Stimulus-responsive materials can act as smart actuators that respond quickly to external stimuli such as light (Kondo, Yu, & Ikeda, 2006; Kwan, 2018; Liu, 2017; Yu, 2003), force (Wu, Wang, Zhang, & Wu, 2021), temperature (Hu et al., 2016), humidity (Dai et al., 2013; Harris, 2005; Islam, Li, Smyth, & Serpe, 2013; Shin et al., 2018; Singamaneni et al., 2007), electrical or magnetic field (Hua, 2004; Mohr, 2006), pH (Cheng, Ren, Yang, & Wei, 2018; Hu et al., 2016), and organic solvents (Abdullah, Li, Braun, Rogers, & Hsia, 2018; Gogoi & Raidongia, 2017). The stimulus-responsiveness is indicated via the change of physical properties, chemical structure or both, and the changing process is reversible or irreversible. In most cases, the responsive process usually exhibits a change in volume or shape, that is, two-dimensional expansion/contraction (Kelly et al., 2013) and three-dimensional bending/unbending processes (Ge et al., 2018; Hu, Zhang, & Li, 1995). Stimulus-responsive materials have been applied in the smart devices field, such as sensors, converters, micro pumps, artificial muscles, and switchgear.
Many stimuli-responsive actuators are developed by using gels, polymer films, or fibers to mimic biological systems. Among them, humidity-responsive materials have received widespread attention. Humidity-responsive actuators are typically based on the materials that have hydrophilic groups which interact with water molecules (Ma, & Sun, 2009). Nan et al. fabricated water vapor responsive composite film by using cellulose nanocrystals (CNC) and graphene (Nan et al., 2016). The resultant CNC/graphene film gave a color change as the water content of the film changes. Khan et al. used phenolic resin and CNC to prepare a bilayer film, which could bend and stretch directionally when the film was exposed to water vapor (Khan, Hamad, & Maclachlan, 2014). Sun et al. fabricated a free-standing film containing thermally cross-linked poly(acrylic acid)/poly(allylamine hydrochloride) (Ma, & Sun, 2009). The film could undergo bending/unbending movements when the environmental humidity and/or temperature changed. Liu et al. prepared cross-linked liquid crystal polymer film with dual responsiveness to humidity and light (Liu et al., 2017). Zhang et al. fabricated an agarose-based hydrogel with triple responsiveness to pH, ultraviolet light and humidity (Zhang & Naumov, 2015). It could be applied in the biomedicine and flexible robotic fields. In addition to the humidity response, some polymers could give organic solvent responsiveness. Zhang et al. utilized poly(vinylidene fluoride) and poly(vinyl alcohol) to fabricate microchannel structures, which could achieve a vapor-mechanical coiling once being exposed to acetone vapor (Zhang, Naumov, Du, Hu, & Wang, 2017). Chang et al. used cholesteric liquid-crystal (CLC) polymer network as an optical sensor to distinguish ethanol and methanol (Chang, Bastiaansen, Broer, & Kuo, 2012). By covalently bonding a hydrogel thin film on a reflective substrate, He et al. fabricated a hydrogel interferometer which could recognize multiple volatile organic compounds (Qin et al., 2018). However, due to the essential difference between water and organic solvents, the fabrication of an actuator with a dual responsiveness to water vapor and organic solvent is a huge challenge.
In this work, inspired by the blooming and shutting of morning glory flower, we prepared a smart dual-responsive flexible actuator with Janus structure from natural cellulose. The ethanol-swellable cellulose 3,5-di-tert-butyl-4 hydroxybenzoate (CBH) and water-swellable carboxymethyl cellulose (CMC) were chosen as the front side and reverse side, respectively, based on precisely controlling the chemical structure of cellulose derivatives. The CBH/CMC Janus film with complete biodegradability exhibited a huge potential in disposable sensor and actuator.