Bio-inspired micropatterned thermochromic hydrogel for rapid stealth and smart solar transmission

Currently, about 15% of the world’s annual electricity energy is used to offset the warming caused by solar heating and create a comfortable operating temperature environment. Thermochromic hydrogel has the function of automatically changing the spectral solar transmittance with the change of temperature, which can realize smart solar photothermal regulation, and has great application potential in the field of energy conservation. However, because traditional thermochromic hydrogels can only regulate the direct solar transmission properties through temperature (thermal) stimulation, they only exhibit visible-light stealth (VLS) ability at high temperatures, and the response time of VLS is up to 180 seconds. In such a slow response process, sensitive information that needs to be hidden by VLS can be compromised, so traditional thermochromic hydrogels cannot meet the rapid stealth needs of military (stealth camouflage) and civilian (privacy protection). In order to realize the dual-utilization of thermochromic hydrogels, it is urgent to realize the simultaneous regulation of rapid VLS and smart solar photothermal regulation of hydrogels.

 

To solve the above problems, in a new paper published in Light：Science & Applications, a team of scientists, led by Professor Fuqiang Wang from Harbin Institute of Technology, and co-workers have proposed a novel micropatterned thermochromic hydrogel (MTH) for concurrent smart solar transmission and rapid VLS at all-working temperatures. The proposed MTH simultaneously has two different optical regulation modes, namely the change of the optical properties of the hydrogel material itself and the regulation of the optical scattering behavior of the hydrogel surface, which can be achieved by temperature (thermal) stimulation and pressure (mechanical) stimulation, respectively. MTH employs PNIPAm hydrogel as a substrate to change its own refractive index through the reversible volume phase transformation of the hydrogel under temperature stimulation, thereby regulating the solar transmission property and realizing smart solar photothermal regulation (sunlight modulation capacity of 61%). In addition, a surface micropattern strategy is introduced, which enables MTH to switch arbitrarily between direct transmission and diffusion of sunlight under pressure stimulation, achieving a rapid VLS within 1 second, which has great advantage compared with 180 seconds in previous study. Furthermore, this newly designed MTH is prepared by a low-cost, mass-producible method, which has the potential for large-scale applications. The proposed MTH can be applied in smart windows, military facilities, smart plant factories and anti-counterfeiting.

 

The MTH is engineered to switch between three working states (Fig. 1C), allowing a single MTH unit to perform the concurrent regulation of smart solar transmission and rapid visible-light stealth on demand. In state 1, the pressure is applied, and the light can transmit directly through the MTH, so the MTH in state 1 exhibits high luminous. In state 2, the pressure is released, and the surface microstructure with random roughness of the MTH modulates and scatters light in various directions, rendering targets imperceptible. Therefore, the MTH in state 2 exhibits low luminous. In state 3, a volume phase transition of the MTH happens under thermal stimulation, resulting in high solar absorption and low reflection, so the MTH in state 3 can block sunlight under high temperature.

 

The MTH can regulate solar-spectrum transmission under thermal stimulation (20–40 °C), enabling a smart solar transmission (from 16% to 77% transmittance) and can realize rapid VLS at all working temperature (luminous modulation capability of 67%). MTH based smart windows maintain indoor air temperature 8 °C lower than Low-E windows at office hours (9:00–17:00) when strong cooling is required, resulting in an annual energy saving of over 200 MJ/m². In addition, the practical utility of the MTH was demonstrated for military equipment requiring rapid VLS. The MTH can also be utilized in smart plant factories and anti-counterfeiting.

 

The research is accomplished by Harbin Institute of Technology (HIT), Nanyang Technological University (NTU), and the Chinese University of Hong Kong (CUHK). HIT is the first communication unit of this paper. Liang Huaxu (post-doctoral fellow of NTU) and Zhang Xinping (PhD candidate of HIT) are the first authors. Professor Wang Fuqiang from HIT and Professor Long Yi from CUHK are the corresponding authors of this paper. Professor Shuai Yong from HIT is the co-author of the paper and leader of the research team at HIT.

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