Biomedical Engineering Reference
In-Depth Information
2.6 Adhesion properties
Adhesion properties were measured using a 90 Degree Adhesive Peel Strength Test (Dimas
et al., 2000), adapted to the rapid evaluation of tubules from a single animal under different
experimental conditions. Individual expelled tubules, as above, were transferred to a wash
solution in a plastic trough containing a wash solution determined by the particular test (see
below). The numbers of samples tested in a given experiment are given in the results Tables;
in each case, tubules from a minimum of 3 separate animals were used. After 60 ± 2 sec, the
tubule was removed from the wash solution and allowed to drain for 5 sec. It was then laid
across the width of a 25 mm wide strip of substratum, selected for the particular test. The
tubule was allowed to adhere to the test substratum under its own weight for 60 ± 2 sec.
This differs from previous studies where a load was applied during adhesion (Flammang et
al., 2002). During the adhesion period the tubule was trimmed to leave <10 mm
overhanging one side of the substratum and about 50 mm on the other side. The flat width
of the tubule was measured and also recorded photographically, with a ruler placed
adjacent, for subsequent verification. At the end of the 60 sec adhesion period, each tubule-
substratum assembly was then transferred to a frame that allowed the substratum to be held
horizontally with the adhered tubule on the underside, i.e. with the free
c.
50 mm length of
tubule hanging below. The load was then increased stepwise (2.5 g/5 sec) to the overhang of
the tubule until the tubule-substratum adhesion failed by peeling. The total load at failure
was recorded. The maximum force tested was 0.2 N (approximately 20 g load) which
equated to about 0.05 N/mm for an average tubule, because higher loads typically took too
long to add and the tubule could have begun to desiccate at that stage, potentially changing
the adhesive strength. A minimum of six determinations was made for each test condition.
Data are presented as the total force at failure (N) divided by tubule width (mm). Although
we did not test values above 0.05 N/mm, our conservative approach did not hinder
examination of conditions that led to reduction of adhesive strength.
Experiments to test adhesion to different substrata used a wash solution of simulated sea-
water comprising 3.5% NaCl, 10 mM sodium phosphate buffer, pH 7.6. Various substrata
were tested, including clean glass (microscope slide), aluminium, polyvinyl chloride,
chitin (from crab), polycarbonate, poly(methyl methacrylate) (PMMA) and
polytetrafluoroethylene (PTFE), all cut to a similar size. As the chitin substratum lacked
stiffness, the samples were first glued with cyanoacrylate onto a glass microscope slide.
The chitin sample also had an irregular surface and was not uniform like the other
materials.
The effects on tubule-glass adhesion of various chloride or sodium salts (50 mM) were
examined by supplementing the 3.5% NaCl, 10 mM sodium phosphate buffer, pH 7.6, before
washing the tubules. The effect of NaCl concentration on tubule-glass adhesion was
examined using different NaCl concentrations in 10 mM sodium phosphate buffer, at final
pH 7.6. The effect of pH on tubule-glass adhesion was examined using solutions prepared
using three salts: Tris/chloride, sodium citrate and sodium acetate, each at 50 mM in 3.5%
NaCl, 10 mM sodium phosphate. Similarly, the effect of urea on tubule-glass adhesion was
examined using different urea concentrations in 3.5% NaCl, 10 mM sodium phosphate at a
final pH of 7.6. Glass was used as the standard substratum as it was readily available in
uniform quality, and had previously been shown to be an excellent material for adhesion of
H. forskåli
tubules (Flammang et al., 2002).
