Faculty Bibliography

This manuscript reports a facile strategy for the modification of polyethylene, a widely used biomaterial, with a thin non-biofouling coating that can act as a platform to introduce biochemical cues to control and direct, e.g., cell adhesion and proliferation. The non-biofouling coating is produced, following a two-step strategy involving photobromination of the polyethylene substrate followed by surface-initiated ATRP of poly(ethylene glycol) methacrylate. The resulting coatings, which are referred to as polymer brushes, are robust and withstand prolonged exposure to aqueous ethanol and phosphate buffered saline and also do not show any signs of detachment or degradation in the in vivo studies over a period of 10 d. Functionalization of these polymer brush coatings with extracellular matrix derived peptide sequences, promotes adhesion and spreading of endothelial cells.

Here we show a method for patterning a thin metal film using self-assembled block-copolymer micelles monolayers as a template. The obtained metallic mask is transferred by reactive ion etching in silicon oxide, silicon and silicon nitride substrates, thus fabricating arrays of hexagonally packed nanopores with tunable diameters, interspacing and aspect ratios. This technology is compatible with integration into a standard microtechnology sequence for wafer-scale fabrication of ultrathin silicon nitride nanoporous membranes with 80 nm mean pore diameter.

The engineering of materials that can modulate the immune system is an emerging field that is developing alongside immunology. For therapeutic ends such as vaccine development, materials are now being engineered to deliver antigens through specific intracellular pathways, allowing better control of the way in which antigens are presented to one of the key types of immune cell, T cells. Materials are also being designed as adjuvants, to mimic specific 'danger' signals in order to manipulate the resultant cytokine environment, which influences how antigens are interpreted by T cells. In addition to offering the potential for medical advances, immunomodulatory materials can form well-defined model systems, helping to provide new insight into basic immunobiology.

We designed block copolymer pro-amphiphiles and amphiphiles for providing very long-term release of nitric oxide (NO). A block copolymer of N-acryloylmorpholine (AM, as a hydrophile) and N-acryloyl-2,5-dimethylpiperazine (AZd, as a hydrophilic precursor) was synthesized. The poly(N-acryloyl-2,5-dimethylpiperazine) (PAZd) is water-soluble, but chemical reaction of the secondary amines with NO to form a N-diazeniumdiolate (NONOate) converts the hydrophilic PAZd into a hydrophobic poly(sodium-1-(N-acryloyl-2,5-dimethylpiperazin-1-yl)diazen-1-ium-1,2-diolate) (PAZd.NONOate), driving aggregation. The PAM block guides this process toward micellization, rather than precipitation, yielding ca. 50 nm spherical micelles. The hydrophobic core of the micelle shielded the NONOate from the presence of water, and thus protons, which are required for NO liberation, delaying release to a remarkable 7 d half-life. Release of the NO returned the original soluble polymer. The very small NO-loaded micelles were able to penetrate complex tissue structures, such as the arterial media, opening up a number of tissue targets to NO-based therapy.

Block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide) (PEG-PPS) have recently emerged as a new macromolecular amphiphile capable of forming a wide range of morphologies when dispersed in water. To understand better the relationship between stability and morphology in terms of the relative and absolute block compositions, we have synthesized a collection of PEG-PPS block copolymers and quantified their critical aggregation concentration and observed their morphology using cryogenic transmission electron microscopy after thin film hydration with extrusion and after solvent dispersion from tetrahydrofuran, a solvent for both blocks. By understanding the relationship between aggregate character and block copolymer architecture, we have observed that whereas the relative block lengths control morphology, the stability of the aggregates upon dilution is determined by the absolute block length of the hydrophobic PPS block. We have compared results obtained with PEG-PPS to those obtained with poly(ethylene glycol)-bl-poly(propylene oxide)-bl-poly(ethylene glycol) block copolymers (Pluronics). The results reveal that the PEG-PPS aggregates are substantially more stable than Pluronic aggregates, by more than an order of magnitude. PEG-PPS can form a wide variety of stable or metastable morphologies in dilute solution within normal time and temperature ranges, whereas Pluronics can generally form only spherical micelles under the same conditions. On the basis of these results, block copolymers of PEG with poly(propylene sulfide) may present distinct advantages over those with poly(propylene glycol) for a number of applications.

The in vitro potential of a synthetic matrix metalloproteinase (MMP)-responsive poly(ethylene glycol) (PEG)-based hydrogel as a bioactive co-encapsulation system for vascular cells and a small bioactive peptide, thymosin beta4 (Tbeta4), was examined. We show that the physical incorporation of Tbeta4 in this bioactive matrix creates a three-dimensional (3D) environment conducive for human umbilical vein endothelial cell (HUVEC) adhesion, survival, migration and organization. Gels with entrapped Tbeta4 increased the survival of HUVEC compared to gels without Tbeta4, and significantly up-regulated the endothelial genes vascular endothelial-cadherin and angiopoietin-2, whereas von Willebrand factor was significantly down-regulated. Incorporation of Tbeta4 significantly increased MMP-2 and MMP-9 secretion of encapsulated HUVEC. The gel acts as a controlled Tbeta4-release system, as MMP-2 and MMP-9 enzymes trigger the release. In addition, Tbeta4 facilitated HUVEC attachment and induced vascular-like network formation upon the PEG-hydrogels. These MMP-responsive PEG-hydrogels may thus serve as controlled co-encapsulation system of vascular cells and bioactive factors for in situ regeneration of ischemic tissues.

One of the major engineering challenges for the implementation of block copolymer vesicles, or polymersomes, as therapeutic drug carriers is obtaining high encapsulation efficiencies for biomolecules. Here we present a novel method for encapsulation of proteins with high encapsulation efficiency within polymersomes formed from block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide). By formulation of the neat block copolymer with a low molecular weight poly(ethylene glycol), direct hydration of the formulated mixture yielded polymersomes. We were able to achieve encapsulation efficiencies for ovalbumin at 37%, bovine serum albumin at 19%, and bovine gamma-globulin at 15% when the proteins were included in the hydration solution. The formulation process and the dispersion of polymersomes from the preparation in phosphate-buffered saline were characterized using confocal microscopy, cryogenic transmission electron microscopy, and fluorimetry. We were also successful in the encapsulation of proteinase K, a proteolytic enzyme, and demonstrated by SDS-PAGE that the enzyme was contained inside polymersomes when dispersed in a solution of ovalbumin.

Nanoarray fabrication is a multidisciplinary endeavor encompassing materials science, chemical engineering, and biology. We formed nanoarrays via a new technique, porphyrin-based photocatalytic nanolithography. The nanoarrays, with controlled features as small as 200 nm, exhibited regularly ordered patterns and may be appropriate for (a) rapid and parallel proteomics screening of immobilized biomolecules, (b) protein-protein interactions, and/or (c) biophysical and molecular biology studies involving spatially dictated ligand placement. We demonstrated protein immobilization utilizing nanoarrays fabricated via photocatalytic nanolithography on silicon substrates where the immobilized proteins are surrounded by a non-fouling polymer background.

We present the formation of collagen-binding mixed micelles and their potential suitability to deliver therapeutic drugs to the vessel wall. We modified poly(ethylene oxide)-bl-poly(propylene oxide)-bl-poly(ethylene oxide) (Pluronic F-127) to display sulfate groups on the terminus of the PEO block to act as a heparin mimics and bind to collagen in the extracellular matrix. This functionalized macroamphiphile was incorporated into a mixed micelle with poly(propylene sulfide)-bl-poly(ethylene oxide), a macroamphiphile that demonstrates improved micellar stability relative to Pluronic F-127 micelles. The mixed micelles were examined using analytical ultracentrifugation, dynamic light scattering, transmission electron microscopy, and measures of the critical micellar concentration using surface tensiometry. Encapsulation and in vitro release of Sirolimus, an immunosuppressant drug of interest in coronary artery treatment, was considered as an example. Mixed micelles with the sulfate functionality demonstrated enhanced binding to collagen I coated surfaces, suggestive of the potential for binding to the extracellular milieu.