Biomedical Engineering Reference
In-Depth Information
Collagen is the primary structure protein of the body found in the extracellular
space (usually in fibril form) as a major constituent of vertebrate connective tissue.
It also is found in blood vessel and organ walls, the cornea and vitreous humor,
basement membranes, and other epithelial and endothelial linings. Fundamental to
all electrochemical interactions with collagen and to our continuum models are the
physiochemical origin and location of charge sites on the molecules. In rat-tail tendon
collagen, there are about 350 acidic and 265 basic groups among the ~3,000 amino
acid residues per tropocollagen molecules (predominantly carboxyl (-COOH) and
amino (-NH
2
) groups, respectively). These groups are capable of ionizing as a
function of electrolyte pH and ionic strength and thereby leaving a net positive or
negative charge fixed to the molecule. Titration methods could be used to determine
the number of these fixed charges in the specimen.
Tanaka and colleagues (1980) of MIT used the Flory and Huggins theory (1953b)
of shrinking-swelling to further formulate the mean free energy needed for phase
transition of electrolytic gels. They used partially hydrolyzed acrylamide gel for
their experimental work. The solvent used was a mixture of acetone and water. The
polymer was found to be highly sensitive to temperature, solvent composition, pH,
added salt concentration, and applied electricity. The degree of volume change of
the gel was found to be 500%. This change is the result of phase transition of the
system of charged polymer network, counter-ions, and fluid composition.
Three major competing forces on the gel in turn cause the phase transition:
1.
positive osmotic pressure of counter-ions
2.
negative pressure due to polymer-polymer affinity
3.
rubber elasticity of the polymer network
Electric forces on the charged sites of the network produce a stress gradient along
the electric field lines in the gel. At the critical stress, the gel shrinks or swells
depending on whether stress developed is above or below critical stress.
In their experiment, they prepared polyacrylamide (PAM) gels by free-radical
polymerization of acrylamide (monomer), using N,N
′
-methylenbisacrylamide (as
cross-linker), ammonium persulfate, and N,N,N
-tetramethylene diamine (TEMED)
(as initiator)—all of which were dissolved in water. Gel formation initiated after 5
min and, to remove excess monomer, gel was immersed in water after 1 h. The gel
was then hydrolyzed in a solution of TEMED (1.2% solution, pH 12) for a period
of one month. Approximately 20% of acrylamide groups were converted to acrylic
acid groups, which in turn were ionized in water:
′
,N
′
-CONH
2
→
-COOH
→
-COO
-
+ H
+
(1.19)
After complete hydrolysis, the gel was immersed in 50% solution of ace-
tone-water to reach equilibrium before cutting into desired dimensions. To energize
the resulting gel, platinum electrodes were used with DC voltage of 0-5 V; the gel
was placed between electrodes. It is noted that PAM is an anionic type of polymer
gel, which means it is negatively charged. The gel reached its equilibrium shape in
one day. At 2.5-V DC the entire gel collapsed. By removal of the voltage, the gel
