nano-fibres for filter materials

7

Nano-Fibres for Filter Materials

K. Schaefer, H. Thomas, P. Dalton, and M. Moeller

Summary. Textile materials are used for a variety of dry and wet filtrationprocesses allowing either the increase of the purity of the material filtered or therecovery of solid particles. Typical examples for textile-based filtration processesare air filtration, process filtration (e.g. solid–liquid separation), industrial effluenttreatment or dehydration of sewage sludges.

Current conventional textile filters consist of natural or human-made fibres withdiameters ranging from a few single to a few ten microns. Small fibres are wellknown to provide better filter efficiency which is related to the increase in surface-area-to-weight ratio. For this reason, nano-fibre filter media enable new levels offiltration performance for several applications in different environments ranging fromindustrial and consumer to defence filtration processes.

Nano-fibres with diameters between 100 nm and 3 µm are readily accessible bythe electrospinning process. Electrospinning uses a high electrical field to draw apolymer solution (or melt) from the tip of a capillary to a collector. By applyingvoltages of approximately 10–50 kV, fine jets of the solution (or melt) can be drawnto a grounded or oppositely charged collector. The evaporating solvent (or coolingof the melt) results in fibres that are collected and formed into nano-fibre mats withadjustable fibre diameters mainly based upon solution viscosity and electrical fieldstrength.

A broad range of polymers ranging from natural and synthetic organic to inor-ganic polymers can be electrospun from the solution or melt allowing the genera-tion of tailored nano-fibre webs for various applications. Furthermore, the nano-fibrewebs may be used as carrier material for subsequent fixation of various substances tofibre surfaces as well as for their direct implementation into the fibre. This increasesthe possibilities for production of, e.g. hygienic functionalised filters or of temper-ature stable filters with catalytic activity. Hygienic filters produced from cationicpolymers or with incorporated silver can reduce the contamination of air or waterfilters with bacteria while temperature stable filters, which can be obtained fromSiO2-precursor or silica hybrid materials and which are loaded with metal/metaloxide nano-particles, are destined for air pollution control.

126 K. Schaefer et al.

7.1 Introduction

Raw materials for non-wovens are generally natural or human-made fibreswith diameters ranging from about 3 to 50 µm. New levels of performance canbe enabled by nano-scaled fibres in all fields of application demanding a highsurface-area-to-weight ratio, e.g. filtration and catalysis.

Nano-fibres with diameters between 100 nm and 3 µm can be made by theelectrospinning process. The technique of electrospinning has been known fromthe work of Formhals [1] since 1934 but received relatively little attention untilrecently. In 1971, Baumgarten [2] performed studies on the electrospinning ofacrylic micro-fibres; he obtained fibres with diameters of 500–1,000 nm. Withincreasing interest in nano-technology and motivated by the reviving workof Reneker’s research group electrospinning has gained exponential researchinterest in the last few years (Fig. 7.1) [3–6]. Since 1990s, the research groupsof Reneker, Vancso, Greiner and Wendorff investigate the electrospinning indetail [3–14]. During the last decade, extensive investigations on the electro-spinning process have been conducted from different viewpoints like aspects oftheoretical simulation [15,16], fibre formation mechanism, influencing factorsfor fibre size and morphology [17] and applications [18,19].

A wide variety of polymers (natural, synthetic, organic and inorganic poly-mers) have been electrospun from the solution and melt phase allowing thegeneration of tailored nano-fibre webs for various areas of application, e.g. fil-tration [18], reinforcement in composite materials [7], protective clothing [21]or biomedical uses [22–24].

Fig. 7.1. Increase in papers on the electrospinning in the last decade [20]

7 Nano-Fibres for Filter Materials 127

7.2 Principle of Electrospinning

Electrospinning (or electrostatic spinning) uses a high-voltage electrical field(10–50 kV) to draw a polymer solution or melt from the tip of a capillaryto a collector (Fig. 7.2). When the electric forces at the surface of a polymersolution or melt overcome the surface tension, an electrically charged fine jetis formed which can be drawn to a grounded or oppositely charged collector.The evaporating solvent (or cooling of the melt) creates fibres that are col-lected and formed into nano-fibre mats. Electrospun fibres are continuous inlength, their diameter ranges from under 3 nm to over 50 µm depending on theelectrospinning conditions. The smallest possible polymer fibre must containone polymer molecule [5].

The fibre diameter of fibres formed during electrospinning is influenced by:

System parameters– Polymer properties

Molecular weight, structure and poly-dispersity of the polymer,concentration, melting point and glass transition point

– Solution propertiesSolvent, volatility, viscosity, conductivity, surface tension, pres-ence of further additives (e.g. salts)

Process parameters– Ambient parameters

Solution temperature, humidity, atmosphere, air velocity in theelectrospinning chamber

– Equipment parameterVoltage, field strength, electrode distance and arrangement,flow rate, delivery volume, needle diameter

HV0–50 kV

Pump

Pressure gauge

GroundElectrode

substrate

Solution

Taylor cone

Fig. 7.2. Setup for electrospinning from polymer solutions


The formation of fibres in the electrospinning process is mainly influenced bythe following forces:

– Surface tension– Electrical-repellent force derived from electrical charged polymer droplets– Visco-elastic force coming from the polymer

Higher polymer concentrations typically result in larger fibre diameters, anincrease of the electrical field strength leads to a decrease of the fibre diame-ter. The fibre diameter shall be consistent and controllable; the fibre surfaceshall be defect-free or defect-controllable. However, in practical electrospin-ning experiments often inhomogeneous fibres with defects and beads canoccur. Splitting of the jet can occur, which results in finer fibres.

Advantages of nano-fibres :– Fibre diameter: <3 nm to >50 µm– High surface area to volume ratio (→ high specific surface)– High aspect (length to diameter) ratio– High bending performance– Flexible surface functionalities– Ability to control pore size in non-woven fabrics– Possibility to insert special functionality

A further advantage of electrospinning compared to conventional solvent spin-ning is that water can be used as solvent. Water-soluble fibres have to becross-linked, e.g. by thermal or by chemical cross-linking [13,20,25].

These advantages result in great application potentials of nano-fibresin broad fields such as separation, adsorption, filtration, catalysis, fibre-reinforced composites, tissue engineering, wound dressings, drug delivery sys-tems, sensors, cleaning tissues, protective textile and other [18–29].

Nano-fibres can be spun from polymer solutions or from polymer melts.Larrondo and Manley [30–32] were the first to carry out and report on meltelectrospinning experiments. Working with PE and PP in the early 1980s,they successfully formed fibres with diameters only as small as the tens ofmicrons range. Electrospinning from polymer melts has the advantage thatno solvents are needed which have to be removed by evaporation. However, themelting temperature of the polymers is an important influencing factor for theapplicability of the procedure to produce nano-fibres. In general, nano-fibreswhich are produced by melt electrospinning have a higher fineness than thoseelectrospun from solutions, achieving nano-dimensions by melt electrospinningis non-trivial (Scheme 7.1).

At DWI, a working group is using melt electrospinning for the productionof nano-fibres or nano-webs for biomedical applications like scaffolds for tissueengineering, in vitro neuron interactions with oriented electrospun fibres orothers [33–36].


Electrospinning

from

Polymer solutions Polymer meltsRestrictive parameters Restrictive parameters • Solubility of polymers Melting point of polymers

• Suitable solvents with Viscosity of melts

regard to viscosity, etc.

volatility, toxicity etc.

⇒ Requirements to ⇒ Requirements to the

polymers and solvents equipment to reach the

right temperature

Results in finer (nanofibres) Results in coarser

and more homogeneous (approx. 1 µm) and less

nanofibres. homogeneous nanofibres.

Scheme 7.1. Comparison of electrospinning from solutions or from polymer melts

7.2.1 Practical Electrospinning

Typical electrospinning equipment consists of three components: a high-voltage source, a spinneret (or nozzle) and a collector (Fig. 7.2).

The polymer solution or melt is applied into a syringe (or a spinneret)which is equipped with a piston and a stainless steel capillary serving as elec-trode and pushed through by a pump with a defined flow rate. The spinneretis connected with the high-voltage source and applies high voltage to thepolymer. This results in the formation of a polymer drop at the end of thespinneret. Under higher voltage the drop changes its shape and turns into aconic form (Taylor cone) (Figs. 7.2 and 7.3) [30–32,37, 38]. At a defined volt-age, the surface tension of the polymer cone at the tip of the spinneret startsto elongate and stretch so that a charged jet is formed. The jet moves in loopsbending and whipping towards the electrode with opposite polarity or to thegrounded target (Fig. 7.3). Recent experiments demonstrate that the rapidlywhipping fluid jet is an essential mechanism of electrospinning [39,40].

Different collection systems are known [35]. For the usually produced non-woven mats metal plates are used as counter electrode and collection sys-tem of the nano-fibres or nano-webs. However, for special applications furthergrounded collectors were developed (Fig. 7.4) [36].


Fig. 7.3. Schematic presentation of the electrospinning process [41]

Fig. 7.4. Different electrospinning collection systems: (a) single plate configuration,(b) rotating drum, (c) triangular frame placed near single plate, (d) parallel dualplate and (e) dual-grounded ring configuration [36]

7.2.2 Nano-Fibres Produced by Electrospinningform Polymer Solutions or Melts

In Figs. 7.5–7.10, nano-non-wovens or nano-fibres which were spun from poly-mer solutions (here: poly(vinyl alcohol) (PVA) in water or polycaprolactone(PCL) in chloroform/ethanol – 3/1, v/v) or from polymer melts (here: ablend of poly(ethylene oxide-block-ε-caprolactone) (PEO-PCL) and PCL)are shown. The melt electrospinning was performed at a temperature of85◦C applying the rotating drum collection system (Fig. 7.4b). High volt-ages of 30 kV were applied during the electrospinning of PCL and 17 kV forthe spinning of PVA solutions. Nano-fibres with average finenesses of about300–600 nm were produced by electrospinning of PVA or PCL solutions, somevery fine fibres with fibre diameters of approximately 100–300 nm were foundin electrospun PCL nano-fibres. The nano-fibres which were obtained aftermelt electrospinning had fibre diameters of about 1 µm.


Fig. 7.5. Nano-non-woven obtained by melt electrospinning of a blend PEO–PCLand PCL

20 µm 10 µm

Fig. 7.6. Nano-fibres produced by electrospinning from aqueous PVA solutions(average fibres in the range of 300–600 nm fineness)

7.2.3 Electrospraying

Electro-driven jets of polymeric fluids undergo instabilities causing eitherbreaking of the jet into droplets (electrospraying) [42–44] or splitting intofiner jets resulting in the production of superfine fibres (electrospinning). Bothprocesses are mechanistically similar with the exception that in electrospin-ning high molecular weight polymers and chain entanglement in more concen-trated polymer solutions stabilise the initial jet towards spraying (Figs. 7.10and 7.11). Electrospraying can be used for the production of multi-functional


100 µm 10 µm

Fig. 7.7. Nano-fibres produced by electrospinning from PCL solutions (PCL chlo-roform/ethanol solution) (average fibres in the range of 300–600 nm fineness and thefine fibres 100–300 nm)

Fig. 7.8. Nano-fibres produced by electrospinning from melts at 85◦C of a blendof PEO–PCL and PCL. Collection times are 1 min (left figure) and 6 h (right fig-ure). The average fibres are approximately 1 µm, however with long collection times,larger fibres are observed. Such impurities are commonly observed for both melt andsolution electrospinning

Fig. 7.9. Nano-fibres electrospun from melts of a blend of PEO–PCL and PCL ontoa conventional PET non-woven


Mainly spraying Spraying/spinning Spinning

2 % PVA 4 % PVA 6 % PVA

10 µm 10 µm 10 µm

Fig. 7.10. Electrospraying or electrospinning in dependence on the PVA concen-tration in solution

10 kV 20 kV

30 kV

200 µm

200 µm

200 µm

Fig. 7.11. Influence of voltage on the particle size obtained during electrosprayingof a non-polymeric organic compound

materials, too. The formation of droplets in the electrospraying process iscaused by breaking up of the jet due to Rayleigh instability [45].

Functional fibre coatings can be obtained by electrospraying, whereas,nano-fibre webs for implementation into non-wovens are produced by elec-trospinning.


7.3 Application of Nano-Fibres or Nano-Websas Filter Media

The large surface area of nano-fibre webs allows rapid adsorption of dust andother particles from air such as micro-organisms or pollen as well as hazardousmolecules. The latter necessitates reactive sites in the polymer or catalyticallyactive additives allowing chemical binding or decomposition of hazardous sub-stances, respectively. Besides fineness and resulting large specific surface areaof nano-fibre webs, their high porosity and small pore size contribute furtherto their high adsorption and filtration efficiency. Pore size and porosity offilter media are determined by the diameter of fibres used for production offilter media. For filter media very thin webs consisting of just a few nano-fibrediameters thickness are effective. The thickness of the nano-web can be lessthan 1–5 µm [46]. While the thinness of the nano-web provides high perme-ability to flow, the nano-web has limited mechanical properties that precludethe use of conventional web handling and filter pleating equipment. The smallfibre diameter of nano-fibres and the thin nano-web layer result in high filterefficiency with minimal pressure drop increases. Furthermore, nano-fibre fil-ter media have demonstrated longer filter lifetimes than conventional filteringmaterials.

The technical requirements for filters are a balancing of the three majorparameters of filter performance: filter efficiency, pressure drop and filter life-time. An improvement in one category generally means a corresponding sac-rifice in another category. It was shown that the proper use of nano-fibres canprovide marked improvements in both filtration efficiency and lifetime, whilehaving a minimal impact on pressure drop [46].

Nano-fibre webs can be applied onto various substrates, e.g. onto conven-tional non-wovens, too. These substrates can be selected to provide appro-priate mechanical properties to allow pleating, filter fabrication, durability inuse, and in some cases, filter cleaning [46,47].

In the beginning of the 1980s, Freudenberg and Weinheim started to applythe electrostatic spinning for the development of non-wovens by arrangementof electrospun fibres between a support layer and a preliminary filter in asandwich-like structure [48–50].

Donaldson Company, Inc. has been using electrospinning technology tomake fine fibres for more than two decades [18,19]. Donaldson produces Ultra-WebTM nano-fibres with sub-half-micron diameters for air filtration in com-mercial, industrial and defence applications [46]. Nano-fibre filter media makenew levels of filtration performance possible in several transportation applica-tions including internal combustion engines, fuel cells and cabin air filtration.According to Luzhansky, Donaldson produces about three pounds of nylon orover 10,000 m2 nano-fibres per day [20,51].

Greiner and Wendorff developed together with Hollingsworth & VoseGmbH/JC Binzer Mill, Hatzfeld/Germany, the so-called NanowebTM, i.e. asuper-filter which is produced by electrospinning of nano-fibres onto a base


material [20]. NanowebTM can be used for air filtration, e.g. for filtering pollenor other particles from the air [52]. The big advantage of NanowebTM is, besideof the optimisation of the filter capacity, the absolutely negligible materialsusage.

In Liberec, the company Elmarco developed in co-operation with theTechnical University of Liberec modified electrospinning technology called“Nano-spider” which is based on electrospinning from non-water-based poly-mer solutions [53–55]. Elmarco presented a pilot line at INDEX 05 inGeneva/Switzerland, to the non-woven industry [55]. The nano-fibre materi-als of Elmarco are developed for wide use in medical, biological and technicalfields.

Apart from using synthetic polymers bearing special functionalities or spe-cific add-ons to the spinning solution, chemical and biological functionalitycan also be achieved from natural polymers accessible from waste materials.For example the chitin-derivative chitosan is known to provide antimicrobialeffectiveness [56] or keratin fibres are known for their propensity in binding airpolluting substances by nucleophilic addition, e.g. formaldehyde [57]. This wasthe basis for us to investigate natural polymers like chitosan and wool keratinsduring electrospinning [25]. Keratins isolated as S -sulpho-keratins cannot onlybe electrospun but also allow the reformation of cystine bridges and thus thefibre stabilisation after reductive removal of the protection group. Chitosan-bearing nano-fibres or nano-fibres post-coated with chitosan can reduce micro-bial growth and are potentially interesting for air filtration uses [25]. Fibreformation with lower molecular weight proteins as well as chitosan needs theaddition of interfering polymers (e.g. PEO) to disturb the rigid association ofchitosan molecules caused by hydrogen bonding. Co-spinning of bio-polymersand water-soluble polymers requires the use of cross-linkers for fibre stabili-sation [25].

7.4 New Developments in Electrospinning

Actual R&D work on electrospinning is focusing on precise control over fibresize and morphology by changing the process parameters, modelling of theelectrospinning process, the development of new structures and functionalitiesof nano-fibres and the development of practical applications of electrospunfibres.

The working group of Greiner and Wendorff developed a co-electrospinningprocedure enabling the production of core–shell nano-fibres with specialtyproperties [13,14,20,58].

Other specialty nano-fibres produced by electrospinning are nanotubes orfibres with very porous surface structure [14].

Another possibility is the incorporation of nano-particles/micro-spheresinto nano-fibres to achieve special functionality (Fig. 7.12) [14,59].


Fig. 7.12. Nano-fibre with incorporated micro-spheres [59]

Big interest in electrospinning of nano-fibres exists in the area of bio-medical applications [33–36]. Yet 1980, ICI patented a “product comprisingelectrostatically spun fibres” produced from polyurethane melts which wereintended to be used as vascular prosthesis [60]. Recently, portable electrospin-ning equipment was developed which can be applied for wound healing [61].

References

1. A. Formhals, US Patent 1-975-504 (1934)2. P.K. Baumgarten, J. Colloid Interf. Sci. 36, 71 (1971)3. J. Doshi, D.H. Reneker, J. Electrostat. 35, 151 (1995)4. J.N. Doshi, G. Shrinivasan, D.H. Reneker, Polym. News 20, 206 (1995)5. D.H. Reneker, I. Chun, Nanotechnology 7, 216 (1996)6. X. Fang, D.H. Reneker, J. Macromol. Sci.: Phys. B 36, 169 (1997)7. M.M. Bergshoef, G.J. Vancso, Adv. Mater. 11, 1362 (1999)8. H. Hou, J. Zeng, A. Reuning, A. Schaper, J.H. Wendorff, A. Greiner, Macro-

molecules 35, 2429 (2002)9. M. Bognitzki, H. Hou, M. Ishaque, T. Frese, M. Hellwig, C. Schwarte,

A. Schaper, J.H. Wendorff, A. Greiner, Adv. Mater. 12, 637 (2000)10. M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig, M. Steinhart,

A. Greiner, J.H. Wendorff, Adv. Mater. 13, 70 (2001)11. M. Bognitzki, T. Frese, J.H. Wendorff, A. Greiner, Polym. Prepr. (ACS, PMSE)

82, 45 (2000)12. M. Bognitzki, T. Frese, M. Steinhart, A. Greiner, J.H. Wendorff, A. Schaper,

M. Hellwig, Polym. Eng. Sci. 41, 982 (2001)13. R. Dersch, A. Greiner, M. Steinhart, J.H. Wendorff, DWI Rep. 127 (2003)14. R. Dersch, M. Steinhart, U. Boudriot, A. Greiner, J.H. Wendorff, Polym. Adv.

Technol. 16, 276 (2005)


15. C.C. Rutledge, M.Y. Shin, S.B. Warner, A. Buer, M. Grimler, S.C. Ugbolue,National Textile Center Annual Report: November 2000, M98-D01 (2000)

16. D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, J. Appl. Phys. 87, 4531(2000)

17. J.M. Deitzel, J. Kleinmeyer, D. Harris, N.C.B. Tan, Polymer 42, 261 (2001)18. T. Grafe, K. Graham, Proceedings of the International Nonwovens Technical

Conference (Joint INDA-TAPPI Conference), 24–26 September 2002, Atlanta,2002

19. T.H. Grafe, K.M. Graham, Nonwovens in Filtration, 5th International Confer-ence, Stuttgart, Germany, March 2003

20. A. Greiner, Lecture on “Electrospinning for Biomedical Applications” at DWIAachen, Germany, 2005

21. H. Schreuder-Gibson, P. Gibson, K. Senecal, M. Senett, J. Walker, W. Yeomans,D. Ziegler, P.P. Tsai, Adv. Mater. 34, 44 (2002)

22. L. Huang, R.A. McMillan, R.P. Apkarian, B. Pourdeyhimi, V. Conticello,E.L. Chaikof, Macromolecules 33, 2989 (2000)

23. G. Verreck, I. Chun, J. Rosenblatt, J. Peeters, A. Van Dijck, J. Mensch,M. Noppe, M.E. Brewster, J. Control. Release 92, 349 (2003)

24. A. Frenot, I.S. Chronakis, Colloid Interf. Sci. 8, 64 (2003)25. H. Thomas, E. Heine, R. Wollseifen, C. Cimpeanu, M. Moller, Int. Nonwovens

J. 14(3), 12 (2005)26. P. Gibson, H. Schreuder-Gibson, D. Rivin, Colloids Surf. A: Physicochem. Eng.

Asp. 187–188, 469 (2001)27. M. Jacobsen, Nonwovens Industry (1991), pp. 36–41; cf. also: Chemiefasern/

Textilindustrie 39/91, 868 (1989)28. E.R. Kenawy, G.L. Bowlin, K. Mansfield, J. Layman, D.G. Simpson,

E.H. Sanders, G.E. Wnek, J. Control. Release 81, 57 (2002)29. J. Zeng, Meso- and Nano-Scaled Polymer Fibers and Tubes Fabrication, Func-

tionalization, and Characterization. Ph.D. Thesis, University Marburg (2003)30. L. Larrondo, R. St. John Manley, J. Polym. Sci.: Polym. Phys. Ed. 19, 909

(1981)31. L. Larrondo, R. St. John Manley, J. Polym. Sci.: Polym. Phys. Ed. 19, 921

(1981)32. L. Larrondo, R. St. John Manley, J. Polym. Sci.: Polym. Phys. Ed. 19, 933

(1981)33. D. Grafahrend, Darstellung von Polyethylenglycol-block-Polyester als Basis fur

elektrogesponnene Nanofasern/Synthesis of polyethyleneglycol-block-polyestersfor electrospun nanofibres, Diploma Thesis RWTH Aachen (2004)

34. K. Feil, Melt Electrospinning of Scaffolds for Tissue Engineering, Diploma The-sis, RWTH Aachen (2005)

35. P. Dalton, T. Kuenzel, D. Klee, M. Moeller, J. Mey, Eur. Cell Mater. 7(1), 52(2004)

36. P.D. Dalton, D. Klee, M. Moller, Polymer 46, 611 (2005)37. G.I. Taylor, Proc. R. Soc. Lond. Ser. A 280, 383 (1964)38. G.I. Taylor, Proc. R. Soc. Lond. Ser. A 313, 453 (1969)39. M. Shin, M.M. Hohman, M.P. Brenner, G.C. Rutledge, Appl. Phys. Lett. 78,

1149 (2001)40. M.M. Hohman, M. Shin, G. Rutledge, M.P. Brennera, Phys. Fluids 13(8), 2201

(2001)


41. G. Muller, Herstellung von wasserbestandigen organischen und anorganis-chen Nanofaservliesen mittels Elektrospinning/Production of Water-ResistantOrganic and Inorganic Nanofibre Nonwovens by Means of Electrospinning,Diploma Thesis, RWTH Aachen (2004)

42. M. Cloupeau, B. Prunet-Foch, J. Electrost. 22, 135 (1989)43. M. Cloupeau, B. Prunet-Foch, J. Aerosol Sci. 25, 1021 (1994)44. A. Jaworek, A. Krupa, J. Aerosol Sci. 30, 873 (1999)45. L. Rayleigh, Phil. Mag. 14, 184 (1882)46. T. Grafe, M. Gogins, M. Barris, J. Schaefer, R. Canepa, Filtration 2001 Inter-

national Conference and Exposition of the INDA (Association of the NowovensFabric Industry), Chicago, IL, 3–5 December 2001

47. K. Graham, M. Ouyang, T. Raether, T. Grafe, B. McDonald, P. Knauf, Proceed-ings of 15th Annual Technical Conference & Expo of the American Filtration &Separations Society, Galveston, TX, 9–12 April 2002

48. A. Weghmann, Nonwovens Ind. 24 (1982)49. A. Weghmann, Schriftenreihe des Deutschen Wollforschungsinstitutes 85,

314–331 (1981)50. K. Schmidt, Melliand Textilber. 61(6), 495–497 (1980)51. D. Luzhansky, Quality Control in Manufacturing of Electrospun Nanofiber Com-

posites, INDA/TAPPI, Baltimore, MD, 16–18 September 200352. http://www.hovo.com

53. http://www.elmarco.cz and http://www.nanospider.cz/aplikace.php?

kategorie=1&h...

54. J. Oldrich, F. Sanetrnik, D. Lukas, V. Kotek, L. Martinova, J. Chaloupek,Process and Apparatus for Producing Nanofibres from Polymer Solution by Elec-trostatic Spinning ; WO 2005024101 A1 (2004)

55. Exhibition at INDEX 05, Geneva, Switzerland, 12–15 April 200556. S. Hirano, Biotechnol. Annu. Rev. 2, 237 (1996)57. G. Wortmann, R. Sweredjuk, G. Zwiener, F. Doppelmayer, F.J. Wortmann,

DWI Rep. 124, 378 (2001)58. Z. Sun, E. Zussman, A. Yarin, J.H. Wenndorff, A. Greiner, Adv. Mater. 22,

1929 (2003)59. R.M. Pereira Paz, Studien zur kontrollierten Freisetzung biologisch aktiver Sub-

stanzen aus resorbierbaren Nano- und Mikrospharen/Investigations on the Con-trolled Release of Biologically Active Compounds from Resorbable Nano- andMicrospheres, Ph.D. Thesis, RWTH Aachen (2004)

60. A. Bornat, R.M. Clarke, Setting a Product Comprising Electrostatically SpunFibres, EP 11437 19800528 (1980)

61. D.J. Smith, D.H. Reneker, A.T. McManus, H.L. Schreuder-Gibson, Ch. Mello,M.S. Sennett, Electrospun Fibers and an Apparatus Therefore, US Patent6.753.454 B1 (2004)

nano-fibres for filter materials

Documents