Recent progress in low hysteresis gels: Strategies, applications, and challenges

Hydrogels are wet, soft materials characterized by a three-dimensional network-like porous structure, formed through the physical or chemical cross-linking of water molecules and polymers (such as polyacrylamide and polyvinyl alcohol) [1], [2], [3], [4], [5]. Due to their high water content, excellent biocompatibility, and adjustable mechanical properties, hydrogels are considered ideal candidates for the design and development of smart wearable devices. Their remarkable conformability allows them to adapt to the complex geometries of the human body, making hydrogels particularly suitable for use as flexible sensors that monitor various health conditions, including heart rate, pulse, oxygen saturation, and joint motion. These sensors operate by converting external stimuli into electrical signals [6], [7], [8], [9], [10], [11], [12], which has also shown great potential in human-computer interfaces, soft robots, and haptic sensing [13], [14], [15], [16], [17].

Low hysteresis in hydrogels refers to their ability to quickly return to their original state after being subjected to external stress or deformation, which is usually highly related to energy dissipation, the more energy dissipated during the cyclic process, the greater the hysteresis [18], [19], [20], resulting in the poor stability of the hydrogels during their service life. Low hysteresis hydrogels always display stable mechanical properties, high resilience, insensitivity to crack propagation and high fatigue resistance [21], [22], [23], [24]. Hence, it is often used in the fields that require high mechanical properties and frequent stress loadings, such as human joint movements, human-computer interaction, soft robotics and haptic perception areas [25], [26], [27]. However, there is always a trade-off between low hysteresis and high toughness of the designed hydrogels. For instance, to toughen the hydrogels, semi-interpenetrating network structures, interpenetrating network structures and double cross-linking have been proposed[28], [29], [30], [31]. Despite their effectiveness in toughening hydrogels, these approaches often result in non-homogeneous structures because of the difficulty of controlling the polymer chain lengths during crosslinking, which significantly increases the hysteresis. Additionally, the dense network structure also hinders the migration rate of ions or electrons, resulting in a significant decrease in the conductivity of the hydrogel and thus affecting the sensing performance of the hydrogel, which is a serious obstacle to the application of hydrogels [32], [33]. Hence, the key to achieving low hysteresis is to minimize the energy dissipation of the hydrogel during the cyclic deformed process and the rapid transfer of stresses to avoid stress concentration [34], [35], [36]. To address that, various mechanisms such as chain entanglement to accelerate stress transfer [37] and slidable cross-linking points to decrease internal friction [38] have been proposed, which greatly contributed to the development of low hysteresis hydrogels.

Additionally, low hysteresis hydrogels have also been used as self-powered devices in the field of triboelectric nanogenerators (TENGs) and new energy batteries as a result of their stabilized mechanical properties and excellent water-absorbing and water-retaining capabilities [39], [40], [41]. Conventional batteries, typically disposable or rechargeable chemical batteries, often rely on liquid electrolytes, which usually display several inherent drawbacks such as limited energy density, poor stability because of dendrite formation during charging and discharging, limited lifespan, potential environmental harm caused by poor degradability, and safety concerns related to potential electrolyte leakage [42], [43], [44], [45], [46]. In recent years, driven by national policies promoting the advancement of new energy technologies, hydrogels have gained significant attention in the field of energy storage contributed by their high safety, excellent biocompatibility, and environmental friendliness [47], [48]. This has led to the emergence of hydrogel-based energy systems, such as hydrogel-based triboelectric nanogenerators (H-TENGs), which use hydrogels as electrodes paired with triboelectric layers to harvest mechanical energies from sources like wind, waves, body motion, and even respiration, converting them into electrical energy [49]. Contributed by its unique working principle, H-TENGs have abundant applications in biomedical devices [50], Internet of things [51], artificial intelligence [52] and wearable devices [53]. However, despite their potential, H-TENGs are currently limited to powering micro-scale devices and are not yet capable of meeting the high power demands of larger devices. As researchers continue to explore hydrogels, scholars have developed hydrogel electrolytes and successfully prepared zinc ion gel batteries, which can greatly satisfy devices with a high power demand [54], [55]. Compared to liquid batteries, gel batteries offer better safety. Even if punctured, gel batteries can continue to provide power with minimal risk of explosion. Notably, numerous studies on hydrogel electrolytes have demonstrated their ability to inhibit dendrite formation, improve charging and discharging efficiency, and extend battery lifespan [56], [57], [58], [59]. As a result, gel electrolytes are anticipated to replace liquid electrolytes as a new standard for energy storage.

However, no comprehensive review has been published on low hysteresis hydrogels until now. Therefore, this review aimed to fill the gap by summarizing the latest strategies in achieving low hysteresis hydrogels. The latest strategies including: chain entanglement, phase separation, optimized molecular structure and designing slidable cross-linking points are summarized as shown in Fig. 1. First, this paper provides an overview of the design principles behind low hysteresis hydrogels, detailing the mechanisms by which low hysteresis is achieved and offering a comprehensive summary of their mechanical properties. Second, it explores the applications of low hysteresis hydrogels across various fields, including flexible sensors, triboelectric nanogenerators (TENGs), human-computer interaction (HCI), and new energy batteries, with a particular focus on the sensing performance of these hydrogels. Finally, the conclusion discusses the future development prospects of low hysteresis hydrogels and highlights the current limitations and challenges in the field. It is hoped that this review will offer valuable insights to researchers working on the development and application of low hysteresis hydrogels, guiding future innovations in this promising area.

Low hysteresis hydrogels (typically, hysteresis <10 %) are usually characterized by tensile cycling tests. In general, larger hysteresis loops represent higher energy dissipation, resulting in a high hysteresis. The following formula was used to calculate the hysteresis of the hydrogel [67]:$\Delta E={\int }_{Loading}\sigma d\epsilon -{\int }_{Unloading}\sigma d\epsilon$$H=\Delta E/{\int }_{Loading}\sigma d\epsilon$Where, $\Delta E$ is the energy dissipation, defined as the area enclosed by the loading-unloading.${\int }_{Loading}\sigma d\epsilon$ is the area enclosed by the loading curve with the X-axis.${\int }_{Unloading}\sigma d\epsilon$ is the area enclosed by the unloading curve with the X-axis. H is the hysteresis.