Limits...
Improved selectivity from a wavelength addressable device for wireless stimulation of neural tissue.

Seymour EÇ, Freedman DS, Gökkavas M, Ozbay E, Sahin M, Unlü MS - Front Neuroeng (2014)

Bottom Line: Optical activation provides a wireless means of energy transfer to the neurostimulator, eliminating wires and the associated complications.We assessed the improved addressability of individual devices via wavelength selectivity as compared to spatial selectivity alone through on-bench optical measurements of the devices in combination with an in vivo light intensity profile in the rat cortex obtained in a previous study.We show that wavelength selectivity improves the individual addressability of the floating stimulators, thus increasing the number of devices that can be implanted in close proximity to each other.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Boston University, Boston MA, USA.

ABSTRACT
Electrical neural stimulation with micro electrodes is a promising technique for restoring lost functions in the central nervous system as a result of injury or disease. One of the problems related to current neural stimulators is the tissue response due to the connecting wires and the presence of a rigid electrode inside soft neural tissue. We have developed a novel, optically activated, microscale photovoltaic neurostimulator based on a custom layered compound semiconductor heterostructure that is both wireless and has a comparatively small volume (<0.01 mm(3)). Optical activation provides a wireless means of energy transfer to the neurostimulator, eliminating wires and the associated complications. This neurostimulator was shown to evoke action potentials and a functional motor response in the rat spinal cord. In this work, we extend our design to include wavelength selectivity and thus allowing independent activation of devices. As a proof of concept, we fabricated two different microscale devices with different spectral responsivities in the near-infrared region. We assessed the improved addressability of individual devices via wavelength selectivity as compared to spatial selectivity alone through on-bench optical measurements of the devices in combination with an in vivo light intensity profile in the rat cortex obtained in a previous study. We show that wavelength selectivity improves the individual addressability of the floating stimulators, thus increasing the number of devices that can be implanted in close proximity to each other.

No MeSH data available.


Related in: MedlinePlus

Cross-sectional schematic of one of the two wafer structures for FLAME stimulators. This particular structure consists of an optical block and two series photodiodes connected through a highly doped tunneling junction. This structure (FLAMES A) is designed to provide maximum responsivity around 860 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3927122&req=5

Figure 2: Cross-sectional schematic of one of the two wafer structures for FLAME stimulators. This particular structure consists of an optical block and two series photodiodes connected through a highly doped tunneling junction. This structure (FLAMES A) is designed to provide maximum responsivity around 860 nm.

Mentions: Our photovoltaic structures are based on custom layered compound semiconductor heterostructures utilizing gallium arsenide/aluminum gallium arsenide (GaAs/AlxGa(1-x)As) on a gallium arsenide substrate. The multilayered GaAs/AlxGa(1-x)As wafers were grown by ITME (Institute of Electronic Materials Technology, Poland). The wafers have eleven epitaxial layers consisting of two p-i-n junctions that are vertically connected through a highly doped tunneling junction region. Two diodes are connected in series to increase the open-circuit voltage across the diode and reduce the requirements on the contact resistance and thus require less optical power to induce neurostimulation. The layer thicknesses are optimized for matching the current in the upper and lower cells and also to provide reasonable etch depths. The top layer serves as an optical block to filter shorter wavelengths. Two custom wafer structures are fabricated, each having a different AlxGa(1-x)As composition in the optical block and photodiode layers: one with a composition of Al0.1Ga0.9As as the optical block and GaAs as the photodiode layer, the other with a composition of Al0.2Ga0.8As as the optical block and Al0.1Ga0.9As as the photodiode layer. The former structure, shown in Figure 2, is designed to have a peak responsivity around 860 nm and the latter was designed to have a peak responsivity around 780 nm. Two different spectral responsivities are used to demonstrate the wavelength addressability. Throughout the paper, these devices will be referred to as FLAME A, and FLAME B stimulators, respectively. Aluminum concentration values were specified when requesting the wafers, however, as it will be explained in the results sections, actual aluminum concentrations were found to differ from these specified values. The designed aluminum concentration of the layers was optimized to provide a good compromise between responsivity and wavelength selectivity. We performed quantum efficiency simulations of the wafer structures with a custom MATLAB program that calculates the reflection and quantum efficiency from a layered structure using film scattering matrix calculations. Real and imaginary values of the refractive index for intermediate aluminum concentrations were calculated based on model functions by Adachi (1985).


Improved selectivity from a wavelength addressable device for wireless stimulation of neural tissue.

Seymour EÇ, Freedman DS, Gökkavas M, Ozbay E, Sahin M, Unlü MS - Front Neuroeng (2014)

Cross-sectional schematic of one of the two wafer structures for FLAME stimulators. This particular structure consists of an optical block and two series photodiodes connected through a highly doped tunneling junction. This structure (FLAMES A) is designed to provide maximum responsivity around 860 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3927122&req=5

Figure 2: Cross-sectional schematic of one of the two wafer structures for FLAME stimulators. This particular structure consists of an optical block and two series photodiodes connected through a highly doped tunneling junction. This structure (FLAMES A) is designed to provide maximum responsivity around 860 nm.
Mentions: Our photovoltaic structures are based on custom layered compound semiconductor heterostructures utilizing gallium arsenide/aluminum gallium arsenide (GaAs/AlxGa(1-x)As) on a gallium arsenide substrate. The multilayered GaAs/AlxGa(1-x)As wafers were grown by ITME (Institute of Electronic Materials Technology, Poland). The wafers have eleven epitaxial layers consisting of two p-i-n junctions that are vertically connected through a highly doped tunneling junction region. Two diodes are connected in series to increase the open-circuit voltage across the diode and reduce the requirements on the contact resistance and thus require less optical power to induce neurostimulation. The layer thicknesses are optimized for matching the current in the upper and lower cells and also to provide reasonable etch depths. The top layer serves as an optical block to filter shorter wavelengths. Two custom wafer structures are fabricated, each having a different AlxGa(1-x)As composition in the optical block and photodiode layers: one with a composition of Al0.1Ga0.9As as the optical block and GaAs as the photodiode layer, the other with a composition of Al0.2Ga0.8As as the optical block and Al0.1Ga0.9As as the photodiode layer. The former structure, shown in Figure 2, is designed to have a peak responsivity around 860 nm and the latter was designed to have a peak responsivity around 780 nm. Two different spectral responsivities are used to demonstrate the wavelength addressability. Throughout the paper, these devices will be referred to as FLAME A, and FLAME B stimulators, respectively. Aluminum concentration values were specified when requesting the wafers, however, as it will be explained in the results sections, actual aluminum concentrations were found to differ from these specified values. The designed aluminum concentration of the layers was optimized to provide a good compromise between responsivity and wavelength selectivity. We performed quantum efficiency simulations of the wafer structures with a custom MATLAB program that calculates the reflection and quantum efficiency from a layered structure using film scattering matrix calculations. Real and imaginary values of the refractive index for intermediate aluminum concentrations were calculated based on model functions by Adachi (1985).

Bottom Line: Optical activation provides a wireless means of energy transfer to the neurostimulator, eliminating wires and the associated complications.We assessed the improved addressability of individual devices via wavelength selectivity as compared to spatial selectivity alone through on-bench optical measurements of the devices in combination with an in vivo light intensity profile in the rat cortex obtained in a previous study.We show that wavelength selectivity improves the individual addressability of the floating stimulators, thus increasing the number of devices that can be implanted in close proximity to each other.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Boston University, Boston MA, USA.

ABSTRACT
Electrical neural stimulation with micro electrodes is a promising technique for restoring lost functions in the central nervous system as a result of injury or disease. One of the problems related to current neural stimulators is the tissue response due to the connecting wires and the presence of a rigid electrode inside soft neural tissue. We have developed a novel, optically activated, microscale photovoltaic neurostimulator based on a custom layered compound semiconductor heterostructure that is both wireless and has a comparatively small volume (<0.01 mm(3)). Optical activation provides a wireless means of energy transfer to the neurostimulator, eliminating wires and the associated complications. This neurostimulator was shown to evoke action potentials and a functional motor response in the rat spinal cord. In this work, we extend our design to include wavelength selectivity and thus allowing independent activation of devices. As a proof of concept, we fabricated two different microscale devices with different spectral responsivities in the near-infrared region. We assessed the improved addressability of individual devices via wavelength selectivity as compared to spatial selectivity alone through on-bench optical measurements of the devices in combination with an in vivo light intensity profile in the rat cortex obtained in a previous study. We show that wavelength selectivity improves the individual addressability of the floating stimulators, thus increasing the number of devices that can be implanted in close proximity to each other.

No MeSH data available.


Related in: MedlinePlus