Fig. 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. [1]. Copyright 2019 The Royal Society of Chemistry. Smart window image: Reprinted from Ref. [146]. Copyright 2018 Nature Publishing Group. Microfluidics image: Reprinted from Ref. [147]. Copyright 2019 The Royal Society of Chemistry. Stretchable conductor image: Reprinted from Ref. [23]. Copyright 2018 Nature Publishing Group.

Fig. 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 μm, and it contained an array of hexagonally spaced through-holes (with a hole diameter of 4 μm). (g–j) Free-standing PDMS beam structures on Si wafer substrates, with the beam thickness of 500 nm and a total beam length of 800 μm. The minimum beam widths were (g) 2 μm, (h) 10 μm, and (i, j) 5 μm, respectively. (Reprinted from Ref. [51]. Copyright 2012 The Royal Society of Chemistry.)

Fig. 3. Microstructuring technique with photopatternable PDMS. (a) Schematic illustrations of the photoPDMS process sequence. (b) SEM image of a 400 μm-wide channel with 200 μm inlets. The image shows vertical channel walls similar to those achieved using conventional photoresists. (c) Photograph of a 30 μm-thick patterned free-standing photoPDMS film with various feature sizes from 0.5 to 2 mm. (d) Digital image of a dual-layered photoPDMS structure fabricated using a two-step lithography process. (Reprinted from Ref. [57]. Copyright 2007 The Royal Society of Chemistry.)

Fig. 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. [78]. Copyright 2006 American Chemical Society.)

Fig. 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. [23]. Copyright 2012 Nature Publishing Group.) (f) Top-view SEM image of the top surface of the 3D polymeric template, which was fabricated with a negative-tone photoresist and (g) the resulting 3D PDMS. (Reprinted from Ref. [24]. Copyright 2017 American Chemical Society.)

Fig. 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. [104]. Copyright 2019 Wiley.)

Fig. 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)

Fig. 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. [23]. Copyright 2012 Nature Publishing Group.)

Fig. 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. [24]. Copyright 2017 American Chemical Society.)

Fig. 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. [15]. Copyright 2016 American Chemical Society.)

Fig. 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. [128]. Copyright 2019 Wiley.)

Fig. 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 μm)/PDMS film. The circles indicate silica nanoparticles. Black regions indicate voids. (e) Transmittance versus applied strain at wavelengths of 500 and 700 nm, respectively. (Reprinted from Ref. [130]. Copyright 2019 Wiley.) (f) Schematic illustration of the procedure to prepare wrinkled-silica composite films. (g) SEM images of the top and bottom surfaces of the wrinkled-silica composite film with embedded 500 nm silica nanoparticles. (h) AFM height profiles of the wrinkled surface and NPs-embedded surface of the composite film in the released state. (i) Normal transmittance of a wrinkled-silica nanoparticle composite film (pre-strain: 10%) at a wavelength of 550 nm as a function of the strain (0–40%). (Reprinted from Ref. [134]. Copyright 2019 Wiley.) (j) Schematic illustrations and SEM images representing the fabrication steps of the 3D nanocomposite which provides optical modulation by the stretching and releasing.

Table 1. Summary of the lithographic techniques to produce micro-/nanostructured PDMS.