Hydrogel fibers for wearable sensors and soft actuators

Empowering artificial muscles with intelligence: recent advancements in materials, designs, and manufacturing

Drawing on foundational knowledge of the structure and function of biological muscles, artificial muscles have made remarkable strides over the past decade, achieving performance levels comparable to those of their natural counterparts. However, they still fall short in their lack of inherent intelligence to autonomously adapt to complex and dynamic environments. Consequently, the next frontier for artificial muscles lies in endowing them with advanced intelligence. Herein, recent works aimed at augmenting intelligence in artificial muscles are summarized, focusing on advancements in functional materials, structural designs, and manufacturing techniques. This review emphasizes memory-based intelligence, enabling artificial muscles to execute a range of pre-programmed movements and refresh stored actuation states in response to changing conditions, as well as sensory-based intelligence, which allows them to perceive and respond to environmental changes through sensory feedback. Furthermore, recent applications benefiting from intelligent artificial muscles, including adaptable robotics, biomedical devices, and wearables, are discussed. Finally, we address the remaining challenges in scalability, dynamic reprogramming, and the integration of multi-functional capabilities and discuss future perspectives of augmented intelligent artificial muscles to support further advancements in the field.

Pattern architected soft magnetic actuation

Bioinspired shape-morphing soft magnetic actuators have potential applications in medicine, robotics, and engineering due to their soft body, untethered control, and infinite degrees of freedom. The shape programming of the soft magnetic actuators (consisting of soft ferromagnetic CI particles in a soft matrix) is an involved task, as it requires a moulding process severely limiting the capability to program complex shapes. The current study explores a shape programming technique that architects the particle pattern configuration in the actuator, mimicking the pattern found in the mould-programmed actuator, thereby eliminating the need for a mould and providing a greater capability of programming complex shapes. At first, actuators with some basic shapes are prepared using the mould programming technique and examined under a microscope to understand the configuration of particle alignment patterns in different shapes. Then, the pattern is architected using magnetic units in the soft matrix to eliminate the need for mould for shape programming. In this study, the programmed soft actuators are characterized for shape morphing and locomotion capability under an external actuating magnetic field. The crawler was found to move at a velocity of 3 mm s-1 under a periodic magnetic field of 1 Hz. The designed actuators are found to quickly respond to the magnetic field thereby generating the desired shapes.

Development and Applications in Intelligent Sports of Hydrogel-Based Triboelectric Nanogenerators

As an emerging self-powered technology, triboelectric nanogenerators have the characteristics of a simple structure, high conversion efficiency, diverse material selection, and stable output. Hydrogels have the advantages of flexibility, extensibility, and shape adaptability, which means that hydrogel-based triboelectric nanogenerators (H-TENGs) have high flexibility, self-healing abilities, conductivity, and fatigue resistance. They can still operate normally in scenarios involving bending, pressing, stretching, and folding. H-TENGs offer a method of versatile and sustainable innovation in sports monitoring. This review elucidates the working principles and modes of H-TENGs, examines H-TENG characteristics that are relevant to intelligent sports, and summarizes their applications in this field. This paper concludes with a discussion on the development and applications of H-TENGs in intelligent sports.

Advancements in Binary Solvent-Assisted Hydrogel Composites for Wearable Sensing Applications

The advancement of wearable sensing technologies has been pivotal in revolutionizing healthcare, environmental monitoring, and personal fitness. Among the diverse materials employed in these technologies, multifunctional hydrogel composites have emerged as critical components due to their unique properties, including high water content, flexibility, and biocompatibility. This review provides a comprehensive overview of the state-of-the-art in binary solvent-assisted hydrogel composites for wearable sensing applications. It begins by defining hydrogel composites and their essential attributes for wearable sensors, specifically focusing on binary solvent-assisted methods that enhance their performance and functionality. The review then delves into the applications of these composites in health monitoring, environmental detection, and sports and fitness, highlighting their role in advancing wearable technologies. Despite their promising features, there are significant challenges related to durability, sensitivity, and integration that need to be addressed to fully exploit these materials in wearable devices. This review discusses these challenges and presents potential solutions, including the development of new materials, improvement in fabrication processes, and strategies for achieving multifunctionality and sustainable design. Looking forward, the paper outlines future directions for research in this field, emphasizing the need for innovative materials and technologies that can lead to more effective, reliable, and eco-friendly wearable sensors. This review aims to inspire further research and development in the field of wearable sensing, paving the way for new applications and advancements in healthcare, environmental monitoring, and personal fitness technologies.

Biomedical Applications and Nutritional Value of Specific Food-Derived Polysaccharide-Based Hydrogels

Food-derived polysaccharide-based hydrogels (FPBHs), which are composed of polysaccharides derived from food sources exhibit great potential for biomedical applications. The FPBHs possess a wide range of biological activities and can be utilized in the treatment of various clinical diseases. However, the majority of research efforts have predominantly focused on nonspecific polysaccharides derived from various sources (most plants, animals, and microorganisms), whereas the exploration of hydrogels originating from specific polysaccharides with distinct bioactivity extracted from natural food sources remains limited. In this review, a comprehensive search was conducted across 3 major databases (PubMed, Web of Science, and Medline) until October 24, 2024 to include 32 studies that employed FPBHs for biomedical applications. This review provides an overview of hydrogels based on specific food-derived polysaccharides by summarizing their types, sources, molecular weight, monosaccharide composition, and biological activities. The crosslinking strategies employed in the fabrication of FPBHs were demonstrated. The attributes and characteristics of FPBHs were delined, including their physical, chemical, and functional properties. Of particular note, the review highlights in vivo and in vitro studies exploring the biomedical applications of FPBHs and delve into the nutritional value of specific food-derived polysaccharides. The challenges encountered in basic research involving FPBHs were enumerated as well as limitation in their clinical practice. Finally, the potential market outlook for FPBHs in the future was also discussed.

Effects of Enteromorpha prolifera sulfated polysaccharide and aluminium ion addition on the multifunctional property of conductive hydrogel for wearable strain sensing

Although introducing Enteromorpha prolifera sulfated polysaccharide (SPEP) enhances the mechanical properties of hydrogels significantly, little is known about the effects of polysaccharide and ion addition on morphological and physicochemical properties of conductive hydrogel. Therefore, the Poly (acrylic acid)/SPEPn/Al3+m (PAA/SPEPn/Al3+m) hydrogels with different SPEP and Al3+ addition were synthesized by simple one-pot method. The porosity, tensile strength, and swelling ration increased, while compressive strength, elongation at break, self-healing, self-adhesion properties increased first and then decreased as SPEP addition increased from 0 % to 3.80 %. The Al3+ addition increased from 0.08 % to 0.30 %, both tensile and compressive strength increased first and then decreased, while elongation at break kept increasing. Unexpectedly, both increasing SPEP and Al3+ addition reduced the electrical conductivity, while SPEP increased the gauge factor of hydrogel. The hydrogel exhibited optimal comprehensive properties when SPEP and Al3+ addition were 2.31 % and 0.24 %, respectively. The PAA/SPEP2.31%/Al3+0.24% hydrogel showed high tensile strength (107.60 kPa), elongation at break (2426.67 %), strain self-healing rate (81.87 %), adhesion strength (21.61 kPa), and conductivity (3.60 S/m). Overall, the properties of PAA/SPEPn/Al3+m hydrogels can be regulated through tailoring SPEP and Al3+ addition, which can be used as on-demand strategy to improve the performance of PAA/SPEPn/Al3+m hydrogels for each application.

Ratiometric Chemosensors That Are Capable of Quantifying Hydrostatic Pressure Stimulus: A Case of Porphyrin Tweezers

Investigating chemosensors that are capable of quantifying pressure in solution, particularly hydrostatic pressure, which is one of the mechanical forces, is an attractive challenge in chemistry from the viewpoint of "mechano"-science. Herein, we report the investigation of chiral porphyrin tweezers, Por-Cy and Por-DPhEt, comprising different flexible linkers; Por-Cy and Por-DPhEt displayed distinct ratiometric signaling by using the higher excited S2 state with a standard excited S1 level. A novel operative mechanism using the S1/S2 fluorescence ratio was revealed using hydrostatic pressure-ultraviolet/visible (UV/vis), fluorescence/excitation, circular dichroism spectroscopy, and lifetime measurements, which can be further controlled by the open-closed conformational change inherent in the tweezer skeleton. Furthermore, the fluorescent chiral tweezers exhibited a promising |g lum| of 2.9 × 10-3, indicating that they are potential candidates for sensory applications in chiral environments. This study provides opportunities for the development of smart pressure-responsive chemosensors.

Repair of hydrogel wearable devices through topological adhesion and cross-shaped sectional enhancement strategy

Hydrogels, recognized for their biocompatibility, are extensively employed in the realm of wearable devices. Nevertheless, their application is often constrained by their low mechanical robustness, rendering them susceptible to damage during operation. The restoration of their load-bearing and sensory functionalities post-damage represents a captivating yet underexplored domain. Conventional repair techniques, reliant on hydrogen bonding or van der Waals forces, falter in the face of hydrogels' high water content. In this study, a novel composite adhesive gel (SGG), integrating sodium alginate, guar gum, and graphene oxide, was engineered to mend impaired hydrogels. Furthermore, an optimized repair approach, utilizing a cross-shaped sectional (CSS) enhancement strategy, was devised to reinstate the hydrogels' load and sensory capabilities. Investigations revealed that the SGG adhesive infiltrated the hydrogel, establishing an intermediary gel stratum, subsequently solidifying to mend the material through topological adhesion. This process reestablished the continuity of the polymer network and the aqueous phase within the hydrogel. Following the application of the CSS augmentation method, the peak tensile strain of the remediated hydrogel exceeded 200 %, with the uppermost observable adhesive energy touching 2526 J/m2. In addition, the ability to respond to strain was significantly rejuvenated, suggesting an effective methodology for the rehabilitation of wearable technology.

Microscopic and Macroscopic Characterization of Hydrogels Based on Poly(vinyl-alcohol)-Glutaraldehyde Mixtures for Fricke Gel Dosimetry

In recent decades, hydrogels have emerged as innovative soft materials with widespread applications in the medical and biomedical fields, including drug delivery, tissue engineering, and gel dosimetry. In this work, a comprehensive study of the macroscopic and microscopic properties of hydrogel matrices based on Poly(vinyl-alcohol) (PVA) chemically crosslinked with Glutaraldehyde (GTA) was reported. Five different kinds of PVAs differing in molecular weight and degree of hydrolysis were considered. The local microscopic organization of the hydrogels was studied through the use of the 1H nuclear magnetic resonance relaxometry technique. Various macroscopic properties (gel fraction, water loss, contact angle, swelling degree, viscosity, and Young's Modulus) were investigated with the aim of finding a correlation between them and the features of the hydrogel matrix. Additionally, an optical characterization was performed on all the hydrogels loaded with Fricke solution to assess their dosimetric behavior. The results obtained indicate that the degree of PVA hydrolysis is a crucial parameter influencing the structure of the hydrogel matrix. This factor should be considered for ensuring stability over time, a vital property in the context of potential biomedical applications where hydrogels act as radiological tissue-equivalent materials.

Soft Fiber/Textile Actuators: From Design Strategies to Diverse Applications

Fiber/textile-based actuators have garnered considerable attention due to their distinctive attributes, encompassing higher degrees of freedom, intriguing deformations, and enhanced adaptability to complex structures. Recent studies highlight the development of advanced fibers and textiles, expanding the application scope of fiber/textile-based actuators across diverse emerging fields. Unlike sheet-like soft actuators, fibers/textiles with intricate structures exhibit versatile movements, such as contraction, coiling, bending, and folding, achieved through adjustable strain and stroke. In this review article, we provide a timely and comprehensive overview of fiber/textile actuators, including structures, fabrication methods, actuation principles, and applications. After discussing the hierarchical structure and deformation of the fiber/textile actuator, we discuss various spinning strategies, detailing the merits and drawbacks of each. Next, we present the actuation principles of fiber/fabric actuators, along with common external stimuli. In addition, we provide a summary of the emerging applications of fiber/textile actuators. Concluding with an assessment of existing challenges and future opportunities, this review aims to provide a valuable perspective on the enticing realm of fiber/textile-based actuators.

UV-Crosslinked Poly(N-isopropylacrylamide) Interpenetrated into Chitosan Structure with Enhancement of Mechanical Properties Implemented as Anti-Fouling Materials

High-performance properties of interpenetration polymer network (IPN) hydrogels, based on physically crosslinked chitosan (CS) and chemically crosslinked poly(N-isopropylacrylamide) (PNiPAM), were successfully developed. The IPN of CS/PNiPAM is proposed to overcome the limited mechanical properties of the single CS network. In this study, the viscoelastic behaviors of prepared materials in both solution and gel states were extensively examined, considering the UV exposure time and crosslinker concentration as key factors. The effect of these factors on gel formation, hydrogel structures, thermal stabilities of networks, and HeLa cell adhesion were studied sequentially. The sol-gel transition was effectively demonstrated through the scaling law, which agrees well with Winter and Chambon's theory. By subjecting the CS hydrogel to the process operation in an ethanol solution, its properties can be significantly enhanced with increased crosslinker concentration, including the shear modulus, crosslinking degree, gel strength, and thermal stability in its swollen state. The IPN samples exhibit a smooth and dense surface with irregular pores, allowing for much water absorption. The HeLa cells were adhered to and killed using the CS surface cationic charges and then released through hydrolysis by utilizing the hydrophilic/hydrophobic switchable property or thermo-reversible gelation of the PNiPAM network. The results demonstrated that IPN is a highly attractive candidate for anti-fouling materials.

Continuous Melt Spinning of Adaptable Covalently Cross-Linked Self-Healing Ionogel Fibers for Multi-Functional Ionotronics

Stretchable conductive fibers play key roles in electronic textiles, which have substantial improvements in terms of flexibility, breathability, and comfort. Compared to most existing electron-conductive fibers, ion-conductive fibers are usually soft, stretchable, and transparent, leading to increasing attention. However, the integration of desirable functions including high transparency, stretchability, conductivity, solvent resistance, self-healing ability, processability, and recyclability remains a challenge to be addressed. Herein, a new molecular strategy based on dynamic covalent cross-linking networks is developed to enable continuous melt spinning of the ionogel fiber with the aforementioned properties. As a proof of concept, adaptable covalently cross-linked ionogel fibers based on dimethylglyoximeurethane (DOU) groups (DOU-IG fiber) are prepared. The resultant DOU-IG fiber exhibited high transparency (>93%), tensile strength (0.76 MPa), stretchability (784%), and solvent resistance. Owing to the dynamic of DOU groups, the DOU-IG fiber shows high healing performance using near-infrared light. Taking advantage of DOU-IG fibers, multifunctional ionotronics with the integration of several desirable functionalities including sensor, triboelectric nanogenerator, and electroluminescent display are fabricated and used for motion monitoring, energy harvesting, and human-machine interaction. It is believed that these DOU-IG fibers are promising for fabricating the next generation of electronic textiles and other wearable electronics.

Pressure-Responsive Polymer Chemosensors for Hydrostatic-Pressure-Signal Detection: Poly-l-Lysine-Pyrene Conjugates

Stimulus-responsive polymer materials are an attractive alternative to conventional supramolecular and polymer assemblies for applications in sensing, imaging, and drug-delivery systems. Herein, we synthesized a series of pyrene-labeled α- and ε-poly-l-lysine conjugates with varying degrees of substitution (DSs). Hydrostatic-pressure-UV/vis, fluorescence, and excitation spectroscopies and fluorescence lifetime measurements revealed ground-state conformers and excited-state ensembles emitting fluorescence with variable intensities. The polylysine-based chemosensors demonstrated diverse ratiometric responses to hydrostatic pressure through adjustments in polar solvents, DSs, and polymer backbones. Additionally, the fluorescence chemosensor exhibited a promising glum value of 3.2 × 10-3, indicating potential applications in chiral fluorescent materials. This study offers valuable insights into the development of smart hydrostatic-pressure-responsive polymer materials.