Julie Hamilton

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.

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.

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.

Microfluidic tools for biological sample preparation

Researchers at Lawrence Livermore National Laboratory are developing means to collect and identify fluid-based biological pathogens in the forms of proteins, viruses, and bacteria. To support detection instruments, we are developing a flexible fluidic sample preparation unit. The overall goal of this Microfluidic Module is to input a fluid sample, containing background particulates and potentially target compounds, and deliver a processed sample for detection. We are developing techniques for sample purification, mixing, and filtration that would be useful to many applications including immunologic and nucleic acid assays. Sample preparation functions are accomplished with acoustic radiation pressure, dielectrophoresis, and solid phase extraction. We are integrating these technologies into packaged systems with pumps and valves to control fluid flow and investigating small-scale detection methods.

A Microfluidic Platform for Precision Small-volume Sample Processing and Its Use to Size Separate Biological Particles with an Acoustic Microdevice.

A major advantage of microfluidic devices is the ability to manipulate small sample volumes, thus reducing reagent waste and preserving precious sample. However, to achieve robust sample manipulation it is necessary to address device integration with the macroscale environment. To realize repeatable, sensitive particle separation with microfluidic devices, this protocol presents a complete automated and integrated microfluidic platform that enables precise processing of 0.15–1.5 ml samples using microfluidic devices. Important aspects of this system include modular device layout and robust fixtures resulting in reliable and flexible world to chip connections, and fully-automated fluid handling which accomplishes closed-loop sample collection, system cleaning and priming steps to ensure repeatable operation. Different microfluidic devices can be used interchangeably with this architecture. Here we incorporate an acoustofluidic device, detail its characterization, performance optimization, and demonstrate its use for size-separation of biological samples. By using real-time feedback during separation experiments, sample collection is optimized to conserve and concentrate sample. Although requiring the integration of multiple pieces of equipment, advantages of this architecture include the ability to process unknown samples with no additional system optimization, ease of device replacement, and precise, robust sample processing.

A Combined Dielectrophoretic and Field-Flow Fractionation Microsystem for Biomedical Separation and Analysis

The ability to separate and identify cells and other particulate matter is a fundamental requirement of microsystems designed for biomedical and other applications. Here we describe a method that combines dielectrophoresis and field-flow fractionation to separate and identify particles in a microfluidic environment. This method is applicable not only to the analysis of cellular and other particulate analytes but also to the detection of toxins using sensitized test particles. We show proof of principle by achieving differential separation of human peripheral blood mononuclear cell subtypes in a microsystem based on the method.

Correlation of UV damage threshold with post-annealing in CVD-grown SiO2 overlayers on etched fused silica substrates

Chemical vapor deposition (CVD) has been used for the production of fused silica optics in high power laser applications. However, relatively little is known about the ultraviolet (UV) laser damage threshold of CVD films and how they relate to intrinsic defects produced during deposition. We present a study relating structural and electronic defects in CVD films to the 355 nm pulsed laser damage threshold as a function of post-deposition annealing temperature (THT). Plasma-enhanced CVD, based on SiH4/N2O under oxygen-rich conditions, was used to deposit 1.5, 3.1 and 6.4 μm thick films on etched SiO2 substrates. Rapid annealing was performed using a scanned CO2 laser beam up to THT~2100 K. The films were then characterized using X-ray photoemission spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and photoluminescence (PL). A gradual transition in the damage threshold of annealed films was observed at THT up to 1600 K, correlating with a decrease in NB silanol and broadband PL emission. An additional sharp transition in damage threshold also occurs at ~1850 K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is proposed, and the potential of using high-THT CVD SiO2 to mitigate optical damage is also discussed.

Microsyringe Arrays in Poly(Dimethyle Siloxane)

In this paper we discuss the development of integrated microsyringe arrays, fabricated using a rapid low-cost process that is easily adapted for many microfluidics systems. A key innovation to the development of the microsyringes is a process for producing microchannels in poly(dimethyl siloxane) (PDMS) with perfectly round cross-sections. The flexible PDMS conforms to an oversized spherical metal piston inserted into the channel, forming a leak-proof seal. This is the reverse of conventional syringes, which use a rigid vessel and compliant plunger.