A flexible tactile sensor prepared based on a laser direct writing graphene method

Abstract

The application relates to a flexible tactile sensor prepared by a laser direct writing graphene method and belongs to the technical field of flexible sensors. The upper half of the sensor is a graphene film, the lower half is a silver electrode film, the middle is a TPU film connecting layer, and the bottom encapsulation layer is encapsulated. The graphene film is generated by scratching the surface of a PI film with a 455nm laser, a mask plate pattern designed in advance in CAD is engraved by laser, a silver electrode film is deposited on the PI film by a physical vapor deposition method, then the two are connected in a face-to-face mode through a high-temperature melted TPU film, and a waterproof tactile sensor array is formed after being combined by pp encapsulation. The sensor has wide application potential in the fields of bionic robots, intelligent prostheses and the like. The sensor has the characteristics of high sensitivity, wide detection range, reliable piezoresistive performance and high real-time performance, the preparation process is simple, the cost is low, and the problems of high cost and complexity in the preparation of previous micro-nano structures are solved.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano material preparation and flexible sensor technology, specifically relating to a flexible tactile sensor prepared based on laser direct writing graphene method. Background Technology

[0002] With the rapid development of electronic skin, robots, and wearable devices, the application of tactile sensors is becoming increasingly widespread. Human-like tactile sensation in robots is achieved through tactile sensors, which often require high-precision, high-performance sensors. Sensitivity is a key parameter of tactile sensors, directly related to the ability to finely distinguish between low detection limits and applied pressure, and crucial for the detection and measurement accuracy of minute pressures. A common strategy to improve sensitivity is to construct sensors with microstructures, such as porous and columnar structures. These microstructures can undergo significant deformation under minute external stimuli, thus significantly enhancing sensor sensitivity. However, most reported tactile sensors cannot analyze and process external forces, cannot provide real-time responses to multiple signals, and suffer from communication interruptions in underwater scenarios. More importantly, these high-performance sensors lack simple manufacturing methods, are costly, and are difficult to implement on a large scale.

[0003] Therefore, designing a sensor with excellent sensing performance is a significant challenge. This is to address the current challenges of tactile sensors in terms of sensitivity, stability, and cost-effectiveness, particularly the balance between performance and manufacturing processes. Utility Model Content

[0004] The purpose of this invention is to fabricate a high-performance and low-cost tactile sensor using laser-induced graphene (LIG) technology, and further apply it as electronic skin for detecting the center of gravity. This invention utilizes the excellent conductivity and simple process of LIG technology to develop a flexible tactile sensor based on the laser-induced graphene (LIG) method. The two key components of this device are a graphene film and a silver electrode film. When pressure is applied to the sensor, the two gradually come into contact. Due to the layered and porous nature of the graphene micro / nano structure, it exhibits a significant piezoresistive effect, thus achieving high sensitivity and a wide detection range. This tactile sensor exhibits excellent sensitivity, displaying different linear piezoresistive effects in different pressure ranges. The sensitivity reaches 0.52453 A / N in the pressure range of 0N to 5N; in the range of 5N to 15N, the sensitivity slightly decreases to 0.01465 A / N; and in the range of 15N to 40N, the sensitivity is 0.00197 A / N. It features a wide detection range up to 40N, reliable piezoresistive performance (up to 50,000 cycles), high real-time performance, and a fast response time (20ms). Importantly, combined with deep learning algorithms, this sensor can visualize the distribution of the center of gravity of a toy figure's foot.

[0005] Specifically, the present invention adopts the following technical solution:

[0006] A flexible tactile sensor based on laser-written graphene method is characterized in that a porous graphene film is formed by laser-written graphene on one side of a polyimide (PI) film A, and a metal Ag electrode is formed on one side of another polyimide (PI) film B; the porous graphene film of polyimide (PI) film A faces downward, and the metal Ag electrode of polyimide (PI) film B faces upward, with the porous graphene film and the metal Ag electrode being parallel and opposite to each other with gaps between them and not in contact; when an external force is applied to polyimide (PI) film A and / or polyimide (PI) film B, the porous graphene film can come into contact with the metal Ag electrode; TPU film is provided on the opposite sides or periphery of polyimide (PI) film A and polyimide (PI) film B for fixation and support.

[0007] The graphene film is formed by laser scribing the surface of a PI film using a 455nm laser, exhibiting a 3D layered porous structure. Preferably, the laser scribing uses a grid structure. The silver electrode film is formed by depositing Ag metal onto another PI film. A TPU film connects these two PI film components, supporting zero standby power consumption. The entire device is encapsulated in PP material, providing waterproofing. The Ag metal electrode is a silver interdigitated electrode, electrically connected to an external data acquisition unit via several wires on its pins.

[0008] This utility model also provides a method for preparing the flexible tactile sensor based on the laser direct writing graphene method, including the following steps: (1) graphene film preparation, (2) silver electrode film preparation, and (3) sensor assembly and packaging.

[0009] (1) Graphene film preparation: According to the laser-induced graphene (LIG) process, a porous graphene film was fabricated by direct writing onto a polyimide (PI) film using a 455nm laser, forming a porous micro-nano structure LIG. The laser parameters were adjusted so that the laser distance from the PI film was 7mm, the weak light power was 25%, and the depth was 20%.

[0010] In step (2), an electrode mask is then fabricated using laser engraving technology, and a silver electrode layer is deposited on the new PI film using physical vapor deposition (PVD) technology.

[0011] Place the mask plate at the bottom and the brand-new PI film on top in sequence on the built-in rack of the thermal evaporation equipment; complete the evaporation of metallic silver according to the operating steps of the thermal evaporation equipment to obtain a silver electrode film; after standing and cooling for 12-24 hours, a stable silver electrode film is obtained.

[0012] Step (3) Sensor assembly and packaging: The silver electrode film and graphene film are placed face to face, and the two gaps of the two polyimide (PI) films are supported by TPU film. The three films are heated at 150°C for 3 minutes by hot pressing to bond them together. The film is then cooled at room temperature for 10 minutes. After encapsulation with PP, a stable and waterproof sensor is formed.

[0013] The preparation of the silver electrode mask includes the following steps:

[0014] Step (1) Design the silver electrode pattern in CDR or CAD software according to the actual requirements, complete the engraving pattern drawing according to the scale, and export the CDR or CAD vector image as "PLT" format.

[0015] Step (2) Import the "PLT" format file into the Autolaser laser machine parameter setting software. Set the parameters such as the engraving mode, speed, and light intensity of the mask material according to the hardness, thickness, and engraving pattern effect requirements of the mask material. Step (3) Turn on the laser to start engraving and generate a hollow silver electrode mask.

[0016] This invention provides a tactile sensor, including a flexible tactile sensor fabricated based on a laser-written graphene method.

[0017] This invention also provides the application of the flexible tactile sensor prepared by the laser direct-writing graphene method described above in detecting changes in the center of gravity position; the test is conducted using a multi-piece flexible tactile sensor array structure.

[0018] The tactile sensor described in this invention utilizes the piezoresistive effect to achieve tactile sensing. When external pressure is applied, the laser-induced graphene thin film electrodes, prepared by physical vapor deposition (PVD) and in contact with silver, deform and compress against each other. This is due to the significant piezoresistive effect generated by the three-dimensional layered porous graphene microstructure. In this case, we assume an equivalent circuit to describe the sensor's working mechanism: R = R1 + R2 + R3, where R1, R2, and R3 represent the resistance of the laser-induced graphene (LIG) thin film, the resistance of the silver electrode, and the contact resistance between the two layers, respectively.

[0019] For piezoresistive tactile sensors, the contact resistance (R3) is mainly affected by the resistivity (ρ) of the conductive material, the electrode contact area (A), and the thickness (t). Contact resistance can generally be expressed as:

[0020] Where ρ is the resistivity of the conductive material (ohm·m), t is the thickness (meters), and A is the electrode contact area (square meters). Here, the contact area A is determined by the number of protrusions forming conductive pathways on the laser-induced graphene (LIG) film. By substituting the calculated values, the resistance equation can be rewritten as:

[0021]

[0022] During sensor manufacturing, the areas of the laser-induced graphene (LIG) film and electrode layer remain constant, ensuring that resistances R1 and R2 remain almost unchanged. Therefore, the change in the total sensor resistance primarily stems from the change in contact resistance (R3). The coupling effect between the multilayer microstructure and the porous laser-induced graphene (TLIG) structure enhances the sensor's sensitivity and expands the resistance variation range, thus endowing the tactile sensor with high sensitivity and an adjustable detection range. Its resistance change can be divided into three stages, exhibiting different linear changes under different pressure ranges. It can be used in different scenarios according to requirements, demonstrating ultra-high sensitivity under low pressure and still possessing a certain current expansion capability under high pressure.

[0023] The beneficial effects of this utility model are as follows:

[0024] (1) This invention presents for the first time a flexible tactile sensor fabricated using a laser-written graphene method. This tactile sensor exhibits excellent sensitivity, displaying different linear piezoresistive effects across various measurement ranges. It boasts a wide detection range up to 40N, reliable piezoresistive performance (up to 50,000 cycles), high real-time performance with a millisecond-level response time (20ms), and a simple and easy fabrication process requiring only common materials and equipment. By improving performance while reducing cost, it is particularly suitable as electronic skin for applications in bionic robots and intelligent prosthetics, providing a new solution to the problem of the limited large-scale application of high-performance sensors and possessing broad market prospects.

[0025] (2) The sensor designed in this utility model is easy to integrate and has the characteristics of miniaturization, integration and wearability. It can measure 4 pixels per square centimeter. The resistance value changes significantly at the corresponding position in the direction of force. The magnitude, direction and position of the applied external force can be inferred from the change of pixel current of the Suger unit, and multi-point measurement within the contact area can be realized.

[0026] (3) By adjusting parameters such as the distance between the laser and the PI film and the laser power, the uniformity and thickness of the graphene, as well as the evaporation thickness and speed of the silver electrode, this invention can make the sensor exhibit different sensitivities in different force ranges, thereby achieving a wide range of measurements. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the fabrication process of the LIG-based single-pixel tactile sensor of this invention, including flowcharts and physical images of each sensor layer.

[0028] Figure 2 The images show physical examples: a) a flexible sensor array; b) graphene with arbitrarily complex three-electrode designs. Figure 2 c. Large-area graphene.

[0029] Figure 3 This is a structural diagram of the tactile sensor of this utility model;

[0030] Figure 4 (ac) is a scanning electron microscope (SEM) image of the LIG thin film surface of this invention; (de) is a cross-sectional SEM image of the LIG thin film; f is a SEM image of the PI surface; g is the Raman spectrum of LIG; h is a three-dimensional morphology diagram of LIG.

[0031] Figure 5 This paper presents a LIG-based tactile sensor sensing mechanism and a zero-standby power consumption sensor model.

[0032] Figure 6The intensity of the red LED indicates the magnitude of pressure applied to the tactile sensor.

[0033] Figure 7 This is the current signal of a LIG-based sensor under 5V voltage and 0 to 40N pressure.

[0034] Figure 8 For sensing performance, a is the current-voltage (IV) scan curve of the LIG-based sensor under a pressure of 4N; Figure 8 b represents a small pressure change at 1.5V for the LIG-based sensor.

[0035] Figure 9 Different sensors are shown: a) a single-pixel tactile sensor; b) a 6x6 array tactile sensor; c) a longitudinal scale diagram of a 6x6 array silver electrode mask; and d) a lateral scale diagram of a 6x6 array silver electrode mask. Detailed Implementation

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the present invention is not limited to the following embodiments.

[0037] Structural diagram as follows Figure 3 Preparation process such as Figure 1 As shown, the fabrication process of the tactile sensor includes the following key steps. First, according to the laser-induced graphene (LIG) process, a porous graphene film is fabricated by direct writing onto a polyimide (PI) film using a 455nm laser. During this process, laser irradiation (adjusting laser parameters to a distance of 7mm from the PI film, a weak light power of 25%, a depth of 20%; laser engraving speed of 5000mm / min; engraving accuracy of 0.01mm) triggers the thermal decomposition reaction of the PI film. PI, as a carbon source, decomposes its main chain at 2000℃, generating gases and an aromatic-rich molten carbon layer. These gases accumulate within the molten carbon layer, generating internal pressure and causing expansion, thus forming a LIG with a porous micro / nano structure. Subsequently, an electrode mask is fabricated using laser engraving technology, and a silver electrode layer is deposited on the new PI film using physical vapor deposition (PVD). Figure 2 As shown in Figures a, b, and c, the laser-induced graphene method can fabricate flexible, complex, and large-area graphene films. The fabricated graphene film is then combined with a silver electrode layer to form a tactile sensor. A complete structural schematic diagram of the tactile sensor is shown below. Figure 3 As shown, the LIG film is used as the piezoresistive layer and the silver interdigitated electrode is used as the electrode layer. The two are bonded together by high-temperature molten thermoplastic polyurethane (TPU). In addition to providing adhesion, TPU also plays a supporting and stress-rebound role.

[0038] Figure 4Images a and b in the figure show scanning electron microscope (SEM) images of the LIG thin film. Figure 4 In the middle a, sub-millimeter-scale periodic stripes can be observed in the LIG thin film, while Figure 4 In section b, a coral-like compressible structure on the scale of hundreds of nanometers is observed. These microstructural features endow the LIG film with unique mechanical properties, supporting its different piezoresistive effects across various pressure ranges. Figure 4 Figure (h) shows the three-dimensional morphology of the LIG film. It can be seen that the surface is indeed uneven, proving that the protrusions are unevenly distributed, thus demonstrating the piezoresistive effect. When external pressure is continuously applied to the sensor, more conductive graphene structures come into contact with the silver electrode, significantly increasing the contact area between the graphene components. This leads to a continuous decrease in total resistance, resulting in a significant piezoresistive effect. This effect enables the sensor to accurately detect pressure changes and exhibits excellent strain capability and high sensitivity. To dispel concerns that the product generated by the PI-laser reaction is not graphene, we measured the Raman spectrum of the laser-written graphene seeding (sample), as shown below. Figure 4 As shown in (g), a D peak was found near 1343, a G peak near 1576, and a 2D peak near 2688.5, which are consistent with the Raman spectrum of graphene, proving that the reaction between the 455nm laser and the PI film did indeed generate graphene.

[0039] Single-pixel real object image as follows Figure 4 As shown in Figure a, the physical image of the 6x6 array is as follows: Figure 4 As shown in b.

[0040] like Figure 5 The diagram shows the zero-power standby model and experimental principle of a LIG-based tactile sensor, as follows: Figure 6 As shown, when the sensor is connected to a circuit with an LED, the LED emitting light indicates that the circuit is conducting, while its absence indicates that the circuit is not conducting. Experiments have shown that the sensor prepared using this method can achieve a switching effect without any delay, and the circuit current, i.e., the brightness of the LED, is controlled by pressure. When pressed slowly, the LED gradually brightens, indicating that the current in the circuit continuously increases and the resistance continuously decreases.

[0041] The resistance change process of a LIG tactile sensor can be divided into three stages. Figure 7 In the diagram, the orange curve corresponds to the data fitting results within the 0-2N pressure range. During this stage, the nanoscale coral-like structure of the LIG film begins to deform, leading to a rapid increase in the conductive path between the graphene and silver electrodes. Due to the high compressibility of the nanostructure and the stress concentration effect of the periodic stripes in the LIG film, the conductive path expands rapidly in this stage, resulting in a significant decrease in resistance. The orange curve reflects the high sensitivity exhibited by the sensor in this stage, with a slope of 0.52453 A / N.

[0042] As the pressure increases to 2-15N, Figure 7 The green curve in the figure corresponds to the data fitting results within this pressure range. At this point, the microstructure compression of the LIG film gradually deepens, the deformation of the nanoscale and microscale structures tends to saturate, and the expansion rate of the conductive paths begins to slow down. Although the conductive pathways continue to increase, their rate of change slows significantly, resulting in a decrease in sensitivity to 0.01465 A / N. The green curve shows the decrease in sensor sensitivity at this stage, reflecting the gradual saturation of the compression. Within the high pressure range of 15-40 N, Figure 7 The blue curve in the figure shows the data fitting results for this stage. At this point, the nanoscale and microscale structures of the LIG film are nearly fully compressed, but the current continues to increase with increasing pressure. The porous structure of the LIG film is further compressed under high pressure, although the expansion rate of the conductive path slows down significantly, and the sensitivity drops to 0.00197 A / N. The blue curve illustrates that under high pressure conditions, the sensor's response speed slows down, but it still maintains a certain ability to expand the conductive path.

[0043] The experimental data were obtained from three repeated measurements using the same sensor sample. Figure 7 The red dots in the graph represent the mean values ​​of the measurement results under various pressure conditions, while the red error bars represent the corresponding standard deviations. As can be seen from the graph, the sensor exhibits small measurement errors across the entire pressure range, indicating good repeatability. This demonstrates that the sensor maintains stable performance even after repeated use and showed no structural damage even under pressures up to 40N, showcasing its excellent mechanical durability.

[0044] Furthermore, the sensor exhibits extremely high sensitivity under minute pressures, effectively capturing minute pressure changes, while maintaining current growth even at higher pressures, demonstrating a wide dynamic detection range. These combined characteristics validate the sensor's stability and reliability across various pressure levels, making it widely applicable in practical applications.

[0045] like Figure 9 Figure a shows a single-pixel tactile sensor. Figure 9 Image (b) shows a 6x6 array of tactile sensors. Figure 9As shown in Figure 1 (cd), the sensor measures 3.4 cm in length and 3.6 cm in width, achieving precise identification of four pixels per square centimeter. This research developed a tactile sensor with high sensitivity, excellent stability, and low cost, and proposed an optimized manufacturing process and performance improvement scheme. It can detect pressure changes of at least four pixels per square centimeter, exhibiting extremely high sensitivity to minute pressure changes. Over a wide pressure range, from minute to higher pressures, the sensor's current response continuously increases, ensuring stability and reliability at various pressure stages. It possesses a wide dynamic detection range, adapting to the detection needs of different pressure levels. A specific optimized manufacturing process scheme is proposed to ensure efficient sensor production and consistent performance, achieving low-cost manufacturing.

[0046] The shape and size of tactile sensors can be designed arbitrarily according to actual application requirements and installed on the surface of humanoid robots or prosthetics.

[0047] This sensor employs a porous micro / nano structure, exhibiting a high sensitivity of up to 0.52453 and a wide dynamic range of 0-40N. Furthermore, the strong adhesion between the graphene layer and the electrode layer endows the sensor structure with high strength and damage resistance. The sensor exhibits small measurement errors across the entire pressure range, indicating good repeatability. This demonstrates that the sensor maintains stable performance even after repeated use and shows no structural damage even under pressures up to 40N, showcasing its excellent mechanical durability. In addition, the sensor exhibits extremely high sensitivity at low pressures, effectively capturing minute pressure changes, while maintaining current growth at higher pressures, demonstrating a wide dynamic detection range. These combined characteristics validate the sensor's stability and reliability across various pressure stages, making it widely applicable in practical applications.

[0048] Although the above embodiments of the present invention provide several implementation methods of the present invention, and the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention patent.

[0049] It should be noted that the above-described embodiments of this utility model are merely explanations and elaborations to enable those skilled in the art to understand the technical essence of this utility model. Therefore, the described technical content is not intended to limit the substantive protection scope of this utility model. The substantive protection scope of this utility model should be determined by the claims. Those skilled in the art should understand that any modifications, equivalent substitutions, and improvements made based on the essential spirit of this utility model should be within the substantive protection scope of this utility model.

Claims

1. A flexible tactile sensor fabricated based on a laser-direct-writing graphene method, characterized in that, A porous graphene film is laser-written on one side of a polyimide (PI) film A, and a metal Ag electrode is formed on one side of another polyimide (PI) film B. The porous graphene film of polyimide (PI) film A faces downward, and the metal Ag electrode of polyimide (PI) film B faces upward. The porous graphene film and the metal Ag electrode are parallel to each other vertically and have gaps between them without contact. When external pressure is applied to polyimide (PI) film A and / or polyimide (PI) film B, the porous graphene film can come into contact with the metal Ag electrode. TPU films are provided on opposite sides or around the perimeter of polyimide (PI) film A and polyimide (PI) film B for fixation and support. The entire device is encapsulated with PP material, which has a waterproof effect.

2. A flexible tactile sensor prepared according to claim 1 using a laser direct-writing graphene method, characterized in that, The graphene film is generated by 455nm laser scribing the surface of the PI film and has a 3D layered porous structure.

3. A flexible tactile sensor prepared according to claim 1 using a laser-direct-writing graphene method, characterized in that, The laser uses a grid structure for marking.

4. A flexible tactile sensor prepared according to claim 3 using a laser direct-writing graphene method, characterized in that, The LIG film has sub-millimeter-level periodic stripes and a surface that is indeed uneven.

5. A flexible tactile sensor prepared according to claim 1 using a laser direct-writing graphene method, characterized in that, The silver electrode film is formed by depositing metallic Ag on another PI film, and the TPU film is used to connect the two parts of the PI film to support zero standby power consumption.

6. A flexible tactile sensor prepared according to claim 1 using a laser direct-writing graphene method, characterized in that, The metal Ag electrode is a silver cross-finger electrode, which is electrically connected to an external data acquisition unit via several wires on its pin.

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