Geckos are exceptional in their ability to climb rapidly up smooth vertical surfaces. Microscopy has shown that a gecko's foot has nearly five hundred thousand keratinous hairs or setae. Each 30-130 microm long seta is only one-tenth the diameter of a human hair and contains hundreds of projections terminating in 0.2-0.5 microm spatula-shaped structures. After nearly a century of anatomical description, here we report the first direct measurements of single setal force by using a two-dimensional micro-electromechanical systems force sensor and a wire as a force gauge. Measurements revealed that a seta is ten times more effective at adhesion than predicted from maximal estimates on whole animals. Adhesive force values support the hypothesis that individual seta operate by van der Waals forces. The gecko's peculiar behaviour of toe uncurling and peeling led us to discover two aspects of setal function which increase their effectiveness. A unique macroscopic orientation and preloading of the seta increased attachment force 600-fold above that of frictional measurements of the material. Suitably orientated setae reduced the forces necessary to peel the toe by simply detaching above a critical angle with the substratum.

Geckos have evolved one of the most versatile and effective adhesives known. The mechanism of dry adhesion in the millions of setae on the toes of geckos has been the focus of scientific study for over a century. We provide the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, and reject the use of mechanisms relying on high surface polarity, including capillary adhesion. The toes of live Tokay geckos were highly hydrophobic, and adhered equally well to strongly hydrophobic and strongly hydrophilic, polarizable surfaces. Adhesion of a single isolated gecko seta was equally effective on the hydrophobic and hydrophilic surfaces of a microelectro-mechanical systems force sensor. A van der Waals mechanism implies that the remarkable adhesive properties of gecko setae are merely a result of the size and shape of the tips, and are not strongly affected by surface chemistry. Theory predicts greater adhesive forces simply from subdividing setae to increase surface density, and suggests a possible design principle underlying the repeated, convergent evolution of dry adhesive microstructures in gecko, anoles, skinks, and insects. Estimates using a standard adhesion model and our measured forces come remarkably close to predicting the tip size of Tokay gecko seta. We verified the dependence on size and not surface type by using physical models of setal tips nanofabricated from two different materials. Both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures.I n the 4th century B.C., Aristotle observed the ability of the gecko to ''run up and down a tree in any way, even with the head downwards'' (1). Two millennia later, we are uncovering the secrets of how geckos use millions of tiny foot-hairs to adhere to even molecularly smooth surfaces. We tested the two currently competing hypotheses (2, 3) of adhesion mechanisms in gecko setae: (i) thin-film capillary forces (or other mechanisms relying on hydrophilicity) and (ii) van der Waals forces. First, we tested the capillary and van der Waals hypotheses experimentally. Second, we used our experimentally measured adhesion forces in a mathematical model (4) to generate an independent prediction of the size of a setal tip. We compared the predicted size with the empirical values measured by electron microscopy (5). Third, we fabricated a physical model of gecko setal tips from two different materials. We then compared the adhesive function of the physical model to predicted force values from the mathematical model. Previously, we showed by calculation that our direct force measurements of a single gecko seta (3) were consistent with adhesion by van der Waals forces, but we could not reject the only other untested mechanism-wet, capillary adhesion that relies on the hydrophilic nature of the surface. Capillary forces contribute to adhesion in many insects (6-13), frogs (14-16), and even some mammals (17). Unlike many insects, geckos lack glands on the surfaces of their feet...

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Micromachined cantilevers have been demonstrated to be effective measuring devices for small forces and displacements. Applications for such micro-electro mechanical systems include imaging of surface topography, data storage, and atomic force microscopy (AFM). Researchers at Stanford University have developed a piezoresistive sensor with independent vertical and lateral force detection capability using oblique ion implantation and high aspect-ratio plasma etching [1–3]. An SEM image of the device is shown in Figure 1. Although this device has been successfully used to obtain data and for servo control of a tracking system for a data storage device, its mechanical behavior has not been carefully studied. This device can be used to measure forces or deflections in various applications. This paper demonstrates how the device performance can be optimized for a specific set of requirements. Applications with different stiffness or bandwidth requirements may be designed using similar methods.

Micromachined piezoresistive cantilevers were fabricated to measure contact resistance and force for low force and small area electrical contacts. The intended application is the evaluation of contact tip geometries and metal films manufactured using standard semiconductor processing techniques. Prototype cantilevers are evaluated for force sensitivity, range, and noise as well as for feasibility of 4 wire contact resistance measurements at the end of the cantilever. The piezoresistors have a gage factor of about 35 and noise of about 1mV/Hz at 1Hz (the lowest frequency of intended measurements). Initial measurements of one cantilever design tested indicate sensitivity of 0.24mV/μN and force resolution of about 0.41mN for a piezoresistor drive voltage of about 9V and 100X gain on the bridge output. Force measurements are noise limited at the 5 Hz sampling rates used. 4-wire contact resistance measurements were made synchronously with piezoresistor bridge output voltage measurements.

There are many interesting biological systems that utilize small mechanical forces to achieve functionality. Protein folding, ligand-receptor binding, cellular adhesion, and others all rely on picoNewton sized mechanical forces. In many of these examples, the fundamental character of the interaction remains controversial. In this paper, we describe work in progress to develop micromechanical force-measuring instruments suitable for measurements of these small biologically derived forces.

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