"Leaf-Inspired Principle" KAIST Develops Autonomous Thermal Regulation Material

An artificial material that mimics the thermal management strategy of poplar leaves has been developed. Poplar trees have a survival strategy in which they roll their leaves to expose the underside and reflect sunlight during hot and dry conditions, while at night, they prevent cold damage by releasing latent heat through moisture condensed on the leaf surface. This is an adaptive approach that regulates heat in response to changes in day and night, as well as temperature and humidity. The artificial material was developed by imitating these principles. When applied to building exteriors, roofs, or temporary shelters, it is expected to enable autonomous thermal management without electricity.

KAIST announced on November 18 that the research team led by Professor Song Youngmin from the Department of Electrical Engineering, in collaboration with Professor Kim Daehyung's team at Seoul National University, has developed a "Latent-Radiative Thermostat (LRT)," a flexible hydrogel-based thermal regulator that mimics the natural thermal regulation mechanism of poplar trees.

(From left) Hyoeun Jung, Integrated MS-PhD Program, Department of Electrical Engineering, KAIST; Hyungrae Kim, PhD Program; Youngmin Song, Endowed Chair Professor; Sehee Jang, Postdoctoral Researcher; Dohyun Kim, Integrated MS-PhD Program; Hyunkyoo Kwak, Master's Program; (From top left) Daehyung Kim, Professor, Seoul National University; Seyun Heo, PhD, SAIT; Yunsu Shin, PhD, MIT. Provided by KAIST

The LRT was developed as a thermal control device capable of autonomously switching between cooling and heating modes. This technology enables the simultaneous control of latent heat-through evaporation and condensation of moisture-and radiative heat-through reflection and transmission of light-within a single device.

The core material is a structure in which lithium ions and hydroxypropyl cellulose (HPC) are combined within a PAAm hydrogel.

Lithium ions absorb and condense ambient moisture to regulate latent heat and maintain warmth, while HPC switches between transparent and opaque states depending on temperature changes, thereby controlling the reflection and absorption of sunlight and toggling between cooling and heating modes.

When the temperature rises, HPC molecules aggregate, making the hydrogel opaque and enhancing the natural cooling effect by reflecting sunlight. Through this mechanism, the LRT can switch among four distinct thermal regulation modes in response to changes in ambient temperature, humidity, and light intensity.

For example, during nighttime or cold environments below the dew point, the device maintains warmth by releasing heat through the absorption and condensation of atmospheric moisture. On cold days with weak sunlight, it allows sunlight to pass through, and the absorbed moisture captures near-infrared rays, providing a heating effect.

In hot and dry conditions, internal moisture evaporates, resulting in strong evaporative cooling. Under intense sunlight and high temperatures, the HPC becomes opaque, reflecting sunlight while evaporative cooling simultaneously lowers the temperature.

These functions enable the device to autonomously switch between cooling and heating modes in response to the surrounding environment, operating as a biomimetic thermal management system without electricity. The LRT thus helps maintain coolness in summer and warmth in winter.

Schematic of the operating concept of a hydrogel-based autonomous temperature regulator mimicking the thermal management strategy of poplar leaves. Provided by KAIST

The joint research team confirmed that by adjusting the concentrations of lithium ions and HPC, the thermal regulation properties can be finely tuned to suit various climate conditions. Additionally, the inclusion of TiO₂ nanoparticles enhanced the material's durability and mechanical strength.

In outdoor experiments, the LRT maintained temperatures up to 3.7 degrees Celsius lower in summer and up to 3.5 degrees Celsius higher in winter compared to conventional cooling materials. Notably, simulations conducted under seven climate zones (according to ASHRAE standards) demonstrated that the LRT could achieve annual energy savings of up to 153 MJ/m² compared to existing roof coatings.

This study represents an engineering realization of the advanced thermal management functions found in nature and is expected to serve as a next-generation thermal management platform for building exteriors, roofs, disaster relief shelters, and outdoor storage facilities, particularly in environments where electricity-based heating and cooling are challenging.

Professor Song defined the research as "an engineering recreation of nature's intelligent thermal regulation strategy," adding, "This technology implements a thermal management device that autonomously adapts to seasonal and climate changes, and it is expected to become an intelligent thermal management platform applicable to a wide range of environments in the future."

Meanwhile, Hyungrae Kim, a PhD student in the Department of Electrical Engineering at KAIST, participated as co-first author, and Professor Song Youngmin served as the corresponding author. The research results (paper) were published online on November 4 in the journal Advanced Materials, a leading publication in the field of materials science.

This content was produced with the assistance of AI translation services.