No abstract

Electrochemical deposition of the conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT) forms thin, conductive films that are especially suitable for charge transfer at the tissue-electrode interface of neural implants. For this study, the effects of counter-ion choice and annealing parameters on the electrical and structural properties of PEDOT were investigated. Films were polymerized with various organic and inorganic counter-ions. Studies of crystalline order were conducted via X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to investigate the electrical properties of these films. X-ray photoelectron spectroscopy (XPS) was used to investigate surface chemistry of PEDOT films. The results of XRD experiments showed that films polymerized with certain small counter-ions have a regular structure with strong (100) edge-to-edge correlations of PEDOT chains at ~1.3 nm. After annealing at 170 °C for 1 hour, the XRD peaks attributed to PEDOT disappeared. PEDOT polymerized with LiClO4 as a counter-ion showed improved impedance and charge storage capacity after annealing at 160 °C.

Nanoscale control of structure in polymer nanocomposites is critical for their performance but has been difficult to investigate systematically due to the lack of a suitable experimental model. In this work, we investigated the role of nanoparticle layer separation in the finite deformation response of layered polyurethane-(PU-) montmorillonite (MTM) nanocomposites. A series of multilayered nanocomposites was manufactured, with alternating PU and MTM nanolayers, using a layer-by-layer manufacturing technique. The systematic variation in MTM nanoparticle volume fraction was achieved by varying the thickness of the PU nanolayer and therefore the MTM layer separation. Traditional polymer nanocomposite blending techniques result in a wide variation in nanoparticle separation for a given nanocomposite. In this investigation, we controlled the MTM nanoparticle layer separation, which allowed us to examine its effect on the nanocomposite response over a broad range in nanoparticle volume fraction. The PU-MTM nanocomposites demonstrated an increasing yield strength and stiffness with increased MTM volume fraction or reduced nanoparticle layer separation. A transition from ductile to brittle behavior in the stress-strain constitutive response was observed at a high volume fraction of MTM nanoparticles. We demonstrate that a critical nanoparticle separation exists, below which brittle behavior dominates the response of PU-MTM nanocomposites.

Most bone tissue-engineering research uses porous three-dimensional (3D) scaffolds for cell seeding. In this work, scaffold-less 3D bone-like tissues were engineered from rat bone marrow stromal cells (BMSCs) and their autogenous extracellular matrix (ECM). The BMSCs were cultured on a 2D substrate in medium that induced osteogenic differentiation. After reaching confluence and producing a sufficient amount of their own ECM, the cells contracted their tissue monolayer around two constraint points, forming scaffold-less cylindrical engineered bone-like constructs (EBCs). The EBCs exhibited alizarin red staining for mineralization and alkaline phosphatase activity and contained type I collagen. The EBCs developed a periosteum characterized by fibroblasts and unmineralized collagen on the periphery of the construct. Tensile tests revealed that the EBCs in culture had a tangent modulus of 7.5 AE 0.5 MPa at 7 days post-3D construct formation and 29 AE 9 MPa at 6 weeks after construct formation. Implantation of the EBCs into rats 7 days after construct formation resulted in further bone development and vascularization. Tissue explants collected at 4 weeks contained all three cell types found in native bone: osteoblasts, osteocytes, and osteoclasts. The resulting engineered tissues are the first 3D bone tissues developed without the use of exogenous scaffolding.