Three-Dimensional-Printed Shape Memory Biomass Composites for Thermal-Responsive Devices

Abstract

In this study, unique three-dimensional (3D)-printed shape memory biomass composites were prepared by the melt blending and extrusion of polyurethane, polycaprolactone (PCL), and wood flour (WF) with adjustable contents. The addition content of PCL was used to adjust the shape memory transition temperature and improve the shape fixing rate of composites. The crystallization, thermal, mechanical, and shape memory properties of different composites were investigated. The results of X-ray diffraction and differential scanning calorimetry tests showed that the crystallization peak and melting temperature of different composites were not obviously changed. As the PCL content increased, the tensile strength of the composites decreased first and then increased, and the elongation at break gradually decreased. Thermal response shape memory test results showed that, when the PCL content was 30 wt.%, the composites had high shape recovery rate and fixed rate (both ∼100%). In addition, carbon black (CB) was added as a photothermal conversion material to the composite with a preferred ratio to achieve the photothermal response shape memory performance. With the addition of CB, the thermal conductivity of composites was improved. Under the same conditions, the thicker the 3D-printed specimens, the longer the specimen shape recovery time; the greater the light intensity, the shorter the specimen shape recovery time. Compared with the composite without CB, the flower model printed with the composites containing CB had a better photothermal response shape memory performance.

Introduction

Stimulus-responsive devices have attracted tremendous scientific interests because of their superior sensing capabilities and being able to recover an obvious deformation from a temporary shape actuated by different environmental stimuli, including heat, light, magnet, and electric current/voltage, which are caused by the conformational changes of their composition stimuli-responsive shape memory polymers.^1–9 At the same time, three-dimensional (3D) printing can offer devices precisely defined geometric shapes and sizes to meet individual needs.^10–13 The stimuli-responsive devices fabricated by 3D printing technology may provide future opportunities of device development in diverse research fields, such as flexible electronics and biomedical and medical devices.^{1–5,10,14–17}

Among the 3D printing approaches, the fused deposition modeling (FDM) usually uses thermoplastic filaments as printing material to print the product layer by layer cost effectively.^14–16 In contrast to traditional manufacturing techniques that subtract materials in a predesigned mold, the 3D printing technique is processed by adding and connecting materials. Therefore, the use of materials is more efficient, and the manufacturing process does not need any mold or template.^10–14,18

The main materials available for FDM to fabricate stimulus responsive devices include polylactic acid (PLA), polycaprolactone (PCL), and thermoplastic polyurethane (TPU).^19,20 However, some mentioned materials, such as PLA, have low toughness, which are difficult to produce flexible stimuli-responsive products.^11,21,22 Currently, the polyurethane polymer has received extensive attention in the field of responsive stimulus equipment materials due to its excellent thermal response shape memory properties.^{1,19,20,23,24} Among them, TPU is a soft 3D printing material with thermal response shape memory performance. It can be converted into flexible products with complex structure, solving the brittleness problem of most traditional materials.^11,21–24

However, the cost of TPU is relatively high, and many researchers have tried to reduce the production cost by adding low-cost fiber materials to obtain composites with better comprehensive performance.²⁵ WF is an environmentally friendly, abundant, and low-cost biomass material, which can be used as an ideal filler. To fabricate 3D printable stimuli-responsive devices by FDM, the TPU/WF composites with thermal response shape memory were developed in our laboratory.²⁶

The transition temperature (T_trans) of the TPU/WF composite used to set the temporary shape was the glass transition temperature (T_g), resulting in poor shape fixation due to the low temperatures. Therefore, to improve the shape fixation rate, it is required a higher temperature to fix the temporary shape for the actuation performance of TPU/WF composites.^19,26–29 PCL is solid state at room temperature and the melting temperature (T_m-pcl) ranges from 45 to 60°C.^3,19 In this study, PCL was chosen for TPU/WF composites due to its excellent shape memory properties caused by its crystalline domains.^5,28,30,31

In addition, to expand the functional application of the thermal response shape memory composites, many studies have designed and manufactured composites with functional stimulus response modes (such as electrical, magnetic, or light) by adding functional particles.^{3,4,6,30,32,33} For example, the light-induced shape memory effect can be achieved indirectly by adding some photothermal media (such as carbon black [CB] particles, gold nanoparticles, and carbon nanotubes), which can convert light into heat.^34–37 Light as an external trigger, with its excellent remote control ability and fast switching performance, has attracted much attention.^1,2,4,32,38 Recently, there have been many studies on the preparation of light response shape memory composites by photothermal conversion materials filled with polymers.

However, in the literature review, no research was found on the preparation of 3D-printed, low-cost, flexible, photothermally responsive shape memory composites by adding such a large amount of WF. CB has excellent photothermal conversion efficiency (92 ± 3%), which is easy to be obtained with a low price.^1,35 Its absorption of visible light is due to the optical transition of π-band.^34,36,37 It reflects only 5–10% amount to the air/medium interface.³⁴

In this study, thermal response shape memory biomass composites comprising TPU, PCL, and WF were prepared by melt blending and FDM technology at different weight ratios to study their shape memory performance. After the satisfactory weight ratio composites was selected, partial CB was added into the TPU/PCL/WF (TWP) composite to make the 3D-printed shape memory devices with photothermal responsive characteristics. An illustration showing the mechanism of the TPU/PCL/WF composite shape memory performance is presented in Figure 1.

FIG. 1.

(a) Shape memory behavior of 3D printing composites based on WF (or CB) with TPU/PCL composite; (b) 3D-printed device fabricated by FDM; (c) shape memory behavior of 3D-printed device. CB, carbon black; 3D, three dimensional; FDM, fused deposition modeling; PCL, polycaprolactone; TPU, thermoplastic polyurethane; WF, wood flour.

Materials and Methods

Material

The TPU with a trade name of Elastollan C85A and PCL with a trade name of CAPA 6500 were purchased from the Dongsheng Plastic Co., Ltd (China). The WF (poplar WF) with an average diameter of 150 μm was supplied by Lingshou County Mineral Processing Plant Co., Ltd (China). The CB (with an average size of 44 nm and a trade name of T-5) was obtained from Xinxiang Deron Chemical Co., Ltd (China). Before they were used, TPU, WF and CB were dried in an oven at 103°C for about 12 h and PCL was dried at 40°C for 12 h. The thermo-variable ink material was purchased from Shenzhen Dongfang Color Technology Co., Ltd (China).

Preparation of shape memory composites

For all blended mixtures, the weight ratio of TPU/PCL and WF remained at 80:20. TPU/PCL/WF ternary blends with different PCL contents (0–40 wt.%) were prepared. The formulations and names of different weight ratio TPU/PCL/WF composites are shown in Table 1. After the satisfactory weight ratio composites were selected, 10 wt.% CB was added to replace part of the WF content at a fixed TPU/PCL/WF ratio, and the composite was named as TWP-CB. The preparation process of composites followed the process of the previous research.²⁶ The printing parameters for the FDM printer are listed in Table 2.

Table 1.

Different Weight Ratio TPU/PCL/WF Composites (%)

Specimens	TPU	PCL	WF
TW	80	0	20
TWP10	70	10	20
TWP20	60	20	20
TWP30	50	30	20
TWP40	40	40	20

PCL, polycaprolactone; TPU, thermoplastic polyurethane; TW, TPU/WF; TWP, TPU/PCL/WF; WF, wood flour.

Table 2.

Printing Parameters Used for Fused Deposition Modeling Printer

Parameter	Value
Nozzle size (mm)	0.4
Infill density (%)	100
Nozzle temperature (°C)	230
Layer height (mm)	0.4
Printing speed (mm/s)	25
Printing plate temperature (°C)	30

Characterizations

The X-ray diffraction (XRD, Philips, and D/max2200) patterns were operated with Cu radiation and at the acceleration voltage of 40 kV, the current of 30 mA. The range of 2θ was 5°–50° and testing speed was 5°/min.

Different TPU/PCL/WF composite melting temperatures were obtained by differential scanning calorimetry (DSC, 214 analyzer) at a heating rate of 10°C/min from 25 to 230°C.

The mechanical properties of TPU/PCL/WF composites were examined by an electronic universal mechanical testing machine (RGT-20A). Tensile tests were carried out using a crosshead speed of 500 mm/min at room temperature according to DIN 53504.

The thermal response shape memory performances of TPU/PCL/WF composites with different PCL contents were examined. 3D-printed rectangular specimens (80 × 8 × 1.2 mm) were fabricated with different TPU/PCL/WF composites. First, the 3D-printed specimens were immersed in a 60°C water bath (>T_m-pcl) for 5 min; second, the 3D-printed specimens were bent into a temporary shape with a deformation angle of 0°, and then, the force was maintained for 5 min at room temperature (<T_m-pcl) to fix the temporary shape. Third, the external forces of the 3D-printed specimens were removed, and the temporary shape angle was recorded as α. Finally, the process of restoring the original shape of 3D-printed specimens was recorded by repositioning them in the 60°C water bath and the recovery shape angle was recorded as β.

The photothermal conversion properties of the optimized TPU/WF/PCL and TWP-CB composites were studied. Circular specimens with a diameter of 20 mm and a thickness of 0.8 mm were printed with the optimized TPU/WF/PCL and TWP-CB composite wires. The circular specimens were fixed on a stable test bed and the light source was placed about 30-cm above the specimens at room temperature. At the 98.47 mW/cm² light intensity, the temperature changes of the different composite circular specimens were recorded as the illumination time increased from 0 to 1200 s.

The effect of the light intensity on the photothermal conversion properties of TWP-CB composites was examined using circular specimens (20 mm in diameter and 0.8 mm in thickness). Temperature changes of the TWP-CB composite specimens were recorded under different light intensities (45.82, 93.41, and 171.88 mW/cm²).

The thermal conductivity of the optimized TPU/WF/PCL and TWP-CB composites was measured by a thermal conductivity meter (TC3000E).

To understand the influence of specimen thickness and light intensity on the photothermal responsive shape memory behavior of composites, the TWP-CB composite was used to print rectangular specimens of different thicknesses (0.8, 1.2, 1.6 mm) with a specification of 50 × 20 mm. In addition, the shape memory behaviors of different 3D-printed composite specimens were explored. First, the specimens were immersed in the 60°C water bath (>T_m-pcl) for 5 min. Second, the specimens were bent into a temporary shape with a deformation angle of 35°, and then, the force was maintained for 5 min at room temperature (<T_m-pcl) to fix the temporary shape. Third, the external force of the 3D-printed specimens was removed. Fourth, the specimens were illuminated by a light source (185.0 mW/cm² light intensity), the process of restoring the original shape of 3D-printed specimens was recorded, and the effect of the specimen thickness on the shape memory behavior was systematically studied. The environmental condition of the test was the same as that of the photothermal conversion test.

Meanwhile, to investigate the effect of light intensity on the photothermal properties of materials, the abovementioned steps were repeated with a rectangular specimen (50 × 20 × 0.8 mm). The effect of light intensity on shape memory performance was observed by taking the light intensity as a variable. In the fourth step, the light intensity was changed to be 46.87, 68.76, 121.85, or 158.18 mW/cm², respectively.

Shape memory behaviors induced by heat and light were recorded by a mobile phone. The light source was a 300 W xenon lamp. The temperature of 3D-printed specimens and models was measured by an infrared thermometer (Smart sensor AR588+). The light intensity of the light source was measured by a power meter (SM206-SOLAR). All specimens and models for the testing were printed with the FDM printer (HORI Z400D).

Results and Discussion

Properties of TPU/PCL/WF composites with different PCL contents

Analysis of the crystalline TPU/PCL/WF composites

The crystalline phase in the TPU/PCL matrix was responsible for fixing the temporary shape, and the amorphous phase of rubber properties was helpful for shape recovery.³⁰ Figure 2 shows XRD curves of the TPU/PCL/WF mixture. TPU itself had a wide diffraction peak from 17° to 25°, which corresponded to a mixture of hard ordered structure and TPU amorphous disordered structure.^19,39 With the increase of PCL content in the blends, the two peaks at 2θ = 21.2° and 23.8° became higher, which was attributed to the diffraction of characteristic lattice planes (110 and 200) of semicrystalline PCL, respectively.^19,40 It was indicated that there was a crystallization zone in the blend. In addition, PCL had no obvious peak shift with the content change, which implied that the lattice orientation of PCL was not affected by the content change of other materials (TPU, WF).

FIG. 2.

The XRD curves of different weight ratio TPU/PCL/WF composites. XRD, X-ray diffraction. Color images are available online.

Analysis of mechanical properties of TPU/PCL/WF composites

The tensile properties of composites were measured to study the mechanical performance of the different TPU/PCL/WF composites. The tensile test curves are shown in Figure 3. It can be seen from Figure 3, with the increase of PCL content, the tensile elongation of TPU/PCL/WF composites decreased gradually, and the tensile strength decreased first and then increased. This phenomenon was mainly attributed to the phase structure and crystal structure of TPU/PCL/WF composites. In the composites, TPU was rubber phase, and PCL was rigid phase. When the PCL content increased gradually, the rigidity of the composite increased and the tensile strength increased, while the toughness decreased. Due to the high crystallinity of PCL and the perfect crystal region, the tensile strength was guaranteed.

FIG. 3.

The tensile properties of different weight ratio TPU/PCL/WF composites. Color images are available online.

The PCL crystal in the composites could maintain sufficient strength to prevent the amorphous region of PCL and the recovery of flexible TPU. This provided composites with the ability to remember temporary shapes at room temperature. This feature is perfect and effective in the practical application.

As shown in Supplementary Figure S1, the TWP30 composite flexibility properties were good and the tension specimen can be bended as shown in Supplementary Figure S1b. These results indicated that the TPU/PCL/WF composites had good flexibility.

Analysis of shape memory mechanism of TPU/PCL/WF composites

It is important to select the appropriate weight ratio of PCL to TPU/WF composites, providing composites with excellent shape memory properties. DSC was used to examine T_m of PCL in different weight ratios of PCL to TPU/WF composites, and the results are presented in Figure 4a. With the increased PCL content, T_trans of shape memory composites was changed to T_m of PCL, and the intensity of the crystallization peak was gradually increased. DSC can also be used to measure the generated heat during the physical transformation of polymer chains, such as phase transformation, which can reflect the movement and rearrangement of polymer chains. Therefore, the shape memory mechanism of composites was explained by DSC. Taking the TWP30 composite as an example, DSC data showed that the T_m of PCL components in composites was 57°C (Fig. 4a).

FIG. 4.

(a) The melting temperature of PCL in different weight ratio TPU/PCL/WF composites. (b) The images of 3D-printed specimens with thermal shape memory behavior. (c) The definition of temporary angle α and recovery angle β. (d) The temporary angle (α) after removing the external force and the recovery angle (β) after thermal induction of different TPU/PCL/WF composites. (e, f) The temporary angle (α) and recovery angle (β) of the TWP30 and the TP composites. TWP, TPU/PCL/WF. Color images are available online.

When the temperature was lower than 57°C, PCL was in the solid state, which was “frozen” because of the crystallization of PCL molecules and the low-molecular motion in composites. When the T_m-pcl was higher than 57°C, it was in the melting state. The PCL crystals were softened by heating, resulting in considerable segmental movement of polymers and changes in the conformation of polymers. When the composites were in a temporary state, the PCL component in the composite was in a crystalline state and kept the shape deformed. After the heat induction, the PCL temperature of composites increased to the one higher than T_m-pcl. PCL crystals softened and the composites shape gradually recovered due to the elasticity of TPU. Finally, the shape recovered from the temporary state to the original state after the heating.

Analysis of shape memory performance of TPU/PCL/WF composites

The images of 3D-printed specimen with thermal shape memory behavior are shown in Figure 4b, and the temporary shape angle and the possible recovery angle after the heating are shown in Figure 4c and d. With the increase in immersion time in the hot water bath (60°C), the shape memory performances of TPU/PCL/WF composites with different weight ratios are shown in Figure 4d. The results showed that, under the same conditions, the shape fixed angle and recovery angle of the composites were improved by the addition of PCL. Thus, the thermal responsive composites' shape memory behavior can be designed by adjusting the weight ratio of PCL.

As for shape memory devices, 30 wt.% PCL was advantageous for TPU/WF thermal responsive composites, because it can achieve high shape recovery rate and fixed rate (both around 100%). The thermal response of TWP40 composites also had a higher recovery rate and fixed rate. However, compared with TWP30 composites, it was observed that the TWP40 composite specimens had the longer recovery time in the recovery process. This may be because PCL had a high viscosity in the molten state and can self-bind when the PCL content was high.

In addition, a composite with TPU: PCL mass ratio of 5:3 without WF was prepared, being named as TP. The TWP30 and TP composite 3D printing wires are shown in Supplementary Figure S2a and b. The shape memory performances of TWP30 and TP were tested and compared (Fig. 4e, f). It was found that the TP composites could not be recovered due to the viscosity of PCL, while the TWP30 composites could be recovered to the original shape generally. It was indicated that WF played a clear role in hindering the increase in viscosity of the material without losing the material's shape memory performance. Compared with other similar studies,^{19,24,26,29,41} the optimized TWP30 biomass composites had improved shape fixation performance and shape recovery capability, as shown in Supplementary Table S1.

Shape memory performance of thermal responsive cube model

A cubic plane model with a thickness of 2 mm (the plane model can be folded into a 20 × 20 × 20 mm cube, as shown in Fig. 5) was printed using TWP30 composite wires by the FDM printer, and the thermal response shape memory behavior of the cubic plane was successfully performed (Fig. 5). The 3D-printed cubic plane (original shape), temporary shape (cube), and the shape recovering process of cube in the 60°C water bath are shown in Figure 5. The composite wires were mechanically forced through a heated nozzle, which liquefied the biomass composites to a melt state. Then the composite was extruded and deposited onto a build-plate, layer by layer, until a cubic plane was generated (Fig. 1b). After the heating, applying external force, and cooling, the 3D-printed cubic plane changed from its original shape to a temporary shape of a cube (Fig. 1c).

FIG. 5.

TWP30 composites of 3D-printed cubic plane model (original shape), temporary shape (cube), and shape recovering process of cube under a 60°C water bath.

It was worth being noted that, because of the thermal responsive shape memory characteristics of the printed composite model, the shape of the 3D-printed cubic plane recovered from the temporary (3D state) to the original shape due to the triggering of external heat (Fig. 5). After being heated for 10 s, the cube completely recovered from the 3D state to the plane state.

Properties of photothermal responsive shape memory composites

Photothermal properties of TWP30 and TWP-CB composites

To obtain FDM 3D printing shape memory composites with excellent photothermal-responsive performance, the TWP-CB composites were fabricated by adding 10 wt.% CB to replace 10 wt.% WF in the thermal responsive TWP30 composites. The TWP-CB composite 3D printing wires are shown in Supplementary Figure S2c.

The temperature change curves of TP, TWP30, and TWP-CB composites under the same light intensity of 98.47 mW/cm² are shown in Figure 6a. The temperature of TWP30 composite specimens increased from 23 to 37°C with the extension of illumination time. When 10 wt.% CB replaced 10 wt.% WF, the temperature of TWP-CB composite specimens increased from 23 to 48°C. The results showed that the addition of CB increased the maximum temperature of photothermal response composites under the same conditions (Supplementary Fig. S3). In addition, the thermal conductivity of TWP30 and TWP-CB composites was tested, as shown in Figure 6b. Obviously, the addition of CB improved the thermal conductivity of composites.

FIG. 6.

(a) The temperature curves of TP, TWP30, and TWP-CB composites under the 98.47 mW/cm² light intensity; (b) thermal conductivity of TWP30 and TWP-CB composites.

Compared with TWP30 composites, the thermal conductivity of TWP-CB composites increased 1.4 times. The thermal conduction test results showed that the addition of CB significantly improved the thermal conductivity of the composites. The above results proved the excellent photothermal performance of TWP-CB composites.

Based on the mentioned findings, the possible mechanism of this phenomenon was analyzed. Figure 7 provides a simple schematic illustration of the physical phenomena associated with light scattering in composites. As for TWP30 composites, WF could absorb light, while WF also scattered a part of light.^33,43,44 The multiple scattering effects of WF in the composites led to multiple changes of the light propagation direction, thus increasing the length of light path in the composites. In TWP-CB composites, CB could absorb almost all lights. In addition, CB may also absorb the light scattered by WF in TWP-CB composites.^33,36 Under the action of light, the contact rate between light and CB in TWP-CB composites was increased. Therefore, the photothermal conversion capability of TWP-CB composites was improved.

FIG. 7.

Photothermal effect mechanism of TWP-CB composites.

Photothermal conversion efficiency of TWP-CB composites

To further understand the mechanism of the photothermal response shape memory properties, control experiments were carried out using 3D-printed circular specimens with TWP-CB composites. The temperature change curves of TWP-CB composites under different light intensities are shown in Figure 8a. In the process of increasing light intensity from 45.82 to 171.88 mW/cm², the maximum temperature of the 3D-printed circular specimens increased from 37.5 to 62.5°C after 1260 s of illumination. With the removal of the light source, the temperature of 3D-printed specimens gradually returned to room temperature. It was suggested that the 3D-printed devices could effectively utilize light energy to generate heat and recover the temporary shape to the original shape.

FIG. 8.

(a) The temperature curves of TWP-CB composites under different light intensities. (b) The external photothermal conversion efficiency of 3D printing materials (TWP-CB) with different light intensities.

In addition, it was also shown that the light intensity was positively correlated with the heat generated by 3D printing devices, affecting the shape memory behavior triggered by light. To further study the photothermal conversion efficiency of composites, a method reported by Yang et al.¹ and Wang et al.⁴² was used to calculate the photothermal conversion efficiency of TWP-CB composites (see the Supplementary Data for details). As shown in Figure 8b, the average external photothermal efficiency of TWP-CB composites was about 17.6% under different light intensities, which looked reasonable as only 10 wt.% CB components in the composites effectively absorbed light to generate heat.

The effects of specimen thickness and light intensity on the photothermal responsive shape memory behavior

To quantitatively study the effects of specimen thickness and light intensity on the photothermal responsive shape memory behavior of composites, the shape memory behaviors of several 3D-printed specimens were investigated. Using the same light intensity, the curves of different thickness specimens' recovery angle (β) that changed with the illumination time are shown in Figure 9a. The results showed that the recovery angle β of the 3D-printed specimens increased with the prolongation of the illumination time.

FIG. 9.

(a) The effect of specimen thickness on the shape memory behavior of 3D-printed specimen. (b) The influence of light intensity on the shape memory behavior of 3D-printed specimen. (c) The recovery angle of the 3D fitting graph decreasing with the increase of illumination time and the thickness of the specimen, obtained by Eq. (1). (d) 3D fitting graph of recovery angle, recovery angle increasing with the increase of illumination time and light intensity, obtained by Eq. (2). Color images are available online.

When the illumination time reached 350 s, the β of 0.8-mm-thick 3D-printed specimens increased from 35° to 180° under 185.0 mW/cm² light intensity (Fig. 9a). It was shown that the 3D-printed specimens had photothermal responsive shape memory characteristics. Furthermore, when the illumination time was 250 s, with the increase of specimen thickness from 0.8 to 1.6 mm, the recovery angle of 3D-printed specimens decreased from 165° to 40° (Fig. 9a). It was indicated that the increase of specimen thickness would prolong the recovery time of shape memory of 3D-printed devices, because thicker specimens would reduce the heat conduction efficiency in composites.

The 3D-printed specimens with a thickness of 0.8 mm were used to explore the effect of light intensity on the shape memory behavior. When the light intensity was low (e.g., 46.87 or 68.76 mW/cm²), the specimens could not be restored to the original shape. When the light intensity was high (e.g., 121.85 or 158.18 mW/cm²), the specimens could be restored to the original shape (β was 180°). When the higher light intensity was used, the shorter shape recovery time was obtained (Fig. 9b). According to the results, it can be inferred that the shape recovery time of 3D-printed specimens under the photothermal response can be shortened by increasing the light intensity.

The above results showed that the specimen thickness, light intensity, and illumination time all affected the shape memory behavior of the 3D-printed devices. The relationship among the recovery angle β, illumination time t, and specimen thickness h is represented by Eq. (1), which takes recovery angle β as a function of illumination time t and specimen thickness h.

$$pc\begin{array}{c}\beta =Z_0+B\times exp\left(-exp\left(\frac{-\left(t-c\right)}{D}\right)\right)+E\\ \times exp\left(-exp\left(\frac{-\left(h-F\right)}{G}\right)\right)+H\\ \times exp\left(-exp\left(\frac{-\left(t-C\right)}{D}\right)-exp\left(\frac{-\left(h-F\right)}{G}\right)\right)\end{array}$$ (1)

In which Z₀ is 35, B is 142.80, C is 273.75, D is 78.96, E is −35.50, F is 1.56, G is 0.10, and H is −133.97. According to Eq. (1), a three-dimensional fitting graph of the recovery angle of the 3D-printed specimens with different thicknesses varying with the illumination time was obtained, and the variation of the recovery angle with the illumination time and specimen thickness is presented in Figure 9c.

The relationship among the recovery angle β, illumination time t, and light intensity i is represented by Eq. (2), which takes recovery angle β as a function of illumination time t and light intensity i.

$$\begin{array}{c}\beta =Z_0+B\times \left(\frac{1}{1+exp\left(-\left(\frac{\left(t-C\right)}{D}\right)\right)}\right)\\ \times \left(\frac{1}{1+exp\left(-\left(\frac{\left(i-E\right)}{F}\right)\right)}\right)\end{array}$$ (2)

In which, Z₀ is 35, B is 219.13499, C is 325.66, D is 91.92, E is 1316.09, and F is 255.42. According to Eq. (2), the three-dimensional fitting graphs of the recovery angle of 3D-printed specimens varying with illumination time under different light intensities were obtained. The variation of recovery angle with illumination time and light intensity is illustrated in Figure 9d.

Photothermal responsive shape memory performance of flower model

As well known, light and temperature are the main factors affecting the flowering of flowers. In this study, the 3D-printed flower model was used to demonstrate the photothermal response shape memory performance of the composites. The FDM printer was combined with the shape memory composites (TWP30 and TWP-CB) to print flower models, as shown in Figure 10. The petals (three layers with 1.2 mm thick in total) of the flower model were printed by TWP-CB shape memory composites, and the stamens were printed by TWP30 shape memory composites (Fig. 10c). As the temperature increased over 57°C, the petals changed from a rigid state to an elastic and flexible state. When the temperature was lower than 57°C, the temporary shape of the closed bud was fixed by applying external force.

FIG. 10.

(a) The true image of flowers. (b) The image of 3D-printed flower. (c) The original shape of the 3D-printed flower with TWP-CB composite. (d) Temporary shape of 3D-printed flower (bud shape) and different deformations of flower under different illumination times at 175.56 mW/cm² light intensity. Flowers change from bracts to blossoms during light. (e) The image of a double-nozzle 3D-printed flower (upper layer printed with TWP30 composite, and lower layer printed with TWP-CB composite). (f) 3D-printed flower's original shape at room temperature was light pink and (g) 3D-printed flowers take on a red color in water at 60°C. (h) 3D-printed flower color changed with illumination time under 175.56 mW/cm² light intensity. After 320 s, it turned rosy red.

As shown in Figure 10d, under the light intensity of 175.56 mW/cm², the flower exhibited different deformations with the prolonged illumination time. As the illumination time increased, the shape of petals returned to its original state, which led to the change of petal shape from bud shape to flowering shape (Fig. 10d). This process was similar with that of actual flowers from germination to flowering. After 260 s of lighting, the flower was completely open.

The photothermal responsive shape memory performance of another 3D printing flower model was studied using the FDM dual-nozzle 3D printer. The thickness of the petals of flower model was also set as three layers (1.2 mm thick in total), the lower two layers (0.8 mm thick in total) were 3D printed with the TWP-CB composites, the upper layer (0.4 mm thick) was 3D printed with TWP30 composites, and the stamens were 3D printed with the TWP-CB composites (Fig. 10e, f). The upper layer was coated with a thermo-variable material (an ink material that was light pink at room temperature [Fig. 10f] and became rose-red at temperatures over 60°C [Fig. 10g]). It can be seen from Figure 10f that the color of the upper layer of the flower was light pink before the illumination, and the temperature was lower than T_m-pcl, thus keeping the flower closed (Fig. 10h).

After the illumination, CB components in flower absorbed light and generated heat. With the extension of the illumination time, the flowers returned to their original shapes (Fig. 10h). At this time, the color of the upper layer of flower was rosy red, which triggered the blooming process of flowers. By comparing Figure 10d and Figure 10h, it was found that it needed a long time for the flower in Figure 10h to return to the original shape, which further proved that the photothermal responsive shape memory performance of TWP-CB composites was improved compared with that of TWP30. In this way, a 3D-printed flower with the photothermal-induced shape memory behavior was obtained, which was similar to that of a real plant flower that responds to sunlight and causes the flower to bloom.

Conclusions

A series of low-cost and thermal response TPU/PCL/WF shape memory composites were manufactured without any surface treatment. XRD and DSC examination results showed that there was no significant change in the crystallization peak and melting temperature of different TPU/PCL/WF composites. With the increase of PCL content, the tensile strength of TPU/PCL/WF composites decreased first and then increased, and the elongation at break gradually decreased. Thermal response shape memory test results showed that, when the PCL content was 30 wt.%, the composites had high shape recovery rate and fixation rate (both around 100%). The low-cost CB was added to the composites to improve the photothermal response shape memory performance of the 3D printing device.

Using the same light intensity, TWP-CB composites increased the maximum temperature of photothermal response composites. The thermal conductivity of TWP-CB composites was significantly higher than that of TWP30 composites. Under the same conditions, the thicker the 3D-printed specimens, the longer the specimen shape recovery time obtained. When the greater light intensity was used, the shorter specimen shape recovery time was achieved. The flower model printed with TWP-CB composites had the improved photothermal response shape memory performance.

By combining the FDM technology, 3D printing biomass composites with photothermal response shape memory characteristics were prepared for application to stimulus-response devices. This not only expands the variety and functionality of 3D printing materials, but also promotes the application of 3D printing technology in the field of stimulus-responsive devices. This simple 3D printing manufacturing method will provide a huge development opportunity for the design and manufacturing of personalized customized stimulus response devices, which can be widely used in the fields of bionic intelligent devices, flexible electronics, and soft robots.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

Thanks for the supported by “the Fundamental Research Funds for the Central Universities (2572019AB01).”

Supplementary Material

Supplementary Figure S1

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Supplementary Table S1