Atomic force microscopy (AFM) is a technique that allows surface topography of structures to be measured with, in principle, sub-nanometre resolution. Various modes of using the microscope are popular. In contact mode, the surface is scanned with the AFM cantilever in continuous contact so as to maintain a constant force between the lever tip and the surface. In non-contact modes, the scanning may involve only intermittent contact during the extreme of a rapid oscillatory excursion of the tip. It is generally thought that such non-contact modes may be more suitable for imaging soft biological samples since the current microfabrication techniques produce cantilevers that are quite stiff (commercially available levers have tip stiffness greater than 0.01 N/m). In biological contexts and in solution, all motions of the cantilevers are severely damped. The resonant excursions of microlevers thus provide less effective information about surface structures, and contact modes are used in preference. To overcome this problem we have followed an evolutionary solution found in the mammalian cochlea to use positive feedback to cancel viscous damping of the oscillating cantilever (Tamayo et al. 2001).

The demonstration is based around a commercially available AFM (‘Nomad’, Quesant Instrument Corp., CA, USA). The circuitry and software of this AFM can be conveniently modified. The AFM has an open architecture that allows access for patch pipettes to record from cells while they are being scanned. The design is such that the cantilever is driven into resonance by means of a small piezoelectric element mounted close to the cantilever and which is secondary to the XYZ piezoelectric scanning elements. We have built an ancillary electronic circuit (or ‘Q box’) that modifies the quality factor of the cantilever resonance. The circuit works by using the position signal of the cantilever (detected by a photodiode as part of the AFM control circuitry) and feeding the signal back with gain and appropriate phase to enhance the resonance. The circuit uses a small number of components and can easily be added to one of the Quesant circuit boards. The ‘Q-box’ is implemented in hardware as the resonant frequencies of some cantilevers can exceed 40 kHz.

Although similar modifications are now beginning to be provided as (costly) add-ons to commercial AFMs, the ‘Q box’ we have designed is instructive and provides insight into the complex dynamics of nanoscale structures. The demonstration will show examples of the biological sample images obtained, as well as progress towards designing an optimal scanning head for studying patch-clamped hair cells.

This work was supported by The Wellcome Trust. J.C. held a Physiological Society Vacation Studentship.