Carbon Nanotubes

The Challenges of the First Syntheses Trials

C. A. Coelho

*, A. T. Sepúlveda

, L. A. Rocha

and A. F. Silva

School of Engineering, University of Minho, Guimarães, Portugal

Institute for Polymers and Nanocomposites/I3N, University of Minho, Guimarães, Portugal

Department of Industrial Electronics, School of Engineering, University of Minho, Guimarães, Portugal

MIT Portugal Program, School of Engineering, University of Minho, Guimarães, Portugal

Keywords: Carbon Nanotubes (CNTs), Biomedical Applications.

Abstract: Carbon nanotubes (CNTs), due to their unique properties, are suitable for application in biomedical devices.

However these devices are not available in the market because of problems associated with biocompatibility

and synthesis reproducibility. Indeed, the production of vertically aligned-carbon nanotubes (VA-CNTs) is

needed for most of these applications. The most common synthesis method, Chemical Vapor Deposition

(CVD), involves a large number of parameters, not all known, and their influence on the process is not fully

understood. Knowing that the synthesis of CNTs is performed in a variety of lab conditions and in different

furnace systems, which makes it impossible to create a universal recipe for use in all labs, this paper aims to

start the design of an universal protocol that all labs can use to generate a suitable recipe for their

environmental conditions and furnace system. The influence of parameters such as best deposition spot and

tube baking was assessed. During this evaluation, the importance of other factors (catalyst uniformity and

climacteric conditions) to the process has been revealed. The universal protocol suggested is in an early

stage and needs to be improved.

1 INTRODUCTION

Nowadays, carbon nanotubes (CNTs) are one of the

most promising and exciting materials in

nanomaterials research field, own to their unique

electrical, mechanical and chemical properties.

These properties make CNTs suitable for many

biomedical applications (Raffa et al., 2011).

Indeed, CNTs found application in scaffolds for

tissue engineering (Tran et al., 2009), namely for

bone (Newman et al., 2013) and nervous tissue

repair and regeneration (Fabbro et al., 2013). CNTs

can also be used as drug and gene delivery devices

(Tran et al., 2009).

There are several reports about electrochemistry

biosensors for medical purposes that include CNTs

on its electrodes (Jacobs et al., 2010). Biofuel cells

work similarly to the enzymatic biosensors, so

naturally CNTs were included in these devices

(Holzinger et al., 2012). Others applications reported

are neuronal electrode interfacing (Bareket-Keren

and Hanein, 2012), microfluidic devices (Chen et al.,

2012), carbon nanotubes-based X-rays devices

(Calderón-Colón et al., 2009) and flexible pressure

sensors (Sepúlveda et al., 2011).

The list of applications of CNTs on the field of

medical devices is vast but despite this growing

research, CNTs still suffer from two main hurdles:

i) the biocompatibility and ii) the synthesis

reproducibility (the focus of this paper).

The majority of the listed devices require the

incorporation of vertically aligned-carbon nanotubes

(VA-CNTs). The most common synthesis process

for the production of CNTs with this arrangement is

Chemical Vapor Deposition (CVD). This process

involves a large number of parameters and some of

them may be still unknown. The ways that these

parameters influence each other and the process are

not completely understood (Oliver et al., 2013).

Furthermore, CVD process is many times

performed in non-cleaning environments so the

ambient conditions of each lab influences the

synthesis as so the furnace system (Oliver et al.,

2013). There are many different furnace systems and

their geometry and function affect the process. So,

these variations between labs prevent the

Coelho C., Sepúlveda A., Rocha L. and Silva A..

Carbon Nanotubes - The Challenges of the First Syntheses Trials.

DOI: 10.5220/0005201000950102

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2015), pages 95-102

ISBN: 978-989-758-071-0

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

reproducibility of results.

Because of the differences in ambient conditions

and in furnace systems between labs, it is not

possible to make a general recipe that can be used in

all labs, achieving the same results. Instead, one

needs to create a universal protocol that can enable

each lab to evaluate the influence of the parameters

in its own environment, and create a recipe

appropriate for its system.

This paper pretends to give an initial insight on

how this protocol can be performed. For that the

influence of the best deposition spot and tube baking

were evaluated. During the evaluation of these

parameters, others factors revealed to be important

for the process such as the catalyst uniformity and

the climacteric conditions.

2 CNT SYNTHESIS

There are several techniques to grow VA-CNTs, but

the most common one among the different research

groups is based on a modified CVD system. In a

broad sense, in this technique, a carbon precursor in

its gaseous form flows inside a tube at elevated

temperature and enters in contact with a catalyst

(Figure 1).

Figure 1: Furnace CVD system. Adapted from Hart, 2007.

Catalysts are used with two purposes: have an

on-site pyrolysis of the precursor and support the

CNTs during and after growth. The most important

properties when selecting a material for catalyst are

its ability to dissolve carbon at elevated

temperatures, the carbon diffusion rate, the melting

point and the equilibrium vapor pressure (Jourdain

and Bichara, 2013). Among the most common

catalysts, one can find Fe, Ni and Co, which are

transition metals that reveal high solubility for

carbon (Hart, 2007).

A thin-film of catalyst is deposited on a substrate

where one desires to grow the CNTs. The most

common one is silicon due to its compatibility with

the catalysts and due to the melting point higher than

the process temperature.

The CVD synthesis process requires the use of

three gases: the carbon precursor, a reducing gas and

an inert one.

The precursor provides the carbon for the CNTs

formation. This gas can be an hydrocarbon (Wang et

al., 2014), an alcohol (Chen et al., 2014), an

aromatic compound (Atiyah et al., 2011) or a natural

carbon source (Qiao-juan et al., 2013).

The reducing gas, as the name implies, reduces

the oxide that forms when the catalyst is not well

stored. Also, it helps the formation of the catalyst

nanoparticles and it keeps the catalytic sites active

during all the growth process (Figure 2).

The inert gas is used during the process to

control the pressure inside the tube if needed, and as

a cleaning gas at the beginning and at the end of the

synthesis process. Furthermore, it helps heat transfer

during heating or cooling stages.

Figure 2: Formation of the catalyst nanoparticles by the

action of hydrogen. Adapted from Hart 2007.

The CNT synthesis process can be divided into

five stages (Figure 3): the cleaning, the dewetting,

the growth, the delamination and the cooling.

The first stage consists on the cleaning of the

lines, with the respective circulating gas, and of the

tube with an inert gas.

The second one involves heating the tube to

operating temperature and the catalyst reduction. In

this stage there is also the formation of the catalyst

nanoparticles, the dewetting step (Figure 2). The size

of these nanoparticles is very important, because it

will determine the diameter, the number of walls and

the alignment of the CNTs (Jourdain and Bichara,

2013; Hart, 2007). The size can be controlled

through the control of the temperature, flow and

exposure time to the reducing gas. After this stage,

the flow of the reducing gas is kept steady to

maintain the catalytic sites active during the next

step.

The third stage is the growth itself. The carbon

precursor starts to flow in the tube and its

degradation occurs by temperature and catalyst

actions. The carbon resulting of these actions is

adsorbed in the catalyst nanoparticles and

precipitates when the particle is saturated, beginning

the growth (Hart, 2007). The growth stops when the

particle is encapsulated by amorphous carbon and

BIODEVICES2015-InternationalConferenceonBiomedicalElectronicsandDevices

Figure 3: Steps of growth process.

other carbon compounds that form during this

process (Brukh and Mitra, 2006).

The previous stages are the most important for

the overall process. There can be an optional step,

the delamination, which eases the removal of CNTs

from the substrate.

The last stage is the cooling. Only the inert gas

flows inside the tube to assist the heat transfer and to

clean the gases and the wastes that remain in the

tube. The CNTs can only be removed from the tube

when the temperature cools down.

The equipment used for CNTs growth was a

furnace Nabertherm, model RS 80/750/11, which is

connected to three valves MKS Instruments, model

RS-485, that control the gases flow. This setup is

connected to a computer which controls the heating

and cooling of the furnace as well as the flows.

The quartz tube used in this study had 25 mm of

internal diameter and a length of 1000 mm. The

selected gases were ethylene as the precursor gas,

hydrogen as the reducing gas and helium.

Thin-films of alumina and iron, 10 and 1 nm

thick respectively, were deposited via e-beam

technique on top of the Si wafer as catalysts.

3 PROCESS PARAMETERS

From the previous section, one easily identifies a

number of parameters that can be tuned to set the

CNTs synthesis. Figure 3 illustrates a standard

recipe for CNT growth via CVD technique.

3.1 Gases Flow/Time

Ethylene, hydrogen and helium are the gases used in

this study.

Helium is the inert gas with the main role of

cleaning and assist the heat transfer inside the tube.

The ethylene is the carbon precursor. Its flow

inside the tube is a key parameter. When selecting

the flow, it must be taken into account that lower

flows mean higher residence time and more

probability of the reactions needed to form CNTs to

occur, than if we had higher flows. However, at the

other hand, it decreases the refresh rate of the gas

inside the tube (Brukh and Mitra, 2006). Another

aspect is the heights of CNTs forests increase with

the growth time but not in a linear fashion. Indeed,

from a certain time, forests are not higher because

the catalyst nanoparticles are encapsulated in

amorphous carbon, preventing them from degrading

more ethylene and adsorb more carbon (Kumar and

Ando, 2010).

The time and the flow of hydrogen influence the

size of catalyst nanoparticles, which in turn

influence the diameter, the number of walls and the

alignment of CNTs (Jourdain and Bichara, 2013;

Hart, 2007). In the growth step, this gas is essential

for keeping the catalytic sites active.

3.2 Temperature

Temperature also affects the process as it is involved

in the reduction process of the oxide and in the

ethylene’s pyrolysis.

It is important to understand the temperature

profile inside the tube and to have it correlated to the

length of the tube. In order to minimize the variation

of the temperature inside the oven towards the tube

endings, the furnace has isolation sleeves at the tube

endings that not only ensure the tube centering

CarbonNanotubes-TheChallengesoftheFirstSynthesesTrials

inside the oven cavity but also minimize the

temperature loss towards the exterior.

Because the synthesis process is based on

chemical reactions, one should not forget that there

are thermal and chemical inertia associated with the

reactions.

3.3 Silicon Pieces

The positioning of the pieces inside the tube also

affects the growth process. The ethylene’s

degradation occurs driven by temperature and

catalyst actions. There is thermal and chemical

inertia, so the initial part of the tube is not the best

place for the positioning of the pieces. As the gases

enter in the tube at room temperature, it is necessary

to consider the time required for the gas to achieve

the required temperature. This requires time, which

in a continuous flow, forces the pieces to be

positioned towards the tube’s downstream side.

However, as the ethylene travels along the tube

undergoes air pyrolysis driven by temperature. The

resulting compounds can be adsorbed on the catalyst

nanoparticles to form CNTs or may continue to

travel along the tube. If the latter is verified, they

begin to recombine into more complex molecules

that can contribute to the encapsulation of the

catalyst nanoparticles, and consequently to the end

of growth (Brukh and Mitra, 2006). This suggests

that the closer the end of the tube pieces are placed,

the lower the height of the CNT forests obtained.

The identification of the growth “sweet spot” in

the tube is key to ensure a reproducible and

consistent synthesis process.

3.4 Other

Tube baking is another parameter that affects the

growth, but its influence is not well established.

After a growth, there are a few residues that still

remain in the tube. If they are left in the tube they

may influence the next growth session. To minimize

the impact of the debris, a tube baking is performed.

To do so, the tube, while open, is raised up to 750 ºC

for 10 minutes. This process burns all the residues

inside the tube, leaving it cleaned. This baking

process can be seen as a strategy to reset the tube.

Figure 4 shows the accumulation of debris after

three growth sessions, as an example.

4 APPROACH/STRATEGY

This paper reports the study to identify the best

positioning inside the tube for CNTs’ synthesis and

the influence of tube baking during the process.

These two parameters are standard to anyone who

begins to synthesize CNTs in a laboratory.

For the realization of such study, a test protocol

was established. In each working day, one would

perform three growths and do a reset (bake) at the

end of the day.

Figure 4: Aspect of the tube a) after the bake, b) after three

growth sessions.

In each session, two pieces would be placed

inside the tube. These pieces were cleaned with

alcohol, acetone and nitrogen prior to their

placement. The silicon pieces had all the same

dimensions (10 mm x 10 mm) and were positioned

with a distance of 10 mm between them. These

pieces provide sufficient catalyst volume to trigger

the local pyrolysis.

In order to start the study, it was necessary to set

a starting recipe for the CNTs growth. The starting

recipe consisted in a dewetting time of 5 min, with a

flow of hydrogen of 200 sccm. At the same time, the

tube was heated until its temperature reached

750 ºC. The growth duration was of 12 min and the

ethylene’s flow in this stage was of 150 sccm. In the

beginning of the recipe, the tube was cleaned with

helium, which was also used at the end while the

tube was cooling down. This recipe was based on

literature review compilation and comparison (Stein

and Wardle, 2013; Wardle et al., 2008; Garcia et al.,

2008).

The height of the CNTs forests produced was

measured using a magnifying glass of the brand

LEICA, model M-80.

For the analysis of morphology of some samples

and their chemical characterization, it was

performed SEM analysis by a microscope Nova

NanoSEM 200, FEI Company. This microscope has

integrated an Energy Dispersive Spectrometer

(EDS), which allows the chemical characterization

of the samples by the analysis of its X-ray spectrum.

BIODEVICES2015-InternationalConferenceonBiomedicalElectronicsandDevices

5 RESULTS AND DISCUSSION

The first parameter to be evaluated was position.

Knowing that there is a certain degree of thermal

and chemical inertia as said before, the pieces were

positioned from the center of the tube (position 0

mm) towards the downstream in the direction of

gases flow. The stopping criterion was the

identification of a deflection point on the height’s

trends along the tube.

The main objective of the evaluation of this

parameter was to identify the location within the

tube where the CNTs obtain the greatest height and

where the results show less variation.

Figure 5 shows the heights of the produced

CNTs forests as a function of pieces position inside

the tube, using the starting recipe.

Figure 5: Height of the obtained CNTs forests in function

of pairs of position of the pieces inside the tube.

Although Figure 5 shows some trend, one can

argue that the variation of heights among the

samples is large for the height average (Table 1).

This non consistency growth led to the thought that

it was not related to the growth parameters but

instead to the catalyst deposition.

Table 1: Pairs samples’ values for average and standard

deviation.

Position

Height Average

(µm)

Standard

Deviation

Standard

Deviation (%)

0-2 410 134.9 32.9

2-4 336 199.0 59.2

4-6 245 131.4 53.6

6-8 346 188.2 54.4

8-10 345 115.6 33.5

10-12 399 103.6 26.0

12-14 168 129.8 77.3

14-16 286 104.0 36.4

16-18 124 66.8 53.9

In order to sort the above issue, a new wafer with

new catalysts films was requested from other

supplier (INESC MN) that had fine control of the

deposition process.

With the new samples, the growths were

repeated for the positions 8-10 and 10-12 (Figure 6),

the ones that were considered from the first set of

growths to be the most relevant.

Figure 6: Height of the obtained CNTs in function of pairs

of position of the pieces inside the tube for the case of the

revised catalysts deposition.

As one can observe, the standard deviation was

drastically reduced (Table 2), which confirms that

the problem verified with the previous study was

caused by the non-uniformity of catalyst deposition.

Since both pairs of position have low standard

deviations and considering that the pair 10-12 has

the highest height average, this pair was chosen has

the best deposition spot for the current system.

Table 2: Pairs samples’ values for average and standard

deviation for the case of the revised catalysts deposition.

Position

Height Average

(µm)

Standard

Deviation

Standard

Deviation (%)

8-10 443 26.3 5.9

10-12 454 43.1 9.5

Probably after position 12, phenomena of

formation of complex molecules are occurring,

consuming the precursor available and leading to the

encapsulation of the catalyst nanoparticles, and

consequently, to the reduction of the height of

forests as seen in the Figure 5 graphic.

Since its entrance on the tube, the ethylene is

suffering aerial pyrolysis. The resulting products

(carbon and/or carbon compounds) can be adsorbed

in the catalyst nanoparticles as well as ethylene

which did not react. These compounds will undergo

CarbonNanotubes-TheChallengesoftheFirstSynthesesTrials

catalyst pyrolysis. The results suggest that the

contribution of the two types of pyrolysis is

maximum in the pair of positions 10-12, reaching in

this place the forests with higher heights.

Next, the influence of the tube baking in the

forests’ height was tested. Figure 7 shows the

relative heights of three growth sessions realized

during three days. Note that the higher heights

obtained compared to the previous runs were due to

the increase of the ethylene time to 25 minutes. This

increase was performed in order to obtain tall forests

for the analysis.

Figure 7: Height (in percentage in respect to the first

growth of the first day) of CNTs in the 3 sessions

performed during 3 days.

The first session of each day presents always the

worst results. After this growth, the forests reach

greater heights. These results suggest that the first

session after cleaning the tube leaves some residues,

which are essential for the next growths.

In order to further evaluate this, it was tested

how long the tube could go in terms of growth

sessions without performing the baking (Figure 8).

In the graphic of Figure 8 it is possible to see that

after the fifth growth the heights of CNTs begin to

decrease. Seeing that there is residues accumulation

in the tube since the first growth, after 125 min of

ethylene (cumulative time from all sessions), the

amount of residues inside the tube becomes

noticeable and starts to deposit on the top of the

forests, preventing the precursor from reaching the

catalyst nanoparticles.

Observing the SEM images (Figure 10a and

Figure 10b), this color change seems to be caused

not only by the deposition of carbon with a different

structural arrangement, but also by the presence of

other substances. The analysis of X-ray spectra in

the regions indicated in Figure 10(c) and (d) was

performed to check if there are other substances

present at the top of the forests. The results are

shown in the Table 3.

Figure 8: Height of CNTs, in percentage in respect to the

first growth, during 10 sessions without cleaning the tube.

The deposition of debris was confirmed by

observing a color change in the forests for a gray

tone (Figure 9) and by the analysis of the X-ray

spectrum held by SEM (Figure 10 and Table 3).

Figure 9: a) Piece with normal color, b) piece with gray

tones.

Table 3: Results of the chemistry analysis performed on

the regions Z1, Z2 e Z3 by the determination of the

respective X-ray spectrum.

Region

Mass percentage (%)

Carbon Oxygen

Z1 97.60 2.40

Z2 97.75 2.25

Z3 92.10 7.90

It was found that, in addition to carbon, there is

also oxygen. This oxygen may result from the

reduction step where the iron oxide is reduced to

iron. In this step occurs the formation of hydroxides,

which eventually settle on the top of the forests.

Despite their different aspect, the regions Z1 and Z2

have similar chemical compositions, suggesting that

the morphological differences between these two

BIODEVICES2015-InternationalConferenceonBiomedicalElectronicsandDevices

100

Figure 10: SEM images of the top view of CNTs forests after: (a) 5 sessions; (b) 7 sessions without performing tube baking.

c) Regions Z1 e Z2 on the top of the forests where were performed X-ray spectrometry analysis. d) Region Z3 where was

performed X-ray spectrometry analysis. e) and f) Lateral view of CNTs. It is possible to observe the: (e) the alignment; (f)

the waving.

regions are due to carbon deposition with a different

structural arrangement. So, the visible

morphological changes are due to not only the

presence of hydroxides but also to the deposition of

carbon with a different structural arrangement.

Despite the effect on the top of the CNTs forest,

a cross section view of the forests shows the vertical

alignment (Figure 10e and Figure 10f).

6 CONCLUSIONS

From the results obtained in the parametric study is

possible to start a universal protocol for generate the

best recipe for each system and for each lab.

Throughout the analysis of the best deposition

site, problems related with the non-uniformity of the

catalyst film deposition were raised. These problems

emphasize the importance of this key step. The

deposition of the catalyst film must be of high

quality in terms of uniformity and thickness to

ensure, respectively, the uniformity of the forest

itself and the CNTs alignment. The catalyst should

be protected of the environment, for example, by

photoresist, to avoid its oxidation and the formation

of undesirable substances during the synthesis that

can affect the morphology and properties of the

forests.

About the protocol itself, first one should

established a starting recipe based on literature

review and accordingly to the system characteristics.

After that, the first parameter to be evaluated should

the best deposition spot, taking into account the

reactions that the precursor suffers since it enters the

tube.

After finding the best deposition spot, the

starting recipe should be adjusted to this position in

terms of fluxes and growth time. Then, one should

evaluate how far the system can go in terms of

growth sessions without doing the baking. After

controlling all these parameters, one should study

the way in which the heights vary with the ethylene

time. This step enables the control of the forest

height in accordance.

This paper presents an initial approach on how to

perform a parametric study.

CarbonNanotubes-TheChallengesoftheFirstSynthesesTrials

101

More work has to be developed for answering

some questions that have risen, namely at the room

humidity and temperature level, which reveled some

influence on the on the process but that was not able

to be monitor at the time.

About the tube baking, there is the need to

understand why, in this system, the residues left on

the tube in the first growth are essential and improve

the results in the next sessions and what residues are

those. An approach to “see” inside the tube should

be created in order to evaluate the substances

remaining in the tube after the growths and how they

behave to contribute to reducing the heights

obtained.

The universal protocol proposed in this paper is

only an initial model, which still needs to be

improved by solving the raised questions and by

enhancing the parametric study developed.

ACKNOWLEDGEMENTS

The authors wish to thank FCT – Fundação para a

Ciência e Tecnologia, Portugal, for the financial

support provided under the project PTDC/EEI-

ELC/1838/2012.

The authors would like to acknowledge INESC-

MN for the deposition of catalysts thin-films.

REFERENCES

Atiyah, M. R. et al., 2011. Low Temperature Growth of

Vertically Aligned Carbon Nanotubes via Floating

Catalyst Chemical Vapor Deposition Method. Journal

of Materials Science & Technology, 27(4), pp.296–

300.

Bareket-Keren, L. & Hanein, Y., 2012. Carbon nanotube-

based multi electrode arrays for neuronal interfacing:

progress and prospects. Frontiers in neural circuits,

6(January), p.122.

Brukh, R. & Mitra, S., 2006. Mechanism of carbon

nanotube growth by CVD. Chemical Physics Letters,

424(1-3), pp.126–132.

Calderón-Colón, X. et al., 2009. A carbon nanotube field

emission cathode with high current density and long-

term stability. Nanotechnology, 20(32), p.325707.

Chen, G. D. et al., 2012. Nanoporous micro-element

arrays for particle interception in microfluidic cell

separation. Lab on a chip, 12(17), pp.3159–67.

Chen, R. et al., 2014. Production of hydrogen-rich gas and

multi-walled carbon nanotubes from ethanol

decomposition over molybdenum modified Ni/MgO

catalysts. Journal of Energy Chemistry, 23(2), pp.244–

250.

Fabbro, A., Prato, M. & Ballerini, L., 2013. Carbon

nanotubes in neuroregeneration and repair. Advanced

drug delivery reviews, 65(15), pp.2034–44.

Garcia, E. et al., 2008. Fabrication and multifunctional

properties of a hybrid laminate with aligned carbon

nanotubes grown In Situ. Composites Science and

Technology, 68(9), pp.2034–2041.

Hart, A. J., 2007. Chemical, Mechanical, and Thermal

Control of Substrate-Bound Carbon Nanotube

Growth. Massachussets Institute of Technology.

Holzinger, M., Le Goff, A. & Cosnier, S., 2012. Carbon

nanotube/enzyme biofuel cells. Electrochimica Acta,

82, pp.179–190.

Jacobs, C. B., Peairs, M. J. & Venton, B. J., 2010. Review:

Carbon nanotube based electrochemical sensors for

biomolecules. Analytica chimica acta, 662(2), pp.105–

27.

Jourdain, V. & Bichara, C., 2013. Current understanding

of the growth of carbon nanotubes in catalytic

chemical vapour deposition. Carbon, 58, pp.2–39.

Kumar, M. & Ando, Y., 2010. Chemical Vapor Deposition

of Carbon Nanotubes: A Review on Growth

Mechanism and Mass Production. Journal of

Nanoscience and Nanotechnology, 10(6), pp.3739–

3758.

Newman, P. et al., 2013. Carbon nanotubes: their potential

and pitfalls for bone tissue regeneration and

engineering. Nanomedicine : nanotechnology, biology,

and medicine, 9(8), pp.1139–58.

Oliver, C. R. et al., 2013. Statistical analysis of variation

in laboratory growth of carbon nanotube forests and

recommendations for improved consistency. ACS

nano, 7(4), pp.3565–80.

Qiao-juan, G. et al., 2013. Catalytic growth of multi-wall

carbon nanotubes on carbon cloth using powdery

ferrous sulfate heptahydrate as catalyst precursor. New

Carbon Materials, 28(6), pp.421–427.

Raffa, V. et al., 2011. Carbon Nanotubes for Biomedical

Applications R. Klingeler & R. B. Sim, eds., Berlin,

Heidelberg: Springer Berlin Heidelberg.

Sepúlveda, a T. et al., 2011. Nanocomposite Flexible

Pressure Sensor for Biomedical Applications.

Procedia Engineering, 25, pp.140–143.

Stein, I. Y. & Wardle, B. L., 2013. Coordination number

model to quantify packing morphology of aligned

nanowire arrays. Physical chemistry chemical

physics : PCCP, 15(11), pp.4033–40.

Tran, P. a, Zhang, L. & Webster, T. J., 2009. Carbon

nanofibers and carbon nanotubes in regenerative

medicine. Advanced drug delivery reviews, 61(12),

pp.1097–114.

Wang, Y. et al., 2014. Quantum chemical simulations

reveal acetylene-based growth mechanisms in the

chemical vapor deposition synthesis of carbon

nanotubes. Carbon, 72, pp.22–37.

Wardle, B. L. et al., 2008. Fabrication and

Characterization of Ultrahigh-Volume- Fraction

Aligned Carbon Nanotube-Polymer Composites.

Advanced Materials, 20(14), pp.2707–2714.

BIODEVICES2015-InternationalConferenceonBiomedicalElectronicsandDevices

102