Skin-mountable Devices Research Articles

Metal-like Stretchable Nanocomposite Using Locally-Bundled Nanowires for Skin-Mountable Devices.

Stretchable conductive nanocomposites have been intensively studied for wearable bioelectronics. However, development of nanocomposites that simultaneously feature metal-like conductivity (> 100,000 S/cm) and high stretchability (> 100%) for high-performance skin-mountable devices is still extremely challenging. Herewepresent a material strategy for such a nanocomposite by using local bundling of silver nanowires stabilized with dual ligands (i.e., 1-propanethiols and 1-decanethiols). When the nanocompositewassolidified via solvent evaporation under a highly humid condition, the nanowires in the organic solution are bundled and stabilized. The resulting locally-bundled nanowires lower contact resistance while maintain their percolation network, leading to high conductivity. Dual ligands of 1-propanethiol and 1-decanethiol further boost up the conductivity. As a result, a nanocomposite with both high conductivity of ∼ 122,120 S/cm and high stretchability of ∼ 200% is obtained. Such superb electrical and mechanical properties are critical for various applications in skin-like electronics, andweherein demonstrated a wearable thermo-stimulation device. This article is protected by copyright. All rights reserved.

Smart electronics based on 2D materials for wireless healthcare monitoring

The demand for wearable electronics in the fields of human healthcare monitoring and disease diagnosis has significantly increased in recent years. In particular, there is a need for light-weight, skin-friendly, soft elastic devices that can attach comfortably to human skin and communicate information via the Internet of Things. Rigorous research has been carried out to find new materials and device designs that can meet the challenging demands of skin-mountable devices. The emergence of atomically thin two-dimensional (2D) materials with exceptional electrical, optical, and mechanical properties, and low cytotoxicity has facilitated the fabrication of low-dimensional electronic devices on flexible/stretchable platforms that can be easily integrated into the human body. Herein, we provide a comprehensive review of recent research progress on 2D material-based wearable sensors that are proposed for a wide range of applications including human health monitoring. Several potential applications based on wearable electronic devices have already been well established and documented, while many others are at a preliminary stage. Based on current research progress, the challenges and prospects toward commercial implementation of such clinical sensors are also discussed.

Triboresistive Touch Sensing: Grid-Free Touch-Point Recognition Based on Monolayered Ionic Power Generators.

Recent growing pursuit of skin-mountable devices has been impeded by the complicated structures of most sensing systems, containing electrode grids, stacked multilayers, and even external power sources. Here, a type of touch sensing, termed "triboresistive touch sensing", is introduced for gridless touch recognition based on monolayered ionic power generators. A homogeneous monolayer, i.e., ionic poly(dimethylsiloxane) (PDMS), generates electricity based on the electric field generated by touch. Voltages generated at each corner of the ionic PDMS rely on resistance between touch points and each corner, ensuring recognition of the touch positions without the need for electrode grid layers and external power sources. With notable advantages of high transparency (96.5%), stretchability (539.1%), and resilience (99.0%) of the ionic PDMS, epidermal triboresistive sensing is demonstrated to express touch position and readily play a musical instrument. A gridless system of triboresistive sensing allows rearrangement of the touch sections according to a given situation without any physical modification, and thus easily completes consecutive missions of controlling position, orientation, and gripping functions of a robot.

Larger, flexible, and skin-mountable energy devices with graphene single layers for integratable, wearable, and health monitoring systems

In recent years, the development of weightless, ultrathin, and non-toxic devices has received tremendous interest owing to their potential applications as building blocks for various flexible, wearable, and skin-mountable systems. In this view, we explored the exceptional performances of flexible energy harvesters and storage devices along with wearable optoelectronic devices by adopting high-quality graphene monolayers and a user-friendly transfer methodology. Flexible and transparent energy generators with an active layer (AL) thickness of ∼20 μm and storage devices (AL≤ 1 μm) were developed by sandwiching the piezoelectric and solid electrolyte materials between two graphene monolayers, respectively. Wearable photosensors, with an AL thickness of ∼30 μm, were designed by integrating an ultrathin zinc oxide layer with a graphene monolayer. Under nominal mechanical movements, typical graphene monolayers-based piezoelectric energy generators exhibited very stable peak voltage and current density of 5.5 V and 0.2 nA/cm2, respectively. Whereas the skin-mountable micro-supercapacitor (mSC) showed slightly lower areal and volumetric capacitances (6.3 μF/cm2 and 91 mF/cm3@100 mV/s scan rate) than that of the flexible mSCs. Interestingly, these mSC devices also showed significant mechanical flexibility, stability, and durability. Further, the as-fabricated photosensors exhibited a strong response to visible light with an On/Off current density ratio of 1.8 and excellent wearability. Based on these demonstrated outcomes, we emphasize that the devices fabricated on different substrates by using graphene single layers could be adopted for various wearable, biocompatible, and skin-mountable devices that are widely being used in various health monitoring systems.

2D Materials for Skin-Mountable Electronic Devices.

Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

In-Line and Off-Line Monitoring of Skin Penetration Profiles Using Confocal Raman Spectroscopy.

Ex-vivo and in-vivo skin analysis has been extensively evaluated by confocal Raman spectroscopy (CRS). The off-line measurement with a CRS-suited skin-mounted device after Franz-cell incubations is the most popular choice. However, real-time monitoring of in-line measurement has clear advantages for obtaining dynamic and more timely results. In our study, a custom-built setup suitable for in-line measurements was implemented, which ensures constant skin incubation and in-situ skin detections. We aim to compare the differences between using in-line and off-line devices for monitoring skin drug penetrations. A well-assessed formulation gel with procaine-HCl as the active ingredient was used as reference. The PEG-23 lauryl ether was added to the formulation as a penetration enhancer to evaluate the enhancement effects of procaine on skin. After incubation times of 14, 20, and 24 h, skin penetration profiles were assessed. Comparable results between off-line and in-line measurements were obtained. Remarkable improvements in penetrated procaine amount and depth were observed. Based on the significant differences of their enhanced penetration amounts, fairly similar estimations were achieved from both methods. A slight difference of 14 h incubation between these two setups can still be found, which may be due to the different detection conditions and affected skin properties. Overall, in-line measurements could provide a more time- and labor-saving alternative for off-line measurements in ex-vivo study.

A flexible skin-mounted wireless acoustic device for bowel sounds monitoring and evaluation

Conventional methods of intestinal inspection play an essential role in the assessment of bowel diseases and other relevant health issues, yet fail to obtain intestinal conditions in real time because of radiation limits and operation inconvenience. Herein, a flexible, skin-mounted device is developed for long-term, real-time monitoring, and for the evaluation of bowel sounds based on the integration of a three-dimensional printed elastomeric resonator with flexible electronics. The device is capable of being flexibly attached to abdominal surfaces without performance degradation during breathing. Clinical tests conduct in a normal volunteer and in patients with mechanical intestinal obstruction or paralytic ileus illustrate the usefulness of the device in capturing the characteristics of bowel sounds. Furthermore, a demonstration of collection and classification of bowel sounds by the flexible device based on machine learning methods can serve as a reference for possible applications of the system in the auxiliary diagnosis of bowel problems.

Skin-Mountable Biosensors and Therapeutics: A Review.

Miniaturization of electronic components and advances in flexible and stretchable materials have stimulated the development of wearable health care systems that can reflect and monitor personal health status by health care professionals. New skin-mountable devices that offer seamless contact onto the human skin, even under large deformations by natural motions of the wearer, provide a route for both high-fidelity monitoring and patient-controlled therapy. This article provides an overview of several important aspects of skin-mountable devices and their applications in many medical settings and clinical practices. We comprehensively describe various transdermal sensors and therapeutic systems that are capable of detecting physical, electrophysiological, and electrochemical responses and/or providing electrical and thermal therapies and drug delivery services, and we discuss the current challenges, opportunities, and future perspectives in the field. Finally, we present ways to protect the embedded electronic components of skin-mountable devices from the environment by use of mechanically soft packaging materials.

Highly Stretchable, Sensitive, and Transparent Strain Sensors with a Controllable In-Plane Mesh Structure.

Highly stretchable and transparent strain sensorsintegrated in-plane meshed structure with silver nanowire conductive networks are rationally designed and prepared by the ultrafast laser cutting technique and facile drop-coating process, which exhibit great promise for efficient large-scale production. The in-plane meshed structure and low-content conductive network enable the sensor to achieve outstanding sensing performance and optical transparency based on simple fabricating processes and simplified components. More impressively, finite element method simulation is used to analyze and predict the influence of multiple structures on properties for selecting optimal meshed structures. Consequently, the strain sensor with optimal meshed structures exhibits high sensitivity in a wide working strain range (gauge factor of 846 at 150% tensile strain), good optical transparency of 88.3%, slight hysteresis, and long-term durability under large-strain cycles. Because of these superior performances, this in-plane mesh-structured strain sensor has great potential as candidates for wearable electronics, especially in skin-mountable devices detecting both subtle physiological signals and large motion information.

Epidermal Microfluidic Electrochemical Detection System: Enhanced Sweat Sampling and Metabolite Detection.

Despite tremendous recent efforts, noninvasive sweat monitoring is still far from delivering its early analytical promise. Here, we describe a flexible epidermal microfluidic detection platform fabricated through hybridization of lithographic and screen-printed technologies, for efficient and fast sweat sampling and continuous, real-time electrochemical monitoring of glucose and lactate levels. This soft, skin-mounted device judiciously merges lab-on-a-chip and electrochemical detection technologies, integrated with a miniaturized flexible electronic board for real-time wireless data transmission to a mobile device. Modeling of the device design and sweat flow conditions allowed optimization of the sampling process and the microchannel layout for achieving attractive fluid dynamics and rapid filling of the detection reservoir (within 8 min from starting exercise). The wearable microdevice thus enabled efficient natural sweat pumping to the electrochemical detection chamber containing the enzyme-modified electrode transducers. The fabricated device can be easily mounted on the epidermis without hindrance to the wearer and displays resiliency against continuous mechanical deformation expected from such epidermal wear. Amperometric biosensing of lactate and glucose from the rapidly generated sweat, using the corresponding immobilized oxidase enzymes, was wirelessly monitored during cycling activity of different healthy subjects. This ability to monitor sweat glucose levels introduces new possibilities for effective diabetes management, while similar lactate monitoring paves the way for new wearable fitness applications. The new epidermal microfluidic electrochemical detection strategy represents an attractive alternative to recently reported colorimetric sweat-monitoring methods, and hence holds considerable promise for practical fitness or health monitoring applications.

VALIDATION OF THE USE OF A SKIN-MOUNTED DEVICE TO MEASURE OUT-OF-PLANE ROTATIONS OF THE SPINE FOR A ROWING ACTIVITY

Objective: To assess the accuracy of a skin-mounted device to measure lumbar spinal rotations in rowers. Methods: Nine subjects were each imaged in 12 extremes of lateral flexion and rotation in an open MR scanner, and the movement between external MR markers fixed to the skin, and the underlying vertebral bodies was quantified. Results: The average error in measuring the changes in the lateral flexion and rotation between the MR markers and the underlying vertebral bodies was ±5.0° (95% CI 3.5–6.4°, S.E. 0.7°) for lateral flexion and 4.4° (95% CI 3.2–5.6°, S.E 0.6°) for lateral rotation (nine subjects). Conclusions: This study has demonstrated the accuracy of using an externally mounted device to measure out-of-plane rotations for the extremes of motion achieved during a rowing activity, allowing this system to be used to quantify deviations in rowing technique. The method could be applied to other repetitive activities of the spine.