Reconfigurable Microlens Array Enables Tunable Imaging Based on Shape Memory Polymers.

Microlens arrays (MLAs) with a tunable imaging ability are core components of advanced micro-optical systems. Nevertheless, tunable MLAs generally suffer from high power consumption, an undeformable rigid body, large and complex systems, or limited focal length tunability. The combination of reconfigurable smart materials with MLAs may lead to distinct advantages including programmable deformation, remote manipulation, and multimodal tunability. However, unlike photopolymers that permit flexible structuring, the fabrication of tunable MLAs and compound eyes (CEs) based on transparent smart materials is still rare. In this work, we report reconfigurable MLAs that enable tunable imaging based on shape memory polymers (SMPs). The smart MLAs with closely packed 200 × 200 microlenses (40.0 µm in size) are fabricated via a combined technology that involves wet etching-assisted femtosecond laser direct writing of MLA templates on quartz, soft lithography for MLA duplication using SMPs, and the mechanical heat setting for programmable reconfiguration. By stretching or squeezing the shape memory MLAs at the transition temperature (80 °C), the size, profiles, and spatial distributions of the microlenses can be programmed. When the MLA is stretched from 0 to 120% (area ratio), the focal length is increased from 116 to 283 µm. As a proof of concept, reconfigurable MLAs and a 3D CE with a tunable field of view (FOV, 160-0°) have been demonstrated in which the thermally triggered shape memory deformation has been employed for tunable imaging. The reconfigurable MLAs and CEs with a tunable focal length and adjustable FOV may hold great promise for developing smart micro-optical systems.

Photothermal responsive slippery surfaces based on laser-structured graphene@PVDF composites.

Photothermal responsive slippery surfaces with switchable superwettability are promising in the fields of biomedicine, self-cleaning, anti-corrosion, and lab-on-a-chip systems. However, the development of a light switchable slippery surface that combines high-performance photothermal materials with hierarchical microstructures of special orientation remains challenging, which limits the applications in anisotropic droplet manipulation. Herein, we demonstrate a photothermal responsive slippery surface based on laser-structured graphene and polyvinylidene difluoride composites (L-G@PVDF) for controllable droplet manipulation. The L-G@PVDF film exhibits high light absorption (∼95.4%) in the visible and NIR region. After lubricating with paraffin, the resultant surface shows excellent self-healing ability and light-responsive wettability change due to the photothermal effect of L-G@PVDF and the hot melting effect of paraffin. Additionally, by introducing anisotropic grooved structures, the paraffin-infused L-G@PVDF surface displays anisotropic wettability that further affects droplet manipulation under light irradiation. Also, the photothermal responsive slippery property endows the paraffin-infused L-G@PVDF surface with excellent anti-frosting and de-icing capability. Moreover, the smart paraffin-infused L-G@PVDF surface can be combined with a microfluidics chip for light-driven automatic sampling. This study offers insight into the rational design of photothermal responsive slippery surfaces for controllable droplet manipulation.

Heterogeneous self-healing assembly of MXene and graphene oxide enables producing free-standing and self-reparable soft electronics and robots.

Self-healing materials (SHMs) with unique mechanical and electronic properties are promising for self-reparable electronics and robots. However, the self-healing ability of emerging two-dimensional (2D) materials, for instance, MXenes, has not been systematically investigated, which limits their applications in self-healing electronics. Herein, we report the homogeneous self-healing assembly (homo-SHA) of MXene and the heterogeneous self-healing assembly (hetero-SHA) of MXene and graphene oxide (GO) under moisture treatments. The self-healing mechanism has been attributed to the hydration induced interlayer swelling of MXene and GO and the recombination of hydrogen bond networks after water desorption. The multiform hetero-SHA of MXene and GO not only enables facile fabrication of free-standing soft electronics and robots, but also endows the resultant devices with damage-healing properties. As proof-of-concept demonstrations, free-standing soft electronic devices including a generator, a humidity sensor, a pressure sensor, and several robotic devices have been fabricated. The hetero-SHA of MXene and GO is simple yet effective, and it may pioneer a new avenue to develop miniature soft electronics and robots based on 2D materials.

Reconfigurable, Reversible, and Redefinable Deformation of GO Based on Quantum-Confined-Superfluidics Effect.

Graphene oxide (GO) films with natural "quantum-confined-superfluidics" (QSF) channels for moisture actuation have emerged as a smart material for actuators and soft robots. However, programming the deformation of GO by engineering QSF nanochannels around 1 nm is extremely challenging. Herein, we report the reconfigurable, reversible, and redefinable deformation of GO under moisture actuation by tailoring QSF channels via moisture-assisted strain-induced wrinkling (MSW). The shape fixity ratio of a general GO film can reach ∼84% after the MSW process, and the shape recovery ratio is ∼83% at room temperature under moisture actuation. The flexible shaping and deformation abilites, as well as the self-healing property of GO make it possible to fabricate soft robots using GO. Besides, as a proof-of-concept, passive electronics and soft robots capable of crawling, turning, switching circuit, and automatic somersault are demonstrated. With unique shaping and deformation abilities, GO may bring great implications for future soft robotics.

Miniature optoelectronic compound eye camera.

Inspired by insect compound eyes (CEs) that feature unique optical schemes for imaging, there has recently been growing interest in developing optoelectronic CE cameras with comparable size and functions. However, considering the mismatch between the complex 3D configuration of CEs and the planar nature of available imaging sensors, it is currently challenging to reach this end. Here, we report a paradigm in miniature optoelectronic integrated CE camera by manufacturing polymer CEs with 19~160 logarithmic profile ommatidia via femtosecond laser two-photon polymerization. In contrast to µ-CEs with spherical ommatidia that suffer from defocusing problems, the as-obtained µ-CEs with logarithmic ommatidia permit direct integration with a commercial CMOS detector, because the depth-of-field and focus range of all the logarithmic ommatidia are significantly increased. The optoelectronic integrated µ-CE camera enables large field-of-view imaging (90°), spatial position identification and sensitive trajectory monitoring of moving targets. Moreover, the miniature µ-CE camera can be integrated with a microfluidic chip and serves as an on-chip camera for real-time microorganisms monitoring. The insect-scale optoelectronic µ-CE camera provides a practical route for integrating well-developed planar imaging sensors with complex micro-optics elements, holding great promise for cutting-edge applications in endoscopy and robot vision.

Multicoating Nanoarchitectonics for Facile Preparation of Multi-Responsive Paper Actuators.

Stimuli-responsive actuators (SRAs) that can harvest environmental energies and convert them to mechanical works without additional energy-supplying systems have revealed great potential for robotic applications. However, at present, the practical usage of SRAs is significantly limited due to the problems with respect to solo responsiveness, simple deformation, and the difficulties for large-scale and cost-effective production. In this paper, multi-responsive paper actuators with multicoating nanoarchitectonics that enable complex deformation have been fabricated through a very simple painting process on common papers. The resultant paper actuator permits large-scale and low-cost production (A4 size: ∼0.5 dollar). The paper actuators that consist of a paper/graphite/polydimethylsiloxane sandwich structure can be actuated by multi-form stimuli, including moisture, temperature, light, and volatile organic compounds. More importantly, the bending deformation of the paper actuators can be further programmed by controlling the pencil drawing orientation, providing the feasibility of performing more complex deformations. Several multi-responsive paper actuators, including organic compound-responsive smart devices working in the liquid environment, moisture-enabled terrestrial crawling actuator, and a light-responsive attitude-control actuator integrated with an airplane model, have been demonstrated. The development of multi-responsive yet cost-effective paper actuators may hold great promise for a wide range of practical applications, for instance, soft micro-electromechanical systems, lab-on-a-chip systems, smart homes, and robotics.

Electro-responsive actuators based on graphene.

Electro-responsive actuators (ERAs) hold great promise for cutting-edge applications in e-skins, soft robots, unmanned flight, and in vivo surgery devices due to the advantages of fast response, precise control, programmable deformation, and the ease of integration with control circuits. Recently, considering the excellent physical/chemical/mechanical properties (e.g., high carrier mobility, strong mechanical strength, outstanding thermal conductivity, high specific surface area, flexibility, and transparency), graphene and its derivatives have emerged as an appealing material in developing ERAs. In this review, we have summarized the recent advances in graphene-based ERAs. Typical the working mechanisms of graphene ERAs have been introduced. Design principles and working performance of three typical types of graphene ERAs (e.g., electrostatic actuators, electrothermal actuators, and ionic actuators) have been comprehensively summarized. Besides, emerging applications of graphene ERAs, including artificial muscles, bionic robots, human-soft actuators interaction, and other smart devices, have been reviewed. At last, the current challenges and future perspectives of graphene ERAs are discussed.

Bioinspired Soft Robots Based on the Moisture-Responsive Graphene Oxide.

Graphene oxide (GO), which has many oxygen functional groups, is a promising candidate for use in moisture-responsive sensors and actuators due to the strong water-GO interaction and the ultrafast transport of water molecules within the stacked GO sheets. In the last 5 years, moisture-responsive actuators based on GO have shown distinct advantages over other stimuli-responsive materials and devices. Particularly, inspired by nature organisms, various moisture-enabled soft robots have been successfully developed via rational assembly of the GO-based actuators. Herein, the milestones in the development of moisture-responsive soft robots based on GO are summarized. In addition, the working mechanisms, design principles, current achievement, and prospects are also comprehensively reviewed. In particular, the GO-based soft robots are at the forefront of the advancement of automatable smart devices.

Light-Driven Magnetic Encoding for Hybrid Magnetic Micromachines.

Remote manipulation of a micromachine under an external magnetic field is significant in a variety of applications. However, magnetic manipulation requires that either the target objects or the fluids should be ferromagnetic or superparamagnetic. To extend the applicability, we propose a versatile optical printing technique termed femtosecond laser-directed bubble microprinting (FsLDBM) for on-demand magnetic encoding. Harnessing Marangoni convection, evaporation flow, and capillary force for long-distance delivery, near-field attraction, and printing, respectively, FsLDBM is capable of printing nanomaterials on the solid-state substrate made of arbitrary materials. As a proof-of-concept, we actuate a 3D polymer microturbine under a rotating magnetic field by implementing γ-Fe2O3 nanomagnets on its blade. Moreover, we demonstrate the magnetic encoding on a living daphnia and versatile manipulation of the hybrid daphnia. With its general applicability, the FsLDBM approach provides opportunities for magnetic control of general microstructures in a variety of applications, such as smart microbots and biological microsurgery.

Dynamics of Strong Coupling Between Free Charge Carriers in Organometal Halide Perovskites and Aluminum Plasmonic States.

We have investigated a strong coupled system composed of a MAPbIxCl3-x perovskite film and aluminum conical nanopits array. The hybrid states formed by surface plasmons and free carriers, rather than the traditional excitons, is observed in both steady-state reflection measurements and transient absorption spectra. In particular, under near upper band resonant excitation, the bleaching signal from the band edge of uncoupled perovskite was completely separated into two distinctive bleaching signals of the hybrid system, which is clear evidence for the formation of strong coupling states between the free carrier-plasmon state. Besides this, a Rabi splitting up to 260 meV is achieved. The appearance of the lower bands can compensate for the poor absorption of the perovskite in the NIR region. Finally, we found that the lifetime of the free carrier-SP hybrid states is slightly shorter than that of uncoupled perovskite film, which can be caused by the ultrafast damping of the SPs modes. These peculiar features on the strong coupled hybrid states based on free charge carriers can open new perspectives for novel plasmonic perovskite solar cells.

Femtosecond laser programmed artificial musculoskeletal systems.

Natural musculoskeletal systems have been widely recognized as an advanced robotic model for designing robust yet flexible microbots. However, the development of artificial musculoskeletal systems at micro-nanoscale currently remains a big challenge, since it requires precise assembly of two or more materials of distinct properties into complex 3D micro/nanostructures. In this study, we report femtosecond laser programmed artificial musculoskeletal systems for prototyping 3D microbots, using relatively stiff SU-8 as the skeleton and pH-responsive protein (bovine serum albumin, BSA) as the smart muscle. To realize the programmable integration of the two materials into a 3D configuration, a successive on-chip two-photon polymerization (TPP) strategy that enables structuring two photosensitive materials sequentially within a predesigned configuration was proposed. As a proof-of-concept, we demonstrate a pH-responsive spider microbot and a 3D smart micro-gripper that enables controllable grabbing and releasing. Our strategy provides a universal protocol for directly printing 3D microbots composed of multiple materials.

Ag nanoparticle ink coupled with graphene oxide cellulose paper: a flexible and tunable SERS sensing platform.

Surface-enhanced Raman scattering (SERS) is highly promising for ultra-sensitive detection in a series of applications. Although extensive advances have been achieved in SERS technologies, the preparation of highly efficient SERS substrates still suffers from several limitations, including complex preparation procedures, high cost, and instability for long time storage. To address these problems, we report a novel, to the best of our knowledge, SERS platform that combines graphene oxide (GO) and cellulose composite paper with colloidal silver nanoparticle (Ag NP) ink. As an efficient substrate, the GO and cellulose composite paper that features hierarchical micro-nanostructures and improved interaction with target molecules can be fabricated on a large scale, and the Ag NP ink can be well stored, avoiding being oxidized in ambient conditions. In this way, our SERS platform not only reduces the cost, but also improved the stability. The sensitivity, reproducibility, and tunable SERS detection performance were evaluated using rhodamine 6G as probing molecules. To demonstrate the capability of our SERS platform in practical analysis, the SERS spectra of two monosodium salt solutions of different concentrations have been collected. The SERS platform has revealed great potential for practical application of SERS technologies.

Laser Fabrication of Bioinspired Graphene Surfaces With Superwettability.

The past decades have seen growing research interest in developing efficient fabrication techniques for preparing bioinspired graphene surfaces with superwettability. Among the various fabrication methods, laser fabrication stands out as a prominent one to achieve this end and has demonstrated unique merits in the development of graphene surfaces with superwettability. In this paper, we reviewed the recent advances in this field. The unique advantages of laser fabricated graphene surfaces have been summarized. Typical graphene surfaces with superwettability achieved by laser fabrication, including superhydrophobic graphene surfaces, oil/ water separation, fog collection, antibacterial surfaces, surface enhanced Raman scattering (SERS), and desalination, have been introduced. In addition, current challenges and future perspectives in this field have been discussed. With the rapid progress of novel laser physical/ chemical fabrication schemes, graphene surfaces with superwettability prepared by laser fabrication may undergo sustained development and thus contribute greatly to the scientific research and our daily life.

Airflow Enhanced Solar Evaporation Based on Janus Graphene Membranes with Stable Interfacial Floatability.

Solar interfacial evaporation has been recognized as a versatile energy conversion protocol for cutting-edge applications such as water treatment and power generation (e.g., hydro voltaic effect). Recently, to enhance water evaporation rates, water temperature and evaporation area have been considered as essential ingredients, and thus photothermal materials and three-dimensional hierarchical structures have been developed to promote light-to-heat conversion efficiency and enhance interfacial evaporation. However, less attention has been paid to the airflow effect, because the interfacial floatability of photothermal membranes should be considered under air blast. Here, inspired from the stable interfacial floatability of lotus leaves, we report the airflow enhanced solar interfacial evaporation approach using a graphene-based Janus membrane. Laser-induced graphene (LIG) film was treated unilaterally by O2 plasma, forming a LIG/oxidized LIG (LIG-O) Janus membrane with distinct wettability on two sides. Higher water evaporation rate of 1.512 kg m-2 h-1 is achieved. The high solar interfacial evaporation performance can be attributed to the two advantages: (i) the combination of microscale capillary water transporting and nanoscale light trapping; (ii) hydrophobic/hydrophilic Janus membrane for stable interfacial floatability under airflow. Our approach is feasible for developing high-performance solar interfacial evaporation devices for practical clean energy utilization.

Bioinspired Zoom Compound Eyes Enable Variable-Focus Imaging.

Natural compound eyes provide the inspiration for developing artificial optical devices that feature a large field of view (FOV). However, the imaging ability of artificial compound eyes is generally based on the large number of ommatidia. The lack of a tunable imaging mechanism significantly limits the practical applications of artificial compound eyes, for instance, distinguishing targets at different distances. Herein, we reported zoom compound eyes that enable variable-focus imaging by integrating a deformable poly(dimethylsiloxane) (PDMS) microlens array (MLA) with a microfluidic chamber. The thin and soft PDMS MLA was fabricated by soft lithography using a hard template prepared by a combined technology of femtosecond laser processing and wet etching. As compared with other mechanical machining strategies, our combined technology features high flexibility, efficiency, and uniformity, as well as designable processing capability, since the size, distribution, and arrangement of the ommatidia can be well controlled during femtosecond laser processing. By tuning the volume of water injected into the chamber, the PDMS MLA can deform from a planar structure to a hemispherical shape, evolving into a tunable compound eye of variable FOV up to 180°. More importantly, the tunable chamber can functionalize as the main zoom lens for tunable imaging, which endows the compound eye with the additional capability of distinguishing targets at different distances. Its focal length can be turned from 3.03 mm to infinity with an angular resolution of 3.86 × 10-4 rad. This zoom compound eye combines the advantages of monocular eyes and compound eyes together, holding great promise for developing advanced micro-optical devices that enable large FOV and variable-focus imaging.

Programmable deformation of patterned bimorph actuator swarm.

Graphene-based actuators featuring fast and reversible deformation under various external stimuli are promising for soft robotics. However, these bimorph actuators are incapable of complex and programmable 3D deformation, which limits their practical application. Here, inspired from the collective coupling and coordination of living cells, we fabricated a moisture-responsive graphene actuator swarm that has programmable shape-changing capability by programming the SU-8 patterns underneath. To get better control over the deformation, we fabricated SU-8 micropattern arrays with specific geometries and orientations on a continuous graphene oxide film, forming a swarm of bimorph actuators. In this way, predictable and complex deformations, including bending, twisting, coiling, asymmetric bending, 3D folding, and combinations of these, have been achieved due to the collective coupling and coordination of the actuator swarm. This work proposes a new way to program the deformation of bilayer actuators, expanding the capabilities of existing bimorph actuators for applications in various smart devices.

Laser Fabrication of Graphene-Based Flexible Electronics.

Recent years have witnessed the rise of graphene and its applications in various electronic devices. Specifically, featuring excellent flexibility, transparency, conductivity, and mechanical robustness, graphene has emerged as a versatile material for flexible electronics. In the past decade, facilitated by various laser processing technologies, including the laser-treatment-induced photoreduction of graphene oxides, flexible patterning, hierarchical structuring, heteroatom doping, controllable thinning, etching, and shock of graphene, along with laser-induced graphene on polyimide, graphene has found broad applications in a wide range of electronic devices, such as power generators, supercapacitors, optoelectronic devices, sensors, and actuators. Here, the recent advancements in the laser fabrication of graphene-based flexible electronic devices are comprehensively summarized. The various laser fabrication technologies that have been employed for the preparation, processing, and modification of graphene and its derivatives are reviewed. A thorough overview of typical laser-enabled flexible electronic devices that are based on various graphene sources is presented. With the rapid progress that has been made in the research on graphene preparation methodologies and laser micronanofabrication technologies, graphene-based electronics may soon undergo fast development.

Nacre-inspired moisture-responsive graphene actuators with robustness and self-healing properties.

Moisture-responsive actuators based on graphene oxide (GO) have attracted intensive research interest in recent years. However, current GO actuators suffer from low mechanical strength. Inspired by the robustness of nacre's structure, moisture-responsive actuators with high mechanical strength and self-healing properties were successfully developed based on GO and cellulose fiber (CF) hybrids. The hybrid paper demonstrated significantly improved tensile strength, ∼20 times higher than that of pure GO paper, and self-healing properties. A broken paper can be well cured under moist conditions, and the mechanical properties of the self-healed hybrid paper can still maintain similar tensile strength to the pristine one. After controllable ultraviolet light photoreduction treatment, a hybrid paper with a photoreduction gradient along the normal direction was prepared, which can act as a moisture-responsive actuator. A maximum bending curvature of ∼1.48 cm-1 can be achieved under high relative humidity (RH = 97%). As a proof-of-concept, a butterfly-like actuator that can deform itself with moisture actuation was demonstrated. Our approach may pave a new way for designing robust and self-healable graphene actuators.

Versatile Electronic Skins with Biomimetic Micronanostructures Fabricated Using Natural Reed Leaves as Templates.

Versatile electronic skin devices that enable detection of multimodal signals have revealed great potential for human health monitoring. To make a versatile electronic skin, hierarchical micronanostructures are essential to obtain improved sensing performance and multisignal detection capability. However, current strategies for developing a nanostructured electronic skin usually involve complex procedures, harsh experimental conditions, and the use of expensive equipment, which limit its practical applications. In this paper, we reported the fabrication of a multifunctional wearable electronic skin with hierarchical micronanostructures by using natural reed leaves as templates. The capacitive-type electronic skin is fabricated by double-sided coating of Au electrodes on an artificial polydimethylsiloxane reed leaf that is duplicated from natural reed leaves via soft lithography. The electronic skin features a very simple device structure yet high sensing performance. It permits multimodal signal detection, including that of pressure, deformation, and proximity, and can serve as surface-enhanced Raman scattering substrates for the detection of metabolites in sweat because of the formation of plasmonic structures. The versatile electronic skin can be attached to the human skin, and it enables effective monitoring of multiphysiological signals, revealing great potential for cutting-edge applications, such as human health monitoring.