The Stickiest *Non-Sticky* Substance

A gecko-inspired adhesive can grip without feeling sticky—holding weight only when pulled in the right direction. That directional “stickiness” matters because it lets small robots climb, pick up fragile objects, and even operate in microgravity without the constant drag and residue associated with conventional tacky materials.

The effect starts with a material that looks nothing like tape. It won’t even hold regular tape, yet it can support a tomato when draped over it. Flip it and it drops, showing the adhesion is highly dependent on how force is applied. The same behavior appears on other smooth surfaces like water bottles and chip bags, as long as the contact area is sufficiently smooth.

The underlying model comes from gecko feet. Geckos stick far more strongly than suction cups, using no hairs or spikes. Instead, their toes contain lamellae that branch into seta, which further split into extremely fine spatula structures—each spatula is less than one micrometer across. The adhesion mechanism is not chemical bonding like ionic or hydrogen bonds. It relies on Van der Waals forces: momentary charge imbalances in neutral atoms induce complementary fluctuations in nearby atoms, creating a weak attraction between the gecko and a surface such as glass.

In everyday life, Van der Waals forces are hard to notice because skin is bumpy, limiting close contact. Gecko feet overcome that by multiplying contact points through their branching microstructures. The artificial version takes a different route: it uses sharp wedge-shaped tips (about one to two micrometers wide) made from a silicone polymer—Sylgard 170—cast from a wax mold. When first pressed onto a surface, only the wedge tips touch, leaving contact too limited to feel sticky. But when shear force loads the material—pulling parallel to the surface—the wedges bend and spread, creating a much larger, nearly continuous contact area. Adhesion then turns on; pull in the wrong direction and it won’t grip, which also makes removal easy.

Researchers built devices to exploit this directional behavior. A tiny robot called MicroTug weighs 17 grams yet can pull a 20 kilogram load. Its gecko adhesive sits on the underside so that pulling creates shear contact; as the load changes, the shear decreases and the robot can detach and move. The team also quantified contact area using a light-through-acrylic method that reveals where contact frustrates an LED beam. Roughly one square inch of contact supports about 4.5 kilograms (10 pounds).

The adhesive has been tested in space on Astrobee, a fan-driven robot inside the International Space Station. With no gravity, the goal was gentle grabbing and easy release, and the same shear-based adhesion worked for about a year and a half. Beyond anchoring and climbing, the material is aimed at robotic grippers—“smart gecko palms”—to lift delicate produce with minimal squeezing. In a dramatic demonstration, multiple adhesive pads anchored a winch setup that could move a car, and a PhD project by Elliot Hawks tackled the challenge of maintaining enough adhesive contact to support a climber’s full weight on glass.