The efficacy of implanted biomedical devices is often compromised by host recognition and subsequent foreign body responses. Here, we demonstrate the role of the geometry of implanted materials on their biocompatibility in vivo. In rodent and non-human primate animal models, implanted spheres 1.5 mm and above in diameter across a broad spectrum of materials, including hydrogels, ceramics, metals, and plastics, significantly abrogated foreign body reactions and fibrosis when compared to smaller spheres. We also show that for encapsulated rat pancreatic islet cells transplanted into streptozotocin-treated diabetic C57BL/6 mice, islets prepared in 1.5 mm alginate capsules were able to restore blood-glucose control for up to 180 days, a period more than 5-fold longer than for transplanted grafts encapsulated within conventionally sized 0.5-mm alginate capsules. Our findings suggest that the in vivo biocompatibility of biomedical devices can be significantly improved by simply tuning their spherical dimensions.

We present experimental results on the phase behavior of block copolymer/salt mixtures over a wide range of copolymer compositions, molecular weights, and salt concentrations. The experimental system comprises polystyrene- block-poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. It is well established that LiTFSI interacts favorably with poly(ethylene oxide) relative to polystyrene. The relationship between chain length and copolymer composition at fixed temperature is U-shaped, as seen in experiments on conventional block copolymers and as anticipated from the standard self-consistent field theory (SCFT) of block copolymer melts. The phase behavior can be explained in terms of an effective Flory-Huggins interaction parameter between the polystyrene monomers and poly(ethylene oxide) monomers complexed with the salt, χ, which increases linearly with salt concentration. The phase behavior of salt-containing block copolymers, plotted on a segregation strength versus copolymer composition plot, is similar to that of conventional (uncharged) block copolymer melts, when the parameter χ replaces χ in segregation strength.

The uncontrollable non-planar electrodeposition of lithium is a significant barrier to the widespread adoption of high energy density rechargeable batteries with a lithium metal anode. A promising approach for preventing the growth of lithium dendrites is the use of solid polymer electrolytes with a high shear modulus. Current density is the key variable in the electrodeposition of lithium. The present study is the first attempt at quantifying the effect of current density on the geometry and density of dendrites and other protrusions during electrodeposition through a solid polymer electrolyte. The geometry and density of defects formed on the lithium electrode were determined by X-ray microtomography. The tomograms revealed protrusions on the electrodeposited lithium electrodes that were either globular or dendritic, or void defects. The range of current densities over which stable, planar deposition was observed is quantified. At higher current densities, globular protrusions were observed. At the highest current density, both globular and dendritic protrusions were observed. The areal density of protrusion defects increased sharply with current density, while the overall defect density is a weak function of current density. Our work enables comparisons between the experimentally determined onset of non-planar electrodeposition and prevailing theoretical predictions with no adjustable parameters.

The limiting current is an important transport property of an electrolyte as it provides an upper bound on how fast a cell can be charged or discharged. We have measured the limiting current in lithium-lithium symmetric cells with a standard polymer electrolyte, a mixture of poly(ethylene oxide) and lithium bis(trifluoromethane) sulfonamide salt at 90°C. The cells were polarized with increasing current density. The steady-state cell potential was a smooth function of current density until the limiting current was exceeded. An abrupt increase in cell potential was taken as an experimental signature of the limiting current. The electrolyte mixture was fully characterized using electrochemical methods to determine the conductivity, salt diffusion coefficient, cation transference number, and thermodynamic factor as a function of salt concentration. We used Newman's concentrated solution theory to predict both cell potential and salt concentration profiles as functions of position in the cell at the experimentally applied current density. The theoretical limiting current was taken to be the current at which the calculated salt concentration at the cathode was zero. We see quantitative agreement between experimental measurements and theoretical predictions for the limiting current. This agreement is obtained without resorting to any adjustable parameters.

The design and engineering of composite materials is one strategy to satisfy the materials needs of systems with multiple orthogonal property requirements. In the case of rechargeable batteries with lithium metal anodes, the system requires a separator with fast lithium ion transport and good mechanical strength. In this work, we focus on the system polystyrene-blockpoly(ethylene oxide) (SEO) with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI). Ion transport occurs in the salt-containing poly(ethylene oxide)-rich domains. Mechanical rigidity arises due to the glassy nature of polystyrene (PS). If we assume that the salt does not interact with the PS-rich domains, we can describe ion transport in the electrolyte by three transport parameters (ionic conductivity, , salt diffusion coefficient, , and cation transference number, + 0) and a thermodynamic factor, f. By systematically varying the volume fraction of the conducting phase, c between 0.29 and 1.0, and chain length, between 80 and 8000, we elucidate the role of morphology on ion transport. We find that is the strongest function of morphology, varying by three full orders of magnitude, while is a weaker function of morphology. To calculate + 0 and f , we measure the current fraction, + , and the open circuit potential, , of concentration cells. We find that + and follow universal trends as a function of salt concentration, regardless of chain length, morphology, or c , allowing us to calculate + 0 for any SEO/LiTFSI or PEO/LiTFSI mixture when and are known. The framework developed in this paper enables predicting the performance of any block copolymer electrolyte in a rechargeable battery. MAIN TEXT

We have measured the effect of added salt on the chain dimensions of mixtures of poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) in the melt state through small angle neutron scattering (SANS) experiments. Scattering profiles from blends of hydrogenated and deuterated PEO mixed with LiTFSI were measured as a function of salt concentration. Scattering profiles from pure deuterated PEO/LiTFSI mixtures were used for background subtraction purposes. The densities of PEO/LiTFSI mixtures of varying salt concentrations were measured to calculate partial molar monomer volumes of PEO and LiTFSI to account for non-ideal mixing, which turned out to be negligible. Kratky plots of the scattering profiles were used to calculate the salt concentration dependence of statistical segment length. At low salt concentrations, segment length decreases with increasing salt concentration, before increasing with increasing salt concentration in the high salt concentration regime. The Random Phase Approximation was used to predict theoretical scattering profiles from the calculated segment lengths and partial molar volumes; there is excellent agreement between the theoretical and measured scattering profiles at all salt concentrations.There appears to be a correlation between chain dimensions and coordination between lithium ions and EO monomers. The scattering profiles of the pure deuterated PEO/LiTFSI mixtures suggested the presence ofhowed ion clusters of characteristic size of 0.58 6 nm at high salt concentrations. The presence of ion clusters is hypothesized to cause the increase in segment length seen in this salt concentration window.

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Understanding the distribution of ionic species in electrolytes is important for predicting the ion-transport properties. Here, a quantitative analysis of wide-angle X-ray scattering (WAXS) profiles was conducted for the first time on a series of mixtures of poly­(ethylene oxide) (PEO) and lithium bis­(trifluoromethanesulfonyl)­imide salt (LiTFSI), PEO/LiTFSI, as a function of salt concentration in the melt state. Abnormal scattering signatures were observed: while WAXS data showed a single peak (Peak 1) in the absence of salt, a second peak (Peak 0) appeared at lower scattering angles with added salt. Molecular dynamics simulations with the standard TraPPE-UA force field were used to uncover the molecular origins of the WAXS peaks. Qualitative agreement was found between the experimental and simulated scattering profiles. Simulations indicated that Peak 1 arises from correlations between EO segments as well as correlations between TFSI– ions and EO segments, while Peak 0 arises from correlations between neighboring TFSI– ions. There were, however, quantitative disagreements between experiment and simulations, which were resolved by the introduction of a charge rescaling factor, R f, to account for the polarization of ions and polymers. Simulations with charge rescaling predicted that the formation of anion-rich clusters occurs at a higher salt concentration than the emergence of Peak 0. While the WAXS data did not directly reflect the presence of anion-rich clusters, they provided a basis for more refined calculation of short-range correlations between ions, correlations that directly affect clustering and ion-transport properties such as ionic conductivities and transference numbers.