Wearable Sensing Systems with Mechanically Soft …download.xuebalib.com/xuebalib.com.41972.pdfPROGRESS REPORT 1700053 (1 of 18) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

PROGRESS REPORT

1700053 (1 of 18) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advmattechnol.de

Wearable Sensing Systems with Mechanically Soft Assemblies of Nanoscale Materials

Youngsik Lee, Jaemin Kim, Hyunwoo Joo, Milan S. Raj, Roozbeh Ghaffari,* and Dae-Hyeong Kim*

DOI: 10.1002/admt.201700053

1. Introduction

Flexible and stretchable electronics technologies have led to the emergence of a unique class of soft biointegrated sensing sys-tems, which are poised to solve many important challenges in healthcare.[1,2] These soft bio-integrated electronics and biosen-sors have driven advances in non-invasive (e.g., wearable),[3–6] minimally-invasive,[7,8] implantable,[9–11] and in vitro sensing capabilities.[12–14] While there are many compelling implantable and minimally invasive applications, the translational viability

Emerging classes of wearable sensing systems that measure motion, physi-ological, electrophysiological, and electrochemical signals emanating from the human body have driven significant advances in clinical and academic research. These wearable systems rely on important breakthroughs in micro/nano-electronics, information technology, and materials science. Compared to conventional bulk materials, nanomaterials with zero, one, and two dimen-sional (0D, 1D, and 2D) architectures exhibit unusual physical properties that could dramatically improve the performance of sensors. By integrating high performance sensors with soft and stretchable electronics, research groups are enabling fully-integrated multifunctional sensing systems in skin-worn for-mats, optimized for managing specific disease models. In this progress report, recent advances in soft wearable sensing systems based on assemblies of 0D, 1D, and 2D nanomaterials, unpackaged integrated circuits, and highly elastic (moisture resistant) encapsulating layers are reviewed. These advanced bioel-ectronic constructs combine multimodal sensor arrays, data storage elements, wireless data transmission modules, and actuators for continuous monitoring. The soft wearable systems that embody these unusual electronic materials and soft packaging strategies are beginning to impact big data analy sis, remote health monitoring, and transdermal drug delivery applications, by tran-sitioning from primary research discoveries to commercial adoption.

Wearable Electronics

Y. Lee, Dr. J. Kim, H. Joo, Prof. D.-H. KimCenter for Nanoparticle ResearchInstitute for Basic Science (IBS)Seoul 08826, Republic of KoreaE-mail: [email protected]. Lee, Dr. J. Kim, H. Joo, Prof. D.-H. KimSchool of Chemical and Biological EngineeringInstitute of Chemical ProcessesSeoul National UniversitySeoul 08826, Republic of Korea

M. S. Raj, Dr. R. GhaffariMC10 Inc.10 Maguire Rd., Lexington, MA 02421, USAE-mail: [email protected]. R. GhaffariCenter for Bio-integrated ElectronicsTechnological InstituteNorthwestern University2145 Sheridan Road, Evanston, IL 60208-3109, USA

of non-invasive/wearable devices has already been established. Here, we report on recent progresses in non-invasive/wearable biosensing technologies, and review their translation to products with commercial implications in the fields of biomedicine and clinical research.

Wearable sensing systems have relied on important advances in nanoscale materials,[15] their unusual assembly techniques,[16] flexible/stretchable elec-tronics,[17,18] soft packaging,[19,20] and novel sensor designs.[21,22] Figure 1 shows representative examples of the wearable bio-electronic systems and their various sub-components, which collect, store, and wirelessly transmit physiological, electro-physiological, and biochemical data to a central server via a mobile device. Func-tional nanomaterials are embedded in these sensors and electronic modules to enhance the performance of the sensing and electronic components. For example, extraordinary sensitivity and selectivity for the sensors, low energy consump-tion in electronic modules, and high effi-ciency in light emitting devices could

be demonstrated owing to the additionally applied functional nanomaterials.[16,23,24]

Recent advances in nanotechnology have led to novel demon strations of soft sensors using functional nanomate-rials, including nanoparticles (0D), nanowires, nanofibers, nanotubes (1D), and nanosheets (2D) to achieve extraordinary physical and biochemical sensing properties and function-alities (Figure 1a).[25–27] The data measured by multifunctional sensor arrays (Figure 1b)[1,28] are stored in data storage mod-ules (Figure 1c)[24,29] and can be wirelessly transferred without

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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700053 (2 of 18)


external connections (Figure 1d).[30,31] Once the collected data is processed, appropriate therapies may be delivered via micro-actuators on the same device, in the form of light, chemical, or electrical stimuli (Figure 1e).[28,32] These individual sub-modules are programmable and could operate together autono-mously (Figure 1f),[33,34] leading to commercialization of fully integrated wearable systems (Figure 1g).[33,34] New products that exploit novel designs and hybrid manufacturing processes (i.e., roll-to-roll and flex/hybrid manufacturing processes) have recently been deployed in both clinical and consumer health-care industries, for example, My UV Patch and BioStamp (MC10 Inc., USA).[35,36]

2. Functional Nanomaterials for Bio-Integrated Sensors

There are distinct classes of nanomaterials differentiated by their structural makeup and complexity as 0D nanoparticles, 1D nanowires/nanofibers/nanotubes, and 2D nanosheets. These nanomaterials each have unique physical properties that could contribute to the enhanced sensor performances, unconventional device functions, and system’s soft mechanical properties.[37,38] By applying novel assembly strategies, several groups have exploited the unusual physical properties of these various nano building blocks to create novel sensors[39–41] and high performance electronics.[16,42,43]

2.1. Zero-Dimensional Functional Nanomaterials and Sensor Applications

Figure 2 presents representative examples of 0D nanoparticles and their sensor applications. Silver nanoparticles (Ag NPs) (Figure 2a, top)[25] are assembled as an array of the curved form to make a strain gauge array (Figure 2a, middle).[44] In this design, Ag NPs are vacuum deposited using a pillar patterned template, which induces Ag NPs to be closely integrated into a wave-shaped structure. When strain is applied to the curved-array, the distance between the assembled Ag NPs increases, thereby increasing resistance due to reduced electrical contact. Because the dimensions of the curved array affect its response to applied strains, the sensitivity and detection range of this strain gauge are tuned by simply changing the dimensions of the curved array. This Ag NPs-based strain sensor was lami-nated on the human face and successfully monitored changes in facial expression by detecting fine movements of facial muscles.

In another example, Ag NPs are used as a performance enhancer of a sensor by being embedded in Al2O3-doped ZnO (AZO) to create highly sensitive temperature sensors (Figure 2a, bottom).[25] Here, the Ag NPs increase the carrier density in the AZO layer because of the deep-level doping of Ag ions, thereby increasing the sensitivity of the AZO layer to external temperature changes. By optimizing the concentration of Ag NPs inside the AZO layer, temperature sensitivity can be markedly enhanced, surpassing that of Pt and pristine AZO-based temperature sensors.

Instead of Ag NPs, gold nanoparticles (Au NPs) (Figure 2b, top)[29] can be used in a form of the closely-packed assembly to

create direction-selective piezoresistive strain sensors (Figure 2b, middle).[45] Strain applied in the parallel direction to the elec-trodes of Au NPs assemblies weakens electrical tunneling effects between the Au NPs. In contrast, strain applied along the per-pendicular direction to the electrodes hardly reduces electrical current because of the disordered architecture of the Au NPs. In the study analyzing the sensitivity and hysteresis of closely packed Au NPs-based strain gauges (Figure 2b, bottom), 15-nm Au NPs show 10-times higher sensitivity, compared to that of metal film-based strain gauges with minimal hysteresis.[46]

Meanwhile, oxide nanoparticles (Figure 2c, top),[47] instead of metal nanoparticles, can serve as an effective chemical sensing

Youngsik Lee received his B.S. degree (2013) from the School of Chemical and Biological Engineering at Seoul National University. Under the supervision of Prof. Dae-Hyeong Kim, he is working on wearable, minimally-invasive, and implantable electronics for bio-medical applications.

Roozbeh Ghaffari received his B.S. (2001) and M.Eng. (2003) degrees from the Electrical Engineering and Computer Science Department at the Massachusetts Institute of Technology. He obtained his Ph.D. (2008) from the Harvard-MIT Division of Health Sciences and Technology in biomedical

engineering. He is co-Founder and Chief Technology Officer at MC10 Inc. where he leads the research and development of epidermal bio-electronics for emerging consumer health and medical systems.

Dae-Hyeong Kim received his B.S. (2000) and M.S. (2002) degrees from the School of Chemical Engineering at Seoul National University. He obtained his Ph.D. (2009) from the depart-ment of Materials Science and Engineering at the University of Illinois, Urbana Champaign. Since joining the faculty of the School of

Chemical and Biological Engineering at Seoul National University in 2011, he has focused on stretchable and wear-able electronics for bio-medical and energy applications.





unit because they contain oxygen, which readily reacts with other functional molecules. For example, a zinc oxide nano-particles (ZnO NPs)-based field effect transistor (FET) gas sensor can detect ethanol gas of the hundreds of parts-per-millions concentration (Figure 2c, middle).[47] When ultra-violet light is irradiated on the ZnO NPs film, electron-hole pairs are produced by the absorption of photons by semiconducting ZnO NPs. These electron-hole pairs create unstable oxygen ions on the film surface, and the oxygen ion reacts with ethanol to emit free electrons that induce a photocurrent. Tungsten oxide nanoparticles (WO3 NPs)-based pH sensors represent another class of the chemical sensor (Figure 2c, bottom).[48] WO3 NPs are electrochemically-deposited on flexible polymer substrates. Because the surface charge of WO3 NPs is changed in accord-ance with the acidity of the solution, the open circuit potential

of the WO3 NPs-based pH sensor changes corresponding to pH changes in the solution.

Nanoparticles can be fused with other types of nanomate-rials to create unusual synergistic effects (Figure 2d, top).[49] Hybrid nanomaterials with 0D NPs show more enhanced car-rier transport characteristics and higher piezoresistivity than homogeneous NPs. For example, ZnO NPs combined with multi-walled carbon nanotubes (MW-CNTs) can create high power nano-generators (Figure 2d, middle).[50] When ZnO NPs are embedded in polydimethylsiloxane (PDMS) without MW-CNTs, ZnO NPs are electrically isolated. However, when MW-CNTs are combined with ZnO NPs, MW-CNTs function as bridges connecting the ZnO NPs, which increases elec-trical conductivity and thereby leads to generation of much higher voltages. This generated voltage and/or current by


Figure 1. Integrated wearable sensing system and its sub-modules. Representative examples of soft bio-electronics modules and fully-integrated wear-able systems. a) Functional nanomaterials are integrated with the soft sensors for enhanced sensor performance and multifunctionality. Reproduced with permission.[25–27] Copyright 2015, Copyright 2014, Wiley-VCH; Copyright 2009, American Chemical Society. b) Multifunctional sensor arrays that exploit different types of sensors, such as mechanical, physiological, electrophysiological, and electrochemical sensors. Reproduced with permis-sion.[1,28] Copyright 2011, American Association for the Advancement of Science; Copyright 2014, Macmillan Publishers Ltd. c) Data storage modules to collect sensor signals in memory cells. Reproduced with permission.[24,29] Copyright 2014, Macmillan Publishers Ltd; Copyright 2016, American Association for the Advancement of Science. d) Wireless data transmission modules to convey sensor signals to external equipment without wires. Reproduced with permission.[30,31] Copyright 2014, Copyright 2015, Wiley-VCH. e) Therapeutic actuators incorporated with sensors. Reproduced with permission.[28,32] Copyright 2014, Copyright 2016, Macmillan Publishers. f) Fully integrated prototypes of wearable systems. Clinically important biosig-nals are continuously monitored and may provide real time feedback for actuators to deliver stimuli. Reproduced with permission.[33,34] Copyright 2016, Macmillan Publishers; Copyright 2014, American Association for the Advancement of Science. g) Recently launched commercial products consisting of fully integrated wearable biosensors associated soft electronics. Reproduced with permission.[35,36] Copyright 2015, MC10 Inc.




external strain and/or pressure can be measured for sensing purposes.

In another example, composite films comprised of Ag NPs and CNTs were used to create highly sensitive electronic whiskers (Figure 2d, bottom).[49] The composite strain sensor exhibits the 10-times higher pressure sensitivity than previ-ously reported capacitive and resistive strain sensors. Using the tunneling effect, Ag NPs create additional current paths to the CNT networks, thus improving electrical conductivity. At the high Ag NPs content, most of the current flows between adjacent Ag NPs via tunneling. Therefore, minute displace-ment changes between Ag NPs can be detected. As a proof of concept, an array of vertical 3D electronic whiskers was used to successfully monitor subtle environmental changes such as the air flow.

Although nanoparticles could significantly enhance sensing performances owing to their large surface area to volume ratio, large number of NPs, and the tunneling effect between NPs, they also have limitations due to their simple spherical archi-tecture. Because of its isotropic structure, they hardly compose complex heterostructure, such as networked structure, and this often limits advanced functionalities. In the case of nan-oparticle-based strain sensors, for example, tunneling current through the nanoparticles can be highly decreased, particularly

in the deformed state of the sensors, due to weak connectivity between neighboring nanoparticles.[46] This may lead to the reduced sensitivity, nonlinear behavior under the certain degree of mechanical deformations and a small sensing range. Alter-natively, anisotropic 1D nanomaterials, which can readily form complex heterogeneous structures and networks between 1D nanostructures, can be used as key sensing materials

2.2. One-Dimensional Functional Nanomaterials and Sensor Applications

Figure 3 shows representative examples of 1D nanowires, nanofibers, nanotubes, and their applications for wearable sensors. These 1D materials show efficient carrier transport characteristics along their longitudinal direction. Zinc oxide nanowires (ZnO NWs) (Figure 3a, top),[51] for example, can be used to build self-powered active skin sensors for monitoring human motion (Figure 3a, middle).[52] ZnO NWs are grown on anodized aluminum oxide (AAO) in an aligned manner. They have dipoles of different polarity depending on the direction of bending due to its piezoelectricity. Therefore, they generate different potentials in response to applied strain, which can be used for a highly sensitive strain sensor. A recent study showed


Figure 2. Zero-dimensional functional nanomaterials and sensor applications. a) Ag NPs: TEM image (top), Ag NPs-based curved strain sensors (middle), and temperature sensor using Ag NPs embedded in AZO (bottom). Reproduced with permission.[25,44] Copyright 2015, Copyright 2016, Wiley-VCH. b) Au NPs: TEM image (top), piezoresistive strain gauge using Au NPs (middle), and a closely packed Au NPs-based strain sensor (bottom). Reproduced with permission.[29,45,46] Copyright 2016, American Association for the Advancement of Science; Copyright 2015, Macmillan Publishers Ltd; Copyright 2013, the American Chemical Society. c) Oxide nanoparticles: SEM image of ZnO NPs (top), ZnO NPs-based ethanol gas sensor (middle), and iron oxide-NPs embedded humidity sensors (bottom). Reproduced with permission.[47,48] Copyright 2015, Macmillan Publishers Ltd; Copyright 2014, the American Chemical Society. d) Nanoparticle hybridized materials: SEM image of Ag NPs and CNT composite film (top), flexible piezoelectric composite using ZnO NPs and CNTs (middle), and electronic whiskers made of an Ag NPs and CNT composite film (bottom). Reproduced with permission.[49,50] Copyright 2014, the National Academy of Sciences; Copyright 2013, Royal Society of Chemistry.




that by attaching these devices on the surface of eyelids, they could effectively monitor the direction and speed of eye move-ments. Unlike ZnO NWs with piezoelectric properties, gold nanowires (Au NWs) are used as conductive networks, which can be applied for highly sensitive and flexible pressure sen-sors. When external pressure is applied to ultrathin Au NWs that are sandwiched between PDMS sheets, the current paths between Au NWs are increased, which induces the resistance decrease. Gong and collaborators have used Au NWs-based pressure sensors to monitor wrist pulse and acoustic vibra-tions (Figure 3a, bottom).[53] The metal NW-based sensors show larger sensing ranges than the metal NPs-based sensors intro-duced in the previous section.

Polymer nanofibers (Figure 3b, top) are generally softer and more flexible than inorganic nanowires.[23] Therefore, highly deformable sensors with high sensitivity can be fabricated. For example, Pt-coated polyurethane nanofibers with a high aspect ratio were used to make highly sensitive strain gauges to monitor pressing, shear, and torsion (Figure 3b, middle).[23] These polymeric nanofibers are interlocked each other and sandwiched by PDMS sheets. Interlocked nanofiber archi-tecture has extremely large contact area between the fibers. Changes in the contact induce the resistance response. The

polyurethane-based strain sensor is sensitive enough to detect the dynamic motion of a water droplet. Another example is an ultrasensitive piezoelectric sensors made up of electrospun fibers of poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-co-TrFe)) (Figure 3b, bottom).[54] Electrospinning onto a fast rotating substrate produces well aligned P(VDF-co-TrFe) fibers. As a result, high piezoactive β-fraction is largely achieved without electrical polling, which dramatically improves the strain gauge sensitivity.

CNTs (Figure 3c, top)[26] have attracted considerable atten-tions because of their unique electrical, mechanical, and physi cal characteristics. Transparent and stretchable strain sen-sors have been established by using CNT films spray-coated on ultrathin PDMS sheets (Figure 3c, middle).[55] By using this, Lipomi and collaborators have reported novel sensors that could detect changes in pressure and strain by measuring capacitance changes in the CNT network. Other representative examples have exploited resistance changes in more complex CNT net-work geometries (Figure 3c, bottom).[56] Aligned CNT films are transferred onto the stretchable substrate to form complex networks composed of islands and gaps. When the device is stretched, cracks are generated in the CNT film. Therefore, when strain is applied, the resistance significantly changes due


Figure 3. One-dimensional functional nanomaterials and sensor applications. a) Inorganic nanowires: SEM image of ZnO NWs (top), motion sensor composed of piezoelectric ZnO NWs (middle), and wearable pressure sensor using Au NWs embedded in tissue paper (bottom). Reproduced with permission.[51–53] Copyright 2010, American Chemical Society; Copyright 2014, Wiley-VCH; Copyright 2014, Macmillan Publishers Ltd. b) Polymer nanofibers: SEM image of polyurethane nanofiber (top), interlocked strain sensor using Pt-coated PU nanofiber (middle), and piezoelectric motion sensor based on P(VDF-TrFe) nanofiber (bottom). Reproduced with permission.[23,54] Copyright 2012, Copyright 2013, Macmillan Publishers Ltd. c) CNTs: SEM image (top), skin-like pressure sensor based on a sprayed CNT network (middle), and CNT-based stretchable strain sensor for human motion detection (bottom). Reproduced with permission.[26,55,56] Copyright 2014, Wiley-VCH; Copyright 2011, Macmillan Publishers Ltd. d) Hybrid nanofibers: SEM image of electrospun nanofibers composed of CNT and graphene (top), bending insensitive pressure sensor based on CNT and graphene composite nanofibers (middle), and piezoresistive strain sensor using ZnO NWs and PS NWs (bottom). Reproduced with permission.[41,57] Copyright 2016, Macmillan Publishers Ltd; Copyright 2011, Wiley-VCH.




to the cracks, thus enabling the sensor to detect small changes of the mechanical strain.

One-dimensional nanomaterials have been combined with other nanomaterials to create 1D hybrid materials (Figure 3d, top).[41] The hybridized nanofibers show improved stability to mechanical deformations and have higher sensitivity due to their larger surface areas, relative to homogeneous 1D nano-materials.[41,57] A bending-insensitive pressure sensor was developed using a nanofiber composite composed of graphene and CNTs (Figure 3d, middle).[41] Many pressure sensors are usually susceptible to unwanted bending-induced noise. How-ever, the pressure sensor made of composite nanofibers is not, since the nanofiber network can change the relative align-ment due to sufficient spacing between each nanofiber during bending deformations, which reduces the bending induced strain. As a result, these pressure sensors can reliably measure pressure changes with minimal noise ever under mechanically bent conditions. Another example of the hybrid-material-based sensors, recently developed by Xiao and collaborators, employed networked films of ZnO NWs and polystyrene nanowires (PS NWs) to fabricate strain gauges (Figure 3d, bottom).[57] This hybridized film of inorganic/organic nanowires and nanofibers is fabricated through electrospinning and hydrothermal pro-cesses. When the external strain is applied, the number of con-tact points between the hybrid nanofibers is reduced, leading to a dramatic increase in resistance. Xiao and collaborators suc-cessfully monitored finger motions using these strain gauges.

2.3. Two-Dimensional Functional Nanomaterials and Sensor Applications

Bio-sensors based on 1D nanomaterial constituents hold great promise because of their remarkable sensitivity and the large sensing range but the spatial arrangement of 1D nanomaterials, sometimes, can be tedious and nonuniform thus limiting their utility in more real-world applications. To address these limita-tions, 2D nanomaterials have recently attracted attentions not only because of their promising electrical/optical properties, but also because of their unique crystalline structures and high uni-formity. The crystalline structure of many 2D hybrid materials has been demonstrated to impart unique physical properties that enable various types of sensors. In addition, conventional fabrication processes can be readily applied due to rapid advances in 2D materials synthesis and/or top-down fabrication processes as film formats. Moreover, 2D materials in ultrathin nanoscale formats support high levels of mechanical defor-mations without compromising their performances, which is desirable for many skin-mounted wearable applications. How-ever, 2D nanomaterials tend to have small surface to volume ratios due to their planar structure, which limits their utility.

Figure 4 shows representative examples of 2D nanosheets and their sensor applications. Nanosheets exhibit transparency, mechanical flexibility, and high electrical and thermal conduc-tivities in the planar direction. Single crystal silicon (Si), first of all, is well-known, widely-used, and high quality inorganic semiconductor. This, in a wafer form, is rigid but becomes very soft and flexible when its thickness is reduced to nanoscale (Figure 4a, top).[27] A flexible and stretchable strain gauge was

developed using single crystal Si nanosheets and integrated for a prosthetic skin to measure applied strains to the prosthetic skin (Figure 4a, middle).[28] The ultrathin and serpentine design of these Si nanosheets facilitates precise monitoring of local-ized mechanical deformations while enduring strain induced by human motion. Si nanosheet-based strain sensors are optimized to accommodate different strain levels observed on different body locations. In addition, Si nanosheet-based p-n junction diodes can be used to characterize the thermal distri-bution of human skin (Figure 4a, bottom).[58] Multiplexed tem-perature sensor arrays composed of p-n junction diodes can precisely monitor the spatio-temporal distribution of tempera-ture gradients on human skin.

Two-dimensional nanomaterials such as graphene and tran-sition metal dichalcogenides are attractive alternatives to Si because of their high performance and unconventional func-tionalities. One of the most widely studied 2D materials is gra-phene that can be achieved by various processes such as exfo-liation,[65] oxidation/reduction,[66] and chemical vapor deposition (Figure 4b, top).[59] Graphene has been implemented in various kinds of sensors including mechanical, electrophysiological, and electrochemical sensors, since intrinsic and/or functionalized graphene shows outstanding electrical and electrochemical per-formances respectively. In one example, highly stretchable strain sensors were developed by combining crumpled graphene and nanocellulose fibrils (Figure 4b, middle).[60] Crumpled graphene can be stretched up to 100% strain without any mechanical fail-ures. Its resistance changes in response to applied strain due to changes of its conductive paths. Yan and collaborators demon-strated a graphene-based piezoresistive strain gauge as a way to monitor finger motions. An effective FET-type electrochemical sensor for biomolecule detection, in another example, can also be fabricated using graphene (Figure 4b, bottom).[61] The gra-phene channel in the FET-type biosensor is treated by chemical dopants for bandgap opening and functionalized by olfactory receptors to selectively detect particular odorant, amyl butyrate, with high sensitivity. Because the liquid gate bias is changed by odorant molecule capture, the current flow extremely changes by the amyl butyrate concentration.

Recently, molybdenum disulfide (MoS2) nanosheets (Figure 4c, top) have also drawn attention as a unique semicon-ductor material with remarkable optical and electrical proper-ties.[62] Monolayer MoS2 nanosheets were chemically grown to create large area conformal tactile sensors (Figure 4c, middle).[63] MoS2 nanosheets on interdigitated electrodes change its resist-ance along external pressure. An ultrathin MoS2-based tactile sensor has numerous advantages including transparency, ability to accommodate mechanical deformations, and high sensitivity. Because these conformal tactile sensors have an ultrathin, flex-ible structure, they could be applied to curved surfaces on body parts such as the fingertip. Moreover, MoS2 materials exhibit piezoelectric behavior, which have been utilized in piezoelectric strain sensors (Figure 4c, bottom).[62] The piezoelectric responses of MoS2 are most sensitive in monolayer formats, making it a promising material for wearable electromechanical sensors.

Two dimensional nanosheets have been integrated with other nanomaterials to create hybridized nanosheets that realize the performance of versatile bio-sensors (Figure 4d, top).[32] Het-erostructures of 2D nanomaterials tune their electrical/optical





carrier transport characteristics and enable various kinds of biosensing functionalities. For example, graphene has been combined with Au NPs and gold microparticles to reduce inter-facial electrical impedances (Figure 4d, middle).[32] Polymer nanosheets that respond to specific chemicals are then com-bined to detect specific biomolecules. The modified graphene-hybrid-based bio-sensor can continuously monitor glucose and pH levels in sweat. Another example is dual-gate photodetectors composed of vertically integrated graphene–MoS2–graphene heterostructures (Figure 4d, bottom).[64] These heterostruc-ture enables band slope tuning and photocurrent generation because of the weak screening effect of graphene. Yu and col-laborators demonstrated that the polarity and amplitude of the photocurrent can be controlled by an external gate to optimize quantum efficiency. The properties of 0D, 1D, and 2D materials and their advantages/disadvantages in device applications are summarized in Table 1.

3. Multifunctional Sensor Arrays

The aforementioned classes of nanomaterials have ena-bled a variety of mechanical, electrical, optical, chemical, and

piezoelectric sensors. Investigations in this area have usually focused on increasing sensitivity, specificity, sensing range, and deformability.[37] Another important research direction is to increase the diversity of measurable physiological and/or elec-trophysiological quantities in one platform through the integra-tion of multiple sensor modalities.[72] By integrating different types of sensors into a single substrate, many research groups have enabled multifunctional sensing within a single platform, and simultaneous analysis of different sensor data provides novel and useful information.[73–75]

Since individual sensors in multifunctional sensors are based on functional nanomaterials, it is possible to measure various biometric information with high resolution and good signal quality on the dynamically deformable human skin. A variety of biometric information can be measured from the skin, including electrophysiological signals such as electromyo-graphy (EMG) and electrocardiography (ECG) signals as well as blood flow, water content in soft tissues, pH of the body fluids, glucose concentrations, and body temperature.

Figure 5a,b shows a skin-mounted device that includes an oximetry sensor to measure pulse and blood flow, a thermal sensor to measure the thermal conductivity of the skin, and an electrophysiological sensor to measure EMG or ECG


Figure 4. Two-dimensional functional nanomaterials and sensor applications. a) Si nanosheet: SEM image (top), Si nanosheet-based stretchable strain gauge for the prosthetic skin (middle), and conformal temperature sensor array for continuous thermal monitoring of human skin (bottom). Reproduced with permission.[27,28,58] Copyright 2014, Copyright 2013, Macmillan Publishers Ltd. b) Graphene: SEM image (top), graphene-based strain sensor for finger motion detection (middle), and graphene FET-based bioelectronics nose (bottom). Reproduced with permission.[59–61] Copyright 2009, Macmillan Publishers Ltd; Copyright 2014, Wiley-VCH; Copyright 2012, the American Chemical Society. c) MoS2 nanosheet: AFM image of a single layer MoS2 nanosheet (top), flexible and transparent tactile sensor based on MoS2 nanosheets (middle), and piezoelectric sensor using MoS2 nanosheets (bottom). Reproduced with permission.[62,63] Copyright 2014, Macmillan Publishers Ltd; Copyright 2016, Wiley-VCH. d) Hybrid nanosheets: SEM image of graphene hybrids (top), graphene hybrid-based glucose and pH sensors for sweat analysis (middle), and gate-tunable photodetector using vertical heterostructures of graphene and MoS2 nanosheets (bottom). Reproduced with permission.[32,64] Copyright 2016, Copyright 2013, Macmillan Publishers Ltd.




(Figure 5c,d).[76] This multifunctional sensor array is capable of measuring three independent quantities, and can be used to predict a user’s health status based on the measurement results. Jang and collaborators were able to monitor the user’s muscle activity by measuring the EMG signal with the elec-trophysiological sensor, monitor the skin’s thermal conduc-tivity with a thermal sensor, and predict skin hydration using the linear relationship between the data of skin hydration and thermal conductivity.[76] In addition, an oximetry sensor has been used to monitor user’s blood flow and brain activity. Nanometer-thick metal interconnections and their stretchable layout enable the integrated device to reliably measure multiple disease-related signals under mechanical deformation.[76]

Skin is a highly attractive biological organ for mimicking its functions for variety of sensing applications. Human skin contains mechano and thermo receptors, therefore it can sense various kinds of environmental stimuli.[2,28] There are many ongoing research efforts in the fields of multimodal pressure-temperature sensors,[28,73,77] soft prosthetics,[28,78] and soft cir-cuit integration[2] for mimicking the functions of human skin. In order to mimic human skin, an artificial skin device must first comprehensively detect and measure multiple external stimuli that can be naturally detected by human skin.[79,80] Unu-sual materials are suitable for base functional materials for these highly sensitive sensors, because each sensor composing an artificial skin device needs fast and sensitive responses like the real human skin.[37] Figures 5e and f show the soft artifi-cial skin device equipped with a multifunctional sensor array and the layered structure of the artificial skin, respectively.[28]

The distribution of external pressure and temperature can be respectively monitored using Si nanosheet pressure sensor arrays (Figure 5g) and temperature sensor arrays (Figure 5h).[28] Single crystal Si nanosheets are highly sensitive to mechan-ical deformations because of their high piezoresistivity, thus making it suitable for pressure sensing with comparable sen-sitivity with the real skin.[28] In addition, Si nanosheet tem-perature sensors based on p-n junction diodes show reliable temperature sensing performance. Furthermore, the ultrathin thickness of the Si nanosheet, which is on the scale of hun-dreds of nanometers, makes it robust to mechanical deforma-tion, thus enabling reliable performances as a skin prosthesis.

However, the Si nanosheet-based sensors embedded in the soft skin prosthesis are limited in their ability to measure shear forces because of their flat structure, which are typically sensed by human skin. There are ongoing research efforts underway to develop various types of soft, multi-axial piezoresistive sen-sors,[81,82] piezoelectric sensors,[83] capacitive sensors,[84] optical sensors, and microfluidic sensors.[85] Further optimization of the current version of multi-axial force sensors (e.g., enhanced softness, robustness, facile interfacing with control unit, and large area coverage) could lead to wide spread adoption of this technology.

In addition to physiological and electrophysiological signals, electrochemical information analyzed from human biofluids secreted from human skin is very useful.[32,86,87] Unusual nano-materials are important to build highly sensitive electrochem-ical sensors which can detect tiny amount of bio-chemicals from human biofluids. A multifunctional sensor arrays that can


Table 1. Comparison of functional nanomaterials characteristics.

0D 1D 2D

Structure • Nanoparticle • Nanorod• Nanowire• Nanofiber

• Nanosheet

Intrinsic

properties• Electrical tunneling[44–46]

• Carrier injection[25]

• Piezoelectricity[50] • High surface to volume ratio[47,48]

• High aspect ratio[23,57]

• Complex networks[55,56]

• Piezoelectrisity[52,54]

• High flexibility[62,63]

• Uniform crystallinity[28,58]

• Transparency[32,63]

Assembly

method• Langmuir Blodgett[29]

• Electrodeposition[47,48]

• Blending[49,50]

• Spin coating[16]

• Hydrothermal synthesis[52,57]

• Electrospinning[54]

• Molding[23]

• Spraying[55]

• Blending[41]

• Spin coating[67]

• Transfer printing[28,58]

• Exfoliation[64]

Sensing

principle• Tunneling current[44–46]

• Carrier doping[25]

• Chemical trap[47,48]

• Piezoelectricity[52,54]

• Piezoresistivity[23,53,57]• PN diode[28,58]

• Piezoresistivity[28,58,60]

• Field effect transistor[61,64]

• Electrochemical reaction[32]

Advantages • High surface area[47,48]

• Property tuning by controlling size[16,68]• Easy to form network structure[55,56]

• Tuning electrical properties through alignment[69]

• Ultrathin and flexible[32,62,63]

• High carrier transport[28,58]

• High uniformity[28,58]

• Adaptable to conventional fabrication process[28,58,63]

• Compatible to high temperature process[28,58,63]

• Low roughness[70]

Disadvantages • Hard to form network structure• Easy to aggregate in high temperature process[29]

• Non-uniform spatial arrangement• High roughness compared to 2D

materials[71]

• Low surface to volume ratio




measure multiple signals simultaneously provides high accu-racy in electrochemical sensing.[32,86,87]

Figure 5i shows a device that can estimate blood glucose concentration by using sweat.[32] Graphene electrodes are used as substrates where the functional layers for bio-sensing are deposited. To improve the electrical properties of the gra-phene electrodes, they were doped with a gold chloride (AuCl3) solution, forming gold nano- and micro-particles. The overall electrochemical impedance of the gold-doped graphene was improved, thereby enabling facile and uniform deposition of the functional polymers.[32] Figure 5j shows a magnified image of the pH (ii), humidity (iii), tremor (iv) sensor, and a refer-ence electrode (v) as well as the glucose sensor (i). The glucose sensor signal changes according to the glucose concentration in sweat (Figure 5k). However, the performance of the glucose sensor that is based on glucose oxidase, is dependent on tem-perature and acidity of human sweat. An accurate estimate of the sweat glucose level was derived by accounting for the simul-taneously measured temperature and sweat acidity levels to make appropriate corrections on the raw glucose sensing data (Figure 5l).[32]

Most multifunctional sensor arrays described above require coupling to external data acquisition devices for data retrieval

and storage. These wiring strategies rely on bulky wiring and electrical connectors, which limits seamless integration of soft stretchable sensors and measurement systems.

4. Data Storage Systems with Sensors

The information collected by sensors must be locally stored in data storage modules for post-processing and analysis. The stored data can be used to monitor the long-term trends of biosignal changes. A charge trap flash memory (CTFM) device has been one of the most widely used memory architec-ture.[88] The need for faster data reading and writing, has led to advances in memory technologies, including resistive random access memory (RRAM), magnetoresistive random access memory (MRAM), and phase-change random access memory (PRAM).[89–91] Conventional CTFMs store information by using the change of the threshold voltage of a transistor (TR).[88] This threshold voltage changes as charges build up in the floating gate between the tunneling oxide and the blocking oxide. Mean-while, RRAM stores information using the characteristic that the resistance of an insulator changes depending on its conduc-tive state which can be controlled by external applied biases.[89]


Figure 5. Multifunctional sensor arrays. a) Stretchable multifunctional electronic system attached to human skin and b) schematics showing its constituent sensors. c) EMG measurements from the inside (w/o hair) and outside (w/ hair) of the forearm. d) Thermal conductivity converted from transient heat source measurements. Skin moisture measured by commercial skin moisture meter is linearly correlated with thermal conductivity. a–d) Reproduced with permission.[76] Copyright 2014, Macmillan Publishers Ltd. e) Artificial skin composed of various types of sensors. f) Layered structure of the artificial skin. g) Regional pressure map obtained by using the pressure sensor array in the artificial skin. h) Regional temperature map obtained by using the temperature sensor in the artificial skin. The infrared camera image on the top shows similar results with those obtained by using the temperature sensor array. e–h) Reproduced with permission.[28] Copyright 2014, Macmillan Publishers Ltd. i) Wearable multifunctional sensor array for sweat analysis. j) Enlarged photograph showing a glucose sensor (i), pH sensor (ii), humidity sensor (iii), tremor sensor (iv), and counter electrode (v). k) Response of the glucose sensor when solutions with different glucose concentrations are applied. l) Comparison of the uncorrected and cor-rected glucose data (correlated using simultaneously-measured pH data) with glucose concentration measured by a commercial glucose assay kit. i–l) Reproduced with permission.[32] Copyright 2016, Macmillan Publishers Ltd.




Because its structure is simple and the device size is relatively small, there have been many previous studies on deformable[43] and wearable RRAM devices.[24,92,93] The study of deformable MRAM and PRAM, however, is still at the early stage.[94,95]

In order to collect and store information by a single platform integrated with the multifunctional wearable biosensor array, the data storage modules must be also ultrathin and soft so that they can be co-integrated with the wearable sensors and confor-mally attached to human skin.[92] In addition, low power con-sumption of the wearable memory is highly demanded because of the limited power source in the portable system.[24] In this respect, unusual nanomaterials are suitable for ultrathin, high performance, and low power-consuming wearable data storage devices co-integrated with wearable sensors. Figure 6a shows a wearable RRAM array combined with a strain sensor array and a thermal actuator for the drug delivery.[24] Sensors, a heater, and a RRAM array in single platform can be mounted on human skin with a drug-containing hydrocolloid skin patch. In this configuration, the strain sensor attached on the wrist region first measures tremor of the patients who are suf-fering from motion-related neurological disorders. The meas-ured data is stored in the memory, and the patient’s condition can be diagnosed afterward. As part of a feasible closed-loop

monitoring-therapy solution, the microheater then elevates the temperature to initiate transdermal delivery of drugs pre-loaded in the wearable patch. The upper frame of Figure 6b presents captured tremor signals measured by the strain sensor and the corresponding change in measured current when the information is read from the RRAM. The lower frame of Figure 6b shows a strategy for the sequential storage of wrist tremor frequency changes in different RRAM pixels with respect to elapsed time.

Despite recent advances in next-generation memory tech-nologies, well-established CTFM are also possible solutions for future wearable memory modules because of their proven reli-ability. Figure 6c shows an example of a wearable sensing plat-form, composed of CTFM arrays, stretchable electrode arrays, and voltage amplifiers.[29] The stretchable electrode array meas-ures ECG signals, which are amplified by a voltage amplifier (Figure 6d). The wearable CTFM array captures the processed data reliably even under mechanical strains approaching ≈20%. A floating gate comprised of Au NPs enlarges the memory window and improves the retention property. Au NPs com-posing a floating gate can effectively store injected charges, because ligands of Au NPs electrically separate neighboring Au NPs.[29] Heart rate can be calculated from amplified ECG


Figure 6. Data storage systems with sensors. a) Wearable RRAM array with the co-located strain sensor, temperature sensor, and heater. The inset shows a 10 × 10 RRAM array on the commercial hydrocolloidal skin patch. b) Resistance change of a strain sensor depending on the different tremor frequencies and corresponding current levels used to store the data in the RRAM under multilevel cell operation (top). Images of RRAM pixels in which the specific tremor frequencies are stored (bottom). The dashed black, orange, red, and blue boxes indicate the RRAM pixels that store the measured tremor frequencies of 0.8 Hz, 0.4 Hz, 0.6 Hz, and 1.0 Hz as two digit codes, respectively. a–b) Reproduced with permission.[24] Copyright 2014, Macmillan Publishers Ltd. c) Skin-mountable electronic system composed of an ECG electrode array, voltage amplifiers, and a CTFM array. d) Amplified ECG signal showing an R peak measured by an ECG electrode array (left) and transfer curves of the erased/programmed CTFM pixel after multiple stretching with 20% strain (right). e) Schematic of the CTFM array storing measured heart rate and elapsed time. The heart rate and elapsed time are converted into binary numbers for storage in the CTFM array. Reproduced with permission.[29] Copyright 2016, American Association for the Advancement of Science.




signals, and the history of measurements can be stored in a memory array. Once the heart rate recovery pattern is stored in memory during an exercise stress test, the clinician can retrieve the stored data for post-study analysis. Figure 6e shows a scheme for storing the recorded heart rate and elapsed time in the memory array.

The wearable memory modules reviewed above require connections to external devices. Generally, heat seal connec-tors have been utilized to electromechanically couple memory modules with the external devices because of their robust mechanical flexibility and strain relief. However, these connec-tors still have limitations. Future system in package approaches could enable controller circuits to be co-located adjacent to the memory array, thus solving interfacing challenges.

5. Wireless Data Transmission Modules with Sensors

The information measured or stored by a wearable device must be delivered to the user or healthcare professional. In this case, wireless data transmission is preferred for convenience over wired alternatives. There are passive and active methods

of wireless data transmission for wearable systems, which are being investigated by several groups. Simply, in wearable appli-cations, passive wireless data transmission is highly demanded because it helps to further miniaturize the battery requirements and energy needs of the system.

Wireless passive methods include inductive coupling using an LC resonator[30,96,97] and near field communication (NFC).[31,98] In the case of an LC resonator, an inductor and a capacitor are connected in series. Because the inductance and/or capacitance of the device change when the biosignal changes, the frequency and phase response of the LC reso-nator changes accordingly.[30,96,97] Figure 7a shows a wire-less sweat sensor that can be attached onto human skin. The inductor of this sensor is a coil that consists of four turns of thin copper traces.[30] The capacitor of this sensor is composed of interdigitated electrodes. Both components are stretchable because of their filamentary serpentine design. The sweat sensor is placed on a porous substrate. As sweat is absorbed by the porous substrate, the dielectric property of the substrate changes. This change induces a change in the phase response of the sweat sensor, and the concentration of sodium chloride in the sweat can be wirelessly measured (Figure 7b).


Figure 7. Wireless data transmission modules with sensors. a) Wireless sweat sensor under a primary coil. b) Phase response when a solution of dif-ferent sodium chloride concentration is applied. a–b) Reproduced with permission.[30] Copyright 2014, Wiley-VCH. c) A wireless hydration and strain sensor under flat (left) and deformed (right) conditions. d) Phase response of the wireless strain sensor when different amounts of strain are applied. c–d) Reproduced with permission.[96] Copyright 2014, Wiley-VCH. e) Schematic of a resonant sensor for wireless pressure measurement. f) Resonant frequency change of the wireless pressure sensor with respect to applied pressure. g) Wireless pressure sensor applied on human skin to measure temporal pressure changes due to heartbeats. h) Measured temporal pressure change showing that the heart rate of the subject is 82 beats per minute. e–h) Reproduced with permission.[97] Copyright 2014, Macmillan Publishers Ltd. i) Exploded view of the flexible NFC device. j) Temperature measure-ment by placing the NFC-equipped smartphone near the flexible NFC device. i–j) Reproduced with permission.[31] Copyright 2015, Wiley-VCH. k) Flexible NFC device for measuring PPG signals. l) Measured PPG signal showing systolic peaks and dicrotic notches. k–l) Reproduced with permission.[98] Copyright 2016, American Association for the Advancement of Science.




The effect of strain can also be characterized using an LC resonator-based wireless sensor.[96] Skin strain sensors can monitor body motions and even detect subcutaneous fluid accu-mulation (sign of lymphedema). Figure 7c shows a wireless strain sensor attached onto the skin. As the skin is stretched, the spacing between the interdigitated electrodes changes causing a marked change in the capacitance. As a result, the frequency response of the LC resonator in the strain sensor changes (Figure 7d).

Pressure sensing is another important physiological sensing mode for clinical applications. Figure 7e shows a wireless pres-sure sensor composed of two coils that form both an inductor and capacitor.[97] A microstructured pressure sensitive dielec-tric polymer is located between the coils. When pressure is applied, the pressure sensitive dielectric layer is compressed, and the distance separating the coils is reduced, which increases its capacitance. This capacitance change induces a change in the frequency response of the wireless pressure sensor (Figure 7f). As a biomedical application of this wearable sensor, the wireless pressure sensor can be attached to the skin above the radial artery (Figure 7g) to monitor the arterial pulse (Figure 7h).

An LC resonator-based wireless sensor has the advantage that its structure is small and simple, and practically invisible to the user. However, these sensors require network analyzers to receive and analyze the transmitted data. In order to build a convenient system for consumers, compatibility with NFC-ena-bled smartphones is highly recommended. Recently, a wearable sensing device that communicates via the NFC protocol has been reported (Figure 7i).[31]

NFC is a wireless technology used for short-range com-munication (




sensor and stimulator are attached to the operator’s skin to control motion of a robot arm (Figure 8k). The motion of the robot arm is determined by the motion sensor signal attached on the operator. For example, once the motion sensor gener-ates a signal corresponding to a pressing motion, the gripper of the robot arm grasps an object (Figure 8l). When the pres-sure sensor located on the gripper senses the grasping motion, a feedback electrical stimulation is applied through the electro-tactile stimulator to alert the user of this event.

To synchronize the operation of sensors and actuators as introduced in this section, both elements are normally con-nected to external controllers with heat seal connectors. Although heat seal connector provides stable electromechanical connections, they are still bulky for wearable systems. In future applications, this external controller could be sufficiently minia-turized and co-integrated with sensors and actuators.

7. Fully-Integrated Wearable Prototypes

The novel materials, sensing modules, and actuators that we have reviewed so far highlight advances in functions and form

factors of individual sub-modules. These sub-modules require external controllers and data analyzers to measure data, store or read the data to and from the memory, and apply feedback actuations. In this section, we review fully integrated system prototypes operated independently without needing external instrumentation. To develop such a stand-alone wearable system, sensors, actuators, controllers, and data transmission modules should be seamlessly integrated all together in one soft substrate.

One approach to create a stand-alone wearable system is to electrically connect the soft sensors with a flexible printed cir-cuit board (FPCB)[33,86] or an wearable band[42,101] that houses all of the commercial chipsets, including the signal condi-tioning circuits, microcontroller units, and wireless modules. Figure 9a shows a fully integrated wearable device that can ana-lyze human sweat.[33] Sensors for monitoring temperature, lac-tate, potassium, sodium, and glucose are located on the flexible substrate. This sensor array is interconnected with the FPCB containing the circuits for data acquisition, analysis, and trans-mission. The connection of the sensor array to the FPCB can be easily implemented via an onboard zero-insertion-force con-nector. Sweat can be analyzed in real-time, and the measured


Figure 8. Actuators coupled with sensors. a) PZT sensors and actuators for measuring the modulus of human skin. b) Enlarged view of the PZT sen-sors and actuators indicated by the red dotted box in a). The PZT actuators vibrate in response of the applied voltage, and the PZT sensors generate electrical voltage in response of the vibration of the actuators. c) Theoretical (T) and experimental (E) results showing the output voltage of the PZT sensor when three different frequencies of the voltage are applied to the PZT actuator. d) Measured PZT sensor voltage and skin modulus changes at different times after application of 1% acrylamidomethylpropane sulphonic acid to the skin. a–d) Reproduced with permission.[99] Copyright 2015, Macmillan Publishers Ltd. e–f) Photograph e) and schematic f) of a transdermal drug delivery device with an integrated temperature sensor and a heater. g) Schematic of a microneedle containing drug-loaded PCNs. h) IR image showing the temperature distribution of separately controlled heaters. e–h) Reproduced with permission.[22] Copyright 2017, American Association for the Advancement of Science. i–j) Schematics of a piezoelectric motion sensor i) and electrotactile stimulators j) composed of a graphene heterostructure. k) Image of pressing a piezoelectric motion sensor laminated onto a subject’s wrist. The generated signal (inset) is transmitted to a robot arm (blue dotted arrow), and the feedback from the robot is delivered (green dotted arrow) to the subject through an electrotactile stimulator. l) Image of the robot arm controlled by the piezoelectric sensor. When the gripper grasps an object, the integrated piezoelectric pressure sensor generates signals to alert the subject through the wearable electrotactile simulation. i–l) Reproduced with permission.[100] Copyright 2014, Wiley-VCH.




data can be corrected according to the simultaneously meas-ured temperature (Figure 9b). Figure 9c shows a schematic illustration and photograph of a calcium, pH, and temperature sensor array fabricated on a flexible substrate.[86] The sensor array is also connected with the integrated FPCB for in-situ data analysis and transmission. For example, the measured pH of sweat is delivered to a smartphone so that the user can mon-itor the trends in sweat acidity changes (Figure 9d).

However, the FPCB module is not sufficiently flexible to match its mechanical properties with those of human skin. Furthermore, soldered areas in the FPCB are vulnerable to repetitive deformations, since the applied strain cannot be suf-ficiently relieved because of the low deformability of the FPCB. Therefore, interconnections with high deformability should be employed to achieve a highly reliable, durable, and wearable system. Figure 9e shows such an integrated system for meas-uring electrophysiological signals from human skin.[34] The electric components are connected with stretchable conducting traces using similar procedures to conventional reflow sol-dering, thus enduring mechanical stretching up to the applied strain of 100%. Power can be wirelessly transmitted to the cir-cuits using inductive coupling. An integrated stretchable elec-trode measures EMG signals (Figure 9f), and the integrated

accelerometer measures body motion (Figure 9g). The acquired signals can be transmitted through co-integrated wireless modules.

To supply power to the device shown in Figure 9e, the pri-mary coil must be placed in close proximity to the secondary coil. Wireless power transmission over large distances is a promising goal for this kind of wearable devices, but there are still limitations. They can be powered only in the restricted region where the power transmitters are located. As an alterna-tive, miniaturized batteries can be utilized as a power source for the wearable devices. Figure 9h shows a fully integrated system that employs a chip-scale lithium ion battery as a power source and NFC module as a sensor, memory, and wireless data trans-mitter.[102] All electronic components are interconnected with filamentary serpentine metal traces, thus achieving stretch-ability. Using an NFC-equipped smartphone, temperature can be continuously monitored by the onboard NFC module in the skin-mountable device (Figure 9i).

Passive wearable sensors that do not rely on a power source have several usability advantages. Colorimetric-based sensors are good examples, whereby a change in a bioanalyte concen-tration corresponds to a color change.[103] The reaction is typi-cally spontaneous, which can be readily analyzed with digital


Figure 9. Fully integrated wearable prototypes. a) Sweat sensor integrated with commercial chips for data acquisition and transmission. b) Real-time and simultaneous measurement result of temperature and concentration of chemicals in sweat. a–b) Reproduced with permission.[33] Copyright 2016, Macmillan Publishers Ltd. c) Schematic of the sweat sensor for measuring the calcium concentration and pH of sweat. Inset shows the deformed sensor array. d) Measured change in potential of a pH sensor according to the pH of a sample. c–d) Reproduced with permission.[86] Copyright 2016, American Chemical Society. e) Photographs of undeformed (left) and stretched (right) wearable circuits composed of commercial chips for measuring electrophysiological data. f) Measured surface-EMG signal. g) Acceleration data measured when a subject walks and falls with the device mounted on a forearm. e–g) Reproduced with permission.[34] Copyright 2014, American Association for the Advancement of Science. h) Image of an NFC device integrated with a battery and regulator circuits. i) Response of an NFC device when a stepwise temperature increase is applied. h–i) Reproduced with permission.[102] Copyright 2016, National Academy of Sciences. j) Image of a wearable microfluidic device for the colorimetric sensing of sweat. k) Nor-malized percentage of red, green, blue (RGB) color channels correlated with the lactate concentration. j–k) Reproduced with permission.[103] Copyright 2016, American Association for the Advancement of Science.





image capture instruments (e.g., smart phone). Figure 9j shows a wearable microfluidic device that harvests and monitors local sweat directly from skin pores using quantitative colorimetric analysis.[103] Sweat is harvested and routed into a small chamber that contains colorimetric reagents. The color of the reagents change as water, lactate, chloride, pH, and glucose molecules in the sweat react with the reagent, respectively. The resulting color changes are analyzed using an image processing software that quantifies the red, green, and blue components of the color (Figure 9k). A NFC chipset and antenna coils are used to auto-matically initiate the analysis process on the smart phone. This wireless interface enables users to easily access and handle the sweat sensing system.

Although integration of commercial chipsets with soft sen-sors and actuators enables independently operating system, bulky and mechanically stiff modules reduce the usefulness of the system. This issue can be resolved by integrating functional blocks (i.e., system on chip modules), thus reducing the overall footprint of the rigid chipsets.[104–106]

8. Fully-Integrated Manufacturable Wearable Products

The ability to create fully-integrated wearable sensing sys-tems has laid the foundation for a new class of manufactur-able products, which have recently entered the marketplace. These products have been recently scaled up in volume (up to

millions of units manufactured to date), highlighting the evo-lution of the technology from key nanomaterial innovations, and system-level prototypes to manufacturable products. The new products described in this section still incorporate many of the foundational technologies, including multiple layers of soft elastic materials, ultrathin integrated dies and mechanics optimization for long term wear on human skin. The additional breakthrough over the existing prototypes in Section 1 is in the application of hybrid roll-to-roll (for soft polymeric materials and adhesives) and pick-and-place assembly (for electronics components) strategies used to significantly scale up manufac-turing and volumes at a reasonable cost.

My UV PatchTM is a powerful example of an epidermal device that has NFC functionality and photosensitive UV dyes both integrated in the multilayer design (Figure 10a).[35] The NFC module contains an ultrathin bare die (≈10 µm thick) that is electrically coupled to a coil-shaped antenna (≈5 µm thick). On the bottom surface, a soft ultrathin adhesive layer provides sufficient adhesion strength for multi-day wear on several body locations (Figure 10b). The ultrathin form factor significantly reduces the bending stiffness of the My UV PatchTM system, while the soft stretchable encapsulating polymeric layers enable elasticity on skin. As a result, My UV PatchTM system bends, twists, and contorts with human skin much like a temporary tattoo (Figure 10c) and with little or no change in its original function (Figure 10d,e).

While My UV PatchTM exploits NFC capability and measures environmental signals (i.e., UV light exposure), there are other

Figure 10. Fully integrated manufacturable wearable products. a) Multi-layer design for the My UV Patch that consists of active dyes, elastic polymer layer, ultrathin electronics, and skin adhesive. b) Device is manufacturable at high volume and adheres to skin like a temporary tattoo on various body locations. c) Ultrathin mechanical design imparts softness and ability to deform with skin during normal operation. d) Electrical resonance proper-ties of the antenna have been characterized. e) The tuning properties of the antenna can accommodate strains. Reproduced with permission.[35] Copyright 2015, MC10 Inc. f) The WiSP device is shown laminated on the torso. g) The device captures ECG signals in real time and passively monitors the heart rate over 24–48 hours. h) BioStamp system laminated on the forearm that measures arm motions and electric biopotentials. Reproduced with permission.[107] Copyright 2016, Springer International Publishing Switzerland. i) The BioStamp captures ECG, EMG, and accelerometry data at high configurable sampling rates. j) The wearable sensing system is inductively rechargeable using a proprietary charging station. k) An ultrathin disposable adhesive layer interfaces with the skin for wear at multiple body locations. Repoduced with permission.[36] Copyright 2015, MC10 Inc.





compelling designs for NFC-enabled devices to capture physio-logical data. The wearable interactive stamp (WiSP) is a novel cardiac sensor that captures heart rate and ECG signals from the torso (Figure 10f). This flexible wearable device collects the heart rate continuously for 24–48 hours, and the user can readily download this data to a smart phone (Figure 10g). The WiSP operates with any NFC-enabled smartphones to enable easy use by consumers, which is necessary for wide scale adoption.

In addition to these NFC-enabled wearable biosystems, there are new classes of soft wearable biosensing systems that exploit Bluetooth Low Energy (BTLE) connectivity to collect and transmit vast amounts of biometric data wirelessly in the clinical environment and at home. The BioStamp Research Connect® (Figure 10h) platform contains soft, stretchable silicone material layers that encapsulate stretchable circuits and biosensors housed within the silicone.[36] These designs combine silicone-molding procedures together with flexible electronics processes, thus creating new classes of soft wear-able products at scale (>10000 units). The BioStamp system captures multiple physiological sensing modalities (e.g., surface EMG, ECG, electroencephalography (EEG), linear motion, and angular motion) (Figure 10i), supports data cap-ture with the onboard flash memory, and has an integrated rechargeable battery and charging station for long-term use (Figure 10j). These sub-modules are encapsulated in soft and stretchable silicone, which serves as a moisture barrier and soft interface between disposable adhesive and human skin (Figure 10k).

9. Conclusions and Perspectives

Nanomaterials and semiconductor technologies have broadly driven important advances in performance, biocompatible design, and manufacturing processes required for wearable sensing systems. Novel functional nanomaterials offer unique chemical, mechanical, and electrical properties that impart enhanced conformability and performance for emerging wear-able sensing systems.[93,101] Representative examples of device sub-modules and fully integrated wearable sensors described in this review highlight the rapid adoption of advanced mate-rials breakthroughs. Nevertheless, there remain many chal-lenges and opportunities that require further investigations in the design and synthesis of nanomaterials in biofluid environ-ments. New research directions addressing long-term device stability, biocompatibility, facile manufacturing, and monolithic assembly, are critical for achieving stable electrical, mechanical, and chemical interfaces with soft skin tissues. We anticipate future growth and sustained commitment to research in these areas, as a means to fostering new scientific discoveries that transition to commercial opportunities in wearable sensing systems.

AcknowledgementsY.L. and J.K. contributed equally to this work. This work was supported by IBS-R006-D1.

Keywordsactuators, data storage device, nanomaterials, sensors, wearable electronics, wireless data transmission

Received: February 26, 2017Revised: April 1, 2017

Published online: June 19, 2017

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