Minimizing Hydrophobic Recovery of Polydimethylsiloxane after Oxygen Plasma Treatment
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Polydimethylsiloxane (PDMS) is a popular silicon-based polymer in bioengineering due to its numerous desirable properties, including biocompatibility, optical transparency, chemical inertness, thermal stability, ease of molding, and low manufacturing cost. However, PDMS surface is inherently hydrophobic, which poses a significant challenge to its applications in the life sciences where most biological systems are aqueous-based. Oxygen plasma treatment is a fast and efficient method for transforming the hydrophobic PDMS surface to hydrophilic. Nonetheless, the effect of plasma treatment on PDMS is short-lived as hydrophobic recovery is observed within hours. The rapid hydrophobic recovery is commonly attributed to the migration of low molecular weight (LMW) species from the bulk to the surface. Since hydrophobic recovery was first described in 1990, an effective method to delay or prevent recovery remains elusive. This is partly because hydrophobic recovery is affected by a variety of experimental conditions during the preparation and treatment of PDMS. This project first established that the addition of LMW species sped up hydrophobic recovery. Next, from surveying the scientific literature, several experimental parameters that may contribute to hydrophobic recovery were identified. These parameters include sample thickness, curing conditions (temperature and time), vacuum treatment conditions (pressure and time), and plasma treatment conditions (time). Each parameter was varied and its effect on hydrophobic recovery was recorded. It was found that sample thickness and curing conditions had the most significant effects on hydrophobic recovery. Micrometer-thick samples showed only half the extent of recovery compared to millimeter-thick samples. Curing PDMS samples over several days at room temperature prior to plasma treatment also slowed down hydrophobic recovery and decreased the extent of recovery at equilibrium. Under the optimized conditions, our treated PDMS samples are comparable or superior to the best systems reported in the literature in terms of the lack of hydrophobic recovery.