Cubic hollow structure of using precursor liquid light curing 3D printing and application thereof

Abstract

The application belongs to the technical field of health care, and relates to a cubic hollow structure for 3D printing by using a precursor liquid and application thereof. The cubic hollow structure is in the shape of a regular hexahedron, the edge length is 600-800 microns, the hollow degree is 25-35%, the Poisson ratio is 0.425-0.525, the cubic hollow structure is provided with a through circular air hole, and the hole diameter of the circular air hole is 200-400 microns. The composition of the cubic hollow structure comprises sodium alginate SA, acrylamide AM and methyl acrylate ACMO. The cubic hollow structure increases the material flexibility and air permeability, and is more sensitive to small deformations such as pulse beats. The internal combination ensures that the material mechanical properties are not affected. After multiple cycles of work, the signal output stability is maintained, there is no obvious peak attenuation or initial resistance offset, the long-term monitoring capability and reliability of the sensor are effectively improved, and the problem that the anti-fatigue performance is poor due to the dependence of traditional strain sensors on nanofillers to improve the sensitivity is solved.

Description

Technical Field

[0001] This invention belongs to the field of health and medical technology, and relates to a cubic hollow structure printed by photopolymerization of precursor liquid in 3D printing and its application. Background Technology

[0002] Wearable skin monitoring devices are electronic devices that are directly attached to or in close contact with the surface of human skin for continuous, real-time monitoring of physiological parameters and health status. These devices are typically designed to be very thin, flexible, and biocompatible to ensure comfortable wear and minimize disruption to daily activities. With the rapid development of the healthcare field, wearable skin monitoring devices have received considerable attention due to their strong interaction capabilities with the human body and long-term monitoring capabilities, and are widely used in various fields such as healthcare, sports and fitness, and environmental exposure assessment.

[0003] Strain sensors are used in medical monitoring to detect and quantify minute deformations or stress changes in objects such as human tissue and medical devices. These sensors convert mechanical deformation into electrical signals, enabling precise measurements of various physiological parameters. In medical applications, strain sensors are typically designed to be highly sensitive and biocompatible to ensure safe contact with the human body and to provide reliable data. The working principle of strain sensors is based on the phenomenon that the resistance, capacitance, or other physical properties of a material change with its shape.

[0004] However, strain sensing components, which are key components for signal response in wearable skin monitoring devices, still face challenges. Commonly used nanofiller composite functional materials for strain sensing components have weak fatigue resistance, while filler-free elastomer strain sensing components have poor sensing performance due to their simple shape. High Poisson's ratio hydrogels prepared based on surface exposure 3D printing can make up for the performance deficiencies caused by simple morphology, while avoiding signal fluctuation problems caused by filler tunneling effect. Therefore, we propose a design method for photocurable printing gel precursor liquid system.

[0005] Therefore, a structure, device, or method is needed to address the aforementioned technical problems by compensating for the performance deficiencies caused by simple morphology while avoiding the packing tunneling effect. Summary of the Invention

[0006] The technical solution adopted by this invention to solve the technical problem is: a cubic hollow structure 3D printed using a precursor liquid photopolymerization method. The cubic hollow structure is in the shape of a regular hexahedron, with an edge length of 600-800 micrometers, a hollowness of 25-35%, and a Poisson's ratio of 0.425-0.525. The cubic hollow structure has through-hole circular pores with a diameter of 200-400 micrometers. The components of the cubic hollow structure include: sodium alginate (SA), acrylamide (AM), and methyl acrylate (ACMO). The cubic hollow structure has a cubic hollow geometry with positive high Poisson's ratio, which enhances the sensor's response to strain changes and solves the problem that strain sensors in traditional flexible electronic devices often rely on nanofillers to improve sensitivity, which may lead to poor fatigue resistance. Furthermore, the specific ratio of sodium alginate (SA), acrylamide (AM), and methyl acrylate (ACMO) not only improves the mechanical properties of the material but also reduces transparency, reduces the diffusion of oligomers during the printing process, and achieves more accurate three-dimensional structure construction.

[0007] Preferably, the cubic hollow structure is prepared by photopolymerization 3D printing using a gel precursor solution; in the gel precursor solution, the weight ratio of sodium alginate (SA) to acrylamide (AM) is 3.5–4.5:1; the concentration of sodium alginate (SA) is 0.8–1.2 wt%; and the viscosity of the gel precursor solution is not less than 198 mPa·s. This viscosity is sufficient to suppress the diffusion of methyl acrylate (ACMO) molecules. At the same time, the gel precursor solution has suitable rheological properties, which can effectively suppress the diffusion of oligomers during the printing process and ensure smooth leveling when the printing platform moves.

[0008] More preferably, the viscosity of the gel precursor liquid includes the viscosity value at a shear rate of 0.01 to 0.11 / s.

[0009] This invention also discloses an application of a cubic hollow structure 3D printed using a precursor liquid photopolymerization process. This application utilizes the aforementioned cubic hollow structure to form a sensor for disease monitoring, daily health management, and exercise monitoring. The sensor composed of the cubic hollow structure can clearly sense pulse signals; due to the pre-strain effect, the pulse feedback is more pronounced, and the detected signal peak is higher and clearer. The sensor composed of the cubic hollow structure can also be used for daily heart rate monitoring and can issue timely alarms in critical situations. In exercise health monitoring, it can track heart rate changes in real time, providing exercise intensity references for athletes. Furthermore, the sensor composed of the cubic hollow structure can be used to monitor the movement status of various parts of the human body, such as joint activity and muscle movement, providing data support for fields such as sports rehabilitation, sports training effect evaluation, and human-computer interaction.

[0010] Preferably, the disease monitoring and daily health management include pulse monitoring and heart rate monitoring; the exercise monitoring includes finger flexion movement monitoring.

[0011] More preferably, the sensor includes: a rubber wristband, sensor electrodes, and a cubic hollow structure. The rubber wristband has grooves, and the cubic hollow structure is embedded in the grooves. The sensor electrodes are disposed on both sides of the wristband. The rubber wristband serves as the encapsulation shell for the sensor. The material of the wristband has good flexibility, wear resistance, and biocompatibility, which can adapt to the wearing needs of different parts of the human body, while providing effective protection for the sensor inside the wristband. The hollow part of the wristband is filled with transparent resin. The resin selection focuses on its optical transparency, chemical stability, and adhesion performance with the rubber wristband and the sensor. The filling resin not only provides structural support for the wristband, enabling it to maintain a certain shape and strength, but also protects the internal sensor from external physical impacts and chemical corrosion to a certain extent.

[0012] More preferably, the cubic hollow structure is firmly fixed in the groove and flush with the surface of the rubber wristband; the cubic hollow structure fits tightly with the bottom and sides of the groove; embedding the cubic hollow structure into the groove of the wristband ensures that the hydrogel component is accurately positioned and fits tightly with the bottom and sides of the groove without gaps or offset, so as to ensure that the sensor can accurately transmit stress when subjected to strain and is not affected by poor installation.

[0013] More preferably, the sensor electrode is fabricated using a silver paste printing process, and the electrode thickness is 8-12 micrometers. One end of the sensor electrode is directly connected to the filling resin, while the other end is a floating end. A certain electrode thickness can ensure good conductivity without excessively affecting the flexibility of the sensor. The direct connection of one end of the sensor electrode to the filling resin ensures good electrical contact, while the floating end can adapt to the deformation of the hydrogel during strain, avoiding electrode damage or signal transmission interruption due to excessive constraint.

[0014] More preferably, the sensor electrode surface is coated with a thin and uniform silicone waterproof membrane with a thickness of 80-300 micrometers. The silicone waterproof membrane completely covers the sensor surface without any gaps or bubbles. The material selection of the silicone waterproof membrane focuses on waterproof performance, flexibility and biocompatibility. The silicone waterproof membrane can effectively prevent external moisture from entering the sensor and affecting its performance, without excessively increasing the thickness and hardness of the sensor, thereby ensuring the durability and long-term stability of the sensor.

[0015] More preferably, the groove depth is 0.7 to 1.1 mm, and the length and width of the groove are 0.9 to 1.1 cm × 0.9 to 1.1 cm. The groove within this size range can accurately install the sensor, ensuring that the sensor is firmly fixed in the groove and flush with the surface of the wristband, without affecting the wearing comfort and normal operation of the sensor.

[0016] The beneficial effects of this invention are:

[0017] 1. The cubic hollow geometric sensor of the present invention increases the flexibility and breathability of the material, making it more sensitive to minute deformations such as pulse beats; the internal bonding of the cubic hollow structure is good, ensuring that the mechanical properties of the material are not affected, and it can stably respond to finger bending movements at different angles; after multiple cycles of operation, it still maintains the stability of signal output, without significant peak attenuation or initial resistance shift, effectively improving the long-term monitoring capability and reliability of the sensor, and solving the problem of poor fatigue resistance caused by traditional strain sensors relying on nanofillers to improve sensitivity.

[0018] 2. The cubic hollow structure of this invention uses sodium alginate (SA), acrylamide (AM), and methyl acrylate (ACMO) mixed in a weight ratio to prepare a photocurable printing gel precursor solution. This ratio enhances the mechanical properties of the material and reduces transparency, limiting the penetration depth of ultraviolet light and ensuring that each layer can be fully cured without affecting the printing quality of the next layer. SA provides biocompatibility and viscosity, AM enhances elastic modulus and toughness, and ACMO reduces oligomer diffusion, achieving more precise three-dimensional structure construction. This solves the problem of weak fatigue resistance of nanofiller composite materials and avoids the performance limitations caused by the simple shaping of fillerless elastomers. Attached Figure Description

[0019] Figure 1 This is a flowchart of the photocurable printing gel precursor liquid preparation process for a cubic hollow structure for 3D printing using a precursor liquid and its application, according to the present invention.

[0020] Figure 2 This is a flowchart of the method for photopolymerization 3D printing using a precursor liquid system according to the present invention;

[0021] Figure 3 This is a flowchart illustrating the manufacturing process of the sensor of this invention;

[0022] Figure 4 This is a flowchart of the method for pulse monitoring using a cubic hollow structure according to the present invention;

[0023] Figure 5 This is a partial enlarged view of the sensor fabricated using the cubic hollow structure of the present invention;

[0024] Figure 6This is a diagram of the sensor fabricated using the cubic hollow structure of the present invention;

[0025] Figure 7 This is a schematic diagram of the pulse monitoring results of the present invention;

[0026] Figure 8 This is a schematic diagram of the finger bending test results of the present invention. Detailed Implementation

[0027] The related technologies of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] refer to Figures 1-8 As shown, the cubic hollow structure in this embodiment utilizes a specific ratio of sodium alginate (SA), acrylamide (AM), and methyl acrylate (ACMO) in photopolymerization 3D printing. This not only improves the mechanical properties of the material but also reduces transparency, minimizing oligomer diffusion during printing and achieving more precise three-dimensional structure construction. Simultaneously, a special design was implemented for the sensor structure, employing a cubic hollow geometry with positive high Poisson bit properties. This enhances the sensor's response to strain changes, addressing the issue that traditional flexible electronic strain sensors often rely on nanofillers to improve sensitivity, potentially leading to poor fatigue resistance. It also addresses the limitations of filler-free elastomers, despite their good flexibility, in complex shapes and sensing performance. The specific solution is as follows:

[0029] The photocurable printing gel precursor solution for cubic hollow structures comprises: sodium alginate (SA) and acrylamide (AM) mixed in a weight ratio of 4:1, and methyl acrylate (ACMO) added as a third component to prepare the precursor solution.

[0030] The sodium alginate (SA) concentration is 1 wt%, dissolved in deionized water and ultrasonically treated for 30 minutes to ensure complete dissolution. The precursor solution has a viscosity of not less than 100 mPa·s at a shear rate of 0.01-0.11 / s to inhibit the diffusion of methyl acrylate (ACMO) molecules.

[0031] The preparation process of the precursor solution includes the following steps:

[0032] Step 1: Dissolve sodium alginate (SA) in deionized water to form a homogeneous solution with a concentration precisely controlled at 1 wt%. Stir at room temperature for 30 minutes and then sonicate for 30 minutes.

[0033] Step 2: Add acrylamide AM to the above solution to ensure that the mass ratio of acrylamide AM to sodium alginate SA is strictly 1:4, and continue stirring for at least 60 minutes.

[0034] Step 3: Add methyl acrylate (ACMO) and gradually adjust the total solvent volume until the predetermined viscosity value is obtained, ensuring that the final viscosity is 198 mPa·s;

[0035] Step 4: Use an ultraviolet light source to measure the transparency, ensuring that the transmittance at a wavelength of 405nm does not exceed 0.00536%;

[0036] Step 5: Add the highly efficient photoinitiator Irgacure2959 to the solution to a final concentration of 0.5 wt% to promote the polymerization reaction.

[0037] A method for photopolymer 3D printing of a cubic hollow structure includes a photopolymer printing gel precursor solution and the following steps:

[0038] Step 1: Use a digital light processing (DLP) device with an ultraviolet light source of 405 nanometers wavelength and a power setting of 3 milliwatts per square centimeter;

[0039] Step 2: Set the slice layer thickness to 100 micrometers and the exposure time to 2 seconds to ensure that each layer is fully cured;

[0040] Step 3: The printing platform is moved up and down repeatedly to allow the solution to level itself and ensure that the forming layer is evenly spread. The shear rate range is 0.01-0.11 / s.

[0041] Step 4: Print out hydrogel parts with a cubic hollow structure, where each unit has a side length of 700 micrometers, a hollowness of 29.4%, and a Poisson's ratio of 0.475;

[0042] Step 5: A through-hole circular vent with a diameter of 300 micrometers is provided at the center of each unit to improve air permeability.

[0043] In the cubic hollow structure: each unit has a side length of 700 micrometers, a hollowness of 29.4%, and a Poisson's ratio of 0.475; each unit has a through circular pore with a diameter of 300 micrometers at its center to improve air permeability; the cubic hollow structure has no obvious boundaries inside, and the interlayer bonding is good, ensuring that the mechanical properties of the material are not affected;

[0044] After printing, immerse the cube-shaped hollow structure in a 0.1 MCaCl solution for 20 minutes, then rinse with deionized water and air dry.

[0045] A sensor fabricated from a cubic hollow structure includes a cubic hollow structure encapsulated within a hollow rubber wristband. The wristband has a 1cm×1cm square groove with a depth of approximately 0.9mm for mounting the sensor.

[0046] The sensor electrodes are printed with silver paste, with a thickness of 10 micrometers, and are fixed to both sides of the wristband with conductive silver glue. One end is directly connected to the resin, while the other end floats. A thin and uniform silicone waterproof film is coated on the sensor surface to enhance durability.

[0047] The manufacturing process of the sensor includes the following steps:

[0048] Step 1: Embed the cube-shaped hollow structure into the groove of the bracelet;

[0049] Step 2: Fill with transparent resin to fix the sensor electrodes to both sides of the wristband, with one end directly connected to the resin and the other end floating.

[0050] Step 3: Apply pre-strain to the sensor to make the pulse feedback more obvious, thereby achieving clear heart rate monitoring;

[0051] Step 4: Tests show that the sensor can operate normally within a temperature range of 0-40 degrees Celsius, and its performance is not affected by relative humidity of up to 95%.

[0052] A method for pulse monitoring using a sensor fabricated using a cubic hollow structure, comprising the cubic hollow structure and the sensor, and further comprising the following steps:

[0053] Step 1: Apply pre-strain to the sensor to enhance the pulse feedback signal;

[0054] Step 2: Achieve clear heart rate monitoring and a stable response to finger bending movements at different angles;

[0055] Step 3: After fatigue testing, it was proven that the structure has the potential for multiple cycles of operation, the signal output is stable, and there is no obvious peak attenuation or initial resistance shift.

[0056] Example

[0057] I. Material Preparation

[0058] Sodium alginate (SA): High-purity sodium alginate powder is used to provide biocompatibility and viscosity. It is widely available and has high biosafety, making it one of the ideal basic materials for preparing hydrogels.

[0059] Acrylamide (AM): As an organic compound, it is a white, odorless solid that is readily soluble in water and various organic solvents. Its chemical formula is CH2=CHC(O),NH2. When AM is used in combination with SA, it forms a hydrogel. AM can introduce hydrophilic centers into lipophilic polymers, thereby improving the overall water solubility. At the same time, it plays an important role in enhancing elastic modulus and toughness, resulting in a hydrogel with better mechanical properties.

[0060] Methyl acrylate (ACMO): In this invention, it serves as a key third component, and its main function is to reduce oligomer diffusion. ACMO and SA have a special solvation effect, which has an important impact on the performance of the entire precursor liquid system and can effectively improve some key performance indicators in the printing process.

[0061] Deionized water: Deionized water that has undergone special treatment to remove impurity ions from the water is used to dissolve SA, ensuring that the solution is pure and free of impurities, and avoiding interference from impurity ions on the chemical reaction and physical properties of the precursor solution system.

[0062] Photoinitiator Irgacure2959: A highly efficient photoinitiator with a final concentration of 0.5 wt% in the final precursor solution; its function is to absorb ultraviolet light energy of a specific wavelength during the photocuring printing process, generate active substances such as free radicals, thereby initiating the polymerization reaction of monomers in the precursor solution, promoting the rapid transformation of the liquid precursor solution into a solid gel structure, ensuring the smooth progress of the printing process and the stability of the structure.

[0063] II. Preparation steps of the precursor solution

[0064] Step 1: Accurately weigh a certain amount of sodium alginate (SA) and dissolve it in an appropriate amount of deionized water to form a homogeneous solution, strictly controlling the SA concentration to 1 wt%. Stir continuously for 30 minutes at room temperature using a magnetic stirrer to ensure that the SA is fully dispersed in the water. Then, transfer the solution to an ultrasonic cleaner for ultrasonic treatment for 30 minutes. The ultrasonic frequency and power are set according to the actual equipment parameters. The cavitation effect generated by the ultrasound further promotes the dissolution of SA, ensuring that the SA is completely dissolved, and obtaining solution A.

[0065] Step 2: Accurately weigh AM and add it to solution A according to the mass ratio of acrylamide AM to sodium alginate SA of 1:4; continue to stir with a magnetic stirrer for at least 60 minutes at a moderate speed to ensure that AM and SA in solution A are fully mixed and that intermolecular interactions are sufficient, to obtain solution B;

[0066] Step 3: Slowly add methyl acrylate (ACMO) to solution B and gradually adjust the total solvent volume. During the addition of ACMO, monitor the solution viscosity using a rotational viscometer or other viscosity measurement equipment. Set different shear rates during measurement, focusing on the viscosity change at shear rates of 0.01-0.11 / s. Continue adjusting until the solution viscosity reaches 198 mPa·s within this shear rate range. At this point, the solution has suitable rheological properties, which can effectively suppress oligomer diffusion during printing and smoothly level the solution when the printing platform moves, thus obtaining the precursor solution.

[0067] Step 4: Use an ultraviolet light source device such as an ultraviolet-visible spectrophotometer to measure the transparency of the precursor liquid. During the measurement, the precursor liquid is placed in a quartz cuvette, the measurement wavelength range is set, and the transmittance at a wavelength of 405nm is the key focus. By adjusting the solution composition or preparation process, ensure that the transmittance at a wavelength of 405nm does not exceed 0.00536% to meet the printing accuracy requirements and reduce the seepage problem caused by the solidification of deep hydrogel.

[0068] Step 5: Add the high-efficiency photoinitiator Irgacure2959 to the precursor solution. Use an analytical balance to accurately weigh an appropriate amount of photoinitiator to make the final concentration 0.5wt%. After adding, use a magnetic stirrer to slowly stir evenly to avoid introducing air bubbles. This completes the preparation of the photocurable printing gel precursor solution. At this point, the precursor solution has the basic conditions for photocurable 3D printing.

[0069] III. Photopolymerization 3D Printing Process

[0070] Equipment preparation

[0071] The device uses a digital light processing (DLP) system with a high-precision optical system and a stable mechanical structure. It is equipped with a 405 nm ultraviolet light source, whose luminous intensity and stability have been rigorously calibrated and whose power has been set to 3 milliwatts per square centimeter to ensure that the light source can provide sufficient and stable energy to accurately initiate the photocuring reaction, so that the monomers in the precursor liquid can be polymerized and cured according to the preset pattern and structure.

[0072] Printing parameter settings

[0073] The slice layer thickness was set at 100 micrometers. This parameter was determined based on a comprehensive consideration of printing accuracy and efficiency. A thinner slice layer thickness can improve printing resolution and make the final printed hydrogel structure have finer details, but it will also increase printing time. After multiple comparative experiments, a slice layer thickness of 100 micrometers can complete the printing task in a reasonable time while ensuring printing accuracy, and is suitable for the preparation of hydrogel structures in this invention.

[0074] The exposure time was determined to be 2 seconds. This exposure time was the result of optimization obtained through a large number of tests on precursor liquids of different thicknesses and compositions. At this exposure time, ultraviolet light can penetrate the precursor liquid layer, allowing the monomers to fully absorb light energy and undergo polymerization reaction, ensuring that each layer of precursor liquid can be uniformly and fully cured to form a stable gel structure. At the same time, it avoids changes in material properties caused by overexposure, such as increased brittleness caused by excessive cross-linking.

[0075] Printing operation

[0076] Slowly pour the prepared precursor solution into the printing tank of the DLP equipment, taking care to avoid introducing air bubbles and ensuring a smooth solution surface. Start the printing program, and the printing platform begins to move up and down repeatedly under program control. The moving speed and stroke are set according to the equipment parameters and printing requirements. During the platform movement, the solution self-levels under the action of surface tension, ensuring uniform spreading of the molding layer. The shear rate range is maintained at 0.01-0.11 / s, which matches the viscosity characteristics of the precursor solution and is conducive to the formation of a uniform and stable thin layer on the printing platform. During the printing process, the ultraviolet light source exposes and cures the precursor solution layer by layer according to the preset pattern and parameters. The digital micromirror device (DMD) in the DLP equipment precisely controls the irradiation area and intensity of the ultraviolet light, so that each layer of the precursor solution is cured layer by layer according to the designed three-dimensional structure, gradually forming a hydrogel part with a specific structure. During the printing process, the printing status is monitored in real time to ensure smooth printing. If problems such as uneven solution leveling or incomplete curing are found, parameters are adjusted or the problem is addressed in a timely manner.

[0077] IV. Preparation and Optimization of Cube-Shaped Hollow-Out Hydrogel Parts

[0078] Structural Design and Printing

[0079] Based on the pre-designed structural parameters, a cubic hollow structure model was designed using professional 3D modeling software. Each unit in the model has a side length of 700 micrometers. This side length was determined through extensive simulation calculations and actual printing experiments to ensure structural stability while giving the hydrogel part good flexibility and strain response performance. The 700-micrometer side length is neither too small, which would make the structure too fragile to withstand the stress during printing and use, nor too large, which would reduce the strain response sensitivity and affect the sensor performance.

[0080] The hollowness is designed to be 29.4%, which was determined based on a balance between structural strength and deformation space. Through precise structural design and simulation analysis, the hollowness of 29.4% can provide sufficient space to accommodate deformation while maintaining the overall strength of the structure. This allows the hydrogel component to effectively transmit stress when subjected to strain, generating significant changes in resistance and capacitance, thereby enhancing the sensor's response to strain changes.

[0081] With a Poisson's ratio of 0.475, this positive high Poisson's ratio characteristic is one of the key advantages of the structural design of this invention. When subjected to external pressure or tension, the positive high Poisson's ratio structure will produce unique deformation behavior, making the microstructure changes of the hydrogel component more significant during the strain process, which in turn leads to a greater change in resistance and capacitance, greatly improving the sensitivity and response performance of the sensor.

[0082] A through-hole circular pore with a diameter of 300 micrometers is set at the center of each unit. The design and size of the pore were determined by comprehensive consideration of breathability and structural integrity. The presence of the pores significantly improves the breathability of the hydrogel component, which is beneficial for gas exchange with the external environment in practical applications, especially in wearable device applications, thereby improving user comfort. At the same time, while ensuring breathability, the 300-micrometer diameter will not excessively weaken the structural strength, ensuring that the hydrogel component can maintain stable mechanical properties during use.

[0083] The designed model is imported into the DLP device and printed according to the printing parameters and operating steps in Example 2, finally obtaining a hydrogel part with a cubic hollow structure.

[0084] Post-processing optimization

[0085] After printing, the cubic hollow structure hydrogel part was immersed in a 0.1 M CaCl solution for 20 minutes; during the immersion process, calcium ions (Ca) in the solution... 2The +) reacts with the carboxyl groups in sodium alginate to form stronger chemical bonds, further enhancing the mechanical properties of the hydrogel and improving its structural stability and durability. The soaking time is strictly controlled at 20 minutes, determined through microstructural analysis and mechanical property testing of hydrogel parts soaked for different times. Too short a soaking time results in insufficient cross-linking, failing to achieve the optimal improvement in mechanical properties; too long a soaking time leads to cracks in the covalently cross-linked network, possibly due to internal stress imbalance caused by excessive cross-linking, severely affecting material performance. After soaking, the hydrogel parts are quickly rinsed with plenty of deionized water to remove residual CaCl solution from the surface, and then placed in a well-ventilated environment to air dry naturally, preparing for subsequent encapsulation and testing. During the drying process, the hydrogel parts are protected from external pressure or contamination to ensure structural integrity and performance stability.

[0086] V. Sensor Fabrication and Packaging

[0087] Sensor assembly

[0088] Commercially available rubber wristbands are selected as the encapsulation shell for the sensors. The wristband material has good flexibility, wear resistance, and biocompatibility, which can adapt to the wearing needs of different parts of the human body, while providing effective protection for the sensors inside the wristband. The hollow part of the wristband is filled with transparent resin. The resin selection focuses on its optical transparency, chemical stability, and adhesion performance with the rubber wristband and sensors. The filling resin not only provides structural support for the wristband, allowing it to maintain a certain shape and strength, but also protects the internal sensors from external physical impacts and chemical corrosion to a certain extent.

[0089] A 1cm x 1cm square groove with a depth of about 0.9mm is reserved in the center of the wristband at the sensor installation position. This size is precisely designed according to the size and shape of the cubic hollow structure hydrogel sensor so that the sensor can be installed accurately, ensuring that the sensor is firmly fixed in the groove and flush with the surface of the wristband, without affecting the wearing comfort and normal operation of the sensor.

[0090] The prepared cubic hollow hydrogel component was carefully embedded into the groove of the wristband. During the embedding process, appropriate tools were used to ensure that the hydrogel component was accurately positioned and tightly fitted to the bottom and sides of the groove without gaps or offset. This ensured that the sensor could accurately transmit stress when subjected to strain and was not affected by improper installation.

[0091] Electrode fixing and waterproofing

[0092] The sensor electrodes are made using a silver paste printing process. The silver paste is made of a material with high conductivity and good adhesion. The electrode thickness is 10 micrometers, which ensures good conductivity without excessively affecting the flexibility of the sensor. During the printing process, high-precision screen printing equipment is used to ensure that the electrode pattern is clear and the lines are uniform. The contact area and position between the electrode and the hydrogel component meet the design requirements to achieve the best electrical signal transmission effect.

[0093] The printed electrodes are fixed to both sides of the wristband with conductive silver paste. The conductive silver paste has excellent conductivity and adhesion properties, which can form a stable electrical connection between the electrodes and the wristband. One end is directly connected to the filled resin to ensure good electrical contact, while the other end is designed as a floating end. The floating end design can adapt to the deformation of the hydrogel during the strain process and avoid electrode damage or signal transmission interruption due to excessive constraint.

[0094] A thin, uniform silicone waterproof membrane is coated onto the sensor surface. The selection of the silicone waterproof membrane material focuses on its waterproof performance, flexibility, and biocompatibility. During the coating process, precise spraying equipment or immersion processes are used to control the membrane thickness within an appropriate range, generally between tens and hundreds of micrometers. This effectively prevents external moisture from penetrating the sensor and affecting its performance without excessively increasing the sensor's thickness and hardness, thus ensuring the sensor's durability and long-term stability. The waterproof membrane coating must ensure complete coverage of the sensor surface without gaps or bubbles, especially at the connection between the electrodes and hydrogel components, where reinforcement is needed to prevent moisture from seeping in through these weak points.

[0095] VI. Sensor Performance Testing

[0096] Pulse monitoring test

[0097] Pre-strain is applied to the packaged sensor using a specially designed clamp or tensioning device. The magnitude of the pre-strain is optimized through multiple experiments based on different application scenarios and sensor characteristics, and is generally adjusted within a certain range to enhance the pulse feedback signal. The sensor is then worn on the experimenter's wrist, ensuring close contact between the sensor and the skin without loosening or slippage. Appropriate bandages or fixation devices can be used for additional fixation. The sensor then begins to operate and monitors the pulse signal in real time. High-precision data acquisition equipment is used to record changes in the electrical signal output by the sensor.

[0098] Experimental results show that the sensor can clearly perceive the pulse signal of the experimenter. Due to the pre-strain, the feedback of the pulse is more obvious, and the detected signal peak is higher and clearer. This is because the pre-strain puts the hydrogel sensor in an initial deformation state. When the pulse causes small deformations in the skin, the sensor can more sensitively capture this change and convert it into an electrical signal change. However, due to the presence of pre-strain, the sensor's ability to detect smaller signal feedback is reduced. It can only detect the main peak of arterial pulsation caused by left ventricular ejection, and cannot detect the peak formed by blood rebound due to aortic valve closure. Nevertheless, this sensor still has important application value in the field of commercial wearable health devices. It can be used for daily heart rate monitoring and timely alarms in critical situations. For example, in sports health monitoring, it can track heart rate changes in real time and provide exercise intensity references for athletes. In the daily health management of the elderly or patients with chronic diseases, it can serve as a convenient heart rate monitoring tool to promptly detect abnormal heart rate changes and remind them to seek medical attention.

[0099] Finger flexion movement monitoring test

[0100] The researchers performed finger bending movements at different angles on the hand wearing the sensor. The range of finger bending angles was set according to the actual application requirements, generally covering common bending angles in daily activities. The sensor recorded the signal changes during the movement in real time, and the data acquisition system collected the electrical signals output by the sensor at a high sampling frequency to ensure that the rapid signal changes during the finger bending process could be accurately captured.

[0101] Test results show that the sensor can successfully monitor continuous finger movements and respond to different bending angles with corresponding signal amplitudes. The signal variation is closely related to the finger bending angle and the amplitude of movement. As the finger bending angle increases, the amplitude of the electrical signal output by the sensor increases accordingly. This is because finger bending causes deformation of the sensor's internal structure, leading to changes in resistance and capacitance, which are then reflected in the electrical signal. This indicates that the sensor has good application potential in the field of motion monitoring and can be used to monitor the movement status of various parts of the human body, such as joint activity and muscle movement, providing data support for sports rehabilitation, sports training effect evaluation, and human-computer interaction. For example, in rehabilitation treatment, doctors can use the sensor to monitor the patient's joint movement and develop personalized rehabilitation training plans. In virtual reality (VR) or augmented reality (AR) devices, the sensor can serve as a key component for gesture recognition, enabling more natural and accurate human-computer interaction.

[0102] fatigue test

[0103] The sensor is mounted on a specialized fatigue testing device that can precisely control the applied strain and the number of cycles. A 10% compressive strain is applied to the specimen. This strain is determined based on the strain range that the sensor may withstand in practical applications and is representative. A signal response output test is conducted for 200 cycles. During the test, high-precision data acquisition equipment and analysis software are used to record key parameters such as the signal peak value and initial resistance value of the sensor in each cycle in real time, and to observe the signal change trend.

[0104] During testing, although slight fluctuations were observed between adjacent signal peaks, the overall signal output remained stable without significant peak attenuation or initial resistance shift. This indicates that the cubic hollow structure possesses excellent stability and can withstand the pressure of multiple cycles of operation. Its stability stems from the structure's rational design; the positive high Poisson's ratio structure maintains good mechanical properties during cyclic strain, making it less prone to fatigue cracks or structural damage. Simultaneously, the inherent properties of the hydrogel material and optimization measures during post-processing (such as CaCl soaking and crosslinking) also contribute to improving structural stability. This stability ensures the sensor's reliability during long-term use, making it effective for scenarios requiring long-term continuous monitoring, such as long-term physiological parameter monitoring in the healthcare field and long-term operational status monitoring of industrial equipment. For example, in hospitals, it can be used for long-term ECG and respiratory monitoring of patients, providing doctors with continuous and accurate data on their condition. On industrial automated production lines, it can be used to monitor vibration, deformation, and other parameters of key equipment components, promptly identifying potential faults and ensuring the safety and stability of the production process.

[0105] Therefore, the design method of this photocurable printing gel precursor liquid system involves preparing the photocurable printing gel precursor liquid by mixing sodium alginate (SA), acrylamide (AM), and methyl acrylate (ACMO) in a weight ratio of 4:1. This ratio enhances the mechanical properties of the material and reduces transparency, limiting the penetration depth of ultraviolet light and ensuring that each layer can be fully cured without affecting the printing quality of the next layer. SA provides biocompatibility and viscosity, AM enhances elastic modulus and toughness, and ACMO reduces oligomer diffusion, achieving more precise three-dimensional structure construction. This solves the problem of weak fatigue resistance of nanofiller composite materials and avoids the performance limitations caused by the simple shaping of fillerless elastomers.

[0106] Furthermore, the design method of this photocurable printing gel precursor liquid system, which produces a cubic hollow geometry sensor with each unit having a side length of 700 micrometers, a hollowness of 29.4%, a Poisson's ratio of 0.475, and a central through-hole circular vent with a diameter of 300 micrometers, enhances the material's flexibility and permeability, making it more sensitive to minute deformations such as pulse beats. The cubic hollow structure has good internal bonding, ensuring that the material's mechanical properties are not affected, and it can stably respond to finger bending movements at different angles. After multiple cycles of operation, it still maintains the stability of the signal output, without significant peak attenuation or initial resistance shift, effectively improving the sensor's long-term monitoring capability and reliability. This solves the problem of poor fatigue resistance caused by traditional strain sensors relying on nanofillers to improve sensitivity. It also addresses the issue that strain sensors in traditional flexible electronic devices often rely on nanofillers to improve sensitivity, which may lead to poor fatigue resistance, while filler-free elastomers, although flexible, have limitations in complex shapes and sensing performance.

[0107] In summary, the cubic hollow structure of this invention increases the material's flexibility and breathability, making it more sensitive to minute deformations such as pulse beats. The internal bonding of the cubic hollow structure ensures that the material's mechanical properties are not affected, enabling it to stably respond to finger bending movements at different angles. After multiple cycles of operation, it maintains the stability of the signal output without significant peak attenuation or initial resistance shift, effectively improving the sensor's long-term monitoring capability and reliability. This solves the problem of poor fatigue resistance caused by traditional strain sensors relying on nanofillers to enhance sensitivity.

[0108] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A cubic hollow structure printed by 3D printing using a precursor liquid and photocuring, characterized in that, The cube hollow structure is in the shape of a regular hexahedron, the edge length of the cube hollow structure is 600-800 microns, the hollow degree is 25-35%, the Poisson's ratio is 0.425-0.525, the cube hollow structure is provided with a through circular air hole, and the pore diameter of the circular air hole is 200-400 microns; the composition of the cube hollow structure comprises sodium alginate SA, acrylamide AM and methyl acrylate ACMO; the cube hollow structure is prepared by using a gel precursor liquid through light curing 3D printing; in the gel precursor liquid, the weight ratio of sodium alginate SA to acrylamide AM is 3.5-4.5:1; the concentration of the sodium alginate SA is 0.8-1.2 wt%, and the viscosity of the gel precursor liquid is not less than 198 mPa·s. The viscosity of the gel precursor liquid includes the viscosity value when the shear rate is 0.01-0.11 / s.

2. Use of a cube hollow structure printed by 3D printing using a precursor liquid which is photocured, characterized in that, The application adopts the cube hollow structure of claim 1, and the application uses the cube hollow structure to form a sensor for disease monitoring, daily health management and exercise monitoring.

3. The application of a cube hollow structure using a precursor liquid for light-cured 3D printing according to claim 2, characterized in that, The disease monitoring and daily health management include pulse monitoring and heart rate monitoring, and the exercise monitoring includes finger bending motion monitoring.

4. The application of a cube hollow structure using a precursor liquid for light-cured 3D printing according to claim 2, characterized in that, The sensor comprises a rubber bracelet, a sensor electrode and a cube hollow structure, the rubber bracelet is provided with a groove, the cube hollow structure is embedded in the groove, and the sensor electrode is arranged on both sides of the wristband of the rubber bracelet.

5. The application of a cube hollow structure using pre-liquid light-cured 3D printing according to claim 4, characterized in that, The cube hollow structure is fixed in the groove and flush with the surface of the rubber bracelet; the cube hollow structure is closely attached to the bottom and side of the groove.

6. The application of a cube hollow structure using pre-liquid light-cured 3D printing according to claim 4, characterized in that, The sensor electrode is made of silver paste printing process, the electrode thickness of the sensor electrode is 8-12 microns; one end of the sensor electrode is directly connected with the filling resin, and the other end is a floating end.

7. The application of a cube hollow structure using pre-liquid light-cured 3D printing according to claim 4, characterized in that, The surface of the sensor electrode is coated with a silica gel waterproof film, the thickness of the silica gel waterproof film is 80-300 microns, and the silica gel waterproof film completely covers the surface of the sensor without any leakage or air bubbles.

8. The application of a cube hollow structure using pre-liquid light-cured 3D printing according to claim 4, characterized in that, The depth of the groove is 0.7-1.1 mm, and the length and width of the groove are 0.9-1.1 cm*0.9-1.1 cm.

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