B12-Mediated, Long Wavelength Photopolymerization of Hydrogels
Hydrogels are cross-linked networks of hydrophilic polymers whose softness and high water content mimic natural tissue. This makes them invaluable tools in tissue regeneration for the treatment of back pain, macular degeneration, cartilage tears, myocardial infarctions, or scaffolds for stem implantation. Clinicians prefer hydrogels that solidify and cross-link in vivo from low viscosity solutions in response to endogenous or applied stimuli, because they can arthroscopically inject these gels into patients in a non-invasive manner and mold them into shape without prior knowledge of the treatment location geometry. One method uses light that transmits through skin activating photoinitiators compounds to create gel forming species and cross-link polymers together. However, current photoinitiators requires ultraviolet or short visible light that poorly penetrates tissue due to absorption by bio-chromophores like melanin and hemoglobin. This limits this method to shallow treatment depths (< 5 mm) and pale skin types. As a solution, research has focused on finding initiators that respond to red and near infrared light that falls within tissue’s optical window (600 – 1000 nm) since this light penetrates tissue the most efficiently up to clinically relevant treatment depths (think of a flash light shine through your hand). Herein, we report the first realization of biocompatible, long wavelength photopolymerization by using Vitamin B12 derivatives modified with a red light absorbing dye that induce photopolymerization with tissue penetrating light. The dye serves as a light capturing antenna that transfers the absorbed red light energy to B12 to induce formation of gel forming species. We show that this new photoinitiator works in a range of skin types (pale to very dark) and has comparable biocompatibility to encapsulated cells to a ubiquitous, ultra-violet sensitive photoinitiator currently used in tissue engineering.
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