Physiological temperature varies temporally and spatially Accurate and real-time detection of localized temperature changes in biological tissues regardless of large deformation is crucial to understand thermal principle of homeostasis, to assess sophisticated health conditions, and further to offer possibilities of building a smart healthcare and medical system Additionally, continuous temperature mapping in flexible and stretchable formats opens up many other potential areas, such as artificially electronic skins and reflection of emotional changes This review exploits a comprehensive investigation onto recent advances in flexible temperature sensors, stretchable sensor networks, and platforms constructed in soft and compliant formats for wearable physiological monitoring The most recent examples of flexible temperature sensors are first discussed regarding to their materials, structures, electrical and mechanical properties; temperature sensing network technologies in new materials and structural designs are then presented based on platforms comprised of multiple physical sensors and stretchable electronics Finally, wearable applications of the sensing network are described, such as detection of human activities, monitoring of health conditions, and emotion-related bodily sensations Conclusions are made with emphasis on critical issues and new trends in the field of wearable temperature sensor network technologies

Review of Flexible Temperature Sensing Networks for Wearable Physiological Monitoring

Flexible electronics are a very promising technology for various applications. Several types of flexible devices have been developed, but there has been limited research on flexible electromechanical systems (MEMS). Surface acoustic wave (SAW) devices are not only an essential electronic device, but also are the building blocks for sensors and MEMS. Here we report a method of making flexible SAW devices using ZnO nanocrystals deposited on a cheap and bendable plastic film. The flexible SAW devices exhibit two wave modes - the Rayleigh and Lamb waves with resonant frequencies of 198.1 MHz and 447.0 MHz respectively, and signal amplitudes of 18 dB. The flexible devices have a high temperature coefficient of frequency, and are thus useful as sensitive temperature sensors. Moreover, strong acoustic streaming with a velocity of 3.4 cm/s and particle concentration using the SAW have been achieved, demonstrating the great potential for applications in electronics and MEMS.

/pdf/flexible-surface-acoustic-wave-resonators-built-on-111atwkn57.pdf

Flexible surface acoustic wave resonators built on disposable plastic film for electronics and lab-on-a-chip applications

A human stress monitoring patch integrates three sensors of skin temperature, skin conductance, and pulsewave in the size of stamp (25 mm × 15 mm × 72 μm) in order to enhance wearing comfort with small skin contact area and high flexibility. The skin contact area is minimized through the invention of an integrated multi-layer structure and the associated microfabrication process; thus being reduced to 1/125 of that of the conventional single-layer multiple sensors. The patch flexibility is increased mainly by the development of flexible pulsewave sensor, made of a flexible piezoelectric membrane supported by a perforated polyimide membrane. In the human physiological range, the fabricated stress patch measures skin temperature with the sensitivity of 0.31 Ω/°C, skin conductance with the sensitivity of 0.28 μV/0.02 μS, and pulse wave with the response time of 70 msec. The skin-attachable stress patch, capable to detect multimodal bio-signals, shows potential for application to wearable emotion monitoring.

/pdf/a-flexible-and-wearable-human-stress-monitoring-patch-1loldhb05o.pdf

A Flexible and Wearable Human Stress Monitoring Patch.

The design, fabrication, and applications of flexible biosensing devices

Flexibleelectronicsareaverypromisingtechnologyforvariousapplications.Severaltypesofflexibledeviceshave been developed, but there has been limited research on flexible electromechanical systems (MEMS).Surface acoustic wave (SAW) devices are not only an essential electronic device, but also are the buildingblocks for sensors and MEMS. Here we report a method of making flexible SAW devices using ZnOnanocrystals deposited on a cheap and bendable plastic film. The flexible SAW devices exhibit two wavemodes-theRayleighandLambwaveswithresonantfrequenciesof198.1 MHzand447.0 MHzrespectively,and signal amplitudes of 18 dB. The flexible devices have a high temperature coefficient of frequency, andare thus useful as sensitive temperature sensors. Moreover, strong acoustic streaming with a velocity of3.4 cm/s and particle concentration using the SAW have been achieved, demonstrating the great potentialfor applications in electronics and MEMS.

Flexible surface acoustic wave resonators built on disposable plastic film for electronics and lab-on-a-chip

A novel fabrication process of MEMS devices on polyimide flexible substrates

A new and simple method is demonstrated to achieve size controllable nanochannels by using nanoimprint lithography and low-pressure thermal bonding methods. The flexible free-standing SU-8 allows the bonding of the nanochannels' pattern features at such a low pressure primarily because of ‘sequential’ bonding made possible by the bonding layer flexibility and the conformal contact made between the bonding layer and the structure layer in a large area. The simple geometrical argument shows that the height of the enclosed nanochannel can be determined by the depth of the cross-linked SU-8 trenches as well as by the initial thickness of the thin unexposed SU-8 layer. The height of the nanochannels can also be controlled by adjusting the ratio of the ridge width to the trench width on the SU-8 trenches. These simple relationships can serve as a guideline in choosing the parameters for SU-8 nanochannel fabrication.

Fabrication of size controllable SU-8 nanochannels using nanoimprint lithography and low-pressure thermal bonding methods

The invention provides a flexible focusing MEMS ultrasonic generator and a preparation method thereof. The flexible focusing MEMS ultrasonic generator comprises ultrasonic transducer arrays prepared through the MEMS technology. The ultrasonic transducer arrays are distributed on the surface of a polyimide matrix, and adjacent ultrasonic transducers are connected through electrodes. The ultrasonic transducers are concavely embedded in the polyimide matrix in the shape of a bowl. According to the flexible focusing MEMS ultrasonic generator, the ultrasonic transducers prepared through the MEMS technology are distributed on the flexible polyimide matrix in an array mode and can be bent randomly, and therefore when used, the ultrasonic transducers can be totally attached to the curved surfaces of samples to be detected; and the resonant frequency of the ultrasonic transducers is high, and a higher resolution ratio can be obtained in the fields of ultrasonic imaging, ultrasonic flaw detection and the like. Operation is convenient, and detection precision is improved.

Flexible focusing MEMS ultrasonic generator and preparation method thereof

Polymer-based nano/microfluidic devices are becoming increasingly important for biological applications and fluidic control. Reported is a new etching method for the fabrication of nano/microfluidic channels based on SU-8 using AZ1350 as a sacrificial resist. In contrast to all the previous fabrication routes, this etching method is suitable for fabricating the channel with dimensions ranging from micrometres to nanometres. By this route, the most critical step is to prevent the two mixed photoresists and this problem is solved by sputtering a thin layer of SiO 2 . Furthermore, this is a size-controlled nanochannel fabrication method because the size of the channel is only dependent on the sacrificial layer structure whose size could be controlled by the oxygen plasma process. In addition, the developing etching speed is measured and some methods to accelerate the developing etching rate are proposed. This novel process is simple and inexpensive for mass nano/microchannel manufacturing, which could have wide applications in biomedical and fluidic transport systems.

Xiaojun Li

Papers

A novel fabrication process of MEMS devices on polyimide flexible substrates

Fabrication of size controllable SU-8 nanochannels using nanoimprint lithography and low-pressure thermal bonding methods

Flexible focusing MEMS ultrasonic generator and preparation method thereof

Nano/microchannel fabrication based on SU-8 using sacrificial resist etching method

Fabrication of size-controllable nanofluidic channels using angled physical vapor deposition