Sulfonation of dialdehyde cellulose extracted from sugarcane bagasse for synergistically enhanced water solubility.

Sulfonated cellulose (SC) was successfully derived from microcrystalline cellulose (MCC) extracted from sugarcane bagasse, which is a type of agricultural waste. The obtained MCC was first modified by oxidation using sodium periodate in order to cleave the carbon-carbon bonds at the C2 and C3 of the pyranose ring to form 2,3-dialdehyde cellulose. These activated aldehyde groups significantly facilitated the sulfonation carried out using potassium metabisulfite. The sulfonic acid group contents, surface morphology, and water solubility of the obtained products were characterized by titration, field-emission scanning electron microscopy, UV-vis spectroscopy, and zeta potential. High sulfonic acid group content was achieved for the obtained SC samples (i.e., 305-689 µmol/g). The increase in the sulfonic acid group content resulted in the gradual change in the surface morphology and water solubility of the SC samples. The obtained results imply that sugarcane bagasse is a promising raw material for the production of SC with good water solubility.

Polymerized Ionic Liquids: Correlation of Ionic Conductivity with Nanoscale Morphology and Counterion Volume.

The impact of the chemical structure on ion transport, nanoscale morphology, and dynamics in polymerized imidazolium-based ionic liquids is investigated by broadband dielectric spectroscopy and X-ray scattering, complemented with atomistic molecular dynamics simulations. Anion volume is found to correlate strongly with Tg-independent ionic conductivities spanning more than 3 orders of magnitude. In addition, a systematic increase in alkyl side chain length results in about one decade decrease in Tg-independent ionic conductivity correlating with an increase in the characteristic backbone-to-backbone distances found from scattering and simulations. The quantitative comparison between ion sizes, morphology, and ionic conductivity underscores the need for polymerized ionic liquids with small counterions and short alkyl side chain length in order to obtain polymer electrolytes with higher ionic conductivity.

Environmental stress cracking performance of polyether and PDMS-based polyurethanes in an in vitro oxidation model.

Environmental stress cracking (ESC) was replicated in vitro on Optim™ (OPT) insulation, a polydimethylsiloxane-based polyurethane utilized clinically in cardiac leads, using a Zhao-type oxidation model. OPT performance was compared to that of two industry standard polyether urethanes: Pellethane® 80A (P80A), and Pellethane® 55D (P55D). Clinically relevant specimen configurations and strain states were utilized: low-voltage cardiac lead segments were held in a U-shape by placing them inside of vials. To study whether aging conditions impacted ESC formation, half of the samples were subjected to a pretreatment in human plasma for 7 days at 37°C; all samples were then aged in oxidative solutions containing 0.9% NaCl, 20% H2 O2 , and either 0 or 0.1M CoCl2 , with or without glass wool for 72 days at 37°C. Visual and SEM inspection revealed significant surface cracking consistent with ESC on all P80A and P55D samples. Sixteen of twenty P80A and 10/20 P55D samples also exhibited breaches. Seven of 20 OPT samples exhibited shallow surface cracking consistent with ESC. ATR-FTIR confirmed surface changes consistent with oxidation for all materials. The number average molecular weight decreased an average of 31% for OPT, 86% for P80A, and 56% for P55D samples. OPT outperformed P80A and P55D in this Zhao-type in vitro ESC model. An aging solution of 0.9% NaCl, 20% H2 O2 , and 0.1M CoCl2 , with glass wool provided the best combination of ESC replication and ease of use. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1544-1558, 2017.

Segmental Dynamics and Dielectric Constant of Polysiloxane Polar Copolymers as Plasticizers for Polymer Electrolytes.

Dielectric relaxation spectroscopy was used to investigate the segmental dynamics of a series of siloxane-based polar copolymers combining pendant cyclic carbonates and short poly(ethylene oxide) (PEO) chains. The homopolymer with cyclic carbonate as the only side chain exhibits higher glass transition temperature T(g) and dielectric constant ε(s) than the one with only PEO side chains. For their copolymers the observed T(g) (agreeing well with the predicted values from the Fox equation) and ε(s) decrease with increasing PEO side chain content. These polar polymers exhibit a glassy ß relaxation with Arrhenius character, attributed to local chain motions of side groups attached to the main chain, and a segmental α relaxation, associated with the glass transition with a Vogel temperature dependence. As PEO side chain content increases, narrowing of the local glassy ß relaxation was observed in the copolymers. The segmental α dynamics were observed to be faster, with an increase in breadth and decrease in strength with increasing PEO side chain content. Owing to the trade-off between T(g) and ε(s), copolymers of intermediate composition result in the highest ionic conductivity when these copolymers are used to plasticize Li single-ion conducting ionomers.

The biostability of cardiac lead insulation materials as assessed from long-term human implants.

Accelerated in vitro biostability studies are useful for making relativistic comparisons between materials. However, no in vitro study can completely replicate the complex biochemical and biomechanical environment that a material experiences in the human body. To overcome this limitation, three insulation materials [Optim™ insulation (OPT), Pellethane® 55D (P55D), and silicone elastomer] from cardiac leads that were clinically implanted for up to five years were characterized using visual inspection, SEM, ATR-FTIR, GPC, and tensile testing. Surface cracking was not observed in OPT or silicone samples. Shallow cracking was observed in 17/41 (41%) explanted P55D samples. ATR-FTIR indicated minor surface oxidation in some OPT and P55D samples. OPT molecular weight decreased modestly (∼20%) at 2-3 years before stabilizing at 4-5 years. OPT tensile strength decreased modestly (∼25%) at 2-3 years before stabilizing at 4-5 years. OPT elongation at 4-5 years was unchanged from controls. P55D had no significant changes in molecular weight or tensile properties. Overall, results for OPT and P55D were consistent with and limited to cosmetic surface oxidation. Silicone demonstrated excellent biostability with no identifiable degradation. This study of explanted cardiac leads revealed that OPT, P55D, and silicone elastomer demonstrate similar and excellent biostability through five years of implantation in human patients.

Charge Transport of Polyester Ether Ionomers in Unidirectional Silica Nanopores.

Dielectric relaxation spectroscopy is employed to investigate charge transport properties of two polyester ether ionomers in the bulk state and when confined in unidirectional nanoporous membranes (average pore diameter = 7.5 nm). Under nanometric confinement in nonsilanized pores, the macroscopic transport quantities (dc conductivity and characteristic frequency rate) are lower by about 1.4 decades compared to the bulk. The remarkable decrease of transport quantities in nonsilanized nanoporous membranes can be quantitatively explained by considering the temperature dependence of the interfacial layer between the ionomer and the silica membrane surfaces. On the other hand, an enhancement of dc conductivity is observed when the surfaces of the pores are treated with a nonpolar organosilane. This effect becomes more pronounced at lower temperatures and is attributed to slight changes in molecular packing density caused by the two-dimensional geometrical constraint.

Limitations of predicting in vivo biostability of multiphase polyurethane elastomers using temperature-accelerated degradation testing.

Polyurethane biostability has been the subject of intense research since the failure of polyether polyurethane pacemaker leads in the 1980s. Accelerated in vitro testing has been used to isolate degradation mechanisms and predict clinical performance of biomaterials. However, validation that in vitro methods reproduce in vivo degradation is critical to the selection of appropriate tests. High temperature has been proposed as a method to accelerate degradation. However, correlation of such data to in vivo performance is poor for polyurethanes due to the impact of temperature on microstructure. In this study, we characterize the lack of correlation between hydrolytic degradation predicted using a high temperature aging model of a polydimethylsiloxane-based polyurethane and its in vivo performance. Most notably, the predicted molecular weight and tensile property changes from the accelerated aging study did not correlate with clinical explants subjected to human biological stresses in real time through 5 years. Further, DMTA, ATR-FTIR, and SAXS experiments on samples aged for 2 weeks in PBS indicated greater phase separation in samples aged at 85°C compared to those aged at 37°C and unaged controls. These results confirm that microstructural changes occur at high temperatures that do not occur at in vivo temperatures. In addition, water absorption studies demonstrated that water saturation levels increased significantly with temperature. This study highlights that the multiphase morphology of polyurethane precludes the use of temperature accelerated biodegradation for the prediction of clinical performance and provides critical information in designing appropriate in vitro tests for this class of materials.

Characterizing the structure of organic molecules of intrinsic microporosity by molecular simulations and X-ray scattering.

The design of a new class of materials, called organic molecules of intrinsic microporosity (OMIMs), incorporates awkward, concave shapes to prevent efficient packing of molecules, resulting in microporosity. This work presents predictive molecular simulations and experimental wide-angle X-ray scattering (WAXS) for a series of biphenyl-core OMIMs with varying end-group geometries. Development of the utilized simulation protocol was based on comparison of several simulation methods to WAXS patterns. In addition, examination of the simulated structures has facilitated the assignment of WAXS features to specific intra- and intermolecular distances, making this a useful tool for characterizing the packing behavior of this class of materials. Analysis of the simulations suggested that OMIMs had greater microporosity when the molecules were the most shape-persistent, which required rigid structures and bulky end groups. The simulation protocol presented here allows for predictive, presynthesis screening of OMIMs and similar complex molecules to enhance understanding of their structures and aid in future design efforts.

Novel hard-block polyurethanes with high strength and transparency for biomedical applications.

Novel hard-block-only polyurethanes are prepared from 1,4-butanediol (BDO) and a pre-polymer synthesized separately from 1,3-bis (4-hydroxybutyl) tetramethyl disiloxane (BHTD) and 4,4'-diphenylmethane diisocyanate (MDI). Three (co)polymers using different proportions of these chain extenders were synthesized using reaction injection moulding, and their microstructure, mechanical properties and in vitro oxidative biostability were evaluated. These materials were found to form a single-phase system, and exhibit optical transparency, high elastic modulus and tensile strength, as well as significant in vitro oxidative biostability. By controlling the BDO/BHTD composition, the ductility can be tuned over orders of magnitude. These polyurethanes may be potentially suitable for biomedical applications, like electronic headers for defibrillators, pacemakers and neurostimulators, and orthopedic nail encapsulation.

Dynamics of hydrated polyurethane biomaterials: Surface microphase restructuring, protein activity and platelet adhesion.

Microphase separation is a central feature of segmented polyurethane biomaterials and contributes to the biological response to these materials. In this study we utilized atomic force microscopy (AFM) to study the dynamic restructuring of three polyurethanes having soft segment chemistries of interest in biomedical applications. For each of the materials we followed the changes in near surface mechanical properties during hydration, as well as fibrinogen activity and platelet adhesion on these surfaces. Both AFM phase imaging and force mode analysis demonstrated that these polyurethane biomaterials underwent reorientation and rearrangement resulting in a net enrichment of hard domains at the surface. Fibrinogen activity and platelet adhesion on the polyurethane surfaces were found to decrease with increasing hydration time. The findings suggest that water-induced enrichment of hydrophilic hard domains at the surface changes the local surface physical and chemical properties in a way that influences the conformation of fibrinogen, changing the availability of the platelet-binding sites in the protein. This work demonstrates that the hydrated polyurethane biomaterial interface is a complex and dynamic environment where the surface chemistry is changing, altering the activity of fibrinogen and affecting blood platelet adhesion.

Molecular mobility and Li(+) conduction in polyester copolymer ionomers based on poly(ethylene oxide).

We investigate the segmental and local dynamics as well as the transport of Li(+) cations in a series of model poly(ethylene oxide)-based single-ion conductors with varying ion content, using dielectric relaxation spectroscopy. We observe a slowing down of segmental dynamics and an increase in glass transition temperature above a critical ion content, as well as the appearance of an additional relaxation process associated with rotation of ion pairs. Conductivity is strongly coupled to segmental relaxation. For a fixed segmental relaxation frequency, molar conductivity increases with increasing ion content. A physical model of electrode polarization is used to separate ionic conductivity into the contributions of mobile ion concentration and ion mobility, and a model for the conduction mechanism involving transient triple ions is proposed to rationalize the behavior of these quantities as a function of ion content and the measured dielectric constant.

In vitro oxidation of high polydimethylsiloxane content biomedical polyurethanes: correlation with the microstructure.

The resistance to in vitro metal ion oxidation of a polydimethylsiloxane (PDMS)-containing thermoplastic polyurethane elastomer (Elast-Eon) is compared with that of a polyurethane consisting of the same hard segment chemistry and content, but with aliphatic polycarbonate soft segments (PCU). Scanning electron microscopy and attenuated total reflectance Fourier transform infrared spectroscopy were used to assess changes in surface morphology and chemistry. The extent of bulk degradation was assessed indirectly by dynamic mechanical analysis and small-angle X-ray scattering experiments. The findings indicate that Elast-Eon is more resistant to oxidation than the PCU, because of the presence of the PDMS soft segments as well as its phase separated microstructure. The PCU exhibits a rather high degree of intermixing between hard and soft segments, rendering the hard segments dissolved or trapped in the soft phase more susceptible to oxidative conditions. By contrast, we propose that the existence of a completely phase separated PDMS soft phase in Elast-Eon protects the remainder of the segments from oxidation.

Plasticized single-ion polymer conductors: conductivity, local and segmental dynamics, and interaction parameters.

This paper considers high-quality conductivity data for plasticized ionomers in the context of polymer local and segmental processes. Dielectric spectroscopy was conducted on a neat PEO-based ionomer and six mixtures containing 6 wt % plasticizer with a wide range of dielectric constants. Conductivity increased dramatically but remained Vogel Fulcher Tamman (VFT)-like for all plasticized ionomers, indicating that the mechanism of ion transport was unchanged. Relaxation times of the polymer local beta and segmental alpha processes were analyzed for the plasticized ionomers, providing activation energies and relaxation strengths for the beta process and VFT fitting parameters for the alpha process. The glass transition temperature T(g) of the mixtures was found to be the critical characteristic governing conductivity, based on four criteria: VFT-like behavior of conductivity, decrease in the conductivity-segmental coupling index upon the addition of plasticizer, statistical insignificance of solvent quality (dielectric constant, donor number, and viscosity) on conductivity, and the creation of a conductivity master curve as a function of T(g)-normalized temperature.

Ion conduction and polymer dynamics of poly(2-vinylpyridine)-lithium perchlorate mixtures.

Ion conduction and polymer dynamics of homogeneous mixtures of poly(2-vinylpyridine) (P2VPy) with 0.1 to 10 mol % lithium perchlorate (LiClO(4)) were investigated using broadband dielectric spectroscopy. Interpretation of the relaxation behavior was assisted by findings from differential scanning calorimetry, Fourier transform infrared spectroscopy, dynamic mechanical analysis, and wide-angle and small-angle X-ray scattering experiments. Five dielectric relaxations were observed: a local beta-process in the glassy state, a segmental relaxation, a slow segmental process, an ion-mode relaxation, and electrode polarization. The local P2VPy beta-relaxation was strongly suppressed with increasing LiClO(4) content arising from the formation of transient crosslinks, which lead to a subsequent decrease in the number of free pyridine groups and/or a reduction in the local free volume in the presence of LiClO(4). Ion conduction at low LiClO(4) concentrations (<10 mol %) is governed by the diffusion of anions through the matrix, which is strongly coupled with the segmental relaxation. At relatively high LiClO(4) concentration (10 mol %), partial decoupling between ion motion and the segmental relaxation was observed, leading to increased conductivity.

Characterization of surface microphase structures of poly(urethane urea) biomaterials by nanoscale indentation with AFM.

Atomic force microscopy utilizing both tapping mode and force mode imaging is used to visualize the separated microphases in poly(urethane urea) films under ambient and aqueous conditions. The topography of the PUU surface changed upon hydration with the formation of nanometer-sized features on the surface. The surface becomes enriched in hard domains with hydration time and this enrichment is irreversible after dehydration. Force mode measurements were used to quantify mechanical properties as both indentation and modulus measurements. Analysis of the modulus during indentation reveals the three-dimensional nature of the structures, with the surface being covered by a 2-20-nm-thick soft segment overlayer under ambient conditions, while hydration leads to the loss of this overlayer. The force measurements also reveal the presence of regions having modulus values between those of the hard and soft phases and located spatially near the interface between the hard and soft domains. However, such regions with intermediate modulus were only rarely seen following hydration. Calculation of the Young's modulus from the compression data shows that hydration increases the modulus of the PUU surface by both enrichment of the amount of hard domain present and increasing the modulus of the individual hard and soft phases themselves. Direct visualization of the distribution of these different domains on the surface by nanoscale measurements provides an important path to characterizing the relationships between the surface properties of these materials and subsequent performance in biomedical applications.

Modeling electrode polarization in dielectric spectroscopy: Ion mobility and mobile ion concentration of single-ion polymer electrolytes.

A novel method is presented whereby the parameters quantifying the conductivity of an ionomer can be extracted from the phenomenon of electrode polarization in the dielectric loss and tan delta planes. Mobile ion concentrations and ion mobilities were determined for a poly(ethylene oxide)-based sulfonated ionomer with Li(+), Na(+), and Cs(+) cations. The validity of the model was confirmed by examining the effects of sample thickness and temperature. The Vogel-Fulcher-Tammann (VFT)-type temperature dependence of conductivity was found to arise from the Arrhenius dependence of ion concentration and VFT behavior of mobility. The ion concentration activation energy was found to be 25.2, 23.4, and 22.3+/-0.5 kJmol for ionomers containing Li(+), Na(+), and Cs(+), respectively. The theoretical binding energies were also calculated and found to be approximately 5 kJmol larger than the experimental activation energies, due to stabilization by coordination with polyethylene glycol segments. Surprisingly, the fraction of mobile ions was found to be very small, <0.004% of the cations in the Li(+) ionomer at 20 degrees C.

Amylose crystallization from concentrated aqueous solution.

Maize amylose, separated from granular starch by means of an aqueous leaching process, was used to investigate spherulite formation from concentrated mixtures of starch in water. Amylose (10-20%, w/w) was found to form a spherulitic semicrystalline morphology over a wide range of cooling rates (1-250 degrees C/min), provided it was first heated to >170 degrees C. This is explained through the effect of temperature on chain conformation. A maximum quench temperature of approximately 70 degrees C was required to produce spherulitic morphology. Quench temperatures between 70 and 110 degrees C produced a gel-like morphology. This is explained on the basis of the relative kinetics of liquid-liquid phase separation vis-à-vis crystallization. The possibility of the presence of a liquid crystalline phase affecting the process of spherulite formation is discussed.

Atomic force microscopy visualization of poly(urethane urea) microphase rearrangements under aqueous environment.

Polyurethane biomaterials are a critically important class of polymers used in a variety of medical devices. It has been suggested that the good blood compatibility of polyurethanes arises from nanoscale chemical heterogeneities at the surface as a consequence of the microphase separated morphology. In this study, we used tapping mode atomic force microscopy with phase imaging under aqueous conditions to visualize the distribution of the surface microphases for a series of poly(urethane urea) block co-polymers with varying hard segment content. The surfaces were prehydrated for 24 h under a flow of 1 mM phosphate buffer. Topographic images showed the formation of nanometer-sized raised features on the surface, having lateral dimensions of 50-70 nm and heights of 10-15 nm. Phase images, reflecting the local distribution of the mechanical properties under aqueous conditions, were quite different from those obtained in ambient conditions, consistent with water-induced structural reorientation. Images suggest that there is little soft phase material at the polymer surface in the presence of water, while images acquired after dehydration of the samples show that the surface layer remains rich in hard domains, indicating that the films do not return to their original states over the time period studied.

Spherulitic crystallization in starch as a model for starch granule initiation.

The influence of cooling rate and quench temperature on the formation of spherulitic morphology in heated mung bean starch is reported. Spherulites were obtained for a wide range of cooling rates (2.5-250 degrees C/min), provided the system was heated to 180 degrees C and then cooled below 65 degrees C. Branched crystalline structures were also observed, as was a gellike morphology. The dissolution temperature for spherulitic material ranged between 100 and 130 degrees C. A second dissolution endotherm was observed between 130 and 150 degrees C in systems containing gellike material. Spherulites revealed B-type X-ray diffraction patterns. Spherulitic crystallization of starch following phase separation is proposed as a model for starch granule initiation in vivo.

Human foetal osteoblastic cell response to polymer-demixed nanotopographic interfaces.

Nanoscale cell-substratum interactions are of significant interest in various biomedical applications. We investigated human foetal osteoblastic cell response to randomly distributed nanoisland topography with varying heights (11, 38 and 85 nm) produced by a polystyrene (PS)/polybromostyrene polymer-demixing technique. Cells displayed island-conforming lamellipodia spreading, and filopodia projections appeared to play a role in sensing the nanotopography. Cells cultured on 11 nm high islands displayed significantly enhanced cell spreading and larger cell dimensions than cells on larger nanoislands or flat PS control, on which cells often displayed a stellate shape. Development of signal transmitting structures such as focal adhesive vinculin protein and cytoskeletal actin stress fibres was more pronounced, as was their colocalization, in cells cultured on smaller nanoisland surfaces. Cell adhesion and proliferation were greater with decreasing island height. Alkaline phosphatase (AP) activity, an early stage marker of bone cell differentiation, also exhibited nanotopography dependence, i.e. higher AP activity on 11 nm islands compared with that on larger islands or flat PS. Therefore, randomly distributed island topography with varying nanoscale heights not only affect adhesion-related cell behaviour but also bone cell phenotype. Our results suggest that modulation of nanoscale topography may be exploited to control cell function at cell-biomaterial interfaces.