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Laser Additive Manufacturing of Zinc
the degradation rate in vivo is slower than that in vitro due behaviors, which should undoubtedly influence the cell
to the complex physiological environment. According to response. Wang et al. reported that large pores of metallic
some reports [136,149] , the degradation research implanted scaffold favor nutrient supply, whereas the small pores
into animals can get a more representative scenario to are beneficial for cell growth [158,159] . In this respect, the
evaluate degradation behaviors for Zn implants prepared biocompatibility of LPBF-processed Zn scaffolds can
by other processes. Different Zn-1X pin (X = Mg, Ca, be affected by regulating the pore structure. Up to now,
and Sr, wt.%) are implanted into mouse femora [136] . The some researchers have performed some preliminary
results show that the bone around the implants begins investigations on the impact of geometric design of LPBF-
to change continuously due to new bone formation and processed degradable Zn on the biological behavior. For
remodel, and the corresponding degradation rates are example, Li et al. [160] prepared functionally graded porous
0.17, 0.19, and 0.22 mm/year, respectively. The Zn and Zn with precisely controlled topology using LPBF and
Zn-xAl (x= 1, 3, and 5 wt%) strips are implanted into studied the effects of different pore sizes on the cell
the abdominal aorta of adult rats [149] . It is found that the behavior of scaffolds. The results showed that a smaller
corrosion develops from the surface toward inside, and pore size provided a larger strut surface area for cell-
partial Zn material remains intact after ½ year. On the surface interactions before cells passed through the pores.
other hand, additive manufacturing can prepare complex In addition, the scaffolds designed of radial direction (i.e.,
porous structure, which is also the key to affecting its along the x and y axes) also affected cell growth. It was
degradation rate. In general, increasing the porosity pointed out that the rates of tissue formation increased
will accelerate the degradation rate, but will sacrifice its with the increase of the curvature of porous scaffolds and
mechanical strength to a certain extent. Therefore, how to are faster on concave surfaces than on convex surfaces
find a balance between biomimetic structure, degradation and planes [161] .
behavior, and mechanical stability is the future research The surface morphology of LPBF-processed porous
direction. implants also affects the reaction between implant and
host tissue [162,163] . LPBF-built parts are usually stained
5. Cytocompatibility with unmelted particles, which will increase the risk of
Medical implants need to have good biocompatibility to bacterial colonization [164] . Relevant researches showed
avoid toxic effects on the host. At present, there are very that the nanosurface topology could induce the osteogenic
few literatures reporting the biocompatibility of LPBF- differentiation of stem cells and promote the adhesion of
processed porous biodegradable metal, including Zn, osteoblasts [165] . Thus, nanosurface topology is a future
Mg, and Fe. The biocompatibility evaluation of LPBF- choice to improve the tissue integration performance
processed porous biodegradable metals is still at the of implants in vivo. The conventional surface treatment
cellular and in vitro level [110,150-152] . It is well accepted that methods such as sand blasting and chemical erosion are
the factors affecting the biocompatibility mainly lie in generally used to improve the surface quality of LPBF-
its chemical properties and degradation products. Zn is processed biodegradable metal [135,166] . However, it is not
the main degradation product. As one necessary element able to obtain a uniform and smooth surface inside the
of human body, about 60% of Zn is stored in skeletal scaffolds, which may lead to the difference of degradation
muscle of human body, about 30% in skeleton, about 5% in different sites of scaffolds and the premature loss
in liver and skin, as well as the rest in other important of mechanical integrity. Surface coatings including
organs [153] . The recommended dietary allowance for Zn bioactive ceramics and biopolymers have been widely
is about 8–11 mg/day, which is far low as compared with used to regulate the biological function of bulk degradable
the median lethal dose value of 27 g/day [154] . Thereby, the metals [167,168] . Furthermore, the coatings have been studied
composition design of Zn powder for LPBF in the future with the purposes of improving degradable metal porous
should meet the requirement of biocompatibility [155] . materials. Furthermore, the coatings have been studied
For Zn implants prepared by LPBF, it possesses with the purposes of improving degradable metal porous
relatively fine grain and strong texture as compared materials. For example, Zhuang et al. [169] designed a new
with that obtained by traditional process. Nevertheless, porous Zn scaffolds with Ca-P coating and found that
the current studies proved that they showed similar cell it effectively promoted osteogenic differentiation and
biocompatibility [156,157] . For LPBF-processed porous calcium deposition of rabbit BMSCs in vitro, and new
implants, its advantage is that the porous structure offers bone formation around the scaffold in vivo. However,
channels for nutrient transport and metabolite excretion, how to form a uniform coating inside porous scaffolds is
and is conducive to the growth of new tissue. On the still a challenge.
other hand, the geometry and microstructure of LPBF- In addition, Zn belongs to inorganic antibacterial
processed porous structure are different from conventional materials [170] . Therefore, degradable Zn implants
bulk materials, resulting in different biodegradation exhibited a good antibacterial property. Bacterial

86 International Journal of Bioprinting (2022)–Volume 8, Issue 1