Bioinspired thermochromic transparent hydrogel wood with advanced optical regulation abilities and mechanical properties for windows

The huge heat loss/gain through windows is the cause of great energy consumption in buildings. In addition, the traditional fabrication method for glass causes many environmental problems. Recently, transparent wood has emerged as a promising alternative to traditional glass because of its high transmittance, strong mechanical properties, excellent thermal insulation ability and sustainability. In this study, inspired by jellyfish, a thermochromic transparent hydrogel wood that can smartly regulate solar irradiation is proposed as a smart window material by impregnating Poly(N-isopropylacrylamide)-polyacrylamide hydrogel into delignified wood. The novel thermochromic transparent hydrogel wood inherits the excellent thermochromic properties of PNIPAM and strong mechanical properties of wood, showing advanced optical regulation ability (i.e. T_lum = 82.7% and 39.8% at the cold and hot states & ΔT_sol = 38.1%), low transition temperature (i.e. T_c = 22.9 °C), mechanically robust (i.e. σ = 11.6 MPa along the axial direction) and low thermal conductivity (i.e. K = 0.37 W m⁻¹ K⁻¹ along the perpendicular direction of the wood growth). A field test conducted in October in Hong Kong shows that thermochromic transparent hydrogel wood can reduce the indoor air temperature by 4.3 °C. Furthermore, a computational simulation for an office building proves that 2.6–10.2% energy could be saved by thermochromic transparent hydrogel wood in four different climate-zone cities. Besides, thanks to the flexibility, thermochromic transparent hydrogel wood can be easily fitted on existing windows, demonstrating the great potential for use in energy-efficient buildings.