i might criticize this study by pointing out that the topical hydroxyapatite was prepared with artificial saliva, while the topical fluoride solution was prepared with distilled - that is, mineral-free - water. you wouldn't expect fluoride to do much by it's lonesome, without calcium & phosphorus. the results are consequently perhaps badly skewed.
regardless, i'm not using it as a replacement for fluoride and not intending to replace fluoride with hydroxyapatite - this is intended to be used in addition to fluoride and, eventually, perhaps in conjunction with a high concentration fluoride topical.
so, i may want to create the gel by mixing it with spit, rather than water.