**5. Conclusions**

This chapter reviewed how controlling the biocompatibility and mechanical properties of a microelectrode is critical when implanting deeper within the auditory system. To effectively monitor extracellular spike amplitudes in the cortex, several design considerations should be factored to ensure that the neuronal ensembles lie within a cylindrical radius of the recording electrode. First, penetrating shafts should reduce the initial mechanical trauma during surgical implantation (to minimize the reactive responses caused by the pathway of tissue damage). Second, the indwelling microelectrodes should embed bioactive reagents since chronic neural recording failure is associated with sustained injury responses from persistent inflammatory reactions. Third, the mechanical properties should be controlled since micromotions can cause shearing and compression to the surrounding tissues (due to the large stiffness mismatch of the implanted substrates). This chapter also describes how hydrogels can minimize the cross-sectional area while ensuring the neuronal density is maintained in local areas surrounding the implant. A novel hydrogel coating "Dropping Method" is proposed instead of the Dipping Method to eliminate the problems of lateral movements, approximated cycles, and uniformity.
