Adaptive Quasi-Linear Viscoelastic Modeling - Computational Modeling in Tissue Engineering - page 65

Fig. 4 Reconstituted
collagen test specimen
mounted on the two parallel
bars of the testing apparatus
[ 55 ]. The upper bar was fixed
and connected to a force
transducer. The lower bar
was movable, with motion
controlled by a computer
to apply a specified
displacement strain function
4 Example of the Calibration and Evaluation of Nonlinear
Viscoelastic Models: Reconstituted Collagen
We present here complete examples of characterizing a bioartificial tissue using
quasi-linear viscoelastic methods. The example is the stress-strain relation of
reconstituted type I collagen gel specimens, used in our laboratories as a scaffold
for bioartifical tissues containing prescribed populations of cells [ 67 - 69 ].
A number of protocols for these tissue constructs can be found in the literature [ 70 , 71 ].
The specimens studied in this section were synthesized from 4 mg/ml rat-tail type I
collagen stock solution (Millipore, Inc., Billerica, MA) in 0.02 M acetic acid, with pH
brought to 7.4 using sodium hydroxide. The solution was poured into rectangular molds
and incubated at 37 C for 15 h as it gelled through non-covalent cross-linking of collagen
molecules. Final specimen dimensions were 30 mm long 9 10 mm wide 9 3mm
thick. Molds contained fabric attachments that were folded and stitched over plastic tubes
to facilitate attachment to testing bars (Fig. 4 ). The testing apparatus consisted of two
horizontal bars, one suspended from a force transducer and the other attached to a sliding
element controlled by a stepper motor through a micrometer. The micro-stepping driver
was programmable and allowed prescription of specific displacement functions.
4.1 Test Protocol
Constitutive models were fit to data from an incremental ramp-and-hold protocol.
Four tests were performed on each specimen, as described in [ 55 ]. Each ramp