pH-sensitive gallol-rich chitosan hydrogel beads for on-off controlled drug delivery.

Malignant tumors have emerged as a serious health issue, and the interest in developing pH-sensitive polymers for site-specific drug delivery has increased. The physical and/or chemical properties of pH-sensitive polymers depend on the pH, and thus, drugs can be released by cleaving dynamic covalent and/or noncovalent bonds. In this study, gallic acid (GA) was conjugated to chitosan (CS) to prepare self-crosslinked hydrogel beads containing Schiff base (imine bond) crosslinks. The CS-GA hydrogel beads were formed by the dropwise addition of the CS-GA conjugate solution into a Tris-HCl buffer solution (TBS, pH 8.5). The pH-sensitivity of pristine CS was significantly enhanced following the introduction of GA moiety, and as a result, the CS-GA hydrogel beads swelled more than approximately 5000 % at pH 4.0, indicating an excellent swelling/deswelling ability of the beads at different pH (pH 4.0 and 8.5). The reversible breakage/recovery of the imine crosslinks in the CS-GA hydrogel beads was confirmed through X-ray photoelectron spectroscopic and rheological studies. Finally, Rhodamine B was loaded onto the hydrogel beads as a model drug to investigate the pH-sensitive drug release behavior. At pH 4, the drug was released up to approximately 83 % within 12 h. The findings indicate that the CS-GA hydrogel beads have great potential as a drug delivery system that is sensitive to acidic tumor sites in the body.

Highly gallol-substituted, rapidly self-crosslinkable, and robust chitosan hydrogel for 3D bioprinting.

Although three-dimensional (3D) bioprinting is a promising technology for reconstructing artificial tissues and organs using bioink, there is a lack of a bioink that satisfies all requirements, including printability, gelation, mechanical properties, and cytocompatibility, Herein, a novel self-crosslinkable bioink derived from chitosan (CS) and gallic acid (GA) is presented. 3D printed scaffolds with excellent shape fidelity are realized by systematically analyzing the self-crosslinking mechanism of hydrogel formation from CS-GA conjugates and by optimizing various parameters of the printing process. The CS-GA hydrogel forms rapidly in a physiological pH without any chemical crosslinking agent. In addition, the CS-GA hydrogel exhibited various physical and chemical intermolecular interactions, fast gelation rates, and excellent mechanical properties (>337 kPa). Moreover, the CS-GA hydrogel singificantly improves the cell viability (>92 %) and proliferation of the bioink. Therefore, the self-crosslinkable CS-GA bioink has great potential to overcome the limitations of conventional bioinks.

Hyaluronic acid/tannic acid hydrogel sunscreen with excellent anti-UV, antioxidant, and cooling effects.

Excessive exposure to UV radiation is one of the major factors that causes skin aging, erythema, burns, and skin cancer. Recently, the usage of sunscreens for skin protection has increased because the amount of UV radiation reaching the Earth's surface has increased owing to the destruction of the ozone layer that blocks UV radiation. Hydrogels with a three-dimensional network structure exhibit physical and chemical properties that are similar to those of the extracellular matrix in the human body, a high water content, flexibility, and biocompatibility. Therefore, they are applied in a wide range of fields, such as in cosmetics, medicines, and pharmaceuticals. However, conventional hydrogel-based sunscreens have drawbacks such as complicated process conditions, high cost, and low biocompatibility. In this study, a novel hydrogel-type sunscreen with excellent UV protection and cooling effects was prepared by a very simple process using two natural materials, hyaluronic acid (HA) and tannic acid (TA). The HA/TA hydrogels exhibited broad-spectrum UV protection in the UVA and UVB regions (280-360 nm). In addition, they showed excellent adhesion to the skin surface, antioxidative activity, cooling effect, and high moisture content, demonstrating great application potential as a hydrogel-type sunscreen.

Effect of tannic acid on the mechanical and adhesive properties of catechol-modified hyaluronic acid hydrogels.

Hyaluronic acid (HA) is applied in various fields, including pharmaceutical science, owing to its favorable biological properties such as moisture retention, non-toxicity, biodegradability, biocompatibility and biodegradability. In particular, many studies have aimed at its application in the form of a hydrogel. However, the applications of HA hydrogels are limited owing to their poor mechanical properties. In this study, an HA-catechol conjugate (HA-Cat) was synthesized by reacting the HA polymer with dopamine to improve its adhesion to various substrates. The HA-Cat hydrogel was prepared via oxidative crosslinking using a small amount of NaIO4 as the oxidant, and the hydrogel formation was investigated by rheological and mechanical studies. Further, the effect of tannic acid (TA) on the adhesive strength and compressive strength of the HA-Cat/TA hydrogels was examined according to the amount of NaIO4 used for crosslinking and the TA contents. Both the adhesive and compressive properties of the HA-Cat hydrogels were improved with the addition of TA. The HA-based hydrogels containing TA have great potential as cost-effective and biocompatible medical adhesives.

Self-crosslinkable hyaluronate-based hydrogels as a soft tissue filler.

With increasing interest in aging and skin care, the use of fillers to increase the volume of soft tissue volume is increasing globally. However, the side effects caused by the residual chemical crosslinking agents present in these fillers limit the effective application of commercialized filler products. Therefore, the development of a novel crosslinking system with a non-toxic chemical crosslinking agent is required to overcome the limitations of commercial hyaluronate (HA)-based fillers. In this paper, a new injectable hydrogel with enhanced mechanical properties, tissue adhesion, injectability, and biocompatibility is reported. The HA derivatives modified with catechol groups (HA-DA) were crosslinked by self-oxidation under in vivo physiological conditions (pH 7.4) without chemical crosslinkers to form hydrogels, which can be further accelerated by the dissolved oxygen in the body. The fabricated HA-DA filler showed excellent mechanical properties and could be easily injected with a low injection force. Further, the HA-DA filler stably attached to the injection site due to the tissue adhesion properties of the catechol groups, thus leading to an improved displacement stability. In addition, the HA-DA filler showed excellent cell viability, cell proliferation, and biocompatibility. Therefore, the HA-DA hydrogel is a novel soft tissue filler with great potential to overcome the limitations of commercial soft tissue fillers.