Biological Performance Testing of Bioceramic Coatings

Physiological fluids within the human body can be divided into human intracellular fluid (hICF, with a volume of 271 for a 70 kg person) and human extracellular fluid (hECF, 131). Extracellular fluid can further be subdivided into two sub-compartments, that is human interstitial fluid (hISF, 9.51) and the liquid component of blood (plasma, 3.51 for a 70 kg person) (Tas, 2014). While the extracellular fluid represents the fluid outside cells, and intracellular fluid the fluid within cells, the interstitial fluid is the tissue fluid found between cells. There is a striking difference between the compositions of extracellular and intracellular fluids as presented in Table 7.7. Since the composition of blood plasma is close to that of hECF (Krebs, 1950), synthetic SBFs developed by various research groups frequently attempt to emulate these compositions (Table 7.8). 

There is a significant difference between the Ca2+ concentrations of intracellular (0.01 mM) and extracellular fluids (2.5 mM). The calcium ion is one of the most disruptive substances for normal cell function, with the intracellular concentration of calcium ions carefully regulated (Saul, 2009) and bone matter serving as the major storage sites for excess calcium. 

According to Tas (2014), aqueous solutions simulating the composition and the overall ionic strength of hECF should have compositional ranges as follows  

5mM Mg2+, 22-27mM HC03 , 142-145mM Na+, 5-5.8mM I +, 103-133 mM Cl-, 1.8-3.75 mM Ca2+ and 0.8-1.67 mM HP042-. Simulated body fluids within these compositional ranges are known to form nanospheres of X ray-amorphous calcium phosphate that on ageing may be converted via calcium octaphosphate to bone-like calcium-deficient hydroxyapatite (see below). 

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The specific uses of bioceramics in vivo require their full compatibility with the living tissues. Taking into account the reliability requirements, implantable ceramics, whether massive or in the form of coatings, should be subjected to a number of tests (physical, chemical, mechanical, mineralogical and biological characterization), for ensuring guaranteed performance without failure in future. 

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