There are a number of technological areas where it is desired to deliver a fluid with high spatial precision. In one area of technology, ink jet printers seek to place drops of ink on a page by propelling the ink out of small equally-spaced nozzles. Such printers generally are less expensive and quieter while printing than laser printers. However, their printing quality is generally not as good as that of laser printers, partially as a result of shortcomings in the nozzle structures currently being used. The realization of a low-cost high-resolution nozzle array is critical to the future of ink jet printers.
Presently, nozzle arrays made of glass fibers are widely used. Such nozzle heads are highly uniform and quite robust. They generally are fabricated by aligning cord fibers, assembling fiber arrays, and then bonding the glass array to supporting glass plates. In order to achieve the required precision in aligning the nozzles, notched structures made of silicon or glass are generally used as a supporting substructure. The spacing between neighboring notches and the wall thickness of the cord fibers become the main factors limiting the separation between the nozzles.
Another problem that is associated with glass nozzle heads is in the wiring that connects same to the host computer. Glass nozzles do not permit the direct integration of circuitry on-chip. Without addressing circuitry, each nozzle requires a separate wire for controlling the firing of the ink. Large numbers of wires cause a tethering problem, limiting the number of nozzles on a print head. A print head with only 10 to 60 nozzles then has to move across the paper being printed many times to print a single page. This results in slow printing speeds compared to other approaches.
In another technological area, it is well-known that complex biochemical reactions are the underlying mechanism on which the functionality of the nervous system is based. In order to understand better the behavior of biological neural networks, at the circuit level, it is important to be able to deliver drugs or other chemicals to highly localized areas of neural tissue in precise quantities while monitoring the responses in vivo. By way of example, specific caged molecules, such as calcium, can be delivered to influence cellular behavior, and NMDA (n-methyl-d-aspartate) can be delivered to modify synaptic activity. In these applications, it is important that the injecting device be very small so as not to disturb the neural system and that it be able to inject fluid volumes in the range of 10-1000 pl controllably.
The most commonly used techniques for injecting chemicals into brain tissue have been microiontophoresis and pressure-injection using single-barrel and multiple-barrel glass micropipettes. The responses of nearby neurons are then measured using separately positioned pipettes filled with electrolyte. These approaches typically suffer from relatively poor control in positioning the injecting pipette relative to the monitoring points. Additionally, the complicated procedures required for the assembly of multiple-barrel pipette structures also prevent them from being widely used. There is, therefore, a need for a neural drug-delivery probe that is able to deliver chemicals selectively at the cellular level as well as being able to record electrically from, and stimulate, neurons, in vivo. Such a probe should allow detailed studies of the neural responses to a variety of chemical stimuli, and would represent an important step toward improving scientific understanding of neural systems and treating a variety of neurophysiological disorders.
Microchannels for conducting and expelling a fluid are embedded in a surface of a silicon substrate. A channel seal is made of plural cross structures formed integrally with the silicon substrate. The cross structures are arranged sequentially over each channel, each cross structure having a chevron shape. The microchannel is sealed by oxidizing at least partially the cross structures, whereby the spaces therebetween are filled. A dielectric seal which overlies the thermally oxidized cross structures forms a complete seal and a substantially planar top surface to the silicon substrate. The dielectric seal is formed of a low pressure chemical vapor deposition (LPCVD) dielectric layer. The channel is useful in the production of an ink jet print head, and has a polysilicon heater overlying the dielectric seal. A current passing through the heater causes a corresponding increase in the temperature of the ink in the microchannel, causing same to be expelled therefreom. After expulsion of the fluid, the microchannel is refilled by capillary action. Control circuitry, including bonding pads and sensors, can be formed integrally on the silicon substrate. In drug or chemical delivery systems, sensors and/or stimulation circuitry for sensing or inducing neural and other response can be formed directly in the silicon substrate which contains the microchannel. The sensor is disposed in close proximity to the chemical distribution nozzle, facilitating neural and other studies. Microvalve arrangements can be formed with the microchannel, controlled by the on-chip circuitry.