Development of flexible, highly conductive electrodes or interconnects with excellent mechanical stability has been a challenging issue in device fabrication for wearable electronics, flexible displays, etc. Researchers at College of Chemistry and Environmental Engineering, Shenzhen University, P. R. China have developed 3D stretchable, compressible and electrically conductive conductors by surface modification of poly(dimethylsiloxane) (PDMS) sponges with poly[2-(methacryloyloxy)ethyl-trimethylammoniumchloride] (PMETAC) polymers followed by electroless deposition of metals.
The PDMS sponges were fabricated using sugar templating method. The sugar cubes were immersed in a mixture of Sylgard 184 and curing agent, degassed in a vacuum desiccator for 2 h followed by baking at 65 °C for 3 h, removal of sugar template by immersion in water at 60 °C for 24 h and drying at 100 °C for 2 h. The PDMS sponges were activated by air plasma treatment for 5 min, functionalized with vinyltrimethoxysilane (VTMS) via silanization followed by in situ free radical polymerization with METAC monomer and potassium persulfate as initiator, leading to the formation of PMETAC-modified PDMS sponges with 3D-interconnected porous structures. Electroless deposition of metals enables the formation of metal-coated PDMS sponges. The schematic of the fabrication process and images of electroless Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponges are shown in Fig. 1.
Fig. 1 (a) Schematic illustration of the fabrication of metal-coated PDMS sponges; (b, c) Optical images of the PDMS sponge and Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponge
The PDMS sponges consist of highly interconnected 3-D porous structures and the sponge-like structure is retained even after coating them with Cu, Ag/Cu and Au/Cu by electroless deposition. The coated metal particles exhibit a close-packed arrangement on the surface of PDMS sponges. The elastomeric property of PDMS sponges enable them to be stretched and compressed while metal coating makes them conductive, thus making them suitable for the fabrication of electrically conductive, stretchable and compressible electrodes. The metal-coated PDMS sponges exhibit remarkable mechanical stability and electrical conductivity, which is evidenced by the overlap of I–V characteristics of Ag/Cu-PDMS sponges while stretching or compressing them from 0% to 50% as well as by the continuous glowing of LED lamps at different extents of stretching, bending, and twisting (Fig. 2). In addition, the metal-coated PDMS sponges remain conducting even after cutting them to two pieces, which indicates that the metal coating is uniform not only on the outer side but also on the inner side of PDMS sponges.
Fig. 2 Flexible LED circuits made of Ag/Cu-PDMS sponge interconnects: (a, b) I–V characteristics of the LED circuit at different (a) tensile; and (b) compressive strains; (c-e) Optical images of the LED circuits with two LEDs at different extent of (c) stretching; (d) bending; and (e) twisting.
The ability of metal-coated PDMS sponges to offer no change in resistance at 40% tensile strain and only ≈20% increments at 50% strain after 5000 cycles of stretching suggests that they can be used for stretchable, compressible, and bendable interconnects or soft electronic contacts.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: S.Q. Liang et al., 3D Stretchable, Compressible, and Highly Conductive Metal-Coated Polydimethylsiloxane Sponges, Adv. Mater. Technol. 2016, 1600117, DOI: 10.1002/admt.201600117
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