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Four-dimensional (4D) printing, which enables 3D printed structures to alter shapes over time, is attracting increasing attention because of its exciting potential in various applications. Among all the 4D printing materials, shape memory polymers (SMPs) have a higher stiffness and faster response rate and therefore are considered as one of the most promising 4D printing materials. However, the current studies of SMP-based 4D printing mainly focused on the deformation behavior and structural design of 4D printed structures. An additional function such as color change is desired for 4D printed structure, which would be potentially beneficial to the applications such as anti-counterfeiting, encryption, and bioinspired camouflage. In this paper, we report an ultraviolet (UV)-curable and thermochromic (UVT) SMP system that enables color-changeable 4D printing. The UVT SMP system is acrylate-based, thus highly UV-curable and compatible with PμSL-based high-resolution 3D printing technique. Thermochromism is imparted by adding the thermochromic microcapsules to the UVT SMP system, which allows the printed structures to reversibly change colors upon heating and cooling. To demonstrate its extraordinary thermochromic and mechanical performance, we use UVT SMP to print QR codes and multilevel anti-counterfeiting patterns which can hide the visible information at room temperature and visualize the information by encrypting, decrypting, and encrypting again steps with the shape–color recovery process. The development of UVT SMP will significantly advance current applications of SMP-based 4D printing, especially for anti-counterfeiting and safe data recording.

Smart windows with tunable optical properties in response to external environments are being developed to reduce energy consumption of buildings. In the present study, we introduce a new type of 3D printed hydrogel with amazing flexibility and stretchability (as large as 1500%), as well as tunable optical performance controlled by surrounding temperatures. The hydrogel on a PDMS substrate shows transparent-opaque transition with high solar modulation (ΔTsol ) up to 79.332% around its lower critical solution temperature (LCST) while maintaining a high luminous transmittance (Tlum ) of 85.847% at room temperature. In addition, selective transparent-opaque transition above LCST can be achieved by patterned hydrogels which are precisely fabricated via projection micro-stereolithography (PμSL) based 3D printing technique. Our hydrogel promises great potential applications for next generation of soft smart windows.

Solar water evaporation, which can be utilized for seawater desalination and wastewater treatment, promises freshwater security with renewable energy. Herein, a bionic solar evaporator consisting of a hydrogel solar absorber and bionic microchannels fabricated by the projection microstereolithography based 3D printing technique is proposed. Such a bioinspired solar evaporator achieves a fast water self‐transport speed and a high solar water evaporation rate of 4.12 kg·m−2·h−1 under 1 sun irradiation (1 kW·m−2), and the solar thermal efficiency can be as high as 92.1%. However, both the evaporation rate and solar thermal efficiency are greatly affected by properties of the hydrogel, geometric parameters of microchannels, and intensity of the light source. Most significantly, the bionic solar evaporator is applied for wastewater treatment including the purification of organic dyes, acid and alkali, as well as metal ions. The purification rate can be as high as 99.999%, demonstrating amazing performance for clean water production. The bioinspired water evaporator promises applications for solar energy harvesting, photothermal conversion, as well as environmental engineering, such as solar vapor generation, thermophotovoltaic cell, sewage disposal, and desalination of seawater.

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