Patent Publication Number: US-2018028802-A1

Title: Dry electrode

Description:
BACKGROUND 
     The inventive concept relates to a dry electrode, and more particularly to a dry electrode that contacts skin of a user to electrically stimulate the skin of the user. 
     A low frequency treatment refers to using a low frequency for treatment, for example, of nerves or muscles by allowing a low frequency pulse to flow unto the nerves or muscles, and in this regard, studies on an electrode that transfers a low frequency pulse to a living body have been considerably established. Currently, low frequency treatment devices are being widely used, and products that may be used by the public as well as the medical professionals are being released. 
     Meanwhile, a brain electrical stimulation technology using a transcranial direct current stimulation (tDCS) is a stimulation technology that applies a current to the brain, and is known to be effective to improve a recognition ability and treat mental diseases such as depressions or attention deficit hyperactivity disorders (ADHD). 
     Accordingly, if the brain electrical stimulation technology may be used in everyday lives, the brain function may be improved, and mental diseases may be continuously treated by activating or restraining connections between nerves. However, in contrast to the low frequency stimulation apparatus, the electrical stimulation apparatus using tDCS needs to secure a high level of safety as the head is stimulated. 
     PRIOR TECHNICAL DOCUMENTS 
     Patent Documents 
     (Patent Document 1) Korean Patent Application Publication No. 10-2003-0045730 
     In particular, a proper electrode is not used in the transcranial direct current stimulation, current density on the electrode may become uneven. In detail, due to an edge effect of the electrode, a current density at a peripheral portion of the electrode may increase. Accordingly, as a high current may flow to a specific location of the electrode, the skin of the user may be burned under circumstances. In addition, sometime, the patch attached to the electrode may be delaminated or the patch may be thermally deformed. 
     SUMMARY 
     The inventive concept has been made in an effort to solve the aforementioned problems, and provides a dry electrode that can prevent a current density at a peripheral portion of an electrode from increasing, by providing a patch with a hydrogel layer that has high impedance. 
     The inventive concept also provides a dry electrode that can prevent an over-potential from occurring between skin and an electrode by providing a hydrogel layer for a patch containing chorine ions and thus providing common ions between the skin and the electrode. 
     The inventive concept also provides a dry electrode that has a structure that may be adhered to skin. 
     The technical objects of the inventive concept are not limited to the above-mentioned one, and the other unmentioned technical objects will become apparent to those skilled ones in the art from the following description. 
     In accordance with an aspect of the inventive concept, there is provided a dry electrode that electrically stimulates while contacting skin of a user, the dry electrode including a main body including a conductive body, and a nonconductive cover that surrounds the conductive body such that an area of the conductive body is exposed, and a patch detachably mounted on the area of the conductive body that is exposed, one surface of the patch contacting the conductive body and an opposite surface of the patch contacting the skin of the user, the patch being configured to receive a current from the main body to electrically stimulate the skin of the user, wherein the patch includes a first hydrogel layer contacting the conductive body, and a second hydrogel layer having an impedance that is higher than that of the first hydrogel layer and contacting the skin of the user, and wherein a surface of the conductive body, which contacts the patch, has a shape that is deflected along one direction as a whole. 
     In accordance with another aspect of the inventive concept, there is provided an electrical stimulation apparatus including a frame mountable on a user, and a dry electrode formed on one surface of the frame, the dry electrode being configured to electrically stimulate skin of the user while contacting the skin of the user when the frame is mounted on the user, wherein the dry electrode includes a main body including a conductive body, and a nonconductive cover that surrounds the conductive body such that an area of the conductive body is exposed, and a patch detachably mounted on the area of the conductive body that is exposed, one surface of the patch contacting the conductive body and an opposite surface of the patch contacting the skin of the user, the patch being configured to receive a current from the main body to electrically stimulate the skin of the user, wherein the patch includes a first hydrogel layer that contacts the conductive body, and a second hydrogel layer having an impedance that is higher than that of the first hydrogel layer and contacting the skin of the user, and wherein a surface of the conductive body, which contacts the patch, has a shape that is deflected along one direction as a whole. 
    
    
     
       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 of a dry electrode according to an embodiment of the inventive concept; 
         FIG. 2  is an exploded perspective view of a dry electrode of  FIG. 1 ; 
         FIG. 3  is a sectional view of the dry electrode of  FIG. 1  taken along direction x; 
         FIG. 4  is a sectional view of the dry electrode of  FIG. 1  taken along direction y; 
         FIG. 5  is a sectional view of a dry electrode according to a modified embodiment of the inventive concept taken along direction x or direction y; 
         FIG. 6  is a bottom view of a conductive body of  FIG. 2 ; 
         FIG. 7  is a sectional view taken along direction y for explaining a hinge function of the dry electrode of  FIG. 1 ; 
         FIGS. 8 and 9  is a sectional view for explaining a method for coupling the main body and a patch of  FIG. 1 ; 
         FIG. 10  is a perspective view of a dry electrode according to another embodiment of the inventive concept; 
         FIG. 11  is an electrical stimulation apparatus including a dry electrode according to embodiments of the inventive concept; 
         FIG. 12  is a perspective view of a dry electrode according to another embodiment of the inventive concept; 
         FIG. 13  is an exploded perspective view of the dry electrode of  FIG. 12 ; 
         FIG. 14  is an exploded sectional view of the dry electrode of  FIG. 12  taken along direction x; 
         FIG. 15  is a sectional view of the dry electrode of  FIG. 12  taken along direction x; and 
         FIG. 16  is a sectional view of the dry electrode of  FIG. 12  taken along direction y. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the inventive concept will be described in detail with reference to the accompanying drawings. The above and other aspects, features and advantages of the invention will become apparent from the following description of the following embodiments given in conjunction with the accompanying drawings. However, the inventive concept is not limited to the embodiments disclosed below, but may be implemented in various forms. The embodiments of the inventive concept is provided to make the disclosure of the inventive concept complete and fully inform those skilled in the art to which the inventive concept pertains of the scope of the inventive concept. The same reference numerals denote the same elements throughout the specification. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which the inventive concept pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     The terms used herein are provided to describe the embodiments but not to limit the inventive concept. In the specification, the singular forms include plural forms unless particularly mentioned. The terms “comprises” and/or “comprising” used herein does not exclude presence or addition of one or more other elements, in addition to the aforementioned elements. 
     Hereinafter, a dry electrode according to various embodiments of the inventive concept will be described with reference to the accompanying drawings. 
     The dry electrode  1  according to an embodiment of the inventive concept will be described in the following with reference to  FIGS. 1 to 9 .  FIG. 1  is a perspective view of a dry electrode  1  according to an embodiment of the inventive concept,  FIG. 2  is an exploded perspective view of the dry electrode  1  of  FIG. 1 ,  FIG. 3  is a sectional view of the dry electrode  1  of  FIG. 1  taken along direction x,  FIG. 4  is a sectional view of the dry electrode  1  of  FIG. 1  taken along direction y,  FIG. 5  is a sectional view of a dry electrode according to a modified embodiment of the inventive concept taken along direction x or direction y,  FIG. 6  is a bottom view of a conductive body  11  of  FIG. 2 ;  FIG. 7  is a sectional view for explaining a hinge function of the dry electrode  1  of  FIG. 1  taken along direction y, and  FIGS. 8 and 9  is a sectional view for explaining a method for coupling a main body  10  and a patch  20  of  FIG. 1 . 
     The dry electrode  1  according to an embodiment of the inventive concept may electrically stimulate skin of the user while contact it. For example, a current may be supplied to the dry electrode  1  through a conductive wire  13  extending from the outside of the dry electrode  1  to the inside of the dry electrode  1 . Further, because the dry electrode  1  may in contact with the skin of the user, a current may be transferred to the skin of the user through the dry electrode  1  so that the user may be electrically stimulated. 
     Referring to  FIGS. 1 and 2 , the dry electrode  1  may include a main body  10  and a patch  20  that may be attached to and detached from the main body  10 . Further, the main body  10  may include a conductive body  11 , a nonconductive cover  12 , and a conductive wire  13 , and the patch  20  may include first and second hydrogel layers  21  and  22 . However, the elements of  FIGS. 1 and 2  are not essential, and thus a dry electrode  1  having a larger number of elements or a smaller number of elements may be implemented. 
     First, the main body  10  will be described. The main body  10  may include a conductive body  11  and a nonconductive cover  12  that surrounds the conductive body  11  such that an area of the conductive body  11  is exposed. If necessary, the conductive wire  13  may be connected to the conductive body  11 . 
     Referring to  FIG. 2 , the conductive body  11  may include a first area  11   a  having a first width w 1  and a second area  11   b  having a second width w 2  that is larger than the first width w 1 . Further, the conductive body  11 , for example, may have a shape in which the first area  11   a  is formed on the second area  11   b  and the first area  11   a  may protrude from the second area  11   b  along a third direction (direction z), but the inventive concept is not limited thereto. Further, the shape of the first area  11   a  is not limited to a cylindrical shape. 
     Referring to  FIGS. 1, 3, and 4 , the conductive wire  13  may be connected to the first area  11   a  of the conductive body  11 . Accordingly, a current provided from the conductive wire  13  may flow to the second area  11   b  of the conductive body  11  via the first area  11   a  of the conductive body  11 . That is, because a current flows from the first area  11   a  having a relatively small width to the second area  11   b  having a relatively large width, an electrical stimulus may be applied to skin of the user by using an area that is as wide as the area of a section of the second area  11   b  on the x-y plane. Accordingly, according to the dry electrode  1  according to the present embodiment, an electrical stimulus may be easily applied to a wide area. 
     In detail, the second area  11   b  of the conductive body  11  may have a surface that is bonded to the patch  20 , and may be easily deformed due to flexible characteristics to help the patch  20  be adhered to the skin. Further, the first area  11   a  of the conductive body  11  may be an area that supports the second area  11   b  that is a deformed area and transfers a current to the conductive body  11 . 
     Here, the second width w 2  of the conductive body  11  may be smaller than a width w 5  of the patch  20 . That is, the cross-sectional area of the conductive body  11  on the x-y plane may be smaller than the cross-sectional area of the patch  20  on the x-y plane. According to the structural characteristics, a bonding surface of the second area  11   b  of the conductive body  11  may be completely covered by the patch  20  when the patch  20  is bonded to the second area  11   b  of the conductive body  11 . Accordingly, because the second area  11   b  of the conductive body  11  is covered by the patch  20 , the patch  20  may contact skin of the user when the dry electrode  1  contacts the skin of the user, and because the conductive body  11  may be prevented from directly contacting the skin of the user, a safety of the user who uses the dry electrode  1  may be secured. 
     Further, referring to  FIGS. 2 to 4 , the second area  11   b  of the conductive body  11  may have a rectangular parallelepiped shape as a whole, and for example, may have a deflected (bent) shape along a first direction (direction x). That is, the second area  11   b  of the conductive body  11  may have a shape that is convex towards the first area  11   a,  but the inventive concept is not limited thereto. In detail, referring to the sectional view taken along the first direction (direction x) of  FIG. 3 , it may be identified that the conductive body  11  is deflected (bent) along the first direction (direction x) when the conductive body  11  has a shape that is deflected (bent) along the first direction (direction x), and referring to the sectional view according to the second direction (direction y) of  FIG. 4 , it may be identified that the conductive body  11  is not defected along the second direction (direction y). 
     The dry electrode  1  may be used for an electrical stimulation apparatus that applies an electrical stimulus, for example, to a head, and because the conductive body  11  is curved along the first direction (direction x) as a whole, the dry electrode  1  may be easily adhered to the head in consideration of the shape of the head of a person is not flat but curved. Accordingly, the dry electrode  1  according to an embodiment of the inventive concept may have an excellent adherence property. 
     However, the shape of the conductive body  11  is not limited thereto, and in some embodiments, the second area  11   b  of the conductive body  11  may not be a deflected (curved) along the first direction (direction x) as a whole but a surface of the conductive body  11 , which contacts the patch  20 , may be deflected (curved), for example, along the first direction (direction x). 
     Further, in some embodiments, a central portion of a surface of the conductive body  11 , which contacts the patch  20 , may be concave such that the dry electrode  1  may be adhered to the forehead of a person. For example, referring to  FIG. 5 , it may be identified that a surface of the conductive body  11 , which contacts the patch, may be deflected (curved) along the first direction (direction x) and the second direction (direction y). Through this, it may be seen that a central portion of a surface of the conductive body  11 , which contacts the patch  20  has a depressed shape. Further, referring to  FIGS. 2 to 6 , a plurality of protrusions  11   c  may be formed in an area of the conductive body  11 , which contacts the patch  20 , that is, in an area of the second area  11   b  of the conductive body  11 , which contacts the patch  20 . The protrusions  11   c  may protrude from the second area  11   b  of the conductive body  11  along a third direction (direction z). Although lattice type protrusions are illustrated as the protrusions  11   c  in the drawings, the inventive concept is not limited thereto, but in some embodiments, the protrusions  11   c  may be replaced by protrusions of various shapes, such as straight protrusions, rectangular protrusions, honey-combed protrusions, spike-shaped protrusions, and spiral protrusions. 
     Grooves  11   d  may be formed between the protrusions  11   c.  A plurality of grooves  11   d  may be regularly formed in an area of the conductive body  11 , which contacts the patch  20 , that is, in an area of the second area  11   b  of the conductive body  11 , which contacts the patch  20 . For example, referring to  FIG. 6 , the grooves  11   d  may have a tetragonal shape, but the inventive concept is not limited thereto. However, in some embodiments, the grooves  11   d  may be irregularly formed, but the depth and sectional shape of the grooves  11   d  may be various. 
     Because the protrusions  11   c  may act as cushions or impact absorbers when the dry electrode  1  is adhered to skin of the user to contact the skin of the user, the dry electrode  1  according to an embodiment of the inventive concept may be adhered to the head with an excellent adherence performance. 
     Meanwhile, because the conductive body  11  is conductive and one end of the conductive body  11  is connected to the conductive wire  13 , the conductive body  11  may receive a current from the conductive wire  13  to transfer the current to the patch  20 . 
     Further, the conductive body  11  may be flexible, the shape of the conductive body  11  may be partially changed if necessary. For example, referring to  FIG. 7 , because an angle between the first area  11   a  and the second area  11   b  changed by using flexible characteristics of the conductive body  11 , the conductive body  11  may have a hinge function. However, the dry electrode  1  may have a hinge function in other methods than the method of using the flexible characteristics of the conductive body  11 . Accordingly, by using the dry electrode  1  according to the embodiment of the inventive concept, the dry electrode  1  may be easily adhered to the skin of the user through changing the angle or the shape of the dry electrode  1 . 
     The conductive body  11  may include conductive silicon or flexible silicon to secure the conductivity and flexible characteristics to secure the conductivity and flexible characteristics, but the material of the conductive body  11  is not limited thereto. 
     The nonconductive cover  12  may surround the conductive body  11  such that an area of the conductive body  11  is exposed. In detail, referring to  FIGS. 2 to 4 , almost all the areas of the conductive body  11 , except for a bottom surface of the second area  11   b  having the protrusions  11   c  and a top surface of the first area  11   a  connected to the conductive wire  13 , may be covered by the nonconductive cover  12 . Because the nonconductive cover  12  is formed of a nonconductive material and surrounds an outer surface of the dry electrode  1 , an area of the dry electrode  1 , which may contact a hand of the user, may be insulated. Accordingly, the user may safely use the dry electrode  1  by using the dry electrode  1  according to the embodiment of the inventive concept. 
     In detail, referring to  FIGS. 2 to 4 , the nonconductive cover  12  may include a third area  12   a  having a third width w 3  and a fourth area  12   b  having a fourth width w 3  that is larger than the third width w 3 . Further, the nonconductive cover  12 , for example, may have a shape in which the third area  12   a  is formed on the fourth area  12   b  and the third area  12   a  may protrude from the fourth area  12   b  along the third direction (direction z), but the inventive concept is not limited thereto. Further, the shape of the third area  12   a  is not limited to a cylindrical shape. 
     Meanwhile, the conductive body  11  may be coupled to the nonconductive cover  12 . To achieve this, because an empty space, to which the conductive body  11  may be coupled, may be formed in the interior of the nonconductive cover  12 , the conductive body  11  may be inserted into the nonconductive cover  12 . For example, a hole  12   c  may be formed in the third area  12   a  of the nonconductive cover  12 , and an empty space also may be formed in the fourth area  12   b  of the nonconductive cover  12 . Accordingly, the first area  11   a  of the conductive body  11  may be inserted into the third area  12   a  of the nonconductive cover  12 , the second area  11   b  of the conductive body  11  may be inserted into the fourth area  12   b  of the nonconductive cover  12 , and the protrusions  11   c  of the conductive body  11  may be exposed. To achieve this, the first width w 1  of the first area  11   a  may be made to be narrower than the third width w 3  of the third area  12   a,  and the second width w 2  of the second area  11   b  may be made to be narrower than the fourth width w 4  of the fourth area  12   b.    
     In addition, the conductive wire  13  may extend from the outside of the dry electrode  1  to the inside of the dry electrode  1 , and for example, may contact a portion of the conductive body  11 . 
     Next, the patch  20  will be described. The patch  20  may be attached to and detached from an area of the main body  10 , to which the conductive body  11  is exposed, and one surface of the patch  20  may contact the conductive body  11  and an opposite surface of the patch  20  may contact skin of the user. Further, the patch  20  may receive a current from the main body  10  to electrically stimulate the skin of the user. 
     In detail, referring to  FIGS. 1 to 4 , the patch  20  may be attached to and detached from a bottom surface of the main body  10 , that is, a surface of the conductive body  11 , on which a lattice type structure is formed. Although a bonding layer (not illustrated) may be formed, for example, on one surface of the patch  20  for attachment and detachment of the patch  20 , the method of attaching and detaching the patch  20  is not limited thereto but various methods may be used. 
     For example, the first hydrogel layer  21  of the patch  20  may contact the conductive body  11 , and the first hydrogel layer  21  has a high bonding force of not less than 200 gf/cm. Accordingly, because the first hydrogel layer  21  is attached to one surface of the second hydrogel layer  22  and another surface of the second hydrogel layer  22  is bonded to the conductive body  11 , the patch  20  may be attached to the conductive body  11 , and the patch  20  may be separated from the conductive body  11  if necessary. 
     In addition, referring to  FIG. 8 , magnets  14  and  24  may be formed in at least one of the main body  10  and the patch  20  so that the patch  20  may be attached to and detached from the main body  10  through a magnetic force. Further, referring to  FIG. 9 , coupling structures  15  and  25  may be formed in at least one of the main body  10  and the patch  20  so that the patch  20  may be attached to and detached from the main body  10  through coupling of the coupling structures  15  and  25 . The coupling structures  15  and  25  may be structures for male/female coupling, such as snap buttons (press buttons) or may be structures for mechanical coupling, such as latches, but the inventive concept is not limited thereto. 
     In this way, according to the dry electrode  1  according to the embodiment of the inventive concept, because the patch  20  may be attached and detached, it may be easily replaced when it is deformed or worn. 
     Meanwhile, the patch  20  may include first and second hydrogel layers  21  and  22 . 
     In detail, the first hydrogel layer  21  may contact the conductive body  11 , and in more detail, may contact the protrusions  11   c.  Accordingly, the first hydrogel layer  21  may transfer a current provided from the conductive body  11  to the second hydrogel layer  22 , and one surface of the patch  20 , which contacts the conductive body  11 , may be one surface of the first hydrogel layer  21 . 
     A bonding force of the second hydrogel layer  22  is as low as not more than 200 gf/cm. Accordingly, the second hydrogel layer  22  may not be stuck to skin of the user. Accordingly, the dry electrode  1  may be used for a wearable device that is repeatedly mounted on and dismounted from the user, and in detail, may be used for a transcranial current stimulation apparatus. 
     Further, the second hydrogel layer  22  may contact skin of the user while having an impedance that is higher than that of the first hydrogel layer  21 . For example, a transcranial direct current stimulation is performed by using the dry electrode  1 , the second hydrogel layer  22  may have an impedance of not less than 4 kΩ. Accordingly, according to the dry electrode  1  according to the embodiment of the inventive concept, because the second hydrogel layer  22  has a high impedance, a current density of a peripheral portion of the electrode may be prevented from increasing due to an edge effect of the electrode. Accordingly, because a constant current density may be maintained in an entire area of the patch  20 , an electrical stimulus may be stably applied through the dry electrode  1 . 
     Further, skin includes keratin having a relatively high impedance and other parts having relatively low impedances, and because a current may easily flow through the other parts of the skin having low impedances, the current may be concentrated on an area while flowing. However, according to the dry electrode  1  according to the embodiment of the inventive concept, because an influence by an impedance difference between the keratin and the other parts may be decreased by the second hydrogel layer  22  of a high impedance, a current may be prevented from being concentrated on a portion of the skin while flowing and a uniform current density may be formed in an entire area of skin, which contacts the dry electrode  1 . Accordingly, an electrical stimulus may be stably applied through the dry electrode  1 . 
     Further, the second hydrogel layer  22  may include chlorine ions. Because skin of the user also include chlorine ions, the skin of the user and the second hydrogel layer  22 , which contacts the skin of the user include common icons. Through this, because an over-potential formed between the dry electrode  1  and the skin of the user may be lowered, a stimulus of an unnecessary magnitude may be prevented from being formed in the skin of the user. Accordingly, an electrical stimulus may be stably applied through the dry electrode  1 . However, in some embodiments, the first hydrogel layer  21  as well as the second hydrogel layer  22  may include chlorine ions. 
     Meanwhile, because the first hydrogel layer  21  may function as a compressible cushion when the dry electrode  1  contacts skin of the user, the dry electrode  1  may be adhered to the skin of the user. 
     The dry electrode  2  according to an embodiment of the inventive concept will be described in the following with reference to  FIG. 10 . However, a difference from the dry electrode  1  according to an embodiment of the inventive concept will be mainly described. Referring to  FIG. 10 , a perspective view of a dry electrode  2  according to another embodiment of the inventive concept is illustrated. 
     Referring to  FIG. 10 , the dry electrode  2  may further include a delamination layer  30  that covers the second hydrogel layer  22  of the patch  20 . The delamination layer  30  is a cover for protecting the patch  20 , and may protect the patch  20  before use of the dry electrode  2 . Accordingly, the delamination layer  30  may be removed during use of the dry electrode  2 , and the second hydrogel layer  22  may contact skin of the user. 
     Hereinafter, an electrical stimulation apparatus  40  including the dry electrode  1  according to embodiments of the inventive concept will be described with reference to  FIG. 11 . Hereinafter, referring to  FIG. 11 , the electrical stimulation apparatus  40  including the dry electrode  1  according to embodiments of the inventive concept is illustrated. 
     Referring to  FIG. 11 , the electrical stimulation apparatus  40  may include a frame  41  and a dry electrode  1  attached to the frame  41 . The dry electrode  1  may be attached to an inner surface of the frame  41 , and it may contact the head of the user when the electrical stimulation apparatus  40  is mounted on the head. Meanwhile, although the frame  41  of  FIG. 11  has a shape that may be mounted on the head of the user, any shape that may be mounted on the user is possible while the inventive concept is not limited thereto, and thus the electrical stimulation apparatus  40  may be implemented by various wearable devices. 
     Because the outside of the dry electrode  1  is surrounded by the nonconductive cover  12 , the user may safely use the dry electrode  1 . Further, a portion of the dry electrode  1 , which contacts the frame  41 , has a hinge function by which an angle of the dry electrode  1  may be adjusted, the user may easily adhere the dry electrode  1  to his or her head. 
     The dry electrode  3  according to another embodiment of the inventive concept will be described in the following with reference to  FIGS. 12 to 16 . However, a difference from the dry electrode  1  according to an embodiment of the inventive concept will be mainly described. Referring to  FIG. 12 , a perspective view of a dry electrode  3  according to another embodiment of the inventive concept is illustrated.  FIG. 13  is an exploded perspective view of the dry electrode  3  of  FIG. 12 ,  FIG. 14  is an exploded sectional view of the dry electrode  3  of  FIG. 12  taken along direction x,  FIG. 15  is a sectional view of the dry electrode  3  of  FIG. 12  taken along direction x, and  FIG. 16  is a sectional view of the dry electrode  3  of  FIG. 12  taken along direction y. 
     Referring to  FIGS. 12 to 16 , the dry electrode  3  according to the present embodiment may further include a second conductive body  16  in addition to a first conductive body  11 , unlike the dry electrode  1  according to the previous embodiment of the inventive concept. 
     That is, referring to  FIGS. 12 to 14 , the main body  10  may include a first conductive body  11  one surface of which is opened and that has a first space  11   e  in the interior thereof, a nonconductive cover  12  that surrounds the first conductive body  11  such that the opened surface of the first conductive body  11  is exposed, and a second conductive body  16  that covers the opened surface of the first conductive body  11 . Further, the second conductive body  16  may include a first surface  16   a  that contacts the first conductive body  11  and a second surface  16   b  that is opposite to the first surface  16   a.    
     In detail, referring to  FIGS. 12 to 14 , the first space  11   e  may be formed in the interior of the first conductive body  11 . The first space  11   e  is an empty space, and may be a space for increasing a degree of freedom by which the patch  20  may be deformed according to the head shape of the user when the dry electrode  3  contacts the head of the user. Because one surface (for example, a lower surface) of the first conductive body  11  may be opened, the first space  11   e  may be exposed through the opened lower surface. 
     Further, referring to  FIGS. 13 to 16 , the second conductive body  16  may include a first surface  16   a  and a second surface  16   b,  and the second surface  16   b  may be a surface that is opposite to the first surface  16   a.    
     The second conductive body  16  may be formed to cover the opened surface of the first conductive body  11 , and then the first surface  16   a  of the second conductive body  16  may fact the first conductive body  11 . Referring to  FIGS. 15 and 16 , the first space  11   e  may be preserved while being surrounded by the first conductive body  11  and the first surface  16   a  of the second conductive body  16  even though the opened surface of the first body  11  is covered by the second conductive body  16 . Accordingly, according to the dry electrode  3  according to the present embodiment, because a degree of freedom by which the patch  20  may be deformed according to the head shape of the user as the first space  11   e  of the main body  10  is preserved, the dry electrode  3  may be easily deformed due to the flexible characteristics, which may help the patch  20  to be adhered to skin. 
     As long as the second conductive body  16  may covers the opened surface of the first conductive body  11  while preserving the first space  11   e  of the first conductive body  11 , the shape of the second conductive body  16  is not limited. However, referring to  FIGS. 13 to 16 , the second conductive body  16  may have a shape one surface (for example, an upper surface) of which is opened, and a second space  16   e  that is an empty space may be formed in the interior of the second conductive body  16 . The second space  16   e  may be exposed through the opened upper space, and at least a portion of the second space  16   e  may be surrounded by the first surface  16   a  of the second conductive body  16 . Because the second conductive body  16  includes the second space  16   e,  the first space  11   e  may be preserved by allowing the first space  11   e  and the second space  16   e  to overlap each other even though the opened surface of the first conductive body  11  is covered by the second conductive body  16 . 
     In detail, referring to  FIGS. 15 and 16 , the second conductive body  16  may be inserted into the main body  10  by inserting the second conductive body  16  into the first space  11   e  of the first conductive body  11 , but the coupling method is not limited thereto and various coupling forms such as protrusions may be used. When the second conductive body  16  is inserted into the first space  11   e  of the first conductive body  11 , the second conductive body  16  may cover the opened surface of the first conductive body  11  and the opened surface of the second conductive body  16  may face the inner surface of the first conductive body  11 . Accordingly, the first space  11   e  of the first conductive body  11  and the second space  16   e  of the second conductive body  16  may overlap each other, an empty space may be present in the main body  10 . 
     Meanwhile, referring to  FIGS. 13 to 16 , a plurality of grooves  16   d  may be formed on the first surface  16   a  of the second conductive body  16 . That is, the grooves  11   d  may be regularly formed on the first surface  16   a  that is opposite to the second surface  16   b  of the second conductive body  16 , which contacts the patch  20 . For example, although the groove  16   d  may have a tetragonal sectional shape such that a lattice structure is formed on the first surface  16   a  of the second conductive body  16 , the inventive concept is not limited thereto. However, in some embodiments, the grooves  16   d  may be irregularly formed, but the depth and sectional shape of the grooves  16   d  may be various. 
     As the plurality of grooves  16   d  are formed on the first surface  16   a  of the second conductive body  16 , a plurality of protrusions  16   c  may be formed on the first surface  16   a,  and for example, the protrusions  16   c  may protrude along a third direction (direction z). Although lattice type protrusions are illustrated as the protrusions  16   c  in the drawings, the inventive concept is not limited thereto, but in some embodiments, the protrusions  16   c  may be replaced by protrusions of various shapes, such as straight protrusions, rectangular protrusions, honey-combed protrusions, spike-shaped protrusions, and spiral protrusions. 
     As described above, because a thickness of the second conductive body  16  may decrease as the plurality of grooves  16   d  are formed on the first surface  16   a  of the second conductive body  16 , it may help the patch  20  to be easily deformed according to the head shape of the user when the patch  20  is coupled to the second conductive body  16 . 
     In addition, the first and second conductive bodies  11  and  16  may include conductive silicon or flexible silicon to secure the conductivity and flexible characteristics, but the materials of the first and second conductive body  11  and  16  are not limited thereto. 
     The patch  20  may be attached to and detached from the second surface  16   b  of the second conductive body  16 , and may be a patch  20  one surface of which contacts the second surface  16   b  of the second conductive body  16  and an opposite surface of which contacts skin of the user. Further, the patch  20  may receive a current through the first and second conductive bodies  11  and  16  to electrically stimulate skin of the user, and the descriptions of the embodiments of the inventive concept may be applied. 
     Here, the second width w 2  of the first conductive body  11  and the width w 6  of the second surface  16   b  of the second conductive body  16  may be narrower than the width w 5  of the patch  20 . That is, the cross-sectional area of the first conductive body  11  on the x-y plane and the cross-sectional area of the second surface  16   b  of the second conductive body  16  on the x-y plane may be smaller than the cross-cross-sectional area of the patch on the x-y plane. Due to the structural characteristics, a bonding surface between the second area  11   b  of the first conductive body  11  and the second area  16   b  of the second conductive body  16  may be completely covered by the patch  20  when the patch  20  is bonded to the second area  11   b  of the first conductive body  11  and the second surface  16   b  of the second conductive body  16 . Accordingly, because the patch  20  may come into contact with skin of the user and the first conductive body  11  and the second conductive body  16  may be prevented from coming into direct contact with the skin of the user when the dry electrode  1  contacts the skin of the user because the second area  11   b  of the first conductive body  11  and the second surface  16   b  of the second conductive body  16  are covered by the patch  20 , the safety of the user who uses the dry electrode  3  can be secured. 
     According to the inventive concept, a current density at a peripheral portion of the electrode can be prevented from increasing by providing a hydrogel layer including a patch having a high impedance. 
     Further, according to the inventive concept, because a hydrogel layer for a patch contains chlorine ions, common ions are present between skin and an electrode so that an over voltage can be prevented from occurring between skin and the electrode. 
     Furthermore, according to the inventive concept, because the dry electrode has a structure that may be adhered to skin, an electrical stimulus may be stably applied to a designated location. 
     However, the effects of the inventive concept are not limited to the above-mentioned ones. 
     Although the exemplary embodiments of the inventive concept have been described with reference to the accompanying drawings, it will be understood by those skilled in the art to which the inventive concept pertains that the inventive concept can be carried out in other detailed forms without changing the technical spirits and essential features thereof. Therefore, the above-described embodiments are exemplary in all aspects, and should be construed not to be restrictive.