Elastomeric polypeptide-based biomaterials

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.

Material matters: exploring the interplay between natural biomaterials and host immune system

Biomaterials are widely used for various medical purposes, for instance, implants, tissue engineering, medical devices, and drug delivery systems. Natural biomaterials can be obtained from proteins, carbohydrates, and cell-specific sources. However, when these biomaterials are introduced into the body, they trigger an immune response which may lead to rejection and failure of the implanted device or tissue. The immune system recognizes natural biomaterials as foreign substances and triggers the activation of several immune cells, for instance, macrophages, dendritic cells, and T cells. These cells release pro-inflammatory cytokines and chemokines, which recruit other immune cells to the implantation site. The activation of the immune system can lead to an inflammatory response, which can be beneficial or detrimental, depending on the type of natural biomaterial and the extent of the immune response. These biomaterials can also influence the immune response by modulating the behavior of immune cells. For example, biomaterials with specific surface properties, such as charge and hydrophobicity, can affect the activation and differentiation of immune cells. Additionally, biomaterials can be engineered to release immunomodulatory factors, such as anti-inflammatory cytokines, to promote a tolerogenic immune response. In conclusion, the interaction between biomaterials and the body's immune system is an intricate procedure with potential consequences for the effectiveness of therapeutics and medical devices. A better understanding of this interplay can help to design biomaterials that promote favorable immune responses and minimize adverse reactions.

Identification and characterization of proteins that form the inner core Ixodes scapularis tick attachment cement layer

Ixodes scapularis long-term blood feeding behavior is facilitated by a tick secreted bio adhesive (tick cement) that attaches tick mouthparts to skin tissue and prevents the host from dislodging the attached tick. Understanding tick cement formation is highly sought after as its disruption will prevent tick feeding. This study describes proteins that form the inner core layer of I. scapularis tick cement as disrupting these proteins will likely stop formation of the outer cortical layer. The inner core cement layer completes formation by 24 h of tick attachment. Thus, we used laser-capture microdissection to isolate cement from cryosections of 6 h and 24 h tick attachment sites and to distinguish between early and late inner core cement proteins. LC-MS/MS analysis identified 138 tick cement proteins (TCPs) of which 37 and 35 were unique in cement of 6 and 24 h attached ticks respectively. We grouped TCPs in 14 functional categories: cuticular protein (16%), tick specific proteins of unknown function, cytoskeletal proteins, and enzymes (13% each), enzymes (10%), antioxidant, glycine rich, scaffolding, heat shock, histone, histamine binding, proteases and protease inhibitors, and miscellaneous (3-6% each). Gene ontology analysis confirm that TCPs are enriched for bio adhesive properties. Our data offer insights into tick cement bonding patterns and set the foundation for understanding the molecular basis of I. scapularis tick cement formation.

Genetically Engineered Viral Vectors and Organic-Based Non-Viral Nanocarriers for Drug Delivery Applications

Conventional drug delivery systems are challenged by concerns related to systemic toxicity, repetitive doses, drug concentrations fluctuation, and adverse effects. Various drug delivery systems are developed to overcome these limitations. Nanomaterials are employed in a variety of biomedical applications such as therapeutics delivery, cancer therapy, and tissue engineering. Physiochemical nanoparticle assembly techniques involve the application of solvents and potentially harmful chemicals, commonly at high temperatures. Genetically engineered organisms have the potential to be used as promising candidates for greener, efficient, and more adaptable platforms for the synthesis and assembly of nanomaterials. Genetically engineered carriers are precisely designed and constructed in shape and size, enabling precise control over drug attachment sites. The high accuracy of these novel advanced materials, biocompatibility, and stimuli-responsiveness, elucidate their emerging application in controlled drug delivery. The current article represents the research progress in developing various genetically engineered carriers. Organic-based nanoparticles including cellulose, collagen, silk-like polymers, elastin-like protein, silk-elastin-like protein, and inorganic-based nanoparticles are discussed in detail. Afterward, viral-based carriers are classified, and their potential for targeted therapeutics delivery is highlighted. Finally, the challenges and prospects of these delivery systems are concluded.

Elastomer-Hydrogel Systems: From Bio-Inspired Interfaces to Medical Applications

Novel advanced biomaterials have recently gained great attention, especially in minimally invasive surgical techniques. By applying sophisticated design and engineering methods, various elastomer-hydrogel systems (EHS) with outstanding performance have been developed in the last decades. These systems composed of elastomers and hydrogels are very attractive due to their high biocompatibility, injectability, controlled porosity and often antimicrobial properties. Moreover, their elastomeric properties and bioadhesiveness are making them suitable for soft tissue engineering. Herein, we present the advances in the current state-of-the-art design principles and strategies for strong interface formation inspired by nature (bio-inspiration), the diverse properties and applications of elastomer-hydrogel systems in different medical fields, in particular, in tissue engineering. The functionalities of these systems, including adhesive properties, injectability, antimicrobial properties and degradability, applicable to tissue engineering will be discussed in a context of future efforts towards the development of advanced biomaterials.

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.

Uncovering Non-random Binary Patterns Within Sequences of Intrinsically Disordered Proteins

Sequence-ensemble relationships of intrinsically disordered proteins (IDPs) are governed by binary patterns such as the linear clustering or mixing of specific residues or residue types with respect to one another. To enable the discovery of potentially important, shared patterns across sequence families, we describe a computational method referred to as NARDINI for Non-random Arrangement of Residues in Disordered Regions Inferred using Numerical Intermixing. This work was partially motivated by the observation that parameters that are currently in use for describing different binary patterns are not interoperable across IDPs of different amino acid compositions and lengths. In NARDINI, we generate an ensemble of scrambled sequences to set up a composition-specific null model for the patterning parameters of interest. We then compute a series of pattern-specific z-scores to quantify how each pattern deviates from a null model for the IDP of interest. The z-scores help in identifying putative non-random linear sequence patterns within an IDP. We demonstrate the use of NARDINI derived z-scores by identifying sequence patterns in three well-studied IDP systems. We also demonstrate how NARDINI can be deployed to study archetypal IDPs across homologs and orthologs. Overall, NARDINI is likely to aid in designing novel IDPs with a view toward engineering new sequence-function relationships or uncovering cryptic ones. We further propose that the z-scores introduced here are likely to be useful for theoretical and computational descriptions of sequence-ensemble relationships across IDPs of different compositions and lengths.

Application of Thermoresponsive Intrinsically Disordered Protein Polymers in Nanostructured and Microstructured Materials

Modulation of inter- and intramolecular interactions between bioinspired designer molecules can be harnessed for developing functional structures that mimic the complex hierarchical organization of multicomponent assemblies observed in nature. Furthermore, such multistimuli-responsive molecules offer orthogonal tunability for generating versatile multifunctional platforms via independent biochemical and biophysical cues. In this review, the remarkable physicochemical and mechanical properties of genetically engineered protein polymers derived from intrinsically disordered proteins, specifically elastin and resilin, are discussed. This review highlights emerging technologies which use them as building blocks in the fabrication of highly programmable structured biomaterials for applications in delivery of biotherapeutic cargo and regenerative medicine.

Diclofenac Embedded in Silk Fibroin Fibers as a Drug Delivery System

Silk fibroin is a biocompatible, non-toxic, mechanically robust protein, and it is commonly used and studied as a material for biomedical applications. Silk fibroin also gained particular interest as a drug carrier vehicle, and numerous silk formats have been investigated for this purpose. Herein, we have prepared electrospun nanofibers from pure silk fibroin and blended silk fibroin/casein, followed by the incorporation of an anti-inflammatory drug, diclofenac. Casein serves as an excipient in pharmaceutical products and has a positive effect on the gradual release of drugs. The characteristics of the investigated composites were estimated by scanning electron microscope, transmission electron microscope, thermogravimetric analysis, and a lifetime of diclofenac by electron paramagnetic resonance analysis. The cumulative release in vitro of diclofenac sodium salt, together with the antiproliferative effect of diclofenac sodium salt-loaded silk nanofibers against the growth of two cancer cell lines, are presented and discussed.

A covalently crosslinked silk fibroin hydrogel using enzymatic oxidation and chemoenzymatically synthesized copolypeptide crosslinkers consisting of a GPG tripeptide motif and tyrosine: control of gelation and resilience

A covalently crosslinked silk fibroin hydrogel was successfully formedviaan enzymatic crosslinking reaction using copolypeptides, which consist of a glycine–proline–glycine tripeptide motif and tyrosine, as linker molecules.

Fmoc-diphenylalanine as a suitable building block for the preparation of hybrid materials and their potential applications

Due to its capability to self-assemble in self-supporting hydrogels (HG) under physiological conditions, Fmoc-FF is one of the most studied ultra-short peptide. The structural properties of the resulting hydrogel (mechanical rigidity, entanglement of the fibrillary network, and the thickness of the fibers) strictly depend on the experimental conditions used during the preparation. In the past few years, a broad range of applications in different fields, such as biomedical and industrial fields, have been proposed. However, the research on novel materials with enhanced mechanical properties, stability, and biocompatibility has brought about the development of novel Fmoc-FF-based hybrid systems, in which the ultra-short hydrogelator is combined with others entities such as polysaccharides, polymers, peptides, or organic molecules. The structural features and the potential applications of these novel hybrid materials, with particular attention to tissue engineering, drug delivery, and catalysis, are described here. The aim is to give the readers a tool to design new hybrid nanomaterials based on the Fmoc-FF dipeptide hydrogelator, with appropriate properties for specific applications.

Redox-Responsive Resilin-Like Hydrogels for Tissue Engineering and Drug Delivery Applications

Resilin, a protein found in insect cuticles, is renowned for its outstanding elastomeric properties. The authors' laboratory previously developed a recombinant protein, which consisted of consensus resilin-like repeats from Anopheles gambiae, and demonstrated its potential in cartilage and vascular engineering. To broaden the versatility of the resilin-like protein, this study utilizes a cleavable crosslinker, which contains a disulfide bond, to develop smart resilin-like hydrogels that are redox-responsive. The hydrogels exhibit a porous structure and a stable storage modulus (G') of ≈3 kPa. NIH/3T3 fibroblasts cultured on hydrogels for 24 h have a high viability (>95%). In addition, the redox-responsive hydrogels show significant degradation in a reducing environment (10 mm glutathione (GSH)). The release profiles of fluorescently labeled dextrans encapsulated within the hydrogels are assessed in vitro. For dextran that is estimated to be larger than the mesh size of the gel, faster release is observed in the presence of reducing agents due to degradation of the hydrogel networks. These studies thus demonstrate the potential of using these smart hydrogels in a variety of applications ranging from scaffolds for tissue engineering to drug delivery systems that target the intracellular reductive environments of tumors.

Characterization of resilin-like proteins with tunable mechanical properties

Resilin is an elastomeric protein abundant in insect cuticle. Its exceptional properties, which include high resilience and efficient energy storage, motivate its potential use in tissue engineering and drug delivery applications. Our lab has previously developed recombinant proteins based on the resilin-like sequence derived from Anopheles gambiae and demonstrated their promise as a scaffold for cartilage and vascular engineering. In this work, we describe a more thorough investigation of the physical properties of crosslinked resilin-like hydrogels. The resilin-like proteins rapidly form crosslinked hydrogels in physiological conditions. We also show that the mechanical properties of these resilin-like hydrogels can be modulated simply by varying the protein concentration or the stoichiometric ratio of crosslinker to crosslinking sites. Crosslinked resilin-like hydrogels were hydrophilic and had a high water content when swollen. In addition, these hydrogels exhibited moderate resilience values, which were comparable to those of common synthetic rubbers. Cryo-scanning electron microscopy showed that the crosslinked resilin-like hydrogels at 16 wt% featured a honeycomb-like structure. These studies thus demonstrate the potential to use recombinant resilin-like proteins in a wide variety of applications such as tissue engineering and drug delivery due to their tunable physical properties.

Advances in Protein-Based Materials: From Origin to Novel Biomaterials

Biomaterials play a very important role in biomedicine and tissue engineering where they directly affect the cellular activities and their microenvironment . Myriad of techniques have been employed to fabricate a vast number natural, artificial and recombinant polymer s in order to harness these biomaterials in tissue regene ration , drug delivery and various other applications. Despite of tremendous efforts made in this field during last few decades, advanced and new generation biomaterials are still lacking. Protein based biomaterials have emerged as an attractive alternatives due to their intrinsic properties like cell to cell interaction , structural support and cellular communications. Several protein based biomaterials like, collagen , keratin , elastin , silk protein and more recently recombinant protein s are being utilized in a number of biomedical and biotechnological processes. These protein-based biomaterials have enormous capabilities, which can completely revolutionize the biomaterial world. In this review, we address an up-to date review on the novel, protein-based biomaterials used for biomedical field including tissue engineering, medical science, regenerative medicine as well as drug delivery. Further, we have also emphasized the novel fabrication techniques associated with protein-based materials and implication of these biomaterials in the domain of biomedical engineering .

Designer biomaterials for mechanobiology

Biomaterials engineered with specific bioactive ligands, tunable mechanical properties, and complex architectural features have emerged as powerful tools to probe how cells sense and respond to the physical properties of their material surroundings, and ultimately provide designer approaches to control cell function.

Polymeric Biomaterials: Diverse Functions Enabled by Advances in Macromolecular Chemistry

Biomaterials have been extensively used to leverage beneficial outcomes in various therapeutic applications, such as providing spatial and temporal control over the release of therapeutic agents in drug delivery as well as engineering functional tissues and promoting the healing process in tissue engineering and regenerative medicine. This perspective presents important milestones in the development of polymeric biomaterials with defined structures and properties. Contemporary studies of biomaterial design have been reviewed with focus on constructing materials with controlled structure, dynamic functionality, and biological complexity. Examples of these polymeric biomaterials enabled by advanced synthetic methodologies, dynamic chemistry/assembly strategies, and modulated cell-material interactions have been highlighted. As the field of polymeric biomaterials continues to evolve with increased sophistication, current challenges and future directions for the design and translation of these materials are also summarized.

Tissue engineering-based therapeutic strategies for vocal fold repair and regeneration

Vocal folds are soft laryngeal connective tissues with distinct layered structures and complex multicomponent matrix compositions that endow phonatory and respiratory functions. This delicate tissue is easily damaged by various environmental factors and pathological conditions, altering vocal biomechanics and causing debilitating vocal disorders that detrimentally affect the daily lives of suffering individuals. Modern techniques and advanced knowledge of regenerative medicine have led to a deeper understanding of the microstructure, microphysiology, and micropathophysiology of vocal fold tissues. State-of-the-art materials ranging from extracecullar-matrix (ECM)-derived biomaterials to synthetic polymer scaffolds have been proposed for the prevention and treatment of voice disorders including vocal fold scarring and fibrosis. This review intends to provide a thorough overview of current achievements in the field of vocal fold tissue engineering, including the fabrication of injectable biomaterials to mimic in vitro cell microenvironments, novel designs of bioreactors that capture in vivo tissue biomechanics, and establishment of various animal models to characterize the in vivo biocompatibility of these materials. The combination of polymeric scaffolds, cell transplantation, biomechanical stimulation, and delivery of antifibrotic growth factors will lead to successful restoration of functional vocal folds and improved vocal recovery in animal models, facilitating the application of these materials and related methodologies in clinical practice.

Elastic proteins and elastomeric protein alloys

The elastomeric proteins elastin and resilin have been used extensively in the fabrication of biomaterials for tissue engineering applications due to their unique mechanical and biological properties. Tropoelastin is the soluble monomer component of elastin. Tropoelastin and resilin are both highly elastic with high resilience, substantial extensibility, high durability and low energy loss, which makes them excellent candidates for the fabrication of elastic tissues that demand regular and repetitive movement like the skin, lung, blood vessels, muscles and vocal folds. Combinations of these proteins with silk fibroin further enhance their biomechanical and biological properties leading to a new class of protein alloy materials with versatile properties. In this review, the properties of tropoelastin-based and resilin-based biomaterials with and without silk are described in concert with examples of their applications in tissue engineering.

Recombinant Resilin-Based Bioelastomers for Regenerative Medicine Applications

The outstanding elasticity, excellent resilience at high-frequency, and hydrophilic capacity of natural resilin have motivated investigations of recombinant resilin-based biomaterials as a new class of bio-elastomers in the engineering of mechanically active tissues. Accordingly, here the comprehensive characterization of modular resilin-like polypeptide (RLP) hydrogels is presented and their suitability as a novel biomaterial for in vivo applications is introduced. Oscillatory rheology confirmed that a full suite of the RLPs can be rapidly cross-linked upon addition of the tris(hydroxymethyl phosphine) cross-linker, achieving similar in situ shear storage moduli (20 k ± 3.5 Pa) across various material compositions. Uniaxial stress relaxation tensile testing of hydrated RLP hydrogels under cyclic loading and unloading showed negligible stress reduction and hysteresis, superior reversible extensibility, and high resilience with Young's moduli of 30 ± 7.4 kPa. RLP hydrogels containing MMP-sensitive domains are susceptible to enzymatic degradation by matrix metalloproteinase-1 (MMP-1). Cell culture studies revealed that RLP-based hydrogels supported the attachment and spreading (2D) of human mesenchymal stem cells and did not activate cultured macrophages. Subcutaneous transplantation of RLP hydrogels in a rat model, which to our knowledge is the first such reported in vivo analysis of RLP-based hydrogels, illustrated that these materials do not elicit a significant inflammatory response, suggesting their potential as materials for tissue engineering applications with targets of mechanically demanding tissues such as vocal fold and cardiovascular tissues.

Resilin-PEG Hybrid Hydrogels Yield Degradable Elastomeric Scaffolds with Heterogeneous Microstructure

Hydrogels derived from resilin-like polypeptides (RLPs) have shown outstanding mechanical resilience and cytocompatibility; expanding the versatility of RLP-based materials via conjugation with other polypeptides and polymers would offer great promise in the design of a range of materials. Here, we present an investigation of the biochemical and mechanical properties of hybrid hydrogels composed of a recombinant RLP and a multiarm PEG macromer. These hybrid hydrogels can be rapidly cross-linked through a Michael-type addition reaction between the thiols of cysteine residues on the RLP and vinyl sulfone groups on the multiarm PEG. Oscillatory rheology and tensile testing confirmed the formation of elastomeric hydrogels with mechanical resilience comparable to aortic elastin; hydrogel stiffness was easily modulated through the cross-linking ratio. Macromolecular phase separation of the RLP-PEG hydrogels offers the unique advantage of imparting a heterogeneous microstructure, which can be used to localize cells, through simple mixing and cross-linking. Assessment of degradation of the RLP by matrix metalloproteinases (MMPs) illustrated the specific proteolysis of the polypeptide in both its soluble form and when cross-linked into hydrogels. Finally, the successful encapsulation and viable three-dimensional culture of human mesenchymal stem cells (hMSCs) demonstrated the cytocompatibility of the RLP-PEG gels. Overall, the cytocompatibility, elastomeric mechanical properties, microheterogeneity, and degradability of the RLP-PEG hybrid hydrogels offer a suite of promising properties for the development of cell-instructive, structured tissue engineering scaffolds.

Elastin-like polypeptides as models of intrinsically disordered proteins

Elastin-like polypeptides (ELPs) are a class of stimuli-responsive biopolymers inspired by the intrinsically disordered domains of tropoelastin that are composed of repeats of the VPGXG pentapeptide motif, where X is a "guest residue". They undergo a reversible, thermally triggered lower critical solution temperature (LCST) phase transition, which has been utilized for a variety of applications including protein purification, affinity capture, immunoassays, and drug delivery. ELPs have been extensively studied as protein polymers and as biomaterials, but their relationship to other disordered proteins has heretofore not been established. The biophysical properties of ELPs that lend them their unique material behavior are similar to the properties of many intrinsically disordered proteins (IDP). Their low sequence complexity, phase behavior, and elastic properties make them an interesting "minimal" artificial IDP, and the study of ELPs can hence provide insights into the behavior of other more complex IDPs. Motivated by this emerging realization of the similarities between ELPs and IDPs, this review discusses the biophysical properties of ELPs, their biomedical utility, and their relationship to other disordered polypeptide sequences.

Elastin-like Polypeptide Diblock Copolymers Self-Assemble into Weak Micelles

The self-assembly of synthetic diblock copolymers has been extensively studied experimentally and theoretically. In contrast, self-assembly of polypeptide diblock copolymers has so far been mostly studied experimentally. We discovered that the theory developed for synthetic diblock copolymer does not fully explain the self-assembly of elastin-like polypeptide diblock copolymers, leading us to generalize the theory to make it applicable for these polypeptides. We demonstrated that elastin-like polypeptide diblocks self-assemble into weak micelles with dense cores and almost unstretched coronas, a state not previously observed for synthetic diblock copolymers. Weak micelles form if the surface tension at the core-corona interface is low compared to that expected of a micelle with a dense core. The predictions of the theory of weak micelles for the critical micelle temperature, hydrodynamic radius, and aggregation number of elastin-like polypeptide diblocks are in reasonable agreement with the experimentally measured values. The unique and unprecedented control of amphiphilicity in these recombinant peptide polymers reveals a new micellar state that has not been previously observed in synthetic diblock copolymer systems.

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.

Opportunities for multicomponent hybrid hydrogels in biomedical applications

Hydrogels provide mechanical support and a hydrated environment that offer good cytocompatibility and controlled release of molecules, and myriad hydrogels thus have been studied for biomedical applications. In the past few decades, research in these areas has shifted increasingly to multicomponent hydrogels that better capture the multifunctional nature of native biological environments and that offer opportunities to selectively tailor materials properties. This review summarizes recent approaches aimed at producing multicomponent hydrogels, with descriptions of contemporary chemical and physical approaches for forming networks, and of the use of both synthetic and biologically derived molecules to impart desired properties. Specific multicomponent materials with enhanced mechanical properties are presented, as well as materials in which multiple biological functions are imparted for applications in tissue engineering, cancer treatment, and gene therapies. The progress in the field suggests significant promise for these approaches in the development of biomedically relevant materials.

Biological materials and molecular biomimetics – filling up the empty soft materials space for tissue engineering applications

The discovery and molecular (genetic) characterization of novel biological materials offers great potential to expand the range of soft materials used for biomedical applications.

Silk-elastin-like protein biomaterials for the controlled delivery of therapeutics

INTRODUCTION

Genetically engineered biomaterials are useful for controlled delivery owing to their rational design, tunable structure-function, biocompatibility, degradability and target specificity. Silk-elastin-like proteins (SELPs), a family of genetically engineered recombinant protein polymers, possess these properties. Additionally, given the benefits of combining semi-crystalline silk-blocks and elastomeric elastin-blocks, SELPs possess multi-stimuli-responsive properties and tunability, thereby becoming promising candidates for targeted cancer therapeutics delivery and controlled gene release.

AREAS COVERED

An overview of SELP biomaterials for drug delivery and gene release is provided. Biosynthetic strategies used for SELP production, fundamental physicochemical properties and self-assembly mechanisms are discussed. The review focuses on sequence-structure-function relationships, stimuli-responsive features and current and potential drug delivery applications.

EXPERT OPINION

The tunable material properties allow SELPs to be pursued as promising biomaterials for nanocarriers and injectable drug release systems. Current applications of SELPs have focused on thermally-triggered biomaterial formats for the delivery of therapeutics, based on local hyperthermia in tumors or infections. Other prominent controlled release applications of SELPs as injectable hydrogels for gene release have also been pursued. Further biomedical applications that utilize other stimuli to trigger the reversible material responses of SELPs for targeted delivery, including pH, ionic strength, redox, enzymatic stimuli and electric field, are in progress. Exploiting these additional stimuli-responsive features will provide a broader range of functional biomaterials for controlled therapeutics release and tissue regeneration.

Transient dynamic mechanical properties of resilin-based elastomeric hydrogels

The outstanding high-frequency properties of emerging resilin-like polypeptides (RLPs) have motivated their development for vocal fold tissue regeneration and other applications. Recombinant RLP hydrogels show efficient gelation, tunable mechanical properties, and display excellent extensibility, but little has been reported about their transient mechanical properties. In this manuscript, we describe the transient mechanical behavior of new RLP hydrogels investigated via both sinusoidal oscillatory shear deformation and uniaxial tensile testing. Oscillatory stress relaxation and creep experiments confirm that RLP-based hydrogels display significantly reduced stress relaxation and improved strain recovery compared to PEG-based control hydrogels. Uniaxial tensile testing confirms the negligible hysteresis, reversible elasticity and superior resilience (up to 98%) of hydrated RLP hydrogels, with Young's modulus values that compare favorably with those previously reported for resilin and that mimic the tensile properties of the vocal fold ligament at low strain (<15%). These studies expand our understanding of the properties of these RLP materials under a variety of conditions, and confirm the unique applicability, for mechanically demanding tissue engineering applications, of a range of RLP hydrogels.

Resilin: protein-based elastomeric biomaterials

Resilin is an elastomeric protein found in insect cuticles and is remarkable for its high strain, low stiffness, and high resilience. Since the first resilin sequence was identified in Drosophilia melanogaster (fruit fly), researchers have utilized molecular cloning techniques to construct resilin-based proteins for a number of different applications. In addition to exhibiting the superior mechanical properties of resilin, resilin-based proteins are autofluorescent, display self-assembly properties, and undergo phase transitions in response to temperature. These properties have potential application in designing biosensors or environmentally responsive materials for use in tissue engineering or drug delivery. Furthermore, the capability of resilin-based biomaterials has been expanded by designing proteins that include both resilin-based sequences and bioactive domains such as cell-adhesion or matrix metalloproteinase sequences. These new materials maintain the superior mechanical and physical properties of resilin and also have the added benefit of controlling cell response. Because the mechanical and biological properties can be tuned through protein engineering, a wide range of properties can be achieved for tissue engineering applications including muscles, vocal folds, cardiovascular tissues, and cartilage.

Synthetic biodegradable functional polymers for tissue engineering: a brief review

Scaffolds play a crucial role in tissue engineering. Biodegradable polymers with great processing flexibility are the predominant scaffolding materials. Synthetic biodegradable polymers with well-defined structure and without immunological concerns associated with naturally derived polymers are widely used in tissue engineering. The synthetic biodegradable polymers that are widely used in tissue engineering, including polyesters, polyanhydrides, polyphosphazenes, polyurethane, and poly (glycerol sebacate) are summarized in this article. New developments in conducting polymers, photoresponsive polymers, amino-acid-based polymers, enzymatically degradable polymers, and peptide-activated polymers are also discussed. In addition to chemical functionalization, the scaffold designs that mimic the nano and micro features of the extracellular matrix (ECM) are presented as well, and composite and nanocomposite scaffolds are also reviewed.

Synthesis and characterization of recombinant abductin-based proteins

Recombinant proteins are promising tools for tissue engineering and drug delivery applications. Protein-based biomaterials have several advantages over natural and synthetic polymers, including precise control over amino acid composition and molecular weight, modular swapping of functional domains, and tunable mechanical and physical properties. In this work, we describe recombinant proteins based on abductin, an elastomeric protein that is found in the inner hinge of bivalves and functions as a coil spring to keep shells open. We illustrate, for the first time, the design, cloning, expression, and purification of a recombinant protein based on consensus abductin sequences derived from Argopecten irradians . The molecular weight of the protein was confirmed by mass spectrometry, and the protein was 94% pure. Circular dichroism studies showed that the dominant structures of abductin-based proteins were polyproline II helix structures in aqueous solution and type II β-turns in trifluoroethanol. Dynamic light scattering studies illustrated that the abductin-based proteins exhibit reversible upper critical solution temperature behavior and irreversible aggregation behavior at high temperatures. A LIVE/DEAD assay revealed that human umbilical vein endothelial cells had a viability of 98 ± 4% after being cultured for two days on the abductin-based protein. Initial cell spreading on the abductin-based protein was similar to that on bovine serum albumin. These studies thus demonstrate the potential of abductin-based proteins in tissue engineering and drug delivery applications due to the cytocompatibility and its response to temperature.

Resilin-based Materials for Biomedical Applications

Resilin, an insect structural protein, exhibits rubber-like elasticity characterized by low stiffness, high extensibility, efficient energy storage, and exceptional resilience and fatigue lifetime. The outstanding mechanical properties of natural resilin have motivated recent research in the engineering of resilin-like polypeptide-based biomaterials, with a wide range of applications including use as bio-rubbers, nanosprings, elements in biosensors, and tissue engineering scaffolds.

Resilin-Like Polypeptide Hydrogels Engineered for Versatile Biological Functions

Natural resilin, the rubber-like protein that exists in specialized compartments of most arthropods, possesses excellent mechanical properties such as low stiffness, high resilience and effective energy storage. Recombinantly-engineered resilin-like polypeptides (RLPs) that possess the favorable attributes of native resilin would be attractive candidates for the modular design of biomaterials for engineering mechanically active tissues. Based on our previous success in creating a novel RLP-based hydrogel and demonstrating useful mechanical and cell-adhesive properties, we have produced a suite of new RLP-based constructs, each equipped with 12 repeats of the putative resilin consensus sequence and a single, distinct biologically active domain. This approach allows independent control over the concentrations of cell-binding, MMP-sensitive, and polysaccharide-sequestration domains in hydrogels comprising mixtures of the various RLPs. The high purity, molecular weight and correct compositions of each new polypeptide have been confirmed via high performance liquid chromatography (HPLC), sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and amino acid analysis. These RLP-based polypeptides exhibit largely random-coil conformation, both in solution and in the cross-linked hydrogels, as indicated by circular dichroic and infrared spectroscopic analyses. Hydrogels of various compositions, with a range of elastic moduli (1kPa to 25kPa) can be produced from these polypeptides, and the activity of the cell-binding and matrix metalloproteinase (MMP) sensitive domains was confirmed. Tris(hydroxymethyl phosphine) cross-linked RLP hydrogels were able to maintain their mechanical integrity as well as the viability of encapsulated primary human mesenchymal stem cells (MSCs). These results validate the promising properties of these RLP-based elastomeric biomaterials.

Resilin-Based Hybrid Hydrogels for Cardiovascular Tissue Engineering

The outstanding elastomeric properties of natural resilin, an insect protein, have motivated the engineering of resilin-like polypeptides (RLPs) as a potential material for cardiovascular tissue engineering. The RLPs, which incorporate biofunctional domains for cell-matrix interactions, are cross-linked into RLP-PEG hybrid hydrogels via a Michael-type addition of cysteine residues on the RLP with vinyl sulfones of an end-functionalized multi-arm star PEG. Oscillatory rheology indicated the useful mechanical properties of these materials. Assessments of cell viability via con-focal microscopy clearly show the successful encapsulation of human aortic adventitial fibroblasts in the three-dimensional matrices and the adoption of a spread morphology following 7 days of culture.

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.

Tunable mechanical stability and deformation response of a resilin-based elastomer

Resilin, the highly elastomeric protein found in specialized compartments of most arthropods, possesses superior resilience and excellent high-frequency responsiveness. Enabled by biosynthetic strategies, we have designed and produced a modular, recombinant resilin-like polypeptide bearing both mechanically active and biologically active domains to create novel biomaterial microenvironments for engineering mechanically active tissues such as blood vessels, cardiovascular tissues, and vocal folds. Preliminary studies revealed that these recombinant materials exhibit promising mechanical properties and support the adhesion of NIH 3T3 fibroblasts. In this Article, we detail the characterization of the dynamic mechanical properties of these materials, as assessed via dynamic oscillatory shear rheology at various protein concentrations and cross-linking ratios. Simply by varying the polypeptide concentration and cross-linker ratios, the storage modulus G' can be easily tuned within the range of 500 Pa to 10 kPa. Strain-stress cycles and resilience measurements were probed via standard tensile testing methods and indicated the excellent resilience (>90%) of these materials, even when the mechanically active domains are intercepted by nonmechanically active biological cassettes. Further evaluation, at high frequencies, of the mechanical properties of these materials were assessed by a custom-designed torsional wave apparatus (TWA) at frequencies close to human phonation, indicating elastic modulus values from 200 to 2500 Pa, which is within the range of experimental data collected on excised porcine and human vocal fold tissues. The results validate the outstanding mechanical properties of the engineered materials, which are highly comparable to the mechanical properties of targeted vocal fold tissues. The ease of production of these biologically active materials, coupled to their outstanding mechanical properties over a range of compositions, suggests their potential in tissue regeneration applications.