Examination of the performance characteristics of velostat as an in-socket pressure sensor

Velostat is a low-cost, low-profile electrical bagging material with piezoresistive properties, making it an attractive option for in-socket pressure sensing. The focus of this research was to explore the suitability of a Velostat-based system for providing real-time socket pressure profiles. The prototype system performance was explored through a series of bench tests to determine properties including accuracy, repeatability and hysteresis responses, and through participant testing with a single subject. The fabricated sensors demonstrated mean accuracy errors of 110 kPa and significant cyclical and thermal drift effects of up to 0.00715 V/cycle and leading to up to a 67% difference in voltage range respectively. Despite these errors the system was able to capture data within a prosthetic socket, aligning to expected contact and loading patterns for the socket and amputation type. Distinct pressure maps were obtained for standing and walking tasks displaying loading patterns indicative of posture and gait phase. The system demonstrated utility for assessing contact and movement patterns within a prosthetic socket, potentially useful for improvement of socket fit, in a low cost, low profile and adaptable format. However, Velostat requires significant improvement in its electrical properties before proving suitable for accurate pressure measurement tools in lower limb prosthetics

A data-driven method to reduce excessive contact pressure of hand orthosis using a soft sensor skin

Discomfort under customised hand orthosis has been commonly reported in clinics due to excessive contact pressures, leading to low patient adherence and decreased effectiveness of orthosis. However, the current orthosis adjustment by clinicians to reduce pressures based upon subjective feedback from patients is inefficient and prone to variability. Therefore, a quantitative method to guide orthosis adjustment was proposed here by developing a data-driven method. Firstly, Verbal Protocol Analysis was employed to convert the implicit process of orthosis customisation into working models of clinicians. Relevant data to inform a new solution development to reduce excessive contact pressure were extracted from the working models in terms of time consumption and iterations of tasks. Secondly, a new soft sensor skin with strategically placed sensing units to measure static contact pressures under hand orthoses was developed. Finite element simulations were conducted to reveal the required contact pressure range (0.02 – 0.078 MPa) and the distribution of relatively high pressures in 12 key areas. A new fabrication method was proposed to produce the sensor skin, which was then characterised and tested on the subject. The results show that the sensor unit has a pressure range from 0.01 MPa to 0.1 MPa with the maximum repeatability error of 6.4% at 0.014 MPa, and the maximum measurement error of 8.26% at 0.023 MPa. Thirdly, a new method was proposed to predict contact pressures associated with the moderate level of discomfort at critical spots under hand orthoses. 40 patients were recruited to collect contact pressures under two types of orthoses using the sensor skin, and their discomfort perceptions were measured with a categorical scale. Based on these data, artificial neural networks for five identified critical spots on the hand were built to predict pressure thresholds that clinicians can use to adjust orthoses, thus reducing excessive contact pressures. The neural networks show satisfactory prediction accuracy with R2 values over 0.89 of regression between network outputs and measurements. Collectively, this thesis proposed a novel method for clinicians to adjust orthoses quantitatively and reduce the need for subjective assessment for patients. It provided a platform to further investigate the pressure for patients with other conditions.Open Acces

NASA Tech Briefs, February 2006

Topics discussed include: Nearly Direct Measurement of Relative Permittivity; DCS-Neural-Network Program for Aircraft Control and Testing; Dielectric Heaters for Testing Spacecraft Nuclear Reactors; Using Doppler Shifts of GPS Signals To Measure Angular Speed; Monitoring Temperatures of Tires Using Luminescent Materials; Highly Efficient Multilayer Thermoelectric Devices; Very High-Speed Digital Video Capability for In-Flight Use; MMIC DHBT Common-Base Amplifier for 172 GHz; Modular, Microprocessor-Controlled Flash Lighting System; Generic Environment for Simulating Launch Operations; Modular Aero-Propulsion System Simulation; X-Windows Socket Widget Class; Infrastructure for Rapid Development of Java GUI Programs; Processing Raman Spectra of High-Pressure Hydrogen Flames; X-Windows Information Sharing Protocol Widget Class; Simulating Humans as Integral Parts of Spacecraft Missions; Analyzing Power Supply and Demand on the ISS; Polyimides From a-BPDA and Aromatic Diamines; Making Plant-Support Structures From Waste Plant Fiber; Large Deployable Reflectarray Antenna; Periodically Discharging, Gas-Coalescing Filter; Ion Milling On Steps for Fabrication of Nanowires; Neuro-Prosthetic Implants With Adjustable Electrode Arrays; Microfluidic Devices for Studying Biomolecular Interactions; Studying Functions of All Yeast Genes Simultaneously; Polarization Phase-Compensating Coats for Metallic Mirrors; Tunable-Bandwidth Filter System; Methodology for Designing Fault-Protection Software; and Ground-Based Localization of Mars Rovers

Fiber inline pressure and acoustic sensor fabricated with femtosecond laser

Pressure and acoustic measurements are required in many industrial applications such as down-hole oil well monitoring, structural heath monitoring, engine monitoring, study of aerodynamics, etc. Conventional sensors are difficult to apply due to the high temperature, electromagnetic-interference noise and limited space in such environments. Fiber optic sensors have been developed since the last century and have proved themselves good candidates in such harsh environment. This dissertation aims to design, develop and demonstrate miniaturized fiber pressure/acoustic sensors for harsh environment applications through femtosecond laser fabrication. Working towards this objective, the dissertation explored two types of fiber inline microsensors fabricated by femtosecond laser: an extrinsic Fabry-Perot interferometric (EFPI) sensor with silica diaphragm for pressure/acoustic sensing, and an intrinisic Fabry-Perot interferometer (IFPI) for temperature sensing. The scope of the dissertation work consists of device design, device modeling/simulation, laser fabrication system setups, signal processing method development and sensor performance evaluation and demonstration. This research work provides theoretical and experimental evidences that the femtosecond laser fabrication technique is a valid tool to fabricate miniaturized fiber optic pressure and temperature sensors which possess advantages over currently developed sensors --Abstract, page iii

Development of piezoresistive sensors for biomedical applications

Tese de doutoramento em Engenharia Electrónica Industrial e de ComputadoresIn the last decades there has been an increase in sensing systems applied in a variety of situations with a large variety of sensor ranges. This represents a growing area with high potential. One of the areas of sensor development that require a great deal of attention is the area of sensor for biomedical applications and biosensors. These sensors have to overcome a number of challenges and limitations inherent to the environment where they are introduced. These difficulties lead to the necessity of using new materials and new techniques for their construction together with the more traditional materials, e.g. silicon based, which have already proven their potential in this area. Among the various materials, polymers have proven to be a good choice, due to a set of advantages such as simple processing, flexibility and facility of being obtained in different shapes. Therefore it is interesting to fabricate polymer based piezoresistive sensors for functional coatings of implantable hip prosthesis. These sensors will allow coating the prosthesis and provide new functionalities to the implants such as the possibility to measure forces and deformations between the prosthesis and the bone and therefore improving the postoperative diagnostic. In this works, a model of hip prosthesis with coated sensors was developed. For this purpose, flexible piezoresistive sensors have been developed that allow being implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a polymeric substrate, using the lithographic technique to construct the various layers of the sensors. The sensor has a gauge factor of -28 for low frequency deformation cycles. In the last decades there has been an increase in sensing systems applied in a variety of situations with a large variety of sensor ranges. This represents a growing area with high potential. One of the areas of sensor development that require a great deal of attention is the area of sensor for biomedical applications and biosensors. These sensors have to overcome a number of challenges and limitations inherent to the environment where they are introduced. These difficulties lead to the necessity of using new materials and new techniques for their construction together with the more traditional materials, e.g. silicon based, which have already proven their potential in this area. Among the various materials, polymers have proven to be a good choice, due to a set of advantages such as simple processing, flexibility and facility of being obtained in different shapes. Therefore it is interesting to fabricate polymer based piezoresistive sensors for functional coatings of implantable hip prosthesis. These sensors will allow coating the prosthesis and provide new functionalities to the implants such as the possibility to measure forces and deformations between the prosthesis and the bone and therefore improving the postoperative diagnostic. In this works, a model of hip prosthesis with coated sensors was developed. For this purpose, flexible piezoresistive sensors have been developed that allow being implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a polymeric substrate, using the lithographic technique to construct the various layers of the sensors. The sensor has a gauge factor of -28 for low frequency deformation cycles.In the last decades there has been an increase in sensing systems applied in a variety of situations with a large variety of sensor ranges. This represents a growing area with high potential. One of the areas of sensor development that require a great deal of attention is the area of sensor for biomedical applications and biosensors. These sensors have to overcome a number of challenges and limitations inherent to the environment where they are introduced. These difficulties lead to the necessity of using new materials and new techniques for their construction together with the more traditional materials, e.g. silicon based, which have already proven their potential in this area. Among the various materials, polymers have proven to be a good choice, due to a set of advantages such as simple processing, flexibility and facility of being obtained in different shapes. Therefore it is interesting to fabricate polymer based piezoresistive sensors for functional coatings of implantable hip prosthesis. These sensors will allow coating the prosthesis and provide new functionalities to the implants such as the possibility to measure forces and deformations between the prosthesis and the bone and therefore improving the postoperative diagnostic. In this works, a model of hip prosthesis with coated sensors was developed. For this purpose, flexible piezoresistive sensors have been developed that allow being implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a polymeric substrate, using the lithographic technique to construct the various layers of the sensors. The sensor has a gauge factor of -28 for low frequency deformation cycles. Sensors with larger flexibility were also developed though inkjet printing technique. Various configurations and materials were used to evaluate which materials are most appropriate for these types of sensors. Sensors with a gauge factor of approximately 2.5 for an active layer of PeDOT were obtained. A sensor matrix of 4 x 5 sensors was fabricated with an active area of 1.8 x 1.5 mm2 per sensor. These sensors were subjected to a set of electromechanical tests to evaluate its performance in situations close to end use. So the prosthesis was coated with the various sensors, cemented and subjected to deformation cycles for three levels of force according to standard ISO7206. An adaptive system read-out electronic circuit was developed and built that allows reading piezoresistive sensors with different characteristics. This system allows measuring a matrix of 8x8 sensors, but can be scaled to a large number of sensors. The readable range of the system is between 50 Ω and 100 kΩ according to the needs of the sensors being implanted. The total area of the circuit is 135 mm2, according to the requirements of a circuit to be used in in-vivo applications. An energy management system was also implemented that allows to activate and deactivate parts of the circuit when they are not needed, reducing the energy consumption. The system was validated by measuring a matrix of sensors with different characteristics. Finally, simulations were performed in order to evaluate the best options for the development of a wireless communications system. Three possible operation frequency ranges were used for three types of standard antennas. The communication system was introduced into a model simulating the characteristics of the various layers that constitute the human body. These simulations allow evaluate the frequency range most appropriate for implantable devices, the most appropriate antenna and the best location within the body. So the frequency chosen for the implementation was 868 Mhz for a Inverted- F antenna (IFA). In conclusion, the key elements for the implementations of an instrumented hip prosthesis were development and validated. The developed and/or simulated elements, including sensors, circuits for reading and communication system can also be used in other applications due to characteristics.These simulations allow evaluate the frequency range most appropriate for implantable devices, the most appropriate antenna and the best location within the body. So the frequency chosen for the implementation was 868 Mhz for a Inverted- F antenna (IFA). In conclusion, the key elements for the implementations of an instrumented hip prosthesis were development and validated. The developed and/or simulated elements, including sensors, circuits for reading and communication system can also be used in other applications due to characteristics. Neste trabalho foi desenvolvido um modelo de prótese de anca com implementação de sensores. Para atingir esse objectivo, foram desenvolvidos sensores piezoresitivos flexíveis que permitam ser implantados. Assim foram fabricados sensores de deformação baseados em filmes finos de n+-nc-si.H pela técnica de hot-wire chemical vapor deposition a uma temperatura de 150ºC sobre um substrato polimérico. Recorreu-se a técnica de litografia para construir as várias camadas do sensor. Os sensores apresentam um gauge factor de -28, para ciclos de baixa frequência em testes de four-point-bending. Foram ainda desenvolvidos sensores com uma maior flexibilidade através da técnica de inkjet printing. Para esse desenvolvimento foram usadas várias configurações e materiais, para avaliar quais os materiais mais adequados para este tipo de sensores. Na caracterização destes sensores obteve-se um gauge factor de aproximadamente 2.5 para uma camada ativa de PeDOT. Com os melhores sensores obtidos foram construídas matrizes de 4 x 5 sensores que apresentam uma área ativa de 1.8 x 1.5mm2 por sensor. Estes sensores foram sujeitos a um conjunto de ensaios electromecânicos, para avaliar o seu desempenho em situações próximas da utilização final. Desta forma foi revestida uma prótese com os diferentes sensores, cimentada e sujeita a ciclos de deformação para três níveis de força, segundo a norma ISO7206. Foi desenvolvido e construído um sistema de leitura adaptável que permite medir sensores piezoresistivos com diferentes características entre eles. Este sistema permite medir uma matriz de 8x8 sensores, mas pode ser escalada para um número maior de sensores. A gama de leitura do sistema varia entre 50 Ω e 100 kΩ, de acordo com as necessidades dos sensores a serem implementados. A área total deste circuito é de 135 mm2, de acordo com as necessidades de um circuito a ser utilizado em aplicações in-vivo. Foi também implementado um sistema de gestão de energia que permite ativar e desativar partes do circuito quando estas não são necessárias, permitindo, desta forma, reduzir os consumos de energia. O sistema foi validado através da medição de uma matriz de sensores com diferentes características. foram realizadas simulações de forma a avaliar as melhores opções para o desenvolvimento do sistema de comunicação sem fios. Foram usadas três possíveis gamas de frequência de operação para três tipos de antenas standard. O sistema de comunicação foi introduzido num modelo simulando as características das várias camadas que constituem o corpo humano. Estas simulações permitem aferir a gama de frequências mais adequadas para os dispositivos implantáveis, a antena mais adequada e a sua melhor localização, pois foi verificado como as várias camadas que constituem o corpo humano influenciam a comunicação. Assim, a frequência escolhida para a implementação foi de 868 MHz e a antena foi a IFA. Em conclusão, os elementos principais para a implementação de uma prótese de anca instrumentada, foram desenvolvidos e validados. Os elementos desenvolvidos e/ou simulados, incluindo os sensores, circuitos de leitura e sistema de comunicação, poderão igualmente ser utilizados em outras aplicações devido às suas boas características

Fibre optic pressure sensors in healthcare applications

This PhD thesis provides an extensive description of the development of two fibre optic pressure sensors for applications in health care: (i) a miniature fibre optic Fabry–Perot pressure sensor for fluid pressure measurements in invasive blood pressure monitoring and; (ii) a highly sensitive fibre Bragg grating sensor for contact/interface pressure measurement. The fibre optic Fabry-Perot pressure sensor has a diameter of 125 μm and is created by forming a cavity at the tip of a single-mode optical fibre. Parylene films were used as the pressure-sensitive diaphragm. The performance of three sensors with different aspect ratios has been investigated. The pressure sensing range of ~10 kPa (diastolic pressure)- ~15 kPa (systolic pressure) was targeted; sensor with the cavity of 70 μm in diameter and cavity length of 87 μm is able to sense within a range of 0- 18 kPa with an average sensitivity of 0.12 nm/kPa and response time of 3 seconds. The temperature sensitivity of 0.084 nm/°C was observed. Hysteresis and wavelength drift were observed for the sensors, which may be due to the permeability of the Parylene film to the air. Solutions for reducing hysteresis, wavelength drift and temperature cross-sensitivity are discussed in detail. Fibre Bragg grating (FBG) sensor technology is an ideal candidate for contact pressure measurement in compression therapy, pressure ulcer or prosthetics due to its many advantages such as conforming to body parts, small size, biocompatibility and multiplexing capabilities. A successful mathematical model for an FBG contact pressure sensor for healthcare applications has been presented and experimentally validated. The model has been compared with previous studies reported in the literature and takes into account birefringence. The highest sensitivity was achieved for the disc shape with a sensitivity of 0.8719 nm/MPa for a diameter of 5.5 mm, thickness of 1 mm and Young’s modulus of 20 MPa. This sensor was comprised of a 3 mm long FBG 6 centrally located in the patch. This is a pressure sensitivity of ~270 times increase when compared with a bare FBG reported in the literature. Birefringence effect was observed for the disk patch for pressures larger than 2.6 MPa. Even though FBGs provide high sensitivity in contact pressure sensing in healthcare, the potential applications are limited by the size and cost of commercially available FBG interrogators. A successful first attempt towards the development of a single channel compact FBG interrogation was accomplished. The system consists of a three-section distributed Bragg Reflector (DBR) tuneable laser, microcontroller unit, precision 5 channel current driver IC, photodiode circuit and a temperature controller IC. The tuneable laser was calibrated within 1535-1544 nm wavelength range to produce three current–wavelength lookup tables for wavelength resolution of 1 nm, 0.1 nm, 0.01 nm which is dependent on the current resolution. Futureworkincludesaddingpowercircuitry, a photodiode circuit and a feedback circuit to minimize power fluctuations. The system was tested compared to the commercial Smartscope FBG interrogator

INERTIAL MOTION CAPTURE SYSTEM FOR BIOMECHANICAL ANALYSIS IN PRESSURE SUITS

A non-invasive system has been developed at the University of Maryland Space System Laboratory with the goal of providing a new capability for quantifying the motion of the human inside a space suit. Based on an array of six microprocessors and eighteen microelectromechanical (MEMS) inertial measurement units (IMUs), the Body Pose Measurement System (BPMS) allows the monitoring of the kinematics of the suit occupant in an unobtrusive, self-contained, lightweight and compact fashion, without requiring any external equipment such as those necessary with modern optical motion capture systems. BPMS measures and stores the accelerations, angular rates and magnetic fields acting upon each IMU, which are mounted on the head, torso, and each segment of each limb. In order to convert the raw data into a more useful form, such as a set of body segment angles quantifying pose and motion, a series of geometrical models and a non-linear complimentary filter were implemented. The first portion of this works focuses on assessing system performance, which was measured by comparing the BPMS filtered data against rigid body angles measured through an external VICON optical motion capture system. This type of system is the industry standard, and is used here for independent measurement of body pose angles. By comparing the two sets of data, performance metrics such as BPMS system operational conditions, accuracy, and drift were evaluated and correlated against VICON data. After the system and models were verified and their capabilities and limitations assessed, a series of pressure suit evaluations were conducted. Three different pressure suits were used to identify the relationship between usable range of motion and internal suit pressure. In addition to addressing range of motion, a series of exploration tasks were also performed, recorded, and analysed in order to identify different motion patterns and trajectories as suit pressure is increased and overall suit mobility is reduced. The focus of these evaluations was to quantify the reduction in mobility when operating in any of the evaluated pressure suits. This data should be of value in defining new low cost alternatives for pressure suit performance verification and evaluation. This work demonstrates that the BPMS technology is a viable alternative or companion to optical motion capture; while BPMS is the first motion capture system that has been designed specifically to measure the kinematics of a human in a pressure suit, its capabilities are not constrained to just being a measurement tool. The last section of the manuscript is devoted to future possible uses for the system, with a specific focus on pressure suit applications such in the use of BPMS as a master control interface for robot teleoperation, as well as an input interface for future robotically augmented pressure suits

Multimodal Wearable Sensors for Human-Machine Interfaces

Certain areas of the body, such as the hands, eyes and organs of speech production, provide high-bandwidth information channels from the conscious mind to the outside world. The objective of this research was to develop an innovative wearable sensor device that records signals from these areas more conveniently than has previously been possible, so that they can be harnessed for communication. A novel bioelectrical and biomechanical sensing device, the wearable endogenous biosignal sensor (WEBS), was developed and tested in various communication and clinical measurement applications. One ground-breaking feature of the WEBS system is that it digitises biopotentials almost at the point of measurement. Its electrode connects directly to a high-resolution analog-to-digital converter. A second major advance is that, unlike previous active biopotential electrodes, the WEBS electrode connects to a shared data bus, allowing a large or small number of them to work together with relatively few physical interconnections. Another unique feature is its ability to switch dynamically between recording and signal source modes. An accelerometer within the device captures real-time information about its physical movement, not only facilitating the measurement of biomechanical signals of interest, but also allowing motion artefacts in the bioelectrical signal to be detected. Each of these innovative features has potentially far-reaching implications in biopotential measurement, both in clinical recording and in other applications. Weighing under 0.45 g and being remarkably low-cost, the WEBS is ideally suited for integration into disposable electrodes. Several such devices can be combined to form an inexpensive digital body sensor network, with shorter set-up time than conventional equipment, more flexible topology, and fewer physical interconnections. One phase of this study evaluated areas of the body as communication channels. The throat was selected for detailed study since it yields a range of voluntarily controllable signals, including laryngeal vibrations and gross movements associated with vocal tract articulation. A WEBS device recorded these signals and several novel methods of human-to-machine communication were demonstrated. To evaluate the performance of the WEBS system, recordings were validated against a high-end biopotential recording system for a number of biopotential signal types. To demonstrate an application for use by a clinician, the WEBS system was used to record 12‑lead electrocardiogram with augmented mechanical movement information

Fibre optic pressure sensors in healthcare applications

This PhD thesis provides an extensive description of the development of two fibre optic pressure sensors for applications in health care: (i) a miniature fibre optic Fabry–Perot pressure sensor for fluid pressure measurements in invasive blood pressure monitoring and; (ii) a highly sensitive fibre Bragg grating sensor for contact/interface pressure measurement. The fibre optic Fabry-Perot pressure sensor has a diameter of 125 μm and is created by forming a cavity at the tip of a single-mode optical fibre. Parylene films were used as the pressure-sensitive diaphragm. The performance of three sensors with different aspect ratios has been investigated. The pressure sensing range of ~10 kPa (diastolic pressure)- ~15 kPa (systolic pressure) was targeted; sensor with the cavity of 70 μm in diameter and cavity length of 87 μm is able to sense within a range of 0- 18 kPa with an average sensitivity of 0.12 nm/kPa and response time of 3 seconds. The temperature sensitivity of 0.084 nm/°C was observed. Hysteresis and wavelength drift were observed for the sensors, which may be due to the permeability of the Parylene film to the air. Solutions for reducing hysteresis, wavelength drift and temperature cross-sensitivity are discussed in detail. Fibre Bragg grating (FBG) sensor technology is an ideal candidate for contact pressure measurement in compression therapy, pressure ulcer or prosthetics due to its many advantages such as conforming to body parts, small size, biocompatibility and multiplexing capabilities. A successful mathematical model for an FBG contact pressure sensor for healthcare applications has been presented and experimentally validated. The model has been compared with previous studies reported in the literature and takes into account birefringence. The highest sensitivity was achieved for the disc shape with a sensitivity of 0.8719 nm/MPa for a diameter of 5.5 mm, thickness of 1 mm and Young’s modulus of 20 MPa. This sensor was comprised of a 3 mm long FBG 6 centrally located in the patch. This is a pressure sensitivity of ~270 times increase when compared with a bare FBG reported in the literature. Birefringence effect was observed for the disk patch for pressures larger than 2.6 MPa. Even though FBGs provide high sensitivity in contact pressure sensing in healthcare, the potential applications are limited by the size and cost of commercially available FBG interrogators. A successful first attempt towards the development of a single channel compact FBG interrogation was accomplished. The system consists of a three-section distributed Bragg Reflector (DBR) tuneable laser, microcontroller unit, precision 5 channel current driver IC, photodiode circuit and a temperature controller IC. The tuneable laser was calibrated within 1535-1544 nm wavelength range to produce three current–wavelength lookup tables for wavelength resolution of 1 nm, 0.1 nm, 0.01 nm which is dependent on the current resolution. Futureworkincludesaddingpowercircuitry, a photodiode circuit and a feedback circuit to minimize power fluctuations. The system was tested compared to the commercial Smartscope FBG interrogator