Abstract
Initiated Chemical Vapor Deposition (iCVD) is a free-radical polymerization technique used to synthesize functional polymer thin films. In the context of drug delivery, the conformality of iCVD coatings and the variety of functional chemical moieties make them excellent materials for encapsulating pharmaceutics. Poly(4-aminostyrene) (PAS) belongs to a class of functionalizable materials, whose primary amine allows decoration of the delivery vehicles with biomolecules that enable targeted delivery or biocompatibility. Understanding kinetics of PAS polymerization in iCVD is crucial for such deployments because drug release kinetics in thin-film encapsulation have been shown to be determined by the film thickness. Nevertheless, the effects of deposition conditions on PAS growth kinetics have not been studied systematically. To bridge that knowledge gap, we report the kinetics of iCVD polymerization as a function of fractional saturation pressure of the monomer (i.e., Pm/Psat) in a dual-regime fashion, with quadratic dependence under low Pm/Psat and linear dependence under high Pm/Psat. We uncovered the critical Pm/Psat value of 0.2, around which the transition also occurs for many other iCVD monomers. Because existing theoretical models for the iCVD process cannot fully explain the dual-regime polymerization kinetics, we drew inspiration from solution-phase polymerization and proposed updated termination mechanisms that account for the transition between two regimes. The reported model builds upon existing iCVD theories and allows the synthesis of PAS thin films with precisely controlled growth rates, which has the potential to accelerate the deployment of iCVD PAS as a novel biomaterial in controlled and targeted drug delivery with designed pharmacokinetics.
Highlights
Polymers have been widely used as the encapsulation material to enable controlled release of pharmaceutics due to their desirable properties such as biocompatibility, mechanical flexibility, rich functional moieties, and cost-effective manufacturing (Khlyustova et al, 2020)
The kinetics of the initiated Chemical Vapor Deposition (iCVD) polymerization of AS were studied in great details
Upon increasing the fractional saturation pressure of the monomer, i.e., Pm/Psat, to values above 0.163 ± 0.008, the rate of polymerization/deposition depends linearly on the surface monomer concentration. The existence of these two regimes is consistent with the solution-based free radical polymerization, where the linear rate dependence applies to polymerization reactions terminated via disproportionation or combination and the quadratic rate dependence applies to those terminated by primary radical termination
Summary
Polymers have been widely used as the encapsulation material to enable controlled release of pharmaceutics due to their desirable properties such as biocompatibility, mechanical flexibility, rich functional moieties, and cost-effective manufacturing (Khlyustova et al, 2020). To avoid acutely toxic solvents such as dichloromethane, water and ethanol have been used in encapsulation of albendazole (used for treatment of echinococcosis). This approach is limited to water-soluble polymers such as polyvinyl alcohol, polyvinyl pyrrolidone, and hydroxypropyl methylcellulose (Alanazi et al, 2007), which commonly lead to high dissolution rates and thereby high and hard-to-control release rates in vivo (Ni et al, 2017). The versatile iCVD technique has been used to encapsulate highly soluble antibiotic gentamicin (Decandia et al, 2020), to enable the controlled release of phenytoin and indomethacin (Christian et al, 2018; Werzer et al, 2019), and to fabricate wound dressing that releases clotrimazole (Ghasemi-Mobarakeh et al, 2019). In the aforementioned study of gentamicin encapsulation, copolymers of MAA retained the antibacterial activity (Decandia et al, 2020)
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