There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

Wireless sensor networks for healthcare: A survey

The "Encyclopedia of Human Computer Interaction" is the most thorough and definitive source providing coverage of everything related to the field of human computer interaction (HCI). This encyclopedia covers a wide range of HCI related topics such as concepts, design, usability, evaluation, innovations, and applications of HCI in organizations around the globe. Hundreds of contributors and advisors from around the world have conferred their expertise to this publication, making this encyclopedia a single source of authoritative and contemporary research in field of human computer interaction. The "Encyclopedia of Human Computer Interaction" also includes coverage of real life experiences and cases of HCI and the lessons learned helping readers to learn extensively about this important field of study.

Encyclopedia of Human Computer Interaction

Smart wearable systems: Current status and future challenges

Post-transition elements, together with zinc-group metals and their alloys belong to an emerging class of materials with fascinating characteristics originating from their simultaneous metallic and liquid natures. These metals and alloys are characterised by having low melting points (i.e. between room temperature and 300 °C), making their liquid state accessible to practical applications in various fields of physical chemistry and synthesis. These materials can offer extraordinary capabilities in the synthesis of new materials, catalysis and can also enable novel applications including microfluidics, flexible electronics and drug delivery. However, surprisingly liquid metals have been somewhat neglected by the wider research community. In this review, we provide a comprehensive overview of the fundamentals underlying liquid metal research, including liquid metal synthesis, surface functionalisation and liquid metal enabled chemistry. Furthermore, we discuss phenomena that warrant further investigations in relevant fields and outline how liquid metals can contribute to exciting future applications.

Liquid metals: fundamentals and applications in chemistry

This article focuses on the fabrication and characterization of stretchable interconnects for wearable electronics applications. Interconnects were screen-printed with a stretchable silver-polymer composite ink on 50-μm thick thermoplastic polyurethane. The initial sheet resistances of the manufactured interconnects were an average of 36.2 mΩ/◽, and half the manufactured samples withstood single strains of up to 74%. The strain proportionality of resistance is discussed, and a regression model is introduced. Cycling strain increased resistance. However, the resistances here were almost fully reversible, and this recovery was time-dependent. Normalized resistances to 10%, 15%, and 20% cyclic strains stabilized at 1.3, 1.4, and 1.7. We also tested the validity of our model for radio-frequency applications through characterization of a stretchable radio-frequency identification tag.

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Screen-Printing Fabrication and Characterization of Stretchable Electronics

Accurate, simple and affordable methods for passive indoor tracking of human beings are still missing. In this article, we describe the development of an unobtrusive two-dimensional human positioning system based on low-frequency electric fields. The system's operation is based on measuring the capacitance between multiple floor tiles and a receiving electrode. The presented system is invisible to the user and uses a single-chip solution to measure the capacitances. The implemented system is evaluated with two different types of receiving electrodes and the results are presented. With the used tiles, the system can locate a standing human with at least 15 cm accuracy and track a walking person with at least 41 cm accuracy. The update rate of the system is 10 Hz.

TileTrack: Capacitive human tracking using floor tiles

Microwave doppler radar offers significant improvements for unobtrusive heart and respiration measurement. Radar monitoring enables non-contact measurement, through clothing, of heart and respiration rate, which is desired in several applications ranging from medical sleep laboratory measurements to home health care measurements and stress monitoring. The use of high frequency radar (>; 10 GHz) instead of lower frequencies (~2.4 GHz) increases the signal-to-noise-ratio of the signal and enables the utilization of commercial radar modules. However, if high frequency radar is used, linear combining of quadrature radar channels is inadequate. Instead, a nonlinear channel combining algorithm is needed. The combining can be performed with an arctangent function if center, amplitude error, and phase error are estimated accurately and corrected. In this paper, we show that the Levenberg-Marquardt (LM) center estimation algorithm outperforms the state-of-the-art center estimation algorithm precision-wise and is computationally less complex. The simulated results show that the root mean squared error with the LM method is always less than 1%, while it is around 5%-13% with the compared method, depending on the breathing signal model used. In addition, the computational complexity of the LM method stays almost constant as the size of the data set increases, whereas with the reference method, it increases exponentially. In this paper, the LM method is validated both with simulations and with real data.

Comparison of Center Estimation Algorithms for Heart and Respiration Monitoring With Microwave Doppler Radar

Continuous miniaturisation of electronic components has made it possible to create smaller and smaller electrical devices which can be worn and carried all the time Together with developing fibre and textile technologies, this has enabled the creation of truly usable smart clothes that resemble clothes more than wearable computing equipment These intelligent clothes are worn like ordinary clothing and provide help in various situations according to the application area This paper describes the design and implementation of a survival smart clothing prototype for the arctic environment Concept development, electrical design, and non-electrical features are discussed The suit provides communication, positioning, and navigation aids for the user Depending on the measurements of the human and the environment, the suit decides whether an emergency message should be sent The user can control the system with a user interface called a Yo-Yo The functionality of the suit has been tested in an arctic environment

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Smart Clothing Prototype for the Arctic Environment

Textile electrodes are a new, potential choice for biopotential measurements. In this research different kinds of textile electrodes were compared and good measurement positions were searched. In the first part the properties of four structurally different textile electrodes were compared. Three of the electrodes were industrially manufactured and one was hand-made by embroidering conductive yarn on fabric. Four test persons were used for testing the electrodes. The measurements were made three times with all the four electrode types so that in each measurement the skin was prepared in a different way. Three of the test persons performed a predefined set of motions for five minutes. The purpose of this test was to examine how well the textile electrodes stay put in different positions during movements. The best results were achieved with the embroidered electrodes, which have a large contact area with the skin. According to the measurement results, undisturbed electrode positions are difficult to find. Good positions for one person might not fit for another, because of the different shapes of bodies. In addition, different properties of skin and direction of the axis of the heart may affect the results.

Jukka Vanhala

Papers

Screen-Printing Fabrication and Characterization of Stretchable Electronics

TileTrack: Capacitive human tracking using floor tiles

Comparison of Center Estimation Algorithms for Heart and Respiration Monitoring With Microwave Doppler Radar

Smart Clothing Prototype for the Arctic Environment

Textile Electrodes in ECG Measurement