China: Crosslinking and Post-Polymerization for Composite Hydrogels

Chinese researchers are experimenting with materials for bioprinting, outlining their findings in the recently published ‘Dual-Enzyme Crosslinking and Post-polymerization for Printing of Polysaccharide-Polymer Hydrogel.’

Tissue engineering, while yielding many successful and astounding efforts in research labs today—from the creation of a variety of hydrogels to scaffolds, to kidney organoids—is a complex area requiring much trial and error as cells can be considerably (and understandably) difficult to sustain. As hydrogels go, polymers are thought to be ideal by many because of their extracellular-matrix-like construction.

As with any material that offers great benefit, however, there are also drawbacks—and in this case, it usually points to a lack of strength and inferior properties. Seeking greater tunability for strength, the scientists used the HRP@GOx dual enzyme system in their study to ‘initiate the immediate crosslinking of chondroitin sulfate grafted with tyrosine and the gradual polymerization of monomers to form the composite hydrogels.’

Hydrogels are associated with many different tissue engineering applications today, including:

• Biocatalysis
• Biosensors
• Cell cultures
• Drug delivery systems
• Biomedicines
• Wound healing
• 3D printing

“In general, hydrogels formed by natural polysaccharides are suitable for 3D printing and encapsulating biomolecules. However, they generally have poor elasticity and weak mechanical properties. The covalently bonded crosslinked network (polymeric hydrogel) is elastically deformable, and still maintains strong mechanical properties (Sun et al., 2012; Zhang X. N. et al., 2018),” explained the researchers.

“However, they generally lack a suitable porous structure to diffuse biomolecules, and it is difficult to obtain a good viscidity window for extruded 3D printing, which are important factors in tissue-repair and 3D printing. An effective way to ensure these two advantages is to use a polysaccharide hydrogel in combination with a polymeric material.”

The scientific team employed two methods for creating composite hydrogels, using crosslinkable hydrogel (Gel I) and polymeric/crosslinked hydrogel (Gel II).

(A) Schematic of the preparation of Gel I and Gel II; (B) Enzymatic reaction illustration of HRP@GOx system; (C) Optical image of Gel II under compression; (D) Optical image of Gel II under stretching [Gel II in (C,D) have different diameter, which cause different colors to be reflected under the same shooting conditions].

(A) Optical image of Gel II composed of different monomers (including AAm, DMAA, NIPAM, and PEGMA); (B) Compressive tests of Gel II composed of different monomers (including AAm, DMAA, NIPAM, and PEGMA). (C) The conversion of AAm in Gel II is calculated using the 1H-NMR spectra.

(A) Fluorescence Spectroscopy spectrum of GMA-CS-Ph-OH with and without HRP@GOx; (B) EPR spectrum of the DMPO radical adduct formed in HRP@GOx, GMA-CS-Ph-OH, and AAm reaction system at 30 min (black line) and 3 h (red line); (C) The GPC spectrum of Gel I (black line) and Gel II (red line); (D) Mechanism illustration of catalytic oxidation of tyrosine via HRP@GOx system.

The composites for this research study were created in a ‘straightforward’ manner, and the scientists involved reported no need for added initiators, with the hydrogels structured by combining the mixing solution and keeping it at room temperature successfully.

Pointing out that composite hydrogels usually exhibit an adjustable strength from 3.29 to 86.73, the monomer conversion was found to be 95% via ¹H-NMR.

“The mechanism analyses confirmed the immediate cross-linking at diluted solution and gradually polymerized reinforcement within viscous polysaccharide network,” concluded the researchers toward the end of their study. “Therefore, our polymer composite hydrogels have denser pore and nanoscale network relative to only polysaccharide hydrogels.

“With excellent biocompatible and mechanically adjustable abilities, the composite hydrogel is particularly interesting for 3D printing to fabricate precision structures for tissue repairing and tissue engineering.”

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(A) The printed shape “button” with several layers; (B) The printed shape “ears” with several layers; (C) The printed 3D shape “button” (up) and enhanced 3D shape “button” (bottom); (D) Compressive test of enhanced 3D shape “button”; (E) cyclic compression test of enhanced 3D shape “button”.