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Page 12 of 43 Wang et al. Soft Sci 2024;4:41 https://dx.doi.org/10.20517/ss.2024.53

and graphene-based sensors for temperature and inflation monitoring have been successfully fabricated on
[91]
the surfaces of airbag catheters [Figure 4C]. In addition, Jordan et al. used aerosol jet printing to fabricate
inductively coupled radio frequency coil markers on a polymer catheter with a diameter of approximately
[13]
2 mm, achieving a printed wire width of about 250 μm .

EHD printing and electrospinning techniques involve applying a high-voltage electric field between the
nozzle and the substrate. The high electric field force pulls the ink out of the nozzle, forming a submicron-
scale jet, which improves ink deposition resolution. Zhang et al. proposed a microscale printing technique
based on electric field-driven jetting, which only requires a single potential at the nozzle electrode to form
the strong electric field, thereby eliminating the shape limitation of the substrate . Fang et al. used
[90]
electrospinning to prepare ultrafine polyaniline fibers with diameters below 5 μm, achieving excellent
[89]
performance [Figure 4D]. Peng et al. used electric field-driven printing to fabricate thin-walled tubular
mesh structures on substrates with diameters ranging from 4 to 8 mm, proposing a pre-set eccentric strategy
to mitigate the uneven electric field distribution caused by high-curvature substrates [Figure 4E].
[14]

The aforementioned techniques utilize substrate rotation equipment to adapt traditional printing methods
to high-curvature micro-cylindrical surfaces, improving the positioning accuracy of the nozzle and the
substrate to achieve higher-precision patterned structures.

Plating and coating for layer deposition
The coating/plating process is an important technique to prepare uniform and dense films around on the
surface of micro-cylindrical substrates, aimed at altering surface properties or imparting new
functionalities [9,10,56,84] . The elongated characteristics of micro-cylindrical objects make them well-suited for
coating and plating processes, which can be categorized into physical coating techniques and chemical
plating techniques based on the principle of material deposition [11,18,92-94] . Table 4 provides a comparison of
different types of coating/plating techniques.

Physical coating techniques utilize mechanical, thermal, or other physical methods to apply functional
materials uniformly to the substrate surface. For example, Ham et al. used a direct dip-coating method to
uniformly coat organic ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] material on
a 100 μm diameter Ag wire, fabricating substrate-free ferroelectric organic transistors that achieve one-
[20]
dimensional artificial multi-synapse functions [Figure 5A]. Lee et al. proposed a suspended shear dip-
coating method to attach deformable semi-solid liquid metal particles (LMP) onto fiber surfaces,
significantly enhancing the coating’s durability, conductivity, stretchability, and biocompatibility
[95]
[Figure 5B].

Compared to physical coating techniques, chemical plating involves chemical reactions during the process,
such as electrochemical, redox, and in-situ chemical reduction mechanisms. Woo et al. incorporated
hydrogel ionic diode systems onto fiber electrodes through electroplating and spin-coating, where the
uniform coating ensured device stability [Figure 5C]. Huang et al. synthesized conductive composite
[96]
fibers with AgNPs conductive sheaths via in-situ chemical reduction, embedding AgNPs within the core
fibers to form a novel sensing mechanism, leading to the development of an AgNPs/double covered yarn
(DCY) composite yarn strain sensor [Figure 5D]. Liao et al. synthesized textile-like V6O13 nanomaterials
[41]
onto aligned CNT fibers using a solution-redox method to fabricate self-charging fiber electrodes,
effectively mitigating active material detachment and cracking during deformation [Figure 5E].
[97]