Abstract:
In one implementation, a method of fabrication of stretchable electronic skin is provided. The method may include receiving an elastic material net. An elastic conductor mesh is formed on the elastic material net. A device is electrically bonded to the elastic conductor mesh. The implementation may further include forming a mold comprising a net pattern on a substrate and creating the elastic material net by coating the mold with an elastic material precursor, and then removing the elastic net from the substrate with the elastic conductor thereon. In one embodiment, a stretchable electronic skin including a net structure having a non-conducting elastic material with an elastic conductor mesh formed on the non-conducting elastic material, and a device electrically connected to the elastic conductor mesh.

Description:
BACKGROUND 
     Stretchable circuits enable electronics onto non-planar 3D surfaces. Distinguished from flexible circuits, which can simply be place on a 2D non-planar surface, stretchable circuits can support biaxial strain rather than simply uniaxial strain. A stretchable circuit can be wrapped around 3D objects. Microscale devices such as a mini UAV, need to not only put electronics on non-planar surfaces, but also embed sensors all over a mini-structure to detect temperature, pressure, force, or other input that a mini system may want to detect, i.e. aeronautical, biological, or other related sensors. 
     Systems with coiled interconnects, which are 3D in nature, are not readily created with standard micro-fabrication planar processing techniques and hence are not easily integrated. On the other hand, standard metal thin films of aluminum, wire bonding, and soldering for the electrical connections offer very limited stretchability. Conventional metal wires patterned on continuous sheets of an elastomer offer only uniaxial strain for micro-scale. 
     What is needed is a low cost stretchable electronic skin design and fabrication technique that provides a stretchable electronic circuit skin for integrating electronics over the structure of a microscale system. Moreover, what is needed is an easily manufactured stretchable electronic skin capable of a large amount of stretch. 
     SUMMARY 
     In one implementation, a method of fabrication of stretchable electronic skin is provided. The method may include receiving an elastic material net. An elastic conductor mesh is formed on the elastic material net. A device is electrically bonded to the elastic conductor mesh. The implementation may further include forming a mold comprising a net pattern on a substrate and creating the elastic material net by coating the mold with an elastic material precursor, and then removing the elastic net from the substrate with the elastic conductor thereon. 
     In one embodiment, a stretchable electronic skin including a net structure having a non-conducting elastic material with an elastic conductor mesh formed on the non-conducting elastic material, and a device electrically connected to the elastic conductor mesh. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows an enlarged cut away top view of a stretchable electronic skin in accordance with one embodiment of the present invention. 
         FIG. 2  is a top view of a stretchable electronic skin  200  in accordance with another embodiment. 
         FIG. 3  is an enlarged exploded top view of the stretchable electronic skin along the  3 - 3  line of  FIG. 2 . 
         FIGS. 4 through 8  show an isometric view of a possible implementation for fabricating the stretchable electronic skin of  FIG. 1 . 
     
    
    
     DESCRIPTION 
       FIG. 1  shows an enlarged cut away top view of a stretchable electronic skin  100  in accordance with one embodiment of the present invention. An elastomer net  110  has a stretchable conductor mesh  120  of metal or other elastic conductor thereon. The stretchable conductor mesh  120  may be a two dimensional conductor, deposited/etched on the elastomer net  110  using lithographic techniques. One or more devices  130  may be bonded to the conductor mesh  120  such as by solder  140  or other conductive bond. The elastomer net  110  with the device(s)  130  thereon may be stretched around an object or a complex topographical surface (not shown) to be conformal with it. The spaces  150  within the net  110  facilitate increased reversible stretch. The mesh  120  on net  110  allows greater reversible stretch and may be fabricated using planar processing techniques as discussed further below. 
       FIG. 2  is a top view of a stretchable electronic skin  200  in accordance with another embodiment. In this embodiment, stretchable mesh conductors  220  and  240  on an elastomer net  210  form a row-column addressable circuit  255  to connect to two-terminal devices  230 . 
       FIG. 3  is an enlarged exploded top view of the stretchable electronic skin  200  along the  3 - 3  line of  FIG. 2 . As illustrated in  FIG. 3 , an intermetal dielectric  250  may be interposed between the stretchable mesh conductors  220  and  240  within the elastomer net  210 . Devices  230   a ,  230   b , and  230   c  are secured to, and contact (not shown in  FIG. 2  or  3 ), the stretchable mesh conductors  220  and  240 . 
       FIGS. 4 through 8  show an isometric view of a possible implementation for fabricating the stretchable electronic skin  100  of  FIG. 1 . Referring to  FIG. 4 , in this implementation, silicon wafer is lithographically patterned to form a mold  400  of the reverse of the elastomer net (not shown). 
     The mold may be 160 micron thick on formed on a 4 inch diameter wafer  470  with SU-8, a photoactive epoxy  420 , available from MICROCHEM, located in Newton, Mass. The photoactive epoxy  420  is capable of forming thick films in one application. The mold  400  could also be made from a pattern etched in a suitable substrate material e.g., silicon, or cut, formed, etc. in/of another known mold material. 
     Shown in  FIG. 5 , a suitable elastomer material  430  is applied, such as silicone, for example SYLGARD 184, available from Dow Corning Corp., Midland, Mich. The silicone material  430  may be applied with a pour and spin on technique, to the mold  400 , and the excess removed with a squeegee to remove the elastomer material  430  from the tops  420   t  of the mesas  420   m  (shown in  FIG. 4 ) of the mold  400 . The excess may be removed prior to solidifying the silicone material  430 . This patterns spaces  150  in the elastomer net  110  referred to with reference to  FIG. 1 , and creates a flat surface  430   t  for subsequent lithography. An optional mold coat layer  580  may be applied prior to applying the elastomer material  430 , such as OMNICOAT available from Microchem Corp., located in Newton, Mass., to facilitate later release of the elastomer material  430  from the mold  400 , as discussed below with reference to  FIG. 8 . 
     Turning to  FIG. 6 , a fine conductor mesh  620  is selectively deposited between the mesas  420   m  in rows on the silicone material net  610  ( FIG. 5 ) using a photo-resist lift-off technique. The conductive material of the conductor mesh  620  could be copper, gold, other high conductivity metal or alloy, or other flexible conductive material. Contacts  660  may also be patterned and deposited on the elastomer net  610  when forming the conductor mesh  620 . The dimensions of the conductor mesh are only limited by the photolithographic technique used. Conductor widths on the order of 25 microns are typical. 
     Turning to  FIG. 7 , while the elastomer net  610  is still on the wafer  470 , devices  730  are bonded with a conductive elastomer ink, paste, or other conductive flexible adhesive known in the art (not shown). Examples of conductive elastomer ink are PI-2000 Highly Conductive Silver Ink, by Dow Corning, Midland, Mich., or PI-2310 Conductive Silver Ink, by Dow Corning, Midland, Mich., or ESL 1901-S Polymer Ag Conductor, by ESL ElectroScience, King of Prussia, Pa. Bonding at this stage, while the elastomer net  610  is on the hard mold surface, allows easy transfer to/for pick-and-place machines. 
     Turning to  FIG. 8 , the elastomer net  610  mesh with devices  730 , is then released from the mold  400  (shown in  FIG. 7 ), with a combination of peeling and dissolving of the mold coat layer  580  (shown in  FIG. 5 ). For example, an OMNICOAT mold coat layer  580 , available from Microchem Corp., in Newton, Mass., may be dissolved using isopropanol as the stretchable electronic skin  800  is removed from the mold  400 . 
     For the embodiment of  FIG. 2 , could be fabricated in a similar fashion, with two iterations of the metal patterning, one for the row and one for the column. An elastomer would be added at the cross-points to prevent shorting between the row column interconnect. A photo-patternable silicone, available from Dow Corning Corp., Midland, Mich., is an example of a material that could be used for the dielectric insulator. In alternate embodiments, the metal could be run or routed along the elastomer net to specific devices on the elastomer net similar to PCB traces. 
     Referring to  FIG. 1 , the conductor mesh  120  is a repeating pattern of continuous metal that provides several adjacent connected parallel serpentine paths, i.e. paths repeatedly curving in alternate directions, which form series loops with the adjacent serpentine paths, the loops being connected to adjacent successive loops along a path and to adjacent lateral loop(s). With this pattern, a stretchable planar conductor mesh  120  is formed using conventional deposition techniques that provides a large degree of reversible stretch capability. In one empirical embodiment, the conductor mesh  120  maintained electrical continuity when reversibly stretched by approximately 70% (total area increase) when the stretchable electronic skin  100  was stretched on an inflatable membrane. 
     In another implementation, a suitable elastic net is imprinted with the conductive mesh, such as by silk screening on the conductive mesh. 
     Various embodiments of the stretchable electronic skin fabricated with the techniques described herein could be used to cover complex surfaces with e.g. active sensors and associated electronics. The free standing net of the elastomer, metal mesh, and connected electronic devices can be stretched to accommodate the additional surface of a 3-D shape. The stretchable electronic skin of some embodiments fabricated with the above described techniques offer more reversible stretch than previously reported stretchable conductors, without the need to pre-stretch the elastomer during the fabrication process before metal is applied. Various embodiments of the stretchable electronic skin disclosed herein may be formed using planar processing techniques and can be integrated into electronics packing schemes, e.g., pick and place. 
     Stretchable electronic skin promises unprecedented integrated sensing and local control in applications ranging from microscale tunability of wing surfaces on planes, UAVs, and MUAVS to large scale morphing structures for optimized performance in different use environments, to enhancing human performance with smart clothing and/or engineered exoskeletons. The meso/micro-scale multiple sensors and actuators needed to achieve these systems. require integration of electronics with polymers, specifically elastomers. These skins require more than just flexible wiring; the wiring must be stretchable (even if wireless schemes are used, the antenna coils must be highly conductive and stretchable). The fabrication processes should be scalable to cover large area surfaces and the interconnects must remain conductive under large strains. Shapeable antennas, wings, and the ability to build electronics into the system structure are of keen interest to reduce weight, power consumption, and to offer local sensing and control functionality approaching that of biological skin. This technology is also useful for spherical imaging systems, conformal antenna or RE systems, and retro-modulator systems all of which would benefit from shapeable integrated electronics. Various embodiments provide low interconnect resistance and 3-D shapeability in a scalable design. This is a long-felt need to the realization electronic skins. 
     Various embodiments could be utilized by unmanned air and ground vehicles, where vehicle size and weight are important, or in fly-by-light power and control of vehicle surfaces where electromagnetic interference (EMI) is of concern. Applications may include automotive, search and rescue, exploration, military defense, or any application where battery weight, power consumption, or electromagnetic interference is a problem, or where safety, system reliability, operating time, observability, operating time, or redundancy impose limitations on a system. 
     Having described this invention in connection with a number of embodiments, modification will now certainly suggest itself to those skilled in the art. The example embodiments herein are not intended to be limiting, various configurations and combinations of features are possible. As such, the invention is not limited to the disclosed embodiments, except as required by the appended claims.