Disregarded Free Chains Affect Bacterial Adhesion on Cross-Linked Polydimethylsiloxane Surfaces

Abstract

The surface properties exhibited by chemically cross-linked polydimethylsiloxanes (CPDMS) such as morphology, stiffness, and wettability have garnered great interest in the study of bacteria–material interactions. Nevertheless, the hidden factor of uncross-linked free PDMS chains that dissociate in CPDMS has often been overlooked when studying the biofilm formation on these polymeric elastomer surfaces. Here, we undertake a comparative characterization of the effects of free chains in CPDMS on bacterial adhesion to both flat and textured Sharklet CPDMS surfaces. Surprisingly, compared to unextracted surfaces, removing free chains from flat and textured CPDMS through solvent extraction results in a tremendous increase in bacterial colony-forming units for both Gram-negative and Gram-positive bacteria up to 2–3 orders in the initial adhesion stage of 2 h. These findings demonstrate that the solvent extraction of free chains from CPDMS is essential in studying the interactions between bacteria and silicone elastomer materials when focusing on a single variable.

Stiffness and Oligomer Content Affect the Initial Adhesion of Staphylococcus aureus to Polydimethylsiloxane Gels

“…As calculated via eq , the uncrosslinked oligomer content in the unextracted gels was ∼46, 33, 11, and 1% for the ultrasoft, soft, standard, and stiff gels (Figure and Table ). While the ultrasoft is being examined for the first time by our group, our findings align with previous studies, where the ethanol and hexane extraction processes determined the oligomer content to be 24 and 38% for soft gels (40:1), 8 and 3% for standard gels (10:1), and 5 and 3% for stiff gels (5:1).…”

“…These manuscripts all report different findings, possibly due to the different bacteria and microbiology analysis techniques used. Sun et al used a plate-counting process that removed bacterial cells from the PDMS with a saline solution prior to a serial dilution and redepositing the cells onto an agar plate for growth overnight. In contrast, we used fluorescence microscopy, which, after a light rinse to remove loosely adhered bacteria, immediately imaged and quantified the bacteria grown directly on the PDMS gels.…”

“…aureus DSM 20231 adhered similarly to gels with Young’s moduli ranging from 60 to 4520 kPa (measured using AFM nanoindentation) prepared from B:C ratios of 5:1, 10:1, 20:1, and 40:1. Recently, Sun et al, reported that more S. aureus ATCC 25923 (a strain commonly used for quality control tests for antibiotics) grew after exposure to extracted 10:1 PDMS gels more than unextracted PDMS gels.…”

“…Uncrosslinked oligomers are a concern because they can diffuse out of the PDMS gels over time and lead to negative biological effects, such as transport into mammalian cell membranes, protein transport interference, and cytotoxicity. ,,,,, The presence of uncrosslinked oligomers also impacts PDMS material properties, as greater uncrosslinked oligomer content increases the associated adhesion force and lowers the deformation resistance of PDMS gels. , Despite the breadth of studies demonstrating the biological and mechanical impacts of uncrosslinked oligomers, to the best of our knowledge, their effects on the bacterial adhesion to PDMS have only been assessed in one study that came online as we were submitting this manuscript. Sun et al tested the proliferation of S. aureus ATCC 25293 after exposure to the as-prepared 10:1 PDMS gels, as well as those that underwent a hexane extraction process.…”

Impact of Molecular Weight on Anti-Bioadhesion Efficiency of PDMS-Based Coatings

“…However, these values were in good accordance with previous studies, as elastic moduli for fouling release coatings are typically ranged between 0.2 and 9 MPa [3,[23][24][25]. In their recent study, similar results were observed by Sun et al [26]. They observed that when the crosslinking density of the polymeric network of PDMS decreases, Young's Coatings 2024, 14, 149 9 of 15 consequently decreases from 8.90 to 2.99 MPa.…”