Shape-Memory Hydrogels: Evolution of Structural Principles To Enable Shape Switching of Hydrophilic Polymer Networks

Abstract

The ability of hydrophilic chain segments in polymer networks to strongly interact with water allows the volumetric expansion of the material and formation of a hydrogel. When polymer chain segments undergo reversible hydration depending on environmental conditions, smart hydrogels can be realized, which are able to shrink/swell and thus alter their volume on demand. In contrast, implementing the capacity of hydrogels to switch their shape rather than volume demands more sophisticated chemical approaches and structural concepts. In this Account, the principles of hydrogel network design, incorporation of molecular switches, and hydrogel microstructures are summarized that enable a spatially directed actuation of hydrogels by a shape-memory effect (SME) without major volume alteration. The SME involves an elastic deformation (programming) of samples, which are temporarily fixed by reversible covalent or physical cross-links resulting in a temporary shape. The material can reverse to the original shape when these molecular switches are affected by application of a suitable stimulus. Hydrophobic shape-memory polymers (SMPs), which are established with complex functions including multiple or reversible shape-switching, may provide inspiration for the molecular architecture of shape-memory hydrogels (SMHs), but cannot be identically copied in the world of hydrophilic soft materials. For instance, fixation of the temporary shape requires cross-links to be formed also in an aqueous environment, which may not be realized, for example, by crystalline domains from the hydrophilic main chains as these may dissolve in presence of water. Accordingly, dual-shape hydrogels have evolved, where, for example, hydrophobic crystallizable side chains have been linked into hydrophilic polymer networks to act as temperature-sensitive temporary cross-links. By incorporating a second type of such side chains, triple-shape hydrogels can be realized. Considering the typically given light permeability of hydrogels and the fully hydrated state with easy permeation by small molecules, other types of stimuli like light, pH, or ions can be employed that may not be easily used in hydrophobic SMPs. In some cases, those molecular switches can respond to more than one stimulus, thus increasing the number of opportunities to induce actuation of these synthetic hydrogels. Beyond this, biopolymer-based hydrogels can be equipped with a shape switching function when facilitating, for example, triple helix formation in proteins or ionic interactions in polysaccharides. Eventually, microstructured SMHs such as hybrid or porous structures can combine the shape-switching function with an improved performance by helping to overcome frequent shortcomings of hydrogels such as low mechanical strength or volume change upon temporary cross-link cleavage. Specifically, shape switching without major volume alteration is possible in porous SMHs by decoupling small volume changes of pore walls on the microscale and the macroscopic sample size. Furthermore, oligomeric...

Programmable Macroporous Photonic Crystals Enabled by Swelling‐Induced All‐Room‐Temperature Shape Memory Effects

“…[1][2][3] Photons with frequencies lying in the PBGs cannot propagate through the crystals, providing enormous opportunities in controlling the flow of light in miniature volumes for photo nic information technology, such as nextgeneration all-optical integrated circuits. [12][13][14][15][16][17] Smart shape memory polymers (SMPs), which can memorize and transit between a permanent shape and one or multiple temporary configurations in responding to various stimuli, such as heat, light, solvents, and electromagnetic fields, [18][19][20][21][22][23][24][25] may hold the key to truly reconfigurable photonic crystals. [12][13][14][15][16][17] Smart shape memory polymers (SMPs), which can memorize and transit between a permanent shape and one or multiple temporary configurations in responding to various stimuli, such as heat, light, solvents, and electromagnetic fields, [18][19][20][21][22][23][24][25] may hold the key to truly reconfigurable photonic crystals.…”

4D Printing of Shape‐Memory Hydrogels for Soft‐Robotic Functions

“…[5] Smart or stimuliresponsive materials have the unique ability to return from a temporary deformed state, induced by heat, light, pH, ultrasound, chemical substances, [6][7][8][9][10][11][12][13] etc., to their permanent, i.e., original, shape, thus exhibiting advantages for applications in numerous sectors, such as sensors and actuators, [14] tissue engineering, [15] bio-separation devices, and controlled drug delivery. [22] Therefore, mechanically active, self-shaping hydrogels that undergo desired, programmable 3D shape transformations and execute mechanical tasks as soft robots under an external trigger have recently attracted growing interest. SMP-based 4D printing offers structural modification and recovery in response to temperature, which are established through complex functionalities of multiple or reversible shape switching, and such printing may provide inspiration for the molecular architecture of shape memory hydrogels (SMHs).…”

Stimuli‐Responsive Supramolecular Hydrogels and Their Applications in Regenerative Medicine

“…For instance, hydrogels that manifest thermoreversible properties are often composed of molecular moieties exhibiting a balance of hydrophilic and hydrophobic interactions . Beyond volume/phase transitions, molecules with optimal hydrophilic and hydrophobic interactions have been used to endow chemically cross‐linked hydrogels with unique functions such as macroscopic self‐organization (solid to hollow interior and sheet to 3D structures), shape memory, and self‐healing …”

“…[8,14,15] Beyond volume/phase transitions, molecules with optimal hydrophilic and hydrophobic interactions have been used to endow chemically cross-linked hydrogels with unique functions such as macroscopic self-organization (solid to hollow interior and sheet to 3D structures), shape memory, and self-healing. [16][17][18][19][20] Similar to polymer hydrogels, supramolecular hydrogels formed by the assembly of molecular building blocks termed as hydrogelators also show stimuli-responsive properties (gel-sol or sol-gel transitions). [21,22] Although supramolecular hydrogels bear many functional similarities with conventional polymer hydrogels, the network formation in supramolecular hydrogels is different from that of conventional polymerbased hydrogels.…”