Ultralow Modulus Research Articles

A polymer-like ultrahigh-strength metal alloy

Futuristic technologies such as morphing aircrafts and super-strong artificial muscles depend on metal alloys being as strong as ultrahigh-strength steel yet as flexible as a polymer1–3. However, achieving such ‘strong yet flexible’ alloys has proven challenging4–9 because of the inevitable trade-off between strength and flexibility5,8,10. Here we report a Ti–50.8 at.% Ni strain glass alloy showing a combination of ultrahigh yield strength of σy ≈ 1.8 GPa and polymer-like ultralow elastic modulus of E ≈ 10.5 GPa, together with super-large rubber-like elastic strain of approximately 8%. As a result, it possesses a high flexibility figure of merit of σy/E ≈ 0.17 compared with existing structural materials. In addition, it can maintain such properties over a wide temperature range of −80 °C to +80 °C and demonstrates excellent fatigue resistance at high strain. The alloy was fabricated by a simple three-step thermomechanical treatment that is scalable to industrial lines, which leads not only to ultrahigh strength because of deformation strengthening, but also to ultralow modulus by the formation of a unique ‘dual-seed strain glass’ microstructure, composed of a strain glass matrix embedded with a small number of aligned R and B19′ martensite ‘seeds’. In situ X-ray diffractometry shows that the polymer-like deformation behaviour of the alloy originates from a nucleation-free reversible transition between strain glass and R and B19′ martensite during loading and unloading. This exotic alloy with the potential for mass producibility may open a new horizon for many futuristic technologies, such as morphing aerospace vehicles, superman-type artificial muscles and artificial organs.

An Ultra-Low Modulus of Ductile TiZrHfTa Biomedical High-Entropy Alloys through Deformation Induced Martensitic Transformation/Twinning/Amorphization.

Biomedical alloys are paramount materials in biomedical applications, particularly in crafting biological artificial replacements. In traditional biomedical alloys, a significant challenge is simultaneously achieving an ultra-low Young's modulus, excellent biocompatibility, and acceptable ductility. A multi-component body-centered cubic (BCC) biomedical high-entropy alloy (Bio-HEA), which is composed of non-toxic elements, is noteworthy for its outstanding biocompatibility and compositional tuning capabilities. Nevertheless, the aforementioned challenges still remain. Here, a method to achieve a single phase with the lowest Young's modulus among the constituent phases by precisely tuning the stability of the BCC phase in the Bio-HEA, is proposed. The subtle tuning of the BCC phase stability also enables the induction of stress-induced martensite transformation with extremely low trigger stress. The transformation-induced plasticity and work hardening capacity are achieved via the stress-induced martensite transformation. Additionally, the hierarchical stress-induced martensite twin structure and crystalline-to-amorphous phase transformation provide robust toughening mechanisms in the Bio-HEA. The cytotoxicity test confirms that this Bio-HEA exhibits excellent biocompatibility without cytotoxicity. In conclusion, this study provides new insights into the development of biomedical alloys with a combination of ultra-low Young's modulus, excellent biocompatibility, and decent ductility.

Gelatin-Based Metamaterial Hydrogel Films with High Conformality for Ultra-Soft Tissue Monitoring

HighlightsNovel customized metamaterial gelatin-based conductive film (GCF) was developed to ensure good biocompatibility and biodegradability for implantable bioelectronics integrating monitoring with tissue repair. The metamaterial property demonstrated high conformal with soft tissues to promote the signal-to-noise ratio.The GCF revealed elastic modulus regulated from 20 to 420 kPa and Poisson's ratio from − 0.25 to 0.52. It was fabricated by embedding different 3D printing ultrafine fiber networks with metamaterial design into ultra-low modulus Gelatin methacryloyl hydrogel.

Dual-modulus 3D printing technology for magnetorheological metamaterials-part I: Manufacturing and performance

Magnetorheological metamaterial (MRM) with magnetic controllable stiffness based on 3D printing technology have attracted extensive attention in recent years. In this paper, a dual-modulus 3D printing technology is developed to manufacture MRM based on magnetorheological elastomer with high matrix modulus (HMRE) and magnetorheological elastomer with low matrix modulus (LMRE). Due to the ultralow modulus of LMRE and the magnetic-sensitive performance of MRE, the magnetic deformation behavior of dual-modulus printed MRM was prominently improved and compared with HMRE structure. The variable amplitude of MRM equivalent stiffness in magnetic field increases by 328.57%, demonstrating a better adjustable characteristic of MRM. Therefore, owing to the dual-modulus 3D printing technology based on LMRE and HMRE, MRM perform a better adjustable capacity performance and show the application potential in the fields of soft robot and shock absorber.

Efficient phase-field simulation for linear superelastic NiTi alloys under temperature gradients

Engineering the martensitic transformation (MT) with exceptional and controllable properties is essential for the innovative application of shape memory alloys (SMAs) in advanced technologies. Herein, we combine high-throughput (HTP) phase-field simulations and machine learning approaches to propose the concept of “temperature-controlled mechanics” and demonstrate that it is possible to design NiTi alloys with outstanding mechanical properties that integrate ultra-low modulus, linear superelasticity, and no hysteresis in environments with temperature gradients. These nontrivial mechanical properties originate from continuous variations in the critical stress for the MT, which contributes to a gradual and continuous MT rather than a sharp first-order transition as occurs in common SMAs. An active learning workflow based on uncertainty sampling is employed to guide phase-field simulations to efficiently clarify and optimize the temperature in the environment for different NiTi alloys with the desired properties. Furthermore, the SISSO (Sure Independence Screening and Sparsifying Operator) algorithm is applied to the datasets from the HTP simulations to establish an explicit expression for the Young's modulus, which is verified by additional phase-field simulations and is instructive for the inverse design of the temperature field. The present study not only provides fundamental insights into the effects of temperature gradients on MTs and overall mechanical properties, but also offers a promising computational approach for developing advanced materials with extraordinary properties.

Wet-Adhesive Multifunctional Hydrogel with Anti-swelling and a Skin-Seamless Interface for Underwater Electrophysiological Monitoring and Communication.

A stable and seamless adhesion between the human skin and the hydrogel-based electronic skin is necessary for accurate sensing and human health monitoring in aquatic environments. Despite achieving significant progress in this field, it remains a great challenge to design skin-interfaced conductive hydrogels with high electrical conductivity, stablility, and seamless underwater adhesion to skin. Herein, a skin-inspired conductive multifunctional hydrogel is proposed, which has a wet-adhesive/hydrophilic and a non-adhesive/hydrophobic bilayer structure. The hydrogel shows high stretchability (∼2400%) and an ultra-low modulus (4.5 kPa), which facilitate the conformal and seamless attachment of the hydrogel to the skin with reduced motion artifacts. Owing to synergistic physical and chemical interactions, this hydrogel can achieve reliable underwater adhesion and display remarkable underwater adhesion strength (388.1 kPa) to porcine skin. In addition, MXene has been employed to obtain high electrical conductivity, create a route for stable electron transport, and reinforce mechanical properties. The hydrogel also possesses self-healing ability, a low swelling ratio (∼3.8%), biocompatibility, and specific adhesion to biological tissues in water. Facilitated with these advantages, the hydrogel-based electrodes achieve reliable electrophysiological signal detection in both air and wet conditions and demonstrate a higher signal-to-noise ratio (28.3 dB) than that of commercial Ag/AgCl gel electrodes (18.5 dB). Also, the hydrogel can be utilized as a strain sensor with high sensitivity for underwater communication. This multifunctional hydrogel improves the stability of the skin-hydrogel interface in aquatic environments and is expected to be promising for the next-generation bio-integrated electronics.

Flexible Electrodes as a Measuring System of Electrical Impedance Imaging

Electrical Impedance Tomography (EIT) is a detection imaging technology developed 30 years ago. When the conventional EIT measurement system is used, the electrode and the excitation measurement terminal are connected with a long wire, which is easily affected by external interference, and the measurement result is unstable. In this paper, we developed a flexible electrode device based on flexible electronics technology, which can be softly attached to the skin surface for real-time physiological monitoring. The flexible equipment includes an excitation measuring circuit and electrode, which eliminates the adverse effects of connecting long wires and improves the effectiveness of measuring signals. At the same time, the design also uses flexible electronic technology to make the system structure achieve ultra-low modulus and high tensile strength so that the electronic equipment has soft mechanical properties. Experiments have shown that when the flexible electrode is deformed, its function is completely unaffected, the measurement results remain stable, and the static and fatigue performances are satisfactory. The flexible electrode has high system accuracy and good anti-interference.

Quasi‐Linear Superelasticity with Ultralow Modulus in Tensile Cyclic Deformed TiNi Strain Glass

Herein, a quasi‐linear superelasticity (≈2.3%) with the ultralow elastic modulus (≈18 GPa) over a wide temperature range (≈55 K) in tensile cyclic deformed TiNi shape memory alloys is reported. Differential scanning analysis (DSC) and dynamic mechanical analysis (DMA) show the suppression of normal martensitic transformation (MT) and the appearance of strain glass transition behavior. Electron backscatter diffraction (EBSD) analysis shows the existence of high‐density defects involving twin boundaries and dislocations under tensile cyclic deformation. Further transmission electron microscope (TEM) observation reveals stabilized B19' nanodomains after tensile cyclic deformation. The observed deformation defects and stabilized B19' nanodomains impose significant influence on the following transition behavior and mechanical property, yielding the detected quasi‐linear superelasticity with ultralow modulus. Herein, an effective way to design unique phase transitions with unprecedented properties by defect engineering may be suggested.

Linear-superelastic Ti-Nb nanocomposite alloys with ultralow modulus via high-throughput phase-field design and machine learning

The optimal design of shape memory alloys (SMAs) with specific properties is crucial for the innovative application in advanced technologies. Herein, inspired by the recently proposed design concept of concentration modulation, we explore martensitic transformation (MT) in and design the mechanical properties of Ti-Nb nanocomposites by combining high-throughput phase-field simulations and machine learning (ML) approaches. Systematic phase-field simulations generate data of the mechanical properties for various nanocomposites constructed by four macroscopic degrees of freedom. An ML-assisted strategy is adopted to perform multiobjective optimization of the mechanical properties, through which promising nanocomposite configurations are prescreened for the next set of phase-field simulations. The ML-guided simulations discover an optimized nanocomposite, composed of Nb-rich matrix and Nb-lean nanofillers, that exhibits a combination of mechanical properties, including ultralow modulus, linear super-elasticity, and near-hysteresis-free in a loading-unloading cycle. The exceptional mechanical properties in the nanocomposite originate from optimized continuous MT rather than a sharp first-order transition, which is common in typical SMAs. This work demonstrates the great potential of ML-guided phase-field simulations in the design of advanced materials with extraordinary properties.

Effect of N addition on nano-domain structure and mechanical properties of a meta-stable Ti-Zr based alloy

This study showed that a superior combination of high strength and ultra-low modulus can be achieved by the addition of N in a meta-stable β-type Ti–Zr based alloy undergoing stress-induced martensitic transformation. A Ti–18Zr–3Nb–2Mo–3Sn alloy exhibited a very low Young's modulus; however, the stress-induced martensitic transformation occurred at a low stress level. The addition of N facilitated the formation of nano-sized lattice modulation (nano-domain) structure and impeded the stress-induced martensitic transformation, leading to higher apparent yield stress, narrower stress hysteresis and large non-linear elasticity. A Ti–18Zr–3Nb–2Mo–3Sn–1.2N alloy exhibited an excellent combination of a low Young’s modulus of 41 GPa and a high strength of 920 MP along with a large recovery strain with negligible hysteresis.

Shuffle-nanodomain regulated strain glass transition in Ti-24Nb-4Zr-8Sn alloy

The unprecedented properties of multi-functional metastable β-Ti alloys, including superelasticity over a wide temperature range, ultra-low modulus, and Invar and Elinvar anomalies, have attracted a great deal of attention. Persistent research efforts have been made towards the understanding of the origins of these unique properties. In this article we report a novel shuffle-nanodomain regulated strain glass transition in a metastable β-Ti alloy, Ti-24Nb-4Zr-8Sn (wt.%, Ti2448), which could be the dominant transformation pathway that offers these unique properties. Using the ex-situ aberration-corrected scanning transmission electron microscopy and in-situ cooling transmission electron microscopy, we find that randomly distributed {011}〈01¯1〉β O′ phase (orthorhombic, shuffle only) nanodomains embedded in the β phase (BCC) matrix at room temperature transform to α″ phase (orthorhombic) with a continuous increase in the amount of {21¯1}〈1¯1¯1〉β shear upon cooling or loading. Crystallographic analysis shows that the shuffle of the O′ phase will restrain the twelve possible shears that transform a BCC lattice to α″ martensite to only two. Thus, the randomly distributed O′ nanodomains prevent the formation of long-range-ordered, self-accommodating transformation-strain domain patterns seen in normal martensitic transformations and suppress completely the sharp first-order, auto-catalytic and avalanche-like martensitic transformation into a high-order-like (continuous) strain glass transition. Such a continuous β → O′ → α″ strain glass transition has been confirmed by dynamic mechanical analysis, resistivity and differential scanning calorimetric measurement. This unique transition pathway allows us to offer new insights into the unique properties found in this alloy.

Linear-superelastic metals by controlled strain release via nanoscale concentration-gradient engineering

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4–7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to ∼13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (∼4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress–strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.

Hydraulic fracture geometry in ultrasoft polymer networks

Determination of failure properties becomes extremely challenging when materials approach very low moduli ( $$< 100$$ kPa) and toughness values. New techniques are emerging to address these challenges in order to facilitate advancement of accurate tissue phantoms and medical diagnostics. We report on needle-induced ‘cavitations’ within a series of silicone elastomer formulations to demonstrate the ability of hydraulically-induced fracture morphology to differentiate fracture behavior in ultrasoft solids. We show, in accordance with previous results by Lefevre et al. (Int J Fract 192:1–23. https://doi.org/10.1007/s10704-014-9982-0 . ISSN 15732673, 2015), that cavities first grow elastically and then transition to a penny-shape with an associated ring crack. Further growth occurs predominantly at the ring crack, leading to variations in cavity anisotropy quantified via the cavity’s sphericity. Sphericity differentiates between materials of identical, ultralow modulus, but distinct formulation. We find that materials for which modulus is lowered by decreasing the crosslinker to pre-polymer ratio are tougher than materials in which modulus is lowered through the addition of unreactive silicone oil. We conclude that cavity shape provides a promising avenue for fracture energy determination and that the ultrasoft silicone/diluted-silicone system studied here provides a material platform for altering fracture properties while holding modulus constant.

Study of knitted fabrics with ultra-low modulus based on geometrical deformation mechanism

In some cases, the value of clothing pressure on the human body is more than desired which results in discomfort, especially for those designed for infants, aged people and professionals. In order to reduce the pressure so that it is at a comfortable level, this investigation developed a knitted fabric with ultra-low modulus based on the geometrical deformation mechanism. Experimental results show that the tensile deformation is due to the geometrical deformation and physical deformation. This paper focuses on the former, which is greatly improved by reknitting of the unroved yarn with remarkable crimps. The tensile results indicate that compared with the normally rib-knitted fabric, which the elongation of the reknitted fabric reaches at 50%, the tensile modulus decreases by 96%. It is proved that increasing the geometrical deformation is an effective method for reducing the elastic modulus. In addition, the pressure of men's cotton socks and ladies' stocking and tights on skin is measured. Experimental results show that the value of the pressure is as high as 2.959 kPa, which far exceeds the comfort pressure threshold of 0.667 kPa. Furthermore, this paper also studies the application of this developed reknitted fabric. The study of geometrical deformation mechanism-based ultra-low-modulus fabrics with knitting of the yarn with remarkable crimps has important significance, and provides scientific guidance for improving clothing pressure.

Transparent and Waterproof Ionic Liquid-Based Fibers for Highly Durable Multifunctional Sensors and Strain-Insensitive Stretchable Conductors.

Ionic liquids (ILs) are regarded as ideal components in the next generation of strain sensors because their ultralow modulus can commendably circumvent or manage the mechanical mismatch in traditional strain sensors. In addition to strain sensors, stretchable conductors with a strain-insensitive conductance are also indispensable in artificial systems for connecting and transporting electrons, similar to the function of blood vessels in the human body. In this work, two types of ILs-based conductive fibers were fabricated by developing hollow fibers with specific microscale channels, which were then filled with ILs. Typically, the ILs-based fiber with straight microchannels exhibited a high strain sensitivity and simultaneously rapid responses to strain, pressure, and temperature. The other ILs-based fiber with helical microchannels exhibited a good strain-isolate conductance under strain. Due to the high transparency of ILs along with the sealing process, the as-prepared ILs-based fibers are both highly transparent and waterproof. More importantly, owing to the low modulus of ILs and the core-shell structure, both conductive fiber prototypes demonstrated a high durability (>10 000 times) and a long-term stability (>4 months). Ultimately, the ILs-based fibrous sensors were successfully woven into gloves, flaunting the ability to detect human breathing patterns, sign language, hand gestures, and arm motions. The ILs-based strain-insensitive fibers were successfully applied in stretchable wires as well.

Soft, stretchable, epidermal sensor with integrated electronics and photochemistry for measuring personal UV exposures.

Excessive ultraviolet (UV) radiation induces acute and chronic effects on the skin, eye and immune system. Personalized monitoring of UV radiation is thus paramount to measure the extent of personal sun exposure, which could vary with environment, lifestyle, and sunscreen use. Here, we demonstrate an ultralow modulus, stretchable, skin-mounted UV patch that measures personal UV doses. The patch contains functional layers of ultrathin stretchable electronics and a photosensitive patterned dye that reacts to UV radiation. Color changes in the photosensitive dyes correspond to UV radiation intensity and are analyzed with a smartphone camera. A software application has feature recognition, lighting condition correction, and quantification algorithms that detect and quantify changes in color. These color changes are then correlated with corresponding shifts in UV dose, and compared to existing UV dose risk levels. The soft mechanics of the UV patch allow for multi-day wear in the presence of sunscreen and water. Two evaluation studies serve to demonstrate the utility of the UV patch during daily activities with and without sunscreen application.

Soft Elastomers with Ionic Liquid-Filled Cavities as Strain Isolating Substrates for Wearable Electronics.

Managing the mechanical mismatch between hard semiconductor components and soft biological tissues represents a key challenge in the development of advanced forms of wearable electronic devices. An ultralow modulus material or a liquid that surrounds the electronics and resides in a thin elastomeric shell provides a strain-isolation effect that enhances not only the wearability but also the range of stretchability in suitably designed devices. The results presented here build on these concepts by (1) replacing traditional liquids explored in the past, which have some nonnegligible vapor pressure and finite permeability through the encapsulating elastomers, with ionic liquids to eliminate any possibility for leakage or evaporation, and (2) positioning the liquid between the electronics and the skin, within an enclosed, elastomeric microfluidic space, but not in direct contact with the active elements of the system, to avoid any negative consequences on electronic performance. Combined experimental and theoretical results establish the strain-isolating effects of this system, and the considerations that dictate mechanical collapse of the fluid-filled cavity. Examples in skin-mounted wearable include wireless sensors for measuring temperature and wired systems for recording mechano-acoustic responses.

Resistive behaviour of silver nanoparticle films on ultra-low modulus polydimethylsiloxane with trench type roughness

The resistive behaviour of silver (Ag) nanoparticle based thin films at different loading conditions (axial stretching, bending and repeated bending) has been studied on rough ultra-low modulus polydimethylsiloxane substrates with scratch or trench type roughness. These films have been fabricated using a non-vacuum and scalable thin film deposition technique, rod coating. The fabricated films showed minute changes in their resistance when stretched up to an average of almost 27% axial strain, and when bent on circular surfaces with diameters in the range 10–30 mm. The films were also found to remain conductive even after 1000 bending cycles. These results suggest that the as-fabricated silver nanoparticle films could be potentially useful in areas like stretchable, flexible and epidermal electronics.

Fabric-based stretchable electronics with mechanically optimized designs and prestrained composite substrates

A mechanically rugged form of stretchable electronics can be achieved through integration of functional materials and devices with composite substrates consisting of an ultralow modulus silicone adhesive layer on a strain-limiting fabric framework. The resulting system is sufficiently soft to enable extreme levels of deformation and non-invasive use on the skin, yet sufficiently robust for repetitive application/detachment. This letter introduces theoretical and experimental studies of mechanical designs, with optimization for a representative island-bridge device configuration to yield high levels of elastic stretchability. The physics of prestrain conversion and its role in enhancing the stretchability are systematically explored.

Rugged and breathable forms of stretchable electronics with adherent composite substrates for transcutaneous monitoring.

Research in stretchable electronics involves fundamental scientific topics relevant to applications with importance in human healthcare. Despite significant progress in active components, routes to mechanically robust construction are lacking. Here, we introduce materials and composite designs for thin, breathable, soft electronics that can adhere strongly to the skin, with the ability to be applied and removed hundreds of times without damaging the devices or the skin, even in regions with substantial topography and coverage of hair. The approach combines thin, ultralow modulus, cellular silicone materials with elastic, strain-limiting fabrics, to yield a compliant but rugged platform for stretchable electronics. Theoretical and experimental studies highlight the mechanics of adhesion and elastic deformation. Demonstrations include cutaneous optical, electrical and radio frequency sensors for measuring hydration state, electrophysiological activity, pulse and cerebral oximetry. Multipoint monitoring of a subject in an advanced driving simulator provides a practical example.