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IMPLANTABLE BIOSENSING
TRANSPONDER
BACKGROUND AND SUMMARY OF THE 5
INVENTION
Passive transponders, i.e., transponders containing no on-board power sources, while available for many years, have only recently been applied to humans. These transponders may be encoded and implanted in a patient, and 10 subsequently accessed with a hand held electromagnetic reader in a quick and non-invasive manner. Due to the small size of the transponder, the patient does not sense any discomfort or even the presence of the transponder. The companion hand held electromagnetic reader emits a low 15 frequency magnetic field to activate the passive transponder and thereby cause it to transmit its encoded data to the reader. Hence, no battery or other source of electrical power need be included in the passive transponder, further reducing the size of the transponder and making it more suitable for 20 implantation.
As disclosed in prior filed U.S. patent applications including Ser. No. 08/461,117 filed on Jun. 5,1995 (now U.S. Pat. No. 5,725,578), Ser. No. 08/375,811 filed on Jan. 20, 1995, Ser. No. 07/938,833 filed on Aug. 31, 1992 (now abandoned), Ser. No. 08/221,706 filed on Apr. 1, 1994 (now U.S. Pat. No. 5,678,288), and Ser. No. 934,785 filed on Aug. 24, 1992 (now U.S. Pat. No. 5,300,120), the disclosures of all of which are incorporated herein by reference, passive 3Q transponders can be implanted in the patient and encoded with data pertaining to the patient's identification and medical history or conditions, as well as data pertaining to the transponder itself. Passive transponders can also be used with other implants, including temporary implants, living 3J tissue implants, and medical prostheses, for storing data pertaining to the other implant. The stored data may be directly representative of the patient or implant data, or may be a unique tag for accessing an external database to obtain the desired information.
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Although passive transponders have not previously been used for remotely accessing physiological information related to a patient or organism, a team at the University of Michigan has developed implantable intramuscular stimulators based on electromagnetically coupled energy and 45 partial glass encapsulation. A hemicylinder of glass is bonded to a silicon substrate containing electronics and stimulating electrodes. After implantation, electromagnetic energy is transmitted through a patient's tissue to energize the device. The coupled energy is stored in the device in an 50 on-board capacitor which discharges upon receiving a coded instruction to thereby stimulate a local muscle. However, this device is incapable of detecting, measuring, and relaying physiologic or other measurements to an external reader, and is solely concerned with delivering stimulation pulses to 55 muscle tissues.
There is also a virtually limitless variety of sensors presently available for detecting almost any physical property related to an organism, including optical, chemical, and electrochemical properties. Examples include temperature go sensors for detecting the temperature of an organism and chemical sensors for detecting the presence or absence of particular chemicals, including direct chemical sensors and dye based chemical sensors that operate by detecting, with a photosensor, changes in an optical property of a dye. 55
Another type of sensor is the spectrophotometer for performing precise optical measurements. Aregion of tissue,
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gas, or fluid of interest is illuminated with an optical energy source having a known spectrum, and the light is broken into its constituent wavelengths, such as with a prism or grating. An array of one or more photosensors is positioned at the output of the prism or grating so that each photosensor corresponds to a predetermined range of wavelengths. By comparing the detected spectrum and the spectrum of illumination, the optical spectrum of absorption or reflection (and thus related chemical or physiologic properties), depending upon the path of light, can be determined for the tissue, gas, or fluid of interest.
An integrated spectrophotometer, designed by Stanford University, is described in U.S. patent application Ser. No. 377,202 filed Jan. 24, 1995, now abandoned, the disclosure of which is incorporated herein by reference. As described therein, a spectrophotometer grating is fabricated by electron beam exposure of a thin layer of polymethalmethacrylate (PMMA) on a thin, i.e., approximately 100 angstrom, layer of chromium on a quartz or glass optical substrate. When the PMMA is developed, unwanted chromium is exposed and can be removed using a chemical etch. The desired optical pattern is then present in the form of patterned chromium. This fabrication process is carried out such that a single substrate can readily yield dozens to hundreds of gratings. The optical substrate is sawed into individual gratings, which are then mounted above linear arrays of photosensors, such as photodiodes or phototransistors for visible and near-infrared light, or infrared sensors for longer wavelengths. Each detector thus receives light in a particular wavelength range, as determined by the grating.
A key aspect of the integrated spectrophotometer, in addition to being fabricated through micromachining techniques, is that a given grating pattern can be computed using the known absorption spectrum of the compound of interest. The essential process comprises computing the inverse Fourier transform of the absorption spectrum and generating a pseudo-continuous-tone grating function using half-toning techniques and/or shallow etching of the quartz substrate to provide phase information, the former often used in laser printers, to simulate continuous tones. Agrating pattern computed in this manner can serve as a matched filter to the optical spectrum of transmission or reflection, depending upon whether light is passing through or is reflected from a substance or tissue before reaching the spectrophotometer, respectively.
As technology progresses, and a greater number of sensors of decreasing sizes become available, more and more sensors become amenable to implantation into living organisms, including both animals and humans, so that remote assessments of physical properties can be accomplished. For example, U.S. Pat. No. 4,854,328 to Pollack discloses an animal monitoring telltale and information system comprising an implantable transmitter which includes a temperature sensor and a power supply. The sensor monitors the temperature of the animal and, upon sensing a predetermined threshold value, transmits a signal indicative thereof to a remote receiver. However, because the implantable transmitter requires a power source, such as a battery, to power the transmitter and sensor, the device is useful for only a limited period of time after implantation. Moreover, after the on-board power source is depleted, an invasive operation, in addition to the initial implantation, will have to be made to remove the device from the animal and to replace it with a like device.
What is needed is an implantable device that can sense one or more physiologic parameter values, and that can be remotely accessed by, for example, a hand held reader to
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obtain the sensed parameter values in a non-invasive manner. No on-board power sources should be utilized so that the device will never need to be removed from an implantation site in order to replace an electrical power source, and can therefore remain implanted for an indefinite period of 5 time. Instead, the remote reader should be used to energize the device, such as with electromagnetic energy, to thereby cause the device to sense the physiologic parameter values and transmit data representative thereof to the remote reader.
The inventors herein have succeeded in designing and 1° developing a biosensing transponder, and methods of use thereof, for implantation in an organism. The biosensing transponder includes a biosensor for detecting one or more physical properties related to the organism, including optical, mechanical, chemical, and electrochemical :5 properties, and a transponder for energizing the device with a remote reader and for transmitting data corresponding to the sensed physical property to the remote reader. Due to its small size and the absence of a need for an on-board electrical power source, the biosensing transponder of the 20 present invention is particularly suitable for human implantation, and can remain implanted for an indefinite period of time.
Nearly any type of sensor can be utilized in the biosensing transponder of the present invention, limited only by the size 25 of the particular sensor and the available space at the implantation site. The biosensing transponder can also be used for sensing physical parameter values related to other implants in a patient, including temporary implants, living tissue implants, and medical prostheses, in addition to 30 sensing physical parameter values directly related to the patient's organs and tissue.
Some examples of implantable biosensing transponders constructed according to the present invention include 3J devices for monitoring blood chemistry, such as sugar, pH, oxygenation, and hemoglobin levels, devices for monitoring blood circulation through transplanted living tissue, devices for monitoring accelerations of tissues, devices for monitoring therapeutic or unintentional doses of ionizing 4Q radiation, devices for monitoring strain forces on prostheses such as artificial heart valves and joint replacement systems, and devices for monitoring the degree of fibrosis around cosmetic implants. In every case, the biosensing transponder is energized by a remote reader for performing the particular 4J sensing or monitoring function, and for transmitting data indicative thereof to the remote reader.
By providing a means for remotely and non-invasively obtaining physiological data related to a patient, the biosensing transponder of the present invention contributes to 50 lower patient and healthcare provider risks, and should contribute to lower patient and healthcare provider costs. The biosensing transponder is particularly suitable for patients having a personal or family history of health problems for which early detection of an onset can be extremely 55 beneficial and oftentimes lifesaving.
As stated above, the biosensing transponder of the present invention includes a biosensor and a transponder. The transponder includes an energy coupler for wirelessly coupling energy from a remote energy source, and for wirelessly 60 transmitting data corresponding to the parameter value sensed by the biosensor to the remote reader. Preferred energy couplers include: inductive circuits for energizing the device with a remote electromagnetic energy source and for electromagnetically transmitting data to a remote reader; 65 photoelectric transducers for energizing the device with a remote optical energy source and LEDs for optically trans
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mitting data to a remote reader; and piezoelectric transducers for energizing the device with a remote ultrasonic energy source and for ultrasonically transmitting data to a remote reader. Such transducers can also be used in combination where it is desirable to receive power through one type of transducer and transmit data with another.
These various types of energy couplers are also suitable for use with previously designed transponders, in addition to the biosensing transponders of the present invention. Where a piezoelectric transducer is utilized along with a transponder capsule, the transducer can be bonded to an interior wall of the capsule, or can simply be placed within the capsule and the capsule filled with an incompressible fluid for coupling ultrasonic energy to the piezoelectric transducer.
The transponder utilized in the present invention also includes a control circuit for performing a variety of functions depending upon the specific implementation. For example, the control circuit can be configured to delay, either randomly or by a fixed or programmed period of time, the transmission of data to the remote reader to prevent the biosensing transponder from transmitting data at the same time as an adjacent like device. Similar to previously developed transponders, the control circuit can also be encoded with data pertaining to the biosensing transponder, the patient, and/or other implanted devices within the patient, and can transmit this data to the remote reader when energized. Where a biosensing transponder utilizes optical emitters for illuminating an implant site, or for optically transmitting data to a remote reader, the control circuit can also be configured to control activation of the optical emitters, such as by alternately illuminating a pair of optical emitters by controlling pulsatile discharges of a storage capacitor.
Disclosed embodiments of the biosensing transponder of the present invention include temperature sensors, strain sensors, ultrasonic sensors, pressure sensors, chemical sensors including direct chemical sensors and dye based chemical sensors, magnetic sensors, acoustic wave sensors, ionizing radiation sensors, acceleration sensors, and photosensors including spectrophotometers. However, these embodiments are merely intended to be exemplary of the various implementations that are available, and are not intended to be an exhaustive list.
Sensors having one or more electrodes can be implemented in the present invention for a number of purposes, including for measuring biopotentials, for detecting wear or failure of mechanical prostheses, and for detecting the presence or levels of specific chemicals. When desired, the electrodes can be coated with ion-selective membranes, or can be separated from an external environment by a selectively permeable membrane such as a gas permeable membrane. Typically, sensors having electrodes are positioned within a sealed capsule with one or more of the electrodes extending through the capsule to an external environment. However, the sensor itself can be positioned in the external environment such as by sealing the sensor over an opening in a capsule containing the energy coupler and control circuit.
In other embodiments, the biosensor can be completely positioned within a capsule containing the energy coupler and control circuitry, and sense parameter values from the external environment through the capsule. Examples include biosensing transponders utilizing temperature sensors, strain sensors, ultrasonic sensors, acceleration sensors, ionizing radiation sensors, magnetic sensors, optical sensors and pressure sensors. In the case of biosensing transponders
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