Biomedical Engineering Reference
In-Depth Information
digest the fibrin while producing their own extracellular matrix to
form the regenerated tissue.
67
Fibrin hydrogels based on the natural fibrin clot are soft
poroelastic materials
69
with mechanical properties that can be tai-
lored. Human fibrin glue is used clinically as a two-component
sealant (72-110 mg/ml fibrinogen, 500 IU/ml thrombin, purified
from human plasma [Baxter]), making it attractive for regulatory
reasons.Autologousfibrincanbealternativelypreparedfromblood,
which contains
5 mg/ml fibrinogen.
70
Finally, individual compo-
nentsofthissystemcanbepreparedfrompurifiedsourcestobetter
control the properties of the gel.
Themechanicalstrengthoffibringelsincreasesasthefibrinogen
concentrationincreases;however,anoverlydensenetworkprevents
cell proliferation.
71
The concentrations of fibrinogen and thrombin
also influence the thickness and homogeneity of the fibrin strands.
According to Zhao
et al.
, the optimum fibrinogen concentration is
20 mg/ml, with up to 40 mM CaCl
2
added to facilitate fibrin cross-
linking. Meanwhile, thrombin concentration can be varied between
0.5-5 U/ml, depending on the required mechanical properties and
reactionkinetics.
68
Generally,higherthrombinconcentrationresults
in thinner fibers, higher water content, and faster gel formation. At
thrombin concentration
>
100 U/ml, fibrin will be formed within
seconds.
68
This condition is not favorable for homogenous encap-
sulation of cells in a matrix of controlled shape, as is desired for
in
vitro
cartilage tissueengineering.
One potential drawback of fibrin for use in cartilage tissue engi-
neeringisitsrapiddegradation.Fibrinmadeof20mg/mLoffibrino-
gen almost completely degrades in phosphate buffered saline (PBS)
andonlyretains
∼
∼
70%ofitsweightaftersixdaysinDulbecco'sMod-
ifiedEagle'sMedium(DMEM).
68
Further,thepresenceofFBS,which
inherentlycontainsplasmin,inthemediasignificantlyincreasesthe
degradation rate.
68
To slow down the degradation process, a pro-
teaseinhibitorsuchasaprotinin,tranexamicacid,or
ε
-aminocaproic
acid, may be added to the culture medium.
16
,
66
However,
accordingtoZhao
et al.
,
68
aprotinin only slightly reduces the
degradation rate and is not completely effective during the first few
days.Enhancedstabilitycanbeattainedbyproducingfibringelwith
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