Micropump

The invention relates to a micropump as used especially for the delivery of small liquid drops from the air in laboratory apparatuses for chemical, in particular biochemical and medical investigations.

In the case of known micropumps of the generic type by means of which liquid drops are delivered from the air, the ejection orifice is usually located eccentrically in a larger, substantially flat, generally rectangular surface. Although EP 0 725 267 A2 proposed making the tip of a micropump pyramidal, no further information is given and it is to be assumed that it is actually intended to form a truncated pyramid in the relatively large flat front surface of which the ejection orifice is eccentrically located.

However, it has been found that a larger front surface surrounding the ejection orifice has a disadvantageous effect on the reliability of the ejection of the liquid drops and on the reproducibility of the drop size. This is due in particular to the fact that an accumulation of liquid concentrated around the ejection orifice readily forms on the front surface and influences the drop ejection in an uncontrollable manner and in certain circumstances even prevents said ejection.

It is the object of the invention to provide a micropump of the generic type in which the ejection of liquid drops takes place with high reliability and little variation of the drop size.

This object is achieved by the features in the claimed invention. The accumulation of liquid in the vicinity of the ejection orifice is as a rule avoided. However, even if such an accumulation of liquid nevertheless forms—for example because regulation of the liquid feed is not very precise—said accumulation remains slight and does not reach a troublesome amount since the liquid flows away laterally. The ejection of liquid drops thus takes place undisturbed and the drop size depends only on the properties of the liquid, on the size of the ejection orifice and on the control of the actuator.

The invention is explained in more detail below with reference to Figures which show only an embodiment and wherein:

FIG. 1 is a side view of a micropump according to the invention in a holder;

FIG. 2 a is a longitudinal section through the front part of the micropump of FIG. 1 on a larger scale;

FIG. 2 b is a bottom view of that part of the micropump which is shown in FIG. 2 a;

FIG. 3 a is, once again on a larger scale, a side view of the tip of that front part of the micropump which is shown in FIG. 2 a,b;

FIG. 3 b is a bottom view of the tip shown in FIG. 3 a ; and

FIG. 4 is, once again on a larger scale, a front view of the tip shown in FIG. 3 a,b.

FIG. 1 shows a micropump 1 according to the invention in a holder 2 which can be suspended, for example, in a three-dimensionally movable mount of a micropipetting apparatus. The micropump 1 has (FIG. 2 a,b ) a baseplate 3 of glass, e.g. boron glass, having a flat top which is connected to the bottom of a thinner cover plate 4 of silicon. A pump chamber 5 about 50 μm high and a straight ejection channel 6 of the same height connected to said chamber at the front and tapering to a segment of constant cross-section which ends in an ejection orifice 7 are etched from the otherwise likewise flat bottom of the cover plate 4 . An inflow channel 8 which ends in an inflow orifice (not shown) which can be connected to a storage vessel or a pump, e.g. a dilutor, is connected via a constriction to the opposite end of the pump chamber 5 . A plate-like piezoelectric element 9 which is controlled via lines 10 is mounted as an actuator on a part of the top of the cover plate 4 which is opposite the pump chamber 5 . The piezoelectric element 9 and the major part of the cover plate 4 are protected by a cover 11 . As shown in FIG. 2 a , it may be in the form of a plastics cover or may consist of a casting material—preferably of plastic—with which the piezoelectric element 9 and the lines 10 are embedded.

The ejection orifice 7 is (FIGS. 3 a,b , 4 ) approximately circular. It is surrounded concentrically by a flat annular front surface 12 . A lateral surface 13 which is rotationally symmetrical, in particular in the form of an envelope of truncated cone, and concentric with the front surface 12 is adjacent to the edge bounding the outside of the front surface 12 . The diameter of the ejection orifice 7 is about 50 μm and that of the front surface 12 is about 150 μm. The area of the front surface 12 is thus about 8 times the area of the ejection orifice 7 . It has been found that, with such ratios, no troublesome accumulations of liquid can form on the front surface 12 .

The smaller the area ratio, the more advantageous in principle are the properties of the micropump with regard to the avoidance of troublesome accumulations of liquid in the region of the ejection orifice. The lower limit for the diameter of the front surface is determined by manufacturing tolerances and requirements with respect to robustness. Micropumps which can be produced without a great deal of expense are those having front surfaces of about 200 μm to about 250 μm in diameter, which, assuming a diameter of about 50 μm for the ejection orifice, leads to area ratios of about 1:15 to 1:25. A diameter of 150 μm for the front surface, as stated above, can also readily be realized. In most circumstances, the properties of the micropump are well within the permitted ranges even with very much larger area ratios than those stated above. The limit is about 1:250, which corresponds approximately to a diameter of 750 μm for the front surface, and is of course dependent on other parameters and on the requirements with respect to the function of the micropump.

The angle between the axis and the generator of the lateral surface 13 is about 30°. It should as far as possible not be larger than 60° so that if necessary liquid flows away from the front surface over its outer edge onto the lateral surface to a sufficient extent. It has been found that the described formation of the lateral surface is also suitable for avoiding the formation of accumulations of liquid on the front surface on withdrawal of the tip of the micropump from a liquid level if said tip is used, for example, as a pipette tip and is occasionally also immersed for drawing up liquid, for example by means of a downstream pump.

Slight roughness of the surface on the front surface 12 and at least one adjacent region of the lateral surface 13 , which as far as possible should be N4 or less, is also advantageous for avoiding accumulations of liquid in the region of the ejection orifice 7 . Said roughness can be reduced by a suitable coating which, owing to its chemical composition, can also reduce the adhesion of liquid. It can be hydrophobic or hydrophilic, depending on the properties of the liquid. In the case of micropumps for the delivery of aqueous liquids, for example, a hydrophobic plasma polymer coating has proved advantageous.

The micropump according to the invention is produced substantially in a manner known per se, by connecting a rectangular silicon plate, from which the pump chamber, the ejection channel and the inflow channel have been etched, by anodic bonding to a glass plate which is likewise rectangular. The lateral surface 13 is then created by grinding by means of a diamond grinding disc, and the tip of the micropump is thus completed. The piezoelectric element 9 and the electrical lines 10 to said element, and the cover 11 , are then mounted. Finally, the front surface 12 and also at least part of the lateral surface 13 are coated.

The delivery of liquid drops by means of the micropump takes place as in known micropumps of the generic type, in such a way that the cover plate 4 is bent abruptly downwards of the piezoelectric element 9 . The reduction in the f the pump chamber 5 causes a pressure wave which runs the ejection channel 6 to the outlet orifice 7 and liquid drop to be ejected from said orifice.

1 micropump

2 holder

3 baseplate

4 cover plate

5 pump chamber

6 ejection channel

7 ejection orifice

8 inflow channel

9 piezoelectric element

10 lines

11 cover

12 front surface

13 lateral surface