Microfluidic probe head for processing a sequence of liquid volumes separated by spacers

Microfluidic probe head for processing a sequence of separate liquid volumes separated by spacers. The microfluidic probe head includes: an inlet, an outlet, a first fluid channel and a second fluid channel and a fluid bypass connecting the first fluid channel and the second fluid channel. The first fluid channel delivers the sequence of separate liquid volumes from the inlet toward a deposition area, the fluid bypass allows the spacers to be removed from the first fluid channel obtaining a free sequence of separate liquid volumes without spacers, the first fluid channel delivers the free sequence of separate liquid volumes to the deposition area, and the second fluid channel delivers the removed spacers from the fluid bypass to the outlet. The present invention also provides a microfluidic probe and method for processing a sequence of separate liquid volumes.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 from European Patent Application No. 15155054.8 filed Feb. 13, 2015, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a microfluidic probe head. More particularly, a microfluidic probe head for processing a sequence of separate liquid volumes separated by spacers.

BACKGROUND

Microfluidics deals with the behavior, precise control and manipulation of small volumes of fluids that are typically constrained to micrometer-length scale channels and to volumes typically in the sub-milliliter range. Here, fluids refer to liquids and either term may be used interchangeably in the rest of the document. In particular, typical volumes of liquids in microfluidics range from 10-15 L to 10-5 L and are transported via microchannels with a typical diameter of 10-7 m to 10-4 m.

At the microscale, the behavior of the liquids can differ from that at a larger, macroscopic scale. In particular, surface tension, viscous energy dissipation and fluidic resistance are dominant characteristics of the flow. For example, the Reynolds number, which compares an effect of momentum of a fluid to the effect of viscosity, can decrease to such an extent that the flow behavior of the fluid becomes laminar rather than turbulent.

In addition, liquids at the microscale do not necessarily mix in the traditional, chaotic sense due to the absence of turbulence in low-Reynolds number flows and interfacial transport of molecules or small particles between adjacent liquids often takes place through diffusion. As a consequence, certain chemical and physical properties of liquids such as concentration, pH, temperature and shear force are deterministic. This provides more uniform chemical reaction conditions and higher grade products in single and multi-step reactions.

A microfluidic probe is a microfabricated scanning device for depositing, retrieving, transporting, delivering, and/or removing liquids, and in particular liquids containing chemical and/or biochemical substances. For example, the microfluidic probe can be used on the fields of diagnostic medicine, pathology, pharmacology and various branches of analytical chemistry. Here, the microfluidic probe can be used for performing molecular biology procedures for enzymatic analysis, deoxyribonucleic acid (DNA) analysis and proteomics.

Many of chemical and biochemical processes require multiple steps that are performed sequentially, involving exposure of a target surface to different liquids including (bio)chemicals, solvents and buffers under various conditions such as different temperatures, different concentrations and/or different durations.

Accordingly, the microfluidic probe should enable the delivery of a sequence of liquids in small volumes to a surface with low or no mixing between the sequential liquids. During transport of the liquids, these sequential sections of liquids inside a capillary or microfluidic channel are often termed as ‘plugs’. Typically, in microfluidic capillaries and channels, mixing between subsequent plugs containing different liquids due to (Taylor) dispersion decreases the concentration gradient between these subsequent plugs. In order to deliver a sequence of small-volume plugs to a surface, the microfluidic probe should be capable of rapidly switching between different liquids that form a sequence of small-volume plugs. In the meantime, the dispersion of plugs during the delivery to the surface should be limited in order to prevent subsequent plugs from mixing with one another.

Microfluidic probe heads are know which are suitable for patterning continuous and discontinuous patterns of biomolecules on surfaces and processing resist materials on a surface. However, liquids that are sequentially delivered to the target surface tend to mix with one another due to advective and diffusive effects. As a result, the sequence of plugs delivered to the surface may no longer be identical in terms of solute or particle concentration, viscosity and plug volume by the time it reaches the surface as compared to its initial state shortly after the point where the sequence is generated.

One approach to prevent sequentially delivered liquids from mixing with one another is made by inserting spacers of an immiscible-phase fluid between sequential plugs that have different continuous-phase liquids. For instance, the sequential plugs could be aqueous, while the immiscible-phase spacers are constituted by an oil or a gas. The immiscible-phase spacers prohibit a diffusion of solutes and/or particles between sequential plugs. “The chemistrode: A droplet-based microfluidic device for stimulation and recording with high temporal, spatial, and chemical resolution”, D. Chen et al., PNAS, 2008 (105), 16843-16848, discloses a tool that delivers aqueous stimulus plugs separated by segments of an immiscible phase to a target surface and retrieves response plugs. However, the tool and the immiscible-phase spacers come into direct contact with the target surface.

A drawback of many prior art solutions is that they are not applicable to local chemistry performed in wet environments, in particular when willing to use hydrodynamic flow confinement. Therefore, the deposition of droplets cannot be localized due to spreading of the liquid using this device and method.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a microfluidic probe head for processing a sequence of separate liquid volumes separated by spacers. The microfluidic probe head includes: an inlet, an outlet, a first fluid channel and a second fluid channel and a fluid bypass connecting the first fluid channel and the second fluid channel. The first fluid channel delivers the sequence of separate liquid volumes from the inlet toward a deposition area, the fluid bypass allows the spacers to be removed from the first fluid channel obtaining a free sequence of separate liquid volumes without spacers, the first fluid channel delivers the free sequence of separate liquid volumes to the deposition area, and the second fluid channel delivers the removed spacers from the fluid bypass to the outlet.

A second aspect of the present invention provides a microfluidic probe for processing a sequence of separate liquid volumes separated by spacers. The microfluidic probe includes: a microfluidic probe head including an inlet, an outlet, a first fluid channel, a second fluid channel, and a fluid bypass connecting the first fluid channel and the second fluid channel; a plurality of liquid supplies fluidly connectable to the inlet of the microfluidic probe; and a control unit for selectively fluidly connecting the inlet of the microfluidic probe to one of the plurality of liquid supplies.

A third aspect of the present invention provides a method for processing a sequence of separate liquid volumes separated by spacers. The method includes: delivering via a first fluid channel the sequence of separate liquid volumes from a liquid inlet and toward a deposition area; removing from the first fluid channel the spacers separating the separate liquid volumes from one another via a fluid bypass that connects the first fluid channel and a second fluid channel, thereby obtaining a free sequence of separate liquid volumes without spacers; delivering the free sequence of separate liquid volumes to the deposition area; and delivering the removed spacers from the fluid bypass to an outlet via the second fluid channel.

Similar or functionally similar elements in the figures have been allocated the same reference signs if not otherwise indicated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some preferred embodiments will be described in more detail with reference to the accompanying drawings, in which the preferable embodiments of the present invention have been illustrated. However, the present invention can be implemented in various manners, and thus should not be construed to be limited to the embodiments disclosed herein. On the contrary, those embodiments are provided for the thorough and complete understanding of the present invention, and to completely convey the scope of the present invention to those skilled in the art.

FIG. 1shows a schematic perspective view of microfluidic probe1for performing sequential chemistry using hydrodynamic flow confinement.

Microfluidic probe1includes: microfluidic probe head2attached to robotic arm3. Robotic arm3is configured for positioning microfluidic probe head2at a specific location and above each of deposition areas4a-4g. Preferably, microfluidic probe1further integrates an x, y and z positioning stage in order to perform an arbitrary three-dimensional movement. In particular, microfluidic probe1is configured for performing sequential chemistry.

Microfluidic probe head2is fluidly connected to liquid supplies5a-5c, spacer supply6and disposal unit7. Liquid supplies5a-5cprovide microfluidic probe head2with different liquids such as liquids that contain biochemical substances. Spacer supply6supplies microfluidic probe head2with oil. Disposal unit7collects waste fluids from microfluidic probe head2.

FIG. 2shows a partial view of a first embodiment of microfluidic probe head2illustrating the fluid flows and the formation of hydrodynamic flow confinement.

Generally speaking, hydrodynamic flow confinement (HFC) relates to a phenomenon that a laminar flow of an injection liquid is spatially confined within a liquid bath containing a background liquid. Embodiments of the invention may advantageously rely on hydrodynamic flow confinement, as discussed in detail below. For the sake of illustration, embodiments discussed herein mostly assume hydrodynamic flow confinement. For instance, in the embodiment ofFIG. 2, an injection microchannel injects the injection liquid into the liquid bath with an injection flow rate and an aspiration microchannel aspirates the injection liquid and some of the background liquid with an aspiration flow rate. By keeping the aspiration flow rate higher than the injection rate, the laminar flow of the injection liquid from the injection channel to the aspiration channel is formed and confined inside a volume within the surrounding liquid bath.

In the embodiment ofFIG. 2, deposition areas4a-4care located on top of bottom surface8of Petri dish9or the like that is at least partly filled with immersion liquid10such that deposition areas4a-4care covered with immersion liquid10. For example, deposition areas4a-4cmay include biological and biochemical substances, such as cells or tissue, and/or a device (e.g. a chip) to detect viral and/or bacterial infections or allergies.

Microfluidic probe head2includes: body11having end face12. First fluid channel13and second fluid channel14are formed in body11. First aperture15and second aperture16are formed in end face12. First and second apertures15,16are fluidly connected to first and second fluid channels13,14, respectively. For example, distance T1 between first and second apertures15,16is less than 2.0 mm.

Body11of microfluidic probe head2acts as housing or carrier. All elements, parts and/or devices integrated in body11may be manufactured on-chip (using lithography, for example) and are movable therewith.

Fluid bypass17is located inside body11. Fluid bypass17is fluidly connected to first fluid channel13at first bypass junction18and to second fluid channel14at second bypass junction19so as to connect first fluid channel13and second fluid channel14.

Microfluidic probe head2is positioned above deposition area4binFIG. 2. End face12of microfluidic probe head2is spaced from deposition area4bsuch that a distance between end face12and deposition area4bis 1-100 μm. At this distance, end face12is immersed in immersion fluid10covering deposition areas4a-4c. A sequence of separate liquid volumes, including liquid volumes23a,23b, separated by spacers20is delivered via first fluid channel13to first bypass junction18, where blocking elements21redirect spacers20into fluid bypass17.

If spacers20, in particular spacers that include an oil-phase, come into contact with deposition area4a-4c, surface properties of deposition area4a-4ccan be altered and deposition areas4a-4ccan be contaminated and the stability of the hydrodynamic flow confinement can be disrupted, thereby disturbing the deposition of the liquid volumes at required deposition areas4a-4c. In particular, biochemical substances such as proteins, cells and biological tissues on deposition areas4a-4cmight be denatured and/or damaged by coming in contact with spacers20. On the other hand, lipophilic analytes such as lipids, therapeutic molecules, hormones, non-polar dyes or tracers may be carried away by spacers20. Furthermore, if spacers20that are discharged through first aperture15they may exert a shear stress on objects below and thereby damage and/or shift them.

By removing spacers20that separate liquid volumes23a,23bof the sequence of liquid volumes from first fluid channel13, spacers20are prevented from reaching deposition areas4a-4cand a free sequence of separate liquid volumes, that is the sequence of separate liquid volumes with spacers20being removed therefrom, is delivered toward first aperture15. Only confinement volume22and, hence, laminar flow C of the free sequence of separate liquid volumes come into contact with deposition area4a-4cduring operation.

The free sequence of separate liquid volumes discharges into immersion liquid10through first aperture15with first flow rate Q1. At the same time, part of immersion liquid10and the free sequence of separate liquid volumes that is discharged into immersion liquid10are aspirated through second aperture16into second fluid channel14with second flow rate Q2. For example, first and second flow rates Q1, Q2can be generated using corresponding pumps (not shown).

If second flow rate Q2is higher than first flow rate Q1, and a ratio of second flow rate Q2to first flow rate Q1is, for example, 1.2-10, laminar flow C can be obtained from first aperture15to second aperture16. Achieving such a laminar flow allows for hydrodynamic flow confinement. The laminar flow C is hydrodynamically confined by immersion liquid10within confinement volume22that extends from below first aperture15to below second aperture16. The size of confinement volume22and the shape of laminar flow C are defined by first flow rate Q1, second flow rate Q2and the ratio of second flow rate Q2to first flow rate Q1, the distance between the first and second apertures and/or the distance between end face12and respective target area4a-4c. For example, the first flow rate Q1may be chosen to be 1.0 fL/s-1.0 mL/s and the second flow rate Q2may be chosen to be 0.2 fL/s-4.0 mL/s.

In order to deposit first liquid volume23aonto deposition area4b, microfluidic probe head2is positioned such that confinement volume22is in contact with deposition area4b. A part of liquid volume23aand/or substances that are carried in liquid volume23amay adhere to deposition area4band/or react with is substance that is located on top of deposition area4b. A remaining part of liquid volume23amoves along with laminar flow C and is aspirated through second aperture16into second fluid channel14.

After the deposition of first liquid volume23a, microfluidic probe head2can be positioned above the next deposition area4cin order to deposit second liquid volume23bonto it. The steps of positioning microfluidic probe head2above the respective deposition area and depositing a liquid volume onto it can be repeated as many times as required.

FIG. 3shows a schematic cross-sectional view of the first embodiment of microfluidic probe head2and the fluid connections fromFIG. 1.

Liquid supplies5a-5care located outside of body11of microfluidic probe head2. Valve device24fluidly connects liquid supplies5a-5cand inlet25. Liquid supplies5a-5ccontain different liquids. Liquid supplies5a-5cmay contain a single liquid, an emulsion, a suspension and/or other mixtures of same phase or different phases, e.g. solid-liquid, liquid-gas and/or liquid-liquid mixtures. In particular, at least one of liquid supplies5a-5cmay contain a liquid that includes a biochemical substance. In particular, at least one of liquid supplies5a-5cmay contain a liquid that is of use in (bio)chemical analysis and/or can be judged as such by those familiar with the field, e.g. any organic and/or inorganic fluids and/or substances that are related to lifeforms. Accordingly, liquid supplies5a-5cmay supply microfluidic probe head2with one or more liquids containing biological substances (e.g. cells, proteins, DNA, drugs, antibodies, chemical stimulants and/or chemical responses). The liquids supplied by liquid supplies5a-5cmay differ, for example, in terms of chemical composition or a concentration of one or more substances contained therein. Liquid supplies5a-5cfeed the liquids to valve device24.

Valve device24is fluidly connected to first fluid channel13and is capable of feeding the liquids received from liquid supplies5a-5cinto first fluid channel13. In particular, valve device24is configured for selectively feeding a specific amount of one of the liquids from liquid supplies5a-5cinto first fluid channel13. Inlet control unit26is located inside body11of microfluidic probe head2and configured for selectively, fluidly connecting inlet25to one of liquid supplies5a-5cby controlling valve device24. For this purpose, valve device24is operable for consecutively and/or alternately feeding liquid volumes from liquid supplies5a-5cinto first fluid channel13via inlet25thereby forming a free sequence of separate liquid volumes. The volume of each separate liquid volume and the order of the separate liquid volumes can be specified using inlet control unit26. For example, a fluorescence readout (a corresponding sensor is not shown) from deposition areas4a-4gmay be interpreted to control the time of exposure to a certain chemical. Once the desired time of exposure is reached, liquid supplies5a-5care switched using valve device24.

Alternatively, it is possible that only one liquid supply5ais fluidly connected to microfluidic probe head2. In this case, liquid from liquid supply5aflows into first fluid channel13instead of a plurality of different liquids.

Spacer supply6, located outside of body11of microfluidic probe head2, is fluidly connected to spacer insertion unit27via inlet27aand supplies it with oil that is immiscible with any of the liquids from liquid supplies5a-5cand immersion liquid10and is thereby suitable for providing spacers20. Instead of oil, other non-polar spacer fluids, such as fats, lipids, hexane and/or toluene that are immiscible with the liquids from liquid supplies5a-5cand the immersion liquid may be employed. In this way, the separation of the liquid volumes from one another is facilitated due to an interfacial tension between adjacent spacers20and liquid volumes.

The free sequence of separate liquid volumes flows along first fluid channel13toward spacer junction28, where spacer insertion unit27is fluidly connected to first fluid channel13. Spacer junction28is located between inlet25and first bypass junction18in first fluid channel13. Spacer insertion unit27is configured for inserting spacers20into first fluid channel13, thereby forming a sequence of separate liquid volumes separated by spacers20. In particular, the insertion of spacers20by spacer insertion unit27is timed such that the liquid volumes of the free sequence of separate liquid volumes are separated from one another by spacers20. In case of one single liquid delivered to spacer junction28, spacers20divide the liquid into separate liquid volumes23a,23b. Control unit29located inside body11of microfluidic probe head2controls the insertion of spacers20into first fluid channel13by spacer insertion unit27.

After deposition of a liquid volume the remaining part of the liquid volume, retrieved spacers20and a part of immersion liquid10are delivered to disposal unit7via second fluid channel14. Outlet30fluidly connects second fluid channel14located inside body11of microfluidic probe head2to disposal unit7outside of it. Disposal unit7can be configured for recycling, re-using, storing and/or properly disposing the retrieved liquids. Alternatively or in addition, it is possible that second fluid channel14is fluidly connected via outlet30to an analyzing device that analyzes the liquid and/or spacers provided at outlet30.

First detector31and second detector32are installed in vicinity of first fluid channel13and second fluid channel14, respectively, inside body11of microfluidic probe head2. Both first and second detectors31,32are configured for detecting and identifying spacer20. Upon detection of spacer20, first and second detectors31,32generate a first detection signal and a second detection signal, respectively, and transmit it to control unit29. Based on the first and second detection signals control unit29may synchronize the insertion rate of spacers20into first fluid channel13and the aspiration rate at which they are aspirated through second fluid channel14. To this end, control unit29may control insertion unit29and/or the aforementioned pumps accordingly.

In particular, first and second detectors31,32can be configured for detecting spacers20by optical, electrical and/or magnetic means. Properties of spacers20such as hydrophilicity and surface tension can be detected and/or measured by first and second detectors31,32.

In the following, achieving laminar flows allows for hydrodynamic flow confinement.

InFIG. 4A, sequence of separate liquid volumes23a-23cseparated by spacers20a,20bis delivered via first fluid channel13to first bypass junction18. Due to the specific ratio of second flow rate Q2to first flow rate Q1as described above, laminar flow C from first aperture15to second aperture16is formed and confined by immersion liquid10within confinement volume22that extends from below first aperture15to below second aperture16.

InFIG. 4B, first liquid volume23aflows past blocking elements21and discharges through first aperture15into confinement volume22, where first liquid volume23ais driven toward second aperture16by laminar flow C.

InFIG. 4C, first spacer20athat separates first liquid volume23aand second liquid volume23bfrom each other flows into fluid bypass17rather than passing through narrow sub-channels formed by blocking elements21.

After first spacer20ais removed from first fluid channel13, second liquid volume23bmoves toward preceding first liquid volume23aand comes into contact with it, as shown inFIGS. 4C and 4D. At the same time, an overpressure is built inside the fluid bypass due to first spacer20abeing added to the volumes of spacer fluid in fluid bypass17.

The overpressure inside fluid bypass17is reduced by releasing spacer20pfrom fluid bypass17into second fluid channel14at bypass junction19, as illustrated inFIGS. 4D to 4F.

First liquid volume23a, discharged into confinement volume22, moves with laminar flow C. Since confinement volume22is in a surface contact with deposition area4b, first liquid volume23acomes into contact with deposition area4b, and a part of first liquid volume23aand/or substances that are carried by the first liquid volume23aadheres to and/or reacts with deposition area4a. A remaining part of first liquid volume23areaches second aperture16and is aspirated into second fluid channel14, as shown inFIG. 4F.

Following first liquid volume23a, second liquid volume23bis discharged through first aperture15into confinement volume22and flows toward second aperture16, as shown inFIGS. 4G-4K. In the meantime, microfluidic probe head2is positioned above the next deposition area4c, e.g. by means of robotic arm3, such that confinement volume22is in a surface contact with the next deposition area4c. During flowing along laminar flow C within confinement volume22, a part of second liquid volume23band/or substances that are carried by second liquid volume23badhere to and/or react with the next deposition area4c. A remaining part of second liquid volume23breaches second aperture16and is aspirated into second fluid channel14, as shown inFIG. 4L.

InFIGS. 4G and 4H, second spacer20bthat separates second liquid volume23bfrom third liquid volume23creaches first bypass junction18and flows into fluid bypass17rather than passing through the narrow channels formed by blocking elements21. Third liquid volume23cmoves toward second liquid volume23band comes into contact with it. The procedure described so far for both first and second liquid volumes23a,23bapplies to the third liquid volume23cin the same manner.

First and second flow rates Q1, Q2can be synchronized (by way of control unit29, for example) such that bypassing spacer23pis inserted from fluid bypass17into second fluid channel14just when retrieved first liquid volume23aarrives in second bypass junction19, as illustrated inFIGS. 4I-4K. The retrieved first liquid volume23ais thereby separated from the preceding liquid volumes moving along second fluid channel14toward outlet30.

Accordingly, the subsequently retrieved/aspirated first and second liquid volumes23a,23bcan be separated by properly phasing the insertion of spacers20into second fluid channel14.

FIG. 5shows a schematic cross-sectional view of a second embodiment of microfluidic probe head2and the fluid connections fromFIG. 1.

The second embodiment of microfluidic probe head2includes all elements of the first embodiment ofFIG. 3. In addition, third fluid channel33is provided inside body11, and third aperture34that is fluidly connected to third fluid channel33is formed in end face12. Further, third fluid channel33is fluidly connected to additional disposal unit35that is capable of containing fluids via outlet35a.

Third flow rate Q3is applied to third fluid channel33such that liquids are aspirated through third aperture34into third fluid channel33using a corresponding pump (not shown), for example. The aspirated liquids are delivered via third fluid channel33to additional disposal unit35. Third flow rate Q3is preferably greater than first flow rate Q1in order to generate and sustain laminar flow C′ from first aperture15to third aperture34. For example, first flow rate Q1may be chosen to be 1.0 fL/s-1.0 mL/s. A ratio of third flow rate Q3to first flow rate Q1may be chosen to be 1.2-10. Third flow rate Q3may be 1.2 fL/s-10 mL/s.

Additionally, the device is preferably configured such that a distance T1 (seeFIG. 6) between first aperture15and second aperture16is greater than a distance T2 between first aperture15and third aperture34, in order to favor and sustain laminar flow C′ between first and third apertures15,34over of a flow between first and second aperture15,16. In particular, distance T2 may be less than 2.0 mm. Again, achieving such a laminar flow allows for hydrodynamic flow confinement.

With confinement volume22located between first and third apertures15,34, second fluid channel14can mainly be used for retrieving spacers20. The remaining part of liquid volumes23a-23cthat does not adhere to respective deposition areas4b-4dcan be retrieved via third fluid channel33. Accordingly, retrieving spacers20and the remaining part of liquid volumes23a-23cis carried out separately at different locations.

FIG. 6illustrates the fluid flows and the hydrodynamic flow confinement using the second embodiment of microfluidic probe head2. Immersion liquid10and body11of microfluidic probe head2are not shown.

A sequence of separate liquid volumes separated by spacers20flows along first fluid channel13toward first aperture15with first flow rate Q1. At first bypass junction18, spacers20are redirected from first fluid channel13via fluid bypass17into second fluid channel14. At second aperture16, immersion liquid10is aspirated into second fluid channel14. Aspirated immersion liquid10and redirected spacers20are delivered to outlet30via second fluid channel14.

Laminar flow C′ from first aperture15to third aperture34is formed and confined by immersion liquid10within confinement volume22′ that extends from below first aperture15to below third aperture34. Microfluidic probe head2is positioned such that confinement volume36is in a surface contact with deposition area4.

While liquid volume23amoves with laminar flow C′ inside confinement volume22′, a part of liquid volume23aand/or substances that are carried by liquid volumes23adhere to deposition area4and/or react with substances located on top of deposition area4.

The operation steps of positioning microfluidic probe head2and depositing a part of liquid volume23aand/or substances carried by liquid volume23can be repeated arbitrarily in order to deposit sequence of separate liquid volumes23a-23conto the respective deposition areas4.

FIGS. 7-9show a portion of microfluidic probe head2in enlarged views VII, VIII, IX fromFIG. 3orFIG. 5in accordance with three different embodiments. In particular, parts of first and second fluid channels9,11, blocking elements23and fluid bypass18are shown.

In all three embodiments, blocking elements21a-21care shaped as three bars each having a width W and different lengths L1-L3. For example, W is 5-80 μm, L1is 120-160 μm, L2is 80-120 μm and L3is 40-80 μm. Sub-channels36are formed between two neighboring blocking elements21and/or an inner wall of first fluid channel13, with a width of each subchannel36being the distance D between the respective blocking elements21a-21cand/or an inner wall of first fluid channel13. For example, D is 5-30 μm.

If a viscosity of the spacers is higher than the viscosity of liquid volumes23a-23c, spacers20are redirected off a front face of blocking elements21a-21crather than enter sub-channels36due to an interfacial tension between the spacers and the liquid volumes. Consequently, blocking element21a-21ccan redirect spacers20from first fluid channel13into fluid bypass17while liquid volumes23a-23cflow through sub-channels36.

In all three embodiments, first fluid channel13has a diameter A1between spacer junction28and first bypass junction18. First fluid channel13opens up to a diameter A2at the position of blocking elements21. Downstream of blocking element21until first aperture15, first fluid channel13tapers to a diameter A3at first aperture15. For example, A1is 50-200 μm, A2is 70-400 μm and A3is 20-100 μm.

InFIG. 7, fluid bypass17has a width P1at first bypass junction18and tapers down continuously toward second bypass junction19to a width P2, thereby increasing a pressure at second bypass junction19. For example, P1is 50-500 μm and P2is 10-100 μm.

InFIG. 8, the width of fluid bypass17increases from P1at first bypass junction18to P3at plane37, then decreases to P2at second bypass junction19. In this way, a greater holding capacity of fluid bypass17for containing spacers is provided. For example, P3is 100-1000 μm.

InFIG. 9, the width P1of fluid bypass17is constant up to plane38and decreases to P2at second bypass junction19. In addition or alternatively, second fluid channel11is constricted to a width B3at second bypass junction19in order to increase the pressure. For example, B3is 20-80 μm.

In all three embodiments of fluid bypass17, the pressure at second bypass junction19is increased by geometry. Accordingly, spacers20can be inserted into second fluid channel14instead of leaking into it.

The suggested method and microfluidic probe involving the suggested microfluidic probe head provide possibilities of consecutive deposition of a plurality of liquid volumes separated by spacers and enables a user to effectively remove the spacers in order to prevent them from reaching the deposition area. In particular, a physical contact of the microfluidic probe and/or the microfluidic probe head with the deposition are can be avoided.

The suggested method and microfluidic probe involving the suggested microfluidic probe head could be applied to locally performing immunohistochemistry on a surface. Multiple liquids could be delivered sequentially onto immobilized cells, formalin fixed tissue sections and/or frozen tissues. In particular, sequential exposure of biological surfaces to a primary antibody, a biotin-labeled secondary antibody and/or streptavidin/HRP with intermittent buffer washing steps could be performed in order to generate a colorimetric signal that indicates the presence (or absence) of a disease marker.

The suggested method and microfluidic probe involving the suggested microfluidic probe head could be applied for surface-based immunoassays that require high efficiency in terms of space and reagent usage, in particular for batch processing in mass manufacturing. In particular, patterning different types of proteins and/or antibodies on a surface, e.g. for the research of allergic reactions, detection of viral and/or bacterial infections, could be performed by repeating operation steps of loading the microfluidic probe with one antibody solution and purging subsequently. The suggested technology could reduce a reagent usage and time requirements for these assays.

The suggested method and microfluidic probe involving the suggested microfluidic probe head could be applied for secretome analysis by delivering a sequence of chemical stimulation pulses to cells immobilized on a surface and exposing the surface to a sequence of liquid sections. The microfluidic probe could further be used for retrieving a response of the cells to the delivered stimulation.