Page 7 - IJB-3-2

Yi Zhang

Surface smoothing is a seemingly trivial but actually a types can also serve mechanical and structural func-
very important process in microfluidics. Because 3D tions. But the biological and chemical functions of
printing produces components layer by layer, the sur- 3D-printed components are frequently ignored. Cur-
face usually appears uneven with evident layer lines. rent research on the biofunctionality of 3D-print-
The resulting rough surface significantly alters the ed components still focuses on several convention-
flow characteristics in microfluidics and leads to un- al biocompatible materials that are adopted for
desired outcome [18] . Commonly used surface smooth- 3D-printing, such as PGA, PLA and titanium. Due to
ing techniques include sanding, blasting and vapor the limited selection of 3D-printable materials and
smoothing. These techniques have been readily ap- fabrication techniques, we believe that post-3D print-
plied to smoothen the exterior surface of 3D-print- ing modification offers more options to functionalize
ed components for aesthetic purposes. However, 3D-printed components for biomedical applications.
smoothen an interior surface such as microfluid- Depending on the purpose, different techniques
ic channels proves challenging. Vapor smoothing is may be employed to either activate or passivate the
perhaps the only technique applicable to microfluidics surface of 3D-printed components.
so far, but vapor cannot effectively enter the micro- 2.2.1 Surface Activation
fluidic channels on its own. Future work needs to look
into how to actively pump vapor or liquid through the Surface activation is accomplished by conjugating or
microfluidic channels to achieve desired surface ar- depositing functional materials to the 3D-printed com-
chitecture. ponents. The functional materials include biologically
active molecules such as growth factors, antibodies,
2.1.2 Microscopic peptides, nucleic acids or their derivatives. Alterna-
The microscopic architecture of 3D-printed compo- tively, they can also be biocompatible or chemically
nents has been extensively studied, especially for active coating that promotes interaction between
3D-printed tissue engineering scaffolds. These scaf- 3D-printed components and biological entities.
folds are designed with microscopic porous architec- Surface activation techniques have already been
ture. The pore size, porosity and other microscopic used in 3D-printed tissue engineering scaffold. Lee et
topographic features of the scaffold are crucial for cell al. immobilized a growth factor called human bone
[3]
growth and scaffold degradation . Techniques adop- morphogenic protein-2 (rhBMP-2) on the 3D-printed
ted from microfabrication, such as reaction ion etching polycaprolactone (PCL) scaffold to promote osteo-
and thin film deposition, can be applied to further genic differentiation [33] . Yeh et al. immobilized an
fine-tune the microscopic architecture of 3D-print- active compound extracted from a traditional Chinese
ed components. The etching process could partially medicine (TCM) herb known as Xu Duan onto 3D-
remove scaffold material on the surface and increases printed PLA scaffold and discovered that both osteo-
the pore size. The thin film deposited by e-beam genic and angiogenic markers of bone marrow
[34]
evaporation or sputtering partially fills the microscop- stem cells were up-regulated . Both groups used
ic pores and decreases the pore size. Chemical deposi- polydopamine chemistry to immobilize the biologi-
tion techniques can also be employed to graft nano- cally active materials. Polydopamine is bio-inspired
scale structures on the surface. These nanostructures material that has a strong adhesion to virtually any
[35]
produce more hierarchical microscopic architecture type of surface . The polydopamine coating contains
that could enhance interactions with biological entities. hydroxyl and amine functional groups which could
covalently conjugate proteins and other active com-
In addition to the techniques mentioned above, gas pounds. Polydopamine itself is also a biologically ac-
foaming, solvent casting, particulate leaching, freeze tive material. Kao et al. coated PLA scaffold with
drying and other techniques can also be implemented polydopamine and noticed enhanced cell adhesion and
to reconfigure microscopic architecture during post- proliferation on the scaffold [36] . Polydopamine also
3D printing processing for various types of materials [31] . has sufficient reducing capability for electrodeless

2.2 Surface Functionalization plating [35,37] . By dipping the 3D-printed components
with a polydopamine coating in the solution contain-
Currently, many 3D-printed components are merely ing gold salt, one can easily plate them with gold for
models that provide a more intuitive way of visualiz- the biomolecular self-assembly via gold-thio interact-
ing the architectural design. Some 3D-printed proto- ions [38] . The same process can be used to metalize the

International Journal of Bioprinting (2017)–Volume 3, Issue 2 95