Compositional Alterations of Fibrin-Based Materials for RegulatingIn VitroNeural Outgrowth

A combined cell and growth factor delivery for the repair of a critical size tibia defect using biodegradable hydrogel implants

The ability to repair critical‐sized long‐bone injuries using growth factor and cell delivery was investigated using hydrogel biomaterials. Physiological doses of the recombinant human bone morphogenic protein‐2 (rhBMP2) were delivered in a sustained manner from a biodegradable hydrogel containing peripheral human blood‐derived endothelial progenitor cells (hEPCs). The biodegradable implants made from polyethylene glycol (PEG) and denatured fibrinogen (PEG‐fibrinogen, PF) were loaded with 7.7 μg/ml of rhBMP2 and 2.5 × 10⁶ cells/ml hEPCs. The safety and efficacy of the implant were tested in a rodent model of a critical‐size long‐bone defect. The hydrogel implants were formed ex‐situ and placed into defects in the tibia of athymic nude rats and analyzed for bone repair after 13 weeks following surgery. The hydrogels containing a combination of 7.7 μg/ml of rhBMP2 and 2.5 × 10⁶ cells/ml hEPCs were compared to control hydrogels containing 7.7 μg/ml of rhBMP2 only, 2.5 × 10⁶ cells/ml hEPCs only, or bare hydrogels. Assessments of bone repair include histological analysis, bone formation at the site of implantation using quantitative microCT, and assessment of implant degradation. New bone formation was detected in all treated animals, with the highest amounts found in the treatments that included animals that combined the PF implant with rhBMP2. Moreover, statistically significant increases in the tissue mineral density (TMD), trabecular number and trabecular thickness were observed in defects treated with rhBMP2 compared to non‐rhBMP2 defects. New bone formation was significantly higher in the hEPC‐treated defects compared to bare hydrogel defects, but there were no significant differences in new bone formation, trabecular number, trabecular thickness or TMD at 13 weeks when comparing the rhBMP2 + hEPCs‐treated defects to rhBMP2‐treated defects. The study concludes that the bone regeneration using hydrogel implants containing hEPCs are overshadowed by enhanced osteogenesis associated with sustained delivery of rhBMP2.

Advanced approaches to regenerate spinal cord injury: The development of cell and tissue engineering therapy and combinational treatments

Spinal cord injury (SCI) is a central nervous system (CNS) devastate event that is commonly caused by traumatic or non-traumatic events. The reinnervation of spinal cord axons is hampered through a myriad of devices counting on the damaged myelin, inflammation, glial scar, and defective inhibitory molecules. Unfortunately, an effective treatment to completely repair SCI and improve functional recovery has not been found. In this regard, strategies such as using cells, biomaterials, biomolecules, and drugs have been reported to be effective for SCI recovery. Furthermore, recent advances in combinatorial treatments, which address various aspects of SCI pathophysiology, provide optimistic outcomes for spinal cord regeneration. According to the global importance of SCI, the goal of this article review is to provide an overview of the pathophysiology of SCI, with an emphasis on the latest modes of intervention and current advanced approaches for the treatment of SCI, in conjunction with an assessment of combinatorial approaches in preclinical and clinical trials. So, this article can give scientists and clinicians' clues to help them better understand how to construct preclinical and clinical studies that could lead to a breakthrough in spinal cord regeneration.

Application of fibrin-based hydrogels for nerve protection and regeneration after spinal cord injury

Traffic accidents, falls, and many other events may cause traumatic spinal cord injuries (SCIs), resulting in nerve cells and extracellular matrix loss in the spinal cord, along with blood loss, inflammation, oxidative stress (OS), and others. The continuous development of neural tissue engineering has attracted increasing attention on the application of fibrin hydrogels in repairing SCIs. Except for excellent biocompatibility, flexibility, and plasticity, fibrin, a component of extracellular matrix (ECM), can be equipped with cells, ECM protein, and various growth factors to promote damage repair. This review will focus on the advantages and disadvantages of fibrin hydrogels from different sources, as well as the various modifications for internal topographical guidance during the polymerization. From the perspective of further improvement of cell function before and after the delivery of stem cell, cytokine, and drug, this review will also evaluate the application of fibrin hydrogels as a carrier to the therapy of nerve repair and regeneration, to mirror the recent development tendency and challenge.

Morphogenesis Guided by 3D Patterning of Growth Factors in Biological Matrices

Three-dimensional (3D) control over the placement of bioactive cues is fundamental to understand cell guidance and develop engineered tissues. Two-photon patterning (2PP) provides such placement at micro- to millimeter scale, but nonspecific interactions between proteins and functionalized extracellular matrices (ECMs) restrict its use. Here, a 2PP system based on nonfouling hydrophilic photocages and Sortase A (SA)-based enzymatic coupling is presented, which offers unprecedented orthogonality and signal-to-noise ratio in both inert hydrogels and complex mammalian matrices. Improved photocaged peptide synthesis and protein functionalization protocols with broad applicability are introduced. Importantly, the method enables 2PP in a single step in the presence of fragile biomolecules and cells, and is compatible with time-controlled growth factor presentation. As a corollary, the guidance of axons through 3D-patterned nerve growth factor (NGF) within brain-mimetic ECMs is demonstrated. The approach allows for the interrogation of the role of complex signaling molecules in 3D matrices, thus helping to better understand biological guidance in tissue development and regeneration.

Growth Factor Delivery for the Repair of a Critical Size Tibia Defect Using an Acellular, Biodegradable Polyethylene Glycol-Albumin Hydrogel Implant

Growth factor delivery using acellular matrices presents a promising alternative to current treatment options for bone repair in critical-size injuries. However, supra-physiological doses of the factors can introduce safety concerns that must be alleviated, mainly by sustaining delivery of smaller doses using the matrix as a depot. We developed an acellular, biodegradable hydrogel implant composed of poly(ethylene glycol) (PEG) and denatured albumin to be used for sustained delivery of bone morphogenic protein-2 (BMP2). In this study, poly(ethylene glycol)-albumin (PEG-Alb) hydrogels were produced and loaded with 7.7 μg/mL of recombinant human BMP2 (rhBMP2) to be tested for safety and performance in a critical-size long-bone defect, using a rodent model. The hydrogels were formed ex situ in a 5 mm long cylindrical mold of 3 mm diameter, implanted into defects made in the tibia of Sprague-Dawley rats and compared to non-rhBMP2 control hydrogels at 13 weeks following surgery. The hydrogels were also compared to the more established PEG-fibrinogen (PEG-Fib) hydrogels we have tested previously. Comprehensive in vitro characterization as well as in vivo assessments that include: histological analyses, including safety parameters (i.e., local tolerance and toxicity), assessment of implant degradation, bone formation, as well as repair tissue density using quantitative microCT analysis were performed. The in vitro assessments demonstrated similarities between the mechanical and release properties of the PEG-Alb hydrogels to those of the PEG-Fib hydrogels. Safety analysis presented good local tolerance in the bone defects and no signs of toxicity. A significantly larger amount of bone was detected at 13 weeks in the rhBMP2-treated defects as compared to non-rhBMP2 defects. However, no significant differences were noted in bone formation at 13 weeks when comparing the PEG-Alb-treated defects to PEG-Fib-treated defects (with or without BMP2). The study concludes that hydrogel scaffolds made from PEG-Alb containing 7.7 μg/mL of rhBMP2 are effective in accelerating the bridging of boney defects in the tibia.

Hydrogel-assisted neuroregeneration approaches towards brain injury therapy: A state-of-the-art review

Recent years have witnessed the development of an enormous variety of hydrogel-based systems for neuroregeneration. Formed from hydrophilic polymers and comprised of up to 90% of water, these three-dimensional networks are promising tools for brain tissue regeneration. They can assist structural and functional restoration of damaged tissues by providing mechanical support and navigating cell fate. Hydrogels also show the potential for brain injury therapy due to their broadly tunable physical, chemical, and biological properties. Hydrogel polymers, which have been extensively implemented in recent brain injury repair studies, include hyaluronic acid, collagen type I, alginate, chitosan, methylcellulose, Matrigel, fibrin, gellan gum, self-assembling peptides and proteins, poly(ethylene glycol), methacrylates, and methacrylamides. When viewed as tools for neuroregeneration, hydrogels can be divided into: (1) hydrogels suitable for brain injury therapy, (2) hydrogels that do not meet basic therapeutic requirements and (3) promising hydrogels which meet the criteria for further investigations. Our analysis shows that fibrin, collagen I and self-assembling peptide-based hydrogels display very attractive properties for neuroregeneration.

Hydrogel composition and laser micropatterning to regulate sciatic nerve regeneration

Treatment of peripheral nerve injuries has evolved over the past several decades to include the use of sophisticated new materials endowed with trophic and topographical cues that are essential for in vivo nerve fibre regeneration. In this research, we explored the use of an advanced design strategy for peripheral nerve repair, using biological and semi-synthetic hydrogels that enable controlled environmental stimuli to regenerate neurons and glial cells in a rat sciatic nerve resection model. The provisional nerve growth conduits were composed of either natural fibrin or adducts of synthetic polyethylene glycol and fibrinogen or gelatin. A photo-patterning technique was further applied to these 3D hydrogel biomaterials, in the form of laser-ablated microchannels, to provide contact guidance for unidirectional growth following sciatic nerve injury. We tested the regeneration capacity of subcritical nerve gap injuries in rats treated with photo-patterned materials and compared these with injuries treated with unpatterned hydrogels, either stiff or compliant. Among the factors tested were shear modulus, biological composition, and micropatterning of the materials. The microchannel guidance patterns, combined with appropriately matched degradation and stiffness properties of the material, proved most essential for the uniform tissue propagation during the nerve regeneration process.

Fundamentals of Laser-Based Hydrogel Degradation and Applications in Cell and Tissue Engineering

The cell and tissue engineering fields have profited immensely through the implementation of highly structured biomaterials. The development and implementation of advanced biofabrication techniques have established new avenues for generating biomimetic scaffolds for a multitude of cell and tissue engineering applications. Among these, laser-based degradation of biomaterials is implemented to achieve user-directed features and functionalities within biomimetic scaffolds. This review offers an overview of the physical mechanisms that govern laser-material interactions and specifically, laser-hydrogel interactions. The influences of both laser and material properties on efficient, high-resolution hydrogel degradation are discussed and the current application space in cell and tissue engineering is reviewed. This review aims to acquaint readers with the capability and uses of laser-based degradation of biomaterials, so that it may be easily and widely adopted.

Poly(ethylene glycol) diacrylate/hyaluronic acid semi-interpenetrating network compositions for 3-D cell spreading and migration

To serve as artificial matrices for therapeutic cell transplantation, synthetic hydrogels must incorporate mechanisms enabling localized, cell-mediated degradation that allows cell spreading and migration. Previously, we have shown that hybrid semi-interpenetrating polymer networks (semi-IPNs) composed of hydrolytically degradable poly(ethylene glycol) diacrylates (PEGdA), acrylate-PEG-GRGDS and native hyaluronic acid (HA) support increased cell spreading relative to fully synthetic networks that is dependent on cellular hyaluronidase activity. This study systematically investigated the effects of PEGdA/HA semi-IPN network composition on 3-D spreading of encapsulated fibroblasts, the underlying changes in gel structure responsible for this activity, and the ability of optimized gel formulations to support long-term cell survival and migration. Fibroblast spreading exhibited a biphasic response to HA concentration, required a minimum HA molecular weight, decreased with increasing PEGdA concentration and was independent of hydrolytic degradation at early time points. Increased gel turbidity was observed in semi-IPNs, but not in copolymerized hydrogels containing methacrylated HA, which did not support cell spreading. This suggests that there is an underlying mechanism of polymerization-induced phase separation that results in HA-enriched defects within the network structure. PEGdA/HA semi-IPNs were also able to support cell spreading at relatively high levels of mechanical properties (∼10kPa elastic modulus) compared to alternative hybrid hydrogels. In order to support long-term cellular remodeling, the degradation rate of the PEGdA component was optimized by preparing blends of three different PEGdA macromers with varying susceptibility to hydrolytic degradation. Optimized semi-IPN formulations supported long-term survival of encapsulated fibroblasts and sustained migration in a gel-within-gel encapsulation model. These results demonstrate that PEGdA/HA semi-IPNs provide dynamic microenvironments that can support 3-D cell survival, spreading and migration for a variety of cell therapy applications.

Molecular sequelae of topographically guided peripheral nerve repair

Peripheral nerve injuries cause severe disability with decreased nerve function often followed by neuropathic pain that impacts the quality of life. Even though use of autografts is the current gold standard, nerve conduits fabricated from electrospun nanofibers have shown promise to successfully bridge critical length nerve gaps. However, in depth analysis of the role of topographical cues in the context of spatio-temporal progression of the regenerative sequence has not been elucidated. Here, we explored the influence of topographical cues (aligned, random, and smooth films) on the regenerative sequence and potential to successfully support nerve regeneration in critical size gaps. A number of key findings emerged at the cellular, cytokine and molecular levels from the study. Higher quantities of IL-1α and TNF-α were detected in aligned fiber based scaffolds. Differential gene expression of BDNF, NGFR, ErbB2, and ErbB3 were observed suggesting a role for these genes in influencing Schwann cell migration, myelination, etc. that impact the regeneration in various topographies. Fibrin matrix stabilization and arrest of nerve-innervated muscle atrophy was also evident. Taken together, our data shed light on the cascade of events that favor regeneration in aligned topography and should stimulate research to further refine the strategy of nerve regeneration using topographical cues.

Development and analysis of semi-interpenetrating polymer networks for brain injection in neurodegenerative disorders

PURPOSE

Our aim was to assess the use of injectable, biocompatible and resorbable, hydrogel-based tools for innovative therapies against brain-related neurodegenerative disorders like Alzheimer's (AD) and Parkinson's (PD) diseases.

METHODS

Two compositions of semi-interpenetrating polymer networks (semi-IPNs) based on collagen and poly(ethylene glycol) (PEG) were investigated. We examined their viscoelastic properties, flow behavior, functional injectability, as well as in vitro biocompatibility with SH-SY5Y human neuroblastoma cells and murine primary neurons. We also evaluated the in vivo biological performance after subcutaneous and brain injection in mice.

RESULTS

selected semi-IPNs showed a gel-like behavior and were injectable through a 30 G needle, with the maximum load ranging from 3.0 to 3.9 N. In vitro results showed that immortalized cells kept their proliferative potential and neurons maintained their viability after embedding in both materials, with better performances for the gel with the higher collagen content. For both semi-IPNs, after subcutaneous injection, the inflammatory response was negligible; after brain injection, the tissue did not show any signs of damage or degeneration.

CONCLUSIONS

The results suggest that the selected semi-IPNs not only represent a proper environment for cells, but also, once injected in vivo, do not induce damage/inflammation in the surrounding brain tissue. These findings represent a crucial starting point for the development of minimally invasive and injectable hydrogel-based tools for innovative drug/cell-based therapeutic strategies against AD, PD, or other severe brain-related neurodegenerative pathologies.</p>

Time-dependent cellular morphogenesis and matrix stiffening in proteolytically responsive hydrogels

Mesenchymal stromal cells residing in proteolytically responsive hydrogel scaffolds were subjected to changes in mechanical properties associated with their own three-dimensional (3-D) morphogenesis. In order to investigate this relationship the current study documents the transient degradation and restructuring of fibroblasts seeded in hydrogel scaffolds undergoing active cell-mediated reorganization over 7days in culture. A semi-synthetic proteolytically degradable polyethylene glycol-fibrinogen (PF) hydrogel matrix and neonatal human dermal fibroblasts (NHDF) were used. Rheology (in situ and ex situ) measured stiffening of the gels and confocal laser scanning microscopy (CLSM) measured cell morphogenesis within the gels. The assumption that the matrix modulus systematically decreases as cells locally begin to enzymatically disassemble the PF hydrogel to become spindled in the material was not supported by the bulk mechanical property measurements. Instead, the PF hydrogels exhibited cell-mediated stiffening concurrent with their dynamic morphogenesis, as indicated by a four-fold increase in storage modulus after 1week in culture. Fibrin hydrogels, which were used as the control biomaterial, proved similarly adaptive to cell-mediated remodeling only in the presence of the exogenous serine protease inhibitor aprotinin. Acellular and non-viable hydrogels also served as control groups to verify that transient matrix remodeling was entirely associated with cell-mediated events, including collagen deposition, cell-mediated proteolysis, and the formation of multicellular networks within the hydrogel constructs. The fact that cell network formation and collagen deposition both paralleled transient stiffening of the PF hydrogels, further reinforces the notion that cells actively balance between proteolysis and ECM synthesis when remodeling proteolytically responsive hydrogel scaffolds.

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

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

Photopolymerization of cell-encapsulating hydrogels: crosslinking efficiency versus cytotoxicity

Cell-encapsulating hydrogels used in regenerative medicine are designed to undergo a rapid liquid-to-solid phase transition in the presence of cells and tissues so as to maximize crosslinking and minimize cell toxicity. Light-activated free-radical crosslinking (photopolymerization) is of particular interest in this regard because it can provide rapid reaction rates that result in uniform hydrogel properties with excellent temporal and spatial control features. Among the many initiator systems available for photopolymerization, only a few have been identified as suitable for cell-based hydrogel formation owing to their water solubility, crosslinking properties and non-toxic reaction conditions. In this study, three long-wave ultraviolet (UV) light-activtied photoinitiators (PIs) were comparatively tested in terms of cytotoxicity, crosslinking efficiency and crosslinking kinetics of cell-encapsulating hydrogels. The hydrogels were photopolymerized from poly(ethylene glycol) (PEG) diacrylate or PEG-fibrinogen precursors using Irgacure® PIs I2959, I184 and I651, as well as with a chemical initiator/accelerator (APS/TEMED). The study specifically evaluated the PI type, PI concentration and UV light intensity, and how these affected the mechanical properties of the hydrogel (i.e. maximum storage modulus), the crosslinking reaction times and the reaction's cytotoxicity to encapsulated cells. Only two initiators (I2959 and I184) were identified as being suitable for achieving both high cell viability and efficient crosslinking of the cell-encapsulating hydrogels during the photopolymerization reaction. Optimization of PI concentration or irradiation intensity was particularly important for achieving maximum mechanical properties; a sub-optimal choice of PI concentration or irradiation intensity resulted in a substantial reduction in hydrogel modulus. Cytocompatibility may be compromised by unnecessarily prolonging exposure to cytotoxic free radicals or inadvertently enhancing the instantaneous dose of radicals in solution, both of which are dependent on the PI type/concentration and irradiation intensity. In the absence of a radical initiator, the short exposures to long-wave UV light irradiation (up to 5 min, 20 mW cm(-2), 365 nm) did not prove to be cytotoxic to cells. Therefore, it is important to understand the relationship between PIs, light irradiation conditions and crosslinking when attempting to identify a suitable hydrogel formation process for cell encapsulating hydrogels.

Injectable hydrogel materials for spinal cord regeneration: a review

Spinal cord injury (SCI) presents a complex regenerative problem due to the multiple facets of growth inhibition that occur following trauma to the cord parenchyma and stroma. Clinically, SCI is further complicated by the heterogeneity in the size, shape and extent of human injuries. Many of these injuries do not breach the dura mater and have continuous viable axons through the injury site that can later lead to some degree of functional recovery. In these cases, surgical manipulation of the spinal cord by implanting a preformed scaffold or drug delivery device may lead to further damage. Given these circumstances, in situ-forming scaffolds are an attractive approach for SCI regeneration. These synthetic and natural polymers undergo a rapid transformation from liquid to gel upon injection into the cord tissue, conforming to the individual lesion site and directly integrating with the host tissue. Injectable materials can be formulated to have mechanical properties that closely match the native spinal cord extracellular matrix, and this may enhance axonal ingrowth. Such materials can also be loaded with cellular and molecular therapeutics to modulate the wound environment and enhance regeneration. This review will focus on the current status of in situ-forming materials for spinal cord repair. The advantages of, and requirements for, such polymers will be presented, and examples of the behavior of such systems in vitro and in vivo will be presented. There are helpful lessons to be learned from the investigations of injectable hydrogels for the treatment of SCI that apply to the use of these biomaterials for the treatment of lesions in other central nervous system tissues and in organs comprising other tissue types.

Biological and mechanical implications of PEGylating proteins into hydrogel biomaterials

Protein PEGylation has been successfully applied in pharmaceuticals and more recently in biomaterials development for making bioactive and structurally versatile hydrogels. Despite many advantages in this regard, PEGylation of proteins is also known to alter biological activity and modify biophysical characteristics in ways that may be detrimental to cells. The aim of this study was to evaluate the relative loss of biological compatibility associated with PEGylating a fibrinogen precursor into a hydrogel scaffold, in comparison to thrombin cross-linked fibrin hydrogels. Specifically, we investigated the consequences of conjugating fibrinogen with linear polyethtylene glycol (PEG) polymer chains (10 kDa) on the ability to cultivate neonatal human foreskin fibroblasts (HFFs) in 3-D. For this purpose, thrombin cross-linked fibrin (TCL-Fib) and PEGylated fibrinogen (PEG-Fib) gels were prepared with HFFs and cultured for up to seven days. The benchmark biological compatibility test was based on a combined assessment of cellular morphology, proliferation, actin expression, and matrix metalloproteinase (MMP) expression in the 3-D culture systems. The results showed correlations between modulus and proteolytic biodegradation in both materials, but no correlation between the mechanical properties and the ability of HFFs to remodel the microenvironment. A slight reduction of actin, MMPs, and spindled morphology of the cells in the PEG-Fib hydrogels indicated that the PEGylation process altered the biological compatibility of the fibrin. Nevertheless, the overall benchmark performance of the two materials demonstrated that PEGylated fibrinogen hydrogels still retains much to the inherent biofunctionality of the fibrin precursor when used as a scaffold for 3-D cell cultivation.

Polymer-conjugated albumin and fibrinogen composite hydrogels as cell scaffolds designed for affinity-based drug delivery

Serum albumin was conjugated to poly-(ethylene glycol) (PEG) and cross-linked to form mono-PEGylated albumin hydrogels. These hydrogels were used as a basis for drug carrying tissue engineering scaffold materials, based on the natural affinity of various drugs and compounds for the tethered albumin in the polymer network. The results of the drug release validation experiments showed that the release kinetics of the drugs from the mono-PEGylated albumin hydrogels were controlled by the molecular weight (MW) of PEG conjugated to the albumin protein, the drug MW and its inherent affinity for albumin. Composite hydrogels containing both mono-PEGylated albumin and PEGylated fibrinogen were used specifically for three-dimensional (3D) cell culture scaffolds, with inherent bioactivity, proteolytic biodegradability and controlled drug release properties. The specific characteristics of these complex hydrogels were governed by the ratio between the concentrations of each protein, the addition of free PEG diacrylate (PEG DA) molecules to the hydrogel matrix and the MW of the PEG conjugated to each protein. Comprehensive characterization of the drug release and degradation properties, as well as 3D cell culture experiments using these composite materials, demonstrated the effectiveness of this combined approach in creating a tissue engineering scaffold material with controlled drug release features.

The role of matrix metalloproteinases in regulating neuronal and nonneuronal cell invasion into PEGylated fibrinogen hydrogels

Injured peripheral nerve tissue could benefit from biomaterial nerve guidance conduits (NGCs) that are designed to promote neuronal regeneration. Nerve regeneration is a complex multi-step process that involves the remodeling of the ECM surrounding the regenerating neural tissue. Hydrogel biomaterials have been used as provisional matrices to regulate this regeneration process by providing the desired physical properties and controllable degradation characteristics. The purpose of this investigation was to understand the mechanism by which nerve cells penetrate into a hydrogel made from PEGylated fibrinogen. In this context, the dorsal root ganglion (DRG) assay was used as an in vitro model to study the cellular invasion behavior of both neural and nonneuronal cells. Our hypothesis stipulated that DRG cells employ matrix metalloproteinases (MMPs) in order to degrade the dense hydrogel matrix and penetrate the biomaterial. Three dimensional (3D) DRG-hydrogel constructs were cultured with MMP inhibitors (MMPi) and the effect of the inhibitors on DRG cell outgrowth was investigated. We also examined the effect of inhibitors on two dimensional (2D) DRG cell outgrowth on PEGylated fibrinogen hydrogels and on tissue culture polystyrene (TCP). Our results demonstrate that DRG cell outgrowth into and onto PEGylated fibrinogen hydrogels was inhibited by MMPi and that the outgrowth characteristics was dependent on the type of inhibitor and its concentration. MMP-3i and MMP-8i decreased both neuronal and nonneuronal outgrowth, where MMP-3i had a stronger inhibitory effect on nonneuronal cells. MMP-2/9i, on the other hand, affected the neuronal outgrowth much more than the others. We concluded that MMPs play a central role in the process of DRG cell penetration into PEGylated fibrinogen hydrogels and may also regulate the adhesion, migration and elongation of neuronal cells on the surface of these hydrogel biomaterials.

Characterization of poly(ethylene glycol) gels with added collagen for neural tissue engineering

Over the past decade, it has been increasingly recognized that both chemical and mechanical properties of scaffolds influence neural cell behavior, ranging from growth to differentiation to migration. However, mechanical properties are difficult to control for in the design of scaffolds for nerve regeneration, as properties change over time for most biologically derived scaffolds. The focus of this project was to examine how the mechanical properties of a nondegradable scaffold, poly(ethylene glycol) (PEG) gels, influenced nerve cell behavior. Low concentration PEG gels, of 3, 4, or 5% PEG, with added collagen to alter chemical properties were examined for both their mechanical properties and their ability to support nerve expression and extension. Stiffness (G*) significantly increased with increased PEG concentration. The addition of chemically conjugated collagen significantly decreased the stiffness compared to plain gels. This phenomenon was confirmed to be an effect of the conjugate, and not the protein itself, as G* of gels containing conjugate, but no protein, was not significantly different than G* of gels with conjugated protein. PC12 cell neurite expression increased with decreasing PEG and increasing collagen concentration. At its best, the expression approached the value on collagen-coated tissue culture plastic, which is a substantial improvement over previous studies on PEG. Neurite extension of dorsal root ganglia was also improved on these same gels over gels with either higher PEG concentration or lower collagen amount. Overall, these results suggest that exploration of lower stiffness materials is necessary to improve neurite growth and extension in three-dimensional synthetic scaffolds.

Protein- and peptide-modified synthetic polymeric biomaterials

This review presents an overview on bio-hybrid approaches of integrating the structural and functional features of proteins and peptides with synthetic polymers and the resulting unique properties in such hybrids, with a focus on bioresponsive/bioactive systems with biomaterials applications. The review is divided in two broad sections. First, we describe several examples of bio-hybrids produced by combining versatile synthetic polymers with proteins/enzymes and drugs that have resulted in (1) hybrid materials based on responsive polymers, (2) responsive hydrogels based on enzyme-catalyzed reactions, protein-protein interactions and protein-drug sensing, and (3) dynamic hydrogels based on conformational changes of a protein. Next, we present hybrids produced by combining synthetic polymers with peptides, classified based on the properties of the peptide domain: (1) peptides with different conformations, such as alpha-helical, coiled-coil, and beta-sheet; (2) peptides derived from structural protein domains such as silk, elastin, titin, and collagen; and (3) peptides with other biofunctional properties such as cell-binding domains and enzyme-recognized degradation domains.

Laser photoablation of guidance microchannels into hydrogels directs cell growth in three dimensions

Recent years have seen rapid progress in the engineering and application of biomaterials with controlled biological, physical, and chemical properties, and the development of associated methods for micropatterning of three-dimensional tissue-engineering scaffolds. A remaining challenge is the development of robust, flexible methods that can be used to create physical guidance structures in cell-seeded scaffolds independently of environmental constraints. Here we demonstrate that focal photoablation caused by pulsed lasers can generate guidance structures in transparent hydrogels, with feature control down to the micron scale. These photopatterned microchannels guide the directional growth of neurites from dorsal root ganglia. We characterize the effect of laser properties and biomaterial properties on microchannel formation in PEGylated fibrinogen hydrogels, and the effect of photoablation on neural outgrowth. This strategy could lead to the development of a new generation of guidance channels for treating nerve injuries, and the engineering of structured three-dimensional neuronal or nonneuronal networks.

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.