Mandheerej S. Nandra

A parylene-based microelectrode array implant for spinal cord stimulation in rats

The design and fabrication of an epidural spinal cord implant using a parylene-based microelectrode array is presented. Rats with hindlimb paralysis from a complete spinal cord transection were implanted with the device and studied for up to eight weeks, where we have demonstrated recovery of hindlimb stepping functionality through pulsed stimulation. The microelectrode array allows for a high degree of freedom and specificity in selecting the site of stimulation compared to wire-based implants, and triggers varied biological responses that can lead to an increased understanding of the spinal cord and locomotion recovery for victims of spinal cord injury.

A MEMS intraocular origami coil

This work presents a MEMS intraocular coil which is designed to be flexible, biocompatible and electrically efficient as the secondary power coil of inductive coupling links for retinal prostheses applications. A microfabricated structure of parylene-gold-parylene is combined with a folding technique to create a conducting strip with increased cross-section and length than is possible without folding, which is then wound to form a single-layer coil. Compared with the wire-wound coil and MEMS planar coil, our strip-fold-and-wound origami coil can better fulfill the size, mass, and flexibility requirements of an intraocular implant, and also reduce the proximity effect and achieve high quality factor at the targeted frequency. As an example, a 5-turn prototype coil of 10 mm outer diameter, 1 mm thickness and 10 mg weight in saline, has an inductance of 480 nH and reaches a maximum Q factor of 12.5 at 30 MHz. A further increase of Q factor can be achieved by increasing the equivalent cross-sectional area by depositing thicker metal.

Human Blood Cell Sensing with Platinum Black Electroplated Impedance Sensor

AC impedance sensing is an important method for biological cell analysis in flow cytometry. For micro impedance cell sensors, downsizing electrodes increases the double layer impedance of the metal-electrolyte interface, thus leaves no sensing zone in frequency domain and reduces the sensitivity significantly. We proposed using platinum black electroplated electrodes to solve the problem. In this paper, using this technique we demonstrated human blood cell sensing with high signal to noise ratio.

Reduction of AC resistance in MEMS intraocular foil coils using microfabricated planar Litz structure

The next-generation implantable high-power prosthetic devices, such as retinal prostheses, will be powered by inductively coupled coils of high efficiency. However, the use of high-frequency power carriers almost always induces significant AC resistance due to skin and proximity effects, which limits the overall coupling efficiency. Inspired by the widely used round Litz wire, planar coils with planar Litz structures were investigated on the printed circuit board (PCB) and showed the reduced AC resistances in some specific frequency ranges. Herein, various planar Litz structures were introduced into previous MEMS foil coils, which have been proved to be superior over the conventional planar counterparts. The experimental results provided by the aforementioned prototypes showed a reduction of AC resistance up to 13.3% compared to a solid foil coil. A microfabrication process, which combines a single layer metal deposition and patterning with a post folding step, was developed to construct the planar Litz structure.

Cantilever actuated by piezoelectric Parylene-C

We developed the first MEMS cantilever resonator actuated by piezoelectric Parylene-C, which is a new piezoelectric material for its integration into/with MEMS because it can be deposited at room temperature and electrically poled at 200°C with 400V of DC bias. After our first report on Parylene-C as a new piezoelectric material, we demonstrated here the first MEMS cantilever resonator actuated by piezoelectric PA-C.

Resonance Induced Impedance Sensing of Human Blood Cells

A challenging problem in AC impedance sensing of particles (e.g., blood cells in plasma) with micro electrodes is that with the shrinking of electrode surface area the electrode double layer capacitance decreases. Combined with the parallel stray capacitance, the system impedance is dominated by these capacitive components. Hence the sensitivity for particle sensing decreases. In this paper, we propose a new approach to solve the problem. The idea is to use resonant sensing by connecting an external parallel inductor to the system. At the resonant frequency, the capacitive components in the system were nullified by the inductor, leaving the electrolyte and particle impedance to be a major component in the system impedance. We then successfully demonstrated this idea by sensing 5 mum polystyrene beads. More important, this technique was extended to sensing blood cells in diluted human whole blood and leukocyte rich plasma. The measured signal pulse height histogram matched well with known volume distribution of erythrocytes and leukocytes.