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Page 6 of 33 Ren et al. Soft Sci. 2026, 6, 6

SLA
SLA is a vat photopolymerization process that transforms liquid photopolymer resin into solid components
by selectively curing each layer with a focused ultraviolet (UV) laser. A typical system includes a resin vat, a
recoating mechanism for creating a uniform free surface, a vertically translating build platform, beam
delivery optics for precise exposure, and control electronics for printing stability [Figure 2C] . During
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printing, the laser scans a two-dimensional (2D) pattern across the resin surface, exceeding the programmed
layer step height to chemically bond newly exposed resin with the partially cured layer below. The platform
then inverts along the Z-axis, serially repeating this process to form a 3D object . Part quality depends on
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machine parameters and resin formulation, particularly laser spot size, exposure dose, scan speed, layer
thickness, recoating kinetics, and the resin’s absorption and reactivity. Since functional group conversion is
rarely complete during layer-by-layer exposure, post-curing with UV light is typically employed to increase
crosslink density and enhance mechanical properties . Due to its fine in-plane resolution and smooth
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surfaces, SLA has become the leading approach for manufacturing complex geometries. It is increasingly
utilized for 4D printing when photoswitchable or stimulus-responsive chemicals are incorporated into the
resin.

Zhao et al. synthesized a polyurethane-acrylate-based photosensitive polymer for SLA and printed high-
precision shape-memory components . These components exhibited high fixation and recovery properties
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along with robust mechanical performance, establishing a material and process baseline for enabling 4D
responses in SLA. Shan et al. leveraged SLA to fabricate epoxy-acrylate SMPs with high resolution and
optical transparency, demonstrated tunable toughness under tension, and built a fast thermally or electrically
triggered valve actuator, linking printable resin to device-level actuation . Advancing toward clinical
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relevance, Paunović et al. used SLA to 4D-print biodegradable shape memory elastomers whose transition
points are set near physiological temperature . This enabled drug-loaded stents capable of room-
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temperature fixation, body-temperature recovery, and controlled drug release.

SLA offers multiple advantages, including high resolution and exceptional surface quality, outstanding
feature fidelity, rapid fabrication of complex internal cavities and thin-walled structures, and excellent
dimensional repeatability. With the advent of biodegradable and biocompatible resins, SLA has
demonstrated unique appeal in biomedical applications . Its limitations stem from the reliance on
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photocurable resins, whose toughness, heat resistance, and long-term stability often fall short of engineering
thermoplastics or metals. Polymerization shrinkage and residual stress may cause warpage, dimensional
deviations, and overcuring defects. Additionally, cleaning and post-curing processes are typically required,
increasing time and cost. Consequently, SLA is well-suited to high-precision, high-surface-quality parts and
master molds fabrication, but it proves less effective for high-temperature load-bearing components, high-
toughness structures, or direct micro/nano-scale manufacturing.

DLP
DLP is a vat-based photopolymerization method similar to SLA [Figure 2D]. It projects a pixelated 2D image
via a digital micromirror device or liquid crystal display, curing an entire resin layer in a single exposure .
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This full-layer exposure reduces build time, and since curing typically occurs at the bottom of the tank, the
process exhibits low sensitivity to oxygen inhibition . Curing depth and lateral resolution are jointly
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regulated by exposure dose, resin absorption rate, initiator concentration, and layer thickness. Through
parameter tuning, complex 3D components with smooth surfaces can be rapidly produced. Leveraging the
advantages of DLP technology alongside optimized and expanded ink formulations, multifunctional
microrobots have been developed for tasks such as targeted cargo delivery or environmental remediation .
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TPP
TPP, as a direct laser writing technique, utilizes a highly focused femtosecond pulsed laser to induce