The development of strong, self-healable, and stretchable materials is highly desirable for applications in electronic skin, wearable devices, and artificial muscles. Self-healing in polymers is achieved through reversible dynamic bonds in cross-linked polymer chains, extending service life and improving device reliability. However, materials relying on single noncovalent bonds often suffer from low strength, particularly hydrogels and elastomers (strength usually <3.0 MPa). Creating flexible composites with high stretchability, outstanding mechanical strength, and high self-healing ability simultaneously remains a significant challenge. Most self-healing materials have tensile strength limited to <10.0 MPa due to the inherent weakness of noncovalent bonds and weak interfacial interactions between fillers and the polymer matrix. Strategies involving multiple dynamic bonds or a combination of reversible noncovalent and permanent covalent cross-links have been explored, but mechanical strength often remains insufficient for structural materials. Natural cartilage tissue, with its high mechanical strength and self-healing capabilities, serves as a compelling bio-inspired model. Its hierarchical structure, composed of collagen fibers interconnected by hydrogen bonds, provides mechanical strength and toughness. Previous attempts to mimic this structure using woven fiber networks or hard plastic skeletons embedded in a soft matrix achieved high mechanical properties but often lacked high stretchability. Two-dimensional (2D) tungsten disulfide (WS2) nanomaterials offer excellent properties but their high rigidity and poor interfacial interaction with elastomer matrices hinder their use in flexible devices. This research presents a cartilage-inspired microscale/nanoscale assembly method to fabricate ultrarobust self-healing materials based on the noncovalent bonding-driven self-assembly of 2D nanosheets into an interwoven network. The aim is to create a composite with significantly improved mechanical properties and self-healing efficiency.

The existing literature highlights the challenges in creating self-healing materials that combine high strength, stretchability, and self-healing ability. Many studies focus on single noncovalent bonds, leading to materials with low mechanical strength, typically below 3 MPa. Others have attempted to improve mechanical properties by introducing multiple dynamic bonds or combining noncovalent and covalent bonds, achieving higher strengths but often compromising other desirable properties like stretchability and self-healing efficiency. Bio-inspired approaches, mimicking the structure of natural materials like cartilage, offer a promising avenue. While previous attempts have shown success in creating high-strength composites, achieving high stretchability simultaneously has been problematic. The use of 2D nanomaterials like WS2 presents another promising direction, but effective incorporation into polymer matrices to create self-healing composites has been limited.

The researchers employed a tannic acid (TA)-assisted exfoliation strategy to obtain WS2 nanosheets. Tannic acid, with its polyhydroxy structure, facilitates exfoliation through hydrophobic interactions with WS2 and subsequent adsorption onto the nanosheets' surface. This modification introduces multiple hydrogen-bonding sites, enabling the controlled assembly of WS2 nanosheets within a waterborne polyurethane (PU) matrix. The TA-modified WS2 nanosheets are incorporated into the PU matrix, creating a cartilage-like interwoven network structure. The high density of hydrogen bonds at the interface between the TA-modified WS2 and the PU matrix is key to the material's properties. The morphology of the resulting nanocomposites was characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and thermogravimetric analysis (TGA). The formation of hydrogen bonds was confirmed by temperature-dependent Fourier transform infrared (FTIR) spectroscopy. Molecular dynamics simulations were used to quantify the binding energy between the PU and the TA-modified WS2 nanosheets. Mechanical testing was performed using an Instron 5560 machine to determine tensile strength, toughness, elongation at break, and self-healing efficiency. Small-angle X-ray scattering (SAXS) was used to investigate the nanostructure's behavior under stretching. The self-healing ability was assessed by cutting the samples, allowing them to heal at room temperature, and then performing tensile tests again. To demonstrate the functional self-healing ability, near-infrared (NIR) actuators were fabricated using the nanocomposite. The actuating performance and self-healing of the actuators were evaluated by measuring bending angles and actuation stress under NIR light irradiation. The fabrication of actuators involved creating bilayer films with cellulose nanofibers (CNF) and the TA-WS2/PU nanocomposite.

The resulting cartilage-inspired nanocomposite material showed remarkably enhanced mechanical properties and self-healing capabilities compared to the pristine PU and other existing self-healing materials. The 16 wt% TA-WS2/PU composite exhibited a tensile strength of 52.3 MPa, toughness of 282.7 MJ m⁻³, and elongation at break of 1020.8%. These values are significantly higher than those of the pristine PU and comparable materials. The self-healing efficiency reached 80.6% for the 16 wt% composite and 105.1% for the 8 wt% composite. SAXS analysis revealed that the interwoven WS2 network oriented along the stress direction during stretching, contributing to the material's high strength and toughness. The study also demonstrated the effective self-healing of the material's functionality, using NIR-actuated devices. The fabricated actuators showed rapid and repeatable actuation with bending angles up to 137° and actuation stress up to ~6.9 kPa. Even after damage and self-healing, the actuators retained their original actuation speed and amplitude, showcasing the functional self-healing capability of the material. The high toughness of the nanocomposite (282.7 MJ m⁻³) is notably higher than that of spider silk (177 MJ m⁻³) and various plastics, including polyether-ether-ketone, polyamide, and high-density polyethylene.

The findings demonstrate a successful strategy for developing ultrarobust self-healing materials by combining bio-inspired structural design with advanced nanomaterials. The cartilage-inspired interwoven network of TA-modified WS2 nanosheets within a PU matrix leads to significantly improved mechanical properties and self-healing efficiency. The high-density hydrogen bonding at the interface plays a crucial role in both the material's strength and self-healing ability, enabling the reformation of bonds after damage. The demonstration of functional self-healing in NIR actuators further highlights the material's potential for applications in flexible electronics and soft robotics. The superior mechanical properties, especially toughness, exceed those of most existing self-healing polymers and several engineering plastics, suggesting broad applicability in structural materials.

This research successfully demonstrates the fabrication of an ultrarobust, tough, and highly stretchable self-healing material using a cartilage-inspired design. The material's exceptional mechanical properties and self-healing capabilities, along with its demonstrated functional self-healing in NIR actuators, highlight its significant potential for flexible electronics, soft robotics, and other applications requiring high performance and durability. Future research could explore the material's performance under diverse environmental conditions and investigate its potential for biointegration and biomedical applications.

While the study demonstrates excellent self-healing properties, further investigation is needed to fully understand the long-term durability and stability of the material under continuous cyclic loading and various environmental conditions. The scalability of the fabrication process for large-scale applications also needs to be addressed. The current study focused on specific applications like NIR actuators; further exploration of the material's applicability in a broader range of devices is necessary.