A climbing robot needs to use its adhesive patches over and over again as it scales a slope. Replacing the adhesive at each step is generally impractical. If the adhesive or attachment mechanism cannot be used repeatedly, then the robot must carry an extra load of this adhesive to apply a fresh layer with each move. Common failure modes include tearing, contamination by dirt, plastic deformation of fibers, and damage from loading/unloading. A gecko-like fibrillar adhesive has been developed that has been shown useful for climbing robots, and may later prove useful for grasping, anchoring, and medical applications.
The material consists of a hierarchical fibrillar structure that currently contains two levels, but may be extended to three or four levels in continuing work. The contacting level has tens of thousands of microscopic fibers made from a rubberlike material that bend over and create intimate contact with a surface to achieve maximum van der Waals forces. By maximizing the real area of contact that these fibers make and minimizing the bending energy necessary to achieve that contact, the net amount of adhesion has been improved dramatically.
The suspension structure consists of millimeter-scale fibers that are bonded to the contacting level through a wet assembly step. These millimeter-sized fibers serve as a discretized way of both conforming to roughness on the surface and distributing the overall climbing loads down to the individual contacts. These structures have been tested on an experimental testbed meant to determine the contact forces very exactly, and have also been demonstrated by hanging weights off of a patch adhering to a variety of walls (glass, metal, wood, plastic, drywall, etc).
This material is fabricated via a molding process. A new process has been developed at JPL to make this process simpler, more reliable, and to allow new geometries not previously possible. These new geometries will make the adhesive and the reliability significantly better, and will drive down cost and development time.
The process involves using optical lithography to make a master pattern, and from this master pattern, making a reusable master mold that is used to cast the adhesive strips. To create the master photoresist pattern that will be used to make the master mold, a self-aligned double exposure technique was used. Two different angled UV exposures are performed using a single opaque pattern on a transparent wafer. This simplifies fabrication considerably.
A second advantage of this technique is the ability to achieve right-angle wedgeshaped structures with both sides of the wedge leaning to the same side, i.e., an actual overhang of the fibers, which is more like the arrangement of a gecko foot’s fibers. A third critical difference is the use of a standard positive Novalac photoresist, which has a wide process latitude.
The new microfabrication process has allowed the shape of the wedge-like fibers to be controlled. Prior to these process improvements, only right-angle wedges had been fabricated. Now, the process not only allows for increased control over the angle of these fibers, but is also much more reliable, manufacturable, and cost-effective.
This work was done by Aaron Parness and Victor E. White of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48156