Carbon Nanotube Biofiber Formation in a Polymer-Free

DOI:10.1002/adfm.200700822

Carbon Nanotube Biofiber Formation in a Polymer-Free Coagulation Bath**

By Joselito M.Razal ,Kerry J.Gilmore ,and Gordon G.Wallace *

1.Introduction

Several approaches for creating continuous macroscopic fibers in the form of ribbons,ropes,or yarns of carbon nano-tubes (CNTs)have been demonstrated.[1–15]Most recently,a variety of pure CNT yarn structures have been fabricated by twist-based dry-spinning methods from aligned [3,4]and super-aligned [5]CNT arrays or directly from a chemical vapor deposi-tion synthesis reactor.[6]Although promising for the production of pure CNT macrostructures,the integration of other func-tional molecules into these structures remains challenging.An alternative method that allows effective integration of useful molecules within the CNT fiber structure is wet spinning.In this method,spinning solutions containing CNTs are dispersed using surfactants [1,2,11]or biomolecules [7]and then exposed to a coagulating medium (typically an insulating polymer such as polyvinyl alcohol (PVA))to induce fiber formation.

The most common approach for achieving homogeneous and relatively concentrated CNT dispersions involves the use of low-molecular-weight surfactants as dispersing agents.[1,2,9,10,16–18]A significant amount of surfactant is required to stabilize the sus-pension in order to achieve efficient repulsion between the hy-

drophobic CNT interfaces.Typically,1.2wt %of the sodium or lithium salt of dodecylsulfate (SDS or LDS)is used to disperse 0.4wt %single-walled carbon nanotubes (SWNTs).Other nonmicelle forming molecules such as conjugated poly-mers [19,20]and synthetic biological molecules [21]have been used as alternatives.More recently,naturally occurring biomolecules such as single-stranded DNA [7,22,23]and chitosan (CHI)[24]have been shown to be excellent dispersants for SWNTs.Interfacing biomolecules with CNTs has been of interest as a method for rendering them biocompatible for use as platforms to support the growth of mammalian cells.DNA was found to be particu-larly efficient in stabilizing up to 1wt %SWNTs and required a proportional equivalent of DNA to make the dispersion suit-able for fiber spinning.[7]CHI was found to preferentially dis-perse CNTs over other impurities and was selective in the non-covalent wrapping of different diameter nanotubes.[24]

When using polymer coagulants in the bath,incorporation of PVA into the fiber has a beneficial effect on mechanical prop-erties but compromises the electronic conductivity.[1,2]By using nonbiological dispersants,others have prepared CNT fibers using a polymer-free coagulation bath.[8–10]For example,a CNT–superacid suspension was used to form fibers by spinning into ether.[8]CNT–surfactant dispersions have been spun into acids [9]or ethanol/glycol/glycerol mixtures.[10]Fibers from bio-molecule–nanotube dispersions have also been reported using either PVA [7]or oppositely charged biomolecules as the coagu-lating medium.[12]

Here we present a different approach to produce CNT fibers with added properties (biofunctionality)and enhanced electri-cal conductivities.In place of using molecular surfac-tants [1,2,9,10,16–18]or superacids,[8,25]we used the biomolecules hyaluronic acid (HA),CHI,or DNA to generate stable and homogeneous dispersions of CNTs.These biomolecules have been shown to be effective components of platforms for a wide range of applications,including nerve regeneration,[26]wound

–

[*]Prof.G.G.Wallace,Dr.J.M.Razal,Dr.K.J.Gilmore

ARC Centre of Excellence for Electromaterials Science Intelligent Polymer Research Institute,University of Wollongong,

Northfields Avenue,Wollongong,NSW 2522(Australia)

E-mail:gwallace@https://www.wendangku.net/doc/7a4214288.html,.au

[**]The authors thank Dr.Simon Moulton and Dr.Carol Lynam for help-ful discussions and the Australian Research Council for financial sup-port of this work.Supporting Information is available online at Wiley InterScience or from the author.

A novel solution spinning method to produce highly conducting carbon nanotube (CNT)biofibers is reported.In this process,carbon nanotubes are dispersed using biomolecules such as hyaluronic acid,chitosan,and DNA,and these dispersions are used as spinning solutions.Unlike previous reports in which a polymer binder is used in the coagulation bath,these dispersions can be converted into fibers simply by altering the nature of the coagulation bath via pH control,use of a crosslinking agent,or use of a biomolecule-precipitating solvent system.With strength comparable to most reported CNT fibers to date,these CNT biofi-bers demonstrate superior electrical conductivities.Cell culture experiments are performed to investigate the cytotoxicity of these fibers.This novel fiber spinning approach could simplify methodologies for creating electrically conducting and biocom-patible platforms for a variety of biomedical applications,particularly in those systems where the application of an electrical field is advantageous-for example,in directed nerve and/or muscle repair.

FULL PAPER

dressing,[27]and biosensors.[28]In contrast to surfactants,only small concentrations of these biomolecules were required to prepare CNTs suitable for fiber preparation by wet spinning.For example,only ca.0.3wt %of HA was required to disperse 0.4wt %CNTs,whereas ca.1.2wt %SDS was required.We have now devised simple approaches to wet-spin fibers from these dispersions without the use of a polymer in the coagulat-ing medium.In the first approach,we promoted gel formation by injecting the novel CNT biodispersions into an aqueous so-lution of dilute acid.Alternatively,we used a coagulation solu-tion that contained divalent cations,such as calcium,to form inter-and intrachain ionic crosslinks.A third approach was to utilize a coagulation bath containing a solvent system that was miscible with but precipitated the biomolecules in the spinning solution.The use of a polymer-free coagulation medium means that only the particular dispersing agent (i.e.,biomolecule)and the CNTs remain within the fiber structure (as opposed to the inclusion of surfactant,PVA,and the combinations of biomole-cules,as indicated in previous reports).

2.Results and Discussion

It was confirmed by optical microscopy that homogeneous HA–SWNT dispersions were obtained.Moulton et al.[29]re-cently reported well-dispersed single-phase isotropic disper-sions of SWNTs using HA as the dispersant.By using just 0.27wt %of HA and a relatively short sonication time of 18min,it was possible to form stable and spinnable dispersions containing 0.40wt %SWNTs in water,achieving a concentra-tion ratio of 0.68:1.This composition is superior to reported CNT dispersions using molecular surfactants,where concentra-tion ratios of at least 2:1,and in some cases 3:1,were re-quired.[1,2]

A rotating coagulation bath [1]was used to carry out prelimi-nary experiments,whereas a continuous fiber spinning set-up [2]was used to produce continuous meter-long fibers when re-quired.Preliminary results indicate that for the CNT disper-sions described above,fiber formation could be induced by modifying the coagulation medium by 1)varying the pH condi-tions,2)integrating an ionic crosslinker,or 3)using a precipi-tating solvent system.2.1.pH-induced Coagulation

It was found that the quality of the initial gel fiber formed in the coagulation bath was best when the pH of the coagulation solutions was lowered to approximately 1using dilute strong acids (hydrochloric,sulfuric,or nitric).Above pH 1,the gel fi-bers had reduced mechanical integrity,whereas below this so-lution pH,rapid coagulation resulted in blob formation at the tip of the needle.Favorable spinnability in terms of gel fiber quality and flexibility was obtained when concentrations be-tween 0.1and 1.0M for the strong acids and 5to 20vol %for acetic acid were used for HA–CNT and DNA–CNT disper-sions.By using the optimized spinning conditions (i.e.,an ap-propriate spinning solution injection rate that matched the co-agulation solution rotation speed),this spinning approach always formed flat ribbons of gel fibers similar to images shown in Figure 1a,irrespective of the acid used as the coagulating medium.We observe that the width of these ribbons depended primarily on the diameter of the needle and the injection rate

of the spinning solution.Wider ribbons that tended to curl (Fig.1a,inset)were produced using larger needle diameters,whereas thicker ribbons were obtained using higher injection rates (Fig.1a).

Fibers produced from CNT dispersions containing the poly-cationic biomolecule CHI prepared by spinning into 0.1to 1.0wt %NaOH solutions were circular in diameter (Fig.S1b).The gel fiber properties were improved by using lower NaOH concentrations (0.1wt %)and by using 70–90vol %ethanol as the solvent rather than pure water.2.2.Ionic Crosslinking

Fiber coagulation can also be induced by ionic crosslinking.We used this approach for HA–SWNT spinning solutions by

Figure 1.Varying shapes and surface morphologies of HA–CNT fibers spun using different coagulating media.a)0.3M HNO 3(scale bar 20l m),b)5%(w/v)CaCl 2in 70%(v/v)ethanol solution (scale bar 200l m),and c)ethanol (scale bar 30l m).Inset in (a)shows similar fiber composition to fibers in (b),but they were spun using different spinning conditions (scale bar 20l m).

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J.M.Razal et al./Carbon Nanotube Biofibers

incorporating calcium chloride into the coagulating medium. Controlled injection of the spinning solution into the coagula-tion solution afforded a gel fiber structure because of the for-mation of calcium bridges between D-glucuronic acid residues on adjacent chains of HA.We found that increasing the cal-cium chloride concentration in the bath increased the rate of coagulation process.To prevent the formation of a core–sheath fiber structure as a result of the rapid thickening of the skin, we used5wt%calcium chloride in the coagulation solution. Improvement in gel fiber strength and flexibility was observed when70vol%ethanol was used as the solvent for CaCl2solu-tions rather than pure water.By employing similar spinning pa-rameters to values used for other coagulants reported in this paper,we successfully prepared several meters of these fibers in continuous lengths.We found that crosslinking with calcium chloride allowed the most flexibility in spinning parameters and was therefore the most promising strategy for the scaled-up production of CNT biofibers.This approach produced ro-bust and flexible gel fibers that were suitable for subsequent processing.Fibers formed using any of the described spinning conditions were uniformly circular in both the wet and dry states and had relatively smooth surfaces,as shown by the rep-resentative images in Figure1b.

2.3.Solvent-Induced Fiber Formation

The use of a coagulation bath containing a solvent that is poor for the polar biomolecules but is highly miscible with the solvent used in the spinning solution(water in this case)was also a viable approach to CNT fiber spinning.We selected common organic solvents to investigate the coagulation pro-cess and found that the use of alcohols or acetone as the coagu-lating medium effectively promoted fiber formation.For HA–CNT dispersions,using methanol,ethanol,and isopropanol as the coagulation media produced flexible gel fibers,whereas ac-etone and butanol produced weak and brittle gels that were difficult to handle for subsequent processing and characteriza-tion.The use of ethanol or methanol as the solvent for the pH-induced and ionic crosslinking approaches dramatically im-proved the flexibility of HA–CNT and CHI–CNT gel fibers. The solvent-induced coagulation approach always produced very thin and curled ribbons(Fig.1c)independent of the spin-ning conditions,unlike those fibers spun using acidic or basic coagulants.The DNA–CNT and CHI–CNT gel fibers spun from ethanol were weak and easily broken when

handled during drying.As for fibers spun

from these dispersions in acidic coagulation

baths,this approach resulted in distinct fi-

ber surface morphologies(additional scan-

ning electron microscopy(SEM)images in

Fig.S1).Different surface roughness and

textures have been shown to induce the

organization of extracellular matrices.[30,31]

In our case,the longitudinal grooves and

porous surface of the fibers(Fig.S1)may

provide contact guidance and increased

surface area for cell adhesion.2.4.Processability and Properties

The quality of the gel fibers produced at the initial spinning stage is critical to successful CNT wet-spinning.The gel fiber strength dictates the endurance of the fibers throughout the spinning process and in any subsequent treatment for improv-ing fiber properties(i.e.,improvement in orientation,crystal-linity,and strength).[13]Although some of the gel fibers report-ed here were observed to be significantly stronger and more flexible than others,these characteristics were not necessarily reflected in the mechanical properties of the dried fibers.For example,dry HA–CNT fibers spun from all coagulants had similar strength and modulus values(Table1),but the gel fi-bers prepared using the ionic crosslinking approach were sig-nificantly stronger and more flexible.By exploiting the precipi-tating solvent approach,flexible gel fibers were produced using methanol,ethanol,and isopropanol as the coagulant but not from acetone or butanol.In contrast,brittle HA–CNT gels were produced using the pH-induced coagulation system.Flex-ibility of HA–CNT and CHI–CNT gel fibers was achieved when70vol%ethanol was used as the solvent for5wt% CaCl2and0.1wt%NaOH,respectively.Most gel and dry fi-bers from DNA–CNT dispersions were relatively weak,irre-spective of the coagulation solution used.Also,unlike the PVA-spun CNT fibers with CNT-enriched inner core and PVA-enriched outer skin,none of the collected SEM images suggested a skin–core fiber microstructure.Homogenous distri-bution of the biomolecule and CNTs was observed on the fiber cross section(Figs.1and S1).

Table1enables comparison of the mechanical properties, conductivity,and capacitance values for HA–SWNT fibers coa-gulated under different conditions.As shown in Table1,under similar spinning conditions,the as-spun HA–CNT dry fibers had tensile strengths,Young’s modulus values,and failure strains around110MPa,15GPa,and7%,respectively.In contrast,CHI–CNT gel and dry fibers had a better tensile strength(ca.155MPa)when spun into0.1wt%NaOH in 90vol%ethanol,while DNA–CNT fibers had a tensile strength of ca.85MPa.

The best energy-to-break(toughness)value of6–10J g–1 measured for HA–CNT fibers spun by ionic crosslinking was comparable to values for previously reported CNT fibers con-taining poly(ethylene imine)(5–6J g–1)[14]but far lower than values for supertough SWNT–PVA fibers(up to870J g–1).[2,13]

Table1.Summary of the mechanical properties,conductivity,and capacitance values for the differ-ent as-spun HA–CNT fibers.

Spinning solution Coagulation

solution Tensile strength

[Mpa]

Young’s modulus

[GPa]

Conductivity

[S cm–1]

Capacitance

[F g–1]

HA–SWNT5%Acetic Acid123±2812±5270±4427 HA–SWNT0.3M H2SO4105±1512±1158±1836 HA–SWNT0.3M HCl111±2417±5229±4632 HA–SWNT0.3M HNO3110±313±1537±5644 HA–SWNT5%CaCl2in70%

Methanol

115±1714±466±732 HA–SWNT Ethanol89±1512±4199±3727FULL PAPER

J.M.Razal et al./Carbon Nanotube Biofibers

Although these fibers did not compare favorably with commer-cial high-strength fibers used in structural composites,they pos-sess adequate mechanical properties for the fabrication of con-duits for nerve and muscle repair.This feature,combined with the inherent electrical properties of the CNTs and the molecu-lar functionality of the biomolecules,provides a unique set of material characteristics.

Closely related CNT fibers were reported by Barisci et al.[7]and Kozlov et al.,[9]who achieved electrical conductivities up to 167S cm –1after annealing fibers at high temperatures.The measured conductivity compared favorably with values for the pure CNT-twisted yarns reported by M.Zhang et al.[3](ca.300S cm –1)and X.Zhang et al.[4](ca.410S cm –1).Lynam et al.[12]have shown measured conductivities up to 135S cm –1for CNT–biofibers prepared by spinning CNT–biomolecule dispersions into a coagulating solution containing an oppositely charged biomolecule (to that of the spinning dispersion).

We have measured conductivity values up to ca.537S cm –1for our as-spun HA–CNT biofibers,which varied with the co-agulation method used (Table 1).The most conducting HA–SWNT fibers were produced using the pH-induced coagulation method.Our DNA–CNT biofibers displayed conductivities up to 150S cm –1without annealing.We attribute the high conduc-tivities to the absence of the supporting polymer matrix that was usually required in the coagulation medium for most CNT wet-spinning processes.It appears that the amount of biomole-cule material remaining in the fiber structure was low enough to permit intimate contact between the nanotube junctions,al-lowing electrical conductivity;yet it was sufficient to provide mechanical support.In support of this,less conducting CHI–CNT biofibers (ca.21S cm –1)were obtained,presumably be-cause of the higher biomolecule concentration required to achieve a homogeneous spinning dispersion (0.6wt %CHI for 0.3wt %CNTs).It was difficult to accurately measure the CNT content of the fibers.Overlap in the decomposition tem-perature of each individual component (CNTs and biomole-cules)of the composite fiber was observed during thermogravi-metric analysis (results not shown).However,assuming no CNT and biomolecule loss during the wet-spinning process,the CNT composition could be estimated based on the concentra-tion of the spinning solution (CNT dispersion).This assessment translates to 67wt %,50wt %,and 33wt %CNT content for the HA–CNT,DNA–CNT,and CHI–CNT fibers,respectively.All electrochemical characterization experiments were per-formed in the physiological medium phosphate-buffered saline (PBS,pH 7.4).Cyclic voltammetry (CV)measurements of HA–CNT biofibers gave responses that indicate capacitive be-havior predominates,irrespective of the coagulation system used (Fig.2a).The linear dependence of the current flows on scan rate confirms this behavior (Fig.2b).There was no ob-servable redox activity,even for acid spun fibers.All biofibers displayed some degree of swelling when cycled in PBS but maintained their structural integrity,even after multiple scans.Specific capacitance values as high as 44F g –1were measured for the HA–CNT biofibers.The DNA–CNT bio-fibers dis-played similar capacitive behavior to HA–CNT biofibers;how-ever,resistive CV measurements were observed for CHI–CNT biofibers because of their reduced electrical conductivity (re-sults not shown).2.5.Cell Growth Studies

In vitro cell culture testing of materials for which biomedical applications are envisaged allows assessment of cytotoxicity and can form the initial step in a more detailed evaluation of the biocompatibility of the materials.Fibroblast cells,including L-929cells,are commonly used as cell lines for cytotoxicity testing.

Cell culture studies were performed to investigate the adhe-sion and proliferation of L-929cells on assemblies of biofibers over a period of 72h.The cells were visualized on the opaque materials using Calcein staining.Calcein AM diffused across the plasma membrane of cells and was cleaved by intracellular esterases in metabolically active cells to yield a bright green fluorescent product.After staining with Calcein AM,the pres-ence of fluorescent green,metabolically active cells was as-sessed under an inverted fluorescent microscope.

Control experiments were carried out on films (obtained by casting dispersions directly into cell culture dishes)and bucky papers (obtained by the filtration method)produced from CNT dispersions using the biomolecules DNA,CHI,and HA or the surfactant Triton-X.L-929adhesion and proliferation were compared on these materials and on fabricated scaffolds from the biofibers.L-929cells adhered to and proliferated as well on the biomolecule-containing materials (Fig.3)as they

-4-2024-0.1

0.10.30.50.7

Potential (V) vs Ag/AgCl

C u r r e n t (A /g

)

0123450

20

40

60

80

100Scan Rate (mV/s)

C u r r e n t (A /g )

Figure 2.Electrochemical behavior of HA–CNT fibers spun in 0.3M HNO 3(solid line),ethanol (dashed line),and 5wt %CaCl 2in 70vol %methanol (dotted line).a)Cyclic voltammograms in 0.2M PBS (pH 7.4)at 50mV s –1and b)plot of current versus scan rate,showing specific capacitances of fi-bers.

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J.M.Razal et al./Carbon Nanotube Biofibers

did on polystyrene tissue culture plastic,but they did not per-form well on materials prepared in the absence of biomaterials (i.e.,dispersions prepared using Triton-X).These results indi-cate that the fiber materials were not toxic to these cells and formed suitable substrates to support L-929cell adhesion and growth.The presence of the biomolecules in the CNT disper-sion and the products generated from them had a significant ef-fect on the compatibility of the materials with L-929cells.Im-portantly,the presence of the biomolecule did not have an adverse effect on the physical properties,mechanical strength, and electrical conductivity.Although further studies would be required for detailed assessment of the biocompatibility of these materials,the current cell growth studies establish that the materials were not cytotoxic to fibroblasts.

3.Conclusions

We have demonstrated novel approaches for spinning fibers from CNTs.Biofunctionality,enhanced electrical conductivity, and control of fiber composition were achieved by using biopo-lymer dispersions without the use of polymer binding agents in the coagulation systems.We also demonstrated proliferation of L-929cells on assemblies constructed from these fibers,which indicates the biocompatibility of these novel materials.There-fore,the CNT biofibers may find application in the develop-ment of new biomedical devices,particularly in those systems where the application of an electrical field is advantageous,for example,in directed nerve repair.The excellent electrical and capacitive behavior of our novel CNT biofibers render them particularly suitable for these applications.

4.Experimental

Purified SWNTs produced by the HiPco process were obtained from Carbon Nanotechnologies,Inc.(Lot P0279and Lot P0314).HA(potas-sium salt from human umbilical cord)and medium molecular weight CHI were obtained from Sigma–Aldrich.Salmon sperm DNA was ob-tained from Nippon Chemical Feed Co.Ltd.,Japan.All materials were used as received.Homogeneous SWNT dispersions were prepared by the probe sonication process in an iced water bath(Digital Branson So-nifier)utilizing a power output of180W for18min in a pulse mode (0.5s on/off).Different compositions of spinning solution and coagu-lating medium were prepared as described in the text.Coagulation

spinning by the rotation method was used to

initially test the fiber spinnability while the con-

tinuous set-up was employed to produce fibers

several meters in length.A5mL syringe with a

detachable needle(0.60mm inner diameter)

controlled by a syringe pump(KDS Scientific-

100)was used to deliver the spinning solution

to the coagulation bath.With many spinning

parameters possible,we maintained a spinning

solution injection rate of45mL hr–1,a coagula-

tion bath rotation speed of25rpm(rotation

bath method),and a coagulation flow rate of

105mL min–1(continuous flow method).The

gel fibers produced were washed in ethanol or

methanol and then dried in air under tension.

The surface morphology and broken end fea-

tures of the fibers were characterized by SEM (Hitachi S-900)to evaluate the effects of different spinning conditions. Electrochemical measurements were performed using an EDAQ e-cor-der(401)and potentiostat/galvanostat(EA160)with Chart v5.1.2/ EChem v2.0.2software(ADInstruments).CV experiments in0.2M PBS(pH7.4)were performed using a standard3-electrode cell set-up utilizing the fiber as a working electrode,Ag/AgCl as a reference elec-trode,and a platinum mesh as a counter electrode.Electrical conduc-tivity at room temperature was measured by the four-point probe method.Tensile property measurements were carried out using a dy-namic mechanical analyzer(Texas Instruments)at a strain rate of 1%min–1.Samples were mounted on aperture cards(1cm length win-dow)with commercial superglue and allowed to air dry.

In order to facilitate cell growth experiments,fibers were fabricated into assemblies as described previously[32].Briefly,the gel fibers from the coagulation bath were grouped,rinsed with ethanol,and then placed in parallel on a Teflon substrate.This procedure was repeated until a4×10cm area was produced.The assemblies were allowed to dry overnight to obtain a free-standing film.For comparison,1.5mL of each spinning solution was cast onto a Teflon substrate and subse-quently dried by solvent evaporation.These fiber assemblies were as-sessed for cytotoxicity by monitoring the growth of L-929(mouse fi-broblast)cells on the materials.Fiber samples were cut into6mm diameter discs and placed into96-well polystyrene cell culture plates. The discs were soaked in2changes of cell culture media(without Fetal Bovine Serum,FBS)for24h and then rinsed with water to remove any soluble impurities.The samples were sterilized by rinsing in and drying from70%ethanol and placed under UV light for20min.Each well was seeded with5×103L-929mouse fibroblast cells and cultured in a DMEM:F12medium supplemented with5%(v/v)FBS for72h at 37°C in5%CO2.As a control,L-929cells were seeded onto tissue culture plates.5l M Calcein AM(Molecular Probes)was added to cells in the culture medium and incubated for15min at37°C before washing twice with PBS and visualizing using a Leica DMIL inverted fluorescence microscope equipped with a Leica DC500camera.

Received:July23,2007

Revised:September18,2007

Published online:December18,2007–

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