Introducing Pure Calcium Orthophosphate Gels and Aerogels

Abstract

Calcium orthophosphates (CaPs) are extensively studied across multiple scientific disciplines, ranging from chemistry and materials science to biomedicine and pharmacy. In the form of apatite, they constitute the mineral component of bone and teeth and are thus probably the most important mineral for the humankind. Although various CaP phases and morphologies are known, wet gel-like states, and, in particular, aerogels remain uncharted territory for this biomineral. Here, a method is introduced for synthesizing ethanol-based gels of anhydrous amorphous CaP that can be further converted into aerogels. In the anhydrous wet state, the gels partially crystallize into spherulites of dicalcium phosphate anhydrate (DCPA) upon drying in air. The CaP aerogel, on the other hand, can be prepared as an amorphous form via supercritical drying, exhibiting specific BET surface areas of ≈180 m2 g−1. It can be stabilized under argon, but the aerogels transform into DCPA over time when exposed to air. However, this transformation does not yield spherulites. The high surface area, alongside other advantages of amorphous CaP aerogels like high porosity, low density, and high strength-to-weight ratio, present significant potential for applications in drug delivery systems or as bone replacement materials.