Transformer Hydrogels: A Review

Abstract

Hydrogels, which are hydrophilic soft porous networks, are an important class of materials of broad relevance to bioanalytical chemistry, soft‐robotics, drug delivery, and regenerative medicine. Transformer hydrogels are micro‐ and mesostructured hydrogels that display a dramatic transformation of shape, form, or dimension with associated changes in function, due to engineered local variations such as in swelling or stiffness, in response to external controls or environmental stimuli. This review describes principles that can be utilized to fabricate transformer hydrogels such as by layering, patterning, or generating anisotropy, and gradients. Transformer hydrogels are classified based on their responsivity to different stimuli such as temperature, electromagnetic fields, chemicals, and biomolecules. A survey of the current research progress suggests applications of transformer hydrogels in biomimetics, soft robotics, microfluidics, tissue engineering, drug delivery, surgery, and biomedical engineering.

Pros and Cons: Supramolecular or Macromolecular: What Is Best for Functional Hydrogels with Advanced Properties?

“…Stimuli‐responsiveness is one of the attractive properties of hydrogels, which is useful for the fabrication of hydrogel actuators, dosing devices, and sensors. The classical way to implement stimuli‐responsiveness into MHGs is the incorporation of temperature‐responsive (leading to lower critical solution temperature (LCST) or upper critical solution temperature (UCST) behavior) or pH‐responsive monomers in the polymer chain . Recently, the utilization of light as stimulus emerged, by the incorporation of photo‐switchable molecules (azobenzenes, spiropyrans, donor–acceptor Stenhouse adduct (DASAs), etc.)…”

Materials as Machines

“…Conventional hydrogels are isotropic and undergo a homogenous volume phase transition. Recent reviews detail the synthesis, mechanism, and utility of hydrogel actuators 446–452. Of relevance to this review, demonstrations focused on the preparation of anisotropic hydrogels and programming of the mechanical response are presented as an emerging area of research with increasing relevance to robotic implementations.…”

Viewpoint: Pavlovian Materials—Functional Biomimetics Inspired by Classical Conditioning

“…Stimuli‐responsive materials refer to synthetic material systems whose properties can be modulated externally through, e.g., chemical, thermal, optical, electrical, mechanical, or magnetic stimuli . The response can take various forms, e.g., changes in the macroscopic shape and optical, mechanical, or surface properties . Characteristic examples of stimuli‐responsive materials are thermoresponsive polymers such as hydrogels made of poly( N ‐isopropylacrylamide), which undergo a thermally driven reversible volumetric switching between the extended and collapsed states .…”

“…This has attracted an increasing interest for scientists to pursue advanced biomimetic material systems. Still, so far, the mainstream research has dealt with mimicking the extraordinary equilibrium biological properties, such as the mechanical properties of silk, nacre, muscle, bone, or phenomena such as wetting, structural colors, and catalysis . However, biological systems are inherently dissipative, i.e., out‐of‐equilibrium (Figure 1d), allowing, for instance, chemotaxis, adaptation, learning, reproduction, and evolution .…”