Ionic liquid modified electroactive polymer-based microenvironments for tissue engineering
Graphical abstract
Introduction
Muscle loss is a particularly serious case of tissue damage, due to the low regenerability of muscle tissue in the case of large volumetric loss, and the complexity of the tissue itself, rendering many therapies ineffective or with less-than-desirable results [1]. In this scope, tissue engineering (TE) emerges as an effective approach to muscle repair. TE proposes, among other strategies, to grow new tissue in vitro from samples taken from the patient itself, to be implanted afterwards in the patient as required, without rejection issues nor the need of compatible donors [2]. However, the specific tissue growth and development requirements of skeletal muscle fibers mean that, to our knowledge, no suitable solutions have yet been developed.
One of the main challenges relies on the fact that muscle cell growth and development is greatly enhanced by active stimuli of cells, such as mechanical and/or electric, by means of scaffolds based on oriented fibers [3,4] mimicking the natural growth conditions of muscle tissue [5]. For this, muscle TE requires the use of biocompatible materials with a controllable mechanic- and electroactive response, being also desirable materials that are biodegradable as well, degrading as the new functional tissue is generated. This will minimize possible long-term health issues, related to the implantation of permanent extrinsic materials within the body.
Electroactive smart materials can be processed into different shapes and morphologies such as fibers, films, and microspheres [6], and have demonstrated potential applicability in a variety of TE applications including bone [7], cardiac [8], and skeletal muscle tissues [3,4,9], where it has verified that an active scaffold with a specific morphology resembling the morphology of the tissue to be regenerated is a suitable approach for functional tissue regeneration.
In particular, electrospinning is a method for obtaining oriented polymer fibers that allows a fine control of fiber properties, and can tune them to the individual needs of specific tissues and microenvironments, such as skeletal muscle tissue, improving both biomimicry and cellular development and regeneration [4].
Among the different electroactive polymers, poly(vinylidene fluoride) (PVDF) has been extensively studied as scaffold for TE applications, due to its biocompatibility, processing versatility and high piezoelectric response associated to the presence of the β-crystalline phase [7,10]. In order to be used in skeletal muscle TE, it has been combined with different organic and inorganic materials, such as cobalt ferrite nanoparticles (CFO) [9], silica nanoparticles [5], and ionic liquids (ILs) [10], among others [11]. However, it is a non-biodegradable material, which restricts its applicability. For this reason, electroactive and biodegradable polymers such as polyhydroxybutyrate-co-hydroxyvalerate (PHBV) are of increasing interest.
PHBV is a biocompatible and biodegradable co-polymer of polyhydroxybutyrate (PHB), with good bioactivity and piezoelectric properties [12], and is an excellent candidate to replace PVDF for TE applications where biodegradability is an in vivo requirement. PHBV has also been processed into different morphologies, including films, fibers and microspheres [13], and presents improved mechanical properties when compared to PHB [12]. PHBV has been used as polymer matrix for the development of composite materials, with tailored functional properties, in combination with ILs and graphene oxide-ZnO for selective barriers in advanced packaging [14], or for TE, combined with CFO [12] and iron oxide-graphene oxide [15], as well as silver nanoparticles, graphene or other polymers [13].
As a material for tailoring the functional response of polymer matrices, ILs have been gaining attention for TE [16,17]. ILs are defined as salts with a low melting temperature (under 100 °C), high electrochemical and thermal stability and their main properties can be tailored by the proper selection of anions and cations. Thus, ILs and IL based polymer hybrid materials have been explored for batteries, actuators, biosensors and biomedical devices [17]. Furthermore, due to the biocompatibility and biodegradability of many ILs, these have been used for different TE applications [16]. This is the case of 1-butyl-3-methylimidazolium chloride and 2-hydroxyethyl-trimethylammonium dihydrogen phosphate for muscle tissue engineering [10], 1-butyl-3-methylimidazolium acetate for skin regeneration [16], or 1-ethyl-3-methylimidazolium acetate for neural TE [16], combined with polymers such as PVDF [10,18] or cellulose blends [19].
One of these biodegradable and biocompatible ILs is choline acetate ([Chol][Ac]), that displays high ionic conductivity [[20], [21], [22]] and has previously been evaluated as an electrolyte in Zn/air rechargeable batteries [22], as an actuator in combination with polypyrrole-PVDF [20], and for biomedical applications when combined with α-chitin [21]. However, the overall research on the combination of ILs and PHBV is limited, and to the best of our knowledge, there are no reports about ILs being used with PHBV polymeric scaffolds for TE applications, nor on the use of [Chol][Ac] for TE applications. It is to notice that, together with the intrinsic characteristics of each of the components of the hybrid materials, IL-polymer composites show, for specific filler concentrations, piezo-ionic effect particularly suitable for the mimicking of muscle tissue [10,16].
In this context, the present work focuses on the development of PVDF and PHBV electrospun fibers and films to be applied as scaffolds for tissue engineering applications, in order to study the possibilities of both as biostable and biodegradable platforms, respectively. The morphology, physico-chemical, thermal and conductive properties of the PVDF and PHBV hybrid fibers and films with the IL [Chol][Ac] were comparatively studied and their potential to be applied as scaffolds for muscle TE assessed.
Section snippets
Materials
PHBV (Mw = 460.64 g mol−1; HV = 3%, mole fraction), 99% purity, was supplied from Natureplast. PVDF with different molecular weights (PVDF 5130 (Mw = 1000–1100 kDa) and PVDF 6010 (Mw = 300–320 kDa)) purchased from Solef and Solvay, respectively, were used. PVDF 5130 was selected for electrospun fiber mats preparation, as high PVDF molecular weights favors the production of homogeneous fibers without the presence of beads [23]. [Chol][Ac] was acquired from Iolitec. Chloroform (Chromasolv 99.8%)
Morphological characterization of the fibers and films
The morphology of the PHBV and PVDF samples with different IL contents was analyzed by SEM. Fig. 1 shows representative SEM images of PVDF and PHBV electrospun and film samples.
Regardless of polymer type, and for the same processing conditions, the inclusion of IL into the polymer matrix induces an increase in the randomness of the fiber orientation (Fig. 1 a-f), with this effect being more noticeable for the PHBV + IL electrospun fibers, an effect already reported in the literature [27].
For
Conclusions
PVDF and PHBV with different [Chol][Ac] IL contents (0, 5, 10 and 15 wt%) were processed into electrospun fibers and films, to support advanced muscle tissue repair and regeneration strategies.
The morphology, physico-chemical and thermal properties of the samples were evaluated. It was verified that the IL was homogeneously distributed in all samples. Increasing IL wt.% results in a smaller electrospun PHBV fiber diameter, from 1.73 ± 0.21 μm for neat PHBV to 0.18 ± 0.05 μm for PHBV + IL 10%
CRediT authorship contribution statement
B. Hermenegildo: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. R.M. Meira: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. A.G. Díez: Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. D.M. Correia: Methodology, Conceptualization, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019-106099RB-C43/AEI/10.13039/501100011033. Financial support from the Basque Government Industry departments under the ELKARTEK program is also acknowledged. The authors acknowledge funding by the Fundação para a Ciência e Tecnologia (FCT) and by ERDF through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) in the framework
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