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Concealed intelligent switches have significant potential for use in security and encryption applications. Recently, several four-dimensional (4D) printed intelligent devices have been designed and fabricated using 3D printing technologies and intelligent materials. These devices have demonstrated stupendous capabilities in smart switches and actuators. In this study, a concealable smart switch that combines a double spiral structure and smart materials is proposed. To achieve the desired performances, the fixity ratio is optimized to values more than 88% by comparing selected structures printed with common shape-memory materials. Furthermore, applications for non-contact circumstances without any control electric circuits (for example, hidden water valves) are also proposed. Results demonstrate that the outputs of the intelligent switches are related to their different input conditions, and the implementation rules can be further used as black box rules for security and encryption applications. Thus, intelligent devices manufactured using 4D printed shape-memory polymers can be applied in various fields such as in intelligent actuators and soft robots.

In recent years, 4D printing has gained increasing attention in the tissue engineering field since this advanced manufacturing platform can create stimulus-responsive structures, which can change their shapes, functions, and/or properties when appropriate external stimulus/stimuli is/are applied. A number of hydrogels with swellable/shrinkable abilities have been explored for 4D printing to fabricate different shape-morphing structures for tissue engineering. Among them, gelatin methacryloyl (GelMA) has been 4D printed, which can self-fold into microtubular structures. Currently, the self-folding ability of 4D printed GelMA hydrogels is mainly based on the different cross-linking degrees (which control and govern the swelling degrees) across the thickness of hydrogels. However, this strategy alone can only form self-folding GelMA tubes with diameters at the micrometer level and cannot create self-folding GelMA tubes with diameters at the millimeter level, which is mainly due to the insufficient internal force generated in 4D printed GelMA hydrogels when they are exposed to water. To overcome this limitation, this study has investigated a new strategy to fabricate self-folding GelMA tubes with large diameters at the millimeter level for tissue engineering applications. The new strategy introduced a cross-linking degree gradient across the GelMA plane in addition to its thickness by printing a second layer of strips on the first 4D printed GelMA film. In the aqueous environment, under the current fabrication condition, such bilayer GelMA hydrogels could self-fold into tubes of larger diameters up to 6mm. The in vitro release behavior of heparin incorporated into the 4D printed GelMA was also studied. It was shown that heparin release could be controlled by the GelMA concentration and heparin content in 4D printed GelMA. The 4D printed GelMA hydrogels with the improved self-folding ability and controlled release of a drug are promising for targeted tissue engineering applications.

Today, in the medical field, innovative technological advancements support healthcare systems and improve patients’ lives. 4D printing is one of the innovative technologies that creates notable innovations in the medical field. For the COVID-19 pandemic, this technology proves to be useful in the manufacturing of smart medical parts, which helps treat infected patients. As compared to 3D printing, 4D printing adds time as an additional element in the manufactured part. 4D printing uses smart materials with the same printing processes as being used in 3D printing technology, but here the part printed with smart materials change their shape with time or by the change of environmental temperature, which further creates innovation for patient treatments. 4D printing manufactures a given part, layer by layer, by taking input of a virtual (CAD) model and uses smart material. This paper studies the capability of smart materials and their advancements when used in 4D printing. We have diagrammatically presented the significant parts of 4D printing technology. This paper identifies 11 significant applications of 4D printing and then studies which one provides innovative solutions during the COVID-19 pandemic.

4D printing is a fast-developing technique which enables the transformation of shape, property, and function after a structure is manufactured. Here the ’fourth dimension’ refers to a time-dependent deformation, and thus 4D printing technique is closely related to the mechanical design strategies of materials and structures. This review concentrates on the recent progress of fundamental mechanical theories, analytical methods, and designing tools, for three categories of designing principles in 4D printing. The first type of 4D printing relies on active materials that respond to external stimuli. The second type includes a wide range of 4D-printed innovative structures, where the automatic actuation mainly comes from a combination of different deformation mechanisms. The third type of 4D printing focuses on mechanical designs related to the manufacturing process. The classification bridges the gaps between materials, microarchitectures, and large-scale structures, while some 4D printing strategies might involve more than one aforementioned design principle. This review provides reference and guidance for future 4D printings with customized deformation modes and multiple functionalities.

4D printing technology endows printed samples with self-driven performance that increasingly show strong application prospects. Polyurethane, as a typical shape memory polymer, is widely used in 4D printing. Current researches on 4D printed polyurethane materials are focused on investigating polyurethane composites or novel printing techniques to optimize the shape memory properties of the printed samples. In this study, the effects of pre-programmed 4D printing process parameters on the shape memory properties of polyurethane were systematically investigated. The higher printing speed, higher printing temperature, and lower fill rate result in faster response time of the biomimetic samples with thermal stimulation. Based on the programming of process parameters (e.g., printing temperature, printing speed and filling rate), the biomimetic flowers and hands were processed to achieve a controlled behavior of sequential deformation. It was successfully achieved that only one printing material could demonstrate the shape memory effects with a sequential response process. The adjustable sequential deformations further enlighten the application of 4D printing technology in specific engineering fields such as aerospace, biomedicine, robot and military engineering, where parts must undergo a sequence of deformations to serve practical requirements.

This study explores the control of the microstructure and properties of NiTi alloy through the adjustment of 4D printing process parameters. The results indicate that increasing the scanning speed from 450 mm/s to 750 mm/s leads to a change in the phase composition from near full martensitic to a two-phase structure of austenite and martensite. The increase in scanning speed can also reduce the evaporation of Ni element and lower the phase transformation temperature. $M_s$ temperature and $A_f$ temperature decrease from 93°C and 63°C at 450 mm/s to 37°C and 12°C at 750 mm/s, respectively. Moreover, the scanning speed can alter the texture characteristics and mechanical behavior of NiTi alloy. The hardness value and critical stress increase with the increase in scanning speed, while the strain distribution during tensile deformation becomes uneven at high scanning speed due to the unevenly distributed dual-phase microstructure. These findings provide insights for the optimization of 4D printing parameters to tailor the properties of NiTi alloy for various applications.

Liquid crystal elastomers (LCEs) have attracted much attention because of their large, reversible, and anisotropic deformation, fast response to various external stimuli and excellent mechanical properties. LCE ink was prepared by a catalyst-free aza-Michael addition chemistry. A multi-material four-dimensional (4D)-printed laminated LCE actuator including Polydimethylsiloxane (PDMS), conductive circuits and LCEs was fabricated by direct ink writing (DIW) technology. The influence of the input current and resistance values on the thermal effect of conductive circuits was studied. The functional relationship between the LCE actuator’s bending angle and printing parameters was obtained. The LCE actuator with a bending angle controllable at 0-410° was fabricated. This research is expected to bring about new possibilities in novel intelligent LCE devices with programmable stimuli-responsive properties and optimal actuation capacities.

4D printing of biocompatible shape memory polymer materials is of great significance in the biomedical field. The addition of PCL material into PLA matrix is capable of modulating the elastic modulus of the material while remaining a reduced degradation rate. However, the influence of 4D printing parameters and shape programming parameters on their shape memory properties had not been systematically investigated. Here, we examine the effects of 4D printing process parameters and shape programming parameters on the shape memory properties of PLA/PCL composites. We first explore the effects of key printing parameters on the print quality and shape memory properties of pure PLA materials, and select composites with a PCL mass fraction of 15% to fully characterize their thermodynamic properties. The effects of programming stress and programming temperature on the shape memory properties of the PCL15 composites are analyzed. Finally, the artificial neural network algorithm is employed to optimize its programming parameters, and brings 5.76% enhancement of shape recovery ratio compared with that before optimization. This work provides a systematic study of the shape memory properties of 4D printed PLA/PCL composites, paving the way for promoting their application in the biomedical industry.