Stimuli-Responsive Peptide-Based Biomaterials: Design, Synthesis, and Applications

Peptide-based biomaterials have gained much interest in various applications in drug delivery and tissue engineering in recent years, in large part due to their typically excellent biocompatibility and biodegradability. Composed of different amino acids, peptides can be designed with numerous sequences, providing flexibility and tunability in biomaterials. Peptides are easy to modify with small molecule drugs, inorganic components, and polymer chains to access multiple functions and tune properties relevant to biology and medicine. Stimuli-responsive peptide-based biomaterials can respond to environmental stimuli, such as light and ultrasound, in addition to local environmental factors, such as temperature, enzyme activity, and pH. Under environmental changes, these materials can be triggered to release therapeutic payloads, change conformations, or induce self-assembly in the target sites.

In this work, I introduce the design, synthesis, and potential applications of several stimuli-responsive peptide-based biomaterials. The first half of this dissertation is based on enzyme-responsive, peptide-based biomaterials as extracellular matrix (ECM) mimics in tissue engineering. We synthesized linear and dendritic elastin-like peptides (ELPs) as crosslinkers and conjugated them with hyaluronic acid (HA) to form hydrogels. Trypsin was used as the enzyme trigger for cleaving the C-terminal lysine and to study how crosslinker topology affects enzymatic degradation. Hydrogels with dendritic ELPs degraded more slowly than linear ELPs, providing a novel strategy to tune the degradation rate of hydrogels as ECM mimics by the molecular design of crosslinker topology. Building on this peptide-polysaccharide platform for synthetic ECM design, we subsequently prepared hydrogels embedded with bioactive cryptic sites. These novel polymeric hydrogels mimicked native ECM cryptic sites by using depsipeptides that undergo an enzyme-triggered molecular rearrangement, "switching" from a non-functional epitope to a bioactive sequence. Mass spectrometry, 1H and 13C NMR spectroscopy, and fluorescence studies were applied to track structural changes in the peptide. SEM was used to image these polymer-peptide hybrid hydrogels. Finally, in vitro studies were conducted to evaluate cell interactions with the hydrogels. Switch peptide-modified alginate hydrogels showed increased cell adhesion upon induction of enzymatic activity, which provided a "gain of function" of the synthetic ECM. Critically, enzymes associated with the cells themselves could trigger the peptide switch and change in synthetic ECM behavior.

With knowledge of stimuli-responsive peptide-based biomaterials applied in tissue engineering, I then studied how this system could be used in drug delivery by designing peptide-hydrogen sulfide (H2S) donor conjugates (PHDCs). H2S is a gasotransmitter that is produced endogenously, which has been explored in recent years with many potential therapeutical applications. We studied H2S release profiles in dual-enzyme-responsive PHDCs, with a further investigation into PHDC–Fe2+ complexes for potential tumor treatments via chemodynamic therapy. The PHDC–Fe2+ complexes were examined in a C6 glioma cell line, exhibiting an improved cell-killing effect compared with controls, by inducing toxic hydroxyl radical generation (•OH) via a Fenton reaction. To this end, we further discovered how side chains influence self-assembling nanostructures, H2S release profiles, and biological activities via three constitutionally isomeric PHDCs. Different morphologies and varied H2S release rates were observed, paving the way for tuning the properties of PHDCs by simple changes in molecular design. Finally, this dissertation discloses conclusions and future directions on stimuli-responsive peptide-based biomaterials using similar platforms with different designs in the drug delivery and tissue engineering fields.