A Cellulose ionogel with rubber-like stretchability for low-grade heat harvesting

Ionogels, composed of cross-linked polymer networks and ionic pairs, are flexible with ionic conductivity, rendering them increasingly relevant in applications such as electronic skins (e-skins), flexible electronics, and human–machine interactions. Recently, the stretchability of ionogels has attracted considerable attention. This is attributed to its essential role in providing a seamless interface with biological tissues and enabling the detection of high-fidelity signals. However, most network molecules in stretched ionogels are sourced from petrochemical polymers, such as polyacrylamide, polydimethylsiloxane, and polyurethane. Environmental and sustainability issues related to non-biodegradable petrochemical polymers have underscored the urgent need to develop sustainable, stretchable ionogels using biomass macromolecules as the structural framework.

Inspired by the intrinsic coiled molecular structure of natural rubber, the groups led by Pro. Dawei Zhao from Shenyang University of Chemical Technology, Pro. Haipeng Yu from Northeast Forestry University, and Pro. Jianfei Zhou from Sichuan University developed an innovative strategy that partially replaced the -OH groups in the cellulose chains with -CH₂CH₂CN groups. This approach successfully achieved the design of coiled molecular configuration in cellulose, resulting in the construction of an ultra-stretchable S-ionogel (Figure 1). The developed S-ionogel exhibited excellent rubber-like stretchability, with a stretching strain exceeding 990%, which is over 10 times its original length. Notably, this stretchability characteristic of the S-ionogel was achieved without compromising mechanical strength.

The S-ionogel exhibited a tensile strain of up to 998.23%, comparable to the extensibility of polyacrylamide (PAAm) hydrogels, but its tensile strength was significantly higher than that of PAAm, exceeding that of PAAm hydrogels by more than 77 times. Additionally, the S-ionogel demonstrated excellent toughness (9.015 MJ^-3) and mechanical shock absorption, outperforming other stretchable polymer gels such as polyvinyl alcohol (PVA), polyacrylic acid (PAA), and PAAm. The S-ionogel also had high transparency, smoothness and elasticity similar to human skin, and excellent skin affinity. Even under significant joint bending and deformation, the S-ionogel can seamlessly conform to the wrist and fingers without any interface defects such as lifting or detachment, which was crucial for information fidelity transmission and perceptual recognition (Figure 2).

In-depth research on the stretchable molecular mechanism of the S-ionogel revealed that the introduction of the -CH₂CH₂CN group enhanced the flexibility and coiling deformation tendencies of the Cellulose-CY molecular chains. This unique chain coiling structure imparted excellent stretchability to the S-ionogel, allowing it to rival most petrochemical polymer gel materials. In the S-ionogel, the -OH groups energies between cellulose molecules, between cellulose and [Bmim]⁺, between cellulose and H₂O, as well as between [Bmim]⁺ and H₂O, were superior to those observed in Cel-gel. This enhanced -OH groups led to strong mechanical strength and excellent structural integrity during stretching. The S-ionogel exhibited a homogeneous molecular assembly structure, enabling its unique stretchability and robust mechanical properties to remain unaffected by stress orientation (Figure 3).

The stretchable S-ionogel designed through molecular configuration not only possessed excellent mechanical properties, self-healing capabilities, and skin affinity but also showed promising potential for efficiently converting low-grade thermal energy into high-value electrical energy. The cellulose-CY molecules in the S-ionogel engaged in differential hydrogen bonding interactions with [Bmim]⁺ and Cl⁻. This disparity in interaction strength established a gradient distribution of positive and negative charges within the S-ionogel when subjected to thermal stimulation, thereby enhancing the conversion of lowgrade thermal energy into electrical energy. The S-ionogel exhibited a stable open-circuit voltage of over 80 mV, indicating its excellent application prospects in self-powered and stretchable electronic skin devices. Additionally, the S-ionogel demonstrated outstanding biocompatibility and the potential for implantation in flexible devices (see Figure 4).

A meaningful and scalable molecular configuration design strategy by partially replacing the -OH groups in cellulose with -CH₂CH₂CN groups, resulting in an ionogel with ultra-high stretchability was presented. Through the modification of functional groups, the linear chain structure of cellulose was transformed into a coiled spatial architecture, imparting rubber-like super-stretchable characteristics to the ionogel. Furthermore, this ionogel exhibited a skin-like modulus of 63 kPa, a high ionic conductivity of 21.35 mS cm⁻¹, excellent biocompatibility, and thermoelectric properties superior to existing ionogels, with a Seebeck coefficient reaching 67.64 mV K⁻¹. This stretchable cellulose ionogel can directly convert human body heat into electrical energy, making it suitable for the development of self-powered, flexible, and stretchable electronic skin devices, with broad applications in human-machine interfaces, healthcare, and implantable electronics. The significance of this study lay in extending and validating the immense potential of molecular configuration design in the development of functional materials, while also elucidating the relationship between molecular structure replacement and the spatial structure-configuration pattern-performance design.