4D printing of shape memory polymer via liquid crystal display Stereolithography

In this study, we report a new epoxy acrylate based shape memory polymer(SMP) fabricated by Liquid crystal display (LCD) Stereolithographic 3D printing. The printed 3D object has a high resolution and high transparency in visible light region. The uniaxial tensile tests showed enhanced tensile toughness and tunable mechanical properties. The fix-recovery and cycle tests indicated high shape recovery properties including high shape recovery rate and excellent cycling stability. In addition, a smart electrical valve actuator was fabricated that can be used in fast heat or electricity responsive electrical circuits. LCD 3D printing provides a low-cost and high efficient way to fabricate fast responsive SMP, which can be used in wide applications in various fields on aerospace engineering, biomedical devices, soft robots and electronic devices.


Introduction
Shape memory polymer (SMP), also known as smart material, has gained Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 9 July 2020 doi:10.20944/preprints202007.0163.v1 increasing research interest and considerable progress in recent years. SMP have the capacity to recover their deformed shape or properties to original state under the external stimuli, such as heat, light, electricity, as well as humidity [1][2][3][4][5][6][7] . Heat-activated stimuli for SMP is the most important type due to the intrinsic glass phase transition of polymer 8 . SMP could be easily fixed to temporary shape under a force when temperature is lower than glass transition temperature (Tg), and recover to their original shape after temperature increases over Tg [8][9][10] . SMP have been widely applied in soft robots, vascular stents, aerospace structures, sensors, as well as electronic devices 2,4,[11][12][13][14][15] . However, the significant drawback of SMP is that it is difficult to fabricate complex geometries of shape memory structures by conventional molding and machining techniques, which limit the advanced applications in various areas 16,17 .
Three-dimensional (3D) printing, also known as additive manufacturing, has drawn a considerable amount of attention due to its capability for fabrication of complicate structures. 3D printing technology has been widely used in various field, including robots, microfluidics, wearable electronics, as well as biomedical devices 14,[18][19][20][21][22]  m with a 2D screen LED and enable a way to fabricate complex 3D objects. In this study, we prepared a type of liquid resin that was applied to print 3D object with shape memory properties though LCD 3D printing. Compared with previous reported shape memory polymer, the fabrication of shape memory polymer with high resolution and complex structure can be achieved easily. The shape memory polymer shows super transparency in visible light region. Furthermore, the enhanced tensile toughness and tunable mechanical properties of shape memory polymer are studied. The shape memory performance of printed samples is evaluated through fix-recovery and cycle test, which shows high shape recovery properties and cycling stability. Thus, this shape memory polymer is a significant candidate of material for 4D printing. Using these technology and material, a smart electrical valve actuator is fabricated that can be used in heat or electricity responsive electrical circuits. We believe the shape memory polymer based 4D printing has potential for application in various filed including aerospace, biomedical device, soft robots as well as electronic devices.

Material
Epoxy acrylate (EA) was purchased from Guangzhou Runao Chemical Co. Ltd.

Preparation of liquid resin
The liquid resin was prepared by mixing epoxy acrylate (EA) with Isobornyl acrylate (IBOA) and trimethylolpropane triacrylate (TMPTA) with different ratios. Then 3 wt% of diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide (TPO) as photoinitiator was added into the mixture and stirred for 30 min. Once finished, the mixture was degassed in a vacuum oven for 15 min to remove bubbles for following 3D printing.

3D printing fabrication
The LCD 3D printer (Photon-s, Shenzhen Anycubic Technology Co., Ltd., China) with"bottom-up" approach was used to fabricate shape memory polymer. A designed 3D model was first slice into a series of 2D images. Using the LCD image principle of liquid crystal display, selective transparent areas was modified by these 2D images. The 405nm of light from LED array was irradiated to liquid resin though transparent areas, while the opaque areas of LCD screen block the light. The irradiated liquid resin was solidified to form a layer, and the part of resin that was not irradiated still maintains the liquid. The substrate with fabricated structure was lifted, followed by next exposure and solidification. The process continues until the whole 3D model was fabricated. During the printing, the sliced thickness of each layer was 10 μm, and the exposed time of each layer was 5 s. After printing, the obtained structure was washed by ethanol to remove uncured resin followed by secondary curing in UV oven for 30 min.

Characterization
The resolution of printed sample was measured by electron microscope (SUPEREYES, China). The transparent spectrum was performed on an UV-vis spectrophotometer (NOVA-EA, China) from 400 to 800 nm. The FTIR spectrum was acquired on a Nicolet 5700 FTIR spectrometer (China). The TG-DSC curves were measured using differential scanning calorimetry analysis (TA Q600, USA).The tensile tests were performed on an electronic universal mechanical testing machine (ZQ-990A, 50 N, China).

Results and discussion
3.1 3D printing procedures for shape memory polymer.
The 3D printing procedure for shape memory polymer is schematically in Figure 1a.
The patterned 405 nm of light though the LCD screen is illuminated onto the liquid resin, then the exposed liquid resin is solidified to form a solid layer, followed by rise of substrate to solidify a new one. Layer by layer solidification proceeds until the whole structure fabricated (details in part of Experiment). As for chemistry of liquid resin, epoxy acrylate (EA) was used as oligomer, and isobornyl acrylate (IBOA) and

Transparency of printed sample.
As shown in Figure 2, the printed sample with 0.25mm thickness was completely transparent, and photograph covered by sample still can be observed clearly (inset figure of Figure 2). To further investigate the transmittance in visible range, Figure 2 shows the transmittance spectra of printed sample in the wavelength range from 400 to 800nm. Remarkably, the printed sample exhibit 95.6% average transmittance in the visible light region. photograph can be seen clearly through the top of printed sample.

Thermal properties
TG analysis was used to study the thermal properties of shape memory polymer. As shown in Figure 4a,

Mechanical properties
The mechanical properties of shape memory polymer were evaluated by the uniaxial tensile tests. These tests were conducted on an electronic mechanical testing machine with an extension rate of 1 mm min -1 . All tensile strength measurements were carried out at 25 ℃ (room temperature), which is below the glass transition temperature of shape memory polymer. Stress−strain curves was generated as shown in Figure 5. It illustrates that the modulus of shape memory polymer with 60 wt% of epoxy acrylate

Shape memory effect
To demonstrate the excellent shape memory effect of printed sample, the test was carried out as follows: Firstly, the printed sample was heat to temperature above Tg and then bend into shape of "U", then the printed sample was cooled down to 25°C(below Tg) to fix the shape of "U" and the angle was measured as θf. Lastly, the printed sample was reheat to temperature above Tg to recover its original shape and the angle was measured as θr. The shape recovery ratio (Rr) is determined using equation: Where θr is the recovery angle, θf is the fixed angle. Figure 6a shows the change of shape recovery ratios of printed 0.2mm of thickness with 40 wt% of epoxy acrylate (EA) at different temperature, it can be seen that the sample recovered to its original shape within 6s at higher temperature, which was Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 9 July 2020 doi:10.20944/preprints202007.0163.v1 faster than one at lower temperature. This phenomenon is because that greater mobility of polymer chains in higher temperature, which lead to excellent shape memory effect.
The images of recovery progress of printed sample were shown in Figure 6d, the printed sample rapidly recovered to its original shape within 21 s at temperature of 80°C. Similarly, the printed sample with different content of epoxy acrylate (EA) also has effect on shape recovery ratio and recovery speed (Figure 6b). With decreasing of content of epoxy acrylate (EA), the recovery ratio and speed decreased. We further studied the repeated shape memory properties of printed sample. Figure 6c shows the value of shape recovery ratio (Rr) during 15 cycles. It can be seen that shape recovery ratio (Rr) was 97.8% for repeated shape memory process, which was nearly the same with the original one, indicating stability of shape memory effect.

3D printed structures and devices
Using LCD 3D printing technique, we printed a series of structures including logo "SMP", vascular stents, and robot arm to exhibit shape memory behavior. Figure 7a shows shape recovery process of the printed logo "SMP", which could rapidly recover back to its original shape within 13s. In addition, vascular stents with high complexity and resolution are a challenge to fabricate by traditional manufacturing approaches.

Figure 7b
shows that the 3D-printed vascular stents with high transparency, the stents was programmed to shape with small diameter and recovered to its original shape with a larger diameter within 16s. This phenomenon indicate that such responsive objects could be used in fabrication of soft robotics, minimal invasive surgery.
By integrating the printed sample with conductive materials such as paste with silver nanoparticle, the shape memory polymer could be applied in electrical devices. We fabricated a smart electrical valve actuator. This device was composed of printed sample with conductive silver paste, this silver paste acts as electrical contacts could print on shape memory polymer due to its solidify at room temperature, and two individual copper electrode was attached to device. As shown in Figure 7c, the original shape was an open electrical circuit, the programed shape was a closed electrical circuit with LED bulb lighted. When the temperature was above the glass transition temperature (Tg), The temperature enables recovery of shape memory polymer and

Conclusion
In summary, we fabricate a shape memory polymer LCD 3D printing, which shows high resolution and high transparency (95.6 %). The uniaxial tensile tests show enhanced tensile toughness and tunable mechanical properties of shape memory polymer. The shape memory properties have been proven through fix-recovery and cycle tests. The deformed sample could recover to its original shape at temperature of