Shape memory alloy working fluid subassembly and device incorporating same

Abstract

The present invention is specific to the SMA actuation fluid subassembly (10) and an apparatus incorporating same as a dispensing device, whereby actuation of the SMA wires (16, 16') causes a fluid-tight reservoir (17") to be compressed by the lid (18) reducing its volume from a maximum volume Vo to a minimum volume V1, which reduces the pressure and opens the outlet flap (13") to dispense fluid through the outlet channel (13).

Description

[Technical field]

[0001] The present invention is specific to a shape memory alloy (SMA) actuated fluidic subassembly. [Background technology]

[0002] Generally speaking, the use of SMA wire as the actuation element offers various advantages over other actuation systems in terms of weight, power consumption, and cost, with the actuation element taking advantage of the ability of a suitably trained SMA wire to shorten when heated, most typically by the Joule effect via an appropriate current supply.

[0003] An area in which the advantages offered by the use of SMA wires have long been recognised is fluidic valve control, as described in references 1 and 2, and particular applications for which these advantages are particularly relevant are microfluidic valves and so-called "lab-on-a-chip" applications, as described in "Electronics for Microelectronics and Microelectronics", Vol. 1, No. 1, 2008, in which an SMA wire is looped around a flexible channel, the diameter of which is controlled until its closure. The solution described in Vol. 1 is far from ideal with regard to the stresses on the channel when it needs to be completely closed.

[0004] The use of SMA wires in fluidic modules is also described in US Pat. No. 5,399,436 to drive a plunger to reduce the fluid dispensed volume to zero, but such a solution is not optimal since repeated contact and impact of the plunger can lead to the release of particulates that contaminate the dispensed fluid. In the same field, a similar solution is disclosed in US Pat. No. 5,499,436, but this does not describe how to effectively achieve an SMA-based system capable of reliable operation, i.e., capable of receiving suitable loads, and also relies on spring-loaded poppets to close the inlet and outlet valves via flexible membranes in contact with the dispensed fluid. More specifically, as outlined in US Pat. No. 5,399,436, a poorly designed SMA-based actuator leads to excessive fatigue of the SMA wire and its breakage / failure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Pat. No. 3,835,659 [Patent Document 2] U.S. Pat. No. 4,973,024 [Patent Document 3] European Patent Application Publication No. 1552146 [Patent Document 4] US Patent Application Publication No. 2012 / 0209189 [Patent Document 5] U.S. Pat. No. 9,068,561 [Patent Document 6] U.S. Patent No. 6,835,083 [Patent Document 7] U.S. Patent No. 8,739,525 [Non-patent literature]

[0006] [Non-Patent Document 1] “Electronic control of elastomeric microfluidic circuits with shape memory actuators” by Vyawahare et al, published in 2008 on labchip, number 8, pages 1530-1535 [Non-Patent Document 2] “Thermomechanical fatigue of shape memory alloys” by Lagoudas et al, published in 2009 in Smart Materials and Structures, Volume 18, Number 8 [Non-Patent Document 3] “A Study of the Properties of a High Temperature Binary Nitinol Alloy Above and Below its Martensite to Austenite Transformation Temperature” by Dennis W. Norwich presented at the SMST 2010 conference [Non-Patent Document 4] “Fabrication Process and Characterization of NiTi Wires for Actuators” Tuissi at al. Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE PRESENT EMBODIMENT It is an object of the present invention to overcome the drawbacks of prior art SMA-based fluid actuators. [Means for solving the problem]

[0008] The first aspect resides in an SMA actuation fluid subassembly as recited in the claims of this application. [Brief description of the drawings]

[0009] [Figure 1A] FIG. 2 is a schematic diagram of a cross-sectional view of an SMA actuation fluid subassembly according to the present invention in an actuated state at rest position. [Figure 1B] FIG. 2 is a schematic diagram of a cross-sectional view of an SMA actuating fluid subassembly in accordance with the present invention in a first operating state. [Figure 1C] FIG. 2 is a schematic diagram of a cross-sectional view of an SMA actuating fluid subassembly in accordance with the present invention in a second operating state. [Figure 2A] FIG. 2 is a schematic diagram of a second embodiment of an SMA actuating fluid subassembly in accordance with the present invention, viewed from above. [Figure 2B] 2B is a schematic diagram of a second embodiment of an SMA actuating fluid subassembly according to the present invention, taken along the line AA' of FIG. 2A in an actuated state at rest position. [Figure 2C] 2B is a schematic diagram of a second embodiment of an SMA actuating fluid subassembly in accordance with the present invention, taken along cross section AA' of FIG. 2A, in a first actuating state. [Figure 2D] 2B is a schematic diagram of a second embodiment of an SMA actuating fluid subassembly in accordance with the present invention, taken along cross section AA' of FIG. 2A, in a second actuating state. [Diagram 3] FIG. 11 is a schematic diagram from above of a preferred method for making the inlet and outlet flaps used in the second embodiment. [Figure 4] FIG. 11 is a schematic view from above of a modified example of the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The invention is further explained with the aid of the following drawings.

[0011] In the drawings, the sizes and dimensional ratios of the various elements shown in some cases have been altered to facilitate understanding of the drawings, with particular but not limited reference to the diameter of the SMA wires, the size of the biasing elements such as return springs, and the length / thickness / width of the fluid-tight sealing elements. Furthermore, crimping means for the SMA wires as well as current conducting elements for operating them are not shown, as they are well known to those skilled in the art and are not necessary for understanding the invention.

[0012] A schematic diagram of a cross-sectional view of a first embodiment of an SMA actuation fluid subassembly according to the present invention is shown in Figure 1A. Subassembly 10 has a base 11 with an inlet channel 12 and an outlet channel 13 separated by a wall 14. Inlet channel 12 and outlet channel 13 communicate via inlet port 12' and outlet port 13', respectively, with a fluid-tight reservoir defined by a shell 17", preferably made of a rubber material, compressible from a maximum volume Vo to a minimum volume V1 by a lid 18.

[0013] The movement of the lid 18 is driven in compression from Vo to V1 by a pair of SMA wires 16, 16' connecting the base 11 to the lid 18, while being driven in expansion from V1 to Vo by return springs 17, 17'.

[0014] The inlet port 12' and the outlet port 13' are normally closed by flaps 12" and 13", respectively, meaning that a certain force is required to open them. The closing force can be provided by various means, for example in the embodiment shown in FIG. 1A, the flaps 12" and 13" abut against and pivot on the base 11, such force can be provided by a torsion-loaded spring which also acts as an anchoring element for the flaps 12", 13" on the base 11. In other words, it is preferred to have the ports 12', 13', respectively, closed by flaps 12", 13", longer than the port width, defined as the width of the opening between the inlet / outlet channels 12, 13 and the fluid-tight compressible reservoir 17.

[0015] FIG. 1A is a cross-sectional schematic view of subassembly 10 from a structural standpoint, while FIGS. 1B and 1C are cross-sectional schematic views of a first operating state of subassembly 10, such as fluid distribution (FIG. 1B), and a second operating state of subassembly 10, such as fluid loading (FIG. 1C).

[0016] In particular, during the dispensing phase, actuation and resulting shortening of the SMA wires 16, 16' causes the lid 18 to move down towards the base 11, compressing the shell 17" and springs 17, 17' and thus reducing the volume Vo towards V1. The pressure increase is such that it overcomes the resistance of the torsion spring of the flap 13" which is pivoted under the outlet port 13', thus rotating the flap 13" downwards and allowing a precise amount of fluid (essentially Vo-V1) to be dispensed through the port 13' in a very reproducible manner. Conversely, as the flap 12" pivots over the port 12', the pressure increase in the shell 17" keeps the port 12' tightly closed preventing backflow of fluid into the inlet channel 12.

[0017] After the dispense phase, the SMA wires 16, 16' are deactivated, whereby the return springs 17, 17' move the lid 18 towards its starting position to restore the volume Vo. This movement causes a pressure drop in the reservoir 17'', leading to the opening of the inlet port 12' through upward rotation of the flap 12" against the resistance of its torsion spring, leading to the filling of the reservoir 17" through fluid suction. Conversely, as the flap 13" pivots under the port 13', the pressure drop in the shell 17" keeps the port 13' tightly closed, preventing backflow of fluid from the outlet channel 13.

[0018] Of course, for the system to perform cyclical dispensing and filling, a first cycle is required with only air exiting the outlet port 13' while the pressure drop due to deactivation of the SMA wire and action of the return springs 17, 17' fills the reservoir 17" with the fluid to be dispensed.

[0019] 2A-2D, a subassembly 20 has a base 21 having an inlet channel 22 and an outlet channel 23, separated by a wall 24, which communicate with a fluid-tight reservoir via an inlet port 22' and an outlet port 23', respectively, that are normally closed by respective flaps 22" and 23", as in the first embodiment described above. The closing force can be provided by various means, for example in the embodiment of FIGS. 2A-3, a preferred method is shown in FIG. 3 in which the flaps 22", 23" are continuous elastic membranes 25 with two cut-outs in an inverted C-shape, which are located at the ceilings of the channels 22, 23 and penetrate the wall 24, and the closing force is provided by the resistance to deformation of the material of the membrane 25.

[0020] As in the first embodiment, the inlet / outlet ports are closed by flaps longer than the port width and abut the base, but in this case both flaps are located below their respective ports, whereby wall 24 extends below port 22' towards inlet channel 22 to provide an abutment for flap 22".

[0021] In this second embodiment, the fluid-tight reservoir is defined by a dome-shaped elastically deformable element 28, the periphery of which is blocked by a gasket 281 held in an annular sheet 280 fixed to the base 21. The SMA wire 26 passes through the top of the dome 28 and through a suitable groove provided in the sheet 280 for connection to fixing pillars 26', 26" fixed to the base 21.

[0022] FIG. 2B shows a cross-sectional schematic of subassembly 20 from a structural point of view, while FIGS. 2C and 2D show the subassembly during the stages of fluid dispensing (FIG. 2C) and fluid loading (FIG. 2D).

[0023] In particular, during the dispensing phase, actuation of the SMA wire 26, preferably via a suitable coupler 29 located at the dome apex, will push the deformable elastic element 28 downwards, thus reducing the volume Vo towards V1, due to the deformation of the element 28 allowed by the compressible gasket 281 housed in the seat 280, i.e. the periphery of the deformable element 28 is pushed deeper into the elastic gasket 281.

[0024] As in the first embodiment, an increase in pressure in the reservoir results in flap 23" being pushed down against the resistance of membrane 25, opening outlet port 23' and allowing fluid communication between the reservoir and outlet channel 23 (Figure 2C). Conversely, as flap 22" abuts against wall 24 from above, an increase in pressure keeps port 22' tightly closed, preventing backflow of fluid into inlet channel 22.

[0025] When the SMA wire 26 is deactivated, the element 28 returns to its original shape by its elasticity and by pushing the gasket 281 along its periphery, thus recovering the maximum volume Vo. This is due to the elasticity of the flap 23″ returning to its undeformed position and the opening of the inlet port 22′ by the upward deformation of the flap 22″, resulting in a pressure drop in the reservoir leading to the closing of the outlet port 23′.

[0026] As with the first embodiment, the subassembly again needs to undergo a first loading cycle to prepare it for the subsequent series of dispensing operations.

[0027] A variation of the second embodiment is shown in Figure 4, which shows a subassembly 40 having substantially the same structure and operation as the subassembly 20 shown in Figures 2A-2D, but with a different number of SMA wires. Indeed, in the subassembly 40, actuation is provided by four SMA wires 461, 462, 463, 464, each connected at a first end to a common coupler 49 located at the top of the dome 28 and at a second end to a respective fixed pillar.

[0028] Other possible variations not shown in the drawings could use only two aligned SMA wires, i.e. one of the pairs 461 / 463 or 462 / 464, or two orthogonally intersecting SMA wires, i.e. SMA wire 26 and another wire orthogonal to it. Furthermore, the pivoted flaps 12", 13" of the first embodiment could be used in place of the membrane flaps 22", 23" in the second embodiment and vice versa. Also, the SMA wires 16, 16' of the first embodiment could be replaced and / or integrated with one or more SMA wires passing through the lid 18, similar to the SMA wire 26 of the second embodiment.

[0029] It is emphasized that the term "flap" is not limited to a particular geometric shape or configuration, but rather encompasses any equivalent element capable of opening and closing the inlet and outlet ports under the action of the pressure change caused by the decrease in reservoir volume from Vo to V1 and the increase in reservoir volume from V1 to Vo, according to the mechanism described above.

[0030] It should also be emphasized that the total force of actuation should be evaluated taking into account all contributions acting on the SMA wire, which is the only active element causing the compression of the fluid-tight reservoir: in the case of a single SMA wire it is the sum of all restoring forces acting on the SMA wire, and in the case of multiple SMA wires it is the sum of all restoring forces acting separately or in common on the different SMA wires.

[0031] Thus, in the embodiment of FIG. 4 the force should be considered as the sum of all restoring forces acting on the wires 461, 462, 463, 464 from a common restoring element, i.e. the elastically deformable element 28 and the peripheral gasket 281, whereas in the first embodiment the restoring forces are provided separately by the return springs 17, 17′ and the shell 17″. It is also possible to have additional elastic restoring means, for example the second embodiment could include a return spring arranged between the wall 24 and the dome 28.

[0032] The present invention sets the right limits on the restoring force exerted by the elastic means, such that the total restoring force F of said elastic means, expressed in Newtons, is given by:

[0033]

number

[0034] Here, Vo / V1 is between 2 and 5. For example, for a system operating at a compression ratio of 3.5 (i.e., Vo / V1=3.5), the total restoring force F acting on the SMA wire should have an upper limit not exceeding 2.625 N and a lower limit not falling below 0.35 N.

[0035] Operating with too low a return force, e.g., 0.1 N, results in limited pressure levels that are insufficient to pump fluid through the channels and, particularly in the case of membrane flaps, too low a pressure differential to reliably operate the inlet and outlet flaps. Operating with too much force, e.g., 6 N, limits the speed of actuation and increases fixation requirements, necessitates high-strength SMA wires with voltage requirements in excess of 5V, and precludes the use of standard USB power.

[0036] The preferred maximum volume Vo of the SMA actuation fluid subassembly according to the present invention is between 100 and 500 μl.

[0037] With regard to the inlet and outlet flaps, preferred are the bendable flaps shown in Figures 2B to 2D having a thickness of 50 µm to 250 µm, more preferably the flaps are obtained from cut-outs of a membrane made from a material having a Young's modulus of 0.001 GPa to 0.05 GPa.

[0038] Although the invention is not limited to a specific SMA, the use of a Ni-Ti based alloy such as Nitinol, which can alternately exhibit superelastic or SMA behavior depending on the processing, is preferred. The properties of Nitinol and the methods making it possible to achieve them are widely known to those skilled in the art, see for example "Nitinol: a new class of alloys for mechanical properties and mechanical properties of silicon-based materials", Journal of Mechanical Engineering, Vol. 1, No. 1, 2002, pp. 1171-1175, 2002.

[0039] Nitinol can be used as is, or its transition temperature characteristics can be adjusted by adding elements such as Hf, Nb, Pt, Cu, etc. The appropriate selection of material alloys and their characteristics are generally known by those skilled in the art, see, for example, "Nitrinol: A New Approach to the Study of the Properties of Nitinol," IEEE Transactions on Materials Science and Engineering, Vol. 13, No. 1, pp. 1111-1115, 2002.

[0040] SMA wires can also be used "by themselves" or with coatings / sheaths to improve their thermal management, i.e., their cooling after deactivation. The coating sheath can be uniform, as described in US Pat. No. 5,399,633, which teaches how to manage residual heat by relying on an electrically insulating coating that is a thermal conductor, but US Pat. No. 5,499,633 discloses that the SMA wire is equipped with an enclosing sheath that can improve cooling after every actuation cycle. Also, coatings made of or including a phase changing material can be used to advantage, as described in US Pat. No. 5,499,633.

[0041] The diameter of the SMA wire is preferably 50 to 150 μm.

[0042] In its second aspect, the present invention comprises an apparatus incorporating a dispensing apparatus including an SMA actuating fluid subassembly as described above. Preferably, such a device is a drug (consumable) cartridge or an analytical instrument, more preferably for lab-on-a-chip applications.

Claims

1. 1. A shape memory alloy working fluid subassembly comprising: a base having an inlet channel, an outlet channel and a channel separation wall; a compressible, fluid-tight reservoir in communication with the inlet channel through an inlet port closed by an inlet flap and in communication with the outlet channel through an outlet port closed by an outlet flap; At least one shape memory alloy wire configured to, upon actuation, act on a fluid-tight reservoir lid to compress said fluid-tight reservoir from a maximum volume Vo to a minimum volume V1; elastic return means for restoring the volume of the compressible fluid-tight reservoir from V1 to Vo upon deactivation of the at least one shape memory alloy wire; Equipped with The total return force F of said elastic return means, expressed in Newtons, is given by: [0010] and Vo / V1 is 2 to 5. A shape memory alloy working fluid subassembly comprising:

2. 2. The shape memory alloy working fluid subassembly of claim 1, wherein Vo is comprised between 100 μl and 500 μl.

3. The shape memory alloy working fluid subassembly according to claim 1, wherein the inlet and outlet flaps have a thickness comprised between 50 μm and 250 μm.

4. The shape memory alloy working fluid subassembly of claim 1 , wherein said inlet flap is longer than a width of said inlet port and said outlet flap is longer than a width of said outlet port.

5. 10. The shape memory alloy working fluid subassembly of claim 1, wherein the lid is a rigid lid in contact with one or more elastic return elements acting in opposition to one or more shape memory alloy wires.

6. The shape memory alloy working fluid subassembly of claim 5 , wherein the elastic return element comprises one or more springs and / or a rubber shell that defines a maximum volume Vo in an uncompressed state.

7. 2. The shape memory alloy working fluid subassembly of claim 1, wherein the lid is a dome-shaped deformable elastic element, the periphery of which is blocked by a gasket held in an annular sheet fixed to the base, and at least one shape memory alloy wire passes through the top of the lid and acts thereon through a coupler located at the top.

8. 8. The shape memory alloy working fluid subassembly of claim 7, wherein the action is provided by four shape memory alloy wires, each having a first end connected to a common coupler located at the top of the dome-shaped lid and a second end connected to a respective fixed pillar.

9. 10. An appliance incorporating a dispensing apparatus including a shape memory alloy working fluid subassembly according to claim 1, said appliance being a fluid dispenser.

10. An analytical system that is a lab-on-a-chip incorporating the device according to claim 9.

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