Smart Skin Research Articles

Materials, Structures, and Functions for Flexible and Stretchable Biomimetic Sensors.

Currently, flexible and stretchable biomimetic sensing electronics have obtained a great deal of attention in various areas, such as human-machine interfaces, robotic smart skins, health care monitoring, and biointegrated devices. In contrast with the traditional rigid and fragile silicon-based electronics, flexible and stretchable sensing electronics can efficiently capture high-quality signals when integrated on curved surfaces due to their elastic and conformal characters, which are expected to play many important roles in the foreseeable age of intelligence. Its realization strongly relies on rapid advances in the development of high-performance and versatile flexible and stretchable sensors, and effective ways to achieve high performance are rational designs of the sensing materials and microstructural configurations. This Account showcases the recent progress in flexible and stretchable biomimetic sensors covering several critical aspects of materials, structures, and applications. Nature-inspired active matter and architectures, which have been well-tuned by evolution through millions of years of optimization, provide us the best learning choices to overcome the restrictions of current sensor techniques such as low sensitivity, instability, and delayed response time. Biomimetic sensing materials and microstructural patterns can efficiently acquire synthetic response abilities, endowing the new-type flexible sensors considered as "smart" electronic components on account of the counterparts to living organisms. Moreover, the developments of diverse functions and multifunctional applications become more and more important in the creation of novel flexible electronics beyond those existing technologies. For instance, flexible and stretchable sensors with the capability of mimicking various human behavioral patterns can be developed to boost the emergence of artificial robots, which can take the place of human beings in strenuous activities, enabling progress in social science, technology, and productivity to improve the quality of human life. For the above purpose, inspired by the in-depth understanding of working principles of living organisms how to operate their natural characteristics, sensing materials with stimuli response (light, humidity, mechanics, etc.) and multifunctionalities (superhydrophobicity, degradation, self-healing, etc.) provide distinctive and multiple detection features generally encountered in their traditional counterparts. In addition, artificial micro- to nanostructures derived from naturally existing high sensitivity structures (such as insect crack or leaves) and stretchable configurations (wrinkle, texture, mesostructures, etc.) offer additional feasible strategies for producing favorable sensitivity and stretchability. Flexible and stretchable biomimetic sensors with analogous senses to those of human beings (such as tactile and auditory senses) have attracted tremendous attention for their diverse applications for next generation smart electronics. The long-term progress of these novel sensors influencing the next generations of bioinspired intelligence systems and medical electronics are also envisioned.

On a silver platter

on a silver platter

Deformation sensing and electrical compensation of smart skin antenna structure with optimal fiber Bragg grating strain sensor placements

Skin antenna structure is a kind of multifunctional composite structure in which printed antennas are integrated. This paper proposes a new smart skin antenna structure with embedded fiber Bragg grating (FBG) strain sensors. Utilizing the measured strains from limited FBG strain sensors, the deformation shapes are reconstructed by using a strain–displacement transformation, and the deteriorated radiation patterns caused by the deformations are compensated by using a strain-electromagnetic coupling mode which is derived and validated for the first time in the paper. Moreover, a two-step sensor placement method considering the information redundancy is proposed to determine optimal FBG strain sensor locations. The comparisons show that the two-step sensor placement method is advantageous in terms of the sensor distribution and computational cost. The fabrication process of the smart skin antenna structure was presented, and an experimental system with optimal sensor placements was developed. The measured results demonstrate that the reconstructed shapes agree well with the real ones, and that the deteriorated radiating patterns were almost completely restored to the desired ones. The proposed smart skin antenna structure which can provide the electromagnetic receiving-sending, structural deformation monitoring and electrical correction functions has a potential application prospect in the future mobile vehicle.

냉방부하 저감을 위한 스마트 스킨의 에너지 성능 평가에 관한 연구

The objective of this study was to evaluate and analyze energy performance using TRNSYS 18 for smart skin and typical windows. The results are as follows : The base model for smart skin types in Seoul area was 24% to 37.6%, and the demonstration model was 14.6% to 25.5%, which reduced energy as compared to the target. Also, the reference model in Gwangju was 26.6% to 41.3%, and the demonstration model was 16.0% to 27.4%, which also reduced energy as compared to the target. The base model in the Seoul area was 108.8% to -35.5% and the demonstration model was 105.5% to -25.2%. In addition, the base model in Gwangju was 106.7% to -40.9% and the demonstration model 103.3% to -27.4%, which increased and reduced energy compared to the target. A smart skin and typical type, base model and demonstration model, and energy requirements in Seoul and Gwangju were reduced as the overall u-value was lower. On the other hand, 0.76 (W/㎡․K) showed higher energy consumption than 1.07 (W/㎡․K). In conclusion, energy savings in work facilities are high when the thermal perfusion rate is 1.07 (W/㎡․K).

Innovative Strain Sensing for Detection of Exterior Wall Tile Lesion: Smart Skin Sensory System.

Tiles are commonly used to cover the exteriors of buildings in Taiwan. However, older buildings in Taiwan encounter the problem of tiles falling off due to natural deterioration, which is unsightly, and more importantly, a threat to public safety. Nevertheless, no current method exists that can effectively detect flaws in building tiles in real time. This study combined the fields of civil engineering and automatic control to reduce risks caused by falling tiles by improving real-time detection of at-risk areas. Micro-resistance was combined with fuzzy theory as the logical foundation for evaluating tile status. String-type strain gauges were adopted as sensors to design a smart skin sensory system that could measure signs of deterioration in tile surface lesions. The design was found to be feasible. In the future, it can be further developed for facile real-time assessment of tile status.

Polyelectrolyte Dielectrics for Flexible Low‐Voltage Organic Thin‐Film Transistors in Highly Sensitive Pressure Sensing

AbstractOrganic thin‐film transistors (OTFTs) can provide an effective platform to develop flexible pressure sensors in wearable electronics due to their good signal amplification function. However, it is particularly difficult to realize OTFT‐based pressure sensors with both low‐voltage operation and high sensitivity. Here, controllable polyelectrolyte composites based on poly(ethylene glycol) (PEG) and polyacrylic acid (PAA) are developed as a type of high‐capacitance dielectrics for flexible OTFTs and ultrasensitive pressure sensors with sub‐1 V operation. Flexible OTFTs using the PAA:PEG dielectrics show good universality and greatly enhanced electrical performance under a much smaller operating voltage of −0.7 V than those with a pristine PAA dielectric. The low‐voltage OTFTs also exhibit excellent flexibility and bending stability under various bending radii and long cycles. Flexible OTFT‐based pressure sensors with low‐voltage operation and superhigh sensitivity are demonstrated by using a suspended semiconductor/dielectric/gate structure in combination with the PAA:PEG dielectric. The sensors deliver a record high sensitivity of 452.7 kPa−1 under a low‐voltage of −0.7 V, and excellent operating stability over 5000 cycles. The OTFT sensors can be built into a wearable sensor array for spatial pressure mapping, which shows a bright potential in flexible electronics such as wearable devices and smart skins.

Noncontact Strain Mapping Using Laser-Induced Fluorescence from Nanotube-Based Smart Skin

AbstractStress fields around structural discontinuities such as cracks usually cause complex but distinct strain contours/maps when structures are subjected to load. Hence, mechanical strain on str...

On buckling and free vibration studies of sandwich plates and cylindrical shells: A review

Many review articles were published on free vibration and buckling of laminated composites, sandwich plates, and shells. The present article reviews the literature on the buckling and free vibration analysis of shear deformable isotropic and laminated composite sandwich plates and shells using various methods available for plates in the past few decades. Various theories, finite element modeling, and experimentations have been reported for the analysis of sandwich plates and shells. Few papers on functionally graded material plates, plates with smart skin (electrorheological, magnetorheological, and piezoelectric), and also viscoelastic materials were also reviewed. The scope for future research on sandwich plates and shells was also accessed.

Dual‐layer nanotube‐based smart skin for enhanced noncontact strain sensing

Dual‐layer nanotube‐based smart skin for enhanced noncontact strain sensing

A Study of Methodology for Optimizing Cooling, Heating and Lighting Loads by Applying Smart Skins in Buildings

A Study of Methodology for Optimizing Cooling, Heating and Lighting Loads by Applying Smart Skins in Buildings

A Flexible Wearable Pressure Sensor with Bioinspired Microcrack and Interlocking for Full-Range Human-Machine Interfacing.

Flexible wearable pressure sensors have drawn tremendous interest for various applications in wearable healthcare monitoring, disease diagnostics, and human-machine interaction. However, the limited sensing range (<10%), low sensing sensitivity at small strains, limited mechanical stability at high strains, and complicated fabrication process restrict the extensive applications of these sensors for ultrasensitive full-range healthcare monitoring. Herein, a flexible wearable pressure sensor is presented with a hierarchically microstructured framework combining microcrack and interlocking, bioinspired by the crack-shaped mechanosensory systems of spiders and the wing-locking sensing systems of beetles. The sensor exhibits wide full-range healthcare monitoring under strain deformations of 0.2-80%, fast response/recovery time (22 ms/20 ms), high sensitivity, the ultrasensitive loading sensing of a feather (25 mg), the potential to predict the health of patients with early-stage Parkinson's disease with the imitated static tremor, and excellent reproducibility over 10 000 cycles. Meanwhile, the sensor can be assembled as smart artificial electronic skins (E-skins) for simultaneously mapping the pressure distribution and shape of touching sensing. Furthermore, it can be attached onto the legs of a smart robot and coupled to a wireless transmitter for wirelessly monitoring human-motion interactivities.

Full Thickness Skin Expansion ex vivo in a Newly Developed Reactor and Evaluation of Auto-Grafting Efficiency of the Expanded Skin Using Yucatan Pig Model.

Skin grafts are required in numerous clinical procedures, such as reconstruction after skin removal and correction of contracture or scarring after severe skin loss caused by burns, accidents, and trauma. The current standard for skin defect replacement procedures is the use of autologous skin grafts. However, donor-site tissue availability remains a major obstacle for the successful replacement of skin defects and often limits this option. The aim of this study is to effectively expand full thickness skin to clinically useful size using an automated skin reactor and evaluate auto grafting efficiency of the expanded skin using Yucatan female pigs. We developed an automated bioreactor system with the functions of real-time monitoring and remote-control, optimization of grip, and induction of skin porosity for effective tissue expansion. We evaluated the morphological, ultra-structural, and mechanical properties of the expanded skin before and after expansion using histology, immunohistochemistry, and tensile testing. We further carried out in vivo grafting study using Yucatan pigs to investigate the feasibility of this method in clinical application. The results showed an average expansion rate of 180%. The histological findings indicated that external expansion stimulated cellular activity in the isolated skin and resulted in successful grafting to the transplanted site. Specifically, hyperplasia did not appear at the auto-grafted site, and grafted skin appeared similar to normal skin. Furthermore, mechanical stimuli resulted in an increase in COL1A2 expression in a suitable environment. These findings provided insight on the potential of this expansion system in promoting dermal extracellular matrix synthesis in vitro. Conclusively, this newly developed smart skin bioreactor enabled effective skin expansion ex vivo and successful grafting in vivo in a pig model.

Smart electronic skin having gesture recognition function by LSTM neural network

Rapid growth of soft electronics has enabled various approaches for developing artificial skin. However, currently existing electronic skin is still facing some problems such as high fabrication complexity, high production cost, and smartness of recognizing the stimulus automatically. In this work, we report a simple, low-cost Polydimethylsiloxane (PDMS)-based smart electronic skin system, consisting of a sensor array and a data processing system. The sensor array can be easily mounted on the human body or robot hand as a result of excellent softness, stretchability, and bendability of PDMS. Signals from the sensor array are processed by a Long and Short Term Memory neural network algorithm in the data processing system. The trained data processing system can recognize four types of gestures at an accuracy of 85 ± 5%, even taking into account environmental variations including folding, curvature, tensile strength, temperature, and endurance cycles. This work proves that this type of skin can be endowed with intelligence with a proper neural network algorithm and fabricated at low cost and reduced complexity.

Control of a high‐voltage bidirectional dc–dc flyback converter for driving DEAs

This study presents modelling and control of a high-voltage ratio flyback converter for driving capacitive loads including smart material dielectric elastomer actuators (DEAs). These actuators find various applications including artificial muscles, optical devices, smart skin, and acoustics but require high actuation voltages. To this end, a high-voltage bidirectional flyback converter for driving a capacitive load is studied in terms of modelling and control and applied to a DEA. The state-space model of the converter is obtained for the capacitor charge and discharge modes when the converter operates in the continuous conduction mode and used to obtain a load voltage controller using the feedback linearisation method. The converter can be used to regenerate the capacitor charge into the dc source. The proposed hardware and control strategy was built and validated by driving DEA capacitive loads operating at high voltages around 4 kV. The experimental results are presented which validate the performance of the proposed converter and its control strategy.

(Invited) Soft Electronics and Machines with Tough Hydrogels

Introducing methods for instant strong bonding between hydrogels and antagonistic materials – from soft to hard – allows us to demonstrate elastic, yet tough biomimetic devices and machines with a high level of complexity [1]. Tough hydrogels strongly attach, within seconds, to plastics, elastomers, leather, bone and metals reaching unprecedented interfacial toughness exceeding 2000 J/m2. Our approach is applicable in rapid prototyping and in delicate environments inaccessible for extended curing and cross-linking. The combination of ionic hydrogels with antagonistic materials such as elastomers, polymers and metals allows to create soft electronics and hybrid machines. We demonstrate stretchable batteries for self-powered soft devices, adaptive lenses, and autonomous electronic skin for triggered drug delivery (Figure 1). We further introduce a new family of biodegradable hydrogels that are able to self-heal and are resistant to dehydration. Soft machines and robots – built from hydrogels with tuned mechanical properties – are designed to be operated in ambient conditions and degrade after use. Besides progressing standalone soft machines, our advances in the synthesis of biodegradable hydrogels bring bionic soft robots a step closer to nature. [1] D. Wirthl, R. Pichler, M. Drack, G. Kettlgruber, R. Moser, R. Gerstmayr, F. Hartmann, E. Bradt, R. Kaltseis, C. M. Siket, S. E. Schausberger, S. Hild, S. Bauer, M. Kaltenbrunner, „Instant tough bonding of hydrogels for soft machines and electronics”, Science Advances, 3(6), e1700053 (2017). Caption Figure 1: Hydrogel electronic skin. (left) Concept of a hydrogel smart skin, with a flexible unit bearing power supply, control, readout and communication units, and a stretchable transducer batch. (right) Photograph of an untethered electronic hydrogel with four stretchable heating elements and adjoined temperature sensors strongly bonded to a PVA hydrogel. Battery, control, readout and Bluetooth Low Energy communication electronics are hosted on a flexible circuit board. Figure 1

(Keynote) Ferroelectric Polymers for Flexible Electronics

Ferroelectric polymers from the PVDF-family have proven to be multifunctional and self-sustaining materials with a broad deployment in printed and flexible electronics. They can be used in large and flexible form factors for detecting mechanical excitations such as pressure variations, force touch and impact, for sensing human-body radiation and proximity, as vibration sensors for structure-borne sound detection and acoustics, as fast and high-precise strain sensors, as stretchable vital parameter sensors for movement, ECG and respiratory rate monitoring, as well as piezoelectric energy harvesting elements, just to name a few. This talk will provide an overview of the most attractive applications of PVDF-based devices such as pressure-sensitive human-machine interfaces on planar, flexible and 3D-shaped surfaces (also in combination with flexible displays), large-area impact sensing films for material and crash testing, sensors for structural health monitoring, ultrathin microphones smoothly integrated on versatile, arbitrary shaped object surfaces, smart skin patches for vital monitoring and intelligent floor for smart home applications and energy supply for low-power sensor networks. The sensors are entirely fabricated by screen printing which is one of the most common techniques used in printed electronics for the fabrication of large-area flexible components and multifunctional devices. Screen printing is highly tolerant to the type and form factor of substrates, the rheology of ink materials, provides sufficient alignment accuracy for multilayer printing and can be done in a sheet-to-sheet or roll-to-roll scheme. The printed ferroelectric polymer sensors come in two versions; (i) either they have a sandwich-type structure of four layers that are printed onto a flexible or stretchable substrate (e. g. plastic films, paper, and textiles up to A3) and response accurately, fast and reproducibly to pressure and temperature changes over large dynamic ranges or (ii) a two layer structure that is highly sensitive to lateral strain and vibrations. By optimizing the design, the printing and annealing process as well as the poling conditions and the source material, functional sensors with a soft yield of more than 98% with less than ± 5% deviation in the remnant polarization were demonstrated. It is also interesting to note that extended aging tests under definite climate and shock conditions revealed more than 98% preservation of the remnant polarization for high molecular weight PVDF-TrFE polymers. Based on these sensors applications such as flexible 3D user interfaces [1,2] (Fig. 1) and large-area force, impact and proximity sensors, ultrathin object-integrated microphones as well as medical patches will be presented either in a passive-matrix or an active-matrix ferroelectric sensor configuration with OTFT- or OECT-backplane. Finally, a novel printable nanocomposite material, which allows reducing the cross-sensitivity between the pyro- and piezoelectric sensing modes, will be presented. This material is composed of inorganic ferroelectric nanoparticles blended in a ferroelectric polymer matrix. By exploiting the fact that the piezoelectric coefficient in inorganic ceramics has an opposite sign to that one of the ferroelectric polymer either the piezo- or the pyroelectric activity can be suppressed by independently defining the poling direction of particles and matrix in a clever poling procedure [3]. M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, P. Bodö, P. Andersson Ersman, D. Platt, S. Bauer, G. Domann, and B. Stadlober, Adv. Mat. 23, 2069 (2011)C. Rendl, D. Kim, P. Parzer, S. Fanello, M. Zirkl, G. Scheipl, M. Haller, S. Izadi, FlexCase: Enhancing Mobile Interaction with a Flexible Sensing and Display Cover, Proceedings of the 2016 CHI Conference on Human Factors in Computing Systems, San Jose, California, USA, pp. 5138-5150, doi: 10.1145/2858036.2858314 (2016)I. Graz, M. Krause, S. Bauer-Gogonea, S. Bauer, S. P. Lacour, B. Ploss, M. Zirkl, B. Stadlober, and S. Wagner, J. Appl. Phys. 106, p. 034503 (2009) Figure 1

Design of Inkjet-Printed RFID-Based Sensor on Paper: Single- and Dual-Tag Sensor Topologies.

The detailed design considerations for the printed RFID-based sensor system is presented in this paper. Starting from material selection and metallization method, this paper discusses types of RFID-based sensors (single- & dual-tag sensor topologies), design procedures, and performance evaluation methods for the wireless sensor system. The electrical properties of the paper substrates (cellulose-based and synthetic papers) and the silver nano-particle-based conductive film are thoroughly characterized for RF applications up to 8 GHz. The reported technology could potentially set the foundation for truly “green”, low-cost, scalable wireless topologies for autonomous Internet-of-Things (IoT), bio-monitoring, and “smart skin” applications.

Thermosensory micromapping of warm and cold sensitivity across glabrous and hairy skin of male and female hands and feet.

The ability of hands and feet to convey skin thermal sensations is an important contributor to our experience of the surrounding world. Surprisingly, the detailed topographical distribution of warm and cold thermosensitivity across hands and feet has not been mapped, although sensitivity maps exist for touch and pain. Using a recently developed quantitative sensory test, we mapped warm and cold thermosensitivity of 103 skin sites over glabrous and hairy skin of hands and feet in male (M; 30.2 ± 5.8 yr) and female (F; 27.7 ± 5.1 yr) adults matched for body surface area (M: 1.77 ± 0.2 m2; F: 1.64 ± 0.1 m2; P = 0.155). Findings indicated that warm and cold thermosensitivity varies by fivefold across glabrous and hairy skin of hands and feet and that hands (warm/cold sensitivity: 1.25/2.14 vote/°C) are twice as sensitive as the feet (warm/cold sensitivity: 0.51/0.99 vote/°C). Opposite to what is known for touch and pain sensitivity, we observed a characteristic distal-to-proximal increase in thermosensitivity over both hairy and glabrous skin (i.e., from fingers and toes to body of hands and feet), and found that hairy skin is more sensitive than glabrous. Finally, we show that body surface area-matched men and women presented small differences in thermosensitivity and that these differences are constrained to glabrous skin only. Our high-density thermosensory micromapping provides the most detailed thermosensitivity maps of hands and feet in young adults available to date. These maps offer a window into peripheral and central mechanisms of thermosensory integration in humans and will help guide future developments in smart skin and sensory neuroprostheses, in wearable, energy-efficient personal comfort systems, and in sport and protective clothing. NEW & NOTEWORTHY We provide the most detailed thermosensitivity maps across glabrous and hairy skin of hands and feet in men and women available to date. Our maps show that thermosensitivity varies by fivefold across hands and feet, distal regions (e.g., fingers, toes) are less sensitive than proximal (e.g., palm, sole), hands are twice as sensitive as feet, and men and women present small thermosensitivity differences. These findings will help guide developments in sensory neuroprostheses, wearable comfort systems, and sport/protective clothing.

Magnetically Responsive Superhydrophobic Surface: In Situ Reversible Switching of Water Droplet Wettability and Adhesion for Droplet Manipulation.

A smart, magnetically responsive superhydrophobic surface was facilely prepared by combining spray coating and magnetic-field-directed self-assembly. The surface comprised a dense array of magnetorheological elastomer micropillars (MREMPs). Benefitting from the magnetic field-stiffening effect of the MREMPs, the surface exhibited reversible switching of the wettability and adhesion that was responsive to an on/off magnetic field. The wettability and adhesion properties of the surfaces with MREMPs were investigated under different magnetic fields. The results revealed that the adhesion force and sliding behaviors of these surfaces were strongly dependent on the intensity of the applied magnetic field and the mixing ratio of poly(dimethylsiloxane) (PDMS), iron particles, and solvent (in solution) used for preparation of the magnetically responsive superhydrophobic surfaces. The adhesion transition was attributed to the tunable mechanical properties of the MREMPs, which was easily controlled by an external magnetic field. It was also demonstrated that the magnetically responsive superhydrophobic surface can be used as a "mechanical hand" for no-loss liquid droplet transportation. This magnetically responsive superhydrophobic surface not only provides a novel interface for microfluidic control and droplet transportation, but also opens up new avenues for achieving smart liquid-repellent skin, programmable fluid collection and transport, and smart microfluidic devices.

Design of Engineered Elastomeric Substrate for Stretchable Active Devices and Sensors

AbstractIn the field of flexible electronics, emerging applications require biocompatible and unobtrusive devices, which can withstand different modes of mechanical deformation and achieve low complexity in the fabrication process. Here, the fabrication of a mesa‐shaped elastomeric substrate, supporting thin‐film transistors (TFTs) and logic circuits (inverters), is reported. High‐relief structures are designed to minimize the strain experienced by the electronics, which are fabricated directly on the pillars' surface. In this design configuration, devices based on amorphous indium‐gallium‐zinc‐oxide can withstand different modes of deformation. Bending, stretching, and twisting experiments up to 6 mm radius, 20% uniaxial strain, and 180° global twisting, respectively, are performed to show stable electrical performance of the TFTs. Similarly, a fully integrated digital inverter is tested while stretched up to 20% elongation. As a proof of the versatility of mesa‐shaped geometry, a biocompatible and stretchable sensor for temperature mapping is also realized. Using pectin, which is a temperature‐sensitive material present in plant cells, the response of the sensor shows current modulation from 13 to 28 °C and functionality up to 15% strain. These results demonstrate the performance of highly flexible electronics for a broad variety of applications, including smart skin and health monitoring.