Synthesis of biodegradable hydroxyethylcellulose cryogels by UV irradiation Petar Petrov a, * , Elissaveta Petrova a , Bozidar Tchorbanov b , Christo B. Tsvetanov a a Institute of Polymers, Bulgarian Academy of Sciences, ‘‘Akad. G. Bonchev‘‘ str. 103A, 1113 Sofia, Bulgaria b Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Received 12 March 2007; received in revised form 7 June 2007; accepted 15 June 2007 Available online 21 June 2007 Abstract Biodegradable macroporous hydroxyethylcellulose (HEC) cryogels of good quality and high gel fraction yield (95%) were synthesized via a facile method. The latter involved a relatively fast preparation of homogeneous semidilute solution of polymer and photoinitiator, (4-benzoyl- benzyl)trimethylammonium chloride, followed by freezing at a defined negative temperature, an extremely short UV irradiation and subsequent thawing. HEC cryogels were successfully prepared also by using H 2 O 2 as a photoinitiator. The effects of the temperature of freezing, the HEC molecular weight and the concentration of HEC solution on the cross-linking efficiency, the swelling ratio and the enzymatic degradation of HEC cryogels were investigated. Due to the cryoconcentration phenomenon, cryogels are formed at substantially low initial concentrations of the studied polymers. The highest values of gel fraction yield are achieved in the 1e2 wt.% concentration range at 20  C. As a rule, the higher the molecular weight, the greater the gel fraction yield of the resulting cryogels. Scanning electron microscopy (SEM) analysis reveals that the interior structure of HEC cryogels is completely different from the conventional HEC hydrogels. HEC cryogels undergo decomposition by the action of cellulase enzyme, however, due to their specific morphology, the rate of degradation is slower compared to the conventional HEC hydrogel of similar gel fraction yield. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Cryogels; UV irradiation; Hydroxyethylcellulose 1. Introduction Hydrogels based on both natural and synthetic polymers have found various applications in medicine and pharmacy as drug and cell carriers, tissue engineering matrices, mem- branes for biosensors, contact lenses, etc. [1e3]. Due to envi- ronmental issues, there is a growing interest in developing natural polymer-based hydrogels that guarantee biodegradabil- ity. In particular, the cellulose derivatives have received con- siderable attention because of their water solubility, easy biodegradation and low costs. Hydrogels of cellulose deriva- tives can be obtained either by reaction with chemical reagents [4e6] or by ionizing radiation [7e11]. Generally, two compet- ing processes, degradation and cross-linking, take place at high-energy irradiation of cellulose derivatives with either electron beam or gamma rays [7,8,10]. It has been found that the examined polymers undergo degradation when exposed to ionizing radiation at ambient temperature in solid state and in aqueous solutions of low concentration (less that 10 wt.%), while the best results of cross-linking have been obtained at paste-like conditions (25e40 wt.%, depending on the polymer). Noteworthy, in the case of highly concentrated polymer solutions several days are required for complete dissolution of the cellulose derivative in water [7]. Nowadays, there is an increasing interest on macroporous polymer gels prepared by cryotropic gelation [3,12e18]. The cryogels obtained are macroporous materials with an open po- rous structure, which significantly increases their equilibrium * Corresponding author. Tel.: þ35929792281; fax: þ35928700309. E-mail address: ppetrov@polymer.bas.bg (P. Petrov). 0032-3861/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2007.06.027 Polymer 48 (2007) 4943e4949 www.elsevier.com/locate/polymer