Public defence, Engineering Physics, MSc Yuhuang Fang

Title of the thesis: Strengthening hydrogels through fibrillar double network

Thesis defender: Yuhuang Fang
Opponent: Professor Wim Thielemans, KU Leuven, Belgium 
Custos: Aalto Distinguished Professor Olli Ikkala, Aalto University School of Science

Hydrogels are soft polymeric materials that contain water and are widely studied for applications such as soft robotics, biomedical devices and flexible technologies. However, combining softness with high strength, stiffness and toughness remains a major challenge. This doctoral thesis investigates how the internal structure of hydrogels can be designed to improve their mechanical performance.

The study focuses on fibrillar double networks based on agarose. Double-network structures are already widely used to strengthen hydrogels, but this thesis explores new ways of controlling how the different networks and reinforcing components are connected and organised.

One approach chemically connects an agarose fibrillar network with a second polymer network. These connections allow the two networks to work together more effectively under deformation, resulting in improved fracture strength, stretchability and toughness.

A second strategy introduces clay nanosheets into the hydrogel. By controlling their organisation and phase separation inside the agarose network, nanosheet-rich hard domains can be formed within a softer polymer matrix. This combination substantially increases the stiffness and strength of the material and also gives the hydrogel notable resistance to high-speed impact.

The thesis further shows that controlling the organisation and connections between nanosheets can create efficient pathways for transferring mechanical stress through the material, enabling hydrogels with high stiffness and self-healing capability.

The main conclusion is that the mechanical properties of hydrogels can be greatly improved not only by changing their chemical composition, but also by carefully designing their internal architecture. The strategies developed in this thesis provide new possibilities for creating mechanically robust soft materials for applications such as soft robotics and flexible devices.

Keywords: hydrogels, fibrillar double network, clay nanosheets, phase separation, mechanical properties

Thesis available for public display 7 days prior to the defence at Aalto University's public display page.