Some polymers react to their environment with conformational changes: one of these is the polymer PNIPAM, short for poly(N-isopropylacrylamide). It is water-soluble below around 32 degrees Celsius, but above this temperature it precipitates and becomes hydrophobic. This qualifies it for smart sensor applications. But what actually happens between PNIPAM and the solvent water? Researchers at Ruhr University Bochum, Germany, and the University of Illinois Urbana Champaign collaborated with sound production specialists from Symbolic Sound Corporation to investigate this question. Using sound representation, they were able to decipher the interaction of water molecules with PNIPAM for the first time. They report their findings in the journal Proceedings of the National Academy of Sciences PNAS on February 4, 2026.

The role of the solvent, water, often takes second billing when researchers analyze the motion of polymers such as proteins or human-engineered PNIPAM. “And yet, water forms hydrogen bonds between water molecules and water and polymer, and these hydrogen bonds organize both the water structure and the polymer structure” says Professor Martina Havenith-Newen, Holder of the Chair of Physical Chemistry II at Ruhr University Bochum and spokesperson for the Ruhr explores Solvation RESOLV Cluster of Excellence. “Water plays an important role in how polymers expand or contract or fold inside of the solvent.”

Scientists from Ruhr University Bochum, sonification specialists from Symbolic Sound Corporation, and researchers from the University of Illinois Urbana Champaign teamed up to see how PNIPAM contracts and expands in water, the universal solvent in the human body, and what the role of water is in this process. To do so, the postdoctoral researcher Wanlin Chen founded with the Henriette Scout program of the Alexander von Humboldt foundation, ran long supercomputer simulations that follow the motion of PNIPAM in water for billions of time steps.

The conundrum was in the data analysis: there are so many thousands of water molecules around PNIPAM, and they form and break hundreds of hydrogen bonds with PNIPAM all the time, so it was hard to visualize using conventional computer visualization. The team brought on board Carla Scaletti and Kurt Hebel who are developing Auditory Analytics, a technique that uses sonification (turning data into sound) in a way complementary to visualization (turning data into videos or images). “Sonification has the advantage with complex time series of data that the human brain is very good at discerning patterns in audio waveforms that arise from many near-simultaneous events”, Martin Gruebele, a guest professor also funded by the Alexander von Humboldt foundation explains.

Using sonification, the researchers found that PNIPAM, as it contracts, does not form many direct hydrogen bonds, but rather its structure is organized by 'water bridges', where a water molecule makes links together two parts of PNIPAM. PNIPAM itself also forms an unusual bond where to hydrogen atoms stuck to nitrogen atoms lign up. All this could be heard and differentiated when different sounds were assigned to different bond types, even when dozens of those bonds form and dissolve at any given time.

The follow-up quantitative analysis of the computer simulation revealed that water bridges are not random, but correlate with one another as PNIPAM collapses to a compact form, but with water pulling the strings in the process, not so much direct PNIPAM contacts. “The research helps us understand how the unusual behavior of some polymers, so useful in biomedical applications and sensing, arises” says Martina Havenith.
Video: https://youtu.be/tnQVaFgKprw