Mechanical and piezoelectric characterizations of electrospun PVDF-nanosilica fibrous scaffolds for biomedical applications

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ScienceDirect Materials Today: Proceedings 5 (2018) 15710–15716

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NTNM2017

Mechanical and piezoelectric characterizations of electrospun PVDF-nanosilica fibrous scaffolds for biomedical applications Seyyed Arash Haddadi, Saeed Ghaderi, Majed Amini, Ahmad Ramazani S.A.* a

Chemical & Petroleum Engineering Department, Sharif University of Technology, Tehran, Iran

Abstract The effects of hydrophilic and hydrophobic nanosilica (SiO2) on the morphology, mean diameter distribution of fibers, mechanical and piezoelectric properties of poly (vinylidene fluoride) (PVDF) nanofibers were studies. We prepared Nanofibers by the electrospinning of PVDF solutions containing 1.5 wt. % both hydrophilic and hydrophobic nano-SiO2 loadings. Morphology and diameter distribution of the electrospun nanofibers were studied using field emission scanning electron microscopy (FE-SEM) analysis. Tensile test was used to study the effect of both types of nanosilica on the tensile strength, young’s modulus and strain at break. Piezoelectric characterization of the electrospun fibers were determined using calculation of the output voltages under certain loading force. Results showed that the fibers diameter and mechanical properties of the nanofibers increase in presence of hydrophilic SiO2 nanoparticles. Furthermore, the output voltages of the electrospun fibers demonstrated that both types of silica could considerably amplify electroactive properties of PVDF fibers. However, the results for PVDF fibers containing the hydrophilic silica nanoparticle were more highlighted. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INN International Conference/Workshop on “Nanotechnology and Nanomedicine’’ NTNM2017. Keywords: Piezoelectric; Nanosilica; Poly(vinylidene fluoride); Scaffold; Biomedical applications

1. Introduction

Poly (vinylidene fluoride) (PVDF) with a simple chemical formula, -CH2-CF2-, has useful special properties such as the remarkable antioxidation, outstanding mechanical properties, high chemical resistance, maintaining cellular

* Corresponding author. Tel.: +9821-66166405; Fax: +9821-66166404. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INN International Conference/Workshop on “Nanotechnology and Nanomedicine’’ NTNM2017.

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functions with promotion of adhesive functionality, higher thermal and hydrolytic stabilities and excellent piezoelectric [1,2]. This semi-crystalline ferroelectric polymer is an important material which has a wide variety of applications in the biomedicine and industries [3]. This polymer is known by several crystallization phases such as α, β, γ and δ that each of these phases are represent a special characteristic and application [4–6]. The nonpolar phase α is most commonly found in commercial films which is formed when melt of PVDF is cooled. The polar β phase, in a same direction has dipole moments oriented, where the polymer chain whit has got the most technologically interesting because this phase displays the strongest pyro and piezoelectric properties [7]. Recently, many researcher have worked on addition of nanoparticles to PVDF, not only using nanofillers is simple but also is very effective. Presence of nanoparticle with a determine charge can increase the value of β phase [4,8,9]. According to the literature, a great number of fillers such as graphene oxide (GO) [10], nanoclay [4], MWCNT [8], TiO2 [11], SiO2 [12,13] and so on, were used to fabricate PVDF composites with unique properties. Among these fillers, silica nanoparticles can be used to promote the total characterization of PVDF matrix. Silica nanoparticles is an inorganic filler and wildly used in many industries. There are two main type of silica. Hydrophobic silica can be applied for non-polar matrix such as polyolefin, while hydrophilic type of silica is proper for polar matrix because of the existence of hydroxyl groups onto the surface [14]. Consequently, reorientation of partially polar polymer chains such as PVDF and its properties can be altered in presence of the hydrophilic and hydrophobic silica nanoparticles [5,6,9]. By considering the noticeable properties of silica nanoparticles such as easily loaded by different drugs and growth factors, high mechanical stiffness, capability of grafting with different functionalities, biocompatibility and biofunctionality to stem cells, easy to synthesis in different sizes [15,16] we loaded SiO2 nanoparticles on the PVDF fibers which resulted in a suitable nanocomposite electrospun fibers to use in biomedical engineering by improving specific properties of PVDF, such as mechanical characteristics, biocompatibility, cell differentiation in a growth free approach, ionic conductivity, etc. [15,17]. In this paper, the mechanical and piezoelectric properties of PVDF in presence of 1.5 wt. % hydrophilic and hydrophobic silica nanoparticles have been evaluated using FE-SEM, tensile test and piezoelectric measuring setup. The results demonstrated that addition of hydrophilic silica nanoparticles has the higher influence on the mechanical and piezoelectric characterization of PVDF. 2. Materials and Methods 2.1. Raw Materials Dimethyl formamide (DMF) as PVDF solvent and Polyvinylidene fluoride (PVDF) as matrix were prepared from Merck and Sigma-Aldrich, respectively. The average density and molecular weight of purchased PVDF were about 1.78 g ml-1 and Mw=270000 g mol-1, respectively. Hydrophilic nanoparticles of SiO2 with average particle size of 20-30 nm and purity of ≥98 % was provided from Nano-Sany Company (Iran). In order to prepare the hydrophobic grade of silica nanoparticles, the hydrophilic silica nanoparticles were placed in a tubular tube at 550 ºC for 12 h for the complete dehydration. 2.2. Preparation of PVDF-Silica Fibers Before electrospinning of PVDF-SiO2 solutions, silica nanoparticles were dispersed into DMF at a concentration of 1.5 wt. % based on the PVDF weight using sonication process. Then, PVDF pellets were slowly added to the stirring DMF-SiO2 solutions at room temperature for 12 h, resulted in a solution with 15 % (w/v) PVDF content. We used the neat PVDF (PPVDF) as a control sample. Then, by sucking composite solutions into two syringes, where needles were connected to the high voltage of 17 kV and then with a mass flow rate of 0.5 ml h-1 for each needle,

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these solutions were pumped and spinned onto a rotational drum (rotation speed=2000 rpm). The distance between the drum and each needle tips was 15 cm. All the experiments were performed at the same environmental conditions. 2.3. Characterization of PVDF-Silica Fibers Images from different regions of each sample were used to investigate the morphology of the electrospun fibers and obtain the mean fibers’ diameter by a TESCAN FE-SEM (Mira 3, Czech Republic). Mechanical characteristics of the electrospun fibers were tested using a universal tensile machine (H10KS, HOUNSFIELD, Germany) at a loading velocity of 1 mm min-1 at room temperature. Measuring the mean fibers’ diameter distribution was performed using an open source image processing software (Fiji). The voltage outputs of the electrospun fibers were evaluated using a ROHDE & SCHWARZ digital oscilloscope (HMO-3522, Germany) under constant loading force of 15±2 N. for more validation, triplicates were prepared for each samples. 3. Results and Discussion In Fig. 1, the morphology and average diameter of the electrospun nanofibers are illustrated.

Fig. 1. FE-SEM images and mean fiber diameter for (a) neat PVDF and PVDF containing 1.5 wt. % (b) hydrophilic and (c) hydrophobic silica nanoparticles.

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In presence of SiO2 nanoparticles due to formation of Si-O-Si network in polymer matrix solvent evaporation of composite nanofibers was decreased, which led to more beads and the higher mean fiber diameter for the electrospun fibers. It is clearly observed that hydrophilic SiO2 nanoparticles were uniformly distributed both in PVDF matrix and onto the surface of fibers. As it can be seen in Fig. 1, addition of both hydrophilic and hydrophobic SiO2 nanoparticles in PVDF matrix increased the average diameter of the electrospun fibers which is more evidence for the PVDF fibers containing hydrophobic silica nanoparticles and also changed the topology of the electrospun fibers. The hydrophobic silica nanoparticles are less compatible with PVDF matrix. Thus, more silica nanoparticles can migrate to the surface of PVDF fibers led to increase in the mean fiber diameter. In addition, the viscosity of PVDF solutions in presence of both hydrophilic and hydrophobic silica nanoparticles increases considerably resulted in increment of the diameter of electrospun fibers. The difference between the dispersion of hydrophilic and hydrophobic silica nanoparticles trough PVDF matrix has been illustrated in Fig. 2. Addition of the hydrophobic silica nanoparticles leads to the agglomeration of nanoparticles onto the surface of PVDF fibers.

Fig. 2. Dispersion of (a) hydrophilic and (b) hydrophobic silica nanoparticles through PVDF matrix.

The mechanical properties of the electrospun PVDF fibers were studied using tensile test. Fig. 3 and Table 1 show the typical stress-strain curves and obtained results for the electrospun fibers.

Fig. 3. Typical stress-strain curves of the neat PVDF and PVDF electrospun fibers containing hydrophilic and hydrophobic silica nanoparticles.

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According to the result of the tensile test in Fig. 3 and Table 1, by using both hydrophilic and hydrophobic SiO2 nanoparticles, tensile strength (MPa) increased and strain at break (%) from about 1.8 and 102 for the neat PVDF fibers to 2.9 and 67 and 5 and 88 for the PVDF fibers containing 1.5 wt. % of hydrophobic and hydrophilic SiO2 nanoparticles, respectively. Reinforcing the mechanical performance of the polymer can be provided by incorporating of inorganic nanoparticles with polymer chains due to the chemical and physical interactions. These interactions provide enhancement regions through the polymer matrix [5]. Strain at break for the neat PVDF fibers containing both hydrophilic and hydrophobic SiO2 nanoparticles showed the smaller value which it means that these PVDF fibers have lees flexibility (about 14 and 34 % lower than that of the neat PVDF fibers) [18]. Considering the higher mechanical performance of the electrospun nanofibers make them appropriate to be used in other applications such as fabrication of scaffolds for cell culture and battery separators. But, less flexibility lead to more brittleness and this means that these membranes are not suitable for applying in the polymer electrolytes for batteries and fuel cell. Table 1. Results of tensile test for the electrospun PVDF fibers. Sample Neat PVDF PVDF-Hydrophilic Silica PVDF-Hydrophobic Silica

Tensile strength (MPa) 1.84±0.25 5±0.35 2.9±0.3

Young’s modulus (MPa) 0.55±0.1 11.4±0.8 5.16±0.5

Strain at break (%) 102.6±18 88.2±12 67±9

Addition of the hydrophilic nanoparticles into PVDF matrix lead to the formation of Si-O-Si bridges because of the formation of H-bonds between the hydroxyl groups onto the surface of the hydrophilic silica nanoparticles. Also, this phenomenon makes the hydrophilic silica nanoparticles more compatible to disperse uniformly in PVDF matrix. So, the regions for stress concentration decrease due to the better dispersion of the hydrophilic nanoparticles compared to the hydrophobic ones and enhance remarkably the total mechanical characterization of PVDF fibers. Fig. 4 illustrates a schematic for the used setup for the measuring the piezoelectric properties of the electrospun nanofibers. In order to measure the output voltages of the electrospun fibers under a certain force, the electrospun fibers were placed between the two aluminum sheets and the force was applied on the top sheet according to Fig. 4. Simultaneously, the output voltages were detected. The effect of addition of SiO2 nanoparticles on the output voltages of the electrospun nanofibers are tabulated in Table 2.

Fig. 4. Schematic of the used setup for the evaluation of piezoelectric characterization of electrospun fibers.

As it can be observed in Table 2, addition of both hydrophilic and hydrophobic silica nanoparticles increases the output voltages of the PVDF fibers. In fact, the reorientation of PVDF chains in form of β phase is responsible for

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increase in the total output voltages of the composite electrospun nanofibers. Existence of hydroxyl groups onto the surface of the hydrophilic silica nanoparticles amplified the tendency of PVDF chains to form β phase near the silica nanoparticles. Thus, as it is expected, the output voltage and piezoelectric performance of PVDF fibers containing the hydrophilic silica nanoparticles is more highlighted in comparison with PVDF fibers containing hydrophobic silica nanoparticles. Table 2. Output voltages of the electrospun fibers. Sample

Applied force/ N

Output Voltage/ V

Sensibility/ V/N

Neat PVDF

15±2

9.3±1.3

0.62

PVDF-Hydrophilic Silica

15±2

11.5±1.8

0.76

PVDF-Hydrophobic Silica

15±2

14.1±1.9

0.94

4. Conclusions

This work presented the fabrication and characterization of the electrospun PVDF fibers containing 1.5 wt. % of the hydrophilic and hydrophobic SiO2 nanoparticles. The FE-SEM results revealed that addition of both hydrophilic and hydrophobic silica nanoparticles led to increase in average fiber diameter. In addition, PVDF fibers containing hydrophilic SiO2 nanoparticles illustrated superior tensile strength compared to other electrospun PVDF fibers because of the good dispersion and the higher compatibility of the hydrophilic silica nanoparticles. Piezoelectric characterization of the electrospun PVDF fibers showed that the addition of both hydrophilic and hydrophobic SiO2 nanoparticles into PVDF matrix can increase piezo-voltages. This increment in the output voltage is more highlighted for the PVDF fibers containing hydrophilic silica nanoparticles. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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