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One strategy for assessing efficacy of a liver transplant is to monitor perfusion and oxygenation after transplantation. An implantable optical sensor is being developed to overcome inadequacies of current monitoring approaches. To facilitate sensor design while minimizing animal use, a polydimethylsiloxane (PDMS)-based liver phantom was developed to mimic the optical properties of porcine liver in the 630-1000 nm wavelength range and the anatomical geometry of liver parenchyma. Using soft lithography to construct microfluidic channels in pigmented elastomer enabled the 2D approximation of hexagonal liver lobules with 15mm sinusoidal channels, which will allow perfusion with blood-mimicking fluids to facilitate the development of the liver perfusion and oxygenation monitoring system.

Abstract:In an effort to develop an implantable optical perfusion and oxygenation sensor, based on multiwavelength reflectance pulse oximetry, we investigate the effect of source-detector separation and other sourcedetector characteristics to optimize the sensor's signal to background ratio using Monte Carlo (MC) based simulations and in vitro phantom studies. Separations in the range 0.45 to 1.25 mm were found to be optimal in the case of a point source. The numerical aperture (NA) of the source had no effect on the collected signal while the widening of the source spatial profile caused a shift in the optimal source-detector separation. Specifically, for a 4.5 mm flat beam and a 2.4 mm × 2.5 mm photodetector, the optimal performance was found to be when the source and detector are adjacent to each other. These modeling results were confirmed by data collected from in vitro experiments on a liver phantom perfused with dye solutions mimicking the absorption properties of hemoglobin for different oxygenation states. Frank, "Development of an implantable oximetry-based organ perfusion sensor," Conf. Proc. IEEE Eng. Med.

Luminescence-based sensors have been developed in microparticle formats for biochemical targets such as glucose, enabling use of dermal implants for on-demand monitoring. For these to be deployed and interrogated in vivo, a matched optoelectronic system for delivery of excitation, collection and analysis of luminescence response is needed. In this work, simulations based on Monte Carlo ray-tracing were performed for models of luminescent microparticle materials embedded in skin. The spectral and spatial distribution of luminescence escaping the skin was determined for different concentrations, implantation depths, and input beam sizes. Results indicate that the implant environment does not significantly alter the measured spectral intensity ratios. The escaping emission light possesses measurable power and spectral information for quantitative analysis. Using these findings, an optical system has been designed specifically for sensor interrogation and response acquisition, and is currently implemented in hardware. Following benchtop validation and signal-to-noise maximization with tissue phantoms, the instrument will be used for measurement on sensors in rat subjects.

An optical system for monitoring glucose with a means of compensating for local oxygen variations is described. Measurements of sensor response to glucose under varying oxygen levels indicate a strong dependence of the optical response on ambient oxygen. The two-probe approach enables simultaneous monitoring of luminescence lifetime of microparticle sensors with a compact optical system. Glucose tracking experiments with both probes reveal a modulation of oxygen in the vicinity of sensors, even when external oxygen is constant. These results prove the concept of simultaneous monitoring of multiple sensor types in the same matrix.