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. 2019 Dec 24;25(1):75.
doi: 10.3390/molecules25010075.

The Effect of Calcium on the Cohesive Strength and Flexural Properties of Low-Methoxyl Pectin Biopolymers

Christine Byun  1 , Yifan Zheng  1 , Aidan Pierce  1 , Willi L Wagner  1   2 , Henrik V Scheller  3 , Debra Mohnen  4 , Maximilian Ackermann  5 , Steven J Mentzer  1
Affiliations

The Effect of Calcium on the Cohesive Strength and Flexural Properties of Low-Methoxyl Pectin Biopolymers

Christine Byun et al. Molecules. .

Abstract

Abstract: Pectin binds the mesothelial glycocalyx of visceral organs, suggesting its potential role as a mesothelial sealant. To assess the mechanical properties of pectin films, we compared pectin films with a less than 50% degree of methyl esterification (low-methoxyl pectin, LMP) to films with greater than 50% methyl esterification (high-methoxyl pectin, HMP). LMP and HMP polymers were prepared by step-wise dissolution and high-shear mixing. Both LMP and HMP films demonstrated a comparable clear appearance. Fracture mechanics demonstrated that the LMP films had a lower burst strength than HMP films at a variety of calcium concentrations and hydration states. The water content also influenced the extensibility of the LMP films with increased extensibility (probe distance) with an increasing water content. Similar to the burst strength, the extensibility of the LMP films was less than that of HMP films. Flexural properties, demonstrated with the 3-point bend test, showed that the force required to displace the LMP films increased with an increased calcium concentration (p < 0.01). Toughness, here reflecting deformability (ductility), was variable, but increased with an increased calcium concentration. Similarly, titrations of calcium concentrations demonstrated LMP films with a decreased cohesive strength and increased stiffness. We conclude that LMP films, particularly with the addition of calcium up to 10 mM concentrations, demonstrate lower strength and toughness than comparable HMP films. These physical properties suggest that HMP has superior physical properties to LMP for selected biomedical applications.

Keywords: fracture mechanics; hydration; material properties; methoxylation; polysaccharides.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Preparation of pectin polymers. (A) Powered pectin with low methyl esterification (less than 50%) was prepared in a high-shear mixer and poured into a mold for curing and subsequent analysis. (B) After curing to a 10% water content, the low-methoxyl pectin (LMP) films were grossly transparent. (C) With the addition of calcium, microscopic examination demonstrated an increased number of microaggregates with an increasing calcium concentration (Bar = 250 µm). When compared to pectin films with high methyl esterification, the thickness of the LMP was similar to high-methoxyl pectin (HMP) at a range of water contents (LMP R2 = 0.855; HMP R2 = 0.949) (D). The addition of 10 mM calcium to the LMP resulted in a slightly thicker film than the HMP film at a water content of 30 ± 2% (w/w) (p < 0.05) (E). Error bars = 1 SD.
Figure 2
Figure 2
Burst strength of low-methoxyl pectin (LMP) films. (A) To test the cohesive properties of the LMP films, a uniaxial constant velocity was applied normal to the plane of the film until rupture. In these experiments, a 5 mm stainless steel probe was used to assess the burst strength, extensibility, stiffness, and work of cohesion (WoC) (B).
Figure 3
Figure 3
Effect of calcium concentration and water content on the burst strength and extensibility of LMP films. (A) In films with identical degrees of methoxylation and a similar hydration state, the addition of calcium resulted in decreased burst strength. (B) The burst strength of the LMP films was generally less than that of HMP, regardless of the calcium concentration or water content. (C) Increasing the water content of LMP films (without calcium) resulted in a decreased burst strength, but increased extensibility. (D) Similar to burst strength, the extensibility of the HMP films was greater than that of LMP films, regardless of the calcium concentration or water content.
Figure 4
Figure 4
Flexural properties of LMP films. (A) To test the flexural properties of the LMP films, a 3-point custom bend fixture with 12 mm separation (double arrow) of the support blades was used. The upper blade was positioned equidistant from the support blades and was applied at constant velocity (2 mm/s) for a distance of 5 mm after film contact. (B) The applied force required for a 5 mm displacement was recorded in Area 1. The force recorded on blade withdrawal (storage modulus) was recorded in Area 2. Resilience was calculated as the ratio of Area 2/Area 1.
Figure 5
Figure 5
Effect of the calcium concentration on LMP film 3-point bend testing. (A) Representative 3-point bend tracings of LMP films with 0, 1, 5, and 10 mM calcium concentrations. (B) The peak force required for 5 mm blade displacement was significantly greater in LMP films with 5 and 10 mM calcium concentrations (p < 0.001). (C) Toughness, here measuring the resistance to deformation, was significantly greater in the 10 mM calcium films (p < 0.01). (D) The resilience, reflecting the ratio of the work of displacement and its corresponding stored energy, demonstrated that the resilience of 0 mM calcium films was significantly greater than that of the 10 mM calcium film (p < 0.01). Error bars = 1 SD.
Figure 6
Figure 6
Effect of the calcium concentration on the physical properties of LMP films. The LMP films were cured to a 15.7 ± 6.6% (w/w) water content; mean values of a minimum of 10 films are shown. (A) Burst strength of the films, reflecting the fracture resistance to a uniaxial load applied at a velocity of 0.5 mm/s (see Figure 3), decreased with an increasing calcium concentration (R2 = 0.938). (B) The spring constant, measured as the film displacement and applied force in the 3-point bend test, increased with an increasing calcium concentration (R2 = 0.9813). (C) Similarly, the stiffness, measured as the resistance to both the fracture probe and the 3-point bend test (see Figure 2 and Figure 4), increased with an increasing calcium concentration (R2 = 0.9427). (D) The bend threshold, measured as the probe distance at the yield point in a modified 3-point bend test, decreased with an increasing calcium concentration (R2 = 09229). Error bars = 1 SD.
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