Highly Stretchable, Strain Sensing Hydrogel Optical Fibers

Hydrogel for light delivery in biomedical applications

Traditional optical waveguides or mediums are often silica-based materials, but their applications in biomedicine and healthcare are limited due to the poor biocompatibility and unsuitable mechanical properties. In term of the applications in human body, a biocompatible hydrogel system with excellent optical transparency and mechanical flexibility could be beneficial. In this review, we explore the different designs of hydrogel-based optical waveguides derived from natural and synthetic sources. We highlighted key developments such as light emitting contact lenses, implantable optical fibres, biosensing systems, luminating and fluorescent materials. Finally, we expand further on the challenges and perspectives for hydrogel waveguides to achieve clinical applications.

hydroSIM: super-resolution speckle illumination microscopy with a hydrogel diffuser

Super-resolution microscopy has emerged as an indispensable methodology for probing the intricacies of cellular biology. Structured illumination microscopy (SIM), in particular, offers an advantageous balance of spatial and temporal resolution, allowing for visualizing cellular processes with minimal disruption to biological specimens. However, the broader adoption of SIM remains hampered by the complexity of instrumentation and alignment. Here, we introduce speckle-illumination super-resolution microscopy using hydrogel diffusers (hydroSIM). The study utilizes the high scattering and optical transmissive properties of hydrogel materials and realizes a remarkably simplified approach to plug-in super-resolution imaging via a common epi-fluorescence platform. We demonstrate the hydroSIM system using various phantom and biological samples, and the results exhibited effective 3D resolution doubling, optical sectioning, and high contrast. We foresee hydroSIM, a cost-effective, biocompatible, and user-accessible super-resolution methodology, to significantly advance a wide range of biomedical imaging and applications.

Self-Healable Multifunctional Fibers via Thermal Drawing

The development of soft electronics and soft fiber devices has significantly advanced flexible and wearable technology. However, they still face the risk of damage when exposed to sharp objects in real-life applications. Taking inspiration from nature, self-healable materials that can restore their physical properties after external damage offer a solution to this problem. Nevertheless, large-scale production of self-healable fibers is currently constrained. To address this limitation, this study leverages the thermal drawing technique to create elastic and stretchable self-healable thermoplastic polyurethane (STPU) fibers, enabling cost-effective mass production of such functional fibers. Furthermore, despite substantial research into the mechanisms of self-healable materials, quantifying their healing speed and time poses a persistent challenge. Thus, transmission spectra are employed as a monitoring tool to observe the real-time self-healing process, facilitating an in-depth investigation into the healing kinetics and efficiency. The versatility of the fabricated self-healable fiber extends to its ability to be doped with a wide range of functional materials, including dye molecules and magnetic microparticles, which enables modular assembly to develop distributed strain sensors and soft actuators. These achievements highlight the potential applications of self-healable fibers that seamlessly integrate with daily lives and open up new possibilities in various industries.

Control of polymers' amorphous-crystalline transition enables miniaturization and multifunctional integration for hydrogel bioelectronics

Soft bioelectronic devices exhibit motion-adaptive properties for neural interfaces to investigate complex neural circuits. Here, we develop a fabrication approach through the control of metamorphic polymers' amorphous-crystalline transition to miniaturize and integrate multiple components into hydrogel bioelectronics. We attain an about 80% diameter reduction in chemically cross-linked polyvinyl alcohol hydrogel fibers in a fully hydrated state. This strategy allows regulation of hydrogel properties, including refractive index (1.37-1.40 at 480 nm), light transmission (>96%), stretchability (139-169%), bending stiffness (4.6 ± 1.4 N/m), and elastic modulus (2.8-9.3 MPa). To exploit the applications, we apply step-index hydrogel optical probes in the mouse ventral tegmental area, coupled with fiber photometry recordings and social behavioral assays. Additionally, we fabricate carbon nanotubes-PVA hydrogel microelectrodes by incorporating conductive nanomaterials in hydrogel for spontaneous neural activities recording. We enable simultaneous optogenetic stimulation and electrophysiological recordings of light-triggered neural activities in Channelrhodopsin-2 transgenic mice.

Flexible hydrogels connecting adhesion and wetting

Raindrops falling on window-panes spread upon contact, whereas hail can cause dents or scratches on the same glass window upon contact. While the former phenomenon resembles classical wetting, the latter is dictated by contact and adhesion theories. The classical Young-Dupre law applies to the wetting of pure liquids on rigid solids, whereas conventional contact mechanics theories account for rigid-on-soft or soft-on-rigid contacts with small deformations in the elastic limit. However, the crossover between adhesion and wetting is yet to be fully resolved. The key lies in the study of soft-on-soft interactions with material properties intermediate between liquids and solids. In this work, we translate adhesion to wetting by experimentally probing the static signature of hydrogels in contact with soft PDMS of varying elasticity of both the components. Consequently, we probe this transition across six orders of magnitude in terms of the characteristic elasto-adhesive parameter of the system. In doing so, we reveal previously unknown phenomenology and a theoretical model which smoothly bridges adhesion of glass spheres with total wetting of pure liquids on any given substrate.

Intelligent perceptual textiles based on ionic-conductive and strong silk fibers

Endowing textiles with perceptual function, similar to human skin, is crucial for the development of next-generation smart wearables. To date, the creation of perceptual textiles capable of sensing potential dangers and accurately pinpointing finger touch remains elusive. In this study, we present the design and fabrication of intelligent perceptual textiles capable of electrically responding to external dangers and precisely detecting human touch, based on conductive silk fibroin-based ionic hydrogel (SIH) fibers. These fibers possess excellent fracture strength (55 MPa), extensibility (530%), stable and good conductivity (0.45 S·m-1) due to oriented structures and ionic incorporation. We fabricated SIH fiber-based protective textiles that can respond to fire, water, and sharp objects, protecting robots from potential injuries. Additionally, we designed perceptual textiles that can specifically pinpoint finger touch, serving as convenient human-machine interfaces. Our work sheds new light on the design of next-generation smart wearables and the reshaping of human-machine interfaces.

High-throughput measurement of elastic moduli of microfibers by rope coiling

There are many fields where it is of interest to measure the elastic moduli of tiny fragile fibers, such as filamentous bacteria, actin filaments, DNA, carbon nanotubes, and functional microfibers. The elastic modulus is typically deduced from a sophisticated tensile test under a microscope, but the throughput is low and limited by the time-consuming and skill-intensive sample loading/unloading. Here, we demonstrate a simple microfluidic method enabling the high-throughput measurement of the elastic moduli of microfibers by rope coiling using a localized compression, where sample loading/unloading are not needed between consecutive measurements. The rope coiling phenomenon occurs spontaneously when a microfiber flows from a small channel into a wide channel. The elastic modulus is determined by measuring either the buckling length or the coiling radius. The throughput of this method, currently 3,300 fibers per hour, is a thousand times higher than that of a tensile tester. We demonstrate the feasibility of the method by testing a nonuniform fiber with axially varying elastic modulus. We also demonstrate its capability for in situ inline measurement in a microfluidic production line. We envisage that high-throughput measurements may facilitate potential applications such as screening or sorting by mechanical properties and real-time control during production of microfibers.

Differentiating aliphatic and aromatic alcohols using triazine-based supramolecular organogelators: end group-specific selective gelation with chain length of alcohols

Supramolecular gels have an extensive range of potential applications, out of which stimuli-responsive materials are a topic of contemporary research. Gels being kinetically entrapped materials can be tuned to different forms using external chemical stimuli. In this context, three different triazine gelators, each containing a unique end group, were examined for gelation in various solvent systems. Nevertheless, the gelation was limited to only alcoholic solvents, suggesting that the hydrogen bonds between the gelating solvent and gelator play a crucial role in gelation. Further, it was found that these gelators could gelate only with aliphatic alcohols, which could be degelled easily using aromatic alcohols. The three gelators exhibited distinct gelation of aliphatic alcohols based on their end groups. The gelator with the polar-aromatic end group (C5H4N) was found to gelate with lighter alcohols, whereas that with the nonpolar aromatic end group (C6H5) was found to prefer higher alcohols. The MGC and Tgel values were also found to depend on the alkyl chain length/branching of the alcohols. The crystal structure of one of the gelators provides insights into the model structure of the gels. Cyclohexanol was the only solvent that could produce gels with all three of the as-synthesised gelators. The process of degelation by aromatic alcohols was monitored at different points of the disassembly process by rheological and morphological measurements to understand the extent of controlled degelation. These gels have great potential for use in controlled drug delivery and chemical sensing, among other areas.

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Hydrogel-Based Sensors for Human-Machine Interaction

In the past decades, remarkable progress has been made in the field of human-machine interaction. The need for accurate sensing devices with satisfactory user experiences has propelled the development of flexible, stretchable, biocompatible, and imperceptible hydrogel-based interfaces. These innovative interfaces facilitate direct interactions between humans and machines while receiving detected input signals from sensors and giving output commands to controllers, thus motivating accurate real-time responsiveness. This Perspective discusses the sensing mechanisms for the two categories of hydrogel-based sensors and summarizes the recent progress in the development of different representations of human-machine interactions, including intelligent identification, information secrecy, interactive control, and virtual reality and augmented reality technologies. The advantages of hydrogel-based systems over conventionally used rigid electrical components are explicitly discussed. The conclusion provides a perspective on current challenges and outlines a future roadmap for the realization of state-of-the-art hydrogel-based smart systems.

High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors

Design of admirable conductive hydrogels combining robust toughness, soft flexibility, desirable conductivity, and freezing resistance remains daunting challenges for meeting the customized and critical demands of flexible and wearable electronics. Herein, a promising and facile strategy to prepare hydrogels tailored to these anticipated demands is proposed, which is prepared in one step by homogeneous cross-linking of acrylamide using hydrophobic divinylbenzene stabilized by micelles under saturated high-saline solutions. The influence of high-saline environments on the hydrogel topology and mechanical performance is investigated. The high-saline environments suppress the size of hydrophobic cross-linkers in micelles during hydrogel polymerization, which weaken the dynamic hydrophobic associations to soften the hydrogels. Nevertheless, the homogeneous cross-linked networks ensure antifracture during ultralarge deformations. The obtained hydrogels show special mechanical performance combining extremely soft deformability and antifracture features (Young's modulus, 5 kPa; stretchability, 10200%; toughness, 134 kJ m-2; and excellent anticrack propagation). The saturated-saline environments also endow the hydrogels with desirable ion conductivity (106 mS cm-1) and freezing resistance (<20 °C). These comprehensive properties of the obtained hydrogels are quite suitable for flexible electronic applications, which is demonstrated by the high sensitivity and durability of the derived strain sensors.

Multi-modal deformation and temperature sensing for context-sensitive machines

Owing to the remarkable properties of the somatosensory system, human skin compactly perceives myriad forms of physical stimuli with high precision. Machines, conversely, are often equipped with sensory suites constituted of dozens of unique sensors, each made for detecting limited stimuli. Emerging high degree-of-freedom human-robot interfaces and soft robot applications are delimited by the lack of simple, cohesive, and information-dense sensing technologies. Stepping toward biological levels of proprioception, we present a sensing technology capable of decoding omnidirectional bending, compression, stretch, binary changes in temperature, and combinations thereof. This multi-modal deformation and temperature sensor harnesses chromaticity and intensity of light as it travels through patterned elastomer doped with functional dyes. Deformations and temperature shifts augment the light chromaticity and intensity, resulting in a one-to-one mapping between stimulus modes that are sequentially combined and the sensor output. We study the working principle of the sensor via a comprehensive opto-thermo-mechanical assay, and find that the information density provided by a single sensing element permits deciphering rich and diverse human-robot and robot-environmental interactions.

Recent Insights about the Role of Gels in Organic Photonics and Electronics

This review article provides an in-depth exploration of the role of gels in the fields of organic electronics and photonics, focusing on their unique properties and applications. Despite their remarkable potential, gel-based innovations remain relatively uncharted in these domains. This brief review aims to bridge the knowledge gap by shedding light on the diverse roles that gels can fulfil in the enhancement of organic electronic and photonic devices. From flexible electronics to light-emitting materials, we delve into specific examples of gel applications, highlighting their versatility and promising outcomes. This work serves as an indispensable resource for researchers interested in harnessing the transformative power of gels within these cutting-edge fields. The objective of this review is to raise awareness about the overlooked research potential of gels in optoelectronic materials, which have somewhat diminished in recent years.

Fatigue-resistant hydrogel optical fibers enable peripheral nerve optogenetics during locomotion

We develop soft and stretchable fatigue-resistant hydrogel optical fibers that enable optogenetic modulation of peripheral nerves in naturally behaving animals during persistent locomotion. The formation of polymeric nanocrystalline domains within the hydrogels yields fibers with low optical losses of 1.07 dB cm-1, Young's modulus of 1.6 MPa, stretchability of 200% and fatigue strength of 1.4 MPa against 30,000 stretch cycles. The hydrogel fibers permitted light delivery to the sciatic nerve, optogenetically activating hindlimb muscles in Thy1::ChR2 mice during 6-week voluntary wheel running assays while experiencing repeated deformation. The fibers additionally enabled optical inhibition of pain hypersensitivity in an inflammatory model in TRPV1::NpHR mice over an 8-week period. Our hydrogel fibers offer a motion-adaptable and robust solution to peripheral nerve optogenetics, facilitating the investigation of somatosensation.

Integrated Flexible Microscale Mechanical Sensors Based on Cascaded Free Spectral Range-Free Cavities

Photonic mechanical sensors offer several advantages over their electronic counterparts, including immunity to electromagnetic interference, increased sensitivity, and measurement accuracy. Exploring flexible mechanical sensors on deformable substrates provides new opportunities for strain-optical coupling operations. Nevertheless, existing flexible photonics strategies often require cumbersome signal collection and analysis with bulky setups, limiting their portability and affordability. To address these challenges, we propose a waveguide-integrated flexible mechanical sensor based on cascaded photonic crystal microcavities with inherent deformation and biaxial tensile state analysis. Leveraging the advanced multiplexing capability of the sensor, for the first time, we successfully demonstrate 2D shape reconstruction and quasi-distributed strain sensing with 110 μm spatial resolution. Our microscale mechanical sensor also exhibits exceptional sensitivity with a detected force level as low as 13.6 μN in real-time measurements. This sensing platform has potential applications in various fields, including biomedical sensing, surgical catheters, aircraft and spacecraft engineering, and robotic photonic skin development.

Titania Nanoparticle-Stimulated Ultralow Frequency Detection and High-Pass Filter Behavior of a Flexible Piezoelectric Nanogenerator: A Self-Sustaining Energy Harvester for Active Motion Tracking

A significant driving force for the fabrication of IoT-compatible smart health gear integrated with multifunctional sensors is the growing trend in fitness and the overall wellness of the human body. In this work, we present an autonomous motion and activity-sensing device based on the efficacious nucleation of the polar β-phase in an electroactive polymer. Representatively, we investigate the nucleating effect of TiO2 nanoparticles on weight-modulated PVDF-HFP films (PT-5, PT-10, and PT-15) and subsequently prototype a sensing device with the film that demonstrates superior β-phase nucleation. The PT-10 film, with an optimal polar β-phase, shows the highest remnant polarization (2Pr) and energy density of 0.36 μC/cm2 and 22.3 mJ/cm3, respectively, at 60 kV/cm. The films mimic a high pass filter at frequencies above 10 KHz with very low impedance and high ac conductivity values. The frequency-dependent impedance studies reveal an effective interfacial polarization between TiO2 nanoparticles and PVDF-HFP, explicitly observed in the low-frequency region. Consequently, the sensor fabricated with PT-10 as the sensing layer exhibits ultralow frequency detection (25 Hz) resulting from the blood flow muscle oxygenation. The device successfully senses voluntary joint movements of the human body and actively tracks a range of motions, from brisk walking to running. Additionally, through repetitive human finger-tapping motion, the nanogenerator lights up multiple light-emitting diodes in series and charges capacitors of varying magnitudes under 50 s. The real-time human motion sensing and movement tracking modalities of the sensor hold promise in the arena of smart wearables, sports biomechanics, and contact-based medical devices.

Perceptual Soft End-Effectors for Future Unmanned Agriculture

As consumers demand ever-higher quality standards for agricultural products, the inspection of such goods has become an integral component of the agricultural production process. Unfortunately, traditional testing methods necessitate the deployment of numerous bulky machines and cannot accurately determine the quality of produce prior to harvest. In recent years, with the advancement of soft robot technology, stretchable electronic technology, and material science, integrating flexible plant wearable sensors on soft end-effectors has been considered an attractive solution to these problems. This paper critically reviews soft end-effectors, selecting the appropriate drive mode according to the challenges and application scenarios in agriculture: electrically driven, fluid power, and smart material actuators. In addition, a presentation of various sensors installed on soft end-effectors specifically designed for agricultural applications is provided. These sensors include strain, temperature, humidity, and chemical sensors. Lastly, an in-depth analysis is conducted on the significance of implementing soft end-effectors in agriculture as well as the potential opportunities and challenges that will arise in the future.

A nanoscale inorganic coating strategy for stabilizing hydrogel neural probes in vivo

Hydrogels with adaptable optical and mechanical characteristics show considerable promise for light delivery in vivo with neuroengineering applications. However, the unlinked amorphous polymer chains within hydrogels can cause volumetric swelling after water absorption under physiological conditions over time. Chemically cross-linked poly(vinyl alcohol) (PVA) hydrogels showcase fatigue-resistant attributes and promising biocompatibility for the manufacture of soft neural probes. However, possible swelling of the PVA hydrogel matrix could impact the structural stability of hydrogel-based bioelectronics and their long-term in vivo functionality. In this study, we utilized an atomic layer deposition (ALD) technique to generate an inorganic, silicon dioxide (SiO2) coating layer on chemically cross-linked PVA hydrogel fibers. To evaluate the stability of SiO2-coated PVA hydrogel fibers mimicking the in vivo environment, we conducted accelerated stability tests. SiO2-coated PVA hydrogel fibers showed improved stability over a one-week incubation period under a harsh environment, preventing swelling and preserving their mechanical and optical properties compared to uncoated fibers. These SiO2-coated PVA hydrogel fibers demonstrated nanoscale polymeric crystalline domains (6.5 ± 0.1 nm), an elastic modulus of 73.7 ± 31.7 MPa, a maximum elongation of 113.6 ± 24.2%, and minimal light transmission loss (1.9 ± 0.2 dB cm-1). Lastly, we applied these SiO2-coated PVA hydrogel fibers in vivo to optically activate the motor cortex of transgenic Thy1::ChR2 mice during locomotor behavioral tests. This mouse cohort was genetically modified to express the light-sensitive ion channel, channelrhodopsin-2 (ChR2), and implanted with hydrogel fibers to deliver light to the motor cortex area (M2). Light stimulation via hydrogel fibers resulted in optogenetically modulated mouse locomotor behaviors, including increased contralateral rotation, mobility speeds, and travel distances.

Stretchable and sensitive sodium alginate ionic hydrogel fibers for flexible strain sensors

Ionic conductive hydrogel fibers based on natural polymers provide an immense focus for a new generation of electronics due to their flexibility and knittability. The feasibility of utilizing pure natural polymer-based hydrogel fibers could be drastically improved if their mechanical and transparent performances satisfy the requirements of actual practice. Herein, we report a facile fabrication strategy for significantly stretchable and sensitive sodium alginate ionic hydrogel fibers (SAIFs), by glycerol initiating physical crosslinking and by CaCl2 inducing ionic crosslinking. The obtained ionic hydrogel fibers not only show significant stretchability (tensile strength of 1.55 MPa and fracture strain of ∼161 %), but also exhibit wide-range sensing, satisfactorily stable, rapidly responsive, and multiply sensitive abilities to external stimulus. In addition, the ionic hydrogel fibers have excellent transparency (over 90 % in a wide wavelength range), and good anti-evaporation and anti-freezing properties. Furthermore, the SAIFs have been easily knitted into a textile, and successfully applied as wearable sensors to recognize human motions, by observing the output electrical signals. Our methodology for fabrication intelligent SAIFs will shed light on artificial flexible electronics and other textile-based strain sensors.

An Anti-Fracture and Super Deformable Soft Hydrogel Network Insensitive to Extremely Harsh Environments

Design of hydrogels with superior flexible deformability, anti-fracture toughness, and reliable environment adaption is fundamentally and practically important for diverse hydrogel-based flexible devices. However, these features can hardly be compatible even in elaborately designed hydrogels. Herein soft hydrogel networks with superior anti-fracture and deformability are proposed, which show good adaption to extremely harsh saline or alkaline environments. The hydrogel network is one-step constructed via hydrophobic homogenous cross-linking of poly (sodium acrylate), which is expected to provide hydrophobic associations and homogeneous cross-linking for energy dissipation. The obtained hydrogels are quite soft and deformable (tensile modulus: ≈20 kPa, stretchability: 3700%), but show excellent anti-fracture toughness (10.6 kJ m-2 ). The energy dissipation mechanism can be further intensified under saline or alkaline environments. The mechanical performance of the hydrophobic cross-linking topology is inspired rather than weakened by extremely saline or alkaline environments (stretchability: 3900% and 5100%, toughness: 16.1 and 17.1 kJ m-2 under saturated NaCl and 6 mol L-1 NaOH environments, respectively). The hydrogel network also shows good performance in reversible deformations, ion conductivity, sensing strain, monitoring human motions, and freezing resistance under high-saline environments. The hydrogel network show unique mechanical performance and robust environment adaption, which is quite promising for diverse applications.

Hydrogel fibers for wearable sensors and soft actuators

Owing to superior softness, wetness, responsiveness, and biocompatibility, bulk hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, optics, and coatings. The one-dimensional (1D) hydrogel fibers possess the metrics from both the hydrogel materials and structural topology, endowing them with extraordinary mechanical, sensing, breathable and weavable properties. As no comprehensive review has been reported for this nascent field, this article aims to provide an overview of hydrogel fibers for soft electronics and actuators. We first introduce the basic properties and measurement methods of hydrogel fibers, including mechanical, electrical, adhesive, and biocompatible properties. Then, typical manufacturing methods for 1D hydrogel fibers and fibrous films are discussed. Next, the recent progress of wearable sensors (e.g., strain, temperature, pH, and humidity) and actuators made from hydrogel fibers is discussed. We conclude with future perspectives on next-generation hydrogel fibers and the remaining challenges. The development of hydrogel fibers will not only provide an unparalleled one-dimensional characteristic, but also translate fundamental understanding of hydrogels into new application boundaries.

AI-Assisted Disease Monitoring Using Stretchable Polymer-Based Sensors

Flexible sensors have attracted significant attention for medical applications. Herein, an AI-assisted stretchable polymer-based (AISP) sensor has been developed based on the Beer-Lambert law for disease monitoring and telenursing. Benefiting from the use of superior polymer materials, the AISP sensor features a high tensile strain of up to 100%, durability of >10,000 tests, excellent waterproofness, and no effect of temperature (1.6-60.9 °C). Such advantages support the capability that the AISP can be flexibly pasted on the skin surface as a wearable device for real-time monitoring of multiple physiological parameters. An AISP sensor-based swallowing recognition technique has been proposed with a high accuracy of up to 88.89%. Likewise, it has been expanded to a remote nursing assistance system to meet critical patients' physiological requirements and daily care. The hands-free communication experiment and robot control applications have also been successfully conducted based on the constructed system. Such merits demonstrate its potential as a medical toolkit and indicate promise for intelligent healthcare.

Visualizing Drug Release from a Stimuli-Responsive Soft Material Based on Amine-Thiol Displacement

In this research, we developed a photoluminescent platform using amine-coupled fluorophores, generated from a single conjugate acceptor containing bis-vinylogous thioesters. Based on the experimental and computational results, the fluorescence turn-on mechanism was proposed to be charge separated induced energy radiative transition for the amine-coupled fluorophore, while the sulfur-containing precursor was not fluorescent since the energy internal conversion occurred through vibrational 2RS- (R represents alkyl groups) as energy acceptor(s). Further utilizing the conjugate acceptor, we establish a new fluorogenic approach via a highly cross-linked soft material to selectively detect cysteine under neutral aqueous conditions. Turn-on fluorescence emission and macroscopic degradation occurred in the presence of cysteine as the stimuli, which can be visually tracked due to the generation of an optical indicator and the cleavage of linkers within the matrix. Furthermore, a novel drug delivery system was constructed, achieving controlled release of sulfhydryl drug (6-mercaptopurine) which was tracked by photoluminescence and high-performance liquid chromatography. The photoluminescent molecules developed herein are suitable for visualizing polymeric degradation, making them suitable for additional "smart" material applications.

Extremely Robust and Multifunctional Nanocomposite Fibers for Strain-Unperturbed Textile Electronics

Textile electronics are needed that can achieve strain-unaltered performance when they undergo irregular and repeated strain deformation. Such strain-unaltered textile electronics require advanced fibers that simultaneously have high functionalities and extreme robustness as fabric materials. Current synthetic nanocomposite fibers based on inorganic matrix have remarkable functionalities but often suffer from low robustness and poor tolerance against crack formation. Here, we present a design for a high-performance multifunctional nanocomposite fiber that is mechanically and electrically robust, which was realized by crosslinking titanium carbide (MXene) nanosheets with a slide-ring polyrotaxane to form an internal mechanically-interlocked network. This inorganic matrix nanocomposite fiber featured distinct strain-hardening mechanical behavior and exceptional load-bearing capability (toughness approaching 60 MJ m^-3 and ductility over 27%). It retained 100% of its ductility after cyclic strain loading. Moreover, the high electrical conductivity (>1.1 × 10⁵ S m^-1 ) and electrochemical performance (>360 F cm^-3 ) of the nanocomposite fiber can be well retained after subjecting the fiber to extensive (>25% strain) and long-term repeated (10 000 cycles) dimensional changes. Such superior robustness allowed for the fabrication of the nanocomposite fibers into various robust wearable devices, such as textile-based electromechanical sensors with strain-unalterable sensing performance and fiber-shaped supercapacitors with invariant electrochemical performance for 10 000 strain loading cycles.

Bioinspired Self-Growing Hydrogels by Harnessing Interfacial Polymerization

The production of natural materials is achieved through a bottom-up approach, in which materials spontaneously grow and adapt to the external environment. Synthetic materials are specifically designed and fabricated as engineered materials; however, they are far away from these natural self-growing attributes. Thus, design and fabrication of synthetic material systems to replicate the self-growing characteristics of those natural prototypes (i.e., hairs and nails) remains challenging. Inspired by the self-growing behaviors of keratin proteins, here the fabrication of synthetic hydrogels (i.e., polyacrylamide (PAAm)) from the free radical polymerization at the interface between AAm precursor solution and liquid metals (i.e., eutectic gallium-indium (EGaIn)) is reported. The newly formed hydrogel materials at the EGaIn/AAm precursor interface gradually push the whole hydrogel upward, enabling the self-growing of these synthetic hydrogel materials. This work not only endows the fabrication of synthetic materials with unprecedented self-growing characters, but also broadens the potential applications of self-growing materials in actuation and soft robotics.

Tough and stretchable all-κ-carrageenan hydrogel based on the cooperative effects between chain conformation transition and stepwise mechanical training

The traditional κ-carrageenan (κCG)-based hydrogel obtained from hot water can rupture easily under mechanical loading. To address this vulnerability, here we presented a robust all-κCG hydrogel without employing the second synthetic network. By simply regulating the polymer chains from random coil to stiff chain conformation in NaOH/urea solvent system via the freeze-thawing process, the as-prepared hydrogel with homogeneous structure can display an enhanced stretchability from 42.1 to 156 %, while maintaining the similar fracture stress. Moreover, upon the stepwise mechanical training and subsequent incubation in KCl aqueous solution, more helical segments of κCG were aligned and involved into the association domains, thus leading to the increment in both the crystallinity and anisotropy. Consequently, a fast self-strengthening behavior occurred, and a more stretchable (fracture strain up to 396 %), strong (stress ∼ 0.55 MPa) and tough (∼1.52 MJ m^-3) κCG hydrogel was obtained. In comparison to the traditional one, the fracture strain and toughness are increased by 8.5 and 11.5 times, respectively. In addition, this κCG hydrogel can demonstrate good recovery and shape-memory behaviors under medium deformation. Hence, this tough all-κCG hydrogel is expected to be tailored into the biomaterials as the wearable device, artificial tendon, and cartilage in the future.

Difunctional Hydrogel Optical Fiber Fluorescence Sensor for Continuous and Simultaneous Monitoring of Glucose and pH

It is significant for people with diabetes to know their body's real-time glucose level, which can guide the diagnosis and treatment. Therefore, it is necessary to research continuous glucose monitoring (CGM) as it gives us real-time information about our health condition and its dynamic changes. Here, we report a novel hydrogel optical fiber fluorescence sensor segmentally functionalized with fluorescein derivative and CdTe QDs/3-APBA, which can continuously monitor pH and glucose simultaneously. In the glucose detection section, the complexation of PBA and glucose will expand the local hydrogel and decrease the fluorescence of the quantum dots. The fluorescence can be transmitted to the detector by the hydrogel optical fiber in real time. As the complexation reaction and the swelling-deswelling of the hydrogel are all reversible, the dynamic change of glucose concentration can be monitored. For pH detection, the fluorescein attached to another segment of the hydrogel exhibits different protolytic forms when pH changes and the fluorescence changes correspondingly. The significance of pH detection is compensation for pH errors in glucose detection because the reaction between PBA and glucose is sensitive to pH. The emission peaks of the two detection units are 517 nm and 594 nm, respectively, so there is no signal interference between them. The sensor can continuously monitor glucose in 0-20 mM and pH in 5.4-7.8. The advantages of this sensor are multi-parameter simultaneous detection, transmission-detection integration, real-time dynamic detection, and good biocompatibility.

Data-driven multi-joint waveguide bending sensor based on time series neural network

Due to the bulky interrogation devices, traditional fiber optic sensing system is mainly connected by wire or equipped only for large facilities. However, the advancement in neural network algorithms and flexible materials has broadened its application scenarios to bionics. In this paper, a multi-joint waveguide bending sensor based on color dyed filters is designed to detect bending angles, directions and positions. The sensors are fabricated by casting method using soft silicone rubber. Besides, required optical properties of sensor materials are characterized to better understand principles of the sensor design. Time series neural networks are utilized to predict bending position and angle quantitatively. The results confirm that the waveguide sensor demodulated by the data-driven neural network algorithm performs well and can be used for engineering applications.

Progression of Quantum Dots Confined Polymeric Systems for Sensorics

The substantial fluorescence (FL) capabilities, exceptional photophysical qualities, and long-term colloidal stability of quantum dots (QDs) have aroused a lot of interest in recent years. QDs have strong and wide optical absorption, good chemical stability, quick transfer characteristics, and facile customization. Adding polymeric materials to QDs improves their effectiveness. QDs/polymer hybrids have implications in sensors, photonics, transistors, pharmaceutical transport, and other domains. There are a great number of review articles available online discussing the creation of CDs and their many uses. There are certain review papers that can be found online that describe the creation of composites as well as their many different uses. For QDs/polymer hybrids, the emission spectra were nearly equal to those of QDs, indicating that the optical characteristics of QDs were substantially preserved. They performed well as biochemical and biophysical detectors/sensors for a variety of targets because of their FL quenching efficacy. This article concludes by discussing the difficulties that still need to be overcome as well as the outlook for the future of QDs/polymer hybrids.

High-Performance Zwitterionic Organohydrogel Fiber in Bioelectronics for Monitoring Bioinformation

Bioinformation plays an imperative role in day-to-day life. Wearable bioelectronics are important for sensing bioinformation in real-time and conductive hydrogel fibers are a key component in next generation wearable bioelectronics. However, current conductive hydrogel fibers have remarkable disadvantages such as insufficient conductivity, stability, and bioinformation sensing ability. Here, we report the synthesis of a zwitterionic organohydrogel (ZOH) fiber by the combination of the mold method and solvent replacement strategy. The ZOH fiber shows transparency (92.1%), stretchability (905.8%), long-term stability, anti-freezing ability (-35-60 °C), and low light transmission loss (0.17 dB/cm). Then, we integrate the ZOH fiber into fabric for use as a bioinformation sensor, the results prove its capability as a bioinformation monitor, monitoring information such as motion and bioelectric signals. In addition, the potential of the ZOH fiber in optogenetic applications is also confirmed.

Concurrent stiffening and softening in hydrogels under dehydration

Hydrogels are an extraordinary soft matter system that serve as a laboratory for a rich plethora of multiphysical phenomena and find applications that range from biocompatible sensors to soft robots. Here, we report a peculiar experimental observation suggesting concurrent stiffening and softening in hydrogels during the dehydration process. Theories based on Flory's work fail to capture the scaling of mechanical behavior with water content, observed in our experiments. We perform coarse-grained molecular dynamics simulations to elucidate the mechanisms underpinning the odd softening-stiffening behavior during dehydration and propose a theoretical model to correctly represent the underlying physics and the divergence from Flory-based theories.

Design, Fabrication and Characterisation of Multi-Parameter Optical Sensors Dedicated to E-Skin Applications

For many years there has been a strong research interest in soft electronics for artificial skin applications. However, one challenge with stretchable devices is the limited availability of high performance, stretchable, electrical conductors and semiconductors that remain stable under strain. Examples of such electronic skin require excessive amounts of wires to address each sensing element-compression force and strain-in a conventional matrix structure. Here, we present a new process for fabricating artificial skin consisting of an optical waveguide architecture, enabling wide ranging sensitivity to external mechanical compression and strain. The manufacturing process allows design of a fully stretchable polydimethylsiloxane elastomer waveguide with embedded gratings, replicated from low cost DVD-Rs. This optical artificial skin allows the detection of compression forces from 0 to 3.8 N with controllable sensitivity. It also permits monitoring of elongation deformations up to 135%. This type of stretchable optical sensor is highly robust, transparent, and presents a large sensing area while limiting the amount of wires connecting to the sensor. Thus, this optical artificial skin presents far superior mechanical properties compared to current electronic skin.

Non-Specific Responsive Nanogels and Plasmonics to Design MathMaterial Sensing Interfaces: The Case of a Solvent Sensor

The combination of non-specific deformable nanogels and plasmonic optical probes provides an innovative solution for specific sensing using a generalistic recognition layer. Soft polyacrylamide nanogels that lack specific selectivity but are characterized by responsive behavior, i.e., shrinking and swelling dependent on the surrounding environment, were grafted to a gold plasmonic D-shaped plastic optical fiber (POF) probe. The nanogel-POF cyclically challenged with water or alcoholic solutions optically reported the reversible solvent-to-phase transitions of the nanomaterial, embodying a primary optical switch. Additionally, the non-specific nanogel-POF interface exhibited more degrees of freedom through which specific sensing was enabled. The real-time monitoring of the refractive index variations due to the time-related volume-to-phase transition effects of the nanogels enabled us to determine the environment's characteristics and broadly classify solvents. Hence the nanogel-POF interface was a descriptor of mathematical functions for substance identification and classification processes. These results epitomize the concept of responsive non-specific nanomaterials to perform a multiparametric description of the environment, offering a specific set of features for the processing stage and particularly suitable for machine and deep learning. Thus, soft MathMaterial interfaces provide the ground to devise devices suitable for the next generation of smart intelligent sensing processes.

Propagation Losses Estimation in a Cationic-Network-Based Hydrogel Waveguide

A method based on the photographic recording of the power distribution laterally diffused by cationic-network (CN) hydrogel waveguides is first checked against the well-established cut-back method and then used to determine the different contributions to optical power attenuation along the hydrogel-based waveguide. Absorption and scattering loss coefficients are determined for 450 nm, 532 nm and 633 nm excitation. The excellent optical loss values obtained (0.32-1.95 dB/cm), similar to others previously described, indicate their potential application as waveguides in different fields, including soft robotic and light-based therapies.

Temperature-adaptive hydrogel optical waveguide with soft tissue-affinity for thermal regulated interventional photomedicine

Photomedicine has gained great attention due to its nontoxicity, good selectivity and small trauma. However, owing to the limited penetration of light and difficult monitoring of the photo-media therapies, it is challenging to apply photomedical treatment in deep tissue as they may damage normal tissues. Herein, a thermal regulated interventional photomedicine based on a temperature-adaptive hydrogel fiber-based optical waveguide (THFOW) is proposed, capable of eliminating deeply seated tumor cells while lowering risks of overtemperature (causes the death of healthy cells around the tumor). The THFOW is fabricated by an integrated homogeneous-dynamic-crosslinking-spinning method, and shows a remarkable soft tissue-affinity (low cytotoxicity, swelling stability, and soft tissue-like Young's modulus). Moreover, the THFOW shows an excellent light propagation property with different wavenumbers (especially -0.32 dB cm^-1 with 915 nm laser light), and temperature-gated light propagation effect. The THFOW and relevant therapeutic strategy offer a promising application for intelligent photomedicine in deep issue.

Liquid-Core Hydrogel Optical Fiber Fluorescence Probes

This paper first reports a liquid-core hydrogel optical fiber fluorescence probe. It is composed of a liquid core, a high-refractive-index hydrogel fiber core, and a low-refractive-index hydrogel fiber cladding, which is completely different from many existing optical fiber fluorescence probes. The sensing solution with sensitive materials is sealed as a liquid core, and it can sufficiently react with small-molecule targets penetrating through the hydrogel fiber cladding and core, thus inducing variations in the fluorescence signals. These fluorescence signals can be localized and transmitted within the probe and finally collected and quantified for target detection. This proposed probe can be simply and rapidly fabricated and reused, and it was proven to have high sensitivity, accuracy, and selectivity in practical applications. Therefore, this liquid-core hydrogel optical fiber fluorescence probe will enable a novel sensing platform for small-molecule analyte detection that faces on-site detection challenges.

Flexible Aggregation-Induced Emission-Active Hydrogel for On-Site Monitoring of Pesticide Degradation

Benefiting from the stimuli-responsive property and powerful loading capacity, functionalized hydrogels are favorable for the fabrication of sensing devices. Herein, we design aggregation-induced emission (AIE)-active hydrogel discs by embedding gold nanoclusters@zeolite-like imidazole framework (AuNCs@ZIF) composites in double-network hydrogels to build a sensitive pesticide biosensor. The hydrogel discs integrate an AIE effect of AuNCs, a stimuli-responsive property of ZIF, and a porous network structure of the hydrogel, which enhances the sensing sensitivity via boosting the stable fluorescent signal and antifouling performance. In conjunction with a homemade device, the fluorescence images of hydrogel discs could be transduced into data information for accurate quantification of chlorpyrifos pesticide with a detection limit of 0.2 ng/mL. The dynamic degradation of chlorpyrifos in Chinese cabbage is demonstrated to confirm the practical application of hydrogel discs. Such AIE-active hydrogel discs could be a plant health sensor for the on-site quantification of pesticide residues on crops, holding great promise for precision agriculture.

High-Strength and Extensible Electrospun Yarn for Wearable Electronics

Stretchable conductive yarns have received significant consideration in the direction of wearable and flexible electronics. Wearable electronic structures need strong materials to assure stability, durability, and an extensive range of strain to develop their applications. Therefore, manufacturing high-performance yarn-based devices with ultrarobustness and great stretchability with a simple, cost-effective, and scalable method remains a great challenge for wearable electronics. Here, a highly stretchable yarn with high performance is fabricated, which comprises a core TPU nanoyarn, successively decorated with a liquid metal (LM) layer, and a protective outer nanofiber layer. The ultrarobust (40 MPa) and high-strain (548%) conducting yarn presents potential applications in assembling strain sensors. Moreover, such a unique conductive yarn can be used as a highly deformable, stretchable conductor to charge a mobile phone or for data transfer, a sensor to monitor human activities, and as an effective control for a hand robot as well as for smart thermal management textile application. This research gives promising applications in the field of flexible and wearable electronics.

Unbreakable Hydrogels with Self-Recoverable 10 200% Stretchability

Design of tough hydrogels maintaining structural integrity under multivariable mechanical loads remains hugely challenging because the anticipated characteristics such as stretchability, strength, toughness, and fracture resistance can hardly be compatible. Herein, a simple but robust hydrogel network formed by copolymerization of divinyl benzene with acrylamide in micellar solutions for ultra-high fracture resistance and self-recoverable stretchability is proposed. The network provides dynamic association of hydrophobic domains and homogeneous crosslinking of hydrophilic chains, which shows step-by-step deformation process. The dynamic associations allow recoverable small deformations, then the homogeneous crosslinking ensures reversible unfolding and alignment of polymer chains to self-strengthen for ultra-large deformations without crack propagations. The resultant hydrogels exhibit comprehensive unbreakable feature with self-recoverable ultra-high stretchability (100% recovery from 10 200% strain), superior fracture resistance (toughness > 26 kJ m^-2 ), and anticrack propagation and fatigue (fatigue threshold: ≈2.5 kJ m^-2 ). Even the prenotched hydrogels can undergo tens cyclic loads at 10 200% strain and thousands cyclic loads at 200% strain without noticeable changes in mechanical performance. The robust network prepared from homogeneous hydrophobic crosslinking provides a facile approach and a new mechanism to explore tough hydrogels with superior antifracture and extreme self-recoverable deformability for diverse applications.

Efficient taper optical hydrogel fiber coupler drawn from suspended photocuring 3D printing

Integrating bio-friendly optical hydrogel fibers (HFs) with solid-state fibers (SFs) could expand the horizons of fiber-optic technology for bio-photonics. However, methods for coupling HF and SF-based systems are inefficient due to the mode field mismatch. Here, a hydrogel fiber coupler with a taper core-cladding structure is demonstrated for efficiently coupling HF to SF and fabricated through suspended photocuring 3D printing. Coupling efficiencies of 8.3 and 9.4 dB are obtained at 632 and 473 nm, respectively, which are 22% better than those of conventional couplers. The working bandwidth covers visible wavelengths, satisfying bioengineering requirements. This research removes obstacles to optical fiber applications in bioscience.

Nanoscale Kevlar Liquid Crystal Aerogel Fibers

Aerogel fibers, the simultaneous embodiment of aerogel porous network and fiber slender geometry, have shown critical advantages over natural and synthetic fibers in thermal insulation. However, how to control the building block orientation degree of the resulting aerogel fibers during the dynamic sol-gel transition process to expand their functions for emerging applications is a great challenge. Herein, nanoscale Kevlar liquid crystal (NKLC) aerogel fibers with different building block orientation degrees have been fabricated from Kevlar nanofibers via liquid crystal spinning, dynamic sol-gel transition, freeze-drying, and cold plasma hydrophobilization in sequence. The resulting NKLC aerogel fibers demonstrate extremely high mechanical strength (41.0 MPa), excellent thermal insulation (0.037 W·m^-1·K^-1), and self-cleaning performance (with a water contact angle of 154°). The superhydrophobic NKLC aerogel fibers can cyclically transform between aerogel and gel states, while gel fibers involving different building block orientation degrees display distinguishable brightness under polarized light. Based on these performances, digital textiles woven or embroidered with high- and low-orientated NKLC aerogel fibers enable up to 6.0 Gb information encryption in one square meter and on-demand decryption. Therefore, it can be envisioned that the tuning of the building blocks' orientation degree will be an appropriate strategy to endow performance to the liquid crystal aerogel fibers for potential applications beyond thermal insulation.

Influence of Light-Intensity-Dependent Droplet Directionality on Dimensions of Structures Constructed Using an In Situ Light-Guided 3D Printing Method

As an alternative to conventional 3D printing methods that require supports, a new 3D printing strategy that utilizes guided light in situ has been developed for fabricating freestanding overhanging structures without supports. Light intensity has been found to be a crucial factor in modifying the dimensions of structures printed using this method; however, the underlying mechanism has not been clearly identified. Therefore, the light-intensity-dependent changes in the structure dimensions were analyzed in this study to elucidate the associated mechanism. Essentially, the entire process of deposition was monitored by assessing the behavior of photocurable droplets prior to their collision with the structure using imaging analysis tools such as a high-speed camera and MATLAB®. With increasing light intensity, the instability of the ejected falling droplets increased, and the droplet directionality deteriorated. This increased the dispersion of the droplet midpoints, which caused the average midpoints of the deposited single layers to shift further away from the center of the structure. Consequently, the diameter of the structure formed by successive stacking of single layers increased, and the layer thickness per droplet decreased. These led to light-intensity-dependent differences in the diameter and height of structures that were created from the same number of droplets.

Augmented tactile-perception and haptic-feedback rings as human-machine interfaces aiming for immersive interactions

Advancements of virtual reality technology pave the way for developing wearable devices to enable somatosensory sensation, which can bring more comprehensive perception and feedback in the metaverse-based virtual society. Here, we propose augmented tactile-perception and haptic-feedback rings with multimodal sensing and feedback capabilities. This highly integrated ring consists of triboelectric and pyroelectric sensors for tactile and temperature perception, and vibrators and nichrome heaters for vibro- and thermo-haptic feedback. All these components integrated on the ring can be directly driven by a custom wireless platform of low power consumption for wearable/portable scenarios. With voltage integration processing, high-resolution continuous finger motion tracking is achieved via the triboelectric tactile sensor, which also contributes to superior performance in gesture/object recognition with artificial intelligence analysis. By fusing the multimodal sensing and feedback functions, an interactive metaverse platform with cross-space perception capability is successfully achieved, giving people a face-to-face like immersive virtual social experience.

Current wearable solutions for Virtual Reality (VR) have limitations of complicated structures and large driven power. Here, the authors report a highly integrated ring consisting of multimodal sensing and feedback units for augmented interactions in metaverse.

Porosity-based heterojunctions enable leadless optoelectronic modulation of tissues

Homo- and heterojunctions play essential roles in semiconductor-based devices such as field-effect transistors, solar cells, photodetectors and light-emitting diodes. Semiconductor junctions have been recently used to optically trigger biological modulation via photovoltaic or photoelectrochemical mechanisms. The creation of heterojunctions typically involves materials with different doping or composition, which leads to high cost, complex fabrications and potential side effects at biointerfaces. Here we show that a porosity-based heterojunction, a largely overlooked system in materials science, can yield an efficient photoelectrochemical response from the semiconductor surface. Using self-limiting stain etching, we create a nanoporous/non-porous, soft-hard heterojunction in p-type silicon within seconds under ambient conditions. Upon surface oxidation, the heterojunction yields a strong photoelectrochemical response in saline. Without any interconnects or metal modifications, the heterojunction enables efficient non-genetic optoelectronic stimulation of isolated rat hearts ex vivo and sciatic nerves in vivo with optical power comparable to optogenetics, and with near-infrared capabilities.