Wireless Intraocular Pressure Sensing Using Microfabricated Minimally Invasive Flexible-Coiled LC Sensor Implant

This paper presents an implant-based wireless pressure sensing paradigm for long-range continuous intraocular pressure (IOP) monitoring of glaucoma patients. An implantable parylene-based pressure sensor has been developed, featuring an electrical LC-tank resonant circuit for passive wireless sensing without power consumption on the implanted site. The sensor is microfabricated with the use of parylene C (poly-chlorop- xylylene) to create a flexible coil substrate that can be folded for smaller physical form factor so as to achieve minimally invasive implantation, while stretched back without damage for enhanced inductive sensor–reader coil coupling so as to achieve strong sensing signal. A data-processed external readout method has also been developed to support pressure measurements. By incorporating the LC sensor and the readout method, wireless pressure sensing with 1-mmHg resolution in longer than 2-cm distance is successfully demonstrated. Other than extensive on-bench characterization, device testing through six-month chronic in vivo and acute ex vivo animal studies has verified the feasibility and efficacy of the sensor implant in the surgical aspect, including robust fixation and long-term biocompatibility in the intraocular environment. With meeting specifications of practical wireless pressure sensing and further reader development, this sensing methodology is promising for continuous, convenient, direct, and faithful IOP monitoring

Microfabricated Implantable Parylene-Based Wireless Passive Intraocular Pressure Sensors

This paper presents an implantable parylene-based wireless pressure sensor for biomedical pressure sensing applications specifically designed for continuous intraocular pressure (IOP) monitoring in glaucoma patients. It has an electrical LC tank resonant circuit formed by an integrated capacitor and an inductor coil to facilitate passive wireless sensing using an external interrogating coil connected to a readout unit. Two surface-micromachined sensor designs incorporating variable capacitor and variable capacitor/inductor resonant circuits have been implemented to realize the pressure-sensitive components. The sensor is monolithically microfabricated by exploiting parylene as a biocompatible structural material in a suitable form factor for minimally invasive intraocular implantation. Pressure responses of the microsensor have been characterized to demonstrate its high pressure sensitivity (> 7000 ppm/mmHg) in both sensor designs, which confirms the feasibility of pressure sensing with smaller than 1 mmHg of resolution for practical biomedical applications. A six-month animal study verifies the in vivo bioefficacy and biostability of the implant in the intraocular environment with no surgical or postoperative complications. Preliminary ex vivo experimental results verify the IOP sensing feasibility of such device. This sensor will ultimately be implanted at the pars plana or on the iris of the eye to fulfill continuous, convenient, direct, and faithful IOP monitoring

Biotelemetric Wireless Intracranial Pressure Monitoring: An In Vitro

Assessment of intracranial pressure (ICP) is of great importance in management of traumatic brain injuries (TBIs). The existing clinically established ICP measurement methods require catheter insertion in the cranial cavity. This increases the risk of infection and hemorrhage. Thus, noninvasive but accurate techniques are attractive. In this paper, we present two wireless, batteryless, and minimally invasive implantable sensors for continuous ICP monitoring. The implants comprise ultrathin (50 μm) flexible spiral coils connected in parallel to a capacitive microelectromechanical systems (MEMS) pressure sensor. The implantable sensors are inductively coupled to an external on-body reader antenna. The ICP variation can be detected wirelessly through measuring the reader antenna’s input impedance. This paper also proposes novel implant placement to improve the efficiency of the inductive link. In this study, the performance of the proposed telemetry system was evaluated in a hydrostatic pressure measurement setup. The impact of the human tissues on the inductive link was simulated using a 5 mm layer of pig skin. The results from the in vitro measurement proved the capability of our developed sensors to detect ICP variations ranging from 0 to 70 mmHg at 2.5 mmHg intervals

The development of a wireless LCP-based intracranial pressure sensor for traumatic brain injury patients

Raised intracranial pressure (ICP) in traumatic brain injury (TBI) patients can lead to death. ICP measurement is required to monitor the condition of a patient and to inform TBI treatment. This work presents a new wireless liquid crystal polymer (LCP) based ICP sensor. The sensor is designed with the purpose of measuring ICP and wirelessly transmitting the signal to an external monitoring unit. The sensor is minimally invasive and biocompatible due to the mechanical design and the use of LCP. A prototype sensor and associated wireless module are fabricated and tested to demonstrate the functionality and performance of the wireless LCP-based ICP sensor. Experimental results show that the wireless LCP-based ICP sensor can operate in the pressure range of 0 - 60.12 mmHg. Based on repeated measurements, the sensitivity of the sensor is found to be 25.62 µVmmHg-1, with a standard deviation of ± 1.16 µVmmHg-1. This work represents a significant step towards achieving a wireless, implantable, minimally invasive ICP monitoring strategy for TBI patients

An Implantable Low Pressure, Low Drift, Dual BioPressure Sensor and In-Vivo Calibration Methods Thereof

The human body’s intracranial pressure (ICP) is a critical component in sustaining healthy blood flow to the brain while allowing adequate volume for brain tissue within the rigid structures of the cranium. Disruptions in the body’s autoregulation of intracranial pressure are often caused by hemorrhage, tumors, edema, or excess cerebral spinal fluid resulting in treatments that are estimated to globally cost up to approximately five billion dollars annually. A critical element in the contemporary management of acute head injury, intracranial hemorrhage, stroke, or other conditions resulting in intracranial hypertension, is the real-time monitoring of ICP. Currently, such mainstream clinical monitoring can only take place short-term within an acute care hospital. The monitoring is prone to measurement drift and is comprised of externally tethered pressure sensors that are temporarily implanted into the brain, thus carrying a significant risk of infection. To date, reliable, low drift, completely internalized, long-term ICP monitoring devices remain elusive. The successful development of such a device would not only be safer and more reliable in the short-term but would expand the use of ICP monitoring for the management of chronic intracranial hypertension and enable further clinical research into these disorders. The research herein reviews the current challenges of existing ICP monitoring systems, develops a new novel sensing technology, and evaluates the same for potentially facilitating long-term implantable ICP sensing. Based upon the findings of this research, this dissertation proposes and evaluates a dual matched-die piezo-resistive strain sensing device, with a novel in-vivo calibration system and method thereof, for application to long-term implantable ICP sensing

Inductively Powered Implantable System with Far-field Data Transmitter for an Intracranial Pressure Monitoring Application: Design and Performance Validation

Monitoring of the intracranial pressure (ICP) is an essential activity for many brain diseases and injuries. For an adult, ICP value is between 7 mmHg to 15 mmHg . However, for a critically ill patient, the ICP should be maintained below 20 mmHg. Therefore, continuous monitoring of ICP is a life-saving activity. Several invasive and non-invasive methods have been proposed for monitoring of the ICP. However, invasive methods cannot be used for continuous monitoring of the ICP due to the risk of infection. Moreover, non-invasive methods lack in accuracy.Therefore, many researchers reported battery-powered or fully passive implantable systems. However, a battery-powered implant has limited life and large size. On the other hand, in a fully passive implant the readout distance is relatively small in comparison with a battery-powered implant due to its zero-power operation.In contrast, this work presents the development of an inductively powered implantable system equipped with a data transmission unit for an ICP monitoring application. The developed system has three main parts: an implant or in-body unit, an on-body unit and an off-body unit. The on-body unit powers the implant through inductive near-field link. After the activation, the implant, consists of a piezoresistive pressure sensor and a data transmission unit, transmits the pressure signal at the industrial, scientific, and medical radio (ISM) band of 2.45 GHz. The off-body unit receives the transmitted signal from the implant and estimates the pressure value.The simulation and the measurement results of both near-filed and far-field links are presented. After the development of the system, the pressue readout measurement results have been presented in the air, water and in a setting mimicking the human head dielectric properties. For biocompatibility, the implant is coated with biocompatible adhesive silicone. The effect of coating on both wireless links has also been studied.Finally, this work also presents the effect of misalignment between the inductively coupled antennas on the pressure readout accuracy of the developed ICP monitoring system and discusses the solution to overcome this impact. The thesis also presents the response of the developed ICP monitoring system with the change in the temperature

Design, manufacturing and characterisation of a wireless flexible pressure sensor system for the monitoring of the gastro-intestinal tract

Ingestible motility capsule (IMC) endoscopy holds a strong potential in providing advanced diagnostic capabilities within the small intestine with higher patient tolerance for pathologies such as irritable bowel syndrome, gastroparesis and chronic abdominal amongst others. Currently state-of-the art IMCs are limited by the use of obstructive off-the-shelf sensing modules that are unable to provide multi-site tactile monitoring of the Gastro-Intestinal tract. In this work a novel 12 mm in diameter by 30 mm in length IMC is presented that utilises custom-built flexible, thin-film, biocompatible, wireless and highly sensitive tactile pressure sensors arrays functionalising the capsule shell. The 150 μm thick, microstructured, PDMS flexible passive pressure sensors are wirelessly powered and interrogated, and are capable of detecting pressure values ranging from 0.1 kPa up to 30 kPa with a 0.1 kPa resolution. A novel bottom-up wafer-scale microfabrication process is presented which enables the development of these ultra-dense, self-aligned, scalable and uniquely addressable flexible wireless sensors with high yield (>80%). This thesis also presents an innovative metallisation microfabrication process on soft-elastomeric substrates capable to withstand without failure of the tracks 180o bending, folding and iterative deformation such as to allow conformable mapping of these sensors. A custom-built and low-cost reflectometer system was also designed, built and tested within the capsule that can provide a fast (100 ms) and accurate extraction (±0.1 kPa) of their response. In vitro and in vivo characterisation of the developed IMC device is also presented, facilitated respectively via the use of a biomimetic phantom gut and via live porcine subjects. The capsule device was found to successfully capture respiration, low-amplitude and peristaltic motility of the GI tract from multiple sites of the capsule.UK Engineering & Physical Sciences Research Council (EPSRC) through the Programme Grant Sonopill (EP/K034537/2)James Watt Scholarshi

Sensors for Wireless Body Monitoring Applications

Body monitoring systems have recently drawn great attention to modern electronic consumers due to their various health−care and security applications. However, most of the existing monitoring systems need wire connections that prevent free body movements. Complementary metal−oxide−semiconductor (CMOS) technology based wireless sensor systems need integration of different components that make the device volume and production cost high. In adition, their dependency on on−sensor power source limits the continuous monitoring capability. In the thesis, to demonstrate the feasibility of low cost and simple body monitoring systems, we propose a near−infrared (NIR) photodetector (PD) and a humidity sensor (HS) using low−temperature thin−film processes suitable for large−area electronics application. For NIR detection, a novel lateral metal−semiconductor−metal (MSM) PD architecture is proposed using low−temperature nanocrystalline silicon (nc−Si) as a NIR absorption layer and organic polyimide (PI) as a blocking layer. Experimental results show that addition of PI layer reduces the dark current (ID) up to 103−105 times compared with the PDs without PI layer. Fabricated devices exhibit a low ID of ~10−10 A, a response time of <1.5 ms, and an external quantum efficiency (EQE) of 35−15% for the 740−850 nm wavelengths of light under 100−150 V biasing conditions. Unlike the standard p−i−n PD, our high−performance lateral PD does not require doped p+ and n+ layers. Thus, the reported device is compatible with industry standard amorphous silicon (a−Si) thin−film transistor (TFT) fabrication process, which makes it promising for large−area full hand biometric imagers suitable for various non−invasive body monitoring applications. For humidity detection, a 30 mm diameter passive LC (p−LC) HS is formed by joining an octagonal planer inductor and a moisture sensitive interdigital zinc oxide (ZnO) capacitor in series. A PCB reader coil is also designed, which is able to sense the HS from <25 mm distance. The HS reads 30−90% of relative humidity (RH) by interrogating change of the resonance frequency (fR) of the reader−sensor system. The reading resolution is ±2.38%RH and the sensitivity is 53.33−93.33 kHz/1%RH for the above 45% RH measurements. Experimental results show that the proposed HS is operational in a range of 0−75 oC as long as recalibration is performed for a temperature drift of above ±3 oC, which makes it suitable for various promising applications operated at different temperatures. Above all, the presented results are promising for the continuous body monitoring applications to observe the humidity wirelessly without any power source on the sensor

Inductively Coupled Passive Resonance Sensors: Readout Methods and Applications

Measurement systems are used to acquire information from the surrounding world. The requirements of the measurement system depend on the application, and the acquired information is used in different ways. For example, measurements are taken as part of the control systems of industrial processes. Alternatively, the information obtained from the measurements can be used to provide answers to scientific questions. Each measurement has a case-specific importance for the user and a certain cost in terms of time and money. Therefore, the same measurement approach is not optimal in every case. The design process of the measurement systems always includes a compromise between performance, viability, and cost. These factors are, in turn, strongly dependent on the implementation of the measurement system in each separate case. Inductively coupled passive resonance sensors provide a measurement method that has two notable benefits: the simple structure of the sensors and the possibility to take short-range wireless measurements. However, the limitations of the available readout devices have often impeded the use and development of these sensors in many demanding applications. In addition, uncertainty in the measurement results due to inductive coupling hinders the use of this method.This work concerns the development and implementation of a measurement system based on inductively coupled passive resonance sensors. A custom-made readout device to improve the feasibility of the readout in applications where continuous field measurements are performed was both specified and produced. The readout device was implemented using a simplified version of the method used in conventional impedance analyzers. In addition, signal processing methods were developed which can extract resonance characteristics from the measured data. A special algorithm was developed to compensate for the effects of the changes in the inductive coupling when the measurement distance varies. The operation of the developed readout methods was studied using simulations, and several realistic measurement configurations were tested. Competing readout methods published in the literature were also simulated. The accuracy of all the studied methods depended on the configuration of the measurement system. The inductive coupling coefficient also had a significant influence on the accuracy of the tested methods.The newly-developed readout methods and the inductively coupled passive resonance sensor were then utilized in a medical application to monitor the pressure between the skin and compression garments. These garments are used, for example, to improve the healing of burns and reduce swelling in the legs. Effective medical treatment of such conditions requires that the appropriate pressure is applied. With this system, the pressure reading under the compression garment can be obtained by using simple disposable sensors that can be read wirelessly through a thin fabric. Using our inductive coupling compensation method, the sensor enabled the monitoring of the pressure with the required level of precision.Inductively coupled resonance sensors can also be used to monitor the properties of the materials around the sensor. This monitoring is possible because the permittivity of the environment near to the sensor affects the sensor’s resonance characteristics. This method was tested in two applications. In the first application, the manufacturing process of ceramic slurry was monitored by a sensor that was installed inside the container where the slurry was mixed. The resonance characteristics of the sensor were measured as the manufacturing process was incrementally carried out. The results indicated that the method could be used to control the composition of the slurry. In the second application, the inductively coupled sensors were tested in monitoring the degradation processes of two different polymers during hydrolysis. In this application, the sensors were encapsulated into the tested polymers. The polymer samples were kept inside containers filled with buffer solution and the resonance characteristics of the encapsulated sensors were then measured wirelessly from outside. The results showed a clear difference in degradation profiles between the tested polymers. The method may provide a novel way to continuously monitor the degradation processes of certain materials.In summary, the developed readout methods improved the applicability of inductive coupled passive resonance sensors in the tested applications and created novel ways to acquire information. This new technology provides a good starting point for the development of a new generation of inductively coupled passive resonance sensors