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Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 9 of 35

PEDOT:PSS is the most researched CP due to its electrical and mechanical performance [85,86] . Jiang et al.
designed a modified version of PEDOT:PSS to achieve high conductivity, stretchability, and
[87]
photopatternability by introducing a topological supramolecular network . A supramolecular additive with
a polyrotaxane structure composed of a polyethylene glycol (PEG) backbone and cyclodextrins (CDs) side
chains was blended into PEDOT:PSS [Figure 3A, left]. The PEG backbone induced aggregation of PEDOT,
which improved electrical conductivity. The CDs improved mobility along the direction of the polymer
chain, enhancing stretchability. Consequently, PEDOT:PSS blended with the supramolecular additive
exhibited two orders of magnitude higher conductivity [Figure 3A, right] and could be stretched up to over
100% without cracking. A stretchable electrode array could be fabricated, which was conformally attached to
human skin to capture EMG signals.

In another example, Tan et al. prepared a self-adhesive conductive polymer (SACP) composite based on
PEDOT:PSS . The SACP was fabricated by homogeneously mixing an elastic polymer (PVA crosslinked
[83]
with glutaraldehyde) and a supramolecular solvent (citric acid and CD) with an aqueous solution of
PEDOT:PSS [Figure 3B, left top]. Strong interfacial adhesion of the SACP arose from multiple interactions,
including hydrogen bonds, ionic interactions, and Van der Waals interactions [Figure 3B, left bottom]. The
SACP could be prepared as a conductive film by spin-coating or drop-casting. The SACP film exhibited
-1
conductivities ranging from 1 to 37 S·cm . Also, the SACP film could be conformally attached to the skin,
thereby offering conductive human-device interfaces [Figure 3B, right].

Rapid solidification of CPs in solvents was a major obstacle to fabricating CP fibers using wet spinning
[88]
techniques . Fang et al. reported wet-spun, ultrafine PANI fibers by adopting a solvent exchange strategy
[Figure 3C, left] . Generally, PANI was doped with camphor sulfonic acid in cresol for conductivity .
[84]
[89]
However, doped PANI was incompatible with conventional wet spinning protocols due to its rapid
solidification into thick gels, which is induced by strong interactions between doped CPs. By replacing
cresol with dimethyl formamide (DMF), however, PANI could be well dispersed. PANI gel protofibers with
a low viscosity below 3,000 cP could be obtained, and subsequently, PANI fibers with a diameter below
5 μm were drawn. Ultrafine PANI fibers with maximized electroactive surfaces exhibited superb CSCs and
unprecedented mechanical strength of 1,080 MPa [Figure 3C, right].

Further conductivity enhancement was achieved by synthesizing 1D nanostructured CP hydrogel, as
demonstrated by Wang et al. . They developed a PPy hydrogel with nanofiber-like structures using a disc-
[90]
shaped dopant, copper phthalocyanine-3,4′,4″,4′′′-tetrasulfonic acid tetrasodium salt (CuPcTs). Steric and
electrostatic interactions between CuPcTs and PPy induced in situ self-assembly of PPy, resulting in 1D
growth of PPy chains [Figure 3D, left]. The 1D morphology and porous structure promoted the transfer of
electrons and ions, which improved the electrochemical property and electrical conductivity. The
PPy-CuPcTs hydrogel exhibited lower impedance and two orders of magnitude higher conductivity
-1
(7.8 S·cm ) than pristine PPy [Figure 3D, right]. Moreover, considering that the water content of the
hydrogel was as high as 94 wt%, CP hydrogels showed great potential as biosensors owing to their excellent
solution processability and high permeability to ions and molecules.

The use of conductive polymers may raise several concerns regarding their biocompatibility. These include
the potential cytotoxicity of the polymers and their degradation byproducts, the risk of allergic reactions in
some individuals, the long-term stability of the polymers and their impact on biocompatibility over
extended periods, the mechanical compatibility of the materials with the skin to avoid discomfort or injury,
and the influence of surface characteristics on interactions with the skin, such as bacterial adhesion or tissue
irritation [91,92] . Addressing these concerns requires a multidisciplinary approach involving material design,