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Shpichka, et al.
2.4 Mechanical properties dependent on the polymerization conditions,
including a concentration of fibrinogen, thrombin,
Pure fibrinogen solutions show a nonlinear increase additional factors such as Factor XIII and CaCl , and
in viscosity with increasing concentration, with physical factors such as temperature and external
2
the values ranging from ones to hundreds of cP . tension or compression forces. Several models for
[19]
Moreover, the concentration of the fibrinogen in fibrin mechanics have been suggested that take
blood plasma correlates with plasma viscosity . into account its filamentous nature and interactions
[20]
The drastic changes in mechanical properties between the fibers at different hierarchy levels [7,8,17] .
occur with the onset of the fibrin clot formation The storage modulus of the gel only weakly
(gelation), which could be traced by a change of depends on frequency, while the loss modulus
turbidity and an increase in the elastic or shear increases with frequency [23] . Thus, at low
[21]
modulus in rheological measurements [22-24] . In frequencies (<10 – 100 Hz), the behavior
vitro, gelation time which can take from several is mostly elastic and could be efficiently
seconds to several minutes is mostly controlled by characterized by elastic modulus only, but
the concentration of thrombin and temperature [21,25] . the viscous component is pronounced at high
The resulted fibrin gel has a set of remarkable frequencies. The shear and elastic moduli show
and unique viscoelastic properties among polymers non-linear behavior with relation to strains, the
which are related to its molecular structure with so-called strain hardening or stiffening [23] . Shear
complex multi-scale hierarchy . Fibrin fibers modulus increases up to a factor of twenty-
[7]
might constitute <1% of the gel volume, yet it will fold at large strains [18] . The elastic modulus
have measurable elastic modulus and strength. initially decreases (up to strain = 0.5), but
The gel also has a high water-uptake ratio of 30 – then dramatically increases by a factor of 100
50 . The fibrin fibers of the gel can have different (compressive strains >0.8) [26] . Strain hardening
[21]
length, thickness, and density and type of branching might be of biological importance since it
points, which generally made up of three fibers allows fibrin clots to sustain larger deformations
at a junction [17,23] . These parameters are strongly without significant integrity loss.

Table 1. Mechanical properties of pure fibrinogen and fibrin.

Components concentrations Viscosity E(Pa) G’ (Pa) Comments Ref
Fibrin Thrombin Ca+ Factor XIII Buffer (cP)
(mg/ml) (U/ml) (mM) (µg/ml)
10–150 – – – PBS 2–43 n/a n/a – [19]
25 100 – – PBS n/a 580-640 – – [27]
1, 2, 4, 8 0.1–6.4 – – – n/a n/a 3.1-247.5 – [28]
6, 7, 8, 9 – – – – n/a n/a 4-147 PEGylated [29]
fibrinogen,
polymerized
by photo-
initiator
using a UV
light
2–50 2–100 40 – – n/a 0.058– n/a – [39]
4000
2 1 2 0–20 HEPES n/a n/a 33-150 – [6]
23 mM
NaCl
175 mM pH 7.4
Ref.: References; E: Young’s modulus; n/a: Not available; PBS: Phosphate buffer saline; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
International Journal of Bioprinting (2020)–Volume 6, Issue 3 29