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Page 14 of 43 Wang et al. Soft Sci 2024;4:41 https://dx.doi.org/10.20517/ss.2024.53

fabrication.

Equivalent technologies for efficient production
Transfer printing [21,35,45,99,100] and nanoimprinting [101,102] techniques are advanced equivalent fabrication
methods that focus on the precise replication and transfer of functional structures or flexible electronics
onto high-curvature micro-cylindrical surfaces. Both techniques offer scalable, high-fidelity reproduction of
intricate structures and are essential for integrating multifunctional electronic devices on curved substrates,
overcoming the limitations of traditional planar manufacturing processes. They enable the creation of
sensors, actuators, and other functional devices, providing a pathway for the mass efficient production of
high-performance electronics on non-planar surfaces.

Transferring flexible electronics from planar onto curved surfaces
Transfer from planar to high-curvature micro-cylindrical surfaces is based on preparing planar thin-film
devices using microelectromechanical systems (MEMS) processes for flexible electronics, which are then
wrapped onto the micro-cylindrical surfaces using molds, adhesives, and other means [21,35,45,99,100,103] . Lee et al.
used a conventional method to prepare planar flexible thin-film electronics, relying on the good ductility of
[44]
the substrate , nested with an implantable micro-cylindrical device of about 1 mm in diameter to achieve
fit by conformal bending [Figure 6A]. Liu et al. first fabricated a 1024-channel Neuroscroll probe electronic
structure using planar MEMS processes and then wrapped it around a micro-cylindrical surface using a
micro-tungsten wire as a carrier , achieving a theoretical diameter of the micro-cylindrical structure of 84
[32]
and 138 μm [Figure 6B]. Pothof et al. transferred a 64-channel circuit structure onto a SEEG probe with an
800 μm diameter using a specialized rolling mold, and employed a Cytop adhesive layer to enhance the
[104]
conformal attachment of the flexible circuit . Fiath et al. also used the same process to fabricate probe
[49]
electrodes , but innovatively designed the transfer mold to reduce processing errors when wrapping
flexible circuits onto micro-cylindrical surfaces [Figure 6C]. Schwaerzle et al. designed an optical
[105]
stimulation electrode by covering a micro-cylindrical substrate with flexible circuits , with mechanical
assembly and adhesive connections between thin-walled multilayer structures [Figure 6D]. The above
studies designed different processing molds to achieve conformal transfer to micro-cylindrical substrates,
aiming to minimize wrapping attachment errors and enhance structural adhesion. Overall, transfer
techniques demonstrate remarkable replication capabilities and offer an effective means for integrating
multifunctional electronic devices onto micro-cylindrical substrates. However, future efforts should
prioritize enhancing transfer accuracy and device consistency on high curvature surfaces to further advance
this technology.

Nanoimprinting for rapid 3D fabrication
The nanoimprinting process can replicate micro-nanostructures from molds onto high-curvature micro-
cylindrical surfaces, with imprint quality depending on the mold design and imprinting method [101,102] .
Mekaru et al. designed a sliding planar mold technique suitable for polyester fibers with a diameter of
180 μm by utilizing rolling and sliding movements [Figure 7A]. This technique addresses the fiber
[106]
twisting issue by clamping the fiber between heated metal cylinders and dynamically rotating the setup to
match the fiber’s rotation. During the process, the fiber rolls between two planar molds while being
thermally imprinted with a 50 μm deep diffraction grating pattern, achieving a manufacturing precision of
50 μm.

The cylindrical mold technique includes two innovative approaches. Wang et al. used the draw-induced
thermal drawing (DITD) method, in which a pair of rollers with the desired surface structures serve as a
template [Figure 7B]. In this process, the softened material is stretched into fibers at a high drawing speed
[38]