A Freeze-Thawing Method to Prepare Chitosan-Poly(vinyl alcohol) Hydrogels Without Crosslinking Agents and Diflunisal Release Studies

Summary

The freezing-thawing method is used to produce chitosan-poly(vinyl alcohol) hydrogels without crosslinking agents. For this method, it is important to consider the freezing conditions (temperature, number of cycles) and polymer ratio, which can affect the properties and applications of the obtained hydrogels.

Abstract

Chitosan-poly(vinyl alcohol) hydrogels can be produced by the freeze-thawing method without using toxic crosslinking agents. The applications of these systems are limited by their characteristics (e.g., porosity, flexibility, swelling capacity, drug loading and drug release capacity), which depend on the freezing conditions and the kind and ratio of polymers. This protocol describes how to prepare hydrogels from chitosan and poly(vinyl alcohol) at 50/50 w/w % of polymer composition and varying the freezing temperature (-4 °C, -20 °C, -80 °C) and freeze-thawing cycles (4, 5, 6 freezing cycles). FT-IR spectra, SEM micrograph and porosimetry data of hydrogels were obtained. Also, the swelling capacity and drug loading and release of diflunisal were assessed. Results from SEM micrographs and porosimetry show that the pore size decreases, while the porosity increases at lower temperatures. The swelling percentage was higher at the minor freezing temperature. The release of diflunisal from the hydrogels has been studied. All the networks maintain the drug release for 30 h and it has been observed that a simple diffusion mechanism regulates the diflunisal release according to Korsmeyer-Peppas and Higuchi models.

Introduction

Recently, hydrogels have attracted great interest in the biomedical field because they are three-dimensional networks with high water content and are soft and flexible, so they can mimic natural tissues easily¹. Also, they do not dissolve in aqueous medium at physiological temperature and pH but present a large swelling². Hydrogels can act as tissue engineering scaffolds, hygiene products, contact lenses, and wound dressings; because they can trap and release active compounds and drugs, they are used as drug delivery systems³. Depending on their application, hydrogels can be made from natural or s....

Protocol

1. Preparation of chitosan-PVA hydrogels

1. Prepare 2% (w/w) chitosan and 10% (w/w) PVA solutions. Dissolve 0.2 g of chitosan in 10 mL of 0.1 M CH₃COOH solution (previously filtered) at room temperature and maintain continuous mechanical stirring overnight. Dissolve 1 g of PVA in 10 mL of distilled water and stir at 80 °C for 1 h.
2. Mix both solutions 1:1 using a magnetic stirrer until they are homogeneous at room temperature, and pour the mixtures on Petri dishes. Leave the samples for 2 h at atmospheric pressure to degas.
3. Freeze the hydrogels at -4 °C, -20 °C or -80 °C for 20 h and 4 cycles (samples CP4-4, CP4-20 and CP4-80, res....

Results

Hydrogels preparation
Chitosan-PVA hydrogels were obtained at -4 °C, -20 °C and -80 °C with 4 freezing cycles and at -80 °C with 5 and 6 freezing cycles by the previously reported freeze-thawing method². All hydrogels were homogeneous, semi-transparent, flexible and resistant against manipulation.

FT-IR characterization
The FT-IR spectra are shown in

Discussion

The freeze-thawing method is a suitable process to prepare biocompatible hydrogels focused in biomedical, pharmaceutical or cosmetical applications^34,35,36. The most important advantage of this method, compared with other well-known methods to prepare hydrogels, is that crosslinking agent use is avoided, which could cause an inflammatory response or adverse effects in the human body³⁴. This is a versatile.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Materials:
Chitosan medium molecular weight	Sigma-Aldrich	448877	Mw determined by capillary viscometry (637,000 Da) and deacetylation degree of 70%
Diflunisal (2'-4'-difluoro-4-hydroxy-3-biphenyl-carboxylicacid)	Merck
Glacial acetic acid	Sigma-Aldrich	1005706
Poly(vinyl alcohol)	Sigma-Aldrich	341584	Mw 89,000-98,000, 99+% hydrolyzed
Equipment:
Cressington Sputter Coater 108 auto	TED PELLA INC
Cryodos Lyophilizator	Telstar
Falcon tubes	Thermo Fisher Company
FT-IR spectroscopy	Nicolet iS50	in ATR mode
Lyophilizator	LABCONCO
Micromeritics Autopore IV 9500	Micromeritics
Scanning electron microscope	Pemtron SS-300LV
UV-visible spectrophotometer	Agilent 8453