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The electron paramagnetic resonance (EPR) spectra of some paramagnetic materials exhibit a pO2 (partial pressure of oxygen)-dependent linewidth. By recording the EPR linewidth in vivo using low-frequency EPR spectrometers, it is possible to measure the partial pressure of oxygen in tissues. It has been found, however, that some of the paramagnetic materials with optimal spectroscopic properties in vitro may lose or change their responsiveness to oxygen in tissues. The aim of this study was to microencapsulate paramagnetic particles by biopolymers in order to stabilize their responsiveness to oxygen. Carbohydrate char particles (Bubinga) were encapsulated with different biopolymers: cellulose acetate or cellulose triacetate, silicone and polyurethane. The performance of the materials was evaluated in vitro and in vivo. X-band EPR spectroscopy was used to test the variation of the calibration curve (EPR linewidth as a function of the pO2) after incubation in saline and after prolonged residence in tissues. The stability of the responsiveness to PO2 in vivo was carried out by L-band EPR spectroscopy using mice that received injection of the oxygen sensors in the muscles. After residence in saline and prolonged residence in tissues, only the calibration curve of the silicone-coated (coating weight of 0.5% (w/w)) paramagnetic materials remained unchanged, while those of oxygen sensors coated with cellulose acetate, cellulose triacetate and polyurethane changed.

The measurement of pO2 in vivo using EPR has some features that provide potential advantages for many applications. There is a strong possibility that EPR oximetry also will have significant applications in clinical medicine. The characteristics of EPR oximetry that appear to be especially useful often are complementary to existing techniques for measuring oxygen in tissues. These characteristics include capabilities of making repeated measurements from the same site, high sensitivity for low levels of oxygen, and non-invasive options. The existing EPR techniques are especially useful for studies in small animals, where the depth of measurements is not an over-riding issue. In larger animals and potentially in human subjects, the non-invasive techniques seem to be immediately applicable to study phenomena very near the surface (within 10 mm), while invasive techniques have some very promising uses for other sites. There also is a possibility of extending the non-invasive approach to greater depths by using lower frequency EPR. The clinical uses of EPR oximetry that seem especially promising are: long term monitoring of the status and response to treatment of peripheral vascular disease and optimizing cancer therapy by enabling it to be modified on the basis of the pO2 measured in the tumor. EPR oximetry is based on the effect of oxygen on EPR spectra, which provides a sensitive and accurate means to measure pO2 quantitatively in vivo. The in vivo applications of this technique have occurred because of the development of oxygen sensitive paramagnetic materials that are very sensitive to changes in the pO2 and, in some cases quite stable in tissues for long periods of time. These advances have made it feasible to make EPR measurements in intact animals. The physical basis and biological applications of in vivo EPR oximetry are reviewed, with particular emphasis on the use of EPR spectroscopy at 1200 MHz using particulate paramagnetic materials for the repetitive and non-invasive measurement of pO2 in tissues.