New pH-neutral peritoneal dialysis solution, low in glucose degradation products, in a double-chamber bag.

Fresenius Medical Care-North America has developed a neutral-pH version of its Delflex peritoneal dialysis (PD) solution with low glucose degradation products (GDPs): Delflex Neutral pH. The Delflex Neutral pH system stores the PD solution in a dual-chamber bag. The product is admixed by the patient before use. The new design facilitates GDP reduction in two ways. First, GDPs are reduced because the dextrose solution is stored at a pH that minimizes degradation during sterilization and that optimizes dextrose stability over time. Second, the design minimizes generation of acetaldehyde by separating dextrose from lactate during heat sterilization of the product. Mixing the contents of the two chambers before use produces a physiologically compatible pH of approximately 7.0, with minimal GDPs. Analysis of GDP content was conducted by high-performance liquid chromatography. The GDP reduction across all sizes and formulations of Delflex Neutral pH ranged between 74% and 93% as compared with conventional Delflex PD solution. Testing of the new delivery system by prevalent PD patients demonstrated that, with minimal training, patients can obtain a homogeneous PD solution low in GDPs with a physiologically compatible pH of approximately 7.0.

Relationship between scattered intensity and separation for particles in an evanescent field.

We describe measurements of the scattering of visible light from an evanescent field by both spherical particles (R = 1-10 mum) that are glued to atomic force microscopy (AFM) cantilevers, and by sharp tips (R < 60 nm) that were incorporated onto the cantilevers during manufacture. The evanescent wave was generated at the interface between a flat plate and an aqueous solution, and an atomic force microscope was used to accurately control the separation, h, between the particle and the flat plate. We find that, for sharp tips, the intensity of scattered light decays exponentially with separation between the tip and the plate all the way down to h approximately 0. The measured decay length of scattered intensity, delta, is the same as the theoretical decay length of the evanescent intensity in the absence of the sharp tip. For borosilicate particles, (R = 1-10 mum), the scattering also decays exponentially with separation at large separations. However, when the separation is less than roughly 3delta, the measured scattering intensity is smaller in magnitude than that which would be predicted by extrapolating the exponential decay observed at large separations. For these particles, the scattering approximately fits the sum of two exponentials. The magnitude of the deviation from exponential at contact was roughly 10-15% for R = 1 mum particles and about 30% for larger particles and is larger for s-polarized light. Preliminary experiments on polystyrene particles shows that the scattering is also smaller than exponential at small separations but that the deviation from exponential is larger for p-polarized light. In evanescent wave AFM (EW-AFM) the scattering-separation can be calibrated for situations where the scattering is not exponential. We discuss possible errors that could be introduced by assuming that exponential decay of scattering continues down to h = 0.

Atomic force microscopy colloid-probe measurements with explicit measurement of particle-solid separation.

We describe the use of evanescent wave scattering to measure the separation between the surface of a solid and a particle that is attached to an atomic force microscope (AFM) cantilever. Termed evanescent wave atomic force microscopy, our approach involves measuring the intensity of the light scattered from an evanescent field formed by the total internal reflection of a laser beam at a solid/fluid interface. In a conventional AFM "colloid probe" measurement, this separation must be inferred from an examination of the surface forces. Direct measurement of this separation with an evanescent wave atomic force microscope (EW-AFM) removes some ambiguity in the surface force measurement and, in addition, allows new types of measurements. For example, the force can be monitored at a constant separation. Our evanescent scattering apparatus is essentially identical to that used in total internal reflection microscopy (TIRM), except that we collect the light that scatters back into the incident medium, because the AFM partly obscures the forward scattered light (i.e., light scattered into the transmitted region). Compared to a conventional TIRM measurement, where the particle moves freely, attaching the particle to the cantilever in an EW-AFM gives much greater control of the particle position.