that its organic semiconductor can be processed in low temperature and atmospheric environment, and because the organic semiconductor molecule has Alky Group side chain, if this Ink is printed on the processed PI The molecular arrangement is relatively regular and directional, and has the characteristics of Self-assembling. Its component characteristic mobility is 0.2 cm2/V-sec and the on/off ratio is 108.
Transfer technology through glass substrate as transfer medium
The transfer technology is another method to avoid the variation of the size of the plastic substrate when manufacturing the thin film transistor. First, the thin film transistor is fabricated on the glass substrate and then transferred onto the plastic substrate. The entire process consists of the following steps:
â—† Make a Sacrificial Stopper Layer on the glass substrate.
â—†Making thin film transistors on this layer
â—†Glue the glass substrate with thin film transistor
Temporary plastic carrier
â—†Remove the glass substrate
Remove Sacrificial Stopper Layer
Attach another plastic substrate
Remove plastic carrier
The entire process is shown in Figure 3. Seiko-Epson and Sony both made a-Si TFTs on plastic substrates using the above method, but no commercial products have been available.
LCD, OLED and electrophoretic display are mainstream flexible display technologies
At present, three technologies, such as LCD, OLED and electrophoretic display, can be applied to flexible displays. At this stage, LCD-related research and machine equipment are more mature; the characteristics of the OLED display mechanism are very suitable for display applications; electrophoretic display has bistable and power-saving advantages, and is used for specific purposes ( Such as electronic paper, e-books, electronic labels, etc.) have more market.
OLED is the first challenge for flexible display of optimal medium water-blocking oxygen performance
There are many companies around the world investing in OLED display technology, in which Japan focuses on organic light-emitting displays (OLEDs) of Small Moleculer Materials, while Europe and the United States focus on organic light-emitting displays of Polymer Materials. PLED).
OLEDs are considered to be the best display medium for flexible displays because they have no viewing angle and gap problems on the display, and have good color performance and suitability for Solution-Processing. Despite this, OLEDs still have problems to overcome before they are applied to flexible displays. First of all, the lifetime of OLED is very sensitive to the presence of water vapor and oxygen. The biggest disadvantage of plastic substrate is the poor barrier property of water and oxygen. Therefore, how to handle it on plastic substrate has a good ability to block water and oxygen. The primary challenge of OLEDs for flexible displays.
At present, the OLED process is not yet mature, and many companies developing OLED products have problems of low quality. In addition, portable products are an important market for flexible displays. Power consumption has always been an important consideration in the selection of flexible display technologies. Compared with LCD technologies that require backlights or color filters, OLEDs consume relatively high power and OLEDs. It is a current-driven component that requires active drive in large-area or high-resolution displays. In view of this, before the maturity of the existing TFT flexible backplane technology, OLED still cannot truly enter the market of flexible displays.
The current development of OLED/PLED flexible displays is as follows: DaiNippon Printing develops PLEDs on plastic substrates using the Roll-to-Roll process; Dupont Display develops 1.5-inch 96×64 PMOLEDs; Seiko-Epson turns OLEDs in 2000 The LTPS TFT plastic backsheet produced by India has released the first AMOLED. Currently, the company has also invested in the development of PLED by inkjet method; Pioneer has released 2 inch 128×64 OLED; Universal Display Corp. (UDC) has invested in OLED. Research, Figure 4 is an organic light emitting display of a plastic substrate exhibited by UDC.
Liquid crystal display should pay attention to the influence of the substrate after bending
Liquid crystals generally modulate the intensity of light by several physical mechanisms: changing the phase difference of the light (Phase Retardation), the polarization of the rotating light (Polarization Rotation), the absorption (Absorption), the scattering (Scattering), and the Bragg reflection (Bragg). Reflection). The first two display modes require a polarizer, and the latter three are reversed. As far as the application of the flexible display is concerned, the gap between the substrates during bending is easily changed by deformation. Therefore, when the liquid crystal display mode is selected, display principles that are not affected by the gap, such as absorption, scattering, and Bragg reflection, can be used. . If a display mode in which the phase difference or the polarization state of the light is changed is selected, it is necessary to increase the structure of the support gap in the display component. In addition, since the liquid crystal is different from the OLED and cannot emit light by itself, if the reflective display mode is selected, the components of the backlight are not required for operation. In terms of driving, if you can provide a bistable (Bistable), it will greatly improve the power saving effect. Here are a few of the most promising LCD modes:
Cholesterol liquid crystal with bistable characteristics
Cholesteric liquid crystal is a deformed structure of "multilayer nematic liquid crystal" (Nematic). With the addition of optically active liquid crystal molecules (Chiral), the orientation of the molecular guide axis is spiraled in a certain direction perpendicular to the space. (P) Variation, when the incident wavelength conforms to the Bragg reflection, the left-handed or right-handed light in the incident light will be reflected.
The use of "Polymer Stabilized" or "Surface Stabilized" can achieve bistable PSCT or SSCT, that is, two stable states of Planar State and Focal Conic state can be achieved without an applied electric field (Figure 5 ). In Planar State, the periodic arrangement of cholesteric liquid crystals is like a regular lattice arrangement of crystals. The wavelength of light in the incident light that satisfies the Bragg diffraction condition will form constructive interference, and the incident light of this wavelength will be reflected back. Bright state. In the focal conic state, the incident light is scattered because the cholesteric liquid crystals will appear in an irregular arrangement. The image display can be maintained without voltage after driving, and the power consumption is very low. At the same time, this display mechanism is less affected by the spacing of the upper and lower plates and has the potential to evolve into a bistable bendable display. Figure 6 shows a flexible cholesteric liquid crystal display exhibited by Philips at SID 2002 with a total thickness of 250 μm.
Polymer distribution has solid material reliability
At present, the main method of the polymer distributed display mode is to mix a polymer monomer and a liquid crystal into an isotropic solution, and to polymerize the polymer monomer by heat or light, and the solubility between the monomer and the liquid crystal during the polymerization. The separation is separated to form a separation, and finally the liquid crystal is uniformly dispersed in the polymer substrate in the form of droplets, and its structure is as shown in FIG. Appropriate selection of the refractive index of Polymer and LC can achieve the bright and dark display effect in the milky white scattering state (no viewing angle problem) and the transparent state (plus back absorption plate) when voltage is applied.
Since the polymer dispersed liquid crystal film is a solid-state display component, it has the reliability of a solid material, and the damage does not affect its display function, and there is no packaging problem. In addition, the display does not require a polarizing plate and does not need to properly align the liquid crystal molecules, but the disadvantages of the display mode, such as excessive driving voltage, low contrast and slow reaction speed, still need to be solved. Eastman-Kodark Company published a polymer-dispersed liquid crystal film of microencapsulated cholesteric liquid crystal by printing and coating in the SID of 2004, which has the advantages of bistable and condensed liquid crystal film processing combined with cholesteric liquid crystal. Convenience, Figure 8 and Figure 9 are schematic views of the products and processes.
Master-slave LCD mode
The Guest-Host Mode is based on liquid crystal, with a small amount of dichroic dye added. The generalized rod-shaped dichroic dye (Dichroic Dyes) molecules absorb almost no polarization of the vertical molecular axis, but are parallel to the molecule. The polarized light of the shaft can absorb the specific color light. When the white light passes, only the complementary color light can pass. When there is no applied voltage, as shown in Fig. 10(a), after the incident white light passes through the liquid crystal and the dye layer, the polarization direction and the molecular axis The parallel partial color light is absorbed and passed through a complementary color light; when a bias voltage is applied, as shown in FIG. 9(b), both the liquid crystal molecules and the dye molecules are converted into a vertical panel, and the molecular axis is perpendicular to the polarization direction of the incident light, and is not absorbed. The outgoing light is still white.
The Dainippon Printing Company has proposed a microcapsule master-slave technology that can be printed on a plastic substrate via a thick film. The I Institute of Electronics and the Institute of Chemical Industry have also successfully used the capsule technology to match the master-slave liquid crystal display technology to produce a black and white flexible display, as shown in Figure 11.
Polymer wall LCD solves the dilemma of high-end product development
In the above liquid crystal mode applied to a flexible display, although the display mechanism is less affected by the gap change, the comparison is about 10-20, so it is only suitable for low-end product applications. In order to achieve better display quality, it is still necessary to select a display mode with a polarizing plate, but usually the display quality of this mode is greatly affected by the gap change. In order to overcome this problem, the concept of using a polymer wall as a gap for supporting a liquid crystal cell (LC Cell) has emerged. The production process is that the LC and the polymer monomer are mixed and dissolved into the Cell with the existing alignment function, and the assembled Cell is subjected to UV exposure by using a photomask, and the phase separation method is initiated by polymerization to form a Polymer- Rich's Polymer Wall and LC-Rich areas. At the SID venue in 2002, NHK published the use of the phase separation mechanism between FLC and Polymer to create a Flexible Display with Polymer Wall and Polymer Network. The overall architecture is shown in Figure 12. Since the Polymer Wall will affect the LC arrangement, the overall comparison is only 100:1, its display effect is shown in Figure 13.
ITRI ​​Electronics also successfully developed a transmissive thin/flexible liquid crystal display (Film-Like Display) using nematic liquid crystal with polymer wall. The components have excellent softness. The display quality performance comparison can be greater than 100, as shown in FIG.
In 2004, Philips used the same concept to propose a new process, using the Offset Printing method to make Adhesion Promoter on the alignment film, as shown in Figure 15(a), followed by coating a layer of liquid crystal and polymer monomer. The mixed solution, as shown in Fig. 15(b), finally forms a single-substrate liquid crystal display with a polymer wall and a liquid crystal domain without full exposure, as shown in Fig. 15(c), Fig. 16 is an exhibit thereof. .
In addition, Philips also published a colorized STN flexible liquid crystal display in the SID of 2004, which has the same structure as the STN liquid crystal display of a general glass substrate, but in the process, the color filter is first transferred on the glass and then transferred to the plastic. On the (PC) substrate, and the gap in the liquid crystal cell is replaced by a conventional sprinkling process using a Photo Spacer. Its structure and products are shown in Figure 17.
Electrophoretic display achieves display by charged colloidal suspension
The concept of making displays with electrophoretic effects emerged in the late 1960s as a non-self-illuminating reflective display. Here we first understand what a colloidal suspension is.
The two-phase system is the most simple colloidal dispersion solution, and the dispersed phase composed of colloidal particles (particles having a diameter ranging from 10-6 to 10-9 m) and the medium in which the dispersed particles are distributed are called a dispersion medium or a continuous phase. The dispersed phase and dispersion medium will have different names depending on the state, as shown in Table 3.
At present, the display that is being developed in the industry uses a colloidal suspension system in which the solid phase is a dispersed phase and the liquid is a dispersion medium, which is called an electrophoretic display (EPD), and a latex system in which the dispersed phase and the dispersion medium are both liquid. It is called Reverse-Emulsion Electrophoretic Display (REED). The principle is similar, firstly preparing a colloidal dispersion solution of a dispersed phase and a dispersion medium having different colors, and then utilizing the interaction between the surface characteristics of the dispersed particles and the dispersion medium to charge the surface of the particle, since the entire system must satisfy the electrical neutral condition, Therefore, there must be an electrically opposite but equal amount of electricity near the interface between the dispersion medium and the particles. This surface is fixed to the electron cloud structure of the adjacent medium and is called an electric double layer. The speed and position of the particle movement can be controlled by controlling the magnitude and direction of the applied electric field. The prepared colloidal dispersion solution is encapsulated between the upper and lower substrates having the electrode design, and can be driven by the electric field. If the particles are on the visible surface, the color of the particles is seen, and if the particles are located on the invisible surface, What you see is the color of the dispersion medium. With this modulation mechanism, you can use it to make a display.
Microencapsulated electronic ink technology
Eink's electrophoretic display technology has the fastest and most mature research and development progress. The main key technologies are derived from the electronic ink manufacturing technology that they published at the 1997 SID conference. The principle is shown in Figure 18.
The microencapsulated electronic ink technology is a technique in which a colloidal suspension containing two kinds of dispersed particles which are respectively black and white and electrically opposite is encapsulated, and then the capsule and the adhesive are mixed to form an electronic ink. The microcapsules have a volume average diameter of about 70 μm, and the electronic ink is fabricated between the upper and lower electrode plates by a precision coating technique, and the thickness of the electronic ink is controlled to be 100 μm, and the visible surface can be changed by the modulation of the electric field direction. Colored particles, you can see different color changes. The electronic ink encapsulation technology not only simplifies the process, the resolution can reach 200ppi, the white state reflectance is 40%, the contrast is between 10 and 15. When the driving voltage is 20V, the image switching time is 250ms, and also has gray scale ( Gray) shows the ability.
Sony launched an e-book in April 2004 (Figure 19) that combines E-ink's microencapsulated electronic ink technology with Toppan's front panel assembly and Phillips' TFT backplane technology. With a size of 6 inches and a resolution of 170dpi, it has four gray levels. This display mechanism has a bistable characteristic, and four 3rd batteries can use 10,000 pages. E-ink's display technology has reached the level of paper in reflectivity and contrast performance, but to achieve dynamic display, the reaction speed needs to be strengthened.
Micro-cup technology with roll-to-roll process characteristics
The Microcup technology was developed by Sipix, and its display mechanism (Fig. 20) was used to modulate the color of dispersed particles and dispersion media of different colors for precise coating by Roll-to-Roll Precision Coating. Fabric technology directly completes the microcup process and panel assembly on a single production line (Figure 21).
The function of the microcup is to provide mechanical strength, so that the panel can withstand bending deformation without affecting the gap between the upper and lower substrates, and also can limit the flow range of the fluid in the microcup, maintain the uniformity of the display image, and when the large area is cut There will be no leakage when it is small. The reel-type process provides rapid and large-scale production efficiency, and is easy to manufacture large-area products and is highly competitive. Referring to the sample of Fig. 22, the contrast ratio is up to 15, and the reaction time is 200 ms at a driving voltage of 45 V. The figure also shows that the sample can still perform normally after cutting.
Particle hiding technology
The particle concealment technology consists of a colloidal suspension composed of colored particles and a transparent dispersion medium, and the lower substrate is used in a color contrasting with the particles. The design of the electrode pattern is used to control the area of ​​the particles distributed on the visible surface, if the particles are scattered throughout the visible On the surface, the color of the particles can be seen; if the particles are squeezed in a relatively narrow area or adsorbed on the side walls, the color of the lower substrate can be felt.
Related technologies include Canon's In-Plane Electrophoretic Display (IP-EPD) and IBM's Lines/Plate Electrophoretic Display and Wall/Post Electrophoretic Display. The structural design of Lines/Plate EPD and Wall/Post EPD is shown in Figure 23. The Lines/Plate has a contrast ratio of 9.7, a maximum reflectance of 61%, a Wall/Post contrast ratio of 11.3, and a maximum reflectance of 71%. This pattern is already close to the soft film level relative to the 65% reflectivity of newspapers.
The prototype structure design of IP-EPD is shown in Figure 24. The electrodes are designed in the lower substrate and the side walls, respectively. When the black particles are driven to the lower substrate electrode (Displaying Electrode), black is seen; when the black particles are driven to the side wall electrode (Collecting Electrode), a white lower substrate is seen.
The photoelectric characteristics of this design are medium contrast ratio of 8 and white state reflectivity of 50%. At the IDRC conference in 2003, the samples published by Canon can be driven by 14V voltage to reach a reaction time of less than 100ms, and they also The residual image defect is effectively solved by the change of the height of the sidewall electrode.
Reverse latex electrophoresis display
The reverse latex electrophoresis display (REED) is a new display mode developed by Zikon, which mainly uses the electrophoretic properties of the inverse latex to achieve the purpose of display.
In general, the liquid-liquid colloidal dispersion system is called latex, but the system in which the dispersed phase is an aqueous solution and the dispersion medium is an oily solution (ie, water in oil) is called an inverse latex; the dispersed phase is an oily solution. The system in which the dispersion medium is an aqueous solution (ie, the oil is in water) is called a latex. In addition to the different relative contents of the aqueous phase and the oil phase, the two systems have different forces depending on the direction in which the molecules are arranged.
Latex systems are complex superatomic structures formed by the agglomeration of Amphiphilic Compounds in aqueous or oily solutions. Amphiphiles refer to long, refined molecules having a lipophilic group at one end and a hydrophilic group at the other end. In the hydrophilic system, due to the hydrophobic effect, the oleophilic ends of the amphiphilic molecules aggregate with each other to form micelles that are only contacted by the hydrophilic end and the aqueous solution; conversely, in the lipophilic system, the amphiphilic pro The water ends gather together to form a microcell that is contacted by the oleophilic end and the oily solution. That is, the microcells at this time are mostly hydrophilic ionic groups, and for them, the electrostatic force is much more important.
The geometry of the inverse latex can be varied (Fig. 25), such as spherical, cylindrical, worm-shaped, double-layer or multi-layer structures. When applying the inverse latex system to the display technology, it must be noted that it must be thermodynamically stable, the microcells will not settle or decompose, and be able to be driven by the modulation of the electric field.
The structure of the REED consists of two glass substrates plated with ITO electrodes, with a reverse emulsion solution injected in between, and a polar dye selected to give the polar phase (ie, inside the cells) a color. At the appropriate electric field strength and frequency, the control cells are evenly distributed over a wide electrode or evenly distributed in the solution, so that the display can display the color of the micro-cell dye; the intensity and frequency of the electric field can also be used to control the micelles. Gathering on the narrower electrodes makes the display panel transparent. The test piece produced by Zikon Company has a pitch of 50 to 80 μm, a driving voltage of 30 to 60 V, a maximum transmittance of 70%, a comparison of 5 and a reaction time of about 50 ms.
OLED, electrophoretic medium characteristics, LCD industry matures more development advantages
In general, flexible displays are currently classified into four types of displays, such as Flat Thin Displays, Curved Displays, Display on Flexible Devices, and Rollable Displays (Roll- Up Display), its final display tends to be a curlable display. In view of the above display technology, since the development and basic research of the liquid crystal display in the process and equipment are quite complete, and it is already a mature industry on the glass substrate, and some products are applied on the rigid plastic substrate, it is necessary to The display mode of the liquid crystal requires relatively few resources when applied to a flexible plastic substrate.
Manufacturers mainly developing flexible displays in Japan still choose liquid crystals as display media. However, due to the inherent limitations of the liquid crystal display mechanism, it is still possible to replace the self-illuminating and colorized OLED/PLED or the simpler electrophoretic display on the final flexible display development date. OLED/PLED has great opportunities in the application of flexible displays due to self-illumination, rapid response, colorization and no viewing angle, but since the manufacturers of OLED/PLED are still working on the mass production of glass substrates, Relatively less energy is invested in the development of flexible displays, so it is not easy to see commercial products in the short term.
The development of electrophoretic displays has gradually matured. Without the limitation of viewing angle and good image memory, it is quite suitable for flexible display applications. Almost all companies developing electrophoretic displays value this part of the market for flexible displays, but the weakness is that the contrast is flat and the reaction time is Relatively slow, colorization technology still has no mature solution.
Figure 26 shows Stanford Resources' prediction and analysis of the flexible display application market. In the short to medium term, flexible display applications are still dominated by low-end and low-priced products such as electronic tags, billboards, automotive displays and smart cards (Smart). Card). In the long run, if the display quality of flexible displays is comparable to that of today's displays, in addition to stimulating more design concepts and emerging applications, it is possible to replace existing markets or achieve cost and price considerations. When it comes to the characteristics of paper, it will also be possible to replace the paper market. Such a huge business opportunity will be the biggest driving force for the continuous improvement of flexible display technology.
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