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.

Synthetic hydrogels have been molecularly engineered to mimic the invasive characteristics of native provisional extracellular matrices: a combination of integrin-binding sites and substrates for matrix metalloproteinases (MMP) was required to render the networks degradable and invasive by cells via cell-secreted MMPs. Degradation of gels was engineered starting from a characterization of the degradation kinetics (kcat and Km) of synthetic MMP substrates in the soluble form and after crosslinking into a 3D hydrogel network. Primary human fibroblasts were demonstrated to proteolytically invade these networks, a process that depended on MMP substrate activity, adhesion ligand concentration, and network crosslinking density. Gels used to deliver recombinant human bone morphogenetic protein-2 to the site of critical defects in rat cranium were completely infiltrated by cells and remodeled into bony tissue within 4 wk at a dose of 5 g per defect. Bone regeneration was also shown to depend on the proteolytic sensitivity of the matrices. These hydrogels may be useful in tissue engineering and cell biology as alternatives for naturally occurring extracellular matrix-derived materials such as fibrin or collagen. extracellular matrix ͉ biomaterials ͉ proteolytic degradation A dvances in the field of tissue engineering are connected to the performance of biomaterials that help in guiding tissue formation or regeneration. Design principles in biomaterials have typically been influenced by the function of the extracellular matrix (ECM). Because the ECM has been demonstrated to play a key role in signal transduction (1-3), the development of materials that can specifically and molecularly interact with cells has become an emerging area of research (4, 5). The regulation of cell behavior through receptor-mediated adhesion [e.g., by functionalizing materials with integrin-binding oligopeptides (6, 7)] and by growth factors (8) has thus been targeted.We and others have been focusing on the development of synthetic materials that are targeted to assist tissue regeneration (9-11). Placed at the site of a defect, such materials should actively and temporarily participate in the regeneration process by providing a platform on which cell-triggered remodeling could occur. Consequently, these matrices must display some key characteristics of the provisional ECM. One of the critical initial functions of the fibrin-rich network that fills tissue defects after trauma, almost regardless of the injury site, lies in its ability to foster the invasion of inflammatory cells. These cells then initiate the remodeling process by partially degrading the matrix and by secreting molecular signals for attraction and differentiation of other cell types such as fibroblasts that build up new ECM (12). Because the provisional ECM presents itself in such situations often as a biophysical barrier to these cells, invasion and remodeling depend on the action of cell-secreted proteases enabling cell migration by clearing of a path (13,14). Thereby, matrix met...

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The generation of surfaces and interfaces that are able to withstand protein adsorption is a major challenge in the design of blood-contacting materials for both medical implants and bioaffinity sensors. Poly(ethylene glycol)-derived materials are generally considered to be particularly effective candidates for the fabrication of protein-resistant materials. Most metallic biomaterials are covered by a protective, stable oxide film; converting such oxide surfaces, which are known to strongly interact with proteins, into noninteractive surfaces requires a specific design of the surface/interface architecture. A class of copolymers based on poly(L-lysine)g-poly(ethylene glycol) (PLL-g-PEG) was found to spontaneously adsorb from aqueous solutions onto several metal oxide surfaces, such as TiO 2 , Si 0.4 Ti 0.6 O 2 , and Nb 2 O 5 , as measured by the in situ optical waveguide lightmode spectroscopy technique and by ex situ X-ray photoelectron spectroscopy. The resulting adsorbed layers are highly effective in reducing the adsorption both of blood serum and of individual proteins such as fibrinogen, which is known to play a major role in the cascade of events that lead to biomaterial-surfaceinduced blood coagulation and thrombosis. Adsorbed protein levels as low as <5 ng/cm 2 could be achieved for an optimized polymer architecture. The modified surfaces are stable to desorption under flow conditions at 37 °C and pH 7.4 in HEPES [4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid] and PBS (phosphatebuffered saline) buffers. The adsorbed layer of copolymer is thought to form a comblike structure at the surface, with positively charged primary amine groups of the PLL bound to the negatively charged metal oxide surface, while the hydrophilic and uncharged PEG side chains are exposed to the solution phase.Copolymer architecture is an important factor in the resulting protein resistance; it is discussed on the basis of packing-density considerations and the corresponding radii of gyration of the different PEG chain lengths studied. This surface functionalization technology is believed to be of value for use in both the biomaterial and biosensor areas, as the chosen macromolecules are biocompatible and the application is straightforward and cost-effective. † Part of the special issue "Gabor Somorjai Festschrift".

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The synthesis of novel hybrid hydrogels by stepwise copolymerization of multiarm vinyl sulfone-terminated poly(ethylene glycol) macromers and alpha-omega cysteine oligopeptides via Michael-type additions is described. Cross-linking kinetics, studied by in situ rheometry, can be controlled by pH and the presence of charged amino acid residues in close proximity to the Cys, which modulates the pK(a) of the thiol group. These end-linked networks were characterized by their equilibrium swelling in water, by their viscoelastic properties in the swollen state, and by their soluble fraction. It was demonstrated that structure and properties are very sensitive to the preparation state including stoichiometry and precursor concentration and less sensitive to the pH during cross-linking. For each network the concentration of elastically active chains (nu) was calculated from experimentally determined sol fractions using Miller-Macosko theory and compared to values obtained from swelling and rheometry studies and by calculation from Flory's classical network models. Hydrogels were also prepared with varying macromer structures, and their properties were shown to respond to both macromer functionality and molecular weight.

Photopolymerized crosslinked networks of poly(ethylene glycol; PEG) diacrylate (MW 8000) were derivitized throughout their bulk with Arg-Gly-Asp (RGD)-containing peptide sequences. Incorporation was achieved by functionalizing the amine terminus of the peptide with an acrylate moiety, thereby enabling the adhesion peptide to copolymerize rapidly with the PEG diacrylate upon photoinitiation. PEG diacrylate hydrogels derivitized with RGD peptide at surface concentrations ranging from 0.001 to 1 pmol/cm2 were studied in vitro for their ability to promote spreading of human foreskin fibroblasts over 24 h. Hydrogels not derivitized with peptides were poor substrates for adhesion, permitting spreading of only 5% of the seeded cells. When immobilized with no spacer arm, both RGD and RDG (inactive control) supported spreading of approximately 50% and approximately 15% of cells at 1 and 0.1 pmol/cm2 surface concentrations respectively; lower concentrations did not promote spreading. When a MW 3400 PEG spacer arm was incorporated between the hydrogel and the peptide linkage, incorporation of 1 pmol/cm2 RGD promoted 70% spreading whereas RDG at the same concentration did not promote spreading. In addition, when cells were seeded in serum-free medium, only RGD peptides incorporated with a spacer arm were able to promote spreading. Thus peptide incorporated into PEG 8000 diacrylate hydrogels without a spacer arm nonspecifically mediated cell spreading whereas incorporation via a MW 3400 PEG spacer arm was required to permit cell spreading to be specifically mediated.

Vesicles formed in water by synthetic macro-amphiphiles have attracted much attention as nanocontainers having properties that extend the physical and chemical limits of liposomes. We sought to develop ABA block copolymeric amphiphiles that self-assemble into unilamellar vesicles that can be further oxidatively destabilized. We selected poly(ethylene glycol) (PEG) as the hydrophilic A blocks, owing to its resistance to protein adsorption and low toxicity. As hydrophobic B blocks, we selected poly(propylene sulphide) (PPS), owing to its extreme hydrophobicity, its low glass-transition temperature, and most importantly its oxidative conversion from a hydrophobe to a hydrophile, poly(propylene sulphoxide) and ultimately poly(propylene sulphone). This is the first example of the use of oxidative conversions to destabilize such carriers. This new class of oxidation-responsive polymeric vesicles may find applications as nanocontainers in drug delivery, biosensing and biodetection.