**6. Biomorphic transformation of natural structures: a new way to obtain biomimetic scaffolds for regeneration of load-bearing segmental bones**

Among the bone diseases, those affecting portions of long bone subjected to mechanical loads are the ones which most seriously impact on the quality of life of sufferers. The incidence of such pathologies is particularly relevant among the aged people (osteoporosis); anyway, more recently the number of relatively young patients affected by bone diseases has increased mainly owing to modern lifestyles (e.g., intense sport activity, obesity, etc.). In this case, pain and disability also impact on the psychological well-being, leading to anxiety, depression, fear for the future, and altered perception of the social role. Such feeling is nowadays shared by the aged people also, because of the increased expectation of an active life and well-being even among the elderly. For this reason, the abovementioned numbers in terms of socioeconomic costs and number of hospitalized people are likely to increase in the next future.

Due to the inability of the current manufacturing technologies to form mechanically strong porous inorganic structures with a hierarchic pore organization and complex morphological details in the submicron scale, the healing of load-bearing bone segments still relies on bioinert dense implants based on alumina, titanium, etc.

A significant change in engineering and ceramic processing is needed, thus greatly expanding the existing tools enabling the development of porous and massive ceramic bodies with designed smart functions. The current manufacturing approach in ceramic development is based on powder synthesis, forming, and thermal consolidation (sintering); the idea is to surpass the existing approach, by developing new "one-step synthesis/consolidation processes" to obtain new 3D ceramics with properties and functions not achievable with the current manufacturing approach. In particular, this is relevant when the ceramic phases with desired functional properties have low thermodynamic stability such as nanosized and atomic position, so that the existing ceramic process, particularly sintering, destroys labile phases increasing their stability but deleting its smart functional properties. Particularly, the sintering process, which is fundamental to consolidate ceramic bodies, impairs the maintenance of ceramic phases characterized by low crystallinity, nanosize, and nonstoichiometric composition. These features, relying on low thermodynamic stability, are very often the source of functions that cannot be expressed by a stable, sintered ceramic phase [20].

*Bio-Inspired Technology*

regeneration, biomaterials provide the template for tissue development that can be

For this reason, several authors have highlighted the need to modulate a multilayered scaffold capable to reproduce different biological and functional environments of osteochondral region to promote regeneration [80, 81]. To create construct with more favorable integrative properties in the osteochondral site, bilayer or tri-layer composite is developed such as a polylactide-co-glycolide copolymer, the first scaffold reported for clinical use; however, it showed poor repair tissue quality at imaging, as well as unsatisfactory clinical outcomes [82, 83]. One of the difficult points in the osteochondral regeneration is the interface between material's layer and host tissue and between layers of host tissue; the cartilage repair should be followed by an adequate regeneration of the subchondral structure and by the effective union with surrounding host tissue [84]. Tampieri et al. designed a composite scaffold consisting of three different but integrated layers, corresponding to cartilage, calcified cartilage, and bone components [69]. It was developed to better mimic structure and composition of the whole osteochondral unit, showing promising clinical results even in challenging conditions, such as complex lesions or osteoarthritic knees [85, 86]. Exploiting biomineralization process, a different extent of mineralization was nucleated on collagen fibers developing a tri-layer with a gradient of hydroxyapatite ranging from a mineral content of 60–70% corresponding to subchondral bone and 30–40% corresponding to mineralized cartilage up to 0% corresponding to hyaline cartilage (**Figure 3**). Furthermore, in the top layer (hyaline cartilage), hyaluronic acid was added to create microstructural features improving the hydrophilic ability to reproduce columnar-like structure converging toward the external surface, where it formed horizontal flat ribbons,

adjusted in shape, size, and orientation according to defect features [80].

thus resembling the morphology of the *lamina splendens*.

Chemical-physical investigation highlighted that chemotactic information provided by collagen-induced unique features in the inorganic phase, promoting the nucleation of a biomimetic apatite very close to the biological one present in the bone [87]. In vivo evaluation demonstrates that it differentially supports cartilage and bone tissue formation in the different histological layers [88]. After 6 months from implantation of graded hybrid composites on femoral condyles of

*Representation of multilayered hybrid scaffold obtained by in-lab biomineralization and its application in* 

**42**

**Figure 3.**

*osteochondral defect.*

The main goal is the implantation of osteoinducting and osteoconducting scaffolds with spatially organized macroporosity and mechanical strength sufficient for early in vivo loading upon implantation and elastic properties close to those of the bone. This may enable scaffolds to respond to the biomechanical loads and activate mechano-transduction mechanisms, yielding remodeling and formation of new functional bone [90]. The complex structure of bones, hierarchically organized from the nano- to the macro-scale, and the interaction taking place across all levels of organization are the reasons of the outstanding mechanical performances of bones. For this reason, long-bone regeneration should be assisted by scaffolds endowed with bone-like composition and similar structural complexity; however, the common manufacturing methods do not produce mechanically resistant scaffolds with the required hierarchical pore organization and bioactivity. The chemical biomimesis in scaffolds for long-bone regeneration is influenced by the mechanical strength of HA-based materials. There are several studies about scaffolds based on composite materials, making use of strong bioactive or bioinert phases [36, 37] that were dispersed in a calcium-phosphate matrix. However, the limitation in the achievement of hierarchically organized structures still remains [8].

This problem can reside in nature, so the attention of scientists has been moved to find and observing complex morphologies that exist in nature, and then try to reproduce them. In particular, the ligneous structures strongly resemble bones in their structural organization and morphology which affect the mechanical performances [8].

Like bone, wood can be considered as a cellular material at the scale of hundred micrometers to centimeters (**Figure 4**). At the cell level, the mechanical properties are governed by the shape and diameter of the cell cross section, as well as by the thickness of the cell wall. In particular, the apparent density of wood, which in turn is a determining factor for the performance of lightweight structures, is directly related to the ratio of cell wall thickness to cell diameter. The particular hierarchical architecture of the cellular microstructure gives wood an exceptional combination of high stiffness, toughness, and strength at low density [91]. The alternation of channel-like porous and fiber bundle areas makes the wood an elective material to be used as a template in the preparation of a new bone substitute that is characterized by a biomimetic hierarchical structure [20].

**45**

*Nature-Inspired Processes and Structures: New Paradigms to Develop Highly Bioactive Devices…*

A subject of investigation in the late 1990s was the transformation of wood into inorganic, hierarchically organized materials (e.g., oxidic ceramics such as Al2O3, ZrO2, TiO2, and MnO and nonoxidic ceramics such as SiC, TiC, and ZrC) [92–96]. The synthesis of hierarchically organized bone scaffolds made of SiC is a result of these studies [94], which have the advantage of offering bio-tolerated surfaces and very high fracture strength. Other kinds of biomorphic transformations, conceived recently, were used to manufacture hierarchically organized scaffolds made of HA [3]. The complexity of the apatite phase, in comparison with nitrides, carbides, and oxides, required the settling of a multistep process transformation, where the native wood was sequentially transformed into pure carbon, calcium carbide, calcium oxide, calcium carbonate, and finally HA. Due to their bone-mimicking hierarchical organization, microstructure and composition such a new generation of bioceramics scaffolds promise to offer enhanced integration, osteogenesis, and biomechani-

Woods such as rattan have strong similarities to 3D structure and morphology of cortical and spongy bone. Rattan is characterized by channel-like pores (simulating the Haversian system in bone), interconnected with a network of smaller channels

There is a precise control of the microstructure, crystallinity, and phase composition, during the multistep transformation process, in which different gas-solid reactions occur where the solid is the template. Calcium, oxygen, carbonate, and phosphate ions were progressively added in the different steps to finally get the HA molecules. The reaction kinetic is controlled throughout the different steps of the transformation process in order to have a precise control of the scaffold microstructure, composition, and bioactivity [95]. Importantly, even in the absence of thermal consolidation treatments, the scaffolds exhibit mechanical strengths comparable to those of spongy bone (~4 MPa) when measured along the channel direction, thanks

The establishment of biomorphic transformations that are able to transform woods into biomimetic bone scaffolds can provide solutions for long-bone regeneration and can be designed in a custom-made fashion. Selected wood structures could reproduce different bone portions that are characterized by different porosities and pore distributions, as occurring in cortical and spongy bones. Such devices may implement the formation of a biological chamber in vivo that contain a suitable environment that allows to promote and enhance bone formation and remodeling. The implant will thus function as an in vivo bioreactor, thus facing an unsolved clinical problem related to the disappearing of the regenerative process at distances

The progressive population aging and the younger people modern behaviors, which expose to serious injuries and traumas, are concerns of large and continuously increasing socioeconomic impact. The continual advances in materials science and nanotechnology allowed great progress in biomedical device development for bone regeneration. Nevertheless, the development of bio-devices mimicking biological tissue structure and composition with high complexity and load-bearing properties, such as extended bone and osteochondral parts or segmental bones, still presents serious limitations. In fact, the related clinical needs remain unmet because of the absence of well-established regenerative devices for such applications. Many possibilities for solving these concerns are offered by the recent advances in materials science: the new-generation smart multifunctional device development could

*DOI: http://dx.doi.org/10.5772/intechopen.82740*

cal behavior when implanted in vivo [8].

to the maintenance of the original wood microstructure.

far from the bone-implant interface [20].

**7. Conclusions and future perspectives**

(such as the Volkmann system) [3].

**Figure 4.** *Scheme of the multi-scale structure of wood and bone tissue.*
