Patent Publication Number: US-2022229967-A1

Title: Method and apparatus for printing electrical circuit directly on target surface having 3-dimensional shape, 3D printer used for the same and electrical device having electrical circuit printed by the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for printing an electric circuit directly on a target surface having a three-dimensional shape, a 3D printing apparatus used therefor, and an electrical device having an electric circuit printed thereby. 
     2. Description of the Related Art 
     In Human-Computer Interface (HCI), on-body electronics are recognized as a new interface medium. Existing studies have developed technologies that enable on-body electronic fabrication by considering various wearability factors. Most of these technologies follow the same design and fabrication flow. The electronics are made as two-dimensional stickers or tattoos on a flat surface and later attached to the body. The manufacturing process usually requires several steps so that the conductive material can be attached to a flexible substrate and subsequently attached to the body. These manufacturing processes are inherently difficult and error-prone. This is because it usually takes several iterations to create a sticker and apply it correctly to the body as desired. More generally, there is a disconnect between the prototype and the final form of the circuit, since design and fabrication run in two-dimensional space whereas the body is essentially a three-dimensional surface. 
     There are a few studies that have shown the possibility of printing directly on the user&#39;s body, but these examples are largely limited by the type of ink used and the form factor of the printing device that allows printing only to a limited location on the human body. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fast and accessible method and apparatus capable of directly manufacturing an electric circuit on a target surface having a three-dimensional shape, such as skin in various body positions, in order to solve the above problems. 
     Another object of the present invention is to provide a 3D printing apparatus used in the above method. 
     Another object of the present invention is to provide an electronic device having a three-dimensional electric circuit manufactured by the above method. 
     In order to solve the above technical problem, according to an aspect of the present invention, there is provided a method for printing an electric circuit directly on a target surface having a three-dimensional shape using a three-dimensional printing apparatus, the method comprising: (a) receiving two-dimensional information about the shape of the electric circuit to be printed; (b) receiving information about the three-dimensional shape of the target surface; (c) generating three-dimensional information on the electric circuit to be printed by adjusting the two-dimensional information about the shape of the electric circuit to be printed on the basis of the information about the three-dimensional shape of the target surface; and, (d) generating a tool path for controlling the three-dimensional printing apparatus based on the three-dimensional information on the electric circuit to be printed which generated in the step (c). 
     According to another aspect of the present invention, there is provided an apparatus for printing an electric circuit directly on a target surface having a three-dimensional shape, comprising: at least one processor; and at least one memory for storing computer-executable instructions; wherein the computer-executable instructions stored in the at least one memory includes: (a) receiving two-dimensional information about the shape of the electric circuit to be printed; (b) receiving information about the three-dimensional shape of the target surface; (c) generating three-dimensional information on the electric circuit to be printed by adjusting the two-dimensional information about the shape of the electric circuit to be printed on the basis of the information about the three-dimensional shape of the target surface; and, (d) generating a tool path for controlling a three-dimensional printing apparatus based on the three-dimensional information on the electric circuit to be printed which generated in the step (c). 
     According to another aspect of the present invention, there is provided a 3D printing apparatus for printing an electric circuit along a given tool path directly on a target surface having a three-dimensional shape, comprising: an ink supply unit supplying conductive ink onto the target surface; a three-dimensional driving unit for moving the ink supply unit along the tool path; and, a control unit for controlling the operation of the ink supply unit and the three-dimensional driving unit, wherein the tool path is generated by adjusting two-dimensional information about the shape of the electric circuit based on information about the three-dimensional shape of the target surface. 
     According to another aspect of the present invention, there is provided an electronic apparatus, comprising: one or more electric circuits attached to an object surface having a three-dimensional shape; and, a control module connected to the one or more electric circuits, wherein the one or more electric circuits are formed by printing directly on the object surface. 
     According to another aspect of the present invention, there is provided a system for printing an electric circuit directly on a target surface having a three-dimensional shape, comprising: a 3D printing apparatus for printing the electric circuit directly on the target surface having a three-dimensional shape; a tool path generator for generating three-dimensional information about the electric circuit by adjusting two-dimensional information about the shape of the electric circuit based on the information about the three-dimensional shape of the target surface and generating a tool path based on the three-dimensional information about the electric circuit; and, a printing apparatus control module for controlling the operation of the 3D printing apparatus according to the tool path. 
     In accordance with the present invention, a fast and accessible method and apparatus for fabricating electrical circuits directly on a target surface having a three-dimensional shape are provided. 
     Also provided is a three-dimensional printing apparatus used in the method according to the present invention. 
     Also provided is an electronic device having a three-dimensional electric circuit manufactured by the method according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a 3D electric circuit printing system according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a printing apparatus control module in the system shown in  FIG. 1 . 
         FIG. 3  is a diagram illustrating a tool path generating apparatus in the system shown in  FIG. 1 . 
         FIG. 4  is a flow chart illustrating a method performed by the system shown in  FIG. 1 . 
         FIG. 5  is a diagram schematically illustrating a state in which an electric circuit is printed on a user&#39;s arm using the system shown in  FIG. 1 . 
         FIG. 6  is a diagram schematically illustrating an electric circuit printed on two parts of a user&#39;s arm and an electronic device having the same. 
         FIG. 7  is a diagram illustrating a control module in the electronic device shown in  FIG. 6 . 
         FIG. 8  is a diagram illustrating a computer connected to the electronic device shown in  FIG. 6 . 
         FIGS. 9A and 9B  are diagrams illustrating output screens according to an application example of the electronic device illustrated in  FIG. 6 . 
         FIG. 10  is a diagram illustrating a 3D printing apparatus according to an embodiment of the present invention. 
         FIG. 11  shows a formula for correcting a 2D tool path into a 3D tool path in a system using the 3D printing apparatus shown in  FIG. 10 . 
         FIGS. 12A and 12B  show graphic interfaces of a system using the 3D printing apparatus shown in  FIG. 10 . 
         FIG. 13  is a diagram illustrating a state in which electric circuits are printed on various parts of the human body using the 3D printing apparatus shown in  FIG. 10 . 
         FIGS. 14A to 14C  show outputs according to various motions of an arm as an application example of an electronic device according to the present invention. 
         FIG. 15  shows a state in which a capacitance electric circuit is printed on a user&#39;s arm. 
         FIG. 16  is another application example of the electronic device according to the present invention, showing an output according to contact with another person. 
         FIG. 17  shows outputs according to various operations as another application example of the electronic device according to the present invention. 
         FIGS. 18A to 18C  show the operation of the slider controller as another application example of the electronic device according to the present invention. 
         FIGS. 19A to 19C  show another application example of an electronic device according to the present invention, in which output is changed according to a user&#39;s posture quality. 
         FIGS. 20A and 20B  show various LED displays printed on a user&#39;s skin as another application example of the electronic device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description of the present invention, it will be noted that the terms and wordings used in the specification and the claims should not be construed as general and lexical meanings, but should be construed as the meanings and concepts that agree with the technical spirits of the present invention, based on the principle stating that the concepts of the terms may be properly defined by the inventor(s) to describe the invention in the best manner. Therefore, because the examples described in the specification and the configurations illustrated in the drawings are merely for the preferred embodiments of the present invention but cannot represent all the technical sprints of the present invention, it should be understood that various equivalents and modifications that may replace them can be present. 
       FIG. 1  schematically shows a system for printing an electric circuit directly on a target surface having a three-dimensional shape according to an embodiment of the present invention, and  FIG. 2  shows a control module for controlling the operation of the 3D printing apparatus, and  FIG. 3  shows a tool path generating apparatus for generating a tool path for printing an electric circuit on a target surface using the 3D printing device shown in  FIG. 1 . 
     As shown, the 3D electric circuit printing system  1  includes a 3D printing apparatus  100 , a printing apparatus control module  200 , and a tool path generating apparatus  300 . 
     The 3D printing apparatus  100  supplies a conductive ink on a target surface having a three-dimensional shape to form a trace of the conductive ink. The conductive ink may be a commercially available material, and it is preferable to have a property of being able to flow during printing and curing after printing. 
     The 3D printing apparatus  100  includes an ink supply unit  120  for supplying conductive ink to a target surface and a 3D driving unit  110  for moving the ink supply unit  120  along a 3D tool path. The ink supply unit  120  supplies the conductive ink on the target surface while moving by the 3D driving unit  110 . The ink supply unit  120  may control the amount of conductive ink supplied on the target surface. As the ink supply unit  120 , for example, an extruder, a brush, or any other device capable of supplying conductive ink to a target surface having a three-dimensional shape may be used. The 3D driving unit  110  may be, for example, a Cartesian platform that independently moves the ink supply unit  120  in three directions orthogonal to each other. The 3D printing apparatus  100  preferably maintains a state attached to the target surface while printing is performed in order to ensure good printing of the electric circuit. 
     The printing apparatus control module  200  controls the operation of the 3D printing apparatus  100 . The printing apparatus control module  200  controls the operations of the 3D driving unit  110  and the ink supply unit  120  of the 3D printing apparatus  100  according to a given tool path. The printing apparatus control module  200  may be, for example, an electronic device having a microcontroller such as Arduino.  FIG. 2  schematically shows an embodiment of the printing apparatus control module  200 . The printing apparatus control module  200  includes the processor  210 , the memory  220  storing programs being executed, an input/output unit  230  inputs or outputs information between the 3D printing apparatus  100  or a tool path generating apparatus  300  to be described later, and internal communication paths therebetween. In the present embodiment, a control program for controlling the 3D printing apparatus  100  is loaded in the memory  220 , and the 3D printing apparatus  100  is controlled by the control program. The control program may be provided to the printing apparatus control module  200  from the outside through the input/output unit  230 . A non-volatile memory (not shown) for storing the provided control program may be additionally provided. Also, the tool path may be transmitted from the tool path generating apparatus  300  through the input/output unit  230 . The 3D printing apparatus control program mounted in the memory  220  of the printing apparatus control module  200  generates a signal to control the ink supply unit  120  and the 3D driving unit  110  of the 3D printing apparatus  100  based on the received tool path. The signal for controlling the 3D printing apparatus  100  is transmitted to the 3D printing apparatus  100  through the input/output unit  230   
       FIG. 3  schematically shows a tool path generating apparatus  300 . The tool path generating apparatus  300  may be, for example, a computer. The tool path generating apparatus  300  includes a processor  310 , a non-volatile storage unit  320  for storing programs and data, etc., a volatile memory  330  for storing programs being executed, and an input/output unit  340  for inputting and outputting information between a user, a user interface  350 , and internal communication paths therebetween. The running program may include an operating system and various applications. Although not shown, it includes a power supply unit. In the present embodiment, a tool path generating program is loaded in the memory  330 , thereby generating a tool path for controlling the 3D printing apparatus  100 . The generated tool path is transmitted to the printing apparatus control module  200  through the input/output unit  340 , and the printing apparatus control module  200  generate a signal for controlling the 3D printing apparatus  100  based on the received tool path. 
       FIG. 4  illustrates a process performed by a computer program for generating a tool path of the tool path generating apparatus  300 . In the illustrated embodiment, tool path generator program receives two-dimensional information about the shape of the electrical circuit to be printed and information about the three-dimensional shape of the target surface are input, generates three-dimensional information on the electrical circuit to be printed by adjusting the two-dimensional information about the shape of the electrical circuit to be printed based on the information about the three-dimensional shape of the target surface, and, based on the generated three-dimensional information, generates a tool path is created. Information on the shape of the electrical circuit to be printed may be input through the input/output unit  340  from the outside of the apparatus or may be input through the user interface  350 . The input information may be stored in the non-volatile storage unit  320 . The information on the shape of the electrical circuit to be printed may be any information that can define the two-dimensional shape of the electrical circuit. For example, it may be simple spatial coordinate information about an electric circuit, or it may be two-dimensional tool path information. In addition, the tool path generating apparatus  300  receives information about the three-dimensional shape of the target surface on which the electric circuit is to be printed. This information may be input using, for example, the 3D printing apparatus  100 , or may be input using an external device (not shown) such as a camera or a laser scanner. Finally, three-dimensional information about the electric circuit to be printed is generated by adjusting the two-dimensional information about the shape of the electric circuit to be printed on the basis of the information about the three-dimensional shape of the target surface, and based on this, creates a toolpath to control the 3D printing apparatus. The adjusting process may be performed, for example, by projecting two-dimensional information regarding the shape of the electrical circuit to be printed onto a target surface having a three-dimensional shape to obtain additional information about each point of the electrical circuit. When the three-dimensional information on the electrical circuit to be printed is obtained by the adjusting operation, a tool path for controlling the 3D printing apparatus is generated based on the obtained three-dimensional information. 
       FIG. 5  shows a state in which an electric circuit is printed using the 3D electric circuit printing system  1  shown in  FIG. 1  at the concave point of the user&#39;s elbow. In the illustrated embodiment, for convenience of explanation, the 3D printing apparatus  100 , the printing apparatus control module  200 , and the tool path generating apparatus  300  are each illustrated and described as separate apparatuses, but these apparatuses may also be combined with each other. For example, the 3D printing apparatus  100  and the printing apparatus control module  200  are combined to form one apparatus, or the printing apparatus control module  200  and the tool path generating apparatus  300  are combined to form one apparatus. 
       FIG. 6  shows an example of an electronic device printed by the 3D electric circuit printing system  1  in  FIG. 5 . In the illustrated embodiment, an electric circuit is printed on the concave point of the user&#39;s elbow and the wrist. In this embodiment, each electronic device is configured to operate as a strain gauge. Since the two electronic devices differ only in size and have the same technical configuration, only the electronic device disposed on the elbow will be described below and the description of the electronic device disposed on the wrist will be omitted. 
     The electronic device includes an electric circuit  10  printed by the 3D electric circuit printing system  1  described above, electric elements  21  and  22  connected to the electric circuit  10 , and a control module  30  electrically connected to the electric circuit, and an insulating layer  40  attached to the user&#39;s skin. Reference numeral  400  denotes a computer and is connected to the control module  30 . 
     The electric circuit  10  is printed directly on the user&#39;s skin by the 3D electric circuit printing system  1  shown in  FIG. 1  by the method described above. In the illustrated embodiment the electrical circuit  10  is printed in a serpentine pattern. The electric circuit  10  is formed by being printed with a conductive ink having fluidity and then cured. In the cured state, the electric circuit has elasticity and flexibility. 
     Preferably, in order to provide electrical insulation to increase safety and improve adhesion of the electrical circuit, the insulating layer  40  is first attached to the user&#39;s skin, and then the electric circuit  10  may be printed thereon. In addition, an insulating layer (not shown) may be additionally attached on the electric circuit  10  to insulate between the electric circuit  10  and other objects. The durability of the electric circuit  10  may also be increased thereby. The insulating layer  40  may be formed using, for example, a liquid bandage such as Nexcare manufactured by  3 M, which is commercially available. 
     The electronic device may include electric elements  21  and  22  electrically connected to the electrical circuit  10 . In the illustrated embodiment, a resistance element  21  and a connector  22  are connected to the electrical circuit  10 . Since the electric circuit  10  has elasticity and flexibility, it is deformed together with the skin at the point where the electric circuit  10  is attached. The resistance element  21  is provided so that the illustrated electronic device operates as a voltage divider, and the connector  22  is provided to connect the electric circuit  10  with the control module  30 . In the illustrated embodiment, the electronic device operates as a strain gauge, converts the strain applied to the skin according to the user&#39;s change of the arm posture into an electrical signal, and provides it to the control module  30 . 
     The control module  30  is electrically connected to the electric circuit  10 , and receives and processes electric signals from the electric circuit  10  or operates the electric circuit  10 . The control module  30  may be, for example, an electronic device having a microcontroller such as Arduino.  FIG. 7  shows an embodiment of the control module  30 . The control module  30  includes a processor  31 , a memory  32  for storing programs being executed, an input/output unit  33  for inputting or outputting information between an electric circuit  10  or a computer  400  to be described later, and an internal communication paths therebetween. In this embodiment, the memory  32  is loaded with a control program for controlling the electric circuit  10 , and the signal of the electric circuit  10  is processed or the electric circuit  10  is operated by the control program. In the illustrated embodiment, when the user changes the posture of the arm, the control module  30  may detect this through a change in the resistance of the electric circuit  10 . 
     For a richer application, the control module  30  may cooperate with the computer  400 .  FIG. 8  shows a computer  400  cooperating with the control module  30 . The computer  400  includes a processor  410 , a non-volatile storage unit  420  for storing programs and data, etc., a volatile memory  430  for storing programs being executed, and an input/output unit  440  for inputting and outputting information between the user, a user interface  450 , and an internal communication paths therebetween. In the illustrated embodiment, an application program is loaded in the volatile memory  430  of the computer  400 . The application program may process the electrical signal received from the control module  30  and provide it in a form for the user to understand. For example, the application program may receive from the control module  30  a change in resistance of an electric circuit disposed on the elbow and wrist according to a change in the posture of the user&#39;s arm and display it graphically on the screen. The application program applies a low-pass filter to the electrical signal provided from the control module  30  and maps it to the rotation angle of the joint of the simplified skeletal model. The graphical user interface renders the arm movements on the screen in real time.  FIGS. 9A and 9B  show an example of a graphical user interface provided by an application program of the computer  400 .  FIG. 9A  shows a posture when the user slightly bends the elbow, and  FIG. 9B  shows a state in which the user bends the elbow by about 90 degrees and slightly bends the wrist. 
     Examples 
     Hereinafter, an embodiment of the 3D electric circuit printing system described above with reference to the drawings will be described. 
       FIG. 10  shows a 3D printing apparatus according to the present embodiment. The 3D printing apparatus  100  has a structure in which the ink extruder  120  is mounted on the Cartesian plotter base  110 . The 3D printing apparatus  100  includes a total of four bipolar stepper motors  114   a ,  114   b ,  115  and  116 . Two stepper motors  114   a  and  114   b  respectively connected to the screw rod are mounted on the edge of the Cartesian plotter base  110  to move the ink extruder  120  in the x-axis direction. Two additional stepper motors  115  and  116  and their associated screw rods move the extruder in the y-axis and z-axis directions, respectively. The maximum resolution of motion in the xy plane is 0.3 mm and the maximum velocity is 8.3 mm/s. The resolution of the Z-axis is 0.1 mm. The ink extruder  120  is mounted on the head of the plotter. The head of the plotter consists of a geared DC motor  123  and a screw rod and is used to extrude ink. The ink extruder  120  has a plunger attached to a nut and connected to a screw rod driven by a motor. The size of the nozzle  122  of the ink extruder  120  is 00.5 mm, and the ink contained in the extruder barrel  121  is extruded. Barrel  121  contains up to 1.5 mL of commercially available, skin-safe conductive ink. All parts of the Cartesian plotter base  110  and the ink extruder  120  are manufactured by a three-dimensional printing method, and the overall size is 120×80×140 mm, and the print area has a size of 74×39 mm. Through the holes  111   a ,  111   b ,  111   c  provided in the Cartesian plotter base  110 , a mechanism such as a string for fixing the 3D printing apparatus to the target surface is disposed. 
     The control module contains an Arduino Uno microcontroller, a CNC Arduino shield V3 with four A4988 stepper motor drivers, and one L293D H-bridge integrated circuit. The entire system is powered via an 8V DC power supply. The Arduino Uno runs the CNC software and is used to control the position of the ink extruder  120  with a motor. In particular, the two stepper motors  114   a ,  114   b  disposed along the x-axis are synchronized so as not to be structurally twisted along the y-axis. The H-bridge IC chip is on a separate breadboard (45 mm×35 mm) and controls the depth and direction of motion of the plunger in the ink extruder  120 . Normally the plunger is pushed out to eject the ink, but it can also be pulled when the extruder needs to be refilled or there are deliberate gaps in the circuit. The tool path generating program is written in Java and runs on the PC. 
     The user designs the desired electrical circuit using CAD software. After completion, the electrical circuit is converted into a tool path and exported as a two-dimensional G-code. The user loads the generated G-code using a tool path generating program and then visualizes it as a two-dimensional toolpath. Any required additions to the circuit can be implemented directly using the mouse click and drag gestures. 
     Next, the user puts on the 3D printing apparatus  100  and starts computer-assisted z-axis calibration. After setting the calibration grid resolution, the user moves the ink extruder  120  to each calibration point and adjusts the z-axis values. The tool path generating program integrates the z-axis data into the imported two-dimensional G-code and transforms it into a three-dimensional tool path that is sent to the hardware control unit. Calibration requires manual adjustment of the vertical height of the ink extruder  120  at each point until the ink extruder  120  touches the skin. Only points close to the circuit need calibration. During calibration, the ink extruder  120  can move with a precision of 0.1 mm and a speed of 8.3 mm/s. Calibration points are the result of subdividing the print area into any number of rows and columns between 6 and 11, depending on the complexity of the body part to be printed. Given a limited set of calibration points, the shape of the print surface can be inferred. The equidistant vertices due to the subdivision constitute the correction points. Therefore, a calibration grid consisting of a minimum of 36 vertices and a maximum of 121 vertices is supported. At full resolution, the calibration points are 6.7 mm and 3.6 mm apart in the x and y directions, respectively. All points within the region divided by these four vertices (Q 11  to Q 22 ) can be calculated using the formula in  FIG. 11 , which shows the bilinear interpolation for the height of the trace f at the point (x, y). For relatively fat body parts, using 6×6 points for calibration is sufficient and takes less than 5 minutes to complete. Once calibration is complete, the user can adjust the ink extruder settings to control the thickness of the printed traces. 
     The graphical interface of the tool path generating program is shown in  FIGS. 12A and 12B . The graphical interface displays the circuits corresponding to the two-dimensional G-code commands loaded by the tool path generating program (see  FIG. 12A ). Users can draw custom shapes that can be printed together. The graphical interface shows the numerical values of the z-axis displayed as a grid overlay over the tool path (see  FIG. 12B ). On the right side of the screen is a control panel with buttons and sliders. This panel allows precise control of the operations of the 3D printing apparatus such as: 1) moving the nozzle in one of three axes, 2) print initialization, calibration and start, 3) moving vertically the plunger of the extruder to fill the extruder or extrude the ink, and 4) setting the thickness of the printed circuit. For example, setting the plunger to full force (100% duty cycle) will print a line approximately 2 mm thick. 
     Finally, the tool path generating program uses the height of each vertex combined with the 2D G-code information to construct a new path including the coordinates of the electric circuit along with the height value of the body surface in three dimensions. A set of three-dimensional G-code commands is entered into the control module and used to control the position of the extruder in three axes. 
     Pressing the “PRINT” button in the graphical interface starts printing the circuit. The ink extruder moves along the curvature of the skin according to the generated three-dimensional tool path. After printing is complete, the user can remove the 3D printing device from the skin surface and prepare the circuit wiring with the necessary components. Printed circuits typically dry in less than 10 minutes, during which time the necessary electronic components are attached to the circuit. 
     The user can apply a liquid band to the skin of the desired printed area to insulate the electric circuit, thereby improving skin safety and improving the adhesion of the electric circuit. This also results in long lasting of the printed circuit. When the electrical circuit dries, an additional layer of liquid band is applied to further insulate the circuit and increase its durability. 
     The 3D electric circuit printing system according to the present embodiment can print electric circuits on various parts of the human body. Referring to  FIG. 13 , in a clockwise direction from the upper left, an electric circuit could be printed on areas such as forearms, wrists, fingers, forehead, neck, back, stomach, thighs, and ankles. 
       FIGS. 14 to 20  show various electronic devices including an electric circuit printed on a body part by the 3D electric circuit printing system according to the present embodiment and application examples thereof. 
       FIGS. 14A to 14C  show, respectively, that various input gestures such as bending, twisting, and gripping the skin of an electric circuit part can be detected by the strain gauge printed on the concave part of the elbow. The graphs show how strongly the resistance changes according to the bending, twisting, and gripping input motions. 
       FIG. 15  shows an example of an electronic device for measuring capacitance. Using the CapactiveSensor library and the printed sensor&#39;s known 1 MΩ resistor, the time required to charge the capacitor was measured with the control module. This electronic device was used to create two simple applications shown in  FIGS. 16 and 17 . When the user touches another person, the relative capacitance increases. As shown in  FIG. 16 , the relative capacitance appears differently, when the user is in a non-contact state with another person (I), when the index finger is in contact with another person (II), and when the middle finger is in contact with another person (III), respectively. This application demonstrates reliable detection of each other&#39;s touch. As the contact area between the user and ground decreases, the body capacitance decreases accordingly. The second application uses this to show motion detection such as standing (I), walking (II), and jumping (III). Depending on these applications, it is easy to imagine other opportunities for different types of detection, such as step counters, fitness trackers, etc. 
     In the application shown in  FIG. 18 , a slider controller is printed on the user&#39;s finger using the concept of a voltage divider. Two parallel lines cross the index finger, while a circular switch is printed on the thumb. When your thumb touches the slider&#39;s rail, it effectively closes the circuit, allowing the control module to sense the corresponding resistance between the start of the slider and where the touch occurred. We then mapped this value to the volume level of the music player. 
     In the application example shown in  FIG. 19 , the quality of the user&#39;s sitting posture was detected using two strain gauge sensors. Electric circuits are printed on the upper  10   a  and lower back  10   b  and their combined readings are used to infer the user&#39;s posture and provide visual feedback to the user. This application also shows how to cover electric circuits with clothing without causing interference. 
     Using the 3D electric circuit printing system according to the present embodiment, it is possible to make a functional and aesthetically satisfactory LED display.  FIG. 20A  shows a flame tattoo printed on the forehead with 6 LEDs to create an animation effect. The LEDs are individually controlled using charlieplexing, and up to eight (8) LEDs can be controlled with three wires connected to the control module.  FIG. 20B  shows various examples of LED displays printed on the forearm. 
     The foregoing detailed description should not be construed as restrictive in any way but as illustrative. For example, in the specification of the present invention, it has been described that an electric circuit is printed on the skin of the human body as an embodiment. The target surface is not limited to the skin of the human body, and any surface to which the 3D printing apparatus can be attached may be the target surface. The scope of the present invention should be determined by a reasonable interpretation of the appended claims, and all modifications within the equivalent scope of the present invention are included in the scope of the present invention.