Cardiovascular diseases are the leading cause of death worldwide. New treatments are continuously developed but require an in-depth understanding of cardiovascular morphology. Vascular corrosion casting provides 3D knowledge of anatomical structures by injecting a polymer resin and subsequently removing the surrounding tissue via chemical maceration, i.e. corrosion. [1] This process often leads to deformations or fragmentation of the fragile cast, resulting in a loss of information. In-situ high-resolution computed tomography (micro-CT) scans could provide detailed information on the vascular architecture without corroding the tissue. Unfortunately, distinguishing the polymer cast from the animals’ surrounding soft tissue is impossible due to a lack of CT contrast. To improve this, we introduce hafnium oxide nanocrystals (HfO₂ NCs) as contrast agents to the polymer resin. Here we communicate our insights on HfO₂ NC synthesis, their surface chemistry and their application as CT contrast agents.

We synthesize 5 – 10 nm HfO₂ NCs starting from HfCl₄.2THF in benzyl alcohol. Initially identified as a purely nonaqueous sol-gel route [2], we find the in-situ water formation to be responsible for gelation of the reaction mixture prior to particle crystallization. Through mechanistic investigation using in-situ Pair Distribution Function (PDF) analysis, Nuclear Magnetic Resonance (NMR), Extended X-ray Absorption Fine Structure (EXAFS) and rheology measurements we study this rapid precursor-to-gel conversion and subsequent crystallization. Using our new-found insight we gain better reaction control over the reaction and scale-up the synthesis, yielding gram-scale HfO₂ NCs with narrow size distribution.

To obtain a stable and homogeneous dispersion of the synthesized NCs in the casting resin, we optimized the particle’s surface chemistry. The ideal ligand is found to be a combination of a strong binding group (phosphonate), while matching the resin’s polarity via its organic tail (ethylene glycol oligomers). We demonstrate that the NCs remain stable during resin curing and homogeneously improve the contrast with concentrations as low as 5 m% of NCs.

Finally, we perform ex-vivo injections of both zebrafish and mouse models with the NC-doped resin and obtain high-quality cast visualization via segmentation of the obtained scans without having to adapt the existing injection methods. We are able to differentiate even µm scale details. This confirms the application potential of HfO₂ NCs as CT contrast agents in corrosion casting, while paving the way to obtain full cardiovascular information from casts without the need for tissue corrosion. Our results emphasize that through mechanistic insight and control over the nanocrystal synthesis, combined with an optimized surface chemistry, we can create high quality stable nanocomposites.