A METHOD TO PRODUCE A MICROFLUIDIC DEVICE AND A DEVICE OBTAINED FROM IT
The present invention refers to a method to produce a microfluidic device and to a device obtained from such a method, which is used particularly but not exclusively in the fields of bioengineering and energetics .
Nowadays, indeed, different types of microfluidic devices are known, that are used in the field of biotechnology and, in particular, of diagnostics and pharmaceutics, in chemical processes and in energy systems, like for example fuel cells.
Such devices are made up from microchannels, formed inside a material in the form of grooves, or "open channels" , and subsequently made to stick to a support structure (substrate) that makes it watertight, that contain at least one electrode. In such microchannels it is therefore possible to inject organic and inorganic fluids that can be electrically stimulated through signals applied to the aforementioned electrode. Another typical application can be that related to the detection of electric currents in the fluids in transit through the microchannels, as well as the presence of ions or of electrically charged molecules.
Most of the microfluidic devices capable of housing connections of the electrical type {^electrical microfluidic devices" or EMD) is manufactured by using methods developed previously in the field of microelectronics. The microchannels, indeed, are generally formed by using rapid prototyping methods, standard photolithography protocols and soft- lithography methods.
In particular, the structure of the microchannels with micrometric dimensions is commonly obtained through moulding in a polymer material, like for example polydimethylsiloxane (PDMS) . Such a structure is then subsequently made to stick on a substrate in which there is a network of electrodes .
Some of the known production techniques foresee forming electrodes positioned on one of the faces of such a substrate, at the microchannels. The electrodes are made through per se known metal deposition methods, like for example sputtering, or through methods based on lithography, such as lift-off and etching.
The arrangement of such electrodes however, leads to some drawbacks. The extension of the electrodes along one of the faces of the substrate indeed limits the natural flexibility of the substrate itself, making it more delicate and therefore less suitable for being deformed, by possible mechanical stimulation or mechanotransduction.
Moreover, since the aforementioned electrodes face into the microchannels at the face of the substrate in turn facing the microchannels themselves, at the moment in which a potential difference is applied, an electric field that is not evenly arranged inside the microchannels is generated. Such an electric field indeed tends to be mostly located in the region of space delimited by the two opposing electrodes which, in the case in which the electrodes are positioned as above, only takes up a small area inside the microchannels . The arrangement of a non even electric field inside the microchannels ensures that the portion of fluid that is closest to the electrodes is electrically stimulated in a substantially different manner with respect to the corresponding portion of fluid farthest from the electrodes.
It is to avoid these drawbacks that different methods have been developed to produce electrodes in two positions that are opposite and perpendicular with respect to the surface of the substrate, the extension of which covers, inside the microchannel , the vertical portions of the microchannels partially or for their entire height . Forming vertical electrodes that face towards the inside of the microchannels makes it possible to obtain an electric field that is evenly arranged inside the microchannels themselves, but does not overcome the aforementioned drawback of the lower flexibility of the microfluidic device.
Moreover, it is important to underline that the known methods of producing these types of microfluidic devices consist of at least two production phases and require the use of complex sputtering technology, thus being quite long and expensive. Indeed, according to the methods described thus far, the manufacturing of the microfluidic devices passes through a first step in which the microchannels are formed and a second phase in which the electrodes are formed.
In addition, the manufacturing methods of the electrodes are very expensive, since they require the use of very sophisticated and complex machines.
The purpose of the present invention is therefore that of providing a method to produce a microfluidic device that is capable of overcoming the aforementioned drawbacks of the prior art in an extremely simple, cost-effective and particularly functional manner.
In particular, one purpose of the present invention is that of providing a method to produce a microfluidic device that is capable of obtaining vertical electrodes that face into the microchannels.
Another purpose of the present invention is that of providing a method to produce a microfluidic device that has a small number of manufacturing steps .
A further purpose of the present invention is that of providing a microfluidic device made from plastic material that is flexible and suitable for simultaneously receiving electric and mechanical stimulations .
These and other purposes according to the present invention are achieved by making a method to produce a microfluidic device as outlined in claim 1.
Further characteristics of the invention are highlighted by the dependent claims, which are an integrating part of the present description.
The characteristics and the advantages of a method to produce a microfluidic device, as well as a device obtained from such a method, according to the present invention shall become clearer from the following description, given as an example and not for limiting purposes, with reference to the attached schematic drawings, in which:
figure 1 is a flow chart that illustrates the steps of the method to produce a microfluidic device according to the present invention;
figures 2a, 2b, 2c and 2d are four schematic views of the structures that respectively come from four of the steps of the method of figure 1;
figure 3 is a schematic perspective view of a first embodiment of a microfluidic device obtained with the method of figure 1;
figure 4 is a schematic perspective view of the microfluidic device of figure 3 and of the corresponding method for injecting electrically conductive material, as one of the possible methods that can be used; and
figure 5 is a schematic perspective view of a second embodiment of a microfluidic device obtained with the method of figure 1.
With reference to the figures, a method to produce a microfluidic device 50, 60, 70, is shown, such a method and the relative steps being wholly indicated with reference numeral 100.
The method 100 comprises a step 10 of forming the microchannels in a watertight structure. According to the present invention, such a step 10 not only obtains a first fluidic microchannel network 51, 62, 72 i.e. the microchannels that can be filled with one or more fluids, organic and inorganic, to be stimulated or controlled, but also advantageously obtains a suitable second network of microchannels 52, 61, 73 that can be filled with an electrically conductive material that can act as impedance or conductor and interacts with such a fluid to be stimulated or controlled. In particular, such electrically conductive material can be intended to create electrodes which, through the application of a potential difference, generate an electric field inside the first fluidic microchannel network 51, 62 72 for the stimulation or the control of the fluids contained in it. Such electrically conductive material can also be intended to create electrodes that are suitable for detecting an electric field inside the first fluidic microchannel network 51, 62, 72.
Such a step 10 to produce both the fluidic microchannels 51, 62, 72, and those suitable for forming the electrodes 52, 61, 73 comprises an initial step 20 of manufacturing a die 30, 31 for creating the microchannels 51, 62, 72 and 52, 61, 73 themselves. This initial manufacturing step 20 in turn comprises a step 21 consisting of the deposition of a layer 31 of photoresistive material on a sample comprising a substrate 30, like for example a substrate of silicon or of polymer material. On such a layer 31 of photoresistive material an impression is obtained corresponding to the positions of the microchannels 51, 62, 72 and 52, 61, 73 through for example the controlled exposure of the die 30, 31 to a light beam. In a step 22, a photoresistive mask, which is generated thanks to the subsequent step 23 of exposure of the photoresistive mask itself to UV rays, is thus formed.
As illustrated in figure 2a, the layer 31 of photoresistive material is removed from the upper surface of the substrate 30, thanks to the action of the UV rays, only at the areas identified by the aforementioned impression. Following such a development process, a die 30, 31 is obtained to produce the microchannels 51, 62, 72 and 52, 61, 73.
At this point the method 100 to produce the microchannels 51, 62, 72 and 52, 61, 73 comprises a step 11 of casting a plastic material 32, like for example PDMS, as visible in figure 2b, on such a die 30, 31 forming a provisional structure 30, 31, 32. From such a provisional structure 30, 31, 32, in a subsequent step 14, the die 30, 31, is removed thus forming a portion of a microfluidic device, illustrated in figure 2c, formed by the single layer of plastic material 32 in which the fluidic microchannels 51, 62, 72 and the microchannels to produce the electrodes 52, 61, 73, have been formed.
Such a^ layer of plastic material 32 is joined (step 15) to a closure substrate 33 in a manner such as to close the already previously formed microchannels 51, 62, 72 and 52, 61, 73, in a watertight structure. The network of microchannels 52, 61, 73 dedicated to the creation of the electrodes is then advantageously filled, in a specific step 16, with an electrically conductive material in the fluid state, like for example a conductive polymer, an ionic solution or molten metal, so as to act as impedance or conductor. Such electrically conductive material is injected, in the same manner as the work fluid, through injection means 74, like for example syringes, in cavities 71 formed at the ends of the microchannels 51, 62, 72 and/or 52, 61, 73. Such electrically conductive material can thus operate both in the fluid state, and can be a material (like for example the same polymer, suitably doped, used in order to obtain the microchannels, or low-melting alloys) configured so as to solidify after the aforementioned injection step.
Alternatively, the electrically conductive material injected into the network of microchannels 52, 61, 73 can be a non conductive polymer doped with conductive material, like for example carbon nanotubes, or micrometric metal spheres or a combination thereof .
In a first embodiment of the method according to the present invention, illustrated in figure 3, a microfluidic device 50 is created in which the microchannels 52, 73 filled with electrically conductive material intersect the microchannels 51, 72 containing the work fluid. Such a configuration is possible since the electrically conductive material polymerises, thus passing from the fluid state to the solid state, and therefore forms some electrodes that come into contact with the work fluid without mixing with it.
In a second embodiment of the method according to the present invention, illustrated in figure 5, a network of microchannels 61 is created, to produce electrodes, that is on a different plane with respect to that on which there is the microfluidic microchannel 62. In such a case, the two types or networks of microchannels 61, 62 develop separately, but preferably at the same time, on two portions of substrate that are then joined to form the microfluidic device 60. In the case in which the two networks of microchannels 61, 62 are arranged next to one another but not in direct communication or, in other words, when the two networks of microchannels 61, 62 do not intersect, the stimulation or the control action carried out by the electrically conductive material on the work fluid is obtained through capacitive effect.
It is important to underline that according to the present invention it is possible to form networks of microchannels having different thicknesses and shapes. The different configurations that can be obtained through the application of the method 100 according to the present invention make it possible to couple different combinations of electrochemical and fluid dynamic fields.
From the description made the characteristics of the method and of the device object of the present invention should be clear, just as the relative advantages should also be clear.
Indeed such a method makes it possible to reduce the production costs and the production time of the mentioned microfluidic devices, thanks to a lower number of production phases than that provided by the prior art.
Moreover, the microfluidic device, obtained through the application of such a manufacturing method, is completely flexible, in a manner such as to allow, for example at the same moment in time, a stimulation of mechanical and electrical type . The electrodes formed due to injecting electrically conductive material in the fluid state inside the microchannels suitably obtained do not indeed jeopardise the flexibility of the device .
The possibility of creating electrodes that are parallel to one another and that are positioned perpendicular with respect to the direction in which the fluid is injected inside the microchannels also makes it possible to develop, inside such fluidic microchannels, an evenly distributed electric field during application of a potential difference between the electrodes themselves .
It should finally be clear that the method and the device thus conceived can in any case undergo numerous modifications and variants, all covered by the same inventive concept; moreover, all the details can be replaced by technically equivalent elements. In practice the materials used, as well as the shapes and sizes, can be any according to the technical requirements . The scope of protection of the invention is thus defined by the attached claims.