Biomimetic system design and material selection. Traditionally, textiles have been made from a limited material set. Natural or synthetic fibers are spun and woven into thick fabrics that inhibit heat diffusion between the body and the environment. Perhaps due to the lack of suitable materials, traditional textile design has largely overlooked the management of radiative heat (i.e. energy carried by light). Recently, this has begun to change with, for example, the development of radiative heating textiles enabled by reflective metallic coatings(17) and radiative cooling textiles enabled by IR transparent polyethylene fibers(24). A performance gap still exists for more efficient personal heating in a comfortable, familiar textile format. One way forward is to not only limit dissipation of radiant body heat outward, but to also optimize absorption of ambient radiant energy inward. The power density of sunlight, for example, is sufficient (100–1000 W/m2) to augment typical metabolic heat production (~ 70–120 W/m2). It is therefore unsurprising to see both directions of radiative energy management in adaptations of polar animals.
Two natural structures inspire the design of our solar thermal textile. The first is a type of structural coloration in some species of cold-adapted moths and butterflies that enables selective absorption of visible-NIR light and suppression of thermal (IR) emission (Fig. 1a).(3, 6) In this case, melanin is the optically active material. With an electron-dense pi-pi stacked structure, melanin behaves as a disordered organic semiconductor and has a significantly higher refractive index than most biopolymers.(4, 5) A variety of donor-acceptor units enable broadband light absorption, while a high optical density enables light interference effects (IR reflection) in melanin-enriched wing coatings. The second source of inspiration is the pelt of the polar bear which, key to the animal’s survival in the extreme cold, simultaneously provides high thermal resistance (R) and high solar utilization (Fig. 1b). The dual function arises from a specific material set and optical structure. Lacking melanin pigment, hollow hair fibers have a low optical density and efficiently forward-scatter light, permitting photothermal capture at the melanized skin while inhibiting heat loss to the environment.(2, 9, 11, 25, 26) The essential optical structure of the pelt - a transmitter-insulator (transparent hair) stacked above an absorber (darkened skin) - efficiently harvests solar thermal energy to support thermoregulation in the extreme cold. While several groups have reported personal heating materials nominally mimicking polar bear fur, none have demonstrated this crucial optical structure nor a transmitting insulator fabric.(27–29) More accurate polar bear inspired textiles and structures have been reported for building-level solar collectors,(25, 30, 31) but here we analyze the energetic significance of direct radiative capture at the body-environment interface and show the high thermoregulation efficiency of a wearable solar thermal textile.
To mimic such adaptations, we engineered a bilayer textile using optical polymer materials (Fig. 1c). The bottom layer is a nylon fabric robustly vapor coated with the conjugated polymer PEDOT, enabling selective absorption of visible light and suppression of IR emission. In structure and function, the soft material PEDOT bears resemblance to melanin. This pi-pi stacked electronic polymer interacts strongly with light due to high free-electron density associated with long conjugated chains.(32) While PEDOT is an organic conductor, it shares the same broadband vis-NIR light absorption of melanin due to plasmon (surface electron) resonance near 1000 nm with a high optical density κ.(22) At longer wavelengths into the IR, PEDOT is a reflector (a weak emitter). Like the melanin-enriched microstructure of moth wings, a PEDOT coating can therefore be used to efficiently manage radiative heat transfer between the body and the environment. Photothermal heat generated at the PEDOT-nylon absorber is further trapped by the top layer, a semi-transparent fabric mimicking the light colored fur of polar animals. This lightweight fabric (Agribon AG-19)(33) is made of low optical density polypropylene fibers that forward-scatter visible light with about 85% transmission and more weakly transmit IR light with about 60% transmission (Figure S1). By confining solar thermal heat as both a diffusion and IR radiation barrier, AG-19 essentially acts like a breathable greenhouse material. Indeed, it is used in the agricultural industry for this purpose. Individual fibers are visualized in an optical transmission micrograph in Fig. 1e. Fiber diameter is in the range of 9–10 microns, comparable to IR wavelengths and thus capable of Mie scattering to trap IR radiation.(34) As with polyethylene and other polyolefins, polypropylene is an attractive material for textiles for its potential sustainability, durability, and ultra-light weight.(24) Along with the spectrally selective PEDOT-nylon fabric, the fabric transmitter layer operates differently than traditional textiles and has the potential to provide highly efficient thermoregulation.
Optical and thermoregulation characterization. A one-micron thick coating of PEDOT onto nylon fabric dramatically changes the surface optical properties due to the high optical density of PEDOT. Figure 2a visualizes this change in photographs and thermal images. Under a commercial lightbulb, the coated fabric absorbs more photothermal energy relative to the reflective uncoated nylon. Over an IR source (human body), the coated fabric emits less thermal energy (appears colder) than the high emissivity surface of the uncoated nylon. These effects are quantified in Fig. 2b, where emissivity of the coated fabric (blue) and uncoated (black) is plotted against wavelength. The uncoated nylon fabric shows a behavior typical of traditional textiles: low emissivity (absorbance) in the visible and high emissivity in the IR. With a PEDOT coating, this optical behavior is reversed, and the fabric behaves more like an ideal photothermal absorber.
With the ability to control the optical properties of the fabric, we next evaluate the thermoregulating performance of a set of traditional and unconventional fabrics. A simple way to conceptualize this task is by considering the lowest temperature rating for which a certain textile can maintain the wearer’s thermal comfort. Warmer textiles are needed for colder environments (Fig. 3a). More specifically, a textile should limit the loss of body heat (~ 75 W/m2 for an average adult at rest) to the environment so that a comfortable skin temperature (33oC) is maintained at steady state. Excluding factors like wind chill and humidity, this situation is simulated in a chamber depicted in Fig. 3b. For a given textile sample, the temperature of a skin heater with a constant output is monitored by a controller that lowers the environmental temperature until thermal comfort (Tskin = 33oC) is reached at steady state. A similar chamber design was previously used to evaluate passive heating solutions,(17) except here a window allows for environmental light (radiative energy) input to the textile-skin system.
Uncoated and PEDOT-coated nylon were tested, as well as a range of traditional and non-traditional textile comparators of varying weights, as shown in Fig. 3c. A cotton jersey sample represents typical T-shirt material, while cotton terry is typical of heavier, warmer garments like a sweatshirt. The other optically-active textiles include commercial Omni-Heat and spun-bonded polypropylene (AG-19). The Omni-Heat fabric is tested with the reflective face up (R up) and down (R down). The AG-19 is held by a 5 mm thick plastic frame for consistency across measurements. The low temperature ratings of the textiles were measured in the environmental chamber and results for dark conditions are shown in Fig. 3d. The low emissivity fabrics – PEDOT-nylon and Omni-Heat (R up) – perform similarly to the cotton jersey fabric as has been previously demonstrated.(17) The thicker cotton terry and AG-19 fabrics offer more insulation and have lower environmental temperature ratings in the range of 18oC.
Under moderate illumination of 130 W/m2, the performance of the textile samples varies more widely. While the PEDOT-nylon and Omni-Heat (R-up) fabrics perform similarly in dark conditions due to comparable thermal emissivity values, the PEDOT-nylon has a greater visible absorbance and so performs significantly better (9.6oC versus 11.5oC) under illumination. Compared to PEDOT fabrics, other metal-coated textiles may show poor solar thermal heating due to the higher resonant frequencies (in the UV-visible range) of commonly used metals like aluminum and silver.(35) Individually, AG-19 also performs well (8.2oC) but when stacked in the absorber-transmitter structure, the complementary optical functions of the PEDOT-nylon and AG-19 yield a more dramatic heating effect. The bilayer textile has a temperature rating that extends 10oC lower than the cotton jersey fabric (4.2oC/~40oF versus 14.1oC/~60oF) while weighing 30% less. With an additional layer of AG-19, the performance improves more modestly (extending to 2.9oC), suggesting that the sacrifice of solar utilization (light transmission) for thermal insulation becomes less favorable to thermoregulation.
Taking the temperature rating of the cotton jersey fabric as a baseline for the other measurements allows comparison of textile performance in both light and dark conditions (Fig. 3f). While the thick, insulating cotton terry fabric shows good performance in dark conditions (+ 1.6oC relative to cotton jersey), it has relatively weak performance in light conditions (+ 1.3oC). This is representative of traditional textiles – thick, opaque insulation that limits heat dissipation outward also necessarily limits photothermal heat transfer inward. On the other hand, the bilayer textile excels in both dark (+ 2.7oC) and light (+ 9.9oC) conditions due to the insulating yet light-transmitting AG-19 layer.
Other wearable photothermal mimics comprise outward-facing darkly colored surfaces,(27–29) which may explain the reduced solar heating effect (relative to cotton T-shirt, + 5.5oC versus + 9.9oC) of one such report.(Figure S1) We confirm the importance of optical structure by reversing our absorber-transmitter stack, i.e. facing the dark colored PEDOT surface outward, which significantly reduces relative performance (+ 7.1oC versus + 9.9oC).(Figure S2) This is in agreement with counterintuitive observations that darker colored pelt features, due to inhibited light transmission, may achieve less solar utilization than light colored pelts. (9–11)
Modeling textile thermoregulation under illumination. While previous work in radiative heating textiles has studied the impact of IR surface properties, here we study the combined impact of IR and visible optical properties on personal heating under moderate light intensity. As a simple case, we focus on an absorber-only system which, due to the broad IR and visible transparency of the transmitter layer (Figure S3), may also inform the bilayer textile design. (Figure S4) Our steady-state heat transfer model (Supplementary Note 1) of the skin-textile system includes incident radiation (130 W/m2) and a natural convection coefficient that varies with the skin-environment temperature difference (Fig. 4a). The results of the simulation are shown in Fig. 4b and experimental data points of uncoated and PEDOT coated nylon are overlaid. The performance of the radiative heated textile is optimized at maximum solar absorbance and minimum thermal emissivity, and we note that the PEDOT coating brings the nylon fabric closer to this target. At 130 W/m2, the dependence of environmental temperature on solar absorbance is roughly comparable to that of thermal emissivity; however, at greater radiance (325 W/m2), solar absorbance becomes the dominant contributor. (Figure S5).
To understand realistic sunlight utilization, the human body is approximated as a vertical cylinder(36) and normal incident radiance per total surface area (typically 1.8 m2 for adults) is calculated across the year assuming half of the body is illuminated. For solar-powered personal heating, a convenient coincidence is that the coldest season may correspond with the maximum solar radiance on a vertical body due to the reduced solar elevation. In Boston, MA, for example, the calculated on-body radiance of direct sunlight increases from about 108 W/m2 at the summer solstice to about 325 W/m2 at the winter solstice. The presence of snow fields can also dramatically increase overall solar insolation values, as is the case in polar environments, which may further double the incident solar contribution.(8) With a combined body-surface irradiance of 650 W/m2, the available wintertime solar thermal power at high latitudes may therefore be up to 10 times larger than the body heat generated by a moderately active adult (70–120 W/m2).(37) The bulky, opaque nature of winter outerwear assures low utilization of this power source, as the weak solar heating of the thick cotton terry fabric demonstrated above suggests. On the other hand, the bilayer textile described here harnesses sunlight to provide remarkable heating for its weight. Exposed to the calculated wintertime radiance of 650 W/m2, the textile supports thermal homeostasis as low as -28oC (Figure S6), approaching the temperature extremes of the polar environment. Indoors, a light capturing textile can support the development of passive solar architectures(38) as personal heating and design elements, as well as be powered by existing indoor light fixtures capable of the lower radiance levels modeled here.(39)
Wearability characterization. Everyday clothing is expected to be comfortable, breathable, and washable. The optically-active textiles presented herein were also evaluated for these functions. Despite being nonwoven, the AG-19 transmitter fabric has many familiar textile qualities that make it suitable for garments and apparel, upholstery, and décor. The nonwoven material has a similar drape to traditional woven textiles (Fig. 5a) and may be sewn and ironed without damage. Advancing from the frame-supported textile presented earlier, we next demonstrate the use of such materials in a self-supported garment made by sewing two layers of AG-19 and ironing pleats to form insulating baffles. PEDOT-nylon is then sewn to the bottom to complete the solar thermal textile (Fig. 5b). This textile performs similarly to the previously characterized bilayer structure and, importantly, is stable after three washing cycles using common laundry detergent and after two total hours of light exposure (130 W/m2)(Fig. 5c). Optical microscopy of the PEDOT coating across washings shows a small change in color and no damage to the mechanical stability of the films (Figure S7). A water vapor transmission test across an AG-19/PEDOT-nylon stack reveals that this bilayer is as breathable as other common fabrics used in the study (Fig. 5d). This is unsurprising given the diffuse open mesh of the AG-19 fabric, designed to be breathable, and the hydrophilic nature of PEDOT.(23)