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A multi-scale porous scaffold fabricated by a combined additive manufacturing and chemical etching process for bone tissue engineering

these methods show significant limitations due to the 2. Materials and methods
use of organic solvents or poor control over the porous
structure (size, shape and interconnectivity). Recently, 2.1 Materials
additive manufacturing (AM) has gained considerable
attention because of its capacity to fabricate scaffolds Medical-grade PLLA powder was purchased from
with tailored architecture [16–18] . The process is conducted Jinan Daigang Biomaterial Co., Ltd. It had an average
in a layer-by-layer fashion enabling the formation of a molecular weight of 10,000, a glass transition
well-defined and highly controlled porous structure [19–21] . temperature of 60~65 °C, a melting temperature of
On the other hand, chemical etching can create micro 175~185 °C and a purity ≥99%. GO powder (diameter
pores on the scaffold surface by using inorganic of 100~200 nm, thickness of 0.8~1.2 nm, single layer
[22]
etchants . As some of the material is etched away, pits ratio >99% and purity >99%) was produced by Nanjing
and protrusions are created on the native smooth surface, JCNANO Tech Co., Ltd., China. Analytical-grade NaOH
resulting in a porous surface topography. However, it used in this study was obtained from Xilong Chemical
is difficult to fabricate the required porous structure by Co., Ltd., China. Ultrapure water was used throughout
using AM or chemical etching alone, as the smallest pore the experimental process.
size by the former method is always tens to hundreds of 2.2 AM and chemical etching process
microns while the porous structure by the latter method
only distributes on the scaffolds surface. In view of this As illustrated in Figure 1, the fabrication process of
deficiency, the efforts in this study were oriented towards multi-scale porous scaffolds consisted of three steps,
investigating the possibility of combining these two including powder preparation, scaffold fabrication
techniques to fabricate multi-scale porous scaffolds. and chemical etching. For the first step, GO/PLLA
Poly(L-lactic acid) (PLLA) is widely recognized composite powder was prepared as the raw material for
as a promising scaffold material by virtue of its good composite scaffolds. More specifically, certain amounts
biocompatibility and process ability [23,24] . It is able to of PLLA and GO powders with a mass ratio of 99:1
degrade in vivo into nontoxic products, which makes it were ultrasonically dispersed with continuous stirring
one of the few polymers that obtain the approval of Food in separate containers containing ethanol for 30 min.
and Drug Administration (FDA). However, PLLA is semi- The resulting suspensions were then mixed together and
crystalline and highly hydrophobic, resulting in very slow subject to ultra-sonication and stirring for additional 30
degradation kinetics. It is known that PLLA is susceptible min. Afterwards, the mixed suspension was filtered using
to chemical etching in sodium hydroxide (NaOH) Millipore filter, followed by vacuum-drying at 40 °C
solution [25] . This motivates us to hypothesize that the for 24 h. Finally, the powder was scraped off the filter
slow degradation rate of PLLA scaffolds may be adjusted and crushed in a mortar to obtain GO/PLLA composite
by chemical etching due to the changes of porosity and powder.
topography on the scaffolds surface. Moreover, the For the second step, the PLLA or GO/PLLA composite
etching process may also introduce hydrophilic hydroxyl powders were used to prepare scaffolds on a self-
[29]
and carboxyl groups to the scaffolds surface, which developed laser AM system , with laser power of 5 W,
–1
is beneficial for apatite nucleation. The mechanical scanning speed of 500 mm•min and layer thickness
strength, on the other hand, inevitably becomes victim of 0.15 mm. The detailed fabrication process could
to the etching process due to the increased porosity and be described as follows: firstly, a layer of powder
formation of pits or even cracks on the scaffolds surface. was laid on the platform, then a laser was controlled
To compensate the mechanical losses and maintain the to selectively scan and bond the powder particles
porous structure, we endeavor to reinforce the porous according to a predetermined path, after the bottom
scaffold by introducing graphene oxide (GO), which has layer was completed the platform would drop down
shown great potential as reinforcing agent because of by a layer’s thickness, and a new layer of powder was
excellent mechanical properties and high surface area [26– laid and printed on the former layer, this process would
28] . To our knowledge, research on the combined process be repeated until the scaffold was obtained. With this
of AM and chemical etching for the fabrication of multi- process, the pore properties (size, shape, distribution,
scale porous scaffold is still a blank area. interconnectivity, etc.) of scaffolds could be highly
In this work, a combined process of AM and chemical controlled by altering the laser and scanning parameters
etching was developed to fabricate multi-scale porous (laser spot size, scanning space, etc.).
structure for GO/PLLA scaffold. The porous structure, For the third step, NaOH solution (concentration of 1
–1
mechanical properties and degradability of scaffolds were mol•L ) was prepared by dissolving the aforementioned
systematically investigated. NaOH in ultrapure water under continuous stirring

2 International Journal of Bioprinting (2018)–Volume 4, Issue 2