Mechanical Properties of Artificial Protein Matrices Engineered for Control of Cell and Tissue Behavior

Engineered Protein Hydrogels as Biomimetic Cellular Scaffolds

The biochemical and biophysical properties of the extracellular matrix (ECM) play a pivotal role in regulating cellular behaviors such as proliferation, migration, and differentiation. Engineered protein-based hydrogels, with highly tunable multifunctional properties, have the potential to replicate key features of the native ECM. Formed by self-assembly or crosslinking, engineered protein-based hydrogels can induce a range of cell behaviors through bioactive and functional domains incorporated into the polymer backbone. Using recombinant techniques, the amino acid sequence of the protein backbone can be designed with precise control over the chain-length, folded structure, and cell-interaction sites. In this review, the modular design of engineered protein-based hydrogels from both a molecular- and network-level perspective are discussed, and summarize recent progress and case studies to highlight the diverse strategies used to construct biomimetic scaffolds. This review focuses on amino acid sequences that form structural blocks, bioactive blocks, and stimuli-responsive blocks designed into the protein backbone for highly precise and tunable control of scaffold properties. Both physical and chemical methods to stabilize dynamic protein networks with defined structure and bioactivity for cell culture applications are discussed. Finally, a discussion of future directions of engineered protein-based hydrogels as biomimetic cellular scaffolds is concluded.

Discovery and development of tyrosine-click (Y-click) reaction for the site-selective labelling of proteins

With the versatile utility of bio-conjugated peptides and proteins in the fields of agriculture, food, cosmetics and pharmaceutical industry, the design of smart protocols to conjugate and modulate biomolecules becomes highly desirable. During this process, the most important consideration for biochemists is the retention of configurational integrity of the biomolecules. Moreover, this type of bioconjugation of peptide and protein becomes frivolous if the reaction is not performed with precise amino acid residues. Hence, chemo-selective, as well as site-selective reactions, that are biocompatible and possess an appropriate level of reactivity are necessary. Based on click chemistry, there are so many tyrosine (Y) conjugation strategies, such as sulfur-fluoride exchange (SuFEx), sulfur-triazole exchange (SuTEx), coupling with π-allyl palladium complexes, diazonium salts, diazodicarboxyamide-based reagents etc. Among these techniques, diazodicarboxyamide-based Y-conjugation, which is commonly known as the "tyrosine-click (Y-click) reaction", has met the expectations of synthetic and biochemists for the tyrosine-specific functionalization of biomolecules. Over the past one and a half decades, significant progress has been made in the classical organic synthesis approach, as well as its biochemical, photochemical, and electrochemical variants. Despite such progress and increasing importance, the Y-click reaction has not been reviewed to document variations in its methodology, applications, and broad utility. The present article aims to provide a summary of the approaches for the modulation of biomolecules at the hotspot of tyrosine residue by employing the Y-click reaction. The article also highlights its application for the mapping of proteins, imaging of living cells, and in the fields of analytical and medicinal chemistry.

Biomaterials via peptide assembly: Design, characterization, and application in tissue engineering

A core challenge in biomaterials, with both fundamental significance and technological relevance, concerns the rational design of bioactive microenvironments. Designed properly, peptides can undergo supramolecular assembly into dynamic, physical hydrogels that mimic the mechanical, topological, and biochemical features of native tissue microenvironments. The relatively facile, inexpensive, and automatable preparation of peptides, coupled with low batch-to-batch variability, motivates the expanded use of assembling peptide hydrogels for biomedical applications. Integral to realizing dynamic peptide assemblies as functional biomaterials for tissue engineering is an understanding of the molecular and macroscopic features that govern assembly, morphology, and biological interactions. In this review, we first discuss the design of assembling peptides, including primary structure (sequence), secondary structure (e.g., α-helix and β-sheets), and molecular interactions that facilitate assembly into multiscale materials with desired properties. Next, we describe characterization tools for elucidating molecular structure and interactions, morphology, bulk properties, and biological functionality. Understanding of these characterization methods enables researchers to access a variety of approaches in this ever-expanding field. Finally, we discuss the biological properties and applications of peptide-based biomaterials for engineering several important tissues. By connecting molecular features and mechanisms of assembling peptides to the material and biological properties, we aim to guide the design and characterization of peptide-based biomaterials for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: Engineering peptide-based biomaterials that mimic the topological and mechanical properties of natural extracellular matrices provide excellent opportunities to direct cell behavior for regenerative medicine and tissue engineering. Here we review the molecular-scale features of assembling peptides that result in biomaterials that exhibit a variety of relevant extracellular matrix-mimetic properties and promote beneficial cell-biomaterial interactions. Aiming to inspire and guide researchers approaching this challenge from both the peptide biomaterial design and tissue engineering perspectives, we also present characterization tools for understanding the connection between peptide structure and properties and highlight the use of peptide-based biomaterials in neural, orthopedic, cardiac, muscular, and immune engineering applications.

Designed protein- and peptide-based hydrogels for biomedical sciences

Proteins are fundamentally the most important macromolecules for biochemical, mechanical, and structural functions in living organisms. Therefore, they provide us with diverse structural building blocks for constructing various types of biomaterials, including an important class of such materials, hydrogels. Since natural peptides and proteins are biocompatible and biodegradable, they have features advantageous for their use as the building blocks of hydrogels for biomedical applications. They display constitutional and mechanical similarities with the native extracellular matrix (ECM), and can be easily bio-functionalized via genetic and chemical engineering with features such as bio-recognition, specific stimulus-reactivity, and controlled degradation. This review aims to give an overview of hydrogels made up of recombinant proteins or synthetic peptides as the structural elements building the polymer network. A wide variety of hydrogels composed of protein or peptide building blocks with different origins and compositions - including β-hairpin peptides, α-helical coiled coil peptides, elastin-like peptides, silk fibroin, and resilin - have been designed to date. In this review, the structures and characteristics of these natural proteins and peptides, with each of their gelation mechanisms, and the physical, chemical, and mechanical properties as well as biocompatibility of the resulting hydrogels are described. In addition, this review discusses the potential of using protein- or peptide-based hydrogels in the field of biomedical sciences, especially tissue engineering.

pH-Sensitive Mechanical Properties of Elastin-Based Hydrogels

Ionizable amino acids in protein-based hydrogels can confer pH-responsive behavior. Because elastin-like polypeptides (ELPs) have an established sequence and can crosslink to form hydrogels, they are an ideal system for creating pH-sensitive materials. This study examines different parameters that might affect pH-sensitive behavior and characterizes the mechanical and physical properties between pH 3 and 11 of three ELP-based crosslinked hydrogels. The first finding is that varying the amount of crosslinker affects the overall stiffness and resilience of the hydrogels but does not strongly affect water content, swelling ratio, or pH sensitivity. Second, the choice of two popular tag sequences, which vary in histidine and aspartic acid content, does not have a strong effect on pH-sensitive properties. Last, selectively blocking lysine and tyrosine residues through acetylation significantly decreases the pH-sensitive zeta potential. Acetylated hydrogels also demonstrate different behavior at low pH values with reduced swelling, reduced water content, and higher stiffness. Overall, this work demonstrates that ELP hydrogels with ionizable groups are promising materials for environmentally-responsive applications such as drug delivery, tissue engineering, and microfluidics.

Elastin-like recombinamers in collagen-based tubular gels improve cell-mediated remodeling and viscoelastic properties

The partial substitution of collagen with elastin-like recombinamers in tubular gels improves cell-mediated remodeling, elastic moduli and strength during maturation.

Emerging Trends in Information-Driven Engineering of Complex Biological Systems

Synthetic biological systems are used for a myriad of applications, including tissue engineered constructs for in vivo use and microengineered devices for in vitro testing. Recent advances in engineering complex biological systems have been fueled by opportunities arising from the combination of bioinspired materials with biological and computational tools. Driven by the availability of large datasets in the "omics" era of biology, the design of the next generation of tissue equivalents will have to integrate information from single-cell behavior to whole organ architecture. Herein, recent trends in combining multiscale processes to enable the design of the next generation of biomaterials are discussed. Any successful microprocessing pipeline must be able to integrate hierarchical sets of information to capture key aspects of functional tissue equivalents. Micro- and biofabrication techniques that facilitate hierarchical control as well as emerging polymer candidates used in these technologies are also reviewed.

Protein-Engineered Functional Materials

Proteins are versatile macromolecules that can perform a variety of functions. In the past three decades, they have been commonly used as building blocks to generate a range of biomaterials. Owing to their flexibility, proteins can either be used alone or in combination with other functional molecules. Advances in synthetic and chemical biology have enabled new protein fusions as well as the integration of new functional groups leading to biomaterials with emergent properties. This review discusses protein-engineered materials from the perspectives of domain-based designs as well as physical and chemical approaches for crosslinked materials, with special emphasis on the creation of hydrogels. Engineered proteins that organize or template metal ions, bear noncanonical amino acids (NCAAs), and their potential applications, are also reviewed.

An elastin-like recombinamer-based bioactive hydrogel embedded with mesenchymal stromal cells as an injectable scaffold for osteochondral repair

The aim of this study was to evaluate injectable, in situ cross-linkable elastin-like recombinamers (ELRs) for osteochondral repair. Both the ELR-based hydrogel alone and the ELR-based hydrogel embedded with rabbit mesenchymal stromal cells (rMSCs) were tested for the regeneration of critical subchondral defects in 10 New Zealand rabbits. Thus, cylindrical osteochondral defects were filled with an aqueous solution of ELRs and the animals sacrificed at 4 months for histological and gross evaluation of features of biomaterial performance, including integration, cellular infiltration, surrounding matrix quality and the new matrix in the defects. Although both approaches helped cartilage regeneration, the results suggest that the specific composition of the rMSC-containing hydrogel permitted adequate bone regeneration, whereas the ELR-based hydrogel alone led to an excellent regeneration of hyaline cartilage. In conclusion, the ELR cross-linker solution can be easily delivered and forms a stable well-integrated hydrogel that supports infiltration and de novo matrix synthesis.

Bioactive scaffolds based on elastin-like materials for wound healing

Wound healing is a complex process that, in healthy tissues, starts immediately after the injury. Even though it is a natural well-orchestrated process, large trauma wounds, or injuries caused by acids or other chemicals, usually produce a non-elastic deformed tissue that not only have biological reduced properties but a clear aesthetic effect. One of the main drawbacks of the scaffolds used for wound dressing is the lack of elasticity, driving to non-elastic and contracted tissues. In the last decades, elastin based materials have gained in importance as biomaterials for tissue engineering applications due to their good cyto- and bio-compatibility, their ease handling and design, production and modification. Synthetic elastin or elastin like-peptides (ELPs) are the two main families of biomaterials that try to mimic the outstanding properties of natural elastin, elasticity amongst others; although there are no in vivo studies that clearly support that these two families of elastin based materials improve the elasticity of the artificial scaffolds and of the regenerated skin. Within the next pages a review of the different forms (coacervates, fibres, hydrogels and biofunctionalized surfaces) in which these two families of biomaterials can be processed to be applied in the wound healing field have been done. Here, we explore the mechanical and biological properties of these scaffolds as well as the different in vivo approaches in which these scaffolds have been used.

Tyrosine-Selective Functionalization for Bio-Orthogonal Cross-Linking of Engineered Protein Hydrogels

Engineered protein hydrogels have shown promise as artificial extracellular matrix materials for the 3D culture of stem cells due to the ability to decouple hydrogel biochemistry and mechanics. The modular design of these proteins allows for incorporation of various bioactive sequences to regulate cellular behavior. However, the chemistry used to cross-link the proteins into hydrogels can limit what bioactive sequences can be incorporated, in order to prevent nonspecific cross-linking within the bioactive region. Bio-orthogonal cross-linking chemistries may allow for the incorporation of any arbitrary bioactive sequence, but site-selective and scalable incorporation of bio-orthogonal reactive groups such as azides that do not rely on commonly used amine-reactive chemistry is often challenging. In response, we have optimized the reaction of an azide-bearing 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) with engineered elastin-like proteins (ELPs) to selectively azide-functionalize tyrosine residues within the proteins. The PTAD-azide functionalized ELPs cross-link with bicyclononyne (BCN) functionalized ELPs via the strain-promoted azide-alkyne cycloaddition (SPAAC) reaction to form hydrogels. Human mesenchymal stem cells and murine neural progenitor cells encapsulated within these hydrogels remain highly viable and maintain their phenotypes in culture. Tyrosine-specific modification may expand the number of bioactive sequences that can be designed into protein-engineered materials by permitting incorporation of lysine-containing sequences without concern for nonspecific cross-linking.

Fabricated Elastin

The mechanical stability, elasticity, inherent bioactivity, and self-assembly properties of elastin make it a highly attractive candidate for the fabrication of versatile biomaterials. The ability to engineer specific peptide sequences derived from elastin allows the precise control of these physicochemical and organizational characteristics, and further broadens the diversity of elastin-based applications. Elastin and elastin-like peptides can also be modified or blended with other natural or synthetic moieties, including peptides, proteins, polysaccharides, and polymers, to augment existing capabilities or confer additional architectural and biofunctional features to compositionally pure materials. Elastin and elastin-based composites have been subjected to diverse fabrication processes, including heating, electrospinning, wet spinning, solvent casting, freeze-drying, and cross-linking, for the manufacture of particles, fibers, gels, tubes, sheets and films. The resulting materials can be tailored to possess specific strength, elasticity, morphology, topography, porosity, wettability, surface charge, and bioactivity. This extraordinary tunability of elastin-based constructs enables their use in a range of biomedical and tissue engineering applications such as targeted drug delivery, cell encapsulation, vascular repair, nerve regeneration, wound healing, and dermal, cartilage, bone, and dental replacement.

Biomolecular functionalization for enhanced cell-material interactions of poly(methyl methacrylate) surfaces

The integration of implants or medical devices into the body tissues requires of good cell-material interactions. However, most polymeric materials used for these applications lack on biological cues, which enhanced mid- and long-term implant failure due to weak integration with the surrounding tissue. Commonly used strategies for tissue-material integration focus on functionalization of the material surface by means of natural proteins or short peptides. However, the use of these biomolecules involves major drawbacks such as immunogenic problems and oversimplification of the constructs. Here, designed elastin-like recombinamers (ELRs) are used to enhance poly(methyl methacrylate) surface properties and compared against the use of short peptides. In this study, cell response has been analysed for different functionalization conditions in the presence and absence of a competing protein, which interferes on surface-cell interaction by unspecific adsorption on the interface. The study has shown that ELRs can induce higher rates of cell attachment and stronger cell anchorages than short peptides, being a better choice for surface functionalization.

Solvent-free liquid crystals and liquids based on genetically engineered supercharged polypeptides with high elasticity

A series of solvent-free elastin-like polypeptide liquid crystals and liquids are developed by electrostatic complexation of supercharged elastin-like polypeptides with surfactants. The smectic mesophases exhibit a high elasticity and the values can be easily tuned by varying the alkyl chain lengths of the surfactants or the lengths of the elastin-like polypeptides.

Microfluidic gradients reveal enhanced neurite outgrowth but impaired guidance within 3D matrices with high integrin ligand densities

The density of integrin-binding ligands in an extracellular matrix (ECM) is known to regulate cell migration speed by imposing a balance of traction forces between the leading and trailing edges of the cell, but the effect of cell-adhesive ligands on neurite chemoattraction is not well understood. A platform is presented here that combines gradient-generating microfluidic devices with 3D protein-engineered hydrogels to study the effect of RGD ligand density on neurite pathfinding from chick dorsal root ganglia-derived spheroids. Spheroids are encapsulated in elastin-like polypeptide (ELP) hydrogels presenting either 3.2 or 1.6 mM RGD ligands and exposed to a microfluidic gradient of nerve growth factor (NGF). While the higher ligand density matrix enhanced neurite initiation and persistence of neurite outgrowth, the lower ligand density matrix significantly improved neurite pathfinding and increased the frequency of growth cone turning up the NGF gradient. The apparent trade-off between neurite extension and neurite guidance is reminiscent of the well-known trade-off between adhesive forces at the leading and trailing edges of a migrating cell, implying that a similar matrix-mediated balance of forces regulates neurite elongation and growth cone turning. These results have implications in the design of engineered materials for in vitro models of neural tissue and in vivo nerve guidance channels.

Human epidermal keratinocyte cell response on integrin-specific artificial extracellular matrix proteins

Cell-matrix interactions play critical roles in regulating cellular behavior in wound repair and regeneration of the human skin. In particular, human skin keratinocytes express several key integrins such as alpha5beta1, alpha3beta1, and alpha2beta1 for binding to the extracellular matrix (ECM) present in the basement membrane in uninjured skin. To mimic these key integrin-ECM interactions, artificial ECM (aECM) proteins containing functional domains derived from laminin 5, type IV collagen, fibronectin, and elastin are prepared. Human skin keratinocyte cell responses on the aECM proteins are specific to the cell-binding domain present in each construct. Keratinocyte attachment to the aECM protein substrates is also mediated by specific integrin-material interactions. In addition, the aECM proteins are able to support the proliferation of keratinocyte stem cells, demonstrating their promise for use in skin tissue engineering.

NANO AND MICRO-STRUCTURES OF ELASTIN-LIKE POLYPEPTIDE-BASED MATERIALS AND THEIR APPLICATIONS: RECENT DEVELOPMENTS

Elastin-like polypeptide (ELP) containing materials have spurred significant research interest for biomedical applications exploiting their biocompatible, biodegradable and nonimmunogenic nature while maintaining precise control over their chemical structure and functionality through genetic engineering. Physical, mechanical and biological properties of ELPs could be further manipulated using genetic engineering or through conjugation with a variety of chemical moieties. These chemical and physical modifications also achieve interesting micro- and nanostructured ELP-based materials. Here, we review the recent developments during the past decade in the methods to engineer elastin-like materials, available genetic and chemical modification methods and applications of ELP micro and nanostructures in tissue engineering and drug delivery.

Elastomeric Recombinant Protein-based Biomaterials

Elastomeric protein-based biomaterials, produced from elastin derivatives, are widely investigated as promising tissue engineering scaffolds due to their remarkable properties including substantial extensibility, long-term stability, self-assembly, high resilience upon stretching, low energy loss, and excellent biological activity. These elastomers are processed from different sources of soluble elastin such as animal-derived soluble elastin, recombinant human tropoelastin, and elastin-like polypeptides into various forms including three dimensional (3D) porous hydrogels, elastomeric films, and fibrous electrospun scaffolds. Elastin-based biomaterials have shown great potential for the engineering of elastic tissues such as skin, lung and vasculature. In this review, the synthesis and properties of various elastin-based elastomers with their applications in tissue engineering are described.

Enhanced cell-material interactions through the biofunctionalization of polymeric surfaces with engineered peptides

Research on surface modification of polymeric materials to guide the cellular activity in biomaterials designed for tissue engineering applications has mostly focused on the use of natural extracellular matrix (ECM) proteins and short peptides, such as RGD. However, the use of engineered proteins can gather the advantages of these strategies and avoid the main drawbacks. In this study, recombinant engineered proteins called elastin-like recombinamers (ELRs) have been used to functionalize poly(lactic) acid (PLA) model surfaces. The structure of the ELRs has been designed to include the integrin ligand RGDS and the cross-linking module VPGKG. Surface functionalization has been characterized and optimized by means of ELISA and atomic force microscopy (AFM). The results suggest that ELR functionalization creates a nonfouling canvas able to restrict unspecific adsorption of proteins. Moreover, AFM analysis reveals the conformation and disposition of ELRs on the surface. Biological performance of PLA surfaces functionalized with ELRs has been studied and compared with the use of short peptides. Cell response has been assessed for different functionalization conditions in the presence and absence of the bovine serum albumin (BSA) protein, which could interfere with the surface-cell interaction by adsorbing on the interface. Studies have shown that ELRs are able to elicit higher rates of cell attachment, stronger cell anchorages and faster levels of proliferation than peptides. This work has demonstrated that the use of engineered proteins is a more efficient strategy to guide the cellular activity than the use of short peptides, because they not only allow for better cell attachment and proliferation, but also can provide more complex properties such as the creation of nonfouling surfaces.

Self-assembly of elastin-mimetic double hydrophobic polypeptides

We have constructed a novel class of "double-hydrophobic" block polypeptides based on the hydrophobic domains found in native elastin, an extracellular matrix protein responsible for the elasticity and resilience of tissues. The block polypeptides comprise proline-rich poly(VPGXG) and glycine-rich poly(VGGVG), both of which dehydrate at higher temperature but form distinct secondary structures, β-turn and β-sheet respectively. In water at 45 °C, the block polypeptides initially assemble into nanoparticles rich in β-turn structures, which further connect into long (>10 μm), beaded nanofibers along with the increase in the β-sheet content. The nanofibers obtained are well-dispersed in water, and show thermoresponsive properties. Polypeptides comprising each block component assemble into different morphologies, showing that the conjugation of poly(VPGXG) and poly(VGGVG) plays a role for beaded fiber formation. These results may provide innovative ideas for designing peptide-based materials but also opportunities for developing novel materials useful for tissue engineering and drug delivery systems.

Design of three-dimensional engineered protein hydrogels for tailored control of neurite growth

The design of bioactive materials allows tailored studies probing cell-biomaterial interactions, however, relatively few studies have examined the effects of ligand density and material stiffness on neurite growth in three-dimensions. Elastin-like proteins (ELPs) have been designed with modular bioactive and structural regions to enable the systematic characterization of design parameters within three-dimensional (3-D) materials. To promote neurite out-growth and better understand the effects of common biomaterial design parameters on neuronal cultures we here focused on the cell-adhesive ligand density and hydrogel stiffness as design variables for ELP hydrogels. With the inherent design freedom of engineered proteins these 3-D ELP hydrogels enabled decoupled investigations into the effects of biomechanics and biochemistry on neurite out-growth from dorsal root ganglia. Increasing the cell-adhesive RGD ligand density from 0 to 1.9×10(7)ligands μm(-3) led to a significant increase in the rate, length, and density of neurite out-growth, as quantified by a high throughput algorithm developed for dense neurite analysis. An approximately two-fold improvement in total neurite out-growth was observed in materials with the higher ligand density at all time points up to 7 days. ELP hydrogels with initial elastic moduli of 0.5, 1.5, or 2.1kPa and identical RGD ligand densities revealed that the most compliant materials led to the greatest out-growth, with some neurites extending over 1800μm by day 7. Given the ability of ELP hydrogels to efficiently promote neurite out-growth within defined and tunable 3-D microenvironments these materials may be useful in developing therapeutic nerve guides and the further study of basic neuron-biomaterial interactions.

New directions in thermoresponsive polymers

Interest in thermoresponsive polymers has steadily grown over many decades, and a great deal of work has been dedicated to developing temperature sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of previously reported temperature-responsive polymers are based on poly(N-isopropylacrylamide) (PNIPAM), despite the fact that a wide range of other thermoresponsive polymers have demonstrated similar promise for the preparation of adaptive materials. Herein, we aim to highlight recent results that involve thermoresponsive systems that have not yet been as fully considered. Many of these (co)polymers represent clear opportunities for advancements in emerging biomedical and materials fields due to their increased biocompatibility and tuneable response. By highlighting recent examples of newly developed thermoresponsive polymer systems, we hope to promote the development of new generations of smart materials.

Tetrakis(hydroxymethyl) phosphonium chloride as a covalent cross-linking agent for cell encapsulation within protein-based hydrogels

Native tissues provide cells with complex, three-dimensional (3D) environments comprised of hydrated networks of extracellular matrix proteins and sugars. By mimicking the dimensionality of native tissue while deconstructing the effects of environmental parameters, protein-based hydrogels serve as attractive, in vitro platforms to investigate cell-matrix interactions. For cell encapsulation, the process of hydrogel formation through physical or covalent cross-linking must be mild and cell compatible. While many chemical cross-linkers are commercially available for hydrogel formation, only a subset are cytocompatible; therefore, the identification of new and reliable cytocompatible cross-linkers allows for greater flexibility of hydrogel design for cell encapsulation applications. Here, we introduce tetrakis(hydroxymethyl) phosphonium chloride (THPC) as an inexpensive, amine-reactive, aqueous cross-linker for 3D cell encapsulation in protein-based hydrogels. We characterize the THPC-amine reaction by demonstrating THPC's ability to react with primary and secondary amines of various amino acids. In addition, we demonstrate the utility of THPC to tune hydrogel gelation time (6.7±0.2 to 27±1.2 min) and mechanical properties (storage moduli ∼250 Pa to ∼2200 Pa) with a recombinant elastin-like protein. Lastly, we show cytocompatibility of THPC for cell encapsulation with two cell types, embryonic stem cells and neuronal cells, where cells exhibited the ability to differentiate and grow in elastin-like protein hydrogels. The primary goal of this communication is to report the identification and utility of tetrakis(hydroxymethyl) phosphonium chloride (THPC) as an inexpensive but widely applicable cross-linker for protein-based materials.

Photoreactive elastin-like proteins for use as versatile bioactive materials and surface coatings

Photocrosslinkable, protein-engineered biomaterials combine a rapid, controllable, cytocompatible crosslinking method with a modular design strategy to create a new family of bioactive materials. These materials have a wide range of biomedical applications, including the development of bioactive implant coatings, drug delivery vehicles, and tissue engineering scaffolds. We present the successful functionalization of a bioactive elastin-like protein with photoreactive diazirine moieties. Scalable synthesis is achieved using a standard recombinant protein expression host followed by site-specific modification of lysine residues with a heterobifunctional N-hydroxysuccinimide ester-diazirine crosslinker. The resulting biomaterial is demonstrated to be processable by spin coating, drop casting, soft lithographic patterning, and mold casting to fabricate a variety of two- and three-dimensional photocrosslinked biomaterials with length scales spanning the nanometer to millimeter range. Protein thin films proved to be highly stable over a three-week period. Cell-adhesive functional domains incorporated into the engineered protein materials were shown to remain active post-photo-processing. Human adipose-derived stem cells achieved faster rates of cell adhesion and larger spread areas on thin films of the engineered protein compared to control substrates. The ease and scalability of material production, processing versatility, and modular bioactive functionality make this recombinantly engineered protein an ideal candidate for the development of novel biomaterial coatings, films, and scaffolds.

Multifunctional materials through modular protein engineering

The diversity of potential applications for protein-engineered materials has undergone profound recent expansion through a rapid increase in the library of domains that have been utilized in these materials. Historically, protein-engineered biomaterials have been generated from a handful of peptides that were selected and exploited for their naturally evolved functionalities. In recent years, the scope of the field has drastically expanded to include peptide domains that were designed through computational modeling, identified through high-throughput screening, or repurposed from wild type domains to perform functions distinct from their primary native applications. The strategy of exploiting a diverse library of peptide domains to design modular block copolymers enables the synthesis of multifunctional protein-engineered materials with a range of customizable properties and activities. As the diversity of peptide domains utilized in modular protein engineering continues to expand, a tremendous and ever-growing combinatorial expanse of material functionalities will result.

Building stem cell niches from the molecule up through engineered peptide materials

The native stem cell niche is a dynamic and complex microenvironment. Recapitulating this niche is a critical focus within the fields of stem cell biology, tissue engineering, and regenerative medicine and requires the development of well-defined, tunable materials. Recent biomaterial design strategies seek to create engineered matrices that interact with cells at the molecular scale and allow on-demand, cell-triggered matrix modifications. Peptide and protein engineering can accomplish these goals through the molecular-level design of bioinductive and bioresponsive materials. This brief review focuses on engineered peptide and protein materials suitable for use as in vitro neural stem cell niche mimics and in vivo central nervous system repair. A key hallmark of these materials is the immense design freedom to specify the exact amino acid sequence leading to multi-functional bulk materials with tunable properties. These advanced materials are engineered using rational design strategies to recapitulate key aspects of the native neural stem cell niche. The resulting materials often combine the advantages of biological matrices with the engineering control of synthetic polymers. Future design strategies are expected to endow these materials with multiple layers of bi-directional feedback between the cell and the matrix, which will lead to more advanced mimics of the highly dynamic neural stem cell niche.

Elastomeric polypeptides

Elastomeric polypeptides are very interesting biopolymers and are characterized by rubber-like elasticity, large extensibility before rupture, reversible deformation without loss of energy, and high resilience upon stretching. Their useful properties have motivated their use in a wide variety of materials and biological applications. This chapter focuses on elastin and resilin - two elastomeric biopolymers - and the recombinant polypeptides derived from them (elastin-like polypeptides and resilin-like polypeptides). This chapter also discusses the applications of these recombinant polypeptides in the fields of purification, drug delivery, and tissue engineering.

Mechanical property characterization of electrospun recombinant human tropoelastin for vascular graft biomaterials

The development of vascular grafts has focused on finding a biomaterial that is non-thrombogenic, minimizes intimal hyperplasia, matches the mechanical properties of native vessels and allows for regeneration of arterial tissue. In this study, the structural and mechanical properties and the vascular cell compatibility of electrospun recombinant human tropoelastin (rTE) were evaluated as a potential vascular graft support matrix. Disuccinimidyl suberate (DSS) was used to cross-link electrospun rTE fibers to produce a polymeric recombinant tropoelastin (prTE) matrix that is stable in aqueous environments. Tubular 1cm diameter prTE samples were constructed for uniaxial tensile testing and 4mm small-diameter prTE tubular scaffolds were produced for burst pressure and cell compatibility evaluations from 15 wt.% rTE solutions. Uniaxial tensile tests demonstrated an average ultimate tensile strength (UTS) of 0.36±0.05 MPa and elastic moduli of 0.15±0.04 and 0.91±0.16 MPa, which were comparable to extracted native elastin. Burst pressures of 485±25 mm Hg were obtained from 4mm internal diameter scaffolds with 453±74 μm average wall thickness. prTE supported endothelial cell growth with typical endothelial cell cobblestone morphology after 48 h in culture. Cross-linked electrospun rTE has promising properties for utilization as a vascular graft biomaterial with customizable dimensions, a compliant matrix and vascular cell compatibility.

Enzymatic cross-linking of human recombinant elastin (HELP) as biomimetic approach in vascular tissue engineering

The use of polymers naturally occurring in the extracellular matrix (ECM) is a promising strategy in regenerative medicine. If compared to natural ECM proteins, proteins obtained by recombinant DNA technology have intrinsic advantages including reproducible macromolecular composition, sequence and molecular mass, and overcoming the potential pathogens transmission related to polymers of animal origin. Among ECM-mimicking materials, the family of recombinant elastin-like polymers is proposed for drug delivery applications and for the repair of damaged elastic tissues. This work aims to evaluate the potentiality of a recombinant human elastin-like polypeptide (HELP) as a base material of cross-linked matrices for regenerative medicine. The cross-linking of HELP was accomplished by the insertion of cross-linking sites, glutamine and lysine, in the recombinant polymer and generating ε-(γ-glutamyl) lysine links through the enzyme transglutaminase. The cross-linking efficacy was estimated by infrared spectroscopy. Freeze-dried cross-linked matrices showed swelling ratios in deionized water (≈2500%) with good structural stability up to 24 h. Mechanical compression tests, performed at 37°C in wet conditions, in a frequency sweep mode, indicated a storage modulus of 2/3 kPa, with no significant changes when increasing number of cycles or frequency. These results demonstrate the possibility to obtain mechanically resistant hydrogels via enzymatic crosslinking of HELP. Cytotoxicity tests of cross-linked HELP were performed with human umbilical vein endothelial cells, by use of transwell filter chambers for 1-7 days, or with its extracts in the opportune culture medium for 24 h. In both cases no cytotoxic effects were observed in comparison with the control cultures. On the whole, the results suggest the potentiality of this genetically engineered HELP for regenerative medicine applications, particularly for vascular tissue regeneration.

Emerging applications of multifunctional elastin-like recombinamers

Elastin-like recombinamers have grown in popularity in the field of protein-inspired biomimetic materials and have found widespread use in biomedical applications. Modern genetic-engineering techniques have allowed the design of multifunctional materials with an extraordinary control over their architecture and physicochemical properties, such as stimuli-responsiveness, monodispersity, biocompatibility or self-assembly, amongst others. Indeed, these materials are playing an increasingly important role in a diverse range of applications, such as drug delivery, tissue engineering and 'smart' systems. Herein, we review some of the most interesting examples of recent advances and progressive applications of elastin-like recombinamers in biomaterial and nano-engineering sciences in recent years.

Multivalent protein polymers with controlled chemical and physical properties

In this review, we describe our work on the design, characterization, and modification of a series of alanine-rich helical polypeptides with novel functions. Glycosylation of the polypeptides has permitted investigation of polymer architecture effects on multivalent interactions. One of the members of this polypeptide family exhibits polymorphological behavior that is easily manipulated via simple changes in solution pH and temperature. Polypeptide-based fibrils formed at acidic pH and high temperature were shown to direct the one-dimensional organization of gold nanoparticles via electrostatic interactions. As a precursor to fibrils, aggregates likely comprising alanine-rich cores form at low temperatures and acidic pH and reversibly dissociate into monomers upon deprotonation. PEGylation of these polypeptides does not alter the self-association or conformational behavior of the polypeptide, suggesting potential applications in the development of assembled delivery vehicles, as modification of the polypeptides should be a useful strategy for controlling assembly.

Applications of elastin-like polypeptides in tissue engineering

Elastin-like polypeptides (ELPs) have found utility in tissue engineering applications, not only because they are biocompatible, biodegradable, and non-immunogenic, but also because their amino acid sequence and molecular weight can be precisely controlled at the genetic or synthetic level, affording exquisite control over final protein functionality. This review presents a basic overview of ELP properties and modifications that are relevant to tissue engineering, as well as a discussion of the application of ELPs to cartilage, intervertebral disc, vascular graft, liver, ocular, and cell sheet engineering.

Boundary crossing in epithelial wound healing

The processes of wound healing and collective cell migration have been studied for decades. Intensive research has been devoted to understanding the mechanisms involved in wound healing, but the role of cell-substrate interactions is still not thoroughly understood. Here we probe the role of cell-substrate interactions by examining in vitro the healing of monolayers of human corneal epithelial (HCE) cells cultured on artificial extracellular matrix (aECM) proteins. We find that the rate of wound healing is dependent on the concentration of fibronectin-derived (RGD) cell-adhesion ligands in the aECM substrate. The wound closure rate varies nearly sixfold on the substrates examined, despite the fact that the rates of migration and proliferation of individual cells show little sensitivity to the RGD concentration (which varies 40-fold). To explain this apparent contradiction, we study collective migration by means of a dynamic Monte Carlo simulation. The cells in the simulation spread, retract, and proliferate with probabilities obtained from a simple phenomenological model. The results indicate that the overall wound closure rate is determined primarily by the rate at which cells cross the boundary between the aECM protein and the matrix deposited under the cell sheet.

Elastomeric polypeptide-based biomaterials

Elastomeric proteins are characterized by their large extensibility before rupture, reversible deformation without loss of energy, and high resilience upon stretching. Motivated by their unique mechanical properties, there has been tremendous research in understanding and manipulating elastomeric polypeptides, with most work conducted on the elastins but more recent work on an expanded set of polypeptide elastomers. Facilitated by biosynthetic strategies, it has been possible to manipulate the physical properties, conformation, and mechanical properties of these materials. Detailed understanding of the roles and organization of the natural structural proteins has permitted the design of elastomeric materials with engineered properties, and has thus expanded the scope of applications from elucidation of the mechanisms of elasticity to the development of advanced drug delivery systems and tissue engineering substrates.

On the biomechanical function of scaffolds for engineering load-bearing soft tissues

Replacement or regeneration of load-bearing soft tissues has long been the impetus for the development of bioactive materials. While maturing, current efforts continue to be confounded by our lack of understanding of the intricate multi-scale hierarchical arrangements and interactions typically found in native tissues. The current state of the art in biomaterial processing enables a degree of controllable microstructure that can be used for the development of model systems to deduce fundamental biological implications of matrix morphologies on cell function. Furthermore, the development of computational frameworks which allow for the simulation of experimentally derived observations represents a positive departure from what has mostly been an empirically driven field, enabling a deeper understanding of the highly complex biological mechanisms we wish to ultimately emulate. Ongoing research is actively pursuing new materials and processing methods to control material structure down to the micro-scale to sustain or improve cell viability, guide tissue growth, and provide mechanical integrity, all while exhibiting the capacity to degrade in a controlled manner. The purpose of this review is not to focus solely on material processing but to assess the ability of these techniques to produce mechanically sound tissue surrogates, highlight the unique structural characteristics produced in these materials, and discuss how this translates to distinct macroscopic biomechanical behaviors.