Designing ECM-mimetic materials using protein engineering

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.

Genetic Functionalization of Protein-Based Biomaterials via Protein Fusions

Proteins implement many useful functions, including binding ligands with unparalleled affinity and specificity, catalyzing stereospecific chemical reactions, and directing cell behavior. Incorporating proteins into materials has the potential to imbue devices with these desirable traits. This review highlights recent advances in creating active materials by genetically fusing a self-assembling protein to a functional protein. These fusion proteins form materials while retaining the function of interest. Key advantages of this approach include elimination of a separate functionalization step during materials synthesis, uniform and dense coverage of the material by the functional protein, and stabilization of the functional protein. This review focuses on macroscale materials and discusses (i) multiple strategies for successful protein fusion design, (ii) successes and limitations of the protein fusion approach, (iii) engineering solutions to bypass any limitations, (iv) applications of protein fusion materials, including tissue engineering, drug delivery, enzyme immobilization, electronics, and biosensing, and (v) opportunities to further develop this useful technique.

Synergy between Laminin-Derived Elastin-like Polypeptides (LELPs) Optimizes Cell Spreading

A major component of the extracellular matrix (ECM), laminins, modulates cells via diverse receptors. Their fragments have emerging utility as components of "ECM-mimetics" optimized to promote cell-based therapies. Recently, we reported that a bioactive laminin peptide known as A99 enhanced cell binding and spreading via fusion to an elastin-like polypeptide (ELP). The ELP "handle" serves as a rapid, noncovalent strategy to concentrate bioactive peptide mixtures onto a surface. We now report that this strategy can be further generalized across an expanded panel of additional laminin-derived elastin-like polypeptides (LELPs). A99 (AGTFALRGDNPQG), A2G80 (VQLRNGFPYFSY), AG73 (RKRLQVQLSIRT), and EF1m (LQLQEGRLHFMFD) all promote cell spreading while showing morphologically distinct F-actin formation. Equimolar mixtures of A99:A2G80-LELPs have synergistic effects on adhesion and spreading. Finally, three of these ECM-mimetics promote the neurite outgrowth of PC-12 cells. The evidence presented here demonstrates the potential of ELPs to deposit ECM-mimetics with applications in regenerative medicine, cell therapy, and tissue engineering.

Hyperspectral Mapping of Human Primary and Stem Cells at Cell-Matrix Interfaces

Extracellular matrices interface with cells to promote cell growth and tissue development. Given this critical role, matrix mimetics are introduced to enable biomedical materials ranging from tissue engineering scaffolds and tumor models to organoids for drug screening and implant surface coatings. Traditional microscopy methods are used to evaluate such materials in their ability to support exploitable cell responses, which are expressed in changes in cell proliferation rates and morphology. However, the physical imaging methods do not capture the chemistry of cells at cell-matrix interfaces. Herein, we report hyperspectral imaging to map the chemistry of human primary and embryonic stem cells grown on matrix materials, both native and artificial. We provide the statistical analysis of changes in lipid and protein content of the cells obtained from infrared spectral maps to conclude matrix morphologies as a major determinant of biochemical cell responses. The study demonstrates an effective methodology for evaluating bespoke matrix materials directly at cell-matrix interfaces.

Characterization of recombinant humanized collagen type III and its influence on cell behavior and phenotype

Collagen made a tremendous impact in the field of regenerative medicine as a bioactive material. For decades, collagen has been used not only as a scaffolding material but also as an active component in regulating cells' biological behavior and phenotype. However, animal-derived collagen as a major source suffered from problems of immunogenicity, risk of viral infection, and the unclear relationship between bioactive sequence and function. Recombinant humanized collagen (rhCol) provided alternatives for regenerative medicine with more controllable risks. However, the characterization of rhCol and the interaction between rhCol and cells still need further investigation, including cell behavior and phenotype. The current study preliminarily demonstrated that recombinant humanized collagen type III (rhCol III) conformed to the theoretical amino acid sequence and had an advanced structure resembling bovine collagen. Furthermore, rhCol III could facilitate basal biological behaviors of human skin fibroblasts, such as adhesion, proliferation and migration. rhCol III was beneficial for some extracellular matrix-expressing cell phenotypes. The study would shed light on the mechanism research of rhCol and cell interactions and further understanding of effectiveness in tissue regeneration.

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Directed Self-Assembly of Dimeric Building Blocks into Networklike Protein Origami to Construct Hydrogels

Engineering proteins to construct self-assemblies is of crucial significance not only for understanding the sophisticated living systems but also for fabricating advanced materials with unexplored functions. However, due to the inherent chemical heterogeneity and structural complexity of the protein surface, designing complex protein assemblies in an anisotropic fashion remains challenging. Here, we describe a self-assembly approach to fabricating protein origami with a networklike structure by designing dual noncovalent interactions on the different positions of a single protein building block. With dimeric proteins as building blocks, 1D protein filaments were constructed by the designed metal coordination at key protein interfaces. Subsequently, the network superstructures were created by the cross-linking of the 1D protein filaments at branch point linkages through the second designed π-π stacking interactions. Notably, upon increasing the protein concentration, the formed protein networks convert into hydrogels with reversible, injectable, and self-healing properties, which have the ability to promote bone regeneration. This strategy could be used to fabricate other protein-based materials with unexplored functions.

Immobilization of modular peptides on graphene cocktail for differentiation of human mesenchymal stem cells to hepatic-like cells

In this study, two novel biomimetic modular peptide motifs based on the alpha-2 subunit of type IV collagen (CO4A2) were designed and immobilized on a graphene platform to imitate integrin and heparan sulfate- (HS-) binding proteins. The in silico study was used to design 9-mer K[KGDRGD]AG and 10-mer KK[SGDRGD]AG for testing designed Integrin-Binding Peptide (dIBP) and HS-Binding Peptide (dHBP). The virtual docking technique was used to optimize the peptide motifs and their relevant receptors. Molecular dynamic (MD) simulation was used to evaluate the stability of peptide-receptor complexes. The effect of the platform on the differentiation of human mesenchymal stem cells (hMSCs) to hepatic-like cells (HLCs) was evaluated. After differentiation, some hepatic cells’ molecular markers such as albumin, AFP, CK-18, and CK-19 were successfully followed. Graphene-heparan sulfate binding peptide (G-HSBP) enhances the mature hepatic markers’ expression instead of control (p ≤ 0.05). The pathological study showed that the designed platform is safe, and no adverse effects were seen till 21 days after implantation.

Application of elastin-like polypeptide (ELP) containing extra-cellular matrix (ECM) binding ligands in regenerative medicine

Extracellular matrix (ECM) molecules play an important role in regulating molecular signaling associated with proliferation, migration, differentiation, and tissue repair. The identification of new kinds of ECM mimic biomaterials to recapitulate critical functions of biological systems are important for various applications in tissue engineering and regenerative medicine. The use of human elastin derived materials with controlled biological properties and other functionalities to improve their cell-response was proposed. Herein, we reported genetic encoded synthesis of ELP (elastin-like polypeptide) containing ECM domains like RGD (integrin binding ligand) and YIGSR (laminin-selective receptor binding ligand) to regulate cell behaviour in more complex ways, and also better model natural matrices. Thermal responsiveness of the ELPs and structural conformation were determined to confirm its phase transition behaviour. The fusion ELPs derivatives were analysed for mechanical involvement of growth mechanism, regenerative, and healing processes. The designed fusion ELPs promoted fast and strong attachment of fibroblast cells. The fusion ELP derivatives enhanced the migration of keratinocyte cells which of crucial for wound healing. Together it provides a profound matrix for endothelial cells and significantly enhanced tube formation of HUVEC cells. Thus, strategy of using cell adhesive ELP biopolymer emphasizing the role of bioactive ELPs as next generation skin substitutes for regenerative medicine.

Vascular Polyurethane Prostheses Modified with a Bioactive Coating-Physicochemical, Mechanical and Biological Properties

This study describes a method for the modification of polyurethane small-diameter (5 mm) vascular prostheses obtained with the phase inversion method. The modification process involves two steps: the introduction of a linker (acrylic acid) and a peptide (REDV and YIGSR). FTIR and XPS analysis confirmed the process of chemical modification. The obtained prostheses had a porosity of approx. 60%, Young's Modulus in the range of 9-11 MPa, and a water contact angle around 40°. Endothelial (EC) and smooth muscle (SMC) cell co-culture showed that the surfaces modified with peptides increase the adhesion of ECs. At the same time, SMCs adhesion was low both on unmodified and peptide-modified surfaces. Analysis of blood-materials interaction showed high hemocompatibility of obtained materials. The whole blood clotting time assay showed differences in the amount of free hemoglobin present in blood contacted with different materials. It can be concluded that the peptide coating increased the hemocompatibility of the surface by increasing ECs adhesion and, at the same time, decreasing platelet adhesion. When comparing both types of peptide coatings, more promising results were obtained for the surfaces coated with the YISGR than REDV-coated prostheses.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

Research Progress of Chitosan-Based Biomimetic Materials

Chitosan is a linear polysaccharide produced by deacetylation of natural biopolymer chitin. Owing to its good biocompatibility and biodegradability, non-toxicity, and easy processing, it has been widely used in many fields. After billions of years of survival of the fittest, many organisms have already evolved a nearly perfect structure. This paper reviews the research status of biomimetic functional materials that use chitosan as a matrix material to mimic the biological characteristics of bivalves, biological cell matrices, desert beetles, and honeycomb structure of bees. In addition, the application of biomimetic materials in wound healing, hemostasis, drug delivery, and smart materials is briefly overviewed according to their characteristics of adhesion, hemostasis, release, and adsorption. It also discusses prospects for their application and provides a reference for further research and development.

Molecular Regulators of Cellular Mechanoadaptation at Cell-Material Interfaces

Diverse essential cellular behaviors are determined by extracellular physical cues that are detected by highly orchestrated subcellular interactions with the extracellular microenvironment. To maintain the reciprocity of cellular responses and mechanical properties of the extracellular matrix, cells utilize a variety of signaling pathways that transduce biophysical stimuli to biochemical reactions. Recent advances in the micromanipulation of individual cells have shown that cellular responses to distinct physical and chemical features of the material are fundamental determinants of cellular mechanosensation and mechanotransduction. In the process of outside-in signal transduction, transmembrane protein integrins facilitate the formation of focal adhesion protein clusters that are connected to the cytoskeletal architecture and anchor the cell to the substrate. The linkers of nucleoskeleton and cytoskeleton molecular complexes, collectively termed LINC, are critical signal transducers that relay biophysical signals between the extranuclear cytoplasmic region and intranuclear nucleoplasmic region. Mechanical signals that involve cytoskeletal remodeling ultimately propagate into the nuclear envelope comprising the nuclear lamina in assistance with various nuclear membrane proteins, where nuclear mechanics play a key role in the subsequent alteration of gene expression and epigenetic modification. These intracellular mechanical signaling cues adjust cellular behaviors directly associated with mechanohomeostasis. Diverse strategies to modulate cell-material interfaces, including alteration of surface rigidity, confinement of cell adhesive region, and changes in surface topology, have been proposed to identify cellular signal transduction at the cellular and subcellular levels. In this review, we will discuss how a diversity of alterations in the physical properties of materials induce distinct cellular responses such as adhesion, migration, proliferation, differentiation, and chromosomal organization. Furthermore, the pathological relevance of misregulated cellular mechanosensation and mechanotransduction in the progression of devastating human diseases, including cardiovascular diseases, cancer, and aging, will be extensively reviewed. Understanding cellular responses to various extracellular forces is expected to provide new insights into how cellular mechanoadaptation is modulated by manipulating the mechanics of extracellular matrix and the application of these materials in clinical aspects.

Nanomechanical Properties and Phase Behavior of Phenylalanine Amyloid Ribbon Assemblies and Amorphous Self-Healing Hydrogels

Phenylalanine was the minimalistic and first of numerous nonproteinaceous building blocks to be demonstrated to form amyloid-like fibrils. This unexpected organization of such a simple building block into canonical architecture, which was previously observed only with proteins and peptides, has numerous implications for medicine and supramolecular chemistry. However, the morphology of phenylalanine fibrils and their mechanical properties was never characterized in solutions. Here, using electron and atomic force microscopy, we analyze the morphological and mechanical properties of phenylalanine fibrils in both air and fluids. The fibrils demonstrate an exceptionally high Young's modulus (up to 30 GPa) and are found to be composed of intertwined protofilaments in a helical or twisted ribbon morphology. In addition, X-ray scattering experiments provide convincing evidence of an amyloidal cross-β-like secondary structure within the nanoassemblies. Furthermore, increasing the phenylalanine concentration results in the formation of highly homogenous, noncrystalline, self-healing hydrogels that display storage and loss moduli significantly higher than similar noncovalently cross-linked biomolecular nanofibrillar scaffolds. These remarkably stiff nanofibrillar hydrogels can be harnessed for various technological and biomedical applications, such as self-healing, printable, structural, load-bearing 3D scaffolds. The properties of this simple but quite remarkable hydrogel open a possibility to utilize it in the biomaterial industry.

Hierarchically structured hydrogels utilizing multifunctional assembling peptides for 3D cell culture

Approaches for the creation of soft materials, particularly hydrogels, with hierarchical structure are of interest in a variety of applications owing to their unique properties. In the context of tissue mimics, hydrogels with multiscale structures more accurately capture the complexities of tissues within the body (e.g., fibrous collagen-rich microenvironments). However, cytocompatible fabrication of such materials with hierarchical structures and independent control of mechanical and biochemical properties remains challenging and is needed for probing and directing cell-microenvironment interactions for three-dimensional (3D) cell encapsulation and culture applications. To address this, we have designed innovative multifunctional assembling peptides: these unique peptides contain a core block that mimics the structure of collagen for achieving relevant melting temperatures; 'sticky' ends to promote assembly of long fibrils; and a biocompatible reactive handle that is orthogonal to assembly to allow the formation of desired multiscale structures and their subsequent rapid, light-triggered integration within covalently crosslinked synthetic hydrogels. Nano- to micro-fibrils were observed to form in physiologically-relevant aqueous solutions, where both underlying peptide chemical structure and assembly conditions were observed to impact the resulting fibril sizes. These assembled structures were 'locked' into place and integrated as linkers within cell-degradable, bioactive hydrogels formed with photoinitiated thiol-ene 'click' chemistry. Hydrogel compositions were identified for achieving robust mechanical properties like those of soft tissues while also retaining higher ordered structures after photopolymerization. The utility of these innovative materials for 3D cell culture was demonstrated with human mesenchymal stem cells, where cell morphologies reminiscent of responses to assembled native collagen were observed now with a fully synthetic material. Using a bottom-up approach, a new materials platform has been established that combines the advantageous properties of covalent and assembling chemistries for the creation of synthetic hydrogels with controllable nanostructure, mechanical properties, and biochemical content.

Targeted Drug Delivery via the Use of ECM-Mimetic Materials

The use of drug delivery vehicles to improve the efficacy of drugs and to target their action at effective concentrations over desired periods of time has been an active topic of research and clinical investigation for decades. Both synthetic and natural drug delivery materials have facilitated locally controlled as well as targeted drug delivery. Extracellular matrix (ECM) molecules have generated widespread interest as drug delivery materials owing to the various biological functions of ECM. Hydrogels created using ECM molecules can provide not only biochemical and structural support to cells, but also spatial and temporal control over the release of therapeutic agents, including small molecules, biomacromolecules, and cells. In addition, the modification of drug delivery carriers with ECM fragments used as cell-binding ligands has facilitated cell-targeted delivery and improved the therapeutic efficiency of drugs through interaction with highly expressed cellular receptors for ECM. The combination of ECM-derived hydrogels and ECM-derived ligand approaches shows synergistic effects, leading to a great promise for the delivery of intracellular drugs, which require specific endocytic pathways for maximal effectiveness. In this review, we provide an overview of cellular receptors that interact with ECM molecules and discuss examples of selected ECM components that have been applied for drug delivery in both local and systemic platforms. Finally, we highlight the potential impacts of utilizing the interaction between ECM components and cellular receptors for intracellular delivery, particularly in tissue regeneration applications.

Bio-instructive materials for musculoskeletal regeneration

The prevalence and cost of disorders affecting the musculoskeletal system are predicted to rise significantly in the coming years due to the aging global population and the increase of associated risk factors. Despite being the second largest cause of disability, the clinical options for therapeutic intervention remain limited. The clinical translation of cell-based therapies for the treatment of musculoskeletal disorders faces many challenges including maintenance of cell survival in the harsh in vivo environment and the lack of control over regulating cell phenotype upon implantation. In order to address these challenges, the development of bio-instructive materials to modulate cell behavior has taken center stage as a strategy to increase the therapeutic potential of various cell populations. However, the determination of the necessary cues for a specific application and how these signals should be presented from a biomaterial remains elusive. This review highlights recent biochemical and physical strategies used to engineer bio-instructive materials for the repair of musculoskeletal tissues. There is a particular emphasis on emerging efforts such as the engineering of immunomodulatory and antibacterial materials, as well as the incorporation of these strategies into biofabrication and organ-on-a-chip approaches. STATEMENT OF SIGNIFICANCE: Disorders affecting the musculoskeletal system affect individuals across the lifespan and have a profound effect on mobility and quality of life. While small defects in many tissues can heal successfully, larger defects are often unable to heal or instead heal with inferior quality fibrous tissue and require clinical intervention. Cell-based therapies are a promising option for clinical translation, yet challenges related to maintaining cell survival and instructing cell phenotype upon implantation have limited the success of this approach. Bio-instructive materials provide an exciting opportunity to modulate cell behavior and enhance the efficacy of cell-based approaches for musculoskeletal repair. However, the identification of critical instructive cues and how to present these stimuli is a focus of intense investigation. This review highlights recent biochemical and physical strategies used to engineer bio-instructive materials for the repair of musculoskeletal tissues, while also considering exciting progress in the engineering of immunomodulatory and antibacterial materials.

The ZT Biopolymer: A Self-Assembling Protein Scaffold for Stem Cell Applications

The development of cell culture systems for the naturalistic propagation, self-renewal and differentiation of cells ex vivo is a high goal of molecular engineering. Despite significant success in recent years, the high cost of up-scaling cultures, the need for xeno-free culture conditions, and the degree of mimicry of the natural extracellular matrix attainable in vitro using designer substrates continue to pose obstacles to the translation of cell-based technologies. In this regard, the ZT biopolymer is a protein-based, stable, scalable, and economical cell substrate of high promise. ZT is based on the naturally occurring assembly of two human proteins: titin-Z1Z2 and telethonin. These protein building blocks are robust scaffolds that can be conveniently functionalized with full-length proteins and bioactive peptidic motifs by genetic manipulation, prior to self-assembly. The polymer is, thereby, fully encodable. Functionalized versions of the ZT polymer have been shown to successfully sustain the long-term culturing of human embryonic stem cells (hESCs), human induced pluripotent stem cells (hiPSCs), and murine mesenchymal stromal cells (mMSCs). Pluripotency of hESCs and hiPSCs was retained for the longest period assayed (4 months). Results point to the large potential of the ZT system for the creation of a modular, pluri-functional biomaterial for cell-based applications.

Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Given their durability and long-term stability, self-healable hydrogels have, in the past few years, emerged as promising replacements for the many brittle hydrogels currently being used in preclinical or clinical trials. To this end, the incompatibility between hydrogel toughness and rapid self-healing remains unaddressed, and therefore most of the self-healable hydrogels still face serious challenges within the dynamic and mechanically demanding environment of human organs/tissues. Furthermore, depending on the target tissue, the self-healing hydrogels must comply with a wide range of properties including electrical, biological, and mechanical. Notably, the incorporation of nanomaterials into double-network hydrogels is showing great promise as a feasible way to generate self-healable hydrogels with the above-mentioned attributes. Here, the recent progress in the development of multifunctional and self-healable hydrogels for various tissue engineering applications is discussed in detail. Their potential applications within the rapidly expanding areas of bioelectronic hydrogels, cyborganics, and soft robotics are further highlighted.

Stimuli-Responsive, Pentapeptide, Nanofiber Hydrogel for Tissue Engineering

Short peptides are uniquely versatile building blocks for self-assembly. Supramolecular peptide assemblies can be used to construct functional hydrogel biomaterials-an attractive approach for neural tissue engineering. Here, we report a new class of short, five-residue peptides that form hydrogels with nanofiber structures. Using rheology and spectroscopy, we describe how sequence variations, pH, and peptide concentration alter the mechanical properties of our pentapeptide hydrogels. We find that this class of seven unmodified peptides forms robust hydrogels from 0.2-20 kPa at low weight percent (less than 3 wt %) in cell culture media and undergoes shear-thinning and rapid self-healing. The peptides self-assemble into long fibrils with sequence-dependent fibrillar morphologies. These fibrils exhibit a unique twisted ribbon shape, as visualized by transmission electron microscopy (TEM) and Cryo-EM imaging, with diameters in the low tens of nanometers and periodicities similar to amyloid fibrils. Experimental gelation behavior corroborates our molecular dynamics simulations, which demonstrate peptide assembly behavior, an increase in β-sheet content, and patterns of variation in solvent accessibility. Our rapidly assembling pentapeptides for injectable delivery (RAPID) hydrogels are syringe-injectable and support cytocompatible encapsulation of oligodendrocyte progenitor cells (OPCs), as well as their proliferation and three-dimensional process extension. Furthermore, RAPID gels protect OPCs from mechanical membrane disruption and acute loss of viability when ejected from a syringe needle, highlighting the protective capability of the hydrogel as potential cell carriers for transplantation therapies. The tunable mechanical and structural properties of these supramolecular assemblies are shown to be permissive to cell expansion and remodeling, making this hydrogel system suitable as an injectable material for cell delivery and tissue engineering applications.

Antibiofilm elastin-like polypeptide coatings: functionality, stability, and selectivity

Antimicrobial peptides (AMPs) are currently receiving interest as an alternative to conventional antibiotics to treat biomaterial-associated infection. However, the inherent instability of such peptides often limits their efficacy in intended clinical applications. Covalent immobilization of AMPs to surfaces is one strategy to increase the long-term stability and minimize the toxicity. In this work, an antimicrobial peptide, RRPRPRPRPWWWW-NH2 (RRP9W4N), was used to modify elastin-like polypeptide (ELP) surface coatings containing cell-adhesive peptide domains (RGD) using covalent chemistry. The AMP retained its antibacterial activity against Staphylococcus epidermidis, Staphylococcus aureus, and Pseudomonas aeruginosa when covalently bonded to ELP surfaces. Simultaneously, the AMP functionalization had insignificant effect on the viability, function, and differentiation of human osteosarcoma MG63 cells and human mesenchymal stem cells (hMSCs). Furthermore, stability of the immobilized AMP in human blood serum was investigated, and the results suggested that the AMP preserved its antibacterial activity up to 24 h. Combined, the results show that covalently attached AMPs onto RGD-containing ELP are an excellent candidate as an antimicrobial coating for medical devices. STATEMENT OF SIGNIFICANCE: Biomaterial associated infection, caused by adherent biofilm, is usually difficult to treat. There is a high demand for new materials and treatments to decrease the infection rates, especially with increasing threats concerning resistant bacteria. Formation of biofilms on medical devices lowers the bacteria susceptibility towards traditional antibiotics and also circumvent our immune system often resulting in revisional surgery and extensive use of antibiotics. One promising strategy is to develop surfaces having low bacterial attractiveness or bacterial killing properties, but still retaining the main function of the device. In this study, we have developed an implant coating that demonstrates a high antimicrobial effect and at the same time showing no negative affect on human cells.

Hydrazone-Linkage-Based Self-Healing and Injectable Xanthan-Poly(ethylene glycol) Hydrogels for Controlled Drug Release and 3D Cell Culture

Polymeric hydrogels have been extensively explored for controlled drug-delivery applications, but there is an increasing demand for smart drug delivery combined with tunable physicochemical attributes and tissue engineering potential. In this work, novel xanthan-poly(ethylene glycol) (PEG) hydrogels were developed by cross-linking polysaccharide, oxidized xanthan, and 8-arm PEG hydrazine through dynamic, pH-responsive, and biodegradable hydrazone linkages. Aqueous solutions (pH 6.5) of oxidized xanthan and PEG hydrazine were mixed together at 37 °C to obtain hydrogels within minutes, and the formation of hydrazone linkages was ascertained using Fourier transform infrared spectroscopy. Fabrication of xanthan-PEG hydrogels using hydrazone linkages has not been reported previously. The 3% hydrogels exhibited the storage modulus of 194 Pa, which increased to 770 Pa for 5% hydrogels. When subjected to alternating cycles of varying strains of 1 and 800% (5 cycles), hydrogels demonstrated instant recovery each time the extreme strain was relieved, thus suggesting excellent self-healing capabilities. Doxorubicin (DOX), chemotherapeutic agent, was loaded onto hydrogels, and release studies were carried out at pH 5.5 (tumoral) and 7.4 (physiological). The cumulative release from 3, 4, and 5% hydrogels at pH 5.5 was 81.06, 61.98, and 41.67%, whereas the release at pH 7.4 was 47.43, 37.01, and 35.34% at 30 days. MTT assay showed that oxidized xanthan and PEG hydrazine are not toxic to mammalian cells (NIH-3T3), as the cell viabilities were found to be 84.66 and 102% for concentrations up to 1 mg/mL. The live/dead assay with encapsulated NIH-3T3 cells showed no significant dead cell population, suggesting excellent compatibility of hydrogels in 2D and 3D culture. DOX-loaded hydrogels exhibited cytotoxicity against A549 cells when exposed to media released from hydrogels. Overall, hydrogels developed in this work may have potential applications in drug delivery and 3D cell culture for cell delivery.

Biomaterial Scaffolds in Regenerative Therapy of the Central Nervous System

The central nervous system (CNS) is the most important section of the nervous system as it regulates the function of various organs. Injury to the CNS causes impairment of neurological functions in corresponding sites and further leads to long-term patient disability. CNS regeneration is difficult because of its poor response to treatment and, to date, no effective therapies have been found to rectify CNS injuries. Biomaterial scaffolds have been applied with promising results in regeneration medicine. They also show great potential in CNS regeneration for tissue repair and functional recovery. Biomaterial scaffolds are applied in CNS regeneration predominantly as hydrogels and biodegradable scaffolds. They can act as cellular supportive scaffolds to facilitate cell infiltration and proliferation. They can also be combined with cell therapy to repair CNS injury. This review discusses the categories and progression of the biomaterial scaffolds that are applied in CNS regeneration.

Laser processing of protein films as a method for accomplishment of cell patterning at the microscale

In this study, we propose a photostructuring approach for protein films based on a treatment with nanosecond pulses of a KrF excimer laser. As a model protein we used an amyloid fibril-forming protein. Laser treatment induced a foaming of the sample surface exhibiting an interconnected fibrous mesh with a high degree of control and precision. The surface foaming was well characterized by scanning electron microscopy, atomic force microscopy, laser induced fluorescence and contact angle measurements. The laser irradiated areas of the protein films acquired new morphological and physicochemical properties that could be exploited to fulfill unmet challenges in the tissue engineering field. In this context we subsequently evaluated the response of NIH/3T3 fibroblast cell line on the processed film. Our results show a strong and statistically significant preference for adhesion and proliferation of cells on the irradiated areas compared to the non-irradiated ones. We propose that this strategy can be followed to induce selective cell patterning on protein films at the microscale.

Covalently adaptable elastin-like protein - hyaluronic acid (ELP - HA) hybrid hydrogels with secondary thermoresponsive crosslinking for injectable stem cell delivery

Shear-thinning, self-healing hydrogels are promising vehicles for therapeutic cargo delivery due to their ability to be injected using minimally invasive surgical procedures. We present an injectable hydrogel using a novel combination of dynamic covalent crosslinking with thermoresponsive engineered proteins. Ex situ at room temperature, rapid gelation occurs through dynamic covalent hydrazone bonds by simply mixing two components: hydrazine-modified elastin-like protein (ELP) and aldehyde-modified hyaluronic acid. This hydrogel provides significant mechanical protection to encapsulated human mesenchymal stem cells during syringe needle injection and rapidly recovers after injection to retain the cells homogeneously within a 3D environment. In situ, the ELP undergoes a thermal phase transition, as confirmed by Coherent anti-Stokes Raman scattering microscopy observation of dense ELP thermal aggregates. The formation of the secondary network reinforces the hydrogel and results in a 10-fold slower erosion rate compared to a control hydrogel without secondary thermal crosslinking. This improved structural integrity enables cell culture for three weeks post injection, and encapsulated cells maintain their ability to differentiate into multiple lineages, including chondrogenic, adipogenic, and osteogenic cell types. Together, these data demonstrate the promising potential of ELP-HA hydrogels for injectable stem cell transplantation and tissue regeneration.

Induction of Chondrogenic Differentiation of Human Mesenchymal Stem Cells by Biomimetic Gold Nanoparticles with Tunable RGD Density

Nanostructured materials have drawn a broad attention for their applications in biomedical fields. Ligand-modified nanomaterials can well mimic the dynamic extracellular matrix (ECM) microenvironments to regulate cell functions and fates. Herein, ECM mimetic gold nanoparticles (Au NPs) with tunable surface arginine-glycine-aspartate (RGD) density are designed and synthesized to induce the chondrogenic differentiation of human mesenchymal stem cells (hMSCs). The biomimetic Au NPs with an average size of 40 nm shows good biocompatibility without affecting the cell proliferation in the studied concentration range. The RGD motifs on Au NPs surface facilitate cellular uptake of NPs into monolayer hMSCs through integrin-mediated endocytosis. The biomimetic NPs have a promotive effect on cartilaginous matrix production and marker gene expression in cell pellet culture, especially for the biomimetic Au NPs with high surface RGD density. This study provides a novel strategy for fabricating biomimetic NPs to regulate cell differentiation, which holds great potentials in tissue engineering and biomedical applications.

New advances in probing cell-extracellular matrix interactions

The extracellular matrix (ECM) provides structural and biochemical support to cells within tissues. An emerging body of evidence has established that the ECM plays a key role in cell mechanotransduction - the study of coupling between mechanical inputs and cellular phenotype - through either mediating transmission of forces to the cells, or presenting mechanical cues that guide cellular behaviors. Recent progress in cell mechanotransduction research has been facilitated by advances of experimental tools, particularly microtechnologies, engineered biomaterials, and imaging and analytical methods. Microtechnologies have enabled the design and fabrication of controlled physical microenvironments for the study and measurement of cell-ECM interactions. Advances in engineered biomaterials have allowed researchers to develop synthetic ECMs that mimic tissue microenvironments and investigate the impact of altered physicochemical properties on various cellular processes. Finally, advanced imaging and spectroscopy techniques have facilitated the visualization of the complex interaction between cells and ECM in vitro and in living tissues. This review will highlight the application of recent innovations in these areas to probing cell-ECM interactions. We believe cross-disciplinary approaches, combining aspects of the different technologies reviewed here, will inspire innovative ideas to further elucidate the secrets of ECM-mediated cell control.

Biological activity of peptide-conjugated polyion complex matrices consisting of alginate and chitosan

Peptide-conjugated polysaccharide matrices using bioactive laminin-derived peptides are useful biomaterials for tissue and cell engineering. Here, we demonstrate an easy handling preparation method for peptide-polysaccharide matrices using polyion complex with both alginate and chitosan. First, aldehyde-alginate was synthesized by oxidization of alginate using NaIO₄ , and then, reacted with Cys-peptides. Next, the peptide-alginate solution was added to a chitosan-coated plate, and the peptide-polyion complex matrices (peptide-PCMs) were prepared. The peptide-PCMs using an integrin αvβ3-binding peptide (A99a: ALRGDN, mouse laminin α1 chain 1145-1150) and an integrin α2β1-binding peptide (EF1XmR: RLQLQEGRLHFXFD, X = Nle, mouse laminin α1 chain 2751-2763) showed strong cell attachment activity in a dose-dependent manner. When we examined the effect of various spacers on the biological activity of A99a-PCM, hydrophobic and long spacers enhanced the cell attachment activity. Further, the A99a-PCM with the spacers strongly promoted neurite outgrowth. The polyion complex method is an easy way to obtain insolubilized matrix and is widely applicable for various polysaccharides. The peptide-PCM is useful as a biomaterial for cell and tissue engineering.

Myoblast adhesion, proliferation and differentiation on human elastin-like polypeptide (HELP) hydrogels

BACKGROUND

The biochemical, mechanical and topographic properties of extracellular matrix are crucially involved in determining skeletal muscle cell morphogenesis, proliferation and differentiation. Human elastin-like polypeptides (HELPs) are recombinant biomimetic proteins designed to mimic some properties of the native matrix protein; when employed as myoblast adhesion substrates, they stimulate in vitro myogenesis. Given the influence that the biophysical properties of extracellular matrix have on skeletal muscle cells, the aim of this work was to investigate the effects of HELP hydrogels on myoblasts' viability and functions.

METHODS

We recently synthesized a novel polypeptide, HELPc, by fusing the elastin-like backbone to a 41aa sequence present in the α2 chain of type IV collagen, containing two arginyl-glycyl-aspartic acid (RGD) motifs. To obtain hydrogels, the enzymatic cross-linking of the HELPc was accomplished by transglutaminase. Here, we employed both non-cross-linked HELPc glass coatings and cross-linked HELPc hydrogels at different monomer densities, as adhesion substrates for C2C12 cells, used as a myoblast model.

RESULTS

By comparing cell adhesion, proliferation and differentiation, we revealed several striking differences. Depending on support rigidity, adhesion to HELPc substrates dictated cell morphology, spreading, focal adhesion formation and cytoskeletal organization. Hydrogels greatly stimulated cell proliferation, particularly in low-serum medium, and partially inhibited myogenic differentiation.

CONCLUSIONS

On the whole, the results underline the potential of these genetically engineered polypeptides as a tool for dissecting crucial steps in myogenesis.

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.

Diversification and enrichment of clinical biomaterials inspired by Darwinian evolution

Regenerative medicine and biomaterials design are driven by biomimicry. There is the essential requirement to emulate human cell, tissue, organ and physiological complexity to ensure long-lasting clinical success. Biomimicry projects for biomaterials innovation can be re-invigorated with evolutionary insights and perspectives, since Darwinian evolution is the original dynamic process for biological organisation and complexity. Many existing human inspired regenerative biomaterials (defined as a nature generated, nature derived and nature mimicking structure, produced within a biological system, which can deputise for, or replace human tissues for which it closely matches) are without important elements of biological complexity such as, hierarchy and autonomous actions. It is possible to engineer these essential elements into clinical biomaterials via bioinspired implementation of concepts, processes and mechanisms played out during Darwinian evolution; mechanisms such as, directed, computational, accelerated evolutions and artificial selection contrived in the laboratory. These dynamos for innovation can be used during biomaterials fabrication, but also to choose optimal designs in the regeneration process. Further evolutionary information can help at the design stage; gleaned from the historical evolution of material adaptations compared across phylogenies to changes in their environment and habitats. Taken together, harnessing evolutionary mechanisms and evolutionary pathways, leading to ideal adaptations, will eventually provide a new class of Darwinian and evolutionary biomaterials. This will provide bioengineers with a more diversified and more efficient innovation tool for biomaterial design, synthesis and function than currently achieved with synthetic materials chemistry programmes and rational based materials design approach, which require reasoned logic. It will also inject further creativity, diversity and richness into the biomedical technologies that we make. All of which are based on biological principles. Such evolution-inspired biomaterials have the potential to generate innovative solutions, which match with existing bioengineering problems, in vital areas of clinical materials translation that include tissue engineering, gene delivery, drug delivery, immunity modulation, and scar-less wound healing.

STATEMENT OF SIGNIFICANCE

Evolution by natural selection is a powerful generator of innovations in molecular, materials and structures. Man has influenced evolution for thousands of years, to create new breeds of farm animals and crop plants, but now molecular and materials can be molded in the same way. Biological molecules and simple structures can be evolved, literally in the laboratory. Furthermore, they are re-designed via lessons learnt from evolutionary history. Through a 3-step process to (1) create variants in material building blocks, (2) screen the variants with beneficial traits/properties and (3) select and support their self-assembly into usable materials, improvements in design and performance can emerge. By introducing biological molecules and small organisms into this process, it is possible to make increasingly diversified, sophisticated and clinically relevant materials for multiple roles in biomedicine.

Design of Injectable Materials to Improve Stem Cell Transplantation

Stem cell-based therapies are steadily gaining traction for regenerative medicine approaches to treating disease and injury throughout the body. While a significant body of work has shown success in preclinical studies, results often fail to translate in clinical settings. One potential cause is the massive transplanted cell death that occurs post injection, preventing functional integration with host tissue. Therefore, current research is focusing on developing injectable hydrogel materials to protect cells during delivery and to stimulate endogenous regeneration through interactions of transplanted cells and host tissue. This review explores the design of targeted injectable hydrogel systems for improving the therapeutic potential of stem cells across a variety of tissue engineering applications with a focus on hydrogel materials that have progressed to the stage of preclinical testing.

Advancing biomaterials of human origin for tissue engineering

Biomaterials have played an increasingly prominent role in the success of biomedical devices and in the development of tissue engineering, which seeks to unlock the regenerative potential innate to human tissues/organs in a state of deterioration and to restore or reestablish normal bodily function. Advances in our understanding of regenerative biomaterials and their roles in new tissue formation can potentially open a new frontier in the fast-growing field of regenerative medicine. Taking inspiration from the role and multi-component construction of native extracellular matrices (ECMs) for cell accommodation, the synthetic biomaterials produced today routinely incorporate biologically active components to define an artificial in vivo milieu with complex and dynamic interactions that foster and regulate stem cells, similar to the events occurring in a natural cellular microenvironment. The range and degree of biomaterial sophistication have also dramatically increased as more knowledge has accumulated through materials science, matrix biology and tissue engineering. However, achieving clinical translation and commercial success requires regenerative biomaterials to be not only efficacious and safe but also cost-effective and convenient for use and production. Utilizing biomaterials of human origin as building blocks for therapeutic purposes has provided a facilitated approach that closely mimics the critical aspects of natural tissue with regard to its physical and chemical properties for the orchestration of wound healing and tissue regeneration. In addition to directly using tissue transfers and transplants for repair, new applications of human-derived biomaterials are now focusing on the use of naturally occurring biomacromolecules, decellularized ECM scaffolds and autologous preparations rich in growth factors/non-expanded stem cells to either target acceleration/magnification of the body's own repair capacity or use nature's paradigms to create new tissues for restoration. In particular, there is increasing interest in separating ECMs into simplified functional domains and/or biopolymeric assemblies so that these components/constituents can be discretely exploited and manipulated for the production of bioscaffolds and new biomimetic biomaterials. Here, following an overview of tissue auto-/allo-transplantation, we discuss the recent trends and advances as well as the challenges and future directions in the evolution and application of human-derived biomaterials for reconstructive surgery and tissue engineering. In particular, we focus on an exploration of the structural, mechanical, biochemical and biological information present in native human tissue for bioengineering applications and to provide inspiration for the design of future biomaterials.