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.
Aqueous and polymer‐based inks with high cellulose nanocrystal (CNC) loading are developed for 3D printing of textured cellular architectures. Alignment of CNC particles within the 3D printed filaments leads to enhanced mechanical properties along the printing direction, akin to wood and other biological composites.
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.
The orientation and distribution of reinforcing particles in artificial composites are key to enable effective reinforcement of the material in mechanically loaded directions, but remain poor if ...compared to the distinctive architectures present in natural structural composites such as teeth, bone, and seashells. We show that micrometer-sized reinforcing particles coated with minimal concentrations of superparamagnetic nano parti des (0.01 to 1 volume percent) can be controlled by using ultralow magnetic fields (1 to 10 milliteslas) to produce synthetic composites with tuned three-dimensional orientation and distribution of reinforcements. A variety of structures can be achieved with this simple method, leading to composites with tailored local reinforcement, wear resistance, and shape memory effects.
Numerous examples of material systems that dynamically interact with and adapt to the surrounding environment are found in nature, from hair‐based mechanoreceptors in animals to self‐shaping seed ...dispersal units in plants to remodeling bone in vertebrates. Inspired by such fascinating biological structures, a wide range of synthetic material systems have been created to replicate the design concepts of dynamic natural architectures. Examples of biological structures and their man‐made counterparts are herein revisited to illustrate how dynamic and adaptive responses emerge from the intimate microscale combination of building blocks with intrinsic nanoscale properties. By using top‐down photolithographic methods and bottom‐up assembly approaches, biologically inspired dynamic material systems have been created 1) to sense liquid flow with hair‐inspired microelectromechanical systems, 2) to autonomously change shape by utilizing plantlike heterogeneous architectures, 3) to homeostatically influence the surrounding environment through self‐regulating adaptive surfaces, and 4) to spatially concentrate chemical species by using synthetic microcompartments. The ever‐increasing complexity and remarkable functionalities of such synthetic systems offer an encouraging perspective to the rich set of dynamic and adaptive properties that can potentially be implemented in future man‐made material systems.
Dynamic material systems have been created to replicate the interactivity and adaptive response of hierarchical biological systems. Selected examples of bio‐inspired hairlike sensors, shape‐changing objects, and interactive microcompartments are reviewed to showcase the increasing level of complexity and the dynamic functionalities that can be achieved by using top‐down fabrication technologies and bottom‐up assembly approaches.
Edible solid particles constitute an attractive alternative to surfactants as stabilizers of food-grade emulsions for products requiring a long-term shelf life. Here, we report on a new approach to ...stabilize edible emulsions using silica nanoparticles modified by noncovalently bound chitosan oligomers. Electrostatic modification with chitosan increases the hydrophobicity of the silica nanoparticles and favors their adsorption at the oil–water interface. The interfacial adsorption of the chitosan-modified silica particles enables the preparation of oil-in-water emulsions with small droplet sizes of a few micrometers through high-pressure homogenization. This approach enables the stabilization of food-grade emulsions for more than 3 months. The emulsion structure and stability can be effectively tuned by controlling the extent of chitosan adsorption on the silica particles. Bulk and interfacial rheology are used to highlight the two stabilization mechanisms involved. Low chitosan concentration (1 wt % with respect to silica) leads to the formation of a viscoelastic film of particles adsorbed at the oil–water interface, enabling Pickering stabilization of the emulsion. By contrast, a network of agglomerated particles formed around the droplets is the predominant stabilization mechanism of the emulsions at higher chitosan content (5 wt % with respect to silica). These two pathways against droplet coalescence and coarsening open up different possibilities to engineer the long-term stabilization of emulsions for food applications.
Soft actuation allows robots to interact safely with humans, other machines, and their surroundings. Full exploitation of the potential of soft actuators has, however, been hindered by the lack of ...simple manufacturing routes to generate multimaterial parts with intricate shapes and architectures. Here, we report a 3D printing platform for the seamless digital fabrication of pneumatic silicone actuators exhibiting programmable bioinspired architectures and motions. The actuators comprise an elastomeric body whose surface is decorated with reinforcing stripes at a well-defined lead angle. Similar to the fibrous architectures found in muscular hydrostats, the lead angle can be altered to achieve elongation, contraction, or twisting motions. Using a quantitative model based on lamination theory, we establish design principles for the digital fabrication of silicone-based soft actuators whose functional response is programmed within the material's properties and architecture. Exploring such programmability enables 3D printing of a broad range of soft morphing structures.
The design and fabrication of synthetic structural components often result in homogeneous materials with uniform microstructures and properties. In contrast, nature has evolved structural composites ...exhibiting rich heterogeneous architectures and tunable site‐specific properties. Creating synthetic systems with the heterogeneous nature of biological materials should enable the fabrication of composites with extended durability under severe mechanical demands or with adequate properties using, for example, a more restricted selection of bioresorbable or environmental‐friendly basic building blocks. Here, the heterogeneous structures of the tooth and of the tendon‐bone interface are revisited to identify design strategies that have been naturally selected to best respond to the non‐uniform stresses distributions typically found in such load‐bearing structures. Recent attempts to replicate some of these strategies in man‐made materials are also shown to illustrate the variety of unusual properties that can be achieved through the proposed bioinspired approach. Finally, the creation of heterogeneous architectures with local microstructure and properties deliberately tuned to match non‐uniform loading conditions is suggested as a new pathway towards the development of “material systems” with unprecedented functionalities and durability in mechanically challenging applications.
Load bearing materials in biology and in engineering differ markedly in their microstructural design. While biological materials exhibit highly heterogeneous microstructures (left), their synthetic counterparts are often designed to be homogeneous and isotropic (middle). Creating heterogeneous architectures with properties locally tuned to match non‐uniform loading conditions should lead to bioinspired composites with extended durability using bioresorbable or environmental‐friendly building blocks (right).
Bulk nacre‐like composites with mineral nano‐interconnectivity at the same length scale as in the biological material are produced using magnetic alignment and selective sintering techniques. These ...materials display stiffness and strength levels comparable to that of continuous fiber composites with the advantage of easier processability inherent of discontinuous composites. This opens new possibilities to produce parts with more complex designs.
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.
3D printing of aqueous‐based cellulose inks followed by wet densification and monomer infiltration processes offers a versatile approach for the digital manufacturing of complex‐shaped composites with high concentrations of a sustainable material resource. Wet densification is induced by a solvent exchange process that modifies the cohesive energy of the liquid medium to promote attractive interactions between the cellulose particles.
Materials combining optical transparency and mechanical strength are highly demanded for electronic displays, structural windows and in the arts, but the oxide-based glasses currently used in most of ...these applications suffer from brittle fracture and low crack tolerance. We report a simple approach to fabricate bulk transparent materials with a nacre-like architecture that can effectively arrest the propagation of cracks during fracture. Mechanical characterization shows that our glass-based composites exceed up to a factor of 3 the fracture toughness of common glasses, while keeping flexural strengths comparable to transparent polymers, silica- and soda-lime glasses. Due to the presence of stiff reinforcing platelets, the hardness of the obtained composites is an order of magnitude higher than that of transparent polymers. By implementing biological design principles into glass-based materials at the microscale, our approach opens a promising new avenue for the manufacturing of structural materials combining antagonistic functional properties.
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