Cellulose Nanocrystal‐Based Gradient Hydrogel Actuators with Controllable Bending Properties

Abstract

Stimuli‐responsive hydrogel actuators are being increasingly used in microtechnology, but typical bilayer hydrogel actuators have significant drawbacks due to weak adhesive interface between the two layers. In this study, thermoresponsive single‐layer hydrogel actuators are produced by generating a gradient distribution of cellulose nanocrystals (CNCs) in a poly(N‐isopropylacrylamide) (PNIPAAm) hydrogel network by electrophoresis. Tunable bending properties of the composite hydrogels, such as the thermoresponsive bending speed and angle, are realized by varying the electrophoresis time, applied voltage, and CNC concentration. By varying these conditions, the gradient distribution of the CNCs can be optimized, leading to fast bending and large bending angles of the hydrogels. Bending properties are attributed to the gradient distribution of CNCs causing different deswelling rates across the hydrogel network owing to reinforcing effects. Bending ability is also influenced by differences in the CNC dimensions based on the sources of cellulose, which determine the rigidity of the CNC‐rich layer of the polymer composite. It is thus shown that thermoresponsive single‐layer gradient hydrogels with tunable bending properties can be realized.

Cellulose-Based Intelligent Responsive Materials: A Review

“…(1) The construction of gradient structures is realized by generating an asymmetric distribution of polymer chains in vivo. For example, Roop sung et al [35] employed electrophoresis to manufacture a single gradient hydrogel actuator utilizing cellulose nanocrystals (CNCs). As shown in Figure 2a, this hydrogel actuator showed temperature-responsive bending behavior, whereby the bending angle progressively increased over time, resulting in the formation of a semicircular shape, as the temperature rose from 25 °C to 50 °C within four minutes.…”