January 21, 2009 15:4 00107
Biomedical Engineering: Applications, Basis and Communications, Vol. 21, No. 1 (2009) 39–50
IMPROVED FLEXIBLE MICROWIRE ARRAY ELECTRODE FOR INTRACORTICAL SIGNALS RECORDING Rong-Chin Lo∗ and Yao-Ming Yu†
∗Department
of Electronic Engineering National Taipei University of Technology, Taiwan †Institute of Computer and Communication Engineering National Taipei University of Technology, Taiwan §
[email protected] Accepted 17 September 2008
ABSTRACT Implanted electrodes are the first piece of hardware in an intracortical signals recording pathway. This work presents an improved flexible microwire array electrode for intracortical recordings. Only ready-made materials and general mechanical tools are used to fabricate a microelectrode. The proposed procedure is relatively simple, even for a novice worker to implement in-house. Many key steps in producing a good microwire array electrode are facilitated. These main steps include selecting materials, preparing for fabrication, and assembling the electrode. The assembly of the microwire array electrode includes connecting and positioning PCB pattern, arraying and fixing microwires, and soldering and packaging the electrode. A practiced researcher can assemble the microelectrode in about 2 h and implant it in approximately three. The mass of this assembled microelectrode is 1.96 g. The cost of the materials in the entire array is less than US$1.5, and the array is suitable for implantation in the cortex of rats for invasive studies. In this study, electrochemical impedance spectroscopy is also applied to measure the impedance and the phase between the electrode and the electrolyte, and then to obtain an equivalent circuit. The improved microwire array electrode is adopted to record the intracortical signal of cerebrum. The microwire array electrode can be fabricated and used for multi-site, multiple single-unit recording experiments. Several experimental results are presented, along with applications that demonstrate the feasibility and advantages of the proposed approach. Keywords: Microwire array electrode; Printed circuit board; Intracortical; Electrochemical impedance spectroscopy; Artificial cerebral spinal fluid.
INTRODUCTION
utilized in multi-site, multiple single-unit recording experiments.5,6 For example, Chapin et al.,7 constructed 24 microwires to use in rats and Wessberg et al.,8 designed 96 microwires to apply in monkeys, which can be feasibly used to record neural signal in conscious animals. Many multi-electrode arrays can be bought from commercial vendors to suit various experimental designs. They involve various cost, time,
Many metal microelectrodes have been used to record and stimulate neural tissue activity for more than 40 years. Numerous neural probes have been developed, including bundles of microwires,1–3 microwires embedded in neurotrophic assemblies,4 polymer substrate probes, and several silicon-substrate probes. Various microwire array electrodes have been
§
Corresponding author: Rong-Chin Lo, Department of Electronic Engineering, National Taipei University of Technology, Taiwan. Tel.: 886-2-2771-2171x2239; E-mail:
[email protected]. 39
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R.-C. Lo & Y.-M. Yu
materials, tools, and technique of microelectrode manufacture, offering flexibility of the experimental design. Developing in-house fabrication methods remains very worthwhile especially for those research teams that are limited by funds. This work describes an improved flexible microwire array electrode for intracortical recording. The main steps of the method comprise selecting materials, preparing for fabrication, and assembling the electrode. The assembly of the microwire array electrode includes connecting and positioning the printed circuit board (PCB), arraying and fixing microwires, and soldering and packaging the electrode. A six-channel microwire array electrode for multi-site and another 16-channel microwire array electrode for multiple single-units are described to elucidate the fabrication procedure. Electrochemical impedance spectroscopy (EIS) is used to measure the impedance and the phase between the electrode and the electrolyte, and then to obtain an equivalent circuit. Finally, the proposed microelectrode is applied successfully to record intracortical signals at the right S1HL of the primary somatosensory cortex (SI) upon mechanical stimulation using a brush.
MATERIALS AND METHODS In the presented approach, the microwire array electrode is composed of teflon-insulated tungsten microwires, a PCB pattern, a flexible flat cable (FFC) and connector and viscose of epoxy A+B that is proposed herein. Fabricating a microelectrode requires under US$1.5 in materials. The assembly of the microwire array electrode includes connecting and positioning PCB pattern, arraying and fixing microwires, and soldering and packaging the electrode. Implementing a microelectrode requires about 2 h. A sixchannel microwire array electrode for a multi-site and another 16-channel microwire array electrode for multiple single-units are described to illustrate the fabrication procedure.
Materials Microwire General microwires are bundles of 8–32 wires (25–50 µm diameter), normally of stainless steel, tungsten, or platinum, and insulated with Teflon, polyimide, or S-isonels.2 In this study, the microwire array electrode consists of 50 µm-diam teflon-insulated tungsten wires (A-M Systems, Carlsberg, WA, #795500). Tungsten wire offers the highest strength and stiffness of all A-M system wires. Teflon-insulated tungsten wires
are typically 25 feet (7.5 m) long per spool. They are cut every 20 mm into microwire probes. Every spool costs $60. Therefore, the average cost of each section is $0.157. This material is chosen because it is cheap, hard and, above all, easily processed by an individual.
Printed circuit board The PCB is double-sided, 1ounce of copper and FR-4 epoxy glass with a thickness 0.064. The market cost of an area of 100 cm2 of glass-epoxy double-sided photoresist board. Therefore, a PCB with an area of around 1 cm2 costs under US$0.012. Such a PCB after design and sculpturing is regarded as the base of the microelectrode and is called the PCB pattern. The PCB is used because it is light and thin, universal, inexpensive and easy to obtain. In particular, the PCB pattern has a flat surface and a convenient workbench can be used to fabricate the microwire array electrode. The user can conveniently and rapidly revise it to various experiments.
Flexible flat cable In this work, the FFC is B-type (with conductor exposed on one side), with a 1.0 mm pitch, a total length of 100 mm, and conductor pins. A connector, mounted to one or both ends of the FFC, is generally employed with a set of electrical receptacles or sockets that are designed to receive terminal posts or contact pads on the PCB. The FFC connector has a 1.0 mm pitch and 10 conductor pins. The price of one set is US$ 0.358; each set includes both FFC and connector. The advantages of selecting this FFC and connector include precision, recyclability, flexibility and various styles, among others. In particular, plugging in and pulling out can be repeated easily. These advantages reduce injury to the rat during the experiment.
Preparations Before an electrode fabrication can work, the circuit layout and the sculpturing of the PCB must be completed, and the microwires straightened. The procedures of preparation and techniques of operation are introduced separately below.
PCB design and sculpturing In the experiment, a two-sided PCB is used. The PCB for each microelectrode is 11.0 mm long, 9.0 mm wide, and approximately 1.6 mm thick. A specially designed PCB is developed to bond the microwire array to
