This Review summarizes recent progress in the development of polymer solar cells. It covers the scientific origins and basic properties of polymer solar cell technology, material requirements and device operation mechanisms, while also providing a synopsis of major achievements in the field over the past few years. Potential future developments and the applications of this technology are also briefly discussed.

Polymer solar cells

Conventionally, engineers have employed rigid materials to fabricate precise, predictable robotic systems, which are easily modelled as rigid members connected at discrete joints. Natural systems, however, often match or exceed the performance of robotic systems with deformable bodies. Cephalopods, for example, achieve amazing feats of manipulation and locomotion without a skeleton; even vertebrates such as humans achieve dynamic gaits by storing elastic energy in their compliant bones and soft tissues. Inspired by nature, engineers have begun to explore the design and control of soft-bodied robots composed of compliant materials. This Review discusses recent developments in the emerging field of soft robotics.

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Design, fabrication and control of soft robots

Wearable sensor technologies are essential to the realization of personalized medicine through continuously monitoring an individual's state of health. Sampling human sweat, which is rich in physiological information, could enable non-invasive monitoring. Previously reported sweat-based and other non-invasive biosensors either can only monitor a single analyte at a time or lack on-site signal processing circuitry and sensor calibration mechanisms for accurate analysis of the physiological state. Given the complexity of sweat secretion, simultaneous and multiplexed screening of target biomarkers is critical and requires full system integration to ensure the accuracy of measurements. Here we present a mechanically flexible and fully integrated (that is, no external analysis is needed) sensor array for multiplexed in situ perspiration analysis, which simultaneously and selectively measures sweat metabolites (such as glucose and lactate) and electrolytes (such as sodium and potassium ions), as well as the skin temperature (to calibrate the response of the sensors). Our work bridges the technological gap between signal transduction, conditioning (amplification and filtering), processing and wireless transmission in wearable biosensors by merging plastic-based sensors that interface with the skin with silicon integrated circuits consolidated on a flexible circuit board for complex signal processing. This application could not have been realized using either of these technologies alone owing to their respective inherent limitations. The wearable system is used to measure the detailed sweat profile of human subjects engaged in prolonged indoor and outdoor physical activities, and to make a real-time assessment of the physiological state of the subjects. This platform enables a wide range of personalized diagnostic and physiological monitoring applications.

/pdf/fully-integrated-wearable-sensor-arrays-for-multiplexed-in-4nphg673vl.pdf

Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis

Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

Transparent films of carbon nanotubes can accommodate strains of up to 150% and demonstrate conductivities as high as 2,200 S cm−1 in the stretched state.

Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes

https://onlinelibrary.wiley.com/doi/pdf/10.1002/eom2.12073

Fully transient stretchable fruit-based battery as safe and environmentally friendly power source for wearable electronics

Mechanical properties of hydrogels are crucial to emerging devices and machines for wearables, robotics and energy harvesters. Various polymer network architectures and interactions have been explored for achieving specific mechanical characteristics, however, extreme mechanical property tuning of single-composition hydrogel material and deployment in integrated devices remain challenging. Here, we introduce a macromolecule conformational shaping strategy that enables mechanical programming of polymorphic hydrogel fiber based devices. Conformation of the single-composition polyelectrolyte macromolecule is controlled to evolve from coiling to extending states via a pH-dependent antisolvent phase separation process. The resulting structured hydrogel microfibers reveal extreme mechanical integrity, including modulus spanning four orders of magnitude, brittleness to ultrastretchability, and plasticity to anelasticity and elasticity. Our approach yields hydrogel microfibers of varied macromolecule conformations that can be built-in layered formats, enabling the translation of extraordinary, realistic hydrogel electronic applications, i.e., large strain (1000%) and ultrafast responsive (~30 ms) fiber sensors in a robotic bird, large deformations (6000%) and antifreezing helical electronic conductors, and large strain (700%) capable Janus springs energy harvesters in wearables. 

/pdf/macromolecule-conformational-shaping-for-extreme-mechanical-34lekxgy.pdf

Macromolecule conformational shaping for extreme mechanical programming of polymorphic hydrogel fibers

Stretchable conductors are critical building blocks for enabling new forms of wearable and curvilinear electronics. In this paper, we introduce a new method using the interfacial design to enable stretchable conductors with ultra-high conductivity and robustness to strain using three-dimensional helical copper micro-interconnects embedded in an elastic rubber substrate (eHelix-Cu). We studied the interfacial mechanics of the metal-elastomer to achieve highly reversible conductivities with strains. The stretchable eHelix-Cu interconnect has an ultra-high conductivity (∼105 S cm−1) that remains almost invariant when stretched to 170%, which is significantly higher than in other approaches using nanomaterials. The stretchable conductors can withstand strains of 100% for thousands of cycles, demonstrating remarkable durability for exciting potential wearable electronic applications.

Highly conductive 3D metal-rubber composites for stretchable electronic applications

DOI: 10.1002/aelm.201800274 properties.[1–3] The utilization of 2DLMs for nanoelectronic devices offers many benefits for overcoming the scaling limits governed by Moore’s Law.[4,5] Graphene, which is attractive for its high mobility, is unsuitable for circuit application due to high off-state leakage current arising from its zero-bandgap property.[6,7] To overcome the shortcoming of graphene circuits with low on/off ratio, TMDs and more recently black phosphorus (BP), have been explored as the new channel materials for digital circuits because of their finite bandgap and superior transport properties. BP is becoming one of the promising 2DLMs for electronic applications due to its superior properties.[8] First, the unique property of BP is that it has a high mobility compared with other 2D materials, such as TMDs, thus ensuring a higher transistor current density. Besides, it shows a tunable bandgap ranging from 0.3 (bulk) to 2 eV (single layer), which enables a wide range of applications in electronics and optoelectronics. On the contrary, the main disadvantage of BP is that it is unstable and the material properties degrade quickly under ambient condition, thus an encapsulation layer, such as Al2O3 and poly (methyl methacrylate) (PMMA),[10] is necessary to protect BP from ambient degradation. Numerous field-effect transistors (FETs) based on BP[11–13] have been reported recently, in which the on/off ratio of the BP FETs is up to ≈105. In addition, BP is also demonstrated to be a superior candidate for nonvolatile memories applications.[8] However, to date, complementary 2D inverters are mostly fabricated by hybrid integration approach where two different types of 2D semiconductor are used as the channel materials. Such hybrid inverters are typically made of n-type FET, such as MoS2 n-FET,[12,14–21] and MoSe2 n-FET,[22] while typical p-type FET comprise of BP p-FET,[12,21] MoTe2 p-FET,[17] and WSe2 p-FET (Table S1, Supporting Information).[18,22] These hybrid inverters are usually achieved by either a transfer or external wiring process, which is complicated and expensive to implement for large scale manufacturing. Thus, monolithically integrated complementary inverter circuits are highly desirable and actively sought after for their simple fabrication process. Arising from the feasibility to achieve complementary doping, BP emerges as a promising material for realizing complementary integrated circuits. For instance, metal atoms have been effective in transforming pristine p-type BP into n-type conductivity,[23–26] thus allowing complementary inverters to be demonstrated on a single BP flake.[28–30] In a recent Black phosphorus (BP) has attracted enormous interest for logic applications due to its unique electronic properties. However, pristine BP exhibits predominant p-type channel conductance, which limits the realization of complementary circuits unless an effective n-type doping is found. Here, a practical approach to transform the conductivity of BP from p-type to n-type via a spatially controlled aluminum (Al) doping is proposed. Symmetrical threshold voltage for the pair of p-type and n-type BP field-effect transistors can be achieved by tuning the Al doping concentration. The complementary inverter circuit shows a clear logic inversion with a high voltage gain of up to ≈11 at a supply voltage (VDD) of 1.5 V. Simultaneously, a high noise margin of 0.27 × VDD is achieved for both low (NML) and high (NMH) input voltages, indicating excellent noise immunity. Moreover, a three-stage ring oscillator with a theoretical frequency above 1.8 GHz and microwatt level power dissipation is modeled, which shows a low propagation delay per stage. This study demonstrates a practical approach to fabricate high performance complementary integrated circuits on a homogenous BP channel material, paving the way toward complex cascaded circuits and sensor interface applications.

Gigahertz Integrated Circuits Based on Complementary Black Phosphorus Transistors

Skin-like sensors that transduce tactile pressures and vibrations with minimal environment variation on performance are crucial in robotic sensing and prosthetic skins. However, sensor performance ...

Benjamin C. K. Tee

Papers

Fully transient stretchable fruit-based battery as safe and environmentally friendly power source for wearable electronics

Macromolecule conformational shaping for extreme mechanical programming of polymorphic hydrogel fibers

Highly conductive 3D metal-rubber composites for stretchable electronic applications

Gigahertz Integrated Circuits Based on Complementary Black Phosphorus Transistors

Environment-Resilient Graphene Vibrotactile SensitiveSensors for Machine Intelligence