Self‐Propelled Micro/Nanomotors for On‐Demand Biomedical Cargo Transportation

Micro/nanomotors (MNMs) are miniaturized machines that can perform assigned tasks at the micro/nanoscale. Over the past decade, significant progress has been made in the design, preparation, and applications of MNMs that are powered by converting different sources of energy into mechanical force, to realize active movement and fulfill on‐demand tasks. MNMs can be navigated to desired locations with precise controllability based on different guidance mechanisms. A considerable research effort has gone into demonstrating that MNMs possess the potential of biomedical cargo loading, transportation, and targeted release to achieve therapeutic functions. Herein, the recent advances of self‐propelled MNMs for on‐demand biomedical cargo transportation, including their self‐propulsion mechanisms, guidance strategies, as well as proof‐of‐concept studies for biological applications are presented. In addition, some of the major challenges and possible opportunities of MNMs are identified for future biomedical applications in the hope that it may inspire future research.     

Self‐Propelled Micro/Nanomotors for Sensing and Environmental Remediation

Self‐propelled micro/nanomotors have gained attention for successful application in cargo delivery, therapeutic treatments, sensing, and environmental remediation. Unique characteristics such as high speed, motion control, selectivity, and functionability promote the application of micro/nanomotors in analytical sciences. Here, the recent advancements and main challenges regarding the application of self‐propelled micro/nanomotors in sensing and environmental remediation are discussed. The current state of micro/nanomotors is reviewed, emphasizing the period of the last five years, then their developments into the future applications for enhanced sensing and efficient purification of water resources are extrapolated.     

Micro‐/Nanomotors toward Biomedical Applications: The Recent Progress in Biocompatibility

Inspired by the highly versatile natural motors, artificial micro‐/nanomotors that can convert surrounding energies into mechanical motion and accomplish multiple tasks are devised. In the past few years, micro‐/nanomotors have demonstrated significant potential in biomedicine. However, the practical biomedical applications of these small‐scale devices are still at an infant stage. For successful bench‐to‐bed translation, biocompatibility of micro‐/nanomotor systems is the central issue to be considered. Herein, the recent progress in micro‐/nanomotors in biocompatibility is reviewed, with a special focus on their biomedical applications. Through close collaboration between researches in the nanoengineering, material chemistry, and biomedical fields, it is expected that a promising real‐world application platform based on micro‐/nanomotors will emerge in the near future.     

Self‐Assembly of Hyperbranched Polymers and Its Biomedical Applications

Hyperbranched polymers (HBPs) are highly branched macromolecules with a three‐dimensional dendritic architecture. Due to their unique topological structure and interesting physical/chemical properties, HBPs have attracted wide attention from both academia and industry. In this paper, the recent developments in HBP self‐assembly and their biomedical applications have been comprehensively reviewed. Many delicate supramolecular structures from zero‐dimension (0D) to three‐dimension (3D), such as micelles, fibers, tubes, vesicles, membranes, large compound vesicles and physical gels, have been prepared through the solution or interfacial self‐assembly of amphiphilic HBPs. In addition, these supramolecular structures have shown promising applications in the biomedical areas including drug delivery, protein purification/detection/delivery, gene transfection, antibacterial/antifouling materials and cytomimetic chemistry. Such developments promote the interdiscipline researches among surpramolecular chemistry, biomedical chemistry, nano­technology and functional materials.     

Catalytic Nanomotors: Self‐Propelled Sphere Dimers

Experimental and theoretical studies of the self‐propelled motional dynamics of a new genre of catalytic sphere dimer, which comprises a non‐catalytic silica sphere connected to a catalytic platinum sphere, are reported for the first time. Using aqueous hydrogen peroxide as the fuel to effect catalytic propulsion of the sphere dimers, both quasi‐linear and quasi‐circular trajectories are observed in the solution phase and analyzed for different dimensions of the platinum component. In addition, well‐defined rotational motion of these sphere dimers is observed at the solution–substrate interface. The nature of the interaction between the sphere dimer and the substrate in the aqueous hydrogen peroxide phase is discussed. In computer simulations of the sphere dimer in solution and the solution–substrate interface, sphere‐dimer dynamics are simulated using molecular‐dynamics methods and solvent dynamics are modeled by mesoscopic multiparticle collision methods taking hydrodynamic interactions into account. The rotational and translational dynamics of the sphere dimer are found to be in good accord with the predictions of computer simulations.     

Bioinspired Photonic Pigments from Colloidal Self‐Assembly

The natural world is a colorful environment. Stunning displays of coloration have evolved throughout nature to optimize camouflage, warning, and communication. The resulting flamboyant visual effects and remarkable dynamic properties, often caused by an intricate structural design at the nano‐ and microscale, continue to inspire scientists to unravel the underlying physics and to recreate the observed effects. Here, the methodologies to create bioinspired photonic pigments using colloidal self‐assembly approaches are considered. The physics governing the interaction of light with structural features and natural examples of structural coloration are briefly introduced. It is then outlined how the self‐assembly of colloidal particles, acting as wavelength‐scale building blocks, can be particularly useful to replicate coloration from nature. Different coloration effects that result from the defined structure of the self‐assembled colloids are introduced and it is highlighted how these optical properties can be translated into photonic pigments by modifications of the assembly processes. The importance of absorbing elements, as well as the role of surface chemistry and wettability to control structural coloration is discussed. Finally, approaches to integrate dynamic control of coloration into such self‐assembled photonic pigments are outlined.     

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‐Organized Multiconstituent Catalytic Nanomotors

Self‐organized catalytic nanomotors consisting of more than one individual component are presented. Tadpole‐like catalytic nanomotors fabricated by dynamic shadowing growth (DSG) self‐organize randomly to form two‐nanomotor clusters (≈1–3% yield) that spin as opposed to circular motion exhibited by the individual structures. By introducing magnetic materials to another system, self‐assembled “helicopter” nanomotors consisting of a V‐shaped nanomotor and a microbead are formed with ≈25% yield, showing a significantly higher yield than the control (0%). A flexible swimmer system that performs complex swimming, such as maneuvering around stationary objects, is also presented. These nanomotor systems are inherently more complex than those previously studied and may be the next step towards building sophisticated multifunctional nanomachinery systems.     

Hierarchical and Heterogeneous Bioinspired Composites—Merging Molecular Self‐Assembly with Additive Manufacturing

Biological composites display exceptional mechanical properties owing to a highly organized, heterogeneous architecture spanning several length scales. It is challenging to translate this ordered and multiscale structural organization in synthetic, bulk composites. Herein, a combination of top‐down and bottom‐up approach is demonstrated, to form a polymer‐ceramic composite by macroscopically aligning the self‐assembled nanostructure of polymerizable lyotropic liquid crystals via 3D printing. The polymer matrix is then uniformly reinforced with bone‐like apatite via in situ biomimetic mineralization. The combinatorial method enables the formation of macrosized, heterogeneous composites where the nanostructure and chemical composition is locally tuned over microscopic distances. This enables precise control over the mechanics in specific directions and regions, with a unique intrinsic–extrinsic toughening mechanism. As a proof‐of‐concept, the method is used to form large‐scale composites mimicking the local nanostructure, compositional gradients and directional mechanical properties of heterogeneous tissues like the bone‐cartilage interface, for mechanically stable osteochondral plugs. This work demonstrates the possibility to create hierarchical and complex structured composites using weak starting components, thus opening new routes for efficient synthesis of high‐performance materials ranging from biomaterials to structural nanocomposites.     

Active Self‐Assembly of Train‐Shaped DNA Nanostructures via Catalytic Hairpin Assembly Reactions

Biomolecular self‐assembly is a powerful approach for fabricating supramolecular architectures. Over the past decade, a myriad of biomolecular assemblies, such as self‐assembly proteins, lipids, and DNA nanostructures, have been used in a wide range of applications, from nano‐optics to nanoelectronics and drug delivery. The method of controlling when and where the self‐assembly starts is essential for assembly dynamics and functionalization. Here, train‐shaped DNA nanostructures are actively self‐assembled using DNA tiles as artificial “carriages,” hairpin structures as “couplers,” and initiators of catalytic hairpin assembly (CHA) reactions as “wrenches.” The initiator wrench can selectively open the hairpin couplers to couple the DNA tile carriages with high product yield. As such, DNA nanotrains are actively prepared with two, three, four, or more carriages. Furthermore, by flexibly modifying the carriages with “biotin seats” (biotin‐modified DNA tiles), streptavidin “passengers” are precisely arranged in corresponding seats. The applications of the CHA‐triggered self‐assembly mechanism are also extended for assembling the large DNA origami dimer. With the creation of 1D architectures established, it is thought that this CHA‐triggered self‐assembly mechanism may provide a new element of control for complex autonomous assemblies from a variety of starting materials with specific sites and times.     

Bionanosphere Lithography via Hierarchical Peptide Self‐Assembly of Aromatic Triphenylalanine

A nanolithographic approach based on hierarchical peptide self‐assembly is presented. An aromatic peptide of N‐(t‐Boc)‐terminated triphenylalanine is designed from a structural motif for the β‐amyloid associated with Alzheimer's disease. This peptide adopts a turnlike conformation with three phenyl rings oriented outward, which mediate intermolecular π–π stacking interactions and eventually facilitate highly crystalline bionanosphere assembly with both thermal and chemical stability. The self‐assembled bionanospheres spontaneously pack into a hexagonal monolayer at the evaporating solvent edge, constituting evaporation‐induced hierarchical self‐assembly. Metal nanoparticle arrays or embossed Si nanoposts could be successfully created from the hexagonal bionanosphere array masks in conjunction with a conventional metal‐evaporation or etching process. Our approach represents a bionanofabrication concept that biomolecular self‐assembly is hierarchically directed to establish a straightforward nanolithography compatible with conventional device‐fabrication processes.