Buckling assisted and lithographically micropatterned fully flexible sensors for conformal integration applications

Carbon nanomaterials for sweat-based sensors: a review

Sweat is easily accessible from the human skin's surface. It is secreted by the eccrine glands and contains a wealth of physiological information, including metabolites and electrolytes like glucose and Na ions. Sweat is a particularly useful biofluid because of its easy and non-invasive access, unlike other biofluids, like blood. On the other hand, nanomaterials have started to show promise operation as a competitive substitute for biosensors and molecular sensors throughout the last 10 years. Among the most synthetic nanomaterials that are studied, applied, and discussed, carbon nanomaterials are special. They are desirable candidates for sensor applications because of their many intrinsic electrical, magnetic, and optical characteristics; their chemical diversity and simplicity of manipulation; their biocompatibility; and their effectiveness as a chemically resistant platform. Carbon nanofibers (CNFs), carbon dots (CDs), carbon nanotubes (CNTs), and graphene have been intensively investigated as molecular sensors or as components that can be integrated into devices. In this review, we summarize recent advances in the use of carbon nanomaterials as sweat sensors and consider how they can be utilized to detect a diverse range of analytes in sweat, such as glucose, ions, lactate, cortisol, uric acid, and pH.

Sericin coated thin polymeric films reduce keratinocyte proliferation via the mTOR pathway and epidermal inflammation through IL17 signaling in psoriasis rat model

Therapeutic treatment forms can play significant roles in resolving psoriatic plaques or promoting wound repair in psoriatic skin. Considering the biocompatibility, mechanical strength, flexibility, and adhesive properties of silk fibroin sheets/films, it is useful to combine them with anti-psoriatic agents and healing stimulants, notably silk sericin. Here, we evaluate the curative properties of sericin-coated thin polymeric films (ScF) fabricated from silk fibroin, using an imiquimod-induced psoriasis rat model. The film biocompatibility and psoriatic wound improvement capacity was assessed. A proteomics study was performed to understand the disease resolving mechanisms. Skin-implantation study exhibited the non-irritation property of ScF films, which alleviate eczema histopathology. Immunohistochemical and gene expression revealed the depletion of β-defensin, caspase-3 and -9, TNF-α, CCL-20, IL-1β, IL-17, TGF-β, and Wnt expressions and S100a14 mRNA level. The proteomics study suggested that ScF diminish keratinocyte proliferation via the mTOR pathway by downregulating mTOR protein, corresponding to the modulation of TNF-α, Wnt, and IL-1β levels, leading to the enhancement of anti-inflammatory environment by IL-17 downregulation. Hematology data demonstrated the safety of using these biomaterials, which provide a potential therapeutic-option for psoriasis treatment due to desirable effects, especially anti-proliferation and anti-inflammation, functioning via the mTOR pathway and control of IL-17 signaling.

Advances in wearable electrochemical antibody-based sensors for cortisol sensing

Cortisol is a crucial hormone involving many physiological processes. Hence, cortisol detection is essential. This review highlights the key progress made on wearable electrochemical sensors using antibodies. It covers the design, principle, and electroanalytical methodology for detecting cortisol noninvasively. This article also analyzes and collects the analytical performances of electrochemical cortisol sensors. The development of these sensors continues to face challenges such as biofouling, sample management, sensitivity, flexibility, stability, and recognition layer performance. It is also necessary to develop a sensitive electrode and material. This article also presents potential strategies for designing antibody electrodes and provides examples of sensing systems. Additionally, it discusses the challenges in translating research into practical applications.

Electroplating and Ablative Laser Structuring of Elastomer Composites for Stretchable Multi-Layer and Multi-Material Electronic and Sensor Systems

In this work we present the concept of electroplated conductive elastomers and ablative multi-layer and multi-material laser-assisted manufacturing to enable a largely automated, computer-aided manufacturing process of stretchable electronics and sensors. Therefore, the layers (conductive and non-conductive elastomers as well as metal layers for contacting) are first coated over the entire surface (doctor blade coating and electroplating) and then selectively removed with a CO₂ or a fiber laser. These steps are repeated several times to achieve a multi-layer-structured design. Is it not only possible to adjust and improve the work previously carried out manually, but also completely new concepts such as fine through-plating between the layers to enable much more compact structures become possible. In addition, metallized areas allow the direct soldering of electronic components and thus a direct connection between conventional and stretchable electronics. As an exemplary application, we have used the process for manufacturing a thin and surface solderable pressure sensor with a silicone foam dielectric and a stretchable circuit board.

Soft and Deformable Sensors Based on Liquid Metals

Liquid metals are one of the most interesting and promising materials due to their electrical, fluidic, and thermophysical properties. With the aid of their exceptional deformable natures, liquid metals are now considered to be electrically conductive materials for sensors and actuators, major constituent transducers in soft robotics, that can experience and withstand significant levels of mechanical deformation. For the upcoming era of wearable electronics and soft robotics, we would like to offer an up-to-date overview of liquid metal-based soft (thus significantly deformable) sensors mainly but not limited to researchers in relevant fields. This paper will thoroughly highlight and critically review recent literature on design, fabrication, characterization, and application of liquid metal devices and suggest scientific and engineering routes towards liquid metal sensing devices of tomorrow.

Tailored topography: a novel fabrication technique using an elasticity gradient

A facile methodology to create a wrinkled surface with a tailored topography is presented herein. The dependency of the elasticity of poly(dimethyl)siloxane (PDMS) on the curing temperature has been exploited to obtain a substrate with an elasticity gradient. The temperature gradient across the length of PDMS is created by a novel set-up consisting of a metal and insulator connected to a heater and the highest usable (no degradation of PDMS) temperature gradient is used. The time-dependent temperature distributions along the substrate are measured and the underlying physics of the dependence of the PDMS elasticity on the curing temperature is addressed. The PDMS substrate with the elasticity gradient is first stretched and subsequently oxidized by oxygen plasma. Upon relaxation, an ordered wrinkled surface with continuously varying wavelength and amplitude along the length of PDMS is obtained. The extent of hydrophobicity recovery of this plasma oxidized PDMS with varying elasticity has been studied. The change in the wavelength and amplitude of the regular patterns on the substrate can be controlled by varying operational parameters like applied pre-strain, plasma power and the heater temperature. It has been found that the spatial distributions of the topography and the hydrophobicity collectively decide the resultant wettability of the substrate. Such surfaces with gradients in the substructure dimensions demonstrate different wetting characteristics that may lead to a wide gamut of applications including droplet movement, cell adhesion and proliferation, diffraction grating etc.

Anisotropic Electrowetting on Wrinkled Surfaces: Enhanced Wetting and Dependency on Initial Wetting State

Electrowetting on dielectric (EWOD) on unidirectional microstructured surfaces has recently evoked significant interest as they can modulate the effect of electrowetting, and can thus find applications in directional wetting in microfluidic systems. However, the dependency of such EW phenomenon on their initial state of wetting and anisotropy is far from being well understood. The current study addresses the initial wetting states and their implication on the anisotropic electrowetting using a wrinkled EWOD platform. Herein we demonstrate a facile stampless and maskless structure generation technique to fabricate wrinkles of varying topography. Further, we have demonstrated alteration in the interfacial wetting conditions by modulating the wrinkle topography, and its effect on the droplet behavior during electrowetting. The capillary wicking-assisted electrowetting on these wrinkled surfaces is in specific direction dictated by the ordered wrinkles and prompts enhanced spreading of the droplet. We also demonstrate that while the enhancement of unidirectional electrowetting is stronger in conformal wetting state surfaces, composite wetting state surfaces depict a reversal in anisotropy.

Atomic force microscopy and nanoindentation investigation of polydimethylsiloxane elastomeric substrate compliancy for various sputtered thin film morphologies

Crack free electrically continuous metal thin films over soft elastomeric substrates play an integral part in realization of modern day flexible bioelectronics and biosensors. Under nonoptimized deposition conditions, delamination, and/or cracking of the top film as well as the underlying soft substrate hinders optimal performance of these devices. Hence it is very important to understand and control not only the various deposition factors like power, time, or deposition pressure but also investigate the various interfacial physics playing a critical role in assuring thin film adhesion and substrate compliancy. In the present study, various nanomechanical information of the underlying substrate, namely, crack profile, average roughness, Young's modulus, and adhesion force were studied for uncracked and cracked polydimethylsiloxane (PDMS) surfaces along with pristine and conventional plasma treated PDMS samples as control. Quantification of the above parameters were done using three-dimensional surface profiler, scanning electron microscopy, nanoindentation, and atomic force microscopy techniques to elucidate the modulus range, average roughness, and adhesion force. Comparative analysis with control revealed remarkable similarity between increased modulus values, increased surface roughness, and reduced adhesion force accounting for reduced substrate compliancy and resulting in film cracking or buckling which are critical for development of various bioflexible devices. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 725-737, 2018.

Unconventional micro-/nanofabrication technologies for hybrid-scale lab-on-a-chip

Micro-/nanofabrication-based lab-on-a-chip (LOC) technologies have recently been substantially advanced and have become widely used in various inter-/multidisciplinary research fields, including biological, (bio-)chemical, and biomedical fields. However, such hybrid-scale LOC devices are typically fabricated using microfabrication and nanofabrication processes in series, resulting in increased cost and time and low throughput issues. In this review, after briefly introducing the conventional micro-/nanofabrication technologies, we focus on unconventional micro-/nanofabrication technologies that allow us to produce either in situ micro-/nanoscale structures or master molds for additional replication processes to easily and conveniently create novel LOC devices with micro- or nanofluidic channel networks. In particular, microfabrication methods based on crack-assisted photolithography and carbon-microelectromechanical systems (C-MEMS) are described in detail because of their superior features from the viewpoint of the throughput, batch fabrication process, and mixed-scale channels/structures. In parallel with previously reported articles on conventional micro-/nanofabrication technologies, our review of unconventional micro-/nanofabrication technologies will provide a useful and practical fabrication guideline for future hybrid-scale LOC devices.