Characterization of Chitosan Hydrogels Obtained through Phenol and Tripolyphosphate Anionic Crosslinking

Abstract

Chitosan is a deacetylated polymer of chitin that is extracted mainly from the exoskeleton of crustaceans and is the second-most abundant polymer in nature. Chitosan hydrogels are preferred for a variety of applications in bio-related fields due to their functional properties, such as antimicrobial activity and wound healing effects; however, the existing hydrogelation methods require toxic reagents and exhibit slow gelation times, which limit their application in biological fields. Therefore, a mild and rapid gelation method is necessary. We previously demonstrated that the visible light-induced gelation of chitosan obtained through phenol crosslinking (ChPh) is a rapid gelation method. To further advance this method (<10 s), we propose a dual-crosslinked chitosan hydrogel obtained by crosslinking phenol groups and crosslinking sodium tripolyphosphate (TPP) and the amino groups of chitosan. The chitosan hydrogel was prepared by immersing the ChPh hydrogel in a TPP solution after phenol crosslinking via exposure to visible light. The physicochemical properties of the dual-crosslinked hydrogels, including Young's moduli and water retentions, were subsequently investigated. Young's moduli of the dual-crosslinked hydrogels were 20 times higher than those of the hydrogels without TPP ion crosslinking. The stiffness could be manipulated by varying the immersion time, and the water retention properties of the ChPh hydrogel were improved by TPP crosslinking. Ion crosslinking could be reversed using an iron chloride solution. This method facilitates chitosan hydrogel use for various applications, particularly tissue engineering and drug delivery.

Keywords: chitosan; hydrogel; phenol group; tripolyphosphate.

Figure 1

Dual-crosslinked chitosan hydrogel obtained by phenol and TPP crosslinking.

Figure 2

Comparison of chitosan hydrogel obtained using four different crosslinking methods: (a) sodium TPP crosslinking for 5 min (left top), (b) phenol crosslinking and exposure to visible light for 20 min (right top), (c) phenol crosslinking after TPP crosslinking for 5 min (left bottom), and (d) TPP crosslinking for 5 min after phenol crosslinking with exposure to visible light for 20 min (right bottom). Scale bar = 5 mm.

Figure 3

FTIR spectra of chitosan, ChPh, ChPh–TPP1, ChPh–TPP5, and ChPh–TPP10.

Figure 4

Effect of TPP immersion time on the Young’s modulus. The ChPh aqueous solutions containing SPS and Ru(bpy)₃ were gelated by exposure to visible light and were subsequently immersed in TPP solution for 1, 5, and 10 min (denoted as ChPh–TPP1, ChPh–TPP5, and ChPh–TPP10, respectively). Data: ±mean S.D. (n = 3–5), * p < 0.05.

Figure 5

Effect of TPP immersion time on the swelling degree after 5 h of immersion in PBS. Data: ±mean S.D. (n = 3), * p < 0.05, n.s. > 0.1.

Figure 6

Effect of TPP immersion time on water retention. Data: ±mean S.D. (n = 5), * p < 0.05.

Figure 7

Effect of TPP immersion time on antimicrobial activity. Data: ±mean S.D. (n = 3), * p < 0.05, n.s. > 0.1.

Figure 8

Removal and reversibility of TPP crosslinking using FeCl₃ (scale bar = 5 mm).