Abstract:
A sensor system for dialysis applications includes a plurality of pressure sensors, wherein each pressure sensor can be provided as an LC type sensor, and/or an RLC type sensor. Each sensor among the plurality of pressure sensors can be inductively coupled with a respective antenna among a plurality of antennas for the wireless transmission of pressure data. A dialysis machine is generally connected to the plurality of antennas, wherein the plurality of pressure sensors monitors pressure during operation of the dialysis machine to generate pressure data that is wirelessly transmitted to at least one antenna among the plurality of antennas.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This patent application is a Continuation-In-Part (CIP) of U.S. patent application Ser. No. 11/242,271, entitled “Wireless Pressure Sensor and Method Forming the Same,” which was filed on Oct. 3, 2005, and is incorporated herein by reference in its entirety. This patent application is also a Continuation-In-Part (CIP) of U.S. patent application Ser. No. 11/226,085, entitled “Wireless Capacitance Pressure Sensor,” which was filed on Sep. 13, 2005, and is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     Embodiments are generally related to sensing devices and methods. Embodiments are also related to wireless sensors. Embodiments are additionally related to dialysis applications and pressure sensors for use in monitoring pressure during a dialysis application.  
       BACKGROUND OF THE INVENTION  
       [0003]     Sensors are utilized in a number of applications, including various medical, commercial and industrial applications. For example, it is often necessary to monitor pressure and/or to detect flow rates in medical applications and processes.  
         [0004]     One area where pressure sensors, for example, find particular usefulness is in the area of hemodialysis applications. In such medical procedures, a dialysis machine is utilized to clean wastes from the blood after the kidneys have failed. The blood travels through tubes to a dialyzer, a machine that removes wastes and extra fluid. The cleaned blood then goes back into the body.  
         [0005]     A known-type dialysis machine comprises a first blood circulation circuit and a second circulation circuit for the dialysate liquid. The first circuit and the second circuit are connected to a filter for conveying, respectively, the blood and dialysate liquid through the filter, which is provided with a semi-permeable membrane separating the blood from the dialysate liquid. The first circuit is provided with a container, known as a drip chamber, into which the blood is supplied from a first tract of the first circuit, and drips and collects on the bottom of the container, thence to enter a second tract of the first circuit.  
         [0006]     The container has the function of preventing air from becoming trapped in the blood in the form of bubbles, which might cause embolisms once the treated blood is returned to the cardiovascular system of the patient. To guarantee the safest possible treatment the blood level in the container must be maintained within an optimum range of values, below which the possibility of creating air bubbles in the blood returning to the patient exists, and above which the pressure increases to unacceptable values which are dangerous for the patient. Thus, the ability to monitor pressure in such a setting is critical to a proper, safe, and successful dialysis treatment.  
         [0007]     One type of dialysis application is disclosed in U.S. Pat. No. 6,695,806, entitled “Artificial Kidney Set with Electronic Key,” which issued to Gefland et al on Feb. 24, 2004 and is incorporated herein by reference. Another type of dialysis application is disclosed in U.S. Pat. No. 6,887,214, entitled “Blood Pump Having a Disposable Blood Passage Cartridge with Integrated Pressure Sensors,” which issued to Levin et al on May 3, 2005 and is incorporated herein by reference. It can be appreciated that U.S. Pat. Nos. 6,695,806 and 6,887,214 are referenced herein for general background and edification purposes only and are not considered limiting features of the embodiments described herein.  
         [0008]     Dialysis machines historically have utilized sets of disposable components that are assembled from various parts produced by different manufacturers. This allowed flexibility, but also resulted in certain disadvantages. Joints between component parts, for example, may leak, allow ingress of air and facilitate blood clotting. A high skill was required by hospital nurses and technicians to assemble the tubes, connectors, filters and accessories and then load them correctly into pumps, bubble detectors, pressures sensors and other elements of a dialysis machine. In the setting of a chronic dialysis center such practices were acceptable. In an acute setting, however, such an Intensive Care Unit (ICU) of a hospital, the complexities of dialysis machines can become an impediment.  
         [0009]     As a result, the use of mechanical fluid removal in the ICU, emergency rooms and general floors of a hospital has been limited. Some manufacturers have released sophisticated dialysis equipment based on the use of an integrated set of disposable dialysis components in which the tubing, filter and accessories are bonded together and no assembly is required. In such a device, the filter, sensor interfaces and four dedicated pump segments (for blood, dialysate, replacement solution and effluent) can be mounted on a flat plastic cartridge to simplify the loading of the dialysis pumps. Such a dialysis system has been marketed as offering an integrated system for continuous fluid management and automated renal replacement therapy blood.  
         [0010]     While such devices do offer significant advantages, such equipment also has a number of deficiencies. One deficiency is that although such systems provide for a set of disposable dialysis components that are continuous and bonded together, the system does not present a smooth blood path, but incorporates elements that create stagnant and slow moving blood zones. In such blood zones clots are likely to form. Such devices may also employ an interface to pressure sensors that is relatively inaccurate, unreliable and requires maintenance. There is thus a need for an improved design of the blood flow dialysis set that is simple to use, requires no maintenance or special training, and also has an improved performance over existing sets of disposable components utilized in such dialysis machines.  
         [0011]     Additionally, such dialysis machines do not integrate pressure sensors. Instead, these types of dialysis devices integrate pressure “pods” shaped as domes. The interface surface of a pod can be made from a silicon membrane approximately one inch in diameter. When mounted on such a dialysis machine, the pods interface with the permanently installed pressure sensors that form a part of the machine. The interface is sealed by a rubber gasket so that the pod membrane serves as a lid on the pressure transducer cavity. When in operation, blood and other fluids flow through the pods and come in contact with the membrane.  
         [0012]     Pressure pods provide a means to measure the pressure of blood and other fluids flowing outside an interface surface. When the pressure inside the pod is increased, the diaphragm stretches and thereby compresses the air inside a transducer cavity. As a result, pressure in the bloodline or a fluid line can be measured. The pod membrane serves as a barrier between the blood and potential contamination from the environment, as is similar to the clinical invasive vascular blood pressure measurements. This method, although functional, has several deficiencies.  
         [0013]     First, to be accurate such pods need to be positioned perfectly when the pressure inside is atmospheric. Over time, if there is even a miniscule leak on the transducer side of the membrane, the pod will creep and gradually stop transmitting pressure accurately because of the tension in the membrane. Second, stretchable membranes and air filled transducer cavities add compliance to the circuit. Compliance is a delay in a pressure measurement due to the time required to stretch the pods and compress the air inside the pod cavity. Compliance is not desired since it makes the system less responsive to controls.  
         [0014]     Third, pods filled with blood increase the blood-plastic contact surface and create stagnant zones with low blood flow velocity that facilitate clot formation. Because the clots may form in the pods, the use of pods also necessitates the use of clot capture devices. Fourth, pod domes have a significant volume that increases the time that blood spends in contact with foreign materials. Altogether this increases the risk of blood loss, hypotension and clotting.  
         [0015]     In order to address the needs of fluid removal and dialysis in acute emergency settings and to eliminate significant limitations of existing designs, it is believed that an improved sensor system should be adapted for use with dialysis machines. It is believed that the improved multiple sensor system disclosed herein can address these and other continuing needs.  
       BRIEF SUMMARY  
       [0016]     The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole.  
         [0017]     It is, therefore, one aspect of the present invention to provide for an improved sensor system.  
         [0018]     It another aspect of the present invention to provide for an improved pressure sensor system for use in dialysis applications.  
         [0019]     It is yet another aspect of the present invention to provide for a sensor system that avoids the need for both careful mechanical alignment and electrical connection between the sensor and dialysis machine. A further aspect of the present invention is to provide for a reduced sensor size that permits reduced contact volume and dead-space in a sensing application.  
         [0020]     The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A sensor system for dialysis applications is disclosed, which includes a plurality of passive resonant circuit pressure sensors inductively coupled to a plurality of antennas. Wherein each sensor among the plurality of pressure sensors is implemented as inductive-capacitive (LC) resonant circuit (tank) sensors associated with a respective antenna among the plurality of antennas for the wireless transmission of pressure data. A dialysis machine is generally connected to the plurality of pressure sensors and the plurality of antennas, wherein the plurality of pressure sensors monitors pressure during a dialysis operation of the dialysis machine to generate pressure data that is wirelessly transmitted from at least one antenna among the plurality of antennas.  
         [0021]     A plurality of oscillator circuits is also associated with the plurality of pressure sensors and the plurality of antennas. Additionally, a plurality of low frequency switches is associated with the plurality of oscillator circuits. An electronics processing module is also provided for processing the pressure data generated by one or more of the pressure sensors, while each oscillator circuit among the plurality of oscillator circuits can be implemented as a Grid Dip Oscillator (GDO). Each GDO can be configured to include an oscillator component that produces an AC output signal that is input to a level shifter, which in turn produces an output signal that has either the negative or positive signal peak clamped to a fixed reference level. This signal is then input to a low-pass filter, which in turn produces a DC output signal. The DC output signal from the filter is thus proportional to the peak-to-peak signal from the oscillator. In this way the use of RF switch is avoided for multiple sensor concepts.  
         [0022]     Each antenna can be provided as a planar coil surrounded by a shielding ring. The shielding ring can be configured in the form of metalized plastic with an electrical connection to ground within the dialysis machine. Each pressure sensor can be implemented as an LC tank sensor and can be located in at least one of the following positions within the dialysis machine: an arterial line, a dialyzer line, or a venous line, depending upon design considerations. Each sensor may operate within different resonant frequency bands from one another or within the same or overlapping frequency bands, depending on design goals and considerations.  
         [0023]     An alternative embodiment involves the use of wireless LC tank multiple sensors in the context of a sensor system in which the sensors share a single antenna. Multiple capacitors, each of which forms a variable C component in the LC tank sensor, can be linked with a single planar coil, such that each associated variable capacitor results in a pressure dependent signature frequency (i.e., spurs). Multiple frequencies can exist in such a system through prudent design. The amplitudes of the spurs can be maximized for ease of detection.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein.  
         [0025]      FIG. 1  illustrates a high-level view of the left side of a conventional kidney dialysis machine, which can be adapted for use in accordance with one or more embodiments;  
         [0026]      FIG. 2 ( a ) illustrates a system of reader antennas reader antenna, which can operate in overlapping frequency bands about the same ‘zero pressure differential’ resonant frequency, f 0 , in accordance with a first embodiment;  
         [0027]      FIG. 2 ( b ) illustrates a graph depicting how the system illustrated in  FIG. 2 ( a ), can operate at different resonant frequencies, in accordance with an alternative version of the first embodiment;  
         [0028]      FIG. 3  illustrates a block diagram depicting a sensor system, which can be implemented in accordance with a preferred embodiment;  
         [0029]      FIG. 4  illustrates a block diagram depicting components that can be utilized to implement an example oscillator circuit in accordance with an alternative, but first embodiment;  
         [0030]      FIG. 5  illustrates a block diagram of a multiple sensor system for use in dialysis applications, in accordance with an alternative first embodiment;  
         [0031]      FIG. 6  illustrates a block diagram of a multiple sensor system for use in dialysis applications, in accordance with another version of the embodiment depicted in  FIG. 5 ;  
         [0032]     .  FIG. 7  illustrates a graph depicting a variety of frequencies in the context of a sensor system for dialysis applications, in accordance with a second embodiment;  
         [0033]      FIG. 8  illustrates a sensor system based on a plurality of circular electrodes forming a plurality of variable capacitors, in accordance with the second embodiment; and  
         [0034]      FIG. 9  illustrates a schematic diagram of an example equivalent circuit of the configuration depicted in  FIG. 8 .  
     
    
     DETAILED DESCRIPTION  
       [0035]     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.  
         [0036]      FIG. 1  illustrates a high-level view of the left side of a conventional kidney dialysis machine  110 , which can be adapted for use in accordance with one or more embodiments. The dialysis machine  110  generally includes a machine housing  111  that contains a membrane apparatus (not shown) for performing dialysis. The illustrated dialysis machine  110  can also include a threaded shaft  112  extending from the back of the housing  111 . The shaft  112  can be located near the top of the housing and a knob  114  can be threaded on the shaft  112 .  
         [0037]     The housing is generally mounted on wheels  115  that support the housing on the floor of a patient station. It can be appreciated that the dialysis machine  110  depicted in  FIG. 1  represents one of many possible dialysis machines that can be utilized in accordance with the embodiments disclosed herein. As such, the dialysis machine  110  illustrated in  FIG. 1  is not considered a limiting feature of the disclosed embodiments. Rather, dialysis machine  110  is presented for general edification and exemplary purposes only. It can be appreciated that the embodiments disclosed herein can be practiced not only in the context of dialysis applications, but also in the context of non-dialysis applications, such as, for example, external blood treatment applications.  
         [0038]      FIG. 2 ( a ) illustrates a system  200  of antenna  201 ,  203 ,  205 , which can operate at the same zero pressure resonant frequency, f 0 , in accordance with a first possible embodiment. System  200  depicted in  FIG. 2 ( a ) can be adapted for use with the dialysis machine  110  depicted in  FIG. 1 . Sensors (not shown in  FIG. 2 ( a ), but illustrated in  FIGS. 3, 5  and  6 ) can be implemented as LC tank sensors in association with system  200  with either L varying with pressure or C or both L and C varying with pressure depending upon design considerations. Such sensors are illustrated in greater detail herein with respect to  FIGS. 3-6 .  
         [0039]     In the configuration of system  200 , shielding rings  202 ,  204 ,  206  respectively surround and screen one side of one or more of the coil antennas  210 ,  208 , and  212 . When utilized in the context of a system that includes a Radio Frequency (RF) switch  228 , the set of three antennas  201 ,  203 ,  205  can make use of the same frequency range for sensing applications. A graph  220  depicted in  FIG. 2 ( a ) illustrates a representative x-ordinate frequency range. In graph  220 , a central zero pressure frequency f 0  is shown with pressure values  222 ,  224 , and  226  (i.e., P 1 , P 2 , P 3 ) varying the resonant frequency associated with three different sensors. Thus,  FIG. 2  illustrates detection of resonant frequencies about a common zero-pressure frequency using multiple interrogation coil antenna  210 ,  208 ,  212 .  
         [0040]      FIG. 2 ( b ) illustrates a graph  231  depicting how a system of sensors (e.g., see sensors  402 ,  404 ,  406  in  FIGS. 5-6  and sensors  330 ,  344  in  FIG. 3 ) can operate in different resonant frequency bands, in accordance with an alternative version of the first embodiment. Note that in FIGS.  2 ( a )- 2 ( b ) identical or similar elements or components are generally indicated by identical reference numerals. Thus, zero pressure frequencies, f 0 , f 02 , and f 03  are shown along the x-ordinate frequency range  230  in  FIG. 2 ( b ).  
         [0041]      FIG. 2 ( b ) indicates that multiple LC tank sensors can operate in different resonant frequency bands to avoid interference between different sensor and antenna signals. Single or multiple interrogation electronics, such as, for example, Grid Dip Oscillator (GDO) circuits, can be utilized depending on the available dynamic range of the GDO circuit. In other words, GDO circuits can be utilized if the frequency range over which the oscillator circuit operates can sustain oscillations. In either case, single or multiple antenna configurations can be implemented, depending on the strength of the inductive coupling of each of the sensors to the antenna(s). The embodiment depicted in graph  220  of  FIG. 2 ( a ) uses sensors operating in the same frequency resonant frequency band and multiple interrogation antenna  201 ,  203 ,  205  separated from one another and preferably with the respective shielding  202 ,  204 ,  206  around each coil  210 ,  208 ,  212 .  
         [0042]     Note that the RF signal from the antenna coils  210 ,  208 ,  212  can be focused and/or limited to respective sensors directly facing the antenna, while signals radiated to other sensors not directly facing the antenna coil can be completely shielded or significantly reduced. Where necessary RF switches such as RF switch  228  depicted in  FIG. 1  can be used to switch the supply and/or output signals from utilized GDO circuits.  
         [0043]     Assuming that it is desired to implement a system in which the sensors operate within different frequency bands (e.g., see graph  231  of  FIG. 2 ( b )); one or more GDO circuits should be utilized, which operate over the widest range frequencies. Such a configuration can be implemented with a single antenna coil and a GDO with a wide dynamic range. In order to obtain the detection sensitivity required, however, the change in resonant frequency of a single sensor (i.e., with pressure) may already reach the limits of the GDO&#39;s dynamic range. Thus, multiple antennas with multiple GDO circuits may be required. Alternatively, where sensors operate within the same frequency band, multiple antennas may also be required to physically differentiate between the sensors (e.g., see  FIG. 2 ( a )).  
         [0044]     Each antenna coil  210 ,  208 ,  212  can take the form of a planar coil based, for example, on a Printed Circuit Board (PCB) or polymer substrate, or the form of a multi-layer PCB coil, wound Litz wire, wound copper wire, or other similar structure. Shielding can be implemented in, for example, metalized plastic or sheet metal, with electrical connections to ground in the dialysis equipment of, for example, the dialysis machine  110  depicted in  FIG. 1 . Preferably, this shielding would be implemented in the same process as the coil manufacture itself. Alternatively, a material with high permeability could be attached, deposited and/or located nearby, such as, for example, mu-metal, in order to stop or reduce the field due to limited skin depth at measurement frequencies. The diameter and height of the shielding rings  202 ,  204 , and/or  206  can be determined by the relative distance and the angle between sensors and their respective antenna coils  210 ,  208 ,  212 .  
         [0045]     In the embodiments described above wherein the GDO dynamic range is large enough, multiple sensors can be connected to one GDO with three different antennas. In such a scenario, an RF switch such as, for example, RF switch  228  depicted in  FIG. 2 ( a ), can be utilized to switch between the interrogation antennas  201 ,  203 ,  205 . In embodiments where the dynamic range of the GDO is limited, multiple GDO circuits can be linked to a single antenna using an RF switch. Alternatively, wherein both multiple GDO circuits and antenna are required to provide both the required operating frequency range and coupling between sensors that are spatially separated along with their respective antenna, two different arrangements can be implemented, as indicated herein with respect to  FIGS. 5 and 6 , which are described in greater detail below.  
         [0046]      FIG. 3  illustrates a block diagram depicting a sensor system  300 , which can be implemented in accordance with a preferred embodiment and in association with the antenna embodiments depicted in FIGS.  1 - 2 ( a )/( b ) and  FIGS. 4-6 . System  300  generally incorporates the use of multiple wireless LC tank pressure sensors for use with a hemodialysis machine such as, for example, the dialysis machine  110  depicted in  FIG. 1 . System  300  includes a disposable cartridge  335  which can support one or more pressure sensors  330 ,  344 . The pressure sensor  330  includes a variable capacitance sensing element  332  and a sensor coil  346 . Similarly, the pressure sensor  344  includes a sensing element  342  and a sensor coil  336 . Inductive coupling (electromagnetic field show schematically as  328  and  326 ) are also provided, wherein the primary inductive coupling is between the sensor coils  346  and  336  and reader coils  322  and  324  respectively.  
         [0047]     The dialysis machine  110  can also include reader coils  322  and  324 , which are located respectively proximate to the sensor coils  346  and  336 . Importantly the relative position of the sensor and reader coils need not be precisely maintained in order to achieve wireless transfer of pressure data, thus allowing ease of placement and attachment of the disposable cartridge by the hospital nurses and technicians. The dialysis machine  110  can also incorporate various measurement and control electronics  315  which communicate with reader electronics  314  that include a GDO  318 , a GDO  320  and a microcontroller  316 . Note that each GDO  318 ,  320  are respectively similar to the GDO  400  illustrated in  FIG. 4 . The system  300  is illustrated as a two sensor configuration. It can be appreciated, however, that system  300  can be modified to operate with additional sensors, GDO circuits, and so forth.  
         [0048]      FIG. 4  illustrates a block diagram depicting components that can be utilized to implement an example oscillator circuit  302  in accordance with an alternative, but first embodiment. Note that in  FIGS. 4, 5  and  6 , the illustrated configurations generally depict a DC/low frequency switch arrangement.  FIGS. 5-6  generally relate to a three sensor configuration. It can be appreciated, however, that the system  300  depicted in  FIG. 3  can be modified to operate in the context of a three sensor configuration, such as that depicted in  FIGS. 5-6 , rather than the two sensor configuration of  FIG. 3 . The oscillator circuit or GDO  302  is generally composed of an oscillator  306 , which in turn generates AC signal that is sent to a level shifter  308 . The level shifter  308  ensures the signal from oscillator is available to the low-pass filter  310  without the influence of the DC bias voltage of the oscillator circuit  302  and also that the output signal has either the negative or positive signal peak clamped to a reference level. The signal strength may be further increased by using a peak detector circuit (not shown) and output to the low pass filter  310 . The low-pass filter  310  finally generates a DC output  311  which is thus proportional to the peak-to-peak signal from the oscillator circuit  302 . The GDO  302  can be connected to an antenna  304  via connecting lines  305 ,  307 .  
         [0049]     Note that the antenna  304  depicted in  FIG. 4  is analogous to each of the antenna  201 ,  203 ,  205  depicted in  FIG. 2 ( a ). In other words, one or more GDO circuits can be implemented in association with one or more antennas  201 ,  203 ,  205 , depending upon design considerations. Note that as utilized herein, the term “oscillator” may refer to the GDO or GDO circuit itself or may simply refer to the oscillator component, such as component  306 , which makes up one portion of the overall GDO, such as, for example, GDO  302 . Sensors  402 ,  404 ,  406  and can be implemented by LC tank sensors, depending upon design considerations.  
         [0050]      FIG. 5  illustrates a block diagram of a multiple sensor system  500  for use in dialysis applications, in accordance with an alternative first embodiment. System  500  includes multiple GDO circuits  414 ,  416 ,  418  (i.e., respectively, GDO 1 , GDO 2 , GDO 3 ). Each GDO  414 ,  416 ,  418  is analogous to the GDO  302  depicted in  FIG. 4 . GDO  414  is connected to a first antenna  408 , which in turn is inductively coupled to a first sensor  402 . GDO  416  is connected to a second antenna  410 , which in turn is inductively coupled to a second sensor  404 . GDO  418  is connected to a third antenna  412 , which in turn is inductively coupled to a third antenna  406 . GDO  414  is also connected to ground  415  and to a voltage supply  421 . GDO  416  is connected to ground  417  and also to the voltage supply  421 . Similarly, GDO  418  is connected to ground  419  and to voltage supply  421 . The antennas  408 ,  410  and  412  are analogous to the antennas  201 ,  203 ,  205  depicted in  FIG. 2 ( a ).  
         [0051]     GDO  414  is also connected to a low frequency switch  420 , which in turn can in a closed position permit an electrical connection of GDO  414  to a processing electronics module  426 . Similarly, GDO  416  is connected to a low frequency switch  422 , which in turn can in a closed position permit an electrical connection of GDO  416  to the processing electronics module  426 . Likewise, GDO  418  can be connected to a low frequency switch  424 , which in turn can in a closed position permit an electrical connection of GDO  418  to the processing electronics module  426 . Note that a pressure output signal  428  can be obtained from the processing electronics module  426 . It is also significant to note that each of the low frequency switches  420 ,  422 , and  424  can be in some embodiments, perform an analogous function to the RF switch  228  depicted in  FIG. 2 ( a ).  
         [0052]     In system  500 , multiple GDO circuits  414 ,  416 ,  418  are utilized. Both the GDO circuits  414 ,  416 ,  418  and the antenna  408 ,  410 ,  412  are always powered up (i.e., oscillations continuously set up in the circuit and antenna). One or more low frequency switches  420 ,  422 ,  424  can be operated by the processing electronics  426 , forming a multiplexer to select the output from each sensor in turn.  
         [0053]     In the configuration depicted in  FIG. 5 , three separate GDO circuits  414 ,  416 ,  418  are respectively associated with three separate sensors  402 ,  404 ,  406 . The three GDO circuits  414 ,  416 , and  418  share the processing electronics module  426 . The output from each GDO  414 ,  416 ,  418  can comprise a DC voltage. Thus, the resulting multiplexer can be composed of low frequency switches  420 ,  422 ,  424 , which are simple in structure and typically are of a low cost. It can be appreciated that although only three sensors  402 ,  404 ,  406  and three GDO circuits  414 ,  416 ,  418  along with three antenna  408 ,  410 ,  412  are depicted in  FIG. 4 , alternative embodiments with more or fewer such sensors, antenna or GDO components may be implemented, depending upon the sensing application requirements.  
         [0054]     Sensors  402 ,  404 ,  406  depicted in  FIGS. 5-6  and sensors  330 ,  344  depicted in  FIG. 3  can be implemented for example as inductance-capacitance resonant circuit (LC tank) sensors such as those disclosed in U.S. patent application Ser. No. 11/242,271, entitled “Wireless Pressure Sensor and Method Forming the Same.” Alternatively, such pressure sensors can be implemented as wireless capacitance pressure sensors, such as those described in U.S. patent application Ser. No. 11/226,085, entitled “Wireless Capacitance Pressure Sensor.  
         [0055]      FIG. 6  illustrates a block diagram of a multiple sensor system  600  for use in dialysis applications, in accordance with another version of the embodiment depicted in  FIG. 5 . Note that in  FIGS. 5-6 , identical or similar parts or elements are generally indicated by identical reference numerals. System  600  is similar to system  500  depicted in  FIG. 5 , with some variations to the overall circuit structure. In the configuration depicted in  FIG. 6 , the switches  420 ,  422 , and  424  are respectively located between the voltage supply  421  and respective GDO circuits  414 ,  416 , and  418 . Switches  420 ,  422 , and  424  can be implemented as low frequency switches. A GDO can be selected by powering it up in order to ensure that there is no interference from a neighboring GDO. Additionally, the power drawn in the configuration depicted in  FIG. 6  may be lower than that of system  500  illustrated in  FIG. 5 .  
         [0056]     In system  600  depicted in  FIG. 6  the GDO circuits  414 ,  416 ,  418  can be powered up in turn by the processing electronics  426 , thereby removing or reducing interference between the antenna  408 ,  410 ,  412 . The response time of system  600  is however reduced based on the need for the GDO circuits  414 ,  416 , and/or  418  to warm-up (i.e., time for oscillations in the GDO&#39;s LC oscillator circuit to build up to their full amplitude).  
         [0057]     The various first embodiments of  FIGS. 1-6  solve the need for multiple wireless pressure sensor systems for hemodialysis applications. Between three and six sensors, for example, can be utilized to make up the whole range of pressures and locations for use in a dialysis machine, such as the dialysis machine  110  depicted in  FIG. 1 . Such sensors can be located on the arterial line (i.e., after blood out of patient, before blood pump), the dialyzer line (i.e., after blood pump, before dialyzer), and/or on the venous line (i.e., after dialyzer, before patient), or any of a number of other possible locations on or in association with a dialysis machine or another medical application, such as, for example, external blood treatment or separation applications.  
         [0058]     The typical pressure range over which such sensors (e.g., sensors  402 ,  404 ,  406  of  FIGS. 5-6 ) preferably (although not necessarily) operate is between −700 mmHg and +1000 mmHg. This is, of course, only a suggested range and other ranges are also possible, depending upon design considerations and specific application requirements.  
         [0059]     In general, size limitations for sensors utilized in hemodialysis applications are problematic. It would be beneficial to design a multiple-sensor system with the lowest cost, small size and fewest parts.  FIGS. 1-6  represent one possible embodiment. A second embodiment involves the use of wireless LC tank multiple sensors in the context of a sensor system in which the sensors share a single antenna. Multiple capacitors can be linked with a single planar coil, such that each associated variable capacitor results in a signature frequency (i.e., spurs). Multiple characteristic resonant frequencies can be detected in such a system through prudent design.  
         [0060]      FIG. 7  illustrates a graph  700  depicting a variety of frequencies in the context of a sensor system for dialysis applications, in accordance with a second embodiment. In graph  700 , f 0  represents the fundamental frequency of the sensor system that will not be detected, while f 1 , f 2 , and f 3  are spurs related to each sensor in, for example, the three sensor system.  
         [0061]      FIG. 8  illustrates a sensor system  800  based on a plurality of circular electrodes forming variable capacitors, in accordance with the second embodiment. In the configuration of system  800 , two sub-systems  802  and  818  are illustrated. Sub-system  802  includes a group of electrodes  804 ,  806 ,  808 , while sub-system  818  includes a group of electrodes  812 ,  814 ,  816 . In the lower level configuration of sub-system  802 , the three circular electrodes  804 ,  806 , and  808  can be associated with three respective variable capacitors (not shown in  FIG. 8 ). Each electrode  804 ,  806 , and  808  is connected to an antenna  810 . At the higher level of sub-system  818 , the three electrodes  812 ,  814 ,  816  can be located on a pressure diaphragm (not shown in  FIG. 8 ) and respectively correspond to each electrode  805 ,  806 ,  808  of the lower level of sub-system  802 . Note that the dashed line  809  in  FIG. 8  represents the interconnection between sub-systems  802  and  818 .  
         [0062]      FIG. 9  illustrates a schematic diagram of an example equivalent circuit  900  of the configuration depicted in  FIG. 8 . The configuration depicted in  FIG. 9  is presented in order to assist in explaining the functioning of the configurations depicted in  FIG. 7 . Note that in  FIGS. 7 and 9 , the variables f 1 , f 2 , and f 3  generally represent the same functionality. In  FIG. 9 , R 0 , C 0 , R 1 , L 1 , R 2 , L 2 , R 3  and L 3  represent small electrical values. Note that equations  902  depicted in  FIG. 9  depicted general formulations for determining f 1 , f 2 , and f 3 . In general, the equivalent circuit  900  can be composed of an inductor  928  connected to a capacitor  926 , which in turn is connected to a resistor  924  that in turn can be connected to ground  930 . Similarly, a capacitor  910  is connected to a resistor  908 , which in turn is connected to an inductor  906  that in turn can be connected to ground  930 .  
         [0063]     A capacitor  916  can be connected to a resistor  914 , which in turn is connector to an inductor  912  that in turn is connected to ground  930 . A capacitor  918  can be further connected to a resistor  920  that in turn is connected to an inductor  922 . Note that the inductor  928 , and the capacitors  910 ,  916  and  918  are generally connected to an antenna  904 .  FIGS. 8-9  thus generally indicate that the pressure sensors discussed herein can be implemented in the context of an LC type sensor (e.g., LC tank sensor), an RLC type sensor, or a combination thereof, depending upon design considerations.  
         [0064]     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.