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Stretchable, conductive polymer fibers of varying stiffness for auxetic strain sensors
It was recently shown that helical auxetic structure allows one to achieve strain sensitive fibres with high sensitivity. Rigidity/stiffness of components of these sensors also plays a role in sensitivity to strain. To further improve our recently published textile strain sensing modality—helical auxetic yarn capacitive sensors—we are seeking a thesis/project student to develop a conductive composite formulation and process to wet-spin fibres of different stiffnesses. Employed as sensors, the effect of stiffness on sensitivity can be tested experimentally. The outcome of this project will further the development of highly sensitive, scalable textile strain sensors.
Keywords: polymer engineering, process engineering, conductive nanoparticles, materials science, wet-spinning
Our group has recently published a new type of capacitive strain sensing fibre [1]. This fibre has a helical auxetic yarn structure formed from an inextensible copper wire wrapped around a stretchable, conductive core. Under strain, the copper wire helical diameter decreases, causing the arrangement to “flip”. During the “flipping”, the overall outer diameter increases with strain, an example of auxetic material behaviour. The sensor has greater sensitivity than expected from a simple parallel-conductor capacitance model. We postulate that the auxetic behaviour combined with “engulfment” of the copper wire into the soft inner conductor contributes to the high sensitivity. From this it follows that the sensitivity could be optimized by changing the stiffness of the core.
A first step involves testing various stiffnesses of conductive cores to evaluate how the engulfment affects the sensitivity. To do this, we hope to develop a robust process to manufacture soft conductive sensors. We are especially interested in thermal curing wet-spinning which shows promise [2] and allows the stiffness to be easily tuned by using different thermoset polymers. We still require a robust formula to produce conductive polymer composites with reasonable conductivity while maintaining mechanical strength. We envision the project having three parts, as shown below.
**References**
[1] Cuthbert, T. J., Hannigan, B. C., Shokurov, A., & Menon, C. (2023). HACS: Helical Auxetic Yarn Capacitive Sensors that Go Beyond the Theoretical Sensitivity Limit. 2209321. https://doi.org/10.1002/adma.202209321
[2] Tang, Z., Jia, S., Wang, F., Bian, C., Chen, Y., Wang, Y., & Li, B. (2018). Highly Stretchable Core-Sheath Fibers via Wet-Spinning for Wearable Strain Sensors. ACS Applied Materials and Interfaces, 10(7), 6624–6635. https://doi.org/10.1021/acsami.7b18677
Our group has recently published a new type of capacitive strain sensing fibre [1]. This fibre has a helical auxetic yarn structure formed from an inextensible copper wire wrapped around a stretchable, conductive core. Under strain, the copper wire helical diameter decreases, causing the arrangement to “flip”. During the “flipping”, the overall outer diameter increases with strain, an example of auxetic material behaviour. The sensor has greater sensitivity than expected from a simple parallel-conductor capacitance model. We postulate that the auxetic behaviour combined with “engulfment” of the copper wire into the soft inner conductor contributes to the high sensitivity. From this it follows that the sensitivity could be optimized by changing the stiffness of the core.
A first step involves testing various stiffnesses of conductive cores to evaluate how the engulfment affects the sensitivity. To do this, we hope to develop a robust process to manufacture soft conductive sensors. We are especially interested in thermal curing wet-spinning which shows promise [2] and allows the stiffness to be easily tuned by using different thermoset polymers. We still require a robust formula to produce conductive polymer composites with reasonable conductivity while maintaining mechanical strength. We envision the project having three parts, as shown below.
**References**
[1] Cuthbert, T. J., Hannigan, B. C., Shokurov, A., & Menon, C. (2023). HACS: Helical Auxetic Yarn Capacitive Sensors that Go Beyond the Theoretical Sensitivity Limit. 2209321. https://doi.org/10.1002/adma.202209321
[2] Tang, Z., Jia, S., Wang, F., Bian, C., Chen, Y., Wang, Y., & Li, B. (2018). Highly Stretchable Core-Sheath Fibers via Wet-Spinning for Wearable Strain Sensors. ACS Applied Materials and Interfaces, 10(7), 6624–6635. https://doi.org/10.1021/acsami.7b18677
**Goals**
- Improve the wet-spinning method to produce stretchable polymer composite fibres with consistent diameters and adequate conductivity.
- Test the materials for stiffness, conductivity, and as a capacitive strain sensor.
- Apply the developed materials as helical auxetic capacitors and experimentally determine the relationship between sensitivity and core stiffness.
**Tasks**
1. Short literature review. (10%)
2. Formulation optimization to obtain conductive stretchable polymer-carbon nanotubes/carbon black. composites (40%)
3. Process development to wet-spin such fibres. (40%)
4. Application of the fibres as strain sensors and characterization of the stiffness-sensitivity relationship. (10%)
**Goals**
- Improve the wet-spinning method to produce stretchable polymer composite fibres with consistent diameters and adequate conductivity.
- Test the materials for stiffness, conductivity, and as a capacitive strain sensor.
- Apply the developed materials as helical auxetic capacitors and experimentally determine the relationship between sensitivity and core stiffness.
3. Process development to wet-spin such fibres. (40%)
4. Application of the fibres as strain sensors and characterization of the stiffness-sensitivity relationship. (10%)
Prof. Carlo Menon and doctoral candidate Brett Hannigan will supervise the student and the research will be performed at ETH Zürich’s Biomedical and Mobile Health Technology research group (www.bmht.ethz.ch) in the Balgrist Campus in Zürich, Switzerland.
**Your Profile**
Background in applied physics, process control, chemistry, chemical engineering, material science, or related fields. Some experience in laboratory environment.
Prof. Carlo Menon and doctoral candidate Brett Hannigan will supervise the student and the research will be performed at ETH Zürich’s Biomedical and Mobile Health Technology research group (www.bmht.ethz.ch) in the Balgrist Campus in Zürich, Switzerland.
**Your Profile**
Background in applied physics, process control, chemistry, chemical engineering, material science, or related fields. Some experience in laboratory environment.
