Paclitaxel (PTX) was purchased from Shanghai Hao-Sheng Biotechnol. Co. Ltd. (Shanghai, China). Polyvinylpyrrolidone (PVP K90, M = 360,000), polyoxyethylene castor oil (PCO, as emulsifier), and triglyceride (TG, as oil phase) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Anhydrous ethanol was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Water was double-distilled just before usage.

Four working fluids were prepared, which are included in Table 1. The first working fluid was a mixed solution in anhydrous ethanol, which contained 8% (w/v) of the polymeric matrix PVP K90, 1.61% (w/v) of the drug PTX, 4% (w/v) of the emulsifier PCO and 2.5% (w/v) of the oil phase TG. A single fluid blending process was exploited to transfer it into solid composite nanofibers, which are denoted as S1 with a drug content of 10% (w/w).

The second sample was prepared using a coaxial electrospinning. A solution containing 3.22% (w/v) of drug and 8% (w/v) of PVP K90 was exploited as a core fluid. A solution containing 8% (w/v) of the emulsifier PCO, 5% (w/v) of the oil phase TG and 8% (w/v) of PVP K90 was exploited as a sheath fluid. The resultant core-shell nanofibers are denoted as S2, whose drug content is 10% (w/w). The third sample was similarly prepared using a coaxial electrospinning. A solution containing 5.56% (w/v) of drug and 8% (w/v) of PVP K90 was exploited as a core fluid. A solution containing 8% (w/v) of the emulsifier PCO, 5% (w/v) of the oil phase TG and 8% (w/v) of PVP K90 was exploited as a sheath fluid. The resultant core-shell nanofibers are denoted as S3, whose drug content is 10% (w/w).

These arrangements are aimed to disclose the influences of two factors (the core-shell structure, and its shell thickness) on the self-emulsifying processes.

The experimental apparatus was a homemade electrospinning system. A homemade concentric spinneret was exploited to conduct both the single-fluid electrospinning for preparing S1 and also the coaxial electrospinning for fabricating S2 and S3. To keep a continuous and stable working process, the applied voltage values during the fabrication was between 12 and 14 kV. The deposition distance was fixed at 15 cm. The environmental parameters included a room temperature of 25°C ± 2°C and a relatively humidity of 51% ± 6%. The collected fibrous films were placed in a dryer to a constant weight, and then were folded and sealed in a self-sealing bag for preservation.

The fibrous films were cut into small patches, which were fixed on the double-sided conductive adhesive. After sprayed with Pt under a nitrogen atmosphere for 60 s, the samples were assessed using a scanning electron microscope (SEM, Quanta FEG-450, FEI corporation, United States). The nanofibers’ sizes were estimated on the SEM images through about 100 places using the ImageJ software (NIH, United States).

The inner structures of the prepared nanofibers S1, S2, and S3 were evaluated using a transmission electron microscope (TEM, JEM 2200-F, JEOL, Japan). The samples were collected by placing the copper grid-supported carbon films above the collectors for 2 min to collect some nanofibers.

In general, SEDDS is an anhydrous pre-nanoemulsion dosage form, which mainly contain isotropic mixtures of oil, surfactant, co-surfactant, drug and polymer molecules traditionally (Chouhan et al., 2015; Nikmaram et al., 2018; Zaghloul et al., 2019). When introduced into aqueous phase of gastric motility, SEDDS is expected to rapidly converted into lipophilic globules, which are often at a nanoscale. Through this way, the solubility and adsorption through gastrointestinal mucosa of a poorly water-soluble drug can be significantly enhanced (Patel and Sawant, 2009; Gao et al., 2020). Electrospinning is able to provide a series of strategies for developing some types of SEDDS. Shown in Figure 2 are diagrams about two different nanofibers-based SEDDS. The first one is the monolithic nanofibers that all the components including drug PTX, emulsifying agent, oil are homogeneously distributed all over the polymeric matrix, which can be easily fabricated from a single-fluid or uniaxial blending electrospinning. The second one is the core-shell nanofibers, in which drug is loaded in the core section, whereas the emulsifying agent and oil are located at the shell section. These components’ arrangement are expected to ensure a better self-emulsifying process.

Although electrospinning is facile to implement, its process can be improved from several aspects such as the capability of creating complicated nanostructures, production on a large scale, creation of novel functional nanofibers and energy-saving (Kang et al., 2020; Wang et al., 2020; Zhao et al., 2021a; Zhang et al., 2021b; Liu et al., 2022c; Liu et al., 2022d). Shown in Figure 3 are records about the implementations of the single-fluid and coaxial electrospinning in this investigation. The diagram in Figure 3A tells the most fundamental four parts in an electrospinning system, i.e., a spinneret, a grounded collector, a high voltage generator, one or more syringe pumps. The key symbol of the commence of an electrospinning process is the formation of Taylor cone. For coaxial electrospinning, the Taylor cone should be a typical core-shell compound one (as indicated by the bottom-right inset of Figure 3A).

A digital picture about the homemade spinneret is given in Figure 3B. The most significant characteristic is that the spinneret was made from a combination of metal, adhesive (epoxy resin), and plastics. Different with the commercial concentric spinneret, in which all section are made of stainless steel, the homemade spinneret left only a small section of the metal capillary open to the environment for the input of the electrostatic energy. This kind of spinneret was demonstrated to be useful for saving energy. The mechanism is diagrammed in Figure 3C. At the nozzle of spinneret, the charges will move to the surface of shell working fluid naturally to form the compound Taylor cone. During the transportation process, the energy dispersed to the environment will be reduced as smaller as possible.

Figures 3D, E are digital images taken during the preparations of samples S1 and S2, respectively. In Figure 3D, the sheath section had no working fluid, thus, a concentric spinneret was exploited to conduct a single-fluid blending process. In Figure 3E, the double-layer compound Taylor cone could be clearly observed when both core and shell working fluids were pumped to the nozzles of spinneret.

Figure 4 shows the SEM images of the three types of nanofibers. All of them have a straight linear morphology with smooth surface. In Figure 4A, the monolithic nanofibers S1 have an average diameter of 1.03 ± 0.26 μm. In Figure 4B, the average diameter of core-shell nanofibers S2 is 1.17 ± 0.34 μm. Both S1 and S2 were fabricated with a total fluid flow rate of 1.0 ml/h. When the total flow rate was increased from 1.0 ml/h to 1.5 ml/h for fabricating nanofibers S3, the applied voltage was also elevated from 12 kV to 14 kV for avoiding the dropping of working fluid on the fiber collector. Meanwhile, the average diameters of the resultant core-shell nanofibers S3 had a significant increase to 1.53 ± 0.48 μm.

The TEM images of the three kinds of nanofibers are included in Figure 5. Just as anticipated, nanofibers S1 from the single-fluid blending electrospinning have a homogeneous structure, as indicated by the similar gray levels all over the nanofibers. In contrast, both nanofibers S2 and S3 have the obvious core-shell structures. By estimations, S2 and S3 fibers have a thickness of 180 and 320 nm, respectively. The increase of sheath fluid flow rate obviously increased the diameters of the whole nanofibers and also the thicknesses of their shell sections.

The TEM observations about the nano emulsion particles assembled from the three sorts of nanofibers are included in Figure 6. All the assembled particles have a diameter around 100 nm. Figures 6A, B are particles self-assembled from the homogeneous nanofibers S1 under different magnifications. These particles are round and have a clear boundary, suggesting that the hydrophobic and insoluble components PTX, TG and PCO could aggregate effectively within the nanofibers after the dissolution of the polymeric matrix PVP K90 during the self-emulsifying processes.

Similarly, the particles self-assembled from the core-sheath nanofibers S2 (Figures 6C, D) and those from the nanofibers S3 (Figures 6E, F) have a round morphology with obvious boundaries. Although the “core-sheath” templates (nanofibers S2 and S3) are different with the monolithic composites (nanofibers S1), the self-assembled particles are similar. The intentional distributions of components on the cores-shell nanostructures exhibited no significant influences on the assembled particles’ morphology. However, the real molecular behaviors during the self-emulsifying processes should be different between the monolithic S1 and the core-sheath S2 and S3.

The particle size and size distribution of self-emulsified particles were tested by the dynamic light scattering. The results are included in Figure 7. The average sizes of emulsion particles self-assembled from nanofibers S1, S2, and S3 were 83.35 nm (Figure 7A), 95.96 nm (Figure 7B) and 102.00 nm (Figure 7C), respectively.

It can be seen from Figures 4, 6 that the average diameter of nanofibers and the average particle size of emulsion are positively correlated. That is to say, the smaller the diameter of self-emulsifying fiber, the smaller the emulsion particle size and the better the emulsification results. Two key elements influence the self-emulsification. One is the components that loaded into the nanofibers and their reasonable ratios, which can draw supports from the traditional pharmaceutical knowledge. The other is the nanofibers’ diameter and inner structures. As for controlling the nanofibers’ diameter for a better self-emulsification. The components and their ratios can be kept constant, whereas the fibers’ diameters and inner structures can be manipulated for a better self-emulsifying process.

The small emulsion particles need to pass through the gastrointestinal mucosa for the final adsorption for oral administration. Thus, in general, the smaller the particle size is, the easier it is to be absorbed by the body; and the smaller the particle size, the more the number of particles can pass through, thereby the higher the content of the drug absorbed by the human body. One of the reasons is that when the diameter of the fiber is smaller, its specific surface area is larger, and it can disperse quickly and well when it contacts with water. Another reason is due to the physical and chemical properties of the components themselves, which have an effect on the diameter of the electrospun nanofibers. In this study, the aim is to disclose the influences of different kinds of electrospun inner structures (i.e., monolithic and core-sheath) on the self-emulsifying. As for how to manipulate the diameters of nanofibers, and in turn the size of self-assembled particles, they will be investigated in another study, where modified coaxial and modified triaxial electrospinning with outer fluids of pure solvents will be exploited.

The standard curve of PTX for UV-vis spectroscopic measurements was A = 2.6801C-0.0137 (R² = 0.99755), where A represents the absorbance of PTX at 228 nm and C represents the PTX concentration in the sample solutions (μg/ml). The measured contents from the self-emulsified emulsions for the nanofibers S1, S2, and S3 are 10.15 ± 0.27%, 9.76 ± 0.42%, and 9.81 ± 0.34%, respectively. These values are closely with the theoretical calculation data, i.e., 10% PTX in the solid nanofibers, suggesting that the electrospinning can completely encapsulate the PTX into the nanofibers, regardless of a single-fluid or a coaxial process. The reason is that the electrospinning is just a physical drying process, and the drying rate is extremely fast. Meanwhile, the drug PTX is stable without sublimation property. Thus, there is no any loss of PTX during the different electrospinning procedures.

After self-emulsifying, 22.1%, 11.6%, and 8.3% of the PTX were dissolved into the supernatant for nanofibers S1, S2, and S3, respectively. These data mean that the EE (%), i.e. the ratio of drug encapsulated into the emulsion particles during self-emulsification, are 77.9%, 88.4% and 91.7% for S1, S2, and S3, respectively. The monolithic nanostructure of S1 released the most PTX to the environment. This is because of the homogeneous distribution of PTX all over the nanofibers S1, which means that there were many PTX molecules were distributed on the surface of S1. These molecules were easy to free into the environment during the formation of emulsion particles during the self-emulsification processes. In sharp contrary, the core-shell nanostructures S2 and S3 have no PTX on the surfaces. All the PTX molecules were located into the core section of nanofibers. Thus, during the self-emulsifying processes, the sheath components TG and PCO are easy to assemble around the PTX to form the emulsion particles. Meanwhile, the thicker the sheath section is, the better the encapsulation effect has. Thus, core-shell fibers S3 has a slightly higher value of 91.7 than S2 of 88.4%.

The results from the in vitro dissolution tests are included in Figure 8, in which the raw drug particles’ dissolution behaviors were exploited as a control. During the first 10 min, the drug released contents from the self-emulsified solutions of S1, S2, S3, and PTX powders are 47.5 ± 4.5%, 36.5 ± 3.8%, 31.6 ± 4.3%, and 3.4 ± 2.1%, respectively. Thus, the PTX release ratios from the self-emulsified emulsion particles during the first 10 min were 25.4%, 24.9%, and 23.3% for S1, S2, and S3, respectively (subtracting the sections during self-emulsification, i.e., 22.1%, 11.6% and 8.3%). After 4 h, the PTX released contents for S1, S2, S3 and PTX powders are 96.8 ± 5.2%, 93.2 ± 4.7%, 90.5 ± 5.2%, and 11.2 ± 4.2%, respectively. It can be concluded that, on one hand, the electrospun nanofibers-based SEDDS are able to enhance the dissolution of PTX. On the other hand, the SEDDS are able to provide an extended release profiles for eliminating the possible toxicity resulted from pulsatile release. Meanwhile, the cores-sheath structures S2 and S3 had a better sustained release profiles than the monolithic S1 in terms of initial burst release, and S3 with a thicker sheath layer showed a better result than S2.

In order to reveal the drug release mechanism of PTX from the self-emulsified particles, Peppas Eq. 2 was used to fit the experimental data (Peppas and Narasimhan, 2014). Q t = k t n   o r   L o g Q t = n L o g Q t + A (2)

In the equation, Q is the accumulative drug release percentage when the time is t, k, and A are constant values, and the index n is an important parameter to indicate the drug release mechanism. When n < 0.45, it indicates that the drug is released through the typical Fickian mechanism; when n > 0.89, it indicates that the drug is released through the skeleton dissolution mechanism; when n is between them, it indicates that the drug is released through the mixed mechanism. It can be seen from Figure 9 that the diffusion indexes of n for nanofibers S1, S2 and S3 are 0.23, 0.27, and 0.32 respectively. All these values are smaller than the critical value of 0.45, indicating that the typical Fickian mechanism has played its role in manipulating the PTX molecules release from the self-emulsified particles in the emulsions.

A diagram showing the engineering mechanism on a molecular scale of the electrospun nanofibers-based SEDDS is proposed in Figure 10. Firstly, the electrospinning, both the single-fluid blending process and the coaxial process, is a very fast “top-down” conversion process, by which solutions are converted into solid polymer-based composites. The homogeneous distribution state of components in the working fluids are propagated into the nanofibers. Thus the electrospun nanofibers can be viewed as a mixture on a molecular scale (in pharmaceutics, medicated nanofibers are often called molecular solid dispersion) (Xie et al., 2021; Ziyadi et al., 2021; Afshar et al., 2022; Zhou et al., 2022).

When these nanofibers are placed into water, the soluble polymeric matrix will absorb water, swell, disentangle, and free into the bulk solution. During these processes, the anchored insoluble molecules such as PTX, PCO and TG will free from the polymeric restriction but within the confined region regulated by the nanofibers’ diameter. The hydrophobic interactions between themselves and from the surrounding water molecules will promote the self-aggregation to form the emulsion particles. These emulsion particles not only enhance the dissolution of PTX, but also benefit a useful trans-membrane for adsorption with smaller gastrointestinal irritation. The cytotoxicology tests and animal experiments of these nanofiber-based SEDDS will be further conducted in future.

In this nano era, more and more “top-down” and “bottom-up” techniques are developed for bioengineering and biotechnology, accompanied with a series of novel functional materials such as hydrogels and new types of polymers (Li et al., 2022b; Zhang et al., 2022b; Wang et al., 2022c; Wang et al., 2022d Song et al., 2022; Wang and Feng, 2022; Zhu et al., 2022) and even inorganic nanoparticles (Tang et al., 2022b; Chen et al., 2022c; Wu et al., 2022). The present protocols showed a frontier, in which the combination of a top-down electrospinning and a bottom-up of molecular self-emulsifying was explored to fabricate biomedical materials for resolving one of the most difficult challenges, i.e. the therapeutic delivery of poorly water-soluble drugs.