Hybrid Bonding Bottlebrush Polymers Grafted from a Supramolecular Polymer Backbone.

Bottlebrush polymers, macromolecules consisting of dense polymer side chains grafted from a central polymer backbone, have unique properties resulting from this well-defined molecular architecture. With the advent of controlled radical polymerization techniques, access to these architectures has become more readily available. However, synthetic challenges remain, including the need for intermediate purification, the use of toxic solvents, and challenges with achieving long bottlebrush architectures due to backbone entanglements. Herein, we report hybrid bonding bottlebrush polymers (systems integrating covalent and noncovalent bonding of structural units) consisting of poly(sodium 4-styrenesulfonate) (p(NaSS)) brushes grafted from a peptide amphiphile (PA) supramolecular polymer backbone. This was achieved using photoinitiated electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization in water. The structure of the hybrid bonding bottlebrush architecture was characterized using cryogenic transmission electron microscopy, and its properties were probed using rheological measurements. We observed that hybrid bonding bottlebrush polymers were able to organize into block architectures containing domains with high brush grafting density and others with no observable brushes. This finding is possibly a result of dynamic behavior unique to supramolecular polymer backbones, enabling molecular exchange or translational diffusion of monomers along the length of the assemblies. The hybrid bottlebrush polymers exhibited higher solution viscosity at moderate shear, protected supramolecular polymer backbones from disassembly at high shear, and supported self-healing capabilities, depending on grafting densities. Our results demonstrate an opportunity for novel properties in easily synthesized bottlebrush polymer architectures built with supramolecular polymers that might be useful in biomedical applications or for aqueous lubrication.

Systemic peptide amphiphile nanofiber delivery following subcutaneous injection.

Peptide amphiphile (PA) nanofibers have been shown to target and deliver drugs when administered via an intravenous (IV) injection. Subcutaneous administration can broaden the applicability of PA nanofibers in the medical field. The ability of PA nanofibers to be absorbed into systemic circulation after subcutaneous administration was investigated. Four PA molecules with different amino acid sequences were designed to understand the effect of nanofiber cohesion and charge on uptake. Solution small-angle X-ray scattering confirmed nanostructure morphology and provided characteristic lengths for co-assemblies. Circular dichroism and solution wide-angle X-ray scattering confirmed PA secondary structure and molecular order. PAs were co-assembled in a 95 %:5 % molar ratio of unlabeled PA to fluorescently labeled PA. Male and female Sprague Dawley rats were injected in the nape of the neck with PA co-assemblies. In vivo normalized abdominal fluorescence was measured 1-72 h after injection. PA nanofibers with a negative charge and low internal order showed the highest amount of systemic absorption at 1, 6, and 24 h. At 24 h after injection, white blood cell count decreased and glucose was elevated. Glucose began to decrease at 48 h. These data indicate that PA nanofibers can be absorbed into the systemic circulation after subcutaneous injection.

Stick-Slip Friction Reveals Hydrogel Lubrication Mechanisms.

The lubrication behavior of the hydrated biopolymers that constitute tissues in organisms differs from that outlined by the classical Stribeck curve, and studying hydrogel lubrication is a key pathway to understand the complexity of biolubrication. Here, we have investigated the frictional characteristics of polyacrylamide (PAAm) hydrogels with various acrylamide concentrations, exhibiting Young's moduli (E) that range from 1 to 40 kPa, as a function of applied normal load and sliding velocities by colloid probe lateral force microscopy. The speed-dependence of the friction force shows an initial decrease in friction with increasing velocity, while, above a transition velocity V*, friction increases with speed. This study reveals two different boundary lubrication mechanisms characterized by distinct scaling laws. An unprecedented and comprehensive study of the lateral force loops reveals intermittent friction or stick-slip above and below V*, with characteristics that depend on the hydrogel network, applied load, and sliding velocity. Our work thus provides insight into the closely tied parameters governing hydrogel lubrication mechanisms, and stick-slip friction.