**3. Nature-derived biomaterials: biomorphic bone scaffolds with hierarchic architecture**

The regeneration of load-bearing bone parts is still a high demanding challenge. Particularly, therapies to solve serious diseases involving the limbs, due to trauma or tissue degeneration, are today restricted to reconstructive approaches based on multiple surgery and the use of metallic parts that often give rise to secondary effects such as infections, pseudoarthrosis, and non-unions which can also lead to the complete loss of the limb functionality and also to amputation [20–22]. The incidence of such events is great and steadily increasing globally due to the modern lifestyle and also to the progressive ageing of the population, thus leading to high disability with huge impact on the healthcare costs and the patient's life.

oriented channel-like porosity mimicking the tubular organization of dentine can be obtained

Preliminary research shows that the application of bio-inspired synthesis techniques can enable the development of new implantable devices for the complete regeneration of dental tissues. This is a major and highly demanding clinical need and a target of high impact for

In perspective, the in-lab biomineralization process may be in principle translated to wider applications, possibly extending the range of natural polymers that can be combined to form composite matrices activating self-assembly and mineralization with specific inorganic phases. Non-mineralized constructs can be used as scaffolds for soft tissues and organs, where the biologic and mechanical performance can be tailored by combining various raw materials such as gelatin, nanocellulose, chitosan, alginate, and fibroin characterized by different hydrophilic behaviour and stiffness. On the other hand, the simultaneous mineralization of composite polymeric matrices with nano-apatites can generate scaffolds with improved mechanical performance, thus enabling wider applications in bone surgery, particularly referred to loadbearing applications where the soft nature of hybrid scaffolds does not allow to withstand

**3. Nature-derived biomaterials: biomorphic bone scaffolds with hierarchic**

The regeneration of load-bearing bone parts is still a high demanding challenge. Particularly, therapies to solve serious diseases involving the limbs, due to trauma or tissue degeneration, are today restricted to reconstructive approaches based on multiple surgery and the use of metallic parts that often give rise to secondary effects such as infections, pseudoarthrosis, and

strong biomechanical loads in the early stages of new bone formation.

by ionotropic gelation techniques applied to the as-synthesized hydrogels (**Figure 6**).

**Figure 6.** Dentin-like scaffolds.

136 Advanced Techniques in Bone Regeneration

**architecture**

materials science and medicine.

Today the use of grafts to assist the regeneration of long segmental bones is considered as a promising approach, with different alternatives including autologous vascularized bone grafts, homologous bone graft, heterologous bone graft (xenograft), or prostheses, each one of them dealing with both specific advantages and drawbacks, such as: donor site morbidity and limited available amount, possible immune response and viral transmission, possible animalderived pathogen transmission and risk of immunogenic rejection, high invasiveness and surgery-related systemic risks, long recovery time and need of prostheses revision [23–27].

Due to these very serious drawbacks, the use of synthetic bone substitutes with osteogenic and osteoconductive ability may offer clear benefits compared to natural bone grafts. Adequate osteogenic ability is required to stimulate the formation of new bone by exhibiting highly exposed active surfaces, favouring cell adhesion and proliferation. Also, osteoconductivity enables the penetration of the scaffold by cells which is a key aspect to achieve early osseoin‐ tegration in turn enabling adequate stability of the bone/implant construct and the possibility for the patient to stimulate bone regeneration by progressively increasing loading [28, 29]. Adequate osteoconductivity is provided by the presence of open and interconnected porosity in the bone scaffold, in association with high surface affinity with bone cells. However, most of the bio-devices today developed exhibits tortuous porosity that hampers the development of extensive angiogenesis and penetration of blood vessels in the inner parts of the scaffold; in consequence, even though a good surface integration occurred, bone penetration is limited thus penalizing the stability of the bone/implant construct and the mechanical performance. [30–32].

HA, and particularly ion-doped apatites are the golden materials for bone scaffolding. However, the feasibility of synthesizing large porous HA bodies with high bioactivity, osteoconductivity, and mechanical strength is hampered by the need of thermal consolidation that destroys the bioactivity features of HA, that means: segregation of the foreign ions outside the HA lattice, thermally-induced grain growth with strong reduction of the specific surface area and reduction of the hydrophilic character and surface reactivity. Moreover, the weak mechanical properties of HA make it difficult to develop large scaffolds with high porosity extent. However, it can be envisaged that early and extensive penetration of new bone into the scaffold pores may significantly enhance the strength of the bone/biomaterial construct and enable mechanical loading. This process may lead to the complete recovery of limb function‐ ality by progressive and assisted stimulation of the implanted part [32].

Since the unique biomechanical properties of bone mainly depend on its hierarchically organized structure ranging from the molecular to the nano-, micro-, and macro-scales, only scaffolds endowed with a 3D structure capable of exhibiting complex biomechanical perform‐ ances may activate mechano-transduction processes in a biological-like fashion and yield regeneration of well-organized bone [33–39].

In consideration to the limits imposed by the ceramic technology (particularly by means of the existing forming techniques and the sintering), new manufacturing approaches are required for synthesis of scaffolds with adequate requisites for regeneration of long segmental bones. In this respect, the complex structural organization exhibited by living beings such as woods and plants is an interesting source of inspiration for material scientists towards the generation of smart devices with strongly improved performances. Indeed, these structures possess a hierarchic organization on multiple size scales that provide high strength and lightness (**Figure 7**).

**Figure 7.** Structure of various woods and plants.

Among these, ligneous structures endowed with open porosity and suitable interconnection enabling extensive permeability to cells and fluids, as well as, at the same time, with adequate anisotropic mechanical behaviour, may be investigated as templates to develop new porous scaffolds with bone-like structural features [40–42].

Porous woods like pine and rattan were recently transformed into HA scaffolds with hierarchic organization, by a multi-step biomorphic transformation process (**Figure 8**) enabling precise control of phase composition and crystal ordering, as well as of the microstructure, since the different reactions occurred between a gas and the solid template at a molecular level, where calcium, oxygen, carbonate, and phosphate ions were progressively added, while building the HA molecules [43].

Nature-Inspired Nanotechnology and Smart Magnetic Activation: Two Groundbreaking Approaches Toward a New Generation of Biomaterials for Hard Tissue Regeneration http://dx.doi.org/10.5772/63229 139

**Figure 8.** Biomorphic transformation process generating hydroxyapatite scaffolds.

By this process, it was possible to incorporate foreign ions, such as carbonate, in the final consolidated apatite scaffold [43] and to control the chemico-physical features related to the scaffold bioactivity and resorbability, such as the Ca/P ratio and the extent of crystal ordering of the HA phase. Among the existing ligneous sources rattan possesses a structure particularly suitable for bone scaffolding, i.e. a channel-like porosity very close to the Haversian structure with wide pores having diameter adequate for enhanced cell hosting and 3D colonization (**Figure 9**), thus being very promising for the activation of extensive vascularization through‐ out large volumes.

**Figure 9.** Bone mimicry of rattan wood.

In consideration to the limits imposed by the ceramic technology (particularly by means of the existing forming techniques and the sintering), new manufacturing approaches are required for synthesis of scaffolds with adequate requisites for regeneration of long segmental bones. In this respect, the complex structural organization exhibited by living beings such as woods and plants is an interesting source of inspiration for material scientists towards the generation of smart devices with strongly improved performances. Indeed, these structures possess a hierarchic organization on multiple size scales that provide high strength and lightness (**Figure**

Among these, ligneous structures endowed with open porosity and suitable interconnection enabling extensive permeability to cells and fluids, as well as, at the same time, with adequate anisotropic mechanical behaviour, may be investigated as templates to develop new porous

Porous woods like pine and rattan were recently transformed into HA scaffolds with hierarchic organization, by a multi-step biomorphic transformation process (**Figure 8**) enabling precise control of phase composition and crystal ordering, as well as of the microstructure, since the different reactions occurred between a gas and the solid template at a molecular level, where calcium, oxygen, carbonate, and phosphate ions were progressively added, while building the

**7**).

138 Advanced Techniques in Bone Regeneration

**Figure 7.** Structure of various woods and plants.

HA molecules [43].

scaffolds with bone-like structural features [40–42].

Moreover, the endowment of the bone scaffolds with the channel-like structure of rattan resulted into anisotropic mechanical properties with values in the range of the trabecular bone, that reflect the complex bone response to directional loading. Preliminary biologic tests reported an outstanding affinity with cells, with complete coverage of the scaffolds by well spread cells (i.e. MG63 osteoblast-like cells) after 1 week and enhanced osteogenic ability compared to sintered HA scaffolds (**Figure 10**). Also, preliminary in vivo tests reported extensive bone formation and colonization in femoral bone defects, also showing good morphological organization after one month from implantation [20].

**Figure 10.** MG63 cells morphology in contact with wood-derived HA scaffold.

The first results obtained with this new type of bone scaffolds are very promising for further development and application into more clinically-relevant models, for assessing the feasibility of regenerating long segmental bone parts. In this respect the exploitation of natural sources as models for generation of new hierarchically organized scaffolds can be considered as a completely new synthesis approach that may open to still unexplored applications in the incoming years.
