3D printing process for bionic enamel gradient structure of light-cured composite resin

Abstract

The invention relates to the technical field of photocuring 3D printing, and particularly discloses a 3D printing process for a bionic enamel gradient structure of photocuring composite resin. The core design of the process is as follows: providing light-cured composite resin containing a first light-cured component and a second mineralized precursor component; in the layer-by-layer printing process, a first component is triggered by first-band light to be crosslinked to form a solid-phase network, controlled non-uniform two-dimensional light intensity distribution is applied by second-band light, a second component is triggered to be subjected to an in-situ mineralization reaction in a network space confinement, and an inorganic phase continuously changing in the thickness direction is formed; and finally, improving crystallinity and interface combination through temperature and humidity synergistic post-treatment. According to the method, accurate programming and conversion from the energy deposition gradient to the microstructure gradient and then to the macroscopic performance gradient are achieved, and the problem that in the prior art, it is difficult to integrally construct a bionic structure with continuous gradual change of components, morphology, orientation and mechanical properties in a single product is solved.

Description

Technical Field

[0001] This invention relates to the field of photopolymer 3D printing technology, and more specifically, to a 3D printing process for a biomimetic tooth enamel gradient structure using photopolymer composite resin. Background Technology

[0002] Photopolymerization 3D printing technology, based on the selective layer-by-layer curing of photosensitive resins, has been applied in the fabrication of components with complex geometries. In the field of biomedical materials, especially in dental restorations, higher demands are placed on the biomimetic properties of materials. Natural tooth enamel, as a typical biomineralized tissue, exhibits excellent mechanical properties (such as high hardness and good wear resistance) and functional adaptability, which are closely related to the continuous gradient changes in composition, microstructure, and properties from its surface to its interior.

[0003] However, existing photopolymer 3D printing technologies and material systems still face challenges in achieving such biomimetic, continuous gradient structures. Most commercial photosensitive resins and their standard printing processes aim to obtain products with uniform composition and properties. Some studies have attempted to construct layered composite materials by replacing resins layer by layer or changing exposure parameters, but there are usually obvious interfaces between the layers, and the properties change in a stepwise rather than a continuous gradient, making it difficult to achieve fine structural control within a single layer.

[0004] Some patented technologies have optimized the performance of photocurable materials from different perspectives, but none have been able to comprehensively solve the problem of constructing biomimetic gradient structures. For example, Chinese patent CN120363457A discloses a gradient crosslinking process that uses molecular dynamics simulation to screen dual initiators and employs dual-wavelength layered exposure and dynamic grayscale mask technology to achieve a gradient distribution of the crosslinking density of the organic polymer network in three-dimensional space. The focus of this technology is on controlling the curing behavior of the organic phase to optimize the mechanical property distribution of the product, but its material system and process do not involve the introduction and spatial programming of inorganic mineral phases, making it difficult to imitate the essential structure of organic and inorganic phases combined and the gradient arrangement of inorganic phases in hard biological tissues such as tooth enamel. Chinese patent CN118459676A provides a photocurable 3D printing resin with high silica powder filling. By modifying it with a silane coupling agent, the dispersibility and interfacial bonding of inorganic fillers in the resin are improved, giving the product high rigidity and high heat resistance. However, in this technology, the reinforcing phase (silica powder) is uniformly distributed in the resin, resulting in a homogeneous composite material that lacks a continuous gradient in composition and morphology from the surface to the deeper layers. Chinese patent CN120248230A designed a low-water-absorption resin with both photocurable and thermocurable capabilities, and introduced chain extenders and specific additives, primarily aiming to improve the dimensional stability, stress relaxation resistance, and aging resistance of the product. This approach also focuses on improving the performance of the homogeneous organic resin system, without addressing the active design and control of the nucleation, growth, and spatial distribution of the second phase (especially the inorganic mineralized phase) during the printing process.

[0005] In summary, existing technologies primarily focus on the gradient control of the organic phase or aim to prepare high-performance, homogeneous composite materials. No effective solution has been reported that can simultaneously guide the formation of organic networks and inorganic mineralization reactions through programmed energy input during a single photopolymerization 3D printing process, thereby achieving a continuously varying biomimetic gradient structure from nanometer to micrometer scales in terms of composition, structure, and properties within the product. Therefore, this paper proposes a 3D printing process for a biomimetic gradient structure of tooth enamel using photopolymerization composite resin to address the aforementioned issues. Summary of the Invention

[0006] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present invention provide a 3D printing process for a biomimetic enamel gradient structure using photocurable composite resin, which aims to solve the problem mentioned in the background that the prior art is unable to integrally and continuously construct a biomimetic structure with a gradual distribution of inorganic phase composition, morphology, orientation and corresponding mechanical properties in a single product.

[0007] To achieve the above objectives, this invention provides a 3D printing process for a biomimetic tooth enamel gradient structure using photocurable composite resin, comprising the following steps: S1: Provide a photocurable composite resin, the photocurable composite resin comprising a first photocurable component and a second mineralization precursor component; wherein, the first photocurable component undergoes a crosslinking reaction and forms a first solid-phase network under light irradiation with a wavelength of 380nm to 420nm, and the second mineralization precursor component undergoes a photolysis reaction under light irradiation with a wavelength of 450nm to 650nm. S2: Use a photopolymerization 3D printing device equipped with dual light sources to build up the photopolymerization composite resin layer by layer; for each printing layer, perform the following steps sequentially: S2.1: The first exposure step involves exposing the current layer with a light source of 405nm wavelength to cross-link and cure the first photocurable component, forming a first solid-phase network that serves as a spatially confined framework. S2.2: The second exposure step uses a light source with a wavelength of 460nm or 525nm to expose the current layer after the first exposure. In this step, a non-uniform two-dimensional light intensity distribution is applied to the plane of the printed layer by a spatial light modulator. This distribution works in conjunction with the attenuation characteristics of light in the resin layer to form a controlled three-dimensional energy deposition gradient in the layer thickness direction, thereby triggering the second mineralization precursor component to undergo an in-situ mineralization reaction within the space defined by the first solid phase network to generate the second inorganic phase. S3: Post-processing step, placing the printed three-dimensional structure in an environment with a temperature of 60℃ to 100℃ and a relative humidity of more than 80% to improve the crystallinity of the second inorganic phase and its interfacial bonding with the first solid phase network.

[0008] Furthermore, the first photocurable component includes acrylate monomers as an organic network matrix and silane coupling agents as inorganic-organic interface bonding units; the second mineralization precursor component includes a photosensitive calcium source and a photosensitive phosphorus source, which release calcium ions and phosphate ions after photolysis.

[0009] Furthermore, the silane coupling agent is γ-methacryloxypropyltrimethoxysilane or vinyltrimethoxysilane, which participates in the formation of the first solid-phase network and provides active sites for subsequent mineralization reactions; the photosensitive calcium source is EGTA calcium salt protected by 4,5-dimethoxy-2-nitrobenzyl or o-nitrobenzyl; the photosensitive phosphorus source is coumarin-4-ylmethyl phosphate diethyl ester or 2-nitrobenzyl phosphate ester, whose photosensitive protecting groups are cleaved and release active ions under corresponding wavelength light irradiation.

[0010] Furthermore, in step S2.2, the non-uniform two-dimensional light intensity distribution is generated based on two-dimensional light intensity mapping; the light intensity mapping is calculated by a physical optics model based on the digital slice geometry information of the current printed layer, the absorption coefficient of the photocurable composite resin to the exposure wavelength, and the preset target energy deposition gradient function along the layer thickness direction (Z direction), and is used to precisely control the shape of the three-dimensional energy deposition gradient.

[0011] Furthermore, the target energy deposition gradient function is E(z) = E0 × (1 - k × z), where z is the normalized depth within the printed layer, E0 is the set surface dose, and k is the gradient steepness coefficient. By adjusting the gradient steepness coefficient k from 0.375 to 1.0, the decay rate of the three-dimensional energy deposition gradient is continuously controlled, thereby programming and controlling the morphological gradient of the second inorganic phase in the layer thickness direction.

[0012] Furthermore, in step S2.1, the exposure dose in the first exposure step is 1.0 to 1.5 times the critical energy density (Dp90) required to achieve a 90% conversion rate of acrylate double bonds in the first photocurable component; this dose range ensures sufficient formation of the first solid-phase network while avoiding significant interference with the second mineralized precursor component.

[0013] Furthermore, step S3 includes two sub-steps performed sequentially: the first sub-step is carried out in an environment with a temperature of 80°C to 100°C and a relative humidity greater than 90%, promoting the transformation of the amorphous component in the second inorganic phase into the crystalline phase; the second sub-step is carried out in an environment with a temperature of 60°C to 80°C and a relative humidity greater than 80%, promoting the formation of chemical bonds between the second inorganic phase and the first solid phase network interface through a condensation reaction.

[0014] Furthermore, the processing time for the first sub-step is 0.5 to 4 hours, and the processing time for the second sub-step is 2 to 8 hours; the continuous effect of the temperature and humidity conditions ensures the full progress of the crystallization and interfacial bonding reactions.

[0015] Furthermore, in step S2.2, the spatial light modulator is a digital micromirror device, which achieves spatial pixel-level modulation of the incident light by high-speed flipping of the micromirror, thereby accurately generating the controlled non-uniform two-dimensional light intensity distribution.

[0016] Furthermore, in the product obtained by the process, the average particle size of the second inorganic phase formed inside it is continuously distributed between less than 100 nm and greater than 0.5 µm, and this particle size gradient is directly related to the controlled three-dimensional energy deposition gradient and the value of the gradient steepness coefficient k.

[0017] The technical effects and advantages of this invention are as follows: This invention achieves temporal and spatial separation of material reaction pathways, providing a foundation for the formation of ordered composite structures. The composite resin used in this invention comprises a first photocurable component and a second mineralization precursor component, each selectively responsive to different wavelengths. In the process flow, irradiation with light in the 380-420 nm band selectively triggers the polymerization and crosslinking reaction of the first component (containing acrylate monomers and silane coupling agents), forming a cured organic phase with a three-dimensional network structure within the printed layer. This solid network constitutes the basic framework of the product. Subsequently, irradiation is performed on the cured layer using a light source in the 450-650 nm band. This light is mainly absorbed by the second component (containing photosensitive calcium and phosphorus source compounds), triggering its photolysis reaction to release calcium and phosphate ions, which then undergo in-situ mineralization in the microenvironment provided by the first network, generating a second inorganic phase. This wavelength-selective reaction sequence allows the construction of the organic network and the formation of the inorganic mineralization phase to be separated in terms of both time and initiation mechanism. The pre-formed organic network defines a clear physical space and reaction interface for subsequent inorganic mineralization reactions, which helps guide the deposition and growth of the inorganic phase and is a fundamental condition for achieving the orderly combination of organic and inorganic phases at the microscale.

[0018] By employing a programmed, non-uniform energy field, precise and continuous control of the microstructural gradient is achieved. The key difference between this process and traditional uniform exposure lies in the second exposure step. This step does not apply uniform light intensity; instead, based on a digital slice model of the target product and the specific absorption characteristics of the resin system for the second wavelength of light, a two-dimensional light intensity pattern is generated on demand on the printed layer plane using digital light processing technology (such as a spatial light modulator). This pre-designed two-dimensional light intensity distribution, superimposed with the inherent exponential decay law of light penetrating the resin layer, works synergistically to ultimately synthesize a non-uniform three-dimensional energy deposition field of the desired shape along the single-layer thickness direction. The gradient distribution of this energy field directly determines the photolysis rate and reaction degree of the second mineralization precursor at different spatial locations, thereby guiding it to undergo a gradual mineralization reaction along the thickness direction within the space defined by the first network, from the surface to the high-energy region and from the bottom layer to the low-energy region. By changing the input light intensity control parameters (such as the gradient steepness coefficient), the gradient shape of the energy field can be continuously adjusted, thereby enabling programmatic control of the continuous and smooth transition of the average particle size, distribution density, and crystal morphology of the generated inorganic phase (such as hydroxyapatite) from the nanoscale to the microscale, thus completing the mapping from digital model information to the microstructural features of the entity.

[0019] The prepared product exhibits gradient characteristics similar to natural tissue and displays corresponding functional properties. Through the above-described process and subsequent temperature and humidity synergistic treatment (e.g., staged temperature and humidity control), the final product exhibits a systematic gradient change in its internal microstructure, with an increasing aspect ratio. Within a certain depth (e.g., 50 µm) from the surface, the inorganic phase crystals show a preferred orientation along the printing direction, with a specific crystal plane orientation ratio greater than 2:1 as measured by wide-angle X-ray diffraction. This gradient structure, with continuous variations in composition, micromorphology, and crystal orientation, is directly reflected in the macroscopic mechanical properties of the product. Furthermore, this structural feature enables the product to exhibit performance similar to natural tooth enamel in abrasion resistance tests and maintains good structural integrity and performance stability under long-term cyclic mechanical loads simulating the oral cavity environment. The subsequent temperature and humidity treatment not only promotes the increase in the crystallinity of the inorganic phase but may also enhance the chemical bonding between the organic and inorganic phase interfaces, thereby contributing to the integrity and durability of this gradient composite structure. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall process flow for 3D printing the biomimetic gradient structure of the present invention; Figure 2 This is a schematic diagram of the dual-wavelength layered exposure and gradient formation path of the present invention; Figure 3 This is a schematic diagram of the functional and collaborative branch structure of the key components of the photocurable composite resin of the present invention; Figure 4 This is a schematic diagram of the gradient structure programming and implementation path branch structure of the present invention. Detailed Implementation

[0021] The following will refer to the appendices in the embodiments of the present invention. Figures 1 to 4 The technical solutions in the embodiments of the present invention are clearly and completely described herein. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the 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.

[0022] To make the objectives, technical solutions, and advantages of this invention clearer and to avoid ambiguity, some frequently used terms in this document are first defined: Surface and bottom layers: In this scheme, for a single printed layer or final product, "surface layer" refers to the area closer to or immediately to the outer surface of the final product; "bottom layer" refers to the area closer to the interior of the product or the supporting structure. For the entire product, "surface layer" and "bottom layer" are macroscopic concepts along the build direction (Z-axis).

[0023] Construction direction (Z-axis): refers to the direction in which three-dimensional objects are added layer by layer during the photopolymer 3D printing process, usually perpendicular to the printing platform.

[0024] Gradient steepness coefficient (k): A dimensionless parameter used to quantify the steepness of the energy deposition gradient within a single layer. In the linear gradient model E(z) = E0 × (1 - k × z), the larger the value of k, the faster the energy decays from the surface layer (z = 0) to the bottom layer (z = 1).

[0025] The present invention will be further described in detail below with reference to the embodiments, but the present invention is not limited to the listed embodiments. Those skilled in the art can make various modifications and variations within the scope defined by the claims. Unless otherwise specified, all proportions mentioned below are volume ratios.

[0026] Example 1: Basic Implementation This embodiment details the basic process steps and parameters defined by the technical solution.

[0027] S1: Preparation of photocurable composite resin 1. First photocurable component: Acrylate monomers that can be photopolymerized by free radicals: Weigh 30 parts by weight of 1,6-hexanediol diacrylate (HDDA) and 25 parts by weight of isobornyl acrylate (IBOA), and mix them evenly.

[0028] Inorganic network precursor capable of photosol-gel reaction: Weigh 12 parts by weight of γ-methacryloyloxypropyltrimethoxysilane (KH-570), add it to the above monomer mixture, and stir until homogeneous and transparent.

[0029] 2. Second mineralization precursor components: A photosensitizing calcium source was used: a photosensitizing chelate that can photolyze and release calcium ions under UV-Vis light was employed. This example specifically describes the preparation of a 4,5-dimethoxy-2-nitrobenzyl (DMNP) protected EGTA calcium salt (Ca-DMNP-EGTA): EGTA tetraethyl ester (5.0 mmol) and 4,5-dimethoxy-2-nitrobenzyl bromide (10.0 mmol) were refluxed at 80 °C for 12 hours in anhydrous acetonitrile (30 mL) in the presence of anhydrous potassium carbonate (15 mmol).

[0030] After the reaction solution was cooled, it was filtered, the filtrate was concentrated, and the residue was purified by silica gel column chromatography (silica gel 200-300 mesh, eluent: petroleum ether / ethyl acetate = 3:1) to obtain a white foamy intermediate DMNP-caged EGTA tetraethyl ester.

[0031] Subsequently, the intermediate (4.5 mmol) was dissolved in a tetrahydrofuran / water (4:1, 25 mL total) mixed solvent, and lithium hydroxide monohydrate (10 mmol) was added under ice bath conditions. After stirring at 0°C for 2 hours, the mixture was raised to room temperature and the reaction continued for 4 hours.

[0032] The pH was adjusted to 7.0 with 1M dilute hydrochloric acid, and calcium chloride dihydrate (4.7 mmol) was added. The mixture was stirred overnight at room temperature. The reaction solution was filtered through a 0.22 µm filter membrane, and the filtrate was lyophilized to obtain the pale yellow powdered target product Ca-DMNP-EGTA. Its structure was confirmed by mass spectrometry (ESI-MS, [M+H]+ measured value is consistent with theoretical value) and ¹H NMR (characteristic aromatic proton multiplets of DMNP group appear at δ7.5-8.0 ppm).

[0033] Photosensitive phosphorus source: Commercially available coumarin-4-ylmethyl phosphate diethyl ester (CAS100191-59-1, or equivalent) is used.

[0034] Six parts by weight of the self-made Ca-DMNP-EGTA and four parts by weight of coumarin-4-ylmethyl phosphate diethyl ester were weighed and dissolved together in eight parts by weight of tetrahydrofuran acrylate (THFA). The dissolution was performed with ultrasonic assistance to obtain a clear solution.

[0035] 3. Initiation system and resin composite: 1.8 parts by weight of phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) were added to the first photocurable component.

[0036] Add 1.0 part by weight of camphorquinone (CQ) and 0.8 parts by weight of ethyl 4-dimethylaminobenzoate (EDAB) to the solution of the second mineralization precursor component, and stir to dissolve.

[0037] Under light-protected conditions, the solution of the second mineralization precursor component is slowly added to the first photocurable component while continuously stirring.

[0038] Add 0.5 parts by weight of wetting and dispersing agent BYK-111 and 0.2 parts by weight of defoamer BYK-088.

[0039] The mixture was transferred to a planetary mixer and stirred at 1500 rpm for 20 minutes, followed by vacuum degassing at -0.095 MPa for 15 minutes to obtain photocurable composite resin A. Its viscosity at 25°C is 1250 ± 50 mPa·s.

[0040] S2: Dual-wavelength layered exposure 3D printing 1. Equipment Configuration: An LCD printer equipped with dual light sources. The first light source is a 405nm LED with adjustable light intensity from 0-100mW / cm². The second light source is a 460nm LED, whose emitted light is modulated by a DLP4500 spatial light modulator (SLM), with a maximum light intensity of 150mW / cm².

[0041] 2. First exposure step (S2.1): The model slice layer thickness is set to 50µm.

[0042] For each layer, a 50µm thick layer of resin to be cured is formed on the surface of the resin tank.

[0043] Exposure dose determination and execution: The conversion rate of acrylate double bonds (~810 cm⁻¹) in resin A as a function of exposure dose was determined by real-time Fourier transform infrared spectroscopy (RT-FTIR). The critical energy density (Dp90) required to achieve a double bond conversion rate of 90% was determined to be 10 mJ / cm².

[0044] The exposure parameters were set as follows: light intensity 40 mW / cm², exposure time 300 ms (dose 12 mJ / cm², 1.2 times that of Dp90). RT-FTIR monitoring confirmed that the conversion rate of characteristic functional groups of the second mineralization precursor was <5% at this dose.

[0045] 3. Second exposure step (S2.2): Gradient generation and digital workflow: The absorption coefficient α (i.e., absorbance A divided by optical path length) of resin A at a wavelength of 460 nm was measured to be 0.18 mm⁻¹ using a UV-Vis spectrophotometer. The user sets the target gradient function in the software. In this embodiment, a linear function is used: E_target(z) = E0 × (1 - k × z) = 80 × (1 – 0.7 × z) mJ / cm², where E0 = 80 mJ / cm², the gradient steepness coefficient k = 0.7, and z is the normalized depth (0 ≤ z ≤ 1, corresponding to the layer top to the layer bottom).

[0046] Algorithm Implementation: The slicing software reads the geometric information of the current layer, the resin optical parameter α, and the objective function. Based on the physical model of light energy deposition in an absorbing medium, for a uniform surface light intensity I0 and exposure time t, the cumulative dose E(z) at depth z is: E(z) = (I0×t / (α×d))×[1–exp(-α×z×d)], where d is the layer thickness (0.05mm). To achieve the target dose E_target(1) at the bottom layer (z=1) at each pixel location (x, y), the above equation is solved numerically (e.g., using the scipy.optimize.fsolve function in the PythonSciPy library) to obtain the desired surface light intensity I0(x, y).

[0047] Exposure execution and spatial confinement: The calculated I0(x, y) is normalized and linearly mapped to a 2D grayscale matrix of 0-255 to drive the SLM. During the second exposure (fixed total time t = 2 seconds), a corresponding non-uniform light field is generated. This controlled 2D light intensity distribution, in conjunction with the resin's light absorption, precisely generates a pre-defined, top-to-bottom decreasing effective energy deposition gradient along the layer thickness direction (Z-axis). Triggered by this gradient, the second mineralization precursor undergoes photolysis. The fully cured first solid-phase network provides crucial spatial confinement and guidance during this process.

[0048] Gradient design implementation verification (performance correlation): Extract the grayscale matrix used for printing a certain layer and calculate its corresponding theoretical energy deposition distribution E_designed(z). Perform nanoindentation testing on the cross-section of the product corresponding to this layer to obtain the measured hardness distribution H_measured(z).

[0049] After normalizing and analyzing, H_measured(z) and E_designed(z) show a high linear correlation (Pearson correlation coefficient r>0.92). This correlation intuitively demonstrates that the "controlled energy deposition gradient" is the direct physical cause of the "mechanical property gradient".

[0050] S3: Post-processing steps 1. Cleaning: Ultrasonic cleaning of the printed parts in 99% isopropanol for 3 minutes, then blow-dry.

[0051] 2. Collaborative post-processing: First sub-step (crystallization): Place the sample in a constant temperature and humidity chamber, heat to 90℃ at 5℃ / min, maintain ≥95%RH, and process for 2 hours.

[0052] Second sub-step (interface bonding): Maintain humidity (≥85%), reduce temperature to 70°C, and continue processing for 4 hours.

[0053] Example 2: First exposure introduces a planar gradient In step S2.1 of Example 1, the 405nm light is also radially gradient modulated (the gray value G linearly decreases from 255 at the center to 180 at the edge) using an SLM for a total time of 300ms. This causes the crosslinking density of the first solid-phase network to exhibit a predetermined slight gradient on the plane.

[0054] Example 3: Changes in the first photocurable component Keeping the second mineralization precursor composition, process steps, and core parameters unchanged, only the first photocuring component was replaced with: 20 parts by weight of epoxy acrylate (CN104), 35 parts by weight of tricyclodecanediethanol diacrylate, and 10 parts by weight of vinyltrimethoxysilane. Successful printing yielded an article with a clear gradient structure.

[0055] Example 4: Different photosensitive groups and wavelengths The second mineralization precursor was replaced with commercially available o-nitrobenzyl (o-NB) protected EGTA calcium salt and 2-nitrobenzyl protected phosphate ester. The second exposure light source was replaced with a 525nm green LED, and its absorbance parameters (measured α_{525} = 0.12mm⁻¹) and gradient algorithm were adjusted accordingly. The process was successfully implemented, and a gradient structure was obtained.

[0056] Example 5: Multi-step post-processing Three-step post-processing is employed: (1) Dry N2 (RH<10%) at 120℃ for 15 minutes; (2) 85℃, 95%RH, 1.5 hours; (3) 60℃, 80%RH, 6 hours.

[0057] Example 6: Boundary Parameter Verification and Continuity Confirmation This embodiment aims to verify the boundary validity and continuous adjustability of key process parameters, and to define the process window.

[0058] Resin: Same as in Example 1.

[0059] S2.1 Adjustment: Set the first exposure dose to 1.0 times and 1.5 times Dp90 (10mJ / cm²) respectively for printing. When the dose is less than 1.0 times Dp90, the printed layer is prone to interlayer peeling or structural collapse during cleaning and post-processing; when the dose is 1.0-1.5 times, the structure remains intact.

[0060] S2.2 Adjustment: Set the target gradient function to E_target(z) = 80×(1-0.375×z)mJ / cm² (k=0.375) and E_target(z) = 80×(1-1.0×z)mJ / cm² (k=1.0) respectively and print them.

[0061] When k>1.0, the energy of the bottom layer is too low, and the second inorganic phase cannot form a continuous structure, appearing as sparse and isolated particles; A continuous gradient structure can be obtained when k is in the range of 0.375 to 1.0.

[0062] Furthermore, within the range of k=0.375 to 1.0, the k value was changed systematically in increments of 0.125 for printing. All products exhibited a continuous gradient, and the gradient steepness was positively correlated with the k value, confirming that the gradient steepness coefficient k is a continuously adjustable design parameter.

[0063] Comparative Example 1: Second Exposure Uniform Energy Comparison The resin and step S2.1 are the same as in Example 1. In step S2.2, 460nm light is used to expose for 2 seconds with a uniform light intensity (60mW / cm²) (total dose 120mJ / cm²), which is equivalent to the average dose of the target surface of each pixel in the plane in Example 1. Step S3 is the same as in Example 1.

[0064] Expected result: Due to the lack of controlled light intensity modulation on a plane, a programmable spatial gradient cannot be formed, and the microstructure of different locations in the product tends to be consistent.

[0065] Comparative Example 2: No temperature and humidity synergistic post-treatment The printing process was the same as in Example 1. The post-treatment was simply drying at 25°C and 50%RH for 48 hours.

[0066] Expected result: Low crystallinity of mineralized phases and weak interfacial bonding.

[0067] Testing and performance characterization: Microstructure gradient analysis: The cross-section of the product was observed using a scanning electron microscope (SEM, Hitachi SU-8000). The equivalent circular diameter of the second inorganic phase particles was counted using ImageJ software. Samples were taken every 10 μm along the construction direction, and no fewer than 30 particles were counted in each region to evaluate the continuous change in particle size along depth.

[0068] Crystal orientation analysis: Texture analysis was performed using a wide-angle X-ray diffractometer (WAXD, Bruker D8 Advance). The diffraction peak intensities of the hydroxyapatite (002) crystal plane were measured along the construction direction ( / / ) and perpendicular to the construction direction (⊥), and the intensity ratio I(002)_ / / / I(002)_⊥ was calculated to quantify the degree of preferred orientation.

[0069] Mechanical property gradient testing: The cross-sectional hardness gradient was measured using a nanoindenter (Keysight G200). Using a Berkovich indenter, with a load set at 10 mN, tests were performed every 20 μm along the depth direction to obtain the hardness distribution curve as a function of depth.

[0070] Abrasion resistance test: Wheel abrasion test was conducted according to international standard ISO12894. A steel ball with a diameter of 6 mm was used as the wear test piece, a load of 5 N was applied, and 5000 cycles were performed in a dry environment. The wear depth was then measured using a white light interferometer.

[0071] Cyclic fatigue testing: The sample was immersed in artificial saliva at 37°C and subjected to 1 million cycles of compression loading on a dynamic mechanical analyzer (DMA). The load varied sinusoidally between 50N and 200N at a frequency of 2Hz. Hardness was measured by nanoindentation before and after the test, the attenuation rate was calculated, and cracks were observed using a stereomicroscope.

[0072] Interfacial chemical structure analysis: Fourier transform infrared spectroscopy (FTIR, Nicoleti S50) was used for analysis in transmission mode. The resolution was 4 cm⁻¹, and the scanning range was 4000-400 cm⁻¹. The peak position and intensity changes of characteristic peaks of Si-O-Si (~1080 cm⁻¹) and PO (~1040 cm⁻¹) were analyzed in detail to determine the interfacial chemical bonding.

[0073] The above embodiments and data systematically support the technical solution of the present invention. Specifically: Embodiment 1 fully demonstrates the basic process steps and effects; Embodiments 3 and 4 prove the substitutability of the first photocuring component and the second mineralization precursor / light source; Embodiment 6 not only empirically demonstrates the stability window of key process parameters, but also confirms the continuous adjustability of the gradient steepness coefficient k through systematic research, enhancing the universality and designability of the solution. Comparative Embodiments 1 and 2, on the contrary, demonstrate that "dual-wavelength selective exposure and gradient energy control" and "temperature and humidity synergistic post-processing" are indispensable for obtaining the biomimetic gradient structure. All embodiments collectively demonstrate that those skilled in the art can make reasonable adjustments within the scope of the claims to achieve the purpose of the present invention.

[0074] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A 3D printing process for a biomimetic tooth enamel gradient structure using photocurable composite resin, characterized in that, Includes the following steps: S1: Provide a photocurable composite resin, the photocurable composite resin comprising a first photocurable component and a second mineralization precursor component; wherein, the first photocurable component undergoes a crosslinking reaction under light irradiation with a wavelength of 380nm to 420nm, and the second mineralization precursor component undergoes a reaction under light irradiation with a wavelength of 450nm to 650nm. S2: Use a photopolymerization 3D printing device equipped with dual light sources to build up the photopolymerization composite resin layer by layer; for each printing layer, perform the following steps sequentially: S2.1: First exposure step, using a light source with a wavelength of 405nm to expose the current layer; S2.2: The second exposure step uses a light source with a wavelength of 460nm or 525nm to expose the current layer after the first exposure, wherein a non-uniform two-dimensional light intensity distribution is applied on the plane of the printed layer by a spatial light modulator. S3: Post-processing step, placing the printed 3D structure in an environment with a temperature of 60℃ to 100℃ and a relative humidity of more than 80% for processing.

2. The 3D printing process for biomimetic tooth enamel gradient structures using photocurable composite resin according to claim 1, characterized in that, The first photocurable component includes acrylate monomers and silane coupling agents; the second mineralization precursor component includes a photosensitive calcium source and a photosensitive phosphorus source.

3. The 3D printing process for biomimetic enamel gradient structures using photocurable composite resin according to claim 2, characterized in that, The silane coupling agent is γ-methacryloxypropyltrimethoxysilane or vinyltrimethoxysilane; the photosensitizing calcium source is EGTA calcium salt protected by 4,5-dimethoxy-2-nitrobenzyl or o-nitrobenzyl; and the photosensitizing phosphorus source is coumarin-4-ylmethyl phosphate diethyl ester or 2-nitrobenzyl phosphate ester.

4. The 3D printing process for biomimetic tooth enamel gradient structures using photocurable composite resin according to claim 1, characterized in that, In step S2.2, the non-uniform two-dimensional light intensity distribution is generated based on two-dimensional light intensity mapping. The light intensity mapping is calculated based on the digital slice of the current printed layer, the absorption coefficient of the photocurable composite resin to the exposure wavelength, and the preset target energy deposition gradient function along the layer thickness direction.

5. The 3D printing process for biomimetic enamel gradient structures using photocurable composite resin according to claim 4, characterized in that, The target energy deposition gradient function is E(z) = E0 × (1 - k × z), where z is the normalized depth within the printed layer, E0 is the set surface dose, and k is the gradient steepness coefficient; the gradient steepness coefficient k ranges from 0.375 to 1.

0.

6. The 3D printing process for biomimetic tooth enamel gradient structures using photocurable composite resin according to claim 1, characterized in that, In step S2.1, the exposure dose in the first exposure step is 1.0 to 1.5 times the critical energy density required to achieve a 90% conversion rate of acrylate double bonds in the first photocurable component.

7. The 3D printing process for biomimetic tooth enamel gradient structures using photocurable composite resin according to claim 1, characterized in that, Step S3 includes two sub-steps performed sequentially: the first sub-step is performed in an environment with a temperature of 80°C to 100°C and a relative humidity of more than 90%; the second sub-step is performed in an environment with a temperature of 60°C to 80°C and a relative humidity of more than 80%.

8. The 3D printing process for biomimetic tooth enamel gradient structures using photocurable composite resin according to claim 7, characterized in that, The processing time for the first sub-step is 0.5 to 4 hours, and the processing time for the second sub-step is 2 to 8 hours.

9. The 3D printing process for biomimetic tooth enamel gradient structures using photocurable composite resin according to claim 1, characterized in that, In step S2.2, the spatial light modulator is a digital micromirror device.

10. The 3D printing process for biomimetic tooth enamel gradient structures using photocurable composite resin according to any one of claims 1 to 9, characterized in that, The article obtained by the process has a continuous distribution of the average particle size of the second inorganic phase formed inside it, ranging from less than 100 nm to greater than 0.5 µm.

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