Biomimetic reconstruction of butterfly wing scale nanostructures for radiative cooling and structural coloration

Bioinspired Low-Angle-Dependent Photonic Crystal Elastomer for Highly Sensitive Visual Strain Sensor

Photonic crystals (PCs) possess unique photonic band gap properties that can be used in the field of sensors and smart displays if modulated on the micronano structure. Both nonclose-packed (NCP) structure and high refractive index (RI) contrast of PC play important roles in response sensitivity during stretching. Herein, we constructed an NCP-structured PC strain sensor with high RI by a novel coating-etching strategy. Stretch-induced changes in structural color correspond to the strength of the force, enabling the detection of the strength of the acting force by the naked eye. The flexible 3D cross-linked network constructed by poly(ethylene glycol) phenyl ether acrylate and pentaerythritol tetrakis(3-mercaptopropionate) endows the sensor with excellent elasticity and robustness. The designed PC strain sensor achieves high mechanochromic sensitivity (∼8.3 nm/%, 0.02 to 4.21 MPa) and a substantial reflection peak shift (Δλ = 249 nm). More importantly, the high RI contrast (Δn = 0.43) between CdS and polymers imparts isotropic optical properties, ensuring a broad viewing angle while avoiding misleading signals. The research provides a novel fabrication strategy to construct sensitive PC strain sensors, expanding the prospective applicability to human movement monitoring and secure message encryption.

Residual Stress Mitigation in Perovskite Solar Cells via Butterfly-Inspired Hierarchical PbI2 Scaffold

Residual stress in metal halide perovskite films intimately affects the photovoltaic figure of merit and longevity of perovskite solar cells. A delicate management of the crystallization kinetics is critical to the preparation of high-quality perovskite films. Only very limited methods, however, are available to regulate the residual stress of a perovskite film in a controllable manner, particularly for a perovskite film fabricated by a two-step method. Here, we demonstrate the construction of a hierarchical PbI2 scaffold inspired by Archaeoprepona demophon butterfly by combining an interlayer guided growth of porous structure and nanoimprinting. The hierarchically structured PbI2 that emulates the physical structure of the butterfly wing scale permits unimpeded permeation of organic amine salts and sufficient space for volume expansion during the crystallization process, accompanied by preferential perovskite growth of a defectless (001) crystal plane. The optimized perovskite film outperforms the control with reduced residual stress and defect density. Consequently, perovskite solar cells with a respectable power conversion efficiency reaching 23.4% (certified 23%) and an impressive open-circuit voltage of 1.184 V can be achieved. The target device can maintain 80% of initial efficiency after maximum power point tracking under illumination for 700 h. This work expands the range of engineering toward PbI2 by exploring a simultaneously tailored morphology and crystallinity and highlights the significance of a hierarchical PbI2 scaffold as an alternative choice to mitigate residual stress in a two-step processed perovskite active layer and boost the longevity of perovskite solar cells.

Bioinspired Polymer Films with Surface Ordered Pyramid Arrays and 3D Hierarchical Pores for Enhanced Passive Radiative Cooling

Passive radiative cooling (PRC) has been acknowledged to be an environmentally friendly cooling technique, and especially artificial photonic materials with manipulating light-matter interaction ability are more favorable for PRC. However, scalable production of radiative cooling materials with advanced biologically inspired structures, fascinating properties, and high throughput is still challenging. Herein, we reported a bioinspired design combining surface ordered pyramid arrays and internal three-dimensional hierarchical pores for highly efficient PRC based on mimicking natural photonic structures of the white beetle Cyphochilus' wings. The biological photonic film consisting of surface ordered pyramid arrays with a bottom side length of 4 μm together with amounts of internal nano- and micropores was fabricated by using scalable phase separation and a quick hot-pressing process. Optimization of pore structures and surface-enhanced photonic arrays enables the bioinspired film to possess an average solar reflectance of ∼98% and a high infrared emissivity of ∼96%. A temperature drop of ∼8.8 °C below the ambient temperature is recorded in the daytime. Besides the notable PRC capability, the bioinspired film exhibits excellent flexibility, strong mechanical strength, and hydrophobicity; therefore, it can be applied in many complex outdoor scenarios. This work provides a highly efficient and mold replication-like route to develop highly efficient passive cooling devices.

Magnetic field-responsive graphene oxide photonic liquids

Modifying the environment around particles (e.g., introducing a secondary phase or external field) can affect the way they interact and assemble, thereby giving control over the physical properties of a dynamic system. Here, graphene oxide (GO) photonic liquids that respond to a magnetic field are demonstrated for the first time. Magnetic nanoparticles are used to provide a continuous magnetizable liquid environment around the GO liquid crystalline domains. In response to a magnetic field, the alignment of magnetic nanoparticles, coupled with the diamagnetic property of GO nanosheets, drives the reorientation and alignment of the nanosheets, enabling switchable photonic properties using a permanent magnet. This phenomenon is anticipated to be extendable to other relevant photonic systems of shape-anisotropic nanoparticles and may open up opportunities for developing GO-based optical materials and devices.

Strain-Insensitive Outdoor Wearable Electronics by Thermally Robust Nanofibrous Radiative Cooler

Stable outdoor wearable electronics are gaining attention due to challenges in sustaining consistent device performance outdoors, where sunlight exposure and user movement can disrupt operations. Currently, researchers have focused on integrating radiative coolers into wearable devices for outdoor thermal management. However, these approaches often rely on heat-vulnerable thermoplastic polymers for radiative coolers and strain-susceptible conductors that are unsuitable for wearable electronics. Here, we introduce mechanically, electrically, and thermally stable wearable electronics even when they are stretched under sunlight to address these challenges. This achievement is realized by integrating a polydimethylsiloxane nanofibrous cooler and liquid metal conductors for a fully stable wearable device. The thermally robust architecture of nanofibers, based on their inherent properties as thermoset polymers, exhibits excellent cooling performance through high solar reflection and thermal emission. Additionally, laser-patterned conductors possess ideal properties for wearable electronics, including strain-insensitivity, nonsmearing behavior, and negligible contact resistance. As proof, we developed wearable electronics integrated with thermally and electromechanically stable components that accurately detect physiological signals in harsh environments, including light exposure, while stretched up to 30%. This work highlights the potential for the development of everyday wearable electronics capable of reliable operation under challenging external conditions, including user-activity-induced stress and sunlight exposure.

Controllable-morphology polymer blend photonic metafoam for radiative cooling

Incorporating radiative cooling photonic structures into the cooling systems of buildings presents a novel strategy to mitigate global warming and boost global carbon neutrality. Photonic structures with excellent solar reflection and thermal emission can be obtained by a rational combination of different materials. The current preparation strategies of radiative cooling materials are dominated by doping inorganic micro-nano particles into polymers, which usually possess insufficient solar reflectance. Here, a porous polymer metafoam was prepared with polycarbonate (PC) and polydimethylsiloxane (PDMS) using a simple thermally induced phase separation method. The metafoam exhibits strong solar reflectivity (97%), superior thermal emissivity (91%), and low thermal conductivity (46 mW m-1 K-1) due to the controllable morphology of the randomly dispersed light-scattering air voids. Cooling tests demonstrate that the metafoam could reduce the average temperature by 5.2 °C and 10.2 °C during the daytime and nighttime, respectively. In addition, the simulation of a cooling energy system of buildings indicates that the metafoam can save 3.2-26.7 MJ m-2 per year in different cities, which is an energy-saving percentage of 14.7-41%. The excellent comprehensive performances, including the passive cooling property, thermal insulation and self-cleaning of the metafoam makes it appropriate for practical outdoor applications, exhibiting its great potential as an energy-saving building cooling material.

Functional Materials and Innovative Strategies for Wearable Thermal Management Applications

Highlights

This article systematically reviews the thermal management wearables with a specific emphasis on materials and strategies to regulate the human body temperature. Thermal management wearables are subdivided into the active and passive thermal managing methods. The strength and weakness of each thermal regulatory wearables are discussed in details from the view point of practical usage in real-life.

Abstract

Thermal management is essential in our body as it affects various bodily functions, ranging from thermal discomfort to serious organ failures, as an example of the worst-case scenario. There have been extensive studies about wearable materials and devices that augment thermoregulatory functionalities in our body, employing diverse materials and systematic approaches to attaining thermal homeostasis. This paper reviews the recent progress of functional materials and devices that contribute to thermoregulatory wearables, particularly emphasizing the strategic methodology to regulate body temperature. There exist several methods to promote personal thermal management in a wearable form. For instance, we can impede heat transfer using a thermally insulating material with extremely low thermal conductivity or directly cool and heat the skin surface. Thus, we classify many studies into two branches, passive and active thermal management modes, which are further subdivided into specific strategies. Apart from discussing the strategies and their mechanisms, we also identify the weaknesses of each strategy and scrutinize its potential direction that studies should follow to make substantial contributions to future thermal regulatory wearable industries.

Photonic structures in radiative cooling

Radiative cooling is a passive cooling technology without any energy consumption, compared to conventional cooling technologies that require power sources and dump waste heat into the surroundings. For decades, many radiative cooling studies have been introduced but its applications are mostly restricted to nighttime use only. Recently, the emergence of photonic technologies to achieves daytime radiative cooling overcome the performance limitations. For example, broadband and selective emissions in mid-IR and high reflectance in the solar spectral range have already been demonstrated. This review article discusses the fundamentals of thermodynamic heat transfer that motivates radiative cooling. Several photonic structures such as multilayer, periodical, random; derived from nature, and associated design procedures were thoroughly discussed. Photonic integration with new functionality significantly enhances the efficiency of radiative cooling technologies such as colored, transparent, and switchable radiative cooling applications has been developed. The commercial applications such as reducing cooling loads in vehicles, increasing the power generation of solar cells, generating electricity, saving water, and personal thermal regulation are also summarized. Lastly, perspectives on radiative cooling and emerging issues with potential solution strategies are discussed.

Design, Manufacturing and Functions of Pore-Structured Materials: From Biomimetics to Artificial

Porous structures with light weight and high mechanical performance exist widely in the tissues of animals and plants. Biomimetic materials with those porous structures have been well-developed, and their highly specific surfaces can be further used in functional integration. However, most porous structures in those tissues can hardly be entirely duplicated, and their complex structure-performance relationship may still be not fully understood. The key challenges in promoting the applications of biomimetic porous materials are to figure out the essential factors in hierarchical porous structures and to develop matched preparation methods to control those factors precisely. Hence, this article reviews the existing methods to prepare biomimetic porous structures. Then, the well-proved effects of micropores, mesopores, and macropores on their various properties are introduced, including mechanical, electric, magnetic, thermotics, acoustic, and chemical properties. The advantages and disadvantages of hierarchical porous structures and their preparation methods are deeply evaluated. Focusing on those disadvantages and aiming to improve the performance and functions, we summarize several modification strategies and discuss the possibility of replacing biomimetic porous structures with meta-structures.

Pruney Finger-Inspired Switchable Surface with Water-Actuated Dynamic Textures

Switchable surfaces play an important role in the development of functional materials. However, the construction of dynamic surface textures remains challenging due to the complicated structural design and surface patterning. Herein, a pruney finger-inspired switchable surface (PFISS) is developed by constructing water-sensitive surface textures on a polydimethylsiloxane substrate by taking advantage of the hygroscopicity of the inorganic salt filler and the 3D printing technology. Like human fingertips, the PFISS shows high water sensitivity with obvious surface variation in wet and dry states, which is actuated by water absorption-desorption of the hydrotropic inorganic salt filler. Besides, when the fluorescent dye is optionally added into the matrix of the surface texture, water-responsive fluorescent emitting is observed, providing a feasible surface-tracing strategy. The PFISS shows effective regulation of the surface friction and performs a good antislip effect. The reported synthetic strategy for the PFISS offers a facile way for building a wide range of switchable surfaces.