Thermochromic Nanocellulose Films for Temperature-Adaptive Passive Cooling.

Energy efficiency in habitation spaces is a pivotal topic for maintaining energy sufficiency, cutting climate impact, and facilitating economic savings; thus, there is a critical need for solutions aimed at tackling this problem. One viable approach involves complementing active cooling methods with powerless or passive cooling ones. Moreover, considerable scope remains for the development of passive radiative cooling solutions based on sustainable materials. Cellulose, characterized by its abundance, renewability, and biodegradability, emerges as a promising material for this purpose due to its notable radiative cooling potential exploiting the mid-infrared (MIR) atmospheric transmission window (8-13 µm). In this work, we propose the utilization of thermochromic (TC) materials in conjunction with cellulose nanofibrils (CNF) to confer temperature-dependent adaptivity to hybrid CNF films. We employ a concept where high reflection, coupled with MIR emission in the heated state, facilitates cooling, while high visible light absorption in the cold state allows heating, thus enabling adaptive thermal regulation. CNF films were doped with black-to-leuco TC particles, and a thin silver layer was optionally applied to the films. The films exhibited a rapid transition (within 1 s) in their optical properties at ∼22 °C, becoming transparent above the transition temperature. Visible range transmittance of all samples ranged from 60 to 90%, with pronounced absorption in the 8-13 µm range. The cooling potential of the films was measured at 1-4 °C without any Ag layer and ∼10 °C with a Ag layer. In outdoor field testing, a peak cooling value of 12 °C was achieved during bright sunshine, which is comparable to a commercial solar film. A simulation model was also built based on the experimental results. The concept presented in this study extends beyond applications as standalone films but has applicability also in glass coatings. Overall, this work opens the door for a novel application opportunity for green cellulose-based materials.

Thermal Nanoimprint Lithography-A Review of the Process, Mold Fabrication, and Material.

Micro- and nanopatterns perform unique functions and have attracted attention in various industrial fields, such as electronic devices, microfluidics, biotechnology, optics, sensors, and smart and anti-adhesion surfaces. To put fine-patterned products to practical use, low-cost patterning technology is necessary. Nanoimprint lithography (NIL) is a promising technique for high-throughput nanopattern fabrication. In particular, thermal nanoimprint lithography (T-NIL) has the advantage of employing flexible materials and eliminating chemicals and solvents. Moreover, T-NIL is particularly suitable for compostable and recyclable materials, especially when applying biobased materials for use in optics and electronics. These attributes make T-NIL an eco-friendly process. However, the processing time of normal T-NIL is longer than that of ultraviolet (UV) NIL using a UV-curable resin because the T-NIL process requires heating and cooling time. Therefore, many studies focus on improving the throughput of T-NIL. Specifically, a T-NIL process based on a roll-to-roll web system shows promise for next-generation nanopatterning techniques because it enables large-area applications with the capability to process webs several meters in width. In this review, the T-NIL process, roll mold fabrication techniques, and various materials are introduced. Moreover, metal pattern transfer techniques using a combination of nanotransfer printing, T-NIL, and a reverse offset are introduced.

Bicomponent Cellulose Fibrils and Minerals Afford Wicking Channels Stencil-Printed on Paper for Rapid and Reliable Fluidic Platforms.

Flexible and easy-to-use microfluidic systems are suitable options for point-of-care diagnostics. Here, we investigate liquid transport in fluidic channels produced by stencil printing on flexible substrates as a reproducible and scalable option for diagnostics and paper-based sensing. Optimal printability and flow profiles were obtained by combining minerals with cellulose fibrils of two different characteristic dimensions, in the nano- and microscales, forming channels with ideal wettability. Biomolecular ligands were easily added by inkjet printing on the channels, which were tested for the simultaneous detection of glucose and proteins. Accurate determination of clinically relevant concentrations was possible from linear calibration, confirming the potential of the introduced paper-based diagnostics. The results indicate the promise of simple but reliable fluidic channels for drug and chemical analyses, chromatographic separation, and quality control.

Thermoresponsive Nanocellulose Films as an Optical Modulation Device: Proof-of-Concept.

Flexible optoelectronic technologies are becoming increasingly important with the advent of concepts such as smart-built environments and wearable systems, where they have found applications in displays, sensing, healthcare, and energy harvesting. Parallelly, there is also a need to make these innovations environmentally sustainable by design. In the present work, we employ nanocellulose and its excellent film-forming properties as a basis to develop a green flexible photonic device for sensing applications. Cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) were used as matrix materials along with a black thermochromic pigment to prepare thermoresponsive hybrid films. Optical properties of nanocellulose films such as transparency and haze were tuned by varying pigment loading. Nearly 90% transparent CNF and CNC films could be tuned to reduce the transmission to as low as 4 and 17%, respectively. However, the films regained transparency to up to 60% when heated above the thermochromic transition temperature (31 °C). The thermoresponsive behavior of the prepared films was exploited to demonstrate an all-optical modulation device. Continuous infrared light (1300 nm) was modulated by using a 660 nm visible diode laser. The laser intensity was sufficient to cause a localized thermochromic transition in the films. The laser was pulsed at 0.3 Hz and a uniform cyclic modulation depth of 0.3 dB was achieved. The demonstrated application of functional nanocellulose hybrid films as a light switch (modulator) could be harnessed in various thermally stimulated sensing systems such as temperature monitoring, energy-saving, and anti-counterfeiting.

High-Throughput Tailoring of Nanocellulose Films: From Complex Bio-Based Materials to Defined Multifunctional Architectures.

This paper demonstrates a high-throughput approach to fabricate nanocellulose films with multifunctional performance using conventionally existing unit operations. The approach comprises cast-coating and direct interfacial atmospheric plasma-assisted gas-phase modification along with the microscale patterning technique (nanoimprint lithography, NIL), all applied in roll-to-roll mode, to introduce organic functionalities in conjunction with structural manipulation. Our strategy results in multifunctional cellulose nanofibrils (CNF) films in which the high optical transmittance (∼90%) is retained while the haze can be adjusted (2-35%). Concomitantly, the hydrophobic/hydrophilic balance can be tuned (50-21 mJ/m2 with the water contact angle ranging from ∼20 up to ∼120°), while intrinsic hygroscopicity of CNF films is not significantly compromised. Therefore, a challenge to produce multifunctional bio-based materials with properties defined by various high-performance applications conjoined to the lack of efficient processing strategies is elucidated. Overall, economically and ecologically viable strategy, which was realized by facile and upscalable unit operations using the R2R technology, is introduced to expand the properties' spaces and thus offer a vast variety of interesting applications for CNF films.