The current in the receiving coil can then be transformed into a power source for the implanted hardware or data signals can be extracted. Several

limiting factors in this approach complicate the design of wireless stimulating implants of any kind, neural prostheses included. The first is that the most efficient transfer of electromagnetic energy between the Tacrolimus supplier primary and secondary coils occurs when the coils directly appose each other; physical separation and misalignment therefore impose an efficiency penalty due to the “uncoupling” of the transmitting and receiving coils (Rasouli and Phee, 2010). In particular, rapid reductions in power transfer efficiency are seen with relative angles >20° between the transmitting and receiving coils (Ng et al., 2011). This is particularly Pexidartinib cell line problematic for retinal implants, in which eye movement may require the use of additional coil pairs to ensure consistent coupling (Ng et al., 2011). In a cortical prosthesis the implanted electrode arrays may be self-contained, including inductive coils for power and data transmit/receive (Lowery, 2013 and Rush et al., 2011), or the power/data transfer electronics and coil may be separate from the arrays themselves (Coulombe et al., 2007). An advantage of the self-contained

array approach is the lack of any requirement for tethering, which may reduce damage to the cortex from relative motion of the brain and arrays in the long term (see Section 6.3.1). However, a disadvantage of the self-contained coils is the variation in coupling between the individual implanted array coils and the external coil. For example, arrays implanted on the medial surface of the occipital pole may be at a greater angle to the transmitting coil than those on the more lateral surface. Furthermore, if arrays are implanted more anteriorly onto medial calcarine cortex, these would be more distant from, and orthogonal to the external coil than the more

superficial arrays, resulting in poor or zero coupling and energy transfer. Aside from tethering medial arrays to a more superficially-mounted Aldehyde dehydrogenase coil, alternatives may include the aforementioned optical or ultrasonic approaches to power and/or data transfer. Another consideration in the use of wireless power and data transfer derives from the absorption of electromagnetic energy by tissue, which increases exponentially with frequency (Al-Kalbani et al., 2012); the need to transfer sufficient power while maintaining high data transfer rates therefore introduces competing constraints that complicate the design process. Moreover, the separate wire coils used for data and power transfer can interfere with each other, introducing complexity to the design of receiving hardware (Kiani and Ghovanloo, 2014 and Rush et al., 2011).