Nanocrystalline Yttria-Stabilized Zirconia Ceramics for Cranial Window Applications

Cranial windows are essential for long-term optical monitoring of brain activity. Traditional materials face challenges in maintaining optical clarity and biocompatibility over time. Prior research has shown that ceramic materials offer mechanical strength and chemical stability. However, the long-term performance of these materials in vivo remains uncertain. This gap motivated the investigation of yttria-stabilized zirconia (YSZ) as a potential cranial window material. YSZ has already been known for its optical transparency and mechanical durability. But its suitability for cranial applications had not been fully resolved. This study aimed to address the lack of data on YSZ's microstructure and biological interactions. The results may help refine material selection for cranial implants.

The goal was to assess the suitability of YSZ ceramics for cranial window use. The researchers focused on how yttria concentration and processing methods affect material properties. They examined the relationship between microstructure and biocompatibility. The motivation stemmed from the need for long-term transparent cranial implants. The study aimed to compare CAPAD and SPS processing techniques. It also aimed to evaluate the impact of surface polishing on cell behavior. The researchers wanted to determine the optimal yttria content for optical and mechanical performance. The findings could guide future material development for cranial applications.

The study involved fabricating YSZ discs using two sintering methods: CAPAD and SPS. Commercial and in-house synthesized nanoparticles were used for these processes. The yttria content in the discs ranged from 3 to 8 mol%. Microstructural analysis was conducted using nanocrystalline grain and domain size measurements. Surface polishing was applied to some samples to assess its effect on cell behavior. Cytocompatibility was evaluated using bone-marrow-derived stem cells (BMSCs). Cell adhesion density and spreading area were measured under direct and indirect contact conditions. Y³⁺ ion release was quantified to assess potential toxicity.

CAPAD-processed 6YSZ and 8YSZ discs had 200-350 nm grains with 20-30 nm crystallite domains. SPS-densified 4YSZ_P discs showed 200-350 nm grains with 29 ± 5 nm crystallite domains. SPS-densified 8YSZ_P discs had significantly larger grains at 819 ± 155 nm. CAPAD-processed 3YSZ discs had the smallest grain size at 39 ± 9 nm. Polished YSZ discs supported greater BMSC spreading than unpolished ones. Under direct contact, polished 8YSZ, 4YSZ_P, and 8YSZ_P discs had lower cell adhesion. Unpolished 8YSZ_R, 4YSZ_PR, and 8YSZ_PR discs also showed lower adhesion. Y³⁺ ion concentrations were higher in polished 4YSZ_P and 8YSZ_P groups and their unpolished counterparts.

The study suggests that 8YSZ may be the most suitable YSZ composition for cranial windows. This conclusion is based on its balance of optical transparency, mechanical properties, and cytocompatibility. The results indicate that processing method and yttria content significantly influence microstructure. Polishing improved BMSC spreading but reduced cell adhesion in some cases. Y³⁺ ion release varied by processing and surface condition. No mineral deposition was observed on polished YSZ discs after cell culture. The findings support further evaluation of 8YSZ for long-term cranial applications. The authors propose that future studies should focus on long-term biocompatibility and optical stability.