Additive Manufacturing of Biomedical Constructs with Biomimetic Structural Organizations

Additive manufacturing (AM), sometimes called three-dimensional (3D) printing, has attracted a lot of research interest and is presenting unprecedented opportunities in biomedical fields, because this technology enables the fabrication of biomedical constructs with great freedom and in high precision. An important strategy in AM of biomedical constructs is to mimic the structural organizations of natural biological organisms. This can be done by directly depositing cells and biomaterials, depositing biomaterial structures before seeding cells, or fabricating molds before casting biomaterials and cells. This review organizes the research advances of AM-based biomimetic biomedical constructs into three major directions: 3D constructs that mimic tubular and branched networks of vasculatures; 3D constructs that contains gradient interfaces between different tissues; and 3D constructs that have different cells positioned to create multicellular systems. Other recent advances are also highlighted, regarding the applications of AM for organs-on-chips, AM-based micro/nanostructures, and functional nanomaterials. Under this theme, multiple aspects of AM including imaging/characterization, material selection, design, and printing techniques are discussed. The outlook at the end of this review points out several possible research directions for the future.

FABRICATION AND CHARACTERIZATION OF CALCIUM SILICATE SCAFFOLDS FOR TISSUE ENGINEERING

Calcium silicate ( CaSiO 3) is a promising material due to its favorable biological properties. However, it was difficult to fabricate ceramic scaffolds with interconnected porous structure via conventional technology. In present study, CaSiO 3 scaffolds with totally interconnected pores were fabricated via selective laser sintering (SLS). The microstructure, mechanical and biological properties were examined. The results revealed that the powder gradually fused together with the reduction of voids and the elimination of particle boundary as the laser power increased in the range of 3–15 W with scanning electron microscope. Meanwhile the low-temperature phase (β- CaSiO 3) transformed into high-temperature phase (α- CaSiO 3) gradually, which decreased the mechanical properties of the obtained scaffolds. Besides, the compressive strength increased from 12.9 ± 2.34 MPa to 18.19 ± 1.24 MPa (the laser power is 12 w) and then decreased gradually with increasing laser power. In vitro biological properties of CaSiO 3 scaffolds sintered under optimal conditions indicated that the distribution of apatite mineralization became uniform as the amount of them increased after being immersed in simulated body fluids. In the meantime, the thin cytoplasmic extensions of MG-63 cells increased until formed a dense cell layer after 1–5 days of cell culture. The results suggested that the CaSiO 3 scaffold fabricated via SLS has potential application for bone tissue engineering.

DEVELOPMENT OF COMPLEX POROUS POLYVINYL ALCOHOL SCAFFOLDS: MICROSTRUCTURE, MECHANICAL, AND BIOLOGICAL EVALUATIONS

Complex three-dimensional (3D) porous scaffolds with macro-pore size of 400–800 μm based on polyvinyl alcohol (PVA) powder were successfully developed by selective laser sintering (SLS) technology. The PVA scaffolds had customizable shape, controlled and totally interconnected porous structure, and high porosity. The microstructure and mechanical property were performed for their suitability for tissue engineering (TE). The results showed that PVA did not decompose while the degree of crystallization decreased in a given sintering condition. Moreover, there were micro-pores with sizes of 20–100 μm in the scaffold. The actual porosity of sintered scaffolds could be up to 82.35%, which was higher than the value of the designed models. An in vitro biocompatibility test showed MG-63 cells could well spread on the scaffold surface. The presented work demonstrates the favorable potential of PVA powder as TE scaffolds fabricated via SLS.

Hydrodynamic modulation of pluripotent stem cells

Controlled expansion and differentiation of pluripotent stem cells (PSCs) using reproducible, high-throughput methods could accelerate stem cell research for clinical therapies. Hydrodynamic culture systems for PSCs are increasingly being used for high-throughput studies and scale-up purposes; however, hydrodynamic cultures expose PSCs to complex physical and chemical environments that include spatially and temporally modulated fluid shear stresses and heterogeneous mass transport. Furthermore, the effects of fluid flow on PSCs cannot easily be attributed to any single environmental parameter since the cellular processes regulating self-renewal and differentiation are interconnected and the complex physical and chemical parameters associated with fluid flow are thus difficult to independently isolate. Regardless of the challenges posed by characterizing fluid dynamic properties, hydrodynamic culture systems offer several advantages over traditional static culture, including increased mass transfer and reduced cell handling. This article discusses the challenges and opportunities of hydrodynamic culture environments for the expansion and differentiation of PSCs in microfluidic systems and larger-volume suspension bioreactors. Ultimately, an improved understanding of the effects of hydrodynamics on the self-renewal and differentiation of PSCs could yield improved bioprocessing technologies to attain scalable PSC culture strategies that will probably be requisite for the development of therapeutic and diagnostic applications.