Biomedical Engineering Reference
In-Depth Information
together, overhanging structures are of no diffi culty. However, a drawback of the powder-supported
and powder-fi lled structure is that the open pores must be able to allow the internal unbound pow-
ders to be removed, if the part is designed to be porous such as a scaffold for tissue engineering
applications. The surface roughness and the aggregation of the powdered materials also affect the
effi ciency of removing the trapped materials. The resolution of the printer is limited by the speci-
fi cation of the nozzle size and position controller, which defi nes the print head movement. Another
factor is the particle size of the powder, which simultaneously determines the layer thickness.
A layer thickness between 100 and 400 µm can be achieved depending on the printer. The versatility
of using a powdered material is advantageous, however, an obvious constraint of the 3-DP process
is that most of the available biomaterials do not come in powder-form and need special processing
conditions to get a powder that fulfi ls the requirements for 3-DP. Chen et al. used reverse SSF to
create a negative mold (negative molds were fi rst designed and then converted into SL data followed
by 3-D printing of the mold) into which a poly-L-lactic acid (PLLA) solution could be poured and
then phase-separated thermally to create 3-D nanofi brous scaffolds.
26
Taylor et al. used 3-D ink-jet
printing in combination with CAD software to create sacrifi cial molds, bovine collagen was then
cast into the mold, and the resulting scaffolds were shown to support the attachment and prolifera-
tion of human aortic valve interstitial cells.
27
More recently, tissue engineers were able to print cells
in combination with hydrogels by simple modifi cation of offi ce ink-jet printers showing the proof of
principle to one day create a tissue-engineered construct in a fully automated system.
2.3.3 S
YSTEMS
B
ASED
ON
E
XTRUSION
/D
IRECT
W
RITING
A number of groups have developed SFF machines, which can perform extrusion of strands/
fi laments and dots all employ extrusion of a material in a layered fashion to build a scaffold.
28
Depending on the type of machine, a variety of biomaterials can be used for scaffold fabrication.
Schantz et al.
29
used FDM-fabricated polycaprolactone (PCL) scaffolds as burr hole plugs in a pilot
study for cranioplasty. The clinical outcome after 12 months was positive, with all patients tolerating
the implants with no adverse side effects reported, and good cosmetic and functionally stable cranio-
plasty observed in all cases. The second-generation scaffolds produced by FDM for bone engineer-
ing of Hutmacher's group are based on composites and have been evaluated
in vitro
and
in vivo.
30
The traditional defi nition of a composite material is a material with at least two phases, a con-
tinuous phase and a dispersed phase. The continuous phase is responsible for fi lling the volume and
transferring loads to the dispersed phase. The dispersed phase is usually responsible for enhancing
one or more properties of the composite. Most of the composites target an enhancement of mechani-
cal properties such as stiffness and strength, but other properties may be of interest such as transport
properties or density.
Matrix materials for composites can be metal, ceramic, polymeric, or biologic. It can be observed
that metals and ceramics are always stiffer and can have larger strength than biologic hard tissue.
Polymers are mostly more compliant (lower modulus) than hard tissue and can have strengths of
the same order of magnitude than hard tissue. Biologic tissues show larger spectra of mechanical
properties than the other materials. This picture clearly illustrates the great interest of compound-
ing polymers and other materials to obtain composites that attain combinations of mechanical and
biological properties similar to those of biologic hard tissue.
As in other areas of biomedical research, nature is seen as a guide to design new scaffold mate-
rials in the area of biocomposites. Mimicking the solutions found in natural materials is one of the
most promising ways to reach the target set of properties needed for biomaterials. The development
of materials for any replacement application should be based on the understanding of the structure
to be substituted. This is true in many fi elds, but particularly exigent in scaffold-based tissue engi-
neering. The demands upon the properties of the scaffold material largely depend on the site of
implantation and the tissue function it has to restore. Ideally, a scaffold material should mimic the
host tissue from a mechanical, chemical, biological, and functional points of view.
