At Politecnico di Torino, Ho-BG powders were ground for 20 minutes by using a zirconia ball-milling machine (ANALYSETTE 3 SPARTAN-FRITSCH) in order to produce fine powders (Figure 3). In such system, grinding was mechanically performed by a zirconia ball, moving in the jar (Figure 3a) under the action of the vibrating plate (Figure 3b).
Figure 3: (a) Alumina jar containing Ho-BG after grinding. (b) Vibrating plate grinder (ANALYSETTE 3 SPARTAN).
After that, Ho-BG powders were sieved using a stainless-steel sieve with a mesh of 32 μm diameter (Figure 4) in order to obtain all particles of controlled size.
Ground and sieved powders were identified as Ho-BG32.
Figure 4: Sieve with mesh of 32 μm diameter containing Ho-BG powders.
Ho-BG and Ho-BG32 powders were then characterized in terms of compositional and physicochemical properties. Results were compared ad discussed according to the preliminary
4.3.1 Scaffold Manufacturing
For the scaffold production, the foam replica method was chosen, because it offers a number of advantages over other scaffold fabrication methods, including the ability to produce foams with a highly porous structure with adjustable pore dimensions [10]. In addition, this method allows producing scaffolds with a microstructure similar to that of human trabecular bone [11], and with irregular shapes to match the size and shape of the bone defect. It does not involve the use of toxic chemicals, and it is more rapid and cost-effective than other techniques [10].
Ho-doped BG-based scaffolds were produced by foam replica method, using polyurethane sponges as sacrificial template.
The main stages of the process are shown in Figure 5.
Figure 5: Schematic representation of foam replica method for scaffold’s fabrication.
4.3.1.1 Materials
The starting material was Ho-BG32, while cube shaped polyurethane PU foams (45 ppi) of 1 cm in size were used as sacrificial templates.
4.3.1.2 Scaffold Fabrication Stages 1. Choice of the slurry composition
The first step for scaffold production was the definition of the slurry composition.
The original idea was to use the composition reported in Table 2 [12], previously used for the production of melt-derived BG-based scaffolds.
Table 2: Original slurry composition.
Component wt%
Ho-BG32 30
Bi-distilled water 64
PVA 6
However, this composition was considered inadequate for the present study, since the suggested quantity of bioactive glass occupied an enormous volume compared to that of bi-distilled water and PVA, as a result of the low density of Ho-BG. According to this, a new slurry composition was
designed by modifying the percentage by weight of glass and bi-distilled water, while maintaining the same percentage by weight of PVA. The adapted slurry composition is shown in Table 3.
Table 3: Adapted slurry composition.
Component wt%
Ho-BG32 15
Bi-distilled water 79
PVA 6
2. Preparation of the slurry
In order to produce 8 scaffolds, 10 g of slurry were needed.
0.6 g of PVA were dissolved into 7.9 g of bi-distilled water at 60 °C under continuous magnetic stirring at 200 rpm, until a clear and transparent solution was obtained. In order to limit water evaporation during this phase, the beaker was covered with a glass lid. After PVA dissolution, some additional water was added dropwise to restore the initial PVA:H2O ratio and the solution was left to cool at room temperature for about 30 min.
1.5 g of Ho-BG32 powders were then added to the solution and stirred at room temperature (200 rpm) for at least 5 min in order to obtain a homogeneous milky suspension (Figure 6).
Figure 6: The beaker containing PVA, BG and bi-distilled water placed on the magnetic stirrer.
3. Green bodies production
PU foam sheets of 1 cm thickness and 45 ppi were cut into cubes and used as sacrificial template for the replica process.
The experimental set-up used for the preparation of the green bodies is shown in Figure 7.
Figure 7: (a) Front, and (b) top view of the experimental set-up used for the preparation of the green bodies.
The polymer template was immersed in the slurry, which subsequently infiltrated the structure and glass particles adhered to the surface of the foam thanks to the action of PVA, used as binder. Then, the PU foam was compressed to 60% of its size, so as to remove the exceeding slurry.
Each sample underwent 3 complete immersion-compression cycles, in order to obtain a continuous and resistant coating. Once the 3th cycle was completed, the green bodies were turned 180° for at least 2h so that the slurry could be evenly distributed in the whole foam volume.
4. Drying process
Finally, the samples, called green bodies, were placed onto a plastic Petri dish and left to dry.
Half of the green bodies were air-dried at room temperature overnight, while the other half was dried inside an incubator at 37 °C for 4 h.
5. Heat treatment
A heat treatment program was properly set for PU template burning-out and glass sintering on the basis of thermal characterization analyses performed on the glass. In order to optimize the scaffolds mechanical resistance, eight different tests were carried out, varying, one at a time, the drying method and the sintering temperature, as shown in Table 4. Sintering conditions were set as follows: 800 for 3h, 850 °C for 3h, 950 °C for 3h, 1000 °C for 3h and 1050 °C for 3h. The heating rate was 5 °C/min for all the sintering treatments.
Table 4: Summary of the test performed for the optimization of Ho-BG32 scaffolds.
Test Sample Name
Drying Method Sintering Temperature (°C)
Heating Rate (°C/min)
Sintering Time (h)
1 Ho-I-800 Incubator/37 °C 800 5 3
2 Ho-AD-850 Air drying/room temperature
850 5 3
3 Ho-I-850 Incubator/37 °C 850 5 3
4 Ho-AD-950 Air drying/room temperature
950 5 3
5 Ho-I-950 Incubator/37 °C 950 5 3
6 Ho-AD-1000 Air drying/room temperature
1000 5 3
7 Ho-I-1000 Incubator/37 °C 1000 5 3
8 Ho-AD-1050 Air drying/room temperature
1050 5 3
The Ho-AD-1050 (Test 8) was measured in order to make some considerations on the density and porosity of the structure. Scaffold dimensions are summarized in Table 5.
Table 5: Physical properties of Ho-AD-1050 scaffold.
Test Sample Side 1 (mm)
Side 2 (mm)
Side 3 (mm)
Mass (g)
Volume (cm3)
Density (g/cm3)
8 Ho-AD-1050 6.34 6.52 6.83 0.1 0.28 0.36
The percentage of porosity related to the sample was evaluated using the Equation 1:
% porosity = 31 − $$
!"#6 ∗ 100 (1) Where:
𝜌 = density of the sample (g/cm3) 𝜌%&' = 2.6 g/cm3 (glass skeletal density).