Patent Publication Number: US-2022231104-A1

Title: Stretchable display device

Description:
STATEMENT REGARDING GOVERNMENT SPONSORED RESEARCH OR DEVELOPMENT 
     The present invention is derived from research conducted as part of Nano Future Material Source Technology Development (R&amp;D) by Ministry of Science and ICT (Project No.: 1711119795, Project No.: 2020M3H4A1A02084896, Research project name: Biaxial stretch-sensitive AMLED display backplane material/element technology, project management institution: National Research Foundation of Korea, task performing institution: Yonsei University, research period: 2020 Jul. 1.˜2021 Dec. 31.). Meanwhile, there is no property interest of the Korean government in any aspect of this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     A claim for priority under 35 U.S.C. § 119 is made to Korean Patent Application No. 10-2021-0006458 filed on Jan. 18, 2021, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Embodiments of the inventive concept described herein relate to a stretchable display device capable of correcting resolution depending on a stretching degree applied to a stretchable substrate. 
     A display technology which brings about changes in various form factors has been commercialized from fixed flexible displays such as first-stage curved displays to second-stage rollable and foldable single-axis variable flexible displays. A stretchable display device called a free-form display as a three-stage next-generation form factor innovation display is emerging. 
     The stretchable display device is a next-generation display in which a display screen is stretched flexibly and is a field that still requires a lot of research. According to the results of future research, the stretchable display device is expected to be applied to various products such as a wearable device attached to the body or clothes, a head-up display inserted into the windshield of a vehicle, which slightly expands or contracts depending on external temperature, or the like, and other display devices that require adjustment of the display screen. 
     One of the technical difficulties of the stretchable display device is that the pixel resolution and luminance of the stretchable display device is changed before and after the stretchable display device is stretched. In other words, when the stretchable display device is stretched, as spacing between pixels increases compared to the same number of pixels, the number of pixels per unit area decreases, resulting in a decrease in resolution and luminance. As such, when a change in resolution/luminance occurs depending to a stretching degree, an image may be distorted. 
     Separate pixel driving is needed to keep resolution and luminance of the stretchable display device constant irrespective of a stretching degree. To this end, as the driving system recognizes the display state before and after being stretched, there is a need to implement a sensor which and feedback-drives the pixel in response to it. For example, because a separate block, such as a strain sensor, for measuring a stretching state of the stretchable display device is required and because stretch should be measured for each pixel area when a stretching degree differs for each pixel area, an element such as a strain sensor should be added per area which should be measured. 
     SUMMARY 
     Embodiments of the inventive concept provide a stretchable display device capable of correcting display resolution and luminance depending to a stretching degree. 
     Embodiments of the inventive concept provide a stretchable display device capable of adjusting display resolution and luminance without the necessity of a separate strain sensor or a switch which operates by a control signal. 
     Embodiments of the inventive concept provide a stretchable display device capable of implementing a display depending on a stretching state of a substrate having a wavy structure based on a sensing line disposed on the substrate and proceeding with detecting stretch for each pixel to much more precisely detect stretch and supplement resolution and luminance. 
     According to an exemplary embodiment, a stretchable display device may include a stretchable substrate, first light emitting pixels including first light emitting elements arranged spaced apart from each other on the stretchable substrate, deformation switch parts arranged between the first light emitting pixels on the stretchable substrate and stretched and deformed according to stretch of the stretchable substrate, and second light emitting pixels including second light emitting elements arranged between the first light emitting pixels on the stretchable substrate. The second light emitting pixels may be controlled to emit light depending on physical deformation of the deformation switch parts. 
     Each of the deformation switch parts may have an electrical contact point having conductivity. Each of the second light emitting pixels may have a pixel contact points having conductivity. Each of the deformation switch parts may be deformed such that the electrical contact point moves between a first position being a position where the electrical contact point is spaced apart from the pixel contact point and a second position being a position where the electrical contact point is in contact with the pixel contact point depending on stretch of the stretchable substrate. Each of the second light emitting pixels may be controlled to emit light depending on whether there is a contact between the electrical contact point and the pixel contact point or may be controlled to emit light depending on a change in resistance and/or capacitance according to deformation of each of the deformation switch parts. 
     Each of the deformation switch parts may be electrically connected with at least one of a light emitting element, an active element, a passive element, and a power wire part of each of the second light emitting pixels. 
     Each of the deformation switch parts may be disposed in a convex shape on the stretchable substrate or may be disposed in a concave portion on the stretchable substrate. 
     Opposite ends of each of the deformation switch parts may be fixed on the stretchable substrate, and a middle area between the opposite ends of each of the deformation switch parts may be spaced apart from an upper surface of the stretchable substrate in a state where the stretchable substrate is not stretched. 
     The deformation switch parts may include a first deformation switch part having a first stretching rate and a second deformation switch part having a second stretching rate different from the first stretching rate. 
     Stretch may be detected and pixel light emission may be controlled, as a contact state with the pixel contact point is changed according to deformation of each of the deformation switch parts according to a stretching force or as resistance and/or capacitance is changed. 
     In the case where the pixel light emission is controlled according to the change in the contact state with the pixel contact point, a first electrical contact point of the first deformation switch part may switch from a non-contact state to a contact state with a first pixel contact point, when a first stretching force (e.g., a stretching force in the direction of an X-axis, a Y-axis, or multiple axes) is applied to the stretchable substrate in one direction or a plurality of directions. 
     A second electrical contact point of the second deformation switch part may be maintained in a non-contact state with a second pixel contact point, when the first stretching force is applied to the stretchable substrate. 
     The second electrical contact point of the second deformation switch part may switch to a contact state with the second pixel contact point, when a second stretching force greater than the first stretching force is applied to the stretchable substrate. 
     An embodiment of the inventive concept is not limited to that the pixel light emission is controlled as the contact state with the pixel contact point is changed according to the deformation of each of the deformation switch parts according to the stretching force. Stretch may be detected and pixel light emission may be controlled, as resistance and/or capacitance is changed. 
     The stretchable display device may further include a power wire part formed on the stretchable substrate and configured to apply power. The pixel contact point may be an area corresponding to a lower position of the electrical contact point of each of the deformation switch parts in the power wire part. 
     Each of the second light emitting pixels may further include a first transistor driven according to scan power applied through a scan line and a second transistor that supplies power applied through the power wire part to a second light emitting element depending on data power applied through a data line, when the first transistor is driven. Each of the deformation switch parts may be electrically connected with a first node corresponding to a drain terminal or a source terminal of the second transistor. 
     A first node corresponding to any one of a drain terminal or a source terminal of the second transistor may be electrically connected with the power wire part. Each of the deformation switch parts may be electrically connected with the second light emitting element. 
     The second light emitting element may be disposed on each of the deformation switch parts. A pixel circuit part including the first transistor and the second transistor may be formed on an upper surface of the stretchable substrate. 
     The first node corresponding to the any one of the drain terminal or the source terminal of the second transistor may be electrically connected with the power wire part. 
     The pixel contact point may be a second node corresponding to the other of the drain terminal or the source terminal of the second transistor. 
     The second light emitting pixel may be electrically short-circuited with the pixel circuit part in a state where the stretchable substrate is not stretched and may be electrically connected with the pixel circuit part through each of the deformation switch parts, in a state where the stretchable substrate is stretched and where the electrical contact point is in contact with the pixel contact point. 
     The deformation switch parts may include a first deformation switch part and a second deformation switch part, each of which has a different stretching rate. The first deformation switch part and the second deformation switch part may have different sizes, may have different shapes, or may be formed of different materials. 
     The number of pixels which emit light among the second light emitting pixels may be changed according to a stretching force applied to the stretchable substrate. 
     The deformation switch parts may be arranged along a direction where the stretchable substrate is stretchable or are arranged in the form of a grid. 
     The stretchable substrate may be implemented in a pre-stretch scheme to be provided on a substrate part having a wavy structure, and the deformation switch parts may be formed in concave portions of the stretchable substrate The stretchable display device may further include a sensing line part located an upper side of the substrate part and a stretching state detection part that detects a stretching state of the substrate part depending on a change in distance between the sensing line part and the deformation switch parts according to stretch of the substrate part. Light emission of each of the second light emitting pixels may be controlled according to the stretching state of the substrate part. 
     According to an exemplary embodiment, a stretchable display device may include a stretchable substrate, first light emitting pixels including first light emitting elements arranged spaced apart from each other on the stretchable substrate, deformation switch parts arranged between the first light emitting pixels on the stretchable substrate and having electrical contact points, each of which has conductivity, and second light emitting pixels including second light emitting elements arranged between the first light emitting pixels on the stretchable substrate and having pixel contact points, each of which has conductivity. 
     The deformation switch parts may be provided to be deformable such that each of the electrical contact points moves from a first position to a second position. The first position may be a position where each of the electrical contact points is spaced apart from the corresponding pixel contact point, and the second position may be a position where each of the electrical contact points is in contact with the corresponding pixel contact point. An upper surface of each of the pixel contact points may be the same in height as an upper surface of the stretchable substrate or may be higher in height than the upper surface of the stretchable substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above and other objects and features will become apparent from the following description with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified, and wherein: 
         FIG. 1  is a perspective view illustrating a stretchable display device according to an embodiment of the inventive concept; 
         FIG. 2  is a side view illustrating a stretchable display device according to an embodiment of the inventive concept; 
         FIGS. 3, 4, and 5  are drawings illustrating an operation state of a stretchable display device according to an embodiment of the inventive concept; 
         FIGS. 6 and 7  are circuit diagrams illustrating an operation of a second light emitting pixel making up a stretchable display device according to an embodiment of the inventive concept; 
         FIG. 8  is a plan view illustrating a stretchable display device according to another embodiment of the inventive concept; 
         FIG. 9  is a circuit diagram of a second light emitting pixel making up a stretchable display device according to an embodiment of  FIG. 8 ; 
         FIG. 10  is a block diagram illustrating a stretchable display device according to another embodiment of the inventive concept; 
         FIGS. 11 and 12  are drawings illustrating an operation of a stretchable display device according to another embodiment of the inventive concept; 
         FIGS. 13 and 14  are drawings for describing an example of a stretchable display device illustrated in  FIG. 11 ; 
         FIGS. 15 and 16  are drawings for describing another example of a stretchable display device illustrated in  FIG. 12 ; and 
         FIG. 17  is a flowchart for describing a method for detecting a stretching state of a stretchable display device according to an embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     Advantages, features, and methods of accomplishing the same will become apparent with reference to embodiments described in detail below together with the accompanying drawings. However, the inventive concept is not limited to the embodiments described below, but may be implemented in various forms. The embodiments are provided so that the disclosure of the inventive concept will be complete and thorough and to fully convey the scope of the inventive concept to those skilled in the art to which the inventive concept pertains and the inventive concept is only defined by the scope of claims. 
     Unless otherwise defined herein, all terms (including technical and scientific terms) used in the specification may have the same meaning that is generally understood by a person skilled in the art. Also, terms which are defined in a dictionary and commonly used should be interpreted as not in an idealized or overly formal detect unless expressly so defined. The terms such as “a first”, “a second” in the specification are used to distinguish one element from another element, and the claims of the inventive concept are not limited to these terms. For example, a first element may be termed a second element. Similarly, a second element may be termed a first element. 
     Furthermore, the term such as “˜unit” described in the specification refers to software or a hardware element such as a field-programmable gate array (FPGA) or an ASIC. The “˜unit” performs certain roles. However, the “˜unit” does not refer to being not limited to software or hardware. The“˜unit” may be configured to be included in an addressable storage medium or to reproduce one or more processors. Therefore, as an example, “˜nit” may include various elements such as software elements, object-oriented software elements, class elements, and task elements, processes, functions, attributes, procedures, subroutines, program code segments, drivers, firmware, microcodes, circuits, data structures, and variables. Functions provided in “units” and elements may be combined with a smaller number of elements and ‘units’ or may be further be divided into additional elements and “units”. 
       FIG. 1  is a perspective view illustrating a stretchable display device according to an embodiment of the inventive concept.  FIG. 2  is a side view illustrating a stretchable display device according to an embodiment of the inventive concept. 
     Referring to  FIGS. 1 and 2 , a stretchable display device  10  according to an embodiment of the inventive concept may include a stretchable substrate  20 , first light emitting pixels  30 , deformation switch parts  40 , second light emitting pixels  50 , and a power wire part  60 . 
     In the stretchable display device  10  according to an embodiment of the inventive concept, as the number of pixels which emit light among the second light emitting pixels  50  is changed according to a stretching degree (or a stretching force) of the stretchable substrate  20 , pixel resolution may be kept constant irrespective of the stretching degree of the stretchable substrate  20 . 
     The stretchable display device  10  according to an embodiment of the inventive concept may be used as various application products, for example, a display device of a wearable electronic device, which is used by being attached to or worn on the body, a head-up display applied to the glass of the vehicle, or a display device capable of adjusting a display screen size. 
     The stretchable substrate  20  may be a substrate having stretchable properties, which includes a stretchable material to be stretchable. The stretchable substrate  20  may be a substrate which is stretchable in one direction parallel to a substrate surface or in several directions. 
     An example of the stretchable material forming the stretchable substrate  20  may be, but is not limited to, polydimethylsiloxane (PDMS), styrene ethylene/butylene styrene (SEBS), rubber, polyethylene terephthalate (PET), polystyrene (PS), polyimide (PI), polyvinyl chloride (PVC), polyethylene (PE), or the like. 
     The stretchable substrate  20  is not necessarily limited to including the stretchable material. The stretchable substrate  20  may be composed of, for example, a structure formed of a fibrous structure, a lattice structure, or the like and may be implemented to be stretchable. Furthermore, the stretchable substrate  20  may be implemented in a structurally stretchable structure, such as an auxetic structure, although there is no stretch due to material characteristics. 
     The stretchable substrate  20  may be a substrate which is stretched when a stretching force is applied to the substrate and is restored to its original state when the stretching force disappears.  FIG. 1  illustrates the stretchable substrate  20  which is stretched in a first direction X parallel to the substrate surface. 
     The stretchable substrate  20  may be stretched in the first direction X. Furthermore, the stretchable substrate  20  may be provided as a flexible substrate which has flexibility capable of being bent in at least one direction at the same time as having stretch. 
     In an example of  FIG. 1 , a second direction Y may be a direction perpendicular to the first direction X on the substrate surface, and a third direction Z may be a direction perpendicular to both the first direction X and the second direction Y, that is, a direction perpendicular to the substrate surface. 
     The first light emitting pixels  30  may be arranged spaced apart from each other on the stretchable substrate  20 . The first light emitting pixel  30  may include a first light emitting element and a pixel circuit part which drives the first light emitting element. 
     The first light emitting element of the first light emitting pixel  30  may be provided as, for example, a light emitting diode (LED), or an organic light emitting diode (OLED), or the like, but not limited thereto. 
     In the example illustrated, the first light emitting pixels  30  may form rows and columns in the first direction X and the second direction Y to be arranged in a matrix structure, but the arrangement structure of the first light emitting pixels  30  is not limited to that illustrated. 
     The first light emitting pixels  30  may be pixels which may be mounted on the stretchable substrate  20  and may emit light in a state where the stretchable substrate  20  is not stretched, as well as a state where the stretchable substrate  20  is stretched. 
     The first light emitting pixels  30  may sequentially and repeatedly emit light in units of rows or columns according to power applied through scan lines. Furthermore, the first light emitting pixels  30  may emit light depending on a high/low level power applied through data lines. 
     In the stretchable display device  10 , display resolution (or the number of pixels per unit area) in the state where the stretchable substrate  20  is not stretched may be determined according to the arrangement of the first light emitting pixels  30 . 
     The stretchable display device  10  according to an embodiment of the inventive concept may have the deformation switch parts  40  and the second light emitting pixels  50  to prevent the number of pixels per unit area (or resolution) from decreasing as an area of the stretchable substrate  20  increases when the stretchable substrate  20  is stretched. 
     The deformation switch part  40  may be disposed in an area between the first light emitting pixels  30  which are adjacent to each other on the stretchable substrate  20 . The deformation switch part  40  may be stretched and deformed as the stretchable substrate  20  is stretched. 
     In an embodiment, the deformation switch part  40  may be provided in a dome-shaped structure which is convex upward. Opposite ends of the deformation switch part  40  may be fixed on the stretchable substrate  20 . Thus, the deformation switch part  40  may be stretched together when the stretchable substrate  20  is stretched. 
     A middle area between the opposite ends of the deformation switch part  40  may be disposed spaced apart from an upper surface of the stretchable substrate  20  in a state where the stretchable substrate  20  is not stretched. 
     The deformation switch part  40  may be electrically connected with at least one of a light emitting element making up the second light emitting pixel  50 , an active element (e.g., a thin film transistor), a passive element (e.g., a capacitor), and the power wire part  60 . 
     The deformation switch part  40  may be formed of a conductive material partially or entirely. The deformation switch part  40  may have electrical contact points  42   a ,  44   a , and  46   a , each of which has conductivity. Each of the electrical contact points  42   a ,  44   a , and  46   a  may be a central portion of a lower surface of the deformation switch part  40 . 
     The conductive material of the deformation switch part  40  may be provided as a stretchable conductive material, for example, liquid metal, an amorphous alloy, double structure polymer metal, or the like, but various stretchable conductive materials other than the listed materials may be used in the deformation switch part  40 . 
     The deformation switch part  40  may be formed of, for example, an organic-based material, such as a conductive polymer, a metal-based material, such as Al, Au, or Ti, an inorganic-based material, such as ITO, IZO, or GZO, a liquid metal-based material, or the like. 
     Various schemes for connecting the deformation switch part  40  on the stretchable substrate  20  may be used. As an example, a scheme which separately manufactures the deformation switch part  40  and bonds the deformation switch part  40  on the stretchable substrate  20  may be used. 
     As another example, a scheme which places a mold for forming the deformation switch part  40  on the stretchable substrate  20  forms the deformation switch part  40  on the mold, and bonds opposite ends of the deformation switch part  40  on the stretchable substrate  20  may be used. 
     The second light emitting pixels  50  may be arranged between the first light emitting pixels  30  on the stretchable substrate  20 . The second light emitting pixels  50  may be pixels capable of emitting light only when the stretchable substrate  20  is stretched to keep resolution of the stretchable display device  10  constant. 
     The second light emitting pixels  50  may have pixel contact points  60   a , each of which has conductivity, to control operations of the second light emitting pixels  50  without measuring a stretching force applied to the stretchable substrate  20 . 
     In other words, the second light emitting pixels  50  may be pixels which do not emit light in a state where the stretchable substrate  20  is not stretched. The second light emitting pixels  50  may be arranged to form arrangement such that resolution of the stretchable display device  10  is kept constant when the stretchable substrate  20  is stretched. 
     The power wire part  60  may be formed on the stretchable substrate  20 . In an embodiment of  FIG. 1 , the power wire part  60  may extend in the second direction Y to be formed to apply power to the second light emitting pixels  50  in units of rows or columns. 
     The second light emitting pixels  50  may be controlled to emit light depending on whether there is a contact between the electrical contact points  42   a ,  44   a , and  46   a  and the pixel contact points  60   a  according to physical deformation of the deformation switch parts  40  when the stretchable substrate  20  is stretched. 
     In case of an embodiment of  FIG. 1 , the pixel contact points  60   a  of the second light emitting pixels  50  may be areas corresponding to lower positions of the electrical contact points  42   a ,  44   a , and  46   a  of the deformation switch parts  40  among the power wire parts  60 . 
     For contact points with the deformation switch parts  40 , heights of upper surfaces of the pixel contact points  60   a  of the second light emitting pixels  50  may be designed to be the same as the upper surface of the stretchable substrate  20  or be higher than the upper surface of the stretchable substrate  20 . 
     Like the first light emitting pixels  30 , the second light emitting pixels  50  may be arranged spaced apart from each other on the stretchable substrate  20 . The second light emitting pixel  50  may include a second light emitting element and a pixel circuit part which drives the second light emitting element. 
     The second light emitting element of the second light emitting pixel  50  may be provided as, for example, a light emitting diode LED, or an organic light emitting diode (OLED), or the like, but not limited thereto. 
     In the example illustrated, the second light emitting pixels  50  may form rows and columns in the first direction X and the second direction Y to be arranged in a matrix structure, but the arrangement structure of the second light emitting pixels  50  is not limited to that illustrated. 
     The second light emitting pixels  50  may be pixels which may be mounted on the stretchable substrate  20 , may not emit light in a state where the stretchable substrate  20  is not stretched and when a stretching force of less than a threshold stretching force is applied to the stretchable substrate  20 , and may emit light in a state where the stretchable substrate  20  is stretched above the threshold stretching force. 
     As such, the second light emitting pixels  50  may act as resolution compensation pixels which emit light when the stretchable substrate  20  is stretched and keep resolution of the stretchable display device  10  uniform. 
     The second light emitting pixels  50  may sequentially or repeatedly emit light in units of rows or columns depending on power supplied through scan lines, when the stretchable substrate  20  is stretched above the threshold stretching force, and may emit light depending on a high/low level power applied through data lines. 
     The deformation switch parts  42 ,  44 , and  46  may be deformed to move in an upward and downward direction parallel to the third direction Z between a first position P 1 , which is a position where the electrical contact points  42   a ,  44   a , and  46   a  are spaced apart from the pixel contact points  60   a , and a second position P 2 , which is a position where the electrical contact points  42   a ,  44   a , and  46   a  are in contact with the pixel contact points  60   a , as the stretchable substrate  20  is stretched. 
     Each of the deformation switch parts  42 ,  44 , and  46  may be located at the first position P 1  where each of the electrical contact points  42   a ,  44   a , and  46   a  is spaced apart from the corresponding pixel contact point  60   a  in a state where stretch is not applied to the stretchable substrate  20 . 
     In this case, as the electrical contact points  42   a ,  44   a , and  46   a  of the deformation switch parts  42 ,  44 , and  46  are electrically short-circuited with the pixel contact points  60   a  of the second light emitting pixels  52 ,  54 , and  56 , working power may not be applied to second light emitting elements of the second light emitting pixels  52 ,  54 , and  56  and the second light emitting pixels  52 ,  54 , and  56  may operate as inactive pixels. 
     On the other hand, in a state where stretch above the threshold stretching force is applied to the stretchable substrate  20 , the deformation switch parts  42 ,  44 , and  46  may be deformed to be located at the second position P 2  where each of the electrical contact points  42   a ,  44   a , and  46   a  is in contact with the corresponding pixel contact point  60   a.    
     In this case, as the electrical contact points  42   a ,  44   a , and  46   a  of the deformation switch parts  42 ,  44 , and  46  are electrically connected with the pixel contact points  60   a  of the second light emitting pixels  52 ,  54 , and  56 , working power may be applied to the second light emitting elements of the second light emitting pixels  52 ,  54 , and  56  and the second light emitting pixels  52 ,  54 , and  56  may operate as active pixels. 
       FIGS. 3 to 5  are drawings illustrating an operation state of a stretchable display device according to an embodiment of the inventive concept. In embodiments of  FIGS. 3 to 5 , deformation switch parts  42 ,  44 , and  46  are designed to have different stretching rates. 
     A stretching rate of the first deformation switch part  42  among the first deformation switch part  42 , the second deformation switch part  44 , and the third deformation switch part  46  may be largest, and a stretching rate of the third deformation switch part  46  may be smallest. The second deformation switch part  44  may have a stretching rate which is less than the first deformation switch part  42  and is greater than the third deformation switch part  46 . 
     When a stretching force corresponding to a first threshold stretching rate (e.g., a stretching rate of 20%) is applied to the stretchable substrate  20 , the first electrical contact point  42   a  of the first deformation switch part  42  may be in contact with the first pixel contact point  60   a  associated with the second light emitting pixel  52  (or may switch from a non-contact state to a contact state). 
     When a stretching force corresponding to a second threshold stretching rate (e.g., a stretching rate of 50%) greater than first threshold stretching rate is applied to the stretchable substrate  20 , the second electrical contact point  44   a  of the second deformation switch part  44  may be in contact with the second pixel contact point  60   a  associated with the second light emitting pixel  54  (or may switch from a non-contact state to a contact state). 
     When a stretching force corresponding to less than the second threshold stretching rate is applied to the stretchable substrate  20 , the second deformation switch part  44  may be maintained in a state where the second electrical contact point  44   a  is not in contact with the pixel contact point  60   a  associated with the second light emitting pixel  54 . 
     When a stretching force corresponding to a third threshold stretching rate (e.g., a stretching rate of 75%) greater than the second threshold stretching rate is applied to the stretchable substrate  20 , the third electrical contact point  46   a  of the third deformation switch part  46  may be in contact with the third pixel contact point  60   a  associated with the second light emitting pixel  56  (or may switch from a non-contact state to a contact state). 
     When the stretching force corresponding to the third threshold stretching rate is applied to the stretchable substrate  20 , the third deformation switch part  46  may be maintained in a state where the second electrical contact point  46   a  is not in contact with the pixel contact point  60   a  associated with the second light emitting pixel  56 . 
     In case of embodiments of  FIGS. 2 to 5 , when a low stretching rate of less than the first threshold stretching rate is applied to the stretchable substrate  20 , as shown in  FIG. 2 , because all the three deformation switch parts  42 ,  44 , and  46  are not in contact with the pixel contact points  60   a , all the three second light emitting pixels  52 ,  54 , and  56  operate as inactive pixels. 
     When a stretching rate of greater than or equal to the first threshold stretching rate and less than the second threshold stretching rate is applied to the stretchable substrate  20 , as shown in  FIG. 3 , as only the one deformation switch part  42  among the three deformation switch part  42 ,  44 , and  46  is in contact with the pixel contact point  60   a , only the one second light emitting pixel  52  among the three second light emitting pixels  52 ,  54 , and  56  operates as an active pixel. 
     Furthermore, when a stretching rate of greater than or equal to the second threshold stretching rate and less than the third threshold stretching rate is applied to the stretchable substrate  20 , as shown in  FIG. 4 , as the two deformation switch parts  42  and  44  among the three deformation switch part  42 ,  42 ,  44 , and  46  are respectively in contact with the corresponding pixel contact points  60   a , only the two second light emitting pixels  52  and  54  among the three second light emitting pixels  52 ,  52 ,  54 , and  56  operate as active pixels. 
     Furthermore, when a stretching rate of greater than or equal to the third threshold stretching rate is applied to the stretchable substrate  20 , as shown in  FIG. 5 , as all the three deformation switch parts  42 ,  44 , and  46  are respectively in contact with the corresponding pixel contact points  60   a , all the three second light emitting pixels  52 ,  54 , and  56  operate as active pixels. 
     As such, the number of the second light emitting pixels  50  which operate as active pixels increases in proportion to the stretching rate applied to the stretchable substrate  20 . Thus, as the number of the second light emitting pixels  50  is adjusted according to a stretching degree applied to the stretchable substrate  20 , constant resolution may always be maintained. 
     Various schemes for changing stretching rates of the deformation switch parts  40  may be used. For example, the deformation switch parts  40  may be formed of different materials or the deformation switch parts  40  may be designed to have different sizes, shapes, and the like to differently adjust a stretching rate. 
       FIGS. 6 and 7  are circuit diagrams illustrating an operation of a second light emitting pixel making up a stretchable display device according to an embodiment of the inventive concept.  FIG. 6  illustrates a state where a second light emitting pixel operates as a deactivated pixel.  FIG. 7  illustrates a state where a second light emitting pixel operates as an activated pixel. 
     Referring to  FIGS. 1, 2, 6, and 7 , a second light emitting pixel  50  may include a second light emitting element D and a pixel circuit part for driving the second light emitting element D. The pixel circuit part may include a first transistor T 1 , a second transistor T 2 , and a capacitor C st . 
     In embodiments of  FIGS. 6 and 7 , the second light emitting element D and the pixel circuit part (or the transistors and the capacitor) of the second light emitting pixel  50  may be formed on a deformation switch part  40 . A pixel contact point  60   a  associated with the second light emitting pixel  50  may be formed on a power wire part  60  on an upper surface of the stretchable substrate  20 . 
     The first transistor T 1  may be driven according to scan power applied to a gate terminal through a scan line SL. The first transistor T 1  may be implemented as a P-channel metal oxide semiconductor (PMOS) transistor or an N-channel metal oxide semiconductor (NMOS) transistor. 
     One end of a drain terminal and a source terminal of the first transistor T 1  may be connected with a data line DL, and the other may be connected with a gate terminal of the second transistor T 2 . Furthermore, one end of the capacitor C st  may be connected with the other end of the first transistor T 1 . 
     The second transistor T 2  may supply power ELVDD applied through the power wire part  60  to the second light emitting element D depending on data power supplied through the data line DL when the first transistor T 1  is driven. The gate terminal of the second transistor T 2  may be connected with the other end (or the drain terminal or the source terminal) of the first transistor T 1 . 
     One end  50   a  (or a first node) between a drain terminal and a source terminal of the second transistor T 2  may be connected with the other end of the capacitor C st . The other end (or a second node) between the drain terminal and the source terminal of the second transistor T 2  may be connected with one end of a second light emitting element D. The second transistor T 2  may be implemented as a PMOS transistor or an NMOS transistor. 
     According to a stretching degree of a stretchable substrate  20 , an electrical contact point  40   a  of a deformation switch part  40  may be spaced apart from a pixel contact point  60   a  of a second light emitting pixel  50  provided in the power wire part  60  as shown in  FIG. 6  or to be in contact with the pixel contact point  60   a  as shown in  FIG. 7 . 
     The deformation switch part  40  may be electrically connected with the one end  50   a  of the second transistor T 2  making up a pixel circuit part of the second light emitting pixel  50  and the other end of the capacitor C st  through a connection node  40   b.    
     The capacitor C st  may provide an active driving function of maintaining a voltage of the gate terminal of the second transistor T 2  during a scan period. The one end of the capacitor C st  may be connected with the other end of the first transistor T 1  and the gate terminal of the second transistor T 2 . 
     The other end of the capacitor C st  may be electrically connected with the one end  50   a  of the second transistor T 2  and the connection node  40   b  of the deformation switch part  40 . The second light emitting element D may be connected between the other end of the second transistor T 2  and the ground. 
     As shown in  FIG. 6 , when the electrical contact point  40   a  of the deformation switch part  40  is spaced apart from the pixel contact point  60   a  of the second light emitting pixel  50  provided in the power wire part  60 , a power ELVDD of the power wire part  60  may not be transferred to the pixel circuit part of the second light emitting pixel  50 . Thus, the second light emitting pixel  50  may fail to emit light. 
     On the other hand, as shown in  FIG. 7 , when the electrical contact point  40   a  of the deformation switch part  40  is in contact with the pixel contact point  60   a  of the second light emitting pixel  50  provided in the power wire part  60 , the power ELVDD of the power wire part  60  may be transferred to the pixel circuit part of the second light emitting pixel  50 . 
     At this time, as contact resistance between the pixel circuit part and the power wire part  60  has a low value due to conductivity of the deformation switch part  40 , the power ELVDD may be efficiently supplied to the pixel circuit part. Thus, the second light emitting pixel  50  may smoothly emit light by means of the pixel circuit part. 
     As described above, a stretchable display device  10  according to an embodiment of the inventive concept may be stretched together with the stretchable substrate  20  to apply power to the second light emitting pixel  50  when the stretchable substrate  20  is stretched by the deformation switch part  40  which is physically deformed. 
     Thus, the stretchable display device  10  according to an embodiment of the inventive concept may maintain constant display resolution irrespective of a stretching degree of the stretchable substrate  20  without the necessity of a separate strain sensor or a switch which operates by a control signal. 
       FIG. 8  is a plan view illustrating a stretchable display device according to another embodiment of the inventive concept. Although a stretchable substrate  20  is stretched in any direction of a substrate surface as deformation switch parts  40  are arranged in the form of a grid, a stretchable display device  10 ′ according to an embodiment of  FIG. 8  may be implemented to facilitate resolution compensation by second light emitting pixels  50 ′. 
     Each of the deformation switch parts  40  may be disposed between first light emitting pixels  30  adjacent to each other in a first direction X and between the first light emitting pixels  30  adjacent to each other in a second direction Y and may be formed in the first direction X and the second direction Y. 
     Thus, when stretch is applied to the stretchable substrate  20  in the second direction Y, display resolution of the second direction Y may be adjusted by the deformation switch parts  40  arranged in the second direction Y. 
     In describing an embodiment of the  FIG. 8 , a duplicated description of the configuration which is the same as or correspond to the embodiment described above will be omitted. The second light emitting pixel  50  making up the stretchable display device  10 ′ according to an embodiment of  FIG. 8  may include a second light emitting element D and a pixel circuit part  70  for driving the second light emitting element D. 
     The second light emitting element D of the second light emitting pixel  50 ′ may be formed on the deformation switch part  40 . A power wire part  60  and the pixel circuit part  70  may be formed on an upper surface of the stretchable substrate  20  rather than the deformation switch part  40 . 
       FIG. 9  is a circuit diagram of a second light emitting pixel making up a stretchable display device according to an embodiment of  FIG. 8 . Referring to  FIGS. 8 and 9 , a pixel circuit part  70  may include a first transistor T 1 , a second transistor T 2 , and a capacitor C st . A duplicated description of the pixel circuit part  70  will be omitted because the pixel circuit part  70  is the same as the embodiment described above. 
     In an embodiment of  FIG. 9 , a deformation switch part  40  may be electrically connected with a second light emitting element D. In other words, the deformation switch part  40  may be connected with one end  50   b  of the second light emitting element D through a connection node  40   b . The second light emitting element D may be connected between the connection node  40   b  of the deformation switch part  40  and the ground. 
     An electrical contact point  40   a  of the deformation switch part  40  may be in contact with or spaced apart from a pixel contact point  70   a  of a pixel circuit part  70  formed on a stretchable substrate  20  depending on a stretching degree of the stretchable substrate  20 . In an embodiment of  FIG. 9 , the pixel contact point  70   a  of a second light emitting pixel  50 ′ is a second node corresponding to the other end (or a drain terminal or a source terminal) of the second transistor T 2 . 
     A first node corresponding to any one of the drain terminal or the source terminal of the second transistor T 2  may be electrically connected with each of the other end of the capacitor C st  and a power wire part  60 . 
       FIG. 9  illustrates a state where the stretchable substrate  20  is stretched and the electrical contact point  40   a  of the deformation switch part  40  is in contact with the pixel contact point  70   a  of the second light emitting pixel  50 ′. In this state, contact resistance C R  of the deformation switch part  40  between the pixel circuit part  70  and the second light emitting element D indicates a low value and the second light emitting element D may operate as an active element. 
     In embodiments of  FIGS. 8 and 9 , the second light emitting element D may be electrically short-circuited in a state where stretch of less than a threshold stretching rate is applied to the stretchable substrate  20 . 
     When stretch of greater than or equal to the threshold stretching rate is applied to the stretchable substrate  20 , the electrical contact point  40   a  may be in contact with the pixel contact point  70   a  by stretch deformation of the deformation switch part  40 . Thus, as the second light emitting element D is electrically connected with the pixel circuit part  70  by using the deformation switch part  40  as a contact point, the second light emitting element D may operate as an active element. 
     In a modified embodiment of the inventive concept, a second light emitting pixel  50  may be formed on the stretchable substrate  20 , and the power wire part  60  may be formed on the deformation switch part  40 . In this case, the electrical contact point  40   a  of the deformation switch part  40  may be a pixel area of the power wire part  60 , and a pixel contact point of the second light emitting pixel  50  may be a drain terminal and a source terminal of the second transistor T 2 . 
       FIG. 10  is a block diagram illustrating a stretchable display device according to another embodiment of the inventive concept.  FIGS. 11 and 12  are drawings illustrating an operation of a stretchable display device according to another embodiment of the inventive concept.  FIG. 11  illustrates a state before a stretchable display device is stretched.  FIG. 12  illustrates a state after a stretchable display device is stretched. 
     Referring to  FIG. 10 , a stretchable display device  100  according to a preferred embodiment of the inventive concept may detect a stretching state of a substrate having a wavy structure based on a change in voltage or capacitance of a sensing line disposed on the substrate, without a separate strain sensor, and may implement a display depending on the stretching state of the substrate. The stretchable display device  100  according to an embodiment of the inventive concept may be applied to stretchable display modules of various structures, for example, an active-matrix LED, an active-matrix OLED, an LCD, and an e-paper. 
     Meanwhile, the stretchable display device  100  according to an embodiment of the inventive concept may be implemented physically independently of a control module (not shown) of a stretchable display and may provide the detected stretching state of the substrate to a control module of the stretchable display. Then, the control module of the stretchable display may operate a pixel disposed to operate only after being stretched based on the stretching state of the substrate, which is provided from the stretchable display device  100 . Of course, the stretchable display device  100  according to an embodiment of the inventive concept may be integrated and implemented into the control module of the stretchable display. 
     To this end, the stretchable display device  100  may include a substrate part  110 , a sensing line part  150 , and a stretching state detection part  170 . The substrate part  110  may be provided in a structure where deformation switch parts are arranged in concave portions on the stretchable substrate. The substrate part  110  may be implemented in a pre-stretch scheme to have a wavy structure. 
     Such a wavy structure serves to help the stretchable display be stretched and is deformed when stretched. Herein, the substrate part  110  may include a plurality of pixel elements  111  (e.g., first light emitting pixels and second light emitting pixels, each of which has a pixel contact point disposed between the first light emitting pixels), which are formed on the substrate part  110 . The plurality of pixel elements  111  may be separately turned on/off according to a signal of the control module of the stretchable display and/or stretch of the stretchable display. 
     In embodiments of  FIGS. 11 and 12 , deformation switch parts may be provided to be stretched and deformed according to stretch of the stretchable substrate on the stretchable substrate making up the substrate part  110 . The deformation switch parts may be provided to have electrical contact points, each of which has conductivity. The sensing line part  150  may be located at an upper side of the substrate part  110 . 
     The sensing line part  150  may be disposed on an upper portion of the substrate part  110  having the wavy structure to be adjusted in interval from the deformation switch parts provided on a concave area of the substrate part  110  according to a deformation pattern of the wavy structure, when the stretchable display is stretched. Thus, light emission of the second light emitting pixel provided for each area may be independently controlled based on stretching information detected for each area of the stretchable substrate. 
     The control module of the stretchable display device may detect stretching information for each area depending on whether there is a contact between the substrate part  110  and the sensing line part  150  or a change in voltage and/or capacitance between the substrate part  110  and the sensing line part  150  and may control light emission of the second light emitting pixel for each corresponding area. When the stretchable display device is implemented to control light emission of the second light emitting pixel depending on a change in voltage and/or capacitance between the substrate part  110  and the sensing line part  150 , an insulating part  130  which is filled with insulators may be provided between the substrate part  110  and the sensing line part  150 . To smooth a height of the substrate part  110  having the wavy structure, the insulating part  130  may be formed of various insulators, each of which is formed of a material having a characteristic in which electricity does not flow. 
     The sensing line part  150  may be formed of an organic-based material, such as a conductive polymer, a metal-based material, such as Al, Au, or Ti, an inorganic-based material, such as ITO, IZO, or GZO, a liquid metal-based material, or the like. In this case, the sensing line part  150  may be connected with a signal wire of each of a plurality of pixel elements  111  formed on the substrate part  110 . Herein, the signal wire may be one of a gate signal line, a data line, or a power line. 
     In other words, because the signal wire of the pixel element  111  and the sensing line part  150  are opened before the substrate part  110  is stretched, a voltage of the sensing line part  150  may not be changed. Because the signal wire of the pixel element  111  and the sensing line part  150  are changed to a short-circuited state after the substrate part  110  is stretched, a voltage of the sensing line part  150  may be changed. Furthermore, the sensing line part  150  may be connected with the signal wire of the pixel element  111  through a capacitor, and a capacitance of the sensing line part  150  may be changed after the substrate part  110  is stretched. 
     The stretching state detection part  170  may detect a stretching state of the substrate part  110  based on a change in voltage or capacitance of the sensing line part  150  according to stretch of the substrate part  110 . In other words, as a distance between the pixel element  111  formed on the substrate part  110  and the sensing line part  150  is changed according to the stretch of the substrate part  110 , when a change in voltage or capacitance of the sensing line part  150  is detected, the stretching state detection part  170  may detect a stretching state of the substrate part  110  based on the detected change in voltage or capacitance of the sensing line part  150  using stretching state information for each predetermined voltage or capacitance. 
     Herein, the stretching state information for each predetermined voltage or capacitance may include information about a voltage value or a capacitance value for each stretching degree according to a stretching degree of the substrate part  110 . For example, before the substrate part  110  is stretched, as shown in  FIG. 11 , the pixel element  111  formed on the substrate part  110  and the sensing line part  150  are away from each other. When the substrate part  110  is stretched, as shown in  FIG. 12 , the pixel element  111  formed on the substrate part  110  and the sensing line part  150  are close to each other. 
     Due to this, a voltage or capacitance of the sensing line part  150  may be changed. The stretching state detection part  170  may detect a stretching state of the substrate part  110  based on the change in voltage or capacitance of the sensing line part  150 . Then, a description will be given in detail of an operation of a stretchable display device according to a preferred embodiment of the inventive concept with reference to  FIGS. 13 and 14 . 
       FIGS. 13 and 14  are drawings for describing an operation of a stretchable display device illustrated in  FIG. 11 . Before a substrate part  110  is stretched, as shown in  FIG. 13 , because a signal wire of a pixel element  111  formed on the substrate part  110  and a sensing line part  150  are opened, a voltage of the sensing line part  150  is not changed. Thereafter, when the substrate part  110  is stretched, as shown in  FIG. 14 , the signal wire of the pixel element  111  formed on the substrate part  110  and the sensing line part  150  are changed to a short-circuited state. Due to this, a voltage of the sensing line part  150  is changed. 
       FIGS. 15 and 16  are drawings for describing another example of a stretchable display device illustrated in  FIG. 12 .  FIGS. 15 and 16  are drawings for describing an operation another example of a stretchable display device illustrated in  FIG. 10 . In a state where a sensing line part  150  is connected with a signal wire of a pixel element  111  formed on a substrate part  110  through a capacitor as shown in  FIG. 15 , when the substrate part  110  is stretched, as shown in  FIG. 16 , a capacitance of the sensing line part  150  is changed. 
     According to embodiments of  FIGS. 10 to 16 , the stretchable substrate of a wrinkle substrate structure may be formed in a lower portion of the sensing line part  150  and the deformation switch parts may be formed in concave portions on the stretchable substrate to implement the stretchable display device. According to the present embodiment, because an electrical contact point is formed between the stretchable substrate (a wrinkle substrate) of the substrate part  110  and the sensing line part  150 , a stable element characteristic may be ensured because the electrical contact is not exposed to the outside, and a manufacturing process of the stretchable substrate and the deformation switches may be facilitated. 
     Hereinafter, a description will be given of a method for detecting a stretching state of a stretchable display device according to a preferred embodiment of the inventive concept with reference to  FIG. 17 .  FIG. 17  is a flowchart for describing a method for detecting a stretching state of a stretchable display device according to a preferred embodiment of the inventive concept. Referring to  FIG. 17 , in operation S 110 , a substrate part  110  of a stretchable display device  100  may be stretched by an operation of a user. Herein, the substrate part  110  may be implemented in a pre-stretch scheme to have a wavy structure and may include a plurality of pixel elements  111  formed on the substrate part  110 . 
     Thereafter, the stretchable display device  100  may detect a stretching state of the substrate part  110  based on a change in voltage or capacitance of a sensing line part  150  located at an upper side of the substrate part  110  according to stretch of the substrate part  110 . Herein, the sensing line part  150  may be connected with a signal wire of each of the plurality of pixel elements  111  formed on the substrate part  110 . 
     In other words, because the signal wire of the pixel element  111  and the sensing line part  150  are opened before the substrate part  110  is stretched, a voltage of the sensing line part  150  may not be changed. Because the signal wire of the pixel element  111  and the sensing line part  150  are changed to a short-circuited state after the substrate part  110  is stretched, a voltage of the sensing line part  150  may be changed. Furthermore, the sensing line part  150  may be connected with the signal wire of the pixel element  111  through a capacitor, and a capacitance of the sensing line part  150  may be changed after the substrate part  110  is stretched. 
     As a distance between the pixel element  111  formed on the substrate part  110  and the sensing line part  150  is changed according to the stretch of the substrate part  110 , when a change in voltage or capacitance of the sensing line part  150  is detected, the stretchable display device  100  may detect a stretching state of the substrate part  110  based on the detected change in voltage or capacitance of the sensing line part  150  using stretching state information for each predetermined voltage or capacitance. 
     Meanwhile, it is described that the one sensing line part  150  according to an embodiment of the inventive concept is located at the upper side of the substrate part  110 , that is, that the one sensing line part  150  is connected with the signal wire of each of the plurality of pixel elements  111  formed on the substrate part  110 , but the inventive concept is not limited thereto. According to an embodiment, a plurality of sensing line parts  150  may be located at the upper side of the substrate part  110 . 
     For example, the first sensing line part  150  may be located on an upper left surface of the substrate part  110 , and the second sensing line part  150  may be located at an upper right surface of the substrate part  110 . The first sensing line part  150  may detect a stretching state of the left surface of the substrate part  110 , and the second sensing line part  150  may detect a stretching state of the right surface of the substrate part  110 . 
     In this case, the first sensing line part  150  may be connected with signal wires of the plurality of pixel elements  111  located on the left surface of the substrate part  110 , and the second sensing line part  150  may be connected with signal wires of the plurality of pixel elements  111  located on the right surface of the substrate part  110 . Thus, a stretching state of the substrate part  110  according to the stretch of the substrate part  110  may be detected for each area (or each left surface or each right surface). 
     Furthermore, when the substrate part  110  is composed of the first substrate part  110  of an area which is not stretched and the second substrate part  110  of an area which is stretched, the one sensing line part  150  according to an embodiment of the inventive concept may be located at an upper side of the second substrate part  110 . In other words, the one sensing line part  150  may be connected with a signal wire of each of the plurality of pixel elements  111  formed on the second substrate part  110 . Of course, the plurality of sensing line parts  150  may be located at an upper side of the second substrate part  110 . Thus, the plurality of sensing line parts  150  may detect a stretching state for only an area which is stretched in the entire area of the substrate part  110 . 
     In the above, even though all elements constituting an embodiment of the inventive concept are described as being combined to one or operating in combination, the inventive concept is not necessarily limited to the embodiment. That is, all the elements may also be selectively combined and operated with each other as one or more elements without departing from the scope of the inventive concept. In addition, although each of all the elements may be implemented by one independent hardware, some or all of the respective elements which are selectively combined with each other may be implemented by a computer program having a program module performing some or all of functions combined with each other in one or plural hardware. Furthermore, such a computer program may be stored in computer readable media such as a USB memory, a CD disk, or a flash memory and may be read and executed by the computer to implement an embodiment of the inventive concept. The computer readable media may include magnetic storage media, optical storage media, or the like. 
     According to an embodiment of the inventive concept, the stretchable display device may be provided to correct display resolution and luminance depending on a stretching degree applied to a stretchable substrate. 
     Furthermore, according to an embodiment of the inventive concept, the stretchable display device may adjust display resolution and luminance without the necessity of a separate strain sensor or a switch which operates by a control signal. 
     Furthermore, according to an embodiment of the inventive concept, the stretchable display device may implement a display depending on a stretching state of a substrate having a wavy structure based on a sensing line disposed on the substrate and may proceed with detecting stretch for each pixel, thus much more precisely detecting stretch and supplementing resolution and luminance. 
     While a few embodiments have been shown and described with reference to the accompanying drawings, it will be apparent to those skilled in the art that various modifications and variations can be made from the foregoing descriptions. Therefore, other implements, other embodiments, and equivalents to claims are within the scope of the following claims.