Yoonsu Choi

A wide-band reflection-type phase shifter at S-band using BST coated substrate

The design and experimental results of a wide-band monolithic reflection-type phase shifter are presented in this paper. The phase shifter fabricated on Ba/sub 0.6/Sr/sub 0.4/TiO/sub 3/ (BST)/sapphire consists of a coplanar waveguide (CPW) Lange coupler, a series resonated LC termination, and a bias network. The CPW Lange coupler results in a power split of 3.5 dB/spl plusmn/0.5 dB in the range of 1.6-3.2 GHz. The BST interdigital capacitor has a tunability (C/sub max//C/sub min/) of 3.1 with 140 V. Measured and simulated performance of a series resonated LC termination was described. The phase shifter has achieved a phase-shift range of over 90/spl deg/ with an insertion loss of better than 2.0 dB and a return loss of higher than 14 dB in the frequency range of 1.9-2.5 GHz over a bias voltage range from 0 to 160 V. A figure-of-merit of maximum 72/spl deg//dB at 2 GHz was obtained. The smaller phase shifter using the folded-type CPW Lange coupler, which maintains a smaller aspect ratio for easier packaging, has an insertion loss of better than 2.3 dB with a phase-shift range of over 130/spl deg/ at 2.5 GHz. Two-tone measurements of the phase shifter indicate an input IP/sub 3/ of 32 dBm with 0 V and 41.9 dBm with 60 V. Total size of the monolithic BST phase shifter is 11.2 mm /spl times/ 4.9 mm /spl times/ 0.43 mm for the straight coupler design and 5.4 mm /spl times/ 6.5 mm /spl times/ 0.43 mm for the folded-type design.

SU-8 2000 rendered cytocompatible for neuronal bioMEMS applications

Microfabrication advances have resulted in small, cheap, and precise devices for biological microelectromechanical systems (bioMEMS). SU-8/SU-8 2000 is an attractive material for these applications because of its high-aspect ratio fabrication capability, dielectric properties, and thermochemical stability. Despite these advantages, the potential toxicity of SU-8 2000 may limit its use in cell-based applications. We show that <10% of primary neurons survived when cultured adjacent to or on top of untreated SU-8 2000. We evaluated the efficacy of various detoxification and surface treatments for SU-8 2000 in neuronal cultures after 7-21 days in vitro. Viability was improved to 45.8% +/- 4.5% (mean +/- standard error of the mean) following 3-day heat treatment (150 degrees C) under vacuum, while UV exposure and CO2 supercritical extraction did not improve survival. Furthermore, parylene coating (25 microm), in combination with heat and sonication (in isopropanol) treatments effectively masked the SU-8 2000 and led to 86.4% +/- 1.9% viability. Glow discharge (oxygen plasma) treatment rendered the SU-8 2000 surface more hydrophilic and improved neuronal viability, possibly through improved cell adhesion. No organic leachants were detected by mass spectrometry before or after heat treatment or after sonication. However, XPS analysis revealed the presence of potentially neurotoxic elements, fluorine and antimony. Strategies to improve the cytocompatibility of SU-8 2000 with primary neurons will allow longer culture times and have applications for cell-based microfabrication.

A wide bandwidth monolithic BST reflection-type phase shifter using a coplanar waveguide Lange coupler

A monolithic reflection-type phase shifter fabricated on BST/sapphire is presented here. The phase shifter consists of a coplanar waveguide (CPW) Lange coupler, meandered line inductors, and ferroelectric tunable capacitors. The CPW Lange coupler has a bandwidth of about 52% for 1-dB amplitude balance with center frequency of 2.7 GHz and the isolation is better than 18 dB in the frequency range of 1-3.7 GHz. The phase shifter using the CPW Lange coupler has a phase shift range of more than 90/spl deg/ with an insertion loss of better than 2 dB and a return loss of better than 14 dB in the frequency range of 1.85-2.56 GHz over a bias voltage range from 0 V to 160 V. A figure of merit of 89/spl deg//dB at 1.87 GHz and better than 44/spl deg//dB from 1.52 to 2.56 GHz was obtained with 160 V. Total size of the monolithic BST phase shifter is 11.2 mm/spl times/4.9 mm/spl times/0.43 mm.

Imaging of genetically engineered T cells by PET using gold nanoparticles complexed to Copper-64

Adoptive transfer of primary T cells genetically modified to have desired specificity can exert an anti-tumor response in some patients. To improve our understanding of their therapeutic potential we have developed a clinically-appealing approach to reveal their in vivo biodistribution using nanoparticles that serve as a radiotracer for imaging by positron emission tomography (PET). T cells electroporated with DNA plasmids from the Sleeping Beauty transposon-transposase system to co-express a chimeric antigen receptor (CAR) specific for CD19 and Firefly luciferase (ffLuc) were propagated on CD19(+) K562-derived artificial antigen presenting cells. The approach to generating our clinical-grade CAR(+) T cells was adapted for electro-transfer of gold nanoparticles (GNPs) functionalized with (64)Cu(2+) using the macrocyclic chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, DOTA) and polyethyleneglycol (GNP-(64)Cu/PEG2000). MicroPET/CT was used to visualize CAR(+)EGFPffLucHyTK(+)GNP-(64)Cu/PEG2000(+) T cells and correlated with bioluminescence imaging. These data demonstrate that GNPs conjugated with (64)Cu(2+) can be prepared as a radiotracer for PET and used to image T cells using an approach that has translational implications.

Highly inclined electrodeposited metal lines using an excimer laser patterning technique

This paper presents a simplified fabrication method to make highly elevated electrical signal lines up to 700 /spl mu/m in height from the substrates. The immediate application is a bioMEMS device that requires high aspect ratio three-dimensional (3D) multi-electrode arrays (MEA) to detect signals from deep within 3D networks of neurons. The demonstrated method generated thick electroplated metal lines on excimer laser patterned parylene molds. The directionality of the laser beam makes it possible to achieve practically straight patterns on the inclined sidewalls in addition to the bottom surface, which are several hundred microns below. 3D multi-electrodes on various elevation surfaces and insulation of parylene coating were efficiently implemented by using the directionality of excimer lasers and the conformal coating of parylene.

High Aspect Ratio SU-8 Structures for 3-D Culturing of Neurons

This paper presents a simplified fabrication method for high aspect ratio micromachined towers for culturing 3-D networks of neurons. The photosensitive epoxy SU-8 was used as both the tower material and the substrate. This material was particularly suitable because of its mechanical stability, biocompatibility, and potential for high aspect ratio fabrication. Fabrication is effectively simplified by a double exposure technique, allowing tower structures and an integral SU-8 substrate to be formed from the same epoxy deposition, thereby addressing column-to-substrate adhesion issues that have traditionally limited the aspect ratios achieved with conventional fabrication techniques. Aspect ratios up to 35:1 have been achieved, and successful cell culturing has been demonstrated.Copyright

Mechanically driven microtweezers with integrated microelectrodes

This paper presents a method for fabricating a fundamental MEMS tool—microtweezers. Microtweezers offer an attractive option to meet the increasing need to grasp, manipulate and excise microstructures or biological components. The microtweezers presented here augment a standard micromanipulator, allowing precise positioning in three dimensions. An additional micro-drive control knob, which is affixed to the micromanipulator, allows actuation of the tweezer tips through the use of a tether-cable drive system. This drive actuates the tweezer tips by the reciprocating motion of two microfabricated parts: the tweezers and tweezer box. A simple three-layer planar fabrication scheme allows for a broad range of tweezer styles (straight and serrated tips) and sizes (microns to millimeters). For these studies, 20 µ mw ide and 10 µm thick nickel beams were developed for the tweezer tips, which could endure 20 mN of force. To demonstrate the concept of microassembly, pick and place operations were performed on 10 µm thick film structures. Additional functionality was achieved by integrating platinum-black microelectrodes into parylene-coated tweezers to allow electrophysiological functions such as cellular stimulation and recording. Ultimately, this unique and simple design affords extraordinarily delicate control that is potentially beneficial for applications in microassembly, electrophysiology and microsurgery. (Some figures in this article are in colour only in the electronic version)

Functional recordings from awake, behaving rodents through a microchannel based regenerative neural interface

OBJECTIVE Neural interface technologies could provide controlling connections between the nervous system and external technologies, such as limb prosthetics. The recording of efferent, motor potentials is a critical requirement for a peripheral neural interface, as these signals represent the user-generated neural output intended to drive external devices. Our objective was to evaluate structural and functional neural regeneration through a microchannel neural interface and to characterize potentials recorded from electrodes placed within the microchannels in awake and behaving animals. APPROACH Female rats were implanted with muscle EMG electrodes and, following unilateral sciatic nerve transection, the cut nerve was repaired either across a microchannel neural interface or with end-to-end surgical repair. During a 13 week recovery period, direct muscle responses to nerve stimulation proximal to the transection were monitored weekly. In two rats repaired with the neural interface, four wire electrodes were embedded in the microchannels and recordings were obtained within microchannels during proximal stimulation experiments and treadmill locomotion. MAIN RESULTS In these proof-of-principle experiments, we found that axons from cut nerves were capable of functional reinnervation of distal muscle targets, whether regenerating through a microchannel device or after direct end-to-end repair. Discrete stimulation-evoked and volitional potentials were recorded within interface microchannels in a small group of awake and behaving animals and their firing patterns correlated directly with intramuscular recordings during locomotion. Of 38 potentials extracted, 19 were identified as motor axons reinnervating tibialis anterior or soleus muscles using spike triggered averaging. SIGNIFICANCE These results are evidence for motor axon regeneration through microchannels and are the first report of in vivo recordings from regenerated motor axons within microchannels in a small group of awake and behaving animals. These unique findings provide preliminary evidence that efferent, volitional motor potentials can be recorded from the microchannel-based peripheral neural interface; a critical requirement for any neural interface intended to facilitate direct neural control of external technologies.

Continuously-varying, three-dimensional SU-8 structures: fabrication of inclined magnetic actuators

A fabrication approach using SU-8 epoxy photoresist to create three-dimensional structures with continuous variation in the third dimension has been developed. Using this approach, a ramp structure 300 /spl mu/m tall with an angle of inclination of 36 degrees has been fabricated. In an extension of the process, both metal and dielectric materials can be deposited and patterned on the inclined surface. The current application for these ramp structures is the fabrication of magnetic switches of well-defined inclination relative to the substrate. Such switches were produced using the ramp structures as an underlying mechanical support. Switch arrays with inclined orientation are able to actuate consistently in the same direction in response to external magnetic fields. Under magnetic actuation, the switch produced a contact resistance of 5.1 /spl Omega/.

Polymeric microcombustors for solid-phase conductive fuels

This paper presents a micromachined SU-8 combustor for the ignition and reaction of solid conductive fuels. Solid fuels can be made conductive by doping the fuel with conductive powder. The conductive solid fuel serves to simplify device fabrication by allowing ignition to occur by passing current directly through the fuel sample from two MEMS-fabricated electrodes as opposed to relying on the transfer of heat from external or imbedded igniters. An array of high-aspect-ratio electrodes were patterned from SU-8 by multiple-exposure. The devices were filled with 3.97 mm/sup 3/ of graphite-doped conductive Glycidyl-Azide-Polymer (GAP). Nozzles with varying sizes are fitted on the microcombustors. Approximately 600 mN was generated within 20 ms. The easy fabrication and low cost/weight of these microcombustors or gas generators lend themselves to disposable, one-use, or short duration applications.