Flexible and stretchable physical sensors that can measure and quantify electrical signals generated by human activities are attracting a great deal of attention as they have unique characteristics, such as ultrathinness, low modulus, light weight, high flexibility, and stretchability. These flexible and stretchable physical sensors conformally attached on the surface of organs or skin can provide a new opportunity for human-activity monitoring and personal healthcare. Consequently, in recent years there has been considerable research effort devoted to the development of flexible and stretchable physical sensors to fulfill the requirements of future technology, and much progress has been achieved. Here, the most recent developments of flexible and stretchable physical sensors are described, including temperature, pressure, and strain sensors, and flexible and stretchable sensor-integrated platforms. The latest successful examples of flexible and stretchable physical sensors for the detection of temperature, pressure, and strain, as well as their novel structures, technological innovations, and challenges, are reviewed first. In the next section, recent progress regarding sensor-integrated wearable platforms is overviewed in detail. Some of the latest achievements regarding self-powered sensor-integrated wearable platform technologies are also reviewed. Further research direction and challenges are also proposed to develop a fully sensor-integrated wearable platform for monitoring human activity and personal healthcare in the near future.

Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoringand Personal Healthcare.

Nanofluids, the fluid suspensions of nanomaterials, have shown many interesting properties, and the distinctive features offer unprecedented potential for many applications. This paper summarizes the recent progress on the study of nanofluids, such as the preparation methods, the evaluation methods for the stability of nanofluids, and the ways to enhance the stability for nanofluids, the stability mechanisms of nanofluids, and presents the broad range of current and future applications in various fields including energy and mechanical and biomedical fields. At last, the paper identifies the opportunities for future research.

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A review on nanofluids: preparation, stability mechanisms, and applications

Multifunctional capability, flexible design, rugged lightweight construction and self-powered operation are desired attributes for electronics that directly interface with the human body or with advanced robotic systems. For these applications, piezoelectric materials, in forms that offer the ability to bend and stretch, are attractive for pressure/force sensors and mechanical energy harvesters. Here, we introduce a large area, flexible piezoelectric material that consists of sheets of electrospun fibres of the polymer poly[(vinylidenefluoride-co-trifluoroethylene]. The flow and mechanical conditions associated with the spinning process yield free-standing, three-dimensional architectures of aligned arrangements of such fibres, in which the polymer chains adopt strongly preferential orientations. The resulting material offers exceptional piezoelectric characteristics, to enable ultra-high sensitivity for measuring pressure, even at exceptionally small values (0.1 Pa). Quantitative analysis provides detailed insights into the pressure sensing mechanisms, and establishes engineering design rules. Potential applications range from self-powered micro-mechanical elements, to self-balancing robots and sensitive impact detectors.

/pdf/high-performance-piezoelectric-devices-based-on-aligned-4aoqiy29fh.pdf

High performance piezoelectric devices based on aligned arrays of nanofibers of poly(vinylidenefluoride-co-trifluoroethylene)

By virtue of their wide applications in personal electronic devices and industrial monitoring, pressure sensors are attractive candidates for promoting the advancement of science and technology in modern society. Flexible pressure sensors based on organic materials, which combine unique advantages of flexibility and low-cost, have emerged as a highly active field due to their promising applications in artificial intelligence systems and wearable health care devices. In this review, we focus on the fundamentals of flexible pressure sensors, and subsequently on several critical concepts for the exploration of functional materials and optimization of sensing devices toward practical applications. Perspectives on self-powered, transparent and implantable pressure sensing devices are also examined to highlight the development directions in this exciting research field.

Advances of flexible pressure sensors toward artificial intelligence and health care applications

Polymer based MEMS and microfluidic devices have the advantages of mechanical flexibility, lower fabrication cost and faster processing over silicon based ones. Also, many polymer materials are considered biocompatible and can be used in biological applications. A valuable class of polymers for microfabricated devices is piezoelectric functional polymers. In addition to the normal advantages of polymers, piezoelectric polymers can be directly used as an active material in different transduction applications. This paper gives an overview of piezoelectric polymers based on their operating principle. This includes three main categories: bulk piezoelectric polymers, piezocomposites and voided charged polymers. State-of-the-art piezopolymers of each category are presented with a focus on fabrication techniques and material properties. A comparison between the different piezoelectric polymers and common inorganic piezoelectric materials (PZT, ZnO, AlN and PMN?PT) is also provided in terms of piezoelectric properties. The use of piezopolymers in different electromechanical devices is also presented. This includes tactile sensors, energy harvesters, acoustic transducers and inertial sensors.

https://iopscience.iop.org/article/10.1088/0964-1726/23/3/033001/pdf

A review of piezoelectric polymers as functional materials for electromechanical transducers

We report a thin PVDF–TrFE (polyvinyledenedifluoride–trifluoroethylene) copolymer film pressure sensor, fabricated using standard lithography process for cost-effective batch process, film uniformity, and high resolution of polymer patterning. PVDF–TrFE copolymer, a semi-crystalline material, was spin-coated into thin films (1 μm thick) to tap the near β-phase formation. Pressure measurements demonstrated that the thin film (1 μm) show better performance compared to thick film (6 μm) with no electrical poling or mechanical stretching. Thin film devices showed higher β phase formation from Raman spectroscopy measurements, which translate into high piezoelectricity for sensing. The sensors can operate over a broad pressure range of 0–300 mmHg, with fast recovery time of 0.17 s, suitable for real time flow measurements in catheter applications.

http://nanolitesystems.org/wp-content/uploads/2015/08/012012SENSOR-ACTUAT-A-PHYS_Patterning-piezoelectri-PVDF%E2%80%93TrFE-based-pressure-sensorcatheter_1.90.pdf

Patterning piezoelectric thin film PVDF–TrFE based pressure sensor for catheter application

Development of carbon nanotubes and nanofluids based microbial fuel cell

We demonstrate the design of thin flexible pressure sensors based on piezoelectric PVDF-TrFE (polyvinyledenedifluoride-tetrafluoroethylene) co-polymer film, which can be integrated onto a catheter, where the compact inner lumen space limit the dimensions of the pressure sensors. Previously, we demonstrated that the thin-film sensors of one micrometer thickness were shown to have better performance compared to the thicker film with no additional electrical poling or mechanical stretching due to higher crystallinity. The pressure sensors can be mass producible using standard lithography process, with excellent control of film uniformity and thickness down to one micrometer. The fabricated pressure sensors were easily mountable on external surface of commercial catheters. Elaborate experiments were performed to demonstrate the applicability of PVDF sensors towards catheter based biomedical application. The resonant frequency of the PVDF sensor was found to be 6.34 MHz. The PVDF sensors can operate over a broad pressure range of 0-300 mmHg. The average sensitivity of the PVDF sensor was found to be four times higher (99 μV/mmHg) than commercial pressure sensor while the PVDF sensor (0.26 s) had fivefold shorter response time than commercial pressure sensor (1.30 s), making the PVDF sensors highly suitable for real-time pressure measurements using catheters.

Flexible thin-film PVDF-TrFE based pressure sensor for smart catheter applications.

We demonstrate that nanostructured thin films play a significant role in compact energy harvesting polyvinylidene fluoride (PVDF) nanogenerators. In this work, we introduced a surface control technique for preparing mesoporous PVDF thin films toward high piezoelectric output. We demonstrated that the morphology of the film could be controlled by varying the solvent evaporation rate. The material crystallinity was experimentally confirmed by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) measurement. The multilayer PVDF structure with porous surface significantly increased the compressibility and charge collecting area which contributes to the significant output enhancement. The piezoelectric output increased 100% with the modified mesoporous layer. The charging capacity increased 107% compared with the same thickness solid PVDF film.

Mesoporous surface control of PVDF thin films for enhanced piezoelectric energy generation

Nanostructures of polyvinyledenedifluoride-tetrafluoroethylene (PVDF-TrFE), a semicrystalline polymer with high piezoelectricity, results in significant enhancement of crystallinity and better device performance as sensors, actuators, and energy harvesters. Using electrospinning of PVDF to manufacture nanofibers, we demonstrate a new method to pattern high-density, highly aligned nanofibers. To further boost the charge transfer from such a bundle of nanofibers, we fabricated novel core-shell structures. Finally, we developed pressure sensors utilizing these fiber structures for endovascular applications. The sensors were tested in vitro under simulated physiological conditions. We observed significant improvements using core-shell electrospun fibers (4.5 times gain in signal intensity, 4000 μV/mmHg sensitivity) over PVDF nanofibers (280 μV/mmHg). The preliminary results showed that core-shell fiber-based devices exhibit nearly 40-fold higher sensitivity, compared to the thin-film structures demonstrated earlier.

Tushar Sharma

Papers

Patterning piezoelectric thin film PVDF–TrFE based pressure sensor for catheter application

Development of carbon nanotubes and nanofluids based microbial fuel cell

Flexible thin-film PVDF-TrFE based pressure sensor for smart catheter applications.

Mesoporous surface control of PVDF thin films for enhanced piezoelectric energy generation

Aligned PVDF-TrFE nanofibers with high-density PVDF nanofibers and PVDF core–shell structures for endovascular pressure sensing.