Microfluidic printing without image reversal

A microfluidic printing apparatus including at least one ink reservoir; a moveable front plate of transparent material having a structure including a plurality of chambers arranged so that the chambers form an array which can be viewed by an observer, each such chamber being arranged to form an ink pixel; a plurality of microchannels connecting the reservoir to a chamber; a plurality of microfluidic pumps each being associated with a single microchannel for supplying ink from an ink reservoir through a microchannel for delivery to a particular chamber for viewing; apparatus for moving the front plate to a printing or ink disposal position; and apparatus for transferring the ink from the fill side of the chambers to a receiver when the front plate is in the printing position.

FIELD OF THE INVENTION 
The present invention relates to printing high quality images by 
microfluidic pumping of inks into receivers such as paper. 
BACKGROUND OF THE INVENTION 
Microfluidic pumping and dispensing of liquid chemical reagents is the 
subject of three U.S. Pat. Nos. 5,585,069, 5,593,838, and 5,603,351, all 
assigned to the David Sarnoff Research Center, Inc. The system uses an 
array of micron sized reservoirs, with connecting microchannels and 
reaction cells etched into a substrate. Electrokinetic pumps comprising 
electrically activated electrodes within the capillary microchannels 
provide the propulsive forces to move the liquid reagents within the 
system. The electrokinetic pump, which is also known as an electroosmotic 
pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A 
Reliable Fluid Propulsion System for Flow Injection Analysis", Anal. Chem. 
66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a 
reservoir, mixed in controlled amounts, and them pumped into a bottom 
array of reaction cells. The array may be decoupled from the assembly and 
removed for incubation or analysis. When used as a printing device, the 
chemical reagent solutions are replaced by dispersions of cyan, magenta, 
and yellow pigment, and the array of reaction cells may be considered a 
viewable display of picture elements, or pixels, comprising mixtures of 
pigments having the hue of the pixel in the original scene. When contacted 
with paper, the capillary force of the paper fibers pulls the dye from the 
cells and holds it in the paper, thus producing a paper print, or 
photograph, of the original scene. One problem with this kind of printer 
is the accurate control of the print density. The problem comes about 
because the capillary force of the paper fibers is strong enough to remove 
all the ink from the device, draining it empty. If the paper is not 
removed from contact with the ink cells at the correct time, the print 
density will be too high or too low. Moreover, the correct paper contact 
time varies with the ambient temperature, making the timing problem more 
difficult. One solution to this problem is given in the above mentioned 
copending application entitled "Microfluidic Printing on Receiver", where 
a special paper is employed which will absorb only a limited amount of 
ink. Nevertheless, it would be cheaper and simpler if plain paper can be 
employed for this kind of printing. Another solution to this problem is 
given in the above mentioned copending application entitled "Microfluidic 
Printing Array Valve", wherein an array of microvalves, each individually 
addressed, controls the flow of ink to the paper. The complexity of 
individually addressed valves leads to a high cost printing apparatus. In 
would be cheaper and easier to manufacture a device that did not have many 
individually addressed valves. In one configuration of the microfluidic 
printer, the image composed of pixels of ink before being transferred to 
the receiver is viewed before printing. A problem with this printer is 
that the viewed image is reversed from the printed image. 
SUMMARY OF THE INVENTION 
It is an object of this invention is to provide a microfluidic printer 
which can rapidly print high quality images on receivers such as plain 
paper with good control of the density and tone scale of the images. 
Another object of this invention is to provide a compact, low power, 
portable printer. 
Another object of this invention is to provide a viewable image that is not 
reversed when printed. 
These objects are achieved by a microfluidic printing apparatus comprising: 
a) at least one ink reservoir; 
b) a moveable front plate of transparent material having a structure 
including a plurality of chambers arranged so that the chambers form an 
array which can be viewed by an observer, each such chamber being arranged 
to form an ink pixel; 
c) a plurality of microchannels connecting the reservoir to a chamber; 
d) a plurality of microfluidic pumps each being associated with a single 
microchannel for supplying ink from an ink reservoir through a 
microchannel for delivery to a particular chamber for viewing; 
e) means for moving the front plate to a printing or ink disposal position 
; and 
f) means for transferring the ink from the fill side of the chambers to a 
receiver when the front plate is in the printing position. 
A feature of the present invention is that it provides apparatus which 
produces high quality prints of the correct density on plain paper. 
Another feature of the invention is that the printer is low power, compact, 
and portable. 
Another feature of the invention is that the printing process is fast, 
because all the pixels are printed simultaneously. 
Another feature of the invention is that the image may be viewed before 
being printed. 
Another feature of the invention is that there is no image reversal between 
the viewed and printed image.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is described in relation to a microfluidic printing 
apparatus which can print computer generated images, graphic images, line 
art, text images and the like, as well as continuous tone images. 
Referring to FIG. 1, a schematic diagram is shown of a printing apparatus 8 
in accordance with the present invention. Reservoirs 20, 30, and 40 are 
respectively provided for holding cyan ink, magenta ink, and yellow ink. 
An optional reservoir 80 is shown for black ink. Microchannel capillaries 
50 respectively connected to each of the reservoirs conduct ink from the 
corresponding reservoir to an array of ink chambers 60. In the present 
invention, the ink chambers 60 deliver the inks directly to a receiver; 
however, other types of ink delivery arrangements can be used such as 
microfluidic channels, and so when the word chamber is used, it will be 
understood to include those arrangements. The colored inks are delivered 
to ink chambers 60 by electrokinetic pumps 70. The amount of each color 
ink is controlled by microcomputer 110 according to the input digital 
image. For clarity of illustration, only one set of electrokinetic pumps 
is shown for the yellow ink channel. Similar pumps are used for the other 
color channels, but these are omitted from the figure for clarity. 
Finally, a reflective receiver 100 is transported by a transport mechanism 
115 to come in contact with the microfluidic printing apparatus. The 
receiver 100 receives the ink and thereby produces the print. Receivers 
may include common bond paper, made from wood fibers, as well as synthetic 
papers made from polymeric fibers. In addition the receiver can be of 
non-fibrous construction, provided the receiver can absorb and hold the 
ink used in the printer. 
FIG. 2 depicts a top view of an arrangement of chambers 60 shown in FIG. 1. 
Each ink chamber 60 is capable of producing a mixed ink having any color 
saturation and hue within the color gamut provided by the set of cyan, 
magenta and yellow inks used in the apparatus. 
The inks used in this invention are dispersions of colorants in common 
solvents. Examples of such inks may be found is U.S. Pat. No. 5,611,847 by 
Gustina, Santilli and Bugner. Inks may also be found in the following 
commonly assigned U.S. patent application Ser. Nos. 08/699,955; 08/699,962 
and 08/699,963 by McInerney, Oldfield, Bugner, Bermel and Santilli, and in 
U.S. patent application Ser. No. 08/790,131 by Bishop, Simons and Brick, 
and in U.S. patent application Ser. No. 08/764,379 by Martin. In a 
preferred embodiment of the invention the solvent is water. Colorants such 
as the Ciba Geigy Unisperse Rubine 4BA-PA, Unisperse Yellow RT-PA, and 
Unisperse Blue GT-PA are also preferred embodiments of the invention. 
The microchannel capillaries, ink pixel chambers 60 and microfluidic pumps 
are more fully described in the references listed above. 
FIG. 3 illustrates the arrangement of a second pattern of color pixels in 
the present invention. The ink chambers 60 are divided into four groups 
cyan ink orifice 200; magenta ink orifice 202; yellow ink orifice 204; and 
black ink orifice 206. Each chamber 60 is connected only to the respective 
colored ink reservoir. When the inks are transferred to the reflective 
receiver 100 some of the inks can mix and blend on the receiver. Inasmuch 
as the inks are in distinct areas on the receiver, the size of the printed 
pixels should be selected to be small enough so that the human eye will 
integrate the color and the appearance of the image will be that of a 
continuous tone photographic quality image. 
Cross-sections of the color pixel arrangement shown in FIG. 3 are 
illustrated in FIG. 4 and FIG. 5. The colored ink supplies 300, 302, 304, 
and 306 are fabricated in channels parallel to the removable printer front 
plate 120. The cyan, magenta, yellow and black inks are respectively 
delivered by colored ink supplies 300, 302, 304, and 306 into each of the 
colored ink chambers 60. 
A detailed view of the cross-section in FIG. 4 is illustrated in FIG. 6. 
The colored inks are delivered to the ink chambers 60 respectively by 
cyan, magenta, yellow, and black ink microchannels 400, 402, 404, and 406. 
(404 and 406 do not show up in the plan shown in FIG. 6, but is 
illustrated in FIG. 8) The colored ink microchannels 400, 402, 404, and 
406 are respectively connected to the colored ink supplies 300, 302, 304, 
and 306 (FIGS. 4 and 5). 
A cross-section view of the plane containing the micronozzles in FIG. 6 is 
shown in FIG. 7. The cyan, magenta, yellow, and black ink micronozzles 
600, 602, 604, and 606 are distributed in the same arrangement as the 
colored ink supply lines 300-304 and the termination of the chambers 60 
which are colored ink orifices 200-206. The column electrodes 650 are 
shown connected to the conducting circuit 550, which is further connected 
to microcomputer 110. 
A cross-section view of the plane containing the microchannels 400, 402, 
404, and 406 in FIG. 6 is shown in FIG. 8. The color ink channels 400-406 
are laid out in the spatial arrangement that corresponds to those in FIGS. 
3 and 7. The lower electrodes in the electrokinetic pumps for delivering 
the colored inks are not shown for clarity of illustration. The row 
electrodes 670 are connected to lower electrodes of the electrokinetic 
pumps. The row electrodes 670 are shown connected to the conducting 
circuit 500, which is further connected to microcomputer 110. 
The operation of a microfluidic printer comprises the steps of activating 
the electrokinetic pumps to pump the correct amount of each color ink to 
the chamber 60 to provide a pixel of the correct hue and intensity 
corresponding to the pixel of the scene being printed. The removable 
printer front plate 120 preferable backed by a white reflecting material 
so that the ink chambers 60 which correspond to the pixels of the image 
render an accurate impression of the image when viewed by the operator. 
After viewing the image, the operator may desire to make a correction in 
the image. For example, if the overall image was deficient in yellow, more 
yellow ink might be pumped into the ink chambers. After the image is 
viewed and corrected and a print is desired, the removable printer front 
plate is separated from the ink supply microchannels 300, 302, 304, and 
306 and placed in contact with the receiver. The capillary forces of the 
receiver fibers draw the ink from the ink chambers into the receiver, 
completing the printing operation. It should be noted that the ink must be 
transferred from the fill side of the ink chambers to the receiver if the 
viewed and printed image are both to be right reading, if the receiver is 
of the kind where the ink is viewed on the side it is printed. 
FIG. 9 illustrates the removable printer front plate in the viewing mode, 
as indicated by the viewing direction "x", where the ink chambers are in 
contact with the ink supply lines 300, 302, 304, and 306. The image as 
viewed is right reading, as indicated by the letter "F". 
FIG. 10 illustrates one method of separation of the removable printer front 
plate 120 by swinging through the radius "y" from the ink supply lines 
wherein the removable printer front plate 120 is attached to the rest of 
the assembly on one side by a hinge 700. Note that the image as viewed in 
the direction "x" on the open removable printer front plate 120 is 
reversed, as indicated by the reversed letter "F". 
FIG. 11 illustrates the transfer of the ink from the ink chambers 60 to the 
receiver 100 and removal of the receiver from the printing plate 120, thus 
completing the printing process. It should be noted that both the printed 
image indicated by the direction "x" is right reading and not reversed, as 
indicated by the letter "F". 
FIG. 12 is an enlargement of the circled area of FIG. 9, showing the 
details of the separation of the removable printer front plate 120 which 
contains the ink chambers 60. In FIG. 12 the removable printer front plate 
120 is partially separated from the rest of the assembly as indicated by 
the distance "a". 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention. 
TS LIST 
8 microfluidic printing system 
20 cyan ink reservoir 
30 magenta ink reservoir 
40 yellow ink reservoir 
50 microchannel capillaries 
60 ink chambers, or printing nozzles 
70 electrokinetic pumps 
80 black ink reservoir 
100 receiver 
110 microcomputer 
115 transport mechanism 
120 removable printer front plate 
130 ink supply plate 
200 colored ink orifices 
202 colored ink orifices 
204 colored ink orifices 
206 colored ink orifices 
300 colored ink supply lines 
302 colored ink supply lines 
304 colored ink supply lines 
306 black ink supply 
400 cyan ink microchannel 
402 magenta ink microchannel 
404 yellow ink microchannel 
406 black ink microchannel 
500 conducting circuit 
550 conducting circuit 
600 cyan ink micro-orifice 
602 magenta ink micro-orifice 
Parts List cont'd 
604 yellow ink micro-orifice 
606 black ink micro-orifice 
650 column electrodes 
670 row electrodes 
700 hinge