text: For many reasons known and unknown, protein adsorption varies from material to material. Two of the biggest determining factors that have been observed are surface roughness and surface free energy. In the case of exposed electrodes, it is desirable to have the adsorbed protein layer as thin as possible to increase sensitivity and performance. Noble metals are an obvious choice for achieving biocompatibility; however, when acting as electrodes, some of these noble metals will actually participate in the reaction, deteriorate, and trigger adverse effects via lost particles. The most (noble metals) are gold (Au), platinum (Pt), and iridium (Ir).
  *  = Surface free energy
  * E = Total energy per unit cell at surface
  * E = Total energy per unit cell in bulk of material
  * A = Area of surface
  The properties of titanium were also investigated in the study that produced the data for the above table, however, its properties are not listed here since its poor conductive properties make it unsuitable for neural implants. Insight on titaniums surface chemistry may give direction to future research. Titanium has the roughest and most hydrophilic surface of any metal described thus far (the importance of protein adsorption, its mechanisms, and the interplay of hydrophilic properties are discusses further in the hydrogels section of the page). Titanium adsorbed the thickest protein layer after the first day and still after the seventh day, but actually had its thickness reduced by the 28th day. Gold, platinum and iridiums protein layers all continued to grow up until the 28th day, but the rates slowed over time.
  Two more conductive materials that have notable characteristics are tungsten and indium tin oxide. Tungsten is electrically conductive and can be manufactured down to a very fine point, and for this reason has been used in intraspinal microstimulation (ISMS) for mapping out spinal cords during terminal surgeries. Tungsten electrodes, however, can corrode and form tungstic ions in the presence of HO and/or O. Tungstic acid has been seen to be highly toxic to cat motorneurons, and for this reason, does not currently make a suitable material for chronic implants. Indium tin oxide (ITO) has been used as electrode material for in vitro studies. ITO electrodes are very precise when stimulating and recording and when placed amongst plasma proteins, develop and maintain the thinnest protein layer compared to the other materials so far mentioned. It may have potential for acute in vivo usage, but over time, it has been observed to let go of particles producing highly toxic effects.