Conductive Fiber‐Based Ultrasensitive Textile Pressure Sensor for Wearable Electronics

Background

Textile capacitive pressure sensors are usually made from two conductive fabrics separated by foams, spacer fabrics or soft polymers, and they suffer from poor resilience, signal drift and limited sensitivity because of the low conductivity of the fibres and the mechanical properties of the dielectric. Electro- and electroless-plated metal fibres conduct well but are less stable, carbon-coated fibres show the opposite trade-off, and an earlier chemical reduction process that forms connected Al nanoparticles on fibres deposits poorly on polymer fibres lacking groups that bind the metal precursor.

Design and approach

A poly(styrene-block-butadiene-styrene) (SBS) layer was coated on Kevlar fibre, silver trifluoroacetate (AgCF3COO) was absorbed into the SBS from ethanol, and the precursor was reduced with hydrazine to form Ag nanoparticles on and inside the SBS layer, which acts as an elastic scaffold for the particle network. The fibre was then coated with PDMS, and two PDMS-coated fibres stacked perpendicularly form a capacitor at the crossing point. Under pressure, the PDMS thickness between the electrodes decreases while the contact area between the two fibres increases, and the empty space around the contact lets the PDMS deform with little viscoelastic delay.

How it was evaluated

The authors measured the resistance per unit length of the fibre over repeated absorption and reduction cycles, the Ag content by thermogravimetric analysis, resistance during 3,000 folding cycles between 180° and 0°, and the breaking load relative to Kevlar. The sensor was characterized under loads of 0.05, 0.1 and 0.5 N, over a range of pressures, under 10 000 cycles at 1.7 N and in loading–unloading hysteresis tests, and its response was compared with a finite-element simulation in Abaqus. Demonstrations included a woven 6 × 6 array, a fabric wristband, a glove with sensors on four fingertips and a sleeve with four channels connected to a Bluetooth data acquisition circuit.

Key results

Ag nanoparticles 70–90 nm in diameter were densely connected on the ≈320 µm fibre and also formed inside the SBS layer. After eight absorption and reduction cycles, the resistance reached 0.15 Ω cm–1, compared with 1 Ω cm–1 for a commercial conductive thread and 0.2 Ω cm–1 for previously reported Al threads, as the Ag content rose from 62.8 to 82.3 wt%. After 3,000 folding cycles, the resistance increased by a factor of 1.25 (0.150–0.187 Ω cm–1), the breaking load was slightly higher than that of Kevlar, and the fibre powered a 220 V AC light bulb.

With a PDMS layer averaging 40.5 µm, the sensor gave stable responses to 0.05, 0.1 and 0.5 N loads, a response time of about 40 ms and a relaxation time of about 10 ms. The sensitivity was 0.21 kPa−1 below 2 kPa and 0.064 kPa−1 above 2 kPa; the authors attribute the higher low-pressure value to simultaneous changes in contact area and PDMS thickness, and the finite-element simulation reproduced both regimes. The sensor detected the placing and removal of an 8 mg grain, had a maximum detection limit of 3.9 MPa, kept its output over 10 000 cycles at 1.7 N and showed negligible hysteresis.

In a woven 6 × 6 array of 36 pixels over 2 × 1.5 cm2 with pixels of about 0.16 mm2, two beads of 0.25 and 0.87 g placed on different pixels were located by the capacitance map. A wristband detected grabbing, opening of the hand and wrist bending. Sensors made from ten PDMS-coated fibres to raise the baseline capacitance against parasitic capacitance were sewn into a glove and a sleeve, gave distinguishable signals without considerable interference between channels, and were used to fly a quadrotor drone wirelessly and to steer a wired hexapod robot.

Open questions

The sensitivity fell to 0.064 kPa−1 above 2 kPa, which the authors attribute to lower effective stress as the contact area grows. Wearable use required sensors built from ten fibres to reduce the effect of parasitic capacitance from the body and clothing, and the authors expect the 3.9 MPa detection limit to rise with a thicker PDMS layer.

How it connects to other research

The precursor absorption and reduction chemistry follows the ion-dipole interaction cited from Park et al. (Nat. Nanotechnol. 2012), which formed Ag nanoparticle networks in SBS fibre mats for stretchable circuits; here the Ag/SBS layer is formed on single Kevlar fibres, and the crossed, PDMS-coated fibres themselves serve as capacitive pressure sensors.