Self-healing materials in neutron light

A material that repairs itself after tearing – sounds like science fiction, but it is already being used in the form of hydrogels for wound healing and in agriculture. Researchers at the Heinz Maier-Leibnitz Zentrum (MLZ) have used neutrons to investigate how these self-healing polymers are structured and how they behave.

polymer_AI_eng Artistic representation of self-healing hydrogels with highlighted internal structure: micelle-like core (orange) surrounded by free polymer areas (blue) on the outside. © generated with AI, Gemini

Artistic representation of self-healing hydrogels with highlighted internal structure: micelle-like core (orange) surrounded by free polymer areas (blue) on the outside. © generated with AI, Gemini

Self-healing hydrogels are gels that consist of polymer networks with temporary bonds. Due to their water-retaining properties, they are popular in medical wound care and for water and fertiliser storage in agriculture. Since the bonds within self-healing hydrogels are only temporary and constantly reform, they can repair defects, such as cracks, themselves.

Structure and movement: insights into micelles in hydrogels

Using neutron measurements at the MLZ, a research team has deciphered how this self-healing process works inside certain hydrogels. “As their concentration increases, polymers form flower-like micelles with a compact core, which are connected to each other by polymer chains to form a gel,” explains co-author Dr. Olaf Holderer, instrument scientist at Forschungszentrum Jülich at the MLZ. Their investigations focused on how the movement of the outer polymer chains compares with that of the dense network core of the hydrogels. This was investigated using small-angle neutron scattering (SANS) experiments at KWS-2 and neutron spin echo (NSE) at J-NSE at the MLZ, among other methods.

Pheonix In 2017, the J-NSE ‘PHOENIX’ device was upgraded. The instrument is now equipped with superconducting main coils that can generate a magnetic field approximately three times stronger than before. © Tobias Hase

In 2017, the J-NSE ‘PHOENIX’ device was upgraded. The instrument is now equipped with superconducting main coils that can generate a magnetic field approximately three times stronger than before. © Tobias Hase

Differences between the core region and outer polymer chains

The investigations showed that the outer polymer chains behave similarly to long chains in liquids. However, when the researchers compared this with data from the core, they found significant differences in behaviour, highlighting the need to consider the core and the outer regions separately. The two regions also differ significantly in terms of dynamics. Olaf Holderer explains: “The core region exhibits significantly slower diffusion-like movement, restricted by the network and neighbouring structures.”

Stability and self-healing

This important difference in movement explains why these materials are stable and can repair themselves quickly. The rapid movement of the outer chains enables self-healing, while the slow movement in the core area ensures the material’s stability. This knowledge is important for improving numerous applications, such as the use of such gels for wound healing in the biomedical field.