Reversible Frictional Adhesion of Natural and Bio-Inspired Multi-Scale Structures

1
Reversible Frictional Adhesion of Natural and
Bio-Inspired Multi-Scale Structures (NIRT
0708367) Kimberly Turner, Thomas Kenny, Jacob
Israelachvili, Mark Cutkosky Dept. of
Mechanical Engineering, UC Santa Barbara, CA
93106 Dept. of Mechanical Engineering, Stanford
University, CA 94305 Dept. of Chemical
Engineering, UC Santa Barbara, CA 93106
Introduction
Improved Roughness Adaptation and Patch Scale-Up
with Bi-Layer Hierarchical Adhesive Structures
Geckos have the remarkable ability to climb and
suspend from several types of rough and smooth
surfaces. The hierarchical gecko adhesive system
is characterized by a high degree of
repeatability, reversibility and control,
durability and self-cleaning. The fact that van
der Waals forces are always present between any
two surfaces in close contact has motivated the
fabrication of dry adhesives inspired by the
gecko. Application of synthetic adhesive systems
to climbing robots requires the development of an
adhesive with both high adhesion and friction
force components, as also with the ability to
both stick and peel off rapidly. Hierarchical
multi-scale bio-inspired adhesive systems have
been developed to satisfy these requirements.
Structural Hierarchy in the Gecko Adhesion System
(A) Macrostructure ventral view of a tokay
gecko climbing vertical glass. (B)
Mesostructure ventral view of the foot, with the
adhesive lamellae visible as overlapping pads.
(C) Microstructure proximal portion of a single
lamella, with individual setae in an array
visible. Nanostructures single seta with
branched structure at its upper right (D),
terminating in hundreds of spatular tips (E).
Adaptation to a wide variety of surface
roughnesses is crucial for applications in
climbing robots. Figure (A) on the left shows an
SEM image of a hierarchical adhesive made of a
polymeric microwedge fine layer supported over
compliant directional polymer stalks (DPS). The
advantages afforded by structural hierarchy are
evident in the increased adhesion measured when
compared with non-hierarchical structures on
rough surfaces such as granite and wood (Figure
B). Improved conformality with rough surfaces
using hierarchical adhesive structures also
enables more efficient patch scaling to larger
areas (Figure C), enabling support of higher
payloads.
Broader Impacts

• Each hierarchical level works synergistically to
  produce high adhesion.
• 14,400 per mm2 setae, 110 µm in length, 5 µm in
  diameter.
• 100-1000 spatulae per setae, 200 nm in length.
• Each seta withstands 200 µN.

Microfabrication of Synthetic Polymer Setae

• International symposium on gecko-inspired
  adhesive design conducted in July 2008 at UCSB to
  promote closer collaboration and idea exchange
  between research groups in the area at UCSB,
  Stanford University and the Leibnitz Institute
  for New Materials, Germany
• Development of an educational documentary on
  adhesion and friction, focusing on their everyday
  life effects on animal and human behavior, is in
  progress

Microfabrication of angled gecko seta-like
polymeric stalks has been performed by the
following method
Autumn, K. at al. Nature, 405, 681-685.
(ii) PDMS mold fabricated using silicon masters.
Desired final pillar angle built into mold by
shearing before fully curing
(i) Microfabrication of silicon masters for
molding
(iii) Angled polymer micro-pillar structures
fabricated using PDMS mold
Contact Dynamics Measurements of Gecko
Setae Using Surface Forces Apparatus (SFA)
Future Work

• Design and construction of a test apparatus for
  precision friction and adhesion force testing on
  large meso-scale adhesive patch samples
• Development of synthetic adhesives focusing on
  mimicry of adhesion control observed in the
  natural system using shear forces (Frictional
  Adhesion)
• Design of synthetic adhesives capable of
  conforming and adhering to an even wider variety
  of surface roughnesses
• Design of robotic systems capable of systematic
  actuation to engage and disengage the dry
  adhesives rapidly

Future Hierarchical Fabrication A further
refinement to the angled polymeric synthetic
setae already fabricated is the ongoing
fabrication of a hierarchical structure with both
micrometer and sub-micron elements, to obtain
larger contact areas and adhesion when dealing
with surfaces of differing roughness. Further
similarity with the gecko adhesive structure will
be achieved with future integration of nanometer
and micrometer scale elements in the future.

• Gecko setal arrays are structurally anisotropic,
  exhibiting strong directional adhesion and
  friction properties.
• Shearing in the gripping direction leads to
  maximum adhesion and friction shearing in the
  opposite releasing direction leads to
  detachment.
• Geckos do not slide, except under severe
  conditions.
• To mimic gecko adhesive pads and functionalities,
  anisotropic curved structures are necessary.

Acknowledgements
Zhao, B. et al. Langmuir, (in press).
This research is supported by the Nanoscale
Interdisciplinary Research Teams (NIRT) Grant No.
0708367 through the National Science Foundation