Smart textiles offer many potential wearable technology applications, from therapeutics to sensing to communication. For such intelligent textiles to function effectively, they need to be strong, stretchable, and electrically conductive. However, fabricating fibres that possess these three properties is challenging and requires complex conditions and systems.
Drawing inspiration from how spiders spin silk to make webs, a team of researchers led by Assistant Professor Swee-Ching Tan from the Department of Materials Science and Engineering under the National University of Singapore’s College of Design and Engineering, together with their international collaborators, have developed an innovative method of producing soft fibres that possess these three key properties, and at the same time can be easily reused to produce new fibres. The fabrication process can be carried out at room temperature and pressure, and uses less solvent as well as less energy, making it an attractive option for producing functional soft fibres for various smart applications.
“Technologies for fabricating soft fibres should be simple, efficient and sustainable to meet the high demand for smart textile electronics. Soft fibres created using our spider-inspired method of spinning has been demonstrated to be versatile for various smart technology applications — for example, these functional fibres can be incorporated into a strain-sensing glove for gaming purposes, and a smart face mask to monitor breathing status for conditions such as obstructive sleep apnea. These are just some of the many possibilities,” said Asst Prof Tan.
Their innovation was demonstrated and outlined in their paper that was published in scientific journal Nature Electronics on 27 April 2023.
Spinning a web of soft fibres
Conventional artificial spinning methods to fabricate synthetic fibres require high pressure, high energy input, large volumes of chemicals, and specialised equipment. Moreover, the resulting fibres typically have limited functions.
In contrast, the spider silk spinning process is highly efficient and can form strong and versatile fibres under room temperature and pressure. To address the current technological challenges, the NUS team decided to emulate this natural spinning process to create one-dimensional (1D) functional soft fibres that are strong, stretchable, and electrically conductive. They identified two unique steps in spider silk formation that they could mimic.
Spider silk formation involves the change of a highly concentrated protein solution, known as a silk dope, into a strand of fibre. The researchers first identified that the protein concentration and interactions in the silk dope increase from dope synthesis to spinning. The second step identified was that the arrangement of proteins within the dope changes when triggered by external factors to help separate the liquid portion from the silk dope, leaving the solid part — the spider silk fibres. This second step is known as liquid-solid phase separation.
The team recreated the two steps and developed a new spinning process known as the phase separation-enabled ambient (PSEA) spinning approach.
The soft fibres were spun from a viscous gel solution composed of polyacrylonitrile (PAN) and silver ions — referred to as PANSion — dissolved in dimethylformamide (DMF), a common solvent. This gel solution is known as the spinning dope, which forms into a strand of soft fibre through the spinning process when the gel is pulled and spun under ambient conditions.
Once the PANSion gel is pulled and exposed to air, water molecules in the air act as a trigger to cause the liquid portion of the gel to separate in the form of droplets from the solid portion of the gel, this phenomenon is known as the nonsolvent vapour-induced phase separation effect. When separated from the solid fibre, the droplets of the liquid portion are removed by holding the fibre vertically or at an angle for gravity to do its work.
“Fabrication of 1D soft fibres with seamless integration of all-round functionalities is much more difficult to achieve and requires complicated fabrication or multiple post-treatment processes. This innovative method fulfils an unmet need to create a simple yet efficient spinning approach to produce functional 1D soft fibres that simultaneously possess unified mechanical and electrical functionalities,” said Asst Prof Tan.
Three properties, one method
The biomimetic spinning process combined with the unique formulation of the gel solution allowed the researchers to fabricate soft fibres that are imbued with three key properties — strong, stretchable, and electrically conductive.
Researchers tested the mechanical properties, strength, and elasticity, of the PANSion gel through a series of stress tests and demonstrated that this remarkable innovation possessed excellent strength and elasticity. These tests also allowed the researchers to deduce that the formation of strong chemical networks between metal-based complexes within the gel is responsible for its mechanical properties.
Further analysis of the PANSion soft fibres at the molecular level confirmed its electrical conductivity and showed that the silver ions present in the PANSion gel contributed to the electrical conductivity of the soft fibres.
The team then concluded that PANSion soft fibres fulfils all the properties that would allow it to be versatile and potentially be used in a wide range of smart technology applications.
Potential applications and next steps
The team demonstrated the capabilities of the PANSion soft fibres in a number of applications, such as communication and temperature sensing. PANSion fibres were sewn to create an interactive glove that exemplified a smart gaming glove. When connected to a computer interface, the glove could successfully detect human hand gestures and enable a user to play simple games.
PANSion fibres could also detect changes in electrical signals that could be used as a form of communication like Morse code. In addition, these fibres could sense temperature changes, a property that can potentially be capitalised to protect robots from environments with extreme temperatures. Researchers also sewed PANSion fibres into a smart face mask for monitoring the breathing activities of the mask wearer.
On top of the wide range of potential applications of PANSion soft fibres, this innovative discovery earns points in sustainability. PANSion fibres could be recycled by dissolving in DMF, allowing it to be converted back into a gel solution for spinning new fibres. A comparison with other current fibre-spinning methods revealed that this new spider-inspired method consumes significantly lower amounts of energy and requires lower volume of chemicals.
Further to this cutting-edge discovery, the research team will continue to work on improving the sustainability of the PANSion soft fibres throughout its production cycle, from the raw materials to recycling the final product.
