Patent Publication Number: US-2022219333-A1

Title: System and method for fabricating soft sensors that conform to arbitrary smooth geometries

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 63/136,428 filed on Jan. 12, 2021, which is expressly incorporated herein by reference. 
    
    
     BACKGROUND 
     The need for soft tactile sensors that conform to arbitrary smooth geometries has been a bottleneck for developing robot hands with dexterous manipulating capabilities. The field requires the sensor to be soft, skin-like, and to conform to the shape of a fingertip and/or a palm. Although, there has been significant development in the field of soft sensors, however, most of them are all in academia. In actual commercial applications several other requirements need to be met especially in the readout electronics segment. For example, adhering soft sensors for robotic purposes may often be a challenge and delamination is often an issue. 
     BRIEF DESCRIPTION 
     According to one aspect, a system for fabricating soft sensors that conform to arbitrary smooth geometries that includes a top stretchable layer that includes a set of electrodes of soft sensors that are made of an elastic material. The system also includes a bottom flexible layer that is composed of a thin sheet of suitable metal that is patterned using photolithography. The bottom flexible layer is configured to be in conformity with the arbitrary smooth geometries. The top stretchable layer is bonded to the bottom flexible layer to form a sensor substrate. The sensor substrate is configured as a stretchable adhesive film which enables robust adhesion to the arbitrary smooth geometries. 
     According to another aspect, a method for fabricating soft sensors that conform to arbitrary smooth geometries that includes fabricating a top stretchable layer that includes a set of electrodes of soft sensors that are made of an elastic material. The method also includes fabricating a bottom flexible layer that is composed of a thin sheet of suitable metal that is patterned using photolithography. The bottom flexible layer is configured to be in conformity with the arbitrary smooth geometries. The method further includes bonding the top stretchable layer to the bottom flexible layer to form a sensor substrate. The sensor substrate is configured as a stretchable adhesive film which enables robust adhesion to the arbitrary smooth geometries. 
     According to yet another aspect, a system for fabricating soft sensors that conform to arbitrary smooth geometries that includes a sensor substrate that is configured to as a stretchable adhesive film which enables robust adhesion to a robotic device that includes a top stretchable layer that includes a set of electrodes of soft sensors that are made of an elastic material. The sensor substrate also includes a bottom flexible layer that is composed of copper films that are patterned using photolithography. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed to be characteristic of the disclosure are set forth in the appended claims. In the descriptions that follow, like parts are marked throughout the specification and drawings with the same numerals, respectively. The drawing figures are not necessarily drawn to scale and certain figures can be shown in exaggerated or generalized form in the interest of clarity and conciseness. The disclosure itself, however, as well as a preferred mode of use, further objects and advances thereof, will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a cross-section view of a sensor substrate according to an exemplary embodiment of the present disclosure; 
         FIG. 2A  is an illustrative overview of the fabrication of a top stretchable layer of the sensor substrate according to an exemplary embodiment of the present disclosure; 
         FIG. 2B  is a cross-section view of the top stretchable layer of the sensor substrate according to an exemplary embodiment of the present disclosure; 
         FIG. 3A  is an illustrative overview of a bottom flexible layer of the sensor substrate according to an exemplary embodiment of the present disclosure; 
         FIG. 3B  is a cross-section view of the bottom flexible layer of the sensor substrate according to an exemplary embodiment of the present disclosure; 
         FIG. 4  is an illustrative overview of bonding of the top stretchable layer and the bottom flexible layer according to an exemplary embodiment of the present disclosure; 
         FIG. 5  is an illustrative overview of the bonding of the sensor substrate to an arbitrary smooth geometry according to an exemplary embodiment of the present disclosure; 
         FIG. 6  is a process flow diagram of a method for fabricating the sensor substrate and attaching the sensor substrate to a robotic device according to an exemplary embodiment of the present disclosure; and 
         FIG. 7  is a process flow diagram of a method for fabricating soft sensors that conform to arbitrary smooth geometries according to an exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     I. System Overview 
     Referring now to the drawings, wherein the showings are for purposes of illustrating one or more exemplary embodiments and not for purposes of limiting same,  FIG. 1  includes a cross-section of the sensor substrate  100  according to an exemplary embodiment of the present disclosure. In one embodiment, a fabrication system may be configured to fabricate the sensor substrate  100  that includes soft sensors to conform to arbitrary smooth geometries to provide a high mechanical robustness and a high level of electronic sensor signal integrity with respect to sensor signals output by the soft sensors. 
     The fabrication system may leverage the advantages of making devices/circuit boards and soft-sensor technology that enables the fabrication of state-of-the-art conformal tactile sensors. In one embodiment, a set of electrodes of soft sensors that may be bonded upon a sensor substrate may be made of flexible material that provides a conformity needed for proper robotic device sensing (e.g., robotic finger sensing) with conventional materials that are solder-able. This configuration may also provide an interface with readout electronics. 
     As described in more detail below, the fabrication system may be configured to utilize an additional set of top electrodes that may be made of stretchable conductor material that renders a top segment of the sensor as soft and compliant. The system may also be configured to form a bottom flexible layer  104  that may be composed of a thin sheet of suitable metal that is patterned using photolithography. In one embodiment, the thin sheet of suitable metal may include copper films that are patterned using photolithography. The bottom flexible layer  104  is configured to be in conformity with the arbitrary smooth geometries with smooth segments of small radii of curvatures, to which a high level of conformity may be achieved with a suitable copper film thickness and copper pattern size and shape. 
     Photolithography is known in the art to have also been implemented in fabricating passive electronic components such as surface mount resistors directly on the circuit boards. The fabrication system may use photolithography as a patterning process to provide a benefit of the process being easily scalable since it may be used to make features in the range of nanometers (used in microchips) all the way to centimeters or larger. Also, the device sizes fabricated using photolithography technology may be scaled all the way from a few millimeters in size to a few meters. The substrate for this fabrication process may be configured as a stretchable adhesive film which enables easy implementation and robust adhesion on a robotic device such as a robot finger/hand. Accordingly, the use of the fabrication method executed by the fabrication system and described in more detail below allows the fabrication of soft sensors that may easily interface with electronics and may provide mechanical and electrical integrity that may be required by a commercial grade product. 
     As shown in  FIG. 1 , the sensor substrate  100  may include a top stretchable layer  102  that may be bonded to a bottom flexible layer  104 . A bottom portion of the sensor substrate  100  may be configured to include an adhesive portion, such that a bottom face of the bottom flexible layer  104  allows the sensor substrate  100  to robustly adhere one or more types of sensors to/upon any arbitrary geometry. As such, the sensor substrate  100  may be configured to adhere one or more types of sensors on robotic applications, such as robotic hands, fingers, and/or additional types of geometries. 
       FIG. 2A  is an illustrative overview of the fabrication of the top stretchable layer  102  of the sensor substrate  100  according to an exemplary embodiment of the present disclosure. As shown, a dielectric layer  202  may be cast as a bottom portion of the top stretchable layer  102 . The dielectric layer  202  may be cast in a mold using an elastic material. Accordingly, the top portion of the top stretchable layer  102  may provide a level of elasticity and pliability that may be useful for various robotic sensing actions. 
     It is appreciated that a wide range of such materials available in the market that range in elastic modulus of 100 kPa (very soft) to 1˜2 MPa (fairly rigid) may be utilized to cast the dielectric layer  202  of the top stretchable layer  102 . Such materials may closely simulate the mechanical properties of human skin. For example, soft elastic materials such as Ecoflex, Dragon Skin, and the like may be utilized to cast the dielectric layer  202  of the top stretchable layer  102 . In some configurations, the dielectric layer  202  may also have structures such as pillars, pyramids, or domes and therefore air gaps, to fine tune the mechanical properties as desired. 
     With continued reference to  FIG. 2A , once the dielectric layer  202  is cast, the fabrication system may pattern the stretchable electrode material into a stretchable electrode pattern  204  with a material using a patterning process of choice. Non-limiting exemplary materials that may be used may include, but may not be limited to, carbon nanotubes, silver nanowires, conducting polymer and/or conducting particle composites. Non-limiting exemplary patterning processes that may be used, but may not be limited to, spray coating shadow masking, and/or screen printing. 
     In one embodiment, upon the patterning of the stretchable electrodes into the stretchable electrode pattern  204 , an encapsulating layer  206  may be cast upon the stretchable electrode pattern  204  using the same or similar elastic material used to cast the dielectric layer  202  of the top stretchable layer  102 . For example, the encapsulating layer  206  may be cast in a mold using Ecoflex, Dragon Skin, or other elastic materials. 
     As shown in  FIG. 2B , a cross-section of the top stretchable layer  102  of the sensor substrate  100  according to an exemplary embodiment of the present disclosure, the encapsulating layer  206  is cast upon the stretchable electrode pattern  204 . As discussed above, the stretchable electrode pattern  204  is disposed atop of the dielectric layer  202  that may be composed of elastic material. 
     With reference to the bottom flexible layer  104 , of the sensor substrate  100 ,  FIG. 3A  includes an illustrative overview of the bottom flexible layer  104  of the sensor substrate  100  according to an exemplary embodiment of the present disclosure. In one embodiment, a bottom portion of the bottom flexible layer  104  and consequently the sensor substrate  100  may be configured as a soft adhesive sheet  302 . In one configuration, the soft adhesive sheet  302  may be configured as a pliable adhesive sheet that may be flexible for robust adhesion to various arbitrary smooth geometries. The soft adhesive sheet  302  may be configured as a double sided acrylic tape sheet adhesive substrate. As an illustrative example, the soft adhesive sheet  302  may include tape dimensions of 12″×12″. It is appreciated that many different sizes of sheets and tape dimensions may be utilized that may include varying properties of mechanical stiffness and chemical stability. 
     With continued reference to  FIG. 3A , a thin sheet of copper film  304  may be laminated upon a top side portion of the soft adhesive sheet  302 . Upon the lamination of the copper film  304  upon the soft adhesive sheet  302 , a dry film photoresist  306  may be laminated upon a top portion of the copper film  304 . In one configuration, the fabrication system may be configured to send instructions to utilize a thermal laminator (not shown) to laminate the dry film photoresist  306  upon the copper film  304 . In one embodiment, the dry film photoresist  306  may be exposed through a mask using an ultraviolet source. 
     In an exemplary embodiment, the fabrication system may be configured to use a developing solution to develop exposed portions of dry film photoresist pattern  308 . The exposed portions of dry film photoresist pattern  308  may be utilized as a mask to etch undesired portions of copper of the copper film  304  previously laminated upon the soft adhesive sheet  302 . Accordingly, the soft adhesive sheet  302  may include etched copper  310  with respective exposed portions of dry film photoresist pattern  308  that remain upon the soft adhesive sheet  302 . 
     In one embodiment, upon the etching of the undesired copper of the copper film  304  to allow the etched copper  310  to remain upon the soft adhesive sheet  302 , the fabrication system may remove the photoresist from the dry film photoresist pattern  308  that remains upon the etched copper  310 . Upon the removal of the photoresist, a patterned copper film may remain upon the etched copper  310  of the bottom flexible layer of a sensor substrate. The patterned copper film may be configured as patterned copper electrodes  312  that may be operably connected to a control board (not shown) that is associated with the sensor substrate  100 . 
       FIG. 3B  is a cross-section view of the bottom flexible layer  104  of the sensor substrate  100  according to an exemplary embodiment of the present disclosure. In an exemplary embodiment, a top layer of elastic material  314  may be cast upon of the soft adhesive sheet  302  that may be configured as an adhesive substrate. As discussed above, the soft adhesive sheet  302  may include the patterned copper electrodes  312  that may remain upon the etched copper  310  of the bottom flexible layer  104  of the sensor substrate  100 . 
       FIG. 4  includes an illustrative overview of the bonding of the top stretchable layer  102  and the bottom flexible layer  104  according to an exemplary embodiment of the present disclosure. In an exemplary embodiment, upon the fabrication of the top stretchable layer  102  of the sensor substrate  100  and the bottom flexible layer  104  of the sensor substrate  100 , the fabrication system may be configured to bond the top stretchable layer  102  to the bottom flexible layer  104  to form the sensor substrate  100 . As shown, the casting of the top layer of elastic material  314  upon the adhesive substrate of the bottom flexible layer  104  may enable a strong adhesion between the dielectric layer  202  that may be composed of elastic material and the layer of elastic material  314  of the bottom flexible layer  104 . Accordingly, the top layer of elastic material  314  may be bonded to the bottom flexible layer  104  to form the sensor substrate  100 . 
       FIG. 5  includes an illustrative overview of the bonding of the sensor substrate  100  to an arbitrary smooth geometry according to an exemplary embodiment of the present disclosure. As represented in  FIG. 5 , upon the fabrication of the sensor substrate  100 , the bottom portion that is configured as a soft adhesive sheet  302  may be configured for robust adhesion to a robotic device such as a robot finger/hand  502 . In other words, once the sensor fabrication is complete, the sensor substrate  100  which is adhesive on a bottom face is used to adhere the sensor substrate  100  on any arbitrary smooth geometry such as to the robot finger/hand  502 . In order to interface with readout electronics, the patterned copper electrodes  312  may include traces that run to the circuit board that is associated with the sensor substrate  100 . In one configuration, the traces may be soldered to consequently form a robust connection between the patterned copper electrodes  312  and the circuit board to communicate sensor signals. 
     In some embodiments, the patterned copper electrodes  312  may be interfaced with copper tape, using crimp connectors on each respective electrode connection trace, and/or using a flexible flat cable connection (exemplary connections not shown). In alternate embodiments, the copper tape and/or the flexible flat cable connection may be soldered on the circuit board. However, it is appreciated that various types of connection techniques may be utilized to operably interface the patterned copper electrodes  312  with the control board that is associated with the sensor substrate  100 . 
     The fabrication system may enable a reduction of the number of interconnects requiring a soft electrode-rigid circuit interface by at least half or even more in case of an asymmetric circuit to enhance the signal integrity by a significant amount. In some configurations, if the sense terminals of the readout hardware are connected using the copper soldered connection, this functionality may provide an additional increase in signal to noise ratio. The utilization of photolithography may enable the fabrication of very complex and dense bottom electrode patterns in asymmetric designs where only one set of electrode patterns need to be more complex than the other. 
     In one embodiment, a stretchable conductor material with electrode materials (such as copper) may be utilized to achieve a higher signal integrity by moving one half or more of the sensor(s) into the solid electrode material domain. The connection of excitation terminals of the readout electronics to the top stretchable layer  102  and the sense terminals to the patterned copper electrodes  312  of the bottom flexible layer  104  is thereby completed. This functionality ensures a higher signal integrity and clean sense signal which delivers a better signal to noise ratio, when compared to a sensor that has both top and bottom electrodes made of stretchable conductor materials. 
     II. Methods Implemented to Fabricate Soft Sensors that Confirm to Arbitrary Geometries 
       FIG. 6  is a process flow diagram of a method  600  for fabricating the sensor substrate  100  and attaching the sensor substrate  100  to a robotic device according to an exemplary embodiment of the present disclosure.  FIG. 6  will be described with reference to the components of  FIG. 1 - FIG. 5  though it is to be appreciated that the method  600  of  FIG. 6  may be used with other systems/components. In one embodiment, the method  600  may be included as computer implemented instructions that are stored within an electronic memory and may be accessed and executed by a processor of a computing system to operably control mechanical equipment (e.g., machinery) to fabricate the top stretchable layer  102  of the sensor substrate  100  and the bottom flexible layer  104  of the sensor substrate  100 , and to attach the bonded layers that comprise the sensor substrate  100  to a robotic device. 
     The method  600  may begin at block  602 , wherein the method  600  may include casting a dielectric layer  202 . In one embodiment, the fabrication system may begin the fabrication process to fabricate the top stretchable layer  102  of the sensor substrate  100  by casting the dielectric layer  202  as a bottom portion of the top stretchable layer  102 . As discussed above, the dielectric layer  202  may be cast in a mold using an elastic material. In some configurations, the dielectric layer  202  may have structures such as pillars, pyramids or domes to fine tune mechanical properties as desired. 
     The method  600  may proceed to block  604 , wherein the method  600  may include fabricating a stretchable electrode pattern  204 . In one embodiment, the fabrication system may fabricate the stretchable electrode pattern  204  with a material of choice using a patterning process of choice. For example, spray coating, shadow mask, and/or screen printing may be utilized to pattern carbon nanotubes, silver nanowires, conducting polymer and/or conducting particle composites as materials of the stretchable electrode pattern  204 . 
     The method  600  may proceed to block  606 , wherein the method  600  may include casting an encapsulating layer  206  to complete fabrication of the top stretchable layer  102 . In one embodiment, the encapsulating layer  206  may be cast upon the stretchable electrode pattern  204  using the same or similar elastic material used to cast the dielectric layer  202  of the top stretchable layer  102 . Accordingly, as shown in  FIG. 2B , the encapsulating layer  206  may be cast atop the stretchable electrode pattern  204  which is included upon the dielectric layer  202  to complete fabrication of the top stretchable layer  102 . 
     The method  600  may proceed to block  608 , wherein the method  600  may include fabricating a soft adhesive sheet  302 . In one embodiment, the fabrication system may begin the fabrication process to fabricate the bottom flexible layer  104  of the sensor substrate  100  by fabricating the soft adhesive sheet  302  as a bottom portion of the bottom flexible layer  104 . The soft adhesive sheet  302  may be configured as a double sided acrylic tape sheet that is configured with varying properties of mechanical stiffness and chemical stability. 
     The method  600  may proceed to block  610 , wherein the method  600  may include laminating a copper film  304  upon the soft adhesive sheet  302 . In one embodiment, the fabrication system may laminate the thin sheet of copper film  304  upon a top side portion of the soft adhesive sheet  302 . In one embodiment, the copper film  304  may be patterned using photolithography. This may enable the fabrication of passive electronic components such as surface mount resistors directly on the circuit board associated with the sensor substrate  100 . This functionality enables the ability to design complex electrode designs which otherwise may not be possible to fabricate using alternative processes such as shadow mask patterning of composites. 
     The method  600  may proceed to block  612 , wherein the method  600  may include laminating a dry film photoresist  306  upon a top potion of the copper film  304 . Upon the lamination of the copper film  304  upon the soft adhesive sheet  302 , the fabrication system may utilize a thermal laminator to laminate a dry film photoresist  306  upon a top portion of the copper film  304 . 
     The method  600  may proceed to block  614 , wherein the method  600  may include etching undesired portions of copper of the copper film  304 . In an exemplary embodiment, the fabrication system may utilize the exposed portions of dry film photoresist pattern  308  as a mask to etch undesired portions of copper of the copper film  304  previously laminated upon the soft adhesive sheet  302 . Accordingly, the soft adhesive sheet  302  may include the etched copper  310  with respective portions of dry film photoresist  306  that remain upon the soft adhesive sheet  302 . 
     The method  600  may proceed to block  616 , wherein the method  600  may include enabling patterned copper electrodes to remain upon the etched copper  310  to complete fabrication of the bottom flexible layer  104 . In one embodiment, the fabrication system may remove the photoresist, leaving the patterned copper electrodes  312 . The patterned copper electrodes  312  may include traces that run to the circuit board that is associated with the sensor substrate  100 . Accordingly, as shown in  FIG. 3B , the bottom flexible layer  104  may be fabricated with the soft adhesive sheet  302  that includes the patterned copper electrodes  312  that are included upon the etched copper  310 . The bottom flexible layer  104  may be configured to be in conformity with the arbitrary smooth geometries with smooth segments of small radii of curvatures, to which a high level of conformity may be achieved with a suitable copper film thickness and copper pattern size and shape. 
     The method  600  may proceed to block  618 , wherein the method  600  may include bonding the top stretchable layer  102  to the bottom flexible layer  104  to form the sensor substrate  100 . In one embodiment, upon the fabrication of the top stretchable layer  102  of the sensor substrate  100  (at block  606 ) and the bottom flexible layer  104  of the sensor substrate  100  (at block  616 ), the fabrication system may be configured to bond the top stretchable layer  102  to the bottom flexible layer  104  to form the sensor substrate  100 . In one embodiment, the top layer of elastic material  414  may include an adhesive coating that may enable a strong adhesion between the dielectric layer  202  of the top stretchable layer  102  and the layer of elastic material  414  of the bottom flexible layer  104 . Accordingly, the top stretchable layer  102  may be bonded to the bottom flexible layer  104  to form the sensor substrate  100 . 
     The method  600  may proceed to block  620 , wherein the method  600  may include attaching the sensor substrate  100  to a robotic device. In an exemplary embodiment, upon the completion of the sensor fabrication, the sensor substrate  100 , which is adhesive on the bottom face may be placed upon any arbitrary geometry to be robustly adhered to the arbitrary geometry. As discussed above with respect to  FIG. 5 , once the sensor fabrication is complete, the sensor substrate  100  which is adhesive on a bottom face is used to adhere the sensor substrate  100  on any arbitrary smooth geometry such as to the robot finger/hand  502 . 
     Since the fabrication system utilizes the soft adhesive sheet  302  as the base of the sensor substrate  100 , robust adhesion arbitrary smooth geometry such as to the robot finger/hand  502  is achieved with little risk of delamination as the robotic device interacts with the physical world. Additionally, this functionality also ensures robust adhesion of the patterned copper electrodes  312  on the sensor substrate  100  and also the top elastic layers that are built upon it. 
       FIG. 7  is a process flow diagram of a method for fabricating soft sensors that conform to arbitrary smooth geometries according to an exemplary embodiment of the present disclosure.  FIG. 7  will be described with reference to the components of  FIG. 1 - FIG. 5  though it is to be appreciated that the method  700  of  FIG. 7  may be used with other systems/components. The method  700  may begin at block  702 , wherein the method  700  may include fabricating a top stretchable layer  102  that includes a set of electrodes of soft sensors that are made of an elastic material. 
     The method  700  may proceed to block  704 , wherein the method  700  includes fabricating a bottom flexible layer  104  that is composed of copper films that are patterned using photolithography. In one embodiment, the bottom flexible layer  104  is configured to be in conformity with the arbitrary smooth geometries. The method  700  may proceed to block  706 , wherein the method  700  includes bonding the top stretchable layer  102  to the bottom flexible layer  104  to form a sensor substrate  100 . In one embodiment, the sensor substrate  100  is configured as a stretchable adhesive film which enables robust adhesion to the arbitrary smooth geometries. 
     It should be apparent from the foregoing description that various exemplary embodiments of the disclosure may be implemented in hardware. Furthermore, various exemplary embodiments may be implemented as instructions stored on a non-transitory machine-readable storage medium, such as a volatile or non-volatile memory, which may be read and executed by at least one processor to perform the operations described in detail herein. A machine-readable storage medium may include any mechanism for storing information in a form readable by a machine, such as a personal or laptop computer, a server, or other computing device. Thus, a non-transitory machine-readable storage medium excludes transitory signals but may include both volatile and non-volatile memories, including but not limited to read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and similar storage media. 
     It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in machine readable media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     It will be appreciated that various implementations of the above-disclosed and other features and functions, or alternatives or varieties thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.