Patent Publication Number: US-11392816-B2

Title: Implantable passive RFID tag

Description:
RELATED APPLICATIONS 
     The present application is a National Phase entry of PCT Application No. PCT/EP2020/030138, filed Apr. 27, 2020, which claims priority from U.S. Application No. 62/839,106, filed Apr. 26, 2019, each of which is hereby fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to radio frequency identification (RFID) animal marking/tracking implants. More particularly, the present disclosure relates to an improved ultra-high frequency (UHF) passive RFID tag configured for implantation in a tail of a rodent that provides for increased effectiveness. 
     BACKGROUND OF THE INVENTION 
     Animal research and experimentation benefits from accurate identification and monitoring of each of a plurality of animals. Animal research involving rodents, such as mice or rats, has increased with the advent of genetically engineered strains of these test animals. 
     A passive radio-frequency identification (RFID) tag may be used to mark/identify an animal. RFID tags are designed to be small to reduce discomfort to the animal. Passive RFID tags are powered by an externally generated electromagnetic wave in the form of an interrogation radio wave. RFID tags have a radio receiver for receiving the interrogation radio wave and a radio transmitter for transmitting a radio wave comprising identification information in response to the received interrogation radio wave. 
     Most implantable passive RFID tags for animals are in the form of a rigid capsule of a non-conductive material like glass. Examples of these kinds of animal implantable passive RFID tags are shown in U.S. Pat. Nos. 4,262,632, 5,211,129, and 6,974,004, and U.S. Publ. No. US2008/0042849 A1. Unfortunately, both the size and rigid nature of these kinds of passive RFID tags in a capsule make implantation difficult, painful, and ineffective in rodents. 
     A solution to these problems is provided by the microelectronic animal identification tags developed by the assignee of the present disclosure that are marketed as the Digitail™ tag (www.mysensalab.com/products/digitail-tag/), various aspects of which are described in U.S. Publ. No. US2016/0037749 A1 and PCT Publ. No. WO2017/136898 A1. Although these relatively flexible UHF RFID tags are one of the smallest implantable passive RFID tags available, the read distances of these RFID tags can be limited by the small dimensions required for needle-implantation of these RFID tag. 
     The maximum distance that a passive RFID tag may be read is dependent on the frequency of the radio wave, the medium through with the radio wave is propagated, the power of the interrogation radio wave, and the size and design of the RFID tag antenna. The power of the interrogation radio wave is typically limited by regulation in the context of animal research and experimentation to about 30 dB. Conventional approaches to RFID antenna design as described in  RFID Tag Antenna Design , IMPINJ Whitepaper Ver. 1.0 (2017) (available at https://support.impinj.com/hc/article_attachments/360000130460/TagAntennaDesignOverview-20170606.pdf) can include increasing the size of the antenna by increasing the length of a meander type antenna or modifying the inductive loop that couples the antenna to the RFID chip. 
     In the context of the significant size constraints imposed for needle-implantation of the RFID tag in a rodent, together with the complex characteristics of the living tissue through which the radio waves are propagated, conventional solutions to improve antenna design or increase power in order to improve read effectiveness of the RFID tag are neither practical nor predictable. Accordingly, there is an opportunity to improve on the design of this kind of relatively flexible, implantable passive UHF RFID tag in terms of improved read effectiveness. 
     SUMMARY OF THE INVENTION 
     A passive UHF RFID tag configured for implantation in a tail of a rodent using a small diameter (20-22 AWG) needle in accordance with embodiments as disclosed provides for increased read effectiveness. In various embodiments, the passive UHF RFID tag is comprised of an elongated flexible substrate having a pair of opposed surfaces with a RFID chip positioned on a first of the opposed surfaces. The RFID chip is directly electrically connected to a closed-loop multi-layer folded dipole antenna that is disposed on both of the opposed surfaces of the substrate. The antenna is electrically connected to the RFID chip and includes at least an inductor as part of the closed-loop antenna. With a biocompatible insulative coating having a maximum thickness of 25 μm, the passive UHF RFID tag can have maximum dimensions of 1 mm wide, 0.6 mm high, and 10 mm long that are suitable for needle implantation in a tail of a rodent. In embodiments, the RFID tag can be read with at least 90 percent effectiveness by a 30 dB RFID tag reader at least 5 cm from a tail of the rodent. 
     In embodiments, the inductor is one component of an antenna matching circuit that is electrically part of the closed-loop antenna and has an equivalent inductance of 5-50 nH and an equivalent resistance of less than 50 ohms. In some embodiments, the component(s) of the antenna matching circuit are provided in a surface mount device (SMD) that is physically mounted on a same surface of the substrate as the RFID chip. In other embodiments, the SMD is physically mounted on a surface of the substrate opposite the surface of the substrate on which the RFID chip is mounted. In some embodiments, all of the components of the antenna matching circuit are electrically connected to the closed-loop antenna location at one gap between portions of the closed-loop antenna. In some embodiments, different components of the antenna matching circuit are electrically connected to the closed-loop antenna at different gaps on the same or opposite surfaces of the pair of opposed surfaces of the substrate. 
     In embodiments, the closed-loop multi-layer folded dipole antenna is formed of a layer of non-ferrous conductive material, such as copper or silver, deposited on each of the pair of opposed surfaces. In some embodiments, each conductive layer covers substantially all of an exposed portion of the opposed surface other than a first gap for mounting the RFID chip and at least a second gap for mounting the inductor with an end opposite the first gap on each of each conductive layer on each of the opposed surfaces being soldered together and terminated to form one of a pair of opposed ends of the strip that provides an electrical connection between the conductive layers on the pair opposed surfaces. In other embodiments, each conductive layer may include at least one strip that does not extend substantially across the opposed surface from one side to another side. In some embodiments, the at least one strip of conductive material is configured as a split antenna portion on at least one of the opposed surfaces. In some embodiments, the at least one strip of conductive material is configured as a meander antenna portion on a least one of the opposed surfaces. 
     In embodiments, the elongated flexible substrate is a strip of polyimide material having a maximum thickness of 100 μm and a dielectric constant in the range of 2.75-3.5. In some embodiments, the RFID chip is secured to the first of the opposed surfaces by a pair of solder pads under the RFID chip and an ultraviolet adhesive on top of at least a portion of the RFID chip. In some embodiments, the elongated flexible substrate of the RFID tag is configured to form a curved arc between the pair of opposed ends of the substrate up to a 45-degree arc angle with less than a ten percent failure rate of the RFID tag. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a photographic view of an RFID tag in accordance with an embodiment. 
         FIG. 1B  illustrates a cross-sectional side view of an embodiment of RFID tag similar to  FIG. 1A . 
         FIGS. 2A and 2B  illustrate a lengthwise mid-point cross-sectional view of an RFID tag positioned within a 21 AWG needle and a 21.5 AWG needle, respectively. 
         FIGS. 3A and 3B  illustrate a top and bottom view, respectively, of an embodiment of a flexible substrate for an RFID tag of  FIG. 1B  with the closed-loop multi-layer folded dipole antenna prior printed on the substrate prior to laser cutting and mounting of the RFID chip and matching circuit. 
         FIGS. 4A, 4B, and 4C  illustrate configurations of an antenna layer of the closed-loop multi-layer folded dipole antenna in different embodiments. 
         FIG. 5A  illustrates a cross-sectional side view of an embodiment of an RFID tag with a closed-loop multi-layer folded dipole, an RFID chip on a first surface of the RFID tag substrate and a matching circuit on a second surface of the RFID tag substrate. 
         FIG. 5B  illustrates a cross-sectional side view of an embodiment of an RFID tag with a closed-loop multi-layer folded dipole, an RFID chip on a first surface of the RFID tag substrate and a pair of inductors on the first surface of the RFID tag substrate. 
         FIG. 6  illustrates an idealized electrical schematic of an embodiment of an antenna matching circuit. 
         FIG. 7A  illustrates an electrical schematic of an embodiment of an RFID tag with a closed-loop multi-layer folded dipole, an RFID chip on a first surface of the RFID tag substrate and a matching circuit on the first surface of the RFID tag substrate. 
         FIG. 7B  illustrates an electrical schematic of an embodiment of an RFID tag with a closed-loop multi-layer folded dipole, an RFID chip on a first surface of the RFID tag substrate and a matching circuit on a second surface of the RFID tag substrate. 
         FIG. 7C  illustrates an electrical schematic of an embodiment of an RFID tag with a closed-loop multi-layer folded dipole, an RFID chip on a first surface of the RFID tag substrate and a pair of inductors on the first surface of the RFID tag substrate. 
         FIG. 7D  illustrates an electrical schematic of an embodiment of an RFID tag with a closed-loop multi-layer folded dipole, an RFID chip on a first surface of the RFID tag substrate and a pair of matching circuits on the first surface of the RFID tag substrate. 
         FIGS. 8A and 8B  illustrate example embodiments of SMD chips incorporating the components of matching circuits. 
         FIG. 9  illustrates a flowchart of the manufacturer and verification steps, according to an embodiment. 
         FIG. 10  illustrates an x-ray view of an RFID tag used to confirm alignment of the RFID chip mounting and laser cuts, according to an embodiment. 
         FIG. 11A  illustrates a cross-sectional end view of a tail of a rodent showing target locations for possible implantation sites. 
         FIG. 11B  illustrates a cross-sectional side of a tail of a rodent showing target locations for possible implantation sites. 
         FIGS. 12A-12C  are photographs showing a sequence of steps for implantation of an RFID tag in the tail of a rodent using a manual needle injection embodiment. 
         FIG. 13  is a table showing the results of experimentations on read effectiveness with different embodiments of a passive RFID tag implanted in a tail of a rodent. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Disclosed herein is an improved passive RFID tag configured for needle implantation in a tail of a rodent. Embodiments disclosed herein provide for increased read distances and identification effectiveness. In various embodiments, an RFID tag includes an elongated flexible substrate having a pair of opposed major surfaces: a first surface and a second surface. An RFID chip configured to operate in the ultra-high frequency (UHF) range is mounted on the first surface. The RFID chip is directly electrically connected to a closed-loop multi-layer folded dipole antenna that is disposed on both of the opposed surfaces of the substrate. In various embodiments, RFID tag also includes one or more matching circuits arranged on one or both of the opposed surfaces that include at least an inductor. 
     As shown in  FIG. 1A  and  FIG. 1B , embodiments of an RFID tag  100  include an elongated flexible substrate  102  having a pair of opposed major surfaces: a first surface  103  and a second surface  104 . An RFID chip  106  is mounted on the first surface  103 . The RFID chip  106  is directly electrically connected by a pair of solder pad joints  108  to a closed-loop multi-layer folded dipole antenna  110  that is disposed on both of the opposed surfaces  103 ,  104  of substrate  102 . Flexible substrate  102  provides a dielectric support structure for RFID chip  106  and antenna  110 . 
     RFID tag  100  also includes a matching circuit  112  including at least an inductor electrically coupled in series with the antenna  110 . In  FIG. 1A , matching circuit  112  is a single discrete inductor component that is directly soldered to antenna  110 . In  FIG. 1B , matching circuit  112  is a surface mount device (SMD) component having an inductive element and a resistive element and is electrically connected to antenna  110  by a pair of solder pad joints  114 . 
     In embodiments, RFID tag  100  is coated with a biocompatible insulative coating (not shown) having a maximum thickness of 25 μm covering the RFID tag  100 . The coating should be thick enough to provide a barrier for enhancing biocompatibility of the RFID tag  100  and also provide an insulation barrier for the antenna  110 . In embodiments, the coating is a parylene C coating that is applied by a tumble coating process to the completed tag. In other embodiments, the coating is a silicon or similar biocompatible coating that is sprayed onto RFID tag  100  once the tag is assembled. 
     In embodiments, RFID chip  106  and matching circuit  112  are further secured to the RFID tag  100  by the use of a bioinert adhesive or potting material  116 , such as an ultraviolet light curable adhesive available from Dymax. In addition to providing further structural integrity, the use of an adhesive material  116  can also reduce the exposed edges of RFID chip  106  and SMD matching circuit chip  112  to provide a more tapered or smoother interface between the exterior of the RFID tag  100  and the tissue in the rodent tail. This can reduce abrasion, adhesions, inflammation, and infection that might otherwise be initiated as a physiological reaction to the needle implantation of the RFID tag  100 . 
     Flexible substrate  102  can comprise a dielectric polyimide material, such as Kapton™ by DuPont. In embodiments, flexible substrate  102  has a maximum thickness of 100 μm and a dielectric constant in the range of 2.75-3.5. Flexible substrate  102  includes a first major surface  103  arranged on what may be referred to as a top or upper portion of flexible substrate  102 . As depicted in  FIG. 1B , flexible substrate  102  also includes a second major surface  104  arranged on what may be referred to as a bottom or lower portion of flexible substrate  102  such that first surface  103  is opposite second surface  104 . In embodiments, at each end of the length dimension of flexible substrate  102 , an upper portion of antenna  110  and a lower portion of antenna  110  are electrically connected by soldered ends  118 . 
     In embodiments, flexible substrate  102  is configured to have a length L of between 4-10 mm. Length L may be determined based on a variety of factors. For example, one parameter of length L may be the intended RFID transmission wavelength, such that an effective length of the antenna  110  is close to a whole fraction of the intended RFID transmission wavelength. In embodiments, the effective length may be adjusted based on the changes in wavelength of the signal propagating through the tissue of the rodent because that wavelength is smaller than that of the same signal travelling in free space. In embodiments, the dimensions of the antenna  100 , including the length and width are configured to more closely match a resonant frequency/wavelength fraction of this reduced wavelength in order to increase the read range. 
     Another parameter of length L may be the type of animal that RFID tag  100  is intended to be implanted. For example, smaller rodents, such as mice, have typical tail lengths that are more readily suited for RFID tags  100  having a length L of around 4-6 mm. In larger rodents, such as rats, the length L of RFID tag may be up to 10-12 mm. Increasing the length L can result in some improvements in the read strength and read effectiveness of the RFID tag  100  but can also impact the ease and effectiveness of needle implantation as well as the structural viability and integrity of the RFID tag  100 . 
     In embodiments, a parameter of length L may include whether the length for a given flexibility of the RFID tag  100  interferes with the ability of the animal to curve the portion of its tail into which the RFID tag  100  is implanted without impacting the structural or electrical integrity of the RFID tag  100 . In embodiments, the elongated flexible substrate  102  is configured to form a curved arc between the pair of opposed ends of the substrate  102  that can be up to a curve defining a 45 degree arc angle without producing more than a ten percent failure rate of the RFID tags  100  during a representative sample of initial flat bend testing (without rotational flex) after manufacture. In embodiments, the flexibility and structural integrity of the RFID tag  100  are influenced by the material and thickness of the substrate  102 , the material and thickness of each layer of the antenna  100 , the size of the solder pads  114  and formulation of the corresponding solder used, and the size of the RFID chip  106  and SMD matching circuit  112 . 
     In various embodiments, RFID chip  106  is a passive-type RFID chip that does not include a battery. RFID chip  106  includes an integrated circuit and a transceiver for receiving and transmitting a UHF signal. Example of a suitable RFID chip  106  include the Monza™ RP-6 by Impinj and the Higgs EC IC by Alien, although other suitable passive RFID chips can be used. 
     The integrated circuit of RFID chip  106  is powered by the incoming UHF signal and thus the UHF signal is transmitted at a power-transmission power level, as opposed to signal-only power level. Due to radio frequency power transmission regulations in the United States and Europe, power-transmission level power is limited to 30 decibels for human exposure and 33 decibels for non-human exposure. For passive RFID chips configured for UHF bandwidths, these power transmission limitations can otherwise limit the overall amount of power that can be transferred to the RFID chip  106 . This limitation is one reason why lower frequency systems have been used in conventional animal RFID tag implants; however, lower frequency systems are inherently limited in the read rate for reading and distinguishing among different RFID tags in a common area. 
     In embodiments, matching circuit  110  can comprise a variety of passive matching components such as resistors, inductors, and capacitors. When RFID chip  106  and antenna  110  are assembled to form RFID tag  100 , there is a resultant complex impedance. Matching circuit  110  is configured to match the resultant complex impedance of RFID chip  106  and antenna  110 , together with any additional impedance, capacitance or inductance values that may be introduced by the inductor(s) or SMD chip(s) for matching circuit  110  and the solder joints for solder pads  108  and  114 , as well as soldered ends  118 . In embodiments, the values for matching circuit  110  are selected to maximize the power transfer of reception/transmission for the particular passive RFID chip  106  such that a read distance between a reader (not shown) and the RFID tag  100  as implanted in the rodent tail is optimized as described in further detail below. 
     In various embodiments, antenna  110  is arranged in a closed-loop, folded dipole antenna configuration. Antenna  110  includes a lower portion arranged on second surface  104  and an upper portion arranged on first surface  103 . Antenna  110  may comprise printable copper layers or layers formed of other printable or depositable electrically conductive materials. In embodiments, antenna  110  is not formed of any ferrous metals such that RFID tag  100  is magnetic resonance imaging (MRI) compatible. 
     In embodiments, antenna  110  creates a closed-loop, folded dipole antenna configuration by coupling the lower portion of antenna  110  to the upper portion of antenna  110  at each end of substrate  102  via solder joints  118 . In this way, antenna  110  effectively wraps around flexible substrate  102  lengthwise. 
     In other embodiments, antenna  110  may form a single continuous printed band of conductive material that covers both the upper surface  104  and lower surface  103  of flexible substrate  102  other than gaps between the solder pads  108  and  114 . In this embodiment, the antenna  110  may be adhered to the flexible substrate  102  or may be deposited onto the flexible substrate  102  by vapor deposition or the like. 
     Referring to  FIG. 2A  and  FIG. 2B , RFID tag  100  is configured for implantation in a tail of a rodent by use of a needle  120 . Needle  120  is configured for loading and delivery of the RFID tag  100 . In embodiments, needle  120  can include a 20-22 AWG gauge needle with a relatively small diameter lumen. In embodiments, needle  120  has a thin-walled tubular or cannula like structure that facilitates an increased inner diameter relative to the desired outer diameter optimized for needle insertion of the RFID tag  100  without the need to use sutures, glue or the like to close the opening in the skin made by needle  102 . In embodiments, a thickness of the tubular wall of needle  120  has dimensions ranging from 0.1-0.5 mm. 
     As shown by the various dimensions in mm in  FIG. 2A  and  FIG. 2B , RFID tag  100  is sized and shaped to be delivered via a relatively small diameter lumen of needle  120 . In embodiments, flexible substrate  102 , RFID chip  106 , antenna  110 , and matching circuit  112  have a total combined width of the RFID tag  100  of less than 1 mm and a total combined height of RFID tag  100 , which includes the stacked thicknesses of flexible substrate  106 , RFID chip  108  or matching circuit  110 , and two thicknesses of antenna  112 , is less than 0.6 mm. 
       FIGS. 3A and 3B  depict a top and bottom view, respectively, of an embodiment of the closed-loop multi-layer folded dipole antenna  110  for an RFID tag  100  as shown in  FIG. 1B . These figures show an upper layer  122  and a lower layer  124  closed-loop multi-layer folded dipole antenna  110  printed on the flexible substrate  102  prior to laser cutting and mounting of the RFID chip  106  and matching circuit  112 . In embodiments, upper layer  122  of antenna  110  is printed onto flexible substrate  102  on first upper surface  103  as depicted in  FIG. 3A , and lower layer  124  is printed on second lower surface  104 , as depicted in  FIG. 3B . In various embodiments, an array of multiple antenna blanks for RFID tags  100  are printed on both sides of a single sheet of the flexible substrate  102  with each being laser cut from the sheet to form an individual RFID tag  100 . 
     In embodiments, antenna  110  includes a pair of ring structures  126 , each ring formed at one of the longitudinal ends of upper layer  122  and lower layer  124 . In embodiments, the corresponding pairs of rings  126  at each longitudinal end of the antenna layers  122 ,  124  are used to define a vertical interconnected access (via) aperture in flexible substrate  106 . After upper portion and lower portion of antenna  112  have been printed onto first surface  113  and second surface  114 , respectively, a via aperture is created between corresponding rings  126  and the via aperture are configured to receive solder to form a thru-connect between the corresponding layers of the rings  126  at each longitudinal end of antenna  110 . In other embodiments, the vias may be formed at locations other than the longitudinal ends of the antenna  110 , for example, as smaller thru-vias completely surrounded by portions of the flexible substrate  102 . 
     In embodiments, a flow soldering step electrically couples upper antenna layer  122  and lower antenna layer  124  by solder within the rings  126 . In embodiments, the flow solder step can occur as a single process after the RFID chip  106  and matching circuit  112 , such as an SMD chip, are mounted on the corresponding solder pads  108 ,  114 . In other embodiments, the flow solder step can occur as a multi-step process. Once soldering is completed, the solder ends  118  are formed by cutting away and/or mechanically or thermally removing any excess solder and corresponding portion of the rings  126  to form the corresponding longitudinal ends of RFID tag  100  where antenna  110  wraps between the upper surface  103  and the lower surface  104  of flexible substrate  102 . In embodiments, solder ends  118  may be finished, polished, coated, or layered over to form relatively smoother end structures that have a rounded, tapered or graduated height and width smaller than the portion of the RFID tag  100  where RFID chip  106  is located. 
     In embodiments, at least one of upper antenna layer  122  and lower antenna layer  124  are printed so as to include an RFID chip gap  128  and one or more matching circuit gaps  129 . RFID chip gap  128  and the one or more matching circuit gaps  129  are printed to allow RFID chip  106  and one or more matching circuits  112  to be soldered into a portion of the closed-loop multi-layer dipole antenna  110 . 
     Various embodiments of the antenna layers  122 ,  124  of antenna  110 , as depicted in  FIGS. 4A-4C , can also be used to vary the RFID system frequency used, or to enhance read distance. It will be understood that various combinations of these antenna layer designs may be used for all of part of one or both of the antenna layers  122 ,  124  of antenna  110 . 
       FIG. 4A  depicts an embodiment of the RFID tag  100  of  FIG. 1B  having a layer  122 ,  124  of antenna  110   a  comprises a solid width of copper or conductive material that is substantially equal to a width of substrate  102 . This embodiment can be printed on substrate  102  and provides ample surface area for connecting to solder pads  108 ,  114 . 
       FIG. 4B  depicts an embodiment of in which a layer of antenna  112   b  comprising a split antenna design having two strips of copper or conductive material separated by a central channel. The central channel creates portions of antenna  112   b  having two conduits, instead of one. As antenna  112   b  includes two conduits in some portions, the effective length of antenna  112   b  may be increased without increasing the actual physical length of RFID tag  100  due to a resonating effect of the two parallel conduits. 
       FIG. 4C  depicts an embodiment of in which a layer  122 ,  124  of antenna  112   c  comprises a relatively thin, meandering strip of copper or conductive material conduit. The meandering conduit of antenna  112   c  may increase the antenna length without increasing the actual physical length of RFID tag  100 . 
     Matching circuits  110  can be arranged in a variety of different physical arrangements on the different surfaces, as well as different variations of circuit components. 
     As depicted in  FIG. 5A , an embodiment of RFID tag  200 , includes matching circuit  110  arranged on second surface  104 , instead of first surface  103 . In this embodiment, matching circuit  110  is arranged such that it bisects antenna lower layer  124  on the lower, second surface  104 . In embodiments, adhesive material  116  is applied as generally indicated on one or both of the first surface  103  and second surface  104  as shown in  FIG. 4A . 
     As depicted in  FIG. 5B , an embodiment of RFID tag  300 , includes a pair of matching circuits  110  arranged on first surface  103 . In embodiments, the matching circuits  110  are positioned symmetrically about the RFID chip  106 . In embodiments, adhesive material  116  is applied as generally indicated on the first surface  103  in  FIG. 4B , with no adhesive material needed on the second surface  104 . In embodiments, the corresponding matching circuit gaps  129  in the upper layer  122  of antenna  110  may be arranged to further facilitate the flexibility of the RFID tag. 
       FIG. 6  illustrates an idealized electrical schematic for a matching circuit  112  in accordance with various embodiment. The Thevenin equivalent for the matching circuit can includes an inductor L 1 , resistor R 1  and equivalent capacitance CEQ 1  that represents the net capacitance of the physical and electrical configuration of the various components and connections for RFID tag  100 . The matching circuit  112  is configured to match the complex impedance of RFID chip  106  (RFchip 1 ) and antenna  110 . Inductor L 1  includes the equivalent inductance of the matching circuit as well as any residual inductance created by antenna  110 . Resistor R 1  includes the equivalent resistance of the matching circuit  112 , as well as any residual resistance created by antenna  110 . Likewise, capacitance CEQ 1  includes the equivalent capacitance of the matching circuit  112 , as well as any residual capacitance created by antenna  110 . Matching circuit  112 , as it is shown in  FIG. 6  is configured to optimally match the complex impedance of RFID chip  106  and antenna  110  such that power transfer is optimized. Due to various cost and manufacturing limitations, matching circuits for RFID tag  100  can be varied and still achieve significant power transfer optimization. 
     In embodiments, the antenna  110  should have an impedance that matches the conjugate of the impedance calculated from the circuit model in  FIG. 6  where the values of parameters are provided in the datasheet of the RFID chip  106 .
 
 C mount=0.21 pF,  Cp =1.23 pF,  Rp =1.2 kΩ  Eq. 1
 
     In this example, the impedance of the antenna expected by the chip is calculated considering the frequency of operation is at 915 Mhz, which is the center frequency of the US ISM UHF band. Because the capacitors in this configuration are in parallel,
 
 C total= Cp+C mount  Eq. 2
 
     Considering the angular frequency and capacitive reactance, the imaginary resistance component is determined as
 
=1.44 pFω=2π f ω= 2×π×915×10 6 =5749114556.069 rad/s Xc=   1   jωC  total= 1 −125749114556.069×1.44×10× j=− 120.7915 jΩ   Eq. 3
 
     Thus, the Resistance,
 
 X   R   =Rp= 1.2 kΩ1=1+1 ZX   R   X   C     Z     =X   R   Xc X   R   +Xc= 12.03687−119.5799 jΩ   Eq. 4
 
     Therefore, the equivalent theoretical impedance at the chip is 12.03687-119.5799j Ω where, in order to match the chip impedance, the antenna must satisfy an impedance equivalent of the total equivalent impedance, which is inductive. The antenna resistance, and the antenna inductance, 
     
       
         
           
             
               
                 
                   
                     
                       
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     Thus, the matching circuit  112  can be configured to generally match the inductive reactance (see attached calculation) required by the chip calculated above, by using an inductor with an inductance of 20 nH. 
     For example, as depicted in  FIG. 7A , a matching circuit  112  can include an inductor L 1  and a resistor R 1  mounted on first surface  103 .  FIG. 7A  is an equivalent circuit diagram to the physical embodiment of RFID tag  100  depicted in  FIG. 1B . In this embodiment, inductor L 1  and resistor R 1  are configured to match the equivalent impedance of RFID chip  106  and antenna  110 . In one example, inductor L 1  can have an inductance of about 20 nH and resistor R 1  can have a resistance of about 12Ω. 
     In an alternative example, as depicted in  FIG. 7B , a matching circuit  112  can include an inductor L 1  and an optional resistor R 1  mounted on second surface  104 .  FIG. 7B  is an equivalent circuit diagram to the physical embodiment of RFID tag  200  depicted in  FIG. 5A . In this embodiment, inductor L 1  and resistor R 1  are configured to match the equivalent impedance of RFID chip  106  and antenna  110  of RFID tag  200 . 
     In yet another alternative example, and as depicted in  FIG. 7C , a matching circuit  112  that includes a pair of inductors L 1  and L 2  mounted on first surface  103 .  FIG. 7C  is an equivalent circuit diagram to the physical embodiment of RFID tag  300  depicted in  FIG. 5B . In this embodiment, the equivalent inductance of inductors L 1  and L 2  are configured to match the complex impedance of RFID chip  106  and antenna  110 . Because the complex portion of the equivalent impedance of RFID chip  108  and antenna  112  may be larger, the use of only matching with inductors can be a cost-effective solution at achieving greater power transfer efficiency. 
     In another alternative example, and as depicted in  FIG. 7D , a matching circuit  112  can include a pair of inductors L 1  and L 2  and a pair of resistors R 1  and R 2  mounted on first surface  103 .  FIG. 7D  is an equivalent circuit diagram to the physical embodiment of RFID tag  300  as depicted in  FIG. 5B . In this embodiment, the equivalent inductance of inductors L 1  and L 2  and the equivalent resistance of resistors R 1  and R 2  are configured to match the impedance of RFID chip  106  and antenna  110 . Using a pair of inductors L 1  and L 2  and a pair of resistors R 1  and R 2  mounted on first surface  103  allows flow soldering to be applied to a single side, i.e. first surface  103 , during the manufacturing process. 
     In  FIG. 8A , for example, an embodiment of a matching circuit  112   a  includes a resistor  140 , an inductor  142 , and conductor  144  within a SMD chip. In matching circuit  112   a , resistor  140  and inductor  142  are arranged in parallel physical orientation, but conductor  144  is configured to electrically couple resistor  140  and inductor  142  in series. In matching circuit  112   b  as shown in  FIG. 8B , resistor  140  and inductor  142  are arranged in series physical orientation and conductor  144  is configured to electrically couple resistor  140  and inductor  142  in series within a SMD chip. In some embodiments, matching circuits  110  can comprise inductor  142  only, and optionally include a resistor  130 , or a capacitor. 
     In one example of manufacturing RFID tag  100 , and as depicted in  FIG. 9 , RFID tag  100  starts with preparing a sheet of flexible substrate material at  302 . Step  302  includes cleaning and preparing flexible substrate sheet that can fit a large array of flexible substrates  102  once cut to size. At step  304 , the lower portion of antenna  110 , including solder rings  126 , is printed in an array patter on one side of the flexible substrate sheet. This side of the flexible substrate sheet will form second surface  104  once each RFID tag  100  is cut to size. At step  306 , the flexible substrate sheet is inverted and the upper portion of antenna  110 , including rings  126 , RFID chip gap  128  and one or more matching circuit gaps  129 , is printed in an array pattern on one side of the flexible substrate sheet. This side of the flexible substrate sheet will form first surface  103  once each RFID tag  100  is cut to size. 
     At this point during the manufacturing process, surface mount components can be placed at step  308 . The surface mount components can include RFID chip  106  and matching circuit(s)  112 . At step  310 , flow solder paste can be placed at coupling points around RFID chip  106  and matching circuit  112 . Flow solder paste is also placed at each ring  126 . The flexible substrate sheet is then heated to melt the flow solder to form soldered vias for soldered ends  118 . Alternatively, flexible substrate sheet can be placed in a vapor phase reflow machine so as to reduce chip exposure to high temperatures. 
     At step  312 , each RFID tag  100  is cut from the flexible substrate sheet using a laser cutter or other cutting precision cutting method. Step  312  can include cutting only the basic envelope of RFID tags  100 , including rings  126 , or can include laser or die cutting where rings  126  are also trimmed. Prior to cutting, after cutting, or both, RFID tags  100  can be inspected at step  314  to confirm alignment of the RFID chip  106  and matching circuit  112  as well as the integrity of soldered ends  118 .  FIG. 10  depicts an X-ray inspection that can be performed at step  314  after individual RFID tags  100  are cut, but prior to ring  120  finishing. 
     At step  316 , RFID tag  100  is coated in a protective dielectric coating such as acrylic or chemical vapor deposited polymers. In embodiments, the thickness of the protective coating is important. Enough protective coating is needed to protect RFID chip  100 , but the RF transmission and emission qualities can be negatively affected by a protective coating that is too thick. For example, a target Parylene C™ thickness could be between 7.5 um to 25 um. Prior to or immediately after protective coating at  316 , an adhesive material  116 , such as a Dymax™, can be placed over RFID chip  106  and matching circuit  112  in order to smooth edges of RFID chip  106  and matching circuit  112 . The smoothed edges ease insertion of RFID tag  100  and reduce wearing against the tissue of the rodent. 
     In use, RFID tag  100  is configured for insertion into a tail of a rodent via needle  120  as shown in  FIG. 11A . In embodiments, as depicted in  FIG. 11B , RFID tag  100  can be placed within an upper half of the rodent&#39;s tail medial to the dermis and hair follicles but lateral to the bone, tendons and muscles. In this position, the physiology of the rodent is relatively unhindered. Further, the flexibility of flexible substrate  102  allows RFID tag  100  to move with the rodent&#39;s tail as opposed to restricting the same movement. 
     In one embodiment the sequence for manual needle implantation of RFID tag  100  is shown in  FIGS. 12A-12C . A tag injector with a 20-22 AWG needle  120  into which the RFID tag  100  is positioned is inserted into the tail of a rodent which is restrained. In embodiments, the needle includes a user-visible mark or indication at a distance of about 1.5 times the length of the RFID tag  100  from the distal tip of the needle  120  as a guide for how far the user should insert the needle. In one embodiment for an RFID tag having a length of 6 mm, the mark is located 9 mm from the distal tip of the needle  120 . In embodiments, the mark may be printed, embossed, or etched on the exterior of the needle. In embodiments, the tag injector includes a stop or other structure to temporarily hold the RFID tag  100  in the implanted position within the upper half of the rodent&#39;s tail medial to the dermis and hair follicles but lateral to the bone, tendons, and muscles while the needle  120  is withdrawn. Various embodiments of a manual injector for insertion of an RFID tag in accordance with the present disclosure are described in U.S. Prov. Appl. No. 62/571,762, U.S. Publ. No. US2016/0037749 A1 and PCT Publ. No. WO2017/136898 A1, the disclosures of which are each hereby incorporated by reference. 
     The tables below show the results of an experiment measuring the read range of Unmodified RFAi.D Tag (A), Uninsulated Tag with Inductor (B), Uninsulated Tag with Inductor+Resistor (C), Insulated Tag with Inductor (D), Insulated Tag with Inductor+Resistor (E). 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Tag Characteristics 
                 Modification 
                 Approximate Range 
               
               
                   
               
             
            
               
                 Somark RFAi.D Tag, Somark  
                 None 
                 No reads 
               
               
                 Parylene C Insulation 
                   
                   
               
               
                 Somark RFAi.D Tag,  
                 20 nH Inductor 
                  6 mm 
               
               
                 Uninsulated 
                   
                   
               
               
                 Somark RFAi.D Tag,  
                 12 Ω Resistor +  
                  4 mm 
               
               
                 Uninsulated 
                 20 nH Inductor 
                   
               
               
                 Somark RFAi.D Tag, Acrylic  
                 20 nH Inductor 
                 10 mm 
               
               
                 Spray Insulation 
                   
                   
               
               
                 Somark RFAi.D Tag, Acrylic  
                 12 Ω Resistor +  
                  6 mm 
               
               
                 Spray Insulation  
                 20 nH Inductor 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
               
               
                 Mouse Characteristics 
                 Modification 
                 Approximate Range 
               
               
                   
               
             
            
               
                 Somark Digital Mouse, Agar  
                 None 
                 30 mm 
               
               
                 Tail, Hollow Body 
                   
                   
               
               
                 Somark Digital Mouse, Agar  
                 20 nH Inductor 
                 85 mm 
               
               
                 Tail, Hollow Body 
               
               
                   
               
            
           
         
       
     
     As shown in the graph illustrated in  FIG. 13 , the results of this experiment demonstrate that the modified RFAi.D tag with an inductor+resistor combination demonstrated the greatest range (112 mm), followed by 85 mm and finally the unmodified RFAi.D tag at 30 mm. The advantage of using near field magnetic coupling is that it is more resilient to any changes caused by dielectric materials within a 30 cm range. 
     One potential explanation for the improvement in read range demonstrated in the Table above may be due to the antenna getting closer to the recommended impedance for near field communications. In this experiment, the modified tags were insulated using an acrylic coating similar to the Parylene coating on the unmodified RFAi.D tag tags to avoid any effects that could be caused by the electrical characteristics of agar. The inductors and the resistors used were 0402 components which were available in-house. There are 20 nH inductors with smaller form factors (01005) which are 0.23 mm H×0.20 mm W×0.40 mm L in size as it is generally the physically largest component. Furthermore, the resistor used (10Ω) does not satisfy the required theoretical resistance from the tag chip. Assuming the Kapton substrate of the tag is at the center of the needle that is used to insert the RFAi.D Tag to the mouse tail, a 21.5 AWG needle would provide enough room for a 01005 form factor inductor to fit with the modified tag being within the diameter of a 21 AWG needle. 
     Persons of ordinary skill in the relevant arts will recognize that embodiments may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the embodiments may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, embodiments can comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. Moreover, elements described with respect to one embodiment can be implemented in other embodiments even when not described in such embodiments unless otherwise noted. Although a dependent claim may refer in the claims to a specific combination with one or more other claims, other embodiments can also include a combination of the dependent claim with the subject matter of each other dependent claim or a combination of one or more features with other dependent or independent claims. Such combinations are proposed herein unless it is stated that a specific combination is not intended. Furthermore, it is intended also to include features of a claim in any other independent claim even if this claim is not directly made dependent to the independent claim. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     For purposes of interpreting the claims, it is expressly intended that the provisions of Section  112 , sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in a claim.