Dielectric and acid-base bifunctional effects are elucidated in heterogeneous aminocatalysis using a synthetic strategy based on bulk silica imprinting. Acid-base cooperativity between silanols and amines yields a bifunctional catalyst for the Henry reaction that forms alpha,beta-unsaturated product via quasi-equilibrated iminium intermediate. Solid-state UV/vis spectroscopy of catalyst materials treated with salicylaldehyde demonstrates zwitterionic iminium ion to be the thermodynamically preferred product in the bifunctional catalyst. This product is observed to a much lesser extent relative to its neutral imine tautomer in primary amine catalysts having outer-sphere silanols partially replaced by aprotic functional groups. One of these primary amine catalysts, consisting of a polar outer-sphere environment derived from cyano-terminated capping groups, has activity comparable to that of the bifunctional catalyst in the Henry reaction, but instead forms the beta-nitro alcohol product in high selectivity (approximately 99%). This appears to be the first observation of selective alcohol formation in primary amine catalysis of the Henry reaction. A primary amine catalyst with a methyl-terminated outer-sphere also produces alcohol, albeit at a rate that is 50-fold slower than the cyano-terminated catalyst, demonstrating that outer-sphere dielectric constant affects catalyst activity. We further investigate the importance of organizational effects in enabling acid-base cooperativity within the context of bifunctional catalysis, and the unique role of the solid surface as a macroscopic ligand to impose this cooperativity. Our results unequivocally demonstrate that reaction mechanism and product selectivity in heterogeneous aminocatalysis are critically dependent on the outer-sphere environment.

We present a new class of architected materials that exhibit rapid, reversible, and sizable changes in effective stiffness.

Recent studies in nonlinear electrokinetics reveal the standard theory to generally overpredict measured velocities, sometimes dramatically. Contamination of the driving surface provides a natural mechanism for electrokinetic suppression. We measure induced charge electro-osmosis over gold electrodes "contaminated" with silica layers of controlled thickness for nearly a thousand distinct conditions, in a system that enables direct comparisons between theoretical predictions and experimental measurements. Both the magnitude and frequency dependence of the measured slip velocity are captured quantitatively over the entire range of experiments by accounting for the physical capacitance and surface chemistry of the dielectric layer. More generally, the quantitative characterization enabled by our apparatus will prove invaluable for the rational design and prediction of electrokinetic systems.

Early examples of computers were almost exclusively based on mechanical devices. Although electronic computers became dominant in the past 60 years, recent advancements in three-dimensional micro-additive manufacturing technology provide new fabrication techniques for complex microstructures which have rekindled research interest in mechanical computations. Here we propose a new digital mechanical computation approach based on additively-manufacturable micro-mechanical logic gates. The proposed mechanical logic gates (i.e., NOT, AND, OR, NAND, and NOR gates) utilize multi-stable micro-flexures that buckle to perform Boolean computations based purely on mechanical forces and displacements with no electronic components. A key benefit of the proposed approach is that such systems can be additively fabricated as embedded parts of microarchitected metamaterials that are capable of interacting mechanically with their surrounding environment while processing and storing digital data internally without requiring electric power.

Flexible electronic skin with features that include sensing, processing, and responding to stimuli have transformed human–robot interactions. However, more advanced capabilities, such as human‐like self‐protection modalities with a sense of pain, sign of injury, and healing, are more challenging. Herein, a novel, flexible, and robust diffusive memristor based on a copolymer of chlorotrifluoroethylene and vinylidene fluoride (FK‐800) as an artificial nociceptor (pain sensor) is reported. Devices composed of Ag/FK‐800/Pt have outstanding switching endurance >106 cycles, orders of magnitude higher than any other two‐terminal polymer/organic memristors in literature (typically 102–103 cycles). In situ conductive atomic force microscopy is employed to dynamically switch individual filaments, which demonstrates that conductive filaments correlate with polymer grain boundaries and FK‐800 has superior morphological stability under repeated switching cycles. It is hypothesized that the high thermal stability and high elasticity of FK‐800 contribute to the stability under local Joule heating associated with electrical switching. To mimic biological nociceptors, four signature nociceptive characteristics are demonstrated: threshold triggering, no adaptation, relaxation, and sensitization. Lastly, by integrating a triboelectric generator (artificial mechanoreceptor), memristor (artificial nociceptor), and light emitting diode (artificial bruise), the first bioinspired injury response system capable of sensing pain, showing signs of injury, and healing, is demonstrated.

Versatile methods are described for the fabrication of micrometer‐scale particles with defined shape, ferro‐ or paramagnetic properties, and amphiphilic Janus surface chemistry. Examples of their utility include measuring the rheological properties of complex, two‐dimensional fluid interfaces and reversible, directed self‐assembly at fluid interfaces.

In recent years, 3D printing has led to a disruptive manufacturing revolution that allows complex architected materials and structures to be created by directly joining sequential layers into designed 3D components. However, customized feedstocks for specific 3D printing techniques and applications are limited or nonexistent, which greatly impedes the production of desired structural or functional materials. Colloids, with their stable biphasic nature, have tremendous potential to satisfy the requirements of various 3D printing methods owing to their tunable electrical, optical, mechanical, and rheological properties. This enables materials delivery and assembly across the multiple length scales required for multifunctionality. Here, a state-of-the-art review on advanced colloidal processing strategies for 3D printing of organic, ceramic, metallic, and carbonaceous materials is provided. It is believed that the concomitant innovations in colloid design and 3D printing will provide numerous possibilities for the fabrication of new constructs unobtainable using traditional methods, which will significantly broaden their applications.

patterned deposits from collections of particles, nearly universally, these methods rely on AC electric fi elds and the particles are manipulated through dielectrophoresis, as with optoelectronic tweezers, [23][24][25] or electrohydrodynamic fl ows. [ 26 ] Perhaps the closest technique to light-directed EPD described in the literature is the report of Hayward et al. [ 27 ] on the assembly of 2D patterned colloidal crystals using DC fi elds and UV light, but the method of action points to near-surface electrohydrodynamic fl ows, [ 28 ] which limits the ability to form thick 3D deposits. To our knowledge, there have been no reports that demonstrate fabrication of 3D patterned multimaterial composites using DC electric fi elds.Light-directed EPD has the potential to elevate EPD from its traditional role as a coating process to a true additive manufacturing technique. By automating the fl uid handling and light delivery systems, light-directed EPD will allow rapid patterning of multiple materials over large areas and can produce composites to near net shape from a diverse set of materials, including metals, ceramics, polymers, and biological materials. Controlled voids can also be incorporated into the composite by utilizing a fugitive material that is removed with subsequent post-processing. These capabilities are unique among current additive manufacturing processes. [ 29 ] A typical multimaterial light-directed EPD process is depicted in Figure 1 . The space between the photoconductive electrode and counter electrode is fi lled with a suspension of particles that will be deposited. The photoconductive electrode consists of a photoconductive layer of titania nanorods hydrothermally grown on an fl uorine-doped tin oxide (FTO) coated glass substrate. A photomask is placed on the back side of the photoconductive electrode and illuminated with a light source. An electrical potential difference is imposed between the FTO layer of the photoconductive electrode and the counter electrode. In the regions illuminated by the light source defi ned by the clear regions of the photomask, the conductivity of the photoconductive fi lm increases. Electric fi eld lines are concentrated in these regions and particles in suspension move long the fi eld lines and attach to the surface of the electrode forming a deposit (Figure 1 a). The suspension composition and the photomask pattern can be changed to deposit another material in a different location within the same layer (Figure 1 b). The process can be repeated to build subsequent layers to form an arbitrarily patterned 3D composite (Figure 1 c). Finally, the suspension can be removed from the chamber yielding a dry deposit while retaining the precise patterning.Electrophoretic deposition (EPD) is a process in which electrically charged colloids are forced to deposit onto the surface of an electrode due to an electric fi eld. EPD was originally developed nearly a century ago for applying paint to metal surfaces. [ 1 ] Since then, EPD has been used to deposit a wide range of materials...