Electrophysiologic recording provides inexpensive and non-invasive monitoring of both normal and pathological human brain function. With a saline-sponge technology, a dense array of 64, 128, or 256 electrodes can be applied in a few minutes, allowing electroencephalographic (EEG) recording in emergency and intensive care settings. However, EEG evaluations in demanding clinical settings often require long-term monitoring, and the current sponge-saline electrode technology provides usable EEG for only 2-3 hours before re-hydration is required. Long-term recordings can be accomplished by the use of conventional electrode pastes or glues, but the discomfort and preparation time become major impediments in many medical settings for which brain monitoring may be useful. This Phase I project proposes the creation of a novel Phase-Reversible Electro-Physiological Sensor (PREPS) that allows long-term recording to become as fast and comfortable as our sponge-saline technology. The PREPS is a hydrogel whose viscosity is changed by a stimulus (such as electrical current or temperature). Based on our promising pilot data, this Phase I project will perform systematic feasibility studies to optimize the viscosity of the hydrogel, determine practical ranges for phase-reversible operation with electrical current and temperature manipulations, and achieve both skin bonding and skin hydration to produce high quality extended EEG monitoring with the PREPS technology.