**3. Resins in SLA**

the use of expensive fs pulsed lasers [37]. This process has as of yet not been commercialized,

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 fabrica-

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

**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

and very little research is invested in pinpoint solidification-SLA.

*2.2.3. Bulk lithography*

8 3D Printing

tion of microstructures [39].

energy dosage [40].

**2.3. Digital light processing stereolithography**

crystal analysis). Copyright (2017) American Chemical Society.

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 can be present.

#### **3.1. Precursors**

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.

Especially for complex geometries with undercuts, this cure depth needs to be precisely defined in order to prevent excessive curing in z-direction and loss in feature development [14, 17, 40]. The most commonly used UV absorbers are benzotriazole derivatives [58].

Stereolithography

11

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

Fabrication of metal or ceramic materials via SLA has been implemented by filling resins with powder, printing the parts, and subsequently debinding and sintering the printed specimens [4, 5], as shown in **Figure 6**. During debinding, the organic resin components are removed by pyrolysis. This binder burnout is easier for thin structures with high filler content, as otherwise, defects such as cracks can form [5]. In the subsequent sintering step, the metal or ceramic powder, which remains, is further thermally treated to achieve dense structures [21]. In order to attain geometrically accurate parts, the material specific shrinkage coefficients need to be taken into account, and high filler content is beneficial to reduce shrinkage [60]. Variations of the thermal treatment include implementing an additional drying step prior to debinding to remove solvents or combining debinding and sintering into one single but correspondingly longer thermal process to eliminate potential defect sources during transporta-

Particles smaller than the layer height need to be utilized, and as previously mentioned, maximum particle content is desirable. In highly filled resins with particles in size range of the wavelength of light, scattering is the main interaction mechanism with light and consequently determines cure depth and affects resolution. Reducing the refractive index difference between filler and matrix is a common approach to minimizing scattering [61]. It is noteworthy that composite materials, where the organic component is not removed but retains its matrix function in the final part, have also been manufactured with SLA [3, 62]. By adding (nano)-particles to the SLA resin, mechanical, thermal, optical, or even electrical

**Figure 6.** Scheme of the formation of dense ceramic components from filled resins by SLA, redrawn and adapted from [59].

**3.4. Filled resins**

tion of the fragile brown parts [59].

properties can be further amended [63–65].

**Figure 5.** Resin components in SLA.

Acrylate-based resins are common in the SLA process, as they exhibit high reactivities, which is advantageous for fast building speeds [7]. Different types of acrylates are readily available to tune mechanical properties and thermal resistance for example by altering the number of reactive groups [8] or by employing different oligomers such as urethane acrylates [50]. One disadvantage of acrylate resins is their high-shrinkage during printing, causing potential distortion of the printed part. As a solution, the combination with methacrylates is often implemented [51]. The resin's sensitivity to oxygen, which inhibits the polymerization reaction, is another challenge.

Epoxy systems have a different curing mechanism than acrylates. They are based on cationic rather than radical photopolymerization and need longer reaction times, are inhibited by moisture, but have the advantage of stability against oxygen [52]. Additionally, epoxy resins exhibit significantly lower shrinkage than their acrylate counterparts [53]. In order to exploit the advantages of both alternatives, hybrid systems have been created. Combination of acrylate and epoxy-based resins lead to fast curing, low-shrinkage materials and are nowadays the standard in most commercial systems [54, 55].

#### **3.2. Photoinitiators (PIs)**

The PI is the resin component, which reacts to light. Once irradiated at the correct wavelength, it is excited and can initiate the curing reaction. A suitable PI, depending on the nature of utilized precursor needs to be selected. Type and amount of PI can substantially influence reaction kinetics, necessary light dosage, conversion, cross-linking density, and by extent, mechanical properties of the printed parts [8, 56, 57].

#### **3.3. Absorbers**

Another component that is essential in most SLA processes is a light absorber, which reduces the penetration of light into the resin and limits the depth until, which the resin is cured. Especially for complex geometries with undercuts, this cure depth needs to be precisely defined in order to prevent excessive curing in z-direction and loss in feature development [14, 17, 40]. The most commonly used UV absorbers are benzotriazole derivatives [58].

#### **3.4. Filled resins**

Acrylate-based resins are common in the SLA process, as they exhibit high reactivities, which is advantageous for fast building speeds [7]. Different types of acrylates are readily available to tune mechanical properties and thermal resistance for example by altering the number of reactive groups [8] or by employing different oligomers such as urethane acrylates [50]. One disadvantage of acrylate resins is their high-shrinkage during printing, causing potential distortion of the printed part. As a solution, the combination with methacrylates is often implemented [51]. The resin's sensitivity to oxygen, which inhibits the polymerization reac-

Epoxy systems have a different curing mechanism than acrylates. They are based on cationic rather than radical photopolymerization and need longer reaction times, are inhibited by moisture, but have the advantage of stability against oxygen [52]. Additionally, epoxy resins exhibit significantly lower shrinkage than their acrylate counterparts [53]. In order to exploit the advantages of both alternatives, hybrid systems have been created. Combination of acrylate and epoxy-based resins lead to fast curing, low-shrinkage materials and are nowadays

The PI is the resin component, which reacts to light. Once irradiated at the correct wavelength, it is excited and can initiate the curing reaction. A suitable PI, depending on the nature of utilized precursor needs to be selected. Type and amount of PI can substantially influence reaction kinetics, necessary light dosage, conversion, cross-linking density, and by extent,

Another component that is essential in most SLA processes is a light absorber, which reduces the penetration of light into the resin and limits the depth until, which the resin is cured.

tion, is another challenge.

**Figure 5.** Resin components in SLA.

10 3D Printing

**3.2. Photoinitiators (PIs)**

**3.3. Absorbers**

the standard in most commercial systems [54, 55].

mechanical properties of the printed parts [8, 56, 57].

Fabrication of metal or ceramic materials via SLA has been implemented by filling resins with powder, printing the parts, and subsequently debinding and sintering the printed specimens [4, 5], as shown in **Figure 6**. During debinding, the organic resin components are removed by pyrolysis. This binder burnout is easier for thin structures with high filler content, as otherwise, defects such as cracks can form [5]. In the subsequent sintering step, the metal or ceramic powder, which remains, is further thermally treated to achieve dense structures [21]. In order to attain geometrically accurate parts, the material specific shrinkage coefficients need to be taken into account, and high filler content is beneficial to reduce shrinkage [60]. Variations of the thermal treatment include implementing an additional drying step prior to debinding to remove solvents or combining debinding and sintering into one single but correspondingly longer thermal process to eliminate potential defect sources during transportation of the fragile brown parts [59].

Particles smaller than the layer height need to be utilized, and as previously mentioned, maximum particle content is desirable. In highly filled resins with particles in size range of the wavelength of light, scattering is the main interaction mechanism with light and consequently determines cure depth and affects resolution. Reducing the refractive index difference between filler and matrix is a common approach to minimizing scattering [61]. It is noteworthy that composite materials, where the organic component is not removed but retains its matrix function in the final part, have also been manufactured with SLA [3, 62]. By adding (nano)-particles to the SLA resin, mechanical, thermal, optical, or even electrical properties can be further amended [63–65].

**Figure 6.** Scheme of the formation of dense ceramic components from filled resins by SLA, redrawn and adapted from [59].

#### **3.5. Additives**

A high-volume fraction of solid loading can cause certain disadvantages. Especially for smaller particles with the large surface area, the viscosity of the slurry rises with particle content [4]. This changes the flow behavior of the resin, interferes with coating mechanisms, and increases the mechanical force necessary for the elevation of the building platform in constrained surface setups [19]. Approaches to reduce viscosity include the application of temperature [66], the use of diluents [67], or evoking shear thinning behavior [68].

reduced surface roughness, and faster production times in this application [75]. Channels with dimensions below 100 μm and valves, pumps, as well as multiplexers for mixing [76],

Stereolithography

13

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

Applications in medicine, where patient-specific designs are often necessary to accommodate for individual anatomies, can greatly benefit from AM as well. Some examples are depicted in **Figure 7**. CT or MRI scans can be employed to determine the geometrical specifications, from which devices are then manufactured. Craniofacial implants out of porously structured hydroxyapatite have for example been implanted in patients with large bone defects [80].

In dentistry, CAD modeling has been applied since the 70s in the creation of crowns, which are used to cover a damaged tooth, and dentures, which are removable or fixed devices to replace lost teeth [81]. Now, many AM technologies including SLA can be employed to speed up the process between the acquisition of the geometrical data and implantation of the device into the patient [82]. A second application in healthcare, where AM has become the norm is the manufacturing of hearing aids. SLA can reduce the manufacturing time of these custom-made devices from more than a week to less than a day while also improving wear comfort [83, 84]. Medical applications of SLA are not limited to the fabrication of implants, prostheses, or other medical devices, but drug delivery systems such as micro-needles, capable of administering drugs by painlessly penetrating the skin [78], or 3D printed tablets for individual dosage

**Figure 7.** Top row: CAD model of skull defect (A), SLA fabricated cranial implant (C), and an implant placed into skull model (D), reprinted and modified from [77]. Middle row: SEM images of SLA printed microneedle structures for transdermal drug delivery, modified and reprinted from [78]. Bottom row: SEM images of tissue engineering scaffolds

for bone regeneration by SLA, modified and reprinted from [79] by permission of Springer Nature.

can be fabricated rapidly and easily.

Rheological additives and stabilizers can increase solid loading and are necessary for extended shelf-life of slurries as well as for stability during longer printing jobs [68]. Agglomeration and sedimentation of particles need to be avoided to ensure continuous, homogeneous ceramic or metal powder distribution. To that end, dispersants such as oligomeric surfactants [69], long chained acids like oleic acid [70], or phosphine oxides with aliphatic chains [71] have been used.

#### **3.6. Post-processing**

After removal of the built part from the platform, any support structures that had been necessary for the printing process need to be cut from the green part. Cleaning in suitable solvents and drying of the structure is often followed by sanding of support residues. Post-curing in a UV chamber can be implemented to complete conversion of the polymerization reaction and thereby attain improved mechanical properties [72]. In the case of filled resins, debinding and sintering are the final post-processing steps.
