Page 90 - Read Online

Page 2 of 30 Kim et al. Soft Sci 2023;3:16 https://dx.doi.org/10.20517/ss.2023.07

Keywords: Flexible, stretchable, three-dimensional (3D), soft electronics

INTRODUCTION
The importance of flexible/stretchable devices in next-generation soft electronics such as emerging displays,
[1-4]
smart sensors, wearable devices, and wireless communication devices is growing . Accordingly, several
flexible/stretchable device fabrication technologies have been reported, and these fabrication methodologies
produce soft electronic devices that can not only be bent, twisted, or stretched but allow them to be freely
transformed into a desired shape . In addition to the diversity of device design, various advantages,
[5-8]
[12]
[11]
[10]
including ultra-lightweight , unbreakability , overcoming space constraints , and low cost can be
[9]
realized, and in particular, mechanical stability against deformation caused by human motion or
displacement of these devices can be achieved without deterioration in performance [13-15] . Based on the
aforementioned superior characteristics, high-functional flexible/stretchable devices such as electronic
skin , smart fabric , and wearable energy harvesters are being developed in various fields, including
[17]
[18]
[16]
microelectromechanical systems (MEMS) [19-27] , optoelectronics [7,28-38] , actuators [39-46] and micro-fluidic
systems [47-54] .
However, most of the reported flexible/stretchable devices operate with a two-dimensional (2D) planar or
stacked device structure, and there are several challenges in the implementation of novel functionality and
application to various fields due to the dimensional constraints of the structure. For example, devices that
can detect external fields only in small spatial resolutions with one or two dimensions may have difficulty in
detecting vector fields in three-dimensional (3D) space, and sensing an in vivo cell with a complex
morphology requires a device with a 3D contact surface area. In this regard, several studies have shown that
forming or embedding 3D structural components in flexible/stretchable devices can overcome the
dimensional limitations of device operations as well as allow for the implementation of fundamentally new
properties and functionalities that are difficult to realize with 2D structures in various application fields
[Figure 1]. For example, broadband electromagnetic radiation can be achieved by constructing a
geometrically reconfigurable 3D mesostructure on a soft substrate , allowing the narrowband resonant
[55]
optical reconfigurations through the transformable optical nano-kirigami , and implementing
[56]
frequency-selective surfaces with stable electromagnetic wave transmission performance by buckling a
periodic array of Jerusalem 2D precursors .
[57]

In this review, we first briefly discuss representative manufacturing methods of 3D structures, and introduce
the implementation of MEMS capable of unique functions or multimodal operations by rearranging 3D
structures such as antennas and sensors in the second section. The third section describes light-emitting
diodes (LEDs) and photodetectors, which can achieve operational stability against deformation or overcome
spatial constraints by using 3D architectures. In the next section (fourth section), we introduce energy
devices such as energy harvesters and batteries that can achieve high performance with high surface areas
and electrochemical stability resulting from the formation of 3D structures. Then, the fifth section discusses
a methodology for fabricating 3D structures that exquisitely mimic biological tissues and tiny biomedical
devices that can accurately monitor the physiological properties of 3D biological systems. The sixth section
covers several sensors that detect various factors such as temperature, strain, and magnetic field and achieve
spatial expansion of the sensing area through 3D geometry. The seventh section introduces 3D actuators
that can perform unique movements or behaviors within a 3D space in response to multiple external
stimuli, and metamaterials that exhibit unique physical behaviors through elaborately designed geometries
in 3D are discussed in the eighth section. In the ninth section, we cover 3D flexible/stretchable microfluidic
systems obtained by introducing soft materials into the fabrication process of 3D complex networks. Finally,