Femtosecond Laser 4D Printing of Light-Driven Intelligent Micromachines

Intelligent micromachines that respond to external light stimuli have a broad range of potential applications, such as microbots, biomedicine, and adaptive optics. However, artificial light-driven intelligent micromachines with a low actuation threshold, rapid responsiveness, and designable and precise 3D transformation capability remain unachievable to date. Here, a single-material and one-step 4D printing strategy are proposed to enable the nanomanufacturing of agile and low-threshold light-driven 3D micromachines with programmable shape-morphing characteristics. The as-developed carbon nanotube-doped composite hydrogel simultaneously enhanced the light absorption, thermal conductivity, and mechanical modulus of the crosslinked network, thus significantly increasing the light sensitivity and response speed of micromachines. Moreover, the structural design and assembly of asymmetric microscale mechanical metamaterial unit cells enable the highly efficient additive nanomanufacturing of 3D shape-morphable micromachines with large dynamic modulation and spatiotemporal controllability. Using this strategy, the world's smallest artificial beating heart with programmable light-stimulus responsiveness for the cardiac cycle is successfully printed. This 4D printing method paves the way for the construction of multifunctional intelligent micromachines for bionics, drug delivery, integrated microsystems, and other fields.     

Biomimetic 4D-Printed Breathing Hydrogel Actuators by Nanothylakoid and Thermoresponsive Polymer Networks

Shape-morphing actuators, which can breathe with the accompany of morphology changes to mimic botanical events, are challenging to fabricate with soft hydrogel materials. Herein, 4D printed-smart hydrogel actuators are reported that can not only dynamically deform but also generate oxygen (O₂) upon external stimulations. The printed breathing actuators featured with spinach leaf-derived thylakoid membrane (nanothylakoid) for photothermal conversion and catalytical O₂ evolution, a poly(N-isopropylacrylamide) (PNIPA) thermoresponsive polymer network for generating deformation forces by swelling/shrinkage (rehydration/dehydration), and an asymmetric bilayer poly(N-isopropylacrylamide)/polyacrylamide (PNIPA/PAA) structure to amplify the mechanical motions. Upon thermal stimulation or laser irradiation, the actuator can reversibly bend/unbend because of the photothermal conversion of nanothylakoid and the printed thermoresponsive asymmetric bilayer structure. Additionally, the catalase-like property of nanothylakoid imparts the actuator with O₂ evolution capability to breathe for further mimicking botanical systems. Notably, 4D printing can greatly facilitate and simplify the actuator fabrication process, including adjusting the size and layer compositions. This artificial breathing actuator with photothermal and catalytical properties provides a strategy in designing intelligent hydrogel systems and proves to be a highly promising material candidates in the fields of 3D/4D printing, automated robotics, and smart biomedical devices.     

Laser-Induced Graphene Tapes as Origami and Stick-On Labels for Photothermal Manipulation via Marangoni Effect

Direct light-to-work conversion enables remote actuation through a non-contact manner, among which the photothermal Marangoni effect is significant for developing light-driven robots because of the diversity of applicable photothermal materials and light sources, as well as the high energy conversion efficiency. However, the lack of nanotechnologies that enable flexible integration of advanced photothermal materials with actuators of complex configurations significantly restricts their practical applications. In this paper, laser-induced graphene (LIG) tape is reported as stick-on photothermal labels for developing light-driven actuators based on the Marangoni effect. With the help of direct laser writing technology, graphene patterns with superior photothermal properties are prepared on the PI tape. The patterned LIG tape can be stuck on any desired objects and generates an asymmetric photothermal field under light irradiation, forming a photothermal Marangoni actuator. Additionally, the PI tape with LIG patterns can be folded into 3D origami actuators that permit photothermal Marangoni actuation including both translation and rotation. The graphene-based photothermal Marangoni actuators feature biocompatibility, which is confirmed by MDA-MB-231 cells proliferation experiments. Owing to the excellent photothermal property of LIG patterns, the as-produced photothermal actuators can be manipulated by a variety of light sources, holding great promise for developing light-driven soft robots.     

Carbon‐Based Photothermal Actuators

Actuators that can convert environmental stimuli into mechanical work are widely used in intelligent systems, robots, and micromechanics. To produce robust and sensitive actuators of different scales, efforts are devoted to developing effective actuating schemes and functional materials for actuator design. Carbon‐based nanomaterials have emerged as preferred candidates for different actuating systems because of their low cost, ease of processing, mechanical strength, and excellent physical/chemical properties. Especially, due to their excellent photothermal activity, which includes both optical absorption and thermal conductivities, carbon‐based materials have shown great potential for use in photothermal actuators. Herein, the recent advances in photothermal actuators based on various carbon allotropes, including graphite, carbon nanotubes, amorphous carbon, graphene and its derivatives, are reviewed. Different photothermal actuating schemes, including photothermal effect–induced expansion, desorption, phase change, surface tension gradient creation, and actuation under magnetic levitation, are summarized, and the light‐to‐heat and heat‐to‐work conversion mechanisms are discussed. Carbon‐based photothermal actuators that feature high light‐to‐work conversion efficiency, mechanical robustness, and noncontact manipulation hold great promise for future autonomous systems.     

Photothermal-Enhanced Uranium Extraction from Seawater: A Biomass Solar Thermal Collector with 3D Ion-Transport Networks

Access to uranium resources is critical to the sustainable development of nuclear energy. The ocean contains abundant uranium resources, but the marine biological pollution and the low concentration of uranium make it a giant challenge to extract uranium from seawater. On the foundation of selective uranium adsorption using high uranium-affinity groups, realizing the external-field improved uranium capture without extra energy consumption is highly attractive. A solar thermal collector with 3D ion-transport networks based on environmentally friendly biomass adsorption material is reported, which contains antibacterial adsorption ligands and photothermal graphene oxide. The antibacterial ability through an easy one-step reaction and the fast mass transfer caused by photothermal conversion collaboratively improve the original adsorption capacity of the hydrogel by 46.7%, reaching 9.18 mg g⁻¹ after contact with natural seawater for 14 days. This study provides a universal strategy for the design of physical-fields-enhanced hydrogel adsorbents.     

Multi-Stimuli-Responsive Weldable Bilayer Actuator with Programmable Patterns and 3D Shapes

Soft actuators can harvest environmental energy and convert it into kinetic energy for motions like bending, twisting, stretching, and contracting. However, it remains challenging to design soft film actuators for complex and programmable deformation in three dimensions. Herein, a weldable and patternable multi-stimuli-responsive bilayer soft actuator is developed by a mask-assisted spraying coating process, and its 3D geometries are achieved by welding the sodium alginate (SA) layer using water. The intrinsic hygroscopicity of SA film and the magnetic and photothermal properties of Fe₃O₄ nanoparticles enable reversible deformation of the bilayer actuator under three different external stimuli: moisture, magnetic field, and sunlight. Based on these properties, a variety of multi-stimuli-responsive intelligent devices are developed including smart curtains, smart grippers, biomimetic walkers, rolling actuators, swimmers, and windmill rotators. All these actuating stimulations are derived from naturally renewable energy without the consumption of any artificial energy, providing important enlightenment for green and sustainable applications of soft actuators.     

Monolithic Dual‐Material 3D Printing of Ionic Skins with Long‐Term Performance Stability

Artificial “ionic skin” is of great interest for mimicking the functionality of human skin, such as subtle pressure sensing. However, the development of ionic skin is hindered by the strict requirements of device integration and the need for devices with satisfactory performance. Here, a dual‐material printing strategy for ionic skin fabrication to eliminate signal drift and performance degradation during long‐term use is proposed, while endowing the ionic skins with high sensitivity by 3D printing of ionic hydrogel electrodes with microstructures. The ionic skins are fabricated by alternative digital light processing 3D printing of two photocurable precursors: hydrogel and water‐dilutable polyurethane acrylate (WPUA), in which the ionically conductive hydrogel layers serve as soft, transparent electrodes and the electrically insulated WPUA as flexible, transparent dielectric layers. This novel dual‐material printing strategy enables strong chemical bonding between the hydrogel and the WPUA, endowing the device with designed characteristics. The resulting device has high sensitivity, minimal hysteresis, a response time in the millisecond range, and excellent repetition durability for pressure sensing. The results demonstrate the potential of the dual‐material 3D printing strategy as a pathway to realize highly stable and high‐performance ionic skin fabrication to monitor human physiological signals and human–machine interactions.     

Construction of Chitosan‐Based Hydrogel Incorporated with Antimonene Nanosheets for Rapid Capture and Elimination of Bacteria

To elaborately construct a novel and efficient photothermal antibacterial nanoplatform is a promising strategy for treating bacterial wound infections. In this work, a composite hydrogel (CS/AM NSs hydrogel) with outstanding antibacterial ability is constructed by incorporating antimonene nanosheets (AM NSs) with extraordinary photothermal properties into the network structure of chitosan (CS). When cultured with bacteria, the CS/AM NSs hydrogel can gather bacteria on the surface through the interaction of CS with the bacterial cell membrane. Subsequently, the intrinsic bactericidal property of CS will kill some of the bacteria. After the introduction of near‐infrared laser, the AM NSs effectively convert light energy into localized heat to eliminate residual bacteria. By virtue of the synergistic action between the capture effect of CS and the photothermal effect of AM NSs, the CS/AM NSs hydrogel shows predominant antibacterial behavior against Escherichia coli and Staphylococcus aureus. In vitro assay and in vivo tests of infected full‐thickness defect wound healing confirm the satisfactory biocompatibility and antibacterial ability. Overall, this work reveals that the CS/AM NSs hydrogel holds great potential as a broad‐spectrum antibacterial wound dressing for treating bacteria‐infected wounds. Additionally, this is the first report of the application of AM NSs in the field of antibacterial treatment.     

Bioinspired Anisotropic Hydrogel Actuators with On–Off Switchable and Color‐Tunable Fluorescence Behaviors

An effective approach to develop a novel macroscopic anisotropic bilayer hydrogel actuator with on–off switchable fluorescent color‐changing function is reported. Through combining a collapsed thermoresponsive graphene oxide‐poly(N‐isopropylacrylamide) (GO‐PNIPAM) hydrogel layer with a pH‐responsive perylene bisimide‐functionalized hyperbranched polyethylenimine (PBI‐HPEI) hydrogel layer via macroscopic supramolecular assembly, a bilayer hydrogel is obtained that can be tailored and reswells to form a 3D hydrogel actuator. The actuator can undergo complex shape deformation caused by the PNIPAM outside layer, then the PBI‐HPEI hydrogel inside layer can be unfolded to trigger the on–off switch of the pH‐responsive fluorescence under the green light irradiation. This work will inspire the design and fabrication of novel biomimetic smart materials with synergistic functions.     

Highly Salt-Resistant 3D Hydrogel Evaporator for Continuous Solar Desalination via Localized Crystallization

The emerging solar desalination by interfacial evaporation shows great potential for alleviating the global freshwater crisis. However, salt deposition on the whole evaporation surface during steam generation leads to a deterioration in the evaporation rate and long-term stability. Herein, it is demonstrated that a hydrogel-based 3D structure can serve as an efficient and stable solar evaporator by salt localized crystallization for high-salinity brine desalination. Under the function of micron-grade brine transport management and edge-preferential crystallization promoted by this novel design, this 3D hydrogel evaporator exhibits a superior salt-resistant property without salt deposition on the photothermal surface even in 20 wt% brine for continuous 24-h illumination. Moreover, by virtue of the synergistic effect of the promising 3D structure and excellent water transport of hydrogel, the proposed evaporator possesses an excellent evaporation performance achieving 2.07 kg m⁻² h⁻¹ on average in a high-salinity brine (from 10 to 25 wt% NaCl) under 1 sun irradiation, among the best values reported in the literature. With stable and efficient evaporation performance out of high-salinity brine, this design holds great potential for its applications in sustainable solar desalination.     

Actuation of Three-Dimensional-Printed Nanocolloidal Hydrogel with Structural Anisotropy

Polymer hydrogels exhibit actuation properties that result in reversible shape transformations and have promising applications in soft robotics, drug delivery systems, sensors, and microfluidic devices. Actuation occurs due to differential hydrogel swelling and is generally achieved by modulating hydrogel composition. Here a different approach to hydrogel actuation that originates solely from its structural anisotropy is reported. For 3D-printed single-layer hydrogels formed by cellulose nanocrystals (CNCs) and gelatin methacryloyl it is shown that shear-induced orientation of CNCs results in anisotropic mechanical and swelling properties of the hydrogel. Upon swelling in water, planar hydrogels acquire multiple complex 3D shapes that are achieved by i) varying CNC orientation with respect to the shape on the hydrogel sheet and ii) patterning the hydrogel with the regions of shear-mediated and random CNC orientation. This study shows the capability to generate multiple shapes from the same hydrogel actuator based on the degree of its structural anisotropy. In addition, it introduces a biocompatible nanocolloidal ink with shear-thinning and self-healing properties for additive manufacturing of hydrogel actuators.     

Printing Patterned Fine 3D Structures by Manipulating the Three Phase Contact Line

The preparation of fine 3D microstructures is an attractive issue; however, it is limited at large‐area fabrication process and fineness morphology manipulation. Here, we propose a strategy to fabricate controllable 3D structures and morphologies from one single droplet via ink‐jet printing. Based on the surface energy difference between the hydrophilic patterns and hydrophobic surface, the three phase contact line of a droplet contained nanoparticles is forced to pin on the patterned hydrophilic points and asymmetrically dewets on the hydrophobic surface, which leads to various morphologies. Through the regulation of pinning patterns and solution properties, the 3D morphology can be well manipulated. This strategy to control the 3D morphology of nanoparticle assembly based on hydrophilic patterns would be of great importance for fabricating controllable 3D structures.     

An Autonomous Soft Actuator with Light‐Driven Self‐Sustained Wavelike Oscillation for Phototactic Self‐Locomotion and Power Generation

Mimicking the intelligence of biological organisms in artificial systems to design smart actuators that act autonomously in response to constant environmental stimuli is crucial to the construction of intelligent biomimetic robots and devices, but remains a great challenge. Here, a light‐driven autonomous carbon‐nanotube‐based bimorph actuator is developed through an elaborate structural design. This curled droplet‐shaped actuator can be simply driven by constant white light irradiation, self‐propelled by a light‐mechanical negative feedback loop created by light‐driven actuation, time delay in the photothermal response along the actuator, and good elasticity from the curled structure, performing a continuously self‐oscillating motion in a wavelike fashion, which mimics the human sit‐up motion. Moreover, this autonomous self‐oscillating motion can be further tuned by controlling the intensity and direction of the incident light. The autonomous actuator with continuous wavelike oscillating motion shows immense potential in light‐driven biomimetic soft robots and optical‐energy‐harvesting devices. Furthermore, a self‐locomotive artificial snake with phototaxis is constructed, which autonomously and continuously crawls toward the light source in a wave‐propagating manner under constant light irradiation. This snake can be placed on a substrate made of triboelectric materials to realize continuous electric output when exposed to constant light illumination.     

Sustainable Droplet Manipulation on Ultrafast Lubricant Self-Mediating Photothermal Slippery Surfaces

Designing intelligent slippery surfaces for droplet manipulation is critical for many applications from drug delivery to bio-analysis, while is of great challenging in sustainability for inescapable wastage of lubricant layer. Herein, an ultrafast lubricant self-mediating (self-replenishing/-absorbing) photothermal slippery surface is designed that achieves sustainable transport of droplet under the irradiation of near infrared light (NIL) even if the lubricant layer is wiped clean completely, as well as at other man-made extreme conditions. The ultrafast lubricant self-mediating performance is caused by synergistic effects of interconnection of porous structure and photothermal expansion of the material. When lubricant on surface is lost, photothermal expansion of material can quickly squeeze the lubricant inside the base to flow into and out of the interconnected porous structure to generate a fresh lubricant layer. Attractively, when the NIL is turned off, the rebuilt lubricant layer can be swiftly self-absorbed into the porous to inhibit unnecessary wastage. Moreover, an arbitrary split of droplet in desired configurations can be achieved by controlling the NIL irradiating route. This sustainable droplet manipulation induced by ultrafast lubricant self-mediating can be extensively applied in microfluidics and micro-reactor settings.     

Broad-Wavelength Light-Driven High-Speed Hybrid Crystal Actuators Actuated Inside Tissue-Like Phantoms

Research on molecular crystals exhibiting light-driven actuation has made remarkable progress through the development of various molecules and the identification of driving mechanisms. However, crystals developed to date have been driven mainly by ultraviolet (UV) or blue light irradiation, and driving by red or near-infrared (NIR) light has not been attempted yet. Herein, a broad-wavelength light-driven molecular crystals that exhibit high-speed bending by photothermal effect is developed. Titanium carbide (Ti₃C₂T_x) MXene nanosheets are integrated into salicylideneaniline crystals to extend the wavelength range that causes photothermally driven bending to UV, visible, and NIR light. In addition, unlike the thin pristine molecular crystals that show slow photoisomerization-induced bending only under UV light, the MXene layer enables the molecular crystals to be actuated rapidly regardless of their thickness over a wide range of wavelengths. The hybridization of molecular crystals with MXene, which exhibits strong biocompatibility as well as NIR light-driven photothermal effect, allows for the bending of the hybrid crystals inside agar phantoms mimicking biological tissue. Last, it is confirmed that MXene hybridization can be extended to common molecular crystals including various salicylideneaniline and anisole derivatives.     

Vertical 3D Printed Forest-Inspired Hierarchical Plasmonic Superstructure for Photocatalysis

Efficient light-harvesting is of significant importance to achieve high solar energy utilization efficiency for various solar-driven technologies. Compared with a 2D planar structure, a 3D plasmonic structure can largely increase the light adsorption/interaction areas and also utilizes the plasmonic effect to achieve much higher light utilization efficiency. However, this remains challenging in terms of structural design, reliable manufacturing, and ability to scale up. Herein, inspired by the light absorption strategy of natural forests, a hierarchical plasmonic superstructure is demonstrated composed of vertical TiO₂ pillar arrays (as tree trunks), dense nanorod arrays (as branches), and a large number of plasmonic Au nanoparticles (as leaves). Such a forest-like plasmonic superstructure can effectively absorb light from the surface plasmonic resonance effects of Au nanoparticles and the multiple scattering of light in the hierarchical branched structure. The strong light absorption and abundant photocatalytic active sites help yield a 15-fold higher nitrogen photo-fixation activity than that of the flat TiO₂ films decorated with Au nanoparticles. The study provides an effective strategy to construct 3D plasmonic superstructures with excellent light-harvesting efficiency and high stability and can be readily applied to a range of light-driven applications     

Bio-Inspired Stimuli-Responsive Ti3C2Tx/PNIPAM Anisotropic Hydrogels for High-Performance Actuators

Near-infrared (NIR) light-responsive hydrogels have the advantages of high precision, remote control and excellent biocompatibility, which are widely used in soft biomimetic actuators. The process by which water molecules diffuse can directly affect the deformation of hydrogel. Therefore, it remains a serious challenge to improve the response speed of hydrogel actuator. Herein, an anisotropic photo-responsive conductive hydrogel is designed by a directional freezing method. Due to the anisotropy of the MXene-based PNIPAM/MXene directional (PMD) hydrogel, its mechanical properties and conductivity are enhanced in a specific direction. At the same time, with the presence of the internal directional channels and the assistance of capillary force, the PMD hydrogel can achieve a volume deswelling of 70% in 2 s under light irradiation, further building a hydrogel actuator with a fast response performance. Additionally, the hydrogel actuator can lift an object 40 times its weight by a distance of 6 mm, realizing the advantages of both rapid responsiveness and high driving strength, which makes the hydrogel actuator have important application significance in remote control, microflow valve, and soft robot.     

Physicochemical Properties in 3D Hydrogel Modulate Cellular Reprogramming into Induced Pluripotent Stem Cells

Understanding the biophysical relationships between stem cells and applied biomaterials can facilitate the ability to control the functions and behaviors of stem cells. However, the role of 3D microenvironment in stem cell biology remains largely unexplored, compared with that of 2D cell-culture environment. Here, a new strategy that improves the efficacy of Yamanaka's four-factor-induced cellular reprogramming into induced pluripotent stem cells (iPSCs) by incorporating cues derived from the 3D microenvironment and biophysical ligands is reported. Among the various 3D hydrogel systems tested, methacrylated hyaluronic acid (HA) hydrogel significantly improves cellular reprogramming into iPSCs. Additionally, the initial upregulation of CD44 in encapsulated cells in low-level methacrylated soft HA hydrogel accelerates the reprogramming. In conclusion, the reported HA hydrogel with low modulus accelerates reprogramming into iPSCs and thus offers potential advantages for translational applications.     

A Multidirectional Locomotion Light-Driven Soft Crawling Robot

Soft robots based on bionics with multi-freedom and communication abilities have attracted extensive attention in recent years. However, the solutions for soft robots with multidirectional locomotion currently concentrate on complex drive modes and exhibit application unfriendliness. In this work, an untethered multidirectional locomotion light-driven soft crawling robot is proposed with the integration of communication module, which can traverse in four directions with a fixed near-infrared (NIR) light source and is also capable of positioning and perception. Owing to the photothermal response of graphene oxide and ingenious structural design, the critical states of robot deformation can be determined simply by controlling the duration of NIR light, ultimately resulting in different crawling directions. Furthermore, a communication module is integrated into the robot enabling the robot to locate and sense humidity by magnetic coupling. The proposed robot provides an innovative strategy for the design and integration of multidirectional locomotion soft crawling robots, showing great potential in intelligent robots.     

An inchworm mobile robot using electromagnetic linear actuator

The capability of motion is one of the important aspects for a micro-robot to fulfill its given tasks. Micro-autonomous systems usually require large force, large displacement and less power consumption. Among different actuation schemes, electromagnetic actuator shows the benefit in a combination of force, displacement and cost effective control. A bristle-based inchworm mobile robot using a short stroke electromagnetic linear actuator is described in the paper. The main body and movable unit of the robot are joined by using a sealed bellows and the bristle legs are designed so that it can operate both on plane surfaces and in liquid. The actuator designed for the robot is a tubular type linear machine with an overall size of Φ7 × 10 mm. The key dimensions of the actuator were determined through magnetic field analysis to achieve optimum force output and necessary travel stroke within the limited space. The predicted actuation force of the actuator is 20 mN and the stroke length is 1.2 mm. Two working prototypes of the actuator were constructed and the performance tests show the effectiveness of the design. A sensorless control scheme with a novel start-up strategy for the designed actuator was developed based on the robotic system modeling and the analyzed results show the satisfactory performance of the system.