Multilayered optofluidics for sustainable buildings

Indoor climate control is among the most energy-intensive activities conducted by humans. A building facade that can achieve versatile climate control directly, through independent and multifunctional optical reconfigurations, could significantly reduce this energy footprint, and its development represents a pertinent unmet challenge toward global sustainability. Drawing from optically adaptive multilayer skins within biological organisms, we report a multilayered millifluidic interface for achieving a comprehensive suite of independent optical responses in buildings. We digitally control the flow of aqueous solutions within confined milliscale channels, demonstrating independent command over total transmitted light intensity (95% modulation between 250 and 2,500 nm), near-infrared-selective absorption (70% modulation between 740 and 2,500 nm), and dispersion (scattering). This combinatorial optical tunability enables configurable optimization of the amount, wavelength, and position of transmitted solar radiation within buildings over time, resulting in annual modeled energy reductions of more than 43% over existing technologies. Our scalable “optofluidic” platform, leveraging a versatile range of aqueous chemistries, may represent a general solution for the climate control of buildings.

Buildings are the costliest energy sinks on the planet (1, 2). For their daily operation, which largely entails trying to heat, cool, and light the indoor environment as exterior conditions change, buildings require 32% (32.4 PWh) of the energy and 50% of the electricity consumed globally (2), corresponding to about 25% (9.18 GtCO₂) of our greenhouse gas emissions (1).Moreover, the emissions associated with buildings may double or triple by mid-century with increased urbanization (1). Global air conditioning demand is set to triple by 2050 (3). Heating and cooling energy use is expected to grow by 79% and 84% in the same timeframe (1). In addition, electricity-based emissions from residential and commercial buildings have already quintupled and quadruped, respectively, in the last four decades (1).Underpinning this alarming and growing footprint is a fundamental unmet challenge in building design: existing facades cannot achieve selective, reconfigurable responses to their solar environment; no window, sunshade, or chromogenic technology is able to independently tune the amount (intensity), wavelength (spectrum), and dispersion (scattering) of incident sunlight as solar conditions change (a comprehensive review of available technologies and their optical properties is provided in SI Appendix, Table S1).Static windows, with or without permanent reflective coatings, for instance, cannot dynamically modulate solar intensity, spectrum, nor scattering (4–6). Manual or automated venetian blinds, on the other hand, can partially control intensity and scattering (with changing slat angle), but not independently (7). Adjustable window shades, which can be bent, rotated, or otherwise translated under mechanical (6, 8–12), electrical (13), hygroscopic (14, 15), or thermal (16) stimuli, can usually tune only total intensity (17), while more experimental chromogenic windows, including reorientable liquid crystal (18–20), suspended particle (21–23), as well as electro- (24–34), photo- (26, 28, 29, 31, 35), and thermochromic (26, 28, 29, 31, 35–37) devices, can generally only regulate total sunlight intensity or spectrum (32–34, 38).At the building interface, independent, synergistic control over transmitted light intensity, spectrum, and scattering is necessary to achieve optimized and climate-responsive functions (4, 6, 31, 39). Control of total light intensity would enable modulation of solar heat gain and illumination; control of spectrum and, in particular, switchable transmission of near-infrared-selective (NIR) sunlight would decouple infrared heating from visible daylighting (38, 40); while control of scattering would allow for the spatial tuning of transmitted photons within a room. In the current absence of a building material with this combinatorial functionality, interior heating, cooling, and lighting systems must bear the brunt of temperature and illumination control, compensating entirely for exterior environmental fluctuations and interior occupant changes, and by themselves consume more than 25% of the energy used in the developed world (4, 6, 31, 39, 41).To alter this energy-intensive paradigm (41), buildings would benefit from independent, switchable control over total transmittance, NIR-selective absorption, and scattering by the outer facade. Developing this scalable, combinatorial optical platform, with the ability to separately tune each of these three properties, might be considered the “holy grail” for building material design (42).Multilayered Optical Mechanisms in Biology.As a possible source of inspiration, certain biological organisms have evolved multilayered mechanisms within their skin to tune independent optical properties at their interface. In a few species of squid (e.g., Loligo plei), for instance, active camouflage is achieved through the independent and cooperative action of a pigmentary layer of chromatophore organs and a structural layer of protein cells (43–45), mediating surface color, spectral reflectance, and spatial patterning (46–50) (Fig. 1B). Large shifts in spectral reflection peaks occur along surface regions where both pigmentary and structural layers are overlaid (SI Appendix, Fig. S3 C–J), enabling combinatorial, additive, optical responses. The panther chameleon has also evolved a multilayered infrastructure within its skin, leveraging a two-tiered system of photonic crystals, each with an independent morphology and function (51) (Fig. 1A). Color change is regulated through the uppermost photonic layer, as chameleons actively manipulate the periodicity of guanine nanocrystals to selectively reflect light. Thermoregulation, on the other hand, is achieved through the lowermost photonic layer, where populations of regularly arranged iridophore cells strongly reflect radiation in the NIR region.Open in a separate windowFig. 1.Biological inspiration for fluidic multilayer. (A) Color change in the panther chameleon, achieved using a multilayer architecture of active photonic crystals. (B) Color change in the squid, achieved using coordinated actuations within a multilayer of pigmentary and structural elements. The top images are of Sepioteuthis lessoniana, whereas the bottom images are of Loligo pealeii. (C and D) Schematic for achieving independent multilayered switchable responses in building facades, where switchable fluid flow within distinct layers can enable multiple distinct optical functions. (E) Schematic exemplifying several functional or multifunctional states, achieved through coordinated fluid injections within a bilayer. The fluid multilayer acts as an additive light filter for incoming light. Scale bars: (A) white, 20 μm; black, 200 nm; (B) 1 mm; (D) 1 cm. Images in (A) reproduced from ref. 51, published under a Creative Commons license (http://creativecommons.org/licenses/by/4.0/). Images in (B) reproduced with permission from ref. 48, and under license from refs. 52 and 53.Independently tunable and multifunctional multilayer interfaces enable combinatorial physiological responses in organisms. We hypothesize that building-scale analogues of these multilayered, multifunctional biological systems might be capable of similarly dynamic optical behaviors (SI Appendix, Fig. S3). To achieve a scalable and sustainable material platform with general optical tunability, we propose moving away from traditional solid-state approaches and instead suggest the integration of confined aqueous fluids: a class of replenishable, recyclable, nontoxic materials with remarkable spatial configurability (54–57) and broadly tailorable optical programmability (58, 59). Conventional microfluidic devices are typically applied to chemical (60, 61), diagnostic (62, 63), and computational (64, 65) “lab on a chip” applications, but few examples take advantage of the optical characteristics of confined fluids, and over large planar dimensions. We suggest that a system of stacked channeled layers capable of confining a selectively designed collection of fluids can achieve versatile, multifunctional, and combinatorial optical functionality.Here, we demonstrate a large-area “optofluidic” platform for buildings, capable of achieving independent and combinatorial control of total light transmission, spectrally selective light absorption, and spatially directable light dispersion through coordinated digital fluid flows therein (Fig. 1 C–E). In simulation, we show that independent control over three sequential aqueous fluid layers within a building facade—to regulate optimal degrees of total light transmission, NIR light transmission, and visible light scattering in response to fluctuating solar conditions—can accomplish savings of 75% on heating energy, 20% on electric lighting energy, and 43% on total operational energy, compared with the best available electrochromic technology. These results point toward an exciting design paradigm for buildings, where confined, switchable fluid layers within a facade can behave in concert as functionally programmable optical and solar filters. Dynamic control of this versatile fluidic platform could significantly improve the way we build, operate, and interact with buildings.