Hierarchical textiles with optical design and fiber nanomanufacturing: breakthrough innovation in personal thermal management

As global warming and energy issues intensify, there is a growing demand for low-energy personal thermal management (PTM) approaches. In contrast to air conditioners, which consume high amounts of energy and are not controllable outdoors, high-performance textiles for personal thermal comfort demonstrate increasing potential with their portability and low cost^[1]. Among the design strategies for PTM textiles (such as phase change materials, thermally conductive materials, thermal insulators, sweat evaporation and radiative cooling)^[2-5], radiative cooling has received widespread attention recently due to its great heat transfer efficiency, ease of regulation and environmental friendliness.

By means of material selection and optical design, radiative cooling textiles are capable of achieving a sub-ambient temperature decrease of more than 6 °C during the daytime by the conduction of thermal radiation into outer space through an atmospheric transmission window (ATW, 8-13 µm) while reflecting sunlight (0.3-2.5 µm)^[6-8]. However, when studying daytime radiative cooling materials, researchers tend to ignore the heat gains generated by the heated ground and urban buildings, which has a significant impact on the cooling efficiency of textiles in urban heat island areas. Additionally, radiative cooling textiles typically have broadband emissivity in the mid-infrared (MIR) region and are described by a horizontal geometry. From the perspective of practical application, about 97% of the textiles on the human body are used vertically on standing^[9], indicating that large amounts of radiation from the heated ground or urban buildings will be absorbed by the textiles, particularly in urban areas.

To address these problems, the challenge of achieving desirable cooling performance lies in the precise modulation of the spectral selectivity of textiles; that is, emission only dominates in the ATW while suppressing all other parasitic heat from the surroundings^[10]. Recently, Wu et al. exhibited a hierarchical textile with optical design targeting urban heat islands^[11]. Through multilayer structural design and spectral engineering, the textiles exhibited precise spectral selectivity and excellent optical performance in the MIR region.

As shown in Figure 1, a hierarchical textile [spectrally selective hierarchical fabric (SSHF)], comprising an emission layer composed of wool fabric, a reflection layer built of silver nanowires (AgNWs) and a polymethylpentene (PMP) nano-micro hybrid fiber layer, was designed with a solar reflectance of 0.97 and an average emissivity of 0.85 in ATW. With its selective and asymmetric design principles^[12], the SSHF efficiently radiated heat into outer space while significantly curbing radiative heat absorption and promoting the loss of body heat from humans.

Figure 1. (A and B) Schematic representation of textiles in the (A) open environments and (B) urban environments; (C) The theoretical cooling power and emissivity selective ratio for PMP, common polymer films, and textile materials; (D) The radiated power from a perfect broadband or spectrum-selective textile emitter at T_skin of 34 °C and hemispherical irradiances from the terrestrial features at T_terr of 50 °C; (E) Structure of the hierarchical SSHF; (F) Radiative heat transfer networks of SSHF in open scenario; (G) Schematic representation of the effect of wool fabrics on the absorption of thermal radiation from human skin. From Ref.^[11]. Reprinted with permission from AAAS. PMP: Polymethylpentene; SSHF: spectrally selective hierarchical fabric.

Owing to its low MIR reflectivity and low MIR transmittance, the AgNW layer effectively shields the environmental thermal radiation within the MIR spectrum from reaching the wool fabric. Therefore, it can significantly curtail all other parasitic heat from the surroundings^[13]. Simultaneously, the AgNW layer leverages its high thermal conductivity to facilitate the conduction of heat from the wool fabric to the surface, thereby enhancing the overall radiative cooling effect. Moreover, the mechanical strength and durability of the textiles were enhanced by embedding AgNWs within a layer of poly(ethylene-ran-butylene)-block-polystyrene (SEBS). Additionally, the breathability of SSHF was enhanced by its porous structure. Given that both the PMP fabric and the AgNW layer exhibit isotropic properties, the SSHF demonstrated superior mechanical performance during tensile tests.

The researchers tested the outdoor cooling performance of SSHF via thermal measurement under different conditions. The temperature of SSHF was 12.6 °C lower than the environment during the nighttime and 6.2 °C lower under the solar irradiance of 1,010 W·m^-2. Even in conditions of high relative humidity (RH), the textile still achieved a sub-ambient temperature reduction of approximately 2.5 °C during the daytime. Furthermore, the cooling effects were assessed using wallpaper to simulate an urban setting, with the temperatures recorded for cotton, rubber, silk, and SSHF being 44.1, 44.6, 42.5, and 40.9 °C, respectively. Beyond its exceptional radiative cooling capabilities, the novel textile also excelled in terms of wearability, featuring durability, washability, breathability, and robust resistance to mechanical stress and anti-UV aging properties.

In summary, the study by Wu et al. provides a strategic approach to textile design, featuring an upper layer that emits heat selectively through the ATW, an AgNW layer for repelling undesired thermal radiation, and a wool base layer that facilitates heat transfer from the skin to the intermediate layer^[11]. This design yields a textile that offers passive cooling even in the presence of urban heat islands, while also exhibiting excellent durability, mechanical strength, and washability.

Building upon the existing progress in PTMs [Table 1], it is evident that further improvements in wearability, breathability, flexibility, and meticulous optical engineering are essential to address the complexities of the microclimates between the human body and textiles^[14]. To advance this field, several comprehensive strategies for enhancing radiative cooling textiles have been proposed and are discussed in detail:

(1) Great optical properties are fundamental for radiative cooling textile applications; thus, developing materials with high solar spectrum reflectivity and high ATW emissivity is vital.

(2) For composite textiles, the cooling mechanism of the textiles and the interaction of the different materials need to be further investigated explicitly.

(3) The creation of textiles with both color and excellent cooling effects is a topic that needs further exploration.

(4) Unidirectional cooling methods may result in excessively low temperatures, compromising human comfort in cold environments; thus, it is essential to explore radiant heat transfer textiles with dual cooling and heating modulation capabilities to address personal radiant heat management in all-weather conditions.

(5) The application of radiative cooling materials to vertical surfaces is a recent research hotspot. Notably, the Ag layer, known for its robust reflection in the visible and near-infrared range, demonstrates an undeniable potential that cannot be overlooked in suppressing non-ATW parasitic heat^[15].

(6) The application of wearable electronics via radiative coolers is an important direction in the development of radiative cooling textiles^[16].

(7) Further improvements in wearability, breathability, and flexibility are essential; methods such as constructing Janus structures to orient humidity, incorporating PCM for temperature control, and applying abrasion-resistant coatings can be used to enhance their wearability^[14,17,18].