We have demonstrated that coaxial electrospinning with organic solvent as the sheath fluid is a feasible way to tailor the diameters of composite nanofibers composed of PVP, tristearic and naproxen. The composite nanofibers self-assembled into drug-loaded nanoparticles in water, whose size had a linear relationship with the fiber diameter.
Coaxial electrospinning has been proved in numerous researches that it is a easy and feasible way to control secondary structures of nanofibers, 1-3 to encapsulate drugs or biological agents into polymer nanofibers, 4-6 to prepare nanofibers from materials lack of filament-forming property, 7 and to enclose the functinal liquids in the naofibers. 8 In all these researches, the coaxial electrospinning processes were conducted under the similar conditions that the sheath fluid had good electrospinnability, whereas the core fluid could either be or not be electrospinnable.
Here a totally different coaxial electrospinning process is presented. The process was conducted under the reserve conditions, namely that the core fluid is electrospinnable whereas the sheath fluid is not electrospinnable -- a pure solvent. Further, the diameters of composite nanofibers composed of multiple components could be tailored through adjusting the flow rates of the sheath solvent, by which the sizes of self-assembled naoparticles could be manipulated.
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In the coaxial electrospinning processes with sheath solvents, the formation of the Taylor cone, the thinning of the the jet will be surrounded by the sheath solvents but not the ambient atmosphere in the traditional electrospinning processes. During the electrospinning process, the evaporation of organic solvents mainly takes place at the instability region from the huge surface of the bending and whiping jet fluids,9 which is identified as an important factor affecting the diameter and morphology of electrospun nanofibers.10 Thus the presence of sheath solvent in the coaxial electrospinning process would exert big influence on the evaporation of the solvents of the core solutions and in turn the solidification of the charged jets when they travel from the instability region to the collector. Most probably, the surounding sheath solvents should let the core electrospinable liquid jets to be subjected to a relatively longer time drawing process and thus resulting in even smaller nanofibers.
To implement the coaxial electrospinning process, a co-dissolving solution of polyvinylpyrrolidone K60 (PVP K60), tristearin (GTS) and naproxen (NAP) in chloroform with a ratio of 12 % : 2% : 0.5 % (w/v) were prepared as the core electrospinnable liquid, and anhydrous ethanol was used as the sheath fluid. The flow rate of the core solutions was fixed at 2.0 ml/h, and the high voltage and fibers accepted distance were fixed at 15 kV and 20 cm, respectively. Different flow rates of the sheath ethanol were taken as 0, 0.5, 1.0 and 2.0 mol/h, and the typical images of the coaxial electrospinning processes are shown in Fig. 1A to D respectively. The corresponding nanofibers were denoted as F1, F2, F3 and F4, and their mopholgies under field emission scanning electron microscope (FESEM) are exhibited in Fig. 2A to D, respectively.
Fig. 1 Co-electrospinning processes with ethanol as sheath fluid and co-dissolving solutions of PVP K60, GTS and NAP in chloroform as core flow liquids, the core flow rate was fixed at 2.0 ml/h, while the shell ethanol flow rate was (A) 0, (B) 0.5, (C) 1.0 and (D) 2.0 ml/h, respectively.
As far as Fig. 1A is concerned, the electrospinning process was solely carried out in an open atmosphere without any sheath fluid. The core spinnable solutions gave a typical bending and whipping process, as demonstrated and elucidated by Reneker and Rutledge group. 9, 11 The jet experiences a whipping instability, leading to bending and stretching of the jet, observed as loops of increasing size as the instability grows. The whipping jet thins gradually, while traveling the short distance between the electrodes. The presence of polymer in solutions leads to the formation of fine solid fibers as the solvent evaporates and the other components are anchored on the matrix polymer synchronously to form the composites in the manner of fiber mats.
Although the electrospinning processes were conducted under the same voltage and the same fiber collected distance, coaxial electrospinning with sheath ethanol exhibited different processes characteristics. Copmared Fig. 1A with Fig. 1B, C and D, the different characteristics are obvious as follows: 1) the elongating and thin straight jet before the onset of the bending instability became shorter, and even shorter as the flow rate of the sheath ethanol gradually increased; 2) at the the onset point of the bending instability, splitting of the straight jets occurred; 3) the envelope cone angles of the instability regions became bigger and bigger as the sheath ethanol flow rate increased.
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Just as anticipated, the diameters of the resulted nanofibers decreased when coaxial electrospinning with sheath ethanol, and the diameters became smaller and smaller as the flow rate of ethanol increase, as indicated by Fig. 2A, B, C and D. The average diameters of F1 to F4 are 1368, 717, 584 and 476 nm, respectively. Fig. 2E showed the diameter distributions of F4, over 90% of them fall in the range of 400nm to 600 nm. Additionally, further increasing the flow rate of sheath ethanol would result in even smaller nanofibers but with spindle-on-a-string morphology.
Control over the electrospun fiber diameter remains a technological bottleneck and numerous efforts have been spent on this during the past decade. Often lowering concentration of polymer in the solutions is taken to thin the electrospun fibers. 11-13 However, this approach is limited by the narrow window of spinnable solution concentration, and the goal of obtaining finer fibers is often compromised by the change of the fiber uniformity. 10
Here the developed coaxial electrospinning process with sheath solvents could reduce the nanofiber diameters, not only effectly, but also gradually and controllablely. The reasons should mainly attribute to the following aspects: 1) the sheath solvent has little influence on the entagelment of the filament-forming polymer in the core fluid, but facilitate the formation of Tayloy cone (owing to smaller interface tension between two liquids) and enlongate the drawing time in the instability regions; 2) The chloroform is aprotic solvent whereas the ethanol is protonic solvent with better charges-carried capability and electrical conductivity. When they were subjected to the electronic fields, the ethanol would initiate the instability region faster by increasing advection current, and would reduce the fibers by splitting the straight jet besides bending and whipping, as indicated by Fig. 1; and 3) Chloroform is more volatile than ethanol. In single electrospinning process (Fig. 1A), faster evaporation of chloroform changed the viscoelastic properties of the co-dissolving solutions, and quickly stopped the elongation. When coaxial electrospinning with ethanol, the sheath ethanol acted as a holder to hinder the fast evaporation of chloroform, and allowed a longer time for the elongation process to further reduce the nanofibers in the instability region. Thus, through adjusting the flow rate of the concentric organic solvent, it is easy and feasible to tailor the diameters of the electrospun nanofibers.
Shown in Fig. 2F is the cross-section morphology of F4, which demonstrates that the nanofibers had a homogeneous inner structure without any nanoparticles resulted from phase sepearation during the coaxial electrospinning process. Shown in Fig. 2G is the transmission electron microscopy (TEM) images of F4. The uniform grey-level of fiber reflected the even distribution of the building blocks (GTS and NAP) in the PVP matrix. The X-ray diffraction (XRD) patterns in Fig. 2H indicated that the drug and GTS had lost their original crystallity but converted to an amphorous state with the polymer matrix PVP. The FESEM, TEM and XRD results similarly demonstrated that the components in the composite nanofibers were highly mixed, most probably in a molecular way. The coaxial electrospinning process with sheath ethanol had no influence on the structural uniformity of the resulted nanofibers and the homogeneous distributions of little molecules on the polymer fiber matrix, just as mutiple components composite nanofibers prepared by a single electrospinning process. 14, 15
Fig. 2 Characterizations of the prepared composite nanofibers: FESEM images of (A) F1, (B) F2, (C) F3, (D) F4, (E) Diameter distributions of F4; (F) cross-section of F4; (G) TEM images of F4; (H)XRD patterns of F4 and the components.
Self-assembly has a central role in life, is a ubiquitous process in nature, and a common phenomenon happening at all scales.16-18 Inspired by nature, this concept can provide a simple and flexible approach to fabricate multifunctional nanostructures for technological applications via self-assembly of functional materials.19 Nanofabrication through self-assembly has drawn attentions in many fields, and new strategies and templates are desired for manipulating molecular self-assembly.
Electrospun nanofibers have unique characteristics that make them good templates for manipulating molecular self-assembly. These characteristics include: 1) The electrospun fibers have a ultrafine diameter, often ranging from a few nanometers to several micrometers, which can give a constraint at nano or micro scale for molecular self-assembly; 2) The nanofiber assemblies, shown as a non-woven mat, have a three-dimensional continuous web structure with very large surface area-to-volume ratio, high porosity with very small pore size, which would faciliates the fast dissolution, permeation and diffusion of solvent and solute molecules; 3) The components in the resulted composite nanofibers can be mixed at a molecular scale due to the very fast drying processes, which is comparable to the liquid solutions. 14, 15, 20 Based on the favorable secodary interactions, the highly mixture of the components in the polymer nanofibers would enable them to contact and co-aggregate into nanoparticles after the leave of the matrix polymer molecules.
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Since self-assembly relies primarily on diffusive transport in solutions and the difficulty of agitating a viscous solution on a small scale, beyond changes in solution temperature and viscosity, limits our ability to control the transport of building blocks. 21 To pre-position the building blocks homogeneously on the polymer matrix at a nano or micro scale through a simple and one-step electrospinning process would provide an approach to overcome this obstacle for fine control over molecular self-assembly.
Fig. 3 (A) TEM images of self-assembld nanoparticles from F4; (B) SPM images of the self-assembled scene when a drop of water was placed on the collected F4 nanofiber mats; (C) Images of polarized microscopy of the self-assembled scene when a drop of water was placed on the collected F1 fibers mats; (D) the change trends of fiber diameters and the self-assembled nanoparticles size with the ratio of sheath to core flow rate.
Shown in Fig. 3A is TEM images of the self-assembled nanoparticles from F4. TShown in Fig. 3B are the scanning probe microscopy (SPM) images when a drop of water was placed on the collected F4 fibers on a glass slide. They all demonstrated that the composite nanofibers had good self-assembly properties and could be used as templates for manipulating molecualr self-assembly. Static and dynamic laser scanning (SDLC) results exhibited that the self-assembled nanoparticles from F1 to F4 had an average diameters of 155.4, 249.2, 372.4 and 880.8 nm, respectively.
To further investigate the self-assembly mechanism, a polarization micrpscope was used to observe the self-assembly process of F1. Shown in Fig. 3C is the scene when a drop of water was placed on the collected fibers on a glass slide. Through this, the self-assembly process can be spectulated as follows: 1) The self-assembly processes began in the polymer swelling process when the fibers absorbed water, and endowed the molvibility of the contained-building blocks in them; 2) As the hydrophilic fiber matrix further absorbed water and swelled, the compact structure of fibers became looser and looser to liberate the contained building blocks. The free building blocks spontaneously co-aggregated into hybrid nanoparticles locally and in the diametrical constraint of the nanofibers, as verified by the dots in the swelling fibers; 3) As the hydrophilic polymer molecules disentangled and dissolved into the dissolution medium, the co-assembled nanoparticles were also free into the dissolution media from the swollen fibers, as indicated by the dots in the regions devoild of fibers fingers.
Thus the "dissolution" process of the electrospun composite fiber mats in water is essentially a nano self-assembly process. When the fiber mats were put into water, they disintegrated very quickly due to fast dissolution of the polymer matrix PVP, as a result of PVP's highly hygroscopic and hydrophilic properties, the small diameter of fibers and the continuous web structure of the mats. Meanwhile, during the PVP dissolution process, GTS and NAP molecules spontaneously co-assembled into nanoparticles, as a result of the hydrophobic interactions repelling them from water and the favorable interactions between them (hydrogen bonding and hydrophobic interactions). Thus it is clear that the self-assembly process is induced by a "dissolution-hydrophobicity" mechanism on the diameterial constrain of nanofibers, which suggests a possible strategy to manipulate the size of the self-assembled naoparticles through tailoring the diameters of the electrospun nanofibers.
Fig. 3D gives the change trends of fiber diameters and the corresponding self-assembled nanoparticles size as the ratio of the sheath to core flow rate increase. The trends are similar in that the fiber diameters and the nanoparticles sizes have a drastic decrease, followed by a gentle downsizing. The "drastic decrease" projects the different effects in thinning nanofibers between traditional electrospinning process and coaxial electrospinning process with sheath solvent. Just as anticipated, the size of self-assembled nanoparticles (Dp, nm) has a fine linear relationship with the electrospun fibers diameter (Df, nm). The regressed equation is Dp=0.8077Df -220.6 with a correlation coefficent of 0.9996.
In summary, we have developed a new coaxial electrospinning process, by which the flow rates of pure sheath solvents were used to tailor the diameters of the resulting nanofibers. We also demonstrated that the one dimensional composite nanofibers were good templates for manipulating drug-loaded self-assembled nanoparticles' size. It can be anticipated that coaxial electrospinning with sheath solvents is not only an effective process to control the diameters of electrospun nanofibers, but may also provide an approach to expand the electrospun windows of polymers, and may offer a possible strategy to prepare nanofibers from polymers without electrospinability owing to lack of suitable solvents. It can also be anticipated that a "top-down" one dimensional composite nanofibers electrospinning process combined with a "bottom-up" molecular self-assembly process may break new ground in nanofbrication of novel functional materials.