Patent Application
20240138688
The invention belongs to the field of active implantable medical devices (AIMDs). More specifically, the invention relates to an implantable pressure sensor for monitoring intravascular pressure, which can be powered and excited by volume conduction (also known as galvanic coupling).
Nowadays, the use of implantable sensing systems is known in the clinical practice, so as to measure relevant magnitudes in a living body (either animal or human) like the blood pressure. Compared to external sensing systems, implantable systems can advantageously detect the stimuli closer to where they originate, thereby providing more accurate measurements. These implantable sensing systems are normally used for diagnosis and for determining treatment dosage and timing.
Millions of patients could benefit from continuous monitoring of biomedical parameters that currently can only be measured at discrete times. A remarkable case is that of intravascular pressure in heart failure monitoring. Regular pulmonary artery pressure measurements, along with clinical signs and symptoms, allow to improve the management of patients with heart failure, reducing the rate of hospitalization.
Non-invasive external measurement systems are obviously preferable over implantable or semi-implantable systems. However, for most biomedical parameters, external systems require the patient to perform some specific actions and hence do not allow continuous monitoring. In addition, since non-invasive external measurement systems typically rely on indirect parameters or sense the magnitudes away from the relevant inner body regions, they usually exhibit reliability and accuracy issues. All this explains the need for semi-implantable or implantable sensing systems. Semi-implantable systems consisting of an external electronic module connected to an implanted sensor or catheter are an option in some cases (e.g., glucose monitoring). However, semi-implantable systems are not feasible in other cases due to risk of infection (e.g., for intravascular pressure monitoring) and in those cases fully implantable systems are required.
A known type of electronic sensing implants is based on passive electronics, which do not include a mechanism to power the implant. Most of the commercially available sensors of this type are based on combinations of inductors and capacitors (LC systems) that resonate at a specific frequency when an alternating magnetic field is applied. Such frequency is typically determined by a capacitor which acts as the sensor, as its capacitance depends on the magnitude of interest (i.e., pressure). The main disadvantage of these systems is that they require coils with a relatively large diameter, both at the implant and at the external unit, especially if the device is intended for deep implantation.
As an alternative to passive electronics, other sensors are based on active electronics. These implants incorporate a mechanism to generate electric energy to power an electronic circuit capable of reading and processing signals from a sensor and transmitting the result to an external unit for further processing or graphical representation. In some cases, the electric power is entirely generated internally (e.g., with electrochemical batteries) and in other cases the electric power is either generated by transforming a source of energy already present in the body (e.g., the so-called energy harvesters that can transform kinematic energy into electric energy), or by wireless power transfer (WPT) from an external reading unit (e.g., by ultrasound power transmission or by inductive coupling power transmission).
Some wireless systems for interrogating intravascular sensors have been proposed and demonstrated in the past. For instance, patent application WO 2017200769 A2 refers to a method to power and interrogate implanted stents using a “touch probe” antenna (e.g., external electrodes) through a near-field electric connection, which makes direct contact with skin. Here the touch probe is always described as a planar (rectangular) and rigid structure and the excitation is provided continuously. However, this application is silent about hermeticity and the construction of a pressure sensor.
In the state of the art, the system CardioMEMS HF (manufactured by Abbott) is also known, which is a microelectromechanical sensor (MEMS) for monitoring pulmonary artery pressure in order to anticipate heart failure. The implant consists of a LC resonant circuit whose capacitance changes depending on the pressure. The size of this sensor is 45 mm×3.4 mm×2 mm. Nevertheless, the interrogation system of CardioMEMS requires a bulky paddle and a desktop system. Furthermore, CardioMEMS technology is adequate only for discrete measurements (e.g., once a day), which is an important limitation.
Furthermore, patent U.S. Pat. No. 7,353,711 B1 discloses an electronic capacitive pressure sensor formed in a hermetically sealed cavity by using part of a housing as a diaphragm, thereby providing enough room for fitting sensing electronics (i.e., an integrated circuit, IC). However, the manufacturing of this sensor is rather complex.
More recently, so-called smart intravascular stents have been proposed by integrating various sensors for detecting vascular restenosis or monitoring medical conditions such as blood pressure or blood flow velocity, as the ones presented in the research article “A Single-Connector Stent Antenna for Intravascular Monitoring Applications” (Liu et al., published in Sensors, vol. 19, no. 21, p. 4616). The information about acquired pressure signals obtained with these sensors is then transmitted to external monitoring systems via wireless communications. In said article, a stent acting as an electrical inductor or antenna for signal transfer from integrated sensors to an external monitoring system is also disclosed, wherein the antenna consists of multiple conductive rings containing crowns and struts. Although this stent provides an antenna with good electromagnetic radiation efficiency, the placement of stent antenna with said rings might lead to complications during the surgical implantation. In this way, it would be preferable providing a stent which could be implanted without surgery. Moreover, embedding wide and rigid coils in the implants hampers the miniaturization. Patent application US 2016/0029956 A1 discloses wireless blood-vessel implants containing a housing, a pressure sensor, internal electronics, and thin wall comprising flexible membranes which communicate pressure to the internal electronics, by means of an incompressible fluid or gel that fills the cavity formed by the housing and the thin wall. The thin wall may be a flexible membrane which is part of a sensing electronic circuit, thus transducing pressure directly into an electronic signal of a sensing circuit. However, this document does not disclose any means for powering such implants but refers, instead, to passive sensors without any internal power supply elements. As such, for their operation they make use of elements such as LC resonant tank circuits, antennae, and coils.
The use of the volume conduction property of human tissues it is also known as a natural medium for the delivery of energy. For example, patent application WO 2006105245 A2 discloses the use of the volume conduction for energy delivery to implants, as an alternative to batteries. The disclosure of this application is focused on an external antenna design, which consists of an array of electrodes arranged to receive voltage and work collaboratively to transmit electrical energy to a target site. Under this arrangement, the external delivery of electrical energy from outside the human body to a target site within the human body is carried out providing electrical stimulation to muscles and delivering power to implanted devices. However, the application remains silent about the structure of the sensors and how said volume conduction currents must be processed. A further example of several systems comprising the use of volume conduction for energy delivery to muscle implants is disclosed in L. Becerra Fajardo et al., “Microcontrolled injectable stimulators based on electronic rectification of high frequency current bursts” (2016). However, this document does not propose any application or adaptation of these implants for their use in blood vessels or body cavities other than muscles.
Furthermore, patent U.S. Pat. No. 8,515,559 B2 refers exclusively to the use of volume conduction for communications in intravascular sensors. This document remains silent regarding the use of volume conduction for a purpose different from the communications. However, in the sensor disclosed in this patent, three major drawbacks are found:
Therefore, all the known implanted sensors (and more particularly, intravascular pressure sensors) present one or more of the following drawbacks:
All the aforementioned problems are addressed, simultaneously, by the sensor, system and method proposed by this invention.
A first object of the invention is to provide an implantable intravascular pressure sensor to obtain reliable measurements of pressure related parameters, without requiring bulky powering systems (e.g., batteries) and therefore enabling miniaturization. This sensor provides measurements which may be clinically relevant per se (e.g., arterial blood pressure) or may be relevant for continuous monitoring the condition of a device, for instance, a therapeutic device attached to the pressure sensor (e.g., to monitor the pressure drop across a stent). This pressure sensor is adapted to be interrogated by a reading unit such that both devices cooperate to obtain reliable measurements of the pressure when the pressure sensor is deployed in a living body.
Preferably, this reading unit is configured for applying high-frequency bursts of alternating current to the tissues through the skin electrodes, and additionally, for interrogating the implant by modulating those bursts of high frequency electric currents. The measurement is obtained by processing the voltage or current signals that result in the electrodes during or after the delivery of the bursts, as it will be explained later.
As it has been outlined in the background section, one technical problem present in the field is how to provide miniaturized implantable intravascular pressure sensors enabling minimal invasiveness. For instance, the possibility of percutaneous deployment (by injection or catheterization) is an important advantage of this system, instead of requiring a surgical procedure. To avoid the need of batteries, wireless power transfer (WPT) by volume conduction (also known as galvanic coupling) is a suitable alternative for powering said pressure sensors, and at the same time, for enabling intrabody communications and communications with external devices.
In a first inventive aspect, the invention refers to a pressure sensor, adapted to be implanted inside a human or animal vessel (i.e., a vein, artery, or lymphatic vessel) for measuring pressure, comprising:
Advantageously, the electronic circuit of said pressure sensor comprises:
In this way, in the present invention the electronic circuit of the pressure sensor is active, and digital, and it is not only interrogated by WPT but also it is powered by WPT based on volume conduction. The use of digital electronics allows implementing volume-conduction interrogating and powering functionalities under a single electronic architecture.
In particular embodiments of the invention, the capsule body of the pressure sensor is made, at least in part, of metallic material. Preferably, this material is a biocompatible metal, such as titanium or stainless steel, adequate for permanent implantation in an animal or human body.
In order to achieve that the hermetic capsule (or at least a portion of it) exhibits some degree of flexibility, the capsule walls must be thin (preferably, a thickness lower than 0.5 mm). Alternatively, the capsule (3) body geometry may facilitate the flexion of the capsule, for instance, by implementing corrugated walls or by using capsules with elongated cross sections, such as rectangular or oval cross sections. In this sense, in more preferred embodiments of the invention, the capsule exhibits an ellipsoidal or tubular shape.
The electromechanical architecture of the hermetic capsule also plays a relevant role in the field of active implantable medical devices (AIMDs). In the context of the invention, two main alternatives can be used for this architecture:
The first of the above alternatives would lead to pressure sensors of larger dimensions in comparison with the second alternative, but it is easier to manufacture. These two alternative designs for the capsule, without limitation to other embodiments covered by the scope of the claims, are described as follows.
In some embodiments of the invention, the pressure sensor further comprises a pressure transmission fluid housed inside the capsule, wherein the fluid is arranged between the flexible portion of the capsule and the electronic circuit, and wherein the electrical circuit further comprises a pressure transducer (e.g., a capacitive or piezoresistive MEMS sensor). Preferably, the fluid is non-compressible (e.g., silicone oil) and serves for maximizing pressure coupling. Other alternatives may use compressible fluid (e.g., air) contained within the capsule. In this way, this embodiment corresponds to the aforementioned first alternative.
In other advantageous embodiments of the pressure sensor, the partially flexible portion of the capsule is electrically connected to the electronic circuit and capacitively coupled to said electronic circuit through a conductor, forming a pressure transducer whose capacitance depends on the deformation of the partially flexible portion of the capsule. Advantageously, in this embodiment the capsule is adapted to work as the diaphragm of the pressure sensing mechanism. In this way, this embodiment corresponds to a possible implementation of the aforementioned second alternative.
In additional embodiments of the pressure sensor, said sensor comprises two conductors and the electronic circuit is capacitively coupled to the capsule through said conductors, forming two pressure-dependent capacitive elements (transducers) connected in series. Advantageously, in this embodiment the capsule is also adapted to work as the diaphragm of the pressure sensing mechanism. In this way, this embodiment corresponds to a possible implementation of the aforementioned second alternative.
In further preferred embodiments of the invention, the pressure sensor comprises fixation means adapted for attaching it to a vessel. Preferably, the fixation means comprise the electrode pair. In even further embodiments, the electrode pair or the fixation means comprise at least one of the following: a stent structure, a cable structure (which serves only for the electrodes, not for fixation purposes), or a wire loop structure. Preferably, the fixation means comprise flexible structures adapted to anchor the pressure sensor to the vessel. In certain embodiments, the fixation means (e.g., a portion of a stent) can be further used as the electrode pair.
In certain embodiments, the fixation means of the pressure sensor comprise at least partially non-insulated flexible wire loops, or even said wire loops are completely uninsulated.
In accordance with a further preferred embodiment of the invention, the exterior of the capsule is coated with a thin and flexible layer of dielectric material (e.g., parylene). In the case of metallic capsules, by preventing short circuiting of the electrode pair, this insulator maximizes power transfer by volume conduction. Advantageously, the insulating layer minimizes interferences during the pressure measurement process caused by the high frequency current bursts delivered by the interrogation units or caused by physiological biopotentials. Besides its dielectric properties, this coating may be also used to provide some other functional features such as dry lubricity, which may be advantageous during the deployment of the pressure sensor by minimally invasive means. In this way, this insulating layer provides enhanced biocompatibility (e.g., for minimizing thrombogenesis) and/or additional therapeutic functionalities (e.g., sustained release of embedded therapeutic drugs for blocking cell proliferation and thus preventing in-stent restenosis).
In a more preferred embodiment of the invention, the external electrodes are also coated with a very thin layer of insulating material (e.g., parylene). Again, this coating may provide some advantageous features such as dry lubricity, enhanced biocompatibility, or therapeutic functionalities. However, in this case, if the material is a dielectric, the coating will be disadvantageous for volume conduction. Therefore, the thickness of the dielectric coating will have to be of less than 1 micrometre to ensure that power transfer by volume conduction at high frequency is effective. Alternatively, if a thick coating of dielectric material is required, or if low frequency or dc currents must be able to flow through the pressure sensor, openings can be created in the dielectric coating by different methods (e.g., by laser ablation).
In certain embodiments, the power transferring stage of the electronic circuit comprises a blocking capacitor connected in series with the electrode pair, adapted so as to prevent passage of direct current (dc) and to enable the flow of high frequency current to and from the electronic circuit. The blocking capacitor is typically required to prevent the passage of very low frequency currents, essentially dc, that would cause electrochemical reactions at the electrode pair and could damage both the surrounding living tissues and the electrodes themselves. In said embodiments, the interrogation stage comprises a demodulator unit adapted to compare low pass filtered signals obtained with a rectifier and a digital converter connected to a pressure sensor, either a sensing capacitor or a piezoresistive sensor. Furthermore, in these embodiments the digital control unit is adapted to generate an electrical signal corresponding to the pressure measurement by modulating the load of the pressure sensor.
In more preferred embodiments, the blocking capacitor does not need to have a large capacitance value (hundreds of nanofarads, or even microfarads). Instead, in these preferred embodiments, it is enough arranging a capacitor, preferably in the order of tens of nanofarads (or even less), which is dimensioned for allowing the flow of high frequency currents (higher than 1 MHz). Advantageously, the lower capacitance value of the blocking capacitor is due to the fact that the pressure sensor does not perform stimulation. Moreover, this enables scaling down the size of the capacitor and further favours the miniaturization of the pressure sensor capsule.
A second object of the invention refers to a system comprising one or more pressure sensors as previously described, along with a reading unit which in turn comprises:
Optionally, the reading unit may also include a display or other graphical representation means to show information to a user.
Preferably, the reading unit is a battery powered hand-help unit, embodied as an external device. Alternatively, the reading unit is formed by two parts, namely: an implantable part for reading an implant inside a body, and an external part wirelessly communicated with the implantable part. In particular, the implantable part can consist of a subcutaneously implanted sub-unit capable of generating current bursts and also for taking measurements. In turn, the external part is capable of processing data wirelessly transmitted by the implanted part, and for representing the measurements for example in a display and for generating alarms, and further transmitting the measurements to a third device (e.g., another computer, a smartphone, a server, etc.) comprising storage means suitable for saving historical data record of pressure measurements and associated information (time at which the measure is taken, etc.).
Advantageously, multiple pressure sensors (also referred to as implants) can be independently interrogated by the reading unit because they can be addressed individually.
In preferred embodiments of the system, the reading unit is adapted to be implanted inside a human or animal body. More preferably, in these embodiments the processing means are further adapted to wirelessly transmit the pressure measurement to an external device. Alternatively, the reading unit electrically is placed out of the body. The reading unit is preferably powered by batteries.
Optionally, the reading unit is adapted to deliver bursts or an alternating current at a carrier frequency 1-100 MHz, with burst duration between 0.1 μs and 10 ms, and a repetition rate between 0-100 kHz. More particularly, delivering the bursts at a frequency 1-20 MHz is particularly advantageous. In a further preferred embodiment, the bursts delivered by the reading unit are sinusoidal.
A third object of the invention refers to an interrogation and powering method for operating the system described above in order to obtain pressure measurements, characterised in that it comprises the realization of the following steps:
In preferred embodiments of the previous method, the interrogation signal generated by the reading unit is a high-frequency alternating current comprising one or more bursts which encode the address of at least one pressure sensor. Said current reaches the at least one pressure sensor by volume conduction through the medium.
In further preferred embodiments of the method, the step of transmitting the pressure measurement comprises modulating the load that the at least one pressure sensor exhibits to the passage of high-frequency current flowing through the medium by volume conduction.
In general terms, the pressure sensor and the system comprising it provide the following additional advantages in comparison with known alternatives in the field:
In the scope of the invention, the following definitions must be considered about certain technical concepts. Throughout the description, digital electronics and active electronics are treated as synonyms. High frequency refers to frequencies above 1 MHz, and more preferably, in the range 1-100 MHz. The medium preferably corresponds to a biological fluid (e.g., blood), flowing within a vessel (e.g., an artery or vein). Moreover, the pressure sensor is also referred to as implant in the description. Finally, the term intravascular pressure refers to the pressure of any intrabody cavity or vessel, i.e., blood or lymphatic vessels.
To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:
In these figures, the following reference numbers are used:
Different possible embodiments of the pressure sensor, the system and the method of the invention will be now described for illustrative purposes.
As disclosed in the preceding sections, the invention refers to an implantable intravascular pressure sensor (1) for measuring pressure due to the presence or flow of a fluid, such as blood, in the vessel (2). A major advantage of the pressure sensor (1) is that it does not require a bulky system for feeding its electronics and, thereby, the miniaturization and the operational life of the implantable device is not limited by this feature. To achieve this, volume conduction (also known as galvanic coupling) is used as a wireless power transfer (WPT) mechanism for the sensor (1), and active (digital) electronics is required. The pressure sensors (1) obtained with this technology based on volume conduction are much thinner (<0.5 mm), therefore less invasive than other implant technologies. Advantageously, this provides further miniaturization of the whole pressure sensor (1) is required to avoid surgical procedures for its deployment within the vessel (2).
The structure of the sensor (1) includes a hermetic capsule (3) comprising at least a partially flexible portion, which enables pressure transmission from the outside to the inside of said capsule (3), and an electronic circuit (4) housed in the capsule (3), adapted to measure a pressure signal resulting in the vessel (2). The hermetic capsule (3) also serves as an external housing for the sensor (1), thereby protecting the electronic circuit (4). The sensor (1) further comprises an electrode pair (5, 5′) electrically coupled to the electronic circuit (4) and passing through the capsule (3), such that each electrode of the electrode pair (5, 5′) is, at least in part, arranged externally to the capsule (3) and adapted to receive power by volume conduction through a medium, such as the blood or muscle tissues. Moreover, in a preferred embodiment of the invention, at least one electrode of the electrode pair (5, 5′) or a structure attached to the capsule (3) is preferably a flexible structure configured for anchoring the implant to a vessel (2), for instance, an artery or vein.
To obtain a pressure measurement, the pressure sensor (1) is preferably used in combination with a reading unit (6), which is adapted for interrogating and powering said pressure sensor (1). In this way, as shown in
The reading unit (6) stores measurements in a memory means or transmits those measurements by radio communication to a nearby computerized device, such as a smartphone. Advantageously in the invention, volume conduction is suitable for powering the implants from the reading unit (6) and for enabling bidirectional digital communications between the pressure sensor (1) and said reading unit (6). In particular, in order to meet the requirements of safety standards and to be able to operate deep implants, the use of volume conducted bursts of alternating (ac) currents at frequencies in the range from 1 MHz to 100 MHz is preferable. In this way, the use of volume conduction is crucial for the invention as it avoids the need of any bulky component within the pressure sensor (1) for generating power or for receiving power sent by the reading unit (6).
The reading unit (6) can produce measurements of pressure by processing either the signals emitted by the sensor during the interrogation signal or after it. The reading unit (6) can also combine measurements acquired during and after the interrogation signal in order to improve the accuracy of the pressure measurement.
As shown in the embodiment of
A further preferred embodiment of the invention is shown in
An optional aspect of the invention is taking advantage of the flexible and hermetic capsule (3) to implement the mechanism of pressure transduction, as displayed in the preferred embodiment of
Preferably, as illustrated in
Alternatively, the capsule (3) can be connected directly to the electronics thus forming a single capacitance. This embodiment is illustrated in
Another optional embodiment is a variant of the one illustrated in
Other preferred embodiments of the pressure sensor (1) comprise one or more piezoresistive elements (e.g., strain gauges) that are attached (e.g., glued) to the inner wall of the flexible portion (3′) of the capsule (3) thus forming a resistive pressure sensor which is connected with conductors (19, 19′) (e.g., wires) to the electronic circuit (4).
An alternative for pressure transduction within the embodiments of the invention is to include a conventional MEMS pressure transducer (1″) integrated in the electronics of the implant (e.g., in the same printed circuit board or in the same integrated circuit) and wherein the external pressure surrounding the capsule (3) is transmitted to the pressure sensor via one of the following possibilities:
The conventional MEMS pressure transducer (1″) can be, for example, a MEMS piezoresistive or capacitive pressure sensor.
In a preferred embodiment of the invention, the capsule (3) body has an elliptical disk shape, as illustrated in
An essential feature of the invention relates to how the wireless power transfer takes place.
The obtained dc power is maximized by delivering interrogation signal (which is an ac field) in the form of short bursts rather than continuously. The maximum attainable dc power depends on the conductivity of the medium (15), e.g., tissue or a body fluid. Such dependency is minimized by delivering the ac field in the form of short bursts rather than continuously, which enhances the wireless power transfer.
The fundamental structure of the powering and interrogation method comprises the following steps:
Both the absolute maximum attainable power and the maximum attainable dc power exhibit a distinctive maximum for a specific load resistance (i.e., optimum load resistance). If the ac field is delivered in the form of bursts rather than continuously, it is possible to set the value of the optimum load resistance by adjusting the duty cycle of the bursts.
Different circuit topologies are known that can be used to implement the CDC, which provides a digital value proportional to the capacitance value of a capacitor. These CDC architectures are often classified by the type of Analog-to-Digital Converter (ADC) used internally. Examples of such CDC architectures are the Sigma Delta (ΣΔ) CDCs, the Successive Approximations (SAR) CDCs or the double slope CDC. Another relevant group of CDC architecture converts the capacitance value into a time varying signal and the measurement of the timing parameters are then converted to digital domain. Examples of such time-based CDC architectures are the Pulse Width Modulation. Any of the aforementioned CDCs, or equivalent ones, can be used in the context of this invention.
Optionally, the electronic circuit (4) further includes a voltage regulator (31) to obtain a suitable voltage to feed the DCU (25).
Alternatively, other implementations of the electronic circuit (4) may use piezoresistive sensing of pressure variations instead of capacitance changes. For instance, in the circuit from
To obtain an efficient wireless power transfer, the parameters of the interrogation signal (frequency, burst rate, burst duration, etc.) must be properly established. Preferably, the pressure sensor (1) electronics operates with a burst frequency below 100 MHz. In more preferred embodiments, the burst frequency range is 1-20 MHz. Advantageously, this range provides a more isotropic behaviour of the tissues (e.g., muscle tissues can be considered isotropic above 1 MHz) and, at the same time, minimizes the skin effect.
For all the embodiments of the invention, the injected alternating currents for interrogation, which are either current controlled or voltage controlled, are specially selected to be innocuous to the body. This is accomplished by ensuring that these currents are of sufficient frequency to prevent unsought stimulation of excitable tissues (first requirement), and their power is low enough to prevent excessive heating of tissues due to Joule heating (second requirement).
Furthermore, it is required that the interrogation signal encode the address of the pressure sensor (1) to be reached (e.g., a physical (MAC) or a logical (IP) address). More preferably, the bursts encode the target pressure sensor (1) at which the interrogation signal is sent.by using known techniques of digital addressing, thereby enabling a selective interrogation. This is particularly convenient when multiple pressure sensors (1) are read, excited (interrogated) and powered by the same reading unit (1) or said sensors (1) are implanted close to each other.
The first requirement can be readily met by using currents whose power spectral density is well above 100 kHz. For instance, currents with a frequency (f) above 1 MHz, are desired. Furthermore, frequencies below 100 MHz are preferred, to prevent that the skin effect becomes significant, and the operation of implants at deep locations is hindered.
The second requirement is achieved by delivering short bursts. For a given voltage gradient (E) required to ensure operation of the implants (e.g., 200 V/m), burst duration (B) and burst repetition frequency (F), are selected to ensure that power dissipation in the tissue where the implants are located does not reach a safety threshold.
Safety thresholds for electromagnetic power dissipation in tissues are usually specified by the so-called Specific Absorption Rate (SAR), which is measured in W/kg. SAR can be calculated with the following expression:
SAR=σ(ERMS)2/ρ
where σ is the electrical conductivity of the tissue (S/m), ρ is the mass density of the tissue (kg/m3) and ERMS is the root mean square value of the electric field in the tissue (V/m).
From that expression it can be obtained the following requirement for F and B in the case of sinusoidal bursts:
FB<2ρSARMAX/σE2
A SAR value of 2 W/kg is considered to be a safe threshold in all circumstances according to different standards. If the above expression is particularized for the case of muscle tissue at 10 MHz (σ10 MHz=0.62 S/m, ρ=1060 kg/m3) and it is assumed that the field required for operation is 200 V/m, the 2 W/kg limit yields FB<0.17 s/s. Thus, for instance, if the burst duration, is 10 μs, the maximum repetition frequency would be 17 kHz. If the above expression is particularized for the case of blood at 6.67 MHz, which is the central frequency of an ISM (Industrial, Scientific and Medical) band and hence it is a suitable frequency to electromagnetic compatibility issues, (σ6.67 MHz=1.066 S/m, ρ=1060 kg/m3) and it is assumed that the field required for operation is 200 V/m, the 2 W/kg limit yields FB<0.099 s/s. Thus, for instance, if the burst duration, is 10 μs, the maximum repetition frequency would be 9.9 kHz.
It must be noted that the SAR limitation must not only be met where the implants are located but also in all tissue regions where the interrogation currents flow through. Therefore, since current densities (and voltage gradients) will be probably higher in the vicinity of the current injecting electrodes of the reading unit, the FB product will have to be scaled down.
In a preferred embodiment of the system, the interrogation signal consists of a modulated sinusoidal waveform with a carrier frequency between 100 kHz and 100 MHz which is delivered as bursts with a duration between 0.1 μs and 10 ms, and a repetition frequency between 0 Hz (i.e., single burst interrogation) and 100 kHz.
In some preferred embodiments, the digital modulation scheme employed in the communications guarantees that throughout the whole transmission of the interrogation signal, power is delivered to the pressure sensor (1). Particularly, even in the case of a long burst transmission containing only 0 values, it would be desirable that a high-frequency current is transferred to the pressure sensor (1) in order to feed the electronic circuit (4). For addressing this case, the use of Manchester codification is convenient as it provides a constant average amplitude of the signal, which is required by the amplitude demodulator unit (26), as well as self-clocking capabilities. Alternatively, the communication can comprise only a fraction of the high frequency burst and during said fraction, the powering of the electronic circuit (4) can be kept by means of a capacitive element associated with power regulator as it is commonly performed in voltage regulators.
A preferred use of the invention is the detection and monitorization of in-stent restenosis. When in-stent restenosis occurs, that is, when excess tissue or plaque builds up on the inner walls of the stent, blood flow is obstructed thus changing the pressure distribution along the stent. An alteration of local pressure, which can be detected with the system of the invention, may be indicative of restenosis. In general, upon the onset of restenosis, the average pressure during the cardiac cycle will increase pressure upstream the stent and will decrease downstream.
In an optional embodiment of the system of the invention, a single pressure sensor (1), which can be located upstream in the stent, suffices to detect an abnormality in blood flow and thus warns about a possible complication. However, in another preferred embodiment of the system of the invention, two pressure sensors (1, 1′) are embedded in a stent structure (16, 16′, 16″), one sensor (1) upstream and another sensor (1′) downstream, as displayed in
15.—The method according to any of claims 13-14, wherein the step of transmitting the electrical transduction pressure signal comprises modulating the load that the at least one pressure sensor (1) exhibits to the passage of high-frequency current flowing through the medium (15) by volume conduction.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21382183.8 | Mar 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/055331 | 3/3/2022 | WO |
