The creation of a tissue engineering scaffold via electrospinning that has minimal toxicity and uses a solvent system composed of solvents with low toxicity and different cross-linking agents was investigated. First, a solvent system of acetic acid/ethyl acetate/water (50:30:20) with gelatin as a solute was evaluated. The optimum system for electrospinning a scaffold with the desired properties resulted from a gelatin concentration of 10 wt %. Several different methods were used to cross-link the electrospun gelatin fibers, including vapor-phase glutaraldehyde, aqueous phase genipin, and glyceraldehyde, as well as reactive oxygen species from a plasma cleaner. Because glutaraldehyde at high concentrations has been shown to be toxic, we explored other cross-linking methods. Using reactive oxygen species from a plasma cleaner is an easy alternative; however, the degradation reaction dominated the cross-linking reaction and the scaffolds degraded after only a few hours in aqueous medium at 37 °C. Glyceraldehyde and genipin were established as good options for cross-linking agents because of the low toxicity of these cross-linkers and the resistance to dissolution of the cross-linked fibers in cell culture medium at 37 °C. MG63 osteoblastic cells were grown on each of the cross-linked scaffolds. A proliferation assay showed that the cells proliferated as well or better on the cross-linked scaffolds than on traditional two-dimensional polystyrene culture plates.
Electrospinning of biologically significant polymers (natural and synthetic polypeptides) has increased since electrospun membranes were identified as candidates for tissue engineering constructs. These materials have a specific secondary structure, which influences their properties. The effect of electrospinning on the secondary structure of nylon-6 and nylon-12 is examined using Raman spectroscopy in order to identify and quantify any conformational changes that occur due to processing. Nylon-6 and nylon-12 were chosen because they possess a specific chain conformation and have a backbone chemical structure similar to the amino acid sequence in polypeptides. Results indicate that a change in the chain conformation due to electrospinning occurs, implying that a high stress is induced on the electrospinning jet as the fibers are being formed, and this stress alters the chain conformation of the nylon backbone.
Polymer and life science applications of a technique that combines atomic force microscopy (AFM) and infrared (IR) spectroscopy to obtain nanoscale IR spectra and images are reviewed. The AFM-IR spectra generated from this technique contain the same information with respect to molecular structure as conventional IR spectroscopy measurements, allowing significant leverage of existing expertise in IR spectroscopy. The AFM-IR technique can be used to acquire IR absorption spectra and absorption images with spatial resolution on the 50 to 100 nm scale, versus the scale of many micrometers or more for conventional IR spectroscopy. In the life sciences, experiments have demonstrated the capacity to perform chemical spectroscopy at the sub-cellular level. Specifically, the AFM-IR technique provides a label-free method for mapping IR-absorbing species in biological materials. On the polymer side, AFM-IR was used to map the IR absorption properties of polymer blends, multilayer films, thin films for active devices such as organic photovoltaics, microdomains in a semicrystalline polyhydroxyalkanoate copolymer, as well as model pharmaceutical blend systems. The ability to obtain spatially resolved IR spectra as well as high-resolution chemical images collected at specific IR wavenumbers was demonstrated. Complementary measurements mapping variations in sample stiffness were also obtained by tracking changes in the cantilever contact resonance frequency. Finally, it was shown that by taking advantage of the ability to arbitrarily control the polarization direction of the IR excitation laser, it is possible to obtain important information regarding molecular orientation in electrospun nanofibers.
Ultrathin films of biodegradable poly[(R)-3-hydroxybutyrate] (PHB) and its random copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx) were prepared by spin-coating onto aluminum substrates with a naturally oxidized aluminum oxide (AO) surface layer or, alternatively, on gold substrates. The opposite surface of the film was in contact with ambient air. Isothermal crystallization kinetics of these films at room temperature were studied using infrared reflection absorption spectroscopy. The overall crystallization rate for all the polymers when crystallizing on AO is significantly retarded compared with the same polymer crystallizing on gold. It was found that the retardation effect was not due to a confinement effect. The crystallization retardation effect was especially enhanced for PHBHx with a higher (R)-3-hydroxyhexanoate content. Avrami analysis showed that the crystallization rate constant k (min–1) for all of the polymers on AO is approximately 3 to 4 orders of magnitude less than that found for the same polymer on gold. Grazing incident wide-angle X-ray diffraction showed that polymers on gold have both flat-on and edge-on crystallite orientations, whereas polymers on AO have a dominating edge-on crystallite orientation. Infrared studies on a quasi-monolayer film revealed no detectable H-bonding between PHB/PHBHx and the AO surface. The crystallization retardation mechanism was explained as being a sum of the dipole–dipole interactions of −C═O of PHB or PHBHx and the −O–Al–O– groups of AO coupled with the rigid disordered amorphous nature of the AO surface.
Raman scattering measures vibrational frequencies in materials, and is used to identify and characterize species and structures. Generally, scattering of laser radiation from a surface upon which molecules are deposited leads only to very weak Raman signals because the number of molecules which are able to interact with the beam is very small. We have recently used integrated optics, i.e., in the form of light propagation in dielectric waveguides, and plasmon surface polaritons (surface electromagnetic waves) on metals, to increase the number of interacting molecules, in the first case, and to greatly increase the strength of the optical field, in the second. The details of the techniques will be described and illustrated with spectra from interfacial molecules. First, the optical properties of a film are determined from measurements of the angular dependence of the reflectivity or transmission of light. Analysis of these results can give the film thickness and refractive index. Second, the scattered light is focused on the slit of a double monochromator and the Raman spectrum recorded as a spectral shift from the exciting light. Frequency shifts, intensity changes and polarization variations are observed in the spectra of surface molecules compared to bulk molecules. Scattering from the substrate and from defects in the films are also observed. Measurements of this type aid in the characterization of surfaces and our ability to engineer its properties for specific uses.
Vibrational spectroscopy has provided significant insights into both the organization and order of long chain molecules in thin film geometries. Understanding the role of molecular architecture in determining orientation is also essential if the ultimate goal of establishing a design protocol for thin film structures is to be achieved. For more than a decade our work has focused on the use of chemistry to design novel molecules which can organize into unique morphologies on a surface and at interfaces. Rigid and flexible side chains and spacer groups, intermolecular bonding moieties and functional end groups have all been incorporated into long chain molecules and their effect on the structural organization studied by polarized IR and Raman spectroscopy as well as other surface sensitive techniques (XPS, neutron reflectivity, NEXAFS, ellipsometry and wetting).
Abstract Integrated optical techniques and resonance Raman spectroscopy have been combined to investigate the intermolecular interactions at dye/polymer and dye/glass interfaces. Frequency shifts and intensity changes of bands assigned to the stretching vibrations of the bridged quinoline rings of the cyanine dye chromophore have been utilized to gain insight into the relative strength of adhesive forces at the surface. Polarized Raman measurements were made to determine the orientation of the chromophores on a poly(vinyl alcohol) surface. This was done to assess the possibility of hydrogen bond formation between the ring nitrogen atoms and the polar hydroxyl groups at the surface.
