LETTERS
Microfabricated adhesive mimicking
gecko foot-hair
A. K. GEIM*1, S. V. DUBONOS1,2, I. V. GRIGORIEVA1, K. S. NOVOSELOV1, A. A. ZHUKOV2
AND S. YU. SHAPOVAL2
1
Department of Physics & Astronomy, University of Manchester,Manchester M13 9PL, UK
2
Institute for Microelectronics Technology, 142432 Chernogolovka, Russia
*e-mail: geim@man.ac.uk
a
Published online: 1 June 2003; doi:10.1038/nmat917
he amazing climbing ability of geckos has attracted the interest of
philosophers and scientists alike for centuries1 3.However,only in
Tthe past few years2,3 has progress been made in understanding the
mechanism behind this ability,which relies on submicrometre keratin
hairs covering the soles of geckos.Each hair produces a miniscule force
H"10 7N (due to van der Waals and/or capillary interactions) but millions
of hairs acting together create a formidable adhesion of H"10 N cm 2:
sufficient to keep geckos firmly on their feet,even when upside down on
a glass ceiling. It is very tempting3 to create a new type of adhesive by
mimicking the gecko mechanism. Here we report on a prototype of
such gecko tape made by microfabrication of dense arrays of flexible
plastic pillars, the geometry of which is optimized to ensure their
b
collective adhesion. Our approach shows a way to manufacture self-
cleaning, re-attachable dry adhesives, although problems related to
their durability and mass production are yet to be resolved.
In principle, any submicrometre object whether it is the tip of an
atomic force microscope (AFM), a small piece of dust or a single gecko
hair sticks to a solid surface with an adhesive force in the range 10 to
1,000 nN, depending on the exact geometry and materials involved2 6.
In the case of hydrophilic materials, it is often an atomic layer of
absorbed water (capillary force) that is responsible for the adhesion4,5.
The capillary force decreases with decreasing a characteristic size R of an
object, and can be estimated as Fc H" ÃR where à is the surface tension of
water. On a submicrometre scale, van der Waals interaction is also no
longer negligible and can compete with the capillary force. A typical
value of van der Waals forces for a submicrometre object is H"100nN,and
this force becomes dominant in the case of hydrophobic surfaces2 5.It is
interesting to note that diameters of gecko foot-hair (0.2 to 0.5 µm)
fall exactly in the range where the two forces become comparable3.
This probably indicates that, in the course of evolution, geckos have
developed their foot hairs to be of the most appropriate diameter to
exploit both the van der Waals and capillary forces,and to climb surfaces
of various hydrophilicities. Figure 1 Scanning electron micrographs of microfabricated polyimide hairs.
Based on the understanding of the gecko s climbing mechanism,an a,A small area near the edge of a 1 cm2 array of polyimide hairs.This array was later
AFM tip has been used to produce a set of dimples on a wax surface, used to evaluate macroscopic adhesive properties of the mimetic material. b, Bunching
which was then used as a mould for making a number of mesoscopic is found to be one of the mechanisms responsible for the reduction of adhesive strength
polymer pyramids3.With the help of another AFM cantilever with a flat of the artificial hair.This micrograph also demonstrates the high flexibility of polyimide
tip,the adhesive force to an individual pyramid was measured.The force pillars. Both scale bars are 2 µm.
nature materials | VOL 2 | JULY 2003 | www.nature.com/naturematerials 461
© 2003 Nature Publishing Group
LETTERS
1.0
0.2/0.15
5
0.4/1.5
0.3/0.15
0.6/1.5
0.4/0.15
1
0.6/2.0
0.8/2.0
0.5
1.2/0.15
4.0/0.15
0.1
0.5 1 2 4
P (µm)
0
0 10 20 30
Figure 2 The perpendicular force F required for detaching various samples of
S (mm2)
polyimide hairs from a silicon surface. The experimental points are marked by D/H,
indicating the hairs diameters D and heights H, respectively.The solid curve is the best
fit to F " P 2.
Figure 3 The adhesive force F exhibited by gecko tape as a function of contact
area S. Squares are experimental data; the solid line is the best linear fit.
was found to be H"200 nN, which is comparable to the average adhesive
force estimated per individual gecko hair2,3.
It would be natural to assume that large arrays of tip-like All data obtained in the above measurements (and repeated for
submicrometre objects would mimic gecko s foot-hair and provide a another preload) fall on a single smooth curve if plotted against the
similar adhesion. However, one has to take into account that real separation between pillars P. Figure 2 clearly shows that F is
surfaces are never ideally flat.In fact,our earlier attempts (unpublished) proportional to the density of hairs (P 2) and depends only weakly on H
to imitate gecko foot-hair by making large arrays of plastic tips (similar and D.Although intuitively plausible, this finding is rather unexpected,
to the pyramids reported previously3) have failed. The reason was that because both capillary and van der Waals forces depend2 6 on D.
only a small fraction of tips based on a solid substrate could make We speculate that large polyimide pillars do not attach over the whole
physical contact with the opposite surface (see below). Thus, to create a top surface, but that each pillar makes a point-like mechanical contact,
gecko adhesive, one also has to find a way to make hairs sufficiently which results in a fixed value of force per pillar, almost independent
flexible and to place them on a soft, flexible substrate, so that individual of its diameter.
tips can act in unison and attach to uneven surfaces all at the same time. These results suggest that, for maximum adhesion, one has to
To meet these non-trivial objectives, we have chosen to micropattern maximize the number of hairs capable of attaching to a surface,and their
thin polyimide films,which are both robust and flexible and allow a wide particular geometry is of less importance. One also has to consider
range of microfabrication procedures. limitations on the maximum density of hairs.For example,hairs should
Figure 1 shows two examples of polyimide hairs microfabricated be flexible enough to attach to uneven surfaces but should not break,
using electron-beam lithography and dry etching in oxygen plasma (see curl or tangle. Examination in a scanning electron microscope (SEM)
Methods). While searching for the most suitable design for such revealed that very thin pillars (D < 0.3 µm) tend to fall down, whereas
artificial hairs, we first tested how their adhesion depends on geometry. long, closely spaced hairs tend to bunch after being in contact with the
To this end, we prepared ten relatively small (H"50 × 50 µm2) arrays of opposite surface (Fig. 1b).For an optimal geometry,we have eventually
hairs having diameters Dranging between 0.2 and 4µm,heights H from chosen hairs to be as long as we could make them (HH"2µm),reasonably
0.15 to 2 µm, and periodicity P from 0.4 to 4.5 µm (parameters of all dense (P H" 1.6 µm) and not too thin (D H" 0.5 µm) (Fig. 1a).
samples are given in Fig. 2).Then,we measured the perpendicular force To test the adhesive properties of the optimized mimetic material on
F required to detach the samples from a SiO2 surface. This was done by a macroscopic scale, we microfabricated a large (1 cm2) sample of
using an AFM (in force mode) with a home-made cantilever having a polyimide pillars with the above parameters. With the polyimide
flat silicon tip of H"50 µm in size.The adhesion between our samples and microstructure remaining on a silicon wafer, we pressed the sample
the flat tip was found to depend strongly on the initial preload and,for all against a microscope glass slide with a preload force of H"20 kg. Rather
the measurements in Fig. 2, we used the same preload H"10 mg (the discouragingly, this resulted in a very small adhesive force of H"0.01 N or
maximum allowed by our cantilever).We note that this preload was not 1 g. This shows that less than 1% of hairs were in actual contact with
sufficient to provide an optimum contact and, even for densest arrays, the glass surface. To increase the number of active , attaching hairs, the
the adhesive force reached only H"10 µN. Comparison of this value with microfabricated polyimide film was peeled off the wafer and transferred
a typical adhesion of H"200 nN measured between a standard AFM tip onto a soft bonding substrate (see Methods), so that the resulting
and the top of an individual polyimide pillar shows that only a small material could be handled just as an adhesive tape. The use of a soft
number (<100) of hairs could actually make contact with the flat tip. rather than solid base has dramatically (by nearly 1,000 times) improved
462 nature materials | VOL 2 | JULY 2003 | www.nature.com/naturematerials
© 2003 Nature Publishing Group
(arbitrary units)
(N)
LETTERS
gecko tape to support the weight of a suitably light familiar object
(a toy in Fig.4).
We found that the gecko tape could go through several
detachment attachment cycles before we noticed a degradation of its
adhesive properties. A subsequent SEM analysis revealed that this was
related to fallen and broken hairs. Many hairs were lying on the
polyimide surface, presumably kept in this position by the capillary
force. We have also tested the tape on different surfaces and found that
the adhesion force varied only by a factor of three between hydrophobic
GaAs and hydrophilic SiO2. This indicates that our microfabricated
adhesive makes use of both capillary and van der Waals forces.
Regarding possible applications, we believe that, instead of
expensive and slow electron-beam lithography, other techniques (for
example,based on the electrical instability of polymers7) can be applied
to produce a similar material in large quantities. Our major concern is
the durability of such microfabricated adhesives, which limits the
number of successful re-attachments. Durability can probably be
improved by trying other materials,which would be sufficiently flexible
similar to hydrophilic polyimide but strongly hydrophobic
(keratin, as in gecko hair, is a possible candidate). In this case, the hairs
would not stick to each other or to the base surface, which should
improve the resistance to attachment detachment cycles. In addition,
it would be possible to use denser arrays of pillars, thereby further
increasing their adhesive strength.
METHODS
MICROFABRICATION
Arrays of submicrometre hairs were microfabricated by using the following set of procedures. First, we
prepared a 5-µm-thick polyimide film (pyromellitic dianhydride-oxydianiline polyimide; baked at
250 °C) on top of a silicon wafer. Next, by using electron-beam lithography, thermal evaporation of an
aluminium film (H"150 nm thick) and lift-off, we prepared an array of submicrometre aluminium disks.
The resulting aluminium pattern was then transferred in the polyimide film by dry etching in oxygen.
The oxygen plasma etching was essential, because it provided a large difference between etching rates of
aluminium oxide and polyimide, so that several micrometres of polyimide could be removed before the
aluminium mask disappeared. In addition, we applied a negative d.c. bias to the substrate, which allowed
us to achieve large aspect ratios (for example, see ref. 8), making the pillars sufficiently tall.
Figure 4 Re-attachable dry adhesives based on the gecko principle can find a
PEELING OFF AND TRANSFER OF POLYIMIDE FILMS ONTO ANOTHER SUBSTRATE
variety of applications. The photo illustrates this point by showing a spider-man toy
Polyimide films are very robust even when a few micrometres thick, and their peeling from the silicon
clinging with one of its hands to a horizontal glass plate.The toy (15 cm high; weighing
wafer does not require any special skills or accuracy. Several support materials and bonding methods were
40 g) has its hand covered with the microfabricated gecko tape, which provides a tried (using unstructured films) before we finally opted for the use of scotch tape as the simplest and most
reliable scheme for placing the peeled-off microstructures on a soft base. Note that transfer onto another
H"0.5 cm2 contact with the glass and a carrying capacity of >100 g. Note that the toy
substrate would not be required, if thicker (>50 µm) polyimide films were used. In our case of thin
was already re-attached several times to various surfaces before this photo was taken.
(5 µm) films, the use of a thicker flexible substrate was advantageous because it simplified handling of the
microstructured material and made this more reliable.
Received 13 January 2003; accepted 27 April 2003; published 1 June 2003.
the adhesive capacity of the microstructure (Fig. 3).We believe that the
References
added flexibility of the base allowed the mimetic tape to compensate for
1. Aristotle Historia Animalium Book IX (trans. Thompson, D. A. W.) (Clarendon, Oxford, 1918);
unevenness and dust particles on the surface, which could not be
http://classics.mit.edu/Aristotle/history_anim.html.
compensated for by the flexibility of polyimide pillars alone.
2. Autumn, K. et al. Adhesive force of a single gecko foot-hair. Nature 405, 681 685 (2000).
3. Autumn, K. et al. Evidence for van der Waals adhesion in gecko setae. Proc. Natl Acad. Sci. USA 99,
Adhesive properties of the resulting gecko tape were characterized
12252 12256 (2002).
by measuring the dependence of its adhesion on contact area S by using
4. Cappella, B. & Dietler, G. Force-distance curves by atomic force microscope. Surf. Sci. Rep. 34, 1 104
millimetre-sized glass wedges and a laboratory balance. The force was
(1999).
found to vary linearly with S (Fig. 3),and to be virtually independent of
5. Yamamoto, K., Tanuma, C. & Gemma, N. Competition between electrostatic and capillary forces
preload (for preloads e"50Ncm 2),which proves that practically all hairs
acting on a single particle. Jpn J. Appl. Phys. 34, 4176 4184 (1995).
on the gecko tape could attach to macroscopic surfaces simultaneously.
6. Stork, N. E. Experimental analysis of adhesion of Chrysolina polita (Chrysomelidae: Coleoptera) on a
variety of surfaces. J. Exp. Biol. 88, 91 107 (1980).
The average force per hair was found to be H"70 nN,and the whole 1 cm2
7. Schäffer, E., Thurn-Albrecht, T., Russell, T. P. & Steiner, U. Electrically induced structure formation
patch was able to support 3 N. This number is comparable to the
and pattern transfer. Nature 403, 874 877 (2000).
estimated adhesive force of 10 N cm 2 for gecko foot-hair2,3.Note that,in
8. Sekine, M. Dielectric film etching in semiconductor device manufacturing: Development of SiO2
control experiments, similar but unstructured polyimide films (both
etching and the next generation plasma reactor. Appl. Surf. Sci. 192, 270 298 (2002).
freshly prepared and subjected to dry etching without patterning)
exhibited negligibly small adhesion (<10 3 Ncm 2).Figure 3 also proves
Acknowledgements
that our experiment can be scaled up, that is, that larger areas of gecko This work was supported by EPSRC (Engineering and Physical Sciences Research Council) UK. S.V.D.,
A.A.Z. and S.Y.S. also acknowledge the financial support of the Russian Academy of Sciences.
tape would support much heavier objects. For example, human palms
Correspondence and requests for materials should be addressed to A.K.G.
have a total area of more than 200 cm2 and,if covered by such a material,
would be able to support the weight of an average human.To emphasise
Competing financial interests
this point,we used the available (unfortunately,rather small) amount of
The authors declare that they have no competing financial interests.
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© 2003 Nature Publishing Group
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