(Nanowerk Spotlight) The demand for materials that combine high conductivity with flexibility has grown sharply, particularly in fields like bioelectronics and wearable technology. Traditional materials either provide conductivity, as metals do, or flexibility, as seen in gels, but achieving both properties in a single material has been a difficult task. Researchers have experimented with merging liquid metals and polymers to create conductive, soft materials, but these efforts often fell short. Existing metal-polymer mixtures struggled to keep the metal contained, resulting in leakage, instability, or a drop in conductivity under strain.
Recent advances in material science, however, are now making it feasible to integrate metal properties into flexible structures without sacrificing durability. A recent study published in Advanced Materials (“A Metalgel with Liquid Metal Continuum Immobilized in Polymer Network”) introduces a new material called metalgel, a gel that incorporates a high volume of liquid metal within a polymer framework, providing a stable, soft, and conductive material that could transform applications in flexible electronics, bioelectronics, and beyond.
Metalgel is a product of a novel manufacturing process that replaces the typical fluid in a gel with liquid metal. The research team began with iota-carrageenan, a polysaccharide widely used in biomedicine due to its compatibility with human tissues. By forming a three-dimensional network in water, iota-carrageenan creates a stable gel-like structure that can immobilize fluids. Next, the team introduced gallium-based liquid metal into the gel and mechanically sheared it into tiny droplets, which formed a stable oxide layer that binds securely with the polymer network. As the water evaporated, the liquid metal dominated the gel’s volume, creating a structure with 92.4% liquid metal and a resilient polymer framework interwoven throughout. This fluid replacement strategy transformed the gel into a conductive, metal-like structure with flexible, gel-like properties.
This unique structure gives metalgel several impressive qualities. Its conductivity reaches values up to 3.18 x 106 siemens per meter, close to that of traditional metals and significantly higher than other conductive gels. Meanwhile, its Young’s modulus – a measure of how much a material resists bending † is as low as 70 kilopascals, similar to the softness of human tissues. This combination of high conductivity and low stiffness positions metalgel as a strong candidate for implantable devices, where materials need to move naturally with body tissues while maintaining stable electrical performance.
The resilience of metalgel under mechanical stress further sets it apart from previous materials. Unlike earlier metal-polymer composites, which lost conductivity when stretched or compressed, metalgel demonstrated outstanding durability. For instance, in one test, a 4.5-metric-ton truck drove over a sample of metalgel. Despite the extreme pressure, the material’s resistance increased by only 9%, and it returned to its original state once the load was removed. Even after thousands of cycles of bending, stretching, and compressing, metalgel maintained its conductivity, showing minimal variation in resistance. This robustness against repetitive motion makes it well-suited for flexible electronics and wearables that require materials capable of repeated deformation.
Electrical and mechanical properties of the metalgel. a), Electrical conductivities and Young’s moduli of samples prepared with various volume fractions of liquid metal. All data are shown as mean ± SD. Error bars, n = 3. The dashed line represents the theoretical conductivity of EGaIn (3.40 × 106 S·m‒1). b), Ashby-style plot comparing the electrical conductivity and Young’s modulus of metalgel to other electrically conductive gels and common metals. Metallic filler-doped gels, carbon material-doped gels, and conductive polymer-doped gels are shown in the blue, orange, and purple circle ranges, respectively. Pt//Ir represents platinum and iridium alloy, Ti represents titanium, SS represents stainless steel, and Cu//Ni represents copper and nickel alloy. c), Optical images of the truck run-over test. Metalgel sample connected to a resistance meter was run over by a truck weighing 4.5 metric tons, and the pressure applied to metalgel was ≈960 N. d), Relative resistance changes of metalgel subjected to the truck run-over. e), SEM images of the metalgel before and after 1000 compression cycles at 0.5 MPa. (Image: Reprinted with permission from Wiley-VCH Verlag)
A key feature behind metalgel’s durability is the electrostatic interaction between the liquid metal droplets and the polymer network. The negatively charged polymer binds securely to the liquid metal’s oxidized surface, preventing the metal from moving within the gel or leaking out. The research team optimized this interaction by adjusting the polymer composition to strengthen the electrostatic bond. This modification led to a continuous, stable phase of liquid metal embedded within the gel, which flexes without tearing or losing conductivity. The electrostatic bonds in metalgel also make it resilient in varied environments, as it remains stable in air, water, and high-humidity conditions without any loss of conductivity or leakage.
Metalgel’s unique properties offer notable benefits for bioelectronic devices, especially implantable electrodes. Rigid electrodes can irritate surrounding tissue, while softer materials often lack sufficient conductivity or stability. Metalgel addresses both challenges by providing a conductive, flexible interface that minimizes irritation while maintaining signal quality.
In experiments with animal models, metalgel electrodes implanted near nerves consistently delivered stable electrical signals and showed low impedance, which is crucial for effective nerve stimulation. These electrodes remained effective over weeks of testing, causing minimal inflammation, similar to the natural response of tissues that had not been altered by implantation.
In addition to bioelectronics, metalgel shows potential as a material for sensors in complex biological environments, like the intestines. The wrinkled surface of the intestinal lining makes it challenging for sensors to establish reliable contact. Metalgel’s flexibility allows it to conform closely to such irregular surfaces without leaving gaps. This property allows continuous and accurate monitoring of electrical signals and ion concentrations in the gut, both of which are essential for understanding digestive health. Tests showed that metalgel adhered smoothly to intestinal tissues, did not disrupt normal muscle movements, and left inflammatory markers at levels similar to those found in unaltered tissue, indicating that it could provide sustained monitoring without causing harm.
Beyond bioelectronics, metalgel’s combination of flexibility and conductivity makes it promising for electromagnetic interference (EMI) shielding and gas barrier applications. Metalgel is effective at blocking EMI across a range of frequencies, performing comparably to traditional metal shields but without the rigidity. This ability makes metalgel useful for wearable technology that must withstand movement without losing shielding performance.
Additionally, its exceptionally low gas permeability, which rivals that of industrial sealing materials, means it could protect sensitive components in electronics or medical devices from environmental contaminants. Unlike other flexible materials that lose their barrier properties under strain, metalgel maintains its impermeability even when stretched, further expanding its potential applications.
With the development of metalgel, researchers have opened a path to a new class of materials that successfully merge the conductive and structural properties of metals with the adaptability of gels. This unique combination has already shown value in initial studies, and as metalgel technology matures, its uses could expand into new areas such as advanced medical implants, flexible sensors, and wearable technology. By offering a stable, conductive, and flexible alternative to traditional materials, metalgel could meet the growing need for versatile, high-performance materials in next-generation electronics and biomedical devices.
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