Electrospun Polymeric Nanofibers: Fundamental Aspects of Electrospinning Processes, Optimization of Electrospinning Parameters, Properties, and Applications

  • Sowmya Sankaran
  • Kalim Deshmukh
  • M. Basheer AhamedEmail author
  • S. K. Khadheer Pasha
Part of the Lecture Notes in Bioengineering book series (LNBE)


Nanotechnology is a novel interdisciplinary field of science which has captured profound attention in all research areas due to its unique applications. Polymer nanofibers (PNFs) are one-dimensional (1D) fibers having diameters less than 1000 nanometers (nm). Electrospinning (ES) has been recognized as one of the most efficient, simple, versatile, and cost-effective methods for the fabrication of PNFs, among various other techniques such as phase separation, template synthesis, and self-assembly. The electrospun PNFs are being increasingly applied to biomedical fields due to its high surface-area-to-volume ratio, high porosity, and easy tuning of their structures, functionalities, and properties. Hence, these electrospun PNFs owing to their high specific surface area create a three-dimensional (3D) porous structure that mimics the native extracellular matrix (ECM), vitally useful in biomedical applications. In this chapter, we briefly discuss the fundamental aspects of the ES process and the properties of electrospun PNFs. This chapter also attempts to highlight the applications and importance of nanofibers in various fields of biomedicine such as tissue engineering, drug delivery, and wound healing.


Electrospinning processes Nanofibers Polymers Biomedical applications 


  1. Abrigo M, McArthur SL, Kingshott PL (2014) Electrospun nanofibres as dressings for chronic wound care: advance, challenges and future prospects. Macromol Biosci 14(6):772–792CrossRefGoogle Scholar
  2. Agarwal S, Wendorff JH, Greiner A (2008) Use of electrospinning technique for biomedical applications. Polymer 49(26):5603–5621CrossRefGoogle Scholar
  3. Ahmad J, Deshmukh K, Hagg MB (2013) Influence of TiO2 on the chemical, mechanical and gas separation properties of polyvinyl alcohol—titanium dioxide (PVA-TiO2) nanocomposite membrane. Int J Polym Anal Charact 18:287–296CrossRefGoogle Scholar
  4. Ahmad J, Deshmukh K, Habib M, Hagg MB (2014) Influence of TiO2 nanoparticles on morphological thermal and solution properties of PVA/TiO2 nanocomposite membranes. Arab J Sci Eng 39:6805–6814CrossRefGoogle Scholar
  5. Asghari F, Samiei M, Adibkia K, Akbarzadeh A, Davaran S (2017) Biodegradable and biocompatible polymers for tissue engineering application: a review. Artif Cells Nanomed Biotechnol 45(2):185–192CrossRefGoogle Scholar
  6. Awang N, Ismail AF, Jaafar J, Matsuura T, Junoh H, Othman MHD, Rahman MA (2015) Functionalization of polymeric materials as a high performance membrane for direct methanol fuel cell: a review. React Funct Polym 86:248–258CrossRefGoogle Scholar
  7. Aytimur A, Uslu I (2014) Promising materials for wound dressing: PVA/PAA/PVP electrospun nanofibers. Polym Plast Technol Eng 53(7):655–660CrossRefGoogle Scholar
  8. Azuma K, Izumi R, Osaki T, Ifuku S, Morimoto M, Saimoto H, Minami S, Okamoto Y (2015) Chitin, chitosan, and its derivatives for wound healing: old and new materials. J Funct Biomater 6(1):104–142CrossRefGoogle Scholar
  9. Baptista AC, Ferreira I, Borges JP (2013) Electrospun fibers in composite materials for medical applications. J Compos Biodegradable Polym 1(1):56–65CrossRefGoogle Scholar
  10. Deshmukh K, Ahmad J, Hagg MB (2014) Fabrication and characterization of polymer blends consisting of cationic polyallylamine and anionic polyvinyl alcohol. Ionics 20:957–967CrossRefGoogle Scholar
  11. Deshmukh K, Ahamed MB, Deshmukh RR, Bhagat PR, Pasha SKK, Bhagat A, Shirbhate R, Telare F, Lakhani C (2015a) Influence of K2CrO4 doping on the structural, optical and dielectric properties of polyvinyl alcohol/K2CrO4 composite films. Polym Plast Technol Eng 55:231–241CrossRefGoogle Scholar
  12. Deshmukh K, Ahamed MB, Pasha SKK, Deshmukh RR, Bhagat PR (2015b) Highly dispersible graphene oxide reinforced polypyrole/polyvinyl alcohol blend nanocomposites with high dielectric constant and low dielectric loss. RSC Adv 5:61933–61945CrossRefGoogle Scholar
  13. Deshmukh K, Ahamed MB, Deshmukh RR, Pasha SKK, Chidambaram K, Sadasivuni KK, Ponnamma D, AlMaadeed MAA (2016a) Eco-friendly synthesis of graphene oxide reinforced hydroxypropyl methyl cellulose/polyvinyl alcohol blend nanocomposites filled with zinc oxide nanoparticles for high-k capacitor applications. Polym Plast Technol Eng 12:1240–1253CrossRefGoogle Scholar
  14. Deshmukh K, Ahamed MB, Sadasivuni KK, Ponnamma D, Deshmukh RR, Pasha SKK, AlMaadeed MAA, Chidambaram K (2016b) Graphene oxide reinforced polyvinyl alcohol blend composites as high performance dielectric materials. J Polym Res 23:159CrossRefGoogle Scholar
  15. Deshmukh K, Ahamed MB, Deshmukh RR, Pasha SKK, Sadasivuni SKK, Ponnamma D, Chidambaram K (2016c) Synergistic effect of vanadium pentoxide and graphene oxide in polyvinyl alcohol for energy storage applications. Eur Polymer J 76:14–27CrossRefGoogle Scholar
  16. Deshmukh K, Ahamed MB, Polu AR, Sadasivuni KK, Pasha SKK, Ponnamma D, AlMaadeed MAA, Deshmukh RR, Chidambaram K (2016d) Impedance spectroscopy, ionic conductivity and dielectric studies of new Li+ ion conducting polymer blend electrolytes based on biodegradable polymers for solid state battery applications. J Mater Sci: Mater Electron 27:11410–11424Google Scholar
  17. Deshmukh K, Ahamed MB, Deshmukh RR, Pasha SKK, Bhagat PR, Chidambaram K (2017a) Biopolymer composites with high dielectric performance: interface engineering. Biopolymer composites in electronics, vol 1. Elsevier Publications, Amsterdam, pp 27–128CrossRefGoogle Scholar
  18. Deshmukh K, Ahamed MB, Deshmukh RR, Pasha SKK, Sadasivuni KK, Ponnamma D, AlMaadeed MAA (2017b) Striking multiple synergies in novel three-phase fluoropolymer nanocomposites by combining titanium dioxide and graphene oxide as hybrid fillers. J Mater Sci Mater Electron 28:559–575CrossRefGoogle Scholar
  19. Deshmukh K, Ahamed MB, Sadasivuni KK, Ponnamma D, Deshmukh RR, Trimukhe AM, Pasha SKK, Polu AR, AlMaadeed MAA, Chidambaram K (2017c) Solution processed white graphene reinforced ferroelectric polymer nanocomposites with improved thermal conductivity and dielectric properties for electronic encapsulation. J Polym Res 24:27CrossRefGoogle Scholar
  20. Deshmukh K, Ahamed MB, Sadasivuni KK, Ponnamma D, AlMaadeed MAA, Pasha SKK, Deshmukh RR, Chidambaram K (2017d) Graphene oxide reinforced poly (4-styrenesulfonic acid)/polyvinyl alcohol blend composites with enhanced dielectric properties for portable and flexible electronics. Mater Chem Phys 186:188–201CrossRefGoogle Scholar
  21. Deshmukh K, Ahamed MB, Deshmukh RR, Pasha SKK, Sadasivuni KK, Polu AR, Ponnamma D, AlMaadeed MAA, Chidambaram K (2017e) Newly developed biodegradable polymer nanocomposites of cellulose acetate and Al2O3 nanoparticles with enhanced dielectric performance for embedded passive applications. J Mater Sci: Mater Electron 28:973–986Google Scholar
  22. Deshmukh K, Sankaran S, Ahamed MB, Sadasivuni KK, Pasha SKK, Ponnamma D, Sreekanth PSR, Chidambaram K (2017f) Dielectric spectroscopy. Instrumental techniques to the characterizations of nanomaterials. Elsevier Publications, Amsterdam, pp 237–299Google Scholar
  23. Deshmukh K, Ahamed MB, Sadasivuni KK, Ponnamma D, AlMaadeed MAA, Deshmukh RR, Pasha SKK, Polu AR, Chidambaram K (2017g) Fumed SiO2 nanoparticle reinforced biopolymer blend nanocomposites with high dielectric constant and low dielectric loss for flexible organic electronics. J Appl Polym Sci 134(5):44427CrossRefGoogle Scholar
  24. Dhandayuthapani B, Yoshida Y, Maekawa T, Kumar DS (2011) Polymeric scaffolds in tissue engineering application: a review. Int J Polym Sci 2011, Article ID290602, 19 pagesGoogle Scholar
  25. Elahi F, Lu W, Guoping G, Khan F (2013) Core-shell fibers for biomedical applications—a review. J Bioeng Biomed Sci 3(1):1–14CrossRefGoogle Scholar
  26. Fang J, Wang X, Lin T (2012) Functional applications of electrospun nanofibers. In: Nanofibers-production, properties and functional applications. InTech, pp 287–326Google Scholar
  27. Focarete ML, Gualandi C (2013) Potentialities of electrospun polymeric nanofibers in the biomedical field. J Tissue Sci Eng 4(1):1–3Google Scholar
  28. Fong H, Reneker DH (1999) Elastomeric nanofibers of Styrene-butadiene-styrene triblock copolymer. J Polym Sci, Part B: Polym Phys 37(24):3488–3493CrossRefGoogle Scholar
  29. Gao Y, Bach TY, Zhu Y, Kyratzis IL (2014) Electrospun antibacterial nanofibers: production, activity and in vivo applications. J Appl Polym Sci 131(18):40797–40810CrossRefGoogle Scholar
  30. Gupta B, Agarwal R, Alam MS (2010) Textile based smart wound dressings. Indian J Fibre Text Res 35(2):174–187Google Scholar
  31. Gupta KC, Haider A, Choi Y, Kang IK (2014) Nanofibrous scaffolds in biomedical applications. Biomater Res 18(2):27–38Google Scholar
  32. Haider A, Haider S, Kang IK (2015) A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology. Arab J Chem. 11(8):1165–1188. Scholar
  33. Hasan MM, Alam AKMM, Nayem KA (2014) Application of electrospinning techniques for the production of tissue engineering scaffolds: a review. Eur Sci J 10(15):1857–7431Google Scholar
  34. Huang ZM, Zhang YZ, Kotaki M, Ramakrishna S (2003) A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos Sci Technol 63(15):2223–2253CrossRefGoogle Scholar
  35. Illa MP, Khandelwal M, Sharma CS (2018) Bacterial cellulose – derived carbon nanofibers as anode for lithium – ion batteries. Emergent Mater 1(3–4):1–6Google Scholar
  36. Kanani AG, Bahrami SH (2010) Review on electrospun nanofibers scaffold and biomedical applications. Trends Biomater Artif Organs 24(2):93–115Google Scholar
  37. Khadka DB, Haynie DT (2012) Protein—and peptide—based electrospun nanofibers in medical biomaterials. Nanomedicine 8(8):1242–1262CrossRefGoogle Scholar
  38. Khan N, Misra M, Koch T, Mohanty A (2012) Applications of electrospun nanofibers in the biomedical field. Stud Undergraduate Researchers Guelph 5(2):63–73Google Scholar
  39. Kim SE, Heo DN, Lee JB, Kim JR, Park SH, Jeon SH, Kwon IK (2009) Electrospun gelatin/polyurethane blended nanofibers for wound healing. Biomed Mater 4(4):044106CrossRefGoogle Scholar
  40. Kishan AP, Cosgriff-Hernandez EM (2017) Recent advancements in electrospinning design for tissue engineering applications: a review. J Biomed Mater Res, Part A 105(10):2892–2905CrossRefGoogle Scholar
  41. Kumar NS, Santhosh C, Sudakaran SV, Deb A, Raghavan V, Venugopal V, Bhatnagar A, Bhat S, Andrews NG (2018) Electrospun polyurethane and soy protein nanofibres for wound dressing applications. IET Nanobiotechnol 12(2):94–98CrossRefGoogle Scholar
  42. Lee HJ, Lee SJ, Uthaman S, Thomas RG, Hyun H, Jeong YY, Cho CS, Park IK (2015) Biomedical applications of magnetically functionalized organic/inorganic hybrid nanofibers. Int J Mol Sci 16(6):13661–13677CrossRefGoogle Scholar
  43. Li Z, Wang C (2013) Effects of working parameters on electrospinning. In: One-dimensional nanostructures: electrospinning technique and unique nanofibers. Springer Ltd., IX: 141–145. ISBN: 978-3-642-36426Google Scholar
  44. Liu W, Thompoulos S, Xia Y (2012) Electrospun nanofibers for regenerative medicine. Adv Healthc Mater 1(1):10–25CrossRefGoogle Scholar
  45. Ma Z, Kotaki M, Ramakrishna S (2006) Surface modified non – woven Polysulphone (PSU) fiber mesh by electrospinning: a novel affinity membrane. J Membr Sci 272(1–2):179–187CrossRefGoogle Scholar
  46. Mohanapriya MK, Deshmukh K, Ahamed MB, Chidambaram K, Pasha SKK (2015) Structural, morphological and dielectric properties of multiphase nanocomposites consisting of polycarbonate, barium titanate and carbon black nanoparticles. Int J Chem Tech Res 8:32–41Google Scholar
  47. Mohanapriya MK, Deshmukh K, Ahamed MB, Chidambaram K, Pasha SKK (2016a) Influence of cerium oxide (CeO2) nanoparticles on the structural, morphological, mechanical and dielectric properties of PVA/PPy blend nanocomposites. Mater Today: Proc 3:1864–1873Google Scholar
  48. Mohanapriya MK, Deshmukh K, Ahamed MB, Chidambaram K, Pasha SKK (2016b) Zeolite 4A filled poly (3, 4-ethylenedioxythiophene): (polystyrenesulfonate) and polyvinyl alcohol blend nanocomposites as high-k dielectric materials for embedded capacitor applications. Adv Mater Lett 7:996–1002CrossRefGoogle Scholar
  49. Mohanapriya MK, Deshmukh K, Chidambaram K, Ahamed MB, Sadasivuni KK, Ponnamma D, AlMaadeed MAA, Deshmukh RR, Pasha SKK (2017) Polyvinyl alcohol (PVA)/Polystyrene sulfonic acid (PSSA)/carbon black nanocomposites for flexible energy storage device applications. J Mater Sci: Mater Electron 28:6099–6111Google Scholar
  50. Muzaffar A, Ahamed MB, Deshmukh K, Faisal M, Pasha SKK (2018) Enhanced electromagnetic absorption in NiO and BaTiO3 based polyvinylidene fluoride nanocomposites. Mater Lett 218:217–220CrossRefGoogle Scholar
  51. Pasha SKK, Deshmukh K, Ahamed MB, Chidambaram K, Mohanapriya MK, Nambiraj NA (2015) Investigation of microstructure, morphology, mechanical and dielectric properties of PVA/PbO nanocomposites. Adv Polym Techonol 36:352–361CrossRefGoogle Scholar
  52. Pattanashetti NA, Heggannavar GB, Kariduraganavar MY (2017) Smart biopolymers and their biomedical applications. Procedia Manuf 12:263–279CrossRefGoogle Scholar
  53. Pawde SM, Deshmukh K (2008a) Characterization of polyvinyl alcohol/gelatin blend hydro gel films for biomedical applications. J Appl Polym Sci 109:3431–3437CrossRefGoogle Scholar
  54. Pawde SM, Deshmukh K (2008b) Influence of γ irradiation on the properties of polyacrylonitrile films. J Appl Polym Sci 110:2569–2578CrossRefGoogle Scholar
  55. Pawde SM, Deshmukh K (2009) Investigation of the structural, thermal, mechanical and optical properties of polymethyl methacrylate (PMMA) and polyvinylidene fluoride (PVDF) blends. J Appl Polym Sci 114:2169–2179CrossRefGoogle Scholar
  56. Pawde SM, Deshmukh K, Parab S (2008) Preparation and characterization of polyvinyl alcohol and gelatin blend films. J Appl Polym Sci 109:1328–1337CrossRefGoogle Scholar
  57. Pertici G (2017) Introduction to bioresorbable polymers for biomedical applications. In: Bioresorbable polymers for biomedical applications, pp 3–29Google Scholar
  58. Pillai CKS, Sharma CP (2009) Electrospinning of chitin and chitosan nanofibres. Trends Biomater Artif Organs 22(3):179–201Google Scholar
  59. Ponnamma D, Chamakh MM, Deshmukh K, Ahamed MB, Alper E, Sharma P, AlMaadeed MAA (2017) Ceramic based polymer nanocomposites as piezoelectric materials. Smart polymer nanocomposites, vol 1. Springer Publications AG, Switzerland, pp 77–93CrossRefGoogle Scholar
  60. Ponnamma D, Erturk A, Parangusan H, Deshmukh K, Ahamed MB, Al – Maadeed MA (2018) Strechable quaternary phasic PVDF – HFP nanocomposite films containing graphene – titania – SrTiO3 for mechanical energy harvesting. Emergent Materi 1(1–2):55–65Google Scholar
  61. Powell HM, Supp DM, Boyce ST (2008) Influence of electrospun collagen on wound contraction of engineered skin substitutes. Biomaterials 29(7):834–843CrossRefGoogle Scholar
  62. Preethi GU, Joseph MM, Unnikrishnan BS, Shiji R, Sreelekha TT (2015) Biomedical applications of natural polymer based nanofibrous scaffolds. Int J Med Nano Res 2(2):1–9Google Scholar
  63. Pulapura S, Kohn J (1992) Trends in the development of bioresorbable polymers for medical applications. J Biomater Appl 6(3):216–250CrossRefGoogle Scholar
  64. Qu H, Wei S, Guo Z (2013) Coaxial electrospun nanostructures and their applications. J Mater Chem A 1(38):11513–11528CrossRefGoogle Scholar
  65. Rath G, Hussain T, Chauhan G, Garg T, Goyal AK (2016) Collagen nanofiber containing silver nanoparticles for improved wound-healing applications. J Drug Target 24(6):520–529CrossRefGoogle Scholar
  66. Rosic R, Pelipenko J, Kristl J, Kocbek P, Baumgartner S (2012) Properties, engineering and applications of polymeric nanofibers: current research and future advances. Chem Biochem Eng Q 26(4):417–425Google Scholar
  67. Rosic R, Kocbek P, Pelipenko J, Krist J, Baumgartner S (2013) Nanofibers and their biomedical use. Acta Pharmaceutica 63:295–304CrossRefGoogle Scholar
  68. Sadeghi-Avalshahr A, Nokhasteh S, Molavi AM, Khorsand-Ghayeni M, Mahdavi-Shahri M (2017) Synthesis and characterization of collagen/PLGA biodegradable skin scaffold fibers. Regenerative Biomater 4(5):309–314CrossRefGoogle Scholar
  69. Sathapathy KD, Deshmukh K, Ahamed MB, Sadasivuni KK, Ponnamma D, Pasha SKK, AlMaadeed MAA, Ahmad J (2017) High-quality factor poly (vinylidene fluoride) based novel nanocomposites filled with graphene nanoplatelets and vanadium pentoxide for high-Q capacitor applications. Adv Mater Lett 8:288–294CrossRefGoogle Scholar
  70. Sharma J, Lizu M, Stewart M, Zygula K, Lu Y, Chauhan R, Yan X, Guo Z, Wujcik EK, Wei S (2015) Multifunctional nanofibers towards active biomedical therapeutics. Polymers 7(2):186–219CrossRefGoogle Scholar
  71. Shin SH, Purevdorj O, Castano O, Planel JA, Kim HW (2012) A short review: recent advances in electrospinning for bone tissue regeneration. J Tissue Eng 3(1):1–11CrossRefGoogle Scholar
  72. Sill TJ, Recum HAV (2008) Electrospinning: applications in drug delivery and tissue engineering. Biomaterials 29(13):1989–2006CrossRefGoogle Scholar
  73. Sridhar R, Sundarrajan S, Venugopal JR, Ravichandran R, Ramakrishna S (2013) Electrospun inorganic and polymer composite nanofibers for biomedical applications. J Biomater Sci Polym Ed 24(4):365–385CrossRefGoogle Scholar
  74. Stevens MM, George JH (2005) Exploring and engineering the cell surface interface. Science 310(5751):1135–1138CrossRefGoogle Scholar
  75. Sundaramurthi D, Krishnan UM, Sethuraman S (2014) Electrospun nanofibers as scaffolds for skin tissue engineering. Polym Rev 54(2):348–376CrossRefGoogle Scholar
  76. Thangamani GJ, Deshmukh K, Chidambaram K, Ahamed MB, Sadasivuni KK, Ponnamma D, Faisal M, Nambiraj NA, Pasha SKK (2018) Influence of CuO nanoparticles and graphene nanoplatelets on the sensing behavior of poly (vinylalcohol) nanocomposites for the detection of ethanol and propanol vapors. J Mater Sci: Mater Electron 29:5186–5205Google Scholar
  77. Turon P, del Valle LJ, Aleman C, Puiggali J (2017) Biodegradable and biocompatible systems based on hydroxyapatite nanoparticles. Appl Sci 7(1):60CrossRefGoogle Scholar
  78. Uttayarat P, Jetawattana S, Suwanmala P, Eamsiri J, Tangthong T, Pongpat S (2012) Antimicrobial electrospun silk fibroin mats with silver nanoparticles for wound dressing application. Fibers Polym 13(8):999–1006CrossRefGoogle Scholar
  79. Venugopal J, Ramakrishna S (2005) Applications of polymer nanofibers in biomedicine and biotechnology. Appl Biochem Biotechnol 125(3):147–157CrossRefGoogle Scholar
  80. Wang X, Ding X, Li B (2013) Biomimetic electrospun nanofibrous structures for tissue engineering. Mater Today 16(6):229–241CrossRefGoogle Scholar
  81. Winter GD (1962) Formation of the scab and the rate of epithelization of superficial wounds in the skin of the young domestic pig. Nature 193:293–294CrossRefGoogle Scholar
  82. Winter GD (1965) A note on wound healing under dressings with reference to perforated—film dressings. J Invest Dermatol 45(4):299–302CrossRefGoogle Scholar
  83. Yu DG, Zhu LM, White K, White CB (2009) Electrospun nanofiber-based drug delivery systems. Health 1(2):67–75CrossRefGoogle Scholar
  84. Yuan TT, Jenkins PM, Foushee AMD, Jockheck-Clark AR, Stahl JM (2016) Electrospun chitosan/polyethylene oxide nanofibrous scaffolds with potential antibacterial wound dressing applications. J Nanomat 2016, Article ID 6231040, 10 PagesGoogle Scholar
  85. Zafar M, Najeeb S, Khurshid Z, Vazirzadeh M, Zohaib S, Najeeb B, Sefat F (2016) Potential of electrospun nanofibers for biomedical and dental applications. Materials 9(73):1–21Google Scholar
  86. Zeng J, Xu X, Chen X, Liang Q, Bian X, Yang L, Jing X (2003) Biodegradable electrospun fibers for drug delivery. J Controlled Release 92(3):227–231CrossRefGoogle Scholar
  87. Zhang Y, Lim CT, Ramakrishna S, Huang ZM (2005) Recent development of polymer nanofibers for biomedical and biotechnological applications. J Mater Sci—Mater Med 16(10):933–946CrossRefGoogle Scholar
  88. Zhang W, Ronca S, Mele E (2017) Electrospun nanofibres containing antimicrobial plant extracts. Nanomaterials 7(2):42CrossRefGoogle Scholar
  89. Zong X, Kim K, Fang D, Ran S, Hsiao BS, Chu B (2002) Structure and process relationship of electrospun bioabsorbable nanofiber membranes. Polymer 43(16):4403–4412CrossRefGoogle Scholar

Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Sowmya Sankaran
    • 1
  • Kalim Deshmukh
    • 1
  • M. Basheer Ahamed
    • 1
    Email author
  • S. K. Khadheer Pasha
    • 2
  1. 1.Department of PhysicsB. S. Abdur Rahman Crescent Institute of Science and TechnologyChennaiIndia
  2. 2.Department of PhysicsVIT-AP UniversityGunturIndia

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