Sonia Regina Biaggio

Titanium (Ti) and its alloys are the most used biomaterials in dental and orthopedic implant applications. However, despite the good performance of these materials, implants may fail; therefore, several surface modification methodologies have been developed to increase the bioactivity of the metal surface, accelerating the osseointegration process while promoting improved corrosion performance. In this work, the production of a TiO₂ coating on titanium through a short-time micro-arc oxidation (MAO) in a green electrolyte (obtained by a mixed solution of K₃PO₄ and Ca(CH₃COO)₂.H₂O) is proposed, aiming at obtaining a porous oxide layer with Ca and P incorporation through an environmentally friendly experimental condition. The morphology, chemical composition, crystalline structure, wettability, hardness and bioactivity of the modified Ti surfaces were characterized. The MAO process at 250 V for 1 min in the green electrolyte solution allowed the production of a highly porous oxide surface in the anatase crystalline phase, with effective Ca and P incorporation. Pre-osteoblastic cells were used in in-vitro assays to analyze viability, adhesion, proliferation and ability to perform extracellular matrix mineralization on the Ti surfaces (polished and MAO-treated Ti). The MAO-treated Ti surface exhibited better results in the bioactivity tests, presenting more calcium phosphate precipitates. This surface also presented higher hardness, lower hydrophilicity and better performance in biological tests than the polished surface. The here-reported MAO-treated Ti surface is promising for dental implants, especially in patients having poor bone quality that requires greater stimulation for osteogenesis.

In the field of Li-ion batteries (LIBs), one prevalent expectation is that smaller sized electrode materials lead to better battery performances. Here, we show that this reasoning fails for nanoparticles of LiMn₂O₄ (LMO) below a certain critical average size. For this, using a microwave-assisted hydrothermal methodology and heat treatment, we obtained spinel LMO nanoparticles with genuinely small average sizes (4, 9, and 14 nm). When the performance of these LMOs for the Li-ion extraction/insertion processes was assessed by voltammetry and impedance spectroscopy, contrary to expectations, the decrease of the nanoparticle size caused a progressive hindrance of these processes. Using an ab initio DFT methodology, this phenomenon could be accounted for with a theoretical explanation for the significantly decreased electrochemical activity of the smallest LMO nanoparticles. We found that the redox voltage associated with the Li-ion insertion process becomes significantly higher as the average size of the nanoparticles decreases from 14 to 1.3 nm, changing from 3.93 to 4.64 V, thus hindering the process. We could also show that the redox voltage increase is due to a consequent significant increase of the energy barrier for Li-ion diffusion within the smaller LMO nanocrystallite, from 0.33 to at least 0.59 eV, caused by atomic relaxations (mainly of oxygen atoms) occurring on the surface of such a nanosystem. Clearly, when very small LMO nanocrystallites are intended for use as cathode materials in LIBs, the average nanoparticle size should not be smaller than the critical value of ∼15 nm; otherwise Li-ion extraction–insertion becomes significantly hindered or unfeasible.