1. O'Leary, Stephen J., Rachael R. Richardson and Hugh J. McDermott. "Principles of design and biological approaches for improving the selectivity of cochlear implant electrodes." Journal of Neural Engineering (2009): 1-11. Acrylate and methacrylate monomers are polymerized by UV light after it passes through a photomask. The mask contains chrome strips which reflect away the light in regular intervals of varying periodicities. In order for the polymers to adhere to the glass slide, the slide must first be activated by attaching methacrylate moieties to its surface. The activation process involves plasma cleaning the slides, then soaking them in a solution containing a silicon based ingredient that has the desired methacrylate moieties covalently bonded to the silicon. The monomer is then exposed to light for a specified amount of time under a photomask. Once the polymer is prepared, auditory nerve cells from rat pups are placed on the polymer to observe the directed neurite growth. The efficacy of the neurite growth is then related to the depth and periodicity of the channels, and also the composition of the polymer. Photopolymerization of Micropatterned Polymers to Improve the Design of Cochlear Implants Scott P. White, Bradley W. Tuft, Dr. C. Allan Guymon, Joseph Clark, and Dr. Marlan Hansen Department of Chemical & Biochemical Engineering, University of Iowa, Iowa City, IA 52242, USA Motivation Cochlear implants are used to treat hearing loss in many patients suffering from damaged cochlea. The current design works well but could be improved by the discovery of a biocompatible material that allows for the regeneration of auditory neurites while simultaneously directing their growth. Photopolymers are an ideal candidate for the application due to their proven biocompatibility, versatility and mild energy requirements. The ideal topographical pattern and composition of photopolymers are investigated in this project to determine which material most efficiently controls the growth of peripheral neurites from auditory nerve cells. Materials A B C A: Hexyl Methacrylate (HMA) -Monomer B: 1,6 – Hexanediol dimethacrylate (HDDMA) - Monomer C: dimethoxy – phenylacetophenone (DMPA) - Photoinitiator Experimental Methods Theory Results Figure 2: The above graph shows the relationship between channel amplitude and exposure time under different photomasks. It shows the optimal behavior and the increasing maximum with wider periodicities. Figure 5: Directed growth of peripheral neurites from an explant of auditory nerve cells. Conclusions and Future Plans The spatial control of the polymerization reaction results in repeating channels on the surface of the photopolymer. The resulting photopolymers of methacrylate and acrylate monomers can effectively direct the growth of auditory nerve cell neurites with the topographical features being the main driving force for this growth. In the future, the dependence between neurite growth on channel depth and channel periodicity will be well characterized, initial results indicate that deeper, wider channels better direct the growth. Future experiments will determine the potential for free-standing photopolymers as a biocompatible material therefore allowing tests to be performed in vivo. The initial hypothesis is that they will work just as well as adhered polymers. AcknowledgementsReferences Acrylate and methacrylate molecules have commonly been used in biomaterials such as contact lens, dental fillings and bone cement. These monomers polymerize by the well understand mechanism known as chain-addition polymerization. The reaction is initiated by a free radical on the photoinitiator generated by UV light. The free radical is then transferred to the carbon-carbon double bond in the acrylate or methacrylate functional group which can in turn react with the same double bond functional group on another molecule (and so on). The polymerization process is dependent upon the intensity of UV light and the amount of time the solution is exposed to that light. In order to make regular patterns on the final polymer, the light is reflected away from parts of the monomer solution by a glass photomask with chrome strips. This procedure allows for the spatial control of the polymerization reaction and corresponding topographical structure. The portions that are directly exposed to light polymerize immediately while the other areas lag behind. This results in the formation of a pattern of channels on the surface of the polymer. The amplitude of the resulting channels depends on both the exposure time and the periodicity of the photomask. This periodicity is varied between 10 and 50 microns. As the solution is hit with UV light, the exposed portions build up to a maximum height which is followed by the polymerization of the unexposed portion. This behavior leads to the existence of a maximum amplitude versus exposure time at each periodicity. This maximum increases at larger periodicities since the polymer can build up higher when the ridges can expand wider. The resulting micro- patterned channels direct the growth of peripheral neurites by providing them with specified paths that offer little resistance. O2+O2+ O2+O2+ O2+O2+ O2+O2+ O2+O2+ O2+O2+ Dirty slide Plasma Clean slide in solution Activated slide Active Ingredient Methacrylate Moieties Figure 1: Image by interferometry of 0.77 um channels with a periodicity of 33.3 um. Composition of 40/60 HMA/HDDMA with DMPA photoinitiator UV Photomask Monomer Solution Activated Slide Micropatterned Polymer Activated Slide Figure 3: Uniformity and control of channel depth across a sample with a periodicity of 10 microns. Figure 4: Free standing patterned photopolymer used to direct the growth of peripheral neurites. I would like to acknowledge Bradley Tuft and Dr. C. Allan Guymon for their work on this project along with Joseph Clark and Dr. Marlan Hansen from the Otolaryngology department.