Development of Stimuli-responsive Materials for Direct Laser Writing for the Realisation of Microactuators

Abstract

Two-photon polymerisation (2PP) is a laser-based fabrication technology which can be used to fabricate complex 3D microstructures with feature sizes ~100 nm. This thesis assesses the state of the art in 2PP and presents an overview of the microstructures that can be achieved and their functionalities and applications. Critical for the development of 2PP-based fabrication is the expansion of the available library of photoresists which will allow for the incorporation of a wider range of materials and stimuli-responsive moieties. Therefore, the aim of this work is to produce and characterise new photoresists compatible with 2PP that allow for the realisation of novel, stimuli-responsive hydrogel microstructures. The drive to achieve such systems at the micro- and submicron scale comes from the generally slow response times recorded for macroscale hydrogel structures. The reason for these typically slow response times is that hydration is a diffusion-controlled process. The characteristic swelling time of a hydrogel are typically proportional to the square of the linear dimension of the hydrogel, therefore a reduction in the dimension of the hydrogel structure from mm to ?m scale can lead to a dramatic reduction in response time. Driven by this prospect, new hydrogel photoresists were developed in this thesis, that respond to changes in temperature (Chapter 2), magnetic field (Chapter 3) or the local chemical environment (Chapters 4 and 5).

Chapter 2 highlights a facile approach for the rapid development of a new library of photoresists, by employing ionic liquids (ILs) as photoresist solvents. While ILs have long been proposed as solvents for radical polymerisation, this is the first reported instance where ILs have been used as solvents for the 2PP fabrication process. Their influence on the polymer network is characterised and exemplified for poly(N-isopropylacrylamide) based hydrogels that exhibit thermo-response.

In Chapter 3, multi-core maghemite nanoparticles were incorporated into photoresists for fabrication via both digital light processing (DLP) and 2PP, for the fabrication of macro- and micro-scale structures. The excellent dispersibility of the magnetic nanoflowers (MNFs) in the trifunctional crosslinker trimethylolpropane ethoxylate triacrylate (Mn ~ 912g/mol) (TMPET-912) allowed for the realisation of photoresists of high MNFs loading, to produce structures that showed a magnetic hyperthermia response.

Chapter 4 showcases a novel sugar responsive, phenyl boronic acid (PBA)-based photoresist for 2PP. This allows, for the first time, the fabrication of hydrogel microstructures showing sugar-induced actuation. The ability to fabricate structures on the microscale overcomes the traditionally slow response times associated with responsive phenylboronic hydrogels on the macroscale, to produce fast actuators. Microstructures with programable actuation are also demonstrated.

In Chapter 5, a new PBA-based photoresist is developed, to fabricate sugar responsive photonic structures which display reversible colour changes in PBS and sugar-containing solutions.