Two-photon polymerization-based 4D printing and its applications

Two-photon polymerization (TPP) is a cutting-edge micro/nanoscale three-dimensional (3D) printing technology based on the principle of two-photon absorption. TPP surpasses the diffraction limit in achieving feature sizes and excels in fabricating intricate 3D micro/nanostructures with exceptional resolution. The concept of 4D entails the fabrication of structures utilizing smart materials capable of undergoing shape, property, or functional changes in response to external stimuli over time. The integration of TPP and 4D printing introduces the possibility of producing responsive structures with micro/nanoscale accuracy, thereby enhancing the capabilities and potential applications of both technologies. This paper comprehensively reviews TPP-based 4D printing technology and its diverse applications. First, the working principles of TPP and its recent advancements are introduced. Second, the optional 4D printing materials suitable for fabrication with TPP are discussed. Finally, this review paper highlights several noteworthy applications of TPP-based 4D printing, including domains such as biomedical microrobots, bioinspired microactuators, autonomous mobile microrobots, transformable devices and robots, as well as anti-counterfeiting microdevices. In conclusion, this paper provides valuable insights into the current status and future prospects of TPP-based 4D printing technology, thereby serving as a guide for researchers and practitioners.


Introduction
Additive manufacturing, commonly referred to as three-dimensional (3D) printing, signifies a progressive manufacturing technology enabling the creation of 3D physical Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.objects from the digital models through the layer-by-layer addition of materials [1].This revolutionary approach has given rise to various 3D printing technologies that have revolutionized manufacturing and prototyping methodologies [2].Among these, technologies such as fused deposition modeling (FDM), digital light processing (DLP), direct ink writing (DIW), selective laser sintering (SLS), stereolithography (SLA), and two-photon polymerization (TPP) have emerged.Notably, the demand for high-resolution printing has thrust TPP technology into prominence [3].TPP achieves submicrometer resolution by selectively polymerizing a liquid As shown in figure 1(b), TPP is such a technology that fulfills this need, possessing the capability to fabricate intricate 3D structures with an exceptional printing resolution ranging from 90 nm to 500 nm [37,38].It also exhibits compatibility with a diverse range of photopolymerizable materials, including normal photoresists [39,40], hydrogels [41,42], liquid crystal elastomers (LCEs) [43][44][45], and shape memory polymers (SMPs) [46].The intricate structures exemplified in figure 1(b) underscore TPP's proficiency in extending fabrication to the micro/nanoscale.Through the integration of 4D printing and TPP technology (figure 1(c)), the resolution of printed responsive structures can be elevated to the micro/nanoscale, opening avenues for the creation of deformable or multifunctional micro/nanostructures.This expansion broadens the scope of 4D printing applications, including biomedicine [47][48][49], microrobotics [30,36], and anti-counterfeiting devices [50,51].Noteworthy applications encompass the use of TPP to fabricate hydrogel-based micromachines that can swim in biological fluids for targeted drug delivery [52,53], liquid crystalline elastomer-based walkers for autonomous surface locomotion [54], and tunable photonic devices employing SMPs for anti-counterfeiting measures [51].Evidently, TPP-based 4D printing has significantly impacted practical applications at the micro/nano scales.While certain works have summarized specific aspects or focused on specific applications of TPP-based 4D printing [34,36,55,56], systematic reviews synthesizing the latest developments and advancements in this field remain scarce.
This article comprehensively reviews the latest advancements on TPP-based 4D printing, as well as its breakthrough in applications (figure 2).In section 2, we introduce the evolution of TPP, including its historical development, working principles, and recent enhancements in printing resolution.Section 3 discusses the smart materials employed in TPP-based 4D printing, including magnetic materials, SMPs, hydrogels, and LCEs.Section 4 highlights typical applications of TPP-based 4D printing, including biomedical microrobots, bio-inspired micro actuators, autonomous mobile microrobots, transformable devices and microrobots, as well as anticounterfeiting devices.Finally, section 5 concludes the article by offering considerable perspectives on the future advancement of TPP-based 4D printing technologies.

TPP technology
TPP (2PP), also known as DLW, dip-in laser lithography, femtosecond laser writing, multiphoton SLA, and 3D laser lithography [4], represents a cutting-edge 3D printing technology that achieves sub-micrometer resolution by utilizing ultrafast lasers to selectively polymerize a liquid resin.This process relies on the theory of two-photon absorption (TPA/2PA), a nonlinear optical phenomenon initially proposed by Göppert-Mayer in 1931 [59] and experimentally confirmed by Kaiser and Garrett in 1961 [60].In its early stages, during the late 1980s and early 1990s, 2PP was employed for basic structure writing with the advent of solid-state femtosecond lasers [61,62].Presently, innovative multi-photon polymerization techniques continue to evolve [63], opening new avenues for the polymerization process.The energy level diagram for the photopolymerization process is depicted in figure 3(a), illustrating the need for a molecule to absorb sufficient energy to transition from the ground state to the excited state and subsequently return to the vibrational state, leading to molecular bond and photopolymerization.
The energy excitation of one-photon absorption (OPA/1PA) process is realized by absorbing a single photon from ultraviolet (UV) light, which occurs in the conventional photopolymerization-based 3D printing such as SLA and DLP.In contrast, the 2PA process involves the simultaneous absorption of two-photons from near-infrared (NIR) light at equal low frequency (v/2), lifting a molecule from its ground state to a higher energy level.This unique transition is not allowed for the 1PA transition, the 2PA spectrum peak in terms of frequency is usually higher than half of that of the 1PA peak.The three-photon absorption process and other multiphoton absorption are also following similar rules [63,64].Figure 3(b) illustrates that while the absorbed energy in the OPA process is linear with respect to UV light intensity I, 2PA follows a nonlinear relationship where the absorbed energy is proportional to the square of NIR light intensity I 2 .This nonlinearity empowers the laser to be tightly focused, allowing for the creation of the smallest building block of a 3D structure, referred to as the 'voxel' (volumetric pixel).Through this voxel-by-voxel process, nano/micro 3D structures can be fabricated by scanning a precisely focused laser beam.
In comparison to OPA-based 3D printing methods such as SLA and DLP, the nonlinear 2PA process is not restricted by diffraction limitations, enabling TPP to achieve subwavelength fabrication.Consequently, TPP boasts significantly higher printing resolution than SLA or DLP.Alongside 2PA, other mechanisms, such as linear absorption, super-linear absorption, avalanche effects, and thermal accumulation, can also be harnessed in photopolymerization processes.These diverse mechanisms offer varied means to initiate and control the polymerization reaction, affording versatility in the design and fabrication of 3D structures [63,64].Additionally, the scope of photopolymerization techniques has expanded beyond the use of solely NIR light, with visible (VIS) light oscillators being extensively utilized for 3D micro/nanolithography [64].
Micromachines of exceptional resolution can be realized through the application of TPP [73,74].A pivotal achievement in 2001 by Kawata and Sun demonstrated the creation of micro-bull sculptures with dimensions of 10 µm × 7 µm, marking it as the smallest artificial model produced at that time (figure 3(c)).This work demonstrated the ability of TPP to surpass the diffraction limit through nonlinear effects, achieving a sub-diffraction-limit resolution of 150 nm [66].Subsequent to this breakthrough, significant progress has been made in TPP technology, enabling the fabrication of diverse micro/nanostructures across various length scales.These include cylinder arrays (1.6 µm in height) [67], intricately designed alphabets and numbers (10 µm) [37], trefoil knot structures (36 µm) [68], buckyball structures (80 µm in diameter) [70], metastructures (80 µm) [37], stacked chiral metamaterials (80 µm) [69], compound objectives (170 µm thickness) [71], London Bridge replicas (120 µm in length) [38], and microneedle arrays with side channels (700 µm in height) [72].These advancements hold the potential to facilitate the creation of even more intricate and precise micro/nanostructures, catering  [7], with permission from Springer Nature.Adapted from [64], with permission from Springer Nature.Reprinted from [65], Copyright (2020), with permission from Elsevier.(c) Different demonstrations of available length scales for TPP technology.Reproduced from [37], with permission from Springer Nature.Reproduced from [38], with permission from Springer Nature.Reproduced from [66], with permission from Springer Nature.Reproduced from [67], with permission from Springer Nature.Reproduced from [68], with permission from Springer Nature.Reproduced from [69], with permission from Springer Nature.Reproduced from [70], with permission from Springer Nature.Reproduced from [71], with permission from Springer Nature.Reproduced from [72], with permission from Springer Nature.
The resolution, quality, and performance of a printed structure can be influenced by multiple factors.The intrinsic properties of photoresists not only affect the behavior of the resultant 3D micro/nanostructures [5,78] but also the printing parameters (e.g.laser power, exposure dose, scanning modes, speed, etc) [79,80], which in turn affect specific physical properties.Exposure conditions dictate the produced feature sizes and various characteristics, including mechanical, thermal expansion [29], and optical properties [81].To optimize the printing parameters for a given TPP printer, a characterization routine specific to different photoresists should be followed.Conversely, the formulation of a photoresist governs the performance and functionality of printed structures [13,82].A conventional photoresist formulation typically consists of photoinitiators, cross-linkers, monomers, and solvents for component dissolution [65].A recent study outlines five pivotal characteristics for desirable TPP photoresists [46]: (1) photoinitiators with sufficient two-photon absorptivity for triggering photopolymerization; (2) prompt curability within the laser focal spot to prevent overheating; (3) optical transparency within the same spectral window as the incident irradiation to avoid blockage, linear absorption, and excess heat generation; (4) optimal viscosity to prevent structural deformation during TPP; (5) capacity to create mechanically robust structures capable of enduring post-TPP development and solvent washing post-TPP.

4D printing materials for TPP
TPP technology empowers the fabrication of multifunctional micro/nanostructures by selecting appropriate photoresist materials tailored to the desired functions of the target application [83].Currently, a diverse range of photoresists is available for 4D printing [34,84], with an increasing number satisfying the five fundamental requirements for TPP outlined in the previous section.These specialized photoresists enable the creation of micro/nanostructures exhibiting dynamic properties such as stimulus responsiveness, biomimetic self-actuation, color-changing, and shape-morphing capabilities, which are beyond the reach of commercial photoresists.This section emphasizes the use of 4D printing materials suitable for TPP, and introduces them according to four categories: magnetic materials, SMPs, hydrogels, and LCEs.Table 1 presents the different material types in detail, including the specific materials, their microstructures, corresponding stimulus responses, and transformation types.

Magnetic materials
Magnetic materials, also known as magnetically responsive materials, belong to a subset of materials that exhibit a response or change in their properties when exposed to a magnetic field [126].These materials have gained prominence due to their rapid, precise, and remote response in various environments [127].A particularly intriguing application lies in magnetic microswimmers, where rotating magnetic fields induce rotational forces and torques, translating into the rotational motion of the magnetic components.Inspired by the structure of bacterial flagella, helical microswimmers are typically designed with corkscrew-like or spiral shapes.Utilizing a rotating magnetic field, magnetic microswimmers convert rotational motion into translational motion through self-helical propulsion, a phenomenon observed in low Reynolds number regimes [128].The evolution of TPP-printed magnetic fielddriven micro/nanostructures has progressed from simple helical designs to complex motion structures with flexible links and rigid segments [129], demonstrating significant potential in biomedical applications [130].Given that many TPPsuitable materials (e.g.photoresists, hydrogels, etc) lack inherent magnetic properties, achieving magnetically responsive micro/nanostructures through TPP involves integrating lightcurable soft materials with magnetic components.There are two approaches to achieving magnetically responsive micro/nanostructures through TPP incorporating magnetic materials: (1) the sequential post-processing approach: coating the magnetic material on the TPP-printed structure's surface; (2) the direct TPP printing approach: mixing magnetic nanoparticles with photoresist, and directly printing them via TPP technology.

Coating of magnetic materials.
Coating magnetic materials process involves depositing a magnetic material onto the desired substrate using various techniques.The magnetic layer adds an additional magnetic response to the coated surface, enhancing the performance in applications that require magnetic functionality.The process of selectively coating magnetic materials typically involves two steps: (1) utilizing TPP technology to create microstructures, and (2) depositing magnetic material selectively through physical vapor deposition (PVD).Tottori et al developed a straightforward fabrication approach for creating spiral micromachines by 3D DLW and PVD, enabling the creation of helical devices in versatile shapes [39].The manufacturing process is outlined in figure 4(a).First, a negative-tone photoresist (such as SU-8 or IP-L) was employed with DLW to construct helical microswimmers.In the second step, the unpolymerized photoresist was removed through development.Finally, thin bilayers of Ni/Ti were deposited onto the surface of the polymer helical micromachine using electron beam evaporation.This coating not only facilitated magnetic actuation but also enhanced the biocompatibility of the surface.Similarly, Huang et al presented a deposit-based method for fabricating micro transporters [40].As depicted in figure 4(b), utilizing SU-8 photoresist with TPP, these micromachines were fabricated horizontally.After the development, only the outer propeller was coated with Ni/Ti bilayers (300/5 nm thickness) via PVD.Meanwhile, non-actuated components were shielded by printing a sacrificial structure post-assembly.Notably, microfluidic channels and helical microswimmers were independently printed using this approach.Yasa et al employed TPP from a prepolymer solution containing PEGDA and a photoinitiator to create 3D microswimmers (figure 4(c)) [52].After printing,   the structures underwent magnetization through sequential sputter coating with nickel and gold.Subsequently, the structures were modified with thiol-modified PEG.These devices exhibit corkscrew motion when exposed to a sufficiently high input frequency from a rotating field.Introducing magnetic properties into materials can also be achieved by mixing photoresists with magnetic nanoparticles.This technique involves incorporating magnetic nanoparticles, usually at the nanoscale, into the formulation of the photoresist.These magnetic nanoparticles become dispersed within the photoresist matrix, resulting in a composite material endowed with magnetic characteristics.This strategy of combining photoresists with magnetic nanoparticles broadens the spectrum of materials capable of exhibiting magnetic behavior.It offers a versatile means to imbue photoresist-based systems with magnetic functionality, facilitating the production of magnetic microstructures with tailored attributes.The integration of magnetite nanoparticles with TPP has enabled the creation of intricate 3D soft-magnetic microdevices boasting shape-independent magnetic features.Peters et al illustrated the fabrication of single-and double-twist microswimmers featuring controlled magnetic anisotropy utilizing a superparamagnetic polymer composite comprised of Fe 3 O 4 nanoparticles embedded in commercial SU-8 negative tone photoresist [85].The fabrication process involves spin coating, particle alignment, and TPP (figure 4(d)).Additionally, Ceylan et al harnessed TPP to craft a structure with dual helical configurations designed for cargo loading and responsive swimming under the influence of a rotating magnetic field [98].As displayed in figure 4(e), these microswimmers are 3D printed from a magnetic precursor mixture containing GelMA and biofunctionalized superparamagnetic Fe 3 O 4 nanoparticles.A continuous magnetic field is applied throughout the fabrication process to maintain nanoparticle alignment.

SMPs
SMPs represent a class of smart materials that undergo controlled shape changes in response to external stimuli [131].Among these, thermos-responsive SMPs exhibit a thermally induced shape memory effect (SME) when the temperature surpasses their glass transition temperature (T g ).This remarkable property facilitates controlled transformations between permanent and temporary shapes, with the ability to return to the original permanent state via temperature adjustments.The exploration of shape transformation within SMPs has ventured into complex 3D geometries with nanoscale attributes, supporting diverse applications ranging from miniaturized deployable biomedical devices to stimuli-responsive mechanical metamaterials.An innovative breakthrough by Zhang et al introduced a novel shape memory photoresist that achieves an impressive ∼300 nm half-pitch resolution in printed features through TPP lithography [51].Their work involved the development and characterization of an SMP photoresist based on Vero Clear, formulated by combining Vero Clear with an elastomeric resist composed of HPPA, BPA, and TPO at varying mass fractions.Figure 5(a) illustrates their approach, where submicron-scale grids were programmed atop a foundation layer.These structures, featuring vertical grids, acted as architectural color filters, selectively transmitting specific visible light wavelengths.Upon heating to elevated temperatures, the structures underwent deformation, flattening the nanostructures and eliminating coloration.Cooling them to room temperature maintained the structures in an invisible state.Reheating the structure restored the original geometry and color of the nanostructures, demonstrating 4D printing at the submicron scale.
Furthermore, researchers have investigated the TPP-printed SMPs at the microscale.Spiegel et al devised a versatile ink system with a shape memory effect suitable for 4D printing at both the macro-and micro-scales, utilizing DLP and DLW technologies, respectively (figure 5(b)) [58].They identified a common functional core system, comprising appropriate monomers, crosslinkers (IsobA, PEGDA 575, and TcddA), and the photoinitiator (Irg819), that exhibited an SME and was adaptable to both printing technologies.A variety of 3D structures were printed using TPP, including basic strips, frames, and more intricate designs such as infinity rings, double platform geometries, and cubic grids at the centimeter scale.These printing endeavors achieved efficient high-resolution microscale results, with a layer height of 8-20 µm.Moreover, both the large-and small-scale printed structures demonstrated a remarkable SME.Elliott et al introduced an ink composed of acrylate/methacrylate-based compounds [96].Their ink formulation featured BMA as the first chain builder, a functionalized amine-methacrylate as the second chain builder, and synthetically converted pentaerythritol triacrylate as the crosslinker.The authors successfully demonstrated the SME in printed microstructures employing this particular material (figure 5(c)).

Hydrogels
Hydrogels are 3D crosslinked polymer networks with softness, biocompatibility, and multifunctionality, making them highly valuable in various biomedical applications [132].In the realm of 4D printing, the use of smart hydrogels is particularly advantageous due to their responsiveness to external stimuli such as temperature, humidity, pH, light, and solutions with varying ionic strengths or concentrations [133].With their swelling properties, hydrogels can give rise to deformable structures capable of intricate shape changes such as bending, folding, twisting, and multiple deformations.It is noteworthy, however, that achieving complex shape transformations requires a swelling mismatch within the actuating region of the hydrogel to generate internal stress.This is because isotropic hydrogels expand uniformly, leading to structural linear expansion [134].Moreover, in addition to the hydrogel application mentioned in section 3.1, where it serves as a matrix for magnetic materials in response to magnetic fields, this section will delve into the detailed design and fabrication of hydrogels responsive to other stimuli for TPP-based 4D printing.Among these, pH-responsive hydrogels merit attention for their ability to undergo structural or chemical changes in response to shifts in pH levels within the surrounding environment, leading to altered swelling behaviors.This characteristic renders them suitable for TPP-based 4D printing due to their ease of preparation [103][104][105][106][107][108][109][110].An advanced strategy for 4D microprinting was introduced by Jin et al, involving the creation of shape-morphing micromachines utilizing stimulusresponsive hydrogels [103].The key polymerization reaction driving the printing process is depicted in figure 6(a), while the fabrication process is illustrated in figure 6(b).By controlling parameters such as the exposure dose of the femtosecond laser, crosslinking densities, stiffness, swelling/shrinking degrees, and other properties can be finely tuned.Finiteelement methods were utilized to predict characterization and shape-changing behaviors (figure 6(b)) [104].Furthermore, Chen et al demonstrated that bilayer-based microbeams crafted from pH-responsive smart materials via TPP could induce nonmonotonic bending deformations through a sequential size-dependent layer-by-layer swelling effect [105].
Light-responsive hydrogels [112,113], thermo-responsive hydrogels [113][114][115][116], and solvent-responsive hydrogels [117][118][119] are also pivotal components in two-photon 4D printing.Light-responsive hydrogels respond to light, enabling remote and precise spatial and temporal control.Deng et al devised a 4D printing methodology to create light-triggered micromachines with unique 3D shape-changing capabilities [112].Through an investigation of carbon nanotube-doped NIPAM composite smart hydrogels, they highlighted distinct responsiveness degrees among structural units to light under a constant laser power (figure 6(c)).Additionally, they designed micropillars with a predetermined 45 • bending angle, arranged them circularly, and thereby fabricated a smart microclamp device (figure 6(d)).Thermo-responsive hydrogels offer ease of operation and tunable temperature range responses.An innovative approach by Spratte et al employed pNIPAM-based microactuator systems designed with precision via DLW technology [114].Their study comprehensively explored the shrinkage and swelling properties of these systems, placing emphasis on actuator size, design variations, and DLW parameter influence on material actuation.Figure 6(e) illustrates the thermally driven shrinkage of pNIPAM hydrogel microstructures with different architectures.Solvent-responsive hydrogels, while allowing reversible shape and size changes, can exhibit slow response times unsuitable for certain applications.Ennis and colleagues leveraged the flexibility of 2PP to develop a novel photoresist based on phenylboronic acid that responds to sugar [118].As shown in figure 6(f), successful microstructure creation from sugar-responsive hydrogels using TPP is demonstrated,  while figure 6(g) elucidates the mechanism behind hydrogel swelling.

LCEs
LCEs stand out as a class of smart materials that exhibit substantial, anisotropic, and reversible shape changes in response to multiple stimuli.With their lightly crosslinked networks boasting oriented mesogenic orientation, LCEs have become a favored choice for realizing responsive structures [135].Notably, LCEs have gained momentum in research due to their capacity to function without reliance on an aqueous environment or external loads [136].These materials hold promise across diverse applications, including soft actuators and robots, artificial muscles, active structures, adaptive optics, and energy dissipators.Since the advent of TPP-based 4D printing technology, LCE actuators with more intricate 3D geometries, higher resolution, and an extended size range [137] have come to fruition.
Among the various stimuli, heat has emerged as a widely employed method, rendering thermos-responsive LCEs an optimal candidate for diverse applications.Del Pozo et al pioneered the development of a liquid crystalline photoresist, yielding a densely crosslinked polymer network that facilitates the creation of 4D-fabricated microactuators with predetermined shape changes (figure 7(a)) [50].These 3D microstructures respond to temperature fluctuations by demonstrating reversible, anisotropic shape expansions and distinct polarization colors that hinge on the structure's geometry rather than the DLW-TPP parameters (figure 7(b)).Guo et al introduced a method for crafting microscale heterogeneous LCEs with controllable and uncoupled 3D structures through the assembly of microscale LCE voxel building blocks [120].The anisotropic properties of LCEs in distinct directions enable the creation of devices with diverse physical properties, facilitated by employing 3D initial structures and director fields (figure 7(c)).Moreover, Del Pozo et al detailed the use of a supramolecular cholesteric LCE photonic photoresist to fabricate 4D photonic microactuators [121].The integration of self-ordering smart materials with DLW-TPP facilitates the realization of dualresponsive 3D microstructures, exhibiting controlled expansion and corresponding color changes in response to variations in humidity and temperature.
Light serves as a popular stimulus due to its precise spatial and temporal control.TPP enables the intricate design of 3D robots with nanometer-scale precision.Martella et al introduced a light-driven microhand capable of remote control or autonomous action based on optical properties (figure 7(d)) [122].The chosen splayed alignment prompts the bending of the four orthogonal fingers upon stimulus application (figure 7(e)).Chen et al developed a direct laser printable photoresist incorporating gold nanorods to enhance mechanical properties and achieve NIR-responsive mechanical deformation [124].Zeng et al fabricated an artificial microwalker equipped with LCE muscle (figures 7(f) and (g)) using acrylic resin (IP-Dip) for the limbs due to its high Young's modulus.The LCE muscle exhibits a fully reversible mechanical response to light, contracting by approximately 20% along the direction of the nematic LCE network [54].Hsu et al proposed a straightforward strategy (figure 7(h) for crafting multi-photoresponsive 3D microstructures activated by different light wavelengths, leveraging an aligned LC ink formulation and dyes with orthogonal absorptions [125].By successfully integrating five distinct dyes into the LC microstructure, they demonstrated the versatility of their approach, enabling actuation across varying wavelength ranges.The combination of different dyes allows the fabrication of multiresponse LC microactuators, offering the flexibility to tailor responses as needed.

Applications of TPP-based 4D printing
4D printing is a programmed transformation of the 3D printed structure in shape, property, and functionality.It has the capability of realizing shape-morphing, multi-functionality, self-assembly, and self-repair.It is printer-independent, timedependent, and programmable [14].The emergence of TPP-based 4D printing technology holds the promise of revolutionizing various fields, including robotics, biomedicine, and nanotechnology, in the near future.This section classifies applications based on structural evolutions and changes, such as shape morphing, color alteration, state switching, and locomotion.Consequently, the potential applications of TPPbased 4D printed structures can be grouped into five categories: biomedical micromachines, bioinspired microactuators, autonomous mobile microrobots, transformable devices and robots, and anti-counterfeiting microdevices.
Magnetically propelled micro/nanorobots have ushered in new possibilities for diverse biomedical applications [127,129].Of particular interest are magnetic helical micromachines, renowned for their capacity to generate corkscrew motion under the influence of rotating magnetic fields.The combination of self-propelled mechanisms and controlled navigation renders them highly suitable for a multitude of environments and applications [128].A significant advancement is the helical micromachine developed by Tottori et al, featuring a microholder and helical body capable of controlled swimming and cargo transport in a frequency magnetic field [39].The transportation process of colloidal microparticles involves four stages: approaching, loading, transporting, and releasing (figure 8(a)).Cabanach et al created zwitterionic 3D-printed microrobots with nonimmunogenic properties to evade immune cell recognition [53].Through functionalization with inorganic magnetic nanoparticles and surface modification for various functionalities (figure 8(b), these microrobots interact with immune cells, leading to either capture upon detection or release if not detected (figure 8(c)).Ceylan et al presented an integrated strategy for designing and fabricating a biodegradable microswimmer capable of therapeutic and diagnostic release in vitro [98].Responsive to the pathological marker matrix metalloproteinase-2 (MMP-2), this microswimmer rapidly releases embedded cargo molecules upon detecting pathological enzyme concentrations (figure 8(d)).A theranostic application scenario for biomedical imaging post-therapeutic transfer procedures utilizing microswimmers is envisioned in figure 8(e).
Furthermore, beyond magnetically actuated microswimmers, several other micro/nanorobots have found utility in active biomedical delivery systems.Huang et al devised wirelessly controlled microtransporters employing an Archimedes screw pumping mechanism for controlled microparticle and magnetic nanohelix transport within microfluidic channels [40].The process of loading and releasing is succinctly depicted in figure 8(f), where the revolving screw draws fluid and particles into the capsule while the piston seals the capsule's back to trap the particles.Upon reverse actuation, the piston slides into an open state, and the ensuing reverse flow expels the trapped particles.Figure 8(g) outlines the four stages of transporting and delivering helical microstructures: (I) loading, (II) transporting, (III) releasing, and (IV) deployment.Villangca et al introduced an advanced light-triggered microrobot for cargo loading and release, employing photothermally driven convection currents [91].Designed as a miniature container for material transportation, this microtool features an anterior opening for cargo manipulation (figure 8(h).The spherical handles enable optical selection, trapping, manipulation, and real-time reorientation.Through the traversal of a heating beam across the microtool, cargo ejection is facilitated via the perturbation of the microbubble, leading to the expulsion of fluid containing particles (figure 8(i)).Figure 8(j) illustrates the collection and release of captured particles, showcasing the potential of this novel light-driven robot in nanofluidic injection and targeted drug delivery applications.

Bioinspired microactuators
TPP-based 4D printing technology has facilitated the creation of bioinspired microactuators that emulate the movements and functions of natural organisms to accomplish specific tasks.These intelligent actuators incorporate stimuliresponsive materials, enabling them to perform precise and controlled grasping and releasing actions.Drawing inspiration from the human hand, Martella et al developed a lightpowered microhand capable of remote or autonomous operation through optical illumination [122].This microhand selectively captures target microelements based on their unique absorption properties.Figure 9(a) illustrates the optical control that transitions the hand from an open to a closed state.This concept can be extended to object recognition based on distinct absorption spectra.Similarly, Zhang et al devised a TPP-based approach for crafting intelligent and flexible 3D microactuators using a common photopolymer material [117].By encoding 3D microstructures with carefully designed networks, these actuators demonstrate predictable deformations in response to specific stimuli.This is achieved through meticulous control of voxel size and distribution at the nanoscale.The manipulation of microclaws for collecting and releasing target microspheres is shown in figure 9(b).Wang et al drew inspiration from the flytrap mechanism to demonstrate an asymmetric 4D-printed hydrogel microactuator with intelligent response properties [107].By directly writing a responsive photoresist with a femtosecond laser, they fabricated a biometric microactuator.The adjustment of manufacturing parameters such as power density and scanning route yields controllable driving behavior.As depicted in figure 9(c), pH-triggered shape changes are harnessed to achieve and finetune the microactuator's gripping and releasing behavior for micro-objects.
Liu et al utilized TPP-based 4D microprinting to achieve bidirectional and reversible self-assembly of microstructures driven by capillary force [92].The engineered curvature, thickness, and smooth morphology of the fabricated microstructures enable transitions between a vertical and curved state.These microstructures exhibit three reversible states in diverse evaporating solvents: (I) a curved state, (II) distance-dependent coalescing clusters through conventional self-assembly, and (III) distance-independent random selfassemblies oriented in curving directions.The foundational concept of programmed self-assembly is illustrated through the 4D-printed micro butterfly wings in figure 9(d).Deng et al introduced a 4D printing method utilizing NIPAMbased hydrogels and asymmetric mechanical metamaterial unit cells to achieve precise and complex 3D geometry deformation of light-driven micromachines [112].This technique enables the creation of high-performance smart micromachines without intricate shape programming and material integration.Figure 9(e) depicts a biomimetic aortic valve microstructure, demonstrating rapid and reliable lightstimulus response and control.The valve remains closed when hydrated and promptly opens upon light stimulation due to the hydrogel's photothermal conversion efficiency and thermal conductivity.Figure 9(f) indicates that the amplitude of the semilunar valve opening can be controlled by varying the power of light stimulation.These bioinspired microactuators have the potential to revolutionize various fields by offering advanced capabilities for precise and efficient manipulation and control of microscale objects and systems.

Autonomous mobile microrobots
Autonomous mobile microrobots are gaining attention due to their inherent advantages, such as miniaturization, autonomous movement, high efficiency, and remote control.These microrobots demand sophisticated functionalities, including navigation, localization, path planning, and motion control, which can be realized through TPP-based 4D printing.This technology empowers the creation of intricate structures with precise control over size and shape, leading to microrobots with advanced capabilities such as self-propulsion and navigation.Moreover, the utilization of biocompatible materials renders them suitable for biomedical applications.The potential of autonomous mobile microrobots to revolutionize diverse industries by offering an efficient microscale task execution avenue is undeniable.
Biomimetic-legged locomotion serves as one strategy to achieve autonomous mobility in microrobots, drawing inspiration from natural organisms such as insects or arachnids to develop microrobots capable of walking, crawling, or jumping.These microrobots generally feature multiple legs that coordinate to generate propulsion.Inspired by inchworm crawls, Chen et al designed bionic multilegged microcrawlers with enhanced locomotion capabilities, incorporating smart microjoints (SMJs) via 4D DLW technology [105].These microcrawlers exhibit adjustable locomotion through autonomous swelling and shrinking, as well as sequential swelling effects layer-by-layer, as illustrated in figure 10(a).Figure 10(b) shows the locomotion range versus time curves for four crawling cycles.With programmable structures and the integration of SMJ functions, the microcrawler demonstrates versatile motion types and exceptional crawling performance.Figure 10(c) illustrates the crawling steps of microcrawlers with three SMJs sequentially responding to timedependent pH stimuli, transitioning from an acidic to an alkaline environment.Zeng et al introduced a light-propelled microscopic walker with LCE muscles crafted through DLW using acrylic resin to create limbs with high elastic moduli E [54].This synthetic walker can autonomously move on diverse surfaces and undergo reversible shape-morphing under modulated laser beams.Its locomotion is governed by design and environmental interactions, enabling arbitrary or directional walking, rotation, and jumping (figure 10(d)).
Modular assembly presents another avenue for achieving autonomous mobility in microrobots.This method involves integrating external modules, such as magnetic propulsion and electrostatic actuation, to achieve the desired motion.Such an approach allows for greater flexibility in designing and customizing microrobots for specific tasks and adaptability to changing environments.Hu et al engineered soft magnetic micromachines composed of interconnected microactuator networks, linking Janus microparticle-based magnetic microactuators using TPP [144].Through magnetic field-based torques, each microactuator is positioned at an intended location and orientation and then linked with temporarily fixed counterparts via 3D-printed soft or rigid linkages.These connected 2D microactuator networks facilitate programmable 2D and 3D shape changes, as demonstrated by limbless and limbed untethered micromachine models showcasing diverse robotic gaits for surface movement.Figure 10(e) illustrates the movement of a lizard-inspired walking microrobot with rigid legs, displaying surface movement under an oscillating field.Meanwhile, the soft-legged robot exhibits programmed robotic gaits under the alternating field.Alapan et al demonstrated how shape-encoded programmed interactions between structural and motor components could dynamically assemble mobile micromachines into expected configurations [145].By encoding dielectrophoretic interactions in the 3D geometry of each unit, they successfully assembled micromachines featuring magnetic and self-propelled motor parts.Various micromachines are hierarchically assembled through shapeencoded dielectrophoretic (DEP) interactions, with a twostage assembly between micromachines (figure 10(f)).The second unit assembles with the first unit in the second stage due to the weak electric field beneath its ledges (figure 10(g)).

Transformable devices and robots
The emergence of transformable devices and robots introduces a category of intelligent machines that can modify both shape and function in response to varying environmental stimuli.Jin et al pioneered 4D-DLW technology utilizing phototunable stimuli-responsive hydrogels to fabricate reconfigurable micromachines with exceptional mechanical properties [103].Unlike bilayered structures, this approach permits extensive shape transformations.Figure 11(a) illustrates diverse reconfigurable micromachines, such as microcages, microstents, and microumbrellas, demonstrating their capacity for rapid, precise, uniaxial and biaxial contraction, reversible changes, and articulated bar folding.The 4D-DLW process involves (1) integrating deformation amplification mechanisms, (2) design, (3) prediction, (4) laser writing, and (5) shape morphing.In response to environmental pH alterations, these structures can function as actuators, transitioning between open and closed positions.Inspired by modular robots and Lego-like blocks, Huang et al introduced a programmable design approach to directly construct 3D reconfigurable microstructures, enabling sophisticated shape transformations using stimuli-responsive hydrogel micro-blocks created through TPP-based 4D printing [104].Varying crosslinked polymer networks are achieved via  casting, and electrodeposition processes [146].They devised a novel cage (or helix) configuration of interconnected rods, resulting in structures with predictable behaviors.These structures are adaptable and can be reshaped through passive or preprogrammed means initiated by an external magnetic field.Figure 11(b) depicts the interlinked helical cage-rod rings that exhibit both in-plane rotation and out-of-plane tumbling motions.An aspect ratio structure with multiple magnetic elements (figure 11(c)) allows for advancing, rolling, and trajectory alteration upon manipulation of the external magnetic field.The rolling mechanism enables out-of-plane tumbling and rolling motion in silicone oil and deionized (DI) water (figure 11(d)).Similarly, in cases where the cage intersects with the bar through in-plane rotation, the structure undergoes rotation without significant net locomotion.The agility of rolling robots in maneuvering obstacles is depicted in figure 11(e), highlighting their adeptness in combining forward rolling and out-of-plane axis rotation.This innovative approach demonstrates the potential of transformable devices and robots in navigating complex and dynamic scenarios.

Anti-counterfeiting microdevices
4D printing based on TPP technology offers remarkable precision, flexibility, and customization capabilities, positioning it as a pivotal player in the advancement of anti-counterfeiting microdevices.These microdevices are designed to safeguard products and brands from counterfeit activities through the incorporation of distinctive attributes such as specific patterns, colors, or concealed messages.Del Pozo et al devised liquid crystalline photoresist microstructures endowed with controlled deformation capabilities and distinct polarization colors for real-time identification and authentication [50].The hexagonal arrays engineered in this study exhibited altered anisotropy upon heating within the 20 • C-220 • C range, resulting in perpendicular expansion and parallel shrinkage to the alignment direction (figure 12(a)).Figure 12(b) displays microstructures demonstrating distinct polarization colorswooden pile structures exhibiting yellow hues and spiral disks displaying a diverse array of colors.The thermal response of the 3D woodpile and spiral disk microstructures led to variations in structure thickness and birefringence, corresponding to changes in optical path difference responsible for polarized color (figure 12(c)).These findings hold promise for the integration of these microstructures into sensing and anticounterfeiting microdevices.
Zhang et al introduced a method to manipulate the optical response of nanostructures by modifying the refractive index, exemplified by grids with size-tunable multi-colors designed to explore shape memory effects through customized nanoscale structure deformation [51].Diverse colors can be achieved by adjusting the printing parameters.Figure 12(d) shows the printed color palette in its printed, compressed, and recovered states, each exhibiting distinct colors.SEM images of the original, programmed, and recovered states are illustrated in figure 12(e).As a demonstration, the authors printed artwork featuring an octopus and a mountainous landscape.The programmed print transitioned from translucent and featureless to its original state upon heating (figure 12(f)).This platform's exceptional resolution printing capabilities and remarkable reversibility offer potential applications in the creation of temperature-sensitive labels, anticounterfeiting devices, and adaptable photonic products.Moreover, TPP technology has enabled the generation of angle-insensitive coloration in biomimetic nanostructures, inspired by the blue Morpho butterfly [95].Through the design of disordered structures, researchers achieved reduced angle dependence, setting them apart from traditional photonic crystals.The manipulation of processing parameters facilitated hue control.Given that these biomimetic structures exhibited structural colors comparable to biological and bioinspired structures in distinct liquid environments [147] or demonstrated heightened selectivity in vapor response [148], this technology provides a potential application for the development of anticounterfeiting microdevices.

Summary and outlook
This paper provides a comprehensive overview of TPP-based 4D printing technology, which can fabricate high-resolution and transformable 3D structures at the micro/nanoscale.This paper aims to summarize the current advancements of the TPP-based 4D printing technology and its associated applications.It commences by elucidating the technological advancements of TPP-based 4D printing, delineating its foundational working principle and recent progress.Additionally, the review encapsulates the strides achieved in smart materials harnessed for TPP-based 4D printing.Last, the paper accentuates the quintessential applications of TPP-based 4D printing, including the realms of biomedical microrobots, bioinspired microactuators, autonomous mobile microrobots, transformable microrobots, and anti-counterfeiting devices.
Despite the recent rapid development, the evolution of TPP-based 4D printing into a more potent 3D micro/nanofabrication tool faces several challenges.(1) Manufacturing capability: augmenting manufacturing efficiency and capacity stands as a pivotal prerequisite for the widespread integration of TPP-based 4D printing technology across diverse industries.The development of equipment capable of cross-scaling and multimaterial micro/nanomanufacturing is a central challenge.This necessitates enhancements in printing speed, scalability, and accuracy.(2) Material performance: the selection and characteristics of photoresists wielded in TPP-based 4D printing bear paramount importance in realizing desired structural transformations and functional proficiencies.Innovating and refining photoresists with elevated chemical, thermal, and mechanical attributes, including robustness, pliancy, and endurance, constitutes a critical endeavor.Moreover, the formulation of functional composite materials can broaden the scope of applications.Ensuring harmonious coexistence between materials and the printing process is pivotal for achieving top-tier, dependable printed structures.(3) Design methodology: synthesizing process−material−structure−function design emerges as a pivotal impetus for advancing TPP-based 4D printing technology.Methodologies such as topological optimization and machine learning can be harnessed to simultaneously refine the printing process, material choice, and structural design.This holistic approach facilitates the creation of adeptly tailored micro/nanostructures replete with desired functionalities.Through the application of advanced design techniques, researchers can chart novel trajectories toward the attainment of intricate, multifunctional structures via TPP-based 4D printing.Overcoming these challenges emerges as a critical enabler for the widespread adoption and progression of TPP-based 4D printing.
Undoubtedly, the merits intrinsic to TPP-based 4D micro/nanofabrication herald revolutionary prospects for basic research and product engineering across diverse domains.In light of this perspective, we anticipate that further research endeavors will be directed toward surmounting current technical hurdles.This includes the exploration of novel printing strategies and optimization methodologies to bolster manufacturing velocity, pioneering innovative TPP-compatible materials, refining material formulations to enhance performance, and devising ingenious approaches to enhance the structural robustness of printed objects.Overcoming these obstacles holds the key to unlocking the maximal potential of TPP-based 4D printing, propelling its assimilation in myriad domains.Furthermore, interdisciplinary synergies with other fields possess the potential to unveil novel frontiers and applications, spanning from healthcare and robotics to electronics and aerospace.

Figure 2 .
Figure 2. Overview of TPP-based on 4D printing methodologies, materials, stimuli and their relevant applications.

Figure 3 .
Figure 3. Working principle and demonstrations of TPP.(a) Energy level diagram for the photopolymerization process of one-photon excitation, two-photon excitation and multi-photon excitation.(b) Comparison of the excitation volume by OPA from UV light versus 2PA from NIR light.[4] John Wiley & Sons.© 2022 The Authors.Advanced Science published by Wiley-VCH GmbH.Adapted from[7], with permission from Springer Nature.Adapted from[64], with permission from Springer Nature.Reprinted from[65], Copyright (2020), with permission from Elsevier.(c) Different demonstrations of available length scales for TPP technology.Reproduced from[37], with permission from Springer Nature.Reproduced from[38], with permission from Springer Nature.Reproduced from[66], with permission from Springer Nature.Reproduced from[67], with permission from Springer Nature.Reproduced from[68], with permission from Springer Nature.Reproduced from[69], with permission from Springer Nature.Reproduced from[70], with permission from Springer Nature.Reproduced from[71], with permission from Springer Nature.Reproduced from[72], with permission from Springer Nature.

Figure 5 .
Figure 5. TPP-based 4D printing shape memory polymer.(a) Illustration of color and shape changes in 'invisible ink' nanostructured elements.Reproduced from [51], with permission from Springer Nature.(b) SMP ink system for 4D printing at the macro/microscale.[58] John Wiley & Sons.© 2022 The Authors.Advanced Functional Materials published by Wiley-VCH GmbH.(c) A diagram and SEM images of the cylinder's shape memory programming cycle, Scale bar 5 µm.[96] John Wiley & Sons.© 2020 Wiley-VCH GmbH.

Figure 6 .
Figure 6.TPP-based 4D printing hydrogels.(a) The main materials and polymerization reactions involved in the TPP process.(b) Illustration of the 4D-DLW process of pH-responsive hydrogel.Reprinted from [103], Copyright (2020), with permission from Elsevier.(c) Microcolumnar cilia have different bending elongations under different light stimulation powers.(d) The transformation process of the printed microclamp with different light stimulation powers in an aqueous environment.[112] John Wiley & Sons.© 2023 Wiley-VCH GmbH.(e) Thermal shrinkage of pNIPAM hydrogel microstructures with different structures.[114] John Wiley & Sons.© 2022 The Authors.Advanced Materials Technologies published by Wiley-VCH GmbH.(f) Chemical structures of photoresist components and illustration of the fabrication procedure of the sugar-triggered hydrogel.(g) Swelling mechanism of a sugar-responsive hydrogel.[118] John Wiley & Sons.© 2023 The Authors.Advanced Functional Materials published by Wiley-VCH GmbH.

Figure 7 .
Figure 7. TPP-based 4D printing Liquid crystal elastomers.(a) Illustration of the fabrication procedure for thermal-driven uniaxially aligned 3D microstructures.(b) Hexagonal plate array in 3D profiles at 20 • C and 220 • C. [50] John Wiley & Sons.© 2021 The Authors.Small Structures published by Wiley-VCH GmbH.(c) Thermal actuation of a cuboid LCE frame with 80 voxels.Reproduced from [120], with permission from Springer Nature.(d) Schematic of a microhand and mesogen alignment before and after stimulus applied.[122] John Wiley & Sons.© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(e) Mechanism of the reversible NIR-driven shape morphing behavior of the AuNR/LCE.Reprinted with permission from [124].Copyright (2019) American Chemical Society.(f) Fabrication of the microscopic walker.(g) SEM image of a microwalker and their behavior of light trigger on and off.The scale bar is 10 µm.[54] John Wiley & Sons.© 2015 The Authors.Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(h) Fabrication process of multiphoto-responsive actuators.[125] John Wiley & Sons.© 2022 The Authors.Advanced Materials Technologies published by Wiley-VCH GmbH.

Figure 8 .
Figure 8. TPP-based 4D printing micromachines for biomedical applications.(a) Schematic of the transport process by a spiral micromachine with a microholder.[39] John Wiley & Sons.© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(b) 3D-printed multifunctional zwitterionic microrobots.(c) Illustration of cell-robot interaction.[53] John Wiley & Sons.© 2020 The Authors.Published by Wiley-VCH GmbH.(d) Cargo release of microswimmers in response to pathological concentrations.(e) Envisioned theranostic application scenario.Reprinted with permission from [98].Copyright (2019) American Chemical Society.(f) Illustration of the loading and releasing mechanism.(g) Different phases of the transfer and delivery of spiral microstructures.[40] John Wiley & Sons.© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(h) The structure and optical operation of the micro-tools.(i) Loading of cargo inside the micro-tool.(j) Eject the trapped particles through the microbubble as the light-controlled piston.Reproduced from [91], with permission from Springer Nature.

Figure 10 .
Figure 10.TPP-based 4D printing autonomous mobile microrobots.(a) The fundamentals of designing inchworm-like crawling actuators.(b) Structural deformation driven by the pH stimulation.(c) Movement of the reptile-inspired soft microcrawler.[105] John Wiley & Sons.© 2020 The Authors.Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(d) Locomotion behaviors of light-fueled microscopic walkers.[54] John Wiley & Sons.© 2015 The Authors.Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(e) Surface locomotion of the lizard-inspired walking microrobot with rigid legs.From [144].Reprinted with permission from AAAS.(f) Concept of the 3D micro-vehicle body with four-wheel compartments.(g) Arrangement of the magnetic microactuators into the wheel compartments.(h) The micro-vehicle undergoes translation in response to a vertically rotating magnetic field.Reproduced from [145], with permission from Springer Nature.

Figure 11 .
Figure 11.TPP-based 4D printing transformable devices and robots.(a) Demonstrations of 3D-to-3D reconfigurable micro-structures (microstent, microcage and micro-umbrella).Reprinted from [103], Copyright (2020), with permission from Elsevier.(b) Rotated in-plane and tumble out-of-plane movement of a cage-bar-ring structure connected to five helices.(c) The aspect ratio structure with multiple magnetic elements can move forward, roll and change direction by switching the external magnetic field.(d) Implementation of an out-of-plane tumbling and rolling motion in silicone oil and DI water.(e) Rolling robot can quickly change direction and use different locomotion modes to avoid obstacles.Reproduced from [146], with permission from Springer Nature.

Figure 12 .
Figure 12.TPP-based 4D printing anti-counterfeiting microdevices.(a) Hexagonal plate array 3D profiles at 20 • C and 220 • C. The orientation of the mesogens is shown by the red arrow.The 20 • C plate contour is depicted by the blue dashed lines.(b) Wooden pile and spiral disk 3D microstructures with uniaxial alignment.The CAD designs are on the left.Optical micrographs of the constructed structures without and with crossed polarizers are shown on the right.(c) The 3D wooden pile (top) and spiral disk (bottom) microstructures' temperature responses.The CAD design and collections of crossed polarized optical micrographs taken at various temperatures are shown from left to right.[50] John Wiley & Sons.© 2021 The Authors.Small Structures published by Wiley-VCH GmbH.(d) With the objective lens in transmission mode, different colors are observed when printing, compressing, and recovering.(e) Tilted and top views of SEM images of the original state, programmed state, and after-recovery state.(f) The painting as printed, compressed, and recovered, respectively.Reproduced from [51] with the permission of Macmillan Publishers Limited, part of Springer Nature, Copyright 2021.

Table 1 .
Summary of the optional materials for TPP-based 4D printing.