An overview of polyurethane biomaterials and their use in drug delivery.

Polyurethanes are a versatile and highly tunable class of materials that possess unique properties including high tensile strength, abrasion and fatigue resistance, and flexibility at low temperatures. The tunability of polyurethane properties has allowed this class of polymers to become ubiquitous in our daily lives in fields as diverse as apparel, appliances, construction, and the automotive industry. Additionally, polyurethanes with excellent biocompatibility and hemocompatibility can be synthesized, enabling their use as biomaterials in the medical field. The tunable nature of polyurethane biomaterials also makes them excellent candidates as drug delivery vehicles, which is the focus of this review. The fundamental idea we aim to highlight in this article is the structure-property-function relationships found in polyurethane systems. Specifically, the chemical structure of the polymer determines its macroscopic properties and dictates the functions for which it will perform well. By exploring the structure-property-function relationships for polyurethanes, we aim to elucidate the fundamental properties that can be tailored to achieve controlled drug release and empower researchers to design new polyurethane systems for future drug delivery applications.

Selective endothelial cell attachment to peptide-modified terpolymers.

In a previous report we screened a combinatorial peptide library to identify novel ligands that bind with high affinity and specificity to human blood outgrowth endothelial cells (HBOEC). In this study we demonstrate the use of the phage display-selected-HBOEC-specific peptides as a tool to direct and modulate endothelial cell (EC) behavior with a focus on designing functional biomaterials intended for use in cardiovascular applications. First, we ensured that our peptide ligands did not interfere with EC function as tested by proliferation, migration, tube formation, and response to vascular endothelial growth factor. Second, peptides that supported EC function were incorporated into methacrylic terpolymers via chain transfer free radical polymerization. The HBOEC-specific peptide, TPSLEQRTVYAK, when covalently coupled to a terpolymer matrix, retained binding affinity towards HBOEC in a serum-free medium. Under the same binding conditions, the attachment of human umbilical vein endothelial cells (HUVEC) was limited, thus establishing HBOEC specificity. To our knowledge, this is the first report demonstrating specificity in binding to peptide-modified biomaterials of mature EC, i.e., HUVEC, and EC of progenitor origin such as HBOEC. The findings from this work could facilitate the development of autologous cell therapies with which to treat cardiovascular disease.

The development of polymeric biomaterials inspired by the extracellular matrix.

The field of biomaterials science has exhibited astounding growth during the last half century. This growth can be attributed to a more thorough understanding of cellular behaviours, new mechanistic insights into cell-materials interactions, advances in polymer synthesis, and the development of innovative fabrication techniques. This progress has enabled biomaterials to transition from materials that fill in for damaged or diseased tissues to implants that actively work with biology to aid in the healing process. In this review, we will discuss several of these advances including emerging knowledge on mechanotransduction, the use of substrate topography in modulating cellular behaviour, and the incorporation of bioactive molecules to foster specific cellular adhesion, promote tissue morphogenesis, and enable biologically driven degradation. We will then consider future challenges and directions in the field.

Oxidative and hydrolytic stability of a novel acrylic terpolymer for biomedical applications.

Oxidative and hydrolytic biostability assessment was carried out on a novel acrylic material made of hexamethyl methacrylate (HMA), methyl methacrylate (MMA), and methacrylic acid (MAA). To simulate the in vivo microenvironment, solutions of H2O2/CoCl2 and buffered solutions of cholesterol esterase (CE) and phospholipase A2 (PLA) were used. As controls, film specimens were incubated in deionized water. Samples were incubated in these solutions at 37 degrees C for 10 weeks before physical and mechanical properties were evaluated by size exclusion chromatography (SEC), 1H- nuclear magnetic resonance (1H-NMR), acid-base titration, and Instron tensile testing. The results from this study indicate excellent biostability of HMA-MMA-MAA terpolymers and thus their potential for use in biomedical devices for long-term implantation.

Interactions between dendrimer biocides and bacterial membranes.

Dendrimer biocides have been shown to be more potent than their small molecule counterparts. In this study, several techniques were utilized to investigate the interactions between quaternary ammonium functionalized poly(propylene imine) dendrimers and bacterial membranes. Both Gram-positive and Gram-negative bacteria were tested. The techniques employed include UV-Vis spectroscopy, differential scanning calorimetry, and bioluminescence. When treated with dendrimer biocides, release of 260nm adsorbing materials from Escherichia coli strains quickly increased and reached a plateau afterwards, while release of 260 nm absorbing materials from Staphylococcus aureus increased monotonically with the concentration due to the difference in cell structures. The different release behavior also correlates with the antimicrobial properties against these two types of bacteria. Bioluminescence experiments using bacteria containing stress-responsive bioluminescent reporter gene fusions provided information suggesting that damage to the cell membranes is the primary mechanism of the antimicrobial action for dendrimer biocides. High concentrations of calcium ions can limit the efficacy of the dendrimer biocides, although the tested concentration range is much higher than most practical applications. Differential scanning calorimetry studies showed at high concentrations that dendrimer biocides formed precipitates with phospholipid vesicles, suggesting a strong interaction with this model of bacterial membrane. These results provide insight about the antibacterial action of dendrimer biocides and establish the basis for their mode of action.

Synthesis and characterization of acrylic terpolymers with RGD peptides for biomedical applications.

The goal of this research was to design a biomaterial, using acrylic terpolymers, which could support endothelial cells and function in small diameter vascular graft applications. Hexyl methacrylate (HMA) and octyl methacrylate (OMA) were used as comonomers to produce a material with a low glass transition temperature (T(g)). Methacrylic acid (MAA) was used to provide ionic character, and methyl methacrylate (MMA) was selected because of its wide usage in biomedical applications. Cation neutralization was employed to modify the mechanical properties. RGD-based peptide sequences were attached to promote endothelial cell adhesion, because vascular grafts seeded with endothelial cells have fewer problems with thrombosis. The two methods used to incorporate peptide sequences were a chain transfer reaction during polymerization, and a coupling reaction attaching the peptides to carboxyl groups on the polymer after polymerization. The compositions that produced T(g)s of approximately 0 degrees C were 75 mol% OMA and 92 mol% HMA. The Young's modulus of the HMA copolymer was approximately 0.37 MPa, well below the desired value of 0.9 MPa. Likewise, the Young's modulus of approximately 0.50 MPa for the OMA copolymer was also below the desired value. After partial neutralization with sodium cations, the Young's moduli increased to approximately 0.93 and 0.99 MPa, respectively. The chain transfer reaction lowered the molecular weights and mechanical properties of the copolymers, while the coupling reaction method had little effect on these properties. The chain transfer method appears to be a promising one-step method to produce polymers with a wide range of peptide concentrations.

Endothelial cell adhesion on RGD-containing methacrylate terpolymers.

Hexyl methacrylate (HMA), methyl methacrylate (MMA), and methacrylic acid (MAA) were used as comonomers to produce a low glass transition temperature material, potentially useful in fabricating a small diameter vascular graft. Because it has been shown that grafts seeded with endothelial cells have better resistance to thrombosis, RGD-based peptide sequences were incorporated into the terpolymer. The two methods used for incorporating peptide sequences were a chain transfer reaction during polymerization, and a coupling reaction between the amine terminus of the peptide and the carboxyl groups of the MAA. Polymers were synthesized using the chain transfer reaction with peptide concentrations ranging from 1.7 to 7.0 micromol/g. Weight-average molecular weights decreased with increasing peptide concentration from 310,000 g/mol for the terpolymer without peptide, to 110,000 g/mol for a peptide concentration of 7.0 micromol/g. As a result, Young's modulus decreased with increasing peptide concentration. Terpolymers with peptides attached through a coupling reaction showed no decrease in molecular weight or mechanical properties. Confocal microscopy showed cells seeded on the RGD surfaces adhered and spread, while terpolymers with RGE sequences showed cells that were rounded and not spreading. Cell density on RGD surfaces increased with increasing peptide concentration up to a bulk peptide concentration of approximately 5 micromol/g and reached a plateau, which indicated the minimum peptide concentration necessary for maximum cell adhesion.

In vitro endothelialization of electrospun terpolymer scaffolds: evaluation of scaffold type and cell source.

A family of methacrylic terpolymer biomaterials was electrospun into three-dimensional scaffolds. The glass transition temperature of the polymer correlates with the morphology of the resulting scaffold. Glassy materials produce scaffolds with discrete fibers and large pore areas (1531±1365 µm(2)), while rubbery materials produce scaffolds with fused fibers and smaller pore areas (154±110 µm(2)). Three different endothelial-like cell populations were seeded onto these scaffolds under static conditions: human umbilical vein endothelial cells (HUVECs), adult human peripheral blood-derived outgrowth endothelial cells, and umbilical cord blood-derived human blood outgrowth endothelial cells. Cellular behavior depended on both cell type and scaffold topography. Specifically, cord blood-derived outgrowth endothelial cells showed more robust adhesion and growth on all scaffolds in comparison to other cell types as measured by the density of adherent cells, the number of proliferative cells, and the enzymatic activity of the adherent cells. Peripheral blood-derived outgrowth cells exhibited less ability to inhabit the terpolymer interfaces in comparison to their cord blood-derived counterparts. HUVECs also exhibited less of a capacity to colonize the terpolymer interfaces in comparison to the cord blood-derived cells. However, the mature endothelial cells did show scaffold-dependent behavior. Specifically, we observed an increase in their ability to populate the low-porosity scaffolds. All cells maintained an endothelial phenotype after 1 week of culture on the electrospun scaffolds.

Design and characterization of sulfobetaine-containing terpolymer biomaterials.

A methacrylic terpolymer system with non-fouling interfacial properties was synthesized by the random copolymerization of hexyl methacrylate, methyl methacrylate and sulfobetaine methacrylate (a monomer bearing a zwitterionic pendant group). Polymers were synthesized from feeds containing 0-15 mol.% of the zwitterion-containing methacrylate. Proton nuclear magnetic resonance verified the incorporation of sulfobetaine methacrylate into the polymer structure. Water absorption studies illustrate that the hydrophilicity of the material increases with increasing zwitterion concentration. The biological properties of the polymer were probed by fibrinogen adsorption, human umbilical vein endothelial cell adhesion and growth, and platelet adhesion. Strong resistance to protein adsorption and cell and platelet attachment was observed on materials synthesized from 15 mol.% sulfobetaine methacrylate. Results were compared to the non-fouling behavior of a PEGylated terpolymer formulation and it was observed that the poly(ethylene glycol)-containing materials were slightly more effective at resisting human umbilical vein endothelial cell adhesion and growth over a 7 day incubation period, but the zwitterion-containing materials were equally effective at resisting fibrinogen adsorption and platelet adhesion. The zwitterion-containing materials were electrospun into three-dimensional random fiber scaffolds. Materials synthesized from 15 mol.% of the zwitterion-containing monomer retained their non-fouling character after fabrication into scaffolds.

Endothelial cell adhesion and proliferation to PEGylated polymers with covalently linked RGD peptides.

A nonfouling peptide grafted polymer was synthesized that can promote endothelial cell (EC) binding. The polymer was composed of hexyl methacrylate, methyl methacrylate, poly(ethylene glycol) methacrylate, and CGRGDS peptide. The peptide was incorporated into the polymer system either by a chain transfer reaction or by coupling to an acrylate-PEG-N-hydroxysuccinimide (NHS) comonomer. The introduction of PEG chains minimizes protein adsorption. Human umbilical vein ECs and endothelial colony forming cells were cultured on these surfaces in short term and long-term studies. A difference in number and morphology of ECs was observed depending on the method of peptide incorporation. Both cell types adhered better to polymer films containing NHS coupled RGD peptide after 2 h even in the presence of albumin but significant cell detachment occurred after 4 days. Polymer solutions were electrospun into fibrous scaffolds. Both nonfouling and peptide binding characteristics were retained after processing.

Design and characterization of PEGylated terpolymer biomaterials.

A terpolymer copolymerized from hexyl methacrylate, methyl methacrylate, and poly(ethylene glycol) methacrylate (PEGMA) was synthesized. Polymers containing 0-25 mol % PEGMA were studied. As the mole fraction of PEGMA in the polymer chains increased, the material becomes more hydrophilic as observed by a decrease in the contact angle of water (81 degrees -68 degrees) and an increase in the equilibrium water absorption (0.7-237 wt %). Furthermore, the material shows nonfouling interfacial properties through resistance to protein adsorption and cellular attachment. A total of 1.2 microg/cm(2) fibrinogen, 18,000 HUVECs/cm(2), and 3,000,000 platelets/cm(2) adsorbed or adhered on non-PEGylated materials, whereas very low amounts of protein or cells were observed on materials containing >or=15 mol % PEGMA. Being thermoplastic, the polymer can be processed postsynthesis. To illustrate the processing capabilities of the material, polymer solutions were electrospun into nonwoven fibrous scaffold, which also retained their nonfouling character.

Interaction of endothelial cells with methacrylic terpolymer biomaterials.

A biomaterial system with tunable mechanical properties was synthesized through random copolymerization of hexyl methacrylate, methyl methacrylate, and methacrylic acid. The composition, molecular weight, water absorption, glass transition temperature, and tensile properties of the polymer system were characterized, and the biocompatibility of the material was demonstrated by the ability of human umbilical vein endothelial cells to adhere, proliferate, and spread on the polymer surface. No difference was observed in the behavior of the endothelial cells on the materials regardless of material stiffness.

Electrospun scaffold topography affects endothelial cell proliferation, metabolic activity, and morphology.

A family of methacrylic terpolymer biomaterials was electrospun into three-dimensional fibrous scaffolds. The glass transition temperature of the polymer correlates with the morphology of the resulting scaffold. Glassy materials produce scaffolds with discrete fibers and a high percent void space (84%) while the rubbery materials produced scaffolds with fused fibers and a much lower percent void space (18%). By electrospinning onto a rotating mandrel, aligned fiber scaffolds were fabricated, and it was shown that controlling the rotation speed of the collector allowed for control over the degree of fiber alignment. The electrospinning was shown to not degrade the number average molecular weight of the polymer chains. Human umbilical vein endothelial cells (HUVECs) were seeded onto the electrospun scaffolds under static conditions and it was found that the morphology of the scaffold controlled the cellular proliferation, the metabolic activity, and the morphology of adherent cells. In particular, HUVECs seeded onto low void space scaffolds exhibited enhanced cellular spreading, enzymatic activity, and proliferation. HUVECs seeded onto aligned fiber scaffolds did not demonstrate increased proliferation; however, the cells did organize themselves in the direction of fiber alignment resulting in cells with elongated morphology.

Selection and initial characterization of novel peptide ligands that bind specifically to human blood outgrowth endothelial cells.

Using phage display technology, we have isolated 12-mer peptide ligands that bind to human blood outgrowth endothelial cells (HBOEC). To avoid non-specific binding we apply negative-positive selection approach by pre-incubating the library with non-HBOEC. The selected phage clones bind to their target cell population with high recovery. Moreover, the isolated clones display outstanding cell specificity as no significant binding is observed on a panel of other cell types. We anticipate the findings from this work to be exploited in the development of future cell-based therapeutic revascularization approaches to ischemic disease and endothelial injury or in combination with biomedical devices.