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Measurements of biological signals, such as those emanating from electrocardiography (ECG) and electromyography, are required in such fields as medical science, sports science, and robotics. There have been many studies that biological signals are measured using conductive cloth or textiles. Although the conductive materials were contact with the body, there have been few studies that considered the deterioration caused by deformation and moisture. We describe here an "undershirt" that we developed for measuring ECG signals. Its electrodes and wires are composed of conductive ink. We expected that even if the subject moved and perspired, measurement performance would not decrease. In our first experiment, we investigated characteristics of electrical resistance when the samples were deformed or became wet. The results indicated that the conductive ink wire was not broken by a 30% stretch or by bending it at a 1-mm curvature radius. The wire also maintained sufficient insulation resistance in the presence of 0.5 wt% NaCl aqueous solution. In the second experiment, ECG signals were measured via the undershirt with the ink wiring. The results showed that deformation of the ink wire on the undershirt is difficult to affect the ECG data.

Stretchable wires were printed on fabrics using an acrylic-based paste loaded with Ag flakes, and their fatigue properties examined. The electrical conductivity of the wires significantly decreased during a cyclic tensile test, because of a decrease in their elastic moduli (Mullins softening) as well as fatigue cracking. Because the electrical resistance and elastic moduli of the damaged samples were partially recovered by annealing at 100 °C, fatigue damage introduced to the wires was divided into reversible and irreversible components, where cracking is the irreversible damage. Although crack bridging by fibrils could occur during the fatigue test, no crack healing was observed during annealing. In contrast, fatigue damage from Mullins softening of the wires could be recovered during annealing. The recovery of electrical conductivity occurs mostly in the initial stage of rearrangement of polymer structure during annealing.