Needle-Shape Multifunctional Neural Probe Integrated with Light-Emitting Diodes and Fluidic Channel

Optical manipulation has been attractive because light can selectively control neuronal activity. Although optogenetic technology is a very powerful tool for proceeding of neuroscience, its scope is limited to specific laboratory animals due to the use of genetic engineering. If signal transduction can be photo-manipulated with organic compounds such as caged compounds, the scopes of optical manipulation technology would be expanded. To develop such a new method, development of the device for continuously injecting a drug locally and irradiating light with specific wavelength on the local region is required. In this study, a needle-shape neural probe integrated with micro-light-emitting diodes (μLED) and fluidic channel was developed.<br/>Prior to fabrication of multifunctional probe, the availability of μLED probe in optical manipulation was physiologically investigated. A blue-emitting μLED neural probe was prepared for verification by optogenetic technology using ChR2, which the technology has been authorized. The optical neural probe with three μLEDs of 50 μm in diameter was fabricated by the following procedure. An InGaN based LED structure on a (111) Si substrate was prepared. After forming mesa structure by dry-etching for the n-contact, transparent conductive electrode (ITO) for p-GaN and Ti/Au electrode for n-GaN were deposited by EB evaporator. Then, SiO₂ insulated layer was covered on the chip. Subsequently, Ti/Au wiring layer was deposited to connect the μLEDs. Finally, a needle shape structure was formed by deep reactive ion etching to achieve smooth insertion to the brain. The μLED probe was designed to be 3.6 mm in length and 200 μm in width. The probe was mounted on a PCB board. A clear rectification characteristic with turn on voltage of 3.2 V was observed. Additionally, blue emission with a peak wavelength of 450 nm was observed. A light output of 10 mW/mm² was obtained at 500 μA. To confirm the availability of μLED probe, mice expressing ChR2 in the CA3 region of the hippocampus was prepared by adeno-associated virus-mediated gene transfer. The CA3 neurons were activated by optical stimulation of the μLED probe, while excitatory postsynaptic local field potentials (LFP) evoked by the optical stimulation were recorded from the contralateral CA1 regions by a tungsten electrode. The recorded signal was analyzed by signal-averaging technique and wavelet transform approach. When μLED was driven at 1 kHz, responses corresponding to this optical manipulation were obtained in specific frequency band. This result indicates that neural activity was manipulated by μLED light.<br/>Next, multifunctional probe integrated with μLED and fluidic channel was developed. The channel was designed to drain the drug from the tip of the needle structure. The length and the crosssection of channel were designed to be 6.6 mm and 880 μm². First, a fluidic channel was fabricated on the back surface of the LED wafer. After 1 μm SiO₂ layer was deposited on Si surface of the backside, the region to be the channel was etched. The channel was spread laterally by isotropic etching of XeF₂. Then, 5 μm SiO₂ layer was deposited to cover the top of the channel. Subsequently, μLEDs were fabricated on the surface of LED wafer as in the previous procedure. When the flow rate of water through the channel was controlled by the air pressure, a flow rate of 2 μL/min was observed at a pressure of 50 kPa. Simultaneous operation of light output from μLED and delivery of water through the channel was realized. The development of this device would contribute to the construction of new application of physiological optical manipulation.