Poster P6 - Natural aging of polydimethylsiloxane (PDMS): Change in PDMS hydrophobicity and stiffness at various mixing ratios and storage conditions

Introduction: Polydimethylsiloxane (PDMS) is an elastomeric polymeric rubber most commonly used in the manufacturing of microfluidic devices in applications including cell cultures, drug screening, drug delivery, DNA sequencing, clinical diagnosis, and chemical synthesis. It is also widely used in other biomedical engineering applications, such as catheter coatings, microvalves, dressings, and implants. Despite its common use, some of its characteristics are severely understudied. For instance, it is still not known whether and how mechanical and chemical properties of PDMS change naturally in ambient conditions, although PDMS is known to undergo induced aging at high temperatures, under UV radiation, in the presence of harsh chemicals, or through plasma treatment. Since changes in mechanical stiffness and surface hydrophobicity could potentially impact the behavior of cells cultured on PDMS substrates, there is a critical need to characterize how these properties change over time. In addition, no consensus has been made on the optimal storage conditions of PDMS. Therefore, in this study, we investigate the change of stiffness (for small strain ratios) and surface contact angle of PDMS at various mixing ratios in various storage environments at ambient conditions over time.

Materials and

Methods: PDMS (Sylgard 184) base and curing agent were mixed at 5:1, 10:1, 15:1, 20:1, and 30:1 ratios. Cast molding against unpatterned silicon wafers and baking at 90 °C for two hours were used to fabricate PDMS layers at each mixing ratio, and each PDMS layer was cut into 20 × 10 × 3 mm slices. Four slices from each mixing ratio were stored in each of six environments: directly exposed to air, stored in covered Petri dishes, tightly wrapped in plastic wraps, stored in a desiccator under vacuum, submerged in DI water, and submerged in mineral oil. Every two weeks, the contact angle of each sample was measured using an optical tensiometer (Biolin Scientific), and the Young’s modulus of each sample was measured using a mechanical testing system (Instron 5944 Universal Testing System) with a maximum strain ratio ε≤0.1. A two-tailed, two-sample t-test was used to compare change in the contact angle and Young’s modulus at each time point compared to the beginning of the study.

Results, Conclusions, and Discussions:

Results: Through the four weeks of the study, we found that there was a trend of increased contact angle and increased Young’s modulus of most PDMS samples. At the beginning of the study (Week 0), the contact angle increased as the mixing ratio increased from 5:1 (94.36°) to 30:1 (108.29°). However, the contact angles of the PDMS samples with lower mixing ratios increased more significantly. Overall, the contact angle increase over the four weeks was the most pronounced for samples stored in the desiccator and the Petri dish, with an average increase of 10.25° and 8.94°, respectively. On the other hand, the contact angle exhibited a very mild increase of 1.52° and 3.55° in DI water and plastic wraps, respectively. The Young’s modulus exhibited a mild increase of 9.5% and 10.7% for the 5:1 and 10:1 samples, respectively. For the 20:1 samples, however, the Young’s modulus experienced a significant increase of 37% (from 0.38 to 0.52 kPa). On average, samples exposed to air and stored in the Petri dish experienced the most pronounced increases of 28% and 24%, respectively, and the Young’s modulus of samples wrapped in plastic, stored in the desiccator, and stored in DI water experienced the least average increases of 9%, 19%, and 19%, respectively.
Discussion and

Conclusions: We have characterized the changes in surface hydrophobicity and mechanical stiffness of PDMS without subjecting PDMS to non-ambient, harsh treatments. We have found that PDMS experiences mechanical stiffening over the course of four weeks with the most pronounced effects on softer samples with higher mixing ratios. PDMS at lower mixing ratios, although exhibiting a less significant change in stiffness, exhibited a more hydrophobic surface over time. Over the four weeks of the study, we found plastic wraps and water to be preferable storage environments for PDMS due to the relatively small changes in surface hydrophobicity and mechanical stiffness resulting from these storage methods. Due to a lack of similar studies, potential reasons that have caused these changes are still unknown and are to be investigated in the future.

Acknowledgements (Optional): We thank Virginia Tech graduate student Mrigank Dhingra for his assistance with this project. This work was supported by the Virginia Tech Student Engineers' Council and the Interdisciplinary Graduate Education Program in Regenerative Medicine at Virginia Tech.