SPM Applications in Biology

     The last decade of using AFM and related Scanning Probe Microscopy techniques in biology showed that their popularity and power continue growing. Numerous reviews both comprehensive and specialized cited in the reference list and articles devoted to biological applications of Atomic Force Microscopy prove this fact. The state of today's activities in this field can be seen from the collection of latest abstracts from Biophysical Journal: What Biologists Are Working On Using SPM?

     In biology Atomic Force Microscopy has been used traditionally to measure topography [22, 357, 721, 905, 959, 1045, 1068, 1574, 1575, 1584] and nanomechanical properties of biological samples, such as elasticity [256, 327, 360, 593, 676, 992, 995, 1569, 1736]. Now applications of AFM probing is far beyond of these approved ones. It is found to be useful in pharmacology [346, 505] biotechnology [360], microbiology [1755, 1768], structural biology [1764, 1787], molecular biology [1375], genetics [1568, 1781] and other biology related fields.

     The variety of objects investigated using Atomic Force Microscopy in biology spans smallest biomolecules encompassing proteins, lipids, DNA, RNA and other nucleic acids, as well as rather "big" human's platelets, viruses and living cells. The main advantage of AFM in biology as compared with other methods is that it usually doesn't require specific sample preparation and allow measuring in most physiological conditions the most of biological objects are susceptible to. It is the most universal method in the sense that all the media including vacuum can be used for probing. The reason of choosing liquid media instead of the air is not only because it is the natural physiological media for biological objects, but also due to the fact that all the interaction forces including unwanted ones are in order of magnitude less than in the air allowing, for instance, to raise resolution power and to diminish image distortion. Although, it should be noted that measuring in liquids is much more complicated rather than in the air [357].

     Because of softness it is recommended that biological samples be investigated in intermittent-contact or Tapping Mode™ AFM. In this case the mode lowers drastically the probability of sample damage not excluding, though, it's displacement. Nevertheless contact mode imaging in the beginning of 21^th century successfully deals with extralow loadings in order of 100 piconewton (see, for instance [381]). And perfectness of AFM apparatus as well as imaging technique and data acquisition improves year after year [22, 905, 979, 984, 1045, 1569, 1572, 1575].

     Along with direct imaging of biological objects Atomic Force Microscopy plays a significant role among numerous biophysical methods for investigation of specific and non-specific molecular interactions all the biological processes governed by [1713, 1728]. These are protein-protein, enzyme-substrate, antigen-antibody, receptor-ligand interactions, drug-target associations, a diverse number of biocomplexes and many others. Being an instrument of choice for the investigation of biomolecule activities Atomic Force Microscopy is an ideal means for the visualization and real-time imaging of nucleation and crystallization of macromolecular crystals [118, 545, 546, 1703], processes involved in the cell living cycle [357, 959, 960, 968, 969, 975, 979, 992, 1068], "working" of biomolecules [427, 1765].

     Being able to monitor biomolecular interactions on biosensor surfaces Atomic Force Microscopy successfully used in biosensing applications [346, 355, 1181, 1592]. Extended force range (theoretical limit estimated to be of 10^-3 pN, cited from [360]) allows reaching of unbelievable extrasensitivity at nanoscale. For example, a special biosensor can be manufactured that is capable of detecting biological species at concentrations of 10^-18 mol/l, which is approximately eight orders of magnitude more sensitive as compare with conventional techniques [360].

     High sensitivity reached so far allows force measurements between individual biomolecules and complexes that have been substantial technical challenge a decade ago. So single-molecule atomic force spectroscopy now are becoming a rather usual practice. These advances gave rise to developing a novel direction in a biosensing techniques based on AFM always mentioned above. Actually, AFM cantilever itself can be used to serve as a main sensitive element of biosensor. Such force measurements are usually performed using the AFM probe functionalized with a biomolecule of interest and its complementary molecule immobilized onto the sample surface [346, 1725].

     AFM by itself is, no doubt, a powerful instrument to explore micro- and nanoscopic biological objects. But since early times of AFM the necessity of combining with another methods were considered (see for instance [997]). For example, in the applications to biology initial location of target object is of great importance and this task can be easily performed by conventional optical or fluorescence microscope. It prevents from unnecessary scans and tip contamination while locating, say, cells or subcell organelles such as chromosomes.

     It isn't much of an exaggeration to say that AFM prospects in biology are unbelievable and who knows, may be some day in the future gene engineering will be provided with a useful tool for easy manipulating of genes inside chromosome.

     Cumulative list of articles devoted to biological applications of Atomic Force Microscopy and placed in "Biology" section including subsections can be downloaded in PDF format:

Biological Applications of Atomic Force Microscopy (Updated on January 14, 2003)

     By now it exceeds 1000 items.
     Excellent bibliography on biological applications of AFM is accumulated by Sophia Hohlbauch, biologist at Digital Instruments: http://www.di.com/Library/Bibliographies/BiologicalBib.html