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Wang et al. Soft Sci 2024;4:41 https://dx.doi.org/10.20517/ss.2024.53 Page 7 of 43

on manufacturing strategies to transition from planar substrates to high-curvature micro-cylindrical
substrates. Due to the high curvature, high aspect ratio, and flexibility of micro-cylindrical surfaces,
researchers have developed various techniques, including conformal lithography, laser processing, inkjet
printing, coating and plating technologies, flexible electronics transfer, and nanoimprinting. These
techniques enable the fabrication of different components, such as pixels, wires, and films, on high-
curvature micro-cylindrical surfaces, combining them into functional structures for active stimulation,
passive sensing, and insulating encapsulation. Table 2 summarizes the pros and cons of each method,
highlighting their suitability for specific applications and detailing the comparative advantages and
limitations of each technique. Based on material and structural formation, micro-cylindrical-specific
manufacturing technologies can be further categorized into three types: precision material subtractive
techniques based on exposure etching and high-energy laser ablation, enabling ultra-high-resolution
microstructure formation; additive deposition techniques through dipping, coating, physical/chemical
plating, and 3D printing, suitable for large-scale conformal manufacturing; and 3D structure equivalent
technologies using transfer and imprinting, offering advantages for efficient production.

Subtractive technologies for high-resolution microstructures
Photolithography [9,20,72-74] and laser processing [75-77] are the common subtractive techniques to achieve
microstructure fabrication and patterning, for their ability to precisely manipulate light or laser beams to
create high-resolution features on substrates, particularly in complex applications such as high-curvature
micro-cylindrical surfaces. They share a common focus on accuracy and high spatial resolution, and can be
adapted for diverse material systems by fine-tuning process parameters such as energy intensity,
wavelength, and beam focus. As key enablers of precision patterning and functionalization, these processes
are vital for manufacturing intricate electronic structures on high-curvature surfaces.

Conformal lithography by selective exposure
Photolithography is a viable strategy for fabricating ultra-high-resolution features, and adapting traditional
planar photolithography to high-curvature micro-cylindrical surfaces is of significance and challenge. The
key to micro-cylindrical surface photolithography lies in minimizing the patterning mapping errors on
high-curvature surfaces [9,20,72,73] . Various photolithography schemes have been developed for both mask-
based and mask-less photolithography, enabling selective exposure and removal of photosensitive materials
to meet the patterning requirements of high-curvature micro-cylindrical objects.

Mask-based photolithography utilizes high-precision masks and optimized exposure setups to reduce
exposure mapping errors on micro-cylindrical surfaces, leveraging rotational exposure systems for effective
lithography. Some studies focus on improving exposure equipment and strategies to adapt to micro-
[40]
cylindrical objects. For example, Yang et al. used a cylindrical projection lithography system to pattern the
surface of a 1 mm diameter capillary substrate, developing rotational exposure equipment to scan the mask
projection along and around the substrate [Figure 2A]. Similarly, Toshiyuki et al. used rotary scanning
projection lithography on a 2.5 mm diameter substrate with an edge-width slit mask to project high-
[78]
performance imaging , scanning a spiral pattern on the substrate [Figure 2B]. Other studies have taken
advantage of the conformal properties of flexible masks and substrates. Doll et al. patterned an 8 mm
diameter small cylindrical surface using a chromium-plated polymer flexible mask , reducing exposure
[79]
errors through the conformal characteristics of the elastic polymer with the substrate [Figure 2C]. Park et al.
successfully fabricated high aspect ratio copper structures on a 20 mm diameter micro-cylindrical object
using a flexible SU-8 lithographic mask, incorporating a planar micro-slit mask to prevent undesirable
exposure from soft lithography [Figure 2D]. High-resolution and conformal mask design and fabrication
[80]
were applied to minimize mapping errors.