Self-healable hyaluronic acid/chitosan polyelectrolyte complex hydrogels and multilayers

Highlights

• Hyaluronic acid and chitosan assembled as hydrogels and nanometric multilayers.

• Formation of the complexes was demonstrated in hydrogels and multilayers.

• HA/CHI hydrogels and multilayers showed self-healing properties in a few minutes.

• Self-healed hydrogels maintained the rheological properties of the initial material.

Abstract

In this work, polyelectrolyte complexes (PEC) were obtained in form of nanometric polyelectrolyte multilayers (PEMs) and macroscopic hydrogels by electrostatic interactions between two natural polysaccharides: hyaluronic acid (HA) and chitosan (CHI). PEM was developed onto poly(ethylene terephthalate) (PET) surface by layer-by-layer approach. The characterization of both systems was carried out simultaneously by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), X ray diffraction (XRD), thermogravimetric analysis (TGA) and transmission and scanning electronic microscopy (TEM and SEM). The rheological properties of PEC hydrogels were also analysed. The formation of the complexes was demonstrated due to electrostatic interactions, which in turn resulted being responsible for intrinsic self-healing ability. This is a highly demanded property in reducing replacements costs. Both, PEC hydrogels and multilayers showed self-healing properties within few minutes, this fact proves the versatility of HA/CHI complexes to easily obtain different forms of self-healable materials interesting as functional biomedical supports and coatings.

Introduction

Self-healing is the ability showed by some materials, which are able to repair themselves after being damaged by restoring broken bonds or interactions [1]. When an implantable and biodegradable material is designed, self-healing is an essential property since materials suffer from mechanical stress derived from movement, cell growth or proliferation, which could damage and even degrade materials before completing its objective. Thus, self-healing property improves biomaterial’s safety, reduces replacement costs and increases its lifetime, as well as, allows recovering their original shape [2], [3]. Materials showing this ability can be classified in extrinsic or intrinsic. The extrinsic implies the release of a self-healing agent embedded in the polymeric matrix by crack propagation; the intrinsic, repairs itself owing to the reversibility of chemical bonds or non-covalent bonds, such as, hydrogen bonds, electrostatic interactions and metal-ligand coordination. Lately, especial attention in intrinsic self-healing materials has been paid, owing to the fact that they do not require the use of healing-agents, which can cause side interaction and biocompatibility problems associated [4].

Autonomous self-healing materials repair themselves by a two-step mechanism, which is independent of the interactions between damaged parts. Firstly, interdiffusion of polymer chains between injured zones occurs; subsequently, bonds are restored. Diffusion rate, which depends on the temperature and the length of the free chain, has to be considered when designing self-healing materials. In this way, augmenting chain length will produce a greater interpenetration; just as increasing temperature will enhance diffusion [5], [6].

Regarding to non-covalent bonds, systems based on hydrogen bonds and electrostatic interactions are the most widely studied. Few works, which based their healability only on hydrogen bonds are found. This is due to the fact that water can also form these interactions with polymer chains weakening polymer-polymer interactions and subsequently, the healing process. Even so, it can be found self-healing of polyvinyl acetate (PVA) hydrogels by hydrogen bonds reported by Zhang et al. [7]. On the other hand, ionic bonds formed between oppositely charged ions, also showed repairing process; for example iron(III)/poly(acrylic acid) (Fe³⁺/PAA) hydrogel, which heals due to the diffusion of Fe³⁺ and the following interaction with carboxylic groups of PAA [8], [9] Concerning dynamic covalent bonds, many types are found, to mention imine bonds. These are also known as Schiff base and are formed between aldehyde and amine groups. Yang et al. [10] reported healability resulting from the association between benzaldehyde groups of difunctional poly(ethylene glycol) and amine groups of glycol chitosan.

Interpolymer complexes are formed as a result of polymers association, which can be driven by an specific interaction between polymer chains, such as hydrogen bond or electrostatic interactions [11]. The latter are known as polyelectrolyte complexes (PECs) and could give rise to self-healing materials due to the dynamic nature of electrostatic interactions between polyions. In order to form polyelectrolyte complexes, compounds must present functional groups capable of interacting with each other by means of electrostatic interactions. Their formation depends on many factors, such as, molecular weight and concentration of polyelectrolytes and mixing ratio [12]. Moreover, it is also pH dependent, due to the nature of the polyions; strong polyions dissociate in all pH values, whereas, weak ones only dissociate in specific range of pH [13]. Among their applications, it is worth mentioning the use of most PECs obtained from natural polymers as biomaterials, such as wound-dressing, drug delivery or bioadhesives [14].

Biopolymers, rather than synthetic polymers, are a suitable option to develop self-healing polycomplexes because they provide materials with biocompatibility and non-toxicity. Many combinations have been reported, such as alginate-chitosan complexes for drug or gene delivery [15], chitosan and γ-poly(glutamic acid) for wound healing [14] or chitosan-xanthan for controlled delivery of encapsulated products [16]. In this work, hyaluronic acid (HA) and chitosan (CHI) based complexes were studied, since their combination has been proven to enhance antimicrobial ability [14]. Hyaluronic acid (HA), a linear glycosaminoglycan with high molecular weight, is composed of repeating disaccharide molecules of β-(1,4)-D-glucuronic acid and β-(1,3)-N-acetil-D-glucosamine. It is a weak polyanion with pKa = 2.9, biocompatible, viscoelastic and non-toxic; it also shows a high hydrophilicity, lubricant and moisturising nature which helps in enhancing biocompatibility and avoiding biofilm formation [17]. In fact, some research has shown the ability of hydrophilic surfaces to reduce biofilm formation due to hydrophobic nature of bacteria [18]. Chitosan (CHI) is a natural and linear polysaccharide, formed by two randomly distributed units, N-acetyl-2-amino-2-deoxy-D-glucose and 2-amino-2-deoxy-D-glucose. It derives from the partial deacetylation of chitin, which is founded in crustaceans’ exoskeletons. CHI is a weak polycation with pKa = 6.5, biocompatible, biodegradable and non-toxic; its cationic nature causes the disruption of negatively charged cell membranes of bacteria, showing great antibacterial properties [19], [20].

Although electrostatic interactions between HA and CHI has been well-studied [21], [22], few works have reported polyelectrolyte complexes based on this system. Among them, Lee et al. [23] synthetized hyaluronic acid and chitosan sponges in different pH in order to prepare a wound healing material. On the other hand, Ma et al. [24] prepared HA-CHI nanofibers as a suitable material for tissue engineering. Although systems based on these natural polymers have been prepared in the last years with the aim of developing suitable materials for medical applications, to the best of our knowledge, self-healing ability of HA/CHI polyelectrolyte complexes has not been studied yet.

These electrostatic interactions between HA and CHI have also been reviewed to develop biodegradable coatings that provide synthetic materials with enhanced biocompatibility and valuable properties, such as, wound dressing or antibacterials. For that, polyelectrolyte multilayers (PEMs) could be constructed, formed by the alternate adsorption of polycations and polyanions on a substrate surface. As a result, a very stable coating can be formed due to the crosslinking of the polyelectrolytes which can be controlled changing pH values; so, according to pH values, polyelectrolyte charges change and so do the crosslinking [25]. PEMs could be constructed by layer-by-layer (LbL) method, developed by Decher et al. [26] in 1992, which allows the control of layers thickness, molecular architecture and surface chemistry. The use of this technique is widely spread nowadays due to the simplicity to adsorb different layers on materials surface as well as being versatile and cost-effective [25].

Within all the types of substrates that can be used for PEMs construction, polymers have attracted great interest for being ideal candidates to develop biomaterials as they show great versatility comparing to other materials, such as metals or ceramics. Among them, a commercial low-cost polymer with great mechanical properties and biocompatibility is found: poly(ethylene terephthalate) (PET) [27], which can be used in catheters, heart-valves and implants [18]. PET surface modification allows to change its hydrophobic nature, providing the surface with good wettability, lubrication and biocompatibility [28]. Tough many attempts of surface modifications have been reported for PET with the aim of improving its features, nowadays layer-by-layer approach is the most widely used procedure. In the last decades, polyelectrolyte multilayers based on interactions between hyaluronic acid and chitosan has been widely studied on different substrates like titanium [21] or polydimethylsiloxane [29]. Nevertheless, few attempts have been made onto PET using layer-by-layer approach [28], [30], which will provide PET with the above mentioned characteristics, as well as with an additional advantage: self-healing.

Surface self-healing ability has been studied after modifying the surface of different materials. For example, Zu et al. [31] constructed chitosan and polyacrylic acid layers onto a glass substrate using different pH values for the chitosan solution in order to study the influence of the pH in the self-healing. Wang et al. constructed different layers of poly(ethylenimine) and polyacrilic acid onto a silicon substrate and [4] showed that self-healing depends on the pH of polyelectrolyte solutions, thickness of the layers and the width of the cuts made to the substrate.

This work aims to synthesize and characterize polyelectrolyte complex hydrogels based on the interaction of HA/CHI polysaccharides to develop a self-healing material suitable for medical applications. Although few works have studied the complexation between these biopolymers [23], [32], to the best of our knowledge, self-healing ability has not been studied yet. Furthermore, HA/CHI complexation was also carried out onto PET surface by the built-up of a multilayered system with a well known biocompatibility and antibacterial properties. Additionally, surface self-healing provided by HA/CHI multilayers was also studied, to design a long-lasting and secure biomaterial.