Cell migration through defined, synthetic ECM analogs

Integration column: artificial ECM: expanding the cell biology toolbox in 3D

Many crucial cellular processes in our tissues are governed by complex, spatio-temporally regulated interactions between cells and their extracellular matrix (ECM). These interactions can be studied using well-established 3D in vitro model systems such as collagen gels or Matrigel generated from native ECM macromolecular components. Recent advances in the molecular design of 'smart' synthetic biomaterials have generated artificial ECM (aECM) that mimic some of the key structural and biochemical characteristics of their naturally derived counterparts and, thanks to their synthetic origin, promise to overcome some complexities of the latter. Here I will discuss emerging approaches in aECM design, hopefully inspiring cell biologists to apply these systems to address their specific biological question.

Beyond molecular recognition: using a repulsive field to tune interfacial valency and binding specificity between adhesive surfaces

Surface-bound biomolecular fragments enable "smart" materials to recognize cells and other particles in applications ranging from tissue engineering and medical diagnostics to colloidal and nanoparticle assembly. Such smart surfaces are, however, limited in their design to biomolecular selectivity. This feature article demonstrates, using a completely nonbiological model system, how specificity can be achieved for particle (and cell) binding, employing surface designs where immobilized nanoscale adhesion elements are entirely nonselective. Fundamental principles are illustrated by a model experimental system where 11 nm cationic nanoparticles on a planar negative silica surface interact with flowing negative silica microspheres having 1.0 and 0.5 microm diameters. In these systems, the interfacial valency, defined as the number of cross-bonds needed to capture flowing particles, is tunable through ionic strength, which alters the range of the background repulsion and therefore the effective binding strength of the adhesive elements themselves. At high ionic strengths where long-range electrostatic repulsions are screened, single surface-bound nanoparticles capture microspheres, defining the univalent regime. At low ionic strengths, competing repulsions weaken the effective nanoparticle adhesion so that multiple nanoparticles are needed for microparticle capture. This article discusses important features of the univalent regime and then illustrates how multivalency produces interfacial-scale selectivity. The arguments are then generalized, providing a possible explanation for highly specific cell binding in nature, despite the degeneracy of adhesion molecules and cell types. The mechanism for the valency-related selectivity is further developed in the context of selective flocculation in the colloidal literature. Finally, results for multivalent binding are contrasted with the current thinking for interfacial design and the presentation of adhesion moieties on engineered surfaces.

The effect of photopolymerization on stem cells embedded in hydrogels

Photopolymerizable hydrogels, formed by UV-exposure of photosensitive polymers in the presence of photoinitiators, are widely used materials in tissue engineering research employed for cellular entrapment and patterning. During photopolymerization, the entrapped cells are directly exposed to polymer and photoinitiator molecules. To develop strategies that prevent potential photoexposure-damage to osteoprogenitor cells, it is important to further characterize the effects of photopolymerization on the exposed cells. In this study we analyzed the viability, proliferation and osteogenic differentiation of multipotent stromal cell (MSC) monolayers after exposure to UV-light in the presence of Irgacure 2959, a frequently used photoinitiator in tissue engineering research. Cell cycle progression, apoptosis and osteogenic differentiation of encapsulated goat MSCs were studied in photopolymerized methacrylate-derivatized hyaluronic acid hydrogel and methacrylated hyperbranched polyglycerol gel. We demonstrate adverse effects of photopolymerization on viability, proliferation and reentry into the cell cycle of the exposed cells in monolayers, whereas the MSCs retain the ability to differentiate towards the osteogenic lineage. We further show that upon encapsulation in photopolymerizable hydrogels the viability of the embedded cells is unaffected by the photopolymerization conditions, while osteogenic differentiation depends on the type of hydrogel used.

Construction of multifunctional proteins for tissue engineering: epidermal growth factor with collagen binding and cell adhesive activities

The development of different techniques based on natural and polymeric scaffolds are useful for the design of different biomimetic materials. These approaches, however, require supplementary steps for the chemical or physical modification of the biomaterial. To avoid such steps, in the present study, we constructed a new multifunctional protein that can be easily immobilized onto hydrophobic surfaces, and at the same time helps enhance specific cell adhesion and proliferation onto collagen substrates. A collagen binding domain was fused to a previously constructed protein, which had an epidermal growth factor fused to a hydrophobic peptide that allows for cell adhesion. The new fusion protein, designated fnCBD-ERE-EGF is produced in Escherichia coli, and its abilities to bind to collagen and promote cell proliferation were investigated. fnCBD-ERE-EGF was shown to keep both collagen binding and cell growth-promoting activities comparable to those of the corresponding unfused proteins. The results obtained in this study also suggest the use of a fnCBD-ERE-EGF as an alternative for the design of multifunctional ECM-bound growth factor based materials.

Elastase-sensitive elastomeric scaffolds with variable anisotropy for soft tissue engineering

PURPOSE

To develop elastase-sensitive polyurethane scaffolds that would be applicable to the engineering of mechanically active soft tissues.

METHODS

A polyurethane containing an elastase-sensitive peptide sequence was processed into scaffolds by thermally induced phase separation. Processing conditions were manipulated to alter scaffold properties and anisotropy. The scaffold's mechanical properties, degradation, and cytocompatibility using muscle-derived stem cells were characterized. Scaffold in vivo degradation was evaluated by subcutaneous implantation.

RESULTS

When heat transfer was multidirectional, scaffolds had randomly oriented pores. Imposition of a heat transfer gradient resulted in oriented pores. Both scaffolds were flexible and relatively strong with mechanical properties dependent upon fabrication conditions such as solvent type, polymer concentration and quenching temperature. Oriented scaffolds exhibited anisotropic mechanical properties with greater tensile strength in the orientation direction. These scaffolds also supported muscle-derived stem cell growth more effectively than random scaffolds. The scaffolds expressed over 40% weight loss after 56 days in elastase containing buffer. Elastase-sensitive scaffolds were complete degraded after 8 weeks subcutaneous implantation in rats, markedly faster than similar polyurethanes that did not contain the peptide sequence.

CONCLUSION

The elastase-sensitive polyurethane scaffolds showed promise for application in soft tissue engineering where controlling scaffold mechanical properties and pore architecture are desirable.

Three-dimensional micropatterning of bioactive hydrogels via two-photon laser scanning photolithography for guided 3D cell migration

Micropatterning techniques that control three-dimensional (3D) arrangement of biomolecules and cells at the microscale will allow development of clinically relevant tissues composed of multiple cell types in complex architecture. Although there have been significant developments to regulate spatial and temporal distribution of biomolecules in various materials, most micropatterning techniques are applicable only to two-dimensional patterning. We report here the use of two-photon laser scanning (TPLS) photolithographic technique to micropattern cell adhesive ligand (RGDS) in hydrogels to guide cell migration along pre-defined 3D pathways. The TPLS photolithographic technique regulates photo-reactive processes in microscale focal volumes to generate complex, free from microscale patterns with control over spatial presentation and concentration of biomolecules within hydrogel scaffolds. The TPLS photolithographic technique was used to dictate the precise location of RGDS in collagenase-sensitive poly(ethylene glycol-co-peptide) diacrylate hydrogels, and the amount of immobilized RGDS was evaluated using fluorescein-tagged RGDS. When human dermal fibroblasts cultured in fibrin clusters were encapsulated within the micropatterned collagenase-sensitive hydrogels, the cells underwent guided 3D migration only into the RGDS-patterned regions of the hydrogels. These results demonstrate the prospect of guiding tissue regeneration at the microscale in 3D scaffolds by providing appropriate bioactive cues in highly defined geometries.

The effects of matrix stiffness and RhoA on the phenotypic plasticity of smooth muscle cells in a 3-D biosynthetic hydrogel system

Studies using 2-D cultures have shown that the mechanical properties of the extracellular matrix (ECM) influence cell migration, spreading, proliferation, and differentiation; however, cellular mechanosensing in 3-D remains under-explored. To investigate this topic, a unique biomaterial system based on poly(ethylene glycol)-conjugated fibrinogen was adapted to study phenotypic plasticity in smooth muscle cells (SMCs) as a function of ECM mechanics in 3-D. Tuning the compressive modulus between 448 and 5804 Pa modestly regulated SMC cytoskeletal assembly in 3-D, with spread cells in stiff matrices having a slightly higher degree of F-actin bundling after prolonged culture. However, vinculin expression in all 3-D conditions was qualitatively low and was not assembled into the classic focal adhesions typically seen in 2-D cultures. Given the evidence that RhoA-mediated cytoskeletal contractility represents a critical node in mechanosensing, we molecularly upregulated contractility by inducing SMCs to express constitutively active RhoA. In these cells, F-actin bundling and total vinculin expression increased, and focal adhesion-like structures began to emerge, consistent with RhoA's mechanism of action in cells cultured on 2-D substrates. Furthermore, SMC proliferation in 3-D did not depend significantly on matrix stiffness, and was reduced by constitutive activation of RhoA irrespective of ECM mechanical properties. Conversely, the expression of contractile markers globally increased with constitutive RhoA activation and depended on 3-D matrix stiffness only in cells with heightened RhoA activity. Combined, these data suggest that the synergistic effects of ECM mechanics and RhoA activity on SMC phenotype in 3-D are distinct from those in 2-D, and highlight the importance of studying the mechanical role of cell-matrix interactions in tunable 3-D environments.

The emergence of ECM mechanics and cytoskeletal tension as important regulators of cell function

The ability to harvest and maintain viable cells from mammalian tissues represented a critical advance in biomedical research, enabling individual cells to be cultured and studied in molecular detail. However, in these traditional cultures, cells are grown on rigid glass or polystyrene substrates, the mechanical properties of which often do not match those of the in vivo tissue from which the cells were originally derived. This mechanical mismatch likely contributes to abrupt changes in cellular phenotype. In fact, it has been proposed that mechanical changes in the cellular microenvironment may alone be responsible for driving specific cellular behaviors. Recent multidisciplinary efforts from basic scientists and engineers have begun to address this hypothesis more explicitly by probing the effects of ECM mechanics on cell and tissue function. Understanding the consequences of such mechanical changes is physiologically relevant in the context of a number of tissues in which altered mechanics may either correlate with or play an important role in the onset of pathology. Examples include changes in the compliance of blood vessels associated with atherosclerosis and intimal hyperplasia, as well as changes in the mechanical properties of developing tumors. Compelling evidence from 2-D in vitro model systems has shown that substrate mechanical properties induce changes in cell shape, migration, proliferation, and differentiation, but it remains to be seen whether or not these same effects translate to 3-D systems or in vivo. Furthermore, the molecular "mechanotransduction" mechanisms by which cells respond to changes in ECM mechanics remain unclear. Here, we provide some historical context for this emerging area of research, and discuss recent evidence that regulation of cytoskeletal tension by changes in ECM mechanics (either directly or indirectly) may provide a critical switch that controls cell function.

Review: Photopolymerizable and Degradable Biomaterials for Tissue Engineering Applications

Photopolymerizable and degradable biomaterials are finding widespread application in the field of tissue engineering for the engineering of tissues such as bone, cartilage, and liver. The spatial and temporal control afforded by photoinitiated polymerizations has allowed for the development of injectable materials that can deliver cells and growth factors, as well as for the fabrication of scaffolding with complex structures. The materials developed for these applications range from entirely synthetic polymers (e.g., poly(ethylene glycol)) to purely natural polymers (e.g., hyaluronic acid) that are modified with photoreactive groups, with degradation based on the hydrolytic or enzymatic degradation of bonds in the polymer backbone or crosslinks. The degradation behavior also ranges from purely bulk to entirely surface degrading, based on the nature of the backbone chemistry and type of degradable units. The mechanical properties of these polymers are primarily based on factors such as the network crosslinking density and polymer concentration. As we better understand biological features necessary to control cellular behavior, smarter materials are being developed that can incorporate and mimic many of these factors.

Biomolecular Hydrogels Formed and Degraded via Site-Specific Enzymatic Reactions

We present polymeric hydrogel biomaterials that are biomimetic both in their synthesis and degradation. The design of oligopeptide building blocks with dual enzymatic responsiveness allows us to create polymer networks that are formed and functionalized via enzymatic reactions and are degradable via other enzymatic reactions, both occurring under physiological conditions. The activated transglutaminase enzyme factor XIIIa was utilized for site-specific coupling of prototypical cell adhesion ligands and for simultaneous cross-linking of hydrogel networks from factor XIIIa substrate-modified multiarm poly(ethylene glycol) macromers. Ligand incorporation is nearly quantitative and thus controllable, and does not alter the network's macroscopic properties over a concentration range that elicits specific cell adhesion. Living mammalian cells can be encapsulated in the gels without any noticeable decrease in viability. The degradation of gels can be engineered to occur, for example, via cell-secreted matrix metalloproteinases, thus rendering these gels interesting for biomedical applications such as drug delivery systems or smart implants for in situ tissue engineering.

Mechanisms of 3-D migration and matrix remodeling of fibroblasts within artificial ECMs

The elucidation of molecular cell-extracellular matrix (ECM) interactions regulating tissue dynamics necessitates straightforward model systems that can dissect the associated physiological complexity into a smaller number of distinct interactions. Here we employ a previously developed artificial ECM model system to study dynamic cell-matrix interactions involved in proteolytic three-dimensional (3-D) migration and matrix remodeling at the level of single cells. Quantitative time-lapse microscopy of primary human fibroblasts exposed to exogenous physiological matrix metalloproteinase (MMP) inhibitors revealed that 3-D migration is dependent on cell seeding density and occurred via highly localized MMP- and tissue inhibitor of metalloproteinases-2-dependent processes. Stimulation of cells by tumor necrosis factor alpha led to a striking augmentation in fibroblast migration that was accompanied by induction of alphaVbeta3 integrin expression. In long-term cultures, extensive localized cellular matrix remodeling resulted in the morphogenesis of single cells into interconnected multicellular networks. Therefore, these tailor-made artificial ECMs can replicate complex 3-D cell-matrix interactions involved in tissue development and regeneration, an important step in the design of next-generation synthetic biomaterials for tissue engineering.

Synthetic biomimetic hydrogels incorporated with ephrin-A1 for therapeutic angiogenesis

Eph receptors and ephrin ligands are essential for vascular development and angiogenic remodeling. In this work, we developed biomimetic poly(ethylene glycol)-diacrylate hydrogels incorporated with ephrin-A1 and examined their angiogenic properties. Ephrin-A1 was covalently immobilized on the surface of hydrogels by chemical modification and photopolymerization. Ephrin-A1 immobilized on hydrogels was found to retain its capacity to stimulate endothelial cell adhesion in a dose-dependent manner as similar findings were observed on polystyrene culture wells pre-adsorbed with ephrin-A1. Cell adhesion stimulated by ephrin-A1 was abolished by treatment with soluble RGDS and anti-alpha(v)beta3 integrin but not anti-alpha(v)beta5 integrin antibodies, suggesting that ephrin-A1 activates cell adhesion through alpha(v)beta3 integrins. Also, surface immobilized ephrin-A1 was found to induce endothelial tubule formation with luminal diameters ranging 5-30 microm on hydrogels. The results of these studies demonstrate that pro-angiogenic properties of ephrin-A1 are preserved in hydrogels and suggest potential applications of this hydrogel system in regenerative medicine and tissue engineering.

Hydrogels as Extracellular Matrices for Skeletal Tissue Engineering: State-of-the-Art and Novel Application in Organ Printing

Organ printing, a novel approach in tissue engineering, applies layered computer-driven deposition of cells and gels to create complex 3-dimensional cell-laden structures. It shows great promise in regenerative medicine, because it may help to solve the problem of limited donor grafts for tissue and organ repair. The technique enables anatomical cell arrangement using incorporation of cells and growth factors at predefined locations in the printed hydrogel scaffolds. This way, 3-dimensional biological structures, such as blood vessels, are already constructed. Organ printing is developing fast, and there are exciting new possibilities in this area. Hydrogels are highly hydrated polymer networks used as scaffolding materials in organ printing. These hydrogel matrices are natural or synthetic polymers that provide a supportive environment for cells to attach to and proliferate and differentiate in. Successful cell embedding requires hydrogels that are complemented with biomimetic and extracellular matrix components, to provide biological cues to elicit specific cellular responses and direct new tissue formation. This review surveys the use of hydrogels in organ printing and provides an evaluation of the recent advances in the development of hydrogels that are promising for use in skeletal regenerative medicine. Special emphasis is put on survival, proliferation and differentiation of skeletal connective tissue cells inside various hydrogel matrices.

The Stiffness of Three-dimensional Ionic Self-assembling Peptide Gels Affects the Extent of Capillary-like Network Formation

Improving our ability to control capillary morphogenesis has implications for not only better understanding of basic biology, but also for applications in tissue engineering and in vitro testing. Numerous biomaterials have been investigated as cellular supports for these applications and the biophysical environment biomaterials provide to cells has been increasingly recognized as an important factor in directing cell function. Here, the ability of ionic self-assembling peptide gels to support capillary morphogenesis and the effect of their mechanical properties is investigated. When placed in a physiological salt solution, these oligopeptides spontaneously self-assemble into gels with an extracellular matrix (ECM)-like microarchitecture. To evaluate the ability of three-dimensional (3D) self-assembled peptide gels to support capillary-like network formation, human umbilical vein endothelial cells (HUVECs) were embedded within RAD16-I ((RADA)4) or RAD16-II ((RARADADA)2) peptide gels with various stiffness values. As peptide stiffness is decreased cells show increased elongation and are increasingly able to contract gels. The observation that capillary morphogenesis is favored in more malleable substrates is consistent with previous reports using natural biomaterials. The structural properties of peptide gels and their ability to support capillary morphogenesis in vitro make them promising biomaterials to investigate for numerous biomedical applications.

Tissue Engineering for Cutaneous Wounds

Skin, the largest organ in the body, protects against toxins and microorganisms in the environment and serves to prevent dehydration of all non-aquatic animals. Immune surveillance, sensory detection, and self-healing are other critical functions of the skin. Loss of skin integrity because of injury or illness may result acutely in substantial physiologic imbalance and ultimately in significant disability or even death. It is estimated that, in 1992, there were 35.2 million cases of significant skin loss (US data) that required major therapeutic intervention. Of these, approximately 7 million wounds become chronic. Regardless of the specific advanced wound care product, the ideal goal would be to regenerate tissues such that both the structural and functional properties of the wounded tissue are restored to the levels before injury. The advent of tissue-engineered skin replacements revolutionized the therapeutic potential for recalcitrant wounds and for wounds that are not amenable to primary closure. This article will introduce the reader to the field of tissue engineering, briefly review tissue-engineered skin replacement from a historical perspective and then review current state-of-the-art concepts from our vantage point.

Synthesis of enzyme-degradable, peptide-cross-linked dextran hydrogels

Hydrogels derived from synthetic polymers have been previously engineered to degrade under the activity of matrix metalloproteinases (MMPs). It is believed that these systems can act as extracellular-matrix (ECM) equivalents mimicking the degradation and remodeling of the ECM through the activity of cell-secreted enzymes. In this study, MMP-sensitive hydrogels derived from dextran were developed. In order to avoid the incorporation of hydrolyzable esters often introduced in dextran modification strategies, the polysaccharide was modified with p-maleimidophenyl isocyanate (PMPI) thereby introducing maleimide functionalities in the backbone and resulting in dextran derivatized with p-maleimidophenyl isocyanate (Dex-PMPI). This strategy was favored to separate out the effects of random hydrolysis and enzymatic digestion in the degradation of the dextran hydrogels. A peptide cross-linker, derived from collagen and susceptible to gelatinase A (MMP-2) digestion, was synthesized with bifunctional cysteine termini and used to cross-link the Dex-PMPI. These hydrogels were found to be hydrolytically stable for more than 200 days yet degraded either within 30 h when exposed to bacterial collagenase or within 16 days when exposed to human MMP-2, demonstrating enzymatic-mediated digestion of the peptide cross-links. Further modification of the cross-linked hydrogels with laminin-derived peptides enhanced cell adhesion and survival, demonstrating the potential of these materials for use in tissue engineering applications.

Recombinant protein-co-PEG networks as cell-adhesive and proteolytically degradable hydrogel matrixes. Part II: biofunctional characteristics

We present here the biological performance in supporting tissue regeneration of hybrid hydrogels consisting of genetically engineered protein polymers that carry specific features of the natural extracellular matrix, cross-linked with reactive poly(ethylene glycol) (PEG). Specifically, the protein polymers contain the cell adhesion motif RGD, which mediates integrin receptor binding, and degradation sites for plasmin and matrix-metalloproteinases, both being proteases implicated in natural matrix remodeling. Biochemical assays as well as in vitro cell culture experiments confirmed the ability of these protein-PEG hydrogels to promote specific cellular adhesion and to exhibit degradability by the target enzymes. Cell culture experiments demonstrated that proteolytic sensitivity and suitable mechanical properties were critical for three-dimensional cell migration inside these synthetic matrixes. In vivo, protein-PEG matrixes were tested as a carrier of bone morphogenetic protein (rhBMP-2) to heal critical-sized defects in a rat calvarial defect model. The results underscore the importance of fine-tuning material properties of provisional therapeutic matrixes to induce cellular responses conducive to tissue repair. In particular, a lack of rhBMP or insufficient degradability of the protein-PEG matrix prevented healing of bone defects or remodeling and replacement of the artificial matrix. This work confirms the feasibility of attaining desired biological responses in vivo by engineering material properties through the design of single components at the molecular level. The combination of polymer science and recombinant DNA technology emerges as a powerful tool for the development of novel biomaterials.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

Fabrication of three-dimensional tissues

The goal of tissue engineering is to restore or replace the lost functions of diseased or damaged organs. Ideally, engineered tissues should provide nutrient transport, mechanical stability, coordination of multicellular processes, and a cellular microenvironment that promotes phenotypic stability. To achieve this goal, many engineered tissues require both macro- (approximately cm) and micro- (approximately 100 microm) scale architectural features. In recent years, techniques from the manufacturing world have been adapted to create scaffolds for tissue engineering with defined three-dimensional architectures at physiologically relevant length scales. This chapter reviews three-dimensional fabrication techniques for tissue engineering, including: acellular scaffolds, cellular assembly, and hybrid scaffold/cell constructs.

Demystifying the effects of a three-dimensional microenvironment in tissue morphogenesis

Tissue morphogenesis and homeostasis are dependent on a complex dialogue between multiple cell types and chemical and physical cues in the surrounding microenvironment. The emergence of engineered three-dimensional (3D) tissue constructs and the development of tractable methods to recapitulate the native tissue microenvironment ex vivo has led to a deeper understanding of tissue-specific behavior. However, much remains unclear about how the microenvironment and aberrations therein directly affect tissue morphogenesis and behavior. Elucidating the role of the microenvironment in directing tissue-specific behavior will aid in the development of surrogate tissues and tractable approaches to diagnose and treat chronic-debilitating diseases such as cancer and atherosclerosis. Toward this goal, 3D organotypic models have been developed to clarify the mechanisms of epithelial morphogenesis and the subsequent maintenance of tissue homeostasis. Here we describe the application of these 3D culture models to illustrate how the microenvironment plays a critical role in regulating mammary tissue function and signaling, and discuss the rationale for applying precisely defined organotypic culture assays to study epithelial cell behavior. Experimental methods are provided to generate and manipulate 3D organotypic cultures to study the effect of matrix stiffness and matrix dimensionality on epithelial tissue morphology and signaling. We end by discussing technical limitations of currently available systems and by presenting opportunities for improvement.

Can anti-migratory drugs be screened in vitro? A review of 2D and 3D assays for the quantitative analysis of cell migration

The aim of the present review is to detail and analyze the pros and cons of in vitro tests available to quantify the anti-migratory effects of anti-cancer drugs for their eventual use in combating the dispersal of tumor cells, a clinical need which currently remains unsatisfied. We therefore briefly sum up why anti-migratory drugs constitute a promising approach in oncology while at the same time emphasizing that migrating cancer cells are resistant to apoptosis. To analyze the pros and cons of the various in vitro tests under review we also briefly sum up the molecular and cellular stages of cancer cell migration, an approach that enables us to argue both that no single in vitro test is sufficient to characterize the anti-migratory potential of a drug and that standardization is needed for the efficient quantitative analysis of cell locomotion in a 3D environment. Before concluding our review we devote the final two parts (i) to the description of new prototypes which, in the near future, could enter the screening process with a view to identifying novel anti-migratory compounds, and (ii) to the anti-migratory compounds currently developed against cancer, with particular emphasis on how these compounds were selected before entering the clinical trial phase.

Fabrication of 3D hepatic tissues by additive photopatterning of cellular hydrogels

We have fabricated a hepatic tissue construct using a multilayer photopatterning platform for embedding cells in hydrogels of complex architecture. We first explored the potential of established hepatocyte culture models to stabilize isolated hepatocytes for photoencapsulation (e.g., double gel, Matrigel, cocultivation with nonparenchymal cells). Using photopolymerizable PEG hydrogels, we then tailored both the chemistry and architecture of the hydrogels to further support hepatocyte survival and liver-specific function. Specifically, we incorporated adhesive peptides to ligate key integrins on these adhesion-dependent cells. To identify the appropriate peptides for incorporation, the integrin expression of cultured hepatocytes was monitored by flow cytometry and their functional role in cell adhesion was assessed on full-length extracellular matrix (ECM) molecules and their adhesive peptide domains. In addition, we modified the hydrogel architecture to minimize barriers to nutrient transport for these highly metabolic cells. Viability of encapsulated cells was improved in photopatterned hydrogels with structural features of 500 microm in width over unpatterned, bulk hydrogels. Based on these findings, we fabricated a multilayer photopatterned PEG hydrogel structure containing the adhesive RGD peptide sequence to ligate the alpha5beta1 integrin of cocultured hepatocytes. Three-dimensional photopatterned constructs were visualized by digital volumetric imaging and cultured in a continuous flow bioreactor for 12 d where they performed favorably in comparison to unpatterned, unperfused constructs. These studies will have impact in the field of liver biology as well as provide enabling tools for tissue engineering of other organs.

Physiologic pulsatile flow bioreactor conditioning of poly(ethylene glycol)-based tissue engineered vascular grafts

Mechanical conditioning represents a potential means to enhance the biochemical and biomechanical properties of tissue engineered vascular grafts (TEVGs). A pulsatile flow bioreactor was developed to allow shear and pulsatile stimulation of TEVGs. Physiological 120 mmHg/80 mmHg peak-to-trough pressure waveforms can be produced at both fetal and adult heart rates. Flow rates of 2 mL/sec, representative of flow through small diameter blood vessels, can be generated, resulting in a mean wall shear stress of approximately 6 dynes/cm(2) within the 3 mm ID constructs. When combined with non-thrombogenic poly(ethylene glycol) (PEG)-based hydrogels, which have tunable mechanical properties and tailorable biofunctionality, the bioreactor represents a flexible platform for exploring the impact of controlled biochemical and biomechanical stimuli on vascular graft cells. In the present study, the utility of this combined approach for improving TEVG outcome was investigated by encapsulating 10T-1/2 mouse smooth muscle progenitor cells within PEG-based hydrogels containing an adhesive ligand (RGDS) and a collagenase degradable sequence (LGPA). Constructs subjected to 7 weeks of biomechanical conditioning had significantly higher collagen levels and improved moduli relative to those grown under static conditions.

Transendothelial migration enhances integrin-dependent human neutrophil chemokinesis

Transendothelial migration of neutrophils induces phenotypic changes that influence the interactions of neutrophils with extravascular tissue components. To assess the influence of transmigration on neutrophil chemokinetic motility, we used polyethylene glycol hydrogels covalently modified with specific peptide sequences relevant to extracellular matrix proteins. We evaluated fMLP-stimulated human neutrophil motility on peptides Arg-Gly-Asp-Ser (RGDS) and TMKIIPFNRTLIGG (P2), alone and in combination. RGDS is a bioactive sequence found in a number of proteins, and P2 is a membrane-activated complex-1 (Mac-1) ligand located in the gamma-chain of the fibrinogen protein. We evaluated, via video microscopy, cell motility by measuring cell displacement from origin and total accumulated distance traveled and then calculated average velocity. Results indicate that although adhesion and shape change were supported by hydrogels containing RGD alone, motility was not. Mac-1-dependent motility was supported on hydrogels containing P2 alone. Motility was enhanced through combined presentation of RGD and P2, engaging Mac-1, alpha(V)beta(3), and beta(1) integrins. Naïve neutrophil motility on combined peptide substrates was dependent on Mac-1, and alpha(4)beta(1) while alpha(6)beta(1) contributed to speed and linear movement. Transmigrated neutrophil motility was dependent on alpha(v)beta(3) and alpha(5)beta(1), and alpha(4)beta(1), alpha(6)beta(1), and Mac-1 contributed to speed and linear motion. Together, the data demonstrate that efficient neutrophil migration, dependent on multi-integrin interaction, is enhanced after transendothelial migration.

Bioactive hydrogel substrates: probing leukocyte receptor-ligand interactions in parallel plate flow chamber studies

The binding of activated integrins on the surface of leukocytes facilitates the adhesion of leukocytes to vascular endothelium during inflammation. Interactions between selectins and their ligands mediate rolling, and are believed to play an important role in leukocyte adhesion, though the minimal recognition motif required for physiologic interactions is not known. We have developed a novel system using poly(ethylene glycol) (PEG) hydrogels modified with either integrin-binding peptide sequences or the selectin ligand sialyl Lewis X (SLe^X) within a parallel plate flow chamber to examine the dynamics of leukocyte adhesion to specific ligands. The adhesive peptide sequences arginine–glycine–aspartic acid–serine (RGDS) and leucine–aspartic acid–valine (LDV) as well as sialyl Lewis X were bound to the surface of photopolymerized PEG diacrylate hydrogels. Leukocytes perfused over these gels in a parallel plate flow chamber at physiological shear rates demonstrate both rolling and firm adhesion, depending on the identity and concentration of ligand bound to the hydrogel substrate. This new system provides a unique polymer-based model for the study of interactions between leukocytes and endothelium as well as a platform to develop improved scaffolds for cardiovascular tissue engineering.

The use of poly(ethylene glycol) hydrogels to investigate the impact of ECM chemistry and mechanics on smooth muscle cells

Hydrogels based on poly(ethylene glycol) (PEG) are of increasing interest for regenerative medicine applications and are ideal materials to direct cell function due to the ability to confer key functionalities of native extracellular matrix (ECM) on PEG's otherwise inert backbone. Given extensive recent evidence that ECM compliance influences a variety of cell functions, PEG-based hydrogels are also attractive due to the ease with which their mechanical properties can be controlled. In these studies, we exploited the chemical and mechanical tunability of PEG-based gels to study the impact of ECM chemistry and mechanics on smooth muscle cells (SMCs) in both 2-D and 3-D model systems. First, by controlling the extent of crosslinking and therefore the mechanical properties of PEG-based hydrogels (tensile moduli from 13.7 to 423.9kPa), we report here that the assembly of F-actin stress fibers and focal adhesions, indicative of the state of actin contractility, were influenced by the compliance of 2-D PEG gels functionalized with either short adhesive peptides or full-length ECM proteins. Varying ECM ligand density and identity independent of gel compliance affected the physical properties of the focal adhesions, and also influenced SMC spreading in 2-D. Furthermore, SMCs proliferated to a greater extent as gel stiffness was increased. In contrast, the degree of SMC differentiation, which was qualitatively assessed by the extent of smooth muscle alpha-actin bundling and the association of calponin and caldesmon with the alpha-actin fibrils, was found to decrease with substrate stiffness in 2-D cultures. In 3-D, despite the fact that their viability and degree of spreading were greatly reduced, SMCs did express some contractile markers indicative of their differentiated phenotype when cultured within PEG-RGDS constructs. Combined, these data suggest that the mechanical and chemical properties of PEG hydrogels can be tuned to influence SMC phenotype in both 2-D and 3-D.

Protease-activated quantum dot probes

We have developed a novel nanoparticulate luminescent probe with inherent signal amplification upon interaction with a targeted proteolytic enzyme. This construct may be useful for imaging in cancer detection and diagnosis. In this system, quantum dots (QDs) are bound to gold nanoparticles (AuNPs) via a proteolytically degradable peptide sequence to non-radiatively suppress luminescence. A 71% reduction in luminescence was achieved with conjugation of AuNPs to QDs. Release of AuNPs by peptide cleavage restores radiative QD photoluminescence. Initial studies observed a 52% rise in luminescence over 47 h of exposure to 0.2 mg/mL collagenase. These probes can be customized for targeted degradation simply by changing the sequence of the peptide linker.

Integrin interactions with immobilized peptides in polyethylene glycol diacrylate hydrogels

This study employs tissue-engineering technologies to evaluate neutrophil interactions with extracellular matrix (ECM)-mimetic peptides. We have used a polyethylene glycol (PEG) diacrylate derivative to form a hydrogel as a biologically inert surface. Covalent attachment of bioactive moieties to the hydrogel makes it bioactive. The goal is to define the mechanisms by which these moieties influence the interactions of neutrophils with this bioactive hydrogel, and thus understand the likely effects of similar ligands in the ECM. The current experiments analyze the interactions of isolated human neutrophils with PEG hydrogels modified with Arg-Gly-Asp-Ser (RGDS), a known ligand for some beta(1) and beta(3) integrins, and Thr-Met-Lys-Ile-Ile-Pro-Phe-Asn-Arg-Leu-Thr-Ile-Gly-Gly (TMKIIPFNRLTIGG), a ligand for Mac-1, a beta(2) integrin. Our results demonstrate that neutrophils, independent of chemotactic stimulation, show little ability to adhere to unmodified PEG hydrogels. However, cell adhesion and spreading are robust on peptide-modified hydrogels. Incorporating distinct bioactive peptides, either alone or in combination, has enabled recognition of differential functions of alpha(v)beta(3), beta(1), and beta(2) integrins on neutrophil adhesion and spreading. Combined interactions result in activity that differs markedly from that seen with either integrin independently engaged. This model allows investigation of specific ligand-induced leukocyte functions and the development of engineered matrices with defined bioactive properties.

Recombinant Protein-co-PEG Networks as Cell-Adhesive and Proteolytically Degradable Hydrogel Matrixes. Part I: Development and Physicochemical Characteristics

Toward the development of synthetic bioactive materials to support tissue repair, we present here the design, production, and characterization of genetically engineered protein polymers carrying specific key features of the natural extracellular matrix, as well as cross-linking with functionalized poly(ethylene glycol) (PEG) to form hybrid hydrogel networks. The repeating units of target recombinant protein polymers contain a cell-binding site for ligation of cell-surface integrin receptors and substrates for plasmin and matrix metalloproteinases (MMPs), proteases implicated in wound healing and tissue regeneration. Hydrogels were formed under physiological conditions via Michael-type conjugate addition of vinyl sulfone groups of end-functionalized PEG with thiols of cysteine residues, representing designed chemical cross-linking sites within recombinant proteins. Cross-linking kinetics was shown to increase with the pH of precursor solutions. The elastic moduli (G') and swelling ratios (Q(m)) of the resulting hydrogels could be varied as a function of the stoichiometry of the reacting groups and precursor concentration. Optima of G' and Q(m), maximum and minimum, respectively, were obtained at stoichiometry ratios r slightly in excess of 1 (r = cysteine/vinyl sulfone). The pool of technologies utilized here represents a promising approach for the development of artificial matrixes tailored for specific medical applications.

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 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.

Breast tissue engineering

Tissue engineering has the potential to redefine rehabilitation for the breast cancer patient by providing a translatable strategy that restores the postmastectomy breast mound while concomitantly obviating limitations realized with contemporary reconstructive surgery procedures. The engineering design goal is to provide a sufficient volume of viable fat tissue based on a patient's own cells such that deficits in breast volume can be abrogated. To be sure, adipose tissue engineering is in its infancy, but tremendous strides have been made. Numerous studies attest to the feasibility of adipose tissue engineering. The field is now poised to challenge barriers to clinical translation that are germane to most tissue engineering applications, namely scale-up, large animal model development, and vascularization. The innovative and rapid progress of adipose engineering to date, as well as opportunities for its future growth, is presented.

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.

Attachment, proliferation, and migration of marrow stromal osteoblasts cultured on biomimetic hydrogels modified with an osteopontin-derived peptide

We prepared oligo(poly(ethylene glycol) fumarate) (OPF) hydrogels modified with a rat osteopontin-derived peptide (ODP), Asp-Val-Asp-Val-Pro-Asp-Gly-Arg-Gly-Asp-Ser-Leu-Ala-Try-Gly (DVDVPDGRGDSLAYG), as well as Gly-Arg-Gly-Asp-Ser (GRGDS) and investigated the modulation of marrow stromal osteoblast function on the peptide-modified hydrogels. Osteoblast attachment was competitively inhibited by a soluble peptide suggesting that the interaction of osteoblasts with the hydrogel was ligand specific. The proliferation index of osteoblasts relative to the initial seeding density was similar on the hydrogels modified with ODP (1.18+/-0.13) and GRGDS (1.27+/-0.12). However, fibroblasts proliferated faster on GRGDS-modified hydrogels than on ODP-modified hydrogels as evidenced by the proliferation indices of 4.89+/-0.03 and 2.42+/-0.16, respectively. A megacolony migration assay conducted for 3 days with a seeding density of 53,000 cells/cm(2) showed that osteoblasts migrated to a longer distance on ODP-modified hydrogels (0.23+/-0.06 mm/day) than on hydrogels modified with GRGDS (0.15+/-0.02 mm/day). In addition, osteoblasts migrated faster than fibroblasts seeded at the same density on ODP-modified hydrogels (0.15+/-0.11 mm/day). The migration of osteoblasts on the peptide-modified hydrogels was dependent on the peptide concentration of the hydrogels resulting in an increased migration distance with increasing the peptide concentration for the concentrations tested. These results show that OPF-based biomimetic hydrogels hold promise for modulating cell proliferation and migration for specific applications by altering the specific ligand and its concentration in the hydrogels.

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.

Human microvascular endothelial cell growth and migration on biomimetic surfactant polymers

Successful engineering of a tissue-incorporated vascular prosthesis requires cells to proliferate and migrate on the scaffold. Here, we report on a series of "ECM-like" biomimetic surfactant polymers that exhibit quantitative control over the proliferation and migrational properties of human microvascular endothelial cells (HMVEC). The biomimetic polymers consist of a poly(vinyl amine) (PVAm) backbone with hexanal branches and varying ratios of cell binding peptide (RGD) to carbohydrate (maltose). Proliferation and migration behavior of HMVEC was investigated using polymers containing RGD: maltose ratios of 100:0, 75:25 and 50:50, and compared with fibronectin (FN) coated glass (1 microg/cm2). A radial Teflon fence migration assay was used to examine HMVEC migration at 12 h intervals over a 48 h period. Migration was quantified using an inverted optical microscope, and HMVEC were examined by confocal microscopy for actin and focal adhesion organization/ arrangement. Over the range of RGD ligand density studied (approximately 0.19-0.6 peptides/nm2), our results show HMVEC migration decreases with increasing RGD density in the polymer. HMVEC were least motile on the 100% RGD polymer (approximately 0.38-0.6 peptides/nm2) with an average migration of 0.20 mm2/h in area covered, whereas HMVEC showed the fastest migration of 0.48+/-0.06 mm2/h on the 50% RGD surface ( approximately 0.19-0.30 peptides/nm2). In contrast, cell proliferation increased with increasing surface peptide density; proliferation on the 50% RGD surface was 1.5%+/-0.06/h compared with 2.2%+/-0.07/h on the 100% RGD surface. Our results show that surface peptide density affects cellular functions such as growth and migration, with the highest peptide density supporting the most proliferation but the slowest migration.

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.

Development of photocrosslinkable hyaluronic acid-polyethylene glycol-peptide composite hydrogels for soft tissue engineering

Hyaluronic acid (HA; also called hyaluronan) is a naturally derived, nonimmunogenic, nonadhesive glycosaminoglycan that has important roles in several wound-healing processes. In previous work, we created photocrosslinkable glycidyl methacrylate-HA (GMHA) hydrogel biomaterials that were cytocompatible, biologically active, and had a decreased rate of hyaluronidase degradation compared with native HA. The goal of the studies presented herein was to explore peptide conjugation techniques to further adjust the material and biological properties of the GMHA hydrogels. We conjugated GMHA with acrylated forms of polyethylene glycol (PEG) and PEG-peptides to yield GMHA-PEG-peptide composite hydrogels. By varying the reactant concentrations, we created stable hydrogels with high peptide conjugation efficiencies (up to 80%), controllable peptide concentrations (in the range of 1-6 micromol peptide per milliliter of hydrogel), and defined physicochemical properties (e.g., swelling ratio, enzymatic degradation rate). These composite hydrogels may prove to be a promising scaffolding biomaterial for a variety of soft tissue engineering applications.

Quantification of ligand surface concentration of bulk-modified biomimetic hydrogels

This study describes a method for the quantification of active ligand surface concentration for bulk-modified hydrogels. Two poly(propylene fumarate-co-ethylene glycol) (P(PF-co-EG)) block copolymers were synthesized with terminal poly(ethylene glycol) (PEG) chains of number average molecular weight 1960 and 5190 g/mol. Hydrogels were synthesized with bulk-modified biotin as a model ligand, making use of a PEG spacer arm with a molecular weight of 3400 g/mol. Bulk concentration of biotin was calculated from the initial concentration of biotin, sol fraction, equilibrium water content, and relative incorporation of the polymers to the hydrogel. Surface concentration of biotin bulk-modified hydrogels was quantified with an enzyme linked immunosorbent assay using mouse monoclonal anti-biotin antibody (IgG), horseradish peroxidase-conjugated anti-mouse IgG, and a chemiluminescent substrate. The larger size of the IgG relative to the mesh size of the hydrogels allowed for the quantification of the active biotin at the surface of the hydrogels. Luminescent imaging was used to qualitatively show the isolation of the horseradish peroxidase-conjugated antibodies to the surface of the bulk-modified hydrogel. The active biotin ligands at the surface of hydrogels synthesized with terminal PEG chains of 1960 g/mol were at the top 7.2 nm while for those synthesized with terminal PEG chains of 5190 g/mol were at the top 4.4 nm of the bulk-modified hydrogel. The relationship between bulk ligand concentration and the active ligand concentration at the surface was dependent on the hydrogel composition. The relative magnitude of the PEG spacer arm of the ligand compared to the PEG block length of the copolymer affected the surface availability of the ligand. The results suggest that steric hindrances caused by mobile PEG chains of the copolymer of molecular weight greater than that of the PEG spacer arm contributed to the decreased surface concentration of ligand. This work relates the bulk concentration of a ligand to its surface concentration, an important parameter for the adhesion, migration, and function of anchorage dependent cells.

RGD modified polymers: biomaterials for stimulated cell adhesion and beyond

Since RGD peptides (R: arginine; G: glycine; D: aspartic acid) have been found to promote cell adhesion in 1984 (Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule, Nature 309 (1984) 30), numerous materials have been RGD functionalized for academic studies or medical applications. This review gives an overview of RGD modified polymers, that have been used for cell adhesion, and provides information about technical aspects of RGD immobilization on polymers. The impacts of RGD peptide surface density, spatial arrangement as well as integrin affinity and selectivity on cell responses like adhesion and migration are discussed.

Biologic gels in tissue engineering

There are many different types of scaffold materials now available for tissue engineering applications. Hydrogels form one group of materials that have been used in a wide variety of applications. These hydrogels can be formed using natural materials, synthetic materials, or some combination of the two. There are advantages and disadvantages to using each type of material, and detailed investigations into the effects on various aspects of cell behavior of chemical and physical properties of the materials are needed to make an informed decision as to which material is best suited for a given application. By combining appropriate scaffold materials, such as hydrogels, with cells and proper signaling for those cells, more commercial tissue engineering products will become available for general use.

Tissue engineered small-diameter vascular grafts

Arterial occlusive disease remains the leading cause of death in western countries and often requires vascular reconstructive surgery. The limited supply of suitable small-diameter vascular grafts has led to the development of tissue engineered blood vessel substitutes. Many different approaches have been examined, including natural scaffolds containing one or more ECM proteins and degradable polymeric scaffolds. For optimal graft development, many efforts have modified the culture environment to enhance ECM synthesis and organization using bioreactors under physiologic conditions and biochemical supplements. In the past couple of decades, a great deal of progress on TEVGs has been made. Many challenges remain and are being addressed, particularly with regard to the prevention of thrombosis and the improvement of graft mechanical properties. To develop a patent TEVG that grossly resembles native tissue, required culture times in most studies exceed 8 weeks. Even with further advances in the field, TEVGs will likely not be used in emergency situations because of the time necessary to allow for cell expansion, ECM production and organization, and attainment of desired mechanical strength. Furthermore, TEVGs will probably require the use of autologous tissue to prevent an immunogenic response, unless advances in immune acceptance render allogenic and xenogenic tissue use feasible. TEVGs have not yet been subjected to clinical trials, which will determine the efficacy of such grafts in the long term. Finally, off-the-shelf availability and cost will become the biggest hurdles in the development of a feasible TEVG product. Although many obstacles exist in the effort to develop a small-diameter TEVG, the potential benefits of such an achievement are exciting. In the near future, a nonthrombogenic TEVG with sufficient mechanical strength may be developed for clinical trials. Such a graft will have the minimum characteristics of biological tissue necessary to remain patent over a period comparable to current vein graft therapies. As science and technology advance, TEVGs may evolve into complex blood vessel substitutes. TEVGs may become living grafts, capable of growing, remodeling, and responding to mechanical and biochemical stimuli in the surrounding environment. These blood vessel substitutes will closely resemble native vessels in almost every way, including structure, composition, mechanical properties, and function. They will possess vasoactive properties and be able to dilate and constrict in response to stimuli. Close mimicry of native blood vessels may aid in the engineering of other tissues dependent upon vasculature to sustain function. With further understanding of the factors involved in cardiovascular development and function combined with the foundation of knowledge already in place, the development of TEVGs should one day lead to improved quality of life for those with vascular disease and other life-threatening conditions.