Drug Delivery Systems for Personal Healthcare by Smart Wearable Patch System.

Smart wearable patch systems that combine biosensing and therapeutic components have emerged as promising approaches for personalized healthcare and therapeutic platforms that enable self-administered, noninvasive, user-friendly, and long-acting smart drug delivery. Sensing components can continuously monitor physiological and biochemical parameters, and the monitoring signals can be transferred to various stimuli using actuators. In therapeutic components, stimuli-responsive carrier-based drug delivery systems (DDSs) provide on-demand drug delivery in a closed-loop manner. This review provides an overview of the recent advances in smart wearable patch systems, focusing on sensing components, stimuli, and therapeutic components. Additionally, this review highlights the potential of fully integrated smart wearable patch systems for personalized medicine. Furthermore, challenges associated with the clinical applications of this system and future perspectives are discussed, including issues related to drug loading and reloading, biocompatibility, accuracy of sensing and drug delivery, and largescale fabrication.

Crack-inducing Strain Sensor Array using Inkjet-Printed Silver Thin Film for Underplate and Off-centered Force Sensing Applications.

The change in resistance upon bending in metal films as thick as 1 mm used for underpanel force touch applications is limited by the low sensitivity, thus requiring high-performance readout circuitry. In this paper, we report inkjet-printed silver thin films having crack-inducing underlayers, which further increases the sensitivity of their resistance changes under deformation. This allows for detecting weak vertical forces even through the plates (force-receiving layer), such as 0.4 or 1.2 mm thick polyethylene terephthalate or 0.4 mm thick glass. The underplate sensors will detect a force level as low as 10 gf, which corresponds to the amount of force required for fingerprint recognition. Furthermore, such highly sensitive strain sensors can potentially solve the inaccuracy issue of wearable devices, which can occur when misplaced sensors detect relatively weak biosignals, such as heart rate and blood pressure. The sensor detects the accurate pulse patterns of the wrist artery even though it is off-centered from the artery by 6 mm or larger. The crack-based strain sensor and its usage as a hidden underplate force sensing device will create various wearable and user-machine interface applications.

Nonpatterned Soft Piezoresistive Films with Filamentous Conduction Paths for Mimicking Multiple-Resolution Receptors of Human Skin.

Soft pressure sensors play key roles as input devices of electronic skin (E-skin) to imitate real human skin. For efficient data acquisition according to stimulus types such as detailed pressure images or macroscopic strength of stimuli, soft pressure sensors can have variable spatial resolution, just like the uneven spatial distribution of pressure-sensing receptors on the human body. However, previous methods on soft pressure sensors cannot achieve such tunability of spatial resolution because their sensor materials and read-out electrodes need to be elaborately patterned for a specific sensor density. Here, we report a universal soft pressure-sensitive platform based on anisotropically self-assembled ferromagnetic particles embedded in elastomer matrices whose spatial resolution can be facilely tuned. Various spatial densities of pressure-sensing receptors of human body parts can be implemented by simply sandwiching the film between soft electrodes with different pitches. Since the anisotropically aligned nickel particles form independent filamentous conductive paths, the pressure sensors show spatial sensing ability without crosstalk, whose spatial resolution up to 100 dpi can be achieved from a single platform. The sensor array shows a wide dynamic range capable of detecting various pressure levels, such as liquid drops (∼30 Pa) and plantar (∼300 kPa) pressures. Our universal soft pressure-sensing platform would be a key enabling technology for actually imitating the receptor systems of human skin in robot and biomedical applications.

Recent progress in strain-engineered elastic platforms for stretchable thin-film devices.

Strain-engineered elastic platforms that can efficiently distribute mechanical stress under deformation offer adjustable mechanical compliance for stretchable electronic systems. By fully exploiting strain-free regions that are favourable for fabricating thin-film devices and interconnecting with reliably stretchable conductors, various electronic systems can be integrated onto stretchable platforms with the assistance of strain engineering strategies. Over the last decade, applications of multifunctional stretchable thin-film devices simultaneously exhibiting superior electrical and mechanical performance have been demonstrated, shedding light on the realization of further reliable human-machine interfaces. This review highlights recent developments in enabling technologies for strain-engineered elastic platforms. In particular, representative approaches to realize strain-engineered substrates and stretchable interconnects in island-bridge configurations are introduced from the perspective of the material homogeneity and structural design of the substrate. State-of-the-art achievements in sophisticated stretchable electronic devices on strain-engineered elastic platforms are also presented, such as stretchable sensors, energy devices, thin-film transistors, and displays, and then, the challenges and outlook are discussed.

Electronic Skin Based on a Cellulose/Carbon Nanotube Fiber Network for Large-Area 3D Touch and Real-Time 3D Surface Scanning.

Electronic skin (E-skin) based on tactile sensors has great significance in next-generation electronics such as biomedical application and artificial intelligence that requires interaction with humans. To mimic the properties of human skin, high flexibility, excellent sensing capability, and sufficient spatial resolution through high-level sensor integration are required. Here, we report a highly sensitive pressure sensor matrix based on a piezoresistive cellulose/single-walled carbon nanotube-entangled fiber network, which forms its own porous structure enabling a superior pressure sensor with a high sensitivity (9.097 kPa-1), a fast response speed (<2 ms), and orders of magnitude detection range with a detection limit of 1 Pa. Furthermore, the remarkable device expandability based on the ease of patterning and scalability allows easy implementation of a large-area pressure sensor matrix which has 2304 (48 × 48) pixels. Combined with a real-time pressure distribution monitoring system, a flexible 3D touch sensor that simultaneously displays plane coordinates and pressure information and a scanning device that detects the morphology of the soft body 3D surface are successfully demonstrated.

Inkjet-Printing-Based Density Profile Engineering of Single-Walled Carbon Nanotube Networks for Conformable High-On/Off-Performance Thin-Film Transistors.

Random networks of single-walled carbon nanotubes (SWCNTs) offer new-form-factor electronics such as transparent, flexible, and intrinsically stretchable devices. However, the long-standing trade-off between carrier mobility and on/off ratio due to the coexistence of metallic and semiconducting nanotubes has limited the performance of SWCNT-random-network-based thin-film transistors (SWCNT TFTs), hindering their practical circuit-level applications. Methods for high-purity separation between metallic and semiconducting nanotubes have been proposed, but they require high cost and energy and are vulnerable to contamination and nanotube shortening, leading to performance degradation. Alternatively, additional structures have been proposed to reduce the off-state current, but they still compromise carrier mobility and suffer from inevitable expansion in device dimensions. Here, we propose a density-modulated SWCNT network using an inkjet-printing method as a facile approach that can achieve superior carrier mobility and a high on/off ratio simultaneously. By exploiting picoliter-scale drops on demand, we form a low-density channel network near the source and drain junctions and a high-density network at the middle of the channel. The modulated density profile forms a large band gap near the source and drain junctions that efficiently blocks electron injection under the reverse bias and a narrow band gap at the high-density area that facilitates the hole transport under the on-state bias. As a result, the density-modulated SWCNT TFTs show both high carrier mobility (27.02 cm2 V-1 s-1) and a high on/off ratio (>106). We also demonstrate all-inkjet-printed flexible inverter circuits whose gain is doubled by the density-modulated SWCNT TFTs, highlighting the feasibility of our approach for realizing high-performance flexible and conformable electronics.

Underwater maneuvering of robotic sheets through buoyancy-mediated active flutter.

Falling leaves flutter from side to side due to passive and intrinsic fluid-body coupling. Exploiting the dynamics of passive fluttering could lead to fresh perspectives for the locomotion and manipulation of thin, planar objects in fluid environments. Here, we show that the time-varying density distribution within a thin, planar body effectively elicits minimal momentum control to reorient the principal flutter axis and propel itself via directional fluttery motions. We validated the principle by developing a swimming leaf with a soft skin that can modulate local buoyancy distributions for active flutter dynamics. To show generality and field applicability, we demonstrated underwater maneuvering and manipulation of adhesive and oil-skimming sheets for environmental remediation. These findings could inspire future intelligent underwater robots and manipulation schemes.

High-performance compliant thermoelectric generators with magnetically self-assembled soft heat conductors for self-powered wearable electronics.

Softening of thermoelectric generators facilitates conformal contact with arbitrary-shaped heat sources, which offers an opportunity to realize self-powered wearable applications. However, existing wearable thermoelectric devices inevitably exhibit reduced thermoelectric conversion efficiency due to the parasitic heat loss in high-thermal-impedance polymer substrates and poor thermal contact arising from rigid interconnects. Here, we propose compliant thermoelectric generators with intrinsically stretchable interconnects and soft heat conductors that achieve high thermoelectric performance and unprecedented conformability simultaneously. The silver-nanowire-based soft electrodes interconnect bismuth-telluride-based thermoelectric legs, effectively absorbing strain energy, which allows our thermoelectric generators to conform perfectly to curved surfaces. Metal particles magnetically self-assembled in elastomeric substrates form soft heat conductors that significantly enhance the heat transfer to the thermoelectric legs, thereby maximizing energy conversion efficiency on three-dimensional heat sources. Moreover, automated additive manufacturing paves the way for realizing self-powered wearable applications comprising hundreds of thermoelectric legs with high customizability under ambient conditions.

Ultraflexible and transparent electroluminescent skin for real-time and super-resolution imaging of pressure distribution.

The ability to image pressure distribution over complex three-dimensional surfaces would significantly augment the potential applications of electronic skin. However, existing methods show poor spatial and temporal fidelity due to their limited pixel density, low sensitivity, or low conformability. Here, we report an ultraflexible and transparent electroluminescent skin that autonomously displays super-resolution images of pressure distribution in real time. The device comprises a transparent pressure-sensing film with a solution-processable cellulose/nanowire nanohybrid network featuring ultrahigh sensor sensitivity (>5000 kPa-1) and a fast response time (<1 ms), and a quantum dot-based electroluminescent film. The two ultrathin films conform to each contact object and transduce spatial pressure into conductivity distribution in a continuous domain, resulting in super-resolution (>1000 dpi) pressure imaging without the need for pixel structures. Our approach provides a new framework for visualizing accurate stimulus distribution with potential applications in skin prosthesis, robotics, and advanced human-machine interfaces.

Soft Modular Electronic Blocks (SMEBs): A Strategy for Tailored Wearable Health-Monitoring Systems.

Precise monitoring of human body signals can be achieved by soft, conformal contact and precise arrangement of wearable devices to the desired body positions. So far, no design and fabrication methodology in soft wearable devices is able to address the variations in the form factor of the human body such as the various sizes and shapes of individual body parts, which can significantly cause misalignments and the corresponding inaccurate monitoring. Here, a concept of soft modular electronic blocks (SMEBs) enabling the assembly of soft wearable systems onto human skin with functions and layouts tailored to the form factors of individuals' bodies is presented. Three types of SMEBs are developed as fundamental building blocks for functional modularization. The physical design of SMEBs is optimized for a mechanically stable island-bridge configuration. The prepared SMEBs can be integrated onto a target body part through rapid, room-temperature (RT) assembly (<5 s) using an oxygen plasma-induced siloxane bonding method. A soft metacarpophalangeal (MP) joints flexion monitoring system that is tailored to allow for accurate monitoring for multiple individuals with unique joint and hand sizes is demonstrated.

Printed cylindrical lens pair for application to the seam concealment in tiled displays.

Seamless tiling of displays is one of the key enabling technologies for the next-generation large-area electronics applications. In this paper, we propose a facile method to demonstrate a seamless display using cylindrical lens pair (CLP) fabricated by dispenser printing method. Optical properties of the printed CLP and corresponding capability of concealing seam in the display are analyzed by a set of luminance simulation and measurement in terms of geometric parameters of the lens. The seamless display with an optimized CLP features a viewing angle of the seam concealment of 40°.

Electronic skins for soft, compact, reversible assembly of wirelessly activated fully soft robots.

Designing softness into robots holds great potential for augmenting robotic compliance in dynamic, unstructured environments. However, despite the body's softness, existing models mostly carry inherent hardness in their driving parts, such as pressure-regulating components and rigid circuit boards. This compliance gap can frequently interfere with the robot motion and makes soft robotic design dependent on rigid assembly of each robot component. We present a skin-like electronic system that enables a class of wirelessly activated fully soft robots whose driving part can be softly, compactly, and reversibly assembled. The proposed system consists of two-part electronic skins (e-skins) that are designed to perform wireless communication of the robot control signal, namely, "wireless inter-skin communication," for untethered, reversible assembly of driving capability. The physical design of each e-skin features minimized inherent hardness in terms of thickness (<1 millimeter), weight (~0.8 gram), and fragmented circuit configuration. The developed e-skin pair can be softly integrated into separate soft body frames (robot and human), wirelessly interact with each other, and then activate and control the robot. The e-skin-integrated robotic design is highly compact and shows that the embedded e-skin can equally share the fine soft motions of the robot frame. Our results also highlight the effectiveness of the wireless inter-skin communication in providing universality for robotic actuation based on reversible assembly.

Fully printable, strain-engineered electronic wrap for customizable soft electronics.

Rapid growth of stretchable electronics stimulates broad uses in multidisciplinary fields as well as industrial applications. However, existing technologies are unsuitable for implementing versatile applications involving adaptable system design and functions in a cost/time-effective way because of vacuum-conditioned, lithographically-predefined processes. Here, we present a methodology for a fully printable, strain-engineered electronic wrap as a universal strategy which makes it more feasible to implement various stretchable electronic systems with customizable layouts and functions. The key aspects involve inkjet-printed rigid island (PRI)-based stretchable platform technology and corresponding printing-based automated electronic functionalization methodology, the combination of which provides fully printed, customized layouts of stretchable electronic systems with simplified process. Specifically, well-controlled contact line pinning effect of printed polymer solution enables the formation of PRIs with tunable thickness; and surface strain analysis on those PRIs leads to the optimized stability and device-to-island fill factor of strain-engineered electronic wraps. Moreover, core techniques of image-based automated pinpointing, surface-mountable device based electronic functionalizing, and one-step interconnection networking of PRIs enable customized circuit design and adaptable functionalities. To exhibit the universality of our approach, multiple types of practical applications ranging from self-computable digital logics to display and sensor system are demonstrated on skin in a customized form.

Revisit to three-dimensional percolation theory: Accurate analysis for highly stretchable conductive composite materials.

A percolation theory based on variation of conductive filler fraction has been widely used to explain the behavior of conductive composite materials under both small and large deformation conditions. However, it typically fails in properly analyzing the materials under the large deformation since the assumption may not be valid in such a case. Therefore, we proposed a new three-dimensional percolation theory by considering three key factors: nonlinear elasticity, precisely measured strain-dependent Poisson's ratio, and strain-dependent percolation threshold. Digital image correlation (DIC) method was used to determine actual Poisson's ratios at various strain levels, which were used to accurately estimate variation of conductive filler volume fraction under deformation. We also adopted strain-dependent percolation threshold caused by the filler re-location with deformation. When three key factors were considered, electrical performance change was accurately analyzed for composite materials with both isotropic and anisotropic mechanical properties.

F-number matching method in light field microscopy using an elastic micro lens array.

In light field microscopy (LFM), the F-number of the micro lens array (MLA) should be matched with the image-side F-number of the objective lens to utilize full resolution of an image sensor. We propose a new F-number matching method that can be applied to multiple objective lenses by using an elastic MLA. We fabricate an elastic MLA with polydimethylsiloxane (PDMS) using a micro contact printing method and address the strain for the F-number variation. The strain response is analyzed, and the LFM system with the elastic MLA is demonstrated. Our proposed system can increase the F-number up to 27.3% and can be applied to multiple objective lenses.