Biomedical Engineering Reference
In-Depth Information
3.4 Adhesion characteristics
In the present study, a 90 Degree Peel Test was used to evaluate the adhesion of freshly
expelled Cuvierian tubules. This method was chosen as we had encountered problems when
tensile testing the H. dofleinii tubules following the approach used by Flammang and
colleagues (Flammang et al., 2002). Specifically, when H. dofleinii tubules were sandwiched
between two materials to which there was good adhesion, e.g. glass or metals, testing could
lead to strength values which reflected the structural failure of the Cuvierian tubule rather
than the failure of the adhesive, especially if some drying had occurred (data not shown).
This method would only allow the determination of a minimum value for the adhesive
strength as the latter exceeded the break strength of the tubule material itself.
The present test was suitable for rapidly examining numerous, freshly expelled samples,
thus allowing ready comparison between the effects of various treatment solutions. The
various treatments (i.e., incubations of tubules in the appropriate wash solutions) prior to
adhesive testing were rapid (1 min) as it appeared that the adhesion could decline if tubules
were left soaking for lengthy periods (data not shown). With H. forskåli , a lag period of about
60 min at 16 C was recorded before adhesion started to decline, decreasing to about 15 min
at 26 C (Müller et al., 1972). In another study (Flammang et al., 2002) a longer lag phase was
observed, and an initial increase in adhesive strength was reported. Yet others have
reported adhesive strength to fall after 20 min (Zahn et al., 1973). The present approach,
therefore, used short incubations in order to minimise time-based variations and to mimic
the timescale over which tubules would be required to act in the natural environment.
Previous studies (Flammang et al., 2002) have shown that a compressive force of 2-10 N
during adhesion led to a 6- to 8-fold increase in the resulting bond strength. In the present
case, no compressive load was added so as to better simulate the natural process of
ensnaring a predator.
Tubule widths showed little variation between individual samples, the average size being
4.0 mm. Tubules that were not fully expelled, and which therefore had a lesser diameter,
were discarded. The observed width is larger than that found for H. forskåli (Flammang et
al., 2002; Zahn et al., 1973) and H. leucospilota (Flammang et al., 2002), the species previously
studied in detail, and also larger than for H. impatiens and H. maculosa; these other species
generally have tubule diameters of 1-2 mm (Flammang et al., 2002). Although there are
many potential tubules within the body cavity (Figure 1A) H. dofleinii expels only a few,
typically 8 - 12 for organisms stimulated in the holding tanks compared with the more
numerous thin tubules expelled by H. leucospilota or H. forskali (Flammang et al., 2002).
Adhesive strength was also found to vary when different substrata were examined, all after
washing the tubules in 3.5% NaCl 10 mM Na/PO 4 , pH 7.6. There was a trend for strongest
adhesion to be observed with hydrophilic substrata, glass and aluminium (Table 2).
Adhesion to polycarbonate, PMMA, and PTFE was very poor; indeed, for PMMA and PTFE,
the load required for peel was barely more than the weight of the 50 mm of tubule
overhang. Intermediate adhesion values were observed with polyvinyl chloride and crab
chitin surfaces (Table 2). The chitin samples were unusual in having a textured surface
rather than a smooth one. Previously, Zahn, Flammang and colleagues had shown strong
adhesion to hydrophilic surfaces such as glass and stainless steel, and poor adhesion to
hydrophobic ones such as paraffin wax, polystyrene and polyethylene (Zhan et al., 1973;
Flammang et al, 2002). In general our results are consistent with this trend: the best adhesion
was observed with glass whilst the poorest was observed with PTFE.
Solution
Force/width
S.D.
n
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