This Article describes a simple two-step synthesis of comb block copolymers with molecular weights exceeding 1,000,000 g mol(-1) and their assembly into ordered morphologies in the solid state. This work is exciting because these polymers assembled into morphologies with domain sizes exceeding 100 nm and, in some examples, 200 nm without the use of additives. These materials reflected selected wavelengths of visible light, and these wavelengths could be affected by swelling with methylene chloride vapor. A comparison between the structures of comb block copolymers and linear block copolymers is presented with a discussion of important parameters affecting their assembly in the solid state. This Article will first describe the synthesis of comb block copolymers using ring opening metathesis polymerization and atom transfer radical polymerization and their subsequent characterization. The comb block copolymers, backbone polymers, and polystyrene arms were all characterized independent of each other and had low polydispersities. The comb block copolymers were assembled by dissolving in methylene chloride and allowing the solvent to evaporate. After thermal annealing, the polymers were characterized by scanning electron and optical microscopy. These polymers assembled into spherical, lamellar, and cylindrical arrays with domain sizes from 132 to 258 nm. This work extends the molecular architectures of polymers that can be assembled in the solid state, the molecular weights of block copolymers that were assembled, and the domain sizes that can be realized without the use of additives.

This article reports the synthesis of comb block copolymers with backbones from exo-norbornene esters via ring-opening metathesis polymerizations (ROMP) and arms composed of polystyrene and polylactide. These polymers represent an exciting new architecture of polymers that have applications in the fabrication of photonic materials and nanofluidic systems. The living polymerization of block copolymers by ROMP with degrees of polymerization up to 2000 units and polydispersities less than 1.2 are described. This result is important as it extends the range of block copolymers that can be synthesized by ROMP to include those with high molecular weights. Comb block copolymers were grown from these block copolymers as they displayed initiators for the ring-opening polymerization of lactide and the atom transfer radical polymerization of styrene. Comb block copolymers with polystyrene and polylactide arms were synthesized with molecular weights up to 63 000 000 g mol -1 . The polystyrene arms had narrow polydispersities and molecular weights in excess of 10 000 g mol -1 ; this result showed that the polymerization of styrene was well controlled. The sizes and shapes of these comb polymers were characterized by multiangle laser light scattering and scanning probe microscopy and demonstrated that some of these polymers were shaped as rigid rods with lengths in excess of 300 nm. To demonstrate their potential as photonic materials, an example of a comb block copolymer was assembled in the solid state with domain sizes exceeding 100 nm and characterized by scanning electron microscopy.

Backbone Polymer Synthesis and Characterization. The backbone block copolymers were synthesized by ROMP according to our previously reported procedure (JACS, 2007, 10551-10560). The backbone polymers were characterized by size exclusion chromatography with a flow rate of 1 mL min -1 with chloroform as the mobile phase. A Waters 515 HPLC pump and two columns (styragel HMW7 and HMW7 or HR4 and HR5E) were used. The detectors were a multi-angle laser light scattering detector (Wyatt DAWN EOS), a quasi elastic light scattering detector (Wyatt QELS), and a refractive index detector (Wyatt Optilab DPS). The absolute molecular weights were found and compared to the predicted molecular weights based on the monomer to catalyst loadings and assuming consumption of all monomer. The results are shown in Table 1.

Electrically conductive hydrogel composites consisting of oligo(polyethylene glycol) fumarate (OPF) and polypyrrole (PPy) were developed for applications in nerve regeneration. OPF-PPy scaffolds were synthesized using three different anions: naphthalene-2-sulfonic acid sodium salt (NSA), dodecylbenzenesulfonic acid sodium salt (DBSA), and dioctyl sulfosuccinate sodium salt (DOSS). Scaffolds were characterized by ATR-FTIR, XPS, AFM, dynamic mechanical analysis, electrical resistivity measurements, and swelling experiments. OPF-PPy scaffolds were shown to consist of up to 25 mol% polypyrrole with a compressive modulus ranging from 265 to 323 kPa and a sheet resistance ranging from 6 to 30 × 103 Ohms/square. In vitro studies using PC12 cells showed OPF-PPy materials had no cytotoxicity and PC12 cells showed distinctly better cell attachment and an increase in the percent of neurite bearing cells on OPF-PPy materials compared to OPF. The neurite lengths of PC12 cells were significantly higher on OPF-PPyNSA and OPF-PPyDBSA. These results show that electrically conductive OPF-PPy hydrogels are promising candidates for future applications in nerve regeneration.

Mechanical and electrical properties of polycaprolactone fumarate-polypyrrole (PCLF-PPy) scaffolds were studied under physiological conditions to evaluate their ability to maintain material properties necessary for application as conductive nerve conduits. PC12 cells cultured on PCLFPPy scaffolds were stimulated with regimens of 10 μA of constant or 20 Hz frequency current passed through the scaffolds for 1 h/day. PC12 cellular morphologies were analyzed by fluorescence microscopy after 48 h. PCLF-PPy scaffolds exhibited excellent mechanical properties at 37°C which would allow suturing and flexibility. The surface resistivity of the scaffolds was 2kΩ and the scaffolds were electrically stable during application of electrical stimulation (ES). In vitro studies showed significant increases in percentage of neurite bearing cells, number of neurites per cell and neurite length in the presence of ES compared to no ES. Additionally, extending neurites were observed to align in the direction of the applied current. This study shows that electrically conductive PCLF-PPy scaffolds possess material properties necessary for application as nerve conduits. Additionally, the capability to significantly enhance and direct neurite extension by passing electrical current through PCLF-PPy scaffolds renders them even more promising as future therapeutic treatments for severe nerve injuries.

In this work, we have investigated the development of a synthetic hydrogel that contains a negatively charged phosphate group for use as a substrate for bone cell attachment and differentiation in culture. The photoreactive, phosphate-containing molecule, bis(2-(methacryloyloxy)ethyl)phosphate (BP), was incorporated into oligo(polyethylene glycol) fumarate hydrogel and the mechanical, rheological and thermal properties of the resulting hydrogels were characterized. Our results showed changes in hydrogel compression and storage moduli with incorporation of BP. The modification also resulted in decreased crystallinity as recorded by differential scanning calorimetry. Our data revealed that incorporation of BP improved attachment and differentiation of human fetal osteoblast (hFOB) cells in a dose-dependent manner. A change in surface chemistry and mineralization of the phosphate-containing surfaces verified by scanning electron microscopy and energy dispersive X-ray analysis was found to be important for hFOB cell attachment and differentiation. We also demonstrated that phosphate-containing hydrogels support attachment and differentiation of primary bone marrow stromal cells. These findings suggest that BP-modified hydrogels are capable of sustaining attachment and differentiation of both bone marrow stromal cells and osteoblasts that are critical for bone regeneration.

Vascular endothelial growth factor (VEGF) is a potent angiogenic stimulator. Controlled release of such stimulators may enhance and guide the vascularization process, and when applied in a nerve conduit may play a role in nerve regeneration. We report the fabrication and in vitro characterization of VEGF encapsulating poly-lactic-co-glycolic acid (PLGA) microspheres and the in vivo application of nerve conduits supplemented with VEGF-containing microspheres. PLGA microspheres containing VEGF were prepared by the double emulsion-solvent evaporation technique. This yielded 83.16% of the microspheres with a diameter < 53 µm. VEGF content measured by ELISA indicated 93.79 ±10.64% encapsulation efficiency. Release kinetics were characterized by an initial burst release of 67.6±8.25% within the first 24 hours, followed by consistent release of approximately 0.34% per day for 4 weeks. Bioactivity of the released VEGF was tested by human umbilical vein endothelial cell (HUVEC) proliferation assay. VEGF released at all time points enhanced HUVEC proliferation confirming that VEGF retained its bioactivity through the 4-week time period. When the microsphere delivery system was placed in a biosynthetic nerve scaffold, robust nerve regeneration was observed. This study established a novel system for controlled release of growth factors and enables in vivo studies of nerve conduits conditioned with this system.

No abstract