阅读说明：本技术 一种基于分形曲线可拉伸加热电路打印的4d打印方法 (Fractal curve stretchable heating circuit printing-based 4D printing method ) 是由 朱晓阳 李政豪 章圆方 李红珂 兰红波 于 2020-12-21 设计创作，主要内容包括：本发明提出了一种基于分形曲线可拉伸加热电路打印的4D打印方法,包括两种方法,其中一种方法是SMP层通电加热后拉伸,冷却定型,再粘贴一层弹性体层,当再次通电加热后,样件由于应变失配发生弯曲变形；另外一种方法是SMP层与弹性体层通电加热后共同拉伸,冷却后撤销外力,由于受到残余应力的软弹性体层与刚性SMP层之间的应变失配,样件发生弯曲变形。当再次加热至玻璃化转变温度后,样件恢复至初始打印状态。本发明提出的分形曲线加热线路具有较强的拉伸性能,同蛇形电路相比较在单轴和双轴拉伸下,电阻变化率很小,便于保证4D打印结构变形过程中稳定的加热性能。(The invention provides a 4D printing method based on fractal curve stretchable heating circuit printing, which comprises two methods, wherein one method comprises the steps of stretching after an SMP layer is electrified and heated, cooling and shaping, then pasting an elastomer layer, and after the SMP layer is electrified and heated again, bending deformation occurs to a sample piece due to strain mismatch; in another method, the SMP layer and the elastomer layer are heated by electricity and then stretched together, and the external force of the pin is removed after cooling, so that the sample piece is bent and deformed due to the strain mismatch between the soft elastomer layer and the rigid SMP layer which are subjected to residual stress. When heated again to the glass transition temperature, the sample returns to the original printing state. The fractal curve heating circuit provided by the invention has stronger tensile property, has small resistance change rate under uniaxial and biaxial tension compared with a snake-shaped circuit, and is convenient for ensuring stable heating property in the deformation process of a 4D printing structure.)

1. A4D printing method based on fractal curve stretchable heating circuit printing is characterized in that: the method comprises the following steps:

(1)3D printing an SMP substrate;

(2) printing a fractal curve heating circuit on an SMP substrate;

(3) heating and sintering the SMP substrate printed with the heating circuit;

(4) attaching an SMP sample piece made of the same material to the sintered SMP substrate, and filling SMP solution between the SMP substrate and the SMP sample piece to fully cure the SMP substrate and the SMP sample piece to obtain an initial 4D printing sample piece;

(5) placing the 4D printing sample piece obtained in the step (4) in a stretching instrument for clamping, then switching on direct current voltage, adjusting the direct current voltage until the steady-state heating temperature is higher than the SMP glass transition temperature, adjusting the parameters of the stretching instrument to start stretching, stretching the part printed with the heating circuit, and taking down the part when the temperature is reduced to the set temperature;

(6) attaching the elastic body sample piece to one side of the stretched 4D printing sample piece to ensure that the elastic body sample piece cannot fall off in a heating state and the 4D printing sample piece is manufactured;

(7) and (4) switching on the 4D printing sample piece in the step (6) by a direct current power supply, generating Joule heat by a heating circuit, and automatically recovering and stretching the SMP layer in the heated area when the temperature is higher than the glass transition temperature of the SMP, but automatically bending and deforming the sample piece under the constraint action of the elastomer layer.

2. The fractal-curve stretchable heating-circuit-based 4D printing method of claim 1, wherein: the fractal curve circuit in the step (2) can print a single-layer or multi-layer circuit according to the required resistance value.

3. The fractal-curve stretchable heating-circuit-based 4D printing method of claim 1, wherein: in the step (3), the sintering temperature is 30-160 ℃, and the sintering time is 10-120 min.

4. The fractal-curve stretchable heating-circuit-based 4D printing method of claim 1, wherein: the filling solution in the step (4) is the same substance as the SMP on the upper layer and the SMP on the lower layer, the curing mode can be UV curing machine or hand-held curing lamp irradiation, and the curing time is 1-10 min.

5. The fractal-curve stretchable heating-circuit-based 4D printing method of claim 1, wherein: and (5) adjusting the direct current voltage according to the resistance value until the steady-state temperature is higher than the glass transition temperature, and starting stretching after adjusting the stretching speed and the stretching amount of a stretching instrument.

6. The fractal-curve stretchable heating-circuit-based 4D printing method of claim 1, wherein: the elastic body sample in the step (6) can be manufactured by 3D printing or formed by pouring an elastic body material in a template and curing; the thickness of the elastomer sample is determined according to the required bending deformation, and the thinner the thickness of the elastomer layer is, the larger the bending angle of the sample is.

7. The 4D printing method with the fractal curve stretchable heating circuit based on multi-material 3D printing is characterized by comprising the following steps:

(1)3D printing a multi-material 4D printing sample piece;

(2) printing a fractal curve heating circuit on a multi-material 4D printing sample substrate;

(3) heating and sintering the multi-material 4D printing sample wafer printed with the heating circuit;

(4) after the fractal curve heating circuit is connected with direct-current voltage, the generated Joule heat enables the temperature of the heated part of the SMP layer in the multi-material 4D printing sample sheet to be higher than the glass transition temperature, and stretching is started after the stretching parameters of a stretching instrument are adjusted; after the sample piece is stretched, the sample piece is cooled to room temperature and then taken down, and automatically generates bending deformation;

(5) and (3) switching on the power supply again for the sample piece in the bending state, and automatically restoring the SMP layer to the state before stretching when the heating temperature is higher than the glass transition temperature, so that the whole sample piece is restored to the original printing state.

8. The multi-material 3D printing-based 4D printing method with fractal-curve stretchable heating circuits according to claim 7, wherein: the thickness of the SMP layer and the thickness of the elastomer layer in the step (4) are determined by the bending deformation of the sample structure.

9. The multi-material 3D printing-based 4D printing method with fractal-curve stretchable heating circuits according to claim 7, wherein: the sintering temperature range in the step (3) is 30-180 ℃.

10. The multi-material 3D printing-based 4D printing method with fractal-curve stretchable heating circuits according to claim 7, wherein: and (4) stretching parameters in the step (4) comprise stretching amount and stretching speed, the part printed with the heating circuit is heated to the glass transition temperature and then begins to be stretched, and the stretching is finished and the temperature is cooled to room temperature.

Technical Field

The invention relates to the technical field of 4D printing, in particular to a novel 4D printing method based on fractal curve stretchable heating circuit printing.

Background

Four-dimensional (4D) printing technology has developed rapidly in recent years, adding a fourth dimension "time" to 3D printed structures that can actively change shape in response to environmental stimuli (e.g., heat, electricity, light, magnetism, water, and sound). Among the materials capable of achieving such behavior, thermoresponsive Shape Memory Polymers (SMPs) are the most widely used because they are compatible with multi-material 3D printing, having the advantages of fast response time, large deformable volume, etc. The basic mechanism of Shape Memory Polymer (SMP) -based 4D printing is to return from a programmed temporary shape to the original structural shape upon printing when the SMP material is heated above its glass transition temperature. Therefore, the effective heating regime is critical to control the "time" dimension of this process.

Currently, most heating methods of SMP based 4D printing adopt an external heating method. However, the heating temperature of the 4D printing structure cannot be precisely controlled by adopting the external heating method, and meanwhile, partial areas cannot be selectively heated, which hinders the application of 4D printing in various complex environments. For SMP based actuators or shape deforming structures, there are other heating means such as attaching local heaters to the desired locations or making the SMP material electrically conductive with additives. However, these solutions either lack compatibility with 3D printing or destroy the mechanical structural properties of the SMP material. Thus, embedded heating circuits have been proposed as a more efficient solution. However, hard and thick resistance wires are manually inserted into the 3D printing structure, so that the manufacturing difficulty is increased, and the mechanical performance of the structure is reduced. In addition, due to the poor stretching ability of joule heating elements, 4D printed structures are limited to bending deformation, thereby limiting the 4D printed design space. For SMP-based thermal 4D printing, the ideal heating method should internally generate heat in an efficient, uniform and predictable manner, while having sufficient extensibility and flexibility to accommodate various deformation modes during shape programming.

Disclosure of Invention

Aiming at some problems existing in heating in the existing 4D printing technology, the invention provides a novel 4D printing method based on a fractal curve stretchable heating circuit, the fractal curve stretchable heating circuit is directly deposited on the surface of a Shape Memory Polymer (SMP) substrate responding to thermal stimulation through an electric field driven jet deposition micro-nano 3D printing technology, a layer of SMP material can be selectively wrapped above the substrate, and the formed embedded heating circuit can realize uniform and rapid heat transfer in the thickness direction. The fractal curve stretchable heating circuit has not only uniaxial stretching ability but also biaxial and radial stretching ability, compared with the serpentine heating circuit. The resistance value changes little during stretching, so that stable heating performance can be achieved during programming and activation of the 4D printed structure.

In order to achieve the purpose of the invention, the invention adopts two technical schemes, which are specifically as follows:

in the first aspect, the 4D printing method based on the fractal curve stretchable heating circuit, provided by the invention, comprises the following steps of electrifying and heating an SMP layer, stretching, cooling and shaping, adhering an elastomer layer, and bending and deforming a sample piece due to strain mismatch after electrifying and heating again:

(1) printing the SMP substrate;

(2) printing a fractal curve heating circuit on an SMP substrate;

(3) heating and sintering the SMP substrate printed with the heating circuit;

(4) attaching an SMP sample piece made of the same material to the sintered SMP substrate, and filling SMP solution in the middle of the SMP sample piece to fully cure the SMP sample piece to obtain an initial 4D printing sample piece;

(5) placing the 4D printing sample piece obtained in the step (4) in a stretching instrument for clamping, then switching on direct current voltage, adjusting the direct current voltage until the steady-state heating temperature is higher than the SMP glass transition temperature, then adjusting the parameters of the stretching instrument, then starting stretching, stretching the part printed with the heating circuit, and taking down the part after the temperature is reduced to the room temperature;

(6) attaching an elastomer sample to one side of the stretched 4D printing sample piece, and fully sticking the elastomer sample piece to ensure that the elastomer sample piece cannot fall off in a heating state, so that the 4D printing sample piece is manufactured;

(7) the 4D printing sample piece is connected with a direct-current power supply, the heating circuit generates Joule heat, when the temperature is higher than the glass transition temperature of the SMP, the SMP layer in the heated area automatically returns to stretch, but the sample piece automatically bends and deforms under the constraint action of the elastic body layer.

As a further technical solution, the fractal curve circuit in step (2) may print a single-layer or multi-layer circuit according to a required resistance value.

As a further technical scheme, in the step (3), the sintering temperature is 30-160 ℃, and the sintering time is 10-120 min;

as a further technical scheme, the filling solution in the step (4) and the SMP at the upper layer and the lower layer are the same material, the curing mode can be irradiation of a UV curing machine or a handheld curing lamp, and the curing time is 1-10 min.

As a further technical scheme, in the step (5), the direct current voltage is adjusted according to the resistance value until the steady state temperature is higher than the glass transition temperature, and after the stretching speed and the stretching amount of the stretching instrument are adjusted, stretching is started.

As a further technical scheme, in the step (6), the elastic body sample can be manufactured by 3D printing or formed by casting an elastic body material in a template and curing; the thickness of the elastomer sample is determined according to the required bending deformation, and the thinner the thickness of the elastomer layer is, the larger the bending angle of the sample is.

In a second aspect, the invention further provides a 4D printing method with a fractal curve stretchable heating circuit based on multi-material 3D printing, which includes the following steps:

(1)3D printing a multi-material 4D printing sample piece; (the multi-material 4D printing sample piece consists of an SMP layer and an elastomer layer, and the thickness of the SMP layer and the thickness of the elastomer layer can be determined according to the required bending deformation)

(2) Printing a fractal curve heating circuit on a multi-material 4D printing sample substrate;

(3) heating and sintering the multi-material 4D printing sample wafer printed with the heating circuit;

(4) after the fractal curve heating circuit is connected with direct-current voltage, the generated Joule heat enables the temperature of the heated part of the SMP layer in the multi-material 4D printing sample sheet to be higher than the glass transition temperature, and stretching is started after the stretching parameters of a stretching instrument are adjusted; after the sample piece is stretched, the sample piece is cooled to room temperature and then taken down, and automatically generates bending deformation;

(5) and (3) switching on the power supply again for the sample piece in the bending state, and automatically restoring the SMP layer to the state before stretching when the heating temperature is higher than the glass transition temperature, so that the whole sample piece is restored to the original printing state.

As a further technical solution, the thickness of the SMP layer and the thickness of the elastomer layer in step (4) are determined by the bending deformation amount of the sample structure.

As a further technical scheme, the sintering temperature range in the step (3) is 30-180 ℃.

As a further technical scheme, the stretching parameters in the step (4) comprise a stretching amount and a stretching speed, the portion printed with the heating line is heated to the glass transition temperature, then stretching is started, and the stretching is finished and cooled to room temperature.

Compared with the prior art, the invention has the following beneficial effects:

(1) according to the fractal curve heating circuit of the shape memory polymer sample, the conductive slurry is directly deposited on the shape memory polymer substrate through an electric field driven jet deposition micro-nano 3D printing technology. The conductive circuit printed by the electric field driven jet deposition micro-nano 3D printing technology has the advantages of freely adjusting the line width of the circuit, freely printing a complex graph structure and the like, and the resistance value of the fractal curve heating circuit can be controlled by adjusting the line width of the printed conductive wire and the number of printing layers (height-to-width ratio) so as to accurately control the steady-state heating temperature.

(2) Most of the heat-driven shape memory polymers at the present stage are heated by external heating such as heating by sticking a heating sheet, heating by a heat gun and the like, and have the defects of incapability of accurately controlling the heating temperature, easy falling of external sticking and the like. By adding conductive material to the SMP material, the compatibility of the 3D printer is lacking. The resistance wire is inserted into the 3D printing structure, so that the manufacturing difficulty is increased, the mechanical property of the structure is reduced, and the resistance wire cannot be stretched. The SMP sample heating circuit can be an embedded circuit and a relief circuit, if the SMP sample is thicker in the thickness direction, the embedded circuit can be adopted, and uniform and stable heating in the thickness direction can be realized. If the SMP sample is thin in the thickness direction, stable and uniform heating can be achieved by using a relief circuit. Compared with the traditional external heating, the fractal curve heating circuit can realize the accurate control of the temperature by adjusting the direct-current voltage, only the part printed with the heating circuit is heated, and the mechanical structure state of other parts is not influenced; compared with the method that a conductive material is added into an SMP material for self-heating, the fractal curve heating circuit has wider process adaptability; compared with resistance wire heating, the fractal curve heating circuit has stretchability and flexibility, and can adapt to various deformation modes in the shape programming process.

(3) The fractal curve heating circuit provided by the invention has stronger tensile property, has small resistance change rate under uniaxial and biaxial tension compared with a snake-shaped circuit, and is convenient for ensuring stable heating property in the deformation process of a 4D printing structure.

(4) The fractal curve heating circuit provided by the invention can control the area coverage rate by printing fractal curves of different orders.

(5) The 4D printing method based on fractal curve stretchable heating circuit printing provided by the invention has the advantages of simple process steps, no toxicity or waste in the whole process and environmental friendliness. The production process does not need special expensive equipment and has low cost.

Drawings

FIG. 1 is a process flow diagram of example 1. Wherein a is a schematic diagram of printing an SMP prototype (VeroWhite material) using a Polyjet 3D printer; b, directly printing a fractal curve heating circuit on an SMP (symmetrical multi-processing) sample by using an electric field driven jet deposition micro-nano 3D printing technology, randomly adjusting the line width of the heating circuit by using the diameter reduction effect of a Taylor cone, and printing a single-layer or multi-layer circuit (adjusting the height-to-width ratio) to control the resistance value of the heating circuit.

In fig. 2, a is a four-layer fifth-order hilbert curve printed on an SMP substrate and an enlarged scanning electron microscope thereof; fig. 2 b is a macroscopic view of the hilbert curve before and after stretching by 5mm, and fig. 2 c is a comparison of the hilbert curve infrared thermal imaging graph before and after stretching by 5mm, and it can be seen from the infrared thermal imaging graph that the temperature before and after stretching is almost unchanged, and thus it can be judged that the heating line resistance change rate is small (fig. 5 a).

FIG. 3 is a change in shape of a 4D printed bi-layer sheet triggered by heating by an embedded stretchable circuit as in example 1, where a is the initial unheated state and b is Joule heating generated by the heating circuit after power is applied, and free shrinkage of the SMP pattern in the heated region is triggered when the temperature is above the glass transition temperature. Due to the constraining action of the elastomer layer, bending deformation occurs after strain mismatch of the heated region.

In fig. 4: a is the temperature change curve of the SMP sample piece under different voltages, the applied voltage is 5V, the maximum temperature is about 37 ℃ when the temperature curve is relatively flat, the achievable steady-state temperature is increased along with the increase of the external voltage, and the achievable steady-state temperature can reach 125 ℃ when the voltage of 25V is applied. b is the change in temperature over time during transient heating with 25V applied.

In fig. 5: and a is the comparison of the relative resistance change rate of the fractal pattern circuit and the serpentine pattern circuit in uniaxial stretching. At a stretch ratio of 20%, the resistance change rate of the serpentine circuit was 239%, while the resistance change rate of the fractal circuit was only 6%. The fractal circuit is proved to have stronger stretchability. In fig. 5: b is a fractal heating circuit infrared thermal imaging graph which changes along with the resistance when the uniaxial stretching SMP sample piece is heated.

Fig. 6 shows that three SMP samples are attached to the base, and the samples are bent and deformed after being respectively energized, so that the transformation from the planar structure to the three-dimensional structure is completed. The final three-dimensional structure can stand stably, and the load can reach 500 g.

FIG. 7 is a comparison of the relative resistance changes of fractal pattern circuits and serpentine pattern circuits when biaxially stretched, with the substrate clamped at four ends while stretched at two perpendicular speeds. At 17.5% tensile strain, the serpentine circuit resistance rate of change was 234%, while the fractal circuit rate of change was only 6%.

Fig. 8 shows a 4D proof print of example 2 that can be altered between a planar and a doubly curved three-dimensional shape. The sample was clamped on a biaxial stretching apparatus, and after energization, it was heated in a steady state, and 3mm biaxial stretching was carried out in two perpendicular directions at a rate of 0.05 mm/s. The structure is then cooled and, after release of the external constraint at room temperature, the structure deforms into a double curvature bowl shape, the bending being caused by the strain mismatch between the soft elastomer layer with residual stress and the rigid SMP layer. And electrifying and heating the sample piece again, and finally recovering the double-curvature structure sample piece to the initial state of the plane.

Fig. 9 is a schematic structural view of a grasping mechanism manufactured in embodiment 4.

Detailed Description

It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an", and/or "the" are intended to include the plural forms as well, unless the invention expressly state otherwise, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof;

based on the problems in the background art, the present embodiment mainly provides two 4D printing methods based on a fractal curve stretchable heating circuit, the first method is to stretch an SMP layer after being electrically heated, cool and shape the SMP layer, and then stick an elastomer layer, and after being electrically heated again, a sample piece is bent and deformed due to strain mismatch, and the steps are as follows:

(1) preparing a printing substrate: designing the shape and size of the SMP substrate, directly preparing a designed SMP sample by using a commercial 3D printer, carrying out pretreatment such as cleaning after printing the sample, taking out and drying for later use. Including, but not limited to, verowite (a photocurable "digital material" with shape memory behavior manufactured by Stratasys corporation), a photosensitive hybrid shape memory polymer material, etc., SMP samples may be printed in a manner including, but not limited to, Stereolithography (SLA), polymer jet (PolyJet) techniques, etc., for example, verowite materials may be printed into desired SMP samples by polymer jet (PolyJet) techniques. The pretreatment of cleaning the SMP sample comprises the steps of carrying out ultrasonic treatment on the SMP sample by using an ethanol solution, then carrying out ultrasonic cleaning by using deionized water for a period of time to remove the residual ethanol solution, and finally carrying out blow-drying by using nitrogen or other inert gases.

(2) Printing a heating line: and directly depositing the conductive paste on the SMP substrate by an electric field driven jet deposition micro-nano 3D printing technology to print a fractal curve heating circuit. Wherein the fractal curve includes but is not limited to hilbert curve, and different orders can control different area coverage rates through the designed specific fractal curve heating circuit. The conductive paste includes, but is not limited to, conductive silver paste, conductive copper paste, conductive gold paste, metal nanowire paste, and the like. The fractal curve circuit may print a single layer or multiple layers of circuits depending on the desired resistance value.

(3) Heating and sintering: the SMP substrate printed with the heating circuit is subjected to heating sintering treatment, so that the organic solvent in the conductive paste can be fully volatilized, and the conductive performance of the heating circuit can be effectively improved. Wherein the sintering temperature is 30-160 ℃, and the sintering time is 10-120 min. In the sintering process, the organic solvent in the conductive slurry is fully volatilized, and the resistivity is stabilized at a certain value.

(4) Covering a layer of SMP sample wafer: and (3) attaching an SMP sample piece made of the same material to the sintered SMP substrate, filling the SMP sample piece with SMP solution in the middle, and irradiating the SMP sample piece by using an ultraviolet lamp for a period of time to fully cure the SMP sample piece so as to obtain an initial 4D printing sample piece. Wherein the filling solution and the SMP layers are the same material, the curing mode can be irradiation of a UV curing machine or a handheld curing lamp, and the curing time is 1-10 min.

(5) Heating and stretching: and (3) placing the 4D printing sample piece in the step (4) in a stretching instrument, clamping, then switching on direct current voltage, adjusting the direct current voltage until the steady state heating temperature is higher than the SMP glass transition temperature (Tg), then adjusting the parameters of the stretching instrument, then starting stretching, stretching the part printed with the heating circuit, and taking down the part after the temperature is reduced to the room temperature. The direct current voltage is adjusted according to the resistance value until the steady state temperature is higher than the glass transition temperature, and stretching is started after the stretching speed and the stretching amount of the stretching instrument are adjusted.

(6) Pasting an elastomer layer: an appropriate elastomer sample is selected, the stretched 4D printing sample piece is attached to one side of the sample piece, the sample piece is made to be fully pasted, the sample piece cannot fall off in a heating state, and the 4D printing sample piece is manufactured. Wherein the elastomeric layer may be manufactured by 3D printing or by casting an elastomeric material in a mold and curing. The thickness of the elastomer layer depends on the required bending deformation, and the thinner the thickness of the elastomer layer, the larger the bending angle of the sample.

(7) Heating to generate bending deformation: the 4D printing sample piece is connected with a direct-current power supply, the heating circuit generates Joule heat, when the temperature is higher than the glass transition temperature of the SMP, the SMP layer in the heated area automatically returns to stretch, but the sample piece automatically bends and deforms under the constraint action of the elastic body layer.

Furthermore, the invention also provides a second 4D printing method based on the fractal curve stretchable heating circuit, the method is based on the 4D printing method of the fractal curve stretchable heating circuit, the SMP layer and the elastomer layer are stretched together after being electrified and heated, the pin external force is removed after cooling, and the sample piece is bent and deformed due to the strain mismatch between the soft elastomer layer and the rigid SMP layer which are subjected to the residual stress. And when the sample is heated to the glass transition temperature again, the sample returns to the initial printing state, and the specific steps are as follows:

(1) printing a multi-material 4D printing sample sheet: according to the multi-material 4D printing sample wafer structure of design, the multi-material 3D printer is utilized to directly print out the required structure, and the printed multi-material 4D printing wafer is cleaned and subjected to other pretreatment. The multi-material 4D print sheet consists of an SMP layer and an elastomeric layer. Wherein the SMP layer includes, but is not limited to, verowite (a photo-curable "digital material" having a shape memory behavior manufactured by Stratasys corporation), a photosensitive hybrid shape memory polymer material, etc., and the 3D printing manner includes, but is not limited to, Stereolithography (SLA), polymer jet (PolyJet) technology, etc. The VeroWhite material can be printed into the desired SMP master by, for example, polymer jet (Polyjet) techniques. Elastomeric layer materials include, but are not limited to, the Tango series, such as TangoBlack FLX973, TangoGray FLX950 (produced by Stratasys corporation), and the like.

(2) Printing a fractal curve heating line: the conductive paste is directly deposited on a multi-material 4D printing sample wafer substrate according to a specific graph structure through an electric field driven jet deposition micro-nano 3D printing technology, and after printing, post-treatment such as sintering is carried out. Wherein the fractal curve includes but is not limited to Hilbert curve and the like, the conductive paste includes but is not limited to nano silver conductive paste, nano copper conductive paste and nano silver wire conductive paste, and the viscosity range is 500-100000 cps. The sintering temperature range is 30-180 ℃.

(3) Heating and stretching: after the fractal curve heating circuit is connected with direct current voltage, the temperature of the heated part of the SMP layer of the sample is higher than the glass transition temperature by the generated Joule heat, and stretching is started after the stretching parameters of the stretching instrument are adjusted. After the sample piece is stretched, the sample piece is cooled to room temperature and then taken down, and automatically generates bending deformation. Wherein the stretching parameters comprise stretching amount and stretching speed, the part printed with the heating circuit is heated to the glass transition temperature and then begins to be stretched, and the part is cooled to room temperature after the stretching is finished. The SMP layer is stretch-set and the elastomeric layer is bent in the direction of the SMP layer under the influence of the stretching force.

(4) And (3) recovery deformation: and (3) switching on the power supply again for the sample piece in the bending state, and automatically restoring the SMP layer to the state before stretching when the heating temperature is higher than the glass transition temperature, so that the whole sample piece is restored to the original printing state. The sample piece undergoes the processes of initial state-heating stretching-bending deformation-heating recovery to the initial state and the like.

The two methods are further described below with reference to specific examples:

example 1

The embodiment provides a 4D printing method based on fractal curve stretchable heating circuit printing, which comprises the following steps:

step 1: preparation of printing substrates

(1) VeroClear material (photo-curable "digital material" with shape memory behavior produced by Stratasys) was selected as the shape memory polymer material. Printing was performed using a Polyjet multi-material 3D printer (model J750 3D printer by Stratasys), the printing principle being shown as a in fig. 1.

(2) The SMP sample piece has a length of 90mm, a width of 25mm and a thickness of 0.9 mm. And after printing of the SMP sample piece is finished, placing the SMP sample piece in alcohol for ultrasonic treatment for 2min, taking out nitrogen and drying.

Step 2: heating circuit for printing fractal curve

(1) The Zhongbonitong TL-201S is selected as conductive paste, and a fifth-order Hilbert curve (the area is 23mm multiplied by 23mm) is selected as a fractal curve.

(2) And starting the electric field to drive the jet deposition micro-nano 3D printer, opening a five-order Hilbert curve printing program, and starting printing after adjusting printing parameters (voltage, air pressure, speed and printing height). The printing principle is shown in fig. 1 b. The printing parameters in this embodiment are: the voltage was 1200V, the air pressure was 150Kpa, the speed was 10mm/s, the printing height was 0.3mm, and the nozzle diameter was 300. mu.m.

(3) And (3) placing the SMP sample piece printed with the fractal curve heating circuit in a vacuum drying oven, and heating and sintering for 1 hour at 120 ℃ to fully volatilize the solvent in the silver paste and effectively increase the conductivity.

And step 3: adhesive SMP overlay

(1) And (3) coating a layer of VeroClear liquid material on the sintered SMP sample, attaching a SMP sample, and curing in a UV curing machine for 3min to fully bond the SMP sample and the SMP sample together. The fractal curve heating circuit is embedded in the middle of the SMP sample piece, so that uniform and stable heating in the thickness direction can be realized.

And 4, step 4: drawing by electric heating

(1) Firstly, placing an SMP sample in a clamp of a stretching instrument for clamping, then switching on direct current voltage, controlling the steady state temperature to be about 65 ℃ by adjusting the voltage, and controlling the maximum stretching rate of the SMP sample to be more than 30% at the temperature.

(2) The stretching was started by adjusting the stretching parameters of the stretching apparatus (in this example, the stretching speed was 0.05mm/s, and the stretching amount was 5 mm).

(3) And after the stretching is finished, the power supply is disconnected. And taking down the sample after the sample is cooled to room temperature. The SMP sample will remain in the post-stretched state.

And 5: conformable elastomeric layer

(1) Elastomer material a TangoGray FLX950 (manufactured by Stratasys) material was selected and printed by a Polyjet multi-material 3D printer (model J750 3D printer manufactured by Stratasys), the SMP elastomer layer in this example being 1mm thick, 95mm long and 25mm wide.

(2) And coating a layer of photoresist on one side of the SMP sample piece, and attaching the elastomer layer on the SMP sample piece. The hand-held curing lamp is irradiated for a period of time to fully cure the material.

Step 6: is heated to generate bending deformation

When the temperature is higher than the glass transition temperature after the external voltage is switched on, the free shrinkage of the heated area of the SMP layer is triggered, and the heated area is subjected to strain mismatch and then generates bending deformation due to the constraint action of the elastomer layer. As shown in fig. 3.

Example 2

The embodiment also provides a 4D printing method based on fractal curve stretchable heating circuit printing, which includes the following steps:

step 1: preparation of printing substrates

(1) VeroWhite material (Stratasys) was selected as the shape memory polymer material, and printing was performed using a Polyjet multi-material 3D printer (model J750 3D printer by Stratasys).

(2) 6 SMP samples were printed in total, the length of the sample being 90mm, the width 25mm and the thickness 0.9 mm.

Step 2: heating circuit for printing fractal curve

(1) Selecting Zhongtongton TL-201S as conductive paste, selecting a fifth-order Hilbert curve as a printing graph (the area is 23mm multiplied by 23mm) according to the required heating area, and selecting a Wucang spray head with the inner diameter of 300 mu m as the printing spray head.

(2) And starting the electric field to drive the jet deposition micro-nano 3D printer, and starting printing after adjusting printing parameters (voltage, air pressure, speed and printing height). A total of three SMP samples were printed with hilbert fractal curve heating lines. The printing principle is shown in fig. 1 b. The printing parameters in this embodiment are: the voltage is 1000V, the air pressure is 120Kpa, the speed is 5mm/s, and the printing height is 0.25 mm.

(3) And (3) placing the SMP sample piece printed with the fractal curve heating circuit in a vacuum drying oven, and heating and sintering for 1 hour at 120 ℃ to fully volatilize the solvent in the silver paste and effectively increase the conductivity.

And step 3: covered with a layer of SMP

(1) And (3) coating a layer of liquid Verowhite material on the three sintered SMP sample pieces, respectively attaching a SMP sample piece as a covering layer, compacting and placing in a UV curing machine for curing for 3min to fully bond the SMP sample pieces into a whole. The fractal curve heating circuit is embedded in the middle of the SMP sample piece, so that uniform and stable heating in the thickness direction can be realized.

And 4, step 4: drawing by electric heating

(1) Firstly, respectively placing SMP (symmetrical multi-processing) sample pieces in special clamps of a stretching instrument for clamping, switching on direct current voltage, and controlling the steady state temperature to be about 65 ℃ by adjusting the voltage (the glass transition temperature of the SMP sample pieces is 60 ℃), wherein the maximum stretching rate of the SMP sample pieces is more than 30% at the temperature.

(2) The stretching was started by adjusting the stretching parameters of the stretching apparatus (in this example, the stretching speed was 0.05mm/s, and the stretching amount was 5 mm).

(3) And after the stretching is finished, the power supply is disconnected. And taking down the sample after the sample is cooled to room temperature. The SMP sample will remain in the post-stretched state.

And 5: conformable elastomeric layer

(1) Elastomer material a Tango Black FLX973 (Stratasys) material was selected and printed by a Polyjet multi-material 3D printer (J750 type 3D printer, Stratasys), in this example the SMP elastomer layer was 0.5mm thick, 95mm long and 25mm wide.

(2) And coating a layer of photoresist on one side of the SMP sample piece, and attaching the elastomer layer on the SMP sample piece. The hand-held curing lamp is irradiated for a period of time to fully cure the material.

Step 6: is heated to generate bending deformation

And respectively sticking the 3 prepared SMP sample pieces on a triangular desktop, respectively connecting with an external power supply, triggering the free shrinkage of the heated area of the SMP layer when the temperature is higher than the glass transition temperature, and generating bending deformation after strain mismatch of the heated area due to the constraint action of the elastomer layer. And the three SMP sample pieces are respectively subjected to bending deformation to complete the transformation from a plane structure to a three-dimensional structure. The final three-dimensional structure can stand stably, and the load can reach 500 g, see the attached figure 6.

Example 3

The embodiment provides a method for printing a multi-material 4D printing sheet, and the method can realize the random change between a plane and a double-curvature three-dimensional structure.

Step 1: printing multi-material 4D printing sample sheet (1) the multi-material 4D printing sample sheet is composed of an SMP layer and an elastomer layer, wherein a shape memory polymer material is VeroClear material (a photo-curing digital material with shape memory behavior produced by Stratasys), and an elastomer material is Tango Black FLX973 (produced by Stratasys). Printing is carried out by using a Polyjet multi-material 3D printer (a J750 type 3D printer produced by Stratasys), and required multi-material 4D printing sample sheets are directly printed.

(2) The planar structure of the multi-material 4D printing sample sheet consists of a double-layer square (side length 30mm) SMP substrate (thickness 1.6mm, SMP layer thickness 0.2mm and elastomer layer thickness 1.4mm) in the middle and rigid plates at the edges. The rigid plates at the edges facilitate gripping when stretched as shown in figure 8.

Step 2: heating circuit for printing fractal curve

(1) Selecting Zhongtongton TL-201S as conductive paste, selecting a fifth-order Hilbert curve as a printing graph (the area is 30mm multiplied by 30mm) according to the required heating area, and selecting a Wucang spray head with the inner diameter of 250 mu m as the printing spray head.

(2) And (3) starting an electric field to drive the jet deposition micro-nano 3D printer, and after printing parameters (voltage, air pressure, speed and printing height) are adjusted, printing can be directly performed on one side of the SMP layer due to the thin SMP layer. A total of three biaxial tensile samples were printed with hubert fractal curve heating lines. The printing principle is shown in fig. 1 b. The printing parameters in this embodiment are: the voltage is 1000V, the air pressure is 130Kpa, the speed is 8mm/s, and the printing height is 0.25 mm.

(3) And (3) placing the biaxial stretching sample piece printed with the fractal curve heating circuit in a vacuum drying oven, and heating and sintering at 120 ℃ for 1 hour to fully volatilize the solvent in the silver paste and effectively increase the conductivity.

And step 3: drawing by electric heating

(1) Firstly, placing a multi-material 4D printing sample piece in a special clamp of a biaxial stretching instrument for clamping, connecting a direct current voltage, and controlling the steady state temperature to be about 65 ℃ by adjusting the voltage (the glass transition temperature of the SMP sample piece is 60 ℃), wherein the maximum stretching rate of the SMP layer is more than 30% at the temperature.

(2) After adjusting the stretching parameters of the stretching apparatus (in this example, the stretching speed was 0.05mm/s and the stretching amount was 3mm), biaxial simultaneous stretching was started in two perpendicular directions.

(3) And after the stretching is finished, the power supply is disconnected. And taking down the sample after the sample is cooled to room temperature. Due to the strain mismatch between the soft elastomer layer with residual stress and the rigid SMP layer, the sample piece undergoes bending deformation, and the structure deforms into a double curvature bowl-shaped structure, as shown in fig. 8.

And 4, step 4: heating to recover deformation

And after the biaxial stretching sample piece is connected with an external direct current voltage, heating, and when the heating temperature is higher than the glass transition temperature, the SMP layer begins to recover and shrink, so that the residual stress on the soft elastic body disappears. The biaxially stretched sample was restored to the original state after printing.

Example 4

The embodiment further provides a 4D printing method based on fractal curve stretchable heating circuit printing, which comprises the following steps:

step 1: preparation of printing substrates

(1) VeroWhite material (Stratasys) was selected as the shape memory polymer material, and printing was performed using a Polyjet multi-material 3D printer (model J750 3D printer by Stratasys).

(2) A total of 8 SMP samples were printed, the length of the sample being 80mm, the width 20mm and the thickness 1 mm.

Step 2: heating circuit for printing fractal curve

(1) Selecting Zhongtongton TL-201S as conductive paste, selecting a fifth-order Hilbert curve as a printing graph (the area is 20mm multiplied by 20mm) according to the required heating area, and selecting a Wucang spray head with the inner diameter of 200 mu m for the printing spray head.

(2) And starting the electric field to drive the jet deposition micro-nano 3D printer, and starting printing after adjusting printing parameters (voltage, air pressure, speed and printing height). In total, hubert fractal curve heating lines were printed on four SMP samples. The printing principle is shown in fig. 1 b. The printing parameters in this embodiment are: the voltage was 1100V, the air pressure was 160Kpa, the speed was 5mm/s, and the printing height was 0.3 mm.

(3) And (3) placing the SMP sample piece printed with the fractal curve heating circuit in a vacuum drying oven, and heating and sintering for 1 hour at 120 ℃ to fully volatilize the solvent in the silver paste and effectively increase the conductivity.

And step 3: adhesive SMP overlay

(1) And (3) coating a layer of liquid Verowhite material on the four sintered SMP sample pieces, respectively pasting a SMP sample piece as a covering layer, compacting, and then placing in a UV curing machine for curing for 3min to fully bond the SMP sample pieces into a whole. The fractal curve heating circuit is embedded in the middle of the SMP sample piece, so that uniform and stable heating in the thickness direction can be realized.

And 4, step 4: drawing by electric heating

(1) Firstly, respectively placing SMP (symmetrical multi-processing) sample pieces in special clamps of a stretching instrument for clamping, switching on direct current voltage, and controlling the steady state temperature to be about 65 ℃ by adjusting the voltage (the glass transition temperature of the SMP sample pieces is 60 ℃), wherein the maximum stretching rate of the SMP sample pieces is more than 30% at the temperature.

(2) The stretching was started by adjusting the stretching parameters of the stretching apparatus (in this example, the stretching speed was 0.05mm/s, and the stretching amount was 4 mm).

(3) And after the stretching is finished, the power supply is disconnected. And taking down the sample after the sample is cooled to room temperature. The SMP sample will remain in the post-stretched state.

And 5: conformable elastomeric layer

(1) Elastomer material a Tango Black FLX973 (Stratasys) material was selected and printed by a Polyjet multi-material 3D printer (J750 type 3D printer, Stratasys), in this example the SMP elastomer layer was 0.2mm thick, 84mm long and 20mm wide.

(2) And coating a layer of photoresist on one side of the SMP sample piece, and attaching the elastomer layer on the SMP sample piece. And (5) irradiating by a handheld curing lamp for 5min to fully cure the resin.

Step 6: is heated to generate bending deformation

And respectively sticking the four prepared SMP sample pieces on a square desktop, switching on an external power supply, triggering the free shrinkage of the heated area of the SMP layer when the temperature is higher than the glass transition temperature, and generating bending deformation after the strain mismatch of the heated area due to the constraint action of the elastomer layer. The four SMP sample pieces are subjected to bending deformation to complete the conversion from a plane structure to a three-dimensional structure, and the final three-dimensional structure can realize the function of automatic grabbing. As shown in fig. 9.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.