Flexible and Stretchable Sensors

Dear Colleagues,

Stretchable and flexible electronics represent an emerging class of devices that can be compressed, twisted, and conform to curved and sometimes complicated shapes. The mechanical and electrical compliances of the technology open up applications for healthcare, energy, and entertainment purposes.

Stretchable and flexible sensors can be subdivided in two main classes of devices, namely: (a) devices that operate by reacting to mechanical stimuli produced by the strain applied by or to the substrate they are fabricated onto, and translate these stimuli into measurable signals, and (b) devices that operate by sensing physical, chemical, or optical stimuli, and are largely unaffected by the deformation induced in the substrate or by the strain applied to mechanically compliant substrates.

Indeed, the role played by the substrate and/or encapsulation layers used to package stretchable and flexible sensor devices is one of the fundamental research aspects in sensor devices, when they are also required to provide accurate, selective, and consistent information about the parameter being transduced, while being subject to a set of mechanical stimuli.

Conventional fabrication methods adopted for electronics- and semiconductor-based devices and integrated circuits have been adapted to accommodate mechanical flexibility with a specific bending radii, when it comes to thinned silicon wafer device manufacturing (thickness <50 µm) and their integration onto flexible printed circuit boards, as well as thin or ultra-thin polymeric substrates, such as poly-ethylene naphthalate (PEN), polyimmide (PI), polyester terephthalate (PET), parylene, and others.

The emergence of stretchable substrates made of elastomeric materials, such, as but not limited to, thermoplastic polyurethanes (TPU) or poly-dimethyl siloxanes (PDMS), to name a few, has led to truly body conformal electronic systems. These systems allow for sensing personalised body parameters directly from skin, as in the case of smart patch-like on-body sensor devices, or from organs, as in the case of implantable sensing devices.

Indeed, studies on the mechanical compliance of human skin revealed that devices that interact with human skin must undergo strains of up to 30%, without constraining natural motions. Reaching this level of stretchability or higher also ensures that the elastic behaviour of human skin can be mimicked, so that a wearable device in the form of an electronic tattoo or smart patch can perform reliably when the substrate is subjected to the same level of strain.

Implantable sensor devices designed to interact directly with organs, such as the heart muscle, spinal cord, or brain, are often required to show the same mechanical compliance as the organ itself. This means that the actual electronic interfaces need to be fabricated with biocompatible materials having the same values of the Young modulus of the target organ.

This, in practice, justifies the scientific and industrial interests in looking at soft electronic devices that can be stretched up to 100% or more of their initial dimensions within their elastic range, so that any changes in terms of their physical (i.e., mechanical, electrical, and optical) characteristics are reversible under multiple cycles. 

The technological hurdles that hinder the commercialization of stretchable electronics include the use of elastomeric materials and the fabrication processes of these devices. Moreover, aspects of the thermal, mechanical, and chemical stability of flexible and stretchable sensor devices, along with their connectivity to existing infrastructures for data collection and processing, play a key role in applications of flexible and stretchable sensor use cases.

This Special Issue of Sensors aims to publish original high-quality research contributions and comprehensive review articles to inform the scientific and technology community, and to achieve a high impact in the field of flexible and stretchable sensors.

The contributed manuscripts will be peer-reviewed so as to select the papers for publication in this Special Issue.

Dr. Luigi G. Occhipinti
Guest Editor

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