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Hierarchical self-assembly of synthetic polypeptides has attracted increasing interests due to its protein-mimetic structure and great potential in nanotechnology and biomedical applications. However, controlling the morphology and function of polymeric nanostructures via secondary structures remains largely unexplored. Here, we report an unusual micelle-to-vesicle transformation of cholesterol-decorated poly(l-cysteine) copolymer assemblies in response to reactive oxygen species (ROS). We found that the interesting morphological transition correlates with the alteration in conformations from β-sheet to α-helix, which grants an attractive "on-off" switch for triggered release and cellular interaction. We further demonstrated the usefulness of the conformation-regulated assembly strategy both in vitro and in vivo, taking cancer treatment as a model. The work offers a new insight on the folding and hierarchical assembly of polypeptides and a novel approach for the development of smart platforms in biosensing, disease treatment, and diagnostic applications.

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The self-assembly of biodegradable polyurethanes constitutes an important area of research for the development of polymeric materials in biomedicine. In particular, colloidal polyurethane assemblies can increase the solubility and stability of hydrophobic compounds, and improve the specificity and efficiency of drug action. Their nanoscale size and modular functionality make them promising for the injectable, targeted and controlled delivery of various therapeutic agents and imaging probes into required cells. Additionally, cationic polyurethanes are able to self-assemble with nucleic acids into nanoparticles to enter cells for efficient gene transfection. These emerging nanocarriers open the door for addressing the failure of traditional localized delivery systems, and present a compelling future opportunity to achieve personalized therapy as versatile candidates. This review article highlights the research progress in the self-assembly of biodegradable polyurethanes for controlled delivery applications, with particular attention being paid to some representative vehicles such as self-assembled polyurethane micelles, nanogels, and polyurethane/DNA complexes, which have emerged as the focus of interest in recent years.

To obtain a pH-sensitive multifunctional polyurethane micelle drug carrier, a novel pH-sensitive macrodiol containing acid-cleavable hydrazone linkers, poly(ε-caprolactone)−hydrazone−poly(ethylene glycol)−hydrazone−poly(ε-caprolactone) diol (PCL−Hyd−PEG−Hyd−PCL), was synthesized and characterized with proton nuclear magnetic resonance spectra (1H NMR). A series of pH-sensitive biodegradable polyurethanes (pHPUs) were designed and synthesized using pH-sensitive macrodiol, l-lysine ethyl ester diisocyanate (LDI) and l-lysine derivative tripeptide as chain extender, which can provide an active reaction site for the development of positive target polyurethane micelles for drug delivery. The bulk structures of the prepared polyurethanes were carefully characterized with 1H NMR, gel permeation chromatograph (GPC), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The polyurethanes could be cleaved in acidic media (pH ∼ 4−6) as well as degraded in PBS and enzymatic solution, as demonstrated by 1H NMR and weight loss, respectively. The cytotoxicity of their degradation products was evaluated using methylthiazoletetrazolium (MTT) assay in vitro, resulting in no apparent inhibition effect on the fibroblasts. These polyurethanes could self-assemble into micelles in aqueous solutions, as verified using dynamic light-scattering (DLS). Our present work provides a new method for the preparation of amphiphilic multiblock polyurethanes with pH-sensitivity and biodegradability. It could be a good candidate as biodegradable multifunctional carrier for active intracellular drug delivery.