In a study published in Nature Communications, Prof. JIA Di's group from the Institute of Chemistry of the Chinese Academy of Sciences reported the first experimental evidence of a novel universality class in hierarchical dynamics using dynamic light scattering (DLS). This study provides insights into reversible physical gel’s complex structures and dynamics, and expands the 50-year legacy of DLS applications beyond conventional chemical gels.

Dynamically adaptable reversible gels with crosslinks based on weak, dynamic physical bonds rather than permanent chemical bonds have emerged as a versatile material class across biological and synthetic systems. These physically bonds endow gel materials with remarkable properties like self-healing, time-programmable with a memory effect and recycling properties. However, it remains a challenge to accurately quantify the strength and fraction of weak physical cross-links and to understand how they lead to large-scale collective dynamical properties.

Based on a newly developed theory by Prof. Murugappan Muthukumar from the University of Massachusetts Amherst, researchers addressed the physical origin of the universal hierarchical gel dynamics, which is in stark contrast with the standard chemical gels whose correlation function is pure exponential decay. For all the physical gels their correlation function can be fitted by a stretched exponential decay with a constant stretched exponent β=1/3.

Researchers found that the stretched exponent β, which equals 1/3, originates from the hierarchical relaxation processes of reversible physical crosslinks as they dissociate and reassociate. This constant β value and the associated characteristic relaxation time τ_β provide a universal law for all reversible physical gels, allowing for the quantification of local energetics within the gel structure.

Moreover, researchers confirmed this universal law across various complex gels, including artificial mucus, self-assembled DNA gel, and natural silk fibroin gels. This discovery offers a rapid and effective method to determine whether chemical or dynamic physical bonds dominate in a gel’s crosslinks. It also enables the extraction of binding energies for complex physical crosslinks when multiple types of bonds are active simultaneously.

This study differentiates physical/chemical bonds governing crosslinking in complex gels and natural systems (e.g., mushy okra, and bat tongue papillae), and enables tunable mechanisms to program large-scale functional properties for smart hydrogel-based applications.