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Atomic force microscopy (AFM) is a popular imaging and characterization technique. AFM uses a probe to scan the surface of a material—sometimes tapping it, sometimes scanning above it—and it is a technique that can determine both the topography of a surface as well as some of its properties.
Any AFM mode can determine the structure of the surface, but the different imaging modes often provide specific property characterizations. The versatility and many different modes available with an AFM mean that it is helping to understand the materials used in many industries, with the molecules, pharmaceuticals and native biomaterials used in healthcare being no exception.
The progression of healthcare to utilize nanomedicines as an effective therapy route has meant that nanoscale characterization methods have had to be used in the healthcare industry for imaging biomolecules, mechanistic processes, diseases and their cellular interactions, and pharmaceutical formulations, as well being a way of detecting various pathological conditions at an early stage.
AFM is now used in the healthcare sector to investigate the properties of the biological matter that will interact with nanomedicine, as well as some of the therapies themselves, and is helping to progress the sector.
What is AFM?
AFM is a type of microscopy that scans the surface of a material with a probe. The probe is commonly made of a cantilever and a sharp tip. The tip scans the surface and the intermolecular forces present between the tip and the surface causes the tip to move towards the surface (and in most cases tap it). A laser beam is always directed towards the cantilever, and as the cantilever is deflected, the laser beam is also deflected but on to a position-sensitive photodiode (PSPD)—which enables the relative position of the atoms to be determined. The system resets itself using a feedback loop, which returns the probe to its original position.
The process is repeated until the scan is finished. If the properties are to be deduced, sometimes it can be done in the same scan, but sometimes a secondary scan is performed for the properties, and the topographic maps generated are superimposable so that the atomic regions and properties can be matched.
Non-Contact Mode for Biological Samples
Most AFM modes are not suitable for biological materials. There is one mode though, known as non-contact mode, which can be used on soft biological samples without damaging them. Where other methods touch the sample to image it, non-contact mode doesn’t, and the cantilever hovers over the sample but is still deflected—it is placed at such a distance away from the sample that it can still interact with the sample and move towards it, but it is far enough away that it won’t touch it when it bends towards the sample. This way, deflections are still recorded which means that the surface and its properties can be mapped. If it wasn’t for the realization of non-contact mode, AFM would not be a useful technique in the healthcare sector.
There are many different biomolecules and biological structures that can be imaged using AFM. AFM was the technique that first enabled biologists to see single biomolecules in their native environments, and everything from DNA to protein complexes has been imaged with AFM. But larger biomolecular systems can also be imaged, including membrane proteins and cell membranes. In terms of imaging membrane proteins, it is not just the molecules themselves which can be imaged, but also the 3D structures of the channels and pores within them. AFM has also been shown to be able to manipulate the properties of the membrane proteins as well. In terms of complete cell membranes, AFM can be used to look at their structures, ion channels, and receptors and how they change when the membrane is diseased—which enables AFM to act as a way of determining the differences between healthy and diseased cells.
Understanding Cell Mechanics
Cells rely on various biochemical processes, as well as the reorganization and regulation of the cytoskeletal structure, to function effectively. These reorganization processes change the mechanical properties of the cell and AFM is one technique that can extrapolate the mechanical properties of a sample with relative ease. AFM does it with a high resolution and offers a way to diagnose pathologies due to abnormal cell mechanics and monitor the efficacy of therapeutic treatments and how they treat any mechanical abnormalities that a cell has.
Understanding the Nanostructures of Tissues
The different fibers which make up the various tissues within the body all have different mechanical properties, and it is these properties that often lead to tissues having specific functions. AFM can be used to probe the various mechanical properties of tissues, and this can lead to diagnosing and monitoring the progression of diseases by the elasticity of the tissue (and how well treatments are working on diseased tissue). Recent advances in this area use AFM to diagnose bone diseases.
Pharmaceutical Formulations and Drug Delivery
AFM is involved in many different stages of drug development and delivery and is especially useful for understanding how any drugs will behave in the body. AFM is first used in the drug discovery process to study any potential drug targets, including proteins and DNA, by showing what effects any possible drug candidate could have on the structure of the biomolecules. This can include testing any potential interactions that could exist between the surface receptors and the drug candidates.
AFM is also used to identify any species, such as diseases, which are present in specimens and provides insight into the physical changes that occur in the body at the cellular/tissue level when there is a disease. This helps to develop drugs that can accommodate these changes to ensure maximum effectiveness. Finally, AFM can be used in the pharmaceutical formulations themselves and their carrier vessels (if applicable) to better understand their physical properties and whether their properties are going to be effective in neutralizing the diseased area without affecting the healthy cells. AFM is involved in the whole drug development cycle and is helping to progress many aspects of healthcare and pharmaceutical development.
- “Atomic Force Microscopy: In Sickness and in Health”- Stylianou A. et al, Scanning, 2019, DOI: 10.1155/2019/6149247
- “Scope of atomic force microscopy in the advancement of nanomedicine”- Lal R. and Ramachandran S., Indian Journal of Experimental Biology, 2010
- “Recent progressive use of atomic force microscopy in biomedical applications”- Maver U. et al, Trends in Analytical Chemistry, 2016, DOI: 10.1016/j.trac.2016.03.014