Page 393 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 387

additional research is required before they can be considered for clinical use.
Primarily understanding chronic effects related to biocompatibility and
long-term stability of these materials in the biological environment should
be explored (Polikov et al., 2005; Schwahn et al., 2001; Cogan et al., 2016).
For example, CPs have low stability under continuous electrical stimulation,
exposure to ultraviolet (UV) light or heat which may gradually deplete their
conductive elements AND degrade their mechanical properties. In some
approaches, toxic residues arising from the synthesis process may exist.
While CNT, Si NWs, and NCDs are highly stable, owing their nano-
dimensionality, they may penetrate into the living cells. This may be an
advantage achieving intracellular connection, but may also lead to damage
to the integrity and operation of the cell. In particular, in case the
nanomaterials are not properly anchored to the substrate and may disinte-
grate and freely float in the tissue and promote cellular uptake. On the other
hand, compared with CPs, these materials are still relatively stiff relative to
the tissue. With micro-magnetic stimulation, it should be noted that the
region of activation is confined to a near-field region around the implanted
coil (Lee et al., 2016). Beyond the materials and design of the electrodes,
implementation of high-density prostheses with more than hundreds of
stimulating channels requires additional challenges to be overcome, includ-
ing high-resolution connections between the stimulation circuitry and the
electrode array, design, and integration of the stimulator chip to individually
control each electrode, and wireless transfer of data and power to the elec-
trodes. For example, the connecting leads can tether the device and increase
its chance of migration and also the chance of device failure by lead fracture
or disconnection. Designing wireless transmission should consider that cou-
pling of the transmitter and receiver coils is most efficient with the coils
apposing each other in parallel (Rasouli and Phee, 2010). The internal coil
may be integral to the electrode array housing or may be a separate unit
receiving wired connections from the electrode arrays. With cortical pros-
theses, the distance between the coils and the absorption of electromagnetic
energy by the scalp and skull are also important design considerations
(RamRakhyani et al., 2011; Schwarz et al., 2014).
When trying to artificially replace the biological function, additional
considerations, beyond engineering of materials and stimulation paradigms
should be taken into account. For example, the postoperative rehabilitation
process and evaluation of visual performance in individuals implanted with
prosthesis. As part of the rehabilitation process, the recipients will need to be
trained to use the bionic device and then engage in daily practice sessions to