To date, various SFF-based indirect methods of scaffold fabrication and tissue engineering applications of these methods have been reported.17,27–32
Sachlos et al.27,28
introduced a collagen scaffold fabricated by an indirect method using an ink-jet printer. Moreover, Taboas et al.17
investigated an indirect construction method of dual-pore/composite scaffolds. In contrast, although the SL technology has many advantages in terms of fabrication speed and resolution, the indirect methods based on the SL technology reported thus far have used agarose, silicone, and ceramic powders.19,33,34
In this research, the indirect SL technology was studied for the production of scaffolds with various biomaterials, such as PLGA, PLLA, PCL, chitosan, alginate, and bone cement, which are generally used in tissue engineering. The process for the indirect SL technology consists of three steps: fabrication of the sacrificial mold, injection molding, and selective removal of the mold. First, this research has demonstrated the processability of alkali-soluble photopolymer through the fabrication of 3D porous molds using the SL technology (). The molding process required no equipment other than a syringe because the mold had both an inlet and an outlet. Second, the sacrificial molding processes were designed to take into account the intrinsic physical and chemical properties of each biomaterial. Chloroform, acetic acid, and distilled water were used to produce injectable material. After injection molding, solvent/nonsolvent exchange was used to extract organic solvent from the injected PLGA, PCL, and PLLA solutions. Chitosan and alginate gel formation was obtained using NaOH and CaCl2
solutions, respectively. Third, selective removal of the mold, the last and most important process, must not result in any defects in the biomaterial structure in the mold. In this research, dissolution tests were conducted for alkali-soluble photopolymer and the other biomaterials. The cured alkali-soluble photopolymer was perfectly dissolved after 3–4
h at 65°C (), whereas significant dissolution of PCL, alginate, chitosan, and bone cement did not occur even after 3 days (). On the other hand, PLGA and PLLA dissolved in 1–2 days in the NaOH solution. The accelerated dissolution of PLGA and PLLA is related to the pH level of the NaOH solution. Burkersroda et al.35
examined the degradation mechanisms of PLGA and PLLA at various pH levels. They showed that surface erosion was generated in NaOH solutions of high pH, but the material properties were unaffected. In addition, many researchers have investigated the use of surface treatment with NaOH solutions of high pH to enhance the cell affinity of scaffolds composed of synthetic polymers such as PLGA, PCL, and PLLA.36–41
Various properties were measured, such as molecular weight, glass transition temperature, melting temperature, enthalpy of melting, and atomic components. NaOH treatment did not significantly affect these properties.36–38
Our data regarding GPC and mechanical properties of PLGA scaffolds fabricated by the indirect SL technology showed a similar trend. No significant changes in biomaterial properties were observed.
An experiment was conducted to investigate dissolution trends of alkali-soluble photopolymer and PLGA. The results showed that the difference in the dissolution rates of these two materials can be maximized by reducing both the concentration of NaOH and the temperature. This result is significant for minimizing structural defects due to high pH in the mold removal process; 0.2
N NaOH at 0°C was used for the mold removal process in PLGA and PLLA scaffold fabrication. This work has led to a well-designed molding process and successful fabrication of scaffolds using PLGA, PLLA, PCL, chitosan, alginate, and bone cement. The technology has a wide range of biomaterial selectivity and high resolution. Various factors affect fabrication resolution. The highest achievable mold resolution using the SL technology was ~50–70
μm. Furthermore, the achieved minimum pore size and strut size were also 50 and 65
μm, respectively (). The properties of biomaterials used in the molding process also affect the fabrication results. Synthetic polymers showed results comparable with mold dimensions. In contrast, an isotropic shrinkage of 40%–60% was observed during fabrication of scaffolds with natural polymers, which occurred during freeze-drying.
This technology can also be combined with traditional technologies, such as salt leaching and phase inversion, to fabricate dual-pore scaffolds.17
The size of the large pores in the inner architecture of the dual-pore scaffold can be controlled to optimize the mechanical strength as well as the fluid dynamics for nutrient and oxygen supplies to seeded cells. Based on micro-CT measurement results, the fabrication of a 3D tooth-shaped scaffold demonstrated that this technology can be applied to the construction of scaffolds with a 3D organ shape.
The results of cytotoxicity testing, NMR, and GPC showed that the scaffolds fabricated with the indirect SL technology are quite suitable for tissue engineering applications. NMR results and cytotoxicity measurements showed that the scaffolds did not have any mold residue and generate any cytotoxic effect. GPC results indicated that the fabrication procedures did not significantly affect the molecular weight of the biomaterial.
Compared with other SFF methods,1–3
the indirect method presented here requires an additional molding process. However, it offers notable advantages in material selectivity, as a wide range of biomaterials can be applied to this technology. Sophisticated adjustment of biomaterial composition is central to achieving proper degradability and physical strength.1
The indirect SL technology developed in the current study makes it possible to fabricate a 3D structure composed of well-designed biomaterials. Above all, this technology has much potential for improving the fabrication resolution. Among various 3D fabrication technologies, the SL technology not only shows the highest resolution but also offers fast fabrication speed.4–13
The reported highest resolution was nanoscale in 3D space. We will continue to examine methods for fabricating high-resolution molds and molding processes on a nano/micrometer scale using various biomaterials. Further development of this technology will provide a new paradigm for the construction of 3D structures composed of biomaterials.