Liquid metal as reconnection agent for peripheral nerve injury

A Metalgel with Liquid Metal Continuum Immobilized in Polymer Network

Gels are formed by fluids that expand throughout the whole volume of 3D polymer networks. To unlock unprecedented properties, exploring new fluids immobilized in polymer networks is crucial. Here, a new liquid metal-polymer gel material termed "metalgel" is introduced via fluid replacement strategy, featuring 92.40% vol liquid metal fluid as a continuum immobilized by interconnected nanoscale polymer network. The unique structure endows metalgel with high electrical conductivity (up to 3.18 × 106 S·m‒1), tissue-like softness (Young's modulus as low as 70 kPa), and low gas permeability (4.50 × 10‒22 m2·s‒1·Pa‒1). Besides, metalgel demonstrates electrical stability under extreme deformations, such as being run over by a 4.5-metric-tonne truck, and maintains its integrity in various environments for up to 180 days. The immobilization of high-volume-fraction liquid metal fluid is realized by electrostatic interactions is further revealed. Potential applications for metalgel are diverse and include soft electromagnetic shielding, hermetic sealing, and stimulating/sensing electrodes in implantable bioelectronics, underscoring its broad applicability.

Liquid metal biomaterials: translational medicines, challenges and perspectives

Until now, significant healthcare challenges and growing urgent clinical requirements remain incompletely addressed by presently available biomedical materials. This is due to their inadequate mechanical compatibility, suboptimal physical and chemical properties, susceptibility to immune rejection, and concerns about long-term biological safety. As an alternative, liquid metal (LM) opens up a promising class of biomaterials with unique advantages like biocompatibility, flexibility, excellent electrical conductivity, and ease of functionalization. However, despite the unique advantages and successful explorations of LM in biomedical fields, widespread clinical translations and applications of LM-based medical products remain limited. This article summarizes the current status and future prospects of LM biomaterials, interprets their applications in healthcare, medical imaging, bone repair, nerve interface, and tumor therapy, etc. Opportunities to translate LM materials into medicine and obstacles encountered in practices are discussed. Following that, we outline a blueprint for LM clinics, emphasizing their potential in making new-generation artificial organs. Last, the core challenges of LM biomaterials in clinical translation, including bio-safety, material stability, and ethical concerns are also discussed. Overall, the current progress, translational medicine bottlenecks, and perspectives of LM biomaterials signify their immense potential to drive future medical breakthroughs and thus open up novel avenues for upcoming clinical practices.

Engineered biomimetic micro/nano-materials for tissue regeneration

The incidence of tissue and organ damage caused by various diseases is increasing worldwide. Tissue engineering is a promising strategy of tackling this problem because of its potential to regenerate or replace damaged tissues and organs. The biochemical and biophysical cues of biomaterials can stimulate and induce biological activities such as cell adhesion, proliferation and differentiation, and ultimately achieve tissue repair and regeneration. Micro/nano materials are a special type of biomaterial that can mimic the microstructure of tissues on a microscopic scale due to its precise construction, further providing scaffolds with specific three-dimensional structures to guide the activities of cells. The study and application of biomimetic micro/nano-materials have greatly promoted the development of tissue engineering. This review aims to provide an overview of the different types of micro/nanomaterials, their preparation methods and their application in tissue regeneration.

Liquid Metal-Based Electrode Array for Neural Signal Recording

Neural electrodes are core devices for research in neuroscience, neurological diseases, and neural-machine interfacing. They build a bridge between the cerebral nervous system and electronic devices. Most of the neural electrodes in use are based on rigid materials that differ significantly from biological neural tissue in flexibility and tensile properties. In this study, a liquid-metal (LM) -based 20-channel neural electrode array with a platinum metal (Pt) encapsulation material was developed by microfabrication technology. The in vitro experiments demonstrated that the electrode has stable electrical properties and excellent mechanical properties such as flexibility and bending, which allows the electrode to form conformal contact with the skull. The in vivo experiments also recorded electroencephalographic signals using the LM-based electrode from a rat under low-flow or deep anesthesia, including the auditory-evoked potentials triggered by sound stimulation. The auditory-activated cortical area was analyzed using source localization technique. These results indicate that this 20-channel LM-based neural electrode array satisfies the demands of brain signal acquisition and provides high-quality-electroencephalogram (EEG) signals that support source localization analysis.

Toxicity and Biocompatibility of Liquid Metals

Recently, room-temperature liquid metals have attracted increasing attention from researchers owing to their excellent material properties. Systematic interpretation of the potential toxicity issues involved is essential for a wide range of applications, especially in the biomedical and healthcare fields. However, even with the exponential growth of related studies, investigation of the toxicological impact and possible hazards of liquid metals to organisms is still in its infancy. This review aims to provide a comprehensive summary of the current frontier of knowledge on liquid metal toxicology and biocompatibility in different environments. Based on recent studies, this review focuses on Ga and Bi-based in different states. It is necessary to evaluate their toxicity considering the rapid increase in research and utilization of such liquid metal composites. Finally, existing challenges are discussed and suggestions are provided for further investigation of liquid metal toxicology to clarify the toxicological mechanisms and strategies are provided to avoid adverse effects. In addition to resolving the doubts of public concern about the toxicity of liquid metals, this review is expected to promote the healthy and sustainable development of liquid metal-based materials and their use in diverse areas, especially those related to health care.

Emerging Liquid Metal Biomaterials: From Design to Application

Liquid metals (LMs) as emerging biomaterials possess unique advantages including their favorable biosafety, high fluidity, and excellent electrical and thermal conductivities, thus providing a unique platform for a wide range of biomedical applications ranging from drug delivery, tumor therapy, and bioimaging to biosensors. The structural design and functionalization of LMs endow them with enhanced functions such as enhanced targeting ability and stimuli responsiveness, enabling them to achieve better and even multifunctional synergistic therapeutic effects. Herein, the advantages of LMs in biomedicine are presented. The design of LM-based biomaterials with different scales ranging from micro-/nanoscale to macroscale and various components is explored in-depth to promote the understanding of structure-property relationships, guiding their performance optimization and applications. Furthermore, the related advanced progress in the development of LM-based biomaterials in biomedicine is summarized. Current challenges and prospects of LMs in the biomedical field are also discussed.

Strategies for interface issues and challenges of neural electrodes

Neural electrodes, as a bridge for bidirectional communication between the body and external devices, are crucial means for detecting and controlling nerve activity. The electrodes play a vital role in monitoring the state of neural systems or influencing it to treat disease or restore functions. To achieve high-resolution, safe and long-term stable nerve recording and stimulation, a neural electrode with excellent electrochemical performance (e.g., impedance, charge storage capacity, charge injection limit), and good biocompatibility and stability is required. Here, the charge transfer process in the tissues, the electrode-tissue interfaces and the electrode materials are discussed respectively. Subsequently, the latest research methods and strategies for improving the electrochemical performance and biocompatibility of neural electrodes are reviewed. Finally, the challenges in the development of neural electrodes are proposed. It is expected that the development of neural electrodes will offer new opportunities for the evolution of neural prosthesis, bioelectronic medicine, brain science, and so on.

Liquid metal biomaterials for biomedical imaging

Liquid metals (LMs) not only retain the basic properties of metallic biomaterials, such as high thermal conductivity and high electrical conductivity, but also possess flexibility, flowability, deformability, plasticity, good adhesion, and so on. Therefore, they open many possibilities of extending soft metals into biomedical sciences including biomedical imaging. One of the special properties of LMs is that they can provide a controllable material system in which the electrical, thermal, mechanical, and chemical properties can be controlled on a large scale. This paper reviews the preparation and characteristics of LM-based biomaterials classified into four categories: LM micro/nanoparticles, surface modified LM droplets, LM composites with inorganic substances, and LM composites with organic polymers. Besides, considering the most important requirement for biomaterials is biocompatibility, the paper also analyzes the toxicity results of various LM biomaterials when used in the biomedical area, from different levels including body weight measurement, histology evaluation, and blood biochemistry tests. Next, the applications of LMs in X-ray, CT, MRI, photoacoustic imaging, and molecular imaging are introduced in detail. And finally, the challenges and opportunities of their application in medical imaging are also discussed.

Liquid metal polymer composite: Flexible, conductive, biocompatible, and antimicrobial scaffold

Gallium and its alloys, such as eutectic gallium indium alloy (EGaIn), a form of liquid metal, have recently attracted the attention of researchers due to their low toxicity and electrical and thermal conductivity for biomedical application. However, further research is required to harness EGaIn-composites advantages and address their application as a biomedical scaffold. In this research, EGaIn-polylactic acid/polycaprolactone composites with and without a second conductive filler, MXene, were prepared and characterized. The addition of MXene, into the EGaIn-composite, can improve the composite's electrochemical properties by connecting the liquid metal droplets resulting in electrically conductive continuous pathways within the polymeric matrix. The results showed that the composite with 50% EGaIn and 4% MXene, displayed optimal electrochemical properties and enhanced mechanical and radiopacity properties. Furthermore, the composite showed good biocompatibility, examined through interactions with fibroblast cells, and antibacterial properties against methicillin-resistant Staphylococcus aureus. Therefore, the liquid metal (EGaIn) polymer composite with MXene provides a first proof-of-concept engineering scaffold strategy with low toxicity, functional electrochemical properties, and promising antimicrobial properties.

Biocompatibility Testing of Liquid Metal as an Interconnection Material for Flexible Implant Technology

Galinstan, a liquid metal at room temperature, is a promising material for use in flexible electronics. Since it has been successfully integrated in devices for external use, e.g., as stretchable electronic skin in tactile sensation, the possibility of using galinstan for flexible implant technology comes to mind. Usage of liquid metals in a flexible implant would reduce the risk of broken conductive pathways in the implants and therefore reduce the possibility of implant failure. However, the biocompatibility of the liquid metal under study, i.e., galinstan, has not been proven in state-of-the-art literature. Therefore, in this paper, a material combination of galinstan and silicone rubber is under investigation regarding the success of sterilization methods and to establish biocompatibility testing for an in vivo application. First cell biocompatibility tests (WST-1 assays) and cell toxicity tests (LDH assays) show promising results regarding biocompatibility. This work paves the way towards the successful integration of stretchable devices using liquid metals embedded in a silicone rubber encapsulant for flexible surface electro-cortical grid arrays and other flexible implants.

A Soft and Absorbable Temporary Epicardial Pacing Wire

Existing temporary epicardial pacing wires (TPWs) are rigid and non-absorbable, such that they can cause severe complications after cardiac surgery. Here, a soft and absorbable temporary epicardial pacing wire (saTPW) for effectively correcting abnormal heart rates in a rabbit model, such as bradycardia and ventricular premature beat, is developed. The saTPW exhibits excellent conductivity, flexibility, cycling stability (>100 000 cycles), and less inflammatory response during two-month subcutaneous implantation in a rat model. The saTPW which consists of poly(l-lactide-co-ε-caprolactone) and liquid metal, can degrade about 13% (mass loss) in the rats over a two-month subcutaneous implantation. It can be absorbed over time in the body. The cytocompatibility and absorbability avoid secondary injuries caused by remaining wires which are permanently left in the body. The saTPW will provide a great platform for diagnosis and treatments in cardiovascular diseases by delivering the physiological signal and applying electrical stimulation for therapy.

Liquid Metal-Based Soft Electronics for Wearable Healthcare

Wearable healthcare devices have garnered substantial interest for the realization of personal health management by monitoring the physiological parameters of individuals. Attaining the integrity between the devices and the biological interfaces is one of the greatest challenges to achieving high-quality body information in dynamic conditions. Liquid metals, which exist in the liquid phase at room temperatures, are advanced intensively as conductors for deformable devices because of their excellent stretchability and self-healing ability. The unique surface chemistry of liquid metals allows the development of various sensors and devices in wearable form. Also, the biocompatibility of liquid metals, which is verified through numerous biomedical applications, holds immense potential in uses on the surface and inside of a living body. Here, the recent progress of liquid metal-based wearable electronic devices for healthcare with respect to the featured properties and the processing technologies is discussed. Representative examples of applications such as biosensors, neural interfaces, and a soft interconnection for devices are reviewed. The current challenges and prospects for further development are also discussed, and the future directions of advances in the latest research are explored.

Ga Based Particles, Alloys and Composites: Fabrication and Applications

Liquid metal (LM) materials, including pure gallium (Ga) LM, eutectic alloys and their composites with organic polymers and inorganic nanoparticles, are cutting-edge functional materials owing to their outstanding electrical conductivity, thermal conductivity, extraordinary mechanical compliance, deformability and excellent biocompatibility. The unique properties of LM-based materials at room temperatures can overcome the drawbacks of the conventional electronic devices, particularly high thermal, electrical conductivities and their fluidic property, which would open tremendous opportunities for the fundamental research and practical applications of stretchable and wearable electronic devices. Therefore, research interest has been increasingly devoted to the fabrication methodologies of LM nanoparticles and their functional composites. In this review, we intend to present an overview of the state-of-art protocols for the synthesis of Ga-based materials, to introduce their potential applications in the fields ranging from wearable electronics, energy storage batteries and energy harvesting devices to bio-applications, and to discuss challenges and opportunities in future studies.

Photothermal photodynamic therapy and enhanced radiotherapy of targeting copolymer-coated liquid metal nanoparticles on liver cancer

The maximized therapeutic efficacy in tumor treatment can be achieved with combination therapy. Herein, a metronidazole (MN) and RGD peptides were linked with the copolymer chains of polyacrylic acid (PAA) and polyethylene glycol (PEG) by condensation and Michael addition reactions, respectively, named as RGD-PEG-PAA-MN. Subsequently, liquid-metal (LM) nanoparticles broken by ultrasonication were coated with modified copolymer, forming RGD-PEG-PAA-MN@LM nanoparticles. These nanoparticles with the degradation under an acidic condition could target to tumor cells, and LM of these composited nanoparticles could not only efficiently convert the photoenergy of near infrared (NIR) into thermal energy, but also produce more reactive oxygen species under NIR or X ray irradiation. Furthermore, MN in the composited nanoparticles could enhance their radiation sensitivity of tumor tissues with hypoxia condition. The synergic effect of these nanoparticles on cancer limitation after the sequential radiations of NIR and X ray was significantly higher than the single radiation. In the experiments of tumor bearing mice, the volume of the tumor in RGD-PEG-PAA-MN@LM group at 14th day after two radiations of NIR and X-ray were significantly smaller than LM group, and the tumor of RGD-PEG-PAA-MN@LM group at 14th day after two radiations almost disappeared, suggesting better synergistic effect of RGD-PEG-PAA-MN@LM nanoparticles on photothermal conversion, photodynamics under two irradiations and their enhanced sensitization of X-ray radiation. Our results indicated that the prepared nanoparticles would be applied in the combinational therapy of liver tumor by the photothermal, photodynamic and sensitized radiation.

Biomedical Applications of Metal 3D Printing

Additive manufacturing's attributes include print customization, low per-unit cost for small- to mid-batch production, seamless interfacing with mainstream medical 3D imaging techniques, and feasibility to create free-form objects in materials that are biocompatible and biodegradable. Consequently, additive manufacturing is apposite for a wide range of biomedical applications including custom biocompatible implants that mimic the mechanical response of bone, biodegradable scaffolds with engineered degradation rate, medical surgical tools, and biomedical instrumentation. This review surveys the materials, 3D printing methods and technologies, and biomedical applications of metal 3D printing, providing a historical perspective while focusing on the state of the art. It then identifies a number of exciting directions of future growth: (a) the improvement of mainstream additive manufacturing methods and associated feedstock; (b) the exploration of mature, less utilized metal 3D printing techniques; (c) the optimization of additively manufactured load-bearing structures via artificial intelligence; and (d) the creation of monolithic, multimaterial, finely featured, multifunctional implants.

Highly Stretchable Metal-Polymer Conductor Electrode Array for Electrophysiology

Narrowing the mechanical mismatch between biological tissues (typically soft) and neural interfaces (hard) is essential for maintaining signal quality for the electrical recording of neural activity. However, only a few materials can satisfy all requirements for such electronics, which need to be both biocompatible and sufficiently soft. Here, a highly stretchable electrode array (SEA) is introduced, based on the liquid metal-polymer conductor (MPC), achieving high mechanical flexibility and good cytocompatability for neural interfaces. By utilizing the MPC, the SEA exhibits high stretchability (≈100%) and excellent cycling stability (>400 cycles). The cytocompatability of the SEA can allow for long-term culturing of primary neurons and enable signal recording of primary hippocampal neurons. In the future, the SEA could serve as a reliable and robust platform for diagnostics in neuronal tissues and greatly advance brain-machine interfaces.

Liquid Metal Based Flexible and Implantable Biosensors

Biosensors are the core elements for obtaining significant physiological information from living organisms. To better sense life information, flexible biosensors and implantable sensors that are highly compatible with organisms are favored by researchers. Moreover, materials for preparing a new generation of flexible sensors have also received attention. Liquid metal is a liquid-state metallic material with a low melting point at or around room temperature. Owing to its high electrical conductivity, low toxicity, and superior fluidity, liquid metal is emerging as a highly desirable candidate in biosensors. This paper is dedicated to reviewing state-of-the-art applications in biosensors that are expounded from seven aspects, including pressure sensor, strain sensor, gas sensor, temperature sensor, electrical sensor, optical sensor, and multifunctional sensor, respectively. The fundamental scientific and technological challenges lying behind these recommendations are outlined. Finally, the perspective of liquid metal-based biosensors is present, which stimulates the upcoming design of biosensors.

Self-Powered Gallium-Based Liquid-Metal Beating Heart

Mercury beating heart is a well-known phenomenon that consists of a mercury droplet covered with aqueous acid and an iron nail. However, mercury is highly poisonous, and its vapor is especially dangerous. Thus, related studies and applications on mercury have often been hindered. Here, we proposed another beating heart but employed a different material, i.e., GaIn alloy with low toxicity. A stainless steel wire was utilized to touch the side of the liquid-metal droplet in basic solution. Based on this method, periodic oscillation could be kept continuous and steady. This finding suggests a more feasible and safer way to realize beating behaviors, which would shed light on a variety of future applications, such as pump and mixer for the mini device.

Novel contrast media based on the liquid metal gallium for in vivo digestive tract radiography: a feasibility study

The barium sulfate has been playing an important role as the contrast medium in gastrointestinal radiography and disease diagnosis. However, its application has been gradually reduced due to the limitation of its imaging effect and the progress of other imaging techniques. Here, the liquid metal gallium was proposed as an improved contrast agent, which was applied in the in vivo digestive tract radiography of the mice for the first time. Under the CT scanning, the gallium produced excellent contrast effect intuitively. According to the records of discharge time, the tissue sections of organs, the survival state and body weight, the liquid metal was proven to be capable and safe for gastrointestinal radiography. Further, the mixture of the gallium and the barium sulfate has been tested, which showed better performance in both contrast and detail. Therefore, with the characteristics of better imaging contrast effect and acceptable safety, the gallium and its mixture with the barium sulfate might be useful as potential candidates for digestive tract contrast agent in animal experiments, even possibly as alternative contrast agents for clinical use.

Rewritable, Printable Conducting Liquid Metal Hydrogel

The development of high-performance printable electrical circuits, particularly based on liquid metals, is fundamental for device interconnection in flexible electronics, motivating numerous attempts to develop a variety of alloys and their composites. Despite their great potential, rewritable and printable electronic circuits based on liquid metals are still manufactured on demand. In this study, we demonstrate liquid metal-based hydrogels suitable for rewritable, printable electrical circuits. Our liquid metal hydrogels are based on sedimentation-induced composites of eutectic gallium-indium (EGaIn) particles in poly(ethylene glycol) diacrylate (PEGDA). The EGaIn particles are vertically phase-segregated in the PEGDA. When a composite surface with high EGaIn content is gently scratched, the surface covering PEGDA is removed, followed by the rupture of the native oxide layers of the particles, and the exposed EGaIn becomes conductive. The subsequent water-driven swelling of PEGDA on the scratched surface completely erases the conductive circuit, causing the system to reset. Our friction-responsive liquid metal hydrogel exhibits writing-erasing endurance for 20 cycles, with a dramatic change in the electrical resistance from metal (∼1 Ω) to insulator (∼10⁷ Ω). By employing surface friction pen printing, we demonstrate mechanically flexible, rewritable, printable electrical conductors suitable for displays.

A Liquid-Metal-Based Magnetoactive Slurry for Stimuli-Responsive Mechanically Adaptive Electrodes

Electrical communication between a biological system and outside equipment allows one to monitor and influence the state of the tissue and nervous networks. As the bridge, bioelectrodes should possess both electrical conductivity and adaptive mechanical properties matching the target soft biosystem, but this is still a big challenge. A family of liquid-metal-based magnetoactive slurries (LMMSs) formed by dispersing magnetic iron particles in a Ga-based liquid metal (LM) matrix is reported here. The mechanical properties, viscosity, and stiffness of such materials rapidly respond to the stimulus of an applied magnetic field. By varying the intensity of the magnetic field, regulation within a factor of 1000 of the Young's modulus from ≈kPa to ≈MPa, and the ability to reach GPa with more dense iron particles inside the LMMS are demonstrated. With the advantage of high conductivity of the LM matrix, the functions of the LMMS are not only limited to the soft implanted electrodes or penetrating electrodes in biosystems: the electrical response based on the LMMS electrodes can also be precisely tuned by simply regulating the applied magnetic field.

Recent Advancements in Liquid Metal Flexible Printed Electronics: Properties, Technologies, and Applications

This article presents an overview on typical properties, technologies, and applications of liquid metal based flexible printed electronics. The core manufacturing material-room-temperature liquid metal, currently mainly represented by gallium and its alloys with the properties of excellent resistivity, enormous bendability, low adhesion, and large surface tension, was focused on in particular. In addition, a series of recently developed printing technologies spanning from personal electronic circuit printing (direct painting or writing, mechanical system printing, mask layer based printing, high-resolution nanoimprinting, etc.) to 3D room temperature liquid metal printing is comprehensively reviewed. Applications of these planar or three-dimensional printing technologies and the related liquid metal alloy inks in making flexible electronics, such as electronical components, health care sensors, and other functional devices were discussed. The significantly different adhesions of liquid metal inks on various substrates under different oxidation degrees, weakness of circuits, difficulty of fabricating high-accuracy devices, and low rate of good product-all of which are challenges faced by current liquid metal flexible printed electronics-are discussed. Prospects for liquid metal flexible printed electronics to develop ending user electronics and more extensive applications in the future are given.