Methacrylate-Modified Nanoparticles for Noninvasive Monitoring of Photocrosslinked Hydrogel Scaffolds

Abstract

Photocrosslinked hydrogels, such as methacrylate-modified gelatin (gelMA) and hyaluronic acid (HAMA), are widely utilized as tissue engineering scaffolds and/or drug delivery vehicles, but lack a suitable means for non-invasive, longitudinal monitoring of surgical placement, biodegradation, and drug release. Therefore, the objective of this dissertation was to investigate a photopolymerizable radiographic contrast agent, methacrylate-modified gold nanoparticles (AuMA NPs), for contrast-enhanced radiographic imaging of photopolymerized hydrogels. AuMA NPs, ~12 nm in diameter, were covalently linked to gelMA hydrogels in one-step during photocrosslinking. GelMA-Au NP hydrogels containing at least 5 mM AuMA NPs provided sufficient X-ray contrast for monitoring hydrogel degradation. The enzymatic degradation kinetics of gelMA-Au NP hydrogels were longitudinally monitored by micro-computed tomography (CT) and validated by concurrent measurements with optical spectroscopy and gravimetric analysis. AuMA NPs did not disrupt the hydrogel network and properties compared with gelMA alone. AuMA NPs, ~5 nm in diameter, were subsequently investigated for contrast-enhanced radiographic imaging in mice and renal  clearance. GelMA-Au hydrogel scaffolds implanted subcutaneously in mice exhibited

sufficient X-ray contrast against surrounding tissues to monitor degradation by micro-CT. The in vivo biodistribution after 28 days verified renal clearance of AuMA NPs. Thus, AuMA NPs enabled noninvasive imaging and longitudinal monitoring of hydrogel degradation by micro-CT, as well as in vivo renal clearance upon release.

An analytical method was developed to quantify the mixed ligand density on AuMA NPs, including carboxylate and methacrylate (MA) ligands, using inductively coupled plasma optical emission spectroscopy (ICP-OES) and total organic carbon/nitrogen analysis (TOC-TNM). The mixed ligand density was measured for AuMA NPs prepared with varying MA/Au ratios, enabling the preparation of AuMA NPs with tailored MA ligand density. AuMA NPs with tailored MA ligand density exhibited tunable surface charge, hydrodynamic size, and aqueous stability. Tailoring the MA ligand density and NP concentration further enabled a tunable swelling ratio and compressive modulus of gelMA and HAMA hydrogels.

Gadolinium-doped hafnium oxide (Gd-HfO₂) NPs were investigated as a new contrast agent for photopolymerized hydrogels to enable the use of higher NP concentrations while not interfering with photocrosslinking. Gd-HfO₂ and HfO₂ NPs exhibited negligible ultra-violet (UV) absorption compared to Au NPs, while also exhibiting strong UV emission, promoting UV crosslinking and the preparation of gelMA hydrogels with a nondisrupted network. Therefore, Gd-HfO₂ NPs are a promising alternative to Au NPs for photopolymerized hydrogels.