Stretchable Electrodes with High Conductivity and Photo‐Patternability

Abstract

Electrically conductive materials capable of bending and stretching are useful for a wide range of applications such as smart clothing, [1] flexible displays, [2] stretchable circuits, [3] strain gauges, [4] implants , [5] high-deformation microelectromechanical systems (MEMS), [4] and dielectric elastomer actuators (DEAs), [6] to name a few. The necessary strain varies from minimal (< 5 %) for applications such as retinal prostheses that only require conformal contact to a surface [5] to high (> 60 %) for large-strain DEAs.[7]A number of approaches have previously been used to produce stretchable electrodes. [8] Printing with carbon grease [6,9,10] or graphite powder [11,12] has been the preferred method for use with DEAs because of the low modulus of these materials, which allows large strains, but they have a relatively low conductivity and can readily be rubbed off the surface. Thin metal films deposited on elastomeric substrates, [13] such as by thermal evaporation or sputtering, can be stretched 10-20 % before failure, [14] or higher if the metal is patterned. [15] In special cases when it is possible to stretch the elastomer during deposition, so that the metal forms corrugations upon polymer relaxation, the achievable strains can be increased even further. [14,[16][17][18][19] Loading an elastomeric host polymer with a conducting material allows some degree of elasticity, but the modulus of the composite increases dramatically with loading, often before the electrical percolation threshold is reached. [20,21] Electrostatic layer-by-layer assembly has been successful in achieving highly conducting, robust, free-standing sheets capable of large strain. [22] Integrating these electrodes with other devices, however, which would include patterning the electrodes, has not yet been demonstrated. Another promising method for producing compliant electrodes is metal ion implantation, [23] which has been used in the fabrication of miniature DEAs. The advantages include patternability and an expectation of a low modulus, while the drawbacks are a rather high sheet resistance (∼ 100 kX/ square), a limited substrate size, a requirement for a low-energy ion implanter, and post-implantation bonding of the elastomer to the Si wafer. The goal of the present work was therefore to develop a new approach to achieving robust and truly elastomeric conductors that can be patterned with standard microfabrication techniques. This communication presents a method for making compliant electrodes that results in materials with a sheet resistance (R s is related to electrical conductivity r through r = 1/R s t, where t is the film thickness, so this corresponds to a conductivity of 50 S cm -1 for our 0.01 cm thick films. R s of only 2 X/& when unstretched that do not lose conductivity until they are elongated by up to 150 %. The electrodes have a Young's modulus of under 10 MPa, and they survive thousands of strain cycles. The electrodes are also photopatternable ( Fig. 1), making them compatible with microfabrication of electronic dev...

Flexible and stretchable electrodes for dielectric elastomer actuators

“…The composite is then immersed into an aqueous reducing solution, causing the Pt salt to diffuse to the surface, reducing to the metal at the polymer/liquid interface. The percolation threshold is observed for 6-7% vol Pt salt at which point the surface resistance of the electrode is ∼ 200 Ω/square, and it drops down to 2.2 Ω/square for 15% vol [80]. The Young's modulus of the composite is shown to be unaffected by the quantity of filler and remains close to that of pure Loctite 3108 (10 MPa) [82].…”

“…However, intrinsically patternable compliant electrodes are very desirable, for they can greatly simplify the structuring process. Delille, Urdaneta et al have reported on photosensitive electrodes based on platinum salts mixed in a photosensitive elastomer (Loctite 3108) [80][81][82]. The ingenuity of the process resides in the use of a filler (Pt salt) which is transparent to UV light.…”

Tunable Nanowrinkles on Shape Memory Polymer Sheets

“…[7,8] Metal wrinkles, thin films of metal on polymer substrates, have promise for applications in molecular detection, optical devices, filters and sorters, high-surface-area conductors and actuators, and even metrology. [9][10][11][12] We have developed a rapid approach to create metal nanowrinkles of tunable size and demonstrable utility on a shape memory polymer, [13,14] pre-stressed polystyrene (PS) sheets commercially available as the children's toy Shrinky-Dinks.…”

Solderable and electroplatable flexible electronic circuit on a porous stretchable elastomer

“…To this end, the normalized resistance (R/R 0 , where R 0 is the initial resistance and R is the final resistance) was measured under application of an external force (R 0 is shown in Supplementary Table S2). Electrical hysteresis in the stretchable electronics is important because electrical conductance should ideally be reproducible under strain cycling to maintain a high-accuracy operation 25 . The hysteresis of the metal on the PDMS layer was measured as a function of the stretching ratio ( Fig.…”

“…Several modified materials have been tested for their utility in flexible electronics, including nanoporous gold films suffused with nanometer-sized clusters of a platinum (Pt) salt 25 , aligned singlewalled carbon nanotubes (SWCNT) 26 , concentrated silver nanoparticle inks 27 , dissolvable films of silk fibroin 28 and elastic conductors comprising uniformly dispersed SWCNTs 29 . Many of these methods were developed to realize stretchable soft electronics that could accommodate large applied strains without fracturing.…”