Supercapacitive Sensors for Force/Strain Measurements in Biomedical Applications

University of Minnesota Ph.D. dissertation. August 2019. Major: Mechanical Engineering. Advisor: Rajesh Rajamani. 1 computer file (PDF); xvi, 117 pages.This dissertation develops a new class of flexible force and strain sensors based on the principle of supercapacitive sensing. The sensing mechanism consists of a change in capacitance in a double-layer supercapacitor in response to an applied force or strain by inducing a change in the contact area between an electrolyte and a pair of electrodes. The new sensors can provide a measurement sensitivity several orders of magnitude higher than traditional capacitive sensors, and have other advantages such as flexibility, soft material construction, ability to operate in liquid environments, tremendous ease of fabrication and unprecedented configurability. As a key component of the new sensors, a paper-based solid-state electrolyte with high deformability is developed. The paper substrate can be easily cut and shaped into complex three-dimensional geometries on which ionic gel can be coated. Paper dissolves in the ionic gel after determining the shape of the electrolyte, leaving behind transparent electrolyte structures with micro-structured fissures responsible for their high deformability. Exploiting this simple paper-based fabrication process, this dissertation develops diverse sensors of different configurations and demonstrates their operation and their advantages. First, force sensors in multiple configurations involving electrolytes that are arch-shaped, corrugated and dome-shaped are fabricated. They have sensitivity that is 1000 times larger than similarly sized capacitive sensors and have negligible parasitic capacitance when used in immersive liquid environments. The use of such force sensors on a urethral catheter which can be used to diagnose the cause of urinary incontinence in a Urology application is demonstrated. A urethral catheter with five distributed force sensors is fabricated that can be used to measure distributed urethral closure pressure in a human subject. Experimental results with the catheter, including cuff tests and ex vivo tests are presented. Next, their high sensitivity allows the use of multiple supercapacitive sensors together in a quad structure to enable a sensor in which normal and shear forces can be simultaneously measured. Such a sensor can have multiple applications in robotics and in wearable monitoring systems which can benefit from measurement of multi-axis forces. The performance of the multi-axis normal-shear force sensor is evaluated using extensive experimental data with a wide range of force combinations. Due to manufacturing imperfections, the sensor does not have uniform axisymmetric sensitivity. Hence, a learning algorithm which utilizes a deep neural network to model the sensor response to multi-axis forces is developed and implemented. The learning algorithm allows the sensor system to provide highly accurate normal and shear force estimates, no matter what the alignment of the forces applied on the sensor. Finally, the use of the supercapacitive sensors for strain measurement is evaluated. The paper-based electrolyte is strengthened with silicate nanoparticles to allow it to withstand over 110% stretch without failure. The strengthened electrolyte is used in a unique strain sensor design. The strain sensor is shown to have ultra-high sensitivity and its performance in a wearable home-monitoring application to measure the size of the leg and thus monitor leg-swelling is demonstrated. The contributions of this dissertation include the development of a new soft deformable electrolyte, the development of a paper-based supercapacitive sensor system, and the development of novel sensor configurations such as a simultaneous normal-shear force sensor, a distributed urethral catheter with multiple pressure sensors and a highly stretchable strain sensor. The developed class of new sensors provides extremely high sensitivity and other advantages in spite of easy fabrication with no requirement for clean room facilities

Wireless Power Transfer Techniques for Implantable Medical Devices:A Review

Wireless power transfer (WPT) systems have become increasingly suitable solutions for the electrical powering of advanced multifunctional micro-electronic devices such as those found in current biomedical implants. The design and implementation of high power transfer efficiency WPT systems are, however, challenging. The size of the WPT system, the separation distance between the outside environment and location of the implanted medical device inside the body, the operating frequency and tissue safety due to power dissipation are key parameters to consider in the design of WPT systems. This article provides a systematic review of the wide range of WPT systems that have been investigated over the last two decades to improve overall system performance. The various strategies implemented to transfer wireless power in implantable medical devices (IMDs) were reviewed, which includes capacitive coupling, inductive coupling, magnetic resonance coupling and, more recently, acoustic and optical powering methods. The strengths and limitations of all these techniques are benchmarked against each other and particular emphasis is placed on comparing the implanted receiver size, the WPT distance, power transfer efficiency and tissue safety presented by the resulting systems. Necessary improvements and trends of each WPT techniques are also indicated per specific IMD

A thin monocrystalline diaphragm pressure sensor using silicon-on-insulator technology.

The sensors market is huge and growing annually, of this a large sector is pressure sensors. With increasing demands on performance there remains a need for ultraminiature, high performance pressure sensors, particularly for medicai applications. To address this a novel capacitive pressure sensor consisting of an array of parallel connected diaphragms has been designed and fabricated from SIMOX substrates. The benefits of this include single crystal silicon diaphragms, small, well controlled dimensions, single sided processing and the opportunity for electronics integration. Theoretical modelling of this structure predicts a high sensitivity and low stress device with opportunities for scaling to suit alternative applications. A novel, process technology was developed to achieve the required structure with the inclusion of procedures to address the specific issues relating to the SIMOX material. The sensor was fully characterised and the results demonstrated high performance compared with similar reported devices. Alternative structures such as cantilevers, bridges and resonators were fabricated as a demonstrative tool to show the feasibility of this technology in a wider field of applications

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

Acoustic sensing as a novel approach for cardiovascular monitoring at the wrist

Cardiovascular diseases are the number one cause of deaths globally. An increased cardiovascular risk can be detected by a regular monitoring of the vital signs including the heart rate, the heart rate variability (HRV) and the blood pressure. For a user to undergo continuous vital sign monitoring, wearable systems prove to be very useful as the device can be integrated into the user's lifestyle without affecting the daily activities. However, the main challenge associated with the monitoring of these cardiovascular parameters is the requirement of different sensing mechanisms at different measurement sites. There is not a single wearable device that can provide sufficient physiological information to track the vital signs from a single site on the body. This thesis proposes a novel concept of using acoustic sensing over the radial artery to extract cardiac parameters for vital sign monitoring. A wearable system consisting of a microphone is designed to allow the detection of the heart sounds together with the pulse wave, an attribute not possible with existing wrist-based sensing methods. Methods: The acoustic signals recorded from the radial artery are a continuous reflection of the instantaneous cardiac activity. These signals are studied and characterised using different algorithms to extract cardiovascular parameters. The validity of the proposed principle is firstly demonstrated using a novel algorithm to extract the heart rate from these signals. The algorithm utilises the power spectral analysis of the acoustic pulse signal to detect the S1 sounds and additionally, the K-means method to remove motion artifacts for an accurate heartbeat detection. The HRV in the short-term acoustic recordings is found by extracting the S1 events using the relative information between the short- and long-term energies of the signal. The S1 events are localised using three different characteristic points and the best representation is found by comparing the instantaneous heart rate profiles. The possibility of measuring the blood pressure using the wearable device is shown by recording the acoustic signal under the influence of external pressure applied on the arterial branch. The temporal and spectral characteristics of the acoustic signal are utilised to extract the feature signals and obtain a relationship with the systolic blood pressure (SBP) and diastolic blood pressure (DBP) respectively. Results: This thesis proposes three different algorithms to find the heart rate, the HRV and the SBP/ DBP readings from the acoustic signals recorded at the wrist. The results obtained by each algorithm are as follows: 1. The heart rate algorithm is validated on a dataset consisting of 12 subjects with a data length of 6 hours. The results demonstrate an accuracy of 98.78%, mean absolute error of 0.28 bpm, limits of agreement between -1.68 and 1.69 bpm, and a correlation coefficient of 0.998 with reference to a state-of-the-art PPG-based commercial device. A high statistical agreement between the heart rate obtained from the acoustic signal and the photoplethysmography (PPG) signal is observed. 2. The HRV algorithm is validated on the short-term acoustic signals of 5-minutes duration recorded from each of the 12 subjects. A comparison is established with the simultaneously recorded electrocardiography (ECG) and PPG signals respectively. The instantaneous heart rate for all the subjects combined together achieves an accuracy of 98.50% and 98.96% with respect to the ECG and PPG signals respectively. The results for the time-domain and frequency-domain HRV parameters also demonstrate high statistical agreement with the ECG and PPG signals respectively. 3. The algorithm proposed for the SBP/ DBP determination is validated on 104 acoustic signals recorded from 40 adult subjects. The experimental outputs when compared with the reference arm- and wrist-based monitors produce a mean error of less than 2 mmHg and a standard deviation of error around 6 mmHg. Based on these results, this thesis shows the potential of this new sensing modality to be used as an alternative, or to complement existing methods, for the continuous monitoring of heart rate and HRV, and spot measurement of the blood pressure at the wrist.Open Acces

Biosensors for Diagnosis and Monitoring

Biosensor technologies have received a great amount of interest in recent decades, and this has especially been the case in recent years due to the health alert caused by the COVID-19 pandemic. The sensor platform market has grown in recent decades, and the COVID-19 outbreak has led to an increase in the demand for home diagnostics and point-of-care systems. With the evolution of biosensor technology towards portable platforms with a lower cost on-site analysis and a rapid selective and sensitive response, a larger market has opened up for this technology. The evolution of biosensor systems has the opportunity to change classic analysis towards real-time and in situ detection systems, with platforms such as point-of-care and wearables as well as implantable sensors to decentralize chemical and biological analysis, thus reducing industrial and medical costs. This book is dedicated to all the research related to biosensor technologies. Reviews, perspective articles, and research articles in different biosensing areas such as wearable sensors, point-of-care platforms, and pathogen detection for biomedical applications as well as environmental monitoring will introduce the reader to these relevant topics. This book is aimed at scientists and professionals working in the field of biosensors and also provides essential knowledge for students who want to enter the field

Ultra-thin and flexible CMOS technology: ISFET-based microsystem for biomedical applications

A new paradigm of silicon technology is the ultra-thin chip (UTC) technology and the emerging applications. Very thin integrated circuits (ICs) with through-silicon vias (TSVs) will allow the stacking and interconnection of multiple dies in a compact format allowing a migration towards three-dimensional ICs (3D-ICs). Also, extremely thin and therefore mechanically bendable silicon chips in conjunction with the emerging thin-film and organic semiconductor technologies will enhance the performance and functionality of large-area flexible electronic systems. However, UTC technology requires special attention related to the circuit design, fabrication, dicing and handling of ultra-thin chips as they have different physical properties compared to their bulky counterparts. Also, transistors and other active devices on UTCs experiencing variable bending stresses will suffer from the piezoresistive effect of silicon substrate which results in a shift of their operating point and therefore, an additional aspect should be considered during circuit design. This thesis tries to address some of these challenges related to UTC technology by focusing initially on modelling of transistors on mechanically bendable Si-UTCs. The developed behavioural models are a combination of mathematical equations and extracted parameters from BSIM4 and BSIM6 modified by a set of equations describing the bending-induced stresses on silicon. The transistor models are written in Verilog-A and compiled in Cadence Virtuoso environment where they were simulated at different bending conditions. To complement this, the verification of these models through experimental results is also presented. Two chips were designed using a 180 nm CMOS technology. The first chip includes nMOS and pMOS transistors with fixed channel width and two different channel lengths and two different channel orientations (0° and 90°) with respect to the wafer crystal orientation. The second chip includes inverter logic gates with different transistor sizes and orientations, as in the previous chip. Both chips were thinned down to ∼20m using dicing-before-grinding (DBG) prior to electrical characterisation at different bending conditions. Furthermore, this thesis presents the first reported fully integrated CMOS-based ISFET microsystem on UTC technology. The design of the integrated CMOS-based ISFET chip with 512 integrated on-chip ISFET sensors along with their read-out and digitisation scheme is presented. The integrated circuits (ICs) are thinned down to ∼30m and the bulky, as well as thinned ICs, are electrically and electrochemically characterised. Also, the thesis presents the first reported mechanically bendable CMOS-based ISFET device demonstrating that mechanical deformation of the die can result in drift compensation through the exploitation of the piezoresistive nature of silicon. Finally, this thesis presents the studies towards the development of on-chip reference electrodes and biodegradable and ultra-thin biosensors for the detection of neurotransmitters such as dopamine and serotonin