**Glossary**

FEM and histological analyses have shown that elevated local strains correspond to increases in gliosis (with a three-fold increase in gliosis at the probe tips compared to other areas of the explanted arrays [19]). In addition, FEM pressure profiles predict that the strain at the tip can be reduced by utilizing more flexible or softer substrates, reducing the opening angle of the probe face, and by promoting tissue integration to reduce excessive adhesion. Therefore, the Dropping Method is recommended for coating hydrogel at the tips of the microelectrodes to optimize the mechanical effects for improved chronic recording stability. Applying hydrogel coatings on the probe tips should stabilize the mechanical interface and lower the interfacial tension with the surrounding biological environment. Constraining hydrogel coating depths to just the tips will also avoid the electrode coverage issue [12]. Finally, the initial penetrating profile should be reduced since only the dehydrated probe tips will reswell in thickness upon water absorption (unlike previous studies that coated the hydrogel layers along the entire length of the neural electrode shanks in efforts to reduce the

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,

In this chapter, some parts of this work was performed when the first author was with the Centre for Biosystems, Neuroscience, and Nanotechnology, City University of Hong Kong, Hong Kong, with Stella Pang. The author is now working at the Center for Information

Technology Innovation (CITI), Academia Sinica, Taipei, Taiwan with Yu Tsao.

sustained injury responses).

**5. Conclusions**

190 An Excursus into Hearing Loss

and uniformity.

**Acknowledgements**

*Auditory cortex*: region of the temporal lobe that is responsible for processing auditory information.

*Biocompatibility*: ability of a biomaterial to perform with an appropriate host response in the body. *Brain machine interface*: technology that allows direct communication pathways between the brain. *Chronic neural recordings*: long-term neural recordings by implantable (e.g. intracortical) electrodes. *Critical surface area model*: theory of minimizing electrode surfaces to reduce tissue encapsulations. *Cylindrical radius*: neuronal ensembles must lie within ~140 μm of the recording electrode [12]. *Dipping method*: hydrogel coating as a function of the number of dips (via optical microscopy). *Dropping Method*: hydrogel coating automated on Dropmeter stage (via programmable software). *Electrode micromotion*: induced strain from relative movements between the probe and the tissue. *Finite element model*: numerical analysis that can approximate the behavior of mechanical systems. *Glial encapsulation*: formation of a fibrotic encapsulation layer or a glial scar surrounding an implant. *Hydrogel coating*: biomaterials with many versatile properties such as gelling and film-forming. *Inflammatory reactions*: complex biological responses of tissues that protect from harmful stimuli. *Insertion killzone*: region around the shaft where the local neuron density is lower than expected. *Utah electrode array*: 3D arrays consisting of conductive needles (designed at University of Utah). *Mechanical property difference*: theory of minimizing micromotions to reduce tissue encapsulation. *Michigan probe*: planar shanks from semiconductor platforms (designed at University of Michigan). *Microelectrode*: electrical conductor used for recording neural representations in the brain.
