W. Richard Bowen and Nidal Hilal 4

278 10. FuTuRE PRosPECTs
targeted drug/gene delivery. Further, AFM is becoming an indispensable
characterisation tool of miniaturised device components, which increasingly
involve the use of (bio)polymers, ultrathin films and nanostructures
on surfaces.
To date, the successful use of AFM-based techniques to determine the
viscoelastic properties of materials is exemplified by the successful use of
indentation or compression experiments, for which established theoretical
models adequately describe the rheological properties. Of particular
note are the developments in biomechanical studies, wherein the determination
of the viscoelastic response of cells and biological tissues using a
combination of an AFM and a confocal laser scanning microscope (CLSM)
is an interesting development, especially so with the use of modern fastscanning
CLSMs.
For fluids, although experimental studies have shown that an AFM is
certainly capable of emulating the rheological capabilities of the surface
forces apparatus (SFA), the use of an AFM to quantitatively describe the
rheology of thin fluid films remains the subject of considerable research
interest. Although, the viability of AFM as a micro-rheometrical tool
has been established, the development of mathematical models capable
of interpreting the response of microcantilevers in viscoelastic fluids is
still evolving, and further development is vital to the successful application
of this technique. Nevertheless, the integration of AFM cantilevers in
microfluidic and other fluid sensor devices is a realistic option, enabled
most significantly by advances in the description of viscous effects upon
resonance characteristics. From a rheometrical perspective, this is particularly
encouraging as few other technologies can address the associated
issues of scale. The development of this area will in part rely upon the
fabrication of appropriate cantilever geometries, such that the resonance
characteristics and sensitivity of the device are optimized for the fluid of
interest. In this respect, the fabrication of multilevers on common substrates
may extend the measuring capabilities and represents a potential
enabling technology for the study of more complex, multicomponent fluidic
systems.
The adaptation of dynamic AFM methods for the routine rheological
characterisation of thin fluid films is more problematical. However,
qualitative results suggest that the technique has potential merits. The
dynamic response of a vibrating probe is clearly able to detect the onset
of viscoelastic behaviour, but quantitative rheological information as yet
remains elusive. Surprisingly, many dynamic studies favour oscillation
in the direction normal to the surface, and as such it may be anticipated
that significant compressional wave components can influence oscillation
of the probe. For the analogous study of macroscale oscillatory shear
and microscale rheometrical parameters, further studies on oscillatory
microscale shear deformations would be beneficial.