Biomedical Engineering Reference
In-Depth Information
Chapter 1
Atomic force microscopy is an amazing technique that allows us to see and measure
surface structure with unprecedented resolution and accuracy. An atomic force micro-
scope (AFM) allows us, for example, to get images showing the arrangement of individual
atoms in a sample, or to see the structure of individual molecules. By scanning in ultra-
high vacuum at cryogenic temperatures the hopping of individual atoms from a surface has
been measured [1]. On the other hand, AFM does not need to be carried out under these
extreme conditions, but can be carried out in physiological buffers at 37
C to monitor
biological reactions and even see them occur in real time [2-4]. Very small images only
5 nm in size, showing only 40-50 individual atoms, can be collected to measure the
crystallographic structure of materials, or images of 100 micrometres or larger can be
measured, showing the shapes of dozens of living cells at the same time [5-9]. AFM has a
great advantage in that almost any sample can be imaged, be it very hard, such as the
surface of a ceramic material, or a dispersion of metallic nanoparticles, or very soft, such
as highly flexible polymers, human cells, or individual molecules of DNA. Furthermore,
as well as its use as a microscope, which is to say as an imaging tool, AFM has various
'spectroscopic' modes, that measure other properties of the sample at the nanometre scale.
Because of this, since its invention in the 1980s, AFM has come to be used in all fields of
science, such as chemistry, biology, physics, materials science, nanotechnology, astron-
omy, medicine, and more. Government, academic and industrial labs all rely on AFM to
deliver quantitative high-resolution images, with great flexibility in the samples that can
be studied.
An AFM is rather different from other microscopes, because it does not form an image
by focusing light or electrons onto a surface, like an optical or electron microscope. An
AFM physically 'feels' the sample's surface with a sharp probe, building up a map of the
height of the sample's surface. This is very different from an imaging microscope, which
measures a two-dimensional projection of a sample's surface. Such a two-dimensional
image does not have any height information in it, so with a traditional microscope, we
must infer such information from the image or rotate the sample to see feature heights. The
data from an AFM must be treated to form an image of the sort we expect to see from a
microscope. This sounds like a disadvantage, but the treatment is rather simple, and
furthermore it's very flexible, as having collected AFM height data we can generate
images which look at the sample from any conceivable angle with simple analysis
software. Moreover, the height data makes it very simple to quickly measure the height,
length, width or volume of any feature in the image.
The fact that the AFM operates differently from most microscopes, and that the AFM
probe physically interacts with the sample, means however that it is not as intuitive to use
as optical microscopes. While most people understand the basic principles of light
microscope use, i.e. focusing, illumination, depth of field, and so on, the use of AFM
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