Manufacturing Techniques of Auxetic Structures: Comparison
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Please note this is a comparison between Version 1 by Pedro Henrique Poubel Mendonça da Silveira and Version 2 by Dean Liu.

This text will address the additive manufacturing of auxetic materials. As it is known, auxetic structures are materials with unique mechanical behavior associated with negative Poisson ratio. Although AXS configurations combined with various types of materials have a wide range of applications, the characteristic re-entrant geometric model of AXSs imposes significant limitations and difficulties in assembling the lattice structure using conventional manufacturing methods. However, additive manufacturing has shown to be a promising option for producing auxetic materials, overcoming the challenges presented by conventional manufacturing. Therefore, the development of auxetic materials through additive manufacturing is an ever-evolving field that presents great potential for a variety of industrial sectors.

 
  • honeycomb
  • sandwich panel
  • auxetic structures

1. Self-Propagating Photopolymer Waveguides (SPPW)

The self-propagating photopolymer waveguides (SPPW) technique assembles a 3D polymeric structure in a cellular microscale arrangement, interconnected periodically [1]. It consists of an ultraviolet (UV) light beam enlightening a 2D mask, which has an aperture inside a photomonomer container [2]. The SPPW is based on the incident light self-trapping effect upon the polymer, being induced by a change in the refraction index among the liquid monomers and the rigid polymeric material. Owing to this event, the SPPW is able to create high aspect ratio beams having a continuous cross-section, being perfect for the 3D topologies construction based on beams. Furthermore, it is possible to control the topologies by an aperture pattern on the mask and orientation of the incident UV light. While the unit cell dimension and the smaller size details, regarding the topologies, rely on the aperture diameter and mask space. The material thickness is controlled mostly by the maximum waveguide length propagation. Moreover, a multilayer strategy could be applied in order to acquire structures displaying a higher thickness [3]. However, in contrast to other AM techniques, the SPPW is limited regarding the topologies randomness, since it is just possible to manufacture variations of beam-based topologies. The extended UV light exposure requested to achieve the maximum waveguide length often enlarges the original diameter, developing thicker materials [138]. On the other hand, the great advantage of it is the high manufacture speed, being able to produce microscale AXS in a few minutes in addition to a great scalability capacity, yielding manufacture rates superior to 1 m2/min [141]. The aforementioned characteristics make it an attractive method for large-scale fabrication.

2. Microstereolithography

Microstereolithography is an AM technique classified according to the ISO/ASTM 52900:2021 standard [4] as belonging to the Vat Photopolymerization (VPP) category. υSLA uses a UV light beam to cure the resin, resulting in the rapid assembly of structures. The operational mechanism of the SLA technique is based on the local polymerization of a photosensitive resin. Each layer is obtained by moving the UV light beam over a surface, which is guided on the x- and y-axes by galvanometric mirrors. The solidified object is then immersed in a resin reservoir, and a new layer is reflected onto the already polymerized layer, allowing the manufacture of the next layer [5][6][7].
Furthermore, the AM techniques yielded a strong influence on the production of industrial parts because they significantly reduced the processing time, entailing a manufacturing economy regarding objects with complex geometry. The υSLA was one of the first to be developed, and due to its velocity, is, hitherto, the most applied in industry. The manufacturing costs for the υSLA pieces are affordable, and the surface finishing is appropriate, without a requirement for further operations. The typical resolution range for υSLA is 150 mm for the three space directions. However, some parameters, such as: resolution, precision, and surface roughness may change since they are based on over layering. In particular, the vertical resolution (over the construction axis) is associated with the overlapped layer thickness, whereas the lateral resolution (on the plane) is determined by the light beam dimension used to design each layer shape on the resin surface.
In comparison to others’ AM, the paramount υSLA advantage is the production of 3D microscale objects holding complex structures and supporting a high velocity fabrication. On account of these compelling characteristics, υSLA is assigned to several fields such as: biomedicine, tissue engineering, micro-optic devices, bioinspired materials, micro-electromechanical, among other systems [8][9][10][11].
The υSLA is relevant regarding the development of AXSs since it can produce with ease auxetic cores for sandwich panels, and therefore apply them on a large scale. Alomarah et al. [12] manufactured re-entrant AXSs and re-entrant chiral auxetic (RCA) composed of a photopolymer composed by polypropylene (PP) by υSLA. In a more recent work, Varas et al. [13] also used PP to produce different types of AXSs and evaluate the properties of each structure.

3. Direct Laser Writing (DLW)

Direct laser writing (DLW) is a well-known AM method for the fabrication of complex structures on a nanoscale up to 100 nm [14]. In this method, a beam laser is focused through an objective lens in order to cure photopolymers by means of a single or multi-photon absorption [15][16]. The DLW by single-photon absorption is limited to the manufacture of 2D structures because its absorption occurs inside a whole area of photopolymers exposed to light. The multi-photon absorption, also known as two-photon polymerization, takes place in a small voxel on the focal point of the laser beam, where the light intensity is remarkably high [17]; thus, arbitrary 3D structures possessing details excessively smaller could be fabricated by the voxel [18][19]. As a consequence, this process is an attractive tool for applications in several fields, such as micro-optics, supercapacitors, microfluidics, biomedical implants and tissue engineering [20][21][22][23][24][25][26]. 
The principle of the DLW is based on a laser beam that is pulsed at a wavelength close to infrared (λ = 780 nm). This laser is focused on a photoresist material, transparent to the wavelength, through a high aperture objective lens. The photoresist contains photoinitiators for the absorption of two-photons. Due to the focus through the objective lens, the photons are absorbed within the focal volume. Thus, initiators cause a chemical reaction when excited, which enables the formation of chemical bonds, for example, by polymerization or bond breaking. During sample manufacturing, moving the focal spot in different directions enables the fabrication of complex 3D structures at high fabrication speeds [27].
The DLW is able to manufacture materials without the necessity of supporting matrices or a layer by layer process, as the SPPW, owing to the capacity to precisely induce the polymerization within a particular spatial position of a photoresist thick-film [28]. Despite the aforementioned benefits, the DLW is not widely applied for mass production in all industries because it is much less scalable than other methods. Additionally, the costs of acquiring and conserving DLW systems are very high because accessories such as optical systems and femtosecond lasers are expensive and difficult to obtain. Thus, the process becomes unfeasible for large-scale fabrication of AXSs. Another factor that makes this technique inaccessible in the production of AXSs is the choice of photoresist materials. Because the transparency to the infrared beam is necessary for the processing to occur, and the selection of materials is limited to transparent polymers, other materials such as metallic and ceramic particles are not attainable in the process because they inhibit the penetration of the laser.

4. Self-Assembly

The Self-Assembly technique is characterized by the mechanism of polymeric phase separation, which can be in the form of emulsion or colloidal suspension. Although previously not considered an AM technique, due to its technological unfeasibility for such classification, since 2018, research involving this manufacturing method has been growing exponentially, allowing the technique to gain technological maturity to be classified as an AM technique [29]. This is due to its ability to rapidly produce complex structures in a distributed manner, making it a method of great importance during the COVID-19 pandemic between 2020 and 2022 [30][31][32]. The interactions between the components in the fabrication present van der Waals bonds, hydrogen bonding, electrostatic attraction, as well as hydrophobic and hydrophilic interactions. These interactions can create a self-assembly system owing to a thermodynamic condition of non-equilibrium, establishing a stable state within well arranged hydrophobic structures [33][34][35]. It is possible to develop materials with organized nanostructures from the separation of the microphases present in the material. Self-assembly is very competitive when compared to other AM techniques owing to the ability to produce parts with customizable morphology and functionalities [5].
In order to manufacture materials on a large scale, a block copolymer (BCP) phase separation technique was developed. BCPs are divided by the management of chemical properties and molecular weight of each polymer. The self-assembly by BCP separation occurs by the thermodynamic incompatibility of the copolymer phases, where a separation of microphases with a variety of nanoscale morphologies is generated, such as: lamellas, spheres, cylinders and others [36][37][38]. 
Manufacturing nanostructured materials by self-assembly is not limited only to polymers. Metallic and ceramic structures can also be processed by this technique; for instance, it has already been developed nanostructures of gold [178], CeO2/MnOx@C [179] CoPC/CNTs [180], Fe3O4/rGO [181] among others for several applications.
In comparison to other AM, self-assembly yields a unique low-cost opportunity, highly scalable and a fast manufacture for 3D structures possessing micro and nanoscale. However, this process’s ability for the fabrication of several morphologies, such as octet or cubic plate lattices, is still very limited. In addition, up to now, it is difficult to manage a self-assembled final topology [35]. As a consequence, defects are acquired in large-scale fabrications, and further investigations are required to solve these problems.

5. Selective Laser Melting (SLM)

The selective laser (SLM) is another method for the construction of 3D metallic structures. It uses computational support and a high energy laser in order to bond the metallic particles [39]. SLM makes viable the manufacture of metallic components layer by layer following a 3D-computer aided design (CAD) model. Thus, this enables an almost unlimited fabrication of complex geometries without the necessity for pre-production costs or specific tools [40]. The SLM mechanism occurs according to the following steps: (i) the 3D-CAD model is decomposed in layers and sent to a selective fusion laser equipment; (ii) a powder material is laid upon the substrate, creating a thin layer; (iii) each piece of geometry information is transferred by the laser beam to the powder bed where the regions having the solid materials are scanned under an inert atmosphere; thus, another layer can be manufactured; (iv) after a layer is constructed upon the substrate, the equipment bases connected to a plunger go down, so a new layer is inserted above the first one. Therefore, the manufacturing steps are repeated layer by layer until the sample is constructed [41][42]. The printed samples can reach a density value close to 100% because metallic powders are used, disclosing suitable mechanical properties combined with a rigorous management of the material composition [43].
The AXSs production by SLM allows the acquisition of materials with superior mechanical properties compared to the auxetic ones composed of polymers. A great majority of AM techniques process polymer structures. Although they present acceptable properties, they still exhibit limited characteristics, in comparison to metals in terms of a greater thermal and mechanical resistance. Singh et al. [44] processed re-entrant AXSs by SLM. They adjusted parameters and mechanical tests and were able to attain a Poisson’s Ratio of −3.1. Gao et al. [45] brought on auxetic contacted cube (ACC) structures by SLM using an AlSi10Mg alloy. The authors observed a superior density to 99% for the specimens; in addition, they displayed NPR and a good deformation distribution for all the samples.

6. Other Techniques

In addition to the previously mentioned techniques, there are several others for the production of AXSs. Each method has its own process particularities, making it viable to construct complex structures fast and with ease. As mentioned in the text, AXSs can be produced by both conventional techniques and more advanced techniques, such as AM techniques. Among all the available techniques, it is worth mentioning: extrusion [46][47][48], fused deposition modeling (FDM) [49][50][51], inkjet printing [52][53], selective electron beam melting (SEBM) [54][55][56][57], selective laser sintering (SLS) [58][59][60][61], aerosol jet printing [62][63][64], chemical vapor deposition (CVD) [65][66][67], and others. Classifying these techniques is a complex task because the research areas for the AM and AXSs present an accelerated expansion. However, certain methods may show similar features but manufacture distinct materials classes. Therefore, it is not possible to classify them in order to expand a range of applications and materials.

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