Being able to measure something plays a vital role in our ability to understand many phenomena. But the action of measuring can often affect what we are trying to measure; this is particularly the case when trying to measure substances that are very small, soft, or fragile.
In a recent study published in PRX Life, a research team led by the University of Osaka successfully used acoustic tweezers as a contactless method to investigate an important type of fragile material called biopolymer condensates.
Biopolymer condensates are liquid-like droplets composed of proteins and/or nucleic acids, and are involved in regulating a variety of physiological functions within cells to keep them healthy. However, problems with these droplet systems can result in a variety of diseases, including neurodegenerative diseases. The mechanical properties of biopolymer droplets, such as fluidity and stiffness, are important for biological activity. Hence, better knowledge of the mechanical properties of these droplets will help us to understand their roles in healthy cells and disease states. However, these droplets are very small and fragile and are difficult to investigate using conventional techniques.
A multi-institutional team led by researchers from the University of Osaka has developed an analytical tool called acoustic tweezers that uses ultrasound to enable the mechanical properties of condensates to be investigated without needing to physically touch the material. "We fabricated a device that creates an acoustic force that can trap condensates at a specific point," explains lead author Kichitaro Nakajima.
As a proof of concept, the group investigated biopolymer condensates made of polyadenylic acid. These condensates are sensitive to salt concentration, so the research team expected that changing the salt concentration would cause changes in the mechanical properties of the droplets, which could then be measured using the acoustic trapping method.
"We found that these condensates could be efficiently trapped and aligned based on the acoustic force in a contactless manner" says Nakajima. "In addition, two condensates could be trapped to analyze what occurs when droplets merge."
The team found that when acoustic trapping was applied, the changes in the natural movement of a droplet in solution could be used to provide information on the stiffness of the droplet and the state of the molecules inside the droplet. They then developed a framework to essentially estimate the stiffness of a droplet from its behavior in their sound-based trapping system. "This information is useful for understanding the mechanical properties of these droplets and hence their biological activity" explains Nakajima.
These acoustic tweezers are useful tools for investigating soft materials, such as biomolecular droplets, and allow better understanding of their role in living cells and disease states. This knowledge may lead to the discovery of new methods for treating diseases associated with the dysfunction of biomolecular droplets.
Fig. 1
Caption: Acoustic trapping of biomolecular droplets
Credit: K. Nakajima et al, PRX Life（American Physical Society）
Movie. 1
Caption: RNA droplets trapped by acoustic tweezer
Credit: K. Nakajima et al, PRX Life（American Physical Society）
Movie. 2
Caption: Microparticles aligned by acoustic tweezer
Credit: K. Nakajima et al, PRX Life（American Physical Society）
Note
The article, "Mechanical profiling of biopolymer condensates through acoustic trapping," was published in PRX Life at DOI: https://doi.org/10.1103/kl9v-5ywv
