highly ordered mwnt-based matrixes: topography at the nanoscale conceived for tissue engineering

Highly Ordered MWNT-Based Matrixes: Topography at theNanoscale Conceived for Tissue Engineering

I. Firkowska,† M. Olek,† N. Pazos-Pere´z,† J. Rojas-Chapana,† and M. Giersig*,†,‡

Center of AdVanced European Studies and Research (CAESAR), Ludwig-Erhard-Allee 2,53175 Bonn, Germany, and Poznan UniVersity of Technology, Department of Technical Physics,

Poznan´ , Poland

ReceiVed NoVember 14, 2005. In Final Form: January 25, 2006

A method based on the conventional lithographic technique combined with the layer-by-layer (LBL) assemblyprocess is applied to the construction of free-standing micro- and nanostructured matrixes. The method enablescontrolled shaping and considerable chemical and mechanical stability of the self-assembled monolayers, allowingfor high reproducibility in manufacturing. The matrixes are characterized by controlled geometry, surface topography,and chemical composition. The complete architecture is made up of successive layers of intercrossed carbon nanotubesthat self-assemble into orderly structures. In particular, the present method aims to create architectures and topographiesthat mimic those occurring naturally (native tissue structures). In addition, nanoindentation and nanoscratch techniqueswere used to evaluate the mechanical properties of the carbon nanotube-based matrixes.

Introduction

The state of the art in the area of tissue engineering scaffoldsis based on biodegradable matrixes that try to mimic the structuraland functional profile found in the native extra cellular matrix(ECM). This biomimetic approach is critical in order to promotecell-matrix interaction and cell growth and differentiation in3D biocompatible structures. Taking into account the growingimportance of “neotissue” regeneration, artificial tissue substitutescan be engineered chiefly from a biophysical standpoint.Subsequently, the engineering will be influenced not only by thebiochemical surface composition but also by the surface texture,microporosity, pore size, density, connectivity, and 3D config-uration. However, because the interactions between cells and theECM involve nanoscale adhesive cues, texture and roughness onthe nanoscale can directly influence cell attachment and function.However, despite substantial existing literature on this subject,the optimal relationship between scaffold architecture and tissueregeneration remains doubtful or uncertain.

Data referring to the fact that nanomaterials may be optimalfor tissue engineering applications already exist. This is due notonly to their ability to simulate dimensions of proteins thatcomprise tissues but also to their higher reactivity for interactionsthat control cell attachment and thus the ability to regeneratetissues. Therefore, the engineering of the surface properties of3D structures by using nanoscaled materials will enable the designof a new generation of scaffolds. They will provide sufficientsurface roughness for cells to attach while simultaneouslypromoting the spreading of the cells. The ultimate aim is to attainthe desired (or required) tissue type(s) to create tissue formationthroughout the entirety of the scaffold.1

One of the most promising material types for the developmentof scaffolds on the nanoscale is carbon nanotubes. Because oftheir unique propertiesshigh mechanical strength, excellentflexibility, and low densityscarbon nanotubes are attractive forthe design of lightweight, high-strength materials such as bone.Thus, carbon nanotubes have been tested in fiber-reinforced

composite materials comprising a collagen matrix with embeddedCNTs.2 Other composites consisting of blends of polylactic acidand carbon nanotubes are used to expose cells to electricalstimulation promoting the osteoblast function.3 Furthermore, abone imitation based on the self-assembly of hydroxyapatite onchemically functionalized single-walled carbon nanotubes hasrecently been reported.4 The potential of nanotubes to mimic therole of collagen as the scaffold for the growth of hydroxyapatitein bone has been proposed by Haddon.5

The combination of tissue engineering polymer scaffolds withcarbon nanotubes offers unique architectures comprising bio-mimetic design characteristics. Multiwalled carbon nanotubes(MWNTs) have already been employed to fabricate high-strength,light-weight films utilizing the layer-by-layer assembly tech-nique.6,7 Correa-Duarte and co-workers have recently shownpreliminary results of the assembly of carbon nanotubes onhexagonally ordered arrays of latex particles. In the workpresented here, similar techniques have been used to preparenanotube/polymer-based matrixes with a controllable porousstructure and well-defined topography comprising biomimeticdesign characteristics. Besides the morphological and structuralproperties, the mechanical parameters on our scaffold have alsobeen proved. Nanoindentation and nanoscratch tests wereperformed using the TriboScope instrument to evaluate theYoung’s modulus, hardness, and coefficient of friction.

Experimental Section

Materials. Silicon substrates were obtained from CEMAT Silicon,Germany, and cut into 1.5× 5 cm2 pieces. Polystyrene spheres (Ps)with diameters of 1.71µm (CV ) 2.2%) and 7µm (CV ) 1.2%)were purchased from microParticles GmbH. Multiwalled carbonnanotubes (CVD method, purity>95%, diameter 10-20 nm, length1-20 µm) were obtained from NanoLab (Boston, MA). Branched

* Corresponding author. E-mail: [email protected].† Center of Advanced European Studies and Research (CAESAR),

Germany.‡ Poznan University of Technology.(1) Andersson, A.-S.; Brink, J.; Lidberg, U.; Sutherland, D. S.IEEE Trans.

Nanobiosci.2003, 2, 49-57.

(2) MacDonald, R. A.; Laurenzi, B. F.; Viswanathan, G.; Ajayan, P. M.;Stegemann, J. P.J. Biomed. Mater. Res. A2005, 74, 489-496.

(3) Supranowicz, P. R.; Ajayan, P. M.; Ullmann, K. R.; Arulanandam, B. P.;Metzger, D. W.; Bizios, R.J. Biomed. Mater. Res. A2002, 59, 499-506.

(4) Zhao, B.; Hu, H.; Mandal, S. K.; Haddon, R. C.Chem. Mater.2005, 17,3235-3241.

(5) http://nanotechwire.com/news.asp?nid)2122&ntid)130&pg)1.(6) Olek, M.; Ostrander, J.; Jurga, S.; Mo¨hwald, H.; Kotov, N.; Kempa, K.;

Giersig, M.Nano Lett. 2004, 4, 1889-1895.(7) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; Liz-

Marzan, L. M.Chem. Mater. 2005, 17, 3268.

5427Langmuir2006,22, 5427-5434

10.1021/la053067e CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 05/04/2006

polyethyleneimine (PEI,Mw ) 700 000) and poly(styrenesulfonate)sodium salt (PSS,Mw ) 70 000) were obtained from Aldrich andused as received. Tetrahydrofuran (THF) used to dissolve particleswas purchased from Sigma-Aldrich. Deionized (DI) water (>18.2MΩ‚cm, Barnstead, E-pure system, pH) was used in all solutionsand rinsing procedures.

MWNT Oxidation. Carbon nanotubes were oxidized by thefollowing procedure.8 MWNTs (16 mg) were sonicated for 2 h in80 mL of a 3:1 mixture of H2SO4/HNO3. Then the sample waswashed with a dilute NaOH aqueous solution and subsequentlywashed three times with water via centrifugation/redispersion cycles.Finally, the MWNTs were dispersed in water, and a stable dispersionof oxidized MWNTs with carboxylic groups was obtained, providinga negative surface charge.

Preparation of 3D MWNT Free-Standing Matrixes. Siliconsubstrates were cleaned in a solution of H2O2/H2SO4 (1:4 by volume)for 10 min and then rinsed in DI water and dried in a stream of air.Subsequently, a polystyrene solution (300 mg of latex in 4 mL ofchloroform) and a positive photoresist (AZ1518) were deposited ina spin-coating process on silicon substrates. Coating was performedat 2850 rpm and gave a 2.5µm film thickness. Additionally, substratescovered with AZ1518 were exposed to a UV lamp. The aim wasto facilitate the lift-off of the LBL assembly in formed layers.

The polystyrene particles monolayer was deposited on a substrateprepared in this way. Colloidal crystals were fabricated using ananosphere lithography technique described in detail elsewhere.9

Briefly, mixtures of particles in organic solvent (Ps/methanol, 1:1by volume) were slowly applied to the surface of water using a glasspipet. All suspensions were spread inside a Ø 15 cmPetri dish insuch amounts as to yield complete coverage of the surface by thecolloids. After a thickening process, which leads to the largest crystalformation, the monolayer was deposited on the substrate by slowwater evaporation. To better fix the Ps mask to the substrate and toavoid sample damage during multilayer film formation, the obtainedsamples were annealed at 175° for 60 s. Throughout the annealingprocedure, samples were cooled simultaneously by using a nitrogenstream.

The LBL assembly composites were prepared with an automaticdipping machine (Dipping Robot DR3, Kirstein GmbH, Germany).The silicon slides already covered with a monolayer of microparticleswere arranged vertically in a custom holder, which was first immersedin a polyelectrolyte solution (PDDA, 1 mg/mL, containing 0.5 MNaCl). The slides were subsequently rinsed successively in threedifferent beakers containing deionized water for 5, 2, and 1 min.They were then dipped into MWNT solution followed by the samerinsing procedure. After every fifth deposition cycle, a layer ofMWNTs was replaced with a layer of PSS (1 mg/mL, containing0.5 M NaCl). A deposition time of 10 min was used for thepolyelectrolyte, and 20 min was used for MWNTs. All obtainedMWNT/polyelectrolyte multilayer films had the structure [(PEI/MWNTs)5(PEI/PSS)]n. Depending on particle diameter, structurescontainingn ) 30 (for 1.71µm particles) with a total of 150 PEI/MWNTs bilayers andn ) 50 (for 7µm particles) with a total of 200PEI/MWNTs bilayers were used.

After depositing an appropriate number of layers, the films werepeeled off the silicon substrates, resulting in free-standing matrixes.To separate multilayers films from the underlying substrate, it wasnecessary to immerse the samples in tetrahydrofuran for 5 min. Thesamples were then washed in deionized water. The final step wasthe removal of the polystyrene through THF treatment. Even thoughTHF is a very powerful solvent, polystyrene debris can remainattached to the surface. To overcome this problem, the reactive ionetching (RIE) process was employed. The RIE was performed in anoxygen atmosphere with the addition of argon. The flow rate ratiowas controlled by changing the flow rate of each gas while the totalflow rate was held constant at 20 sccm. The total gas pressure was100 mTorr, and the input power was 80 W.

Nanomechanical tests were evaluated on the nanoscale using anatomic force microscope (AFM) (NanoScope IV Digital Instruments)with a conjugated TriboScope nanomechanical test instrument fromHysitron Inc. Diamond conical and Berkovich tips were employedin this study as indenters. The total included angle on the Berkovichtip is 142.3°, with a half angle of 65.35°. This makes the tip veryflat and efficient for a wide range of materials, including polymers.A conical tip with a nominal radius of curvature equal to 1µm wasused for scratching experiments because of nondirectional geometry.

Nanoindentation. The hardness (H) and elastic modulus (Er)were calculated from the recorded unloading step of the depth-displacement curve based on the method of Oliver and Pharr.10 Thetip calibration was carried out on poly(methyl methacrylate) (PMMA)with an elastic modulus equal to 3.6 GPa.11The calibration procedurewas repeated for three independent PMMA samples, and a highreproducibility was observed. The typical indentation test was carriedout with conical and Berkovich tips using a triangular force profilewith an indentation force ranging from 25 to 250µN and a loading/unloading rate of 30µN/s. In general, indents with a contact depthranging from 50 to 500 nm were performed for each sample. Tominimize the effect of the material creep, the 20 s hold time wasincorporated at the maximum load. Prior to the indentation, the tipwas used for surface scanning to find reasonably smooth areas andto avoid large roughness effects on the mechanical properties. Theindentation depth was maintained to less than 15% of the filmthickness to avoid and minimize the substrate contribution tomeasured nanomechanical properties. Our experiments have shownnegligible substrate effects up to contact depths of 20% of the entiresample thickness. Similar observations have been described by Pavooret al.12 The average roughness (Ra) of the samples was estimated tobe in the range of 15-20 nm, which is low enough to avoid theinfluence of the evaluation of the calculated values ofH andEr. Atleast eight indents were performed for each maximum applied loadthroughout the whole area of the sample but at a reasonable distancefrom the sample edges to avoid any boundary influence on themechanical properties of tested composites. The data from the indents,performed under the same maximum load, were averaged to obtainthe mean and standard deviations for all samples.13

Nanoscratch.In the nanoscratch experiment, we used a conicaldiamond tip. The coefficient of friction (µ) was measured as theratio of the lateral force to the normal force.11 In all tests, the scratchlength was set to 5µm. In the ramp force test, a maximum normalforce of 200µN was applied. The load was applied for 20 s, givinga scratching rate of 0.5µN/s. Each sample was tested repeatedly,and a plot of the coefficient of friction versus lateral displacementwas used to characterize the film properties. In the constant loadexperiment, the lateral force and normal displacement were obtainedby applying a constant normal force ranging from 15 to 100µN anda scratch displacement of 5µm. At least four scratches were conductedfor different normal force values. The average values of the coefficientof friction of four scratches at the same maximum constant normalload were used to estimate the friction behavior of the samples.

Nanomechanical tests were also performed on polymer filmsproduced by the layer-by-layer (LBL) assembly method withoppositely charged polyelectrolytes PSS and PEI (polystyrene-sulfonate and polyethyleneimine, respectively). The film contained150 bilayers (PSS/PEI) and was prepared in a manner similar to thatfor MWNT-based LBL scaffolds utilizing an automated dippingrobot. The thickness of the film was estimated to be 1.5µm usinga profilometer (Surface Profiler Kla-Tencor P-10). In this study, weanalyzed and compared nanomechanical properties of PEI/MWNTsmultilayer composites and PSS/PEI films.

The surface of the microstructure films was characterized byscanning electron microscopy (SEM, Supra 55, Bonn, Germany)

(8) Yu, R.; Chen, L.; Liu, Q.; Lin, J.; Tan, K.-L.; Ng, S. C.; Chan, H. S. C.;Xu, G.-Q.; Hor, T. S. A.Chem. Mater. 1998, 10, 718.

(9) Kosiorek, A.; Kandulski, W.; Chudzinski, P.; Kempa, K.; Giersig, M.Nano Lett. 2004, 4, 1359.

(10) Oliver, W. C.; Pharr, G. M.J. Mater. Res. 1992, 7, 1564.(11) Rau, K.; Singh, R.; Goldberg, E.Mater. Res. InnoVations2002, 5, 151-

161.(12) Pavoor, P. V.; Bellare, A.; Strom, A.; Yang, D.; Cohen, R. E.

Macromolecules2004, 37, 4865-4871,(13) Olek, M.; Kempa, K.; Jurga, S.; Giersig, M.Langmuir2005, 21, 3146-

3152.

5428 Langmuir, Vol. 22, No. 12, 2006 Firkowska et al.

with an Oxford exL X-ray system and a cryostage operating at anacceleration voltage of 20 kV and a field depth of 8.5.

Results and Discussion

The method used throughout this work is based on the self-organization of colloidal particles. This method affords theproduction of a hexagonally packed (hpc) monolayer of latexspheres. For instance, to fabricate 1.5× 5 cm2 samples withwell-ordered 2D periodic areas of latex spheres, we used aprocedure published recently.9 Figure 1 shows SEM images ofthe ordered crystals with sphere diameters of 1.71 and 7µm.With the obtained masks, it is possible to use them as templatesfor the assembly of MWNTs.

The growth of the PEI/MWNT multilayer film was examinedusing SEM microscopy. The image (Figure 2) shows that afterthe first layer deposition MWNTs follow the morphology of thesphere monolayer, thereby maintaining the 2D ordered structure.7

As a consequence, the deposition process leads to the formationof a layer with well-defined nanotopography. This layer possessesa sufficient depth to wrap the entire surface around the spheres.In addition, only a few of the carbon nanotubes provide bridgeconnections between neighboring spheres. As confirmed by SEMimages (Figure 2c), MWNT bridge connections increase withgrowth in PEI/MWNT multilayers. After the tenth CNT layer,the gaps between the spheres are fully covered.

The LBL-assembled MWNTs films were lifted off from thesubstrate through chemical delamination.14However, in the case

of silicon substrates lacking a polystyrene or photoresist layer,separation of the LBL film from the substrate was not possible.In this case, the multilayer composite could be peeled off withthe aid of adhesive tape only. Figure 3A shows an SEM imageof the nanostructure of the MWNT-based film after dissolutionof the Ps mask and having been attached to the tape. The resultingnanostructure corresponds to an inverted replica of a colloidalcrystal, which has the same degree of order as the original templatewith perfectly spherical and smooth cavities. The diameter of thecavities correlates with the diameter of the starting colloids; thevolume, however, corresponds to the half-particle volume (Figure3B). An AFM image and depth profile of a cavity are shown inFigure 3B.

The presence of a polyelectrolyte pattern (Figure 4) on thesilicon substrate highlights the importance of using the precursor(14) Mamedov, A. A.; Kotov, N. A.Langmuir2000, 16, 5530.

Figure 1. SEM images of a patterned colloidal crystal monolayerassembled using (A) 1.7 and (B) 7µm PS spheres.

Figure 2. SEM images of polystyrene particles coated with (A) 1,(B) 4, and (C) 10 layers of carbon nanotubes. All images correspondto the same sample.

Highly Ordered MWNT-Based Matrixes Langmuir, Vol. 22, No. 12, 20065429

layer as a convenient way to obtain free-standing films. However,it verifies that the deposition of CNTs takes place only on theupper surface, without infiltration into the interspaces. It is clearthat during PEI/MWNT deposition only polyelectrolytes permeateinside the interstices and bind the LBL film with the siliconsubstrate. To facilitate the lift-off of the LBL film, the substrateswere supported with a precursor thin layer (Experimental Section).This spin-coated layer isolates the Ps mask from the siliconsubstrate and thereby precludes the deposition of polyelectrolyteson the mentioned substrate (Figure 5A).

It was possible to pick up the free-standing films peeled offthrough chemical delamination from the suspended state withtweezers and subsequently to dry, transfer, and finally cut it intopieces of the desired size or wrap it around a solid substrate(Figure 5B). The thickness of the free-standing films wasdetermined from SEM images and was found to be 3.00µm for

n ) 30 (1.7µm Ps-latex mask) and 1.5µm for n ) 50 (7µmPs-latex mask).

Figure 6 shows the influence of the precursor layers on thetopography of the film side, which was attached to the template.A photoresist layer spin-coated onto the silicon substratecontributed to the creation of ordered rings or so-called“nanodonuts” (Figure 6A). One explanation is that thesenanodonuts occur as a direct result of the annealing process,which causes a mobilization of particles from the infrastructureinto the photoresist layer. Chemical delamination, using tet-ramethylammonium hydroxide (developer AZ 726), results inphotoresist film dissolution and the separation of the LBLcomposite from the silicon substrate. After the AZ 726 treatment,Ps particles are still covered with a thin layer of photoresist filmwith the exception of a small part that protruded out of thephotoresist layer. This part is subsequently exposed to the actionof THF. As etching proceeds, it moves deeper into the particlesand finally reaches the bottom. The inset in Figure 6A shows thatonce this process is completed an array of circular orifices isetched into all particles, which in turn are surrounded by a thinshell of the photoresist film. A second noteworthy feature of thisstructure is the fact that the holes at the top of the photoresistshell have the same diameter.

In the case of the polystyrene layer, the resulting nanostructurediffers from the nanodonut structure described above. As shownin Figure 6B, THF etching completely removed both the latexparticles and the polymer precursor layer but left a very thinpolyelectrolyte membrane, which was created by the infiltrationof the polyelectrolytes.

Figure 3. (A) Scanning electron micrograph of MWNT multilayerswith an inverted replica of the PS-latex mask. (B) AFM image anddepth profile of a cavity.

Figure 4. SEM image of the polyelectrolyte pattern formed on thesilicon substrate.

Figure 5. (A) SEM image of a PS mask deposited on a precursorpolystyrene layer and covered with a PEI/MWNT film. (B) Free-standing MWNT-based film after dissolution of the PS mask andwrapped around a glass wire.


Despite THF treatment, the removal of the polyelectrolytemembrane was not possible. To dislodge residual material fromthe surface, the reactive ion etching process (RIE) was used.During RIE, gas species from the plasma react with the surfaceatoms, forming compounds or molecules that then leave thesurface thermally or as a result of ion bombardment. For this partof the work, we use a mixture of oxygen and argon with the totalflow rate held constant at 20 sccm. The etch rates were determinedby controlling the structure changes in the film after 40 s ofetching. Scanning electron micrographs during various stages ofO2 and O2 + Ar are shown in Figure 7. For the O2 + 75% Arreactive ion etched surface, the initially porous membranedisappeared, leaving polystyrene residue on the edges of thecavities. Progressively decreasing the argon flow rate and therebyincreasing the oxygen atom concentrations lead to efficientpolymer etching. As shown in Figure 7D, optimal etch conditionsfor which a residue-free nanostructure was obtained were foundfor O2 flow rates equal to 20 sccm. Moreover, as the same imagerevealed, the RIE process induces morphological changesaffecting the cavity surfaces. The originally smooth surface (insetsin Figure 7B) disappeared and was replaced with a completelyrough one with the carbon nanotubes sticking out at random. Adetailed examination of these exposed MWNTs indicates thatoxygen plasma removed not only polymer residue but also thesuperficial polyelectrolyte layer. Figure 7D also indicates thatthe MWNTs can withstand the etching process, which leavesthem practically undamaged.15,16 Because the effect of plasmatreatment on the MWNTs has been dealt with on a number of

occasions, it is well known that the mentioned process resultsin the formation of a high-polarity carbon nanotube surface17

and does not reduce the nanotube tensile strength.18 The latterinformation is quite important to the work that we are doing.Thus, we can assume that oxygen etching under the experimentalconditions employed results in no change in the mechanicalproperties of the MWNT/PEI composite.

The LBL assembly process facilitates the assembly of carbonnanotubes onto spherical ordered particles of various sizes fromthe nanometer to the micrometer range. It affords the creationof free-standing matrixes with a wide variety of cavity structures.Figure 8 shows an MWNT-based film with a cavity diameter ofaround 7µm. The final structure after the RIE process, performedunder the same conditions as described in Figure 7, is depicted.The MWNT-based matrixes prepared on templates provided withdifferent test particle diameters are almost identical in nano-topography, with the exception of one remarkable structuralcharacteristic. As shown in Figure 8, cavities prepared with 7µm particles are surrounded by pillars as a consequence ofpolyelectrolytes that infiltrate the interstices of the template. Thethickness and length of these pillars are dependent on the sizeof the gap between particles and therefore can be controlled bychanging the diameter of the spheres.

A specific scaffold structure that forms the basis for furtherresearch and development in tissue engineering was selectedbecause of its special features and ability to steer a cellular system,including optimal cavity size and cavity interconnection (Figure9). For this purpose, the mechanical properties of the MWNT-based matrixes as cell scaffolds were characterized. Themechanical properties were tested on the TriboScope instrument(Experimental Section). Nanoindentation experiments wereconducted for all samples under the same fixed experimentalconditions using Berkovich and conical diamond tips. The typicalload-displacement curves are presented in Figure 10. The imageshows two different mechanical responses of the PSS/PEImultilayer film (Figure 10A) and the MWNT-based LBLcomposite (Figure 10B). The absence of steps and discontinuitieson the curves indicates that no cracks or fractures occurred duringindentation.19

These plots clearly demonstrate the softer nature of the PEI/MWNTcompositeand the resulting inferiormechanical propertiesof this sample. At the same maximum indentation force (140µN), the indenter performs a deeper penetration into the MWNT-based material than into the polyelectrolyte structures (300 and120 nm, respectively). This suggests that the presence of carbonnanotubes in a polymer matrix does not improve the mechanicalperformance of the matrix. This indication is confirmed by acomparison of the hardness and the elastic modulus of bothsamples obtained from the depth-sensing nanoindentation tests(Figures11 and 12). Figures 11 and 12 show the elastic modulusand the hardness as a function of contact depth. Several indentswere performed under the same maximum force, and the datawere then averaged. Error bars represent the standard deviationin the calculated values ofH andEr. The indents were carriedout at contact depths ranging from 50 to 450 nm. The resultsobtained using Berkovich and conical tips are consistent foreach sample. Figure 10 reveals that the elastic modulus is relativelyindependent of indent depth for all polymeric composites.However, the hardness exhibits some decreasing trend for small

(15) Schmidt, M. S.; Nielsen, T.; Madsen, D. N.; Kristensen, A.; Bøggild, P.Nanotechnology 2005, 16, 750-753.

(16) El-Aguizy, T. A.; Jeong, J.-h.; Jeon, Y.-B.Appl. Phys. Lett. 2004, 85,5995.

(17) Bubert, H.; Haiber, S.; Brandl, W.; Marginean, G.; Heintze, M.; Bruser,V. Diamond Relat. Mater. 2003, 12, 811.

(18) Pittman, C. U., Jr.; Jiang, W.; He, G.-R.; Gardner, S. D.Carbon1987,25, 36.

(19) Li, X.; Gao, H.; Scrivens, W. A.; Fei, D.; Xu, X.; Sutton, M. A.; Reynolds,A. P.; Myrick, M. L. Nanotechnology2004, 15, 1416.

Figure 6. Scanning electron micrographs of free-standing filmswith (A) nanodonut structure and (B) porous membrane structure.


loads and then smoothly attains a plateau. This behavior ofHandEr as a function of contact depth is consistent with our previousobservation.13Because nanoindentation tests were carried out inat least seven independent areas of the sample, the relativelysmall standard deviation of the data points indicates the highhomogeneity of the composites. The average Young’s modulusof PSS/PEI film is around 8 times higher than that obtained forthe PEI/MWNT assembly (3.9( 0.3 and 0.49( 0.15 GPa,respectively). The average hardness of the PSS/PEI compositeis also about 8 times higher than the hardness of MWNT-basedscaffolds (0.096( 0.005 and 0.015( 0.006 GPa, respectively).These results clearly demonstrate that the presence of carbon

nanotubes strongly affects the nanomechanical properties of com-posites by reducing the mechanical performance of the testedmaterial. These observations are consistent with the previouslyreported study on the nanoindentation of LBL multilayers fromMWNTs and polyallylamine hydrochloride (PAH).20 Pavoor etal. showed that assembled PAH/MWNT composites are signi-ficantly softer than LBL films based on polyelectrolytes (PAH/poly(acrylic acid)). Those results confirm that a high concentrationand a homogeneous distribution of CNTs within a polymer matrix

(20) Pavoor, P.; Gearing, B. P.; Gorga, R. E.; Bellare, A.; Cohen, R. E.J. Appl.Polym. Sci. 2004, 92, 439,

Figure 7. SEM images showing the morphological changes on the film surface after (A) O2 + 75% Ar, (B) O2 + 50% Ar, (C) O2 + 25%Ar, and (D) the O2 reactive ion etching process. The scale bar is 200 nm.

Figure 8. SEM image depicting the nanostructure of a free-standingmatrix with a cavity diameter of about 7µm. The scale bar is 1µm.

Figure 9. SEM image of a final CNT-based matrix obtained afterthe RIE process.


as well as strong adhesion between the structural componentsare insufficient to provide composite reinforcement (in terms ofthe hardness and Young’s modulus obtained from nanoindentationexperiments). (It was shown that LBL assembly composites fromMWNTs exhibit extraordinarily high tensile strength that is ashigh as that observed for ceramics.6) It is suggested that the flex-ibility of carbon nanotubes and their curved morphology mayreduce the reinforcement action. Even strong interconnectivitybetween the CNTs and the host polymer does not lead to a sig-nificant increase in the nanomechanical reinforcement under theindentation load. The indenter can easily displace carbon nano-tubes because of their flexibility and inferior bending proper-ties.13,21

As mentioned above, a conical diamond tip was used fornanoscratch experiments. The coefficient of friction was cal-culated for all samples using data from the ramp force and constantforce tests. Several scratches were made in different areas of thesamples. Data were then averaged, and the coefficient of frictionwas calculated as the ratio of lateral force to normal force. Theramp force and constant force tests resulted in identical valuesfor each investigated sample. In Figure 13, the average valuesof µ (coefficient of friction) are presented as a function of scratchlength. There are no sudden changes in the coefficient of frictionindicating that any cracking or failure of the films occurred.22

The PEI/MWNT film (Figure 13B) displays a friction coefficientthat is significantly higher than that of the PSS/PEI composite(Figure 13A). The average value ofµ of the PEI/MWNT assemblyagainst the diamond conical tip is calculated to be 0.66( 0.06,and the average coefficient of friction of the PSS/PEI film is 0.33( 0.06. These data display considerable adhesion and frictionfor PEI/MWNT films. We assume that the increased value of thecoefficient of friction of the MWNT-based composite is a resultof high composite homogeneity and strong interconnectivitybetween the polymer matrix and carbon nanotubes. Moreover,the LBL method ensures a high concentration of carbon nanotubes,thus they may form a well-tangled network that may play animportant role as a resistant against the diamond tip during scratchexperiments.

For PSS/PEI films, theµ data show variations as a functionof the lateral displacement. This fluctuation in friction coefficientvalues may be promoted by the nanoscale topography, surfaceroughness, and structural inhomogeneities as well as by the layeredstructure of LBL films.23,24The lateral force as a function of thescratch length is shown in Figure 13C and D. Apparently, thelateral force increases linearly with the scratch position for theramp force test (Figure 13C). In the constant force experiment,the lateral force remains relatively constant during the scratchand independent of the scratch position (Figure 13D). Figure 14shows results for the coefficient of friction versus the maximumnormal load. The results demonstrate thatµ is relativelyindependent of the normal force. This indicates that plasticshearing (plowing) is the dominant deformation process.25,26

(21) Wong, E. W.; Sheehan, P. E.Science1971, 277.(22) Wei, G.; Barnard, J. A.J. Appl. Phys.2002, 91, 7565.(23) Charitidis, C.; Logothetidis, S.; Gioti, M.Surface of Coatings and

Technology2000, 125, 201-206.(24) Jyh-Wei Lee; Jenq-Gong Duh.Surf. Coat. Technol.2004, 188-189, 655.(25) Lu, W.; Komvopoulos, K.J. Tribol. 2001, 123, 641.(26) Lu, W.; Komvopoulos, K.J. Tribol. 2001, 123, 717.

Figure 10. Load-displacement data for (A) the PSS/PEI films and(B) the PEI/MWNT composite. The maximum indent load in bothcases was set to 140µN.

Figure 11. Elastic modulus data for two different polymericcomposites at different maximum contact depths, probed withBerkovich and conical diamond tips.

Figure 12. Hardness data for two different polymeric compositesat different maximum contact depths, probed with Berkovich andconical diamond tips.

Figure 13. Coefficient of friction and lateral load as a function oflateral displacement. (A) Average coefficient of friction of the PSS/PEI film, (B) average coefficient of friction of the PEI/MWNTcomposite, (C) typical lateral force behavior in the ramp forceexperiment, and (D) typical lateral force behavior in the constantforce test.


Conclusions

In this article, attention has been focused on the preparationand properties of CNT-based matrixes for potential use in tissueengineering applications. We have shown that the combinationof NSL and LBL techniques enables the assembly of CNTs intohighly ordered matrixes with precise control of the cavity size,

geometry, and distribution. Moreover, the architecture of thematrixes, characterized by well-defined nanotopography, couldbe modified by the precursor layer and by controlling plasmaetching.

Nanomechanical tests revealed that because of the low bendingstrength of CNTs, matrixes based on this material exhibit a softernature than the polyelectrolyte LBL structure. Additionally, dataobtained from the nanoscratch test showed that carbon nanotube-based composites display considerable adhesion and friction,which indicates a strong interconnectivity and adhesion betweenCNTs and polymers as well as the resistive role of tangled carbonnanotubes against the tip.

Our future work will be focused on the biomechanicalcharacterization of free-standing matrixes as cell scaffolds interms of their architecture and surface chemistry. Presently,MWNT-based matrixes are being tested with osteoblast cellculturing to evaluate their biocompatibility and in vitro structuralstability.

Acknowledgment. We thank NanoLab. Inc (www.nano-lab.com) for kindly supplying the MWNTs.

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Figure 14. Coefficient of friction of the PEI/MWNT film versusthe normal load for a conical diamond tip.


highly ordered mwnt-based matrixes: topography at the nanoscale conceived for tissue engineering

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