Microstereolithography

CHAPTER 1.2

Microstereolithography Arnaud Bertsch, Philippe Renaud


cole Polytechnique F
ed
erale de Lausanne (EPFL), Lausanne, Microsystems Laboratory (LMIS4), Institute of Microengineering, E Switzerland

1 Introduction Microstereolithography is a 3D microfabrication technology that is fundamentally different from the techniques commonly used in cleanroom environment for the manufacturing of microelectromechanical system (MEMS) components. Most microfabrication techniques evolved from the microelectronics industry and often use silicon wafers as substrate or carrier material, thin films of metals deposited by evaporation or sputtering, thin layers of polymers deposited by spin coating and patterned by photolithography, chemical and plasma etching to generate various shapes. Microstereolithography is also a microfabrication technique, but it is related to rapid prototyping technologies, and more precisely to stereolithography (SLA), a technique patented in 1986 [1], allowing the fabrication of 3D components by layer-by-layer curing of a photopolymerizable resin with an ultraviolet (UV) laser. Microstereolithography is based on a manufacturing principle very similar to the one of SLA, but implements process improvements that result in a far better resolution. The first developments of the microstereolithography technique have been published in 1993, and a considerable effort has been first made to explore different ways to modify the SLA process to achieve a better lateral and vertical resolution. This resulted in a number of machines being developed by different research teams and using a very diverse range of components: different light sources, optical elements, irradiation configurations, etc. These machines can seem very dissimilar, but all of them have in common the use of a space-resolved light-induced photopolymerization of a liquid resin to create small-size 3D components by a superimposition of a large number of thin polymer layers. If today, the word “microstereolithography” is commonly used to describe this technique, many designations have been used by the research teams who published the first reports, such as IH-process, optical forming, spatial forming, microphotoforming, microstereophotolithography, and 3D optical modeling. All the processes that have been imagined can be classified in three main categories: - Scanning microstereolithography machines in which every layer of the object is made by a vector-by-vector tracing with a light beam, tightly focused on the surface of the photosensitive resin. Three-Dimensional Microfabrication Using Two-Photon Polymerization https://doi.org/10.1016/B978-0-12-817827-0.00050-3

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- Projection microstereolithography machines in which the image of the layer to be built is projected on the surface of the photosensitive medium with a high resolution. - Submicron microstereolithography machines in which the object is built directly inside the photosensitive medium and not on its surface. Today, many different microstereolithography machines based on various principles are still in use, mostly in an academic context, but the focus of research has shifted from process development to the formulation of new polymers and materials and to finding applications of innovative 3D microcomponents made by this technique. Compared to more conventional MEMS fabrication techniques, microstereolithography can generate small objects with complicated shapes and intricate details that are impossible to be built with any other method. This is valuable in a research context, but can also be a limitation in an industrial context as conventional replication methods can generally not be used to massproduce components fabricated by microstereolithography.

2 Rapid prototyping and stereolithography Rapid prototyping technologies, also called additive manufacturing technologies, are a group of processes allowing the rapid production of large-size models and prototypes, and their manufacturing layer-by-layer. Additive manufacturing processes are opposed to subtractive methods, such as CNC (computer numerical control) milling, turning, and grinding, where the object is built by cutting away material from a block and they are also different from formative methods, such as injection molding, imprinting, or casting, where the object is built using a mold in which material is solidified. Additive manufacturing methods have in common that they rely on CAD (computer-aided design) for the design of the object to be built, then this virtual object is oriented in a way that will allow its manufacturing by the chosen method, sliced in a number of 2D layers by a dedicated slicing program (which may add supports if needed) and built layer-by-layer. Additive manufacturing technologies have the advantage to be able to produce objects with extreme geometric complexity and intricate details as well as assembled components, in a relatively fast and simple process. A number of additive manufacturing methods have been developed and some of them have been commercialized. The most popular today are: - Stereolithography (SLA), which is based on the local polymerization of a photosensitive resin. Each layer of the object is obtained by moving an ultraviolet light beam on its surface by steering it in X and Y directions with galvanometric mirrors (Fig. 1.2.1). The solidified layers are then lowered in the resin tank such that a fresh layer of resin can be spread on them to cover the already polymerized part of the object, which allows the fabrication of the next layer. - Fused deposition modeling (FDM), which uses a wire of thermoplastic polymer that is melted in a heated head and deposited by moving that head in X and Y directions to

Microstereolithography

Fig. 1.2.1 Diagram of the two most commonly used rapid prototyping methods: (left) stereolithography and (right) fused deposition modeling.

form every layer of the object. The superimposition of the layers is done by moving down the table on which the object is built (Fig. 1.2.1). - Selective laser sintering (SLS), which is based on the fusion of powdered material with a high-power laser. The laser defines the layer shapes similarly as in SLA and powdered material is spread on the previously built layers to be used for the fabrication of the next layer. In this process, the unsintered powder can serve as support for overhanging layers. 3D Systems, Inc. was a pioneer in the rapid prototyping market, with the commercialization of the first SLA machine in 1988, and still leads the market today. Stratasys, Inc. is the second player on this market and is specialized in FDM. Early adopters of rapid prototyping technologies were mostly large companies in the aerospace and automotive industry as they are heavily relying on prototyping in their product development phase and because the price of the first rapid prototyping machines was very high. In recent years, lower-cost systems have been commercialized, and rapid prototyping machines are often referred to as “3D printers.” These more affordable machines can be used almost as easily as printers by engineers for design confirmation and functional testing and have contributed to spreading rapid prototyping technologies to a wide range of companies and a wide range of applications. Today, the first patents taken in the rapid prototyping field have expired. As a consequence, there has been the rise of new competitors selling rapid prototyping equipment, proposing low-cost machines having only very basic functions but that hobbyists can afford. Of course, these machines do not have an interest in an industrial context, where very expensive systems are still in use, as they provide the accuracy, high speed, and functional materials required by product development cycles.

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Rapid prototyping technologies had a strong impact on industrial product development as they reduce significantly the time to market in manufacturing. If the first domain of application of rapid prototyping technologies was the creation of components used for the physical verification of CAD models, for form fitting and for functional testing, many other applications have been found when these technologies became more affordable, including prototypes for design reviews, models for geology and architecture, mold masters for investment casting, anatomical models for surgical planning and prosthesis design, etc. With hobbyists having now access to this technology, the field of its applications will expand even more. SLA has been the first rapid prototyping method developed and commercialized, and it is still today the most used by the industry, as it is the fastest among additive manufacturing technologies, it is affordable, and the obtained parts have excellent surface finish even without additional operations. The typical resolution of SLA is in the range of 150 μm in the three directions of space, but as it is based on the superimposition of layers, resolution, precision and surface roughness are anisotropic. In particular, the vertical (along the build axis) resolution is related to the thickness of the superimposed layers, whereas the lateral (in-plane) resolution depends on the dimension of the light beam used to draw the shape of each layer, at the surface of the resin. As microstereolithography is directly related to SLA, it is relatively simple to understand the choices that have been made to develop this process by studying how the resolution of SLA can be improved.

3 Improving stereolithography resolution As the resolution of SLA is inherently anisotropic, different factors will affect its resolution in the vertical and horizontal directions.

3.1 Reducing the thickness of the layers Even if the objects made by SLA are formed by the superimposition of layers, the thickness of the layer of fresh liquid deposited on the surface of the object being manufactured is not related to the vertical resolution of SLA process at all, and making thinner layers of fresh resin on the surface of the last layer manufactured does not allow to improve its vertical resolution. In fact, the vertical resolution of the SLA process is only related to the way the light penetrates in the photopolymerizable medium: to make thinner layers, the light beam used to draw each layer on the surface of the resin should be confined to the surface and not penetrate too deeply in the medium. If this is not the case, and the light penetrates deeply in the medium, it will not only start the photopolymerization phenomenon and solidify the next layer of the object at the surface, but may also result in parasitic polymerizations in the already-polymerized layers of the object being built. The control of the thickness of the polymerized layer in SLA is related to the photopolymerization phenomenon. Photopolymerization is a chain reaction in which a

Microstereolithography

polymer grows by the addition of monomers and oligomers. This chemical reaction starts when a certain amount of photons of adequate energy is absorbed by volume unit of resin, which creates reactive species (in general radicals or cations) resulting in the propagation of the chain reaction. The evolution of the thickness of the polymerized layer with time in SLA can be approximated by the following equation:   1 t T e ¼ ln (1.2.1) with t0 ¼ αc t0 αcF0 where e is the thickness of the polymerized layer (m), α is the Napierian coefficient of molar extinction of the photoinitiator (L mol1 m-1), c is the photoinitiator concentration (mol L1), t0 is the threshold irradiation time required to start the photopolymerization phenomenon (s), t is the irradiation time (s), T is the irradiation threshold value (photons m3), and F0 is the light flux arriving on the surface of the resin (photons m3 s1). Zissi et al. [2] have measured the evolution of the polymerized layer thickness with the irradiation time at constant irradiation flux (210 mW cm1), for five monomers in which the same photoinitiator (α,α-dimethoxy-α-phenylacetophenone, abbreviated DMPA) has been added at a concentration of 0.4 mol L1. The different monomers used were tri(ethyleneglycol)diacrylate (TIEGDA); 1,6-hexanediol diacrylate (HDDA); trimethylolpropane triacrylate (TMPTA); pentaerythritol triacrylate (PETIA) and 2,20 -bis[4-(methacryloxy-ethoxy)phenyl] propane (Diacry 101). The logarithmic evolution of the thickness of the polymerized layers with the irradiation time can be seen in Fig. 1.2.2, showing the good agreement of Eq. (1.2.1) with experimental results in the case of resins that do not undergo photobleaching. When a SLA resin undergoes photobleaching, the evolution of the thickness of the fabricated layers with the irradiation time is no longer logarithmic, but follows a more complex curve that can be predicted by numerical simulations [3]. The contribution of the different terms present in Eq. (1.2.1) to the thickness of the polymerized layer indicates two ways to improve the vertical resolution: (a) Irradiating the resin for a small duration, close to the irradiation threshold Reducing the thickness of the polymerized layers can be obtained simply by reducing the irradiation time in such a way that it is just slightly longer than the threshold irradiation time required to start the photopolymerization phenomenon. In that case, Eq. (1.2.1) can be simplified to: effi

1 t  t0 αc t0

(1.2.2)

Reducing the layer thickness by reducing the irradiation time seems a simple and efficient way to produce thinner layers, but the drawbacks of this method largely outweigh the benefits. First, the polymer layer obtained in conditions close to the irradiation threshold has in general poor mechanical properties, but most importantly, the precise control of

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Fig. 1.2.2 Evolution of the thickness of the polymerized layer with the irradiation time in stereolithography for five different monomers. For all curves, the irradiation flux is 210 mW cm1, the photoinitiator is DMPA at concentration of 0.4 mol L1. (Reprinted by permission by SpringerVerlag, ©1995. S. Zissi, A. Bertsch, J.-Y. J
ez
equel, S. Corbel, D.J. Lougnot, J.C. Andr
e, Stereolithography and microtechniques, Microsyst. Technol. 2 (1995) 97–102.)

the layer thickness is extremely challenging, as a small variation in the irradiation time results in a major change in the polymerized thickness. In the case of scanning microstereolithography processes, this induces major errors, as it is very common in this technique that two or more vectors used to draw a layer are secant. In that case, the intersection point is irradiated twice, which results locally in a major increase of the polymerized thickness. (b) Using reactive media having a small optical thickness If the photosensitive media used are strongly absorbing the irradiation wavelength, the light will be confined at the surface and the resulting thickness of the polymerized layer will be small, even for irradiation durations relatively long compared to the threshold irradiation time. When resins are irradiated for durations much longer than the time required to reach the polymerization threshold, Eq. (1.2.1) can be simplified to:   R αcF0 τ e ffi with R ¼ ln αc T

(1.2.3)

Small variations of the irradiation time τ induce only small changes of R when τ ≫ t0. In that case, the layer thickness is strongly dependent on the absorption properties of the reactive medium, described by the term μ ¼ 1/αc, named the optical thickness. Using again the example of the intersection point of two secant vectors in the case of a scanning

Microstereolithography

microstereolithography process, the energetic dose received at such a point is still the double than anywhere else, but the subsequent change in polymerized thickness is limited when using a strongly absorbing resin in conditions far from the irradiation threshold. This is a desirable condition for operating a microstereolithography machine, as there is no need to precisely control the irradiation conditions to obtain a well-controlled layer thickness. To produce a resin for SLA or microstereolithography having a small value of the optical thickness, two main ideas can be implemented. The first is to use a photoinitiator or a photosensitizer that is well tuned to the irradiation wavelength, and to have a large concentration of this compound in the photosensitive resin. This simultaneously increases the reactivity of the resin and reduces its optical thickness, as it becomes more absorbent for the irradiation wavelength (Fig. 1.2.3A). The second is to add a neutral absorber to the resin. Neutral absorbers are nonreactive chemicals that, once added to the photopolymerizable medium, will strongly absorb the light at the irradiation wavelength and dissipate the corresponding energy by ways that do not interfere with the photopolymerization reaction. Neutral absorbers compete with the photopolymerization reaction and reduce the amount of energy available for initiating the photopolymerization phenomenon, which results in a reduced optical thickness of the reactive resins, but also in a reduced reactivity (Fig. 1.2.3B), which can be a serious drawback. Neutral absorbers were first used by Zissi et al. [2] for the formulation of microstereolithography resins. They demonstrated that the cure depth of a resin used for microstereolithography can be drastically reduced by the addition of very small concentrations of an inert UV dye (2-(2-hydroxy-5-methylphenyl)benzotriazole also named Tinuvin P) mixed into an acrylate-based photopolymer. Since then, many others have used neutral absorbers, mostly benzotriazole-based components (derived from the Tinuvin family) for the adjustment of the cure depth of microstereolithography resins [4–8]. The evolution of the polymerized thickness of a photopolymerizable medium with the irradiation time, when neutral absorbers are added to the medium has been described analytically by Bertsch and Renaud [9] and is presented in Eq. (1.2.4)   1 t ðαc + αN cN ÞT e¼ ln 0 with t00 ¼ αc + αN cN t0 α2 c 2 F0

(1.2.4)

where cN is the concentration and αN is the Napierian coefficient of molar extinction of the neutral absorber. The influence of the concentration of neutral absorber on the mechanical properties, density, accuracy, surface roughness, and green-state shrinkage of acrylate components made by microstereolithography was investigated experimentally by Zabti [10].

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Fig. 1.2.3 Evolution of the thickness (e) and width (l) of the polymerized layer: (A) with the photoinitiator concentration, (B) with the concentration of neutral absorber. (Reprinted by permission by SpringerVerlag, ©1995. S. Zissi, A. Bertsch, J.-Y. J
ez
equel, S. Corbel, D.J. Lougnot, J.C. Andr
e, Stereolithography and microtechniques, Microsyst. Technol. 2 (1995) 97–102.)

3.2 Avoiding local degradations of the vertical resolution SLA and microstereolithography techniques rely on an adequate tuning between the irradiation and absorption wavelength of the resin for controlling the polymerized thickness of each layer. If the absorption peak of the resin does not correspond to the irradiation wavelength, the light will penetrate deeply in the medium, resulting in bad manufacturing conditions. Schematically, four cases can be encountered depending on resin reactivity and light penetration in the photosensitive medium: - If the resin has a strong reactivity and a strong absorption for the irradiation wavelength, the light will not penetrate deeply in the medium but where it penetrates the polymerization threshold will be easily reached, resulting in a small and welldefined polymerized layer. The thickness of this layer is easily controlled by adjusting

Microstereolithography

the optical thickness of the resin. These are the most desirable conditions for microstereolithography. - If the resin has a low reactivity and a strong absorption for the irradiation wavelength, the light is confined at the surface of the medium, and most likely there will be no polymerization at all. If some polymerization occurs, it will result in only a very thin polymerized layer at the surface of the medium. This is typically the case of a resin that contains a large concentration of neutral absorber (e.g., a large concentration of carbon-black micro- or nanoparticles). - If the resin has a strong reactivity and a low absorption for the irradiation wavelength, the light penetrates deep in the medium and the photopolymerization reaction propagates quickly, resulting in very thick solidified layers. This is not a favorable condition for SLA as the polymerized layer thickness is strongly dependent on the irradiation time and easily contributes to generate defects, even in the already-polymerized parts of the object being built. Such conditions are typically obtained when resins formulated for UV-LIGA (such as SU-8) are used in a SLA machine. - If the resin has a low reactivity and a low absorption, the light penetrates deeply in the medium, it takes a long time to start the photopolymerization phenomenon, but when the polymerization starts, the threshold is reached everywhere at almost the same time, resulting in unpredictable polymerization conditions. This is the worst possible case for SLA resins and can happen if there is a mismatch between the irradiation wavelength and the absorption spectrum of the resin. A common cause of degradation of the resolution of SLA in the vertical direction is the z-overcure error, also known as “print-though” phenomenon. This happens when irradiating a resin close to its polymerization threshold, or when resins having a low light absorption are used for manufacturing components in which overhanging structures are present. In this case, when the first layer of the overhanging structure is built, a small amount of light penetrates the resin present underneath it, not large enough to reach the polymerization threshold at that place, but resulting in a local sensitization of the liquid resin. As photopolymerization is a cumulative process, more energy will be added underneath that overhanging area if additional layers are built on top of it. When the polymerization threshold energy is reached in the sensitized zone, unwanted polymer structures grow under the first layer of the overhanging structure, in a noncontrolled way, resulting in a degradation of the vertical resolution that can deteriorate features in the fabricated object or result in a local loss of vertical resolution in the object. This phenomenon is known since the early days of SLA, and can be a real problem when building small parts [11] as shown in Fig. 1.2.4, where an unwanted polymerization has occurred under overhanging layers, corresponding in dimensions to the addition of many layers. Both in SLA and microstereolithography, z-compensation algorithms have been created to counterbalance this effect [12, 13], simply by anticipating the magnitude of the unwanted polymer added underneath the overhanging parts and deforming the

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Fig. 1.2.4 SEM picture showing how the z-overcure error can degrade the resolution in an object. Unwanted polymer structures have appeared under overhanging layers in an object made by microstereolithography.

fabricated components such that their final shape is the one expected. However, the best way to solve the z-overcure error is to use adapted resins, showing a high absorbance for the irradiation wavelength and a strong reactivity.

3.3 Improving the lateral resolution The strategies used to improve the vertical resolution of the SLA process rely mainly on adapting the reactivity and optical thickness of the photochemical media. This, however, does not impact much the lateral resolution, as it is not directly related to the chemistry as seen in Fig. 1.2.3: when increasing the photoinitiator concentration, a slight decrease of lateral resolution (the polymerized width increases) can be observed in Fig. 1.2.3A. Increasing the photoinitiator concentration increases the reactivity of the resin and consequently reduces the threshold energy required to start the photopolymerization process. As the light beam used for irradiating the resin has a Gaussian energetic repartition, the width of a polymerized line is slightly larger when the resin is more reactive. When increasing the concentration of neutral absorber (Fig. 1.2.3B), the opposite happens as the reactivity of the resin is decreased. In the SLA process, each layer of the object being built is drawn by scanning a laser beam on the surface of the resin. The laser beam diameter and the accuracy of the

Microstereolithography

scanning system define the limit of the smallest features that can be built and consequently the lateral resolution of the process. In the 3D-systems SLA-250, the most widely used model of SLA machine, the laser beam diameter is 0.25 mm. The first way to improve the resolution of this machine is change the laser, which is an easy way to reduce the beam diameter to about 0.1 mm. This improvement has been implemented by a number of service bureaus worldwide and is named “small-spot stereolithography” [11]. This technique has found applications in the production of high-resolution prototypes for the watch industry, hearing aids, and electronics. Further improvements of the horizontal resolution of SLA have led to the development of various microstereolithography machines: - The first idea implemented to improve the lateral resolution of SLA was simply to focus more precisely the light beam on the surface of the resin, such that the spot size has a diameter of only a few micrometers. This was done by adapting the optics of the system, but also required to know accurately the exact position of the liquid surface. This first category of microstereolithography machines are similar to conventional SLA and perform the fabrication of each layer by scanning a light beam on the resin surface in a vector-by-vector fashion. - Later, another family of microstereolithography processes was developed based on the projection of each layer being built on the surface of the photopolymer rather than scanning a light beam on it. These machines are relatively simple as the use of projection optics rather than a scanning system significantly reduces the number of moving parts in the system and the large depth of focus achieved when projecting an image on a surface makes focusing optics much more simple than in scanning microstereolithography machines. - Finally, microstereolithography machines allowing the fabrication of objects having a submicron resolution were developed, in which the layers are no longer solidified at the surface, but directly inside the reactive medium. This requires the control of the energetic density of the light beam used in the machine, such that it is maximal at a chosen place inside the liquid resin but not at the surface, which can be obtained either by using the two-photon polymerization method or by exploiting nonlinearities of the photopolymerization process.

4 Microstereolithography techniques based on a scanning principle The first created microstereolithography machines were based on the scanning method. Two such machines were presented in scientific publications in 1993, one by Takagi and Nakajima [14] and another by Ikuta and Hirowatari [15]. Both machines are relatively similar: they use a scanning principle for fabricating the different layers of the objects and use a UV light beam focused on the surface of the resin through a transparent

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window. The light beam is occulted by a shutter when a new layer is made or during positioning steps. The vector-by-vector fabrication of each layer is obtained using X-Y translation stages to move the whole photoreactor while focusing the light beam at a fixed location (Fig. 1.2.5A). These first two microstereolithography machines are described in more detail in a series of subsequent publications [16–18]. To obtain an improved lateral resolution compared to conventional SLA machines, these microstereolithography processes rely on a simplified optical system in which the light beam is fixed and tightly focused on one point. When moving the light beam with galvanometric mirrors, the focal point moves on a sphere, and dynamically focusing lenses are necessary to obtain a good focusing of the light on the flat surface of the resin. Removing the galvanometric mirrors and dynamic focusing optics results in a robust and simple focusing system, but X-Y translation stages are required to draw and solidify the shapes of each layer, making the process slow, especially if objects of high geometrical complexity are built. The choice to constrain the surface of the liquid resin and perform the exposure through a transparent window has the advantage to avoid all problems related to spreading the fresh resin on the already polymerized part of the object being built and allows to create even layers of well-controlled thickness; however, it also has for drawbacks that the fabrication yields are very low because the adhesion of the structures being built to the window often causes their destruction [19]. Different variations of the constrained surface technique have been later developed using nonsticking coatings to obtain a better detachment of the polymer layers from the transparent window. To avoid the limitations related to polymerizing through a window, scanning microstereolithography processes where the polymerization takes place at the free surface of the photosensitive resin have been developed by many research teams. Similar to the scanning microstereolithography machines using the fixed surface technique, the ones implementing the free surface technique also rely on moving the resin tank with X-Y-Z stage for solidifying the layers vector by vector, while the light beam is focused statically at a given location in space (Fig. 1.2.5B). The first paper describing a scanning microstereolithography machine using a free surface technique was published by Zissi et al. in 1994 [20]. To create a layer of fresh resin on the surface of the object being built, the object was immersed in the resin tank and brought back close to the surface and time was spent waiting for gravity to level the surface. Even if this method used to spread fresh resin on the workpiece seems simple, the control of the thickness of the layer of liquid deposited is difficult and the time required to get a perfectly flat surface can be very long, and depends both on the geometry of the last layer and on the viscosity of the resin. Other scanning microstereolithography machines using the free surface technique were later developed by various research teams [21–25], but only a few of them made original improvements to the process:

Fig. 1.2.5 Diagrams of scanning microstereolithography machines. (A) Constrained surface technique. (B) Free surface technique. (Reprinted by permission by Springer-Verlag, ©1997. A. Bertsch, S. Zissi, J.Y. Jezequel, S. Corbel, J.C. Andre, Microstereophotolithography using a liquid crystal display as dynamic mask-generator, Microsyst. Technol. 3 (1997) 42–47.)

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Fig. 1.2.6 Diagram of a scanning microstereolithography machine using an array of optical fibers for collective manufacturing, first described by Ikuta et al. in [27].

- Kobayashi and Ikuta investigated the use of a scrapping device for leveling the surface by actively spreading the resin on the previous layer rather than wait for the relaxation of the resin surface [19, 24]. By coating one surface of this scrapping device with a fluorocarbon resin and adjusting its angle to minimize the size of the meniscus of resin created at its contact, the layer of deposited material was lowered to 10 micrometers and complex objects were produced with this resolution. - Kang et al. developed a scanning microstereolithography process using a blu-ray optical pickup unit (using a LED emitting at 405nm as light source), which came from a commercial blu-ray optical disk driver [26]. The optical pickup unit was used without modification and combined to an optical system in order to obtain an extended focal length. An adapted resin was designed for this apparatus, using two monomers (SR499 and SR368), a photoinitiator (Irgacure 819DW), and a polymerization inhibitor to control the polymerized layer thickness. - In 1996, Ikuta et al. presented a collective microstereolithography process (Fig. 1.2.6) based on the use of an array of single-mode optical fibers to focus the UV light from a xenon lamp in many different locations on the surface of the resin [27]. An X-Y-Z translation stage moves the resin tank and the objects being built, which allows the fabrication of the layers composing each object and their superimposition.

5 Microstereolithography techniques based on a projection principle Projection microstereolithography machines (also named integral microstereolithography) do not build each layer of the object vector by vector, but by the projection of their image on the surface of the resin. This way, the irradiation of a layer is performed in one step only, regardless of its complexity, making projection microstereolithography machines generally faster than scanning machines. Starting from a 3D CAD file describing the object to be built, a slicing algorithm generates the layers that will be projected during the manufacturing phase, as a series of blackand-white bitmap files. These are then used sequentially to shape the light beam that is

Microstereolithography

Fig. 1.2.7 Diagram of the first projection microstereolithography machine using an LCD screen as dynamic mask. (Reprinted by permission by Springer-Verlag, ©1997 A. Bertsch, S. Zissi, J.Y. Jezequel, S. Corbel, J.C. Andre, Microstereophotolithography using a liquid crystal display as dynamic mask-generator, Microsyst. Technol. 3 (1997) 42–47.)

projected on the surface of the resin, using a dynamic mask. The components that can be used as dynamic mask in projection microstereolithography machines are the same that are used for video projection applications. Such components comprise liquid crystal displays (LCDs) working in transmittance mode, and Digital Micromirror Devices (DMDs) and Liquid Crystal on Silicon (LCOS) both working in reflectance mode. An appropriate optical system is used to reduce and focus the projected image on the surface of the resin where it induces the solidification of the layer. The superimposition of all the polymer layers creates the physical object, as it is the case in conventional rapid prototyping techniques. The idea of projection microstereolithography was first demonstrated by Bertsch et al. in 1995 using a LCD as dynamic mask (Fig. 1.2.7) [28]. When this first projection microstereolithography machine was built, commercial liquid crystal displays were still relatively archaic compared to these available today. In particular they could not be used with UV light, but only with visible wavelengths and the pixels in their opaque state did not fully stop the light, which required the use of an adapted chemical medium allowing the photopolymerization initiation in the visible range and correcting the low contrast of the LCD with an adapted polymerization threshold. This was obtained by the use of a photopolymerization system based on an organic dye (Eosin Y) as photosensitizer, combined with N-methyl diethanolamine as co-initiator, for initiating the photopolymerization of acrylate monomers. Such photopolymerizable resins can be made strongly absorbing for the irradiation wavelengths by tuning the concentration of dye they contain, but in any case, controlling the thickness of the polymerized layers is challenging

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with this type of resin as they are affected by photobleaching, which induces a propagation of the polymer solidification deeper and deeper in the medium as the dye bleaches [3]. This first projection microstereolithography machine used an argon-ion laser emitting at 515 nm as light source in order to have a good match between the irradiation wavelength and the resin absorption peak, it also required the addition of a complex optical system for eliminating the speckle effect and redistributing the irradiance of the laser beam from a Gaussian to a flat-top profile [29]. Other authors also used LCDs in microstereolithography machines [30–32]. A few years later, Farsari et al. used a spatial light modulator operating in the UV as mask generator in a projection microstereolithography machine. This special LCD component (a polysilicon thin film twisted nematic LCD) was not damaged by wavelengths above 350 nm and was irradiated with an argon-ion laser operating at 351.1 nm [33, 34]. Again, the Gaussian irradiance distribution of the laser had to be reshaped to a rectangular uniform irradiance to achieve a uniform energetic distribution. This was performed using diffractive optical elements. As the machine operated in the UV, it was possible to use epoxy-based commercial SLA resins for the manufacturing of the objects [35]. The Digital Micromirror Devices (DMD™) made by Texas Instruments has an improved contrast compared to LCDs and can be used with a wider range of wavelengths. Its first commercial use was for video projectors but it can also be used as mask in a microstereolithography machine. This component is an array of micromirrors, each of them acting as an independent pixel. Each DMD™ pixel is actuated by electrostatic forces between two states corresponding to two orientations of the mirror, one that reflects the light toward a projection lens system, and the other that projects the light toward an absorbing area. Thus, each pixel of the device can be set independently to an “on” or an “off” state. In most microstereolithography applications, having black and white pixels is sufficient for manufacturing objects layer-by-layer, but for video projection applications gray levels and color images are generated by combining the rapid on/off movement of the mirror with a synchronized rotating color wheel. Many microstereolithography machines were built by different research teams using the DMD™ technology as dynamic mask [4, 36–40]. The first has been developed by Bertsch et al. [41] in 1998. A metal halide lamp was combined with adequate optical filters to tune the irradiation wavelength to the resin absorption peaks. Different acrylatebased resins were used in this machine, reacting either at 515 nm or at 410 nm. Later, the machine was improved to work in the UV at 365 nm, which allowed using commercial SLA resins after adapting them to the specific needs of microstereolithography. A variety of small-scale objects were built with this machine to demonstrate its speed and the freedom in the shapes that can be obtained (Fig. 1.2.8). If most process developments in the field of microstereolithography have been performed between 1993 and 2000, a small number of publications have been made more recently exploring new ways to build projection microstereolithography machines:

Microstereolithography

Fig. 1.2.8 Small cups (left) and fluidic connector (right) made by projection microstereolithography using a DMD™ by Bertsch et al.

- Liquid crystals on silicon (LCOS) have been used as dynamic mask for microstereolithography applications [42, 43]. LCOS can be used with UV light and have a very good contrast ratio in the projected images. Light-emitting diodes (LED) emitting at 395 nm have been used as light source. - Zheng et al. further improved the projection microstereolithography process by using a “step and repeat” method, which consists in moving the platform on which the object is built in X and Y to perform the polymerization of each layer through multiple exposures, leading to larger polymerized layers. This allows to make bigger objects with the same resolution [42], or to produce arrays of identical microstructures as presented by Choi et al. [44]. - Emami et al. presented a new SLA process in which a DMD™ is moved continuously in X and Y over the resin and the image it projects is updated continuously, such that large layers can be built with a good resolution [45]. Objects having dimensions in the centimeter range were built, with a resolution similar to that of some microstereolithography machines. This scanning-projection system allows to overcome the intrinsic problem of projection-based systems, in which the dimensions of the objects are limited by the size of the mask generator, and for which the uniform illumination of the projection array is difficult to achieve. - Microstereolithography using evanescent waves has been investigated by Takahashi et al. and allows to polymerize very thin layers of polymer [46]. Evanescent waves are formed when a light beam undergoes total internal reflection at an interface because the light beam angle is smaller than the critical angle. The evanescent wave controls the vertical propagation of the polymerization phenomenon and layers of submicron thickness can easily be obtained. The lateral light distribution is controlled by a DMD™ and focused with an appropriate set of lenses; consequently, the process only has a resolution in the micro range in the lateral direction. The superimposition of layers can theoretically be obtained, but has not been demonstrated yet.

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- In liquid-bridge microstereolithography, the liquid photosensitive resin forms a liquid bridge between a transparent substrate with low surface energy across which the light beam is applied, and another substrate with high surface energy used to hold the fabricated structure. This process uses a relatively small amount of material, and allows to use highly viscous materials in a microstereolithography process [47]. - The CLIP stereolithography process (CLIP stands for continuous liquid interface production) is conducted above an oxygen-permeable fluoropolymer window. The oxygen-containing photopolymer in close contact to this window becomes unreactive by oxygen inhibition and creates a liquid interface below the part being built, allowing the part to be continuously drawn out of the bath as it is solidified. There is no longer a layer-by-layer fabrication of the component, which makes this process faster than other DLP-based microstereolithography machines. This process has been first used to manufacture large components [48], but has also been adapted to microstereolithography by using adapted optics and to multimaterial microfabrication [5].

6 Microstereolithography processes having a submicron resolution When manufacturing small components, it is possible to exploit nonlinear phenomena to build objects directly inside the resin. There are many obvious advantages in creating an object directly inside the resin rather than at its surface: there is no need to spread the next layer of fresh resin on the part being manufactured, which speeds up the fabrication process, there is no need to fabricate supporting structures nor to remove them once the object is built, assembled objects and feely movable components can be made easily. Various microstereolithography processes based on this principle were developed, based either on two-photon photopolymerization or on under-the-surface photopolymerization with a one-photon process. These microstereolithography machines can build very small components with a submicrometer resolution. (a) Two-photon microstereolithography As three-dimensional microfabrication using two-photon polymerization is the subject of the present book and will be discussed in detail in the subsequent chapters, only a very brief summary of the two-photon technique will be presented here to underline its relations with the other microstereolithography processes. The first machine based on the polymerization of the object inside the resin rather than at its surface was built to push further the limits in resolution of the microstereolithography processes. The resolution of scanning and projection microstereolithography machines is limited to a few microns in the three directions of space, but using the two-photon polymerization principle results in a significant improvement of resolution: the two-photon absorption requires that the combined energy of two photons matches the transition energy between the ground and excited states of the photoinitiator, it has a quadratic dependence on the light intensity, which confines this phenomenon to the area of the focal point, and allows to obtain polymerized volume elements of submicrometer resolutions.

Microstereolithography

Maruo et al. presented the first two-photon microstereolithography machine in 1996 [49] and detailed it in subsequent publications [50, 51]. The light source used in this machine was a mode-locked Ti:Sapphire laser emitting at 770nm, and having a peak power in the resin of about 3 kW with a repetition rate of 76 MHz and a pulse width of 130 fs. A UV-photopolymerizable resin based on urethane acrylate monomers and oligomers, transparent to light at 770 nm, was used. This chemical resin did not attenuate the incident light beam, which could be focused inside the medium without starting unwanted polymerizations at the surface. Polymerization by two-photon absorption was induced at the focal point, which allowed the solidification of submicron volume elements. The use of scanning mirrors allowed deflecting the light in the horizontal plane to build the layers, whereas the manufacturing in the vertical direction was obtained by moving the photoreactor with a positioning stage (Fig. 1.2.9). Other authors further developed two-photon microstereolithography machines [52–54]: Kawata et al. obtained structures having a resolution of 120 nm (Fig. 1.2.10) [53], and various attempts have been made to further improve the resolution of twophoton microstereolithography since that date, using new photoinitiators [55], shorter wavelength [56], by confining the polymerization phenomenon using a quencher molecule in the photopolymerizable system [57, 58] and by developing a new approach to photopolymerization inspired by stimulated emission depletion (STED) microscopy where a multiphoton absorption of pulsed light is performed to initiate crosslinking and a simultaneous one-photon absorption of continuous-wave light is performed to deactivate photopolymerization [59]. Additionally, the replication of components made by two-photon microstereolithography was investigated by various molding techniques [60, 61]. Microstereolithography using a two-photon polymerization process is also named direct laser lithography, direct laser writing, or multiphoton lithography. Collective manufacturing of micro- and nanostructures using two-photon microstereolithography was also investigated by Kato et al., using a microlens array for the simultaneous fabrication of more than 200 identical small 3D structures [62] using a multifocal polymerization approach. (b) One-photon under-the-surface microstereolithography Finally, Maruo et al. presented a microstereolithography machine using a single photon process in a similar way to two-photon machines, and succeeded in performing the local

Fig. 1.2.9 Diagram of a two-photon microstereolithography machine.

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Fig. 1.2.10 Microbull made by two-photon microstereolithography, the scale bar corresponds to 2 μm. (Reprinted by permission by Nature, ©2001. S. Kawata, H.-B. Sun, T. Tanaka, K. Takada, Finer features for functional microdevices, Nature 412 (2001) 697–698.)

polymerization of a conventional UV-initiated resin under the surface [63]. This was made possible by tightly focusing inside the resin a radiation that is only weakly absorbed by the resin. Thus, the light intensity is sufficient to start the polymerization phenomenon only at the focal point, but not in the out-of-focus regions. This is possible if the chosen resin has a nonlinear behavior with the light intensity, which is the case for resins for which the photopolymerization reaction is inhibited by dissolved oxygen: the dissolved oxygen molecules scavenge the radicals produced by the initiators and prevent the propagation of the polymerization reaction in areas where the light intensity is sufficiently low. This microstereolithography machine used a continuous-wave He-Cd laser as light source, as its blue wavelengths at 441.6 nm is weakly absorbed by the UV-initiated urethane-acrylate resin they used. The best lateral resolution obtained with this process was 430 nm, whereas the best vertical resolution was 1.4 μm.

7 Microfabrication with microstereolithography Most of the research and development work on the microstereolithography process has taken place in the 10 years that followed the first scientific publication on this subject. First, scanning processes were developed followed by projection techniques and finally submicron processes were invented. Different ways to create a fresh new layer of resin at the surface of the object being built were tested, resulting in the implementation of two

Microstereolithography

main recoating strategies: free or constrained surface methods. Research on photopolymers and composite resins for microstereolithography has been carried out in parallel. If only minor developments of the microstereolithography process have taken place in recent years, research has been done on the development of materials and applications. Similar to more conventional rapid prototyping methods, microstereolithography is first used for providing small polymer components for prototyping purposes, allowing to create rapidly devices and systems when conventional rapid prototyping technologies reach their limits in terms of size and resolution. Microstereolithography can be successfully used to produce polymer components of dimensions between a fraction of a millimeter and a few centimeters, with small openings, complex shapes and details, with resolutions far better than any other rapid prototyping method. This is not only useful for highresolution prototypes, but also for manufacturing small personalized components, such as hearing-aid parts fitted to the patient morphology. The second main reason to use microstereolithography is the complexity of shapes that can be easily generated, leading to the fabrication of small and complex objects that would be very difficult to fabricate with any other manufacturing method. Being able to produce components that no other method can make is of course an asset as it allows to imagine new products and perform research on new concepts, but this aspect of microstereolithography is a double-edged sword, as such components cannot be mass produced by molding and if a few attempts have been made for fabricating multiple components simultaneously by microstereolithography, this technique is not yet suited for mass production. Consequently, these high-complexity high-resolution components are often successfully used to study new concepts in a research context, but rarely make it into products. The research teams who developed microstereolithography machines demonstrated the possibilities of this technique by manufacturing components of high geometric complexity. In many cases, the fabrication of various components was an actual part of the process development phase as it allowed to identify specific problems in the design of the machines and in the formulation of the resins. Typically, such components include pipes [15, 50], springs [15, 51], conical and pyramidal structures [21], microgears [29, 31, 34, 63], freely movable structures [63], and scaled-down models [11].

7.1 Microstereolithography components containing inserts Microstereolithography can be used to produce small polymer components, similar to other rapid prototyping methods, but more complex components can also be produced by combining inserts and components made by microstereolithography, generally to create active structures. The addition of an insert to a polymer component can be performed during the manufacturing process by interrupting the fabrication between two layers, placing the insert in a cavity and starting the fabrication again, such that the insert will

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be fully encapsulated in the generated polymer structure. It can also be assembled to the polymer structure afterwards. Building components with inserts has been reported in many scientific papers either for fabricating microsystem components or active microfluidic systems: - Microactuators having multiple degrees of freedom have been manufactured by inserting and gluing shape memory alloy wires in mechanical structures. By submitting some of the wires to a mechanical stress while assembling them, mechanical energy is stored in the final component. Joule heating of the wires allows to deform the structures, which return to their original position when the heating is stopped. Different shapes of microactuators have been tested, some having a distributed elasticity, and some based on elastic hinges [64]. - Carrozza et al. used a piezoelectric ceramic disk as insert to fabricate a micropump. SLA was used to build the pump body, channels, and valve housing, and the piezoelectric actuator was glued to the structure. The pump was tested with water, and the flow rate could be modulated between 1 and 50 μL/min by changing the frequency of the applied voltage [65]. - Ikuta et al. demonstrated the fabrication of a series of active and passive microfluidic components made by inserting rubber sheets, ultrafiltration membranes, shape memory alloy wires, and piezoelectric elements in the polymer structures, while they are manufactured by microstereolithography. A microreactor used to perform the separation of Luciferase from other reagents was made by inserting an ultrafiltration membrane and a photodiode in a structure made by microstereolithography, and allowed to monitor the evolution of the concentration of reagents by photoluminescence [66]. A micropump was fabricated by inserting rubber valves and shape memory alloy actuators during the microstereolithography step that created the pump body (Fig. 1.2.11), and achieved water flow rates up to 10 μL/min [67]. Switching valves were also produced to direct a liquid flow between two outlets, using shape memory alloy actuators as inserts [68]. An ultrasonic homogenizer was made inserting piezoelectric elements in a horn-shaped polymer component and was used to disrupt cells [69]. More complex microfluidic systems were obtained by assembling various active and passive fluidic components and interconnecting them without leakage [70].

7.2 Microstereolithography of composite materials The insertion of micro and nanoparticles in photopolymerizable media for microstereolithography allows fabricating components having other properties than these that can be obtained with polymers. The addition of ceramic particles to microstereolithography resins has been studied since the early days of microstereolithography, first using alumina particles in aqueous and nonaqueous photosensitive resins [71] and later extending the range of materials to zircona, silicon oxide, or titanium oxide particles. When the particle loading content

Microstereolithography

Fig. 1.2.11 Micropump made by inserting rubber valves and shape memory alloy wires actuators in the polymer pump chamber during its fabrication by microstereolithography. (Reprinted by permission by Springer-Verlag, ©2001. K. Ikuta, T. Adachi, T. Hasegawa, Multi-micro PuMP chip to flow gases and liquids, in: Micro Total Anal. Syst. 2001, Springer, Dordrecht, 2001, pp. 233–235.)

of the resin is high enough, the photopolymerizable media obtained are no longer liquid, but behave like pastes. In that case, components are needed to stabilize the formulations and prevent the aggregation of the particles. Bertsch et al. adapted a projection-based microstereolithography machine to work with pastes containing up to 80 wt% of alumina nanoparticles [72]. They were able to manufacture objects made of a large number of layers of 10 μm in thickness without noticeable loss of resolution in the transverse direction due to light scattering. Such composite objects, having a very high content in ceramic materials, can be placed in a furnace after manufacturing and be submitted to a debinding and sintering step, using the appropriate temperature ramps. The obtained components show some shrinkage and residual porosity, but keep their original shape. Apart from the classical objects fabricated as a proof of feasibility, different authors also found an interest in creating photonic crystal structures in composite materials. The fabrication of complex periodic 3D structures, of dimensions in the micron range that form photonic crystals can be easily achieved with microstereolithography machines, whereas they are quite difficult to produce with other manufacturing methods. 3D photonic crystals having micrometer-scale features and a diamond structure were manufactured for terahertz (THz) frequencies, using acrylate-ceramic suspensions containing up to 40 vol% of high refractive index ceramics [73]. Similar structures were made out of pastes containing high loads of SiO2 [74], SiO2-Al2O3 [75], ZrO2-Al2O3 [76] and were subsequently submitted to debinding and sintering, leading to the formation of dense ceramic photonic crystals. Metal photonic crystal structures have also been fabricated the same way [77]. The band gaps of all these structures were measured and corresponded well to the expected values. The same type of complex periodic 3D structures demonstrated in the field of photonic crystals are of interest for the production of optical metamaterials, which are materials exhibiting properties not found in nature such as negative refraction or cloaking

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properties. Wu et al. used a projection microstereolithography machine to fabricate metameterials, and produced a plastic component made of a large number of very high aspect-ratio pillars (30 μm in diameter, 1 mm in height) arranged in square lattice [78]. This component, once sputtered with gold, exhibits a sharp change in reflection and transmission at a frequency of 0.7 THz, making this structure a high-pass filter. Similar to optical metamaterials, the concept of metamaterials has been applied to the mechanical properties, leading to the fabrication of polymer 3D network structures exhibiting negative values of the Poisson’s ratio [79, 80] (such materials expand in their crosssection when stretched, in contrast to ordinary materials). With the finer features that can be obtained using three-dimensional two-photon polymerization, the field of photonic crystals and metamaterials has seen considerable developments in recent years. These will be detailed in the following chapters of this book.

7.3 Medical applications Today, rapid prototyping techniques are commonly used in the medical domain, where Computed Tomography-scans and Magnetic Resonance Imaging data can be converted to produce 3D-CAD models, and used to build real-size replicas of patient’s organs and injuries. Microstereolithography is also starting to find applications in the biomedical domain, as it allows to produce high-resolution and high-complexity components that could be used as tissue scaffolds of chosen pore dimensions and geometry, or to study and prototype miniaturized devices such as small prosthesis components tailored to the patient anatomy, or to manufacture microneedles or other devices: - The EnvisionTEC Perfactory® system, a commercial microstereolithography machine, is designed to produce fully functional components for medical and dental applications, such as personalized hearing aid shells made in biocompatible resins. - Different teams developed microneedles made by microstereolithography, aiming at applications such as transdermal drug delivery [81]. Such microneedles must be greater than 100 μm in lengths, in order to penetrate the skin. For withdrawal of blood from dermal blood vessels, microneedles at least 700 μm in length need to be used. Arrays of simple conical microneedles were made [82, 83] to study force required to insert them in human skin. Polymeric microneedles with antimicrobial coatings were also manufactured [84], as well as microneedles that are integrated with microelectrodes and used as transdermal electrochemical sensors for the detection of glucose or lactate [85]. - Stents inserted in blood vessels using catheters are widely used for the treatment of vascular diseases. Microstereolithography has been used for the fabrication of stents with high resolution and complex features, using bioresorbable photopolymerizable materials [86].

Microstereolithography

- Nerve guidance conduits used for the regeneration of peripheral nerve were fabricated by microstereolithography and their use in vivo showed a good reinnervation over 3 mm peripheral nerve lesion at three weeks in a mouse fibular nerve injury model [87, 88].

7.4 Bioprinting with microstereolithography Inkjet and extrusion-based additive manufacturing methods complemented by the laserinduced-forward-transfer (LIFT) technique are currently the most widely used approaches for bioprinting experiments. SLA-based methods are not prevalent in this field, mainly as common photoinitiators present in SLA resins are known to be cytotoxic [89], and the UV-light generally used for initiating the photopolymerization process is harmful for cells. However, these limitations can be circumvented by using visible light irradiation and adapted photoinitiating systems. A number of microstereolithography based bioprinting methods have been developed that are of interest in terms of fabrication speed, complexity of the fabricated biocompatible components, and bioprinting resolution. Different approaches describing high-resolution SLA-based bioprinting processes can be found in the literature, allowing to develop 3D microenvironments suitable for tissue engineering. Scaffolds for engineering soft tissues have been produced by microstereolithography. These are mechanical structures of complex shapes on which cells can attach, proliferate, and grow [90, 91]. Tissue scaffolds are also a three-dimensional microenvironment that is known to affect the cell-proliferation behavior, and allow cells to grow in a way that is closer to the one of a normal living tissue, compared to cell cultures in conventional Petri dishes. Fabricating scaffolds with microstereolithography allows the production of welldefined pore sizes, pore distribution, scaffold porosity, pore connectivity, and pore gradients out of a large number of biocompatible materials. - Photosensitive composite materials have been developed with the aim to manufacture scaffolds for potential applications such as bone [92] and cartilage reconstruction: Lee et al. made scaffolds by microstereolithography in order to enhance bone reconstruction; in vitro experiments showed preosteoblasts to adhere and proliferate on such structures and in vitro experiments on rats showed some bone regeneration after 11 weeks [93]. - Scaffolds for engineering soft tissues have also been produced by microstereolithography, using either synthetic polymers [94, 95] or natural polymers (hyaluronic acid [96], alginate [97], chitosan [98], or gelatin [99]). Scaffolds in elastic materials that can be used as artificial blood vessels have also been studied [100]. - Scaffolds into which cells are seeded result in random distributions of cells, but often a precise arrangement of different types of cells should be created to replicate natural tissues. Various studies demonstrated the potential of SLA-based bioprinting for manufacturing multicellular 3D constructs: Han et al. built a high-resolution SLA

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machine using DMDs, in which switching of monomers during fabrication has been implemented. This allowed to form microstructures with localized variations in material properties, such as scaffolds providing specific protein adhesion sites for cells [101]. Soman et al. demonstrated the bioprinting of gels containing cells using a SLA machine resulting in structures encapsulating different types of cells [102], and Raman et al. demonstrated multimaterial fabrication using cell-laden materials with a microstereolithography machine [103].

7.5 4D printing 4D printing is the process of fabricating through additive manufacturing methods, structures that can change of shape over time when a specific stimulus is applied. This change of shape can be linked to hinge-based folding structures, hydration of hydrogels, shapememory polymers, temperature-based deformations, and often require the fabrication of components using multiple materials of contrasting properties. Multijet modeling (also known as MultiJet printing or PolyJet technology) is often the preferred fabrication method for 4D printing as it is a multimaterial additive technology and allows mixing different resins to obtain the desired properties. It is also possible to use microstereolithography for the fabrication of 4D-printed components, with the advantage of its high resolution and using the large number of materials that can be shaped with this process. Different strategies have been imagined to obtain the 4D effect, either using single material solidification or multimaterial microstereolithography machines. - Various photopolymerizable resins demonstrating shape memory properties in a single print were developed. Shape memory was demonstrated through fold-deploy tests with a thermal activation mechanism for various tailor-made resins [104–106]. - Temperature-responsive swelling hydrogel structures were fabricated by microstereolithography in a single print. By varying locally the applied fluence during fabrication, bilayer beams of different swelling characteristics were obtained. Their thermal actuation caused them to bend inwards to generate a gripping motion [107]. - Multimaterial microstereolithography machines were developed, allowing to exchange the photopolymerizable resin during the manufacturing process. Their use with different shape memory polymerizable resins allowed to perform complex actions, with different temperatures triggering sequentially the deformation of each individual polymer [106].

8 Conclusion Microstereolithography is a technology that has seen its first implementations more than 20 years ago. It has been first developed in an academic context, with many research teams developing different concepts and apparatuses as well as formulating adapted resins dedicated to this technology. First, it has been used in a microsystems context, to fabricate

Microstereolithography

components that were not possible to be built with more conventional wafer-scale cleanroom technologies, giving an access to the third dimension to this field. However, it has only known a marginal success in the microsystems field due to the difficulty to integrate it with other microfabrication methods. Additionally, most complex objects made by microstereolithography have complex shapes and a 3D geometry and cannot be molded easily, making microstereolithography a manufacturing technology rather than a prototyping method. Further developments of microstereolithography focused on the development of new materials dedicated to this technology. A variety of polymers and composite resins have been developed, as well as biocompatible and biodegradable materials. In recent years, only minor developments of the microstereolithography process itself have been performed, but many components that are extremely difficult to produce with other manufacturing techniques have been fabricated to investigate the fields of photonic crystals and metamaterials, and there is a growing interest of the biomedical domain for this technology as it provides a rapid way to provide small components required for personalized medical devices, such as hearing aid components. In the recent years, two new developments of additive manufacturing have emerged: 4D printing aims at adding functions that allow the fabricated components to evolve with time, resulting in components made by additive manufacturing that either change of shape or self-assemble themselves in given conditions. Bioprinting aims at printing living tissues and organs using additive manufacturing principles and is based on 3D printing biomaterials and cells. In both cases, the high resolution that can be obtained with microstereolithography can be an asset to develop new applications. Currently there are only a few microstereolithography machine providers on the market, and many machines are operated in an academic context. However, rapid prototyping technologies are currently gaining popularity as they are made more affordable and reach the general public. This will generate more ideas and new applications of rapid prototyping technologies that require high resolution and highly complex components may emerge, expanding the domain of application of microstereolithography.

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