Effects of Ar–H₂–N₂ microwave plasma on chitosan and its nanoliposomes blend thin films designed for tissue engineering applications

Abstract

This work addresses the functionalization of chitosan thin films and its nanoliposomes blend films by a microwave-excited Ar/N₂/H₂ surface-wave plasma treatment which was found an effective tool to modify surface properties. Changes in the film properties (wettability, chemical composition, morphology) induced by the plasma treatment are studied using water contact angle measurements, X-ray photoelectron spectroscopy and scanning probe microscopy. The results suggest that hydrophilicity of the films is improved by plasma treatment in a plasma condition dependency manner. Water contact angle of chitosan films before and after plasma treatment are, respectively, 101° and 27°. Besides chemical changes on the surface, the nanoliposomes incorporation and plasma treatment also induce morphological modifications. Moreover, a correlation is found between the nanoliposomes composition and size, and the effects of plasma treatment. It is shown that the plasma treatment significantly improves the chitosan film functionalization. The effect of N₂ content (88% and 100%) in the plasma gas mixture on the film etching is also pointed out.

Introduction

As a biomaterial that exhibits an excellent biocompatibility and admirable biodegradability, chitosan has been largely used in the field of tissue engineering, drug delivery, and gene therapy (Bhardwaj and Kundu, 2011, Casettari et al., 2012, Dash et al., 2011, Domard, 2011, Duarte et al., 2010, Hoemann et al., 2005, Ji et al., 2011, Muzzarelli et al., 2012, Prabaharan and Jayakumar, 2009). Nevertheless, chitosan films suffer from low wetting properties in aqueous basic media with pH value higher than 7. This represents a drawback for the production of scaffolds for tissue engineering application.

Previous works show that by adding natural nanoliposomes based on vegetable and marine lecithin, the surface wettability of chitosan and blend films increased because nanoliposomes present polar phospholipids (Zhang et al., 2012). The choice of nanoliposomes has been preferred to liposome because of their higher surface contact area and their potential ability to increase solubility and to enhance bioavailability of the encapsulated material to a greater extent (Mozafari, 2005). The main constituents of nanoliposomes are phospholipids, which are amphiphilic molecules containing a water-soluble hydrophilic head section and a lipid-soluble hydrophobic tail section (Nirmala et al., 2011). The nanoliposomes in this work were prepared based on soya, rapeseed and marine lecithins. Rapeseed and soya lecithins consist mainly of three mono- and poly-unsaturated fatty acids namely oleic (C18:1), linoleic (C18:2), and linolenic acids (C18:3). Linoleic and linolenic acids are considered essential fatty acids because they are important to human health and our body cannot synthesize them (Coonrod, Brick, Byrne, DeBonte, & Chen, 2008). Marine lecithin from salmon (Salmo salar) contains a high percentage of polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) (Belhaj, Arab-Tehrany, & Linder, 2010). Numerous studies, both in humans and in animals, have demonstrated that PUFAs of the n-3 series, especially EPA and DHA, are necessary to several physiological processes (Mirajkar, Jamadar, Patil, & Mirajkar, 2011).

Plasma processes which are well-known tools to modify the properties of polymer surfaces (Belmonte et al., 2005, Hody et al., 2006a, Hody et al., 2006b, Moser et al., 2010, Ogino et al., 2008, Wanichapichart et al., 2009). This process could increase the wettability of chitosan films resulting in an improvement of the cell adhesion on chitosan surface (Morra, Occhiello, & Garbassi, 1989). The most critical parameters of the plasma treatment are the injected power, gas mixture composition, and process duration. In the present study, Ar/N₂/H₂ gas mixture was used, because such a plasma atmosphere is considered to be an effective precursor to introduce amine functional groups onto the material, thus enhancing their hydrophilicity and surface energy (Chen et al., 2010, Inagaki et al., 2007, Kral et al., 2008). Doehlert experimental design was used to obtain a number of distinct levels and the response surface methodology (RSM) was applied as an effective tool to get an optimal response.

The aim of this work is to functionalize chitosan films and its nanoliposomes blend matrix surface by cold plasma treatment for tissue engineering applications. In the first time, we characterized the fatty acid composition, lipid class, size and electrophoretic mobility of nanoliposomes. Then, several surface properties of films before and after cold plasma treatment were analyzed such as morphology and roughness, elemental composition and chemical bonds, water contact angle, and surface energy (Digidrop).