Abstract:
Microfluidic shunt valves are disclosed having a deflectable element capable of being held in a closed position to occlude the passage of fluid between an inlet and outlet and, when not held in the closed position, the deflectable element is adapted to oscillate in response to fluid pressure pulses and thereby facilitate fluid passage through the valve. Controls for activating the deflectable element to permit fluid passage are also included.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to the treatment of hydrocephalus, and more particularly relates to cerebrospinal fluid (“CSF”) shunts.  
         [0002]     Hydrocephalus is a condition in which cerebrospinal fluid accumulates in the ventricles of the brain. This accumulation of fluid increases the pressure within the ventricles and without medical intervention can cause brain damage and/or death to the patient. A common treatment for hydrocephalus is to use a fluid shunt system to drain excess CSF from the cerebral ventricles to a second body cavity, typically the peritoneal cavity. By draining the excess fluid, the elevated intracranial pressure is relieved. CSF shunts are well known and used broadly to treat patients with chronic hydrocephalus.  
         [0003]     Generally, fluid shunt systems include a valve mechanism for controlling or regulating the flow rate of fluid through the system. Shunt systems typically permit fluid flow only when the fluid pressure reaches a threshold pressure that opens the shunt valve. Fluid flow normally continues until the intracranial pressure has been reduced to a level less than the threshold release pressure of the valve.  
         [0004]     Thus, the flow regulating mechanism for most CSF shunts rely on pressure-sensitive valves that open when there is a sufficient pressure difference between the cerebral ventricle and the distal drainage cavity. In theory, this allows the right amount of the CSF to be drained. However, there are a number of problems associated with these shunts, for example, the control of the CSF flow typically is limited to a preset pressure. Although some shunt valves have mechanisms to adjust the pressure difference that triggers the valve to open, such mechanisms are typically cumbersome to use in real situations. In addition, the valves become clogged over time. Moreover, the existing shunts do not take into consideration the effects associated with CSF pulsations. The pressure in the cerebral ventricles will vary, typically in synchrony with the subject&#39;s heart rate. Under-drainage or over-drainage may arise due to the mismatched dynamic characteristics of the valve of the shunt and the CSF pulsations.  
         [0005]     Accordingly, a need exists for a CSF shunt that can regulate the flow of CSF in a more controlled and intelligent manner. A need also exists for CSF shunts in which the dynamics are sensitive to the fluctuations and flow variation that arise due to CSF fluctuations.  
       SUMMARY OF THE INVENTION  
       [0006]     Microfluidic shunt valves are disclosed having a deflectable element capable of being held in a closed position to occlude the passage of fluid between an inlet and outlet and, when not held in the closed position, the deflectable element is adapted to oscillate in response to fluid pressure pulses and thereby facilitate fluid passage through the valve. Controls for activating the deflectable element to permit fluid passage are also included.  
         [0007]     Microfluidic shunt valve arrays are also disclosed with a plurality of valves that control cerebrospinal fluid flow depending on the CSF pulse rate. A subset of the plurality of valves can be kept in reserve so that if, during operation, a valve becomes clogged, the system automatically reacts by replacing the failing valve with another one from the valve reserve. The array of valves can also include valves with different characteristics, such as size and/or resonant frequency, to optimize fluid flow control under various conditions. An impedance sensor can detect the impedance changes caused by valve clogging or less than desired valve performance and provide a signal to controller, which brings a new valve (or valves) from the reserve into play.  
         [0008]     Each valve can contain an oscillating valve element capable of moving in resonance with the CSF pulsations. The operation of each valve can be modulated by AC and DC capacitance forces provided by AC and DC voltages between the oscillating valve element and other conductive layers in a sandwich-like, integrated circuit structure. The DC component can provide a bias voltage controlling the oscillating valve element&#39;s oscillation amplitude and the opening of the valve. The AC component can induce the oscillating valve element to oscillate in phase and/or synchrony with the CSF pulsations. Ideally, the AC signals and the CSF flow pulsations have the same frequency.  
         [0009]     The oscillating valve elements of the present invention are able to provide a fast response to variations in the CSF flow dynamic parameters. An impedance sensor monitors the impedance values of all oscillating valve element&#39;s and provides feedback signals to the controller. Impedance changes caused by valve clogging or perturbations in the CSF pulsations are monitored, and a feedback signal from the impedance controller to the processor triggers a corrective response.  
         [0010]     Accordingly, in one aspect, the invention features a microfluidic shunt valve formed as part of a semiconductor chip having at least one inlet and at least one outlet for fluid passage therebetween. The terms “semiconductor chip” and “chip,” as used herein, is intended to encompass devices fabricated at least in part from a substrate material, e.g., a silicon or silicon-on-insulator (SOI) wafer. Typically two or more valve elements are formed on (or in) such a substrate material by lithographic patterning, etching and similar processes well known to those skilled in the art. In addition, the “chip” need not be monolithic but may consist of a plurality of segments or layers that bonded together or otherwise coupled. All of the components of a “chip” do not need to be electrically conductive (or insulating), but can also include structural or otherwise ancillary elements as well. Illustrative examples are described in more detail below.  
         [0011]     The valve may further comprises at least a first electrode and a second electrode, the first electrode associated with the deflectable element and isolated from the second electrode such a voltage applied between the first and second electrodes can induce movement of the deflectable element. The valve can also be structured such that movement of the deflectable element can cause occlusion of the inlet or outlet.  
         [0012]     A current regulator can be used for applying voltage between the first and second electrodes to bias the deflectable element in one position or to dampen oscillation. The current regulator can also be used for applying an alternating current to at least one of the electrodes. The current regulator can further be adapted to adjust at least one of DC voltage, or alternating current amplitude, frequency or phase in response to a control signal. The valve can also further comprise at least one impedance sensor for monitoring oscillations of the deflectable member.  
         [0013]     In another aspect, the invention provides for an array of valves, each valve being substantially as described above, and a controller. Accordingly, the invention further encompasses microfluidic valve assemblies with a plurality of valves, each valve comprising a channel to guide flow of a fluid and a deflectable valve element disposed within the channel. The deflectable valve elements are capable, when activated, of oscillating in response to fluid pulses to permit fluid passage. The assembly further comprising electrical controls for activating a subset of the plurality of valves, and at least one impedance sensor for monitoring movement of the deflectable member.  
         [0014]     In this aspect, the present invention differs from conventional shunt valves which rely, for the most part, on a threshold pressure differential to open (and in some instances an maximum pressure differential for operation to prevent over drainage due to siphoning effects). In contrast to these static designs, the shunt valves and valve arrays of the present invention permit more precise control over the fluid transport. By controlling the number of valves operational at any given time and/or the oscillation of their deflectable gate elements, the volume and rate of fluid transport can be readily controlled without resort to fixed threshold or maximum pressure parameters. Thus, the present invention permits essentially pressure independent control of cerebrospinal fluid transport, limited only by CSF pulsation and the elastic modulus of the deflectable gate.  
         [0015]     In a further aspect, the invention features a microfluidic shunt valve comprising a micro-machined structure defining a channel to guide flow of a fluid therethrough in a primary direction. A deflectable valve element can be disposed within the channel, and is capable of oscillating in response to fluid pulses to permit fluid passage. In this shunt valve, the deflectable valve element is electrically isolated from at least a portion of the structure to permit impedance measurements of an oscillatory frequency of the deflectable valve element.  
         [0016]     The deflectable valve element can be electrically biased in a normally closed valve position, and a sensor can be used for measuring changes in impedance. In addition, a controller can be used for modulating the oscillatory frequency. The controller can include a regulator adapted to apply a current that alternates at a desired frequency. The regulator can also be adapted to apply a direct current bias.  
         [0017]     In yet another embodiment, the invention features a shunt valve including a valve body with a distal end and a proximal end. A valve assembly can be disposed upon a semiconductor chip, the valve assembly having a plurality of deflectable valve elements, where each valve element is capable of oscillating in response to fluid pulses, and where each valve element is electrically isolated from at least a portion of the chip to permit impedance measurements of an oscillatory frequency of the valve element.  
         [0018]     The array assembly can further comprise a sensor for measuring changes in impedance of at least one valve element, and a controller for modulating the oscillatory frequency. The controller can include a voltage applicator adapted to apply an alternating current that is, preferably, adjustable to a desired frequency and the voltage applicator can further be adapted to also apply a direct current bias to deactivate particular valves (or dampen their movements).  
         [0019]     In yet another aspect, the invention features a method of treating hydrocephalus in a subject by implanting a shunt with a distal end and a proximal end in the subject, where the proximal end is implanted in a brain ventricle and the distal end is implanted in a region other than the brain. The shunt comprises a valve assembly having a plurality of channels to guide flow of cerebral spinal fluid (CSF), with each channel having a deflectable valve element disposed therein, which is capable, when activated, of oscillating in response to fluid pulses to permit fluid passage. At least one of the valve elements is activated and the operation of the activated valve element is monitored. If necessary, at least one additional valve element is activated based on the monitored valve operation.  
         [0020]     Each valve element can further comprise at least a first electrode and a second electrode, the first electrode associated with the deflectable element and isolated from the second electrode, the method further comprising applying an alternating voltage between the first and second electrodes to induce movement of the deflectable element. The applied voltage can maintain a subset of the valve elements in a closed position.  
         [0021]     The method can further comprise applying an alternating current to the first electrode to facilitate oscillation of the deflectable element, or to facilitate impedance measurements. Moreover, at least some of the deflectable elements can have different resonant frequencies and the method can further comprise selecting at least one deflectable element with a desired resonant frequency based on one or more impedance measurements.  
         [0022]     The valve assembly can have a plurality of deflectable valve elements in subsets of a different size. The deflectable valve elements can be activated upon occlusion of another subset of deflectable valve elements, or activated upon a change in pulse rate of the CSF.  
         [0023]     In yet another aspect, the invention features a method of fabricating a micro-electro-mechanical-system (MEMS) valve assembly. The method comprises providing a SIMOX wafer having a first silicon layer, a second silicon layer and a buried oxide disposed therebetween. A plurality of channels can be formed in the first silicon layer, and a plurality of gate structures can be formed in the second silicon layer, each gate structure being aligned with a channel of the first layer. A portion of the oxide layer between the gate structures of the first layer and the second layer can be removed to permit deflection of the gate structures. Electrical leads to each gate structure are also provided, such that a voltage applied between a gate structure and the first layer can induce deflection.  
         [0024]     The method can further comprise p-doping the first layer, and n-doping the second layer. Other doping schemes, such as the use of p-doped and p+-doped layers, will be apparent to those skilled in the art. The step of forming the channels can further comprise patterning a resist and then etching the channels in portions of the first layer not protected by the resist.  
         [0025]     The step of forming the gate structures can further comprise patterning a resist and then etching the gate structures in portions of the second layer not protected by the resist. The method can further include the step of depositing an conductive material to form an electrode on each gate structure, or depositing a conductive material to form an electrode on at least a portion of one or more of the layers. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0026]     A more complete understanding of the invention may be attained by reference to the drawings, in which like reference numerals indicate the same or equivalent element:  
         [0027]      FIG. 1  is a schematic partially cut, perspective view of a shunt valve of the invention;  
         [0028]      FIG. 2  is a schematic sectional view of the shunt valve of  FIG. 1 ;  
         [0029]      FIG. 3  is a schematic perspective view of a portion of the shunt valve of  FIG. 2  showing individual valve elements;  
         [0030]      FIG. 4A  is a schematic cross-sectional view of an individual valve element in an open position;  
         [0031]      FIG. 4B  is a schematic cross-sectional view of the individual valve element of  FIG. 4A  in the closed position;  
         [0032]      FIG. 5  is a schematic cross-sectional view of an alternative embodiment of a valve element;  
         [0033]      FIG. 6A  is a schematic illustration of electrostatic forces utilized in the present invention;  
         [0034]      FIG. 6B  is a schematic illustration of harmonic oscillations utilized in the present invention;  
         [0035]      FIG. 6C  is an illustrative graph of phase and impedance versus frequency for an oscillating gate element (in arbitrary units);  
         [0036]      FIG. 7  is a block diagram of a waveform sensing valve system according to the invention;  
         [0037]      FIG. 8A  is a top view of the valve elements that can be utilized in the invention;  
         [0038]      FIG. 8B  is the bottom view of valve elements of  FIG. 8A ;  
         [0039]      FIG. 9A  is a top view of an alternative embodiment of a valve element;  
         [0040]      FIG. 9B  is a schematic perspective view of the a valve element of  FIG. 9A ;  
         [0041]      FIG. 10A  is an illustration of the initial step in forming the valve of  FIG. 4A ;  
         [0042]      FIG. 10B  is an illustration of the second step in forming the valve of  FIG. 4A ;  
         [0043]      FIG. 10C  is an illustration of the third step in forming the valve of  FIG. 4A ;  
         [0044]      FIG. 10D  is an illustration of the fourth step in forming the valve of  FIG. 4A ;  
         [0045]      FIG. 10E  is an illustration of the fifth step in forming the valve of  FIG. 4A ;  
         [0046]      FIG. 11A  is a schematic perspective view of a tandem valve assembly employing a microfluidic valve array and a conventional pressure-sensitive valve; and  
         [0047]      FIG. 11B  is a schematic perspective view of another embodiment of a tandem valve assembly employing a microfluidic valve array and a conventional pressure-sensitive valve. 
     
    
     DETAILED DESCRIPTION  
       [0048]      FIG. 1  is a schematic view of a shunt valve  10  having a valve body  12 , a proximal end  14 , a distal end  16 , and a power supply and electronic controls  18 . The shunt valve comprises a valve assembly  20  with a plurality of deflectable valve elements  22 , a connector  24 , and associated electrical conduits  26  that serve to couple the valve assembly  20  with the power supply and electrical controls  18 . The term “deflectable” and variants thereof as used in this specification is intended to include bending, shifting, swinging, stretching and elastic deformation as well as other forms of physical movement.  
         [0049]      FIG. 2 a  schematic sectional view of the shunt valve. The valve body  12  houses the shunt valve assembly  20  which has a plurality of deflectable valve elements  22 . The shunt valve assembly  20  is designed to facilitate the passage of cerebrospinal fluid (CSF) from the proximal end  14  to the distal end  16  when the pressure differential between the ventricle and the distal cavity is greater than a threshold value. More importantly, the valve of the invention allows “fine” control of CSF by operating deflectable valve elements at the same oscillation frequency as the CSF pulsation. This provides a valve with dynamics that are sensitive and which are optimized for CSF pulsation in each individual patient.  
         [0050]      FIG. 3  is a schematic perspective view of the shunt valve showing two individual valve elements  22 A and  22 B disposed within the valve assembly  20 . Each valve element  22 A has a gate element  30 , and a chamber  32  having an outlet hole  34  (shown in phantom). The gate element  30  comprises an electrically conductive upper surface  36 A and an electrical lead  38 . The valve element  22 B has comparable structures to element  22 A. A plurality of valve elements provide the valve assembly  20 .  
         [0051]      FIG. 4A  is a more detailed cross-sectional view of a single valve element  22  in an “open” position. The valve element comprises a silicon substrate  40  and a silicon surface layer  42  separated by an oxide layer  44 . The silicon surface layer  42  acts as a cantilever that is sufficiently flexible such that it can respond to naturally occurring CSF pulsation pressure. The deflectable valve element further includes a lower electrode  46 . The “open” position permits the CSF  48  to enter the inlet  28  and exit through the outlet  34 .  FIG. 4B  shows a schematic perspective view of an individual valve element  22  in a “closed” position. In the closed position, the CSF flow is occluded by the silicon surface layer  42  contacting the silicon substrate  40 . The term “occlude” and variants thereof to include both stopping and substantially inhibiting passage of fluid.  
         [0052]     The design facilitates the manufacture of array elements using micro-electro-mechanical systems (MEMS) technology and a “silicon-on insulator” (“SOI”) or “separated-by-insulating-metal-oxide (SIMOX) wafers as a starting material. The manufacturing technique is described in detail below. The term “MEMS” as used herein is not intended to denote a strict size category of devices. MEMS devices can be as large as several millimeters or as small as 10 nanometers, depending upon the techniques used and the desired applications.  
         [0053]      FIG. 5  is an alternative embodiment of the valve element  50  with an electrically conductive gate element  30  (e.g., an oscillating central cantilever) is situated between a lower silicon block  52  and an upper silicon block  54 , separated by oxide layers  56  and  58 . Both upper and lower silicon blocks  52  and  54 , respectively, are electrically conductive and preferably grounded. The design facilitates oscillations of the gate element  30  in response to pulsation of the CSF. The design also facilitates measurement of impedance. The electrically conductive gate element  30  can be biased with a DC voltage by electrode  60 . Additionally, and alternating current (AC) can be applied by electrode  62 . As discussed in more detail below, an impedance sensor  64  can also be deployed to provide feedback signals. The sensor can monitor the valve performance by measuring impedance changes over time.  
         [0054]     It will be appreciated that the terms “upper” and “lower” are used simply for illustrative purposes and do not correspond to any particular orientation. The valves of the present invention are not limited to vertical operation. For a device implanted in a patient, the orientation may change as the orientation of the patient changes.  
         [0055]     As shown in  FIG. 5 , the valve is divided into an upper chamber  51  and a lower chamber  53 . Fluid flows from the upper chamber to the lower chamber whenever the gate  30  is allowed to swing. In one embodiment the depth of each chamber can range from about 10 to 500 micrometers, more preferably from about 25 to 100 micrometers. The width of each chamber can range from about 50 to about 1000 micrometers, more preferably from about 50 to about 250 micrometers. The dimensions of chambers  51  and  53  can be the same or different. As shown in  FIG. 5 , the upper chamber  51  is smaller than lower chamber  53 , which can be advantageous to increase fluid transport (from the upper to the lower chamber as illustrated). On the other hand, a reduction in the size or depth of the lower chamber can be advantageous to provide an anti-siphon effect when an ambulatory patient changes orientation.  
         [0056]     In operation, the threshold pressure for opening the valve can be controlled by electrostatic force, e.g. by applying a DC bias voltage to the central cantilever layer while maintaining upper and lower silicon blocks at relative ground, as shown in  FIG. 5 . As the voltage difference is increased, the threshold necessary for valve opening will likewise increase. A high DC bias can also be employed to maintain certain valves closed, e.g., to hold a number of valve elements in reserve. Moreover, in the case of an array having different subsets of valves, in which one or more subsets are designed for particular conditions, the DC bias voltage can be used to activate only the optimal subset for a particular condition.  
         [0057]     Once the valve is opened, an AC current can also be imposed to help “tune” the oscillations of the cantilever and maintain synchrony with the frequency of CSF pulses. The AC current can also provide additional energy to the oscillating lever cantilever to accommodate changes in the CSF pulse frequency or compensate for fluid resistance effects.  
         [0058]      FIG. 6A  illustrates one mechanism of operation of the present invention. When a DC voltage is applied between an upper plate  66  and a lower plate  68 , electrostatic forces will cause the plates to be attracted to each other. This principle is used in the invention to close the valve as shown in  FIG. 4B . Where the electrostatic forces draw closer to the silicon substrate, contact is established, thereby closing the outlet  28 .  FIG. 6B  shows that additional AC current is used on the plates  66  and  68  to induce harmonic oscillations which correlate with the CSF pulse rate.  
         [0059]     The natural oscillation of a beam or cantilever at its resonant frequency can be defined in part by its quality factor, also known as its “Q factor,” which is a measure of energy loss when a oscillating element experiences energy losses due to friction and the like. A high Q system will maintain it amplitude with minimum intervention while a low Q system is termed “damped.” In the present invention, the effect of a surrounding fluid will inherently have a damping effect of oscillations of the deflectable gate element  30 , as shown in the figures. Impedance measures can be used to monitor such losses and, if necessary, provide a compensatory “boost” or other remedial action.  
         [0060]     Impedance measures the degree to which an electric circuit resists electric-current flow when a voltage is impressed across its terminals. Impedance, expressed in Ohms, is the ratio of the voltage impressed across a pair of terminals to the current flow between those terminals. In direct-current (DC) circuits, impedance corresponds to resistance. In alternating current (AC) circuits, impedance is a function of resistance, inductance, and capacitance. Inductors and capacitors build up voltages that oppose the flow of current. This opposition, called reactance, must be combined with resistance to determine the overall impedance. The reactance produced by inductance is proportional to the frequency of the alternating current, whereas the reactance produced by capacitance is inversely proportional to the frequency.  
         [0061]      FIG. 6C  is an illustrative graph of phase and impedance versus frequency for an oscillating gate element (in arbitrary units). In a damped system, such shown in  FIG. 5 , the actual phase of oscillations will trail the natural resonant frequency, ƒ, by a factor, Δƒ, which can be detected as an impedance. The phase of the driving AC voltage can be adjusted (advanced) to balance out frictional losses. Thus, continued monitoring of the impedance and feedback control adjustments can be used to maintain synchrony as well as to adapt to new conditions, e.g., changes in heart rate or patient activity levels, or simply to “fine-tune” valve operations.  
         [0062]     In the present invention, a controller can monitor the impedance (or phase) of the AC signals transmitted through each valve. At resonance the impedance will exhibit a minimum value (i.e. the resistivity exhibits its minimum value and, conversely, conductivity exhbits its maximum value). The relationship between impedance and phase is shown in the  FIG. 6C . At resonance, the impedance exhibits a minimum value while the phase curve, which has been going up with frequency, exhibits a sign change (inversion) of the slope. In operation, for example, if a valve is blocked by debris, both impedance and phase curves will disappear and the valve will start to behave like a heavily damped resonator. The Q factor of a stuck valve will approach zero, the impedance curve will broaden (large delta f), and the phase curve will flatten. Since the controller is monitoring all valves, if a high impedance value and no phase change for a certain valve is detected, the controller automatically can switch valves, putting another valve with normal resonant characteristics to work.  
         [0063]      FIG. 7  is a block diagram of a feedback control system, showing a power source and current regulator  72 , connected to a processor  74  such as a digital signal processor (DSP), which sends and receives signals from a transponder  76 . An impedance current sensor  78  receives signals from the valve array  20  and communicates these signals to the processor  74 . All elements  72 - 78  can be incorporated into a single body power supply and electrical control system  18 , as shown in  FIG. 1 .  
         [0064]     The DSP controls the operations of all valves in the shunt array through the AC/DC voltage source and an impedance controller. The DSP receives feedback signals from the impedance controller. If a change in the CSF pulsation characteristics occurs the controller detects the resulting impedance changes and sends an alarm signal to the DSP. The DSP makes the necessary corrections in frequency, phase, flow, and number of valves participating in the process at a given moment of time through the AC and DC voltage sources. Valve clogging results in changes in the resonant-cantilever&#39;s impedance. The impedance controller detects the failing valve and submits an alarm signal to the DSP. The DSP automatically responds by replacing the failing valve with another one from the reserve valves. The DSP can bring to operation various numbers of valves at various times depending on the CSF characteristics and the feedback signals from the impedance controller. This ensures accurate monitoring of the CSF flow and avoids over-draining or under-draining of CSF from the ventricles. Over-draining or under-draining CSF occurs because of mismatched dynamic characteristics of the valves and the CSF pulsations.  
         [0065]     Most generally, the invention encompasses microfluidic valve arrays in which fluid conditions, such as pulse frequency and amplitude (e.g., pressure), are detected and monitored as electronic waveforms and such waveforms are used to select particular subsets of valves or modulate the behavior of one or more valve elements.  
         [0066]      FIGS. 8A and 8B  are top and bottom views, respectively of an illustrative valve array having individual valve elements with different shapes and sizes. Each subset of valves operates at an optimal resonant frequency. The geometry of each gate element responds best to the particular resonant frequency based of the pulsation of CSF and the measured impedance. If the patient is likely to have variations in CSF pulse rate, the invention permits electronic monitoring and activation of particular subsets of valves whose resonant frequency is best tuned to the pulse rate of the CSF. For example, valves  81 A,  81 B and  81 C illustrate gate designs of different length, each of which will exhibit a optimal resonant frequency. Further fine-tuning of the resonant frequency can be achieved by also varying the width of the gate, as illustrated by valve elements  82 A-C. Valves  83 A-C illustrate an alternative gate design in which the cantilever gate element is replaced by one that creates a fulcrum-like effect. In operation, each of the illustrated valves will respond differently to a particular pulse frequency of the CSF. Impedance measures permit real-time feedback control of the valve array and selection of particular valves best suited to the patient&#39;s conditions.  
         [0067]      FIG. 9A  is a top view of another embodiment of a valve  90  of the invention showing a gate element  91  and an outlet port  34 .  FIG. 9B  shows a side view of the valve showing the gate element  91  coupled to the silicon substrate with finger elements  92  which permit deformation in response to applied voltages. This permits the outlet  34  to “open” and “close” in resonance with the pulse rate. Again, the illustrated valve  90  can be part of an array of valves having different resonant characteristics, e.g., formed by varying the size of the gate  91  and/or the properties of the finger elements  92 .  
         [0068]     FIGS.  10 A-E provide an illustrative set of manufacturing steps for creating a valve of the invention using wet chemical etch, deep reactive ion etching (DRIE), photolithography, and thin film deposition techniques. The first step, shown in  FIG. 10A , begins with a silicon-on-insulator (SOI) type wafer  100 , e.g., a SIMOX wafer, having a buried silicon oxide layer  102  disposed between an upper silicon layer  101  and lower silicon layer  103 . The upper silicon layer can be an n-type doped silicon region while the lower silicon layer can be a p-type silicon region. The thickness of the SOI silicon film can be, for example, in the range of about 10 microns to 25 microns.  
         [0069]     In  FIG. 10B  a microfluidic input channel  104  is etched into the p-type doped lower silicon layer  103  using, for example, deep reactive plasma etching. In the next step, shown in  FIG. 10C , a portion  105  of the upper n-doped silicon layer is removed by etching (See  FIG. 8A  also). The next step shown in  FIG. 10D  involves removing a portion of the oxide layer  102  between the upper and lower layers to form a resonant cantilever. The final step, shown in  FIG. 10E , involves depositing a electrodes  106  and  107  (e.g., gold electrodes) on the upper and lower doped silicon wafers using techniques such as a gold etch bath. One or both of the electrodes can be grounded. It should be apparent that various intermediate steps such as resist deposition and patterning have been omitted from the foregoing schematic illustrations. Such steps are well known to those skilled in the art.  
         [0070]     A plurality of these valves can be created to produce a valve array that can be housed in a valve body. The valve of the invention replaces the functions of conventional shunts and provides better control of the CSF flow from the ventricles to the distal location than conventional shunts. The valve arrays of the invention has a high-accuracy control over the CSF flow, fast response to various changes in the CSF flow dynamics, and the capability to operate synchronously with the CSF pulsations.  
         [0071]     The present invention can also be used in tandem with a conventional pressure-sensitive shunt valve device to provide an emergency safety feature, in the event the primary valve becomes fouled or is otherwise rendered inoperative. Two such tandem configurations are shown in  FIG. 11A  and  FIG. 11B . In  FIG. 11A , a microfluidic valve array  110  is illustrated as a alternative path which can be activated in the event that a conventional shunt valve  112  becomes clogged or otherwise malfunctions. As shown in  FIG. 11A , the microfluidic valve array  110  has its own drainage catheter  114  that provides an independent drainage pathway. In  FIG. 11B , the microfluidic valve array  110  is shown in a tandem configuration to permit CSF to by-pass the conventional shunt valve  112 . The tandem configuration of  FIG. 11B  utilizes a single drainage catheter but permits the microfluidic valve array to assist the main valve  112 .  
         [0072]     Although the present invention has been described herein primarily in connection with CSF shunt systems, the structures, systems and methods are equally applicable to other biomedical fluid control problems. For example, the microfluidic valves can also be used to control urine, blood or endocrine fluid transport and/or employed in catheters, hemodialysis, drug infusion, tissue engineering and other applications as well as in artificial organs or organ assist devices, e.g., in kidney, liver, bladder and heart replacement or assist devices.  
         [0073]     It is understood that the geometry, dimensions, and materials can vary depending upon the requirements of particular applications and the anatomy of the patient. The housing can be constructed from suitable non-toxic and bioimplantable materials, such as medical grade silicones, polyurethanes, or other polymeric materials. In one embodiment, the housing can be constructed of an injection molded plastic. Although the housing is primarily shown and described as having a rectangular shape, it is understood that other geometries are possible such as cylindrical and polygonal structures.  
         [0074]     One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.