State of the art Various micropump structures are known in the art. Generally, they consist of several layers of silicon and glass as well as piezoelectric elements bonded together.

Valving and passages are arranged in and through silicon layers to achieve a proper functionality. The piezoelectric elements are bonded to a glass membrane to effect the pumping action. Examples of the prior art may be found in EP A1 0 134 614, US 5,259,737, WO 92/01160 and WO 89/07199. The complex structure results in a large internal pump volume or dead volume of these micropumps e. g. in the order of 100 pl. Thus, plugs of liquid of interest that are injected into the fluid being pumped by these micropumps are subjected to dispersion because of the passage through inlets and outlets and valving having corners. Also, the large dead volume results in greater operational costs, due to increased chemical consumption. In many applications the micropumps are required to precisely handle a volume as low as possible, e. g. in the order of 1 pu or less. Moreover, the piezoelectric elements are bonded to glass membranes having a thickness in the order of 200-500 um. Thus, the piezoelectric elements require a high control voltage to generate a sufficient force to actuate the membranes. Also, because of the high dead volume and complex internal structure, there is a problem in priming these micropumps, i. e. filling the micropump completely with liquid. If gas bubbles are trapped within the micropump then it will not function at all, since the gas bubbles are compressed and expanded instead of pumping the liquid.

Summary of the invention The present invention provides a peristaltic piezoelectric micropump comprising a number of, preferably piezoelectric, controlled pump elements to pump liquid from an inlet to an outlet. Each pump element has a pump membrane and at least one actuating element for moving the pump membrane.

According to the invention, a sealing layer provides a seat for the pump elements and the bottom of a channel, which may be present and which forms at least part of a path between the pump inlet and outlet. The main part of any channel and the pump membranes of the pump elements are provided in an actuation layer, and actuating elements, preferably piezoelectric actuating elements, are connected to each pump membrane.

Preferably, the sealing layer is flat (or has a surface shape which is adapted to fit closely to the surface of the actuation layer which it is in contact with) and the pump membranes are arranged to be located against the sealing layer in order to act as check valves when the actuating elements are not energised.

Preferably, the micropump structure comprises only two layers, one flat sealing layer, e. g. made of glass or some other substantially rigid material which it is possible to attach an actuation layer to e. g. metal, plastic, etc., and one actuation layer, e. g. made of silicon or some other material suitable for making microstructures, for example, plastic, quartz, diamond, etc., in which the valving and pump membranes of the micropump are provided. The pump elements are preferably activated by actuating elements, which can work hydraulically, pneumatically, piezoelectrically, by using thermal expansion or in any other suitable way. The pump elements and serve both as elements of the micropump and, when they are not energised, check valves. The non-complex structure of the micropump results in a low internal pump volume and also enables self-priming of the pump.

The present invention is defined in the accompanying claim 1, while preferred embodiments are set forth in the dependent claims.

The present invention solves some of the problems of the prior art devices by providing a simple micropump structure. Preferably, the micropump comprises one sealing layer without any ports or passages and one actuation layer in which the main body of the pump is provided. Thus, inlets and outlets as well as any channels therebetween and the pump membranes and the valving can be provided in the actuation layer. The pumping membranes are preferably actuated by piezoelectric elements. When the piezoelectric elements are not energised to their pump element filling position, the membranes are preferably located against the sealing layer in order to provide a check valve function. Thus, because of the non-complex structure, a micropump according to the present invention has a low dead volume.

This leads to low dispersion and low chemical costs. Since the pumping membranes are provided in the actuation layer they may be made very thin thus requiring only a low force from, and low control voltage for, the actuating elements. Also, a channel may be made very small resulting in that capillary forces may be utilised to prime the micropump. Thus, the micropump of the present invention can have a self- priming capability.

Brief description of the drawings The invention will be described below with reference to the accompanying drawings, in which: figure 1 is a side view of a first embodiment of the present invention; figure 2 is a bottom view of the pump of figure 1;

figure 3 is a side view similar to figure 1 of a second embodiment of the present invention; figure 4 is a bottom view of the second embodiment; figure 5 is a bottom view of a third embodiment; and figure 6 is a bottom view of a fourth embodiment.

Detailed description of preferred embodiments A micropump according to the present invention can be manufactured as a two layer pump body structure comprising a plain sealing layer and an actuation or channel layer. The actuation layer contains the pump elements. The sealing layer is suitably made of glass but other materials such as a crystal layer having a metallized surface may be contemplated as well. The actuation layer is preferably formed from a homogeneous material, which may be provided with a protective coating to prevent reaction with the surroundings or the fluid being pumped. The actuation layer may be manufactured in silicon, diamond, quartz or other materials suitable for forming microstructures. Thus it is also possible to form the actuation layer in plastic materials formed, for example, using a silicon matrix. Many plastics have good chemical properties.

A silicon layer structure can be produced by means of well-known techno- logies. For example, channels and cavities can be produced by means of anisotropic etching. The silicon layer may be protected against etching by an oxide layer, that is by forming a Si02 layer. Patterns may be arranged in the Si02 layer by means of lithographic technologies. Also, etching may be selectively stopped by doping the silicon and using pn etch stop or other etch stop techniques. Glass and silicon layers can be bonded together using anodic bonding. Also the bonding may be performed selectively by arranging a non-bonding Si02 layer on the silicon layer in regions when no bonding is desired. Since all these process steps are well known in the art they are not described in detail here. Suitably adapted bonding methods may be used if the actuating layer and/or sealing layers is/are made of other materials.

As is well known in the art, a peristaltic pump comprises several pump elements co-operating with a predetermined phase relationship, such that liquid is extracted and ejected by the pump elements in a sequence to generate a flow in one direction. Generally, peristaltic pumps also comprise check valves in order to determine the flow direction of the pump.

In figure 1, a first embodiment of the present invention is shown. The pump comprises a silicon wafer 1 bonded or otherwise attached to a thin glass layer 2. The glass layer for bonding is selected to have a coefficient of thermal expansion that matches that of the silicon as well as possible. If the problem with different coefficients of thermal expansion is not addressed then large material stresses will

occur when hot bonded layers are cooled to room temperature. The pump is provided with at least three actuating elements 3, preferably piezoelectric elements 3, for controlling an equal number of membranes 4 of the pump, said membranes 4 being formed in the silicon wafer 1. The piezoelectric elements 3 are attached to a backing or yoke (not shown) which holds the elements 3 in place and takes up the reaction forces i. e. the backing forms a substantially rigid surface for the actuating elements 3 to push or pull against. The piezoelectric elements 3 are of the so-called multi-layer type in order to be able to generate element contractions/expansions that are as large as possible. Bimorph piezoceramic actuating elements may also be used as well as other types of actuating elements such as electromagnet devices. In the pump of the present invention, elements generating small forces and large contraction/expansion have been selected, since the silicon membranes 4 can be made very thin and therefore do not require large actuating forces.

The pump has an inlet 5 and an outlet 6 connecting the pump between the source of liquid and tubing to deliver the liquid (not shown). Between the inlet and outlet the liquid flows in a channel 7, the bottom of which is provided by the glass layer 2, while the main structure of the channel is provided in the silicon layer 1 as will be explained more in detail below. It will be appreciated that all the channel system can be arranged on one side of the silicon wafer only, and in this event no through passages are required in the glass layer.

As may be seen from figures 1 and 2, a narrow channel 7 has been etched between the inlet 5 and the outlet 6. The channel 7 is interrupted only by the membranes 4. In this embodiment, the membranes are generally rectangular in shape, although other shapes are conceivable such as square, circular, oval, etc., and are recessed such that they are in contact with the glass layer 2 when the piezoelectric elements are not energised. The channel 7 projects a small distance, e. g. 0,5 mm into the membrane region as a roof-like or gable-like structure.

Typically, the membrane can have a width of 3,5 mm and a length of 5,0 mm The length of the channel can be 4,0 mm and the depth of the channel can be typically 50-900 pm and the width approximately 0,5 mm. The thickness of the membrane can be w10 pm. These dimensions result in an internal pump volume or dead volume of less than 1 pLI. If the contraction of the piezoelectric element is approximately 2,9 llm then the resulting total stroke volume of the micropump is about 51 nl.

The piezoelectric element may be attached to the membrane via a spacer 8, e. g. made of acrylic resin (Plexiglas), in order to concentrate the lift force of the piezoelectric element to a narrow area of the membrane.

The depth of the channel 7 may be even smaller than the dimensions mentioned above, such as 10-20 m. In this case, capillary forces would assist

filling of the micropump with the primer fluid. Thus, the pump can be filled with liquid by supplying a small amount of liquid at the inlet and opening all the pump elements. The capillary forces will automatically draw the liquid all the way through the pump to the outlet. The micropump may be said to be self-priming.

The function of the pump may be understood with reference to figure 2. The pumping sequence is started by actuating the first (left-hand) piezoelectric element with a voltage of approximately 80 V, whereby this piezoelectric element contracts in height. As the piezoelectric element is bonded to the silicon membrane it lifts the membrane which then extracts a volume corresponding to the stroke volume of the pump. When the piezoelectric element is not energised the membrane is in contact with the glass layer which acts as a valve seat. In order to improve the function of the pump and to obtain a pump without valves, a negative bias can be supplied to the piezoelectric element 3, such that the membrane 4 and the glass layer 2 form a tight seal, thereby eliminating the possibility of back-flow.

When the first piezoelectric element is actuated with a positive voltage, the element contracts, thereby lowering the pressure in the pump chamber which is formed below the membrane, which in turn results in that fluid flows into the pump chamber. After a certain time period, the next piezoelectric element is actuated resulting in that the fluid flows on into the next pump chamber. At the same time as the third piezoelectric element is energised and the fluid flows into pump chamber number three, the first piezoelectric element is deactivated. The pump cycle is completed when the second piezoelectric and third piezoelectric elements are de- activated in sequence. During this pump cycle the pump has pumped its stroke volume and a new cycle may begin. An advantage with this type of pump is that it is relatively easy to change the pumping direction by changing the sequence of operation of the actuating elements. Another advantage with this pump is that the pump flow may be varied using a number of parameters, for example, the stroke, i. e. contraction, of the piezoelectric elements may be controlled by the means of the amplitude of the energising voltage, the frequency of the input signals actuating the piezoelectric element may be varied, and the phase differences between the input signals to the piezoelectric elements may be varied.

In figure 3, a second embodiment of the invention is illustrated. A bottom view of this embodiment is shown in figure 4. The same components have retained the same reference characters. However, it will be seen that a bulk of silicon remains in the centre of each membrane providing a heel 9 in contact with the glass layer 2 when the piezoelectric elements are not energised. Also, the channel 7 extends under the membrane up to the heel 9. This means that the channel 7 does not end in the gable-like structure but the top is level with the membrane 4. The piezoelectric elements are bonded directly on the bulks of silicon. When the

piezoelectric elements contract they lift the bulk of silicon together with the heel 9 and also the top side of the channel 7 resulting in a greater pump stroke. In this embodiment no spacers are necessary.

A third embodiment of the present invention is shown in figure 5. Just like in the second embodiment, a bulk of silicon remains unetched in the centre of the membrane. Here, channels 10 are parallel but staggered under the heel 9. Thus, the channels extend a longer distance under the piezoelectric element and a greater part of the channels are subjected to the lifting force.

A fourth embodiment of the invention is shown in figure 6. In this embodi- ment, channels 11 are partly under the bulk of silicon and partly outside, approxi- mately half the channel. This means that a rotating or torsion force is acting on the channel. Since the channel is more resistant to bending than torsion a greater stroke is achieved.

It will be appreciated by persons skilled in the art that the micropump structure according to the present invention has several advantages over the prior art. The non-complex two-layer structure results in a straight and smooth channel having low dead volume. The lack of corners and unnecessary ports and passages results in low dispersion and the low dead volume. Also self-priming of the micro- pump is achieved, if the dimensions are small enough. The micropump is easily manufactured in small dimensions, also because of the non-complex structure. No check valves need to be provided in the actuating structure since the actuating elements together with the membranes also perform the function of valves. The micropump according to the invention features a very thin membrane that requires only a small force from an actuating element. This also means that the actuating elements may be chosen to perform a greater stroke.

While the present invention has been illustrated by examples of embodiments in which there is a channel connecting adjacent pump elements, it is conceivable to position the pump elements closer together so that no connecting channel is necessary.

A person skilled in the art may produce the micropump according to the invention using standard technologies. It will be appreciated that the process steps may be performed in various ways and various orders without departing from the inventive concept. The scope of the invention is only limited by the claims below.