United States Patent (19)
`Dunphy et al.
`
`54) IMPLANTABLE PRESSURE TRANSDUCER
`75 Inventors: Roderick R. Dunphy, San Jose; Leo
`A. Bullara, Glendora; Robert H.
`Pudenz, Pasadena, all of Calif.
`73) Assignee: Huntington Institute of Applied
`Medical Research, Pasadena, Calif.
`Sept. 16, 1974
`(22 Filed:
`(21) Appl. No.: 506,217
`
`52 U.S. Cl............................. 128/2 P; 128/2.05 E;
`128/2.1 A; 73/398 R; 73/398 C; 73/410
`51) Int. Cl”................... . . . . . . . . . . . . . . . . . . . . . . . . . A61 B 5/02
`58) Field of Search............ 128/2.1 A, 2 P, 2.05 D,
`128/2.05 E; 73/398 R, 398 C, 399, 410
`
`56
`
`2,958,781
`3,034,356
`3,144,017
`3,583,387
`3,638,496
`3,738,356
`
`References Cited
`UNITED STATES PATENTS
`1 1/1960 Marchal et al................. 1282 P Xf
`5/1962 Bieganski et al.................. 73/398 R
`8/1964 Muth......
`... 128/2 P
`6/1971
`Garner................................ 12811 R
`2/1972 King.................................. 73/398 R
`6/1973 Workman ..................... 73/398 R X
`OTHER PUBLICATIONS
`Electronics, Mar. 22, 1963, pp. 58-60.
`
`3,958,558
`11)
`(45) May 25, 1976
`
`Primary Examiner-Kyle L. Howell
`Attorney, Agent, or Firm-Christie, Parker & Hale
`
`ABSTRACT
`57
`A wireless, surgically implantable pressure transducer
`for measuring pressure of fluid or tissue in a body
`chamber such as brain ventricle of a patient suffering
`hydrocephalus or a severe head injury. The transducer
`includes a coaxial variable capacitor electrically con
`nected across an inductor to form a parallel resonant
`L-C circuit. Alternatively, a coaxially variable induc- .
`tor may be connected across a capacitor to form the
`L-C circuit. A bellows is mechanically connected to
`the variable component to vary the value of capaci
`tance or inductance and hence the resonant frequency
`of the L-C circuit in response to pressure changes of
`the fluid in which the bellows is immersed. The trans
`ducer is electromagnetically coupled to an external
`source of variable-frequency oscillatory energy such
`as a grid-dip oscillator which enables external detec
`tion of the transducer resonant frequency which is in
`turn indicative of the level of fluid pressure being
`sensed.
`
`7 Claims, 5 Drawing Figures
`
`
`
`
`
`
`
`W ZZAZAZZZZZZZZZ.C. kN 7A7AAAAAAAAA
`
`27 2.
`Aaaaaaaay Vy
`Zyzzzyzzzzzzzzz!
`
`
`
`
`
`
`
`Abbott
`Exhibit 1015
`Page 001
`
`

`

`U.S. Patent May 25, 1976
`
`Sheet 1 of2
`
`3,958,558
`
`
`
`/
`o\'
`
`
`
`
`K\\\\\\\\\\it“.
`
`
`
`
`
`/MflA//f£fl 76941590552
`
`769.4.
`£x7£,eA/41 £Qo/pMEA/7‘
`
`VAR/4525-
`
`FEéWUEl’C V
`629/0 - PIP
`ago/Lame
`
`
`
`Abbott
`Exhibit 1015
`
`Page 002
`
`Abbott
`Exhibit 1015
`Page 002
`
`

`

`U.S. Patent May 25, 1976
`
`Sheet 2 of 2
`
`3,958,558
`
`
`
`a144444446
`IHE HII
`
`TS iT NG sesserteresareasses >-
`I
`N D. seas
`III PEO
`IDROI
`III ERC II
`III DEC II
`ITI RC II
`
`riverre case
`
`==
`
`me
`
`-
`
`M
`
`"
`
`
`
`
`
`
`
`S
`
`N
`
`SY
`
`N
`
`IRI
`S
`
`in
`
`is
`
`L
`
`it
`
`|
`
`W
`
`
`
`2.
`| E
`
`NS
`
`Abbott
`Exhibit 1015
`Page 003
`
`

`

`1
`IMPLANTABLE PRESSURE TRANSDUCER
`
`10
`
`25
`
`35
`
`BACKGROUND OF THE INVENTION
`Hydrocephalus is a brain condition in which cerebro
`spinal fluid accumulates at abnormally high pressure in
`Ventricles or chambers within the brain. The ventricles
`expand in response to the pressure exerted by the fluid,
`and surrounding brain tissue is compressed between
`the ventricles and the skull. Hydrocephalus usually
`occurs in babies or young children, and, if unchecked,
`results in brain damage, enlargement and deformation
`of the head, and eventual death.
`Modern medical methods are effective in arresting
`many cases of hydrocephalus, but it is often desirable
`to monitor pressure of the cerebrospinal fluid over an
`extended period to detect relapse and to determine
`long-range effectiveness of treatment. In the past, this
`measurement has been made by surgically implanting a
`20
`miniature but generally conventional transducer such
`as a strain-gage-bridge pressure pickup. This technique
`requires that wiring be conducted from the implanted
`transducer to external instrumentation which provides
`excitation voltage to the bridge and detects bridge
`unbalance voltage signals indicative of pressure. Alter
`natively, non-electrical manometric measurement
`methods may be used, but these techniques require
`installation of a conduit extending from the interior of
`30
`the brain ventricle through the skull and scalp to exter
`nal measurement equipment.
`-
`The primary disadvantage of these known techniques
`is that they involve conducting an electrical cable or
`fluid tube through the skull and scalp to enable direct
`electrical or mechanical connection between the inter
`ior of the brain ventricle and external equipment. This
`connection is disturbing and uncomfortable for the
`patient, and the danger of infection of tissue surround
`ing the cable or tube (and the risk of infection spread
`resulting in meningitis, ventriculitis, brain abscess or
`septicemia) requires constant supervision and usually
`full-time hospitalization of the patient. There is accord
`ingly a need for a measuring device which does not
`require direct electrical or mechanical connection
`45
`from the brain to external equipment, and which per
`mits the patient to be ambulatory after the device is
`Connection-free implantable transducers have been
`previously proposed, and they typically function by
`50
`external detection of the resonant frequency of a reso
`nant circuit in the implanted device. For example, the
`prior art includes a biological pressure transducer for
`sensing pressure in the gastrointestinal tract and having
`a resonant circuit with a pressure-controlled inductor.
`55
`Wireless systems are also used for sensing EEG or ECG
`voltages, the implantable part of the system using an
`electrically variable capacitor in a resonant circuit. A
`wireless resonantcircuit transducer has also been used
`for measuring intraocular pressure, the transducer
`using a pair of variably spaced Archimedean-spiral
`coils mounted on pressure-sensitive diaphragms.
`The transducer of this invention operates in wireless
`fashion similar to the instruments described above, but
`provides improved performance and lower drift in im
`plantation applications involving placement in body
`65
`cavities such as brain ventricles or heart chambers
`where only a very small transducer can be tolerated.
`The transducer and is disclosed below in a specific
`
`3,958,558
`2
`form suitable for intracranial implantation to monitor
`pressure of cerebrospinal fluid in a brain ventricle. This
`form is also suitable for mounting on hydrocephalus
`shunt apparatus as often used in treating and control
`ling this disease.
`.
`-
`Our transducer is, however, also suitable for implan
`tation elsewhere in the body, and is believed to be
`useful in any application where a very small, implant
`able and wireless device is needed to measure fluid or
`tissue pressure. For example, the transducer is believed
`useful for either short- or long-term monitoring of ab
`normal intracranial pressure in head-injury patients, or
`for post-surgical monitoring of brain-tumor victims to
`detect possible recurrence of the tumor. When such
`monitoring is no longer needed, the implanted trans
`ducer is removed by a simple re-opening and closure of
`the overlying scalp tissue.
`SUMMARY OF THE INVENTION
`Briefly stated, the transducer of this invention is a
`sealed housing having an outer surface formed of a
`biologically compatible material, the housing having a
`pressure-sensing means such as a bellows extending
`therefrom. Preferably, the bellows is isolated from di
`rect contact with the biological fluid to be monitored
`by a flexible balloon-like enclosure which extends from
`the housing around the bellows, a space between the
`enclosure and bellows being filled with a buffer fluid
`such as distilled water.
`An inductor assembly is mounted within the housing,
`and in one form is a hollow ferrite core having a con
`ductive coil wound on its outer surface. A variable
`capacitor assembly, preferably a coaxial piston-cylin
`der type, is fitted within the core and includes a mov
`able element connected to the pressure-sensing means
`to vary capacitance of the capacitor in response to
`changes in pressure of the fluid being monitored and in
`which the transducer is immersed. The capacitor is
`electrically connected across the inductor to form a
`40
`resonant L-C circuit, the resonant frequency of which
`is varied by changes in the fluid pressure applied to the
`bellows or similar means which in turn drives the ca
`pacitor movable element. In another form of the inven
`tion, a coaxially variable inductor is mechanically cou
`pled to the pressure-sensing means and electrically
`connected across a capacitor to form a pressure-con
`trolled, variable-resonant-frequency L-C circuit. The
`transducer is implanted in the body to measure pres
`sure of surrounding fluid or tissue. In one important
`application, the transducer is positioned within a brain
`ventricle to sense pressure of cerebrospinal fluid or
`surrounding tissue in this body chamber. There is no
`direct electrical connection from the transducer to
`equipment external to the chamber. The resonant fre
`quency of the L-C circuit is monitored by wireless
`transmission of electromagnetic energy from an exter
`nal generator such as a grid-dip oscillator, thereby
`providing resonant-frequency data which is analogous
`to fluid or tissue pressure in the chamber.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is an exploded view of a portion of a pressure
`transducer according to the invention;
`FIG. 2 is a side elevation, partly in section, of the
`assembled transducer;
`FIG. 3 is a side elevation of the transducer mounted
`on a plug for intracranial installation;
`
`installed.
`
`-
`
`60
`
`Abbott
`Exhibit 1015
`Page 004
`
`

`

`5
`
`10
`
`15
`
`35
`
`45
`
`3,958,558
`4
`3
`cement such as a conductive silver-epoxy or gold
`FIG. 4 is a block diagram of external electronic
`epoxy adhesive. The capacitor piston is thus electri
`equipment used with the transducer; and
`cally connected to the right end of coil 13 through
`FIG. 5 is a side sectional elevation of an alternative
`and presently preferred transducer according to the
`bellows 24. Sealing of the transducer is completed by
`placing an annular body 27 of epoxy resin or a similar
`invention and using a variable inductor.
`sealant between the right end of housing 11 and the
`DESCRIPTION OF THE PREFERRED
`bellows.
`-
`.
`. .
`.
`EMBODIMENT
`The transducer interior is hermetically sealed from
`Referring to FIGS. 1-2, a pressure transducer 10
`the outside environment so fluid cannot seep into the
`bellows or variable coaxial capacitor. Preferably, the
`according to the invention includes a housing 11 which
`is preferably a hollow cylindrical cup of cast plastic as
`transducer is evacuated prior to final sealing, and is
`back-filled with dry nitrogen. Back-filling is normally
`sold under the trademark "Hysol." In a typical form,
`done at one atmosphere of pressure to provide a trans
`the housing is 0.550-inch long, and has outside and
`inside diameters of 0.275 and 0.260-inch respectively.
`ducer which functions as a "sealed gage pressure'
`measuring device, but other pressures may be used if a
`A hollow cylindrical ferromagnetic core 12 is sized to
`reference pressure other than one atmosphere is pre
`make a loose fit within housing 11, and the core is
`typically 0.500-inch long, with outside and inside diam
`ferred.
`Preferably, housing 11 is sheathed in a covering 29 of
`eters of 0.250 and 0.125-inch respectively. The core is
`a biologically compatible material such as plastic sold
`preferably made from a sintered ferrite material such as
`under the trademark “Silastic.” In a preferred embodi
`sold by Indiana General Division of Electronic Memo
`20
`ries Magnetics Corporation as "O-2 Ferramic' mate
`ment, covering 29 is extended to form a loose balloon
`like enclosure 30 around bellows 24, and enclosure 30
`rial.
`A coil 13 (FIG. 2) is formed by helically wrapping
`is filled with distilled water 31, or preferably with a
`fluid which approximates the composition of the fluid
`about 10 turns of a conductor such as 0.020-inch-diam
`being monitored (such as Elliot's 'B' solution when
`eter gold wire around the outside surface of core 12.
`cerebrospinal fluid is being monitored) to provide a
`Core 12 and coil 13 form an inductor 14 for the trans
`ducer as shown in the electrical schematic in FIG. 4.
`correct ionic balance on both sides of the enclosure.
`A hollow cylindrical sleeve 17 (FIGS. 1-2) of a non
`Pressure of the fluid being monitored is transmitted
`through enclosure 30 and water 31 to actuate bellows
`ferrous material such as brass forms a fixed electrode of
`a coaxial variable capacitor 18 (FIG. 4) in transducer
`24, but the enclosure and water form a chemical and
`30
`10. The sleeve is sized to make a snug slip fit within
`mechanical buffer preventing tissue encroachment
`core 12, and is typically 0.400-inch long with outside
`which could interfere with free compression and exten
`and inside diameters of 0.124 and 0.100-inch respec
`sion of the bellows.
`tively.
`When used as an intracranial implant in a brain ven
`A rod or piston 20, having a thin, integrally formed
`tricle or in brain tissue, transducer 10 is preferably
`and radially extending flange 21 at one end, is also
`mounted on a flanged plug 33 of a material such as
`Silastic plastic. Surgical installation of this equipment
`made from a non-ferrous material such as brass. A
`involves generally the same procedures used in install
`portion of the outside of the rod is covered with a thin
`ing hydrocephalus shunts or pressure absorbers, these
`dielectric coating 22 of a material such as tantalum
`pentoxide. The piston fits into sleeve 17 in piston-cylin
`procedures being briefly discussed in U.S. Pat. No.
`der fashion, and forms a movable element or electrode
`3,583,387-Garner and Bullara titled “Pressure Ab
`sorbing Appliance for Treating Hydrocephalus.'
`of coaxial variable capacitor 18. The piston has an
`overall length of about 0.500-inch, and the piston and
`The values of inductance and capacitance of the
`parallelconnected inductor and capacitor of transducer
`flange have diameters of about 0.095 and 0.020-inch
`respectively.
`10 can be computed and pre-determined using known
`A generally cylindrical bellows 24 (FIG. 2) provides
`engineering formulae. Circuits having nominal reso
`nant frequencies in the range of about 30 to perhaps
`a force-summing surface for transducer 10, the bellows
`varying in length according to the pressure of fluid in
`100 megaHertz are believed best suited for biological
`applications. Higher frequencies (e.g., 200 mHz) have
`which the transducer is immersed. A typical and suit
`some advantages, but low "O's" typically experienced
`able bellows is sold by Servometer Corporation as a
`Type SK4681. The bellows is made of an electrically
`in tissue at these frequencies tend to obscure the accu
`conductive material which is preferably gold-plated
`rate external detection of resonance of the transducer
`nickel. The ends of the bellows are open, and each end
`L-C circuit.
`defines an axially extending shell or flange 25.
`Pressure range of the transducer is determined pri
`marily by the mechanical performance of bellows 24,
`To assemble the transducer, capacitor sleeve 17 is
`and these displacement-versus-pressure characteristics
`cemented within core 12, the left ends (as viewed in
`FIG. 2) of these components being flush. The left end
`can also be calculated by known engineering formulae.
`Typical units we have tested have had an operating
`of coil 13 is drawn around the end of core 12 and
`pressure range of 0 to 1000 millimeters of water
`soldered into electrical contact with the sleeve. Flange
`(gage), and the transducer L-C circuit has been de
`25 at the left end of bellows 24 is then slipped over the
`signed to have a zero-pressure resonant frequency of
`right end of core 12 and cemented in place. The right
`about 82 mHz. As the fluid pressure is increased, bel
`end of coil 13 is soldered or otherwise bonded into
`lows 24 contracts to drive capacitor piston 20 into
`electrical contact with the bellows flange as shown in
`sleeve 17, thereby increasing the capacity of the coax
`FIG. 2.
`ial capacitor and decreasing the resonant frequency of
`Capacitor piston 20 is then fitted through the bellows
`into sleeve 17, and flange 21 of the piston is secured
`the circuit. A change in resonant frequency of about 20
`mHz is typically obtained in driving the transducer
`within flange 25 at the right end of the bellows, the
`attachment being made with an electrically conductive
`from zero to full-scale pressure.
`
`40
`
`50
`
`55
`
`60
`
`65
`
`Abbott
`Exhibit 1015
`Page 005
`
`

`

`25
`
`30
`
`10
`
`15
`
`20
`
`3,958,558
`6
`5
`A cup-shaped housing 53 made of medical-grade
`in use, the installed transducer is irradiated with
`acrylic plastic is slipped over and secured to sleeve 41.
`electromagnetic energy transmitted through the body
`An enclosure 54 is fitted over and sealed to the open
`and generated by an external variable-frequency oscil
`end of housing 53, and this enclosure is preferably a
`lator. Some of this radio-frequency energy is absorbed
`membrane of Silastic plastic sheet. The space between
`(and also reflected or retransmitted) by the resonant
`the outer surface of the bellows and the inner surfaces
`circuit, depending on how close the incident frequency
`of the membrane and housing is filled with distilled
`is to the resonance frequency of the circuit. The fre
`water or a fluid compatible...with the characteristics of
`quency of the external oscillator is varied or swept until
`the fluid being monitored as described above.
`resonance of the transducer L-C circuit is externally
`The dimensions of housing 53 are about 0.165-inch
`detected. This resonant frequency is in turn indicative
`diameter by 0.445-inch length, and a very compact
`of the internal fluid pressure being sensed by the trans
`assembly is provided which is suitable for implantation.
`ducer.
`A nominal resonant frequency of about 80 mHz is
`A simple and accurate way to detect internal trans
`provided by using a capacitor of 5 picofarads and an
`ducer resonance with an external circuit involves use of
`inductance of about 0.8 microhenries. Installation and
`a grid-dip oscillator 35 (FIG. 4) which shows a sharp
`use of transducer 40 corresponds to the procedures
`drop or "valley' in grid current when the resonant
`point of the "receiving" circuit is swept through by the
`discussed above with respect to transducer 10.
`.
`There has been described a compact variable-reso
`"transmitting" oscillator. The oscillator is preferably
`nance-frequency pressure transducer using a coaxial
`used in conjunction with a conventional electronic
`variable capacitor or inductor controlled by pressure
`frequency counter which provides a direct visual read
`sensing means such as a bellows. The use of coaxial
`out of frequency at the resonant point.
`variable-reactance components permits packaging of
`External phase-sensitive equipment can also be used
`the transducer in a compact size and shape which en
`to detect the characteristic and marked phase shift
`ables wireless implantation in body chambers which
`which occurs when the resonant circuit receives energy
`heretofore were monitored effectively only with at
`at its resonant frequency. Other external detection
`tached-wire measurement systems.
`systems are discussed in the aforementioned article
`We claim: .
`from IEEE Transactions on Bio-Medical Engineering
`1. An implantable pressure transducer, comprising:
`and the references therein cited.
`an elongated housing having an axis of elongation;
`Prior to installation, the transducer is calibrated by
`a capacitor assembly mounted in the housing;
`immersing it in a fluid (e.g., Elliot's 'B' solution) having
`an inductor assembly mounted in the housing and
`characteristics similar to the biological fluid or tissue to
`connected across the capacitor assembly to form a
`be eventually monitored. The pressure of the test fluid
`resonant L-C circuit;
`is then varied under controlled conditions while the
`one of the assemblies being of coaxial form and in
`resonant frequency of the transducer is tracked as de
`cluding an axially movable element for varying the
`scribed above to develop a pressure-versus-frequency
`35
`reactance thereof, the element being movable
`calibration curve.
`along the axis of elongation;
`The transducer of this invention can also be made
`a pressure-sensitive bellows secured and sealed to
`with a variable-reactance element which is a coaxial
`one end of the housing and effective to generate a
`variable inductor connected across a fixed capacitor,
`displacement in response to applied pressure;
`or both the capacitive and inductive components can
`40
`means connecting the bellows and the axial movable
`be variable under control of the pressure-sensitive bel
`element whereby bellows displacement resulting
`lows. A presently preferred embodiment of the inven
`from applied pressure is transmitted by the con
`tion is shown as a transducer 40 in FIG. 5.
`necting means to move the element and thereby
`Transducer 40 includes a cup-shaped hollow cylin
`vary the resonant frequency of the L-C circuit in
`drical coil-supporting sleeve 41 which is preferably
`response to applied pressure, said resonant fre
`made of polytetrafluorethylene plastic or a medical
`quency being externally detectable by wireless
`grade acrylic plastic. The sleeve has an annular recess
`transmission of electromagnetic energy between
`42 in which is wound an inductive coil 43 of say 12
`external equipment and the implanted transducer
`turns of 0.005-inch-diameter copper or gold insulated
`to provide data analogous to pressure; and
`50
`wire. The ends of the coil are fed through a pair of
`a flanged plug secured to an end of the housing oppo
`longitudinal slots 44 at one end of sleeve 41 for connec
`site the end on which the bellows is mounted;
`tion to a miniature fixed capacitor 46 mounted on a
`the transducer being free of external wired connec
`wall 47 which closes one end of the sleeve. The coil and
`tions and having outer surfaces formed of materials
`capacitor are preferably “potted” in a medical-grade
`suitable for human implantation, the inductor and
`55
`paraffin (not shown).
`capacitor assemblies being isolated within the
`A bellows 48 (generally corresponding to bellows 24
`housing against contact with biological material in
`described above) is fitted over and secured to the open
`which the transducer is implantable.
`end of sleeve 41. A solid cylindrical ferrite core 49 is
`2. The transducer defined in claim 1 wherein said one
`positioned within sleeve 41 to form an inductor with
`coil 43. A stiff metal shaft 50 (preferably a length of 60 assembly is a piston-cylinder capacitor assembly in
`cluding a hollow conductive sleeve forming a stationary
`stainless-steel tubing of about 0.009-inch outside diam
`element, and in which the axially movable element is a
`eter as used in hypodermic needles) is secured to the
`rod making a non-conductive slip fit in the sleeve.
`core and extends therefrom through a central opening
`3. The transducer defined in claim 2 wherein the
`51 in the closed end of bellows 48. During assembly of
`inductor assembly comprises a ferromagnetic hollow
`65
`the transducer, the "zero' position of the core is ad
`core, and a conductive coil wound around the outer
`justed to provide a desired inductance of the coil and
`surface of the core, the coil being connected at oppo
`core, and shaft 50 is then permanently secured to the
`site ends to the stationary and movable capacitor ele
`bellows to support the core and seal opening 51.
`
`Abbott
`Exhibit 1015
`Page 006
`
`

`

`10
`
`15
`
`ck
`
`ck
`
`k
`
`:
`
`3,958,558
`8
`7
`pressure-sensing means mounted on the housing and
`ments respectively, the stationary element being fitted
`effective to generate a displacement in response to
`within the core.
`applied pressure;
`4. The transducer defined in claim 1 wherein the
`means connecting the pressure-sensing means and
`inductor assembly comprises a coil connected across
`the axially movable element whereby displacement
`the capacitor assembly, and a ferrous core positioned
`of the pressure-sensing means is transmitted by the
`within the coil and forming said axially movable ele
`connecting means to move the element and
`net.
`thereby vary the resonant frequency of the L-C
`5. The transducer defined in claim 1 wherein the
`circuit in response to applied pressure;
`housing includes a flexible covering extending around
`a flexible covering secured to the housing and ex
`the bellows to form a sealed clearance space between
`tending over the pressure-sensing means to form a
`the covering and bellows, and further including a body
`sealed clearance space between the covering and
`of liquid filling the clearance space.
`pressure-sensing means, and further including a
`6. The transducer defined in claim 5 wherein said
`body of liquid filling the clearance space whereby a
`connecting means comprises a hollow shaft secured at
`biological fluid pressure to be sensed is exerted on
`opposite ends to the axially movable element and a
`the covering and transmitted through the covering
`movable end of the bellows.
`and liquid to the pressure-sensing means;
`7. A system for measuring biological pressures, com
`the implantable housing being free of external wired
`prising:
`connections and having outer surfaces formed of
`an elongated and biologically implantable housing
`materials suitable for human implantation, the in
`having an axis of elongation;
`ductor and capacitor assemblies being isolated
`a capacitor assembly mounted in the housing;
`within the housing against contact with biological
`an inductor assembly mounted in the housing and
`material in which the transducer is implantable,
`connected across the capacitor to form a resonant
`and
`L-C circuit;
`external means for wireless transmission of electro
`magnetic energy to the implantable L-C circuit,
`one of the assemblies being of coaxial form and in
`cluding an axially movable element for varying the
`and for detecting the resonant frequency of the
`reactance thereof, the element being movable
`circuit.
`along the axis of elongation;
`
`30
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`Abbott
`Exhibit 1015
`Page 007
`
`