Atomic Force Microscopy

A nanoscale imaging technique capable of sub-nanometre spatial resolution and piconewton force sensitivity. The AFM can also be used to measure biophysical properties of cells and cellular sub- structures.  

The Atomic Force Microscope is comprised of a cantilever-tip assembly that raster scans across the sample surface using a piezoelectric tube controlled by a computer. The cantilever tip tapers to a very sharp point, typically less than 10nm radius of curvature. The deflection of the cantilever is monitored using an optical detection system in the form of a laser that reflects off the back of the cantilever and onto a four quadrant photodiode. A feedback loop maintains a constant tip-sample force. This can be done by operating in contact mode, where a constant cantilever deflection is maintained, or in intermittent contact mode where the cantilever is oscillated near resonance and constant amplitude is maintained. The AFM is also capable of force spectroscopy measurements.

Atomic Force Microscopy:

  • A Technique capable of measuring topographical images of surfaces with nanoscale spatial resolution.
  • Can be operated under physiologically relevant conditions (pH, temperature, ionic strength of media) and therefore is very useful for the study of biological systems.
  • In addition to imaging capabilities the AFM can be used to study the elasticity of cells and other biomechanical properties.
  • Can be used to measure single molecular interactions in the form of single molecule force spectroscopy measurements and the forces required to unfold proteins. 

 

Cell Elasticity Project

We are able to measure the Young’s modulus of cells, which is a measure of elasticity. Micrometre- scale beads are first attached to regular AFM tips. The cells are then indented and the force versus displacement response monitored. Using the Hertz model we are then able to determine the Young’s modulus. Using beads with a large height minimizes erroneous contact between the cantilever beam and the cell surface and permits greater spatial averaging of the elastic properties. 

 SEM images of AFM cantilever with and without a 15um bead with optical images of a bead attached and approaching fibroblast cells. SEM images from [Harris et al 2011].

SEM images of AFM cantilever with and without a 15um bead with optical images of a bead attached and approaching fibroblast cells. SEM images from [Harris et al 2011].