New generation of interactive platforms based on novel printed smart materials

Programa doutoral em Engenharia Eletrónica e de Computadores (área de Instrumentação e Microssistemas Eletrónicos)The last decade was marked by the computer-paradigm changing with other digital devices suddenly becoming available to the general public, such as tablets and smartphones. A shift in perspective from computer to materials as the centerpiece of digital interaction is leading to a diversification of interaction contexts, objects and applications, recurring to intuitive commands and dynamic content that can proportionate more interesting and satisfying experiences. In parallel, polymer-based sensors and actuators, and their integration in different substrates or devices is an area of increasing scientific and technological interest, which current state of the art starts to permit the use of smart sensors and actuators embodied within the objects seamlessly. Electronics is no longer a rigid board with plenty of chips. New technological advances and perspectives now turned into printed electronics in polymers, textiles or paper. We are assisting to the actual scaling down of computational power into everyday use objects, a fusion of the computer with the material. Interactivity is being transposed to objects erstwhile inanimate. In this work, strain and deformation sensors and actuators were developed recurring to functional polymer composites with metallic and carbonaceous nanoparticles (NPs) inks, leading to capacitive, piezoresistive and piezoelectric effects, envisioning the creation of tangible user interfaces (TUIs). Based on smart polymer substrates such as polyvinylidene fluoride (PVDF) or polyethylene terephthalate (PET), among others, prototypes were prepared using piezoelectric and dielectric technologies. Piezoresistive prototypes were prepared with resistive inks and restive functional polymers. Materials were printed by screen printing, inkjet printing and doctor blade coating. Finally, a case study of the integration of the different materials and technologies developed is presented in a book-form factor.A última década foi marcada por uma alteração do paradigma de computador pelo súbito aparecimento dos tablets e smartphones para o público geral. A alteração de perspetiva do computador para os materiais como parte central de interação digital levou a uma diversificação dos contextos de interação, objetos e aplicações, recorrendo a comandos intuitivos e conteúdos dinâmicos capazes de tornarem a experiência mais interessante e satisfatória. Em simultâneo, sensores e atuadores de base polimérica, e a sua integração em diferentes substratos ou dispositivos é uma área de crescente interesse científico e tecnológico, e o atual estado da arte começa a permitir o uso de sensores e atuadores inteligentes perfeitamente integrados nos objetos. Eletrónica já não é sinónimo de placas rígidas cheias de componentes. Novas perspetivas e avanços tecnológicos transformaram-se em eletrónica impressa em polímeros, têxteis ou papel. Neste momento estamos a assistir à redução da computação a objetos do dia a dia, uma fusão do computador com a matéria. A interatividade está a ser transposta para objetos outrora inanimados. Neste trabalho foram desenvolvidos atuadores e sensores e de pressão e de deformação com recurso a compostos poliméricos funcionais com tintas com nanopartículas (NPs) metálicas ou de base carbónica, recorrendo aos efeitos capacitivo, piezoresistivo e piezoelétrico, com vista à criação de interfaces de usuário tangíveis (TUIs). Usando substratos poliméricos inteligentes tais como fluoreto de polivinilideno (PVDF) ou politereftalato de etileno (PET), entre outos, foi possível a preparação de protótipos de tecnologia piezoelétrica ou dielétrica. Os protótipos de tecnologia piezoresistiva foram feitos com tintas resistivas e polímeros funcionais resistivos. Os materiais foram impressos por serigrafia, jato de tinta, impressão por aerossol e revestimento de lâmina doctor blade. Para terminar, é apresentado um caso de estudo da integração dos diferentes materiais e tecnologias desenvolvidos sob o formato de um livro.This project was supported by FCT – Fundação para a Ciência e a Tecnologia, within the doctorate grant with reference SFRH/BD/110622/2015, by POCH – Programa Operacional Capital Humano, and by EU – European Union

TACTILE SENSING WITH COMPLIANT STRUCTURES FOR HUMAN-ROBOT INTERACTION

This dissertation presents the research on tactile sensing with compliant structures towards human-robot interaction. It would be beneficial for robots working collaboratively with humans to be soft or padded and have compliant tactile sensing skins over the padding. To allow the robots to interact with humans via touch effectively and safely and to detect tactile stimuli in an unstructured environment, new tactile sensing concepts are needed that can detect a wide range of potential interactions and sense over an area. However, most highly sensitive tactile sensors are unable to cover the forces involved in human contacts, which ranges from 1 newton to thousand newtons; to implement area sensing capabilities, there have been challenges in creating traditional sensing arrays, where the associated supporting electronics become more complex with an increasing number of sensing elements. This dissertation develops a novel multi-layer cutaneous tactile sensing architecture for enhanced sensitivity and range, and employs an imaging technique based on boundary measurements called electrical impedance tomography (EIT) to achieve area tactile sensing capabilities. The multi-layer cutaneous tactile sensing architecture, which consists of stretchable piezoresistive strain-sensing layers over foam padding layers of different stiffness, allows for both sufficient sensitivity and an extended force range for human contacts. The role that the padding layer plays when placed under a stretchable sensing layer was investigated, and it was discovered that the padding layer magnifies the sensor signal under indentation compared to that obtained without padding layers. The roles of the multi-layer foams were investigated by changing stiffness and thickness, which allows tailoring the response of multi-layer architectures for different applications. To achieve both extended force range and distributed sensing, EIT technique was employed with the multi-layer sensing architecture. Machine and human touch were conducted on the developed multi-layer sensing system, revealing that the second sensing skin is required to detect the large variability in human touch. Although widely applied in the medical field for functional imaging, EIT applied in tactile sensing faces different challenges, such as unknown number and region of tactile stimuli. Current EIT tactile sensors have focused on qualitative demonstration. This dissertation aims at achieving quantitative information from piezoresistive EIT tactile sensors, by investigating spatial performance and the effect of sensor’s conductivity. A spatial correction method was developed for obtaining consistent spatial information, which was validated by both simulation and experiments from our stretchable piezoresistive EIT sensor with an underlying padding layer

Thermoplastic polyurethane flexible capacitive proximity sensor reinforced by CNTs for applications in the creative industries

Wearable sensing platforms have been rapidly advanced over recent years, thanks to numerous achievements in a variety of sensor fabrication techniques. However, the development of a flexible proximity sensor that can perform in a large range of object mobility remains a challenge. Here, a polymer-based sensor that utilizes a nanostructure composite as the sensing element has been presented for forthcoming usage in healthcare and automotive applications. Thermoplastic Polyurethane (TPU)/Carbon Nanotubes (CNTs) composites are capable of detecting presence of an external object in a wide range of distance. The proximity sensor exhibits an unprecedented detection distance of 120 mm with a resolution of 0.3%/mm. The architecture and manufacturing procedures of TPU/CNTs sensor are straightforward and performance of the proximity sensor shows robustness to reproducibility as well as excellent electrical and mechanical flexibility under different bending radii and over hundreds of bending cycles with variation of 4.7% and 4.2%, respectively. Tunneling and fringing effects are addressed as the sensing mechanism to explain significant capacitance changes. Percolation threshold analysis of different TPU/CNT contents indicated that nanocomposites having 2 wt% carbon nanotubes are exhibiting excellent sensing capabilities to achieve maximum detection accuracy and least noise among others. Fringing capacitance effect of the structure has been systematically analyzed by ANSYS Maxwell (Ansoft) simulation, as the experiments precisely supports the sensitivity trend in simulation. Our results introduce a new mainstream platform to realize an ultrasensitive perception of objects, presenting a promising prototype for application in wearable proximity sensors for motion analysis and artificial electronic skin

Sensors for Robotic Hands: A Survey of State of the Art

Recent decades have seen significant progress in the field of artificial hands. Most of the surveys, which try to capture the latest developments in this field, focused on actuation and control systems of these devices. In this paper, our goal is to provide a comprehensive survey of the sensors for artificial hands. In order to present the evolution of the field, we cover five year periods starting at the turn of the millennium. At each period, we present the robot hands with a focus on their sensor systems dividing them into categories, such as prosthetics, research devices, and industrial end-effectors.We also cover the sensors developed for robot hand usage in each era. Finally, the period between 2010 and 2015 introduces the reader to the state of the art and also hints to the future directions in the sensor development for artificial hands

Flexible sensors—from materials to applications

Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications

Soft Sensors in digital healthcare monitoring

Stretchable sensors are a class of materials with applications across research fields from healthcare to structural engineering. Despite the extensive research aiming to improve the performance of individual materials or components, stretchable sensor devices are difficult to implement because conventional electronic components, mainly used for processing, which are rigid, have to make contact with soft components reliable enough to withstand real-world usage. This thesis introduces a method for creating electrical contacts that can be robustly attached onto soft, stretchable conductive polymer composites on one side and soldered to metal wires on the other side. Mechanically robust electrical contacts were developed to interface (soft) silicone-based strain sensors with conventional (hard) solid-state electronics using a nanoporous silicon-copper contact. Contacts are mounted on custom-made and commercial soft strain sensitive silicone sensors. The contacts are shown to be reliable under large deformations, then compared with a commonly used alternative under real-world strain conditions. The layered structure of the device creates a complex electronic component deriving from the silicon-copper Schottky junction. This thesis tests the versatility of the technology through a series of real-world applications. The silicon-copper contacts were used to produce a series of proof-of-concept devices, including a wearable respiration monitor, leg band for exercise monitoring, and squeezable ball to monitor rehabilitation of patients with hand injuries or neurological disorders. The sensor is shown to operate and detect multiple modes of motion regardless of placement on the body. Next, a proof-of-concept device was employed to measure hand grip strength. The optimized sensor can detect grip strength with high sensitivity. The hardness of the device was shown to increase sensitivity when healthy humans performed manual exercises and completed digital tasks. Providing patients with these devices can help monitor their rehabilitation following hand injuries or neurological disorders. This can be done through self-led at-home therapy which has been shown to improve treatment, engagement, long-term lifestyle adherence, while avoiding repeated visits to clinics which plays an important role in frequency of therapy, effectiveness, and accessibility.Open Acces

Smart Fabric sensors for foot motion monitoring

Smart Fabrics or fabrics that have the characteristics of sensors are a wide and emerging field of study. This thesis summarizes an investigation into the development of fabric sensors for use in sensorized socks that can be used to gather real time information about the foot such as gait features. Conventional technologies usually provide 2D information about the foot. Sensorized socks are able to provide angular data in which foot angles are correlated to the output from the sensor enabling 3D monitoring of foot position. Current angle detection mechanisms are mainly heavy and cumbersome; the sensorized socks are not only portable but also non-invasive to the subject who wears them. The incorporation of wireless features into the sensorized socks enabled a remote monitoring of the foot

Properties and Applications of Graphene and Its Derivatives

Graphene is a two-dimensional, one-atom-thick material made entirely of carbon atoms, arranged in a honeycomb lattice. Because of its distinctive mechanical (e.g., high strength and flexibility) and electronic (great electrical and thermal conductivities) properties, graphene is an ideal candidate in myriad applications. Thus, it has just begun to be engineered in electronics, photonics, biomedicine, and polymer-based composites, to name a few. The broad family of graphene nanomaterials (including graphene nanoplatelets, graphene oxide, graphene quantum dots, and many more) go beyond and aim higher than mere single-layer (‘pristine’) graphene, and thus, their potential has sparked the current Special Issue. In it, 18 contributions (comprising 14 research articles and 4 reviews) have portrayed probably the most interesting lines as regards future and tangible uses of graphene derivatives. Ultimately, understanding the properties of the graphene family of nanomaterials is crucial for developing advanced applications to solve important challenges in critical areas such as energy and health

Materials and processes for 3D printed electronics

Dissertação de mestrado integrado em Engenharia de MateriaisThe traditional manufacturing of electronic components consists of complex and with high environmental impact methods. Those materials are potentially dangerous for both environmental and public health, during the manufacturing process and at the end of the product lifetime when not correctly handled. Thus, the goal consists of producing in a simpler/cheaper way and with lower environmental impact, materials to be used into electronic components. In this work inks based of a natural polymer (carrageenan) and ultrapure water (a “green” solvent) were used to produce more environmentally friendly printable electronic components. To achieve magnetic, conductive and dielectric properties CoFe2O4 (CFO), multiwalled carbon nanotubes (MWCNTs) and BaTiO3 (BTO) nanoparticles were added, respectively. To promote a better dispersion and, therefore, to improve the properties of the final product, Triton X-100 was used as a surfactant. Trition X-100 was selected among other surfactants, since it has shown better results on initial selection tests. For the printing process, the most suitable parameters were selected according to the ink viscosity to improve the process as well as to optimize the method to introduce the ink into the syringe. Morphological, thermal, and mechanical tests were performed in order to evaluate the effects of fillers addition and concentration. Dielectric tests were carried out to the samples with BTO. The higher dielectric constant has been obtained for the sample with 20 wt.% BTO content, reaching 1.3 x 104 at 10 kHz. The electrical conductivity evaluation in the samples with MWCNTs shows that a DC conductivity of 0.026 S.m-1 is achieved for the sample with 5 wt.% MWCTNs content. Vibrating sample magnetometer (VSM) test was performed to analyse the magnetic behaviour of the composite samples with CFO, a saturation magnetization of 11 emu.g-1 being obtained for the samples with 20 wt.% CFO content. The inks developed on this work highlights the relevance of the implementation of natural materials as a base for the development of functional and multifunctional materials. Adding to that, this work can also act as an incentive to the study of materials and manufacturing procedures with lower environmental risks with the capacity of still answering society’s needs.A manufatura tradicional de componentes eletrónicos consiste em métodos complexos, com elevado impacto ambiental, quer por gasto energético quer pelos materiais usados que são potencialmente nocivos para o ambiente e para a saúde pública, durante o processo de fabrico e no final de vida do produto, quando não corretamente processados. Visto isto, o objetivo deste projeto consiste em produzir de um modo simples, com baixo custo e com menor impacto ambiental materiais que possam ser usados em componentes eletrónicos. Assim, neste trabalho foram desenvolvidas tintas à base de um polímero natural (carragenina) e água ultrapura (usada como solvente “verde”) para produzir componentes eletrónicos impressos mais amigos do ambiente. De modo a fornecer propriedades magnéticas, condutivas e dielétricas foram adicionadas nanopartículas de CoFe2O4 (CFO), Multicamadas de Nanotubos de Carbono (MWCNTs) e BaTiO3 (BTO), respetivamente. Para promover uma melhor dispersão foi usado Triton X-100 como surfactante. No processo de impressão foram estudados os parâmetros mais adequados de acordo com a viscosidade da tinta para tornar o processo mais rentável assim como tentar encontrar o melhor método para introduzir a tinta dentro da seringa com a menor formação de bolhas possível. Os testes morfológicos, térmicos e mecânicos foram feitos para todas as amostras para comparar as propriedades fornecidas pela adição das partículas, avaliando a sua interferência com o aumento da concentração de filler. Os testes dielétricos foram realizados para as amostras de BTO. A constante dielétrica com valor mais elevado foi obtido para a amostra com concentração de 20 wt.% BTO, atingindo 1.3 x 104 a 10 kHz. A avaliação dos testes de condutividade elétrica nas amostras de MWCNTs, mostraram uma condutividade DC de 0.026 S.m-1 foi obtida para a mostra com concentração de 5 wt.% MWCTNs. O teste de mapeamento de fluxo de valor (VSM) foi realizado para analisar o comportamento magnético do compósito com partículas de CFO, a magnetização de saturação de 11 emu.g-1 foi obtida para a amostra com concentração de 20 wt.% CFO . As tintas desenvolvidas neste trabalho veem dar relevância à implementação de materiais naturais como base para o desenvolvimento de materiais funcionais e multifuncionais. Vem também promover o estudo de materiais e métodos de produção com menos impacto ambiental e que consigam manter a resposta às necessidades da sociedade

Wearable pressure sensing for intelligent gesture recognition

The development of wearable sensors has become a major area of interest due to their wide range of promising applications, including health monitoring, human motion detection, human-machine interfaces, electronic skin and soft robotics. Particularly, pressure sensors have attracted considerable attention in wearable applications. However, traditional pressure sensing systems are using rigid sensors to detect the human motions. Lightweight and flexible pressure sensors are required to improve the comfortability of devices. Furthermore, in comparison with conventional sensing techniques without smart algorithm, machine learning-assisted wearable systems are capable of intelligently analysing data for classification or prediction purposes, making the system ‘smarter’ for more demanding tasks. Therefore, combining flexible pressure sensors and machine learning is a promising method to deal with human motion recognition. This thesis focuses on fabricating flexible pressure sensors and developing wearable applications to recognize human gestures. Firstly, a comprehensive literature review was conducted, including current state-of-the-art on pressure sensing techniques and machine learning algorithms. Secondly, a piezoelectric smart wristband was developed to distinguish finger typing movements. Three machine learning algorithms, K Nearest Neighbour (KNN), Decision Tree (DT) and Support Vector Machine (SVM), were used to classify the movement of different fingers. The SVM algorithm outperformed other classifiers with an overall accuracy of 98.67% and 100% when processing raw data and extracted features. Thirdly, a piezoresistive wristband was fabricated based on a flake-sphere composite configuration in which reduced graphene oxide fragments are doped with polystyrene spheres to achieve both high sensitivity and flexibility. The flexible wristband measured the pressure distribution around the wrist for accurate and comfortable hand gesture classification. The intelligent wristband was able to classify 12 hand gestures with 96.33% accuracy for five participants using a machine learning algorithm. Moreover, for demonstrating the practical applications of the proposed method, a realtime system was developed to control a robotic hand according to the classification results. Finally, this thesis also demonstrates an intelligent piezoresistive sensor to recognize different throat movements during pronunciation. The piezoresistive sensor was fabricated using two PolyDimethylsiloxane (PDMS) layers that were coated with silver nanowires and reduced graphene oxide films, where the microstructures were fabricated by the polystyrene spheres between the layers. The highly sensitive sensor was able to distinguish throat vibrations from five different spoken words with an accuracy of 96% using the artificial neural network algorithm