**5. Designing porous calcium phosphate/titania scaffolds exploiting direct foaming method**

Regeneration of load-bearing bone segments is still an open challenge due to the lack of biomaterials mimicking natural bone with suitable physicochemical and mechanical performance. Additionally, bone scaffolds should exhibit wide open and interconnected porosity, which however could strongly penalize the mechanical strength. Therefore, the research on adequate methods for porous ceramic development is today a hot topic in materials science [9].

Among the several processes proposed in literature to produce porous ceramics [63, 64], template-free foaming techniques are particularly promising, especially due to the absence of large amounts of organic phases to be eliminated during thermal consolidation. Indeed, forming techniques, making use of sacrificial templates, require long and slow thermal treatments to eliminate the organic component, possibly yielding structural damage and penalization of the final mechanical properties.

In particular, the direct foaming method was stated to be a low-cost and easy process that can provide pore volumes in the range 40–97% by incorporating air into a ceramic suspension that is subsequently dried and sintered. It was also reported that cellular structures prepared by direct foaming usually exhibit considerably higher mechanical strength than those obtained by other template-based techniques, mainly due to the strongly reduced occurrence of flaws in the cell struts [64, 65]. The decisive step in direct foaming methods is related to the development of ceramic slurries with optimal rheological properties, so that they can be dried and physically stabilized upon pouring into preshaped containers, while maintaining the shape, size, and distribution of the air bubbles.

This method was successfully applied to the synthesis of ceramic bone scaffolds made of β-TCP and TiO<sup>2</sup> , developed from hydroxyapatite (HA) and TiO2 powders, on the basis of the approach carried out by [23] exhibiting high and interconnected macroporosity (>70 vol%).

As the foaming process is based on a concentrated ceramic suspension, rheological properties are a critical issue. Indeed, when applied on a simple mixture of HA and TiO2 powders (see previous paragraph), phase separation occurred in the green ceramic body, thus resulting in scaffolds characterized by reduced structural homogeneity. Therefore, an alternative approach was needed to obtain homogeneous blends, for which the mixture was subjected to a thermal treatment at 1000°C with a dwell time of 1 h before applying the foaming process, to obtain a powder with homogeneous composition.

With this, the application of direct foaming process was feasible and successful in obtaining ceramic bone scaffolds exhibiting high and interconnected macroporosity (>70 vol%).

Direct foaming process resulted very interesting to generate highly porous ceramics, as described in Ref. [66]. As observed by scanning electron microscopy, the microstructure of foamed scaffolds was characterized by large pores in the range 700–900 μm, in turn containing smaller pores, which provide interconnection throughout the whole scaffold (**Figure 5**).

Such microstructure is ideal when it comes to enable extensive penetration of new bone and expressing, at the same time, remarkable strength. In fact the spheroidal pore morphology provides enhanced resistance against fracturing, whereas smaller pores can aid to develop an effective vascular network. In this respect, the lack of vascularization in critical-size bone defects was reported as among the most critical issues limiting the extent of bone regeneration [1].

Besides morphology, compositional aspects can play a relevant role in determining the mechanical properties. Indeed the compressive strength of the TCP/TiO2 composite scaffolds resulted about 8 MPa, with 75% porosity, i.e., thrice than the reference HA scaffolds. Together with Young's modulus, these composites thus exhibited mechanical properties in the range of cancellous bones (i.e., compression strength, 2–12 MPa; Young's modulus, 0.05–0.5 GPa). The enhanced mechanical competence was also associated to superior biological performance in vitro. Osteoblast-like cells (MG63) cultivated on the scaffold surface for 7 days covered almost completely the external surfaces of the scaffolds, and most of the macropores were completely infiltrated by cells, demonstrating high biocompatibility and osteointegrative potential as well [33].

Furthermore, an increase in cell proliferation was detected during 2 weeks of analysis, whereas the analysis of alkaline phosphatase (ALP) activity revealed a higher osteogenic activity for β-TCP/TiO<sup>2</sup> scaffolds (**Figure 6**). This improvement could be related to both the higher solubility of TCP that yielded enhanced release of calcium ions to cells and also the presence of TiO2 that in physiological environment can be promptly covered by a layer of HA.

**Figure 5.** Porous microstructure of sintered porous TCP/TiO2 .

In particular, the direct foaming method was stated to be a low-cost and easy process that can provide pore volumes in the range 40–97% by incorporating air into a ceramic suspension that is subsequently dried and sintered. It was also reported that cellular structures prepared by direct foaming usually exhibit considerably higher mechanical strength than those obtained by other template-based techniques, mainly due to the strongly reduced occurrence of flaws in the cell struts [64, 65]. The decisive step in direct foaming methods is related to the development of ceramic slurries with optimal rheological properties, so that they can be dried and physically stabilized upon pouring into preshaped containers, while maintaining the shape,

This method was successfully applied to the synthesis of ceramic bone scaffolds made of

approach carried out by [23] exhibiting high and interconnected macroporosity (>70 vol%). As the foaming process is based on a concentrated ceramic suspension, rheological proper-

(see previous paragraph), phase separation occurred in the green ceramic body, thus resulting in scaffolds characterized by reduced structural homogeneity. Therefore, an alternative approach was needed to obtain homogeneous blends, for which the mixture was subjected to a thermal treatment at 1000°C with a dwell time of 1 h before applying the foaming process,

With this, the application of direct foaming process was feasible and successful in obtaining

Direct foaming process resulted very interesting to generate highly porous ceramics, as described in Ref. [66]. As observed by scanning electron microscopy, the microstructure of foamed scaffolds was characterized by large pores in the range 700–900 μm, in turn containing smaller pores, which provide interconnection throughout the whole scaffold (**Figure 5**). Such microstructure is ideal when it comes to enable extensive penetration of new bone and expressing, at the same time, remarkable strength. In fact the spheroidal pore morphology provides enhanced resistance against fracturing, whereas smaller pores can aid to develop an effective vascular network. In this respect, the lack of vascularization in critical-size bone defects was

ceramic bone scaffolds exhibiting high and interconnected macroporosity (>70 vol%).

reported as among the most critical issues limiting the extent of bone regeneration [1].

cal properties. Indeed the compressive strength of the TCP/TiO2

strating high biocompatibility and osteointegrative potential as well [33].

Besides morphology, compositional aspects can play a relevant role in determining the mechani-

about 8 MPa, with 75% porosity, i.e., thrice than the reference HA scaffolds. Together with Young's modulus, these composites thus exhibited mechanical properties in the range of cancellous bones (i.e., compression strength, 2–12 MPa; Young's modulus, 0.05–0.5 GPa). The enhanced mechanical competence was also associated to superior biological performance in vitro. Osteoblast-like cells (MG63) cultivated on the scaffold surface for 7 days covered almost completely the external surfaces of the scaffolds, and most of the macropores were completely infiltrated by cells, demon-

Furthermore, an increase in cell proliferation was detected during 2 weeks of analysis, whereas the analysis of alkaline phosphatase (ALP) activity revealed a higher osteogenic activity for

powders, on the basis of the

composite scaffolds resulted

powders

, developed from hydroxyapatite (HA) and TiO2

ties are a critical issue. Indeed, when applied on a simple mixture of HA and TiO2

size, and distribution of the air bubbles.

to obtain a powder with homogeneous composition.

β-TCP and TiO<sup>2</sup>

52 Application of Titanium Dioxide

**Figure 6.** Proliferation of MG-63 osteoblast-like cells (a) and alkaline phosphatase (ALP) activity (b) when seeded on β-TCP/TiO<sup>2</sup> scaffolds and HA control [66].

## **6. Conclusions and future perspectives**

The presented results show that porous scaffolds with bone-like composition and strength can be developed, by following approaches based on ceramic composite development. In particular, TiO2 is a promising material as bioactive reinforcing phase for calcium phosphate matrices, giving its high biocompatibility. In respect to the design and development of adequate ceramic compositions, the challenge is still open as the settling of ceramic systems requires optimization of a variety of parameters related to initial composition, preliminary powder processing, forming methods, and sintering, all of which are crucial for the final biologic and mechanical properties. In this respect, direct foaming is a very promising method for porous scaffold development that can be flexibly applied to a variety of compositions. Therefore, the application of such process can be decisive for the development of reinforced scaffolds; in this respect, TiO2 -based scaffolds were still investigated in a limited extent, in spite of their potential to chemically and mechanically assist the regeneration of bone tissue defects, particularly loadbearing bone segments, as no regenerative solutions still exist in this field.

## **Author details**

Massimiliano Dapporto, Anna Tampieri and Simone Sprio\*

\*Address all correspondence to: simone.sprio@istec.cnr.it

Laboratory of Bioceramics and Bio-hybrid Composites, Institute of Science and Technology for Ceramics, National Research Council of Italy, Faenza, Italy

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**6. Conclusions and future perspectives**

lar, TiO2

54 Application of Titanium Dioxide

TiO2

**Author details**

**References**

The presented results show that porous scaffolds with bone-like composition and strength can be developed, by following approaches based on ceramic composite development. In particu-

giving its high biocompatibility. In respect to the design and development of adequate ceramic compositions, the challenge is still open as the settling of ceramic systems requires optimization of a variety of parameters related to initial composition, preliminary powder processing, forming methods, and sintering, all of which are crucial for the final biologic and mechanical properties. In this respect, direct foaming is a very promising method for porous scaffold development that can be flexibly applied to a variety of compositions. Therefore, the application of such process can be decisive for the development of reinforced scaffolds; in this respect,


Laboratory of Bioceramics and Bio-hybrid Composites, Institute of Science and Technology

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