3D-Printed Architectured Silicones with Autonomic Self-Healing and Creep-Resistant Behavior.

Self-healing silicones that are able to restore functionalities and extend the lifetime of soft devices hold great potential in many applications. However, currently available silicones need to be triggered to self-heal or suffer from creep-induced irreversible deformation during use. Here, a platform is proposed to design and print silicone objects that are programmed at the molecular and architecture levels to achieve self-healing at room temperature while simultaneously resisting creep. At the molecular scale, dioxaborolanes moieties are incorporated into silicones to synthesize self-healing vitrimers, whereas conventional covalent bonds are exploited to make creep-resistant elastomers. When combined into architectured printed parts at a coarser length scale, the layered materials exhibit fast healing at room temperature without compromising the elastic recovery obtained from covalent polymer networks. A patient-specific vascular phantom and fluidic chambers are printed to demonstrate the potential of architectured silicones in creating damage-resilient functional devices using molecularly designed elastomer materials.

Fabrication of Three-Dimensional Polymer-Brush Gradients within Elastomeric Supports by Cu0-Mediated Surface-Initiated ATRP.

Cu0-mediated surface-initiated ATRP (Cu0 SI-ATRP) emerges as a versatile, oxygen-tolerant process to functionalize three-dimensional (3D), microporous supports forming single and multiple polymer-brush gradients with a fully tunable composition. When polymerization mixtures are dispensed on a Cu0-coated plate, this acts as oxygen scavenger and source of active catalyst. In the presence of an ATRP initiator-bearing microporous elastomer placed in contact with the metallic plate, the reaction solution infiltrates by capillarity through the support, simultaneously triggering the controlled growth of polymer brushes. The polymer grafting process proceeds with kinetics that are determined by the progressive infiltration of the reaction solution within the microporous support and by the continuous diffusion of catalyst regenerated at the Cu0 surface. The combination of these effects enables the accessible generation of 3D polymer-brush gradients extending across the microporous scaffolds used as supports, finally providing materials with a continuous variation of interfacial composition and properties.

Cellulose-Based Microparticles for Magnetically Controlled Optical Modulation and Sensing.

Responsive materials with birefringent optical properties have been exploited for the manipulation of light in several modern electronic devices. While electrical fields are often utilized to achieve optical modulation, magnetic stimuli may offer an enticing complementary approach for controlling and manipulating light remotely. Here, the synthesis and characterization of magnetically responsive birefringent microparticles with unusual magneto-optical properties are reported. These functional microparticles are prepared via a microfluidic emulsification process, in which water-based droplets are generated in a flow-focusing device and stretched into anisotropic shapes before conversion into particles via photopolymerization. Birefringence properties are achieved by aligning cellulose nanocrystals within the microparticles during droplet stretching, whereas magnetic responsiveness results from the addition of superparamagnetic nanoparticles to the initial droplet template. When suspended in a fluid, the microparticles can be controllably manipulated via an external magnetic field to result in unique magneto-optical coupling effects. Using a remotely actuated magnetic field coupled to a polarized optical microscope, these microparticles can be employed to convert magnetic into optical signals or to estimate the viscosity of the suspending fluid through magnetically driven microrheology.

Dynamics of Cellulose Nanocrystal Alignment during 3D Printing.

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.

Mineral Nano-Interconnectivity Stiffens and Toughens Nacre-like Composite Materials.

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.

Bio-inspired self-shaping ceramics.

Shaping ceramics into complex and intricate geometries using cost-effective processes is desirable in many applications but still remains an open challenge. Inspired by plant seed dispersal units that self-fold on differential swelling, we demonstrate that self-shaping can be implemented in ceramics by programming the material's microstructure to undergo local anisotropic shrinkage during heat treatment. Such microstructural design is achieved by magnetically aligning functionalized ceramic platelets in a liquid ceramic suspension, subsequently consolidated through an established enzyme-catalysed reaction. By fabricating alumina compacts exhibiting bio-inspired bilayer architectures, we achieve deliberate control over shape change during the sintering step. Bending, twisting or combinations of these two basic movements can be successfully programmed to obtain a myriad of complex shapes. The simplicity and the universality of such a bottom-up shaping method makes it attractive for applications that would benefit from low-waste ceramic fabrication, temperature-resistant interlocking structures or unusual geometries not accessible using conventional top-down manufacturing.

Magnetic assembly of transparent and conducting graphene-based functional composites.

Innovative methods producing transparent and flexible electrodes are highly sought in modern optoelectronic applications to replace metal oxides, but available solutions suffer from drawbacks such as brittleness, unaffordability and inadequate processability. Here we propose a general, simple strategy to produce hierarchical composites of functionalized graphene in polymeric matrices, exhibiting transparency and electron conductivity. These are obtained through protein-assisted functionalization of graphene with magnetic nanoparticles, followed by magnetic-directed assembly of the graphene within polymeric matrices undergoing sol-gel transitions. By applying rotating magnetic fields or magnetic moulds, both graphene orientation and distribution can be controlled within the composite. Importantly, by using magnetic virtual moulds of predefined meshes, graphene assembly is directed into double-percolating networks, reducing the percolation threshold and enabling combined optical transparency and electrical conductivity not accessible in single-network materials. The resulting composites open new possibilities on the quest of transparent electrodes for photovoltaics, organic light-emitting diodes and stretchable optoelectronic devices.

Periodically microstructured composite films made by electric- and magnetic-directed colloidal assembly.

Living organisms often combine soft and hard anisotropic building blocks to fabricate composite materials with complex microstructures and outstanding mechanical properties. An optimum design and assembly of the anisotropic components reinforces the material in specific directions and sites to best accommodate multidirectional external loads. Here, we fabricate composite films with periodic modulation of the soft-hard microstructure by simultaneously using electric and magnetic fields. We exploit forefront directed-assembly approaches to realize highly demanded material microstructural designs and showcase a unique example of how one can bridge colloidal sciences and composite technology to fabricate next-generation advanced structural materials. In the proof-of-concept experiments, electric fields are used to dictate the position of the anisotropic particles through dielectrophoresis, whereas a rotating magnetic field is used to control the orientation of the particles. By using such unprecedented control over the colloidal assembly process, we managed to fabricate ordered composite microstructures with up to 2.3-fold enhancement in wear resistance and unusual site-specific hardness that can be locally modulated by a factor of up to 2.5.

Hierarchically roughened microplatelets enhance the strength and ductility of nacre-inspired composites.

Rough interfaces featuring nanoscale asperities are known to play a major role in the mechanics of nacre. Transferring this concept to artificial bioinspired composites requires a detailed understanding about the effect of the surface topography of reinforcing elements on the mechanical performance of such materials. To gain further insights into the effect of asperity size, hierarchy and coverage on the mechanics of nacre-inspired composites, we decorate alumina microplatelets with silica nanoparticles of selected sizes and use the resulting roughened platelets as reinforcing elements (15vol%) in a commercial epoxy matrix. For a single layer of silica nanoparticles on the platelet surface, increased ultimate strain and toughness are obtained with a large roughening particle size of 250nm. On the contrary, strength and stiffness are enhanced by decreasing the size of asperities using 22nm silica particles. By combining particles of two different sizes (100nm and 22nm) in a hierarchical fashion, we are able to improve stiffness and strength of platelet-reinforced polymers while maintaining high ultimate strain and toughness. Our results indicate that carefully designed hierarchically roughened interfaces lead to a more homogeneous stress distribution within the polymer matrix between the stiff reinforcing elements. By enabling the deformation of a larger fraction of the polymer matrix, this design concept improves the mechanical response of bioinspired composites and can possibly also be exploited to enhance the performance of conventional fiber-reinforced polymers.

Ultrahigh magnetically responsive microplatelets with tunable fluorescence emission.

Tuning the optical properties of suspensions by controlling the orientation and spatial distribution of suspended particles with magnetic fields is an interesting approach to creating magnetically controlled displays, microrheology sensors, and materials with tunable light emission. However, the relatively high concentration of magnetic material required to manipulate these particles very often reduces the optical transmittance of the system. In this study, we describe a simple method of generating particles with magnetically tunable optical properties via sol-gel deposition and functionalization of a continuous layer of silica on ultrahigh magnetically responsive (UHMR) alumina microplatelets. UHMR microplatelets with tunable magnetic response in the range of 15-36 G are obtained by the electrostatic adsorption of 2 to 13% of superparamagnetic iron oxide nanoparticles (SPIONs) on the alumina surface. The magnetized platelets are coated with a 20-50 nm layer of SiO2 through the controlled hydrolysis and condensation reactions of tetraethylorthosilicate (TEOS) in an NH3/ethanol mixture. Finally, the silica surface is covalently modified with an organic fluorescent dye by conventional silane chemistry. Because of the anisotropic shape of the particles, control of their orientation and distribution using magnetic fields and field gradients enables easy tuning of the optical properties of the suspension. This strategy allows us to gain both spatial and temporal control over the fluorescence emission from the particle surface, making the multifunctional platelets interesting building blocks for the manipulation of light in colloid-based smart optical devices and sensors.

Mechanics of platelet-reinforced composites assembled using mechanical and magnetic stimuli.

Current fabrication technologies of structural composites based on the infiltration of fiber weaves with a polymeric resin offer good control over the orientation of long reinforcing fibers but remain too cumbersome and slow to enable cost-effective manufacturing. The development of processing routes that allow for fine control of the reinforcement orientation and that are also compatible with fast polymer processing technologies remains a major challenge. In this paper, we show that bulk platelet-reinforced composites with tailored reinforcement architectures and mechanical properties can be fabricated through the directed-assembly of inorganic platelets using combined magnetic and mechanical stimuli. The mechanical performance and fracture behavior of the resulting composites under compression and bending can be deliberately tuned by assembling the platelets into designed microstructures. By combining high alignment degree and volume fractions of reinforcement up to 27 vol %, we fabricated platelet-reinforced composites that can potentially be made with cost-effective polymer processing routes while still exhibiting properties that are comparable to those of state-of-the-art glass-fiber composites.

Stretchable heterogeneous composites with extreme mechanical gradients.

Heterogeneous composite materials with variable local stiffness are widespread in nature, but are far less explored in engineering structural applications. The development of heterogeneous synthetic composites with locally tuned elastic properties would allow us to extend the lifetime of functional devices with mechanically incompatible interfaces, and to create new enabling materials for applications ranging from flexible electronics to regenerative medicine. Here we show that heterogeneous composites with local elastic moduli tunable over five orders of magnitude can be prepared through the site-specific reinforcement of an entangled elastomeric matrix at progressively larger length scales. Using such a hierarchical reinforcement approach, we designed and produced composites exhibiting regions with extreme soft-to-hard transitions, while still being reversibly stretchable up to 350%. The implementation of the proposed methodology in a mechanically challenging application is illustrated here with the development of locally stiff and globally stretchable substrates for flexible electronics.

Improved photocatalytic activity of anisotropic rutile/anatase TiO2 nanoparticles synthesized by the Ti-peroxo complex method.

Anisotropic rutile/anatase TiO2 nanoparticles (AB-TiO2) were synthesized by the Ti-peroxo complex method. Their photocatalytic activity in the degradation of Rhodamine B (RhB) was evaluated and compared to that of commercial TiO2 P25 and TiO2 obtained through the benzyl alcohol route (OB-TiO2). The samples were characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR in DRIFT mode), Field-Emission Scanning Electronic Microscopy (FEG-SEM), N2 physisorption and UV-visible spectroscopy. Photodegradation of RhB was carried out under visible light and the results revealed a remarkable photocatalytic activity of the AB-TiO2 in terms of surface area. The excellent performance of the AB-TiO2 was explained in light of the synergistic effect of the coexistence of anatase/rutile phases, anisotropy and irreversible adsorption of organic species during sol-gel synthesis. UV-visible measurements also indicated that N-deethylation and photobleaching mechanisms occur to different extents, depending on the surface composition of the photocatalyst.

Locally reinforced polymer-based composites for elastic electronics.

A promising approach to fabricating elastic electronic systems involves processing thin film circuits directly on the elastic substrate by standard photolithography. Thin film devices are generally placed onto stiffer islands on the substrate surface to protect devices from excessive strain while still achieving a globally highly deformable system. Here we report a new method to achieve island architectures by locally reinforcing polymeric substrates at the macro- and microscale using magnetically responsive anisotropic microparticles. We demonstrate that the resulting particle-reinforced elastic substrates can be made smooth enough for the patterning and successful operation of thin film transistors with transfer characteristics comparable to state-of-the-art devices.

Composites reinforced in three dimensions by using low magnetic fields.

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 nanoparticles (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.

Unifying model for the electrokinetic and phase behavior of aqueous suspensions containing short and long amphiphiles.

Aqueous suspensions containing oppositely charged colloidal particles and amphiphilic molecules can form fluid dispersions, foams, and percolating gel networks, depending on the initial concentration of amphiphiles. While models have been proposed to explain the electrokinetic and flotation behavior of particles in the presence of long amphiphilic molecules, the effect of amphiphiles with less than six carbons in the hydrocarbon tail on the electrokinetic, rheological, and foaming behavior of aqueous suspensions remains unclear. Unlike conventional long amphiphiles (≥10 carbons), short amphiphiles do not exhibit increased adsorption on the particle surface when the number of carbons in the molecule tail is increased. On the basis of classical electrical double layer theory and the formerly proposed hemimicelle concept, we put forward a new predictive model that reconciles the adsorption and electrokinetic behavior of colloidal particles in the presence of long and short amphiphiles. By introducing in the classical Gouy-Chapman theory an energy term associated with hydrophobic interactions between the amphiphile hydrocarbon tails, we show that amphiphilic electrolytes lead to a stronger compression of the diffuse part of the electrical double layer in comparison to hydrophilic electrolytes. Scaling relationships derived from this model provide a quantitative description of the rich phase behavior of the investigated suspensions, correctly accounting for the effect of the alkyl chain length of short and long amphiphiles on the electrokinetics of such colloidal systems. The proposed model contributes to our understanding of the stabilization mechanisms of particle-stabilized foams and emulsions and might provide new insights into the physicochemical processes involved in mineral flotation.