Biomedical Engineering Reference
In-Depth Information
shown that additional purification steps to remove residual bioactive compo-
nents from the polymeric material (such as lipopolysaccharides (endotoxins)
stemming from bacterial cell walls) significantly reduce complement activa-
tion. Interestingly, the rate of complement activation by the polymers tested
decreased with increasing amounts of 3HV, either before or after additional
purification [165].
Other P3HB-based materials tested included P3HB/PEG blends, which ex-
hibited increasing blood coagulation times and less platelet adhesion with
increasing amount of PEG [103]. In another study, P3HB-3HV (5%HV)films
immobilized with hyaluronic acid had less protein (albumin, fibrinogen) ad-
sorbed and longer coagulation times than unmodified films. Immobilization
with chitosan had the opposite effect [118].
In vivo, a complete coverage of the surface of nonwoven P3HB patches with
endothelial-like cells without any platelet or fibrin adhesions was reported
from a 12-month study of closure of an atrial septal defect in sheep [144]. No
platelet aggregates were seen and fibrin rarely observed on the patch surface
in a 24-month study on the right ventricular outflow tract of sheep [145].
However, severe thrombosis was observed in vivo when testing P3HB-
22%3HV-coated tantalum stents in pigs [158]. The hydrophobic nature of the
polymer surface supporting protein adsorption, followed by platelet adhe-
sion and subsequent thrombi formation, was given as a possible reason for
the stent failure [166]. This is in accordance with in vitro results mentioned
above [162]. Based on these experiments, P3HB-3HV was assessed to be less
suited for implants that are in immediate and extended blood contact [166].
Severe thrombosis, including complete thrombotic occlusion, also occurred
after implantation of P3HB stents but not with tantalum controls in a rabbit
study [159]. It must be considered that the polymer stents used in this study
were plasticized with 30% TEC, which can leach out quickly under physio-
logical conditions leaving behind a more porous polymer structure that may
increase protein adsorption.
3.3
Biodegradability
In Vitro Degradation
In addition to suitable mechanical and biocompatibility properties, a tem-
porary implant material needs to degrade within clinically reasonable time
periods. In vitro degradation studies on P3HB films in buffer solution
(pH 7.4, 37
◦
C) showed no mass loss after 180 days, but a decrease in mo-
lecular weight starting after an induction period of about 80 days [167].
This induction period was attributed to the time required for water to pen-
etrate the polymer matrix. The degradation mechanism was examined in
an accelerated test at 70
◦
C. It was concluded that the hydrolysis of micro-
bial polyesters proceeds in two steps. First, there is a random chain scission
