The Formation Mechanism of Hydrogels

Abstract

Hydrogels are degradable polymeric networks, in which cross-links play a vital role in structure formation and degradation. Cross-linking is a stabilization process in polymer chemistry that leads to the multi-dimensional extension of polymeric chains, resulting in network structures. By crosslinking, hydrogels are formed into stable structures that differ from their raw materials. Generally, hydrogels can be prepared from either synthetic or natural polymers. Based on the types of cross-link junctions, hydrogels can be categorized into two groups: the chemically cross-linked and the physically cross-linked. Chemically cross-linked gels have permanent junctions, in which covalent bonds are present between different polymer chains, thus leading to excellent mechanical strength. Although chemical cross-linking is a highly resourceful method for the formation of hydrogels, the cross-linkers used in hydrogel preparation should be extracted from the hydrogels before use, due to their reported toxicity, while, in physically cross-linked gels, dissolution is prevented by physical interactions, such as ionic interactions, hydrogen bonds or hydrophobic interactions. Physically cross-linked methods for the preparation of hydrogels are the alternative solution for cross-linker toxicity. Both methods will be discussed in this review.

Injectable hydrogels for tendon and ligament tissue engineering

“…Several methods for preparing injectable hydrogels, classified according to their responsiveness or the different connections that they contain, exist. The cross‐linking mechanism of hydrogels can be divided into two types: physical cross‐linking and chemical cross‐linking (Guan et al, 2017; Lu et al, 2018; Mredha et al, 2017; Murray et al, 2007; Sivashanmugam, Kumar, Priya, Nair, & Jayakumar, 2015) Table 3 lists the different synthetic routes and application summary of injectable hydrogels in T/L tissue engineering.…”

Engineering Hydrogel‐Based Biomedical Photonics: Design, Fabrication, and Applications

“…[ 65 ] In fact, crosslinking is also an important parameter, which depending on the experimental setting, might weigh heavily on the polymer choice. [ 67,68 ] While UV crosslinking is useful for obtaining fiber‐like PEGDA structures [ 65 ] or complex structural colored gelatin methacryloyl (GelMA) hydrogels, [ 69 ] thermal crosslinking might be easier to employ for fabricating gelatin layers and films. [ 70,71 ] Ionic crosslinking might be a quick and straightforward way to create alginate cladding layers in optical fibers, bringing the advantage of its high water content and low refractive index.…”

Hydrogels to engineer tumor microenvironmentsin vitro

“…[246][247][248] Importantly, these hydrogels remain relatively inert with low protein adsorption, hence a wide range of bioactive moieties ( proteins, peptides and drugs) or biopolymers (HA, collagen and fibrin) can be conjugated to PEG to mimic the in vivo ECM with controlled and tunable bioactivity. [249][250][251] By modulating the polymer concentration or extent of crosslinking, 252,253 synthetic hydrogels also enable tuning of physical properties such as stiffness to match that of the tissue ECM of interest. [254][255][256] More recently, hybrid hydrogels which are composites of natural and synthetic polymers have been developed.…”