Abstract
In recent years, smart/stimuli-responsive hydrogels have drawn tremendous attention for their varied applications, mainly in the biomedical field. These hydrogels are derived from different natural and synthetic polymers but are also composite with various organic and nano-organic fillers. The basic functions of smart hydrogels rely on their ability to change behavior; functions include mechanical, swelling, shaping, hydrophilicity, and bioactivity in response to external stimuli such as temperature, pH, magnetic field, electromagnetic radiation, and biological molecules. Depending on the final applications, smart hydrogels can be processed in different geometries and modalities to meet the complicated situations in biological media, namely, injectable hydrogels (following the sol-gel transition), colloidal nano and microgels, and three dimensional (3D) printed gel constructs. In recent decades smart hydrogels have opened a new horizon for scientists to fabricate biomimetic customized biomaterials for tissue engineering, cancer therapy, wound dressing, soft robotic actuators, and controlled release of bioactive substances/drugs. Remarkably, 4D bioprinting, a newly emerged technology/concept, aims to rationally design 3D patterned biological matrices from synthesized hydrogel-based inks with the ability to change structure under stimuli. This technology has enlarged the applicability of engineered smart hydrogels and hydrogel composites in biomedical fields. This paper aims to review stimuli-responsive hydrogels according to the kinds of external changes and t recent applications in biomedical and 4D bioprinting.
Highlights
photothermal therapy (PTT) is used with some nanoparticles, which are capable of converting near-infra-red (NIR) electromagnetic spectra to heat, while photodynamic therapy (PDT) is a technique used with some molecules named photosensitizers (PS) for creating reactive oxygen species (ROS) [189,190,191]
Ulag and colleagues used 3D printing technology to create vessel-like constructions made of Poly (-caprolactone) (PCL), low molecular weight chitosan (CS), and alginate-hyaluronic acid-collagen type I hydrogels to overcome the issues associated with past use of traditional grafts [305]
Human umbilical vein endothelial cells (HUVECs) were used to test the biocompatibility of triple-layered vascular scaffold (TVS), and the results showed that the cells could effectively connect to the graft surface while maintaining high vitality
Summary
Stereolithography, or three-dimensional (3D) printing, was first used in 1986 for printing layers or compact 3D lines or shapes by means of light [1] This technology was developed for the rapid, versatile, and digitally-guided production of infinite amounts of objects of different scales via repetitive and well-controlled transmissions [2,3]. (bio)printing technologies are designed to fabricate functional (bio)materials responsive to internal or external stimuli, thereby changing the structure of the 3D printed biomaterials through time (the fourth dimension) [25,26,27]. The smart structure can be of two types—the rigid materials can be completely made from expandable materials or may be connected with expandable elements [33] Once these expandable elements are exposed to certain stimuli, they change shape by moving or rotating, thereby transforming into a new shape. Some of the common challenges and applications of 3D and 4D printed hydrogels for biomedical applications are discussed
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