Bioinspired Stretchable Polymers for Dynamic Optical and Thermal Regulation

Elaborated optical and thermal modulatory systems are of great importance to the survival and evolution of organisms in nature. Inspired by these natural intelligent systems, researchers have made great efforts for developing stretchable polymers and exploring their applications in fields of communication, dynamic camouflage, thermal management, and others. Herein, an up‐to‐date account of the advancements in bioinspired stretchable polymers for dynamic optical and thermal regulation is provided. First, stretchable polymers for dynamic structural colors are presented, including cholesteric liquid crystal elastomers, photonic crystal elastomers, and emerging photonic polymers. Then stretchable polymers for dynamic infrared emissivity are introduced, which are achieved by stretch‐induced wrinkled‐flat surface or stretch‐induced cracked surface. Third, stretchable polymers for dynamic thermal management are discussed, focusing on tunable solar transmittance and dynamic radiative cooling. Moreover, the perspectives on the opportunities and challenges for future research directions of bioinspired stretchable polymers are presented at the end.


Introduction
Light and heat are the essential elements for the living things on the earth.[3][4] For example, chameleons can camouflage themselves by changing skin colors to fit the background colors (Figure 1a). [5]The microstructures of an adult male chameleon skin are found to be guanine crystals packed into highly ordered nanostructures.Upon muscle expansion, the skin color of chameleon changes from green to yellow owing to the increase in the spacing of the guanine crystals.Similarly, Cephalopods show remarkable camouflaging ability based on the rapid color change across the entire visible and infrared (IR) region (Figure 1b). [6]Such remarkable camouflage capability is believed to originate from the contract and expansion of the chromatophore pigment cells by rapid neural and muscle signals.Saharan silver ants can keep cool in hot desert environment by high reflection of sunlight and enhanced thermal radiation in the mid-IR region. [7]t is found that the silver ant's head is covered by dense hairs with delicate triangular cross-sections, which can effectively reflect solar radiation and protect the ants from getting overheated.All of these ingenious photonic structures and the underlying light-matter interaction mechanisms have proved as attractive and feasible avenues to develop various artificial photonic materials for advanced optical and thermal management.
Previous studies on optical and thermal regulation can only work under specific surroundings and temperature conditions due to their fixed and monofunctional properties.These materials are unable to cope with ever-changing environment because the optical camouflage and thermal management are extremely sensitive to spatial and temporal variations as well as diurnal fluctuation and seasonal climate.Therefore, dynamic optical and thermal regulation are highly demanded with the ability to switch in response to a variety of environmental stimuli and achieve tunable functions.Materials, such as metals, [8][9][10] oxides, [11,12] graphene, [13][14][15] and polymers, [16][17][18][19][20][21] have been studied for dynamic optical and thermal regulation.Among these materials, stretchable polymers such as poly(dimethylsiloxane) (PDMS), dielectric elastomers, liquid crystal elastomers, rubbers, hydrogels, celluloses, and so on, are particularly fascinating because their optical properties can be regulated by simple mechanical stress, which is an ideal feature to mimic biological skins and bionic functions.In addition, it is easy to combine stretchable polymers with other functional nanomaterials for desired properties.The mechanisms of dynamic optical and thermal regulation of stretchable polymers are controlling the self-organized nanostructures on the materials' surface or embedded in the stretchable polymers.
4][25][26][27] Y. Yang, Y. Liu, Y. Chen, L. Wang, W. Feng School of Materials Science and Engineering Tianjin University Tianjin 300350, P. R.China E-mail: lwang17@tju.edu.cn;weifeng@tju.edu.cn To the best of our knowledge, there is a lack of a specific classification of the bioinspired stretchable polymers whose optical and thermal properties can be dynamically controlled.Herein, we provide a review on the state-of-the-art of recent advancements in bioinspired stretchable polymers for dynamic optical and thermal regulation (Figure 1c).This review mainly focuses on the following three sections.The first section introduces stretchable polymers for dynamic structural colors, including cholesteric liquid crystal elastomers (CLCEs), photonic crystal elastomers, and emerging photonic polymers.The second section introduces stretchable polymers for dynamic IR emissivity, which is achieved by stretch-induced wrinkled-flat surface or stretchinduced cracked surface.The third section introduces stretchable polymers for dynamic thermal management, focusing on tunable solar transmittance and dynamic radiative cooling.Finally, this review presents a brief conclusion and discusses both the opportunities and challenges for the future development of bioinspired stretchable polymers.

Bioinspired Stretchable Polymers for Dynamic Structural Colors
Structural colors are widespread in nature and observed in chameleons, [5] opals, [28] plant tissues, [29] beetles, [30] butterflies, [31] and similar iridescent materials.Accompanied by the growing understanding of the morphology and mechanism of natural structural-colored materials, their artificial counterparts have been developed with improved functions and superior performance.Especially, the application of elastic polymers is favorable to realize wide-range and reversible mechanochromism on structure-colored materials. [32]42] Stretchable polymers for dynamic structural colors, including CLCEs, photonic crystal elastomers, and emerging photonic polymers, are overviewed systematically as follows.

Cholesteric Liquid Crystal Elastomers
CLCEs are soft photonic materials that couple the structural colors from the chiral helix nanostructures to the elasticity of elastomers.The helical nanostructure can be deformed by applying uniaxial strain in the plane perpendicular to the helix axis, resulting in the unwinding of the helix along the direction of uniaxial stretching.Therefore, the reflection colors of CLCEs can be easily tuned over a broad range by mechanical stretching.The uniformly vertical orientation of the CLCE helix was first reported by the Finkelmann group with the "anisotropic deswelling" method in 2001. [43]A hydrosilylation reaction was involved in their solution during the first step, followed by solvent evaporation during the second step.In 2020, Zhang et al. made CLCEs in a liquid crystal cell with alignment layers based on a two-step crosslinking, including the first-stage thiol-acrylate reaction and the second-stage photoinitiated polymerization reaction. [33]o prepare CLCEs easily and quickly, Kizhakidathazhath et al. proposed a facile anisotropic deswelling method for preparing Guanine crystals in the chameleon's skin are packed into face-centered cubic lattices at the relaxed state and expanded at the excited state.Reproduced with permission. [5]Copyright 2015, Springer Nature.b) A squid changes its skin color through the mechanical action of muscle cells in front of a rocky background.Reproduced with permission. [6]Copyright 2018, The American Association for the Advancement of Science.c) Schematic of bioinspired stretchable polymers for dynamic structural colors, dynamic infrared emissivity, and dynamic thermal management.
large-area CLCEs with uniform structural color. [34]In 2022, Sol et al. prepared CLCEs by direct ink writing method. [35]The writing speed and direction were programmed to obtain a slanted photonic axis, which exhibited atypical iridescent and selective polarization colors.These fabrication methods open windows for large-scale application of CLCEs in advanced sensing, display, and beyond.
Based on the facile anisotropic deswelling method, our research group have integrated dynamic covalent boronic ester bonds into the CLCE polymer network for fabricating CLCEs with mechanochromic, shape-programmable, and self-healable properties (Figure 2a). [44]The reflection color of CLCEs could be dynamically and reversibly tuned across the entire visible range by uniaxial strain.In addition, the CLCEs exhibited shape-reprogrammable and self-healing capabilities owing to the introduction of dynamic boronic ester bonds.After this work, we further developed bioinspired color-changing smart materials based on ionic conductive CLCEs (iCLCEs) for fabricating iCLCEs with excellent mechanochromism, ionic conductivity, which can simultaneously output optical and electrical signals under mechanical stretching. [45]LCEs have strong mechanochromic responses that offer attractive opportunities for mechanically responsive textiles.Geng et al. developed a simple method for making long CLCE fibers that exhibit excellent mechanochromic response across the entire visible spectrum, upon elongational strain up to 200% (Figure 2b). [46,47]The CLCE fibers were sewn into clothes, which could withstand regular washing and repeated stretching.This fabrication method and the resulting CLCE fibers are appealing for applications in wearable technology and other fields benefiting from strong deformation detection or autonomous strain sensing.
The ability to control the separation of biomimetic multicolors is highly desired for dynamic camouflage applications.Kim et al. geometrically programmed the layout and the size of the encapsulated air channels to realize color shifting from ultraviolet (UV) to near-IR (NIR) wavelengths by taking advantage of the elasticity anisotropy and large Poisson's ratio (>0.5) of CLCEs  [44] Copyright 2022, Wiley-VCH.b) Robust CLCE fibers for mechanochromic textiles.Reproduced with permission. [46]Copyright 2022, Springer Nature.
(Figure 3a). [36]Each color "pixel" could be individually controlled via pneumatic actuation to match with surroundings.Nam et al. demonstrated stretchable CLCEs with simultaneous multicolor control by engineering the multiple elastic modulus of the CLCEs (Figure 3b). [37]Stretch-induced multicolor separation was observed using hybrid CLCEs on dielectric elastomers.Invisible camouflage based on CLCE multicolor wavelength control was demonstrated for invisible chameleon-like photonic skin switching.This multicolor control of stretchable CLCE systems paves the way for various electro-optic applications, such as adaptive optics, cryptography, and advanced display applications.
50][51] Schlafmann et al. studied the mechanical properties and optical reconfiguration of blue-phase liquid crystal elastomers. [52]Boott et al. reported stretchable chiral nematic cellulose nanocrystal elastomer composites that exhibit reversible colors under mechanical stress. [53]These chiral photonic liquid crystal elastomers are expected to enable functional implementations in photonics, sensing, and energy applications.

Photonic Crystal Elastomers
Photonic crystals are advanced optical materials comprising periodic arrangements of refractive indexes on sub-microscale and have unique light manipulation properties.Thanks to the periodic nanostructures of photonic crystals within visible wavelength, the reflected light through photonic crystals can be easily tuned.Mechanochromism of photonic crystals are mainly achieved by changing the crystalline interplanar spacing, which can be easily altered via the deformation of elastic nanospheres or the tuning of the intersphere spacing. [54,55]Inspired by opals, a Reproduced with permission. [36]Copyright 2022, Springer Nature.b) Electrically stretchable multicolor separation of modulus-engineered chiral photonic elastomers.Reproduced with permission. [37]Copyright 2023, Wiley-VCH.
kind of stone with photonic crystal nanostructures, various photonic crystal elastomers with mechanochromic response have been prepared based on the opal and inverse opal architectures.Many existing works fill flexible matrix into the blanks of opal for fabricating opal-embedded elastomer.The opal component inside the elastomers can be efficiently deformed during mechanochromism process.Inverse opals are obtained based on opalembedded elastomers.The embedded opal is removed from opal-embedded elastomers to produce a 3D-ordered polymer scaffold.The inverse opal elastomers demonstrate high elasticity and flexibility due to the removal of rigid opal templates, which are highly beneficial to mechanochromism. [56,57]sing the opal-embedded elastomer, Kim et al. prepared a mechanochromic opal organogel which is able to generate tunable reflective colors and acoustic vibrations upon applying alternating current (Figure 4a). [58]Preassembled photonic crystals were filled with ethylene glycol dimethacrylate, acrylamide, and 2-hydroxyethyl methacrylate, and then photopolymerized to prepare the opal-embedded organogel.The strain applied on the opal-embedded organogel increased upon applying the voltage, leading to the blue-shifted structural color.In addition, the vibration of the opal-embedded organogel produced sound waves upon applying alternating current.Such electroactive photonic crystal elastomers open the window for next-generation multimedia devices and smart displays.
[61] Wang et al. prepared a stretchable, conductive, and adhesive photonic crystal elastomer for visually stretchable electronics. [62]The photonic crystal elastomer is fabricated by adding polydopamine filler and conductive carbon nanotubes (CNTs) into an inverse opal scaffold.The photonic crystal elastomers showed brilliant structural colors and stable stretchability.The catechol groups on polydopamine impart elastomers with self-healing capability and high tissue adhesiveness.The introduction of CNTs endowed the elastomers with high electrical conductivity.The resultant photonic crystal elastomer can function as dual-signal sensors for visually and electrically monitoring human-motion signals.
Figure 4. Photonic crystal elastomers for dynamic structural colors.a) Opal-embedded organogel capable to generate tunable reflective colors and acoustic vibrations upon applying alternating current.Reproduced with permission. [58]Copyright 2019, Wiley-VCH.b) Self-driven inverse opal hydrogels implanted with the cardiomyocyte tissue for heartbeat monitoring.Reproduced with permission. [63]Copyright 2018, The American Association for the Advancement of Science.c) Bioinspired photonic crystal elastomer kirigami for color switching between blue and red reversibly under mechanical strain.Reproduced with permission. [38]Copyright 2021, Wiley-VCH.
The photonic crystal hydrogels with mechanochromic response can be used for monitoring the heartbeat of biological tissue and testing drug effects on organs and tissue.Fu et al. have made remarkable efforts to combine living tissue with photonic crystal hydrogels to obtain tissue-driven robots with dynamic structural color. [63]As shown in Figure 4b, inverse opal hydrogels were implanted with the cardiomyocyte tissue, and the spontaneous movement of cardiomyocyte tissue drove the inverse opal hydrogels to repeatedly relax and contract, resulting in the blue-shift and red-shift of its structural color.
Taking inspiration from the unique color modulation of male hummingbird's crowns and gorgets, Lai et al. fabricated a mechanically triggered color switchable film based on a laser-cut photonic crystal-coated PDMS kirigami (Figure 4c). [38]he structural color of the bioinspired photonic crystal elastomer kirigami changes when it is stretched and released.The color variation originated from stretch-induced view angle changes.Similarly, Hou et al. demonstrated a multichannel information encryption method based on the orientation of two-dimensional deformable Kirigami array gratings. [39]hanks to the angular dependence of structural color, the kirigami grating film showed rapid, programmable, and repeatable color change.This method uses topological space deformation to achieve the change in optical properties, providing new possibilities for spectral and spatial encryption, as well as camouflage and mechanical sensing.

Emerging Photonic Polymers
Recently, many advanced photonic polymers have emerged as promising candidates for engineering structural colors, where the outstanding optical properties originate from the wavelength-scale nanostructures and diverse light-matter interaction.Miller et al. used adapting Lippmann photography to produce large-area photonic polymers to design sophisticated color patterns with mechanochromic response and angular scattering characteristics (Figure 5a). [40]The stretchable color patterns were Figure 5. Emerging photonic polymers for dynamic structural colors.a) Optical manufacture of stretchable color-changing materials by adapting Lippmann photography.Reproduced with permission. [40]Copyright 2022, Springer Nature.b) Tunable structural color of diselenide-containing shape memory material based on controllable stress relaxation and birefringence of stretched materials.Reproduced with permission. [42]Copyright 2020, Wiley-VCH.
prepared based on commercially available photosensitive elastomers and a digital projector.The approach was scalable, affordable, fast, and relevant for a wide range of manufacturing settings.The mechanochromic photonic polymers were also demonstrated for stress sensing for human-computer interaction and mechanosensitive healthcare materials.
The structural color can originate from the birefringence of stretched polymers, whose shapes and colors can be fixed when the mechanical stress is maintained. [41]Liu et al. prepared diselenide-containing shape memory materials for creating color patterns via visible light irradiation (Figure 5b). [42]The structural color originated from birefringence of stretched polymers, whose shapes were fixed when the uniaxial strain was maintained.The fixed stress was released by dynamic bond-exchange under visible light stimulus.The patterns with tunable structural colors were achieved by regulating visible light irradiation time with a commercial projector.The structural color patterns could be arbitrarily erased and rewritten for many times.The light stimulus signals were stored as mechanical signals and then transformed into optical signals during the patterning process.The birefringence of stretched polymers for preparing light-responsive structural colors has enormous potential in data storage, anticounterfeit labels, and display devices.

Bioinspired Stretchable Polymers for Dynamic IR Emissivity
IR emissivity is of profound significance to understand the IR thermal radiation of materials or objects.It is worth noting that two materials at the same actual temperature could show different thermal images if their IR emissivity is different.Since emissivity is a material-dependent parameter, changing materials' structure is believed to be one of the most efficient approaches to regulate IR emissivity of materials.The IR radiation characteristic of an object is directly dependent on surface properties.In theory, topographic changes will cause the change of IR emissivity by applying mechanical stretch on the materials.Thus, mechanical deformation of stretchable polymers can directly lead to dynamic IR emissivity changes of materials in a broad IR spectrum region. [1]Dynamic control of IR emissivity is highly appealing in advanced applications, such as adaptive thermal camouflage, [64] optical communication, [65] and IR encryption. [66]here are two types of stretchable polymers in the variation of IR emissivity caused by mechanical stress: stretch-induced wrinkled-flat surface and stretch-induced cracked surface.

Stretch-Induced Wrinkled-Flat Surface for Dynamic IR Emissivity
The skin colors of cephalopods are capable of quickly change for the purposes of concealment or signaling (Figure 6a). [6]Such remarkable camouflaging capability is originated from the elaborate nanostructures of the cephalopod's soft and flexible skin, where chromatophore pigment cells are found in innervated dermal layers.Inspired by the extraordinary camouflage capability of cephalopods, Xu et al. developed electrically driven adaptive IR-reflecting materials and platforms (Figure 6b-d). [6]A typical cephalopod-shaped device had a negligible temperature Figure 6.Cephalopods-inspired dynamic IR emissivity system for IR camouflage.a) Thermal camouflage of cephalopods.b) IR-reflecting device before (left) and after (right) actuation.The active region of its surface morphology changed upon mechanical actuation.c) The IR reflectance spectra of the IR-reflecting device before (left) and after (right) mechanical strain.d) A cephalopod-shaped camouflage device that can alter its thermal appearance under electrical actuation.Reproduced with permission. [6]Copyright 2018, The American Association for the Advancement of Science.
difference with the background, so that it was camouflaged in the environment under thermal camera.The device surfaces were wrinkled and it was covered by reconfigurable arrangement of reflective microstructures analogous to cephalopod's skin.Interestingly, the cephalopod-shaped device exhibited flat surface and a distinct temperature difference with the environment upon electrical actuation, which caused it distinguishable from its surroundings. [67]Similarly, Liu et al. designed advanced materials with hierarchical morphologies for reconfigurable wrinkled surface and dynamic IR emissivity in the visible-to-IR spectrum (Figure 7a). [68]Under negative mechanical strains, the materials exhibited microstructured morphologies because of compression of the hierarchical film.Under zero strain, the materials exhibited intermediate flat morphologies.Under positive mechanical strains, the materials exhibited nanostructured morphologies owing to shifting of the embedded Cu nanostructures.As a result, such promising architectures were able to reversibly reconfigure their morphologies across multiple different length scales and dynamic IR emissivity in a wide spectral range.
Based on elastic substrates, wrinkled MXene, graphene, and some metals have been used for dynamic IR emissivity.Li et al. fabricated a stretchable MXene/elastomer bilayer film with strain-dependent microstructures (Figure 7b). [69]The MXene/ elastomer bilayer film exhibited tunable IR emission under The hierarchically reconfigurable wrinkled surface for dynamic IR emissivity in the visible-to-IR spectrum.Reproduced with permission. [68]Copyright 2021, The American Chemical Society.b) Highly stretchable MXene robotic skins for stimulus sensation, thermal camouflage, and wireless communication.Reproduced with permission. [69]Copyright 2022, Wiley-VCH.
strains and enabled dynamic thermal camouflage for soft robots.Thanks to the mild microcrack propagation behaviors and intrinsic Seebeck effect, the MXene/elastomer bilayer film could be used for strain and thermal sensing.Moreover, with high electrical conductivity, MXene robotic skins were acted as a deformable dipole antenna for the application of wireless microwave communication, enabling the robots to receive and send massive information.Krishna et al. reported a UV to mid-IR emissivity modulatory system with wrinkled graphene by applying mechanical strain. [70]It was found that the optimized pitch distances of the wrinkled graphene for maximally regulating UV emissivity and IR emissivity are 140 nm and 10 μm, respectively.Similarly, Xu et al. developed freestanding, optically transparent sulfonated pentablock copolymer membranes with wrinkled surface and high protonic conductivities. [71]The membranes are used for the preparation of electrically and mechanically actuated camouflage devices that function over visible and IR specular spectral window.
Manganite ceramics with excellent mechanical flexibility can also be used as elastic substrates for dynamic IR emissivity. [72,73]hao et al. coupled thermochromism and thermal deformation in manganite perovskite-based ceramics to synergistically modulate thermal emission. [74]The deformable manganite perovskite-based composite film exhibited a remarkably tunable emittance contrast of 0.61 within the mid-wavlength IR and longwavelength IR region.Only by optimizing the thickness of manganite, the composite film can be developed as a spectral selective absorber, an adaptive camouflage device, or a thermal management device.The design imparts the brittle flexible manganite ceramics, making the composite film facile for integrating with Figure 8. Stretch-induced cracked surface for dynamic IR emissivity.a) Metal/elastomer composites with a stretch-induced cracked surface for adaptive body temperature regulation sleeve.Reproduced with permission. [75]Copyright 2019, Springer Nature.b) Visible-to-microwave emissivity tunability of adaptive multispectral mechano-optical systems at initial and contracted, stretched state.Reproduced with permission. [76]Copyright 2023, The American Chemical Society.
the curved or flat configurations.The proposed strategy may enable a versatile manganite perovskite-based platform for the dynamic thermal emission control.

Stretch-Induced Cracked Surface for Dynamic IR Emissivity
Distributed stretch-induced microcracks of film's topography enable the exposure of the substrate for dynamic IR emissivity.Leung et al. demonstrated a stretch-induced cracked surface to allow IR transmission, leading to the shifting of IR emissivity (Figure 8a). [75]The composites consisted of IR-reflecting nanostructure-anchored metal domains overlaid on an IRtransparent polymer matrix.In such a dynamic optical property regulation system, the metal domain arrangement was switched from densely to sparsely packed by an applied strain, resulting in the modulation of the transmission and reflection of IR radiation.
Stretch-induced wrinkled-to-cracked surface provides adaptive multispectral mechano-optical systems with an effective range extending from the visible to the microwave range.Liang et al. prepared an adaptive multispectral mechano-optical composite film based on a dielectric elastomer coated with silver nanowire (Figure 8b). [76]The composite film could reconfigure the surface morphology between cracks and wrinkles upon mechanical stretching and contraction.The designed system could switch between opacity and transparency in visibleÀIRÀmicrowave wavelength, as well as continuous regulation, wide spectral range, excellent recyclability, and rapid response time.
Similarly, inspired by the skin of cephalopods, Zeng et al. fabricated a hybrid multilayer elastomer comprising a high emissivity substrate and a low emissivity surface metal layer, which could realize high emissivity via mechanical strain and revert back to low emissivity state when the mechanical strain is removed. [77]Through uniaxial strain and bulging strain, the thermal regulating system could be applied for thermal camouflage, encryption, and dynamic display (Figure 9).

Bioinspired Stretchable Polymers for Dynamic Thermal Management
The utilization and regulation of thermal energy are increasingly important in modern society owing to the growing demand for cooling and heating in applications of energy-efficient buildings and personal thermal management.Over millions of years of natural selection, biological organisms have evolved elaborated microstructures to efficiently modulate solar thermal energy and thermal radiation of themselves for cooling or heating.For example, the hairs of polar bears are hollow structure, which is significant to keep them warm by preventing heat transfer to the environment. [78]For biological species living in extremely hot environment, it is crucial to keep their body temperature cool for their survival.Longicorn beetles Neocerambyx gigas evolved the cross-sectional triangle fluffs on their forewings to reduce Figure 9. Hybrid multilayer elastomers for dynamic IR emissivity.a) Scheme of a reversible thermal radiation modulator comprising a substrate with a stretchable polymer substrate with high IR emissivity and a rigid metal surface film with low IR emissivity.The thermal radiation modulator could be used for b) dynamic display, c) encryption, and d) thermal camouflage.Reproduced with permission. [77]Copyright 2021, Elsevier Ltd. absorption of solar thermal radiation and enhance body thermal radiation, allowing them to survive in extreme hot environment with a surrounding temperature above 40 °C. [79]The unique thermal management strategies discovered in biological species provide inspiration for the design and preparation of various high-performance stretchable polymers for solar and thermal radiation modulation, which can dynamically modulate the optical properties under external stretch to accommodate the heating/cooling needs in different seasons.In this section, advanced thermal management based on tunable solar transmittance and dynamic radiative cooling will be introduced.

Stretchable Polymers for Tunable Solar Transmittance
[82][83] Typically, the internal structures or surface morphologies of stretchable polymers are reconfigured or deformed upon the external strain, resulting in a change in light transmittance through light diffraction or scattering.No external power supply is required for control, enabling the cost-effectiveness for mass production.In addition, the optical properties of mechanochromic polymers are capable of being tuned instantly upon mechanical strain.By comparison, thermochromic, photochromic, or electrochromic materials used for smart window generally need more time for response.Therefore, the design of stretchable polymers for tunable solar transmittance that can dynamically regulate light transmittance is considered an effective method for thermal management, which is of paramount importance for the applications of energy-saving buildings, sunglasses, automobiles, etc.By varying the size of silica nanoparticles, the wrinkle geometry, and the stretching strain, Kim et al. achieved a large drop in solar transmittance in the visible-to-NIR range with a relatively small mechanical strain (Figure 10a). [84]The 10% prestrain sample exhibited a low amplitude shallow wrinkle and showed the highest transmittance of 86.4% at 550 nm initially.Stretching beyond the prestrain level caused a 70.8% decrease in the light transmittance with an Figure 10.Stretchable polymers for tunable solar transmittance.a) Fabrication and photographic images of a wrinkle-silica composite film at the transparent state (left) and opaque state (right) with 40% mechanical strain.Reproduced with permission. [84]Copyright 2018, Wiley-VCH.b) Smart additional 30% strain.The large degree of solar transmittance was attributed to combined effects from the nanovoids formed around the particles and the formation of secondary wrinkles.
Combining wrinkled surface with skin layer materials is a common strategy for tunable solar transmittance.Jiang et al. fabricated a bilayer film consisting of elastomeric PDMS substrates with reversible wrinkled surface and various skin layer materials, such as hydroxyethyl cellulose, chitosan, and polyvinyl alcohol (PVA) (Figure 10b). [85]The surface wrinkles perpendicular to the mechanical prestrain direction were obtained on the bilayer films' surface by uniaxial stretching and then releasing the films thanks to the mismatched elastic modulus of the soft compliant PDMS substrate and the skin layer.The bilayer film was reversibly switched between a transparent stretched state and a highly opaque released state with a solar transmittance between ≈91% (transparent state) and ≈6% (opaque state) in the visible wavelength when applied a relatively small strain (less than 20%).
Li et al. reported a simple approach to fabricate surface cracking-wrinkling patterns on PDMS films and achieved large tuning range of optical transmittance. [86]The surface cracking-wrinkling patterns were generated by stretching UV/ozone-treated PDMS films and could effectively decrease the light transmittance of the films.The optical transmittance could be tuned between 9.2% and 92% with a moderate 50% uniaxial tensile strain.The films could be reversibly switched between opaque and transparent for at least 1000 cycles without optical performance degradation, which is promising to be used in smart windows.
Kirigami is considered to be the efficient toolbox of stretchable polymer design.The stretchable polymers are cut with patterns and endowed with extraordinary mechanical properties, which can be used for solar modulation. [87]Ke et al. developed a kirigami-inspired stretchable metamaterial containing phase change nanoparticles vanadium dioxide (VO 2 ) for tunable solar transmittance targeting energy-efficient smart windows (Figure 11). [88]he temperature-dependent localized surface plasmon resonance (LSPR) and the geometrical transition presented excellent solar thermal controls in UV-visible-NIR regions.The active LSPR control was achieved by stretch-induced local dielectric changes, which were mitigated on stretchable metamaterials owing to their unique strain distributions.These kirigamiinspired structures demonstrated improved solar energy modulation capabilities and paved a new way for energy-efficient smart windows.

Stretchable Polymers for Dynamic Radiative Cooling
The temperature on the earth's surface (room temperature) is about 300 K and the temperature of the universe is around 3 K.The heat on the earth can dissipate to the universe for the purpose of cooling.However, thermal radiation is absorbed to different degrees when passing through the earth's atmospheric layer.The absorption of thermal radiation is found to be fairly low in the wavelengths from 8 to 13 μm.Radiative cooling is usually realized by enhancing the materials' thermal radiation in this spectral window, so that the thermal energy can directly pass through the earth's atmosphere to the cold universe.Therefore, this spectral range is called the IR atmospheric transparency window. [89,90]Radiative cooling can be divided into nighttime radiative cooling and daytime radiative cooling.Nighttime radiative cooling means that an object exhibits sub-ambient temperature automatically during nighttime.Without solar thermal radiation at night, nighttime radiative cooling only needs to focus on high IR emissivity in the IR atmospheric transparency window.In contrast to nighttime radiative cooling, daytime radiative cooling is more appealing for researchers owing to broad application prospects in 24 h.Daytime radiative cooling under direct sunlight can be realized by suppressing solar absorption (0.2-2.5 μm) and enhancing thermal radiation within the atmospheric window (8-13 μm). [91,92]Stretchable polymers are a promising class of materials for dynamic radiative cooling, whose solar absorption and thermal emission properties are regulated by mechanical strain.The mechanism of the mechanical response of dynamic radiative cooling performance is mechanically driven reconfiguration and deformation of the scattering points eliminated/formed inside the stretchable polymers or on the material surface.
Zhao et al. casted water droplet emulsions in PDMS precursors as pore templates and successfully prepared switchable cavitation of silica coatings (Figure 12a). [93]The evaporation of water molecules generated negative pressure during the cross-linking of PDMS chains, leading to the water droplets shrinking.After the water droplets evaporated completely, the metastable creases were generated at the position previously occupied by the water Figure 12.Stretchable polymers for dynamic radiative cooling.a) Schematic of the bilayer elastomer composed of a switchable silicone top layer and CBPembedded bottom layer and corresponding heating and cooling mechanism.Reproduced with permission. [93]Copyright 2020, Wiley-VCH.b) Corrugated nickel coated on an elastomer substrate for dynamic radiative cooling.Reproduced with permission. [94]Copyright 2019, Taylor & Francis.c) Schematic of the mechanically reconfigurable photonic structure composed of a PDMS layer embedded with multiple species of nanoparticles (such as boron nitride and silicon nitride) on top of the 1D PDMS grating at the relaxed and stretched states.Reproduced with permission. [95]Copyright 2020, Springer Nature.d) Schematic of a dynamic IR gating textile for personal IR radiative cooling.Reproduced with permission. [96]Copyright 2019, The American Association for the Advancement of Science.
droplets.The coatings could continuous and reversible switch between a highly porous state and a transparent solid state.Under mechanical stretching, these metastable creases became porous structure, leading to outstanding solar reflection and radiative cooling effects.In addition, the metastable creases could be reversibly regulated from the porous state to the transparent solid state upon mechanical compression, accompanied by carbon black particle (CBP)-embedded bottom layer to enhance solar heating.
Inspired by the radiative cooling property of desert silver ants, Sala-Casanovas et al. developed a stretchable selective emitter relying on corrugated nickel that can dynamically control IR emissivity (Figure 12b). [94]The periodic corrugations of nickel increased the absorptivity from 0.3 to 0.7 in the solar spectrum owing to multiple scattering.The optical change was reversible and accompanied by ambient surface temperature variations in 305-315 K. Liu et al. fabricated a reconfigurable photonic structure with multiple species of nanoparticles (such as boron nitride and silicon nitride) in the 1D PDMS grating (Figure 12c). [95]The photonic structure could be used for dynamic thermal radiation by deformation of 1D PDMS grating via different mechanical strains.Variable hysteresis temperatures were obtained with strain ranging between 0 % and 20 %, and more than 20 % of the strain could switch radiative cooling to heating.
Adaptive regulating optical channels of textiles responding to environmental changes is of paramount significance for improving the thermal management abilities of clothing systems.For this purpose, Zhang et al. designed and prepared an IR gating textile to dynamically modulate thermal radiation of human bodies for personal thermal management (Figure 12d). [96]A bundle of conductive metafibers made up of the textile yarn that could adaptively respond to relative humidity or temperature of the skin.Under wet or hot conditions, the yarn collapsed into a tight bundle, which tuned the IR emissivity of the textiles to the IR transparency window, and notably promoted radiative cooling effect of human bodies.Under dry or cold conditions, the yarn expanded and reduced heat dissipation.This unique mechanism made it possible to regulate thermal radiation adaptively through textiles in response to environmental changes.

Conclusion
In this work, we present a state-of-the-art review of bioinspired stretchable polymers for dynamic optical and thermal regulation as well as their potential applications.The functions, materials, mechanisms, and key properties or applications of these advanced achievements are summarized in Table 1.Inspired by the color camouflage of chameleon, stretchable polymers such as CLCEs, photonic crystal elastomers, and emerging photonic polymers, are developed for dynamic structural colors.The mechanochromic response of these stretchable polymers and the underlying mechanism are pointed out in different reports.
Stretchable polymers with intrinsic structural colors have been used in various fields such as displays, healthcare, anticounterfeiting, and sensors.Inspired by cephalopods' skin, stretchable polymers for dynamic IR emissivity are achieved by stretchinduced wrinkled-flat surface or stretch-induced cracked surface.The thermal radiation properties of an object directly rely on its surface properties.Topographic changes by applying mechanical strain on the stretchable polymers can cause an IR emissivity change.Therefore, mechanical deformation can directly lead to dynamic changes in the IR emissivity of materials.Inspired by the thermal management functions of biological species, stretchable polymers for dynamic thermal management are developed based on the mechanisms of dynamic radiative cooling and tunable solar transmittance.The surface morphologies or internal structures of stretchable polymers are reconfigured or deformed upon the mechanical strain, resulting in a change in solar transmittance or thermal emission within atmospheric window.
Through natural selection and evolution over millions of years, numerous ingenious organisms have developed unique and complicated micro/nanostructures for efficiently manipulating light and thermal radiation, two necessary and critical sources for not only natural organisms but also human beings.These organisms provide enlightening examples for us to learn from.From this point of view, stretchable polymers are supposed to be one of the most ideal materials to mimic biological skins and bionic functions.However, related research in this field is still in a preliminary stage.There are many opportunities and challenges in accelerating the development of this exciting field.First, achieving truly smart camouflage in a complex environment is still a formidable task.99][100][101] Second, the long-lasting thermal camouflage performance and response time of stretchable polymers should be improved to satisfy the requirement of camouflage technology.An adaptive system with the dynamic emissivity regulating materials integrated with background emissivity detection devices may realize a smart camouflage system.104] Third, stretchable polymers have not yet achieved the greater purpose of smart thermal management.Development of smart stretchable polymers for dynamic thermal management in response to weather conditions or ambient temperature is expected, which is desired to employ in energy-efficient green buildings or smart textiles for real on-demand thermal management. [82,105,106]It is anticipated that functional stretchable polymers could play an increasingly important role in the prosperous area of dynamic optical and thermal regulation.

Figure 1 .
Figure1.Summary of bioinspired stretchable polymers for dynamic optical and thermal regulation.a) Color camouflage of a chameleon.Guanine crystals in the chameleon's skin are packed into face-centered cubic lattices at the relaxed state and expanded at the excited state.Reproduced with permission.[5]Copyright 2015, Springer Nature.b) A squid changes its skin color through the mechanical action of muscle cells in front of a rocky background.Reproduced with permission.[6]Copyright 2018, The American Association for the Advancement of Science.c) Schematic of bioinspired stretchable polymers for dynamic structural colors, dynamic infrared emissivity, and dynamic thermal management.

Figure 7 .
Figure 7. Stretch-induced wrinkled-flat surface for dynamic IR emissivity.a) The hierarchically reconfigurable wrinkled surface for dynamic IR emissivity in the visible-to-IR spectrum.Reproduced with permission.[68]Copyright 2021, The American Chemical Society.b) Highly stretchable MXene robotic skins for stimulus sensation, thermal camouflage, and wireless communication.Reproduced with permission.[69]Copyright 2022, Wiley-VCH.

Table 1 .
Summary of stretchable polymers for dynamic optical and thermal regulation.