Stretch-Induced Conductive Hydrogels: Enhancing Mechanical Robustness and Positive Piezoconductivity for Next-Generation Electronics

The development of hydrogels for use in flexible electronics, biomedical devices, and soft robotics has gained significant attention because of their excellent biocompatibility, flexibility, and conductivity. However, still a major challenge faced by hydrogels is their tendency to have increased resistance and reduced conductivity when subjected to tensile strain which can limit their utility in stable and reliable electronic devices. Previously, engineers tried traditional strategies such as incorporating conductive fillers like metal particles or carbon-based materials which showed limited improvement but resulted in reduced flexibility and mechanical strength. Moreover, the inability of rigid fillers to deform with the hydrogel matrix further limits their potential for stable, stretchable electronics. To address these challenges, recent research published in Advanced Materials Journal and conducted by Xiaowei Wang, Sijie Zheng, Jiaofeng Xiong, Ziyang Liu, Qingning Li, Weizheng Li, and led by Professor Feng Yan from the School of Chemistry and Chemical Engineering at Soochow University, the researchers developed a novel hydrogel composite made from poly(vinyl alcohol) (PVA), silver nanowires (AgNWs), and liquid metal (LM) that can maintain high conductivity while offering superior mechanical toughness and stretchability. The research team started by synthesizing the PVA-AgNWs-LM (PAL) hydrogels with the focus on the preparation of highly conductive and tough materials. LM particles were sonicated to break them into submicrometer sizes and then mixed with AgNWs in a PVA solution. This precursor solution was cast into molds and left to evaporate to create a hydrogel with well-dispersed LM and AgNWs. The researchers successfully demonstrated that the LM and AgNWs were evenly distributed within the hydrogel which were essential to ensure the material’s conductivity and mechanical integrity and prevented agglomeration or leakage that could compromise the hydrogel’s functionality. The authors tested the mechanical properties of the hydrogels under different strain levels to examine their fracture stress, strain capacity, and toughness and them to have significant improvements with the ultimate fracture strain reaching up to 5300%, and high toughness values. The authors believe the high impressive toughness is because of the interaction between the LM particles and PVA where hydrogen bonds and coordination bonds were formed. These interactions not only reinforced the structure but also allowed the LM particles to deform in response to strain and enabled the hydrogel to remain intact and resist cracking under extreme mechanical stress. Moreover, the authors demonstrated that with optimizing the ratio of LM to PVA, they can finely tune and control the hydrogel’s mechanical strength making it adaptable for various applications.

We think one of the most critical findings in the paper was the hydrogels’ conductivity behavior under strain. The research team’s Initial tests showed that PAL hydrogels had a baseline conductivity of 4.05 × 10⁻³ S m⁻¹, which increased dramatically as the hydrogels were stretched. When subjected to strains of up to 4200%, the conductivity increased by a factor of 6000 and reached a staggering 24 S m⁻¹. This extraordinary enhancement was linked to the stretch-induced orientation of both the LM particles and the AgNWs. As the hydrogel was stretched, these conductive components aligned along the direction of the strain and formed continuous conductive pathways that improved electrical performance. According to the authors, such stretch-induced positive piezoconductivity was a unique feature that set the PAL hydrogels apart from conventional materials which typically experience a drop in conductivity when stretched. To further understand the structural changes occurring during stretching, the researchers conducted in depth small-angle and wide-angle X-ray scattering (SAXS and WAXS) experiments and showed that the PVA nanocrystals within the hydrogel also became oriented during stretching which contributed to the material’s mechanical robustness. The SAXS and WAXS data also indicated that the spacing between neighboring PVA nanocrystals decreased as the strain increased which suggested that the crystalline regions were becoming more tightly packed. This structural reorganization provided improved mechanical properties for the hydrogel while in the same time for the alignment of the LM particles and AgNWs, which was crucial for maintaining high conductivity.

It is worth mentioning that the recyclability of the PAL hydrogels has also been evaluated. Since the hydrogels were crosslinked through physical interactions rather than irreversible covalent bonds, they could be recycled by dissolving the damaged material in water and reforming it. The hydrogels were placed in water at 95°C for one hour, the LM, AgNWs, and PVA segments were re-dispersed. When water was evaporated, the recycled hydrogels retained nearly all their original mechanical and conductive properties. In another experiment, Professor Feng Yan and colleagues compared the performance of their PAL hydrogels to that of ionic conductive hydrogels. The PAL hydrogels maintained stable conductivity even when stretched by 500%, whereas the ionic hydrogels showed a sharp increase in resistance and caused a light bulb connected to the circuit to dim and highlighted the PAL hydrogels’ superior ability to maintain conductive pathways under strain which is a major advantage for applications in flexible electronics which need consistent performance. Additionally, the PAL hydrogels demonstrated minimal changes in resistance during cyclic loading even after 1000 cycles of 100% strain which showed their potential for use in sensors and other devices that experience repeated mechanical deformation. Furthermore, the researchers also used optical microscopy and finite element analysis to visualize the orientation of LM particles during stretching and found that as the strain increased, the LM particles elongated and aligned with the strain direction which contributed to the formation of conductive pathways. Finite element simulations supported as well these observations and showed a decrease in resistance as the hydrogel was stretched which agreed with the experimental data.

In conclusion, Professor Feng Yan and colleagues from Soochow University successfully developed an excellent material that retained both a high mechanical toughness under extreme strain with a 6000-fold increase in conductivity when stretched. We believe this positive piezoconductivity achieved in the new study is a rare and valuable property which can offer a new direction for the development of stretchable electronics, wearable devices, and soft robotics. The ability to maintain, and even enhance, conductivity under strain is critical for real-world applications where materials are subject to continuous deformation, such as in biomedical sensors and flexible circuits. These innovative PAL hydrogels can be the potential solution for creating next-generation electronic devices that require both flexibility and reliable performance and possibly revolutionize wearable electronics and enable better robust and durable sensors that can withstand repeated mechanical stresses without losing functionality. Additionally, the demonstrated ability to reprocess and reuse these hydrogels without loss of performance is especially exciting and significant for industries that want to adopt more eco-friendly practices. Furthermore, it is possible to integrate these new hydrogels in advanced biomedical applications to make excellent implantable sensors and soft robotics for medical procedures.