Abstract:
Disclosed herein are a three-dimensional stretchable electronic device and a manufacturing method comprising the same, wherein the three-dimensional stretchable electronic device is configured such that a connection line is positioned therein and thus can be protected from the outside, and the connection line is made of a liquid metal so there is no change in volume of the connection line upon stretching. Additionally, elements can be transferred to both sides of the substrate, thus increasing the degree of integration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/KR2014/000174 filed on Jan. 8, 2014, which claims priority to Korean Application No. 10-2013-0124090 filed on Oct. 17, 2013, which applications are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a three dimensional stretchable electronic device and a manufacturing method comprising the same. 
       BACKGROUND ART 
       [0003]    Currently, electronic device process techniques are focused on manufacturing flexible or stretchable electronic devices so as to achieve wearable computers. In particular, process techniques for stretchable electronic devices are being ambitiously developed based on the prediction that silicon-based hard electronic devices will be changed in the future into flexible electronic devices that may be utilized in daily lives. 
         [0004]    Accordingly, thorough research into advancements in process techniques for stretchable electronic devices is ongoing. Especially, stretchable electronic devices having elements and connection lines in two-dimensional form have been developed, and the structure thereof where strain is intensively applied to the connection lines is in the spotlight. A manufacturing method of a stretchable electronic device includes forming driving elements and electronic elements having connection lines able to absorb strain on an existing hard substrate, and then transferring the electronic elements to a stretchable polymer-based film. The connection lines of the stretchable electronic device thus manufactured have a zigzag structure or a pop-up structure so as to be adapted for two-dimensional stretching, and may thus absorb most strain applied thereto. 
         [0005]    However, technical limitations are recently imposed on such process techniques. Existing studies have proposed techniques in which elements and connection lines are two dimensionally integrated on a plane so as to be stretchable. As such, the connection lines covered with only a very thin protective layer may undergo intense strain. Hence, such a two-dimensional stretchable electronic device is very weak to external force, and thereby shorting of the connection lines may easily occur and integration is possible only on a plane, making it difficult to achieve integration in a vertical structure and thus exhibiting a low degree of integration. 
       SUMMARY OF DISCLOSURE 
       [0006]    The present invention has been made keeping in mind the above problems, and an object of the present invention is to provide a manufacturing method of a three-dimensional stretchable electronic device, which may reduce strain applied to a connection line and may protect the connection line from the outside. 
         [0007]    Another object of the present invention is to provide a three-dimensional stretchable electronic device, in which elements are transferred to both sides of a substrate to thus increase the degree of integration of the three-dimensional stretchable electronic device. 
         [0008]    In order to accomplish the above objects, the present invention provides a manufacturing method of a three-dimensional stretchable electronic device, comprising: (1) preparing an aluminum mold ( 100 ) for producing a substrate having one or more protrusions on an upper side and a lower side thereof; (2) forming a path for a connection line for connecting the protrusions of the substrate using a wire ( 110 ); (3) introducing a first polymer ( 120 ) for forming the protrusions of the substrate into a predetermined portion of the aluminum mold, semi-curing the first polymer, introducing a second polymer ( 130 ) into a remaining portion of the aluminum mold, and curing the second polymer, thus forming a three-dimensional stretchable substrate having the wire in the aluminum mold; (4) removing the wire and the three-dimensional stretchable substrate from the aluminum mold; (5) injecting a liquid metal ( 140 ) into the path for a connection line from which the wire was removed, thus manufacturing a three-dimensional stretchable substrate having a connection line; and (6) transferring elements ( 150 ) to the protrusions of the three-dimensional stretchable substrate having the connection line and connecting the elements to the connection line, thus connecting the to elements to each other. 
         [0009]    In addition, the present invention provides a three-dimensional stretchable electronic device manufactured by the manufacturing method as above. 
         [0010]    According to the present invention, a three-dimensional stretchable electronic device is configured such that a connection line is positioned in a substrate and thus can be protected from the outside. When a liquid metal is used for the connection line, there are no changes in volume of the connection line upon stretching, so that shorting cannot occur even upon external deformation, making it possible to easily connect the elements to each other. 
         [0011]    In addition, elements can be transferred to both sides of the substrate, thus effectively increasing the degree of integration of the three-dimensional stretchable electronic device. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a schematic view illustrating a process of manufacturing a three-dimensional stretchable electronic device ( 100 : aluminum mold,  110 : wire,  120 : first polymer,  130 : second polymer,  140 : liquid metal, and  150 : element); 
           [0013]      FIG. 2  is an optical image illustrating a three-dimensional stretchable substrate; 
           [0014]      FIG. 3  is an optical image illustrating a lateral side of the three-dimensional stretchable substrate; 
           [0015]      FIG. 4(   a ) is a view illustrating a thin film having a liquid connection line connected with an additional connection line for measurement of electric power, and  FIG. 4(   b ) is a view illustrating a three-dimensional stretchable substrate having a liquid metal connection line connected with an additional connection line for measurement of electric power; and 
           [0016]      FIG. 5  is a graph illustrating resistance of the connection line upon stretching of the three-dimensional stretchable substrate of Example 1 and the thin film. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Hereinafter, a detailed description will be given of the present invention. 
         [0018]    The present invention addresses a manufacturing method of a three-dimensional stretchable electronic device, as described below: (1) preparing an aluminum mold  100  for producing a substrate having one or more protrusions on upper and lower sides thereof; (2) forming a path for a connection line for connecting the protrusions of the substrate using a wire  110 ; (3) introducing a first polymer  120  for forming the protrusions of the substrate into a predetermined portion of the aluminum mold, semi-curing the first polymer, introducing a second polymer  130  having lower hardness than the first polymer  120  in the remaining portion of the aluminum mold, and curing the second polymer, thus obtaining a three-dimensional stretchable substrate having the wire in the aluminum mold; (4) removing the wire and the three-dimensional stretchable substrate from the aluminum mold; (5) injecting a liquid metal  140  into the path for a connection line from which the wire was removed, thus obtaining a three-dimensional stretchable substrate having a connection line; and (6) transferring elements  150  to the protrusions of the three-dimensional stretchable substrate having the connection line, and connecting the elements to the connection line, thus connecting the elements to each other. 
         [0019]    In the present invention, the aluminum mold  100  is employed after coating of the surface thereof with Teflon. When the surface of the aluminum mold is coated with Teflon in this way, the Step (4) for removing the three-dimensional stretchable substrate made of a polymer from the aluminum mold may be easily performed. 
         [0020]    In the Step (2), a wire having a diameter of 200 to 300 μm is inserted into the aluminum mold to make a path for a connection line capable of connecting elements, after which a liquid metal  140  is injected into a space formed by removing the wire, thus forming the connection line. 
         [0021]    In the Step (3), the first polymer  120  and the second polymer  130 , which are introduced into the aluminum mold  100 , may include one or more selected from the group consisting of PDMS (polydimethyl siloxane), Ecoflex, PVA (poly vinyl alcohol) to and Dragon skin  10  slow. The first polymer  120  introduced into the recesses of the aluminum mold may be PDMS, and the second polymer  130  introduced into the remaining portion of the aluminum mold may be a mixture of PDMS and Ecoflex which have lower hardness than the first polymer  120  and may preferably be mixed at a mass ratio of 5:5 to 2:8. The three-dimensional stretchable substrate may be prepared when the second polymer  130  has lower hardness than the first polymer  120 . 
         [0022]    Furthermore, in the Step (3), the first polymer  120  is introduced into the recesses of the aluminum mold and then semi-cured, after which the second polymer  130  is introduced into the remaining portion of the aluminum mold and then cured, thus making a three-dimensional stretchable substrate having the wire therein. If the first polymer  120  is not semi-cured, when the second polymer  2  is introduced into the remaining portion of the aluminum mold, it may be mixed with the first polymer that may flow down. Also, to defoam the first and the second polymer before curing of the introduced second polymer, the polymer may be stored in a vacuum and then cured. Even when the polymer is introduced, the wire in the aluminum mold is not deformed. 
         [0023]    In the Step (4), the three-dimensional stretchable substrate formed in the aluminum mold is removed from the aluminum mold, after which the wire is removed. In the Step (5), the liquid metal  140  is injected into the path for a connection line produced by removal of the wire, thus manufacturing the three-dimensional stretchable substrate having the connection line. 
         [0024]    Since a liquid metal such as EGaIn or EGaInSn is used for the connection line in the present invention, the connection line is not changed in volume even upon stretching the substrate, and may connect the elements even upon external deformation. Also, since the connection line is positioned in the substrate, the connection line may be safely protected from the external environment. 
         [0025]    In the Step (6), the elements  150  are transferred to the protrusions of the three-dimensional stretchable substrate, and then connected to the connection line, finally manufacturing a three-dimensional stretchable electronic device. As such, the elements may be transferred to both sides of the three-dimensional stretchable substrate, so that the three-dimensional stretchable electronic device according to the present invention may exhibit a high degree of integration, compared to existing electronic devices. 
         [0026]    In addition, the present invention addresses a three-dimensional stretchable electronic device manufactured by the above manufacturing method. The three-dimensional stretchable electronic device enables connection of the elements to each other without shorting even upon external deformation, and thus may be applied to a variety of fields including wearable computers or stretchable devices. 
         [0027]    A better understanding of the present invention may be obtained through the following example and test example which are set forth to illustrate, but are not to be construed as limiting the scope of the present invention. 
       Example 1 
     Manufacture of Three-dimensional Stretchable Electronic Device 
       [0028]    An aluminum mold, the surface of which had been coated with Teflon, was connected with a wire having a diameter of 300 μm, thus making a path for a connection line. Subsequently, PDMS was introduced into the inner recesses of the aluminum mold having the wire therein and then semi-cured in a dry oven at 65° C. for 13 min Subsequently, a mixture of PDMS and Ecoflex at a mass ratio of 2:8 was introduced into the remaining portion of the aluminum mold, stored in a vacuum for 30 min so as to be defoamed, and then cured in a dry oven at 65° C. for 1 hr 20 min, thus manufacturing a three-dimensional stretchable substrate having the wire therein. 
         [0029]    Thereafter, the cured three-dimensional stretchable substrate was removed from the aluminum mold, the wire was removed, and a liquid metal (EGaIn) was injected into an empty space formed by removal of the wire, thus forming a connection line. 
         [0030]    Thereafter, elements were transferred to the PDMS protrusions of both sides of the three-dimensional stretchable substrate and then connected to the connection line, thus completing the connection between the elements, ultimately manufacturing a three-dimensional stretchable electronic device. 
       Test Example 1 
     Comparison of Resistance of Connection Line Upon Stretching 
       [0031]    The resistance values of the connection line upon stretching of an existing thin film and the three-dimensional stretchable substrate of Example 1 according to the present invention were compared. The results are graphed in  FIG. 5 . 
         [0032]    To compare the resistance values due to stretching, a liquid connection line was introduced into each of the thin film and the three-dimensional stretchable substrate and an additional connection line for measurement of electric power was connected thereto, followed by stretching ( FIGS. 4(   a ) and  4 ( b )). 
         [0033]    When the three-dimensional stretchable substrate of Example 1 was stretched up to 80%, there were no changes in the resistance, and a low resistance value was maintained. However, even when the existing thin film was stretched by 10%, the resistance was drastically increased. This resistance value was maintained up to 30% stretching and then increased about 10 6  times from 40% stretching. 
         [0034]    Although the resistance of the connection line of the existing thin film was remarkably increased upon stretching, the resistance of the three-dimensional stretchable substrate according to the present invention was not increased even upon stretching, and a low resistance value was uniformly maintained. This is considered to be due to the following: upon stretching of the existing thin film, both the liquid connection line and the portion for measurement of electric power are affected and thus the connection may be easily broken; however, upon stretching of the three-dimensional stretchable substrate according to the present invention, the liquid connection line portion may be significantly stretched and also there is almost no deformation in the protrusions, thus preventing shorting between the portion for measurement of electric power and the liquid connection line.