Non-Toxic Natural Additives to Improve the Electrical Conductivity and Viscosity of Polycaprolactone for Melt Electrospinning

Abstract

Polycaprolactone (PCL) is biodegradable and non-toxic, making it an eco-friendly polymer with various medical applications. In order to increase the stability of PCL used in the field of medical applications, it is necessary to be able to produce fibers with a melt electrospinning method that does not use toxic hydrophobic solvents. However, PCL has very high viscosity and low conductivity, making melt electrospinning difficult. This study presents natural additives enabling the solvent-free melt electrospinning of PCL, wherein the physical properties (i.e., conductivity and viscosity) of the additive-mixed PCL are analyzed. Among the natural additives added to PCL, 7 wt% gallic acid increased conductivity by 81 times and decreased viscosity by 1/8526 times, showing the best results. We believe that our study, improving the physical properties of melt PCL by adding natural additives, will be of great help to the development of the melt electrospinning method of PCL.

1. Introduction

Polycaprolactone (PCL) is a high-molecular-weight aliphatic polyester that is biodegradable and non-toxic, making it an eco-friendly polymer with various medical applications [1,2,3,4,5]. PCL has a low melting point (ca. 60 °C) but is stable and easy to process, even at temperatures of 200 °C or higher. [6]. This is an important characteristic that makes PCL useful in various medical products, such as sutures, dental implants, and wound-covering tapes [7,8,9]. The global market size of PCL was $530 million in 2018 and is expected to grow to $1230 million by 2026 [10]. Among the methods of fabricating fiber products using PCL, electrospinning is the most commonly used. The diameter of a fiber manufactured during electrospinning is determined by various spinning conditions, among which the electrical conductivity and viscosity of the spinning solution are the most decisive variables [11]. If the electrical conductivity of the spinning solution is low, a low surface charge density is formed on the jet during electrospinning, causing a decrease in the mobility of the ions and electrostatic repulsive forces [12]. This phenomenon also reduces the elongation of the solution, resulting in thicker fibers [13]. In addition, because the diameter of the spun fiber is proportional to the square of the viscosity of the polymer solution, thick fibers are generally obtained when the viscosity of the spun solution is high [14,15].

PCL has very high viscosity and low conductivity, making electrospinning difficult without the use of an appropriate solvent [16]. Because PCL is hydrophobic, it does not dissolve well in non-toxic or almost non-toxic solvents, such as water or dimethyl sulfoxide. Currently, the solvents used for electrospinning PCL include tetrahydrofuran [17], N,N-dimethylformamide [18], chloroform/methanol [19], 1,1,1,3,3,3-hexafluoro-2-propanol [20], dichloromethane [21], 1-methyl-2-pyrrolidone [22], and 1,4-dioxane [23]. Most solvents used for the electrospinning of PCL are highly toxic and adversely affect the environment. During the electrospinning process, most of the solvent is removed by evaporation, but a low concentration of toxic solvent remains in the PCL fibers. Although the concentration of toxic solvents remaining in PCL fibers is low, the residual toxic solvents impart a very negative effect on medical products, which is the most important application field of PCL [24]. Despite heat treatment under a vacuum to remove toxic solvents remaining in PCL, studies have reported that toxic solvents are still not completely removed [24,25].

To solve the problem of PCL electrospinning using toxic solvents, a solvent-free melt electrospinning technique has been actively developed [26,27]. During melt electrospinning, because the spinning solution has low conductivity and high viscosity, the diameter of the fiber is larger than that of fibers manufactured by solution electrospinning [28,29,30]. According to recently published papers on electrospinning, despite attempting to reduce the diameter of the fiber as much as possible in the case of melt electrospinning, the realistic limit is from two to several tens of micrometers [26]. However, in the case of solution electrospinning, it is possible to reduce the fiber diameter to several tens of nanometers [31,32], where the diameter of the fibers produced by the two methods differs significantly from approximately 50 to 1000 times. Even in the case of PCL, owing to its hydrophobicity, high viscosity, and low conductivity, the desired fiber diameter (e.g., tens of nanometers) cannot be achieved by melt electrospinning [33].

To solve the problem of fiber diameter limitation in melt electrospinning, a method of increasing the electrical conductivity and lowering the viscosity by mixing a low-molecular-weight polar additive with a spinning polymer solution was proposed [34,35,36]. Several additives have been used for melt electrospinning of PCL; however, to date, no satisfactory results have been reported. In addition, in order to maintain the biocompatibility of PCL nanofibers, it is necessary to logically improve the electrical conductivity and melting viscosity of the spinning solution during melt electrospinning by using a non-toxic natural additive. Therefore, there is an urgent need to discover a new additive group that is harmless to the environment and human body, while improving the physical characteristics of the spinning solution to meet the needs of melt electrospinning of PCL.

This study reveals additives enabling the solvent-free melt electrospinning of PCL, wherein the physical properties (i.e., conductivity and viscosity) of the additive-mixed PCL are analyzed. All substances used as additives are non-toxic natural products. These are FDA-approved substances that are edible and highly unlikely to cause human and environmental problems. All additives are added to the PCL polymer up to the solubility limit, and the degrees of improvement in the conductivity and viscosity are measured for each added mass percentage. In this study, only the degrees of conductivity and viscosity improvement of the PCL spinning solution with additives are studied. Other characteristics including the morphology, uniformity, fiber diameter, hydrophilicity, toxicity, and biodegradability of the PCL fibers, manufactured by melt electrospinning, to which additives are added, will be investigated in further studies.

2. Experimental Section

2.1. Reagents

PCL (37.865 MFI) (Table S1) was procured from ILKWANG Polymer (Hwaseong-si, Gyeongki-do, Republic of Korea) and used as received. The additives used in this study, including quercetin hydrate (QC, ≥95%), oleic acid (OA, ≥99%), alizarin (AZ, 97%), (+)-catechin hydrate (CH, ≥96%), gallic acid monohydrate GA (≥98%), acetone (≥99.5%), and chloroform (≥99.5%), were purchased from Sigma-Aldrich (St. Louis, MO, USA).

2.2. Experimental Setup for Measuring Conductance

The current (I) is proportional to the voltage (E) and inversely proportional to the resistance (R). The reciprocal of the resistance is called conductance (G). The conductivity (κ) can be obtained by multiplying the conductance by the distance between the electrodes (l) and dividing by the electrode area (A). Therefore, if the distance between the electrodes and area of the electrodes are the same under all the experimental conditions, the increase in conductance equals the increase in conductivity. In this study, the conductance was calculated using I–V curves, where the data can be easily measured using a potentiostat. The equipment used to measure the conductance is shown in Figure 1. It consists of four major components: a heating mantle, electrochemical cell, potentiostat, and data station. The temperature controller of the stirred heating mantle (Model MS-DMS; Misung Scientific Co., Ltd., Yangju, Republic of Korea) was able to control the temperature with an accuracy of ±1 °C; the temperature was double-checked using a K-type external temperature sensor to minimize errors. In the electrochemical cells, the additives were completely mixed with PCL polymer powder in glass vials (at an appropriate weight percentage for each additive) and thoroughly mixed on a digital hot plate stirrer before loading into a heating mantle for the conductance measurement. The heating mantle was preheated to 200 °C. After preheating, approximately 5 g of the polymer and additive mixture was added to the heating mantle. The conductance was measured by preparing a homogenous melt after achieving a stable state by maintaining the temperature in the heating mantle for 10 min. Measuring the conductivity of pure PCL and PCL with additives at high temperatures (approximately 200 °C, which is the actual temperature for melt electrospinning) using a commercially available conductivity meter has many limitations. Therefore, in this study, the conductance of the polymer solution was measured using a potentiostat. The I–V plot can be obtained from the linear sweep voltammetry (LSV) data. In the I–V plot, the slope of the line is equivalent to the conductance of the mixture. LSV analysis was performed on a CHI model 617E potentiostat (CH Instrument, Austin, TX, USA) equipped with two glassy carbon electrodes (GCE, a 2 mm diameter disk surface, purchased from Redox.me). GCE1 was used as the working electrode, and GCE2 was used as the counter and reference electrodes. The current flowing between the electrodes was measured by applying a voltage from 0 to 1 V. To obtain the conductance with the minimum error, the two GCEs were fixed at a distance of 5 mm using Teflon tape. All electrochemical tests were conducted in triplicate at 200 °C to ensure reproducibility.

2.3. Measurement of Viscosity

The viscosity of the pure polymers and polymers with additives were measured using a rheometer (Kinexus Ultra+, NETZSCH, Selb, Germany) equipped with parallel plates. Using the shear rate table program, the rheological test was performed over a wide range of shear rates (0.1–100 s^–1) at 200 °C. To determine and minimize the gap errors caused by plate non-parallelism and gap-zeroing procedures, the background solution was re-measured each time the solution was replaced, and the instrument was cleaned with acetone and chloroform.

3. Results and Discussion

3.1. Characteristics of Additives

PCL is a high-molecular-weight aliphatic polyester with very low conductivity (Figure 2). Therefore, adding low-polarity molecules to the spinning solution can increase the conductivity of the overall solution, while lowering the viscosity. In this study, AZ, QC, OA, GA, and CH were selected and tested as additives (Figure 3). AZ, QC, OA, GA, and CH are all low-molecular-weight molecules with masses of 170–302 g/mol, and contain several carboxylic acid or hydroxyl groups. All these additives are substances that can be obtained from nature and can be consumed by humans. Specifically, AZ is an aromatic organic compound that has long been used as a bio-based dye. Because of its ability to bind metal ions, AZ is mainly used in biochemistry for colorimetric analysis to quantify the calcification of bone tissue by staining calcium salt deposits in the body. QC, which is a phytochemical known as an antioxidant and flavonoid, is converted into a substance called isorhamnetin and quercetin 3-glucuronide, which belong to the flavonoid vitamin P group in the body. QC stops the enlargement of smooth muscle cells in blood vessels to prevent blood vessel narrowing. In addition, QC suppresses the inflammatory response of adipocytes; thus, it is harmless to the human body and its consumption is recommended. CH, which belongs to the flavan-3-ols of the flavonoid group, is a polyphenol that imparts the astringent taste of green tea. CH removes free radicals and acts as an antioxidant with a strong active oxygen removal effect, and has also been reported to protect the cardiovascular system by preventing the oxidation of low-density lipoproteins. GA, which is usually obtained by the hydrolysis of tannins, is one of the most abundant phenolic acids in plants. It has antibacterial, antiviral, and antiallergic properties as well as anticancer activity against various types of cancer cells, and is also effective in reducing body fat and improving blood cholesterol. OA is an unsaturated fatty acid with one double bond and is an FDA-approved food additive. Previous studies have confirmed that it is effective in improving the viscosity and electrical conductivity of molten polymers.

3.2. Improvement of Electrical Conductivity by Additives

The current can be increased by adding polar additives, such as AZ, QC, OA, GA, and CH. Figure 4 shows the change in conductance at 200 °C when various amounts of additives were introduced into the molten PCL. To analyze the change in the conductance of the solution, LSV data were acquired in the potential range of 0–1 V at a 0.1 V/s scan rate to construct the I–V plot. During the measurement, the additives were introduced at a ratio (wt%) up to the solubility limit, which is the maximum limit at which the additive and PCL solutions can be homogeneously mixed. The solubility limits for each additive were 1 wt% AZ, 4 wt% QC, 7 wt% GA, 10% CH, and 10% OA. The slope of the obtained I–V plot represents the conductance of the solution. As shown in Figure 4, the slope increased with all additives. Thus, the conductance of the solution increased. In general, the conductance of a solution is determined by the charge carrier, the presence of ions, ion mobility, and polarity of the solvent. Because the polar additives used in the experiment contribute to increasing the number of ions and the polarity of the solution, the conductance of the solution increases.

To compare the effects of the type of additive, we converted the slope of the I–V plot of the PCL solution (obtained in Figure 4) into the electrical conductivity (Figure 5). Pure PCL solution exhibited a conductance of 2 pS at the applied temperature (200 °C). After introducing various additives into the solution, the conductance increased as the content of each additive increased. The numerical values, which are measures of the conductance change according to the type and amount of additive, can be found in the Supplementary Materials, Tables S2–S6. The difference in the degree of conductance improvement for each additive is due to the difference in the size and number of ions generated, and the degree of polarity of the additive, depending on the amount and type of additive used. Because the radius of the generated ions is small and the number of ions increased, the charge density is increased, and thus, the electric conductivity increased, owing to the influence of the external electric field [37,38,39].

First, when 1 wt% AZ was added, the conductance of the PCL solution increased to 85 pS, which is 43 times higher than that of pure PCL. Similarly, the conductance of the solution increased to 86.8 pS with the addition of 10 wt% CH, and up to 120.5 pS with the addition of 4 wt% QC, indicating a 45-fold and 60-fold improvement in conductance compared to that of the pure PCL solution, respectively. Among the analyzed non-toxic natural additives, the additive with the least improvement in conductance was OA (10 wt%). With the addition of OA, the conductance of the solution increased to 11 pS, i.e., approximately six-fold. GA addition at 7 wt.% led to a surprisingly high improvement rate. The conductance of the PCL polymer solution improved to 168 pS, which is an 81-fold increase. This means that when the additives are introduced up to the solubility limit, most of the current can flow in the GA used as the additive. Therefore, if GA is added during melt electrospinning of PCL, it is expected that a stronger elongation force can be applied to the jet compared to the force that can be applied with the other additives, and the finest fibers can be formed during spinning.

3.3. Effect of Additives on Viscosity

As mentioned above, the two factors that have the greatest influence on the quality of the fibers during melt electrospinning are the electrical conductivity and the viscosity of the polymer [40]. The viscosity of a polymer is a major property that must be controlled when optimizing the spinning voltage to obtain the desired fiber size and shape. In general, the lower the viscosity, the smaller the fiber diameter and the easier the processing. Appropriate adjustment of the melt viscosity not only speeds up the formation of fibers, but also enables the formation of stable Taylor cones without the jet sputtering on the entire collector [41,42]. Therefore, in this study, the viscosity improvement was also analyzed to evaluate the suitability of using non-toxic natural additives for melt electrospinning.

The effects of various additives on the shear viscosity of PCL are shown in Figure 6. The viscosity of the pure PCL polymer was approximately 25,580 mPa·s when measured at the applied temperature (200 °C). When additives are introduced into the solution, the entanglement between the polymer chains is reduced and the viscosity gradually decreases [43]. The viscosity of the PCL solution with additives was measured up to the solubility limit of the additives (Tables S2–S6).

As shown in Figure 7, the viscosity of the PCL solution increased to 9338 mPa·s with the addition of 1 wt% AZ, 4508 mPa·s for 10 wt% CH, mPa·s for 4 wt% QC, 975.9 mPa·s for 10 wt% OA, and 3 mPa·s for 7 wt% GA (note that the Y-axis is a logarithmic scale). GA produced the highest viscosity and conductance improvement, where the viscosity improvement rate was as much as 1/8526 times. Therefore, GA is expected to be the most effective additive for controlling melt electrospinning of PCL, leading to the best improvement in its electrical and rheological properties.

4. Conclusions

It was demonstrated that five additives that are harmless to humans and the environment (AZ, QC, OA, GA, and CH) could be used to improve the conductivity and viscosity of the PCL solution for melt electrospinning. The optimal content of each additive was analyzed by measuring the electrical conductance and viscosity of the solution. All the proposed additives are potentially useful for improving the electrical conductivity and viscosity. In particular, GA showed an excellent effect compared to the other additives; it was expected to be an optimal additive for controlling the diameter of the nanofibers during melt electrospinning. We believe that the non-toxic, ecofriendly additives presented in this study would help expand the melt electrospinning field of PCL along with various biocompatible polymers (e.g., PLA, PDO, etc.), and would greatly contribute to research in biomedical applications, such as drug delivery, wound healing, tissue engineering, and scaffold composition.