Using India Ink as a Sensor for Oximetry: Evidence of its Safety as a Medical Device.

Clinical EPR spectroscopy is emerging as an important modality, with the potential to be used in standard clinical practice to determine the extent of hypoxia in tissues and whether hypoxic tissues respond to breathing enriched oxygen during therapy. Oximetry can provide important information useful for prognosis and to improve patient outcomes. EPR oximetry has many potential advantages over other ways to measure oxygen in tissues, including directly measuring oxygen in tissues and being particularly sensitive to low oxygen, repeatable, and non-invasive after an initial injection of the EPR-sensing material is placed in the tumor. The most immediately available oxygen sensor is India ink, where two classes of carbon (carbon black and charcoal) have been identified as having acceptable paramagnetic properties for oximetry. While India ink has a long history of safe use in tattoos, a systematic research search regarding its safety for marking tissues for medical uses and an examination of the evidence that differentiates between ink based on charcoal or carbon black has not been conducted. METHODS Using systematic literature search techniques, we searched the PubMed and Food and Drug Administration databases, finding ~1000 publications reporting on adverse events associated with India/carbon based inks. The detailed review of outcomes was based on studies involving >16 patients, where the ink was identifiable as carbon black or charcoal. RESULTS Fifty-six studies met these criteria. There were few reports of complications other than transient and usually mild discomfort and bleeding at injection, and there was no difference in charcoal vs. carbon black India ink. CONCLUSIONS India ink was generally well tolerated by patients and physicians reported that it was easy to use in practice and used few resources. The risk is low enough to justify its use as an oxygen sensor in clinical practice.