**1. Introduction**

*Brain machine interfaces* connect the human brain to electronic devices and computer software [1]. Machine intelligence can offer the inherent advantages of greater processing speeds, computing power, memory capabilities, and even unrestricted sensory perception (e.g. infrared, ultra violet, X-ray, and ultrasonic spectra). With the emergence of deep brain stimulation, neuroprosthesis, neurofeedback, and exoskeleton technologies, science fiction is being bridged into a modern reality. However, the ultimate realization of a brain machine interface is to have a computer system that can chronically interface with the neural tissues. This neurotechnology will require neuroscientists and engineers to work together to address the technical challenges of accessing neural communication channels (for data routing and transmission), preserving the *biocompatibility* (to interface electronic components within the biological neural tissues), and maintaining the bio-signal processing (for selecting the appropriate control signals).

In Section 2, chronic neural recording failure is examined by reviewing cortical spike patterns, histological analyses, indentation experiments, and *finite element models* (FEM). For instance, probe impedance measures have been shown to increase, histological analyses have indicated that changes in the impedance spectra along the electrode-tissue interface are caused by the increasing spatial distribution of the reactive tissue responses, indentation experiments have shown persistent *inflammatory reactions* on the indwelling microelectrodes, and FEMs have simulated that surrounding tissues are compressed by the large stiffness mismatch of the implanted substrates (with pressure profiles revealing extensive tension at the probe

Controlling the Biocompatibility and Mechanical Effects of Implantable Microelectrodes…

http://dx.doi.org/10.5772/intechopen.72659

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In Section 3, the microfabrication of the *"Utah" electrode array* and the *"Michigan" neural probe* is compared by reviewing how their different sizes, shapes, and geometries can be redesigned to address: (1) the spatial distribution of neurons (as related to improving the recording quality); (2) the initial penetrating profile (as related to minimizing the *insertion killzone*); (3) the reactive cell responses (as related to preventing *glial encapsulation*); (4) the anchoring of a neural probe's position in the tissue (as related to reducing *electrode micromotion*) and (5) the embedding of various bioactive reagents (as related to eluting growth factors,

In Section 4, a *hydrogel coating* layer is proposed for improving the mechanical properties and *biocompatibility* at the electrode-tissue interface. Hydrogel coatings can be tailored to provide a buffer layer (for reducing stiffness mismatch), and to control the swelling properties to anchor the position of the probe in the tissue. In addition, a novel hydrogel coating method called the "*Dropping Method*" is proposed to control the mechanical properties at the electrode-tissue-interface. The

**Figure 2.** (A) Mean SNR across implanted probes over time and (B) mean impedance values. Reproduced from [10].

Dropping Method will be compared versus the traditional "*Dipping Method*."

tip).

anti-inflammatory drugs, etc.).

**2. Chronic neural recording failure**

**2.1. Deterioration of neural recordings over time**

This chapter will focus on the *microelectrode*: a neural probe used in neurophysiology for either recording neural representations in the brain or for electrically stimulating the nervous tissue. Micromachined electrodes and microwires are often used to monitor the neuronal activities by characterizing extracellular field potentials of multiple active neurons. These neural interfaces are artificially-engineered extensions of the nervous system that must coexist in the precise connections of supporting glial cells, oligodendrocytes, astrocytes, and microglia (**Figure 1A**).

**Figure 1.** (A) Depiction of cellular changes induced by an implanted electrode and (B) multichannel neural recordings.

Multichannel microelectrodes (**Figure 1B**) can be used to monitor the activity in the *auditory cortex* and to investigate the functional organization of the auditory system [2–4]. For example, a chronic microelectrode investigation of the human auditory cortex [5] revealed a "tonotopic" pattern where the sound-driven units had excitatory receptive fields with sharply tuned best-frequency response over a range of frequencies. These micro-machined devices are currently being developed to control brain machine interfaces [6–8] and can benefit many applications in medicine, communication, entertainment, military, and education. However, there has been one limiting factor that has obstructed their reliability as a fully-implantable neural prosthesis: *chronic neural recordings* have been shown to deteriorate [9, 10] with time (**Figure 2**).

In Section 2, chronic neural recording failure is examined by reviewing cortical spike patterns, histological analyses, indentation experiments, and *finite element models* (FEM). For instance, probe impedance measures have been shown to increase, histological analyses have indicated that changes in the impedance spectra along the electrode-tissue interface are caused by the increasing spatial distribution of the reactive tissue responses, indentation experiments have shown persistent *inflammatory reactions* on the indwelling microelectrodes, and FEMs have simulated that surrounding tissues are compressed by the large stiffness mismatch of the implanted substrates (with pressure profiles revealing extensive tension at the probe tip).

In Section 3, the microfabrication of the *"Utah" electrode array* and the *"Michigan" neural probe* is compared by reviewing how their different sizes, shapes, and geometries can be redesigned to address: (1) the spatial distribution of neurons (as related to improving the recording quality); (2) the initial penetrating profile (as related to minimizing the *insertion killzone*); (3) the reactive cell responses (as related to preventing *glial encapsulation*); (4) the anchoring of a neural probe's position in the tissue (as related to reducing *electrode micromotion*) and (5) the embedding of various bioactive reagents (as related to eluting growth factors, anti-inflammatory drugs, etc.).

In Section 4, a *hydrogel coating* layer is proposed for improving the mechanical properties and *biocompatibility* at the electrode-tissue interface. Hydrogel coatings can be tailored to provide a buffer layer (for reducing stiffness mismatch), and to control the swelling properties to anchor the position of the probe in the tissue. In addition, a novel hydrogel coating method called the "*Dropping Method*" is proposed to control the mechanical properties at the electrode-tissue-interface. The Dropping Method will be compared versus the traditional "*Dipping Method*."
