ENGINEERING BOTTLEBRUSH POLYMERS AS MUCO-PENETRATING NANOCARRIERS

Abstract

Mucus is a viscoelastic, adhesive hydrogel that protects mucosal epithelia but also traps and rapidly clears conventional nanocarriers, posing a major barrier to effective transmucosal drug delivery. To overcome both adhesive and steric interactions with the mucin network, nanocarriers must be engineered with non-fouling surface chemistries and architectures that can navigate the tight, heterogeneous mesh of mucus without disrupting barrier integrity. This dissertation investigates two complementary design axes for muco‑penetrating delivery systems: (i) surface chemistry and polymer architecture in self‑assembled micelles, and (ii) worm‑like bottlebrush polymers as single‑molecule carriers capable of diffusing and penetrating through native mucus. The first part of this Thesis dealt with examining the roles of surface chemistry and polymer architecture on the ability of spherical polymer-based micelles to transport in a mucus network. The biological fate of polymer-based nanocarriers is known to be sensitive to carrier size, shape, and surface chemistry. Much less is known regarding the role of surface topography on transport through mucus, despite reports confirming that it strongly impacts circulation time and cellular uptake. For this, spherical micelles were constructed from amphiphilic block copolymers of linear and bottlebrush architectures. The hydrophilic coronas examined herein were based on poly(ethylene glycol) (PEG), 2‑methacryloyloxyethyl phosphorylcholine (MPC), carboxybetaine methacrylate (CBM), and 2-(dimethylamino)ethyl methacrylate (DMAE), which contain neutral, zwitterionic, and cationic moieties, respectively. Colloidal stability was initially examined in three different mucin models: pig gastric mucin (PGM), bovine submaxillary mucin (BSM), and sheep small intestine mucin (SSIM). Single particle tracking and Transwell assays were carried out to examine the diffusion behavior and permeation of the different micelles in SSIM. Ensemble‑averaged mean squared displacement (MSD), anomalous diffusion exponents, and apparent permeability coefficients were determined from these measurements and correlated to polymer chemistry and architecture. Of the systems examined, zwitterionic MPC bottlebrush micelles exhibited the highest MSD and apparent permeability, significantly outperforming PEG and CBM analogues as well as DMAE‑containing micelles, highlighting the importance of dense zwitterionic shielding and extended bottlebrush coronas for minimizing mucoadhesion. The second part of this Thesis dealt with examining the ability of worm-like carriers based on bottlebrush copolymers of the core-shell type to transport in a mucus network. This work was inspired by recent reports that properly engineered bottlebrush PEG carriers can traverse airway mucus and epithelium via architecture‑enhanced endocytosis. For this, bottlebrushes were produced by a modular synthetic route based on a combination of controlled radical polymerization methods as well as ring-opening polymerization. A family of nine bottlebrush variants were produced that differ in backbone length and side‑chain length while maintaining narrow dispersity and well‑defined worm‑like geometry, as confirmed by SAXS and AFM. Their diffusion in SSIM was quantified by multiple particle tracking, and anomalous diffusion exponents are extracted from power‑law fits of MSD versus time lag on log–log scales, allowing direct comparison of how molecular geometry and grafting density tune subdiffusive behavior in mucus. Transwell permeability assays performed with reconstituted sheep intestinal mucus demonstrated that the intermediate bottlebrush architecture provided the greatest extent of mucus diffusion and apparent permeability, implying that an optimal ratio of contour length to cross sectional width, together with sufficiently long, highly hydrated side chains, maximizes steric “slipperiness” and enables efficient passage through the mucus mesh without significant adhesive interactions. Together, these studies establish clear design rules for mucus‑penetrating nanocarriers that integrate nonfouling zwitterionic chemistry with bottlebrush architecture, and they demonstrate that worm‑like bottlebrush polymers can serve as efficient single‑polymer carriers across mucus barriers. The resulting structure–transport relationships provide a quantitative framework for optimizing micelle and bottlebrush formulations for inhaled and oral delivery applications and inform the broader use of zwitterionic and bottlebrush platforms in mucosal therapeutics.