Advanced Wearable Microfluidic Sensors for Healthcare Monitoring

Recent Progress in Intelligent Wearable Sensors for Health Monitoring and Wound Healing Based on Biofluids

The intelligent wearable sensors promote the transformation of the health care from a traditional hospital-centered model to a personal portable device-centered model. There is an urgent need of real-time, multi-functional, and personalized monitoring of various biochemical target substances and signals based on the intelligent wearable sensors for health monitoring, especially wound healing. Under this background, this review article first reviews the outstanding progress in the development of intelligent, wearable sensors designed for continuous, real-time analysis, and monitoring of sweat, blood, interstitial fluid, tears, wound fluid, etc. Second, this paper reports the advanced status of intelligent wound monitoring sensors designed for wound diagnosis and treatment. The paper highlights some smart sensors to monitor target analytes in various wounds. Finally, this paper makes conservative recommendations regarding future development of intelligent wearable sensors.

State of Sweat: Emerging Wearable Systems for Real-Time, Noninvasive Sweat Sensing and Analytics

Skin-interfaced wearable systems with integrated colorimetric assays, microfluidic channels, and electrochemical sensors offer powerful capabilities for noninvasive, real-time sweat analysis. This Perspective details recent progress in the development and translation of novel wearable sensors for personalized assessment of sweat dynamics and biomarkers, with precise sampling and real-time analysis. Sensor accuracy, system ruggedness, and large-scale deployment in remote environments represent key opportunity areas, enabling broad deployment in the context of field studies, clinical trials, and recent commercialization. On-body measurements in these contexts show good agreement compared to conventional laboratory-based sweat analysis approaches. These device demonstrations highlight the utility of biochemical sensing platforms for personalized assessment of performance, wellness, and health across a broad range of applications.

Large-Area Biocompatible Random Laser for Wearable Applications

Recently, wearable sensor technology has drawn attention to many health-related appliances due to its varied existing optical, electrical, and mechanical applications. Similarly, we have designed a simple and cheap lift-off fabrication technique for the realization of large-area biocompatible random lasers to customize wearable sensors. A large-area random microcavity comprises a matrix element polymethyl methacrylate (PMMA) in which rhodamine B (RhB, which acts as a gain medium) and gold nanorods (Au NRs, which offer plasmonic feedback) are incorporated via a spin-coating technique. In regards to the respective random lasing device residing on a heterogenous film (area > 100 cm2), upon optical excitation, coherent random lasing with a narrow linewidth (~0.4 nm) at a low threshold (~23 μJ/cm2 per pulse) was successfully attained. Here, we maneuvered the mechanical flexibility of the device to modify the spacing between the feedback agents (Au NRs), which tuned the average wavelength from 612.6 to 624 nm under bending while being a recoverable process. Moreover, the flexible film can potentially be used on human skin such as the finger to serve as a motion and relative-humidity sensor. This work demonstrates a designable and simple method to fabricate a large-area biocompatible random laser for wearable sensing.

Inorganic Perovskite Quantum Dot-Based Strain Sensors for Data Storage and In-Sensor Computing

Although remarkable improvement has been achieved in stretchable strain sensors, challenges still exist in aspects including intelligent sensing, simultaneous data processing, and scalable fabrication techniques. In this work, a strain-sensitive device is presented by fabricating a CsPbBr₃ quantum dots (QDs) floating-gate field-effect transistor (FET) sensing array on thin polyimide (PI) films. The FET exhibits an excellent on/off ratio (>10³) and a large memory window (>2 V). With the introduction of CsPbBr₃ QDs as the trapping layer, an additional UV response is obtained because of the photogenerated charge carriers that significantly enhance the source-drain current (I_DS) of the device. At each electrical state, the I_DS varies with the strains and the sensing range is from compressive +12.5% to tensile -10.8%. Excellent data retainability and mechanical durability demonstrate the high quality and reliability of the fabricated sensors. Furthermore, synapse functions including long-term potentiation (LTP), long-term depression (LTD), etc., are emulated at the device level. Linearity factor changes of LTP/LTD in different sensing scenarios demonstrate the reliability of the device and further confirm the different sensing mechanisms with/without UV illumination. Our results exhibit the potential of transistor-based devices for multifunctional intelligent sensing.

Flexible/Bendable Acoustofluidics Based on Thin-Film Surface Acoustic Waves on Thin Aluminum Sheets

In this paper, we explore the acoustofluidic performance of zinc oxide (ZnO) thin-film surface acoustic wave (SAW) devices fabricated on flexible and bendable thin aluminum (Al) foils/sheets with thicknesses from 50 to 1500 μm. Directional transport of fluids along these flexible/bendable surfaces offers potential applications for the next generation of microfluidic systems, wearable biosensors and soft robotic control. Theoretical calculations indicate that bending under strain levels up to 3000 με causes a small frequency shift and amplitude change (<0.3%) without degrading the acoustofluidic performance. Through systematic investigation of the effects of the Al sheet thickness on the microfluidic actuation performance for the bent devices, we identify the optimum thickness range to both maintain efficient microfluidic actuation and enable significant deformation of the substrate, providing a guide to design such devices. Finally, we demonstrate efficient liquid transportation across a wide range of substrate geometries including inclined, curved, vertical, inverted, and lateral positioned surfaces using a 200 μm thick Al sheet SAW device.

Recent progress, challenges, and opportunities for wearable biochemical sensors for sweat analysis

Sweat is a promising, yet relatively unexplored biofluid containing biochemical information that offers broad insights into the underlying dynamic metabolic activity of the human body. The rich composition of electrolytes, metabolites, hormones, proteins, nucleic acids, micronutrients, and exogenous agents found in sweat dynamically vary in response to the state of health, stress, and diet. Emerging classes of skin-interfaced wearable sensors offer powerful capabilities for the real-time, continuous analysis of sweat produced by the eccrine glands in a manner suitable for use in athletics, consumer wellness, military, and healthcare industries. This perspective examines the rapid and continuous progress of wearable sweat sensors through the most advanced embodiments that address the fundamental challenges currently restricting widespread deployment. It concludes with a discussion of efforts to expand the overall utility of wearable sweat sensors and opportunities for commercialization, in which advances in biochemical sensor technologies will be critically important.

A wearable patch for continuous analysis of thermoregulatory sweat at rest

The body naturally and continuously secretes sweat for thermoregulation during sedentary and routine activities at rates that can reflect underlying health conditions, including nerve damage, autonomic and metabolic disorders, and chronic stress. However, low secretion rates and evaporation pose challenges for collecting resting thermoregulatory sweat for non-invasive analysis of body physiology. Here we present wearable patches for continuous sweat monitoring at rest, using microfluidics to combat evaporation and enable selective monitoring of secretion rate. We integrate hydrophilic fillers for rapid sweat uptake into the sensing channel, reducing required sweat accumulation time towards real-time measurement. Along with sweat rate sensors, we integrate electrochemical sensors for pH, Cl-, and levodopa monitoring. We demonstrate patch functionality for dynamic sweat analysis related to routine activities, stress events, hypoglycemia-induced sweating, and Parkinson's disease. By enabling sweat analysis compatible with sedentary, routine, and daily activities, these patches enable continuous, autonomous monitoring of body physiology at rest.

Microfluidics-Based Sensing of Biospecies

Over the past decades, microfluidic devices based on many advanced techniques have aroused widespread attention in the fields of chemical, biological, and analytical applications. Integration of microdevices with a variety of chip designs will facilitate promising functionality. Notably, the combination of microfluidics with functional nanomaterials may provide creative ideas to achieve rapid and sensitive detection of various biospecies. In this review, focused on the microfluids and microdevices in terms of their fabrication, integration, and functions, we summarize the up-to-date developments in microfluidics-based analysis of biospecies, where biomarkers, small molecules, cells, and pathogens as representative biospecies have been explored in-depth. The promising applications of microfluidic biosensors including clinical diagnosis, food safety control, and environmental monitoring are also discussed. This review aims to highlight the importance of microfluidics-based biosensors in achieving high throughput, highly sensitive, and low-cost analysis and to promote microfluidics toward a wider range of applications.

Coupling of Silk Fibroin Nanofibrils Enzymatic Membrane with Ultra-Thin PtNPs/Graphene Film to Acquire Long and Stable On-Skin Sweat Glucose and Lactate Sensing

The applications of enzymatic biosensors are largely limited by their relatively poor stability and short lifespan. Herein, a bio-active porous enzymatic nanofiber (PEN) membrane composed of silk fibroin nanofibrils (SFNFs) and enzymes is developed to effectively retain the enzymes in the 3D space. The 3D functional scaffolds formed by SFNFs can immobilize enzymes and provide a large surface area for molecular/ion diffusion and biochemical reactions. The PEN membrane is subsequently attached to an ultra-thin PtNPs/graphene (Pt-G) nanocomposite film to facilitate the electron transport between the enzymes and electrodes, permitting highly effective glucose and lactate sensing with long and stable performance. The as-assembled glucose and lactate sensors demonstrate high sensitivity, good cyclic reproducibility, and in particular long-term stability of up to 25 and 23.6 h, respectively. These glucose sensors have a working life that is ≈1.25-times longer than that of the best available sensors reported so far. Moreover, a wearable platform based on the sensors is developed for real-time analysis of sweat during outdoor exercising to transmit signals to a mobile handset. The high sensitivity, comfort and long-term stability of the device can benefit for long-term in-line surveillance of physiological conditions.

Recent Impact of Microfluidics on Skin Models for Perspiration Simulation

Skin models offer an in vitro alternative to human trials without their high costs, variability, and ethical issues. Perspiration models, in particular, have gained relevance lately due to the rise of sweat analysis and wearable technology. The predominant approach to replicate the key features of perspiration (sweat gland dimensions, sweat rates, and skin surface characteristics) is to use laser-machined membranes. Although they work effectively, they present some limitations at the time of replicating sweat gland dimensions. Alternative strategies in terms of fabrication and materials have also showed similar challenges. Additional research is necessary to implement a standardized, simple, and accurate model representing sweating for wearable sensors testing.

MXenes and Their Applications in Wearable Sensors

MXenes, a kind of two-dimensional material of early transition metal carbides and carbonitrides, have emerged as a unique class of layered-structured metallic materials with attractive features, as good conductivity comparable to metals, enhanced ionic conductivity, hydrophilic property derived from their hydroxyl or oxygen-terminated surfaces, and mechanical flexibility. With tunable etching methods, the morphology of MXenes can be effectively controlled to form nanoparticles, single layer, or multi-layer nanosheets, which exhibit large specific surface areas and is favorable for enhancing the sensing performance of MXenes based sensors. Moreover, MXenes are available to form composites with other materials facilely. With structure design, MXenes or its composite show enhanced mechanical flexibility and stretchability, which enabled its wide application in the fields of wearable sensors, energy storage, and electromagnetic shielding. In this review, recent progress in MXenes is summarized, focusing on its application in wearable sensors including pressure/strain sensing, biochemical sensing, temperature, and gas sensing. Furthermore, the main challenges and future research are also discussed.