Patent Publication Number: US-2022211269-A1

Title: Methods and Devices for Implantation of Intraocular Pressure Sensors

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. patent application Ser. No. 16/248,699 filed Jan. 15, 2019 (now U.S. Pat. No. 11,202,568); which is a Continuation of U.S. patent application Ser. No. 14/789,491 filed Jul. 1, 2015 (now U.S. Pat. No. 10,213,107); which claims the benefit of U.S. Provisional Appln No. 62/019,826 filed Jul. 1, 2014; the disclosures which are incorporated herein by reference in their entirety for all purposes. 
     The present application is related to co-assigned U.S. patent application Ser. No. 14/789,839 entitled “Hermetically Sealed Implant Sensors with Vertical Stacking Architecture” filed Jul. 1, 2015 (now U.S. Pat. No. 10,973,425) and U.S. patent application Ser. No. 14/789,942 entitled “Ultra Low Power Charging Implant Sensors With Wireless Interface for Patient Monitoring”; each of which is incorporated herein by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     This application relates generally to devices and methods for implanting an intraocular pressure (IOP) sensor within an eye of a patient, particularly by injecting the IOP sensor device within a patient&#39;s eye for monitoring and management of glaucoma treatment. 
     Glaucoma is a condition resulting in increased pressure within the eye that eventually leads to damage of the optic nerve that transmits images to the brain, which results in gradual vision loss. The increased pressure within the eye causes a loss of retinal ganglion cells in a characteristic pattern of optic neuropathy. A patient suffering from glaucoma typically experiences a build-up of aqueous fluid which increases the pressure inside the eye (i.e. intraocular pressure). Elevated IOP is one of the primary risk factors for developing glaucoma, which must be carefully monitored and controlled in treating glaucoma. As retinal ganglion cells are damaged by glaucoma, the visual signals from at least a portion of visual field are no longer reported to the brain, forming blind spots or scotomas. As glaucoma progresses and increasingly damages more nerve tissue in the optic nerve, vision loss continues as the scotomas increase in size and/or number. Failure to properly treat glaucoma and to reduce and monitor the IOP may cause irreversible vision loss. Untreated glaucoma, which affects one in 200 people under the age of fifty and 10% of those over the age of 80, is the second leading cause of blindness worldwide. As of 2012, about 60 million people suffer from glaucoma world-wide and it is estimated that, by 2020, about 80 million people will suffer from glaucoma. In addition, since a high percentage of people are over the age of 75 years old, and as the world-population ages and life-spans increase, it is expected that glaucoma patient populations will continue to increase. 
     IOP in a healthy human eye is generally between 10 mmHg and 20 mmHg. Glaucoma causes substantial increase in and/or variation in IOP than that experienced in a healthy eye. The IOP is determined largely by the amount of aqueous fluid entering and exiting the eye. Aqueous fluid is produced by the ciliary body to supply the lens and cornea with nutrients and carry away waste products. Normally, aqueous fluid flows between the iris and the lens, through the pupil and to the drainage angle before exiting the eye through a tissue called the trabecular meshwork in the drainage angle. If the aqueous fluid is produced at a rate faster than it drains, then the intraocular pressure will rise. An elevated intraocular pressure is associated with two major types of glaucoma: open-angle glaucoma and closed-angle glaucoma. In open-angle glaucoma, the drainage angle between the cornea and the iris is open and allows the aqueous fluid of the eye to reach the trabecular meshwork, but abnormalities in the trabecular meshwork reduce the outflow of aqueous fluid from the eye. In closed-angle glaucoma, obstructions within the trabecular meshwork prevent the aqueous fluid from draining properly out of the eye. 
     While the progression of glaucoma can be substantially halted in many patients using a variety of treatments, for example, medicines, prescription eye drops, shunts, and surgical procedures, failure to properly diagnose and/or monitor the IOP of a patient can drastically reduce the effectiveness of available treatments. Currently, glaucoma monitoring often uses infrequent IOP measurements obtained by a physician at a medical facility. For example, a typical patient may have their IOP measured on average four to six times per year by non-invasive techniques, such as tonometry. While tonometry techniques are generally low cost, easy, and non-invasive, a number of different types of errors can significantly reduce the accuracy of this diagnostic tool and as such potentially result in inappropriate diagnosis and/or ineffective follow-up medical treatment. 
     For example, at least some of these non-invasive clinical techniques may not detect elevated IOP levels (e.g., pressure spikes) as only a single point measurement is taken during an eye exam. Failure to continuously and/or frequently monitor IOP levels outside the eye clinic (e.g., more than four to six measurements per year) may lead to inaccurate detection of the patient&#39;s real IOP profile (e.g., real IOP may be higher or lower than measured IOP). Non-invasive measurements in some instances also lack accuracy as these devices measure pressure of the eye with an external sensor that provides an indirect measurement of the actual pressure inside the eye and are unable to capture the dynamic state of the disease in which there is a continuously changing IOP at low and high frequency rates with up to 12,000 spikes per hour. For example, factors that affect accuracy may include failure to account for anatomical differences, such as a patient&#39;s cornea thickness, scleral rigidity, or conical curvature, variances due to operator&#39;s use or technique, physiological influences, such as caffeine or alcohol use, or prior refractive surgery that may affect a patient&#39;s IOP, etc. Hence, the indirect IOP measurements from such non-invasive devices may differ from the actual IOP inside the eye (e.g., overestimated or underestimated) which may lead to inappropriate diagnosis and/or follow-up treatment. Further, it often inconvenient and impractical for patients to visit the eye clinic on a strict regular schedule for repeated IOP measurements. 
     Although implantable IOP devices have been proposed for direct IOP measurements on a daily basis, these first generation implants may also suffer from several drawbacks which in turn may result in indirect and/or inaccurate measurement of IOP and inappropriate medical treatment of glaucoma. For example, the IOP devices may be too large or bulky in dimension, size or shape to be safely and effectively placed entirely within a desired location or structure of the eye for direct measurement of IOP. Further, some devices may be extremely invasive, requiring major surgery for implantation and/or complicated positioning of multiple components which are each implanted in different structures or areas of the eye, which unnecessarily increases patient risk and/or injury and total healthcare costs. 
     Further, some IOP implantable devices may utilize pressure ports which are susceptible to sensing inaccuracies or require direct implantation within certain anatomical locations, such as the anterior chamber, posterior chamber, suprachoroidal space, or cornea of the eye which may lead to unanticipated complications. Also, some of these devices may not be well suited for chronic implantation due to IOP implant design issues of water ingress and/or thermal stress (e.g., associated with polymer packaging), which in turn precludes continuous monitoring of IOP. Such proposed flexible sensors also have issued of degraded stability. In some instances, some IOP devices also suffer from poor calibration and/or monitoring is not adjustable so as to further result in inaccurate IOP detection levels. 
     Accordingly, it would be desirable to provide improved implant devices and methods of implantation that overcome at least some the above mentioned shortcomings. In particular, it would be desirable to develop ultra-miniature implantable IOP devices that accurately, continuously, and adjustably monitor IOP levels. Ideally, such devices should directly measure IOP pressure levels and can be safely and effectively implanted entirely within a desired location within the eye quickly and easily in an outpatient environment, such as the physician&#39;s office, without invasive major surgery. Such devices should further allow for chronic implantation so as to provide long-term stable and continuous IOP measurement profiles for appropriate diagnosis and follow-up therapy. In addition, there exists a need for improved methods of implantation for such devices within the eye that do not require surgical intervention and avoid damage to the sensitive structures of the eye. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides devices and methods for implanting an IOP sensor within the eye of a patient. In one aspect, such methods include injecting a sensor device within the eye to provide improved sensing and/or monitoring of IOP for use in glaucoma treatments. 
     Since the mechanisms contributing to the increase in intra-ocular pressure occur within the anterior chamber or adjacent thereto, conventional methods generally focus on measuring intraocular pressure within the anterior chamber. Because the anterior chamber is a particularly sensitive region, great care must be taken to avoid contacting the various parts of the anterior chambers, which may result in damage to the delicate structures therein and degrade visual function. In one aspect, the present invention relates to measuring IOP of the eye by measuring pressure within the vitreous body. Since the pressure within the anterior chamber pushes against and increases the pressure within the vitreous body, measurement of pressure within the vitreous body provides a relatively accurate pressure measurement of IOP of the eye. In certain aspects, the methods of measuring IOP include positioning a pressure sensor within the vitreous body such that the entire pressure-sensing membrane of the pressure sensor is maintained within the vitreous body. In one aspect, the IOP measurement of pressure within the vitreous body may be compared to and correlated with a pressure within the anterior chamber, which may be measured according to various other independent measurement methods. This comparison or correlation can determine any degradation or attenuation of the intraocular pressure, if any, as it is transmitted from the anterior chamber to the vitreous body. Studies suggest that pressure between the anterior and posterior chamber may equalize such that a change in pressure in the anterior chamber will be reflected in the posterior chamber with a slight time delay. Monitoring the anterior chambers directly is not worth the risk of affecting vision significantly or the associated liability. Even if there were a slight degradation or attenuation in IOP when measuring within the vitreous humour, the increased pressure may be detected with a continuous pressure profile that will satisfactorily quantify the increase in pressure in the anterior chamber. The proposed measurement locations can be readily validated across a range of animal models, which may also be used to adjust the sensor sensitivity if necessary. The actual pressure of interest is the pressure seen by the optical nerve head (ONH) which is the pressure that, if excessive, leads to apoptosis or death of retinal ganglion cells and axons. 
     In certain aspects, methods of implanting an IOP sensor within the eye include penetrating a distal tip of an injector or syringe through a conjunctiva and a sclera of the eye into the vitreous body and injecting the IOP sensor through the distal tip of the injector or syringe. Positioning the IOP sensor may include distally advancing the distal tip of the injector or syringe until a distal facing surface of the injector or syringe abuts against the conjunctiva. In some embodiments, the sensor device has a maximum thickness and width of about 600 microns or less to facilitate injection through a needle having a gauge of 19 or higher (e.g. 28 or 29 gauge) along an insertion axis in-plane with the sensor device. 
     In embodiments in which the sensor device is injected with a fluid filled syringe, methods may include positioning the injector or syringe within an or near an ora serrata region of the eye, for example the pars plana region in between the ora serrata and limbus, prior to penetrating the distal tip of the injector or syringe so as to avoid any optic structures of a retina of the eye. In some embodiments, the syringe may be positioned along the pars planar. In some embodiments, a fluid is injected into the conjunctiva prior to penetrating the distal tip of the injector or syringe, thereby creating a ballooned portion of the conjunctiva in the ora serrata region. The fluid may include an anesthetic to reduce sensitivity in the eye of the patient prior to penetrating the sclera with the distal tip of the injector or syringe. Typically, the distal tip of the injector or syringe is then offset by a small distance, such as about 2 mm or less, before penetrating the sclera. The ballooned portion facilitates deployment of the anchor along the sclera and covering of a proximal end of the device with the conjunctiva after implantation. Methods may further include releasing the IOP sensor from the syringe by displacing a fluid within the syringe. This release may be effected by proximally retracting the distal tip into the syringe. This approach allows the user to stabilize a position of the IOP sensor within the vitreous body during release from the syringe. 
     In some embodiments, methods of implantation include penetrating partly through a sclera of the eye with an instrument and advancing the IOP sensor into the partly penetrated sclera so as to complete penetration of the sclera with a distal tip of the IOP sensor. In some embodiments, this allows the release of the IOP sensor without penetrating the distal tip of a needle of the syringe through the sclera. For example, the distal tip may be inserted only partly into the sclera before advancement of the IOP sensor causes the IOP sensor device to penetrate through the remainder of the sclera and into the vitreous body. Such embodiments may include a distally tapered tip on the sensor device of sufficient strength and stiffness to penetrate through the sclera or at least a portion thereof. In some embodiment, the distal penetrating tip may be formed within a same layer or substrate as the one or more anchoring members formed at a proximal end of the sensor device. In other embodiments, the distal penetrating tip may be included within an outer casing, housing or “boat” structure that extends at least partly about the IOP sensor device. 
     In another aspect, methods of implantation further include anchoring of the sensor device within the eye by deploying one or more anchoring members against the sclera. The one or more anchoring members may be defined so as to be resiliently biased to extend laterally outward relative to the insertion axis. The outwardly extending anchors may be sufficiently flexible to be bent toward the insertion axis and constrained within the distal tip of the syringe when injected. Upon release from the distal tip of the syringe, the anchoring members are deployed proximally of the sclera so as to engage an outer surface of the sclera to inhibit movement of the IOP sensor further into the cortical vitreous body at the periphery of the posterior chamber and away from retina and ciliary body. 
     In one aspect, the one or more anchoring members and the IOP sensor are defined portions of a wafer or substrate of the sensor device. At least a portion of the IOP sensor may include a MEMs device formed by a wafer process. In such embodiments, anchoring may include engaging the sclera with the one or more anchoring members that are resiliently deflectable in a direction in-plane with the wafer in which the anchors are formed. In another aspect, the anchoring members may include one or more expandable mechanical features such as memory shaped alloy (e.g. Nitinol loop) that is clamped into a silicon wafer of the sensor device or may include a polymer expandable mechanical layer in the device. 
     In still other embodiments, the sensor device may include at least a first and second anchoring member such that anchoring includes deploying the first anchoring member along the sclera outside of the vitreous body and deploying the second anchoring member along the sclera inside the vitreous body so as to inhibit axial movement of the IOP sensor along the insertion axis in both proximal and distal directions after implantation. Each of the first and second anchoring members may include one or more anchoring members. In another aspect, anchoring against rotation of the sensor device may be provided by the shape of the sensor device itself. For example, in some embodiments, the sensor device may be formed with a cross-sectional shape that is not axi-symmetric along a longitudinal axis of the sensor device, for example, a square or rectangular cross-section, so as to inhibit rotation of the device along its longitudinal axis. This feature allows for improved charging and telemetry by allowing the user to stabilize the position at which the charging and telemetry components of the sensor device are positioned. 
     In another aspect, methods for extracting the implanted sensor device are provided. Such methods may include extracting the IOP sensor by attaching an instrument to an extraction feature coupled to a proximal end of the IOP sensor disposed outside the vitreous body. In an embodiment where the sensor device has first and second anchors, extraction may include pulling the extraction feature until an anchoring force provided by the at least second anchoring member is overcome. 
     Methods of anchoring an implant are also provided. Such methods may include inserting an implantable device through a distal tip of an injector or syringe along an insertion axis into a body tissue or body space within a patient, the device being formed, at least in part, by a wafer process and deploying one or more anchoring members of the device so as to extend laterally outward from the insertion axis to inhibit movement of the implanted device within the body tissue or body space. The one more anchoring members may include a portion of a wafer of the device defined so as to be resiliently deflectable in a direction in-plane with the wafer. Inserting the implantable device may include constraining the one or more anchoring members within the distal tip or a sheath in a position deflected toward the insertion axis, the insertion axis being in-plane with the wafer defining the one or more anchoring members. Deploying the one or more anchoring members may include releasing the one or more anchoring members from the constraining distal tube or sheath when the device is positioned in a desired position within the body tissue space. The sensor may be positioned within a target region in the patient in which a physiological measurement is desired by injecting the sensor into the target region so that the entire sensor portion is disposed within the region. In some embodiments, the method may include penetrating a distal tip of an injector or syringe through a tissue wall of the patient along an insertion axis, wherein the sensor is injected through the distal tip of the injector or syringe. Such injectable sensors may include any of the features described herein and may be implanted, anchored or extracted according to any of the methods set forth herein. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a method of implantation in accordance with embodiments of the invention. 
         FIGS. 2A-2D  illustrate sequential steps of an implantation method in accordance with embodiments of the invention. 
         FIGS. 3A-3C  illustrate an implantable sensor device before, during and after implantation in accordance with embodiments of the invention. 
         FIG. 4A  and  FIG. 4B  illustrate an implantable sensor device constrained within an implantation device and an associated cross-sectional view in accordance with embodiments of the invention. 
         FIGS. 5A-5B  and  FIGS. 5C-5D  illustrate an injector syringe used in two implantation methods, respectively, in accordance with embodiments of the invention. 
         FIGS. 6A-6B through 9A-9B  illustrate example implantable sensor devices and views of the example devices after implantation in accordance with embodiments of the invention. 
         FIGS. 10A-10C  illustrate sequential steps of an explantation of an implanted sensor device using an explantation tool in accordance with embodiments of the invention. 
         FIGS. 11-13  illustrate examples of an implantable sensor device in accordance with embodiments of the invention. 
         FIGS. 14A-14C  illustrate sequential steps of an example implantation method in accordance with embodiments of the invention. 
         FIGS. 15A-17C  illustrate an example implantable sensor devices in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an overview illustration of a sensor implantation method in accordance with embodiments of the present invention. In particular, the depicted method relates to implantation of an IOP sensor device  10  within an eye  1  of a patient by injecting the IOP sensor device  10  into the eye with a fluid-filled syringe  20  or injector. In one aspect, the IOP sensor device is positioned within the vitreous body of the eye  1  by penetrating the conjunctiva and sclera with a distal tip of a needle of a syringe  20  along insertion axis I extending through the ora serrata region  2 . Implanting the sensor device by injection at this location is advantageous over conventional implantation methods as it avoids the potential for damaging the delicate structures within the anterior chambers and as well as damage to the photo-sensitive tissues of the retina. 
     In one aspect, the injectable sensor can be implanted in a physician&#39;s office without surgery, such as by a relatively simple injection procedure using a standard needle size (e.g. 19 gauge). Once implanted the sensor device can provide continuous monitoring, up to one week or several weeks between charges. The system may include an external patient data acquisition unit that is used to charge the implanted sensor, collect and store data from the implanted sensor, and transmit the collected IOP data to a data server for further analysis and monitoring (e.g. the cloud or other server). The data may be available to the patient and/or the patient&#39;s treating physician at any time. The external patient data acquisition unit may be incorporated into a personal mobile device, such as a smart-phone. These aspects are further described in U.S. Provisional patent application Ser. No. 14/789,942 entitled “Ultra Low Power Charging Implant Sensors with Wireless Interface for Patient Monitoring.” 
       FIGS. 2A-2D  illustrate sequential steps of a method of implanting an IOP sensor within a vitreous body of an eye  1  in accordance with the approach described above in  FIG. 1 . As shown in  FIG. 2A , a fluid is injected into the conjunctiva  3  in the ora serrata region of the eyeball through injection needle  40  to form a ballooned portion  4  of the conjunctiva. The fluid may be saline and/or may include a numbing or anesthetic to reduce sensitivity in the eye  1  prior to penetrating the layers of the eye during implantation. The conjunctiva is the mucous membrane that lines the inner surface of the eyelids and is continued over the forepart of the eyeball. The ballooned portion facilitates deployment of the sensor device against the sclera and allows the conjunctiva to cover the proximal anchoring portion of the sensor device after implantation, as will be described in further detail below. 
     As shown in  FIG. 2B , a distal tip  21  of the fluid-filled syringe or injector  20  is penetrated through the ballooned portion  4  of the conjunctiva and through the sclera  5  and the choroid  6  until the distal tip  21  is positioned within the vitreous body  7 . The sclera  5  is the dense fibrous opaque white outer coat enclosing the eyeball except the part covered by the cornea (not shown), while the choroid  6  is the vascular layer extending between the retina  8  and the sclera  5  to the ciliary body and iris (not shown) of the eye  1 . The IOP sensor  10  is disposed within the distal tip  21  of the fluid-filled syringe and may include one or more anchoring members  12  constrained within the distal tip to be deployed upon release (e.g., self-expanding). The distal tip  21  is advanced distally along the insertion axis until a distal facing surface of the syringe  22  abuts against the ballooned portion  4 . The distal tip  21  is then retracted into the syringe  20  such that displacement of fluid within the syringe causes the sensor device  10  to maintain its position while the distal tip  21  is retracted, thereby releasing the sensor device from the syringe or injector  20 . 
     A shown in  FIG. 2C , after release from the distal tip  21  of the syringe  20 , the anchoring members  12  resiliently extend laterally outward from the insertion axis against the outer surface of the sclera  5 . This anchoring configuration substantially maintains the position of the sensor diaphragm s of the device  10  at the desired location so that an IOP sensor near a distal end of the sensor device remains entirely within the vitreous body, such as shown in  FIGS. 2B and 2C . By extending the anchoring members along the sclera outside the vitreous body, the anchoring members  12  prevent the sensor device  10  from potentially slipping into the vitreous body, which could cause damage to the retina or optic nerve  9 . 
       FIG. 2D  shows the sensor device  10  implanted within the eye. In certain aspects, the sensor device  10  is configured such that a particular alignment (e.g. rotational) is not required to facilitate charging and/or wireless communication with the device. For example, so long as the sensor device  10  is implanted into the eye so that the sensor s is disposed within the targeted region and the sensor device  10  is anchored near the sclera, the charging and communication coils are in close enough proximity such that an external device positioned near the eye can establish sufficient magnetic coupling so as to charge and/or communicate with the device. Nonetheless, in another aspect, for certain configurations, it may be useful to provide methods that control alignments/orientation for the sensor device when implanted for various reasons, such as to optimize charging or improve communication. In such cases, the orientation of the can be controlled by providing the sensor device in a substantially fixed or at least a known orientation within the distal tip such that controlling the orientation of the syringe or injector during implantation controls orientation of the sensor  10 . For example, if the sensor orientation is known, the syringe can be marked (e.g. a line or arrow) so that the physician can visually align the syringe in a desired location to control orientation of the implanted sensor device  10 . 
       FIGS. 3A-3C  illustrate an example sensor device  10  having two anchoring members at a proximal end thereof, which have a bias in a lateral direction. As shown in  FIG. 3A , the anchoring members  12  extend laterally outward from an insertion axis i along which the sensor  10  is implanted. Although the anchoring members are shown as extending substantially perpendicular to the insertion axis, it is appreciated that the anchoring members could configured to extend along other angles (e.g. 30, 45 or 60 degrees from the insertion axis) according to a desired anchoring configuration or as needed for a particular application. In one aspect, the sensor device is formed, at least in part, using wafer processing methods, such that the anchoring members may be defined as features of a wafer or substrate of the sensor device. For example, the sensor device may be formed on a silicon substrate and the anchoring members may be defined as features of the silicon substrate using wafer processing methods, such as deep etching. In addition, various wafer fabrication methods may be used to round or soften any sharp corners of the sensor device to avoid damage to eye tissues after implantation. 
     In one aspect, the sensor device is formed, at least in part, on a rigid substrate, such as a silicon wafer. Although the substrate may be substantially rigid, portions of the substrate, such as those portions defining the one or more anchoring members, may be processed so as to alter a mechanical property as desired. For example, a wafer processing method may be used to define the anchoring members with a desired thickness and/or width so that the anchoring members become semi-rigid or flexible. This allows the one or more anchoring members to be sufficiently flexible in a direction in-plane with the device so as to bend alongside the sensor device when constrained within a distal tip of a needle, such as shown in  FIG. 3B . Preferably the sensor device is sufficiently small enough to be injected through a needle (e.g. gage of 19 or higher) along a direction in-plane with sensor device. Once the distal tip  21  of the syringe is retracted, the anchoring members  12  resiliently return to their laterally extended configuration, as shown in  FIG. 3C , so that when pushed distally along the insertion axis, the anchoring members engage an outer surface of the sclera, thereby preventing further distal movement of the sensor device. In one aspect, the conjunctiva covers and heals over the proximal surface of the sensor device such that the conjunctiva inhibits movement of the implanted sensor device in the proximal direction while the anchoring members inhibit movement of the implanted sensor in the distal direction. 
       FIG. 4A  illustrates a detail view of a sensor device  10  having two resiliently deflectable anchoring members  12  at a proximal end of the device  10 , which are constrained within the distal tip  21  of the syringe or injector.  FIG. 4B  illustrates a cross-sectional view of the sensor device of  FIG. 4A  constrained within the distal tip  21  of the syringe. As can be seen, the sensor device  10  comprises a vertically-stacked device formed, at least in part using wafer fabrication methods. The anchoring members  12  may be formed from a portion of a rigid substrate, such as a silicon wafer. Typically, the anchor is defined its own substrate material separate from the MEMS pressure sensor wafer. In one aspect, a MEMs device is supported on a rigid substrate that defines at least a part of the sensor. Forming the sensor on a rigid substrate may improve the integrity, accuracy and longevity of the MEMs device, while defining various other portions of the rigid substrate to be flexible allows for improved anchoring and/or deployment as needed for a given application. While a certain configuration of sensor, in particular a sensor having a vertically stacked architecture, it is appreciated that various other configurations may be utilized in accordance with the implantation methods described herein. For example, the device may include a vertically stacked architecture such as those shown in FIG. 5 of U.S. Non-Provisional patent application Ser. No. 14/789,839 entitled “Hermetically Sealed Implant Sensors with Vertical Stacking Architecture,” filed Jul. 1, 2015. 
     In certain aspects, the anchoring members are defined in a portion of the wafer that is thinned down (e.g. thinned to a 100 μm range) and that passes all electrical connections using TSV (through silicon vias). The electrical connections are sealed from the body fluid/media using a seal ring similar to the one used between MEMS wafer and CMOS wafer. The portion extending through the die area that will define the anchoring members will extend and typically etched using a process called DRIE to create the desired shape. This process may be done when all the wafers are bonded together so as to process them in a batch mode. The anchoring members are formed such that they are extended when the wafers are stacked, which creates a fairly wide spacing between each die. In one aspect, before dicing all edges are rounded with a wet etch (isotropic) to create soft corners and avoid sharp edges that may need to be eliminated to reducing tissue damage. To facilitate the dicing, the anchors are typically pre-etched (DRIE) and formed before the interposer is bonded. After the stack (CSP) is created via bonding, the dicing will cingulated each die but they will be left on blue-tape for pick and place. Each die with extended anchoring members is pushed into the syringe such that the anchoring members fold against the sidewall of the syringe. The angle and length of the anchoring members determines how much anchoring force the anchoring members provide against the tissue. In some embodiments, the anchoring members are formed to provide an anchoring force within a range of 100 to 1,000 μN. It is appreciated that while such configurations are particularly useful for anchoring of the described sensor device  10  within the eye, various other configurations of anchoring members may be utilized, including anchoring members with sharpened edges (e.g. barbs), so as to facilitate anchoring within various other tissues. Such configuration may be particularly useful in sensor devices implanted in various other locations within the human body. 
       FIGS. 5A-5B  depict a syringe  20  or injector used in accordance with the sensor implantation methods described above. The syringe  20  may include an outer cylinder having a distal facing surface  22  and an inner cylinder  23  slidably disposed within. The syringe  20  may be provided to a surgeon pre-sterilized and pre-filled with a fluid and a sensor device fixed within the distal tip  21  of the syringe. The sensor device  10  may be releasably positioned within the distal tip  21  and may be held in place by force of the resilient anchoring members constrained within. After penetration of the sclera with the distal tip, the distal tip  21  as well as the inner cylinder attached thereto can be retracted, such as by pulling a proximal feature  25  (e.g. thumb ring) attached to the inner cylinder in a proximal direction while maintaining the outer cylinder position with the fingers against stabilizer  24 . As the volume of the syringe is reduced, the fluid within the syringe is displaced, which moves the sensor device relative the distal tip during retraction so that the sensor device  10  substantially maintains its position and is released from the distal tip  21  as shown in  FIG. 5B . Upon release of the sensor device  10  from the distal tip  21 , the anchoring members  12  deploy laterally outward thereby anchoring the sensor device against the sclera. In some embodiments, the sensor device  10  may be pushed distally after deployment to facilitate engagement of the anchoring members  12  against the sclera and coverage of the anchoring members with the ballooned portion of the conjunctiva.  FIGS. 5C-5D  depict a syringe  20  or injector used in accordance with another implantation method, which is similar to that described above in  FIGS. 5A-5B  expect the needle or distal tip of the injector does not penetrate through the sclera. Rather, the distal tip is partly inserted into the sclera and the sensor device is advanced, in a similar manner as described above, such that a distal tip of the sensor device continues to advance through the sclera and into the vitreous body. 
       FIGS. 6A-9A  illustrate alternative example sensor devices having differing anchoring configuration and  FIGS. 6B-9B  illustrate each of the examples after implantation. In some embodiments, the sensor device  10  may include anchoring members disposed on opposite sides of the sclera, such as shown in FIGS.  7 A 1 ,  7 A 2 ,  8 A 1 ,  8 A 2  and  9 A, which may provide additional anchoring in both proximal and distal directions, as can be understood by referring to their deployed positions as shown in corresponding FIGS.  7 B 1 ,  7 B 2 ,  8 B 1 ,  8 B 2 , and  9 B, respectively. 
     In another aspect, the sensor device  10  may include an extraction feature  13  that facilitates extraction of the sensor after implantation. The extraction feature  13  may include a hole or loop feature that interfaces with an extraction tool  30  to allow the sensor device  10  to be pulled in a proximal direction and extracted from the eye. The extraction feature  13  may include a separate feature attached to the device  10  or may be integrally formed with the device itself. In embodiments where the anchoring members are defined portions of a substrate or wafer of the sensor device, the extraction  13  may be defined in a different portion of the same substrate or wafer. For example, as shown in  FIG. 10A , the anchoring members  12  are S-shaped portions of a rigid substrate, such as a silicon wafer, and the extraction feature  13  is a hole or opening formed within the same layer. The hole is dimensioned to allow an extraction tool  30 , such as a hook-like tool, to be inserted within the hole to allow the implanted sensor device  10  to be proximally pulled and extracted from the eye. 
       FIGS. 11-13  illustrate additional examples of sensor devices  10  in accordance with implantation methods of the invention.  FIG. 11  illustrates a sensor device  10  having two anchoring members  12  for engaging an outer surface of the sclera and two additional anchors  12 ′ for engaging an inner surface of the sclera, as well as a proximal extraction feature. In one aspect, the anchoring members  12 ,  12 ′ are dimensioned so that the anchoring members  12  that engage an outer surface of the sclera provide an anchoring force F 1  that is greater than an anchoring force F 2  provided by the anchoring members  12 ′ that engage an inner surface of the sclera. This configuration allows for improved anchoring in both distal and proximal directions along the insertion axis, while still allowing for extraction by proximally pulling the extraction feature  13  until force F 2  is overcome.  FIG. 12  illustrates a sensor device  10  having an S-shaped anchoring member  12 , which may be defined so as to provide a spring-like resistance when pushed against the sclera. 
       FIG. 13  illustrates a sensor device  10  having anchoring members  12  and an extraction feature  13  at a proximal end of the device  10  and a penetrating tip  15  formed at the distal tip. In some embodiments, the penetrating tip  15 , anchoring members  12  and extraction feature  13  are each formed within a different portion of the same layer, such as a rigid substrate (e.g. a silicon wafer) of the device. Each of the features may be formed using wafer processing techniques such as deep plasma etching. An example of how such a penetrating tip  15  can be used is shown in the implantation method shown in  FIGS. 14A-14C . 
     In some embodiments, the sensor device  10  can be implanted by injecting with a tool or device other than a fluid-filled syringe. In the example method shown in  FIG. 14A , the sclera is partly penetrated by a tool  50  leaving only a thin or weakened portion of the sclera. The sensor device is then advanced distally in this region until the penetrating tip  15  of the sensor device  10  penetrated through the sclera and into the vitreous body, as shown in  FIG. 14B . The sensor device is advanced until the anchoring members  12  engage the sclera, as shown in  FIG. 14C . The sensor device  10  may be advanced using a pusher tool  51  that interfaces with the extraction feature  13  such that the extraction feature facilitates implantation and extraction. In this embodiment, the extraction/implantation feature  13  may be oblong in shape, such as a slot, so as to improve stability and prevent rotation or twisting of the sensor device when being pushed through the sclera with the pusher tool  51 . In one aspect, this approach allows for a variety of alternative anchoring configurations since it does not require that the anchoring members be constrained within a distal tip of a syringe. For example, the anchoring members  12  could be relatively fixed in a laterally outward configuration, which may allow for thicker or more rigid anchoring members to be used if desired. 
       FIGS. 15A-15D  illustrate views of an alternative example sensor device  10  having a distal penetrating tip  15  for advancing through the sclera such as in the implantation method described above and an explantation feature  13  at the opposite end. This embodiment does not include the anchoring members described above. Such an embodiment may be useful in an application where anchoring is not desired or needed, for example, when used with a shunt that is self-anchoring within the eye. 
       FIGS. 16A-16C  illustrate views of an alternative example sensor device  10  formed with a vertically stacked architecture. A wafer or substrate extending through the device is defined to have a penetrating tip  15  for advancing through the sclera defined at the distal end of a wafer and an explanation feature  13  and anchoring members  12  defined in the same wafer at a proximal end of the device  10 . In one aspect, the vertically stacked architecture of this device may be the same as that shown in FIG. 5 of U.S. Non-Provisional patent application Ser. No. 14/789,839 entitled “Hermetically Sealed Implant Sensors with Vertical Stacking Architecture.” 
       FIGS. 17A-17C  illustrate various view of an alternative design of a sensor device  100  in accordance with embodiments of the invention. In this alternative design, the distal penetrating tip of the sensor device and the anchoring features are integral parts of a support structure or boat in which the wafer-stacked sensor device resides and is bonded thereto. While the distal penetrating tip and/or the anchoring features are described herein as being parts of an interposer layer or a boat support structure, it is appreciated that these components may be configured in various other ways, including separately formed structures that are bonded to the sensor device after fabrication. 
     The implantable device  100  comprises vertically stacked heterogeneous components, namely a first MEMS wafer or die  112  and a second CMOS wafer or die  114 . The first wafer  112  comprises at least a pressure sensor configured to measure IOP on a frequent or desired basis (e.g.,  1  sample per hour, 2-4 samples per day, etc.). The second wafer  114  comprises at least a digitizing ASIC. In some embodiments, the ASIC includes a microcontroller to enable firmware update of the implant, customization of sampling function (rate/window, accuracy, resolution, etc), auto-adaptive sampling to measured pressure, built-in self-test, error detection and correction, embedded diagnostics, broad use models with on-demand sample, streaming data and autonomous mode. The first MEMS wafer  112  is vertically stacked or disposed over the second CMOS wafer  114  so as to form a first hermetic seal. In particular, the vertical stacking of the wafers is configured to create a hermetically sealed cavity between the MEMS  112  and CMOS wafers  114  of the implantable device  100 . In some embodiments, the stack includes one or more additional wafers, for example one or more wafers adapted for use as a power source. Such embodiments may include a third wafer that includes a supercapacitor. In some embodiments, the stack further includes a fourth wafer that includes a battery. Such embodiments may utilize a power management scheme switching between the supercapacitor and battery in order to prove more efficient power discharge from a high impedance thin-film battery, such as a LiPON battery. An example of such a configuration is shown in the embodiment in  FIG. 17A . As can be seen in the cross-sections A-A and B-B in  FIGS. 17B and 17C , respectively, the stacked sensor device of  FIG. 7A  includes the MEMS  112  and CMOS wafers  114 , a decoupling capacitor wafer  113  and a thin film battery/energy storage wafer  115 . In one aspect, the wafers of the stack may be bonded together with low temperature Gold-Indium (Au—In) bond, while the cavities are formed using a silicon-to-silicon fusion bond. This configuration provides improved thermal budget management, while the silicon-to-silicon fusion bond provides long term vacuum stability (e.g. greater than 20 years). In this embodiment, rather than an interposer layer, the stacked device is placed within a support structure or boat  119 . The boat can include integral anchor features  121  for engaging proximal and distal sides of the sclera while the IOP sensor portion is maintained within the vitreous body. 
     In this embodiment, the IOP sensor device  100  includes a reference sensor  123  disposed adjacent the IOP sensor  122 , as shown in  FIG. 17C , the reference sensor  123  being configured for detecting one or more second order effects so as to allow cancellation of the second order effects from the pressure measurement obtained from the IOP sensor  122 . In one aspect, the IOP sensor and the reference sensor are of a substantially similar construction having a cavity, except the cavity of the IOP sensor is under vacuum such that the sensing diaphragm is sensitive to changes of pressure such that a signal from the IOP sensor corresponds to changes in pressure and the corresponding cavity of the reference sensor is filled such that a signal obtained from the reference sensor corresponds to the second order effects. In some embodiments, methods of obtaining IP measurements with such a device include cancelling the second order effects associated with the IOP measurement obtained from the IOP sensor using the signal from the reference sensor. Such methods can further include embedding data within the IOP measurement relating to the second order effects detected by the reference sensor, wherein the second order effects are associated with at least one of temperature and stress in the sensor device. 
     In some embodiments, the anchoring structure is formed in a separate support structure or “boat” in which the diced multi-wafer stack is placed and attached with low temperature metal alloy. An example of such a “boat” can be seen in the embodiment of  FIG. 17A . In some embodiments, this support structure or boat may also include a distally tapered tip  120  to facilitate penetration through the sclera during implantation and may also include one or more anchoring features  121 . Such features may be included as components with a mechanical function that clamps onto the sclera (e.g. a proximal and distal anchor on opposite sides of the sclera). The anchoring feature may also include an anchoring loop or extensions. Such anchoring features may be formed of Silicon, Titanium, shape memory alloy, or other suitable materials. In some embodiments, the boat is formed of a monolithic material and include side-walls that extend upwards, at least partly, along a thickness dimension of the stacked sensor device  100 . 
     In the foregoing specification, the invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. Various features and aspects of the above-described invention can be used individually or jointly. Further, the invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art.