On the influence of surface coating on tissue biomechanics - effects on rat bones under routine conditions with implications for image-based deformation detection

Abstract

Background: Biomechanical testing using image-based deformation detection techniques such as digital image correlation (DIC) offer optical contactless methods for strain and displacement measurements of biological tissues. However, given the need of most samples to be speckled for image correlation using sprays, chemical alterations with impact on tissue mechanicals may result. The aim of this study was to assess the impact of such surface coating on the mechanical properties of rat bones, under routine laboratory conditions including multiple freeze-thaw cycles.

Methods: Two groups of rat bones, highly-uniform and mixed-effects, were assigned to six subgroups consisting of three types of surface coating (uncoated, commercially-available water- and solvent-based sprays) and two types of bone conditions (periosteum attached and removed). The mixed-effects group had undergone an additional freeze-thaw cycle at - 20 degrees. All bones underwent a three-point bending test ranging until material failure.

Results: Coating resulted in similar and non-significantly different mechanical properties of rat bones, indicated by elastic moduli, maximum force and bending stress. Scanning electron microscopy showed more pronounced mechanical alterations related to the additional freeze-thaw cycle, with fewer cracks being present in a bone from the highly-uniform group.

Conclusions: This study has concluded that surface coating with water- or solvent-based sprays for enhancing image correlation for DIC and having an additional freeze-thaw cycle do not significantly alter mechanical properties of rat bones. Therefore, this method may be recommended as an effective way of obtaining a speckled pattern.

Keywords: 3-point bending test; Biomechanical experiment; Chemical fixation; Digital image correlation; Tensile test.

Fig. 1

An overview of the experiment

Fig. 2

Top: coating sequence represented on a femur (note: the alignment of the femur was changed during the coating process for a homogenous distribution of speckles. The same spray stand was used for all three stages hence the inconsistent background). Bottom: setup three-point bending test, with two flexure fins and a plunger. Crosshead displacement was 10 mm/min. (^#note that the left side of the support roller had an additional rotational degree of freedom, allowing to adjust for the bones’ outer shape during the tests, as seen in the image)

Fig. 3

Bending-stress-bending-strain curve in comparison to displacements optically measured using a one-camera setup at different deformations during 3-point bending tests, reaching until material failure, of a water-based (WB) femora with periosteum removed (PR). Note that the fracture line forms distally of the site of the load application (right image)

Fig. 4

Graphs showing mean values with standard deviations for the highly-uniform group of rat femora and humeri undergoing three-point bending tests (one standardized snap freezing cycle). The significantly different result is indicated with * (p = 0.0354)

Fig. 5

Graphs showing mean values with standard deviations for the mixed-effects group of rat femora and humeri undergoing three-point bending tests (two freezing cycles)

Fig. 6

On the left (a, c), a femur from the mixed-effect group is shown with partial disruption in cortical bone integrity, indicated by crack-like cavities. On the right (b, d), a femur is depicted from the highly-uniform group showing fewer cracks, and on the edge of the bone. The asterisk (*) is to indicate the tape used to fix the bones. The bar on the bottom right corner is 1 mm for (a, b), 68 μm (c, d)