July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 2 2 Hatch distance is the space be- tween two adjacent scanning rows and is an important parameter that affects the properties of parts fabricated by SLS. A large part of the laser spot may scan over a previously scanned line and increases the flow and the spread of the liquid, causing an inflation in the inter- line bonding and reducing the porosity. Another phenomenon affecting the po- rosity is balling, which is defined as the conglomeration of the particles, formed by the breakup of liquid phase in order to reduce the surface energy. There are two factors leading to this effect, Gibbs-Marangoni effect and thermo- capillary convection. Balling has an im- mediate effect on the creation of large pores and is not a good way to fabricate tissues with porous features. A homog- enously sintered specimen is required, which serves as a bottleneck to imple- mentation of the approach [19] . STEREOLITHOGRAPHY Stereolithography (SLA) is one of the basic 3D printing technologies. In this approach, a photosensitive liquid polymer is solidified upon illumination from a lamp, digital light processing (DLP), or laser. Mechanical properties are regulated by the degree of cross- linking; structures with microscale fea- tures can be readily achieved. Because it is a non-contact and nozzle-free printing method, there are no limita- tions associated with clogging. SLA is possible with various types of photo- polymers [21-23] . However, the resins and epoxies used in stereolithography may not always be biocompatible or neces- sarily eco-friendly. To solve this prob- lem of biocompatibility, bioceramics can be obtained using this SLA tech- nique [24] with a complex osteoconduc- tive final structure (Fig. 4). It is also possible to modify biocompatible poly- mers to make photosensitive and bio- compatible photoresins [25] . LASER-INDUCED FORWARD TRANSFER In laser-induced forward trans- fer, a laser pulse heats the rear side of a thin ribbon, or donor, supported by a transparent quartz substrate along with Fig. 3 — General schematic of the SLS printing process, which is based on step by step supply of a printing material into the printing chamber for high energy and high intensity laser. Intreraction of the material (polymer, ceramic, or metal) leads to melting and sintering according to the CAD pathway [20] . Fig. 4 — On the left is a schematic of the SLA scaffold-building process. Osteoconductivity can be designed into the structure (a) as shown in the gyroid CADmodel (b). SEM images show macroporous structure (c,d) [26] . BIOPRINTING: OPPORTUNITIES FOR GROWTH The Society of Manufacturing Engineers, led by Lauralyn McDaniel, con- ducted a Medical Additive Manufacturing/3D Printing Survey in 2017, which included the opinions of 181 participants [33] . 47% of respondents considered materials to be a limitation to medical additive manufacturing (AM), 39% of re- spondents considered AM processes to be a limitation to medical AM, and 25% of respondents considered software to be a limitation to medical AM. They spe- cifically mentioned that developing standards for raw material suppliers and developing inexpensive and straightforward segmentation software would fa- cilitate growth of the field. The bioprinting field is also faced with unique technical challenges. One challenge is that the bioprinting process must not damage the drugs, cells, and/ or biological components, also known as the “bio-ink,” between placement of the bio-ink in the bioprinter and patterning of the bio-ink on the surface [34] . An- other challenge is the patterning rates of conventional bioprinting technologies are too slow to create large three-dimensional cell-containing structures [35] . A coordinated effort by industrial and university stakeholders is needed to over- come these obstacles in order to drive additive manufacturing innovation and clinical translation.