ature uses self-assembly to create a widespread variety of complex structures with elaborate geometries and outstanding properties 1 such as hierarchical order, adaptability, selfhealing and bioactivity. Developing new bioinspired processes based on dynamic self-assembly could facilitate the fabrication of synthetic three-dimensional (3D) materials with enhanced complexity, dynamic properties and functionality 2 . Proteins are particularly attractive building blocks because of their versatility and biofunctionality 3 . Elastin-like polypeptides (ELPs) 4 are recombinant proteins that have generated great interest 5 as a result of their modular structure, bioactivity, ease of design and production, and the possibility to create robust and elastic materials 5,6 . ELPs allow for a tunable molecular design 7 and are based on the tropoelastin recurrent motif Val-Pro-Gly-X-Gly (VPGXG), in which X is any amino acid other than proline 7 . This repeating pentapeptide provides ELPs with a thermoresponsive behaviour. Below a critical transition temperature (T t ), the ELP molecule undergoes a reversible-phase transition wherein the protein is soluble in aqueous solution and becomes highly solvated, surrounded by clatharate-like water structures. Above the T t , the hydrophobic domains dehydrate and the protein chain hydrophobically collapses and aggregates to form a phaseseparated state 8 .The use of natural and synthetic proteins to create functional materials has been hindered by the difficulty in controlling their conformation and nanoscale assembly with the precision required to form macroscopic materials. This limitation has driven the development of simpler and more-predictable peptide-based materials 9,10 . Peptide amphiphiles (PAs), for example, are synthetic molecules that can self-assemble into nanofibres and create functional 3D hydrogels that emulate the fibrous architecture of the extracellular matrix (ECM) 11,12 . Nonetheless, most peptide and/or protein materials are formed through equilibrium-based self-assembly approaches that are capable of generating stable supramolecular structures, but with limited hierarchy and spatiotemporal control, which has hindered their functionality 2 .Novel approaches based on the dynamic self-assembly of inorganic building blocks [13][14][15] , actin self-organization 16 and the combination of top-down processes with peptide self-assembly have been reported recently 17 . In particular, Stupp and co-workers have described a self-assembling membrane system obtained through strong electrostatic interactions between PAs and oppositely charged polysaccharides 18 . However, the possibility to exploit the unique structural and functional properties of proteins to create dynamic hierarchical materials remains an elusive target. In this study, we attempt to overcome this hurdle by using self-assembling peptides to promote protein conformational changes and guide their assembly into complex, yet functional, materials. We report the discovery and development of a protein/peptide system t...

A major goal in materials science is to develop bioinspired functional materials based on the precise control of molecular building blocks across length scales. Here we report a protein-mediated mineralization process that takes advantage of disorder–order interplay using elastin-like recombinamers to program organic–inorganic interactions into hierarchically ordered mineralized structures. The materials comprise elongated apatite nanocrystals that are aligned and organized into microscopic prisms, which grow together into spherulite-like structures hundreds of micrometers in diameter that come together to fill macroscopic areas. The structures can be grown over large uneven surfaces and native tissues as acid-resistant membranes or coatings with tuneable hierarchy, stiffness, and hardness. Our study represents a potential strategy for complex materials design that may open opportunities for hard tissue repair and provide insights into the role of molecular disorder in human physiology and pathology.

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ABSTRACT. Although significant progress has been made in the area of injectable hydrogels for biomedical applications and model cell niches, further improvements are still needed, especially in terms of mechanical performance, stability and biomimicry of the native fibrillar architecture found in the extracellular matrix (ECM). This work focuses on the design and production of a silk-elastin-based injectable multiblock co-recombinamer that spontaneously forms a stable physical nanofibrillar hydrogel under physiological conditions. That differs from previously reported silk-elastin-like polymers on a major content and predominance of the elastin-like part, as well as a more complex structure and behavior of such part of the molecule, which is aimed to obtain well defined hydrogels.Rheological and DSC experiments showed that this system displays a coordinated and concomitant dual gelation mechanism. In a first stage, a rapid, thermally driven gelation of the co-recombinamer solution takes place once the system reaches body temperature due to the thermal responsiveness of the elastinlike (EL) parts and the amphiphilic multiblock design of the co-recombinamer. A bridged micellar 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 structure is the dominant microscopic feature of this stage, as demonstrated by AFM and TEM.Completion of the initial stage triggers the second, which comprises a stabilization, reinforcement, and microstructuring of the gel. FTIR analysis shows that these events involve the formation of β-sheets around the silk motifs. The emergence of such β-sheet structures leads to the spontaneous selforganization of the gel into the final fibrous structure. Despite the absence of biological cues, here we set the basis of the minimal structure that is able to display such a set of physical properties and undergo microscopic transformation from a solution to a fibrous hydrogel. The results point to the potential of this system as a basis for the development of injectable fibrillar biomaterial platforms towards a fully functional, biomimetic, artificial extracellular matrix and cell niches.

Genetic engineering techniques were used to design and biosynthesise an extracellular matrix (ECM) analogue. This was designed with a well-defined molecular architecture comprising different functional domains. The structural base is a elastin-derived repeating unit, which confers an adequate elastic characteristic. Some of these elastin domains have been modified to contain lysine; this amino acid can be used for crosslinking purposes. The polymer also contain periodically spaced fibronectin CS5 domains enclosing the well-known cell attachment sequence REDV. Finally, the polymer has target sequences for proteolitic action. These sequences are those found in the natural elastin and are introduced to help in the bioabsorption of the polymer. In addition, these proteolitic sequences were chosen in a way that, after proteolitic action, the released fragments will be bioactive. These fragments are expected to promote cell proliferation activity, angiogenesis and other bioactivities of interest for tissue growing, repairing and healing. After purification, the resulting polymers proved to be of high purity and correct sequence. Glutaraldehyde has shown to be a cross-linking agent for this polymer, yielding insoluble hydrogel matrices. This work is framed in a long term project aimed to exploit the power of genetic engineering for the design and bioproduction of complex ECM analogues showing the rich complexity and multi (bio)functionality of the natural matrix.

This work explores the dependence of the inverse temperature transition of elastin-like polymers (ELPs) on the amino-acid sequence, i.e., the amino-acid arrangement along the macromolecule and the resulting linear distribution of the physical properties (mainly polarity) derived from it. The hypothesis of this work is that, in addition to mean polarity and molecular mass, the given amino-acid sequence, or its equivalent--the way in which polarity is arranged along the molecule--is also relevant for determining the transition temperature and the latent heat of that transition. To test this hypothesis, a set of linear and di- and triblock ELP copolymers were designed and produced as recombinant proteins. The absolute sequence control provided by recombinant technologies allows the effect of the amino-acid arrangement to be isolated while keeping the molecular mass or mean polarity under strict control. The selected block copolymers were made of two different ELPs: one exhibiting temperature and pH responsiveness, and one exhibiting temperature responsiveness only. By changing the arrangement and length of the blocks while keeping other parameters, such as the molecular mass or mean polarity, constant, we were able to show that the sequence plays a key role in the smart behavior of ELPs.

The pH-responsive elastin-like polymers, [(PGVGV) 2-(PGEGV)-(PGVGV)2]n with n ) 5, 9, 15, 30, 45, were obtained using genetic engineering and microbial protein expression. These were intended to study the effects of the molecular weight (MW) on the properties of their inverse temperature transition (ITT) and its dependence on pH. As a result, the transition temperature decreased and the transition enthalpy increased as the molecular weight increased, especially for the lowest MWs. More strikingly, the apparent pK a for the γ-carboxyl residue of the glutamic acid also depends on MW. The apparent pKa is lower for lower MWs. In summary, the modification in the ITT caused by changes in MW is similar to the one caused by changes in the mean polarity of the polymer as described in the literature. A reduction in the molecular weight is equivalent to a decrease in the mean hydrophobicity of the polymer.

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