Galvanic Transdermal Conduction Communication System and Method

1. Field of the Invention

The present invention relates to a transdermal communication system having an internal communication device and an external communication device, and more particularly, to a galvanic transdermal conduction communication system.

2. Description of Related Art

With recent advances in the field of microelectronics, it is now common to subdermally implant semiconductor chips and related circuitry into the body of a patient. The chips and circuitry are used to control a variety of bodily functions, and/or measure any one of a number of physiological attributes of the patient. The control information, or the measured information, is transferred in and out of the body using a transdermal communication device.

One problem confronting biomedical engineers developing transdermal communication devices is providing electrical power to the chronically implanted circuitry inside the body. The majority of implanted devices are powered using a battery. The power of the battery eventually drains, and needs to be replaced. The most common way to replace the battery is through surgery. Prior to the expiration of the battery, an operation is performed on the patient and either the battery is replaced, or a new device is implanted into the patient. Surgery, however, is usually a major ordeal for the patient, is costly, and is generally undesirable.

Another way to provide power to an implanted device is through the use of a split transformer, where one coil of the transformer is located underneath the skin and the other coil is positioned outside the skin. The transformer is used to replenish power to an implanted power supply, such as a battery, when needed. See for example U.S. Pat. No. 5,368,040 issued to Carney. The problem with transformers is that they require a coil to be implanted under the skin, which is typically bulky, and the split transformer provides relatively little power transfer to the internal device.

Another problem confronting biomedical engineers is providing two-way communication through the skin of the patient. It is known to surgically implant wires through the skin of the patient. While this approach facilities two-way communication, it is generally undesirable. Chronically implanted wires piercing the skin tend to be uncomfortable for the patient, are unsanitary, and may cause infection.

Radio telemetry is another known approach for communication between an implanted device and an external device. With radio telemetry, data is transmitted either into or out of the body using radio waves. The problem with radio telemetry is that a transmitter/receiver is needed inside the body of the patient. These transmitter/receivers tend to be very sophisticated and expensive. Furthermore, the transmitter/receiver inside the body consumes a relatively large amount of power, particularly during broadcasting. In battery powered radio telemetry transdermal communication devices, the frequent broadcasting of data from the body to an external receiver tends to significantly reduce the life of the battery.

U.S. Pat. No. 4,571,589 entitled "Biomedical Implant With High Speed, Low Power Two Way Telemetry", issued to Slocum on Feb. 18, 1996 discloses a transdermal communication system that relies on an external coil and an internal coil implanted under the skin of a patient. During data transmission from inside to outside of the body, the external coil generates a carrier signal which causes an impedance on the internal coil. The impedance of the internal coil is then modulated using a switch. For example, in transmitting a binary zero, the carrier signal is modulated for two cycles. With a binary one, the carrier signal is modulated for six cycles. The modulated carrier signal is then re-radiated by the internal coil to the external coil. The re-radiated signal is then demodulated to recover the transmitted data by measuring the length of time in which the impedance of the internal coil has been modulated by the switch. The problem with this arrangement is that modulation of the data, particularly a binary one, takes up to six cycles. As a result, the data transfer from the internal to external communication device is relatively inefficient.

U.S. Pat. No. 4,987,897 entitled "Body Bus Medical Device Communication System", issued to Funke on Jan. 29, 1991, discloses a transdermal communication system that relies on electrolytical-galvan coupling. In the Funke system, the internal device includes a battery, a CPU, I/O circuitry, transmitting and receiving circuitry, and a pair of electrodes coupled to an internal organ, such as the heart of the patient. During external to internal communication, modulated signals generated by an external device are applied to a pair of external electrodes coupled to the wrist of the patient. With internal to external communication, modulated signals generated by the CPU are transmitted by the electrodes coupled to the internal organ. Regardless of the direction of the communication, the modulated signals pass to and from the electrodes coupled to the internal organ to the electrodes coupled to the patient's wrist by way of galvanic coupling. The problem with the system of Funke is that the power required for internal to external communication is provided by the internal battery, and the amount of power required to create the galvanic coupling between the internal and external electrodes is believed to be relatively large.

Accordingly, a galvanic skin conduction communication system is needed wherein the energy required for communication between the internal and external communication devices is substantially provided by the external communication device and wherein the data transfer from the internal communication device to the external communication device is efficient.

The present invention relates to a bi-directional galvanic conduction transdermal communication system. The system includes an internal communication device implanted inside the body of a patient and an external communication device. The external communication device includes an external transmitter which transmits a carrier signal into the body of the patient during communication from the internal communication device to the external communication device. The carrier signal causes galvanic conduction to occur through the skin of the patient. The internal communication device includes an internal modulator that modulates the carrier signal with information stored in the internal communication device by selectively controlling the level of galvanic conduction that occurs through the skin of the patient. An external demodulator then demodulates the information by measuring the levels of galvanic conduction through the skin of the patient, and then provides the information to an external processor. During external to internal communication, an external modulator modulates the carrier signal with information contained in the external communication device. Accordingly, the majority of the energy required for communication between the external and internal communication devices, regardless of the direction, is provided by the external communication device.

The features and advantages of the present invention will be apparent from the following description in which:

FIG. 1 is a block diagram of the transdermal communication device of the present invention.

FIG. 2 is a logic diagram of an internal communication device and an external communication device of the present invention.

FIG. 3 is a circuit that illustrates the internal communication device and the external communication device during external to internal communication according to the present invention.

FIG. 4 is a circuit that illustrates the internal communication device and the external communication device during internal to external communication according to the present invention.

FIG. 5A is an amplitude modulated signal transmitted by the present invention.

FIG. 5B is the amplitude modulated signal after being demodulated and filtered.

Referring to FIG. 1, a block diagram of a galvanic transdermal communication device of the present invention is shown. The communication device 10 includes an internal communication device 12, an external communication device 14, and a processing station 16. The internal communication device 12 is coupled to a measuring and/or controlling medical device 18. The internal communication device 12 and the medical device 18 are both implanted in the body of a patient. According to various embodiments of the invention, the medical device 18 measures and/or controls any physiological attribute of the patient. In one embodiment, the internal communication device 12 receives a measured signal from the medical device 18 and performs one or more data processing operations on the signal. The internal communication device 12 then temporarily stores the processed data. Upon the direction of the patient, the internal communication device 12 transfers the stored data to the external communication device 14 through the skin of the patient. The external communication device 14 transfers the processed data to the processing station 16, where the data is further processed and analyzed. The processing station 16 presents the data related to the measured physiological attribute to a doctor or other medical personnel, who then may monitor the physiological attribute of the patient, and may subscribe a treatment for the patient if needed. The external communication device 14 can also be used to communicate information from the processing station 16 to the internal communication device 12. Such information may include a computer program used to control the internal communication device 12, updates to a computer program previously stored in the internal communication device 12, or control information for controlling the medical device 18.

In one embodiment, the measuring device 18 is a blood pressure sensor implanted into the heart of the patient. The blood pressure sensor generates a signal indicative of the absolute blood pressure in the heart of the patient. The internal communication device 12 contains circuitry that samples the absolute blood pressure signal, and generates a filtered blood pressure signal in response thereto. The circuitry in the internal communication device 12 then analyzes the filtered blood pressure signal, and then generates a set of parameters indicative of the condition and strength of the heart of the patient. The set of parameters are then temporarily stored in the internal communication device 12. The set of parameters are then subsequently transferred to the external communication device 14, and the processing station 16 for further processing and analysis. For more information for regarding this embodiment, see the above-mentioned parent internal communication device 12 may be used to process, filter and generate parameters for any physiological attribute of the patient. Such physiological attributes may include, but are not limited to, e.g., chemical, hormonal, digestive, neural, or any organ in the body including, but not limited to, the brain, heart, lungs, kidneys, liver, blood, bladder, etc.

Referring to FIG. 2, a logic diagram of the internal communication device 12 and the external communication device 14 is shown. The internal device 12 and the external device 14 are separated by the skin of the patient 20.

The internal device 12 includes a microprocessor 30 with an analog-to-digital (A/D) converter 32, low pass filter 34, demodulator 36, modulating transistor 38, and electrodes formed as a conducting disk 40 and a conducting ring 42. The microprocessor 30 is a standard, low powered chip which operates off an internal battery (not shown), such as the PIC 16C9 by Microchip Technology, Inc., Tempe, Ariz. This chip has sufficient on-chip memory for many applications. In alternative embodiments, additional off-chip memory may be used, if needed. The low pass filter 34 is a standard resistor-capacitor filter circuit. The demodulator is a germanium diode with a turn-on threshold voltage of approximately 5 millivolts. The modulating transistor 38 is a standard MOSFET. The source and drain of the modulating transistor 38 are coupled between the conducting disk 40 and the conductive ring 42. The gate of the modulating transistor 38 is coupled to and controlled by the microprocessor 30. The conducting disk 40 is coupled to ground.

In one embodiment, the conductive disk 40 and ring 42 are implanted just beneath the skin of the patient in a location in relatively close approximation to the microprocessor 30 and/or the medical device 18. In alternative embodiments, the conductive disk 40 and ring 42 are implanted anywhere under the skin of the patient.

The external device 14 includes a microprocessor 50 with an on-chip (A/D) converter 52, a modulator 54, an oscillator 56, resistor R1, demodulator 58, low pass filter 60 and electrodes formed as a conductive disk 62 and a conductive ring 64. The microprocessor 50 can be any standard microprocessor, with internal memory, such as the 68HCll from Motorola. The modulator 54 is a standard MOSFET transistor. The oscillator 56 generates a positive to negative carrier signal ranging from 100 KHz to 5 MHz depending on various embodiments of the invention. The resistor R1 has a value of approximately 10k ohms. The demodulator 58 is a germanium diode having a threshold voltage of approximately 5 millivolts.

The filter 60 is a standard resistor-capacitor low pass filter. The conductive disks 40, 62 and ring 42, 64 can be made from any conductive and biocompatable material including but not limited to surgical stainless steel. The disks 40, 62 and rings 42, 64 can be configured in any desired shape. For instance, the conductive disks 40, 62 and rings 42, 64 illustrated in FIG. 2 have a round geometry. The disk 62 and ring 64 "mirror" the same size and shape of the conductive disk 40 and ring 42 internal to the body of the patient regardless of their relative angular displacement. As a result, conductive disks 40, 62 and rings 42, 64 with a round geometry are insensitive to relative angular displacement.

Referring to FIG. 3, a circuit that illustrates the internal device 12 and the external device 14 during external to internal communication is shown. The circuit 70 includes, external to the patient, the oscillator 56, the modulator 54, microprocessor 50, resistor R1, and resistor R2 which is the interstitial resistance of the skin between the conductive disk 62 and ring 64. Internal to the patient, the circuit 70 includes demodulator 36, low pass filter 34 including resistor R3 and capacitor C1, microprocessor 30 including A/D 32, and resistor R4 which is the interstitial resistance of the skin between the conductive disk 40 and ring 42. Resistors R5 represent the "through skin" resistance between the conductive disks 40, 62 and rings 42, 64 respectively.

During operation, the internal communication device 12 and the internal communication device 14 form a transdermal voltage divider network. Resistor R2 forms a first voltage divider element external to the body. Since R2 represents resistance of the skin between conductive disk 62 and ring 64, the resistance is a function of the electrolytical galvanic coupling of the bodily tissue between the conductive disk 62 and ring 64. Resistor R4 forms a second voltage divider element internal to the body. Since R4 represents resistance of the skin between conductive disk 40 and ring 42, the resistance is a function of the electrolytical galvanic coupling of the bodily tissue between the conductive disk 40 and ring 42.

A circuit similar to the circuit of FIG. 3 can also be used to re-charge a power source located within the internal communication device 12. The current passing through the skin is directed to a battery re-charge circuit.

During external to internal communication, the oscillator 56 generates a positive to negative carrier signal. The microprocessor 50 modulates the carrier signal by selectively turning the modulator transistor 54 on and off For example to transmit a logical "1", the transistor 54 is turned on, permitting the carrier signal to pass, creating a voltage across resistor R4. When this occurs, the voltage across R4 exceeds the threshold voltage of demodulating diode 36. The high frequency components of the carrier signal are then removed by R3 and C1 of filter 34, and the carrier signal is provided to the microprocessor 30. The A/D converter converts the relatively high voltage into digital form and the microprocessor 30 interprets the signal as a logic "1". To transmit a logical "0", the microprocessor 50 turns the modulating transistor 54 off, preventing the carrier signal from being transmitted into the body of the patient. The voltage across R4 is therefore insufficient to turn modulator 36 on, and as a result, a relatively low voltage signal is provided to the microprocessor 30. This low voltage signal is interpreted as a logical "0". In an alternative embodiment, receipt of relatively high and low voltages at the microprocessor 30 can be interpreted as a logical "0" and "1" respectively.

Referring to FIG. 4, a circuit that illustrates the internal device 12 and the external device 14 during internal to external communication is shown. The circuit 72 includes, external to the patient, oscillator 56, resistor R1, resistor R2 which represents the interstitial resistance of the skin between the conductive disk 62 and ring 64, demodulator 58, low pass filter 60 including R6 and capacitor C2, and microprocessor 50. An operational amplifier 74 may be included to amplify the signal. The internal portion of the circuit 72 includes microprocessor 30, modulating transistor 38 and resistor R4 which represents the interstitial resistance of the skin between the conductive disk 40 and ring 42. Resistors R2 and R4 again form a voltage divider network. Resistors R5 represent the through skin resistance.

During internal to external communication, the oscillator 56 generates a continuous positive to negative zero bias carrier signal. The carrier signal causes galvanic conduction to occur between the conductive disks 40, 62 and rings 42, 64 respectively. To transmit a logical "1", the microprocessor 30 turns the modulating transistor 38 off. As a result, the voltage across resistors R2 of the voltage divider network is large enough to turn on demodulator diode 58. The low pass filter 60 removes the high frequency components from the output signal of the demodulator 58, and the A/D converter 52 converts the signal to a level that represents a logical "1". To transmit a logic "0", the microprocessor 30 turns on the modulating transistor 38, causing resistor R4 to be shunted to resistor R5' and the ground. As a result, the voltage across resistor R2 of the voltage divider network is significantly reduced, below the turn on voltage of the demodulating diode 58. The A/D converter interprets the relatively low output voltage of the demodulator 58 as a logical "0". The microprocessor 30 and modulating transistor 38 are thus used to selectively control the galvanic conduction through the skin of the patient to either a first level or a second level. In an alternative embodiment, the receipt of relatively high and low voltages at the microprocessor 50 can be interpreted as a logical "0" and "1" respectively.

Referring to FIG. 5A, an amplitude modulated signal transmitted through the skin of the patient is shown. In this example, a data sequence of "0, 1, 0, 0, 1, 1, 0" for time period t.sub.1 through t.sub.7 is shown. The logical "0" signals are represented by the lower amplitude signals, and the logical "1" signals are presented by the higher amplitude signals. This example is illustrative of data transmission in the present invention, regardless of the direction of the data flow. FIG. 5B shows the signal after it has been demodulated and filtered. In alternative embodiments, data transfer can be accomplished using pulse width modulation of the carrier signal.

In accordance with one embodiment, the external device 14 is contained in a hand held unit (not shown). During operation, the conductive disk 62 and ring 64 of the external device 14 are aligned with the conductive disk 40 and ring 42 of the internal device 14. Data transfer, either external to internal, or vice versa, takes place as described above. The external device 14 is also configured to communicate either data from a remote source to the internal device 12, or vice versa, in a similar manner described in the above-identified patent applications. In fact, the external device 14 is intended to include similar peripheral circuitry and components to communicate with the processing station 16 as described in the parent applications.

With the present invention, since the carrier signal is generated external to the body, the majority of the energy required for communication between the internal device 12 and the external device 14 is supplied by the external communication device 14. Further, the alignment of the conductive disks 40, 62 and rings 42, 64 tends to localize the area of galvanic conduction, which reduces the power consumption. Both of these features significantly reduce the power consumption and increase the efficiency of the communication device 10 of the present invention. In an alternative embodiment, the germanium diodes 36 and 58 can be replaced by a silicon transistor detector, or a diode. With such embodiments, an amplifier 74 may be needed to amplify the carrier signal.

It is intended that the specification be only exemplary, and that the true scope and spirit of the invention be indicated by the following claims.