Fabrication of dual network self-healing alginate/guar gum hydrogels based on polydopamine-type microcapsules from mesoporous silica nanoparticles

Highlights

• Self-healing hydrogels of guar gum and sodium alginate were successfully obtained.

• Dual self-healing network were constructed by PDA-type microcapsules from mesoporous silica.

• These nanocomposite hydrogels have high strength (7.3 MPa) and excellent self-healing performance (95.5%).

Abstract

In this manuscript, dual network self-healing alginate/guar gum hydrogels with polydopamine-type microcapsules from mesoporous silica nanoparticles were facilely prepared through one-pot method. Glutaraldehyde (GA) was used as a crosslinking agent to crosslink guar gum and sodium alginate, respectively. Metal−ligand interactions as reversible non-covalent bonds make the dual network hydrogels have a high degree of self-healing ability, which FeCl₃·6H₂O was coordinated with sodium alginate and polydopamine on the surface of microcapsules, respectively. The dual network structure significantly enhances the strength of the hydrogels (up to 7.3 MPa). It has also proved that these hydrogels exhibit excellent self-healing performance at ambient temperatures, which self-healing efficiency can reach to 95.5%. The obtained self-healing hydrogels have a promising application prospect for the design and synthesis of functional self-healing materials.

Introduction

Hydrogel [[1], [2], [3], [4], [5], [6]] is a type of high molecular weight polymers with water as a dispersion medium and has three-dimensional network structure. Depending on different sources of substrates, hydrogels can be divided into biomass hydrogels [7] and synthetic hydrogels. Synthetic hydrogels have good stability, but their biodegradability and biocompatibility are relatively poor. Biomass hydrogels mainly include chitosan [[8], [9], [10]], cellulose [11], starch [12], sodium alginate [[13], [14], [15]], guar gum [[16], [17], [18], [19], [20], [21], [22]], etc. Since these natural materials have excellent biocompatibility and biodegradability, their utilization has become a hot topic of concern. For example, chitosan-type hydrogels, which can be implemented to a cartilage healing project, were successfully prepared by Hong [23] et al. The chondrocytes encapsulated in this hydrogel can maintain superior morphology in vitro for several days. However, the traditional biomass hydrogel has inferior mechanical properties, the structure can be easily destroyed, and the recoverability is low, which limits the potential application.

In recent years, researches have been studied on self-healing hydrogels [24,25] to extend their service life. Based on the using for additional healing agents or not, self-healing materials [[26], [27], [28]] can be divided into two types as intrinsic [29] and extrinsic [30]. Extrinsic self-healing materials mainly include hollow fiber type and microcapsule type [[31], [32], [33], [34]] materials of loading self-healing agent. Compared with the self-healing system of hollow fiber, the microcapsules as loading healing agent have more application values with the advantages of perception of microcrack and easier packaging into the matrix material, which have been industrialized. For example, suryanarayana [35] et al. studied an efficient microcapsule was used to heal the cracks in paint/coatings with loading linseed oil. After the simulated of mechanical action, linseed oil can release from microcapsules to in situ polymerizate with the substrates. However, multiple self-healing functions of microcapsules and their compatibility with materials have yet to be studied. The intrinsic self-healing materials can utilize the reversible and dynamic physicochemical action in the molecular network to achieve self-healing of the crack without external stimulus. These interactions include hydrogen bonds [36], metal coordination [37,38], ionic interactions [[39], [40], [41], [42], [43]], π–π stacking [44], and host-guest interactions [45]. Reversibility, rapidity, directionality and sensitivity make reversible dynamic chemistry particularly attractive.

However, single self-healing hydrogels still have some disadvantages such as low mechanical strength and less healing efficiency. So self-healing under the action of a dual self-healing mechanism has become the focus of current researches. Yang et al. [46] effectively improves the compatibility of graphene oxide (GO) with the substrate and the mechanical properties of the material through the dual action of reversible covalent bond and reversible hydrogen bond between the maleimide functionalized GO and the polyurethane chain. The prepared polyurethane composite materials have excellent self-healing properties and mechanical strength. Strengthening the network structure of self-healing gels is also an effective way to regulate their properties, such as double network hydrogels [47], nanocomposite hydrogels [48], double crosslinked hydrogels [32], and interpenetrating network hydrogels [49,50]. For instance, Li et al. [13] used a chemically-crosslinked polyethylene glycol (PEG) as a network and polyvinyl alcohol (PVA) as another network to achieve the dual self-healing process. Double network hydrogels (PVA-PEG) were successfully synthesized to achieve self-healing after being damaged. Compared with the single network hydrogels (PVA), the mechanical strength of the double network hydrogels increased by nearly 6 times reaching 1.3 MPa, and the dual network structure enhanced the strength of hydrogels obviously.

Mesoporous silica nanoparticles are widely used in the fields of drug delivery [51] and adsorption because of their unique advantages such as high specific surface area, large pore volume, morphology and controllable particle size. Therefore, the application of mesoporous silica nanoparticles will significantly improve self-healing properties. For example, Vijayan et al. [52] fabricated functional materials with excellent self-healing properties, which mesoporous silica was used as a storage medium to load the healing agent. Recently, mussel-inspired chemistry [53] have attracted much attention in cell and tissue engineering applications [54]. Dopamine (DA) and its derivatives can form polydopamine (PDA) and adhere to almost any materials surface under weak base conditions, which can be used for the surface modification of biological materials or for further surface modification after filming on the surface of materials [55]. As a convenient tool of surface modification, biological imaging [56] and adsorption materials [57] were successfully obtained by the development of mussel-inspired chemistry. After integrating the mussel-inspired chemistry and PET-ATRP, Matyjaszewski et al. [58] developed a green strategy for surface engineering of Fe₃O₄. Therefore, PDA and its derivatives [59,60] can be easily coated on the surface of porous materials with a large amount of amino groups on the surface.

Herein, we report a new type of dual network self-healing hydrogels with PDA-type microcapsules. The stellate mesoporous silica (STMS) was used to load of glutaraldehyde (GA) and encapsulated with PDA to control the release of GA as crosslinking agent. The microcapsules (STMS-GA@PDA) were added to the hydrogels for forming a dual self-healing system of Guar gum (GG)/Sodium alginate (SA). High mechanical and healing properties were obtained in these dual self-healing hydrogels, which used GA as crosslinking agent and coordination between SA, PDA and Fe³⁺, simultaneously. When the hydrogel is damaged slightly, self-healing process mainly relies on coordination. Fe³⁺ can coordinate with 5-carboxyl and 2-hydroxyl on the G unit of SA and the hydroxyl of PDA on the surface of the microcapsules, respectively. As the hydrogel is damaged seriously, the microcapsules can release GA to crosslink the damaged area to enhence the self-healing performance. The mussel-inspired chemistry and STMS as eco-friendly strategy for the functionalization for microcapsules can fabricate dual self-healing biomass hydrogel with the potential application for flexible sensor.