Stretchable and self-healing polymers and devices for electronic skin

This review covers some of the most recent advances in stretchable and self-healing polymers and devices for Electronic skin (E-skin) applications. Applications for both stretchable and self-healing materials include, but are not limited to, electronics, displays, energy, the environment, and medicine. While the majority of organic materials can generally be rendered flexible, such materials are not stretchable, which is a key mechanical property necessary to realize applications of E-skin for prosthetics, artificial intelligence, systems for robotics, personal health monitoring, biocompatibility, and communication devices. In our effort to survey materials utilized in various components of an electronic device, we report herein recent advances in stretchable and self-healing conductors, semiconductors, and substrates. We highlight some key technologies recently developed in stretchable organic-based sensors, solar cells, light-emitting diodes, and self-healing electronic devices.     

Organic-based molecular switches for molecular electronics

In a general sense, molecular electronics (ME) is the branch of nanotechnology which studies the application of molecular building blocks for the fabrication of electronic components. Among the different types of molecules, organic compounds have been revealed as promising candidates for ME, due to the easy access, great structural diversity and suitable electronic and mechanical properties. Thanks to these useful capabilities, organic molecules have been used to emulate electronic devices at the nanoscopic scale. In this feature article, we present the diverse strategies used to develop organic switches towards ME with special attention to non-volatile systems.     

Adhesive lithography for fabricating organic electronic and optoelectronics devices

Improvements in organic electronic materials have led to novel device applications, ranging from large-area flexible displays to lightweight plastic electronics. Progress on these applications would benefit from development of low-cost fabrication techniques for organic semiconductors. In this review, several fabrication processes based on adhesion force (i.e. van der Waals forces, thiol-metal reactions, and cold welding) are introduced. These patterning techniques are dry patterning techniques, i.e., the electronic materials are patterned from the raised regions of molds onto a substrate directly by additive or subtractive patterning methods. Patterning of organic small molecule, polymer thin films and metal electrodes by adhesive lithography is demonstrated. The operating properties of patterned organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs) are comparable with the performance of devices fabricated by conventional evaporation deposition methods.     

Fabrication of ultra-fine nanostructures using edge transfer printing

The exploration of new methods and techniques for application in diverse fields, such as photonics, microfluidics, biotechnology and flexible electronics is of increasing scientific and technical interest for multiple uses over distance of 10-100 nm. This article discusses edge transfer printing--a series of unconventional methods derived from soft lithography for nanofabrication. It possesses the advantages of easy fabrication, low-cost and great serviceability. In this paper, we show how to produce exposed edges and use various materials for edge transfer printing, while nanoskiving, nanotransfer edge printing and tunable cracking for nanogaps are introduced. Besides this, different functional materials, such as metals, inorganic semiconductors and polymers, as well as localised heating and charge patterning, are described here as unconventional "inks" for printing. Edge transfer printing, which can effectively produce sub-100 nm scale ultra-fine structures, has broad applications, including metallic nanowires as nanoelectrodes, semiconductor nanowires for chemical sensors, heterostructures of organic semiconductors, plasmonic devices and so forth.     

Solution processable semiconducting organic single crystals

In this review we describe recent progress in fabrication, characterisation and measurements of solution processed organic single crystals based on small molecule semiconductors. We focus on single crystal applications using Field-Effect Transistors as building blocks for organic electronics.     

Superlattice nanowire pattern transfer (SNAP)

During the past 15 years or so, nanowires (NWs) have emerged as a new and distinct class of materials. Their novel structural and physical properties separate them from wires that can be prepared using the standard methods for manufacturing electronics. NW-based applications that range from traditional electronic devices (logic and memory) to novel biomolecular and chemical sensors, thermoelectric materials, and optoelectronic devices, all have appeared during the past few years. From a fundamental perspective, NWs provide a route toward the investigation of new physics in confined dimensions. Perhaps the most familiar fabrication method is the vapor-liquid-solid (VLS) growth technique, which produces semiconductor nanowires as bulk materials. However, other fabrication methods exist and have their own advantages. In this Account, I review a particular class of NWs produced by an alternative method called superlattice nanowire pattern transfer (SNAP). The SNAP method is distinct from other nanowire preparation methods in several ways. It can produce large NW arrays from virtually any thin-film material, including metals, insulators, and semiconductors. The dimensions of the NWs can be controlled with near-atomic precision, and NW widths and spacings can be as small as a few nanometers. In addition, SNAP is almost fully compatible with more traditional methods for manufacturing electronics. The motivation behind the development of SNAP was to have a general nanofabrication method for preparing electronics-grade circuitry, but one that would operate at macromolecular dimensions and with access to a broad materials set. Thus, electronics applications, including novel demultiplexing architectures; large-scale, ultrahigh-density memory circuits; and complementary symmetry nanowire logic circuits, have served as drivers for developing various aspects of the SNAP method. Some of that work is reviewed here. As the SNAP method has evolved into a robust nanofabrication method, it has become an enabling tool for the investigation of new physics. In particular, the application of SNAP toward understanding heat transport in low-dimensional systems is discussed. This work has led to the surprising discovery that Si NWs can serve as highly efficient thermoelectric materials. Finally, we turn toward the application of SNAP to the investigation of quasi-one-dimensional (quasi-1D) superconducting physics in extremely high aspect ratio Nb NWs.     

Graphene‐based electrodes for flexible electronics

As ‘flexibility’ has emerged as an important issue in next‐generation electronics, many efforts to find new classes of materials have been devoted to realizing stretchable, bendable and foldable electronic devices. For these devices to be realized, graphene has been considered as one of the most promising candidates for flexible electrodes due to its extraordinary electrical, optical and mechanical properties. Particularly, recent developments in the fabrication and modification of graphene point to a bright future for graphene electrodes in flexible electronics. This mini‐review summarizes the recent progress in graphene films as flexible electrodes for various applications such as solar cells, organic light‐emitting diodes, touchscreens, transistors and supercapacitors. © 2015 Society of Chemical Industry     

Biomimetic graphene films and their properties

Biomimetic fabrication has long been considered a short cut to the rational design and production of artificial materials or devices that possess fascinating properties, just like natural creatures. Considering the fact that graphene exhibits a lot of exceptional properties in a wide range of scientific fields, biomimetic fabrication of graphene multiscale structures, denoted as biomimetic graphene, is of great interest in both fundamental research and industrial applications. Especially, the combination of graphene with biomimetic structures would realize structural and functional integrity, and thus bring a new opportunity of developing novel graphene-based devices with remarkable performance. In this feature article, we highlight the recent advances in biomimetic graphene films and their structure-defined properties. Functionalized graphene films with multiscale structures inspired from a wide range of biomaterials including rose petals, butterfly wings, nacre and honeycomb have been collected and presented. Moreover, both current challenges and future perspectives of biomimetic graphene are discussed. Although research of the so-called "biomimetic graphene" is still at an early stage, it might become a "hot topic" in the near future.     

Navigating the Biological-Material Interface with the Guide of Chemical Biology

Research at the biological-material interface often has translation in mind, with applications in medical implants, drug delivery, and regenerative medicine. While the clinical impact of this research is undeniable, a clearer picture of the in vivo behavior of materials is needed to address longstanding limitations in performance and function. Advances in chemical biology and biotechnology have propelled our understanding of how small molecules and biologics behave in living systems. Adapting these techniques to the study of synthetic materials, enabled by modern polymer chemistry, will bring molecular resolution to biological-material interactions and guide the development of next-generation biomaterials for therapeutic and diagnostic applications.     

The ultimate diamond slab: GraphAne versus graphEne

In this article, we present a comprehensive characterization of three carbon nanomaterials of technological interest: graphene, graphane, and fluorinated graphene. By means of first principles and tight-binding calculations in combination with analytical methods, we carried out detailed comparative studies of their structural, mechanical, thermal, and electronic properties. The calculated elastic properties of these materials confirm their high mechanical stability and stiffness, which in association with their low dimensionality, translates into a large ballistic thermal conductance. Furthermore, we show that while graphene is a zero gap semi-metal, graphane and fluorinated graphene are wide gap semiconductors. Finally, we discuss designed interfaces between these systems, and show that their physical properties have potential applications in nanoelectronic devices.     

n-Channel semiconductor materials design for organic complementary circuits

Organic semiconductors have unique properties compared to traditional inorganic materials such as amorphous or crystalline silicon. Some important advantages include their adaptability to low-temperature processing on flexible substrates, low cost, amenability to high-speed fabrication, and tunable electronic properties. These features are essential for a variety of next-generation electronic products, including low-power flexible displays, inexpensive radio frequency identification (RFID) tags, and printable sensors, among many other applications. Accordingly, the preparation of new materials based on π-conjugated organic molecules or polymers has been a central scientific and technological research focus over the past decade. Currently, p-channel (hole-transporting) materials are the leading class of organic semiconductors. In contrast, high-performance n-channel (electron-transporting) semiconductors are relatively rare, but they are of great significance for the development of plastic electronic devices such as organic field-effect transistors (OFETs). In this Account, we highlight the advances our team has made toward realizing moderately and highly electron-deficient n-channel oligomers and polymers based on oligothiophene, arylenediimide, and (bis)indenofluorene skeletons. We have synthesized and characterized a "library" of structurally related semiconductors, and we have investigated detailed structure-property relationships through optical, electrochemical, thermal, microstructural (both single-crystal and thin-film), and electrical measurements. Our results reveal highly informative correlations between structural parameters at various length scales and charge transport properties. We first discuss oligothiophenes functionalized with perfluoroalkyl and perfluoroarene substituents, which represent the initial examples of high-performance n-channel semiconductors developed in this project. The OFET characteristics of these compounds are presented with an emphasis on structure-property relationships. We then examine the synthesis and properties of carbonyl-functionalized oligomers, which constitute second-generation n-channel oligothiophenes, in both vacuum- and solution-processed FETs. These materials have high carrier mobilities and good air stability. In parallel, exceptionally electron-deficient cyano-functionalized arylenediimide derivatives are discussed as early examples of thermodynamically air-stable, high-performance n-channel semiconductors; they exhibit record electron mobilities of up to 0.64 cm(2)/V·s. Furthermore, we provide an overview of highly soluble ladder-type macromolecular semiconductors as OFET components, which combine ambient stability with solution processibility. A high electron mobility of 0.16 cm(2)/V·s is obtained under ambient conditions for solution-processed films. Finally, examples of polymeric n-channel semiconductors with electron mobilities as high as 0.85 cm(2)/V·s are discussed; these constitute an important advance toward fully printed polymeric electronic circuitry. Density functional theory (DFT) computations reveal important trends in molecular physicochemical and semiconducting properties, which, when combined with experimental data, shed new light on molecular charge transport characteristics. Our data provide the basis for a fundamental understanding of charge transport in high-performance n-channel organic semiconductors. Moreover, our results provide a road map for developing functional, complementary organic circuitry, which requires combining p- and n-channel transistors.     

Transparent Conducting Oxides: Electronic Structure–Property Relationship from Photoelectron Spectroscopy with in situ Sample Preparation

The various applications of transparent conducting oxides (TCO), e.g., as electrodes in flat panel displays and solar cells or as low‐emissivity coatings have stimulated extensive research on their fabrication and properties. Recent experimental and theoretical studies of defect properties have considerably improved the understanding of the limitations of the electrical conductivity of both n‐ and p‐type transparent conductors and of the structural and electronic surface properties of the most important TCO materials. Development of emerging and future applications in the area of transparent thin film electronics with oxide semiconductors as well as the improvement of existing applications require a detailed control of the Fermi level position in the bulk and at surfaces and interfaces of polycrystalline and amorphous TCO materials. This feature article describes how the important parameters for such control can be identified using photoelectron spectroscopy with in situ sample preparation. The parameters influencing doping, work functions, ionization potentials, and surface band bending as well as energy band alignment at interfaces are described and discussed providing a fundamental understanding of important material properties for tailoring TCOs in electronic devices.     

Bioelectrochemical interface engineering: toward the fabrication of electrochemical biosensors, biofuel cells, and self-powered logic biosensors

Over the past decade, researchers have devoted considerable attention to the integration of living organisms with electronic elements to yield bioelectronic devices. Not only is the integration of DNA, enzymes, or whole cells with electronics of scientific interest, but it has many versatile potential applications. Researchers are using these ideas to fabricate biosensors for analytical applications and to assemble biofuel cells (BFCs) and biomolecule-based devices. Other research efforts include the development of biocomputing systems for information processing. In this Account, we focus on our recent progress in engineering at the bioelectrochemical interface (BECI) for the rational design and construction of important bioelectronic devices, ranging from electrochemical (EC-) biosensors to BFCs, and self-powered logic biosensors. Hydrogels and sol-gels provide attractive materials for the immobilization of enzymes because they make EC-enzyme biosensors stable and even functional in extreme environments. We use a layer-by-layer (LBL) self-assembly technique to fabricate multicomponent thin films on the BECI at the nanometer scale. Additionally, we demonstrate how carbon nanomaterials have paved the way for new and improved EC-enzyme biosensors. In addition to the widely reported BECI-based electrochemical impedance spectroscopy (EIS)-type aptasensors, we integrate the LBL technique with our previously developed "solid-state probe" technique for redox probes immobilization on electrode surfaces to design and fabricate BECI-based differential pulse voltammetry (DPV)-type aptasensors. BFCs can directly harvest energy from ambient biofuels as green energy sources, which could lead to their application as simple, flexible, and portable power sources. Porous materials provide favorable microenvironments for enzyme immobilization, which can enhance BFC power output. Furthermore, by introducing aptamer-based logic systems to BFCs, such systems could be applied as self-powered and intelligent aptasensors for the logic detection. We have developed biocomputing keypad lock security systems which can be also used for intelligent medical diagnostics. BECI engineering provides a simple but effective approach toward the design and fabrication of EC-biosensors, BFCs, and self-powered logic biosensors, which will make essential contributions in the development of creative and practical bioelectronic devices. The exploration of novel interface engineering applications and the creation of new fabrication concepts or methods merit further attention.     

Cyclic Dipeptide: A Privileged Molecular Scaffold to Derive Structural Diversity and Functional Utility

Cyclic dipeptides (CDPs) are the simplest form of cyclic peptides with a wide range of applications from therapeutics to biomaterials. CDP is a versatile molecular platform endowed with unique properties such as conformational rigidity, intermolecular interactions, structural diversification through chemical synthesis, bioavailability and biocompatibility. A variety of natural products with the CDP core exhibit anticancer, antifungal, antibacterial, and antiviral activities. The inherent bioactivities have inspired the development of synthetic analogues as drug candidates and drug delivery systems. CDP plays a crucial role as conformation and molecular assembly directing core in the design of molecular receptors, peptidomimetics and fabrication of functional material architectures. In recent years, CDP has rapidly become a privileged scaffold for the design of advanced drug candidates, drug delivery agents, bioimaging, and biomaterials to mitigate numerous disease conditions. This review describes the structural diversification and multifarious biomedical applications of the CDP scaffold, discusses challenges, and provides future directions for the emerging field.     

Chemical and engineering approaches to enable organic field-effect transistors for electronic skin applications

Skin is the body's largest organ and is responsible for the transduction of a vast amount of information. This conformable material simultaneously collects signals from external stimuli that translate into information such as pressure, pain, and temperature. The development of an electronic material, inspired by the complexity of this organ is a tremendous, unrealized engineering challenge. However, the advent of carbon-based electronics may offer a potential solution to this long-standing problem. In this Account, we describe the use of an organic field-effect transistor (OFET) architecture to transduce mechanical and chemical stimuli into electrical signals. In developing this mimic of human skin, we thought of the sensory elements of the OFET as analogous to the various layers and constituents of skin. In this fashion, each layer of the OFET can be optimized to carry out a specific recognition function. The separation of multimodal sensing among the components of the OFET may be considered a "divide and conquer" approach, where the electronic skin (e-skin) can take advantage of the optimized chemistry and materials properties of each layer. This design of a novel microstructured gate dielectric has led to unprecedented sensitivity for tactile pressure events. Typically, pressure-sensitive components within electronic configurations have suffered from a lack of sensitivity or long mechanical relaxation times often associated with elastomeric materials. Within our method, these components are directly compatible with OFETs and have achieved the highest reported sensitivity to date. Moreover, the tactile sensors operate on a time scale comparable with human skin, making them ideal candidates for integration as synthetic skin devices. The methodology is compatible with large-scale fabrication and employs simple, commercially available elastomers. The design of materials within the semiconductor layer has led to the incorporation of selectivity and sensitivity within gas-sensing devices and has enabled stable sensor operation within aqueous media. Furthermore, careful tuning of the chemical composition of the dielectric layer has provided a means to operate the sensor in real time within an aqueous environment and without the need for encapsulation layers. The integration of such devices as electronic mimics of skin will require the incorporation of biocompatible or biodegradable components. Toward this goal, OFETs may be fabricated with >99% biodegradable components by weight, and the devices are robust and stable, even in aqueous environments. Collectively, progress to date suggests that OFETs may be integrated within a single substrate to function as an electronic mimic of human skin, which could enable a large range of sensing-related applications from novel prosthetics to robotic surgery.     

Effect of different side chains on alternating copolymers of benzodithiophene and thiophene in organic solar cells

Polymeric semiconductors offer the dual advantages of lightness and flexibility, facilitating the large-scale production of organic electronic devices. In the present research, electron donor polymers were synthesized incorporating high electron density aromatic units, specifically benzodithiophene (BDT) and thiophene (Th), to explore their efficacy in organic electronics. This systematic study focused on evaluating the impact of varying side chains on the material properties of these polymers. It was found that polymers with Th side chains exhibited significantly enhanced thermal stability, approximately 100°C higher than their alkoxide side chain counterparts. For the polymer PEHO-BDT3HT, a bandgap value of around 1.6 eV was obtained. Furthermore, binary devices were developed using these novel copolymers, among which PDT-BDT3HT demonstrated superior photovoltaic performance, achieving a power conversion efficiency of 1.56% without any optimization. This work not only sheds light on the influence of side chain variations in polymer properties but also showcases the potential of BDT and Th-based copolymers in the field of organic electronics.     

The evolution of graphene-based electronic devices

Successful isolation of single-layer graphene, the two-dimensional allotrope of carbon from graphite, has fuelled a lot of interest in exploring the feasibility of using it for fabrication of various electronic devices, particularly because of its exceptional electronic properties. Graphene is poised to save Moore's law by acting as a successor of silicon-based electronics. This article reviews the success story of this allotrope with a focus on the structure, properties and preparation of graphene as well as its various device applications.     

Biomedical applications and colloidal properties of amphiphilically modified chitosan hybrids

Chitosan is among the most abundant biopolymers on earth and has been either used or exhibited potential in a wide variety of industrial and biomedical applications. With the advancement of materials technologies, chitosan has been chemically modified to self-assemble into nanoarchitectures that are usable in advanced biomedical applications, such as drug nanocarriers, macroscopic injectables, tissue-engineering scaffolds, and nanoimaging agents. Colloidal amphiphilically modified chitosan (AMC) is a relatively recent material receiving increased attention with numerous publications addressing the medical advantages of specific systems. To date, many reviews have focused on the synthesis and biomedical properties of chitosan-based biomaterials, but a comprehensive study focusing on the colloidal properties of AMC in relation to biomedical performance appears to be lacking. This review provides a survey of the field, critically reviewing the colloidal properties and biomedical performance of AMC systems, such as nanoparticle drug delivery systems and macroscopic medical devices. Finally, the future development, market potential, and clinical implications of these promising colloidal-structured biomaterials are summarised.     

Silk proteins for biomedical applications: Bioengineering perspectives

Biomaterials of either natural or synthetic origin are used to fabricate implantable devices, as carriers for bioactive molecules or as substrates to facilitate tissue regeneration. For the design of medical devices it is fundamental to use materials characterized by non-immunogenicity, biocompatibility, slow and/or controllable biodegradability, non-toxicity, and structural integrity. The success of biomaterial-derived biodevices tends to be based on the biomimetic architecture of the materials. Recently, proteins from natural precursors that are essentially structural and functional polymers, have gained popularity as biomaterials. The silks produced by silkworms or spiders are of particular interest as versatile protein polymers. These form the basis for diverse biomedical applications that exploit their unique biochemical nature, biocompatibility and high mechanical strength. This review discusses and summarizes the latest advances in the engineering of silk-based biomaterials, focusing specifically on the fabrication of diverse bio-mimetic structures such as films, hydrogels, scaffolds, nanofibers and nanoparticles; their functionalization and potential for biomedical applications.     

Versatility of photoalignment techniques: From nematics to a wide range of functional materials

Alignment methods of nematic liquid crystals (LCs) by surface photoreactions on substrate surfaces were initially proposed around 1990, and the photoalignment technology of nematic LCs has recently been integrated into the LC device fabrication industry due to its profitable features. Accumulated efforts in this field have revealed that applications of photoalignment processes are not limited to conventional nematic LCs but that a variety of functional materials can also be manipulated according to this principle. Target materials have now been extended to thermotropic smectic LCs, discotic LCs, LC polymers, block copolymers, gel networks, conjugated polymers, and organic semiconductors and lyotropic systems including chromonic LCs and inorganic–organic mesostructured hybrids. Through these photochemical approaches, many types of photopatterning for both topographical and orientational modulations have become feasible. This article reviews photoalignment processes applied to a wide range of materials, surveying relatively recent work. Some important related alignment and patterning processes are also introduced to clarify the significance of these photoalignment techniques.