Feasibility of Neural Stimulation With Floating-Light-Activated Microelectrical Stimulators

Abstract

Neural microstimulation is becoming a powerful tool for the restoration of impaired functions in the central nervous system. Microelectrode arrays with fine wire interconnects have traditionally been used in the development of these neural prosthetic devices. However, these interconnects are usually the most vulnerable part of the neuroprosthetic implant that can eventually cause the device to fail. In this paper, we investigate the feasibility of floating-light-activated microelectrical stimulators (FLAMES) for wireless neural stimulation. A computer model was developed to simulate the micro stimulators for typical requirements of neural activation in the human white and gray matters. First, the photon densities due to a circular laser beam were simulated in the neural tissue at near-infrared (NIR) wavelengths. Temperature elevation in the tissue was calculated and the laser power was retrospectively adjusted to 325 and 250 mW/cm(2) in the gray and white matters, respectively, to limit ΔT to 0.5 °C. Total device area of the FLAMES increased with all parameters considered but decreased with the output voltage. We conclude that the number of series photodiodes in the device can be used as a free parameter to minimize the device size. The results suggest that floating, optically activated stimulators are feasible at submillimeter sizes for the activation of the brain cortex or the spinal cord.

Fig. 1

The 3-D model geometry used in the heat-transfer model for calculations of temperature rise due to the absorption of the NIR light beam.

Fig. 2

Electrical model of the FLAMES, which includes the photodiodes, the interface with the tissue at the contacts, and the tissue impedance (B). The drawing on top shows the arrangement of the photodiode active area and the contacts on the wafer (A). The aspect ratio of the top surface is 1:5.

Fig. 3

Matlab flow diagram for the calculation of the FLAMES surface area. Boxes with thicker-line frames indicate the input parameters chosen according to the application.

Fig. 4

NIR fluence (W/cm²) inside the (a) gray and (c) white matters due to a 2-mm radius NIR beam and the resulting temperature elevations in tissue (B for gray and D for white). Fluence data are color-coded in the logarithmic scale. The NIR light beam is aimed from top to the center of the volume. Due to cylindrical symmetry, the plots show the fluence and temperature distributions only in one-half of a cross section.

Fig. 5

Active area for a single GaAs photodiode as a function of output current and implantation depth into human gray matter.

Fig. 6

Total contact area (anode+cathode) as a function of: (a) Output current (Depth = 3 mm, ρ = 500 Ωcm, N = 6). (b) Medium-specific impedance (Depth = 3 mm, I = 50 μA, N = 6). (c) Implantation depth (I = 50 μA, ρ = 500 Ωcm [29], N = 6). (d) The number of photodiodes in a device (N). Only one parameter is varied in each plot while the others are fixed at values shown in parentheses.

Fig. 7

Total device area (active areas plus contact areas) as a function of: (a) output current, (b) medium-specific resistivity, and (c) implantation depth. Fixed parameters are the same as in the Fig. 6 legend.

Fig. 8

Total device area as a function of N. The output current increases from top to bottom (25, 50, and 75 μA). Medium-specific resistances are representative of the gray (500 Ωcm) and white matters (longitudinal and transverse resistivity 300 Ωcm and 1200 Ωcm [30]), left and right columns, respectively. Implantation depths are as indicated in the first plot.

Fig. 9

Device output voltage as a function of: (a) output current, (b) medium-specific impedance, (c) implantation depth, and (d) the number of photodiodes in a device. Fixed parameters for each plot are the same as in the Fig. 6 legend.

Fig. 10

Driving function according to [16] for the FLAMES in the last row of Table I for a 7-μm myelinated mammalian axon fiber running parallel along the device above its top surface. The black belt shows the threshold of activation (i.e., axons placed inside this line will be activated).