Device for controlling fluid motion into micro/nanochannels by means of surface acoustic waves

The present invention relates to active control of the fluid motion in micro/nanofluidic structures on chips, and more specifically it relates to a device for controlling fluid motion in micro/nanochannels according to the preamble of claim 1.

The micro/nanofluidic systems, that is with a cross-section of the fluid distribution channels comprised between 1 nanometre and 1 millimetre, represent devices which are important for biological, chemical, biochemical, optofluidic and sensing applications, given that they allow to perform many diagnostic experiments, of chemical and optical type, rapidly and in parallel, integrating many complex analytical functions, while limiting the consumption of reagents and samples, given that the volumes of liquid usually employed are comprised between 1 pico litre and a few nano litres.

In order to obtain a high efficiency and integration of these systems, an active control of the fluid motion through active elements which can be integrated on chips such as pumps, valves and mixers is required.

Unfortunately, a widespread distribution of the micro/nanofluidic devices is strongly limited by the low possibilities to incorporate these elements and ensure an accurate and reproducible control of the fluid motion at the inlet, outlet and along the path of micro/nanofluidic channels.

Other difficulties are due to some basic requirements for devices with practical use: the devices need to be relatively inexpensive, compatible with the fluid analysis processes (for example compatible with electro-osmotic flows, that is non-conductive), portable and compact, and they should have excellent sealing properties among all the elements in a manner such to bear internal or external high pressures.

Typically, polar fluids such as water solutions can fill hydrophilic micro/nanochannels (that is with a contact angle smaller than 90°) by spontaneous capillarity, but this move-

ment cannot be controlled accurately. On the contrary, in case of hydrophobic capillaries or networks of capillaries the same fluids somehow have to be forced into the mi- cro/nanochannels. The pressures required can be generated through external pumping systems such as for example syringe pumps connected to the devices. However, these systems require an ideal sealing and a number of connections to external devices which drastically reduce the portability and the compactness of the entire system.

An alternative known pumping method is based on the electro-osmotic flow control. This allows transporting fluids and it provides an almost constant velocity within the channels and it can be integrated without invasive external connections. However, this approach requires high voltage drops along the channels, amounting to hundreds or thousands of volts, and it is quite sensitive to the properties of the fluid, such as the ionic strength, pH and ionic composition, as well as to the presence of electrical charges on the surface of the channels. Furthermore, the electro-osmotic flow control technique is typically not capable of pumping liquids at high flow rates (greater than 1 mm/s) in any type of mi- cro/nano channel .

Known are other miniaturised fluid handling devices, which include controlling the fluid motion through electrostatic, piezoelectric, thermopneumatic, electromagnetic, bimettalic means and by means of shape-memory alloys. Most of these devices are made employing microelectromechanical systems (MEMS) technologies on silicon, but the poor reliability, and the high times and costs of production are aspects that limit their distribution.

Recently, studies regarding use of polymer materials, whose processing is now possible due to the latest micro/nano manufacturing technologies, such as soft lithography and imprint lithography, which allow to generate plastic micro/nanostructures in a quick manner and at a low cost with high performance, have been carried out with the aim of reducing the overall production costs of fluidic micro/nanodevices.

For example, a structure of conventional miniaturised channels can be transferred to a flexible polymer such as the polydimethylsiloxane (PDMS), with the aim of integrating pneumatically-controllable pumps and valves. The advantage of this approach lies in the

manufacturing simplicity and inexpensiveness, quick prototyping and the general biocom- patibility of the materials. Disadvantageously, the valves and the pumps thus produced are basically passive systems, due to the difficulties encountered when integrating actuator devices, typically operated pneumatically from the external. In particular, these solutions are hardly compatible with an electronic control integrated with a chip bearing a mi- cro/nanofluidic structure in combination with other analysis devices.

A micro-pumping technology worth mentioning is described in WO97/25531. This technology is based on the use an "acoustic streaming flow" mechanism for forcing fluids into capillary pipes. The "acoustic streaming flow" transport technique is based on the transfer of energy and momentum of an acoustic wave to a fluid medium present along the wave propagation path. The interaction determines a net increase of the kinetic energy of the fluid medium and its movement. This phenomenon has been known to scientists since late 1800 and it was recently studied also from the system's quantistic nature point of view. Due to the energy and momentum conservation laws, the "acoustic streaming flow" technique leads to the motion of the fluid medium to which it is applied along a direction matching the direction of propagation of the acoustic wave. Due to the progressive moving of the fluid away from the acoustic source and due to the absorption of the acoustic power by the fluid and the propagation medium, the efficiency of this pumping mechanism decreases along the path of the fluid and practically prevents the creation of long micro fluidic channels, without the inclusion of sets of micro-pumps distributed along the length of the channel.

A different type of sensors and actuators applying the "acoustic streaming flow" physical mechanism mentioned beforehand has been recently developed exploiting the characteristics of the surface acoustic waves (SAW).

A surface acoustic wave is typically generated along the surface of a piezoelectric material through metallic interdigitated transducers (IDT). The interdigitated acoustic transducers are periodic structures which consist of a succession of comb-like interposed metal strips (electrodes), connected in an alternating manner to collection tracks for distributing the electrical signal as shown in figure 1. The transducer period is determined by the distance

between opposite strips of electrodes and it determines the period of the abovementioned surface acoustic wave. The shape of a transducer may vary depending on the spectral characteristics required by the acoustic wave. The periodicity of the transducer and the velocity of the acoustic wave in the piezoelectric medium determine the resonance frequency for SAW excitation. According to the standard configuration of SAW propagation on a lithium niobate substrate, the resonance frequencies of a transducer are typically of the order of magnitude of tens or hundreds of MHz. Different operation frequencies, i.e. different transducer periodicities, can be employed depending on the typical sizes of the mi- cro/nanofluidic network to which the transducer is coupled.

Figure 1 shows a typical configuration of an interdigitated transducer for the generation of a surface acoustic wave.

The acoustic wave is generated at the free surface of a piezoelectric substrate with elastic properties by the application of an alternating voltage at the metal electrodes of the transducer. When a radio frequency voltage is applied across the transducer electrodes, the piezoelectricity properties of the substrate allow the conversion of the electrical signal into a lattice deformation of the substrate material, hence generating a periodic deformation of the free surface of the substrate beneath the transducer, capable of exciting surface acoustic waves which are propagated at the substrate/air interface. Due to the transducer symmetry, two acoustic waves propagated in opposite directions are emitted off the transducer ends. The propagation can be easily made unidirectional by including known reflectors and absorbers in the geometry of the device.

SAWs are also exploited for transporting and moving small amounts (drops) of liquid deposited on free surfaces, along the direction of propagation of the acoustic wave.

This technology also allows the division and mixture of small amounts of fluid, for example for controlling the movement of reagents provided in form of drops on the surfaces having regions with different chemical functions.

This technology was employed to develop efficient micromixers, included in microfluidic

systems to improve mixing operations at the low Reynolds number regimes, strongly improving various biochemical reactions.

The object of the present invention is to provide an improved technique for controlling the fluid motion while entering, exiting and along micro/nanofluidic structures, integrated on chips.

According to the invention such object is obtained due to a device having the characteristics outlined in claim 1.

In brief, the present invention is based on the principle of controlling the motion of a fluid element by means of surface acoustic waves propagated in a direction opposite with respect to the desired direction of movement of the fluid, and it regards the control of the inlet, outlet and motion of the fluids in micro/nanochannel networks through pumping systems based on counter-propagation of surface acoustic waves generated by interdigitated transducers on piezoelectric materials.

This mechanism differs from the traditional fluid motion induction method by means of an "acoustic streaming flow" technique, and it leads to a much more efficient pumping and handling of fluids in nano/microchannels.

Furthermore, this device geometry allows an efficient construction of micro-pumps and microvalves integrated in the microfluidic systems, therefore providing and effective possibility of assembling and integrating systems.

Herein described are also processes for manufacturing micro/nanofluidic devices on chips, including integrated active elements for a complete control of the fluid flow in mi- cro/nanochannels.

A micro/nanofluidic device is made by overlapping two functional layers, respectively a piezoelectric substrate, adapted to support the propagation of surface acoustic waves generated by means of micro fabricated interdigitate transducers, and a structured layer made

of plastic material, in which a micro/nanochannels network required for the intended application is defined.

When implementing the device on chips, the channel structured layer is aligned with the transducers in such a manner that the digital electrodes of the transducers are arranged in an orthogonal manner with respect to the lateral walls of the channels to which they are associated.

The substrate currently preferred for SAWs excitation is Lithium Niobate (LiNbOs), though other substrates like quartz, BiI ₂ GeO ₂ O or LiTaO ₃ or any other piezoelectric material having sufficient piezoelectric coupling can also possibly be used.

SAW generation is obtained by applying an alternating voltage on the interdigitated transducer electrodes deposited on the piezoelectric substrate surface. In a preferred embodiment, transducers whose configuration comprises an array of pairs of electrodes polarised with an alternating polarity are used.

The transducers are fabricated by means of standard metal film deposition techniques such as electron beam lithography or photolithography, metal evaporation, lift-off to name a few examples.

The surface of the piezoelectric substrate can be coated by a thin film made of inert material, for example silicon oxide, whose thickness is much smaller with respect to the length of the surface acoustic wave, adapted to form a base layer suitable for biological and biochemical applications.

The channel structured layer is preferably fabricated by means of replica molding techniques, that is by preparing an elastomeric replica mold by means of casting the prepoly- mer liquid of an elastomer against a mold patterned to form a relief structure similar to the model of the desired channel structure. The subsequent thermal curing leads to the transfer of the configuration from the master mold to the elastomeric replica. The same technique can be used to generate polymer replicas using a solution of a thermoplastic polymer in its

solvent. Other different techniques and materials can be used to obtain a network of micro- channels, including polymers, semiconductors, glasses, ceramics, plastics, etc. Various plastic materials provide an excellent mechanical and thermal stability and high resistance to solvents, acids and bases.

The currently preferred choice of a replica molding technique for implementing polydi- methylsiloxane (PDMS) channels offers various advantages:

- polydimethylsiloxane, due to its elastic characteristics, can be released easily, also by complex and fragile structures;

- polydimethylsiloxane replicas can be typically conformed to the substrate on a relatively large area without chemical treatments of the substrate surface;

- polydimethylsiloxane provides a surface with a lower interface free energy (21 6x10 ³ J/m ² ) and chemically inert, making the related devices suitable for biological applications;

- devices can be quickly prototyped, with low production costs.

Conveniently, the final device is assembled by overlapping the channel structured layer on the piezoelectric substrate bearing the transducers and by subsequent sealing (thermally, by photon irradiation, by exposure to plasma, chemically, etc) reversibly or irreversibly for the definition of closed channels.

In order to generate a surface acoustic wave on the substrate, the transducers are connected to an external alternating voltage source through the collector strips of its electrodes.

Through a suitable choice of the shape of the device, in particular the arrangement of the transducers adapted to generate surface acoustic waves whose wavelength is smaller than the typical dimensions of the channel, the propagation of the surface acoustic waves leads to a pressure gradient along the channel, which produces a net force on the fluid directed in the opposite direction with respect to the wave propagation direction. The fluid is thus propagated towards the opposite direction with respect to the spreading direction of the acoustic wave.

It was ascertained that this effect is more efficient and flexible with respect to the traditional "acoustic streaming flow" technique, which necessarily forces the fluid to move along a direction matching the direction of propagation of the acoustic wave.

Therefore, the configuration proposed does not require the acoustic wave propagation along the entire micro/nanofluidic channel, but a strong interaction is required only at the liquid/air interface (free channel). This allows to achieve full control of the fluid flow by means of a single transducer. As a matter of fact, the propagation of the acoustic wave from the transducer to the fluid element (drop) to be dragged without a substantial deterioration of the wave and energy drop, and at greater distances, given that the propagation of the surface acoustic wave occurs at the interface between the piezoelectric substrate and the air.

Once the surface acoustic wave is turned off, the fluid motion is therefore immediately in- terruptible given that no more energy is transferred to the fluid, which is held within a hydrophobic channel where the spontaneous capillarity is suppressed. The flow can be restored subsequently by reactivating the acoustic wave.

For example, according to the described technique it is possible to control a fluid element (a drop) from the inlet to the outlet of a channel 1 mm long, 500 microns wide and 50 microns high in a few seconds.

Furthermore, the filling or travel velocity of the channel can be tuned by adjusting in a controlled manner the power of the dragging surface acoustic wave, that is the power of the electric signal applied to the interdigitated transducer. This allows using a single mi- cro/nanochannel integrated with a plurality of transducers as a means for dispensing small volumes of fluid (from a few pico litres to a few microlitres).

By using the device described according to the invention, it is possible to make micro/nanofluidic devices with active micro-pumps and valves integrated, each determined by a selective configuration of active transducers and channel ducts, which do not require any external actuator element and can be controlled entirely by electronic means. A single

transducer can be used as a pump or as a valve to achieve fluid flow control into a single micro/nanochannel, in such a manner to obtain a very small packaging volume and an ideal integration into complex micro/nanofluidic devices.

In particular, the approach subject of the invention can be used in hydrophobic mi- cro/nanochannels, which advantageously reduce the adsorption of molecules dissolved in the moving fluids (proteins, enzymes, colorants, and other organic molecules) on the channel surfaces, and eliminate problems related to spontaneous capillary motion, which thus is largely uncontrollable.

Further characteristics and advantages of the invention shall be outlined further in detail in the subsequent detailed description, provided for exemplifying and non-limiting purposes, with reference to the attached drawings, wherein:

Figure 1 is an exemplifying representation of a surface acoustic wave generating in- terdigitated transducer according to the known art;

Figure 2 is a flowchart of an exemplary embodiment of a micro/nanofluidic structure fabrication process according to the invention;

Figures 3a-3i are cross-sectional schematic views of the elements which intervene in the micro/nanofluidic structure fabrication process according to the flow chart of figure 2;

Figures 4a and 4b are axono metric representations of a micro/nanofluidic device comprising a complex network of micro/nanochannels and transducers for generating surface acoustic waves according to a device for controlling the fluid motion which is the subject of the invention; and

Figure 4c is an exemplifying representation of a fluid motion control configuration according to the invention.

Referring to figures 2 and 3 described hereinafter is a succession of steps for a micro/nanofluidic structure fabrication process.

In a first step 102 a resist is applied onto a substrate 10, for example a silicon substrate.

Preferably, the resist is applied on the substrate by means of a spin coating technique, forming a resist layer 20 having a thickness comprised between a few nanometres and hundreds of microns, at the desired height (depth) of the channel (figure 3 a).

In a subsequent step 104 the resist layer 20 is structured, for example by exposure to radiation by means of an aligning mask, leading to a structured resist layer 21 (figure 3b).

In a step 106 the layer 21 is baked in order to selectively cross-link the structured portion of the layer itself and in a step 108 the layer is developed in order to produce a patterned resist layer 22 (figure 3 c).

After a subsequent bake curing process (for example for approximately 15 minutes at a temperature of 180 ⁰ C) in a step 110, the layer 22 is ready to be used as the mold master to replicate the master features on thermoplastic or elastomeric materials as shown in figures 3d-3i.

In a step 112 a liquid prepolymer layer 23, is cast on the master fabricated in the previous steps (figure 3d). In the subsequent step 114 the prepolymeric fluid is polymerized in situ forming a related layer 24 (figure 3e), e.g. by baking at a temperature of 140 ⁰ C for 15 minutes, according to standard replica molding processes. In the subsequent step 116 the polymeric mold is peeled off from the master in such a manner to obtain a patterned polymer layer 25 with the negative shape of the master layer 22 (figure 3f).

A nano-imprinting or hot-embossing process can be used as an alternative to steps 112- 116.

In a step 118 a polymer substrate 28 is inserted into a molding machine, directly in contact with a lower heater layer 26 and with layer 22, then heated to a temperature exceeding the glass transition temperature T _g of the polymer (figure 3g).

In a subsequent step 120 the layer 22, using an upper heater layer 27, is pressed against the polymer substrate 28 transferring its negative configuration therein (figure 3h). After a cer-

tain contact time between layer 22 and layer 28, in step 122, the product is cooled to a temperature below T _g , then the master is separated from the polymeric substrate. As a result a patterned polymeric layer 30 with the negative shape of the master is obtained as shown in figure 3i.

There are categories of polymers (for example, thermoplastic polymers) which can be modelled using both the replica molding techniques (steps 112-116) and the nano- imprinting techniques (steps 118-122) and categories of polymers (for example, elastomers) which can be modelled using only the replica molding technique.

Subsequently, in a step 124 the device is assembled by overlapping the structured layer 25 or 30 bearing the predetermined network of channels on a piezoelectric substrate 31, providing a reversible or irreversible sealing.

Given that the single interdigitated transducers can be used as pumps or valves, different geometries can be exploited successfully to make portable and compact so-called "lab-on- chip".

Shown in figure 4a and 4b is an example of a prototype device comprising six transducers Ti-T ₆ deposited on the piezoelectric substrate 31 for controlling the motion of one or more fluid elements in a complex network of micro/nanochannels defined in the structured layer 30. The transducers are connected to an electronic control circuit P, for example a microprocessor unit, provided for their selective activation and the adjustment of the SAWs propagation energy, depending on the desired application.

This configuration allows various operations, such as mixing fluids at the channel crosses or splitting an amount of fluid by simultaneous activation of the transducers associated to different channels when a fluid element is at the cross region of the abovementioned channels. Microchambers could also be added as reaction chambers or reagent products collection chambers.

The arrangement of figure 4a, 4b is a result of an accurate alignment of the substrate surfaces for supporting the active elements (transducers, serving as pumps and valves) and the structured layer which defines the micro/nanochannels and possible reservoirs. Such alignment can for example be provided with the help of microscopes or specific instruments for aligning two surfaces (for example, mask-aligners).

Elastomeric materials are suitable to generate polymeric stamps incorporating micro/nanochannels and reliably ensuring a good conformal contact with different surfaces, therefore a good sealing of the assembled devices. Other polymer materials can be used to generate substrate 31 , as long as it ensures a good sealing with surface, thermal, optical or chemical bonding techniques between layer 31 and substrate 30. Such bonding process can be reversible if the stamp used is made of elastomeric materials, or irreversible if based on chemical, optical or thermal treatments.

To obtain such reliable bonding between the abovementioned layers and preserve the characteristics of the transducers, deposition of thin films of polymeric or ceramic materials on the piezoelectric surface may also be required.

In the structure illustrated in figure 4a, 4b fluid elements such as drops or the like can be controlled accurately and dragged form one or more inlets IN to one or more outlets OUT along the micro/nanochannels C. As better indicated in figure 4c, the motion of a fluid element occurs towards the direction indicated by arrow F and it is controlled by a surface acoustic wave SAW which is propagated towards the opposite direction, generated by an interdigitated transducer T arranged in proximity to the channel outlet OUT.

Specifically, the travel process of the channels can be interrupted with immediate effect by interrupting the radio frequency supply of one or more corresponding transducers associated to the generation of the surface acoustic waves, and the travel velocity of the channels can be tuned by modifying the power of the signal applied to the transducers.

For example, providing an interdigitated transducer formed by 20 pairs of electrodes with a periodicity of 24 microns and an aperture of 500 microns, deposited on the lithium niobate

substrate, can drag a drop of water in 1.3 seconds into a microchannel made of PDMS 500 microns wide, 50 microns high and 1 mm long, with moderate power supplied to the transducer (20 dBM). A drop of water can be completely dragged from the inlet IN to the outlet OUT of the microchannel C within 2 seconds. Furthermore, the fluid motion can be quickly stopped by "turning off" the surface acoustic wave.

Advantageously, the interdigitated transducers can also be used for localising the position of a fluid element according to the known art of analysing SAW echo signals.

Therefore, the method described offers a straightforward and efficient process for providing micro/nanofluidic devices with integrated active pumps and valves thus improving mi- cro/nanofluidic devices in terms of performances for industrial application.

Conveniently, the micro/nanochannels can be fabricated with different materials to obtain monolithic or hybrid channel structures, for example made of ceramic, glass, inorganic or organic semiconductor, or polymer materials.

A device as shown in figure 4b can be used as a dispensing device for controlling the fluid volume at the outlet or inlet of micro/nanochannels or for dragging a fluid drop from the inlet to the outlet of one or more micro/nanochannels. This allows a quick transport of a small volume of fluid (from a few millilitres to a few pico litres) in different regions of a chip through the travel of one or more micro/nanochannels connected or not connected to a network.

The fluid can be a liquid, a molten polymer or a polymer solution, a solution containing nanoparticles, organic molecules, inorganic molecules or their blends, a solution containing biological molecules, or a gas.

A microstructured device subject of the invention can be used to control biochemical reactions, genomic and proteomic reactions and complex analysis systems, for example for chemical or biochemical applications in which solvent-resistant, acid and/or base resistant, heat-resistant or chemically inert materials are required. Furthermore, it can be used for

biological applications in which the materials used must have a specific gas permeability, or be materials capable of preventing the adsorption of molecules and their unfolding at the surfaces.

Further applications provided for are optofluidic applications, including fabrication of laser devices, LED light-emitting devices, polarisers, filters, and waveguides.

Obviously, subject to the principle of the invention, the implementations and details of embodiment may widely vary with respect to the points argued and illustrated solely for exemplifying and non-limiting purposes, without for this reason departing from the scope of protection of the present invention as defined by the attached claims.