Abstract:
The invention exploits a widely used device in micro-fluidics, the electro-osmotic pump (EOP), to create very low energy micro-scale and macro-scale mechanical actuators. The EOP uses electrical fields to move naturally occurring charged particles (ions) through a fluid medium. As the ions move in response to the applied field, they drag the (non-charged) fluid along, establishing bulk flow. When confined to a narrow chamber, a pressure gradient can be established. The combination of pressure gradient and flow performs mechanical work. With the use of electro-osmotic pumps, the invention enables actuators to be constructed in a variety of embodiments, including for example, a sheet structure, a piston structure, and a cellular structure to name a few.

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/691,025 filed Aug. 20, 2012. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to micro-fluidics and more specifically to a system and methods for creating very low energy micro-scale and macro-scale mechanical actuators through use of one or more electro-osmotic pumps. 
     BACKGROUND OF THE INVENTION 
     Most conventional mechanical pumps have issues with reliability due, in part, to the moving components. However, pumps that do not require moving components such as electro-osmotic pumps (EOPs) make them suitable for a variety of applications, including for example “lab-on-a-chip” devices, diagnostic devices, micro total analysis systems (μMTAS), drug delivery systems, and separation and mixing processes, as well as micro-processor cooling systems, to name a few. 
     Electro-osmosis is used to pump fluids that contain some quantity of charged species, such as positive and negative ions. An electric double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The electric double layer refers to two parallel layers of charge surrounding the object. The first layer comprises ions adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction. 
     Most solid surfaces acquire a surface electric charge when brought into contact with a liquid. When an electric field is applied across the liquid, the ions in the double-layer migrate in the field, which results in viscous drag to create bulk fluid flow and generation of a net pressure. This effect is referred to as electro-osmosic pumping, or EOP pumping. Specifically, EOPs provide fluid flow due to movement of an electric double layer that forms at the solid-liquid interface. Not only do EOPs eliminate moving components, but EOPs also move fluid using the electric field. EOPs can move low conductivity fluids and have a greater pressure and flow rate change when compared to conventional pumps. 
     Thus, there is a need for low energy mechanical actuators that are of a simple design and construction. The invention satisfies this need. 
     SUMMARY OF THE INVENTION 
     The invention exploits a widely used device in micro-fluidics, the electro-osmotic pump (EOP), to create very low energy micro-scale and macro-scale mechanical actuators. The EOP uses electrical fields to move naturally occurring charged particles (ions) through a fluid medium. As the ions move in response to the applied field, they drag the (non-charged) fluid along, establishing bulk flow. When confined to a narrow chamber, a pressure gradient can be established. The combination of pressure gradient and bulk flow performs mechanical work, for example, fluid flow control (i.e. actuators, valves), linear actuators (e.g. for artificial muscles), micro-electro-mechanical system (MEMS) devices (e.g. micro-pistons in bulk silicon), and quasi-sealed actuators that expand and contract to realize both linear and bending motion. 
     The invention relies on individual micro-scale actuators that can be combined in any number including to produce a macro-scale actuator structure with better power density, increased reliability and lower production cost. With the use of electro-osmotic pumps, the invention enables actuators to be constructed in a variety of embodiments, including for example, a sheet structure, a piston structure, and a cellular structure, to name a few. 
     When compared to the small number of other efforts that have applied electro-osmotic pumping to actuation, the invention improves on these efforts in two key areas: (1) production of linear strain that can be scaled down for micro-actuators and scaled up for macro-actuators, and (2) mass-production using commercially available processes that is also cost-effective. 
     When compared to other types of hydraulic actuation, the invention yields high-force, high-strain linear and rotary actuators that are cheap to produce, and are at least an order of magnitude more efficient. These actuators are extremely valuable to the robotics community, since power generation and storage is one of the key limiting factors for the performance of mobile robots. However, these actuators are general purpose and can be applied to any area in which low cost and energy efficiency are priorities. 
     Certain embodiments of the invention are particularly suited to cost-sensitive areas, since they are intended to be produced with a very high-volume reel-to-reel production technique that reduces the individual component cost. Certain other embodiments are intended to be the actuation building block for a new paradigm of modular micro-machines. One goal is to provide a small set of micro-fabricated modular building blocks that form the basis for larger, more complex machines. The key different building block types include computation, power storage and transmission, structure, and actuation. Large numbers of these standardized building blocks may be prefabricated to establish the building blocks in specific locations in order to build an actuator assembly. 
     Certain other embodiments of the invention integrate a plurality of electro-osmotic pumps within a flexible material in order to provide an actuator with uniquely controllable states. For example, certain portions of the actuator can be curled up or curled down while maintaining a somewhat rigid state while other portions of the actuator can be soft and pliable. Therefore, actuators of this embodiment enable low or zero-power rigid portions. 
     Certain other embodiments may be of a more complex structure by incorporating an inherently three-dimensional (3D) mechanism so that the electro-osmotic pumps can move fluid vertically. 
     Specifically, the invention constructs micro-actuators that require very few moving parts, which is far more efficient than other hydraulic actuators, vastly simplifies the actuator design relative to other micro-actuators, offers large force output, and can be readily scaled to create macro-scale actuators that are composed of millions of individual actuators. 
     Some advantages of the invention include, for example, the creation of high efficiency actuators with a very low energy requirement, actuators of simple design with very few moving parts, actuators that can exude a large force output, and actuators that are readily scalable from micro-scale to macro-scale actuators. 
     The present invention and its attributes and advantages may be further understood and appreciated with reference to the detailed description below of contemplated embodiments, taken in conjunction with the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the invention and, together with the description, serve to explain the advantages and principles of the invention: 
         FIG. 1  illustrates an exploded view of one embodiment of an actuator in the form of a sheet structure according to the invention. 
         FIG. 2  illustrates the actuator of  FIG. 1  in a relaxed state according to the invention. 
         FIG. 3  illustrates the actuator of  FIG. 1  in a contracted state according to the invention. 
         FIG. 4  illustrates a cross-section of another embodiment of an actuator in the form of a piston structure according to the invention. 
         FIG. 5  illustrates another embodiment of an actuator in the form of a cellular structure according to the invention. 
         FIG. 6  illustrates the actuator of  FIG. 6  with a portion actuated to bend slightly according to the invention. 
         FIG. 7  illustrates the actuator of  FIG. 6  with a portion actuated to curl upward according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     All electro-osmotic pumps exploit the natural equilibrium-state distribution of ions at a fluid/solid interface. This distribution, known as the electrical double layer, results in a net charge on the solid and an equal, but opposite, net charge in the fluid near the surface of the solid. This charged fluid can be moved when an externally applied electric field is applied. 
     For exemplary purposes, the invention is discussed with respect to three embodiments: a sheet structure, a piston structure, and a cellular structure. However, it should be noted that the invention is not limited to these three embodiments, but that electro-osmotic pumps can be used to construct actuators in a variety of embodiments. 
     In one embodiment of the invention, the actuator is in the form of a sheet structure as shown in  FIG. 1  through  FIG. 3 .  FIG. 1  illustrates an exploded view of one embodiment of an actuator in the form of a sheet structure  100  according to the invention. The sheet structure  100  comprises a plurality of sheets—three sheets  120 ,  150 ,  170  as shown in  FIG. 1  through  FIG. 3 ; however, any number of sheets are contemplated. Each sheet  120 ,  150 ,  170  are constructed from a flexible, inelastic material such as a polymer. 
     The first flexible sheet  120  includes a first top side  122  and a first bottom side  124 . The first top side  122  and the first bottom side  124  are coated with a first conductive layer  130  and the first flexible sheet  120  comprises a plurality of first chambers  128 . 
     The second flexible sheet  150  includes a second top side  152  and a second bottom side  154 . The second top side  152  and the second bottom side  154  are coated with a second conductive layer  160 . Like the first flexible sheet  120 , the second flexible sheet  150  comprises a plurality of second chambers  158 . 
     A third flexible sheet  170  includes a third top side  172  and a third bottom side  174 , wherein the third flexible sheet  170  comprises a plurality of third chambers  178  of a honey-comb arrangement  179 . 
     As shown in  FIG. 2 , the second top side  152  of the second flexible sheet  150  is fused to the third bottom side  174  of the third flexible sheet  170  at a plurality of regular intervals and the first bottom side  124  of the first flexible sheet  120  is fused to the third top side  172  of the third flexible sheet  170  at a plurality of regular intervals to obtain an assembled flexible sheet structure  200  including a plurality of assembled chambers  220 . 
     Each of the sheets  120 ,  150 ,  170  as shown in  FIG. 1  can be mass produced as a rolled sheet. The assembled flexible sheet structure  200  is then formed using a reel to reel process that fuses the sheets  120 ,  150 ,  170  together at specific locations using heat and pressure in order to produce specific chambers  220 . 
     The assembled flexible sheet structure  200  is positioned inside an outer membrane  250  that includes a working fluid  252  to form an actuator  260 . 
     For purposes of this application, a working fluid is any gas or liquid that actuates the actuator. Examples of working fluids include for example, water, steam, pentane, toluene, chlorofluorocarbons, hydro-chlorofluorocarbons, fluorocarbons, propane, butane, isobutene, ammonia, sulfur dioxide, helium, etc. 
     As shown in  FIG. 2  and  FIG. 3 , the assembled flexible sheet structure  200  contracts and expands to form a linear actuator when a voltage is applied to pump the working fluid  252  across the assembled flexible sheet structure  200  and into the plurality of assembled chambers  220 . 
     More specifically, the conductive layer  130 ,  160  enables a charge to be externally applied. When opposite charges are applied to the assembled flexible sheet structure  200 —specifically the conductive layers  130 ,  160 —the working fluid  252  is forced through the assembled chambers  220  and swells the chambers  220  formed by the third flexible sheet  170  as shown in  FIG. 3 . 
     In one embodiment, it is contemplated that the chambers  220  range from  200  nanometers to several micrometers, depending on the design goals and the working fluid  252 . In addition, it is contemplated that the chambers  220  may be approximately one millimeter in each planar dimension with the entire assembled flexible sheet structure  200  being several hundred micrometers thick. Again, these dimensions are not fixed and can be easily changed depending on the specific goals. In general however, a typical sheet structure actuator can employ hundreds of thousands to millions of individual chambers in a single sheet. For example, using the approximate chamber dimensions previously provided, a sheet structure actuator 10 centimeters long that employs a rolled sheet of chambers that is 10 centimeters long before being rolled would contain over 100,000 individual chambers. 
       FIG. 2  illustrates the actuator  260  in a relaxed state whereas  FIG. 3  illustrates the actuator  260  in a contracted state according to the invention. As shown in  FIG. 3 , the swelling of chambers  220  stretches the flexible, inelastic sheets  120 ,  150 , causing a contraction. This motion can be exploited to create low cost, batch-fabricated linear actuators such as artificial muscles. 
     In another embodiment of the invention, the actuator is in the form of a piston structure as shown in  FIG. 4 . In the piston structure embodiment, the electro-osmotic pumps perform two distinct functions: (1) pumping and (2) sealing. The pumping function moves fluid from one side of the piston to the other, forcing the piston to move. The sealing function prevents fluid from slipping past the piston (between the piston and the chamber wall) without also forcing the piston to move. 
     The piston structure according to the invention employs widely used micro-fabrication techniques to create micro-pistons in bulk silicon. This design yields high actuator force with low actuation power, and exploits the electro-osmotic pump to avoid complex seals at the pump/fluid boundaries. 
     In this piston structure embodiment, the invention is intended to be implemented in traditional microelectromechanical systems (MEMS) setting using bulk silicon and various etch and plate stages. 
     The piston structure actuator  400  comprises a piston  402  positioned within an enclosed chamber  404 . Specifically, the piston  402  includes a head  406  and a cylinder  408 . The enclosed chamber  404  includes a plurality of outside surfaces  410  and a plurality of inside surfaces  411 . The head  406  of the piston  402  includes a plurality of perforations  412 . 
     The enclosed chamber  404  is filled with a working fluid  414 , wherein the enclosed chamber  404  includes an aperture  416  through which the piston  402  is positioned within the enclosed chamber  404 . 
     The piston structure actuator  400  includes one or more electrodes  430 . In one embodiment, the one or more electrodes  430  are positioned on either side of one or more perforations  412  of the head  406 . In another embodiment, the one or more electrodes  430  are positioned at two opposing outside surfaces  410  of the enclosed chamber  404 . In another embodiment, the one or more electrodes  430  are positioned at the aperture  416  of the enclosed chamber  404 . It is further contemplated that one or more conductors  432  can be used to connect the one or more electrodes  430  to an outside surface  410  of the enclosed chamber  404 . 
     In the embodiment with one or more electrodes  430  positioned at two opposing outside surfaces  410  of the enclosed chamber  404 , a weaker electric field for the same applied voltage is realized, but this embodiment avoids the need for conductors  432  that connect the electrodes  430  on the piston  402  to the outside surface  410  of the chamber  404 . 
     The one or more electrodes  430  are configured to be charged to create an electric field in order to move the working fluid  414  and actuate the piston  402 . Specifically, when the one or more electrodes  430  are charged, the resulting electric field causes the working fluid  414  to flow from one insides surface  411  to another inside surface  411  of the enclosed chamber  404  causing the piston  402  to move. 
     More specifically, the pumping and sealing functions are both accomplished by the same electro-osmotic pump that uses the one or more electrodes  430  on the piston  402  itself, and is formed by the gap  440  between the cylinder  408  and an inside surface  411  of the chamber  404 . When the electrodes  430  are energized, the working fluid  414  is forced to flow within the gap  440 , causing a bulk flow of the working fluid  414  from one side of the piston  402  to the other. This approach avoids flexible seals which are difficult to fabricate and cause friction losses or stiction—the static friction that needs to be overcome to enable relative motion of stationary objects in contact. 
     It is also contemplated that a second electro-osmotic pump may be implemented, separate and apart from the piston structure actuator  400 , that moves working fluid from one side of the piston to the other via microfluidic channels that lead to either side of the piston (these channels might lead to either end of the chamber, for example). This version is more complicated, but allows greater fluid pressures and flow rates. 
       FIG. 5  through  FIG. 7  illustrate another embodiment of an actuator in the form of a cellular structure  500  according to the invention. In this embodiment, the invention uses an array of identical chambers that are separated by individually addressable permeable layers. Rather than pumping the working fluid from outside the assembly as described in the embodiment of  FIG. 1  through  FIG. 3 , it uses pumps that separate different cellular chambers to move the working fluid between chambers. This motion swells some chambers while shrinking others. Some of these chambers employ inelastic inner struts which bias the contraction/expansion due to fluid motion to cause non-isotropic expansion or contraction. This approach can yield actuators that are sealed, yet realize both linear and bending motion. 
     Similar to the first embodiment described in  FIG. 1  through  FIG. 3 , the cellular structure  500  comprises a plurality of sheets  520 ,  550  that are constructed from a flexible, inelastic material such as a polymer. 
     The first flexible sheet  520  includes a first top side  522  and a first bottom side  524 . The first top side  522  and the first bottom side  524  are coated with a first conductive layer  530  and the first flexible sheet  520  comprises a plurality of first chambers  528 . 
     The second flexible sheet  550  includes a second top side  552  and a second bottom side  554 . The second top side  552  and the second bottom side  554  are coated with a second conductive layer  560 . Like the first flexible sheet  520 , the second flexible sheet  550  comprises a plurality of second chambers  558 . 
     As shown in  FIG. 5 , the second top side  552  of the second flexible sheet  550  is fused to the first bottom side  524  of the first flexible sheet  520  at a plurality of regular intervals to obtain an assembled flexible cellular structure  500  including a plurality of assembled chambers  570 . 
     A working fluid  575  is positioned inside the flexible cellular structure  500  to create the mechanical actuator, wherein the plurality of assembled chambers  570  are separated by individually addressable permeable layers  530 ,  560  such that voltage applied to one or more individually addressable permeable layers  530 ,  560  causes the working fluid  575  to flow from one assembled chamber  570  into an adjacent assembled chamber  570  such that select assembled chambers  570  of the plurality are configured to contract and expand. Furthermore, an inelastic inner strut  580  may be positioned within each assembled chamber  570  to bias the contraction and expansion of the chamber  570 . 
       FIG. 6  illustrates the actuator of  FIG. 6  with a portion actuated to bend slightly according to the invention. Working fluid  575  from four of the chambers  571  has been pumped into the four chambers  571  directly above them causing the chambers  571 ,  572  to swell or contract causing the entire assembled cellular structure  500  to bend. 
       FIG. 7  illustrates the actuator of  FIG. 6  with a portion actuated to curl upward according to the invention. Working fluid  575  pumped from chamber  591  into the chamber  592 , causing chamber  591  to contract and chamber  592  to swell, which causes the actuator in the form of a cellular structure  500  to curl upward. Specifically, the resulting tension draws the ends of the actuator together. 
     Similar to the other embodiments of the invention, the actuator in the form of a cellular structure  500  can be built on a reel-reel process. As shown two sheets are stacked; however, multiple sheets may be stacked in order to create a many-layer stack-up. 
     The described embodiments are to be considered in all respects only as illustrative and not restrictive, and the scope of the present invention is not limited to the foregoing description. Those of skill in the art may recognize changes, substitutions, adaptations and other modifications that may nonetheless come within the scope of the present invention and range of the present invention.