Method and microfluidic structure for dispensing a liquid

Abstract

The invention relates to a microfluidic structure and a method for dispensing a liquid. The microfluidic structure has a dispensing channel (10) with a nozzle (16), a sample channel (22) opening into the dispensing channel (10), and a suction channel (24) branching off from the dispensing channel (10) downstream of the opening of the sample channel (22) and upstream of the nozzle (16).The procedure comprises the steps of providing a microfluidic structure, supplying a liquid from the sample channel (22) into the dispensing channel (10), aspirating the liquid downstream of the orifice of the sample channel from the dispensing channel through a branch into the aspiration channel (24), and applying a pressure pulse on the inlet side (12) of the dispensing channel (10), whereby the liquid is ejected through the nozzle (16) between the orifice of the sample channel (22) and the branch of the aspiration channel (24), the liquid experiencing a higher flow resistance in the sample channel than in the dispensing channel from the orifice of the sample channel to the nozzle.

Description

[0001] The invention relates to a method and a microfluidic structure for use in a microfluidic system, such as a microfluidic chip or cartridge, for dispensing a liquid, in particular a dispersion, in particular a suspension, and in particular small quantities thereof, among other things for the purpose of selectively isolating inclusions in the liquid. The invention relates to the field of microfluidics, in which liquids in volume ranges from 1 µl to 10 ml are processed.

[0002] Microfluidics has long been the subject of biotechnological research and development and is used, for example, in the form of so-called lab-on-a-chips, including for medical diagnostics in point-of-care products. On these microfluidic chips, protocols previously developed in the laboratory are implemented as completely as possible in a microfluidic structure on the lab-on-a-chip, allowing the protocols to run largely automatically with minimal manual intervention. The chips are typically used with operator devices, which are equipped with a slot for the chip and, if necessary, electrical, fluidic, and actuator interfaces to the chip.

[0003] Microfluidic systems, such as chips or cartridges, contain various microfluidic structures with dimensions in the micrometer range, although individual microfluidic structures, particularly fluid chambers or reservoirs, can have larger cross-sections up to the millimeter range. Microfluidic systems often consist of a base plate (substrate) with grooves and depressions formed within it, and a cover film that seals these grooves and depressions. The base plates are molded from plastic using injection molding or embossing processes, and the cover films are bonded to the base plates using adhesive or welding processes to create a fluid-tight seal.

[0004] For the purposes of this document, "liquid" encompasses both liquids with and without inclusions. The term "inclusion," as used here, includes not only extended solids of all kinds, hereinafter referred to as "particles," which also include biological cells and microorganisms, but also droplets of a liquid other than the carrier liquid. The term "object" is also used here instead of "inclusion." If the inclusions are particles, the liquid forms a suspension. If they are droplets, then the liquid is an emulsion.

[0005] Suspensions, in particular, are known to tend towards the sedimentation of particles dispersed in the liquid. Especially in microfluidic structures with capillary dimensions of, for example, 100 µm or less, very short sedimentation distances can occur relative to gravity, depending on the orientation of the transport and storage structures. This allows particles to sediment and adhere to surfaces within short timeframes (seconds or less). Typical biological cell suspensions usually contain different cell types that vary in size, density, shape, and other characteristics. These different cell types typically sediment at different rates. Consequently, neither a constant velocity can be measured nor the exact location of the sedimenting particles predicted in a microfluidic structure, and the dispensing process cannot be precisely controlled for all target particles.This means that not all selected particles can be reliably dispensed. Undispensed particles can remain in the channel of the microfluidic structure and lead to blockages in the dispensing device's microfluidic structure.

[0006] When dispensing biological particles, it is also important to note that biocompatible carrier media must be used. These media, in addition to having a suitable salt content, often do not possess Newtonian flow properties (such as blood) and are highly viscous. Due to their viscosity, these suspensions are often difficult and difficult to dispense consistently with regard to both volume and direction. Furthermore, the high salt content leads to clogging of the microfluidic structure of the dispensing device after a short operating time due to evaporation effects.

[0007] Furthermore, it should be noted that cells in particular are stressed and often destroyed by the conditions prevailing during dispensing. Therefore, non-destructive analysis of biological and other sensitive particles cannot be guaranteed using conventional methods.

[0008] As is well known, particle dispensing usually occurs from a storage volume and, in rare cases, from a continuous (micro)fluidic flow. The pressure for the dispensing pulse is provided by liquid columns, gases, or piezoelectric actuators; see Lin Robert et al., “Novel on-demand droplet generation for selective fluid sample extraction”, Biomicrofluidics 6, 2012.

[0009] The use of non-Newtonian, shear-thinning liquids to prevent sedimentation during the storage of dispersions is also known; see Launiere, Czaplewski et al., “Rheologically biomimetic cell suspensions for decreased cell setting in microfluidic devices”, Biomed Microdevices, 2011.

[0010] A dispensing device with a dispensing line, into which a droplet inlet opens at a first branch and into which an outlet to a waste line opens at a second branch, wherein the dispensing line is connected to a pressure source on one inlet side and opens into a droplet outlet line with a droplet outlet on one outlet side, is known, for example, from EP 3 641 937 B1. Dispensing from a microfluidic flow is thus possible.

[0011] Against this background, the inventors have set themselves the task of providing an improved microfluidic structure and a method of the type mentioned above, which enables the non-destructive and precise dispensing of predetermined target volumes and / or target objects from, preferably sedimentation-free, suspensions in a microfluidic flow. The invention is particularly useful, for example, where volumes need to be selectively dispensed from a microfluidic flow, such as in chromatography or electrophoresis, or where particles need to be extracted from or dispensed from a suspension, such as in flow cytometry.

[0012] The problem is solved by a microfluidic structure with the features of claim 1 and by a method with the features of claim 8. As already explained at the outset, the microfluidic structure is a component of a microfluidic system with a substrate into which the microfluidic structure is formed.

[0013] The microfluidic structure according to the invention for dispensing a liquid has a dispensing channel with an inlet side and, downstream, an outlet side, wherein the dispensing channel has a device for introducing a pressure pulse on the inlet side and wherein the dispensing channel has a nozzle on the outlet side. It further has a sample channel which is connected at the inlet side to a device for sample feeding and which opens at the outlet side into the dispensing channel between the inlet side and the outlet side.It further comprises a suction channel which branches off from the dispensing channel at an inlet at a distance d downstream of the opening of the sample channel and at a distance a upstream of the nozzle, and which is connected at the outlet to a device for sample removal, wherein the sample channel, the suction channel and the dispensing channel are designed such that the liquid in the sample channel or in the suction channel or in the sample channel and in the suction channel each experiences a higher flow resistance than in the dispensing channel from the opening of the sample channel to the nozzle.

[0014] The inventive method for dispensing a liquid comprises the following steps: - Providing a microfluidic structure of the type described above, - Feeding a liquid from the sample channel through an orifice into the dispensing channel, - Extraction of the liquid downstream of the sample channel opening from the dispensing channel through a branch into the extraction channel, - Intermittent application of a pressure pulse on the inlet side of the dispensing channel, whereby the liquid in the dispensing channel between the opening of the sample channel and the branch of the suction channel is expelled from the nozzle, - wherein the liquid in the sample channel or in the extraction channel or in the sample channel and in the extraction channel experiences a higher flow resistance than in the dispensing channel from the opening of the sample channel to the nozzle.

[0015] The section of the dispensing channel between the opening of the sample channel and the branch of the suction channel is hereinafter also referred to as the dispensing section, and the section of the dispensing channel between the branch of the suction channel and the nozzle is referred to as the nozzle section.

[0016] The device for initiating a pressure surge includes, for example, an interface to a pressure generating device, a pressure line, or a pressure reservoir. It may also include a valve configured to briefly connect the dispensing channel to the pressure generating device, the pressure line, or the pressure reservoir.

[0017] The sample delivery system includes, for example, a sample reservoir integrated into the microfluidic structure and / or an interface to an external sample source.

[0018] The sample extraction system includes, for example, a pump integrated into the microfluidic structure, a waste container integrated into the microfluidic structure, or an interface to an external sample extraction or sample repository.

[0019] In its simplest form, a nozzle is the open end of a straight dispensing channel. However, a nozzle can also refer to a specific geometry and / or surface finish at the opening of the dispensing channel, which, for example, promotes the separation of the flowing fluid.

[0020] The microfluidic structure is formed in a microfluidic system, such as a microfluidic chip or cartridge, which essentially comprises a flat substrate into which the microfluidic structure is introduced in the form of depressions and channels, which are closed, for example, by means of a lid film laminated on the top of the substrate.

[0021] The liquid is fed from the sample channel through an opening into the dispensing channel in a continuous feed operation over a specific period t1. The dispensing pulse, measured against this, lasts only for a very short period t2 << t1 and can be repeated several times during t1. In this sense, this is also referred to as an intermittent application of a pressure pulse.

[0022] The microfluidic structure allows, and the method provides, the flow of liquid (with or without inclusions) from the sample channel, preferably continuously, to the dispensing channel and from there, preferably continuously, to the suction channel. The liquid flows in the dispensing channel from the opening of the sample channel to the branch of the suction channel, which is offset downstream by a distance d. As a result, the liquid flows over the length of the distance d essentially in the direction of the dispensing channel; that is, it has no or only a negligible component of movement perpendicular to the dispensing channel. This distinguishes the microfluidic structure according to the invention from known dispensing devices in which the sample channel crosses the dispensing channel (perpendicularly), i.e., in which the opening and branch open into and branch off from the dispensing channel without any longitudinal offset along its axis.Due to this difference, the liquid in the microfluidic structure according to the invention has a longer residence time in the dispensing channel compared to a flow oriented transversely to the direction of the dispensing channel. This simplifies the targeted dispensing of a specific section of the liquid column by applying a pressure pulse, which is particularly beneficial for the precise singulation of objects in the liquid whose position has been determined with sufficient accuracy along the flow path. The longer the distance d, and thus the residence time of the liquid or any inclusions contained therein in the dispensing channel, the lower the requirements for position determination. On the other hand, the distance d can be limited, for example, by the density of individual inclusions in the liquid.For example, it should not exceed the minimum distance between two consecutive objects in the liquid if the task is to separate objects by dispensing. The crucial factor here is therefore the prediction of the particle position. The more accurate this prediction, the smaller the distance d can be.

[0023] The flow resistance of the fluid in the individual channels depends on the geometry of the channels, the properties of the fluid (viscosity and density), and the flow velocities. Since both the fluid and the volumetric flow rate are constant in the system under consideration, the flow resistance in the channels depends, to a first approximation, only on the geometry of the channels. If the flow resistance in the suction channel and the flow resistance in the sample channel are higher than that in the dispensing channel, this has the advantage that, when a pressure surge is applied during the dispensing process, as little fluid as possible is forced into the suction channel on the one hand and the sample channel on the other.In the case of the extraction duct, this also ensures that the flow resistance for air and liquid from the direction of the nozzle to the branch into the extraction duct is so low that only a small pressure drop occurs in the section from the branch of the extraction duct to the nozzle, thus ensuring that this section is always well emptied or kept empty.

[0024] Generally speaking, the flow resistance W in channels with a rectangular cross-section, which can be realized to a good approximation in injection molding, is calculated as follows: W=ΔpQ=12 η lK 3 b, where:h≤b where h: channel height, b: channel width, I: channel length, Q: volume flow rate, Δp: pressure drop across the channel of length l, η: dynamic viscosity. Additionally, a geometry factor K, determined by the aspect ratio h / b, is used: K=1−6hb∑n=1∞1βn5tanh(βnbh),with βn=(2n−1)π2

[0025] Taking these relationships into account, the channel dimensions should preferably be designed so that the following ratios of flow resistances are achieved: The flow resistance in the sample channel is preferably greater than or equal to 2 times, particularly preferably greater than or equal to 10 times, and most preferably greater than or equal to 100 times, than the flow resistance in the dispensing channel from the opening of the sample channel to the nozzle, i.e., in the entirety of the dispensing section and the nozzle section. In principle, the greater the difference in flow resistances, the lower the backflow of liquid due to pressure surges into the sample channel, which ultimately increases process accuracy. - The flow resistance of the sample channel is preferably a maximum of 10,000 times greater than the flow resistance of the dispensing channel from the opening of the sample channel to the nozzle. This limit is essentially only determined by the fact that the transport pressure for moving the liquid through the sample channel and the extraction channel cannot be chosen to be arbitrarily high. - For the reasons mentioned above, the flow resistance of the extraction channel is preferably greater than or equal to 2 times, particularly preferably greater than or equal to 10 times and most preferably greater than or equal to 100 times as much as the flow resistance of the dispensing channel from the opening of the sample channel to the nozzle. - And likewise, the flow resistance of the extraction channel is preferably a maximum of 10,000 mm, i.e., as large as the flow resistance of the dispensing channel from the opening of the sample channel to the nozzle.

[0026] Thus, the flow resistances at both T-junctions (the opening of the sample channel and the branch of the suction channel) ensure that, during the dispensing pulse, the vast majority of the liquid volume is transported in the dispensing channel towards the nozzle, and the proportions that are displaced back into the sample channel or forward into the suction channel are negligibly small.

[0027] An advantageous further development of the microfluidic structure provides that the suction channel has a cross-section whose minimum dimension is smaller than the minimum dimension of the dispensing channel. Similarly, another advantageous embodiment provides that the sample channel also has a cross-section whose minimum dimension is smaller than the minimum dimension of the dispensing channel.

[0028] The minimum cross-sectional area of ​​the channels, i.e., the height h in the approximation above (equations 1 and 2), significantly determines the flow resistance of the liquid in each individual channel. In these embodiments, the flow resistance in the extraction channel and / or the flow resistance in the sample channel is higher than that in the dispensing channel. In the case of the extraction channel, this ensures that the flow resistance for air and liquid from the direction of the nozzle to the branch into the extraction channel is so low that only a small pressure drop occurs in the section from the branch of the extraction channel to the nozzle, thus ensuring that this section is always thoroughly emptied or kept empty. Furthermore, this has the advantage that when a pressure surge is applied during the dispensing process, as little liquid as possible is forced into the extraction channel on the one hand and the sample channel on the other.

[0029] For example, if you compare a square channel with a flat, wide channel with a high aspect ratio (w / h) and the same cross-sectional area, the flat, wide channel has a drastically higher flow resistance. Here are two examples, each with the same volume flow rate (Q) of 8 µl / s and the same cross-sectional area of ​​0.03 mm². 2 Assuming the same channel length of 65 mm, a cross-sectional geometry of 0.06 mm × 0.5 mm results in a pressure drop of 1630 mbar, while a square cross-section of 0.173 mm × 0.173 mm results in a pressure drop of 165 mbar.

[0030] This will be explained again below using an example (calculated for water with a dynamic viscosity η = 1 mPa s): Table 1 Volume flow rate Q during supply and extraction 8 µl / s w(mm) h(mm) l(mm) Q(µl / s) Δp(mbar) W(mbar / (µl / s)) factor Sample channel 0,5 0,06 65 8 625 78 223 Dispensing section + nozzle section 0,3 0,3 10 8 2,8 0,35 extraction duct 0,1 0,1 15 8 341 43 860 Nozzle section 0,3 0,3 1,5 8 0,4 0,05 Volume flow rate Q during the dispensing pulse 113,33 µl / s Sample channel 0,5 0,06 65 113 8829 78 223 Dispensing section + nozzle section 0,3 0,3 10 113 40 0,35 extraction duct 0,1 0,1 15 113 4823 43 860 Nozzle section 0,3 0,3 1,5 113 6,0 0,05

[0031] Here, w denotes the width of the channels, h the height of the channels, and l the length of the channels. Q is the respective volumetric flow rate during continuous feeding and during the pressure surge, Δp the pressure drop, and W the flow resistance. The factor represents the ratio of the flow resistances from the sample channel to the entire dispensing channel, or from the extraction channel to the nozzle section of the dispensing channel.

[0032] Besides the targeted increase in flow resistance, a channel cross-section with a smaller height h and larger width w, especially for the sample channel, offers the advantage of a defined particle position approximately in a plane perpendicular to the height. The microfluidic structure according to the invention is typically formed on a substrate, with the channel height extending perpendicular to its main dimension. Optical detection of the particles flowing in the fluid in such microfluidic systems occurs along a detection axis that runs parallel to the vertical direction of the substrate. If the particles all lie in one plane, it is sufficient to focus the detector in this plane to precisely determine the position of all particles. In other words, the depth of field of the imaging system is not critical for detection. The velocity of the target objects in the wide, shallow detection section remains low to ensure optimal detection overall.

[0033] If an excitation laser is used to illuminate a detection zone within the detection section, a further advantage of a wide, flat channel is that the illuminated zone lies entirely within the channel width. This avoids highly scattering channel edges.

[0034] If, for example, the extraction channel and the dispensing channel each have a cross-section with the same minimum dimension h for manufacturing reasons, then only the channel width b is relevant. Consequently, the sample channel and / or the extraction channel advantageously has a smaller width than the dispensing channel.

[0035] All the above considerations regarding the geometry of the channel cross-sections are not limited to rectangular channel cross-sections, but also apply to rounded, oval or other channel cross-sections where two directions of extension and an aspect ratio can be clearly determined.

[0036] The length of the dispensing section, i.e., the distance d, determines, given the cross-section and volume flow rate of the liquid during feeding and suction, the residence time of the target objects in the dispensing channel and the probability that another object is simultaneously located in the dispensing volume.

[0037] The length of the nozzle section, i.e., the distance a, determines the transport velocity of the liquid upon exiting the nozzle, given a specific acceleration of the liquid during the dispensing pulse. The ratio of the length a of the nozzle section to the maximum cross-sectional area of ​​the dispensing channel, which typically corresponds to the width b, is preferably > 2. A ratio > 5 is particularly preferred, as this ensures that a dispensing pulse that does not damage the microfluidic structure generates a sufficient transport velocity to provide enough kinetic energy to overcome the surface tension and allow the droplet (or jet) to exit the nozzle without leaving any residue.Furthermore, with increasing kinetic energy of the dispensed liquid volume, the directional deviations resulting from asymmetries in the direction of surface tension, which can be caused, for example, by manufacturing tolerances and / or deposits at the nozzle edge, become smaller.

[0038] It is particularly advantageous if the fluid exit velocity at the nozzle is at least 1 m / s for typical channel dimensions. The upper limit for the exit velocity is determined by the stability of the microfluidic system, i.e., the cartridge or chip, and by the mechanical resistance of any objects contained in the fluid, such as cells. For typical channel dimensions, this upper limit is 10 m / s.

[0039] Preferably, the suction channel branches off from the dispensing channel at an acute angle against the flow direction. Equally preferably, the sample channel opens into the dispensing channel at an acute angle in the flow direction.

[0040] At high Reynolds numbers (>> 100), a negative pressure is created in the sample channel and the extraction channel during the dispensing stroke, similar to what occurs with an angled impeller, due to the acute angle. This negative pressure further counteracts the backflow of liquid into the sample channel and forward flow into the extraction channel. The acute angle also offers mechanical advantages. The branch of the extraction channel can be positioned closer to the nozzle outlet than with a perpendicular branch, while still providing sufficient contact area for a durable bond between the lid film and the substrate or base plate.

[0041] Finally, the dispensing channel must also be sufficiently narrow in at least one spatial direction to prevent the surface tension of the liquid from leaking due to gravity. Preferably, for the typical viscosities of the liquids under consideration, the minimum expansion should be less than 0.5 mm.

[0042] A preferred further development of the procedure provides that the extraction of the liquid takes place at a higher flow rate than the supply of the liquid.

[0043] This results in the formation of a relatively small liquid meniscus or lower liquid boundary in the dispensing channel at the upper edge of the branch of the suction channel, so that the liquid column in the dispensing channel has only a small interface with the surrounding air. This minimizes evaporation in the dispensing channel, which counteracts undesirable salt crystallization.

[0044] A valve can be provided on the inlet side of the dispensing channel, at which an externally generated overpressure is applied and which is opened to generate a pressure surge. Alternatively, the pressure surge can also be generated in an external operator device, thus eliminating the need for a valve in the microfluidic structure or the microfluidic system itself.

[0045] Pressure control via valves is both technically simple to implement and very responsive, allowing for the straightforward creation of a pressure build-up of preferably 300 to 900 mbar in the dispensing channel using a fluid, such as a column of liquid, or a gas, particularly air. Solenoid valves have proven particularly suitable for this purpose. The advantage of a liquid column is its incompressibility, unlike that of a gas (air), which reduces the system's time constants. This means that the spring action caused by the pressure wave in the system decreases, and the entire system, including the liquid meniscus, returns to equilibrium—i.e., the initial state before the dispensing pulse—more quickly after the valve closes.With a liquid-driven pressure pulse, a dispensing process from triggering to the breaking of the liquid jet takes approximately 20 msec, depending on the parameters and desired dispensing volume.

[0046] Regardless of these parameters, the dispensing process should preferably be carried out so powerfully and rapidly that the surface tension of the liquid and the exact geometry of the nozzle play no role in the exit of the liquid jet. Even with an asymmetrical nozzle shape and / or manufacturing-related geometric errors, the direction of the liquid jet would remain unchanged.

[0047] Opening the valve causes an immediate pressure surge that forces the liquid from the dispensing channel through the nozzle. By varying the valve opening time and pressure, the dispensing volume can be individually controlled, meaning that targeted portions of the continuously supplied liquid can be dispensed.

[0048] Advantageously, the dispensing channel is straight from the opening of the sample channel to the nozzle.

[0049] This ensures that the flow direction in the dispensing channel, from the opening of the sample channel to the branch of the suction channel, coincides with the direction of the liquid discharge during dispensing.

[0050] Preferably, the sample channel upstream of its opening into the dispensing channel has a cross-channel structure for supplying two enveloping streams. This cross-channel structure is preferably designed such that two enveloping stream supply channels open into the sample channel on opposite sides via the cross-channel structure. Furthermore, the enveloping stream supply channels each have a widened channel cross-section upstream of the cross-channel structure in the flow direction.

[0051] The method accordingly provides that two envelope streams are supplied to the liquid in the sample channel via the cross-channel structure, with the liquid being drawn out of the dispensing channel through the suction channel together with the envelope streams, wherein in this case the suction preferably takes place at a higher flow rate than the supply of the liquid plus the envelope streams. The liquid located in the dispensing channel between the opening of the sample channel and the branch of the suction channel is then also expelled from the nozzle together with the fluid of the envelope streams as a result of the pressure surge.

[0052] A fundamental advantage of using envelope flows is that their properties, particularly their viscosity, can be selected independently of the composition of the carrier fluid (sample), and that wall effects such as friction or wetting do not affect this carrier fluid, which can therefore flow through the channel undisturbed. Furthermore, envelope flows can be used to achieve variable dilution of the sample for better separation of specific target objects (inclusions) from unwanted objects or other target objects.

[0053] The channel geometry of the envelope feed channels, i.e., the section thereof with a widened channel cross-section before the mouth or the cross-channel structure, further ensures that both envelope feed channels can be sufficiently filled with liquid during commissioning, and that any asymmetry between the flow resistances of the two envelope feed channels is minimized, so that both envelope streams are reliably subjected to the same flow rate.

[0054] Preferably, the sample channel has a detection section upstream of its opening into the dispensing channel. In the case of a cross-channel structure, this detection section is particularly preferably located downstream of it. The method involves detecting particles in the liquid within the detection section of the sample channel, with the application of the pressure pulse occurring in response to the detection of a particle. "In response" in this case preferably means with a time interval after detection, corresponding to the time it takes for the particles to enter the dispensing section at a given volumetric flow rate.

[0055] The microfluidic structure designed in this way and the correspondingly further developed method thus offer the possibility of identifying individual inclusions in the flow of liquid and dispensing them individually by targeted application of the pressure pulse.

[0056] A preferred further development of the method provides that the liquid comprises a shear-thinning liquid.

[0057] Under conventional conditions, non-Newtonian sample behavior, such as that exhibited by shear-thinning liquids, or high viscosities can lead to difficulties during the dispensing process. However, in combination with the microfluidic structure and method according to the invention, the non-Newtonian and / or viscoelastic sample behavior can be exploited as an advantage. A shear-thinning medium is highly viscous without the influence of external forces and forms a network between the polymer chains. If this network is strong enough, it can completely suppress the sedimentation of inclusions, particularly particles, by causing the particles to come to a standstill within the network. The force of the network must counteract the gravitational pull of the particles. However, as soon as shear forces act on a shear-thinning medium, its viscosity decreases.This property implies that the pressure drop required to transport a shear-thinning medium decreases as the applied shear force increases. In microfluidic structures, this means that the pressure drop is reduced, particularly at high flow rates and with small channel cross-sections. This allows shear-thinning media to be pumped through small channel systems at high flow rates without risking damage to the channel structure.

[0058] Furthermore, shear-thinning media cause inclusions, especially particles, to be transported at maximum flow velocity during a fluidic application, i.e., at the maximum of the flow profile, and therefore preferentially accumulate in the center of the channel. This offers further advantages, namely a reduction in particle adhesion to the channel walls not only during storage but also during transport. To optimize this effect, it is even advantageous to establish high shear forces in the near-wall regions of the flow profile, i.e., to provide high flow rates and small channel dimensions in at least one direction. In this way, particle adhesion can be efficiently reduced or even prevented, even in complex fluidic structures with bends and intersections.

[0059] Another advantage is that lower shear rates prevail in the center of the channel than at the channel walls, allowing inclusions, especially particles, to be transported gently through the fluidic system. This enables non-destructive analysis with low stress, particularly for biological objects such as cells.

[0060] Furthermore, in many applications, such as flow cytometry, the particles must be focused in at least one plane. This is often achieved mechanically by introducing shear-thinning fluids according to the principle of hydrodynamic focusing. However, this effort is no longer necessary when using shear-thinning media, as the particles are transported solely by the non-Newtonian fluid in the center, even without limiting elements. If shear-thinning fluids are still required for processing, for example, to introduce additional substances, this is possible without affecting the described focusing and sedimentation prevention effect. On the contrary, the method can advantageously combine these properties if the fluid is shear-thinning and the shear-thinning fluid is Newtonian.This ensures particularly gentle transport of the inclusions or particles, because the shear forces in the liquid are minimized by the envelope flow.

[0061] Furthermore, the transport position in the center of the channel, unlike in Newtonian media where objects occupy two separate positions on the flanks of the flow profile, ensures that no two objects are transported side by side in the channel. The objects always follow one another sequentially, which allows for unambiguous detection and sorting.

[0062] Further advantages and features of the invention are explained below with reference to the figures. Fig. Figure 1 shows a schematic representation of an embodiment of the microfluidic structure according to the invention and Fig. 2 three cross-sections through the channels of the microfluidic structure according to Fig. 1.

[0063] The microfluidic structure in Fig.The device 1 has a dispensing channel 10 with an inlet side 12 and an outlet side 14. A nozzle 16 in the form of a straight, open end of the dispensing channel 10 is arranged on the outlet side 14. The flow direction in the dispensing channel 10 is indicated by the arrow 18. On the inlet side, the dispensing channel 10 has a valve 20 with which the inlet side of the dispensing channel can be selectively opened or closed in order to apply a pressure pulse to the dispensing channel 10.

[0064] A sample channel 22 opens into the dispensing channel 10, and a suction channel 24 branches off downstream. The opening and the branch are offset by a distance d in the flow direction 18. The distance is measured by the position at which the central axes of the sample channel and the suction channel intersect the contour of the dispensing channel 10. The suction channel 24 opens into the dispensing channel 10 at a distance a upstream of the nozzle 16. Both the suction channel 24 and the sample channel 22 have a cross-section whose minimum dimension is smaller than the minimum dimension of the cross-section of the dispensing channel 10.

[0065] The Fig.Figure 1 schematically shows the microfluidic structure in a top view, i.e., looking down at the plane of a (not shown) substrate into which the channels of the microfluidic structure are formed as depressions in a manner known per se by injection molding or embossing. The channels are sealed by means of a cover film on the top side of the substrate. They typically have a rectangular cross-section. It should be noted here that channels produced particularly by injection molding actually have a trapezoidal cross-section for demolding purposes. Such cross-sections are also meant in this document when, for the sake of simplicity, a "rectangular cross-section" is mentioned. The dispensing channel 10, without the invention being limited thereto, has, for example, a square cross-sectional geometry with an edge length of 300 × 300 µm.The sample channel 22 and the extraction channel 24 are, as can be seen in the figure, narrower in at least one spatial dimension.

[0066] The opening and closing of valve 20 is automatically controlled in response to a control signal. The necessary actuators, sensors, evaluation and control electronics are part of an operator unit into which the substrate is inserted.

[0067] The dispensing channel 10 runs straight along its entire length, from the inlet side 12 to the nozzle 16. It is particularly important that it runs straight at least from the opening of the sample channel 22 to the nozzle 16, so that the dispensing direction coincides with the flow direction 18 in the section between the opening of the sample channel 22 and the branch of the extraction channel 24, in order to ensure reliable, targeted dispensing.

[0068] Upstream of its mouth, the sample channel 22 has a cross-channel structure 26. In the region of the cross-channel structure 26, a sheathing feed channel 28 opens into the sample channel 22 on both sides. Both sheathing feed channels 28 have a section 30 with a widened channel cross-section upstream of the cross-channel structure 26.

[0069] Between the opening of the sample channel 22 into the dispensing channel 10 and the cross-channel structure 26, the sample channel 22 has a detection section 32. This section is primarily characterized by the fact that the substrate of the microfluidic system at this point exhibits good optical quality, both on the surface and throughout its volume, to ensure error-free optical detection of inclusions in the sample liquid. Inclusions located in this area can, for example, be detected by an optical detection device in the operator unit, and the resulting detection signal can be used to control the valve 20. The valve is typically opened at a time interval after detection, depending on the flow velocity in the dispensing channel, thus ensuring that the inclusion has entered the dispensing channel.

[0070] A sufficiently long length l of the dispensing channel between the opening of the sample channel 22 and the branch of the suction channel 24 provides a relatively large target area for the inclusions to be dispensed and, in conjunction with detection in the detection section 32, ensures reliable positioning of these inclusions and thus increased dispensing reliability (cell yield). Furthermore, the large channel cross-section and short length compared to the sample channel 22 and suction channel 24 result in very low flow resistance, which reduces the recoil β into the sample channel 22 when a dispensing pulse is triggered. At the same time, the loss of dispensing volume into the suction channel 24 is also minimized, as the suction channel itself has a small cross-section and is comparatively long, resulting in a comparatively high flow resistance.

[0071] The sample channel 22 preferably opens into the dispersion channel 10 at an acute angle to the flow direction 18. Likewise, the suction channel 24 branches off from the dispersion channel 10 at an acute angle to the flow direction 18.

[0072] The liquid is extracted continuously through the extraction channel 24 at a higher flow rate than the liquid is supplied through the sample channel 22, including the surrounding fluid streams supplied through the envelope feed channels 28, which are preferably also supplied continuously. This results in the formation of a liquid meniscus 34 in the dispensing channel 10 at the upper edge of the branch of the extraction channel 24.

[0073] During continuous operation, this liquid meniscus 34 positions itself at approximately a distance a from the nozzle 14 between two dispensing cycles. The greater the distance a, the lower the evaporation from the liquid surface. This also reduces the crystallization of salts that could disrupt the dispensing process. Furthermore, with each dispensing pulse, the forward and reverse flow in the dispensing section causes any crystallized salts to dissolve back into the liquid, keeping the outlet area of ​​the dispensing channel essentially free of flow-impeding deposits.

[0074] A particularly safe operating mode exists when the volumetric transport rate in the suction channel 24 is (slightly) higher than the volumetric transport rate in the sample channel 22. In this case, the liquid meniscus at the suction channel 24 will pulsate, and air will be regularly transported from the nozzle 16 into the suction channel 24. This operating mode is easier to control and ensures that liquid never leaks unintentionally from the nozzle 16, since the volumetric transport rate of the suction channel 24 does not accommodate the entire sample volume.

[0075] The length a of the nozzle section of the dispensing channel 10 is selected such that, after a dispensing pulse is triggered, the liquid front reaches a sufficiently high velocity upon arrival at the nozzle opening 16. At this velocity, the kinetic energy of the liquid prevents wetting of the external surfaces of the microfluidic system, i.e., with the exception of the internal surfaces of the dispensing section. This ensures that no contamination occurs on external surfaces around the nozzle and that the jet exits precisely in the direction of the nozzle channel without deflection, regardless of any manufacturing tolerances of the microfluidic system.

[0076] Fig. Figure 2 schematically shows the three cross-sections a) of the dispensing channel 10, b) of the sample channel 22 and c) of the suction channel 24 for size comparison. The sections are along the line shown in Fig.The lines A-A, B-B, and C-C are shown in Figure 1. While the dispensing channel 10 has an almost square cross-section with a large height h and large width w and an aspect ratio of almost 1, the sample channel 22 is significantly smaller, primarily in the height direction, and has an aspect ratio w / h >> 1. The extraction channel 24, on the other hand, is also significantly smaller in the width direction and again has an aspect ratio of almost 1. However, due to their significantly smaller minimum cross-sectional dimensions, both the extraction channel 24 and the sample channel 22 generate significantly higher flow resistances than the dispensing channel 10.

[0077] With the exception of the sample channel, the cross-sections of the channels do not change along their length. The sample channel cross-section only widens in a ramp region 33 in the vertical direction from the one in Fig. 2 b) flat cross-section shown in the depth direction down to the in Fig.2 a) shown depth measurement of the dispensing channel 10. In this way, a step-like transition from the sample channel to the dispensing channel is avoided. Reference symbol list 10 Dispensing channels 12 Entrance side 14 Outlet side 16 nozzle 18 Flow direction arrow 20 valve 22 Sample channel 24 extraction channel 26 Cross-channel structure 28 Envelope flow supply channel 30 Section with widened channel cross-section 32 Detection section 33 Ramp section 34 Fluid meniscus a distance, length of the nozzle section d distance, length of the dispensing section QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 3 641 937 B1

[0010] Cited non-patent literature

[0000] Launiere, Czaplewski et al., “Rheologically biomimetic cell suspensions for decreased cell setting in microfluidic devices”, Biomed Microdevices, 2011

[0009]

Claims

A microfluidic structure for dispensing a liquid, comprising: a dispensing channel (10) with an inlet side (12) and downstream an outlet side (14), wherein the dispensing channel (10) has a device for introducing a pressure pulse on the inlet side (12) and wherein the dispensing channel (10) has a nozzle (16) on the outlet side (14), a sample channel (22) which is connected on the inlet side to a device for sample supply and which opens on the outlet side into the dispensing channel (10) between the inlet side (12) and the outlet side (14), and a suction channel (24) which branches off on the inlet side from the dispensing channel (10) at a distance (1) downstream of the opening of the sample channel (22) and at a distance (a) upstream of the nozzle (16) and which is connected on the outlet side to a device is connected for sample extraction, wherein the sample channel, the suction channel and the dispensing channel are designed in such a way thatthat the liquid in the sample channel or in the extraction channel or in the sample channel and in the extraction channel each experiences a higher flow resistance than in the dispensing channel from the opening of the sample channel to the nozzle. Microfluidic structure according to claim 1, characterized in that the flow resistance in the sample channel or in the extraction channel or in the sample channel and in the extraction channel is greater than or equal to 2 times, preferably 10 times, particularly preferably 100 times, as the flow resistance in the dispensing channel from the opening of the sample channel to the nozzle. Microfluidic structure according to claim 1 or 2, characterized in that the suction channel (24) and / or the sample channel (22) has a cross-section whose minimum extent is smaller than the minimum extent of the cross-section of the dispensing channel (10). Microfluidic structure according to one of the preceding claims, characterized in that the sample channel (22) has a cross-channel structure (26) upstream of its opening into the dispensing channel (10) for supplying two sheathing streams. Microfluidic structure according to claim 4, characterized in that two envelope flow supply channels (28) open into the sample channel (22) on opposite sides by means of the cross-channel structure (26), which envelope flow supply channels (28) each have a section (30) with an expanded channel cross-section in the flow direction (18) upstream of the cross-channel structure (26). Microfluidic structure according to one of the preceding claims, characterized in that the sample channel (22) has a detection section (32) upstream of the opening into the dispensing channel (10). Microfluidic structure according to claim 4 and 6 or 5 and 6, characterized in that the cross-channel structure (26) is arranged upstream of the detection section (32). A method for dispensing a liquid comprising the steps of: - providing a microfluidic structure according to any one of claims 1 to 7, - supplying a liquid from the sample channel (22) through an opening into the dispensing channel (10), - aspirating the liquid downstream of the opening of the sample channel (22) from the dispensing channel (10) through a branch into the aspiration channel (24), - intermittently applying a pressure pulse on the inlet side (12) of the dispensing channel (10), wherein the liquid in the dispensing channel (10) between the opening of the sample channel (22) and the branch of the aspiration channel (24) is ejected through the nozzle (16), wherein the liquid in the sample channel experiences a higher flow resistance than in the dispensing channel from the opening of the sample channel to the nozzle. Method according to claim 8, characterized by detecting inclusions in the liquid in the detection section (32) of the sample channel (22), wherein the application of the pressure pulse is carried out in response to the detection of an inclusion. Method according to claim 8 or 9, characterized in that the application of the pressure pulse is carried out by opening a valve on the inlet side of the dispensing channel. Method according to one of claims 8 to 10, characterized in that the liquid is continuously fed through the sample channel (22) into the dispensing channel (10) and is extracted from the dispensing channel (10) through the suction channel (24) when no pressure pulse is applied. Method according to one of claims 8 to 11, characterized in that the extraction of the liquid is carried out at a higher flow rate than the supply of the liquid. Method according to one of claims 8 to 12, characterized in that two envelope streams are supplied to the liquid in the sample channel (22) via a cross-channel structure (26), wherein the liquid together with the envelope streams is drawn from the dispensing channel (10) through the suction channel (24) when no pressure pulse is applied, and wherein the liquid and the envelope streams in the dispensing channel (10) between the opening of the sample channel (22) and the branch of the suction channel (24) are expelled from the nozzle (16) when a pressure pulse is applied. Method according to one of claims 8 to 13, characterized in that the suction is carried out at a higher flow rate than the sum of the supply of the liquid and the envelope flow. Method according to one of claims 8 to 14, characterized in that the liquid comprises a shear-thinning liquid and the envelope stream comprises a Newtonian liquid.

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