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the imprinting process. When the FTIR spectra of NiHCF-MIP (BE) and NiHCF-MIP (AE) are compared, a
weak peak at ~1,118 cm is observed in the BE sample. This peak, assigned to the C-H bond of secondary
-1
alcohol structures, disappears after the cortisol-elution process . Their XRD peaks [Figure 2H] correspond
[30]
well to PDF#86-0501, which has the chemical formula of Ni[Fe(CN )] 0.667 ·H O 3.333 with a cubic phase (F43m
6
2
space group), characterized by the unit cell parameter of 10.23 Å × 10.23 Å × 10.23 Å. The main diffraction
peaks at 17.3°, 24.6°, 35.1°, and 39.4° are assigned to (200), (220), (400), and (420) planes, respectively. Their
peak positions and intensities are almost identical, indicating that the deposition of polypyrrole and the
elution of cortisol did not adversely affect the crystal structures. This ensures high stability during
subsequent processing, electrochemical measurements, and long-term practical use.

Structures and characteristics of conjugate electrospun yarns
Conjugate electrospinning technology is a promising method for the scalable and cost-effective fabrication of
biosensors. This technology enables one-step, continuous, large-scale fabrication of a ‘conductive
core-sensing cortex’ structure. In this structure, the conductive carbon core and the sensing nanofiber cortex
work synergistically to reduce charge-transfer resistance and enhance the current response of the yarn-type
sensor . This technology also synergistically enhances the interaction between the nanoparticles and the
[31]
electrospun nanofiber network. The small-sized sensing particles and electrospun nanofiber network
construct a high-surface-area architecture. This architecture increases the density of specific recognition sites
and enhances fiber surface hydrophilicity. Consequently, it promotes rapid sweat absorption. This
technology produces a cortisol-sensing yarn that is conductive, flexible, hydrophilic, and sensitive to cortisol.
Thus, the conjugate electrospinning technology was selected, which applies oppositely charged nozzles and a
trumpet-like rotating collector to electrospin nanofiber networks onto the core yarn, winding and twisting .
[32]
Herein, the electrically conductive and mechanically flexible carbon threads are utilized as the core part; the
cortisol-sensing NiHCF-MIP NCs are electrospun to form a nanofiber-network cortex with the assistance of
a hydrophilic polymer matrix [Figure 3A]. This technology enables continuous preparation of
cortisol-sensing yarns at a rate of 7.2 m/h [Figure 3B].

The surface morphology of the NiHCF-MIP cortex is shown in Figure 3C and D, from which nanofiber
networks together with nanoporous microstructure can be observed. In addition, a coaxial structure with the
conductive carbon thread and NiHCF-MIP nanofiber as core and cortex parts is demonstrated in the yarn
cross-section [Figure 3E]. Elemental surface mapping is shown in Figure 3F and Supplementary Figure 1,
from which a large number of Fe, Ni, C, N, and O elements are detected, indicating encapsulation of the
NiHCF-MIP nanofiber network around the core carbon thread.

Fast sweat absorption ability is crucial for sweat-sensing yarns. Herein, we used a type of aqueous fluorescent
dye to demonstrate the rapid water absorption process of this sensing yarn [Figure 3G and Supplementary
Movie 1]. We aspirated 10 µL of fluorescent dye using a pipette and dropped it onto the cortex of the sensor
yarn. Due to the hydrophilic groups and nanoporous structure of the sensing cortex, once 10 µL of aqueous
fluorescent dye was applied to the yarn, it completely permeated 3 cm of the sensor yarn within 5 seconds.
To further test the hydrophilicity of the nanofiber cortex, an electrospun nanofiber film from the spinning
solution was collected for WCA measurement. The WCAs were recorded by a high-speed camera [Figure 3H
and Supplementary Movie 2]. The measured WCAs were 32.55° after 33 ms and 8.52° after 100 ms,
respectively, and the water droplet was completely absorbed after 133 ms. Liquid absorption ability is mainly
related to surface chemistry and physical structures. The spinning solution contains large amounts of PAN
and F127, which are rich in -C≡N and ethylene/propylene oxide structures. Furthermore, the conjugate
electrospun cortex features a nanofiber network-like and micro/nanoporous morphology, which greatly
enhances hydrophilicity and ensures rapid sweat absorption. The Young-Laplace equation (see Eqation 3)
was used to quantitatively characterize the transport and distribution dynamics of sweat within the porous
epidermal layer of cortisol-sensing yarns: