Simulation Results

Regardless of circuit specifics, prosthesis pixels interface with the body through microelectrodes. When SCA is implanted beneath the pigment epithelial layer (PRL) as shown in Fig. 3.8, three main layers are expected to be stimulated: photoreceptor layer, horizontal/bipolar layer and ganglion cell layer. Each layer can be viewed as 2-dimensional spreading resistive network; three layers can be viewed as 3-dimension (3D). In order to simulate current distribution in the retina and to evaluate which kind of return electrodes is the best one for SCA, 3D spreading resistive network model, as shown in Fig. 3.9, has to be involved in the simulation of retinal stimulation current.

SCA directly drives current to the retina from the return electrode to the stimulating electrode.

The surfaces of three main layers of retina are defined as layer 1, layer 2 and layer 3. Layer 1 is closest to the SCA. Impedances of each layer are thought to be equal. where _i represents 3 different layers. Assuming that the stimulating electrode of SCA attaches to 3 x 3 sets of resistances and the retinal tissue is composed of 256 x 256 sets of resistances. Stimulation current flows from the stimulating electrode to the return electrode. Potential of the return electrode is represented by ground. Figure 3.10 shows the simplified schematic of the first tissue layer. The resistances are represented by the lines between two intersections to better understand the relationship among different positions. Elements of the matrix are named by x-coordinate and y-coordinate. Origin of the coordinates locates on the middle point of the matrix, and it is also middle point of the stimulation site and probe region.

Stimulation site is where the stimulating electrode attaches to the retina; 3 x 3 nodes surrounding the origin of the resistive network are involved. We probed the current passing through Rti, and probe region involves 5 x 5 nodes surrounding the stimulation site of the resistive network.

In addition to the proposed local surrounding return electrode in this work, there

are two other kinds of return electrodes as shown in Fig. 3.11. One is remote surrounding return electrode which is far away from the stimulation site in ring shape.

The other one is remote single return electrode which is also far away from the stimulation site in square shape. Electrode distance of local return is 2 columns, which is very close to the stimulation site; whereas electrode distance of remote return is 128 columns. Single remote return is at the position (-128, 0).

The simulation model of each solar cell in the circuit is established based on the previous experimental results. As strong light irradiates a 5μm x 5μm solar cell, generated photocurrent is about 10nA [25] as shown in Fig. 3.12(a). In the simulation, we shunted a current source near a solar cell to be the generated photocurrent as shown in Fig. 3.12(b). In comparison of which design of return electrodes can stimulate higher current at retina tissue, we probed the current passing through Rt1.

Simulation results of one pixel of SSCA stimulation chip with three different kings of return electrodes are shown in Fig. 3.13. With 25.4μA photocurrent, current in the stimulation site, i.e. (-1, -1), (-1, 0), (-1, 1), (0, -1), (0, 0), (0, 1), (1, -1), (1, 0), (1, 1), taking the major portion of the generated photocurrent. The minimum current in the stimulating area is at the (0, 0); the maximum current (Imax) is at the four corner, because the equivalent resistance at the latter node is lower than the former node.

Imax of the remote single return structure is 101.1nA; Imax of the remote surrounding return structure is 344.7nA; Imax of the local surrounding return structure is 1085.4nA. The Imax of local surrounding return is 10 times of remote single return structure and 3 times of remote surrounding return structure. Thereby SCA with local surrounding return electrode has higher current efficiency than other return electrode designs.

The simulation method of TSCA is similar with SSCA. Because the photo-sensing region of each solar cell of TSC is four time larger than SSC, we assumed the zero bias photocurrent of each solar cell is 40nA as shown in Fig. 3.14 (a); then 414 TSCs can generate 16.56μA photocurrent. In the simulation, we shunted a current source near the solar cells as the generated current in Fig. 3.14(b). Simulation results of TSCA with three different kings of return electrodes are shown in Fig. 3.15. Imax of the remote single return structure is 217.1nA; Imax of the remote surrounding return structure is 697.5nA. Imax of the local surrounding return structure is 1917.8nA. The Imax of local surrounding return is 10 times of remote single return structure and 3 times of remote surrounding return structure. These ratios are very similar with SSCA.

Even though the current generated from the SSCA is 1.5 times larger than the TSCA, the stimulation current of the latter is twice larger than the former, because

photovoltage of the latter is twice larger than the former. Thereby TSCA with local surrounding return electrode is more current efficient than SSCA. Stimulation current not only changes with different return electrode designs but also decreases with the depth of retinal layer for both SSCA and TSCA. Figure 3.16 shows the SSCA simulation results of stimulation current passing through three different retina layers with different return electrode designs. No matter what kind of return electrode is, stimulation current decreases 1.5 times with increasing retina layers if impedance of each layer is the same.

The quantity of solar cells decides the electrode distance. More solar cells generate more stimulation current but also result in farer electrode distance. To find the optimum value of solar cells with local surrounding return electrode, simulation on various quantities of solar cells is made. We pretended that SSCA or TSCA covers 256 x 256 retina impedance sets. Then electrode distance of 2540 SSCs or 414 TSCs is equivalent to 128 impedance sets. To make these two solar cell numbers more relate to electrode distance, we viewed them as 18 times and 3 times of 128, that is, 2304 and 384. If the decreasing ratio of electrode distance to solar cell number is 1:1, then for electrode distances 64, 48, 32, 16, 4, the solar cell numbers of SSCA approximate to 1152, 864, 576, 288, 72, respectively, whereas for TSCA approximate to 192, 144, 96, 48, 32, respectively. Figure 3.17 shows electrode distance to current curves of SSCA and TSCA, and maximum current occurs at electrode distances 32 columns and 64 columns, respectively. Simulation on various quantities of solar cells was also made. Figure 3.18(a) shows solar cell numbers to current curve of SSCA and TSCA. Maximum currents occur at 576 and 192 solar cells, respectively. Figure 3.18(b) shows photo-sensing region to current curve of two SCA, Optimum photo-sensing regions are 14400μm² and 19200μm² and maximum stimulation currents are 36.8nA and 60.6nA for SSCA and TSCA, respectively.

Current stimulation efficiency of TSCA is 1.65 times larger than SSCA with the optimum photo-sensing region.

Figure 3.8 Schematic of a SCA implanted beneath the PRL.

Figure 3.9 Schematic of the 3D spreading resistive network model. Thick, column and row resistances are represented by Rt, Rc, and Rr, respectively.

Figure 3.10 Schematic of the first layer of the retinal tissue. The retinal tissue we assumed is composed of 256 x 256 sets of resistances. The middle point of the matrix is at (0, 0). Stimulation site is labeled by the smallest square, 9 sets of resistances are included in a stimulating electrode. Probe region is labeled by the bigger square, 25 sets of resistances are included in the probe region.

(a) (b) (c)

Figure 3.11 Three different kinds of return electrodes: (a) local surrounding return electrode, (b) remote surrounding return electrode and (c) remote single return electrode.

(a) (b)

Figure 3.12 (a) Illumination model and (b) simulation model of P+/N-Well solar cells.

P+ with LV is connected to ground and N-Well with HI is connected to retina tissue.

(a) (b) (c)

Figure 3.13 Simulation results of stimulation current of three different return electrode designs in SSCA: (a) local surrounding return, (b) remote surrounding return, and (c) remote single return.

(a) (b)

Figure 3.14 (a) Illumination model and (b) simulation model of two cascode N+/P-Well solar cells. N+ with LV is connected to ground and P-Well with HV is connected to retina tissue.

(a) (b) (c)

Figure 3.15 Simulation results of stimulation current of three different return electrode designs in TSCA: (a) local surrounding return, (b) remote surrounding return, and (c) remote single return.

Figure 3.16 SSCA simulation results of stimulation current to different return electrode design curves of three different retina layers.

Figure 3.17 (a) Electrode distance to current curves of SSCA and TSCA. Maximum current occurs at electrode distances 32 and 64, respectively.

Figure 3.18 (a) Solar cell numbers and (b) Photo-sensing region to current curves of SSCA and TSCA. Maximum current (36.8nA and 60.6nA) occurs at solar cell numbers of 576 and 192 and photo-sensing regions of 14400μm² and 19200μm² for SSCA and TSCA, respectively.