Patent Grant
7523608
a. Field of Invention
The invention relates generally to actuators, and, more particularly, to an actuation apparatus and method utilizing electrically driven fluidic pumping for generating large stresses and strains.
b. Description of Related Art
Fluids in a channel can be pumped by the application of an electric field across the channel. The physical mechanism depends on the fluid, the size of the field, the geometry of the channel holding the fluid, and the frequency of the electric field. Among the many mechanisms are electro-osmosis (EO), electro-hydro-dynamic (EHD) pumping, and dielectrophoresis (DEP). These will be introduced briefly below. The invention can make use of any electric-field driven pumping mechanism, although electro-osmosis is preferred.
Electro-Osmosis This technique is used to pump fluids that contain some quantity of charged species, such as positive and negative ions produced by dissolving salt in a liquid. It is known in the art that most solid surfaces acquire a surface electric charge when brought into contact with a liquid. The surface charging mechanisms include, for example, ionization, ion adsorption, and ion dissolution. These mechanisms occur naturally, and their strength depends upon both the solid and liquid materials, as well as the surface preparation. The surface charge attracts counter-ions to the solid/liquid interface, thus creating a thin “Debye” electric double-layer. When an electric field is applied across the liquid, the ions in the Debye layer migrate in the field, dragging the remainder of the fluid with them by means of viscous forces. This effect is referred to as electroosmosic pumping, or EOF pumping, and is a phenomena that is well known in the art of microfluidics for pumping liquids, typically for transporting liquids in micro-scale applications where the size of this effect is significant due to the large surface to volume ratio.
Specifically, referring to related-art
The applied field can be either DC (direct current, or constant) or AC (alternating current, or oscillating). Most EO pumping is done at DC, but when high voltages are applied, AC frequencies are advantageous because they reduce or eliminate electrolysis. However, AC pumping is more complex and is frequency-dependent.
In EO pumping, the ratio between the surface forces and the bulk (or volume) forces is important. In general, the higher the surface to volume ratio, the greater the pumping force (provided that the dimensions are not so small that the Debye layers of the walls overlap). Channels can be of arbitrary shape, having cross-sections that are round, oval, square, rectangular, etc. To increase the amount of fluid that can be pumped (and thus increase the actuator speed), while maintaining surface to volume ratios that give high pressure, the number of channels between the chambers can be increased. In addition, one dimension of the channel can be increased. For example, a narrow but deep channel can be used, since the small width dimension will ensure a high surface area to volume ratio. In addition, the channel can be filled with a porous or gel medium, creating, in effect, many parallel convoluted channels.
Electro-Hydro-Dynamic Pumping This technique, also known as ion-drag pumping, can be used to pump dielectric fluids (i.e. ones that do not conduct a current), which do not inherently contain charged species. A very high electric field is used, high enough to inject electrons from the electrodes into the fluid. The injected electrons ionize fluid molecules, making them negatively charged. These ions move toward the positively charged electrode, dragging other fluid molecules with them.
Dielectrophoresis When an electric field is applied to a polarizable particle or molecule, it develops a dipole (plus charge on one side and minus on the other). The dipole can be moved in the field if the field is asymmetric (if it is symmetric, the forces on the plus and minus simply cancel). DEP uses AC voltages. DEP effects will appear if the channel is not of uniform cross-section, or if there are changes in the material making up the channel, or through other effects that cause non-homogeneous fields.
Conventional actuators, such as motors and hydraulic actuators, are in wide use and can provide large forces and displacements. However, they are rigid, discrete units made of discrete parts. In some applications, it would be more advantageous to have actuators that are more like biological muscles, which expand and contract, come in a range of sizes and shapes, and can be conformally placed on other structures, like the muscles of the face are placed over the skull. It is of particular interest to have these artificial muscle-like materials be electrically controlled (rather than, for example, chemically controlled). Such actuators made from existing electroactive materials include piezo-electric ceramics and polymers, shape memory alloys (SMA), and dielectric elastomer actuators (DEA). Neither the conventional actuators nor the electroactive materials employ electroosmotic pumping for generation of stress and/or strain. In addition, conventional actuators and some electroactive materials cease to function if a small portion of their structure is damaged.
For all actuators, two important performance parameters are stress (force per unit area) and strain (change in length divided by the original length). Among the electroactive materials, those that are stiff (have a high Young's modulus) provide higher forces, but those that are soft can undergo large displacements.
It would therefore be of benefit to provide an actuation apparatus and method utilizing electro-osmotic pumping for generating stresses and strains that are capable of exceeding those generated by existing muscle-like actuators. It would also be of benefit to provide an actuation apparatus and method which can be produced in a variety of shapes and sizes, and either used as a stand-alone structural material or be applied over and/or within another material or structure. It would be of benefit to provide an actuation apparatus and method that can continue to function if part of the structure is damaged. It would also be of benefit to provide an actuation apparatus and method which would be applicable in a variety of fields, such as mechanics, aerodynamics etc., and an actuation apparatus and method which is robust in design, and which is relatively simple and economical to manufacture and implement.
Definitions
Stress is force per unit area and has units of Pascals.
Strain is the change in length divided by the original length, or ΔL/L, and has no units; it is often given in percent.
Actuating cell means a single microfluidic pumping unit consisting of at least two chambers, a connecting channel, and electrodes. These shall also be referred to these as building blocks and unit elements.
Fluidic actuator material refers to a device consisting of a group of actuating cells that have been configured in a predetermined way.
Substrate means the material into which the chambers and channels are fabricated.
The present invention achieves the aforementioned exemplary objects by providing a fluid-filled actuator cell (see
If the chambers and the channels are formed in a rigid substrate, then a compliant membrane can be bonded over the substrate. This allows deformation of the membrane perpendicular to the rigid substrate (see
The actuator cell can further include at least two expansion chambers (see
Combining several cells together can produce a fluidic actuator material. The actuating cells can be used as “building blocks” to construct a “smart” material that can change continuously from one shape into another shape, or several other shapes. This fluidic actuator material can have a variety of form factors, for example a plate, cylinder, or a cube, as desired. The cells can be sized, oriented, shaped, and arranged in different ways to allow the fluidic actuator material to perform a variety of deformations, including bending, twisting, shortening, lengthening, bulging, dimpling, etc. Which of the variety of deformations the fluidic actuator material takes on, and how much it deforms, is controlled by the voltages applied to each of those cells. Preferably, the cells are small on the scale of the fluidic actuator material.
The fluidic actuator material can be a free standing structure and it can also be integrated over or within other structures, which can be non-active. The free-standing structure can be load-bearing, such as the wing of an aircraft that is able to change shape upon command. If the fluidic actuator material is integrated over a structure, then it can be used to change surface texture of that structure (for example producing small bumps on the surface in order to change the characteristics of fluid flow over the surface) or to change the shape of the surface (for example to control the expressions of an animatronic face). If the fluidic actuator material is integrated over or within a structure, then it can be used to deform or reconfigure that structure (similar to the way an insect muscle deforms the carapace, or mammalian muscles change the relative orientations of bones).
Since the fluidic actuator material consists of individual sealed actuation cells, if some of those cells are damaged, the others can continue to function. Thus, if the fluidic actuator material is punctured, for example, the rest of the device is unaffected (provided that the wiring layout of the electrodes has been designed with the appropriate redundancy).
The invention further provides a method of actuating that consists of providing an electric field between two or more chambers having compliant elements and being connected by micro- or nano-scale channels, causing fluid to be pumped from one chamber to another. The method can include applying voltages across a pair of chambers and among a plurality of chambers.
Additional features, advantages, and embodiments of the invention can be set forth or will be apparent from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary of the invention and the following detailed description are exemplary and intended to provide further explanation without limiting the scope of the invention as claimed.
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and together with the detail description serve to explain the principles of the invention. In the drawings:
Referring now to the drawings wherein like reference numerals designate corresponding parts throughout the several views,
Generally, the present invention provides an actuator that can exhibit both large stresses (up to tens of giga-Pascals) and large strains (up to tens of percent).
In the particular embodiment illustrated, electrodes 118 are compliant. The electrodes can be compliant (stretchable), flexible (bendable), or rigid, depending on the placement of the electrodes and the materials used to construct the cells.
In operation, when an electric field is applied by means of electric circuit 110, as briefly discussed above, fluid is pumped from the supply chamber 104 via channels 108 by means of the aforementioned electroosmosic or other electrical pumping effect to expansion chamber 106, so that expansion chamber 106 becomes deformed as shown in
As shown in
Referring to
A fluidic actuator material can be formed by roll-to-roll fabrication, and thus include repeated layers of building block actuation cell elements, with each element being continuously (i.e., in an analog manner through e.g. the magnitude of the voltage, rather than simply on-off) and individually addressable to produce a number of configurations. In a particular embodiment of the fluidic actuator material, a plate can be formed in a 0.33 m wide compliant substrate of any length using roll-to-roll fabrication.
The fluidic actuator material may be designed to consist of a plurality of identical actuation cells with top and bottom sections fed by a central supply chamber. Both top and bottom sections may be designed, as in
As an example, referring to the (2,3) quadrant of
The fluidic actuator material may be in the form of a plate composed of repeated 1 cm×1 cm×2 cm actuator cells 203, each of which is able to expand, bend around the x or y axis as shown in
As discussed above, the fluidic actuator material would also be able to withstand punctures by a number of holes since each hole will only cause local “bleeding” and a loss of performance in the punctured cell. Thus, so long as the fluidic actuator material includes redundant electronics (for example parallel connections and redundant micro-processors), the actuator material will continue to function even if a fraction of its volume is damaged.
Yet further, the actuation cells are advantageous in that in the event of channel blocking by particulates and other causes, the other channels can still carry fluid. Thus, even if, for example, 10% of the channels become blocked, the achieved pressure and strain will remain essentially the same. However, the actuation rate will decrease linearly with the percentage of blocked channels. For an extreme value of 30% blocked channels, for example, the plate would still be able to achieve a stress of 107 Pa, a strain of 10-30%, and an expansion rate of 2.6 seconds (up from 2.0 seconds).
Yet further, the actuator cells and the fluidic actuator material are advantageous since, once fabricated, the cells would be sealed. They would thus be essentially impervious to external conditions such as humidity, sand, salt, etc. To avoid detrimental effects due to large temperature variations, materials and fluids may be chosen whose thermal expansion coefficients are similar.
Actuator cells 100 and fluidic actuator materials can be made using microfabrication techniques and other methods known in the art. For example, cells 100 and fluidic actuator materials can be batch fabricated using microelectromechanical system (MEMS) techniques. Channels and reservoirs can be fabricated by etching or by molding, depending on the material. Compliant electrodes can be made by, for example, metal films deposited over accordion-like surface structures, elastomer films containing conducting nanoparticles, and other techniques known in the art. Electrode corrosion and electrolysis can be reduced by using AC fields.
For cells 100 and fluidic actuator materials, the chambers and channels can be shaped and oriented to optimize stress, strain, and/or speed. In addition, the layout of the actuation cells can be optimized, for example, to minimize dead-space and minimize the overall size and/or complexity of the structure.
Referring next to
Specifically, as shown in
The various properties of electroosmotic pumping in actuator cell 100 and the fluidic actuator material will now be described in detail.
Volume Flow Rate: Material Actuation Speed
Referring to
where {dot over (V)} is the volume flow rate (volume per second, units m3/s), ε is the dielectric constant of the liquid (C/Vm), ζ is the zeta potential (V), E is the electric field (V/m), and μ is the viscosity of the liquid (Ns/m2). The zeta potential quantifies the strength of the Debye double layer and varies according to the liquid and solid materials and the surface preparation. The fabrication of micro-channels by chemical etching can create a larger number of dangling bonds and lead to a higher zeta potential then fabrication by heat molding.
For a typical zeta potential of 0.1 volts, using a typical dielectric polar liquid in a one centimeter long, one micrometer diameter channel with an electric field well below dielectric breakdown, the flow rate can be defined by the following Equation (2):
{dot over (V)}=π(d=10−6 m)2(ε=10−9 C/Vm)(ζ=1V)(|{right arrow over (E)}|=106 V/m)/(μ=10−3 Ns/m2)=10−12 m3/s.
At this rate, it takes approximately 104 seconds, or about 3 hours, to fill a 100 μm×1 cm×1 cm =10−8 m3 expansion chamber, such as chamber 106 of
Yet another way to increase the speed of volume flow rate via channels 108 is to use wider/deeper channels. For example, increasing the diameter d by a factor of two will increase the actuation speed and decrease the pressure, both by a factor of four.
Electroosmotic Pressure: Achievable Stress in the Material
For a channel 108 driven by a pressure difference ΔP, the flow rate can be defined by the following Equation (3):
{dot over (V)}=πd4ΔP/8 μl,
where the pressure ΔP would be caused by the elastic restoring force in the material and by any applied external load. Setting the electro-osmotic and pressure flow rates equal to one another yields the following Equation (4) for the pressure required to block the electro-osmotic flow:
ΔP˜lεζ|{right arrow over (E)}|/d2,
This pressure corresponds to the maximum stress level that can be achieved at the cavities. Thus the achievable stress in the material increase linearly with the length of channel 108, the electric field, and the zeta potential, and it is inversely proportional to the cross-sectional area of the channel.
Therefore, for actuator cell 100 described above, it is possible to achieve cavity stresses in the tens of mega-Pascals using channel 108 with one micron diameters. Dielectric polar liquids with dielectric constant ε˜100εo, εo=8.854×10−12 C/Vm are available. In approximately 1 cm long by 1 μm diameter channels, an electric field well below dielectric breakdown (E˜106 V/m), and surface treatments to create reasonable zeta potentials, the achievable pressures can be defined by the following Equation (5):
ΔP˜(l=10−2 m)(ε=10−9 C/Vm)(ζ=1 V)(|E|=106 V/m)/(d=10−6 m)2=107 Pa.
The dielectric constant ε can be further increased by adding cellulose particles to the fluid that will polarize under an electric field. This will further increase the pressures that are generated.
Material Elasticity and Network Configuration: Achievable Strain
The corresponding achievable strain can be determined by the stiffness of the material. The micro-meter diameter channels 108 can generate pressures in the tens of mega-Pascals at the expansion chambers (ΔP˜107 Pa). The deformation at chambers 104, 106 will then scale as d˜ΔP/G where d is the percent strain, ΔP is the stress, and G is the Young's modulus of the material. Assuming that the net strain in the material is about 1/10th the strain generated locally at each expansion chamber, a Young's modulus of G˜107 Pa would yield a 10% material strain. This modulus corresponds to a rubber or to a low density polyethylene. If the Young's modulus is chosen at a lower value, the same stress will achieve an even greater strain. Therefore, for a Young's modulus on the order of G˜106 Pa the expected strain would be between 30% to 100%.
Energy Density Bandwidth, and Power Density
The energy density of actuator cell 100 can be given by σε where σ is the stress (units Pa=N/m2) and ε is the strain (non-dimensional) of the material. As discussed above, the achievable stress can be 107 Pa, and the achievable strain for robust materials such as rubber, polyethylene, or PMMA, can be on the order of 10% to 30%. Thus the energy density can be on the order of 106 J/m3. It should be noted that this is the energy density of an actuator cell 100 as a whole, not the energy density of stand alone chambers.
The bandwidth of actuator cell 100 or the fluidic actuator material is determined by the amount of time it takes to fill and empty the chambers. This time can be on the order of 2 seconds or 0.5 Hz.
Lastly, the power density can be the energy density per unit time. Since cell 100 or the fluidic actuator material actuates in about 2 seconds, the power density can be the energy density divided by 2 seconds. Using an average 20% strain number yields a power density of 106 W/m3.
The fabrication of actuator cell 100 will now be described in detail.
Table I below summarizes some of the pertinent dimensions and metrics of an exemplary actuator cell 100. The depth of the channels and chambers is ˜20 μm.
Actuator cell 100 may be fabricated of materials such as silicon, silicon dioxide, 3108/platinum composites, and PDMS.
As shown in
The second step may include covering the entire front-side 302 of the wafer with hexamethyldisilazane (HMDS) 304. After waiting one minute, the wafer may be spun at 3000 rpm on a spin-coater for 30 seconds. The HMDS serves as an adhesion promoter between the silicon dioxide and photoresist.
Referring to
The fourth step may include aligning a mask (as shown in
Referring to
Referring to
As shown in
Referring to
As shown in
The tenth step may include mixing together ten parts Sylgard 184 elastomer base and one part Sylgard 184 hardening agent, and evacuating the air bubbles in a vacuum chamber at 250 torr for 30 minutes (this will cure in the eleventh step to form polydimethylsiloxane (PDMS)).
The eleventh step may include starting with a second wafer; neither the coating, material, nor the orientation are critical for the following steps. For the eleventh step, ⅔ of the front-side of the wafer can be covered with 1813 photoresist (Shipley, Inc.) 304, spun at 3000 rpm for 30 seconds, and soft baked on a hot plate, front-side up, at 100° C. for one minute. After this, ⅔ of the wafer surface can be covered with the mixture created in the tenth step and spun at 6000 rpm for 30 seconds. This will create a film approximately 7 microns thick. The film can be allowed to cure for 24 hours at room temperature to form PDMS, or for less time at higher temperature.
The twelfth step may include covering ⅔ of the PDMS-covered side of the wafer with a mixture of Loctite 3108 and platinum salt, Pt(NH3)4Cl2. A possible recipe is 1.36 g of 3108 and 0.47 g of the Pt salt to form a 12% by volume mixture. The mixture may be spun over the PDMS at 7000 rpm for 30 seconds. This will yield a film approximately 12 microns in thickness.
As shown in
Referring to
The fifteenth step may include immersing the second wafer with the PDMS membrane and the attached patterned 8108/Pt salt composite into a solution of 450 mL deionized water and 500 mg of sodium borohydride to chemically reduce the platinum salt. A possible recipe includes immersion in the solution for 5 hours at 60° C. This converts the Pt salt into Pt metal, and makes the composite conducting.
The sixteenth step may include oxidizing the surfaces of the PDMS and Loctite 3108 with oxygen plasma by reactive ion etching (RIE) in order to permanently bond the PDMS membrane and composite electrodes onto the surface of the microchannels. A possible recipe can include exposure to oxygen plasma for 20 seconds at a pressure of 1000 millitorr, a power of 20 Watts, and an oxygen concentration of 100 sccm.
Referring to
It should be noted that the aforementioned manufacturing steps for actuator cell 100 are provided for exemplary purposes only, and those skilled in the art would readily appreciate that various modifications may be made without departing from the scope of the present invention.
Referring to
Another important performance metric is the amount of time needed for cell 100 to actuate. For actuator cell 100 described above, as shown in
To summarize, for the present invention, electroosmotic pumping causes large expansions of the expansion chambers, and thus large deformations of cells 100 or the fluidic actuator material at high pressure.
Actuator cell 100 and the fluidic actuator material thus provide an actuation apparatus and method utilizing electric-field driven pumping for generating stresses and strains that are capable of exceeding those generated by conventional muscle-like actuators, such as, piezoelectric, shape memory alloys (SMA), and dielectric elastomers (DEA). Actuator cell 100 and the fluidic actuator material can also be applicable in a variety of fields, such as mechanics (i.e. robust snake-like crawling robots, and quiet/stealthy manta ray swimming vehicles), animatronics, robotics, and aerodynamics (i.e. shape changing wings for unmanned air vehicles), and a host of other fields which utilize stress and/or strain actuators.
Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skilled in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims.
This application claims benefit of priority of Provisional Application Ser. No. 60/608,838 filed Sep. 10, 2004, hereby incorporated by reference in its entirety.
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| Number | Date | Country | |
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| Number | Date | Country | |
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| 60608838 | Sep 2004 | US |
