With recent advancements in the field of wearable devices based on Internet of Things (IoT), the concept of flexible and stretchable electronic systems has become increasingly significant. Although decades of dimensional scaling have led to the miniaturization of the traditional complementary metal oxide semiconductor- (CMOS-) based electronic components, they still remain mechanically rigid and brittle. These rigid devices can be mounted on flexible PCB substrates to obtain a certain degree of flexibility; however, this technique cannot lead to truly body conformal electronic systems. Thus, research teams around the world have been looking at various ways of obtaining completely flexible electronic components at the device level itself. These efforts include various processes to thin down silicon chips to make them flexible or development and use of flexible and stretchable substrate materials to fabricate electronic devices.
The thinning down process for traditional silicon-based electronic chips can be performed before or after the complete transistor fabrication process [1]. These approaches are referred to as “device first” approach or “device last” approach. In case of the device last approach, thin films of single crystal silicon are transfer printed to a flexible substrate and processed further to fabricate CMOS circuitry [2, 3]. However, in this approach, many high-temperature steps have to be avoided due to the limited thermal stability of the flexible substrate, leading to a suboptimal circuit. In case of the device first approach, the circuits are made on the silicon substrate using state-of-the-art CMOS processes as usual. After completion of the process, some additional process steps are employed to thin down the silicon chip to make it flexible. These include the controlled spalling process [4, 5], the trench-protect-etch-release (TPER) process [6, 7], and the soft-etch-back (SEB) process [8, 9].
Complementing the efforts to fabricate conformal silicon chips, efforts have been made to make other components of an electronic system flexible and stretchable. These include the use of novel processes to fabricate flexible and stretchable sensor systems [10, 11], actuator systems [12, 13], communication systems [14, 15], memory modules [16, 17], and batteries [18–20]. Having flexible and stretchable versions of these systems is particularly important because body conformal end gadgets generally have applications in the IoT segment where sensing, actuation, storage, and communication are key processes. The impact of successful fabrication of conformal systems ranges from advanced healthcare and wearable diagnostics to military and aerospace applications. Indeed, several key challenges such as material selection, scalable fabrication, reliability, and cost need to be solved to ascertain ubiquitous adoption of such systems.
The call for papers for this special issue focused on publishing high-quality, high-impact, and original research articles and review articles focusing on flexible and stretchable sensor devices, sensor drive circuitry, and overall systems. In response to the call, papers were submitted from research teams across the world. These papers were reviewed for novelty and quality of research. Because of the complexity of real-life deployment of flexible and stretchable systems, special attention was given to the papers including experimental data and field data. After a rigorous peer-review process, 4 papers were accepted for publication in this special issue.
The paper by G. Prats-Boluda et al. presents a wearable textile ECG sensor electrode. Two sizes of textile concentric ring electrodes (TCREs) are fabricated and tested for monitoring cardiac activity. The electrodes are fabricated using multilayer thick film serigraphic technology. The devices are found to be low-cost and easy to implement, while having the advantages of textiles for being lightweight, stretchable, adjustable, washable, and long-lasting.
The paper by H. Nakamoto et al. presents a wearable lumbar-motion monitoring device using stretchable strain sensors. The strain sensors are fabricated using urethane elastomer and carbon nanotube membranes. Six of these strain sensors form a parallel-sensor mechanism that measures rotation angles of lumbar motion in three axes. The parallel-sensor mechanism calculates rotation angles from the lengths of the strain sensors iteratively.
The paper by M. Li et al. presents an underwater wireless sensor network (UWSN) routing algorithm based on simplified harmony search (SHS).
The paper by Y. C. Manie et al. presents a Fiber Bragg Grating (FBG) sensor using intensity and wavelength division multiplexing (IWDM), Raman amplifier, and extreme learning machine (ELM).
Conflicts of Interest
The editors declare that they have no conflicts of interest regarding the publication of this special issue.
Aftab M. Hussain
Mohamed T. Ghoneim
Jhonathan P. Rojas
Hossain Fahad