Omnidirectional Configuration of Stretchable Strain Sensor Enabled by the Strain Engineering with Chiral Auxetic Metamaterial.

An electromechanical interface plays a pivotal role in determining the performance of a stretchable strain sensor. The intrinsic mechanical property of the elastomer substrate prevents the efficient modulation of the electromechanical interface, which limits the further evolution of a stretchable strain sensor. In this study, a chiral auxetic metamaterial (CAM) is incorporated into the elastomer substrate of a stretchable strain sensor to override the deformation behavior of the pristine device and regulate the device performance. The tunable isotropic Poisson's ratio (from 0.37 to -0.25) achieved by the combination of CAM and elastomer substrate endows the stretchable strain sensor with significantly enhanced sensitivity (53-fold improvement) and excellent omnidirectional sensing ability. The regulation mechanism associated with crack propagation on the deformed substrate is also revealed with finite element simulations and experiments. The demonstration of on-body monitoring of human physiological signals and a smart training assistant for trampoline gymnastics with the CAM-incorporated strain sensor further illustrates the benefits of omnidirectionally enhanced performance.

Skin-Attachable and Stretchable Patch Antenna with Fractal Design for Remote On-Body Motion Sensing.

This article describes the implementation of a wireless human motion detection with interference resistance to untargeted deformations based on a stretchable patch antenna with fractal design. By rationally incorporating the Hilbert fractal pattern in the conductive patch and ground plane, the patch antenna shows a mechanical stretchability of ∼40% and a maximum gain of 2.95 dB at 2.5 GHz. Furthermore, the influence of the fractal order on the mechanical stretchability and radiation properties of the stretchable patch antenna is discussed. The resonant frequency of the stretchable fractal antenna demonstrates highly selective sensitivity to different deformations; i.e., it remains almost unchanged with bending deformations and is linearly dependent on the tensile strain. Remote detection of joint motions is experimentally verified by a wireless on-body strain sensor based on the fractal design-based stretchable microstrip antenna.

Outer Surface-Labeled Bacteria as Live Sensors Accurately Quantitating Interfacial pH: A Smart Technique for Antimicrobial Resistance.

The techniques to quantitatively monitor environmental factors surrounding the bacterial outer surface rather than the host's subcellular regions (e.g., lysosomes) should be the key to evaluate bacterial immune escape behavior. We report wild Staphylococcus aureus (SA) and methicillin-resistant Staphylococcus aureus (MRSA) labeled with a fluorescent resonance energy transfer probe, 4SR-L-BDP, on their outer surfaces as smart live sensors to quantify interfacial pH. The dual emission of 4SR-L-BDP affords high sensitivity to pH change in a ratiometric way in the pH range of 4-8 with high precision. Notably, 4SR-L-BDP possesses an anchoring group to fix on the bacterial surface for sensing the microenvironment encountered. Super-resolution imaging clearly demonstrates the specific labeling of bacterial membranes. These live sensors are applied in two-channel ratiometric imaging to dynamically visualize and quantify their interfacial pH changes during infection of macrophages. It is found that the interfacial pH of MRSA is lower by 0.2 units compared to that of SA. Such small but critical difference in pH reflects MRSA's ability to adapt to microenvironmental pH inside macrophages. These labeled bacteria as live sensors are also proven to be practically applicable in mice models with immune deficiency and immune activation.

Bio-orthogonal Toolbox for Monitoring Nitric Oxide in Targeted Organelles of Live Cells and Zebrafishes.

Nitric oxide (NO), playing crucial roles as a cellular messenger and as a toxic ROS, is highly related to the physiological and pathological states of living systems. The very wide but very uneven distribution of this radical gas in the inhomogeneous biological microenvironment imposes big challenges for specifically detecting its local level in certain subcellular areas, which calls for a long list of NO probes for each target. In order to simplify the syntheses and designs of these probes, herein it is proposed to construct a versatile NO-sensing toolbox based on a bio-orthogonal concept, i.e., inverse electron demand Diels-Alder click reaction between tetrazine and strained alkyne BCN. On the one hand, rhodamine-o-phenylenediamine as the NO-responsive scaffold is coupled with a tetrazine unit to generate a general probe TMR-Tz-NO, which, to our knowledge, is the first case of the tetrazine-coupled analyte-responsive probe. On the other hand, the BCN moiety is connected to different targeting groups, such as TPP, morpholine, and Ac4ManN, targeting to mitochondria, lysosomes, and membranes, respectively. It works well to use TMR-Tz-NO to match with any targetable BCN counterpart in this toolbox to achieve the imaging of NO in the corresponding subcellular area. For example, through metabolism, Ac4ManN-BCN is effectively taken and grows on the cell membranes. The bio-orthogonal reaction between TMR-Tz-NO and Ac4ManN-BCN makes the NO probe anchored to the membrane surface permanently. The zebrafish experiment revealed that this bio-orthogonal pair can track and image the NO produced during inflammation in vivo.

Transfer printing technologies for soft electronics.

Soft electronics have received increasing attention in recent years, owing to their wide range of applications in dynamic nonplanar surface integration electronics that include skin electronics, implantable devices, and soft robotics. Transfer printing is a widely used assembly technology for micro- and nano-fabrication, which enables the integration of functional devices with flexible or elastomeric substrates for the manufacturing of soft electronics. Through advanced materials and process design, numerous impressive studies related to transfer printing strategies and applications have been proposed. Herein, a discussion of transfer printing technologies toward soft electronics in terms of mechanisms and example demonstrations is provided. Moreover, the perspectives on the potential challenges and future directions of this field are briefly discussed.

Super-Resolution Imaging of Autophagy by a Preferred Pair of Self-Labeling Protein Tags and Fluorescent Ligands.

Autophagy is a core recycling process for homeostasis, with its dysfunction associated with tumorigenesis and various diseases. Yet, its subtle intracellular details are covered due to the limited resolution of conventional microscopies. The major challenge for modern super-resolution microscopy deployment is the lack of a practical labeling system, which could provide robust fluorescence with fidelity in the context of the dynamic autophagy microenvironment. Herein, a representative autophagy marker LC3 protein is selected to develop two hybrid self-labeling systems with tetramethylrhodamine (TMR) fluorophores through SNAP/Halo-tag technologies. A systematic investigation indicated that the match of the LC3-Halo and TMR ligand remarkably outperforms that of LC3-SNAP, as the former Halo system exhibited more robust single-molecule brightness (440 vs 247), total photon numbers (45600 vs 13500), and dwell time of the initial bright state (0.82 vs 0.40 s) than the latter. With the aid of this desirable Halo system, for the first time, live-cell ferritinophagy is monitored with a spatial resolution of ∼50 nm, which disclosed reduced sizes of autophagosomes (∼650 nm, ferritinophagy) than those in nonselective (∼840 nm, mammalian target of rapamycin (mTOR)) and selective autophagy (∼900 nm, mitophagy).

Polarity-Sensitive and Membrane-Specific Probe Quantitatively Monitoring Ferroptosis through Fluorescence Lifetime Imaging.

As a new form of regulated cell death, ferroptosis is closely related to various diseases. To interpret this biological behavior and monitor related pathological processes, it is necessary to develop appropriate detection strategies and tools. Considering that ferroptosis is featured with remarkable lipid peroxidation of various cell membranes, it is logical to detect membranes' structural and environmental changes for the direct assessment of ferroptosis. For this sake, we designed novel polarity-sensitive fluorescent probes Mem-C1C18 and Mem-C18C18, which have superior plasma membrane anchorage, high brightness, and sensitive responses to environmental polarity by changing their fluorescence lifetimes. Mem-C1C18 with much less tendency to aggregate than Mem-C18C18 outperformed the latter in high resolution fluorescence labeling of artificial vesicle membranes and plasma membranes of live cells. Thus, Mem-C1C18 was selected to monitor plasma membranes damaged along ferroptosis process for the first time, in combination with the technique of fluorescence lifetime imaging (FLIM). After treating HeLa cells with Erastin, a typical ferroptosis inducer, the mean fluorescence lifetime of Mem-C1C18 displayed a considerable increase from 3.00 to 4.93 ns, with a 64% increase (corresponding to the polarity parameter Δf increased from 0.213 to 0.232). Therefore, our idea to utilize a probe to quantitate the changes in polarity of plasma membranes proves to be an effective method in the evaluation of the ferroptosis process.

Self-Assembly of Podophyllotoxin-Loaded Lipid Bilayer Nanoparticles for Highly Effective Chemotherapy and Immunotherapy via Downregulation of Programmed Cell Death Ligand 1 Production.

Low drug delivery efficiency elevates the cost of medication, lowers the therapeutic efficacy, and appears as a leading reason for unmet needs in anticancer therapies. Herein, we report the development of self-assembled podophyllotoxin-loaded lipid bilayer nanoparticles that inhibit the production of programmed cell death ligand 1 in lung cancer cells and promote tumor-specific immune responses, thus offering a strategy for regulating the immunosuppressive microenvironment of tumors. In addition, encapsulation of podophyllotoxin in the nanoparticles reduced its systemic toxicity, enhanced its penetration into tumors, and increased its antitumor efficacy. Systemic injection of the nanoparticles into tumor-bearing mice not only prevented tumor immune escape but also significantly inhibited tumor growth and extended survival. In general, the podophyllotoxin-loaded nanoparticles exhibited both immunological effects and antitumor effects in addition to having better targeting activity and fewer side effects than free podophyllotoxin. We expect our findings to facilitate the development of therapies for lung cancer.

Three-dimensional transistor arrays for intra- and inter-cellular recording.

Electrical impulse generation and its conduction within cells or cellular networks are the cornerstone of electrophysiology. However, the advancement of the field is limited by sensing accuracy and the scalability of current recording technologies. Here we describe a scalable platform that enables accurate recording of transmembrane potentials in electrogenic cells. The platform employs a three-dimensional high-performance field-effect transistor array for minimally invasive cellular interfacing that produces faithful recordings, as validated by the gold standard patch clamp. Leveraging the high spatial and temporal resolutions of the field-effect transistors, we measured the intracellular signal conduction velocity of a cardiomyocyte to be 0.182 m s-1, which is about five times the intercellular velocity. We also demonstrate intracellular recordings in cardiac muscle tissue constructs and reveal the signal conduction paths. This platform could provide new capabilities in probing the electrical behaviours of single cells and cellular networks, which carries broad implications for understanding cellular physiology, pathology and cell-cell interactions.

A nucleus targetable fluorescent probe for ratiometric imaging of endogenous NO in living cells and zebrafishes.

Nitric oxide (NO) is an important cellular messenger molecule in the cardiovascular, nervous and immune systems. Real-time monitoring of NO activity in specific organelles of live cells is important to understand its biological function. In this work, a nucleus targetable ratiometric NO probe, Hoe-Rh-NO, is developed by linking Hoechst to rhodamine spirolactam. The Hoechst part conducts nucleus targeting and the rhodamine spirolactam senses NO. The two fluorophores constitute a NO switchable FRET system for ratiometric imaging. This newly developed probe Hoe-Rh-NO displays good nucleus targeting ability, high sensitivity and selectivity towards NO, low cytotoxicity and most importantly detects NO through ratiometric fluorescence imaging. By using Hoe-Rh-NO, we confirmed the presence of NO in the nucleus and detected endogenous NO during inflammation in cells and zebrafishes.

Hybrid labeling system for dSTORM imaging of endoplasmic reticulum for uncovering ultrastructural transformations under stress conditions.

The endoplasmic reticulum (ER) transforms its morphology to fit versatile cellular functions especially under stress conditions. Since various ER stresses are critical pathophysiological factors, the precise observations of ER can provide insights into disease diagnoses and biological researches. Live-cell super-resolution imaging is highly expected for uncovering microstructures of ER. However, to achieve this, there remains a big challenge in how to efficiently label ER with advanced fluorophores. Herein, we report a new SNAP-tag fluorescent probe, namely, CLP-TMR, for specific and high-density labeling of the newly constructed dual ER-signal (targeting and retention) peptides fused-SNAP proteins. This hybrid labeling system integrating chemical probes with genetically encoded techniques enables molecular position reconstructions of ER morphologies through direct stochastic optical reconstruction microscopy (dSTORM) imaging. The super-resolution imaging reveals several never-known ultrastructural changes responding to different ER stresses, i.e. the formation of peripheral ER sheets to restore the immunogenicity, and the long flattened ER tubules under inflammation.

Mitochondria-Anchored Molecular Thermometer Quantitatively Monitoring Cellular Inflammations.

Temperature in mitochondria can be a critical indicator of cell metabolism. Given the highly dynamic and inhomogeneous nature of mitochondria, it remains a big challenge to quantitatively monitor the local temperature changes during different cellular processes. To implement this task, we extend our strategy on mitochondria-anchored thermometers from "on-off" probe Mito-TEM to a ratiometric probe Mito-TEM 2.0 based on the Förster resonance energy transfer mechanism. Mito-TEM 2.0 exhibits not only a sensitive response to temperature through the ratiometric changes of dual emissions but also the specific immobilization in mitochondria via covalent bonds. Both characters support accurate and reliable detection of local temperature for a long time, even in malfunctioning mitochondria. By applying Mito-TEM 2.0 in fluorescence ratiometric imaging of cells and zebrafishes, we make a breakthrough in the quantitative visualization of mitochondrial temperature rises in different inflammation states.

Cyclo-Ketal Xanthene Dyes: A New Class of Near-Infrared Fluorophores for Super-Resolution Imaging of Live Cells.

Newly emerging super-resolution imaging techniques provide opportunities for precise observations on cellular microstructures. However, they also impose severe demands on fluorophores. Here, we develop a new series of NIR xanthene dyes, named as KRhs, by replacing the 10-position O of rhodamines with a cyclo-ketal. KRhs display an intense NIR emission peak at 700 nm with fluorescence quantum yields up to 0.64. More importantly, they, without the aid of enhancing buffer, exhibit stochastic fluorescence off-on switches to support time-resolved localization of single fluorophore. KRhs are functionalized into KRh-MitoFix, KRh-Mem and KRh-Halo that demonstrate mitochondria, plasma membrane and fusion protein targeting ability, respectively. Consequently, these KRh probes demonstrate straightforward usage for super-resolution imaging of these targets in live cells. Therefore, KRhs merit future development for fluorescence labeling and super-resolution imaging in the NIR region.

Reflecting Size Differences of Exosomes by Using the Combination of Membrane-Targeting Viscosity Probe and Fluorescence Lifetime Imaging Microscopy.

Exosomes are cell-secreted membrane-coated vesicles with their sizes variable from 30 to 150 nm. So far, there is no simple, fast, and economical way to evaluate the sizes of exosomes in living systems. Here, we put forward a hypothesis in which the sphere sizes (resulting in different curvature) may affect the local mobility/viscosity of exosome membranes. Based on this hypothesis, we propose a novel method to evaluate the exosome sizes by quantifying the membrane viscosity. For this sake, we design a membrane-targeting molecular rotor with its fluorescence lifetime sensitive to viscosity and use it under a fluorescence lifetime imaging microscope (FLIM). Through a multiple-step ultrafiltration technique, we isolate three individual size distributions (10-50, 50-100, and 100-220 nm) with exosomes from HeLa and MCF-7 cell culture media and then perform the FLIM assay on the above two groups. In both cases, we indeed find a regular pattern in which the membrane viscosity reflected by lifetime decreases with exosome sizes. We then perform the assay on exosomes from cancer cells, corresponding normal tissue cells, and serum of breast cancer patients. We find that exosomes from cancer cells have a fluorescence lifetime (larger viscosity) longer than that of normal tissue cells. The average fluorescence lifetime of exosomes from a triple-negative breast cancer patient is longer (or the viscosity is larger) than that of a HER2 positive one. Therefore, our new and simple method may hold application prospects in future cancer diagnosis.

Quaternary Piperazine-Substituted Rhodamines with Enhanced Brightness for Super-Resolution Imaging.

Insufficient brightness of fluorophores poses a major bottleneck for the advancement of super-resolution microscopes. Despite being widely used, many rhodamine dyes exhibit sub-optimal brightness due to the formation of twisted intramolecular charge transfer (TICT) upon photoexcitation. Herein, we have developed a new class of quaternary piperazine-substituted rhodamines with outstanding quantum yields (Φ = 0.93) and superior brightness (ε × Φ = 8.1 × 104 L·mol-1·cm-1), by utilizing the electronic inductive effect to prevent TICT. We have also successfully deployed these rhodamines in the super-resolution imaging of the microtubules of fixed cells and of the cell membrane and lysosomes of live cells. Finally, we demonstrated that this strategy was generalizable to other families of fluorophores, resulting in substantially increased quantum yields.

Biomembrane-Modified Field Effect Transistors for Sensitive and Quantitative Detection of Biological Toxins and Pathogens.

The efforts of detecting bioactive targets with complex, dynamic, and unknown molecular profiles have inspired the development of various biosensor platforms. Herein, we report a cell-membrane-modified field effect transistor (FET) as a function-based nanosensor for the detection and quantitative measurement of numerous toxins and biological samples. By coating carbon nanotube FETs with natural red blood cell membranes, the resulting biomimetic nanosensor can selectively interact with and absorb broad-spectrum hemolytic toxins regardless of their molecular structures. Toxin-biomembrane interactions alter the local charge distribution at the FET surface in an ultrasensitive and concentration-dependent manner, resulting in a detection limit down to the femtomolar (fM) range. Accurate and quantitative measurements are enabled via a built-in calibration mechanism of the sensor, which overcomes batch-to-batch fabrication variations, and are demonstrated using three distinct toxins and various complex bacterial supernatants. The measured signals of bacterium-secreted proteins correlate linearly with the actual bacterial numbers, making the biosensor a nontraditional approach to rapidly detecting bacterial concentrations without a need to count bacterial colonies.

A targetable fluorescent probe for dSTORM super-resolution imaging of live cell nucleus DNA.

HoeSR, a nucleus specific probe for dSTORM super-resolution imaging of nucleus DNA in live cells, was designed by conjugating a rhodamine fluorophore and a Hoechst tag. HoeSR labels the cell nucleus in a wash-free way and emits intensive fluorescence exclusively in the nucleus. With the aid of HoeSR, nucleus nanostructures at different mitosis stages were observed through super-resolution imaging.

Immobilizable fluorescent probes for monitoring the mitochondria microenvironment: a next step from the classic.

Mitochondria play extremely important roles in supplying energy and maintaining the functions of cells. It is essential to monitor mitochondrial microenvironments that reflect the status of this organelle. In this review, we summarize the major progress in the developments and applications of mitochondrial targetable trackers and probes. The newly emerged immobilizable probes for microenvironments are highlighted, and their values are commented. Generally, cationic dyes with appropriate lipophilicity tend to accumulate in the mitochondria due to charge attraction from the negative potential of inner membranes. Following this rationale, numerous fluorescent MitoTrackers have been developed. These probes provide opportunities to investigate mitochondrial morphology and functions. However, such electrostatic interactions are not always reliable since the mitochondrial potential is highly changeable with mitochondria status. The probes will leave when the mitochondria lose their potential. In other words, conventional mitochondrial probes are not suitable for monitoring unhealthy mitochondria that have abnormal mitochondrial potentials. Kim's, Chen's and our teams proposed, separately, new designs of mitochondrial probes that can be immobilized to mitochondria through covalent bonds and monitor microenvironmental factors, including pH, viscosity, polarity and temperature. We believe that the developing of immobilizable mitochondrial fluorescent probes is valuable for tracking and monitoring mitochondrial status accurately. Therefore, this paper reviews and prospects mitochondria immobilizable fluorescent probes of important microenvironmental factors.

Fixable Molecular Thermometer for Real-Time Visualization and Quantification of Mitochondrial Temperature.

A change of mitochondrial temperature can be an important indicator of mitochondrial metabolism that generates considerable heat. For this reason, development of fluorescent probes to detect mitochondrial temperature has become an attractive topic. Previous efforts have successfully addressed the major issues, such as temperature sensitivity and mitochondrial targetability. However, there remains a key obstacle to practical applications. Considering the highly dynamic features of mitochondria, especially the variation of the inner-membrane potential, it is quite necessary to permanently immobilize a temperature probe in mitochondria in order to avoid unstable intracellular localization along with the changes of mitochondrial status. Herein, we report Mito-TEM, the first fixable, fluorescent molecular thermometer. Mito-TEM is based on a positively charged rhodamine B fluorophore that has the tendency of being attracted to mitochondria, which have negative potential. This fluorophore containing rotatable substituents also contributes to the temperature-responsive fluorescence property. Most importantly, a benzaldehyde is introduced in Mito-TEM as an anchoring unit that condenses with aminos of the protein and thus immobilizes the probe in mitochondria. The specific immobilization of Mito-TEM in mitochondria is unambiguously demonstrated in colocalization imaging. By using Mito-TEM, a method of visualizing and quantifying a temperature distribution through grayscale imaging of mitochondria is established and further applied to monitor the temperature changes of live cells under light heating and PMA stimulation.

Materials and Structures toward Soft Electronics.

Soft electronics are intensively studied as the integration of electronics with dynamic nonplanar surfaces has become necessary. Here, a discussion of the strategies in materials innovation and structural design to build soft electronic devices and systems is provided. For each strategy, the presentation focuses on the fundamental materials science and mechanics, and example device applications are highlighted where possible. Finally, perspectives on the key challenges and future directions of this field are presented.