Suleman Ayub

High-density probe with integrated thin-film micro light emitting diodes (μLEDs) for optogenetic applications

We report on the design, fabrication, assembly and characterization of an innovative neural probe with optical functionality for optogenetic applications in neuroscientific research. In contrast to existing systems, thin-film micro light-emitting diodes (μLEDs) are directly integrated in probes based on silicon (Si) substrates using a wafer-level bonding process. It allows to considerably shrink the overall probe dimensions despite an increased density of the integrated light sources. Our new system approach demonstrates the integration of multiple μLEDs with a size of 112×112 μm2 at a minimal pitch of 150 μm that can be controlled individually. The all-electrical interconnection of this optical tool constitutes a distinct advantage over existing optical probes for in vivo experiments with freely behaving animals, as comparably stiff optical fibers interfacing external light sources are avoided. The optical power density of these μLEDs was measured to be up to 3.5 mW/mm2 in air, at a peak wavelength of 450 nm which fulfills the requirements for optogenetic applications.

Led-based optical cochlear implant on highly flexible triple layer polyimide substrates

This paper reports on the design, fabrication, assembly, as well as the optical, mechanical and thermal characterization of a novel MEMS-based optical cochlear implant (OCI). Building on advances in optogenetics, it will enable the optical stimulation of neural activity in the auditory pathway at 10 independently controlled spots. The optical stimulation of the spiral ganglion neurons (SGNs) promises a pronounced increase in the number of discernible acoustic frequency channels in comparison with commercial cochlear implants based on the electrical stimulation. Ten high-efficiency light-emitting diodes are integrated as a linear array onto an only 12-μm-thick highly flexible polyimide substrate with three metal and three polyimide layers. The high mechanical flexibility of this novel OCI enables its insertion into a 300 μm wide channel with an outer bending radius of 1 mm. The 2 cm long and only 240 μm wide OCI is electrically passivated with a thin layer of Cy-top™.

Hybrid intracerebral probe with integrated bare LED chips for optogenetic studies

This article reports on the development, i.e., the design, fabrication, and validation of an implantable optical neural probes designed for in vivo experiments relying on optogenetics. The probes comprise an array of ten bare light-emitting diode (LED) chips emitting at a wavelength of 460 nm and integrated along a flexible polyimide-based substrate stiffened using a micromachined ladder-like silicon structure. The resulting mechanical stiffness of the slender, 250-μm-wide, 65-μm-thick, and 5- and 8-mm-long probe shank facilitates its implantation into neural tissue. The LEDs are encapsulated by a fluropolymer coating protecting the implant against the physiological conditions in the brain. The electrical interface to the external control unit is provided by 10-μm-thick, highly flexible polyimide cables making the probes suitable for both acute and chronic in vivo experiments. Optical and electrical properties of the probes are reported, as well as their in vivo validation in acute optogenetic studies in transgenic mice. The depth-dependent optical stimulation of both excitatory and inhibitory neurons is demonstrated by altering the brain activity in the cortex and the thalamus. Local network responses elicited by 20-ms-long light pulses of different optical power (20 μW and 1 mW), as well as local modulation of single unit neuronal activity to 1-s-long light pulses with low optical intensity (17 μW) are presented. The ability to modulate neural activity makes these devices suitable for a broad variety of optogenetic experiments.

Heterogeneous 3D optrode with variable spatial resolution for optogenetic stimulation and electrophysiological recording

We report on the concept, development, and geometrical, optical as well as electrical characterization of the first three-dimensional (3D) optrode. This new device allows to optically interact with neuronal cells and simultaneously record their response with a high spatial resolution. Our design is based on a single-shank optical stimulation component and a multi-shank recording probe stacked together in a delicate assembly process. The electrical connection of both components is ensured by using flexible polyimide (PI) ribbon cables. The highly accurate relative positioning and precise alignment of the optical and electrical components in 3D with an optical output power at 460 nm well above 5 mW/mm2 and an all-electrical interface makes this device a promising tool for optogenetic experiments in neuroscientific research.

Compact intracerebral probe with yellow phosphor-based light conversion for optogenetic control

This paper reports on the use of phosphor particles to alter the emission spectrum of blue light-emitting diodes (LED) integrated in MEMS-based neural probes. For this purpose, we apply drop-casting to deposit the phosphor particles embedded in a transparent polymer matrix onto LED chips. The light emission is confined to spots as small as 100 μm in diameter by implementing micro-machined apertures in a probe stiffening structure made of silicon. Additionally, stray light emission from the probe rear through the semi-transparent polyimide substrate is effectively blocked by a metallic shielding layer. We demonstrate the optical down-conversion towards yellow wavelengths of 570–590 nm. A time-averaged output intensity of 0.679 mW/mm2 was measured at 5 m A modulated at a duty cycle of 10%. The output intensity strongly depends on the amount of phosphor covering the LED surface.

Low stress flip-chip package for pressure sensors operating at 500 °C

This work presents a method for a reliable assembly and interconnection of MEMS for very high temperatures. A flip-chip concept for resistive micromechanical pressure sensors with a platinum thin film was developed and sensor-assemblies were fabricated. The investigated metallized ceramic substrates were AlN, Si3N4, a Low-Temperature-Cofired-Ceramic (LTCC) and a zirconia-silicate (ZrSiO4). A borosilicate glass-solder was the die-attachment material and gold stud-bumps were the interconnection. The thermal-mechanical stresses in the sensors, induced by the packaging process due to material-dependent mismatches were analyzed with FEM and optical deformation measurements from 20 to 500 °C. The comparison of the obtained experimental and FE-results revealed a strong influence of the applied substrate on the thermal-mechanical stresses in the chip-membrane which is affecting the output-signal and reliability. Both methods were in good accordance. The two specific silicon-matched ceramic substrates LTCC and ZrSiO4 reduced the stresses in the sensor-element significantly. Furthermore, the electrical characterization of assembled test-sensors revealed a correlation between the package-induced stresses in the chip-membrane and the shift of the sensor-signal after the assembly-process.