**2. Synthesis of brushite (CaHPO4·2H2O)**

The typical method synthesis of the brushite powder was performed at ambient according to the following equation:

$$\text{Ca}(\text{NO}\_3\text{)}\_2 + \text{Na}\_2\text{HPO}\_4 \rightarrow \text{CaHPO}\_4\cdot2\text{H}\_2\text{O} + 2\text{NaNO}\_3\tag{1}$$

Two solutions were prepared for the synthesis of brushite. The first solution is prepared by dissolving 0.5 mol of Na2HPO4·2H2O in 1 L of distilled water, and the second solution is a result of dissolving 0.5 mol of Ca(NO3).6H2O in 1 L of distilled water. After preparing the solutions, 200 ml of the Ca(NO3)2 was added dropwise using a glass funnel with a glass stopcock (flow rate is 2 ml/min) to the Na2HPO4·2H2O solution while stirring and adjusting the pH between 6 and 6.5 using ammonia solution (25%, Labochemie, India). Afterward, the resultant solution with precipitates was stirred (400 rpm) at ambient conditions for 1 hour to ensure a homogeneous mixture. The precipitate was vacuum filtered using a qualitative filter paper via a Buchner funnel, washed three times with de-ionized water and another three times with ethanol to reduce the possibility of agglomeration [14, 15], after which it was placed upon a watch glass and dried at 40°C overnight in a dryingoven. After the formation of the precipitate, some of the powder was washed with distilled water, then dried using ethanol at 40°C for a week. The major steps of the experimental design are reported in **Figure 1**.

**Figure 1.**

*preparation of brushite [16].*

## **3. Crystal morphology of brushite**

Two morphologies of brushite crystal layers are reported in our previous work [17]. These brushite crystals consist of [2] platy particles (**Figure 2A**) and a continuous needle-like (**Figure 2B**) dendritic network [18]. The plate-like brushite crystals mostly formed in parallelogram shapes stacked in multiple layers. Their dimensions ranged from 1 μm to a few micrometers, in two directions. The thickness of the platy crystals is measured in the nano-scale. The needle-like brushite particles appeared to have a dendritic network structure. The total length of the crystal is around 40 μm. The EDS measurements (**Figure 2C**) showed that the needle-like brushite crystals are composed of, by atomic percentage: O (76.46%), Ca (12.04%), and P (10.95%). The fact that the percentages of P and Ca are nearly equal and is in good agreement with the theoretical Ca/P atomic percentages of brushite [19, 20]. The brushite morphology depends on the pH of the solution during the precipitation; at acidic pH, around 5, platy crystals of brushite are formed (**Figure 2A**), whereas needle-like (**Figure 2B**) ones are predominant at a higher pH [2].

SEM images of the brushite crystals are shown in **Figure 3**. These plate-like crystals are obtained according to the experimental procedure as reported in **Figure 1**.

**Figure 2.**

*(A) crystal morphologies of brushite; platy structure, (B) needle-like structure, and (C) EDS analysis [17].*

**Figure 3.** *SEM image of monoclinic brushite crystals, see Figure 1 (preparation procedure).*

It is known that the morphology of brushite is characterized by a plate-like or needlelike structure, depending on the solution pH used [11, 13]. The plate-like crystals are thin (~400 nm), while their width and elongation are approximately 10 and 20 μm, respectively, values similar to those reported in other studies [21].

The XRD patterns of brushite, as well as the patterns of standard brushite, are shown in **Figure 4**. The mineralogy of the powder confirms that this precipitate produced after mixing NaH2PO4·2H2O and Ca(NO3)2·4H2O solutions with a Ca:P molar ratio 1:1 (**Figure 1**) is pure brushite, while its crystals grow after nucleation in proportion to the three major planes, namely, (020), (121-), and (141). All peaks of the powder pattern denote the brushite's monoclinic structure [16], while the peak at 11.7° 2-Theta indicates that the crystal growth takes place primarily along the (020) crystallographic plane. Rietveld refined unit cell parameters for brushite are presented in **Table 2**.

**Figure 4.** *XRD patterns of brushite (synthesis details are shown in Figure 1) [16].*


#### **Table 2.**

*Refined unit cell parameters for brushite from XRD data using the Rietveld approach.\**
