X-ray CT Reveals How Twist Architecture Controls Conductive Yarn Resistance

Conductive yarns built for smart clothing owe their electrical performance to an invisible internal architecture, and researchers have now captured that architecture in three dimensions without cutting the yarn apart. In a study published in Fashion and Textiles, a team fabricated three types of conductive bicomponent yarns from PET filament and silver-coated polyamide (AP) filaments under different twisting conditions, then used non-destructive X-ray computed tomography to visualize the intact fiber assemblies. The imaging revealed pronounced differences in packing density, helical structure, and turn length depending on how the first and second twisting steps were combined, and those differences tracked closely with the yarns’ measured electrical resistance.

The motivation comes from a practical problem in electronic textiles. Conductive threads used in smart garments are typically made by coating conventional fibers with conductive particles such as silver, gold, nickel, or copper, leaving the conductive layer exposed to the environment. That exposure gives excellent conductivity, but the particles easily fall off under abrasion from sewing needles and external stresses, degrading durability and performance. Combining ordinary yarns with conductive yarns into composite structures offers a route to threads that behave more like conventional textile yarns during apparel manufacturing processes such as weaving, sewing, embroidery, and knitting, while maintaining electrical function.

The research team, including Hyelim Kim, Kosei Nakamura, KyoungHou Kim, and Wonyoung Jeong, prepared three yarn samples with identical final linear densities of 290 denier. Each was built from PET yarn and silver-coated polyamide filament in different combinations and sequences across two twisting steps: a first twist applied at 500 turns per meter in the S direction and a second twist at 500 TPM in the Z direction. The three configurations were PET2P/AP140, PET-AP70/PET-AP70, and PET-AP140/PET. A twisting machine with a 5 cN pre-load produced the samples, and a universal testing machine plus a two-point probe multimeter characterized their mechanical and electrical behavior.

Conventional cross-sectional analysis posed the very problem the team wanted to avoid. Cutting a yarn to examine it under optical microscopy, confocal microscopy, or field-emission scanning electron microscopy subjects the fiber strands to tension and shifts their positions, producing inconsistent cut surfaces. Flexible threads release their original twist when cut, yielding uneven fiber arrangement or deformation that differs from the actual internal structure. X-ray CT sidesteps this by imaging the yarn intact. The researchers secured specimens to prevent untwisting, centered them on supports, and scanned them with a Skyscan 1272 system operating at 50 kV and 200 microamps with a spatial resolution of 0.5 micrometers per pixel.

Because the field of view was limited at that resolution, the team scanned overlapping regions along the yarn’s length, covering approximately one twist turn, about 2.0 millimeters, per sample. Transmission images of 4,904 by 3,280 pixels were acquired while rotating each specimen in 0.1-degree increments around its longitudinal axis. The reconstructed data distinguished the components by X-ray absorption: the silver coating appeared as bright regions, while the underlying polyamide and the PET filaments appeared as darker or smaller features. This contrast allowed the spatial distribution of conductive and non-conductive components to be mapped without any physical sectioning.

To quantify the helical geometry, the researchers traced a single silver-coated polyamide filament at the outermost region of the AP assembly, extracting its X, Y, and Z coordinates at 1-micrometer intervals along roughly 1,900 positions per sample. The choice of an outermost filament minimized overlap with surrounding filaments and allowed consistent identification across sequential tomographic slices. The team notes that this coordinate-based analysis characterized the trajectory of one selected filament rather than statistically representing all conductive filaments in the yarn.

The tomographic images showed clearly different internal organizations. In PET2P/AP140, the two PET assemblies partially lost their first-twist arrangement during the second Z-twisting step, a rearrangement the authors attribute to the opposing twist directions. PET-AP140/PET showed local separation between the PET yarn added in the second step and the first-twisted PET-AP140 component, possibly linked to the linear density mismatch between the 75-denier PET and the 215-denier composite component. PET-AP70/PET-AP70, by contrast, displayed the most compact and relatively uniform internal arrangement, with its four component assemblies periodically shifting between parallel and compact circular configurations along the yarn axis, a pattern reflecting the helix imposed by the second twist.

Quantitative tracing sharpened the contrast. All three samples showed a turn length of roughly 2,000 micrometers, consistent with the 500 TPM manufacturing condition, but PET-AP70/PET-AP70, with four fiber aggregates and three twisting processes, had a shorter turn length of about 1,700 micrometers. The additional twisting increased internal tension and compression, producing higher packing density. Cross-sections reconstructed from the traced filament were elliptical, and PET-AP70/PET-AP70 and PET-AP140/PET showed smaller diameters and cross-sectional areas than PET2P/AP140. The helical shapes also differed: the two samples using conductive yarn only once formed simple Z- or S-direction spirals, while PET-AP70/PET-AP70 exhibited a mixed S-Z spiral.

These structural differences aligned with the measured properties. Tensile testing showed initial modulus increasing in the order PET2P/AP140, PET-AP140/PET, PET-AP70/PET-AP70, with values of 14.53 plus or minus 0.80, 18.60 plus or minus 1.40, and 20.42 plus or minus 1.68 gf/den. The compact structure of PET-AP70/PET-AP70 likely restricts relative filament movement during early tensile deformation and facilitates load transfer through increased inter-filament interaction. Tenacity, however, varied only narrowly, from 4.23 to 4.31 gf/den, and elongation at break ranged from 53.50 to 57.99 percent, indicating the twisting configurations affected initial stiffness far more than ultimate strength.

Electrically, all samples showed low linear resistance below 5 ohms per centimeter, but the values differed meaningfully. PET-AP70/PET-AP70, with its dense internal packing, smaller cross-section, and shorter helical turn length, exhibited the lowest resistance with minimal variation, while PET2P/AP140 combined the longest conductive helical path with the highest resistance. The authors caution that because the silver-coated polyamide linear density, 70 or 140 denier, varied simultaneously with the twisting configuration, the individual effects of geometry and AP fineness cannot be fully decoupled in this experimental design; the observed structure-property relationships reflect their combined effects. Future work, they write, will extend the single-strand tracing to multi-strand analysis of whole composite yarns, and the findings offer design guidelines for durable, reliable conductive yarns in smart textile applications.

Subject of Research: Structure-property relationships in twist-controlled conductive bicomponent yarns analyzed by X-ray computed tomography

Article Title: Twist-controlled conductive bicomponent yarns: internal structure and properties analyzed by X-ray computed tomography

Article References: Kim, H., Nakamura, K., Kim, K., & Jeong, W. (2026). Twist-controlled conductive bicomponent yarns: internal structure and properties analyzed by X-ray computed tomography. Fashion and Textiles, 13(1), Article 44. https://doi.org/10.1186/s40691-026-00499-2

Image Credits: AI Generated

DOI: 10.1186/s40691-026-00499-2

Keywords: conductive yarns, X-ray computed tomography, bicomponent yarn, silver-coated polyamide, twist structure, smart textiles, e-textiles, packing density, helical geometry, electrical resistance, PET filament, yarn twisting