Femtosecond laser direct writing of functional stimulus-responsive structures and applications

Diverse natural organisms possess stimulus-responsive structures to adapt to the surrounding environment.Inspired by nature,researchers have developed various smart stimulus-responsive structures with adjustable properties and functions to address the demands of ever-changing application environments that are becoming more intricate.Among many fabrication methods for stimulus-responsive structures,femtosecond laser direct writing(FsLDW)has received increasing attention because of its high precision,simplicity,true three-dimensional machining ability,and wide applicability to almost all materials.This paper systematically outlines state-of-the-art research on stimulus-responsive structures prepared by FsLDW.Based on the introduction of femtosecond laser-matter interaction and mainstream FsLDW-based manufacturing strategies,different stimulating factors that can trigger structural responses of prepared intelligent structures,such as magnetic field,light,temperature,pH,and humidity,are emphatically summarized.Various applications of functional structures with stimuli-responsive dynamic behaviors fabricated by FsLDW,as well as the present obstacles and forthcoming development opportunities,are discussed.

Encryption/decryption and microtarget capturing by pH-driven Janus microstructures fabricated by the same femtosecond laser printing parameters

Several natural organism can change shape under external stimuli. These natural phenomena have inspired a vast amount of research on exploration and implementation of reconfigurable shape transformation. The Janus structure is a promising approach to achieve shape transformation based on its heterogeneous chemical or physical properties on opposite sides.However, the heterogeneity is generally realized by multi-step processing, different materials,and/or different processing parameters. Here, we present a simple and flexible method of producing p H-sensitive Janus microactuators from a single material, using the same laser printing parameters. These microactuators exhibit reversible structural deformations with large bending angles of ~31°and fast response(~0.2 s) by changing the p H value of the aqueous environment. Benefited from the high flexibility of the laser printing technique and the spatial arrangements, pillar heights, and bending directions of microactuators are readily controlled,enabling a variety of switchable ordered patterns and complex petal-like structures on flat surfaces and inside microchannels. Finally, we explore the potential applications of this method in information encryption/decryption and microtarget capturing.

Rapid fabrication of reconfigurable helical microswimmers with environmentally adaptive locomotion

Artificial helical microswimmers with shape-morphing capacities and adaptive locomotion have great potential for precision medicine and noninvasive surgery.However,current reconfigurable helical microswimmers are hampered by their low-throughput fabrication and limited adaptive locomotion.Here,a rotary holographic processing strategy(a helical femtosecond laser beam)is proposed to produce stimuli-responsive helical microswimmers(<100μm)rapidly(<1 s).This method allows for the easy one-step fabrication of various microswimmers with controllable sizes and diverse bioinspired morphologies,including spirulina-,Escherichia-,sperm-,and Trypanosoma-like shapes.Owing to their shape-morphing capability,the helical microswimmers undergo a dynamic transition between tumbling and corkscrewing motions under a constant rotating magnetic field.By exploiting adaptive locomotion,helical microswimmers can navigate complex terrain and achieve targeted drug delivery.Hence,these microswimmers hold considerable promise for diverse precision treatments and biomedical applications.

Five-wavelength optical-resolution photoacoustic microscopy of blood and lymphatic vessels

Optical-resolution photoacoustic microscopy(OR-PAM)has been developed for anatomical,functional,and molecular imaging but usually requires multiple scanning for different contrasts.We present five-wavelength OR-PAM for simultaneous imaging of hemoglobin concentration,oxygen saturation,blood flow speed,and lymphatic vessels in single raster scanning.We develop a five-wavelength pulsed laser via stimulated Raman scattering.The five pulsed wavelengths,i.e.,532,545,558,570,and 620∕640 nm,are temporally separated by several hundreds of nanoseconds via different optical delays in fiber.Five photoacoustic images at these wavelengths are simultaneously acquired in a single scanning.The 532-and 620∕640-nm wavelengths are used to image the blood vessels and dye-labeled lymphatic vessels.The blood flow speed is measured by a dual-pulse method.The oxygen saturation is calculated and compensated for by the Grüneisen-relaxation effect.In vivo imaging of hemoglobin concentration,oxygen saturation,blood flow speed,and lymphatic vessels is demonstrated in preclinical applications of cancer detection,lymphatic clearance monitoring,and functional brain imaging.

Review of photoacoustic imaging for microrobots tracking in vivo [Invited]

Microrobots-assisted drug delivery and surgery have been always in the spotlight and are highly anticipated to solve the challenges of cancer in situ treatment. These versatile small biomedical robots are expected to realize direct access to the tumor or disease site for precise treatment, which requires real-time and high-resolution in vivo tracking as feedback for the microrobots’ actuation and control. Among current biomedical imaging methods, photoacoustic imaging(PAI) is presenting its outstanding performances in the tracking of microrobots in the human body derived from its great advantages of excellent imaging resolution and contrast in deep tissue. In this review, we summarize the PAI techniques, imaging systems, and their biomedical applications in microrobots tracking in vitro and in vivo. From a robotic tracking perspective,we also provide some insight into the future of PAI technology in clinical applications.

Investigation of β-Ga2O3 films and β-Ga2O3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201)β-Ga2O3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD).It is revealed that theβ-Ga2O3 film grown on GaN possesses superior crystal quality,material homogeneity and surface morphology than the results of common heteroepitaxialβ-Ga2O3 film based on sapphire substrate.Further,the relevance between the crystal quality of epitaxialβ-Ga2O3 film and theβ-Ga2O3/GaN interface behavior is investigated.Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequentβ-Ga2O3 film.Moreover,the energy band structure ofβ-Ga2O3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained.The results in this work not only convincingly demonstrate the advantages ofβ-Ga2O3 films grown on GaN substrate,but also show the great application potential of MOCVDβ-Ga2O3/GaN heterostructures in microelectronic applications.

Wide-range-adjusted threshold voltages for E-mode AlGaN/GaN HEMT with a p-SnO cap gate

p-GaN cap layer has been recognized as a commercial technology to manufacture enhanced-mode(E-mode)AlGaN/GaN high electron mobility transistor(HEMT);however,the difficult activation of Mg doping and etching damage of p-GaN limit the further improvement of device performance.Thus,the more cost-effective cap layer has attracted wide attention in GaN-based HEMT.In this paper,p-type tin monoxide(p-SnO)was firstly investigated as a gate cap to realize E-mode AlGaN/GaN HEMT by both Silvaco simulation and experiment.Simulation results show that by simply adjusting the thickness(50 to 200 nm)or the doping concentration(3×10^(17)to 3×10^(18)cm^(-3))of p-SnO,the threshold voltage(V_(th))of HEMT can be continuously adjusted in the range from zero to 10 V.Simultaneously,the device demonstrated a drain current density above 120 mA mm^(-1),a gate breakdown voltage(V_(BG))of 7.5 V and a device breakdown voltage(V_(B))of 2470 V.What is more,the etching-free AlGaN/GaN HEMT with sputtered p-SnO gate cap were fabricated,and achieved a positive V_(th) of 1 V,V_(BG) of 4.2 V and V_(B) of 420 V,which confirms the application potential of the p-SnO film as a gate cap layer for E-mode GaN-based HEMT.This work is instructive to the design and manufacture of p-oxide gate cap E-mode AlGaN/GaN HEMT with low cost.

Confocal visible/NIR photoacoustic microscopy of tumors with structural, functional, and nanoprobe contrasts

Distinguishing early-stage tumors from normal tissues is of great importance in cancer diagnosis.We report fiberbased confocal visible/near-infrared(NIR)optical-resolution photoacoustic microscopy that can image tumor microvasculature,oxygen saturation,and nanoprobes in a single scanning.We develop a cost-efficient single laser source that provides 532,558,and 1064 nm pulsed light with sub-microseconds wavelength switching time.Via dual-fiber illumination,we can focus the three beams to the same point.The optical and acoustic foci are confocally aligned to optimize the sensitivity.The visible and NIR wavelengths enable simultaneous tumor imaging with three different contrast modes.Results show obvious angiogenesis,significantly elevated oxygen saturation,and accumulated nanoparticles in the tumor regions,which offer comprehensive information to detect the tumor.This approach also allows us to identify feeding and draining vessels of the tumor and thus to determine local oxygen extraction fraction.In the tumor region,the oxygen extraction fraction significantly decreases along with tumor growth,which can also assist in tumor detection and staging.Fiber-based confocal visible/NIR photoacoustic microscopy offers a new tool for early detection of cancer.

Photo-induced anti-Markovnikov hydroalkylation of unactivated alkenes employing a dual-component initiator

Metal-free anti-Markovnikov hydroalkylation of unactivated alkenes with cyanoacetate has been developed,featuring the use of a dual-component initiator containing an organic photocatalyst and a radical precursor.When combined,the two components can undergo visible light-induced singleelectron transfer,and serve as a versatile and effective alkyl radical generator.