Main routes and conditions to obtain high aspect ratio HA nanostructures precipitated from solutions (CCP: conventional chemical precipitation, HT: hydrothermal, ST: solvothermal; S: solvent; A: additive, AR: aspect ratio, HAR: high aspect ratio).

Synthesis	Ca, Pi precursor	Crystal growth control	Additives (A)/solvent (S)	Conditions	Range of AR	Crystal morphology	Comments	Ref.
CCP	Ca(NO3)2 4H2O, (NH4)2HPO4	pH control with NH4OH or HNO3	(S)H2O	T = 50 to 200 °C, t = 20 to 100 h	n.s.	Rods to nanorods	Mixtures of HA/DCPA/OCP	42 and 43
CCP	CaCl2, Na2HPO4	Organic templates/micelles	(S)H2O, (A)polysorbate, (A)sorbitan monolaurate, (A)sodium polyacrylate, (A)polydiallyldimethylammonium chloride	T = 25 to 60 °C	n.s.	Neuron-like	Neuron-like structures assembled into clusters with growing fibres of 1 to 2 μm length, with few nm of diameter	44
HT	CaCl2, Na5P3O10	Pi concentration, saturation index (SI)	(S)H2O, (A)glutamic acid, (A)(NH4)2CO	T = 180 °C, t = 10 h	n.s.	Nanorods (SI > 0), prism-like nanorods (SI < 0)	Nanorods, hexagonal prisms, microspheres, and flowerlike structures grow governed by SI	45
HT	Ca(NO3)2 4H2O, H3PO4	pH control with NH4OH	(S)CH3CH2OH	T = 140 to 220 °C, t = 1 to 12 h	2.3 to 4.9	Nanorods	↑AR at ↓pH, pH 8–10–12	46
HT	Ca(NO3)2 4H2O, (NH4)2HPO4	pH control with (NH4)2CO	(S)H2O	T = 90 or 200 °C, t = 60 h	3 to 24	Nanorods	↑AR at ↓pH, ↑AR at ↑T	47
HT	Ca(NO3)2 4H2O, (NH4)2HPO4	Solvent and pH control with NH4OH	(S)CH3CH2OH, (S)CH3OH, (S)(CH3)2CHOH	T = 150 °C, t = 24 h	n.s.	Nanorods, hexagonal rods, needle	Variation in concentration with CH3CH2OH yields HAR	48
ST	Ca(NO3)2 4H2O, NH4H2PO4	Oleate template	(S)H2O, (S)CH3CH2OH	T = 120 °C, P = 7 bar, t = 20 to 96 h	100	Nanowires	Nanowires of 37 nm in diameter and 3.7 μm in length	49
ST	CaCl2, NaH2PO4 2H2O	Oleate template	(S)H2O, (S)CH3OH, (A)oleic acid	T = 180 °C, t = 40 h	7 to 10 000	Nanowires	Lengths > 100 μm	50
ST	CaCl2, NaH2PO4 2H2O	Organic template, Pi concentration	(S)H2O, (S)CH3CH2OH, (A)stearic acid, (A)palmitic acid	T = 180 °C, t = 24 h	>1000	Nanofibres	Different sources of Pi precursors: NaPO3/Na5P3O10/Na2HPO4/Na2H2P2O7	51
ST	CaCl2, NaH2PO4 2H2O	Oleate template	(S)H2O	T = 200 °C, t = 36 h	65 to >1000	Nanowires	1D to 2D assembled nanowires after immersion in CH3CH2OH	18
ST	CaCl2, NaH2PO4	Oleate template and alcohols	(S)CH3CH2OH, (S)CH3OH, propanol/butanol/pentanol/hexanol, (A)Oleic acid	T = 180 °C, t = 18 h	100 to 1000	Nanowires	Lengths from 100 μm to mm	52
ST	CaCl2	Pi concentration and source	(A)Oleic acid	T = 180 °C, t = 25 h	10 to 1000	Nanorods, nanowires, nanofibres	Different sources of Pi precursors: NaH2PO4/Na2HPO4/Na3PO4/Na5P3O10/Na4P2O7	53
ST	CaCl2, NaH2PO4 2H2O	Oleate template	(S)CH3CH2OH, (A)oleic acid	T = 180 °C, t = 5 to 23 h	>100	Nanofibres	Self-assembly of nanofibers upon immersion in CH3CH2OH, yields a fibre entanglement 28 mm long	54 and 55
ST	CaCl2, NaH2PO4 2H2O	Oleate template	(S)H2O, (S)CH3OH, (A)oleic acid	T = 180 °C, t = 24 h	>100	Nanofibres	Post-alignment of fibres using ice-templating	56