Article pubs.acs.org/Biomac
Wound Healing Bionanocomposites Based on Castor Oil Polymeric Films Reinforced with Chitosan-Modified ZnO Nanoparticles Ana M. Díez-Pascual*,† and Angel L. Díez-Vicente‡ †
Analytical Chemistry, Physical Chemistry and Chemical Engineering Department, Faculty of Biology, Environmental Sciences and Chemistry, Alcalá University, E-28871 Alcalá de Henares, Madrid, Spain ‡ Airbus Operations S. L., John Lennon s/n, 28906 Getafe, Madrid, Spain ABSTRACT: Castor oil (CO), which is a readily available, relatively inexpensive, and environmentally benign nonedible oil, has been successfully used as matrix material to prepare biocompatible and biodegradable nanocomposite films filled with chitosan (CS)-modified ZnO nanoparticles. The biocomposites were synthesized via a simple and versatile solution mixing and casting method. The morphology, structure, thermal stability, water absorption, biodegradability, cytocompatibility, barrier, mechanical, viscoelastic, antibacterial, and wound healing properties of the films have been analyzed. FT-IR spectra were used to obtain information about the nanoparticle−matrix interactions. The thermal stability, hydrophilicity, degree of porosity, water absorption, water vapor transmission rate (WVTR), oxygen permeability (Dk), and biodegradability of the films increased with the CS-ZnO loading. The WVTR and Dk data obtained are within the range of values reported for commercial wound dressings. Tensile tests demonstrated that the nanocomposites displayed a good balance between elasticity, strength, and flexibility under both dry and simulated body fluid (SBF) environments. The flexibility increased in a moist atmosphere due to the plasticization effect of absorbed water. The nanocomposites also exhibited significantly enhanced dynamic mechanical performance (storage modulus and glass transition temperature) than neat CO under different humidity conditions. The antibacterial activity of the films against Escherichia coli, Staphylococcus aureus, and Micrococcus luteus bacteria was investigated in the presence and the absence of UV light. The biocide effect increased progressively with the CSZnO content and was systematically stronger against Gram-positive cells. Composites with nanoparticle loading ≤5.0 wt % exhibited very good in vitro cytocompatibility and enabled a faster wound healing than neat CO and control gauze, hence showing great potential to be applied as antibacterial wound dressings. hyaluronic acid (HA),4 collagen,5 sodium alginate (SA),6 chitosan (CS) and its derivatives.7 In particular, PU membranes are increasingly popular due to good physical strength, abrasion resistance, and tissue compatibility. However, their dense structure leads to low water vapor and gas permeability, which can cause exudate accumulation and consequence related infections.8 Further, their efficiency against bacteria contamination is typically low, hence modifications are necessary to overcome these limitations. Polymeric materials are widely used in the biomedical field due to their good combination of properties. Among them, vegetable oils are very promising candidates for polymer production due to their biodegradability, sustainability, versatility of chemistry, and cost-effectiveness.9 The main constituents of plant oils are triglycerides which are the product of esterification of glycerol with three fatty acids.10 Depending on the fatty acid distribution, each type of oil has specific
1. INTRODUCTION In their daily lives, human beings are always under threat from various kinds of infections. The skin plays a key role in the prevention of infections from microbes and simultaneously keeping the homeostasis of the body. Once a trauma is suffered, the damaged skin should be immediately covered with a dressing, which should be able to maintain a moderately moist environment for regeneration of the skin, prevent infection, alleviate pain, allow gaseous exchange and remove excessive exudates. Besides, a soft and flexible texture, elasticity and high mechanical strength, chemical and physical stability, biocompatibility, biodegradability, antibacterial activity and easy applicability are required.1 In addition, major desirable characteristics are that wound dressings should be easily sterilized, have a long shelf life, and be cost-effective.2 However, commercially available dressings like gauze, paraffin gauze, or methyl cellulose only satisfy a few of these standards, hence new materials for use in wound healing are sought. In this regard, films/membranes with a homogeneous polymeric network structure have been recently developed. The polymers used include polyurethane (PU), polyvinylpyrrolidone (PVP),3 © 2015 American Chemical Society
Received: April 5, 2015 Revised: August 22, 2015 Published: August 24, 2015 2631
DOI: 10.1021/acs.biomac.5b00447 Biomacromolecules 2015, 16, 2631−2644
Article
Biomacromolecules Table 1. Fatty Acid Composition of Castor Oil fatty acid
structure
%
saturated
palmitic stearic
CH3(CH2)14COOH CH3(CH2)16COOH
C16:0 C18:0
unsaturated
oleic linoleic linolenic ricinoleic
CH3(CH2)7CHCH(CH2)7COOH CH3(CH2)4CHCH−CH2−CHCH(CH2)7COOH CH3−CH2−CHCH−CH2−CHCH−CH2−CHCH(CH2)7COOH CH3(CH2)5CHOH−CH2−CHCH(CH2)7COOH
C18:1 C18:2 C18:3 C18:1−OH
physical and chemical properties affecting the final properties of the synthesized polymer. One of the most frequently used for the synthesis of ecofriendly resins is castor oil (CO). It is obtained by pressing the seeds of the Ricinus communis plant, which belongs to the Euphorbiaceae family, and has attracted a lot of research interest because of its use in coatings, adhesives, paints, sealants, and encapsulating compounds. As a vegetable oil with reactive hydroxyl functional groups, it can be used as a polyol to develop new and “green” polymeric materials.11,12 Over centuries, CO has been recommended for its potent medicinal and curative effects.13 Some modern therapeutic uses of CO include gastrointestinal remedy, antimicrobial, antiinflammatory, analgesic, and lymphatic stimulant. The benefits of the oil can be obtained by topical application, and it seems to be useful for a variety of skin problems such as keratosis, dermatosis, wound healing, acne, ringworm, warts, and other skin infections.13 Further, castor-oil derived polyesters have been reported to exhibit antimicrobial activity against human pathogen bacteria Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus).14 Therefore, CO-based polymers display great potential as wound dressing materials. Metal oxide nanoparticles (NPs) such as zinc oxide (ZnO) have attracted much attention as future materials due to their interesting properties including large surface-to-volume ratio, high surface reaction activity, high catalytic efficiency, and strong adsorption ability,15 which make them suitable candidates for a broad range of applications. With a wide bandgap of 3.4 eV and a large exciton binding energy of 60 meV at room temperature, ZnO is widely used for optical devices.16 It also exhibits intense UV absorption, hence can be used as a UV-shielding material, and displays antimicrobial activity against both Gram-positive and Gram-negative bacteria even in the absence of light.17 Further, it possesses high stiffness and hardness, low coefficient of thermal expansion, and high thermal conductivity. Nano-ZnO can be synthesized in many forms: rods, wires, whiskers, belts, tubes, flowers, bridges, and cages. ZnO−NPs can be prepared by different techniques, such as the sol−gel method, precipitation, hydrothermal synthesis, and spray pyrolysis.18 Chitosan (CS), poly[β-(1 → 4)-2-amino-2-deoxy-D-glucopyranose], is a natural cationic biopolymer obtained from deacetylation of chitin by thermochemical reaction. As a natural polymer, it has many advantages such as nontoxicity, nonallergenic, biodegradability, biocompatibility, inexpensiveness, hydrophilicity, and antibacterial activity.19 CS contains two functional groups, hydroxyl and amino, which are responsible for its powerful adsorptive capacity and has been regarded as a useful material for various purposes such as treatment of wastewater, ion-exchanger and functional matrixes.20 This polysaccharide has been found to be effective against both Gram-negative and Gram-positive bacteria, although its effectiveness depends on several factors including
1.3 1.2 4.0 5.2 0.3 89.0
its molecular weight, degree of deacetylation (DD), and concentration as well as the surface characteristics of the bacterial cell wall (hydrophilicity and charge).21 The exact mechanism of its antimicrobial effect is still under discussion, with a few studies suggesting its ability to penetrate the bacterial cell wall via pervasion and development of a polymer membrane on the surface of the cell wall.22 This biopolymer is a relevant candidate in the field of biomaterials, especially for tissue engineering. In addition, CS has been widely used as a topical dressing in wound management owing to its hemostatic and stimulation of healing properties and its ability to deliver extrinsic antimicrobial agents to wounds and burns.23 Lately, CS hybrid materials including metal and metal oxide NPs have been developed with excellent properties arising from synergistic effects.24−27 It has been shown that combining CS or modified CS with Ag results in hydrogel materials with superior antimicrobial activity, increased tensile strength but decreased water vapor permeability.27 Further, blends of CS, PVP, and nano-TiO225 or Ag2O26 display excellent antimicrobial and wound healing properties. Moreover, ZnO−CS complexes with antimicrobial and antibiofilm characteristics have been very recently developed via nano-spray-drying and precipitation methods.28 More importantly, several papers including in vitro and in vivo studies about chitin/ZnO,29,30 CS/ZnO,31 and alginate/ZnO32 nanocomposites have been reported, demonstrating their great potential as wound dressing materials. In this study, castor oil was used as matrix material for the development of nanocomposite films filled with different amounts of CS-modified ZnO nanoparticles. The bionanocomposites have been characterized through a variety of techniques to obtain information about their morphology, thermal stability, water absorption, cytocompatibility, biodegradability, barrier, mechanical, viscoelastic, antibacterial, and wound healing properties. These novel biomaterials with antimicrobial activity are very promising for use in biomedical applications, particularly as wound dressings.
2. EXPERIMENTAL SECTION Materials. CO was supplied by Sigma-Aldrich and used without purification. Its main characteristics are iodine value = 85; hydroxyl value = 168; saponification value = 180; acid value = 3; d25°C = 0.96 g/ cm3; Mw ∼ 937 g/mol. Its fatty acid composition is shown in Table 1. ZnO nanopowder,
