Thermally Drawn Elastomer Nanocomposites for Soft Mechanical Sensors

Abstract

Stretchable and conductive nanocomposites are emerging as important constituents of soft mechanical sensors for health monitoring, human-machine interactions, and soft robotics. However, tuning the materials' properties and sensor structures to the targeted mode and range of mechanical stimulation is limited by current fabrication approaches, particularly in scalable polymer melt techniques. Here, thermoplastic elastomer-based nanocomposites are engineered and novel rheological requirements are proposed for their compatibility with fiber processing technologies, yielding meters-long, soft, and highly versatile stretchable fiber devices. Based on microstructural changes in the nanofiller arrangement, the resistivity of the nanocomposite is tailored in its final device architecture across an entire order of magnitude as well as its sensitivity to strain via tuning thermal drawing processing parameters alone. Moreover, the prescribed electrical properties are coupled with suitable device designs and several fiber-based sensors are proposed aimed at specific types of deformations: i) a robotic fiber with an integrated bending mechanism where changes as small as 5° are monitored by piezoresistive nanocomposite elements, ii) a pressure-sensing fiber based on a geometrically controlled resistive signal that responds with a sub-newton resolution to changes in pressing forces, and iii) a strain-sensing fiber that tracks changes in capacitance up to 100% elongation.

Keywords: conductive polymer nanocomposites; functional fibers; pressure sensors; soft materials; soft robotics; strain sensors.

Figure 1

Materials, fabrication, and rheological investigation of nanocomposites. a) Schematic of the thermal drawing process. b) Optical images of fiber cross‐section. Each fiber architecture is targeted at specific modes of stimulation such as bending, pressure or stretch sensing. c) Photograph of a fiber of length 10 m to illustrate the scalability of the process. d) Temperature ramp experiment of the thermoplastic elastomer SEBS. A crossing point is observed at ≈110 °C, above which the material behavior is dominated by viscous flow. e) Temperature ramp experiment of the nanocomposite 7 wt% CNT–SEBS. The storage modulus remains larger than the loss modulus for all the temperatures in the range of interest, suggesting a pronounced melt elasticity. f) Frequency sweep experiments at 140 °C for both pure SEBS and 7 wt% CNT–SEBS nanocomposite. The near‐linear relationship in the logarithmic scale between the complex viscosity and the frequency for the nanocomposite is indicative of a pronounced melt elasticity. g) Storage modulus as a function of the oscillation stress amplitude for different conductive composite systems. The onset of the curve indicates the start of irreversible plastic deformation, also defined as the apparent yield stress. h) Transmission electron image of 7 wt% CNT–SEBS. i) Transmission electron image of 7.5 wt% CNT‐ 42.5 wt% PE–SEBS. j) Nanocomposite selection map, quantified by the storage modulus plateau value and the apparent yield stress. The arrow pointing toward lower moduli and apparent yield stresses indicates the improved processability by thermal drawing.

Figure 2

Mechanical and electrical characterization of the nanocomposite fibers. a) Static tensile tests of fibers composed of 7.5 wt% CNT‐ 42.5 wt% PE–SEBS in an SEBS cladding. The curves correspond to multiple fiber samples. b) Dynamic tensile test of a nanocomposite fiber cyclically stretched between 0 and 100% strain for 10 cycles. c) Schematic of the adapted thermal drawing process. A draw‐supporting 1D element, termed spine, is introduced to extend the range of processing temperatures. d) Nanocomposite resistivity in the fiber for different set drawing temperatures. The error bar represents the standard deviation. e) Piezoresistivity of nanocomposite fibers drawn at different temperatures, where fibers processed at higher temperatures exhibit a reduced sensitivity to deformation and increased strain limit of the electrical conduction breakdown.

Figure 3

Piezoresistive feedback in robotic fibers. a) Optical image of a robotic fiber cross‐section, integrating two nanocomposite sheets, a metallic wire, an optical guide, and two lumens that can host tendons to actuate the fiber. b) Schematic of the fiber bending controlled by integrated tendons and quantified by the bending angle θ. c) Sequence of overlaid photographs of the fiber during a bending ramp experiment. d) Bending angle, quantified by a camera and computer vision, as a function of time. e) Schematic of the side view of a fiber with two nanocomposite electrodes, highlighted in black, and a metallic wire in gray (left), and the resulting equivalent electrical circuit (right). f) Relative resistance change versus the fiber curvature for both nanocomposite electrodes. The color coding in the schematics indicates the state of tension or compression for the two curves. The line represents the mean and the shaded area indicates the standard deviation for 10 bending cycles. g) Resistive response of the piezoresistive nanocomposite sheet for a sequence of arbitrarily set bending angles. h) Piezoresistive response of the nanocomposite to a small bending angle step of 5°. i) Resistive response of the nanocomposite during two cycles of bending. In the first cycle, the bending unopposed, whereas, in the second cycle, the motion is hindered by an obstacle, altering the electrical signal. j) Robotic fiber emitting light through the integrated optical guide onto a screen.

Figure 4

Resistive pressure‐sensing and capacitive stretch‐sensing fibers. a) Optical image of a resistive pressure‐sensing fiber cross‐section. Two nanocomposite sheets and metallic wires act as electrodes and are integrated in an SEBS support structure. b) Schematic of the fiber submitted to localized pressure and resulting equivalent circuit. c) Conductance as a function of the applied compression force for a loaded surface area of 80 mm². Schematic of the fiber cross‐section during compression are shown above the graph. d) Effect of the loaded surface area. The conductance versus pressing force is recorded for different loaded surface areas. e) Threshold force and sensitivity as a function of the loaded surface area. From graph (d), the force where the nanocomposite electrodes first touch and the slope of the conductance–pressing force curve, referred as sensitivity, are reported. f) Optical image of a capacitive stretch sensing fiber. Two nanocomposite sheets (black) are rolled and act as electrodes to create a capacitive fiber with SEBS as dielectric layer. g) Stretch‐sensing performance. The current is recorded while the fiber is being stretched between 50% and 100% strain. The line represents the mean and the shaded area indicates the standard deviation for 5 cycles. h) Current flowing through the fiber as a function of the applied compression force. The line represents the mean and the shaded area indicates the standard deviation on 10 cycles. The inset shows the maximum current change $(\max (|\frac{\mathrm{\Delta }I}{I_0}|))$ for different positions with respect to the electrical contacts.