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Direct laser writing of auxetic structures: present capabilities and challenges
View the table of contents for this issue, or go to the journal homepage for more
2014 Smart Mater. Struct. 23 085033
(http://iopscience.iop.org/0964-1726/23/8/085033)
Home Search Collections Journals About Contact us My IOPscience
Direct laser writing of auxetic structures:present capabilities and challenges
Stefan Hengsbach1 and Andrés Díaz Lantada2
1 Institute of Microstructure Technology, Karlsruhe Institute of Technology (KIT),Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany2 Product Development Laboratory, Mechanical Engineering & Manufacturing Department,Universidad Politécnica de Madrid (UPM), c/José Gutiérrez Abascal 2, 28006 Madrid, Spain
Received 1 February 2014, revised 30 May 2014Accepted for publication 6 June 2014Published 9 July 2014
AbstractAuxetic materials (or metamaterials) are those with a negative Poisson ratio (NPR) and thatdisplay the unexpected property of lateral expansion when stretched, as well as an equal andopposing densification when compressed. Such geometries are being progressively employed inthe development of novel products, especially in the fields of intelligent expandable actuators,shape morphing structures and minimally invasive implantable devices. Although severalmicromanufacturing technologies have already been applied to the development of auxeticgeometries and devices, additional precision is needed to take full advantage of their specialmechanical properties. In this study we present a very promising approach for the developmentof auxetic metamaterials and devices based on the use of direct laser writing. The process standsout for its precision and complex three-dimensional (3D) geometries attainable without the needof supporting structures. To our knowledge it represents one of the first examples of theapplication of this technology to the manufacture of auxetic geometries and mechanicalmetamaterials, with details even more remarkable than those shown in very recent studies,almost reaching the current limit of this additive manufacturing technology. We have used somespecial 3D auxetic designs whose remarkable NPR has been previously highlighted.
Keywords: auxetics, negative Poisson ratio, metamaterials, morphing structures, direct laserwriting
(Some figures may appear in colour only in the online journal)
1. Introduction
When a material is stretched there is normally an accom-panying reduction in width. A measure of this dimensionalchange can be defined by Poisson’s ratio, ν =−dεtrans/dεaxial,being εtrans and εaxial the transverse and axial strains when thematerial is stretched or compressed in the axial direction. In amore general case, νij is the Poisson ratio that corresponds to acontraction in direction ‘j’ when an extension is applied indirection ‘i’. For most materials this value is positive andreflects a need to conserve volume. Auxetic materials (ormetamaterials) are those with a negative Poisson ratio (NPR)and that display the unexpected property of lateral expansionwhen stretched, as well as an equal and opposing densifica-tion when compressed [1–4]. Natural (some minerals, skins,
etc) and man-made (foams, Gore-Tex®, polymeric foams)auxetics have been described, and very special attention hasbeen paid, since their discovery, to the search and develop-ment of auxetic structures designed and controlled on amolecular scale [2, 5] and, more recently [6].
It is necessary to note that auxetics, understood asmaterials and models of NPR, are not only geometries butalso interactions with external conditions and constraints,such as negative pressure, proximity of certain phase transi-tions, specially woven materials, living tissues and their sur-roundings, polydispersions, among other possibilitiesdescribed in the seminal papers of this field of study [7–10].In any case, auxetic materials and structures leading to auxeticbehaviour are being progressively employed in the develop-ment of novel products, especially in the fields of intelligent
expandable actuators, shape morphing structures and mini-mally invasive implantable devices. Regarding smart actua-tors based on an auxetic structure, it is important to cite somerecent progress linked to auxetic shape-memory alloys (SMA)for developing deployable satellite antennas [11] and someresearch on the characterization of polyurethane foams withshape-memory behaviour and auxetic properties, which havebeen promoted thanks to several post-processing stages [12].In the area of medical devices, recent research has alsoassessed the behaviour of a few auxetic structures forimplementing expandable stents [13] and, more recently[14, 15], their application to other implantable biodevices isclearly a matter of research.
Several auxetic materials and potentially auxetic struc-tures, normally grouped under the terms ‘re-entrant’ (Almg-ren 1985), ‘chiral’ [16] and ‘rotating’ [17] in relation to thecharacteristics that promote auxetic behaviour, have beensummarized in previous reviews and research. However,precise information regarding the values of Poisson ratios isnot always provided due to difficulties with simulating andmanufacturing such complex geometries. Sometimes just ascheme of their folding process, when submitted to uniaxialstresses, is provided, which proves to be limited for sub-sequent design activities. Recent comparative studies havetried to provide additional information of relevant propertiesof different auxetic structures, in order to assist with material/structure selection tasks for the development of novel folda-ble–morphing actuators, structures and devices [18].
Regarding the manufacture of auxetics, to our knowledgethe first successful attempt to obtain such auxetic structures inthe microscale, for achieving metamaterials with NPR, wasmade by soft lithography, leading to details and pores in the100-micron range [19]. The lithographic process explained inthat reference is interesting and promotes some applications(i.e. microtubular structures were proposed).
However, for adequately exploiting the potential ofauxetic metamaterials, an additional degree of precision isneeded. The manufacture of polymeric sheets with auxeticnanostructures can prove to be useful, indeed, for developingactive selective membranes, whose pore sizes can be real-timecontrolled just by applying uniaxial loads. Applications in thebiomedical field (i.e. dialysis) and in energy (i.e. membranesfor catalytic reactors) are worthy of exploration. By rollingsuch auxetic sheets, even easy-implantable devices (i.e.stents) can be created for minimally invasive surgical proce-dures [19].
Tissue engineering, with interactions at a cellular andeven molecular level, can also benefit from auxetic structures[20], especially if these preliminary approaches are improvedwith more micro- or nano-auxetics for obtaining smallerclearances between the cells being cultured. During cell cul-ture, the uniaxial excitations of an auxetic scaffold lead tobiaxial expansions and compressions of the tissue beinggrown, which promotes growth and can potentially controlcell differentiation and tissue viability. However, using con-ventional photolithography or stereolithography for themanufacture of two-dimensional (2D) auxetics and three-dimensional (3D) auxetics, with typical distances between the
lattices of the auxetic structure of more than 100 microns,leaves important clearances between cells and preventsinteraction at the single-cellular level.
The special properties of auxetic metamaterials extendthe benefits of micromanufacturing to auxetic devices foroptoelectronics and telecommunications, which require moreprecision than those attainable by traditional micromachining.Some additional remarkable proposals for obtaining realmechanical metamaterials, with the finest details reachinghundreds of microns, include both subtractive approaches,such as UV laser ablation [21], and additive manufacturingprocedures, such as stereolithography, digital light processingor direct laser writing [22, 23]. Normally 2D and 2D½ auxeticstructures are obtained by means of surface micromachining,chemical etching, laser ablation and typical mass-productionprocesses imported from electronics, while 3D auxetics, withmore complex geometries and inner details, require 3Dadditive manufacturing, or ‘layer-by-layer’ processes, espe-cially the support-less ones, such as selective laser sintering orselective laser melting.
In this study we present a very promising approach forthe development of 3D auxetic metamaterials and devicesbased on the use of direct laser writing. The process standsout for its precision and for the complex 3D geometries it canattain. To our knowledge it represents one of the first exam-ples of the application of this technology to the manufactureof auxetic geometries and mechanical metamaterials, withdetails even more remarkable than those shown in very recentstudies [23]. We have used some special 3D auxetic designswhose remarkable NPRs have been previously highlighted[18]. In the following sections we try to provide interestingdetails of the design and manufacturing processes we haveused, as well as some discussion about the main results,present capabilities, difficulties and challenges concerningnano-auxetics.
2. Materials and methods
2.1. Design process
The different geometries that are the object of the presentstudy were designed with the help of an NX-8.5 (SiemensPLM Solutions), first by obtaining the different unit cells andsubsequently by using Boolean operations and 3D matrixreplication. In both designs unit cells were repeated four timesin the x and y directions and three or four times in the z(vertical) direction to obtain almost cubic structures. Suchrepetitions were oriented at providing a more exact visualimpression of the aspect of the auxetic structure, becausesometimes using just one unit cell might make it difficult toimagine the final aspect. These 3D auxetics are designed toexperiment transversal contractions along the x and y axes,when compressed along the z direction. In a similar way,tractions along the z direction lead to transversal expansions.
A homogeneous thickness was used for the smaller fea-tures of the structures obtained to promote a more adequatecomparison, especially in terms of the equivalent Young
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modulus. We have tried to obtain structures with similaroverall dimensions, with a≈ b≈ c for both 3D auxetics (witha, b and c being the global length, depth and height of thestructures), although some differences appear due to the useof cell units with different degrees of complexity.
Figure 1 includes images of the two 3D auxetic structuresselected; upper geometry has been taken from our recentlydeveloped library of auxetic geometries [18], and the lowergeometry has been inspired by a US Patent [24]. Theircomplexity and reduced dimensions require 3D support-lessadditive manufacturing technologies for adequate develop-ment, as explained in the following subsection.
The structures have been selected due to their high NPR,which has been previously reported [18] and due to having anintricate 3D geometry requiring special support-less additivemanufacturing technologies for their manufacture. For the
first structure (figure 1(a)) the expected Poisson ratio reacheda value of −1.8, and its maximum volume reduction wasaround 17%. These measurements were taken after theapplication of uniaxial loading levels that lead to the begin-ning of contacts between inner features, thus also promotingthe beginning of buckling and structural collapse. For thesecond structure (figure 1(b)) the expected Poisson ratioreached a value of −1.35, and its maximum volume reductionwas around 35%. The normalized Young moduli (Youngmoduli of the lattice structures/Young modulus of the bulkmaterial) reached values of 0.0002 and 0.0027 for the first andsecond structures, respectively. These results are according tosimulations carried out using the finite-element method andhave been validated with trials upon macroscopic rapid pro-totypes obtained by laser stereolithography. However, theideal boundary conditions of the simulations are impossible toobtain, and the actual Poisson ratios measured were 20–25%lower than ideally expected [18].
The validation of the actual properties of micro- andnano-auxetics is even more complex, as adhesion forcesincrease with part-size reduction, and therefore the idealboundary conditions with free lateral displacements in thedirections normal to the loading forces are much more diffi-cult to obtain. For instance uniaxial loading using nanoin-denters prevents ideally free lateral displacements because thewhole upper and lower surfaces of the structures completelyadhere to the testing bench. The impact of such an adhesion,being a surface force, increases with part-size reduction. Inaddition, the microscopy facilities and procedures needed toadequately view the characterization process of a micro- ornano-auxetic when using a nanoindenter are also complex andcurrently beyond our reach. Consequently, in the presentstudy we concentrated on design and manufacturing pro-cesses, while the characterization of the actual behaviour ofsuch geometries remains a challenge for future research. Asthe mentioned properties (Poisson ratio, maximum volumereduction and normalized Young modulus) are, in principle,scale-independent [25], we hope that the provided valuesobtained by means of simulations may be useful for com-parison with future experimental results once the difficultiesof boundary conditions and characterization have beensolved.
In this respect, as highlighted by Grima [25], it isimportant to mention that the scale-independency of thePoisson ratio has been exploited by several researchers intheir quest towards finding new auxetic materials by firstproposing auxetic macrostructures, which are then down-scaled to the molecular level to produce real auxetic meta-materials that mimic the original macrostructures, as we havealso done here. The next subsection deals with the manu-facturing process, and is followed by a discussion of the mainresults.
2.2. Manufacturing process
Once the designs are prepared and optimized for 3D printing,manufacturing is accomplished by means of direct laserwriting, also called 3D laser lithography, an additive
Figure 1. Three-dimensional auxetic structures selected for thepresent study. Taken from the CAD auxetic library developed byÁlvarez and Díaz [18].
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manufacturing technology based on two-photon polymeriza-tion with ultra-short laser pulses [26] using the PhotonicProfessional System from NanoScribe GmbH, the first com-mercial direct laser writing system. NanoScribe GmbH(www.nanoscribe.de) was founded in 2007 by scientists in thefield of photonics as a spin-off company of the KarlsruheInstitute of Technology (www.kit.edu). The company spe-cializes in the innovative technique of 3D laser lithographyand produces compact and easy-to-operate table-top laserlithography systems (Photonic Professional). These systemsare designed for the fabrication of true 3D micro- and nano-structures in various photoresists, although the finest detailsare still above the conventional 100 nm threshold ofnanotechnology.
The direct laser writing process stands out for its accu-racy and versatility, as several resists and even polymer-ceramic mixtures can be manufactured. It is also noted for itsability to work in an additive way without the need of sup-porting structures, which allows for the manufacture ofespecially complex parts with inner details. In short, whenfocused into the volume of a photosensitive material, the laserpulses initiate two-photon polymerization via two-photonabsorption and subsequent polymerization, normally per-ceived as a change of resist viscosity. Polymerization onlyoccurs in the focal point, where the intensity of the absorbedlight is highest, which promotes accuracy. After illuminationof the desired structures inside the resist volume and finaldevelopment (washing out of the non-illuminated regions),the polymerized material remains in written 3D form [26].
It is important to note that the NanoScribe direct laserwriting technology writes the structures in a different waythan conventional additive or ‘layer-by-layer’ manufacturingtechnologies. In other additive technologies, such as normallaser stereolithography, selective laser sintering, melting orink-jet printing, the manufacturing process starts from a 3Dcomputer-aided design (CAD) file, which is sliced in layerswith the help of ad hoc software. Then, the manufacture isaccomplished layer-by-layer, by photopolimerization ordeposition of material along the boundaries of each layer andsubsequent filling of layers with parallel lines of material. Inthe NanoScribe process, the structures are not written layer-by-layer, but follow 3D paths connected from the beginningto the end of the writing process, so additional programmingfor converting the original CAD files into writable structuresmay be needed to generate writing schemes similar to thoseshown in figures 2 and 3.
In addition, it is important to establish an adequatewriting strategy in order to avoid writing through an alreadypolymerized resist. This can lead to unwanted optical effectsand to accuracy problems because the polymerized resist hasa different refractive index when compared to the unexposedresist. The NanoScribe software is able to convert polygonalbodies stored in.stl (standard tessellation language) format,currently the most common format for additive manufactur-ing, into formats such as.dxf or.gds, which store geometries inthe form of polylines more adequate for the vectorial writingprocess followed by the NanoScribe laser. However, in ourcase Matlab (The Mathworks Inc.) is used as support software
to create the structures, and it is also used to generate infor-mation exchange files that can be used directly in the Nano-Scribe Photonic Professional.
The advantage over using more conventional additive-manufacturing slicing software or NanoScribe’s own softwareis that the structure can be calculated and optimized accordingto the writing strategy, which could save time and energy.Time can be saved by choosing the right order for writinglines. Another advantage is that additional control variablescan be used, while parameter variation is easily promoted bywriting ad hoc programs. As the geometries of our study aremade of repeated 3D patterns of unit cells, programming is
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Figure 2. Design adaptation of the NanoScribe process to the firstauxetic structure. Conversion from typical 3D CAD designs to linearpaths for direct laser writing. Upper image: structure written again.Lower image: view of unit cell.
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easier. At first the unit cells are obtained, by measuring ver-tice positions from the initial CAD files and by connecting thevertices following single closed loops. Then the cells arereplicated in the X and Y direction for creating the lower layerof the structures. Finally, the whole 3D geometry is arrangedby adding layers of unit cells. The writing process follows asimilar approach, so that the layers of unit cells are written ina bottom-up approach, and therefore the laser does not collidewith already-written parts of the structure, which promotesfinal accuracy. Using the above-mentioned approach the lasermoves along the trusses of the structure, and its generation isremarkably straightforward. Using conventional slicing soft-ware from other additive manufacturing resources or evenfollowing the polylines of .dxf or .gds files would create apolyhedron-based description of the trusses and lead to
writing several lines for a simple linear truss, thus increasingwriting time and overall energy consumption. By usingMatlab-based programming it is also easy to define additionalsupporting lines or to add extra overlapping writing paths toincrease structural stability, which will be further discussedbecause it has been indeed relevant in this study because it ledto the formation of the final stable structures. Furthermore,parameter variation (i.e. the distance between lines, structurescales, etc) is especially useful for systematic research, whileMatlab-based designs are helpful for providing versatility andfreedom of design. Finally, mathematical variables can beincorporated into the model and used to create more complexstructures in accordance with recent tendencies linked tominimizing.stl file size, thanks to the use of algorithms [27].
As for the laser source, the NanoScribe system uses anerbium-fiber laser system from Toptica, with two outputs:1560 nm and 780 nm (at double frequency). In our case wehave used the second-highest frequency output to obtain thedesired validation probes. This higher frequency output workswith a single-pass second-harmonic generator (SHG), whichuses a periodically poled Li:NbO3 (PPLN) crystal to divide thewavelength, according to information from the Toptica laserdatasheets. The repetition rate of the oscillator is measured at79.5MHz and the pulse duration is below 100 fs. The setup isbasically a laser system combined with an inverted microscope,which is synchronized and controlled by a PC. The setup alsocombines the laser system with an acoustic optic modulator,which is responsible for power adjustments, as well as for thewriting process by helping to block the light when moving tothe new starting point of a new line. The system also incor-porates a beam expander for widening the laser beam. Thebeam is guided through that objective and focused on a resistplaced upon a substrate. The substrate is mounted in a holder,which can be moved by a piezoactuator in the Cartesiandirections x, y, z. The range of the piezoactuator is 300 μm ineach direction with a resolution of 4 nm. To move larger dis-tances than the mentioned 300 μm there are additional motorstages, which can move the piezoactuator and the sampleholder. For enhanced precision and in order to find the interfaceposition of the substrate—sample, by the change of refractiveindex between the two mediums, the system includes a sub-system called ‘definite focus’ by Carl Zeiss. In addition, thereis a high-precision camera system that allows seeing themanufacturing process in progress, as well as to align themanufacturing process to already-existing structures.
For building the auxetic microstructures a resist called‘IP-Dip’ (from dip-in-lithography or DILL) was used. IP-Dipis a resist developed by NanoScribe GmbH and basicallybehaves similarly to SU-8 and to other acrylate-based nega-tive resists. The resist is specially optimized to be used withNanoScribe devices and in particular to be used with thespecial objective for the DILL-technology (dip-in-litho-graphy) [28]. It is not only optimized to provide a very goodresolution but also a remarkable aspect ratio. At optimalconditions a line width of 150 nm at an aspect ratio of 3.5 canbe reached. The material is directly polymerized upon theobjective, without the need of chemical bonding, and the
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Figure 3. Design adaptation to the NanoScribe process of the secondauxetic structure. Conversion from typical 3D CAD designs to linearpaths for direct laser writing. Upper image: Structure written again.Lower image: View of unit cell.
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lower part of the structure remains adhered to the substrate,which allows the manufacture of the whole geometries.
The choice of laser power depends on the material beingprocessed and has a direct influence on the attainable voxelsize (here defined as the minimal building block in additivemanufacturing approaches). Lower powers lead to smallervoxel sizes, although to start the polymerization at a specificpoint, a minimum threshold has to be overcome. Thisthreshold is the minimum laser power that promotes enoughenergy density in the focal point to start polymerization.Underneath that point the possibility that in the focus pointtwo photons can be absorbed is too low. If the density in thefocal point is too high, inner explosions in the resist areobtained. In our case, for the auxetic structures a laser powerof 6.3 mW leads to the best results and to the finest structures.Further discussion of the initial trials required to reach ade-quate values at different scales, as well as final adjustmentusing different laser powers appears in the results section.
3. Results and discussion
Once the structures have been adequately converted from theoriginal CAD files to the writable lines, the two structures can bemanufactured using different scales to allow analysis of theactual limits of the combination technology–resist used. Theanalysis continues until the final structures have been optimizedto the voxel size of the resist. The trials with 6×6×6 μm3
structures, with details of 200 nm, led to unsatisfying results, asshown in figure 4. In this trial they reached the resolution limitsof the machine and the resist, thus leading to the adjacent voxelsintersecting each other, which affected the lattice structures byreducing both their porosity and the slender features of theirtrusses. The first trials with 60×60×60 μm3 for the ‘cuboid’geometry and 40×40×40 μm3 for the structure inspired by theUS patent also led to inadequate results due to structural col-lapse, although this helped us to verify the adequate resolution ofthe machine and resist for details around 500 nm−1 μm, as canbe appreciated in figure 5.
To increase stability and prevent collapse, several lines werewritten for each truss of the structures, the scale was changedand the influence of laser power was also analyzed. In our case,writing three parallel lines for each truss proved adequate.Figure 6(a) helps to show the relevant influence of laser poweron structural viability; to the right, lower laser powers lead tostructural collapse, while to the left, higher laser powers lead tobetter results. It is also important to control the critical pointdrying process for elimination of the unpolymerized resist, so asto obtain the final structures. In some cases, the drying processalso promotes collapse and shrinkage. Interestingly, the shrink-age from some structures helps to show the auxetic behaviour ofthe designed and manufactured geometries.
When using the laser power at 100%, 20 mW are mea-sured after the system optics, although the real power actingupon the resist and promoting polymerization is alwayslower. This is due to losses that occur when the laser furthergoes through the air, reaching the final objective and resistuntil the focal point. Table 1 includes some calibration results
showing the actual power acting upon the resist, dependingon the percentage of laser power used by the NanoScribesystem. Figure 6(b) includes a schematic representation of thedifferent laser powers used to manufacture the test auxeticstructures, so as to select the most adequate value. Laserpowers below 50% (equivalent to 6.3 mW upon the resistbeing processed, according to the calibrated values of table 1),lead to structural collapse.
In short, higher laser powers lead to more satisfyingresults, as shown in figure 5, even though the whole partconstruction requires more energy and some shrinking duringthe critical drying process (around 4%, better perceived infigures 6(a) and 7) is still present. Such shrinking can bereduced, to values of around 1–2%, by incorporating someadditional outer pillars, connected to the surface, as support-ing structures capable of stress absorbance, as previousresearch has shown [28]. Such beneficial increases of laserpower have a limit, as it is also important to mention thatuncontrolled increases of laser power can promote multi-photon, instead of two-photon absorption, which reducesaccuracy and, causes an unpredictable response of the resistduring polymerization, normally leading to important defects.
After completing the final process and dimensionaladjustments, images from figures 6(a) and 7 show adequateauxetic geometries, measuring 20 × 20 × 20 μm3 and
Figure 4. Analysis of voxel size limit by writing 6 × 6 × 6 μm3
structures. The adjacent written lines voxels intersect each other,which affects the lattice structures, reducing their porosity and theslender features of their trusses.
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10 × 10 × 10 μm3, with unit cells of 10 to 20 μm3 and details(truss widths) of around 500 nm to 1μm. Table 2 summarizesthe main geometrical features of the initially designed andfinally manufactured structures, including a comparisonbetween the preliminary solid CAD designs, the result fromconverting such preliminary polygonal bodies to the polylinesneeded for the NanoScribe system and the manufacturedprototypes. Characterization challenges have already beenmentioned, but we hope that the provided geometrical fea-tures of table 2 and other experimental details may be usefulfor researchers aiming to manufacture other metamaterials.
The detailed views from figures 7 and 8 lets us appreciatethe three parallel lines written for each member of the struc-ture and to verify that the quality of the process is highlyremarkable. The different writing process used by theNanoScribe machine does not provide the ‘stepped’ structures
typically obtained from layer-by-layer additive manufacturingprocesses, thus leading to enhanced surface quality. In addi-tion, the finest details are submicrometric, and the obtainedgeometries are therefore true microstructured metamaterials,which are adequate for several applications in many differentfields of study. This is discussed further on, as severalremarkable properties arise when reaching this level of pre-cision. These examples are not the first true mechanicalmetamaterials obtained by direct laser writing, as previouspioneering research [22, 23] has been a source of inspirationfor us. However, we believe that the structures presented hereare novel and that we have used an interesting dimensionaladjustment process, based on writing structures at differentscales and using different laser powers, in order to reachremarkably precise and stable structures. We hope that thepresented discussion on the effects of scale, laser power andnumber of written lines may be useful for other colleaguesaiming at the additive manufacture of metamaterials.
Figure 5. Difficulties with process adjustment: inadequate laser powerand problems with the critical drying process lead to structural collapse.
Figure 6. (a) Influence of laser power on structural stability. To theright, lower laser powers lead to structural collapse. To the left, thebest structures are obtained using laser powers of 55 mW. (b)Schematic representation of the different laser powers used tomanufacture the test structures. Laser powers below 50% (equivalentto 6.3 mW upon the resist being processed, according to thecalibrated values of table 1), lead to structural collapse.
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A limiting factor for some applications is the difficulty indirectly processing metals through direct laser writing.However, it is important to note that organic photoresists, likeSU-8 (MicroChem Corp.) or the IP-Photoresists (NanoScribeGmbH), hybrid materials, such as the Ormocere® organic-inorganic hybrid polymer family (Fraunhofer-Gesellschaft e.V.), and the amorphous semiconductor As2S3 are capable oftwo-photon polymerization, which provides a wide range ofpossibilities. In addition, through CVD/PVD coating pro-cesses, or just by electroplating, final metallization is possible,and casting processes can also be used for additional versa-tility. Moreover, advanced research groups, as well as com-panies, are focusing on the continuous development of novelmaterials, including photoelastomers, photopolymers andpolymer-ceramic composites. These materials, even whenused for medical applications, can be structured by means ofdirect laser writing [26].
Although in the present study we have focused only onmanufacturing issues, we would like to discuss some impor-tant devices and applications that may benefit from the use ofmicromanufactured auxetic structures and the outstandingdegree of precision attainable by using direct laser writing.Apart from the application fields detailed in the introduction,recent progress in the field of auxetics is focusing on theirapplication to the control deformations induced by thermalgradients, especially in plates, shells, spheres and cylinders[29], and to the control of stiffness, especially in sandwichstructures [30]. Generically focusing on the control of ther-mal-induced deformations and of structure stiffness is one ofthe keys for enabling additional specific applications of
Table 1. Calibrated values of power acting upon the resist, depending on the percentage of laser power used by the NanoScribe system.
% of laser power used 10 20 30 40 50 60 70 80 90 100Power reaching objective (mW) 1.32 2.57 3.83 5.09 6.36 7.63 8.9 10.17 11.43 12.69
Figure 7. Final result after process adjustment: writing three lines foreach lattice member promotes structural stability and leads to theformation of adequate structures.
Figure 8. Final result after process adjustment. Details of auxeticstructure.
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auxetics in several fields. Furthermore, three-dimensionalauxetic structures have the potential to impact on a wide rangeof applications from deployable and morphing structures tospace-filling composite and medical treatments.
The possibility of obtaining 3D micro- and even nano-auxetics can be useful for broadening the applications of theseinteresting metamaterials. Nanometric features are of specialinterest for interacting with microorganisms even at themolecular level, and applications range from enhanced tissue-engineering scaffolds to biological traps and selective filterscapable of capturing pathogens for subsequent studies or uses[20]. As the density and porosity of auxetics can be changedjust by applying uniaxial stresses, such scaffolds, filters andbiological traps can be tuned easily, adapted to specificapplications and controlled in real-time, thus promoting novelways of interacting with microorganisms. Electromagnetic,acoustic and optical waveguides can also benefit from the useof auxetic geometries and from the possibility of obtainingnanofeatures, especially at higher frequencies, and from real-time control of structure porosity and density, as recentresearch has shown [31].
In addition, the ability to fabricate auxetics using smartmaterials can greatly promote their applications, as it mayenhance the control of actuation and deployment processes byusing tunable stiffness responses [32]. We truly believe thatthe process detailed here is very adequate for the manufactureof highly precise 3D auxetics, and that it can even be appliedto the manufacture of auxetics using smart materials, asseveral photoresins apt for additive manufacturing processesare in fact shape-memory polymers. In future studies we hopeto address the manufacture of 3D micro-auxetics using shape-memory polymers. Novel horizons are also opened by thepossibility of adjusting the auxetic behaviour by means ofexternally applied electromagnetic fields [33, 34], whichshould be further explored for the development of magneto-mechanical microsystems, possibly by combining 3D auxe-tics manufactured via direct laser writing and further func-tionalized by means of metallic CVD or PVD coatings.
4. Conclusions
In this study we have presented a very promising approach forthe development of auxetic metamaterials and devices basedon the use of direct laser writing. The process stands out forits precision and the complex 3D geometries attainable. To
our knowledge it represents one of the first examples of theapplication of this technology to the manufacture of auxeticgeometries and mechanical metamaterials. We have usedsome special 3D auxetic designs whose remarkable NPRshave been previously highlighted. We have also tried toprovide interesting details of the design and manufacturingprocess, as well as some discussions about the main results,present capabilities, difficulties and challenges concerningnano-auxetics. Even though the progressive size reduction ofartificially obtained auxetic geometries leads to realmechanical metamaterials and can promote novel applica-tions, other difficulties linked to manipulation and integrationinto complex devices arise, and further research is needed totake advantage of nano-auxetic geometries. Future studieswill be focused on the development of characterization pro-cedures and support devices to address the actual auxeticbehaviour of the geometries obtained. Interesting applicationsmay be based not only on the special stiffness tensors ofauxetics, but also on their natural vibration mode shapes.Using nanomanufactured auxetics, ultra-high resonant actua-tors may be developed by fixing them to piezoactuators, oncepresent manipulation and integration challenges have beenaddressed.
We foresee relevant applications in several fields, such asbiomedical and tissue engineering, i.e. for the development ofactive implantable medical devices, minimally invasive sur-gical actuators or active scaffolds for dynamic cell culture;aerospace and aeronautics, i.e. for the development ofmicroactuators and highly accurate deployable structures;telecommunications and optoelectronics, i.e. for novelantennae designs, special photonic crystals and stress-strainelectromechanical microsensors, among other interestingareas. Their use for controlling deformations induced bythermal gradients and for controlling the stiffness of materialsand structures is also noteworthy and may promote newapplications in the fields of research already described.
We aim to continue our research by searching for newappliances based on these interesting geometries. Our geo-metries are at the disposal of colleagues who would like tofurther explore with us the potential of these geometries. Thedescribed process can be used for many other families ofmetamaterials and smart materials and structures as a way ofincreasing the precision of available microactuators ormicrosensors based on the interesting properties of mechan-ical metamaterials.
Table 2. Geometrical features of the initial CAD designs, of the adapted designs and of the manufactured prototypes.
Geometrical featureInitial CAD and firstcollapsed trials
Smart Mater. Struct. 23 (2014) 085033 S Hengsbach and A D Lantada
Acknowledgements
This work was carried out with the support of the EuropeanCommunity. We appreciate the support of the EuropeanResearch Infrastructure EUMINAfab (funded under the FP7specific programme Capacities, Grant Agreement Number226460) and its partner, the Karlsruhe Institute of Technol-ogy. We also appreciate the continued support from theKarlsruhe Nano Micro Facility (KNMF). We are also gratefulto Dr Dieter Maas and to Dr Thomas Schaller for their kindhelp and support to the EUMINAfab 1139 proposal. Weacknowledge reviewers for their positive opinions, encoura-ging comments and proposals for improvement, which havehelped to enhance the paper’s quality, readability, content andfinal result. Thanks to their feedback we have providedadditional relevant experimental details, hoping they may beuseful for researchers facing the manufacture of similarstructures.
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