3D Printing of Biomimetic Functional Nanocomposites via Vat Photopolymerization

Abstract

The complex structures and functional material systems of natural organisms effectively cope with crisis-ridden living environments such as high temperature, drought, toxicity, and predator. Behind these excellent survival strategies evolved over hundreds of millions of years is a series of effective mechanical, optical, hydraulic, and electromagnetic properties. Bionic design and manufacturing have always attracted extensive attention, but the progress has been limited by the inability of traditional manufacturing techniques to reproduce microscopically complex structures and the lack of functional materials. Therefore, there is an urgent need for a fabrication technique with a high degree of fabrication freedom and using composites derived from biological materials. Vat photopolymerization, an emerging additive manufacturing (aka 3D printing) technology, exhibits high manufacturing flexibility in the integrated manufacturing of multi-material systems and multi-scale structures. Here, biomaterial-inspired heterogeneous material systems based on polymer matrices and nanofillers, and the introduction of magnetic and electric fields on the basis of conventional 3D printing systems to spatially and programmably distribute nanofillers are summarized, which provides a new strategy for fabricating anisotropic structures. The application of this versatile 3D printing system in fabricating mechanically reinforced structures, polymer/metal structures, self-actuating, and superhydrophobic structures is also elaborated.

1. Introduction

Organisms in nature exhibit unique survival strategies due to their special multi-scale and multi-material structures. These biological structures are not only highly hierarchical but also highly flexible and have a clear division of labor in function. The multi-scale or multi-layer structure allows it to exhibit excellent heat conduction efficiency and energy dissipation when it is subjected to external stimuli, such as high temperature and impact force. Biomaterials make it have excellent mechanical, optical, and hydraulic performance. For example, the eggbeater-like structure on the surface of Salvinia molesta has excellent superhydrophobic properties, keeping the leaf surface clean all the time [1]. Nacre’s bricks-and-mortar multilayer structure allows it to effectively disperse the energy shock when it is subjected to external force, showing excellent energy dissipation capability [2]. Due to the mechanical reinforcement of the arrayed nanofiller in the polymer, the limpet teeth, as the hardest structure in nature, make it firmly attached to the rock wall in the turbulence of the ocean [3, 4]. The microneedles on the surface of the cactus can efficiently absorb moisture in the air even in arid environments [5]. These structures with different functions all contribute to improving the survival rate of the natural organisms. Hierarchical structure [1, 6, 7], biomaterial matrix [8, 9], and organic/inorganic fillers [8, 10, 11] are all essential to realize highly functional structures, which also provide broad opportunities for fabricating next-generation highly integrated functional structures and materials.

Bionic design and manufacturing have always been hot spots in scientific research, and a lot of research work has been carried out in this field. Some reported preliminary results also demonstrate the feasibility of artificial biomimetic structures. For example, an anisotropic structure whose deformation can be programmably controlled is produced based on the hygroscopic properties of cellulose [9], and the mechanical reinforcement caused by the electric field arrangement of multiwalled carbon nanotubes (MWCNTs) inspired by shrimp claws [12], and the superhydrophobic structure based on S. molesta leaves showed excellent performance in oil separation [1]. Among the manufacturing technologies corresponding to these artificial structures, vat photopolymerization (VPP) [13], a layer-by-layer projection and selective solidification printing technology, can not only manufacture complex structures but also far outperform 3D printing methods such as fused deposition modeling (FDM) [14] and direct ink writing (DIW) [15] in terms of printing accuracy and efficiency. However, the degree of biomimeticity of current artificial structures and the development of bioinspired materials are still limited.

In order to solve the aforementioned problems, the researchers made the following new attempts based on the previous bionic manufacturing. By learning the material composition of natural structures, a series of heterogeneous hybrid systems with mixtures of polymer matrix and nanofillers have been developed. The introduction of magnetic and electric fields makes it possible to align nanofillers during printing, which further endows the structure with anisotropic behavior. Specifically, vat photopolymerization of mechanically enhanced structures inspired by Limpet teeth and magnetic field-assisted alignment of nanofillers will be described in Section 2. The heterogeneous material system inspired by the multilayer metal-containing shell of scaly-foot snail, polymer/metal structure, and electric field-assisted 3D printing system will be described in Section 3. The anisotropic gradient distribution and deformation-programmable porous structure inspired by the mechanism of Delosperma nakurense’s seed compartment moisture absorption and release of seeds, and liquid crystal templating assisted vat photopolymerization will be described in Section 4. The immersed surface accumulation 3D printing of superhydrophobic structure inspired by S. molesta leaves will be described in Section 5. Finally, the summary and prospect of materials and methods for biomimetic manufacturing will be detailed in the conclusion section.

2. Limpet teeth inspired nanocomposite for mechanical reinforcement

Limpets are a type of aquatic gastropod mollusk with flat, cone-shaped shells that use a specialized organ called the radula to acquire food from hard ocean rocks [16, 17]. As shown in Figure 1, the radula is composed of rows of mineralized teeth that exhibit the highest known mechanical strength, surpassing that of any other naturally occurring materials [16]. This excellent mechanical strength results from biomineralization, a cyclical process, that reinforces the chitin protein matrix through the distribution of aligned iron oxide minerals to create distinctive hierarchical structures [17]. These mineral-based microstructures have evolved to provide limpets with the necessary strength to graze on rough ocean surfaces, enabling them to extract nutrients from their environment without incurring damage [16]. However, the structural rigidity of mature limpet teeth eventually declines due to repeated feedings, which prompts the formation of new rows of biomineralized teeth [17]. The naturally occurring nanocomposite, a combination of soft protein and aligned mineral phase, is primarily dictated by the presence of mineral nanorods, which enhance the mechanical performance of otherwise weak biomaterials [17]. Consequently, limpet teeth provide an ideal design inspiration for the fabrication of a biomimetic nanocomposites with hierarchical microstructures and superior mechanical strength.

Figure 1.

Images of limpet tooth. (a) Top view of limpet shells; (b) microscale image of radula organ with exposed teeth; (c) microscale image of a single limpet tooth showing the base and cusp; (d) scanning electron microscope (SEM) image of limpet teeth microstructure with aligned mineral fibers. (a) Copyright from ref. [18]; (b) copyright from ref. [16]; (c, d) copyright from ref. [17].

2.1 Functional architecture of limpet teeth

Biomaterials found in living organisms have adapted through natural selection processes to develop unique microstructures with enhanced physical properties [2, 12, 16, 17]. For example, the complex microstructures found in mantis shrimp have adapted a twisted plywood structure, known as a Bouligand structure, to enhance its mechanical properties and flexibility in order to withstand high-impact punches using its fist-like clubs [2, 12, 19]. Recent studies have determined that limpet teeth exhibit a linear elastic modulus of 120 GPa under tensile testing conditions [16]. Furthermore, the mineral phase of tooth samples was demonstrated to have a linear elastic of 180 GPa, which, compared to the overall structure, indicates that the observed mechanical strength is primarily attributed to the mineral phase [16]. This superior mechanical strength, through the integration of biomineralized reinforcement of a polymer matrix, relaxes challenges associated with manufacturing technologies to fabricate microstructures with highly complex geometries and excellent mechanical performance. Current fabrication methodologies have certain limitations that must be addressed given the high degree of difficulty in achieving microstructures with aligned mineral biomaterials. Through recent advancements in biomimetic AM technologies, these limitations can be mitigated by rigorously controlling the microstructures using 3D printing methods that were previously unachievable through traditional fabrication methods [3].

Considerable challenges arise from the capability to create uniform mineral nanofillers with adjustable dimensions, such as length and cross-sectional area. The interaction of the mineral nanofillers within the chitin-polymer matrix plays a critical role in the observed tensile strength in the natural composite material based on varying dimensions and orientation. Moreover, the ability to develop mineral nanofillers with desired dimensions, with high repeatability, is of key importance to maximizing the mechanical strength in 3D printed nanocomposite parts. Therefore, these challenges have led to strategies that incorporate physical fields to aid in the alignment of magnetic microbundles during photopolymerization [2, 3, 12].

Integrating a physical field during the 3D printing process allows for high control of the alignment direction of mineral nanofillers in order to reinforce the printed part such that the compressive, tensile, and bending are deflected by the mineral constituents. The controlled alignment of mineral nanofillers within the 3D printed part is imperative to effectively create an AM process capable of fabricating hierarchical microstructures into a fully functional 3D object using a bioinspired nanocomposite. Using methods such as physical field-assisted 3D printing, a bioinspired nanocomposite material can be created and manipulated into a fully functional 3D shape that exhibits a significantly high mechanical strength in comparison to other microscale manufacturing processes as well as highly accurate and controllable micro features.

2.2 Magnetic field-assisted vat photopolymerization

In order to reproduce the microstructures observed in limpets’ teeth, an AM process, known as magnetic field assisted vat photopolymerization (MF-VPP), can be employed because of its flexibility to control the alignment direction of ferromagnetic nanofillers in polymeric resins, depicted in Figure 2a. For example, when a dynamic magnetic field is applied to the printing region, randomly distributed magnetic nanoparticles align along magnetic flux lines and subsequently agglomerate to form magnetic microbundles. Furthermore, the magnetic-field-assisted 3D printing process is highly advantageous because of its capability to rapidly align magnetic nanofillers in any spatial direction without direct contact between the magnet and nanocomposite material. A digital light projector (DLP) selectively cross-links the polymer resin on the printing platform, using a microscale mask image, which as a result constrains the magnetic nanofillers in the structure, as seen in Figure 2b. This process of alignment and photopolymerization is consecutively repeated, in a layer-by-layer fashion, to fabricate the desired 3D printed object.

Figure 2.

Magnetic field assisted vat photopolymerization. (a) Schematic of MF-VPP printer with COMSOL Multiphysics simulation of magnetic flux lines in the printing region; (b) images showing the alignment of different concentrations of iron hydroxide nanoparticles within photopolymer matrix; (c) compression strength of reinforced iron hydroxide with different iron oxide concentrations. All figures’ copyright from ref. [4].

Furthermore, optimization of printing parameters is highly dependent on the strength of the magnetic field and nanofiller concentrations because of light scattering effects that inhibit photopolymerization. As shown inFigure 2c, as the concentration of magnetic nanofiller increases, microbundle length also increases while the gap between adjacent bundles is reduced. Moreover, increasing the strength of the magnetic field significantly increases microbundle length, when compared to microbundle length under weaker magnetic fields, while simultaneously decreasing the bundle diameter and gap. Consequently, nanocomposite material with high concentrations of magnetic nanofiller requires longer exposure times since it is difficult for the projected 2D light beam to penetrate the material to initialize photopolymerization. Taking these printing parameters into consideration, the process of coupling a dynamic magnetic field combined with 2D light projections during cross-linking allows for the creation of bioinspired microstructures with attractive anisotropic mechanical performance greater than that seen in limpet teeth. The 3D-printed microstructures can be fabricated at a low cost with unique features that are modifiable resulting in differing morphologies for various applications.

2.3 Biomimetic material and structures

Recent studies have revealed that the excellent mechanical performance exhibited by limpets’ teeth can be attributed primarily to the reinforcement of a soft protein matrix by the controlled alignment of embedded iron-based minerals, specifically a mineral known as goethite [16]. Thus, the anisotropic mechanical strength of biomimetic hierarchical microstructures should be controlled by managing the spatial orientation of the magnetic nanofiller within the polymer resin. With the purpose of achieving high mechanical performance, a nanocomposite is prepared through the homogenous distribution of goethite nanoparticles in photocurable polymer resin. When initially distributed in the photocurable resin, the goethite nanoparticles have a random spatial orientation and must be coupled with the MF-VPP process in order to reinforce the normal weak polymer material via controlled nanofiller alignment.

Based on the morphologies seen in limpets’ teeth, mechanical reinforcement is strongest when the mineral nanofillers are aligned parallel to the direction of the applied force. For example, this can be clearly observed when comparing the compressive strength of random and aligned magnetic nanofillers with just the pure polymer. As depicted in Figure 2c, the compressive strength of aligned nanofillers outperforms test specimens with random alignment and pure polymer. The maximum compressive load of aligned iron oxide particles-based composite is 80 times greater than that of the pure polymer. Anisotropic mechanical reinforcement of the microstructures is heavily influenced by the magnetic field intensity, dimension of magnetic nanofiller, and the magnetic nanofiller concentration in the 3D printable nanocomposite. However, only compression strength was enhanced due to the microbundle alignments, and the alignment of magnetic particles have no significant effect on the improvement of the tensile strength and bending strength. This is because there are no constraints between two adjacent magnetic particles, and the dissipations may lead to an early failure between each particle and polymer under the bending and tensile load.

Furthermore, bundles of magnetic nanofillers can be annealed, using a high-temperature furnace, to form long fibers that better mimic the microstructures seen in limpets’ teeth. Similarly, these aligned magnetic nanofibers provide anisotropic mechanical reinforcement to the nanocomposite when a compressive load is applied. For example, the reinforced printed microstructures show superior mechanical performance when compared to the pure polymer and randomly oriented magnetic microbundles. However, high annealing temperatures, above the critical temperature for the photocurable resin, can cause cracks in the polymer material, which can lead to a reduction in the overall compressive strength. This can be easily mitigated with different polymers, such as polymer-derived ceramics, to fabricate high-strength composites with enhanced thermal performance. Thus, the reinforcement mechanism coupled with customizable alignment of magnetic nanofillers opens intriguing perspectives for designing high-strength 3D printed material based on bioinspired features with modifiable configurations.

2.4 Applications of anisotropically enhanced structure

Limpets’ teeth provide a promising design inspiration for the creation of a nanocomposite material with anisotropic functions with enhanced mechanical thermal and electrical properties. The MF-VPP method has been demonstrated to have a wide range of capabilities, including the ability to fabricate unique geometries and distinctive microstructures that are difficult to manufacture using conventional manufacturing techniques. These advancements have the potential to open new possibilities for creating intricate structures at the microscale, with exceptional mechanical strength. This technology is expected to have a significant impact in various fields such as aerospace, biomedical, and electronics in the coming years [4, 20, 21, 22, 23]. For instance, rapid prototyping of highly accurate and low-cost scale models can aid in the design and optimization of lightweight and fuel-efficient airplane components [24]. Furthermore, 3D printing of nanocomposites can be used to generate complex components for jet engines that have enhanced damage tolerance and corrosion resistance [24].

This method of 3D printing nanocomposites is also advantageous for electrical components where heat dissipation is crucial for the functionality of key components that are suspectable to overheating. Effective heat transfer in electrical components can be achieved by adjusting the orientation of aligned nanofiller materials with anisotropic thermal properties [23]. Moreover, polymers with enhanced energy capacity and anisotropic electrical conductivity can be realized through the alignment of functional nanofillers for a wide variety of electronics [25]. Additionally, high-resolution and porous biomaterials, such as scaffolds for bone regeneration, can be easily designed and fabricated using the MF-VPP method to create implants capable of withstanding shearing, compressive, and tensile loading [26]. In conclusion, creating complex structures with high mechanical strength on demand, which are both low cost and reliable, can be realized for numerous applications using the biomimetic approach described here.

3. Scaly-foot snail inspired multimaterial for property enhancement

3.1 Functional architecture of scaly-foot snail

Organisms in nature have evolved excellent survival strategies due to their special living environments. Scaly-foot snail (Figure 3a), a creature that lives in a deep-sea crater, is the only organism with metal in its skeleton (Figure 3c) [6, 29]. The multi-layer structure of its shell can effectively deal with the threat of toxic substances and high temperature in the environment (Figure 3b) [6, 29]. The multi-layer multi-material structure also endows shell with extremely high hardness and excellent energy dispersion capability under impact. This outstanding property has widely attracted the attention of researchers, but it is extremely difficult to reproduce this structure in an integrated manner, both in terms of materials and manufacturing. Inspired by the excellent performance of the heterogeneous material system inside the scaly-foot snail’s shell, the researcher Tang et al. developed a heterogeneous mixture that can be both photocured and electroplated. The matrix of the heterogeneous material system is PEGDA with good biocompatibility, PEDOT:PSS with excellent conductivity is used as filler to improve the conductivity of the matrix, photoinitiator is used to initiate the crosslinking of PEGDA, and CuSO4 solution is used as electrolyte to adjust rheological properties to meet the viscosity requirements of DLP printing.

Figure 3.

Structural images of scaly-foot snail. (a) Image of the scaly-foot snail; (b) multilayered structure shown in cross-section of scaly-foot snail shell; (c) columnar channels and dispersed iron sulfide inside the shell. (a) Copyright from ref. [27]; (b, c) copyright from ref. [28].

3.2 Electrical field-assisted vat photopolymerization

The printing system in the reported work is a typical digital light processing (DLP) [30], which is mainly composed of digital micromirror device (DMD), linear stage, solution tank, printing platform, and control system (Figure 4a). The difference is that in order to introduce electroplating in the conventional layer-by-layer photocuring printing, two copper sheets are placed in the solution tank and connected to the negative pole of the power supply as an anode, and one corresponding copper sheet is placed on the bottom of the printing platform for both adhering the cured layer and as a cathode (Figure 4b). As shown in Figure 4c, when curing the polymer matrix, the projector projects a 2D light pattern of a specific shape to selectively cure the local area, and the conductive filler PEDOT:PSS and CuSO₄ electrolyte are sealed in the PEGDA polymer chain. The projection of UV light is stopped during electrodeposition, the power supply is connected to the electric field between the cathode and anode, and the copper particles migrate to the bottom of the polymer layer to obtain electrons and then reduce to copper (Figure 4d). So far, this article has demonstrated the composition of the heterogeneous material system and the working logic of the printing system, and the electroplating results also show the effectiveness of the materials and manufacturing methods [29].

Figure 4.

Manufacturing of bioinspired polymer/metal structures using electrical field-assisted vat photopolymerization and heterogeneous material systems. (a) Schematic diagram of the electrical field-assisted vat photopolymerization set-up; (b) illustration diagram of the electric field and projection system of the printing set-up; (c) demostration diagram of photocuring process in a single printed layer; and (d) schematic diagram of electrical field assisted metal deposition process. All figures’ copyrights from ref. [29].

3.3 Fabrication of polymer/metal structures

Polymer/metal structures are widely used in flexible circuits [31, 32], sensors [33, 34], and soft robots [35, 36] because of their light weight, extraordinary corrosion, and wear resistance. Most of the existing manufacturing methods are to first manufacture the polymer base by injection molding or 3D printing and then perform electroplating [37]. Since most polymers are not conductive, it is necessary to spray a conductive layer before electroplating [37]. Here, electric field-assisted 3D printing avoids the tedious and energy-wasting, time-consuming, and labor-intensive steps of traditional manufacturing [29]. It can print the polymer base and electroplate the metal surface in one step. The specific steps are as follows. The 3D model is first established by SolidWorks, and the output STL format file is imported into the self-built program for slicing. The system automatically assigns different material indexes to different layers during slicing. An index of zero corresponds to the photocuring step, and an index of one corresponds to the electroplating process. The obtained black and white mask images are then loaded into the printing program. Before printing starts, the motion system needs to be initialized, and the motion control program is used to move the printing platform to the zero point. At this time, the ground of the printing platform is in contact with the Teflon film at the bottom of the resin pool. After the printing process starts, the printing platform rises to leave a gap to solidify the base layer. Afterward, the system judges the material index corresponding to the picture. When the index is zero, the system projects UV light to cure polymer. The cured layer is then lifted by the platform to generate a new printing layer gap, and all following layers can be printed in this way. When the index is one, the system automatically progresses to the electroplating step. At this time, the polymer layer is lifted as an anode and only the bottom touches the mixture solution. After the electric field is turned on, the copper ions accept electrons on the surface of the anode and reduce to copper. As shown in Figure 5, the pitchfork polymer base was first printed, and then the structure was placed in the same mixture for electroplating to obtain the metal surface. The copper and polymer layer can be clearly seen in Figure 3c. Due to the polymer base photocuring material and the material used for electroplating being the same material, the copper grows toward the polymer layer, which strengthens the polymer/metal interlayer bonding to avoid delamination.

Figure 5.

Polymer/metal structure. (a) Schematic diagram of the polymer ASU pitchfork with copper surface. (b-e) Electroplating results and SEM images of the printed ASU pitchfork. All figures are copyrighted from ref. [29].

3.4 Applications of polymer/metal structure

Benefiting from the aforementioned photocurable heterogeneous materials that can be used as electrolytes at the same time, it is possible to manufacture polymer/metal structures in an integrated manner, making up for the time-consuming and redundant multi-step process of traditional manufacturing methods. After testing the curing properties and plating properties of the material, the authors printed a series of different structures in Figure 6 [29]. In order to demonstrate the corresponding multi-layer printing logic, the author printed a triangular base, electroplated a layer of copper and continued to print a layer of triangular polymer cylinders, and finally obtained a polymer-metal-polymer sandwich structure (Figure 6a,b). Metals are used in circuit manufacturing because of their excellent conductive properties, and the metal/polymer structure not only makes up for the low conductivity of polymers but also has the advantages of lightweight and flexibility of polymers. Based on this, circuits printed with pure polymers and electroplated circuits were manufactured (Figure 6c-e). The experimental results show that the circuits after metal plating have better electrical conductivity. The LED lamp beads light up when the circuit is connected, but the polymer circuit cannot be lighted because of its high resistivity. As a result, the multi-layered complex polymer-metal-polymer sandwich structure and the enhanced conductivity after electroplating demonstrate the high degree of manufacturing flexibility, structural complexity, and functionality of electric field-assisted printing heterogeneous materials in integrated packaged circuits [38], flexible sensors [38], and electromagnetic interference (EMI) shielding [39, 40].

Figure 6.

Demonstration of the sample made by electric field assisted vat photopolymerization. (a, b) schematic diagram and the manufacturing results of the polymer-metal-polymer sandwich structure. (c-e) Effect of electrodeposition on circuit conductivity. All figures are copyrighted from ref. [29].

4. Delosperma nakurense inspired nanocomposite for shape changing

4.1 Functional architecture of Delosperma nakurense

Delosperma nakurense, a plant that lives in arid environments, chooses to open its seed chambers after rain or when there is high humidity to release its seeds to increase seed survival rate. This specific strategy of hygroscopic deformation is reflected in many plant species, such as pinecones [41], wheat awns [42], seedpods of orchid trees [43], and spikemoss stems [43]. These hygro-responsive structures choose to open the structure under wet or dry conditions to release seeds to complete reproduction. The spontaneous hygroscopic deformation of the seed chamber structure is of great significance to the design and manufacture of flexible devices, self-responsive sensors, and soft robots [44]. Figure 7 describes the process of the seed chamber absorbing moisture and expanding to release the seeds. The researchers found that the three main points of this process are: porous structures are the basis of moisture absorption deformation, arranged cell wall realizes anisotropic deformation ratio, and swellable cellulose fiber is used to absorb water. It is essential to replicate these three elements in principle to bionically manufacture this structure. In order to achieve this goal, the researchers Tang et al. used liquid crystals and nano-fillers as the materials and used the phase separation of the two liquid crystals during curing to obtain a porous structure that can absorb moisture.

Figure 7.

Seed capsule releases the seeds after absorbing moisture. (a, c) copyright from ref. [9]; (b) copyright from ref. [45].

4.2 Liquid crystal templating assisted vat photopolymerization

Liquid crystal, a substance that is both liquid and crystal and has a specific effect on light, has a strong polarization property that enables it to align under the action of electric field. This electro-alignment property is exploited to align nanofillers to impart anisotropic deformation of the structure. Correspondingly, using an electric field to align polymers and nanofillers is a proven approach to obtain anisotropic structures. Unlike extrusion molding, which uses shear force or ultrasonic vibrations to align polymers and nanofillers, ellipsoidal liquid crystal molecules are deflected under the action of an electric field force so that the long axis aligns with the direction of the electric field. At the same time, the deflected liquid crystal molecules drive the nanofillers to align in the direction of the electric field. This indirect arrangement makes up for the deficiency that the electric field can only arrange conductive substances (such as CNT [46], graphene [47]), and makes it possible to use the electric field to arrange non-conductive substances, such as the SiC nanofiller used in this study.

Based on the characteristics of liquid crystal materials, the authors Tang et al. propose an electric field-assisted printing strategy, which places electrode pairs facing in different directions in the resin tank on the basis of traditional digital light processing. The printing system is mainly composed of a projector that projects a specific 2D light pattern, a stage that carries a cured layer and moves linearly, and DC power supply that is used to generate a high-voltage electric field. The electrode pair in a certain direction is first connected during printing, and after the LC and nanofillers are arranged for a period of time under the action of the electric field, the projector projects a beam of light to cure the selected area. By connecting electrode pairs in different directions and printing repeatedly layer by layer, LC and nanofillers with different alignment directions can be obtained by curing in a single layer or inside different layers.

4.3 Dynamic electro-alignment characterization

Controlling the strength and timing of the electric field is critical for aligning liquid crystals and SiC nanofillers. Even though both liquid crystal and SiC are poor conductors, applying a high-intensity electric field for a long time generates a lot of heat, which causes the liquid crystal to undergo a nematic to isotropic phase transition [48]. Once the phase transition occurs, the liquid crystal becomes disordered [49], and the SiC that was previously aligned indirectly is simultaneously driven out of the ordered alignment state by the moving liquid crystal. When the electric field strength is weak, the polarization force applied to the liquid crystal molecules is not enough to drive the liquid crystal to move or it takes a long time to complete the arrangement. Therefore, choosing an appropriate electric field strength and application time is an indispensable condition for obtaining a homogeneous unidirectional alignment. In addition, the researchers also found that the alignment results using an alternating current (AC) electric field are significantly different from those using a direct current (DC) field. Even if the AC electric field is applied for a long time, there is no significant movement once the alignment is completed. In the case of the same voltage value, although the DC electric field increases the rate of arrangement, if the electric field is still applied after the arrangement reaches the highest degree of anisotropy, the order of the arrangement changes significantly, and the mixture undergoes approximately macroscopic disordered turbulence. This is detrimental to the desired result and should be avoided.

Specifically, when an AC voltage of 1 kV is applied, the long axis of the liquid crystal is slowly deflected to the direction of the electric field while driving the dispersed SiC nanofillers to gather and arrange in a line. The Fourier transform results in Figure 8 show that the mixture is not directional when the electric field is not applied (0 s). After 195 s of alignment, the prominent peak curve in the probability distribution curve indicates that the mixture is anisotropic. Continuing to apply electric field, the degree of alignment does not change. In order to demonstrate the sustainability of the arrangement, within 2 mins after the electric field was turned off, the directionality of the arrangement did not decrease significantly, see the relative positions of the red and green curves in the Figure 9. When aligning mixture with 1 kV DC compared to AC, the results were significantly different in terms of time and sustainability of the alignment. The DC electric field makes the alignment more rapid, and an application time of 12 s is sufficient to obtain the most anisotropic alignment, compared to the 195 s required for the AC electric field (Figure 10). However, this directionality is destroyed with the prolongation of time. When the arrangement is 120 s, the mixture again becomes chaotic and enters a turbulent state. The potential reason for this phenomenon is that the direction of the DC electric field is always constant, and the liquid crystal molecules are subjected to the electric field force of a single direction and then always migrate in the same direction. In the AC electric field, the liquid crystal molecules are subjected to alternating electric field forces, and there is no dominant force to move them in a specific direction, so the liquid crystals are only deflected in situ. The alignment state of the LC/SiC nanofiller does not change over time as long as the heat caused by the electrification does not cause the liquid crystal to undergo a nematic to isotropic phase transition. In summary, the AC electric field can make the arrangement more uniform and avoid entering a chaotic state, while the SiC nanofiller under the arrangement of the DC electric field has a more significant bundle, and the degree of anisotropy of the arrangement is higher but need to avoid applying an electric field for a long time to maintain the directionality of the arrangement.

Figure 8.

Probabilistic analysis of the directionality of the alignment under alternating electric field.

Figure 9.

Liquid crystal templating assisted vat photopolymerization. (a) Schematic illustration of the liquid crystal templating assisted vat photopolymerization; (b) schematic diagram of electric field alignment of liquid crystal monomer and SiC nanofiller. All figures are copyrighted from ref. [45].

Figure 10.

Probabilistic analysis of the directionality of the alignment under direct current electric field.

4.4 Photopolymerization induced phase separation

As mentioned in Section 4.1, the core of realizing hygroscopic deformation is an anisotropic porous structure, and liquid crystal as a functional material is well suited for this purpose. The phase separation of the two liquid crystals during solidification provides the possibility to fabricate a porous structure, and the arrangement of the applied electric field allows the pores to be aligned along a specific direction, and finally, an anisotropic and gradient porous structure can be obtained [45]. As shown in Figure 11a, under the action of an electric field in the RM257/5CB/SiC homogeneous mixture, the long axes of the liquid crystals RM257 and 5CB are deflected to be in line with the direction of the electric field, and this microscopic movement then indirectly drives the SiC nanofillers to converge and align in a straight line. Subsequent irradiation of UV light triggers the crosslinking of RM257, and 5CB, which does not participate in the reaction as a solvent, separates from the crosslinked RM257 and gathers together, and the area occupied by it is cleaned by acetone, leaving a large number of pores. The timing of phase separation is different due to the difference in light intensity at the bottom and top of the cured layer. The bottom is rapidly cross-linked under strong light irradiation, and it is difficult for 5CB to gather locally on a large scale, so the pore size at the bottom is smaller. On the contrary, due to the lower cross-linking rate at the top, the liquid crystals phase-separated to a higher degree, eventually leaving larger-sized pores. Figure 11b-(i, ii) show a porous structure with a gradient distribution, and iii and iv show elongated pores with long and short axes and a SiC nanofiller whose arrangement direction is consistent with the long axis of the pores. Therefore, the anisotropic porous structure mechanically enhanced by SiC can be obtained using liquid crystal phase separation and electric field indirect alignment of SiC nanofiller, which opens the possibility of programmable deformations and hygroscopic actuation described in the following section.

Figure 11.

Fabrication process of anisotropic porous structure. (a) Schematic illustration of aligning composite, phase separation during the photopolymerization, and the anisotropic porous structure after extracting unreacted 5CB; (b) SEM images of the gradient anisotropic porous structure with aligned SiC nanofiller. All figures are copyrighted from ref. [45].

4.5 Applications of gradient anisotropic porous structure

As previously conceived, when the LC/SiC mixture is aligned, the cured structure undergoes deformation in a specific direction after cleaning the unreacted 5CB and drying it. Since the major axis of the pores is aligned with the alignment direction, the dry structure curls up along the major axis. As shown in Figure 12a, the curling direction corresponding to the blue area is perpendicular to the red area. The annular structure in Figure 12b is divided into four regions and has a bowl shape when dry. The smaller rectangular region in Figure 12c corresponds to the hinge during deformation, along which the larger rectangle deflects. When two adjacent areas in the same layer are at +45° and −45°, respectively, the deformation directions of the two areas are perpendicular to each other after drying, and the deformations are mutually restrained and folded inward (Figure 12d). In order to demonstrate the deformation constraints between different layers due to the different alignment directions, a double-layer ring structure as shown in Figure 12e was printed, and the deformation directions perpendicular to each other in the two layers lead to a saddle-shaped deformation. However, the structure in Figure 12f is diagonally curled between the respective layers, and the opposite curling directions of the two layers are perpendicular to each other. So far, by setting different alignment directions in different regions in the same layer or between different layers, a series of programmable deformations can be obtained. Furthermore, the final deformation result can be predicted in advance through simulation, and the experimental results also confirm that the simulation is highly consistent with the real deformation.

Figure 12.

Spatially programmable alignment and deformation with the prediction of simulation. All figures are copyrighted from ref. [45].

Soft grippers have attracted widespread attention because of their flexibility and unique driving methods. The common ones are pneumatic drive [50, 51], electromagnetic drive [52, 53, 54], and cable chain drive [55, 56]. However, the structures corresponding to these driving methods are very complex and require the input of external energy. Therefore, designing and manufacturing a self-actuating flexible gripper is the focus of current research. The spontaneous deformation property of the porous structure upon moisture absorption and drying is a favorable way to fabricate self-propelled flexible grippers. The author designed a four-beam gripper, and the liquid crystal alignment direction is perpendicular to the direction of the beams. As shown in Figure 13, the beam is fully bent, and the entire gripper is closed in the dry state. After placing it in the acetone, the gripper opens after the pores absorb moisture and expand, and then moves the gripper over the yellow target. As acetone volatilizes in the air, the pore previously filled with acetone collapses, and the gripper bends and closes to grab the target. Finally, the target is moved into the acetone, and the beam continues to expand hygroscopically to release the target. The entire grasping process is about 206 s, and the deformation of the gripper is reversible, as a hygro-responsive structure stretches in acetone and contracts in the air. In summary, the programmable control of the hygroscopic deformation of the anisotropic porous structure opens up new opportunities for the fabrication of flexible grippers [57], ultrafiltration membranes [58, 59], and flexible sensors [60]. The future research direction is mainly to control the shape and distribution of pores on the micron scale, improve the actuation response rate, and enhance the mechanical properties of composites.

Figure 13.

Dynamic grasping process of hygroscopic gripper. All figures are copyrighted from ref. [45].

5. Salvinia molesta inspired nanocomposite for controllable wettability

5.1 Functional architecture of S. molesta

Since Dettre and Johnson discovered in 1964 that the superhydrophobicity of the lotus leaf is related to the nano/microscale dual-scale structure of its surface [61], a large number of biomimetic structures have been designed and manufactured by researchers, such as the eggbeater-like structure of S. molesta that exhibits long-term air retention capability, which enables water droplets to remain on the structure’s surface (Figure 14a,b). The core of the superhydrophobicity exhibited by this structure is that the hair on the head of the eggbeater is coated with nano-scale wax crystals and the hair ends have hydrophilic patches (Figure 14c). This nano/microscale dual-scale roughness effectively reduces the surface energy of the structure. In order to bionically manufacture this structure, researchers mostly use polymer resin to print structures below the millimeter level and the smallest size is only a few microns [63]. Although this method can reproduce the natural structure excellently and has good superhydrophobic performance, the dual-scale roughness characteristics of the surface of the natural structure cannot be reflected. Therefore, Yang et al. tried to add multiwalled carbon nanotubes (MWCNTs) to the resin to increase the roughness at the nanometer level. The results showed that MWCNTs improved the superhydrophobicity of the polymer structure.

Figure 14.

Superhydrophobic structures of Salvinia molesta in nature. All figures are copyrighted from ref. [62].

5.2 Immersed surface accumulation vat photopolymerization

Different from conventional top-down or bottom-up layer-by-layer printing, immersed surface accumulation innovatively uses optical fiber to guide the light beam, combined with computer numerical control (CNC) multi-axis printing platform to obtain greater degree of printing freedom (Figure 15a). The mask image obtained by slicing the three-dimensional model is projected onto the focusing objective lens by the digital micromirror device (DMD), and the shrunken optical pattern is guided into the photocurable resin solution through the optical fiber. The polymer undergoes photoinitiated polymerization under light, and the mixture is selectively cured. The MWCNTs are sealed in the polymer network, some MWCNTs protrude from the surface of the structure to form nanoscale roughness. The polydimethylsiloxane covering the end of the optical fiber makes the cured polymer easily separated from the optical fiber. At the same time, the printing result can be observed through the beam splitter, so as to detect the printing quality in real time. At the level of printing freedom, immersed surface accumulation can not only realize the layer-by-layer printing of conventional vat photopolymerization on the plane but also can print on complex curved surfaces or sides. The scanning electron microscope image in Figure 16 shows the effectiveness of the printing system. The shape of the structure is similar to the natural structure. The structure below the micron level has a minimum size of only tens of microns. The subsequent contact angle test also fully confirmed the superhydrophobicity of the structure.

Figure 15.

Immersed surface accumulation 3D printing. (a) Schematic diagram of the vat photopolymerization set-up; (b) schematic diagram of the optical system of the immersed surface accumulation strategy. All figures are copyrighted from ref. [1].

Figure 16.

Scanning electron microscope (SEM) images of the eggbeater-like structure. (a) Structures made from polymer; (b) structures made from polymer/MWCNTs mixture. All figures are copyrighted from ref. [1].

5.3 Fabrication of eggbeater-like microstructure

By using pure polymer and polymer/MWCNTs to print the eggbeater structure, the effect of MWCNTs on the surface roughness can be compared. Figure 16a is the top view and side view of the structure made of pure polymer. It is obvious that the entire Salvinia molesta-like structure has been completely reproduced, the surface is very smooth, and there is no overcuring between layers. This demonstrates that it is feasible to replicate micron-scale biomimetic structures using photocurable polymers and digital light processing. When MWCNTs were mixed into the polymer, the surface of the structure was distributed with microscale microgroove (Figure 16b), and the roughness increased. More importantly, part of the MWCNTs protrudes from the surface of the structure (Figure 16b), which constitutes the nanoscale roughness. Experiments prove that the nanoscale roughness caused by the microgrooves on the surface of the structure and the protruding MWCNTs significantly reduces the surface energy of the structure, and the two synergistically enhance the superhydrophobicity of the structure.

5.4 Applications of superhydrophobic structure

In recent years, with the increasingly frequent exploitation and transportation of marine resources, emergencies such as offshore oil spills have occurred more and more frequently, which not only caused huge economic losses but also caused serious damage to the marine ecological environment. Traditionally, spilled crude oil has been dealt with by physical, chemical, and biological methods. For example, a porous lipophilic material absorbs oil through capillary action and stores it in the pores of the material, and finally recovers the crude oil adsorbed by the porous material through extrusion or centrifugation. In addition, the oil-water separation ability of the structure can be greatly improved by changing the lipophilic and hydrophobic properties of the structure. Therefore, through the design and surface treatment of the surface micro-nano structure, the oil absorption and hydrophobicity of the oil-absorbing material can be effectively improved. At the same time, due to the capillary force caused by the surface micro-nano structure, its adsorption capacity to the oil layer is greatly enhanced. As shown in Figure 17, under the action of surface tension and gravity of the droplet, the droplet of the oil-water mixture remains spherical on the surface of the eggbeater-like structure. Since the structure is superhydrophobic and has a strong capillary effect on oil droplets, the oil droplets penetrate into the gaps of the columns while the water droplets remain spherical. After 6 s, the oil droplets were completely absorbed by the biomimetic structure, achieving oil-water separation. When the eggbeater-like structure holds the oil-water droplets in different relative positions, even if the droplets are suspended below the surface of the structure, the result of oil-water separation does not change. At tiny scales, oil-liquid separation will not be affected by gravity but is mainly determined by the surface tension of the droplets and the surface energy of the structure. Therefore, regardless of the relative position of the droplet and the eggbeater-like superhydrophobic structure, the result and rate of the oil-water separation did not change significantly. In summary, the bionic superhydrophobic structure will have a wide range of applications in surface self-cleaning, oil-water separation, and anti-icing.

Figure 17.

Oil/water separation of the printed eggbeater-like structure. Oil/water separation performance under the tilt condition of (a) 0°, (b) 180°, (c) 45°, and (d) 90°. All figures are copyrighted from ref. [1].

6. Conclusion

The implications for biomimetic fabrication of highly functional structures found in nature are both significant and challenging. This paper summarizes the difficulties and shortcomings of existing research in terms of materials, structures, and manufacturing methods required for bionic manufacturing, and proposes a series of preparation methods for bionic materials and highly flexible additive manufacturing methods. Firstly, since a single material is not enough to make the structure highly functional, the bioinspired polymer/nanofiller composite can effectively endow the structure with better mechanical performance, optical characteristics, thermal conductivity, hygroscopic deformation, and superhydrophobicity. Secondly, in view of the fact that there are still some slight differences between the bionic structure and the natural structure, and these differences are decisive for the performance, it is necessary to replicate the natural structure both macroscopically and microscopically, which will play an important role in improving performance. Thirdly, at the level of manufacturing methods, vat photopolymerization provides an effective means for manufacturing multi-scale complex structures, and the introduction of magnetic and electric fields has created the possibility to increase the complexity of the structure, and the nanofillers spatially arranged in a specific direction have brought the structure mechanical, thermal, optical, and deformational manifestations of anisotropy. Although the current research on biomimetic manufacturing is quite effective and the performance of the structure is excellent, there are still many potential research fields worth studying in the future: 1) The study and imitation of the structure, material, function, and interaction of natural organisms must be the result of comprehensive application and cross-learning of biological principles, material chemistry, advanced manufacturing technology, and numerical simulation; 2) Bionic structures need not be limited to natural structures despite their superior performance, the next-generation functional structures can be predicted through big data analysis and numerical simulation; 3) Continue to develop advanced manufacturing methods to cope with multi-scale, multi-material, and multi-physical-field printing needs; 4) The intelligent bionic structure should not only originate from nature but also return to nature, which realizes the purpose of green manufacturing and sustainable development. Overall, deconstructing at the material and structural level and using additive manufacturing to reproduce natural structures is effective in promoting bionic design and manufacturing, and opens new doors for the manufacture of next-generation high-performance mechanical, thermal, optical, and hydraulic structures.

Acknowledgments

The authors acknowledge ASU startup funding, ASU FSE Strategic Interest Seed Funding, and National Science Foundation (NSF grant No. CMMI-2114119).

Conflict of interest

The authors declare no conflict of interest.