Engineering zwitterionic and boronic acid-based nanomaterials to overcome mucosal barriers

Abstract

Mucus is a hydrated, viscoelastic, and biochemically complex barrier that protects epithelial surfaces throughout the optic, respiratory, digestive, and reproductive tracts. While essential for host defense, this barrier severely limits the transport of drug-loaded nanoparticles (NPs), which are often immobilized by interactions with mucin components and rapidly cleared from mucosal surfaces. To address this challenge, drug delivery systems must be carefully engineered to either minimize adhesive interactions with mucus or strategically leverage them to improve retention and localization.This thesis explores the use of zwitterionic and boronic acid chemistries as two complementary strategies for modulating nanoparticle–mucin interactions. Zwitterionic polymers are known for their strong hydration and charge neutrality, which impart antifouling properties and promotes diffusion through mucus. In contrast, boronic acids form dynamic covalent bonds with cis-diols found in sialylated glycans of mucins, facilitating selective mucoadhesion. We hypothesized that combining these opposing chemical motifs could yield materials capable of navigating the mucus barrier more effectively by balancing penetration and retention. To investigate this hypothesis, a series of model systems were developed, increasing in structural complexity from planar surfaces to colloidal and self-assembled nanoparticles. Each platform enabled the study of material–mucus interactions under distinct, physiologically-relevant conditions. Silicon wafers were first modified with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes and terminal 3-(pent-4-ynamido)phenylboronic acid (APBA) groups to study surface-level mucin interactions under static, well-defined conditions. These planar systems allowed decoupling of bulk diffusion from interfacial effects. Characterization by X-ray photoelectron spectroscopy and contact angle measurements confirmed successful functionalization, while mucin-binding assays revealed that APBA–mucin interactions are pH-dependent and glycan-specific. Next, colloidal silica nanoparticles were functionalized with the same moieties to assess dynamic mucin interactions in suspension. PMPC coatings conferred colloidal stability and reduced mucin association, while APBA-functionalized particles exhibited a reduced mobility at acidic pH, consistent with enhanced mucoadhesion under those conditions. These results highlight the critical influence of surface chemistry and environmental context on nanoparticle–mucin interactions. Finally, amphiphilic block copolymers composed of hydrophobic poly(D,L-lactide) (PLA) cores and the zwitterionic coronas of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) or poly(carboxybetaine) (PMCB) were self-assembled into nanoparticles with tunable APBA surface densities. A physiologically-relevant model of sheep small intestinal mucus was developed to evaluate transport behavior. Permeation studies revealed that intermediate APBA densities provided the most favorable balance between mucus penetration and selective binding. Further experiments showed that mucus composition, free sialic acid, and calcium ions could modulate nanoparticle transport and post-exposure stability. Altogether, this work provides a systematic, multiscale investigation of how zwitterionic and boronic acid functionalities can be integrated to modulate interactions with mucus. The findings offer design principles for creating drug delivery systems optimized for transport across mucosal barriers, contributing to the development of more effective nanoparticle-based therapeutics.