58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

The primary efforts of this granting period were focused on the synthesis of photoactive polymers that had unique properties of photoreconfigurability and photodegradation. Inititial efforts focused on the use of photocrosslinkable elastomers using simple, biocompatible monomers. The key innovation in the initial activities was the incorporation of cinnamic acid as a pendant group. These elastomers exhibited unique mechanical properties, biodegradability, and biocompatibility. The second thrust focused on the synthesis and characterization of a class of photodegradable hydrogels. These physically crosslinked hydrogels exhibit a novel photodegradation mechanism based on uncaging of photoactive pendant groups. Detailed descriptions of each thrust are shown below.

Photocrosslinkable Biodegradable Elastomers

Synthetic biodegradable elastomers are an emerging class of materials that play a critical role in supporting innovations in bioabsorbable medical implants. This work describes the synthesis and characterization of poly(glycerol-co-sebacate)-cinnamate (PGS-CinA), a biodegradable elastomer based on hyperbranched polyesters derivatized with pendant cinnamate groups. PGS-CinA can be prepared via photodimerization in the absence of photoinitiator using monomers that are found in common foods. The resulting network exhibits a Young’s modulus of 50.5 to 152.1 kPa and a projected in vitro degradation half-life time between 90 and 140 days. PGS-CinA elastomers are intrinsically cell adherent and support rapid proliferation of fibroblasts. Spreading and proliferation of fibroblasts are loosely governed by the substrate stiffness within the range of Young’s moduli in PGS-CinA networks that were prepared. The thermo-mechanical properties, biodegradability, and in vitro biocompatibility profiles suggest that PGS-CinA networks abroadly applicable for use in next generation bioabsorable materials including temporary medical devices and scaffolds for soft tissue engineering.

Photoreconfigurable Hydrogels

Photodegradable physically crosslinked polymer networks have been prepared from self-assembly of photolabile triblock copolymers. Linear triblock copolymers composed of poly(o-nitrobenzyl methacrylate) and poly(ethylene glycol) (PEG) segments of variable molecular weights were synthesized using atom transfer radical polymerization. Triblock polymers with low molecular weight PEG segments form solid films upon hydration with robust mechanical properties including a Young's modulus of 76 ± 12 MPa and a toughness of 108 ± 31 kJ m-3. Triblock polymers with high molecular weight PEG segments form physically crosslinked hydrogels at room temperature with a dynamic storage modulus of 13 ± 0.6 kPa and long-term stability in hydrated environments. Both networks undergo photodegradation upon irradiation with long wave UV light.

Summary and Outlook

The current grant period was deemed to be very productive. There are several new and exciting directions that this grant application has helped to fund. The next phase of the grant will continue to focus on photoreconfigurability of physically crosslinked hydrogel networks. Specifically, we are interested in conferring the ability to degrade hydrogel networks using light at tissue transparent near infrared wavelengths. These materials would be suitable for a wide range of applications including environmentally degradable materials and biomedical devices.