This document summarizes research on developing gecko-mimetic dry adhesives for potential space applications. The research investigated growing vertically aligned carbon nanotubes (VA-CNTs) on flexible metal foil and lithographic nickel cantilever substrates to create hierarchical, compliant structures capable of van der Waals adhesion forces. Mechanical testing of CNT samples on silicon and nickel substrates showed adhesion in the range of 0.5-2 N/cm2 for small areas and below 1.5 N/cm2 for larger areas, with 15-20% of samples exceeding 0.1 N/cm2. While progress was made in growing CNTs on flexible substrates and measuring adhesion of larger sample areas, significant challenges remain

1. CNR Roadshow 2014
Gecko Mimetic Adhesives for space applications
Elettra – Sincrotrone Trieste, Italy
Luca Gregoratti, Andrea Goldoni, Olexander Trygub, Marco Marazzi
Massimo Tormen, Gianluca Grenci, Simone Dalzilio
CNR – IOM, Trieste, Italy
Alessandro Gasparetto, Renato Vidoni, Sander De Ridder
Università degli studi di Udine, Udine, Italy
A1-6313/09/NL/SFe

2. 2
Presentation of Elettra - Sincrotrone Trieste
Elettra
Synchrotron
Research Center
Trieste
Elettra is an international multisciplinary laboratory specialized in synchrotron radiation
and its use in the science of matter. It hosts more than 35 laboratories.
The mission of Elettra - Sincrotrone Trieste through its Elettra Research Center is to
promote cultural, social and economic growth through basic and applied research in
relevant fields, technical and scientific training, and technology transfer.
Elettra is the coordinator of

3. 3
Local partnership & collaboration
Elettra - Sincrotrone Trieste is
part of
• Multi Sectoral Tech Park
• Technology Transfer
• 65 tenant companies
• 21 research centres
Elettra - Sincrotrone Trieste is associate
Laboratory of
Sincrotrone Trieste has permanent
collaborations with

4. 4
CNT wide research
CNT
X-ray beam spot
C
Au 1.5 µm
CNT diameter: 60-70 nm
From high density
to low density
MCNT arrays
Confined patches on
single nanostructure

5. 5
Outline
• Aim of the project
• State of the art
• Approaches
- model systems
- all metal carbon
- polymer/metal/carbon
• Mechanical and other tests
• Conclusions

6. 6
Aim of the project
The project aims at developing the gecko mimetic tape concept
from its current laboratory origins
Current status
• Production limited to small areas
• Laboratory scale technologies
• High production costs
• Rigid substrates
• Reversibility (?)
Future technologies
• Low cost technologies
• Industrial scale
• High performance
Why a dry adhesive?
• For space business they should overcome the main limitations of conventional
adhesives: operating temperature range, outgassing performance in vacuum
applications.
• Further applications also in daylife applications

7. Literature review - state of the art
• Van der Waals Forces
• Capillary Forces
• Anisotropic adhesion
• Anti-bunching conditions
• Self-Cleaning properties
• A low-to-no adhesion state in
absence of shear
Picture from: Yu et al., Adv. Funct. Mater. 2011, 21, 3010–3018
Picture from web..See: Kesel et al., J Exp Biol206, 2733-2738, 2003
• Relatively soft elastomer fibers with
mushroom-shaped tips
• Stiff, very high aspect ratio fibers, which
exhibit good friction forces, but either
low or zero normal adhesion strength
Natural adhesion mechanisms
Mimicking materials/methods
Fabrication methods
• Micro/nano casting
• Gas phase fabrication methods

8. Manufacturing and processing parameters
• high-density (i.e. pattern and periodicity of the
synthetic setae)
to achieve a high adhesion force
• high aspect-ratio (i.e. length, diameter and
stiffness of the shafts)
to achieve a high adhesion force and an anti-
bunching condition
• mushroom-shaped structures (i.e. size, shape
and stiffness of the spatulae ends)
to increase the contact area and optimize
adhesion
• angled/inclined fibers
to produce directionality, make the detaching
more convenient and tune the mechanical
properties
• hierachical and branched structure
to improve the surface adaptability
The existing results show that necessary conditions and key
parameters for creating an effective dry-adhesive are:
Hierarchical level structures

9. 99
State of the art
• The already developed dry adhesives are typically produced as very small
samples of few squared millimetres of active area. Technical solution for a large
area samples are missing.
• In most of the cases the proposed technologies are lab-sized and cannot be
easily extended to a large scale production. Due to it the available technologies
are also expensive.
• Data available in the literature rarely include reversibility and repetition rate of
the adhesion process with the same sample. This feature must be investigated
more deeply.
• Most of the technical solutions reported in literature use rigid substrates which
are fine for proof of principle experiments but have large limitations when used
on real systems.
The technical solutions developed in this project will hopefully contribute to
overcome these limitations and as will be described later have been selected
and studied having in mind the above mentioned scenario.

10. 10
Approaches
Development of “dry” reversible adhesives main routes
All metal/carbon technology
Compliant
substrates:
Polymer-plastic/carbon technologies
Nanostructures: VA-CNTs
polymer-plastic
Nanostructures:
• thin metal foils (e.g. Ni)
• metal cantilevers
• CNT bushes • VA-CNTs
• polymer-plastic
nanostructures
For the simplest
architectures no need
for lithographic steps
Use of lithographic
approaches
Compliant
substrates:
Need to trasfer CNTs from
growing substrate to
polymer/plastic

11. 11
Model systems
Growth of the Vertically Aligned Multiwall Carbon Nanotubes (VA-CNTs): CVD
• 650-700 °C
• 2 feeding gases (H2, C2H2)
• 30 mbar of C2H2
VA-MWCNTs
Diameter: < 10 – 30 nm
Model system: Vertically Aligned Multiwall Carbon Nanotubes on Si substrates
Major limitations:
• Rigid substrate
• No compliance
• Expensive
HV environment (2 vaccum stages) / near ambient pressure environment

12. 12
Deposition of the catalyst
Selected catalyst for the growth of VA-CNTs: Fe
Ways of deposition investigated
• HV evaporation of pure metallic Fe
• Evaporation of pentacene
• Spin coating of Fe nanoparticles
HV required
Ambient pressure
Spin coating: not very
homogeneous coverage: need
better tuning of the parameters
Need for post-evaporation
thermal treatments

13. 13
Deposition of the catalyst - summary
Results achieved
• The best results have been obtained by evaporating the Fe directly from the
metallic source.
• Use alternative lower cost procedures such as ferrocene evaporation and spin
coating of a solution containing Fe nanoparticles is possible. No uniform
distribution catalyst nanoparticles.
Results not achieved and drawbacks
• Need more experiments (e.g. to define the temperature of the evaporation of
ferrocene or the solvent used to disperse the Fe nanoparticles and their density
in the solution) for trimming ferrocene evaporation and spin coating of a solution
containing Fe nanoparticles.
What’s new with respect to the state of the art technology
• The evaporation of Ferrocene molecules and the spin coating of Fe
nanoparticles represent two valid low cost alternatives for the deposition of the
catalyst needed for the growth of the CNTs. Both solutions are not sufficiently
documented in the literature and have never been used for this application.

14. 14
All metal / Carbon
VA-MWCNTs/Ni thin foil Self compliant CNTs
CNTs bushes + VA-MWCNTs
Ni thin foils (0.05-0.1 mm thk) are:
Flexible and cheap
1st hierarchical level

15. 15
All metal / Carbon
VA-MWCNTs/Ni thin foil Self compliant CNTs
Problems found
• Low adhesion of CNTs on substrate
• Need for buffer layer: Si and Al2O3 tested
• Bad results for Si not easy to make a
uniform layer of Al2O3
Compression test of the CNT bushes
Growth of the second VA-CNTs
hierarchical level under electrical field
Al2O3 deposition:
- Sputtering
- Al dep in O2

16. 16
CNTs on Si/Ni substrates, self compliant CNTs - summary
Results achieved
• The CVD procedure used for the growth of the CNTs is largely used in the scientific community.
Conditions to grow VA-MCNTs on Ni foils have been found
• Large region of samples with the two layers of CNTs were grown. Some regions are covered by
vertically aligned CNTs while other regions look smoother and at a closer view show randomly
distributed CNTs.
Results not achieved and drawbacks
• The improvement of growth of the second layer of CNTs grown on top of the bushes by
trimming the parameters can in principle be possible, but required more resources and efforts.
The resulting adhesion of these samples was not significant.
What’s new with respect to the state of the art technology
• This approach is really innovative; to our knowledge no similar experiment has been published
in peer review journals so far. It needs to be investigated and trimmed to show all its
potentialities but we consider it a valid solution for the generation of a compliant substrate
covered by Van der Waals force generators.

17. 17
Lithographic growth of Ni cantilevers
Hot embossing
Ni cantilever
before mould
release
Ni cantilever
after mould
release

18. 18
Lithographic growth of Ni cantilevers
Array of ‘’clean’’ cantilevers After the growth of VA-CNTs
High temperature!!
After adhesion the
‘’weaker’’ cantilevers tend
to bend: necessity for
better design

19. 19
Lithographic growth of Ni cantilevers - summary
Results achieved
• Samples of Ni cantilevers have been realized; the microstructures have a good
precision and reproducibility.
• VA-MCNTs were grown on top of the Ni cantilevers. The CNTs forms bundles
which are vertically aligned.
• The approach based on micron sized flexible levers for the compliance is in our
opinion valid; other solution are possible.
• Low mechanical adhesion.
Results not achieved and drawbacks
• The lithographic procedure to obtain good Ni cantilevers is very long and
complicated.
• During the adhesion tests performed with these samples we encountered
several problems due to the balance that is required between the adhesion of
the CNTs to the foreign material and that of the Ni cantilevers to the substrate.
No significant results were obtained.
What’s new with respect to the state of the art technology
• Combination of mesoscale compliant substrate with nanoscale structures for
VdW generation. No reference in literature.

20. 20
Mechanical tests
Loading tests Sample holder for pull-off and
shear tests
Surfaces used for
adhesion tests:
Polystyrene
(rigid)
Al foil
• Results obtained by using
patterned/not patterned
surfaces
• Sample stored and used
in normal athmosphere

21. 21
Mechanical tests

22. 22
Mechanical tests
Pull off avg adhesion loads
CNTs / Si CNTs / Ni foil
Small area samples
A < 1cm2
0.5 – 2 N/cm2
Good repetition rate
-
Large area samples
A > 1cm2
< 0.5 N/cm2
Good repetition rate
< 1.5 N/cm2
Bad repetition rate
Shear avg adhesion load
CNTs / Si
Small area samples
A < 1cm2 1.2 – 9 N/cm2
Large area samples
A > 1cm2 < 1 N/cm2
• Percentage of samples which crossed the adhesion threshold (0.1-0.2 N/cm2): 15-20%
• No major differences were noticed while using polystyrene or aluminum as testing substrates.
• No major differences were recorded between patterned and not patterned samples – maybe
already ideal density.

23. 23
Mechanical tests - failures
• Rigid substrates are very sensible to dust/defect
particles
• Damage of CNTs
• Detachment of CNTs from substrate
Ni foil – buffer layers

24. 24
Mechanical tests
Results achieved
• Absolute load values found for the CNTs samples grown on Si and Ni substrates are close to
what typically found in the literature.
• Realization of samples with large active area
• 15-20% of the samples produced for adhesion showed an adhesion load higher than 0.1 N/
cm2. Average loads for Si substrates pull-off 0.5-2 N/cm2.
• The higher adhesion values/cm2 have been achieved for small samples with active areas < 1
cm2 but measurable values have been recorded also for large active area samples (area > 1
cm2).
• No major differences were noticed while using polystyrene or aluminum as testing substrates.
• The use of Ni foils as substrates for the CNTs did not enhanced the adhesion loads. Poor
adhesion of CNTs with the Ni foil. This problem could be fixed in principle by adding a proper
buffer layer.
• As expected adhesion tests of the shear mode produced higher adhesion loads than what
achieved during pull off.
• Samples manipulated and tested in normal environments rather than clean rooms typically
available in the laboratories.
• Good repetition of the adhesion tests that can be performed on the sample with reasonable
load values. The reversibility of the adhesion is a parameter typically not reported in the
literature where only the best adhesion result is provided.

25. 25
Mechanical tests
Results not achieved and drawbacks
• The adhesion loads achieved with the Ni foils did not produce the expected
results. The reason was the poor adhesion of the CNTs to the Ni foil; we tried to
introduce a buffer layer between the CNTs and the Ni in order to improve the
adhesion; among the materials tested the Si did not improved significantly the
adhesion and we did not have enough time to trim the growth/deposition of Al2O3
buffer layer.
• As already discussed the use of cantilevers to perform a compliant substrate
suffered from the complexity of the realization and cannot be considered as a
valid solution.
What’s new with respect to the state of the art technology
• The active area of the sample produced is generally larger than what reported in
the literature.
• We reported the data for the repetition rate of the adhesion that is generally not
provided in literature.

26. 26
Other tests (following the ESA technical requirements)
q Functional & Performance Requirements
v Storage and working conditions of the material
-80 °C
q Environmental Requirements
q Human Factors Requirements
q Degassing UHV mass spectrometry &
quarz microbalance, no
organic pollutants
q Fine chemical characterization:
photoemission
spectromicroscopy

27. 27
Conclusions
Main results
(also partially achieved)
Ø Growth of CNTs on flexible/compliant substrates
Ø Use of larger substrates
Ø Investigation of low cost technical solutions
Main drawbacks
Ø Low reproducibility of the CNTs parameters
Ø Low reproducibility of the adhesion values
Ø Best adhesion values still far from wet adhesives
Possible next steps
§ Micro-punching
§ Self compliant CNT
§ Transfer of CNTs
Obtaining
compliant micro
cantilevers with
micro-punching
masters
§ High level surface finishing
§ Fine alignment for large areas

28. 28
Conclusions / other results

29. 29
Conclusions
• We produced and successfully measured the adhesion properties of large
samples (2.20 cm2 of active area). Results reported in literature are usually
referred to much smaller samples.
• We reported results about reversibility/reproducibility of the adhesion of several
samples showing how samples can be attached and removed from substrates
several times. Again results reported in literature never describe this property
which is very important in case of a spread use of the adhesive.
• We got some good results using as substrate thin Ni foils, which exhibit a first
level of compliance with respect to the Si substrates commonly used. Ni foils do
not represent the ideal substrate like plastic or polymers but the technical
solutions developed represent a step forward with respect to the state of the art
technology mainly based on Silicon. The results obtained demonstrate how the
growth of CNTs can be performed directly on metallic substrates.
• Other technologies such as the growth of vertically aligned CNTs on CNTs
bushes have been explored but need further development to demonstrate their
potentialities.