Thiol-ene UV-curable sponge electrolyte for low-voltage color changing wearable tactile device
Graphical abstract
Introduction
To date, wearable electronic devices with high performance and elastic mechanical responses have garnered significant interest over the last decades. In this regard, considerable developments in wearable technology such as flexibility and sensing are being achieved to enhance the functionalities of devices, including smart watches, medical devices, smart glasses, wrist bands, eye wear, and smart clothing [[1], [2], [3]].
Synthetic design of intrinsically flexible materials and introducing flexibility with a rational architectural design are the most straightforward approaches for providing wearable electronic device flexibility and durability. Thus, it is critical to provide appropriate flexible material depending on the different wearable applications [4].
Moreover, low-power consumption in the electronic device is another critical criterion for ensuring the excellent reliability of futuristic wearable devices [5].
Among the technologies for wearable devices, tactile sensing ability, which is responsiveness to externally applied pressure, is fundamental. Various pressure sensors are classified by the sensing mechanism, such as resistive [6], capacitive [7], piezoelectric [8], and optical type [9]. The tactile sensor, which monitors optical responses other than conventional electrical changes, is a relatively new approach [10].
Recently, an interesting nature-inspired optical type tactile sensor has been reported [11]. The chameleon-like stretchable electronic skin sensor demonstrates interactive color changes and tactile-sensing properties by stacking a thin electrochromic (EC) layer onto the patterned tactile sensing moiety.
Electrochromism is defined as a reversible change in the electronic structure and optical properties of EC materials caused by an applied current or potential [12]. Recently, flexible electrochromic devices (ECDs) have gained significant interest owing to their ability to overcome the conventional rigid structure and promising applications such as flexible display, electronic papers, curved windows, and military camouflage [13].
Because an ECD can present low-power operation by adopting appropriate materials, it is anticipated that a new design of the ECD can be advantageous in terms of energy-saving and easy optical signal readouts if the ECD displays a color change in response to an externally applied pressure [14].
It is inevitable to replace all the rigid components with flexible components to obtain a deformable ECD. Among the flexible ECDs, stretchable [15] and foldable [16] ECDs are currently under intensive investigation.
However, there are few reports on compressible ECDs because of the difficulty in fabricating the electrolyte layer that withstands compressible stress.
In this study, we fabricated a thin-film sponge electrolyte by combining UV crosslinking of thiol-ene polymer and sugar leaching process. This study proposes a new route for fabricating sponge with elastic recovery using thiol-ene chemistry that utilizes photopolymerization of acrylate monomers instead of the conventionally used heat-cured polydimethylsiloxane [17,18]. The ECD equipped with a sponge electrolyte has unique tactile sensing characteristics of a simple structure with easy optical signal readouts by the color change and low operating voltage by introducing a new idea of a compressible electrolyte layer in the existing ECD-based structure.
The flexible and compressible properties of the sponge electrolyte were investigated with the composition and curing time. Furthermore, the possibility of a low-voltage color-changing wearable tactile device was investigated using the compressible ECD equipped with a sponge electrolyte.
Section snippets
Materials
Pentaerythritol tetrakis (3-mercaptopropionate) (>95%, thiol monomer, PETMP) and poly(ethylene glycol) dimethacrylate (Mn = 750, acrylate monomer, PEGDMA) for thiol-ene sponge were obtained from Sigma Aldrich. General-purpose powdered sugar (CJ Cheiljedang, Korea) was used as received (Fig. S1). 4-Vinylbenzyl chloride (VBC, 90%), 4,4-bipyridyl (98%), propylene carbonate (PC, 99.7%), acetonitrile (99.8%), sodium tetrafluoroborate (98%), and ferrocene (98%) were obtained from Sigma-Aldrich.
Fabrication and characterization of thiol-ene sponge
The EC electrolyte layer is a core component that exhibits compressibility. Therefore, we designed a matrix with a porous structure that could be deformed and restored while supporting electrolytes to fabricate a compressible ECD.
In particular, we especially focused on a sponge that can endure high compressive stress and easily recover from deformation as a compressible EC matrix among 3D porous structures. Because good wettability, high porosity, and high flexibility are advantageous for
Conclusions
We demonstrated a compressible sponge-electrolyte layer for compressible ECD applications. The sponge matrix was fabricated using a facile and eco-friendly method by leaching sugar as a pore-creating agent from a UV-cured thiol-ene polymer matrix. The fabrication is advantageous for controlling the size of pores and thickness of the electrolyte. The sponge was compatible with the general ECD and exhibited low-voltage operation properties compared with film-type polymer electrolytes. We also
CRediT authorship contribution statement
Jinhyeok Ahn: Investigation, Data collection and Presentation, Writing – original draft. Youngwoo Lee: Data curation, Visualization. Jihoon Kim: Methodology, Writing – review & editing. Yong-Cheol Jeong: Validation, Writing – review & editing. Kuk Young Cho: Conceptualization, Writing – original draft, Writing – review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) granted Mid-Career Research Program (No. 2021R1A2C2005764), Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government. [22ZB1200, Development of ICT Materials, Components and Equipment Technologies], and Korea Institute of Technology (KITECH) grant joint research program (KITECH UI-21-0002).
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