Microfluid Ejection Device Based on Complementary Metal–Oxide–Semiconductor Technology as an Artificial Synapse Kyosuke Minakawa, Toshihiko Noda, Kiyotaka Sasagawa, Takashi Tokuda, and Jun Ohta Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan Received April 24, 2009; accepted September 24, 2009; published online January 20, 2010 A complementary metal–oxide–semiconductor (CMOS)-based multisite microfluid ejection device was fabricated and demonstrated. An array of microreservoir cells integrated with micropumps was fabricated on this device. Microfluid ejection is based on the actuation of a silicone membrane between the microreservoir and the micropump cell. This membrane was actuated by the gas produced during the electrolysis of water. The device was equipped with a CMOS chip as an addressable electrode array for multisite microfluid ejection. The microfluid ejection cell array was configured on the CMOS chip to realize addressable microfluid ejection. We measured the membrane displacement to characterize the performance of the micropump, and successfully demonstrated microfluid ejection. The volume of the ejected microfluid was nearly proportional to injected charge. These results suggest that the proposed microfluid ejection device is a promising platform for artificial synaptic devices for various scientific and clinical applications. # 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.01AG03 1. Introduction A neural stimulator is widely used for scientific research and for the therapy of the brain and nervous system. Various neural stimulators have been proposed and demonstrated thus far. Currently, most neural stimulators use electricity as a medium for neural stimulation. 1,2) However, considering that the native signal transmission medium in the synapse is a neurotransmitter, 3) it is reasonable to use neural stimulators for chemical stimulation. The use of a neural stimulator for local chemical stimulation will serve as a powerful tool in scientific and clinical applications related to the brain and nervous system. 4) Such a neural simulator can be used as an intelligent drug delivery system in the brain. Many efforts have been made to fabricate devices such as drug delivery devices 5,6) or micro-electro-mechanical systems (MEMS) devices 7,8) for the release of chemical compounds. We aim to realize such an intelligent chemical neural stimulator using complementary metal–oxide–semiconductor (CMOS) technology (Fig. 1). CMOS technology can be used as a platform for realizing a highly integrated addressable neural stimulation device. 9,10) We configured an array of microfluid ejection cells on a CMOS-based addressable electrode cell array using MEMS technology. To realize microfluid ejection within a voltage range compatible with CMOS technology (typically less than 5 V), we utilized the pressure induced by gas production during the electrolysis of water as a driving force. 11,12) This actuation scheme is advantageous in that there are no restrictions on the target reagent (chemical species for stimulation) that can be used since the transportation does not depend on the electrical properties of the solution. In this study, we developed a CMOS-based microfluid ejection device and demonstrated its functionalities. We designed a CMOS chip using standard 0.35 mm CMOS technology. This chip has the basic features required for a microfluid ejection device, such as arrayed electrodes for gas production in separate electrolysis cells, a line selector for addressing the electrodes, and an operating voltage that is compatible with the electrolysis of water. To achieve microfluid ejection, we fabricated an array of two-chamber unit cells on a CMOS chip. Each unit cell had an electrolyte cell (micropump cell) stacked with a reservoir cell separated by a silicone membrane. We determined the basic function- alities of the CMOS chip and the micropump structure used for microfluid ejection. In addition, we successfully demon- strated the microfluid ejection capabilities of the fabricated device. 2. Fabrication of Microfluid Ejection Device 2.1 Design and functionality of CMOS chip A photograph and a block diagram of the CMOS chip used as a microfluid ejection device are shown in Fig. 2. Table I shows the specifications of the CMOS chip. The CMOS chip comprised a main cell array and a test cell array. The main cell array had 4  4 unit cells, each of which had two Al connection pads for current injection. Au bumps were formed on the Al connection pads; these were used in the electrolysis of water as a work electrode and counter electrode pair. The work electrodes are addressable. The addressing func- tion is based on column-parallel injection lines (input: V in1 – V in4 ) with a 2-bit decoder (input: V sig1 ,V sig2 ) for row selection, as shown in Fig. 2(b). Each column injection line was connected to a work electrode selectable using the 2-bit decoder. All counter electrodes on the chip were grounded. Transmission gates were used as switching circuitry for connecting the work electrodes and column injection lines. The chip had a maximum current specification of 1 mA. 2.2 Device structure and fabrication process We fabricated a two-chamber structure on the CMOS chip to realize microfluid ejection. A schematic of the cross I/O pads Controller Microfluid ejection hole Glutamate L-dopa, etc. Device for chemical stimulation Brain Implantation Fig. 1. (Color online) Concept of a CMOS-based device for an artificial synapse that can stimulate neuron cells by releasing chemical com- pounds to mimic a neural synapse. Japanese Journal of Applied Physics 49 (2010) 01AG03 REGULAR PAPER 01AG03-1 # 2010 The Japan Society of Applied Physics