WO2009136167A1

WO2009136167A1 - System for characterising or monitoring implanted devices

Info

Publication number: WO2009136167A1
Application number: PCT/GB2009/001148
Authority: WO
Inventors: Patricia Connolly; Laurie Shedden; Roger M. Wadsworth; Simon Kennedy
Original assignee: University Of Strathclyde
Priority date: 2008-05-07
Filing date: 2009-05-07
Publication date: 2009-11-12
Also published as: GB0808266D0

Abstract

A method for determining the condition of a device that has an electrically conductive surface and is implanted in the human or animal body, the method involving measuring impedance of the implanted device using substantially the whole of the device surface as a measurement electrode.

Description

System for Characterising or Monitoring Implanted Devices

The present invention relates to a system and method for characterising or monitoring the condition of implanted devices. In particular, the invention relates to a system and method for monitoring the condition of a coronary stent, an endovascular stent, a percutaneous heart valve (including anchor stents for percutaneous heart valves) and replacement joints such as hip joints.

Background of the Invention Devices implanted into human or animal bodies are often subject to tissue growth on the surface of the device. Such tissue growth, depending on the application and growth type, may be desirable or undesirable. An example of this type of situation can be seen in the treatment of coronary heart disease.

Coronary heart disease, or atherosclerosis is a progressive disease involving formation of a plaque, which leads to narrowing of the arteries. A common way of treating atherosclerosis is by percutaneous transluminal angioplasty. This involves inserting a balloon mounted on a catheter through the skin on the thigh or arm and into the femoral or brachial artery, from where it is threaded up to the site of a blockage in the coronary artery. The balloon is then inflated to compress the plaque against the artery wall in order to restore blood flow. Whilst this procedure is effective, it leads to damage of the coronary artery wall. The biological response of the artery to this damage is restenosis, which is a renarrowing of the artery.

In order to minimise the restenosis, coronary stents are often used. The stent acts a support against the artery wall. Various designs of stent are available, as described by Lau et. al. in "Clinical Impact of Stent Construction and Design in Percutaneous Coronary Intervention", American Heart Journal Vol. 147(5), Pages 764-773. These include stents formed by self-expanding wire mesh structures, coils, tubular structures, multicellular structures and welded hoops. Stents may be produced from a range of materials such as stainless steel, cobalt alloy and nitinol, which is an alloy of titanium and nickel.

Although the stents described above improve restenosis, it still occurs in 20-30% of cases in the form of in-stent restenosis. The mechanism of in-stent restenosis differs from post-angioplasty restenosis. The stent prevents any arterial remodelling or recoil that is commonly associated with post-angioplasty restenosis and instead the main mechanism of in-stent restenosis is neointimal growth. The neointima is formed of vascular smooth muscle cells and extracellular matrix. The neointimal growth is formed around and through the stent and restricts blood flow through the artery.

A variety of methods for measuring restensosis have been developed. These include angiography, which involves injecting an x-ray opaque material into the coronary vessels and using x-ray methods to analyse the artery structure. Another method uses intra-coronary pressure measurements with a pressure monitoring wire and arterial pressure measurements using a coronary catheter to determine a fractional flow reserve (FFR). The FFR is the ratio of mean distal coronary pressure to mean arterial pressure and it can be used as a measure of restenosis. Magnetic Resonance Imaging (MRI) may also be used to determine the degree of restenosis. These techniques suffer from a range of drawbacks including being invasive, having a risk due to the use of radiation, requiring the use of compatible stents and artefacts resulting from the stent itself. In addition, they suffer from a difficulty in distinguishing between growth of a healthy endothelial layer and onset of in-stent restenosis.

To provide a measure of in-stent restenosis, sensors for attaching to stents have been developed. For example, US 5,967,986 describes pressure sensors, ultrasound sensors or an integrated circuit sensor having dielectric filaments for detecting fatty deposits by measuring the dielectric properties of the fatty deposits. WO 02/058,549 describes a stent that has a sensor for obtaining proliferation parameters. The sensor has a plurality of electrodes, conductor tracks, a semiconductor chip and electronics embedded in a film for registering impedance spectra across the surface of the film. Another sensor for detecting in-stent restenosis is described in US 6,206,835. This sensor is interrogated using an external device. Examples of suitable sensing elements include electrical, piezoelectric, sonic optical, microfluidic, chemical, thermal and magnetohydrodynamic sensing elements.

Summary of Invention

According to a first aspect of the present invention, there is provided a method for characterising the neighbourhood of an implanted device, such as a stent, comprising utilising substantially the whole of the device as an electrode in order to determine the impedance of material in the neighbourhood of the device.

Preferably, the impedance data is used to detect or determine a degree of restenosis associated with the device. The implantable device may be a coronary stent, an endovascular stent, a percutaneous heart valve (including anchor stents for percutaneous heart valves) and replacement joints such as hip joints.

Measurements made using the device are responsive to tissue growth over the whole surface. This makes the method of the invention less susceptible to anomalies due to localised tissue growth. Knowing the total amount of tissue formed along the entire length of the device gives the most accurate indication of the degree of restenosis and the overall device condition.

The method may involve coupling the device with a remote control unit. The coupling may be inductive coupling. The coupling may be radiative coupling. The method may include the steps of using remote induction means to induce an electrical signal in the implantable device. The remote induction means may be a coil or an aerial through which an electrical signal is passed.

The induced electrical signal may be an alternating current signal. The induced electrical signal may have a frequency in the range of from 1 Hz to 500 MHz. For radiative electromagnetic coupling this is preferably in the range of 1 MHz to 500 MHz, whereas for proximity (inductive) coupling this is preferably between 5kHz and 100kHz.

Inductive coupling allows lower frequency characteristics of the tissue and the implantable device to be probed directly, whereas higher frequency characteristics may be probed via radiative coupling. The type of coupling used is dependent on factors such as the implantable device's performance as an aerial and the device or material being characterised.

The implantable device may have a microcircuit on board that is adapted to convert any incoming signal of a specific frequency to another frequency for application to the implantable device. Similarly, frequencies of signals for return transmission from the device to the control unit may be converted to an outgoing signal of a different frequency before transmission. The method may involve monitoring the effect of the implantable device on the induction means to thereby collect data from the device. This may involve monitoring changes in the current and/or voltage and/or inductance of the induction means.

According to a second aspect of the present invention, there is provided a method of collecting AC impedance spectra using at least part of an implantable device as an electrode. The method may involve using the measured impedance to analyse the environment and/or condition of the device. The method may comprise using the measured impedance to determine a degree of restenosis associated with the device.

The implantable device may be a coronary stent, an endovascular stent, a percutaneous heart valve (including anchor stents for percutaneous heart valves) and replacement joints such as hip joints.

The impedance spectrum may be between 1 Hz and 500MHz and most preferably between 5kHz and 100kHz. By operating in this frequency range, the largest changes in impedance associated with common tissue types are observed. In particular, the largest changes in impedance due to smooth muscle tissue and endothelial tissue are seen in the preferred frequency range.

The method may involve characterising material by measuring changes in impedance at characteristic frequencies of the material. The method may involve analysing frequency dependence and/or magnitude of normalized impedance in order to determine a stage of growth of tissue on the device. The normalised impedance may be the ratio of measured impedance to impedance recorded by measurement apparatus without cell growth on the electrode.

The method may involve analysing changes in frequency at which at least one peak in normalised impedance occurs to determine the stage of growth on the electrode. The method may involve determining the frequency of one or more peaks in normalised impedance after the electrode has been covered in cells to characterise cell types.

The method may involve applying a DC bias. This may be done to tune the measurement. The method may involve comparing impedance data with a model representing an equivalent electrical circuit. The equivalent circuit may include resistive and capacitative elements. One or more elements may be representative of electrodes and/or tissue and/or blood and/or fluids. The method may include characterising tissue growth by determining capacitance and/or resistance values of equivalent circuit elements associated with tissue.

By applying the above method steps, not only is it possible to determine the degree of tissue growth on the electrode but also to characterise the growth, for example to differentiate between smooth muscle tissue and endothelial tissue which allows an assessment of whether or not the growth is healthy and desirable or is unacceptable in-device restenosis.

According to a third aspect of the present invention, there is provided an implantable device adapted such that substantially the whole of the device operates as an electrode for measuring the impedance of tissue. Substantially the whole of the surface of the device may comprise a conductive material, for example a metal. The electrode may be an electrode for use in AC impedance spectroscopy. At least a portion of the device may comprise a coil.

The device may be adapted to be inductively linked to a remote coil. The device may be adapted to facilitate electrical signals to be induced in it. The device may be adapted to affect a remote coil to which it is inductively linked. In this way, the device is operable to communicate wirelessly with a remote control unit. This is especially advantageous when implanted within the body, as it minimises the amount of foreign material implanted within the body, which in turn minimises the risk of complications as a result of, for example, bio-incompatibility.

According to a fourth aspect of the present invention, there is provided a system for measuring impedance including an implantable device adapted such that substantially the whole of the device is arranged to operate as an electrode for measuring the impedance of tissue and at least one counter electrode. The implantable device may be a coronary stent, an endovascular stent, a percutaneous heart valve (including anchor stents for percutaneous heart valves) and replacement joints such as hip joints.

The device may be sized and shaped to be implantable within an artery. The counter electrode may be sized and shaped to be implantable within an artery. At least the surface of the device and/or the counter electrode may be composed of a biocompatible metal.

According to a fifth aspect of the present invention, there is provided is a system for measuring impedance comprising an implantable device that can be used as an electrode for measuring impedance of tissue and a control device for wirelessly receiving measurements from the device.

According to a sixth aspect of the present invention, there is provided a characterisation and/or measurement system including a control device, a sensor and an AC impedance analyser, wherein the control device includes means for receiving impedance data collected from the sensor; the sensor at least partly includes an implantable device; and the impedance analyser is adapted to measure changes in impedance of the device at one or more frequencies.

The sensor may use substantially the whole surface of the device as an electrode for use in obtaining impedance data.

The one or more frequencies may be frequencies that are characteristic of at least one material. The system may be arranged to analyse frequency dependence and/or magnitude of normalized impedance in order to determine a stage of growth of tissue on the device. The normalised impedance may be the ratio of measured impedance to impedance recorded by measurement apparatus without cell growth on the electrode. The system may be arranged to analyse change in frequency at which at least one peak in normalised impedance occurs to determine the stage of growth on the electrode. The system may be arranged to determine the frequency of one or more peaks in normalised impedance after the electrode has been covered in cells to characterise the cell types.

The impedance analyser may have means for calculating model impedance data based on an equivalent circuit. The equivalent circuit may comprise at least one circuit element associated with the implantable device and/or a counter electrode and/or the electrolyte and/or tissue growth in the neighbourhood of the device. The electrolyte may be blood or another bodily material or bodily fluid. The circuit elements may include resistive and/or capacitive elements. The impedance analyser may have means for determining parameters associated with at least one circuit element. The means for determining may include means for fitting the model impedance data to the impedance data received from the device.

The means for receiving may be induction means. The induction means may be adapted to inductively link to a device such that AC impedance data is relayed back to the control unit via changes in the current and/or voltage and/or inductance of the induction coil caused by the device.

According to a further aspect of the invention, there is provided a method of comprising recording AC impedance of an implantable device and analysing the data to determine pulsatile changes in characteristics. The pulsatile changes in characteristics may be indicative of the beating of the heart and blood flow through the body. The pulsatile changes in characteristics may be used to determine a degree of in- or on-device restenosis. This method is advantageous when tissue layers are very thin or where tissue has not formed a neointimal layer over the device.

The pulsatile changes may include a rhythmical fluctuation in measured impedance characteristics, the fluctuation being synchronous with the pulsing of an artery to which the implantable device is attached. This can be used to analyse the bloodflow through the artery and properties associated with it. The pulsatile flow analysis may be combined with the impedance analysis of the second aspect to monitor the health and performance of the device in the body.

Brief Description of the Drawings

Various aspects of the invention will now be described by way of example only with reference to the accompanying drawings, of which: Figure 1 is a measurement stent;

Figure 2 is a diagram of a measurement system including the stent of Figure 1 ;

Figure 3 is circuit diagram for the system of Figure 2; Figure 4 shows an equivalent circuit of an implanted stent without restenosis;

Figure 5 shows an equivalent circuit of an implanted stent with restenosis;

Figure 6A shows variation in impedance response as smooth muscle cells grow on an electrode;

Figure 6B shows variation in impedance response as endothelial cells grow on an electrode;

Figure 7 shows the mean ratio absolute impedance |Z| of confluent cell layers in comparison to respective controls, for epithelial, endothelial and smooth muscle cells, and

Figure 8 shows the frequency at which the maximum change in impedance is seen for varying tissue types.

Detailed Description of the Drawings

Figure 1 shows an implantable device, which in this embodiment is a coronary stent 5 for implanting in an artery that is suitable for operating as an electrode in a system 10 that collects information regarding the material in the neighbourhood of the stent 5 by AC impedance spectroscopy. In alternative embodiments, the implantable device may be any implantable device having an at least partially conducting surface, such as an endovascular stent, percutaneous heart valve (including anchor stents for percutaneous heart valves) and replacement joints such as hip joints.

In addition to the stent 5, the measurement system 10 further comprises a control unit 15 and an implanted counter electrode 20, as shown in Figures 2 and 3. The counter electrode can be on the skin of a patient or in direct tissue or blood contact or be replaced by the patient's relative path to electrical earth. The choice of counter electrode and counter electrode location can be selected to achieve optimum measurement conditions. Applications for this system 10 include determining the degree of healthy endothelial tissue growth around the stent 5 and/or determining the degree of in-stent restenosis, which is composed primarily of smooth muscle cells and extracellular matrix.

The stent 5 is made of a conductive material, is generally tubular and formed from an open mesh of stainless steel wires such that substantially the whole of its surface is an exposed metal surface. This arrangement permits the stent 5 to be used as an electrode and also to be inductively coupled with a remote coil through which a signal is passed. The stent is constructed such that its electrical characteristics are optimised depending on the measurements for which it is being used. In one embodiment, the stent is shaped to function as an aerial for radio frequency or other electromagnetic signals, such as by using a stent that is, or is the electrical equivalent of, a conducting single or multi-turn coil.

The control unit 15 is provided with processing means 25 and an inductive control coil or aerial 30. The inductive control coil or aerial 30 is adapted to inductively couple with the stent 5. In use, the control unit 15 is operable to supply an AC electrical signal to the inductive control coil 30, which in turn induces an AC electrical signal between the stent 5 and the counter electrode 20. The frequency of the induced signal is selectable and is controlled by the signal supplied to the inductive control coil 30. The stent 5 and the counter electrode 20 thus form a circuit, with the blood in the artery acting as an electrolyte.

The measurements taken by the stent 5 and counter electrode 20 system are detectable in the inductive control coil or aerial 30 via the inductive link. If the coupling between coil and stent is local and inductive then the measurements reflect the current or voltage in the inductive coil circuit 30, related directly to the coupled stent circuit and surrounding tissue. In embodiments where the coupling is aerial based and radiative, then the measurements will be of power transmitted and received by the aerial, rather than local and inductive, at the probing frequencies or across the frequency spectrum. This is essentially the power of the electromagnetic wave transmitted from the aerial and coupled stent but could also be measured as voltage or current in a suitable transmitter / receiver circuit. In this way, the control unit 15 is operable to perform AC impedance spectroscopy using the stent 5 and the counter electrode 20.

AC impedance spectroscopy involves monitoring the impedance response of a system to a small AC perturbation current applied over a range of frequencies. This involves applying an electrical stimulus between the stent 5 and the counter electrode and measuring the magnitude and phase of the current and voltage between at least two points in the electrical path between them. In this case, the measurement points are at the stent 5 and the counter electrode themselves. The measured voltages and currents can be used to determine the impedance of the system and the magnitude and/or phase and/or phase difference of the impedance, voltage and/or current can be analysed to determine properties of the electrical circuit. The measurements can be expanded to allow local dc or other probing waveforms such as cyclic voltammetry to be induced in the conducting surface of the stent for the purposes of further analysis of the local environment of the stent. A DC bias can be applied when the AC impedance spectra is being collected in order to tailor the measurement for various materials or environments.

The measurement system 10 is further provided with an impedance analyser 35 to analyse the collected AC impedance spectra. In one embodiment, the impedance analyser 35 is adapted to determine resistance and capacitance values associated with the features of the circuit by fitting theoretical AC impedance spectrum generated from a model equivalent circuit 4OA, 4OB to the measured data. As shown in Figures 4 and 5, the equivalent circuit contains capacitive 45-55 and/or resistive 60-85 elements associated with the stent 5, blood, counter electrode 20 and tissue.

Figure 4 shows the equivalent circuit when there is no tissue growth on the stent. In this case, the circuit includes a capacitor 45 and resistor 70 in parallel representing the stent 5, a series resistor 60 representing the blood between the stent and the counter electrode, and another capacitor 50 and resistor 65 in parallel representing the counter electrode. With tissue growth on the stent 5, the equivalent circuit includes, in series with the stent and the counter electrode, resistive 75-85 and capacitive elements 55 associated with the tissue, as shown in Figure 5. These elements include a first tissue resistive element 75 in parallel with a tissue capacitive element 55; a second tissue resistive element 80 in series with the parallel combination of the first resistive element 75 and the capacitive element 55 and a third tissue resistive element 85 in parallel with the other elements 55, 75-80 of the tissue component. Parameters of these resistive 75-85 and capacitive elements 55 can be used to characterise the tissue and quantify the degree of restinosis.

The resistive 75-85 and capacitive 55 elements can be broken down as being representative of cell membrane capacitance, transcellular (through the cell) and paracellular (between the cells) resistances. This is because each of the tissue types that form growth around the stent 5 has differing frequency-impedance characteristics, which are used to characterise the type of tissue on the stent 5. Advantageously, this may be used to determine if a layer of healthy endothelial cells covers the stent 5. It also permits differentiation between the advantageous covering of endothelial cells and restenosis and determination of the extent of restenosis.

The environment of the stent can be characterised by probing the frequency dependent response of the normalised measured impedance. Figure 6A shows the frequency response of smooth muscle cells grown on a gold electrode. The bare electrode (i.e. without cells) impedance (|Z|_Day i) is used as a control. The impedance (Z) is then re-measured over time as the smooth muscle cells grow on the electrode. Normalised impedances are calculated by dividing the measured impedance by the control impedance. Confluence of the cells (complete electrode covering) occurs at Day 10 with a characteristic frequency peak at 2512 Hz in this electrode system. However, when endothelial cells are grown in a similar manner, an impedance response as shown in Figure 6B is obtained. It can be seen that frequency characteristic is markedly different from that corresponding to the smooth muscle tissue, peaking at 22387 Hz rather than 2512 Hz. The peak in frequency for the endothelial tissue growth occurs on Day 6.

Figure 7 shows the mean ratio absolute impedance |Z| of confluent cell layers grown on identical electrode sets in comparison to respective controls, over the frequency range 10Hz to 1 MHz for three cell types, these being: epithelial (EPI), endothelial (END) and smooth muscle cells (SMC). It can be seen from this that the peak frequencies of Z_n allow the three cell types to be clearly differentiated from each other. The inset shows detail of smooth muscle and endothelial cell impedance with their respective frequency peaks. From this, it can be seen that there is a characteristic peak frequency for all of the key cell types involved in both (a) the healthy covering of a stent and (b) arterial plaque formation or in-stent restenosis. Each stage of growth can give a different peak frequency.

When cells have covered the electrode (reached confluence) the peak frequency for Z_n is different for each cell type, so that there are different characteristic peak frequencies for Z_n in stents that have been successfully lined with endothelial cells and different characteristic frequencies for cells with restenosis blockage or unwanted other deposits of cells or molecular build-up. The peak or optimal frequency for each cell type is shown in Figure 8. The peak frequencies of Z_n allow the three cell types to be clearly differentiated from each other. By obtaining frequency spectra or monitoring the impedance response at these frequencies, the cell type and their growth pattern can be determined. This technique can be utilised to perform cell typing and sorting as well as in the measurement of cell growth on any implanted medical device.

Surprisingly, it has been found that at the peak frequency, the phase angles of Z_actu_ai and Z_lnt,_aι are the same. The resistance - capacitance models of the electrodes and the surrounding tissue are linked to this phase angle relationship. Thus, the starting impedance of the electrode/stent influences the peak frequency and should be measured. Hence, the stent monitoring ideally needs a characteristic impedance to be measured immediately after implantation and kept as a reference in order to yield a full tissue electrical model. Such models can be used to differentiate fats, proteins, cellular and other deposits.

The measurement system described above can also contain analysis circuitry and/or software to allow the pulsatile nature of any impedance measurement to be viewed thus monitoring any part of the characteristic dependent on blood flow. In particular, the pulsing of the artery as blood flows through it leads to a pulsatile variation in the impedance measurements, the variation being synchronised with the pulsing of the artery. Hence, by monitoring the impedance as a function of time, this effect can be used to determine blood flow and characteristics, which may be used to provide further information, both on the degree of restenosis and also other blood flow dependent properties.

In use, the stent 5 is implanted within the artery along with the counter electrode 20. When the stent 5 is to be interrogated, the control unit 15 is moved into vicinity of the stent 5. By having the control unit 15 external to the body, the amount of apparatus that needs to be implanted within the body is minimised, which minimises risks due to bio-incompatibility. After implanting the stent 5, at least one AC impedance spectrum is taken by applying an AC current between the stent 5 and the counter electrode 20 and measuring the impedance between the stent 5 and counter electrode 20 as the frequency is swept between 1 Hz and 1 MHz. This forms a baseline data set that corresponds to a restinosis free equivalent circuit of Figure 4, that can be modelled using an equivalent circuit 40A including a stent component, an electrolyte component and a counter electrode component in series. The stent component comprises a stent resistance 70 in parallel with a stent capacitance 45. The electrolyte component comprises a blood resistance 60. The counter electrode component comprises a counter electrode resistance 65 in parallel with a counter electrode capacitance 50. The parameters for these resistive 60-70 and capacitive components 45-50 can be determined by fitting data generated using the equivalent circuit model 4OA to the measured data.

AC impedance spectra may then be taken continuously, at predetermined intervals or on demand in order to monitor the growth of tissue around the stent 5 and thereby monitor its condition. The AC impedance spectra are again collected in the frequency range of between 1 Hz and 1 MHz. Use of this frequency range results in the greatest effect of frequency on the impedance of tissue. As can be seen in Figures 6A and 6B, the collected frequency range is most advantageously between 5kHz and 25kHz, which results in the greatest effect of frequency on the impedance smooth muscle and endothelial cell layers. In one embodiment, the impedance can be measured at certain discreet frequencies corresponding to a maximum impedance response for certain tissue types and degree of growth of each tissue type determined from the change in impedance.

The measured impedance data and the equivalent circuit of Figure 5 are used to determine parameters of the circuit elements 55, 70-85 and thereby characterise and measure the tissue growth. The values of the resistance and capacitance of the tissue resistive 70-85 and capacitive 55 elements can be calculated by fitting generated impedance spectra determined in conjunction with the implantable device and counter electrode capacitance and resistance values and the blood resistance values determined during calculation with the measured impedance spectra. These component values can then be analysed to determine tissue parameters such as layer type and layer thickness.

In accordance with the present invention, cell proliferation and build up of a restenosis plaque on the stent can be simply and effectively be detected merely by monitoring the impedance of the stent, using the stent itself as a measurement electrode, and without disrupting the normal functioning of the stent. This is advantageous. In one embodiment a simple radio aerial approach can be used to couple electrical signals in and out of the stent. Alternatively, the stent could be used as the secondary coil of a transformer, with the primary coil being a diagnostic instrument placed on the patient's chest to couple into the stent itself. In either case, the condition of the stent can be readily determined without requiring any modification to the stent and avoiding the need for complex imaging techniques such as MRI. A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the scope of the invention. For example, rather than just using one of the above-described techniques, a plurality of AC impedance based techniques and/or voltammetry and/or pulsatile methods may be used in combination to provide a wider range of information. Also, whilst the above invention is described in relation to the degree of restenosis, it may be used for a variety of purposes, in particular relating to the characterisation of other tissues, in body scientific investigations, detection of chemical materials, calcification, etc. For example impedance with a pulsatile flow characteristic might be monitored to monitor the performance or operation of a percutaneous flow valve. Although the counter electrode 20 is described as being separate from the stent 5, it may be incorporated into the stent 5. At least one further measurement electrode may be provided for measuring AC impedance spectra in place of the driving and/or counter electrodes. Although the communication between the control unit 15 and the stent 5 is advantageously described as being via inductive coupling, other communications methods may be used, including radio frequency transmission or wired communication.

Claims

1. A method for determining the condition of a device that has an electrically conductive surface and is implanted in the human or animal body, the method involving measuring impedance of the implanted device using substantially the whole of the device surface as a measurement electrode.

2. A method according to claim 1 , wherein the device is a stent, for example a coronary stent or an endovascular stent, or a percutaneous valve or an anchor stent for a pertcutaneous valve.

3. A method according to claim 1 or claim 2 involving using the measured impedance to detect or determine a degree of restenosis associated with the device.

4. A method according to any of the preceding claims comprising analysing the measured impedance to determine the condition of the device.

5. A method according to any of the preceding claims involving inductively coupling the device with a remote control unit.

6. A method according to any of the preceding claims comprising using remote induction means to induce an electrical signal in the device.

7. A method according to claim 5, wherein the induction means comprise a coil through which an electrical signal is passed.

8. A method according to claim 6 or claim 7 involving monitoring the effect of the device on the induction means to determine the impedance.

9. A method according to any of the preceding claims comprising measuring the impedance as a function of ac frequency.

10. A method as claimed in claim 9 comprising analysing the impedance to determine the condition of the device, for example to determine the stage of tissue growth on the device.

11. A method according to claim 9 or claim 10 comprising determining the frequency of one or more peaks in the impedance or normalised impedance and using this to characterise one or more cells or types of cells on the device.

12. A method according to any of the preceding claims, wherein the impedance data is collected using an ac signal having a frequency of between 1 Hz and 1 MHz and preferably between 5kHz and 25kHz.

13. A method according to any of the preceding claims involving characterising at least one material by measuring changes in impedance at one or more characteristic frequencies of the at least one material.

14. A method according to claim 13, wherein the one or more characteristic frequencies are associated with smooth muscle tissue and/or endothelial tissue.

15. A method according to any of the preceding claims involving comparing impedance data with a model representing an equivalent electrical circuit.

16. A method according to claim 15, wherein the equivalent circuit elements are selected from resistive and capacitive elements.

17. A method according to claim 15 or claim 16, wherein one or more elements of the electrical circuit are representative of one or more of the device; a counter electrode; tissue; blood; bodily fluid.

18. A method according to any of claims 15 to 17 involving characterising tissue growth by determining capacitance and/or resistance values of equivalent circuit elements associated with tissue growth.

19. A method according to any of the preceding claims involving using voltage probing of the implanted device in conjunction with the impedance analysis.

20. A method according to claim 19, wherein the voltage probing is dc voltage probing or cyclic voltammetry

21. A method according to any of the preceding claims involving analysing a pulsatile variation in impedance.

22. A system for measuring impedance of an implanted device that has an electrically conductive surface, the system having at least one counter electrode and means for measuring the impedance between the implanted device and the counter electrode.

23. A system according to claim 22, wherein the device and the counter electrode are sized and shaped to be implantable within an artery.

24. A system according to claim 22 or claim 23, wherein at least the surface of the device and/or the counter electrode comprise a biocompatible conductive material, for example a biocompatible metal.

25. A system for measuring impedance comprising an implanted device adapted to operate as an electrode for measuring impedance and means for wirelessly receiving impedance measurements from the device.

26. A characterisation and/or measurement system including a sensor for measuring the ac impedance of an implanted device that has an electrically conductive surface at one or more frequencies and an ac impedance analyser for using the measured impedance to determine the condition of the device.

27. A system according to claim 26 wherein the sensor is operable to measure the impedance as a function of frequency and the impedance analyser is operable to use the frequency dependence of the measured impedance to determine the condition of the device.

28. A system according to claim 26 or claim 27 wherein the one or more frequencies are frequencies characteristic of at least one material.

29. A system according to claim 28, wherein the frequencies are characteristic of smooth muscle tissue and/or endothelial tissue.

30. A system according to any of claims 26 to 29, wherein the impedance analyser is adapted to use the measured impedance and an equivalent circuit model to determine the presence or otherwise of one or more materials on the device.

31. A system according to claim 30, wherein the equivalent circuit comprises at least one circuit element associated with the stent and/or a counter electrode and/or the electrolyte and/or tissue growth in the neighbourhood of the stent.

32. A system according to claim 31 , wherein the circuit elements are resistive and/or capacitive elements.

33. A system according to any of claims 26 to 32, wherein the impedance analyser has means for determining parameters associated with at least one circuit element.