A joint research team has developed a technology that will allow windows to change colors according to the amount of moisture, without the need for electricity. The researchers developed a variable color filter using a metal-hydrogel-metal resonator structure using chitosan-based hydrogel, and combined it with solar cells to make a self-powering humidity sensor. Humidity sensor combining variable filter and solar cells. Courtesy of Junsuk Rho, POSTECH. The team members are professor Junsuk Rho of the mechanical and chemical engineering departments at POSTECH, Jaehyuck Jang and Aizhan Ismukhanova of the chemical engineering department at POSTECH, and professor Inkyu Park of KAIST’s department of mechanical engineering. The research team found that when the chitosan hydrogel is made into the metal-hydrogel-metal structure, the resonance wavelength of light transmitted changes in real time depending on the humidity of the environment. This is because the chitosan hydrogel repeats expansion and contraction as the humidity changes around it. Using this mechanism, the team developed a humidity sensor that can convert light’s energy into electricity by combining a solar battery with a water variable wavelength filter made of metal-hydrogel-metal-structured metamaterial that changes resonance wavelength depending on the external humidity. The design principle is to overlap the filter’s resonance wavelength with the wavelength where the absorption of the solar cells changes rapidly. This filter is designed to change the amount of light absorption of solar cells depending on the amount of moisture, and to lead to electric changes that ultimately detect the surrounding humidity. Unlike conventional optical humidity sensors, this newly developed one works regardless of the type of light. Also, not only does it function without external power, but it can also predict humidity according to the filter’s color. Rho, who led the research, said, “This technology is a sensing technology that can be used in place like nuclear power reactors where people and electricity cannot reach.” He added, “It will create even greater synergy if combined with IoT technology such as humidity sensors that activate or smart windows that change colors according to the level of external humidity.”