Atomic force microscopy (AFM) is part of a broad class of scanning probe microscopes (SPMs) that were originally developed in the 1980s. AFMs physically track samples with a microfabricated probe to generate 3D topographical images. Typical resolution is limited by the probe dimensions and the sample and is on the order of angstroms in Z and nanometers in XY. Scans can be on the order of a few nanometers to several tens of microns in XY, allowing for the investigation of processes at multiple spatial scales. This technique does not require sample fixation or staining, and measurements can be made in near-physiological conditions, in biological buffers and culture media.
In addition to imaging, the physical interaction between the probe and sample can be measured and quantified in terms of force. These data are commonly displayed as a force versus distance or indentation curve. As the probe contacts and pushes into the sample, the force versus indentation data can be used to reveal information on sample stiffness via a variety of mechanical models. As the probe is pulled away from the surface, adhesive interactions between the tip and sample―if any―are measured. These adhesion data provide information on inter- or intra-molecular forces.
AFM is well suited to pharmacological research. Fundamental research includes the surface characterization of tablets and their coatings, growth of crystals as a function of manufacturing parameters (concentration, temperature, pH), and size and form of drug delivery vehicles.
AFM can also provide a better understanding of how a drug affects a target molecule or cell. An example is the effect of cisplatin, an anticancer drug, on DNA structure and mechanics. Cisplatin is an intercalative agent, which covalently binds to a specific location on guanine and adenine bases. After treatment with cisplatin, topographic images reveal an elongation of the DNA molecule and force measurements show a change in its unfolding pattern due to structural changes in the molecule.
Because of its ability to operate in aqueous conditions, time-lapse experiments monitoring the effects of drugs on living systems are routine. For example, AFM has been used to observe the effect of natural toxins such as cytochalasin, latrunculin, and jasplakinolide, as well as plant-derived taxol, on cell structure and stiffness. These drugs alter cytoskeletal dynamics, which has subsequent effects on cell motility, division, and overall mechanical function. Similarly, morphological changes of living cerebral endothelial cells have been studied. AFM results showed a decrease in cell height, the emergence of surface protrusions, and a decrease in Young’s Modulus (stiffness) after exposure to hyperosmotic concentrations of mannitol, which is commonly used to open up the blood-brain barrier.
The use of AFMs in pharmacological research has the potential to progress further as the technology is applied and developed with improved resolution and faster scan speeds.