**2. The stereolithography setup**

SLA is a vat polymerization method [13], where layers of the liquid precursor in a vat are sequentially exposed to ultraviolet (UV) light and thereby selectively solidified. A photoinitiator (PI) molecule in the resin responds to incoming light and upon irradiation, locally activates the chemical polymerization reaction, which leads to curing only in the exposed areas. After developing the first layer in that manner, a fresh resin film is applied, irradiated, and cured. Thus, the part incrementally grows layer-after-layer [1]. This principle spans all SLA processes, which can be classified according to the direction of incident light or irradiation method, see **Figure 1**.

The required light for solidification of the resin can be applied in two distinct manners; either from above in the free surface approach, or from below through a transparent vat in the constrained surface approach. Irradiation can either be implemented by scanning of each point of the desired cross-section with a laser in laser-SLA or by projecting the entire pixelated image onto the layer in digital light processing (DLP) SLA. A more uncommon method is illumination through an liquid crystal display (LCD) photomask.

These systems and some of their adaptations will be explained in detail in the following sections.

**Figure 1.** Classification of SLA according to irradiation method (left) and direction of incident light (right).

### **2.1. Free and constrained surface approach**

of composites [3] and even metallic [4] or ceramic [5] specimens is possible. However, to date, SLA is only being utilized to structure one material at a time, and when comparing to other additive manufacturing (AM) technologies, typical SLA processes exhibit superior resolution

**Table 1.** Comparison of average lateral resolution and surface roughness of AM technologies for polymers and of soft

**Resolution (μm) Surface roughness (μm) Sources**

While soft lithography is not an AM technology, it merits mentioning as a competing method due to its superior resolution to most SLA systems. It utilizes elastomeric stamps or molds to fabricate 3D structures and consequently does not have the characteristic advantages of 3D printing such as the ability of direct and rapid fabrication from a computer-aided design (CAD) model [12]. Nevertheless, it is often employed as an alternative to SLA in for example

SLA is a vat polymerization method [13], where layers of the liquid precursor in a vat are sequentially exposed to ultraviolet (UV) light and thereby selectively solidified. A photoinitiator (PI) molecule in the resin responds to incoming light and upon irradiation, locally activates the chemical polymerization reaction, which leads to curing only in the exposed areas. After developing the first layer in that manner, a fresh resin film is applied, irradiated, and cured. Thus, the part incrementally grows layer-after-layer [1]. This principle spans all SLA processes, which can be classified according to the direction of incident light

The required light for solidification of the resin can be applied in two distinct manners; either from above in the free surface approach, or from below through a transparent vat in the constrained surface approach. Irradiation can either be implemented by scanning of each point of the desired cross-section with a laser in laser-SLA or by projecting the entire pixelated image onto the layer in digital light processing (DLP) SLA. A more uncommon method is illumina-

These systems and some of their adaptations will be explained in detail in the following

and better surface qualities (see **Table 1**) but at slower printing times and higher cost.

SLA 10–150 0.38–0.61 [6–8] Material jetting 25–100 0.47–8.44 [6, 9] Material extrusion 100–400 3.24–42.97 [6, 9] Powder bed fusion 50–100 17–105 [7, 10] Soft lithography <0.01 <0.001 [9, 11]

microfluidic applications [6].

lithography.

4 3D Printing

**2. The stereolithography setup**

or irradiation method, see **Figure 1**.

sections.

tion through an liquid crystal display (LCD) photomask.

In the **free surface approach**, the building platform on which the printed part grows is situated in a tank of resin and coated with a liquid resin film. Illumination of the desired crosssection, which happens from above the resin bath, cures the first layer. After each layer, the platform with the growing part (i.e. the z-stage), is lowered further into the tank, and new resin is coated on top with a mechanical sweeper. This sets the stage for the subsequent layer [14].

The **constrained surface approach** or bottom-exposure approach has a building platform, which can be suspended above the resin bath. Illumination from below, through the transparent floor, cures a layer of resin between building platform and vat floor. This layer adheres to the platform, as the z-stage is raised by a defined distance. As each layer is cured, the building platform with the adhered part is elevated, and the part grows suspended from the platform downward [6]. As with the free surface setup, support structures made from the same printing material are needed in case of overhangs and undercuts to ensure adequate adhesion to the platform.

Recently, there has been a trend toward the bottom-exposure approach, as it has certain advantages [6, 15, 16]. The smooth surface, which is created with a narrowly defined layer height due to the precise movement of the z-stage at an accuracy of down to 0.1–1 μm [17, 18], is the main benefit in the constrained surface setup. Without the need for a mechanical sweeper, this layer can be created faster than in the free surface approach, reducing the printing time [15]. Another advantage, which decreases cost, is the lower amount of resin that is needed because the specimen does not have to be completely submerged in the vat [14].

A main disadvantage of the bottom-exposure setup, however, is that attractive forces between printed part and vat floor need to be overcome for each layer [19]. When pulling up the z-stage, the newly cured layer needs to adhere to the layers above, and may not stick to the vat surface. Attempts to reduce this unwanted interaction include the application of hydrophobic coatings of the material tray [15, 18–20] and modification of the mechanical separation mechanism with tilting steps [21] or application of shear forces [20].

#### **2.2. Laser-stereolithography**

In Laser-SLA, also known as vector-based SLA, or often simply referred to as SLA, each layer is cured by scanning of a UV laser onto the resin film. This x-y motion of the laser is implemented by two galvanometers in combination with a dedicated optical system. An example of setup is given schematically in **Figure 2**. These conventional SLA devices, although more expensive than other AM technologies such as their extrusion-based counterparts, can reach resolutions of 5–10 μm [8].

region of its focal point, called a volume pixel or voxel [30]. A high-intensity femtosecond pulsed laser can cause molecules to absorb two photons simultaneously. As the probability of this phenomenon is proportional to the squared intensity of the laser pulse, the process is limited to the focal point of the laser. Instead of UV light, a laser at twice the wavelength (i.e. half the energy) with near-infrared (NIR) light such as a Titanium-sapphire laser is employed in TPP. The energy necessary for excitation is nevertheless attained by the combination of the

Stereolithography

7

http://dx.doi.org/10.5772/intechopen.78147

As portrayed in **Figure 3**, the spatially constrained 3D voxel in TPP allows for curing of shapes inside the resin bath and not just on its surface. This eliminates the need for layer-wise production and enables fabrication of extremely complex geometries including freely moving

One of the challenges which remain in TPP is the restriction to extremely small geometries in the mm range [34] and low writing speed of the laser lines. At a maximum of a few mm/s, it cannot compare to hundreds of mm/s, which is attainable with conventional laser-SLA methods [8]. Developing a suitable PI could help speed up the process. Conventional UV initiators have the drawback of low-activity in TPP. In order to augment their response, the design of

**Figure 3.** SEM images of microstructures fabricated with TPP (A), reprinted with permission from [32] (Ovsianikov A. et al. ultra-low shrinkage hybrid photosensitive material for two-photon polymerization microfabrication.), copyright (2008) American Chemical Society. Scheme of TPP vs. conventional laser-SLA (B) redrawn and adapted from [33].

A method similar to TPP, pinpoint solidification, was proposed by Ikuta *et al.* in 1998 under the name of super integrated hardened polymer SLA (Super IH) process [36]. A tightly focused laser is used and as with TPP, due to the high intensity in the focal point of the laser, curing of the resin can only be achieved in this voxel. The mechanism, however, is that of conventional single photon polymerization. Thus, resolutions of below 0.4 μm have been reached without

energies of both individual photons [31].

parts without superfluous support structures [28].

molecules with specific structures is necessary [35].

*2.2.2. Pinpoint solidification*

**Figure 2.** Schematic setup of a laser-SLA printer (A), adapted, and modified from [22]. Scanning electron microscope (SEM) images of a bioreactor with capillaries fabricated with laser-SLA (B), reprinted from [23] by permission of springer nature.

In order to attain these resolutions, a number of parameters need to be considered. Besides accuracy of the z-stage and optimized resin composition (see Section 3.3), which are important factors especially in z-resolution, the manner in which the UV laser is scanned across the desired layer is decisive. The geometry of the precisely defined laser lines, which illuminate the entire cross-section [24] and their accuracy, given by movement of the galvano-mirror, determine lateral resolution. Furthermore, the scanning speed and diameter of the laser spot need to be considered. They, respectively, influence the cure depth and width of the exposed laser lines and thereby affect vertical and lateral resolution [25]. Methods to further improve the resolution to sub-micron regions include two-photon polymerization (TPP) and pinpoint solidification.

#### *2.2.1. Two-photon polymerization (TPP)*

TPP was first proposed as an AM method by Strickler *et al.* [26]. As resolutions superior to 100 nm with surface roughness below 10 nm are attainable [27], it has been extensively studied since then [28], and despite its high-cost, TPP was even commercialized by Nanoscribe GmbH in 2007 [29].

In TPP, excitation of the PI in the resin, and thereby activation of the curing reaction, does not occur in the entire illumination path of the laser, as in conventional SLA, but only in the

region of its focal point, called a volume pixel or voxel [30]. A high-intensity femtosecond pulsed laser can cause molecules to absorb two photons simultaneously. As the probability of this phenomenon is proportional to the squared intensity of the laser pulse, the process is limited to the focal point of the laser. Instead of UV light, a laser at twice the wavelength (i.e. half the energy) with near-infrared (NIR) light such as a Titanium-sapphire laser is employed in TPP. The energy necessary for excitation is nevertheless attained by the combination of the energies of both individual photons [31].

As portrayed in **Figure 3**, the spatially constrained 3D voxel in TPP allows for curing of shapes inside the resin bath and not just on its surface. This eliminates the need for layer-wise production and enables fabrication of extremely complex geometries including freely moving parts without superfluous support structures [28].

**Figure 3.** SEM images of microstructures fabricated with TPP (A), reprinted with permission from [32] (Ovsianikov A. et al. ultra-low shrinkage hybrid photosensitive material for two-photon polymerization microfabrication.), copyright (2008) American Chemical Society. Scheme of TPP vs. conventional laser-SLA (B) redrawn and adapted from [33].

One of the challenges which remain in TPP is the restriction to extremely small geometries in the mm range [34] and low writing speed of the laser lines. At a maximum of a few mm/s, it cannot compare to hundreds of mm/s, which is attainable with conventional laser-SLA methods [8]. Developing a suitable PI could help speed up the process. Conventional UV initiators have the drawback of low-activity in TPP. In order to augment their response, the design of molecules with specific structures is necessary [35].

#### *2.2.2. Pinpoint solidification*

**2.2. Laser-stereolithography**

6 3D Printing

resolutions of 5–10 μm [8].

solidification.

nature.

GmbH in 2007 [29].

*2.2.1. Two-photon polymerization (TPP)*

In Laser-SLA, also known as vector-based SLA, or often simply referred to as SLA, each layer is cured by scanning of a UV laser onto the resin film. This x-y motion of the laser is implemented by two galvanometers in combination with a dedicated optical system. An example of setup is given schematically in **Figure 2**. These conventional SLA devices, although more expensive than other AM technologies such as their extrusion-based counterparts, can reach

In order to attain these resolutions, a number of parameters need to be considered. Besides accuracy of the z-stage and optimized resin composition (see Section 3.3), which are important factors especially in z-resolution, the manner in which the UV laser is scanned across the desired layer is decisive. The geometry of the precisely defined laser lines, which illuminate the entire cross-section [24] and their accuracy, given by movement of the galvano-mirror, determine lateral resolution. Furthermore, the scanning speed and diameter of the laser spot need to be considered. They, respectively, influence the cure depth and width of the exposed laser lines and thereby affect vertical and lateral resolution [25]. Methods to further improve the resolution to sub-micron regions include two-photon polymerization (TPP) and pinpoint

**Figure 2.** Schematic setup of a laser-SLA printer (A), adapted, and modified from [22]. Scanning electron microscope (SEM) images of a bioreactor with capillaries fabricated with laser-SLA (B), reprinted from [23] by permission of springer

TPP was first proposed as an AM method by Strickler *et al.* [26]. As resolutions superior to 100 nm with surface roughness below 10 nm are attainable [27], it has been extensively studied since then [28], and despite its high-cost, TPP was even commercialized by Nanoscribe

In TPP, excitation of the PI in the resin, and thereby activation of the curing reaction, does not occur in the entire illumination path of the laser, as in conventional SLA, but only in the A method similar to TPP, pinpoint solidification, was proposed by Ikuta *et al.* in 1998 under the name of super integrated hardened polymer SLA (Super IH) process [36]. A tightly focused laser is used and as with TPP, due to the high intensity in the focal point of the laser, curing of the resin can only be achieved in this voxel. The mechanism, however, is that of conventional single photon polymerization. Thus, resolutions of below 0.4 μm have been reached without the use of expensive fs pulsed lasers [37]. This process has as of yet not been commercialized, and very little research is invested in pinpoint solidification-SLA.

With its pixel-based exposure mechanism, DLP is excellent for illumination of sharp corners but can cause saw-tooth type surface roughness on otherwise curved surfaces [18]. Consequently, when aiming for higher resolution, the pixel size needs to be reduced with the help of designated optics. As the DMD has a fixed amount of mirrors, this leads to shrinkage of the image and reduces maximum geometry size. Large parts are thus often printed at lower resolutions than small ones. While not quite reaching the sub-micron resolutions of laser-

Stereolithography

9

http://dx.doi.org/10.5772/intechopen.78147

Continuous liquid interface production (CLIP) is a type of constrained surface DLP process, where a thin film between building platform and the material tray is not cured and remains liquid. The so-called dead zone at the interface can be generated by utilizing a vat with a floor that is permeable to oxygen. This inhibits curing, and the resin in contact with oxygen remains liquid. Recoating mechanisms are thereby superfluous and continuous elevation of the building platform can be achieved, which improves surface quality and drastically increases printing speed up to 500 mm/h [44]. Similarly, to attain a liquid interface film, a high-density inert and an immiscible liquid layer such as brine has been proposed [45]. CLIP has been commercialized by Carbon Inc., and is establishing itself in the AM market due to its

Since its development in 1997 by Bertsch *et al.* [47], using an LCD device as a dynamic mask for SLA has been almost completely replaced by the DLP counterpart. The latter benefits are from superior switching speeds at higher accuracy [48]. Nevertheless, it merits mentioning as a low-cost alternative to DLP with commercially available LCD printers primarily catering to

Photocurable resins for SLA all have the same essential components, as summarized in **Figure 5**. The liquid precursors, which form the network when polymerized, as well as PIs, which start the reaction, are indispensable. In addition, most resin formulations have inert dyes, which absorb incident light and enhance control over the polymerization. Especially when using filled resins, further additives such as diluents, surfactants, or other stabilizers

The precursors in SLA are liquid molecules, which can be linked together (i.e. polymerized), after exposure to light to form a solid 3D network. Depending on the future application and

desired attributes, a variety of monomers, oligomers, or prepolymers can be utilized.

SLA, DLP retains the advantages of lower cost and higher printing speeds [40, 43].

*2.3.1. Continuous liquid interface production (CLIP)*

**2.4. Liquid crystal display stereolithography**

the laypersons demographic as opposed to the industry [49].

reduced printing times [46].

**3. Resins in SLA**

can be present.

**3.1. Precursors**

#### *2.2.3. Bulk lithography*

In bulk lithography, 3D textures can be created by variation of exposure energy. The cure depth, which is a direct function of laser power or scan velocity (i.e. of the applied energy), thereby defines the depth of the features [38]. One can thus see the entire part as only existing of one layer with varying thickness. This eliminates the sometimes abrupt steps in z-direction, which are generated with conventional SLA methods, and vastly speeds up printing. Although this process is not capable of printing structures with overhangs and is limited to geometries thinner than 0.25 mm, it could have potential future applications in high-throughput fabrication of microstructures [39].

### **2.3. Digital light processing stereolithography**

DLP is a method, which can reach resolutions in the order of 25 μm [7]. Smallest feature sizes of 0.6 μm have also been reported [40], and resins filled with ceramic particles have been printed via DLP with layer heights of 15 μm and with lateral resolutions of 40 μm [18].

In contrast to laser-SLA, the entire cross-section of a layer is illuminated simultaneously by a DLP light engine, as shown in **Figure 4**. The digital micromirror device (DMD) is the key component and functions as a dynamic mask for the DLP process. It is constructed of an array of mirrors, each one representing a single pixel. Individual tilting of every mirror enables fast and reliable switching of pixels [42]. When linked with a computer for image processing, a light source (often LED), and optics, it can project desired cross-sections of light quickly and precisely [42]. The fast switching speed of the DMD is a prerequisite for realizing grayscale illumination, which can be beneficial for precise control over exposure time and by extent energy dosage [40].

**Figure 4.** Setup of a DLP-SLA printer (A) adapted and redrawn from [22]. Printed polymer structures (B) adapted and reprinted with permission from [41] (Macdonald NP. et al., 3D printed micrometer-scale polymer mounts for single crystal analysis). Copyright (2017) American Chemical Society.

With its pixel-based exposure mechanism, DLP is excellent for illumination of sharp corners but can cause saw-tooth type surface roughness on otherwise curved surfaces [18]. Consequently, when aiming for higher resolution, the pixel size needs to be reduced with the help of designated optics. As the DMD has a fixed amount of mirrors, this leads to shrinkage of the image and reduces maximum geometry size. Large parts are thus often printed at lower resolutions than small ones. While not quite reaching the sub-micron resolutions of laser-SLA, DLP retains the advantages of lower cost and higher printing speeds [40, 43].
