Measuring the mechanics of a fragile biological material without disturbing it has long been a major challenge in biophysics. Now, researchers led by the University of Osaka have demonstrated a contactless technique that could make it possible to probe the physical behavior of tiny biomolecular droplets while they remain suspended in liquid. Their method uses ultrasound to trap and manipulate the droplets, offering a new way to study the soft, dynamic structures that help organize life inside cells.
The study, published in PRX Life, focuses on biopolymer condensates, liquid-like droplets formed when proteins, nucleic acids, or combinations of both gather into concentrated compartments. Unlike membrane-bound organelles, these condensates are assembled through physical processes such as liquid-liquid phase separation. Their interiors can concentrate specific molecules, creating temporary reaction environments that help regulate gene expression, signaling, stress responses, and other essential cellular functions. Because condensates can form, merge, dissolve, and change their internal organization rapidly, their material properties are closely connected to their biological roles.
The droplets are not simply passive blobs of liquid. Their viscosity, stiffness, surface tension, internal molecular arrangement, and ability to flow can determine how they interact with other cellular components. A condensate that becomes unusually viscous or rigid may fail to merge correctly, trap molecules that should remain mobile, or persist longer than it should. Such changes have been associated with abnormal cellular states and are of particular interest in research on neurodegenerative diseases, where proteins and nucleic acids can form persistent, dysfunctional assemblies. Yet measuring these properties directly is difficult because the droplets are microscopic, soft, and easily altered by physical contact.
Conventional measurement techniques can introduce precisely the disturbances scientists are trying to avoid. Touching a droplet with a probe may deform it, move molecules within it, or change its shape and composition. Even the process of placing a droplet on a surface can modify its behavior. To overcome this problem, the Osaka-led team developed an acoustic tweezer system that uses ultrasound to exert forces on condensates without a mechanical tool ever contacting them. The device creates a controlled acoustic field, allowing droplets to be captured at a specific location, held in place, and arranged for observation.
“ We fabricated a device that creates an acoustic force that can trap condensates at a specific point,” explains lead author Kichitaro Nakajima. In practical terms, the system uses the pressure distribution generated by sound waves to influence the motion of the droplets in solution. When the acoustic forces are balanced appropriately, a condensate can be confined near a defined position. This form of acoustic manipulation is related to the broader field of acoustic tweezers, in which sound is used to control small objects ranging from cells to particles without the need for physical contact.
To test the approach, the researchers studied condensates made from polyadenylic acid, a nucleic-acid-based polymer. These condensates are particularly useful for a proof-of-concept experiment because their properties respond sensitively to salt concentration. Changing the concentration of dissolved salt can alter the interactions among polymer molecules, affecting how the droplets form and how they behave mechanically. The researchers therefore expected that changes in the chemical environment would produce measurable changes in droplet behavior under acoustic trapping.
The experiments showed that the condensates could be efficiently trapped and aligned using the acoustic force. This allowed the team to observe the droplets while minimizing direct disturbance. The system also made it possible to bring two condensates together and examine what happened as they merged. Droplet fusion is an important physical process: in a simple liquid system, two droplets may rapidly combine into one larger sphere, but the speed and manner of fusion can reveal information about viscosity, interfacial tension, and molecular organization. Watching this process under controlled conditions gave the researchers another way to examine the mechanics of the condensates.
The team’s analysis went beyond simply holding the droplets still. When a condensate is trapped by sound, its natural movement in the surrounding solution changes. That movement includes fluctuations caused by thermal energy and interactions between the droplet and its fluid environment. By analyzing how the droplet moved under the acoustic force, the researchers extracted information about its stiffness and the condition of the molecules inside it. They then developed a framework for estimating droplet stiffness from the observed response of the trapped condensate.
This approach effectively turns microscopic motion into a mechanical measurement. A softer droplet may respond differently to the acoustic field than a stiffer one, while changes in internal molecular interactions can influence both its movement and its response during fusion. The method therefore provides a way to connect visible behavior with properties that are otherwise difficult to measure directly. Importantly, the technique does not require the droplet to be attached to a surface or compressed by a probe, preserving a more natural solution-based environment.
The researchers believe acoustic tweezers could become a broadly useful tool for investigating soft biological materials. By enabling contactless measurements of condensate mechanics, the technology may help scientists understand how these droplets function in healthy cells and how their physical properties change during disease. It could also support studies of other delicate materials whose behavior is easily disrupted by conventional instruments. Although the current work is a proof of concept using polyadenylic-acid condensates, the underlying strategy offers a promising route toward mapping how molecular composition, chemical conditions, and mechanical properties interact inside biomolecular droplets. In the long term, such information could contribute to a clearer understanding of condensate-related dysfunction and guide the search for therapies targeting diseases in which these dynamic cellular compartments go awry.
Subject of Research: Not applicable
Article Title: Mechanical profiling of biopolymer condensates through acoustic trapping
News Publication Date: 17-Aug-2026
Web References: https://doi.org/10.1103/kl9v-5ywv
References: PRX Life, “Mechanical profiling of biopolymer condensates through acoustic trapping,” DOI: 10.1103/kl9v-5ywv
Image Credits: K. Nakajima et al., PRX Life (American Physical Society)
Keywords: Acoustic tweezers, biopolymer condensates, biomolecular droplets, biophysics, acoustic trapping, soft matter physics, cell biology, neurodegenerative diseases, microfluidic droplets, Brownian motion, applied acoustics, natural polymers
Tags: acoustic trapping in biophysicsbiomolecular condensate stiffness measurementbiomolecular droplet mechanicsbiomolecular droplet viscosity and surface tensionbiophysical techniques for fragile biological materialsbiopolymer droplet propertiescellular organelle mimics using acoustic tweezersliquid-liquid phase separation in cellsnon-invasive cellular material analysisstudying internal molecular organization of condensatesultrasound manipulation of cellular dropletsultrasound-based probes for dynamic cellular structures
