Highly sensitive, flexible strain sensor based on direct printing of a mixture of metal nanoparticles and carbon nanotubes and method for manufacturing such a sensor

Abstract

Method for manufacturing a flexible strain sensor, comprising: Placing a flexible substrate on a movable stage in a working chamber, in which the movable stage, suitable for a desired movement according to a control signal, and a nozzle, suitable for spraying towards an upper surface of the movable stage, are installed, and with which a first pressure control unit, suitable for controlling an internal pressure of the working chamber, is combined; and preparing a printing material mixture, which comprises metal nanoparticles and carbon nanotubes (CNTs) in a powder form, which is to be introduced into a printing material tank, which is provided with an upper outlet that communicates with a nozzle via a first communication line, and a lower inlet, with which a second pressure control unit, suitable for controlling pressure, is combined. Control, by means of a control unit, a movement of the movable stage by providing a predetermined motion control signal to the movable stage in order to move the flexible substrate at a desired speed along a path which corresponds to a predetermined conductor pattern of a strain sensor, Forming a relatively low-pressure atmosphere within the working chamber by operating the first pressure control unit and simultaneously a relatively high-pressure atmosphere at a lower inlet of the pressure material tank by operating the second pressure control unit, In parallel with controlling the movement of the movable stage, the printing material mixture is forcibly transmitted in an aerosolized state through the first communication line from the printing material tank to be sprayed through the nozzle towards a surface of the flexible substrate by means of a compression wave, which is caused by a pressure difference between the low-pressure atmosphere and the high-pressure atmosphere. Direct printing of a predetermined conductor pattern of the strain sensor onto the flexible substrate by a process in which the printing material mixture, which is extruded through the nozzle, collides with the surface of the flexible substrate to create cracks on the surface, and the CNTs penetrate into the cracks and are mechanically anchored to the flexible substrate, and then the following metal nanoparticles and CNTs of the printing material mixture are deposited in a predetermined width and height on the surface of the flexible substrate. Connecting a first and a second lead wire, which extend electrically to protrude from the flexible substrate, to both ends of the predetermined conductor pattern of the strain sensor and Binding a flexible cover, which has the same size as the flexible substrate, to the surface of the flexible substrate on which the predetermined conductor pattern is printed, so that the predetermined conductor pattern is inserted between the flexible substrate and the flexible cover.

Description

BACKGROUND 1. Technical field

[0001] The present invention relates to the field of the manufacturing technology of a strain sensor, and in particular the technology of manufacturing a strain sensor by direct printing a mixture of conductive nanoparticles and carbon nanotubes (CNTs) onto a flexible substrate. 2. Discussion of related techniques

[0002] A strain gauge, also called a strain sensor, is a sensor used to measure changes in length, such as displacement or deformation of an object, and is typically used to detect even the smallest displacements. A strain gauge generally consists of a resistor and is attached to the object being measured. This resistor converts the object's physical strain into an electrical signal for measurement. If the strain gauge's resistor becomes long and thin due to deformation of the object being measured, its resistance can increase. Conversely, if it becomes thicker and shorter, its resistance decreases. Therefore, by measuring the change in the strain gauge's resistance, it is possible to detect even the slightest deformation of the object being measured.

[0003] For a strain sensor, sensitivity and measuring range are important performance indicators. It should be able to sensitively measure the degree of deformation of the object being measured. A strain sensor with good sensitivity can accurately measure the degree of deformation. Depending on the properties of the object being measured, the degree of deformation can be high. To be used on such an object, the measuring range of the strain sensor must be large enough to adequately cover the deformation range of the object being measured. A strain sensor must be developed that fulfills both of these factors well.

[0004] Additionally, a technique for printing conductive traces, which form the measuring grid of a strain sensor, onto a substrate using an inkjet printing process is known. However, the method for manufacturing the strain sensor using conventional inkjet printing requires a predetermined chemical post-treatment after printing. This is because, due to the properties of the inkjet printing process, an organic solvent is added to the raw ink material in addition to metal nanoparticles. Furthermore, the conductive trace of the strain sensor must be firmly bonded to the substrate. The deformation of the object being measured is transmitted to the conductive trace through the substrate. Despite frequent or significant deformation of the object being measured, the bond between the conductive trace and the substrate must remain strong.Typically, when liquid ink mixed with an organic solvent is used as a conductive trace material, the liquid ink is simply applied to the substrate surface and then dried through chemical post-treatment and heat treatment, thus bonding the trace to the substrate. With this type of bonding, the trace cannot penetrate the substrate surface, and therefore, a strong bond between the mechanically and firmly anchored structures cannot be achieved. If the number or degree of expansion and contraction of the substrate increases due to deformation of the object being measured, the bond strength between the substrate and the trace can weaken.

[0005] Conventional techniques for manufacturing strain sensors can only partially fulfill the requirements of high sensitivity, a wide measuring range, and high durability due to strong coupling between the conductive traces and the substrate, but not all of these simultaneously. It appears that this is because the optimal material and manufacturing process capable of meeting all these conditions at once has not yet been found.

[0006] The following publications are cited as state of the art: US 2015 / 0 174 909 A1, US 2017 / 0 226 362 A1. SUMMARY

[0007] Some embodiments of the present disclosure provide a method for manufacturing a strain sensor with detection properties which, due to its excellent sensitivity, can accurately measure the deformation of an object to be measured and, because of its wide measurement deformation range, can also be used on an object with a wide degree of deformation. Additionally, the present disclosure will provide the strain sensor manufactured by this method.

[0008] Some embodiments of the present disclosure provide a method for manufacturing a strain sensor that can be easily produced without the need for chemical post-treatment and separate heat treatment, by aerodynamically spraying a powdered printing material mixture, containing only metal nanoparticles and CNTs, without liquid organic solvents, onto a flexible substrate at high speed to directly print a conductor track for a strain sensor onto the substrate, and which has excellent durability due to a mechanical anchoring structure between the conductor track and the substrate. Additionally, the present disclosure will provide a strain sensor manufactured using this method.

[0009] The problem to be solved by means of the present revelation is not limited to those described above and can be extended in various ways without deviating from the nature and scope of the present revelation.

[0010] In one aspect, the present disclosure relates to a method for manufacturing a flexible strain sensor based on direct printing of a mixture of metal nanoparticles and CNTs. The method comprises: placing a flexible substrate on a movable stage in a working chamber, in which the movable stage, which is capable of a desired movement according to a control signal, and a nozzle, which is capable of spraying towards an upper surface of the movable stage, are installed, and with which a first pressure control unit, which is capable of controlling an internal pressure of the working chamber, is combined; and preparing a printing material mixture comprising metal nanoparticles and CNTs in powder form, which is to be introduced into a printing material tank, which is provided with an upper outlet that communicates with a nozzle via a first communication line.and a lower inlet, with which a second pressure control unit, suitable for controlling pressure, is combined; controlling, by means of a control unit, a movement of the movable stage by providing a predetermined motion control signal to the movable stage in order to move the flexible substrate at a desired speed along a path corresponding to a predetermined conductor pattern of a strain sensor; creating a relatively low-pressure atmosphere within the working chamber by operating the first pressure control unit and simultaneously a relatively high-pressure atmosphere at a lower inlet of the pressure material tank by operating the second pressure control unit; in parallel with controlling the movement of the movable stage, forcibly transmitting the pressure material mixture in an aerosolized state through the first communication line from the pressure material tank,to be ejected through the nozzle towards a surface of the flexible substrate by means of a compression wave caused by a pressure difference between the low-pressure and high-pressure atmospheres; direct printing of a predetermined conductor pattern of the strain sensor onto the flexible substrate by a process in which the printing material mixture ejected through the nozzle collides with the surface of the flexible substrate to create cracks on the surface, and the CNTs penetrate the cracks and are mechanically anchored to the flexible substrate, and then the following metal nanoparticles and CNTs of the printing material mixture are deposited on the surface of the flexible substrate in a predetermined width and height; connecting a first and a second lead wire, which extend electrically to protrude from the flexible substrate,with both ends of the predetermined conductor pattern of the strain sensor; and binding a flexible cover, which has the same size as the flexible substrate, to the surface of the flexible substrate on which the predetermined conductor pattern is printed, so that the predetermined conductor pattern is inserted between the flexible substrate and the flexible cover.

[0011] In an exemplary embodiment, the flexible substrate and the flexible cover can have the same Shore hardness of 10 to 70 based on Shore A or the same Shore hardness of 22 or less based on Shore D.

[0012] In an exemplary embodiment, the flexible substrate and the flexible cover can be made of polydimethylsiloxane (PDMS).

[0013] In an exemplary embodiment, the predetermined conductor pattern can have a plurality of linear conductor tracks, each extending linearly with a predetermined length in a first direction, and the plurality of linear conductor tracks are arranged side by side to form a series connection, while a predetermined distance between them is maintained in a second direction perpendicular to the first direction.Additionally, a cross-sectional structure of the majority of linear conductor tracks can have a nucleation layer, which is mechanically anchored to the surface of the flexible substrate and is formed by causing the CNTs to penetrate and fix themselves in cracks that are irregularly formed on the surface of the flexible substrate; and a mixed layer of the metal nanoparticles and the CNTs, which is formed by depositing the metal nanoparticles and the CNTs on the nucleation layer with a predetermined thickness and height.

[0014] In an exemplary embodiment, the direct printing of the predetermined conductor pattern can include the formation of a nucleation layer, which is mechanically anchored to the surface of the flexible substrate, by causing the printing material mixture of metal nanoparticles and CNTs from the nozzle to collide with the surface of the flexible substrate to form irregular cracks, so that the CNTs penetrate into the cracks on the surface of the flexible substrate and become fixed there; and printing the predetermined conductor pattern onto the surface of the flexible substrate by applying the printing material mixture, which is subsequently sprayed at high speed onto the nucleation layer to be deposited on the nucleation layer with a predetermined thickness and height, by bonding with the CNTs of the nucleation layer.

[0015] In an exemplary embodiment, the direct printing of the predetermined conductor pattern can include monitoring, by means of a monitoring unit, of a quantity of the printing material mixture which is deposited on the surface of the flexible substrate to be provided to the monitoring unit; and controlling, by means of the control unit, of a movement speed of the movable stage on which the flexible substrate is placed, based on the monitored quantity from the monitoring unit.

[0016] In an exemplary embodiment, the relative low-pressure atmosphere in the working chamber can be a pressure atmosphere of 1 Torr (133.322 Pa) to 10 Torr (13.3322 Pa).

[0017] In an exemplary embodiment, controlling the movement of the movable stage can include adjusting, by means of the control unit, a distance from the nozzle to the movable stage, so that the printing material mixture is aerodynamically focused on the surface of the flexible substrate when the printing material mixture is ejected through the nozzle.

[0018] In an exemplary embodiment, the mixing ratio between the metal nanoparticles and the CNTs in the printing material mixture can be in a range of 60 wt.% - 90 wt.% to 40 wt.% - 10 wt.%.

[0019] Meanwhile, in another aspect, the present disclosure relates to a flexible strain sensor comprising a flexible substrate; a predetermined conductor pattern printed directly onto a surface of the flexible substrate; and a flexible cover that covers and is connected to the surface of the flexible substrate on which the predetermined conductor pattern is printed, such that the predetermined conductor pattern is inserted between the flexible cover and the flexible substrate. The predetermined conductor pattern comprises a plurality of linear conductors, each extending linearly with a predetermined length in a first direction, and the plurality of linear conductors are arranged side by side to form a series connection, while maintaining a predetermined distance between them in a second direction perpendicular to the first direction.Additionally, a cross-sectional structure of the majority of linear conductor tracks features a seed layer, which is mechanically anchored to the surface of the flexible substrate and is formed by causing carbon nanotubes (CNTs) to penetrate and become fixed in cracks formed on the surface of the flexible substrate; and a mixed layer of metal nanoparticles and CNTs, which is formed by depositing the metal nanoparticles and CNTs onto the seed layer to have a predetermined thickness and height.

[0020] In an exemplary embodiment, the flexible substrate and the flexible cover can have the same Shore hardness of 10 to 70 based on Shore A, or the same Shore hardness of 22 or less based on Shore D.

[0021] In an exemplary embodiment, the flexible substrate and the flexible cover can be made of polydimethylsiloxane (PDMS).

[0022] In an exemplary embodiment, the mixing ratio between the metal nanoparticles and the CNTs in the mixed layer can be in a range of 60 wt.% - 90 wt.% to 40 wt.% - 10 wt.%.

[0023] According to the exemplary embodiments of the present disclosure, the material used for printing the conductive trace onto the flexible substrate is a powdered mixture containing only metal nanoparticles and CNTs, and no liquid chemicals, such as solvents. When printing using only metal nanoparticles, the metal nanoparticles sprayed onto the flexible substrate are not adequately deposited on the substrate's surface and rebound, thus preventing the desired printing. However, when the composite nanomaterial containing metal nanoparticles and CNTs is used as the printing material, the composite nanomaterial can be printed directly onto the surface of the flexible substrate with strong adhesion.

[0024] As the flexible substrate is moved at a suitable speed along a desired conductor pattern, the printing material mixture is blended with a high-speed airflow and sprayed in an aerosolized state through a nozzle onto the flexible substrate. The metal nanoparticles and CNTs, sprayed at high impact velocity, can create cracks on the surface of the flexible substrate. Simultaneously, the CNTs penetrate these cracks and are mechanically anchored to the surface of the flexible substrate, forming a solid nucleation layer between the substrate and the CNTs. The metal nanoparticles and CNTs are then deposited onto this nucleation layer, and the conductor is printed directly onto the flexible substrate.

[0025] Thus, the CNTs penetrate the surface of the flexible substrate to mechanically couple the seed layer firmly to the flexible substrate, and then the metal nanoparticles and CNTs are deposited onto the seed layer. The flexible substrate and the printed conductor track can be very tightly coupled. Accordingly, a very strong bond between the flexible substrate and the conductor track can be maintained for a long time despite frequent expansion and contraction of the flexible substrate.

[0026] In addition, the printing material mixture used in the present disclosure contains only powdered metal nanoparticles and CNTs, without any liquid organic solvent or binder resin. The printing mechanism for the flexible substrate is also completed by deposition via high-speed spraying. Therefore, there is no need for separate chemical post-treatment or heat treatment after deposition by such high-speed impact spraying. Thus, the manufacturing process is simple. Moreover, the printing process can be carried out in a low-pressure atmosphere and at room temperature. In this way, manufacturing costs can be reduced because the printing process is not complicated and the pressure and temperature requirements are not stringent.

[0027] The present disclosure has a very significant advantage in that the conductor track consists of a mixture of metal nanoparticles and CNTs, with regard to the properties of the strain sensor. If the conductor track is made only of metal nanoparticles, and the degree of deformation of the object being measured is large, the metal nanoparticles, which were electrically connected before the deformation, can become electrically separated due to the large gap between them caused by the deformation. Consequently, it is impossible to accurately measure the degree of deformation of the object being measured. That is to say, if the conductor track is made using only metal particles, the measurable deformation range is not large.In contrast, if the conductor is made from a composite nanomaterial mixture of metal nanoparticles and CNTs as described in the present disclosure, the metal nanoparticles may have large gaps between them, but the CNTs can still be interconnected, even if the degree of deformation of the object being measured is large. Thus, the metal nanoparticles and the CNTs can maintain a state of electrical interconnection as a whole and function as a strain sensor. Therefore, the strain sensor made from the composite nanomaterial in which metal nanoparticles and CNTs are mixed can have a much wider measuring range compared to the strain sensor made solely of metal nanoparticles. BRIEF DESCRIPTION OF THE DRAWINGS Fig.Figure 1 illustrates a configuration of a strain sensor manufacturing device according to an exemplary embodiment of the present disclosure. Fig. Figure 2 shows a manufacturing process of a strain sensor by direct printing of a conductor track for a strain sensor on the surface of a flexible substrate using an aerosolized mixture of metal nanoparticles and CNTs according to an exemplary embodiment of the present disclosure. Fig. Figures 3 to 6 conceptually illustrate a principle of direct printing of the mixture of metal nanoparticles and CNTs onto the flexible substrate and a subsequent printing process according to an exemplary embodiment of the present disclosure. Fig.Figure 7 illustrates images taken using a scanning electron microscope and a confocal microscope of a layer of the mixture of metal nanoparticles and CNTs deposited onto the flexible substrate by direct printing, according to an embodiment of the present disclosure. Fig. Figure 8 illustrates a case in which a large deformation force is applied to a conductor track of the strain sensor, which is made only of metal nanoparticles, resulting in an electrical separation between the metal nanoparticles. Fig. Figure 9 shows that a state in which the metal nanoparticles and the CNTs are electrically connected is maintained even when a large deformation force is applied when the conductor track of the strain sensor consists of the nanomaterial mixture of metal nanoparticles and CNTs, according to an exemplary embodiment of the present disclosure. Fig. Figure 10 is a graph showing a performance evaluation result of a strain sensor that was actually manufactured according to an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION OF THE REVELATION

[0028] Preferred exemplary embodiments of the present disclosure are described in more detail below with reference to the accompanying drawings. The same reference numerals are used for identical components in the drawings, and repeated descriptions of identical components are omitted. The inventive concept can be implemented in many different forms and should not be interpreted as being limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure is thorough and complete, and fully conveys the scope of the inventive concept to the person skilled in the art. In the drawings, the sizes and relative sizes of layers and regions may be exaggerated for clarity. The same reference numbers refer to identical elements.

[0029] Although terms such as first, second, third, etc., may be used in this description to describe different elements, it should be understood that these elements are not intended to be limited by these terms. This term is used to distinguish one element from another. Accordingly, a first element discussed below may also be designated as a second element without departing from the teaching of the invention concept. As used herein, the term "and / or" includes any and all combinations of one or more related listed elements.

[0030] When an element is described as "connected" or "coupled" to another element, it will be understood that the element may be directly connected or coupled to the other element, or that intermediate elements may exist. Conversely, when an element is described as "directly connected" or "directly coupled" to another element, there is no intermediate element. Other expressions used to describe the relationship between components should be interpreted in the same way (e.g., "between" vs. "directly," "adjacent" vs. "directly adjacent," etc.). Additionally, the terminology used herein serves the purpose of describing specific exemplary embodiments and is not intended to limit the concept of the present disclosure. As used herein, the singular forms "a" and "the" are intended to include the plural forms, unless the contact specifies otherwise.The term “exhibit” as used herein refers to the presence of a particular feature, integer, step, process, component and / or element, but it shall be understood that this does not exclude the presence or addition of one or more other features, integers, steps, processes, elements, components and / or groups thereof.

[0031] Fig. Figure 1 illustrates a configuration of a strain sensor manufacturing device 100 according to an exemplary embodiment of the present disclosure.

[0032] With reference to Fig. 1 The strain sensor manufacturing device 100 can have a working chamber 10, a first pressure control unit 12, a movable stage 14, a nozzle 20, a pressure material tank 30 and a first communication line 44, a second pressure control unit 70 and a second communication line 54.

[0033] The work chamber 10 can provide a workspace for printing a conductor track 160 of a strain sensor 200 on the surface of a flexible substrate 110a. The nozzle 20 can be installed on the upper inside of the work chamber 10, and the movable stage 14 can be installed on its lower side.

[0034] The movable stage 14 can comprise a stage 15 and a stage drive unit 18. The stage 15 can have a plate structure on which the flexible substrate 110a is placed and supported. The stage drive unit 18 can move the stage 15 according to a motion control signal for positioning movement or control the positioning movement as well as the change of angle with respect to the horizontal plane. That is, the stage drive unit 18 can be configured to move the stage 15 at a desired speed along a desired path in a two-dimensional plane or in three-dimensional space, or to control an angle of inclination with respect to a horizontal plane together with such movement. According to an exemplary embodiment, the stage drive unit 18 can, for example, adopt a linear guide structure which is suitable for 3-axis control of the stage drive unit 18.Furthermore, a ball joint can be mounted at the center of the base surface of the stage 15, and a plurality of cylinders or an expansion and contraction element suitable for length adjustment can be mounted at the edge of the base surface of the stage 15, so that processes for tilt control and rotation in all directions of the stage 15 can be carried out. Using this configuration, it will be possible to realize different lamination states of a printing material mixture 150 on the surface of the flexible substrate 110a.

[0035] The nozzle 20 can be fixed to the ceiling of the working chamber 10, and the nozzle outlet, through which the printing material mixture 150 is injected, can be installed facing the upper surface of the movable stage 14. The nozzle 20 can be configured as a converging nozzle or as a capillary nozzle without a change in cross-section. The nozzle 20 can be aerodynamically focused onto the surface of the flexible substrate 110a to inject the powder of the printing material mixture 150 in a concentrated state. The outlet diameter of the nozzle 20 can range in size from approximately 10 µm to several mm.

[0036] The flexible substrate 110a, which is the object to be printed, can be made of a material with good elasticity and flexibility, whose physical properties are not hard but soft. According to an exemplary embodiment, the flexible substrate 110a can be manufactured using a material with a Shore hardness of 10 or more and 70 or less (i.e., hardness classified as extra-soft or soft) based on Shore A, or a Shore hardness of 22 or less based on Shore D. A typical material with such hardness properties can be PDMS. That is, a substrate made of PDMS can be cut to a desired size and used as the flexible substrate 110a to be printed.

[0037] The interior of the working chamber 10 must maintain a negative pressure atmosphere (e.g., approximately 1 Torr to 10 Torr), lower than normal pressure, during the printing process. The working chamber 10 can be equipped with the first pressure control unit 12, which is suitable for controlling the internal pressure of the working chamber 10. The first pressure control unit 12 can be configured to connect a first vacuum pump, such as a rotary suction pump, to the working chamber 10 to remove air from the working chamber to the outside. The first pressure control unit 12 can extract the air from the working chamber 10 to the outside and create a relatively low-pressure atmosphere with its internal pressure compared to the printing material tank 30.

[0038] The printing material tank 30 can provide an interior containing a mixture, referred to as "printing material mixture" 150, of conductive nanoparticles 152 and CNTs 154. The printing material tank 30 can be configured to discharge printing material mixture 150 through the upper outlet by means of the high-pressure airflow applied through the lower inlet.

[0039] The printing material tank 30 can communicate with the nozzle 20 in the working chamber 10 via the first communication line 40, which is connected to the upper through-hole. The first communication line 44 is a passage through which the printing material mixture 150, which is dispensed from the printing material tank 30, is forcibly transferred to the nozzle 20.

[0040] An aerodynamic filter 40 can also be installed in a predetermined section of the first communication line 44. The aerodynamic filter 40 can control the amount of particles of the printing material mixture 150 that is transferred from the printing material tank 30 to the working chamber 10.

[0041] The pressure material tank 30 can also be connected to the second pressure control unit 70 via the second communication line 54, which is coupled to a lower through-hole. The second pressure control unit 70 can include a pressure unit 50 and a rinsing unit 60. The second communication line 54 can be branched into a second-1 communication line 54-1 and a second-2 communication line 54-2 and connected accordingly to the pressure unit 50 and the rinsing unit 60.

[0042] The pressure unit 50 can be used to provide a high-pressure airflow to the pressure material tank 30 in order to create a high-pressure atmosphere at the lower inlet side compared to that at the upper outlet side of the pressure material tank 30. This high-pressure atmosphere can be at a relatively high pressure compared to the pressure in the working chamber 10. The pressure unit 50 can include an air compressor 51 for generating compressed air and supplying it to the pressure material tank 30, and a flow control valve 52, which is arranged in the second communication line 54, for controlling the pressure applied to the inlet of the pressure material tank 30 by controlling the amount of air compressed by the air compressor 51 and supplied to the pressure material tank 30.The flow control valve 52 can be configured using a type of solenoid valve suitable for controlling the pressure of the applied air according to its degree of opening and closing. Whether the air compressor 51 is operated, as well as the opening and closing of the flow control valve 52, can be controlled by a control unit 80.

[0043] After the printing process of the printing material mixture 150 onto the flexible substrate 110a is complete, the printing material mixture 150 can remain in the working chamber 10, the first communication line 44, the aerodynamic filter 40, etc. The purge unit 60 is a means of returning the transport gas, which is mixed in the aerosolized printing material mixture 150 that remains in the working chamber 10, the filter 40, the first communication line 44, etc., to outside the printing material tank 30 after the printing process on the flexible substrate 110a is complete. To return the remaining printing material mixture 150, the purge unit 60 draws air from the printing material tank 30 to create a relatively low-pressure atmosphere inside the printing material tank 30 compared to the working chamber 10.The purge unit 60 may include a second vacuum pump 61 for drawing air from the lower side of the printing material tank 30 through the second communication line 54 and for expelling the air to the outside, and a purge valve 62, which is arranged in the second communication line 54, for controlling the amount of air drawn in by the second vacuum pump 61. Whether the second vacuum pump 61 should be operated and whether the purge valve 62 should be open or closed can also be controlled by the control unit 80. Here, the intensity of the suction force of the second vacuum pump 61 can be greater than the intensity of the suction force of the first pressure control unit 12, such as a first vacuum pump, in order not to impair the suction of the printing material mixture 150, which remains in the working chamber 10 and the first communication line 44 and the aerodynamic filter 40.

[0044] The pressure unit 50 and the purge unit 60 can be configured to operate alternately. This means that while the pressure unit 50 is operating, the purge unit 60 cannot be operating, and conversely, the pressure unit 50 cannot be operating while the purge unit 60 is operating. In this way, the purge unit 60 can stop the air intake and supply compressed air to the printing material tank 30 simultaneously. Additionally, the pressure unit 50 can simultaneously block the compressed air supply and draw air from the printing material tank 30 by operating the purge unit 60. The transfer of the printing material mixture 150 to the working chamber 10 and the return of the printing material mixture 150 from the working chamber 10 can be immediate, fast, and continuous.Such a control process of the pressure unit 50 and the rinsing unit 60 can be controlled by means of the control unit 80.

[0045] According to an exemplary embodiment, the strain sensor manufacturing device 100 can further include a monitoring unit 75 and the control unit 80.

[0046] The monitoring unit 75 can be installed in the work chamber 10 to monitor in real time the process of spraying the printing material mixture 150 and laminating it onto the flexible substrate 110a. The monitoring unit 75 can be configured with, for example, an optical microscope or a scanning electron microscope.

[0047] The control unit 80 can be electrically connected to the pressure unit 50, the rinsing unit, and the monitoring unit 75. It may be possible to control the start and stop of operation of the pressure unit 50 and the rinsing unit 60 based on image information obtained from the monitoring unit 75. The control unit 80 can also be electrically connected to the first pressure control unit 12 to control its operation.

[0048] The control unit 80 can also be electrically connected to the stage drive unit 18 to control its movement. Based on the pattern of the conductor track 160 to be printed, information about the path along which the stage drive unit 18 is to move can be preset in the control unit 80. The stage drive unit 18 can perform a movement according to a motion control signal provided by the control unit 80. The control unit 80 can control the stage drive unit 18 to regulate the distance between the flexible substrate 110a and the nozzle 20. When the printing material mixture 150 is sprayed through the nozzle 20 in an aerosolized state, it can be aerodynamically focused by adjusting the distance so that it is focused onto the surface of the flexible substrate 110a and sprayed accordingly.

[0049] The control unit 80 can integrally control various processes, such as the operation and stopping of the pressure unit 50, the operation and stopping of the rinsing unit 60, the operation of the first pressure control unit 12, and the movement speed of the stage drive unit 18, based on the image information obtained from the monitoring unit 75. The control unit 80 can include programs for carrying out such control, a storage device suitable for storing and executing the programs, and a processor.

[0050] According to an exemplary embodiment, the printing material mixture 150 can be a composite nanomaterial in which conductive nanoparticles 152 and CNTs 154 are mixed. The conductive nanoparticles 152 can be, for example, nanoscale metal particles. A typical example of the metal nanoparticles can be silver (Ag) nanoparticles (AgNPs) with excellent conductivity. Instead of silver, conductive nanoparticles made of other metals, such as copper, gold, platinum, nickel, aluminum, titanium, etc., can be used. The conductive nanoparticles 152 can have a diameter in the range of several nanometers to several tens of micrometers.

[0051] The CNTs 154 can be single-walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs). MWCNTs may exhibit better bonding with metal nanoparticles than SWCNTs.

[0052] The mixing ratio of the conductive nanoparticles (i.e., metal nanoparticles) 152 and the CNTs 154, which constitute the printing material mixture 150, can be in the range of 60 wt.%–90 wt.% to 40 wt.%–10 wt.%. If the metal nanoparticles 150 are less than 60 wt.%, it may be difficult to achieve a desired level of conductive properties. At more than 90 wt.%, the CNTs 154 are insufficient, so that lamination on the flexible substrate 110a cannot be carried out effectively. Within the above mixing ratio range, a suitable value can be set according to the application objective.That is, within the mixing ratio range, in the application area where sensitivity is more important than the measurable range, it is preferred that the printing material mixture 150 is made such that it contains much more metal nanoparticles 152 compared to a normal mixture, and, conversely, in the application area where the measurable range is more important than sensitivity, it is preferred that the printing material mixture 150 is made such that it contains much more CNTs 154 compared to a normal mixture.

[0053] Fig. Figure 2 shows a manufacturing process of the strain sensor 200 by direct printing of the conductor track for the strain sensor on the surface of the flexible substrate 110a with aerolising of the printing material mixture 150 of metal nanoparticles and CNTs according to an exemplary embodiment.

[0054] With reference to Fig.2. The flexible substrate 110a can first be positioned on stage 15 in the working chamber 10. In this state, the nozzle 20 can be located directly above the flexible substrate 110a placed on stage 15, and its outlet is opposite the upper surface of the flexible substrate 110a (see (a) in Figure 2). Fig. 2).

[0055] The stage drive unit 18 can drive the stage 15, allowing the nozzle outlet 20 to move relative to the pattern of the conductor track 160 of the strain sensor 200. While the nozzle outlet 20 moves relative to the stage 15, the printing material mixture 150 can be sprayed in an aerosolized state through the nozzle outlet 20. Accordingly, the conductor track 160 of the strain sensor 200 can be directly printed onto the surface of the flexible substrate 110a (see (b) in Figure 1). Fig. 2).

[0056] The conductor track 160 of the strain sensor 200 can, for example, consist of long and thin conductor strips arranged parallel to each other in a zigzag pattern to form a series connection. The resulting conductor track 160 can have a waveform-like pattern in which rectangular pulse waves are interconnected. The conductor track 160 can be called a measuring grid. The length (L) and width (W) of the measuring grid can be set to different sizes according to the application.

[0057] After the direct printing of the conductor track 160 onto the flexible substrate 110a is complete, lead wires 162 can be connected to both ends of the conductor track 160 to extend outside the flexible substrate 110a (see (c) in Fig. 2) The supply wires 162 can be connected in such a way that a separate strip-shaped conductor is connected to the conductor track 160 outside the working chamber 10.

[0058] After the conductor track 160 and the connecting wires 162 are connected, they can be covered with a flexible cover 110b, which is to be bonded to the flexible substrate 110a. Accordingly, the conductor track 160 and the connecting wires 162 can be inserted between the flexible substrate 110a and the flexible cover 162 (see (d) in Figure 1). Fig. 2) The flexible cover 110b can be made from the same material as the flexible substrate 110a mentioned above.

[0059] Next we will show Fig. 3 to 6 the principle that the printing material mixture 150 consisting of metal nanoparticles 152 and CNTs 154 is printed directly onto the flexible substrate 110a, and a subsequent printing process (the process according to (b) in Fig.2) according to an embodiment of the present disclosure, which is shown conceptually. The manufacturing process is described in detail below with further reference to these drawings.

[0060] First, the printing material can be prepared. For this purpose, the printing material mixture 150, in a powder state in which the metal nanoparticles 152 and the CNTs 154 are mixed, is fed into the printing material tank 30.

[0061] An object to be printed can be prepared. For this purpose, the flexible substrate 110a can be placed on the stage 15, which can be moved by means of the stage drive unit 18.

[0062] To print the printing material mixture 150 onto the flexible substrate 100a in the work preparation state, the control unit 80 can provide a drive control signal to the first pressure control unit 12 to operate it. When the first pressure control unit 12 is operating, the pressure inside the working chamber 10 can be reduced. The control unit 80 can control the internal pressure of the working chamber 10 to maintain it at a predetermined level of negative pressure. The pressure inside the working chamber 10 can be maintained, for example, from approximately 1 Torr to 10 Torr. Additionally, the interior of the working chamber 10 can be kept at room temperature during the printing process.

[0063] In parallel, the control unit 80 can operate the pressure unit 50 of the second pressure control unit 70 to create a high-pressure atmosphere higher than the internal pressure of the working chamber 10 at the lower inlet of the pressure material tank 30. This means that the flow control valve 52 can be opened while the air compressor 51 is operating to generate high-pressure compressed air. At the same time, the control unit 80 can control the purge unit 60 to be deactivated. This means that the purge valve 62 is controlled to close without the second vacuum pump 61 being operated. Accordingly, a pressure differential can occur between the high pressure at the bottom of the pressure material tank 30 and the low pressure inside the working chamber 10, and the pressure material mixture 150 in the pressure material tank 30 can be excited into an aerosol state by means of this pressure differential and aerodynamically fed to the nozzle 20 for injection.

[0064] More precisely, the high-pressure compressed air generated by the air compressor 51 can be supplied to the lower inlet of the pressure material tank 30 via the second communication line 54 by opening the flow control valve 52. Accordingly, a high pressure (HP) can be applied to the lower inlet of the pressure material tank 30. Conversely, a relative low pressure, which is lower than normal atmospheric pressure, is generated within the working chamber 10 by operating the first pressure control unit 12. The outlet of the nozzle 20 can essentially have the same pressure as the internal pressure of the working chamber 10. Therefore, a pressure differential can occur between the lower inlet of the pressure material tank 30 and the outlet of the nozzle 20.Due to the pressure difference, a high-speed airflow can be generated from the printing material tank 30 to the nozzle 20 via the first communication line 44 in the form of a shock wave (or compression wave). This high-speed airflow, generated by the resulting shock wave, excites the printing material mixture 150 contained in the printing material tank 30 into an aerosolized state as it is mixed and dispersed by the high-speed airflow.

[0065] When the flow control valve 52 is closed, the powder of the printing material mixture 150, which is mixed into the high-speed airflow and excited in the printing material tank 30, rises and returns to its original state by gravity. However, a transfer gas (combination of air and printing material mixture) may remain inside the printing material tank 30 and the first communication line 44 between the opening and closing times of the flow control valve 52. If this residual transfer gas is injected in excess into the nozzle 20, it can cause undesired printing. To prevent the powder of the aerosolized printing material mixture 150 from being injected in excess into the nozzle 20 by stabilizing it, the control unit 80 can purge the excited printing material mixture 150.This means that while the operation of the air compressor 51 is stopped and the flow control valve 52 is closed, the purge valve 62 can be opened while the second vacuum pump 61 of the purge unit 60 is operating. This allows the printing material mixture 150 to be purged in its excited state in the printing material tank 30 and the first communication line 44 and returned to the purge unit 60.

[0066] In this way, by pumping the printing material mixture 150, which can be suitably controlled during the excitation and purging processes, the printing material mixture 150 can be aerosolized and emitted from the upper outlet of the printing material tank 30 to be forcibly transferred through the aerodynamic filter 40 and the first communication line 44 to the nozzle 20. When the printing material mixture 150 is excited, according to Stokes' law, small particles or particles with low density are accelerated first by the transport gas. The printing material mixture 150 in an aerosol state, carried in a high-speed airflow and forcibly transported through the first communication line 44, is aerodynamically focused in the nozzle 20 and can be ejected through the outlet of the nozzle 20.

[0067] While the printing material mixture 150 is being extruded through the nozzle 20, the control unit 80 can provide a predetermined motion control signal to the stage drive unit 18 to control the stage 15 so that it moves at a desired speed along the path corresponding to a predetermined strain sensor conductor pattern. Through such motion control of the movable stage 14, the flexible substrate 110a can also move along a path corresponding to the conductor pattern 160 of the strain sensor 200. The monitoring unit 75 can monitor size (height and width) information of the printing material mixture 150 being deposited on the surface of the flexible substrate 110a and provide this size information to the control unit 80. The control unit 80 can then control the aforementioned motion control based on the information provided by the monitoring unit 75.

[0068] The control unit 80 can also adjust the distance between the flexible substrate 110a and the nozzle 20 by controlling the stage drive unit 18, so that the printing material mixture 150 can be ejected in a state where it is precisely focused onto the surface of the flexible substrate 110a. The printing material mixture 150 can be ejected through the nozzle in an aerosolized state. At this time, the desired conductor pattern can only be printed precisely if the printing material mixture 150 is aerodynamically focused to create a good focus on the surface of the flexible substrate 110a. For this purpose, the control unit 80 can adjust the distance between the flexible substrate 110a and the nozzle 20.

[0069] As the flexible substrate 110a moves within the working chamber 10, the printing material mixture 150, injected through the nozzle 20, collides at high speed with the surface of the flexible substrate 110a, which is positioned on the stage 15. This collision can cause cracks of random shapes to form on the surface of the flexible substrate 110a. The collision of the metal nanoparticles 152 significantly contributes to crack formation. The CNTs 154 of the printing material mixture 150 can penetrate the cracks to mechanically create an anchoring structure on the surface of the flexible substrate 110a. Accordingly, the CNTs 154 can form a very strong bond with the surface of the flexible substrate 110a. This anchoring structure can serve as a nucleation layer onto which the printing material mixture 150, subsequently injected, can be deposited.Accordingly, the printing material mixture 150 can be deposited on the surface of the flexible substrate 110a to have a desired thickness and height, so that the conductor track 160 of the strain sensor can be directly printed with a desired pattern.

[0070] The principle by which the printing material mixture 150 is directly printed in the form of being deposited on the surface of the flexible substrate 110a will be described in more detail.

[0071] Fig. Figure 3 shows a state in which the printing material mixture 150, which is sprayed from the nozzle 20 in an aerosol state, collides with the surface of the flexible substrate 110a. As in Fig.As shown in Figure 3, the printing material mixture 150, which is extruded from the nozzle 20 in an aerosol state, can collide with the flexible substrate 220a at a very high impact velocity while being aerodynamically focused. The impact velocity can be, for example, approximately 200 m / s. Through this high-speed impact printing, the metal nanoparticles 152 and the CNTs 154, which constitute the printing material mixture 150, can possess high kinetic energy. The printing material mixture 150 with such high kinetic energy can generate cracks 112 with random shapes as it collides with the surface of the flexible substrate 110a.

[0072] If the printing material mixture consists only of metal nanoparticles without containing CNTs, the metal nanoparticles can crack the surface of the flexible substrate 110a as they collide with it at high impact velocity. However, there is a strong tendency for the metal nanoparticles to rebound off the flexible substrate 110a rather than grow onto it while being deposited. Consequently, there is a problem with the metal nanoparticles not being effectively printed onto the surface of the flexible substrate 110a.

[0073] However, this problem can be solved by using the composite nanomaterial in which CNTs are mixed with metal nanoparticles as a printing raw material according to the embodiment of the present disclosure. That is, in the initial state of the collision, the metal nanoparticles 152 can significantly contribute to the formation of cracks 112 on the surface of the flexible substrate 110a. Once the cracks 112 have formed on the surface of the flexible substrate 10, the CNTs 154 can then burrow into the cracks 112 and become firmly fixed, thus being mechanically anchored to the flexible substrate 110a.

[0074] Accordingly, as in Fig.As shown in Figure 4, a mechanical anchoring structure can be formed between the CNTs 154 and the flexible substrate 110a. That is, a CNT layer 156, firmly rooted in the surface of the flexible substrate 110a, can be formed by means of this anchoring structure. The CNT layer 156 of this anchoring structure can serve as a nucleation layer 156 to enable the subsequently deposited metal nanoparticles 152 to be firmly deposited onto the flexible substrate 110a.

[0075] Since the printing material mixture 150 continuously deposits onto the nucleation layer 156, which was initially produced using the CNTs 154, the nucleation layer 156 can continue to combine with the CNTs 154 and the metal nanoparticles 152 and grow larger, as shown in Fig.Figure 5 shows that the CNTs 154 of the nucleation layer 156 can retain the metal nanoparticles 152 that penetrate it and can also form a bond with the incoming CNTs. Accordingly, the rebound of the metal nanoparticles 152 can be suppressed as much as possible, and a strong bond can be established between the CNTs 154 and the metal nanoparticles 152.

[0076] This bonding process can be repeated. That is, the CNTs 154 and metal nanoparticles 152, which are deposited on the surface of the flexible substrate 110a, can form an anchoring structure with the CNTs 154 colliding with it, and the metal nanoparticles 152 can combine with the anchoring structure by penetrating it. Through such reciprocal bonding, direct printing onto the flexible substrate 110a becomes possible, which was previously impossible with metal nanoparticles alone.

[0077] The printing material mixture 150 can exhibit various deposition states according to the opening and closing degree of the flow control valve 52 of the printing unit 50. The deposition state can be monitored in real time by the monitoring unit 75 in the working chamber 10. The amount of aerosol fed from the printing material tank 30 to the nozzle 20 can be controlled according to the opening and closing time of the flow control valve 52. Accordingly, the pattern size of the conductive track 160 printed on the flexible substrate 110a can be controlled. If the amount of aerosol fed from the printing material tank 30 to the nozzle 20 can be controlled to be as low as possible, the size of the conductive track pattern printed on the flexible substrate 110a can be reduced, allowing for the printing of a more complex pattern.Accordingly, by varying the time interval between the opening and closing times of the flow control valve 52, the size of the conductor pattern printed onto the flexible substrate 110a, i.e., the height and width of the printed conductor pattern, can also be varied. Such control can be carried out using the control unit 80. By controlling the movement speed of the flexible substrate 110a and the pressure of the printing material mixture 150 per unit of time, the layer of the conductor 160 can be grown to a desired width and height on the surface of the flexible substrate 110a. Fig. Figure 6 conceptually illustrates the conductor track 160 as an example, which was completely deposited onto the surface of the flexible substrate 110a by direct printing up to a desired width and height.

[0078] For testing purposes, the inventors actually manufactured the strain sensor. After preparing the printing material mixture 150 by mixing silver nanoparticles (AgNPs) and MWCTNs, the conductive track 160 for the strain sensor was printed onto a PDMS substrate, specifically the flexible substrate 110a, using the strain sensor fabrication device 100. Fig. 1 printed. Fig. Figure 7 illustrates an image (a) taken using a scanning electron microscope and a confocal microscope image (b) of the conductor track 160, which is actually printed onto the flexible substrate 110a by direct printing a mixture of AgNPs and MWCNTs. Referring to image (a) of Fig. 7 can be seen that the vertical cross-section of the conductor track 160 has an essentially triangular shape and is firmly deposited on the surface of the flexible substrate 110a with almost uniform width and height. [Table 1] Substrate / Material AgNP AgNP / MWCNT PET ◯ × Polyimid ◯ × silicon ◯ × CFRP ◯ △ GFRP ◯ △ PDMS Sylgard184™ × ◯ EcoFlex™ × ◯ <×: no deposition, △: powder deposition (no pattern), ◯: pattern deposition>

[0079] When manufacturing the strain sensor 200 using the direct printing method according to the present disclosure, the properties of the substrate used for printing and the printing nanomaterial can be important factors influencing the printing performance. Table 1 shows the printing performance according to the types of nanomaterials and substrates used for printing.

[0080] Initially, when only AgNPs were used as the printing nanomaterial for high-speed impact printing through nozzle 20, the AgNPs were well deposited on a substrate made of PET film or polyimide film, a silicon substrate, or a substrate made of a relatively stable material, such as carbon fiber reinforced plastic (CFRP) or glass fiber reinforced plastic (GFRP).

[0081] On the other hand, when the mixture of AgNPs and MWCNTs was used as the printing nanomaterial for high-speed impact printing, the mixture did not deposit well on the substrate, which was made of PET film, polyimide film, or silicon. A phenomenon occurred where the mixture fractured due to the impact force caused by the high-speed impact collision against the substrate. In other words, MWCNTs did not deposit properly on the hard substrate because they exhibited a brittle property, unlike AgNPs, when colliding with a solid substrate at high speed, due to their elongated shape compared to their diameter. For the CFRP or SFRP substrate, the mixture of AgNPs and MWCNTs was only partially deposited in the cavity between the fibers.

[0082] However, different results were obtained when a flexible, low-hardness substrate, such as the PDMS substrate, was used. Specifically, when high-speed impact printing was performed using only AgNPs as the printing nanomaterial, the AgNPs were not properly deposited and bounced off due to the stretchable or elastic properties of the PDMS substrate. On the other hand, when high-speed impact printing was performed using a mixture of AgNPs and MWCNTs as the printing nanomaterial, it was well deposited onto the PDMS substrate, and a desired conductive trace was printed.

[0083] In this way, when the mixture of AgNPs and MWCNTs is used as a printing material for high-impact injection molding, it collides with the surface of the PDMS substrate to initially create cracks on the surface of the PDMS substrate and form the anchoring structure, while the MWCNTs mechanically anchor to the surface of the substrate where the cracks are formed. Because the anchoring structure acts as a nucleation layer, the mixture of AgNPs and MWCNTs injected onto the nucleation layer can be readily deposited. The printing method according to the present disclosure performs direct printing using a powder state, without adding a solvent to the printing material, under a low-pressure condition near atmospheric pressure at room temperature.Therefore, it is environmentally friendly because it does not require a separate chemical post-treatment process or heat treatment process to remove the solvent after printing.

[0084] The printing method according to the embodiment described in the present disclosure is a process that can directly print various inorganic materials onto flexible substrates without chemical reaction or heat treatment. The direct printing process is particularly effective for printing microscale patterns. A key advantage is that a porous structure can be printed at a microscale without design limitations. Furthermore, the limitations of traditional processes, such as laser processing, can be overcome because the printed pattern can be modified as required.

[0085] Meanwhile, it shows Fig.8. A case in which a large deformation force is applied to a conductor track of the strain sensor, which is made only of metal nanoparticles, resulting in an electrical separation between the metal nanoparticles. In contrast, [the text abruptly ends here, so the translation stops as well.] Fig. 9, that a state is maintained in which, when the conductor track of the strain sensor is composed of the nanomaterial mixture of metal nanoparticles and CNTs according to the present disclosure, the metal nanoparticles and the CNTs are still electrically connected even when a large deformation force is applied.

[0086] With reference to Fig.8. In the strain sensor whose conductor track is printed using only metal particles, when strain is applied to the conductor track due to the deformation of the object being measured, the electrical connection between the metal nanoparticles 152 is interrupted by the flexible substrate, resulting in a sudden and large change in the conductor track's resistance. In such a condition, the extent of the strain cannot be measured accurately. For this reason, when the strain sensor's conductor track is composed solely of metal nanoparticles 152, the measuring range is inevitably narrow. Although the measurement sensitivity can be high, the limited measuring range is a disadvantage.

[0087] On the contrary, with reference to Fig.9. In the strain sensor, whose conductor track 160 is printed using a composite nanomaterial in which the metal nanoparticles 152 and the CNTs 154 are mixed as the printing mixture according to the embodiment of the present disclosure, the conductor track 160 can be configured in such a way that the CNTs 154 are mixed between the metal nanoparticles 152. The conductor track 160 with this structure can have high measurement sensitivity as well as a wide strain measurement range. When strain is applied to the flexible substrate 110a, even if the metal nanoparticles 152 can be separated from one another, they can still have electrical connectivity through the CNTs 154. In addition, the CNTs themselves also have some elasticity. Accordingly, the resistance of the conductor track 160 of the strain sensor 200 changes comparatively less, so that the strain measurement range is much wider and the sensitive strain sensor can be produced.

[0088] Fig. Figure 10 illustrates various graphs showing performance evaluation results of a strain sensor that was actually manufactured by printing the mixture of AgNPs and CNTs onto the PDMS substrate according to an embodiment of the present disclosure.

[0089] Graph (a) of Fig. Figure 10 shows the result of measuring the change in relative resistance up to the maximum measuring range. The strain gauge was deformed to a strain of approximately 75% of its existing length and ultimately showed a resistance value 45 times the initial resistance. Based on the strain factor (relative change in resistance / strain value), which is a commonly used index to indicate the sensitivity of a strain gauge, it showed a high sensitivity of 50 or more.

[0090] Graph (b) of Fig.Figure 10 shows the results of comparing the actual experimental values ​​with the model's simulation values, which are presented through statistical analysis using a two-variable probit model. The analysis was performed by discretizing the case where electrical connectivity between the AgNPs and the CNTs was maintained and the case where their electrical connection was lost. It was assumed that the probability of transitioning from 1 to 0 at a given displacement value, where 1 represents electrical connectivity between the AgNPs and 0 represents the non-electrical connection between them, follows a normal distribution. One result was that the proposed model accurately represented the behavior of the actual strain sensor.

[0091] Graphs (c) and (d) of Fig.Figure 10 presents the results of experiments conducted to evaluate the life cycle of the manufactured strain sensor. For the experiment, the process of stretching the strain sensor and restoring it to its original state was performed approximately 1,000 times. The experiment revealed that the resistance value of the strain sensor exhibited an error of within 5% when stretched to its maximum displacement.

[0092] Graph (e) of Fig. Figure 10 shows the result of an experiment involving changes in the composition ratio of AgNPs and CNTs. The results showed that as the mass of AgNPs increased compared to CNTs, the sensitivity of the strain sensor increased, while the measuring range decreased. This indicates a trade-off between measuring range and sensitivity. Depending on the application, optimal values ​​for measuring range and sensitivity can be determined.

[0093] For example, in applications that do not require high sensitivity, such as soft robots, but rather a wide measuring range, a lower mass of AgNPs compared to CNTs may be suitable.

[0094] On the other hand, in applications that require high sensitivity, such as vibration sensors, but do not significantly affect the measuring range, a higher mass of AgNPs may be suitable compared to CNTs.

[0095] Graph (f) of Fig.Figure 10 shows the change in sensitivity and measuring range according to the scan time of the manufacturing process. The scan time is a value that indicates the time it takes to print a conductor pattern of a unit length. A long scan time means that the nozzle remains in the same position for a longer period. Accordingly, when the amount of metal nanoparticles and CNTs per unit volume within the conductor pattern is controlled, the electrical properties of the conductor-160 pattern change. As a result of the experiment, it was shown that both the sensitivity and the measuring range of the strain sensor decreased with increasing scan time.

[0096] The present disclosure can be used to fabricate the strain sensor using a flexible substrate. Furthermore, it can be widely used to fabricate other conductor patterns using flexible substrates.

[0097] Although the embodiments have been described with reference to the limited drawings, as described above, with reference to the embodiments of the present disclosure disclosed in the description, specific structural or functional descriptions are only exemplary for the purpose of describing the embodiments of the present disclosure. Embodiments of the present disclosure can be implemented in various forms and should not be considered limited to the embodiments described herein. That is to say, because the present disclosure can have various modifications and different forms, specific embodiments are illustrated in the drawings and described in detail in the description.However, this is not intended to limit the present disclosure to the specific forms disclosed, but should be understood as encompassing all modifications, variations, and substituents that are contained within the essence and scope of the present disclosure. It will be understood by those skilled in the art that various modifications and changes can be made to the present disclosure without deviating from the essence and scope of the present disclosure as set forth in the following claims. Therefore, other implementations, other embodiments, and variations of the claims are also within the scope of the following claims.

Claims

[1] Method for manufacturing a flexible strain sensor, comprising: Placing a flexible substrate on a movable stage in a working chamber, in which the movable stage, suitable for a desired movement according to a control signal, and a nozzle, suitable for spraying towards an upper surface of the movable stage, are installed, and with which a first pressure control unit, suitable for controlling an internal pressure of the working chamber, is combined; and preparing a printing material mixture, which comprises metal nanoparticles and carbon nanotubes (CNTs) in a powder form, which is to be introduced into a printing material tank, which is provided with an upper outlet that communicates with a nozzle via a first communication line, and a lower inlet, with which a second pressure control unit, suitable for controlling pressure, is combined. Control, by means of a control unit, a movement of the movable stage by providing a predetermined motion control signal to the movable stage in order to move the flexible substrate at a desired speed along a path which corresponds to a predetermined conductor pattern of a strain sensor, Forming a relatively low-pressure atmosphere within the working chamber by operating the first pressure control unit and simultaneously a relatively high-pressure atmosphere at a lower inlet of the pressure material tank by operating the second pressure control unit, In parallel with controlling the movement of the movable stage, the printing material mixture is forcibly transmitted in an aerosolized state through the first communication line from the printing material tank to be sprayed through the nozzle towards a surface of the flexible substrate by means of a compression wave, which is caused by a pressure difference between the low-pressure atmosphere and the high-pressure atmosphere. Direct printing of a predetermined conductor pattern of the strain sensor onto the flexible substrate by a process in which the printing material mixture, which is extruded through the nozzle, collides with the surface of the flexible substrate to create cracks on the surface, and the CNTs penetrate into the cracks and are mechanically anchored to the flexible substrate, and then the following metal nanoparticles and CNTs of the printing material mixture are deposited in a predetermined width and height on the surface of the flexible substrate. Connecting a first and a second lead wire, which extend electrically to protrude from the flexible substrate, to both ends of the predetermined conductor pattern of the strain sensor and Binding a flexible cover, which has the same size as the flexible substrate, to the surface of the flexible substrate on which the predetermined conductor pattern is printed, so that the predetermined conductor pattern is inserted between the flexible substrate and the flexible cover. [2] Method according to claim 1, wherein the flexible substrate and the flexible cover have the same Shore hardness of 10 to 70 based on Shore A or the same Shore hardness of 22 or less based on Shore D. [3] Method according to claim 1, wherein the flexible substrate and the flexible cover are made of polydimethylsiloxane (PDMS). [4] Method according to claim 1, wherein the predetermined conductor pattern comprises a plurality of linear conductors, each extending linearly with a predetermined length in a first direction, and the plurality of linear conductors are arranged side by side to form a series connection, while maintaining a predetermined distance between them in a second direction perpendicular to the first direction, and wherein a cross-sectional structure of the plurality of linear conductors comprises a nucleation layer which is mechanically anchored to the surface of the flexible substrate, which is formed by causing the CNTs to penetrate into and become fixed in the irregularly formed cracks on the surface of the flexible substrate;and a mixed layer of metal nanoparticles and CNTs, which is formed by depositing the metal nanoparticles and CNTs on the seed layer to have a specific width and height. [5] The method according to claim 1, wherein the direct printing of the predetermined conductor pattern comprises: forming a seed layer which is mechanically anchored to the surface of the flexible substrate by causing the printing material mixture of metal nanoparticles and CNTs from the nozzle to collide with the surface of the flexible substrate to form irregular cracks, so that the CNTs penetrate into and become fixed in the cracks on the surface of the flexible substrate; and printing the predetermined conductor pattern onto the surface of the flexible substrate by applying the printing material mixture, which is subsequently sprayed onto the seed layer at a high velocity to be deposited on the seed layer in a predetermined width and height by bonding with the CNTs of the seed layer. [6] Method according to claim 5, wherein the direct printing of the predetermined conductor pattern comprises: monitoring, by means of a monitoring unit, a quantity of the printing material mixture deposited on the surface of the flexible substrate to be provided to the control unit; and controlling, by means of the control unit, a speed of movement of the movable stage on which the flexible substrate is placed, based on the monitored information from the monitoring unit. [7] Method according to claim 1, wherein the relative low-pressure atmosphere in the working chamber is a pressure atmosphere of 1 Torr (133,322 Pa) to 10 Torr (13,3322 Pa). [8] Method according to claim 1, wherein controlling the movement of the movable stage comprises adjusting, by means of the control unit, a distance from the nozzle to the movable stage, such that the printing material mixture is aerodynamically focused on the surface of the flexible substrate when the printing material mixture is ejected through the nozzle. [9] Method according to claim 1, wherein the mixing ratio between the metal nanoparticles and the CNTs in the printing material mixture is in a range of 60 wt.% - 90 wt.% to 40 wt.% - 10 wt.%. [10] Flexible strain sensor comprising: a flexible substrate, a predetermined conductor pattern that is printed directly onto a surface of the flexible substrate, and a flexible cover which covers and is bonded to the surface of the flexible substrate on which the predetermined conductor pattern is printed, such that the predetermined conductor pattern is inserted between the flexible cover and the flexible substrate, wherein the predetermined conductor pattern comprises a plurality of linear conductors, each extending linearly with a predetermined length in a first direction, and the plurality of linear conductors are arranged side by side to form a series connection, while maintaining a predetermined distance between them in a second direction perpendicular to the first direction, and wherein a cross-sectional structure of the majority of linear conductor tracks comprises a seed layer which is mechanically anchored to the surface of the flexible substrate and is formed by causing carbon nanotubes (CNTs) to penetrate into and become fixed in cracks formed on the surface of the flexible substrate; and a mixed layer of metal nanoparticles and the CNTs which is formed by depositing the metal nanoparticles and the CNTs onto the seed layer to have a predetermined width and height. [11] Flexible strain sensor according to claim 10, wherein the flexible substrate and the flexible cover have the same Shore hardness of 10 to 70 based on Shore A or the same Shore hardness of 22 or less based on Shore D. [12] Flexible strain sensor according to claim 10, wherein the flexible substrate and the flexible cover are made of polydimethylsiloxane (PDMS). [13] Flexible strain sensor according to claim 10, wherein the mixing ratio between the metal nanoparticles and the CNTs in the mixing layer is in a range of 60 wt.% - 90 wt.% to 40 wt.% - 10 wt.%.

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