whitlockite and fluorapatite

Phosphate is an essential nutrient for life on Earth, present in adenosine triphosphate (ATP), deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and phospholipid membranes. Phosphorus does not have a significant volatile phase, and its release from minerals is therefore critical to its bioavailability. Organic ligands can enhance phosphate release from minerals relative to release in inorganic solutions, and phosphorus depletion in paleosols has consequently been used as a signature of the presence of ligands secreted by terrestrial organisms on early Earth. We performed batch dissolution experiments of the Mars-relevant phosphate minerals merrillite, whitlockite, chlorapatite, and fluorapatite in solutions containing organic compounds relevant to Mars. We also analyzed these phosphate minerals using the ChemCam laboratory instrument at Los Alamos, providing spectra of end-member phosphate phases that are likely present on the surface of Mars. Phosphate release rates from chlorapatite, whitlockite, and merrillite were enhanced by mellitic, oxalic, succinic, and acetic acids relative to inorganic controls by as much as >35 × . The effects of the organic compounds could be explained by the denticity of the ligand, the strength of the complex formed with calcium, and the solution saturation state. Merrillite, whitlockite, and chlorapatite dissolution rates were more strongly enhanced by acetic and succinic acids relative to inorganic controls (as much as >10 ×) than were fluorapatite dissolution rates (≲2 ×). These results suggest that depletion of phosphate in soils, rocks or sediments on Mars could be a sensitive indicator of the presence of organic compounds.

Topics: Apatites; Calcium; Calcium Phosphates; Geologic Sediments; Hydrogen-Ion Concentration; Ligands; Minerals; Organic Chemicals; Phosphates; Soil; Spectrum Analysis

Plasma-sprayed ceramic coatings of fluorapatite (FA), magnesiumwhitlockite (MW), and hydroxylapatite (HA), and noncoated Ti-6Al-4V alloy (Ti) implants were evaluated histologically and histomorphometrically in a goat animal study. Cylindrical Ti-6Al-4V plugs were plasma-spray-coated with FA, MW, and HA. Noncoated, grit-blasted Ti plugs served as controls. The plugs were implanted into the right femur and left humerus of 20 adult goats. The results were evaluated using descriptive histology and histomorphometry. The histomorphometry consisted of measurements of bone apposition and coating thickness. The results demonstrated that FA showed a high amount of bone apposition without signs of degradation or dissolution. MW showed considerable reduction in thickness and at 12 weeks an adverse tissue reaction. However, at 25 weeks the amount of bone apposition was significantly increased compared with the 12-week implants. HA revealed considerable and progressive reduction in thickness and at 25 weeks a lower amount of bone apposition than FA and MW. At 12 weeks the Ti implants did reveal bone apposition, although frequently localized fibrous tissue was visible. At 25 weeks the Ti implants did not differ in bone apposition from the HA implants. Further studies are necessary on the effect of degradation or dissolution of HA on the compatibility with bone.

Topics: Alloys; Animals; Apatites; Biocompatible Materials; Calcium Phosphates; Durapatite; Female; Femur; Foreign-Body Reaction; Goats; Humerus; Hydroxyapatites; Magnesium; Osseointegration; Prostheses and Implants; Titanium

Ceramic coatings of fluorapatite (FA), magnesiumwhitlockite (MW), and hydroxylapatite (HA), and noncoated Ti-6Al-4V alloy (Ti) implants were evaluated before and after implantation in an animal study. Cylindrical plugs were coated by plasma-spraying with FA, MW, and HA. X-ray-diffraction patterns showed for FA and HA a crystalline and for MW an amorphous-crystalline coating structure. The plugs were implanted into the right femora and left humeri of 16 adult goats. Follow-up periods were 12 and 25 weeks. The in vivo results were evaluated using push-out tests and scanning electron microscopy. There were significant differences in push-out strengths between femur and humerus. The FA and HA implants showed significantly higher push-out strengths than the MW and Ti alloy implants, especially for the 12 week follow-up period. Furthermore, at 12 weeks, MW showed significantly lower push-out strengths than Ti alloy. SEM-investigation of the interfaces revealed that FA did not degrade while both MW and HA showed extensive degradation at 12 and 25 weeks. In addition, the interface after push-out testing showed for the MW, HA, and Ti alloy implants to be fractured at the implant-tissue interface and for the FA to be fractured at the coating-titanium interface.

Topics: Animals; Apatites; Biocompatible Materials; Biomechanical Phenomena; Bone and Bones; Calcium Phosphates; Durapatite; Female; Femur; Goats; Humerus; Hydroxyapatites; Microscopy, Electron, Scanning; Prostheses and Implants; X-Ray Diffraction