#
Elastic Mechanical Properties of 45S5-Based Bioactive Glass–Ceramic Scaffolds^{ †}

^{*}

^{†}

## Abstract

**:**

## 1. Introduction

^{2+}ions) that promote bone cell responses towards a path of regeneration and self-repair [5,6,7]. Convincing evidence is also emerging about the suitability of bioactive glasses in contact with soft tissues: in this regard, recent results in the field of wound healing proved the potential of bioactive glasses to promote skin repair and regeneration through the stimulation of angiogenesis [8,9].

^{®}powder as a starting material exhibited poor compressive strength (0.2–0.4 MPa) compared to the typical range of trabecular bone (2–12 MPa [11]). The main “technological” limitation of the famous 45S5 composition is that this glass has a narrow sintering window and tends to crystallize upon thermal treatment [12]. Therefore, the struts of 45S5 glass scaffolds are poorly densified [10] and, if the sintering temperature is increased, the crystalline phase resulting from partial devitrification decrease the material bioactivity [13]. The 45S5-based graded scaffolds having a highly porous outer shell surrounding a less-porous core were produced by combining sponge replication and polyethylene burn-off method in the attempt to increase the compressive strength (0.7 MPa) [14]. Wu et al. [15] fabricated stronger 45S5 Bioglass

^{®}scaffolds by using rice husk as a sacrificial pore-forming agent (compressive strength in the range of 5 to 7 MPa), but the resulting porous structure was poorly interconnected and, thus, not very suitable for tissue engineering applications. Implementation of additive manufacturing methodologies, such as robocasting, led to a significant improvement of the compressive strength of 45S5 glass scaffolds (above 10 MPa [16,17]); however, these structures have a grid-like arrangement of channels that do not properly mimic the trabecular architecture of cancellous bone as glass scaffolds produced by foam replication successfully do.

^{®}, which, however, remains the most commonly used biomedical glass for clinical use worldwide [18]. For example, bone-like glass or glass–ceramic foams based on CEL2 (43.8SiO

_{2}-23.6CaO-15.0Na

_{2}O-4.6MgO-6.1K

_{2}O-6.9P

_{2}O

_{5}wt.%) [19], 13-93 (53SiO

_{2}-20CaO-6Na

_{2}O-5MgO-12K

_{2}O-4P

_{2}O

_{5}wt.%) [20], and SCNA (57SiO

_{2}-34CaO-6Na

_{2}O-3Al

_{2}O

_{3}mol.%) [21] glasses were shown to achieve a compressive strength of 5, 11, and 18 MPa, respectively.

^{®}foams has not been determined in the literature so far. This is probably due to the presence of some technological problems, as the elastic modulus and the other elastic properties may be difficult to determine for brittle porous ceramics without a proper equipment, while the compressive strength is relatively easy to assess from the stress–strain curve.

## 2. Materials and Methods

#### 2.1. Fabrication of Glass-Derived Scaffolds

_{2}–24.5CaO–24.5Na

_{2}O–6P

_{2}O

_{5}wt.%), used as a starting material for making scaffolds, was produced by melting in an electrically heated furnace. The reagents (analytical-grade powders of SiO

_{2}, CaCO

_{3}, Na

_{2}CO

_{3}, and Ca

_{3}(PO

_{4})

_{2}all purchased from Sigma–Aldrich, St. Louis, MO, USA) were homogeneously mixed in a platinum crucible and heated to 1500 °C for 1 h in air (heating rate 5 °C/min). The melt was quenched in deionized water to obtain a frit that was ball milled (Pulverisette 0, Frtisch, Germany) and sieved by a stainless-steel sieve (Giuliani Technology Srl, Italy) to obtain a final particle size below 32 μm. According to previous studies, this particle size range is very suitable to produce porous glass-derived scaffolds by sponge replication [26].

^{3}) as a sacrificial template. The basics and processing schedule of this fabrication method were described in detail elsewhere [19]. The sponge, which was previously cut in the form of discs (dimeter 30 mm, thickness 10 mm) was dipped into an aqueous suspension of 45S5 glass powder. Poly(vinyl alcohol) (PVA), used as a binder, was dissolved in deionized water at 80 °C under magnetic stirring for 30 min prior to the addition of glass powder; the water evaporated during PVA dissolution was re-added to keep constant the solid-to-liquid ratio for each sample during the manufacturing process. As the aim of this work was to study the relationships between elastic properties and porosity of the scaffolds, samples with different porosities were produced by varying the weight ratio of the slurry components (glass: PVA: water = x: 6: (100 – (x + 6))). Specifically, the solid load x was increased from 27 to 45 wt.% with a step of 2 wt.%; as a result, ten samples with different total porosity were obtained and denoted by the code “S-x”. After being extracted from the slurry, the glass-coated sponge was squeezed twice by applying an homogenous pressure (14.1 kPa, generated by a mass of 1.0 kg) on its circular faces in order to remove the excess suspension from the pores and leave a thin layer of glass particles on the template struts. This sequence of dipping–squeezing cycles was repeated thrice. The samples were left to dry overnight at room temperature in air and all were thermally treated under the same condition (1180 °C for 3 h) to remove the sacrificial polymer and sinter the glass particles.

#### 2.2. Characterizations

^{TM}40, Zeiss, Oberkochen, Germany) at an accelerating voltage of 15 kV.

_{0}is the density of the bulk (non-porous) material. The density was calculated as the mass-to-volume ratio of the scaffold (sample geometry was measured using digital calipers).

^{2}) was the resistant cross-sectional area measured prior to the test by using digital calipers.

## 3. Results and Discussion

_{2}CaSi

_{2}O

_{6}, PDF code 01-077-2189), and silico-rhenanite (Na

_{2}Ca

_{4}(PO

_{4})

_{2}SiO

_{4}, PDF code 00-032-1053) was also detected as a secondary phase. These results are in good accordance with previous studies about the sinter-crystallization of 45S5 Bioglass

^{®}powder [26,31,32].

_{0}is the elastic modulus of the non-porous solid, and C and n are constants that depend on the microstructure. For cellular structures with a dense solid network, we have C = 1, while in other cases, this constant can assume different values (for example C = 0.3 for ceramic network with a central empty channel) [36]. The exponent n has a value in the range of 1 to 4, with n = 2 for open-cell structures. The 45S5-based scaffolds can be approximated as open-cell structures in which the solid network was fully dense; therefore, we have C = 1 and n = 2 and Equation (3) can be rewritten as:

_{0}= 78 GPa) was estimated by means of the software SciGlass (AKos, Steinen, Germany) implementing the analytical method Priven 2000. The result of the data fitting is reported in Figure 3; the high value of the correlation coefficient (R

^{2}= 0.8953), calculated by implementing the least squares method in MATLAB (MathWorks, Natick, MA, USA), suggests the good predictive capability of the power-law model.

^{2}= 0).

_{2}–Na

_{2}O–CaO–MgO–K

_{2}O–P

_{2}O

_{5}glass (CEL2) scaffolds produced by the sponge replica method. The results of the data fitting are shown in Figure 4 and, also in this case, the power-law model with n = 2 seems very suitable to describe the relationship between elastic modulus and porosity (R

^{2}= 0.8378), unlike the Pabst–Gregorova approach (R

^{2}= 0).

_{0}) and the Poisson’s ratio of the pore-free material (ν

_{0}) as follows [45]:

_{0}= 0.2651) was estimated by means of the software SciGlass implementing the analytical method Priven 2000. The result of the data fitting is reported in Figure 5: a linear relationship can be suggested, also considering the moderately high value of the correlation coefficient (R

^{2}= 0.5489). This trend is in agreement with the results reported by Arnold et al. [45] for highly porous gel-derived silica. It is interesting to underline that the porosity dependence of Poisson’s ratio varies according to the level of porosity considered. In fact, for p < 0.40, the Poisson’s ration tends to decrease as porosity increases, as shown by De With et al. [46] who prepared and analyzed porous hydroxyapatite with porosity ranging from 0.03 to 0.27: this is an opposite trend compared to that exhibited by highly porous ceramics which still needs to be fully elucidated.

_{0}is the shear modulus of the non-porous solid and $\gamma =\frac{3\left(1+{\nu}_{0}\right)}{4}$.

_{0}= 31 GPa) was estimated by means of the software SciGlass. The result of the data fitting is reported in Figure 6: the high value of the correlation coefficient (R

^{2}= 0.8491) suggests the suitability of the power-law model to describe the experimental data.

^{®}foams produced by sponge replication and sintered at 1100 °C for 1 h [10]. This can be explained considering that the higher temperature (1180 versus 1100 °C) and longer time (3 versus 1 h) used for the sintering process allowed obtaining a better densification of the struts; these results are consistent with those assessed on the same porous material in a previous work [49].

_{0}is the compressive strength of the bulk material, and C

_{1}and m are the model constant.

_{1}and m was carried out using a proper code developed in MATLAB and based on the least squares method. The results of model fitting, carried out assuming σ

_{0}= 500 MPa [50], is reported in Figure 7. The fitted parameters are C

_{1}= 0.0258 and m = 1.84; the coefficient R

^{2}= 0.9165 suggests a good accuracy and predictive capability of the power-law model for the compressive strength, too. The exponent m is also comparable to that assessed for human trabecular bone by other authors [51].

## 4. Conclusions

^{2}coefficients show promise and support these results, which deserve to be confirmed in future studies on a larger scaffold number. In principle, these relations are valuable for the development of mechanically optimized scaffolds: knowing the porosity, which is relatively easy to determine, they can be used to predict the scaffold mechanical properties; alternatively, using the mechanical property as a model input, the scaffold manufacturer can estimate the porosity which is required to achieve that mechanical performance. Hence, the fabrication process can be properly optimized to obtain that mechanical property, thereby saving manufacturing time and cost.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 2.**SEM micrographs showing the morphology of 45S5-derived foam-like scaffolds (S-41 sample): (

**a**) overview of pore-strut architecture (magnification 50×) and (

**b**) detail of a sintered strut (magnification 3000×).

**Figure 3.**Porosity dependence of elastic modulus in 45S5 glass-derived foams with data interpolation by applying the Gibson–Ashby power-law model (Equation (4)) (dashed line) and the Pabst–Gregorova exponential relation (Equation (6)) (solid line).

**Figure 4.**Porosity dependence of elastic modulus in CEL2 glass-derived foams with data interpolation by applying the Gibson–Ashby power-law model (Equation (4)) (dashed line) and the Pabst–Gregorova exponential relation (Equation (6)) (solid line). Raw data from Reference [41].

**Figure 5.**Porosity dependence of the Poisson’s ratio in highly porous 45S5 glass-derived foams with data interpolation by a linear model (Equation (9)).

**Figure 6.**Porosity dependence of shear modulus in 45S5 glass-derived foams with data interpolation by applying the Gibson–Ashby power-law model (Equation (10)).

**Figure 7.**Porosity dependence of compressive strength in 45S5 glass-derived foams with data interpolation by applying the Gibson–Ashby power-law model (Equation (11)).

Scaffold Code (S-x) | p | E (GPa) | G (GPa) | ν | σ_{c} (MPa) |
---|---|---|---|---|---|

S-27 | 0.86 | 1.2 | 0.43 | 0.3953 | 0.58 |

S-29 | 0.82 | 2.0 | 0.73 | 0.3698 | 0.78 |

S-31 | 0.80 | 2.5 | 0.91 | 0.3736 | 0.88 |

S-33 | 0.75 | 6.1 | 2.2 | 0.3863 | 0.95 |

S-35 | 0.71 | 7.3 | 2.8 | 0.3035 | 0.96 |

S-37 | 0.70 | 8.0 | 3.1 | 0.2903 | 1.1 |

S-39 | 0.68 | 11.5 | 4.3 | 0.3372 | 1.2 |

S-41 | 0.65 | 13.0 | 4.8 | 0.3541 | 1.5 |

S-43 | 0.55 | 16.0 | 6.3 | 0.2698 | 2.8 |

S-45 | 0.52 | 16.5 | 6.5 | 02692 | 3.4 |

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Baino, F.; Fiume, E.
Elastic Mechanical Properties of 45S5-Based Bioactive Glass–Ceramic Scaffolds. *Materials* **2019**, *12*, 3244.
https://doi.org/10.3390/ma12193244

**AMA Style**

Baino F, Fiume E.
Elastic Mechanical Properties of 45S5-Based Bioactive Glass–Ceramic Scaffolds. *Materials*. 2019; 12(19):3244.
https://doi.org/10.3390/ma12193244

**Chicago/Turabian Style**

Baino, Francesco, and Elisa Fiume.
2019. "Elastic Mechanical Properties of 45S5-Based Bioactive Glass–Ceramic Scaffolds" *Materials* 12, no. 19: 3244.
https://doi.org/10.3390/ma12193244