Degradation Behaviour and Biocompatibility of Additively Manufactured Porous Metallic Implants

Abstract

Biodegradable metals as orthopaedic implant materials receive
substantial scientific and clinical interest. Marketed cardiovascular
products confirm good biocompatibility of iron. Solid iron biodegrades
slowly in vivo and has got supra-physiological mechanical properties as
compared to bone and porous implants can be optimized for specific
orthopaedic applications. We used Direct Metal Printing (DMP)3 to
additively manufacture (AM) scaffolds of pure iron with fine-tuned
bone-mimetic mechanical properties and improved degradation behavior to
characterize their biocompatibility under static and dynamic 3D culture
conditions using a spectrum of different cell types.
Atomized iron powder was used to manufacture scaffolds with a
repetitive diamond unit cell design on a ProX DMP 320 (Layerwise/3D
Systems, Belgium). Mechanical characterization (Instron machine with a
10kN load cell, ISO 13314: 2011), degradation behavior under static and
dynamic conditions (37ºC, 5% CO2 and 20% O2) for up of 28 days, with μCT
as well as SEM/energy-dispersive X-ray spectroscopy (EDS) (SEM,
JSM-IT100, JEOL) monitoring under in vivo-like conditions.
Biocompatibility was comprehensively evaluated using a broader spectrum
of human cells according to ISO 10993 guidelines, with topographically
identical titanium (Ti-6Al-4V, Ti64) specimen as reference. Cytotoxicity
was analyzed by two-way ANOVA and post-hoc Tukey's multiple comparisons
test (α = 0.05).
By μCT, as-built strut size (420 ± 4 μm) and porosity of 64% ±
0.2% were compared to design values (400 μm and 67%, respectively).
After 28 days of biodegradation scaffolds showed a 3.1% weight reduction
after cleaning, while pH-values of simulated body fluids (r-SBF)
increased from 7.4 to 7.8. Mechanical properties of scaffolds (E =
1600–1800 MPa) were still within the range for trabecular bone, then. At
all tested time points, close to 100% biocompatibility was shown with
identically designed titanium (Ti64) controls (level 0 cytotoxicity).
Iron scaffolds revealed a similar cytotoxicity with L929 cells
throughout the study, but MG-63 or HUVEC cells revealed a reduced
viability of 75% and 60%, respectively, already after 24h and a further
decreased survival rate of 50% and 35% after 72h. Static and dynamic
cultures revealed different and cell type-specific cytotoxicity
profiles. Quantitative assays were confirmed by semi-quantitative cell
staining in direct contact to iron and morphological differences were
evident in comparison to Ti64 controls.
This first report confirms that DMP allows accurate control of
interconnectivity and topology of iron scaffold structures. While
microstructure and chemical composition influence degradation behavior -
so does topology and environmental in vitro conditions during
degradation. While porous magnesium corrodes too fast to keep pace with
bone remodeling rates, our porous and micro-structured design just holds
tremendous potential to optimize the degradation speed of iron for
application-specific orthopaedic implants. Surprisingly, the biological
evaluation of pure iron scaffolds appears to largely depend on the
culture model and cell type. Pure iron may not yet be an ideal surface
for osteoblast- or endothelial-like cells in static cultures. We are
currently studying appropriate coatings and in vivo-like dynamic culture
systems to better predict in vivo biocompatibility.