# Recombinant Proteins-Based Strategies in Bone Tissue Engineering
## 7 Elastin-like Recombanmers (ELRs)
An alternative way to obtain non-cytotoxic and biocompatible hydrogels with application in tissue regeneration from ELRs exploits an innovative strategy in the generation of hydrogels. The biofunctionality of this type of hydrogels lies in the introduction of bioactive sequences in the composition of the biopolymers [67]. Recently, two bioactive ELRs were developed, one including the osteogenic and osteoinductive BMP-2, and the other the Arg-Gly-Asp (RGD) cell adhesion motif. These two ELRs were mixed, obtaining a hydrogel scaffold that was considered to act as an extracellular matrix carrying BMP-2, feasible for bone tissue engineering strategies. This scaffold exhibited excellent cytocompatibility, and the culture of cells on RGD-containing ELRs resulted in optimal cell adhesion. When implanted in a rabbit bone injury model, complete regeneration of the defect--confirmed by radiography, computed tomography, and histology--was demonstrated [51] (Table 1). In recent years, new biomaterials based on ELRs that are genetically engineered using recombinant technology, bioproduced by fermentation in _E.coli_, have been developed with proven qualities in regenerative medicine [68]. One of the most important characteristics of these new ELR polymers is that they can be specifically designed and tailored for each application. In the first place, these materials have demonstrated their bioactivity and efficacy with general adhesion sequences (RGD) [69], and with specific adhesion sequences such as REDV [70]. Their excellent biocompatibility [71] and their ability to induce angiogenesis [72] have also been demonstrated. In addition, when used as implants in a defect, these biomaterials are able to recruit cells from the surrounding tissues and induce tissue regeneration in the area of the lesion, without the need to introduce cells with the implant [73; 74]. It is also noteworthy how its degradation rate can be regulated by specific sequences mediated by elastases [75]. One of the key properties that regenerated tissue might achieve is that newly produced cells would be able to produce an extracellular matrix containing elastin/collagen similar to that of native tissue. Generally, cells embedded in various biomaterials generate collagen but not elastin, and therefore the mechanical properties are not adequate for the final application [76]. There are studies showing how embedded cells in ELRs are able to generate both collagen and elastin distinguishable from that of ELR [77]. The technology required for the creation of hydrogels from ELRs is available in the literature. Chemical modification of these polypeptides is performed by introducing reactive groups to form chemical hydrogels through a \"click chemistry\" reaction and, more specifically, through a Huisgen cycloaddition under fully cytocompatible conditions and without the presence of any initiator or catalyst [67]. Hydrogels formed through this reaction have shown mechanical properties similar to those found in natural tissues, and a high biocompatibility, as mentioned above [67]. More specifically, in relation to skeletal regeneration, nanoparticles have been developed from ELR capable of effectively encapsulating BMPs and controlling their release for 14 days. Moreover, the osteoinductive activity of these growth factors was evidenced by the induction of ALP activity and osteogenic mineralization in C2C12 cells [78]. Hydrogels have displayed also osteoinductive effects in several studies, due to their ability to recruit osteoprogenitor cells from native tissues surrounding the lesion [51]. Finally, different hydrogels have been prepared from ELR with embedded cells, and have recently been used in different studies on the regeneration of osteochondral defects in rabbits, with very promising results [79; 80].