Laser-based bionic manufacturing

Through the study of the super-wetting phenomenon, it is found that the rational design of micro-nano structures plays a crucial role in controlling the surface wetting state. Fine micro/nano structures (below tens of microns) can confer superhydrophobicity to the prepared surface, while sub-millimeter structures (above tens of microns) can modulate the shape of droplets on the bionic surface, allowing the surface to exhibit anisotropic wettability to the droplets [50]. With the discovery of the water mist-collecting capabilities exhibited by desert beetles, cactus spines, spider silk, and Nepenthes mirabilis mouthparts, surfaces with special wettable structures featuring superhydrophobic/superhydrophilic characteristics have emerged as a novel approach to obtaining fresh water [51, 52]. The integration of the multi-level network structure of leaf veins, the hydrophobic–hydrophilic water collection structure observed on beetle backs, and the asymmetric self-driven structure of cactus spines can result in a multifunctional bionic surface characterized by strong self-driven centralization and excellent efficiency (figure 5(a)). The multi-level micro/nano structures play a crucial role in determining the extreme wetting properties of the material, while the distribution and orientation of these structures govern the wettability state and liquid motion. For instance, a tree-shaped layered cone structure fabricated on a superhydrophobic TiO₂ membrane can precisely and continuously drive tiny droplets to the root of the tree-pattern without relying on gravity. In comparison with a substrate lacking micro/nano structures, the efficiency of directional water collection is enhanced by 36 times (figure 5(b)) [53]. The development of bionic surfaces with accurate regional control of superwetting behaviors for on-demand response has potential applications in droplet manipulation, micro-reaction, and other fields. Shape memory polymer arrays with multiple switchabilities between the 'lotus effect' and 'rose petal effect' can be prepared on a large scale by laser templating method (figure 5(c)) [9]. The programmable bionic superhydrophobic surface accurately achieves regional morphology deformations when subjected to near-infrared light, leading to reversible surface transitions between high and low adhesion. This capability allows for precise and remote control of super-wettability. Integrating dynamic tuning of wetting properties into wearable surfaces holds the potential to unlock innovative applications for smart interface materials. The one-step generation of programmable skin-like microstructures and tunable nanostructures on polydimethylsiloxane using laser subtractive fabrication techniques can lead to extreme hydrophobicity, excellent skin conformability and reversible deformability [54]. The surface topography can be finely regulated in a rapid and reversible manner by simple stretching, providing the feasibility of controlling the surface wettability by simple body motions (figure 5(d)). Surfaces with ultra-low ice adhesion strength and superior mechanical durability in extremely cold and wet environments are of great interest. Anti-icing surfaces prepared using femtosecond lasers to mimic the periodic micro-nanostructures of white axle grass exhibit an excellent static/dynamic anti-icing effect and mechanical durability (figure 5(e)) [55]. The surface demonstrates a static anti-icing time of up to 2832 s and a frost protection time of up to 5 h in complex and extreme environments. Moreover, it shows efficient de-icing in motion with an ice adhesion strength as low as 1.60 kPa.

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With the growing concern about the energy crisis and global warming, saving resources has become a hot topic. Drag reduction is the most direct way to reduce energy consumption, however, most studies on drag reduction have focused on the macroscopic scale. Inspired by shark skin, researchers have found that organisms with non-smooth surface structures can achieve drag reduction by altering the flow field near the surface of an object [56]. This challenges the traditional notion that smoother surfaces are the sole effective means of reducing drag, introducing a novel perspective on microstructure-based drag reduction [57]. An array of sharkskin-like grooves prepared on polytetrafluoroethylene surface is shown in figure 6(a) [31]. In the case of dimensionless rib periods of 14–20, the air friction drops by up to 6%. Compared with mechanical grinding, the microgroove structure with a depth of 20 μm–120 μm obtained by laser processing is more conducive to reducing the resistance of the fluid medium. Since Kim and Kim [58] discovered the drag-reducing effect of micro/nano structural surfaces on liquid droplets and proposed the concept of effective slip, scholars have begun to explore the drag-reducing function of superhydrophobic surfaces in depth. Surfaces that are superhydrophobic in air will evolve underwater into surfaces with the ability to adsorb bubbles. Composite surfaces with superhydrophobic and superhydrophilic properties enable self-aggregation and self-assembly of bubbles on specified regions (figure 6(b)) [59]. The aluminum alloy surface demonstrated a transition from highly adherent properties (Wenzel state) to low adherent properties (Cassie state) as the laser scanning pitch varied. However, when the laser scanning spacing exceeds 100 μm, the water contact angle is less than 150° and superhydrophobicity cannot be achieved. Bionic miniature boats based on underwater superaerophobic/superaerophilic properties exhibit excellent drag reduction and load capacity [60]. Compared with the untreated miniature boat, the sliding distance of the bionic miniature boat increased by 52% and the loading capacity increased by 27% (figure 6(c)). Inspired by the character of the water strider, Dai et al [61] fabricated a micro-surfboard device with a dual gradient of geometry and wettability by laser subtractive manufacturing and chemical modification (figure 6(d)). Superhydrophilic wedge grooves with various wedge angles were prepared using a femtosecond laser to facilitate the programmable motion of the micro-surfboard.

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The antibacterial bionic surfaces inspired by dragonfly wings, cicada wings, moth eyes, and gecko skin, featuring regular or disordered micro/nano structures, have demonstrated the ability to reduce bacterial adhesion and even kill attached bacteria [62]. One crucial factor affecting bacterial adhesion on the surface is the depth-to-period aspect ratio of the bionic micro/nano structures. The micro/nanoscale spacing on the antibacterial bionic surface alters the contact state of bacteria, effectively controlling bacterial adhesion and reducing biofilm formation [63]. Richter et al [64] fabricated LIPSS with different period ranges and explored the dependence of bacterial adhesion on the spatial period (figure 7(a)). For the smallest LIPSS feature size, cell adhesion was reduced by more than 90% compared to the unmodified surface. The initial steps of biofilm formation are strongly impeded by the bionic antibacterial structure. However, the study found that flagellated E. coli showed improved adhesion to grooved surfaces with 440 nm-sized structures. This could be attributed to bacterial surface appendages, such as pili and flagella, entering the trenches and enhancing bacterial attachment. For this reason, the feature size of LIPSS must be designed rationally according to the size of the bacteria. However, there is still a risk of adhesion when superhydrophobic surfaces are exposed to wet environments for a long period of time. Preventing bacterial adhesion properties from transforming into mechanically triggered bacterial killing when micro/nanoscale surface structures exhibit high aspect ratios. When bacteria come into contact with the micro/nano structures, some cell membranes suspended between the micro/nano structures break due to stretching, leading to bacterial death (figure 7(b)) [65]. Copper and its alloys have demonstrated the capacity to eliminate a broad spectrum of microorganisms upon direct contact, albeit frequently necessitating prolonged exposure over several hours. Employing successive laser scans, a uniform multi-level micro/nano dual-scale structure is established on the copper surface. The antibacterial bionic surface with multi-level has a strong contact killing effect on many different strains of bacteria, including clinically relevant drug-resistant bacteria [66]. The integration of antibacterial properties, anti-adhesion attributes, and structural sterilization within this bionic surface offers innovative perspectives and methodologies for the advancement of secure and efficacious antibacterial surfaces. Inspired by the superhydrophobic surface structure of the Nepenthes pitcher plant, a large number of interconnected porous microstructures with diameters in the range of several 100 nm can be directly fabricated on various polymer surfaces through a single-step femtosecond laser ablation (figure 7(c)) [67]. After chemical treatment and injection of lubricating fluid, this bionic surface can achieve multiple functionalities such as liquid repellency, anti-contamination, and corrosion resistance. The use of the original laser-induced porous polymer surface promotes the growth of C6 glioma cells, whereas a smooth polymer surface completely inhibits the growth of C6 glioma cells. This bionic superhydrophobic surface not only exhibits excellent liquid-repellent capability but also possesses the ability to rapidly self-repair without the need for any additional treatment, thus regaining its smooth performance.

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The enduring human fascination and pursuit of color has never ceased, and structural colors in nature have inspired researchers to construct new strategies for optical structures with versatility and multiple degrees of freedom. The development of laser manufacturing strongly demonstrates its unique advantages in structural color fabrication, showing flexible, one-step, and contactless procedures and remarkable suitability for a wide scope of materials. Despite these strides, the attainment of single-pixel control and dynamic color adjustment for structural color bionic surfaces poses a formidable challenge. With the high flexibility of femtosecond laser fabrication, the structured color bionic surfaces prepared by Liu et al [69] via the laser template method can adjust the color of each pixel independently and precisely control the color brightness and pattern (figure 8(a)). Furthermore, owing to its Janus nature, this structured color bionic surface exhibits asymmetric optical characteristics, manifesting as distinct 'display' and 'cloaking' states. This duality imparts potential applications in anticounterfeiting technology and color sensors. The assistance of the template often introduces problems such as the inconvenience of the template changing and color blurring at the edges of the pattern. The rapid development of LIPSS has shown great advantages in the efficient production of structural colors. Instead of individual ablation pits or ablation lines, a group of periodic submicron structure arrays is miraculously created within the whole laser spot area. Real-time control of laser intensity, facilitated by a spatial light modulator, enhances the flexibility and efficiency of LIPSS production [70]. This breakthrough in structural color bionic surface fabrication technology delves into the potential applications of information steganography (figure 8(b)). Simultaneously achieving high resolution, high speed, cost-effectiveness, long-term stability, and viewing-angle independence in structural colors poses a formidable challenge [71]. The schematic in figure 8(c) illustrates the process of ultrafast laser surface coloring on TiAlN-TiN films utilizing low-cost pulsed lasers. All resultant colors are attributed to TiAlN oxide films with double resonance absorption characteristics. By modulating the laser fluence, TiAlN-TiN films obtained angle-robust structured colors with an unprecedented ∼90% sRGB large color gamut. In-depth exploration of the interplay between laser parameters and materials is imperative for the development of novel optically functional materials. The subwavelength structures with a moth-eye effect (gradient refractive index) demonstrate excellent anti-reflective properties. However, manufacturing high-performance, uniformly large-area subwavelength anti-reflective structures poses significant challenges. Yin et al [72] proposed a laser surface processing strategy based on the Marangoni effect, and through the optimization of laser parameters, they achieved a multi-scale structure with broadband anti-reflection enhancement (figure 8(d)). When the laser's energy density is 95.5 J·cm⁻² and the scanning distance is 5 µm, a bionic surface with low reflectance was obtained. Compared to the untreated material, the average reflectance of the sample in the 300 nm–800 nm wavelength range decreased by 45.5%.

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The inherent tolerance to frictional wear exhibited by an organism's body surface results from the synergistic coupling of morphology, material, and structure [78]. In recent years, researchers from Jilin University have extensively studied bionic wear-resistant surfaces for heavy machinery components, including machine tool guideways, train tracks, drilling rods, and brake shoes. The microhardness of the strengthening layer is strongly influenced by the laser parameters, and in general the wear resistance of the material increases with increasing laser energy per unit area. However, the microhardness of the bionic unit is only as close to the microhardness of the wear pair as the specimen will exhibit good wear-resistance [79]. The wear-resistance of the bionic strengthening surface at the same process parameters is closely related to the shape of the bionic units, including cross-sectional shape, unit spacing, angle, size, and others play pivotal roles [80]. Among bionic surfaces, the 45° bionic wear-resistance surface demonstrates the most effective hindrance and dispersion effects on wear. It is observed that when the angle between the bionic streak and the wear direction surpasses 45°, trapped particles persistently plow through the soft phase, resulting in an escalation of wear mass loss (figure 10(a)). A small-scale soft zone proves advantageous in absorbing mechanical energy and retaining lubricating oxides. Conversely, a large-scale soft zone may lead to noticeable material damage. The wear resistance is enhanced with a uniformly distributed bionic structure, and the presence of a continuous hard region facilitates structural strengthening and wear resistance, consequently retarding the initiation of cracks [33]. Specific bionic coupling units are prepared according to the parts with different degrees of wear, which improves the fatigue resistance of the die and makes the die surface wear degree converge.

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In harsh environments, equipments such as hot forging dies, exhaust pipes, and turbomachines are prone to thermal fatigue cracking, high-temperature wear, and other failures. Bionic structures inspired by the design principles found in blades and dragonfly wings showcase exceptional crack-arrest and fatigue resistance properties, even when subjected to repeated stress cycles. In order to explore the influence of the bionic unit shape on the fatigue resistance of materials, Meng et al [81] studied the thermal fatigue resistance of various unit shapes ('prolate', 'U', 'V') under the surface of H13 die steel. The 'U'-shaped morphology unit was found to effectively impede crack expansion through a crack tip passivation mechanism. Bionic crack arrest surface increases the heating area of the tooth surface, ensuring a greater temperature gradient on the gear tooth surface with the bionic surface morphology, and in turn, exerts a deterrent effect on the initiation and expansion of gear cracks (figure 10(b)). The service miles of the brake pad with bionic crack arrest surface can reach 70 000 Km and above, and the service life is more than double that of the untreated brake pad [82]. The thermal cycle life of thermal barrier coatings (TBCs) is relatively short due to factors such as the mismatch of thermal expansion coefficients, phase changes causing coating volume changes, and oxide formation. To address this challenge, the preparation of bionic structures with typical shapes of biological structures (dots, stripes, and lattices) on the TBCs by laser equivalent manufacturing technique can effectively improve the bonding strength and limit the extension of cracks (figure 10(c)) [27]. During thermal shock tests, coating failure or peeling typically initiates at the edges and subsequently propagates to adjacent areas. Dotted units with columnar grains and segmented cracks prove advantageous for enhancing strain tolerance. The striated and grid units, owing to their large overlap, result in the formation of more continuous segmented cracks and transverse cracks. The bionic crack-arrest surface achieves the redistribution of surface stresses, elevating the threshold for crack initiation and aiding in limiting the path of crack propagation. Bionic specimens with coupled effects of non-smooth mechanical properties and microstructural features can effectively extend the service life of a die (figure 10(d)) [83].

Porous structures (cellular structures, lattice structures, honeycomb structures, etc) are considered highly promising for the new generation of advanced lightweight load-bearing systems due to their lightweight and multifunctional properties [93, 94]. Benefiting from the ability to fabricate components with complex shape geometry and high precision, LPBF holds a significant advantage in manufacturing bionic porous structures. By simulating the crystal structure and strengthening mechanisms (grain-boundary, precipitation, multiphase strengthening, etc) of crystalline materials, the lattice-like structure exhibits excellent damage resistance characteristics. Engine blades inspired by crystal microstructures are prepared from multi-scale layered crystal structures to enhance the functionality and performance of the material in response to external loads (figure 12(a)). The multi-scale and multi-level crystal structure achieves synergistic strengthening associated with both crystallographic microstructures and designed mesostructures [36]. The combination of AM and topology optimization can produce lightweight porous structures with optimal performance under specific conditions. Xiao et al [95] combined AM and topology optimization for lattice structure design. The face centre cube and vertex cube lattice structures have better mechanical properties than the edge centre cube lattice structure. Topologically optimized lattice structure outperforms most lattice structures and has a maximum lightweight design (figure 12(b)). In structures with the same porosity, other lattice structures are more prone to forming cracks due to stress concentration. It is evident that controlling porosity, optimizing lattice structures, and considering specific stress conditions are crucial aspects of producing bionic porous structures with superior mechanical properties and performance [96]. Previous studies have found that the internal structure of the Boeing 777 wing after topology optimization and the internal structure of the beak skeleton show striking similarities (figure 12(c)) [97]. The combination of AM and topology optimization offers the potential to create lightweight porous structures with optimal performance for specific conditions. Notably, porous structures may provide new ideas for the development of next-generation thermal control structures or materials. Bionic gradient porous materials inspired by Norway spruce can effectively reduce thermal diffusivity, demonstrating the great potential of LAM technology in the field of thermal management [98].

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In various fields such as automotive, aviation, and military, enhancing the impact resistance and energy absorption capacity of lightweight components is a pressing challenge. Organisms exhibit lightweight characteristics through highly complex and adaptable structural forms, inspiring the development of bionic solutions. The tailstock of the Mantis Shrimp can withstand repeated impacts of 1500 N without any catastrophic failure. Inspired by the telson of the mantis shrimp, the bionic bi-directionally corrugated plate (DCP) structure (plate structure) fabricated by researchers through LPBF has excellent energy absorption capacity and high-speed impact stability (figure 13(a)) [99]. Through optimizing the geometric parameters, such as wavelength and amplitude, an effective energy-absorbing structure with superior overall performance was achieved. The helicoidal structure inspired by the mantis shrimp abdominal segment showed remarkable improvements in specific energy absorption (2.4 times) and plateau stress (2.7 times) compared to conventional honeycomb and tubular structures (figure 13(b)) [100]. The helicoidal structure with a rotation angle of 45° and 3 layers has the optimal comprehensive energy absorption performance. The twisted stress conduction path enables the helical structure to withstand greater compressive forces and distribute the stress more evenly, which is the mechanism for improved energy absorption performance. The application of hierarchical concepts such as multiscale, multilayer, and superimposition to DCP structures can further improve load uniformity (figure 13(c)) [101]. The sine-wave structure exhibited a higher maximum crush force efficiency of 73.06%, surpassing most reported energy-absorbing structures. The energy absorption and specific energy absorption values increased with the height and thickness of the sine curve, reaching 37.73 J and 8.45 J·g⁻¹, respectively. The diving bell of the water spider can withstand high velocity impacts from currents in different directions. Wang et al [102] investigated the effect of strut diameter on the dimensional accuracy, densification behavior, and compression properties of mesh shell structures. The load-displacement curves of all the mesh shell members showed similar trends. The member with a strut diameter of 1.50 mm exhibited the maximum load capacity, whereas the member with a strut diameter of 0.75 mm demonstrated the maximum displacement (figure 13(d)).

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Natural materials are generally composites with spatially heterogeneous and tunable properties [2]. To attain properties that closely match those of natural biological structures, particle-reinforced composites with a structure akin to mussels prove highly effective in resisting wear and deformation [103]. By incorporating the unique properties of different materials, the heterogeneous bionic structures exhibit versatility and comprehensiveness. Multi-material AM enables 'the right materials printed in the right positions' and 'unique structures printed for unique functions' [88]. In general, LAM strategies for heterogeneous materials involve direct deposition, functional gradient joining, and intermediate layer joining [104]. Fabrication of crossed-lamellar structures inspired by conch shells by LDED demonstrates the feasibility of depositing multiple materials on the same deposition layer and on different deposition layers (figure 14(a)) [105]. The 'softer' and 'harder' regions of a heterogeneous material are unevenly deformed during plastic deformation. The synergistic effect of these regions leads to heterogeneous deformation-induced strengthening and strain hardening, which greatly enhances the strength-ductility trade-off of the heterogeneous structures [106, 107]. Special material interface design is instrumental in enhancing the diffusion of material elements and improving mechanical properties. Inspired by natural materials such as bamboo, bone, and teeth, advanced functional grading materials are an appropriate response to high performance and multifunctional applications. Figure 14(b) shows the functional gradient materials from 100% Ti6Al4V to 100% Mo. There is satisfactory metallurgical bonding between the deposit and the substrate, but also between each deposited layer [108]. However, intermetallic compounds that exhibit thermal mismatch along the transition zone (interface zone) of different alloys may cause cracks. Direct deposition and functional gradient joining methods may lead to the formation of harmful phases, which can limit the application prospects of heterogeneous materials. Joining with the help of interlayer materials offers a promising approach to overcome these limitations and expand the possibilities of heterogeneous material design. Inspired by the multilayer structure of the Crysomallon squamiferum shell, Wang et al [35] prepared fully dense layered ceramic/metal materials with excellent interfacial bonding. Careful control of the scan speed and energy input during the manufacturing process is crucial to avoid visible defects in the final product (figure 14(c)). The utilization of bio-inspired layered composite and whiskers-structured reinforcement in the TiB₂/Ti6Al4V multi-material has resulted in superior overall performance. This material exhibits outstanding strength, surpassing 2000 MPa, and a high bending strain of approximately 8%.

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Irritability refers to the instinctive response of organisms to environmental stimuli. Many plants possess unique structures in their stems, leaves, fruits, seeds, and other organs that enable spontaneous responses in response to environmental conditions such as light, sound, humidity, and temperature [109, 110]. Smart material-based AM (4D printing), is an advanced manufacturing technology that allows materials to be 'programmed' to respond to external stimuli. This ability endows the material with dynamic properties that allow it to exhibit responsive behavior in response to changes in its surroundings [111]. Inspired by the autonomous movement observed in living organisms, actuatable materials find diverse applications in fields such as biosensors, soft robotics, and bionic actuation. Nishiguchi et al [112] devised a method for direct in-gel laser writing with an impressive resolution of 1/100 nm. When combined with a thermosensitive hydrogel, even minor temperature variations can induce substantial amplitude motions. Addressing the challenge of creating artificial musculoskeletal systems at the micro- and nanoscale, Ma et al [113] introduced femtosecond LAM to fabricate micro-robots. The heightened responsiveness of polymeric materials to ambient humidity induces alterations in the confined volume, resulting in bending and deformation movements of the material. This characteristic creates new opportunities for the application of soft actuators and artificial muscles [114].

NiTi alloy stands out as one of the most frequently employed smart materials, owing to its distinctive functional characteristics [115, 116]. Leveraging laser material manufacturing technology for the creation of intricate shapes, and controlled deformation accuracy of nickel–titanium alloy enjoys unparalleled advantages over traditional methods, providing an ideal means for manufacturing intelligent metal devices [117]. The bionic crab claw structures prepared by utilizing the flexible building strategy of LPBF technology and the track-by-track and layer-by-layer processing modes have strong toughness and damage resistance [118]. Different pore distribution patterns prove instrumental in significantly enhancing the toughness of NiTi parts. Small rotation increments often lead to torsional fracture, while relatively large rotation increments tend to increase torsional rigidity and the tangential force at the interface between adjacent tracks, contributing to an increase in cumulative deformation. The fabricated bionic NiTi alloys using the streak rotation scanning strategy have excellent tensile properties and high shape recovery (figure 15(a)) [34]. By optimizing the laser scanning length in the layer and the laser scanning direction between the layers, the NiTi component prepared has a tensile fracture of (690 ± 15) MPa and a tensile strain of up to (15.2 ± 0.8)%. Moreover, NiTi functional microdevices with extreme scales can be fabricated by optimizing the combination of laser power and scanning rate (figure 15(b)). These microdevices not only showcase high manufacturing quality, with a minimum feature size of 52 µm and less than 2 µm of low surface roughness but also demonstrate desirable mechanical properties seldom reported in other µ-LPBF (micro-scale selective laser melting) micro-components [119]. Bionic honeycomb structures fabricated using NiTi alloys exhibit excellent impact resistance and cushioning properties (figure 15(c)). The utilization of NiTi alloy endows the structure with the capability to exhibit good shape memory recovery after experiencing slight impacts. This feature allows for the repeated utilization of shock-buffering structures under minor deformation [120]. Bionic manipulator fabricated using NiTi alloy helps to accomplish finger rehabilitation without motors (figure 15(d)). The actuation and bending of the device are achieved through resistive heating, which activates the SME in different laser processed regions. Each laser processed region possesses unique phase transformation onset temperatures and thermo-mechanical recovery characteristics, thereby imparting distinct actuation characteristics [121].

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