Showing papers in "Advanced materials and technologies in 2020"

Liquid Metal Direct Write and 3D Printing: A Review

Abstract: Low melting point metal alloys exhibit many of the desirable properties of conventional rigid metals and alloys, such as high electrical and thermal conductivity. Yet, unlike metals in the solid state, molten metals are inherently soft and can be easily dispensed, deformed, and stretched. This positions liquid metals (here, the term “liquid metal” means metals or metal alloys with melting points at or near room temperature) as uniquely suited for a variety of applications where conductive materials must endure varying degrees of stress, such as in stretchable circuitry and strain sensors, or in cases where design constraints require soft materials, such as wearable electronics or soft robotics. For these reasons, the patterning and processing of these materials are an area of active research. Due to their fluid nature, liquid metals can be patterned and processed in ways that simply are not possible with solid metals. Of the many possible methods to pattern liquid metals, one of the most interesting is additive manufacturing, or known colloquially as “3D printing.” 3D printing has become a general term for layer-by-layer deposition of material only in specific patterns on a surface. Here the terms will be used interchangeably. Additive processes promise to use less material and generate less waste by placing material only where needed, as compared to subtractive methods which must start with complete surface coverage and remove material to form the desired patterns. This review discusses methods, challenges, and opportunities for directwrite and 3D printing of low melting point, gallium-based liquid metal alloys at room temperature. Alloys of gallium exhibit high conductivity and high stretchability making them well suited for use in soft circuitry for stretchable electronics and soft robotics. In addition, the liquid nature of the metal enables entirely new ways to pattern metals at room temperature; herein, the focus is placed on additive printing via nozzle-based methods. Room temperature printing of liquid metals enables rapid fabrication of complex geometries (with dimensions as small as 10 μm) on a wide range of materials, such as polymers. These processes can be used to make metallic conductors for devices with self-healing capabilities, soft/stretchable electrodes, and sensors.

Soft Crawling Robots: Design, Actuation, and Locomotion

Abstract: DOI: 10.1002/admt.201900837 high-cost, and unsuitable for interaction with humans as well as intricate environments.[1–3] To overcome these challenges, great efforts have been taken to develop soft robots that are primarily made of compliant elastomers and polymers (silicone rubber, dielectric elastomers, liquidcrystalline elastomers and hydrogels, etc.) and possess unique properties such as lightweight, mechanical compliance, infinite degrees of freedom, continuous deformation, low cost, and easy fabrication.[2–6] Among these soft robotics, the class of soft crawling robots inspired from biological creatures has attracted increasing attention owing to their anticipated effective interaction with humans and uncertain environments, as well as potential capabilities of completing a variety of tasks, like search and rescue, infrastructure inspection, surveillance, drug delivery, and human assistance.[2,5] Various novel actuations and locomotion designs have been explored to drive soft crawling robots, with some of them even equipped with capabilities of the autonomous motion,[7] environment accommodation[4,8] and decision making.[9,10] For instance, soft crawling robots inspired by octopuses, worms, and jellyfishes have been designed to achieve complex motions with multiple gaits at low cost.[10,11] While there have been several excellent review papers on the topic of soft robotics,[3,4,12,13] there has not been a review paper that covers recent advances in soft crawling robots in depth. This review paper aims to fill that void. The locomotion mode, crawling speed, and working efficiency of soft crawling robots are mainly determined by the soft actuators they employed.[14] Generally, the external stimuli drive the actuators to generate the desired strains and/or deformations, and the induced strain and/or deformation will then supply the necessary propulsion for the robots to crawl. Up to now, a few actuation approaches have been successfully employed to drive the soft crawling robots, mainly consisting of pneumatic/hydraulic pressure,[15] chemical reaction,[16,18] and stimuli responses of soft active materials, such as dielectric elastomers (DEs),[19] shape memory alloys (SMAs),[20] magnetoactive elastomers (MAEs),[21] liquid-crystalline elastomers (LCEs),[22] piezoelectric materials (PEMs),[23] ionic polymer– metal composites (IPMCs),[24] and twisted and coiled polymers (TCPs).[25] Despite the inherent drawbacks of these actuation approaches, continuous efforts have been devoted to designing soft crawling robots capable of performing vivid, multigait, effective, and intelligent locomotion. Over the past decade, Soft crawling robots have attracted great attention due to their anticipated effective interactions with humans and uncertain environments, as well as their potential capabilities of completing a variety of tasks encompassing search and rescue, infrastructure inspection, surveillance, drug delivery, and human assistance. Herein, a comprehensive survey on recent advances of soft crawling robots categorized by their major actuation mechanisms is provided, including pneumatic/hydraulic pressure, chemical reaction, and soft active material-based actuations, which include dielectric elastomers, shape memory alloys, magnetoactive elastomers, liquid crystalline elastomers, piezoelectric materials, ionic polymer–metal composites, and twisted and coiled polymers. For each type of actuation, the prevalent modes of locomotion adopted in representative robots, the design, working principle and performance of their soft actuators, and the performance of each locomotion approach, as well as the advantages and drawbacks of each design are discussed. This review summarizes the state-of-the-art progresses and the critical knowledge in designing soft crawling robots and offers a guidance and insightful outlook for the future development of soft robots.

Extrusion and Microfluidic-Based Bioprinting to Fabricate Biomimetic Tissues and Organs

Abstract: Next generation engineered tissue constructs with complex and ordered architectures aim to better mimic the native tissue structures, largely due to advances in three-dimensional (3D) bioprinting techniques. Extrusion bioprinting has drawn tremendous attention due to its widespread availability, cost-effectiveness, simplicity, and its facile and rapid processing. However, poor printing resolution and low speed have limited its fidelity and clinical implementation. To circumvent the downsides associated with extrusion printing, microfluidic technologies are increasingly being implemented in 3D bioprinting for engineering living constructs. These technologies enable biofabrication of heterogeneous biomimetic structures made of different types of cells, biomaterials, and biomolecules. Microfluiding bioprinting technology enables highly controlled fabrication of 3D constructs in high resolutions and it has been shown to be useful for building tubular structures and vascularized constructs, which may promote the survival and integration of implanted engineered tissues. Although this field is currently in its early development and the number of bioprinted implants is limited, it is envisioned that it will have a major impact on the production of customized clinical-grade tissue constructs. Further studies are, however, needed to fully demonstrate the effectiveness of the technology in the lab and its translation to the clinic.

Wearable Electronic Textiles from Nanostructured Piezoelectric Fibers

Abstract: Wearable energy harvesting has been of practical interest for many years and for diverse applications, including development of self-powered wireless sensors within garments for human health monitoring. No commercially available systems currently exist with typical problems including low energy efficiency; short cycle life; slow and expensive manufacturing; and stiff, heavy or bulky componentry that reduce wearer comfort and aesthetic appeal. Herein, a novel approach is reported to create wearable energy generators and sensors using nanostructured hybrid piezoelectric fibers and exploiting the enormous variety of textile architectures (knitting, braiding and weaving). It is found that high performance hybrid piezofiber was obtained using a barium titanate nanoparticle and poly (vinylidene fluoride) with a mass ratio of 1:10. These fibers were knitted to form a wearable energy generator that produced a maximum voltage output of 4 V and a power density 87μW/cm which is 45 times higher than earlier report for piezoelectric textiles. The wearable energy generator charged a 10 μF capacitor in 20 sec which is 4 and 6 times faster than previously reported for PVDF/BT and PVDF energy generators, respectively. It also emerged that the established knitted energy harvester exhibits sensitivity of 6.3 times higher in compare with the piezofibers energy generator. A knee sleeve prototype based on a PVDF/BT wearable device for monitoring real-

Fundamentals, Materials, and Applications for Daytime Radiative Cooling

Abstract: DOI: 10.1002/admt.201901007 by designing various materials with high emissivity in the wavelength of 8–13 μm. Achieving daytime radiative cooling to a temperature below ambient under sunlight is difficult because the incoming solar radiation is intense and can easily counteract the outgoing thermal radiation to outer space. In 2014, Fan’s group first experimentally achieved the daytime radiative cooling below the ambient air temperature under direct sunlight using an integrated photonic solar reflector and thermal emitter.[1] Afterward, other material designs, including metamaterials[2] and porous polymers,[3] have been proposed to achieve daytime radiative cooling. The rapidly developed technique also brings a wide range of practical applications in building, water cooling, air cooling, and clothing. In this review, we systematically discuss the fundamentals for radiative cooling, summarize recent advancements in daytime radiative cooling, and introduce its real-world applications (Figure 1). Meanwhile, we also propose scopes for further developments. Because of its nonenergyconsuming characteristic, we believe passive daytime radiative cooling will be a promising area for energy-saving cooling methods and expect more efforts in the practical applications.

Biocompatible and highly stretchable PVA/AgNWs hydrogel strain sensors for human motion detection

Abstract: DOI: 10.1002/admt.202000426 Piezoresistive sensors, one of the main types of sensors, change their resistance in response to mechanical deformation and have demonstrated a number of applications in monitoring strain,[4–8] pressure,[6,7,9] flow,[10–12] and temperature.[4,13,14] To meet the requirements for both human motion detection and biosafety, flexible sensors should be biocompatible. In this regard, different types of elastomers such as polydimethylsiloxane (PDMS) or hydrogels such as polyvinyl alcohol (PVA) hydrogels have been extensively used as the flexible substrate/matrix.[15–18] While the flexible substrate/matrix, in most cases, determines the mechanical properties of the sensors, conductive materials including carbon nanoparticles,[6] carbon nanofibers (CNFs),[15,19] carbon nanotubes (CNTs),[20] graphene,[5,21] silver nanowires (AgNWs),[19,22] and silver nanoparticles[23] have been successfully used as the sensing elements. Different techniques have been developed to incorporate conductive elements with the flexible substrate/matrix. For instance, direct mixing of polymeric components with conductive nanomaterials has been extensively studied.[4,16,24] More recently, dipcoating/spin-coating and in situ polymerization have been utilized to develop conductive composite sensors with porous structure,[15] double layer or sandwich structures.[4,25] Among different types of flexible sensors, hydrogel-based sensors have gained increasing interest in recent years due to their human tissue-like mechanical properties and excellent biocompatibility. Different types of hydrogel sensors have been demonstrated recently, including composite hydrogels containing conductive nanomaterials[16,17] or conducting polymers (e.g., polyaniline),[26] ionic conductive hydrogels,[25] as well as zwitterionic hydrogel sensors.[27] PVA is a promising candidate for wearable applications where biocompatibility is required.[16,28] Research into the fabrication of PVA-based hydrogel sensors currently focuses on the relationship between the mechanical properties and material design factors such as PVA concentration, preparation process, and the incorporation of conductive nanofillers.[29,30] However, adding crosslinking agents and conductive nanomaterials can affect the mechanical properties of PVA hydrogels. Cai et al. presented an extremely stretchable strain sensor based on PVA, CNTs, and borax, which exhibited a high stretchability of up to 1000% strain with Hydrogel-based strain sensors have attracted considerable interest for applications such as skin-like electronics for human motion detection, soft robotics, and human–machine interfaces. However, fabrication of hydrogel strain sensors with desirable mechanical and piezoresistive properties is still challenging. Herein, a biocompatible hydrogel sensor is presented, which is made of polyvinyl alcohol (PVA) nanocomposite with high stretchability up to 500% strain, high mechanical strength of 900 kPa, and electrical conductivity (1.85 S m-1) comparable to human skin. The hydrogel sensors demonstrate excellent linearity in the whole detection range and great durability under cyclic loading with low hysteresis of 7%. These excellent properties are believed to be contributed by a new bilayer structural design, i.e., a thin, conductive hybrid layer of PVA/silver nanowires (AgNWs) deposited on a pure strong PVA substrate. PVA solution of high concentration is used to fabricate the substrate while the top layer consists of dilute PVA solution so that high content of AgNWs can be dispersed to achieve high electrical conductivity. Together with a rapid response time (0.32 s) and biocompatibility, this new sensor offers great potential as a wearable sensor for epidermal sensing applications, e.g., detecting human joint and muscle movements.

Stability of PEDOT:PSS-Coated Gold Electrodes in Cell Culture Conditions

Abstract: A.L.R. acknowledges support from the Whitaker International Scholars Program and the European Commission’s Horizon 2020 Marie Sklodowska-Curie Individual Fellowship BRAIN CAMO (No. 797506). G.D. acknowledges support from the European Commission through the project of OrgBIO-ITN 607896.

Piezocapacitive Flexible E‐Skin Pressure Sensors Having Magnetically Grown Microstructures

Abstract: DOI: 10.1002/admt.201900934 and other robots.[5] Especially communication of home service robots and artificial limbs with human beings is carried out via friendly human–machine interaction.[6] An ideal e-skin is expected to be highly flexible, sensitive, lightweight, easy to fabricate, inexpensive, and should be capable of performing like the natural skin, which feels tactile pressures ranging from light touches (0–10 kPa) to object handling levels (10–100 kPa).[2,6–12] Generally, e-skin pressure sensors use piezoresistive,[10,13,14] piezocapacitive,[8,15–17] piezoelectric,[2,18] and triboelectric[19] sensing mechanisms to transduce applied pressure into an electrical signal. Particularly, piezocapacitive pressure sensors are quite attractive due to their simple structure, fast response time, low power consumption, and compact circuit layout.[1,17] A capacitive pressure sensor typically consists of two parallel electrodes that sandwich a polymeric dielectric layer. An external force applied to the capacitive sensor changes the thickness of its dielectric layer, which leads to the variation in its capacitance. The sensing performance of such a sensor is determined by mechanical properties of its elastomeric dielectric layer, i.e., greater the compressibility of the material used, the greater the sensitivity of the sensor will be.[20] Polydimethylsiloxane (PDMS) is considered a suitable candidate for pressure-sensitive dielectric thin films,[9] but its high Young’s modulus (≈2 MPa)[21] Flexible pressure sensors are highly desirable in artificial intelligence, health monitoring, and soft robotics. Microstructuring of dielectrics is the common strategy employed to improve the performance of capacitive type pressure sensors. Herein, a novel, low-cost, large-area compatible, and mold-free technique is reported in which magnetically grown microneedles are selfassembled from a film of curable magnetorheological fluid (CMRF) under the influence of a vertical curing magnetic field (Bcuring). After optimizing the microneedles’ fabrication parameters, i.e., magnetic particles’ (MPs’) concentration and Bcuring intensity, piezocapacitive sensors capable of wide range pressure sensing (0–145 kPa) with ultrafast response time (50 ms), high cyclic stability (>9000 cycles), as well as very low detection limit (1.9 Pa) are obtained. Sensor properties are found dependent on microneedles’ fabrication parameters that are controllable, produce variable-sized microneedles, and allow to govern sensing properties according to desired applications. Finally, the sensor is employed in holding a bottle with different weights, human breath, and motion monitoring, which demonstrate its great potential for the applications of human–machine interaction, human health monitoring, and intelligent soft robotics.