Perspectives on recent advancements in energy harvesting, sensing and bio-medical applications of piezoelectric gels

Advancements in membrane technology for efficient POME treatment: A comprehensive review and future perspectives

The treatment of POME related contamination is complicated due to its high organic contents and complex composition. Membrane technology is a prominent method for removing POME contaminants on account of its efficiency in removing suspended particles, organic substances, and contaminants from wastewater, leading to the production of high-quality treated effluent. It is crucial to achieve efficient POME treatment with minimum fouling through membrane advancement to ensure the sustainability for large-scale applications. This article comprehensively analyses the latest advancements in membrane technology for the treatment of POME. A wide range of membrane types including forward osmosis, microfiltration, ultrafiltration, nanofiltration, reverse osmosis, membrane bioreactor, photocatalytic membrane reactor, and their combinations is discussed in terms of the innovative design, treatment efficiencies and antifouling properties. The strategies for antifouling membranes such as self-healing and self-cleaning membranes are discussed. In addition to discussing the obstacles that impede the broad implementation of novel membrane technologies in POME treatment, the article concludes by delineating potential avenues for future research and policy considerations. The understanding and insights are expected to enhance the application of membrane-based methods in order to treat POME more efficiently; this will be instrumental in the reduction of environmental pollution.

Molecular Design of Electron-Rich Polyoxometalates Based Clusters Enabling Intelligent Energy Storage

The fabrication of molecular cluster-based intelligent energy storage systems remains a significant challenge due to the intricacies of multifunctional integration at the molecular level. In this work, low-valent metal atoms are successfully encapsulated within ɛ-type Keggin structures, yielding a novel cluster denoted as CuMo16. This unique structure displayed the characteristic "molybdenum red" coloration, with a high degree of reduction (76.47%), which played a pivotal role in enhancing its electrochemical properties. The specialized configuration significantly enhanced multi-proton-coupled electron transfer kinetics, enabling efficient and rapid electron storage and release, with up to thirteen electrons per molecule. To construct an intelligent energy storage device, CuMo16 is employed as a proton-coupled electron-active material and embedded within a polyvinyl alcohol (PVA) matrix, resulting in the flexible, wearable, rechargeable devices. The flexible electronics not only demonstrate real-time human motion detection but also exhibit remarkable energy storage performance, reaching a peak capacity of 194.19 mAh g-1 and maintaining 68.2% capacity retention after 2500 cycles. Molecular dynamics simulations reveal that integrating CuMo16 significantly enhances the intelligent storage performance of flexible electronics, and molecular regulation of CuMo16 content provides an effective strategy for optimizing flexible electronic devices. This study lays the foundation for the development of cluster-based intelligent energy storage systems.

Sound Wave-Activated Self-Powered Adhesive Dressing for Accelerated Wound Healing

Self-powered wound dressings are effective in treating chronic wounds because of their low toxicity and convenience. However, current self-powered dressings rely on the bending movements of the skin or additional large ultrasonic devices. Herein, a flexible adhesive self-powered wound dressing (FASW) that promotes skin regeneration through daily sound wave driving without relying on skin bending or external sound devices is proposed. The FASW dressing consists of a bioadhesive film (BAF), a unidirectional fluorinated conductive film (UFCF), and a liquid metal (LM) interlayer. Benefiting from the cross-linking of chitosan, the dressing exhibits excellent properties, such as biocompatibility, stretchability, tissue adhesion, and recyclability. In vivo experiments show that the FASW dressing reduced inflammation and stimulated hair follicle regeneration. This wound dressing utilizes previously overlooked natural energies for the treatment of chronic wounds, thereby enhancing the therapeutic effect of traditional self-powered dressings on individuals with movement disorders.

Self-Powered Thermoelectric Hydrogels Accelerate Wound Healing

Electrical stimulation (ES) serves as a biological cue that regulates critical cellular processes, including proliferation and migration, offering an effective approach to accelerating wound healing. Thermoelectrics, capable of generating electricity by exploiting the temperature difference between skin and the surrounding environment without external energy input, present a promising avenue for ES-based therapies. Herein, we developed Ag2Se@gelatin methacrylate (Ag2Se@GelMA) thermoelectric hydrogels with high room-temperature thermoelectric performance and employed them as self-powered ES devices for wound repair. Systematic in vivo and in vitro investigations elucidated their biological mechanisms for enhancing wound healing. Our findings reveal that the Ag2Se@GelMA thermoelectric hydrogels can significantly accelerate the wound closure by amplifying the endogenous electric field, thereby promoting cell proliferation, migration, and angiogenesis. Comprehensive in vitro experiments demonstrated that ES generated by the hydrogels activates voltage-gated calcium ion channels, elevating intracellular Ca2+ levels and enhancing mitochondrial functions through the Ca2+/CaMKKβ/AMPK/Nrf2 pathway. This cascade improves mitochondrial dynamics and angiogenesis, thereby accelerating tissue regeneration. The newly developed Ag2Se@GelMA thermoelectric hydrogels represent a marked progress in wound dressing technology with the potential to improve clinical strategies in tissue engineering and regenerative medicine.

Piezoelectric Biomaterials for Use in Bone Tissue Engineering-A Narrative Review

To examine natural bone's bioelectrical traits, notably its piezoelectricity, and to look into how these characteristics influence bone growth and repair. In the context of exploring the potential of piezoelectric biomaterials, such as biopolymers and bio-ceramics, towards orthopedic and bone regeneration applications, the research seeks to evaluate the significance of piezoelectricity-driven osteogenesis. The paper reviews recent research on bone's electrical and dielectric properties, surface polarization/electrical stimulation effects interacting with cell activity and the effectiveness of piezoelectric biomaterials to support tissues' regenerative process. The study includes a number of materials, such as collagen, polyvinylidene fluoride (PVDF) and barium titanate. The applications of piezoelectric bio-ceramics, piezoelectric organic polymers, and piezoelectric natural polymers are particularly highlighted. Piezoelectric biomaterials are being shown in recent studies to enhance cellular metabolism in vitro as well as promote the regeneration of tissues in vivo, especially when paired with electric field stimulation or interface polarization. Piezoelectric bio-ceramics like magnesium silicate and barium titanate, as well as biopolymers like collagen and PVDF, have shown possibilities for orthopedic applications. However, there are several challenges regarding the manufacturing of bio-ceramics of specific compositions having the desired properties. This review highlighted the potential of piezoelectric biomaterials in orthopedic applications with special emphasis on biopolymers and bioceramics. Therefore, these types of materials have huge potential for bone regeneration because they can mimic the piezoelectric properties of bone and allow better advances in tissue engineering or regenerative medicine. To date, little is known about their mechanism of action, and modifications are needed to improve efficacy for clinical uptake.

Piezoelectric Nanomaterials for Cancer Therapy: Current Research and Future Perspectives on Glioblastoma

Cancer significantly impacts human quality of life and life expectancy, with an estimated 20 million new cases and 10 million cancer-related deaths worldwide every year. Standard treatments including chemotherapy, radiotherapy, and surgical removal, for aggressive cancers, such as glioblastoma, are often ineffective in late stages. Glioblastoma, for example, is known for its poor prognosis post-diagnosis, with a median survival time of approximately 15 months. Novel therapies using local electric fields have shown anti-tumour effects in glioblastoma by disrupting mitotic spindle assembly and inhibiting cell growth. However, constant application poses risks like patient burns. Wireless stimulation via piezoelectric nanomaterials offers a safer alternative, requiring ultrasound activation to induce therapeutic effects, such as altering voltage-gated ion channel conductance by depolarising membrane potentials. This review highlights the piezoelectric mechanism, drug delivery, ion channel activation, and current technologies in cancer therapy, emphasising the need for further research to address limitations like biocompatibility in whole systems. The goal is to underscore these areas to inspire new avenues of research and overcome barriers to developing piezoelectric nanoparticle-based cancer therapies.

3D printed shape-memory piezoelectric scaffolds with in-situ self-power properties for bone defect repair

Electrical stimulation has been shown to regulate early immunity and late-stage osteogenesis in bone repair. However, achieving in-situ electrical stimulation in the form of self-power in vivo during the initial postoperative stages when the patients have limited mobility remains challenging. In this study, we developed a 3D-printed in-situ self-powered composite scaffold composed of shape memory polyurethane elastomers (SMPU) and polyvinylidene fluoride (PVDF) piezoelectric nanofibers. The composite scaffold demonstrates excellent shape memory performance, allowing for minimally invasive implantation. During the shape memory process, the composite scaffold can provide mechanical force stimulation to PVDF nanofibers to generate charge. Therefore, self-power was achieved through the integration of the shape memory process and piezoelectric effects, and it can be used for in-situ electrical stimulation during the initial postoperative period. Additionally, the composite scaffold can output voltage under continuous mechanical force stimulation, indicating that the patients can apply sustained mechanical force stimulation to the composite scaffold to output voltage through rehabilitation exercises when the patients regain mobility. Both cell experiments and animal studies confirmed that this composite scaffold can effectively regulate the immune microenvironment and enhance osteogenesis. This study successfully achieves in-situ electrical stimulation in the form of self-power by integrating the shape memory process and piezoelectric effects, which is expected to be an effective repair strategy for bone tissue engineering.

New Perspectives of Hydrogels in Chronic Wound Management

Chronic wounds pose a substantial healthcare concern due to their prevalence and cost burden. This paper presents a detailed overview of chronic wounds and emphasizes the critical need for novel therapeutic solutions. The pathophysiology of wound healing is discussed, including the healing stages and the factors contributing to chronicity. The focus is on diverse types of chronic wounds, such as diabetic foot necrosis, pressure ulcers, and venous leg ulcers, highlighting their etiology, consequences, and the therapeutic issues they provide. Further, modern wound care solutions, particularly hydrogels, are highlighted for tackling the challenges of chronic wound management. Hydrogels are characterized as multipurpose materials that possess vital characteristics like the capacity to retain moisture, biocompatibility, and the incorporation of active drugs. Hydrogels' effectiveness in therapeutic applications is demonstrated by how they support healing, including preserving ideal moisture levels, promoting cellular migration, and possessing antibacterial properties. Thus, this paper presents hydrogel technology's latest developments, emphasizing drug-loaded and stimuli-responsive types and underscoring how these advanced formulations greatly improve therapy outcomes by enabling dynamic and focused reactions to the wound environment. Future directions for hydrogel research promote the development of customized hydrogel treatments and the incorporation of digital health tools to improve the treatment of chronic wounds.

Piezoelectric-Augmented Thermoelectric Ionogels for Self-Powered Multimodal Medical Sensors

A paradigm ionogel consisting of ionic liquid (IL) and PVDF-HFP composites is made, which inherently possesses dual-function ionic thermoelectric (iTE) and piezoelectric (PE) attributes. This study investigates an innovative "PE-enhanced iTEs" effect, wherein the ionic thermopower exhibits a 58% enhancement while the ionic conductivity arises more than 2× within a PE-induced internal electric field. By harnessing these multifaceted features, fully self-powered, multimodal sensors demonstrate their superior energy conversion capabilities, which possessed minimum sensitivities of 0.13 mV kPa-1 and 0.96 mV K-1 in pressure and temperature alterations, respectively. The PE augmentation of iTEs is maximized by ≈3× under rising water pressure. Their swift and sophisticated responses to various in vivo vital signs simultaneously in a hemorrhagic shock scenario, indicative of good prospects in the clinical medicine field are showcased.

Advancements in GelMA bioactive hydrogels: Strategies for infection control and bone tissue regeneration

Infectious bone defects present a significant clinical challenge, characterized by infection, inflammation, and subsequent bone tissue destruction. Traditional treatments, including antibiotic therapy, surgical debridement, and bone grafting, often fail to address these defects effectively. However, recent advancements in biomaterials research have introduced innovative solutions for managing infectious bone defects. GelMA, a three-dimensional network of hydrophilic polymers that can absorb and retain substantial amounts of water, has attracted considerable attention in the fields of materials science and biomedical engineering. Its distinctive properties, such as biocompatibility, responsiveness to stimuli, and customisable mechanical characteristics make GelMA an exemplary scaffold material for bone tissue engineering. This review aims to thoroughly explore the current literature on antibacterial and osteogenic strategies using GelMA hydrogels for the restoration of infected bones. It discusses their fabrication methods, biocompatibility, antibacterial effectiveness, and bioactivity. We conclude by discussing the existing challenges and future research directions in this field, with the hope of inspiring further innovations in the synthesis, modification, and application of GelMA-based hydrogels for infection control and bone tissue regeneration.

Mechanoelectrical Transduction through Anion Recognition with Naphthalenediimide Monolayers at the Air-Water Interface

In biological systems, various stimuli and energies are transduced into membrane potentials via ion transport or binding. The application of this concept to artificial devices may result in biomimetic signal transmitters and energy harvesters. In this study, we investigated the mechanical control of fluoride anion recognition with naphthalenediimide (NDI) monolayers at the air-water interface. Similar to the mechanosensitive ion channels in biological membranes, mechanical stimuli modulated the packing manner of the NDI monolayers, which reproducibly triggered anion binding and concomitant shifts in the membrane potential. Furthermore, mechanical stimuli resulted in anion binding or release, depending on the structure of the alkyl side chains attached to the NDI core, which was explained by the difference in the packing manner of the NDI monolayers. These findings provide insights into the development of novel mechanoelectrical transduction systems that mimic biological processes.

Flexible mechano-optical dual-responsive perovskite molecular ferroelectric composites for advanced anticounterfeiting and encryption

Hybrid organic-inorganic molecular ferroelectrics have emerged as promising materials for multifunctional piezoelectric devices. However, they present challenges in practical applications because of their inherent brittleness and poor ductility. Herein, we present a flexible mechano-optical dual-responsive molecular ferroelectric composite by incorporating trimethylchloromethyl ammonium (TMCM)-MnCl3 into styrene ethylene butylene styrene (SEBS) matrix. The SEBS/TMCM-MnCl3 exhibits excellent stretchable mechanical properties (tensile strain >1300%, thickness of 30 μm), piezoelectricity, and photoluminescence, enabling advanced visual-tactile-fused anticounterfeiting and encryption applications. Anticounterfeiting and antitampering tags are developed to judge whether the valued items are true or tampered with based on pattern recognition and piezoelectric response, respectively. Additionally, high-security password keyboards featuring triple-layer encryption are designed, offering more password combinations (524,288 times greater than those of traditional password devices relying solely on digital encryption) and enhanced security reliability against cracking attempts. This work can inspire designs of multifunctional optoelectronic materials and enable visual-tactile-fused intelligent applications in human-machine interfaces, information security, and advanced robotics.

Observation of Ferroelectricity in Carbapenem Intermediates Enables Reactive Oxygen Species Generation by Ultrasound

Organic ferroelectrics show great applications in the fields of biomedicine, including disease treatment, biosensors, and tissue engineering. Organosilicon pharmaceutical intermediates generally include chiral centers and have satisfying biosafety, biocompatibility, or even biodegradability, which provide versatile platforms for the design of ferroelectricity. However, their academic values in ferroelectricity have long been long overlooked. Here, we demonstrated the ferroelectric properties of 4-acetoxy-azacyclic butanone (4-AA), a key synthetic organosilicon-based intermediate of carbapenem drugs. This compound undergoes a 222F2-type ferroelectric-ferroelastic phase transition at 326 K. As an organic piezoelectric material, 4-AA can produce reactive oxygen species when subjected to ultrasonic vibrations. Combined with its desirable biocompatibility, this material may contribute to antimicrobial and wound healing, tumor treatment, etc. This work will provide inspiration for the discovery of multifunctional biomedical ferroelectric materials as well as their related application prospects.

Progress in Protein-Based Hydrogels for Flexible Sensors: Insights from Casein

In recent years, the rapid advancement of flexible sensors as the cornerstone of flexible electronics has propelled a flourishing evolution within the realm of flexible electronics. Unlike traditional flexible devices, hydrogel flexible sensors have characteristic advantages such as biocompatibility, adhesion, and adjustable mechanical properties and have similar properties to human skin. Especially, biobased hydrogels have become the preferred substrate material for flexible sensors due to increased environmental pressures caused by the scarcity of petrochemical resources. In this regard, proteins possess advantages such as diverse amino acid compositions, adjustable advanced structures, chemical modifiability, the application of protein engineering techniques, and the ability to respond to various external stimuli. These enable the hydrogels constructed from them to have greater designability, flexibility, and adaptability. As a result, their applications in manufacturing various types of sensors have experienced rapid growth. This work systematically reviews the sensing mechanism of protein-based hydrogels, focusing on the preparation of protein-based hydrogels and the optimization of flexible sensors mainly from the perspective of a typical type of animal-derived protein casein. In addition, while the potential of casein is recognized, the limitations of casein-based hydrogels in flexible sensor applications are explored, and insights are provided into the development trends of next-generation sensors based on casein-based hydrogel materials.

Advanced Materials for Energy Harvesting and Soft Robotics: Emerging Frontiers to Enhance Piezoelectric Performance and Functionality

Piezoelectric energy harvesting captures mechanical energy from a number of sources, such as vibrations, the movement of objects and bodies, impact events, and fluid flow to generate electric power. Such power can be employed to support wireless communication, electronic components, ocean monitoring, tissue engineering, and biomedical devices. A variety of self-powered piezoelectric sensors, transducers, and actuators have been produced for these applications, however approaches to enhance the piezoelectric properties of materials to increase device performance remain a challenging frontier of materials research. In this regard, the intrinsic polarization and properties of materials can be designed or deliberately engineered to enhance the piezo-generated power. This review provides insights into the mechanisms of piezoelectricity in advanced materials, including perovskites, active polymers, and natural biomaterials, with a focus on the chemical and physical strategies employed to enhance the piezo-response and facilitate their integration into complex electronic systems. Applications in energy harvesting and soft robotics are overviewed by highlighting the primary performance figures of merits, the actuation mechanisms, and relevant applications. Key breakthroughs and valuable strategies to further improve both materials and device performance are discussed, together with a critical assessment of the requirements of next-generation piezoelectric systems, and future scientific and technological solutions.

Metal-free small molecule-based piezoelectric energy harvesters

Organic and metal-free molecules with piezoelectric and ferroelectric properties have gained wide interest for their applications in the domain of mechanical energy harvesting due to their desirable properties such as light weight, thermal stability, mechanical flexibility, feasibility to achieve high Curie temperatures, and ease of synthesis. However, the understanding and design of these materials for piezoelectric energy harvesting applications is still in its early stages. This review paper presents a comprehensive overview of the fundamental characterization of piezoelectricity for a range of organic ferro- and piezoelectric materials and their composites. It also discusses the limitations of traditional piezoelectric materials and highlights the advantages of organic materials in this area in the introduction part. In addition, the paper provides a detailed description of peptide-based and other biomolecular piezoelectric materials as a bio-friendly alternative to current materials. This perspective aims to guide researchers in designing functional organic materials and composites for practical mechanical energy harvesting applications and to highlight current limitations and future perspectives in this emerging area of research.

Highly stretchable nanocomposite piezofibers: a step forward into practical applications in biomedical devices

High-performance biocompatible composite materials are gaining attention for their potential in various fields such as neural tissue scaffolds, bio-implantable devices, energy harvesting, and biomechanical sensors. However, these devices currently face limitations in miniaturization, finite battery lifetimes, fabrication complexity, and rigidity. Hence, there is an urgent need for smart and self-powering soft devices that are easily deployable under physiological conditions. Herein, we present a straightforward and efficient fabrication technique for creating flexible/stretchable fiber-based piezoelectric structures using a hybrid nanocomposite of polyvinylidene fluoride (PVDF), reduced graphene oxide (rGO), and barium-titanium oxide (BT). These nanocomposite fibers are capable of converting biomechanical stimuli into electrical signals across various structural designs (knit, braid, woven, and coil). It was found that a stretchable configuration with higher output voltage (4 V) and a power density (87 μW cm-3) was obtained using nanocomposite coiled fibers or knitted fibers, which are ideal candidates for real-time monitoring of physiological signals. These structures are being proposed for practical transition to the development of the next generation of fiber-based biomedical devices. The cytotoxicity and cytocompatibility of nanocomposite fibers were tested on human mesenchymal stromal cells. The obtained results suggest that the developed fibers can be utilized for smart scaffolds and bio-implantable devices.

Blue Laser Triggered Hemostatic Peptide Hydrogel for Gastrointestinal Bleeding Treatment

In an emergency, nonvariceal upper gastrointestinal bleeding (NVUGIB), endoscopic hemostasis is considered the gold standard intervention. However, current endoscopic hemostasis is very challenging to manage bleeding in large-diameter or deep lesions highly prone to rebleeding risk. Herein, a novel hemostatic peptide hydrogel (HPH) is reported, consisting of a self-assembly peptide sequence CFLIVIGSIIVPGDGVPGDG (PFV) and gelatin methacryloyl (GelMA), which can be triggered by blue laser endoscopy (BLE) for nonvariceal upper gastrointestinal bleeding treatment without recurring bleeding concerns. Upon contact with GelMA solution, PFV immediately fibrillates into β-sheet nanofiber and solvent-induced self-assembly to form HPH gel. HPH nanofiber networks induced ultrafast coagulation by enveloping blood cells and activating platelets and coagulation factors even to the blood with coagulopathy. Besides its remarkable hemostatic performance in artery and liver injury models, HPH achieves instant bleeding management in porcine NVUGIB models within 60 s by preventing the rebleeding risk. This work demonstrates an extraordinary hemostatic agent for NVUGIB intervention by BLE for the first time, broadening potential application scenarios, including patients with coagulopathy and promising clinical prospects.

Flexible piezoelectric materials and strain sensors for wearable electronics and artificial intelligence applications

With the rapid development of artificial intelligence, the applications of flexible piezoelectric sensors in health monitoring and human-machine interaction have attracted increasing attention. Recent advances in flexible materials and fabrication technologies have promoted practical applications of wearable devices, enabling their assembly in various forms such as ultra-thin films, electronic skins and electronic tattoos. These piezoelectric sensors meet the requirements of high integration, miniaturization and low power consumption, while simultaneously maintaining their unique sensing performance advantages. This review provides a comprehensive overview of cutting-edge research studies on enhanced wearable piezoelectric sensors. Promising piezoelectric polymer materials are highlighted, including polyvinylidene fluoride and conductive hydrogels. Material engineering strategies for improving sensitivity, cycle life, biocompatibility, and processability are summarized and discussed focusing on filler doping, fabrication techniques optimization, and microstructure engineering. Additionally, this review presents representative application cases of smart piezoelectric sensors in health monitoring and human-machine interaction. Finally, critical challenges and promising principles concerning advanced manufacture, biological safety and function integration are discussed to shed light on future directions in the field of piezoelectrics.

Fabrication of Flexible Wearable Mechanosensors Utilizing Piezoelectric Hydrogels Mechanically Enhanced by Dipole-Dipole Interactions

Conductive hydrogels have been increasingly employed to construct wearable mechanosensors due to their excellent mechanical flexibility close to that of soft tissues. In this work, piezoelectric hydrogels are prepared through free radical copolymerization of acrylamide (AM) and acrylonitrile (AN) and further utilized in assembling flexible wearable mechanosensors. Introduction of the polyacrylonitrile (PAN) component in the copolymers endows the hydrogels with excellent piezoelectric properties. Meanwhile, significant enhancement of mechanical properties has been accessed by forming dipole-dipole interactions, which results in a tensile strength of 0.51 MPa. Flexible wearable mechanosensors are fabricated by utilizing piezoelectric hydrogels as key signal converting materials. Self-powered piezoelectric pressure sensors are assembled with a sensitivity (S) of 0.2 V kPa-1. Additionally, resistive strain sensors (gauge factor (GF): 0.84, strain range: 0-250%) and capacitive pressure sensors (S: 0.23 kPa-1, pressure range: 0-8 kPa) are fabricated by utilizing such hydrogels. These flexible wearable mechanosensors can monitor diverse body movements such as joint bending, walking, running, and stair climbing. This work is anticipated to offer promising soft materials for efficient mechanical-to-electrical signal conversion and provides new insights into the development of various wearable mechanosensors.

Flexible Organic Molecular Single Crystal-Based Triboelectric Device as a Self-Powered Tactile Sensor

Triboelectric nanogenerators (TENGs) have proven to be effective at converting mechanical energy into electrical power, making them a viable technology for operating self-powered electronic devices used in medical diagnostics and environmental monitoring. In the present study, we demonstrate the utility of the flexible single crystals of an organic compound for the fabrication of a TENG as a self-powered tactile sensor. Triboelectrification was attained in single crystals as a result of surface functionalization with positively and negatively charged moieties, viz. Zn2+ and F-, respectively, which resulted in a variable surface potential and reversible adhesion through electrostatic interaction and induction phenomena. TENG incorporating the single crystals showed an output voltage of 2.4 V, a current density of ∼2.2 μA/m2, and a power density of ∼850 mW/m2 and was capable of charging commercial capacitors thereby ensuring its ability to be used as a self-powered touch sensor. Capitalizing on these features, a self-powered tactile sensor was fabricated to demonstrate limb movements. The excellent mechano-electric sensitivity (∼102 mV/kPa until 6 kPa range) and response time (∼38 ms) establish the viability of flexible organic single crystals for mechanical energy harvesting and biosensing applications that could pave the way for their utilization as biomedical wearable devices.

Stretchable, Self-Healing, and Bioactive Hydrogel with High-Functionality N,N'-bis(acryloyl)cystamine Dynamically Bonded Ag@polydopamine Crosslinkers for Wearable Sensors

Hydrogels present attractive opportunities as flexible sensors due to their soft nature and tunable physicochemical properties. Despite significant advances, practical application of hydrogel-based sensor is limited by the lack of general routes to fabricate materials with combination of mechanical, conductive, and biological properties. Here, a multi-functional hydrogel sensor is reported by in situ polymerizing of acrylamide (AM) with N,N'-bis(acryloyl)cystamine (BA) dynamic crosslinked silver-modified polydopamine (PDA) nanoparticles, namely PAM/BA-Ag@PDA. Compared with traditional polyacrylamide (PAM) hydrogel, the BA-Ag@PDA nanoparticles provide both high-functionality crosslinks and multiple interactions within PAM networks, thereby endowing the optimized PAM/BA-Ag@PDA hydrogel with significantly enhanced tensile/compressive strength (349.80 kPa at 383.57% tensile strain, 263.08 kPa at 90% compressive strain), lower hysteresis (5.2%), improved conductivity (2.51 S m-1) and excellent near-infrared (NIR) light-triggered self-healing ability. As a strain sensor, the PAM/BA-Ag@PDA hydrogel shows a good sensitivity (gauge factor of 1.86), rapid response time (138 ms), and high stability. Owing to abundant reactive groups in PDA, the PAM/BA-Ag@PDA hydrogel exhibits inherent tissue adhesiveness and antioxidant, along with a synergistic antibacterial effect by PDA and Ag. Toward practical applications, the PAM/BA-Ag@PDA hydrogel can conformally adhere to skin and monitor subtle activities and large-scale movements with excellent reliability, demonstrating its promising applications as wearable sensors for healthcare.

Piezoelectric Biomaterials Inspired by Nature for Applications in Biomedicine and Nanotechnology

Bioelectricity provides electrostimulation to regulate cell/tissue behaviors and functions. In the human body, bioelectricity can be generated in electromechanically responsive tissues and organs, as well as biomolecular building blocks that exhibit piezoelectricity, with a phenomenon known as the piezoelectric effect. Inspired by natural bio-piezoelectric phenomenon, efforts have been devoted to exploiting high-performance synthetic piezoelectric biomaterials, including molecular materials, polymeric materials, ceramic materials, and composite materials. Notably, piezoelectric biomaterials polarize under mechanical strain and generate electrical potentials, which can be used to fabricate electronic devices. Herein, a review article is proposed to summarize the design and research progress of piezoelectric biomaterials and devices toward bionanotechnology. First, the functions of bioelectricity in regulating human electrophysiological activity from cellular to tissue level are introduced. Next, recent advances as well as structure-property relationship of various natural and synthetic piezoelectric biomaterials are provided in detail. In the following part, the applications of piezoelectric biomaterials in tissue engineering, drug delivery, biosensing, energy harvesting, and catalysis are systematically classified and discussed. Finally, the challenges and future prospects of piezoelectric biomaterials are presented. It is believed that this review will provide inspiration for the design and development of innovative piezoelectric biomaterials in the fields of biomedicine and nanotechnology.

Biodegradable Ferroelectric Molecular Plastic Crystal HOCH2(CF2)7CH2OH Structurally Inspired by Polyvinylidene Fluoride

Ferroelectric materials, traditionally comprising inorganic ceramics and polymers, are commonly used in medical implantable devices. However, their nondegradable nature often necessitates secondary surgeries for removal. In contrast, ferroelectric molecular crystals have the advantages of easy solution processing, lightweight, and good biocompatibility, which are promising candidates for transient (short-term) implantable devices. Despite these benefits, the discovered biodegradable ferroelectric materials remain limited due to the absence of efficient design strategies. Here, inspired by the polar structure of polyvinylidene fluoride (PVDF), a ferroelectric molecular crystal 1H,1H,9H,9H-perfluoro-1,9-nonanediol (PFND), which undergoes a cubic-to-monoclinic ferroelectric plastic phase transition at 339 K, is discovered. This transition is facilitated by a 2D hydrogen bond network formed through O-H···O interactions among the oriented PFND molecules, which is crucial for the manifestation of ferroelectric properties. In this sense, by reducing the number of -CF2- groups from ≈5 000 in PVDF to seven in PFND, it is demonstrated that this ferroelectric compound only needs simple solution processing while maintaining excellent biosafety, biocompatibility, and biodegradability. This work illuminates the path toward the development of new biodegradable ferroelectric molecular crystals, offering promising avenues for biomedical applications.

In vitro and in vivo biocompatibility assessment of chalcogenide thermoelectrics as implants

The ability of thermoelectric materials to generate electricity in response to local temperature gradients makes them a potentially promising solution for the regulation of cellular functions and reconstruction of tissues. Biocompatibility of implants is a crucial attribute for the successful integration of thermoelectric techniques in biomedical applications. This work focuses on the in vitro and in vivo evaluation of biocompatibility for 12 typical chalcogenide thermoelectrics, which are composed of biocompatible elements. Ag2Se, SnSe, Bi2Se3, Bi2Te2.88Se0.12 and Bi2Te3, each with a released ion concentration lower than 10 ppm in extracts, exhibited favorable biocompatibility, including cell viability, adhesion, and hemocompatibility, as observed in initial in vitro assessments. Moreover, in vivo biocompatibility assessment, achieved by hematological and histopathological analyses in the rat subcutaneous model, further substantiated the biocompatibility of Ag2Se, Bi2Se3, and Bi2Te3, with each possessing superior thermoelectric performance at room temperature. This work offers robust evidence to promote Ag2Se, Bi2Se3, and Bi2Te3 as potential thermoelectric biomaterials, establishing a foundation for their future applications in biomedicine.

Synergistic Coassembly of Folic Acid-Based Supramolecular Polymer with a Covalent Polymer Toward Fabricating Functional Antibacterial Biomaterials

Supramolecular biomaterials can recapitulate the structural and functional facets of the native extracellular matrix and react to biochemical cues, leveraging the unique attributes of noncovalent interactions, including reversibility and tunability. However, the low mechanical properties of supramolecular biomaterials can restrict their utilization in specific applications. Combining the advantages of supramolecular polymers with covalent polymers can lead to the fabrication of tailor-made biomaterials with enhanced mechanical properties/degradability. Herein, we demonstrate a synergistic coassembled self-healing gel as a multifunctional supramolecular material. As the supramolecular polymer component, we chose folic acid (vitamin B9), an important biomolecule that forms a gel comprising one-dimensional (1D) supramolecular polymers. Integrating polyvinyl alcohol (PVA) into this supramolecular gel alters its ultrastructure and augments its mechanical properties. A drastic improvement of complex modulus (G*) (∼3674 times) was observed in the folic acid-PVA gel with 15% w/v PVA (33215 Pa) compared with the folic acid gel (9.04 Pa). The coassembled hydrogels possessed self-healing and injectable/thixotropic attributes and could be printed into specific three-dimensional (3D) shapes. Synergistically, the supramolecular polymers of folic acid also improve the toughness, durability, and ductility of the PVA films. A nanocomposite of the gels with silver nanoparticles exhibited excellent catalytic efficiency and antibacterial activity. The folic acid-PVA coassembled gels and films also possessed high cytocompatibility, substantiated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and live-dead assays. Taken together, the antibacterial and cell-adhesive attributes suggest potential applications of these coassembled biomaterials for tissue engineering and wound healing.

Electroactive Biomaterials Regulate the Electrophysiological Microenvironment to Promote Bone and Cartilage Tissue Regeneration

The incidence of large bone and articular cartilage defects caused by traumatic injury is increasing worldwide; the tissue regeneration process for these injuries is lengthy due to limited self‐healing ability. Endogenous bioelectrical phenomenon has been well recognized to play an important role in bone and cartilage homeostasis and regeneration. Studies have reported that electrical stimulation (ES) can effectively regulate various biological processes and holds promise as an external intervention to enhance the synthesis of the extracellular matrix, thereby accelerating the process of bone and cartilage regeneration. Hence, electroactive biomaterials have been considered a biomimetic approach to ensure functional recovery by integrating various physiological signals, including electrical, biochemical, and mechanical signals. This review will discuss the role of endogenous bioelectricity in bone and cartilage tissue, as well as the effects of ES on cellular behaviors. Then, recent advances in electroactive materials and their applications in bone and cartilage tissue regeneration are systematically overviewed, with a focus on their advantages and disadvantages as tissue repair materials and performances in the modulation of cell fate. Finally, the significance of mimicking the electrophysiological microenvironment of target tissue is emphasized and future development challenges of electroactive biomaterials for bone and cartilage repair strategies are proposed.

Advances in magnetoelectric composites for promoting bone regeneration: a review

Repair of large bone defects is one of the clinical problems that have not yet been fully solved. The dynamic balance of bone tissue is regulated by many biological, chemical and physical environmental factors. Simulating the microenvironment of bone tissue in the physiological state through biomimetic materials is an important development direction of tissue engineering in recent years. With the deepening of research, it has been found that when bone tissue is damaged, its surrounding magnetoelectric microenvironment is subsequently destroyed, and providing a magnetoelectric microenvironment in the biomimetic state will be beneficial to promote bone repair. This review describes the piezoelectric effect of natural bone tissue with magnetoelectric stimulation for bone regeneration, provides a detailed account of the historical development of magnetoelectric composites and the current magnetoelectric composites that are most commonly utilized in the field of tissue engineering. Besides, the hypothesized mechanistic pathways through which magnetoelectric composite materials promote bone regeneration are critically examined, including the enhancement of osteogenesis, promotion of cell adhesion and angiogenesis, modulation of bone immunity, and promotion of nerve regeneration.

Piezoionic Elastomers by Phase and Interface Engineering for High-Performance Energy-Harvesting Ionotronics

Piezoionic materials play a pivotal role in energy-harvesting ionotronics. However, a persistent challenge lies in balancing the structural requirements for voltage generation, current conduction, and mechanical adaptability. The conventional approach of employing crystalline heterostructures for stress concentration and localized charge separation, while effective for voltage generation, often compromises the stretchability and long-range charge transport found in homogeneous quasisolid states. Herein, phase and interface engineering strategy is introduced to address this dilemma and a piezoionic elastomer is presented that seamlessly integrates ionic liquids and ionic plastic crystals, forming a finely tuned microphase-separated structure with an intermediate phase. This approach promotes charge separation via stress concentration among hard phases while leveraging the high ionic charge mobility in soft and intermediate phases. Impressively, the elastomer achieves an extraordinary piezoionic coefficient of about 6.0 mV kPa-1, a more than threefold improvement over current hydrogels and ionogels. The resulting power density of 1.3 µW cm-3 sets a new benchmark, exceeding that of state-of-the-art piezoionic gels. Notably, this elastomer combines outstanding stretchability, remarkable toughness, and rapid self-healing capability, underscoring its potential for real-world applications. This work may represent a stride toward mechanically robust energy harvesting systems and provide insights into ionotronic systems for human-machine interaction.

Peptide hydrogen-bonded organic frameworks

Hydrogen-bonded porous frameworks (HPFs) are versatile porous crystalline frameworks with diverse applications. However, designing chiral assemblies or biocompatible materials poses significant challenges. Peptide-based hydrogen-bonded porous frameworks (P-HPFs) are an exciting alternative to conventional HPFs due to their intrinsic chirality, tunability, biocompatibility, and structural diversity. Flexible, ultra-short peptide-based P-HPFs (composed of 3 or fewer amino acids) exhibit adaptable porous topologies that can accommodate a variety of guest molecules and capture hazardous greenhouse gases. Longer, folded peptides present challenges and opportunities in designing P-HPFs. This review highlights recent developments in P-HPFs using ultra-short peptides, folded peptides, and foldamers, showcasing their utility for gas storage, chiral recognition, chiral separation, and medical applications. It also addresses design challenges and future directions in the field.

Three-Dimensional Printing of Hydrogels for Flexible Sensors: A Review

The remarkable flexibility and heightened sensitivity of flexible sensors have drawn significant attention, setting them apart from traditional sensor technology. Within this domain, hydrogels-3D crosslinked networks of hydrophilic polymers-emerge as a leading material for the new generation of flexible sensors, thanks to their unique material properties. These include structural versatility, which imparts traits like adhesiveness and self-healing capabilities. Traditional templating-based methods fall short of tailor-made applications in crafting flexible sensors. In contrast, 3D printing technology stands out with its superior fabrication precision, cost-effectiveness, and satisfactory production efficiency, making it a more suitable approach than templating-based strategies. This review spotlights the latest hydrogel-based flexible sensors developed through 3D printing. It begins by categorizing hydrogels and outlining various 3D-printing techniques. It then focuses on a range of flexible sensors-including those for strain, pressure, pH, temperature, and biosensors-detailing their fabrication methods and applications. Furthermore, it explores the sensing mechanisms and concludes with an analysis of existing challenges and prospects for future research breakthroughs in this field.

Low-dimensional nanostructures for monolithic 3D-integrated flexible and stretchable electronics

Flexible/stretchable electronics, which are characterized by their ultrathin design, lightweight structure, and excellent mechanical robustness and conformability, have garnered significant attention due to their unprecedented potential in healthcare, advanced robotics, and human-machine interface technologies. An increasing number of low-dimensional nanostructures with exceptional mechanical, electronic, and/or optical properties are being developed for flexible/stretchable electronics to fulfill the functional and application requirements of information sensing, processing, and interactive loops. Compared to the traditional single-layer format, which has a restricted design space, a monolithic three-dimensional (M3D) integrated device architecture offers greater flexibility and stretchability for electronic devices, achieving a high-level of integration to accommodate the state-of-the-art design targets, such as skin-comfort, miniaturization, and multi-functionality. Low-dimensional nanostructures possess small size, unique characteristics, flexible/elastic adaptability, and effective vertical stacking capability, boosting the advancement of M3D-integrated flexible/stretchable systems. In this review, we provide a summary of the typical low-dimensional nanostructures found in semiconductor, interconnect, and substrate materials, and discuss the design rules of flexible/stretchable devices for intelligent sensing and data processing. Furthermore, artificial sensory systems in 3D integration have been reviewed, highlighting the advancements in flexible/stretchable electronics that are deployed with high-density, energy-efficiency, and multi-functionalities. Finally, we discuss the technical challenges and advanced methodologies involved in the design and optimization of low-dimensional nanostructures, to achieve monolithic 3D-integrated flexible/stretchable multi-sensory systems.

An Emerging Era: Conformable Ultrasound Electronics

Conformable electronics are regarded as the next generation of personal healthcare monitoring and remote diagnosis devices. In recent years, piezoelectric-based conformable ultrasound electronics (cUSE) have been intensively studied due to their unique capabilities, including nonradiative monitoring, soft tissue imaging, deep signal decoding, wireless power transfer, portability, and compatibility. This review provides a comprehensive understanding of cUSE for use in biomedical and healthcare monitoring systems and a summary of their recent advancements. Following an introduction to the fundamentals of piezoelectrics and ultrasound transducers, the critical parameters for transducer design are discussed. Next, five types of cUSE with their advantages and limitations are highlighted, and the fabrication of cUSE using advanced technologies is discussed. In addition, the working function, acoustic performance, and accomplishments in various applications are thoroughly summarized. It is noted that application considerations must be given to the tradeoffs between material selection, manufacturing processes, acoustic performance, mechanical integrity, and the entire integrated system. Finally, current challenges and directions for the development of cUSE are highlighted, and research flow is provided as the roadmap for future research. In conclusion, these advances in the fields of piezoelectric materials, ultrasound transducers, and conformable electronics spark an emerging era of biomedicine and personal healthcare.