Jonathan Cooper and Manilo Tassieri at the University of Glasgow explain how mechanical phenomena in biological systems can be studied at very small scales.

Studying biological systems at the micro and nanoscale can give important information about cellular processes responsible for respiration, reproduction and signalling. But, while there are many methods for performing measurements at a surface, techniques for measuring interactions between biomolecules in solution are limited. In fact, current methods for measuring biomolecular interactions in solution, such as fluorescent labelling require complex chemistry, and such methods give only limited biophysical data on the molecular mechanics of any interaction.

Optical tweezers can hold and manipulate microscopically small samples and living cells

Microrheology is the study of the flow of materials over small scales. It is based on observing the free or driven motion of micrometer-sized beads suspended in solution. The time dependent positional changes of the beads can be directly related to the mechanical properties of the molecules and their solvent. The technique is an important new addition to bioanalysis as it can characterise the mechanical interactions between complex biomaterials in solution. Biophysicists have been quick to take advantage of this technique, using it to measure the compliance of bacterial tails, the forces exerted by single motor proteins and the stretching of single DNA molecules. 

Combining microrheological techniques and using optical tweezers with lab-on-a-chip technology, has made it possible to produce sensitive sensors for measuring the changes that occur when molecules interact with each other. Optical tweezers use beams of light to hold and manipulate microscopically small objects such as biological samples including living cells. They are now an invaluable tool in biological and physical sciences with wide ranging applications, allowing studies such as protein folding and denaturation in DNA, nucleotide interactions between proteins and RNA amongst others. A particularly exciting emerging area for their use is to study the self-assembly of protein fibres in the brain - an irreversible process that is strongly implicated in Alzheimer's disease. Using microrheology could allow the kinetics of the fibrous assembly in solution to be measured. Also, drug interactions with the fibres could be probed to study both a preventative role inhibiting their formation and in a therapeutic role to reverse those that have already formed. 

Nanotechnology could also be advanced using microrheology, as exploring the dynamics of molecular self-assembly in solution could lead to more well-defined functional nanostructures. An improved understanding of these assembly processes could lead to a wealth of versatile peptide-based self-assembled nanostructures whose properties could be engineered to create artificial lubricants for joints, new tissue scaffolds and drug delivery systems. 

Read more in the review 'Microrheology with optical tweezers' in Lab on a Chip. 

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