Experimental study of PDMS membranes fabricated either by spin coating or transfer bonding to a silicon chip with etched cavity

Nowadays, with no doubt, PDMS, poly(dimethylsiloxane) elastomer is material of choice for microfluidic fabrication because of its unique chemical, optical and mechanical properties. Unfortunately, it is not photo-definable (i.e. not a photoresist) and fabrication of PDMS MEM (micro-electro-mechanical) devices is typically done using soft lithography. Some steps of the process are difficult to perform without manually handling PDMS layers. Next problem to be considered in patterning PDMS membranes is bond strength between membrane and silicon substrate. To investigate this, we fabricated PDMS membranes on silicon either by spin coating Si wafer or transferring previously fabricated PDMS membrane to Si chip with bonding layer on it. PDMS network samples for this research were synthesized with the same composition, which are Sylgard 184 (Dow Corning, USA) silicone elastomer base and silicone elastomer curing agent, volume ratio 10:1. Fabrication of test structures is based on bulk micromachining on ⟨100⟩ oriented Si wafers to fabricate square cavities on which PDMS membranes were realized by one of mentioned procedures. Mechanical testing of PDMS membranes, elastic properties and adhesion strength of membranes with different thicknesses were investigated applying pressurized bulge testing. Pressure was applied to the PDMS membrane via nitrogen gas and the resulting load-deflection curves were monitoring

All-Printed piezoresistive sensor matrix with organic thin-film transistors as a switch for crosstalk reduction

A generation of piezoresistive sensors (force or deformation) fully processed by printing technologies is increasingly being implemented in applications due to advantages as a large-area application, simple device, integration, and high flexibility. This work reports the development of a fully printed piezoresistive (5 × 5 sensor) matrix in which an organic thin-film transistor (OTFT) is placed in each sensor to allow the readout of each sensor independently and thus reducing the crosstalk between individual sensors. The manufacturing was carried out using inkjet printing for the deposition of materials in a thin layer stacked on top of each other to obtain functional OTFTs. The piezoresistive nanocomposite sensors, based on multiwalled carbon nanotubes within an elastomeric styrene-ethylene-butadiene-styrene (SEBS) polymer matrix, were fabricated by screen printing. The fabrication and characterization of both OTFT and sensors are presented and discussed in detail. The inkjet-printed OTFTs (width/length channel ratio of ∼130) show a drain-source current (IDS) of 150 μA with a gate-source voltage of −40 V. Gauge factors of up to 5.9 were obtained for the sensors, resulting in a current variation of 1.5 μA. This corresponds to about 0.7% of the total IDS in a deformation cycleFCT – Fundação para a Ciência e Tecnologia within the Project Scope: UID/CEC/00319/2019, UID/FIS/04650/2019 and projects PTDC/FIS-MAC/28157/2017 and PTDC/BTM-MAT/28237/2017, SFRH/BPD/110914/2015 (PC). V.C. thanks FCT for the junior researcher contract (DL57/2016). We acknowledge funding from the European Union’s Horizon 2020 Programme for Research, ICT-02-2018 - Flexible and Wearable Electronics. Grant agreement no. 824339 – WEARPLEX. Financial support from the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06

Highly sensitive capacitive pressure sensors for robotic applications based on carbon nanotubes and PDMS polymer nanocomposite

Flexible tactile pressure sensor arrays based on multiwalled carbon nanotubes (MWCNT) and polydimethylsiloxane (PDMS) are gaining importance, especially in the field of robotics because of the high demand for stable, flexible and sensitive sensors. Some existing concepts of pressure sensors based on nanocomposites exhibit complicated fabrication techniques and better sensitivity than the conventional pressure sensors. In this article, we propose a nanocomposite-based pressure sensor that exhibits a high sensitivity of 25&thinsp;%&thinsp;N−1, starting with a minimum load range of 0–0.01&thinsp;N and 46.8&thinsp;%&thinsp;N−1 in the range of 0–1&thinsp;N. The maximum pressure sensing range of the sensor is approximately 570&thinsp;kPa. A concept of a 4×3 tactile sensor array, which could be integrated to robot fingers, is demonstrated. The high sensitivity of the pressure sensor enables precision grasping, with the ability to sense small objects with a size of 5&thinsp;mm and a weight of 1&thinsp;g. Another application of the pressure sensor is demonstrated as a gait analysis for humanoid robots. The pressure sensor is integrated under the foot of a humanoid robot to monitor and evaluate the gait of the robot, which provides insights for optimizing the robot's self-balancing algorithm in order to maintain the posture while walking.</p

Flexible Pressure Sensor with Ag Wrinkled Electrodes Based on PDMS Substrate

Flexible pressure sensors are essential components of electronic skins for future attractive applications ranging from human healthcare monitoring to biomedical diagnostics, robotic skins, and prosthetic limbs. Here we report a new kind of flexible pressure sensor. The sensors are capacitive, and composed of two Ag wrinkled electrodes separated by a carbon nanotubes (CNTs)/polydimethylsiloxane (PDMS) composite deformable dielectric layer. Ag wrinkled electrodes were formed by vacuum deposition on top of pre-strained and relaxed PDMS substrates which were treated using an O2 plasma, a surface functionalization process, and a magnetron sputtering process. Ultimately, the developed sensor exhibits a maximum sensitivity of 19.80% kPa−1 to capacitance, great durability over 500 cycles, and rapid mechanical responses (&lt;200 ms). We also demonstrate that our sensor can be used to effectively detect the location and distribution of finger pressure

Highly Sensitive Soft Foam Sensors for Wearable Applications

Due to people’s increasing desire for body health monitoring, the needs of knowing humans’ body parameters and transferring them to analyzable and understandable signals become increasingly attractive and significant. The present body-sign measurement devices are still bulky medical devices used in settings such as clinics or hospitals, which are accurate, but expensive and cannot achieve the personalization of usage targets and the monitoring of real-time body parameters. Many commercial wearable devices can provide some of the body indexes, such as the smartwatch providing the pulse/heartbeat information, but cannot give accurate and reliable data, and the data could be influenced by the user’s movement and the loose wearing habit, either. In this way, developing next-generation wearable devices combining good wearable experience and accuracy is gathering increasing attention. The aim of this study is to develop a high-performance pressure/strain sensor with the requirements of comfortable to wear, and having great electromechanical behaviour to convert the physiological signal to an analyzable signal

A novel approach to the 'pressing problem' of lymphoedema - engineering a foundation for clinical standards and efficacy-based compression therapy

Lymphoedema, particularly as manifested in a secondary form following previous intervention or trauma, is a progressive and debilitating disorder, embodied through the gradual volumetric swelling of a patient’s limb(s), often leading to fibrosis as well as loss of limb function in extreme cases. Compression-based therapy, the most prevalent prophylaxis and post-onset treatment approach for lymphoedema, has shown only limited success. A clear lack of standards, i.e., guidelines insuring consistency of structure and function of lymphoedema compression sleeves, hinders progress toward finding a cure for lymphoedema and prevents testing of sleeve efficacy. The body of work included in this thesis highlights the specific areas of focus necessary to move the field forward and sets a path towards comprehensive profiling of compression sleeves, to understand the underlying mechanisms relevant to efficacious treatment at the interface of the sleeve and skin. Current high-resolution commercial pressure sensors proved ineffective for measurement of spatial and temporal pressure profiles of sleeves in situ; limitations of such compliant sensors, and specific areas of needed improvement, were identified. Promising prototypes of flexible, high-resolution custom sensors were also assessed, with preliminary data and their shortfalls leading to definition of technical specifications for the future. Finally, a Digital Image Correlation (DIC)-based approach was applied to map strains of compression sleeves in high resolution and in situ. The DIC-method platform was tested and validated as a means to provide qualitative and quantitative characterisation of compression sleeves as a function of sleeve size, class, and manufacturer independent of lymphoedema state. Unexpected differences in pressure profiles underscore the need for both standardisation of sleeve design as well as follow on studies testing sleeve function in lymphoedema patients. Looking toward the future, testing of efficacy and head-to-head comparison of standardised and bespoke garments will better enable mechanistic understanding of lymphoedema's aetiology, unraveling how the ‘global’ disease state emerges from ‘local’ events, leading to a basis for lymphoedema prevention in the future

Development of Multifunctional E-skin Sensors

Electronic skin (e-skin) is a hot topic due to its enormous potential for health monitoring, functional prosthesis, robotics, and human-machine-interfaces (HMI). For these applications, pressure and temperature sensors and energy harvesters are essential. Their performance may be tuned by their films micro-structuring, either through expensive and time-consuming photolithography techniques or low-cost yet low-tunability approaches. This PhD thesis aimed to introduce and explore a new micro-structuring technique to the field of e-skin – laser engraving – to produce multifunctional e-skin devices able to sense pressure and temperature while being self-powered. This technique was employed to produce moulds for soft lithography, in a low-cost, fast, and highly customizable way. Several parameters of the technique were studied to evaluate their impact in the performance of the devices, such as moulds materials, laser power and speed, and design variables. Amongst the piezoresistive sensors produced, sensors suitable for blood pressure wave detection at the wrist [sensitivity of – 3.2 kPa-1 below 119 Pa, limit of detection (LOD) of 15 Pa], general health monitoring (sensitivity of 4.5 kPa-1 below 10 kPa, relaxation time of 1.4 ms, micro-structured film thickness of only 133 µm), and robotics and functional prosthesis (sensitivity of – 6.4 × 10-3 kPa-1 between 1.2 kPa and 100 kPa, stable output over 27 500 cycles) were obtained. Temperature sensors with micro-cones were achieved with a temperature coefficient of resistance (TCR) of 2.3 %/°C. Energy harvesters based on micro-structured composites of polydimethylsiloxane (PDMS) and zinc tin oxide (ZnSnO3) nanowires (NWs; 120 V and 13 µA at > 100 N) or zinc oxide (ZnO) nanorods (NRs; 6 V at 2.3 N) were produced as well. The work described herein unveils the tremendous potential of the laser engraving technique to produce different e-skin devices with adjustable performance to suit distinct applications, with a high benefit/cost ratio