Invented in 1986 by Binning et Al., the Atomic Force Microscope has undergone much development. The first AFMs operated in contact mode. (See Binning et al., Physics Review Letters 1986, and Rugar and Hansma, Physics Today 1990). In contact mode, the tip, mounted onto the end of a flexible cantilever, raster scans the surface of the sample. The deflection of the cantilever due to tip-surface interaction reveals the sample surface. Samples can be analyzed in air, liquids or vacuum. In liquids and vacuum, the absence of strong capillary forces due to a thin liquid film on all samples in air can result in higher resolutions. Biological samples are difficult to scan using contact mode because they are often soft and weakly bound to the surface and therefore can be damanged easily.

Non-contact mode was first introduced in 1987. (See Martin et al., 1987). Developed in an effort to more accurately image soft biological samples, in non-contact mode the cantilever oscillates close to its resonant frequency at a small distance (1-10 nm) above the surface. Long-range attractive forces induce changes in the amplitude, frequency and phase of the cantilever and maintain a constant distance during scanning. Because the forces on the sample are much lower than in contact mode, even the softest samples can be imaged without damage. Imaging in attractive mode is also possible.

The Optical AFM Version 4 was also developed in 1989. This fourth prototype of the AFM generated interest in the commercial development and implementation of AFMs.

Microfabricated tips were developed in 1991. (Prater et al., 1991)

In 1993, Tapping Mode® was first introduced. (See Zhong et al., 1993) In this mode, the cantilever oscillates at its resonant frequency, but unlike non-contact mode, the cantilever gently taps the surface during scanning, greatly reducing damaging lateral forces.

Tapping mode in fluids was first introduced in 1994 by the Hansma Lab. (See Hansma et al., 1994) In the first implementation of tapping mode in fluids, the sample, which sits on a piezoelectroc scanner, oscillates up and down and taps the tip at the apex of each oscillation cycle. The amplitude of the piezoelectric is set manually at the beginning of the run, and the tapping force is held constant by a feedback loop.

Smaller cantilevers were developed in 1996, allowing higher resolution and smaller scanning times. The Hansma group began developing a new generation of AFMs that would utilize these smaller, lighter cantilevers. For biological samples, desirable cantilevers have a higher resonant frequency (and therefore a higher scanning speed) and a low spring constant. This is most easily achieved by decreasing the mass of a cantilever. Recently cantilevers have been fabricated on the order of 9-40mm in length with resonant frequencies an order of magnitude higher than commercially available cantilevers. (See Schaeffer et al., 1997b)

The next prototype was a small cantilever AFM Version 5. The major differences between this AFM and previous AFMs lie in the optics design of the microscope. This prototype has a much smaller laser spot allowing for smaller cantilevers to be used. It also includes an integrated illumination source, essentially combining an optical microscope and an atomic force microscope in the same piece of equipment.

Improvements in instrumentation and understanding of the Atomic Force Microscope has led to its wide use in many fields in engineering, materials science and biology. Our laboratory is committed to further improve the instrumentation behind Atomic Force Microscopy and apply its capabilities into new, exciting areas, in particular, in biomineralization and in the life sciences.