Public defence in Engineering Physics, M.Sc. (Tech) Amanda Eklund

Title of the thesis: Switchable hydrogel networks based on natural polysaccharides

Doctoral student: Amanda Eklund
Opponent: Prof. Marleen Kamperman, University of Groningen, The Netherlands
Custos: Prof. Olli Ikkala, Aalto University School of Science, Deparment of Applied Physics

Responsive hydrogels, composed of polymer networks in water, are gaining interest in different applications due to their flexible chemistries, biocompatibility, and softness. This allows utilisation in fields such as biomedicine, and electronics. By modifying the microstructure of the hydrogel, different material properties can be introduced and optimised. 

In this thesis, a natural polysaccharide, agarose, is used to modify the hydrogel network of a thermoresponsive polymer, N-isopropylacrylamide (NIPAm) to enhance its properties. Using two different network architectures, the optical and adhesive properties of the hydrogels are controlled using temperature change as a stimulus. A technique is presented, where agarose is utilised as a primary network that is removed after PNIPAm polymerisation to form channels into the hydrogel. These channels enhance water transportation and enable the hydrogel to undergo phase transitions more quickly compared to traditional PNIPAm. Additionally, the material has a bright white appearance, enabling use in applications such as controllable screens and optical switches. To further improve the white appearance of the hydrogels, chemically modified agarose is utilised as a macro-crosslinker in the PNIPAm network, producing a hydrogel that shows superior whiteness at a smaller thickness of the reflecting layer compared to the channeled PNIPAm. 

The improved water transportation properties of the channeled hydrogel are also used to realise controllable underwater adhesion. Additionally, the hydrogel includes biomimetic catechol groups to enhance adhesive properties. The combination of the improved adhesion and controllable water transportation allows the adhesion to be switched on and off using a change in temperature with a high switching efficiency, both underwater and in dry conditions. This hydrogel system can be used as a controllable gripper for fragile, lightweight, irregular biological systems, showing the potential of the channeling approach in fields utilising controllable underwater adhesion such as biomedicine and soft robotics.

Keywords: Hydrogel, thermoresponsive, polymer, network, poly(N-isopropylacrylamide), agarose, adhesion, whiteness

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus 

Doctoral theses in the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52