Patent Description:
Elevated intracranial pressure (ICP) is a dangerous condition that can be caused by severe head injuries and other pathological problems. Continuous and accurate measurement of intracranial pressure (ICP) is considered a valuable means of management of patients suffering from ICP hypertension. The volume of the intracranial cavity is constant under normal conditions. The maintenance of a steady ICP depends on the volume of its contents, which include brain tissue (~<NUM>%), venous blood (~<NUM> to <NUM>%), arterial blood (~<NUM> to <NUM>%) and cerebrospinal fluid (CSF) (~<NUM>%). As brain tissue is relatively incompressible, steady ICP requires balancing the inflow and outflow of the fluid components. In other words, there must be a balance between the inflow of arterial blood and the outflow of venous blood from the head, as well as between the rate of CSF production and drainage. Some changes in mean ICP are expected under regular physiologic conditions, including changes in posture, brain activity, cardiovascular function, respiratory function and adrenergic tone.

Elevated ICP can result from any mechanism that increases the volume of blood or CSF. Alternatively, ICP can also increase by the addition of a fourth component, such as a mass, intracranial haemorrhage or cerebral oedema that expands beyond the ability of the system to compensate. As ICP increases, mean arterial pressure (MAP) is increased, primarily through a rise in cardiac output, in order to maintain a steady cerebral perfusion pressure (CPP), which represents the pressure gradient driving cerebral blood flow and hence oxygen and metabolite delivery. In the presence of elevated ICP beyond the ability for compensation through elevation of MAP, CPP will be compromised and cerebral ischemia may follow. When ICP is sufficiently elevated, the pressure differential between the intracranial cavity and the spinal canal can cause the downward motion of brain tissue (i.e., herniation), which can compress vital brainstem structures, and subsequently lead to severe neurological outcomes including death. Untreated hydrocephalus has a <NUM> - <NUM>% death rate, while survivors having varying degrees of intellectual, physical, and neurological disabilities.

The most common neurological and neurosurgical pathologies that require ICP monitoring are traumatic brain injury (TBI), subarachnoid hemorrhage (SAH), and hydrocephalus.

Conventional invasive ICP monitoring systems require a wire, optical fibre or tube penetration of the skin. Such wired systems may limit patient transport and movement and may have high risks of infection, which can prevent long term usage. Some commercial telemetry ICP systems may offer the possibility of long term and continuous ICP monitoring. However, the sizes of the implantable and external device components and the cost of the system can limit their applications due to the wireless transmission method of inductive coupling, which, in nature, requires large coils.

Elevated ICP can be treated by intracranial shunts, i.e. tubes that drain CSF into other parts of body (e.g. the abdomen). Shunts may be made from two tubes. One is inserted into the ventricle at one end and connected to a valve at the other end. The valve adjusts CSF flow from the brain into the second tube. However, current technology shunts can be prone to failures because of issues ranging from shunt obstruction, disconnections, fracture, over drainage or underdrainage. Therefore a 'smart shunt', i.e. a shunt integrated with a wirelessly readable pressure sensor, is desirable to improve reliability, control, precision and monitoring.

It is desirable to provide a pressure sensor that can have some or all of the following features: to be fabricated to have a very small size; to be wirelessly readable; to be powered using wireless technology.

As reproduced in <FIG> of this disclosure, <CIT> describes an implantable cardiovascular pressure sensor <NUM> which comprises rigid enclosure <NUM> arranged for holding a compressible fluid or a vacuum <NUM> sealed within the rigid enclosure by a flexible membrane <NUM>. An elongate compliant member <NUM> comprising a piezoelectric material is provided within the enclosure and the flexible membrane <NUM> is coupled to the elongate compliant member <NUM> to transfer external fluid pressure load <NUM> to the elongate compliant member <NUM> to cause deflection of the elongate compliant member <NUM>. The pressure sensor <NUM> comprises a first acoustic wave device <NUM> provided by the piezoelectric material of the elongate compliant member <NUM> for sensing deflection of the elongate compliant member <NUM>.

The membrane <NUM> may include at least one flexible feature arranged to reduce rigidity in the membrane. For example, such a flexible feature may include a corrugation of the membrane <NUM> arranged to reduce strain placed on the membrane by deformation. As seen in <FIG> the corrugation may comprise a ridge <NUM> towards the ends / sides of the membrane <NUM>. The ridge <NUM> may extend around the perimeter of the membrane <NUM> to provide a flexible connection between the compliant member <NUM> and the rigid enclosure <NUM>. As seen in <FIG>, the corrugation may comprise ridges <NUM> (such as folds or bends) in the surface of the membrane <NUM>. These may follow a closed path (which may be curved in places) that circumscribe the elongate compliant member <NUM> about the surface of the sensor.

<CIT> discloses a sensor and insertion assembly for intravascular measurement of pressure in a living body, comprising an insertion means and a sensor chip having a substrate body that comprises a recess covered by a pressure sensitive film thereby forming a cavity. A piezoelectric element, preferably in the form of a piezoelectric film, is arranged in connection with said pressure sensitive film, and an energy feeding means is arranged to apply energy to the piezoelectric element such that acoustic waves are generated in said element, wherein the piezoelectric element is arranged to generate an output signal, representing the pressure at the film, in dependence of measured properties of the acoustic waves related to the deflection of the pressure sensitive film.

<CIT> discloses systems and methods for monitoring physiological parameters such as intracranial pressure ("ICP"), intracranial temperature, and subject head position. For example, in some embodiments, an implantable apparatus for measuring ICP can be implanted into a subject skull. The apparatus can comprise an implant body having a hermetically sealed chamber housing a gas at a reference pressure, and a pressure conduction catheter having a proximal end and a distal end, wherein the distal end is configured to extend into the brain through a burr hole in the skull and includes a plurality of ports. A barrier can cover the ports of the distal end of the pressure conduction catheter, wherein the barrier and pressure conduction catheter are filled with a number of gas molecules so that the barrier is not in tension in a predefined range of ICPs.

It is an object of the invention as set out in appended claim <NUM>, to provide improvements in pressure sensors such as that described in <CIT> as discussed above.

The pressure sensing apparatus may be configured such that inward pressure applied to the flexible membrane at the second face causes inward deflection of the flexible membrane disposed over the deflectable portion of the first sensor device and inward and / or outward displacement of the flexible membrane along said at least one or two sides of the first sensor device and the chamber. The flexible membrane of the envelope may surround the first sensor device, the chamber and the support structure along at least a portion of the longitudinal axis of the first sensor device. The flexible membrane may form a sleeve extending along the longitudinal axis and around the first sensor device, the chamber and at least a portion of the support structure. The support structure may comprise two longitudinal end portions which each close a respective end of the sleeve to form the hermetic seal of the envelope. The elongate first sensor device may be supported at each longitudinal end by the rigid support structure and the deflectable portion may be a deflectable central portion between the opposing longitudinal ends. A base of the rigid support structure may comprise a second sensor device extending parallel to the first sensor device adjacent the chamber. The rigid support structure may further comprise a pair of spacers separating the base and first sensor to form the chamber. The spacers may each comprise an electrically conductive material coupled to a respective electrical terminal of at least one of the first sensor device and the second sensor device. The envelope may comprise an electrically conductive material electrically coupled to a first one of the spacers which may form a ground plane enveloping at least a substantial part of the first sensor device. The longitudinal end portions may each comprise an electrically conductive cap. Each cap may be bonded to a respective end of the sleeve around its circumference to form the hermetic seal.

The electrically conductive material of a second one of the spacers may be electrically connected to an antenna extending away from the envelope. The antenna may comprise a resilient material having an expanded shape memory configuration. The antenna may define a substantially linear axial portion and an off-axis laterally extending portion. The material may be resiliently bendable into a substantially linear configuration for delivery of the apparatus via a catheter.

The flexible membrane may comprise a metal material soldered, welded or otherwise bonded directly to at least one electrically conductive end cap of the envelope. The flexible membrane may comprise a metallised polymer bonded to at least one electrically conductive end cap of the envelope and electrically continuous therewith by an electroplated layer. The flexible membrane may comprise a glass material forming the envelope as a closed-ended capsule sealed around at least one electrical connection passing therethrough. The closed-ended capsule may be sealed around at least two electrical connections passing therethrough, and may further include an electrically conductive sleeve disposed around the capsule electrically connected to one of the electrical connections to form a ground plane around the capsule.

The rigid support structure may comprise a housing having a trench within which the first sensor device is positioned, and the flexible membrane may comprise a polymer which encapsulates the housing to form the envelope. The housing may comprise first and second electrically conductive portions separated from one another by an electrically insulating portion, and each electrically conductive portion may be coupled to a respective electrical terminal of the first sensor device. The first electrically conductive portion of the housing may be substantially longer than the second electrically conductive portion to form a ground plane, and the second electrically conductive portion of the housing may be coupled to an antenna. The trench of the housing may be narrower at the ends to support the respective electrical terminals of the first sensor device and may be wider therebetween to enable unrestricted displacement of the deflectable portion at one or two sides of the first sensor device. The flexible membrane may be coated with one or more layers of material to increase the hermeticity of the envelope.

The pressure sensing apparatus may be incorporated within an intracranial shunt apparatus. The pressure sensing apparatus may further include a valve within the intracranial shunt apparatus. The valve may be configured for control by an output of at least the elongate first sensor device.

Embodiments of the present teaching will now be described by way of example and with reference to the accompanying drawings in which:.

Throughout the present specification, descriptors relating to relative orientation and position, such as "top", "bottom", "horizontal", "vertical", "left", "right", "up", "down", "front", "back", as well as any adjective and adverb derivatives thereof, are used in the sense of the orientation of apparatus as presented in the drawings. However, such descriptors are not intended to be in any way limiting to an intended use of the described or claimed invention.

Pressure sensors according to the present disclosure can be based on an elongate beam that is supported at one or both longitudinal ends by a rigid structure, but which is unsupported where the beam extends into a deflection zone. The deflection zone can be a medial zone between the two rigidly supported longitudinal ends of the beam or, in the case of a cantilever arrangement, the deflection zone can be in an unsupported part of the beam and preferably at an end of the beam proximal to the supported end where strain is maximised. Pressure sensors based on a beam arrangement are particularly attractive as they can be used to fabricate narrow sensors that are dimensionally suited for implantation by catheter or other delivery device into the human or animal body, and can have high sensitivity to pressure. Packaging a beam sensor with long-term hermeticity that is suitable for permanent implantation in the human or animal body, for example, is challenging.

An ideal size for ICP monitoring may be a sensor with of a width less than <NUM>. Many shunt tubes that are inserted into the brain have a diameter of <NUM>, although a larger <NUM> diameter tube may also be used. A similar cross section is appropriate for insertion into brain tissue. A normal ICP pressure is around <NUM>-<NUM> mmHg and ideally a resolution of <NUM> mmHg is desired for diagnosis. Pressure sensors as described above have a deformable / displaceable membrane that seals a cavity at a reference pressure and this ability of the membrane to deform / displace is reduced as the sensor width (or generally the membrane area) gets smaller. One solution to this is to make the membrane thinner and thinner, but there are practical limits to this. Another solution is to provide a flexible corrugation feature as discussed above in connection with <FIG>, but this may still provide limitations on how small the sensor can be made while achieving a desired pressure sensitivity.

<FIG> shows first and second elongate sensor devices <NUM>, <NUM> each having a respective surface acoustic wave (SAW) resonator <NUM>, <NUM>. The first sensor device <NUM> may be provided on a thin piezoelectric substrate <NUM>, e.g. crystalline quartz or aluminium nitride. This first sensor device <NUM> may be configured to function as a pressure sensitive resonator and the substrate <NUM> may have a thickness of approximately <NUM> microns so that it may readily deflect under pressure-induced forces. The second sensor device <NUM> is configured to function as a reference sensor and as such may be provided on a relatively thick substrate <NUM>, such that there is negligible deflection or bending under the pressure-induced forces expected during normal operation.

Each sensor device <NUM>, <NUM> may have at least two input/output terminals <NUM>, <NUM> connected via electrical tracks <NUM>, <NUM> to terminals / pads <NUM>-<NUM> at the ends of the substrates <NUM>, <NUM> as shown. The electrical tracks <NUM>, <NUM> and the pads <NUM>-<NUM> may be formed by metal deposition (e.g. of gold or gold-plated other metal) and suitable patterning process.

As seen in <FIG>, the two substrates <NUM>, <NUM> are bonded together in a face-to-face configuration via metal (e.g. gold) spacers <NUM>, <NUM>, so that the resonators <NUM>, <NUM> are facing each other but spaced apart to define gap <NUM> or cavity therebetween. In this configuration, the two resonators <NUM>, <NUM> can readily be electrically connected in parallel and can be excited through the two electrically conductive spacers <NUM>, <NUM>. The spacers <NUM>, <NUM> may be extended longitudinally to enable external connections, as will be described further below.

As seen in <FIG>, the first sensor device <NUM> exemplifies an elongate first sensor device having a beam configuration which is supported at each longitudinal end <NUM>, <NUM> by a rigid support structure which is exemplified by the thick substrate <NUM> and the spacers <NUM>, <NUM>. The gap <NUM> between the lower face <NUM> of the first sensor device <NUM> / substrate <NUM> and the upper face <NUM> of the second sensor device <NUM> / substrate <NUM> forms part of a chamber <NUM> into which the substrate <NUM> of the first sensor device <NUM> may deflect when pressure is applied to an upper face <NUM> of the substrate <NUM>. The lower face <NUM> may be considered generally as an inwardly- or internally-facing face of the first sensor device and the upper face <NUM> may be considered generally as an inwardly- or internally-facing face of the second sensor device.

The chamber <NUM> and the first sensor device <NUM> must be hermetically sealed from the external ambient of which the pressure is to be measured, while allowing the external pressure to exert a deflecting force on the beam defined by the substrate <NUM> extending between the spacer supports <NUM>, <NUM>.

With reference to figure 6a, hermetic packaging of the structure of <FIG> can be effected using a pre-formed shell, e.g. in the form of a tube <NUM>, of suitable wall thickness, having ends <NUM>. The wall thickness in an ICP pressure sensing apparatus may for example be <NUM> - <NUM>. The shell or tube <NUM> can be made from a suitable material, such as metal (including metal alloys), metallised polymer or glass, and houses the resonators <NUM>, <NUM>. The cross-section of the shell <NUM> is important as it is required to transfer pressure of the external ambient onto the beam-configured substrate <NUM> efficiently. An end cross-sectional view is shown in Figure 6b. The curvature of the shell <NUM> may be configured to minimise the shell resistance to deformation. The shell <NUM> exemplifies a flexible membrane that is disposed over and coupled to the upper face <NUM> of the first sensor device <NUM>. Unlike the prior art examples illustrated in <FIG>, the flexible membrane provided by the shell <NUM> extends around the sides <NUM>, <NUM> of the first sensor device <NUM> and the chamber <NUM> over at least a substantial proportion of their length, rather than only extending across the upper face <NUM> of the first sensor device <NUM> and onto a surrounding rigid support structure. In this context, the expression 'sides' is intended to encompass the longitudinally extending sides <NUM>, <NUM> of the sensor devices and the chamber <NUM>. The longitudinal ends <NUM>, <NUM> (<FIG>) of the sensor devices and chamber <NUM> may be enclosed by a different structure, to be described below.

As best seen in <FIG>, the ends <NUM>, <NUM> of the apparatus may be closed by a pair of end caps <NUM>, <NUM> which engage with longitudinally-extending portions of the respective spacers <NUM>, <NUM> and hermetic seals <NUM> are formed between the interfaces <NUM> of the end caps <NUM>, <NUM> and the inner surfaces <NUM> of the shell / tube <NUM> just inward of the ends <NUM>, to be described in more detail later.

The pressure sensor apparatus as described above using resonators <NUM>, <NUM> takes advantage of a change in resonant frequency of SAW resonators due to induced strain and stress in the substrate <NUM> upon deflection. The first resonator <NUM> on the thin substrate <NUM> functions as a pressure sensitive resonator to deflect under pressure-induced forces transmitted from the ambient environment via the shell <NUM> to compress the gas which exists within chamber <NUM> at some reference pressure, e.g. below ambient pressure. The second resonator <NUM> is on the thick substrate <NUM> which is sufficiently thick that negligible deflection occurs under the pressure-induced forces.

The SAW resonators as described may have a high quality factor and operate at high frequencies (e.g. in the range of <NUM> - <NUM>). These resonators can be made by depositing thin film metal electrodes and acoustic wave reflectors on the piezoelectric substrates <NUM>, <NUM>. They can be excited by a radio frequency (RF) signal such that the resonator picks up the energy and converts it to mechanical vibrations via piezoelectric effect. Upon quenching the excitation, the resonator continues to oscillate at its predetermined resonant frequency. If the SAW resonator substrate is deflected, the induced stress and strain in the substrate causes a shift in the resonant frequency. The first resonator <NUM> on the thin substrate <NUM> lies over the gas-filled or vacuum cavity in chamber <NUM> which defines the reference pressure. Any change in the environmental (ambient) pressure outside the shell <NUM> creates a differential pressure with respect to the cavity pressure and induces strain / stress in the membrane <NUM>, causing a shift in the resonant frequency that is proportional to the pressure. The second resonator <NUM> on the thick substrate <NUM> will not be affected by the pressure change and will not change in resonant frequency as a result of the ambient pressure changes. It can therefore be used as a reference.

The resonators <NUM>, <NUM> can be coupled to an antenna and be excited wirelessly by sending a RF pulse at a frequency close to resonance. Part of the RF pulse energy is stored in the resonator and is radiated back as a weak decaying RF wave, once the transmitting pulse is switched off. This signal is detected by a receiver and its frequency is estimated to obtain pressure. An RF transceiver can be used to interrogate the sensor. In transmit mode, the interrogator may send RF bursts of, e.g., ~<NUM> duration to excite a resonator. At the end of each pulse, the interrogator is switched to its receiving mode to detect the decaying signal. An on-board processor calculates the frequency and converts it to pressure.

Thus, where resonators <NUM>, <NUM> are used for the sensor devices, provision is made for such an RF interface. With further reference to <FIG>, one end cap, e.g. end cap <NUM> may be used to provide a ground plane. The shell <NUM> is formed from an electrically conductive material, as is the end cap <NUM>. The two components are bonded together, e.g. by welding or soldering, such that electrical continuity is maintained. The end cap <NUM> is also electrically connected to the spacer <NUM> and thereby to the terminals <NUM>, <NUM> of the first and second resonators respectively. In this way one of the input / output terminals <NUM> of the resonators <NUM>, <NUM> is connected to a ground plane that surrounds a substantial part of the device.

The other end cap <NUM> may also have an electrically conductive outer surface which is electrically bonded to the shell <NUM>, extending the ground plane to the end <NUM> of the device, as well as providing a hermetic seal. The end cap <NUM> may also have an electrically insulating outer surface which is bonded to the shell <NUM> by an electrically insulating material, such as glass, ceramic or glue to provide a hermetic seal. However, this end cap <NUM> also provides an electrical feed-through <NUM> which is electrically continuous with the spacer <NUM> and thereby to the terminals <NUM>, <NUM> of the first and second resonators <NUM>, <NUM> respectively. The electrical feed-through <NUM> is electrically isolated from the ground plane formed by the shell <NUM>, the end cap <NUM> and the outer surface of the end cap <NUM>.

The electrical feed-through <NUM> is couplable to an antenna <NUM> of a suitable type, examples of which are described below.

The shell <NUM> may generally take the form of a flexible tube or sleeve <NUM> having a flattened circular cross-section as illustrated in <FIG>. The sleeve has first and second end regions <NUM>, <NUM> which, in the assembled pressure sensing apparatus, are coupled to and supported by the end caps <NUM>, <NUM>. The central region <NUM> may be generally unsupported except by its contact with the upper face <NUM> of the deflectable substrate <NUM> and by its contact with the base or underside of the rigid substrate <NUM>. At least the upper parts of the sides <NUM>, <NUM> of the central region <NUM> are free to flex unsupported by, and not in contact with, the support structure defined by the rigid substrate <NUM>, the spacers <NUM>, <NUM> and the end caps <NUM>, <NUM>. In this way, the flexible membrane exemplified by the shell <NUM> is free to flex not only in its top region <NUM> but also in the central part of its side regions <NUM>, <NUM>. Inward deflection of the top region <NUM> which would otherwise tend to be inhibited by the need for the membrane to stretch to enable the required out-of-plane movement is in this instance substantially eased by the ability of the side walls / side regions <NUM>, <NUM> to also deflect inwards and / or outwards thereby reducing tension in the membrane top region <NUM>. <FIG> illustrates total surface displacement and <FIG> illustrates surface von Mises stress indicating the spreading of the stress from the beam deflection to the side regions of the <NUM>, <NUM> of the shell <NUM>. The ability of the side walls / side regions <NUM>, <NUM> of the shell to deflect inwards and / or outwards can assist the top region <NUM> of the shell <NUM> to deflect inwards by (i) a reduced or eliminated stretching requirement of the shell material because the side regions can also deflect inwards, and / or (ii) enabling the shell to reduce the curvature of the side regions <NUM>, <NUM> otherwise required to accommodate inward deflection of the top region, e.g. by allowing an outward bulging of the side regions. The exact extent and relative location of inward and / or outward deflection of the various parts of the side regions will depend on the properties of the shell material, for example, elasticity and stiffness as well as the shell geometry. In a general aspect, the inward and / or outward movement of the side regions <NUM>, <NUM> of the shell enables an overall reduction in stress in the shell undergoing deflection in the top region <NUM>, thereby decreasing resistance to deflection of the top region and increasing sensitivity of the pressure sensor.

The shell forming the flexible membrane may be fabricated using any suitable method. For example, a metal shell may be extruded from, for example, nitinol or other shape memory alloy, and may have a thickness of <NUM> to <NUM> microns for example. This can be thinned, using a suitable technique such as polishing or electropolishing, to between <NUM> and <NUM> microns thickness, for example. The thinning process may be implemented selectively in target areas for optimum flexibility response.

The sensitivity of the pressure sensing device described is improved as the pressure-induced forces required to deflect the beam-configured first sensor device <NUM> via the membrane / shell <NUM> can be reduced from the prior art configurations shown in <FIG>. In the arrangement of <FIG>, the flexible membrane provided by the shell is held rigidly where it contacts the longitudinal ends at the rigid structure and the rigid base of the substrate <NUM>, but not where the flexible membrane extends along sides of first sensor device <NUM> and the chamber <NUM>, to thereby provide additional flexibility.

There are numerous ways of fabricating the shell and connecting it to the rest of the assembly.

In the example of <FIG>, the shell <NUM> may be a metal or metal alloy sleeve sealed to the two end caps <NUM>, <NUM> by, e.g. soldering or welding to form the hermetic seal and thereby defining an envelope which hermetically seals the first and second sensor devices <NUM>, <NUM> and the chamber <NUM> that is disposed between them. The metallic nature of the sleeve <NUM> and the direct soldering or welding to the metal or metallised surfaces of the end caps provides the ground plane for the antenna.

A similar arrangement can be provided where the shell <NUM> is made from a suitable polymer whose outer or inner surface is metallised to provide the ground plane as well as to provide better long term hermeticity. A suitable metallisation layer may be, for example <NUM> - <NUM> microns in thickness. Completing the seal of this shell <NUM> to the end caps <NUM>, <NUM> may be effected differently, as welding or soldering of the polymer is not possible. The shell may instead be bonded to the end caps <NUM>, <NUM> by a suitable adhesive to form an at least temporary seal. The seams between the shell <NUM> and the two end caps <NUM>, <NUM> may be covered by electroplating or vapour deposition to ensure long term hermeticity and electrical continuity. The adhesive may be selected to resist electrolyte ingress during the electroplating process.

In another arrangement, the shell <NUM> may comprise a thin-walled glass tube to provide the hermetic seal while being thin enough to be flexible to provide for deflection under pressure. The assembly of <FIG> may, for example, be inserted into a glass tube of suitable cross section, so that the metal spacers <NUM>, <NUM> are respectively protruding from each end. The hermetic seal can be completed by heating the glass tube at each end. This may soften / melt the glass tube sufficiently to seal around the metal spacers <NUM>, <NUM> to form a capsule while leaving a portion of each spacer extending longitudinally out of the capsule, as shown in <FIG>. The portions <NUM>, <NUM> of the two metal spacers extending out of the hermetically sealed capsule <NUM> form the electrical contacts. As seen in <FIG>, this arrangement may also include a metal cap <NUM> coupled to the contact portion <NUM> and a ceramic cap <NUM> coupled to the contact portion <NUM>.

Also as seen in <FIG>, one of these contacts <NUM> can be electrically connected to an antenna <NUM> similar to the arrangement of <FIG>. The other contact <NUM> can be coupled to a ground plane extending around the capsule by the metal cap <NUM> or <NUM>. The ground plane may also be formed by metallising the outer surface of capsule <NUM>.

The ground plane can generally be formed as a sleeve around the capsule. In the example of <FIG>, a sleeve <NUM> may be a rigid metal tube into which the capsule, e.g. capsule <NUM> or shell <NUM>, is inserted and the ground plane contact <NUM> of the end cap <NUM> can be welded, soldered or otherwise connected thereto. The sleeve <NUM> may be formed of the same material as the spacers <NUM>, <NUM>. Using the same electrically conductive materials for parts of the apparatus that are exposed to, for example, ionic liquids in the body - such as live tissues - may be particularly advantageous to avoid chemical reactions where two dissimilar metallic materials are electrically connected together. If two dissimilar metals are electrically connected and placed into such an ionic medium, a short circuit battery may be formed resulting in chemical reactions as well as corrosion of the parts.

The distal end <NUM> of the metal tube <NUM> that is coupled to the ground plane contact <NUM> (e.g. the non-antenna end) or the distal end of the end cap <NUM> (or both) may be configured with a suitable profile such as rounded or tapered as seen in <FIG> to prevent tissue damage during implantation, where it may form the distal end of the pressure sensing apparatus as it is introduced into the body by a suitable catheter / guidewire arrangement. Part of the metal sleeve <NUM> may be removed or may contain holes <NUM> or other apertures to allow transmission of ambient pressure to the capsule surface.

The glass capsule body may be further encapsulated in a thin polymer layer, e.g. silicone or Parylene C for protection.

<FIG> shows a schematic perspective view of a partially assembled pressure sensing apparatus similar to that shown in cross-sectional view in <FIG>. The tube forming the shell <NUM> may be slid on to or otherwise applied to the body <NUM> incorporating the sensor device <NUM>. End cap <NUM> is soldered or otherwise electrically connected to the distal end <NUM> of the shell <NUM>, as described above, to form the ground plane <NUM>. An antenna <NUM> is connected via the end cap <NUM>.

Although the spacers <NUM>, <NUM> described above have been described as being formed of metal to provide a convenient electrical connection to each of the terminals / pads <NUM>-<NUM>, it will be understood that non-conducting rigid material such as ceramic could be used and separate provision for electrical connection to the first and/or second sensor devices <NUM>, <NUM> made using conductive coatings or conducting tracks / vias extending along or through the spacers. Such an arrangement may be useful, for example where the first sensor device is formed as a cantilever beam structure such that both the first and second input / output terminals <NUM>, <NUM> and their respective tracks <NUM>, <NUM> must be connected to a ground plane and antenna via one end of the apparatus since the other end of the beam is unsupported.

Suitable antenna connection arrangements can be particularly important when using the pressure sensing apparatus for intracranial implants.

An antenna such as antenna <NUM> of <FIG> can be made from gold plated or cladded nitinol wire which can also be used mechanically to insert the implant. As seen in <FIG> and <FIG>, an antenna <NUM> extends from a pressure sensing apparatus <NUM> such as described above, and the end of the antenna <NUM> may be terminated with a continuous or discontinuous loop <NUM> or other laterally-extending portion. The loop may be configured with a suitable diameter such that it can remain on the skull <NUM> subcutaneously, i.e. beneath the cutaneous layer <NUM>. Preferably, the antenna material is chosen to have superelastic shape memory properties such that the loop <NUM> can be elongated and placed in a pusher tube for implantation. Once the implant is inserted, the loop <NUM> then re-expands to its pre-formed shape and sits on the skull <NUM>, positioning the implant at the desired depth. The skin <NUM> can be closed over the antenna <NUM>, <NUM> thereby substantially reducing risks of infection, and the pressure sensing apparatus can be activated and interrogated by a suitable transceiver (e.g. mounted within a cap worn by the patient) in communication with the wireless RF interface provided by the antenna <NUM>.

The arrangement of <FIG> exemplifies an antenna comprising a resilient material having an expanded shape memory configuration which defines a substantially linear axial portion such as shaft <NUM> and an off-axis, laterally-extending portion such as loop <NUM>. The material at least of the loop <NUM> is resiliently bendable into a substantially linear configuration for delivery of the apparatus via a catheter. Other possible forms of antenna are possible. For example, with reference to <FIG>, the antenna <NUM> may comprise a radially extending portion <NUM> coupled to the shaft <NUM> and extending laterally away from the shaft axis, which supports a continuous or discontinuous loop <NUM> around the shaft or device axis as exemplified in <FIG>. The antenna <NUM> may comprise a discontinuous loop <NUM> coupled directly to the antenna shaft <NUM> as exemplified in <FIG>. The antenna <NUM> may comprise a spiral element <NUM> coupled directly to the antenna shaft <NUM> as exemplified in <FIG>. The spiral element <NUM> may be used for antenna tuning. The antenna <NUM> may comprise a loop <NUM> coupled directly to the antenna shaft <NUM> and the antenna shaft may include a coil <NUM> forming part of the shaft <NUM>, e.g. for antenna tuning, as exemplified in <FIG>. In a general aspect, each of the antenna designs described above may be considered to exemplify an antenna with an axially extending portion (e.g. shaft <NUM>) and an off-axis laterally extending portion such as the portion <NUM> or the loop portions that extend away from or around the axially extending portion as found in portions <NUM>, <NUM>, <NUM>. It will be understood that various features of the antennae of <FIG>, <FIG> and <FIG> may be combined in different ways.

In another arrangement, the rigid support structure may comprise a housing having a trench within which the first sensor device is positioned. <FIG> is a simplified cross-sectional side view of the pressure sensing apparatus <NUM> described with reference to <FIG> in which first <NUM> and second <NUM> sensor devices are spaced apart from one another in a face-to-face configuration via metal spacers <NUM>, <NUM> to define a chamber <NUM> therebetween. The metal spacers <NUM>, <NUM> are electrically connected to the SAW resonators of the first <NUM> and second <NUM> sensor devices to enable the formation of external connections. In this respect, the metal spacers <NUM>, <NUM> may be referred to as "electrical terminals" in this example.

<FIG> shows one example of a housing <NUM> suitable for use with the pressure sensing apparatus <NUM> of <FIG>. The housing <NUM> may be made from a suitable insulating material such as sapphire, biocompatible ceramic (e.g. zirconia) or a hard biocompatible polymer, and may comprise first <NUM> and second <NUM> electrically conductive portions separated from one another by an electrically insulating portion <NUM>. This can be achieved by coating the surface of the housing <NUM> with a conductive layer of metal (e.g. gold) and subsequently removing the conductive coating in the area <NUM> shown between the two dark lines <NUM>. Alternatively, if the housing <NUM> is formed from a ceramic, selected portions <NUM>, <NUM> of the housing <NUM> may be made conductive by selectively adding a conductive material to these portions <NUM>, <NUM> before sintering the ceramic.

In the example shown in <FIG>, the first electrically conductive portion <NUM> is substantially longer than the second electrically conductive portion <NUM> to form a ground plane. Furthermore, the trench <NUM> of the housing <NUM> is narrower at the ends <NUM>, <NUM> to support the metal spacers <NUM>, <NUM> and is wider therebetween to enable unrestricted displacement of the deflectable portion at one or two sides of the first sensor device <NUM> (ideally with a clearance of <NUM>-<NUM> between the sides of the first sensor device <NUM> and the sides of the trench <NUM>).

<FIG> shows the housing <NUM> of <FIG> electrically coupled to an antenna <NUM>. In this example, the antenna pole <NUM> is inserted and integrated into the housing <NUM> such that the antenna pole end <NUM> is electrically exposed. This allows electrical connection of the antenna pole <NUM> to the second (shorter) electrically conductive portion <NUM>. In this way, one of the metal spacers <NUM> (and hence one terminal of the SAW resonators that are connected in parallel) can be electrically connected to the ground plane and the other <NUM> can be electrically connected to the antenna pole <NUM>. The combination of the antenna pole <NUM> and the ground plane forms an electromagnetic radiating structure and enables the SAW resonators to receive and transmit electromagnetic waves.

<FIG> show assembly of the pressure sensing apparatus <NUM> of <FIG> with the housing <NUM> of <FIG>. Once the pressure sensing apparatus <NUM> has been positioned within the trench <NUM> of the housing <NUM>, the first <NUM> and second <NUM> electrically conductive portions may be conductively bonded to a respective metal spacer <NUM>, <NUM> to connect the SAW resonators to the ground plane and antenna <NUM>.

As mentioned previously, the chamber <NUM> formed between the first <NUM> and second <NUM> sensor devices must also be hermetically sealed from the external environment whilst allowing the external pressure to exert a deflecting force on the beam of the first sensor device <NUM>. This can be achieved by encapsulating (substantially or completely) the assembled structure <NUM> of <FIG> in a flexible polymer membrane to form the envelope.

<FIG> shows a cross-sectional end view of the assembled structure <NUM> of <FIG> after encapsulation. The flexible polymer membrane <NUM> is preferably made from an elastomer (e.g. silicone or polyurethane), but other polymers may be used instead. As illustrated in <FIG>, the flexible polymer membrane <NUM> extends around all sides of the assembled structure <NUM> (and thus the pressure sensing apparatus <NUM>). In this way, deformation of the flexible polymer membrane <NUM> (by indentation on top <NUM> and stretching or reconfiguration elsewhere) allows for ease of deflection of the beam of the first sensor device <NUM> under pressure.

The envelope may be preformed as a casing into which the assembled structure <NUM> is inserted before being sealed. Alternatively, the envelope may be formed on the assembled structure <NUM> using polymer coating techniques to deposit the flexible polymer membrane <NUM>. For example, the assembled structure <NUM> may be dip-coated in a polymer solution. In this scenario, the ingress of polymer into the trench <NUM> of the housing <NUM> should be prevented otherwise the polymer may restrict the beam deflection and dampen the sensed signal. Prevention is possible by adjusting the viscosity of the polymer solution with respect to the clearance between the sides of the first sensor device <NUM> and the sides of the trench <NUM>. The larger the clearance and lower the viscosity, the easier it is for the solution to enter the trench <NUM>. In practice, the clearance should be as narrove as possible whilst allowing for the full range of displacement.

The flexible polymer membrane <NUM> may be deposited only around the housing <NUM> leaving a substantial part of the antenna <NUM> uncovered, or it may encapsulate the whole structure <NUM> including the antenna <NUM>. Furthermore, once the flexible polymer membrane <NUM> has been deposited, the hermeticity of the resulting envelope may be increased by depositing one or more additional layers of material on top of the flexible polymer membrane <NUM> to cover any pores in the polymer. This can be achieved using thin film coating techniques (e.g. using ALD or PVD) to deposit an additional layer of polymer (e.g. Perylene C), metal (e.g. gold), inorganic material (e.g. an oxide or nitride) or a combination of these.

Providing a sensor of adequate pressure sensitivity and small width is challenging, particularly when such a deeply implanted sensor should be readable wirelessly. This is not least due to the fact that the signal received from the implant is often weak and embedded in noise. The design described above may resolve to <NUM> mmHg pressure changes using the described SAW-based pressure sensor with a SAW resonator formed on a quartz substrate. The wireless operation and the small size enable it to be fitted into the tip of a catheter and to wirelessly communicate with an external device. Since there need be no cable penetration through the scalp <NUM>, the complications of infection, breakage and dislodgement can be eliminated or substantially reduced, resulting in improved patient mobility and the opportunity for continuous ICP measurement in a ventricle or the brain tissue. The pressure sensing apparatus can be implanted as a standalone unit, if required.

The pressure sensing apparatus as described can turn a shunt from a passive device to a smart device with a pressure measurement function, which provides a baseline for adjusting a valve and detecting blockage. The measured pressure may also be used as feedback for active valve control. The device may benefit normal pressure hydrocephalus (NPH) patients. The current methods of NPH diagnosis are based on symptoms as well as MRI and CT scans. Although the "gold standard" of diagnosing NPH is an improvement of symptoms with ventricular shunting, an ICP monitoring device with or without drainage capability is likely to improve the NPH diagnosis and identify the candidates for permanent shunts. If a ventriculoperitoneal (VP) shunt is required, a conventional shunt can be connected to the pressure sensing apparatus as described herein. The device can be removed with minimal operation.

The pressure sensing device can offer a high quality factor and high operating frequency for battery-less SAW based sensors, allowing continuous monitoring using a light weight low power electronic reader. Other resonant sensors (such as MEMS based) may typically work at much lower frequencies (e.g. at least an order of magnitude lower) and have low quality factors (e.g. two orders of magnitude lower). This means that relevant electronic readers cannot operate in radiation mode but rather work by inductive coupling.

Sensor interrogation with this technology requires higher power and, as a consequence, the readers are bulkier and less suitable for use as a wearable technology.

The specific packaging methods for the pressure sensor allow for using a resonator as a beam structure. This means that the sensor width (compared to other resonator sensor packaging technologies) can be significantly smaller, while maintaining a high pressure sensitivity. This is particularly important for integration of the sensor with an intracranial shunt or direct implantation in the brain.

The combination of the above-mentioned features also allows for a low profile sensor antenna that does not require a bulky pick up loop, which would otherwise be required for inductively coupled systems. This is particularly advantageous for integration of the pressure sensing apparatus with shunts, as the antenna can be a very thin and short wire positioned inside a shunt tube in a manner that does not compromise the patency of the shunt tube. In case of a standalone implant, the antenna can be dimensioned for the desired sensor depth in brain tissue and can be easily fixed subcutaneously as described above.

The whole pressure sensing apparatus can be constructed with a form factor suitable for placement at the end of a shunt tube that may be implanted into the brain. A short flexible tube may be inserted into the brain with the wireless and battery-less pressure sensing device therein. The short tube is connected to a long tube that extends subcutaneously into the abdomen where the CSF can be discharged and absorbed. Such a shunt could include a valve arrangement to enable control of the flow of CSF from brain to abdomen. The pressure sensing apparatus as described herein may be configured to control the valve to maintain an appropriate intracranial pressure, e.g. in a closed loop feedback configuration, or the valve may operate under control of the pressure sensing apparatus when initiated from an externally applied signal.

Various modifications and adaptations of the pressure sensing apparatus as described above are possible.

Although the flexible membrane provided by the shell <NUM> has been illustrated as extending around the entire periphery of the first and second substrates <NUM>, <NUM> of the first and second sensor devices <NUM>, <NUM> and the gap <NUM> between them, it will be understood that the shell defining the flexible membrane could extend around the sides of the first substrate and the gap <NUM> between the substrates, and be secured to the rigid substrate <NUM> along its sides. Although optimum performance of the flexible membrane provided by the shell can be provided by having the membrane free to flex inwardly and / or outwardly on both sides of the device in at least the central region as discussed above, it will be understood that having the freedom of movement on only one side may also be advantageous.

Other ways of improving hermeticity of a polymer shell <NUM> may include incorporation of oxide or nitride layers (e. g by Atomic Layer Deposition process) on the polymer layer. Mutilayers of polymer, metal, oxide or nitride may also be advantageous.

It will be understood that deploying the beam sensor in a cantilever configuration may require that the tracks <NUM>, <NUM> connecting to the inputs / outputs <NUM>, <NUM> of the sensors be connected to the same, cantilevered end of the rigid structure with suitable arrangements for antenna and ground plane connections.

Although the examples of a pressure sensing apparatus given above have used an acoustic wave device whose resonant frequency may change as a result of flexing of the device substrate under pressure changes, it will be recognised that other forms of sensing device may be used that can provide a transducer output as a function of flexing / displacement of the substrate on which they are formed. For example, a capacitive sensor or piezo-resistive sensors could be used.

The pressure sensing devices as described herein can readily be formed into capsules of length <NUM> or less, and width / diameter of <NUM>-<NUM> or less.

Although examples have been described in the context of use for intracranial pressure monitoring applications, the pressure sensing apparatus arrangements as described herein can be readily used, modified and/or adapted for use in other applications.

Claim 1:
A pressure sensing apparatus comprising:
an elongate first sensor device (<NUM>) in a beam configuration supported at at least one longitudinal end (<NUM>, <NUM>) by a rigid support structure (<NUM>, <NUM>, <NUM>) and having a deflectable portion;
a chamber (<NUM>) disposed adjacent a first, internally-facing to the chamber, face (<NUM>) of the first sensor device;
an envelope hermetically sealing the first sensor device and chamber from an ambient environment;
the envelope comprising a flexible membrane disposed over and coupled to a second, externally-facing from the chamber, face of the first sensor device, wherein the flexible membrane extends along two sides (<NUM>, <NUM>) of the first sensor device and the chamber.