Exposure to excessive ultraviolet (UV) radiation can have detrimental effects on human health. Inexpensive easy-to-use sensors for monitoring UV radiation can allow broad-scale assessment of UV exposure, but their implementation requires technology that enables rapid and affordable manufacturing of these sensors on a large scale. Herein, we report a novel three-dimensional (3D) printing procedure and printable ink composition that produce robust, flexible, and wearable UV sensors. To fabricate the sensors, a color-changing hydrogel ink was first developed from which standalone constructs were 3D printed. The ink contains alginate, gelatin, photoactive titanium dioxide nanoparticles, and dyes (methyl orange, methylene blue, and malachite green) in which the nanoparticles are used to initiate photocatalytic degradation of dyes, leading to discoloration of the dye. The sensors resemble a color-changing tattoo that loses color upon exposure to UV. The viscosity and ink composition were optimized to achieve printability and tune the mechanical properties (e.g., modulus, hardness) of the sensors. The optimized procedure enabled the one-step fabrication of mechanically stable sensors that can effectively measure outdoor sun exposure by quantifying the decrease in color, visible to the naked eye. Apart from being used as wearable sensors, these sensors have the potential to be used along with UV-based workspace sterilizing devices to ensure that surfaces have been efficiently exposed to UV. The sensors are inexpensive, stable, extremely robust, biodegradable, and easy to use. The tunability, biocompatibility, and printability of the ink offer excellent potential for developing advanced 3D printing methods that, in addition to UV sensors, can be applied more broadly to fabricate other sensing technologies for a variety of other applications.

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Nanomaterials obtained from sustainable and natural sources have seen tremendous growth in recent times due to increasing interest in utilizing readily and widely available resources. Nanocellulose materials extracted from renewable biomasses hold great promise for increasing the sustainability of conventional materials in various applications owing to their biocompatibility, mechanical properties, ease of functionalization, and high abundance. Nanocellulose can be used to reinforce mechanical strength, impart antimicrobial activity, provide lighter, biodegradable, and more robust materials for packaging, and produce photochromic and electrochromic devices. While the fabrication and properties of nanocellulose are generally well established, their implementation in novel products and applications requires surface modification, assembly, and manufacturability to enable rapid tooling and scalable production. Additive manufacturing techniques such as 3D printing can improve functionality and enhance the ability to customize products while reducing fabrication time and wastage of materials. This review article provides an overview of nanocellulose as a sustainable material, covering the different properties, preparation methods, printability and strategies to functionalize nanocellulose into 3D-printed constructs. The applications of 3D-printed nanocellulose composites in food, environmental, and energy devices are outlined, and an overview of challenges and opportunities is provided.

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The ability to manufacture sensors rapidly and cost-effectively is essential for creating devices that would benefit healthcare, medical diagnostics, environmental monitoring and process industries. Because of the inherent complexity of sensing devices, new fabrication methods are needed to create sensors that could be fabricated in large scale and with high efficiency. Additive manufacturing techniques like 3D printing and 3D bioprinting enable printing of different biomaterials into intricate 3D architectures which could serve as sensing platforms. This presentation will describe the development and optimization of 3D printed hydrogel-based (bio)sensors with incorporated receptor molecules and transduction interfaces. In order to 3D print these sensors, an extrusion-based 3D bioprinter, and a computer-aided designing software were used along with a novel bio-ink formulation, which contains enzymes, catalytic and photoactive properties. The optimum hydrogel composite provided excellent mechanical properties to the printed biosensors enabling them to be used in a wide range of wearable applications. Fabrication and performance characteristics of two types of sensors will be presented: a lactate sensor for measuring physiological activity in sweat and a UV sensor for measuring harmful UV radiation exposure. The proposed sensors are reagent-less and highly portable making them ideal for use as wearable devices and printed biocatalytic platforms for portable bioelectronics. Examples of applications of these additively manufactured sensors for monitoring different analytes will be discussed. Figure 1