Biomedical Engineering Reference
In-Depth Information
be considered by using exogenous growth factors or cultured cells [220]. In
a subsequent study using the same animal model, these tubes were filled with
glial growth factor embedded in an alginate/fibronectin matrix. While cross-
sectional staining of axons was greatest in the group containing growth factor,
empty P3HB tubes showed the greatest axonal regeneration distance. Alginate
had an inhibitory effect on the regeneration [221].
P3HB fibers embedded in an alginate/fibronectin matrix were tested as
a scaffold in a spinal cord hemisection model in rats. Scaffolds, 2-3 mm long,
supported neuronal survival after spinal cord injury. In the absence of poly-
mer fibers, implants of alginate and/or fibronectin hydrogel alone were found
to have no effect on neuronal survival [37].
Bone and Cartilage Repair
The long-term degradation profile of P3HB is considered to be of advan-
tage in orthopedic applications [152, 222]. Thus for example, fast-degrading
polyesters are not suitable for load-bearing fracture fixation devices [183],
and bone implants made from those polymers have been coated with P3HB
in order to slow down the degradation [223]. Additionally, the piezoelectric
potential of P3HB has been considered as a special feature, because it is com-
parable with that of natural bone [224, 225]. It is well-known that bone can
be strengthened and repaired by electrical stimulation. Therefore, P3HB com-
posites may stimulate bone growth and healing [226].
P3HB implants have been tested for connecting osteotomies in the tibia
of rabbits. The defect was healed after 12 weeks in most cases and the
implant was completely resorbed after 24 weeks [209]. P3HB was also suc-
cessfully tested as an occlusive membrane for guided bone regeneration in
the mandibula of rats [210, 227]. Osteosynthesis plates made of P3HB and
anchored with two P3HB bolts were examined for repair of a cut through
zygomatic arches of rabbits. The polymer was assessed to be suitable for
covering defects of the osseus skull and as an osteosynthesis material for frac-
tures of the visceral cranium [208]. However, no differences appeared in the
healing pattern of 15 mm diameter rhinobasal skull defects in minipigs when
covered with a 20 mm diameter P3HB/at-P3HB patch, compared to the con-
trolgroupwithoutpatch[39].
P3HB composites were extensively examined for bone repair, such as com-
posites made of P3HB and HA. HA stimulates the formation of new bone,
while P3HB is considered as potentially bioactive due to its degradability
and piezoelectricity. P3HB/HA composites were found to closely match the
mechanical properties of cortical bone, but had insufficient strength and duc-
tility for the construction of major load-bearing components [93]. Phosphate
glass was added to improve mechanical strength and to serve as precursor
for bone-forming osteoblasts [152]. Among composites made from P3HB or
P3HB-3HV (8%, 12%, 24% 3HV) and HA, the P3HB-8%3HV/HA (30
70)
composite had a compressive strength in the order of that of human bone,
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