Programmable Humidity-Responsive Actuation of Polymer Films Enabled by Combining Shape Memory Property and Surface-Tunable Hygroscopicity

Abstract

Most humidity-responsive polymeric actuators can only exhibit shape transformations between a planar shape in the dry state and a bended three-dimensional (3D) shape when exposed to moisture, and it is challenging to design and prepare hygroscopic actuators with programmable actuating behaviors displayed from sophisticated 3D structures. Herein, we demonstrate that the integration of shape memory property and surface treatment enabled hygromorphic responsivity endows a single-component polymer film with programmable moisture-driven actuating behaviors. The solvent-processed polyethylene-co-acrylic acid (EAA) copolymer film is soft and stretchable at room temperature, and has a good thermal-responsive shape memory property. By surface treatment using base/acid solutions, the reversible gradient conversion between carboxyl groups and carboxylate salts along the thickness direction enables the film to exhibit designed hygroscopic actuations. The shape memory property and moisture-driven actuating behaviors can be combined to realize 3D-3D morphing by first programming the films into 3D shapes and then conducting the surface treatments. Both shape programming and surface treatment processes can be reprogrammed to make the actuation behavior readily tunable. We also show that the created surface patterns can act as moisture-sensitive conducting paths to detect human breathes, and the combination of shape memory, moisture-responsive morphing and conductivity change leads to some interesting applications such as smart switch in conducting circuit. This work provides a new and general strategy for the design of advanced humidity-responsive actuators.

A pH-Responsive Liquid Crystal Hydrogel Actuator with Calcium-Induced Reprogrammable Shape Fixing

“…Soft polymeric actuators have garnered much interest over the past decade due to their potential applications, including microfluidic devices, artificial muscles, and soft robotics. − Various stimuli-responsive polymeric systems were developed, which change shape when exposed to light, − electric or magnetic fields, , humidity, − pH, ,− or temperature. , More recently, examples of combined actuators and programmable shape-memory materials have been reported, enabling (re)­programming and reuse of these materials. ,,− …”

External Stimuli-Responsive Characteristics of Poly(N,N′-diethylacrylamide) Hydrogels: Effect of Double Network Structure

“…Stimuli-responsive polymer hydrogels that are able to respond to the changes in temperature [ 1 ], pH [ 2 ], humidity [ 3 ], light [ 4 ], specific ions or molecules [ 5 ], electrical fields [ 6 ], solvent [ 7 ] and ionic strength [ 8 ] have been extensively studied due to potential applications in the areas of drug delivery [ 9 ], microlenses [ 10 ], sensors [ 1 ] and artificial organs [ 11 ].…”

Development and challenges of smart actuators based on water-responsive materials

“…Although the hydrogel materials can respond to humidity changes, it has a slower response speed and less mechanical deformation, 2 and further improvements are needed. Additionally, other polymers containing hydrophilic groups can also respond to humidity changes, such as polyvinyl alcohol (PVA), 51,103 polyethylene oxide (PEO), 32,52,104 chitosan (CS), 105 polyol-borate, 106 polyethylene-co-acrylic acids (EAAs), 107 biaxially oriented polypropylene (BOPP), 108 and some liquid crystal polymers, [109][110][111] which are widely used in various actuation studies. For example, Wang et al demonstrated the continuous response of the actuator to humidity gradients by using PVA films as actuators.…”