Thermally-Induced Triple-Shape Hydrogels: Soft Materials Enabling Complex Movements

Abstract

Shape-memory hydrogels enable directed movements of a specimen in response to temperature, whereby crystallizable switching segments incorporated as side chains resulted in constant degrees of swelling during the shape-memory cycle. Here we report about hydrogels exhibiting a thermally induced triple-shape effect that allows complex movements of soft materials with two almost independent shape changes. Potential applications for those soft triple-shape materials are two-step self-unfolding devices or temperature-sensitive hydrogel actuators, for example, smart valves for flow rate control in aqueous media. Series of hydrogels with two different hydrophobic crystallizable switching segments were prepared. The degrees of swelling of the triple-shape hydrogels were not affected for different shapes or temperatures, which avoided in this way interferences on the shape shifts. During the two-step programming procedure, two distinct shapes can be implemented as reflected by shape fixity ratios of generally >50%. Structural analysis of the switching domains during the triple-shape cycle by means of X-ray scattering indicates that longer side chains gain lower orientation after deformation and that shorter side chains orient perpendicular to the hydrophilic main chain. Furthermore, it is observed that increased orientation of the switching domains is not a key requirement for adequate shape fixity and recovery ratios of the triple-shape effect in hydrogels, thus longer side chains can be utilized as switching segments in other shape-memory hydrogels.

Trends in polymeric shape memory hydrogels and hydrogel actuators

“…During the programming procedure of changing the temperature, two distinct temporary shapes can be memorized and recovered through crystallization and amorphization of hydrophobic side chains. 42 2.2.2 Different stimuli-triggered triple-/multi-shape memory effects. As different stimuli can be introduced step by step, combining non-interfering interactions in one material provides an efficient way to create triple-/multi-shape memory materials.…”

Polyampholyte Hydrogels with pH Modulated Shape Memory and Spontaneous Actuation

“…For the shape memory hydrogels, the special circumstance is that the plenty of water inside promotes the motion of polymer chains and weakens the fixation for the temporary shape. The temporary shape is recovered to the original (permanent) shape induced by some external stimuli, which promises the shape memory hydrogels potential applications in soft actuators, such as smart valves for flow control, sequential “blooming” of artificial flowers, and cell encapsulating and detaching. [3d,e] However, all of these actuations are one‐off and the original shape cannot convert to the temporary shape spontaneously and reversibly .…”

Dual-Stimuli Responsive Carbon Nanotube Sponge-PDMS Amphibious Actuator

“…Therefore, shape memory hydrogels have been investigated in recent years. Lendlein et al have reported a shape memory hydrogel actuated by temperature change through crystallization and amorphization of hydrophobic side chains [30]. Chen et al have fabricated a double layer self-deformed shape memory hydrogel which includes a thermo-responsive actuating layer and a pH-responsive memorizing layer [31].…”