Multiple characterization study on porosity and pore structure of calcium phosphate cements

Abstract

Being able to thoroughly characterize the intricate pore structure of calcium phosphate cements is a key step to successfully link the structural properties of these synthetic bone grafts with their most relevant properties, such as in vitro or in vivo behaviour, drug loading and release properties or degradation over time. This is a challenging task, due to the wide range of pore sizes compared to most other ceramic biomaterials. This work provides a critical assessment of three different techniques based on different physical phenomena, namely Mercury Intrusion Porosimetry (MIP), Nitrogen sorption and Thermoporometry (TPM) for the detailed characterization of four calcium phosphate cements with different textural properties, in terms of total porosity, Pore Size Distribution (PSD) and Pore Entrance Size Distribution (PESD). MIP had the advantage of covering a much wider size range than TPM and Nitrogen sorption, offering a more comprehensive information at the micrometric level. TPM, and specially Nitrogen sorption, besides being non-destructive techniques, and although covering a limited size range, provided complementary information regarding pore structure associated to crystal shape at the nanoscale, recording both PSD and PESD in a single experiment. MIP tended to register smaller sizes, especially at low L/P ratios, due to the network effect, that has a strong influence on the outcome of this technique.¡

In vitro degradation of calcium phosphates: Effect of multiscale porosity, textural properties and composition

“…Thus, the porosity at the nano-microscale and the specific surface area were modified by varying the L/P ratio and the particle size of the starting powder, respectively [41,42]. In turn, macroporosity was introduced through a foaming process [43].…”

“…We took advantage of the possibility offered by the fabrication of hydroxyapatite scaffolds using biomimetic routes, i.e., by low temperature dissolutionprecipitation reactions, to systematically introduce controlled levels of porosity at the nano, micro, and macroscale, as well as to modify the specific surface area [41][42][43].…”

Análise comparativa entre diferentes ensaios porosimétricos em alumina macroporosa

“…Para esse método, foi utilizado um porosímetro AutoPore IV 9500 (Micromeritics), evacuado a uma pressão de aproximadamente 7.10 -6 MPa (~50 µmHg), seguido do preenchimento com mercúrio em uma pressão de 413,68 MPa. A distribuição do tamanho de poros foi determinada segundo o proposto na literatura [34,35].…”

“…Apesar de o PIM ser amplamente utilizado, a distribuição de tamanho de poros é feita considerando-se o formato dos poros cilíndricos, segundo a equação de Washburn [35]. Quando há interconexão entre dois poros, é necessária a aplicação de pressão para o mercúrio superar o efeito de capilaridade e ocupar o segundo poro.…”

“…Esse segundo poro é maior do que a interconectividade e consequentemente o mercúrio irá ocupar um volume maior. Como nesse ensaio assume-se que os poros têm formatos cilíndricos, as interconexões entre os poros são erroneamente considerados como sendo diversos poros do tamanho dessas interconectividades, levando assim a valores de distribuições de poros que não correspondem exatamente à morfologia da amostra analisada [35].…”