Page 72 - IJB-9-1

International Journal of Bioprinting Osteoconduction and scaffold directionality

exclusively dependent on the post-processing procedure . of bone regeneration. Unfortunately, the so-formed pores
[2]
Stereolithography, selective laser sintering , and three- were 0.30 mm in diameter and therefore suboptimal for
[4]
dimensional (3D) printing in a powder bed represent 3D osteoconduction .
[1]
[5]
printing technologies developed over the last decades to Extrusion-based additive manufacturing methodologies
[3]
build bone substitutes from ceramics [6,7] . are widely available and frequently used because of the low
Due to the porous structure of cancellous bone, which cost of such systems. Melt-extrusion additive manufacturing
still is, if autologous, the gold standard bone substitute was established in 1992 and solution/slurry/gel extrusion
[24]
used in the clinic, the porosity and pore size were initially in 2002 [25,26] . The latter has been widely used in bioprinting
the main determinants for the ideal microarchitecture of since gel extrusion allows the embedding of cells and growth
synthetic bone substitutes . Early studies defined rather factors in the extruded hydrogels. In fused deposition
[8]
small pore diameters to be ideal for bone substitutes [9-11] . systems based on poly-e-caprolactone [27,28] and systems
Next, these numbers were elevated based on work with with nozzle-extruded polymers, the so-produced filaments
random pore-based microarchitectures in bone substitutes are deposited layer-by-layer on a building platform. The
formed by leaching . More recently, with the aid of variability of the microarchitecture of filament-based
[12]
additive manufacturing, the ideal pore diameter for methodologies is, however, limited, since it depends on the
osteoconductivity in pore-based scaffolds was increased dimension and the mechanical constrain predefined by the
from 0.80 to 1.20 mm in diameter . material, shape, and diameter of the filament at the time
[13]
Osteoconduction is a 3D process induced by a point of extrusion [1,29] .
scaffold placed in a bone defect. The porous scaffold Here, we used a lithography-based additive
serves in this constellation as guiding cue for sprouting manufacturing system for ceramics and mimicked
capillaries, perivascular tissue, and osteoprogenitor filament-based microarchitectures with filaments between
cells to direct them from the defect margins into the 3D 0.40 and 1.25 mm to study the effect of filament size and
structure (adjusted from [14,15] ) to accelerate the bridging distance on osteoconductivity and bone regeneration.
of the defect with bony tissue . Mesenchymal stem Moreover, we compared two types of scaffolds, which were
[1]
cells are the key players during bone regeneration. Their derived from the same microarchitecture in a rabbit non-
guiding by biophysical and biochemical cues triggered critical calvarial defect model, and studied the influence of
by the microenvironment, which might also affect the directionality of the filament for osteoconduction. For
osteoconduction, has been reviewed recently . Any cell the first type (Fil), all filaments are aligned with the natural
[16]
guiding which enforces a directional migration of cells advancement of bone in a calvarial defect. For the second
is a process that involves cell adhesion, polarization, and type (FilG), only 50% of the filaments are aligned with the
movement into a predefined direction [17-19] . The most advancement of bone and the other 50% are orthogonal
prominent technologies to generate various types of to it. Based on this library of eight distinct filament-based
micro/nano-structured surfaces or substrates include soft scaffolds from tri-calcium phosphate (TCP), we evaluated
lithography, nanolithography (e.g., writing with an e-beam the effect of filament directionality, dimension, and
or dip pen), and electrospinning . The so-generated distance on osteoconductivity and bone regeneration.
[20]
features are, however, in the submicron and low micrometer
range and therefore are much finer and on a lower level 2. Materials and methods
of dimension than the filaments used to build extrusion- 2.1. Library of scaffolds
based bone substitutes with diameters from low-hundreds The scaffolds were assembled by unit cells of cubes of
to thousand micrometers . On the cellular level of in vitro 0.80, 1.00, 1.75, or 2.50 mm in length to build filament-
[21]
methodologies, it has been shown that the directionality based scaffolds mimicking filaments of 0.40 mm, 0.50 mm,
of fibers from electrospun samples in the range of 100 nm 0.83 mm, or 1.25 mm in square (Figure 1).
to 1000 nm guide cell migration . However, information
[22]
on the effect of filaments in the 100 µm to 1000 µm range The TCP scaffolds were produced with TCP slurry
on osteoconductivity and bony bridging is scarce. The first LithaBone™ TCP 300 (Lithoz, Vienna, Austria) using a
[30]
study comparing different orientations of the laydown CeraFab 7500 system (Lithoz, Vienna, Austria). The green
patterns of filaments to form a scaffold showed that in vivo body was assembled from 25-µm layers of slurry solidified
the scaffold with the orientation of the layers of filaments by exposure to blue LED light at a resolution of 50-µm
at 0°/90° performed better in terms of bone formation than in the x/y-plane. The green body was removed from the
the counterpart in which the filaments of the layers followed building platform of the printer with a razor blade, cleaned
the pattern 0°/60°/120° . Since all these filaments were with LithaSol 20™ (Lithoz, Vienna, Austria) and pressurized
[23]
stacked in layers, all of these filaments are in the direction air. The polymeric binder was decomposed by heat and the

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Volume 9 Issue 1 (2023)olume 9 Issue 1 (2023) 64 https://doi.org/10.18063/ijb.v9i1.626