**4. Applications**

SLA is a very versatile method with applications in a variety of industries. The aerospace and automotive industries can, for instance, benefit from rapid manufacturing of high-performance materials. Microfluidics and medicine are furthermore, significant fields where SLA shows great potential and is already being applied successfully.

Fully polymeric materials structured by SLA can range in their properties from highly elastic silicones for applications in soft robotics [45] to high-strength thermally post-cured epoxy resins [73]. Their limited thermo-mechanical stability is, however, an issue for most polymeric materials. Using filled resins to create metal or ceramic structures, is a possibility in SLA, as previously mentioned in Section 3.4. Furthermore, polymer-derived ceramics can be manufactured by using monomers as precursors, which contain the essential components to form ceramics upon pyrolysis. These methods offer superior versatility in geometry than casting or machining processes and can yield components for high-temperature applications such as in propulsion systems or as thermal insulators [74].

Recently, SLA has been extensively investigated in the field of microfluidics, where small fluid volumes need to be precisely manipulated through micro-sized channels for applications such as inkjet print heads or lab-on-a-chip technologies [6]. When compared to material extrusion and jetting, DLP-SLA shows superior resolution, smaller possible feature sizes, 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], can be fabricated rapidly and easily.

**3.5. Additives**

12 3D Printing

**3.6. Post-processing**

**4. Applications**

sintering are the final post-processing steps.

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],

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.

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

SLA is a very versatile method with applications in a variety of industries. The aerospace and automotive industries can, for instance, benefit from rapid manufacturing of high-performance materials. Microfluidics and medicine are furthermore, significant fields where SLA

Fully polymeric materials structured by SLA can range in their properties from highly elastic silicones for applications in soft robotics [45] to high-strength thermally post-cured epoxy resins [73]. Their limited thermo-mechanical stability is, however, an issue for most polymeric materials. Using filled resins to create metal or ceramic structures, is a possibility in SLA, as previously mentioned in Section 3.4. Furthermore, polymer-derived ceramics can be manufactured by using monomers as precursors, which contain the essential components to form ceramics upon pyrolysis. These methods offer superior versatility in geometry than casting or machining processes and can yield components for high-temperature applications such as in

Recently, SLA has been extensively investigated in the field of microfluidics, where small fluid volumes need to be precisely manipulated through micro-sized channels for applications such as inkjet print heads or lab-on-a-chip technologies [6]. When compared to material extrusion and jetting, DLP-SLA shows superior resolution, smaller possible feature sizes,

the use of diluents [67], or evoking shear thinning behavior [68].

shows great potential and is already being applied successfully.

propulsion systems or as thermal insulators [74].

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.

regulation [85] have also been established. SLA can furthermore be used in medical imaging to create 3D models [86] for preoperative planning [87] or educational purposes [88]. 3D cell culture for more accurate in vitro models to study diseased, as well as healthy tissues, can likewise be created [89]. Tissue engineering constructs in regenerative medicine have also been fabricated with SLA methods [90] and even bioprinting, where live cells are incorporated into the printed scaffold is currently being investigated [91].

during the printing process, have also been established [76]. For laser-SLA processes, illumination itself is a limiting factor in reducing printing times and different approaches to increase throughput are necessary. Using a broader scanning pattern for bulk features and applying more precise, narrow lines only in areas where the maximum resolution is required, such as for fine structures and at surfaces, is one method, which is already being implemented [93]. The development of hybrid systems of DLP and laser techniques is currently being investigated as well. Similarly, the inner area should be illuminated via pixel-based DLP, and only round surfaces drawn with the vector-based system of laser-SLA. This could further reduce printing times to rates comparable to DLP while maintain-

Stereolithography

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

Another method, which combines laser and DLP-SLA, addresses the compromise between build size and resolution in DLP. A proposition to retain small pixels and thereby highresolution even when printing large parts is to laterally stitch the projected images. If a layer has a cross-section exceeding the attainable size by the DMD, it can be divided into smaller areas, which are then illuminated one after the other [100]. This combination of scanning and projection-based illumination, also called large area projection micro SLA (LAPμLA) [101], can lead to the low-cost fabrication of cm-sized objects with a μm-range resolution [102].

Modification of available SLA systems to manufacture parts from multiple materials has been attempted. This usually includes a time consuming cleaning step between material changes. Thus, minimum feature sizes and resolution are no more comparable to conventional SLA than required printing time [103–105]. Nevertheless, after thorough investigation and development, these methods could help to further extend the application spectrum of SLA in the future.

[1] Kodama H. Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer. The Review of Scientific Instruments. 1981;**52**(11):1770-1773

ing high-accuracy of laser illumination [22].

Christina Schmidleithner and Deepak M. Kalaskar\* \*Address all correspondence to: d.kalaskar@ucl.ac.uk University College London, London, United Kingdom

**Conflict of interest**

**Author details**

**References**

None.
