Each electrode is made of two stainless-steel threads coated with p-doped poly(3,4-ethylenedioxythiophene) material (PEDOT-Cl) and stacked to form a 3D architecture with 5 mm length, 0.6 mm width, and 1.2 mm height. Then a gel electrolyte is deposited over the electrodes to insulate them and prevent inter-electrode contact (Fig. 1). Before the electrolyte fully solidifies, the pre-stretched substrate is released and the gel wrinkles; this prevents folding-induced microcracks and yields a compact, pliable device. It’s critical to minimize the pacing between the electrode fingers to maximum charge storage. However, the “fuzziness” of the threads makes this difficult, because “microfibirals” of the stainless-steel thread stick out and thus short-circuit to adjacent electrodes. To overcome this problem, the substrate textile is a stretchable material and it’s pre-stretched as the six interdigitated electrode fingers are sewn, with adjacent electrodes separated by two threads of the substrate textile.
The result is an extensive 3D array of compactly aligned electrodes with the electrode dimensions defined by the knit structure of the textile backing (Fig. 2). The supercapacitor has areal capacitance of 80 mF/cm2. Energy density was 11 μW-hr/cm2 with a polymer gel electrolyte and 34 μW-hr/cm2 with an ionic liquid electrolyte, enough to power many wearable biosensors.
Tolerance to extreme mechanical distortions is obviously important for wearable charge storage technologies, which are subjected to constant movements and deformations. These textile MSCs are also super-deformable with relatively unchanged electrochemical performance even after fully rolling-up the device.