Liquid Infiltration of Porous Calcium Hydroxide Solid for Synthesizing Bulk Calcium Phosphate Biomaterials

Bulk calcium phosphate biomaterials without sintering were synthesized at room temperature by infiltrating various orthophosphate aqueous solutions into a porous Ca(OH)2 green powder compact. These orthophosphate aqueous solutions were prepared from a different ratio of Ca(OH)2/H3PO4 up to pH=12. The infiltrated specimens did not show any shape or dimension changes at a lower concentration of H3PO4 or a liquid with pH>2. When the liquid contained some supersaturated solids or precipitates, the infiltrated specimens showed their reacted phases at the surface similar to these supersaturated precipitates, which were Ca(H2PO4)2.H2O (MCPM), CaHPO4.2H2O (DCPD), or Ca10(PO4)6(OH)2 (HA) as pH was increasing. Under a liquid infiltration with a lower concentration of H3PO4 liquid, a functional graded calcium orthophosphate composite was observed. Furthermore, relationships among processing factors like pH plus impurities and liquid concentration, phase formation, and microstructure of infiltrated specimens were discussed in detail.

Based on the previous findings, different calcium orthophosphate powders are very easy to obtain in the Ca(OH) 2 -H 3 PO 4 -H 2 O ternary system. In this study, we will use an alternative way to prepare a bulk calcium orthophosphate composite instead of a powder form. This is because a chunk of reacted or unreacted Ca(OH) 2 solid form is frequently found if the Ca(OH) 2 /H 3 PO 4 aqueous solution is not well mixed [7]. Even though this phenomenon is a shortcoming for powder preparation, but it may be a convenient way to obtain a bulk calcium phosphate composite at room temperature. Therefore, a porous Ca(OH) 2 green powder compact is designed to infiltrate the supersaturated orthophosphate aqueous solution, in which phosphate ions such as H 2 PO 4 -, HPO 4 2-, or PO 4 3are phase equilibrated or coexist with their related calcium orthophosphates. It is hoped that a bulk porous calcium phosphate composite is obtained through this liquid infiltration method for orthopedic or other related applications. Furthermore, phases and microstructures of infiltrated specimens are also observed to explain the possible reaction mechanisms of this liquid infiltration.

Preparations of Porous Ca(OH) 2 Green Powder Compacts and Orthophosphate Aqueous Solutions
A porous Ca(OH) 2 green powder compact, with the cylindrical dimension ofφ1.2cm (D) ×~0.3cm (T) (D, 98 Liquid Infiltration of Porous Calcium Hydroxide Solid for Synthesizing Bulk Calcium Phosphate Biomaterials diameter; T, thickness), was formed from a batch of 0.5g Ca(OH) 2 initial powder inside a cylindrical stainless steel die under a uniaxial compressive stress up to 22Kg/cm 2 , to obtain a porosity up to 30vol%.
As to a specific orthophosphate aqueous solution at a different pH up to pH=12, it was obtained from the different portions of Ca(OH) 2 and 0.06M or 1.2MH 3 PO 4 , as listed in Table 1, by mixing and also staying-put over a day to observe any pH change. The pH values of these orthophosphate aqueous solutions were measured by using a pH meter (PHM 210 Standard pH Meter, Radiometer Analytical SAS, Villeurbanne Cedex, France).

Liquid Infiltration
A 0.5g Ca(OH) 2 green powder compact was submerged into a specific orthophosphate solution at room temperature for 1h up to 1 week to observe its liquid infiltration phenomena and pH changes. From preliminarily observations, there were no any significant pH changes before and after liquid infiltration.

Phase Determination and Microstructure Observations
Phase existence of infiltrated samples or initial powders were determined from X-ray diffraction patterns (XRDs) at room temperature by using an X-ray unit (X'Pert PRO, PANalytical Inc., Netherlands) at 2θ=20-80 o , with a scanning speed of 3 o /min and a sampling interval of ∆θ=0.04 o . For microstructure observations, a field emission scanning electron microscope (FE-SEM, S-4700, Hitachi Inc., Germany) was used to observe the free surface or facture surface of infiltrated specimen.

Macrographs and phase existence of liquid infiltrated specimens.
Macrographs of liquid infiltrated specimens are shown in Fig. 1. From these optical macrographs, the Ca(OH) 2 powder compact is completely dissolved in the HP solution, but slightly dissolved in the HP2 solution because its dimension is a little smaller than the original, as shown in Fig. 1. Therefore, Ca(OH) 2 powder compacts are very easy to dissolve in the higher concentrated H 3 PO 4 , but not in the lower concentration of H 3 PO 4 . However, the rest samples at higher pHs do not show any significant dimension changes. It indicates that the transformed phases inside the infiltrated specimen can be bonded together without any significant dimension change, even though the initial Ca(OH) 2 powder compact is simply mechanically-bonded.  X-ray diffraction patterns (XRDs) at the surface of the infiltrated specimens are selectively shown in Fig. 2. From these XRDs, phases at the infiltrated surfaces are MCPM, DCPD, and HA as the pH value of infiltrated aqueous solution increases. MCPM exists in the infiltrated specimen LP; DCPD is present in the infiltrated specimens HP2-HP6 and LP2-LP6. However, HA is formed in the infiltrated specimens HP8-HP12 and LP8-LP12. There are no any trace of Ca(OH) 2 X-ray diffraction peaks being observed on the surface of these infiltrated specimens. Therefore, the initial Ca(OH) 2 powder on the surface is already completely transformed into new calcium orthophosphate phases after liquid infiltration.

Microstructures of Liquid-infiltrated Specimens
(a) Microstructures on the infiltrated surface.
SEM micrographs of infiltrated specimens at the surface are representatively shown in Fig. 3. It indicates that calcium orthophosphates grow faster at a low pH value. This phenomenon also can be seen from the corresponding X-ray diffraction patterns, as shown in Fig. 2. Calcium hydroxyapatite (HA) crystals shown in Fig. 2(c) have much broader diffraction peaks, which demonstrate HA having much smaller grains or domains, if compared with Figs. 2(a) & 2(b). At lower pHs, calcium orthophosphates are plate-like for MCPM and DCPD. However, at higher pHs, HA seems shows ellipsoid-like morphologies.
(b) Fractographs perpendicular to the infiltration surface.
In the LP solution (Ca(OH) 2 /0.06MH 3 PO 4 system), a fractograph of infiltrated specimen perpendicular to the free surface is shown in Fig. 4(a). Free surface or the infiltrated surface contains several large holes, which were produced from the chemical reaction between the Ca(OH) 2 powders and H 3 PO 4 , but not from the original pores. Except for the free surface, several different facture morphologies, upper area, transition zone, plus lower area in the reacted zone, and unreacted zone are defined in this fractograph. This is because these areas have different contents of Ca, P, and O elements, as shown in Figs. 4(b), (c), & (d). Based on the trace content of P, the unreacted zone is identified to be the location of the original Ca(OH) 2 powders, which are not being liquid infiltrated. As to the reacted zone, the upper area has a higher P content, but a lower Ca content if compared it with the lower area.

Liquid Infiltration
In the Ca(OH) 2 [3,5,6,13]. As to the liquid infiltration of Ca(OH) 2 /H 3 PO 4 in this study, phase formation mechanisms in the infiltrated Ca(OH) 2 specimen are discussed as follows.
(a) Liquids with a low pH value without any supersaturated precipitates Infiltrated liquids such HP, HP2, LP, and LP2 do not contain any apparent supersaturated precipitates; therefore, when a porous Ca(OH) 2 green powder compact are put inside these liquids, these liquids are expected to react with Ca(OH) 2 instantaneously. This is because Ca(OH) 2 is not a stable phase in a low pH aqueous solution. The dissolution rate of Ca(OH) 2 is dependent of its solubility product (K sp ) and the formation mechanisms of a new calcium orthophosphate phase, in which K sp = (Ca 2+ ) aq (OH -) aq 2 , (Ca 2+ ) aq and (OH -) aq are the concentrations of Ca 2+ and OHin the aqueous solution (aq), respectively.
From solubility product (K sp ), when a Ca(OH) 2 powder compact is impregnated in a low pH solution or a not supersaturated solution, such as a H 3 PO 4 solution (LP or HP liquid), (OH -) aq is very low; subsequently for maintaining a constant K sp , more Ca(OH) 2 powders should be dissolved to increase (Ca 2+ ) aq . From the pH value listed in Table 1, the Ca(OH) 2 /0.06M H 3 PO 4 system (LP liquid) has a higher pH=1.89, or a higher concentration of OHthan the Ca(OH) 2 /1.2M H 3 PO 4 system (HP liquid) with an pH=1.03; therefore, the dissolution rate in the HP liquid is expected to be faster than that in the LP liquid. As the dissolution of Ca(OH) 2 proceeds, (Ca 2+ ) aq and (OH -) aq are increasing. Consequently a new equilibrium state of H 3 PO 4 ↔ H + +H 2 PO 4 reaches at the pKa=2.12 in the liquid, and new calcium orthophosphate crystals or precipitates will appear at the solid/liquid interface, in which the solid is the porous Ca(OH) 2 powder compact. After reaching such conditions, except for K sp , the dissolution rate of Ca(OH) 2 is also dependent of the formation mechanisms of these new calcium orthophosphate precipitates.
For an HP/Ca(OH) 2 system, if a 0.5g Ca(OH) 2 powder compact is impregnated in a 100cc HP liquid, it will be completely dissolved in the liquid, as shown in Fig. 1. This is because a new equilibrium state H 3 PO 4 ↔ H + +H 2 PO 4 at pKa=2.12 does not reach yet, which can be seen from the amount of Ca(OH) 2 powder dissolution of liquid HP to HP2 as listed in Table 1. From liquid HP to HP2, the amount of Ca(OH) 2 dissolution is 5g in 165.0cc of 1.2M H 3 PO 4 to reach pH=2; therefore, a complete dissolution of a 0.5g Ca(OH) 2 green powder compact in 100cc of 1.2MH 3 PO 4 should not be able to obtain an pH=2. As to the HP2/Ca(OH) 2 2 powders, Therefore, the infiltrated Ca(OH) 2 specimen HP2 shows some dimension changes in Fig. 1. However, in the LP/Ca(OH) 2 and LP2/Ca(OH) 2 systems, LP and LP2 liquids only need a slight amount of Ca(OH) 2 being dissolved in a 100cc liquid to reach a higher pH, as listed in Table 1; therefore, the infiltrated Ca(OH) 2 powder compacts don't show any apparent dimension changes in Fig. 1.

(b) Liquids with supersaturated precipitates
If an infiltrated liquid reaches the first equilibrium state of H 3 PO 4 ↔ H + +H 2 PO 4 -, during liquid infiltration, the infiltration front in the solid/liquid interface (solid for the porous Ca(OH) 2 powder compact) can be schematically described in Fig. 5. As the liquid infiltration proceeds, Ca 2+ and OHions are released to the solid/liquid interface due to the dissolution of the solid phase Ca(OH) 2 . The dissolution rate of Ca(OH) 2 is initially dependent of pHs. However, the OHion release is not large enough to locally change the pH value of infiltrated liquid at the initial interface of Ca(OH) 2 /liquid. Therefore, precipitates are formed from the reactions between Ca 2+ from dissolution of Ca(OH) 2 and an orthophosphate ion H 2 PO 4 from the incoming infiltrated liquid. Subsequently, the phases of infiltrated specimen at the surface are the same crystal structures as the solid or precipitates in the original supersaturated aqueous solution. Therefore, MCPM, DCPD, and HA appear at the surface of infiltrated specimens as pH increases. Overall, the chemical reactions between Ca(OH) 2 and infiltrated liquid are classified to have three mechanisms: dissolution of Ca(OH) 2 , precipitation, and growth of new precipitates.
Among these three mechanisms for the chemical reactions of Ca(OH) 2 /liquid mentioned above, the most important factor for a successful liquid infiltration is preliminarily related to the dissolution rate of Ca(OH) 2 powders. As a result, for obtaining an intact infiltrated specimen, the dissolution rate of Ca(OH) 2 powders should keep as low as possible, especially at a low pH liquid.

(c) Functional graded materials
For a low pH liquid infiltration, if a porous Ca(OH) 2 can be continuously infiltrated with the supersaturated liquid, the reacted phases along the infiltration direction or in the thickness direction can be proposed as shown in Fig. 6. This layer-like microstructure is much easier to form under a lower phosphate concentration, because a smaller amount of Ca(OH) 2 dissolution can cause a higher pH change, as listed in Table 1.
At the first stage of liquid infiltration, an equilibrium state H 3 PO 4 ↔ H + +H 2 PO 4 of the infiltrated liquid can be reached at pKa=2.12. Therefore, MCPM starts to appear in zone I shown in Fig. 6. As the liquid infiltration proceeds, more Ca(OH) 2 reacts with the incoming infiltrated liquid from the open pore channels to increase its pH up to another equilibrium state of H 2 PO 4 -↔ H + +HPO 4 2at pKa=7.21, and then DCPD begins to form in the zone II. As the liquid is further infiltrated into the porous Ca(OH) 2 powder compact, the reacted infiltration liquid finally reaches to the other equilibrium state of HPO 4 2-↔ H + +PO 4 3at pKa=11.77, and HA is supposed to finally exist in zone III even if the intermediate phase such as OCP (Ca 8 (HPO 4 ) 2 (PO 4 ) 4 ‧5H 2 O) or CDHA(Ca 10-x (HPO 4 ) x (PO 4 ) 6-x (OH) 2-x , 0<x<1) is probably formed in the early transformation period. [1,[8][9][10][11][12]  Microstructures shown in Fig. 4 contain three different morphologies along the thickness direction in the fractographs of the infiltrated specimen LP, upper area and lower area in the reacted zone, and unreacted zone. In this LP/Ca(OH) 2 system, the upper area (zone I) has been identified as MCPM from the corresponding XRD shown in Fig. 2(a). As to the lower area (zone II), it is DCPD because a lower content of P but a higher content of Ca is determined from EDXSs shown in Figs. 4(b)-(d), by comparing it with the upper area (zone I). Except for EDXSs, an XRD for an infiltrated specimen at a higher pH=4 (in the LP4 solution) also shows DCPD crystal structure at the infiltrated surface, as shown in Fig. 2(b). If the liquid can infiltrate further inside the Ca(OH) 2 powder compact, HA or zone III can be obtained. Therefore, we believe a liquid infiltration method in the Ca(OH) 2 /H 3 PO 4 can be used to fabricate a layer-like structure or a functional graded material.

Correlations between Liquid Infiltration and Vapor Infiltration
From SEM micrographs shown in Figs. 7(a), whisker morphologies are frequently being observed in the interface between the reacted and unreacted zones. This is because the chemical reaction between Ca(OH) 2 [5,6,14,15]. This exothermic reaction will locally heat up the surrounding liquids and materials. In the reaction front, the interface between the reacted zone (in the infiltrated liquid) and unreacted zone (Ca(OH) 2 powders), some infiltrated liquids (or orthophosphate solutions) become vapors to vapor infiltrate into the unreacted Ca(OH) 2 powders. Therefore, in the system of Ca(OH) 2 powder and H 3 PO 4 liquid, some whiskers has been found in the reaction front, interface between the reacted zone and unreacted Ca(OH) 2 zone, as shown in Fig. 7(a). From EDXS in Fig.  7(b), these whiskers contain Ca, P, and O elements; however, their morphologies are different from calcium orthophosphates in the liquid infiltration. The same whisker morphologies have been reported in some calcium orthophosphates being obtained from a chemical vapor deposition [16]. Therefore, it is believed that these whiskers are formed from a vapor reaction. While the infiltrated liquid reaches, these whiskers are expected to react with the liquid to transform into another phase, except for the area of closed pores. As a result, whiskers are difficult to find inside the reacted zone after liquid infiltration.

Time Effect of Ca(OH) 2 Green Powder Compact in the Air
If Ca(OH) 2 green powder compacts are put in the air for a period of time, some of Ca(OH) 2 powders are transformed into the CaCO 3 phase, as shown in Fig.8(b). This is because Ca(OH) 2 can absorb CO 2 from the environment to transform into CaCO 3 . From the comparisons of phase existence shown in Figs.8(a)-8(b), it indicates that CaCO 3 in the initial powders is a minor phase, but it becomes a major phase after these Ca(OH) 2 green powder compacts have been left in the air for a week. During a higher pH orthophosphate liquid infiltration, CaCO 3 remains inside the infiltrated specimen and it is not transformed into any calcium orthophosphate phase, as shown in Fig.8(c). From X-ray diffraction patterns in Figs.8(b)-8(c), it shows that hydroxyapatite (HA) is from Ca(OH) 2 not from Ca(CO 3 ) 2 . However, Ca(CO 3 ) 2 can be dissolved in a very low pH orthophosphate liquid; therefore, it does not remain in the liquid infiltrated specimens.

Conclusions
Several aqueous solutions with a different ratio of Ca(OH) 2 /H 3 PO 4 were designed to infiltrate into the Ca(OH) 2 green powder compacts from pH=1 to pH=12. An intact shape of bulk infiltrated specimen was obtained without any significant dimension or shape changes when a low concentration of H 3 PO 4 or a liquid with pH>2 was used.
Upon an infiltrated aqueous solution with the supersaturated calcium orthophosphate solids or precipitates, the infiltrated specimens showed their reacted phases at the surface similar to these supersaturated precipitates, which were MCPM (Ca(H 2 PO 4 ) 2 ．H 2 O) to DCPD(CaHPO 4 ．2H 2 O), and further to HA(Ca 10 (PO 4 ) 6 (OH) 2 ) as pH was increasing.
With a continuing liquid infiltration in the Ca(OH) 2 /0.06MH 3 PO 4 system, the infiltrated specimen contained several different morphologies in its corresponding SEM fractographs. Inside the reacted zone, different concentrations of Ca, O, and P elements were observed in the two different areas: MCPM for the upper reacted area, and DCPD for the lower reacted area. If the liquid could infiltrate deeper enough, a layer structure of HA was proposed. Based on the above findings, a functional graded calcium orthophosphate material was obtained in the liquid with a very low concentration of H 3 PO 4 .
Except for the above SEM micrographs, some whiskers were frequently found at the interface between the reacted and unreacted zones in the Ca(OH) 2 /0.06MH 3 PO 4 system. CaCO 3 was also being observed if the Ca(OH) 2 green powder compact had been exposed in the air for a period of time. This CaCO 3 phase was not transformed into any orthophosphates during a liquid infiltration at higher pHs. Overall, through a liquid infiltration in this study, different bulk calcium orthophosphate biomaterials were easy to synthesize at room temperature without any further sintering.