Patent Publication Number: US-2023147160-A1

Title: Blunt Force Sensor Array

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of prior-filed, co-pending U.S. Provisional Application No. 63/168,503 filed on Mar. 31, 2021, the entire contents of which are hereby incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENTAL INTEREST 
     This invention was made with Government support under contract number N00024-13-D-6400 awarded by the Naval Sea Systems Command (NAVSEA). The Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     Example embodiments generally relate to sensor technologies, and in particular force sensor technologies including pressure and strain forces. 
     BACKGROUND 
     Sensor technologies have become ubiquitous for use in a variety of testing environments, as well as, in completed products. For example, automobiles utilize impact sensors to detect a collision to trigger the deployment of an air bag. Such force sensors are not space-constrained and can therefore be placed within the body of the vehicle with relative ease. However, in many implementations, particularly in test environments, space and other limitations can exist. Additionally, the presence of testing sensors should not change the results of the test. As such, an ideal test sensor is one that is present within the test environment to take the necessary measurements, but otherwise takes up no space and its presence has no impact on the measurements. 
     An example testing environment where strict requirements on sensors exist is in the helmet testing and development space, and, in particular, with respect to testing that is performed to measure behind helmet blunt trauma (BHBT). In this regard, sensors are needed that can measure the forces transferred to, for example, the skull by the interior of a helmet in response to an impact on the helmet. Conventional sensors that could be used for such testing are too large to be able to perform a proper test. Therefore, there is a need for an improved sensor that can be utilized in space-constrained testing environments without having a significant effect on the forces that are to be measured. 
     BRIEF SUMMARY 
     According to some example embodiments, a blunt force sensor array for application to a non-planar surface is provided. The blunt force sensor array may include, i.e. comprise, a flexible thin-film substrate, a plurality of force sensors secured to the flexible thin-film substrate proximate to a center measurement point, a strain gauge secured on the flexible thin-film substrate proximate to the center measurement point, and a sensor interface configured to connect to external measurement and control circuitry. In this regard, the sensor interface may be electrically connected to each of the force sensors and the strain gauge via traces disposed on the flexible thin-film substrate. A flexibility and a shape of the flexible thin-film substrate may permit the blunt force sensor array to be applied to the non-planar surface to detect forces and strains experienced by the non-planar surface in response to a blunt force event on the non-planar surface. 
     According to some example embodiments, a blunt force measurement system is provided. The blunt force measurement system may include a blunt force sensor array configured to be subdermally implanted on a non-planar surface of a bone of a test subject, and measurement and control circuitry disposed external to the test subject and configured to interface with the blunt force sensor array to convert signals provided by the blunt force sensor array into force and strain measurements. The blunt force sensor array may include a flexible thin-film substrate secured to the non-planar surface of the bone, a plurality of force sensors secured to the flexible thin-film substrate proximate to a center measurement point, a strain gauge secured on the flexible thin-film substrate proximate to the center measurement point, and a sensor interface configured to connect to the measurement and control circuitry. The sensor interface may be electrically connected to each of the force sensors and the strain gauge via traces disposed on the flexible thin-film substrate. A flexibility and a shape of the flexible thin-film substrate may permit the blunt force sensor array to be applied to the non-planar surface to detect forces and strains experienced by the non-planar surface in response to a blunt force event on the non-planar surface. 
     According to some example embodiments, a method of subdermally implanting a blunt force sensor array in a test subject is provided. In this regard, the method may include making an incision in a dermal layer of the test subject, and applying a blunt force sensor array onto a non-planar surface of a bone of the test subject. The blunt force sensor array may include a central force sensor, a first peripheral force sensor, a second peripheral force sensor, and a strain gauge disposed on a flexible thin-film substrate. Applying the blunt force sensor array onto the non-planar surface includes applying the blunt force sensor array such that sensing areas of the central force sensor, the first peripheral force sensor, the second peripheral force sensor, and the strain gauge disposed on a flexible thin-film substrate are disposed in different planes. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Having thus described some example embodiments in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG.  1    illustrates an example behind helmet blunt trauma test environment according to some example embodiments; 
         FIG.  2    illustrates a top view of a blunt force sensor array according to some example embodiments; 
         FIG.  3    illustrates a side cross-section view of a blunt force sensor array according to some example embodiments; 
         FIG.  4    illustrates a top view of another blunt force sensor array according to some example embodiments; and 
         FIG.  5    illustrates a block diagram of an example method for subdermally implanting a blunt force sensor array in a test subject according to some example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Some example embodiments now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all example embodiments are shown. Indeed, the examples described and pictured herein should not be construed as being limiting as to the scope, applicability or configuration of the present disclosure. Rather, these example embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like reference numerals refer to like elements throughout. Furthermore, as used herein, the term “or” is to be interpreted as a logical operator that results in true whenever one or more of its operands are true. As used herein, operable coupling should be understood to relate to direct or indirect connection that, in either case, enables functional interconnection of components that are operably coupled to each other. 
     According to various example embodiments, a blunt force sensor array and associated systems are provided. To be useful in a many space constrained environments the blunt force sensor array may be constructed on a flexible thin-film substrate. The thin-film substrate may provide a support structure upon which the blunt force sensor array may be constructed, while still permitting the array to conform to a non-planar surface (e.g., a sphere, the shape of a skull, or the like). Because a number of measurements at different locations may be useful to improve measurement accuracy and to determine force propagation, a plurality of force sensors may be included at different locations on the blunt force sensor array. Additionally, a strain gauge or strain sensor may also be included on the blunt force sensor array. According to some example embodiments, the sensors themselves may be thin (e.g., less than a millimeter scale) and may be formed as thin-film sensors for application to the flexible thin-film substrate. These sensors may be affixed (via an adhesive) to the flexible thin-film substrate in a defined arrangement, and that arrangement may be reproduced for use in a number of blunt force sensor arrays. In doing so, test repeatability can result due to the use of the array&#39;s common, unitary form factor. Because the spacing and placement of the sensors may be known, implementation of a blunt force sensor array within a test subject may simply require rotational orientation measurements relative to the center measurement point. As such, the flexible thin-film substrate may be used as an alignment key for placement of the blunt force sensor array during a procedure to simplify the placement procedure. This is an improvement over procedures that involve placement of individual sensors, which can add error to each placement in repeated tests. 
     According to some example embodiments, the flexibility and shape of the flexible thin-film substrate may permit the blunt force sensor array to be applied to a non-planar surface. Once applied, the blunt force sensor array may operate to detect forces (including strains) experienced by a non-planar surface to which the blunt force sensor array is applied. With respect to shape, the blunt force sensor array may include a number of flexible fingers with respective sensors. The fingers of the blunt force sensor array may be independently bendable at a base or bending region of the finger to permit the sensors to conform to a variety of surface shapes. Such non-planar surface shapes may include spherical shapes and non-uniform shapes such as the shape of a skull bone. In this way, the inclusion of the fingers of the flexible thin-film substrate permits the sensors to be positioned such that the sensors wrap around a region of a non-planar surface to support proper engagement of the sensors with the surface to be measured. 
     Such a blunt force sensor array may be useful in a number of different applications, including behind helmet blunt trauma (BHBT) testing. In such testing environments, the blunt force sensor array may be placed between the helmet and the skull of a cadaver or live subject (e.g., an animal such as a pig). According to some example embodiments, the blunt force sensor array may be subdermally implanted in the cadaver or live subject such that the flexible thin-film substrate and the sensors are in contact with or adhered to the skull bone surface to improve detection of the forces experienced by the skull bone in response to a blunt force test event. To satisfy the biological conditions of such a test, the blunt force sensor array, according to some example embodiments, may be sealed and waterproof to avoid the intrusion of fluids that may, for example, contact the electrical traces and affect signaling and transmission of data associated with sensor control and measurements. 
     Having described some example embodiments, reference will now be made to  FIG.  1   , which illustrates an example BHBT test environment  100 . A helmet  120  is shown as applied to a test subject  110  having a dermal layer  112  and a skull surface  114 . An example blunt force sensor array  150  may be applied beneath the dermal layer  112  and directly onto the skull surface  114 , which may be a non-planer surface. 
     As mentioned above, the blunt force sensor array  150  may be implanted within the test subject via an incision location. Because the sensors of the blunt force sensor array  150  are affixed to the flexible thin-film substrate, all of the sensors may be placed with relative positioning based on the shape of the flexible thin-film substrate (rather than being separately positioned) thereby requiring a simpler placement procedure. As subtly shown in  FIG.  1   , the blunt force sensor array  150  conforms to the non-planar skull surface due to the flexibility of the substrate. A sensor interface extension or finger  152  of the flexible thin-film substrate may be extend a distance away from the sensors and pass out of the dermal layer  112  as a subcutaneous pass through to an external sensor interface  154 . 
     The sensor interface  154  may include a coupler or a plug that facilitates an electrical connection to the measurement and control circuitry  160 . The measurement and control circuitry  160  may be configured to receive and interpret sensor signals supplied by each of the sensors of the blunt force sensor array  150 . The sensors of the blunt force sensor array  150  may include any type of sensors including force sensor strain gauges, or the like. For example, the sensors may be configured to output a voltage that is based on the force that is detected by the force sensors. Similarly, a strain gauge of the blunt force sensor array  150  may output a voltage that is based on the strain (e.g., compression or tension) that has been detected by the strain gauge. The measurement and control circuitry  160  may be configured to receive these voltage signals from the sensors and convert those signals into force measurement values in Newtons or the like and associated positions for analysis. 
     In this regard, the measurement and control circuitry  160  may embody or include processing circuitry that may be configurable to receive and interpret sensor signals. In this regard, the measurement and control circuitry  160  may include a processor, a memory, and various passive components for driving the sensors. According to some example embodiments, measurement and control circuitry  160  may be in operative communication with or embody, the memory and the processor. Through configuration and operation of the memory and the processor, the measurement and control circuitry  160  may be configurable to perform various operations as described herein, including the operations and functionalities described with respect to receipt, interpretation or conversion, and analysis of sensor signals. In this regard, the measurement and control circuitry  160  may be configured to perform computational processing, memory management, and sensor interface control and monitoring, according to some example embodiments. In some embodiments, the measurement and control circuitry  160  may be embodied as a chip or chip set. In other words, the measurement and control circuitry  160  may include one or more physical packages (e.g., chips) including materials, components or wires on a structural assembly (e.g., a baseboard). The measurement and control circuitry  160  may be configured to receive inputs (e.g., sensor signals via the sensor interface  154 ), perform actions based on the inputs, and generate outputs as described herein. In an example embodiment, the measurement and control circuitry  160  may include one or more instances of a processor, associated circuitry, and memory. Further, the measurement and control circuitry  160  may be embodied as a circuit chip [e.g., an integrated circuit chip, such as a field programmable gate array (FPGA)] configured (e.g., with hardware, software or a combination of hardware and software) to perform operations described herein. 
     In an example embodiment, the memory of the measurement and control circuitry  160  may include one or more non-transitory memory devices such as, for example, volatile or non-volatile memory that may be either fixed or removable. The memory may be configured to store information, data, applications, instructions or the like for enabling, for example, the functionalities described with respect to the measurement and control circuitry  160 . The memory may operate to buffer instructions and data during operation of the measurement and control circuitry  160  to support higher-level functionalities, and may also be configured to store instructions for execution by the measurement and control circuitry  160 . The memory may also store various information including conversion algorithms. According to some example embodiments, various data stored in the memory may be generated based on other data and stored or the data may be retrieved. 
     As mentioned above, the measurement and control circuitry  160  may be embodied in a number of different ways. For example, the measurement and control circuitry  160  may be embodied as various processing means such as one or more processors that may be in the form of a microprocessor or other processing element, a coprocessor, a controller or various other computing or processing devices including integrated circuits such as, for example, an ASIC (application specific integrated circuit), an FPGA, or the like. In an example embodiment, the measurement and control circuitry  160  may be configured to execute instructions stored in the memory or otherwise accessible to the measurement and control circuitry  160 . As such, whether configured by hardware or by a combination of hardware and software, the measurement and control circuitry  160  may represent an entity (e.g., physically embodied in circuitry—in the form of measurement and control circuitry  160 ) capable of performing operations according to example embodiments while configured accordingly. Thus, for example, when the measurement and control circuitry  160  is embodied as an ASIC, FPGA, or the like, the measurement and control circuitry  160  may be specifically configured hardware for conducting the operations described herein. Alternatively, as another example, when the measurement and control circuitry  160  is embodied as an executor of software instructions, the instructions may specifically configure the measurement and control circuitry  160  to perform the operations described herein. 
     With the measurement and control circuitry  160  configured according to various example embodiments, the BHBT test environment  100  may be configured to perform a test operation. In this regard, a projectile  191  may be fired or launched at the helmet  120  in alignment with the blunt force sensor array  150 . In this regard, prior to being fired or launched, the path  190  of the projectile  191  may be aligned with a measurement center point for the blunt force sensor array  150 . Upon impact with the helmet  120 , forces in the projectile  191  may be transferred to the helmet  120 . While the helmet  120  may dissipate some of the forces of the impact, some forces may be transferred through the helmet  120  to the test subject  110  and the skull surface  114  as indicated by arrow  192 . The forces and strains that reach the skull surface  114  may be measured by the sensors of the blunt force sensor array  150 . Sensor signals may be provided via the sensor interface  154  in real time (with the exception of circuitry delays) to the measurement and control circuitry  160  for storage and/or analysis. According to some example embodiments, the measurement and control circuitry  160  may be configured to analyze the sensor signals to construct a map of the intensities of the forces and strains over time. The analysis of the sensor signals may also be used to the determine potential health effects of the projectile impact on the helmet  120  (e.g., likelihood of a concussion, loss of consciousness, brain cell bruising, torn tissue, bleeding, and other physical damage to the brain). 
     Having described a test environment and implementation of an example blunt force sensor array,  FIGS.  2  to  4    will now be described which show example constructions of blunt force sensor arrays according to some example embodiments. It is understood that the layouts and positioning of the sensors in the blunt force sensor arrays described herein are exemplary. However, the inclusion of extended portions or fingers facilitates an ability to move or bend the sensors into different relative positions to conform the sensing areas of the sensors in to close engagement with a non-planar surface. 
     Now referring specifically to  FIG.  2   , a blunt force sensor array  200  is shown that may be the same or similar to the blunt force sensor array  150  of  FIG.  1   . In general, the blunt force sensor array  200  may include a flexible thin-film substrate  210 , a plurality of force sensors (e.g., center force sensor  260 , first peripheral force sensor  262 , and second peripheral force sensor  264 ), and a strain gauge  266 . The blunt force sensor array  200  may also include a sensor interface  270 . The sensor interface  270  may be a coupler or connector to electrically connect the blunt force sensor array  200  to external measurement and control circuitry. According to some example embodiments, the sensor interface  270  may include a connector (e.g., a twelve pin connector). 
     The flexible thin-film substrate  210  may include a flexible supportive substance that may be supplied as a sheet for cutting out a desired design for the flexible thin-film substrate  210 . According to some example embodiments, the flexible thin-film substrate  210  may include a polyamide film (e.g., KAPTON®) and may include a metallic layer that may be included or added and etched to form traces  271  of a circuit. According to some example embodiments, the flexible thin-film substrate  210  may be PYRALUX® LF-series flexible circuit board substrate. 
     The layout of the flexible thin-film substrate  210  may include a plurality of extensions or fingers. Each finger of the flexible thin-film substrate  210  may have at least one force sensor, a strain gauge, or the sensor interface  270  affixed thereto. In this regard, the flexible thin-film substrate  210  may include, for example, four fingers that generally extend away from a center region in a radial manner. To form the fingers, voids may be cut between the fingers. As such, the flexible thin-film substrate  210  may include a first finger  211  to which the first peripheral force sensor  262  is affixed, a second finger  212  to which the second peripheral force sensor  264  is affixed, a third finger  213  to which the strain gauge  266  is affixed, and a fourth finger  214  to which the sensor interface  270  is affixed. As mentioned above, to form the fingers void  215  may be disposed between the fourth finger  214  and the first finger  211 , void  216  may be disposed between the first finger  211  and the second finger  212 , void  217  may be disposed between the second finger  212  and the third finger  213 , and void  218  may be disposed between the third finger  213  and the fourth finger  214 . 
     The cutting of the voids can form natural bend lines or regions at a base of each finger where the fingers are most likely to bend. In a substantially symmetric design of the flexible thin-film substrate  210 , bend lines  220 ,  221 ,  222 , and  223  are formed between apexes of the voids. The bend lines allow for bending in addition to the substrate  210  being flexible which permits bending elsewhere as well. As such, if applied to a non-planar surface, the first finger  211  may bend about the bend line  220 , the second finger  212  may bend about the bend line  221 , the third finger  213  may bend about the bend line  222 , and the fourth finger  214  may bend about the bend line  222 . As mentioned earlier, such bend lines are not the only bending that the flexible thin-film substrate  210  may perform, but the existence of the voids tends to create such natural bend lines or bend regions. As such, differently shaped fingers and voids may result is different bend lines and regions. In any event, the fingers  211 ,  212 ,  213 , and  214  may have an ability to bend independent of each other at these bend lines or regions to conform the sensors to a non-planar or non-uniform surface. 
     According to some example embodiments, the blunt force sensor array  200  may include a plurality of force sensors. In the example embodiment shown  FIG.  2   , the blunt force sensor array  200  includes three force sensors, i.e., the central force sensor  260 , the first peripheral force sensor  262 , and the second peripheral force sensor  264 . According to some example embodiments, each of these force sensors may be identically constructed, but placed at different locations on the flexible thin-film substrate  210 . For example, the central force sensor  260  may be a thin film force sensor with a thickness of less than 0.25 millimeters. As such, a thickness of the blunt force sensor array  200  at any of the sensing areas of the plurality of force sensors may be less than about 0.5 millimeters. According to some example embodiments, a voltage biasing scheme may be applied to the central force sensor  260  to determine a range of forces that the sensor is configured to detect. The central force sensor  260  may change a characteristic value (e.g., electrical resistance) when a force is applied to the sensor. Based on the test or application for the blunt force sensor array  200 , an appropriate voltage-biasing scheme may be used. According to some example embodiments, the change in electrical resistance may be indicated in a voltage of a signal that is provided by the central force sensor  260  to the sensor interface  270 . The central force sensor  260  may have a sensing area  261 . The sensing area  261  may be an area where a measurement of force is taken for a force applied to the sensing area  261 . According to some example embodiments, the central force sensor  260  may be a TEKSCAN® FLEXIFORCE® sensor A 301 . As mentioned above, the first peripheral force sensor  262  and the second peripheral force sensor  264  may be the same or similar to the central force sensor  260 . In this regard, the first peripheral force sensor  262  may have a sensing area  263  and the second peripheral force sensor  264  may have a sensing area  265 . 
     The strain gauge  266  may be configured to measure compression or tension in the surface to which the strain gauge  266  is affixed. According to some example embodiments, the strain gauge  266  may be a triaxial strain gauge. More specifically, according to some example embodiments, the strain gauge  266  may be a KYOWA® KFW strain gauge. The strain gauge  266  may change a characteristic value (e.g., electrical resistance) when a strain is applied to the gauge. According to some example embodiments, the change in electrical resistance may be indicated in a voltage of a signal that is provided by the strain gauge  266  to the sensor interface  270 . Similar to the force sensors, the strain gauge  266  may have a sensing area  267 . 
     The sensors may be electrically connected to the sensor interface  270  via traces  271 . The traces  271  may be formed from a thin layer of metal (e.g., copper) that may be etched to define the traces  271 . The traces  271  may carry a control and/or output signal for each of the sensors. In this regard,  FIG.  3    shows a cross-section of the blunt force sensor array  200  taken from the sensor interface  270  to the central force sensor  260  along the fourth finger  214 . As can be seen in  FIG.  3   , the trace  271  sits atop the flexible thin-film substrate  210 . Additionally, according to some example embodiments, a protecting thin film layer  276  may be applied over the traces  271  to protect and waterproof the traces  271 . The traces  271  may be connected to the respective sensors at connection points  272  ( FIGS.  2  and  3   ). In this regard, the traces  271  may form a pad at the connection points  272  and the pins of the sensors, for example, may be soldered to the pads to form the connections. As shown in  FIG.  3   , the pin  273  of central force sensor  260  is connected to trace  271  at connection point  272  via solder  274 . According to some example embodiments, the soldered connection points  272  may be encapsulated with an insulating substance  276  to seal and waterproof the connection points  272 . The configuration of the connection point  272  in  FIG.  3    is provided as an example how the other connection points  272  may also be embodied. 
     Referring back to  FIG.  2   , a description of the placement of the sensors and fingers for the blunt force sensor array  200  will now be provided. In this regard, according to some example embodiments, the plurality of force sensors  260 ,  262 , and  264  may be secured to the flexible thin-film substrate  210  proximate to a center measurement point  250 . Similarly, the strain gauge  266  may be secured on the flexible thin-film substrate  210  proximate to the center measurement point  250 . In this regard, according to some example embodiments, the central force sensor  260  may be disposed such that the center measurement point  250  is centrally located within the sensing area  261  of the central force sensor  260 . According to some example embodiments, the center measurement point  250  may be a location that, in a test environment implementation, is aligned with an impact force, for example, delivered by a projectile. 
     With respect the positioning of the components of the blunt force sensor array  200 , it is noted that the blunt force sensor array  200  is shown in a flat, planar configuration. While not intended to be applied to a flat surface, the relative positions of the components of the blunt force sensor array  200  may be described while in such as configuration. In this regard, when flat, sensing areas of the first peripheral force sensor  262 , the second peripheral force sensor  264 , and the strain gauge  266  may be disposed a common radial distance away from the center measurement point  250 . Additionally, according to some example embodiments, the sensor interface  270  may be disposed a distance, i.e., a sensor interface distance, from the center measurement point  250 . The sensor interface distance may be larger than the common radial distance for the sensors, for example, to limit an effect of the external connections to the sensor interface  270  on measurements performed by the plurality of force sensors and the strain gauge. Additionally, the fourth finger  214  may be used as a subcutaneous pass through, when the blunt force sensor array  200  is implanted into a test subject, and the added length may facilitate such a pass through. In this regard, the sensor interface  270  may be disposed external to the test subject, while the remainder of the blunt force sensor array  200  may be subdermally implanted, for example, onto a bone (e.g., the skull). 
     Additionally, the peripheral force sensors  262  and  264  and the strain gauge  266  may be disposed at the distal ends of their respective fingers (with the opposite proximal ends being more centrally located). In this regard, the first peripheral force sensor  262  may be disposed at a distal end of a first finger  211  and the second peripheral force sensor  264  may be disposed at a distal end of the second finger  212 . Additionally, the strain gauge  266  may be disposed at a distal end of a third finger  213 . Further, according to some example embodiments, the sensing areas of the central force sensor  260 , the first peripheral force sensor  262 , and the strain gauge  266  may be in a linear alignment (as indicated by line  251 ) when the flexible thin-film substrate  210  is flat. Additionally, the second peripheral force sensor  264  may be positioned on a line  252  through the center measurement point  250  that is perpendicular to line  251 . Such linear alignments and relative positioning may assist with data analysis and alignment for determining the positions of force measurements and strain measurements, for example, to map the positions of the forces and strains on the non-planar surface being evaluated. 
     Due to the positioning of the plurality of force sensors and the strain gauge on the flexible thin-film substrate  210 , the blunt force sensor array  200  may be flexed and bent into a shape that conforms with a non-planar surface, such that the plurality of force sensors and the strain gauge are in close engagement with the non-planar surface to measure forces and strains. Additionally, according to some example embodiments, the voids between the first finger, the second finger, and the third finger operate to permit each of the first finger, the second finger, and the third finger to be bent about at a respective finger base and an associated bend line or region relative to each other to conform the blunt force sensor array to a non-planar surface. 
     Now referring to  FIG.  400    another blunt force sensor array  400  is shown that includes the components of the blunt force sensor array  200  (the same reference numerals refer to the same components), however with a differently shaped flexible thin-film substrate  410 . The flexible thin-film substrate  410  may be formed in the same or similar manner as the flexible thin-film substrate  210 , however with a different cut shape. Further, in the construction of the blunt force sensor array  400 , the packaging of the sensors of the array  400  may be clipped or cut to reduce the size of the sensors and thus the blunt force sensor array  400 . 
     As can be seen in  FIG.  4   , the fingers  411 ,  412 , and  413  of the flexible thin-film substrate  410  for the sensors are cut from a substantially circular lower portion (i.e., sensor section) of the flexible thin-film substrate  410  with a finger  414  for the sensor interface  270  extending upwards. According to some example embodiments, a diameter of the substantially circular lower portion may be about 5 centimeters. The first finger  411  forms an extension upon which the first peripheral force sensor  262  is affixed with a sensor area at the first finger&#39;s distal end. The cutting of void  415  between the fourth finger  414  and the first finger  411  and void  415  between the first finger  411  and the second finger  412  can create a natural bending region  420  where the first finger  411  may bend to conform to a non-planar surface. 
     The second finger  412  forms an extension upon which the second peripheral force sensor  264  is affixed with a sensor area at the second finger&#39;s distal end. The cutting of void  416  between the first finger  411  and the second finger  412  and void  417  between the second finger  412  and the third finger  413  can create a natural bending region  421  where the second finger  411  may bend to conform to a non-planar surface. The voids  416  and  417  also extend inwards to be adjacent to the central force sensor  260 . As such, the second finger  412  may have a natural bending region near an intersection of the fingers. Note also that, to maintain a small footprint for the flexible thin-film substrate  410 , the orientation of the pins of second peripheral force sensor  264  is rotated to the left. By rotating the sensor  264  in this way, the overall layout of the flexible thin-film substrate  410  remains compact while also lessening a length of the traces  271  that feed the second peripheral force sensor  264  (i.e. the most distant sensor from the sensor interface  270 ). 
     The third finger  413  forms an extension upon which the strain gauge  266  is affixed with a sensor area at the third finger&#39;s distal end. The cutting of void  417  between the second finger  412  and the third finger  413  and void  418  between the third finger  413  and the fourth finger  414  can create a natural bending region  422  where the third finger  413  may bend to conform to a non-planar surface. Finally, the fourth finger  414  forms an extension upon which the sensor interface  270  is affixed at the fourth finger&#39;s distal end. The cutting of void  418  between the third finger  413  and the fourth finger  414  and void  415  between the fourth finger  414  and the first finger  411  can create a natural bending region  423  where the fourth finger  414  may bend to conform to a non-planar surface. 
     Note that the characteristics (angles, depths, widths, etc.) of the voids between the fingers may determine where and how the fingers may naturally bend. According to some example embodiments, the non-planar surface or the types of non-planar surfaces to which the blunt force sensor array  400  is to be affixed may be analyzed to determine characteristics for the voids that form the fingers (angles, depths, widths, etc.). In this regard, depending on the curvature and/or common features on the non-planar surface, the voids may be determined improve engagement of the sensors with the non-planar for improved measurements. 
     Now referring to  FIG.  5   , a method for subdermally implanting a blunt force sensor array in a test subject is provided. The blunt force sensor array to be implanted may be the blunt force sensor array  150 ,  200 , or  400 , and variations thereof as described herein. In this regard, the example method may include, at  500 , making an incision in a dermal layer of the test subject. The incision is made to provide access to the bone structures to which the blunt force sensor array will be affixed or placed near. Subsequently, at  510 , the example method may include applying the blunt force sensor array onto a non-planar surface of a bone (e.g., a skull) of the test subject. As mentioned above, the blunt force sensor array may be the same or similar to the blunt force sensor array  150 ,  200 , or  400 . In this regard, the blunt force sensor array may include a central force sensor, a first peripheral force sensor, a second peripheral force sensor, and a strain gauge disposed on a flexible thin-film substrate. Further, according to some example embodiments, applying the blunt force sensor array onto a non-planar surface may also include applying the blunt force sensor array such that sensing areas of the central force sensor, the first peripheral force sensor, the second peripheral force sensor, and the strain gauge disposed on a flexible thin-film substrate are on different planes. In this regard, the flexibility of the blunt force sensor array may permit the sensor and, more specifically, the sensing areas of the sensors to be engaged to be bent into positions such that the sensing areas are oriented in different planes (e.g., the sensing areas are engaged with respective positions on a generally spherical shape such as a skull bone). 
     As such, the following provides a description of some example embodiments in light of the subject matter included herein. In this regard, a blunt force sensor array for application to a non-planar surface is provided. The blunt force sensor array may include a flexible thin-film substrate, a plurality of force sensors secured to the flexible thin-film substrate proximate to a center measurement point, a strain gauge secured on the flexible thin-film substrate proximate to the center measurement point, and a sensor interface configured to connect to external measurement and control circuitry. The sensor interface may be electrically connected to each of the force sensors and the strain sensor via traces disposed on the flexible thin-film substrate. A flexibility and a shape of the flexible thin-film substrate permits the blunt force sensor array to be applied to the non-planar surface to detect forces and strains experienced by the non-planar surface in response to a blunt force event on the non-planar surface. 
     According to some example embodiments, the plurality of force sensors may include a central force sensor, a first peripheral force sensor, and a second peripheral force sensor. The sensing area of the central force sensor may be disposed at the center measurement point. The sensing areas of the first peripheral force sensor, the second peripheral force sensor, and the strain gauge may be disposed a common radial distance away from the center measurement point when the flexible thin-film substrate is flat. According to some example embodiments, the first peripheral force sensor may be disposed at a distal end of a first finger of the flexible thin-film substrate. The second peripheral force sensor may be disposed at a distal end of a second finger of the flexible thin-film substrate. The strain gauge may be disposed at a distal end of a third finger of the flexible thin-film substrate. The flexible thin-film substrate may include voids between the first finger, the second finger, and the third finger to permit each of the first finger, the second finger, and the third finger to bend about at a respective finger base relative to each other to conform the blunt force sensor array to the non-planar surface. According to some example embodiments, sensing areas of the first peripheral force sensor, the second peripheral force sensor, and the strain gauge may be disposed a common radial distance away from the center measurement point. The sensor interface may be disposed at a distal end of a fourth finger of the flexible thin-film substrate. The sensor interface may be disposed a sensor interface distance from the center measurement point. The sensor interface distance may be larger than the common radial distance to limit an effect of external connections to the sensor interface on measurements performed by the plurality of force sensors and the strain gauge. According to some example embodiments, the plurality of force sensors may include a central force sensor, a first peripheral force sensor, and a second peripheral force sensor. Sensing areas of the central force sensor, the first peripheral force sensor, and the strain gauge may be in a linear alignment when the flexible thin-film substrate is flat. According to some example embodiments, the traces may be disposed on the flexible thin-film substrate and covered by a protecting film layer. According to some example embodiments, electrical connection points between the plurality of force sensors and the traces may be encapsulated by a waterproof substance to waterproof the blunt force sensor array. According to some example embodiments, the blunt force sensor array may be configured to operate as a subdermal implant that is affixed to bone. According to some example embodiments, each of the plurality of force sensors may include a sensing area. Additionally, a thickness of the blunt force sensor array at any of the sensing areas may be less than about 0.5 millimeters. 
     According to some example embodiments, a blunt force measurement system is provided. The blunt force measurement system may include a blunt force sensor array configured to be subdermally implanted on a non-planar surface of a bone of a test subject, and measurement and control circuitry disposed external to the test subject and configured to interface with the blunt force sensor array to convert signals provided by the blunt force sensor array into force and strain measurements. The blunt force sensor array may include a flexible thin-film substrate secured to the non-planar surface of the bone, a plurality of force sensors secured to the flexible thin-film substrate proximate to a center measurement point, a strain gauge secured on the flexible thin-film substrate proximate to the center measurement point, and a sensor interface configured to connect to the measurement and control circuitry. The sensor interface may be electrically connected to each of the force sensors and the strain sensor via traces disposed on the flexible thin-film substrate. A flexibility and a shape of the flexible thin-film substrate may permit the blunt force sensor array to be applied to the non-planar surface to detect forces and strains experienced by the non-planar surface in response to a blunt force event on the non-planar surface. According to some example embodiments, the plurality of force sensors may include a central force sensor, a first peripheral force sensor, and a second peripheral force sensor. A sensing area of the central force sensor may be disposed at the center measurement point. Sensing areas of the first peripheral force sensor, the second peripheral force sensor, and the strain gauge may be disposed a common radial distance away from the center measurement point when the flexible thin-film substrate is flat. According to some example embodiments, the plurality of force sensors may include a central force sensor, a first peripheral force sensor, and a second peripheral force sensor. The first peripheral force sensor may be disposed at a distal end of a first finger of the flexible thin-film substrate. The second peripheral force sensor may be disposed at a distal end of a second finger of the flexible thin-film substrate. A strain gauge may be disposed at a distal end of a third finger of the flexible thin-film substrate. The flexible thin-film substrate may include voids between the first finger, the second finger, and the third finger to permit each of the first finger, the second finger, and the third finger to bend about at a respective finger base relative to each other to conform the blunt force sensor array to the non-planar surface. According to some example embodiments, sensing areas of the first peripheral force sensor, the second peripheral force sensor, and the strain gauge may be disposed a common radial distance away from the center measurement point. The sensor interface may be disposed at a distal end of a fourth finger of the flexible thin-film substrate. The sensor interface may be disposed a sensor interface distance from the center measurement point. The sensor interface distance may be larger than the common radial distance to limit an effect of external connections to the sensor interface on measurements performed by the plurality of force sensors and the strain gauge. According to some example embodiments, the plurality of force sensors may include a central force sensor, a first peripheral force sensor, and a second peripheral force sensor. Sensing areas of the central force sensor, the first peripheral force sensor, and the strain gauge may be in a linear alignment when the flexible thin-film substrate is flat. According to some example embodiments, the traces may be disposed on the flexible thin-film substrate and covered by a protecting film layer. Electrical connection points between the plurality of force sensors and the traces may be encapsulated by a waterproof substance to waterproof the blunt force sensor array. According to some example embodiments, each of the plurality of force sensors may include a sensing area. A thickness of the blunt force sensor array at any of the sensing areas is less than about 0.5 millimeters. 
     According to some example embodiments, a method for subdermally implanting a blunt force sensor array in a test subject is provided. The method may include making an incision in a dermal layer of the test subject, and applying a blunt force sensor array onto a non-planar surface of a bone of the test subject. The blunt force sensor array may include a central force sensor, a first peripheral force sensor, a second peripheral force sensor, and a strain gauge disposed on a flexible thin-film substrate. Applying the blunt force sensor array onto the non-planar surface may include applying the blunt force sensor array such that sensing areas of the central force sensor, the first peripheral force sensor, the second peripheral force sensor, and the strain gauge, disposed on a flexible thin-film substrate, are disposed in different planes. According to some example embodiments, the first peripheral force sensor may be disposed at a distal end of a first finger of the flexible thin-film substrate. The second peripheral force sensor may be disposed at a distal end of a second finger of the flexible thin-film substrate. The strain gauge may be disposed at a distal end of a third finger of the flexible thin-film substrate. The flexible thin-film substrate may include voids between the first finger, the second finger, and the third finger to permit each of the first finger, the second finger, and the third finger to bend about at a respective finger base relative to each other to conform the blunt force sensor array to the non-planar surface. According to some example embodiments, each of the plurality of force sensors include a sensing area. A thickness of the blunt force sensor array at any of the sensing areas may be less than about 0.5 millimeters. 
     Many modifications and other embodiments of the measuring device set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the measuring devices are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although the foregoing descriptions and the associated drawings describe exemplary embodiments in the context of certain exemplary combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. In cases where advantages, benefits or solutions to problems are described herein, it should be appreciated that such advantages, benefits and/or solutions may be applicable to some example embodiments, but not necessarily all example embodiments. Thus, any advantages, benefits or solutions described herein should not be thought of as being critical, required or essential to all embodiments or to that which is claimed herein. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.