Bioinspired design

We envisioned a personal thermal management technology, which would leverage the static infrared-reflecting configuration of the space blanket and draw inspiration from the dynamic color-changing mechanisms intrinsic to squid skin (Fig. 1e). We thus designed a composite material comprised of (1) a soft and stretchable infrared-transparent polymer matrix and (2) an overlaid array of infrared-reflecting metal domains stably anchored within the matrix via columnar nanostructures (Fig. 1e, inset and Fig. 1f, left). In our design, the polymer matrix emulates the chromatophore-containing transparent dermal layer of squid, while the metal domains emulate the embedded chromatophore organs themselves. Prior to any mechanical actuation, the nanostructure-anchored, infrared-reflecting metal domains are densely packed and completely cover the underlying matrix (Fig. 1f, left), in similar fashion to how expanded plate-like chromatophores are overlapped in squid skin (Fig. 1d, left). The composite material thus reflects nearly all incident infrared radiation (Fig. 1f, left), in analogy to the arrays of overlapped chromatophores reflecting visible light of specific wavelengths (Fig. 1d, left). However, upon mechanical actuation, the anchored metal domains become spread apart and uncover portions of the stretched underlying polymer matrix (Fig. 1f, right), in similar fashion to how contracted point-like chromatophores are separated in squid skin (Fig. 1d, right). The composite material thus transmits a significant fraction of the incident infrared radiation (Fig. 1f, right), in analogy to arrays of contracted chromatophores transmitting relatively more visible light (Fig. 1d, right). In essence, mechanical actuation (stretching) reversibly changes the surface microstructure of our composite material and thus dynamically alters its ability to transmit and reflect infrared radiation (e.g., heat). The general design simultaneously encompasses the desirable technical advantages of the space blanket (i.e., straightforward low-cost manufacturability and outstanding energy efficiency) and the unique natural characteristics of squid skin (i.e., a favorable form factor and on-demand controllability).

Composite fabrication

We began our experiments by fabricating the desired thermoregulatory composite material according to the scheme in Fig. 2a. Here, we prepared samples with areas of >~160 cm2 by using common laboratory techniques and methods that would ultimately allow for larger-area manufacturing on an industrial scale. In brief, we electron-beam evaporated an infrared-reflecting, nanostructured copper film onto a support substrate via a two-step oblique angle deposition process (Fig. 2a, left)33, obtaining arrayed tilted nanoscale columns that emerged from an underlying continuous metal (copper) coating, as confirmed by top–down scanning electron microscopy (SEM) (Fig. 2b and Supplementary Fig. 1). Next, we spincast an infrared-transparent styrene–ethylene–butylene–styrene (SEBS) copolymer34,35 directly onto the nanostructured metal layer (Fig. 2a, middle), stably anchoring the copper columns within this elastomer, as confirmed by cross-sectional SEM (Fig. 2c and Supplementary Fig. 1). Subsequently, we heat-treated and delaminated the resulting composite from the support substrate to obtain a free-standing material (Fig. 2a, right), which featured a fractured, multi-domain copper coating on one side, as confirmed by top–down SEM (Fig. 2d). Overall, the described robust, high-yield procedure furnished a uniform and relatively large-area composite analogous to the one envisioned in Fig. 1e, f.

Fig. 2 Preparation of the thermoregulatory composite material. a Schematic of the general fabrication procedure for the composite material. The steps consist of the electron-beam deposition of a nanostructured copper film onto a support substrate (left), the spincasting of a styrene–ethylene–butylene–styrene block copolymer directly onto this film (middle), and the delamination of the resulting composite from the substrate (right). b Digital camera image of a substrate-bound nanostructured copper film (top). The scale bar is 2 cm. A representative top–down scanning electron microscopy (SEM) image demonstrates that the film consists of arrayed tilted nanoscale columns that emerge from an underlying continuous copper coating (inset). The scale bar is 200 nm. c Digital camera image of a substrate-bound composite material (top). The scale bar is 2 cm. A representative cross-sectional SEM image demonstrates that the tilted copper nanostructures are anchored within the polymer (inset). The scale bar is 500 nm. d Digital camera image of a free-standing composite material in a tape-based holder (top). The scale bar is 2 cm. A representative top–down SEM image demonstrates that the overlaid copper coating is fractured and consists of multiple abutting domains (inset). The scale bar is 20 μm Full size image

Infrared and mechanical characterization

We initially characterized our composites’ mechanical properties via tensile testing, while also evaluating their surface microstructure at different applied strains. For our composites, the engineering stress vs. engineering strain curves revealed that they behaved like soft and stretchable elastomers, with an average elastic modulus of ~2 MPa and an elongation at break of ~700% (Supplementary Fig. 2). In their relaxed state, the composites were visibly opaque and highly reflective, as indicated by digital camera imaging, and their surfaces consisted of a dense arrangement of abutting, irregularly shaped metal domains, which completely covered the underlying polymer matrix, as confirmed by SEM and energy dispersive spectroscopy (EDS) (Fig. 3a, left and Fig. 3b). However, under a strain of 30%, the composites became partially transparent and less reflective, as indicated by digital camera imaging, and their surfaces consisted of a sparser arrangement of metal domains that were proximal to but not directly contacting one another, revealing some of the underlying strained polymer matrix, as confirmed by SEM and EDS (Fig. 3a, middle and Fig. 3c). Under a larger strain of 50%, the composites further increased their visual transparency and appeared even less reflective, as indicated by digital camera imaging, and their surfaces consisted of an arrangement of metal domains that were spread farther apart from each other, uncovering even more of the underlying strained polymer matrix, as confirmed by SEM and EDS (Fig. 3a, right and Fig. 3d). Importantly, the composites could be loaded/unloaded in <1 s, featured fully reversible changes in both their visible appearance and surface microstructure, and readily withstood repeated mechanical cycling. Interestingly, the composites demonstrated properties similar to those reported for various squid skin components, including elastic moduli (~0.5–3 MPa for the chromatophore-containing dermal layer)25, response times (<1 s for chromatophore organs)26, and capacities for extreme elongation (>14-fold diameter expansion for chromatophore pigment cells)27. Therefore, the composites exhibit several advantageous characteristics that are desired for thermal switching applications, including a rapid response time, a soft and stretchable form factor, and a straightforward actuation mechanism (Supplementary Table 2).

Fig. 3 Mechanical actuation of changes in surface microstructure and infrared properties for the composite material. a Schematic of the mechanical actuation of the composite with strains of 0% (left), 30% (middle), and 50% (right). The surface microstructure and the reflection and transmission of infrared radiation change as a function of the applied strain. b Digital camera image of a composite under a strain of 0% above an anteater cartoon (top). The scale bar is 1 cm. A top–down scanning electron microscopy (SEM) image and copper elemental map, where the metal is colored green, for the surface of a representative composite material under a strain of 0% (inset). The scale bars are 100 μm. c Digital camera image of a composite under a strain of 30% above an anteater cartoon (top). The scale bar is 1 cm. A top–down SEM image and copper elemental map, where the metal is colored green, for the surface of a representative composite material under a strain of 30% (inset). The scale bars are 100 μm. d Digital camera image of a composite under a strain of 50% above an anteater cartoon (top). The scale bar is 1 cm. A top–down SEM image and copper elemental map, where the metal is colored green, for the surface of a representative composite material under a strain of 50% (inset). The scale bars are 100 μm. e The total infrared reflectance spectra for a representative composite material under strains of 0% (black trace), 30% (red trace), and 50% (blue trace). The reflectance observed at 0% strain is recovered even after successive actuation with higher strains (pink dotted trace). f The total infrared transmittance spectra for a representative composite material under strains of 0% (black trace), 30% (red trace), and 50% (blue trace). The transmittance observed at 0% strain is recovered even after successive actuation with higher strains (pink dotted trace). g Plot of the decrease in the average total reflectance for representative composites as a function of the applied strain. All error bars represent the standard deviation. h Plot of the increase in the average total transmittance for representative composites as a function of the applied strain. All error bars represent the standard deviation Full size image

We next showed that the composites’ infrared reflectance and transmittance could be dynamically modulated on demand by an applied mechanical stimulus (a uniaxial strain). In its relaxed state, a representative composite featured a high average total reflectance of ~100% (Fig. 3e) and a low average total transmittance of ~1%, as well as no apparent signals associated with specific functional groups (Fig. 3f), due to complete coverage of the infrared-transparent polymer matrix by the infrared-reflecting copper coating (Fig. 3b). However, under a strain of 30%, the representative composite featured a reduced average total reflectance of ~77% (Fig. 3e) and an increased average total transmittance of ~16%, as well as signals corresponding to the chemical functionalities of SEBS (e.g., –C=C–, –CH 2 –, and =C–H) (Fig. 3f), confirming that the underlying polymer matrix was partially uncovered (Fig. 3c)36,37. Under an increased strain of 50%, the representative composite featured an even smaller average total reflectance of ~67% (Fig. 3e) and an even larger average total transmittance of ~25%, as well as increased intensities for the SEBS-associated spectroscopic signals (Fig. 3f), due to additional exposure of the underlying polymer matrix (Fig. 3d). In general, the composites’ reflectance and transmittance both demonstrated a non-linear dependence on the strain, with the reflectance decreasing by ~25 ± 1%, ~35 ± 1%, and ~47 ± 2% at strains of 30%, 50%, and 100%, respectively (Fig. 3g), and the transmittance increasing by ~15 ± 1%, ~24 ± 2%, and ~39 ± 3% at strains of 30%, 50%, and 100%, respectively (Fig. 3h) (all relative to the initial unstretched state). Notably, the composites displayed fully reversible changes in their infrared reflectance and transmittance (Fig. 3e, f) and also exhibited excellent stability, with no degradation in their functionality after >~103 actuation cycles (Supplementary Fig. 3). Moreover, based on their transmittance at strains of 0% and 100%, our mechanically actuated materials possess maximum transmittance on/off ratios of >~25, which exceed the best values reported for radiative thermal switches (Supplementary Table 2). Thus, the composites showcase several additional characteristics desired for thermal switching applications, including reversibility, stability, and high on/off ratios (Supplementary Table 2).

Thermal modeling and evaluation

Having confirmed our composites’ tunable infrared properties, we evaluated their promise for adaptive thermal management within the context of a wearable configuration. To this end, we leveraged our spectroscopic measurements and computationally modeled heat transfer between human skin, the composite material, and the surrounding environment in various states of actuation (for comparison, we performed analogous experiments and calculations for several common types of cloth) (Fig. 4a, Supplementary Note 1, Supplementary Fig. 4, and Supplementary Table 3)38,39. We also determined the environmental setpoint temperatures, i.e., ones at which the body’s skin temperature and outgoing heat flux remain constant, that would ensure individuals maintained their thermal comfort while wearing the unstrained and strained composites (or types of cloth). In their relaxed state, the composites featured a setpoint temperature of ~14.5 °C, which was similar to the value of ~14.3 °C found for the space blanket (Fig. 4b). However, under a moderate strain of 30%, the composites featured a setpoint temperature of ~19.2 °C, which was similar to the value of ~18.9 °C found for a Columbia Omniheat fleece lining (Fig. 4b). In addition, under an increased strain of 50%, the composites featured a setpoint temperature of ~20.9 °C, which was similar to the value of ~20.5 °C found for wool (Fig. 4b). Moreover, under an even greater strain of 100%, the composites featured a setpoint temperature of ~22.7 °C, which was similar to the value of ~22.8 °C found for cotton (Fig. 4b). Taken together, our infrared spectroscopy experiments and subsequent calculations confirmed that the composites could be rapidly actuated with strain to maintain a wearer’s thermal comfort across an environmental setpoint temperature window of ~8.2 °C, which is among the largest dynamic ranges reported for any comparable passive material (Fig. 4b). In essence, the composites possess unprecedented switching capabilities, wherein their infrared reflectance and transmittance can be modulated in real time to emulate various types of cloth (note that common fabrics would typically exhibit small and/or irreversible changes in their infrared properties upon the application of strain). Assuming broad implementation in advanced garments that are widely deployed and adopted, our composites’ significant dynamic temperature setpoint window of >8 °C could enable an estimated reduction in building energy consumption of >30%9,40,41.

Fig. 4 Mechanical control over the setpoint temperature and thermoregulatory properties for the composite material. a Schematic of the heat flux from human skin, through the composite material, and to a variable-temperature environment without (left) and with (right) mechanical actuation. The heat flux from the skin to the surroundings increases in order to maintain the skin temperature at a constant value upon going from a cooler (left) to a warmer (right) environment. b Plot of the environmental setpoint temperatures at which an individual would remain comfortable, i.e., maintain a constant skin temperature and unchanged outgoing heat flux, while wearing either various common types of cloth (black diamonds) or the composite material at different applied strains (blue and red triangles). The calculated setpoint temperature associated with the composite can be dynamically adjusted via mechanical actuation, and the composites’ accessible setpoint temperature range is indicated by the area shaded in red and blue. c Schematic of the heat flux from a sweating-guarded hot plate, through a composite, and to a controlled environment without (left) and with (right) mechanical actuation. d Plot of the steady-state heat flux from the hot plate as a function of time for a representative composite material under strains of 0% (black dots), 30% (red dots), and 50% (blue dots) Full size image

We next proceeded to validate our composites’ ability to adaptively manage heat transfer from a surface that emulated the thermal behavior of human skin. To this end, we mounted the composites in a custom-designed holder that allowed for the application of strain on a sweating guarded hot plate and then tested their thermoregulatory functionality under controlled conditions without and with mechanical actuation (Fig. 4c and Supplementary Fig. 5). In its relaxed state, a representative composite maintained the steady state heat flux from the hot plate at an average value of ~248 W m−2 (Fig. 4d). Under a moderate strain of 30%, the same composite maintained the steady state heat flux from the hot plate at a higher average value of ~291 W m−2 (Fig. 4d). Under a further increased strain of 50%, the composite maintained the steady state heat flux from the hot plate at an even higher average value of ~307 W m−2 (Fig. 4d). In general, the heat flux could be stably, reliably, and repeatedly modulated by average values of ~36 ± 8 and ~51 ± 8 W m−2 upon actuation with applied strains of 30% and 50%, respectively (with a maximum change of ~59 W m−2). Excitingly, based on these measurements, the composites can manage up to one quarter of the metabolic heat generation from the human body, which is expected to be ~70 W m−2 for a sedentary individual (and potentially much higher depending on the type of activity)38,42. In addition, the composites need an estimated input of only ~3 W m−2 for one mechanical switching event (at moderate strain) and do not continuously consume power, making their energy requirements substantially lower than all known active thermal management platforms (Supplementary Note 2 and Supplementary Table 1). Consequently, the composites demonstrate the key advantages of both passive and active systems by exhibiting excellent energy efficiency while simultaneously allowing for real-time user control.

Human subject testing

Last, we assessed the composites’ ability to function as the dynamic switchable components in wearable systems and, more specifically, to locally manage the temperature of the human body. For this purpose, we fabricated custom-designed composite-based sleeves, which allowed for mechanical actuation of the joined composites via a straightforward fastener assembly (for comparative purposes, we prepared analogous space blanket-based sleeves) (Fig. 5a and Supplementary Fig. 6). Subsequently, we outfitted a human subject with these sleeves and then recorded the outgoing heat flux and apparent change in temperature for both the sleeve-covered and bare forearms of the wearer, while actuating the composites with different strains (note that the forearms served as internal standards that facilitate comparisons across all measurements) (Fig. 5b–f and Supplementary Fig. 7). In initial benchmark experiments, a static space blanket-based sleeve trapped (primarily reflected back) the heat emitted by the covered forearm (Fig. 5b) and thus raised its temperature by ~1.0 ± 0.1 °C more than the bare forearm of the same person (Fig. 5g and Supplementary Fig. 7). Similarly, without applied strain, the composite-based sleeve trapped the heat emitted by the covered forearm (Fig. 5c) and raised its temperature by ~0.9 ± 0.1 °C more than the same person’s bare forearm (Fig. 5g and Supplementary Fig. 7), much like the space blanket. However, for an applied strain of 30%, the composite-based sleeve trapped only some of the heat emitted by the covered forearm (Fig. 5e) and raised its temperature by only ~0.3 ± 0.1 °C more than the same person’s bare forearm (Fig. 5g and Supplementary Fig. 7), which constituted a >2-fold reduction in the temperature change measured for the space blanket. Furthermore, for an applied strain of 50%, the composite-based sleeve trapped substantially less of the heat emitted from the covered forearm (Fig. 5f) and raised its temperature by only ~0.1 ± 0.1 °C more than the same person’s bare forearm (Fig. 5g and Supplementary Fig. 7), which represented a ~10-fold reduction in the temperature change measured for the space blanket. Overall, the measurements demonstrated that the composite-based sleeves could be adjusted to manage the heat flux from the wearer to the surrounding environment in real time (Fig. 5b–f and Supplementary Movie 1) and to modulate the wearer’s local changes in body temperature by nearly an order of magnitude with excellent control (Fig. 5g). Importantly, the maximum measured temperature changes were ~2–5-fold greater than the reported temperature perception thresholds for a variety of human subjects, allowing users of the composite-based sleeves to readily perceive the thermal effect of the different actuation states43. In principle, the performance of the sleeves and comfort of the wearer could be improved further through the use of composites from comparable infrared-transparent polymer matrices with enhanced breathabilities. Overall, these exciting preliminary findings lay the groundwork for the development of sophisticated composite material-based garments, which can be site-specifically actuated via more advanced strategies and thus allow for regulation of the local thermal environment across a wearer’s entire body.