Programming Cells for Dynamic Assembly of Inorganic Nano‐Objects with Spatiotemporal Control

Programming living cells to organize inorganic nano‐objects (NOs) in a spatiotemporally precise fashion would advance new techniques for creating ordered ensembles of NOs and new bio–abiotic hybrid materials with emerging functionalities. Bacterial cells often grow in cellular communities called biofilms. Here, a strategy is reported for programming dynamic biofilm formation for the synchronized assembly of discrete NOs or hetero‐nanostructures on diverse interfaces in a dynamic, scalable, and hierarchical fashion. By engineering Escherichia coli to sense blue light and respond by producing biofilm curli fibers, biofilm formation is spatially controlled and the patterned NOs' assembly is simultaneously achieved. Diverse and complex fluorescent quantum dot patterns with a minimum patterning resolution of 100 µm are demonstrated. By temporally controlling the sequential addition of NOs into the culture, multilayered heterostructured thin films are fabricated through autonomous layer‐by‐layer assembly. It is demonstrated that biologically dynamic self‐assembly can be used to advance a new repertoire of nanotechnologies and materials with increasing complexity that would be otherwise challenging to produce.     

Hierarchical Multicomponent Inorganic Metamaterials: Intrinsically Driven Self‐Assembly at the Nanoscale

Increasingly intricate in their composition and structural organization, hierarchical multicomponent metamaterials with nonlinear spatially reconfigurable functionalities challenge the intrinsic constraints of natural materials, revealing tremendous potential for the advancement of biochemistry, nanophotonics, and medicine. Recent breakthroughs in high‐resolution nanofabrication utilizing ultranarrow, precisely controlled ion or laser beams have enabled assembly of architectures of unprecedented structural and functional complexity, yet costly, time‐ and energy‐consuming high‐resolution sequential techniques do not operate effectively at industry‐required scale. Inspired by the fictional Baron Munchausen's fruitless attempt to pull himself up, it is demonstrated that metamaterials can undergo intrinsically driven self‐assembly, metaphorically pulling themselves up into existence. These internal drivers hold a key to unlocking the potential of metamaterials and mapping a new direction for the large‐area, cost‐efficient self‐organized fabrication of practical devices. A systematic exploration of these efforts is presently missing, and the driving forces governing the intrinsically driven self‐assembly are yet to be fully understood. Here, recent progress in the self‐organized formation and self‐propelled growth of complex hierarchical multicomponent metamaterials is reviewed, with emphasis on key principles, salient features, and potential limitations of this family of approaches. Special stress is placed on self‐assembly driven by plasma, current in liquid, ultrasonic, and similar highly energetic effects, which enable self‐directed formation of metamaterials with unique properties and structures.     

Bioinspired Heterogeneous Structural Color Stripes from Capillaries

As an important characteristic of many creatures, structural colors play a crucial role in the survival of organisms. Inspired by these features, an intelligent structural color material with a heterogeneous striped pattern and stimuli‐responsivity by fast self‐assembly of colloidal nanoparticles in capillaries with a certain diameter range are presented here. The width, spacing, color, and even combination of the structural color stripe patterns can be precisely tailored by adjusting the self‐assembly parameters. Attractively, with the integration of a near‐infrared (NIR) light responsive graphene hydrogel into the structural color stripe pattern, the materials are endowed with light‐controlled reversible bending behavior with self‐reporting color indication. It is demonstrated that the striped structural color materials can be used as NIR‐light‐triggered dynamic barcode labels for the anti‐counterfeiting of different products. These features of the bioinspired structural color stripe pattern materials indicate their potential values for mimicking structural color organisms, which will find important applications in constructing intelligent sensors, anti‐counterfeiting devices, and so on.     

Photoaligned Nanorod Enhancement Films with Polarized Emission for Liquid‐Crystal‐Display Applications

Semiconductor nanorods (NR) emit polarized light, which is expected to bring manifold benefits, in terms of brightness and color enhancement, for modern liquid‐crystal displays (LCD). In this regard, photoaligned nanorod enhancement films (NREF) for color and polarization conversion for LCD backlights are introduced here. The photoinduced anchoring forces, by the photoalignment layer, stimulate well‐ordered self‐assembly of NR in the thin polymer films. Green and red emitting NR with a quantum yield of ≈80% are aligned unidirectionally and in‐plane, showing a polarization ratio of >7:1 and a degree of polarization of >0.81. The photoalignment technique facilitates the fabrication of mixed and multiple stacked NREF for LCDs, which improves the color gamut and polarization efficiency, and is thus expected to increase the optical efficiency of conventional LCDs by ≈60%.     

Self‐Regulating Iris Based on Light‐Actuated Liquid Crystal Elastomer

The iris, found in many animal species, is a biological tissue that can change the aperture (pupil) size to regulate light transmission into the eye in response to varying illumination conditions. The self‐regulation of the eye lies behind its autofocusing ability and large dynamic range, rendering it the ultimate “imaging device” and a continuous source of inspiration in science. In optical imaging devices, adjustable apertures play a vital role as they control the light exposure, the depth of field, and optical aberrations of the systems. Tunable irises demonstrated to date require external control through mechanical actuation, and are not capable of autonomous action in response to changing light intensity without control circuitry. A self‐regulating artificial iris would offer new opportunities for device automation and stabilization. Here, this paper reports the first iris‐like, liquid crystal elastomer device that can perform automatic shape‐adjustment by reacting to the incident light power density. Similar to natural iris, the device closes under increasing light intensity, and upon reaching the minimum pupil size, reduces the light transmission by a factor of seven. The light‐responsive materials design, together with photoalignment‐based control over the molecular orientation, provides a new approach to automatic, self‐regulating optical systems based on soft smart materials.     

Self‐Assembly of Graphene Oxide at Interfaces

Due to its amphiphilic property, graphene oxide (GO) can achieve a variety of nanostructures with different morphologies (for example membranes, hydrogel, crumpled particles, hollow spheres, sack‐cargo particles, Pickering emulsions, and so on) by self‐assembly. The self‐assembly is mostly derived from the self‐concentration of GO sheets at various interfaces, including liquid‐air, liquid‐liquid and liquid‐solid interfaces. This paper gives a comprehensive review of these assembly phenomena of GO at the three types of interfaces, the derived interfacial self‐assembly techniques, and the as‐obtained assembled materials and their properties. The interfacial self‐assembly of GO, enabled by its fantastic features including the amphiphilicity, the negatively charged nature, abundant oxygen‐containing groups and two‐dimensional flexibility, is highlighted as an easy and well‐controlled strategy for the design and preparation of functionalized carbon materials, and the use of self‐assembly for uniform hybridization is addressed for preparing hybrid carbon materials with various functions. A number of new exciting and potential applications are also presented for the assembled GO‐based materials. This contribution concludes with some personal perspectives on future challenges before interfacial self‐assembly may become a major strategy for the application‐targeted design and preparation of functionalized carbon materials.     

Smectic Layer Origami via Preprogrammed Photoalignment

Hierarchical architecture is of vital importance in soft materials. Focal conic domains (FCDs) of smectic liquid crystals, characterized by an ordered lamellar structure, attract intensive attention. Simultaneously tailoring the geometry and clustering characteristics of FCDs remains a challenge. Here, the 3D smectic layer origami via a 2D preprogrammed photoalignment film is accomplished. Full control of hierarchical superstructures is demonstrated, including the domain size, shape, and orientation, and the lattice symmetry of fragmented toric FCDs. The unique symmetry breaking of resultant superstructures combined with the optical anisotropy of the liquid crystals induces an intriguing polarization‐dependent diffraction. This work broadens the scientific understanding of self‐assembled soft materials and may inspire new opportunities for advanced functional materials and devices.     

Interface‐Directed Self‐Assembly of Cell‐Laden Microgels

Cell‐laden hydrogels show great promise for creating engineered tissues. However, a major shortcoming with these systems has been the inability to fabricate structures with controlled micrometer‐scale features on a biologically relevant length scale. In this Full Paper, a rapid method is demonstrated for creating centimeter‐scale, cell‐laden hydrogels through the assembly of shape‐controlled microgels or a liquid–air interface. Cell‐laden microgels of specific shapes are randomly placed on the surface of a high‐density, hydrophobic solution, induced to aggregate and then crosslinked into macroscale tissue‐like structures. The resulting assemblies are cell‐laden hydrogel sheets consisting of tightly packed, ordered microgel units. In addition, a hierarchical approach creates complex multigel building blocks, which are then assembled into tissues with precise spatial control over the cell distribution. The results demonstrate that forces at an air–liquid interface can be used to self‐assemble spatially controllable, cocultured tissue‐like structures.     

Molecular Self‐Assembly on Graphene

The formation of ordered arrays of molecules via self‐assembly is a rapid, scalable route towards the realization of nanoscale architectures with tailored properties. In recent years, graphene has emerged as an appealing substrate for molecular self‐assembly in two dimensions. Here, the first five years of progress in supramolecular organization on graphene are reviewed. The self‐assembly process can vary depending on the type of graphene employed: epitaxial graphene, grown in situ on a metal surface, and non‐epitaxial graphene, transferred onto an arbitrary substrate, can have different effects on the final structure. On epitaxial graphene, the process is sensitive to the interaction between the graphene and the substrate on which it is grown. In the case of graphene that strongly interacts with its substrate, such as graphene/Ru(0001), the inhomogeneous adsorption landscape of the graphene moiré superlattice provides a unique opportunity for guiding molecular organization, since molecules experience spatially constrained diffusion and adsorption. On weaker‐interacting epitaxial graphene films, and on non‐epitaxial graphene transferred onto a host substrate, self‐assembly leads to films similar to those obtained on graphite surfaces. The efficacy of a graphene layer for facilitating planar adsorption of aromatic molecules has been repeatedly demonstrated, indicating that it can be used to direct molecular adsorption, and therefore carrier transport, in a certain orientation, and suggesting that the use of transferred graphene may allow for predictible molecular self‐assembly on a wide range of surfaces.     

Self‐Healing of Polarizing Films via the Synergy between Gold Nanorods and Vitrimer

Conventional self‐healing is about the recovery of shape and mechanical properties. In contrast, recovery of functional properties is still a great challenge, especially for optical functional materials, as the known self‐healing methods are incompatible with optical properties. By utilizing the synergistic effect between Au nanorods and vitrimer, the alignment of Au nanorods can be achieved in the crosslinked polymer. The optical properties of the resulting polarizing film, such as light transmittance and polarization degree, can be fully recovered without an external repair agent. With simple laser irradiation to induce the photothermal effect of Au nanorods, the shape‐memory effect of vitrimer returns the Au nanorods to their initial orientation, and the plasticity achieves in situ self‐healing of the cutting area. The self‐healing of polarizing film provides a new research direction and reference for the application of self‐healing systems in functional materials.     

Flash Light Millisecond Self‐Assembly of High χ Block Copolymers for Wafer‐Scale Sub‐10 nm Nanopatterning

One of the fundamental challenges encountered in successful incorporation of directed self‐assembly in sub‐10 nm scale practical nanolithography is the process compatibility of block copolymers with a high Flory–Huggins interaction parameter (χ). Herein, reliable, fab‐compatible, and ultrafast directed self‐assembly of high‐χ block copolymers is achieved with intense flash light. The instantaneous heating/quenching process over an extremely high temperature (over 600 °C) by flash light irradiation enables large grain growth of sub‐10 nm scale self‐assembled nanopatterns without thermal degradation or dewetting in a millisecond time scale. A rapid self‐assembly mechanism for a highly ordered morphology is identified based on the kinetics and thermodynamics of the block copolymers with strong segregation. Furthermore, this novel self‐assembly mechanism is combined with graphoepitaxy to demonstrate the feasibility of ultrafast directed self‐assembly of sub‐10 nm nanopatterns over a large area. A chemically modified graphene film is used as a flexible and conformal light‐absorbing layer. Subsequently, transparent and mechanically flexible nanolithography with a millisecond photothermal process is achieved leading the way for roll‐to‐roll processability.     

Control of spatial polarization by use of a liquid crystal with an optically treated alignment layer and its application to beam apodization

We have investigated the alignment of a liquid crystal whose orientation is controlled by photoisomerization reaction for use in developing optical devices to improve beam quality. A glass window of a liquid-crystal cell that is coated with poly(vinyl alcohol) doped with azo dye was illuminated with a Hg lamp. We confirmed the dependence of the spatially controlled alignment direction of a liquid crystal on the irradiation time of this ultraviolet light. The new azo dye used in this study substantially reduced the illumination energy density required for aligning liquid-crystal molecules. We have demonstrated the control of polarization and successfully fabricated a serrated apodizing aperture and a soft aperture.     

Magnetically Guided Self‐Assembly and Coding of 3D Living Architectures

In nature, cells self‐assemble at the microscale into complex functional configurations. This mechanism is increasingly exploited to assemble biofidelic biological systems in vitro. However, precise coding of 3D multicellular living materials is challenging due to their architectural complexity and spatiotemporal heterogeneity. Therefore, there is an unmet need for an effective assembly method with deterministic control on the biomanufacturing of functional living systems, which can be used to model physiological and pathological behavior. Here, a universal system is presented for 3D assembly and coding of cells into complex living architectures. In this system, a gadolinium‐based nonionic paramagnetic agent is used in conjunction with magnetic fields to levitate and assemble cells. Thus, living materials are fabricated with controlled geometry and organization and imaged in situ in real time, preserving viability and functional properties. The developed method provides an innovative direction to monitor and guide the reconfigurability of living materials temporally and spatially in 3D, which can enable the study of transient biological mechanisms. This platform offers broad applications in numerous fields, such as 3D bioprinting and bottom‐up tissue engineering, as well as drug discovery, developmental biology, neuroscience, and cancer research.     

Photovoltaic–Pyroelectric Coupled Effect Induced Electricity for Self‐Powered Photodetector System

Ferroelectric materials have demonstrated novel photovoltaic effect to scavenge solar energy. However, most of the ferroelectric materials with wide bandgaps (2.7–4 eV) suffer from low power conversion efficiency of less than 0.5% due to absorbing only 8–20% of solar spectrum. Instead of harvesting solar energy, these ferroelectric materials can be well suited for photodetector applications, especially for sensing near‐UV irradiations. Here, a ferroelectric BaTiO₃ film‐based photodetector is demonstrated that can be operated without using any external power source and a fast sensing of 405 nm light illumination is enabled. As compared with photovoltaic effect, both the responsivity and the specific detectivity of the photodetector can be dramatically enhanced by larger than 260% due to the light‐induced photovoltaic–pyroelectric coupled effect. A self‐powered photodetector array system can be utilized to achieve spatially resolved light intensity detection by recording the output voltage signals as a mapping figure.     

Patterning of Discotic Liquid Crystals with Tunable Molecular Orientation for Electronic Applications

The large‐area formation of functional micropatterns with liquid crystals is of great significance for diversified applications in interdisciplinary fields. Meanwhile, the control of molecular alignment in the patterns is fundamental and prerequisite for the adequate exploitation of their photoelectric properties. However, it would be extremely complicated and challenging for discotic liquid crystals (DLCs) to achieve the goal, because they are insensitive to external fields and surface chemistry. Herein, a simple method of patterning and aligning DLCs on flat substrates is disclosed through precise control of the formation and dewetting of the capillary liquid bridges, within which the DLC molecules are confined. Large‐area uniform alignment occurs spontaneously due to directional shearing force when the solvent is slowly evaporated and programmable patterns could be directly generated on desired substrates. Moreover, the in‐plane column direction of DLCs is tunable by slightly tailoring their chemical structures which changes their self‐assembly behaviors in liquid bridges. The patterned DLCs show molecular orientation–dependent charge transport properties and are promising for templating self‐assembly of other materials. The study provides a facile method for manipulation of the macroscopic patterns and microscopic molecular orientation which opens up new opportunities for electronic applications of DLCs.     

Confined Assembly of Asymmetric Block‐Copolymer Nanofibers via Multiaxial Jet Electrospinning

Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene‐b‐isoprene), PS‐b‐PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post‐fabrication annealing of the fibers to obtain equilibrium self‐assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self‐assembled structures. Various confined assemblies such as co‐existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self‐assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self‐assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block‐copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS‐b‐PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS‐b‐PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self‐assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high‐end applications such as drug delivery.     

Directed Self‐Assembly of Liquid‐Crystalline Molecular Building Blocks for Sub‐5 nm Nanopatterning

The thin‐film directed self‐assembly of molecular building blocks into oriented nanostructure arrays enables next‐generation lithography at the sub‐5 nm scale. Currently, the fabrication of inorganic arrays from molecular building blocks is restricted by the limited long‐range order and orientation of the materials, as well as suitable methodologies for creating lithographic templates at sub‐5 nm dimensions. In recent years, higher‐order liquid crystals have emerged as functional thin films for organic electronics, nanoporous membranes, and templated synthesis, which provide opportunities for their use as lithographic templates. By choosing examples from these fields, recent progress toward the design of molecular building blocks is highlighted, with an emphasis on liquid crystals, to access sub‐5 nm features, their directed self‐assembly into oriented thin films, and, importantly, the fabrication of inorganic arrays. Finally, future challenges regarding sub‐5 nm patterning with liquid crystals are discussed.     

Independent Blue and Red Light Triggered Narcissistic Self‐Sorting Self‐Assembly of Colloidal Particles

The ability of living systems to self‐sort different cells into separate assemblies and the ability to independently regulate different structures are one ingredient that gives rise to their spatiotemporal complexity. Here, this self‐sorting behavior is replicated in a synthetic system with two types of colloidal particles; where each particle type independently self‐assembles either under blue or red light into distinct clusters, known as narcissistic self‐sorting. For this purpose, each particle type is functionalized either with the light‐switchable protein VVDHigh or Cph1, which homodimerize under blue and red light, respectively. The response to different wavelengths of light and the high specificity of the protein interactions allows for the independent self‐assembly of each particle type with blue or red light and narcissistic self‐sorting. Moreover, as both of the photoswitchable protein interactions are reversible in the dark; also, the self‐sorting is reversible and dynamic. Overall, the independent blue and red light controlled self‐sorting in a synthetic system opens new possibilities to assemble adaptable, smart, and advanced materials similar to the complexity observed in tissues.     

Layer‐by‐Layer Approach to (2+1)D Photonic Crystal Superlattice with Enhanced Crystalline Integrity

Large‐area polystyrene (PS) colloidal monolayers with high mechanical strength are created by a combination of the air/water interface self‐assembly and the solvent vapor annealing technique. Layer‐by‐layer (LBL) stacking of these colloidal monolayers leads to the formation of (2+1)D photonic crystal superlattice with enhanced crystalline integrity. By manipulating the diameter of PS spheres and the repetition period of the colloidal monolayers, flexible control in structure and stop band position of the (2+1)D photonic crystal superlattice has been realized, which may afford new opportunities for engineering photonic bandgap materials. Furthermore, an enhancement of 97.3% on light output power of a GaN‐based light emitting diode is demonstrated when such a (2+1)D photonic crystal superlattice employed as a back reflector. The performance enhancement is attributed to the photonic bandgap enhancement and good angle‐independence of the (2+1)D photonic crystal superlattice.     

Remotely Light‐Powered Soft Fluidic Actuators Based on Plasmonic‐Driven Phase Transitions in Elastic Constraint

Materials capable of actuation through remote stimuli are crucial for untethering soft robotic systems from hardware for powering and control. Fluidic actuation is one of the most applied and versatile actuation strategies in soft robotics. Here, the first macroscale soft fluidic actuator is derived that operates remotely powered and controlled by light through a plasmonically induced phase transition in an elastomeric constraint. A multiphase assembly of a liquid layer of concentrated gold nanoparticles in a silicone or styrene–ethylene–butylene–styrene elastic pocket forms the actuator. Upon laser excitation, the nanoparticles convert light of specific wavelength into heat and initiate a liquid‐to‐gas phase transition. The related pressure increase inflates the elastomers in response to laser wavelength, intensity, direction, and on–off pulses. During laser‐off periods, heating halts and condensation of the gas phase renders the actuation reversible. The versatile multiphase materials actuate—like soft “steam engines”—a variety of soft robotic structures (soft valve, pnue‐net structure, crawling robot, pump) and are capable of operating in different environments (air, water, biological tissue) in a single configuration. Tailored toward the near‐infrared window of biological tissue, the structures actuate also through animal tissue for potential medical soft robotic applications.