ZIF-8 enriched electrospun ethyl cellulose/polyvinylpyrrolidone scaffolds: The key role of polyvinylpyrrolidone molecular weight

Abstract

A set of zeolitic imidazolate framework-8 (ZIF-8) incorporated ethyl cellulose/polyvinylpyrrolidone scaffolds was prepared by electrospinning method. The impact of polyvinylpyrrolidone molecular weight on characteristics of prepared scaffolds was investigated. The ethyl cellulose/polyvinylpyrrolidone scaffold made of polyvinylpyrrolidone 17,000 showed the narrowest nanofibers (mean diameter = 140 nm), the highest percentages of porosity (62%) and swelling (>130%), the most hydrophilic surface (water contact angle = 74°), the fastest rate of degradation, and the best elongation at break (6.3%) among the samples. Furthermore, the higher capability of this scaffold for cell proliferation was revealed through MTT test (125% after 5 days of culture), Live/Dead assay, and FESEM images of cell attachment. This scaffold also represented the highest released amount of ZIF-8-induced Zn²⁺ ions (20 ppm after 84 h) leading to its greatest antibacterial activity. These findings indicated that the ZIF-8 incorporated ethyl cellulose/polyvinylpyrrolidone scaffold can be used in future skin tissue-engineered constructs.

Introduction

Skin is the largest organ in the human body that establishes a shield between the body and the external world. The main role of the skin is the protection of the body against various dangers including body dehydration, adverse effects of chemicals, and UV irradiation. In addition, skin plays other important roles in the body such as adjusting the body temperature, the sensation of external stimuli (touch, pain, temperature), supplying vitamin D for the body, etc. Therefore, chronic skin injuries caused by trauma, diabetes, burn, congenital anomalies, etc. can put the human in the danger of amputation or even death (Dias, Granja, & Bártolo, 2016; Keirouz, Chung, Kwon, Fortunato, & Radacsi, 2020). Allografting and autografting as common treatments for full-thickness skin damages have respectively faced issues such as immunological rejection and morbidity of the donor site. Recently, tissue engineering has been employed in skin regeneration to overcome the disadvantages of previous methods (Lin, Chen, Chang, & Ni, 2013; Sundaramurthi, Krishnan, & Sethuraman, 2014). Three main factors of tissue engineering are scaffolds, cells, and biological agents that come together to mimic the function of a specific tissue like skin tissue. Scaffolds are key requirements of a successful skin tissue engineering that can not only stimulate the formation of extracellular matrix (ECM) but also prepare a suitable environment for cell adhesion and cell growth (Zhang et al., 2019). Different methods have been used in the literature to fabricate proper scaffolds for skin tissue engineering and most of them have failed to form high-standard scaffolds that can imitate the nanostructure of a native skin (Meyer, Meyer, Handschel, & Wiesmann, 2009; Yang et al., 2008). In this regard, electrospinning as a simple and efficient technique has been used for the production of nanofiber-based scaffolds that can imitate the natural ECM on the nanoscale. Electrospinning is fast developing from a single-fluid process (Krugly et al., 2022) to coaxial (Liu, Chen, Liu, Gao, & Liu, 2022), tri-axial (Wang et al., 2020), side-by-side (Zhang et al., 2022), and other complicated processes. The used blending process in the present study is still the main stream which is facile to be scaled up (Brimo, Serdaroğlu, & Uysal, 2021). Electrospun skin scaffolds represent similar mechanical characteristics to native skin. They provide high surface area and great cell proliferation in comparison to three-dimensional scaffolds fabricated by other methods. Moreover, the high surface area and porosity of the electrospun scaffolds facilitate the permeation of oxygen and nutrition, prevent waste accumulation, and provide an appropriate medium for the delivery of therapeutic agents (Dias et al., 2016; Kumbar, Nukavarapu, James, Nair, & Laurencin, 2008). Cellulose is the most abundant biopolymer on earth that possesses a variety of excellent characteristics such as biocompatibility and low cost. However, an important drawback of pure cellulose is its poor solubility in usual organic solvents which restricts its wide use in different areas. Therefore, using cellulose derivatives like ethyl cellulose (EC) are good replacements that not only preserve the main properties of pure cellulose but also enhance its dissolubility. The reaction of alkali cellulose with ethyl chloride results in a partially ethylated cellulosic ether named EC. EC is not dissoluble in water, however, this hydrophobic polymer is soluble in different organic solvents such as toluene, ethanol, and methanol. In addition to renewability and solubility features, EC represents wonderful mechanical properties that render it a suitable polymer for tissue engineering and sustained drug release (Li et al., 2019; Oprea & Voicu, 2020; Wali et al., 2018). Polyvinylpyrrolidone (PVP) is a hydrophilic and safe polymer that has been used in different areas such as medicine, pharmacy, and food. Since PVP can form fibers and help the spinnability of EC, the combination of PVP and EC can result in uniform electrospun fibers. Both EC and PVP, and their different combined formats are broadly investigated for tissue engineering and drug delivery, however, the influences of PVP are seldom mentioned (Godakanda et al., 2019; Keirouz, Fortunato, Zhang, Callanan, & Radacsi, 2019).

Metal-organic frameworks (MOFs) are crystalline three-dimensional porous structures. MOFs are composed of metal cations or clusters that are connected to polyfunctional organic ligands. Zeolitic imidazolate framework (ZIF) compounds such as ZIF-8 are a new class of MOFs in which divalent metal cations are connected by imidazolate anions and form tetrahedral frameworks that usually express a zeolite topology. ZIF-8 has shown great characteristics such as high surface area, chemical and thermal stability, and antibacterial activity which nominate it as an appropriate choice to be used in biomedical applications (Cravillon et al., 2009; Horcajada et al., 2010; Hoseinpour & Shariatinia, 2021; Kohsari, Shariatinia, & Pourmortazavi, 2016; Zirak Hassan Kiadeh et al., 2021). Multiple components ensure multiple-functional performances. Therefore, the encapsulation of MOFs can endow the scaffolds with fine antibacterial properties. The antibacterial activity of MOFs and ZIFs has been related to the release of metal cations into the surroundings media and several studies have confirmed this fact (Berchel et al., 2011; Hajibabaei, Zendehdel, & Panjali, 2020; Kohsari et al., 2016; Tamames-Tabar et al., 2015).

The main objective of the present study was the preparation of an antibacterial uniform electrospun mat based on EC and PVP with enhanced surface wettability, reasonable degradation rate, good mechanical strength, and high cell proliferation ability for potential use in skin tissue engineering. Therefore, a set of electrospun EC/PVP nanofiber scaffolds with different ratios of EC:PVP (90:10, 80:20, and 70:30) were fabricated using three kinds of PVP with different molecular weights (17,000, 25,000, 30,000 Da). The optimum ratio of EC:PVP for the electrospinning process was selected. Thereafter, morphological aspects, water contact angle, swelling ratio, degradation rate, tensile strength, cell viability, cell proliferation, and cell attachment of the fabricated scaffolds were investigated. Moreover, ZIF-8 nanoparticles were incorporated into the network of scaffolds with different molecular weights of PVP inducing Zn²⁺ ions release and antibacterial activity. Finally, the results of all analyses were considered to indicate the most appropriate PVP molecular weight for the preparation of the ZIF-8 loaded EC/PVP electrospun scaffold with desired characteristics.