Encapsulating Elastically Stretchable Neural Interfaces: Yield,Resolution, and Recording/Stimulation of Neural Activity

A high‐resolution elastically stretchable microelectrode array (SMEA) for interfacing with neural tissue is described. The SMEA consists of an elastomeric substrate, such as poly(dimethylsiloxane) (PDMS), elastically stretchable gold conductors, and an electrically insulating encapsulating layer in which contact holes are opened. We demonstrate the feasibility of producing contact holes with 40 μm × 40 μm openings, show why the adhesion of the encapsulation layer to the substrate is weakened during contact hole fabrication, and provide remedies. These improvements result in greatly increased fabrication yield and reproducibility. An SMEA with 28 microelectrodes was fabricated. The contact holes (100 μm × 100 μm) in the encapsulation layer are only ～10% the size of the previous generation, allowing a larger number of microelectrodes per unit area, thus affording the capability to interface with a smaller neural population per electrode. This new SMEA is used to record spontaneous and evoked activity in organotypic hippocampal tissue slices at 0% strain before stretching, at 5% and 10% equibiaxial strain, and again at 0% strain after relaxation. Stimulus–response curves at each strain level are measured. The SMEA shows excellent biocompatibility for at least two weeks.     

Bioresorbable Wireless Sensors as Temporary Implants for In Vivo Measurements of Pressure

Pressures at targeted locations inside the human body serve as critically important diagnostic parameters for monitoring various types of serious or even potentially fatal medical conditions including intracranial, intra‐abdominal, and pulmonary hypertension, as well as compartment syndromes. Implantable commercial sensors provide satisfactory accuracy and stability in measurements of pressure, yet surgical removal is required after recovery of the patient to avoid infections and other risks associated with long‐term implantation. Sensors that dissolve in biofluids (or, equivalently, bioabsorb or bioresorb) avoid the need for such surgeries, yet current designs involve either hard‐wired connections and/or fail to provide quantitative measurements over clinically relevant lifetimes. Here, a bioresorbable, wireless pressure sensor based on passive inductor‐capacitor resonance circuits in layouts and with sets of materials that overcome these drawbacks is reported. Specifically, optimized designs offer sensitivity as high as ≈200 kHz mmHg^?1 and resolution as low as 1 mmHg. Encapsulation approaches that use membranes of Si₃N₄ and edge seals of natural wax support stable operation in vivo for up to 4 days. The bioresorbable pressure sensing technology reported here may serve as an important solution to temporary, real‐time monitoring of internal pressure for various medical conditions.     

Biodegradable and Highly Deformable Temperature Sensors for the Internet of Things

Recent advances in biomaterials, thin film processing, and nanofabrication offer the opportunity to design electronics with novel and unique capabilities, including high mechanical stability and biodegradation, which are relevant in medical implants, environmental sensors, and wearable and disposable devices. Combining reliable electrical performance with high mechanical deformation and chemical degradation remains still challenging. This work reports temperature sensors whose material composition enables full biodegradation while the layout and ultrathin format ensure a response time of 10 ms and stable operation demonstrated by a resistance variation of less than 0.7% when the devices are crumpled, folded, and stretched up to 10%. Magnesium microstructures are encapsulated by a compostable‐certified flexible polymer which exhibits small swelling rate and a Young's modulus of about 500 MPa which approximates that of muscles and cartilage. The extension of the design from a single sensor to an array and its integration onto a fluidic device, made of the same polymer, provides routes for a smart biodegradable system for flow mapping. Proper packaging of the sensors tunes the dissolution dynamics to a few days in water while the connection to a Bluetooth module demonstrates wireless operation with 200 mK resolution prospecting application in food tracking and in medical postsurgery monitoring.     

Flexible and Stretchable 3ω Sensors for Thermal Characterization of Human Skin

Characterization of the thermal properties of the surface and subsurface structures of the skin can reveal the degree of hydration, the rate of blood flow in near‐surface micro‐ and macrovasculature, and other important physiological information of relevance to dermatological and overall health status. Here, a soft, stretchable thermal sensor, based on the so‐called three omega (i.e., 3ω) method, is introduced for accurate characterization of the thermal conductivity and diffusivity of materials systems, such as the skin, which can be challenging to measure using established techniques. Experiments on skin at different body locations and under different physical states demonstrate the possibilities. Systematic studies establish the underlying principles of operation in these unusual systems, thereby allowing rational design and use, through combined investigations based on analytical modeling, experimental measurements, and finite element analysis. The findings create broad opportunities for 3ω methods in biology, with utility ranging from the integration with surgical tools or implantable devices to noninvasive uses in clinical diagnostics and therapeutics.     

Piezoelectric Sensors Operating at Very High Temperatures and in Extreme Environments Made of Flexible Ultrawide-Bandgap Single-Crystalline AlN Thin Films

Extreme environments are often faced in energy, transportation, aerospace, and defense applications and pose a technical challenge in sensing. Piezoelectric sensor based on single-crystalline AlN transducers is developed to address this challenge. The pressure sensor shows high sensitivities of 0.4–0.5 mV per psi up to 900 °C and output voltages from 73.3 to 143.2 mV for input gas pressure range of 50 to 200 psi at 800 °C. The sensitivity and output voltage also exhibit the dependence on temperature due to two origins. A decrease in elastic modulus (Young's modulus) of the diaphragm slightly enhances the sensitivity and the generation of free carriers degrades the voltage output beyond 800 °C, which also matches with theoretical estimation. The performance characteristics of the sensor are also compared with polycrystalline AlN and single-crystalline GaN thin films to investigate the importance of single crystallinity on the piezoelectric effect and bandgap energy-related free carrier generation in piezoelectric devices for high-temperature operation. The operation of the sensor at 900 °C is amongst the highest for pressure sensors and the inherent properties of AlN including chemical and thermal stability and radiation resistance indicate this approach offers a new solution for sensing in extreme environments.     

Flexible Pressure Sensors for Objective Assessment of Motor Disorders

Monitoring body motion is relevant to motor control disorders as well as assessment of fine motor skills in child development. Furthermore, motion tracking is necessary for rehabilitation monitoring and injury prevention and benefits both sick and healthy individuals. Flexible pressure sensors based on resistors, capacitors, inductors, or transistors are reviewed in the context of healthcare measurements, ranging from physiological signals to body movement characteristics such as grip and gait. To demonstrate the use of flexible pressure sensors for motor assessment, a touch sensing glove that evaluates fine motor skills in autism research is developed. The results show that autistic children perform fewer taps per minute compared to typically developing children, with larger variations in tap durations. In a second example, a force and motion sensing glove is developed to assess spasticity, a neuromuscular disorder that causes muscle stiffness/resistance and jerky movement. Analyses of force versus velocity show movement‐dependent muscle resistance in a patient with spasticity. Through these flexible sensor systems, the shift from subjective scores to objective measurement will promote better diagnosis and dramatically improve the accuracy in tracking patient response to therapy.     

In‐Plane Micro‐Supercapacitors for an Integrated Device on One Piece of Paper

Portable and wearable sensors have attracted considerable attention in the healthcare field because they can be worn or implanted into a human body to monitor environmental information. However, sensors cannot work independently and require power. Flexible in‐plane micro‐supercapacitor (MSC) is a suitable power device that can be integrated with sensors on a single chip. Meanwhile, paper is an ideal flexible substrate because it is cheap and disposable and has a porous and rough surface that enhances interface adhesion with electronic devices. In this study, a new strategy to integrate MSCs, which have excellent electrochemical and mechanical performances, with sensors on a single piece of paper is proposed. The integration is achieved by printing Ni circuit on paper without using a precoating underlay. Ink diffusion is also addressed to some degree. Meanwhile, a UV sensor is integrated on a single paper, and the as‐integrated device shows good sensing and self‐powering capabilities. MSCs can also be integrated with a gas sensor on one‐piece paper and can be charged by connecting it to a solar cell. Thus, it is potentially feasible that a flexible paper can be used for integrating MSCs with solar cell and various sensors to generate, store, and use energy.     

Smart Design of Stable Extracellular Matrix Mimetic Hydrogel: Synthesis,Characterization, and In Vitro and In Vivo Evaluation for Tissue Engineering

The simplicity and versatility of hydrazone crosslinking has made it a strategy of choice for the conjugation of bioactive molecules. However, the labile nature of hydrazone linkages and reversibility of this coupling reaction restricts its full potential. Based on the fundamental understanding of hydrazone stability, this problem is circumvented by resonance‐stabilization of a developing N² positive charge in a hydrazone bond. A novel chemistry is presented to develop a resilient hydrazone bond that is stable and non‐ reversible under physiological conditions. A carbodihydrazide (CDH) type hydrazide derivative of the biomolecule forms intrinsically stabilized hydrazone‐linkages that are nearly 15‐fold more stable at pH 5 than conventional hydrazone. This chemoselective coupling reaction is catalyst‐free, instantaneous, and virtually non‐cleavable under physiological conditions, therefore can serve as a catalyst‐free alternative to click chemistry. This novel crosslinking reaction is used to tailor a hyaluronan hydrogel, which delivered exceptional hydrolytic stability, mechanical properties, low swelling, and controlled enzymatic degradation. These desired characteristics are achieved without increasing the chemical crosslinking. The in vivo evaluation of this hydrogel revealed neo‐bone with highly ordered collagen matrix mimicking natural bone regeneration. The proximity ligation assay or PLA is used to detect blood vessels, which highlighted the quality of engineered tissue.     

Free‐Standing and Eco‐Friendly Polyaniline Thin Films for Multifunctional Sensing of Physical and Chemical Stimuli

Multifunctional flexible sensors that are sensitive to different physical and chemical stimuli but remain unaffected by any mechanical deformation and/or changes still present a challenge in the implementation of flexible devices in real‐world conditions. This challenge is greatly intensified by the need for an eco‐friendly fabrication technique suitable for mass production. A new eco‐friendly and scalable fabrication approach is reported for obtaining thin and transparent multifunctional sensors with regulated electrical conductivity and tunable band‐gap. A thin (≈190 nm thickness) freestanding sensing film with up to 4 inch diameter is demonstrated. Integration of the freestanding films with different substrates, such as polyethylene terephthalate substrates, silk textile, commercial polyethylene thin film, and human skin, is also described. These multifunctional sensors can detect and distinguish between different stimuli, including pressure, temperature, and volatile organic compounds. All the sensing properties explored are stable under different bending/strain states.     

Epidermal Electronic Systems for Measuring the Thermal Properties of Human Skin at Depths of up to Several Millimeters

Monitoring the composition, blood flow properties, and hydration status of human skin can be important in diagnosing disease and tracking overall health. Current methods are largely limited to clinical environments, and they primarily measure properties of superficial layers of the skin, such as the stratum corneum (10–40 µm). This work introduces soft, skin‐like thermal depth sensors (e‐TDS) in designs that seamlessly couple with human skin and measure its thermal properties with depth sensitivity that can extend up to 6 mm beneath the surface. Guidelines for tailoring devices to enable measurements through different effective depths follow from a systematic set of experiments, supported by theoretical modeling. On‐body testing validates the physiological relevance of measurements using the e‐TDS platform, with potential to aid the diagnosis of deep cutaneous and systemic diseases. Specific demonstrations include measurements that capture responses ranging from superficial changes in skin properties that result from application of a moisturizer, to changes in microvascular flow at intermediate depths induced by heating/cooling, to detection of inflammation in the deep dermis and subcutaneous fat in an incidence of a local bacterial infection, cellulitis.     

Inkjet Printing of a Highly Loaded Palladium Ink for Integrated,Low‐Cost pH Sensors

An inkjet printing process for depositing palladium (Pd) thin films from a highly loaded ink (>14 wt%) is reported. The viscosity and surface tension of a Pd‐organic precursor solution is adjusted using toluene to form a printable and stable ink. A two‐step thermolysis process is developed to convert the printed ink to continuous and uniform Pd films with good adhesion to different substrates. Using only one printing pass, a low electrical resistivity of 2.6 μΩ m of the Pd film is obtained. To demonstrate the electrochemical pH sensing application, the surfaces of the printed Pd films are oxidized for ion‐to‐electron transduction and the underlying layer is left for electron conduction. Then, solid‐state reference electrodes are integrated beside the bifunctional Pd electrodes by inkjet printing. These potentiometric sensors have sensitivities of 60.6 ± 0.1 and 57 ± 0.6 mV pH^?1 on glass and polyimide substrates, and short response times of 11 and 6 s, respectively. Also, accurate pH values of real water samples are obtained by using the printed sensors with a low‐cost multimeter. These results indicate that the facile and cost‐effective inkjet printing and integration techniques may be applied in fabricating future electrochemical monitoring systems for environmental parameters and human health conditions.     

Flexible and Transparent Nanocomposite of Reduced Graphene Oxide and P(VDF‐TrFE) Copolymer for High Thermal Responsivity in a Field‐Effect Transistor

A new class of temperature‐sensing materials is demonstrated along with their integration into transparent and flexible field‐effect transistor (FET) temperature sensors with high thermal responsivity, stability, and reproducibility. The novelty of this particular type of temperature sensor is the incorporation of an R‐GO/P(VDF‐TrFE) nanocomposite channel as a sensing layer that is highly responsive to temperature, and is optically transparent and mechanically flexible. Furthermore, the nanocomposite sensing layer is easily coated onto flexible substrates for the fabrication of transparent and flexible FETs using a simple spin‐coating method. The transparent and flexible nanocomposite FETs are capable of detecting an extremely small temperature change as small as 0.1 °C and are highly responsive to human body temperature. Temperature responsivity and optical transmittance of transparent nanocomposite FETs were adjustable and tuneable by changing the thickness and R‐GO concentration of the nanocomposite.     

Scanned Pipette Techniques for the Highly Localized Electrochemical Fabrication and Characterization of Conducting Polymer Thin Films,Microspots, Microribbons,and Nanowires

The limited toolbox for conducting polymer (CP) microscale fabrication and characterization hampers the development of applications such as sensors and actuators. To address this issue, a robust and integrated methodology is presented, capable of electrochemical fabrication and characterization of CPs in a highly localized manner, allowing for CP patterning and spatial mapping of voltammetric response. This is enabled by scanning probe microscopy (SPM) tipped with a single‐barreled micropipette to electrochemically polymerize CP microspot arrays, demonstrated for 3,4‐ethylenedioxythiophene and aniline monomers. Stationary electropolymerization produces individual microspots; lateral movement produces long microribbons; retraction produces extruded microstructures. Subsequently the same SPM setup is tipped with a double‐barreled micropipette to carry out localized cyclic voltammetry. The micropipettes are filled with saline solutions in contact with Ag/AgCl electrodes, forming a thin meniscus of solution at the micropipette tip, which enable an automated approach in air and subsequent contact with the surface. The flexibility of this novel technique is demonstrated by application to 2D poly(3,4‐ethylenedioxythiophene) (PEDOT) microspots, microribbons and nanowires, plus polyaniline (PANI) microstructures and self‐assembled thin films. Finally, setting up a dynamic electrochemical cell allowed for voltammetric–amperometric imaging, simultaneously mapping the morphology and current response of CPs. Future refinements towards the nanoscale through smaller‐tipped pipettes should open up new opportunities for voltammetric response mapping of individual CP nanostructures.     

Self‐Healing,Flexible, and Tailorable Triboelectric Nanogenerators for Self‐Powered Sensors based on Thermal Effect of Infrared Radiation

Self‐healing triboelectric nanogenerators (TENGs) with flexibility, robustness, and conformability are highly desirable for promising flexible and wearable devices, which can serve as a durable, stable, and renewable power supply, as well as a self‐powered sensor. Herein, an entirely self‐healing, flexible, and tailorable TENG is designed as a wearable sensor to monitor human motion, with infrared radiation from skin to promote self‐healing after being broken based on thermal effect of infrared radiation. Human skin is a natural infrared radiation emitter, providing favorable conditions for the device to function efficiently. The reversible imine bonds and quadruple hydrogen bonding (UPy) moieties are introduced into polymer networks to construct self‐healable electrification layer. UPy‐functionalized multiwalled carbon nanotubes are further incorporated into healable polymer to obtain conductive nanocomposite. Driven by the dynamic bonds, the designed and synthesized materials show excellent intrinsic self‐healing and shape‐tailorable features. Moreover, there is a robust interface bonding in the TENG devices due to the similar healable networks between electrification layer and electrode. The output electric performances of the self‐healable TENG devices can almost restore their original state when the damage of the devices occurs. This work presents a novel strategy for flexible devices, contributing to future sustainable energy and wearable electronics.     

Engineering the Thermal Conductivity of Functional Phase‐Change Materials for Heat Energy Conversion,Storage, and Utilization

Thermal energy storage technologies based on phase‐change materials (PCMs) have received tremendous attention in recent years. These materials are capable of reversibly storing large amounts of thermal energy during the isothermal phase transition and offer enormous potential in the development of state‐of‐the‐art renewable energy infrastructure. Thermal conductivity plays a vital role in regulating the thermal charging and discharging rate of PCMs and improving the heat‐utilization efficiency. The strategies for tuning the thermal conductivity of PCMs and their potential energy applications, such as thermal energy harvesting and storage, thermal management of batteries, thermal diodes, and other forms of energy utilization, are summarized systematically. Furthermore, a research perspective is given to highlight emerging research directions of engineering advanced functional PCMs for energy applications.