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o current has been investigated in a ewexperiments7,8. It is thereore natural thatthe Belast team chose a chain o atoms as atest system or their work.

Te next step is to include Jouleheating on an equal ooting with current-induced orces to address stability and

other current-induced eects9. Tere havealready been several theoretical proposalsor molecular rotors driven by current10,11.Perorming more involved moleculardynamics simulations rom rst principles(simulations without any experimentalinput or tting parameters) could turn outto be an invaluable tool or designing andpredicting the behaviour o current-driven

molecular motors, beore testing thesedesigns in real experiments.

Mads Brandbyge is in the Department of Micro

and Nanotechnology (DU-Nanotech), Te

echnical University of Denmark, DK-2800 Kgs.

Lyngby, Denmark.

e-mail: Mads.Brandbyge@nanotech.dtu.dk

Rfr

1. Kohano, K. Electronic Structure Calculations for Solids and

Molecules (Cambridge Univ. Press, 2006).

2. Dundas, D., McEniry, E. J & odorov, . N. Nature Nanotech.

4, 99102 (2009).

3. Sorbello, R. S. Solid State Phys. 51, 159231 (1998).

4. Cunibert i, G., Fagas, G. & Richter, K. (eds) Introducing Molecular

Electronics (Springer, 2005).

5. Di Ventra, M., Chen, Y.-C. & odorov, . N Phys. Rev. Lett.

92, 176803 (2004).

6. Agrat, N., Yeyati, A. L. & van Ruitenbeek, J. M. Phys. Rep.

377, 81279 (2003).

7. Yasuda, H. & Sakai, A. Phys. Rev. B

56, 10691072 (1997).

8. Smit, R. H. M., Untiedt, C. & van Ruitenbeek, J. M.

Nanotechnology15, 472478 (2004).

9. Stella, L., Meister, M., Fisher, A. J. & Horseld, A. P.J. Chem. Phys.127, 214104 (2007).

10. Krl, P. & Seideman, .J. Chem. Phys.123, 184702 (2005).

11. Bailey, S. W. D., Amanatidis, I. & Lambert, C. J. Phys. Rev. Lett.

100, 256802 (2008).

12. Datta, S. ransport in Mesoscopic Systems (Cambridge Univ.

Press, 1995).

13. Brandbyge, M., Stokbro, K., aylor, J., Mozos, J. L. & Ordejn, P.

Phys. Rev. B67, 193104 (2003).

PUBlisheD online: 1 Fbrury 2009

neurons are electrically excitable cellsthat transmit and process inormationin the nervous system. Recently, it has

been shown that neurons continue to growwhen placed on carbon nanotubes, and canstill carry electrical signals when stimulated

by the nanotubes16

(Fig. 1). As a result, severalnanotube-based neural applications arebeing developed, such as neural prosthesisor restoring neurological sensory- andmotor-unctions, and tools or monitoringor modiying neural activity. However, littleis known about the details o the interactionsbetween the nanotubes and the neurons. Onpage 126 o this issue, Laura Ballerini romthe University o rieste and co-workers7rom Italy, Switzerland, Belgium and Irelandreport that carbon nanotubes alter theelectrophysiological responses o neurons byorming tight contacts with their membranes.

A neuron consists o a central part knownas the soma or cell body, and long processescalled axons and dendrites. In general, eachneuron has multiple dendrites that carrysignals into the soma, and a single axon thatcarries signals away rom the soma towardsthe next cell. Driven by electrochemicalgradients, the movement o ions across theion channels ound on the soma and the axongenerate precisely timed electrical signalscalled action potentials, which normallytravel along the axon in one direction awayrom the soma and towards the next neuron.

However, in neurons with extensivedendrites near the soma, such as some neurons

ound in the hippocampus part o the brain,the action potentials can sometimes travel inthe opposite direction. Tis backpropagationrom the soma induces calcium-mediated

action potentials in the dendrites and thiscontributes to an afer-depolarizationevent8,9 an induced depolarization o themembrane potential immediately ollowing anaction potential that is larger than normal atthe soma. A decision to re an action potential

occurs when the inputs rom the dendritesexceed the ring threshold. Te relativecontribution o the afer-depolarization eventto this decision to re depends on the degreeo coupling between the dendrites and soma,which, in turn, determines the strength o theafer-depolarization eect.

Ballerini and co-workers7 addressed theeect o nanotubes on the electrical propertieso isolated neurons by examining the presenceo afer-depolarization events in the cell soma.Tey injected a brie current pulse into thesoma to orce the neurons to re a regulartrain o six action potentials and measured the

presence o membrane depolarization in thesoma at the end o the last action potential.Compared with control neurons grown onglass substrates, hippocampal neurons grownon carbon nanotube substrates showedsignicantly enhanced afer-depolarization.

When the experiment was repeated withneurons grown on a conductive substrate suchas tin oxide and a non-conductive substratecontaining nanoscale eatures similar tocarbon nanotubes, the neurons did not showsignicantly enhanced afer-depolarization,demonstrating that this eect is specic tocarbon nanotubes. Furthermore, as expected,the afer-depolarization depended on the

nanoMeDicine

srt ur wt tubNew insights are emerging about the interactions between brain cells and carbon nanotubes, which couldeventually lead to the development of nanoengineered neural devices.

gbr a. sv

Fi 1 | Scanning electron micrograph of a

neuron (pseudo-coloured in blue) growing on a

carbon nanotube layer. New experiments indicate

that carbon nanotubes enhance the excitability

of neurons by forming tight contacts with the cell

membrane so that electrical activity is diverted

through the nanotubes.

LAU

RAB

ALLERINI

2009 Macmillan Publishers Limited. All rights reserved

8/3/2019 Gabriel A. Silva- Shorting neurons with nanotubes

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Diffractive imaging with X-rays orelectrons could potentially probe thethree-dimensional structure of singlenanoparticles on the atomic scale,but its resolution is limited because

weak coherent scattering cannot bedetected by conventional diffractionimaging procedures. Now, Jian Zuo andcolleagues at the University of Illinoisat Urbana-Champaign report diffractiveimaging of cadmium sulphide quantumdots with sub-ngstrm resolution byintegrating information from a low-resolution image with the diffractionpattern generated by the passage ofelectrons through the sample insidea transmission electron microscope(Nature Phys. 5, 129133; 2009).

Zuo and co-workers achieved a four-fold improvement in the resolution of theelectron diffraction pattern for a cadmiumsulphide quantum dot with a diameter of7 nm (main image), which enabled themto resolve the 0.84 separation betweenthe columns of cadmium and sulphuratoms (inset). As low-resolution imagescan be obtained from various sources, thistechnique provides a general method forthe high-resolution, three-dimensionalimaging of individual nanoparticles.

aDaRsh sanDhU

h from low

iMaging

2

008NPG

degree o dendritic branching: dorsal rootganglion neurons, which have minimaldendritic branching, never displayed aer-depolarization at any spiking requency whengrown on the same carbon nanotube suraces.

Modelling the membrane voltage inputoutput relationship between the soma and

dendrites, the European team ound that thenanotubes may be eectively short-circuitingthe dendrites and soma that is, diverting theelectrical activity through the nanotubes and this may be sufcient to account or theenhanced aer-depolarization o neuronsgrown on nanotubes. Furthermore, in acoupled-network version o the single-neuronmodel, the enhanced aer-depolarizationeect could signicantly prolong thespontaneously recurring action-potential burstevents in the network. In eect, the nanotubeswere able to modulate the physiology o theneurons. ransmission electron micrographs

o neurons grown on nanotubes showed tightcontacts between the nanotubes and the cell

membrane, suggesting that specic changes inthe electrical behaviour o the cell may be dueto these intimate interactions.

Tis new work represents the tip othe iceberg in that it hints at the potentialo carbon nanotubes or interacing withneurons, and eventually the nervous

system. Although the observations and theelectrotonic hypothesis are interesting, itremains unclear how this phenomena maycontribute to a putative nano-engineeredneural device: or example, calcium currentsinduced by backpropagation in dendriteso adult hippocampal neurons rarely resultin calcium action-potentials, because thecurrents are too small9. It will take thedegree o experimentation and modellingillustrated in this paper to answer thesequestions and engineer meaningul nanotubeneural devices.

Understanding the mechanisms that

underlie the eect o carbon nanotubes onneural cells will be critical or designing

unctional neural devices based on empiricaldata and engineering principles, rather thanqualitative trial-and-error approaches. Teengineering challenges involved demandthis, but the potential impacts o successullyinteracing carbon nanotube devices withneural cells will be well worth it.

Gabriel A. Silva is in the Departments of

Bioengineering and Ophthalmology, University of

California, San Diego, California 92037, USA.

e-mail: gsilva@ucsd.edu

Referece

1. Massobrio, G., Massobrio, P. & Martinoia , S. Nano Lett.

8, 44334440 (2008).

2. Malarkey, E. B. et al. Nano Lett. 8, 35383542 (2008).

3. Keeer, E. W. et al. Nature Nanotech. 3, 434439 (2008).

4. Li, J. & Andrews, R. J.Acta Neurochir. Suppl.97, 537545 (2007).

5. Mazzatenta, A. et al.J. Neurosci.27, 69316936 (2007).

6. Nguyen-Vu, . D. et al. Small2, 8994 (2006).

7. Cellot, G. et al. Nature Nanotech.4, 126133(2009).

8. Kepecs, A. & Lisman, Net. Comp. Neural. Sys.

14, 103118 (2003).9. Chen, S. & Yaari, Y.J. Physiol.586, 13511363 (2008).

2009 Macmillan Publishers Limited. All rights reserved