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A superhydrophobic textile inspired by polar bear hair for both in air and underwater thermal insulation

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Abstract

Polar bears have special hairs with porous microstructure and hydrophobic surface, which help them keep warm in both cold air and water. Due to the large difference in temperature and heat transfer process of aquatic and terrestrial environments, it is quite difficult for typical thermal insulating materials to maintain ideal insulation in air and underwater at the same time. To tackle this problem, we report here a superhydrophobic porous textile mimicking the outstanding thermal insulating properties of polar bear hairs. Fibers used to weave this textile are fabricated by a freeze-spinning method, providing them aligned porous microstructure. After a superhydrophobic surface treatment, the obtained textile with porous microstructure and superhydrophobic surface is capable of capturing air in its multi-scale porous structure, promising an excellent thermal insulating ability in both air and water environments. Our study paves a way for the bioinspired engineering of thermal insulation textiles both in air and underwater.

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... The programmable form of the hairy SMP allows the irreversible and adaptable reconfiguration of the hair morphology in response to external environments, resulting in more than 61.4% dynamic control of thermal insulation [82]. Superhydrophobic surface treatment of the textile with porous microstructure (Fig. 4d-e) allows for promising a superior thermal insulation in both air and water ambient, paving the way for the bioinspired thermal insulating materials in both air and underwater [83]. In addition, biomimetic thermalregulation textiles can also have stretchability, high strength, flame retardancy, and acid and alkali resistance [84]. ...
... Besides, the anti-reflection properties of hollow cylinders are superior than shell/hollow and solid cylinder structures (Fig. 5d). Moreover, the white scales with gradient refractive [83]. f-g Thermal stealth properties biomimetic textiles. ...
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... In recent years, anti-icing coatings research has experienced considerable progress by mimicking nature's materials. For example, the polar bears, which live in the world's coldest environments, have a special hair that prevent the penetration of cold sea water to the skin, and their corresponding fur exhibits excellent superhydrophobicity and antiadhesion characteristics needed to repel water microdroplets [4]. In addition, polar bear fur is oily; as a result, an increment of the water repellency is generated. ...
... However, a simple, cost-effective, scalable and highly versatile method for the large-scale fabrication of superhydrophobic micro/nanofibers is electrospinning [32][33][34]. This technique can provide a low surface energy as a function of the selected polymeric precursor [35] as well as a highly rough structure caused by the hierarchical microstructures and nanostructures of micro/nanofibrous mat [36], which could show a similar effect to that of polar bear fur [4]. The morphology surface roughness and the structure of the electrospun mats depend on the intrinsic polymeric precursor properties (concentration, molecular weight, viscosity, surface tension, and nature of solvent), the operational condition parameters (applied voltage, flow rate, and distance of tip to collector), and the environmental conditions (relative humidity and temperature) [37][38][39]. ...
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... Wu et al. [15] studied the thermal insulation performance of silver ants from the aspects of spectrum, wave transmission and thermal conductivity. Some researchers have studied the infrared radiation properties of the porous hair structure of polar bear [16][17][18] and penguin [19], and prepared the fiber structure based on its structure. However, hair materials have superior performance in heat insulation, but the soft hair cannot meet the requirements of labyrinth seal in terms of load-bearing performance. ...
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... Liquid repellent coatings are being broadly studied to explore the fundamental interfacial phenomena, 1-6 and due to their wide range of potential practical applications in self-cleaning, 7 drag reduction, 8 anti-corrosion, 9 anti-fouling, 10 fog harvesting, 11 chemical shielding, 12 thermal insulation, 13 and anti-/deicing 14 to name a few. An ideal superomniphobic coating should ensure stable liquid repellency and should possess easy deposition through a simple process on substrates regardless of their size, shape, or composition. ...
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... Some of the effective thermal heating strategies have been exploited on the animals living in an extremely cold environment [ 214 , 215 ]. For instance, the animals like polar bears keep their body warm by reflecting the IR radiation to the body, ascribed from their membrane-pore structure of the fur and skin [214][215][216] . A biomimetic porous fiber-based woven heating device was developed to keep the body warm and to reduce the indoor heat loss, by freeze spinning of a CNT doped spinning solution, mimicking the hair of a polar bear ( Fig. 10 a) [54] . ...
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... [1][2][3][4][5][6][7] Among them, the functions related to light and heat have attracted extensive attention from researchers because light and heat are indispensable factors for human survival. Inspired by the special functions and microstructures related to light and heat of animals, some artificial materials with multiple functions have been designed and used for light and heat management, including broadband light reflection, [8,9] broadband light absorption, [10][11][12] thermal insulation, [13][14][15] and radiation cooling. [16,17] West African Gaboon viper (Bitis rhinoceros), which features black spots on its dorsal scales (Figure 1a, iii), is a master of camouflage. ...
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... Smaller λ or higher dosage are favorable for building more conductive pathways, yet simultaneously blocks the optical path, leading to poor transmittance. According to our previous work, [39][40][41][42][43][44][45][56][57][58][59] λ can be tuned by changing . From left to right: randomly oriented and parallelly oriented patterns with various spacing (λ) of ≈150, ≈200, and ≈300 µm, respectively. ...
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... As the biggest organ of animals, the skin not only provides protection from the environment but also functions as an efficient sensor to external stimuli, [16][17][18] providing valuable design principles for durable flexible electronic skin. In this context, many strong and stretchable composite films have been developed in the past few years by mimicking the fiber-reinforced structure of the skin. ...
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Fabrication of multifunctional porous fibers with excellent mechanical properties has attracted abundant attention in the fields of personal thermal management textiles and smart wearable devices. However, the high cost and harsh preparation environment of the traditional solution-solvent phase separation method for making porous fibers aggravates the problems of resource consumption and environmental pollution. Herein, a micro-extrusion process that combines environmentally-friendly CO2 physical foaming with fused deposition modeling (FDM) technology is proposed, via the dual features of high gas uptake and restricted cell growth, to implement the continuous production of porous polyetheretherketone (PEEK) fibers with a production efficiency of 10.5 cm per second. The porous PEEK fiber exhibits excellent stretchability (234.8% strain) and good high-temperature thermal insulation property. The open-cell structure on the surface is favorable for the adsorption to achieve superhydrophobicity (154.4°) and high-efficiency photocatalytic degradation of rhodamine B (90.4%). Moreover, the parameterized controllability of the cell structure is beneficial to widening the multifunctional window. In short, the first porous PEEK physical foaming fiber, which opens up a new avenue for the application expansion, especially in the medical field, is realized. This article is protected by copyright. All rights reserved
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Wax deposition and flow obstruction in the petroleum industry cause serious safety problems and economic loss. Superamphiphobic coating with anti-adhesion property can be a potential strategy to solve these problems. However, the integration between non-wettability and durability of the coating remains a formidable challenge. Here, a robust and multifunctional superamphiphobic coating was fabricated by a facile spraying method. The highly fluorinated [email protected]2 ([email protected]2-F) composite fillers were prepared by in-situ growth of SiO2 on the Pal surface and chemical modification, which were integrated into the polyethersulfone (PES) and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to construct unique micro-/nano-hierarchical structures with abundant papillae and intertwined networks. A stable air film on the PES-PVDF-HFP/[email protected]2-F (P-P/[email protected]2-F) coating surface was thereby formed owing to the chemical inertness and strong adhesion of polymers apart from well-designed structures. The air film was adopted as an anti-adhesion layer and a good thermal barrier layer, which endowed the coating with effective anti-adhesion property in crude oil system. The coating exhibited excellent crude oil repellency and almost remained clean after 2000 immersion cycles and 30 d of immersion in crude oil. Moreover, the coating could be applied in pipes and showed an efficient transportation property. Most importantly, the resultant coating demonstrated a high anti-waxing deposition ratio of 90.9%. Additionally, the coating worked effectively even after harsh mechanical damage and chemical corrosion. Therefore, the robust and multifunctional superamphiphobic coating in this work can be an appropriate candidate for anti-adhesion/waxing coating, which has enormous application potential in petrochemical industry and other harsh environments.
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Heart diseases caused by structure and function degradation is troubling millions of people all over the world. With the extension of human lifespan, the demand for heart valve replacement is rapidly increasing. However, the currently used prostheses of heart valve in clinic such as mechanical and tissue valves all have serious limitations over long‐term stability after implanting. Therefore, novel artificial heart valves with outstanding durability and low degradation risk are still in great demands, despite of numerous studies in the literature. The sophisticated, multiscale structure of native heart valve is believed to be the key to its mechanical and biological durability during long‐term cyclic motion. In this review, we discussed the complex structure of the native heart valve as well as their contributions to the valve's cyclic work. In addition, representative and state of the art studies of biomimetic heart valves inspired by the natural valve are also introduced. Based on these, the structural and functional design of future novel biomimetic heart valves are proposed. Summary of the review: • A brief introduction of the complex structure of the native heart valve as well as their contributions to the valve's cyclic work. • Summary of state of the art studies of biomimetic heart valves inspired by native heart valve.
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Encouraged by the porous and stable structure of cold-resist animals’ hair or feather, bio-inspired hierarchical structure yarns combining polyacrylonitrile(PAN) nanofibers and poly-propylene(PP) hollow microfibers have been developed by a modified conjugate electrospinning technology. Physical cross-linking has been built to increase fibers adhesion and construct interlayer support for nanofibrous assembly. The nanofibers and hollow micro-fibers construct a stable porous structure with porosity of 62%, providing excellent thermal insulating ability[temperature difference(∣ΔT∣) between skin and yarn surface is 4.9 °C] as well as good mechanical property. More interestingly, the water transfer ability (infiltrate the yarn in 10 s) of synthetic fibers has been improved greatly by the combination of thin diameter nanofibers to the yarn. It is believed that the research lays the foundation for bio-inspired engineering technology in the manufacture of thermal comfort.
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In this work, the fabrication and characterization of a multifunction cellulose membrane is reported: (і) high stable in harsh environment (acid/alkali solution, high temperature, abrasion, underwater writable, heat-insulated and self-cleaning), (іі) oil-water separation with high efficiency, and (ііі) effective and rapid photocatalytic degradation of hazardous environmental contaminants. This multifunction cellulose membrane was successfully prepared through depositing Fe2O3 particles and subsequent modification of stearic acid (STA). The results showed that this multifunction cellulose membrane had excellent superhydrophobic and self-cleaning properties with a high static water contact angle of 167.2 ± 2°. Moreover, it possessed unusual repellent properties towards acid/alkali solution, abrasion, high temperature and heat-insulated properties. This membrane could also be used for writing underwater and keep satisfactory superhydrophobic performance for a long time with a water contact angle of 154.2 ± 2°. It also showed a high separation efficiency (>89.0 %) and a high separation flux (greater than 80 Lm⁻² h⁻¹) for these three oils (toluene, trichloromethane and n-Hexane). After repeated separation for 6 cycles, the separation flux and separation efficiency of n-Hexane has not changed significantly. It also demonstrated reliable photocatalytic ability, a useful property for resisting organic contaminations. Compared with conventional cellulose membrane, it is anticipated that this multifunction superhydrophobic cellulose membrane is not only really competitive in complex and harsh environment, but also demonstrates great potentials in the field of effective underwater treatment, fire-proof applications, oil-water separation field and photocatalytic property.
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To overcome the poor water-resistance, magnesium oxychloride (MOC) cement, was hydrophobically modified with hydroxy-terminated polydimethylsiloxane (PDMS). MOC cement foam was prepared using hydrogen peroxide as the foaming agent. Scanning electron microscope (SEM) images showed that the MOC cement foams contained visible pores with sizes ranging from 150 μm to 600 μm, and the apertures increase with the increase in foaming agent. A lot of acicular phase 5 were observed in the pores, which conferred strength to the materials. Porosity analyses illustrated that the addition of the foaming agent increased the porosity and enlarged the pores, making the materials less dense. The increase in porosity and pore size significantly reduced the thermal conductivity, thereby improving the thermal insulation of the MOC cement foam. The modification of PDMS not only enhanced the water-resistance, but also achieved the superhydrophobicity of MF-1, while the sample with relatively large pores also had good hydrophobic ability. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to analyse the samples, explaining the mechanism of the water-resistance enhancement from a microscopic perspective.
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Highly efficient infrared (IR) stealth materials with low emissivity and thermal insulation features are urgently demanded in many fields. However, these two characteristics are difficult to be realized by an individual compound due to the high thermal conductivity of low emissivity materials (e.g. metal materials) and the high emissivity of excellent thermal insulation materials (e.g. polymer foam and aerogel). Herein, we demonstrated a new strategy to fabricate low emissivity/thermal insulation IR stealth composites with asymmetric structure based on a combination of silver-plated hollow glass microsphere (HGM-Ag) and waterborne polyurethane (WPU) through one-step density-driven filler separation coupled with freeze-drying method. The composites were constructed from the HGM-Ag concentrated on the top of the composites as low emissivity layer and the bottom layer with low thermal conductivity about 0.044 W m⁻¹ K⁻¹ served as the thermal insulation layer. The emissivity of the composites can be reduced from 0.943 to 0.713 by adding HGM-Ag. Benefitting from the well-interconnected 3-dimensional structure, the HGM-Ag/WPU porous composites also display outstanding mechanical property. We believe this work will provide a new strategy for designing highly efficient IR stealth materials based on synergetic mechanism of reducing emissivity and thermal insulation.
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Liquid absorption and recycling play a crucial role in many industrial and environmental applications, such as oil spill cleanup and recovery, hemostasis, astronauts' urine recycling, and so on. Although many liquid absorbing materials have been developed, it still remains a grand challenge to achieve both fast absorption and efficient recycling in a cost-effective and energy-saving manner, especially for viscous liquids such as crude oil. A smart polyurethane-based porous sponge with aligned channel structure is prepared by directional freezing. Compared to common sponges with random porous structure, the as-prepared smart sponge has larger liquid absorption speed due to its lower tortuosity and stronger capillary ("tortuosity effect"). More importantly, the absorbed liquid can be remotely squeezed out due to a thermally responsive shape memory effect when the sponge is heated up. Such smart sponges with well-defined porous structure and thermal responsive self-squeezing capability have great potential in efficient liquid absorption and recycling.
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Replicating nacre’s multiscale architecture represents a promising approach to design artificial materials with outstanding rigidity and toughness. It is highly desirable yet challenging to incorporate self-healing and shape-programming capabilities into nacre-mimetic composites due to their rigidity and high filler content. Here, we report such a composite obtained by infiltrating a thermally switchable Diels-Alder network polymer into a lamellar scaffold of alumina. The chemical bond switchability and the physical confinement by the filler endows the composite with sufficient molecular mobility without compromising its thermal dimension stability. Consequently, our composite is capable of self-healing internal damages. Additionally, in contrast to the intractable planar shape of other artificial nacres, precise control of the polymer chain dynamics allows the shape of our composite to be programmed permanently via plasticity and temporarily via shape memory effect. Our approach paves a new way for designing durable multifunctional bioinspired structural materials.
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Polymer‐based thermal interface materials (TIMs) with excellent thermal conductivity and electrical resistivity are in high demand in the electronics industry. In the past decade, thermally conductive fillers, such as boron nitride nanosheets (BNNS), were usually incorporated into the polymer‐based TIMs to improve their thermal conductivity for efficient heat management. However, the thermal performance of those composites means that they are still far from practical applications, mainly because of poor control over the 3D conductive network. In the present work, a high thermally conductive BNNS/epoxy composite is fabricated by building a nacre‐mimetic 3D conductive network within an epoxy resin matrix, realized by a unique bidirectional freezing technique. The as‐prepared composite exhibits a high thermal conductivity (6.07 W m−1 K−1) at 15 vol% BNNS loading, outstanding electrical resistivity, and thermal stability, making it attractive to electronic packaging applications. In addition, this research provides a promising strategy to achieve high thermal conductive polymer‐based TIMs by building efficient 3D conductive networks. An anisotropically high thermal conductive boron nitride/epoxy composite is fabricated by building a nacre‐mimetic 3D conductive network within an epoxy resin matrix, realized by a unique bidirectional freezing technique. With a high thermal conductivity (6.07 W m–1 K–1) at 15 vol% boron nitride loading, this composite may find wide applications including as a thermal interface material for advanced electronic packaging technology.
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Animals living in the extremely cold environment, such as polar bears, have shown amazing capability to keep warm, benefiting from their hollow hairs. Mimicking such a strategy in synthetic fibers would stimulate smart textiles for efficient personal thermal management, which plays an important role in preventing heat loss and improving efficiency in house warming energy consumption. Here, a “freeze-spinning” technique is used to realize continuous and large-scale fabrication of fibers with aligned porous structure, mimicking polar bear hairs, which is difficult to achieve by other methods. A textile woven with such biomimetic fibers shows an excellent thermal insulation property as well as good breathability and wearability. In addition to passively insulating heat loss, the textile can also function as a wearable heater, when doped with electroheating materials such as carbon nanotubes, to induce fast thermal response and uniform electroheating while maintaining its soft and porous nature for comfortable wearing.
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Climate change is expected to increase the frequency and duration of long-distance swims by polar bears (Ursus maritimus). The energetic costs of such swims are assumed to be large, however, no estimates of metabolic costs of swimming for polar bears are available. Here, I use data on internal body temperature and external ambient temperature for two swimming polar bears, combined with mathematical modeling of heat production and of heat conduction to the surrounding water, to estimate the metabolic rate of swimming. Using this metabolic rate, I then examine the relative heat production and heat loss for bears of a range of sizes and body conditions. I calculated overall mean metabolic rate for a swimming bear to be 2.75 ml O2 g⁻¹ h⁻¹, which is generally higher than metabolic rates previously reported for walking polar bears. When compared at the same movement rate, the cost of transport for swimming was estimated to be approximately 5× that of walking. I further show that for small bears (less than approx. 145 cm body length or 90 kg) and bears in poor body condition, heat loss while swimming in cold Arctic waters should exceed heat production, and long swims should therefore not be thermodynamically sustainable. These results support previous claims that increasing frequency and duration of long-distance swims in polar bears is energetically stressful. Energetic and thermodynamic costs of long swims may be further exacerbated by recent declines in body condition that have been documented due to climate warming.
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Materials combining lightweight, robust mechanical performances, and multifunctionality are highly desirable for engineering applications. Graphene aerogels have emerged as attractive candidates. Despite recent progresses, the bottleneck remains how to simultaneously achieve both strength and resilience. While multiscale architecture designs may offer a possible route, the difficulty lies in the lack of design guidelines and how to experimentally achieve the necessary structure control over multiple length scales. The latter is even more challenging when manufacturing scalability is taken into account. The Thalia dealbata stem is a naturally porous material that is lightweight, strong, and resilient, owing to its architecture with three-dimensional (3D) interconnected lamellar layers. Inspired by such, we assemble graphene oxide (GO) sheets into a similar architecture using a bidirectional freezing technique. Subsequent freeze-drying and thermal reduction results in graphene aerogels with highly tunable 3D architectures, consequently an unusual combination of strength and resilience. With their additional electrical conductivity, these graphene aerogels are potentially useful for mechanically switchable electronics. Beyond such, our study establishes bidirectional freezing as a general method to achieve multiscale architectural control in a scalable manner that can be extended to many other material systems.
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Through designing hierarchical structures, particularly optimizing the chemical and architectural interactions at its inorganic/organic interface, nacre has achieved an excellent combination of contradictory mechanical properties such as strength and toughness, which is highly demanded yet difficult to achieve by most synthetic materials. Most techniques applied to develop nacre-mimetic composites have been focused on mimicking the 'brick-and-mortar' structure, but the interfacial architectural features, especially the asperities and mineral bridges of 'bricks', have been rarely concerned, which are of equal importance for enhancing mechanical properties of nacre. Here, we used a modified bidirectional freezing method followed by uniaxial pressing and chemical reduction to assemble a nacre-mimetic graphene/poly(vinyl alcohol) composite film, with both asperities and bridges introduced in addition to the lamellar layers to mimic the interfacial architectural interactions found in nacre. As such, we have developed a composite film which is not only strong (up to ~150.9 MPa), but also tough (up to ~8.50 MJ/m(3)), and highly stretchable (up to ~10.44%), difficult to obtain by other methods. This was all achieved by only interfacial architectural engineering within the traditional 'brick-and-mortar' structure, without introducing a third component or employing chemical cross-linker as in some other nacre-mimetic systems. More importantly, we believe that the design principles and processing strategies reported here can also be applied to other material systems to develop strong and stretchable materials.
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Bioinspired materials capable of driving liquid in a directional manner have wide potential applications in many chemical engineering processes, such as heat transfer, separation, microfluidics and so on. Numerous natural materials and systems such as spider silk, cactus, shorebirds, desert beetles, butterfly wing, and Nepenthes alata have been serving as a rich source of inspirations in the area. During the last decades, great efforts have been devoted to design bioinspired smart materials for directional liquid transport. In this review, we will start from introducing several natural materials and systems with surface structural features contributing for their directional liquid transport property, followed by the basic concepts and theories about surface wettability, droplet motion and driving forces with different structural features. Then, we will summarize some typical applications of such bioinspired smart materials in industrial process and chemical engineering, particularly in heat transfer, separation, microfluidic systems. In the end, future perspectives of such bioinspired smart materials for directional liquid transport will be discussed.
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Hairs of a polar bear are of superior properties such as the excellent thermal protection. The polar bears can perennially live in an extremely cold environment and can maintain body temperature at around 37 °C. Why do polar bears can resist such cold environment? Its membrane-pore structure plays an important role. In the previous work, we established a 1-D fractional heat conduction equation to reveal the hidden mechanism for the hairs. In this paper, we further discuss solutions and parameters of the equation established and analyze heat conduction in polar bear hairs.
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Motivated by diving semiaquatic mammals, we investigate the mechanism of dynamic air entrainment in hairy surfaces submerged in liquid. Hairy surfaces are cast out of polydimethylsiloxane elastomer and plunged into a fluid bath at different velocities. Experimentally, we find that the amount of air entrained is greater than what is expected for smooth surfaces. Theoretically, we show that the hairy surface can be considered as a porous medium and we describe the air entrainment via a competition between the hydrostatic forcing and the viscous resistance in the pores. A phase diagram that includes data from our experiments and biological data from diving semiaquatic mammals is included to place the model system in a biological context and predict the regime for which the animal is protected by a plastron of air.
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Unlike many other mammals spending a considerable amount of time in water, river otters (Lutra canadensis (Schreber, 1777)) do not have a thick layer of body fat. Instead, they have a very densely packed layer of thin underhairs. The structure of river otter hair was examined by scanning electron microscopy and polarizing light microscopy. Guard hairs were hollow and became thicker distally and then tapered to a point and had different cuticle scales in proximal and distal regions. The cuticle of the thin underhairs had a striking pattern of sharply sculpted fins with deep grooves between them; usually there were four fins at each level, rotated 45° with respect to those at an adjacent level. Underhairs varied in diameter and the scales were sometimes petal-shaped. Polarizing light microscopy images showed interlocking arrangements of the underhairs that help to impede the penetration of water. Also, these images showed that the grooves between fins or petals of underhairs entrap air bubbles. The structure of the hairs allows them to interact loosely with each other, despite variations in size and structure. Furthermore, the nature of the interactions between the fins and depressions allows space between the hairs that can trap air bubbles to increase the thermal insulation of the otter's coat.
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The purpose of this study was to compare the thermal resistance of a wetsuit fabricated from aerogel-syntactic foam hybrid insulation developed by Bardy et al [1] to a foam neoprene wetsuit. The thermal resistance of the hybrid wetsuit and a foam neoprene wetsuit was measured on a human test subject in water at 0.25 MPa (15.25 msw) of hyperbaric pressure. Measurements showed that although certain body regions of the hybrid wetsuit had a higher thermal resistance than foam neoprene, the overall thermal resistance of the hybrid wetsuit was 41% less than a foam neoprene wetsuit, and 51–88% less than predicted values. This was postulated, based on sample testing in water, to be due, in part, to increased heat flow through the hybrid insulation from water filled surface depressions at higher pressures. Other factors may have included water flow over the skin and the presence of thermal bridges in the insulation. Due to a smooth surface and tighter fit, the measured thermal resistance of the foam neoprene wetsuit was within 2–23% of the values predicted using data from Bardy et al [2]. It was concluded that unless the surface depressions can be eliminated, and alternative methods for a tighter fit achieved, foam neoprene provides more thermal protection.
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The purpose of this study was to present a new underwater thermal insulation designed for flexibility and high thermal resistance. The insulation was a hybrid composite of two constituents: syntactic foam and an insulating aerogel blanket. Methods for treating and combining the constituents into a hybrid insulation of several designs are presented. A final configuration was selected based on high thermal resistance and was tested for thermal resistance and compressive strain to a pressure of 1.2 MPa (107 msw, meters of sea water) for five continuous pressure cycles. The thermal resistance and compressive strain results were compared to foam neoprene and underwater pipeline insulation. It was found that the hybrid insulation has a thermal resistance significantly higher than both foam neoprene and underwater pipeline insulation at atmospheric and elevated hydrostatic pressures (1.2 MPa). The total thermal resistance of the hybrid insulation decreased 32% at 1.2 MPa and returned to its initial value upon decompression. It was concluded that the hybrid insulation, with modifications, could be used for wetsuit construction, shallow underwater pipeline insulation, or any underwater application where high thermal resistance, flexibility, and resistance to compression are desired.
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The purpose of this study was to show that the thermal properties of foam neoprene under hydrostatic pressure cannot be predicted by theoretical means, and that uni-axial pressure cannot simulate hydrostatic compression. The thermal conductivity and compressive strain of foam neoprene were measured under hydrostatic pressure. In parallel, uni-axial compressive strain data were collected. The experimental set-up and data were put into perspective with past published studies. It was shown that uni-axial compression yielded strains 20–25% greater than did hydrostatic compression. This suggests the need for direct hydrostatic pressure measurement. For comparison to hydrostatic experimental data, a series of thermal conductivity theories of two phase composites based on particulate phase geometry were utilized. Due to their dependence on the porosity and constituent thermal conductivities, a model to predict porosity under hydrostatic pressure was used and an empirical correlation was derived to calculate the thermal conductivity of pure neoprene rubber from experimental data. It was shown that, although some agreement between experimental data and thermal conductivity theories was present, no particular theory can be used because they all fail to model the complex structure of the pores. It was therefore concluded that an experimental programme, such as reported here, is necessary for direct measurement.
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Thermoregulating textiles (or protective clothing) are highly demanded for both human health and labor productivity especially in hot working environments. It is challenging to balance wearability and thermal insulating property as they usually demand for opposite porosity. Here, learning from the porous structure of polar bear hair, we report a polyimide aerogel fiber obtained by a freeze-spinning technique. A textile woven with such polyimide fiber is thermally insulating, strong and highly stretchable, fire-retardant (or self-extinguishing), and temperature-resistant. Additionally, it can be readily incorporated with other functions such as acid alkali resistance and thermoregulation by surface modification and infiltration of phase change material. All these properties indicate its potential in protective clothing in hot environments. With the versatility and scalability, our approach paves a new way for fabricating smart textiles with well-defined microstructure and multifunctionality by learning from nature.
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Their hollow, lightweight, and non-wettable hair helps polor bears resist their extremely cold environment. Inspired by the microstructure of polar bear hair, we fabricated the carbon tube aerogel (CTA), which shows all merits of polar bear hair, such as being lightweight, waterproof, and thermally insulating. Briefly, the CTA composed of hollow carbon tube fibers is lightweight with the lowest density of only 8 kg/m3, and is also waterproof with a contact angle of 146°. The lowest thermal conductivity of the CTA is only 23 mW m−1 K−1. Remarkably, the bioinspired CTA materials show better performance than the polar bear hair. The interconnected network structure of the CTA is endowed with super-elasticity and fatigue resistance, which is confirmed by the rebounding speed of 1,434 mm/s and structural integrity after more than 10,000,000 cyclic compressions at 30% strain. The bioinspired design of macroscopic synthesis of CTA opens a window for designing high-performance material.
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A cloth that adapts to the heat Textiles trap infrared radiation, which helps keep us warm in cold weather. Of course, in hot weather, this is less desirable. Zhang et al. constructed an infrared-adaptive textile composed of polymer fibers coated with carbon nanotubes. The yarn itself expanded and collapsed based on heat and humidity, which changed the spacing of the fibers. Wider fiber spacing allowed the textile to breathe but also altered the infrared emissivity of the textile. This allowed for better heat exchange under hot and wet conditions. The self-adjusting emissivity of the textile could help toward wearable thermal-management attire. Science , this issue p. 619
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Infrared (IR) stealth is essential not only in high technology and modern military but also in fundamental material science. However, effectively hiding targets and rendering them invisible to thermal infrared detectors have been great challenges in past decades. Herein, flexible, foldable, and robust Kevlar nanofiber aerogel (KNA) films with high porosity and specific surface area were fabricated first. The KNA films display excellent thermal insulation performance and can be employed to incorporate with phase-change materials (PCMs), such as polyethylene glycol, to fabricate KNA/PCM composite films. The KNA/PCM films with high thermal management capability and infrared emissivity comparable to that of various backgrounds demonstrate high performance in IR stealth in outdoor environments with solar illumination variations. To further realize hiding hot targets from IR detection, combined structures constituted of thermal insulation layers (KNA films) and ultralow IR transmittance layers (KNA/PCM) are proposed. A hot target covered with this combined structure becomes completely invisible in infrared images. Such KNA/PCM films and KNA-KNA/PCM combined structures hold great promise for broad applications in infrared thermal stealth.
Article
High-performance thermally insulating materials is highly desirable for many applications in which heat transfer should be strictly restricted. Traditional organic or inorganic insulation materials are limited by either poor thermal stability or mechanical brittleness. Here, SiO2 nanoparticles crosslinked polyimide aerogels synthesized by one-pot freeze-drying are presented, which show excellent mechanical properties and super-insulating behavior in a wide temperature range. The highly porous structure of the aerogel and nanosized components benefit the significant reduction of thermal conductivity through inhibiting gas conduction and imparting interfacial thermal resistance, respectively. The PI/SiO2 composite aerogel with a density of 0.07 mg cm⁻³ can achieve a low thermal conductivity of 21.8 mW m⁻¹ K⁻¹, which is lower than the most common super-insulating criterion (25 mW m⁻¹ K⁻¹). More importantly, the PI/SiO2 aerogel exhibits good thermal insulation performance at elevated temperatures, with a thermal conductivity still lower than 35 mW m⁻¹ K⁻¹ even at 300 °C. Therefore, the mechanically strong and super-insulating PI/SiO2 composite aerogels are promising candidates for practical thermal insulation applications.
Article
Wearable devices and systems demand multifunctional units with intelligent and integrative functions. Smart fibers with response to external stimuli, such as electrical, thermal, and photonic signals, etc., as well as offering energy storage/conversion are essential units for wearable electronics, but still remain great challenges. Herein, flexible, strong, and self‐cleaning graphene‐aerogel composite fibers, with tunable functions of thermal conversion and storage under multistimuli, are fabricated. The fibers made from porous graphene aerogel/organic phase‐change materials coated with hydrophobic fluorocarbon resin render a wide range of phase transition temperature and enthalpy (0–186 J g−1). The strong and compliant fibers are twisted into yarn and woven into fabrics, showing a self‐clean superhydrophobic surface and excellent multiple responsive properties to external stimuli (electron/photon/thermal) together with reversible energy storage and conversion. Such aerogel‐directed smart fibers promise for broad applications in the next‐generation of wearable systems. A variety of multiresponsive smart fibers with a wide range of tunable phase transition temperatures and enthalpy are produced through impregnation of different types of organic phase‐change materials into graphene aerogel fibers and finished by coating a fluorocarbon resin layer, showing a self‐clean superhydrophobic surface and excellent multiple‐responsive properties to external stimuli (electron/photon/thermal) together with reversible energy storage and conversion.
Article
Thermal management through personal heating and cooling is a strategy by which to expand indoor temperature setpoint range for large energy saving. We show that nanoporous polyethylene (nanoPE) is transparent to mid-infrared human body radiation but opaque to visible light because of the pore size distribution (50 to 1000 nanometers). We processed the material to develop a textile that promotes effective radiative cooling while still having sufficient air permeability, water-wicking rate, and mechanical strength for wearability. We developed a device to simulate skin temperature that shows temperatures 2.7° and 2.0°C lower when covered with nanoPE cloth and with processed nanoPE cloth, respectively, than when covered with cotton. Our processed nanoPE is an effective and scalable textile for personal thermal management.
Article
Solar thermal technology is a promising key strategy for future renewable energy production. Various concepts exist that use solar collectors and heat mirrors, built from rigid materials, to gather thermal energy from solar radiation. A new approach is the utilization of textile materials to build solar thermal collector systems with flexible material properties, lightweight design and improved material-efficiency. A solar collector, based on a multi-layer arrangement of technical textiles and foil membranes, has been realized by the ITV Denkendorf (Institute of Textile Technology and Process Engineering Denkendorf). The proposed collector system allows transparent insulation in textile-based buildings while gathering thermal energy simultaneously. The system is inspired by the transparent insulation and heat harvesting strategies of polar bear fur and can inform textile-based envelopes of future transparent buildings. In this study, different material arrangements and the influence of different parameters on the temperature distribution along the collector were tested. Air temperatures up to 150 °C (302 °F) could be generated inside the collector system. Furthermore, a closer look at the polar bear fur and other related principles in nature delivered additional concepts for energetic optimization.
Article
Inspired by the surface geometry and composition of lotus leaf with self-cleaning behavior, in this work, a composite TiO2@fabric was prepared via a facile strategy for preparing marigold flower-like hierarchical TiO2 particles through a one-pot hydrothermal reaction on cotton fabric surface. In addition, robust superhydrophobic TiO2@fabric were further constructed by fluoroalkylsilane modification as a versatile platform for self-cleaning, and oil-water separation. The results showed TiO2 particles were distributed uniformly on the fibre surface with a high coating density. In comparison with hydrophobic cotton fabric, the TiO2@fabric exhibited a highly superhydrophobic activity with a contact angle of ~160o and a sliding angle lower than 10o. The robust superhydrophobic fabric had high stability against repeated abrasion without apparent reduction of contact angle. The as-prepared composite TiO2@fabric demonstrated good anti-UV ability. Moreover, the composite fabric demonstrated high efficient oil-water separation due to its highly extreme wettability contrast (superhydrophobicity/superoleophilicity). We expect that this facile process can be readily and widely adopted for the design of multifunctional fabrics for excellent anti-UV, effective self-cleaning, efficient oil-water separation, and microfluidic management applications.
Article
High-performance thermally insulating materials from renewable resources are needed to improve the energy efficiency of buildings. Traditional fossil-fuel-derived insulation materials such as expanded polystyrene and polyurethane have thermal conductivities that are too high for retrofitting or for building new, surface-efficient passive houses. Tailored materials such as aerogels and vacuum insulating panels are fragile and susceptible to perforation. Here, we show that freeze-casting suspensions of cellulose nanofibres, graphene oxide and sepiolite nanorods produces super-insulating, fire-retardant and strong anisotropic foams that perform better than traditional polymer-based insulating materials. The foams are ultralight, show excellent combustion resistance and exhibit a thermal conductivity of 15 mW m(-1) K(-1), which is about half that of expanded polystyrene. At 30 °C and 85% relative humidity, the foams retained more than half of their initial strength. Our results show that nanoscale engineering is a promising strategy for producing foams with excellent properties using cellulose and other renewable nanosized fibrous materials.
Article
In the development of next-generation materials with enhanced thermal properties, biological systems in nature provide many examples that have exceptional structural designs and unparalleled performance in their thermal or nonthermal functions. Bioinspired engineering thus offers great promise in the synthesis and fabrication of thermal materials that are difficult to engineer through conventional approaches. In this review, recent progress in the emerging area of bioinspired advanced materials for thermal science and technology is summarized. State-of-the-art developments of bioinspired thermal-management materials, including materials for efficient thermal insulation and heat transfer, and bioinspired materials for thermal/infrared detection, are highlighted. The dynamic balance of bioinspiration and practical engineering, the correlation of inspiration approaches with the targeted applications, and the coexistence of molecule-based inspiration and structure-based inspiration are discussed in the overview of the development. The long-term outlook and short-term focus of this critical area of advanced materials engineering are also presented.
Article
The surprising properties of biomaterials are the results of billions of years of evolution. Generally, biomaterials are assembled under mild conditions with very limited supply of constituents available for living organism, and their amazing properties largely result from the sophisticated hierarchical structures. Following the biomimetic principles to prepare manmade materials has drawn great research interests in materials science and engineering. In this review, we summarize the recent progress in fabricating bioinspired materials with the emphasis on mimicking the structure from one to three dimensions. Selected examples are described with a focus on the relationship between the structural characters and the corresponding functions. For one-dimensional materials, spider fibers, polar bear hair, multichannel plant roots and so on have been involved. Natural structure color and color shifting surfaces, and the antifouling, antireflective coatings of biomaterials are chosen as the typical examples of the two-dimensional biomimicking. The outstanding protection performance, and the stimuli responsive and self-healing functions of biomaterials based on the sophisticated hierarchical bulk structures are the emphases of the three-dimensional mimicking. Finally, a summary and outlook are given.
Article
High-performance thermal insulating materials are desired especially from the viewpoint of saving energy for a sustainable society. Aerogel is the long-awaited material for extended applications due to its excellent thermal insulating ability. These materials are, however, seriously fragile against even small mechanical stress due to their low density, and their poor mechanical properties inhibit their practical use as superinsulators. In this paper, we report relationships between the thermal conductivity, pore size and mechanical properties of organic–inorganic hybrid polymethylsilsesquioxane (PMSQ) aerogels with improved mechanical properties and controllable pore sizes from 50 nm to 3 μm. The dependency of thermal conductivity on gas pressure and pore properties can be well explained by the thermal conduction theory of porous materials. These PMSQ aerogels show improved mechanical properties due to their elastic networks, which enable easier handling compared to conventional aerogels and facile production by simple ambient pressure drying. An aerogel-like “xerogel” monolithic panel has been successfully prepared via ambient pressure drying, which shows a low thermal conductivity (0.015 W m−1 K−1) comparable with those of the corresponding PMSQ aerogel and conventional silica aerogels. These results would open the gate for practical applications of these porous materials.
Article
Aerogel is a kind of synthetic porous material, in which the liquid component of the gel is replaced with a gas. Aerogel has specific acoustic properties and remarkably lower thermal conductivity (≈0.013 W/m K) than the other commercial insulating materials. It also has superior physical and chemical characteristics like the translucent structure. Therefore, it is considered as one of the most promising thermal insulating materials for building applications. Besides its applications in residential and industrial buildings, aerogel has a great deal of application areas such as spacecrafts, skyscrapers, automobiles, electronic devices, clothing etc. Although current cost of aerogel still remains higher compared to the conventional insulation materials, intensive efforts are made to reduce its manufacturing cost and hence enable it to become widespread all over the world. In this study, a comprehensive review on aerogel and its utilization in buildings are presented. Thermal insulation materials based on aerogel are illustrated with various applications. Economic analysis and future potential of aerogel are also considered in the study.
Article
This review is focused on describing the intimate link which exists between aerogels and thermal superinsulation. For long, this applied field has been considered as the most promising potential market for these nanomaterials. Today, there are several indicators suggesting that this old vision is likely to become reality in the near future. Based on recent developments in the field, we are confident that aerogels still offer the greatest potential for non-evacuated superinsulation systems and consequently must be considered as an amazing opportunity for sustainable development. The practical realization of such products however is time-consuming and a significant amount of R&D activities are still necessary to yield improved aerogel-based insulation products for mass markets.
Article
In order to obtain foams possessing flexibility and at the same time heat insulation under high hydrostatic pressure, composite foams with spherical rigid foams filled in flexible rubber foam at certain intervals were prepared and their thermal conductivity and flexural rigidity were studied. The following points were found: (1) With a unit model having a spherical rigid foam in the middle, the thermal conduction of a composite foam was analyzed under the conditions of steady one-dimensional heat flow. Theoretical equations giving overall coefficients of heat transmission under atmospheric and hydrostatic pressures were obtained, and the adequacy of these theoretical equations was confirmed by the measurement of overall coefficients of heat transmission of composite foams in an apparatus so constructed as to allow heat conduction experiments under pressures ranging from atmospheric to the hydrostatic pressure corresponding to 100-m depth in water. (2) The effect of the filled spherical rigid foams on heat insulation is notable under hydrostatic pressures corresponding to a 20-m depth or more in water. Under the hydrostatic pressure corresponding to a 100-m depth in water, the coefficient of heat insulation of the most closely filled composite foam used in the experiment was approximately 35% larger than that of the unfilled foam, while the theoretical most closely filled composite foam gives an approximately 110% increase. (3) Under the hydrostatic pressure corresponding to a 100-m depth in water, the flexural rigidity of the most closely filled composite foam used in the experiment was approximately one half that of an unfilled foam of the same heat insulating property.
Article
A novel superhydrophilic and underwater superoleophobic polyacrylamide (PAM) hydrogel-coated mesh is successfully fabricated in an oil/water/solid three-phase system. Compared to traditional oleophilic materials, the as-prepared hydrogel-coated meshes can selectively separate water from oil/water mixtures with the advantages of high efficiency, resistance to oil fouling, and easy recyclability.
Article
Silk fibroin, derived from Bombyx mori cocoons, is a widely used and studied protein polymer for biomaterial applications. Silk fibroin has remarkable mechanical properties when formed into different materials, demonstrates biocompatibility, has controllable degradation rates from hours to years and can be chemically modified to alter surface properties or to immobilize growth factors. A variety of aqueous or organic solvent-processing methods can be used to generate silk biomaterials for a range of applications. In this protocol, we include methods to extract silk from B. mori cocoons to fabricate hydrogels, tubes, sponges, composites, fibers, microspheres and thin films. These materials can be used directly as biomaterials for implants, as scaffolding in tissue engineering and in vitro disease models, as well as for drug delivery.
Article
A summary of existing passive solar-heat conversion panels provides the basis for a definition of an ideal passive solar-heat converter. Evidence for the existence of a biological greenhouse effect in certain homopolar homeothermic species is reviewed. The thermal and optical properties of homeothermic pelts, in particular those of the polar bear, are described, and a qualitative optical model of the polar bear pelt is proposed. The effectiveness of polar bear and seal pelts as solar-heat converters is discussed, and comparison is made with the ideal converter.
Article
Solar thermal collectors used at present consist of rigid and heavy materials, which are the reasons for their immobility. Based on the solar function of polar bear fur and skin, new collector systems are in development, which are flexible and mobile. The developed transparent heat insulation material consists of a spacer textile based on translucent polymer fibres coated with transparent silicone rubber. For incident light of the visible spectrum the system is translucent, but impermeable for ultraviolet radiation. Owing to its structure it shows a reduced heat loss by convection. Heat loss by the emission of long-wave radiation can be prevented by a suitable low-emission coating. Suitable treatment of the silicone surface protects it against soiling. In combination with further insulation materials and flow systems, complete flexible solar collector systems are in development.