Patent Publication Number: US-10786175-B2

Title: Sensors for measuring skin conductivity and methods for manufacturing the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2017-0003653, filed Jan. 10, 2017, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to a sensor for measuring skin conductivity and a method of manufacturing the same. 
     Description of the Related Art 
     Medical research has shown that stress has a negative effect on persons. Examples of such negative effects include reduced concentration, negative emotions, and increased aggression. Based on this research, it is important to be able to objectively and accurately measure stress levels in order to determine how much stress a person is currently experiencing and provide feedback that recommends appropriate measures. 
     When a person is stressed, the sympathetic nervous system becomes excited, and the sweat glands present in skin are activated in accordance with excitation of the sympathetic nervous system. The electrical characteristics of skin are changed in accordance with activity of the sweat glands present in skin. These changes are called electrodermal activity (EDA), which can be measured to determine a person&#39;s stress levels. 
     The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art. 
     DOCUMENTS OF RELATED ART 
     
         
         
           
             (Patent Document 1) Korean Utility Model Registration No. 20-0416389 (issued May 8, 2006) 
           
         
       
    
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and the present invention is intended to propose a sensor for measuring skin conductivity and a method of manufacturing the same, wherein an uneven structure is provided on an electrode, which is provided on a base board made of a flexible material to come into contact with skin, and is configured to increase an electrical contact area with skin via sweat secreted onto a surface of skin. 
     Further, the present invention is intended to propose a sensor for measuring skin conductivity and a method of manufacturing the same, wherein the uneven structure is provided on a first electrode and a second electrode, and also on the base board at locations between the first and second electrodes. 
     Further, the present invention is intended to propose a sensor for measuring skin conductivity and a method of manufacturing the same, wherein a plurality of through holes is formed through the base board, the electrode, and the uneven structure in a direction perpendicular to a surface of the base board. 
     In order to achieve the above object, according to one aspect of the present invention, there is provided a sensor for measuring skin conductivity, the sensor including: the base board made of a flexible material; the electrode provided on a surface of the base board, and transmitting an electric signal; and the uneven structure provided on the electrode, and configured to increase an electrical contact area with skin via sweat secreted onto the surface of skin. 
     Further, the electrode may include: the first electrode; and the second electrode electrically insulated from the first electrode, wherein each of the first and second electrodes includes a measurement area in contact with skin and a connection area to which an external circuit is connected. 
     Further, the uneven structure may include a Pt-black layer structure provided on the electrode and configured to have a porous nanostructure formed of platinum particles. 
     Further, the uneven structure may include a plurality of pillar structures provided on the electrode. 
     Further, the uneven structure may be provided on the measurement areas of the first and second electrodes, and may be provided on the base board at locations between the measurement areas of the first and second electrodes. 
     Further, the uneven structure may be configured to have protrusions, with gaps defined between the protrusions, so that the sweat secreted onto the surface of skin permeates between the protrusions by a capillary phenomenon. 
     Further, the uneven structure may be configured to have protrusions that have heights determined such that when the sweat secreted onto the surface of skin permeates between the protrusions, the sweat is free from reaching the electrode. 
     Further, the uneven structure may be configured to have protrusions that have heights determined such that when the sweat secreted onto the surface of skin permeates between the protrusions, the sweat reaches the electrode. 
     Further, each of the first and second electrodes includes a plurality of first branches that are configured such that the first branches of the first electrode and the first branches of the second electrode are formed in a comb pattern. 
     Further, the sensor may further include a plurality of through holes formed through the base board, the electrode, and the uneven structure in the direction perpendicular to the surface of the base board. 
     In order to achieve the above object, according to another aspect of the present invention, there is a provided a method of manufacturing a sensor for measuring skin conductivity, the method including: preparing a base board on a carrier board; forming an electrode on the base board by forming and patterning a conductive material; forming an uneven structure on the electrode such that an electrical contact area with skin is increased by sweat secreted onto a surface of skin; and removing the carrier board. 
     Further, the method may further include forming a plurality of through holes through the base board, the electrode, and the uneven structure in a direction perpendicular to a surface of the base board after the forming of the uneven structure. 
     Further, in the forming of the uneven structure, the uneven structure is formed on both the electrode and the base board. 
     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings. 
     All terms or words used in the specification and claims have the same meaning as commonly understood by one of ordinary skill in the art to which inventive concepts belong. 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 relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     According to the sensor for measuring skin conductivity with the above-described configuration, the uneven structure is provided on the electrode provided on the base board made of the flexible material to come into close contact with skin, whereby it is possible to measure that the skin-electrode contact resistance Rct is drastically reduced as the electrical contact area between skin and the electrode is enlarged by the sweat secreted onto the surface of skin. Thus, electrodermal activity can be efficiently measured. 
     In addition, the electrode includes the first and second electrodes, the uneven structure is provided on the first and second electrodes, and also on the base board at the locations between the first and second electrodes, whereby the electrical signal passing between the first and second electrodes can be transmitted through not only the inside of skin but also through the sweat and the uneven structure. Thus, electrodermal activity can be efficiently measured. 
     Moreover, the plurality of through holes is formed through the base board, the electrode, and the uneven structure in the direction perpendicular to the surface of the base board, whereby the sweat secreted onto the surface of skin can be rapidly evaporated by air flowing through the through holes. Thus, in measuring skin electrical activity, it is possible to eliminate measurement errors that occur when the sweat is accumulated on the uneven structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view showing a conventional sensor for measuring skin conductivity; 
         FIG. 2  is a view showing impedance model between skin and an electrode of the sensor for measuring skin conductivity; 
         FIGS. 3A and 3B  are views showing a principle of stress measurement of the conventional sensor for measuring skin conductivity; 
         FIG. 4  is a perspective view showing a sensor for measuring skin conductivity according to an embodiment of the present invention; 
         FIG. 5  is a cross-sectional view taken along line A-A′ of  FIG. 4 ; 
         FIG. 6  is a view showing a Pt-black layer according to the embodiment of the present invention; 
         FIGS. 7A and 7B  are views showing a principle of stress measurement of the sensor for measuring skin conductivity according to the embodiment of the present invention; 
         FIGS. 8A, 8B, and 8C  are views showing various types of an electrode of  FIG. 4 ; 
         FIGS. 9A and 9B  are views showing a difference in accordance with a height of an uneven structure of  FIG. 4 ; 
         FIGS. 10A, 10B, and 10C  are views showing the uneven structure provided at an entire measurement area of  FIG. 4 ; 
         FIGS. 11A and 11B  are cross-sectional views taken along line A-A′ of  FIG. 10A ; 
         FIG. 12  is a cross-sectional view showing a plurality of through holes formed at the sensor for measuring skin conductivity shown in  FIG. 4 ; 
         FIGS. 13A, 13B, and 13C  are cross-sectional views showing a step of forming the electrode in a method of manufacturing the sensor for measuring skin conductivity according to an embodiment of the present invention; 
         FIGS. 14A, 14B, and 14C  are cross-sectional views showing a step of forming the uneven structure in the method of manufacturing the sensor for measuring skin conductivity according to the embodiment of the present invention; and 
         FIG. 15  is a cross-sectional view showing a step of forming a through hole in the method of manufacturing the sensor for measuring skin conductivity according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. Further, it will be understood that, although the terms “one side”, “the other side”, “first”, “second”, etc. may be used herein to describe various elements, these elements should not be limited by these terms. Further, in the following description, when it is determined that the detailed description of the known art related to the present invention might obscure the gist of the present invention, the detailed description thereof will be omitted. 
     Hereinbelow, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Throughout the drawings, the same reference numerals will refer to the same or like parts. 
       FIG. 1  is a view showing a conventional sensor for measuring skin conductivity, and  FIG. 2  is a view showing impedance model between skin and an electrode of the sensor for measuring skin conductivity. 
     As shown in  FIG. 1 , a conventional method of measuring skin conductivity is a method in which a pair of sensors  2  and  3  for measuring skin conductivity are worn on fingers of a user, and an electrical signal having a positive (+) polarity and an electrical signal having a negative (−) polarity are applied to the sensors  2  and  3  respectively. The Electrical signal is transmitted from an electrode  4  of the sensor  2  to the sensor  3  through skin  1 , and is affected by a change in electrical characteristics of skin. Generally, when a person is stressed, the sympathetic nervous system is excited and then sweat glands present in skin are activated in response to excitation of the sympathetic nervous system, and thus electrical characteristics of skin are changed. The change in electrical characteristics of skin due to stress is represented by a change in the electrical signal, and the change in the electrical signal is measured by the electrode  4  coupled to the sensor body  5 , thereby measuring a user&#39;s stress levels. 
     With reference to both an enlarged view of  FIG. 1  and  FIG. 2 , the human skin  1  is composed of a skin layer a, a dermal layer b, a subcutaneous fat layer c, and a muscle layer d. Resistance Rs inside skin, which affects the electrical signal passing through an inside of skin  1 , can be modeled as a parallel connection of resistance Ra of skin layer a, resistance Rb of the dermal layer b, resistance Rc of the subcutaneous fat layer c, and resistance Rd of the muscle layer d. Impedance between skin  1  and the electrode  4  of the sensor for measuring skin conductivity can be modeled as a parallel connection of skin-electrode contact resistance Rct and skin-electrode capacitance C I . 
     In this model, a total of skin-electrode impedance Z is as shown in Equation 1 below, and skin conductivity G is as shown in Equation 2. Since skin conductivity G is measured by using a DC signal (frequency is 0, that is, w=0), measured skin conductivity G can be summarized as shown in Equation 3. 
     
       
         
           
             
               
                 
                   Z 
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                         + 
                         
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                           ⁢ 
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                   Equation 
                   ⁢ 
                   
                       
                   
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                   1 
                 
               
             
             
               
                 
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                   = 
                   
                     
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                       Z 
                     
                     = 
                     
                       1 
                       
                         
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                               1 
                               
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                                 ct 
                               
                             
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                               j 
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                                 C 
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
             
               
                 
                   
                     G 
                     ⁡ 
                     
                       ( 
                       
                         ω 
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                         0 
                       
                       ) 
                     
                   
                   = 
                   
                     1 
                     
                       
                         R 
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                         S 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
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                   3 
                 
               
             
           
         
       
     
       FIGS. 3A and 3B  are views showing a principle of stress measurement of the conventional sensor for measuring skin conductivity. As shown in  FIG. 3A , a surface of the electrode  4  of the conventional sensor for measuring skin conductivity and a surface of skin  1  partially come into contact with each other due to curves of the surface of skin  1 . Accordingly, an area of a skin-electrode contact surface CS is smaller than a surface area of the electrode  4 , and skin-electrode contact resistance Rct is high. 
     When the person is stressed and the sweat glands are activated by excitation of the sympathetic nervous system, the sweat SW is secreted onto the surface of skin  1 . The sweat SW includes an electrolyte and has electrical conductivity, and the sweat SW is filled between skin  1  and the electrode  4 , thereby enlarging the area of skin-electrode contact surface CS. Since resistance is inversely proportional to an area, skin-electrode contact resistance Rct is reduced as the area of skin-electrode contact surface CS is enlarged. Accordingly, electrodermal activity is measured by measuring a reduction in contact resistance Rct. 
     As shown in  FIG. 1 , the conventional sensor for measuring skin conductivity has to be worn on the palm or fingers of a user&#39;s hand, in which the sweat glands are more active due to excitation of the sympathetic nervous system. As a result, the conventional sensor is inconvenient to constantly wear to measure a change in stress levels in daily life. In addition, there is no significant difference in area between skin-electrode contact surface CS in a state where no sweat SW is secreted on to the surface of skin  1  (see  FIG. 3A ) and skin-electrode contact surface CS in a state where the sweat SW is secreted onto the surface of skin  1  (see  FIG. 3B ). Thus, there is a problem in that it is difficult to sensitively measure electrodermal activity because the area of the skin-electrode contact surface CS according to the stress state is not significantly changed. 
     If the sensor for measuring skin conductivity is attached to a body part (where activity of the sweat glands is less active compared to the palm of the hand) that does not cause discomfort in daily life, it is difficult to efficiently measure stress due to such low sensitivity. 
       FIG. 4  is a perspective view showing a sensor for measuring skin conductivity according to an embodiment of the present invention,  FIG. 5  is a cross-sectional view taken along line A-A′ of  FIG. 4 ,  FIG. 6  is a view showing a PT-black layer according to the embodiment of the present invention, and  FIGS. 7A and 7B  are views showing a principle of stress measurement of the sensor for measuring skin conductivity according to the embodiment of the present invention. 
     In an effort to solve the above problems of the conventional sensor for measuring skin conductivity, the sensor for measuring skin conductivity according to the embodiment of the present invention includes a base board  110  made of a flexible material, an electrode  120  provided on a surface of the base board  110  and transmitting an electrical signal, and an uneven structure  130  provided on the electrode  120  and configured to increase an electrical contact area with skin  1  via sweat SW secreted onto the surface of skin  1 . 
     The base board  110  is made of the flexible material, such that the sensor for measuring skin conductivity can be brought into close contact with skin  1 . The base board  110  may be made of a Flexible PCB (FPCB) or the like. The base board  110  supports the electrode  120  provided on the surface of the base board  110 , and serves as a body of the sensor for measuring skin conductivity. 
     As shown in  FIG. 1 , the electrode  120  includes a first electrode  121 , and a second electrode  122  electrically insulated from the first electrode  121 , wherein each of the first and second electrodes  121  and  122  includes a measurement area As in contact with skin  1  and a connection area Ac to which an external circuit is connected. The sensor for measuring skin conductivity is provided with the first electrode  121  and the second electrode  122  at the same time, such that skin conductivity can be measured at one point and an area of the sensor can be reduced unlike the conventional sensor for measuring skin conductivity shown in  FIG. 1 . 
     The first and second electrodes  121  and  122  are made of a conductive material, and the electrical signal is input thereto. The measurement area As is a region where skin  1  comes into contact therewith, thereby measuring electrodermal activity, and the first and second electrodes  121  and  122  are provided with the uneven structure  130 . The connection area Ac is a region where the external circuit is connected thereto such that the electrical signal is input from the external circuit. For example, the positive (+) polarity may be connected to the connection region Ac of the first electrode  121 , and the negative (−) polarity may be connected to the connection region Ac of the second electrode  122 . 
     The uneven structure  130  is provided on the measurement area As of the electrode  120 . For example, as shown in  FIGS. 4  and  5 , the uneven structure  130  may comprise a plurality of pillar structures provided on the electrode  120 . The pillar structures are protrusions  133  provided to be spaced apart from each other at regular intervals, wherein each of the protrusions  133  may have a diameter d of 10 μm and a height h of 1 μm, or may have a diameter d of 5 μm and a height h of 5 μm. 
     As shown in  FIG. 6 , the uneven structure  130  may include a Pt-black layer structure provided on the electrode  120  and configured to have a porous nanostructure formed of platinum particles. The Pt-black layer is formed by electroplating platinum on the electrode  120  to have an irregular porous structure. Since a surface area of the Pt-black layer is much larger than that of the electrode  120  having a flat surface, the sweat SW secreted onto the surface of skin  1  permeates into the Pt-black layer, and thus the area of skin-electrode contact surface CS is significantly changed. 
     The uneven structure  130  may include structures having a conductive property, having the protrusions  133 , and in which a contact area is enlarged by the sweat SW secreted onto the surface of skin  1 , without being limited to the pillar structures or the Pt-black layer. 
     The uneven structure  130 , namely the pillar structures and the Pt-black layer, has a structure in which when the sensor for measuring skin conductivity is brought into contact with skin  1 , upper ends of the protrusions  133  of the uneven structure  130  come into contact with the surface of skin  1 , whereas side surfaces of the protrusions  133  or a surface of the electrode  120  between the protrusions  133  does not come into contact with the surface of skin  1 . Thus, skin-electrode contact resistance Rct measured in the state where no sweat SW is secreted onto the surface of skin  1  is higher than that of the conventional sensor for measuring skin conductivity, in which there is no uneven structure  130  and the electrode  120  directly comes into contact with the surface of skin  1 . 
     In other words, because the area of skin-electrode contact surface CS shown in  FIG. 7A  is smaller than that of skin-electrode contact surface CS shown in  FIG. 3A , skin-electrode contact resistance Rct of the sensor for measuring skin conductivity ( FIG. 7A ) according to the embodiment of the present invention is higher than that of the conventional sensor for measuring skin conductivity ( FIG. 3A ). 
     In addition, skin-electrode contact resistance Rct measured in the state where sweat SW is secreted onto skin  1  is lower than that of the conventional sensor for measuring skin conductivity, in which there is no uneven structure  130  and the electrode  120  directly comes into contact with the surface of skin  1 . 
     In other words, because the area of skin-electrode contact surface CS shown in  FIG. 7B  is larger than that of skin-electrode contact surface CS shown in  FIG. 3B , skin-electrode contact resistance Rct of the sensor for measuring skin conductivity ( FIG. 7B ) according to the embodiment of the present invention is lower than that of the conventional sensor for measuring skin conductivity ( FIG. 3B ). 
     Thus, the uneven structure  130  increases difference between skin-electrode contact resistance Rct measured in the unstressed state (state in which no sweat SW is secreted onto the surface of skin  1 ) and skin-electrode contact resistance Rct measured in the stressed state (state in which the sweat SW is secreted onto the surface of skin  1 ). 
     Accordingly, the sensor for measuring skin conductivity according to the embodiment of the present invention has a larger difference in contact resistance Rct than the conventional sensor for measuring skin conductivity, and thereby has high sensitivity. Thus, even if the sensor is attached to an inner surface of the wrist or the chest of a user, in addition to the palm where activity of the sweat glands is active, it is possible to efficiently measure user&#39;s stress levels. 
       FIGS. 8A, 8B, and 8C  are views showing various types of the electrode  120  of  FIG. 4 . As shown in  FIG. 8A , the measurement area As of the first electrode  121  is formed in a circular shape, and the measurement area As of the second electrode  122  is spaced apart from a circular part of the first electrode  121  by a predetermined distance, thereby being formed in a ring shape to surround the circular part. 
     Alternatively, as shown in  FIG. 8B , the measurement area As of the first electrode  121  and the measurement area As of the second electrode  122  may be formed in a comb shape. A first branch  120   c  of the first electrode  121  is formed in a direction of the second electrode  122 , and a first branch  120   c  of the second electrode  122  is formed in a direction of first electrode  121 , thereby forming the comb shape. In other words, each of the first and second electrodes  121  and  122  includes a plurality of first branches  120   c , such that the first branches  120   c  of the first and second electrodes  121  and  122  are formed in a comb pattern. The first branches  120   c  may be formed in a zigzag shape, measurement points  120   e  having widths larger than that of the first branches  120   c  are provided at the first branches  120   c  at regular intervals, and the uneven structure  130  is provided on the measurement points  120   e.    
     Further alternatively, as shown in  FIG. 8C , the measurement area As of the first electrode  121  and the measurement area As of the second electrode  122  are formed in the comb shape, wherein each of the first branches  120   c  includes a plurality of second branches  120   d , such that the second branches  120   d  of the first and second electrodes  121  and  122  are formed in the comb pattern. The measurement points  120   e  are provided at the first branches  120   c  and the second branches  120   d , and the uneven structure  130  is provided on the measurement points  120   e.    
     When compared to the electrode  120  shown in  FIG. 8A , the electrode  120  shown in  FIG. 8B or 8C  has a structure in which a distance between the first and second electrodes  121  and  122  is small, and the first and second electrodes  121  and  122  are uniformly distributed due to the comb shape. Thus, electrodermal activity can be efficiently measured. 
       FIGS. 9A and 9B  are views showing a difference in accordance with a height of the uneven structure  130  of  FIG. 4 . As shown in  FIG. 9A , the protrusions  133  of the uneven structure  130  may have heights h determined such that even if the sweat SW secreted onto the surface of skin  1  permeates between the protrusions  133 , the sweat SW is free from reaching the electrode  120 . When the heights h of the protrusions  133  are sufficiently high, the sweat SW secreted onto the surface of skin  1  may come into contact with the side surfaces of the protrusions  133  and may not reach the surface of the electrode  120 . Here, the side surfaces of the protrusions  133  are brought into electrical contact with the surface of skin  1  by the sweat SW, such that the area of skin-electrode contact surface CS is enlarged and skin-electrode contact resistance Rct is drastically reduced. Thus, sensitivity of the sensor for measuring skin conductivity is increased. 
     Further, as shown in  FIG. 9B , the protrusions  133  of the uneven structure  130  may have heights h determined such that when the sweat SW secreted onto the surface of skin  1  permeates between the protrusions  133 , the sweat SW reaches on the electrode  120 . When the heights h of the protrusions  133  are sufficiently low, the sweat SW secreted onto the surface of skin  1  comes into contact with both the side surfaces of the protrusions  133  and the surface of the electrode  120 . Accordingly, the area of skin-electrode contact surface CS is enlarged and skin-electrode contact resistance Rct is drastically reduced. Thus, sensitivity of the sensor for measuring skin conductivity is increased. 
     Moreover, the protrusions  133  of the uneven structure  130  may have gaps therebetween, so that the sweat SW secreted onto the surface of skin  1  permeates between the protrusions  133  by a capillary phenomenon. In this case, even when a small amount of sweat SW is secreted onto the surface of skin  1 , the sweat SW can permeate between the protrusions  133  by capillary phenomenon. Accordingly, the area of skin-electrode contact surface CS is enlarged and skin-electrode contact resistance Rct is drastically reduced. Thus, sensitivity of the sensor for measuring skin conductivity is increased. 
       FIGS. 10A, 10B, and 10C  are views showing the uneven structure  130  provided at the entire measurement area As of  FIG. 4 , and  FIGS. 11A and 11B  are cross-sectional views taken along line A-A′ of  FIG. 10A . 
     As shown in  FIGS. 10A to 11B , the uneven structure  130  is provided at the measurement areas As of the first and second electrodes  121  and  122 , and also on the base board  110  at locations between the measurement areas As of the first and second electrodes  121  and  122 . In other words, the protrusions  133  are further provided between the first and second electrodes  121  and  122 . 
     As shown in  FIG. 11A , protrusions  133   b  of an uneven structure  130   b  are provided on the base board  110  at locations between the first and second electrodes  121  and  122 , such that upper ends of the protrusions  133   b  are aligned with upper ends of protrusions  133   a  of an uneven structure  130   a  provided on the electrode  120 . As shown in  FIG. 11B , when the sweat SW is secreted onto the surface of skin  1 , the sweat SW permeates between the protrusions  133   b  provided on the base board  110 , thereby forming a path Rsw by the protrusions  133   b  provided on the base board  110  and the sweat SW. This path Rsw connects the first electrode  121  and the second electrode  122  to each other, whereby the electrical signal can be transmitted along the surface of skin  1  without passing through the inside of skin  1 . 
     Moreover, even if the first and second electrodes  121  and  122  are not connected to each other by the protrusions  133   b  provided on the base board  110  and the sweat SW, the amount of the electrical signal passing through the inside of skin  1  can be minimized by the path Rsw formed by the protrusions  133   b  provided on the base board  110  and the sweat SW. 
     Thus, as the sweat SW permeates between the protrusions  133   b  provided on the base board  110  at locations between the first and second electrodes  121  and  122 , skin-to-electrode contact resistance Rct is drastically reduced. Thus, sensitivity of the sensor for measuring skin conductivity is increased. 
       FIG. 12  is a cross-sectional view showing a plurality of through holes  140  formed at the sensor for measuring skin conductivity shown in  FIG. 4 . As shown in  FIG. 12 , the through holes  140  are formed through the base board  110 , the electrode  120 , and the uneven structure  130  in a direction perpendicular to the surface of the base board  110 . Accordingly, even when the sensor for measuring skin conductivity covers the surface of skin  1 , air can flow through the through holes  140 . Thus, the sweat SW permeating between the surface of skin  1  and the protrusions  133  can be evaporated. 
     If the sweat SW remains between the protrusions  133  after the sweat SW is secreted onto the surface of skin  1  by stress, it is difficult to measure skin-electrode contact resistance Rct due to the sweat SW secreted by the following stress. Accordingly, even if the sensor for measuring skin conductivity is continuously attached to skin  1 , the sweat SW can be discharged into the air through the through holes  140  formed at the sensor for measuring skin conductivity. Thus, it is possible to continuously and accurately measure a user&#39;s stress levels. 
     In the sensor for measuring skin conductivity according to the embodiment of the present invention, the uneven structure  130  is provided on the electrode  120  provided on the base board  110  made of the flexible material to come into close contact with skin  1 , whereby it is possible to measure that skin-electrode contact resistance Rct is drastically reduced as a electrical contact surface CS area between skin  1  and the electrode  120  is enlarged by the sweat SW secreted onto the surface of skin  1 . Thus, electrodermal activity can be efficiently measured. 
     In addition, the electrode  120  includes the first electrode  121  and the second electrode  122 , the uneven structure  130  is provided on the first electrode  121  and the second electrode  122 , and also on the base board  110  at the locations between the first and second electrodes  121  and  122 , whereby the electrical signal passing between the first and second electrodes  121  and  122  can be transmitted through not only the inside of skin  1  but also through the sweat SW and the uneven structure  130 . Thus, electrodermal activity can be efficiently measured. 
     Moreover, the plurality of through holes  140  is formed through the base board  110 , the electrode  120 , and the uneven structure  130  in the direction perpendicular to the surface of the base board  110 , whereby the sweat SW secreted onto the surface of skin  1  can be rapidly evaporated by air flowing through the through holes  140 . Thus, in measuring skin electrical activity, it is possible to eliminate measurement errors that occur when the sweat SW is accumulated on the uneven structure  130 . 
     Hereinafter, a method of manufacturing a sensor for measuring skin conductivity according to an embodiment of the present invention will be described with reference to the accompanying drawings. 
       FIGS. 13A, 13B, and 13C  are cross-sectional views showing a step of forming the electrode  120  in the method of manufacturing the sensor for measuring skin conductivity according to an embodiment of the present invention,  FIGS. 14A, 14B, and 14C  are cross-sectional views showing a step of forming the uneven structure  130  in the method of manufacturing the sensor for measuring skin conductivity according to the embodiment of the present invention, and  FIG. 15  is a cross-sectional view showing a step of forming a through hole  140  in the method of manufacturing the sensor for measuring skin conductivity according to the embodiment of the present invention.  FIGS. 13A to 15  are cross-sectional views taken along line A-A′ of  FIG. 4 . 
     The method of manufacturing the sensor for measuring skin conductivity according to the embodiment of the present invention includes preparing the base board  110  on a carrier board  150 , forming the electrode  120  on the base board  110  by forming and patterning a conductive material, forming the uneven structure  130  on the electrode  120  such that the electrical contact surface CS area with skin  1  is enlarged by the sweat SW secreted onto the surface of skin  1 , and removing the carrier board  150 . 
     As shown in  FIG. 13A , first, the base board  110  is placed on the carrier board  150 . The carrier board  150  serves to support the base board  110  in a manufacturing process. 
     Next, as shown in  FIG. 13B , the conductive material is formed to form the electrode  120  on the base board  110 . For example, the conductive material may be a metal layer, wherein a Ti layer  120   a  may be formed on the base board  110  to a thickness of about 300 Å, and an Au layer  120   b  may be formed on the Ti layer  120   a  to a thickness of about 1000 Å. 
     Next, as shown in  FIG. 13C , the electrode  120  is patterned using a semiconductor manufacturing process such as a photoresist PR and etching. At this time, the electrode  120  may be formed as shown in  FIGS. 8A, 8B, and 8C . 
     Next, as shown in  FIG. 14A , a seed layer  135  is further formed on the base board  110  and the electrode  120 . The seed layer  135  is formed prior to forming the uneven structure  130  on the electrode  120  or the base board  110 , thereby efficiently forming the uneven structure  130  on the electrode  120  or the base board  110 . For example, in the seed layer  135 , a Ti layer  135   a  may be formed to a thickness of about 300 Å on the base board  110 , and an Au layer  135   b  may be formed on the Ti layer  135   a  to a thickness of about 500 Å. 
     Next, as shown in  FIG. 14B , the photoresist PR is formed on the seed layer  135 , and a portion where the protrusions  133  will be formed is patterned. The photoresist PR may be patterned to form the protrusions  133  only on the electrode  120 , or may be patterned to form the protrusions  133  on both the electrode  120  and the base board  110 . Then, the uneven structure  130  is formed on the electrode  120  or the base board  110  by using the patterned photoresist PR as a mask and by plating or vapor-depositing a metal such as Au. 
     Next, as shown in  FIG. 14C , the photoresist PR is removed, the seed layer  135  on which no protrusions  133  are formed is removed, and the carrier board  150  is removed, thereby manufacturing a sensor for measuring skin conductivity. 
     Next, after the forming of the uneven structure  130 , the method may further include forming a plurality of through holes  140  passing through the base board  110 , the electrode  120 , and the uneven structure  130  in the direction perpendicular to the surface of the base board  110 . Since the forming of the through holes  140  is carried out after the electrode  120  and the uneven structure  130  are formed on the base board  110 , the through holes  140  can be easily formed. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 
     Further, simple changes and modifications of the present invention are appreciated as included in the scope and spirit of the invention, and the protection scope of the present invention will be defined by the accompanying claims.