Shear-flow-induced graphene coating microfibers from microfluidic spinning

Flexible Optoelectronic Hybrid Microfiber Long-period Grating Multimodal Sensor

Flexible wearable biosensors have emerged as a promising tool for tracking dynamic glycemic profiles of human body in diabetes management. However, it remains a challenge to balance the shrunken device space and multiple redundant sensing arrays for further advancement in miniaturization of multimodal sensors. Herein, this work proposes an entirely new optoelectronic hybrid multimodal optical fiber sensor which is composed of laser patterning of polydimethylsiloxane (PDMS) to form laser-induced graphene (LIG) as the interdigital electrodes, and a long period grating (LPG) prepared from an optical microfiber encapsulated into the PDMS modulated by periodical structure of LIG electrodes. This operation can simultaneously integrate two heterogeneous sensing mechanisms, optical and electrical, into a single sensor in a compact manner. Combining the LIG electrode with conductive hydrogel, a flexible glucose biosensor based on electrical mechanism is constructed by loading glucose oxidase into the hydrogel. Meanwhile, the microfiber LPG can also be served as a spectroscopically available sensor for biomechanical monitoring. Optical and electrical sensors can work simultaneously but independently of each other, particularly in the scene of wound healing for rat model and movement for human exercise. This platform represents a pivotal step toward multifunctional sensors that enable measurements of biomechanical information and glucose.

Advances in Microfluidics-Enabled Dimensional Design of Micro-/Nanomaterials for Biomedical Applications: A Review

Biomedical materials are of great significance for preventing and treating major diseases and protecting human health. At present, more stringent requirements have been put forward for the preparation methods and dimension control of biomedical materials based on the urgent demand for high-performance biomedical materials, especially the existence of various physiological size thresholds in vitro/in vivo. Microfluidic platforms break the limitations of traditional micro-/nanomaterial synthesis, which provide a miniaturized and highly controlled environment for size-dependent biomaterials. In this review, the basic conceptions and technical characteristics of microfluidics are first described. Then the syntheses of biomedical materials with different dimensions (0D, 1D, 2D, 3D) driven by microfluidics have been systematically summarized. Meanwhile, the applications of microfluidics-driven biomedical materials, including diagnosis, anti-inflammatory, drug delivery, antibacterial, and disease therapy, are discussed. Furthermore, the challenges and developments in the research field are further proposed. This work is expected to facilitate the convergence between the bioscience and engineering communities and continue to contribute to this emerging field.

Microfluidic Electrospinning Core-Shell Nanofibers for Anti-Corrosion Coatings With Efficient Self-Healing Properties

Self-healing materials have been extensively explored in metal anti-corrosion fields. However, improving the self-healing efficiency remains a significant work that severely limits their further development. Here, a strategy to fabricate anti-corrosion coatings with efficient self-healing properties based on microfluidic electrospinning technologies and UV-curable healing agents is reported. The damaged composite coating contains core-shell nanofibers that can be completely healed within only 30 min, indicating an outstanding healing efficiency. The corrosion current density (Icorr) of the composite coatings containing core-shell nanofibers (abbreviated as composite coatings) is lower than the coatings without any fibers (abbreviated as pure resin coatings) during the test of repeated damage and healing cycles, showing superior resistance to corrosion and repeated self-healing property. The composite coating has even better mechanical properties such as tensile strength, bending strength, and impact strength than the pure resin coating, which are explained by simulating the deformation process. These excellent properties greatly improve the practicability of self-healing coatings in the application of anti-corrosion, especially in some special fields.

Improved Pyroelectric Nanogenerator Performance of P(VDF-TrFE)/rGO Thin Film by Optimized rGO Reduction

The pyroelectric nanogenerator (PyNG) has gained increasing attention due to its capability of converting ambient or waste thermal energy into electrical energy. In recent years, nanocomposite films of poly(vinylidene fluoride-co-trifluoro ethylene) (P(VDF-TrFE)) and nanofillers such as reduced graphene oxide (rGO) have been employed due to their high flexibility, good dielectric properties, and high charge mobility for the application of wearable devices. This work investigated the effect of rGO reduction on pyroelectric nanogenerator performance. To prepare rGO, GO was reduced with different reducing agents at various conditions. The resulting rGO samples were characterized by XPS, FT-IR, XRD, and electrical conductivity measurements to obtain quantitative and qualitative information on the change in surface functionalities. Molecularly thin nanocomposite films of P(VDF-TrFE)/rGO were deposited on an ITO-glass substrate by the Langmuir-Schaefer (LS) technique. A PyNG sandwich-like structure was fabricated by arranging the thin films facing each other, and it was subjected to the pyroelectric current test. For various PyNGs of the thin films containing rGO prepared by different methods, the average pyroelectric peak-to-peak current (APC) and the pyroelectric coefficient (p) values were measured. It was found that a more reduced rGO resulted in higher electrical conductivity, and the thin films containing rGO of higher conductivity yielded higher APC and p values and, thus, better energy-harvesting performance. However, the thin films having rGO of too high conductivity produced slightly reduced performance. The Maxwell-Wagner effect in the two-phase system successfully explained these optimization results. In addition, the APC and p values for the thin film with the best performance increased with increasing temperature range. The current PyNG's performance with an energy density of 3.85 mW/cm2 and a p value of 334 μC/(m2∙K) for ΔT = 20 °C was found to be superior to that reported in other studies in the literature. Since the present PyNG showed excellent performance, it is expected to be promising for the application to microelectronics including wearable devices.

Heat source recognition sensor mimicking the thermosensation function of human skin

The human skin maintains a comfortable and healthy somatosensory state by sensing different aspects of the thermal environment, including temperature value, heat source, energy level, and duration. However, state-of-the-art thermosensors only measure basic temperature values, not the full range of the thermosensation function of human skin. Here, we propose a heat source recognition (hsr) sensor of poly(butyl acrylate)-lithium bis(n-fluoroalkylsulfonyl)imide (PBA-Li:nFSI; n = 1, 3, 5), which enables response to temperature, pressure, and proximity stimulus signals based on the relaxation behavior of the ionic gel and distinguished between different types of heat sources (i.e., radiation, convection, and conduction). The hsr sensor was integrated into a prosthetic limb covered by an e-skin with isothermal regulation, and experiments with a robot showed that it could achieve human-like thermosensation function, recognizing multidimensional information about thermal environments, such as temperature value, comfort level, and heat source signal. This work deeply mimics the human body's thermosensation function and provides a reliable solution for the development of bionic e-skin for intelligent robots and prosthetics.

A Novel Implantable Piezoresistive Microsensor for Intraocular Pressure Measurement

Glaucoma is the world's second-leading irreversible eye disease causing blindness. Although the pathogenesis of glaucoma is not particularly well understood, high intraocular pressure (IOP) is widely recognized as a significant risk factor. In clinical practice, various devices have been used to measure IOP, but most of them cannot provide continuous measurements for a long time. To meet the needs of glaucoma patients who experience frequent fluctuations in the IOP and require constant monitoring, we fabricated an implantable piezoresistive IOP sensor based on microfluidic technology. The sensor has a sensitivity of 0.00257 Ω/mbar and demonstrates excellent linearity, stability, and repeatability. According to the calibration data, the average measurement error is ±0.5 mbar. We implanted it into the vitreous of a rabbit and successfully detected its IOP fluctuations. The sensor is simple in design, easy to fabricate, and can be used for long-term continuous IOP measurements. It presents a new approach for microfluidic-based IOP sensors and offers a novel method for the daily care of patients with glaucoma.

Multi-Functional Nano-Doped Hollow Fiber from Microfluidics for Sensors and Micromotors

Nano-doped hollow fiber is currently receiving extensive attention due to its multifunctionality and booming development. However, the microfluidic fabrication of nano-doped hollow fiber in a simple, smooth, stable, continuous, well-controlled manner without system blockage remains challenging. In this study, we employ a microfluidic method to fabricate nano-doped hollow fiber, which not only makes the preparation process continuous, controllable, and efficient, but also improves the dispersion uniformity of nanoparticles. Hydrogel hollow fiber doped with carbon nanotubes is fabricated and exhibits superior electrical conductivity (15.8 S m-1), strong flexibility (342.9%), and versatility as wearable sensors for monitoring human motions and collecting physiological electrical signals. Furthermore, we incorporate iron tetroxide nanoparticles into fibers to create magnetic-driven micromotors, which provide trajectory-controlled motion and the ability to move through narrow channels due to their small size. In addition, manganese dioxide nanoparticles are embedded into the fiber walls to create self-propelled micromotors. When placed in a hydrogen peroxide environment, the micromotors can reach a top speed of 615 μm s-1 and navigate hard-to-reach areas. Our nano-doped hollow fiber offers a broad range of applications in wearable electronics and self-propelled machines and creates promising opportunities for sensors and actuators.

Free-standing conductive nickel metal-organic framework nanowires as bifunctional electrodes for wearable pressure sensors and Ni-Zn batteries

Free-standing metal-organic frameworks (MOFs) with controllable structure and good stability are emerging as promising materials for applications in flexible pressure sensors and energy-storage devices. However, the inherent low electrical conductivity of MOF-based materials requires complex preparation processes that involve high-temperature carbonization. This work presents a simple method to grow conductive nickel MOF nanowire arrays on carbon cloth (Ni-CAT@CC) and use Ni-CAT@CC as the functional electrodes for flexible piezoresistive sensor. The resulting sensor is able to monitor human activity, including elbow bending, knee bending, and wrist bending. Besides, the soft-packaged aqueous Ni-Zn battery is assembled with Ni-CAT@CC, a piece of glass microfiber filters, and Zn foil acting as cathode, separator, and anode, respectively. The Ni-Zn battery can be used as a power source for finger pressure monitoring. This work demonstrates free-standing MOF-based nanowires as bifunctional fabric electrodes for wearable electronics.

Laser-reduced graphene oxide for a flexible liquid sliding sensing surface

Flexible electronic skin is a flexible sensor system that imitates human skin. Recently, flexible sensors have been successfully developed. However, the droplet sliding sensing technology on a flexible electronic skin surface is still challenging. In this Letter, a flexible droplet sliding sensing surface is proposed and fabricated by laser-reduced graphene oxide (LRGO). The LRGO shows porous structures and low surface energy, which are beneficial for infusing lubricants and fabricating stable slippery surfaces. The slippery surface guarantees free sliding of droplets. The droplet sliding sensing mechanism is a combination of triboelectricity and electrostatic induction. After a NaCl droplet slides from lubricant-infused LRGO, a potential difference (∼0.2 mV) can be measured between two Ag electrodes. This study reveals considerable potential applications in intelligent robots and the medical field.

Self-Healing Supramolecular Hydrogels with Antibacterial Abilities for Wound Healing

Wound healing due to skin defects is a growing clinical concern. Especially when infection occurs, it not only leads to impair healing of the wound but even leads to the occurrence of death. In this study, a self-healing supramolecular hydrogel with antibacterial abilities was developed for wound healing. The supramolecular hydrogels inherited excellent self-healing and mechanical properties are produced by the polymerization of N-acryloyl glycinamide monomers which carries a lot of amides. In addition, excellent antibacterial properties are obtained by integrating silver nanoparticles (Ag NPs) into the hydrogels. The resultant hydrogel has a demonstrated ability in superior mechanical properties, including stretchability and self-healing. Also, the good biocompatibility and antibacterial ability have been proven in hydrogels. Besides, the prepared hydrogels were employed as wound dressings to treat skin wounds of animals. It was found that the hydrogels could significantly promote wound repair, including relieving inflammation, promoting collagen deposition, and enhancing angiogenesis. Therefore, such self-healing supramolecular hydrogels with composite functional nanomaterials are expected to be used as new wound dressings in the field of healthcare.

Responsive hydrogel microfibers for biomedical engineering

Responsive hydrogel microfibers can realize multiple controllable changes in shapes or properties under the stimulation of the surrounding environment, and are called as intelligent biomaterials. Recently, these responsive hydrogel microfibers have been proved to possess significant biomedical values, and remarkable progress has been achieved in biomedical engineering applications, including drug delivery, biosensors and clinical therapy, etc. In this review, the latest research progress and application prospects of responsive hydrogel microfibers in biomedical engineering are summarized. We first introduce the common preparation strategies of responsive hydrogel microfibers. Subsequently, the response characteristics and the biomedical applications of these materials are discussed. Finally, the present opportunities and challenges as well as the prospects for future development are critically analyzed.