Abstract
1. Introduction
As micro- and nanoelectromechanical system (MEMS/NEMS) technologies have been rapidly developed, soft electronics are opening new a paradigm in human-machine interfaces[
Figure 1.(Color online) An overview figure of the lithographic fabrication techniques and their applications. Strain sensor image, optoelectronics image, and brain/neural probe image: Reprinted from Ref. [
To date, there have been many efforts to achieve stretchable electrodes by constructing conductive percolation networks on/in elastomeric films[
Among the many classes of elastic foam materials, silicon-based organic elastomers, especially polydimethylsiloxane (PDMS) have received intensive research interest as vital building blocks[
In this review, we aim to provide an overview of the comprehensive lithographic technologies used to produce micro-/nanostructured PDMS, as well as their soft electronic applications such as in stretchable displays, sensors, and bio-sensor platforms and showcase intriguing demonstrations of structural effects. This review begins with a brief introduction to representative fabricating strategies for structured PDMS, and a discussion of their advantages and drawbacks follows. Afterwards, the roles of structured PDMS in various applications are considered, and finally, this review is concluded with personal intuition towards existing technical challenges and a forecast of further research directions.
2. Fabrication methods of micro-/nanostructured PDMS
2.1. Photolithography with PDMS etching procedures
The demand for PDMS as the structural material in MEMS/NEMS technologies has explosively increased due to its excellent structural resolution and accuracy of less than 10 nm[
Figure 2.(Color online) Lithographic surface micromachining of PDMS. (a) Schematic illustrations of photolithographic surface micromachining of PDMS. (b–j) Free-standing PDMS microfiltration membranes and beam structures. (b–f) SEM images of the microstructured PDMS membrane bonded to (e) a PDMS support structure. The membrane had a thickness of 10
2.2. Direct patterning of PDMS
Previous lithographic technologies using PDMS have significantly relied on additional direct etching procedures such as RIE processes and mechanical scrapping with sharp edges. In spite of their active removal processes, it is difficult to achieve precise control of the morphology because of the unavoidable PDMS residue left behind[
Figure 3.Microstructuring technique with photopatternable PDMS. (a) Schematic illustrations of the photoPDMS process sequence. (b) SEM image of a 400
2.3. PDMS replication from a 3D template via unconventional lithography
With the recent development of a variety of new 3D patterning technologies through non-traditional methods, the high-level engineering of elaborate 3D architectures and the fundamental understanding of these structures through in-depth studies have been considered as breakthroughs for a wide range of existing applications over the fields of energy storage devices[
2.3.1. Interference lithography
The use of multi-beam interference lithography offers a rational design opportunity to fabricate well-established 3D structures at the submicron scale[
Figure 4.(Color online) Fabrication of 3D elastomeric network/air structures via interference lithography. (a) Schematic illustration of the fabrication process. (b) Theoretical light intensity model. The brown and green colors indicate the inner and outer surfaces, respectively. (c) Reconstructed confocal image showing a perspective view of the PDMS elastomeric structure. (d) SEM images of the prepared 3D PDMS network/air structure. (Reprinted from Ref. [
2.3.2. Proximity-field nanopatterning (PnP)
The PnP technique is an advanced 3D nanofabrication technology that utilizes high-resolution, conformable phase masks and provides a powerful route to produce classes of 3D nanostructures[
Figure 5.(Color online) Fabrication of highly ordered, 3D nanostructured PDMS by the proximity-field nanopatterning (PnP) technique and material conversion technique. (a) Schematic illustration of the fabrication procedures to produce 3D PDMS. (b) Top-view SEM image of the top surface of the 3D polymeric template, which was fabricated with a positive-tone photoresist, after intentionally removing part of the first layer and (c) the replicated 3D PDMS from the template. (d) Optical image of a supported 3D PDMS film and (e) a folded 3D PDMS film with line patterns (scale bar = 1 cm) (Reprinted from Ref. [
Recently, Cho et al. reported that the 3D nanostructure defined by the PnP using a negative-tone photoresist can also be an appropriate 3D template for conversion to PDMS[
2.4. 3D printing technique
Additive manufacturing, also known as 3D printing techniques, has drawn tremendous attention due to its extraordinarily high freedom of producing intricate or arbitrary architectures[
Figure 6.(Color online) Preparation of hierarchical porous PDMS via a 3D printing technique. (a) Schematic illustration of the 3D printing of trimodal porous PDMS with complex architectures. (b) SEM images of the microscaled porosity inside the extruded filaments. (c) Illustration of hierarchically porous printed objects. (d) Optical images of 3D-printed PDMS foam structures (an octopus, a pyramid, a half of ball, and a butterfly). (Reprinted from Ref. [
2.5. Pen lithography
Another representative example of direct fabrication method is a pen lithography. This method is based on a scanning probe lithography technique, which has made use of an array of tips that includes transparent two-dimensional pyramid-shaped elastomeric tips (as writing pens)[
2.6. Brief summary
The aim of this section is to provide a comprehensive overview of fabricating methods for producing micro-/nanostructured PDMS defined by representative examples of lithographic techniques (Fig. 7). The characteristics of these recently reported strategies are briefly listed in Table 1 in terms of major advantages as well as technical challenges.
Figure 7.(Color online) Schematic illustration of the fabrication of structured PDMS with various lithographic techniques. (a) Photolithography with PDMS etching procedures. (b) Direct patterning of photopatternable PDMS. (c) PDMS replication from pre-structured mold. (d) PDMS replication from a 3D template via interference lithography. (e) Direct writing techniques. (e.g. 3D printing techniques)
Table Infomation Is Not Enable3. Applications
In this section, the applications of materials based on structured PDMS in soft electronics, including stretchable conductors, sensors, health-monitoring systems, mechano-responsive smart windows, etc. are discussed. These promising applications were realized by utilizing the novel features of structured PDMS and its designed structures, including large surface areas, viscoelasticity, mechanical robustness, biocompatibility, processability, ease of handling and low cost. Additionally, the intriguing functions originating from the controlled structure such as dry adhesive properties without the assistance of a wet adhesive and high-optical modulation through simple mechanical deformation are explained and discussed.
3.1. Stretchable electrodes
As the demand for flexible electronics has rapidly increased, the development of electronic devices capable of operating in a stretched state rather than in a simple bending state has been attempted[
Figure 8.(Color online) Stretchable display applications based on the electrical robustness of the 3D stretchable conductor. (a) Electrical conductivity of the sandwich-structured stretchable conductor with strain of up to 220%. (b) Cyclic stretching and releasing of various strains. (c) Schematic illustration of LED devices on the stretchable conductors. (d) Stable LED operation under strains of up to 220%. (Reprinted from Ref. [
3.2. Stretchable sensors
As wearable smart devices are rapidly developed to fuel the revolution of the life quality of human beings, the demand for high-performance soft strain sensors is explosively increasing[
Figure 9.(Color online) Piezoresistive-type strain sensors based on porous PDMS. (a) Schematic illustrations of an experimental procedure to produce 3D strain sensors based on periodic porous PDMS. (b) Top-view SEM images of the SWCNT-coated porous PDMS fabricated by controlling the infiltration cycles of SWCNT solution. (c) Relative resistance changes of the 3D strain sensor and (d) comparison of the resulting gauge factor of recently reported CNT/elastomer-based strain sensors. (e) Cyclic property of the 3D strain sensor. (f–h) Demonstrations of the 3D strain sensor measurement of various human motions in daily life including (f) general phonations, (g) index finger movement, and (h) wrist movement. (Reprinted from Ref. [
3.3. Bio-inspired architectures for biosensor platforms
There has been a need for the development of wearable sensors to detect physical movements of the body and to continuously monitor vital signs or in vivo biomolecule levels[
In 2011, Kwak et al. demonstrated that high-density PDMS-based micropillar arrays inspired by gecko hair can be an alternative to the adhesive parts of ECG electrodes[
Figure 10.(Color online) Gecko-inspired architecture for biosensor applications. (a) A schematic illustration of the fabrication procedure for conductive dry adhesive patches. (b) A digital image of replicated conductive dry adhesive and its cross-sectional SEM image. (c) ECG measurement with conductive dry adhesive skin patches under various operation conditions. (Reprinted from Ref. [
The skin patch with an octopus suction cup structure showed a high adhesive property on the surface of wet as well as dry skin and transmitted the ECG signal stably[
Figure 11.(Color online) Bio-inspired hierarchical architecture for biosensor applications. (a) A schematic illustration of conductive hierarchical architectures inspired by amphibians and octopi. (b) A digital image of an rGO nanoplatelet-coated bio-inspired skin patch and a schematic illustration of its fabrication procedures. (c) ECG measurement with a conductive bio-inspired skin patch under various measuring conditions. (Reprinted from Ref. [
3.4. Mechano-responsive smart windows
The highly stretchable PDMS scaffold plays a significant role in optical modulation through simple mechanical deformations, which is considered to be one of the most common practices used to control light transmission comprehensively, similar to the drawing of blinds or curtains[
Figure 12.(Color online) Optical modulation from various PDMS composite structures. (a) Schematics of fabrication procedures. (b) Cross-sectional SEM image of the prepared composite. (c) SEM image of a stretched composite film with silica nanoparticles with a diameter of 258 nm at ~ 80% strain. Arrows indicate PDMS ligaments. d) Confocal optical image of (i) an unstretched and (ii) a stretched silica nanoparticle (diameter of 5
4. Summary and outlook
The silicone-based elastomer PDMS, which enables the construction of a stretchable platform for soft electronic applications, has received explosive research attention due to its ease of handling, processability, stretchability, flexibility, cost-effectiveness, and high degrees of freedom to incorporate electrically functional additives. Thus, the introduction of finely controlled micro-/nanoarchitectures into bulk PDMS is a promising approach for improving its mechanical and electrical properties by utilizing the synergetic effects among 1) its intrinsic bulk material properties, 2) the significant increase in its surface area due to the introduction of a structure, and 3) its capability to create novel material systems, which provide optimally designed structural features and the facile incorporation of functional materials in/on the PDMS scaffold. Recently, to meet practical needs—especially, integration with conventional MEMS/NEMS technologies—lithographic fabrication methods to produce stretchable platforms have become one of the promising research topics and directions for many researchers. In turn, reliable structuring of PDMS affords characteristics to satisfy practical purposes such as scalability, reproducibility, and high performance and provides great potential for use in various research fields including stretchable displays, sensors, and healthcare system platforms. To achieve these, it is imperative to develop a versatile and highly scalable fabrication method of the structured PDMS in parallel. The studies into structuring the PDMS via the lithographic fabrication techniques has been drastically growing. For further practical production of the structured PDMS, in particular for industrial applications, one of the promising research directions includes pursuit of developing the cost-effective, eco-friendly, and reliable technologies. In this point of view, the lithographic fabrication methods offer a general pathway to create the vital building blocks, not just PDMS, for a broad scope of materials and potential applications. We envision that this review will offer a focused understanding of the lithographic fabrication techniques used for the further adjustment and optimization of the physical structures of PDMS and their compelling features, as well as to address technical challenges. In addition, it can also motivate researchers to design research directions with knowledge of the structure-function correlation for the realization of next-generation stretchable electronics.
Acknowledgements
This research was supported by the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT and Future Planning (MSIP) (2016R1E1A1A01943131).
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