Integrating Substrateless Electrospinning with ... - ACS Publications

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Integrating substrate-less electrospinning with textile technology for creating biodegradable 3D structures

John Joseph, Shantikumar V Nair*, Deepthy Menon* Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham University, Kochi, India - 682 041 ABSTRACT The present study describes a unique way of integrating substrate-less electrospinning process with textile technology. We developed a new collector design that provided a pressuredriven, localized cotton-wool structure in free space from which continuous high strength yarns were drawn. An advantage of this integration was that the textile could be drug/dye loaded and be developed into a core-sheath architecture with greater functionality. This method could produce potential nano-textiles for various biomedical applications.

KEYWORDS: Electrospinning, collector, nanofibers, yarns, textiles, core-sheath

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Electrospinning was investigated in early 1930’s as a fibre fabrication technique1. Subsequently, in the pioneering work of Reneker2, this technique of electrospinning was extended to nanofiber production and the mechanisms of fiber formation and control were also elucidated. Initially, densely packed fibrous mesh in the form of two-dimensional non-woven mats2 were deposited on flat targets/collectors. Subsequently, various collector designs have been explored for generating a variety of fibrous forms (fluffy mass3–7 and yarns8–18), which includes rotating collector drums19, patterned and dual conductive electrodes separated by an air gap20,21, funnel targe17,18 , liquid target12,22, knife-edged collectors23, dual ring electrodes24, hemispherical collector with concentric needular arrays3, etc. Of special importance for commercialization is the requirement of obtaining yarns from either nano or micro electrospun fibers, and their woven products, in a continuous fashion using conventional textile technology. This would provide the opportunity to manufacture a variety of complex shaped components for biomedical use. Amongst various collectors previously studied, dual conductive electrodes, funnel targets, liquid targets and dual ring electrodes have been successfully adapted for making yarns and core-spun yarns25,26. The novelty of this work lies in the rotation of an open collector with peripheral tines that permitted the highly efficient substrate-less formation of fluffy mass largely at a single plane, thereby allowing rapid yarning/core-sheath yarning with controlled yarn architecture. Further, by adjusting the spinneret position, the same rotating collector could also provide a nonwoven two-dimensional sheet or even a three-dimensional mass. In conventional textile technology, cotton is picked as a fluffy mass and from this yarns are drawn and wound into spools for further processing such as weaving. To integrate electrospinning with the textile process technology, it would be necessary to deposit by electrospinning such a fluffy mass within a continuous textile yarn-making process. This is best


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done using a substrate-less electrospinning deposition, the science and the associated technology of which is described in this paper. We have also demonstrated, for the first time, the fabrication of continuous functional core-sheath yarns with high mechanical strength, wherein a dye/drug could be incorporated either in the core or the sheath, providing requisite functionality. Our focus was also to enhance the mechanical properties of yarns so as to process them further into constructs of varied architecture using textile technology techniques of braiding, plying and weaving. The design adopted here (Figure 1a) consisted of an electrospinning source coupled to a substrate-less deposition approach by specially controlling the electric fields so that fluffy deposits formed in a focal plane (Figure 1b). This was made possible by using an array of equidistant point electrodes (conducting tines of length 4 cm and radius, R) at the periphery of an open-ended frame made from flexible metallic rods. The closed end of this geometry was connected to a motor shaft whose rotation speed was variable (100-1100 rpm). Two spinnerets of opposite charge were positioned at 45° with respect to collector axis. The residual charges of opposite polarity present on the fibers get attracted towards the tines23. In fact from previous work, use of point electrodes with small R values resulted in high charge accumulation27 ( = ⁄4 ; : charge density, Q: charge), and all the electric field lines got converged at the tines. In the absence of rotation, fibers formed like webs in advance of the electrodes and on the electrodes themselves (Figure 2a & b). Variations in the number of tines, its length and position on fiber deposition were investigated as detailed in supporting information (Supporting Information Figure S1). These changes in collector design did not impart any significant difference in the amount of fiber deposited, which was quantified in terms of collector efficiency (defined as weight of polymer obtained divided by weight of polymeric mass taken in unit time).


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However, rotation of the collector served two important purposes: i) the electric field was made localized and dynamic, and ii) created an in situ pressure drop inside the collector (Supporting Information Video 1). This pressure drop is primarily generated by the rotating open-ended frame of the collector (and not a closed design as in hemispherical3 and funnel collector17,18) resulting in an inward airflow (depicted by arrows in Figure 1a). This aided in collecting the fibers into a focal plane at the center of the open frame, forming a fluffy ball that was held loosely by other fibers connected to the tines (Figure 2c & d, Supporting Information Video 2). In this case, primarily the deposition and collection occurred without a substrate in free space. The next step was to use a guide wire to bundle fibers from the fluffy mass (Figure 1b inset) and pull it out as a yarn as shown in Figure 1c. This drawing process aligns the fibers (similar to the carding process in conventional textile technology) and results in the formation of a cone with a high fiber density at its apex (Figure 1c inset). The rotation of the tines caused a twisting of this yarn to create a strong thread with good integrity. When a single spinneret was used, instead of two spinnerets of opposite polarity, a conventional 2-D mat was deposited in the plane of the circumference containing the tines (Figure 1d and Supporting Information Figure S2). The new process demonstrated here creates a high efficiency of yarning mainly due to the open-ended geometry of the collector that created an inward laminar airflow, which gathered the fibers effectively. This design is quite unique in comparison to the closed geometries obtained using funnels or hemispherical stationary target containing point electrodes. Furthermore, the specific design developed is critical for continuous yarning process. In this study, biodegradable polymers such as PLLA and PCL were electrospun to obtain yarns. Figures 3a and 3c represent the optical and SEM images of the fluffy mass formed as a first step of yarning process. From this deposit, continuous fibrous PLLA yarns (shown in Figure 3b) were


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obtained through the formation of a cone (Supporting Information Figure S3). Yarn formation occurred predominantly via three steps, viz., (i) alignment of non-woven fibers due to the drawing forces, (ii) twisting of aligned fibers in the cone-forming region owing to collector rotation, and (iii) bundling of fibers. The resultant continuous, interlocked yarns had individual fibers with micro, micro-nano or sub-micron size scales as depicted respectively in Figures 3d, 3e and 3f. To generalize this concept of yarning, polymers such polystyrene, polyurethane and composite sol-gel fibers of polyvinylpyrrolidone/titanium isopropoxide were electrospun into yarns from the fluffy mass as shown in Supporting Information Figure S4.

The nature and properties of the yarn were influenced by the fluid injection and yarn uptake rates, collector rotation and applied voltage as shown in Supporting Information Figure S5. It was noted that nearly 93.96±1.46 % of the injected polymer was uptaken into yarn, giving a good yield for the yarning process as in Table 1. This was high considering that even in nearfield electrospinning process, less than 3% of the spun fibers were lost to the environment and fails to deposit on the target. A constant feed of electrospun fibers into the collector ensured continuity of the yarns for several kilometers, yielding spools with yarn diameter ranging from 200 to 600 µm. Below a critical uptake rate, which would depend on the injection rate of the polymer, continuous yarns could not form. This corresponded in our case to an uptake rate of < 0.2 m.min-1, and collector rotation of < 50 rpm. Uptake rates of upto 1.6 m.min-1 yielded strong yarns of smaller diameters, whereas at very high uptake (> 2 m.min-1) or high collector rotations (> 1100 rpm), there was an increased susceptibility to fiber breakage. When the uptake rate of the yarn was increased while keeping the same polymer injection rate, the yarn contained fewer fibers and hence had lower force at break, as demonstrated in Figure 4a.


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However, the tensile strength of the yarns was nearly the same for all uptake rates. To increase the yarn uptake without loss of strength, the polymer injection rate would need to be increased, such as by the use of multiple syringes. The number of fiber rotations per unit length (N) of the yarn depended on the relative values of the uptake rate (U, in distance per unit time) and the collector rotation rate (C, in revolutions per unit time). In fact by simple geometry: =

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Higher uptake rates and lower collector rotation rates would decrease N. The corresponding twist angle, , in radians, can be given by: = 2 (2) We observed that when or increased, the fibers were more tightly wound together giving higher values of Young’s Modulus and lower values of strain to failure (see Table 2). In the absence of twist, there was a decrease of both tensile strength and force at break as shown in Figure 4b. This may be attributed to the relative slip between fibers in the absence of twist 28. For high strength yarns, tight winding of its constituent fibers with high twist is essential as obtained in our case for a twist angle of 57.49 ± 2.03o corresponding to a collector rotation of 900 rpm (Figure 4c). A lower uptake rate of 0.6 m.s-1 was found optimal for yarn collection, (average yarn diameter: 300 microns), imparting the requisite force for its further processing as detailed below. Additionally, we estimated the number of individual fibers in a 300 micron-sized yarn based on random packing (Packing fraction ~ 64%). This was about 20,000 for a 1 micron sized individual fiber and about 800,000 for 300 nm sized individual fibers. These numbers were on the order of the values obtained from SEM images (Figure 4d) analyzed using Image J software. Provided the fibers are suitably twisted, the large fiber numbers ensured yarns of high integrity/strength, wherein any possible failure of few fibers would not disrupt the entire yarn.


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The process was also readily adapted to the formation of core-sheath yarns. For this, the up-taken PLLA yarn was passed through a second collector as shown in Figure 5a wherein the core was coated by an electrospun PLGA sheath, after a post-processing step. The second collector was also rotated such that the sheath twisted over the existing core, giving rise to a robust coating. In the absence of twisting of the core, the sheath made a uniform coverage over the core yarn. The resultant fibrous bundle was collected on a rotating mandrel whose speed was synchronized with that of the rotating collector. Thickness of the sheath layer was adjusted by varying parameters such as flow rate and uptake rate. SEM images (Figure 5b) further affirmed the formation of a uniform fibrous sheath of typical thickness 25-40 µm around the core. By integrating a postprocessing step of heat stretching prior to sheath deposition on the core yarn, its tensile strength and force were significantly enhanced (Figure 5a inset). In this step, a temperature of 120°C and a 6-times yarn elongation yielded core yarns with high tensile strength and force. The heat stretching process resulted in yarns with minimal twist (longitudinally aligned as in Figure 5b), but with increased strength, mainly due to the alignment of polymeric chains 29. The sheath was also used to encapsulate drug/dye/biomolecules. A representative near infrared dye, Indocyanine Green (ICG), was incorporated within electrospun PLGA fibers. Loading of the dye molecule within the fibrous PLGA sheath did not alter the tensile strength or force of the core-sheath yarns (Figure 5c). Additionally, fluorescence images depicted in Figure 5d confirmed the in vivo distribution of the dye around the implanted site with reference to control yarns. A major drawback reported thus far for electrospun fibers is their significantly reduced mechanical strength upon drug/biomolecule loading, challenging its utility for applications that demand good mechanical strength with concurrent drug loading, such as in drug eluting sutures30. Herein, the use of a core-sheath architecture coupled with the post-processing


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technique adopted, yielded mechanically strong, biomolecule/dye incorporated yarns that can find versatile use. We also demonstrated the utility of these core or core-sheath yarns for developing various kinds of constructs (Figure 6) by exploiting the textile technology techniques of plying, braiding and weaving using industrial machines (Supporting Information Video 3). Braiding of yarns yielded open-ended hollow structures that have potential use as biodegradable stents or vascular grafts, while the woven patch can be a suitable candidate for drug delivering implants, which is currently being explored by us. To summarize, it was possible to integrate conventional textile technology with electrospinning by utilizing a rotating collector design that yielded stable interlocked fibrous yarns, continuously drawn from a fluffy electrospun mass via a substrate-less deposition process. Strong yarns with controlled twist angle were produced at a high rate of several meters per minute and an overall efficiency of ~ 94 %, proving the scalability of the process. An optimal uptake rate of 0.6 m.s-1 and a collector rotation of ~ 900 rpm lead to a 8-fold increase in the tensile strength, with a concurrent 6-fold elongation, by adopting a heat stretching process. This in turn helped to generate a novel high strength core-sheath yarn that enabled the incorporation of other molecules into the sheath for potential drug delivery applications. These yarns could meet the two prime pre-requisites of high tensile strength and force, necessary for processing into complex 3D woven, knitted or braided structures. Method Electrospinning: Micro-Nano fibrous composite PLLA-PCL yarns: Poly-L-Lactic acid (PLLA) (Mw=100 140 kDa, Goodfellow, UK) and Poly(caprolactone) (PCL) (Mw= 45 kDa, Polysciences Inc.,USA) were dissolved in chloroform:acetone- 3:1 and Chloroform:methanol-1:1 (HPLC


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grade, Merck, India) respectively. For this, dual spinnerets were maintained at positive (12 kV) and negative potentials (-14 kV) [ Gamma High Voltage, USA] and a uniform flow rate of 2.5 ml/hr using an infusion pump (KD Scientific, USA) was applied to polymeric solutions taken in a syringe at a concentration of 14% w/v (PLLA and PCL), yielding composite yarns with micron-sized PLLA and nano-sized PCL fibers. Both the spinnerents were maintained at a relative angle of 45° with respect to collector axis. Micro fibrous PLLA yarns: Dual spinnerets were maintained at opposite polarities of ±12 kV and a uniform flow rate of 3.5 ml/hr was applied to the polymeric solution taken in a syringe at a concentration of 14 % w/v of PLLA, yielding yarns with micron-sized PLLA fibers. Nano fibrous yarns were also obtained using the same assembly for applied potentials of ± 14 kV, at a flow rate of 2.5 ml/hr and concentration of 14% w/v of PCL. Fluffy mass: Dual spinnerets of opposite polarities of ±12 kV were positioned at 45° with respect to collector axis. Both the syringes were loaded with PLLA solution (14% w/v) and a flow rate of 3.5 ml/hr resulted in a 3D cotton-like fluffy mass at the rotating collector (100-1100 rpm) as shown in Figure 2a. 2D non-woven mats: Mats of PLLA were developed with a single spinneret positioned along the axis of the non rotating collector (for diameters < 8 cm), wherein the point electrodes were close enough forming a continuous flat electrode as shown in Figure 1d. Core-sheath yarn: A spool of core PLLA yarns hundreds of meters long, with diameters of 200±50 µm were drawn through 12 wt% electrospun PLGA 50-50 (Mw=50 kDa Polysciences Inc.,USA) jet at a voltage of 12 kV and flow rate of 3ml/hr. The resulting PLLA-PLGA coresheath yarns had a final diameter 240±60 µm.


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Functional Core-sheath yarn: Functionality to the core –sheath structure was imparted by loading 0.5 wt% Indocyanine Green (Sigma Aldrich,USA) within 12 wt% PLGA under the above mentioned conditions. Efficiency: Efficiency (which we define as weight of polymer obtained divided by weight of polymeric mass taken in unit time) of static (planar electrode), rotating, near field (all using single spinnerets) and the modified collector (single and dual spinneret) were determined at a constant tip-target distance (20cm, except for near field spinning-2 cm), flow rate (3.5 ml/hr/spinnerets) at a voltage of 12 kV. Areas of all the collector/target were maintained identical (153.86 cm2). After electrospinning for one minute, fibers collected on each target were weighed to calculate efficiency. Heat Stretching (Post-processing of yarns) Yarns were heat-stretched in a custom designed setup consisting of a cylindrical furnace (404000 C), enabling the yarns to be stretched by using an uptake and unwinding motors (not shown) of variable speed (0.4- 4 m.min-1). An optimal heating of 1200C and elongation of 6 times yielded yarns of high tensile strength. Mechanical Testing Tensile strength and maximum force at break were evaluated according to the ASTM protocol by a straight pull test using a Universal Testing Machine (H5KL, Tinius Olsen, USA). All samples were tested using pneumatic grips with a load cell of 10 N. In vivo Studies Functionality of the core-sheath structure was evaluated in vivo according to the protocol approved by Institutional Animal Ethical Committee, AIMS, India. A near infrared dye, viz., Indocyanine Green (ICG), was incorporated within electrospun PLGA fibers deposited over the


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core yarn. A 2 cm incision was made on the dorsal flank of anesthetized Sprague-Dawley (SD) rats. Sub-dermal implantation of 1.5 cm long yarns was carried out (n=4) and the incision was closed using Vicryl 4-0 suture. Fluorescence imaging of SD rats was performed at different time intervals to assess the in vivo distribution of the dye using Kodak Multispectral imaging system, Carestream, USA. FIGURES

Figure 1. Schematic representation of the electrospinning setup with modified collector for fabricating multidimensional constructs. (a) The assembly consists of an infusion pump, syringe that is connected to a high voltage dc power supply through a spinneret and a rotating collector


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with conducting tines at the periphery. Fibers are drawn towards the collector by maintaining sufficient potential difference with the spinneret. Inset represents the planar collector used for traditional electrospinning process. (b), (c) & (d) represents the schematic arrangement for developing 3D fluffy mass, 1D yarn and 2D mats respectively. Inset in (b) depicts the actual photograph of the fluffy mass deposited at the center of the collector. Inset in (c) depicts the actual photograph of the cone formation region where the fiber density is maximum at the apex. Arrows in Figure 1(a), (b) & (c) represent the airflow pattern generated upon collector rotation. (Patent pending 3131/CHE/2014)

Figure 2. Pattern of fiber deposition on to the collector at different time points (30 and 120 seconds) (a) & (b) in absence and (c) & (d) in presence of rotation. 3D fluffy mass was formed only for the rotating collector for t > 120 seconds. Relative positions of spinnerets with opposite polarity have been marked in the figure.


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Figure 3. Optical and SEM micrographs of (a) & (c) 3D PLLA cotton-wool, (b) & (d) 1D PLLA yarn, (e) PLLA-PCL micro-nano yarns and (f) PLLA submicron fibrous yarns fabricated using the modified collector. Inset represents the corresponding magnified SEM images.


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Figure 4. Influence of (a) yarn uptake rate and (b) collector rotation on force and tensile strength, (c)(i-iv) SEM micrographs of yarns with varying twist angles obtained at different collector rotations and (d) Cross sectional view of micro fibrous yarns, inset shows its high magnification image (Magnification: 2000 X).


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Figure 5. (a) Schematic representation of core-sheath yarning using the modified collector; left inset represents the photograph of sheath formation onto the core yarn (blue) and right inset depicts the variations in mechanical properties upon post processing (b) SEM image of PLLAPLGA core-sheath yarn, with uniform sheath thickness of 45 µm. Blue and green arrows indicate the PLLA core and PLGA sheath respectively (c) Mechanical testing of core, core-sheath & core-(sheath+ICG) yarns (2 weeks post implantation) showing no significant change in tensile


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strength or force at break upon dye loading. (d) & (e) Functionality of dye (ICG) loaded coresheath yarns evaluated by in vivo fluorescence imaging in comparison to control.

Figure 6. Optical and SEM micrographs of 3D constructs fabricated using electrospun yarns (a) & (b) braided construct, (c) & (d) helical coil and (e) & (f) woven fabric.


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TABLES Table 1. Efficiency of various Collectors used in Electrospinning Types of Target

% Efficiency

Static (Traditional)

63.79 ± 4.50

Rotating mandrel

78.73 ± 4.97

Near field

100 ± 2.28

- Single spinneret

76.89 ± 2.88

- Dual spinneret

93.96 ± 1.46

Modified

Parameters such as target area, spinneret-target distance, flow rate, voltage, concentration, number for spinnerets and time of fiber deposition were kept identical. # denotes change is spinneret target distance (2 cm).

Table 2. Variation of Young’s modulus and % strain with respect to collector rotation Collector rotation (rpm)

Young’s modulus (MPa)

% Strain

1100

449.0 ± 54.34

63.79 ± 4.50

100

67 ± 9.38

189 ± 28.47

ASSOCIATED CONTENT Supporting Information. Details of variations in collector design, electrospun mat, SEM image of cone formation, yarning parameters, optical and SEM images of Polystyrene, Polyvinylpyrolidone/Titanium Isopropoxide and Polyurethane and videos depicting air suction by the rotating collector, generation of fluffy


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mass and fabrication of woven constructs from electrospun yarns are shown in this section. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest Funding Sources This study is supported by the Department of Science and Technology (DST), Government of India, under the project “Thematic Unit of Excellence” (SR/NM/NS-07/2011). ACKNOWLEDGMENT Authors acknowledge the funding received from Department of Science & Technology, Government of India through a Thematic Unit of Excellence, and Amrita Vishwavidyapeetham for all infrastructural support. John Joseph acknowledges Council of Scientific and Industrial Research (CSIR) for a Senior Research Fellowship. We thank Dr. Krishnaprasad Chennazhi, Dr. Sahadev Shankarappa and Mr. Sajesh K M for all their valuable inputs for this work.


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ABBREVIATIONS PLLA, Poly-L- Lactic Acid; PCL, Poly-ε- Caprolactone; PLGA, Poly-(Lactide-co-Glycolide); ICG, Indocyanine Green; PS, Polystyrene; PVP, Polyvinylpyrolidone; TIP, Titianium isopropoxide; PU, Polyurethane. REFERENCES

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Nano Letters

Graphical abstract


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Integrating Substrateless Electrospinning with ... - ACS Publications

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