title: The electrofabrication of di- and tripeptide hydrogels and their subsequent material properties
Creators: name: Patterson, Courtenay
Publisher: name: University of Glasgow
Year: 2023

description: In this thesis, we report the ability to fabricate hydrogels using low molecular weight gelators (LMWGs) and the subsequent characterisation of their mechanical properties over a variety of different length scales. These materials have been investigated due to their potential use in a wide range of biomedical applications including drug delivery, tissue engineering, cell culture and wound healing. We describe the localised gelation of LMWGs on electrode surfaces via electrochemically generated pH gradients. The electrofabrication of hydrogels on electrode surfaces has shown great potential in the field of biomedicine, with applications ranging from antimicrobial wound dressings, tissue engineering scaffolds and biomimetic materials. First, we describe the largest reported di- and tri-peptide-based hydrogels on electrode surfaces via the electrochemical oxidation of hydroquinone. Expanding upon previous work which focuses on the fabrication of hydrogels on the nanometre to millimetre scale, we deposit hydrogels around 3 cm3 in size. Furthermore, we demonstrate that there is an upper limit to how large the hydrogels can grow which is determined by the size of the pH gradient from the electrode surface. To grow hydrogels of this size, much longer deposition times of two to five hours are required than in previous reports. When the gelator/hydroquinone solution is left exposed to the open atmosphere for this amount of time, the hydroquinone in solution oxidises to benzoquinone/quinhydrone before it can be consumed electrochemically. This inhibits the electrochemical reaction and reduces gelation efficiency. To prevent this, we build a system that can perform the fabrication process under an inert nitrogen atmosphere. Using this system, we show how the choice of gelator affects the mechanical properties of the hydrogel and the resulting material phenomena that cause these changes. As well as this, we show how this approach can be used to grow multi-layered hydrogels, with each layer presenting different chemical and mechanical properties. Secondly, we report the first known example of electrodeposition for a LMWG molecule using an electrochemically generated basic pH gradient at electrode surfaces. This approach has previously been used to fabricate hydrogels of the biopolymer chitosan using the galvanostatic reduction of hydrogen peroxide. During the electrochemical reduction of hydrogen peroxide, hydroxide ions are produced. As a result, a basic pH zone is generated at the electrode, triggering solutions of chitosan to form immobilised hydrogels on the electrode surface. Using this approach, we show how electrodeposition at high pH can be applied to our LMWG system. We then show that we can electrochemically form hydrogels at high pH, with the gel properties being greatly improved by the addition and increased concentration of hydrogen peroxide. Following from this, we then show the simultaneous formation of two low molecular weight hydrogels at acidic and basic pH extremes. To achieve this, we couple the electrochemical reduction of hydrogen peroxide and the electrochemical oxidation of hydroquinone described in the previous chapter. Finally, we report the electrodeposition of five carbazole-protected amino acid hydrogels on electrode surfaces via the electrochemical oxidation of hydroquinone. As well as this, we report the full to partial electropolymerisation of the pre-assembled hydrogels in perchloric acid. For the less bulky carbazole-protected amino acids, the full collapse of the hydrogel to form electrochromic polymers on the electrode surface is achieved. However, for the bulkier gelators, little to no evidence of polymerisation occurs. We believe this is due to the bulky side chain on the gelator backbone preventing the molecular reorganization required for polymerization to occur. To probe the primary self-assembled structures of the carbazole-based hydrogels growing in-situ and their full to partial electropolymerisation in perchloric acid, a first-of-its-kind experiment was performed using small-angle X-ray scattering (SAXS) at Diamond Light Source (I22 beamline, Oxfordshire, UK). We present the novel SAXS set-up discussed as a tool to open up new opportunities to probe and analyse soft materials in realtime.