Development of a completely encapsulated intraocular pressure sensor

Home monitoring for glaucoma

Glaucoma services are overwhelmed and struggling to accommodate current demand. Reducing the need for hospital based services would improve our ability to see those most at risk of vision loss, which could both reduce demand and improve patient outcomes. Digital technologies that provide opportunities for home monitoring of glaucoma progression have potential to contribute to solve these challenges and, potentially, improve glaucoma care. This article will review the literatures of well-established technologies that support home monitoring for glaucoma, specifically home tonometry (with rebound tonometry) and perimetry with Moorfields Motion Displacement Test and Melbourne Rapid Field.

Advances in diagnostic applications for monitoring intraocular pressure in Glaucoma: A review

Continuous intraocular pressure (IOP) monitoring for improving glaucoma diagnosis and treatment has remained a challenge for the past 60 years because glaucoma is the second leading cause of irreversible blindness worldwide. Several devices with different measurement principles and recently developed biosensors with semiconductor materials offer exciting properties. However, none of these devices for continuous IOP monitoring have been fully integrated into clinical practice, primarily due to technical problems. This review summarizes state-of-the-art biosensors developed for IOP monitoring by explaining their basic functions and applications, the main technology (pressure transductors, piezoresistive sensors, capacitive sensors, and resonant sensors), measurement approach (noninvasive, minimally invasive or invasive (surgically implantable)), and telemetry characteristics. To provide updated information for clinicians and researchers, we also describe the advantages and limitations of the application of these new sensors to eye care management. Despite significant improvements in IOP biosensor technology, the accuracy of their measurements must be improved to obtain a clear equivalence with actual IOP (measured in units of mmHg) to facilitate their clinical application. In addition, telemetry systems may be simplified to prevent adverse outcomes for patients and to guarantee the safety of stored data.

Telemetric Intraocular Pressure Monitoring after Boston Keratoprosthesis Surgery Using the Eyemate-IO Sensor: Dynamics in the First Year

PURPOSE

To analyze the dynamics of telemetrically measured intraocular pressure (IOP) during the first year after implantation of a Boston keratoprosthesis type I (BI-KPro) cornea and to compare agreement of telemetric IOP measurements with finger palpations.

DESIGN

Prospective, open-label, multicenter, single-arm clinical trial.

METHODS

In the ARGOS (NCT02945176) study, 12 individuals underwent implantation of an Eyemate-IO intraocular system. Follow-up after surgery took place 12 months later with 13 visits planned per patient. During BI-KPro surgery, an electromagnetic induction sensor ring enabling telemetric IOP data transfer to a hand-held reading device outside the eye was implanted into the ciliary sulcus with or without trans-scleral suture fixation. Comprehensive ophthalmic examinations and IOP assessments through the telemetric system were compared to IOP assessed by finger palpation by 2 experts.

RESULTS

Preoperative IOP measured by Goldmann tonometry was 13.4 ± 6.2 mm Hg. Telemetric IOP peaked at 23.1 ± 16.5 mm Hg at the first postoperative day. On day 5, mean IOP was 16.0 ± 5.2 mm Hg and 20.95 ± 6.5 mm Hg after 6-12 months. IOP estimation by finger palpation was grouped in 4 categories: normal, A; soft/hypotonic, B; borderline, C; and hypertonic, D. Mean telemetric IOP was 18.2 ± 6.1 mm Hg in category A, 8.9 ± 2.8 mm Hg in B, 22.4 ± 4.9 mm Hg in C, and 34.3 ± 11.0 mm Hg in D. Differences in mean telemetric IOPs per category were statistically significant (P < .001). Daily IOP fluctuations and peaks could be identified.

CONCLUSIONS

Telemetric IOP assessment seems to be able to identify postoperative IOP peaks and a longitudinal increase of IOP after BI-KPro surgery. IOP measurements using the telemetric Eyemate-IO sensor showed a satisfactory agreement with those of finger palpations by 2 experts.

Diurnal and 24-h Intraocular Pressures in Glaucoma: Monitoring Strategies and Impact on Prognosis and Treatment

The present review casts a critical eye on intraocular pressure (IOP) monitoring and its value in current and future glaucoma care. Crucially, IOP is not fixed, but varies considerably during the 24-h cycle and between one visit and another. Consequently, a single IOP measurement during so-called office hours is insufficient to characterize the real IOP pathology of a patient with glaucoma. To date IOP remains the principal and only modifiable risk factor for the development and progression of glaucoma. Only by evaluating IOP characteristics (mean, peak and fluctuation of IOP) at diagnosis and after IOP-lowering interventions can we appreciate the true efficacy of therapy. Unfortunately, a major limiting factor in glaucoma management is lack of robust IOP data collection. Treatment decisions, advancement of therapy and even surgery are often reached on the basis of limited IOP evidence. Clearly, there is much room to enhance our decision-making and to develop new algorithms for everyday practice. The precise way in which daytime IOP readings can be used as predictors of night-time or 24-h IOP characteristics remains to be determined. In practice it is important to identify those at-risk glaucoma patients for whom a complete 24-h curve is necessary and to distinguish them from those for whom a daytime curve consisting of three IOP measurements (at 10:00, 14:00 and 18:00) would suffice. By employing a staged approach in determining the amount of IOP evidence needed and the rigour required for our monitoring approach for the individual patient, our decisions will be based on more comprehensive data, while at the same time this will optimize use of resources. The patient’s clinical picture should be the main factor that determines which method of IOP monitoring is most appropriate. A diurnal or ideally a 24-h IOP curve will positively impact the management of glaucoma patients who show functional/anatomical progression, despite an apparently acceptable IOP in the clinic. The potential impact of nocturnal IOP elevation remains poorly investigated. The ideal solution in the future is the development of non-invasive methods for obtaining continuous, Goldmann equivalent IOP data on all patients prior to key treatment decisions. Moreover, an important area of future research is to establish the precise relationship between 24-h IOP characteristics and glaucoma progression.

Advances in Materials for Recent Low-Profile Implantable Bioelectronics

The rapid development of micro/nanofabrication technologies to engineer a variety of materials has enabled new types of bioelectronics for health monitoring and disease diagnostics. In this review, we summarize widely used electronic materials in recent low-profile implantable systems, including traditional metals and semiconductors, soft polymers, biodegradable metals, and organic materials. Silicon-based compounds have represented the traditional materials in medical devices, due to the fully established fabrication processes. Examples include miniaturized sensors for monitoring intraocular pressure and blood pressure, which are designed in an ultra-thin diaphragm to react with the applied pressure. These sensors are integrated into rigid circuits and multiple modules; this brings challenges regarding the fundamental material’s property mismatch with the targeted human tissues, which are intrinsically soft. Therefore, many polymeric materials have been investigated for hybrid integration with well-characterized functional materials such as silicon membranes and metal interconnects, which enable soft implantable bioelectronics. The most recent trend in implantable systems uses transient materials that naturally dissolve in body fluid after a programmed lifetime. Such biodegradable metallic materials are advantageous in the design of electronics due to their proven electrical properties. Collectively, this review delivers the development history of materials in implantable devices, while introducing new bioelectronics based on bioresorbable materials with multiple functionalities.

iDrugs and iDevices Discovery Research: Preclinical Assays, Techniques, and Animal Model Studies for Ocular Hypotensives and Neuroprotectants

Discovery ophthalmic research is centered around delineating the molecular and cellular basis of ocular diseases and finding and exploiting molecular and genetic pathways associated with them. From such studies it is possible to determine suitable intervention points to address the disease process and hopefully to discover therapeutics to treat them. An investigational new drug (IND) filing for a new small-molecule drug, peptide, antibody, genetic treatment, or a device with global health authorities requires a number of preclinical studies to provide necessary safety and efficacy data. Specific regulatory elements needed for such IND-enabling studies are beyond the scope of this article. However, to enhance the overall data packages for such entities and permit high-quality foundation-building publications for medical affairs, additional research and development studies are always desirable. This review aims to provide examples of some target localization/verification, ocular drug discovery processes, and mechanistic and portfolio-enhancing exploratory investigations for candidate drugs and devices for the treatment of ocular hypertension and glaucomatous optic neuropathy (neurodegeneration of retinal ganglion cells and their axons). Examples of compound screening assays, use of various technologies and techniques, deployment of animal models, and data obtained from such studies are also presented.

A Wireless Pressure Sensor for Continuous Monitoring of Intraocular Pressure in Conscious Animals

An important aspect of eye health in humans and animal models of human diseases is intraocular pressure (IOP). IOP is typically measured by hand with a tonometer, so data are sparse and sporadic and round-the-clock variations are not well characterized. Here we present a novel system for continuous wireless IOP and temperature measurement in small animals. The system consists of a cannula implanted in the anterior chamber of the eye connected to pressure sensing electronics that can be worn by rats or implanted in larger mammals. The system can record IOP with 0.3 mmHg accuracy and negligible drift at a rate of 0.25 Hz for 1-2 months on a regulated battery or indefinitely at rates up to 250 Hz via RF energy harvesting. Chronic recordings from conscious rats showed that IOP follows a diurnal rhythm, averaging 16.5 mmHg during the day and 21.7 mmHg at night, and that the IOP rhythm lags a diurnal rhythm in body temperature by 2.1 h. IOP and body temperature fluctuations were positively correlated from moment-to-moment as well. This technology allows researchers to monitor for the first time the precise IOP history of rat eyes, a popular model for glaucoma studies.

Development of a Smart Pump for Monitoring and Controlling Intraocular Pressure

Animal models of ocular hypertension are important for glaucoma research but come with experimental costs. Available methods of intraocular pressure (IOP) elevation are not always successful, the amplitude and time course of IOP changes are unpredictable and irreversible, and IOP measurement by tonometry is laborious. Here we present a novel system for monitoring and controlling IOP without these limitations. It consists of a cannula implanted in the anterior chamber of the eye, a pressure sensor that continually measures IOP, and a bidirectional pump driven by control circuitry that can infuse or withdraw fluid to hold IOP at user-desired levels. A portable version was developed for tethered use on rats. We show that rat eyes can be cannulated for months without causing significant anatomical or physiological damage although the animal and its eyes freely move. We show that the system measures IOP with <0.7 mmHg resolution and <0.3 mmHg/month drift and can maintain IOP within a user-specified window of desired levels for any duration necessary. We conclude that the system is ready for cage- or bench-side applications. The results lay the foundation for an implantable version that would give glaucoma researchers unprecedented knowledge and control of IOP in rats and potentially larger animals.

24-h monitoring devices and nyctohemeral rhythms of intraocular pressure

Intraocular pressure (IOP) is not a fixed value and varies over both the short term and periods lasting several months or years. In particular, IOP is known to vary throughout the 24-h period of a day, defined as a nyctohemeral rhythm in humans. In clinical practice, it is crucial to evaluate the changes in IOP over 24 h in several situations, including the diagnosis of ocular hypertension and glaucoma (IOP is often higher at night) and to optimize the therapeutic management of glaucoma. Until recently, all evaluations of 24-h IOP rhythm were performed using repeated IOP measurements, requiring individuals to be awakened for nocturnal measurements. This method may be imperfect, because it is not physiologic and disturbs the sleep architecture, and also because it provides a limited number of time point measurements not sufficient to finely asses IOP changes. These limitations may have biased previous descriptions of physiological IOP rhythm. Recently, extraocular and intraocular devices integrating a pressure sensor for continuous IOP monitoring have been developed and are available for use in humans. The objective of this article is to present the contributions of these new 24-h monitoring devices for the study of the nyctohemeral rhythms. In healthy subjects and untreated glaucoma subjects, a nyctohemeral rhythm is consistently found and frequently characterized by a mean diurnal IOP lower than the mean nocturnal IOP, with a diurnal bathyphase - usually in the middle or at the end of the afternoon - and a nocturnal acrophase, usually in the middle or at the end of the night.

The Application of a Contact Lens Sensor in Detecting 24-Hour Intraocular Pressure-Related Patterns

Glaucoma is one of the leading causes of blindness worldwide. Recent studies suggest that intraocular pressure (IOP) fluctuations, peaks, and rhythm are important factors in disease advancement. Yet, current glaucoma management remains hinged on single IOP measurements during clinic hours. To overcome this limitation, 24-hour IOP monitoring devices have been employed and include self-tonometry, permanent IOP, and temporary IOP monitoring. This review discusses each IOP measuring strategy and focuses on the recently FDA-approved contact lens sensor (CLS). The CLS records IOP-related ocular patterns for 24 hours continuously. Using the CLS, IOP-related parameters have been found to be associated with the rate of visual field progression in primary open-angle glaucoma, disease progression in primary angle-closure glaucoma, and various clinical variables in ocular hypertension. The CLS has been used to quantify blink rate and limbal strain and measure the circadian rhythm in a variety of disease states including normal-tension glaucoma and thyroid eye disease. The effects of various IOP-lowering interventions were also characterized using the CLS. CLS provides a unique, safe, and well-tolerated way to study IOP-related patterns in a wide range of disease states. IOP-related patterns may help identify patients most at risk for disease progression and assist with the development of tailored treatments.

Ambulatory 24-h intraocular pressure monitoring in the management of glaucoma

PURPOSE OF REVIEW

To review current status and future of ambulatory 24-h intraocular pressure monitoring. Despite important advances in the diagnosis and management of glaucoma during the last decade, the fundamental understanding of intraocular pressure, its only modifiable risk factor, remains elusive. The current practice of single intraocular pressure measurements during a clinic visit does not adequately reflect the variability of intraocular pressure throughout the 24-h day.

RECENT FINDINGS

There has been considerable progress recently with the prototype and commercial introduction of continuous 24-h intraocular pressure monitoring devices. Implantable intraocular pressure sensors have the advantage to directly measure intraocular pressure over many months and years, whereas temporary (contact lens based) approaches provide a noninvasive alternative for repeated 24-h periods. This review provides an overview of implantable devices as well as a critical assessment of a 24-h contact lens sensor.

SUMMARY

Recent advances in microelectromechanical systems and nanoelectromechanical systems have enabled the development of 24-h intraocular pressure monitoring devices. Once these technologies have shown their safety and efficacy, larger questions as to the data interpretation and handling will arise. It is likely that the use of 24-h intraocular pressure monitoring will herald fundamental changes in our understanding and management of glaucoma.

Removal of Silicone Oil From Intraocular Lens Using Novel Surgical Materials

PURPOSE

To design, fabricate, and evaluate novel materials to remove silicone oil (SiO) droplets from intraocular lenses (IOL) during vitreoretinal surgery.

METHODS

Three different designs were fabricated using soft lithography of polydimethylsiloxane (PDMS), three-dimensional (3D) inverse PDMS fabrication using water dissolvable particles, and atomic layer deposition (ALD) of alumina (Al₂O₃) on surgical cellulose fibers. Laboratory tests included static and dynamic contact angle (CA) measurements with water and SiO, nondestructive x-ray microcomputer tomography (micro-CT), and microscopy. SiO removal was performed in vitro and ex vivo using implantable IOLs and explanted porcine eyes.

RESULTS

All designs exhibited enhanced hydrophobicity and oleophilicity. Static CA measurements with water ranged from 131° to 160° and with SiO CA approximately 0° in 120 seconds following exposure. Nondestructive x-ray analysis of the 3D PDMS showed presence of interconnected polydispersed porosity of 100 to 300 μm in diameter. SiO removal from IOLs was achieved in vitro and ex vivo using standard 20-G vitrectomy instrumentation.

CONCLUSION

Removal of SiO from IOLs can be achieved using materials with lower surface energy than that of the IOLs. This can be achieved using appropriate surface chemistry and surface topography. Three designs, with enhanced hydrophobic properties, were fabricated and tested in vitro and ex vivo. All materials remove SiO within an aqueous environment. Preliminary ex vivo results were very promising, opening new possibilities for SiO removal in vitreoretinal surgeries.

TRANSLATIONAL RELEVANCE

This is the first report of an instrument that can lead to successful removal of SiO from the surface of IOL. In addition to the use of this instrument/material in medicine it can also be used in the industry, for example, retrieval of oil spills from bodies of water.

Reliable intraocular pressure measurement using automated radio-wave telemetry

PURPOSE

To present an autonomous intraocular pressure (IOP) measurement technique using a wireless implantable transducer (WIT) and a motion sensor.

METHODS

The WIT optical aid was implanted within the ciliary sulcus of a normotensive rabbit eye after extracapsular clear lens extraction. An autonomous wireless data system (AWDS) comprising of a WIT and an external antenna aided by a motion sensor provided continuous IOP readings. The sensitivity of the technique was determined by the ability to detect IOP changes resulting from the administration of latanoprost 0.005% or dorzolamide 2%, while the reliability was determined by the agreement between baseline and vehicle (saline) IOP.

RESULTS

On average, 12 diurnal and 205 nocturnal IOP measurements were performed with latanoprost, and 26 diurnal and 205 nocturnal measurements with dorzolamide. No difference was found between mean baseline IOP (13.08±2.2 mmHg) and mean vehicle IOP (13.27±2.1 mmHg) (P=0.45), suggesting good measurement reliability. Both antiglaucoma medications caused significant IOP reduction compared to baseline; latanoprost reduced mean IOP by 10% (1.3±3.54 mmHg; P<0.001), and dorzolamide by 5% (0.62±2.22 mmHg; P<0.001). Use of latanoprost resulted in an overall twofold higher IOP reduction compared to dorzolamide (P<0.001). Repeatability was ±1.8 mmHg, assessed by the variability of consecutive IOP measurements performed in a short period of time (≤1 minute), during which the IOP is not expected to change.

CONCLUSION

IOP measurements in conscious rabbits obtained without the need for human interactions using the AWDS are feasible and provide reproducible results.

Fabricated micro-nano devices for in vivo and in vitro biomedical applications

In recent years, the innovative use of microelectromechanical systems (MEMSs) and nanoelectromechanical systems (NEMSs) in biomedical applications has opened wide opportunities for precise and accurate human diagnostics and therapeutics. The introduction of nanotechnology in biomedical applications has facilitated the exact control and regulation of biological environments. This ability is derived from the small size of the devices and their multifunctional capabilities to operate at specific sites for selected durations of time. Researchers have developed wide varieties of unique and multifunctional MEMS/NEMS devices with micro and nano features for biomedical applications (BioMEMS/NEMS) using the state of the art microfabrication techniques and biocompatible materials. However, the integration of devices with the biological milieu is still a fundamental issue to be addressed. Devices often fail to operate due to loss of functionality, or generate adverse toxic effects inside the body. The in vitro and in vivo performance of implantable BioMEMS such as biosensors, smart stents, drug delivery systems, and actuation systems are researched extensively to understand the interaction of the BioMEMS devices with physiological environments. BioMEMS developed for drug delivery applications include microneedles, microreservoirs, and micropumps to achieve targeted drug delivery. The biocompatibility of BioMEMS is further enhanced through the application of tissue and smart surface engineering. This involves the application of nanotechnology, which includes the modification of surfaces with polymers or the self-assembly of monolayers of molecules. Thereby, the adverse effects of biofouling can be reduced and the performance of devices can be improved in in vivo and in vitro conditions.

Implantable electrolyte conductance-based pressure sensing catheter, II. Device construction and testing

Direct measurements of arterial blood pressure most commonly use bulky external instrumentation containing a pressure transducer connected to an ex vivo fluid-filled arterial line, which is subject to several sensing artifacts. In situ blood pressure sensors, typically solid state piezoresistive, capacitive, and interferometric sensors, are unaffected by these artifacts, but can be expensive to produce and miniaturize. We have developed an alternative approach to blood pressure measurement based on deformation of an elastic tube filled with electrolyte solution. Simple measurement of the electrical conductance of this solution as the tube dimensions change allows determination of the external pressure. The sensor is made from inexpensive materials and its miniaturization is straightforward. In vitro static testing of initial sensor prototypes mounted on a catheter tip showed a linear response with applied pressure and a resolution of 1 mmHg. In vivo sensing followed catheterization of the sensor into the femoral artery of a porcine model through a 7F catheter port. The sensor performed comparably to a commercial pressure transducer also connected to the catheter port. Due to its scalability and cost, this sensor has the potential for use in a range of pressure sensing applications, such as measurement of intracranial, spinal, or interstitial pressures.

Is 24-hour intraocular pressure monitoring necessary in glaucoma?

Although intraocular pressure (IOP) is the only treatable risk factor for glaucoma, its 24-hour behavior is poorly understood. Conflicting information is available in the literature with regard to the importance and predictive value of IOP peaks and fluctuations on the risk of glaucoma development and progression. This may be secondary to lack of prospective studies designed to address this issue. This article critically reviews the current evidence for the importance of 24-h IOP measurements in glaucoma and discusses shortcomings of current methods to assess 24-h IOP data, drawing attention to new developments in this field.

Detecting the onset of urinary bladder contractions using an implantable pressure sensor

This study investigates whether signals obtained from an implantable pressure sensor placed in the urinary bladder wall could be used to detect the onset of bladder contractions. The sensor assembly was custom made using a small piezoresistive sensor die. The die was mounted on ceramic substrate (8 mm × 8 mm) and encapsulated in silicone by a two-part moulding process. The final sensor was lens shaped with a diameter of 13.6 mm and height of 2.0 mm. Experiments were performed in six pigs that had one or more sensors placed in the bladder wall. An external reference sensor was used to simultaneously monitor intravesical pressure via a transurethral catheter. Bladder contractions were evoked by unilateral electrical stimulation of the pelvic nerve. Onset latency was computed using both signals. In addition, the correlation between wall pressure and intravesical pressure was calculated. On average, the onset latency was - 307 ms using the wall sensors compared to the intravesical pressure, i.e., the detection occurred earlier using the wall sensors than the intravesical sensor. In 91 of 114 recordings the correlation coefficient was above 0.90. In conclusion, the implantable sensor performs similar to the reference sensor when used to detect the onset of bladder contractions.

Self-tonometry in glaucoma management--past, present and future

Glaucoma is the leading cause of irreversible blindness in the world. Diagnosis and management of glaucoma is significantly associated with intraocular pressure, but contemporary office-based measurements are not sufficient to discover diurnal changes and spikes, nor do they demonstrate the effect of medication and compliance. Patient-directed self-tonometry can be taken throughout the day and is therefore the subject of much discussion and research. In this article we review the history of self-tonometry devices and present technologies for the future.

Continuous monitoring of intraocular pressure: rationale and progress toward a clinical device

Intraocular pressure (IOP) is a dynamic physiologic parameter with regular circadian variations and unpredictable short-term and long-term fluctuations. Current methods of measuring IOP are suboptimal with a typical clinical practice only performing periodic IOP measurements during regular office hours. Diurnal and 24-hour IOP measurements obtained on an in-patient basis can increase measurements but are inconvenient and expensive, and do not allow ambulatory monitoring of IOP. The goal of continuous IOP monitoring is to provide automated 24-hour recording of ambulatory IOP. Continuous IOP monitoring involves 2 complementary paradigms. Temporary noninvasive monitoring, possibly involving a contact lens-based pressure sensor, would be used to measure 24-hour IOP on a periodic basis. Permanent monitoring would be more invasive, using an implantable pressure sensor. Despite numerous previous attempts at continuous IOP monitoring, a device suitable for clinical use is not yet available. However, devices currently in development for permanent IOP monitoring seem to be nearly ready for human testing. The technologic issues for temporary monitoring may be greater than for permanent monitoring.

Wireless contact lens sensor for intraocular pressure monitoring: assessment on enucleated pig eyes

PURPOSE

Assessment on enucleated pig eyes of a novel and minimally invasive method for the continuous monitoring of intraocular pressure (IOP), based on a novel wireless contact lens sensor (CLS).

METHODS

The wireless CLS is a disposable silicone soft contact lens with a sensor embedded in it, allowing the wireless measurement of changes in corneal curvature induced by IOP variations. A CLS was adapted and tested on enucleated pig eyes. To demonstrate the measurement principle of the device, the enucleated pig eye was cannulated, allowing precise control of IOP. The CLS signal was then compared to the imposed IOP.

RESULTS

First, the IOP of enucleated pig eyes was changed between 11 and 14 mmHg, simulating ocular pulsation. Then, IOP was changed with static steps of 1 mmHg between 20 and 30 mmHg to assess the reproducibility and linearity of the CLS. In both cases, measurements from the CLS and IOP showed very good correlation. A calibration graph shows that the CLS is capable of monitoring the IOP of each individual eye with a reproducibility of +/- 0.2 mmHg (95% confidence interval).

CONCLUSION

The wireless CLS shows a good functionality to monitor the IOP on enucleated pig eyes. The device is placed in the same way as a soft disposable contact lens. This device would allow a minimally invasive and continuous monitoring of IOP over prolonged periods of up to 24 hr, regardless of patient activity, thus opening up new diagnostic and therapeutic methods to manage glaucoma.

Development of a fully implantable wireless pressure monitoring system

A fully implantable wireless pressure sensor system was developed to monitor bladder pressures in vivo. The system comprises a small commercial pressure die connected via catheter to amplifying electronics, a microcontroller, wireless transmitter, battery, and a personal digital assistant (PDA) or computer to receive the wireless data. The sensor is fully implantable and transmits pressure data once every second with a pressure detection range of 1.5 psi gauge and a resolution of 0.02 psi. In vitro calibration measurements of the device showed a high degree of linearity and excellent temporal response. The implanted device performed continuously in vivo in several porcine studies lasting over 3 days. This system can be adapted for other pressure readings, as well as other vital sign measurements; it represents the first step in developing a ubiquitous sensing platform for telemedicine and remote patient monitoring.

An Optically Powered Single-Channel Stimulation Implant as Test System for Chronic Biocompatibility and Biostability of Miniaturized Retinal Vision Prostheses

A microsystem based microimplant with an optically powered single-channel stimulator was designed and developed as test system for an epi-retinal vision implant. Biostability of the hybrid assembly and the encapsulation materials were evaluated in pilot experiments in chronic implantations in a cat animal model. The implant was fabricated on a flexible polyimide substrate with integrated platinum electrode, interconnection lines, and contact pads for hybrid integration of electronic components. The receiver part was realized with four photodiodes connected in series. A parylene C coating was deposited on the electronic components as insulation layer. Silicone rubber was used to encapsulate the electronics in the shape of an artificial intraocular lens to allow proper implantation in the eye. Pilot experiments showed the biostability of the encapsulation approach and full electric functionality of the microimplant to generate stimulation currents over the implantation period of three months in two cats. In one cat, electrical stimulation of the retina evoked neuronal responses in the visual cortex and indicated the feasibility of the system approach for chronic use.

Implantation of an inactive epiretinal poly(dimethyl siloxane) electrode array in dogs

The aim of this study is to investigate the long-term, mechanical biocompatibility of a polymer microtechnology that can be used to position electrodes in close proximity to the retina. Poly(dimethylsiloxane) (PDMS) arrays were manufactured by soft-lithography at Lawrence Livermore National Laboratory. The PDMS implant measured 4 mm x 40 mm x 55-60 microm and included 4-8 electrodes. Micromolded ribs were placed at the perimetry for strength and ease of manipulation. The PDMS arrays were implanted epiretinally in four normal dogs, with a single retinal tack used in each case to hold the device on the retina. The mechanical effects of the implant were followed up after surgical implantation by photography, fluorescein angiography, optical coherence tomography (OCT), and electrophysiologic tests. An intraoperative retinal tear occurred in the first implanted dog, causing retinal detachment and necessitating termination. The remaining dogs experienced no gross complications secondary to the array implantation procedure. During the follow-up period of 2 months in one eye and 6 months in three eyes, OCT demonstrated that the arrays were in close contact with the retina. Fluorescein angiography showed good perfusion of the retina under the array. At the end of 6 months, there was no statistical difference from baseline in mean retinal thickness under the array (P=0.43) or peripapillary retinal nerve fibre layer thickness corresponding to the implanted area (P=0.34). The mean distance between the array and the retinal surface varied from 32 to 68 microm throughout the follow-up. Histopathologic evaluation of the retinal implantation site in eyes followed for 6 months showed a general preservation of the normal, layered retinal structure, except for some localized retinal thinning in two eyes, where the array frame had been in direct retinal contact. The PDMS substrate micro array is a new and promising technology that can be scaled to support a high-density retinal stimulating array. Its implantation and handling is surgically manageable, and it forms a mechanically stable, acceptable interface with the inner retinal surface.

[Medical aspects of active implants in the eye]

Due to progress in miniaturizing complex electronic systems the development of intraocular sensors and stimulators is feasible. The Retina Implant will restore vision in patients with progressive retinal degenerations by means of electrical stimulation of retinal neurons, e.g. in Retinitis pigmentosa. After severe destructions of the anterior segment with intact retina, e.g. after explosion trauma an intraocular display will restore vision. In glaucoma patients the implantation of a telemetric pressure sensor will help to optimise the management of each individual patient. Pressure profiles under everyday conditions will become available. These active implants will open new perspectives in treatment and diagnostics in ophthalmology.

Constant-volume hydrogel osmometer: a new device concept for miniature biosensors

A new type of biosensor is proposed that combines the recognition properties of "intelligent" hydrogels with the sensitivity and reliability of microfabricated pressure transducers. In the proposed device, analyte-induced changes in the osmotic swelling pressure of an environmentally responsive hydrogel are measured by confining it within a small implantable enclosure between a rigid semipermeable membrane and the diaphragm of a miniature pressure transducer. Proof-of-principle tests of this device were performed in vitro using pH-sensitive hydrogels, with osmotic deswelling data for the same hydrogels used as a benchmark for comparison. The swelling pressure of the hydrogel was accurately determined from osmotic deswelling measurements against reservoirs of known osmotic stress. Values of swelling pressure vs salt concentration measured with a preliminary version of the sensor agree well with osmotic deswelling results. Through modification of the hydrogel with various enzymes or pendant binding moieties, the sensor has the potential to detect a wide range of biological analytes with good specificity.