Stretchable and Self-Adhesive Humidity-Sensing Patch for Multiplexed Non-Contact Sensing.

The slippage of moisture-sensitive materials from substrates during bending or stretching is a common issue that causes baseline drift and even failure of the flexible humidity sensors, which are essential components of wearable electronic devices. In this study, we report a stretchable, self-adhesive, and transparent humidity-sensing electronic patch comprising liquid metal particle electrodes with a stretchable serpentine structure and a humidity-sensing layer made of Ti3C2Tx MXene/carboxymethyl cellulose. This patch is constructed on a soft-hard integrated heterostructure substrate and demonstrates stable humidity-sensitive response performance at 100% tensile strain, along with autonomous adhesion to human skin. Additionally, it exhibits maximum response (1145.4%) at 90% relative humidity (RH), fast response and recovery time (1.4/5.9 s), elevated sensitivity (64.63%/% RH), and preserved humidity sensing under deformation, as well as easy scalability for multiplexed detection. We further illustrate the patch's potential applications in healthcare and environmental monitoring through a non-contact security door control system and wind monitor system. Our proposed strain-isolation strategy can be extended to other rigid conductive materials and stretchable substrates, providing a feasible mechanism for producing stretchable electronic skin patches.

Overcoming the adhesion paradox and switchability conflict on rough surfaces with shape-memory polymers.

Smart adhesives that can be applied and removed on demand play an important role in modern life and manufacturing. However, current smart adhesives made of elastomers suffer from the long-standing challenges of the adhesion paradox (rapid decrease in adhesion strength on rough surfaces despite adhesive molecular interactions) and the switchability conflict (trade-off between adhesion strength and easy detachment). Here, we report the use of shape-memory polymers (SMPs) to overcome the adhesion paradox and switchability conflict on rough surfaces. Utilizing the rubbery-glassy phase transition in SMPs, we demonstrate, through mechanical testing and mechanics modeling, that the conformal contact in the rubbery state followed by the shape-locking effect in the glassy state results in the so-called rubber-to-glass (R2G) adhesion (defined as making contact in the rubbery state to a certain indentation depth followed by detachment in the glassy state), with extraordinary adhesion strength (>1 MPa) proportional to the true surface area of a rough surface, overcoming the classic adhesion paradox. Furthermore, upon transitioning back to the rubbery state, the SMP adhesives can detach easily due to the shape-memory effect, leading to a simultaneous improvement in adhesion switchability (up to 103, defined as the ratio of the SMP R2G adhesion to its rubbery-state adhesion) as the surface roughness increases. The working principle and the mechanics model of R2G adhesion provide guidelines for developing stronger and more switchable adhesives adaptable to rough surfaces, thereby enhancing the capabilities of smart adhesives, and impacting various fields such as adhesive grippers and climbing robots.

Skin-Interfaced Superhydrophobic Insensible Sweat Sensors for Evaluating Body Thermoregulation and Skin Barrier Functions.

Monitoring sweat rate is vital for estimating sweat loss and accurately measuring biomarkers of interest. Although various optical or electrical sensors have been developed to monitor the sensible sweat rate, the quantification of the insensible sweat rate that is directly related to body thermoregulation and skin barrier functions still remains a challenge. This work introduces a superhydrophobic sweat sensor based on a polyacrylate sodium/MXene composite sandwiched between two superhydrophobic textile layers to continuously measure sweat vapor from insensible sweat with high sensitivity and rapid response. The superhydrophobic textile on a holey thin substrate with reduced stiffness and excellent breathability allows the permeation of sweat vapor, while preventing the sensor from being affected by the external water droplets and internal sensible sweat. Integrating the insensible sweat sensor with a flexible wireless communication and powering module further yields a standalone sensing system to continuously monitor insensible sweat rates at different body locations for diverse application scenarios. Proof-of-concept demonstrations on human subjects showcase the feasibility to continuously evaluate the body's thermoregulation and skin barrier functions for the assessment of thermal comfort, disease conditions, and nervous system activity. The results presented in this work also provide a low-cost device platform to detect other health-relevant biomarkers in the sweat (vapor) as the next-generation sweat sensor for smart healthcare and personalized medicine.

Design of protective and high sensitivity encapsulation layers in wearable devices.

Elastomeric encapsulation layers are widely used in soft, wearable devices to physically isolate rigid electronic components from external environmental stimuli (e.g., stress) and facilitate device sterilization for reusability. In devices experiencing large deformations, the stress-isolation effect of the top encapsulation layer can eliminate the damage to the electronic components caused by external forces. However, for health monitoring and sensing applications, the strain-isolation effect of the bottom encapsulation layer can partially block the physiological signals of interest and degrade the measurement accuracy. Here, an analytic model is developed for the strain- and stress-isolation effects present in wearable devices with elastomeric encapsulation layers. The soft, elastomeric encapsulation layers and main electronic components layer are modeled as transversely isotropic-elastic mediums and the strain- and stress-isolation effects are described using isolation indexes. The analysis and results show that the isolation effects strongly depend on the thickness, density, and elastic modulus of both the elastomeric encapsulation layers and the main electronic component layer. These findings, combined with the flexible mechanics design strategies of wearable devices, provide new design guidelines for future wearable devices to protect them from external forces while capturing the relevant physiological signals underneath the skin. Electronic Supplementary Material: Supplementary material is available in the online version of this article at 10.1007/s11431-022-2034-y.

Surface Wettability for Skin-Interfaced Sensors and Devices.

The practical applications of skin-interfaced sensors and devices in daily life hinge on the rational design of surface wettability to maintain device integrity and achieve improved sensing performance under complex hydrated conditions. Various bio-inspired strategies have been implemented to engineer desired surface wettability for varying hydrated conditions. Although the bodily fluids can negatively affect the device performance, they also provide a rich reservoir of health-relevant information and sustained energy for next-generation stretchable self-powered devices. As a result, the design and manipulation of the surface wettability are critical to effectively control the liquid behavior on the device surface for enhanced performance. The sensors and devices with engineered surface wettability can collect and analyze health biomarkers while being minimally affected by bodily fluids or ambient humid environments. The energy harvesters also benefit from surface wettability design to achieve enhanced performance for powering on-body electronics. In this review, we first summarize the commonly used approaches to tune the surface wettability for target applications toward stretchable self-powered devices. By considering the existing challenges, we also discuss the opportunities as a small fraction of potential future developments, which can lead to a new class of skin-interfaced devices for use in digital health and personalized medicine.

Moisture-resistant MXene-sodium alginate sponges with sustained superhydrophobicity for monitoring human activities.

Wearable mechanical sensors are easily influenced by moisture resulting in inaccuracy for monitoring human health and body motions. Though the superhydrophobic barrier has been extensively explored as passive water repel strategy on the sensor surface, the dense superhydrophobic surface not only limits the sensor working under large deformations but also inevitable degradation in high humidity or saturation water vapor environments. This work reports a superhydrophobic MXene-sodium alginate sponge (SMSS) pressure sensor with a low voltage Joule heating effect to provide sustain moisture-insensitive property for both sensing performance and superhydrophobicity by heating-driven water molecules away. Because of the positive temperature coefficient under pressure applied, the Joule heating can provides a stable temperature to the moisture-insensitivity property during the whole dynamic pressure cycled. Therefore, the pressure sensor with a simple spray-coating superhydrophobic coating on the outer layer demonstrates key capabilities even in extreme use scenarios with high humidity or water vapor and also provides stable and reliable bio-signal monitoring.

Strain-Tunable Microfluidic Devices with Crack and Wrinkle Microvalves for Microsphere Screening and Fluidic Logic Gates.

Mechanical instabilities in soft materials have led to the formation of unique surface patterns such as wrinkles and cracks for a wide range of applications that are related to surface morphologies and their dynamic tuning. Here, we report a simple yet effective strategy to fabricate strain-tunable crack and wrinkle microvalves with dimensions responding to the applied tensile strain. The crack microvalves initially closed before stretching are opened as the tensile strain is applied, whereas the wrinkle microvalves exhibit the opposite trend. Next, the performance of crack and wrinkle microvalves is characterized. The design predictions on the bursting pressure of microvalves and others from the theory agree reasonably well with the experimental measurements. The microfluidic devices with strain-tunable crack and wrinkle microvalves have then been demonstrated for microsphere screening and programmable microfluidic logic devices. The demonstrated microfluidic devices complement the prior studies to open up opportunities in microparticle/cell manipulations, fluidic operations, and biomedicine.

Skin-interfaced microfluidic devices with one-opening chambers and hydrophobic valves for sweat collection and analysis.

Soft, skin-interfaced microfluidic platforms are capable of capturing, storing, and assessing sweat chemistry and total sweat loss, which provides essential insight into human physiological health. However, sweat loss from the outlet of the microfluidic devices often leads to deviation of the measured concentration of the biomarker or electrolyte from the actual value. Here, we introduce hydrophobic valves at the junction of the chamber and the microfluidic channel as a new chamber design to reduce sweat evaporation. Because the advancing front of the liquid in the hydrophilic microchannel is blocked by the hydrophobic valve, the fluid flows into the chambers, forms the initial meniscus, and completely fills the chambers along the initial meniscus. Fluid dynamic modeling and numerical simulations provide critical insights into the sweat sampling mechanism into the chambers. With significantly reduced evaporation and contamination, the sweat sample can be easily stored for a long time for later analysis when in situ analysis is limited. Additionally, the design with multiple chambers can allow sequential generation of sweat collection at different times for long-term analysis. The in situ real-time measurements of the sweat loss and pH value analysis from the human subject demonstrate the practical utility of the devices in collecting, storing, and analyzing the sweat generated from sweat glands on the skin.