Highly porous 3D nanofiber scaffold using an electrospinning technique

Highly Porous 3D Nanofiber Scaffold Using anElectrospinning Technique

GeunHyung Kim, WanDoo Kim

Bio-mechatronics Laboratory, Department of Future Technology, Korea Institute of Machinery and Materials,Yuseong-Gu, Daejeon, Korea

Received 7 December 2005; revised 23 March 2006; accepted 15 May 2006Published online 21 August 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30642

Abstract: A successful 3D tissue-engineering scaffold must have a highly porous structure

and good mechanical stability. High porosity and optimally designed pore size provide

structural space for cell accommodation and migration and enable the exchange of nutrients

between the scaffold and environment. Poly(e-carprolactone) fibers were electrospun using an

auxiliary electrode and chemical blowing agent (BA), and characterized according to porosity,

pore size, and their mechanical properties. We also investigated the effect of the BA on the

electrospinning processability. The growth characteristic of human dermal fibroblasts cells

cultured in the webs showed the good adhesion with the blown web relative to a normal

electrospun mat. The blown nanofiber web had good tensile properties and high porosity

compared to a typical electrospun nanofiber scaffold. ' 2006 Wiley Periodicals, Inc. J Biomed Mater

Res Part B: Appl Biomater 81B: 104–110, 2007

Keywords: poly(e-carprolactone); nanofiber; scaffold; chemical blowing agent

INTRODUCTION

One of the important issues in tissue engineering has been

the development of 3D scaffolds, which guide cells to grow

functional tissues and allow the diffusion of nutrients, metabo-

lites, and soluble factors.1,2 Factors governing scaffold de-

sign include considerations of pore size and morphology,

mechanical properties versus porosity, surface properties, and

appropriate biodegradability.3–5 Of these, pore size and po-

rosity are directly related to the success of a scaffold. High

porosity provides structural space to accommodate cells and

serves as the exchange path for nutrients and metabolic waste

between the scaffold and the environment.3 The pore size of

a scaffold should be optimized to allow cell migration. How-

ever, it is difficult to define the optimum pore size accurately;

rather, a range of pore sizes is used. The pore size is also de-

pendent on the tissue type desired. For example, the effective

pore size for cell growth is between 20 and 60 mm in vascular

grafts, while it is 75–150 mm for bone tissue.6

Materials with a 3D micropore structure have been pro-

duced using various manufacturing methods, including a

solid free-form (SSF) fabrication system (or RP process),7,8

fiber bonding (unwoven meshes), solvent casting or particu-

lar leaching, phase separation, high pressure gas expansion,

and emulsion freeze-drying.9–13 Although various techni-

ques have been developed for manufacturing scaffolds, it

is not clear which method would best fabricate a so-called

ideal scaffold.

As other studies have reported, the criteria for an ideal

polymeric scaffold are also dependent on the type of mate-

rial, which can range from natural biodegradable high-

molecular weight materials, such as collagen, agarose, and

alginate,14–16 to synthetic polymers such as poly(glycolic

acid) (PGA), polylactic acid (PLA), poly(e-carprolactone)(PCL), and poly(D,L-lactide-co-glycolide).4,17 An electro-

spun PCL scaffold coated with collagen has been used to

seed smooth muscle cells to construct vascular tissue.18

Moreover, PLA and PGA have also been used for cell trans-

portation and the regeneration of vascular tissue in vivo.19

Recently, the electrospinning process has been explored

widely as a way to fabricate nanofibers for various applica-

tions, such as in biosensors, implantable scaffolds, and ultra-

thin filtration. With increasing demand for a sophisticated

nanosized hybrid and pore structure based on commercial

polymers, various electrospinning processes have been inves-

tigated and developed.20–25 As a good method for fabricating

nanofiber structures, this technique is preferred because of its

simplicity, allowing the manufacture of nanofibers made from

complex ingredients, and high applicability.

The principle of electrospinning is that a Taylor cone is

formed by applying an electric field to polymer solution

hanging from a capillary tip, which causes jets of electrically

charged solution to be emitted when the applied electrostatic

force is stronger than the surface tension of the solution.20–22

Correspondence to: G. H. Kim (e-mail: [email protected] or [email protected])

Contract grant sponsor: Korea Institute Machinery and Materials (KIMM)

' 2006 Wiley Periodicals, Inc.

104

The jets of solution erupt from the apex of the cone at a noz-

zle and travel toward an electrically grounded target on which

they are stacked as a nonwoven mat.

Electrospun nanofibers overcome the limitations of con-

ventionally fabricated scaffolds because of their high surface

area and high porosity, and the easy fabrication process.4,26

However, the pore size of an electrospun nanofiber web is

so small that cultured cells have difficulty penetrating the

scaffold, which is a 3D structure composed of nanofibers.

Consequently, it is not suitable for fabricating 3D nanofiber

scaffolds because of the low rate of cell diffusion in the

thickness direction.

To overcome these problems, we fabricated nanofiber mats

using an electrospinning technique with a chemical blowing

agent (BA), and investigated the influence of the agent on the

spinning process, pore size, and mechanical properties of the

resulting mats. In addition, to control the unstable electro-

spinning process involving a chemical BA, we introduced an

auxiliary cylindrical electrode to this system.

MATERIALS AND EXPERIMENTAL PROCEDURES

Materials

A biodegradable polymer solution was prepared by dissolving

2.4 g of poly(e-carprolactone) (PCL; Sigma–Aldrich, St. Louis,

MO) in 30 g of a solvent mixture consisting of methylene-

chloride (MC) and dimethylformamide (DMF) in a MC/DMF

wt% ratio of 80/20. The PCL solution was prepared with a

fixed 8 wt% concentration. To observe the effect of a chemi-

cal BA on processability, three different weight percents of

BA were mixed with the PCL solution. The BA was azodi-

carbonamide (Cellcom-AC series; KumYang, Seoul, Korea).

The polymer solution was placed in a 20-mL glass syringe

with a G-20 needle. The flow rate of polymer solution was

controlled using a syringe pump system (KDS 230; NanoNc,

Seoul, Korea).

Electrospinning Setup for PCL Nanofibers

PCL nanofibers were fabricated using an electrospinning

technique with a cylindrical auxiliary electrode connected

to a nozzle. The cylindrical electrode and nozzle were con-

nected by a copper wire and could be charged simultaneously

as the fluid passed through the spinneret. The detailed shape

and geometry of the auxiliary electrode are shown in Figure 1.

The cylindrical electrode served to reduce the instability of

the initial jet leaving the apex of the Taylor cone. More details

on the role of the electrode are described later.

The syringe was subjected to the same applied voltage (15–

20 kV) with a distance of 150–300 mm between the target and

needle tip, and a high-voltage power supply (SHV300RD-

50K; Convertech, Korea) was used to control the applied

voltage.

Figure 1. Schematic of the electrospinning setup with an auxiliary

cylindrical electrode. The cylindrical electrode has a radius of 13 mm

and is 0.45 mm thick.

Figure 2. EFCFs for a single nozzle and a nozzle connected to a cylindrical electrode. (a) Schematic

of a nozzle with an auxiliary electrode in the electrospinning setup used in this work, showing thecalculated EFCFs (b) without and (c) with an auxiliary electrode. [Color figure can be viewed in the

online issue, which is available at www.interscience.wiley.com.]

105HIGHLY POROUS 3D NANOFIBER SCAFFOLD

Journal of Biomedical Materials Research Part B: Applied BiomaterialsDOI 10.1002/jbmb

The initial straight jet in the electrospinning process was

visualized using a 532 nm green laser with an expanding op-

tical lens. The initially spun jet was photographed using a

digital camera (E-300; Olympus, Tokyo, Japan).

The spun fibers were deposited on a poly(ethylene tereph-

thalate) (PET) film covering a copper target and the spun

nanofiber web was blown in 2–3 s at 100 8C in a convection

drying oven.

Cell Culture and Experiment

The nanofiber scaffolds were sterilized with 70% ethanol

and exposed to UV light for 1 h They were prewarmed with

Hank’s balanced salt solution for 2 h at 50 8C. Human der-

mal fibroblasts (HDFs) were cultured in Dulbecco’s modi-

fied eagle medium supplemented with 10% fetal bovine

serum and maintained up to passage 7. Isolated fibroblasts

were seeded on each sample (1 3 1 cm2) at a density of 2 3104 cells and cultured for up to 3 days at 37 8C and 5%

CO2. The cells were fixed with 2.5% glutaraldehyde for 1 h

at room temperature, and dehydrated through a series of

ethanol dilutions. The samples were sputter-coated with Pt.

Cell morphology and growth was observed using a scanning

electron microscopy.

Structural Morphology and MechanicalTesting of PCL Scaffolds

The morphologies of the electrospun PCL mats constructed

with and without a BA were observed under an optical micro-

scope (BX FM-32; Olympus) and scanning electron microscopy

(FE–SEM, Sirion; FEI, Hillsboro, OR). Before observation,

the scaffolds were coated with gold using a sputter coater.

The pore size and porosity were measured from the SEM and

optical micrographs. The tensile properties of nanofiber mats

were characterized using a Universal Tensile Machine (Tytronics,

Lowell, MA) at a 10 mm/min crosshead speed.

RESULTS AND DISCUSSION

Effect of the Auxiliary Electrode and Process Parameters

To analyze the effect of the auxiliary electrode in this elec-

trospinning process, ANSYS/Emag-3D was used to conduct

Figure 3. Image showing the stable initial jet of solution (straight line)from a single nozzle during electrospinning to a target plate coved

with PET film with DC ¼ 18 kV and a distance between the nozzle

and ground of 150 mm. The spun fibers were obtained from 8% solu-

tion of PCL and 1 wt% BA in MC/DMF.

Figure 4. Electrical conductivity, k, and surface tension, g, of 8 wt%

PCL solutions in a mixture of MC and DMF containing various con-centrations of BA.

Figure 5. Transmission optical microscope images of electrospun

nanofiber obtained with (a) 0 wt% or (b) 1 wt% BA in an 8 wt% PCLsolution.

Figure 6. Process diagram of electrospinning using PCL solutions

with or without a BA.

106 KIM AND KIM


an electromagnetic analysis using the finite element method.

The numerical models were constructed using three different

elements: 20-node brick elements, tetrahedron elements that

were degenerate brick elements, and infinite elements. The

radius and length of the nozzle were 0.5 and 15 mm, respec-

tively. For the system shown in Figure 2(a), the inner radius,

R, of the auxiliary electrode and the thickness of the auxil-

iary electrode were 13 and 1 mm, respectively. The distance,

H, between the spinneret and the ground electrode was

250 mm. To consider the effects of an applied electric field

in this electrospinning process, the electric field concentra-

tion factor (EFCF) was defined using the equation27:

EFCF ¼ Ep��E

��

rp��rp

��¼ Er

��E

��

where Ep is the projection vector of the electrical field, E,

into the x–y plane, rp is the projection vector, and Er is the

r-directional component of the electrical field in the cylindri-

cal coordinates. Since the e-spinning system is approximately

axisymmetric, we located the origin of the coordinate system

at the center of the ground electrode. In this case, we used the

Cartesian coordinates system and defined the EFCF as the

rightmost term in the aforementioned equation in the cylin-

drical coordinates system. The EFCF is the normalized value

of the electrical field in the r direction. The normalized

r-directional value of the electric field can vary from �1 to

11, where a positive sign means that the divergence of the

electric field at the calculated position tends toward outside

areas, while a negative sign means that it converges on the

spinning axis. To calculate the EFCF for a single nozzle, we

used an applied electric field of 0.8 kV/cm.

The graphs in Figure 2(b,c) show that using the cylindri-

cal electrode results in a stable initial jet emitted straight

from the nozzle tip. The calculated results were compared to

an experimental test. Figure 3 shows that the initial jet from

the nozzle spinneret with a cylindrical electrode is stable.

This experimental observation is in good agreement with the

numerical prediction.

The variable solution parameters, such as the electric con-

ductivity and surface tension, were characterized to determine

the effect of a chemical BA on the formation of nanosize

fiber mats. Figure 4 shows the electrical conductivity and sur-

face tension of PCL solutions containing various amounts of

BA. Increasing the weight percent of BA in the PCL solution

increased the conductivity of the mixture while decreasing

the surface tension. Optical microscope images of the result-

ant submicron nanofibers prepared using different weight per-

centages of BA are shown in Figure 5. From the figure, the

average diameter of spun fibers with 1 wt% BA was 0.74 mmversus 0.53 mm with no BA. This means that the electrical

conductivity and surface tension play an important role in

nanosize filament formation.

Figure 6 shows a process diagram for several flow rates and

applied electric fields. At a constant flow rate, when the solution

contained a BA, a higher electric field was needed to form nano-

fibers compared to a pure PCL solution, and increasing the con-

tent of BA markedly increased the unstable region where the

spinning process produced a mixture of drops and nanofibers.

Effect of a BA on the Porosity of the Nanofiber Mat

A mat composed of nanofibers forms a good scaffold, with

high porosity and a large surface area, and allows good cell

adhesion, propagation, and growth.4,26 Although a nanofiber

Figure 7. SEM images of PCL scaffolds made with and without a BA.

Figure 8. SEM image of a thin cross section of a slightly melted PCLnanofiber mat.



structure has benefits as a biomedical scaffold, several weak-

ness have been reported. These include poor mechanical prop-

erties, too small an interpore size between the electrospun

nanofibers, and difficulty making a 3D pore structure.4,28

In this section, to fabricate a nanofiber scaffold with a

large interpore structure and enhanced mechanical proper-

ties, a chemical BA and heating process were used. This

method is similar to the gas forming technique in that the

microporosity is generated by decomposition of the BA.29–31

Figure 7(a,b) is scanning electron micrographs of a typical

electrospun PCL nanofiber mat and a blown PCL nanofiber

mat. The nanofiber mat in Figure 7(a) was obtained from con-

ventional electrospinning using 8 wt% of PCL solution under

20 kV with 250 mm between the nozzle tip and target. The

distribution of the interpore size of the mat ranged from 10 to

130 mm. The electrospun mat in Figure 7(b) was fabricated

with the assistance of an auxiliary electrode with 1 wt% of a

BA in the solution. The nanofiber mat was made in 2 s at

1008C in a drying oven. As shown in Figure 7(b), the nano-

fiber mat melted slightly due to the low melting temperature

of PCL, and two blown spots are visible. These blown spots

averaged 50–70 mm in size. From this structure, we postulate

that the pores generated using blowing media might provide

good access to deliver seeded cells into a 3D scaffold.

Although the optimal pore size does not need to be

defined because the pore size is dependent on the tissue type

desired, pore size and the porosity of a scaffold clearly influ-

ence cell division and growth. Figure 8 shows a SEM image

of a thin cross section of a nanofiber mat. This slightly melted

nanofiber mat might have sufficient porosity in the thickness

direction, and the structure has many interpore connections

resulting from the electrospun nanofibers. We think although

the pores are too small to allow vascularization, the pore

structure might be enlarged by degrading the PCL scaffold,

which would allow nutrients to reach cells seeded within the

scaffold.

To observe the effect of the blown nanofiber mat to the

cell proliferation, the HDFs cells were embedded in both the

normal electrospun fiber mat and the mat blown with 1 wt%

of BA and maintained in culture for 3 days. As shown in the

Figure 9, cells were spread on the nanofibers matrixes and

show interactions varied in the different scaffold structures.

When the Figure 9(a,b) were compared, the blown PCL mat

had good attachment with the cells compared with a nor-

mally spun fiber mat. As shown in the Figure 9(b), although

the HDFs cells were well spread in the surface of the blown

nanofiber web, the cells were difficult to penetrate to the

thickness direction of the scaffold. We believed that it was

because of the hydrophobic property of the PCL.

The graph in Figure 10 plots pore size and the porosity of

a nanofiber mat against the amount of BA. In Figure 10, the

increase in porosity with a BA was calculated as (porosity

generated with a BA)/(total porosity of the mat). Both pore

size and porosity increased with increasing amounts of BA.

Tensile Properties of Blown Nanofiber Mats

The mechanical properties of a scaffold are an important

design parameter for maintaining mechanical stability when

the scaffold is used to fill a defect in a host and retain struc-

tural intergrity.4,29 The tensile behavior of the electrospun

mat was measured using a Tytron (MTS) at a 10 mm/min

crosshead speed, and is presented in Figure 11. As Figure

11(a) shows the maximum strength and tensile modulus of a

nanofiber mat that was electrospun with a BA, but not blown

in a drying oven, was lower than that of a nanofiber mat

electrospun with a PCL solution without a BA. This differ-

Figure 9. SEM images of human skin fibroblasts on (a) a normal electrospun PCL nanofiber and (b) the blown nanofiber mat.

Figure 10. Effect of BA content on pore size and the increase in po-rosity using a BA for a PCL scaffold fabricated using electrospinning.

108 KIM AND KIM


ence may have resulted from the diameter of the nanofibers

or the interconnectedness and arrangement of randomly

stacked nanofibers. Figure 11(b) shows the stress–strain

curve obtained from samples of electrospun mats. Compared

to Figure 11(a), all the tensile properties increased due to

the enhanced interconnections between slightly melted nano-

fibers. The detailed tensile properties of several nanofiber

mats are compared in Table I.

CONCLUSIONS

We developed a new method of manufacturing microporous

scaffolds for specific tissue engineering applications that

involved electrospinning with an auxiliary cylindrical elec-

trode and a BA. The auxiliary electrode connected to the

spinneret stabilized the unstable initial jet and was influ-

enced by the BA. The effect of the electrode was validated

theoretically and experimentally. High microporosity and

large pores in the nanofiber scaffold were generated via the

decomposition of a chemical BA and this was improved by

melting the nanofibers slightly. The blown nanofiber mat has

good tensile properties compared to a typical nanofiber mat.

The blown nanofiber web provides a favorable environment

for HDFs cell attachment and proliferation comparable to a

normal electrospun web.

Future studies will examine cell culturing using smooth

muscle cells to mimic natural small-diameter blood vessels

and characterize cell proliferation and measure the penetra-

tion of seeded cells into a blown 3D scaffold.

The authors thank Ms. SunA Song, Mr. HyukSub Han, Mr.JongHa Park, Dr. YoungSam Cho, and SuA Park for their experi-mental and calculation assistances.

REFERENCES

1. Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson JD.Molecular biology of the cell. New York: Garland Publishing;1994. 971p.

2. Langer R, Vacanti JP. Tissue engineering. Science 1993;260:920–926.

3. Hutmacher DW. Scaffold design and fabrication technologiesfor engineering tissue: State of the art and future perspectives.J Biomater Sci 2001;12:107–124.

4. Li W-J, Laurencin CT, Caterson EJ, Tuan RS, Ko FK. Electro-spun nanofibrous structure: A novel scaffold for tissue engi-neering. J Biomed Mater Res 2002;60:613–621.

5. Boyan BD, Hummert TW, Dean DD, Schwartz Z. Role ofmaterial surfaces in regulating bone and cartilage cell response.Biomaterials 1996;17:137–146.

6. von Recum AF, Shannon CE, Cannon CE, Long KJ, van Koo-ten TG. Surface roughness, porosity, and texture as modifiersof cellular adhesion. Tissue Eng 1996;2:241–253.

7. Phan DT, Gault RS. A comparison of technologies. Int J MachTool Manuf 1998;38:1257–1287.

8. Hutmacher DW, Sittinger M, Risbud MV. Scaffold-based tissueengineering: Rationale for computer-aided design and solid free-form fabrication systems. Trends Biotechnol 2004;22:354–362.

9. Mikos AG, Sarakinos AJ, Leite SM, Vacanti JP, Langer R.Laminated three-dimensional biodegradable foams for use intissue engineering. Biomaterials 1993;14:323–330.

10. Mikos AG, Thorsen AJ, Czerwonka LA, Bae Y, Langer R,Winslow DN, Vacanti JP. Preparation and characterization ofpoly(L-lactic Acid) foams. Polymer 1994;35:1068–1077.

11. Freed LE, Marquis JC, Nohria A, Emmanual J, Mikos A,Langer R. Neocartilage formation in vitro and in vivo usingcells cultured on synthetic biodegradable polymers. J BiomedMater Res 1993;27:11–23.

12. Gomes ME, Godinho JS, Tchalamov D, Cunha AM, Reis RL.Alternative tissue engineering scaffolds based on starch: Proc-essing methodologies, morphology, degradation and mechanicalproperties. Mater Sci Eng C 2002;20:19–26.

Figure 11. The tensile properties of a nanofiber PCL scaffold prepared by electrospinning using a chemical BA.

TABLE I. Comparison of the Tensile Properties

Samples

Modulus

(Mpa)

Max. Strength

(Mpa)

Max. Strain

(%)

Before forming

W/O BA 0.25 4.17 37.6

With BA 0.11 3.62 70.9

After forming

W/O BA 0.61 7.42 150.5

With BA 0.41 5.41 97.0



13. Mano JF, Vaz CM, Mendes SC, Reis RL, Cunha AM.Dynamic mechanical properties of hydroxyapatite-reinforcedand porous starch-based degradable biomaterials. J Mater Sci:Mater Med 1999;10:857–862.

14. Dahlberg L, Kreicbergs A. Demineralized allogenic bone matrixfor cartilage repair. J Orthop Res 1991;9:11–19.

15. Watt FM, Dudhia J. Prolonged expression of differentiatedphenotype by chondrocytes cultured at low density on a com-posite substrate of collagen and agarose that restricts cellspreading. Differentiation 1988;38:140–147.

16. Ponticiello MS, Schinagl RM, Kadiyala S, Barry FP. Gelatin-based resorbable sponge as a carrier matrix for human mesen-chymal stem cells in cartilage regeneration therapy. J BiomedMater Res 2000;52:246–255.

17. Agraval CM, Ray RB. Biodegradable polymeric scaffolds formusculoskeletal tissue engineering. J Biomed Mater Res 2001;55:141–150.

18. Venugopal J, Ma LL, Yong T, Ramakrishna S. In vitro studyof smooth muscle cells on polycarprolactone and collagen nano-fibrous matrics. Cell Biol Int 2005;29:861–867.

19. Chu CF, Lu AL, Sipehia R. Enhanced growth of animal andhuman endothelial cells on biodegradable polymer. BiochimBiophys Acta 1999;1472:479–485.

20. Baumgarten PK. Electrostatic spinning of acrylic microfibres.J Colloid Interface Sci 1971;36:71–79.

21. Doshi J, Reneker DH. Electrospinning process and applicationof electrospun fibers. J Electrostatics 1995;35:151–160.

22. Yu JH, Fridrikh SV, Rutledge GC. Production of submicrondiameter fibers from difficult-to-process materials by two-fluidelectrospinning. Adv Mater 2004;16:1562–1566.

23. Kim G-M, Michler GH, Potschke P. Deformation processes ofultrahigh porous multiwalled carbon nanotubes/polycarbonatecomposite fibers prepared by electrospinning. Polymer 2005;46:7346–7351.

24. Han SO, Son WK, Cho D, Youk JH, Park WH. Preparationof porous ultra-fine fibres via selective thermal degradationof electrospun polyetherimide/poly(3-hydroxybutyrate-co-3-hy-droxyvalerate) fibres. Polym Degrad Stab 2004;86:257–262.

25. You Y, Youk JH, Lee SW, Min B-M, Lee SJ, Park WH. Prepara-tion of porous ultrafine PGA fibers via selective dissolution ofelectrospun PGA/PLA blend fibers. Mater Lett 2006;60:757–760.

26. Zong X, Bien H, Chung C-Y, Yin L, Fang D, Hsiao BS, Chu B,Entcheva E. Electrospun fine-textured scaffolds for heart tissueconstructs. Biomaterials 2005;26:5330–5338.

27. Kim GH, Cho YS, Kim WD. Stability analysis for multi-jetselectrospinning process modified with a cylindrical electrode.Eur Polym J. In press.

28. Smith LA, Ma PX. Nano-fibrous scaffolds for tissue engineer-ing. Colloids Surf B: Biointerfaces 2004;39:125–131.

29. Gomes ME, Godinho JS, Tchalamov D, Cunha AM, Reis RL.Alternative tissue engineering scaffolds based on starch: Proc-essing methodologies, morphology, degradation and mechanicalproperties. Mater Sci Eng C 2002;20:19–26.

30. Harris LD, Kim BS, Mooney DJ. Open pore biodegradablematrices formed with gas foaming. J Biomed Mater Res 1998;42:396–402.

31. Gravel M, Gross T, Vago R, Tabrizian M. Responses of mesen-chymal stem cell to chitosan–coralline composites microstruc-tured using coralline as gas forming agent. Biomaterials 2006;27:1899–1906.

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Highly porous 3D nanofiber scaffold using an electrospinning technique

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