A pair of mutually-attractive but self-repulsive decapeptides, with alternating charged/neutral amino acid sequence patterns, was found to co-assemble into a viscoelastic material upon mixing at a low total peptide concentration of 0.25 wt%. Circular dichroism spectroscopy of individual decapeptide solutions revealed their random coil conformation. Transmission electron microscopy images showed the nanofibrillar network structure of the hydrogel. Dynamic rheological characterization revealed its high elasticity and shear-thinning nature. Furthermore, the co-assembled hydrogel was capable of rapid recoveries from repeated shear-induced breakdowns (compliance), a property desirable for designing injectable biomaterials for controlled drug delivery and tissue engineering applications. A systematic variation of the neutral amino acids in the sequence revealed some of the critical design principles involved in this novel class of biomaterials. Lowering the hydrophobicity of the neutral amino acids lowered the elastic modulus and the resilience of the assembled hydrogel, thereby providing a means to fine-tune material property. Replacement of neutral amino acids in the sequence with proline (a β-sheet breaker) impaired the ability of the peptides to co-assemble into a hydrogel.

The therapeutic potential for messenger RNA (mRNA) in infectious diseases and cancer was first realized almost three decades ago, but only in 2018 did the first lipid nanoparticle-based small interfering RNA (siRNA) therapy reach the market with the United States Food and Drug Administration (FDA) approval of patisiran (Onpattro™) for hereditary ATTR amyloidosis. This was largely made possible by major advances in the formulation technology for stabilized lipid-based nanoparticles (LNPs). Design of the cationic ionizable lipids, which are a key component of the LNP formulations, with an acid dissociation constant (pKa) close to the early endosomal pH, would not only ensure effective encapsulation of mRNA into the stabilized lipoplexes within the LNPs, but also its subsequent endosomal release into the cytoplasm after endocytosis. Unlike other gene therapy modalities, which require nuclear delivery, the site of action for exogenous mRNA vaccines is the cytosol where they get translated into antigenic proteins and thereby elicit an immune response. LNPs also protect the mRNA against enzymatic degradation by the omnipresent ribonucleases (RNases). Cationic nano emulsion (CNE) is also explored as an alternative and relatively thermostable mRNA vaccine delivery vehicle. In this review, we have summarized the various delivery strategies explored for mRNA vaccines, including naked mRNA injection; ex vivo loading of dendritic cells; CNE; cationic peptides; cationic polymers and finally the clinically successful COVID-19 LNP vaccines (Pfizer/BioNTech and Moderna vaccines)—their components, design principles, formulation parameter optimization and stabilization challenges. Despite the clinical success of LNP-mRNA vaccine formulations, there is a specific need to enhance their storage stability above 0 °C for these lifesaving vaccines to reach the developing world.

We have previously developed shear-responsive hydrogels assembled from mutually attractive but self-repulsive oligopeptide modules. To explore the mechanism of material assembly, we designed and synthesized a quartet of oligopeptides. The peptide quartet contains two positively charged modules 10 and 15 amino acid residues long and two negatively charged modules 10 and 15 amino acid residues long. Each positive module can pair with one negative module and hence there are four potential pairings, two of equal chain length (blunt-ended pairs) and two of unequal chain length (sticky-ended pairs). Viscoelastic properties and structural features of the hydrogels assembled from these oligo-peptide pairs were evaluated by dynamic rheometry and small-angle X-ray scattering (SAXS) techniques, respectively. It was found that the sticky ends provide no advantage in terms of mechanical properties of the hydrogels. Instead, the hydrogel assembled from the 10:10 peptide pair forms the strongest gel. These results are consistent with a cross-β structural model in which β-strands are stacked against each other in a parallel fashion to form nanofibers, the axes of which are perpendicular to individual β-strands. The optimal chain length of oligopeptide modules for such nanofiber network assembly is around 10 amino acid residues.