Peptide- and protein-mediated assembly of heparinized hydrogels

Injectable smart stimuli-responsive hydrogels: pioneering advancements in biomedical applications

Hydrogels have established their significance as prominent biomaterials within the realm of biomedical research. However, injectable hydrogels have garnered greater attention compared with their conventional counterparts due to their excellent minimally invasive nature and adaptive behavior post-injection. With the rapid advancement of emerging chemistry and deepened understanding of biological processes, contemporary injectable hydrogels have been endowed with an "intelligent" capacity to respond to various endogenous/exogenous stimuli (such as temperature, pH, light and magnetic field). This innovation has spearheaded revolutionary transformations across fields such as tissue engineering repair, controlled drug delivery, disease-responsive therapies, and beyond. In this review, we comprehensively expound upon the raw materials (including natural and synthetic materials) and injectable principles of these advanced hydrogels, concurrently providing a detailed discussion of the prevalent strategies for conferring stimulus responsiveness. Finally, we elucidate the latest applications of these injectable "smart" stimuli-responsive hydrogels in the biomedical domain, offering insights into their prospects.

A polyacrylamide gel containing an engineered hexameric hemoprotein as a cross-linking unit toward redox-responsive materials

Hydrogels containing synthetic polymers and supramolecular cross-linking units are expected to exhibit unique functions and properties. The heme-heme pocket interaction in hemeproteins may be useful for development of a cross-linking unit because heme binding depends on the redox states of the iron center. In this work, hexameric tyrosine-coordinated hemoprotein (HTHP) is employed as a cross-linking unit in a polyacrylamide gel to create redox-responsive hydrogels. First, redox-dependent stability of the heme-heme pocket interaction in HTHP was evaluated, and it was found that the heme affinity dramatically decreases in the Fe(ii) state. Second, the polymerization of acrylamide and engineered HTHP possessing acryloyl group-tethering heme moieties provided a polyacrylamide gel containing HTHP as a cross-linking unit. A reduction-triggered gel-sol transition in the presence of apomyoglobin was observed. Furthermore, the mechanical properties of the gels containing the engineered HTHP and methylene bisacrylamide were evaluated by a tensile test, and the Young's modulus value was determined to be 14 kPa, which is higher than that of the control gel containing only methylene bisacrylamide (8.5 kPa). Compression tests of the gels revealed redox-responsive mechanical behavior, resulting in a decrease in the compressive modulus upon the addition of a reductant. This behavior is qualitatively consistent with the redox-responsive heme binding of HTHP in a solution state. This finding is expected to contribute to the development of redox-responsive materials for biomedical and biological applications.

Synthesis and Hydrogelation of Star-Shaped Graft Copolypetides with Asymmetric Topology

To study the self-assembly and hydrogel formation of the star-shaped graft copolypeptides with asymmetric topology, star-shaped poly(_L-lysine) with various arm numbers were synthesized by using asymmetric polyglycerol dendrimers (PGDs) as the initiators and 1,1,3,3-tetramethylguanidine (TMG) as an activator for OH groups, followed by deprotection and grafting with indole or phenyl group on the side chain. The packing of the grafting moiety via non-covalent interactions not only facilitated the polypeptide segments to adopt more ordered conformations but also triggered the spontaneous hydrogelation. The hydrogelation ability was found to be correlated with polypeptide composition and topology. The star-shaped polypeptides with asymmetric topology exhibited poorer hydrogelation ability than those with symmetric topology due to the less efficient packing of the grafted moiety. The star-shaped polypeptides grafted with indole group on the side chain exhibited better hydrogelation ability than those grafted with phenyl group with the same arm number. This report demonstrated that the grafted moiety and polypeptide topology possessed the potential ability to modulate the polypeptide hydrogelation and hydrogel characteristics.

Meet me halfway: Are in vitro 3D cancer models on the way to replace in vivo models for nanomedicine development?

The complexity and diversity of the biochemical processes that occur during tumorigenesis and metastasis are frequently over-simplified in the traditional in vitro cell cultures. Two-dimensional cultures limit researchers' experimental observations and frequently give rise to misleading and contradictory results. Therefore, in order to overcome the limitations of in vitro studies and bridge the translational gap to in vivo applications, 3D models of cancer were developed in the last decades. The three dimensions of the tumor, including its cellular and extracellular microenvironment, are recreated by combining co-cultures of cancer and stromal cells in 3D hydrogel-based growth factors-inclusive scaffolds. More complex 3D cultures, containing functional blood vasculature, can integrate in the system external stimuli (e.g. oxygen and nutrient deprivation, cytokines, growth factors) along with drugs, or other therapeutic compounds. In this scenario, cell signaling pathways, metastatic cascade steps, cell differentiation and self-renewal, tumor-microenvironment interactions, and precision and personalized medicine, are among the wide range of biological applications that can be studied. Here, we discuss a broad variety of strategies exploited by scientists to create in vitro 3D cancer models that resemble as much as possible the biology and patho-physiology of in vivo tumors and predict faithfully the treatment outcome.

Alginate and alginate composites for biomedical applications

Alginate is an edible heteropolysaccharide that abundantly available in the brown seaweed and the capsule of bacteria such as Azotobacter sp. and Pseudomonas sp. Owing to alginate gel forming capability, it is widely used in food, textile and paper industries; and to a lesser extent in biomedical applications as biomaterial to promote wound healing and tissue regeneration. This is evident from the rising use of alginate-based dressing for heavily exuding wound and their mass availability in the market nowadays. However, alginate also has limitation. When in contact with physiological environment, alginate could gelate into softer structure, consequently limits its potential in the soft tissue regeneration and becomes inappropriate for the usage related to load bearing body parts. To cater this problem, wide range of materials have been added to alginate structure, producing sturdy composite materials. For instance, the incorporation of adhesive peptide and natural polymer or synthetic polymer to alginate moieties creates an improved composite material, which not only possesses better mechanical properties compared to native alginate, but also grants additional healing capability and promote better tissue regeneration. In addition, drug release kinetic and cell viability can be further improved when alginate composite is used as encapsulating agent. In this review, preparation of alginate and alginate composite in various forms (fibre, bead, hydrogel, and 3D-printed matrices) used for biomedical application is described first, followed by the discussion of latest trend related to alginate composite utilization in wound dressing, drug delivery, and tissue engineering applications.

Affinity Hydrogels for Protein Delivery

Proteins have been studied as therapeutic agents for treatment of various human diseases. However, the delivery of protein drugs into the body is challenging. In this review, we summarize and highlight progress in developing affinity hydrogels (i.e., hydrogels functionalized with protein-bound ligands) for controlled protein release. Contrary to traditional hydrogels, which release proteins mainly through diffusion, affinity hydrogels stably retain and sustainably release proteins based mainly on diffusion coupled with a binding reaction. These hydrogels can also be modulated to release proteins in response to defined molecules in a triggered manner. Future research efforts may focus on the development of intelligent affinity hydrogels to mimic the properties of human tissues in sensing different environmental stimuli for on-demand release of single or multiple proteins (i.e., biomimetic intelligence for protein delivery).

Supramolecular polymeric biomaterials

Polymeric chains crosslinked through supramolecular interactions-directional and reversible non-covalent interactions-compose an emerging class of modular and tunable biomaterials. The choice of chemical moiety utilized in the crosslink affords different thermodynamic and kinetic parameters of association, which in turn illustrate the connectivity and dynamics of the system. These parameters, coupled with the choice of polymeric architecture, can then be engineered to control environmental responsiveness, viscoelasticity, and cargo diffusion profiles, yielding advanced biomaterials which demonstrate rapid shear-thinning, self-healing, and extended release. In this review we examine the relationship between supramolecular crosslink chemistry and biomedically relevant macroscopic properties. We then describe how these properties are currently leveraged in the development of materials for drug delivery, immunology, regenerative medicine, and 3D-bioprinting (253 references).

Glycosaminoglycan and Proteoglycan-Based Biomaterials: Current Trends and Future Perspectives

Proteoglycans and their glycosaminoglycans (GAG) are essential for life as they are responsible for orchestrating many essential functions in development and tissue homeostasis, including biophysical properties and roles in cell signaling and extracellular matrix assembly. In an attempt to capture these biological functions, a range of biomaterials are designed to incorporate off-the-shelf GAGs, typically isolated from animal sources, for tissue engineering, drug delivery, and regenerative medicine applications. All GAGs, with the exception of hyaluronan, are present in the body covalently coupled to the protein core of proteoglycans, yet the incorporation of proteoglycans into biomaterials remains relatively unexplored. Proteoglycan-based biomaterials are more likely to recapitulate the unique, tissue-specific GAG profiles and native GAG presentation in human tissues. The protein core offers additional biological functionality, including cell, growth factor, and extracellular matrix binding domains, as well as sites for protein immobilization chemistries. Finally, proteoglycans can be recombinantly expressed in mammalian cells and thus offer genetic manipulation and metabolic engineering opportunities for control over the protein and GAG structures and functions. This Progress Report summarizes current developments in GAG-based biomaterials and presents emerging research and future opportunities for the development of biomaterials that incorporate GAGs presented in their native proteoglycan form.

Multicomponent self-assembly as a tool to harness new properties from peptides and proteins in material design

Nature is enriched with a wide variety of complex, synergistic and highly functional protein-based multicomponent assemblies.

Tissue-specific extracellular matrix promotes myogenic differentiation of human muscle progenitor cells on gelatin and heparin conjugated alginate hydrogels

Myogenic differentiation, cell fusion, and myotube formation of skeletal muscle progenitor cells (SMPCs) have key roles during skeletal muscle development and repair. However, after isolation from living tissue and transition to culture dishes, SMPCs gradually lose their function and stop propagating due to the absence of extracellular matrix (ECM). Despite encouraging results of experiments using ECM components in cell culture for maintenance and propagation of some tissue types, the benefits of this approach on SMPC culture are limited, because the bioactive molecules and proteins instantly release and are degraded during culture. In this study, we developed a novel approach to enhance the proliferation and differentiation of human skeletal muscle progenitor cells (hSMPCs) in vitro with skeletal muscle ECM in combination with a modified alginate hydrogel conjugated with gelatin and heparin (Alg-G-H) as a substrate. This Alg-G-H substrate, together with skeletal muscle ECM, significantly enhanced cell expansion, differentiation, and maturation of hSMPCs compared with individual substrata (i.e. gelatin, Matrigel®, or ECM alone). In Western-blot and immunocytochemical analyses, the Alg-G-H-ECM predominantly enhanced expression of skeletal myogenesis markers (MyoD, Myf5, Myogenin, Desmin and Myosin) and myotube formation in hSMPCs. This study demonstrated that combining Alg-G-H substrates with skeletal muscle ECM modulated homeostasis of cell proliferation, differentiation, and maturation of hSMPCs by releasing signaling molecules and growth factors. This technique could be a cost-effective tool for in vitro skeletal muscle cell differentiation and maturation, with potential applications in tissue regeneration and drug development.

STATEMENT OF SIGNIFICANCE

Alginate based biomaterials are commonly used in tissue engineering and regenerative medicine field, however, the inefficient sequestration of growth factors restricted its utilization. In this study, a novel alginate based substrates was produced covalently modified with gelatin and heparin, in order to capture more effective cytokines and proteins in the culture milieu, keep homeostasis for cell survival and tissue regeneration with growth factor sequestration and long-term release capacities. Combining with skeletal muscle derived ECM, the modified Alginate-Gelatin-Heparin gel could most effectively mimic the tissue specific microenvironment to support skeletal muscle progenitor cells proliferation, differentiation and myotube formation. Additionally, the economical and practical features will make it more promising in high-throughput application for regenerative medicine research.

Biomolecularly stimuli-responsive tetra-poly(ethylene glycol) that undergoes sol–gel transition in response to a target biomolecule

We designed biotin-conjugated four-armed poly(ethylene glycol) (biotinylated Tetra-PEG) as biomolecularly stimuli-responsive polymers that underwent the phase transition from a sol to a gel state in response to avidin as a target biomolecule.

Designing hydrogels for controlled drug delivery

Hydrogel delivery systems can leverage therapeutically beneficial outcomes of drug delivery and have found clinical use. Hydrogels can provide spatial and temporal control over the release of various therapeutic agents, including small-molecule drugs, macromolecular drugs and cells. Owing to their tunable physical properties, controllable degradability and capability to protect labile drugs from degradation, hydrogels serve as a platform in which various physiochemical interactions with the encapsulated drugs control their release. In this Review, we cover multiscale mechanisms underlying the design of hydrogel drug delivery systems, focusing on physical and chemical properties of the hydrogel network and the hydrogel-drug interactions across the network, mesh, and molecular (or atomistic) scales. We discuss how different mechanisms interact and can be integrated to exert fine control in time and space over the drug presentation. We also collect experimental release data from the literature, review clinical translation to date of these systems, and present quantitative comparisons between different systems to provide guidelines for the rational design of hydrogel delivery systems.

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.

Using carbohydrate-based biomaterials as scaffolds to control human stem cell fate

This review describes the current state and applications of several important and extensively studied natural polysaccharide and glycoprotein scaffolds that can control the stem cell fate.

Rational design of a photo-responsive UVR8-derived protein and a self-assembling peptide-protein conjugate for responsive hydrogel formation

Responsive hydrogels hold great potential in controllable drug delivery, regenerative medicine, sensing, etc. We introduced in this study the first example of a photo-responsive protein for hydrogel formation. Based on the first example of the crystal structure of a photo-responsive protein, Arabidopsis thaliana protein UVR8, we designed and expressed its derived protein UVR8-1 with a hexa-peptide WRESAI. We also prepared supramolecular nanofibers with a TIP-1 protein at their surface. The simple mixing of these two components resulted in rapid hydrogel formation through the specific interactions between the protein TIP-1 and the peptide WRESAI. Since the protein could show a reversible dimer-monomer transformation, the resulting gels also showed a reversible gel-sol phase transition which was controlled by photo-irradiation. The photo-controllable gel-sol phase transition could be applied for protein delivery and cell separation.

Thiolated human serum albumin cross-linked dextran hydrogels as a macroscale delivery system

Hydrogels play an important role in macroscale delivery systems by enabling the transport of cells and molecules. Here we present a facile and benign method to prepare a dextran-based hydrogel (Dex-sHSA) using human serum albumin (HSA) as a simultaneous drug carrier and covalent cross-linker. Drug binding affinity of the albumin protein was conserved in the thiolation step using 2-iminothiolane and subsequently, in the in situ gelation step. Oscillation rheometry studies confirmed the formation of a three-dimensional viscoelastic network upon reaction of dextran and the HSA protein. The mechanical properties of Dex-sHSA hydrogel can be tuned by the protein concentration, and the degree of thiolation of sHSA. Sustained release of hydrophobic drugs, such as ibuprofen, paclitaxel and dexamethasone, from the Dex-sHSA network was shown over one week. Hence, this albumin-based dextran hydrogel system demonstrates its potential as a macroscale delivery system of hydrophobic therapeutics for a wide range of biomedical applications.

A repertoire of peptide tags for controlled drug release from injectable noncovalent hydrogel

A repertoire of conjugable tags for controlling the release of drugs from biomaterials is highly interesting for the development of combinatorial drug administration techniques. This paper describes such a system of 11 peptide tags derived from our previous work on a physical hydrogel system cross-linked through peptide-heparin interactions. The release kinetics of the tags correlate well with their affinity to heparin and obey Fick's second law of diffusion, with the exception of the ATIII peptide, which displays a stable release profile close to a zero-order reaction. A system for release experiments over seven months was built, using the hydrogel matrix as a barrier between the reservoirs of tagged compounds and supernatant. The gel matrix can be injected without affecting the releasing properties. A tagged cyclosporin A derivative was also tested, and its release was monitored by measuring its biological activity. This work represents a design of biomaterials with an integral system of drug delivery, where both the assembly process of the matrix and affinity capture/release of tagged compounds are based on the noncovalent interaction of heparin with one class of peptides.

Heparin-functionalized polymeric biomaterials in tissue engineering and drug delivery applications

Heparin plays an important role in many biological processes via its interaction with various proteins, and hydrogels and nanoparticles comprising heparin exhibit attractive properties, such as anticoagulant activity, growth factor binding, and antiangiogenic and apoptotic effects, making them great candidates for emerging applications. Accordingly, this review summarizes recent efforts in the preparation of heparin-based hydrogels and formation of nanoparticles, as well as the characterization of their properties and applications. The challenges and future perspectives for heparin-based materials are also discussed. Prospects are promising for heparin-containing polymeric biomaterials in diverse applications ranging from cell carriers for promoting cell differentiation to nanoparticle therapeutics for cancer treatment.

Designing degradable hydrogels for orthogonal control of cell microenvironments

Degradable and cell-compatible hydrogels can be designed to mimic the physical and biochemical characteristics of native extracellular matrices and provide tunability of degradation rates and related properties under physiological conditions. Hence, such hydrogels are finding widespread application in many bioengineering fields, including controlled bioactive molecule delivery, cell encapsulation for controlled three-dimensional culture, and tissue engineering. Cellular processes, such as adhesion, proliferation, spreading, migration, and differentiation, can be controlled within degradable, cell-compatible hydrogels with temporal tuning of biochemical or biophysical cues, such as growth factor presentation or hydrogel stiffness. However, thoughtful selection of hydrogel base materials, formation chemistries, and degradable moieties is necessary to achieve the appropriate level of property control and desired cellular response. In this review, hydrogel design considerations and materials for hydrogel preparation, ranging from natural polymers to synthetic polymers, are overviewed. Recent advances in chemical and physical methods to crosslink hydrogels are highlighted, as well as recent developments in controlling hydrogel degradation rates and modes of degradation. Special attention is given to spatial or temporal presentation of various biochemical and biophysical cues to modulate cell response in static (i.e., non-degradable) or dynamic (i.e., degradable) microenvironments. This review provides insight into the design of new cell-compatible, degradable hydrogels to understand and modulate cellular processes for various biomedical applications.

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.

Secondary photocrosslinking of injectable shear-thinning dock-and-lock hydrogels

Shear-thinning hydrogels are useful in numerous applications, including as injectable carriers that act as scaffolds to support cell and drug therapies. Here, we describe the engineering of a self-assembling Dock-and-Lock (DnL) system that forms injectable shear-thinning hydrogels using molecular recognition interactions that also possess photo-triggerable secondary crosslinks. These DnL hydrogels are fabricated from peptide-modified hyaluronic acid (HA) and polypeptide precursors, can self-heal immediately after shear induced flow, are cytocompatible, and can be stabilized through light-initiated radical polymerization of methacrylate functional groups to tune gel mechanics and erosion kinetics. Secondary crosslinked hydrogels retain self-adhesive properties and exhibit cooperative physical and chemical crosslinks with moduli as high as ∼10 times larger than moduli of gels based on physical crosslinking alone. The extent of reaction and change in properties are dependent on whether the methacrylate is incorporated either at the terminus of the peptide or directly to the HA backbone. Additionally, the gel erosion can be monitored through an incorporated fluorophore and physical-chemical gels remain intact in solution over months, whereas physical gels that are not covalently crosslinked erode completely within days. Mesenchymal stem cells exhibit increased viability when cultured in physical- chemical gels, compared with those cultured in gels based on physical crosslinks alone. The physical properties of these DnL gels may be additionally tuned by adjusting component compositions, which allows DnL gels with a wide range of physical properties to be constructed for use.

Arginine functionalization of hydrogels for heparin binding--a supramolecular approach to developing a pro-angiogenic biomaterial

Our aim was to synthesize a biomaterial that stimulates angiogenesis for tissue engineering applications by exploiting the ability of heparin to bind and release vascular endothelial growth factor (VEGF). The approach adopted involved modification of a hydrogel with positively charged peptides (oligolysine or oligoarginine) to achieve heparin binding. Precursor hydrogels were produced from copolymerization of N-vinyl pyrolidone, diethylene glycol bis allyl carbonate and acrylic acid (PNDA) and functionalized after activation of the carboxylic acid groups with trilysine or triarginine peptides (PNDKKK and PNDRRR). Both hydrogels were shown to bind and release bioactive VEGF165 with arginine-modified hydrogel outperforming the lysine-modified hydrogel. Cytocompatibility of the hydrogels was confirmed in vitro with primary human dermal fibroblasts and human dermal microvascular endothelial cells (HUDMECs). Proliferation of HUDMECs was stimulated by triarginine-functionalized hydrogels, and to a lesser extent by lysine functionalized hydrogels once loaded with heparin and VEGF. The data suggests that heparin-binding hydrogels provide a promising approach to a pro-angiogenic biomaterial.

Rapid, high resolution screening of biomaterial hydrogelators by μ2rheology

A combination of sample manipulation and rheological characterization at the microscale is used to identify the gelation of poly(ethylene glycol)-heparin hydrogels over a wide range of compositions. A microfluidic device produces 50-100 droplet samples, each with a different composition. Multiple particle tracking microrheology is used to measure the rheological state of each sample. This combination requires little material and enables efficient and rapid screening of gelation conditions. The high resolution data identifies the gelation reaction percolation boundaries and a lower limit of the total hydrogelator concentration for gelation to occur, which can be used for the subsequent engineering, testing, and processing of these materials.

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.

Elastomeric polypeptide-based biomaterials

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

Cell-mediated Delivery and Targeted Erosion of Vascular Endothelial Growth Factor-Crosslinked Hydrogels

We have previously reported a novel polymeric delivery vehicle that is assembled via interaction between heparin and the vascular endothelial growth factor (VEGF). Here, the cell-responsiveness of this hydrogel-including the delivery of VEGF in response to VEGFR-2 overexpressing PAE/KDR cells (porcine aortic endothelial cells (PAE) equipped with the transcript for the kinase insert domain receptor (KDR)), consequent erosion of the hydrogel matrix, and cellular response-are highlighted. The release of VEGF and hydrogel erosion reached 100% only in the presence of PAE/KDR. The [PEG-LMWH/VEGF] hydrogel (PEG = poly(ethylene glycol), LMWH = low molecular weight heparin) correspondingly prompted increases in VEGFR-2 phosphorylation and proliferation of PAE/KDR cells. This study proves that growth factor-crosslinked hydrogels can liberate VEGF in response to specific receptors, causing gel erosion and desired cell responses. The promise of these approaches in therapeutic applications, including targeted delivery, is suggested.

Controlled polymer synthesis--from biomimicry towards synthetic biology

The controlled assembly of synthetic polymer structures is now possible with an unprecedented range of functional groups and molecular architectures. In this critical review we consider how the ability to create artificial materials over lengthscales ranging from a few nm to several microns is generating systems that not only begin to mimic those in nature but also may lead to exciting applications in synthetic biology (139 references).

Gelation of Covalently Cross-Linked PEG-Heparin Hydrogels

We study PEG-heparin hydrogels to identify compositions that lead to gel formation and measure the corresponding gelation kinetics. The material consists of a maleimide-functionalized high molecular weight heparin (HMWH) backbone covalently cross-linked with bis-thiol poly(ethylene glycol) (PEG). Using multiple particle tracking microrheology, we investigate a broad composition space, defined by the number of maleimide functional sites per HMWH (f = 3.9-11.8), the molecular weight of the PEG cross-linker (M(n) = 2000, 5000, and 10 000), and the concentrations of the heparin and PEG polymers. Gelation kinetics are characterized by time-cure superposition, yielding the gel time, t(c), and the critical relaxation exponent, n. Gelation times range from 5 < t(c) ≤ 45 min, with the fastest kinetics occurring for the highest HMWH maleimide functionalities. t(c) depends nonmonotonically on the PEG cross-linker molecular weight, suggesting that gelation is affected by the length of the cross-linker relative to intermolecular interactions between heparin molecules. The critical relaxation exponent decreases from n = 0.52 for PEG 2000 to n = 0.39 for PEG 10 000. Finally, 219 equilibrated samples taken over the entire composition space are identified as liquid or solid, defining the "gelation envelope". The boundaries of this empirical gelation envelope are in good agreement with Flory-Stockmayer theory. In all, microrheological measurements enable characterization over a large parameter space and provide crucial insight into the gelation of complex, multifunctional hydrogelators used in therapeutic applications.

Hybrid multicomponent hydrogels for tissue engineering

Artificial ECMs that not only closely mimic the hybrid nature of the natural ECM but also provide tunable material properties and enhanced biological functions are attractive candidates for tissue engineering applications. This review summarizes recent advances in developing multicomponent hybrid hydrogels by integrating modular and heterogeneous building blocks into well-defined, multifunctional hydrogel composites. The individual building blocks can be chemically, morphologically, and functionally diverse, and the hybridization can occur at molecular level or microscopic scale. The modular nature of the designs, combined with the potential synergistic effects of the hybrid systems, has resulted in novel hydrogel matrices with robust structure and defined functions.

Rheological behavior of self-assembling PEG-beta-cyclodextrin/PEG-cholesterol hydrogels

The rheological properties of a recently developed self-assembling hydrogel system composed of beta-cyclodextrin (betaCD)- and cholesterol-derivatized 8-arm star-shaped poly(ethylene glycol) (PEG8) were investigated. To understand and predict the gel rheological properties, data fitting with the Maxwell model as well as comparing the system's concentration-dependent behavior with Cates' model for reversibly breaking chains were performed. To investigate the influence of the polymer architecture, networks were also prepared by replacing the cholesterol-derivatized 8-arm star-shaped PEG by linear bifunctional PEG-cholesterol or by using 4-arm instead of 8-arm polymers. Rheological analysis showed that the 8-arm polymer-based mixtures yielded tight viscoelastic networks, but their storage and loss moduli significantly deviated from those predicted by the Maxwell model. The scaling of the plateau moduli, relaxation times, and zero-shear viscosities with concentration for gels composed of 8-arm cholesterol- and betaCD-derivatized PEG followed a power law with exponents higher than predicted by Cates' model. On the other hand, hydrogels in which linear bifunctional PEG-cholesterol was used instead of 8-arm star-shaped PEG-cholesterol or which were based on 4-arm polymers showed a substantially better fit with the Maxwell model and reduced differences between empirical and Cates' theoretical scaling exponents. Rheological analysis also showed that the hydrogels were thermoreversible. At low temperatures, the gels showed viscoelastic behavior due to slow overall relaxation of the polymer chains. At higher temperatures, however, a reduced number of betaCD/cholesterol complexes and concomitant faster chain relaxation processes eventually led to liquid-like behavior. The relationship between temperature and the relaxation time was used to determine an activation energy of 46 kJ/mol for breaking and reptation of the polymers.