Tissue engineering - Biomedical Engineering Desk Reference - page 658

layer using the computer-driven laser. The spatial resolu-
tion of such a system is 100 m m( Cooke et al., 2002 ).
In the SLS technique, a thin layer of evenly distrib-
uted fine powder is first laid down ( Bartels et al. , 1993;
Berry et al. , 1997 ). A computer-controlled scanning laser
is then used to sinter the powder within a cross-sectional
layer. The energy generated by the laser heats the powder
into a glassy state and individual particles fuse into
a solid. Once the laser has scanned the entire cross sec-
tion, another layer of powder is laid on top and the whole
process is repeated.
In the 3DP process, each layer is created by adding
a layer of polymer powder on top of a piston and cylinder
containing a powder bed and the part being fabricated.
This layer is then selectively joined where the part is to
be formed by ink-jet printing of a binder material such as
an organic solvent. The printed droplet has a diameter of
50-80 m m. The printhead position and speed are con-
trolled by computer. The piston, powder bed, and part
are lowered and a new layer of polymer powder is laid on
top of the already processed layer and selectively joined.
The layered printing process is repeated until the desired
part is completed.
The local microstructure within the component can be
controlled by varying the printing conditions. The resolu-
tion of features currently attainable by 3DP for degradable
polyesters is about 200 m m( Griffith et al. , 1997 ). Using
this technique, scaffolds with complex structures may be
fabricated ( Giordano et al. , 1996 ). A model drug (dye)
with a concentration profile specified by a CAD model has
been successfully incorporated into a scaffold during the
3DP process, demonstrating the feasibility of producing
complex release regimes using a single drug-delivery device
( Wu et al., 1996 ). By mixing salt particles in the polymer
powder and their subsequent leaching after 3DP process,
porous PLGA scaffolds with an intrinsic network of inter-
connected branching channels have been fabricated for cell
transplantation ( Kim et al. ,1998 ). This network of chan-
nels and micropores could provide a structural template to
guide cellular organization, enhance neovascularization, and
increase the capacity for mass transport. Furthermore,
multiple printheads containing different binder materials
can be used to modify local surface chemistry and struc-
ture. Patterned PLA substrates with selective cell-adhesion
domains have been fabricated by 3DP ( Park et al. ,1998 ).

Table 7.1.4-4 Characterization of bioresorbable polymer scaffolds

Properties Techniques

Bulk properties

Molecular weight GPC

Polydispersity index GPC

Chemical composition,
structure

NMR, X-ray diffraction, FTIR, FTR

Thermal properties
(T g , T m , X c , etc.)

DSC

Porosity, pore size Mercury intrusion porosimetry

Morphology SEM, confocal microscopy

Mechanical properties Mechanical testing

Degradative properties In vitro, in vivo

Surface properties

Surface chemistry ESCA, SIMS

Distribution of chemistry Imaging methods (e.g., SIMS)

Orientation of groups Polarized IR, NEXAFS

Texture SEM, AFM, STM

Surface energy and
wettability

Contact-angle measurement

be obtained by nuclear magnetic resonance (NMR)
spectroscopy, X-ray diffraction, Fourier transform in-
frared (FTIR), and FT-Raman (FTR) spectroscopy. The
thermal properties of the polymer such as glass transition
temperature ( T g ), melting temperature ( T m ), and crys-
tallinity ( X c ) can be determined by differential scanning
calorimeter (DSC). Porosity and pore size distribution of
a porous scaffold are measured by mercury intrusion
porosimetry. Scanning electron microscopy (SEM) is the
most common method to view the pore structure and
morphology. The 3D microstructure of porous PLGA
matrices has been analyzed by confocal microscopy ( Tjia
and Moghe, 1998 ). Mechanical properties of the scaf-
folds such as tensile strength and modulus, compression
strength and modulus, compliance/hardness, flexibility,
elasticity, and stress and stain at yield can be measured
using mechanical testing equipment. Some tests require
the processing of scaffolds into a particular shape and
dimensions specified by ASTM standards.
The in vitro degradation properties can be evaluated by
placing the bioresorbable scaffolds in simulated body
fluid, typically pH 7.4 phosphate-buffered saline (PBS).
Changes in sample weight, molecular weight, morphol-
ogy, and thermal and mechanical properties can then be
measured at various time points until degradation process

Characterization of processed
scaffolds

Various techniques are available to characterize the fab-
ricated polymer scaffolds ( Table 7.1.4-4 ). The molecular
weight and polydispersity index of the polymer can be
measured by gel permeation chromatography (GPC).
Information on chemical composition and structure can