Organosilicon uptake by biological membranes

Abstract

Organosilicon compounds are ubiquitous in everyday use. Application of some of these compounds in food, cosmetics and pharmaceuticals is widespread on the assumption that these materials are not systemically absorbed. Here the interactions of various organosilicon compounds (simeticone, hexamethyldisilazane and polydimethylsiloxane) with cell membranes and models thereof were characterized with a range of analytical techniques, demonstrating that these compounds were retained in or on the cell membrane. The increasing application of organosilicon compounds as replacement of other plastics calls for a better awareness and understanding of these interactions. Moreover, with many developments in biotechnology relying on organosilicon materials, it becomes important to scrutinize the potential effect that silicone leaching may have on biological systems.

Fig. 1. Imaging spectroscopic analysis of individual cells.

a Normalized mean Raman spectra of: (1) 5 negative control cells (green), (2) 5 HMDS-dried cells (magenta), (3) neat (liquid) HMDS (black). b Raman images of HMDS-dried cells (top row) and negative control cells (bottom row) obtained by integration of the band between 450 and 550 cm⁻¹, the region in which the peak assigned to Si–C bonds is located. Raman images were acquired with 35 mW laser excitation power, 100 ms illumination time and 0.31-µm scanning step size. c AES/SEM inspection of the silicon (yellow) and carbon (green) content of cells incubated with silicones compared to non-incubated samples (negative controls). In the same locations, overlapping with cells as shown by SEM, both silicon and carbon are found (overlapping dilated pixels corresponding to both Si and C are indicated in red). The original AES spectra revealing also the presence of N 1s in all cells are provided in SI2.

Fig. 2. Spectroscopic analysis by XPS and IR of biological membranes in the form of SLBs and biological cells.

a XPS wide scan showing all atomic species present; carbon content (285 eV) increases after SLB formation and again after silicone introduction. The relative contribution of In 3d (444 eV) decreases after incubation steps. b XPS narrow scan in the Si 2p region, showing increase after silicone introduction. The presence of a trace amount of Si 2p in DOPC supported lipid bilayers (magenta) indicates a minor impurity of the chemicals. c Si-C region in IR spectra obtained from pure PDMS (pink) and cells incubated with various organosilicon compounds (green, HMDS; blue, PDMS; red, Infacol; black, negative control). Incubated cells show a shift in a peak (~1250 cm⁻¹ → ~1230 cm⁻¹) compared to negative control cells.

Fig. 3. Models proposed with different possible conformations for silicone oligomers and polymers in lipid membranes.

Embedding of the polymer within the membrane decreases the solvation energy but is balanced by the entropic cost of uncoiling the oligomer molecule.

Fig. 4. Overview of the sample preparation and characterization steps.

Cells and SLBs were first incubated with PDMS, Infacol, or HMDS on substrates conforming to the requirements of the intended characterization technique. The organosilicon compounds are presented as “red wavy lines” and their hypothetical interaction with membranes (vide infra) is here suggested. The excess of organosilicon compounds was removed before samples were characterized with various techniques.