NANO LETTERS
Electrospinning of Three-Dimensional Nanofibrous Tubes with Controllable Architectures
2008 Vol. 8, No. 10 3283-3287
Daming Zhang and Jiang Chang* Biomaterials and Tissue Engineering Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China Received June 10, 2008; Revised Manuscript Received August 6, 2008
ABSTRACT This paper reports a novel static method to fabricate three-dimensional (3D) fibrous tubes composed of ultrafine electrospun fibers. By using this unique technique, micro and macro single tubes with multiple micropatterns, multiple interconnected tubes, and many tubes with the same or different sizes, shapes, structures, and patterns can be prepared synchronously. Parameters that could influence the order degree of patterned architectures have also been investigated. It is expected that electrospun tubes with controllable patterned architectures and 3D configurations may be attractive in many biomedical and industrial applications.
Electrospinning is currently the only technique that allows fabrication of nanoscale continuous fibers. Electrospun ultrafine fibers with extremely long length and high specific surface area1,2 have found extensive applications in many biomedical and industrial fields.3-6 For example, electrospun fibrous tubes have shown great potential in vascular, neural, and tendinous tissue engineering.7-10 However, electrospun fibers are generally collected as two-dimensional (2D) membranes with randomly arranged structures, which has greatly limited their applications. In order to fully realize the potential of electrospun fibers, it is important to fabricate fibrous assemblies with controllable three-dimensional (3D) microstructures as the fiber arrangement will significantly affect the performance of devices.11 Considering specific requirements of tubular scaffolds in tissue engineering applications, such as the variation in anatomic location and biological environment,12 it is important to design and control microscopic and macroscopic 3D structures of tubes to create desired cellular responses.13,14 Intensive studies on assembly of electrospun fibers have met with some success both in microscopic arrangements of fibers15-17 and macroscopic 3D tubular structures.8,9 Li and co-workers have demonstrated that nanofibers can be uniaxially aligned by introducing insulating gaps into conductive collectors.15 Furthermore, our group has successfully fabricated electrospun mats with controllable architectures and patterns.17 These specific architectures might promote favorable biological responses in tissue regeneration, such as enhanced protein adsorption as well as enhanced cell attachment and proliferation in tissue * To whom correspondence should be addressed. E-mail: jchang@ mail.sic.ac.cn. 10.1021/nl801667s CCC: $40.75 Published on Web 09/04/2008
2008 American Chemical Society
regeneration.18,19 However, these electrospun mats with specific fiber arrangements were usually collected as 2D membranes. Generally, rotating devices could be used to collect 3D fibrous tubes. Nevertheless, there still remain some disadvantages of this collecting method. For example, it is difficult to control the arrangements of fibers and the architectures of electrospun tubes, except for the circumferential well-aligned arrangement because of the rotational movements of the collectors. Meanwhile, failures in fabrication of tiny tubes (less than 0.3 mm), tubes with one closed end, and tubes with multiple interconnected tubular structures, may also limit the application of fibrous tubes, and there still remains considerable difficulties in fabricating fibrous tubes with controllable micropatterns and macroscopic 3D tubular structures synchronously, and complex tissue structure and fiber orientation still cannot be mimicked adequately.14 To overcome various limitations of the current preparation methods, a unique static collecting method with combinatorial electric fields was designed and nanofibrous tubes with different microscopic architectures and macroscopic 3D tubular structures were fabricated in the present study. The novel method using novel 3D collecting templates is based on manipulation of electric field and electric forces, and micro and macro single tubes with multiple micropatterns, multiple interconnected tubes and many tubes with same or different sizes, shapes, structures and patterns can be prepared synchronously using this unique technique. In addition, effects of parameters such as voltage, feeding rate, and solvents ratio on architectures of the tubes were investigated, which demonstrated the possibility to adjust the order degree of fiber deposition and arrangement.
Figure 1. (a) Schematic illustration of fabrication of fibrous tubes by electrospinning technique using 3D columnar collectors. 1: 3D columnar collectors. 2: relevant fibrous tubes. (w, working collector; pa, plane assistant collector; sa, stick assistant collector) (b) Fibrous tube with diameter of 500 µm (inset is the cross-section image). (c) SEM image of fiber assemblies of tube shown in panel b.
Figure 1a shows a schematic illustration of electrospinning technique combined with a unique collecting method. Part 1 shows designed 3D columnar collectors, and relevant fibrous tubes with similar configurations could be generated after electrospinning as schematically shown in part 2. It is found that the macroscopic structures of the tubes are controllable by controlling the configurations of collectors. Tubular structures with different lengths, diameters, as well as cross-section shapes could be fabricated. Meanwhile, by using this unique method, tubes with multiple interconnected tubular structures could also be fabricated. Basically, the collector could be divided into two parts in the collecting process, the working collector (“w” in Figure 1a) with similar configuration of desired fibrous tubes and the assistant collectors (“sa” and “pa” in Figure 1a). With effect of electric forces, fiber loops could deposit on both working and assistant collectors. When an individual columnar template is used as the collector, fibers tend to converge toward the top part of the collector because of the concentration of electric field pointing to the top of the collector,20 especially when the top part is in a sharp shape. It has been reported that fiber deposition could be influenced even with a slight variation in electric field profile.12,21-23 Thus, a plane assistant collector (pa) is used to alter the electric field and further extend the deposition areas of fibers, and as a result convergent deposition of fibers on the top of the collector could be avoided. Meanwhile, it has been found that fibers may suspend between the root of columnar collectors and assistant collectors in some situations, which could cause inhomogeneous collections. Thus, another stick assistant collector (sa) is introduced. Polycaprolactone (PCL) and D,L-poly(lactic acid) (PDLLA) were dissolved in dimethylformamide (DMF) and tetrahy3284
Figure 2. (a) Schematic illustration of collecting process using a cylindrical collector with equally spaced circular protrusions (es, electrospinning process; pc, patterned collector). (b) A fibrous tube with patterned architectures (scale bar ) 5 mm). (c) Magnified image of panel b (scale bar ) 200 µm). (d) Schematic illustration of collectors with two different patterns and relevant fibrous tube (pc, patterned collector; ft, fibrous tube). (e) A fibrous tube with two different patterns (scale bar ) 5 mm). (f,g) Magnified images of two different patterns of panel e (scale bar ) 200 µm).
drofuran (THF), and the solutions were used in a typical electrospinning process (Supporting Information, Materials and Methods). Figure 1b shows optical photograph of a tiny fibrous tube fabricated using this method. The diameter of the tube is 0.50 mm, and the relevant cross section image is shown in the inset. It could be found from Figure 1c that the 3D fibrous tube is composed of randomly arranged ultrafine fibers. By using this method, 3D fibrous tubes with different diameters, lengths, and various cross-section shapes, as well as tubes with one closed end, can be fabricated (Supporting Information, Figure S1). Many applications require patterned architectures of the materials. For example, materials with parallel orientation or specific patterned structure may have specific biological effects on tissue regeneration.24,25 In the previous work, we demonstrated that nanofibrous materials with patterned architectures could be fabricated using 2D flat collectors.17 In the present work, our study is focused on electrospinning using 3D collectors with patterned structures. One of the most important advantages of the unique technique is the fabrication of tubes with controllable patterned architectures. A schematic illustration of collecting process using a cylindrical collector with equally spaced circular protrusions is shown in Figure 2a. Fibers tend to deposit randomly on the protrusions and suspend in a parallel manner between the protrusions with the effect of electric forces F. Figure 2b shows the generated fibrous tube with patterned architectures. Nano Lett., Vol. 8, No. 10, 2008
Partly magnified image of the tube is shown in Figure 2c. Fibers deposited on the protrusions are much denser than those aligned in parallel between the protrusions, and the suspended fibers tend to adhere together to form fiber bundles; as a result, a specific patterned architecture is generated. In many situations, different patterned structures or architectures may be required in one tube, and two or more different patterns can be generated in one tube by the new technique presented in this study. Figure 2d shows a schematic illustration of fabrication of a tube with two different patterned structures. A 3D collector with two different patterns is illustrated in Part 1, while the generated tube with two relevant patterned architectures is shown in part 2. Figure 2e shows a fibrous tube with two different patterned architectures. It could be found that, besides the structure aligned in parallel and shown in Figure 2f, a gridlike patterned architecture was also generated in the same tube (Figure 2g). It is believed that fibrous tubes with various controllable patterned architectures in one tube could be generated by design of appropriate 3D collectors, and a typical example of one fibrous tube with four different patterns on four sides of the tube is presented in Supporting Information (Figure S2). The deposition and arrangement of fibers, as well as the formation of patterned architectures are determined by electric forces.15,17 Fibers move toward the 3D collectors with the effect of electrostatic forces resulting from the altered electric field. When fibers get close to a patterned collector, the deposition and arrangement of fibers are primarily affected by Coulomb interactions and deposit on the protrusions by the stronger Coulomb interactions with the protrusions as opposed to the bottom part of the collectors.17 When a patterned plane template is used as a collector, fiber loops move vertically toward the 2D collector. The arrangement of fibers on the patterned collectors is primarily influenced by the patterned architecture, and generally if a symmetrical patterned collector is used preferential symmetrical deposition of the fibers between protrusions will occur. In contrast, when a 3D patterned template is used as collector the collecting sides, which can be regarded as separated 2D plane collectors, are parallel along the moving direction of the fiber loops at the beginning of the spinning process, and the fibers will be attracted toward the patterned surface of the collectors only when they are getting close to the templates. Apparently, it is expected that the fiber deposition on a 3D patterned collector might be different from that on a 2D collector, since the moving direction of fiber loops toward the collector is different between the 3D and 2D collectors. However, no any obvious change of symmetry in the patterns formed on 3D collectors was observed as compared to the patterns formed on 2D collectors. This may be explained by the much smaller distance between the protrusions of the patterned collectors as compared to moving distance of the fiber loops; the formation of the patterned fibers is determined mainly by the fiber fragments close to the protrusions, but not by fiber loops. Nano Lett., Vol. 8, No. 10, 2008
It is known that the structures with patterns of different ordering could affect the cell activities in tissue engineering.26 Thus, it is necessary to control the order degree of the fibrous architectures. In the previous study, we have shown the fabrication of patterned fibrous structures using flat patterned collectors. 17 In the present study, a patterned collector with protrusions was used to investigate the influences of electrospinning parameters on the order degree of the patterns. Figure 3 shows the influence of voltage, feeding rate, and volume ratio of solvents on the patterned architectures. It was found that with proper parameters, a fully patterned structure could be generated.17 However, with the increase of voltage (7.5 kv), some fibers adhere together, and the ordered orientation of suspended crossing fibers has been changed. When the voltage is as large as 10 kv, no obvious ordered fiber arrangement could be found. Similar phenomena is found with increased feeding rate as shown in Figure 3b. The volume ratio of the solvents also showed significant influence on the ordering of the patterned structures. When THF volume ratio in solvents increased, both the ordered fiber arrangement and specific fiber deposition on protrusions changed, and patterned architectures tended to disappear (Figure 3c). There are two factors that directly determine the patterning of the fibers: velocity of fibers, and surface charge density of fibers. When the velocity of fibers is relatively small, surface charge density of fibers is the key factor. Higher charge density induces stronger Coulomb interactions, which will result in the increase of the order degree of fiber arrangement. However, when the velocity of fibers is relatively large, it turns to be more effective as compared to surface charges. Many fibers will deposit on the substrates before they are drawn to the patterned positions. With the increase of voltage, the fiber movement is affected by increased electrical forces, so that the fibers arrive at the collectors with high velocity,1 which leads to disordered structures on patterned collectors. It has been reported that charge density on the fibers decreases with increased feeding rate.27 Thus, fibers will experience smaller Coulomb interactions, and the order degree will be reduced. Meanwhile, as more liquid jets are drawn out of the syringe, the vapor pressure of solvents around the fiber loops becomes rather higher, and the evaporation of solvents will be resisted. Therefore, many fibers adhere together as they are wet when deposited on the collectors, which could also influence the motion of fibers because of adhesion. The permittivity of THF (7.4) is about one fifth of that of DMF (36.7), and with the increase of THF/DMF volume ratio the permittivity of solvent is reduced obviously. As a result, the charge density on the fibers is highly decreased, which leads to smaller Coulomb interactions and random fiber deposition. In summary, by adjusting the process parameters of the electrospinning, patterned architectures with different order degree could be fabricated, and lower voltage, smaller feeding rate, and solvents with high permittivity could promote the formation of highly patterned architectures. However, it is worth indicating that the relationship between the parameters and order degree of the patterns identified in our system may 3285
Figure 3. (a) Influence of voltage on patterned architectures (v, voltage). (b) Influence of feeding rate on patterned architectures (FD, feeding rate). (c) Influence of volume ratio of solvents (DMF/THF) on patterned architectures (VR, volume ratio; D, DMF; T, THF). (scale bar ) 100 µm).
not be suitable for other systems, and optimization of the parameters needs to be carried out for each specific system in engineering applications. Besides simple tubes, this static collecting method can also be used to fabricate fibrous tubes with multi-interconnected tubular structures, which seems to be difficult by using rotating devices. Figure 4a shows a schematic illustration of a typical collecting process, and the generated crossing tube with interconnected tubular structure is shown in Figure 4b. The combinatorial collector 1 is composed of two parts, a removable part C1 and a basal part C2 with a circular hole that has the same diameter as C1. In the process 2, a tube with crossing structure was formed on the surface of the combinatorial collector during electrospinning. After electrospinning, C1 was then removed as shown in process 3, followed by the removal of the crossing tube from C2 as shown in process 4. Some similar complex tubes fabricated using this method are shown in Figure 4c. By designing combinatorial collectors with complex tubular structures, interconnected tubes with many branches can be fabricated. This kind of interconnected tubes may find important applications in blood vessel reconstruction.28,29 Besides fabrication of fibrous tubes with controllable patterned architectures and 3D configurations, another important feature of the new technique is the batch fabrication of tubes with different size, shape, wall structure, and pattern. Figure 4d shows a schematic illustration of a typical batch 3286
fabrication process. 3D collectors with the same or different configurations and microscopic architectures could be fixed in one plane assistant collector, and various tubes could be generated synchronously. Figure 4e shows an optical photograph of nine tiny tubes fabricated in one combinatorial collector. Distance between the individual 3D collectors is a key parameter in this process, and excessively small distance may cause fiber suspension between collectors. However, the exact value is influenced by lots of parameters such as materials used and applied electrospinning parameters, and may vary in different experimental situations. In summary, electrospinning with unique 3D collecting method is a versatile technique to fabricate fibrous tubes for various industrial and biomedical applications. By designing 3D collectors, fibrous tubes with different macroscopic configurations (length, diameter, shape) can be fabricated, and multiple tubes with various interconnected tubular structures could be fabricated using removable collecting templates. Furthermore, fabrication of tubes with patterned architectures is synchronously controllable in this process, and two or more different patterns could be generated in one tube. Voltage, feeding rate, and volume ratio of solvents could influence the order degree of patterned architectures. In addition, this method has been proven to be effective in batch fabrication of tubes with the same or different configurations and architectures at one time. Manipulation of electric field is the basic principle of this method, and Nano Lett., Vol. 8, No. 10, 2008
attractive in many biomedical and industrial applications such as tissue engineering and filtration applications. Acknowledgment. This work is supported by the National Basic Science Research Program of China (973 Program) (Grant 2005CB522704) and the Natural Science Foundation of China (Grant 30730034). Supporting Information Available: Materials and methods and figures. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 4. (a) Schematic illustration of process for fabrication of tubes with multiple interconnected tubular structure (C1, removable collector; C2, basal collector; T, tubes with interconnected tubular structure). (b) A crossing tube (scale bar ) 5 mm). (c) Tubes with various interconnected tubular structures (scale bar ) 5 mm). (d) Schematic illustration of the batch fabrication of tubes with same or different configurations and microscopic architectures. (e) Image of batch fabrication of nine micro tubes.
many parameters including the distance between tip of spinneret and the top part of 3D collector, as well as the specific properties of materials and the enviromental conditions, may affect the tube structures. The influence of these parameters might mutually be dependent, and further studies are required to determine the role of all of these parameters. It is expected that electrospinning with the static collecting method using 3D collectors with specifically designed patterns and configurations has great potential for fabrication of fibrous tubes with controllable architectures and 3D configurations, and these kinds of fibrous tubes may be
Nano Lett., Vol. 8, No. 10, 2008
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