## 3 Results and Discussion  
This section includes results and discussion of CMOS retina neuron IC that supports photoreceptors for retinal image acquisition. The four main challenges incur when slotted loop antenna, RF power harvesting IC, and retina circuit are inducted in an integration manner as well as interfacing approach: (1) to choose appropriate antenna, which can receive RF signals in the desired direction; (2) RF power harvester circuit should meet the biocompatibility requirements for retina neuron IC; (3) it is important to deliver sufficient energy to the photodiode of neuron circuit for appropriate selection in magnitude of read-out spikes; and (4) optimized process for heterogeneous integration of components. It is important to note that the area of slotted antenna and RF power harvester IC are obtained according to general dimensions of contact lens. Generally, the area of standard contact lens is about 1 cm\({}^{2}\) with total thickness of about 200 \(\upmu\)m. The calculated area of small electrical antenna is about 4 mm with thickness of 0.12 mm, while the area of power harvester IC is 1.2 mm\({}^{2}\). The measurement set-up of interfacing between the wireless power devices and the CMOS neuron IC is shown in Figure 9a where performance of antenna is measured using Agilent vector network analyzer with port 1 and achieves wide band performance from 3.1 GHz to 10.7 GHz. Another port 2 of the network analyzer is used for input selection of the power harvesting circuit where we choose resonating frequencies for each band at the input port of the RF harvester chip. The harvested power measured from the power meter using channel A and B are delivered to the neuron circuit IC via off-chip \(\upmu\)-LED that provides light to the photodiode. The neuron retina chip provides read-out spikes varied according to light intensity is controlled by the control voltage. The measured read-out spikes is shown in Figure 9b where 50 mV of amplitude is an appropriate choice for better resolution of image acquisition while the power or light intensity is varied from 0 W/cm\({}^{2}\) to 2 W/cm\({}^{2}\), respectively. The considered image is aligned along XY plane that shows the alphabetical letters. It is observed that better resolution of letters has been received by retina when 50 mV amplitude of read-out spikes is tuned, otherwise it would provide low resolution if other amplitude is selected. For example, low resolution of image can be seen in Figure 9c when 110 mV of amplitude is taken. Consequently, it is clear at the observation point that selection of amplitudes of read-out spikes is very important parameter for retinal image acquisition. Moreover, the power consumption of retina neuron IC is illustrated in Figure 9d. The lowest value of power consumption 110 nW has been achieved, which made good agreement with simulated one. The performance comparison of current works with other reported ones are shown in Table 2.  
Figure 8: CMOS retina neuron IC with its (**a**) schematic, (**b**) layout, and (**c**) microchip photograph.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Design-I, II \& III** & **Current Work** & [27] & [28] \\ \hline CMOS process (\(\mu\)m) & 0.065 & 0.18 & 0.35 \\  & Bio-compatible telemetry & & \\ UWB Antenna & Frequency range = 3.1 GHz to & DPSK data telemetry & \\  & 10.7 GHz & Data rate = 2 Mpbs & - \\  & Data rate = 2 Mpbs & & \\ RF Power harvesting IC & On-chip transistor, LV & & On-chip transistor, H.V & - \\  & Efficiency = 40\% to 80\% & & \\  & SoC (System on a chip) & SoC & SoC \\ CMOS neuron retina & Chip area = 1.1 x 2.1 mm\({}^{2}\) & Chip area = 5.7 x 6.6 mm\({}^{2}\) & Chip area = 4.3 x 3.2 mm\({}^{2}\) \\  & Power consumption = 110 nW & & \\ Application & Retinal Image Acquisition & Epi-Retinal and Neural Prostheses & Subretinal Implants \\ \hline \end{tabular}
\end{table}
Table 2: Comparison performance of current work with reported ones.  
Figure 9: _Cont._