The self-assembly peptide hydrogels used as tissue culture scaffolds have drawn great attention in recent years. They have the advantages of natural polymer hydrogels including biocompatibility, biodegradability and the advantages of synthetic materials such as controlled structural properties and mechanical properties. Furthermore, the bioactive ligands which can promote bioactivities and control cell behaviours can be easily introduced to the peptide backbone through peptide synthesis. One particular self-assembly FEFEFKFK peptide was chosen in this project.FEFEFKFK peptide used in this project has been reported to self-assemble in solution, forming hydrogels with a 3D fibrous network structure above a critical gelation concentration. In this project, the self-assembly and gelation properties of FEFEFKFK peptide were further investigated, assessing the effect of pH and ionic strength on the self-assembly and gelation behaviour. The biomimetic nanofibrous hydrogels of FEFEFKFK were also assessed for their ability to support human dermal fibroblast cells. The protocols of gel preparation were developed for both 2 dimensional (2D) and 3 dimensional (3D) cell culture. A short peptide sequence homoarginine-glycine-aspartate (hRGD) has been introduced onto the amide end of the self-assembly peptide instead of bioactive ligand arginine-glycine-aspartate (RGD), creating hydrogels with a fibrous network with functionalised groups at the fibre surface. The functionalised peptide hydrogels enhanced cell adhesion on gel surface, with cell interaction assessed using various imaging and spectroscopic techniques. A preliminary 3D cell culture study also showed potential of these peptide gels to be used for encapsulated human dermal fibroblast cell studies.

The use of low-molecular weight peptide-based hydrogelators (LMWGs) for the immobilisation of enzymes is presented in this thesis. Low-molecular weight hydrogelators are a class of materials which are highly suitable for increasing enzyme lifetimes as they create a suitable biomimetic environment. Immobilised enzymes can be utilised in enzyme fuel cells, providing low-energy conversion routes for chemical processes. The hydrogels also possess tunable properties which allow their structure to be manipulated to give desirable properties. This work begins with an exploration of dipeptide hydrogelators by investigating the effect of varying salt solutions and concentrations of dipeptide on final hydrogel structures. A wide range of characterisation techniques are employed to provide information about the micro- and macro-structure of the hydrogels. The creation of dipeptide hydrogel materials via an electrochemical method is explored, which allows the production of nanometre thick, membrane-like materials. These layers are measured using Surface Plasmon Resonance techniques. The electrochemical technique for dipeptide gelation is expanded in later chapters to produce a range of novel materials. Finally, an exploration into the effect of additives on dipeptide hydrogels is conducted, where the effect of adding chiral molecules is investigated. This provides interesting information regarding the self-assembly processes involved with hydrogelation processes, which has important implications for studying the folding and unfolding processes of peptides.

Hydrogels based on self-assembling peptides are believed to have potential for use in a wide range of biomedical and biodiagnostic applications. For many of these, control over various properties of the gels is essential for tuning the gels to fit certain constraints or requirements in terms physical properties such as diffusive properties and swelling. One important property to control for applications such as cell culture and drug delivery is its mechanical strength, and this study investigates three different strategies by which the individual peptide monomers can be modified in order to effect a change in the macromolecular self-assembled structure and therefore a bulk change in the mechanical stiffness. In chapter 4, two ionic-complementary octapeptides, FEFKFKFK and FEFQFKFK are described, with monomer charges of +2 and +1, respectively at physiological pH. FEFKFKFK was observed to form largely discrete fibrils, characteristic of similar systems, while FEFQFKFK formed fibril bundles – believed to be a limited form of an aggregation effect frequently seen in similar peptides with neutral charge. As a result of this structural change, FEFQFKFK was found to have values for the elastic and viscous moduli (which are often used to measure the ‘strength’ of a gel) between 5 and 10 times larger than those of FEFKFKFK at the same concentration. The same behaviour was seen in FEFKFKFK when the monomer charges were reversed by adjusting pH, suggesting that the monomer charge is indeed responsible for the bundling effect. In chapter 5, two branched peptides were designed and synthesized: KG17, with two arms consisting of self-assembling FKFEFKFK-motifs, and KG28 which had three such arms. Each branched peptide was doped into pure FKFEFKFK and the resulting gels investigated. While no obvious structural changes were observed for either dopant (save for a potential fibril parallelisation effect with KG17 observed in Small-Angle Neutron Scattering (SANS)), both were observed to increase the elastic and viscous moduli of the gels at overall peptide concentrations of 30 and 50 mg mL-1 (gels), but not at 10 mg mL-1 (viscous liquid). The most dramatic change was observed in the 50 mg mL-1 gels, suggesting that higher concentrations could enhance the effect of the dopants. In chapter 6, three thermo-responsive polymers (pTEGMA), of Degrees of polymerisation (DPs) 17, 47 and 142 were conjugated to CGFKFEFKFK and incorporated into a peptide hydrogel. Gels containing the non-conjugated versions of each polymer were also tested. While no changes in morphology were observed at the fibillar level, the polymer Lower Critical Solution Temperature (LCST) behaviour could be observed in SANS in all samples apart from the DP17 conjugate. However, in rheological tests gels doped with this conjugate appeared to show the strongest the elastic and viscous moduli. In general the conjugates appeared to increase the elastic and viscous moduli, particularly at temperatures above ca. 50°C. Rather than this being LCST behaviour, it was suggested that the polymers can act to enhance a natural thermo-response that was observed in the peptide, with the shortest polymer (DP17) experiencing the least steric hindrance and therefore having the strongest effect. It was postulated that this interaction could involve the screening of charge on the peptide fibril. Non-conjugated polymer appeared to have little effect on the mechanical properties, with elastic modulus values correlating strongly to the overall peptide concentration.

Tissue engineering is a promising approach that combines the interactions of biomaterials, cells, and growth factors to stimulate tissue growth and regeneration. As such, selecting a suitable biomaterial is vital to the success of the procedure. Ideally, the material should show similarity to the extracellular matrix in the structure and relative stiffness, and biofunctionality beside others to provide a comfortable environment for the cells. Additionally, the biomaterial properties should allow for the effective diffusion of relevant growth factors and nutrients throughout the material to enable cell growth. Because peptides are composed of amino acids found naturally within the human body, they are considered non-toxic and biocompatible. Ultrashort peptides are peptides with three to seven amino acids that can be self-assembled into helical fibers forming scaffolds of supramolecular structures. These peptide hydrogels formed a highly porous network of nanofibers which can quickly solidify into nanofibrous hydrogels that resemble the extracellular matrix (ECM) and provide a 3D environment for cells with suitable mechanical properties. Furthermore, we can easily tune the stiffness of these peptide hydrogels by just increasing peptide concentration, thus providing a wide range of peptide hydrogels with different stiffness for 3D cell culture applications. Herein we describe the use of ultrashort peptide hydrogels for the maintenance and the differentiation of human mesenchymal stem cells into the osteogenic lineage. Furthermore, we develop a three dimensional (3D) biomimicry acute myeloid leukemia (AML) disease model using biomaterial from a tetramer ultrashort self-assembling peptide. In addition, we evaluate the potential application of peptide hydrogels as a hemostatic agent. The results presented in this study suggest that our biomimetic ultrashort tetrapeptide hydrogels are an excellent candidate for tissue engineering and biomedical applications.