Peter Krulevitch

Thin film shape memory alloy microactuators

Thin film shape memory alloys (SMAs) have the potential to become a primary actuating mechanism for mechanical devices with dimensions in the micron-to-millimeter range requiring large forces over long displacements. The work output per volume of thin film SMA microactuators exceeds that of other microactuation mechanisms such as electrostatic, magnetic, thermal bimorph, piezoelectric, and thermopneumatic, and it is possible to achieve cycling frequencies on the order of 100 Hz due to the rapid heat transfer rates associated with thin film devices. In this paper, a quantitative comparison of several microactuation schemes is made, techniques for depositing and characterizing Ni-Ti-based shape memory films are evaluated, and micromachining and design issues for SMA microactuators are discussed. The substrate curvature method is used to investigate the thermo-mechanical properties of Ni-Ti-Cu SMA films, revealing recoverable stresses up to 510 MPa, transformation temperatures above 32/spl deg/C, and hysteresis widths between 5 and 13/spl deg/C. Fatigue data shows that for small strains, applied loads up to 350 MPa can be sustained for thousands of cycles. Two micromachined shape memory-actuated devices-a microgripper and microvalve-also are presented.

A practical microgripper by fine alignment, eutectic bonding and SMA actuation

Abstract A microgripper with a large gripping force, a relatively rigid structural body, and flexibility in functional design is presented. The actuation is generated by NiTiCu shape memory alloy (SMA) films and the stress induced can deflect each side of the microgripper up to 55 μm for a total gripping motion of 110 μm. When fully open, the force exerted by the film corresponds to a 13 mN gripping force on the tip of the gripper.

In vitro and in vivo measurements of fiber optic and electrochemical sensors to monitor brain tissue pH

Abstract We report herein the development of fiber optic and electrochemical pH sensors that could become part of an arsenal to quickly and aggressively treat people undergoing a stroke as well as people who have suffered traumatic brain injury. The fiber optic pH sensor design consists of the immobilization of a pH sensitive dye, seminaphthorhodamine-1 carboxylate (SNARF-1C) within a silica sol–gel matrix. A miniature optoelectronics package was developed to acquire data from the fiber optic sensor. The electrochemical sensor consists of a thin film multilayer coating sputtered on a kapton substrate. The sensors were tested in vitro and in vivo. For both sensors, the in vitro results show linear and reproducible responses in human blood in the pH range 6.8–8.0. The results of the in vivo studies which were performed in Spraque–Dawley rats indicate that both the fiber optic and electrochemical sensors monitor pH with very little drift. It was concluded that both types of sensors would be useful in tracking brain tissue pH.

Mixed-sputter deposition of Ni-Ti-Cu shape memory films

Abstract Ni-Ti-Cu shape memory films were mixed-sputter deposited from separate nickel, titanium, and copper targets, providing increased compositional flexibility. Shape memory characteristics, examined for films with 7 at.% Cu and 41–51 at.% Ti, were determined with temperature-controlled substrate-curvature measurements, and the microstructure was studied with transmission electron microscopy. The Ni-Ti-Cu films were found to have shape memory properties comparable with bulk materials, with transformation temperatures between 20 and 62 °C, a 10–13 °C hysteresis, and up to 330 MPa recoverable stress.

Vertical-actuated electrostatic comb drive with in situ capacitive position correction for application in phase shifting diffraction interferometry

This research utilizes the levitation effect of electrostatic comb fingers to design vertical-to-the-substrate actuation for optical phase shifting interferometry applications. For typical polysilicon comb drives with 2 /spl mu/m gaps between the stationary and moving fingers, as well as between the microstructures and the substrate, the equilibrium position is nominally 1-2 /spl mu/m above the stationary comb fingers. This distance is ideal for most phase shifting interferometric applications. A parallel plate capacitor between the suspended mass and the substrate provides in situ position sensing to control the vertical movement, providing a total feedback-controlled system. The travel range of the designed vertical microactuator is 1.2 /spl mu/m. Since the levitation force is not linear to the input voltage, a lock-in amplifier capacitive sensing circuit combined with a digital signal processor enables a linearized travel trajectory with 1.5 nm position control accuracy. A completely packaged micro phase shifter is described in this paper. One application for this microactuator is to provide linear phase shifting in the phase shifting diffraction interferometer (PSDI) developed at LLNL which can perform optical metrology down to 2 /spl Aring/ accuracy.

Microfabricated Multi-Frequency Particle Impedance Characterization System

We have developed a microfabricated flow-through impedance characterization system capable of performing AC, multi-frequency measurements on cells and other particles. The sensor measures both the resistive and reactive impedance of passing particles, at rates of up to 100 particles per second. Its operational bandwidth approaches 10 MHz with a signal-to-noise ratio of approximately 40 dB. Particle impedance is measured at three or more frequencies simultaneously, enabling the derivation of multiple particle parameters. This constitutes an improvement to the well-established technique of DC particle sizing via the Coulter Principle. Human peripheral blood granulocyte radius, membrane capacitance, and cytoplasmic conductivity were measured (r = 4.1 μm, Cmem = 0.9 μF/cm2, σint = 0.66 S/m) and were found to be consistent with published values.

Stretchable micro-electrode array [for retinal prosthesis]

This paper focuses on the design considerations, fabrication processes, and preliminary testing of a retinal prosthesis that has the potential to aid in vision restoration to millions of blind patients. We are developing an implantable, stretchable micro-electrode array using polymer-based microfabrication techniques. The device will serve as the interface between an electronic imaging system and the human eye, directly stimulating retinal neurons via thin film conducting traces and electroplated electrodes. The metal features are embedded within a thin (/spl sim/50 /spl mu/m) substrate fabricated using poly (dimethylsiloxane) (PDMS), a biocompatible elastomeric material that has high oxygen permeability and low water permeability. The conformable nature of PDMS is critical for ensuring uniform contact with the curved surface of the retina. To fabricate the device, we developed unique processes for metalizing PDMS to produce robust traces capable of maintaining conductivity when stretched (strain = 7%, SD 1), and for selectively passivating the conductive elements. An in situ substrate curvature measurement taken while curing the PDMS revealed a tensile residual strain of 10%, explaining the stretchable nature of the thin metalized devices.

A Practical Microgripper By Fine Alignment, Eutectic Bonding And Sma Actuation

A silicon microgripper with a large gripping force, a relatively rigid structural body, and flexibility in functional design is presented. The actuation is generated by Ni-Ti-Cu shape memory alloy (SMA) films and the stress induced can deflect each side of the microgripper up to 55 {mu}m for a total gripping motion of 110 {mu}m. When fully open, the force exerted by the film corresponds to a 40 mN gripping force on the tip of the gripper.

Polymer-Based Packaging Platform for Hybrid Microfluidic Systems

A polymer-based packaging platform for creating hybrid microfluidic systems is presented. Polydimethylsiloxane (PDMS) is cast into an acrylic mold frame with suspended elements that are removed after curing to form chip cavities, inlet and outlet ports, microchannels, and reservoirs. The packaging approach enables the integration of off-the-shelf components such as pumps and valves with glass microfluidic devices, electronic chips, sample reservoirs, and flow channels. A particle pre-concentration module with a glass capture chip and integrated micropump is shown as an example. A pneumatically driven microfluidic pumping module is also shown. Custom microfluidic interconnects for interfacing to micro-scale fluidic systems are presented. The connectors are capable of withstanding more than 1000 psi and allow microdevices to be rapidly connected to macroscopic devices and systems, without the use of tools.

Electrostatic comb drive for vertical actuation

The electrostatic comb finger drive has become an integral design for microsensor and microactuator applications. This paper reports on utilizing the levitation effect of comb fingers to design vertical-to-the-substrate actuation for interferometric applications. For typical polysilicon comb drives with 2 micrometers gaps between the stationary and moving fingers, as well as between the microstructures and the substrate, the equilibrium position is nominally 1-2 micrometers above the stationary comb fingers. This distance is ideal for many phase shifting interferometric applications. Theoretical calculations of the vertical actuation characteristics are compared with the experimental result, and a general design guideline is derived from these result. The suspension flexure stiffness, gravity forces, squeeze film damping, and comb finger thicknesses are parameters investigated which affect the displacement curve of the vertical microactuator. By designing a parallel plate capacitor between the suspended mass and the substrate, in situ position sensing can be used to control the vertical movement, providing a total feedback-controlled system. Fundamentals of various capacitive position sensing techniques are discussed. Experimental verification is carried out by a Zygo distance measurement interferometer.