Material properties gradually degrade under cyclic loading, leading to catastrophic failure. It results in large costs for inspection, maintenance, and downtime. Besides, materials require combinations of performance such as load bearing and energy dissipation. However, improving one performance of a material often sacrifices another performance, making it difficult to create materials with optimal performance profiles. Here we report a liquid-infused porous piezoelectric scaffold (LIPPS) that simultaneously enhances its load-bearing and energy dissipation capability under cyclic loading. For example, after 12 million loading cycles, LIPPS increases its modulus by 3600% and hysteresis by 3000%. From a CT study, this behavior is attributed to the self-recoverable mineralization under mechanical loading. Moreover, LIPPS shows a reprogrammable stiffness distribution based on the loading distribution, which enables the material to generate multiple shapes by self-folding. Our findings can contribute toward unprecedented opportunities in soft robotics, vehicles, infrastructure, and tissue engineering and contribute to the new paradigm of material selection with improved resilience and sustainability.

Controlling the growth of material is crucial in material processing for desired properties. Current approaches often involve sophisticated equipment for controlling precursors and monitoring material formation. Here we report a self-limiting material growth mechanism controlled by the experienced mechanical loading without the need for precise control over precursors or monitoring material growth. Material formation that reduces the driving force for growth is hypothesized to result in a saturation thickness that is dependent on the maximum driving force. Analytical relations based on the growth model are derived and verified using a piezoelectric substrate immersed in an electrolyte solution under fixed frequency cyclic loading to attract surrounding mineral ions to form mineral layers. Accumulating mineral layers decrease the driving force for further growth and the material eventually reaches a saturation thickness. This allows for loading force to control the saturation thickness of the self-limiting material growth. Experimental data supports the predicted exponential relations, offering guides to predict the saturation thickness and control the growth profile. The findings are envisioned to contribute to the fundamental understanding of the self-limiting material growth mechanism and could benefit a range of applications including coatings for orthopedic implants as well as marine surface and underwater vehicles. We report a self-limiting growth mechanism controllable by force based on an analytical model that is verfied experimentally using a piezoelectric substrate immersed in an electrolyte solution under cyclic loading.

The natural world contains a diverse range of solutions that allows for living organisms to dynamically adapt their structure and mechanical properties to meet environmental demands. For example, coral reef is able to accumulate reinforcing calcium carbonate from wave agitation and water current that stabilizes gaps in the structure and increases the reef density and strength through diagenetic reef cementation. Bone responds to repeated stress by translating deformations and fluid movement in the bone matrix into cellular signals that trigger bone formation through mechanotransduction. Utilizing these mechanisms as inspiration, synthetic materials have been developed that utilize stress-generated piezoelectric charges to attract mineral ions to form reinforcing mineral layers that can repair defects and damage over time and extend material lifetime. In this article, we examine natural adaptive processes that give inspiration for new synthetic materials with similar dynamic adaptive properties. We also introduce the capabilities of existing bioinspired synthetic materials, current challenges these systems face, potential application areas of this technology, and future research opportunities of these adaptive materials.

Conventional synthetic materials have fixed mechanical properties and suffer defects, damage, and degradation over time. This makes them unable to adapt to changing environments and leads to limited lifecycles. Recently, self-adaptive materials inspired by natural materials have emerged as a solution to address these problems. With the ability to change their mechanical properties based on changing mechanical environments, repairing defects, and maintaining their mechanical properties, these materials can lead to improved performance while decreasing waste. In this review, we explore self-adaptive phenomena found in nature that have inspired the development of synthetic self-adaptive materials, and the mechanisms that have been employed to create the next generation of materials. The potential applications of these materials, the challenges that existing approaches face, and future research opportunities are also discussed.

Inspired by the mineralization process of bone, we have investigated mineralization on piezoelectric samples immersed in a solution with mineral ions. We have utilized polyvinylidene fluoride as a piezoelectric material and 10× simulated body fluid as a mineral solution. Three synthetic material systems were developed and characterized using scanning electron microscopy, X-ray diffraction, nanoindentation, and scratch testing. With these techniques, we provide insights into how the characteristics of the mineralization protocol affect the microstructure, chemical composition, crystal structure, and mechanical properties of the minerals. Increasing the solution temperature from 25 to 50 °C resulted in a greater packing density, roughly 10 times the stiffness and 4 times the fracture toughness. Collagen surface treatment resulted in roughly 7 times the stiffness along with potential anisotropy in the fracture toughness. Lastly, calcium phosphate minerals appear to pack in low-density and high-density phases on the piezoelectric scaffolds.

A material with asymmetric mechanical responses offers diverse potential applications.

Continuous technological advances resulted in an ever-growing need for novel materials. In conventional engineering materials, cyclic loadings weaken materials by initiating cracks or degrading materials, which leads to premature failure. Contrarily, natural materials such as bones address the issue elegantly. Stress is utilized as a stimulus for material synthesis to strengthen the material. The salient difference between these natural material systems and their synthetic counterparts is their adaptive behavior. Adaptive materials can self-tune their properties in response to stimuli from the environment. Recently, growing attention has been directed towards adaptive materials owing to their autonomous “smart” behavior and multifunctionality. The goal of this article is to discuss the recent efforts in adaptive materials with a focus on mechanically adaptive materials based on piezoelectricity. The rationale behind these material systems is explained as well as an overview of different material systems and their implications. Lastly, we will discuss the challenges to overcome and prospects for future studies.

Robots with submillimeter dimensions are of interest for applications that range from tools for minimally invasive surgical procedures in clinical medicine to vehicles for manipulating cells/tissues in biology research. The limited classes of structures and materials that can be used in such robots, however, create challenges in achieving desired performance parameters and modes of operation. Here, we introduce approaches in manufacturing and actuation that address these constraints to enable untethered, terrestrial robots with complex, three-dimensional (3D) geometries and heterogeneous material construction. The manufacturing procedure exploits controlled mechanical buckling to create 3D multimaterial structures in layouts that range from arrays of filaments and origami constructs to biomimetic configurations and others. A balance of forces associated with a one-way shape memory alloy and the elastic resilience of an encapsulating shell provides the basis for reversible deformations of these structures. Modes of locomotion and manipulation span from bending, twisting, and expansion upon global heating to linear/curvilinear crawling, walking, turning, and jumping upon laser-induced local thermal actuation. Photonic structures such as retroreflectors and colorimetric sensing materials support simple forms of wireless monitoring and localization. These collective advances in materials, manufacturing, actuation, and sensing add to a growing body of capabilities in this emerging field of technology.

Microscale programmable shape-memory actuators based on reversible electrochemical reactions can provide exciting opportunities for microrobotics.

The recent advances in wearable electronics and intelligent human-machine interface systems have garnered great interests in electromechanical sensors, which can measure and quantify physical stimuli. Among different types of electromechanical sensors, piezoresistive sensors have been extensively investigated due to the excellent sensitivity, simple construction, and durability. Especially, there have been remarkable developments of flexible and stretchable piezoresistive sensors for wearable devices by investigating novel material/structural strategies to obtain highly sensitive piezoresistive sensors with skin-like flexibility. Here, we give a comprehensive overview of the recent progress in flexible and stretchable piezoresistive sensors and their applications. Based on the material composition and structural characteristics, the piezoresistive sensors are categorized into three types—conductive polymeric composite, porous conductive material, and architected conductive material. Subsequently, we have summarized their transduction mechanisms, fabrication processes, sensing performances, and applications. Finally, we have discussed current challenges and future opportunities for piezoresistive sensors.

The rigidity and relatively primitive modes of operation of catheters equipped with sensing or actuation elements impede their conformal contact with soft-tissue surfaces, limit the scope of their uses, lengthen surgical times and increase the need for advanced surgical skills. Here, we report materials, device designs and fabrication approaches for integrating advanced electronic functionality with catheters for minimally invasive forms of cardiac surgery. By using multiphysics modelling, plastic heart models and Langendorff animal and human hearts, we show that soft electronic arrays in multilayer configurations on endocardial balloon catheters can establish conformal contact with curved tissue surfaces, support high-density spatiotemporal mapping of temperature, pressure and electrophysiological parameters and allow for programmable electrical stimulation, radiofrequency ablation and irreversible electroporation. Integrating multimodal and multiplexing capabilities into minimally invasive surgical instruments may improve surgical performance and patient outcomes.

Mechanically guided, 3D assembly has attracted broad interests, owing to its compatibility with planar fabrication techniques and applicability to a diversity of geometries and length scales. Its further development requires the capability of on-demand reversible shape reconfigurations, desirable for many emerging applications (e.g., responsive metamaterials, soft robotics). Here, the design, fabrication, and modeling of soft electrothermal actuators based on laser-induced graphene (LIG) are reported and their applications in mechanically guided 3D assembly and human-soft actuators interaction are explored. Over 20 complex 3D architectures are fabricated, including reconfigurable structures that can reshape among three distinct geometries. Also, the structures capable of maintaining 3D shapes at room temperature without the need for any actuation are realized by fabricating LIG actuators at an elevated temperature. Finite element analysis can quantitatively capture key aspects that govern electrothermally controlled shape transformations, thereby providing a reliable tool for rapid design optimization. Furthermore, their applications are explored in human-soft actuators interaction, including elastic metamaterials with human gesture-controlled bandgap behaviors and soft robotic fingers which can measure electrocardiogram from humans in an on-demand fashion. Other demonstrations include artificial muscles, which can lift masses that are about 110 times of their weights and biomimetic frog tongues which can prey insects.

In addition to mechanical compliance, achieving the full potential of on-skin electronics needs the introduction of other features. For example, substantial progress has been achieved in creating biodegradable, self-healing, or breathable, on-skin electronics. However, the research of making on-skin electronics with passive-cooling capabilities, which can reduce energy consumption and improve user comfort, is still rare. Herein, we report the development of multifunctional on-skin electronics, which can passively cool human bodies without needing any energy consumption. This property is inherited from multiscale porous polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) supporting substrates. The multiscale pores of SEBS substrates, with characteristic sizes ranging from around 0.2 to 7 µm, can effectively backscatter sunlight to minimize heat absorption but are too small to reflect human-body midinfrared radiation to retain heat dissipation, thereby delivering around 6 °C cooling effects under a solar intensity of 840 W⋅m-2 Other desired properties, rooted in multiscale porous SEBS substrates, include high breathability and outstanding waterproofing. The proof-of-concept bioelectronic devices include electrophysiological sensors, temperature sensors, hydration sensors, pressure sensors, and electrical stimulators, which are made via spray printing of silver nanowires on multiscale porous SEBS substrates. The devices show comparable electrical performances with conventional, rigid, nonporous ones. Also, their applications in cuffless blood pressure measurement, interactive virtual reality, and human-machine interface are demonstrated. Notably, the enabled on-skin devices are dissolvable in several organic solvents and can be recycled to reduce electronic waste and manufacturing cost. Such on-skin electronics can serve as the basis for future multifunctional smart textiles with passive-cooling functionalities.

Three-dimensional (3D) cellular graphene structures have wide applications in energy storage, catalysis, polymer composites, electromagnetic shielding, and many others. However, the current strategies to form cellular graphene are only able to realize limited structure control and are hard to achieve the construction of 3D hierarchical architectures with complex, programmed configurations, limiting the design capabilities to satisfy various next-generation device applications. In addition, cellular graphene usually exhibits limited electromechanical properties, and its electrical and electrochemical performances are dramatically affected by mechanical deformations, constraining its applications in emerging wearable electronics and energy devices. Herein, we report a simple, general, and effective route to 3D hierarchical architectures of cellular graphene with desired geometries through the use of a mechanically guided, 3D assembly approach to overcome the aforementioned two challenges. Demonstrations include more than 10 3D hierarchical architectures with diverse configurations, ranging from mixed tables and tents, to double-floor helices, to kirigami/origami-inspired structures, and to fully separated multilayer architectures. The LED arrays interconnected with 3D helical coils and 3D interdigital supercapacitors fabricated with solid-state electrolytes provide prototypic examples of wearable devices that exhibit outstanding electromechanical properties and can maintain stable performances with little change in the electrical and electrochemical responses under extreme deformations, in both the static and cyclic loading conditions.

Soft on-skin electronics have broad applications in human healthcare, human-machine interface, robotics, and others. However, most current on-skin electronic devices are made of materials with limited gas permeability, which constrain perspiration evaporation, resulting in adverse physiological and psychological effects, limiting their long-term feasibility. In addition, the device fabrication process usually involves e-beam or photolithography, thin-film deposition, etching, and/or other complicated procedures, which are costly and time-consuming, constraining their practical applications. Here, a simple, general, and effective approach for making multifunctional on-skin electronics using porous materials with high-gas permeability, consisting of laser-patterned porous graphene as the sensing components and sugar-templated silicone elastomer sponges as the substrates, is reported. The prototype device examples include electrophysiological sensors, hydration sensors, temperature sensors, and joule-heating elements, showing signal qualities comparable to conventional, rigid, gas-impermeable devices. Moreover, the devices exhibit high water-vapor permeability (≈18 mg cm-2 h-1 ), ≈18 times higher than that of the silicone elastomers without pores, and also show high water-wicking rates after polydopamine treatment, up to 1 cm per 30 s, which is comparable to that of cotton. The on-skin devices with such attributes could facilitate perspiration transport and evaporation, and minimize discomfort and inflammation risks, thereby improving their long-term feasiblity.