# Recombinant Proteins-Based Strategies in Bone Tissue Engineering
## 7 Elastin-like Recombanmers (ELRs)  
Certain polymers can be produced as recombinant chimeric proteins from a synthetic DNA construct, so called recombinamers [59]. Due to recent advances in molecular biology and biotechnology, the degree of complexity and control achieved in the final composition of recombinamers can be much higher than that achieved by the most advanced chemical methods in polymer science [60]. This allows the incorporation of any function present in natural proteins, such as cell adhesion sequences, specific protease targets, or higher molecular weight domains. The family of ELRs have proven great application and relevance [59; 60]. The composition of ELRs, based on the repetition of natural elastin sequences, L-Val-L-Pro-Gly-X-Gly, where X can be any amino acid except L-Pro, endows the material with a series of properties that are not easy to find in other polymer families: extreme biocompatibility, strong \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Scaffolds** & **In Vitro** & **In Vivo** & **Results** & **References** \\ \hline Recombinant human type I & & & Osteogenic differentiation of stem cells. Mineralization & \\ collagen achieved by & & & increased with the scaffolds. & \\ cross-linking followed by & & & High biocompatibility in vivo. & \\ lyophilization, forming a 3-D & & & & \\ porous structured scaffold & & & & \\ \hline Recombinant human type I & & & Osteogenic differentiation. & \\ collagen-nanohydroxapatite- & & & & Similar effects to collagen of animal origin, but without potential hazards. & [55] \\ poly(lactic acid) composite & & & & & \\ \hline Recombinant type I & & & & Genipin crosslinking recombinant type I collagen scaffolds & \\ collagen-based scaffolds, & & & & supported the lowest MSC & \\ obtained by three cross-linking & & & & adhesion. All the cross-linking & [35] \\ procedures, using & & & & methods produced scaffolds that & \\ dehydrothermal, heasamethylene & & & support osteoblast differentiation & \\ diisocyanate or genipin & & & & and mineralization. & \\ \hline Recombinant collagen and & & & Collagen-ELP hydrogels had a & \\ elastin-like polypeptide & & & significantly higher modulus of & \\ (ELP)-based bone regenerative & & & \(35\pm 5\) kPa compared to & \\ hydrogels loaded with & & & collagen-only hydrogels. In vitro & [57] \\ recombinant human bone & & & osteogenic markers, alkaline & \\ morphogenetic & & & & phosphatase and osteoclast, \\ protein-2 (rhBMP-2) & & & & were expressed. \\ \hline Recombinant collagen type I was & & & & \\ functionalized with & & & & \\ photo-cross-linkable & & & & \\ methacrylamide (RCPhC1-MA), & & & & \\ norbornene (RCPhC1-NB), or & & & & \\ thiol (RCPhC1-SH) functionalities & & & & \\ to enable high-resolution 3D & & & & \\ printing via two-photon & & & & \\ polymerization (2 PP) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Outstanding work applying recombinant human collagen in tissue engineering strategies. responsiveness to external stimuli, self-assembly, and adequate mechanical properties of its hydrogels. ELRs provide transient mechanical support and act as a vehicle that introduces a number of specific tissue repair bioactivities. The combination of these properties allows these materials to mimic the rich and complex functionality of the extracellular matrix [60; 61; 62; 63; 64; 65]. With a different approach, amphiphilic systems based on ELR copolymers of order four or higher gave rise, through self-assembly processes, to the formation of physical gels with excellent mechanical properties. These gels have a marked elastic nature, and the gelation temperature can be adjusted to the desired value by controlling the polymer composition. In addition, the gelation time of this type of system is very short, which is very important to avoid dilution of the implant in the biological environment, which would diminish its effectiveness [66].