Synthetic matrix metalloproteinase-sensitive hydrogels for the conduction of tissue regeneration: engineering cell-invasion characteristics

Osteoblast-like cell attachment to and calcification of novel phosphonate-containing polymeric substrates

In an attempt to interact natural bone and bone cells with biomaterials and to begin to develop modular tissue engineering scaffolds, substrates containing phosphonate groups were identified to mimic mineral-protein and natural polymer-protein interactions. In this study, we investigated poly(vinyl phosphonic acid) copolymer integration with existing materials as a graft-copolymer surface modification. Phosphonate-containing copolymer-modified surfaces were created and shown to have varying phosphate content within different polymeric surfaces. As the phosphonate content in the monomer feed approached 30% vinyl phosphonic acid, increased osteoblast-like cell adhesion (3- to 8-fold increase in adhesion) and proliferation (2- to 10-fold increase in proliferation rate) was observed. Since surfaces modified with 30% vinyl phosphonic acid in the feed exhibited a maximal cell adhesion and proliferation (9.4 x 10(4) cells/cm(2)/day), it was hypothesized that this copolymer composition was optimal for protein-polymer interactions. Osteoblast-like cells formed confluent layers and were able to differentiate on all surfaces that contained vinyl phosphonic acid. Most importantly, cells interacting with these surfaces were able to significantly mineralize the surface. These results suggest that phosphonate-containing polymers can be used to integrate biomaterials with natural bone and could be used for tissue engineering applications.

Macroporous interconnected dextran scaffolds of controlled porosity for tissue-engineering applications

Dextran hydrogels have been studied as drug delivery vehicles but not as scaffolds for tissue-engineering likely because previously synthesized dextran hydrogels had pores too small for cell penetration. Our goal was to create macroporous, interconnected dextran scaffolds. To this end, we took advantage of the liquid-liquid immiscibility of poly(ethylene glycol) and methacrylated dextran during radical crosslinking of the methacrylated moieties. By controlling the degree of methacrylate substitution on dextran, dextran molar mass and PEG concentration, macroporous hydrogels were created. The presence of PEG in solution had a significant effect on the final morphology of the dextran hydrogel leading to the formation of different types of structures, from microporous gel to macroporous gel-wall to a macroporous interconnected-beaded structure. A series of formulation diagrams were prepared which allowed us to determine which conditions led to the formation of macroporous interconnected-beaded scaffolds. Dextran macroporous interconnected-beaded gels had a high water content, between 89% and 94%, a homogeneous morphology, determined by scanning electron microscopy, with interconnected macroporous pores, as determined by protein diffusivity where the effective diffusion coefficients of BSA were calculated to be 3.1 x 10(-7)cm2/s for Dex-MA 40 kDa DS 5 and 1 x 10(-7)cm2/s for Dex-MA 6 kDa DS10, which are similar to that of BSA in water, 5.9 x 10(-7)cm2/s. Mercury intrusion porosimetry showed that the macroporous interconnected-beaded scaffolds had a bimodal distribution of macropores, with a median diameter of 41 microm, interconnected by throats, which had a median diameter of 11 microm. Together, these data suggest that the dextran scaffolds will be advantageous in applications that require an interconnected macroporous geometry, such as those of tissue engineering where cell penetration and nutrient diffusion are necessary for tissue regeneration.

Exploring and engineering the cell surface interface

Cells are inherently sensitive to local mesoscale, microscale, and nanoscale patterns of chemistry and topography. We review current approaches to control cell behavior through the nanoscale engineering of materials surfaces. Far-reaching implications are emerging for applications including medical implants, cell supports, and materials that can be used as instructive three-dimensional environments for tissue regeneration.

Inductive tissue engineering with protein and DNA-releasing scaffolds

Cellular differentiation, organization, proliferation and apoptosis are determined by a combination of an intrinsic genetic program, matrix/substrate interactions, and extracellular cues received from the local microenvironment. These molecular cues come in the form of soluble (e.g. cytokines) and insoluble (e.g. ECM proteins) factors, as well as signals from surrounding cells that can promote specific cellular processes leading to tissue formation or regeneration. Recent developments in the field of tissue engineering have employed biomaterials to present these cues, providing powerful tools to investigate the cellular processes involved in tissue development, or to devise therapeutic strategies based on cell replacement or tissue regeneration. These inductive scaffolds utilize natural and/or synthetic biomaterials fabricated into three-dimensional structures. This review summarizes the use of scaffolds in the dual role of structural support for cell growth and vehicle for controlled release of tissue inductive factors, or DNA encoding for these factors. The confluence of molecular and cell biology, materials science and engineering provides the tools to create controllable microenvironments that mimic natural developmental processes and direct tissue formation for experimental and therapeutic applications.

Extracellular stimulation in tissue engineering

The field of tissue engineering has created a need for biomaterials that are capable of providing biofunctional and structural support for living cells outside of the body. Most of the commonly used biomaterials in tissue engineering are designed based on their physicochemical properties, thus achieving precise control over mechanical strength, compliance, porosity, and degradation kinetics. Biofunctional signals are added to the scaffold by tethering, immobilizing, or supplementing biofunctional macromolecules, such as growth factors, directly to the scaffold material. The challenge in tissue engineering remains to find the correct balance between the biofunctional and the physical properties of the scaffold material for each application. Moreover, the ability to modulate communication between cells and the extracellular environment using the engineered scaffold as the actuator can provide a significant advantage in tissue engineering. In this study, a unique scaffold material is presented. The material interchangeably combines biofunctional and structural molecules by fusing the two into a single backbone macromolecule. This integration provides the basis for practical, effective, and high-resolution control of both the biofunctional and the physical properties of the scaffold material. This new scaffold material has proven effective with smooth muscle, cardiac, cartilage, and human embryonic stem cell cultures. The advantages of this approach as well as the potential applications of this unique scaffold material are discussed.

Synthesis, characterization and cytocompatibility of polyurethaneurea elastomers with designed elastase sensitivity

In designing a synthetic scaffold for engineering soft, mechanically active tissues, desirable properties include elasticity, support of cell adhesion and growth, ease of processability, and responsiveness to in vivo remodeling. To achieve these properties, we have developed a family of thermoplastic elastomers, polyurethaneureas (PUs), that possess enzymatic remodeling capabilities in addition to simple hydrolytic lability. PUs were synthesized using either polycaprolactone or triblock copolymer polycaprolactone-b-poly(ethylene glycol)-b-polycaprolactone as the soft segment, 1,4-butanediisocyanate as the hard segment, and the peptide Ala-Ala-Lys as a chain extender. The synthesized PUs had high molecular weights, low glass transition temperatures (< -54 degrees C), and were flexible with breaking strains of 670-890% and tensile strengths of 15-28 MPa. Incubation in buffered saline without elastase for 8 weeks resulted in mass loss from 12% to 18% depending on soft segment composition. The degradation significantly increased (p < 0.05) in the presence of elastase, ranging from 19% to 34% with degradation products showing no cytotoxicity. To encourage cell adhesion, PUs were surface-modified with radio frequency glow discharge followed by coupling of Arg-Gly-Asp-Ser (RGDS). Endothelial cell adhesion was >140% of tissue culture polystyrene on PU surfaces and >200% on RGDS-modified surfaces. The synthesized PUs thus combine mechanical, chemical, and bioresponsive properties that might be employed in soft-tissue engineering applications.

Biodegradable and photocrosslinkable polyphosphoester hydrogel

A new biodegradable, photocrosslinkable and multifunctional macromer, poly(6-aminohexyl propylene phosphate) (PPE-HA)-ACRL, was synthesized by conjugation of acrylate groups to the side chains of PPE-HA. By controlling the synthetic conditions, different weight fractions of acrylate in the macromers were achieved as confirmed by 1H NMR. The hydrogels obtained from PPE-HA-ACRL through photocrosslinking were dominantly elastic. With increasing acrylate contents in the macromers, the hydrogels exhibited a lower swelling ratio and higher mechanical strength. The hydrogels with different crosslinking densities lost between 4.3% and 37.4% of their mass in 84 days when incubated in phosphate-buffered saline at 37 degrees C. No significant cytotoxicity of the macromers against bone marrow-derived mesenchymal stem cells from goat (GMSC) was observed at a concentration up to 10mg/ml. Finally, GMSCs encapsulated in the photopolymerized gel maintained their viability when cultured in osteogenic medium for three weeks. Clear mineralization in the hydrogel scaffold was revealed by Von Kossa staining. This study suggests the potential of these biodegradable and photocrosslinkable as injectable tissue engineering scaffolds.

Degradable thiol-acrylate photopolymers: polymerization and degradation behavior of an in situ forming biomaterial

Degradable thiol-acrylate photopolymers are a new class of biomaterials capable of rapidly polymerizing under physiological conditions upon exposure to UV light, with or without added photoinitiators, and to depths exceeding 10 cm. These materials are formed in situ, and the versatility of their chemistry affords a high degree of control over the final material properties. For example, variations in monomer mole fractions directly affect the final network molecular structure, varying the time required to achieve complete mass loss from 25 to 100 days, the molecular weight distributions of the degradation products, and the swelling ratios and compressive moduli throughout degradation. Additionally, varying the mole fraction of multifunctional thiol monomer in the initial reaction mixture controls the concentration of reactive sites in the network available for post-polymerization modification of the polymer.

Influence of fracture gap size on the pattern of long bone healing: a computational study

Following fractures, bones restore their original structural integrity through a complex process in which several cellular events are involved. Among other factors, this process is highly influenced by the mechanical environment of the fracture site. In this study, we present a mathematical model to simulate the effect of mechanical stimuli on most of the cellular processes that occur during fracture healing, namely proliferation, migration and differentiation. On the basis of these three processes, the model then simulates the evolution of geometry, distributions of cell types and elastic properties inside a healing fracture. The three processes were implemented in a Finite Element code as a combination of three coupled analysis stages: a biphasic, a diffusion and a thermoelastic step. We tested the mechano-biological regulatory model thus created by simulating the healing patterns of fractures with different gap sizes and different mechanical stimuli. The callus geometry, tissue differentiation patterns and fracture stiffness predicted by the model were similar to experimental observations for every analysed situation.

Smart materials as scaffolds for tissue engineering

In this review, we focused our attention on the more important natural extracellular matrix (ECM) molecules (collagen and fibrin), employed as cellular scaffolds for tissue engineering and on a class of semi-synthetic materials made from the fusion of specific oligopeptide sequences, showing biological activities, with synthetic materials. In particular, these new "intelligent" scaffolds may contain oligopeptide cleaving sequences specific for matrix metalloproteinases (MMPs), integrin binding domains, growth factors, anti-thrombin sequences, plasmin degradation sites, and morphogenetic proteins. The aim was to confer to these new "intelligent" semi-synthetic biomaterials, the advantages offered by both the synthetic materials (processability, mechanical strength) and by the natural materials (specific cell recognition, cellular invasion, and the ability to supply differentiation/proliferation signals). Due to their characteristics, these semi-synthetic biomaterials represent a new and versatile class of biomimetic hybrid materials that hold clinical promise in serving as implants to promote wound healing and tissue regeneration.

Bioresponsive Hydrogel Microlenses

We report investigations of bioresponsive hydrogel microlenses as a new protein detection technology. Stimuli-responsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels have been synthesized via free-radical precipitation polymerization. These hydrogel microparticles were then functionalized with biotin via EDC coupling. Hydrogel microlenses were prepared from the particles via Coulombic assembly onto a silane-modified glass substrate. Arrays containing both pNIPAm-co-AAc microgels (as an internal control) and biotinylated pNIPAm-co-AAc microgels were then used to detect multivalent binding of both avidin and polyclonal anti-biotin. Protein binding was determined by monitoring the optical properties of the microlenses using a brightfield optical microscopy technique. The microlens method is shown to be very specific for the target protein, with no detectable interference from nonspecific protein binding. Finally, the reversibility of the hydrogel microlens assay has been studied in the case of anti-biotin to determine the potential application of the microlens assay technology in a displacement-type assay. These results suggest that the microlens method may be an appropriate one for label-free detection of proteins or small molecules via displacement of tethered protein--ligand pairs.

Localized delivery of growth factors for bone repair

Delivery of growth factors for tissue (e.g. bone, cartilage) or cell repair (e.g. nerves) is about to gain important potential as a future therapeutic tool. Depending on the targeted cell type and its state of differentiation, growth factors can activate or regulate a variety of cellular functions. Therefore, strictly localized delivery regimens at well-defined kinetics appear to be logical prerequisites to assure safe and efficacious therapeutic use of such factors and avoid unwanted side effects and toxicity, a major hurdle in the clinical development of growth factor therapies so far. This review summarizes various approaches for localized growth factor delivery as focused on bone repair. Similar considerations may apply to other growth factors and therapeutic indications. Considering the vast number of preclinical studies reported in the area of growth factor-assisted bone repair, it surprises though that only two medical products for bone repair have so far been commercialized, both consisting of a collagen matrix impregnated with a bone morphogenetic protein. The marked diversity of the reported growth factors, delivery concepts and not yet standardized animal models adds to the complexity to learn from past preclinical studies presented in the literature. Nonetheless, it is now firmly established from the available information that the type, dose and delivery kinetics of growth factors all play a decisive role for the therapeutic success of any such approach. Very likely, all of these parameters have to be adapted and optimized for each animal model or clinical case. In the future, systems for localized growth factor delivery thus need to be designed in such a way that their modular components are readily adaptable to the individual pathology. To make such customized systems feasible, close cooperative networks of biomedical and biomaterials engineers, pharmaceutical scientists, chemists, biologists and clinicians need to be established.

The roles of tissue engineering and vascularisation in the development of micro-vascular networks: a review

The construction of tissue-engineered devices for medical applications is now possible in vitro using cell culture and bioreactors. Although methods of incorporating them back into the host are available, current constructs depend purely on diffusion which limits their potential. The absence of a vascular network capable of distributing oxygen and other nutrients within the tissue-engineered device is a major limiting factor in creating vascularised artificial tissues. Though bio-hybrid prostheses such as vascular bypass grafts and skin substitutes have already been developed and are being used clinically, the absence of a capillary bed linking the two systems remains the missing link. In this review, the different approaches currently being or that have been applied to vascularise tissues are identified and discussed.

Molecularly engineered PEG hydrogels: a novel model system for proteolytically mediated cell migration

Model systems mimicking the extracellular matrix (ECM) have greatly helped in quantifying cell migration in three dimensions and elucidated the molecular determinants of cellular motility in morphogenesis, regeneration, and disease progression. Here we tested the suitability of proteolytically degradable synthetic poly(ethylene glycol) (PEG)-based hydrogels as an ECM model system for cell migration research and compared this designer matrix with the two well-established ECM mimetics fibrin and collagen. Three-dimensional migration of dermal fibroblasts was quantified by time-lapse microscopy and automated single-cell tracking. A broadband matrix metalloproteinase (MMP) inhibitor and tumor necrosis factor-alpha, a potent MMP-inducer in fibroblasts, were used to alter MMP regulation. We demonstrate a high sensitivity of migration in synthetic networks to both MMP modulators: inhibition led to an almost complete suppression of migration in PEG hydrogels, whereas MMP upregulation increased the fraction of migrating cells significantly. Conversely, migration in collagen and fibrin proved to be less sensitive to the above MMP modulators, as their fibrillar architecture allowed for MMP-independent migration through preexisting pores. The possibility of molecularly recapitulating key functions of the natural extracellular microenvironment and the improved protease sensitivity makes PEG hydrogels an interesting model system that allows correlation between protease activity and cell migration.

Computational model for cell migration in three-dimensional matrices

Although computational models for cell migration on two-dimensional (2D) substrata have described how various molecular and cellular properties and physiochemical processes are integrated to accomplish cell locomotion, the same issues, along with certain new ones, might contribute differently to a model for migration within three-dimensional (3D) matrices. To address this more complicated situation, we have developed a computational model for cell migration in 3D matrices using a force-based dynamics approach. This model determines an overall locomotion velocity vector, comprising speed and direction, for individual cells based on internally generated forces transmitted into external traction forces and considering a timescale during which multiple attachment and detachment events are integrated. Key parameters characterize cell and matrix properties, including cell/matrix adhesion and mechanical and steric properties of the matrix; critical underlying molecular properties are incorporated explicitly or implicitly. Model predictions agree well with experimental results for the limiting case of migration on 2D substrata as well as with recent experiments in 3D natural tissues and synthetic gels. Certain predicted features such as biphasic behavior of speed with density of matrix ligands for 3D migration are qualitatively similar to their 2D counterparts, but new effects generally absent in 2D systems, such as effects due to matrix sterics and mechanics, are now predicted to arise in many 3D situations. As one particular sample manifestation of these effects, the optimal levels of cell receptor expression and matrix ligand density yielding maximal migration are dependent on matrix mechanical compliance.

Biosynthetic hydrogel scaffolds made from fibrinogen and polyethylene glycol for 3D cell cultures

Tissue engineering scaffolds are fabricated from either biological materials, which provide biofunctional signals and interact well with cells, or from synthetic polymers, which provide precise control over their structural properties. We describe a biosynthetic hybrid scaffold comprised of a fibrinogen backbone and crosslinked with difunctional polyethylene glycol (PEG) side chains. Denatured fibrinogen fragments are PEGylated with PEG-diacrylates, mixed with photoinitiator and exposed to UV light to form a hydrogel material in the presence of a cell suspension. This unique hydrogel material provides a distinct advantage over other scaffold materials because its mechanical properties are highly malleable while the biological functionality is maintained by the backbone of the polymeric network. The elastic modulus of the PEG-fibrinogen hydrogel is dependent on the molecular weight of the PEG constituent and proportional to the percent polymeric composition. The biological domains in the fibrinogen backbone provide attachment motifs for endothelial cell and smooth muscle cell adhesion as well as proteolytic sensitivity for biodegradation. Smooth muscle cells demonstrate the ability to proteolytically penetrate through the hydrogel material and form interconnecting networks of cells. Our efforts to develop novel biodegradable scaffolds for cultivating cells in a 3D environment are beneficial for tissue regeneration therapies.

Synthetic extracellular matrices for tissue engineering and regeneration

The need for replacement tissues or organs requires a tissue supply that cannot be satisfied by the donor supply. The tissue engineering and regeneration field is focused on the development of biological tissue and organ substitutes and may provide functional tissues to restore, maintain, or improve tissue formation. This field is already providing new therapeutic options to bypass the limitations of organ?tissue transplantation and will likely increase in medical importance in the future. This interdisciplinary field accommodates principles of life sciences and engineering and encompasses three major strategies. The first, guided tissue regeneration, relies on synthetic matrices that are conductive to host cells populating a tissue defect site and reforming the lost tissue. The second approach, inductive strategy, involves the delivery of growth factors, typically using drug delivery strategies, which are targeted to specific cell populations in the tissues surrounding the tissue defect. In the third approach, specific cell populations, typically multiplied in culture, are directly delivered to the site at which one desires to create a new tissue or organ. In all of these approaches, the knowledge acquired from developmental studies often serves as a template for the tissue engineering approach for a specific tissue or organ. This article overviews the development of synthetic extracellular matrices (ECMs) for use in tissue engineering that aim to mimic functions of the native ECM of developing and regenerating tissues. In addition to the potential therapeutic uses of these materials, they also provide model systems for basic studies that may shed light on developmental processes.

Bovine primary chondrocyte culture in synthetic matrix metalloproteinase-sensitive poly(ethylene glycol)-based hydrogels as a scaffold for cartilage repair

A poly(ethylene glycol) (PEG)-based hydrogel was used as a scaffold for chondrocyte culture. Branched PEG-vinylsulfone macromers were end-linked with thiol-bearing matrix metalloproteinase (MMP)-sensitive peptides (GCRDGPQGIWGQDRCG) to form a three-dimensional network in situ under physiologic conditions. Both four- and eight-armed PEG macromer building blocks were examined. Increasing the number of PEG arms increased the elastic modulus of the hydrogels from 4.5 to 13.5 kPa. PEG-dithiol was used to prepare hydrogels that were not sensitive to degradation by cell-derived MMPs. Primary bovine calf chondrocytes were cultured in both MMP-sensitive and MMP-insensitive hydrogels, formed from either four- or eight-armed PEG. Most (>90%) of the cells inside the gels were viable after 1 month of culture and formed cell clusters. Gel matrices with lower elastic modulus and sensitivity to MMP-based matrix remodeling demonstrated larger clusters and more diffuse, less cell surface-constrained cell-derived matrix in the chondron, as determined by light and electron microscopy. Gene expression experiments by real-time RT-PCR showed that the expression of type II collagen and aggrecan was increased in the MMP-sensitive hydrogels, whereas the expression level of MMP-13 was increased in the MMP-insensitive hydrogels. These results indicate that cellular activity can be modulated by the composition of the hydrogel. This study represents one of the first examples of chondrocyte culture in a bioactive synthetic material that can be remodeled by cellular protease activity.

Network Formation and Degradation Behavior of Hydrogels Formed by Michael-Type Addition Reactions

Hydrolytically labile poly(ethylene glycol)-based hydrogels are fabricated via a Michael-type addition reaction between unsaturated acrylate moieties and nucleophilic thiols. Although these gels offer the advantage of selective, in situ polymerization and potential as biocompatible matrixes for cell and protein encapsulation, a thorough understanding of the complex structure-property relationships that control the macroscopic behaviors of these cross-linked networks before and during hydrolytic degradation does not exist. Therefore, in this work, a novel theoretical model is presented to describe the formation and hydrolytic degradation of the step-polymerized gels. The model accounts for variations in hydrolysis kinetics as well as structural effects such as precursor functionality and the presence of primary cycles or other structural nonidealities that lower the cross-linking efficiency of the networks. Comparison of model predictions and experimental data validate this methodology for optimizing biomaterial design and reveal that structural nonidealities play a key role in determining the degradation behavior of real cross-linked systems. Decreasing precursor concentration and functionality during network formation generate high concentrations of network nonidealities that ultimately lead to higher initial swelling ratios and faster apparent rates of degradation.

Synthetic MMP-13 degradable ECMs based on poly(N-isopropylacrylamide-co-acrylic acid) semi-interpenetrating polymer networks. I. Degradation and cell migration

Thermoresponsive and injectable semi-interpenetrating polymer networks (sIPNs) containing a biospecific cell-adhesive signal and proteolytically degradable domains were developed as a synthetic equivalent of the extracellular matrix (ECM). The sIPNs synthesized define a modular hydrogel ECM where different properties of the matrix can be manipulated independently, thus creating a system where parametric analysis of the effect of hydrogel properties on cell proliferation and differentiation is possible. sIPNs composed of poly(N-isopropylacrylamide-co-acrylic acid) [p(NIPAAm-co-AAc)] and RGD-grafted poly(acrylic acid) linear chains [p(AAc)-g-RGD] were synthesized with peptide crosslinkers containing a matrix metalloproteinase-13 (MMP-13, collagenase-3) degradable domain. The lower critical solution temperature (LCST) of peptide-crosslinked p(NIPAAm-co-AAc) sIPNs was not influenced by the addition of either linear p(AAc) or peptide-modified p(AAc) chains ( approximately 34 degrees C) in PBS. Degradation of peptide-crosslinked hydrogels and sIPNs was enzyme specific and concentration dependent. Exposure of rat calvarial osteoblast (RCO) culture to the degradation products from the peptide-crosslinked hydrogels did not significantly affect cell viability. Migration of RCOs into the sIPNs was dependent upon the presence of both a cell-adhesive RGD peptide (Ac-CGGNGEPRGDTYRAY-NH2) and proteolytically-degradable crosslinks; however, there was greater dependence on the latter. The sIPNs synthesized are versatile materials for assessing cell fate in synthetic ECM constructs in vitro and tissue regeneration in vivo.

Evolving concepts in bone tissue engineering

The field of tissue engineering integrates the latest advances in molecular biology, biochemistry, engineering, material science, and medical transplantation. Researchers in the developing field of regenerative medicine have identified bone tissue engineering as an attractive translational target. Clinical problems requiring bone regeneration are diverse, and no single regeneration approach will likely resolve all defects. Recent advances in the field of tissue engineering have included the use of sophisticated biocompatible scaffolds, new postnatal multipotent cell populations, and the appropriate cellular stimulation. In particular, synthetic polymer scaffolds allow for fast and reproducible construction, while still retaining biocompatible characteristics. These criteria relate to the immediate goal of determining the ideal implant. The search is becoming a reality with widespread availability of biocompatible scaffolds; however, the desired parameters have not been clearly defined. Currently, most research focuses on the use of bone morphogenetic proteins (BMPs), specifically BMP-2 and BMP-7. These proteins induce osteogenic differentiation in vitro, as well as bone defect healing in vivo. Protein-scaffold interactions that enhance BMP binding are of the utmost importance, since prolonged BMP release creates the most osteogenic microenvironment. Transition into clinical studies has had only mild success and relies on large doses of BMPs for bone formation. Advances within the field of bone tissue engineering will likely overcome these challenges and lead to more clinically relevant therapies.

Synthetic biomaterials as instructive extracellular microenvironments for morphogenesis in tissue engineering

New generations of synthetic biomaterials are being developed at a rapid pace for use as three-dimensional extracellular microenvironments to mimic the regulatory characteristics of natural extracellular matrices (ECMs) and ECM-bound growth factors, both for therapeutic applications and basic biological studies. Recent advances include nanofibrillar networks formed by self-assembly of small building blocks, artificial ECM networks from protein polymers or peptide-conjugated synthetic polymers that present bioactive ligands and respond to cell-secreted signals to enable proteolytic remodeling. These materials have already found application in differentiating stem cells into neurons, repairing bone and inducing angiogenesis. Although modern synthetic biomaterials represent oversimplified mimics of natural ECMs lacking the essential natural temporal and spatial complexity, a growing symbiosis of materials engineering and cell biology may ultimately result in synthetic materials that contain the necessary signals to recapitulate developmental processes in tissue- and organ-specific differentiation and morphogenesis.

Matrix metalloproteinases and their clinical applications in orthopaedics

Imbalance in the expression of matrix metalloproteinases and their inhibitors contribute considerably to abnormal connective tissue degradation prevalent in various orthopaedic joint diseases such as rheumatoid arthritis and osteoarthritis. Matrix metalloproteinase expression has been detected in ligament, tendon, and cartilage tissues in the joint. They are known to contribute to the development, remodeling, and maintenance of healthy tissue through their ability to cleave a wide range of extracellular matrix substrates. Their role has been extended to cell growth, migration, differentiation, and apoptosis. In orthopaedics, their clinical applications constantly are being explored. The multiple steps in matrix metalloproteinase regulation offer potential targets for inhibition, useful in drug therapy. The correlation between matrix metalloproteinases and progression in joint erosion presents potential prognostic and diagnostic tools in rheumatoid arthritis. Matrix metalloproteinases also can be incorporated into scaffold design to control the degradation rate of engineered tissue constructs. This current review aims to summarize and emphasize the importance of matrix metalloproteinases and their natural inhibitors in the maturation of musculoskeletal tissue through matrix remodeling and, therefore, in the generation of a new clinical potential in orthopaedics.

Protein- and gene-based tissue engineering in bone repair

A tissue engineering approach to bone regeneration includes the use of a scaffold, cells and bioactive factors alone or in various combinations. Several investigators have demonstrated enhanced bone formation when the tissue-engineered construct possesses traits inherent to autogenic bone grafts, namely osteoconductivity, osteoinductivity and osteogenicity. Use of the biodegradable polymer poly(lactide-co-glycolide) in combination with bone morphogenetic protein or primary cells genetically modified to release osteogenic protein have demonstrated the ability to induce osteogenic differentiation of, and subsequent mineralization by, muscle-derived cells and mesenchymal stem cells in both in vitro and in vivo applications.

Three-dimensional tissue fabrication

In recent years, advances in fabrication technologies have brought a new dimension to the field of tissue engineering. Using manufacturing-based methods and hydrogel chemistries, researchers have been able to fabricate tissue engineering scaffolds with complex 3-D architectures and customized chemistries that mimic the in vivo tissue environment. These techniques may be useful in developing therapies for replacing lost tissue function, as in vitro models of living tissue, and also for further enabling fundamental studies of structure/function relationships in three dimensional contexts. Here, we present an overview of 3-D tissue fabrication techniques based on methods for: scaffold fabrication, cellular assembly, and hybrid hydrogel/cell methods and review their potential utility for tissue engineering.

Light-Activated Immobilization of Biomolecules to Agarose Hydrogels for Controlled Cellular Response

We describe a new method of synthesizing photolabile hydrogel materials for convenient photoimmobilization of biomolecules on surfaces or in 3-D matrixes. Dissolved agarose was modified with photolabile S-(2-nitrobenzyl)cysteine (S-NBC) via 1,1'-carbonyldiimidazole (CDI) activation of primary hydroxyl groups. S-NBC-modified agarose remained soluble and gelable with up to 5% S-NBC substitution, yet gelation was slower and the elastic modulus of the resulting gel was lower than those of unmodified agarose. Irradiating S-NBC-grafted agarose resulted in the loss of the protecting 2-nitrobenzyl groups, thereby exposing free sulfhydryl groups for biomolecular coupling. When appropriately activated with sulfhydryl-reactive groups, either peptides or proteins were effectively immobilized to the photoirradiated hydrogel matrixes, with the irradiation energy dose (i.e., irradiation time) used to control the amount of biomolecule immobilization. When the GRGDS peptide was immobilized on agarose, it was shown to be cell-adhesive and to promote neurite outgrowth from primary, embryonic chick dorsal root ganglion neurons. The immobilized GRGDS surface ligand concentration affected the cellular response: neurite length and density increased with GRGDS surface concentration at low adhesion ligand concentration and then plateaued at higher GRGDS concentration. Grafting 2-nitrobenzyl-protected compounds to hydrogel materials is useful for creating new photolabile hydrogel substrates for light-activated functional group generation and biomolecular immobilization.

Biopolymeric delivery matrices for angiogenic growth factors

The development of new therapeutic approaches that aim to help the body exert its natural mechanisms for vascularized tissue growth (therapeutic angiogenesis) has become one of the most active areas of tissue engineering. Through basic research, several growth factor families and cytokines that are capable to induce physiological blood vessel formation have been identified. Indeed, preclinical and clinical investigations have indicated that therapeutic administration of angiogenic factors, such as the prototypic vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF), to sites of ischemia in the heart or the limb can improve regional blood flow. For new and lasting tissue vascularization, prolonged tissue exposure to these factors could be critical. Furthermore, as shown for VEGF, dosage must be tightly controlled, as excess amounts of VEGF can cause severe vascular leakage and hypotension. This review emphasizes natural and synthetic polymer matrices with respect to their development as vehicles for local and controlled delivery of angiogenic proteins, such as VEGF and bFGF, and their clinical applicability. In the dawn of experimental vascular engineering, new biomaterial schemes for clinical growth factor administration that take better account of biological principles of angiogenic growth factor function and the cell biological basis necessary to produce functional vasculature are evolving. Alongside their base function as protective embedment for angiogenic growth factors, these new classes of bioactive polymers are engineered with additional functionalities that better preserve growth factor activity and more closely mimic the in vivo release mechanisms and profiles of angiogenic growth factors from the extracellular matrix (ECM). Consequently, the preparation of both natural or completely synthetic materials with biological characteristics of the ECM has become central to many tissue engineering approaches that aim to deliver growth factors in a therapeutically efficient mode. Another promising venue to improve angiogenic performance is presented by biomaterials that allow sequential delivery of growth factors with complementary roles in blood vessel initiation and stabilization.

Materials as morphogenetic guides in tissue engineering

Within native tissues cells are held within the extracellular matrix (ECM), which has a role in maintaining homeostasis, guiding development and directing regeneration. Efforts in tissue engineering have aimed to mimick the ECM to help guide morphogenesis and tissue repair. Studies have not only looked at ways to mimick the structure and characteristics of the ECM, but have also considered ways to reproduce its molecular properties including its bioadhesive character, proteolytic susceptibility and ability to bind growth factors.

Engineering biological structures of prescribed shape using self-assembling multicellular systems

Self-assembly is a fundamental process that drives structural organization in both inanimate and living systems. It is in the course of self-assembly of cells and tissues in early development that the organism and its parts eventually acquire their final shape. Even though developmental patterning through self-assembly is under strict genetic control it is clear that ultimately it is physical mechanisms that bring about the complex structures. Here we show, both experimentally and by using computer simulations, how tissue liquidity can be used to build tissue constructs of prescribed geometry in vitro. Spherical aggregates containing many thousands of cells, which form because of tissue liquidity, were implanted contiguously into biocompatible hydrogels in circular geometry. Depending on the properties of the gel, upon incubation, the aggregates either fused into a toroidal 3D structure or their constituent cells dispersed into the surrounding matrix. The model simulations, which reproduced the experimentally observed shapes, indicate that the control parameter of structure evolution is the aggregate-gel interfacial tension. The model-based analysis also revealed that the observed toroidal structure represents a metastable state of the cellular system, whose lifetime depends on the magnitude of cell-cell and cell-matrix interactions. Thus, these constructs can be made long-lived. We suggest that spherical aggregates composed of organ-specific cells may be used as "bio-ink" in the evolving technology of organ printing.

Synthetic extracellular matrices for in situ tissue engineering

Cell interactions with the extracellular matrix play important roles in guiding tissue morphogenesis. The matrix stimulates cells to influence such things as differentiation and the cells actively remodel the matrix via local proteolytic activity. We have designed synthetic hydrogel networks that participate in this interplay: They signal cells via bound adhesion and growth factors, and they also respond to the remodeling influence of cell-associated proteases. Poly(ethylene glycol)-bis-vinylsulfone was crosslinked by a Michael-type addition reaction with a peptide containing three cysteine residues, the peptide sequence being cleavable between each cysteine residue by the cell-associated protease plasmin. Cells were able to invade gel networks that contained adhesion peptides and were crosslinked by plasmin-sensitive peptides, while materials lacking either of these two characteristics resisted cell infiltration. Incorporated bone morphogenetic protein-2 (BMP-2) induced bone healing in a rat model in materials that were both adhesive and plasmin-sensitive, while materials lacking plasmin sensitivity resisted formation of bone within the material. Furthermore, when a heparin bridge was incorporated as a BMP-2 affinity site, mimicking yet another characteristic of the extracellular matrix, statistically improved bone regeneration was observed.

Microenvironmental VEGF concentration, not total dose, determines a threshold between normal and aberrant angiogenesis

Use of long-term constitutive expression of VEGF for therapeutic angiogenesis may be limited by the growth of abnormal blood vessels and hemangiomas. We investigated the relationship between VEGF dosage and the morphology and function of newly formed blood vessels by implanting retrovirally transduced myoblasts that constitutively express VEGF164 into muscles of adult mice. Reducing VEGF dosage by decreasing the total number of VEGF myoblasts implanted did not prevent vascular abnormalities. However, when clonal populations of myoblasts homogeneously expressing different levels of VEGF were implanted, a threshold between normal and aberrant angiogenesis was found. Clonal myoblasts that expressed low to medium levels of VEGF induced growth of stable, pericyte-coated capillaries of uniform size that were not leaky and became VEGF independent, as shown by treatment with the potent VEGF blocker VEGF-TrapR1R2. In contrast, clones that expressed high levels of VEGF induced hemangiomas. Remarkably, when different clonal populations were mixed, even a small proportion of cells with high production of VEGF was sufficient to cause hemangioma growth. These results show for the first time to our knowledge that the key determinant of whether VEGF-induced angiogenesis is normal or aberrant is the microenvironmental amount of growth factor secreted, rather than the overall dose. Long-term continuous delivery of VEGF, when maintained below a threshold microenvironmental level, can lead to normal angiogenesis without other exogenous growth factors.

MMP-2 sensitive, VEGF-bearing bioactive hydrogels for promotion of vascular healing

We sought to develop bioactive hydrogels to facilitate arterial healing, e.g., after balloon angioplasty. Toward this end, we developed a new class of proteolytically sensitive, biologically active polyethylene glycol (PEG)-peptide hydrogels that can be formed in situ to temporarily protect the arterial injury from blood contact. Furthermore, we incorporated endothelial cell-specific biological signals with the goal of enhancing arterial reendothelialization. Here we demonstrate efficient endothelial cell anchorage and activation on PEG hydrogel matrices modified by conjugation with both the cell adhesive peptide motif RGD and an engineered variant of vascular endothelial growth factor (VEGF). By crosslinking peptide sequences for cleavage by MMP-2 into the polymer backbone, the hydrogels became sensitive to proteolytic degradation by cell-derived matrix metalloproteinases (MMPs). Analysis of molecular hallmarks associated with endothelial cell activation by VEGF-RGD hydrogel matrices revealed a 70% increase in production of the latent MMP-2 zymogen compared with PEG-peptide hydrogels lacking VEGF. By additional provision of transforming growth factor beta1 (TGF-beta1) within the PEG-peptide hydrogel, conversion of the latent MMP zymogen into its active form was demonstrated. As a result of MMP-2 activation, strongly enhanced hydrogel degradation by activated endothelial cells was observed. Our data illustrate the critical importance of growth factor activities for remodeling of synthetic biomaterials into native tissue, as it is desired in many applications of regenerative medicine. Functionalized PEG-peptide hydrogels could help restore the native vessel wall and improve the performance of angioplasty procedures.

Molecular diffusion in tissue-engineered cartilage constructs: Effects of scaffold material, time, and culture conditions

Diffusion is likely to be the primary mechanism for macromolecular transport in tissue-engineered cartilage, and providing an adequate nutrient supply via diffusion may be necessary for cell proliferation and extracellular matrix production. The goal of this study was to measure the diffusivity of tissue-engineered cartilage constructs as a function of scaffold material, culture conditions, and time in culture. Diffusion coefficients of four different-sized fluorescent dextrans were measured by fluorescence recovery after photobleaching in tissue-engineered cartilage constructs seeded with human adipose-derived stem cells or acellular constructs on scaffolds of alginate, agarose, gelatin, or fibrin that were cultured for 1 or 28 days in either chondrogenic or control conditions. Diffusivities in the constructs were much greater than those of native cartilage. The diffusivity of acellular constructs increased 62% from Day 1 to Day 28, whereas diffusivity of cellular constructs decreased 42% and 27% in chondrogenic and control cultures, respectively. The decrease in diffusivity in cellular constructs is likely due to new matrix synthesis, which may be enhanced with chondrogenic media, and matrix contraction by the cells in the fibrin and gelatin scaffolds. The increase in diffusivity in the acellular constructs is probably due to scaffold degradation and swelling.

Protein-based signaling systems in tissue engineering

Tissue engineering aims to replace damaged tissues or organs using either transplanted cells or host cells recruited to the target site. Protein signaling is crucial to regulate cell phenotype and thus engineered tissue structure and function. Biomaterial vehicles are being designed to incorporate and locally deliver various molecules involved in this signaling, including both growth factors and peptides that mimick whole proteins. Controlling the concentration, local duration and spatial distribution of these factors is key to their utility and efficacy. Recent advances have been made in the development of polymeric delivery systems intended to achieve this control.