Arrays for delivering tumor treating fields (tt fields) with individually accessible electrode elements and temperature sensors

Abstract

To provide a first device for applying an alternating electric field to a subject's body.SOLUTION: Tumor treating fields (TT Fields) can be delivered to a subject's body at higher field strengths by switching off one or a plurality of electrode elements in a transducer array that are overheating. This may be accomplished by using thermistors that sense the temperature of each electrode element. Portions of the wiring of each transducer array is shared between the electrode elements and the thermistors by using a plurality of conductors, each of which electrically connects (a) a pin of a connector, (b) each electrode element, and (c) each thermistor. In some embodiments, all of the thermistors are wired in series. In other embodiments, all the thermistors share a common connection.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 62 / 955,664, filed December 31, 2019, which is incorporated herein by reference in its entirety. [Background technology]

[0002] TT Fields therapy is a well-established technique for treating tumors. Figure 1 is a schematic diagram of a prior art Optune® system for delivering TT Fields. TT Fields are delivered to a patient via four transducer arrays 21-24 positioned on the patient's skin in close proximity to the tumor (e.g., as depicted in Figures 2A-2D for an individual with glioblastoma). The transducer arrays 21-24 are arranged in two pairs, with each transducer array connected to an AC signal generator 20 via a multi-wire cable. The AC signal generator (a) sends an AC current through one pair of arrays 21, 22 for a first time period, which induces an electric field having a first direction through the tumor, and then (b) sends an AC current through the other pair of arrays 23, 24 for a second time period, which induces an electric field having a second direction through the tumor, and then repeats steps (a) and (b) for the duration of the treatment.

[0003] Each transducer array 21-24 is configured as a set of capacitively coupled electrode elements E (e.g., a set of nine electrode elements, each approximately 2 cm in diameter) interconnected via a flex circuit. Each electrode element includes a conductive substrate having a dielectric layer (more specifically, a layer of ceramic material with a high dielectric constant) disposed thereon. Each electrode element is sandwiched between a layer of conductive medical gel and adhesive tape. When the array is placed on the patient, the medical gel conforms to the contours of the patient's skin, ensuring good electrical contact between the body and the device. The adhesive tape holds the entire array in place on the patient as the patient goes about their daily activities.

[0004] The amplitude of the alternating current delivered through the transducer array is controlled so that the skin temperature (as measured on the skin beneath the transducer array) does not exceed a safe threshold of 41°C. Temperature measurements on the patient's skin are obtained using thermistors T placed beneath some of the disks of the transducer array. In existing Optune® systems, each array contains eight thermistors, one thermistor positioned beneath each disk in the array. (Note that most arrays contain more than eight disks, in which case temperature measurements are taken beneath only a subset of the disks in the array.)

[0005] AC signal generator 20 obtains temperature measurements from all 32 thermistors (4 arrays x 8 thermistors per array), and a controller in the AC signal generator uses the temperature measurements to control the current sent through each pair of arrays to maintain the temperature on the patient's skin below 41°C. The current itself is sent to each array via additional wires extending from AC signal generator 20 to each array (i.e., one wire 28 for each of arrays 21-24). An additional wire (not shown) for each of arrays 21-24 also serves as a common return wire for all eight thermistors. Thus, each of the four cables terminating on arrays 21-24 in existing Optune systems has a total of 10 conductors. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] US 16 / 686,918 [Patent Document 2] US 2018 / 0050200 Summary of the Invention [Means for solving the problem]

[0007] One aspect of the present invention relates to a first device for applying an alternating current electric field to a subject's body. The first device includes a plurality of electrode elements, a support configured to hold the electrode elements against the subject's body, and a plurality of thermistors. Each of the thermistors has a first terminal and a second terminal, and each thermistor is positioned to sense temperature at a corresponding one of the electrode elements. The first device also includes a connector having a plurality of first and second pins, and a plurality of first conductors, each of which provides a conductive path between (a) a respective one of the first pins, (b) a respective one of the electrode elements, and (c) a corresponding first terminal of the thermistor, and a second conductor providing a conductive path between a second pin and the second terminal of at least one of the thermistors.

[0008] In some embodiments of the first device, the second terminals of all of the thermistors are wired together. Optionally, in these embodiments, the second terminals of all of the thermistors can be wired together using at least one of: (a) at least one trace on the flex circuit; and (b) at least one wire.

[0009] In some embodiments of the first device, the multiple thermistors are arranged in series, starting with the first of the thermistors and ending with the last of the thermistors, with the second terminal of each of the thermistors except the last one wired to the first terminal of the respective subsequent thermistor, and with a second conductor providing a conductive path between the second pin of the connector and the second terminal of the last thermistor.

[0010] In some embodiments of the first device, a second conductor provides a conductive path between the second pin and a second terminal of only one of the thermistors.

[0011] In some embodiments of the first device, the plurality of electrode elements comprises at least four electrode elements and the plurality of thermistors comprises at least four thermistors.

[0012] In some embodiments of the first device, the plurality of electrode elements comprises at least nine electrode elements and the plurality of thermistors comprises at least nine thermistors.

[0013] In some embodiments of the first device, each of the electrode elements comprises a conductive plate and a dielectric layer disposed on the conductive plate, and the support is configured to hold the electrode elements against the subject's body so that the dielectric layer of each of the electrode elements faces the subject's body.

[0014] In some embodiments of the first aspect, the second terminals of all of the thermistors are wired together, and the second conductor provides a conductive path between the second pin and the second terminals of all of the thermistors.

[0015] Another aspect of the present invention relates to a second device for applying an AC electric field to a subject's body using multiple electrode elements. Each of the multiple electrode elements is disposed in thermal contact with a respective thermistor. The second device includes an AC signal generator that generates an AC output signal and a connector including multiple pins. Each of the pins corresponds to a respective one of the multiple electrode elements. The second device also includes a first multiple switches. Each of the first multiple switches is configured to selectively apply or not apply the AC output signal to a respective one of the pins. The second device also includes an amplifier configured to receive an electrical signal from each of the thermistors and generate a corresponding output. The electrical signal from the thermistor arrives via the same pin corresponding to the multiple electrode elements. The second device also includes a controller configured to control the first multiple switches to individually adjust the duty cycle of the AC signal applied to each of the multiple pins based on the output of the amplifier.

[0016] In some embodiments of the second apparatus, the controller is configured to (a) determine, based on the output of the amplifier, when at least one of the electrode elements is hotter than the other electrode elements, and (b) control the first plurality of switches to reduce a duty cycle of the AC signal applied to the at least one respective pin. In some embodiments of the second apparatus, the controller is configured to (a) determine, based on the output of the amplifier, when at least one of the electrode elements is hotter than a threshold level, and (b) control the first plurality of switches to reduce a duty cycle of the AC signal applied to the at least one respective pin.

[0017] In some embodiments of the second device, the plurality of electrode elements comprises at least four electrode elements, the plurality of pins comprises at least four pins, and the first plurality of switches comprises at least four switches. In some embodiments of the second device, the plurality of electrode elements comprises at least nine electrode elements, the plurality of pins comprises at least nine pins, and the first plurality of switches comprises at least nine switches.

[0018] Some embodiments of the second device further comprise a second plurality of switches, each of which is arranged to route a signal from a respective one of the plurality of pins to a first input of the amplifier. In these embodiments, the controller is further configured to control the second plurality of switches to sequentially select each of the plurality of pins to sequentially obtain temperature readings from each of the thermistors. Optionally, in these embodiments, the connector includes an additional pin, and the device further comprises an additional switch arranged to route a signal from the additional pin to a second input of the amplifier.

[0019] Some embodiments of the second device further include a second plurality of switches, each of which is arranged to route a signal from a respective one of the plurality of pins to a first input of the amplifier. In these embodiments, the controller is further configured to control the second plurality of switches to sequentially select each of the plurality of pins and sequentially obtain temperature readings from each of the thermistors. The connector includes an additional pin, and the device further includes (a) an additional switch arranged to route a signal from the additional pin to the second input of the amplifier, and (b) a third plurality of switches, each of which is arranged to route a signal from a respective one of the plurality of pins to the additional pin. In these embodiments, the controller is further configured such that when a given switch from the first plurality of switches is open, the controller closes a corresponding switch from the third plurality of switches.

[0020] Some embodiments of the second device further include a second plurality of switches, each arranged to route a signal from a respective one of the plurality of pins to a first input of the amplifier, and a third plurality of switches, each arranged to route a signal from a respective one of the plurality of pins to a second input of the amplifier. In these embodiments, the controller is further configured to control the second plurality of switches and the third plurality of switches to sequentially route signals from pairs of the plurality of pins corresponding to both terminals of each of the thermistors in turn to the first and second inputs of the amplifier to sequentially obtain temperature readings from each of the thermistors.

[0021] Another aspect of the present invention relates to a first method for applying an AC electric field to a subject's body using multiple electrode elements, each of which is disposed in thermal contact with a respective thermistor. The first method includes applying an AC signal to each of the electrode elements at a respective duty cycle so that an AC electric field is induced in the subject, inputting signals from each of the thermistors, determining the temperature of each of the electrode elements based on the input signals, and adjusting the duty cycle of the AC signal applied to the electrode elements based on the determined temperature. After the adjusting step, the duty cycle of the AC signal applied to at least one of the electrode elements is different from the duty cycle of the AC signal applied to another one of the electrode elements.

[0022] In some cases of the first method, the adjusting step includes reducing a duty cycle of an AC signal applied to at least one of the electrode elements that is hotter than at least one other electrode element. In some cases of the first method, the adjusting step includes reducing a duty cycle of an AC signal applied to the hottest of any of the electrode elements. In some cases of the first method, the adjusting step includes reducing a duty cycle of an AC signal applied to any of the electrode elements whose temperature exceeds a threshold level.

[0023] In some instances of the first method, the adjusting step includes reducing the duty cycle of the AC signal applied to any of the electrode elements whose heating rate exceeds a threshold level. In some instances of the first method, the adjusting step includes switching off the AC signal applied to any of the electrode elements whose temperature exceeds a first threshold level and switching on the AC signal applied to at least one of the electrode elements whose temperature is below a second threshold level, the second threshold level being below the first threshold level. [Brief explanation of the drawings]

[0024] [Figure 1]FIG. 1 is a schematic diagram of a prior art Optune® system for delivering TT Fields. [Figure 2A] FIG. 1 illustrates the positioning of a transducer array above a person's head to treat a brain tumor. [Figure 2B] FIG. 1 illustrates the positioning of a transducer array above a person's head to treat a brain tumor. [Figure 2C] FIG. 1 illustrates the positioning of a transducer array above a person's head to treat a brain tumor. [Figure 2D] FIG. 1 illustrates the positioning of a transducer array above a person's head to treat a brain tumor. [Figure 3] FIG. 1 depicts a first embodiment of a transducer array that provides an individual conductor for each individual electrode element. [Figure 4] FIG. 4 is a block diagram of a system using four copies of the transducer array of FIG. 3 to apply TT fields to a subject. [Figure 5] FIG. 10 depicts a second embodiment of a transducer array that provides an individual conductor for each individual electrode element. [Figure 6] FIG. 6 is a block diagram of a system using four copies of the transducer array of FIG. 5 to apply TT fields to a subject. [Figure 7] 7 is a schematic diagram of a circuit suitable for implementing each of the switches in banks 1L and 1R in the embodiments of FIGS. 4 and 6. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0025] Various embodiments are described in detail below with reference to the accompanying drawings, in which like reference numerals represent like elements, and in which:

[0026] 1 described above is very effective in delivering TTFields to tumors, but the effectiveness of the treatment will be reduced if good electrical contact is not maintained between each of the elements in the four transducer arrays 21-24 and the body. This can occur, for example, if the hydrogel underneath one or more elements of the transducer array dries out over time or due to hair growth underneath one or more of the elements.

[0027] For example, suppose there are nine electrode elements E in each of the transducer arrays 21-24, and the hydrogel beneath a single electrode element E on the front transducer array 21 dries, leaving enough hydrogel beneath (a) all other electrode elements in that transducer array 21 and (b) all electrode elements E in the other transducer arrays 22-24. In this situation, the resistance between the single electrode element E and the human body will be higher than the resistance between any of the other electrode elements and the human body. This increased resistance will also cause the temperature of the single electrode element E to rise more than the other electrode elements.

[0028] In this situation, because all of the electrode elements E in each of the transducer arrays 21-24 are wired in parallel, the AC signal generator 20 must limit the current applied to all anterior / posterior pairs of the transducer arrays 21, 22 to keep the temperature of any single electrode element E on the anterior array 21 below 41° C., even though the temperatures of all of the remaining electrode elements E on the anterior and posterior transducer arrays 21, 22 can be well below 41° C. This reduction in current also causes a corresponding reduction in the strength of the electric field at the tumor, which may reduce the effectiveness of the treatment.

[0029] The embodiments described herein advantageously provide the ability to control the current routed through individual electrode elements without unduly increasing the number of conductors in the cables that terminate on the transducer array, and without relying on active elements positioned on or near the transducer array.

[0030] Unlike prior art configurations in which all of the electrode elements E in each of the transducer arrays 21-24 are wired in parallel, the embodiments described herein have individual conductors for each of the multiple electrode elements in each of the transducer arrays. This configuration allows current to be independently switched on and off for any given individual electrode element in any one of the arrays.

[0031] One possible approach to controlling current on a per-element basis is to start with the prior art configuration shown in FIG. 1 and rewire the electrode elements so that they are not connected in parallel but rather have individual wires leading to each individual electrode element. The problem with this approach, however, is that it requires nearly doubling the number of conductors in each of the cables running to the transducer array. For example, a transducer array with nine electrode elements would require a total of 19 wires in each cable (i.e., nine to provide individual access to each of the nine electrode elements, an additional nine for the signal from the thermistors, plus one additional wire to serve as a common return for all nine thermistors). This dramatic increase in the number of wires in each cable also makes the cables more difficult to bend and more cumbersome, which may make the system more difficult to use and reduce patient compatibility.

[0032] One way to reduce the total number of wires in each cable is to position active elements (e.g., electrically controlled switches) on or near the transducer array. For example, a set of switches can be used to control which of the electrode elements are on at any given moment (e.g., as described in US 16 / 686,918, filed November 18, 2019, which is incorporated herein by reference in its entirety). Alternatively or additionally, an analog multiplexer can be used to multiplex temperature readings obtained from thermistors into a cable with a reduced number of conductors (e.g., as described in US 2018 / 0050200, which is incorporated herein by reference in its entirety). However, positioning active elements on or near the transducer array brings its own set of drawbacks (e.g., added weight and complexity, plus potential issues with sterilization).

[0033] 3 depicts a first embodiment of a transducer array 50 that provides individual conductors to each individual electrode element 51 without a dramatic increase in the number of wires in each cable extending to a given transducer array and without requiring active elements to be positioned on or near the transducer array. As described below in connection with FIG. 4, preferably, four copies of the transducer array 50 are used to administer TT Fields treatment to a person's head (or other body part).

[0034] Each transducer array 50 includes a plurality of electrode elements 52, labeled E1 through E9 for ease of reference in the embodiment of FIG. 3. Each of these electrode elements 52 has a conductive substrate (e.g., a circular metal substrate) on which a dielectric layer is disposed. In some preferred embodiments, each of these electrode elements 52 is a disk-shaped, capacitively coupled electrode element (e.g., having a diameter of 2 cm), similar to the prior art electrode elements used in the Optune® system, where the dielectric layer comprises a thin layer of ceramic material having a high dielectric constant. However, unlike the Optune® system and FIG. 1 (where all elements are wired in parallel), in this embodiment of FIG. 3, individual conductors extend from each of the electrode elements 52 to a connector 57. These conductors are numbered 1 through 9 immediately above the “wire routing” block 55 (which converges the individual conductors together into a single cable 56). In some preferred embodiments, the electrical connection to each of the electrode elements 52 includes one or more traces on a flex circuit and / or one or more conductive wires.

[0035] In the embodiment depicted in FIG. 3 , all of the capacitively coupled electrode elements 52 are held in place by a support structure 59. The support structure is configured to hold the electrode elements against the subject's body so that the dielectric layers of the electrode elements 52 face and are positioned in contact with the subject's body. Optionally, the support structure may include a flexible backing 59 (e.g., a layer of foam material). Preferably, a layer of hydrogel is disposed between the dielectric layers of the electrode elements 52 and the subject's body when the transducer array 50 is placed against the subject's body. Construction of the support structure 59 may be performed using any of a variety of conventional techniques apparent to those skilled in the art, including, but not limited to, adhesive cloth, foam, or plastic sheeting.

[0036] Each transducer array 50 also includes a plurality of thermistors 54, one thermistor positioned at each of the electrode elements 52 so that the thermistors 54 can sense the temperature of the respective electrode element 52. This can be achieved, for example, by incorporating a hole or recess in the center of each electrode element 52 and positioning a respective one of the thermistors 54 within the hole or recess. Each of the thermistors 54 has a first terminal (i.e., the bottom terminal of the thermistor in FIG. 3 ) and a second terminal (i.e., the top terminal of the thermistor in FIG. 3 ).

[0037] Each transducer array 50 also has a connector 57 that is used to transmit electrical signals into and out of the transducer array 50. The connector 57 has a plurality of first and second pins. In the illustrated embodiment, the number of first pins is the same as the number of electrode elements 52, and each of the first pins corresponds to a respective one of the electrode elements 52. Also, in the illustrated embodiment, there is only a single second pin, labeled C. It should be noted that, as used herein, the term "pin" can refer to either a male or female pin of the connector 57.

[0038] Each transducer array 50 also has a plurality of first conductors, the number of which will depend on the number of electrode elements 52. In the embodiment depicted in FIG. 3 , which includes nine electrode elements 52, these conductors are labeled 1 through 9. Each of these first conductors provides a conductive path between (a) a respective one of the first pins in the connector 57, (b) the conductive substrate of a respective one (E1 through E9) of the electrode elements 52, and (c) a first terminal of a corresponding thermistor 54. Note that each of these first conductors may optionally be implemented using multiple segments of wire and / or multiple traces on a flex circuit.

[0039] Each transducer array 50 also has a second conductor that provides a conductive path between a second pin of the connector 57 and a second terminal of at least one of the thermistors 54 (i.e., the top terminal in FIG. 3 ). In the embodiment depicted in FIG. 3 , all of the second terminals of the thermistors are wired together. In this embodiment, the second conductor provides a conductive path between the second pin of the connector 57 and all of the second terminals of the thermistors 54. The second conductor may optionally be implemented using multiple segments of wire and / or multiple traces on a flex circuit.

[0040] The connector 57 has a respective first pin corresponding to each of the respective electrode elements 52, and because a conductive path exists between each of the first pins and a respective one of the electrode elements 52, a system mating with the connector 57 can individually and selectively energize or de-energize each of the electrode elements 52 by either applying or not applying a signal to the respective first pin on the connector 57.

[0041] Connector 57 also has a respective first pin corresponding to the first terminal of each of the thermistors 54, and a conductive path exists between each of the first pins and a respective one of the thermistors 54, allowing a system mated with connector 57 to access the first terminal of each of the thermistors 54. In addition, because the second terminals of all of the thermistors 54 are all wired together and connected to a second pin (labeled C), a system mated with connector 57 also has access to the second terminal of each of the thermistors 54. As a result, a system mated with connector 57 can measure the resistance of any of the thermistors 54. This can be accomplished, for example, by routing a known current through each thermistor 54 and measuring the voltage that appears across each thermistor.

[0042] In particular, each of the first pins on connectors 57 serves two functions, since any given first pin on connector 57 corresponds to a respective one of the individual electrode elements 52 and also corresponds to a respective one of the individual thermistors 54. This reduces the number of wires that must be included in each of cables 56, which in turn advantageously makes the cables more flexible and less cumbersome.

[0043] FIG. 4 is a block diagram of a system that uses four copies of transducer array 50 (described above in connection with FIG. 3) to apply TT fields to a subject. In FIG. 4, these four copies are labeled 50A, 50P, 50L, and 50R, where A, P, L, and R stand for anterior, posterior, left, and right, respectively. The bottom of FIG. 4 depicts AC voltage generator 35 and “CAD box” 30 as separate blocks, the latter of which includes temperature measurement block 32, controller 34, and banks of switches 1L, 2L, 3L, 1R, 2R, and 3R. In some embodiments, the components in these two blocks 35, 30 can be physically separated into two separate housings. However, in alternative embodiments, the components in these two blocks 35, 30 are combined into a single housing.

[0044] For ease of viewing, only the left and right channels are depicted in FIG. 4. However, the remaining channels (i.e., the front and rear channels) operate in the same manner as the left and right channels, respectively. In addition, each of the transducer arrays 50 in FIG. 4 is depicted with only four electrode elements 52 and four thermistors 54 for ease of viewing. However, it is expected that an actual system will have a greater number of electrode elements and thermistors (e.g., between 9 and 30), as well as a greater number of certain other components (e.g., switches, conductors, etc.), depending on the number of electrode elements 52 actually used in each of the transducer arrays 50.

[0045] The system of FIG. 4 can measure the temperature of the thermistors 54 in the left channel 50L by controlling the electronically controlled switches in bank 2L (which can be implemented using bidirectional analog switches) to select each of the thermistors in turn. For example, to select thermistor T1, switches C and 1 must be closed; to select thermistor T2, switches C and 2 must be closed; and so on. After any given one of thermistors T1-T4 in transducer array 50L is selected, temperature measurement block (TMB) 32 can determine the temperature of the thermistor by measuring the resistance of the thermistor. This can be accomplished, for example, by using a current source generating a known current (e.g., 150 μA) positioned within TMB 32 so that a known current is routed to whichever thermistor is selected by bank 2L of switches at any given time. The known current causes a voltage to appear across the selected thermistor (T1-T4), and by measuring this voltage, the temperature of the selected thermistor can be determined. Controller 34 runs a program that selects each of the thermistors T1-T4 in turn and measures the voltage appearing across each of the thermistors in turn (indicating the temperature at the selected thermistor). Examples of suitable hardware and procedures that can be used to obtain temperature readings from each of the thermistors are described in US 2018 / 0050200, which is incorporated herein by reference in its entirety.

[0046] Measuring the temperature of the thermistor 54 in the right channel 50R is accomplished using the same technique as described above in connection with the left channel 50L, except that bank 2R of switches is used instead of bank 2L. Corresponding banks of switches (not shown) are also provided for the other channels 50A, 50P, and a similar technique is also used in those channels.

[0047] Based on temperature readings obtained from thermistors 54 (T1-T4), controller 34 controls the switches in bank 1L to either turn on or off the current (coming from AC voltage generator 35) to each of the corresponding electrode elements 52 (E1-E4). For example, to keep the current on in all four of the electrode elements 52, all four of the switches in bank 1L must be closed. To interrupt the current reaching electrode element E1, switch 1 in bank 1L must be open, to interrupt the current reaching electrode element E2, switch 2 in bank 1L must be open, and so on.

[0048] If a few electrode elements begin to overheat, controlling the current routed through the individual electrode elements can be used to reduce or eliminate the reduction in the average current coupled into the body. This, in turn, can advantageously reduce or eliminate the reduction in the strength of the electric field at the tumor. This can be accomplished by programming the controller 34 to alternately switch the current on and off for each individual electrode element that begins to approach 41°, in order to reduce the average current for those electrode elements without affecting the current passing through the remaining electrode elements (which are not approaching 41°).

[0049] For example, assume a situation in which 500 mA of current is passing through a transducer array containing 10 electrode elements and only one of those electrode elements begins to approach 41°C. Further assume that a 10% reduction in current through the single electrode element is required to keep the temperature of that single electrode element below 41°C. Instead of achieving this 10% reduction in current by reducing the current through the entire transducer array from 500 mA to 450 mA (as in the prior art), the controller 34 can reduce the average current through the single electrode element by 10% by controlling the switches in bank 1L to switch the current through that single electrode element on and off at a 90% duty cycle while leaving the current on full time for all of the remaining electrode elements. Note that, in view of the thermal inertia of the electrode elements, the switching speed must be fast enough so that the instantaneous temperature of the single electrode element never exceeds 41°C. For example, a 90% duty cycle can be achieved by switching the current on for 90 ms and off for 10 ms.

[0050] When this approach is used, the current through the remaining nine electrode elements can remain unchanged (i.e., 50 mA per electrode element), and the current through only a single electrode element is reduced to an average of 45 mA. The average net total current through the transducer array is now 495 mA (i.e., 9 x 50 + 45), which means that significantly more current can be coupled into the body without exceeding 41° on any of the electrode elements.

[0051] The controller 34 can even be configured to increase the current through the remaining nine electrode elements to compensate for the decrease in current through a single electrode element. For example, the current through the remaining nine electrode elements can be increased to 50.5 mA per electrode element (e.g., by the controller 34 sending a request to the AC voltage generator 35 to increase the voltage by 1%). If this solution is implemented, the average net total current through the entire transducer array is (9 electrodes × 50.5 mA + 1 electrode × 50.5 mA × 0.9 duty cycle) = 499.95 mA, which is very close to the original 500 mA current.

[0052] If at some later time (or even the same time) the temperature of the second electrode element begins to approach 41°, a similar technique (i.e., reducing the duty cycle from 100% to some point below 100%) can be used to prevent the temperature of the second electrode element from exceeding 41°.

[0053] In some embodiments, this technique can be used to individually customize the duty cycle at each of the electrode elements to maximize the current flowing through each of those electrode elements while keeping the temperature of each of those elements below 41°. Optionally, instead of taking remedial action to reduce the duty cycle only when the temperature of a given electrode element begins to approach 41°, the controller 34 can be configured to individually preset the duty cycle at each of the electrode elements in a given transducer array to equalize the temperature across all of the electrode elements in the array. For example, the controller 34 can be configured to individually set the duty cycle at each electrode element to maintain a temperature at each of the electrode elements that does not stray from around 40.5°. Optionally, the controller 34 can be configured to send requests to the AC voltage generator 35 to increase or decrease the voltage as necessary to achieve this result.

[0054] Using this approach, it is possible to ensure that every electrode element delivers the maximum possible average current (without exceeding 41°), thereby achieving an increase in field strength in the tumor and correspondingly improved treatment.

[0055] In some embodiments, the controller 34 can be programmed to keep the temperatures of all of the electrode elements below a safety threshold (e.g., below 41°C) as follows: Start by closing all of switches 1-4 in bank 1L so that the current is continuously on (i.e., 100% duty cycle). Then, based on the signal arriving through the TMB 32, the controller 34 determines whether the temperature of each of the electrode elements exceeds an upper threshold (e.g., 40°C) below the safety threshold. If the controller 34 detects this condition, the controller 34 reduces the duty cycle for the corresponding switch in bank 1L by toggling the corresponding digital output at the desired duty cycle. This will interrupt the current to the corresponding electrode element 52 at the same duty cycle, thereby reducing the average current in the particular electrode element 52 whose temperature exceeds its upper threshold. The level of current reduction is determined by the duty cycle. For example, using a 50% duty cycle will reduce the current by half, while using a 75% duty cycle will reduce the current by 25%.

[0056] In particular, this procedure interrupts current only to certain ones of the electrode elements 52 on the transducer array 50, but does not interrupt current to the remaining electrode elements 52 on that transducer array 50. This provides a very significant advantage over the prior art because it eliminates or reduces the need to reduce the current routed through electrode elements when only a few of those electrode elements heat up.

[0057] A numerical example will be useful to illustrate this point. In the embodiment of FIG. 4 , assume that the left and right transducer arrays 50L and 50R are positioned on the left and right sides of the subject's head, respectively, that all of the switches in banks 1L and 1R are on with a 100% duty cycle, and that the AC voltage generator 35 initially outputs 500 mA of current. An AC voltage appears between the electrode elements 52 of the left transducer array 50L and the electrode elements 52 of the right transducer array 50R, causing 500 mA of AC current to be capacitively coupled through the electrode elements 52 and through the subject's head. The controller 34 monitors the temperature of each of the electrode elements 52 in each of the transducer arrays 50L and 50R by receiving signals from each of the temperature sensors 54 via the temperature measurement block 32. Now assume that the temperature of a given one of the electrode elements 52 in the left transducer array 50L rises to 40° C. This condition will be reported to the controller 34 via a signal from the corresponding temperature sensor 54. When the controller 34 recognizes that the temperature of a given electrode element 52 has risen to 40° C., the controller 34 will toggle the control signal going to the corresponding switch in bank 1L at a desired duty cycle in order to periodically interrupt the current to the given electrode element 52 to maintain a lower average current.

[0058] This is in stark contrast to prior art devices, which require current flow through all of the electrode elements to be reduced as soon as the temperature of even just one of the electrode elements 52 approaches 41°C.

[0059] Note that if the duty cycle of just one of the remaining electrode elements 52 is reduced, it is possible to maintain the original 500 mA current (and enjoy the benefits that come from using all of the current). However, if the duty cycle of a sufficient number of electrode elements 52 is reduced, the original 500 mA current may have to be reduced. To accomplish this, the controller 34 can send a command to the AC voltage generator 35. When the AC voltage generator 35 receives this request, it will reduce its output voltage, thereby reducing the current.

[0060] Optionally, the duty cycle selected by the controller 34 may be controlled based on the rate at which a given electrode element 52 increases in temperature (as measured via the temperature sensor 54 and the TMB 32) after current is applied to the given electrode element 52. More specifically, if the controller 34 determines that a given electrode element 52 increases in temperature twice as fast as expected, the controller 34 may select a 50% duty cycle for that electrode element. Similarly, if the controller 34 determines that a given electrode element 52 increases in temperature 10% faster than expected, the controller 34 may select a 90% duty cycle for that electrode element.

[0061] In other embodiments, instead of deterministically reducing the average current by reducing the duty cycle, the controller 34 can reduce the average current at a given electrode element 52 based on real-time temperature measurements by using a switch in bank 1L to turn off the current to the given electrode element 52 as described above and waiting until the temperature measured using the temperature sensor 54 falls below a second temperature threshold (e.g., below 38°C). Once the temperature falls below this second temperature threshold, the controller 34 can restore the current to the given electrode element 52. This can be accomplished, for example, by controlling the state of a switch in bank 1L that was previously turned off so that the switch returns to an on state, thereby allowing current to flow between the electrical conductor and the respective electrode element 52. In these embodiments, the current to a given electrode element 52 can be repeatedly switched off and on based on real-time temperature measurements to keep the temperature of the given electrode element 52 below a safe threshold.

[0062] Individually switching current to each of the electrode elements 52 in the right channel 50R is achieved using the same technique as described above in connection with the left channel 50L, except that bank 1R of switches is used instead of bank 1L. Corresponding banks of switches (not shown) are also provided for the other channels 50A, 50P, and a similar technique is also used in those channels.

[0063] Note that the examples described above (referring to duty cycles of 100%, 90%, 75%, 50%, etc.) refer to the duty cycle within the window of time during which a given channel (e.g., left / right channel) is activated. In some preferred embodiments, the AC signal generator 35 (a) sends an AC current through the anterior / posterior arrays 50A / 50P for a first time period (e.g., 1 second), which induces an electric field in a first direction through the tumor in the subject's body, and then (b) sends an AC current through the left / right arrays 50L / 50R for a second time period (e.g., 1 second), which induces an electric field in a second direction through the tumor, and then repeats steps (a) and (b) for the duration of the treatment. In these embodiments, the overall duty cycle for any given channel (i.e., either the A / P or L / R channels) will be half the value in the examples described above. This is because operating at 100% duty cycle for a 1 s time window and then holding off for the next 1 s time window results in an overall duty cycle of 50%. Similarly, operating at 90% duty cycle for a 1 s time window and then holding off for the next 1 s time window results in an overall duty cycle of 45%.

[0064] Optionally, an additional bank of switches 3L may be provided, each of the switches in this bank wired in parallel with a corresponding one of the thermistors T1-T4, so that when a given one of switches 1-4 is closed, a respective one of the thermistors T1-T4 is shorted out.

[0065] The reason for including the additional bank of switches 3L is that (a) current from the AC voltage generator 35 flows through the electrode elements 52 of the left channel 50L, the subject's body, and the electrode elements 52 of the right channel 50L, and (b) when power to any of the electrode elements 52 of the left channel 50L is switched off by a corresponding one of the switches in bank 1L, current can leak through the thermistor 54 in the left channel 50L. For example, assume that only switch #2 in bank 1L is switched off (i.e., open). Because switches #1, #3, and #4 are switched on (i.e., closed), the AC voltage generator 35 will impose a voltage on electrode elements E1, E3, and E4. Thermistors T1 and T2 provide a path for current to flow from E1 to E2, thermistors T3 and T2 provide a path for current to flow from E3 to E2, and thermistors T4 and T2 provide a path for current to flow from E4 to E2. This is equivalent to a parallel combination of E1, E3, and E4 wired in series with E2. Because the number of thermistors in this parallel combination increases linearly with the number of electrode elements 52, the current in a single thermistor E2 (wired in series with the parallel combination) can become significant. The inclusion of an optional additional bank 3L of switches provides the system with the ability to prevent power loss in that single thermistor E2 by closing the corresponding switch #2 in bank 3L.

[0066] To accomplish this (in those embodiments including an additional bank 3L of switches), the controller 34 can be programmed so that whenever a given one of the switches in bank 1L is open, the corresponding switch in bank 3L is closed, which will prevent the thermistors 54 associated with the turned-off electrode elements 52 from dissipating too much power, as described in the previous paragraph.

[0067] In those embodiments including an additional bank of switches 3L, individually bypassing each of the thermistors 54 in the right channel 50R is accomplished using the same technique as described above for the left channel 50L, except that bank of switches 3R is used instead of bank 3L. Corresponding banks of switches (not shown) are also provided for the other channels 50A, 50P, and a similar technique is also used in those channels.

[0068] 5 depicts a second embodiment of a transducer array 150 that provides individual conductors for each individual electrode element 152 without a dramatic increase in the number of wires in each cable extending to a given transducer array and without requiring active elements to be positioned on or near the transducer array. As described below in connection with FIG. 6, preferably, four copies of the transducer array 150 are used to administer TT Fields therapy to a person's head (or other body part).

[0069] Each transducer array 150 includes a plurality of electrode elements 152, labeled E1 through E9 for ease of reference. The electrode elements 152 are similar to the electrode elements 52 described above with respect to the embodiment of FIG. 3. In this embodiment of FIG. 5, individual conductors extend from each of the electrode elements 152 to a connector 157. These conductors are numbered 1 through 9 immediately above a "wire routing" block 155 (which converges the individual conductors together into a single cable 156). In some preferred embodiments, the electrical connection to each of the electrode elements 152 includes one or more traces on a flex circuit and / or one or more conductive wires.

[0070] In some preferred embodiments, all of the capacitively coupled electrode elements 152 are held in place by a support structure 159, similar to support structure 59 in the embodiment of FIG.

[0071] Each transducer array 150 also includes a plurality of thermistors 154, one thermistor positioned at each of the electrode elements 152, so that the thermistors 154 can sense the temperature of the respective electrode element 152. This can be accomplished as described above in the embodiment of Figure 3. Each of the thermistors 154 has a first terminal (i.e., the bottom terminal of the thermistor in Figure 5) and a second terminal (i.e., the top terminal of the thermistor in Figure 5).

[0072] Each transducer array 150 also has a connector 157 that is used to transmit electrical signals into and out of the transducer array 150. The connector 157 has a plurality of first pins and second pins. In the illustrated embodiment, the number of first pins is the same as the number of electrode elements 152, and each of the first pins corresponds to a respective one of the electrode elements 152. Also, in the illustrated embodiment, there is only a single second pin, labeled N. It should be noted that, as used herein, the term "pin" can refer to either a male or female pin of the connector 157.

[0073] Each transducer array 150 also has a plurality of first conductors, the number of which will depend on the number of electrode elements 152. In the embodiment depicted in FIG. 5 , which includes nine electrode elements 152, these conductors are labeled 1 through 9. Each of these first conductors provides a conductive path between (a) a respective one of the first pins in the connector 157, (b) the conductive substrate of a respective one (E1 through E9) of the electrode elements 152, and (c) a first terminal of a corresponding thermistor 154. As in the embodiment of FIG. 3 , each of these first conductors may optionally be implemented using multiple segments of wire and / or multiple traces on a flex circuit.

[0074] The multiple thermistors 154 are arranged in series, starting with the first one of the thermistors (i.e., the top left in FIG. 5) and ending with the last one of the thermistors (i.e., the bottom right in FIG. 5), with the second terminal of each of the thermistors except the last one wired to the first terminal of the respective succeeding thermistor.

[0075] Each transducer array 150 has a second conductor that provides a conductive path between the second pin of the connector 157 and the second terminal of the last thermistor 154 (i.e., the top terminal of the bottom right thermistor in FIG. 5 ). The second conductor may optionally be implemented using multiple segments of wire and / or multiple traces on a flex circuit.

[0076] Connector 157 has a respective first pin corresponding to each of the respective electrode elements 152, and because a conductive path exists between each of the first pins and a respective one of the electrode elements 152, a system mated with connector 157 can individually and selectively energize or de-energize each of the electrode elements 152 by either applying or not applying a signal to the respective first pin on connector 157. Also, because the two terminals of any given one of the thermistors 154 are wired to different pins on connector 157, a system mated with connector 157 can access both terminals of each of the thermistors 154. As a result, a system mated with connector 157 can measure the resistance value of either of the thermistors 154.

[0077] In particular, each of the first pins on the connectors 157 serves two functions, since any given first pin on the connectors 157 corresponds to one or two of the individual electrode elements 152, and also corresponds to one of the individual thermistors 154. This reduces the number of wires that must be included in each of the cables 156, which in turn advantageously makes the cables more flexible and less cumbersome.

[0078] FIG. 6 is a block diagram of a system that uses four copies of the transducer array 150 (described above in connection with FIG. 5) to apply TT fields to a subject. In FIG. 6, these four copies are labeled 150A, 150P, 150L, ​​and 150R, where A, P, L, and R stand for anterior, posterior, left, and right, respectively. The bottom of FIG. 6 depicts the AC voltage generator 35 and the “CAD box” 130 as separate blocks, the latter of which includes a temperature measurement block 132, a controller 134, and banks of switches 1L, 2L, 3L, 1R, 2R, and 3R. In some embodiments, the components in the two blocks 35, 130 can be physically separated into two separate housings. However, in alternative embodiments, the components in the two blocks 35, 130 are combined into a single housing.

[0079] For ease of viewing, only the left and right channels are depicted in Figure 6. However, the remaining channels (i.e., the front and rear channels) operate in the same manner as the left and right channels, respectively. In addition, each of the transducer arrays 150 in Figure 6 is depicted with only four electrode elements 152 and four thermistors 154 for ease of viewing. However, an actual system would have a greater number of electrode elements and thermistors (e.g., between 9 and 30), as well as a greater number of certain other components (e.g., switches, conductors, etc.), depending on the number of electrode elements 152 actually used in each of the transducer arrays 150.

[0080] The system of Figure 6 can measure the temperature of thermistors 154 in left channel 150L and select each of the thermistors in turn by controlling electronically controlled switches in banks 2L and 3L (which can be implemented using bidirectional analog switches). For example, to select thermistor T1, switches 1 and 2 must be closed; to select thermistor T2, switches 2 and 3 must be closed; to select thermistor T3, switches 3 and 4 must be closed; and to select the last thermistor (i.e., T4 in Figure 6), switches 4 and N must be closed. After any given one of thermistors T1-T4 in transducer array 150L is selected, temperature measurement block 132 can determine the temperature of that thermistor by measuring the resistance of the thermistor, as described above with respect to Figure 4.

[0081] Measuring the temperature of the thermistor 154 in the right channel 150R is accomplished using the same technique as described above in connection with the left channel 150L, ​​except that banks of switches 2R and 3R are used instead of banks 2L and 3L. Corresponding banks of switches (not shown) are also provided for the other channels 150A, 150P, and a similar technique is also used in those channels.

[0082] 4, based on temperature readings obtained from thermistors 154 (T1-T4), controller 134 controls the switches in banks 1L and 1R (and corresponding switches in the preceding and succeeding channels, not shown) to either turn on or off current (coming from AC voltage generator 35) to each of the corresponding electrode elements 152 (E1-E4). For example, to keep current on in all four of the electrode elements 152, all four of the switches in bank 1L must be closed. To interrupt current reaching electrode element E1 in channel 150L, ​​switch 1 in bank 1L must be open; to interrupt current reaching electrode element E2, switch 2 in bank 1L must be open; and so on.

[0083] If a small number of electrode elements begin to overheat as described above with respect to FIG. 4, steps can be used to control the current routed through individual electrode elements (e.g., by reducing the duty cycle to particular electrode elements) as described above to reduce or eliminate the reduction in average current coupled to the body.

[0084] FIG. 7 is a schematic diagram of a circuit suitable for implementing each of the switches in banks 1L and 1R in the embodiments of FIGS. 4 and 6 described above, as well as the corresponding banks (not shown) for the front and rear channels. The circuit includes two field-effect transistors 66, 67 wired in series, configured to allow current to pass in either direction. An example of a suitable FET for this circuit is the BSC320N20NSE. (Note that the diodes illustrated in FIG. 7 are inherently contained within the FETs 66, 67 themselves.) The series combination of the two FETs 66, 67 will either conduct or block power flow, depending on the state of a control input arriving from one of the digital outputs of the controller 34 described above. When the series combination is conductive, current can flow between the common conductor and the respective electrode element 52. On the other hand, when the series combination of FETs 66, 67 is not conductive, current does not flow between the common conductor and the respective electrode element 52.

[0085] 3 and 5, all of the electrode elements 52 are capacitively coupled, and the support structure 59 is configured to hold the electrode elements 52 against the subject's body so that the dielectric layers of the electrode elements 52 face and are positioned in contact with the subject's body. However, in alternative embodiments, electrode elements that are not capacitively coupled may be used. In this case, the dielectric layer of each electrode element is omitted, and the support structure 59 holds the electrode elements 52 against the subject's body so that the conductive surfaces of the electrode elements 52 face and are positioned in contact with the subject's body. Optionally, in this embodiment, a layer of hydrogel may be disposed between the conductive surfaces of the electrode elements 52 and the subject's body when the transducer array 50 is placed against the subject's body.

[0086] While the present invention has been disclosed with reference to certain embodiments, many modifications, substitutions, and variations to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but rather have the full scope defined by the language of the following claims, and equivalents thereof. [Explanation of symbols]

[0087] 20 AC signal generator 21 Transducer Array 22 Transducer Array 23 Transducer Array 24 transducer array 28 wires 30 CAD box 32 Temperature measurement block, TMB 34 Controller 35 AC voltage generator 50 Transducer Array 50A Transducer Array 50P transducer array 50L transducer array 50R Transducer Array 52 electrode elements 54 Thermistor, temperature sensor 55 Wire Routing Blocks 56 Cable 57 Connector 59 Support structure, flexible backing 66 Field-effect transistor, FET 67 Field-effect transistor, FET 130 CAD box 132 Temperature measurement block 134 Controller 150 Transducer Array 150A Transducer Array 150P Transducer Array 150L transducer array 150R Transducer Array 152 electrode elements 154 Thermistor 155 Wire Routing Blocks 156 Cable 157 Connector 159 Support structures

Claims

1. 1. An apparatus for applying an alternating electric field to the body of a subject, comprising: a plurality of electrode elements; a support configured to hold the electrode element against the subject's body; a plurality of thermistors, each thermistor having a first terminal and a second terminal, each thermistor positioned to sense temperature at a corresponding respective one of the electrode elements; a connector having a plurality of first pins and a plurality of second pins; a plurality of first conductors, each of which provides a conductive path between (a) a respective one of the first pins, (b) a respective one of the electrode elements, and (c) the first terminal of the corresponding thermistor; a second conductor providing a conductive path between the second pin and the second terminal of at least one of the thermistors; An apparatus comprising:

2. 2. The apparatus of claim 1, wherein the second terminals of all of the thermistors are wired together.

3. 3. The apparatus of claim 2, wherein the second terminals of all of the thermistors are wired together using at least one of: (a) at least one trace on a flex circuit; and (b) at least one wire.

4. 2. The apparatus of claim 1, wherein the plurality of thermistors are arranged in series, beginning with a first one of the thermistors and ending with a last one of the thermistors, the second terminal of each of the thermistors except the last one being wired to the first terminal of a respective subsequent thermistor, and the second conductor providing a conductive path between the second pin of the connector and the second terminal of the last thermistor.

5. 2. The apparatus of claim 1, wherein the second conductor provides a conductive path between the second pin and the second terminal of only one of the thermistors.

6. The apparatus of claim 1 , wherein the plurality of electrode elements comprises at least four electrode elements and the plurality of thermistors comprises at least four thermistors.

7. The apparatus of claim 1 , wherein the plurality of electrode elements comprises at least nine electrode elements and the plurality of thermistors comprises at least nine thermistors.

8. 2. The device of claim 1, wherein each of the electrode elements comprises a conductive plate and a dielectric layer disposed on the conductive plate, and the support is configured to hold the electrode elements against the subject's body such that the dielectric layer of each of the electrode elements faces the subject's body.

9. 2. The apparatus of claim 1, wherein the second terminals of all of the thermistors are wired together, and the second conductor provides a conductive path between the second pin and the second terminals of all of the thermistors.

10. 1. An apparatus for applying an alternating current electric field to a body of a subject using a plurality of electrode elements, each of the plurality of electrode elements being disposed in thermal contact with a respective thermistor; an AC signal generator for generating an AC output signal; a connector including a plurality of pins, each of the pins corresponding to a respective one of the plurality of electrode elements; a first plurality of switches, each of the first plurality of switches configured to selectively either apply or not apply the AC output signal to a respective one of the pins; an amplifier configured to receive an electrical signal from each of the thermistors and generate a corresponding output, the electrical signals from the thermistors arriving via the same pins corresponding to the plurality of electrode elements; a controller configured to control the first plurality of switches to individually adjust a duty cycle of the AC signal applied to each of the plurality of pins based on the output of the amplifier; and An apparatus comprising:

11. 11. The apparatus of claim 10, wherein the controller is configured to: (a) determine, based on the output of the amplifier, when at least one of the electrode elements is hotter than other electrode elements; and (b) control the first plurality of switches to reduce the duty cycle of the AC signal applied to at least one respective pin.

12. 11. The apparatus of claim 10, wherein the controller is configured to: (a) determine, based on the output of the amplifier, when at least one of the electrode elements is hotter than a threshold level; and (b) control the first plurality of switches to reduce the duty cycle of the AC signal applied to at least one respective pin.

13. The apparatus of claim 10 , wherein the plurality of electrode elements comprises at least four electrode elements, the plurality of pins comprises at least four pins, and the first plurality of switches comprises at least four switches.

14. The apparatus of claim 10 , wherein the plurality of electrode elements comprises at least nine electrode elements, the plurality of pins comprises at least nine pins, and the first plurality of switches comprises at least nine switches.

15. a second plurality of switches, each configured to route a signal from a respective one of the plurality of pins to a first input of the amplifier; 11. The apparatus of claim 10, wherein the controller is further configured to control the second plurality of switches to sequentially select each of the plurality of pins to sequentially obtain a temperature reading from each of the thermistors.

16. the connector includes an additional pin; 16. The apparatus of claim 15, further comprising an additional switch arranged to route a signal from the additional pin to a second input of the amplifier.

17. a third plurality of switches each arranged to route a signal from a respective one of said plurality of pins to said additional pin; 17. The apparatus of claim 16, wherein the controller is further configured such that when a given switch from the first plurality of switches is open, the controller closes a respective corresponding switch from the third plurality of switches.

18. a second plurality of switches, each configured to route a signal from a respective one of the plurality of pins to a first input of the amplifier; a third plurality of switches, each configured to route a signal from a respective one of the plurality of pins to a second input of the amplifier; and Furthermore, 11. The apparatus of claim 10, wherein the controller is further configured to control the second plurality of switches and the third plurality of switches to sequentially route signals from pairs of the plurality of pins corresponding to both terminals of each of the thermistors in order to the first and second inputs of the amplifier to sequentially obtain temperature readings from each of the thermistors.

19. 1. A method of applying an alternating current electric field to a body of a subject using a plurality of electrode elements, each of the plurality of electrode elements being disposed in thermal contact with a respective thermistor; applying an AC signal to each of the electrode elements at a respective duty cycle such that an AC electric field is induced in the subject; inputting a signal from each of said thermistors; determining a temperature of each of the electrode elements based on the input signal; adjusting the duty cycle of the AC signal applied to the electrode elements based on the determined temperature; Including, A method wherein after the adjusting step, the duty cycle of the AC signal applied to at least one of the electrode elements is different from the duty cycle of the AC signal applied to another one of the electrode elements.

20. 20. The method of claim 19, wherein the adjusting step comprises reducing the duty cycle of the AC signal applied to at least one of the electrode elements that is hotter than at least one other electrode element.

21. 20. The method of claim 19, wherein the adjusting step comprises reducing the duty cycle of the AC signal applied to the hottest of any of the electrode elements.

22. 20. The method of claim 19, wherein the adjusting step comprises reducing the duty cycle of the AC signal applied to any of the electrode elements whose temperature exceeds a threshold level.

23. 20. The method of claim 19, wherein the adjusting step comprises reducing the duty cycle of the AC signal applied to any of the electrode elements whose heating rate exceeds a threshold level.

24. 20. The method of claim 19, wherein the adjusting step comprises the steps of: switching off the AC signal applied to any of the electrode elements whose temperature exceeds a first threshold level; and switching on the AC signal applied to at least one of the electrode elements whose temperature is below a second threshold level, the second threshold level being lower than the first threshold level.

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