Abstract:
A microscale polymer-based apparatus comprises a substrate formed from a first polymer material and at least one active region integrated with the substrate. The at least one active region is patterned from a second polymer material that is modified to perform at least one function within the at least one active region.

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/810,799, filed Jun. 2, 2006, under 35 U.S.C. §119. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government assistance under Defense Advanced Research Projects Agency (DARPA) Grant No. FA9550-05-1-0459. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of microscale devices. 
     BACKGROUND OF THE INVENTION 
     It is known to incorporate polymer materials, such as elastomers, into microscale devices for structural purposes, while relying on non-polymer materials, such as metals, in such devices for performing active device functions. In some applications, however, use of metal components introduces problems, for example, due to a lack of robustness or conformity of the devices used. 
     SUMMARY OF THE INVENTION 
     According to example embodiments of the invention, a microscale polymer-based apparatus is provided. The apparatus comprises an insulating substrate formed from a first polymer material and at least one active region embedded in the substrate. The at least one active region is patterned from a second polymer material that is modified to be conductive and perform at least one function within the at least one active region. 
     According to other embodiments of the invention, a method of forming a microscale polymer-based device is provided. In an example method, a photoresist is patterned on a substrate to provide a photoresist mold. A modified polymer is applied to the photoresist mold to form at least one modified polymer structure. The patterned photoresist is removed, and nonmodified polymer is formed around the at least one modified polymer structure to provide a nonmodified polymer structure and capture the at least one modified polymer structure within the nonmodified polymer structure. The at least one modified polymer structure and the nonmodified polymer structure are released from the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1F  show steps in an example process for fabricating a single layer of a device integrating modified and non-modified polymers, according to an embodiment of the present invention; 
         FIG. 2  is a photograph of a polydimethylsiloxane (PDMS) tactile sensor sheet, according to an embodiment of the present invention; 
         FIG. 3  is a micrograph of a PDMS microfluidic channel with embedded conductive elastomer devices crossing the channel, according to an embodiment of the present invention; 
         FIG. 4  is a photograph of a PDMS sheet with an embedded strain gauge, according to an embodiment of the present invention; 
         FIG. 5  is a schematic view of the PDMS sheet of  FIG. 4 ; 
         FIG. 6  is a graph showing sample data for a multiwall carbon nanotubes (MWNT) strain gauge undergoing large (˜25%) strain; 
         FIG. 7  is a micrograph of embedded flow sensors in a microfluidic channel filled with dye, according to an embodiment of the present invention; 
         FIG. 8  is a schematic view of an embedded flow sensor as shown in  FIG. 7 , in which an external reference resistor R ref  is connected in series with a PDMS sensor Ro and a current heats the sensor above ambient; 
         FIG. 9  is a graph showing change in output voltage (6V excitation) when fluid flows into the channel shown in  FIGS. 7-8 ; 
         FIG. 10  is a photograph of orthogonal embedded PDMS electrode layers forming an array of capacitive pressure sensors, according to an embodiment of the present invention; 
         FIG. 11  is a schematic view of the capacitive device of  FIG. 10 ; and 
         FIG. 12  is a graph showing change in capacitance for the array shown in  FIGS. 10-11  when loaded by a 3 mm spherical indenter, where X and Y axes are the row and columns of the capacitive array. 
     
    
    
     DETAILED DESCRIPTION 
     It is desirable to create devices that can be handled directly, take advantage of the unique characteristics of composite elastomers, and allow applications that require conformal and robust materials. Preferred embodiments of the present invention provide preferably all-polymer devices with embedded sensors or other active components. Such devices provide improved robustness, and reduce or eliminate the need for silicon or metal parts in particular sensors. 
     Generally, a preferred structure includes a monolithic integration of at least two polymer-based materials, differing in at least one property. The two types of polymer materials may be, for example, a modified polymer (e.g., a polymer composite) and the same polymer without modification. In an exemplary embodiment, a polymer is mixed or otherwise combined or treated with another material to provide the modified polymer (e.g., polymer composite) with new properties, and thus making the modified polymer functional. Such properties, as nonlimiting examples, may make the polymer conductive or sensing (e.g., signal producing). Exemplary, nonlimiting materials that may be mixed with a polymer to provide additional properties include metal powder (e.g., nickel) for increasing conductivity, multi-walled carbon nanotubes (MWNT) for conductivity, carbon black, aluminum nitrate for thermal sensitivity, silicon dioxide to make the polymer porous, molecules that combine to induce swelling or electrical charge, a biological agent, an enzyme, chemical markers, and others. 
     In a preferred method for forming a structure, a polymer is combined (e.g., mixed) with one or more materials to provide new properties for a modified polymer, and one or more different regions of the modified polymer are integrated with, e.g., formed, in or on the other (e.g., non-modified) polymer to provide active regions. For example, the polymer composite may be micropatterned. Generally, a region including the polymer having the additional property (such as the modified polymer) is encapsulated or separated by the other, non-modified or nonfunctional, polymer. 
     An exemplary method for forming regions in a single layer fabrication process includes patterning a photoresist mold on a substrate, applying the composite or modified polymer to the mold, removing excess composite or modified polymer, removing the mold to leave patterned modified polymer behind, spin casting the unmodified polymer, and peeling off the structure from the substrate. However, other elastomer patterning techniques may be used, such as techniques for defining microfluidic or pneumatic channels. Further, multiple layers may be combined to create more complex devices. 
     Electrical connections between the formed active regions may be provided by, for example, forming conductive paths using the modified (e.g., composite) material between the active regions and outside devices. Alternatively, liquid metals may be applied to the device, such as but not limited to Gallium, Indium, tin, mercury, etc. to improve localization of the active regions. 
     It will be appreciated by one of ordinary skill in the art that various types of structures may be formed according to embodiments of the present invention, and that the specific structures, polymer modifications, polymers, patterns, etc., described in examples herein are not intended to limit the scope of the present invention to such examples. Forming active regions may, as nonlimiting examples, include forming a series of freestanding and/or embedded wires on or in a substrate, gaskets, valves, heaters, sensors, or any of various layers and/or structures. The modified polymer may be used in an integrated polymer-based structure to provide any of various applications, including, but not limited to, capacitive plates, tactile sensors, strain gauges, etc. 
     Such integration, for example, can result in strips or other patterned pieces of polymer that are functionalized or active. For example, clear polymer sheets may be provided having strips, functioning like metal, but made of polymers. The particular modifications, including the material(s) to be used, the ratio for combining or treating, etc., and the pattern or region formed from the modified polymer with respect to the non-modified polymer may vary depending on the desired application. 
     Particular example devices according to the present invention use modified and unmodified PDMS (polydimethylsiloxane) elastomer. PDMS is widely used in MEMS. However, PDMS is non-conductive, and as a result elastomers such as PDMS have played a large but mainly structural role in MEMS, serving as protective layers, encapsulants, valve diaphragms, fluidic channel structures, etc. 
     A number of “active” devices have been made using modified elastomers, including organic vapor sensors, liquid sensors, force sensitive resistors, and ultrasonic emitters. Such devices use elastomers mixed with solid fillers, such as carbon black, MWNT, or metallic powders to give the resulting composite material desired properties. Researchers have also captured metal films in PDMS layers to create elastomer tactile sensors. More recently, work has been done to capture in-situ grown MWNT in PDMS to create strain gauges and field emission devices. 
     According to embodiments of the present invention, wholly polymer-based devices with embedded elastomer wires, electrodes, heaters, sensors, etc. can be realized by monolithic integration of modified (e.g., conductive) and unmodified (e.g., non-conductive) PDMS. Using such integration, elastomer strain gauges, capacitive pressure sensors, microfluidic channels with integrated heaters and sensors, and various other devices are possible, providing MEMS capabilities in soft devices. An example device fabrication process according to embodiments of the present invention uses a series of PDMS patterning, micromolding, and bonding techniques with conductive PDMS features made by mixing with multiwall carbon nanotubes (MWNT). 
     Exemplary embodiments of the present invention provide devices that can be handled directly, take advantage of the unique characteristics of composite elastomers, and allow applications that require conformal and robust materials. By combining spin casting and molding, one can provide a number of all-elastomer devices with embedded conductors and sensors. 
     In example devices, conductive PDMS is incorporated within structural insulating PDMS substrates to realize devices having functional regions of conductive PDMS. An exemplary fabrication process is provided for such incorporation according to embodiments of the present invention. Referring now to the drawings, the example process begins with the vapor coating of chlorotrimethylsiloxane (CTMS)  10  on a substrate  12  to assist in the release of the final elastomer assembly ( FIG. 1A ). Next, photoresist (PR)  14  is spun and patterned to define the molds for the conductive PDMS ( FIG. 1B ). 
     PDMS is then mixed with multiwalled carbon nanotubes (MWNT) to make a conductive composite. The ratio of MWNT to PDMS elastomer is chosen depending on the desired application and performance of the device. In example devices having simple conductors for capacitive sensors or resistive heaters, a large amount of MWNT may be added to increase the conductivity of the composite. In the case of strain or force sensitive devices, a lower loading of MWNT may be desired to increase sensitivity. Details on the conductivity of PDMS and MWNT composites can be found in J. M. Engel, J. Chen, N. Chen, S. Pandya, and C. Liu, “Multi-Walled Carbon Nanotube Filled Conductive Elastomers: Materials and Application in Micro Transducers”, IEEE International Conference on MEMS, 2006. In an exemplary device, 10% by weight MWNT is mixed with Sylgard-184 PDMS to form the conductive PDMS. 
     Once the MWNT and PDMS have been mixed, the conductive PDMS  16  is applied to the PR mold and patterned ( FIG. 1C ) to form one or more conductive PDMS structures  18 . An example patterning method is described in K. Ryu, X. Wang, K. Shaikh, and C. Liu, “A method for precision patterning of silicon elastomer and its applications”, Journal of Microelectromechanical Systems, vol. 13, pp. 568-575, 2004. Excess conductive PDMS  16  after application may be removed, for example, using a blade. The patterned elastomer is cured at 90° C. for 10 minutes, and the PR mold  14  is removed, for example in acetone ( FIG. 1D ). The conductive PDMS structures  18  are then captured by either spin or pour casting unmodified PDMS  20  around them ( FIG. 1E ). The resulting assembly  24  is cured at 90° C. for 30 minutes in a leveled oven before being removed, for example, peeled, from the substrate ( FIG. 1F ). 
     This exemplary process can be combined with other traditional elastomer patterning techniques such as those used for defining microfluidic or pneumatic channels. Multiple layers can also be combined to create complex devices such as, but not limited to, capacitive pressure sensors. 
     For example,  FIG. 2  shows a PDMS tactile sensor sheet. The sensor sheet includes an array of electrodes (shown in black) made of conductive PDMS and embedded in transparent, nonconductive PDMS skin. It will be appreciated that the number, specific pattern, etc. of the individual electrodes can be varied. The device provides, for example, a soft capacitive tactile sensor sheet. 
       FIG. 3  shows another device including conductive and nonconductive PDMS. Within a nonconductive PDMS sensor skin, a microfluidic channel is formed extending along the length of the device. An array of embedded conductive elastomer devices cross the microfluidic channel so that the microfluidic channel substantially bisects the embedded conductive elastomer devices. These conductive elastomer devices may include, for example, heaters, conductors, sensors (e.g., flow rate sensors), etc. 
     Other nonlimiting examples of devices include elastomer strain gauges.  FIG. 4  shows a sheet of unmodified clear PDMS with an embedded strain gauge of conductive PDMS. The strain gauge extends generally along the length of the unmodified PDMS sheet, shown in perimeter by dashed white lines. In an example operation of the strain gauge, shown in  FIG. 5 , applied strain alters the average spacing between conductive particles  30  in conductive PDMS  32  within unmodified clear PDMS  34  and therefore alters a resistance reading. Thus, the strain is transduced as a change in resistance. In general, tensile strain causes increased resistance, while compressive strain decreases mean particle spacing and decreases resistance. In contrast to existing semiconductor and metal strain gauges, the exemplary all-elastomer strain gauge shown in  FIG. 4  can repeatably measure large strains (e.g., &gt;1%). Embedded in the insulating elastomer, the strain gauge undergoes the same strain as the bulk PDMS, surviving large deformations typical of elastomers.  FIG. 6  shows sample data collected with an Agilent 34401A multimeter from an elastomer strain gauge such as that shown in  FIG. 4  undergoing large (˜25%) strain while being manually stretched. 
     Another example device using conductive and nonconductive PDMS, shown in  FIG. 7 , includes a nonconductive PDMS sheet having embedded conductive flow sensors. Conventionally, a rigid substrate is needed to bring heaters or sensors into close proximity to microfluidic channels and reaction chambers. Compliant total analysis systems such as required for implantation or use with wearable labs cannot be easily implemented in this way. To overcome these limitations, the device shown in  FIG. 7  embeds conductive elastomer sensors along with microfluidic channels to allow detection of liquids, flow, organic solvents, as well as localized heating. 
     In the device shown in  FIG. 7 , which may be capable of one or several functions, an embedded sensor array (shown in dotted shading in  FIG. 7 ) formed of conductive PDMS is disposed along a microfluidic channel (shown as a horizontal line) formed from nonconductive PDMS. The conductive portion serves as both a heater and a sensor. 
     The exemplary operating principle generally is that of a heated flow meter  40 , as schematically represented in  FIG. 8 . The flow meter  40  includes a plurality of sensors  42  formed of conductive PDMS (with connectors  43 ) generally surrounding a fluidic channel  44  formed from nonconductive PDMS within a substrate  45 . When the sensor  42  is heated above ambient and the resistance monitored, a change in output signifies a change in heat lost to the environment. With an excitation voltage of 6V, and only 1 μW input power, a large (e.g., ˜10%) change in output voltage is measured when a fluid such as water  46  is introduced into the channel  44  ( FIG. 9 ). This change is due to heat loss to the fluid  46 . Measurements are made using an Agilent 34401A multimeter. This exemplary sensor  40  can be used to detect fluid fronts or analyte plugs as commonly used in micro total analysis systems. 
     By using multiple layers, devices that are more complex may also be provided. For example, by combining two layers of elastomer with embedded electrodes, such as in the tactile sensor sheet shown in  FIG. 2 , and orienting them orthogonal to each other, a collapsible capacitive polymer device  50  having a matrix of capacitive pressure sensors  52  can be created, an example of which is shown in  FIGS. 10-11 . The capacitance of a flat plate capacitor is proportional to electrode area and inversely proportional to electrode gap. Thus, large area and small gaps are desired, but using soft materials like PDMS presents a significant challenge for maintaining a small electrode gap. Previous efforts to make collapsible capacitive PDMS devices required larger gaps, numerous bonding steps, and subsequently a large area. 
     The exemplary collapsible capacitive polymer device  50  uses a PDMS-filled capacitive gap  54  of 4 μm, which gives it high stiffness compared to air-gap capacitive devices. However, the filled gap  54  gives increased robustness to stiction, particles, and mechanical overload, as well as increasing the baseline capacitance of the sensors. Air gap capacitors are possible using similar techniques to those used to create the microfluidic channels shown in  FIG. 7 . 
     Testing reveals that interrogating the row and column capacitance of the array of sensors  52  allows imaging of contact with other objects. For example, when loaded by a 3 mm spherical indenter  58  under a 500 g load, and the capacitance measured with an Agilent 4263B LCR meter, the array  52  changes capacitance, as shown in  FIG. 12 . Additionally, the negative-valued artifacts observed in  FIG. 12  can be eliminated by, for example, using electronics designed to interrogate multiplexed capacitive arrays. These circuits switch non-interrogated rows and columns to ground to minimize parasitic parallel capacitance. 
     Thus, exemplary fabrication techniques according to embodiments of the present invention allow realization of all-elastomer MEMS devices (soft MEMS). In some example embodiments, this is accomplished by combining micropatterning of modified (e.g., conductive) elastomer features with traditional spin casting and molding of nonmodified (e.g., insulating) elastomers. Conductive elastomers are made functional by methods including, but not limited to, mixing with multi-walled carbon nanotubes. In this way, example embodiments of the present invention can provide various new devices, including but not limited to all-elastomer strain gauges, microfluidic systems with embedded elastomer sensors and heaters, and robust stretchable capacitive elastomer tactile sensors. Other applications of exemplary fabrication techniques include soft biomedical applications, such as interocular pressure measurements, large strain measurements for smart-prosthetics and robotics, and compliant pathogen detection systems for wearable deployment. 
     While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions, and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions, and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
     Various features of the invention are set forth in the appended claims.