TEXT OF THE DESCRIPTION

The present invention relates to an actuator device comprising at least one chamber containing a liquid actuation medium, said chamber comprising at least one semi-permeable wall portion, which separates said liquid actuation medium from a medium external to said chamber.

Applications of the device described herein refer to the medical sector, for example actuators for endoscopy.

Prior art

In the microactuation sector, i.e., in the sector of actuators of microscopic dimensions, for example sub-millimetric dimensions, different principles and approaches are exploited. In general, high actuation speeds are achieved via actuators that exploit electrical energy, for example dielectric-elastomer actuators (DEAs). An alternative to the direct use of electricity is represented by the use of actuators driven by pressure. This occurs, for example, in “soft robotics”, where fluidic or pneumatic actuators are frequently used.

The main problem of the above category of actuators is that they require external devices for generating appreciable pressure variations. Pumps and compressors are cumbersome and heavy. Also known are spontaneous actuators, which respond to physical/chemical conditions of an environmental nature and thus usually operate with low energy levels. There exist different types of spontaneous actuators that belong to this category, a typical example being actuators based upon osmosis. Such actuators present, however, the drawback of low rapidity of actuation, typically on a time scale of minutes or hours. The fastest known osmosis actuators exploit bistable structures. In this way, it is possible to operate with actuation speeds on the scale of seconds. Another drawback of osmosis actuators is that they do not have dimensions smaller than one millimetre. An example of such actuators is contained in the document by Sinibaldi E. et al., “Another Lesson from Plants: The Forward Osmosis- Based ActuatoF, PlosOne, 2014, which describes a low-energy- consumption actuator capable of generating adequate forces during a characteristic actuation time of minutes. The device is a forward-osmosis actuator with a typical size of 10 mm and a characteristic time of 2 to 5 minutes. The actuator is made up of four main elements: a reservoir chamber, an osmotic membrane that separates it from an actuation chamber with a rigid boundary portion and a compliant or elastic boundary portion. The latter is used to transduce the actuation force. The work of osmotic actuation is garnered/transduced through swelling of an elastomeric disk.^Hence, in general there is not available in the prior art an actuation device that is fast and at the same time capable of operating with contained levels of power consumption.

The object of the present invention is to provide an actuation device that will overcome the above drawbacks.

According to the present invention, the above object is achieved thanks to an actuation device, as well as to a corresponding actuation method, that present the characteristics recalled specifically in the ensuing claims.

The invention will now be described with reference to the annexed drawings, which are provided purely by way of non-limiting example and in which:

- Figures 1 , 2, and 3 show the operating principle of the actuation device described herein in three operating steps;

- Figures 4A and 4B are schematic illustrations of an embodiment of the actuation device described herein in cross-sectional and perspective view;

- Figure 5 is a perspective view of the actuation device described herein;

- Figures 6 to 9 are schematic illustrations of the actuation device of Figures 4A and 4B in four operating steps;

- Figures 10 to 12 show diagrams of quantities regarding operation of the actuation device of Figures 4A and 4B in different geometrical proportions;

- Figures 13 and 14 show a structure of actuation devices in two different operating steps;

- Figure 15 are schematic illustrations of the actuation device described herein in a further embodiment;

- Figure 16 are schematic illustrations of the actuation device described herein in a further embodiment;

- Figures 17A, 17B, and 17C show an application in a device that comprises the actuation device described herein; and

- Figure 18 shows a perspective view of a constructional detail of the device of Figures 15, 16, and 17A-17C.

The solution described herein envisages using the conversion of chemical energy contained in a confined liquid into mechanical energy, exploiting the properties of a metastable liquid for sustaining negative pressures up to the consequent phase change.

In brief, the actuator device according to the solution described herein comprises a microcavity, or microchamber, filled with a substance or medium in the liquid state, for example water. The above microcavity is defined by a casing, having a continuous walled structure, i.e., a walled structure that is as a whole without openings, defining a hollow space inside it. The casing comprises a substantially rigid portion of said walled structure, but also at least a portion of said walled structure that has the behaviour of an elastic membrane that is semi-permeable to the liquid; i.e., it is selectively permeable to ions or molecules of the liquid contained in the casing, which can traverse it through a process of diffusion. The actuator device is set in an external environment, or external medium, hence external to the casing of the microcavity, which may be a liquid or gas. An inner face or surface of the semi-permeable elastic membrane faces the liquid contained in the casing; an outer face or surface of the semi-permeable elastic membrane faces the external environment. In the case where in the external environment it happens that, at the moment when the device is set in the external environment or at a subsequent instant when it has been in this environment, the concentration of the substance, or medium, in the liquid state inside the microcavity is higher than the concentration of the substance outside the microcavity, or that the atmosphere outside the microcavity is not saturated with the vapour of the liquid inside the microcavity, a diffusion flow is activated, for example on account of evaporation of water from the inside towards the air in the external environment through the semi-permeable elastic membrane, which causes a reduction in the pressure of the substance in the liquid state in the microcavity and progressively brings about deformation of the elastic membrane inwards. Since the microcavity has micrometric dimensions such as to cause a high degree of confinement of the liquid, the decrease in pressure is such as to drop below the value of vapour pressure of the actuation liquid, bringing about negative pressures in the liquid, this causing phase changes in the liquid, for example in the form of nucleation of one or more vapour bubbles.

Bubble nucleation brings about fast relaxation of the elastic membrane; consequently, the elastic energy stored in the membrane is converted into kinetic energy. At the end of the actuation process, in general the microcavity no longer contains liquid, but can be filled again.

The above solution enables fast actuation, brought about by the movement of the elastic membrane itself. Moreover, it can be used in combined systems of actuators, for example an array of contiguous microcavities, as illustrated more fully in what follows.

As regards setting-up of negative pressures in the liquid inside the microcavity, it may be considered how, on account of evaporation, the volume of the liquid in the microcavity decreases, causing deformation of the elastic membrane inwards on account of the presence of the forces of cohesion of the liquid. To this deformation, the membrane responds with a force of elastic return that tends to draw the liquid towards its initial configuration. In this state - referred to also as “stretched” state - the liquid has a pressure with a direction opposite to that of the standard condition, and hence has a negative pressure. If the liquid inside the microcavity is stretched excessively can break up, i.e., be subject to nucleation of a vapour bubble and to relaxation of the system. The smaller the volume of the liquid, the higher the negative pressure that the liquid is able to sustain before breaking up, i.e., causing nucleation of a vapour bubble. For this reason, according to the solution described herein, it is necessary to have a microconfined liquid to be able to generate pressures in the region of at least some megapascals that can be converted in actuation systems.

The membrane is in general a semi-permeable elastic membrane; in particular, if on the outside of the microcavity there is a medium represented by an osmotic solution, such as water or CaCh, it is an osmotic semi-permeable elastic membrane. In some embodiments, instead, the external medium or environment is represented by air, in particular in atmospheric conditions, where the flow from the inside of the microchamber towards the outside depends upon the humidity; hence, the membrane is a semi-permeable elastic membrane, but is not osmotic.

In variant embodiments, the wall portion made up of an elastic membrane and the casing portion that is semi-permeable to the actuation liquid may not coincide; i.e., the elastic membrane may not be semi- permeable. In this case, there is another portion of the walled structure of the cavity that is such as to allow, for the purposes of operation of the device, a flow of liquid outwards through a wall that will make it possible to reduce the pressure inside the casing, the elastic wall portion undergoing deformation accordingly.

Figure 1 is a schematic illustration of the operating principle. Designated by the reference number 10 is the actuation device described herein, which comprises a microcavity 11 filled with an actuation liquid 12, for example water. The microcavity 11 is defined by a casing, with a walled structure 13, which defines a hollow solid without openings; however, it comprises a portion 14 of said walled structure 13 that is represented by an elastic membrane permeable to the confined liquid 12. Hence, the microcavity 11 is closed, delimited by a confinement casing 13 comprising a wall portion 14 obtained via an elastic membrane.

Represented in Figure 1 is the actuation device 10 in a first operating step, in which the diffusion process has not substantially started. The microcavity 11 is immersed in an external environment or medium 19, which may be liquid or gaseous. In the example illustrated, it is gaseous; in particular, it is air at atmospheric pressure. The above first operating step corresponds to a moment when either the concentration of the actuation liquid 12 in the microcavity 12 is in equilibrium with the concentration in the external environment 19 or else the evaporation through the membrane 14 by diffusion has just begun. It should be noted that the concentration of the actuation liquid 12 in the external environment 19 may even be zero, if the latter liquid or gaseous external environment 19 does not contain any.

In general, the process of diffusion from inside the cavity 11 towards the outside can occur in air or in a different liquid. In air as external environment 19, for the flow to be activated, the air must not be saturated with gases of the actuation liquid. In the case where this liquid is water, the relative humidity must be lower than 100%. The device described herein functions also in a liquid external environment, for example in the actuation liquid itself, provided that the concentration outside is lower than the concentration inside the microcavity 11 . In some embodiments, the microcavity contains water, whereas outside it an osmotic solution of water and CaCh is present. In this condition, the membrane may be referred to as being osmotic. In variant embodiments, it is possible to introduce a solution of water and CaCh also inside the microcavity 11 provided that there is a difference of molarity between the inside and the outside, with the former having a lower molarity than the latter.

Based on the above, the relationship between the actuation liquid in the microcavity 11 and the liquid or gaseous external environment or medium 19 in embodiments may be considered also in terms of solvent and solute. Indeed, the actuation liquid in embodiments may be regarded as a solvent, while the external environment can be considered in general as a solution of solvent and solute. Under this view, in embodiments the actuation liquid as a solvent as higher concentration in the cavity 11 rather than in the external environment. If a solute can be defined in the external environment, the solute inside the cavity 11 , if any, has a lower concentration than the solute outside the cavity 11 .

Illustrated in Figure 2 is the actuation device 10 in a second step, where the pressure inside the microcavity 11 , which is lower than the pressure outside the microcavity 11 , decreases towards negative values. The concentration of the liquid 12 in the microcavity 11 is higher than that of the liquid 12 in the environment 19 external to the microcavity 11 , and consequently diffusion of the liquid 12 from inside the microcavity 11 to the external environment 19 through the semi-permeable elastic membrane 14 is favoured, this bringing about a reduction in the volume of liquid inside the microcavity 11 and hence a deformation D of the elastic membrane 14 towards the inside of the microcavity 11 with respect to the resting position. Indicated by arrows N in Figure 2 is the pressure exerted by the liquid on the walls of the microcavity (towards the inside, and hence negative) due to the force of return of the elastic membrane. The deformation D of the above elastic membrane 14 is measured via a maximum depth Ax, i.e., the depth of the lowest point of the surface of the elastic membrane 14 towards the inside with respect to the resting position, in the direction normal to the membrane in the resting configuration. This movement of deformation D corresponds to storing energy in the form of elastic potential energy.

Figure 3 then shows a third step in which a nucleation bubble 15, i.e., a vapour bubble of the liquid 12, that is formed in the microcavity 11 on account of the high negative pressures N, which reach, by increasing in absolute value, values sufficient for nucleation, brings about, by expanding, restoration of the pressure inside the microcavity 11 to the value of the initial state, bringing about a movement of return R of the membrane 14 back into the position of the first step. The elastic energy stored in the second step in the membrane 14 is thus converted into kinetic energy by said movement of return R. The nucleation bubble 15, as it grows, in general tends to exhaust the liquid 12 inside the microcavity 11 ; consequently, it becomes necessary, for re-use, to fill the microcavity 11 again with actuation liquid 12.

In Figures 1 to 3, the walled structure 13 of the microcavity 11 is illustrated as having an irregular spheroidal shape in order to represent a generic geometrical shape; however, according to an important aspect of the solution described herein, the microcavity may assume specific geometrical shapes, with internal dimensions of the cavity in the region of tens or hundreds of microns such as to determine the confinement necessary to obtain setting-up of negative pressures in the liquid and nucleation of the vapour bubble in the microcavity 11 .

In this connection, Figure 4A is a schematic cross-sectional illustration of an embodiment of the actuation device 10, in which the microcavity 11 has a hollow parallelepipedal shape. Hence, shown in Figure 4A is a continuous walled structure 13 having a parallelepipedal shape, the major dimension of which, i.e., the height, is oriented along a vertical axis of the figure and rests on a substrate 20, which lies in a horizontal plane. The purpose is to define the relative orientation between the microcavity and the substrate, but, in use, the device 10 can be oriented differently. Hence, a base wall 13a of the parallelepiped, having a square shape, rests on said substrate 20, which may, for example, be made of silicon, glass, or metal. In some production modalities, a thin layer of reflecting material may be deposited on the glass substrate, for example a layer of ITO (Indium Tin Oxide). Designated by 13c are the four side walls of the parallelepiped. The cross section in the figure is taken along a parallel plane passing through the main vertical axis of the parallelepiped and parallel to one of the side walls 13c, clearly one of the two non-visible walls of the four walls 13c. The walled structure 13 further comprises a wall portion 13b that corresponds to the wall opposite to the base wall 13a, the wall portion 13b being open (as may be seen more clearly in Figure 4B), i.e., that has an opening in the material of the walls 13a, 13c, which, however, is closed by an elastic membrane 14, semi-permeable in regard to the liquid 12, in the example in question water. The material of the walls 13a, 13c is, for example, photoresist, as will be described in greater detail in what follows, whereas the material of the membrane 14 is PDMS (PolyDiMethyl Siloxane).

Illustrated in perspective view in Figure 4B is the walled structure 13, in particular the side walls 13c and the roof wall portion 13b, which is open and on which the membrane 14 is applied. It may be noted that in the example the opening is square with a side /; i.e., I is the distance between the inner walls of the cavity. The membrane 14 is not shown in Figure 4B so that the opening corresponding to the roof wall 13b is visible.

Illustrated, instead, in perspective view in Figure 5 is an embodiment of the actuation device 10 with microcavity 11 , which reproduces graphically the image that can be obtained via a SEM (Scanning Electron Microscope) photograph of a real device 10, where it may be noted how the membrane 14, made, for example, of PDMS, is deposited on a walled structure 13 like the one illustrated in Figure 4B, for example made of photoresist, which rests on a substrate 20. In this embodiment, the membrane 14 covers the entire walled structure 13.

With reference to the geometry of Figure 4B, provided hereinafter is the behaviour of the microchamber 11 as a function of the ratio between the dimension I of the side of the square opening 13b (/ being variable, for example, between 10 pm and 250 pm) and the depth d ot the structure ( being fixed at 105 pm): l/d< 0.25: deformation too modest to obtain effective actuation. There is found deformation of the elastic membrane and nucleation of the vapour bubble. The deformation of the membrane 14 appears too modest to obtain effective actuation;

0.25 < l/d< 0.75: deformation at the limit of touching the bottom. The membrane 14 undergoes deformation up to a depth of approximately 1.4 times the value of I, without touching the bottom wall 13a of the microchamber 11 . Bubble nucleation then triggers the movement of return of the membrane 14; l/d< 2: deformation with adhesion that leaves pockets of liquid. The membrane 14 reaches the bottom 13a of the microchamber 11. In any case, the water present at the corner edges of the structure, i.e., in residual pockets of liquid, is sufficient for nucleation and expansion of a vapour bubble and for triggering the actuation mechanism;

2 < l/d: deformation with complete adhesion. The membrane 14 adheres completely to the bottom of the microchamber. Neither nucleation of a bubble nor return of the membrane 14 are noted in this case.

From what has been set forth above, the operating range of the device in some embodiments may be 0.25 < // < 2; namely, the ratio between the side I and the depth d may be greater than or equal to 0.25 and smaller than or equal to 2, in particular, but in non-limiting way, in relation to the embodiment of example of Figures 4A and 4B, with a square opening of side I and dimensions in the ranges indicated. The value of depth d may have proportional variations according to the aforesaid range, 0.25 < l/d < 2.

The microcavity 11 is obtained, for example via two-photon polymerization, for example using the instrument Nanoscribe Photonic Professional GT, which enables 3D printing of photoresist structures on different substrates. An internal depth d between the base wall 13a and the top of the opening 13b is, for example, 105 pm, while the base walls 13a and the opening 13b are square with length I of the inner side of the microcavity 11 , which, as has been said, may vary in a range comprised, for example, between 10 pm and 250 pm, preferably between 10 pm and 200 pm. In the example described the depth is 105 pm and the side / is approximately 75 pm (l/d= 0.71 approximately).

The walled structure 13, i.e., the walls 13a, 13c, in the example have a thickness of 5 pm. For sizing of the microcavity 11 , reference may be made to side-to-depth ratio l/d, which in the example illustrated may range between 0.25 and 2. It is, however, clear that these ranges refer to shapes of the microcavity similar to that of the example, whereas for microcavities with different geometries the principles discussed in regard to Figure 4B apply, in the phases referred to, namely, deformation too modest to obtain effective actuation, deformation to the limit of touching the bottom (or a wall portion, in the geometry, to which the membrane can adhere, in particular can first adhere), deformation with adhesion that leaves pockets of liquid, e.g., at the corner edges, deformation with complete adhesion, whilst the dimensions and ranges selected as corresponding to such phases may be different.

The microcavity 11 , i.e., its walls 13a, 13c, is obtained, for example, via IP-S photoresist (Young’s modulus E = 4.6 GPa). As illustrated, the membrane 14 covers the opening 13b; however, it is obtained via a microfilm designated by 14a, for example made of PDMS, which is applied over the entire microcavity 11 , including the side walls 13c and the adjacent substrate portion 20, closing the opening of the wall 13b, and hence identifying therein the elastic membrane 14, where the opening 13b enables deformation towards the inside. The elastic membrane 14 has a thickness, for example in the case of PDMS, of 3 pm, which yields a suitable Young’s modulus of approximately 1 MPa.

Illustrated in Figures 6 to 9 is the behaviour of the actuation device 10, filled with water as actuation liquid 12 inside the microcavity 11 , and set in air as external environment 19. In Figure 6, the liquid (water) 12 is in the first step, where evaporation due to the difference of concentration of the liquid 12 has just started. The pressure in the microcavity 11 , as a result of evaporation, starts to be lower than the atmospheric pressure of the air in the environment 19; thus the membrane 14 undergoes slight deformation. Denoted by EV is the evaporation of the water 12 towards the air of the external environment 19.

Figure 7 represents the microcavity 11 in a second step, after a time much longer than the time that has elapsed in the first step (basically a few seconds), for example 50 s from start of the process, during which the membrane 14 undergoes a deformation D with a very large depth Ax, while the water 12 is in a stretched condition, i.e., subject to mechanical tensile stress; namely, the pressure of the water 12 is lower than its own vapour pressure, having reached negative pressure values. This renders the liquid metastable with respect to its vapour, and hence liable to nucleation and expansion of bubbles of its vapour.

Figure 8 then represents a third step, immediately following the second step, during which the water 12 is unable to sustain the high internal negative pressures, and nucleation of the bubble 15 occurs in a cavitation time At. The second and third steps here substantially correspond to the second step of Figure 1 .

Figure 9 represents a subsequent fourth step during which the bubble 15 has grown, leading to relaxation of the membrane 14, with generation of a movement of return R, and an associated force. In general, the microcavity 11 in the fourth step is emptied of water or liquid by continuation of the evaporation process that contributes to the growth of the bubble 15.

Since the water tends to evaporate, with a structure like the one described, having, for example, a ratio l/d of between 0.25 and 1 , it is possible to obtain a marked deformation of the membrane 14 and a consequent fast relaxation of the membrane 14, due to nucleation of the vapour bubble 15. For lower values of ratio l/d, for example 0.1 , the deformation D brings about a smaller maximum depth Ax, but nucleation still occurs. This may represent a lower operating limit. An upper operating limit of the ratio l/d may be obtained for values, for example, greater than or equal to 2, or in any case for values of ratio l/d that bring the membrane 14 into contact with the base wall 13a, to which it tends to adhere. Tests conducted by the present applicant indicate a duration of the process from the first to the third step, and then back again to the first step, of approximately 1 min, in the example illustrated. The speed of the movement of return R of the membrane 14 is at least in the region of 10 cm/s.

Figure 10 is a diagram showing, as a function of the ratio l/d, the initial speed a of suction of the membrane 14. It presents a substantially decreasing slope from 1.2 pm/s for l/d = 0.25 to less than 0.1 pm/s for l/d = 1.

Figure 11 illustrates, instead, the time interval At expressed in seconds between start of deformation of the membrane 14 and nucleation of the bubble 15. In this case, the time interval At is substantially increasing from the lowest values of the ratio l/d {l/d = 0.25, approximately 23 s) to the highest values l/d = 1 , approximately 155 s) in the diagram of Figure 11 .

Illustrated, instead, in Figure 12 is the maximum depth Ax to which the membrane 14 is sucked, i.e., the deformation along the main (vertical) axis of the microcavity 11 , substantially increasing from the lowest values of the ratio l/d (l/d = 0.25, approximately 37 pm) to the highest values (// =1 , approximately 99 pm) in the diagram of Figure 12.

Hence, in some embodiments, as mentioned previously, a minimum value of ratio // of the microchamber 11 may be the one that brings about a deformation of the membrane 14 of 1 .4. This minimum value enables the membrane 14 to have the space necessary to undergo deformation inside the microcavity 11 without adhering completely to its walls, considering that even though the membrane 14 touches the bottom of the microcavity, the device functions as long as there is a volume of water at the corner edges adequate for nucleation of a bubble; i.e., the membrane 14 does not completely adhere to the bottom wall 13a. This minimum value of l/d defines the ratio between the internal dimensions of the parallelepiped. For instance, the parallelepiped must have an internal volume comprised between 10 ³ and 10 ⁸ pm ³ , as indicated in what follows, and a ratio l/d smaller than 1 , or in other embodiments a ratio l/d comprised between 0.25 and 2.

Appearing in Table 1 below are values of speed of the membrane 14 in the movement of return R as a function of the ratio l/d. This speed increases with the ratio l/d in the range 0.2-1 adopted also for the measurements of Figures 10 to 12. Table 1

The actuation device 10 may be used in combination with other devices 10 in larger structures. For instance, illustrated in Figure 13, once again in cross-sectional view according to the plane of the cross section of Figure 4, is an array 30 of nine devices 10, with corresponding microcavities 11 , arranged adjacent to one another; i.e., the respective parallelepipeds are arranged in a rectilinear array adjacent to one another. With the exception of the first and last microcavities 11 of the array 30, the microcavities 11 have two opposite side walls 13c on the sides of adjacent devices 10 shared with the aforesaid adjacent microcavities 11. In the shared side walls 13c holes 17 are present passing between the shared walls, in the proximity of the bottom wall 13a, which enable synchronization of the deformation of the membranes 14, as illustrated in Figure 13, when they are in the second step; i.e., negative pressure is created in the liquid 12. Figure 14 shows that in the array 30 also simultaneous nucleation of bubbles in at least two microcavities 11 is then obtained.

Synchronization of the deformation of the membranes 14 makes it possible to obtain synchronization of the nucleation of vapour bubbles in different chambers. Moreover, in the case where the contiguous chambers do not adhere to the substrate, i.e., they are in some way suspended, the negative pressure generated within each of them can contribute to the movement of the whole array, enabling a greater control of the movement of the entire structure.

Further tests conducted by the present applicant, by setting the microcavity 11 in an osmotic solution of CaCh of known molarity, with water and CaCh in the external environment and pure water, in particular deionized water, in the microcavity 11 , have made it possible to estimate the value of the negative pressure reached in the microcavity 11 , which is in the region of -10 ⁷ Pa. This indicates also the possibility for the device 10 to operate with a liquid external environment 19.

The liquid 12 may be any liquid that is able to withstand high negative pressures at the microscale (e.g., around -10 ⁷ Pa). For instance, ethanol may be used as an alternative to water, since it presents a bubble- nucleation pressure in the region of -10 ⁷ Pa.

Hence, on the basis of what has been discussed so far, the solution described refers to a microactuator device comprising at least one closed cavity, or microcavity 11 , containing an actuation liquid 12, for example water or ethanol, the aforesaid closed cavity being delimited by a confinement casing 13 comprising a wall portion 14 obtained via an elastic membrane, where this membrane is semi-permeable in regard to the aforesaid actuation liquid 12.

According to one embodiment, the casing 13 of the above closed cavity 11 comprises a structure having a parallelepipedal shape. This parallelepipedal structure comprises a base wall 13a, side walls 13c, and an opening 13b at a wall opposite to the base wall closed by said elastic membrane 14.

In general, the geometrical shape of said closed cavity 11 has submillimetric characteristic dimensions, preferably comprised between 10 and 500 pm, or a volume comprised between 10 ³ and 10 ⁸ pm ³ , i.e. cubic micrometers, approximately, such as to set up in the confined liquid 12, subject to the progressive reduction in pressure of the liquid by diffusion outwards through the membrane 14, a pressure such as to bring about deformation of the membrane 14 and nucleation of vapour bubbles. By “characteristic dimension or length” is here meant for example the side, in particular the major side, of a parallelepiped or the equivalent diameter, for example defined as diameter of a hypothetical spherical particle that has the same geometrical, optical, electrical, or aerodynamic behaviour of a real non-spherical particle.

The parallelepipedal structure has a ratio between the dimension of the side / of the square base wall and the depth d comprised between 0.25 and 2, in which case the maximum characteristic dimension can be calculated in proportion to this range of the ratio l/d, which, as has been said, is, however, merely indicative, in particular for geometries other than the parallelepipedal ones with a square face. As has been mentioned, in variant embodiments, the elastic membrane 14 may not be semi-permeable, but another portion of the casing is. For instance, a portion of a side wall or an entire side wall may be semi-permeable, while the elastic membrane remains set over the opening 13b. In this connection, Figure 15 is a schematic illustration of an actuation device 10’, where a portion of the walled structure 13, designated by 23, is semi-permeable and enables evaporation EV of the water 12 towards the air of the external environment 19. Designated, instead, by 24 is an elastic membrane 24 that is not semi-permeable but functions as actuation terminal.

It may be noted that in general in this case there may be at least one permeable wall portion, but there may also be even more, for example two portions of two surfaces of two opposed side walls in the case of the parallelepipedal microchamber 10.

Figure 16 illustrates a possible solution in which the elastic confinement portion, i.e., the membrane 24, and the semi-permeable portion do not coincide. It envisages obtaining the same parallelepipedal geometry of the microchamber 11 simply using different materials. For instance, the walled structure 13 may be made (with the same two-photon polymerization technique) of PDMS. The walls 13c, with a thickness in the region of 10-20 pm, would be adequately permeable to the vapour to ensure emptying of the microchamber 11 in a few minutes, remaining, however, too stiff to undergo deformation in an appreciable way. The elastic membrane 14 may, for example, be obtained, once again with the spin-coating technique, using polyurethane or silicone rubbers, such as the known Dragon Skin, having properties of impermeability and marked elasticity.

Consequently, the actuator device described herein comprises at least one closed cavity 11 containing a liquid actuation medium 12, in the example water, the closed cavity 11 being delimited by a confinement casing 13 having dimensions smaller than one millimetre, in particular smaller than 500 pm, comprising a semi-permeable wall portion 14 or else 23, which separates said liquid actuation medium 12 from an external medium 19, where said confinement casing 13 of the closed cavity 11 further comprises a second elastic wall portion, which also separates said second liquid actuation medium 12 from said external medium 19, which may correspond to said semi-permeable wall portion, thus being a semi- permeable elastic membrane 14, or else may correspond to a different wall portion of the confinement casing 13 of said closed cavity 11 with respect to said semi-permeable wall portion; for example, the semi- permeable wall portion corresponds to a portion 23 of side wall, and the aforesaid different wall portion of the casing 13 on which the membrane 24 that is only elastic is positioned corresponds to the wall that closes the opening 13b.

The actuation device 10 described may be used, for example, for drug-delivery applications as microcannon or microcatapult. The movement of return of the membrane can be used in vitro to expel microagents close to tissues or individual cells. In this regard, even though the material described for making the microcavity is photoresist, it may be different, and in this context the adoption of biocompatible materials may enable its application in vivo. In the latter case, the device 10 may form part of more complex implantable devices, in which it can function as sensor or have an active role. It may enable a spontaneous release of therapeutic material guided by external conditions (for example, pH or blood pressure) that affect the concentration of the liquid in the microcavity, thus not requiring any external triggering and ensuring fast and local action.

Actuators based upon the combination of a number of microcavities, like the array 30, can be used in endoscopy applications. The possibility of designing in this way, by combining microcavities, multicavity actuation devices with a different characteristic size for each dimension, for example long and narrow geometries, is advantageous to obtain components that are highly compatible with endoscopy devices for the human body. For instance, it is possible to obtain microtweezers to pick up biological material during a biopsy, which can, for example, be inserted closed and then opened if there arises a condition of difference of concentration for the liquid inside the microcavity, as described previously. Subsequently, bubble nucleation leads to closing of the microtweezers, which remove a small part of tissue or organ. The closed position is maintained in so far as the microcavity no longer contains liquid after bubble nucleation.

For instance, in an embodiment of a microtweezer 40 illustrated in Figures 17A to 17C, two arrays 30 of microchambers 11 are set on two arms 40 constituted by adjacent flexible surfaces 41 attached to a fulcrum 42, for example strips of light metals or rubbers, or directly printed en bloc with monolithic flexible structures made of the same resin (two-photon polymerization), thus forming part of a compliant mechanism, i.e., a flexible mechanism that obtains transmission of the force and motion via deformation of an elastic body. In these embodiments, to reduce the flexural stiffness of the arms 41 of the tweezers 40, the microchambers 11 may be printed by removing the side walls 13c not shared between two adjacent chambers, thus obtaining a comb-like configuration, as illustrated in the perspective view of Figure 18, which shows the parallel walls 13c arranged in comb-like fashion, perpendicular on a common plate, which forms the bottom walls 13a. The microcavity 11 then comes to form once the membrane 14 is applied on the structure. With reference to each individual parallelepiped, in addition to the top wall 13b, also two of the opposed side walls 13c are open, i.e., the ones that are not visible in Figure 18. All three of these open faces of the structure are then closed by the microfilm that forms the membrane 14, in a way similar to what is illustrated in Figure 5. This solution of Figure 18 can be applied to the arrays 30 for example of Figures 15, 16, and 17A to 17C.

With reference to Figures 17A, 17B, and 17C, the three operating steps of a microtweezer 40 thus obtained are described. Figure 17A shows the initial condition, before start of evaporation of the water from the microchambers, where the microtweezer 40 is closed. Figure 17B shows how, following upon evaporation of the water, the pressure in the microchambers 11 decreases, causing opening of the tweezers 40 by elastic bending of its arms 41. Figure 17C shows, instead, a subsequent step, in which rapid closing of the arms 41 of the microtweezer 40 occurs, triggered by nucleation of the vapour bubbles in the microchambers 11 and sudden increase in pressure. Given complete emptying of water from the microchambers 11 at the end of the third step of Figure 17C, maintenance of the closed configuration of the microtweezer 40 is ensured, with consequent exertion of a grip on objects or removal of material. The microtweezer 40, in variant embodiments, could comprise more than two arms, for example three or four.

The solution described may be used to obtain sensors and actuators in microfluidic devices, thanks to the possibility of varying the configuration of the combined structures of microcavities, for example by modifying their geometry to create passive valves. A further possible application is the instantaneous transmission of hydraulic signals through connected microcavities. This application can exploit the simultaneous nucleation of the bubbles, as shown in Figure 14.

As illustrated, the actuation device described is governed by variations of concentration of chemical species, which brings about diffusion through the elastic membrane from inside to the outside, creating negative pressures in the liquid inside the microcavity. Such variations of concentration may be due to gradients of humidity, which can for example be used for environmental monitoring. For instance, the solution may be used for detecting traces of water (humidity) in the ground in order to search for water in the subsoil. In another application, it is possible to carry out monitoring of polluting contaminations in various environments, such as air, liquid, or soil, a threshold of bubble nucleation being set that represents a preset concentration of chemical species.

Hence, from what has been described above, the advantages of the solution proposed emerge clearly.

Advantageously, the solution described, based upon a spontaneous physical process, is able to generate high negative pressures at the microscale. Moreover, the solution described is able to function in different environments, whereas known actuators usually operate in a specific liquid or in air.

In general, reaching of a high actuation speed requires a large amount of power. This is particularly true in the field of “soft” robotics. The solution described is intrinsically fast (membrane speed in the region of 10’ ¹ m/s for actuators having a size of 10’ ⁴ m) and based upon a spontaneous process (the process does not require external supply).

Generation of high negative pressures usually entails the use of cumbersome systems (pumps, compressors). In the solution described the pressure in the region of -10 ⁷ Pa, or a negative pressure reaching at least -10 ⁷ Pa, is generated without any type of external system.

Since the solution described can function in liquid environments, it can be exploited for biological applications.

The microactuator is re-usable once it has been filled again with the liquid. Immersion of the microchambers in the liquid for a period of time in the region of a few hours is sufficient for the process of recharging the microcavity with the liquid.

The possibility of combining a number of microactuators in larger structures of any size (ranging from 100 pm to the millimetric scale) bestows versatility of application on the solution described, just as the possibility of choosing different liquids and materials renders the solution described configurable for different applications.

As compared to other known solutions based upon osmosis that comprise elastic membranes, the solution described comprises a membrane that is at one and the same time elastic and permeable and closes a cavity of micrometric dimensions, i.e., with characteristic dimensions of less than one millimetre, in particular comprised between 10 pm and 500 pm, or a volume comprised between 10 ³ pm ³ and 10 ⁸ pm ³ , bringing about simultaneously passage of the liquid and actuation on account of the microcavity of micrometric dimensions that enables creation of a liquid in a state of mechanical tensile stress, which makes possible accumulation of elastic energy in the membrane and thus enables creation, by nucleation, of vapour bubbles, which bring about return of the membrane and conversion of the elastic energy stored into kinetic energy.