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Microtubule architecture: inspiration for novel carbon nanotube-based biomimetic materials Francesco Pampaloni 1 and Ernst-Ludwig Florin 2 1 Cell Biology and Biophysics Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany 2 Center for Nonlinear Dynamics, University of Texas, Austin, TX 78712, USA Microtubules are self-assembling biological nanotubes that are essential for cell motility, cell division and intra- cellular trafficking. Microtubules have outstanding mechanical properties, combining high resilience and stiffness. Such a combination allows microtubules to accomplish multiple cellular functions and makes them interesting for material sciences. We review recent experiments that elucidate the relationship between molecular architecture and mechanics in microtubules and examine analogies and differences between micro- tubules and carbon nanotubes, which are their closest equivalent in nanotechnology. We suggest that a long- term goal in bionanotechnology should be mimicking the properties of microtubules and microtubule bundles to produce new functional nanomaterials. Introduction Microtubules are cytoskeletal biopolymers that, along with actin and intermediate filaments, accomplish essential functions at each stage of the cell’s life cycle. They ensure the mechanical stability of the mitotic spindle, provide oriented tracks for intracellular trafficking of organelles and support the cell’s shape during migration [1]. At the cell length scale, microtubules are very stiff filaments. The average Young’s modulus of a microtubule, considered as a simple isotropic tube, is 2 GPa. Thus, microtubules are as stiff as hard plastic and about one hundred times stiffer than the other cytoskeleton components, actin and inter- mediate filaments [1]. Interestingly, microtubules are not only stiff, but also highly resilient. Their efficient combi- nation of high stiffness (relative to the other cytoskeletal filaments) and resilience is due to the anisotropic molecu- lar architecture of microtubules and allows them to accom- plish multiple tasks in the cell. On the one hand, high stiffness is required to resist the large pushing forces occurring during mitotic spindle elongation at the end of anaphase. On the other hand, high resilience allows micro- tubules to search the cellular space laterally for binding partners and to keep growing in a different direction with- out breaking when encountering obstacles. As microtubules are extraordinarily versatile struc- tures, the question arises of what could be learned from them for the design of novel structural and multifunctional materials for applications in material sciences and biona- notechnology. Carbon nanotubes (CNTs), one of the most promising products of nanotechnology, are the closest technological counterpart of microtubules. CNTs are an extremely stiff material; their Young’s modulus is 1 TPa, about five times higher than that of steel (210 GPa) [2]. Similar to microtubules, CNTs are also highly resilient. It is generally acknowledged that CNTs will play a major role in the development of new materials, with applications ranging from ‘super-tough’ composite fibers [3] to drug-delivery systems [4]. Molecular control over the CNT assembly process would be desirable for the full exploitation of their nanoscale properties and for the reproducible fabrication of CNT-based materials. How- ever, although CNTs aggregate into bundles or sheets spontaneously, their assembly into designed composite structures remains difficult to understand and to direct at molecular level. Microtubules and CNTs are surprisingly similar in their mechanical behavior despite their very different chemical composition (proteins and non-covalent interactions in the case of microtubules, carbon and covalent bonds in the case of CNTs) and elastic moduli. Here, we describe key structural aspects of microtubules and review recent results on their mechanics. We then compare CNTs and microtubules side-by-side with respect to structural and elastic properties. Finally, we discuss examples of how microtubules and microtubule-based structures could provide insights for the design and assem- bly of novel CNT-based biomimetic materials. Structure of microtubules The basic building unit of microtubules is the heterodi- meric protein tubulin, consisting of a and b subunits. Tubulin dimers self-assemble head-to-tail (-ab-ab-) into linear protofilaments (PFs) (Figure 1). The cylindrical wall most commonly comprises 13 PFs in vivo, but this number can vary from 9 to 16 in vitro [5]. Tubulins of adjacent PFs are laterally linked through homologous monomer con- tacts a-a, b-b (except at the ‘seam’, Figure 1). In the microtubule, stacked PFs are slightly longitudinally dis- placed with respect to each other (0.9 nm in a 13-PF microtubule). This displacement results in a left-handed helical surface lattice. Although the biological function of Opinion Corresponding authors: Pampaloni, F. ([email protected]); Florin, E.-L. ([email protected]). 302 0167-7799/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2008.03.002 Available online 21 April 2008

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Opinion

Microtubule architecture: inspirationfor novel carbon nanotube-basedbiomimetic materialsFrancesco Pampaloni1 and Ernst-Ludwig Florin2

1 Cell Biology and Biophysics Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany2 Center for Nonlinear Dynamics, University of Texas, Austin, TX 78712, USA

Microtubules are self-assembling biological nanotubesthat are essential for cell motility, cell division and intra-cellular trafficking. Microtubules have outstandingmechanical properties, combining high resilience andstiffness. Such a combination allows microtubules toaccomplish multiple cellular functions and makes theminteresting for material sciences. We review recentexperiments that elucidate the relationship betweenmolecular architecture and mechanics in microtubulesand examine analogies and differences between micro-tubules and carbon nanotubes, which are their closestequivalent in nanotechnology. We suggest that a long-term goal in bionanotechnology should be mimickingthe properties of microtubules and microtubule bundlesto produce new functional nanomaterials.

IntroductionMicrotubules are cytoskeletal biopolymers that, along withactin and intermediate filaments, accomplish essentialfunctions at each stage of the cell’s life cycle. They ensurethe mechanical stability of the mitotic spindle, provideoriented tracks for intracellular trafficking of organellesand support the cell’s shape during migration [1]. At thecell length scale, microtubules are very stiff filaments. Theaverage Young’s modulus of a microtubule, considered as asimple isotropic tube, is�2 GPa. Thus, microtubules are asstiff as hard plastic and about one hundred times stifferthan the other cytoskeleton components, actin and inter-mediate filaments [1]. Interestingly, microtubules are notonly stiff, but also highly resilient. Their efficient combi-nation of high stiffness (relative to the other cytoskeletalfilaments) and resilience is due to the anisotropic molecu-lar architecture of microtubules and allows them to accom-plish multiple tasks in the cell. On the one hand, highstiffness is required to resist the large pushing forcesoccurring during mitotic spindle elongation at the end ofanaphase. On the other hand, high resilience allows micro-tubules to search the cellular space laterally for bindingpartners and to keep growing in a different direction with-out breaking when encountering obstacles.

As microtubules are extraordinarily versatile struc-tures, the question arises of what could be learned fromthem for the design of novel structural andmultifunctional

Corresponding authors: Pampaloni, F. ([email protected]);Florin, E.-L. ([email protected]).

302 0167-7799/$ – see front matter � 2008 Elsevie

materials for applications in material sciences and biona-notechnology.

Carbon nanotubes (CNTs), one of the most promisingproducts of nanotechnology, are the closest technologicalcounterpart of microtubules. CNTs are an extremely stiffmaterial; their Young’s modulus is �1 TPa, about fivetimes higher than that of steel (�210 GPa) [2]. Similarto microtubules, CNTs are also highly resilient.

It is generally acknowledged that CNTs will play amajor role in the development of new materials, withapplications ranging from ‘super-tough’ composite fibers[3] to drug-delivery systems [4]. Molecular control over theCNT assembly process would be desirable for the fullexploitation of their nanoscale properties and for thereproducible fabrication of CNT-based materials. How-ever, although CNTs aggregate into bundles or sheetsspontaneously, their assembly into designed compositestructures remains difficult to understand and to directat molecular level.

Microtubules and CNTs are surprisingly similar in theirmechanical behavior despite their very different chemicalcomposition (proteins and non-covalent interactions in thecase of microtubules, carbon and covalent bonds in the caseof CNTs) and elastic moduli.

Here, we describe key structural aspects ofmicrotubulesand review recent results on their mechanics. We thencompare CNTs and microtubules side-by-side with respectto structural and elastic properties. Finally, we discussexamples of how microtubules and microtubule-basedstructures could provide insights for the design and assem-bly of novel CNT-based biomimetic materials.

Structure of microtubulesThe basic building unit of microtubules is the heterodi-meric protein tubulin, consisting of a and b subunits.Tubulin dimers self-assemble head-to-tail (-ab-ab-) intolinear protofilaments (PFs) (Figure 1). The cylindricalwallmost commonly comprises 13 PFs in vivo, but this numbercan vary from 9 to 16 in vitro [5]. Tubulins of adjacent PFsare laterally linked through homologous monomer con-tacts a-a, b-b (except at the ‘seam’, Figure 1). In themicrotubule, stacked PFs are slightly longitudinally dis-placed with respect to each other (�0.9 nm in a 13-PFmicrotubule). This displacement results in a left-handedhelical surface lattice. Although the biological function of

r Ltd. All rights reserved. doi:10.1016/j.tibtech.2008.03.002 Available online 21 April 2008

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Figure 1. Polymerization of microtubules. Tubulin dimers assemble ‘head-to-tail’, forming oligomers that elongate into protofilaments. As the protofilaments reach an

estimated critical length of 12 � 2 dimers [65] they start to interact laterally, forming sheets with a characteristic intrinsic inward curvature. At a typical number of 13

protofilaments, the tubulin sheet closes into a tube, forming a microtubule. The tubulin lattice has a left-handed helical symmetry. The microtubule closes at the seam

(black arrows), where there is a discontinuity point in the helical lattice.

Opinion Trends in Biotechnology Vol.26 No.6

the helical symmetry of microtubule lattice has not yetbeen clearly explained, it has been speculated that helicitycould be a necessary geometrical requirement for thecorrect self-assembly of microtubules [6].

Microtubules, although often regarded as a load-bearingcomponent of the cytoskeleton, are far from just beingstatic struts. Instead, they are highly dynamical struc-tures, able to switch rapidly between phases of assembly(growth) and disassembly (shrinkage) on the time scale ofseconds. Because of this process, the microtubule is said tohave an inherent ‘dynamic instability’. This property ofmicrotubules is interesting for bionanotechnology becausecontrol over assembly and disassembly of supramoleculararchitectures is a very active area of research in this field[7]. A discussion on dynamic instability is beyond the scopeof this opinion. An excellent overview on microtubulepolymerization dynamics is provided, for instance, byDesai and Mitchison [8].

Figure 2. Protofilament–protofilament lateral contacts in microtubules. (a) Protofilamen

lateral interaction is the M-loop. It establishes contacts with the a-helix H3 and the lo

interactions are H9/H3 and H10-S9 loop/H4, located at different radial positions with res

1TUB. Molecular graphics images were produced using the UCSF Chimera package from

California, San Francisco, supported by NIH P41 RR-01081).

Protofilament architecture and mechanical anisotropy

Structural investigations and theoretical calculationsshow that lateral and longitudinal tubulin interactionshave different properties [9–11]. Whereas the lateralinter-PF contacts (a-a, b-b) are mostly electrostatic, theintra-PF (-ab-ab-) interactions are prevalently hydro-phobic [11,12]. Interestingly, facilitating lateral associ-ation with electrostatic forces and axial self-assemblywith the hydrophobic effect is a principle exploited inbionanotechnology for building-up peptide nanoropes[13]. The inter-PF tubulin bonds are weaker (�7 kcal/mol [14]) and more compliant than the longitudinal onesalong individual PFs [14,15]. X-ray crystallography andcryo-electron microscopy have shown that the main struc-turalmotif involved in inter-PF contacts is the so-called ‘M-loop’ [11,12,15–17] (Figure 2a,b). TheM-loop of one tubulininteracts with the H1-S2 loop of the adjacent tubulin(Figure 2a,b) [17]. The N- and C-terminal parts of the

ts associate laterally with a �0.9 nm offset. The main structural feature involved in

ops H2-S3/H1-S2 of the neighboring protofilament. (b) Further important lateral

pect to the center of the cross-section. (Tubulin structure: Protein Data Bank entry

the Resource for Biocomputing, Visualization and Informatics at the University of

303

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Opinion Trends in Biotechnology Vol.26 No.6

M-loop are considerably flexible and appear to work like ahinge, allowing relative motion between PFs, such assliding [12].

It has been shown by electron microscopy that PFs slidepast each other longitudinally when the microtubule isbent by thermal forces [18,19]. The relative sliding is verytiny (� <0.2 nm), but easily resolved by electron micro-scopy. Inter-PF bonds are stretched as a consequence ofsliding. The resulting lattice tension can be relieved by atwisting of the PF along the microtubule’s axis [5]. Theability to accommodate larger deformations of the inter-PFbonds by PF twisting has important structural con-sequences because it allows the existence of microtubuleswith a different number of PFs (from 10 to 16) (Figure 3)[5,19]. Such PF-number polymorphism has physiologicalsignificance [20].

The PF structure has also an important effect on micro-tubule mechanics. The different strengths of intra- andinter-PF bonds imply that the elastic modulus along themicrotubule axis is different from the moduli along theaxes parallel to the cross-section.

Mechanics of microtubulesIn cells, microtubules are often highly bent because of theaction of strong internal cytoskeletal forces [21,22]. Severalapproaches have been developed to measure the bendingstiffness of microtubules. One method is the analysis ofthermal shape fluctuations via fluorescence light micro-scopy [23]. Alternatively, themicrotubule bending stiffnesshas been determined by applying controlled forces. Opticaltweezers [24], atomic force microscopes (AFMs) [25,26] or

Figure 3. Polymorphism of microtubules. (a) Schematic representation of the helical su

from [18]). Because dimers in adjacent protofilaments are axially shifted (for a 3-start 13-

closes exactly three monomers above its starting point. (b) In a 14-protofilament microtu

most favorable way to compensate this mismatch is to skew the protofilaments at a sma

microtubule with a shallow pitch of �2 mm. (c) Schematic representation of microtubul

protofilament microtubules and the left-handedness (14-protofilament microtubule) and

The 16-protofilament microtubule has a 4-start helix and no seam. (d) Isosurface renderin

handed in 15-protofilament microtubules (blue arrow). The protofilaments’ skew angle

microscopy and helical three-dimensional reconstruction, courtesy of Linda Sandblad,

304

hydrodynamic flow [27] have been employed to this pur-pose. The persistence lengths (Figure 4, Box 1) of micro-tubules obtained by these techniques fall within the rangeof 1 to 6millimeters, implying that they are very stiff on thecell length scale (�10 mm).

Most of the previous work has modeled microtubules asisotropic tubes with a single Young’s modulus. However,such a simplified picture is inadequate for describingmicrotubulemechanics [23]. Recent experiments show thatthe elastic modulus parallel to the microtubule’s axisdiffers drastically from that of the cross-section or, in otherwords, microtubules are mechanically strongly anisotropic(Figure 4) [28]. Mechanical anisotropy is found in nearly allmacroscopic biological materials, such as bones, wood,stalks, stems and bamboo culms. In most cases, the moststiff direction lies parallel to the long axis [2]. Interestingly,at a completely different length scale, the exact sameobservations are made for micrometer-sized microtubules.Needleman et al. [29] probed the radial mechanical proper-ties of microtubules by varying the osmotic pressure actingon the microtubule’s wall. They found that the walldeforms from a circular to an elliptic shape above a criticalpressure, Pcr = 600 Pa, which is four orders of magnitudesmaller than the one predicted by modeling the microtu-bule as an isotropic tube [29]. Such unexpectedly softresponse along the cross-section is likely to be derivedfrom the deformability of inter-PF lateral contacts. Thedata by Needleman et al. [29] have been successfullyexplained by an orthotropic elastic shell model for micro-tubules that accounts for the different contributions ofintra- and inter-PF bonds [30]. In a further study, the

rface lattice of a 3-start 13-protofilament microtubule (modified, with permission,

protofilament microtubule the subunit rise is �0.9 nm), the 3-start left-handed helix

bule, the tubulin dimers at the helix closure are not registered (i). The energetically

ll angle of 0.758 (ii). This produces a left-handed superhelix in the 14-protofilament

e’s surface for 13, 14 and 16 protofilaments. Notice the skew angle for 14- and 16-

right-handedness (16-protofilament microtubule) of the superhelices (blue arrows).

g of the electron density in a 15-protofilament microtubule. The superhelix is right-

and the superhelix’s right-handedness are clearly visible (data from cryo-electron

European Molecular Biology Laboratory).

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Figure 4. Anisotropic mechanical properties of microtubules. (a) A flexible polymer bends over relatively short lengths, coiling up on itself. The ‘persistence length’ (Lp) is

the length separating bends along the biopolymer contour. The shorter the persistence length compared to the total length (or ‘contour length’), the more flexible is the

polymer. The filament in (i) is flexible: Lp is shorter than the contour length. By contrast, the filament in (ii) is stiff: Lp is larger than the contour length. Microtubules have a

length up to tens of micrometers and a persistence length in the millimeter range. Thus, microtubules are extremely stiff polymers. (b) Long microtubules are stiffer than

shorter ones, which can be shown by plotting the persistence length (Lp) of microtubules against increasing contour length (L). This counter-intuitive behavior is a

consequence of bending-associated shear. (c) In a bending microtubule, adjacent tubulins shear (shear strain g1) and stretch the lateral links between protofilaments (i,iii).

From electron microscopy studies it is known that stretching of the lateral links is relieved by protofilaments skewing (shear strain g2) (ii,iv) and that protofilament skewing

induces a twist in the lattice of the microtubule. This implies that protofilament shearing and twisting are coupled deformations in the microtubule (iii,iv).

Opinion Trends in Biotechnology Vol.26 No.6

dependence of the microtubule’s persistence length on thecontour length was investigated by analysing thermalfluctuations of the free tip of grafted microtubule(Figure 4a) [31,32]. The data showed a surprising depen-dence of the persistence length on the total length(Figure 4b). For total microtubule lengths shorter than5 mm, the persistence length is on the order of 500 mm [32].For longer microtubules, the persistence length increasesproportionally to L2. A qualitatively similar dependencewas also observed in AFM experiments [26]. To model thedata in [31] and [26], an additional shear modulus wasincluded in the bending equations. This modulus is ameasure for the tendency of elements of the microtubuleto slide past each other (Figure 4c) [33,34]. The inter-PFbonds (in the M-loop region) can be identified as theprobable shear-bearing elements because they can beeasily deformed by thermal fluctuations [18,19]. Inter-PF sliding releases the elastic energy stored in a bentmicrotubule, thereby avoiding a structural collapse dueto stress concentration in the lattice [2,18,19]. Thus, inter-PF sliding improves the resiliency of microtubules bypreventing the formation of ‘kinks’ and other types offailures under bending stresses. Consequently, microtu-bules can be bent elastically to small radii of curvaturewithout rupture [22]. For instance, the average radius ofcurvature of microtubules in a blood platelet is�1 mm [35].Moreover, buckling microtubules to radii of curvatures<1 mm by using optical tweezers does not result in break-ing, even if high curvature is maintained for up to one hour[22]. These examples show microtubules’ remarkable resi-liency, which allows the cell to resist damage from thestrong contractile forces involved, for example, in cellmigration [36].

Microtubules are often not found individually but arearranged in bundles. Bundling is essential for mechanicalreinforcement of the cell. The bending stiffness of a bundlecan vary by orders of magnitude depending on the strengthof cross-linkers between individual filaments. The bendingstiffness of a bundle with weak cross-linkers is approxi-mately proportional to the number of filaments, whereas,

in the case of strong cross-linkers, the bending stiffness isapproximately proportional to the square of the number offilaments [34]. Thus, bundling allows a ‘fine-tuning’ of themechanical properties of microtubule-based organelles.

Specialized microtubule bundles are involved inchromosome segregation, motility and mechanosensing.Examples of these organelles are the mitotic spindle individing cells [37], cilia and flagella [38] and the axostyle insome protists [39].

Parallels with CNTsConsidering their remarkable mechanical properties, thequestion arises of how microtubules and microtubule bun-dles could be exploited in material science and bionano-technology. Elegant proof-of-principle experiments havebeen performed by combining microtubules with micro-fluidic devices. For example, nanotransport and nanosort-ing systems have been realized with microtubulestraveling on microfluidic lanes coated with the motorprotein kinesin [40,41]. Although this approach is extre-mely promising for laboratory-on-a-chip systems, it doesnot directly rely on the mechanical properties of micro-tubules. A further possibility is using microtubules as atemplate, for example, for the production of structurallydefined and monodisperse metallic nanowires [42]. Thedrawback in this case is that the sophisticated interplaybetween architecture andmechanics found inmicrotubulesis not preserved in these fully metallic nanoobjects. A thirdand more viable option would be to apply our understand-ing of microtubule architecture for the creation of fullysynthetic biomimetic nanofibers. Microtubule-inspirednanofibers should be as ‘smart’ as microtubules but chemi-cally stable over a much wider range of environmentalconditions. Toward this goal, CNTs are obvious candidatesas suitable building blocks.

CNTs (Figure 5) are in several aspects the technologicalcounterpart of nature’s microtubules. A side-by-side com-parison with microtubules (see also Table 1) could provideuseful insights for the fabrication of novel CNT-based nanomaterials. Ideally, such novel materials would

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Box 1. Mechanical properties of microtubules

Stress

Stress is defined as the load per unit area: s = P/A (where s = stress,

P = load, A = area). It is expressed in Newton per square meter

(N/m2).

Strain

Strain expresses the deformation of a material under load. A rod

with original length L stretched by an amount DL by the action of a

stress on it, is subject to a strain e = DL/L.

Young’s modulusIn linear elasticity theory, it is a constant (E) that expresses the

stiffness of a material and is defined as the ratio of stress to strain:

E = s/e.

Shear

The shear stress measures the tangential load per unit area needed

to let one part of a material slide past the neighboring part. It is

expressed in Newton per square meter (N/m2). The shear strain, g, is

angular and measured in radians. In linear elasticity theory, the

shear modulus, G (units N/m2), is defined as the ratio between shear

stress and shear strain.

Bending stiffness

A measure of how a certain structure can be deflected under load.

The bending stiffness is defined as the bending moment per

curvature necessary to bend a filament. For isotropic materials,

the bending stiffness, El, is the product of the Young’s modulus,

E, and a geometric parameter, l, the momentum of inertia of the

cross section.

Persistence length

Lp – this parameter (used particularly in polymer physics) also

defines the bending stiffness of a filament. It is the length along the

filament over which the tangent to the contour remains correlated

when a filament fluctuates under thermal forces. It is related to the

bending stiffness, El, by the relation Lp = El/KBT (where KB =

Boltzmann’s constant, T = temperature).

Resilience

This is the quality of a material to be deflected elastically to small

radii of curvature without breaking by storing strain energy.

Microtubules are highly resilient but not ‘floppy’. In fact, they have

a large Young’s modulus of �2 GPa, but they can be bent to a radius

as small as rc = 86 nm in the sporozoite of Plasmodium berghei (a

rodent malaria parasite) [68]. This corresponds to a local strain of e�0.14 (e = rMT/rc, where rMT = the microtubule’s radius, rc = the

radius of curvature of a bent microtubule) and represents a

difference in length of 14% between the two sides. In living

fibroblasts, an average bending radius of 0.4 mm was measured

on intact microtubules [22]. This corresponds to a length difference

of �3% between the two sides. For comparison, elementary

calculations show that a hypothetical nylon nanotube of the same

scale and comparable Young’s modulus would already have broken

apart at a radius of curvature of 2 mm (�0.6% difference between the

two sides). Carbon nanotubes (CNTs) are also highly resilient. For

example, an 850 nm long, 10.5 nm diameter CNT was bent with an

atomic force microscope (AFM) to a radius of curvature of �20 nm

(corresponding to a local strain e �0.25) repeatedly and without

breakage [51].

Opinion Trends in Biotechnology Vol.26 No.6

combine the high mechanical and chemical stability ofCNTs with the self-assembling ability and multifunction-ality of microtubule structures.

Structure and mechanics of CNTs

CNTs are members of the structural family of fullerene, anallotropic form of carbon. Single-wall CNTs (SWCNTs) canbe essentially considered as two-dimensional graphite

306

sheets wrapped onto a tube surface [43]. The outerdiameter is 1–2 nm, and the wrapping direction deter-mines the possible lattice symmetries (Figure 5a) [44].Other types of CNTs are multi-walled CNTs (MWCNTs)and carbon nanoropes. Whereas MWCNTs are onion-likearrangements of concentric SWCNTs (Figure 5b), carbonnanoropes are bundles of SWCNTs tightly packed in hex-agonal order (Figure 5c,d).

The Young’s modulus of individual SWCNTs has beenobtained from the amplitude of thermally excitedvibrations measured with transmission electron micro-scopy. An average Young’s modulus of 1.25 TPa (higherthan diamond) has been found [45]. The same method hasgiven a modulus of 1.8 TPa for MWCNTs [46]. By laterallydeflecting MWCNTs with AFM, a Young’s modulus of1.3 TPa has been measured [47]. A similar approachapplied to individual SWCNTs and SWCNT ropes hasgiven a Young’s modulus of �1 TPa [48,49]. These exper-iments show that SWCNTs, SWCNT ropes, and MWCNTsare extremely stiff structures. Their high bending stiffnessis mainly due to the rigid s-bonds between carbon atoms.However, CNTs are also remarkably resilient. Even afterextreme bending to a local strain of e �16%, MWCNTsreversibly come back to their original shape [47,50,51].This behavior results from the high anisotropy of graphiteand the nested structure of MWCNTs [50,52]. Carbonnanoropes are mechanically anisotropic as well [52].AFM experiments with carbon nanoropes revealed thatthe low intertube shear stiffness dominates their bendingbehavior [49]. In fact, the weak Van derWaals interactions(deriving from out-of-plane p-bonds) allow the individualSWCNTs to slide with respect to each other. As for micro-tubules, the CNT bending properties can be appropriatelymodeled by inserting the contribution of the shearmodulusin the bending equation [48,49].

The high stiffness, high resiliency and low density ofCNTs, are extremely promising for the development ofmaterials with superior properties. A dramatic increaseof the strength of a nanorope has been achieved by cross-linking SWCNTs with electron beam irradiation. Thismethod could allow the fabrication of macroscopic ribbons,fibers and strands made entirely of CNTs [53]. A layer ofupright CNTs (nanotube ‘forests’) has been used to coatmicro-fabric fiber cloths and to obtain multifunctionalthree-dimensional composites [54]. The aforementionedfabrication strategies pave the way for future technologicalapplications of CNTs. However, these approaches do notexploit ‘bottom-up’ molecular self-assembly. This limits thefabrication of structurally well-defined functionalnanoarchitectures based on CNTs [55].

Microtubules versus CNTs

Although their elastic moduli differ by orders of magni-tude, microtubules and CNTs have similar mechanicalbehaviors. First, both microtubules and CNTs have atubular structure that ensures structural efficiency. Hol-low, thin-walled sections are in general more efficient thansolid ones for carrying loads. Circular tubes are moreefficient than other shapes under bending loads that areapplied from every possible direction. Additionally, hollowsections use less material than solid ones. Therefore, they

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Figure 5. Structure and polymorphism of carbon nanotubes (CNTs). (a) Models of single-walled CNTs (SWCNTs). There are three possible patterns along the circumference

of the CNT: ‘armchair’ (i), ‘zigzag’ (ii) and ‘chiral’ (iii). (b) A multi-wall CNT (MWCNT). The inner diameter is 10 nm. (c) Example of an SWCNT rope with defect-free and

parallel walls (modified, with permission, from [66]). (d) Example of an SWCNT rope ‘cross-section’ (modified, with permission, from [66]). (e,f) CNTs dissolved in water via

surfactants (e) (modified, with permission, from [67]) and peptides (f) (modified, with permission, from [58]). (Structures in Figure 5a produced with CoNTub v1.0. 5-b from

http://www.nano-laboratory.com/nanotube-image3.html).

Opinion Trends in Biotechnology Vol.26 No.6

are lighter and more ‘economical’ while resisting the samebending or torsional load [2]. Second, both microtubulesand CNTs are exceptionally resilient, that is, they can bebent to a small radius of curvature and are able to restoretheir original shape without permanent damage. Theresilience of microtubules and CNTs originates from theirarchitecture, which allows the dissipation of part of thebending energy through internal rearrangements. Inmicrotubules, the ‘M-loop’ that links single PFs laterallyis deformable and can relieve local bending stress byallowing a limited sliding of PFs past each other.MWCNTsand carbon nanoropes exploit a similar mechanism anddissipate bending energy through sliding of neighboringCNTs. The third similarity between microtubules andCNTs is the ability to form large bundles. CNTs formnested structures, such asMWCNTs, and parallel bundles,such as carbon nanoropes. As with microtubules, CNTbundles have improved stiffness and resiliency.

Despite numerous similarities in mechanical behavior,a fundamental difference between microtubules and CNTsis that the former are ‘soft’ materials that self-assemble atmild pH and temperature conditions, whereas the latterare typical hard-matter objects that are fabricated by

spinning, layering and further shaping procedures [54].However, in recent years, strategies have been developedto enable the self-assembly of CNTs in solution. Dispersionof CNTs in water has been obtained by their non-covalentstabilization with surfactants or polymers or by wrappingwith oligopeptides [56,57] (Figure 5e-f). Dispersion inliquid media, and particularly water, is critical for theachievement of molecular-level, ‘bottom-up’ assembly ofCNTs because it allows the separation of heterogeneousmixtures of CNTs and the controlled association of func-tionalized CNTs by hydrophobic, electrostatic or covalentbonding [57,58]. Realization of molecular-level assembly ofCNTs could open the way to biomimetic nanomaterials.Toward this goal, microtubules can provide useful insightsand inspiration.

Insights from microtubules for bionanotechnology andnanomedicineBiomimetic approaches are of great interest for the syn-thesis of nanomaterials because biological architecturesoften resist large mechanical stresses much better thanman-made ones. This phenomenon is likely to be the resultof evolution, which has selected structures with highmech-

307

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Table 1. Comparison between carbon nanotubes and microtubules

Carbon nanotubes Microtubules

Synthesis � Arc discharge � MTs self-assemble at physiological temperature and pH.

Guanosine triphosphate (GTP) is required� Laser ablation

� Chemical vapor deposition

Linear dimensions � CNT outer diameter: �1–2 nm � Tube diameter: �25 nm

� CNT inside diameter: �0.8–1.6 nm � PF diameter: �5–6 nm

� Wall thickness: �diameter of a carbon atom � Wall thickness: 5–6 nm

� Length-to-diameter ratio: �103 � Length-to-diameter ratio: �103

Lattice � Hexagonal lattice with ‘armchair’, ‘zigzag’ or ‘chiral’

arrangements of carbon atoms

� Tubular bundle of PFs

� Polarity (plus/minus ends)

� Tubulin is packed into a helical lattice with 12 nm pitch in

the MT wall

� PFs twist along the MT, forming left- or right-handed

superhelixes

Structural

polymorphism

� CNTs are a seamless graphene cylinder � MTs are a protein cylinder with a seam (13 PF) and without

a seam (15 PF)� CNTs form bundles (ropes) and multi-walled CNTs

through Van der Waals interactions � MTs can accommodate different numbers of PFs (from 9

to 18)

� MTs linked by proteins form bundles

Mechanical properties � Young’s modulus: >1 TPa � Young’s modulus: �2 GPa

� Highly resilient. CNTs can be bent considerably without

damage

� Highly resilient. MTs can be bent considerably without

damage

� Low shear modulus (in the case of carbon nanoropes) � Low shear modulus

Applications and

functions

� Structural composites � Essential functions in cells (mitotic spindle, intracellular

transport, cell motility)� Fibers and fabric

� Catalysts

� Biomedical applications

Abbreviations: CNT, carbon nanotube; MT, microtubule; PF, protofilament.

Opinion Trends in Biotechnology Vol.26 No.6

anical efficiency. Association into bundles is the basicstrategy exploited by microtubules for the creation ofreinforced structures and sophisticated ‘active nanomater-ials’, which are able to exert and sense forces. A promisingfuture perspective could be applying this ‘blueprint’ ofmicrotubules to the design of fibrous nanomaterials con-sisting of CNT-based building blocks.

As mentioned above, controlled molecular self-assemblyof CNTs is already possible [58]. Coating CNTs with oli-gopeptides enables the production of self-assembled com-posite structures. By varying the factors that influencepeptide–peptide interactions (e.g. salt concentration),these structures can assume a wide range of shapes andsizes, such as cylindrical microfibers and flat ribbons [58].In addition, DNA could be used as an alternative cross-linker [59], as well as synthetic organic molecules [60]. Thelength and flexibility of these macromolecular linkers canbe precisely determined by chemists, and this will allow forcontrol over the final properties of functional bundledarchitectures. Flexible and long cross-linkers, such asDNA, would produce bundles that are highly resilientunder bending deformations. In this case, the bundle’sbending stiffness would be approximately proportional tothe number of CNTs. By contrast, stiff and short linkers,such a small organic molecules, would produce more rigidbundles with bending stiffness approximately proportionalto the square of the CNT number [34]. Thus, the mechan-ical properties of CNT bundles could be varied at willthrough the choice of the cross-linkers.

Microtubules could inspire the realization of active CNTnanomaterials in the form of force sensors and transdu-cers. By considering a CNT as a ‘protofilament’, artificialmicrotubules composed of individual CNTs could be builtby choosing appropriate linkers. Such ‘synthetic microtu-bules’ could be used to replicate microtubule-based

308

mechanosensory organelles, such as cilia in epithelial cells.Cilia sense mechanical stimuli and trigger a cell response.Synthetic mechanosensors inspired by cilia and based onCNTs are conceivable [61].

The creation of nanostructures that are sensitive tomechanical forces could lead to novel responsive drugand gene delivery systems. For example, a drug could becaged inside a synthetic microtubule. The carbon ‘proto-filaments’ could be cross-linked by molecules sensitive tobending strains, thereby programming the syntheticmicrotubules to disassemble beyond a certain bendingcurvature. Thus, the caged drug could be released if shearforces increase, which occurs, for instance, in a blood vesselat the onset of thrombosis [62]. Sensitivity to bending couldbe achieved by cross-linking CNTs with short peptides,which are cleavable by proteases. Bending of these syn-thetic microtubules would expose the target peptide andthus trigger the enzymatic disassembly. A similar mech-anism is exploited in the cell for the severing of micro-tubules. A strongly bent microtubule lattice increases theactivity of the microtubule-severing enzyme katanin.Through this mechanism, the enzyme ‘feels’ the curvatureof eachmicrotubule and specifically cuts highly bentmicro-tubules, thereby triggering a cellular mechanochemicalresponse [22]. Another possibility is the use of light-sensi-tive cross-linkers, which could trigger the disassembly ofsynthetic microtubules upon illumination. This approachpresents a promising strategy for photodynamic cancertherapy [63].

ConclusionsMicrotubules are one of nature’s best examples in terms ofdynamic ‘smart materials’. Their primary role in livingsystems is reflected in their ubiquity and the multitude offunctions they fulfil in unicellular as well as higher level

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Opinion Trends in Biotechnology Vol.26 No.6

organisms. They are self-assembling structures with out-standing mechanical properties, which combine high stiff-ness and high resilience. Microtubule mechanics isstrongly determined by the PF architecture. Long-rangesliding and twisting of PFs dissipates bending energyefficiently. Moreover, the PF structure and the associationwith microtubule-associated proteins and molecularmotors generate a stunning variety of microtubule-basedhierarchical structures, from simple bundles to highlyspecialized cilia and flagella. Each one of these structureshas a distinct functional role in the cell. CNTs, as ananotechnological counterpart of microtubules, are idealcandidates for realizing biomimetic functional equivalentsof microtubules and microtubule-based organelles. One ofthe major challenges associated with this goal is theunspecific hydrophobic aggregation of CNTs in liquidphase. However, new promising approaches for the self-assembly of CNTs in fluid environments can overcomethese problems [58,64].

Understanding the relation between molecular archi-tecture and function in microtubules can provide a wealthof insights and inspiration for material scientists. Mimick-ing the principles learned from microtubules can movebionanotechnology a step closer toward a next generationof materials with improved mechanical efficiency and awide range of functional properties.

AcknowledgementsWe thank Katja Taute for providing a critical review of the manuscriptand numerous valuable suggestions. We gratefully acknowledge A.Hoenger, A. Kruljac-Letunic, T. Surrey, and Z. Yao for helpfuldiscussions and comments on the manuscript and L. Sandblad forproviding cryo-electron microscopy data. We thank the National ScienceFoundation and the Landesstiftung Baden-Wurttemberg for financialsupport. F.P. thanks E.H.K Stelzer for support and interestingdiscussions.

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