Chemical Gradients in Polymer-Modified Paper Sheets—Towards Single-Layer Biomimetic Soft Robots

Abstract

Biomimetic actuators are typically constructed as functional bi- or multilayers, where actuating and resistance layers together dictate bending responses upon triggering by environmental stimuli. Inspired by motile plant structures, like the stems of the false rose of Jericho (Selaginella lepidophylla), we introduce polymer-modified paper sheets that can act as soft robotic single-layer actuators capable of hygro-responsive bending reactions. A tailored gradient modification of the paper sheet along its thickness entails increased dry and wet tensile strength and allows at the same time for hygro-responsiveness. For the fabrication of such single-layer paper devices, the adsorption behavior of a cross-linkable polymer to cellulose fiber networks was first evaluated. By using different concentrations and drying procedures fine-tuned polymer gradients throughout the thickness can be achieved. Due to the covalent cross-linking of polymer with fibers, these paper samples possess significantly increased dry and wet tensile strength properties. We furthermore investigated these gradient papers with respect to a mechanical deflection during humidity cycling. The highest humidity sensitivity is achieved using eucalyptus paper with a grammage of 150 g m−2 modified with the polymer dissolved in IPA (~13 wt%) possessing a polymer gradient. Our study presents a straightforward approach for the design of novel hygroscopic, paper-based single-layer actuators, which have a high potential for diverse soft robotic and sensor applications.

Pinecone-Inspired Humidity-Responsive Paper Actuators with Bilayer Structure

“…For the formation of CMC-paper composites, eucalyptus paper is impregnated with an aqueous CMC solution and dried afterwards on a Teflon ® plate in an oven, as described Polymers 2024, 16, 1402 6 of 18 in Section 2.3. During the drying of the CMC-paper composite, i.e., the evaporation of the solvent, polymer mass transport takes place [41]. Since paper strips impregnated with CMC are placed on a Teflon plate during the drying process, most of the water evaporation takes place only on the upper side of the paper strip.…”

Cellulose-Based Intelligent Responsive Materials: A Review

“…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. As shown in Figure 2c, Schäfer et al [36] developed a single-layer actuator consisting of polymermodified cellulose paper, which shows a bending response when exposed to moisture, as shown in Figure 2d. The researchers showed that this design not only minimizes the likelihood of delamination, but also simplifies the overall design and construction processes in contrast to multilayer structures.…”

Four-Dimensional Printing of Temperature-Responsive Liquid Crystal Elastomers with Programmable Shape-Changing Behavior

“…Certain organisms and materials in nature are capable of adapting to changes in their surroundings by responding to different stimuli and transforming their structural forms [ 1 , 2 , 3 , 4 , 5 , 6 ]. Inspired by the stimuli-responsive behavior of biological materials, smart soft materials with shape-shifting capabilities have attracted increasing attention in numerous fields [ 7 , 8 , 9 , 10 , 11 , 12 ]. Among them, liquid crystal elastomers (LCEs) are a kind of exemplary smart material with soft elasticity that can produce reversible and high actuating strain under different stimuli, including heat, light, humidity, electricity and magnetism [ 13 , 14 , 15 ].…”