Abstract: The last 20 years have seen considerable
interest in bioinspired dry adhesives, based on discoveries regarding the
adhesive system of the gecko and some arthropods. Such adhesives typically
have the advantage of being reusable, leaving no residue, and allowing
control of the adhesion through loading states. However, the number of
practical applications of these adhesives remains small. One possible reason
is that unlike in mechanical design, where design, simulation, and testing
methodologies are all well established, there are significant gaps in all of
these phases of engineering as applied to gecko-inspired adhesives. There
are a variety of methods and metrics used for evaluating adhesives, often
giving differing results, and even in some cases results that do not
accurately reflect those observed in practical applications. Even with an
accurate evaluation of an adhesive material, refining the design is
challenging, as the design and manufacturing methods are typically
time-consuming, highly constraining, or both. At the same time, there
continues to be growing interest in the use of these adhesives in
wide-ranging applications including reusable tapes and bandages; improved
and more gentle industrial grippers; and grasping objects in space, where
the combination of large objects, low contact forces, and lack of atmosphere
make adhesives of particular interest. To address this growing need for
improved ability to design and manufacture adhesives tailored to these
applications, a three-pronged approach is taken. An improved method for
testing gecko-inspired adhesives is presented. Unlike the common testing
paradigms published in the literature, which impose a fixed displacement
between the adhesive material and a test surface, the proposed testing
method uses a series elastic configuration to apply forces to the adhesive.
This shift in test control from displacement-space to force-space allows the
testing conditions to be aligned to those seen in applications; whether for
climbing, grasping, or adhesive tapes, nearly all applications of
gecko-inspired adhesives fundamentally involve force-space constraints in
normal conditions. It is shown that by testing the adhesives in similar
conditions to those observed in use, the measured limit curves better
reflect those seen in practice. Further, in cases where the adhesive
structures are more complicated, or more integral to the performance of the
adhesive---such as the directional, controllable adhesives at the core of
this work---force-space testing enables measuring the full capabilities of
the adhesive, which in many cases are impossible to measure in
displacement-space. With the ability to accurately measure more complex
limit curves, spatial variation is investigated as a means to improve the
ability to create adhesives with novel parameters. In this case, the
property of interest is a high friction ratio, the ratio of friction in a
preferred direction to friction in the opposite direction, a property of the
natural gecko adhesive system. Taking inspiration from the spatial variation
found on the gecko's feet, an adhesive structure with wedges of varying
length is developed, modeled, and analyzed. The friction ratio of this
adhesive is measured, indicating an improvement of orders of magnitude over
the current state of the art. Further, this adhesive structure also
demonstrates the possibility of simplifying the adhesive design problem.
Rather than developing a single complex feature to provide all of the
desired properties, spatial variation permits the development of multiple
features that are individually simpler but interact to provide more complex
behavior. A discussion of the manufacturing process and associated
fabrication constraints for these designed adhesive geometries follows. The
process is an extension of a previous manufacturing process developed for
making uniform adhesives. This is coupled with methods for directly
incorporating adhesives into larger assemblies to create tightly coupled
adhesive and sensing systems. Finally, a simplified design framework is
presented, synthesizing many of the concepts from the prior sections. The
current state of the art in adhesive simulation and modeling, while useful
for understanding and explaining various specific aspects of adhesive
design, is not adequate for directly analyzing the adhesion of complex
adhesive geometries. The framework is intended to be a heuristic that
synthesizes concepts from the various models of adhesion to provide useful
guidance for thinking about adhesive designs for particular applications.
Suresh, S.A., Hajj-Ahmad, A., Hawkes, E.W., and Cutkosky, M.R., "Forcing the
issue: testing gecko-inspired adhesives,"
Journal of the Royal Society Interface. 18 174, January 2021.
doi:10.1098/rsif.2020.0730.
Abstract: Materials are traditionally tested either by imposing controlled displacements and measuring the corresponding forces, or by imposing controlled forces. The first of these approaches is more common because it is straightforward to control the displacements of a stiff apparatus and, if the material suddenly fails, little energy is released. However, when testing gecko-inspired adhesives, an applied force paradigm is closer to how the adhesives are loaded in practice. Moreover, we demonstrate that the controlled displacement paradigm can lead to artefacts in the assumed behaviour unless the imposed loading trajectory precisely matches the deflections that would occur in applications. We present the design of a controlled-force system and protocol for testing directional gecko-inspired adhesives and show that results obtained with it are in some cases substantially different from those with controlled-displacement testing. An advantage of the controlled-force testing approach is that it allows accurate generation of adhesive limit curves without prior knowledge of the expected behaviour of the material or the loading details associated with practical applications.
Suresh, S.A., Kerst, C.F., Cutkosky, M.R., and Hawkes, E.W., "Spatially
variant microstructured adhesive with one-way friction,"
Journal of the Royal Society Interface. 16(150), January 2019.
doi:10.1098/rsif.2018.0705.
Abstract: Surface microstructures in nature enable
diverse and intriguing properties, from the iridescence of butterfly wings
to the hydrophobicity of lotus leaves to the controllable adhesion of gecko
toes. Many artificial analogues exist; however, there is a key
characteristic of the natural materials that is largely absent from the
synthetic versions—spatial variation. Here we show that exploiting spatial
variation in the design of one class of synthetic microstructure,
gecko-inspired adhesives, enables one-way friction, an intriguing property
of natural gecko adhesive. When loaded along a surface in the preferred
direction, our adhesive material supports forces 100 times larger than when
loaded in the reverse direction, representing an asymmetry significantly
larger than demonstrated in spatially uniform adhesives. Our study suggests
that spatial variation has the potential to advance artificial
microstructures, helping to close the gap between synthetic and natural
materials.
Suresh, S.A., Christensen, D.L., Hawkes, E.W., and Cutkosky, M.R., "Surface
and Shape Deposition Manufacturing for the Fabrication of a Curved Surface
Gripper," ASME Journal of Mechanisms and Robotics7(2), May
2015. doi:10.1115/1.4029492.
Abstract: Biological systems such as the gecko are
complex, involving a wide variety of materials and length scales.
Bio-inspired robotic systems seek to emulate this complexity, leading to
manufacturing challenges. A new design for a membrane-based gripper for
curved surfaces requires the inclusion of microscale features, macroscale
structural elements, electrically patterned thin films, and both soft and
hard materials. Surface and shape deposition manufacturing (S2DM)
is introduced as a process that can create parts with multiple materials, as
well as integrated thin films and microtextures. It combines SDM techniques,
laser cutting and patterning, and a new texturing technique, surface
microsculpting. The process allows for precise registration of sequential
additive/subtractive manufacturing steps. S2DM is demonstrated
with the manufacture of a gripper that picks up common objects using a
gecko-inspired adhesive. The process can be extended to other integrated
robotic components that benefit from the integration of textures, thin
films, and multiple materials.
Other Publications
Chen, T.G., Cauligi, A., Suresh, S.A., Pavone, M., Cutkosky, M.R. "Testing Gecko-Inspired Adhesives with Astrobee Aboard the International Space Station: Readying the Technology for Space."
IEEE Robotics and Automation Magazine. 2022.
doi:10.1109/mra.2022.3175597.
Abstract: Gecko-inspired adhesives can allow free-flying space robots to grasp and manipulate large items or anchor themselves on smooth surfaces. In this article, we report on the first tests conducted using gecko-inspired adhesives on a gripper attached to an Astrobee free-flying robot operating inside the International Space Station (ISS). We present results from on-ground testing as well as two on-orbit sessions conducted during early 2021. Recorded data demonstrated that an adhesive gripper for Astrobee could provide 3.15 N of force for manual perching. The adhesives functioned as anticipated, despite a lengthy storage period. The results also highlighted some considerations for future adhesive gripping in space with free-flying robots. We discuss these topics along with system design considerations for successful implementation aboard the ISS, raising the readiness of this technology for a space-grade environment.
Hajj-Ahmad, A., Suresh, S.A., Cutkosky, M.R. "Cutting to the Point: Directly Machined Metal Molds for Directional Gecko-Inspired Adhesives."
ASME Journal of Micro and Nano-Manufacturing. 2021.
doi:10.1115/1.4051406.
Abstract: Fabrication techniques for gecko-inspired adhesives generally target mold durability, adhesive performance, and process efficiency and simplicity. With these goals in mind, we present a micromachining process for creating reusable aluminum molds used to fabricate directional dry adhesives. The molds require deep, narrow and overhanging grooves to create sharp and angled adhesive features. This geometry precludes most traditional machining and lithographic material removal processes. The presented process is a hybrid of indenting and orthogonal machining, using a diamond-coated microtome blade as the tool. An finite element analysis reveals the local extent of work hardening as each groove is created, and helps to define a trajectory that reduces the effects of tool deflection and chip build up. The results of a series of experiments agree with predictions from the analysis and reveal a range of blade approach angles and a lower bound on groove spacing to achieve the desired geometry. This range is narrower than for molds machined from wax in previous work. Nonetheless, adhesive samples cast from the new metal molds achieve comparable performance to those previously cast from wax.
Kerst, C.F., Suresh, S.A., Cutkosky, M.R. "Creating Metal Molds for
Directional Gecko-Inspired Adhesives."
ASME Journal of Micro and Nano-Manufacturing. 2019.
doi:10.1115/1.4045764.
Abstract: We describe a process for creating durable
metal molds for the fabrication of directional, gecko-inspired dry
adhesives. The adhesives require microscopic inclined features with a
challenging combination of tapered geometry, high aspect ratio, and smooth
surface finish. Wedge-shaped features produced by the new metal mold exhibit
the same geometry and surface finish as those cast from single-use wax molds
and epoxy molds in previous fabrication methods. They also produce the same
levels of adhesion and shear stress. The metal molds, and the adhesives cast
from them, show no degradation after repeated molding cycles.
Wu, X.A., Huh, T.M., Sabin, A., Suresh, S.A., Cutkosky, M.R. "Tactile
Sensing and Terrain-Based Gait Control for Small Legged Robots."
IEEE Transactions on Robotics. 2019.
doi:10.1109/TRO.2019.2935336.
Abstract: For small legged robots, ground contact
interactions significantly affect the dynamics and locomotion performance.
In this article, we designed thin, robust capacitive tactile sensors and
applied them to the feet of a small hexapod with C-shaped rotating legs. The
sensors measure contact forces as the robot traverses different types of
terrain including hard surfaces with high or low friction, sand, and grass.
Different gaits perform best on different types of terrain. Useful measured
parameters include the magnitude and timing of the peak normal forces, in
combination with the leg rotational velocity. The measured parameters were
used in a support vector machine classifier to identify terrain types with
82.5% accuracy. Based on gait performance studies, we implemented a
terrain-based gait control using real-time terrain classifications. A
surface transitioning test shows 17.1% increase in body speed and 13.2%
improvement in efficiency as the robot adjusts its gait.
Naclerio, N., Kerst, C., Division, D.A., Suresh, S.A., Singh, S., Ogawa, K.,
Miyazaki, S., Cutkosky, M., Hawkes, E.W. "Low-cost, Continuously Variable,
Strain Wave Transmission Using Gecko-inspired Adhesives."
Robotics and Automation Letters. 2019.
doi:10.1109/LRA.2019.2893424.
Abstract: In robotics, high gear reductions are
often required when using an electromagnetic motor to drive revolute joints,
as in a humanoid robot. Strain wave gears (SWGs), also known as harmonic
drives, are often used. However, these transmissions are relatively
expensive and have a fixed gear ratio. Here, we present a low-cost
transmission that maintains the high reduction of the SWG and enables a
continuously variable gear ratio. We achieve this by, first, replacing the
teeth of an SWG with a smooth frictional contact to enable continuous gear
variation and, second, inverting the gear elements to enable high frictional
torque transfer with a low preload via the capstan effect. We choose a
gecko-inspired dry adhesive as the frictional material because it has high
friction at low normal forces, and is not inherently tacky. We present a
model describing the transmission, as well as experimental data gathered
from a prototype device that verifies this model and establishes a proof of
concept. Our transmission expands the possibilities for low-cost robotic
systems.
Hashizume, J., Huh, T.M., Suresh, S.A., Cutkosky, M.R. "Capacitive Sensing
for a Gripper with Gecko-Inspired Adhesive Film."
Robotics and Automation Letters. 2019.
doi:10.1109/LRA.2019.2893154.
Abstract: We present a capacitive sensor suitable
for a gripper that uses thin films of gecko-inspired adhesives. The sensor
is fabricated directly on the films and measures the area over which the
adhesive makes intimate contact. In experiments, a new underactuated gripper
uses adhesive films to acquire and hold objects having a variety of shapes
and textures. Using the adhesive films, the gripper achieves 2.6 × greater
pullout force on rough surfaces as compared to using soft rubber. For a good
grip, as the applied load increases, the films adhere more tightly to object
surfaces and the local capacitance increases at contact regions. With six
taxels per finger, the sensor can also detect whether the contact pattern of
a grasp matches the expectations.
Huh, T.M., Liu, C., Hashizume, J., Chen, T.G., Suresh, S.A., Chang, F.K. and
Cutkosky, M.R. "Active Sensing for Measuring Contact of Thin Film
Gecko-Inspired Adhesives." Robotics and Automation Letters. 2018.
doi:10.1109/LRA.2018.2851757.
Abstract: Active sensing provides a way to assess
whether a thin film of gecko-inspired adhesive has made good contact with a
surface. This knowledge is useful for applications like gripping objects in
space where a failed grasp could lead to loss of the object. Our active
sensing approach uses Lamb waves in thin bilayers, excited, and detected by
piezoelectric strips. From the theory, we describe how attenuation increases
with contact boundary condition changes. We validated the theory using
frustrated total internal reflection imaging, showing that attenuation
increases as the contact area grows. Pull tests on different textures of
acrylic plate show that the slope change of the signal can predict the
maximum adhesion limit with a 10 N window and predict impending failure with
a detection rate >80%. Lifting a cylindrical object shows that the sensor
can signal different types of failures with a detection rate >85%,
associated with unstable grasping.
Glick, P., Suresh, S.A., Ruffatto III, D., Cutkosky, M.R., Tolley, M.T., and
Parness, A. "A soft robotic gripper with gecko-inspired adhesive."
Robotics and Automation Letters. 2018.
doi:10.1109/LRA.2018.2792688.
Abstract: Previous work has demonstrated the
versatility of soft robotic grippers using simple control inputs. However,
these grippers still face challenges in grasping large objects and in
achieving high-strength grasps. This work investigates the combination of
fluidic elastomer actuators and gecko-inspired adhesives to both enhance
existing soft gripper properties and generate new capabilities. On rocky or
dirty surfaces where adhesion is limited, the gripper retains the
functionality of a pneumatically actuated elastomer gripper with no measured
loss in performance. Design strategies for using the unique properties of
the gecko-inspired adhesives are presented. By modeling fluidic elastomer
actuators as a series of joints with associated joint torques, we designed
an actuator that takes advantage of the unique properties of the
gecko-inspired adhesive. Experiments showed higher strength grasps at lower
pressures compared to nongecko actuators, in many cases enabling the gripper
to actuate more quickly and use less energy. The gripper weighs 48.7 g, uses
$7.25 of raw materials, and can support loads of over 50 N. A second
gripper, using three fingers for a larger adhesive surface, demonstrated a
grasping force of 111 N (25 lbf) when actuated at an internal pressure of 40
kPa.
Jiang, H., Hawkes, E.W., Fuller, C., Estrada, M.A., Suresh, S.A., Abcouwer,
N., Han, A.K., Wang, S., Ploch, C.J., Parness, A. and Cutkosky, M.R. "A
Robotic Device Using Gecko-Inspired Adhesives can Grasp and Manipulate Large
Objects in Microgravity." Science Robotics. 2017.
doi:10.1126/scirobotics.aan4545.
Abstract: Grasping and manipulating uncooperative
objects in space is an emerging challenge for robotic systems. Many
traditional robotic grasping techniques used on Earth are infeasible in
space. Vacuum grippers require an atmosphere, sticky attachments fail in the
harsh environment of space, and handlike opposed grippers are not suited for
large, smooth space debris. We present a robotic gripper that can gently
grasp, manipulate, and release both flat and curved uncooperative objects as
large as a meter in diameter while in microgravity. This is enabled by (i)
space-qualified gecko-inspired dry adhesives that are selectively turned on
and off by the application of shear forces, (ii) a load-sharing system that
scales small patches of these adhesives to large areas, and (iii) a
nonlinear passive wrist that is stiff during manipulation yet compliant when
overloaded. We also introduce and experimentally verify a model for
determining the force and moment limits of such an adhesive system. Tests in
microgravity show that robotic grippers based on dry adhesion are a viable
option for eliminating space debris in low Earth orbit and for enhancing
missions in space.
Wu, X.A., Christensen, D.L., Suresh, S.A., Jiang, H., Roderick, W.R.T., and
Cutkosky, M.R. "Incipient Slip Detection and Recovery for Controllable
Gecko-Inspired Adhesion," Robotics and Automation Letters. 2016.
doi:10.1109/LRA.2016.2636881.
Abstract: We present work on incipient slip sensing
and recovery for controllable gecko-inspired adhesives. The approach is
based on the relationship between changes in real contact area and maximum
shear force. Using signals from an on-board tactile sensor, we detect the
onset of adhesive failure and execute recovery behavior. Results show the
system using tactile sensor feedback is able to achieve >92% of the peak
adhesion performance achieved with a force plate and commercial load cell.
Results are consistent over a variety of common smooth surfaces, with the
system achieving repeatable force loading behavior independent of varying
materials and surface conditions.
Christensen, D.L., Suresh, S.A., Hahm, K. and Cutkosky, M.R., "Let’s All
Pull Together: Principles for Sharing Large Loads in Microrobot Teams."
Robotics and Automation Letters. 2016.
doi:10.1109/LRA.2016.2530314.
Abstract: We present a simple statistical model to
predict the maximum pulling force available from robot teams. The expected
performance is a function of interactions between each robot and the ground
(e.g., whether running or walking). We confirm the model with experiments
involving impulsive bristlebots, small walking and running hexapods, and 17
g μTug that employ adhesion instead of friction. With attention to load
sharing, each μTug can operate at its individual limit so that a team of six
pulls with forces exceeding 200 N.
Wu, X.A., Suresh, S.A., Jiang, H., Ulmen, J.V., Hawkes, E.W., Christensen,
D.L., and Cutkosky, M.R. "Tactile Sensing for Gecko-Inspired Adhesion,"
IEEE/RSJ International Conference on Intelligent Robots and Systems
(IROS), 2015. Best Conference Paper Award. doi:10.1109/IROS.2015.7353566.
Abstract: Adhesion quality sensing is critical to
the performance of any robot that utilizes gecko-inspired dry adhesives for
climbing, perching, or grasping. We present a 3-axis tactile sensor designed
for this application that demonstrates performance on par with a large
commercial load cell while being compact enough to integrate into a robot
foot. The sensor can measure spatially distributed force loads and
demonstrates high sensitivity in both shear and normal components. Results
showcase the sensor’s ability to detect a variety of unreliable contact and
loading conditions before the onset of adhesion failure.
Christensen, D.L., Hawkes, E.W., Suresh, S.A., Ladenheim, K., and Cutkosky,
M.R. "µTugs: Enabling Microrobots to Deliver Macro Forces with Controllable
Adhesives,"
IEEE International Conference on Robotics and Automation (ICRA),
2015.
doi:10.1109/ICRA.2015.7139765.
Abstract: The controllable adhesives used by insects
to both carry large loads and move quickly despite their small scale
inspires the μTug robot concept. These are small robots that can both move
quickly and use controllable adhesion to apply interaction forces many times
their body weight. The adhesives enable these autonomous robots to
accomplish this feat on a variety of common surfaces without complex
infrastructure. The benefits, requirements, and theoretical efficiency of
the adhesive in this application are discussed as well as the practical
choices of actuator and robot working surface material selection. A robot
actuated by piezoelectric bimorphs demonstrates fast walking with a no-load
rate of 50 Hz and a loaded rate of 10 Hz. A 12 g shape memory alloy (SMA)
actuated robot demonstrates the ability to load more of the adhesive
enabling it to tow 6.5 kg on glass (or 500 times its body weight).
Continuous rotation actuators (electromagnetic in this case) are
demonstrated on another 12 g robot give it nearly unlimited work cycles
through gearing. This leads to advantages in towing capacity (up to 22 kg or
over 1800 times its body weight), step size, and efficiency. This work shows
that using such an adhesive system enables small robots to provide truly
human scale interaction forces, despite their size and mass. This will
enable future microrobots to not only sense the state of the human
environment in which they operate, but apply large enough forces to modify
it in response.
Murman, S. and Suresh, S.A. "Modeling Effective Stiffness Properties of IAD
Fabrics."
21st AIAA Aerodynamic Decelerator Systems Technology Conference and
Seminar. 2011.
doi:10.2514/6.2011-2568.
Abstract: A model for the mechanical stiffness
properties of bladder fabrics for inflatable decelerators under high stress
conditions is developed. This planar orthotropic model uses understanding of
the fabric behavior, analytical modeling, numerical simulations, and
available experimental data to characterize the fabric stiffness (elastic
modulus), contraction (Poisson’s ratio), and shear modulus. The derived
model is designed to integrate with standard finite-element methods and is
validated against available static test data for two types of
silicone-coated Kevlar fabric using the commercial LS-DYNA solver
Newman, J., Zhu, H., Partridge, B.A., Szocs, L.J., Abiola, S.O., Corey,
R.M., Suresh, S.A., and Yu, D.D. "Phobetor: Princeton University's entry in
the 2010 Intelligent Ground Vehicle Competition."
IS&T/SPIE Electronic Imaging. International Society for Optics and
Photonics, 2011.
doi:10.1117/12.872642.
Abstract: In this paper we present Phobetor,
an autonomous outdoor vehicle originally designed for the 2010 Intelligent
Ground Vehicle Competition (IGVC). We describe new vision and navigation
systems that have yielded 3x increase in obstacle detection speed using
parallel processing and robust lane detection results. Phobetor also
uses probabilistic local mapping to learn about its environment and Anytime
Dynamic A* (AD*) to plan paths to reach its goals. Our vision software is
based on color stereo images and uses robust, RANSAC-based algorithms while
running fast enough to support real-time autonomous navigation on uneven
terrain. AD* allows Phobetor to respond quickly in all situations
even when optimal planning takes more time, and uses incremental replanning
to increase search efficiency. We augment the cost map of the environment
with a potential field which addresses the problem of "wall-hugging" and
smooths generated paths to allow safe and reliable path-following. In
summary, we present innovations on Phobetor that are relevant to
real-world robotics platforms in uncertain environments.
