# Light-Controlled Microbots in Biomedical Application: A Review
## 4 Aspects of Design
### Fabrication Process
#### 4.2.1 Material Selection  
Being the main constituent of the substrate to be manufactured, it is crucial that the material be biocompatible, biodegradable, and have suitable mechanical, physical and chemical properties. Photopolymer inks are widely used for laser printing into complex 3D structures. Monomers, oligomers, and macromers undergo photopolymerization, underlining the core of the photochemical reaction process. Predefined 3D structures are created as the initiators in ink absorb laser energy, subsequently initiating layer-by-layer polymerization. IP-resist photoresists are used in applications that require high precision and structural integrity. Easy handling features along with submicrometer resolution make this group of resins very desirable for the fabrication of microbots. There are a wide variety of resins available, IP-PDMS, IP-n162, IP-Visio, IP-Q, IP-S, IP-Dip, IP-G, and IP-L [156]. Each resin has its own specialty, with possible applications of all resins hovering around material science, cell and tissue engineering, microfluidics, drug delivery, and microdots. [PERSON] et al. used the negative tone photoresist IP-G for fabricating multifunctional Janus helical microswimmers, with the resin providing autofluorescence and magnetic properties [157].  
Figure 7: Different printing processes of microbots using light: (i) Steps involved in a direct laser writing fabrication process. The design process begins with a 3D CAD model (STL format) integrated into the 2 PP system programming software. After converting the file to the 2 PP specialized format, it is divided into multiple layers (depending on the desired resolution and structure size), followed by a hatching process to fill in the majority of the structure. Finally, the 3D model is a near-exact of the initial CAD design. (ii) (a) traditional 2D photolithography, which produces 2D structures with the same heights, (b) gray-tone photolithography, which produces non-freestanding 2.5D structures with different heights, (c) two-photon polymerization approach with two different possible writing methods (DLW and Dip-in Laser Lithography DiLL) to fabricate 3D tiny structures, and (d) 4D lithography, which produces 3D structures made of materials that respond to an external stimulus [155]. Reproduced under the terms of the CC-BY license.  
The latter was incorporated using the shadowing of the metallization process using electron beam evaporations. The ability of the microswimmers to respond to both fluorescence and weak magnetic fields could be useful in theranostic applications.  
Along with flexibility, thermal and high mechanical stability is a crucial requirement for a microbot. A fully cross-linked SU-8 photoresist has approximately 4-5 GPa Young's modulus, \(\sim\)200 \({}^{\circ}\)C Tg (glass transition temperature), and \(\sim\)380 \({}^{\circ}\)C degradation temperature [158]. For in vitro and in vivo biomedical applications, [PERSON] et al. designed a microbot system using SU-8 and polydimethylsiloxane (PDMS), which included an untethered magnetic tumbling microbot, a rotating permanent magnet of two degrees of freedom and an ultrasound imaging system has been designed [159]. When studying the movement of sperm flagella in the bovine oviduct, microtubes using IP-Dip were manufactured for maneuvering in complex biological environments [160]. These microtubes could be used to release sperm cells at a certain location, ensuring the start of fertilization.  
In addition to the above-mentioned photopolymer inks, hydrogels that can be photocross-linked can be utilized for 3D printing. Gelatin methacrylate (GelMA) hydrogels were manufactured using a 3D bioplotter for bone tissue engineering applications [161]. The incorporation of gold nanoparticles ensured the attenuation of the substrates during imaging. Furthermore, the GelMA hydrogel consisting of platelet-rich plasma was 3D printed and used for the repair of osteochondral defects [162].