Key Points
• Electrospinning (espinning) appears to be
moving into the mainstream through its ability to create sub- 1 gsm fibrous
layers with high functionality due to the very high surface areas
achieved.
• Centrifugal spinning of 0.1 to 0.7 micron fibres from commercially available paint sprayers is possible. Fibre sizes are more variable than from espinning but the process is simple and relatively easy to scale up.
• Needleless espinning from rollers rotating in polymer solutions is allowing a wide range of nanofibre types to be produced at far higher productivity than the original needle process. Elmarco's pioneering work in this field is attracting much interest.
• Elmarco is now collaborating with Oerliken-Neumag on a production line for sound absorbtion materials, and with Alltracel on nanopeutics and tissue engineering. Swiss private equity has now taken a 26% stake.
• Elmarco are continuing contract research, selling the technology, selling laboratory test machines, doing commission coating with nanofibres and developing antimicrobial products. They continue to seek partners in specified market and technology areas.
• Chitosan, alginate, and collagen can now be converted into nanofibres using the roller process, and many hygiene and biomedical applications become possible.
• A PP nanofibre layer can make a surface super-hydrophobic, and a PVA nanofibre layer can meke it superhydrophilic.
• Cross-linked water soluble polymer nanofibres (Nano-superabsorbents?) can be made.
• Centrifugal spinning of 0.1 to 0.7 micron fibres from commercially available paint sprayers is possible. Fibre sizes are more variable than from espinning but the process is simple and relatively easy to scale up.
• Needleless espinning from rollers rotating in polymer solutions is allowing a wide range of nanofibre types to be produced at far higher productivity than the original needle process. Elmarco's pioneering work in this field is attracting much interest.
• Elmarco is now collaborating with Oerliken-Neumag on a production line for sound absorbtion materials, and with Alltracel on nanopeutics and tissue engineering. Swiss private equity has now taken a 26% stake.
• Elmarco are continuing contract research, selling the technology, selling laboratory test machines, doing commission coating with nanofibres and developing antimicrobial products. They continue to seek partners in specified market and technology areas.
• Chitosan, alginate, and collagen can now be converted into nanofibres using the roller process, and many hygiene and biomedical applications become possible.
• A PP nanofibre layer can make a surface super-hydrophobic, and a PVA nanofibre layer can meke it superhydrophilic.
• Cross-linked water soluble polymer nanofibres (Nano-superabsorbents?) can be made.
Introduction
This predominantly academic meeting, to Elmarco's surprise, attracted 140 visitors from all over the world. The papers were generally good, but the printed papers from the Universities often bore little resemblance to and contained less information than the slides presented.Techniques, Functions and Applications
Prof Joachim Wendorff of the Phillips University in Marburg ( Germany ) illustrated the wide range of espin research underway with a collection of slides not available in the printed text, and some from unpublished papers:
• 5-7 nm PLA filaments
• Barbed PLA filaments
• Porous PLA and polycarbonate “honeycomb” nanofilaments
• Beaded polyethylene oxide filaments
• Composites of PLA nanofibres and Polyamide ribbon filaments
• Bicomponents with enzymes, proteins and viruses in the core.
• Hollow carbon fibres obtained from polyacrylonitrile nanofibres.
• Superparamagnetic hybrid nanofibres based on bico, PLA sheath with iron oxide-loaded PP in the core.
• Copper nanowires from copper dinitrate in polyvinyl butyrate (converted to copper oxide by pyrolysis and copper by reduction in a hydrogen atmosphere at 300 o C)
• Green fluorescent protein which maintains its fluorescence (indicating its structure is intact) in the core of a bico nanofibre.
• Micrococcus luteus bacteria which maintains its viability in the core of a polyethylene oxide nanofibre. (E coli fails to reproduce after spinning).
• A 20/80 PLA/PEO nanofibre (400nm diameter) had had the PEO dissolved out to make a sponge fibre with high absorbency and a density of 0.2 g/cc.
• Barbed PLA filaments
• Porous PLA and polycarbonate “honeycomb” nanofilaments
• Beaded polyethylene oxide filaments
• Composites of PLA nanofibres and Polyamide ribbon filaments
• Bicomponents with enzymes, proteins and viruses in the core.
• Hollow carbon fibres obtained from polyacrylonitrile nanofibres.
• Superparamagnetic hybrid nanofibres based on bico, PLA sheath with iron oxide-loaded PP in the core.
• Copper nanowires from copper dinitrate in polyvinyl butyrate (converted to copper oxide by pyrolysis and copper by reduction in a hydrogen atmosphere at 300 o C)
• Green fluorescent protein which maintains its fluorescence (indicating its structure is intact) in the core of a bico nanofibre.
• Micrococcus luteus bacteria which maintains its viability in the core of a polyethylene oxide nanofibre. (E coli fails to reproduce after spinning).
• A 20/80 PLA/PEO nanofibre (400nm diameter) had had the PEO dissolved out to make a sponge fibre with high absorbency and a density of 0.2 g/cc.
Applications in homogeneous catalysis, tissue
engineering, inhalation therapies, agriculture and oceanography are being
developed.
• Catalysts when immobilised in the core of
a polystyrene nanofibre allow synthesis of compounds completely free of the
catalyst. 100% recovery of the catalyst is possible. (Scandium triflate
mentioned as the catalyst for a Aza-Diels-Alder reaction).
• Growing stem cells on a chitosan/collagen nanofibre nonwoven scaffold until they differentiate. They are then used for implants. Oriented nanofibre webs are used to grow oriented muscle and bone tissues.
• Inhalation therapies are based on the ability of ~200nm-long nanotubes loaded with drugs (RNA and glucocorticoides mentioned) to penetrate deeper into the lungs than particles of the same diameter (this is what asbestos fibres could do.). Cutting the filaments to 200nm had been a challenge, but was now done with lasers.
• Water-resistant fibres can be spun from water emulsions of polystyrene latex: the PS nanosheres forming a water repellent surface.
• Hand-held espinning guns have been developed to spray nanofibres directly onto wounds to aid healing.
• Electospinning heads were being towed behind a tractor to cover crops in a “spider-web” of pheromone-loaded nanofibres to protect them from pests.
• Growing stem cells on a chitosan/collagen nanofibre nonwoven scaffold until they differentiate. They are then used for implants. Oriented nanofibre webs are used to grow oriented muscle and bone tissues.
• Inhalation therapies are based on the ability of ~200nm-long nanotubes loaded with drugs (RNA and glucocorticoides mentioned) to penetrate deeper into the lungs than particles of the same diameter (this is what asbestos fibres could do.). Cutting the filaments to 200nm had been a challenge, but was now done with lasers.
• Water-resistant fibres can be spun from water emulsions of polystyrene latex: the PS nanosheres forming a water repellent surface.
• Hand-held espinning guns have been developed to spray nanofibres directly onto wounds to aid healing.
• Electospinning heads were being towed behind a tractor to cover crops in a “spider-web” of pheromone-loaded nanofibres to protect them from pests.
Prof Wendorff commented that scale-up issues were now
the key development area, and waved a large, apparently heavyweight sample of
nanofibre nonwovens (around 2 m long by 1.5m wide) to indicate progress was
being made. Asked about current production rates he said his large sample took
about a minute to make, but fibre speeds at laydown were about 50 m/sec. Solvent
removal, even water, was not an issue. The surface area was so great, the fibres
were dry before they reached the conveyor. As an aside he commented that
Freudenberg have been spraying nanofibre polyamide from formic acid solutions
for over 20 years.
Espinning Molten Polymer
Paul Dalton of the University of Southampton
(UK) claimed this to be an under-researched area with only 10 papers
published since 1981. He was working on this route for biomedical applications,
earlier work having focussed on filtration, breathable barriers and tissue
engineering. For tissue engineering, cell cultures are sprayed with a nanoweb
which forms a scaffold to direct further growth. Because solvents tend to be
toxic and kill the cells, he uses a polypropylene melt, the viscosity of which
has been reduced to “treacle-like” (i.e. higher than the “honey-like” solution
viscosities). He claimed to get down to 1-2 micron fibre diameter when spraying
onto a single collector, and down to 0.25 microns when making aligned nanofibres
by spraying into the gap between two parallel collectors. Here draw-down caused
by stretching between the plates caused the further reduction. (The illustrative
slide showed fibres from 190nm to 50 micron in diameter).
Comparisons of solution espinning of polyethylene
glycol/poly caprolactone block copolymers with melt spinning the same polymer at
10 times the viscosity showed that good quality solid fibres could be produce,
but only at about one-twentieth of the productivity. Melt spinning gave benefits
in more controllable focussed fibre laydown but only at the critical temperature
which might cause degradation. Solvent espinning gave unfocussed laydown but did
not degrade the polymers. Varying the relative size of the PEG/PCL blocks in the
polymer allowed a wide range of hydrophilic/hydrophobic behaviour to be
obtained.
Nanosurface Engineering
Prof Akihiko Tanioka of the Tokyo Institute of
Technology (Japan) positioned nano-surface engineering amongst the
other surface sciences and defined a nanofibre as having a diameter less than
100nm and a length to diameter ratio of more than 100 – considerably more
stringent than those claiming nanofibres from melt-blowing. Nanowebs made from
such fibres have pore sizes below 100nm also, can become effective virus
filters. His laboratory is working on a variety of applications for electrospun
nanofibres:
• Ion exchange materials, either cationic or
anionic with colossal surface areas. These could be for making ultra-pure water
for semiconductor processing, or for purifying pharmaceuticals or biological
products. For cationics, the polymer would be sulphonated, and for anioics,
quats could be added. Catalysts could be added to create highly absorbtive
materials with high catalytic power. Polystyrene spun from 10-17% solutions in
THF/DMF was preferred for cationics, and poly-4-vinyl pyridine spun from
ethanol/water for the anionics.
• A bipolar composite of both anionic and cationic nano-webs with a third unspecified ionexchange membrane in the center was being developed for electrodialysis of water to give improved efficiency of hydrogen production. It also allowed the direct production of acids and bases without bi-products.
• Aligned nanofibres of hydrophobic acrylics gave super hydrophobicity when sprayed onto a surface and the equivalent hydrophilic acrylic resin gave a more hydrophilic surface, both with reference to a cast film control. In the hydrophobic example, a random web had a 92 o contact angle and the same polymer parallel laid had a 130 o contact angle. Applications in textile finishing were sought to replace fluorochemicals with microstructured polyolefins. (148 o contact angle was the highest so far)
• Protein chips were being prepared by espinning a purified solution of alpha-lacalbumin from cows milk, followed by crosslinking it with glutaraldehyde. No further details provided.
• Nano-tubes were being used to make a microtubular fuel cell for micro-power sources. The nano tubes were espun from a coaxial bico needle using Nafion in a sol-gel precursor for the sheath and mineral oil in the core. Removal of the mineral oil yields a hollow fibre with a 2 micron outer diameter.
• Aligned nanofibre webs which diffract light to create colours without dyeing are under development.
• Flexible webs of carbon fibre are made by carbonising blends of phenolics and PAN chosen to give the best balance of strength and flexibility. Electrodes for batteries and capacitors, catalyst supports, sorbents and reinforcements were targeted. Fabrics with a density of 0.15 g/cc and a surface area of 500 m2/gm had been made so far.
• A bipolar composite of both anionic and cationic nano-webs with a third unspecified ionexchange membrane in the center was being developed for electrodialysis of water to give improved efficiency of hydrogen production. It also allowed the direct production of acids and bases without bi-products.
• Aligned nanofibres of hydrophobic acrylics gave super hydrophobicity when sprayed onto a surface and the equivalent hydrophilic acrylic resin gave a more hydrophilic surface, both with reference to a cast film control. In the hydrophobic example, a random web had a 92 o contact angle and the same polymer parallel laid had a 130 o contact angle. Applications in textile finishing were sought to replace fluorochemicals with microstructured polyolefins. (148 o contact angle was the highest so far)
• Protein chips were being prepared by espinning a purified solution of alpha-lacalbumin from cows milk, followed by crosslinking it with glutaraldehyde. No further details provided.
• Nano-tubes were being used to make a microtubular fuel cell for micro-power sources. The nano tubes were espun from a coaxial bico needle using Nafion in a sol-gel precursor for the sheath and mineral oil in the core. Removal of the mineral oil yields a hollow fibre with a 2 micron outer diameter.
• Aligned nanofibre webs which diffract light to create colours without dyeing are under development.
• Flexible webs of carbon fibre are made by carbonising blends of phenolics and PAN chosen to give the best balance of strength and flexibility. Electrodes for batteries and capacitors, catalyst supports, sorbents and reinforcements were targeted. Fabrics with a density of 0.15 g/cc and a surface area of 500 m2/gm had been made so far.
Factors affecting fibre formation
Dr Tong Lin of Deakin University ( Australia )
has studied the formation of PAN nanofibres spun from different
concentrations in dope, over various field strengths and collection distances,
and with and without the use of an ethanol coagulation bath on the grounded
electrode. He also used high speed video to investigate the draw down process
and assessed the number of non-fibrous beads in the final web. He concluded:
• 7% dope is better than 5%
• The ethanol bath improves formation and reduces beading.
• The depth of ethanol affects the field strength at its surface, so higher voltages are needed to compensate for deeper baths.
• Finer fibres and fewer beads were obtained at the higher die-collector distances (10 cm much better than 2 cm)
• There is a 400:1 draw-down in the first 2 cms after leaving the nozzle.
• The ethanol bath improves formation and reduces beading.
• The depth of ethanol affects the field strength at its surface, so higher voltages are needed to compensate for deeper baths.
• Finer fibres and fewer beads were obtained at the higher die-collector distances (10 cm much better than 2 cm)
• There is a 400:1 draw-down in the first 2 cms after leaving the nozzle.
Nanofibre Yarns
Marjeet Jassal of the Indian Institute of
Textiles ( Delhi ) has been spinning PAN nanofibres into the gap
between two conveyors, sometimes through a rotating annular magnet to insert
false twist, and sometimes through an intermediate annular electode which was
said to improve filament alignment. Both of these claims were hard to understand
as only a single filament was present at this stage of the process. Both the
verbal descriptions of the equipment and the slides were unclear. She was
however positive that the electric field had to be twice the normal strength,
this being achieved by the conveyors being at an equal but opposite voltage to
the nozzle, and not simply grounded as is usual. While at one point 8% PAN
appeared to be the best concentration, at the end she claimed 22% PAN spun into
a 70C chamber with a field strength of 36kV. The resulting “yarn” collected from
the conveyor is too delicate and shows too high surface friction ever to be used
in a textile process, so reinforcement is the suggested application. The longest
yarn produced to date? 10 inches.
Engineering and Medical Nanofibres
Masaya Kotaki of the Kyoto Institute of
Technology ( Japan ) described the Nanon laboratory scale
electrospinner developed by MECC Co. Ltd. This produces an A4 size sheet of web
on a rotating cylinder in a sealed chamber. The rotational speed of the cylinder
defines the orientation of the fibres and SEM's of a range of random to fully
oriented results were shown. The webs could be removed from the cylinder on a
cardboard frame. So far he had made webs from:
• Porous nanofibres which looked like an
open-celled foam and must have a very high specific surface area.
• 10nm nanofibres
• PCL nanofibres loaded with 75% calcium carbonate
• Hollow fibres and hollow nano-beads.
• Epoxy, polyimide and polyaniline nanofibres.
• Bico nanofibres with a polyaniline core and a poly methylmethacrylate sheath. The bico route allows non fibre-forming materials to be espun if they can be inserted into a suitable sheath.
• 10nm nanofibres
• PCL nanofibres loaded with 75% calcium carbonate
• Hollow fibres and hollow nano-beads.
• Epoxy, polyimide and polyaniline nanofibres.
• Bico nanofibres with a polyaniline core and a poly methylmethacrylate sheath. The bico route allows non fibre-forming materials to be espun if they can be inserted into a suitable sheath.
He has also measured the tensile strength of single
nano-filaments made at cylinder surface speeds between 70 and 700 m/min and has
shown that this speed change causes a big increase in orientation which can be
further enhanced by hot drawing. The as-spun (700 m/min ) PLLA single filament
broke at a stress of 175 MPa and a strain of 0.5mm/mm compared with 65MPa/1.6
mm/mm at 70m/min. Nozzle sizes have been reduced from around 0.1mm to 1 micron
to further improve orientation.
Target applications were coatings for stents (to reduce
porosity), electronic devices and sensors, and a filter receptor for
spectroscopic analysis, the latter being used to carry liquids into FTIR
analysis where the high transparency of PAN allowed good spectra to be obtained.
Espinning Emulsions
E. Klimov of BASF ( Germany ) explained
how aqueous dispersions of polymers from emulsion suspension polymerisation
could be espun into water resistant nanofibres. The polymer emulsion was first
thickened with a water soluble polymer, preferably one with some fibre-forming
capability, and then espun. The polymer particles coalesced as the water
evaporated, then deformed and finally their surface molecules interdiffused to
form a coherent, if fragile, fibre. 50nm emulsions could give 200nm fibres while
160nm emulsions could give 500nm fibres. The best fibres are obtained when the
spinning temperature and the Tg of the polymer were matched, but cross-linking
is also needed to increase strength and thermal stability. To achieve this
allylmethacrylate is added as a co-monomer. Asked for the ratio of emulsion to
thickener, Mr Klimov said they were spinning dopes with from 1 to 20% of
emulsion solids.
Disruptor™ Nanofibre filters
Rodney Komlenic VP Business Development of
Ahlstrom Filtration (USA) described their use of nanoalumina, in
reality the naturally occurring mineral boehmite - AlO(OH), the major
constituent of bauxite – as a filter aid. The mineral is refined using a
proprietary technique into nanofibrils said to be 2nm in diameter and 250nm
long. These fibrils have a specific surface area of 500m 2 /gm and are grafted
onto microglass and wet-laid to make depth filters 0.8mm thick, with an average
pore size of 2 microns. The filter papers have 42,000m 2 of nanofibre surface
per m 2 of paper.
The fibrils work well in water between 5 and 9 pH where
they develop a “huge” electropositive charge and can attract particles which
would normally miss an uncharged fibre by a micron. This means that the 2 micron
pore size paper can filter out submicron particles with minor increases in
pressure drop. Log 3 reduction values for removal of 27nm MS2 phage virus had
been found and Log 4 reductions of 300nm bacteria (Diminutia) and cysts
(simulated by 3 micron latex spheres). They are also capable of absorbing
dissolved heavy metals such as copper, lead and iron. The main application
envisaged is as a polishing filter for drinking water. Having hydroxyl
functionality it can attach to cellulose and can be formed into 3D filters using
the “egg-box” techniques. 32% of Boemite is typically added to the filters.
Powdered activated carbon (27 micron) can also be added to the paper to remove
halogens and soluble orgainics without reducing the filter's efficiency at
removing viruses.
Apparently sensitive to the fact that asbestos used to
be fibrillated into nanofibres for filters, Mr Komlenic was keen to point out
that the nanofibre emulsion version of Boehmite, “Alhydrogel” is approved by the
FDA for use in formulating vaccines to treat diphtheria, tetanus, polio and
pertussis. It is also commonly used as a thickener for gastric medicines and
tablets. Asked about Boemite particles in the filtered water, they could be
found, but only during the initial flushing out process.
Cabin Air Filters
Nico Behrendt of Helsa Automotive GmbH ( Germany
) mentioned their continuous bipolar nanofibre coating process used to
treat both sides of a nonwoven simultaneously while being very cost effective
and allowing great flexibility of polymer. The process was not described in any
detail but from the schematic shown, it used a dip roll to pick up the polymer
solution, this being scraped off the surface by a charged doctor blade, the
other edge of which provided the electospinning edge. The benefits of the
process were illustrated with reference to filtration efficiency versus pressure
drop on cabin air filters with various coatings. At a mid-range pressure drop of
20 Pa, state of the art cabin-air filters were 25% efficient. This could be
increased to 45% using single side coated uncharged nanofibres, and further to
65% with charged nanofibres. Their bipolar process, presumably with the
electrical charge gave 90% efficiency at the same pressure drop.
Ion Exchange Nanofibres
Ludek Jelinek of the Institute of Chemical
Technology (Czech Rep.) has been collaborating with Elmarco on
producing ion-exchange nanofibres using the Nanospider equipment. Polystyrene
was espun onto a PP spunbond, its stability increased by cross linking to allow
mild sulphonation using 80% sulphuric acid at 25C for 5 minutes. Stronger
sulphonation dissolved the nanofibre. Unfortunately the ion-exchange properties
were too weak for industrial applications, so work continues with other
polymers. Polymethacrylates, cellulose and chitosan were mentioned. Chitosan
itself can adsorb heavy metals but uncrosslinked it dissolves in acids. It can
however be phosphorylated and functionalised with other chelating groups.
Inorganic Nanofibres
Vit Chudoba of Elmarco (Czech Rep.)
described Elmarco's development of inorganic nanofibres for
electrochemical devices (batteries, capacitors, fuel cells etc), composites,
thermal barriers, water purification and catalysts. The additives used,
presumably loaded into a polymer solution were elements such as copper, carbon
and nickel; metal oxides (NiO, TiO 2 , ZrO 2 , and CuO); ceramics (SiC, SiO 2 ,
Al 2 O 3 ) and spinels, particularly lithium titanate. The lithium titanate
nanofibre webs were for anodes of high power Lithium ion batteries, the metal
oxides were for composite reinforcement or thermal insulation, and the TiO 2 was
for photocatalysis where the sub-500nm fibres had a surface area of 40-60m 2 /gm
and outperformed Degussa's P25 TiO 2 powder.
Solvent-free electospinning
Wiebke Voigt replaced Helga Thomas
of the German Wool Research Institute (RWTH Aachen - Germany )
to talk about espinning water solutions of polymers. Clearly the
waterbased process gives water-soluble nanofibres, and to be useful these have
to be stabilised by cross-linking. Presumably this yields a nanofibre
superabsorbent intermediate, but Ms Voigt did not mention this.
• Lupamine, a poly(n-vinylamine) from BASF
had been modified with BHBP and MAC so that it could be stabilised by UV-induced
crosslinking to give a fibre with intrinsic antimicrobial activity.
• PVA nanofibres had been stabilised by heat treatment. Versions with silver nitrate had been made for antimicrobial use.
• PVA in 50/50 blend with polyethylene glycol dimethylamine and spun at 15% concentration in water gave a water stable mixture of beads and fibres, less concentrations of PEGDMA giving better looking fibres with inadequate water stability.
• Silicon dioxide nanofibres (600 nm) had been prepared by spinning solutions via a sol-gel process with tetraethyl orthosilicate as the precursor.
• Silica/PVA composites with a diameter of 380nm and excellent water stability had also been made.
• PVA nanofibres had been stabilised by heat treatment. Versions with silver nitrate had been made for antimicrobial use.
• PVA in 50/50 blend with polyethylene glycol dimethylamine and spun at 15% concentration in water gave a water stable mixture of beads and fibres, less concentrations of PEGDMA giving better looking fibres with inadequate water stability.
• Silicon dioxide nanofibres (600 nm) had been prepared by spinning solutions via a sol-gel process with tetraethyl orthosilicate as the precursor.
• Silica/PVA composites with a diameter of 380nm and excellent water stability had also been made.
Needleless Espinning
David Lukas of the Technical University of
Liberec (Czech Rep.) pointed out that liquid surfaces deformed into
multiple tiny cones when subjected to a strong electric field, and if the
liquids contained polymers this deformation could lead to fibre formation. This
was the basis of TUL's spinning of clouds of near invisible nanofibres from the
surface of a smooth roller rotating in bath of polymer solution, a process which
was now being commercialised by Elmarco and is the reason for the conference. Mr
Lukas explained the physics involved in incomprehensible detail.
Centrifugal Spinning
Martin Dauner of ITV Denkendorf ( Germany )
observed that while molten glass, pitch (carbon-fibre precursor) and
Basofil have been converted into micron sized fibres by centrifugal spinning for
many years, little work has been done using the process to make polymeric
fibres. Commercial centrifugal sprayers (“Center Bells”) have been developed for
paint and varnish application by Reiter (not Rieter), these being hand held with
rotors running at 45,000 rpm. These have now been adapted to handle fibre
forming solutions, and each head can process about a third of a litre of
solution per hour to give fibres in the range 0.1-0.7 micron diameter. Each head
has coverage of about one-third of a metre, and they can easily be lined up to
make wider widths. Gear pumps are used for accurate polymer feed. Fibrous webs
shown were:
• Polyethylene oxide nanofibres from a 5%
solution in water.
• Polyimide nanofibres from a 15% solution in DMAC/DMF.
• Cellulose acetate spun into filters with ~1 micron fibre size from a 13% solution in acetic acid spun at 12 mls/min. 0.2 micron fibres were obtained from a 5% solution spun at 60 mls/min.
• Others claimed but not illustrated were polyurethane, PAN, PVA and PLA.
• Polyimide nanofibres from a 15% solution in DMAC/DMF.
• Cellulose acetate spun into filters with ~1 micron fibre size from a 13% solution in acetic acid spun at 12 mls/min. 0.2 micron fibres were obtained from a 5% solution spun at 60 mls/min.
• Others claimed but not illustrated were polyurethane, PAN, PVA and PLA.
Compared with espinning, the centrifugal approach gave
similar fibre sizes at higher productivity (0.5 kg/m.hr claimed at this point),
easy cleaning of spinning heads, and easy scale up by replication of
commercially available heads. The downside? Filament diameter variability was
higher and the system worked less well with solvents of low boiling point (80C
minimum). Furthermore, fibre diameters below 100nm appeared impossible, and
melts were too viscous to work. A 1.5 metre line running at 7 m/min was being
planned.
Medical Applications
Gary Wnek of the Case Western Reserve University
( Ohio USA ) is growing cells on espun nanofibre webs. He has shown
that the inherent charge on such webs attracts cells and encourages tissue
formation. Random webs form random tissues (e.g for skin) and oriented webs form
oriented tissues (e.g for muscles). The cells can be grown separately on the
opposite sides of the web and eventually link up to form a bi-layer structure –
like skin. Webs tried were listed as:
• Poly lactic and glycolic acids and
blends.
• Polycaprolactone
• Poly(ethylene-co-vinyl acetate) and EVOH
• PVA and PVOH
• Collagen
• Elastin
• Polycaprolactone
• Poly(ethylene-co-vinyl acetate) and EVOH
• PVA and PVOH
• Collagen
• Elastin
Collagen nanofibres were particularly successful for
skin regeneration because they mimicked natural collagen scaffolding and
exhibited excellent cell infiltration. The best solvent for espinning collagen
was hexafluoroisopropanol. Asked if nanofibres were really necessary, Dr Wnek
said that it depends on the cell types. For cartilage, micro fibres are best:
for liver cells, even coarser fibre are better. If the cells are much larger
than the fibres, the fibres are incorporated in the cells. If the cells are much
smaller, they grow on the fibre surface.
Multi functional scaffolds for retina regeneration were
using suspensions (rather than solutions) of poly(lactic-co-glycollic) acid, the
water phase carrying the MMP2 gelatinase protein. The protein domains could be
seen along the nanofibres. Asked if the lack of transparency of PLGA webs was a
problem, it was not. Once the cells had colonised the web, web opacity was
irrelevant.
Elmarco Developments
Jana Svobovoboda of Elmarco R&D (Czech Rep.)
described the work being done with chitosan, collagen and alginate on
the Nanospider™ roller espinner to make antimicrobial and antiviral filters for
face masks.
• Chitosan's haemostatic, bacteriocidal,
fungistatic and antitumoric properties make it a natural for biomedical fabrics,
but it has proved hard to espin on its own. Now, Elmarco has a patent on
blending chitosan with a little (<10%) polyethylene oxide which makes it work
better. PVA can also be used in this support role.
• Similary, sodium alginate has been converted into 50-250nm fibres using a PVA polymer support.
• Collagen too has proved difficult on the roller system, and needs a special solvent and support polymer. A successful combination to give collagen (>98.5%pure) nanofibres of diameters in the range 30-120nm is now patent pending.
• Nanospider™ AntimicrobeWeb™ uses a polyamide (6/12) structure of 100-700nm fibres with a basis weight of 0.4 gsm and an air permeability of ~60Pa to carry antimicrobials into a filter structure. Chitosan, and quats were the additives mentioned. Nelson Labs (USA) have tested face-masks using the web and found both viral and bacterial filtration efficiencies above 99.9%. S. aureus; E.coli; P. aeruginosa, C. albicans; A. niger and Penicillium aurantiogriseum were the challenges.
• Similary, sodium alginate has been converted into 50-250nm fibres using a PVA polymer support.
• Collagen too has proved difficult on the roller system, and needs a special solvent and support polymer. A successful combination to give collagen (>98.5%pure) nanofibres of diameters in the range 30-120nm is now patent pending.
• Nanospider™ AntimicrobeWeb™ uses a polyamide (6/12) structure of 100-700nm fibres with a basis weight of 0.4 gsm and an air permeability of ~60Pa to carry antimicrobials into a filter structure. Chitosan, and quats were the additives mentioned. Nelson Labs (USA) have tested face-masks using the web and found both viral and bacterial filtration efficiencies above 99.9%. S. aureus; E.coli; P. aeruginosa, C. albicans; A. niger and Penicillium aurantiogriseum were the challenges.
Applications in waste water treatment, HVAC filters,
cabin air filters and cleanroom filters are now being sought.
1 O 2 from Nanofibres
J Mosinger of Charles University in Prague
(Czech Rep.) is producing polyurethane nanofibre webs doped with
5,10,15,20-tetraphenylporphyrin (TPP) photosensitizer. The 2 gsm webs contain
0.12% of the TPP, which on exposure to light releases singlet oxygen, a highly
cytotoxic bacteriocide at the fibre surface. E.coli can be grown on the webs in
the dark, but exposure to light kills the colony and prevents further growth.
Auto disinfecting, sterile materials are therefore possible. Asked how much
light is needed to trigger the kill, Dr Mosinger said about 2 hours in daylight
or half an hour under a 100w halogen lamp. What was the shelf life of the
fabric? About a year if kept in the dark. How transparent was the web? 90%: it
was only 0.03mm thick.
Electrochemical Storage Devices
Lukas Rubacek of Elmarco (Czech Rep.)
has demonstrated that the Nanospider™ roller system can make lithium
titanate spinel nanofibres (LTO) for use in batteries, capacitors and
supercapacitors.
As an anode in a lithium battery, the LTO fibre allows
higher potential leading to better safety than the conventional lithium ion
battery which can ignite if the charging circuit malfunctions. It's theoretical
capacity is 176 mAh/g (c.f. 300 mAh/g of the lithium carbide anode batteries)
but it can be discharged and charged at much higher rates safely and without
damage. 3 minute recharge times with a 5000 cycle life were quoted. So, high
power rapidly rechargeable batteries which will not catch fire can be developed.
The very high surface area of the LTO webs also allows
electrical double layer capacitors to be made at higher capacities with lower
internal resistance.
Air Filters
Stanislav Petrik of Elmarco (Czech Rep.)
has been investigating the effects of nanofibres spun from the
Nanospider™ machine on the performance of cellulosic air filter media. He has
discovered that the Relative Fibre Length, i.e. the number of kilometres of
nanofilament per square metre of surface correlates well with filtration
performance for a wide range of fibre sizes and basis weights. In fact RFL and
Pressure Drop correlate better than Basis Weight and Pressure Drop, so image
analysis to establish fibre diameter and RFL can predict filtration performance
where basis weights in the 0.01 to 0.1 gsm range are much harder to determine.
Filtration efficiencies of submicron particles is increased by >>100%
while air permeability is only reduced by >10%.
Sound Absorbtion
Klara Kalinova of the Technical University of
Liberec (Czech Rep.) stressed that here we were considering absorbtion,
not insulation, of sound. Absorbtion, being the difference between incident and
reflected sound, is very dependent on sound frequency for any material. Most
absorbtion occurs at the resonant frequency of the material, in reality the
resonant frequency of the free fibres in the wadding or nonwoven. The remainder
of absorbtion occurs due to friction between the vibrating air and the fibres.
Layered nanofibres work best, and if a density gradient can be set up sound
absorbtion is further improved. As fibre size is reduced the resonant frequency
falls, so the finer the fibre, the better the absorbtion of usually
problematical low frequencies. An acoustic nanofibre is now being developed as
part of the recently announced collaboration between Elmarco and
Oerlikon-Neumag.
Aligning Nanofibres
Yousef Mohammadi of the Stem Cell Technology
Company, Tehran ( Iran ) is using the dynamic gap electospinning method
to precisely control the alignment of filaments in a nanofibre web. In this
method the electodes attracting the fibres are two vertical rotating discs space
about 10 inches apart below the vertically downwards extruding nozzle. Both
discs are at the same voltage and the nanofibres align themselves between them.
As the synchronised rotation of the pair of discs carries the aligned filament
away from the forming zone they are collected on the surface of a drum rotating
between the lower half of the electrodes. The text referenced an EU patent
application. (no details)
Calvin Woodings
29 th October 2007