Flexible Strain Sensors Based on an Interlayer Synergistic Effect of Nanomaterials for Continuous and Noninvasive Blood Pressure Monitoring.

The continuous, noninvasive monitoring of human blood pressure (BP) through the accurate detection of pulse waves has extremely stringent requirements on the sensitivity and stability of flexible strain sensors. In this study, a new ultrasensitive flexible strain sensor based on the interlayer synergistic effect was fabricated through drop-casting and drying silver nanowires and graphene films on polydimethylsiloxane substrates and was further successfully applied for continuous monitoring of BP. This strain sensor exhibited ultrahigh sensitivity with a maximum gauge factor of 34357.2 (∼700% sensitivity enhancement over other major sensors), satisfactory response time (∼85 ms), wide strange range (12%), and excellent stability. An interlayer fracture mechanism was proposed to elucidate the working principle of the strain sensor. The real-time BP values can be obtained by analyzing the relationship between the BP and the pulse transit time. To verify our strain sensor for real-time BP monitoring, our strain sensor was compared with a conventional electrocardiogram-photoplethysmograph method and a commercial cuff-based device and showed similar measurement results to BP values from both methods, with only minor differences of 0.693, 0.073, and 0.566 mmHg in the systolic BP, diastolic BP, and mean arterial pressure, respectively. Furthermore, the reliability of the strain sensors was validated by testing 20 human subjects for more than 50 min. This ultrasensitive strain sensor provides a new pathway for continuous and noninvasive BP monitoring.

Bubbles-Induced Porous Structure-Based Flexible Piezoresistive Sensors for Speech Recognition.

Flexible piezoresistive sensors with a porous structure that are used in the field of speech recognition are seldom characterized by both high sensitivity and ease of preparation. In this study, a piezoresistive sensor with a porous structure that is both highly sensitive and can be prepared by using a simple method is proposed for speech recognition. The preparation process utilizes the interaction of bubbles generated by ethanol evaporation and active agents with polydimethylsiloxane to produce a porous flexible substrate. This preparation process requires neither templates nor harsh experimental conditions such as a low temperature and a low pressure. Furthermore, the prepared piezoresistive sensor has excellent properties, such as a high sensitivity (27.6 kPa-1), a satisfactory response time (800 µs), and a good stability (10,000 cycles). When used for speech recognition, more than 1500 vocalizations and silent speech signals obtained from subjects saying numbers from "0" to "9" were collected by the sensor for training a convolutional neural network model. The average accuracy of the recognition reached 94.8%. The simple preparation process and the excellent performance of the prepared flexible piezoresistive sensor endow it with a wide application prospect in the field of speech recognition.

Enhanced Hemocompatibility of a Direct Chemical Vapor Deposition-Derived Graphene Film.

A wide range of biomedical devices are being used to treat cardiovascular diseases, and thus they routinely come into contact with blood. Insufficient hemocompatibility has been found to impair the functionality and safety of these devices through the activation of blood coagulation and the immune system. Numerous attempts have been made to develop surface modification approaches of the cardiovascular devices to improve their hemocompatibility. However, there are still no ideal "blood-friendly" coating materials, which possess the desired hemocompatibility, tissue compatibility, and mechanical properties. As a novel multifunctional material, graphene has been proposed for a wide range of biomedical applications. The chemical inertness, atomic smoothness, and high durability make graphene an ideal candidate as a surface coating material for implantable devices. Here, we evaluated the hemocompatibility of a graphene film prepared on quartz glasses (Gra-glasses) from a direct chemical vapor deposition process. We found that the graphene coating, which is free of transfer-mediating polymer contamination, significantly suppressed platelet adhesion and activation, prolonged coagulation time, and reduced ex vivo thrombosis formation. We attribute the excellent antithrombogenic properties of the Gra-glasses to the low surface roughness, low surface energy (especially the low polar component of the surface energy), and the negative surface charge of the graphene film. Given these excellent hemocompatible properties, along with its chemical inertness, high durability, and molecular impermeability, a graphene film holds great promise as an antithrombogenic coating for next-generation cardiovascular devices.

Soft transparent graphene contact lens electrodes for conformal full-cornea recording of electroretinogram.

Visual electrophysiology measurements are important for ophthalmic diagnostic testing. Electrodes with combined optical transparency and softness are highly desirable, and sometimes indispensable for many ocular electrophysiology measurements. Here we report the fabrication of soft graphene contact lens electrodes (GRACEs) with broad-spectrum optical transparency, and their application in conformal, full-cornea recording of electroretinography (ERG) from cynomolgus monkeys. The GRACEs give higher signal amplitude than conventional ERG electrodes in recordings of various full-field ERG responses. High-quality topographic mapping of multifocal ERG under simultaneous fundus monitoring is realized. A conformal and tight interface between the GRACEs and cornea is revealed. Neither corneal irritation nor abnormal behavior of the animals is observed after ERG measurements with GRACEs. Furthermore, spatially resolved ERG recordings on rabbits with graphene multi-electrode array reveal a stronger signal at the central cornea than the periphery. These results demonstrate the unique capabilities of the graphene-based electrodes for in vivo visual electrophysiology studies.