What is Directional Adhesive (Gecko Tape)?

Directional adhesion is a new innovation resulted from years of learning from gecko's multi-hierarchical nano dry adhesive structures as well as pondering about 'Stickiness'. It has Anisotropic structure featured controllable adhesion with directionality in adhesion force. The movie clip  demonstrates directional adhesion compared to conventional double sided tape. Unlike conventional tape, it sticks on smooth surface with very small preload and is also able to detach with by reducing load. If it is loaded in desired direction, it creates maximum contact minimizing stress concentration along the contact area. if it is loaded in wrong direction, the adhesion force is very low. (This capability is also in contrast to the isotropic materials using microfibers, nanotubes, etc. that a number of researchers have been developing.- Prof. Cutkosky)


This directional adhesive allows Stickybot to climb a range of smooth surfaces like glass, acrylic and ceramic tile with relatively fast speed. For a climbing robot, the role of controllable adhesive is essential primarily because light-weight of robot limits the maximum torque that actuators can produce. First version was made in Sept. 2005. I did not understand essentials of adhesion and it had very low adhesion. Alternations between design iterations and better understanding about adhesion leads to current design turned out to be successful. Directional adhesive is directly inspired by the work of Prof. Kellar Autumn on gecko adhesion. It can clean without any additional process and has directionality in its adhesion. Figure I shows the range of adhesive solutions ordered in terms of feature size and effective modulus. A material is considered tacky when the effective modulus is less than 100kPa (Dahlquist Criterion). Using higher modulus material which is not tacky , the hierarchical geometry of the gecko dry adhesive lowers the effective stiffness enough to make the system function like a tacky material. Same principle has been adapted on Gecko tape (Directional polymer adhesion).

Adhesion system model (What is stickiness?)

Many people asked me about what kind of material do I use for sticky pads. As I learned more about molecular mechanics, however, it is getting more convincing to me that Adhesion is depends more on geometry of structures than the type of material. This is also consistent with literature. In the beginning of RiSE (Robots in Scansorial Environment) project, winter of 2003, I was wondering about the reason how sticky material can stick on surfaces. What feature makes things sticky? My hypothesis was that the more compliant the structure of a system, the stickier it is. I came up with simple idea to test my hypothesis. Which is very simple Adhesion system model that I made in Jan 2004.

 Bottom line question is 'what is direct correlation between stiffness of the material and the overall sticky force'. In the figure above, yellow bar represents linear spring and red tip presents sticky elements that remain in contact if pulling force caused by deformation of sticky structure (yellow). After engaging every single element on substrate, we can measure the contact force increasing distance from substrate (see blue curve in graph above). It reaches a peak when the amount of force lost my detached elements and the amount of force gained by stretched structures. After this peak overall adhesion force decreases rapidly and reaches zero when all element are detached. We can measure maximum adhesion force at a certain condition (substrate roughness, structure stiffness, adhesion force of element).

An interesting result yielded from this simulation is that, with same sticky element, as the stiffness of the structure increases, maximum adhesion force drops drastically. This concludes that softer material fundamentally stickier than stiffer material with same stickiness of lower level elements. We can apply this concept to each level of hierarchical system. I apply this intuition on directional adhesive design.