Changes in chemical and ultrastructural composition of ameroid constrictors following in vitro expansion

Abstract

Objective: To (1) characterise the chemical and ultra-structural composition of ameroid constrictors, at a native state and during in vitro expansion and (2) determine the presence of irritant compounds at the surface or within the bulk of the constrictor.

Methods: Twelve sterile, commercially packaged ameroid constrictors (3 repeats of 3.5 mm, 5 mm, 6 mm and 7 mm internal diameter) were analysed by time-of-flight secondary ion mass spectrometry, Raman spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy and scanning electron microscopy.

Results: Ameroid constrictors have a composition commensurate with casein with little-to-no intra- or inter- constrictor variation. Microscopic analysis indicated that the topographical features of the constrictor surfaces were consistent between all constrictors. Following in vitro expansion there was a reproducible decrease in Ca+ ion content, little-to-no variation in secondary protein structure and morphological changes including the presence of surface aggregates present only at the inner surface of the ameroid constrictor. The potential irritant polydimethylsiloxane was found on the constrictor surface. A trace quantity of an ion fragment assigned as formaldehyde was detected; however, the extremely low level is thought highly unlikely to play a role as an inflammatory trigger clinically.

Discussion: There is a high degree of inter- and intra-constrictor homogeneity from different batches, and reproducible ultrastructural changes following in vitro expansion. Variations occur in both the surface chemistry and topography of the device during closure, which can potentially affect the biomaterial-host interface. Ameroid constrictor closure mechanism is likely involving calcium-mediated inter-protein interactions rather than the imbibition of water only.

Fig 1

ToF-SIMS depth analysis: (a) positive and (b) negative polarity profiles of the native 5 mm ARC; (c) positive and (d) negative polarity profiles of the 5 mm ARC subsequent to exposure to canine plasma. It is important to note that the dominant ion in the positive polarity profile of all the ARCs was Na⁺ (m/z = 22.99). However, this has been deliberately excluded from the corresponding profiles to allow visualisation of the other important mass fragments, which would be scaled to suppression if included. The variation of Na⁺ intensity with depth was seen to be directly analogous to the trend observed for NaO₃H⁺, and this has therefore been used as a surrogate.

Fig 2

Ca⁺ profiles for ARCs: (a) 3.5 mm, (b) 5 mm, (c) 6 mm and (d) 7 mm. For all constrictors in the native state calcium was a dominant cation associated with the bulk of the constrictor. Following plasma immersion there is an overall depletion from the bulk from all constrictors with an enrichment at the subsurface, indicating movement from the bulk to the superficial layers.

Fig 3

Raman spectra of the different-sized ARCs (a) 3.5 mm, (b) 5 mm, (c) 6 mm and (d) 7 mm, measured in triplicate to evaluate the homogeneity of the constrictors, in the native state and post-immersion in canine plasma. Spectra have been shifted on the vertical axis for clarity.

Fig 4

Scanning electron microscopic images of representative 5 mm ARC sections: (a) Native ARC at 40x magnification, (b) Post-Immersion ARC at 40x magnification, (c) Native ARC at 2000x magnification and (d) Post-Immersion ARC at 2000x magnification.

Fig 5

ATR-FTIR spectra of the 5 mm ARC in the Native state (A, B) and Post-Immersion (C, D) in canine plasma.