Switchable hydrogel networks based on natural polysaccharides

Responsive hydrogels are gaining interest in different applications due to their flexible chemistries, biocompatibility, and softness. This has allowed 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 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. In Publication I, agarose is utilised as a primary network that is removed after PNIPAm polymerisation to create 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. Publication II utilises chemically modified agarose as a macro-crosslinker in the PNIPAm network, producing a hydrogel that shows superior whiteness at smaller thickness of the reflecting layer compared to the channeled PNIPAm. Publication III utilises the water transportation properties of the channeled hydrogel to realise controllable underwater adhesion. Additionally, the hydrogel includes biomimetic catechol groups to enhance adhesive properties. The combination of the 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 as demonstrated, showing the potential of the channeling approach in fields utilising controllable underwater adhesion such as biomedicine and soft robotics.