Piezoelectric ceramics are envisioned as cell-stimulating materials for in-vivo load-bearing orthopedic implants. However, perovskite materials that have made up the bulk of piezoceramics in applications are generally thermodynamically unstable in aqueous solutions [1]. As such, perovskites are prone to lower their energy when exposed to aqueous solution by releasing their A-site cations into their surroundings. For in-vivo applications, this could cause major issues as exemplified by the principal perovskite Pb(Zr,Ti)O₃ whose toxic Pb²⁺ ions can readily accumulate in surrounding bone, organs, and muscle tissues [2]. Furthermore, releasing those ions possibly risks altering the material’s dielectric properties and reducing its functional properties over time. As such, it is vital to better understand the time-dependent dissolution chemistry of perovskites in aqueous solutions and ascertain if their functional lifetime is compatible with biomedical applications.

To achieve this, powders of the perovskite BaTiO₃ were prepared in-house and submerged in aqueous solutions of different pH values for up to 31 days. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) measurements were conducted on the solution at specific time steps to monitor the evolving Ba²⁺ concentration. An accompanying numerical shrinking core model was created to derive kinetic rate constants and offer a low-cost tool for predicting dissolution behavior in the future. The experiments revealed that contrary to existing literature [3], the leaching process does not halt within the first few hours of exposure but continues at a decreasing rate up to and possibly past 31 days. The findings highlight the importance of considering changes to the driving thermodynamic forces with time in combination with diffusion limitations within the solid phase. Previous studies have reported that precipitating BaCO₃ forms a physical barrier around the particles which stops dissolution, yet no clear evidence of this process was found in the current study. Rather, the mounting presence of Ba²⁺ in solution and the reduction of the chemical potential gradient appear to contribute more than previously believed. The numerical model was able to predict the experimental dissolution behavior with high accuracy up to 31 days, cementing the validity of the shrinking core model in predicting perovskite dissolution and solution kinetics.

References

[1] H. W. Nesbitt et al. Nature, 1989, 289, 358-362.

[2] J. Rödel et al. Journal of the American Ceramic Society, 2009, 92, 1153-1177.

[3] S. S. Tripathy, Last name; J.S. Smith Journal of Experimental Nanoscience, 2011, 6, 127-137.