After billions of years of evolution and survival competition, the diversity and various unique functions exhibited by organisms in nature have inspired human imagination and creativity. Geckos, which can stay motionless or walk effortlessly on cliffs, tree trunks, walls, ceilings, and even vertical smooth glass surfaces, have long been the focus of human attention. Ancient Greek philosopher Aristotle attributed this adhesive force to a supernatural power. Later, scientists suggested that the adhesion mechanism of gecko feet might involve capillary action, microscopic locking, friction, or electrostatic attraction. Considering that geckos can stay motionless or climb effortlessly on vertical smooth glass walls even in a vacuum, the suction force between their feet and surfaces cannot be a vacuum suction force. Air ionization has no effect on the adhesive force of gecko feet, ruling out electrostatic attraction. Gecko feet do not have glands and do not secrete any liquids, so it cannot be a conventional adhesive. In the 19th century, scientists observed that gecko feet have unusual microstructures. In 1965, optical microscopy revealed that gecko feet have millions of setae, each composed of 100 to 1,000 villi with a radius of 0.2 to 0.4 micrometers. Since 2000, American scientists such as Autumn have used microelectromechanical systems (MEMS) to measure the adhesive force of gecko setae and found that when the villi on the setae adhere closely to the surface of objects, a weak intermolecular force, known as van der Waals force in physics, can be generated. Although the force at each contact point is very weak, the array of millions of setae can provide adhesive force far exceeding the weight of a gecko. Geckos rely on the van der Waals force between their setae and surfaces to adhere to objects. The secret to their effortless and rapid walking lies in the rapid formation and disappearance of van der Waals forces.
Autumn and colleagues also found that setae must have relative sliding against the contact interface to measure adhesion. This sliding may involve constantly adjusting the angle of the setae to promote full contact with the contact surface. Moreover, the muscle and nerve tissues at the base of the setae regulate the gecko's foot setae, enabling active adhesion, anti-adhesion, and their conversion functions. Of course, bioelectricity may also be involved, allowing geckos to climb quickly and freely. In 2005, scientists Hu Bo and colleagues also found that capillary action plays an important role in the adhesive force of gecko feet. Even the presence of a single layer of water molecules can greatly affect the adhesive force results, and this water molecule layer is easily formed by the setae absorbing water from the air. Experiments show that the adhesive force increases with increasing air humidity. When the air humidity increases from 0 to 70%, the adhesive force generated by a single villus nearly doubles. Scientists are very interested in replicating these setae. Although the currently replicated "artificial setae" have some adhesive ability, they are plagued by mutual "sticking" and have difficulty converting between adhesion and detachment from surfaces. It seems that there are still many mysteries yet to be unraveled about gecko feet!