Self-assembly of graphene oxide flakes for smart and multifunctional coating with reversible formation of wrinkling patterns.

One of the main purposes of smart and multifunctional coatings is to have the versatility to be applied in a wide range of applications. However, the functions of smart materials are often highly limited. In particular, the stimuli-responsive lateral expansion of coatings based on 2D materials has not been reported before. This manuscript describes small two-dimensional graphene oxide (GO) flakes (e.g., thin sheets with a thickness of a few nanometers and much larger lateral dimensions) that act as elementary agents for the formation of smart and multifunctional coatings. The coating can be self-assembled from the GO flakes and disassembled flexibly when required. The coating is stimuli-responsive: upon localized contact with water, it expands and forms wrinkling patterns throughout its whole surface. Evaporating the water allows the wrinkles to disappear; hence, the process is reversible. This stimuli-responsiveness can be controlled to be reduced or completely switched off by temperature or pressure. These features are fundamentally due to the reversible intermolecular interactions among the flakes and favorable packing structure of the coating. The smart coating is shown to be useful for patterned fluidic systems of the desired shapes and the development of channels between fluidic reservoirs via the shortest path. Importantly, these results showed that a simple collection of uniquely 2D elementary agents with small nanoscale thickness can self-assemble into macroscopic materials that perform interactive and multifunctional operations.

Static charge is an ionic molecular fragment.

What is static charge? Despite the long history of research, the identity of static charge and mechanism by which static is generated by contact electrification are still unknown. Investigations are challenging due to the complexity of surfaces. This study involves the molecular-scale analysis of contact electrification using highly well-defined surfaces functionalized with a self-assembled monolayer of alkylsilanes. Analyses show the elementary molecular steps of contact electrification: the exact location of heterolytic cleavage of covalent bonds (i.e., Si-C bond), exact charged species generated (i.e., alkyl carbocation), and transfer of molecular fragments. The strong correlation between charge generation and molecular fragments due to their signature odd-even effects further shows that contact electrification is based on cleavage of covalent bonds and transfer of ionic molecular fragments. Static charge is thus an alkyl carbocation; in general, it is an ionic molecular fragment. This mechanism based on cleavage of covalent bonds is applicable to general types of insulating materials, such as covalently bonded polymers. The odd-even effect of charging caused by the difference of only one atom explains the highly sensitive nature of contact electrification.

Functional polymeric molecules for performing autonomous synthesis of particles with core-shell structures and customizable shapes.

Self-organization by the directed migration of components within a system is an important process in many applications, such as the unidirectional migration of motor proteins for transporting items to specific sites in a cell. This manuscript describes a class of functional polymeric molecules that have a set of instructions written by specific chemical moieties. These instructions allow the functional polymeric molecules to be used for autonomous synthesis of particles: particles with both functional core-shell structure and customizable shapes are fabricated for the first time. The functional polymeric molecules direct the large-scale migration of the liquid molecules to specific sites for forming the required customized structure of the particle, thus overcoming previous challenges of fabricating this class of particles. This first synthesis of this class of particles enables the development of novel applications: the concept of shape specificity for targeting sites. Both the basic structural properties (core-shell structure and customizable shape) are used in the specific applications of targeted drug delivery and imaging. The secure physical fit due to the complementary shapes enables the particles to remain locked in position for the targeting. Polymeric molecules are first shown to be highly capable of being encoded with instructions for autonomous synthesis of structured materials.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications.

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

Performing calculus: Asymmetric adaptive stimuli-responsive material for derivative control.

Materials (e.g., brick or wood) are generally perceived as unintelligent. Even the highly researched "smart" materials are only capable of extremely primitive analytical functions (e.g., simple logical operations). Here, a material is shown to have the ability to perform (i.e., without a computer), an advanced mathematical operation in calculus: the temporal derivative. It consists of a stimuli-responsive material coated asymmetrically with an adaptive impermeable layer. Its ability to analyze the derivative is shown by experiments, numerical modeling, and theory (i.e., scaling between derivative and response). This class of freestanding stimuli-responsive materials is demonstrated to serve effectively as a derivative controller for controlled delivery and self-regulation. Its fast response realizes the same designed functionality and efficiency as complex industrial derivative controllers widely used in manufacturing. These results illustrate the possibility to associate specifically designed materials directly with higher concepts of mathematics for the development of "intelligent" material-based systems.

Nonconductive Noncharging Composites: Tunable and Stretchable Materials for Adaptive Prevention of Charging by Contact Electrification.

Static charge generated by contact electrification can cause a wide range of undesirable consequences in our lives and in industry (e.g., adhesion of particles on surfaces, damage to electronics, and explosions). It has, however, been challenging to develop methods to prevent charging due to the vast types of materials that charge easily by contact electrification and the frequent changes in process and environmental conditions. The most common method is to use conductive materials for dissipating charge away; however, it is ineffective for many circumstances. Here, we propose a general and effective materials framework that involves a two-level consideration for preparing noncharging materials: (1) the variation of the proportion of a two-material composite and (2) the extent of stretching the composite material. This materials strategy is achieved by infusing particles within a stretchable bulk material. Importantly, the preparation of the noncharging surface for (1) is based on a novel fundamental mechanism that involves combining an appropriate amount of a material (e.g., the particles) that tends to charge positively with another material (e.g., the bulk material) that tends to charge negatively. This mechanism does not rely on conductivity; both the contacting materials naturally prevent the generation of static charge even when only nonconductive materials are involved. When the composite material is stretchable, the change in proportion of the surface coverage of the particles allows the charging response to be changed. Therefore, the variation in composition and stretching provide a wide two-dimensional parameter space for achieving noncharging response for the vast range of contacting materials that are used in industry and our lives. In addition, stretchability allows the composite material to flexibly adapt to changes in process and environmental conditions. This stretchable composite material was also demonstrated to be capable of preventing the adhesion of particles and separating particles of different materials.

On-demand fully customizable drug tablets via 3D printing technology for personalized medicine.

Personalized medicine should ideally be prescribed to every individual because of the unique characteristics (e.g., biological, physical, and medical) of each individual. It is, however, challenging to provide personalized medicine for the mass population of specific individuals effectively and efficiently. This manuscript describes a method of fabricating fully customizable drug tablets for personalized medicine by the 3D printing technology. This method involves the versatile fabrication of the tablets via the specifically designed 3D printed molds of different shapes and sizes, and an intuitive 1-dimensional release of drug that relates the shape of the drug-containing matrix to the release profile. The customization includes all the aspects of varying dosage, duration, release profiles, and combination of multiple drugs. In particular, it has previously been technically difficult to devise a single platform that fabricates carriers that release drug with any desired type of release profiles. This method of fabricating fully customizable tablets is simple, inexpensive, and efficient. Detailed selection and investigation of the materials ensured that the tablet and the method of fabrication are safe (e.g., biocompatible, FDA-approved ingredients used) and other desirable features (e.g., sustained release and high dosage) are achieved. These desirable characteristics of the method thus allow fully customizable drug tablets to be fabricated efficiently on the spot after the diagnosis of individual patients; at the same time, the method can be made widely accessible to the mass population. Hence, the concept of personalized medicine can truly be realized.

The Relationship between Static Charge and Shape.

The amount of charge of a material has always been regarded as a property (or state) of materials and can be measured precisely and specifically. This study describes for the first time a fundamental physical-chemical phenomenon in which the amount of charge of a material is actually a variable-it depends on the shape of the material. Materials are shown to have continuously variable and reversible ranges of charge states by changing their shapes. The phenomenon was general for different shapes, transformations, materials, atmospheric conditions, and methods of charging. The change in charge was probably due to a dynamic exchange of charge from the material to the surrounding atmosphere as the shape changed via the reversible ionization and deposition of air molecules. Similar changes in charge were observed for self-actuating materials that changed their shapes autonomously. This fundamental relationship between geometry and electrostatics via chemistry is important for the broad range of applications related to the charge of flexible materials.