# Recombinant Proteins-Based Strategies in Bone Tissue Engineering
## 3 Tissue Engineering  
Tissue engineering is a biotechnological area whose term emerged approximately 27 years ago, to define a multidisciplinary field of study encompassing material engineering and biomedical sciences. Therefore, it involves knowledge from various disciplines such as physics, chemistry, and biology, and seeks to reconstitute, replace and/or regenerate specific tissues or organs through the implementation of effective and practical materials, trying to maintain the existing residual structure, as well as to make tissue growth viable while acting as scaffolds that promote the growth of living tissues [20; 21; 22; 23; 24].  
A variety of biodegradable materials have been developed, with great versatility to modify various parameters such as degradation rate, mechanical properties, and porosity, according to the characteristics required for each application. The scaffolds thus developed are called third generation scaffolds; they exhibit biological and physical properties compatible with the in vitro and in vivo physiological conditions of the damaged tissue. In other words, they are intended to provide a temporary support with sufficient mechanical integrity to maintain the structure, and then lead to proliferation, differentiation, and cellular biosynthesis specific to the site where it was implanted. To do so, it must exhibit properties such as being non-toxic, biocompatible, and biodegradable, promote and reorganize the desired cells, have adequate porosity for supporting cell seeding, and be interconnected. Third generation biomaterials to be considered as such must combine properties of biodegradation and bioactivity within the same material, the latter meaning that they must possess the capacity to induce, stimulate, provoke, or modulate a defined biological action in the host tissue. A bioactive material is one that enables a specific biological response at its interface with the surrounding tissues, by favoring their binding. These materials also seek to stimulate different cellular responses based on their surface characteristics, so that their function is therefore temporary, as the material is intended to be reabsorbed once the tissue function is restored. Tissue engineering applied to bone tissue repair must ensure that the materials used as scaffolds are potentially osteoinductive, i.e., capable of promoting the differentiation of progenitor cells into osteoblastic cells, and have osteointegrating properties, i.e., enabling integration with adjacent bone tissue, aiming to repair bone tissue with an intact biomechanical status [25].  
An implant scaffold must present an architecture in which the cells are organized, promoting an initial biomechanical profile for tissue replacement until the cells produce an adequate extracellular matrix and the initial scaffold is degraded or metabolized to produce de novo tissue. The idea is to synthesize or implement a scaffold that is capable of supporting the initial loads and degrades gradually, transferring the loads progressively to the new bone. The degradation rate of a matrix used in bone tissue engineering should be slow, maintaining the mechanical resistance of the tissue itself until the bone tissue is regenerated and avoiding a second surgical intervention for implant removal. The scaffold materials must stimulate the appropriate cellular responses that will be gradually replaced by the new tissue [22; 26].  
Several research groups have developed scaffolds to be used as implants in bone lesions, some of them derived from natural products such as chitosan [27] and collagen [28; 29; 30], while other scaffolds have been produced by chemical synthesis [31; 32]. Composite materials based on biodegradable polymers associated with other types of scaffolds have begun to be synthesized in the last decade, and they are of particular interest due to the fact that they offer an adequate balance between strength and toughness, beyond the properties of each of their individual components [33; 34].