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Oxygen transfer in the implant environment

  • Author(s): Goor, Jared Braden
  • et al.
Abstract

Oxygen mass transfer in the subcutaneous tissue environment was studied using implanted oxygen sensors and quantitative histology. The impact of biomaterials on mass transfer was also observed through analysis of materials encased sensor performance and measurement of serum proteins. Dynamic sensor challenges were used to gain insight into the oxygen stasis mechanisms of subcutaneous tissue and to estimate diffusion of oxygen through the tissue and biomaterials. Five hamsters were implanted with planar arrays consisting of 16 symmetrically distributed oxygen sensors. Each array was half encased by smooth PDMS and half by a microporous cellulose membrane. The sensor signals were measured over 14 days, including two sessions of hypoxic challenges. Steady state sensor signals normalized to pre-surgical calibrations demonstrated elevated signal magnitudes for PDMS encased sensors, significant for the first 7 days (P<0.015). Noise levels for PDMS encased sensors were observed to be lower except during hypoxia when the trend was reversed. At the hypoxic levels observed, sensitivity of the sensors to oxygen remained linear. In vitro and in vivo, cellulose encased sensors had smaller time constants, a measure of the sensor's ability to respond to change. However, time constants were more strongly dependent in vivo on the proximity of vasculature to the sensor, observed to be closer in tissues adjacent to cellulose. Comparison of the time constants for oxygen increases versus decreases demonstrated a tissue resistance to oxygen loss inversely dependent on tissue volume and thus likely vascular in origin. Diffusivity for the array environments was 1.91±0.86*10⁻⁵ cm²/s on average, with greater diffusion resistance in the cellulose membranes. In vivo, loss of sensor signal magnitude and gain in time constants following hypoxic challenge was independent of biomaterials, but dependent on the tissue response to the implant. A method of analyzing digital images of histological sections using color and morphological filters was refined for the quantification and spatial mapping of specific tissue features with potential effect on oxygen diffusion. Haptoglobin measurements over 7 days following biomaterials implantation in hamsters showed that microporous polytetrafluoroethylene and cellulose in window chambers elicited elevated responses versus a control and versus a window chamber only implantation

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