Profound hypothermia, a medical procedure where the body is cooled to low temperatures, has limited clinical applications due to the time constraints of approximately one hour. Major benefits for certain neurosurgical and pediatric cardiovascular procedures would be realized if the time limitations imposed by present technologies for cardiac arrest during hypothermia could be extended 3-4 hours without significant ischemic injury. One approach towards achieving this is to design an aqueous blood substitute that protects the brain and visceral organs during controlled hypothermia. The specific aims of this SBIR are: I. To develop and characterize an aqueous blood substitute for profound hypothermia. II. To evaluate the effectiveness of the developed substitute in an ex- vivo rat heart model. III. To evaluate the comparative efficacy of the substitute in an in-vivo animal model. If phase 1 is successful, in phase 2, we will perform additional studies to optimize the composition of the substitute using the models established in phase 1 and perform profound hypothermia assisted surgery using the developed substitute in a select human population.