3D printing of renewable building blocks like cellulose nanocrystals offers an attractive pathway for fabricating sustainable structures. Here, viscoelastic inks composed of anisotropic cellulose nanocrystals (CNC) that enable patterning of 3D objects by direct ink writing are designed and formulated. These concentrated inks are composed of CNC particles suspended in either water or a photopolymerizable monomer solution. The shear-induced alignment of these anisotropic building blocks during printing is quantified by atomic force microscopy, polarized light microscopy, and 2D wide-angle X-ray scattering measurements. Akin to the microreinforcing effect in plant cell walls, the alignment of CNC particles during direct writing yields textured composites with enhanced stiffness along the printing direction. The observations serve as an important step forward toward the development of sustainable materials for 3D printing of cellular architectures with tailored mechanical properties.

The alignment of anisotropic particles during ink deposition directly affects the microstructure and properties of materials manufactured by extrusion-based 3D printing. Although particle alignment in diluted suspensions is well described by analytical and numerical models, the dynamics of particle orientation in the highly concentrated inks typically used for printing via direct ink writing (DIW) remains poorly understood. Using cellulose nanocrystals (CNCs) as model building blocks of increasing technological relevance, we study the dynamics of particle alignment under the shear stresses applied to concentrated inks during DIW. With the help of in situ polarization rheology, we find that the time period needed for particle alignment scales inversely with the applied shear rate and directly with the particle concentration. Such dependences can be quantitatively described by a simple scaling relation and qualitatively interpreted in terms of steric and hydrodynamic interactions between particles at high shear rates and particle concentrations. Our understanding of the alignment dynamics is then utilized to estimate the effect of shear stresses on the orientation of particles during the printing process. Finally, proof-of-concept experiments show that the combination of shear and extensional flow in 3D printing nozzles of different geometries provides an effective means to tune the orientation of CNCs from fully aligned to core-shell architectures. These findings offer powerful quantitative guidelines for the digital manufacturing of composite materials with programmed particle orientations and properties.

With the development of the internet‐of‐things for applications such as wearables and packaging, a new class of electronics is emerging, characterized by the sheer number of forecast units and their short service‐life. Projected to reach 27 billion units in 2021, connected devices are generating an exponentially increasing amount of electronic waste (e‐waste). Fueled by the growing e‐waste problem, the field of sustainable electronics is attracting significant interest. Today, standard energy‐storage technologies such as lithium‐ion or alkaline batteries still power most of smart devices. While they provide good performance, the nonrenewable and toxic materials require dedicated collection and recycling processes. Moreover, their standardized form factor and performance specifications limit the designs of smart devices. Here, exclusively disposable materials are used to fully print nontoxic supercapacitors maintaining a high capacitance of 25.6 F g−1 active material at an operating voltage up to 1.2 V. The presented combination of digital material assembly, stable high‐performance operation, and nontoxicity has the potential to open new avenues within sustainable electronics and applications such as environmental sensing, e‐textiles, and healthcare.

Cellulose is an attractive material resource for the fabrication of sustainable functional products, but its processing into structures with complex architecture and high cellulose content remains challenging. Such limitation has prevented cellulose‐based synthetic materials from reaching the level of structural control and mechanical properties observed in their biological counterparts, such as wood and plant tissues. To address this issue, a simple approach is reported to manufacture complex‐shaped cellulose‐based composites, in which the shaping capabilities of 3D printing technologies are combined with a wet densification process that increases the concentration of cellulose in the final printed material. Densification is achieved by exchanging the liquid of the wet printed material with a poor solvent mixture that induces attractive interactions between cellulose particles. The effect of the solvent mixture on the final cellulose concentration is rationalized using solubility parameters that quantify the attractive interparticle interactions. Using X‐ray diffraction analysis and mechanical tests, 3D printed composites obtained through this process are shown to exhibit highly aligned microstructures and mechanical properties significantly higher than those obtained by earlier additively manufactured cellulose‐based materials. These features enable the fabrication of cellulose‐rich synthetic structures that more closely resemble the exquisite designs found in biological materials grown by plants in nature.

Recent development for cellulose inks have provided methods to deal with important volume shrinkage resulting from the drying and evaporation of the dispersing solvent. [14] In order to make a reliable 3D printable ink, the relevance of understanding parameters such as the rheological properties (controlling the printability) [15] and the conductivity was evidenced by previous research. [16] Rheological aspects involve a shear-thinning behavior and the existence of a yield stress to enable a proper extrusion and shape fidelity of prior and after 3D printing. [17] The conductivity of the ink can be controlled by the addition of conductive fillers such as silver flakes, silver nanowires (AgNWs) as earlier research suggested. [18,19] Among other electrochemical sensor platforms, ion selective membrane sensors have been reported as efficient tools to detect ion analytes selectively from ion mixed solutions such as pH, [20][21][22][23] glucose, [24] or urea. [25] Furthermore, by integrating with Field Effect Transistors (FETs), the analyte information is quantitatively analyzed using Ion Selective Field Effect Transistors (ISFETs). [26][27][28] Recyclable ISFETs based on oxide semiconductors were also studied for sustainability. [29] High selectivity and stability enable ISFETs to be investigated for potential applications in biomedical and environmental fields. ISFETs are mainly controlled by the generated potential from ion selective electrodes and the generated potential influences the electrical performance of FETs. Therefore, variation of ion concentration is reflected by the fluctuation of drain current, drain voltage, or output voltage. Specially, wireless ion selective sensors are demonstrated to extract remotely ion concentration signals out of analyte solution. Consequently, a great interest was demonstrated over the past years in the fields of real-time monitoring and internet-of-things (IoT) applications. Research was conducted in integrating novel wireless technologies, such as RFID, [30][31][32][33][34] Bluetooth, [35,36] Zigbee, [37] or NFC [38] together with electrochemical sensors for wireless analytes information recovery. A conventional approach for wireless sensing with silicon-based chip technology has been already developed. [31] As the wireless communication technologies are becoming more crucial for internet-of-things (IoT) electronic devices, sensors have also been equipped with wireless data collection. A conventional way to make wireless sensor systems is to develop active sensor devices with silicon-based chip technologies integrated with an amplifier, a battery, a converter, among others. However, it is difficult to generate disposable inexpensive flexible sensors with all these rigid components. Here, 3D printed disposable wireless ion selective sensor systems with unique form factors, high sensitivity, and flexibility are reported. A 3D printable conductive ink is designed and optimized with cellulose nanofibers by addition of silver nanowires for sustainable and biocompatible sensor app...

No abstract

Polysaccharides are attractive sustainable resources for the fabrication of advanced materials, but the assembly of these building blocks into complex-shaped structures combining the high strength and low weight required in many applications remains challenging. We have investigated and optimized the rheological and mechanical properties of polysaccharide-based composite foams based on mixtures of methylcellulose (MC), cellulose nanofibrils (CNF), montmorillonite (MMT), and glyoxal and tannic acid. Such foams were found to be stabilized by the coadsorption of MC, CNF, and MMT at the air−water interface, while the complexation of the polysaccharides with tannic acid improved the foam stability. Tannic acid could also be used to tune and optimize the microstructure and the viscoelastic properties of the wet foam for direct ink writing of robust cellular architectures. Glyoxal had no noticeable effect on the properties of the wet foams but significantly enhanced the water resilience and stiffness of the lightweight material obtained after drying at ambient pressure and elevated temperatures with minimum shrinkage. The foams possessed a high porosity and displayed a specific Young's modulus and yield strength that outperformed other biobased foams and commercially available expanded polystyrene. The strong and water-resilient 3D printed foams can be surface modified using, for example, aminosilanes, which opens up applications for air purification and thermal insulation.

SummaryWe review the process rates and energy intensities of various additive processing technologies and focus on recent progress in improving these metrics for laser powder bed fusion processing of metals, and filament and pellet extrusion processing of polymers and composites. Over the last decade, observed progress in raw build rates has been quite substantial, with laser metal processes improving by about 1 order of magnitude, and polymer extrusion processes by more than 2 orders of magnitude. We develop simple heat transfer models that explain these improvements, point to other possible strategies for improvement, and highlight rate limits. We observe a pattern in laser metal technologies that mimics the development of machine tools; an efficiency plateau, where faster rates require more power with no change in energy nor rate efficiency.