Multi-fluid Ink-Jet Array for Manufacturing of Chip-Based Microarray Systems

David Wallace1, Hans-Jochen Trost1, and Udo Eichenlaub2

1 MicroFab Technologies, Inc.

2 Boehringer Mannheim, GmbH
Werk Tutzing
Bahnofstraße 9-15
D 82324 Tutzing/Obb
Germany

Abstract

A fast, reliable method for dispensing picoliter fluid volumes is needed in many emerging areas of biotechnology. All of the lab-on-a-chip diagnostic designs will ultimately require some robust method for applying the reagents onto the chip substrates. The use of ink-jet technology can make picoliter scale dispensing of diagnostic fluids a practical, production-line reality. The development of multiple fluid (10), high density, integrated array printhead dispensers will enable the manufacturing of microspot array-based immunoassays and DNA diagnostic assays. The array printhead being developed uses microfabication techniques to produce 40-140um fluid channels and piezoelectric actuators. A ten fluid printhead was selected as the initial design, and the initial results the printhead development effort are discussed. Extension of the design to a larger number of fluids is planned.
 

PRINT HEAD DESCRIPTION
 

Fabrication
 

The drop-on-demand, multi-fluid, array ink-jet printhead design was derived from one developed for the office printer market.(1) (2) (3) (4) The fabrication process is briefly described briefly below.
 

Figure 1: Printhead fabrication process: piezoelectric material preparation.
 
As illustrated in Figure 1, piezoelectric material (usually lead-zirconium-titanate, or PZT) is prepared initially in a standard fashion: a large rectangular block is formed and a temporary metallization is applied to two sides of the block. A high voltage is then applied across the two metallization layers to orient the piezoelectric properties of the material. This is referred to as poling. At this point, the preparation of the material departs from standard processing. Because the design uses the piezoelectric material in shear mode, the poling metallization must be removed. The block of piezoelectric material is then sliced into rectangular pieces that will be formed into individual print heads. These pieces are much larger in two of their three dimensions: 25 x 25 x 1.8mm and 20 x 25 x 0.2mm being typical for this design, where the former is referred to as the "bottom" PZT and the later is referred to as the "thin" PZT. [PZT is lead-zirconium-titanate, the most common piezoelectric material, and is used here as shorthand for piezoelectric material.] Metallization is applied in a batch (i.e., multiple parts at the same time) deposition process to the two large areas of both the bottom and thin blocks. This is the functional metallization that will be used to actuate individual jets.
 
Figure 2: Printhead fabrication process: bonding and sawing.
 
Next, a bottom and a thin block of piezoelectric material are laminated together with electrically conductive epoxy such that their poling directions are aligned. This "sandwich" structure is then machined with a precision diamond saw, creating both fluid channel openings and channel actuator structures. Figure 2 illustrates the laminating and sawing operations. The channel width, wall width, and channel depth for devices used in conventional ink-jet printing are 85, 85, and 360µm, respectively. For the ten fluid printheads, channel and wall widths of 140µm and 360µm were utilized. The channels are sawn the entire length of the piezoelectric block (25mm). Print heads for conventional ink-jet printing have 120 channels and the initial multiple fluid printhead has ten fluid channels interspersed between eleven dummy channels. The tops of the ink channels are created by attaching a cover plate to the machined sandwich structure. The cover plate is solid except for holes that are machined in it to form fluid manifolds.
 
Figure 3: Printhead fabrication process: cover & orifice plate attachment, and channel sealing.
 An excimer laser ablation process is used to machine precision orifices into a polymer film that attaches to one end of the printhead. The orifices are machined to align to the ink channels when assembled. The back of the channels are sealed with an acoustic energy absorbing polymer. Figure 3 illustrates these process steps.
 
Figure 4: Assembled ten fluid printhead.

When the sandwich structure is formed, the lower piezoelectric block is longer at the rear (i.e., the surface opposite from the orifice plate) of the printhead, thus leaving a metallization layer exposed. The sawing process thus creates individual metallized bonding pads. A flex-circuit is attached to these pads, allowing in the metallization layer in the middle of each channel wall (i.e., actuator) to be individual addressed electrically. The metallization layer at the top of each wall is electrically connected to a common potential through the cover plate metallization. In the final assembly operation, the other end of the flex-circuit is attached to the drive electronics. Figure 4 shows a fully assembled ten fluid printhead and Figure 5 shows the printhead assembled with fluid reservoirs.
 

Figure 5: Ten fluid printhead with fluid reservoirs.
 
 Actuator Operating Principle
 
Figure 6 illustrates the actuator operating principle of the printhead. Each ink channel requires its two walls to move, in opposite directions, to create the pressure waves that causes a droplet to be ejected. This is accomplished by placing a voltage difference on the inner electrical layers of each wall while holding the metallization layer at the top of both walls at ground. This creates equal and opposite uniform electrostatic fields in the upper sections of the two walls (formed from the thinner of the two piezoelectric blocks) . Since this field is at a 90 angle to the poling field, the upper wall sections deform in shear mode (5) and the middle of each wall is displaced outward from the channel being actuated.
 
Figure 6: Principle of operation.
 
The electric field in the lower section of the walls and in the floor of the channel originates in a much more complex fashion. Since the dielectric constant of most "soft" piezoelectric materials (e.g., PZT) is 3-4 orders of magnitude greater than typical fluids and insulating materials, (6) an electric field is established between the inner metallization layers of the two walls through the floor of ink channels, as shown in Figure 6. This complex "U-shaped" field creates a mostly shear mode displacement in the bottom section (below the inner metallization layer) of each wall, again causing the middle of each wall to be displaced outward from the channel being actuated. 
Figure 7: 40µm drops being formed by every fourth channel in a 120 channel array printhead. Distance between drops is 510µm.
 
The electric fields in the upper and lower section of the walls cause the channel to widen and the floor to drop when the field is applied, thus acting to increase the cross-sectional area of a channel when a voltage difference is applied to the inner metallization layers of the two walls, as indicated in Figure 6. This change in area is what creates the pressure waves that lead to ejection of a droplet. Figure 7 illustrates a 120 jet array printhead in operation.
 
Configuration for Printing in Wells
 Figures 8-9: Ten fluid printhead with end-effector

The printhead configurations shown in Figures 4 and 5 are designed for printing onto flat substrates where the channel pitch does not have to match the printed array pitch. For printing into wells, such as Boehringer Mannheim's Microspot® system, (7) the printhead channel pitch and the printed array pitch must match for maximum throughput, due to the z-axis motion requirement. For this configuration, an "end effector" is being generated that translates the 1mm pitch channels of the ten fluid printhead back to the 170µm pitch of the 120 channel configuration. This end effector acts as a passive waveguide. The first prototype is currently being fabricated and is shown in Figures 8 and 9.
 

PRINTHEAD PERFORMANCE DATA
 
In the initial printhead testing, bipolar voltage pulses were applied to the walls of one channel at a time, at pulse frequencies of a few hundreds to 2,000 Hz. Good jetting (i.e., stable drop velocities, no satellite formation observable within about 1 mm from the orifice) performance was established for each channel. In addition, an exploration of the pulse shape parameter space was performed. Isopropanol was used as the test fluid in these experiments to be able to compare the results with the results for the 120 channel configuration used for ink-jet printing.
 

Figure 10: Droplets dispensed at 2 kHz from one channel of the first 10-channel head.
 
The pulse shapes applied are simple piecewise linear bipolar pulses [4]. Droplets dispensed at 2 kHz from the first printhead built and tested are shown in Figure 10. A comparison of drop velocities and volumes on a channel-by-channel basis from a later printhead, using very little variation of the pulse shapes between the channels, indicates a fairly homogenous performance across the print head (Figure 11). Compared to previous work with array printheads for conventional ink-jet printing, these results are unique in that we have looked for lower velocities (< 2.5 m/s instead of 3-8 m/s) and larger volumes (> 120 pL instead of < 70 pL). The peak-to-peak pulse amplitudes could be kept well below 30 V, well within the range of drive voltages that we are applying to other jetting devices routinely. The tolerance for channel-to-channel variation for the 120 channel printhead is ±5%, which is also expected for the ten fluid configuration when the manufacturing processes mature.
 
Figure 11: Performance data from initial 10-channel head testing.
 
REFERENCES
 

1. J. Pies, D. Wallace, and D. Hayes, "Sidewall Actuator for a High Density Ink Jet Printhead," U.S. Patent 5,227,813, July 13, 1993.

2. J. Pies, D. Wallace, and D. Hayes, "High Density Ink Jet Printhead," U.S. Patent 5,235,352, August 10, 1993.

3. D. Wallace "High Density Ink Jet Printhead," U.S. Patent 5,400,064, March 21, 1995.

4. D. Wallace, V. Shah, D. Hayes, and M. Grove, "Photo-Realistic Ink Jet Printing Through Dynamic Spot Size Control," Journal of Imaging Science & Technology, Vol. 40, no. 5, p. 390, 1996.

5. R. Holland and E. EerNisse, "Accurate Measurement of Coefficients in a Ferroelectric Ceramic," IEEE Trans. of Sonics and Ultrasonics, Vol. SU-16, No. 4, October, 1969.

6. R. Lerch, "Simulation of Piezoelectric Devices by Two- and Three-Dimensional Finite Elements," IEEE Trans. On Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 35, No. 2, May 1990.

7. U. Eichenlaub et al., "Microspot® - A Highly Integrated Ligand Binding Assay Technology," Proceedings, Second International Conference on Microreaction Technology, New Orleans, March, 1998.